Microalgal Biomass for Greenhouse Gas Reductions; Potential for Replacement of Fossil-Fuels and Animal Feeds

Microalgal biomass production offers a number of advantages over conventional biomass production including, higher productivities, use of otherwise non­ productive land, reuse and recovery of waste nutrients, use of saline or brackish waters, and reuse of CO2 from power-plant flue-gas or similar sources. Microalgal biomass production and utilization offers potential for greenhouse gas (GHG) avoidance by providing biofuel replacement of fossil fuels and carbon-neutral animal feeds. This paper presents an initial analysis of the potential for GHG avoidance using a proposed algal biomass production system coupled to recovery of flue-gas CO2 combined with waste sludge and/or animal manure utilization. A model is constructed around a 50 megawatt (MW) natural gas fired electrical generation plant operating at 50% capacity as a semi base-load facility. This facility is projected to produce 216 million kWh/240-day season while releasing 30.3 million kg-C/season of GHG-CO2. An algal system designed to capture 70% of flue-gas CO2 would produce 42,400 metric tons (dry wt.) of algal biomass/season, and require 880 ha of high-rate algal ponds operating at a productivity of 20 g-dry-wt/m-day. This algal biomass is assumed to be fractionated into 20% extractable algal oil, useful for biodiesel, with the 50% protein content providing animal feed replacement, and 30% residual algal biomass digested to produce methane gas, providing gross GHG avoidances of 20%, 8.5% and 7.8% respectively. The total gross GHG avoidance potential of 36.3% results in a net GHG avoidance of 26.3% after accounting for 10% parasitic energy costs. Parasitic energy is required to deliver CO2 to (1) Professor and Newman Endowed Chair, Department of Agri. and Biol. Engr., Clemson Univ., Clemson, S.C., 29634, Assistant Professor, California Polytechnic State University, San Luis Obispo, CA., 93407, (3) Institute for Environmental Management Inc., 3434 Tice Creek, Walnut Creek, CA., 94595 the algal culture and to harvest and process algal biomass and algal products. At CO2 utilization efficiencies predicted to range from 60% to 80%, net GHG avoidances are estimated to range from 22% to 30%. To provide nutrients for algal growth and to ensure optimal algae digestion, importation of 53 metric tons/day of waste paper, municipal sludge, or animal manure would be required. This analysis does not address the economics of the processes considered. Rather, the focus is directed at determination of the technical feasibility of applying integrated algal processes for fossil-fuel replacement and power-plant GHG avoidance. The technology discussed remains in early stages of development, with many important technical issues yet to be addressed. Although theoretically promising, successful integration of waste treatment processes with algal recovery of flue-gas CO2 will require pilot-scale trials and field demonstrations to more precisely define the many detailed design requirements.