Experimental and modeling study of SO2 behavior during oxy combustion in fluidized beds

SO2 emissions from coal combustion have several harmful effects on the environment, including their contribution to acid rain. A variety of approaches have been developed to control SO2 pollution. Direct dry sorbent (limestone) injection is a relatively simple and low-cost process. Fluidized beds have been one of the most popular furnaces for the application of direct sorbent injection. In addition, oxy fuel combustion is a promising, practical method to reduce greenhouse gas emissions. Thus, studies on the mechanisms related to the production of SO2 emissions under oxy fuel conditions are important. The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. Direct sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an indirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually , in fluidized bed, in fluidized bed, in fluidized bed , in fluidized bed, in fluidized bed, in fluidized bed , in fluidized bed, in fluidized bed, in fluidized bed, in fluidized bed, in fluidized bed , in fluidized bed, in fluidized bed, in fluidized bed combustion combustion combustion combustion combustion combustion combustion combustion combustion conditions,conditions,conditions,conditions,conditions, conditions,conditions,conditions,conditions, direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in oxyoxyoxy fuel combustion due to COfuel combustion due to COfuel combustion due to CO fuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to CO fuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to CO fuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to CO2 inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. A bench-scale bubbling fluidized bed (BFB) and a 330 KW pilot-scale circulating fluidized bed (CFB) were used to investigate SO2 behavior in air and oxy coal combustion. SO2 release with and without limestone sorbent were investigated in N2/O2 and CO2/O2 environments for the following conditions: temperature range (765 902℃), O2 concentration range (10 30%), and a wide range of Ca/S ratios. The bench-scale experiments without recycled flue gas (RFG) iv and the equilibrium calculations of NASA Chemical Equilibrium with Applications (CEA) show no effect of combustion diluent (N2, CO2) on SO2 emissions. Limestone addition shows greater SO2 capture efficiency in air firing than that in oxy firing. In addition, In addition, In addition, In addition, In addition, In addition, In addition, In addition, In addition, In addition, sulfation behavior of limestone in N2/O2/SO2 and CO2/O2/SO2 atmospheres was studied, exploring the mechanisms of indirect and direct sulfation in a wide range of temperatures (765 874℃). A significant temperature effect on sulfation behavior of limestone was seen during direct sulfation reactions. However, limited effect was seen for an indirect sulfation reaction. A scanning electron microscope (SEM) and energy dispersive x-ray spectrometer (EDS) were used to exam the microstructure of sulfated limestone and their sulfur distribution. A mathematical and computational framework for a single particle model was developed to understand the differences between indirect and direct sulfation and single coal particle combustion processes. The model framework presented here, however, provides a broader flexibility that could be used to address a range of particle sizes. The shrinking core particle model and fine single particle model are two specific applications of my general single particle model.

EXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SOEXPERIMENTAL AND MODELING STUDY OF SOEXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SOEXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SOEXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SOEXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SO EXPERIMENTAL AND MODELING STUDY OF SOEXPERIMENTAL AND MODELING STUDY OF SO 2 BEHAVIOBEHAVIO BEHAVIO BEHAVIOR DURING URING URING URING URING OXY COMBUSTIONOXY COMBUSTION OXY COMBUSTIONOXY COMBUSTION OXY COMBUSTION OXY COMBUSTIONOXY COMBUSTION IN FLUIDIZIN FLUIDIZIN FLUIDIZIN FLUIDIZ IN FLUIDIZ IN FLUIDIZ IN FLUIDIZED BEDS ED BEDSED BEDS by Liyong Wang A dissertation submitted to the faculty of The University of Utah in partial fulfillment of the requirements for the degree of Doctor of Philosophy Department of Chemical Engineering The University of Utah December 2012Copyright © Liyong Wang 2012 All Rights Reserved The University of Utah Graduate School STATEMENT OF DISSERTATION APPROVAL The dissertation of Liyong Wang has been approved by the following supervisory committee members: Eric G. Eddings , Chair 08/16/2012 Date Approved Jost O.L. Wendt , Member 08/16//2012 Date Approved JoAnn Lighty , Member 08/16//2012 Date Approved Geoffrey Silcox , Member 08/16//2012 Date Approved Lawrence E. Bool III , Member 08/16/2012 Date Approved and by JoAnn Lighty , Chair of the Department of Chemical Engineering and by Charles A. Wight, Dean of The Graduate School. ABSTRACT SO2 emissions from coal combustion have several harmful effects on the environment, including their contribution to acid rain. A variety of approaches have been developed to control SO2 pollution. Direct dry sorbent (limestone) injection is a relatively simple and low-cost process. Fluidized beds have been one of the most popular furnaces for the application of direct sorbent injection. In addition, oxy fuel combustion is a promising, practical method to reduce greenhouse gas emissions. Thus, studies on the mechanisms related to the production of SO2 emissions under oxy fuel conditions are important. The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether The sulfation mechanisms (direct or indirect) of limestone depend on whether the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. the limestone is calcined. Direct sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an irect sulfation takes place in an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an uncalcined state while an indirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usually indirect sulfation happens with calcined limestone. Usuallyindirect sulfation happens with calcined limestone. Usually , in fluidized bed, in fluidized bed, in fluidized bed , in fluidized bed, in fluidized bed, in fluidized bed , in fluidized bed, in fluidized bed, in fluidized bed, in fluidized bed, in fluidized bed , in fluidized bed, in fluidized bed, in fluidized bed combustion combustion combustion combustion combustion combustion combustion combustion combustion conditions,conditions,conditions,conditions,conditions, conditions,conditions,conditions,conditions, direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in direct sulfation occurs in oxyoxyoxy fuel combustion due to COfuel combustion due to COfuel combustion due to CO fuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to CO fuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to CO fuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to COfuel combustion due to CO2 inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and inhibition, and indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. indirect sulfation occurs in air combustion. A bench-scale bubbling fluidized bed (BFB) and a 330 KW pilot-scale circulating fluidized bed (CFB) were used to investigate SO2 behavior in air and oxy coal combustion. SO2 release with and without limestone sorbent were investigated in N2/O2 and CO2/O2 environments for the following conditions: temperature range (765 902℃), O2 concentration range (10 30%), and a wide range of Ca/S ratios. The bench-scale experiments without recycled flue gas (RFG) iv and the equilibrium calculations of NASA Chemical Equilibrium with Applications (CEA) show no effect of combustion diluent (N2, CO2) on SO2 emissions. Limestone addition shows greater SO2 capture efficiency in air firing than that in oxy firing. In addition, In addition, In addition, In addition, In addition, In addition, In addition, In addition, In addition, In addition, sulfation behavior of limestone in N2/O2/SO2 and CO2/O2/SO2 atmospheres was studied, exploring the mechanisms of indirect and direct sulfation in a wide range of temperatures (765 874℃). A significant temperature effect on sulfation behavior of limestone was seen during direct sulfation reactions. However, limited effect was seen for an indirect sulfation reaction. A scanning electron microscope (SEM) and energy dispersive x-ray spectrometer (EDS) were used to exam the microstructure of sulfated limestone and their sulfur distribution. A mathematical and computational framework for a single particle model was developed to understand the differences between indirect and direct sulfation and single coal particle combustion processes. The model framework presented here, however, provides a broader flexibility that could be used to address a range of particle sizes. The shrinking core particle model and fine single particle model are two specific applications of my general single particle model. CONTENTSCONTENTS CONTENTSCONTENTS ABSTRACT ······················································································································ iii LIST OF TABLES ············································································································ viii ACKNOWLEDGEMENTS ·································································································· x 1. INTRODUCTION ············································································································· 1 1.1. Carbon capture and storage (CCS) ·········································································· 1 1.2. Precombustion CO2 capture and postcombustion CO2 capture ····························· 3 1.2.1. Gas srubbing ········································································································· 3 1.2.2. Integrated gasification combined cycle ································································ 4 1.2.3. Oxy fuel combustion ····························································································· 5 1.2.4. CCS in postcombustion: carbonate looping ·························································· 8 1.3. Fluidized bed technology ······················································································ 10 1.4. SO2 emissions ········································································································ 15 1.4.1. SO2 formation mechanism during oxy and air firing combustion ······················ 16 1.4.2. The mechanisms of SO2 removal by limestone ················································· 19 1.5. The application of a single particle model ···························································· 24 1.5.1. Shrinking core particle model and grains model ··············································· 25 1.5.2. Single coal/char particle combustion model ······················································ 26 1.5.3. SO2/CaCO3/CaO particle model ·········································································· 27 2. EXPERIMENTAL APPARATUS ······················································································· 33 2.1. Bench-scale bubbling fluidized bed (BFB) ····························································· 33 2.1.1. Bed materials ······································································································ 34 2.1.2. Electrical heating system ···················································································· 35 2.1.3. Gas mixture system: mass flow controller ························································· 36 2.2. Pilot-scale circulating fluidized bed (CFB) ····························································· 36 2.3. Gas measurement system ···················································································· 37 2.3.1. Fourier transform infrared spectroscopy (FTIR) ················································· 38 2.4. Thermo gravimetric analyzer (TGA Q600) ···························································· 40 2.5. Scanning electron microscope (SEM) ··································································· 40 3. SO2 EMISSIONS IN OXY VS AIR COAL COMBUSTION ··················································· 45 vi 3.1. Technical approach ······························································································· 45 3.2. Materials and methods ························································································· 47 3.2.1. Experimental installation ···················································································· 47 3.2.2. Preparation of single coal particles ···································································· 49 3.2.3. Design of experimental matrix··········································································· 49 3.2.4. Equilibrium model: CEA programs ······································································ 50 3.3. Results and discussion ·························································································· 50 3.3.1. SO2 emission in bench-scale BFB ········································································ 51 3.3.2. SO2 emission in a pilot-scale CFB ········································································ 53 3.3.3. Comparison of SO2 emission between BFB and CFB with RFG ··························· 54 3.3.4. SO2 emission estimated by an equilibrium model ·············································· 54 3.4. Summary ··············································································································· 55 4. SO2 REMOVAL BY LIMESTONE DURING COAL COMBUSTION ····································· 68 4.1. Objectives·············································································································· 69 4.2. Materials and methods ························································································· 69 4.2.1. Experimental installation ···················································································· 69 4.2.2. Preparation of limestone particles ····································································· 70 4.2.3. Design of experimental matrix ··········································································· 70 4.3. Results and discussion ·························································································· 71 4.3.1. SO2 removal by limestone during oxy and air firing combustion ······················· 71 4.3.2. With and without limestone ··············································································· 72 4.3.3. SO2 removal by limestone in an oxy firing CFB ··················································· 72 4.4. Summary ··············································································································· 74 5. INDIRECT SULFATION AND DIRECT SULFATION ·························································· 84 5.1. Introduction ·········································································································· 84 5.2. Objectives·············································································································· 86 5.3. Materials and methods ························································································· 87 5.3.1. Experimental installation ···················································································· 88 5.3.2. Design of experimental matrix ··········································································· 89 5.4. Results and discussion ·························································································· 90 5.4.1. Preliminary experiment: calcination and carbonation ······································· 90 5.4.2. Indirect sulfation and direct sulfation in TGA ····················································· 93 5.4.3. Mechanism Identification by microstructure ····················································· 95 5.5. Summary ··············································································································· 98 6. DEVELOPMENT AND APPLICATION OF A SINGLE PARTICLE MODEL ························ 112 6.1. Introduction ········································································································ 112 6.2. Objectives············································································································ 113 6.3. Single particle model methodology ···································································· 114 vii 6.3.1. Classification of particle models ······································································· 115 6.3.2. Single coal particle combustion model ····························································· 116 6.3.3. SO2 limestone particle model ··········································································· 122 6.4. Results and discussion ························································································ 124 6.4.1. Multigrains model in single char particle combustion ····································· 124 6.4.2. SO2 limestone model ························································································ 126 6.5. Summary ············································································································· 130 7. SUMMARY AND CONCLUSION ·················································································· 138 7.1. Conclusion ··········································································································· 138 7.2. Recommendations for future research ······························································ 139 APPENDICES A: CALIBRATION CURVES FOR FTIR ··········································································· 141 B: MATLAB SULFATION MODEL SCRIPT ···································································· 147 REFERENCES ·············································································································· 157 LIST OF TABLES 1. Estimated storage capacities and retention times for CO2 in different types of sinks (adapted from reference [5]) ....................................................................... 28 2. Comparison of main characteristics of CCS √[6] ................................................ 28 3. Design parameters for BFB and CFB (adapted from reference [51]) ................... 28 4. Rate expression and rate constant of direct sulfation reaction (adapted from reference [128]) .................................................................................................... 29 5. Technical properties of zirconium silicate (ZrSiO4) ceramic beads (BSLZ-3) ........ 42 6. Scan numbers, resolution, and collection time .................................................... 42 7. Ultimate and proximate analyses of Illinois #6 coals (wt.%). ............................... 57 8. Analysis of ash from Illinois #6 coal (wt.%). .......................................................... 57 9. Experimental matrix for SO2 formation in air vs. oxy combustion ....................... 57 10. Simulation matrix for SO2 formation in air vs. oxy firing ...................................... 57 11. Experimental matrix for SO2 formation in air vs. oxy combustion ....................... 75 12. Experimental matrix for known SO Experimental matrix for known SO Experimental matrix for known SO Experimental matrix for known SOExperimental matrix for known SO Experimental matrix for known SOExperimental matrix for known SO Experimental matrix for known SOExperimental matrix for known SOExperimental matrix for known SO Experimental matrix for known SOExperimental matrix for known SOExperimental matrix for known SOExperimental matrix for known SO Experimental matrix for known SOExperimental matrix for known SOExperimental matrix for known SOExperimental matrix for known SOExperimental matrix for known SOExperimental matrix for known SOExperimental matrix for known SO 2 concentration in Nconcentration in Nconcentration in Nconcentration in Nconcentration in N concentration in Nconcentration in Nconcentration in Nconcentration in Nconcentration in Nconcentration in Nconcentration in Nconcentration in Nconcentration in N concentration in Nconcentration in N2/CO/CO/CO2 with limestone.with limestone. with limestone.with limestone.with limestone. with limestone.with limestone.with limestone.with limestone. .. 100 13. The conditions of equilibrium calculation for CaCO3-CaO .................................. 100 14. The results of equilibrium calculation for CaCO3-CaO ........................................ 100 15. The parameters of equilibrium calculation for CaCO3/CaO-SO2 ........................ 101 16. Nomenclature for single char particle combustion model ................................. 131 ix 17. Nomenclature for sulfation model ..................................................................... 132 18. The parameters in the sulfation model .............................................................. 132 ACKNOWLEDGEMENTSACKNOWLEDGEMENTS ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS ACKNOWLEDGEMENTS As with any accomplishment, this work is truly the result of the efforts of many people. The author wishes to thank the many people who have lent their time and talents. First and foremost, I would like to extend my sincere appreciation to my advisor, Dr. Eric G. Eddings, for his continuous support, kindness, endless patience, invaluable guidance, and inspiration. I wish to express my appreciation to my committee members for serving on my committee and for their insightful comments and critiques of my work. Thanks to Dana Overacker and Ryan Okerlund for all their advice, guidance, and help. I am very grateful to Dr. Hongzhi Zhang, Dr. Daniel Sweeney, Dr. Charles Reid, Dr. Randy Pummill, , and Keith Gneshin for all their help. And finally, great appreciation to my wife, Dr. Xiaoxia Jin, for her deep love, great patience, full understanding, and steadfast support through all these years. You are the most important person in my life. This work was supported by the US Department of Energy (DE-NT0005015) and Praxair. CHAPTER 1 INTRODUCTIONINTRODUCTION INTRODUCTION INTRODUCTION INTRODUCTION T a g mp a u of Ea 's atmosphere, land, and oceans (global warming) leads to serious consequences: for example, changes in precipitation patterns. It also results in a rising sea level, the expansion of subtropical deserts, and the retreat of glaciers, etc. Global warming also brings extreme-weather occurrences, i.e., droughts, heavy rainfall, heat waves, species extinctions, and reduction of crop yields. Since the early 20th century, the average global surface temperature has been raised by 0.8 °C, two thirds of which have been gained since 1980. Many believe that global warming is the result of human activities, i.e., the increasing greenhouse gas emissions, especially CO2, which was released from burning fossil fuel, compounded by deforestation around the world. According to the US Environmental Protection Agency (EPA), the concentrations of CO2 and CH4 have been raised by 36% and 148% since 1750 [1], respectively. . 1.1. Carbon capture and storage (CCS) In order to reduce the effect of human activities on global warming, scientists have proposed various approaches to control greenhouse gases emissions, especially CO2, into the atmosphere from fossil fuel utilization in power plants. Carbon capture and 2 storage (CCS) is one of the most effective approaches to prevent greenhouse gas emissions from entering the carbon cycle. A CCS approach usually includes two consecutive cycles: i. CO2 capture: Separation of CO2 from the flue gas stream ii. CO2 storage: It is often called carbon sequestration A few approaches have been proposed to capture CO2 emissions from fossil fuel power plants, including postcombustion capture (PCC), oxy fuel combustion (OFC), integrated gasification combined cycle (IGCC), and chemical looping combustion (CLP). The costs of CO2 separation, capture, and compression before injection into a sink account for about 75% of an entire geologic sequestration process [2]. Following the capture and scrubbing, CO2 is injected into a geological formation for long-term storage. The CO2 storage locations are usually either in deep geological formations (deep ocean) or in the form of mineral carbonates. There are two types of CO2 disposal [3] based on the length of time period. Sequestration means permanent disposal, while storage is defined as a significant time period. Possible sinks include gas reservoirs, depleted oil fields, coal beds, deep ocean, and deep saline aquifers [3, 4]. Table 1 shows storage capacity and retention time of various disposal sites [5]. Deep ocean disposal seems to be less attractive due to the limitation in retention times, acidification of oceans, and the great risk of CO2 escape from the site. Deep aquifers are considered to be the most promising sequestration locations. However, the risk of CO2 leak from the mineral carbonate to the atmosphere is still a challenging area in the research and development. 3 1.2. Precombustion CO2 capture and postcombustion CO2 capture Currently, there are several broad approaches to capture CO2 emissions, including precombustion capture, postcombustion capture, oxy fuel combustion (OFC), and chemical looping combustion (CLC) [6]. i. Postcombustion capture (PCC): CO2 is captured from coal firing power plants by scrubbing of the flue gas. ii. Precombustion capture: for example, the integrated gasification combined cycle (IGCC) converts coal into synthesis gas and enhances H2 content using the water-gas shift reaction. CO2 is captured after syngas production. iii. Oxy fuel combustion (OFC): This approach uses pure O2 rather than air during the combustion. Often, O2 is diluted by recycled flue gas (RFG) to avoid overheating. Relatively pure CO2 stream is captured with no gas separation. iv. Chemical looping combustion (CLC): metal oxide is employed as an oxygen carrier material to separate O2 from air using a dual fluidized beds system. Table 2 compares main characteristics of the CCS technologies [6]. PPC and OFC are feasible to retrofit current coal firing power plants. IGCC and PCC have been applied to partial capturing of CO2 from flue gas emissions. 1.2.1. Gas scrubbing Figure 1 illustrates a precombustion capture. Retrofit to existing coal firing power plants using PCC requires no significant modifications to the original plant [7, 8] . 4 As long as there is space available, the CO2 capture unit can be added downstream of the boiler. Prior to the CO2 capture unit, particulate matter, NOx, and SOx in the flue gas are required to be removed using electron spin resonance (ESP), selective catalytic reduction (SCR), and flue gas desulfurization (FGD), respectively. A CO2 capture unit usually includes an absorber and a regenerator. In the absorber, CO2 is captured by a chemically active agent, such as an amine-based compound (e.g., monoethanolamine (MEA), or methyldiethanolamine (MDEA)). At the same time, the captured CO2 is released from the chemically active agent in the regenerator, where the chemically reactive agent is regenerated. Although using amine-based chemically reactive agents is a proven technology [9, 10], its cost is not cheap. Recently, limestone was proposed to be a chemically active agent due to its relative lower cost and lower energy penalty. It is introduced as carbonate looping, which has a great potential. However, carbonate looping demands a higher standard of impurity clean up (i.e., particulate matter, mercury, SOx, NOx) prior to the CO2 capture, since impurities will significantly deactivate chemically active agents. 1.2.2. Integrated gasification combined cycle Figure 2 shows how IGCC works. In an IGCC plant, synthesis gas (CO, CO2, and H2) is generated from gasification of coal. CO is converted into CO2 by the water-gas shift conversion reaction. CO2 thus can be removed prior to combustion, while H2 is injected into a combustor. At the same time, impurities are removed from the synthesis gas prior to combustion. Alternatively, H2 can be separated from synthesis gas, followed by 5 burning CO in a CO2/O2 atmosphere [11]. IGCC leads to lower pollution emissions (SOx emissions, particulate matter, and mercury) [12]. Excess heat is then passed to a steam cycle, similar to a omb y l ga u b Ba o om a ' al a economic calculations [9, 13-15], IGCC has a high efficiency and is financially feasible [12]. However, IGCC requires high upfront capital expenses. In addition, IGCC-CCS is not a viable option for retrofit of existing coal firing power plants. Since the first plant was established in 1984, only a few IGCCs have been built for electricity generation, none of which has applied CCS. In the US, the Department of Energy (DOE) clean coal demonstration project helped to set up three IGCC power plants: Wabash River Power Station in West Terre Haute (IN), Polk Power Station in Tampa (FL), and Pinon Pine in Reno (NV) l o, Pola ' Kę y w ll oon build a zero-emission power and chemical plant that combines gasification technology with CCS [16]. Industrial applications have been accumulating rapidly, although IGCC is still in the early stages of commercialization. 1.2.3. Oxy fuel combustion Many researchers have studied oxygen fuel combustion (OFC) [17-35], which is described in Figure 3. In OFC, a fuel is burned with pure O2, instead of air as the primary oxidant. The temperature will be excessively hot using pure O2. In order to avoid overheating, the mixture of recycled flue gas (RFG) and pure O2 is used to control flame temperature. The flame temperature is reduced to that in the conventional combustion. 6 Typically, 60-80% of the flue gas is recycled [5, 6, 21, 24]. The CO2 concentration in the flue gas can reach 95%, and can be captured directly [5, 6, 24, 31]. During oxy fuel combustion, CO2 and H2O in RFG replace N2 in the air. CO2 and H2O are termolecular gases, which present significant differences in heat transfer. Termolecular gases absorb and emit radiation much more than dimolecular nonpolar molecules, and lead to higher heat capacities, resulting in a lower temperature in the convective section of the boiler. Buhre et al. [24] described the difference between OFC with RFG and conventional air combustion: i. To attain a similar adiabatic flame temperature, the O2 concentration of the injection gases is typically 26-30%, higher than that (21%) in air combustion. ii. The volume of gases passing through the boiler is reduced by about 80%. iii. Concentrations of gaseous species (i.e., SOx, water vapor) are higher, compared to air firing conditions. iv. Oxy fuel combustion combined with CO2 sequestration must provide power to several new units, such as flue gas compression. These compressors will result in an energy penalty. In addition, pure O2 separation from N2 requires excessive energy. Buhre et al. [24] and Toftegaard et al. [5] in a more recent paper reviewed the effect of oxy fuel combustion on gaseous pollutant formation and emissions. Buhre et al. concluded that the SOx emissions per ton of coal combusted is significantly unchanged [24]. Toftegaard et al. [5] pointed out a lot of contradictory observations on SOx during air combustion and oxy fuel combustion . 7 Both reviews agreed that NOx emissions per unit of energy are reduced during OFC. There are three main NOx formation paths, as shown in Figure 4: thermal, prompt, and fuel [36-38]. i. Thermal: N2 can react with O2 at above 1873 K to form NO; the three principal reactions are well known as the extended Zeldovich. ii. Prompt: Prompt NO tends to be formed in the fuel-rich zone. In addition, prompt NO mechanism can happen at lower temperatures. iii. Fuel: The fuel NO mainly comes from either volatile-N or char-N. The first principal path is the oxidation of volatile-N species during the initial combustion. N released from the volatile will be decomposed into cyanide and amine species, which can form N2 or NO, depending on reaction conditions. Volatile-N is oxidized into NO with oxidants. In comparison, volatile-N tends to form N2 under a reducing atmosphere. The second path involves char-N, whose reaction is much slower than that of volatile-N. Only around 20% of the char-N eventually leads to NO, since 80% of the NOx produced is reduced to N2 by the char. In conventional air combustion, about 20% of NOx comes from thermal NOx, and 80-100% of NOx from fuel-N. NOx from the prompt mechanism is negligible. It has been proved that OFC reduces NOx emissions [28, 30, 31, 39-41]. The reduction of NOx emissions has been a key motivation to develop OFC-CCS technology. Buhre et al. summarized possible reasons for the reduced NOx emission from OFC [24], and pointed out the major reduction coming from reburning; the interactions among recycled NOx, fuel-N, and hydrocarbons released from coal may further decrease NOx formation. 8 1.2.4. CCS in postcombustion: carbonate looping Carbonate looping is a PCC technology. Although substantial research and development for carbonate looping are still needed prior to a full-scale realization in coal firing power plants, the technology has its advantages. Firstly, carbonate looping has a relatively lower energy penalty. Secondly, limestone contributes to its overall low cost. Furthermore, it is noted that limestone reserves are enormous worldwide and usually very easy to be acquired. In general, PCC is one of the most straightforward ways to add CCS capability to coal firing power plants without extensive alteration of existing processes. Despite these advantages, carbonate looping processes are still not well understood, including their kinetic reactions, micromechanisms of carbonate and calcination, and a novel heat transfer design between two solid flows to reduce thermal energy dissipation. The potential of carbonate looping was first proposed by Shimizu [42]. A carbonate looping system uses two interconnected fluidized beds (carbonator and calciner), as shown in Figure 5. CO2 in a flue gas stream is removed in a carbonator at operating temperatures between 873 and 1023 K. The solids leaving the carbonator are injected into a calciner, in which free lime (CaO) is regenerated. The regenerated free lime is then fed back into the carbonator, to form a complete loop. In contrast, thermal energy is needed in a calciner (1173-1273 K) to support an endothermic reaction. When CO2 reacts to form free lime, substantial heat is released and removed to maintain the carbonator at 923-1023 K. 9 Pressure has a significant effect on the carbonation reaction rate. Yu et al. [43] studied carbonation kinetics of CaO derived from three different calcium-based sorbents using a magnetic suspension balance (MSB) analyzer at different pressures (1000-15000 torr) and with a wide range of CO2 concentrations (10-30%). With a 20% concentration of CO2, the reaction rate increases with pressure until it reaches 4000 torr; however, the reaction rate decreases with pressure beyond 4000 torr. Deactivation of natural limestone poses another challenge of carbonate looping. Deactivation can be delayed by modifying natural limestone or synthesizing CaO-based sorbent. Florin et al. [44, 45] developed a synthetic CaO-based sorbent with 85 wt. % CaO, which showed a CO2 uptake that was three times higher than that of a natural limestone after 30 cycles. In order to increase the reactivity of calcium-based sorbents, Blamey et al. [46] investigated the carbonation mechanism of a Ca(OH)2-CaO system. They found a " up a y a o " ff Ca(OH)2 pellets and hydrated calcined limestone, but not in Ca(OH)2 powder or hydrated calcined dolomite. Abonades et al. [47] presented their experimental results from a small test facility (30 KW) using dual circulating fluidized beds (CFB). CO2 absorption efficiencies in their carbonator have achieved between 70 and 97%. In addition to experiments, some models have been developed to investigate the carbonate process [48-50]. Lasheras et al. [48] implemented a 1D fluidized bed model using data from a 1052 MW coal firing power plant. CO2 absorption efficiency achieved about 80% in the carbonator, leading to 88% CO2 removal. A sensitivity analysis shows that the CO2 absorption rate increases significantly with bed inventory and circulating 10 solid flow. The effect of the reactor height, the particle size, and make-up flow on the rate is negligible. Hawthorne et al. [50] modeled carbonate looping using Aspen plus, showing a total CO2 absorption efficiency of 88%. 1.3. Fluidized bed technology Fluidized bed combustion (FBC) was introduced in the 1970s to provide cleaner energy [51], and has become more and more popular throughout the world. It also provides fuel flexibility and reduces emissions (SOx, NOx). Countries with abundant quantities of low-grade coal, such as the USA, UK, China, and Germany, have been the main drivers in research and development. Countries with abundant biomass resources (peat, wood waste, sludge, and bark), especially in Northern Europe, are also very interested in FBC. FBC provides an improved heat and mass transfer by fluidization of a solid particulate substance. Fluidization is a process in which granular materials (e.g., bed materials, coal particle, biomass, and limestone) are converted from a static solid-like state (103 kg/m3) to a dynamic liquid-like state (0.1 kg/m3) [52] subject to an upward high-velocity gas flow stream. A reduction of solid particle concentration will greatly improve heat and mass transfer. In the combustion zone of a fluidized bed, the heat transfer coefficient ranges from 250 to 750 W/m2K, in comparison to less than 100 W/m2K in the freeboard [52]. Therefore, fluidized bed combustors are often used to burn low-quality fuels with low calorific value, high moisture content, and high ash content. They also provide fuel flexibility, and reduced emissions (SOx, NOx). 11 As a mature technology, FBC was reviewed by many authors [53-62]. A primary upward air or O2 stream passes through a distributor plate or bubble caps to achieve high velocities. A significant pressure drop also occurs as Bernoulli's principle describes. The velocity of the fluid increases with pressure or potential energy. Bed materials (usually sand) are fluidized by the high velocity gas stream, and a secondary gas stream is often injected from the side of the bed to ensure a complete combustion. Simultaneously, fuels (e.g., coal particle, biomass) or optional sorbents (e.g., limestone, dolomite) are injected into the fluidized bed from the feeders. However, there are also some disadvantages of fluidized beds: i. Increasing the combustor height: FBC usually has thin and tall shapes that require more initial capital expenses. ii. Pressure drop: more pump power is demanded to achieve a higher gas velocity. iii. Particle entrainment and agglomeration: collision between particles, particles and bed walls, especially at a high velocity, generates fine particles. It is very difficult to separate and collect them. Moreover, agglomeration happens under high operating temperatures, which leads to serious accidents (defluidization) [56]. iv. Erosion: particle collision into combustor walls also leads to severe erosion that requires expensive maintenance and repairs. 12 v. Limited theoretical understanding: most of current FBC designs are based on empirical experiences; it is still a very challenging task to simulate a fluidized bed. The general development trend of a novel technology follows a standard pattern (S shape) [63]. This curve can be divided into three stages: innovation, commercialization and maturation. The innovation stage has no physical application to market, and takes from 10 to 60 years [63]. The development slope in this stage is not steep. During the commercialization stage, spending in research and development (R&D) leads to dramatic improvements in quality and cost. The rate of R&D slows down when potential improvements start to exhaust, the technology is widely used, and markets become saturated. Finally, the technology achieves the matured phase. i. Innovation In 1922, the research of FBC began with the Winkler patent [51]. In the 1960s, R&D accelerated the development of FBC. In 1965, the first experimental bubbling fluidized bed (BFB) combustor was constructed. The test proved the concept and showed potential for reducing SOx emissions. In the same year, an atmospheric FBC programs was initiated in the US. The Environmental Protection Agency (EPA) considered FBC technology as a low pollution combustion technology. The first R&D on circulating fluidized beds (CFB) started in Germany in the mid-1970s [64]. In 1979, the first industrial-scale CFB was developed by Foster Wheeler. ii. Commercialization 13 Northern European countries were interested in the FBC technology since they would like to burn low quality biomass (e.g. peat, wood waste, and sludge). The wide use of FBC in Finland and Sweden started in the 1980s and 1990s, respectively. iii. Maturation Today, FBC is a mature technology. Potential future R&D directions include fuel flexibility, boiler reliability, and pollution control. Compared to CFB, future development of BFB is likely to be limited, with no major industrial interest. Currently, Alstom and Foster Wheeler are the two largest CFB manufacturers in the world. Both of them are active worldwide and in North America with Foster Wheeler dominating 5 of the 7 defined markets. Kvaerner is the market leader of BFB technology, followed by Foster Wheeler; both of them have their market in Scandinavia. Kvaerner is the market leader in Europe, and Foster Wheeler dominates the market in Asia and North America. There are two approaches to scale up FBC. The first approach is to use multiboilers, with identical single boilers. The second way is to increase single boiler capacity. The motivation for scaling up is to achieve higher efficiency for CFB. The following considerations are important in CFB scaling up: i. Fluidization in large cross-section areas ii. Increasing efficiency of particle separation systems iii. Optimization of fuel and air distribution iv. Heat management (e.g., flue gas cooling, external heat exchanger) E gy R& of fo u o " ff ": ombu o ff y, bo l efficiency, and thermal efficiency. The combustion efficiency is defined as the ratio 14 between burned fuel and injected fuel. The boiler efficiency is the ratio between the amounts of heat absorbed by the working fluid (water/steam). The thermal efficiency is the percentage of the amount of electricity produced over electricity requirement in a total energy input. The combustion efficiency for CFB can reach up to 99%, higher than that for BFB, since CFB has a better mass and heat transfer efficiency compared to BFB. The range of boiler efficiency for FBC is from 75% to 92% (HHV) [65]. BFB and CFB share a similar principle, but their design parameters are quite different. The main design parameters are summarized by Koornneef et al. [51] in Table 3. The design of an FBC depends mainly on the type of fuel, required gas steam velocity, pollution emission requirements, and manufacturing site. For example, the gas stream velocity in a CFB is much higher than that in a BFB. Moreover, particle size in a CFB is smaller than that in a BFB. As a result, the particle concentration is much lower in a CFB. The higher velocities and vigorous mixing in a CFB have a significant effect on heat transfer patterns. The heat and particle concentration distributions show a gradual decrease along with combustor height in a CFB, while there is a significant decrease in particle concentration with the height in BFBs. Thus, the temperature and mass profile in a CFB shows much more homogeneity. The combustion temperature in a BFB is lower due to poor fuel quality, greater particle size, and high moisture content. Particle collection and recirculation is required in CFB. Due to a higher gas stream velocity, high particle concentrations are also found in the upper regions of CFB (e.g., freeboard section, convective pass). A cyclone has to be used to separate particles (e.g., bed materials, unburned solid fuel particles) from the flue gas stream for 15 recirculation. The recirculation greatly increases the residence time so as to achieve high combustion efficiency. Combustion temperatures were also adjusted by varying solid particle and flue gas circulating ratio. However, a cyclone cannot provide enough centrifugal force to capture super fine particles. The remaining gas stream with super fine particles is separated by ESP, bag house, or other particle collection facilities prior to discharge. 1.4. SO2 emissions According to the EPA [66], the majority of SO2 emissions come from power plants (73%) and other industrial facilities (20%). SO2 is harmful to the environment; e.g., the occurrence of acid rains. EPA started to regulate SO2 emissions in 1971. Recently, mechanisms of SO2 reduction were also studied under oxy fuel conditions due to the combined benefit of simultaneous reduction of greenhouse gases. The main sulfur source in coal can be categorized as organic or inorganic sulfur (pyritic, sulfide sulfate). i. Organic sulfur Organic sulfur Organic sulfur Organic sulfur Organic sulfur Organic sulfur Organic sulfur Organic sulfur Organic sulfur Organic sulfur Organic sulfur Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30Organic sulfur is directly bonded with carbon atoms and accounts for about 30 -70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical 70% in total sulfur content of US coals. Organic can only be removed by chemical separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches separation approaches [67 -69 ]. Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but . Some of these approaches are greatly effective, but relatively expensive. relatively expensive. relatively expensive. relatively expensive. relatively expensive. relatively expensive. relatively expensive. relatively expensive. relatively expensive. relatively expensive. relatively expensive. relatively expensive. ii. Pyritic or sulfide sulfurPyritic or sulfide sulfur Pyritic or sulfide sulfurPyritic or sulfide sulfur Pyritic or sulfide sulfur Pyritic or sulfide sulfurPyritic or sulfide sulfurPyritic or sulfide sulfur Pyritic or sulfide sulfur Pyritic or sulfide sulfur Pyritic or sulfide sulfur Pyritic or sulfide sulfurPyritic or sulfide sulfur The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as The majority of inorganic sulfur exists as pyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeS pyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeS pyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeS pyritic (sulfide sulfur, FeS pyritic (sulfide sulfur, FeS pyritic (sulfide sulfur, FeS pyritic (sulfide sulfur, FeS pyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeS pyritic (sulfide sulfur, FeSpyritic (sulfide sulfur, FeS 2). Different ). Different ). Different ). Different ). Different ). Different ). Different ). Different ). Different ). Different ). Different from organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfur from organic sulfur, pyritic sulfurfrom organic sulfur, pyritic sulfur can be partially removed bycan be partially removed bycan be partially removed bycan be partially removed bycan be partially removed bycan be partially removed bycan be partially removed bycan be partially removed bycan be partially removed bycan be partially removed by can be partially removed bycan be partially removed by can be partially removed bycan be partially removed bycan be partially removed by can be partially removed bycan be partially removed bycan be partially removed bycan be partially removed bycan be partially removed bycan be partially removed by conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity conventional gravity 16 flotation flotation flotation flotation flotation flotation flotation flotation coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. coal cleaning prior to combustion. iii. Sulfate sulfur Sulfate sulfur Sulfate sulfur Sulfate sulfur Sulfate sulfur Sulfate sulfur Sulfate sulfur Sulfate sulfur Sulfate sulfur Sulfate sulfur Sulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorganiSulfate sulfur represents only a few percent of the total inorgani Sulfate sulfur represents only a few percent of the total inorganic sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate c sulfur. Sulfate salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with salts such as gypsum are present in small quantities (typically less than 0.01%), with the the the the exception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, which exception of iron sulfates, whichexception of iron sulfates, which exception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, which exception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, which exception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, which exception of iron sulfates, which exception of iron sulfates, whichexception of iron sulfates, whichexception of iron sulfates, which exception of iron sulfates, which are all all formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion.formed by oxidation of pyrite during combustion. formed by oxidation of pyrite during combustion. Some of the most useful industrial approaches to contro Some of the most useful industrial approaches to contro Some of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to contro Some of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to contro Some of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to contro Some of the most useful industrial approaches to contro Some of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to contro Some of the most useful industrial approaches to contro Some of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to contro Some of the most useful industrial approaches to contro Some of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to controSome of the most useful industrial approaches to control SO l SO x emission emission emissions are are are summarized here: summarized here: summarized here:summarized here:summarized here: summarized here:summarized here:summarized here:summarized here:summarized here: i. Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separation Pyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separationPyritic sulfur is removed from coal using physical separation (flotation)(flotation)(flotation) (flotation)(flotation)(flotation)(flotation)(flotation)(flotation)(flotation). ii. SO x during combustion during combustion during combustion during combustion during combustion during combustion during combustion during combustion during combustion during combustion during combustion during combustion during combustion is captured by injecting limestone directly.captured by injecting limestone directly. captured by injecting limestone directly.captured by injecting limestone directly.captured by injecting limestone directly.captured by injecting limestone directly. captured by injecting limestone directly.captured by injecting limestone directly.captured by injecting limestone directly. captured by injecting limestone directly. captured by injecting limestone directly. captured by injecting limestone directly.captured by injecting limestone directly.captured by injecting limestone directly. captured by injecting limestone directly. captured by injecting limestone directly. captured by injecting limestone directly. captured by injecting limestone directly.captured by injecting limestone directly.captured by injecting limestone directly.captured by injecting limestone directly. captured by injecting limestone directly.captured by injecting limestone directly. captured by injecting limestone directly. captured by injecting limestone directly.captured by injecting limestone directly. captured by injecting limestone directly. iii. SO x in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using in the flue gas is removed using flue gas desulfurizationflue gas desulfurizationflue gas desulfurizationflue gas desulfurization flue gas desulfurizationflue gas desulfurization flue gas desulfurizationflue gas desulfurizationflue gas desulfurization flue gas desulfurizationflue gas desulfurizationflue gas desulfurizationflue gas desulfurizationflue gas desulfurization flue gas desulfurizationflue gas desulfurizationflue gas desulfurization flue gas desulfurization after combustionafter combustionafter combustionafter combustion after combustionafter combustionafter combustionafter combustionafter combustionafter combustionafter combustionafter combustionafter combustionafter combustion. iv. Co al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption.al gasification or liquefaction, followed by removal absorption. al gasification or liquefaction, followed by removal absorption. v. High suHigh su High suHigh su lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low lfur coals are blended with low sulfur coals sulfur coalssulfur coalssulfur coalssulfur coals sulfur coalssulfur coalssulfur coals . 1.4.1. SO2 formation mechanism during oxy and air firing combustion Different operating conditions between oxy and air firing combustion can affect SO2 emissions. In the past decades, theoretical and experimental studies have been performed to understand the mechanism of SO2 emissions. Efforts have focused on simulation [20, 70, 71] , thermodynamics [71], flame characteristics [18, 21, 22, 32, 33, 72-74], and NOx/SOx formation [39, 75-81]. Contradictory observations were reported on SO2 emissions under air and oxy firing conditions. Some researchers showed that SO2 emissions were reduced during oxy firing [41, 76, 82]. In contrast, others found no difference in experimental studies or concluded so using equilibrium models [40, 71, 75, 17 79, 83, 84]. In particular, most studies [75, 76] have shown that oxy fuel combustion with recycled flue gas would result in a higher SO2 concentration (in ppm) but a lower SO2 emission per energy content (mg/MJ). Some researchers [24] suggested that the lower SO2 emission be the result of a high conversion of SO2 compared to other sulfur species, such as SO3, in oxy fuel combustion. Sarofim [85] indicated that a high SO2 concentration could result in a high sulfur removal by sulfation of ash under oxy fuel combustion. Furthermore, it was suggested that the difference in SO2 emission between wet and dry recycled flue gases comes from SO3 deposition onto cooling transport lines. A wet flue gas recycle would have a higher SO2 concentration in the furnace, compared to a dry flue gas recycle, because returning SO3 will inhibit SO2 conversion to SO3, especially through the high temperature reaction pathway. Ahn et al. [86] investigated SOx formation during oxy coal combustion using a 330 kW pilot-scale circulating fluidized bed (CFB) and a 1.5 MW pilot-scale pulverized coal combustor (L1500). Both combustors were equipped with RFG. Two different bituminous coals (a high sulfur Illinois coal and a low sulfur Utah coal) were studied. The concentration of SO2 was found to be much higher during oxy fuel combustion, with the SO3 concentration 4-6 times greater than that in air combustion when high sulfur coal (Illinois #6) was used. In contrast, the SO3 concentration was similar for oxy vs. air firing using a low sulfur Utah bituminous coal. Kiga et al. [82] investigated ignition characteristics, flame propagation, and NOx and SOx emissions using a pulverized coal combustor with wet RFG. They fired three types of coals with O2/CO2 and air, and accounted for all sulfur content, including SO2 and sulfur in ash. Their results showed 18 that the SO2 emission in oxy fuel combustion is lower than that in air combustion. Croiset et al. [76] studied the implications of O2/CO2 recycle on NOx and SOx emissions. They used a 0.21 MW combustor with a recycled flue gas stream containing 5% excess O2 (dry basis). The experiments were carried out for both wet and dry flue gases. When SO2 emissions were based on mass, the SO2 emission with RFG was 280 ng/J, which was lower than that (~320 ng/J) without RFG under O2/CO2 conditions. The SO2 emission with wet RFG was found to be close to that with dry RFG, although a lower SO2 emission was expected for the dry RFG case. When SO2 emissions were based on concentration (ppm), the SO2 concentration with RFG (~1750 PPM) was twice that without RFG (700-950 PPM) under O2/CO2 conditions. Croiset et al. [76] also reported that the SO2 emission increased with O2 concentration, although the effect was not significant. Zheng et al. [71] simulated combustors under air and oxy firing using the Facility for Analysis of Chemical Thermodynamics (FACT); no RFG was considered in these studies. They found no difference in SO2 emission during oxy fuel combustion. The SO2 emission depends on the O2 concentration, not on the CO2 concentration. Moreover, the SO3 concentration increases with SO2. Liu et al. [79] also compared oxy fuel combustion with air combustion using a 20 kW down-firing combustor in the absence of RFG. Conversion of coal-S to SO2 ranged from 86-90%. The authors concluded that SO2 emissions are independent of the nitrogen presence. The authors suggested that the slight difference in observed SO2 emission was probably caused by the difference in the overall char burnout for different combustion diluent (N2 or CO2). 19 1.4.2. The mechanisms of SO2 removal by limestone The operating temperature of FBC can remove SO2 directly with added limestone. Despite many earlier efforts, e.g., Borgwardt et al. [87-94], Dan-Johansen et al. [95-101], Silcox et al. [102-104], and Anthony et al. [105-109], the mechanisms for SO2 removal by limestone is still not fully understood. Weisweiler et al. [110] listed the following parameters related to the SO2 removal efficiency. i. Chemical composition of limestone. ii. Limestone calcination time and temperature. iii. Physical properties of calcined limestone (free limes). iv. Sulfation reaction temperature. v. Sulfation mechanism (direct or indirect sulfation). vi. Miscellaneous properties. 1.4.2.1. Direct sulfation and indirect sulfation Figure 6 shows the shows the shows the shows the shows the shows the shows the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the current understanding of the overall mechanism of SOoverall mechanism of SOoverall mechanism of SO overall mechanism of SO overall mechanism of SOoverall mechanism of SO overall mechanism of SOoverall mechanism of SOoverall mechanism of SO overall mechanism of SOoverall mechanism of SOoverall mechanism of SOoverall mechanism of SO 2 capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of capture by limestone. Generally, the sulfation mechanisms (direct or indirect) of limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in limestone depend on whether the is calcined. A direct sulfation takes place in an uncalcined state while indirect sulfation happens with an uncalcined state while indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens with an uncalcined state while indirect sulfation happens with an uncalcined state while indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens with an uncalcined state while indirect sulfation happens with an uncalcined state while indirect sulfation happens with an uncalcined state while indirect sulfation happens withan uncalcined state while an indirect sulfation happens with an uncalcined state while indirect sulfation happens withan uncalcined state while an indirect sulfation happens with an uncalcined state while indirect sulfation happens with an uncalcined state while indirect sulfation happens withan uncalcined state while an indirect sulfation happens with an uncalcined state while indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens with an uncalcined state while indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens withan uncalcined state while an indirect sulfation happens with an uncalcined state while indirect sulfation happens withan uncalcined state while an indirect sulfation happens with an uncalcined state while indirect sulfation happens with calcined calcined calcined calcined limestone limestonelimestonelimestonelimestonelimestone. Usually. Usually. Usually . Usually. Usually , in fluidized bed in fluidized bedin fluidized bedin fluidized bed in fluidized bedin fluidized bedin fluidized bed in fluidized bed in fluidized bedin fluidized bedin fluidized bedin fluidized bed combustioncombustioncombustioncombustioncombustioncombustioncombustioncombustion conditions,conditions,conditions,conditions,conditions,conditions,conditions, conditions, a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in a direct sulfation occurs in oxy fueloxy fueloxy fuel oxy fueloxy fueloxy fuel combustion combustion combustion combustion combustion combustion combustion combustion combustion combustion due to COdue to COdue to COdue to COdue to COdue to COdue to COdue to COdue to CO2 inhibition inhibitioninhibition inhibitioninhibitioninhibition inhibition of the calcination stepof the calcination stepof the calcination stepof the calcination stepof the calcination stepof the calcination stepof the calcination stepof the calcination stepof the calcination step of the calcination stepof the calcination stepof the calcination step of the calcination stepof the calcination stepof the calcination step , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air , and an indirect sulfation occurs in air combustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCOcombustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCO combustion. During an indirect sulfation, CaCO3 de composes into CaO and COcomposes into CaO and COcomposes into CaO and CO composes into CaO and COcomposes into CaO and CO composes into CaO and COcomposes into CaO and COcomposes into CaO and COcomposes into CaO and COcomposes into CaO and COcomposes into CaO and CO composes into CaO and COcomposes into CaO and CO composes into CaO and COcomposes into CaO and COcomposes into CaO and COcomposes into CaO and CO2, followed , followed , followed , followed , followed , followed , followed , followed by the reaction of CaO and SOby the reaction of CaO and SO by the reaction of CaO and SOby the reaction of CaO and SO by the reaction of CaO and SOby the reaction of CaO and SO by the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SO by the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SO by the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SOby the reaction of CaO and SO 2 to form CaSOto form CaSOto form CaSOto form CaSOto form CaSOto form CaSO to form CaSO 4. In . In direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of direct sulfation, a high concentration of 20 CO2 and/or a relatively low operating temperature inhibits the decomposition of CaCO3. CaCO3 directly reacts with SO2 to form CaSO4. i. Indirect sulfation (CaO-SO2) o Step 1 Calcination of CaCO3 : 3 2CaCO CaOCO (g)  o Step 2 Formation of sulfite : 2 3 CaO SO CaSO o Step 3 Oxidation of sulfite: 3 2 4 1 2 CaSO  O CaSO ii. Direct sulfation (CaCO3-SO2) o Step 1 Formation of sulfite: 3 2 3 2CaCO  SO CaSO CO (g)  o Step 2 Oxidation of sulfite : 3 2 4 1 2 CaSO  O CaSO 1.4.2.2. Calcination and carbonation Direct or indirect sulfation is decided by the occurrence of calcination, which in turn is determined by operating temperature, CO2 concentration, and pressure. A number of researchers have studied the equilibrium CO2 pressure over limestone [111, 112] under thermal treatment. A local high CO2 concentration can inhibit calcination, such as in pores of a large particle or in interstices of a packed bed. Correspondingly, operating temperature can be increased to promote calcination. Similarly, at operating temperatures of FBC, the partial pressure of CO2 plays an important role on calcination. 21 Properties of limestone and calcined limestone (free lime) have been studied extensively [113-120]. In natural limestone, CaCO3 accounts for more than 90% of the weight, and void volume ranges from 3% to 35%. If particle shrinkage is negligible during calcination, the porosity of free lime is much greater than that of limestone. Sintering tends to occur at high operating temperatures, which clogs pores and decreases porosity and surface area. Also, kinetics of calcination was also studied [102-104, 118, 120]. The relevant parameters include: i. CO2 concentration, which promotes the reverse reaction, called recarbonation. ii. Operating temperature. iii. A total pressure, which increases CO2 partial pressure and inhibits calcination. iv. Particle size, which determines thermal and mass transfer limitations [121-123]. v. Catalysis and/or inhibition by impurities. The operating temperature is determined by balancing calcination and sintering. The degree of calcination increased at high operating temperatures, which also promotes the occurrence of sintering. At certain temperatures, recarbonation is possible, which is the reverse reaction of calcination. 1.4.2.3. Sulfation mechanisms A few mechanisms have been proposed [101, 102, 124-127]. The mechanism of direct sulfation is not well understood. Little else can be confirmed except for CaSO4 as the final product [128]. Direct injection of limestone to the furnace is a very attractive option to control SOx emissions using oxy firing. The efficiency of sulfation with a high 22 CO2 concentration differs from air combustion. Tullin [129] observed in experiments the inhibition effect of a high CO2 concentration during direct sulfation. Currently, the mechanism of direct sulfation is described by the same models used for indirect sulfation, i.e., shrinking core particle model, CaSO4 production layer, and diffusion limited control. Liu et al. [77, 78, 127] presented a drastic reduction of SO2 emissions during oxy firing. They argued that limestone can maintain a high reactivity under a high CO2 partial pressure. They made two assumptions: the high concentration of SO2 inhibits CaSO4 decomposition; secondly, the diffusion resistance through the CaSO4 layer is minimized due to suppressed limestone calcination. The point was cited in several review papers [5, 130]. Their points are debatable. (1) The typical temperatures in CFBs (1073-1173 K) are relatively low for significant decomposition of CaSO4. (2) Intrinsic sulfation kinetics of limestone with a high CO2 concentration is much lower than that in N2, a o g o L u' paper [127] om of L u' xp m al ul a p o u Figure 7 If L u' second argument is correct, the crossover point of sulfation degree for his data takes place after 4500 seconds (1123 K), as shown in Figure 7. This is the time elapsed before SO2 removal in oxy firing exceeds that in air firing u o ffu o lay ' ff T effect should not be significant since SO2 residence time in CFBs is only a few seconds. Moreover, the limestone injection is continuous in practical industrial applications. A sulfation process can be both kinetically controlled and diffusion controlled, depending upon the temperature. The rate of SO2 removal by limestone is determined by the intrinsic reaction rate and the CaSO4 effective diffusivity, which are competing 23 factors. Which factor dominates depends on experimental conditions; e.g., properties of the limestone, operating temperature, environmental gas concentrations, particle size, the properties of the CaSO4 product layer, and the change of porosity. The formation of a CaSO4 layer inside the particle will block or shrink pores because CaSO4 has a larger molar volume than CaO or CaCO3. At the beginning, the system is dominated by the intrinsic rate. After a CaSO4 layer is formed, diffusion control competes with kinetic control. The diffusion barrier becomes more important and finally dominates the process. Therefore, sulfation in smaller particles is likely to be dominated by intrinsic rate, while that in larger particles is dominated by diffusion. Sulfation is usually dominated