Biomass Storage Options Influence Net Energy and Emissions of Cellulosic Ethanol

Abstract

Incremental biomass losses during the harvest and storage of energy crops decrease the effective crop yield at the biorefinery gate. These losses can affect the environmental performance of biofuels from cellulosic feedstocks by indirectly increasing agricultural inputs per unit of fuel and increasing direct emissions of pollutants during biomass decomposition in storage. In this study, we expand the Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREETTM) model to include parameters for harvest and storage of dry bales, bale silage, and bulk silage and examine the potential impact of the biomass supply chain on energy use and air pollutants from cellulosic ethanol from corn stover, switchgrass, and miscanthus feedstocks. A review of storage methods shows substantial differences in expected losses (4.2 to 16.0 %) and variability. Model results indicate that inclusion of feedstock harvest and storage pathways increases net fossil energy consumption (0.03–0.14 MJ/MJ) and greenhouse gas emissions (2.3–10 g CO2e/MJ) from cellulosic ethanol compared to analyses that exclude feedstock losses, depending on the storage scenario selected. Greenhouse gas emissions were highest from bulk ensiled silage and bale silage pathways, driven by direct emissions of greenhouse gasses during storage and material use, respectively. Storage of dry bales indoors or under cover minimizes emissions. This report emphasizes the need to increase the detail of biofuel production models and address areas of great uncertainty in the biomass supply chain, such as biomass decomposition emissions and dry matter losses.