Direct and indirect greenhouse gas emissions from biomass storage: Implications for life cycle assessment of biofuels
Ethanol and other biofuels from cellulosic feedstocks are currently the most promising candidates to replace a large fraction of gasoline consumption in the United States and reduce greenhouse gas emissions. Gaps in current approaches to estimating the net greenhouse gas emissions from second-generation biofuels may lead to underestimation of the carbon intensity of these fuels. Current life cycle assessment models of biofuels do not sufficiently account for biomass losses and emissions associated with the harvest and storage of biomass feedstocks, which can require additional fuel and materials use on the farm as well as reducing the effective yield of a crop at the biorefinery gate. The goal of this dissertation is to quantify the range of likely impacts of feedstock storage on the net greenhouse gas emissions from biofuel production. A broad survey of published forage and bioenergy studies was used to assess the range of likely feedstock dry matter losses during storage by several methods. These loss distributions, as well as updated parameters for biomass harvesting processes and potential direct emissions of non-CO2 greenhouse gases during biomass decomposition were incorporated into the Greenhouse gases, Regulated Emissions, and Energy use in Transportation (GREET) model to determine the effects on life cycle global warming impact. Methods for laboratory-scale storage experiments were developed using a variety of potential bioenergy feedstocks harvested at Purdue University. Experiments with corn stover and switchgrass under controlled temperature and moisture conditions were conducted to determine rates of dry matter loss and methane and nitrous oxide emissions during storage. Results show that updating biofuels life cycle analysis models to include harvest and storage of biomass feedstocks can substantially increase net greenhouse gas emissions from 2.0–10.0 gCO2e/MJ ethanol. Differences between storage methods are significant: materials use and direct emissions of methane may lead to greater emissions during wet storage, while covering dry bales reduced average emissions and variability. Both methane and nitrous oxide are produced during aerobic biomass storage at the laboratory scale, though at low rates which may not substantially affect the carbon intensity of cellulosic biofuels. Incorporating harvest and storage parameters into biofuels life cycle assessment models significantly alters both point estimates and stochastic analyses of greenhouse gas emissions. While ethanol from cellulosic feedstocks still provides a greater than 60% reduction in greenhouse gases compared to gasoline, storage processes should be considered when assessing the extent to which biofuels reduce net fossil energy use and climate change emissions.
Mosier, Purdue University.
Agricultural engineering|Environmental science
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