|Title||Evaluation of Electric Power Generation from Sugar Cane Waste|
|Contributor||Gilfillan, W.N.; Dixon, T.J.|
|Spatial Coverage||Houston, Texas|
|Subject||2014 AFRC Industrial Combustion Symposium|
|Description||Paper from the AFRC 2014 conference titled Evaluation of Electric Power Generation from Sugar Cane Waste by J. Pohl.|
|Abstract||First, this paper begins with discussion of the properties of the waste products and location of major sugar cane growing areas in Queensland, Australia. This includes chemical analysis of sugar cane biogases and waste products, photographs of the waste to be burned, biogases (sugar can sheaf) and trash (leaves and waste left in the field). Use of the energy from sugar cane waste will displace CO2 emissions in Queensland by up to 100 %. Second this paper evaluates using a life cycle economic analysis the cost and efficiency of three techniques to use the sugar cane waste to generate electricity; 1) improve the efficiency of exiting low pressure steam boilers, 2) use a high pressure steam cycle, and 3) use an advanced gasification combined cycle. The fluidized bed co-generation technique was found to be the most cost effective. Finally, this paper presents the design and construction of a 5 MWt pilot scale combustor which will be used to evaluate the potential problems of burning sugar cane waste and converting that energy to electricity to export to the grid.|
|Rights||No copyright issues exist.|
EVALUATON OF ELECTRIC POWER GENERATION FROM SUGAR CANE WASTE John H. Pohl, Energy International, Laguna Woods, CA, USA William N. Gilfillan, Queensland Institute of Technology, Brisbane, QLD, Australia Terry J. Dixon, Dixon Solutions, Blackbutt, QLD, Australia Presented at the AFRC 2014 Symposium, Houston, TX, September 7, 2014. ABSTRACT First, this paper begins with discussion of the location of sugar cane fields in Queensland, Australia and the properties of sugar cane waste products. The properties include chemical analysis of sugar cane bagasse and waste products, photographs of the wastes to be burned, bagasses (sugar cane sheaf) and trash (leaves and waste left in the field). Next, we review the available worldwide technology to burn and use such wastes for electricity generation. The technology are reviewed for cost and efficiency. The systems reviewed are proven systems to generate steam to be used in refining sugar and generate electricity power through-out the world. The energy from sugar cane waste can provide, when supplemented with natural gas, up to 100 % of the electricity and displace CO2 emissions by up to 100 % in Queensland, Australia. Second, this paper evaluates using a life cycle economic analysis of the cost and efficiency of three techniques used to burn sugar cane waste to generate electricity; 1) improve the efficiency of exiting low pressure steam boilers, 2) use of a high pressure steam cycle, and 3) use an advanced gasification combined cycle. We found the fluidized bed co-generation technique to be the most efficiency and cost effective technique. This paper describes the combustion systems used to burn the sugar cane and other waste. In this system exhaust heat is used to dry the 50 % moisture content of incoming sugar cane wastes. The efficiency is improved by using a preheating system developed for burning brown coal in Australia. The water content of brown coal is similar to the water content of sugar cane wastes, ~50 % compared to 60 % for brown coal. Finally, this paper presents the design and construction of a 5 MWt pilot scale combustor which will be used to evaluate the potential problems in burning sugar cane wastes and converting that energy to electricity for export to the grid. In particular, the rig will be used to evaluate deposits formed from the high K content of sugar cane waste. 1.0 INTRODUCTION Queensland grows a large amount of sugar cane. The waste from this sugar cane is currently burned inefficiently to raise steam for processing sugar in the mill. The waste products of sugar cane, sugar cane bagasse and sugar cane trash, trash and leaves left in the field, can be used to generate electric power. This use of sugar cane wastes to generate electric power reduces the Greenhouse gases produced from the coal, which is typically burned in Australia to generate electricity. Use of sugar cane wastes also reduce the emissions of greenhouse gases. The reduction in greenhouse emissions arise from two factor. First, sugar cane waste has a H/C ratio of about 1.5 compared to coal which has a H/C ratio of 1/1. The high heating value of sugar cane waste is about 4000 BTU/lb while coal is about 12000 BTU/lb. Sugar cane waste is usually inefficiently burned to produce steam for processing sugar. If sugar cane wastes are not burned, the wastes will still degenerate to produce Greenhouse Gases (CO2 and CH4) and emit these gases into the atmosphere. During the crushing season, using sugar cane waste to generating electric power will reduce CO2 emissions in Australia by 16.5 Mt/yr or 60 % of the greenhouse gases generated in Australia. If supplemented outside the crushing season with natural gas firing in the same unit, burning sugar cane waste will produce 34,500 GWe/yr of electric power or approximately 100 % of the electric requirement of state of Queensland, Dixon et al. (1998). This paper is based on Terry Dixon et al. (1998) who, at the time, worked for the Sugar Research Institute in Mckay, QLD Australia and William Glfillan (2002) who completed a Master of Engineering Science under the other author, John Pohl at the University of Queensland, St. Lucia, QLD, Australia. The paper reviews the available co-generation technology in the world at the time this work was performed, the authors used life cycle analysis of the cost of generating power from sugar cane waste and reviewed the possible improvements to the chosen technology. It also designs and builds a 5Mwt pilot scale combustor to assess the problems of generating electric power from sugar cane waste. Sugar cane waste has an sodium concentration of 0.9 % and potassium concentration of 2.3 % K2O for bagasse and sodium concentration 1. 6 % for bagasse and potassium concentration of 5.6 % for trash. Based on the research on coal these alkali concentrations are expected to cause deposition problems. 2.0 GEOGRAPHY The geography of Queensland including sugar cane fields and sugar mills is shown in Figure 1.. Additionally, Queensland was the home territory of the authors at the time this work was done also shown in Figure 1. is the coal reserves and several central power plants, Coxhead (1999). 2.1 SUGAR CANE Queensland, Australia contains abundant sugar cane fields, mills and, power generation plants. In addition, the sugar cane waste needs to be destroyed and greenhouse gases reduced. This seemed like the ideal situation in which to generate electricity from biomass, in this case sugar cane waste. Figure 1., Coxhead (1999) shows the coal deposit, the power plant, Stanwell, who supported this work, the sugar research institute (SRI) where Terry Dixon worked, and the location of the other authors, when this work was done was University of Queensland, and Standwell Power Station, and the current location of the authors Queensland Institute of technology, Dixon Solutions, Blackbutt, QLD, and California USA. Fig. 1. Location of Authors, Support, and Power Supplies in Queensland, Australia, Dixon, et al. (1998). 2.2 SUGAR MILLS Figure 2., Dixon et al. (1998), shows the location of 26 sugar mills in Queensland Australia and 2 in northern New South Wales, Australia. Queensland produces and mills 46 % of the sugar cane in Australia, Juniper and Joseph (1996). The sugar mills are concentrated along the coast in Queensland. Two of the Author's, Dixon Sugar Research Institute University of Queensland, Queensland Institute of Technology Queensland Standwell Power Plant and Gilfillan, worked, after this work was completed at the Sugar Research Insittute in Mckay. The other author, Pohl, worked at the University of Queensland in St. Lucia outside of Brisbane at the time of the work. Figure 2. Location of Sugar Mills in Queensland and Northern NSW. 3.0 PROPERTIES OF SUGAR CANE BAGASSE AND TRASH. a) SUGAR CANE BAGASSE b) SUGAR CANE TRASH Fig. 3. Photographs of Sugar Cane Wastes. Gilfillan(2001). Photographs of bagasse and trash particles are shown in figure 2. The bagasse particles are approximately 5 mm in diameter by 37 mm long: The trash particles are slightly larger. VELOCITY We chose a co-generation fluidized bed gasification system. With a fluidized bed, the velocity must be great than that required to fluidized the particles. Figure 4. shows the estimated velocity required to fluidize the sugar cane waste particles. We chose a co-generation fluidized bed gasification system. The calculated velocity used to fluidize the sugar cane bagasse is 2.12 m/s, 9.96 ft/s. The velocity to fluidize sugar cane trash will be slightly higher. Figure 4. Velocity Required to Fluidize the Bagasse Particles, Gilfillan (2001). 3.0 CHOOSEN SYSTEM Table 1. and 2. show a number of systems reviewed that are installed through-out the world, Dixon, et al. (1999). This review resulted in selection of a fluidized bed gasifier. This system was chosen because of the efficiency of operation and the resulting cost of power generation. The system reviewed are shown in Table 1 and 2, Dixon, et al. (1999). 3.0 SUGAR CANE WASTE PROPERTIES This section presents the chemical analyses in Table 3., of sugar cane bagasse and trash. The composition of the trash may cause more problems than the Bagasse. Both have moisture contents approaching 50 %, which will cause loss of efficiency. The trash has an ash concentration of about 10 % compared to ~5 % in the bagasse, typical composition of ash in coals is about 15 %.. The ultimate analysis is about the same. However, the alkali concentration contained in the trash is considerably higher, 5,6 % K2O in the trash, compared to 2.3 % in the bagasse TABLE 1. Installed World Systems Reviewed. Dixon, et al. (1999). Table 2. Continuations of review of World Systems Reviewed, Dixon, et al. (1999). 4.0 Co-Generation Systems, Gilfillan (2001). Three generation systems used worldwide were reviewed for power output- efficiency, and cost. The three systems were 1) maximizing output from existing low steam pressure boilers, 2) a high pressure steam system, and 3) advanced gasification combined cycle system. For the base scenario of bagasse, used only during the six month crushing season for a 600 tonne/hr mill, export power increased from 29-59 Mwe to 116 Mwe for the co-generation plant reviewed. The application of advanced technology, gasification combined cycle system, will generate an estimated mean annual 3400 Mwe (20,722 Gwh pa) of power or approximately 60 % of Queensland power demand. This system will also reduce CO2 by 16.5 Mt/yr or nearly 10 % of Australia's greenhouse emission. In order to maximize the power output, natural gas will be burned in the off-season. in this case, the system will generate 5800 Mwe (34,000 Gwh pa or nearly 100 % of the then current Queensland generating capacity. The capital costs for the systems reviewed ranged from $ 1,400 /kwe to $2,100 /kwe and total costs was between $0.044-$0.069/ kwe-h these figures should be compared with coal-fired power generation at the time, with a capital cost of ~ 700-1000 $/kwe and a generating cost of 3-3.5 cents/kwe-h. While the costs are higher for use of sugar cane wastes than coal. The waste exists and must be destroyed beneficially. The sugar cane will create greenhouse gases whether they are used beneficially or not. TABLE 3. Proximate Analyses of Sugar Cane Bagasse and Trash, Dixon et al. (1999). The Na2O concentration is 1.6 % in the trash compared to and 0.9 % in the bagasse. The higher alkali concentration can lead to the conclusion that sugar cane wastes will create greater deposits that other fuels. Higher Alkali concentration usually lead to more deposits of the fouling (super heater deposits) and slagging (water wall deposits) than other fuels. Table 2. shows analyses of the ash of sugar cane waste composition. The composition of the sugar cane bagasse and trash are similar. However, the important elements for performance are higher in the trash. Fe2O3 (a indication of forming slagging, deposits on the water walls) is 6.3 % in bagasse. Table 4. Ash Analysis ( d % ) of Sugar Cane Waste, Gilfillan, 2001. Bagasse Trash SiO2 84.0 % 76.7 % Al2O3 3.6 % 7.1 % Fe2O3 1.8 % 6.3 % TiO2 0.3 % 0.4 % K2O 2.3 % 5.6 % MgO 1.7 % 4.0 % Na2O 0.9 % 1.6 % CaO 1.2 % 4.2 % SO3 0.9 % 2.3 % Cl n.d. n.d. CO2 n.d. n.d. Other (by difference) 3.3 % 0.8 % Ash Deformation Temp. Reducing 1218 C Ash Softening Temp. Reducing 1302 C Hemispherical Temp. Reducing 1372 C Fluid Temperature Reducing 1480 C Sintering Temperature Reducing 800-1000 C 625-670 C n.d.= not detected compared to 1.8 % in trash; K2O ( an indication of fouling, deposits on the super heater tubes) is 5.6 % in trash compared to 2.3 % in bagasse; and Na2O, also an indication of fouling is 0.9 % in bagasse compared to 1.6 % in trash. These higher alkali levels in the wastes indicated promotes deposits; another indication of deposits is lower ash sintering temperature. The composition of the wastes results in a significantly lower sintering temperature for the trash than for other fuels, 625-670 C compared to 800-1000 C for the bagasse. Deposits form are proposed to be formed by equation 1. and 2. K2O(silicate)+Fe=FeO(silicate)+K(g) (1) Na2O(silicate)+Fe+FeO(silicate)+Na(g) (2) The high alkali concentrations encourage iron deposits at the temperature of the heat transfer surfaces. These deposits can be molten at the temperature of the walls (~1400 C) or solid at the temperature of the super heater (~450 C). 3.0 REVIEW OF CO-GENERATING SYSTEMS A co-generation system was selected to burn sugar cane waste and generate power. This system has worldwide were reviewed in Dixon, et al. (1998). The system designed is shown in Fig. 6. 5.0 SELECTED CO-GENERATED SYSTEM Figure 6. shows the gasification co-generation system chosen for utilizing sugar cane waste to generate electricity. The system gasifies the sugar cane waste in a pressurized (25 atmos.) fluid bed combustor. The hot gases contain combustible gases (CH4, H2, and CO). The hot gases are used to dry the high moisture fuel. The cooled gases are then passed on to a flue gas clean up system. The combustible cleaned gases are passed to and burned in a gas turbine. These gases exchange heat with water and the resulting steam is passed into a high pressure steam turbine to generate electricity. The chosen system for 35% steam-on-cane (SOC) and a capital cost of $1150 A$/kwe generates electricity at 4.2 A cents/kwe-h for high pressure steam system, at 35% SOC and capital cost of 1400 A$ /kwe generates at 4.5 A cents/kwe-h for combined cycle for burning sugar cane waste, for 35% SOC, and capital cost of 1400 A$/kwe and generating cost of 5.2 A cents/kwe-h, for a system burning sugar cane waste supplemented with natural gas, the cost of generation electricity with coal has a capital cost of approximately 700 A$/kwe-h and a generating cost 3.5 A cents/kwe-h. Figure 6. Design of System Chosen to Generate Electric Power, Gilfillan (2001). Figure 7. Shows the equilibrium concentrations of the gas products produced in the fluidized gasification bed, Gilfillan (2001). At a stoichiometry of 0.35 ( rich) is expected to produce under equilibrium concentration of combustible gas products at about 800 C of 17 % H2, 6% CO, and 3 % CH4, Gilfillan (2001). Approximately, 0 % solid carbon remains at these conditions. Figure 7. Equilibrium Concentration from Fluidized Bed, Gilfillan (2001). 6.0 PILOT SCALE COMBUSTION TESTS The performance of fuels can be approximately estimated by the chemical analyses supplemented by empirical formed from full scale or pilot scale data. However, these techniques yield only an approximately indication of performance of fuels. The authors chosen, instead, to determine the combustion performance of sugar cane wastes based on techniques and procedures developed at EER by one of the authors, Pohl (2008) using data from a 5 Mwt combustor. Parameters that can affect to performance of sugar cane wastes are: 1. preparation and feeding of the fuel 2. ignition of the fuel 3. Flame Stability 3. Burnout of the fuel 4. Deposits - Slagging - Fouling 5. Emissions - particulate ESP, CO and Burnout - HC's - NOx 6.1 SCALING The pilot scale combustor is designed to maintain the parameters that control the above properties as near as possible. This requires that the gas velocity be sufficient to fluidized the particles and carry the particle through the reactor as close to the full scale units temperature-time profile. Additionally, the temperature of the heat transfer surface must be similar to the that in full size equipment. The electrostatic preceptors must have similar properties to the full scale precipitator. This is accomplished by reducing the velocity in the unit until the gas resident time is approximate the value of the full sized unit, but not below the velocity necessary to fluidize the particles. The time-temperature profile is achieved by insulating the pilot scale unit. The heat transfer surfaces are controlled by insulating and/or heating and cooling fluids. The heat transfer surfaces should be monitoring in time. The heat flux through the surface as an indication of deposits. 6.2 CONSTRUCTION OF 5 Mwt PILOT SCALE COMBUSTOR Figure 8. shows the design of the 5 Mwt co-generation pilot scale unit. Figure 9. Design of 5Mwt Pilot scale Combustor, Gilfillan (2001). Figure 10. shows the heat transfer and estimated temperature profile of the pilot scale unit. Multiple layers of insulation were required to reduce heat loss. The inner layer has a high temperature limit, but high conductivity. This is followed by layer of progressively lower temperature layers with lower conductive. This design also includes a annulus with a heat transfer fluid. This yields an inside wall temperature of 1600 C and outside wall temperature approaching 100 C. Figure 10. Design of the Heat Transfer Walls of the 5 Mw Pilot Scale Combustor, Gilfillan (2001). 6.3 PHOTOGRAPH of 5 Mwt Pilot Scale Rig. Figure 11. shows a photograph of the 5 Mwt pilot scale Rig and Bill Gilfillan, Gilfillan (2001) . Figure 11. Photograph of 5 Mwt Rig with Bill Gilfillan, Gilfillan (2001). 6.3 Design of Deposition Collection Probe. Two deposit collection probe are required, one for slagging and one for fouling. 6.3.1 FOULING PROBE Figure 12. shows the design of the super heater heat transfer surface. This probe has a temperature of ~650 C=1200 F of the surface by controlling the cooling fluid of the surface. The change in heat transfer is then monitored as the deposits builds. The probe is position across the gas flow. The probe is also used to collect and remove deposits for examination by scanning electron microscopy (SEM). Figure 12. Design of Fouling Deposit Probe for the 5 Mw Pilot Scale Combustor, Gilfillan (2001). 6.3.2 SLAGGING PROBE The slagging probe also monitors heat transfer and collects a deposit sample for viewing with an SEM. The slagging probe forms a wall surface with the flow parallel to the surface. The temperature is again controlled by varying the cooling fluid flow. Once the wall temperature is achieved, ~1400 C=2550 F, the cooling flow is held constant and the change in heat transfer with time is monitored. The probe also collects a sample of the slagging deposit. Figure 13. DESIGN OF 5 Mwe SLAGGING PROBE, Gilfillan (2001). 7.0 CONCLUSIONS This paper has presented the program conducted in Australia to review the design around the world and select one for a unit to burn sugar cane wastes to generate electricity. The design is selected and test for a system to beneficially burn sugar cane wastes to produce electric. This project was supported by Queensland Biomass Integration Gasification (GBIG) project, Department of Mines and Energy of Queensland, and the Stanwell Corporation. The work was performed at the Sugar Research Institute (SRI), and The University of Queensland. The authors selected a co-generation system for burning sugar cane waste and generate electric. The reviewed capital costs varied from 1150 A$/Kwe to A$ 2500 /Kwe. the calculated generating costs varied from 4.0 to 6.9 A cents/Kwe-hr compared to a comparable price for coal (most of the electric is generated from coal in Australia) capital cost 700 A$/Kwe and generating cost of 3.5 A cents/Kwe-h. Therefore at the time and place this study was done generating electric for sugar cane waste was more expensive that generating electric from coal and the project was abandoned. Generation from sugar cane wastes is the most cost effective of the biomass fuels. None-the-less, sugar cane waste is the most effective of all fuels except coal. Use of sugar cane wastes also reduces the amount of greenhouse gases emitted to the atmosphere, the sugar cane wastes will degenerates to greenhouse gases (CO2 and CH4) if not burned. REFERENCES 1. Dixon, T.F., P.A. Hobson, J.A. Joyce, J.H. Pohl, B.R. Stanmore, and C Spero, , August, 1998 "Electricity Cogeneration and Greenhouse Gas Abatement in the Sugar Industry", Queensland Biomass Energy Group, Confidential Report, Queensland, Australia, August, 1998. 2. Juniper, L.A. and S. Joseph, Biomass Resources and Their Use for Power Generation in Queensland, Study for Queensland Transmission and Supply Corporation, August 1996. 3. Gilfillan, William N.,(2001) , Design of a Laboratory Rig to determine Slagging of Biomass Fuels,MEngSc, University of Queensland, St. Lucia, QLD, Australia 4. Coxhead, B.A., "Queensland Coals: Physical and Chemical; Properties, Colliery and Company Information", Department of Mines and Energy, Brisbane, Qld., Australia, 1999. 5. Pohl, J.H., Evaluation of the Combustion Properties of Coal, Presented at ASME Southern California Division, 2009, Los Angeles, CA.