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Ethanol fuel is ethanol (ethyl alcohol), the same type of alcohol found in alcoholic beverages. It can be used as a fuel, mainly as a biofuel alternative to gasoline, and is widely used in cars in Brazil. Because it is easy to manufacture and process, and can be made from very common materials, such as corn, it is steadily becoming a promising alternative to gasoline throughout much of the world.

Anhydrous ethanol (ethanol with less than 1% water) can be blended with gasoline in varying quantities up to pure ethanol (E100), and most spark-ignited gasoline style engines will operate well with mixtures of 10% ethanol (E10).[1] Most cars on the road today in the U.S. can run on blends of up to 10% ethanol,[2] and the use of 10% ethanol gasoline is mandated in some cities where harmful levels of auto emissions are possible.[3]

Ethanol can be mass-produced by fermentation of sugar or by hydration of ethylene from petroleum and other sources. Current interest in ethanol mainly lies in bio-ethanol, produced from the starch or sugar in a wide variety of crops, but there has been considerable debate about how useful bio-ethanol will be in replacing fossil fuels in vehicles. Concerns relate to the large amount of arable land required for crops,[4] as well as the energy and pollution balance of the whole cycle of ethanol production.[5][6] Recent developments with cellulosic ethanol production and commercialization may allay some of these concerns.[7]

According to the International Energy Agency, cellulosic ethanol could allow ethanol fuels to play a much bigger role in the future than previously thought.[8] Cellulosic ethanol can be made from plant matter composed primarily of inedible cellulose fibers that form the stems and branches of most plants. Dedicated energy crops, such as switchgrass, are also promising cellulose sources that can be produced in many regions of the United States.[9]

Chemistry

In this 3-d diagram of ethanol, the lines represent single bonds.

During ethanol fermentation, glucose is decomposed into ethanol and carbon dioxide.

C6H12O6 → 2C2H6O + 2CO2

During combustion ethanol reacts with oxygen to produce carbon dioxide, water, and heat: (other air pollutants are also produced when ethanol is burned in the atmosphere rather than in pure oxygen)

C2H6O + 3O2 → 2CO2 + 3H2O

Together, these two equations add up to the following:

C6H12O6 + 6O2 → 2CO2 + 4CO2 + 6H2O + heat

This is the reverse of the photosynthesis reaction, which shows that the three reactions completely cancel each other out, only converting light into heat without leaving any byproducts:

6CO2 + 6H2O + light → C6H12O6 + 6O2

Sources

Main article: Energy crop
Sugar cane harvest
Cornfield in South Africa
Switchgrass

About 5% of the ethanol produced in the world in 2003 was actually a petroleum product.[10] It is made by the catalytic hydration of ethylene with sulfuric acid as the catalyst. It can also be obtained via ethylene or acetylene, from calcium carbide, coal, oil gas, and other sources. Two million tons of petroleum-derived ethanol are produced annually. The principal suppliers are plants in the United States, Europe, and South Africa.[11] Petroleum derived ethanol (synthetic ethanol) is chemically identical to bio-ethanol and can be differentiated only by radiocarbon dating.[12]

Bio-ethanol is obtained from the conversion of carbon based feedstock. Agricultural feedstocks are considered renewable because they get energy from the sun using photosynthesis, provided that all minerals, required for growth (such as nitrogen and phosphorus), are returned to the land. Ethanol can be produced from a variety of feedstocks such as sugar cane, bagasse, miscanthus, sugar beet, sorghum, grain sorghum, switchgrass, barley, hemp, kenaf, potatoes, sweet potatoes, cassava, sunflower, fruit, molasses, corn, stover, grain, wheat, straw, cotton, other biomass, as well as many types of cellulose waste and harvestings, whichever has the best well-to-wheel assessment.

Production process

The basic steps for large scale production of ethanol are: microbial (yeast) fermentation of sugars, distillation, dehydration (requirements vary, see Ethanol fuel mixtures, below), and denaturing (optional). Prior to fermentation, some crops require saccharification or hydrolysis of carbohydrates such as cellulose and starch into sugars. Saccharification of cellulose is called cellulolysis (see cellulosic ethanol). Enzymes are used to convert starch into sugar [13].

Fermentation

Main article: Ethanol fermentation

Ethanol is produced by microbial fermentation of the sugar. Production of ethanol from sugarcane (sugarcane requires a tropical climate to grow productively) returns about 8 units of energy for each unit expended compared to corn which only returns about 1.34 units of fuel energy for each unit of energy expended.[14]

Carbon dioxide, a greenhouse gas, is emitted during fermentation and combustion. However, this is canceled out by the greater uptake of carbon dioxide by the plants as they grow to produce the biomass.[15] When compared to gasoline, depending on the production method, ethanol releases less or even no greenhouse gases.[16][17]

Distillation

Ethanol plant in West Burlington, Iowa
Ethanol plant in Sertãozinho, Brazil.

For the ethanol to be usable as a fuel, water must be removed. Most of the water is removed by distillation, but the purity is limited to 95-96% due to the formation of a low-boiling water-ethanol azeotrope. The 96% m/m (93% v/v) ethanol, 4% m/m (7% v/v) water mixture may be used as a fuel.

Dehydration

Currently, the most widely used purification method is a physical absorption process using a molecular sieve, for example, ZEOCHEM Z3-03 (a special 3A molecular sieve for EtOH dehydration). Another method, azeotropic distillation, is achieved by adding the hydrocarbon benzene which also denatures the ethanol (to render it undrinkable for duty purposes). A third method involves use of calcium oxide as a desiccant.

Technology

Ethanol-based engines

Ethanol is most commonly used to power automobiles, though it may be used to power other vehicles, such as farm tractors and airplanes. Ethanol (E100) consumption in an engine is approximately 34% higher than that of gasoline (the energy per volume unit is 34% lower)[18][19][20]. However, higher compression ratios in an ethanol-only engine allow for increased power output and better fuel economy than would be obtained with the lower compression ratio.[21][22] In general, ethanol-only engines are tuned to give slightly better power and torque output to gasoline-powered engines. In flexible fuel vehicles, the lower compression ratio requires tunings that give the same output when using either gasoline or hydrated ethanol. For maximum use of ethanol's benefits, a much higher compression ratio should be used,[23] which would render that engine unsuitable for gasoline usage. When ethanol fuel availability allows high-compression ethanol-only vehicles to be practical, the fuel efficiency of such engines should be equal or greater than current gasoline engines. However, since the energy content (by volume) of ethanol fuel is less than gasoline, a larger volume of ethanol fuel would still be required to produce the same amount of energy.[24]

A 2004 MIT study,[25] and an earlier paper published by the Society of Automotive Engineers,[26] describing tests, identify a method to exploit the characteristics of fuel ethanol that is substantially better than mixing it with gasoline. The method presents the possibility of leveraging the use of alcohol to even achieve definite improvement over the cost-effectiveness of hybrid electric. The improvement consists of using dual-fuel direct-injection of pure alcohol (or the azeotrope or E85) and gasoline, in any ratio up to 100% of either, in a turbocharged, high compression-ratio, small-displacement engine having performance similar to an engine having twice the displacement. Each fuel is carried separately, with a much smaller tank for alcohol. The high-compression (which increases efficiency) engine will run fine on ordinary gasoline under low-power cruise conditions. Alcohol is directly injected into the cylinders (and the gasoline injection simultaneously reduced) only when necessary to suppress ‘knock’ such as when significantly accelerating. Direct cylinder injection raises the already high octane rating of ethanol up to an effective 130. The calculated over-all reduction of gasoline use and CO2 emission is 30%. The consumer cost payback time shows a 4:1 improvement over turbo-diesel and a 5:1 improvement over hybrid. In addition, the problems of water absorption into pre-mixed gasoline (causing phase separation), supply issues of multiple mix ratios and cold-weather starting are avoided.

Ethanol's higher octane allows an increase of an engine's compression ratio for increased thermal efficiency according to the formula given at [27]. In one study, complex engine controls and increased exhaust gas recirculation allowed a compression ratio of 19.5 with fuels ranging from neat ethanol to E50. Thermal efficiency up to approximately that for a diesel was achieved.[28] This would result in the MPG of a dedicated ethanol vehicle to be about the same as one burning gasoline.

Engines using fuel with from 30% to 100% ethanol also need a cold-starting system. For E85 fuel at temperatures below 11 °C (52 °F) a cold-starting system is required for reliable starting and to meet EPA emissions standards.[29]

Ethanol fuel mixtures

Further information: Common ethanol fuel mixtures

To avoid engine stall, the fuel must exist as a single phase. The fraction of water that an ethanol-gasoline fuel can contain without phase separation increases with the percent of ethanol. This is shown for 25 C (77 F) in a gasoline-ethanol-water phase diagram, Fig 13 of [1]. This shows, for example, that E30 can have up to about 2% water. If there is more than about 71% ethanol, the remainder can be any proportion of water or gasoline and phase separation will not occur. However, the fuel mileage declines with increased water content. The increased solubility of water with higher ethanol content permits E30 and hydrated ethanol to be put in the same tank since any combination of them always results in a single phase. Somewhat less water is tolerated at lower temperatures. For E10 it is about 0.5% v/v at 70 F and decreases to about 0.23% v/v at -30 F as shown in Figure 1 of [2].

In many countries cars are mandated to run on mixtures of ethanol. Brazil requires cars be suitable for a 25% ethanol blend, and has required various mixtures between 22% and 25% ethanol, as of October 2006 23% is required. The United States allows up to 10% blends, and some states require this (or a smaller amount) in all gasoline sold. Other countries have adopted their own requirements.

File:Gas x álcool - 70%.svg
Hydrated ethanol × gasoline type C price table for use in Brazil

Beginning with the model year 1999, an increasing number of vehicles in the world are manufactured with engines which can run on any fuel from 0% ethanol up to 100% ethanol without modification. Many cars and light trucks (a class containing minivans, SUVs and pickup trucks) are designed to be flexible-fuel vehicles (also called dual-fuel vehicles). Their engine systems contain alcohol sensors in the fuel and/or oxygen sensors in the exhaust that provide input to the engine control computer to adjust the fuel injection to achieve stochiometric (no residual fuel or free oxygen in the exhaust) air-to-fuel ratio for any fuel mix. The engine control computer can also adjust (advance) the ignition timing to achieve a higher output without pre-ignition when higher alcohol percentages are present in the fuel being burned.[citation needed]

Fuel economy

All vehicles have a fuel economy (measured as miles per US gallon -MPG- , or liters per 100 km) that is directly proportional to energy content.[30] Ethanol contains approx. 34% less energy per unit volume than gasoline, and therefore will result in a 34% reduction in miles per US gallon.[18][19][20] For E10 (10% ethanol and 90% gasoline), the effect is small (~3%) when compared to conventional gasoline,[31] and even smaller (1-2%) when compared to oxygenated and reformulated blends.[32] However, for E85 (85% ethanol), the effect becomes significant. E85 will produce lower mileage than gasoline, and will require more frequent refueling. Actual performance may vary depending on the vehicle. The EPA-rated mileage of current USA flex-fuel vehicles[33] should be considered when making price comparisons, but it must be noted that E85 is a high performance fuel and should be compared to premium.In one estimate [34] the US retail price for E85 ethanol is 2.62 US dollar per gallon or 3.71 dollar corrected for energy equivalency compared to a gallon of gasoline priced at 3.03 dollar. Brazilian cane ethanol (100%)is priced at 3.88 dollar against 4.91 dollar for E25 (figures july 2007).

Use by country

The top five ethanol producers in 2005 were Brazil (4.35 billion US gallons per year), the United States (4.3 billion US gallons per year), China (530 MMgy), the European Union (250 MMgy) and India (80 MMgy). Brazil and the United States accounted for 90 percent of all ethanol production. Also, it should be noted that the United States, now producing at a rate of about 4.6 billion US gallons per year, is widely considered the world’s largest ethanol producer. Strong incentives, coupled with other industry development initiatives, are giving rise to fledgling ethanol industries in countries such as Thailand, the Philippines, Columbia, the Dominican Republic and Malawi. Nevertheless, ethanol hasn't yet made much of a dent in world oil consumption.[35]

Brazil

Main article: Ethanol fuel in Brazil
Gasoline on the left, alcohol on the right at a filling station in Brazil

Brazil has one of the largest bio-fuel programs in the world, involving production of ethanol fuel from sugar cane, and ethanol now provides 18 percent of the country's automotive fuel. As a result of this, together with the exploitation of domestic deep water oil sources, Brazil, which years ago had to import a large share of the petroleum needed for domestic consumption, recently reached complete self-sufficiency in oil.[36][37][38]

Brazil produced around 16.4 billion liters of ethanol in 2004 and used 2.7 million hectares of land area for this production (4.5% of the Brazilian land area used for crop production in 2005[39]). Of this, around 12.4 billion liters were produced as fuel for ethanol-powered vehicles in the domestic market. In Brazil, ethanol-powered and flexible-fuel vehicles are manufactured for operation with hydrated ethanol, an azeotrope of ethanol (around 93% v/v) and water (7%).

Production and use of ethanol has been stimulated through:

  • Low-interest loans for the construction of ethanol distilleries
  • Guaranteed purchase of ethanol by the state-owned oil company at a reasonable price
  • Retail pricing of neat ethanol so it is competitive if not slightly favorable to the gasoline-ethanol blend
  • Tax incentives provided during the 1980s to stimulate the purchase of neat ethanol vehicles.[40]

Guaranteed purchase and price regulation were ended some years ago, with relatively positive results. In addition to these other policies, ethanol producers in the state of São Paulo established a research and technology transfer center that has been effective in improving sugar cane and ethanol yields.[41]

United States

A Ford Taurus "fueled by clean burning ethanol" owned by New York City.
Main article: Ethanol fuel in the United States

Most cars on the road today in the U.S. can run on blends of up to 10% ethanol, and motor vehicle manufacturers already produce vehicles designed to run on much higher ethanol blends. Portland, Oregon, recently became the first city in the United States to require all gasoline sold within city limits to contain at least 10% ethanol.[42] Several motor vehicle manufacturers, including Ford, DaimlerChrysler, and GM, sell “flexible-fuel” cars, trucks, and minivans that can use gasoline and ethanol blends ranging from pure gasoline up to 85% ethanol (E85). By mid-2006, there were approximately six million E85-compatible vehicles on U.S. roads.[43]

Fuel ethanol as it is currently produced in the United States is variously criticized for its dependence on high subsidies, its consumption of more energy than is contained in the resulting fuel, and its (usually) consuming a food crop to produce fuel.[44] The subsidies have resulted in the conversion of considerable land to corn (maize) production, which generally consumes more fertilizers and pesticides than many other land uses.[citation needed] Recent developments with cellulosic ethanol production and commercialization may allay some of these concerns.[45]

Sweden

Bioethanol production (milj. liter*)[46]
Country 2006 2005
United States 431 165
Spain 402 303
France 250 144
Sweden 140 153
Italy 128 8
Poland 120 64
Hungary 34 35
Lithuania 18 8
Netherlands 15 8
Czech Republic 15 0
Latvia 12 12
Finland 0 13
Total 1 565 813
* 100 l bioethanol = 79,62 kg
Consumption of Bioethanol (GWh)[46]
No Country 2006 2005
1 United States* 3 573 1 682
2 Sweden* 1 895 1 681
3 France 1 747 871
4 Spain 1 332 1 314
5 Austria 920 0
6 Poland 611 329
7 UK 561 502
11 Finland 9 0
27 EU 10 210 6 481
*Total includes vegetable oil in Germany and biogas in Sweden
225 GWh (2006) ja 160 GWh (2005)
Main article: Ethanol fuel in Sweden

All Swedish gas stations are required by an act of parliament to offer at least one alternative fuel, and every fifth car in Stockholm now drives at least partially on alternative fuels, mostly ethanol.[47]

Stockholm will introduce a fleet of Swedish-made electric hybrid buses in its public transport system on a trial basis in 2008. These buses will use ethanol-powered internal-combustion engines and electric motors. The vehicles’ diesel engines will use ethanol.[48]

The Netherlands

Regular petrol with no bio-additives is slowly outphased, since EU-legislation has been passed that requires the fraction of nonmineral origin to become minimum 5,75% of the total fuel consumption volume in 2010. This can be realised by substitutions in diesel or in petrol of any biological source; or fuel sold in the form of pure biofuel. (2007:) There are only a few gas stations where E85 is sold, which is a 85% ethanol, 15% petrol mix. [49] Directly neighbouring country Germany is reported to have a much better biofuel infrastructure and offers both E85 and E50. Biofuel is taxed equally as regular fuel. However, fuel tanked abroad cannot be taxed and a recent payment receipt will in most cases suffice to prevent fines if customs check tank contents. (Authorities are aware of high taxation on fuels and cross-border fuel refilling is a well-known practice.)

Australia

Main article: Ethanol fuel in Australia

Legislation imposes a 10% cap on the concentration of fuel ethanol blends. Blends of 90% unleaded petrol and 10% fuel ethanol are commonly referred to as E10. E10 is available through service stations operating under the BP, Caltex, Shell and United brands as well as those of a number of smaller independents. Not surprisingly, E10 is most widely available closer to the sources of production in Queensland and New South Wales. E10 is most commonly blended with 91 RON "regular unleaded" fuel. There is a requirement that retailers label blends containing fuel ethanol on the dispenser.

China

China is promoting ethanol-based fuel on a pilot basis in five cities in its central and northeastern region, a move designed to create a new market for its surplus grain and reduce consumption of petroleum. The cities include Zhengzhou, Luoyang and Nanyang in central China's Henan province, and Harbin and Zhaodong in Heilongjiang province, northeast China. Under the program, Henan will promote ethanol-based fuel across the province by the end of this year. Officials say the move is of great importance in helping to stabilize grain prices, raise farmers' income and reducing petrol- induced air pollution.[50]

Iceland

On Monday, September 17th, 2007 the first ethanol fuel pump was opened in Reykjavik, Iceland. This pump is the only one of its kind in Iceland. The fuel is imported by Brimborg, a Volvo dealer, as a pilot to see how ethanol fueled cars work in Iceland. In a few weeks, the pump will be opened for public use.[citation needed]

Environment

Energy balance

Main article: Ethanol fuel energy balance

All biomass needs to go through some of these steps: it needs to be grown, collected, dried, fermented, and burned. All of these steps require resources and an infrastructure.

Opponents of corn ethanol production in the U.S. often quote the 2005 paper [51] of David Pimentel, a retired Entomologist, and Tadeusz Patzek, a Geological Engineer from Berkeley. Both have been exceptionally critical of ethanol and other biofuels. Their studies contend that ethanol, and biofuels in general, are "energy negative", meaning they take more energy to produce than is contained in the final product.

A 2006 report by the U.S. Department Agriculture compared the methodologies used by a number of researchers on this subject and found that the majority of research showed that the energy balance for ethanol is positive.[52] A 2006 study published in Science analyzed six studies, normalizing assumptions for comparison; Pimental and Patzek's studies still showed a net energy loss, while four others showed a net energy gain.[53] Furthermore, fossil fuels also require significant energy inputs which have seldom been accounted for in the past.

Ethanol is not the only product created during production, and the energy content of the by-products must also be considered. Corn is typically 66% starch and the remaining 34% is not fermented. This unfermented component is called distillers grain, which is high in fats and proteins, and makes good animal feed.[54]

In Brazil where sugar cane is used, the yield is higher, and conversion to ethanol is somewhat more energy efficient than corn.[14] Recent developments with cellulosic ethanol production may improve yields even further.[55]

Air pollution

Compared with conventional unleaded gasoline, ethanol is a particulate-free burning fuel source that combusts cleanly with oxygen to form carbon dioxide and water. The Clean Air Act requires the addition of oxygenates to reduce carbon monoxide emissions in the United States. The additive MTBE is currently being phased out due to ground water contamination, hence ethanol becomes an attractive alternative additive.

Use of ethanol, produced from current (2006) methods, emits a similar net amount of carbon dioxide but less carbon monoxide than gasoline.[56] Current production methods includes air pollution from the manufacturer of macronutrient fertilizers. The production of ammonia to produce nitrogen fertilizer consumed about 5% of the world natural gas consumption while China uses coal for 60% of their nitrogen fertilizer production.[57]

If ethanol-production energy came from non-fossil sources the use of ethanol as a fuel would add less greenhouse gas.[58]

A study by atmospheric scientists at Stanford University found that E85 fuel would increase the risk of air pollution deaths relative to gasoline.[59]

Manufacture

In 2002, monitoring of ethanol plants revealed that they released VOCs (volatile organic compounds) at a higher rate than had previously been disclosed.[60] The Environmental Protection Agency (EPA) subsequently reached settlement with Archer Daniels Midland and Cargill, two of the largest producers of ethanol, to reduce emission of these VOCs. VOCs are produced when fermented corn mash is dried for sale as a supplement for livestock feed. Devices known as thermal oxidizers or catalytic oxidizers can be attached to the plants to burn off the hazardous gases. Smog causing pollutants are also increased by using ethanol fuel in comparison to gasoline.[citation needed]

Greenhouse gas abatement

Corn ethanol has received much support on environmental grounds primarily because of its role in reducing greenhouse gas emissions. However, the evidence for this claim is mixed.

Land use

Large-scale 'energy farming', necessary to produce agricultural alcohol, requires substantial amounts of cultivated land. University of Minnesota researchers report that if all corn grown in the U.S. were used to make ethanol it would displace 12% of current U.S. gasoline consumption.[61] Some have claimed that land is acquired through deforestation, while others have observed that areas currently supporting forests are usually not suitable for growing any sort of crops.[62][63] Related concerns have been raised regarding a decline in soil fertility due to reduction of organic matter[64], a decrease in water availability and quality, an increase in the use of pesticides and fertilizers, and potential dislocation of local communities.[65]

As demand for ethanol fuel increases, food crops are replaced by fuel crops, driving food supply down and food prices up. Growing demand for ethanol in the United States has been discussed as a factor in the increased corn prices in Mexico.[66] Average barley prices in the United States rose 17% from January to June 2007 to the highest in 11 years. Prices for all grain crops trend upward, reflecting a progressive increase in farm land devoted to corn for the production of produce ethanol fuel.[67] Prices for U.S. corn-based products, including animal feed, also rise. This translates to higher prices for animal products like chicken, beef, and cheese. June 2007 cheese prices rose to $2 per pound on average, increasing 65% over the same period in 2006. As milk prices in the United States, approached $4.00 per US gallon,[68] many American restaurant franchises announced price increases for their products to compensate for rising food costs.[69][70][71] Alternatively, cellulosic ethanol can be produced from any plant material, potentially doubling yields, in an effort to minimize conflict between food needs versus fuel needs.[72] Instead of utilizing only the starch by-products from grinding wheat and other crops, cellulosic ethanol production maximizes the use of all plant materials, including gluten. This approach would have a smaller carbon footprint because the amount of energy-intensive fertilisers and fungicides remain the same for higher output of usable material.[72] While the enzyme technology[73] for producing cellulosic ethanol is currently in developmental stages, it is not expected to be available for large-scale production in the near future.[74] Moreover, the production of ethanol for fuel raises a number of land scarcity issues, regardless of what production method is employed. Many analysts suggest that biofuel strategies must be accompanied by fuel conservation restrictions.[75]

Renewable resource

Ethanol is considered "renewable" because it is primarily the result of conversion of the sun's energy into usable energy. Creation of ethanol starts with photosynthesis causing the feedstocks such as switchgrass, sugar cane, or corn to grow. These feedstocks are processed into ethanol (see production).

Current, first generation processes for the production of ethanol from corn use only a small part of the corn plant: the corn kernels are taken from the corn plant and only the starch, which represents about 50% of the dry kernel mass, is transformed into ethanol. Two types of second generation processes are under development. The first type uses enzymes to convert the plant cellulose into ethanol while the second type uses pyrolysis to convert the whole plant to either a liquid bio-oil or a syngas. Second generation processes can also be used with plants such as grasses, wood or agricultural waste material such as straw.

Efficiency of common crops

The United States Department of Energy, finds that for every unit of energy put towards ethanol production, 1.3 units are returned.[76] Another study found that corn-grain ethanol produced 1.25 units of energy per unit put in.[77] As yields improve or different feedstocks are introduced, ethanol production may become more economically feasible in the US. Currently, research on improving ethanol yields from each unit of corn is underway using biotechnology. By utilizing hybrids designed specifically with higher extractable starch levels, the energy balance is dramatically improved. Also, as long as oil prices remain high, the economical use of other feedstocks, such as cellulose, become viable. By-products such as straw or wood chips can be converted to ethanol. Fast growing species like switchgrass can be grown on land not suitable for other cash crops and yield high levels of ethanol per unit area [34]

Crop Annual yield (Liters/hectare) Annual yield (US gal/acre) Greenhouse-gas savings (% vs. petrol) Comments
Miscanthus 7300 780 37 - 73 Low-input perennial grass. Ethanol production depends on development of cellulosic technology.
Switchgrass 3100 - 7600 330 - 810 37 - 73 Low-input perennial grass. Ethanol production depends on development of cellulosic technology. Breeding efforts underway to increase yields. Higher biomass production possible with mixed species of perennial grasses.
Poplar 3700 - 6000 400 - 640 51 - 100 Fast-growing tree. Ethanol production depends on development of cellulosic technology. Completion of genomic sequencing project will aid breeding efforts to increase yields.
Sugar cane 5300 - 6500 570 - 700 87 - 96 Long-season annual grass. Used as feedstock for most bioethanol produced in Brazil. Newer processing plants burn residues not used for ethanol to generate electricity. Only grows in tropical and subtropical climates.
Sweet sorghum 2500 - 7000 270 - 750 No data Low-input annual grass. Ethanol production possible using existing technology. Grows in tropical and temperate climates, but highest ethanol yield estimates assume multiple crops per year (only possible in tropical climates). Does not store well. [78] [79] [80] [81]
Corn 3100 - 3900 330 - 420 10 - 20 High-input annual grass. Used as feedstock for most bioethanol produced in USA. Only kernels can be processed using available technology; development of commercial cellulosic technology would allow stover to be used and increase ethanol yield by 1,100 - 2,000 litres/ha.
Source (except sorghum): Nature 444 (Dec. 7, 2006): 670-654.

Reduced petroleum import

One rationale given for extensive ethanol production in the U.S. is its benefit to energy security, by shifting the need for some foreign-produced oil to domestically-produced energy sources. [82] Production of ethanol requires significant energy, but current U.S. production derives most of that energy from coal, natural gas and other sources, rather than oil. [83] Because 60% of oil consumed in the U.S. is imported, compared to a net surplus of coal and just 16% of natural gas (2006 figures), [84], the displacement of oil-based fuels to ethanol produces a net shift from foreign to domestic U.S. energy sources.

Recent patents

In 2006-2-23, Veridium Corporation announced the technology to convert exhaust carbon dioxide from the fermentation stage of ethanol production facilities back into new ethanol and biodiesel. The bioreactor process is based on a new strain of iron-loving blue-green algae discovered thriving in a hot stream at Yellowstone National Park.[85]

In 2006-11-14, US Patent Office approved Patent 7135308, a process for the production of ethanol by harvesting starch-accumulating filament-forming or colony-forming algae to form a biomass, initiating cellular decay of the biomass in a dark and anaerobic environment, fermenting the biomass in the presence of a yeast, and the isolating the ethanol produced.[86]

Fuel system problems

  • Fuels with more than 10% ethanol are not compatible with non E85-ready fuel system components and may cause corrosion of ferrous components.[87][88]
  • Can negatively affect electric fuel pumps by increasing internal wear[88] and undesirable spark generation. [89]
  • Is not compatible with capacitance fuel level gauging indicators and may cause erroneous fuel quantity indications in vehicles that employ that system.[90]
  • Not always compatible with marine craft, especially those that use fiberglass tanks.[91][92]
  • Decreases fuel-economy by 15-30%; this can be avoided using certain modifications that would, however, render the engine inoperable on regular petrol without the addition of an adjustable ECU, or use of multiple ECUs to run the engine on multiple fuel types. [93][20]
  • Tough materials are needed to accommodate a higher compression ratio to make an ethanol engine as efficient as it would be on petrol; these would be similar to those used in diesel engines (which typically run at a CR of 20:1[94], versus about 8-12:1 for petrol engines [95].)


References

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  25. ^ MIT Study
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See also

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