The search for alternate fuels encompasses three separate and distinct goals:
Since each type of fuel can affect each of these goals differently, the "best" fuel often depends on the goals being sought.
Alternate fuel sources are often classified into the following categories (a fuel may be listed in more than one category):
Best near term chances for success:
All three use existing engine technology and, in the case of alcohols, the same fuel distribution system. A gasoline engine requires surprisingly little modification to burn compressed natural gas.
Gasoline and most alternative fuels are hydrocarbons (compounds of hydrogen and carbon). For all hydrocarbon fuels, the products of complete combustion are water (H2O) and carbon dioxide (C02). Until the recent concern over global warming, of which the principal culprit is CO2, complete combustion that produced solely these two gases was considered a desirable goal.
Current and proposed automotive emission regulations primarily set limits on unburned hydrocarbons (HC), partially oxidized carbon in the form of carbon monoxide (CO), and oxides of nitrogen (NOX). Unburned HC and NOX react in the presence of sunlight to form smog. Emission regulations do not address the CO2 problem.
The catalytic converter on automobile engines today recombines about 90% of the base components of HC, CO, and NOX in the engine exhaust into CO2, water, and atmospheric nitrogen. (By 1990, exhaust emission of HC, CO, and NOX had been reduced 96%, 96%, and 76% respectively compared to an equivalent vehicle in 1970.) However, the Clean Air Act ammendments of 1990 establishes turn-of-the-century deadlines that will force strict new emission standards plus ultra-clean, non-gasoline vehicles for high air pollution areas like Los Angeles.
In the new legislation, Congress, as part of a two-step plan, specified a moderately reformulated fuel for nine metropolitan areas (including Los Angeles, New York, Houston, and Chicago) that have high ozone levels. In the first phase of the plan, which will start in 1995, all gasoline sold in those nine areas must incorporate a formula that takes full advantage of the least costly method of reducing HC emissions - lowering the vapor pressure of gasoline, which basically reduces fumes that otherwise would evaporate. (Currently nearly half of the total hydrocarbons in the air come from evaporation.) This is expected to reduce car and light truck emissions by 15% from 1990 levels at a cost of about 6 cents per gallon.
Lowering gasoline vapor pressure is achieved by adding oxygenates, such as MTBF (methyl tertiary butyl ether). These increase the oxygen content of the fuel and encourage the formation of CO2, thereby decreasing the formation of CO. On the negative side, these new fuels cause a slight increase in NOX. In addition, it's unclear how much variation there will be in the burn efficiency of the new fuel in different car models. Lower efficiency can easily offset the gains otherwise achieved.
The second phase of the legislation requires that, starting in the year 2000, total vehicle emission be reduced by an additional 10% from the 1990 base, through more extreme gasoline reformulations. This will require a radical change in how gasoline is manufactured - so radical that it's not clear that it can be made. This new formulation will add about 15 to 20 cents a gallon to fuel costs.
It has been calculated that the best reformulated fuel will reduce the automotive contribution to Los Angeles ozone from the 33% level it is today to 7% by the year 2010. However, opponents of this second phase note that the gradual disappearance of older, more-polluting vehicles is projected to reduce the level to 9% over the same period without any fuel changes. (Cars built before 1975 accounted for 7% of all the miles driven in this country in 1991, but produced 25% of the automobile emissions.) Moreover, by 1995 only 10% of hydrocarbon emissions will be emitted as tailpipe exhaust, and more than half of that will come from heavy trucks and off-road equipment such as bulldozers. The remaining 4%-5% emitted by light car and truck tailpipe emissions is the major target of the costly second phase of the 1990 Clean Air Act. And finally, opponents argue that hydrocarbon emissions could be reduced by nearly 10% by fully implementing the U.S. EPA's enhanced inspection and maintenance program. As of 1992, not a single state had fully implemented this program.
Starting in 1998, California will require all car builders doing business in the state to offer zero-emission (ZEV) electric vehicles for sale. In 1998, 2% of a manufacturer's car and light truck sales must be ZEV; and by 2003, 10%. In addition, by 2003 15% of all new vehicles sold must must meet the so-called ultra-low emissions vehicle (ULEV) standard which further reduce allowable hydrocarbons to 10% of current 1990 levels. Since Californians buy 1 of every 10 cars made in the U.S., and since currently fifteen other states have voted to adopt the California standards, this legislation is having a major impact on the automotive industry.
These new regulations are aimed at the long-chain hydrocarbons that predominate in gasoline. Reducing these hydrocarbons to ULEV levels using today's catalytic converter technology is considered difficult if not impossible. But alternative fuels offer a different scenario.
The new standards, which regulate only non-methane hydrocarbons, is based on the premise that methane (natural gas) hydrocarbons produce much less ozone and smog. Methane is therefore considered "clean" even though the total amount of hydrocarbons produced may not be significantly different from gasoline. Likewise, while unburned methanol is more reactive than methane, it still produces fewer non-methane hydrocarbons than gasoline.
| Fuel | Chemical Formula | % Oxygen by Weight | Net Heat of Combustion MBTU/gallon | Air/Fuel Ratio |
|---|---|---|---|---|
| Liquids | ||||
| Methanol | CH2OH | 50 | 57 | 6.4 |
| Ethanol | C3H5OH | 35 | 76 | 9.0 |
| Isopropyl alcohol | C3H7OH | 27 | 86 | 10.3 |
| Tertiary butyl alcohol | C3H9OH | 22 | 93 | 11.1 |
| Gasoline (typical) | C8H15 | 0 | 115 | 14.6 |
| Liquefied natural gas | 84 | |||
| Hydrogen | 27 | |||
| Propane | 90 | |||
| Gases (BTU/ft3) | ||||
| Natural gas | 1,000 | |||
| Hydrogen | 3,300 | |||
| Propane | 2,500 | |||
| Reactive Hydrocarbons | Nitrogen Oxides | Carbon Monoxide | |
|---|---|---|---|
| Dedicated natural gas (new technology) | 0.1 | 0.75 | 0.1 |
| Natural gas (current technology) | 0.2 | 1.0 | 0.6 |
| Methanol | 0.75 | 1.0 | 3.75 |
| Gasoline | 1.25 | 1.0 | 3.5 |
Natural gas offers the largest potential savings, at only 1/3 to 1/2 the cost of gasoline, and is plentiful. Total U.S. gas resources are estimated to be enough to last the country 60 years at current rates of use. In addition, natural gas burns cleanly, with essentially no particulate emissions and with significant reductions in unburned hydrocarbons. Since CNG and propane are stored in completely sealed tanks, there are no evaporative losses, and they don't pollute engine oil. Their combustion characteristics also help reduce CO. On the negative side, natural gas uses a pressure system that requires special fueling equipment and a large pressure container. It also has a lower energy content than gasoline, which means that it requires a larger fuel tank for an equivalent range.
New natural gas technologies produce 30% to 60% less CO2 than the same amount of energy derived from oil or coal. Moreover, an efficient natural gas-based energy system is a logical step to a hydrogen economy, in which solar-generated electricity would be used to produce hydrogen that is fed into pipelines similar to today's natural gas network.
Natural gas is the simplest of the hydrocarbons. Its main ingredient is methane, CH4. The methane generally is accompanied by CO2 and other useful gases such as ethane and propane. Unfortunately, any unburned methane is a powerful heat-trapping gas that contributes greatly to global warming. However, natural gas contains little or no sulphur or particulates, the two major pollutants from coal fired plants and the source of acid rain. NOX is reduced by 85% compared to coal.
Multinational oil companies own about half of U.S. gas resources, but most oil companies earn a larger profit importing, refining, and selling oil than they do selling their own natural gas.
| SO2 | NOX | |
|---|---|---|
| Coal-Steam (with scrubbers) | 2.1 | 4.3 |
| Oil-Steam | 1.6 | 1.4 |
| Gas Combined-Cycle | 0.0 | 0.3 |
Found in most natural gas, 65% of America's propane comes from gas wells and processing plants. The remainder is a by-product of gasoline refining. Propane is readily liquified under pressure at ordinary temperatures.
Propane is C3H8, while gasoline is C8H15. The fewer carbon atoms per molecule in propane compared to gasoline effectively give it a higher octane rating (better knock resistance). However, propane's heat of combustion is only 92,000 BTU per gallon compared to gasoline's 120,000 BTU per gallon, so less energy is available from an equivalent amount of propane. Functionally, a propane system is similar to a gasoline/carburetor arrangement.
Methanol, a liquid that is derived from natural gas, is toxic and produces as much CO2 as gasoline. Methanol and ethanol both have a lower energy content than gasoline, although the increased volumetric efficiencies partially make up for it. Methanol is potentially plentiful; however, increased demand would utilize overseas sources where natural gas is virtually free and hugh production plants are already built. It can be made from sources like natural gas, coal or biomass - garbage or wood chips - and is about 40% cheaper than gasoline. However, the miles per gallon is lower because of its lower energy content per gallon. The net result is that fuel costs for methanol are about equal to that of premium unleaded gasoline. The Ford Taurus can burn pure gasoline, or 85% methanol and 15% gasoline, or any mixture in between.
Ethanol is generally distilled from crops. While ethanol (and methanol) fuel produces about 40% lower emissions than gasoline, the growing, fertilizing, harvesting, drying and transportation of these crops produces considerable CO, CO2 and some NOX, about six times as much as burning a gallon of gasoline.
All alcohols have a common feature: Their molecular structure contains a hydroxy radical - OH. This extra oxygen gives alcohols a high solubility in water and high latent heat of vaporization compared to gasoline. Generally, alcohol's high water affinity is a disadvantage when used as a motor fuel, since it pulls water from the air. On the plus side, high latent heat value means more heat is required to change it from a liquid to a vapor, which cools the fuel/air charge going to the cylinder for increased volumetric efficiency.
At maximum power, the fuel air ratio for alcohols must be significantly richer than gasoline. This tends to reduce thermal efficiency and increase air pollutants due to incomplete burning. Alcohol fuels produce emissions just about on par with gasoline fuels, except that methanol produces significantly more formaldehyde (CH2O) during combustion than does gasoline. At this time, little is known about the health implications of formaldehyde, it's just one more pollutant to worry about.
Alcohols are also more active chemically than gasoline and will corrode some plastics and rubbers in existing fuel systems, making conversions difficult. Although alcohol comes from almost any agriculture product, it is not very cost effective. Cost estimates to replace only 10% of present motor fuel demands from in-country sources, exclusive of any new investments, project a methanol cost twice that of gasoline, while ethanol would be five times more expensive.
When hydrogen is burned as a fuel, which in chemical terms means when it is rapidly oxidized (combined with oxygen), the waste product of the reaction is simply H2O (water). Burning it releases no CO2, CO, HC, sulphur, or particulates. If the oxygen source is air, the burning process will also produce a small amount of NOX (from the nitrogen in the air), but these can be easily filtered out with existing technology.
Sources of hydrogen include water, fossil fuels, and organics such as wood, crop residues, garbage, or any biomass. Processing fossil fuels to get hydrogen emits only minimal pollutants. Even CO2 emissions - inevitable with fossil fuels - can be around 40% lower than emissions from comparably sized coal or oil systems due to the high overall fuel efficiency of hydrogen-based systems. Natural gas, or methane, is the fuel of choice for stationary power plants, while a liquid fuel like methanol may be more practical for cars. The hydrocarbon fuel is heated and mixed with steam to produce the hydrogen-rich gas that fuel cells use to make electricity.
Hydrogen can be stored as a gas in pressurized tanks, as an ultracold liquid, or by combining it with metals into powders or pellets known as hydrides. The latter technique is based on the atomically small size of hydrogen which allows it to be interspersed in the lattice of larger molecular structures. The hydrides then release hydrogen as a gas when they are heated.
Hydrogen can also be derived from water by passing an electric current through it. This causes the hydrogen and oxygen atoms to separate. If the process is reversed, and the hydrogen and oxygen atoms are allowed to recombine in a controlled way, water is produced along with an electrical voltage and current. This source of electrical power, equivalent to a battery, is called a fuel cell. While the fuel cell can deliver energy, it is more correct to consider it as an energy storage device. The energy that it stores is the energy required to separate the hydrogen and oxygen atoms that comprise it.
The ultimate environmental fuel and energy system is to use solar energy as the power source to generate hydrogen from water. In such a system, hydrogen provides a non-polluting fuel, and also serves as a means of storing solar energy.
First-generation electric vehicles will go only about 100 miles before they must be plugged in for several hours to be recharged, and they will cost several thousand dollars more than equivalent gasoline cars. Because of their higher price, electrics over their lifetime will cost the consumer somewhat more than gasoline cars, even though the cost of electric power for recharging will run only 2-3 cents per mile. Heating and cooling is a major problem since it cuts into the driving range. However, electrics lack complex engines and therefore have the potential to be very reliable, low maintenance vehicles.
Even when the emissions from power plants used to charge batteries are considered, an electric car adds less than 5% of the pollution of a typical gasoline-powered car.
John Schwarz
7-22-92