Effects of co-produced biochar on life cycle greenhouse gas emissions of pyrolysis-derived renewable fuels

Biochar is a co-product from biomass pyrolysis that can sequester carbon when applied to soils. It may also reduce N2O and CH4 emissions from soils, increase fertilizer efficiency, increase soil organic carbon, and increase crop yields. Treatment of these additional agricultural effects in life cycle analyses (LCAs) of pyrolysis-based liquid fuels could significantly influence LCA results. In this study, we include these effects in analyses of fast and slow pyrolysis. We also consider scenarios in which biochar is combusted to produce electricity. Probability distribution functions are developed for biochar yield and carbon content whereas average, minimum, and maximum values for biochar's stability factor and agricultural effects are developed from a thorough literature review and used in baseline and sensitivity analyses. Overall, life-cycle greenhouse gas (GHG) emissions for pyrolysis-based gasoline are lower when biochar is applied to soil than when it is combusted. Carbon abatement (CA) values of fast and slow pyrolysis fuel production systems are comparable. CA is reduced for an alternative fast pyrolysis system in which the pyrolysis oil is combusted for heat and electricity generation rather than upgraded to a hydrocarbon fuel. In the baseline case with biochar soil application, inclusion of agricultural effects reduces GHG emissions by 2.1 g CO2e/MJ from 16 g CO2e/MJ. Biochar carbon content and yield exert the strongest influence on GHG emissions results. Results are also sensitive to biochar's ability to suppress N2O emissions and increase soil organic carbon, which are subject to high uncertainty. © 2013 Society of Chemical Industry and John Wiley & Sons, Ltd