The use of
alcohol as a fuel for internal combustion engines, either alone or in combination with other fuels, has been given much attention mostly because of its possible environmental and long-term economical advantages over fossil fuels.
Both ethanol and methanol have been considered for this purpose. While both can be obtained from petroleum or natural gas, ethanol may be the most interesting because many believe it to be a renewable resource, easily obtained from organic material such as grain or sugarcane.
Fuel alcohols Proposals to use alcohol as a fuel are generally concerned with its use in transportation, chiefly as a total or partial replacement for gasoline in cars and other road vehicles. However, other less conventional approaches have been advanced, such as the use of alcohol in fuel cells, either directly or as a feedstock for hydrogen production.
Fuel alcohols can be produced from a variety of crops, such as sugarcane, sugar beets, maize, barley, potatoes, cassava, sunflower, eucalyptus, etc. Two countries have developed significant bio-alcohol programmes: Brazil (ethanol from sugarcane) and Russia (methanol from eucalyptus). Alcohol can also be obtained synthetically, via ethene or acetylene, from calcium carbide, coal, oil gas, and other sources.
Agricultural alcohol for fuel requires substantial amounts of cultivable land with fertile soils and water. It is hardly an option for densely occupied and industrialized regions like Western Europe. For example, even if Germany were to be entirely covered with sugarcane plantations, it would get only half of its present energy needs (including fuel and electricity). However, if the fuel alcohol is made of the stalks, wastes, clippings, straw, corn cobs, and other crop field trash, then no additional land is needed. However using these sources for this purpose would require additional replacement animal feedstock, fertilizers and electric power plant fuels.
Ethanol Ethanol can be derived from corn, wheat, potato wastes, cheese whey, rice straw, sawdust, urban wastes, paper mill wastes, yard clippings, molasses, sugar cane, seaweed, surplus food crops, and other cellulose waste. Petroleum is also used to make industrial ethanol.
Ethanol, which is the same chemical as the alcohol in alcoholic beverages, can approach 96% purity by distillation, and is as clear as water. Higher purities require different industrial processes. It is flammable and burns more cleanly than many other fuels. When fully combusted its combustion products are only carbon dioxide and water. For this reason, it is favoured for environmentally conscious transport schemes and has been used to fuel public buses. However, pure ethanol attacks certain rubber and plastic materials and cannot be used in unmodified car engines. Additionally, ethanol has a much higher octane rating than ordinary gasoline, requiring changes to the spark timing in engines. To change a gasoline-fueled car into an ethanol-fueled car, larger carburetor jets (about 50% larger) are needed. Also, a system is added to inject a little warmed ethanol into the carburetor to solve the cold starting problem. If 10% - 30% ethanol is blended with gasoline, then no engine modification is needed.
A mixture containing gasoline with at least 10% ethanol is known as gasohol. It is commonly available in the Midwest of the United States and is required by the state of Minnesota. One common gasohol variant is "E15", containing 15% ethanol and 85% gasoline. These concentrations are generally safe for regular automobile engines, and some regions and municipalities mandate that the locally-sold fuels contain limited amounts of ethanol. One way to measure alternative fuels is the "gasoline-equivalent gallons" (GEG). In 2002, the U.S. used as fuel an amount of ethanol equal to the energy of 1.13 billion gallons of gasoline. This was less than one percent of the total fuel used that year.
[1] (http://www.eia.doe.gov/cneaf/alternate/page/datatables/table10.html)
The term "E85 ethanol" is used for a mixture of 15% gasoline and 85% ethanol. Beginning with the model year 1999, a number of vehicles in the U.S. were manufactured so as to be able to run on E85 fuel without modification. Most of the vehicles are officially classified as light trucks (a class containing minivans, SUVs, and pickup trucks). These vehicles are often labeled dual fuel or flexible fuel vehicles, since they can automatically detect the type of fuel and change the engine's behavior to compensate for the different ways that they burn in the engine cylinders.
When farmers distilled their own ethanol, they sometimes used radiators as part of the still. The radiators often contained lead, which would get into the ethanol. Lead entered the air during the burning of contaminated fuel, possibly leading to neural damage. However this was a minor source of lead since tetraethyl lead was used as a gasoline additive.
In Brazil and the United States, the use of ethanol from sugar cane and grain as car fuel has been promoted by government programs. Some individual U.S. states in the corn belt began subsidizing ethanol from corn (maize) after the Arab oil embargo of 1973. The Energy Tax Act of 1978 authorized an excise tax exemption for biofuels, chiefly gasohol. The excise tax exemption alone has been estimated as worth US$1.4 billion per year. Another U.S. federal program guaranteed loans for the construction of ethanol plants, and in 1986 the U.S. even gave ethanol producers free corn.
Methanol Methanol, too, has been considered as a fuel, mainly in combination with gasoline. It has received less attention than ethanol, however, because it has a number of problems of its own. Its main advantage is that it can be easily manufactured from methane (the chief constituent of natural gas) as well as by pyrolysis of many organic materials. Pure methanol has been used in indy cars since the mid-1960's.
However, unlike ethanol, it is a toxic product; extensive exposure to it could lead to permanent health damage, including blindness. US maximum allowed exposure in air (40 h/week) are 1900 mg/m³ for ethanol, 900 mg/m³ for gasoline, and 260 mg/m³ for methanol. It is also quite volatile and therefore would increase the risk of fires and explosions.
Nevertheless, a drive to add a significant percentage of methanol to gasoline got very close to implementation in Brazil. A pilot experiment that was to be conducted in São Paulo was vetoed at the last minute by the city's mayor, out of concern for the health of gas station workers (who are mostly illiterate and could not be trusted to follow safety precautions). The idea has not been heard of since.
Alcohol and hydrogen There is an emerging view that current consumers of fossil fuels should move to using hydrogen as a fuel, creating a new so-called hydrogen economy. One theory holds that hydrogen should not be considered to be a fuel source in and of itself. In this view, hydrogen is merely an intermediate energy storage medium existing between an energy source (be it solar power, biofuels, and even fossil fuels) and the place where the energy will be used. Because hydrogen in its gaseous state takes up a very large volume when compared to other fuels, logistics becomes a very difficult problem. One possible solution is to use ethanol to transport the hydrogen, then liberate the hydrogen from its associated carbon in a hydrogen reformer and feed the hydrogen into a fuel cell. Alternatively, some fuel cells can be directly fed by ethanol.
In early 2004, researchers at the University of Minnesota announced that they had invented a simple ethanol reactor that would take ethanol, feed it through a stack of catalysts, and output hydrogen suitable for a fuel cell. The device uses a rhodium-cerium catalyst for the initial reaction, which occurs at a temperature of about 700°C. This initial reaction mixes ethanol, water vapor, and oxygen and produces good quantities of hydrogen. Unfortunately, it also results in the formation of carbon monoxide, a substance that "chokes" most fuel cells and must be passed through another catalyst to be converted into carbon dioxide. The ultimate products of the simple device are roughly 50% hydrogen gas and 30% nitrogen, with the remaining 20% mostly composed of carbon dioxide. Both the nitrogen and carbon dioxide are fairly inert when the mixture is pumped into an appropriate fuel cell. Once the carbon dioxide is released back into the atmosphere, it is reabsorbed by plant life.
Alternate sources Sugar cane grows in the southern United States, but not in the cooler climates where corn is dominant. However, many regions that currently grow corn are also appropriate areas for growing sugar beets. Some studies indicate that using these sugar beets would be a much more efficient method for making ethanol in the U.S. than using corn.
In the 1980s, Brazil seriously considered producing ethanol from cassava, a major food crop with massive starchy roots. However yields were lower than sugarcane, and the processing of cassava was considerably more complex, as it would require cooking the root to turn the starch into fermentable sugar. The babaçu plant was also investigated as a possible source of alcohol.
There is also growing interest in the use of biomass as a source for ethanol and other types of fuel. This is a broad-ranging idea, using various types of organic matter including purpose-grown crops of plants and trees as well as leftover waste products — even including animal waste.
At this time, most of the different processes for converting biomass into ethanol and other fuels are very complicated and not particularly efficient. A few processes have seen increasing buzz, including thermal depolymerization (though that process produces what is described as light crude oil).
Net fuel energy balance To be viable, an alcohol-based fuel economy should have positive net fuel energy balance. Namely, the total fuel energy expended in producing the alcohol — including fertilizing, farming, harvesting, transportation, fermentation, distillation, and distribution, as well as the fuel used in building the farm and fuel plant equipment — should not exceed the energy contents of the product.
Switching to a system with negative fuel energy balance would only increase the consumption of non-alcohol fuels. Such a system would only be worth considering as a way of exploiting non-alcohol fuels that may not be suitable for transportation use, such as coal, natural gas, or biofuel from crop residues. (Indeed, many U.S. proposals assume the use of natural gas for distillation.) However, many of the expected environmental and sustainability advantages of alcohol fuels would not be realized in a system with negative fuel balance.
Even a positive but small energy balance would be problematic: if the net fuel energy balance is 50%, then, in order to eliminate the use of non-alcohol fuels, it would be necessary to produce two gallons of alcohol for each gallon of alcohol delivered to the consumer.
In this regard, geography is the decisive factor. In tropical regions with abundant water and land resources, such as Brazil, the viability of production of ethanol from sugarcane is no longer in question; in fact, the burning of sugarcane residues (bagasse) generates far more energy than needed to operate the ethanol plants, and many of them are now selling electric energy to the utilities. Also, in countries with abundant hydroelectric power, the net fuel energy balance of the cycle could be improved to some extent by using electricity in the production, e.g. for milling and distillation.
The picture is quite different for other regions, such as the United States, where the climate is too cool for sugarcane. In the U.S., agricultural ethanol is generally obtained from grain, chiefly maize, and the net fuel energy balance of that route is still critical.
Energy balance in the United States Many early studies concluded that the use of corn ethanol for fuel would have a negative net energy balance. Namely, the total energy needed to produce ethanol from grain — including fermentation, fertilizing, fuel for farm tractors, harvesting and transporting the grain, building and operating an ethanol plant, and the natural gas used to distill corn sugars into alcohol — exceeds the energy content of ethanol. Critics have argued that since production energy comes mostly from fossil fuels, gasohol isn't just wasting money but hastening the depletion of nonrenewable resources. Most such studies were based on data collected in the 1970s and early 1980s, but some analyses in 2001, continued to indicate that ethanol has a negative energy balance. A peer-reviewed study by Cornell University ecology professor David Pimentel seemed to confirm this conclusion. Pimentel's study was disputed by other specialists, forcing him to revise his figures. Still, in August 2003, he stated in a Cornell bulletin that production of ethanol from corn only takes 29% more energy than it produces.
However, continuous refinements to ethanol production procedures have much improved the benefit/cost ratio, and most studies of modern systems indicate that they now have a positive net energy balance.
Many other studies of corn ethanol production have been conducted, with greatly varied net energy estimates. Most indicate that production requires energy equvialent to 1/2, 2/3, or more of the fuel produced is required to run the process. A 2002 report by the United States Department of Agriculture concluded that corn ethanol production in the U.S. has a net energy value of 1.34, meaning 34% more energy was produced than what went in. This means that 75% (1/1.34) of each unit produced is required to replace the energy used in production.
MSU Ethanol Energy Balance Study: (http://www.ethanol.org/pdfs/msu_ethanol_study.pdf) Michigan State University, May 2002. This comprehensive, independent study funded by MSU shows that there is 56% more energy in a gallon of ethanol than it takes to produce it.
Arguments and criticisms Template:NPOVThe use of alcohol as fuel is advocated with various arguments, mainly relating to its beneficial effects on the local and global environment, its independence from foreign oil, and its economic advantages. Critics generally dispute those arguments, claim that the switch would be expensive, and object to perceived need for increased government subsidies, taxes, and regulations.
Air pollution There has long been widespread acknowledgement that ethanol is a cleaner-burning fuel than gasoline. Ethanol has far fewer standard regulated pollutants such as carbon monoxide and hydrocarbons, compared with plain gasoline in equivalent tests. See, for example, the air pollution and environmental studies listed at the Renewable Fuels Association website
http://www.ethanolrfa.org/pubs.shtml
There has been concern about increased evaporative smog-forming hydrocarbon emissions. For example, the conservative organization RPPI claims that "adding ethanol to gasoline will at best have no effect on air quality and could even make it worse. Studies show ethanol could even increase emissions of nitrogen oxides and volatile organic compounds, which are major ingredients of smog."
[2] (http://www.rppi.org/ethanolmandates.html) Other critics have argued that the beneficial effects of ethanol can be achieved with other cheaper additives made from petroleum.
It is important to distinguish the issues. Ethanol in a blend with gasoline replaces tetra ethyl lead, benzene and MTBE -- all of which are additives that are meant to raise octane levels. Ethanol, with an octane rating of 110, far surpasses regular gasoline and precludes any need for other additives that are dangerous. However, ethanol can increase vapor pressure of gasoline causing increased evaporative emissions which, on balance, are far less serious than lead, benzene or MTBE.
Ethanol as a straight fuel is far cleaner than gasoline in its own right and this has been recognized from the dawn of the automotive age. See, for instance, Kovarik's "Fuel of the Future"
http://www.radford.edu/~wkovarik/lead
Fire safety Ethanol appears to be less of a fire hazard than gasoline; while methanol, being more volatile, is somewhat more prone to fire and explosions. However, since ethanol and methanol dissolve in water (rather than floating on it like gasoline) their fires can be extinguished with ordinary water hoses.
Greenhouse gases A separate (and perhaps more important) benefit of switching to an ethanol fuel economy would be the decreased net ouptut of the greenhouse gas carbon dioxide (CO
2), since all the CO
2 that would be liberated in the manufacture and consumption of ethanol would have to be absorbed by the plantations. In constrast, the burning of fossil fuels injects massive amounts of "new" CO
2 into the atmosphere, without creating a corresponding sink.
Needless to say, this advantage will be accrued only with agricultural ethanol, not with ethanol derived from petroleum — which, due to its much smaller cost, presently accounts for most of the alcohol produced for industrial consumption. This point must be taken into account when estimating the cost of the switch.
Renewable resource According to its proponents, another advantage of (agricultural) alcohol as a fuel is that it is a renewable energy source that will never be exhausted; whereas an economy based on fossil fuels will sooner or later collapse when the world runs out of oil.
However, David Pimentel disputes that "ethanol production from corn" is a renewable energy source.
Dependency on foreign oil and international crime A somewhat related (but more compelling) argument is that developed regions like the United States and Europe consume much more fossil fuels than they can extract from their territory. Those countries have therefore become dependent on foreign suppliers, and their economies have thus become hostage to international events. The dependency has also been a major cause of wars, coups d'etat, and attendant misery and human rights violations. For each U.S. dollar spent on gasloine by customers at the pump, 15 cents of the dollar is spent on terrorism in the Middle East. Oil dollars build terrorist schools that turn young arabian men into terrorists. A switch to alchohol fueled cars in western nations would bankrupt much of the terrorist activities in the Middle East. Thus switching to an agricultural ethanol economy, by lessening that dependency, would stabilize the economies of consumer countries, reduce terrorism, and make the world a better place for all.
Statism Some critics, mainly on ideological grounds, dislike the idea of an ethanol economy because they see it as leading to increased government subsidy for corn-growing agribusiness. The Archer Daniels Midland Corporation of Decatur, Illinois, better known as ADM, the world's largest grain processor, produces 40% of the ethanol used to make gasohol in the U.S.. The company and its officers have been eloquent in their defense of ethanol and generous in contributing to both political parties.
Tax Incentives for ethanol and petroleum: (http://www.ethanol.org/pdfs/oil_incentive_study.pdf) U.S. General Accounting Office, September 2000. This study examines subsidies historically given to the oil industry and to the ethanol industry and finds that the amounts of those to the oil industry are far higher. At the same time, this study applies only to historical subsidies and doesn't not investigate the question of what the case would be if petroleum fuels were substantially replaced by ethanol.
The Brazilian experiment In Brazil, ethanol is produced from sugar cane which is a more efficient source of fermentable carbohydrates than corn as well as much easier to grow and process. Brazil has the largest sugarcane crop in the world, which, besides ethanol, also yields sugar, electricity, and industrial heating. Sugar cane growing requires little labor, and government tax and pricing policies have made ethanol production a very lucrative business for big farms. As a consequence, over the last 25 years sugarcane has become one of the main crops grown in the country.
Ethanol production basics Sugarcane is harvested manually or mechanically and shipped to the distillery (
usina) in huge specially built trucks. There are several hundred distilleries throughout the country; they are typically owned and run by big farms or farm consortia and located near the producing fields. At the mill the cane is roller-pressed to extract the juice (
garapa), leaving behind a fibrous residue (bagasse). The juice is fermented by yeasts which break down the sucrose into CO2 and ethanol. The resulting "wine" is distilled, yielding hydrated ethanol (5% water by volume) and "fusel oil". The acidic residue of the distillation (
vinhoto) is neutralized with lime and sold as fertilizer. The hydrated ethanol may be sold as is (for ethanol cars) or be dehydrated and used as a gasoline additive (for gasohol cars). In either case, the bulk product was sold until 1996 at regulated prices to the state oil company (Petrobras). Today it is not regulated anymore.
One ton (1,000 kg) of harvested sugarcane, as shipped to the processing plant, contains about 145 kg of dry fiber (bagasse) and 138 kg of sucrose. Of that, 112 kg can be extracted as sugar, leaving 23 kg in low-valued molasses. If the cane is processed for alcohol, all the sucrose is used, yielding 72 liters of ethanol. Burning the bagasse produces heat for distillation and drying, and (through low-pressure boilers and turbines) about 80 kWh of electricity, of which 50 kWh is used by the plant itself and 30 kWh sold to utilities.
The average cost of production, including farming, transportation and distribution, is US$ 0.63 per gallon; gasoline prices in the world market is about US$ 1.05 per gallon. The alcohol industry, entirely private, was invested heavily in crop improvement and agricultural techniques. As a result, average yearly ethanol yield increased steadily from 3,000 to 5,500 liter/hectare (0.30 to 0.55 liter/m
2) between 1978 and 2000, or about 3.5% per year.
Electricity from bagasse Sucrose accounts for little more than 30% of the chemical energy stored in the mature plant; 35% is in the leaves and stem tips, which are left in the fields during harvest, and 35% are in the fibrous material (bagasse) left over from pressing.
Part of the bagasse is currently burned at the mill to provide heat for distillation and electricity to run the machinery. This allows ethanol plants to be energetically self-sufficient and even sell surplus electricity to utilities; current production is 600 MW for self-use and 100 MW for sale. This secondary activity is expected to boom now that utilities have been convinced to pay fair price (about US$ 30-40/MWh) for 10 year contracts. The energy is especially valuable to utilities because it is produced mainly in the dry season when hydroelectric dams are running low. Estimates of potential power generation from bagasse range from 1,000 to 9,000 MW, depending on technology. Higher estimates assume gasification of biomass, replacement of current low-pressure steam boilers and turbines by high-pressure ones, and use of harvest trash currently left behind in the fields. For comparison, Brazil's Angra I nuclear plant generates 600 MW (and it is often off line).
Presently, it is economically viable to extract about 80 kWh of electricity from the residues of one ton of sugarcane, of which about 50 kWh are used in the plant itself. Thus a medium-size distillery processing 1 million tons of sugarcane per year could sell about 5MW of surplus electricity. At current prices, it would earn US$ 18 million from sugar and ethanol sales, and about US$ 1 million from surplus electricity sales. With advanced boiler and turbine technology, the electricity yield could be increased to 180 kWh per ton of sugarcane, but current electricity prices do not justify the necessary investment. (According to one report, the World bank would only finance investments in bagasse power generation if the price were at least US$ 70/MWh.)
Bagasse burning is environmentally friendly compared to other fuels like oil and coal. Its ash content is only 2.5% (against 30-50% of coal), and it contains no sulfur. Since it burns at relatively low temperatures, it produces little nitrous oxides. Moreover, bagasse is being sold for use as a fuel (replacing heavy fuel oil) in various industries, including citrus juice concentrate, vegetable oil, ceramics, and tyre recycling. The state of São Paulo alone used 2 million tons, saving about US$ 35 million in fuel oil imports.
Program statistics Except where noted, the following data apply to the 2003/2004 season.
| land use: | 4.5 million hectares = 45,000 km2 in 2000 |
| labor: | 1 million jobs (50% farming, 50% processing) |
| sugarcane: | 344 million metric tons (50-50 for sugar and alcohol) |
| sugar: | 23 million tons (30% is exported) |
| ethanol: | 14 billion liters = 14 million m3 (7.5 anhydrous, 6.5 hydrated; 2.4% is exported) |
| dry bagasse: | 50 million tons |
| electricity: | 1350 MW (1200 for self use, 150 sold to utilities) in 2001 |
The labor figures are industry estimates, and do not take into account the loss of jobs due to replacement of other crops by sugarcane.
Effect on oil consumption Most cars in Brazil run either on alcohol or on gasohol; only recently dual-fuel ("Flex Fuel") engines have become available. Most gas stations sell both fuels. The market share of the two car types has varied a lot over the last decades, in response to fuel price changes. Ethanol-only cars were sold in Brazil in significant numbers between 1980 and 1995; between 1983 and 1988, they accounted for over 90% of the sales. They have been available again since 2001, but still account for only a few percent of the total sales.
Ethanol-fueled small planes for farm use have been developed by giant Embraer and by a small Brazilian firm (Aeroálcool), and are currently undergoing certification.
Domestic demand for alcohol has grown from 4 to 12 billion liters between 1982 and 1998, and has remained roughly constant since then. In 1989 more than 90% of the production was used by ethanol-only cars; today that percentage has fallen to about 40%, the remaining 60% being used with gasoline in gasohol-only cars. Both the total consumption of ethanol and the ethanol/gasohol ratio are expected to increase again with deployment of dual-fuel cars.
Presently the use of ethanol as fuel by Brazilian cars - as pure ethanol and in gasohol - replaces about 10 billion liters of gasoline per year, or about 40% of the fuel that would be needed to run the fleet on gasoline alone. However, the effect on the country's oil consumption was much smaller than that. Although Brazil is a major oil producer and now exports gasoline (7 billion liters/year), it still must import oil because of internal demand for other oil byproducts, chiefly diesel fuel (which cannot be easily replaced by ethanol).
Environmental effect The improvement the air quality in big cities in the 1980s, following the widespread use of ethanol as car fuel, was evident to everyone; as was the degradation that followed the partial return to gasoline in the 1990s.
However, the ethanol program also brought a host of environmental and social problems of its own. Sugarcane fields are traditionally burned just before harvest, in order to remove the leaves and kill snakes. Therefore, in sugarcane-growing parts of the country, the smoke from burning fields turns the sky gray throughout the harvesting season. As winds carry the smoke into nearby towns, air pollution goes critical and respiratory problems soar. Thus, the air pollution which was removed from big cities was merely transferred to the rural areas (and multiplied). This practice has been decreasing of late, due to pressure from the public and health authorities; but the powerful sugarcane growers' lobby has managed to prevent a total ban.
Many nations have produced alchohol fuel with no destruction to the environment. Advancements in fertilizers and natural pesticides have eliminated the need to burn fields. With condensed agriculture, like hydroponics and greenhouses, less land is used to grow more crops. Now it is possible to grow crops in the desert and other unarable lands, where there are much fewer native plants and animals to disturb.
Social implications The ethanol program also led to widespread replacement of small farms and varied agriculture by vast seas of sugarcane monoculture. This led to a decrease in biodiversity and further shrinkage of the residual native forests (not only from deforestation but also through fires caused by the burning of adjoining fields). The replacement of food crops by the more lucrative sugarcane has also led to a sharp increase in food prices over the last decade.
Since sugarcane only requires hand labor at harvest time, this shift also created a large population of destitute migrant workers who can only find temporary employment as cane cutters (at about US$3–5 per day) for one or two months every year. This huge social problem has contributed to political unrest and violence in rural areas, which are now plagued by recurrent farm invasions, vandalism, armed confrontations, and assassinations.
Politics The Brazilian alcohol program has been often criticized for many motives, including excessive land use, environmental damage, displacement of food crops, reliance on misery-wage temporary labor, statism and dependency on government subsidies, etc..
Until 1996, the brazilian oil company (Petrobras) was forced to buy ethanol from the private distilleries and sell it to gas station chains, both as pure (hydrated) ethanol and gasohol. Nowadays Petrobras only buy ethanol as a anti-knocking additive. However, for lack of internal demand, Petrobras is virtually forced to sell its surplus gasoline in the international market at a rather low price, US$ 0.13/liter. Since the domestic market price is about US$ 0.50/liter, Petrobras could increase its revenue by over 1 billion US$ per year if the ethanol program were cancelled. Petrobras also produces methyl-tert-butyl ether (MTBE), a compound that could replace ethanol in gasohol as an anti-knocking and anti-pollution additive.
On the other hand, the sugarcane agribusiness sector is politically powerful and so far it has successfully defended the program from its critics. The positive effect of the program on Brazil's overstrained foreign trade speaks louder than all its environmental and social problems.
Petrol (
gasoline in the United States and Canada) is a petroleum-derived liquid mixture consisting primarily of hydrocarbons, used as fuel in internal combustion engines. The term
gasoline is the common usage within the oil industry, even within companies that are not American. Often the term
mogas (short for
motor gasoline, for use in cars) is used to distinguish it from avgas, used in light aircraft.
Chemical analysis and production Petrol is produced in oil refineries. These days material that is simply separated from crude oil via distillation, called natural gasoline, will not meet the required specifications (in particular octane rating; see below) for modern engines, but these streams will form part of the blend.
The bulk of a typical petrol consists of hydrocarbons with between 5 and 12 carbon atoms per molecule.
The various refinery streams that are blended togther to make petrol all have different characteristics. Some important streams are:
- Reformate, produced in a catalytic reformer with a high octane and high aromatics content, and very low olefins (alkenes).
- Cat Cracked Gasoline or Cat Cracked Naphtha, produced from a catalytic cracker, with a moderate octane, high olefins (alkene) content, and moderate aromatics level.
- Natural Gasoline (has very many names), directly from crude oil with low octane, low aromatics (depending on the crude oil), some naphthenes (cycloalkanes) and zero olefins (alkenes).
- Alkylate, produced in a Alkylation unit, with a high octane and which is pure paraffin (alkane), mainly branched chains.
- Isomerate (various names) which is made by isomerising Natural Gasoline to increase its octane.
(The terms used here are not always the correct chemical terms, typically they are old fashioned, but they are the terms normally used within the industry. The exact terminology for these streams varies by oil company and by country.)
Overall a typical petrol is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), aromatics and olefins (alkenes). The exact ratios can depend on
- the oil refinery that makes the petrol, as not all refineries have the same set of processing units
- the crude oil used by the refinery on a particular day
- the grade of gasoline, in particular the octane
These days petrol in many countries has tight limits on aromatics in general, benzene in particular, and olefins (alkene) content. This is increasing the demand for high octane pure paraffin (alkane) components, such as Alkylate, and is forcing refineries to add processing units to reduce the benzene content.
Petrol can also contains some other organic compounds: such as organic ethers, (deliberately added) plus small levels of contaminants, in particular sulfur compounds such as disulphides and thiophenes. Some contaminants, in particular mercaptans and hydrogen sulphide must be removed because they cause corrosion in engines.
Volatility Petrol is a more volatile fuel than diesel or kerosene. The reason for this is not only the base constituents, but the additives that are put into it. The final control of volatility is often via blending of butane. The desired volatility depends on the ambient temperature: the hotter the weather, the lower the volatility. In Australia the volatility limit changes every month and differs for each main distribution centre, but most countries simply have a summer, winter and perhaps intermediate limit.
The maximum volatility of petrol in many countries has been reduced in recent years to reduce the fugitive emissions during refuelling.
Octane rating The most important characteristic of petrol is its
Research Octane Number (RON) or octane rating, which is a measure of how resistant petrol is to premature detonation (knocking). It is measured relative to a mixture of isooctane (2,2,4-trimethylpentane) and n-hepta ne. So an 87-octane petrol has the same knock resistance as a mixture of 87% isooctane and 13% n-heptane.
There is another type of Octane, called "Motor Octane Number" (MON), which is a better measure of how the fuel behaves when under load. Its definition is also based on the mixture of isooctane and n-heptane that has the same performance. Depending on the composition of the fuel, the MON of a modern petrol will be about 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.
In most countries (including all of Europe and Australia) the 'headline' octane that would be shown on the pump is the RON: but in the United States and some other countries the headline number is in fact the average of the RON and the MON, sometimes called the "Road Octane Number" or DON. Because of the 10 point difference noted above this means that the octane in the United States will be about 5 points lower than the same fuel elsewhere: 87 octane fuel in the United States would be 92 in Europe.
It is possible for a fuel to have a RON greater than 100. This reflects the fact that isooctane is not the most knock-resistant substance available. Racing fuels, Avgas and LPG typically have octane ratings of 110 or significantly higher.
It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. Using a fuel with a higher octane allows an engine to be run at a higher compression ratio without having problems with knock. Compression is directly related to power, so engines that require higher octane usually deliver more power. Some high-performance engines are designed to operate with a compression ratio associated with high octane numbers, and thus demand high-octane petrol. It should be noted that the power output of an engine also depends on the energy content of its fuel, which bears no simple relationship to the octane rating. Some people believe that adding a higher octane fuel to their engine will increase its performance or lessen its fuel consumption. This is false - engines perform best when using fuel with the octane rating they were designed for.
The octane rating was developed by the chemist Russell Marker. The selection of n-heptane as the zero point of the scale was due to the availability of very high purity n-heptane, unmixed with other isomers of heptane or octane, distilled from the resin of Jeffrey Pine. Other sources of heptane produced from crude oil contain a mixture of different isomers with greatly differing ratings, which would not give a precise zero point.
Dangers Many of the non-aliphatic hydrocarbons naturally present in petrol (especially aromatic ones like benzene), as well as many anti-knocking additives, are carcinogenic. Because of this, any large-scale or ongoing leaks of petrol pose a threat to the public's health should the petrol reach a public supply of drinking water. The chief risks of such leaks come not from vehicles, but from petrol delivery truck accidents and leaks from underground storage tanks. Because of this risk, most underground storage tanks now have extensive measures in place to detect and prevent any such leaks, such as sacrificial anodes. Petrol is rather volatile (it readily evaporates), meaning that storage tanks on land and in vehicles must be properly sealed. But the high volatility also means that it will easily ignite in cold weather conditions, unlike diesel for example. However, certain measures must be in place to allow appropriate venting to ensure the level of pressure is similar on the inside and outside. Petrol also reacts dangerously with certain common chemicals; for example, petrol and crystal Drano® react together in a spontaneous combustion.
Energy content Energy content of some fuels compared to petrol:
| Fuel Type | BTU/gallon | RON |
| Petrol | 125000 | 87-98 |
| LPG | 95475 | 110 |
| Diesel fuel oil | 138690 | |
| Residential fuel oil | 149690 | |
| Ethanol | 84400 | |
| Methanol | 62800 | |
| Gasohol (10% ethanol + 90% petrol) | 120900 | |
A high octane fuel such as LPG actually has a lower energy content than lower octane petrol, resulting in an over all lower power output. However, with an engine tuned to the use of LPG this lower power output can be overcome.
Additives
Oxygenate blending
Oxygenate blending adds oxygen to the fuel in the form of oxygen bearing compounds such as MTBE or ethanol. This oxygen in the fuel reduces the amount of carbon monoxide and unburned fuel in the exhaust gas, thus reducing smog. In many areas throughout the U.S. oxygenate blending is mandatory. For example in Southern California, fuel must contain 2% oxygen by weight.
MTBE use is being phased out due to issues with contamination of ground water. In some places it is already banned. Ethanol is a common replacement, especially ethanol derived from biomatter such as corn. An ethanol petrol mix is called gasohol. The most extensive use of ethanol takes place in Brazil, where the ethanol is derived from sugar cane.
MMT Methylcyclopentadienyl manganese tricarbonyl (MMT) has been used for a long time in Canada and recently in Australia to boost octane. It also helps old cars designed for leaded fuel run on unleaded fuel without need for additives to prevent valve stem problems.
History
Lead additives
Because the mixture known as petrol has a tendency to explode early (causing a disturbing "knocking" noise in internal combustion engines), lead additives were first blended with fuel in the 1920s. This practice continued through the 1980s. The most popular one was tetra-ethyl lead. However, with the recognition of the environmental damage caused by the lead, and the incompatibility of lead with catalytic converters, most countries are in the process of phasing out the sale of leaded fuel, and different additives to reduce knocking are now used. Among the most popular ones are aromatics, ethers and alcohol (usually ethanol or methanol).
There are also additives to reduce internal engine carbon buildups, to improve combustion, and to allow easier starting in cold climates.
Octane rating Romania is a supplier of "light-sweet" crude oil, which, when distilled, resulted in a petrol with an 87 rating (DON). 87 octane was the general benchmark for much of the world, and is the current standard rating for "normal" petrol in the US and Canada.
World War II and octane One interesting historical issue involving octane rating took place during WWII. Germany received the vast majority of her oil from Romania, and set up huge distilling plants in Germany to produce petrol from it. In the US the oil was not "as good" and the oil industry instead had to invest heavily in various expensive boosting systems. This turned out to be a huge blessing in disguise. US industry was soon delivering fuels of ever-increasing octane ratings by adding more of the boosting agents, with cost no longer a factor during wartime. By war's end American aviation fuel was commonly 130 to 150 octane, which could easily be put to use in existing engines to deliver much more power by increasing the compression delivered by the superchargers. The Germans, relying entirely on "good" petrol, had no such industry, and instead had to rely on ever-larger engines to deliver more power. The result is that British and US engines consistently outperformed German ones during the war, playing no small part in the defeat of the Luftwaffe
