Fuels: Biodiesel
A transit bus, running on 100% biodiesel, would reduce CO2 emissions by nearly 80% across the life cycle of the fuel, in comparison with diesel. A transit bus running on the more commonly used mixture of 20 percent biodiesel and 80 percent diesel, called “B20,” would reduce CO2 life cycle emissions by about 16 percent in comparison with diesel. For both the pure form of biodiesel, and the 20 percent mix, less petroleum is required to make and distribute the product. The bulk of biodiesel's greenhouse gas benefit, however, follows from its renewability as a biomass fuel.
Unlike the carbon stored in fossil fuels, the carbon in biodiesel is renewable: carbon released in combustion of the fuel will, theoretically, be absorbed in next year's soybean crop. Biodiesel can be made from any kind of fatty oil (derived from peanuts, mustard seeds, canola, soybeans, or even used cooking oil). Rather than being released into the atmosphere after millions of years of sequestration beneath the surface of the earth, the life cycle of biodiesel introduces no more carbon than is already circulating in the biosphere from season to season. The same is true for ethanol; the difference lies in the greater amount of energy needed to turn corn—ethanol's most common feedstock—into fuel. The manufacture of ethanol is, in fact, second in energy intensiveness only to gasoline.
As stated in the National Renewable Energy Laboratory’s life cycle study, “biodiesel’s life cycle emissions of CO2 are substantially lower than those of petroleum diesel…[U]se of biodiesel to displace petroleum diesel in urban buses is an extremely effective strategy for reducing CO2 emissions.” Biodiesel has the added advantage of reducing methane emissions, together with all regulated pollutants except oxides of nitrogen (NOx).
In contrast to both natural gas and the alcohol fuels, biodiesel has an energy content equivalent to diesel, resulting in comparable mileage in transit buses applications. Using B20 requires no modification of conventional diesel burning engines; higher blends of biodiesel require replacement of rubber with synthetic engine seals. Aside from the seals, biodiesel actually increases engine lubricity, and helps to clean out the fuel system.
Biodiesel, like the alcohol fuels, is currently at a competitive disadvantage with diesel due to its relatively high cost of production. Pure biodiesel, or “B100,” can cost nearly $2 per gallon with taxes. Biodiesel is no longer a demonstration project, however; a competitive market in B20 has emerged over the last two to three years, with municipal school districts and the U.S. military two of the biggest consumers.
A 1998 amendment to the Energy Policy Act authorized use of biodiesel “as a way for federal, state, and public utility fleets to meet requirements for using alternatives fuels.” The biodiesel industry hopes that continued use of the fuel by various private and public fleets will expand the market and lower production costs.
Impending air quality regulations may soon be working in favor of biodiesel as well. Transit agencies must meet an EPA deadline of 2006 to reduce the sulfur content of their diesel fuel to 15 parts per million. As more expensive, low-sulfur diesel comes to market in response to this demand, biodiesel will become more competitive.
Some analysts point out that another cost advantage lies in the easy convertibility of the existing petroleum distribution system, which could support biodiesel with “little or no modification.” Several municipalities are running, or have run, a portion of their fleets on biodiesel, often with financial assistance from agencies such as US Department of Energy Congestion & Mitigation and Air Quality Program. Cincinnati’s Metro experimented with soy-based biodiesel in the early 1990’s, and cooking-oil/animal fat-based biodiesel in 2000. It is currently nearing the end of a 2001 trial running 150 buses on B20. In the case of Cincinnati, cost rather than performance is the obstacle to long-term adoption of biodiesel.
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