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Gasification
Like direct combustion, gasification is a high-temperature thermochemical conversion process, but the desired result in this case is the production of a combustible gas, instead of heat. This is achieved through the partial combustion of the biomass material in a restricted supply of air or oxygen, usually in a high-temperature environment of around 1200-1400 oC. The product of gasification - producer gas - can, after appropriate treatment, be burned directly for cooking or heat supply, or it can be used in secondary conversion technologies such as gas turbines and engines for producing electricy or mechanical work.
Electricity production from gasification is a more complex process then using the direct combustion/steam cycle process, resulting in biomass gasification power systems having higher investment costs. However, the advatage of gasification is that it can achieve considerably higher efficiencies: most small and medium combustion power plants have overall efficiencies in the range of 15-20%, wheras their gasification counterparts can achieve ovarall efficiences of around 35%, with 45-50% as a near-term possibility. As a further attempt to improve the efficiency the gas turbine can be integrated with a steam turbine, in which case the system is termed biomass integrated gasification combined cycle - BIGCC. An important thing to keep in mind is that higher efficiencies mean also lower emissions.
The main types of gasifier designs are the following:
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| McNeil Generating Station in Burlington, Vermont. Credit:
Parsons, Dave – NREL/DOE |
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Gas production module. Credit: Yost, Jim – NREL/DOE |
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Thermochemical Users Facility (TCUF) main reactor and
feed hopper. Credit: Yost, Jim – NREL/DOE |
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