The world community is getting increasingly concerned at the sustained high price of crude and its continuous rising trend. The US$100+ price of crude is knocking many people back with hard realization that fossil fuels are getting costlier .The price hike do not seem to be temporary. The sky rocketing oil price has already triggered slowing down of stronger economies which is directly impacting the economic growth of developing and underdeveloped countries as well.
Burning of some fossil fuel derivatives in huge transport fleets of the world is among the major reasons for serious green house gas emissions. World community is getting desperate to cut these emissions. Something needs to be done and that too very soon regarding appropriate fuel choice in the context of economic and environmental considerations. Feasible, practicable and commercially more attractive alternate fuel options need to be adopted and made available to world market as soon as possible. Rising oil price has already created energy insecurity and adversely impacting upon food security. Developing and underdeveloped nations are the worst sufferers.Dealys or indecisions may cause colossal damage to nations like
It has been estimated that about 80 % of the world’s demand of transport fuels – road, river and sea are met from the derivatives of fossil fuel, petroleum. Petrol, one of major petroleum derivatives is the leading motor vehicle fuel all over the world.Diesel; LPG and CNG are also used as alternative to Petrol. These are also refined or reformed derivatives of petroleum products .Some alternatives are also derived from non fossil fuels, or partially from renewable sources such as grain or other agricultural crops. But these require taking away of some lands now being used to cultivate crops and corns to feed the people. Some experts believe that use of agricultural land for growing crops for fuel may cause food insecurity. However, crops need fertilizers to cultivate which mostly come from non renewable fossil fuels. So the terms partially renewable has been attached to them.
The other fossil fuel alternatives to petrol are:
· Diesel
· Liquefied 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 are :
- Ethanol
- Hydrogen.
Although about eight million vehicles worldwide currently run on blends containing alternative fuels, yet 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 at this moment.
However the concerns about the impact of fossil fuels on the environment and the rising price of crude which basically is the source of petrol are driving the quest for suitable alternatives.
We have two challenges to encounter- environmental impacts of burning petrol and the other is the rising price of crude. Countries which have vast petroleum resources of their own or have access to major petroleum resources abroad either through ownership of petroleum resources or long term contract with producers are in relatively safer position. But countries which exclusively depend on imported petroleum products and have very little or no major alternative like natural gas face major challenge to keep their transport fleet running to move the wheels of economy. They must have to think about environmental impacts of burning petrol and other fossil fuels. Global warming from Green House gas emission has exposed many third world relatively les affluent countries to environmental disasters. Matters in Bangladeshi context need to be viewed with deep attention.
Crude oil is found in underground deposit in many countries of the world both offshore and onshore areas. It comprises of about 300 compounds of hydrogen and carbon ie hydrocarbons, as well as sulphur and nitrogen. Composition is fairly constant all over:
· Carbon - 83 -87%
· Hydrogen – 10-14%
· Nitrogen - 0.1-2%
· Oxygen - 0.05-1.5%
· Sulphur- 0.05- 6%
In the early days the intial refining process involved simple separation of components of crude by distillation process according to boiling points. The petrol was one of such component. But the growing demand of motor spirit made scientists to device thermal cracking from 1913. Severe heating of higher boiling point hydrocarbons cracked them into smaller low boiling hydrocarbons more suitable for petroleum production. Catalysts also came into play for supplementing thermal cracking as catalysts sped up the reaction to produce greater yield of higher octane petrol.
Now let us see how petrol burns. Petrol is a complex mixture of hydrocarbons that boils below 180 Degree Centigrade. The hydrocarbon constituents are those that have 4-12 carbon atoms and fall into three general types:
· Paraffin , such as hexane (C6 H 14) , and Octane ( C8 H 18)
· Olefin, such as Hexane (C6 H12).
· Aromatics, such as Benzene (C6 H6) and Tolune.
Petrol consists of a blend of more than 200 such hydrocarbons either occurring naturally in petroleum or derived from it. The variety of petrol depends on source of mother crude and process applied to derive it.
Now how does various Petrol type burn?
The equations for complete combustion in presence of sufficient oxygen for various categories are:
· For Hexane 2 C6H14 + 19O2 - > 12 CO2 + 14 H2O
· For Octane 2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O
· For Benzene 2 C6H6 + 15 O2 -> 12 CO2 +6H20.
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.
Advantages and Disadvantages:
Whether it is complete or incomplete combustion, burning of petrol in vehicles give off obnoxious green house gases. Transport emissions are one of major cause of environmental pollution of city and urban areas.Hydrocarbon, Nitrogen oxide and Carbon Monoxide emissions can be effectively reduced by fitting a three way catalytic converter that converts these undesirable exhaust components to less reactive substances.
Volatile organic compounds are also emitted into the atmosphere through evaporation from fuel tanks, carburetors and refueling stations. These emissions can be reduced by using carbon canisters containing activated charcoal which absorbs these vapors.
Octane number is an important element in the efficiency of petrol combustion. 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
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.
The following graph shows the fuel mix for transports.
World Transportation Fuels Demand
Diesel as Automotive Fuel:
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.
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 vapor 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 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.
Advantages and Disadvantages:
The diesel combustion system is very efficient -- CO and hydrocarbon emissions are much lower compared to petrol engines although the use of catalytic converters in petrol vehicles has significantly reduced this advantage. Diesel fuels also emit less CO2 per kilometers traveled 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 oxidized 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.
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) as Automotive Fuel:
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.
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 odorless 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, | |
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 however a non-renewable resource.
. 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).
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.
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 refueling 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.
About half a million vehicles currently run on CNG, mostly in
Advantages and Disadvantages:
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 odorized 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 refueling 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 refueled centrally, their use can be attractive to bus operators.
For Bangladesh CNG is the preferred option. It has its own Gas resource to support the sill not that massive transport fuel. Most of the country will come under National gas Transmission Grid in not too distant future. But safety of conversion and use of proper CNG cylinders must be ensured.ADB assisted Dhaka Clean Fuel Project may have been completed already. There must a be a post project implementation assessment of the impact of the various project components. If necessary similar project may be taken up in areas where Natural gas will penetrate through Gas Sector Development Project currently under implementation.
Methanol as Transport Fuel:
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 | |
Natural gas is by far the most economically and environmentally viable source.
Methanol is the simplest alcohol. It is a clear, colorless 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.
Advantages and Disadvantages:
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.
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.
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 as Transport Fuel:
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 | |
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 vaporization 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.
Advantages and Disadvantages:
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
- 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.
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
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.