The environmental impacts of switching from petroleum-based fuels to those from other feedstocks must include the impacts across the complete life cycle of primary energy production, feedstock logistics, conversion to useful fuel, distribution, and storage of the fuel, and fuel end use.
For biofuels, the feedstock production and logistics impacts are the same as those discussed for biomass above: impacts on land use (and the potential for GHG emissions due to such changes), water quality and quantity issues, potential impacts to soil quality, air emissions from feedstock collection and transport, and potential ecosystem impacts due to changes in the biomass being grown and the disruption during production and harvest.
A significant increase in the ethanol content of fuel for motor vehicles will result in environmental impacts during the distribution and storage phase of the full biofuel life cycle. Increased transport of ethanol, either through pipelines or by truck or rail will result in increased spills of the fuel. The impacts of such spills include potential major fish kills if spilled into open water bodies, contamination of ground-water, mobilization of inorganic compounds such as iron and manganese, and the potential for generating noxious odors during decomposition . When released into groundwater, the presence of ethanol and gasoline can result in increased benzene concentrations compared to gasoline alone due to the changes in degradation chemistry caused by the ethanol [75-77].
Increasing levels of ethanol in gasoline also changes the profile of compounds emitted to the air from engine operation. In general, higher ethanol concentrations tend to result in higher emissions of aldehydes, particularly formaldehyde and acet-aldehyde . Although emissions of other pollutants, including organic compounds, may decrease due to the higher oxygen content of ethanol-gasoline mixtures, some studies have estimated that wide-scale use of high-ethanol content fuel such as E85 (85% ethanol, 15% gasoline) could result in higher ambient ozone and aldehyde concentrations and therefore higher mortality rates compared to those that would be projected for conventional gasoline or low-ethanol content fuels (E10) .
Given the potential environmental impacts associated with biofuel production and use, it is crucial to determine whether biofuels result in a net reduction in fossil fuel consumption and fuel-related greenhouse gas emissions. The current technology used for the majority of biofuel production, ethanol from corn, appears to provide a slight net gain in energy compared to petroleum, and can provide a net reduction in GHG emissions [80-83]. Although the gain in total energy is relatively modest for corn-based ethanol (20-25%), corn ethanol does displace a significant (80-90%) volume of petroleum, because the majority of energy inputs to ethanol production are natural gas and electricity [81, 84]. The remaining fossil energy input is from coal and natural gas used in the feedstock production and fuel conversion stages of the life cycle, and these uses have the same impacts as noted above. In general, the extent to which life-cycle GHG emissions are reduced when using corn-based ethanol may depend upon two key factors: the amount of coal-derived energy used to power the conversion plant and the net changes in land use (noted above).
The Energy Independence and Security Act of 2007 (EISA) has provided considerable incentives for accelerated development of technologies to convert biomass to motor vehicle fuels . Technologies to convert biomass to "advanced biofuels" (i.e., fuels that have 50% or greater reduction in life-cycle GHG emissions compared to petroleum fuels) are being developed at a particularly rapid rate due largely to the lack of any technology with a clear competitive advantage. For ethanol, those technologies are focusing on conversion of cellulosic biomass through either thermochemical or biochemical processes. Most new process designs appear to be working toward minimization of effluents from their particular process, but true zero-emissions systems are unlikely to be available at a commercial scale in the near term.
Effluents from cellulosic ethanol plants are likely to include conventional air pollutants as well as CO2,4 wastewater, and solid residues. Lignin-based residues will likely be one of the more substantial byproducts of cellulosic ethanol production, although it also has considerable value as a feedstock for non-fuel bio-based products or a as a fuel for heat and power generation. Another potential solid effluent is gypsum, which results from the use of lime to neutralize sulfuric acid used to hydrolyze the biomass feedstock to separate the lignin from the cellulose and hemi-cellulose. Although other hydrolyzation processes may be used that would avoid the generation of gypsum, acid-based processes will generate significant quantities of gypsum that will require disposal.
Biochemical processes have been developed to use organisms and enzymes that have, in many cases, been developed specifically to enhance ethanol production [86, 87]. There is also considerable interest in feedstocks that have been bred or modified to maximize their energy production potential [88-90]. These biological materials include genetically modified organisms (GMOs) and other modified bio-logicals that are not found naturally in the environment. The impacts of these materials if released into the environment are not understood, and therefore represent a key gap in our ability to evaluate the environmental risks and possible mitigation approaches to such releases. The industry is changing rapidly, and this pace of development is likely to continue for the foreseeable future. It is therefore likely that the number and types of these new biological materials will continue to increase, with unknown potential for adverse environmental impacts.
The existing regulatory and technical infrastructure is much better developed relative to thermochemical processing. Many, if not most, petrochemical processes rely on thermochemical processing, and these processes are technically mature and have been subject to regulatory oversight for many years. On the other hand, the biochemical processes now being developed for large-scale biofuel production are not as well developed and are less well characterized from a regulatory perspective. Although thermochemical cellulosic ethanol plants may well increase emissions of air and water pollutants and solid wastes, at least in locations that currently have little, if any, previous industrial process emissions, these changes are likely to be similar in kind to emissions from other thermochemical processes that have been
4 In this case the CO2 emissions would be a mixture of fossil- and biomass-based CO2. The fermentation process generates significant levels of CO2, but this CO2 is biomass-based and therefore not a net addition to the atmospheric carbon cycle.
used commercially for decades. Beyond the question of emissions into water across the biofuel life cycle, biofuels (and particularly ethanol) will also impact water quantity. Recent life-cycle studies suggest significant increases in water consumption of up to 20 times that required for production of petroleum fuels [91, 92], with one study concluding that corn-based ethanol will require over 1,000 gal of water to produce 1 gal of ethanol in the US .
A full life cycle assessment of the environmental impacts of biofuels will also need to include issues such as water consumption by feedstock production and conversion to fuel, soil productivity, and other parameters that are impacted by intensive agricultural practices. Finally, there have been considerable advances in the area of conversion of biomass to hydrocarbon fuels . These processes may have different environmental impacts than the biochemically-based fermentation processes that produce alcohols, and both process developers and regulatory agencies need to be aware of such changes and how they may need to be addressed.
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