Sugar and starch crops are converted through a fermentation process to form bioalcohols, including ethanol, butanol, and propanol. Oils and animal fats can be processed into biodiesel. Ethanol is the most widely used bioalcohol fuel. Most vehicles can use gasoline-ethanol blends containing up to 10 percent ethanol by volume. Flexible fuel vehicles can use E85, a gasoline-ethanol blend containing up to 85 percent ethanol.
Second generation biofuels, or cellulosic biofuels, are made from cellulose, which is available from non-food crops and waste biomass such as corn stover, corncobs, straw, wood, and wood byproducts.
Third generation biofuels use algae as a feedstock. Commercial cellulosic biofuel production began in the US in , while algae biofuels are not yet produced commercially. Replacing fossil fuels with biofuels has the potential to generate a number of benefits. In contrast to fossil fuels, which are exhaustible resources, biofuels are produced from renewable feedstocks.
Thus, their production and use could, in theory, be sustained indefinitely. Academic studies using other economic models have also found that biofuels can lead to reductions in lifecycle GHG emissions relative to conventional fuels Hertel et al. Second and third generation biofuels have significant potential to reduce GHG emissions relative to conventional fuels because feedstocks can be produced using marginal land.
Moreover, in the case of waste biomass, no additional agricultural production is required, and indirect market-mediated GHG emissions can be minimal if the wastes have no other productive uses. Biofuels can be produced domestically, which could lead to lower fossil fuel imports Huang et al. If biofuel production and use reduces our consumption of imported fossil fuels, we may become less vulnerable to the adverse impacts of supply disruptions US EPA Reducing our demand for petroleum could also reduce its price, generating economic benefits for American consumers, but also potentially increasing petroleum consumption abroad Huang et al.
Biofuels may reduce some pollutant emissions. Ethanol, in particular, can ensure complete combustion, reducing carbon monoxide emissions US EPA It is important to note that biofuel production and consumption, in and of itself, will not reduce GHG or conventional pollutant emissions, lessen petroleum imports, or alleviate pressure on exhaustible resources. Biofuel production and use must coincide with reductions in the production and use of fossil fuels for these benefits to accrue.
These benefits would be mitigated if biofuel emissions and resource demands augment, rather than displace, those of fossil fuels. Biofuel feedstocks include many crops that would otherwise be used for human consumption directly, or indirectly as animal feed. Diverting these crops to biofuels may lead to more land area devoted to agriculture, increased use of polluting inputs, and higher food prices. Cellulosic feedstocks can also compete for resources land, water, fertilizer, etc.
As a result, some research suggests that biofuel production may give rise to several undesirable developments. Changes in land use patterns may increase GHG emissions by releasing terrestrial carbon stocks to the atmosphere Searchinger et al. Biofuel feedstocks grown on land cleared from tropical forests, such as soybeans in the Amazon and oil palm in Southeast Asia, generate particularly high GHG emissions Fargione et al.
Even use of cellulosic feedstocks can spur higher crop prices that encourage the expansion of agriculture into undeveloped land, leading to GHG emissions and biodiversity losses Melillo et al. Biofuel production and processing practices can also release GHGs. Fertilizer application releases nitrous oxide, a potent greenhouse gas.
Most biorefineries operate using fossil fuels. Some research suggests that GHG emissions resulting from biofuel production and use, including those from indirect land use change, may be higher than those generated by fossil fuels, depending on the time horizon of the analysis Melillo et al.
Regarding non-GHG environmental impacts, research suggests that production of biofuel feedstocks, particularly food crops like corn and soy, could increase water pollution from nutrients, pesticides, and sediment NRC Increases in irrigation and ethanol refining could deplete aquifers NRC Air quality could also decline in some regions if the impact of biofuels on tailpipe emissions plus the additional emissions generated at biorefineries increases net conventional air pollution NRC Economic models show that biofuel use can result in higher crop prices, though the range of estimates in the literature is wide.
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