Greenhouse Gas Effects

Life cycle GHG emissions from a fuel source refer to the aggregate GHG production from both direct and significant indirect sources. The definition of life cycle greenhouse gas emissions established by the USEPA states that:

The term ‘life cycle greenhouse gas emissions’ means the aggregate quantity of greenhouse gas emissions (including direct emissions and significant indirect emissions such as significant emissions from land use changes),… related to the full fuel life cycle, including all stages of fuel and feedstock production and distribution, from feedstock generation or extraction through the distribution and delivery and use of the finished fuel to the ultimate consumer, where the mass values for all greenhouse gases are adjusted to account for their relative global warming potential [3].

Greenhouse gas emissions are expressed in mass of CO2 equivalent emissions per unit of fuel energy (typically kg CO2 eq per mmBTU), and are generally compared to a gasoline or diesel baseline scenario. This explains the “well to wheel” GHG reference within the RFS2 regulations, and is the reason approved biofuels must achieve at least a 60% GHG reduction compared to baseline scenarios. For cellulosic based biofuels, life cycle GHG emissions include those associated with growing, transporting, and converting feedstocks, as well as effects due to indirect land use changes and changes in soil carbon. Because cellulosic-based processes are just coming on-line in 2013 as this chapter is being written, the best projections for the United States are based on environmental analyses prepared by the USEPA [1] for the RFS2 mandate. Based on this analysis, switchgrass had net GHG emissions of —110% or -72%, for biochemical or thermochemical conversion, respectively. By comparison, corn stover had net GHG emission reductions of —129% or —92%, for biochemical or thermochemical conversion, respectively. Overall, a reduction in net GHG emissions of greater than 100% means the entire system will store carbon. Compared to the 2006 baseline, diesel from a Fischer-Tropsch process had net GHG emissions of —71% for switchgrass and —91% for corn stover. Estimates were made by the USEPA only for these two feedstocks because there was insufficient data for other cellulosic feedstock sources.

Although net GHG emissions of other crops are not as well understood as switchgrass, estimates have been made for other perennials that may be used for cellulosic feedstocks. In a study of 10 Midwestern states, Gelfand et al. [10] estimated that about 25% of the RFS2 mandated supply of liquid biofuels could be produced on 11 million ha of marginal and under-utilized land by growing successional mixes of herbaceous plants or hybrid poplar (Populus spp.) plantations. Both crops had net GHG emissions of —105% or less. Modeling of these areas indicated that enough feedstock could be grown within 80 km of potential biorefineries to produce these fuel amounts using secondary succession vegetation. Using marginal agricultural lands that are not in current crop production or which have low yields for conventional crops reduces both the carbon debt and the indirect land use effects of conversion of land use to cellulosic bioenergy crops [10].

In many cases, net GHG reduction depends on the type of energy sources used in the conversion facility. For instance, ethanol production from switchgrass can emit fewer GHGs by combusting the lignin component to produce the facility’s steam and electricity needs and thus eliminating the need for external energy sources [11,12].

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