Environmental Engineering Reference
In-Depth Information
The Searchinger et al. (2008) study has been criticized regarding assumptions on crop prices and
co-product displacements, land-use change locations, and oversimplified modeling of free market
interactions. Indirect land-use change caused by biofuel production is an active area of research that
will continue to evolve through research on modeling of global market interactions and allocation
methods. For example, Kendall et al. (2009) developed a method for reporting the life-cycle emis-
sions intensity of a large initial CO
2
emission pulse, such as from clearing new cropland, which
reflects the relative effect of those early emissions. As will be discussed later in this section, the
U.S. Renewable Fuel Standard policy—a central driver of U.S. biofuel production—does include
indirect land-use in life-cycle GHG calculations.
Fargione et al. (2008) examined land-use change to determine upper bounds on potential GHG
emissions due to conversion of various types of habitats. The authors estimated that conversion of
rainforest could incur a carbon debt that would take from dozens to hundreds of years to repay,
whereas using abandoned or marginal cropland for agriculture would incur little to no carbon debt.
The fraction of the carbon debt allocated to biofuels produced from these lands (allocation to co-
products was included) was as low as 39% for soybeans and as high as 100% for sugarcane on the
basis of a relative market value allocation approach. However, Kim et al. (2009) contend that better
modeling of land management practices, which are more representative of the current state of tillage
practices, significantly reduces the carbon debt payback period.
The Gallagher review concluded that Europe has sufficient land available for biofuel production,
but the current production growth rate should be reduced until the impacts of increased produc-
tion are better understood (RF Agency 2008). The review also recommended that marginal and
idle lands be targeted for biofuel crop production and expressed concern over the findings of the
Searchinger et al. (2008) study, although they found the results to be unreliable because of numerous
assumptions that compounded uncertainty. Evaluation of marginal lands for feedstock crop produc-
tion is an active area of research (Gutierrez and Ponti 2009).
The U.S. Energy Independence and Security Act of 2007 (EISA 2007) is federal legislation
that includes mandates for biofuel production through year 2022, known as the Renewable Fuel
Standard. In the EISA (2007), biofuels are categorized according to their ability to reduce life-
cycle GHG emissions relative to a 2005 petroleum baseline, as shown in Table 11.11. For example,
the advanced biofuels category is defined as renewable fuels other than ethanol derived from corn
starch with 50% lower life-cycle GHG emissions. The EPA was charged with implementing a
life-cycle system model to calculate the life-cycle GHG emissions of various biofuel production
pathways.
taBle 11.11
life-cycle GhG emission criteria and example Biofuels
GhG
reduction (%)
a
Fuel category
example qualifying Fuels
Renewable fuel
20
Ethanol produced from corn starch at a new natural gas-fired facility
using advanced efficient technologies
Advanced biofuel
50
Ethanol produced from sugarcane
Biomass-based diesel
50
Biodiesel from soy oil and renewable diesel from waste oils, fats, and
greases; diesel produced from algal oils
Cellulosic biofuel
60
Cellulosic ethanol and cellulosic diesel (based on currently modeled
pathways)
Source:
EPA,
EPA Fuels and Additives—Renewable Fuel Standard Program
, 2010. U.S. Environmental Protection
Agency, available at http://www.epa.gov/OMS/renewablefuels/index.htm#regulations
a
Life-cycle GHG reduction requirement compared with 2005 petroleum baseline.
