Ethanol and methanol can be used as fuels or may be blended with petrol and used in a spark type internal combustion engine. These alcohols can be produced from biomass and agricultural residues, etc. Fermentation is the process involved in ethanol production whereas methanol is produced by thermal treatment followed by catalytic synthesis from the resultant gas.
Two major alcohol production programmes from biological feed-stocks are the Pro-alcohol Programme in Brazil and the Gasohol Programme in the USA. In Brazil, it is intended to produce ethanol by yeast fermentation mainly from sugar cane. This process involves the two preliminary steps, and bagasse is a by-product.
The clarified juice is then subjected to fermentation, distillation and dehydration, and about 95 per cent ethanol is produced.
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In the USA, maize is the usual starting material. This is subjected to cleaning, steeping in water, grinding and separation. The residue left after the separation of germ cake is further ground and sieved to remove the fibrous material and then centrifuged to obtain starch and gluten. The starch is hydrolyzed and then subjected to yeast fermentation. The subsequent steps are the same as for sugar cane.
Several other countries are planning to start ethanol production from molasses.
The cane based systems appear to involve a net energy gain since all the energy requirements of the process can be met by burning the fibrous bagasse. The maize-based process needs an external source of fuel but has the advantage that valuable by-products such as com oil, protein and animal feed are produced.
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Doubtless, the production of power alcohol from biomass, whether cane or corn, is a very welcome and feasible programme. The only limitation is that the diversion of sugar or starch crops to alcohol production may further aggravate the already serious food problem in many countries.
The ultimate deciding factor for the developing countries may be a comparison of the cost of producing fuel alcohol from biomass with the cost of importing oil. A recent study suggests that production of ethanol from sugarcane may be quite economic at the 1980 oil price level of the US $ 31 per bbl oil fob. provided that the cost of sugar cane delivered at the factory does not exceed $ 14 per ton (Anonymous, 1980).
According to Coombs (1981), the most promising systems for the tropics and subtropics are those based on sugar cane whereas for the temperate regions, the preferred raw material for ethanol production is lignocellulose.
The latter can come from such diverse sources as agricultural residues, garbage, by-product from forestry, timber and paper industries, or from coppiced hard-wood trees or tree plantations that are purposely grown for this specific use. The main biological breakthrough needed now is to reduce the costs of processing lignocellulose.
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The temperate regions offer great prospects for producing methanol from wood. In this case the required technology for methanol production by catalytic synthesis for CO and H20 already exists. Attempts are currently underway to assess the relative merits and demerits of the alternative routes (pyrolysis or gasification) and to work out the economics of designing the actual gas producing plant.
According to some current estimates and projections for the future, ethanol production from molasses or food crops is expected to progressively rise until around 1988-1990 and then level off. In contrast, the production of ethanol from purpose-grown sugar and starch crops will continue to show a rising trend late beyond 1990s, and the industrial production of methanol from biomass are expected to rise into the year 2000 and beyond (Coombs, 1981).
These projections suggest the superiority of methanol to ethanol for conversion of biomass into liquid fuel in Europe. For other countries there are several options available for consideration.
One important byproduct generated during biomass fermentation into ethanol is stillage. This can be utilized for producing algae and other aquatic plants which are then fermented to methane. Current estimates are that one billion cubic meters of biogas can be produced from the still age of an ethanol plant producing one million m3 of ethanol per day.