Biobased Fuel Gas and Syngas

Syngas Plant in Maryland

Fuel gas and syngas are products of either black liquor gasification or biomass gasification. The mixture of raw product gases vary according to the feedstock and the gasification approach utilized. Regardless of the process, fuel gas and syngas must be separated from mineral ash and char, alkali compounds, tars, nitrogen components, and sulfur (Stevens 2001). For use in fuel synthesis or fuel cells, the gases must also be reduced to remove methane, ethane, and other hydrocarbons.

Bio-based fuel gas is also known as producer gas or wood gas and contains a relatively low energy density. Over 50 percent of its content is nitrogen and 40 percent hydrogen and carbon monoxide. This produces a heating value of about 5 MJ/m3 in a biomass gasification process. Bio-based syngas is a medium energy mixture that is richer in hydrogen and carbon monoxide with a heating value of 10 to 20 MJ/m3 (Ross 1996).

Low energy fuel gas is most suitable for combustion to produce thermal energy, however research by FlexEnergy is examining its potential for combustion in microturbines. Medium energy syngas, once cleaned, can be used as fuel in boilers or to fuel electricity and steam generation via gas turbines or fuel cells. However, some fuel cell designs should not be used due to the limited tolerances for carbon monoxide. Steam reformed syngas has a high content of hydrogen and is used in applications that demand high levels of gas cleanliness. Methanol and DME or dimethylether are liquid fuels that can be used in automotive systems. They have a hydrogen to carbon monoxide ratio of 2.1 to 1 and, with the Fischer-Tropsch process, utilize metal catalysts to widen the range of hydrocarbons that can be produced. At the present time this process is not yet competitive with coal or petroleum but as prices continue to climb for those fossil fuels, F-T plants are becoming more of an option (Rauch 2002).

The fuel gas is fed to microorganisms that can convert the gas to ethanol, methane, or other fuel forms. Research is ongoing to look at the effectiveness of producing bio-based feedstocks for plastics, wax oils, methane, and ethanol from these organisms (Larson et al. 2003, Paisley and Overend 2002).