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Show PREDICTING THE COMPLETE DISTRIBUTIONS OF VOLATILE PRODUCTS FROM DIVERSE FUEL TYPES WITH FLASHCHAIN™ Stephen Niksa Niksa Energy Associates, 1745 Terrace Drive, Belmont, CA 94002, USA A B S T R A CT P L A S H C H A I N is a reaction mechanism for the rapid devolatilization of solid fuels. This paper illustrates how the theory was recently expanded to predict the complete distribution of all major devolatilization products for a vanety of fuel types, including any kind of coal, biomass, and petroleum coke Noncondensible gases are now resolved mto the primary hydrocarbon species (CH4, C2H9, C2H4, C3H9. C3H8). HCN. Hi, H2S, H 20. C 0 2 . and C O . Tars are charactenzed by their complete elemental compositions (C/H/O/N/S) and their molecular weight distributions. Chars are characterized by their complete elemental compositions (C/H/O/N/S) and their sizes and densities. The theory also predicts the partitioning of chlorine and alkali species (Na and K). This paper also introduces a mechanism to descnbe the secondary pyrolysis of the pnmary devolatilization products, as occurs naturally at elevated temperatures in all combustion and gasification systems. During secondary volatiles. the volatiles are radically transformed, with complete conversion of tar into soot and conversion of all gases into H2, CH4, C2H2, HCN, H2S, CO, C 0 2 , and H 20. INTRODUCTION Engineers in the utility industry are being called upon to manage power production with a wide spectrum of solid fossil fuels, including coal, cokes, biomass, and vanous waste materials. They usually use either test data and/or C F D simulations to characterize fuel quality impacts dunng combustion However, traditional fuel quality parameters such as the fuel ratio, proximate volatile matter content, and fuel-nitrogen content are often unable to correlate the most important macroscopic combustion characteristics, such as fuel-nitrogen conversion to NOx, char burnout times, and carbon carryover Measured pyrolysis and combustion behavior performs better, but testing entails a substantial investment of professional staff and is usually not used routinely. This paper demonstrates a computational approach for fuel quality impacts based on F L A S H C H A I N ™ (Niksa, 1995a & 1995b) that achieves the accuracy of laboratory testing for a fraction of the expense. Evaluations cover a vanety of solid fossil fuels, including coals, petroleum cokes, and various forms of biomass. In all cases, the only sample-specific information in the simulations is the ultimate and proximate analyses of the coals. No model parameters were fine-tuned to match the predictions to measured values. COAL DEVOLATILIZATION BEHAVIOR Pnmary Devolatilization FLASHCHAIN™ predicts the devolatilization behavior of any coal at any operating conditions. Well over 1000 coals from all geographical regions worldwide have been analyzed to date. Recent publications descnbe the theory's mechanisms to predict the yields of the major volatile products, tar and gas, plus the yields of the gases containing oxygen and nitrogen (Niksa, 1995a). The theory was recently expanded to predict the release of all major hydrocarbons, H2, H 2 S and the elemental compositions of char and tar. Consequently, FLASHCHAIN™ now predicts the yields of char, tar, CH4, C2H9, C2H4, C3H8, C3H9, HCN, H2S, H2, H 20, CO. C02 , plus the C/H/N/O/S compositions of tar and char and the molecular weight distribution of tar. Table 1. Evaluation of predicted hydrocarbon and H2 yields for test conditions of Xu and Tomita (1989) CH4 C2's C3's H2 Subbit Meas. 2.00 0.78 0.49 0.40 (SY) Pred. 3.00 1.36 0.90 0.31 Hv Bit. Meas. 3.20 1.43 0.85 040 (HV) Pred 3.50 1.58 1.05 0.39 Lv Bit. Meas. 3.90 089 0.37 0.50 (KS) Pred 3.50 1 00 035 0 42 An evaluation of the predicted yields of hydrocarbons and H2 for subbituminous, hv bituminous and Iv bituminous coals appears in Table 1. These data were reported by Xu and Tomita (1989) for heating at 3000 K7s to 1037 K at atmospheric pressure with a 4 s reaction period at the ultimate temperature. The predicted hydrocarbon yields are within 15 % of the measured values for the bituminous coals, but about 50 percent too high for the subbituminous coal Hydrogen yields are generally within experimental uncertainty for all coal types. Predicted char compositions for hv bituminous coal chars prepared by rapid heating are evaluated in Fig. 1. in these tests, three similar hv bituminous coals were heated under atmospheric pressure at 1000 K/s to progressively increasing temperatures with a 2 s reaction period at each temperature (Cai, 1995). The total weight loss is evident in Fig. 1 from the reported char yields. The char yields are generally predicted within experimental uncertainty throughout pyrolysis, except for a 5 % discrepancy with the Pit. 8 sample. The predicted char carbon contents increase throughout devolatilization, in accord with the tendency in all three datasets. Predicted C-contents exceed the measured values in all cases, although the overestimation is significant only for the Gedling coal. The predicted H-contents are within experimental uncertainty throughout all stages of devolatilization with all three coals. Predicted char-N levels are within experimental uncertainty for all but the highest temperature with the Linby and Gedling coals. But predicted values exceed the data at all temperatures for the Pit. 8. Whereas the predicted char- N level exceeds the measured values at 1500 DC m this dataset, other datasets do not indicate a similarly large extent of N-release under comparable conditions, as explained previously (Niksa, 1995b). Although the char-0 levels were not measured, the predicted values appear in Fig. 1 to illustrate that oxygen (and sulfur) are the only elements that are completely expelled dunng devolatilization Such behavior makes them suitable, in principle, for tracing the extent of devolatilization, although laboratory determinations of char-0 are currently problematic. |