What is Biomass Gasification?
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Biomass gasification is a thermal process that converts plant matter into mixtures of combustible gases (hydrogen, carbon monoxide, methane, ethylene, etc). It is accomplished through direct or indirect heating of the biomass in the presence of limited oxygen, causing partial oxidation of the biomass polymers. The basic process has been known since 1792, when a Scottish engineer first gasified coal to light his home. A similar gas, also known as “town gas,” was distributed throughout Europe and the U.S. beginning in the latter half of the 18th century for street lighting and home heating.
Gasification can convert 90% of the chemical energy contained in dry plant material into chemical fuel gas plus recoverable waste heat. Direct gasification can be accomplished with air or steam and oxygen. When using air, it is called "air blown" gasification and generates “producer gas” with an energy density of about 150 Btu/scf. The nitrogen in the air is not utilized in the reaction and dilutes the fuel gas. When using steam and oxygen instead of air, "synthesis gas” is produced. Synthesis gas or “syngas” is a richer fuel gas with an energy content of 300 Btu/scf.
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Gasification differs from combustion in several ways. When oxygen is limited in the gasification reaction, combustible products like hydrogen, carbon monoxide, and methane are produced. If sufficient oxygen is provided, as it is in combustion, the products fully oxidize to water vapor and carbon dioxide. While combustion is useful for providing immediate heat, the produced gases have little chemical energy remaining.
