(28a) Impacts of CO2 Supply Systems for Algal-Based Biorefineries on Biofuel Life Cycle Assessments

Biofuels have great potential as low-carbon alternatives to fossil-based transportation fuels. This talk will focus on utilization of CO₂ in an advanced biofuel system and the integration of that system with CO₂ generation sources. The biofuel system is the Algenol Direct to Ethanol^®technology, which provides an efficient, cyanobacteria-based system for producing ethanol and bio-crude. The Algenol process consumes CO₂ in a process that yields a biofuel with much lower greenhouse gas emissions than gasoline.^1-3 In this talk, different options for CO₂ capture and utilization (CCU) are considered and their impact on the overall system operation evaluated in terms of carbon footprint reduction. The boundary of the life cycle analysis includes the CO₂ source. Overall 13 scenarios are described including an artificial reference case with CO₂ arriving at the boundary limits of a 2000 acre, model biorefinery with zero carbon footprint burden and zero CO₂ cost at a 10% IRR. Most scenarios involve combined heat and power (CHP) systems with no power supplied from the grid. Both natural gas and cellulosic fuel sources are considered for the CHP system. Five of the scenarios are “stand-alone” in that they do not require co-location with an anthropogenic CO₂ source. This is achieved by generating enough CO₂ on site to supply the needs of the biorefinery and selling excess power back to the grid. Four of the five stand-alone systems have carbon footprints and economics that are fully competitive with the commonly considered coal flue gas case. Overall carbon footprint reductions range from 23% to 116%. Equivalent CO₂ costs (based on comparisons to the reference case) range from $35/-$96/tonne with coal flue gas being $45/tonne.

¹D. Luo et al, “Lifecycle Energy and Greenhouse Gas Emissions from Ethanol Produced by Algae”, Environmental Science and Technology 44, 8670 (2010).

²Notice of EPA pathway approval: http://epa.gov/otaq/fuels/renewablefuels/new-pathways/approved-pathways.htm

³R. P. Lively et al, “Anthropogenic CO₂ as a feedstock for the production of algal-based biofuels”, Biofuels, Bioproducts, and Biorefining  9, 72-81 (2015)