(634f) Algal Feedstocks for Biogas Production: What Are We Waiting for?

Biogas energy (mainly hydrogen and methane) from renewable sources can replace fossil fuels and address issues of environmental pollution and energy security but has been scarcely developed in the USA. The choice of biomass feedstock is being debated nationally as it is a major determinant of energy conversion efficiency (ECE = [heating value of gas]/[heating value of biomass]). Herein we illustrate that microalgal biomass feedstocks offer significant ECE advantage over terrestrial plant feedstocks as they are devoid of recalcitrant biopolymers (cellulose, hemicellulose & lignin). A novel two-stage dark fermentation process for cogenerating H2 and CH4 from microalgae biomass was used in this paper, ultilizing hydrogenogenic and methanogenic bacteria. The theoretical maximum yields of H2 and CH4 from cyanobacterial biomass (A. maxima) by two-phase fermentation are 16.6% (ECE only to H2) and 61.9%. The carbohydrate content in A. Maxima biomass harvested at different growth time, light condition, nitrate and sodium media contents were measured. The energy conversion efficiency increases dramatically from 1.2% for hydrogen-only production to 15.7% for cogenerating H2 and CH4, using young cells with high protein/low carbohydrate content obtained by growth on excess nitrate. The total carbohydrate in A. Maxima cells increases from 14.5% to 41.0% when the nitrate dosage decreases from 30 to 10 mM. Nutrient deprivation is a proven means to shift cellular composition to obtain more carbohydrates as precursor to further increasing the yields of fermentative H2 and CH4 and ECE. Supported by the DOE-GTL program and AFOSR-MURI program.