Like fossil fuels, biofuels release carbon dioxide emissions during combustion. However, some of these emissions can be partly offset by reductions in greenhouse gas emissions elsewhere in the fuel’s lifecycle. For crop-based feedstocks, plants absorb carbon dioxide from the atmosphere as they grow, which can offset some of the emissions released when the fuel is used. For fuels made from residues or waste materials, using these feedstocks can avoid some greenhouse gas emissions that would occur if the materials were instead left to decompose.
Assessing the emissions performance of biofuels, therefore, requires considering emissions across the full fuel lifecycle rather than from combustion alone. This performance is summarized using a metric known as carbon intensity, which estimates net greenhouse gas emissions per unit of energy. Carbon intensity is estimated using an approach known as lifecycle analysis, which accounts for emissions associated with a fuel’s production, transportation, and use. Several US biofuel policies tie incentives to the carbon intensity of fuels relative to conventional petroleum fuels, with lifecycle emissions estimated using models such as Argonne National Laboratory’s Greenhouse gases, Regulated Emissions, and Energy use in Technologies (GREET) model.
