08-20-2013, 05:01 AM
Ethanol produced by microorganisms, e.g., Saccharomyces cerevisae, from biomass is called bioethanol. It is the most widely used biofuel used for transport purposes, especially in Brazil and USA. 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:
1. It should be portable in sufficient quantities in the vehicle
2. It should burn in the internal combustion engines
3. It should be roughly equivalent to petrol in energy content
Ethanol has certain features, which make it more desirable than petrol, but it also has some undesirable features.
Advantages of bioethanol over petrol:
1. Ethanol has a much higher latent heat of vaporization (855MJ/kg) than petrol (293kJ/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 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, preignition does not occur when ethanol is used in engines set for petrol. Preignition denotes ignition before the piston has reached the correct position during compression cycle. It 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.
4. Ethanol is burnt more completely so that hydrocarbon emission is drastically lower as compared to that in case of petrol.
5. Ethanol has a much higher flash point (45 degrees C) than petrol (13 degrees C).
6. It can be mixed with petrol; this increases the octane rating of petrol. However, in these cases the ethanol has to be 100% pure otherwise it won’t mix properly with petrol.
Any biofuel proposed to be used for transport purposes should have the following features:
1. It should be portable in sufficient quantities in the vehicle
2. It should burn in the internal combustion engines
3. It should be roughly equivalent to petrol in energy content
Ethanol has certain features, which make it more desirable than petrol, but it also has some undesirable features.
Advantages of bioethanol over petrol:
1. Ethanol has a much higher latent heat of vaporization (855MJ/kg) than petrol (293kJ/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 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, preignition does not occur when ethanol is used in engines set for petrol. Preignition denotes ignition before the piston has reached the correct position during compression cycle. It 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.
4. Ethanol is burnt more completely so that hydrocarbon emission is drastically lower as compared to that in case of petrol.
5. Ethanol has a much higher flash point (45 degrees C) than petrol (13 degrees C).
6. It can be mixed with petrol; this increases the octane rating of petrol. However, in these cases the ethanol has to be 100% pure otherwise it won’t mix properly with petrol.
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