Biomass Gases

Biomass gases may be produced from anaerobic digestion of any type of biomass from methane-producing organisms. Common feedstocks for biomass gases are wood waste, landfill, and sewer sludge. Types of biomass gases include sanitary landfill gas and digester gas.

Sanitary landfill gas is produced from digesting landfill. Biological degradation of the organic material produces large quantities of methane, which can be captured under a non-permeable cap over the site, pumped out of the landfill, filtered, and used commercially. This gas has a moderate heating value of about 500 to 600 Btu/cf (18,600 to 22,400 kJ/m3) due to a high content of CO2 (about 35 to 45%).

The presence of hydrogen sulfide (H2S) can damage equipment, even in small quantities. Levels as low as 0.1% of H2S must be treated to avoid corrosion of certain equipment metals. The term "sweet" refers to sulfur-free gas and "sour" refers to gas containing a large proportion of sulfur compounds.

Digester (sewage or sludge) gas is a mixture of several component gases produced in digester tanks where biodegrading takes place. It can be made from sewage, animal waste, and liquid effluent from vegetable oil mills and alcohol mills. Generally, these gases consist of about two-thirds methane and one-third carbon dioxide, with a few percent nitrogen and minute quantities of other gases such as oxygen, hydrogen, and hydrogen sulfide.

Methane is the primary combustible component that makes this gas compatible with equipment designed for natural gas usage. At 66% methane, the HHV is 668 Btu/cf (24,891 kJ/m3). Of course, if a significant amount of H2S (0.01%) is present, the gas must be treated to avoid corrosion of certain equipment metals.

Another technique for producing combustible gas from biomass is similar to the process used for producer gas. In this case, biomass, most commonly wood waste materials, are used instead of coal or coke.

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