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Biomass Gasification

Authored By: D. Cassidy

The gasification process occurs at temperatures between 600-1000 degrees Celsius and decomposes the complex hydrocarbons of wood. Gasification products can not be easily stored and thus the system is often integrated with other conversion processes to utilize the outputs in the form of various bio-based syngases. The gasification process produces ash and char, tars, methane, charcoal and other hydrocarbons. Most of the time these byproducts are acceptable if the gases are to be combusted, but in fuel synthesis or fuel cells, they must be reduced by using scubbers or filters (Stevens 2001).

Studies have shown that gasification systems can be as much as 20 percent more efficient than direct combustion systems (Bain and Amos 2003) thus potentially making them more economical for power production. Biomass gasification is an emerging technology; most developers are still in the prototype or first commercial demonstration stage, but research is ongoing worldwide. The Community Power Corporation of Littleton, Colorado is currently testing a small scale unit and larger systems are coming online in Skive, Denmark. SilvaGas is being promoted worldwide by FERCO, the Atlanta-based Future Energy Resources Corporation.

The chemical composition of the feedstock influences the constituents of the product gas, the gasification design, and the clean-up methods that must be used, thus some types of biomass may prove more costly to gasify than others. For example, wood residues high in sodium or potassium will require pre-cleaning prior to utilization. In general, feedstocks should have a high carbon-to-nitrogen ratio, relatively low sulfur content, and a low moisture content (Bain and Amos 2003).

Gasification is a relatively old technology that was widely used during World War II. The Germans, when faced with a lack of petrol, gasified coal using the Fischer-Tropsch process to produce diesel. The pollution from this created vast forest destruction in eastern Germany, Poland and the Czech Republic.