Ethanol and Methanol are well known as fuels for spark ignition engines and can be used to a very limited extent with diesel-like fuels – about 5 to 10 % for reducing the emissions. Use of larger amounts will reduce the power delivered by the engine since the calorific value of the alcohols is about half of diesel.
Hence, this discussion is limited to liquid fuels that can be used in compression ignition engines.
Compression ignition engines are very robust and can accept a wide variety of fuels like Diesel, Light Diesel Oil (LDO), Low Sulphur Heavy Stock (LSHS), Fuel oil or Furnace oil.
We also consider edible and non-edible oils from plant seed origin – Coconut oil, Pea nut oil, etc and Jatropha oil, Rape seed oil, Castor oil, Pongemia oil, etc.
The edible oils are eliminated from consideration as fuels as they are required for the survival of the society. If and when they become available in plenty – in excess of human consumption, they will qualify for use as engine fuels.
The non-edible oils are obtained by screw extraction of dried seeds of selected plants. The output of 15 to 35 % through this method can be improved by another 10 % by solvent extraction of the residue from screw extraction process. The liquid so obtained should be filtered to remove any particulate matter and can be further upgraded by performing transesterification. The crude itself will function better than furnace oil and for transportation vehicles, esterification is desirable to preserve the longevity of the engine components and reduce the emissions.
Another kind of oil is sought to be obtained from fast heating of biomass solids. The idea of fast heating arises from the following consideration of the pyrolisis process. As biomass gets heated to about 400 C (± 50), biomass will lose volatiles. If the loss of volatiles occurs slowly – at a rate much smaller than 1000 C/s, then the principal products are gaseous components and less than 3 % will be liquid fuel.
If the heating rate is around 1000 C/s and the volatile residence time is less than 0.5 to 1 s, then one gets a liquid called pyrolitic liquid up to 60 % of the dry weight of the biomass. To achieve these, one needs to reduce the size of the biomass to about a millimeter or less and arrange special reactor operational procedures to ensure the non-exceedance of the residence time and quick quench to prevent break down of the components to permanent gases.
The calorific value of non-edible oils is 85 to 95 % of diesel (whose calorific value is 42 MJ/kg or 10 kcal/kg), where as those of pyrolitic oils is around 40 % of diesel.
Nonedible oils as well as pyrolitic oils are composed of oxygenated compounds. Pyrolitic oils have non-separable water of 20 to 24 % and is the principal cause of the reduction in calorific value.
Pyrolitic oils are highly acidic and therefore corrosive. The choice of materials for storage and plumbing must account for this property.
Pyrolitic oils are difficult to ignite due to water content in the oil. Nonedible oils behave virtually like diesel in respect of ignition and combustion. The process cost for generating the oils is Rs. 6 to 10 / kg (in reasonable scale) for non-edible oils making use of existing industrial infrastructure.
It would cost Rs. 25 to 30 per kg for pyrolitic oils. The technology itself is successfully tried out for a few biomass types. Even these costs are valid for large throughputs (> a tonne per hour)
The process of extraction of non-edible oils generates valuable cake that can be used as a animal feed or a biogas fuel where required. This additional revenue will imply equivalent reduction in the cost of the non-edible oil to an extent of two rupees. In comparison, there is no additional revenue stream for pyrolitic oil.
On-edible oils have been known by the society for a long time as oils for wick lamps and other applications. Pyrolitic oil is thermochemical discovery of modern times.