ALTERNATIVE AUTO FUELS POSE COST OR TECHNICAL CHALLENGE

A special study has examined automotive emissions and the possible outcome of efforts to improve air quality by altering the mix of motor vehicle fuels.

The study, prepared by Stanford Field, director of energy programs for SRI International, is titled "1991 Oil Report, Chapter 13: Motor Vehicle Fuels of the Future."

The study focuses on U.S. activities in this arena and evaluates the projected costs and viabilities of the alternative fuels and the potential environmental improvements resulting from their use.

As the U.S. enters the 1990s with little worry of energy shortages, there is renewed concern about the environment. People increasingly recognize that concentrated population centers are becoming more polluted and less habitable, resulting in a growing tendency to legislate limits on pollution.

AIR POLLUTION SOURCES

The major air pollutants are:

• Lead

• Carbon monoxide (CO)

• Volatile organic compounds (VOCs)

• Nitrogen oxides (NOx)

• Sulfur oxides (SOx)

• Particulates.

These materials are produced by the following sources:

• Transportation

• Fuel combustion

• Industrial processing

• Solid waste disposal.

Lead - arguably the most harmful pollutant and probably the easiest to eliminate over time - has been reduced by 96% since 1970. Other pollutants have been reduced, but they are more difficult to control because emissions of them tend to increase with growth in population and economic and industrial activity.

MOTOR VEHICLE EMISSIONS

The passage of the Air Pollution Control Act in 1962 established air pollution standards for the U.S. Since then, steady progress has been made in cleaning the air despite growing industrialization.

As part of the overall improvement, emissions from motor vehicles have been declining and are likely to continue a downward trend, although progress will be difficult to achieve over the next two decades (Table 1).

Field says anticipated reductions in future emissions will be achieved mainly by a "systems approach" involving coordinated actions in the following areas:

• Fuels - change in composition

• Combustion system - balance of emissions and efficiency to take advantage of the composition of new fuels

• Emission control - improvement in effectiveness and life of catalysts; improvement in refueling connection to reduce emissions

• Vehicle design - improvement in vehicle efficiency and mileage.

One of the most important political responses to the oil shock of 1973/74 was the mandated higher mileage ratings for new cars sold in the U.S. The mandate, known as Corporate Average Fuel Economy (CAFE) standards, improved fleet mileage.

Current legislation to raise CAFE standards to 34-40 mpg by 2001 is opposed by the automobile industry and by the U.S. administration. Environmentalists and conservationists favor raising CAFE standards. Opening of oil development in the Alaska National Wildlife Refuge (ANWR) is involved in this political controversy,

SRI projections of gasoline use over the next two decades are shown in Fig. 1. In projecting gasoline use, it was assumed that the standoff will be resolved by developing ANWR and raising CAFE standards.

FUELS AND AIR POLLUTION

Although the complex chemical and photochemical reactions occurring in the atmosphere have been studied for many years, much uncertainty exists about the requirements for controlling man-made pollutants.

Unlike most air pollutants, ozone is not emitted as a combustion product. Rather, it is formed in the atmosphere by reactions of precursor pollutants.

The chemistry of ozone formation involves several steps. Air combustion processes produce NOx (Equations 1 and 2). Nitrogen dioxide reactions subsequently produce ozone (Equations 3 and 4). Therefore, limiting ozone requires reducing emissions of NOx.

However, ozone is very reactive with nitric oxide (NO), as shown in Equation 5, producing oxygen (O2). But Scientists believe that NO will react with hydrocarbons preferentially to ozone. This means that atmospheric concentrations of hydrocarbons need to be limited to reduce ozone concentrations.

Catalytic converter systems can be designed to react CO and NO in exhaust gases to minimize emissions of NOx in accordance with Equation 6.

Emissions of NOx come predominantly from high-temperature sources such as motor-vehicle engines, electricity-generation boilers, and industrial furnaces. VOC (hydrocarbon) emissions are mainly a result of processes such as gasoline evaporation, incomplete gasoline combustion, and solvent evaporation.

A recent report by Conservation of Clean Air and Water, Europe (Concawe), indicates that the on-board vehicle canister route to limiting VOC evaporative emissions would be more effective and less costly than limiting the vapor pressure of gasoline at the pump.

It is estimated that most CO emissions come from automobiles, despite considerable reductions achieved by catalytic converters. Catalytic converters react oxygen from air with CO and unburned hydrocarbons to form CO2.

Another approach to reducing CO is to blend oxygenates into gasoline. These alcohols and ethers enhance unleaded gasoline octanes and reduce CO emissions. In the U.S., the 1990 Clean Air Act Amendments (CAAA) mandate the use of gasoline containing 2.7 wt % oxygen in certain areas for the winter months, beginning in November 1992.

Regulatory pressure has also limited the concentration of benzene in gasoline. Benzene is considered hazardous to personnel handling gasoline and breathing its vapors. The CAAA limits benzene to 1.0 vol % of gasoline by 1995.

This reduction of benzene, as compared to 1990 levels of about 1.6 vol %, will free as much as 660 million gal/year for use in the chemical market. This additional extracted benzene could eventually shut down all toluene hydrodealkylation facilities in the U.S., according to the SRI study.

SO2 emissions from diesel fuel are controlled by limiting diesel sulfur content. In the U.S., diesel fuel sulfur limits will be reduced from 0.50 to 0.05 wt % on Oct. 1, 1993 (small refiners have until 1995).

At the same time, diesel aromatics (particulates precursors) concentration will be controlled by limiting the cetane index to a minimum of 40. Diesel particulate emissions are controlled by using traps on vehicle exhaust-gas. In California, diesel aromatics content will be limited to a maximum of 10 wt % on Oct. 1, 1993.

MOTOR VEHICLE FUELS

The current initiative to reduce automotive emissions centers on the reformulation of gasoline and the use of alternative fuels. As usual, the advantages gained are offset by new disadvantages, says the report.

The trade offs are still being analyzed and in many cases the net benefits are difficult to assess. Knowledge of the physical properties of the various fuels is necessary for understanding the complexity of the environmental gains and losses (Table 2).

Emissions data suggest that using new fuels in new cars will lead to slow progress as long as old cars are on the road. The ability of new fuels to clean the air will also be constrained by increasing urban traffic congestion.

A car stuck in rush-hour traffic gets little more than 0 mpg, and the pollution generated per mile traveled is increased enormously. Field sees smoothing traffic flows as a vital part of a campaign to clean the air.

PUMP PRICE: 1990-2010

SRI has estimated future pump prices of various fuels based on delivery to the Los Angeles area. The estimate assumes modest real increases in fuel and sales taxes over the next two decades. SRI's projections are summarized in Table 3.

The estimates for CNG and propane assume only moderate demand for natural gas and propane and no significant change in the structure of hydrocarbon pricing.

REGULAR UNLEADED GASOLINE

The actions to be taken as a result of the 1990 Clean Air Act are:

• Vapor pressure reduction

• Benzene reduction

• Olefins reduction

• Oxygen addition

• Aromatics reduction

• End point reduction.

The cost of making these comprehensive changes in gasoline formulation is estimated to be about 32/gal (1990 $) in the year 2010 (Table 4). Estimating passenger car gasoline use at 64 billion gal/year in 2010, the added cost to the public will be about $20 billion/year (1990 $).

Reformulations will have the most benefit in older cars without catalytic converters. Other benefits will be more difficult to achieve.

METHANOL

About 100 metropolitan areas in the United States have failed, as of 1987, to meet the federal ambient air quality standards for ozone or CO.

Studies have suggested that the use of methanol-powered cars, trucks, and buses would significantly improve air quality in the Los Angeles area. Consequently, politicians and regulators are actively promoting the use of methanol as a motor vehicle fuel.

California began its clean fuels program in 1978 by testing a few cars running on alcohol blends. From 1981 to 1988, the focus was on "dedicated" (designed, operated, and optimized to use one fuel) passenger cars using 100% methanol.

The methanol-powered vehicles showed excellent drivability and acceleration. The average fuel economy was 413% better than that of gasoline-powered vehicles, on an energy input basis. But a gallon of methanol has only 57% as much energy as a gallon of gasoline.

The gain in fuel economy is largely attributable to higher compression ratios (i.e., higher work efficiency) with methanol. Methanol has an (R+M)/2 of 115, whereas gasoline's is 87-93. Higher compression ratios produce higher peak temperatures of combustion, resulting in higher emissions of NOx.

The methanol cars required frequent oil changes to avoid excessive engine wear caused by the corrosive properties of methanol and the formic acid it produces.

It also became apparent that consumers would not buy dedicated methanol cars because of the lack of filling stations offering methanol. Even if the Los Angeles area offered methanol, drivers would be unlikely to buy dedicated methanol vehicles that could not be refueled outside Los Angeles.

Thus, a new phase of the clean fuels program began in 1986 and is continuing today. The program's current emphasis is on the development of a flexible-fueled vehicle that can operate on gasoline, methanol, ethanol, or mixtures of those fuels.

A distinct disadvantage of the flexible-fueled vehicle is that it cannot be optimized to take advantage of methanol's high octane number. However, technology improvements are expected to increase its operational efficiency to 5-10% over that of a conventional gasoline-powered car.

A major advantage of methanol is that exhaust and evaporative emissions are less photochemically reactive than the hydrocarbons emitted by gasoline-fueled vehicles.

But the combustion of methanol produces formaldehyde - a toxic substance and probable human carcinogen. Formaldehyde emissions from M85 exceed the U.S. Environmental Protection Agency (EPA) standard. Formaldehyde is also photochemically reactive in forming smog.

EPA considers 0.25 ug/cu m to be the formaldehyde concentration in ambient air at which concern about health impacts should be raised. This is equivalent to 1 part per 5 billion, or 1 person out of the entire earth's population. This stringent level of concern poses questions of attainability and even measurability.

EPA has also indicated that the widespread use of methanol-fueled vehicles could cause the threshold concentration of formaldehyde to be exceeded by a factor of two. Research is under way to develop pollution-control equipment to capture formaldehyde from methanol-engine exhaust gases.

Until that problem is resolved, says Field, it seems prudent to be cautious about using methanol, to avoid solving one problem and simultaneously creating a new set of possibly more serious ones.

The pump price of M85 in Los Angeles derived from natural gas in Saudi Arabia is estimated to be 34% (or 71/gal, in 1990 $) higher than the price of reformulated regular unleaded gasoline in 2010. To give consumers an economic incentive to purchase M85, it would be necessary to eliminate all taxes (amounting to 80/gal in 1990 $). For the methanol case, it was assumed that natural gas prices in Saudi Arabia would remain relatively unchanged despite rising crude oil prices.

COMPRESSED NATURAL GAS

CNG was first used in significant quantities in Italy in the 1930s.

It is estimated that in 1990, 500,000 CNG-powered vehicles were operating worldwide.

Almost all CNG-powered vehicles on the road today are conventional gasoline-fueled vehicles that have been converted to "dual-fuel" operation. These vehicles can burn either gasoline drawn from a standard tank or CNG drawn from a high-pressure storage cylinder.

A number of manufacturers have concluded that a fully optimized, dedicated CNG vehicle will perform better than its conventional gasoline counterpart. To take full advantage of the high octane number (130) of natural gas, the compression ratio of the combustion chamber should be increased to about 12:1 (87-93 octane number gasoline engines operate at compression ratios of 8 or 9:1).

Dedicated natural gas-powered vehicles would have longer engine lives and lower maintenance costs than gasoline-fueled vehicles because CNG is cleaner-burning and generates less dirt to foul and abrade engine parts. And spark plugs would need to be changed less frequently.

Although a given weight of natural gas provides more energy than the same weight of gasoline, the compressed volumetric energy content of natural gas is one-sixth that of an equal volume of gasoline. Thus, to hold an energy content equal to a given amount of gasoline, a CNG storage tank must be six times as large as a gasoline storage tank. This results in a weight penalty of about 400 lb for a CNG vehicle.

The CO emissions from a CNG vehicle are one half to one tenth of those from a gasoline-powered vehicle. EPA estimates that existing dual-fueled CNG vehicles operating on CNG reduce reactive hydrocarbon emissions by 50-80% compared to gasoline-powered vehicles. When substituted for diesel fuel, CNG vehicles can be expected to virtually eliminate particulate emissions.

Emissions of NOx, however, are likely to be higher for a dedicated CNG vehicle than for a gasoline-powered vehicle. NOx is a function of the peak temperature of combustion in the presence of excess oxygen. For a given compression ratio, combustion temperature in spark-ignition engines is generally lower for natural gas than for gasoline.

However, the higher compression ratio used to optimize the potential of the higher-octane natural gas results in higher peak temperatures and greater NOx emissions than those that characterize the lower-compression-ratio gasoline engines.

Also, the higher stoichiometric air-to-fuel ratio for natural gas tends to increase NOx emissions as compared to gasoline.

The cost of CNG at a hypothetical fueling station in Los Angeles is estimated to be almost the same as the cost of reformulated regular unleaded gasoline. The natural gas was assumed to be shipped from Alberta, Canada, at prices related to the price of crude oil.

This price mechanism assumption is compatible with the historical pricing of natural gas throughout the world.

PROPANE

Propane-sometimes called liquefied petroleum gas-has been used as a motor vehicle fuel since the early 1920s. Propane's market share is larger than that of any other alternative transportation fuel.

About 3.9 million vehicles are operating on propane worldwide, about 300,000 of which are in the U.S.

Propane contains about 75% of the energy content of gasoline on a volume basis. At ambient temperatures, propane can be liquefied at relatively low pressure. Thus, in contrast to natural gas, enough liquid propane can be stored in a relatively lightweight tank to provide a driving range similar to that of gasoline.

Propane has an octane rating of 100 (R+M)/2, which allows for higher compression ratios and better engine performance than in gasoline-fueled vehicles.

But propane can be quite hazardous. In case of a leak or a spill, propane vapor is heavier than air and readily forms explosive mixtures in poorly ventilated spaces (such as garages).

Propane-vehicle emissions contain low concentrations of CO and particulates, which explains the widespread use of propane to fuel forklifts and other industrial trucks. The NOx output from a propane engine is normally higher than that from a gasoline engine because of the higher peak flame temperature caused by a higher compression ratio. The pump price of propane is estimated to be lower than that of any other alternative motor vehicle fuel being considered.

ELECTRICITY

The electric vehicle (EV) has the potential for eliminating almost all automotive pollution. If the electricity used to charge the batteries for EVs were generated by nuclear power (a technology that produces almost zero air emissions), rapid progress in cleaning the air of polluted cities throughout the world could result from widespread use of EVs.

However, the struggle for the EV/nuclear power option faces its own unique set of problems-namely battery storage capacity and nuclear waste disposal.

The SRI study concludes that the compelling EV/nuclear power combination would eventually cause the demand for oil to decline. The world's dependence on oil from the unstable Middle East would decline, and violent and economically upsetting price upheavals would give way to stable or steadily declining oil prices.

The first phase of commercialization of the electric vehicle may be achieved by the development of a hybrid that uses a small gasoline engine (about 15 hp) to do what it does best: run at constant speed while driving a generator to recharge the car's batteries.

The electric motor would provide variable-speed power (what it does best) directly to the driving wheels. The small, efficiently used gasoline engine would increase the EV driving range between battery recharges. Thus, the hybrid EV promises to shorten the time needed to introduce the EV.

An all-electric car will go only as far as its battery pack will take it. The battery is the key to making the entire concept of EVs a viable alternative to gasoline engines. The perfect battery would be lightweight, able to store enough energy for long-range use between charges, capable of quick energy release for acceleration, and environmentally safe.

Several battery systems are being developed: lead-acid, nickel-iron, sodium-sulfur, nickel-cadmium, and zinc-air.

The average price of electricity for residential consumers in the U.S. in 1991 is about 80/kw-hr.

If it is assumed that residential electricity generated from nuclear power would be sold in 2010 for 15/kw-hr (in 1990 $), then the cost of electricity for the EV would be: 528/gal of gasoline equivalent.

HYDROGEN

The advantages of hydrogen as a motor vehicle fuel are similar to the advantages of EVs. Hydrogen could be made by passing an electric current (generated from nuclear or solar energy) through water, which would split into atoms of hydrogen and oxygen. Burning the hydrogen in a motor vehicle would produce only emissions of water vapor (steam) and some NOx.

A key technical question is how to store the hydrogen. One German auto company has opted to use gaseous hydrogen that has been bonded with powdered metals (mainly Ti, Va, and Mn) at a pressure of 725 psi to form metallic hydrides. Heating the hydride (probably using waste engine heat) releases the trapped hydrogen. A Japanese company is pursuing the storage of hydrogen as a liquid at - 253 C.

All feasible hydrogen storage systems consume a great deal of space, and their costs are high. The cost of dissociating water with electricity can be used as a rough measure of the fuel cost. Using the cost of electricity previously mentioned, the cost of dissociating hydrogen in 2010 (in 1990 $) is equivalent to a gasoline price of 350/gal.

Copyright 1991 Oil & Gas Journal. All Rights Reserved.