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Alternative Transport Fuels - Courtesy of AIP

At present, about 80% of the world's demand for transportation fuels -- road, rail, air and sea -- are met by derivatives from the fossil fuel, petroleum. Petrol, one of the major derivatives of petroleum, is used throughout the world as a motor vehicle fuel.

Other petroleum derivatives including diesel and liquid petroleum gas can be used in motor vehicles as alternatives to petrol as can compressed natural gas, which often occurs in conjunction with petroleum deposits. Some alternatives are derived from non-fossil, or partly renewable, sources such as grain or other agricultural crops. However, these need fertilisers made from fossil fuels etc. and are not, therefore, totally renewable.

The major fossil fuel alternatives to petrol are:

  • diesel
  • liquid petroleum gas (LPG)
  • compressed natural gas (CNG)
  • ethers -- methyl tertiary butyl ether (MTBE) produced from natural gas and butane
  • electricity from coal/oil/gas and
  • methanol produced from natural gas or coal,

and the major non-fossil alternative fuels:

  • ethanol
  • hydrogen.

Although about eight million vehicles worldwide currently run on blends containing alternative fuels, it is unlikely that any one of these fuels will achieve the worldwide usage of petrol in the foreseeable future, primarily because they are too expensive.

However the concerns about the impact of fossil fuels on the environment, is driving the quest for suitable alternatives.

alt1

Petroleum

Petroleum which is also called crude oil, is found in underground deposits throughout the world and contains up to 300 compounds of hydrogen and carbon, or hydrocarbons, as well as sulphur and nitrogen. Its elemental composition is fairly constant:

  • Carbon -- 83 to 87%
  • Hydrogen -- 10 to 14%
  • Nitrogen -- 0.1 to 2%
  • Oxygen -- 0.05 to 1.5%
  • Sulphur -- 0.05 to 6%

Petrol as a fuel

Production

Early refineries used a simple distillation process to separate crude oil into its components according to their boiling points. The petrol produced by this method was only that naturally occurring in the crude oil.

As demand for motor spirit grew, engineers and chemists found that more severe heating of the higher boiling points hydrocarbons broke them down, or 'cracked' them, into smaller, lower boiling hydrocarbons more suitable for petrol production. From 1913, thermal cracking was used to increase petrol production.

Substances known as 'catalysts' were later found to do a better job of cracking hydrocarbons than heat alone, by speeding up the reaction and producing a greater yield of higher octane petrol.

Chemistry

Petrol is a derivative of petroleum. It is essentially a complex mixture of hydrocarbons that boils below 180o C. The hydrocarbon constituents are those that have 4 - 12 carbon atoms in their structure and fall into three general types:

  • Paraffins, such as hexane (C6H14), and octane (C8H18)
  • Olefins, such as hexene (C6H12)
  • Aromatics, such as benzene (C6H6) and toluene

Petrol consists of a blend of more than 200 such hydrocarbons either occurring naturally in petroleum or manufactured from it. Petrol's can vary considerably in composition, depending upon the source of the original crude oil, and the processes used in production.

When there is enough oxygen, hydrocarbons can be burnt to form CO2 and water vapour, releasing HEAT.

The equation for the complete combustion of hexane is:

  • 2C6H14 + 19O2 -> 12CO2 + 14H2O,

for octane:

  • 2C8H18 + 25O2  -> 16CO2 + 18H2O,

for hexene:

  • 2C6H12 + 18O2 -> 12CO2 + 12H2O,

for benzene:

  • 2C6H6 + 15O2 -> 12CO2 + 6H2O,
If insufficient oxygen is available, incomplete combustion occurs, forming carbon moNOxide CO, nitrogen oxides and carbon, as well as carbon dioxide and water.

The energy value of petrol is 31.9 MJ/L.

alt2Environmental considerations

Exhaust emissions from petrol-driven cars include, in addition to CO2 and water vapour, hydrocarbons, nitrogen oxides and CO. These latter emissions may be effectively reduced by fitting a three-way catalytic converter that converts these three types of exhaust components into less reactive substances.

Volatile organic compounds are also emitted into the atmosphere through evaporation from fuel tanks, carburetors and refuelling stations. These emissions can be reduced by using carbon canisters containing activated charcoal which absorbs these vapours. Evaporation can also be controlled during manufacturing and distribution with double tank roofs, improved tank seals and vapour recovery units.

An important element in the efficiency of petrol combustion is the octane number. This indicates the ability of the fuel to resist detonation, which is referred to as engine pinging or knocking. Such detonation is caused by the spontaneous igniting of the fuel and air in the engine cylinders before the spark is fired. Higher octane fuels are less susceptible to detonation and thus prevent engine knock and in turn maintain engine power.

Lead has traditionally been added to petrol as an effective and economic method of boosting octane quality. However, concerns have recently arisen about the possible health effects of lead in vehicle exhaust emissions. Concerns also about atmospheric 'smog' pollution have led to the desire to remove up to 90% of the smog precursors present in engine exhaust gases by the use of catalytic converters. This in turn requires that the petrol be lead free if the catalyst is to function properly. In Australia this resulted in a decision to change to cars which operate on unleaded petrol with a lower octane than previously used, so that changes to refinery configurations, to make up for the octane loss upon the removal of the lead, would not be too extensive.

This change is not without its disadvantages, since a lower octane fuel results in a less efficient engine, and an overall increase in carbon dioxide emissions. Some additional CO2 emissions also arise from the changed refining processes. Thus, although the move to unleaded petrol may be successful on a local level from a smog point of view, it is likely to have an increased impact upon global air quality in terms of CO2.

Economic considerations

Most cars today run on petrol because it is a relatively cheap, convenient, safe and reliable fuel that yields good vehicle performance complete with a good vehicle range capability. It can also be stored and handled easily.

alt3World Transportation Fuels Demand

Diesel fuels

Production

Diesel fuels are derived partly from the distillation of crude oil and from other processing operations such as catalytic cracking units. Diesel is made up of hydrocarbons which boil at temperatures between 150 and 400o C. Diesel is normally produced by blending two or more refinery streams such as light gas oil, heavy gas oil and kerosene.

Chemistry

Diesel fuels comprise a mixture of paraffinic, aromatic and olefinic hydrocarbons. Diesel is chiefly composed of hydrocarbons containing 12 or more carbon atoms per molecule. They are 'heavier' than the components of petrol and thus it is a less volatile fuel. Diesel generally will also have a lower aromatic component as this reduces the cetane value. Cetane number is a measure of the tendency of a diesel fuel to knock in a diesel engine

The combustion of diesel is similar to petrol, with the variety of hydrocarbons reacting with oxygen to produce CO2 plus water vapour and releasing heat. The difference lies in the type of engine required for the combustion process. Diesel fuel is injected into the combustion chamber as a fine liquid spray. It requires a higher compression ratio and thus has a higher potential thermal efficiency than petrol. Consequently, diesel engines have a lower fuel consumption than an equivalent petrol engine. The energy value of diesel is 35.6 MJ/L.

Efficient engine operation requires the diesel fuel to have a good ignition quality -- in particular, it should have a short ignition delay period. The higher the cetane number, the shorter the ignition delay period and the better the fuel.

Environmental considerations

The diesel combustion system is very efficient -- CO and hydrocarbon emissions are much lower than from petrol engines although the use of catalytic converters in petrol vehicles has significantly reduced this advantage. Diesel fuels also emit less CO2 per kilometre travelled than any other fuel of fossil origin. Emissions of benzene, butadiene and formaldehyde are also very low.

One of the major drawbacks to diesel-fuelled cars can be their cold weather performance. Temperature is critical in order for the engine to start and continue running without wax forming on the filters and fuel lines. In cold climate regions a fuel heater or special low wax diesel fuel is sometimes required.

The sulphur content of diesel fuels is of increasing interest in terms of the effects of fuel quality on emissions. Most sulphur is oxidised and expelled in the exhaust gases. Some, however, is emitted as particles, and this can add to any environmental problems. On the other hand any radical reduction in sulphur levels creates difficulties such as fuel pump failure, reduced engine durability, more expensive fuel and an increase in CO2 emissions from the refining operations necessary to remove the sulphur.

Economic considerations

Diesel cars have better fuel economy than petrol-driven cars and are cheaper to maintain, however the capital costs of a diesel are greater because the higher compression ratio demands larger, stronger and hence more costly components than an equivalent power output petrol engine.

If additional emission control equipment to reduce the particulate emissions, from diesel engines is ultimately required to meet new standards, the extra engine costs and the costs of producing cleaner diesel fuel may render diesel cars financially unattractive, unless fuel price differential is established between diesel and petrol.

Liquid petroleum gas (LPG)

Production

LPG is often produced from raw natural gas when this is processed into pipeline quality natural gas. LPG is also produced when crude oil is refined.

Chemistry

LPG is a mixture of light hydrocarbons which are gaseous at normal temperatures and pressures, and which liquefy readily at moderate pressures or reduced temperature. It is odourless and so, for safety reasons, a pungent compound, mercaptan, is added to make any leaks easily detectable.

The main component gases of LPG are:

  • Propane (C3H8)
  • Propylene (C3H6)
  • Butane (C4H10)

Each gas undergoes a separate reaction during combustion:

Propane: C3H8 + 5O2 -> 3CO2 + 4H2O,

Propylene: 2C3H6 + 9O2 -> 6CO2 + 6H2O,

Butane: 2C4H10 + 13O2 -> 8CO2 + 10H2O,

Environmental considerations

The use of LPG is widespread, with an estimated 250,000 vehicles running on it in Australia. Of these, around 180,000 are privately owned.

Estimates are that exhaust and evaporative greenhouse emissions are approximately 15 per cent lower from LPG than from petrol vehicles. It does not need lead or other additives to boost its octane rating.

Comparisons of the levels of NOxious gas emissions from LPG and petrol vehicles are inconclusive, with test results indicating both higher and lower levels that petrol vehicles. Some recent tests suggest that NOxious emissions are worse from LPG vehicles.

LPG is a non-renewable resource.

Economic considerations

LPG is available Australia-wide through the service station networks. When converted to a gas, LPG expands up to 270 times. This means that the liquid form -- which is easily achieved -- is a very efficient way of carrying large amounts of gas. In general economic terms it is unattractive, requiring a subsidy, in the form of an excise exemption as an incentive to consumers who must cover the costs of conversion of the vehicle to operate on LPG.

Compressed natural gas (CNG)

Production

Natural gas is comprised of a mixture of gases, mainly hydrocarbons, found in geological formations. Methane is the principal component, generally comprising from 87 per cent to 97 per cent by volume of the hydrocarbons depending on the source of the gas.

Chemistry

In addition to methane (CH4), natural gas also contains small percentages of:

  • ethane (C2H6)
  • propane (C3H8)
  • butane (C4H10)
  • pentane (C5H12)
  • nitrogen, oxygen and carbon dioxide

It can be compressed and used as an automotive fuel.

Its combustion is given by:

CH4 + 2O2 -> CO2 + 2H2O

Environmental considerations

Because of its high octane number, CNG is an excellent fuel for spark ignition engines. Older cars are not difficult to convert from petrol to CNG. However, as engine management systems become more complicated, conversions are becoming more difficult or involve non-optimal engine operation. As a gas it can pose safety hazards during necessarily frequent refuelling operations. Although when properly operated and maintained, leakage of CNG is minimal it should be noted that methane is an even more active greenhouse gas than CO2.

Emissions from CNG-powered vehicles depend on the quality of the vehicle's conversion. In older cars without catalytic converters, non-methane hydrocarbon, CO and nitrogen oxides in exhausts from CNG-fuelled cars are much less than from petrol-driven vehicles.

There is less difference between emissions from petrol and CNG in cars with catalytic converters -- in both instances emissions are greatly reduced. CO emissions are the same while nitrogen oxide emissions may be slightly higher from CNG. Overall there appears to be slightly less greenhouse gas emission from CNG vehicles compared to petrol vehicles.

Use of CNG substantially reduces particulate emissions, particularly from the new, dedicated CNG engines now available for buses and trucks. These new engines reduce particulate emissions to very low levels and are expected to rapidly penetrate the city bus fleet sector because of their cleaner image. Many new CNG buses are in operation or on order for several Australian capital cities.

Economic considerations

About half a million vehicles currently run on CNG, mostly in Italy, New Zealand and Canada. Most converted cars, however, retain their fuel tanks and are actually dual-fuelled. The benefits of CNG are thus greatly reduced, because the compression ratio and engine efficiency of dual-fuelled cars cannot be increased to take advantage of CNG's high octane number.

Storage of CNG is also a problem. Because of its low boiling point, natural gas must be stored in high pressure tanks. These are heavy, reducing payload and space in smaller vehicles. A CNG-fuelled car with a 75 litre tank is about 150kg. heavier than a petrol-driven car of the same size. This is not such a problem with large vehicles such as buses.

Natural gas is lighter than air, and will dissipate into the atmosphere if leakage occurs. Like LPG, it is usually odourised to make it detectable. It is non-toxic and non-reactive.

The major problems with CNG are that it is uneconomic because the cost of converting cars is high and the short range between refuelling is inconvenient.

At present CNG buses are more expensive than diesel buses, however this price differential can be expected to reduce with time. The subsidy provided by the current excise exemption means that, where they can be refuelled centrally, their use can be attractive to bus operators.

Methanol

Production

Methanol (CH3OH) is a clear liquid alcohol that can be produced from natural gas, coal, crude oil and biomass crops such as wood and wood residues as well as directly from catalytic synthesis:

CO + 2H2 -> CH3OH

At present, however, natural gas is by far the most economically and environmentally viable source.

Chemistry

Methanol is the simplest alcohol. It is a clear, colourless liquid.

Combustion of methanol:

2CH3OH + 3O2 -> 2CO2 + 4H2O

Currently, pure methanol can be used in purpose-designed engines such as some racing cars, since its very high octane rating allows for the use of very high compression engines producing significantly more power than an equivalent petrol engine.

Pure methanol, can be mixed with petrol for use in flexible-fuelled vehicles (FFV) capable of measuring the methanol:petrol ratio being delivered to the engine. This is so that the engine management system can adjust the air:fuel ratio and timing to match the requirements of whatever mixture is being used.

The water solubility of methanol poses a problem. Methanol cannot be used in blends with petrol above 5% in normal cars, and then only with co-solvents, because of the fear of phase separation.

Environmental considerations

Methanol has the potential to reduce greenhouse gas emissions but would need to be produced from biomass to make a possible contribution. Methanol derived from natural gas using current technology offers at best only a small greenhouse gas emission benefit over petrol.

Although the emissions of CO, hydrocarbons and nitrogen oxides are lower in methanol-dedicated cars, the exhaust of these vehicles contains more formaldehyde, a known carcinogen. Methanol can also lead to greater unburnt fuel emissions of methanol and methane which, however, are usually more readily degraded than unburnt hydrocarbons. Methane is a major greenhouse gas. Under combustion, methanol produces neither soot particles nor sulphur oxides. It also yields less nitrogen oxides than any other fuel.

Economic considerations

Methanol is a high cost fuel compared with petrol, but relatively cheap compared with other options.

Methanol is extremely toxic and therefore hazardous to handle. It is also corrosive requiring modification of a conventional vehicle's fuel system.

It has only half the energy content of petrol, which results in greater fuel consumption per unit volume and shorter travelling range -- compensated to some extent by its suitability for use at a higher compression ratio and its ability to deliver more power.

Ethanol

Production

Ethanol is presently the most widely used alternative fuel in the world. It is mostly produced from crops which contain sugar (e.g. sugar cane or sugar beet), or by pretreatment of starch crops (e.g. corn or wheat) or cellulose to produce sugars. The fermentation process uses the conversion of sugars by yeast into ethanol and CO2:

C6H12O6 -> 2C2H5OH + 2CO2

Chemistry

Ethanol (C2H5OH).

As with methanol, ethanol requires less oxygen for combustion than petrol:

C2H5OH + 3O2 -> 2CO2 + 3H2O

Ethanol can be used straight but, since both ethanol and methanol have a higher heat of vaporisation than petrol, cold starting an engine can be a problem. However, this does not appear to be a problem using petrol blended with up to 20% ethanol.

Ethanol has about two-thirds the energy and heat value of petrol (21.2 MJ/L), but exhibits different burning characteristics to petrol, which may be more efficient. It is less toxic and corrosive than methanol, although its technical performance and emission levels are similar.

Environmental considerations

A positive environmental aspect is that ethanol is a renewable resource, unlike oil, gas or coal, and in some cases may even be produced from waste material.

However, there are drawbacks:

  • As an alcohol, ethanol contains the hydroxyl group (OH), giving it a high affinity with water and making it more difficult to separate from water. This can cause environmental problems, e.g. if an ethanol/petrol blend is spilt in a small watercourse or drain, the petrol may be able to be skimmed off the top but the ethanol will dissolve and be almost impossible to recover. Ethanol is however, more easily biodegraded or diluted to non-toxic concentrations than is petrol.
  • Because ethanol is produced from crops, large areas of land are be required for its production. In Australia, for example, it has been estimated that the amount of land readily available would provide only 10% of our fuel needs.
  • While CO emissions are reduced with alcohol fuels, aldehydes, which irritate the eyes, are increased.
  • As with methanol, the potential greenhouse gas savings depend on the feedstock and process used for production. Ethanol's full fuel cycle greenhouse gas emissions are said to range from 30 - 180% from maize and 0 - 115% from wood, of the emissions from the petrol it replaces. CO2 from the combustion process alone is similar for alcohol fuels and petrol on an energy equivalent level.

Economic considerations

To be able to achieve any significant reductions in emissions of greenhouse gas by using alcohol fuels, the ethanol or methanol will need to be produced from the lignocellulose fractions of biomass. However, it has yet to be demonstrated that large-scale production of this type is technically or economically viable.

At present ethanol production is 2 - 3 times more expensive than petrol production. Australian production costs are usually estimated to be 50 to 65 c/L, making it an uneconomic proposition. Currently, ethanol use in Australia is being supported by both a production bounty payment and total relief from excise.

Hydrogen

Production

There are two common feedstocks for hydrogen production -- water and hydrocarbons such as methane.

  • Hydrogen is produced from water by hydrolysis, using electricity. The major positive aspect of hydrogen is that there is an almost limitless supply of it in water (if the supply of electricity is limitless), and that it is non-toxic.
  • Hydrogen is produced when hydrocarbons react with steam. While this is a very simple process, it relies upon the earth's finite reserves of hydrocarbons, making hydrogen, in this case, not a true non-fossil alternative. If, however, vegetable oils/plants are used as a source of hydrocarbons, hydrogen becomes a renewable, if expensive, alternative.

Chemistry

Hydrogen is the lightest element in the universe. Under normal conditions, it is a colourless, odourless and tasteless gas. The complete combustion of hydrogen is very clean, provided the peak temperature is limited:

2H2 +O2-> 2H2O

If it burns at high temperatures, nitrogen in the air is also heated, forming nitrogen oxides. However, the temperature can be controlled by introducing water to the hydrogen/air mixture while still obtaining good combustion. It is also possible to cool the combustion by using excess air since hydrogen will burn even in dilute mixtures.

Environmental considerations

Because hydrogen produced by electrolysis is an indirect user of electricity, which is most often derived from fossil fuel-powered stations, the complete production process may indirectly involve considerable CO2 emissions. For the total environmental effect of hydrogen to be positive, the electricity used in its production should be generated from renewable sources such as solar, wind or hydro-power.

Economic considerations

Currently hydrogen is used as a fuel only in space rockets. However, some vehicle manufacturers are developing hydrogen powered engines which may be tested as prototypes in about three years' time.

In Australia, Ford has been working with the University of Melbourne on development of a hydrogen-powered vehicle. The vehicle used initially was a Ford Cortina and the results were adapted later to the Capri.

The main technical difficulty with hydrogen is storage. In compressed or liquid form, it needs a heavy and expensive tank. Another alternative is to utilise the ability of metal hydrides to absorb hydrogen, and to desorb it when it is needed, as used in the prototype being trialed by Mazda.

Other disadvantages of the use of hydrogen gas include:

Liquefying it is costly in terms of energy use,

Safety is a major concern, in use and distribution. Hydrogen is very flammable over a wide range of air:fuel ratios, and it burns rapidly with a high temperature, colourless flame.

Conclusion

Petrol is undoubtedly a convenient fuel for cars. It is easy to store and handle, and a petrol fuel tank takes up little space in a car. It is, like all other fuels, highly combustible and therefore potentially dangerous, particularly if a fuel tank ruptures.

On the cost side, with the exception of diesel fuels, alternative fuels at present are not commercially viable for use in cars in Australia, nor indeed in other countries without some form of government assistance such as subsidies or tariffs. The cost of using the alternatives is high compared with petrol. Petrol's current competitors in Australia -- CNG and LPG -- are subsidised through exemption from government excise, reducing their retail price by as much as 40 cents a litre.

Using alternative fuels also involves specific problems. As gases, hydrogen, LPG and CNG pose hazards in storage and in refuelling operations. Methanol is toxic and is therefore a possible health hazard. It also corrodes engines. With respect to safety, all fuels are hazardous but when correctly engineered the risk can be minimised and is probably similar for all.

At first sight, the blending of ethanol with petrol seems to offer the best combination of convenience and safety but it is uneconomic and restricted in supply. It decreases the vehicle range, poses some problems for existing car engines and produces levels of smog precursor emissions similar or greater to those of petrol. In reality, CNG is the fuel which is likely to penetrate into the transport fuel market in the near future, as city bus fleets are progressively replaced by CNG powered versions in order to reduce particulate emissions.

While these alternative fuels have not made a significant impact worldwide, mainly because they involve more compromises than does petrol, some have a potential role to play in areas of special requirements, such as cities with extreme air pollution, or in undeveloped countries with no indigenous petroleum deposits and an inability to participate in normal world trade.

Nevertheless, the lower cost of petrol and diesel and availability of new technologies to improve the emission performance of engines using these fuels, will ensure that for some time to come, petrol will continue to be the preferred and most widely used motoring fuel in the world closely followed by diesel.

alt4Main Users of Alternative Fuels

Main Physical/Chemical Properties

PROPERTY PETROL DIESEL METHANOL ETHANOL
Formula C4 - C12
Hydrocarbons
C12 -C19
Hydrocarbons
CH3OH C2H5OH
Composition (wgt %)
  Carbon
  Hydrogen
  Oxygen


85 - 88
12 - 15
negl.


85 - 88
12 - 15
negl.


37.5
12.5
50.0


52.2
13.1
34.7
Stoichiometric
Air/Fuel Ratio
14.5 to 1 14.5 to 1 6.5 to 1 9 to 1
Heat of combustion (Btu/lb) 18,900 18,500 8,570 11,500
Heating Value (MJ/L) 31.9 35.6 15.8 21.2
Boiling Temp. (oC) 27 - 225 185 - 380 65 78
Research Octane Number (RON) 91 - 97 not appl. 106 - 115 105 - 121*
Motor Octane Number (MON) 82 - 88 not appl. 82 - 92 90 - 95*
Cetane Number not appl 45 - 55 not appl not appl

* Laboratory engine Research and Motor octane rating procedures are not suitable for neat alcohols. The effective octane number in blends is highly dependent on the composition of the petrol to which it is added.

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