Life-Cycle Analysis of Energy Use, Greenhouse Gas Emissions, and Water Consumption in the 2016 MYPP Algal Biofuel Scenarios

The Department of Energy (DOE) Bioenergy Technologies Office (BETO) Multi-year Program Plan (MYPP) describes the bioenergy objectives pursued by BETO, the strategies for achieving those objectives, the current state of technology (SOT), and a number of design cases that explore cost and operational performance required to advance the SOT towards middle and long term goals (MYPP, 2016). Two options for converting algae to biofuel intermediates were considered in the MYPP, namely algal biofuel production via lipid extraction and algal biofuel production by thermal processing. The first option, lipid extraction, is represented by the Combined Algae Processing (CAP) pathway in which algae are hydrolyzed in a weak acid pretreatment step. The treated slurry is fermented for ethanol production from sugars. The fermentation stillage contains most of the lipids from the original biomass, which are recovered through wet solvent extraction. The process residuals after lipid extraction, which contain much of the original mass of amino acids and proteins, are directed to anaerobic digestion (AD) for biogas production and recycle of N and P nutrients. The second option, thermal processing, comprises direct hydrothermal liquefaction (HTL) of the wet biomass, separation of aqueous, gas, and oil phases, and treatment of the aqueous phase with catalytic hydrothermal gasification (CHG) to produce biogas and to recover N and P nutrients.