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Bioethanol
Ethanol produced by micro-organisms, e.g. Saccharomyces cerevisiae, from biomass is called bioethanol. Bioethanol is the most widely used biofuels for transport purposes, especially in Brazil and U.S.A. The interest in use of bioethanol as petrol replacement began in Brazil and USA during 1980s. At present, bioethanol is not cost-competitive as compared to petrol, but is being used for transport due to government subsidies. Any biofuel proposed to be used for transport purposes should have the following features: (i) it should be portable in sufficient quantities in the vehicle, (ii) it should burn in the internal combustion engines, and (iii) it should be roughly equivalent to petrol in energy content. Ethanol possesses certain features which make it more desirable than petrol, but it also has some undesirable features.

Bioethanol vs. petrol

Ethanol has the following advantages over petrol.

1. Ethanol has a much higher latent heat of vaporization (855 MJ/kg) than petrol (293 kJ/kg). This energy is obtained from the air in the carburetor. As a result, the fuel mixture entering the cylinder is much cooler and hence denser in case of ethanol than in the case of petrol. Even though ethanol has only 61.8% (27.2 kJ/kg) energy content of petrol (44 kJ/kg), the energy produced by combustion of ethanol during each stroke is only slightly lower than that released from petrol.

2. Ethanol has a higher octane number (99) than petrol (80-100). As a result, ‘pre-ignition’ denotes ignition before the piston has reached the correct position during compression cycle. Pre-ignition leads to power loss and damage to valves and piston.

3. Higher octane rating of ethanol allows the compression ratio of the engines to be increased; this results in increased production of power. In view of the above points, ethanol burning engines have only 10% more fuel consumption than petrol ones although ethanol has only about 62% as much energy content as petrol.

4. Ethanol is burnt more completely so that hydrocarbon omission is drastically lower as compared to that in case of petrol.

5. Ethanol has a much higher flash point (45˚C) than petrol (13˚C). flashpoint is the temperature at which a substance catches fire. Therefore, ethanol is much less likely to catch fire and explode in cases of fuel leakage, e.g. during accidents.

6. It can be mixed with petrol; this increases the octane rating of petrol. In USA a 20% ethanol: 80% petrol mixture is being marketed as ‘gasohol’. However, in such cases the ethanol must be 100% pure otherwise it will not mix in petrol and separation would occur.

However, ethanol suffers from several disadvantages as compared to petrol; these are briefly summarized below.

1. Engines run on ethanol may give starting problems when the air is cool; this is because of the higher latent heat of evaporations of ethanol. This can be overcome by electrically heating the carburetor.

2. Ethanol is highly hydrophilic. As a result, it can absorb moisture from atmosphere etc. Presence of water interferes with combustion and also causes corrosion in storage tanks and engines.

3. It reacts with metals used in the alloys of carburetors etc. e.g. aluminium and magnesium. It can be remedied by using nickel in the engine alloys.

4. The downstream processing for ethanol recovery is costly as it requires lots of energy. This is perhaps the chief reason for limiting the use of bioethanol as a fuel.

5. Ethanol run engines use about 10% more fuel than petrol, which means a proportionately larger tank.

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