Chapter 16 : Biomethane from Biomass, Biowaste, and Biofuels

Abstract:

Biogas can be made from most biomass and waste materials regardless of the composition and over a large range of moisture contents, with limited feedstock preparation. Methane-producing communities are very stable and resilient, but they are also complex and largely undefined. The methanogenic Archaebacteria uniquely catabolize acetic acid and one-carbon compounds to methane. The methanogens are obligate anaerobes that can pick up electrons from dead-end fermentations, through interspecies hydrogen transfer, and shuttle these electrons through a unique form of respiration which results in the reduction of carbon dioxide to methane. Codigestion with manure often enhances the conversion of other biomass and waste feedstocks through balancing micronutrients. Organic acids, pH, and alkalinity are related parameters that influence digester performance. The major alkalis contributing to alkalinity are ammonia and bicarbonate. Biowastes and biomass crops can be gasified in a reduced atmosphere combustion process to convert the biomass into a mixture of CH4, CO2, CO, and H2. Any improvement in conversion efficiency that enhances cellulosic ethanol yields is equally applicable for biomass conversion to methane. Methane yields from seaweeds, grasses, and crops all approach theoretical yields, such that as much as 80% of biomass energy content could be recovered in methane. Processing of terrestrial and marine energy crops to biomethane can result in higher energy yields than that of other biofuels.