Alternative fuels: LPG, NGV, synthetic fuels, and biofuel
Alternatives to petroleum are readily available, such as natural gas or coal. Natural gas can be used as is (Natural Gas for Vehicles - NGV) or in liquid form (LPG - Liquefied Petroleum Gas).
Liquid fuels can also be synthesized from natural gas (Gas-to-Liquid - GTL) or coal (Coal-to-Liquid - CTL).
Renewable fuels made from biomass or biofuel can be distributed in liquid form (usually mixed with fossil fuels) or in gaseous form (essentially methane from the fermentation of organic matter).
Natural gas for vehicles: NGV, LPG, and Hythane®
These are very light hydrocarbons, distributed in either compressed or liquid form. Natural gas for vehicles (NGV) is made mostly of methane (CH4) with traces of ethane (C2H6), propane (C3H8) and butane (C4H10). The methane molecule is the smallest amongst the hydrocarbon molecules and has the lowest carbon content. For the same amount of energy delivered during combustion, natural gas emits 25 % less CO2 than diesel fuel or gasoline and doesn't release any particulate matter. However it must be compressed to 200 bars to compensate for its low density. Natural gas tanks (CNG for Compressed Natural Gas) are relatively heavy and therefore limit the vehicle's range. Liquefaction at very low temperatures, a very common industrial process, is not appropriate for road transportation.
Thanks to its positive environmental balance, especially in terms of point source pollution, CNG is being increasingly used in bus fleets and urban vehicles that can carry a large tank and don't need a long range. In Argentina, a big natural gas producer, many vehicles run on CNG. Italy and the Netherlands are also big CNG users. Some Asian cities also use CNG vehicles, especially for taxis (New Delhi, Shanghai). Yet overall CNG suffers from lack of distribution structures.
However, vehicle owners with access to domestic gas networks can refill their tanks at home using a small compressor. Though most engines are still not optimized for this kind of fuel, progress is being made.
Liquefied Petroleum Gas (LPG) has been used for a number of years. It's a mix of propane and butane, so a bit "heavier" than natural gas. It is produced by refining petroleum (40 %) and extracting the natural gas (60 %), which is considered an "impurity." At room temperature, it liquefies at a pressure of several bars. The tank technology is therefore very simple: domestic gas cylinders. It's affordable, emits less CO2 than conventional fuel (but a bit more than CNG), and can be used in slightly modified combustion engines. Though well established in some countries, it is nevertheless hampered by a limited distribution network that prevents LPG from taking off, as it is still only available in limited quantities. Italy remains the primary consumer of LPG, particularly for captive fleets such as taxis. South Korea is also a big consumer.
Hythane® is a more recent invention, a mix of natural gas (typically 80 %) and hydrogen. It is distributed by the same networks and used by the same vehicles as natural gas. The addition of hydrogen facilitates combustion and reduces NOx emissions. The vehicle emits around 10 % less CO2 than if it were to run on pure natural gas. Hythane® is also seen as a necessary step paving the way for the introduction of hydrogen to the transportation market. The general public is also becoming more familiar with hydrogen, since Hythane® can be used for domestic heating or cooking purposes. Hythane® is being increasingly tested in buses (Montreal, Las Vegas, Palm Springs) and all over the world. India recently launched its first station in New Delhi.
GTL and CTL synthetic fuels
These are synthetic liquid hydrocarbons made from natural gas (GTL for Gas-to-Liquid) or coal (CTL for Coal-to-Liquid).
They are obtained through a process patented in 1923 by German chemists Franz Fischer and Hans Tropsch. They were used extensively in the Second World War, first by the Germans and then the Japanese. After the war, they were developed in South Africa, which was isolated by the embargo and had major coal reserves. They are a blend of carbon monoxide (CO) and hydrogen (H2), called "syngas", made by subjecting coal, natural gas, or biomass to very high-temperature steam. In the presence of a metal catalyst, water and a liquid hydrocarbon (methanol, gasoline, diesel fuel, kerosene, petrochemical products, etc.) are obtained. The fuel produced in this way is purer than petroleum derivatives. The tank-to-wheel CO2 emissions are the same (with a slight reduction in consumption thanks to more efficient combustion), but CO2 emissions from production are greater than with conventional refining. This disadvantage can only be overcome through carbon capture and sequestration.
While they provide an alternative to oil dependency, synthetic fuels do have limitations. However, GTL could be of environmental interest if it were produced from natural gas derived from oil drilling. Synthesis of CTL, on the other hand, releases a great deal of CO2, and also consumes a lot of water. These inconveniences have prompted China to back off CTL development, despite initial interest. Widespread use of CTL will therefore only be possible once methods to capture and store CO2 are more developed. Another limitation: even though known gas and especially coal reserves will last far longer than conventional oil reserves, synthetic fuels made from primary fossil energy sources are not renewable. This disadvantage doesn't apply to biomass energy sources such as carbon, which is not a fossil fuel.
Hydrogen has been seen as a promising energy carrier for a number of years; however it's not a very efficient fuel for internal combustion engines (still in the demonstration stage), mostly due to its very low density.
Biofuel is not a recent invention, even thought it has come to the fore in recent years because of the expected decline in petroleum reserves and increasing concern about global warming. In 1903, a Gordon-Brillié running on ethanol fuel established the world record for speed at 177 km/h. Biofuels are compatible with existing fuels and distribution systems, and their well-to-wheel CO2 output is low. In fact, CO2 released by biofuel combustion was in large part already "extracted" from the atmosphere through plant photosynthesis.
First generation biofuels
The most common biofuel in the world is ethanol (and its derivatives) for spark-ignition engines. Biodiesel production is much more limited. Ethanol fuel is obtained by fermenting sugar from either hydrolyzed starch (grains, cassava, potatoes) or directly from fermentable sugar (sugar cane, sugar beets), followed by distillation. In Europe, ETBE (ethyl tert-butyl ether) is often synthesized by mixing ethanol with isobutylene. Another possibility is producing butanol by fermenting sugar with bacteria rather than "traditional" yeasts.
A non-modified gasoline engine can generally run on gasoline that contains up to 10 % ethanol or 15 % ETBE, but modifications are necessary for higher percentages. Nevertheless, there is ongoing research in the US to evaluate the feasibility of using 10-15 % or even 20 % ethanol in existing gasoline engines. One of the characteristics of ethanol is its high octane rating, also called the anti-knock index, which is much higher than gasoline's and greatly contributes to engine efficiency. Experiments have shown that the efficiency of an engine designed to run on ethanol and adapted to its key features would without a doubt be at least equivalent to diesel engine efficiency. However combustion parameters would need to be adjusted and components such as joints and fuel hoses replaced, since ethanol is more corrosive.
E85, composed of 85 % ethanol and 15 % gas, is already sold in some European countries as well as the United States. Brazil uses E100, or pure ethanol (made of 95% ethanol and water), and E25 (a mix of 25 % anhydrous ethanol and 75 % gasoline) in flexible-fuel engines that can run on all of these types of fuels.
Ethanol fuel must be anhydrous, yet alcohol produced by distillation is around 8 % water and would thus require a specially designed engine. The absence of water prohibits transportation of the blend via pipelines, and consequently requires that the gasoline-alcohol blend be made near distribution points.
Diesel engines can run on vegetable oil (canola, sunflower, soy, palm, etc.) or their derivatives (fatty acid methyl esters (FAME) or ethyl esters) obtained by combining a fatty acid with methanol or ethanol. Diesel engines can run on pure vegetable oil under certain conditions, but they will emit acrolein, a toxic (carcinogenic) product. NAD+ (nicotinamide adenine dinucleotide) can form significant deposits in the combustion chamber.
Biodiesel blends like B30, made of 30 % fatty acid methyl esters and petroleum diesel, can be used in some unmodified diesel engines. The B5 to B20 blends (5 to 20 % FAME) are the most common.
The well-to-wheel CO2 emissions for vehicles can currently be improved by up to 90 % by replacing gasoline with E100. Diesel engines using B30 also emit much less CO2 as compared to gasoline engines. These are the best estimates, based on ideal production conditions.
Additionally, the combustion process for biofuels (whether using ethanol or vegetable oil derivatives) emits fewer pollutants (particulate matter, sulfur dioxide) than petroleum-based fuels.
There is a third "traditional" sector which has received less attention than ethanol and biodiesel: the production of biogas (mostly made of methane) through fermentation of organic material (sludge from waste water purification plants, liquid manure, liquid waste from the agro-food industry, household waste, and energy crops). Even though biogas is more commonly used in gasoline engines, it can also be used in specially adapted diesel engines. However this solution has limited use for transportation purposes, because it's hard to stock small, scattered production units, and it has a relatively low energy content. Nevertheless, it is very well suited to local use, such as generators and captive fleets with access to centralized refueling areas.
The current biofuel network has some limitations, however, and could be problematic if questions of sustainability are not taken into account. Some raw materials, such as grain crops or oleaginous plants, could compete with crops destined for human or animal consumption. That's why it is essential to establish priorities for the use of arable land, and choose appropriate biofuel crops.
The social and ecological advantages of producing and using biofuel must therefore be evaluated according to different criteria: the type of crop that is chosen, productivity, market value, the available stock, possible financial speculation, climatic conditions, and demand. Consequently, we should be cautious and put biofuel use in perspective before drawing simplistic conclusions or condemning its use.
Biofuels are not all the same, and are not all produced in the same way. While the production of ethanol from corn in the US or wheat in Europe has prompted much debate, the Brazilian use of sugar cane has been an undeniable success. Though biofuels have often been criticized for costing more than their traditional equivalents, larger-scale, more effective production systems and technological innovations have considerably increased their competitiveness.
The case of ethanol in Brazil is a good example. In Brazil, ethanol made from sugar cane is cheaper than gasoline and has become the fuel of choice for light vehicles. Even though conditions in Brazil are ideally suited to large-scale ethanol production, it is fair to say that the Brazilian example could be reproduced in other countries, particularly in tropical regions where agricultural yields are higher than in temperate climates.
Countries in Central and South America, Southeast Asia, and certain parts of Africa are ideally suited to sugar cane ethanol production. If programs that remain compatible with food production are developed, they could also increase the electricity supply in isolated rural areas by using bagasse (fibrous residue from sugar cane) as a source of renewable energy. Countries including Colombia, Venezuela, Peru, Jamaica, Thailand, and India have developed this approach.
The Brazilian example
In the 1930s, Brazil was faced with an overproduction of sugar and thus increased its production of ethanol in order to develop new markets for the sugar cane industry. It became mandatory to add 5 % ethanol to gasoline (10 % for government vehicles). During the Second World War, gas shortages led to the addition of a high percentage of ethanol (around 40 %) to gasoline. With the end of the war and the availability of cheap gas, ethanol's importance waned. The first oil crisis led to renewed interest in ethanol, and in 1975 Brazil presented a national program to gradually replace gasoline with pure ethanol and ethanol-gasoline blends. The government charged the automobile industry with developing cars with engines that could run on ethanol, and they succeeded in doing so in 1979. Since then, more than 5 million ethanol cars have been sold in Brazil. In 2003, these vehicles were replaced with flexible-fuel cars, which can run on gasoline, 100 % ethanol, or any other mix of fuel. In 2009, close to 90 % of new cars sold in Brazil used flex-fuel technology. Since 2003, around 9 million flex-fuel cars have been sold and consumers tend to prefer pure ethanol for daily use. The ethanol content of gas has its own story. In 1977, after decades of experiments with different ethanol blends, Brazil adopted an ethanol content of 5 % at the national level. Thanks to continually increasing ethanol production, the ethanol content gradually increased to between 20 and 25 %. Cars that are produced for the Brazilian market are designed to work with this high-ethanol gasoline, while imported cars are adapted to accept the local gas.
Thanks to its climate and the cutting-edge technology used to grow sugar cane and produce ethanol, Brazil grows 8 million hectares of sugar cane and dedicates about half of the harvest to the production of close to 30 billion liters of ethanol per year, mostly for domestic consumption. The remaining sugar cane is used to produce sugar for the local and export markets. The massive use of ethanol in flex-fuel cars prevented 80 million tons of CO2 from being released between 2003-2009. Estimates from the federal government indicate that using ethanol has prevented more than 600 million tons of carbon dioxide from being emitted since 1976, when ethanol became a major part of Brazil's energy matrix.
Second generation biofuels
In the medium term (2015-2020), a second generation of biofuels should become available that will potentially resolve different environmental and agricultural problems. These fuels may be produced biochemically or thermo chemically from lignocellulosic biomass: agricultural residues, hedge or lawn scraps, plants that are rich in cellulose and lignin (grasses and trees), wood residues, etc. As their name indicates, these plants or plant parts contain macromolecules of cellulose and lignin. These two types of macromolecules make up the majority of the Earth's total biomass. Cellulose is essentially a chain of glucose molecules, and thus a sugar. Lignin is a phenolic compound.
There are currently two main processes to produce these fuels. The first is the enzyme hydrolysis of cellulose into glucose, which is then used to produce standard ethanol. A potential enzyme has been identified in a bacteria living in the digestive tube of termites that can assimilate cellulose. The drawback to this system is that it can't use lignin and can only produce gas substitutes. The other process transforms biomass into syngas (synthetic gas containing a variable mix of carbon monoxide (CO) and hydrogen (H2)) by means of steam-cracking, followed by synthesis using the Fischer-Tropsch process. This process is abbreviated as BTL (Biomass-to-Liquid) and is similar to synthetic fuels. However, purifying syngas requires very high temperatures and burns part of the biomass. Otherwise an external heat source, preferably carbon neutral, must be available (for example, residual biomass, wind turbine energy, even fourth generation nuclear reactors). Alternative solutions such as catalysis or membrane separation are also being studied. When second generation biofuels are used in pure form, there is theoretically a 90 % reduction in CO2 emissions from well-to-wheel as compared to conventional fuels. But their cost remains high. Amyris, a company based in California, has developed a new technology that will be commercialized in Brazil by 2015.
It uses genetically modified yeast to transform sugar into hydrocarbon molecules. Sugar cane juice is used to produce different products that could replace diesel fuel, kerosene, and some chemical products.
Along with the development of second generation technologies, there is ongoing research on non-food biomass. Non-arable land could potentially be used to cultivate inedible oleaginous shrubs like Jatropha curcas (Barbados nut), Millettia pinnata (Indian beech) Madhuca longifolia (butter tree).
Third generation biofuels
Certain saltwater microalgae can be cultured in photo bioreactors and produce a very high yield. This kind of fuel is sometimes referred to as third generation. Companies such as Shell and Exxon Mobil are currently investing in this line of research, which has also attracted a number of young companies.
Comparative characteristics of spark-ignition engine fuels
Since ethanol's volumetric energy density is lower than that of gasoline, the energy content of a liter of a blend of both will also be lower than a liter of gasoline. The vehicle's range will therefore be affected, even if a difference of 3.4 % in the case of SP 95 E10 is barely noticeable.
% Ethanol volume
% Ethanol energy
Liter for 1 l gasoline equivalent
SP 95 E10
Comparative characteristics of diesel engine fuels
As the difference in volumetric energy density between diesel fuel and vegetable oil methyl ester (VOME) is very small, there is no noticeable difference in fuel consumption between pure diesel fuel and a 7 % blend.
% VOME volume
% VOME energy
Liter for 1 l gasoline equivalent
Diesel fuel + 7 % VOME
Diesel + 30 % VOME*
*VOME: vegetable oil methyl ester
Comparison between gasoline, LPG, and NGV
The differences between their specific energies are relatively weak, around 10 %. However the difference in volumetric energy density between a liquid and a gas is obviously enormous. The liquid form is therefore preferable since it is less bulky and the tank is easy to fill.
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