The effectiveness with which greenhouse gas emissions (GHGs, including CO2, CH4, and others) can be avoided using biofuels is related to the amount and carbon intensity of the fossil fuel inputs needed to produce the biofuel, as well as to what fossil fuel is substituted by use of the biofuel. A proper GHG accounting considers the full life cycle of the biofuel, from planting and growing the biomass to conversion of the biomass to biofuel, to combustion of the biofuels at the point of use. (In the case of vehicle applications, this full life cycle analysis is sometimes referred to as a “well-towheels” analysis.) If the harvested biomass is replaced by new biomass growing year-on-year at the same average rate at which it is harvested, then CO2 is being removed from the atmosphere by photosynthesis at the same rate at which the already-harvested biomass is releasing CO2 into the Atmosphere – carbon-neutral situation. However, typically some fossil fuel is consumed in the course of producing or converting the biomass or delivering the biofuels to the point of use, resulting in net positive GHG emissions on a life cycle basis. These emissions will offset to some degree the emissions that are avoided when the biofuel is used in place of a fossil fuel.
There could also be net GHG emissions associated with converting land from its current use to use for biomass energy feedstock production. The net emissions might be positive if existing forests were to be removed to establish energy crops. The net emissions might be negative if perennial energy crops (which can build soil carbon) are established, replacing annual row crops that were being grown on carbon-depleted soil. Emissions associated with land use change can be significant, but are very much dependent on local factors. Therefore, as a simplifying assumption for the discussion presented in this publication, no GHG emissions associated with such land use changes are Considered.
There is a rich literature on GHG life cycle analyses (LCAs) of biofuels. Most published LCAs have been undertaken in a European or North American context, with an excellent study of Brazilian sugar cane ethanol being an exception. There is considerable context-specific variability and uncertainty around input parameter values in LCA analysis, which may explain the wide-ranging results from different studies for the same biofuel and biomass source. The estimated range in reductions of GHG emissions per vehicle kilometre (v-km) for rape methyl ester (RME) compared to conventional diesel fuel (for which RME can substitute) is 16 per cent on the low end and 63 per cent on the high end – a range of a factor of four. The range in reduction indicated for SME (soy methyl ester) is 45 per cent to 75 per cent. The range for ethanol from sugar beets is somewhat smaller (but complicated by three alternative sets of assumptions about how GHG emission credits are assigned to the residual pulp co-product of ethanol production). Ethanol from wheat shows anywhere from a 38 per cent GHG emissions benefit to a 10 per cent penalty relative to gasoline.
Understanding such diversity in LCA results requires examining details of each analysis, including analytical boundaries, numerical input assumptions and calculation methodologies. However, without delving into that level of detail, it is possible to draw a few firm conclusions. Higher GHG savings with biofuels are more likely when sustainable biomass yields are high and fossil fuel inputs to achieve these are low, when biomass is converted to fuel efficiently, and when the resulting biofuel is used efficiently. Conventional grain- and seed-based biofuels can provide only modest GHG mitigation benefits by any measure (per megajoule of fossil fuel displaced, per v-km driven, or per hectare of land use) and will be able to provide only modest levels of fuel displacement in the long term in any case due to the relatively inefficient land use associated with these fuels.
The fundamental reason for the poor performance of grains and seeds is that they represent only a portion (typically less than 50 per cent of the dry mass) of the above-ground biomass, so they are disadvantaged from the yield point of view. Higher efficiency in converting seeds/grains to fuel compensates the lower biomass yield to some extent. For example, some 380 litres of ethanol can be produced from a dry ton of corn grain using current technology. This compares to today’s known technology for cellulosic biomass conversion to ethanol, which can only yield some 255 litres/dry ton (at least on paper – no commercial-scale plant has been built). Future improvements in cellulosic ethanol production are expected to eliminate the conversion efficiency advantage currently enjoyed by corn ethanol: yields from lignocellulose of 340 litres/ton are projected for 2010 and 437 litres/ton for 2030. Technology for production of Fischer-Tropsch fuels from lignocellulose (which could be commercially ready in the 2010/2015 timeframe) can yield some 280 litres of diesel equivalent, which corresponds to 471 litres of ethanol equivalent.
More efficient land use in mitigating GHG impacts can be achieved in the longer term by dedicated high-yielding lignocellulosic energy crops. Decades of experience with development of food crop yields, together with recent experience with developing lignocellulosic energy crops, suggests that major yield gains can be expected (probably with lower inputs per ton of biomass produced) with concerted development efforts. While historically there have been relatively low levels of research and development support provided for energy crop development, recent major private sector investments in research and development are likely to accelerate the pace of progress in improving yields. Assuming high yields are sustainable and acceptable from biodiversity and other perspectives, land requirements to achieve GHG emission reductions with biofuels will be reduced. There is also the possibility for some by-product CO2 to be captured (for long-term underground storage) during the process of making biofuels, especially via thermochemical conversion, which could lead to negative GHG emissions for a biofuel system. Proposals have also been made for thermochemically co-processing coal and biomass to make carbon-neutral liquid fuels by capturing and storing some CO2 produced during the conversion process.
Finally, it is worth noting that biomass can be converted into heat or electricity as well as into liquid fuel. GHG emissions per unit land area that are avoided in this way may be greater than when making liquid fuel. However, for electricity or heat production, a variety of renewable resources is available (hydro, solar, geothermal, wind, etc.). Biomass is the only renewable source of carbon, which makes it the only renewable resource for producing carbon-bearing liquid fuels.