Climate mitigation potential of biofuels from residues and wastes
This paper conducts a novel analysis of the carbon intensity of several advanced biofuel pathways and processes. Using the U.K. Department for Transport biofuels carbon calculator as an LCA framework, adding modules to capture additional GHG emissions in the expanded system boundary and considering a project time frame of 20 years, the authors analyze eight types of agricultural residues, one type of forest residue (“slash”: tree tops, twigs and branches), biogenic municipal solid waste, and sawdust. These feedstocks are based on the potentially sustainably available feedstocks in the European Union. For each feedstock, the authors consider three biofuel pathways: biochemical ethanol, Fischer-Tropsch diesel, and pyrolysis diesel.
The study shows that cellulosic biofuels derived from agricultural residues offer substantial GHG savings (exceeding 60%) for all three biofuel pathways, even given the expanded system boundary. The GHG benefits are even higher for biogenic municipal solid waste, where feedstock collection can result in avoided methane emissions, and for biofuel from sawdust (despite appreciable indirect emissions in that case).
The GHG emission savings of cellulosic biofuels produced from slash are dependent on the magnitude of soil carbon loss due to residue harvest, as well as on expected residue decomposition rates. Still, provided that soil carbon loss is minimized, biofuel from slash offers 20-year GHG savings greater than 50%, even considering these additional emissions sources.
This study demonstrates that creating further incentives for the production of biofuel from cellulosic wastes and residues in general through the Renewable Energy Directive has potential to deliver carbon savings without the same level of indirect land use change concerns or sustainability effects associated with production of biofuel from food commodities. However, not all wastes and residues will provide substantial GHG emission reductions. As Europe considers the support mechanisms for cellulosic fuels, it is important that the criteria for wastes and residues not over-incentivize biofuels from these feedstocks whose carbon footprints are comparable to those of fossil fuels. Additional incentives for using wastes and residues will be most effective if targeted at the feedstocks that deliver the largest environmental benefit when comprehensive carbon accounting is undertaken.