The US EPA estimated 5,500MW of power could be produced from US landfill, wastewater treatment, and animal waste digester biogas. These biogas resources are an important component of improving distributed generation and reducing emissions of greenhouse gases such as methane. Siloxanes and Hydrogen sulfide (H2S) are the most troublesome biogas contaminants. During combustion, siloxanes are converted into silicates (sand), which damage turbine blades; form harmful scales that increase engine wear, and foul post combustion catalysts. The H2S and halogenated compounds in the gas form corrosive acids that poison catalytic emission control systems. Commercially available media adsorbents will adsorb H2S, siloxanes, organic halides, and long chain hydrocarbons in biogas but are expensive and difficult to regenerate. Current media regeneration methods are complicated, expensive and can involve additional waste streams or flares.
The CHA biogas-upgrading system consists of contaminant adsorption and microwave regeneration system. It uses two-stage adsorption to remove H2S with impregnated activated carbon and siloxanes with silica gel adsorbents. When the adsorbents are fully saturated, they are removed and replaced with regenerated media.
The saturated adsorbents are regenerated with microwaves which causes rapid desorption of the contaminants. The regenerated media is recycled back to the adsorber. The contaminant gas from the regeneration step is first chilled to condense water and hydrocarbons. The remaining contaminant gas then flows through a microwave reactor where air is added so that H2S is converted to elemental sulfur and H2O. The sulfur is filtered out for disposal. A second microwave reactor is used to rapidly oxidize the remaining contaminants forming water vapor, carbon dioxide and solid silica. This gas flows to a final activated carbon adsorber to remove any remaining trace contaminants. There is no flare needed to burn waste gas.
CHA Corporation and the Sacramento Municipal Utility District (SMUD) were awarded a California Energy Commission grant to field-test the biogas upgrading technology. A two-month long field-demonstration was successfully completed in October 2009 at Carson Energy Group’s Elk Grove, CA power plant using a side stream of biogas produced from the Sacramento Waste Water Treatment Plant.
Following this demonstration SMUD provided additional funding demonstrate the technology’s ability to remove contaminants from landfill gas produced at the Yolo County California Landfill. A three month test campaign was successfully completed in December 2009.