Substituting Low-Carbon Fuels for Carbon-Intensive Fuels
Taking Climate Change into Account in U.S. Transportation
Substituting Low-Carbon Fuels for Carbon-Intensive Fuels
Alternative and replacement fuels can lower GHG emissions. Alternative fuels are those that are used as a complete substitute for petroleum and require specifically designed engines and fueling structure. Replacement fuels are those that are blended with gasoline in proportions that do not require specialized technology. Declining conventional oil reserves may motivate a shift to such low-carbon fuels. The other option would be to shift to unconventional petroleum, but its production involves significantly more GHG emissions than conventional petroleum.
Alternative fuels are promising long-term solutions, but economic and technological hurdles must first be overcome. Alternative fuels—such as liquefied petroleum gas (LPG), compressed natural gas (CNG), hydrogen (H2), solar and electricity—offer reductions in GHG emissions from 10 to 100 percent over the full fuel cycle depending on how they are produced and used. These fuels, however, often suffer from higher costs, limited driving range, a lack of fuel supply and refueling infrastructure, and the need for specifically designed engines. Largely unaffected by these constraints, replacement fuels, particularly biofuels, can reduce GHG emissions by up to 20 percent. Ethanol and other fuels that can be blended with petroleum fuels offer the greatest promise for reducing transportation GHG emissions during the next 15 years.
Biofuels. About 1.76 billion gallons of ethanol were produced in 2001, almost all from the fermentation of corn. At present, corn-based ethanol reduces full fuel-cycle GHG emissions by slightly more than 30 percent in comparison with gasoline. In the long term, ethanol production may come mainly from cellulosic feedstocks, such as switch grass, hardwoods and softwoods, agricultural residues, and municipal waste. Production of ethanol from cellulose requires much less energy and fertilizer than production from corn and allows for co-production of electricity, resulting in approximately zero net GHG emissions. Other advantages of ethanol and methanol include the ability of the gasoline infrastructure to convert to handling alcohol fuels and the hundreds of thousands of flexible-fuel vehicles already on U.S. roads that are capable of running on any blend of gasoline with up to 85 percent ethanol. An important limitation of these fuels is that land available to grow biofuel feedstock is limited.
Liquefied Petroleum Gas (LPG) and Compressed Natural Gas (CNG). LPG vehicles can reduce GHG emissions by almost 20 percent compared to gasoline vehicles. However, LPG’s potential as a large-scale transportation fuel is limited because reserves are much smaller than those of crude oil and natural gas. CNG can reduce CO2 emissions by about 30 percent, and although a complete network of refueling stations is lacking, a nearly ubiquitous natural gas supply infrastructure exists.
Hydrogen and electricity. Among the alternative fuels, only hydrogen and electricity could be produced on a sufficiently large scale to fuel the entire U.S. transportation system. Hydrogen and electricity can be produced from a variety of sources, including carbon-free renewables. If fossil fuels are used, CO2 emissions can be nearly eliminated if the CO2 can be captured and permanently sequestered. Hydrogen can be burned in internal combustion engines or oxidized in fuel cells. The most efficient hydrogen-fueled internal combustion engines reduce energy use by roughly 15 percent compared to gasoline engines.7 Fuel cells are a more promising long-term option for using hydrogen because they have potential for a two-fold efficiency increase. High-temperature fuel cells may be a promising longer-term option for marine and rail transport.
Electric vehicles consume significantly less energy per mile than internal combustion engine vehicles. An even lower rate can be achieved by recovering kinetic energy during braking and by shutting off the motor during vehicle stops, approaches that are already being used in hybrid electric cars on the market today. Obstacles to commercializing hydrogen and electric vehicles include their higher cost, the lack of associated infrastructure, and technological barriers such as fuel storage.
Policies for substituting low-carbon fuels for carbon-intensive fuels include:
- Incentives for ethanol use. Continuing the federal highway tax exemption for ethanol would promote ethanol use and reduce GHG emissions. A renewable fuel content standard — a requirement that gasoline contain a specified volume of renewable fuel — would also be an incentive to use ethanol.
- Incentives for alternative fuels and vehicles. These could take the form of tax breaks to private and institutional owners of low-carbon alternative fueling facilities, as well as to purchasers of low-carbon alternative fuel vehicles.
- Laying the groundwork for a hydrogen-based transportation system. The Bush Administration’s proposed hydrogen research initiative is an important first step. There are opportunities through the reauthorization of the Transportation Equity Act and through energy legislation to fund research and pilot demonstrations of hydrogen production, storage and delivery systems, and vehicles. Also, the U.S. Department of Transportation could evaluate hydrogen safety issues and develop codes and standards for pipelines, vehicles, and refueling facilities.