CO2 Life-Cycle Assessment of the Production of Algae-Based Liquid Fuel Compared to Crude Oil to Diesel

Life-cycle assessment for carbon dioxide emission was performed for an open pond microalgae based process to grow, harvest, extract, and produce a paraffinic hydrotreated algae oil (HTAO). The process was assumed to also produce a co-product of animal feed. A base case was defined using generally accepted parameter values for present technology and does not represent an optimized design. This base case process was compared to the CO2 equivalent emissions from conventional crude oil refining through the middle distillate (MD) of the atmospheric column of a refinery. Both MD and HTAO products were viewed as equivalent being diesel or nearly diesel. CO2 emissions from the HTAO process were strongly dominated by drying of lipid extracted biomass for animal feed containing 85% moisture and the CO2 emissions in producing purified CO2 for algal growth. Ammonia as a nutrient was also a major CO2 contributor followed by energy required for pond flow. In aggregate, utilities (heat and electric) were a greater contributor than the combination of raw materials and nutrients. A sensitivity analysis showed that the most impactful operating parameters on CO2 emission from the base case HTAO process are, in order of importance, (a) moisture content of the lipid extracted biomass feeding the animal feed dryer, (b) oil content of algae, (c) algae doubling time, (d) feed broth algae concentration, (e) pond depth, and (f) retention time in the pond. On the basis of well-(or pond)-to-wheel (WTW), it was found that the base case algal route to HTAO of this work did not reduce CO2 emission compared to production of conventional diesel, being worse than MD. However, if moisture content in the wet cake going to the animal feed dryer was reduced from 85% to 75% moisture and if a 10% improvement in operating parameters over the base case was made, the HTAO process was estimated to have WTW CO2 emissions which are similar to 34% lower than MD. Since the base case is not optimized with respect to process integration and energy economization, still further improvement in the HTAO process is possible.