Every year the farmers of Punjab, Haryana and other adjoining States of the national capital Delhi burn their rice straw after the harvest of the crop. They have hardly less than one month to prepare their land for sowing of the wheat crop.
Burning appears to be the quickest method to dispose of the straw, which are left in their field due to machine harvesting. The smoke generated moves southward and almost chokes the national capital Delhi during the winter. This has become a major environmental hazard for the capital region.
Burning a substantial part of cereal straw is practised around the world, since cost effective methods to utilise them are yet to be discovered. Cost of collection and transport due to their bulk to end users is high. Alternate uses so far on a limited scale have been confined to their use as a fuel in nearby thermal plants and in manufacture of straw-boards.
locally farmers use them as cattle feed. The local demand on straw in India has been considerably reduced as brick houses have increasingly replaced thatched mud houses and improved breeds of cattle are given more nutritious fodder. Rice straw constitutes a major part of crop residues generated every year in India and needs to be properly utilised. About 150 million tons of rice straw is produced annually in India and about 650-975 million tons globally (Binod and co-researchers, 2009). Proper utilisation of rice straw is a global problem.
Currently, research work is in progress to use agricultural wastes such as rice and wheat straw, corn stover, corn stalks, sugarcane bagasse as well as municipal solid waste to produce bioethanol.
Forestry wastes like wood chips, branches, and saw dusts have also been used as bioethanol feed stocks.
Bioethanol is same as ethyl alcohol present in all alcoholic beverages. It is a renewable source of energy and can be produced from agricultural feed stock. It is now used as a nonconventional source of fuel to replace fast depleting fossil fuel reserves. Bioethanol as fuel has significant characteristics like high octane number, low cetane number and high heat of vaporization. Its drawbacks are the corrosiveness, low flame luminosity, lower vapor pressure, miscibility with water, and toxicity to ecosystems.
About 1.5 volume of ethanol is required to replace energy equivalent to 1 volume of gasoline. In 2014 global production of bioethanol was 24.5 billion gallons (Achinas and co-researchers, 2016). About 70 per cent of bioethanol is produced by USA and Brazil. While USA produces bioethanol from corn starch, Brazil uses sugarcane juice. The Brazilian Government has made it mandatory since 2007 to blend 25 per cent of bioethanol with gasoline. Many cars in USA can run with blend of up to 10 per cent ethanol.
A lot of research work is currently in progress to produce bio-ethanol from the lignocellulosic rice straw. lignocellulose is a component of cell wall of all plants and consists of three chemical compounds Cellulose, Hemicellulose and lignin, which are intimately bound to each other along with other compounds and minerals to provide strength to the plant.
The first step therefore is to loosen their grip from each other and separate them. This is achieved by a preliminary treatment with acids or alkalis at an elevated temperature. A microbial route is preferred for subsequent steps to produce bioethanol.
Cellulose, which constitutes 40-50 per cent of treated residue, can be readily converted to six carbon atom glucose by specific microorganisms. Glucose is then fermented by yeast to produce ethanol. Hemicellulose (25-30 per cent) consists of five carbon atom molecules called pentoses, which cannot be converted to glucose. Recently, microorganisms have been found, which can convert both pentoses and hexoses to bioethanol (Okamoto and co-researchers, 2014). lignin (10 per cent) has no use and is to be burnt.
These processes are still confined to research laboratories. They have to be suitably modified for use on industrial scale. Bioethanol produced from rice straw will be initially costlier than the current market price of ethanol. Since it is linked with disposal of waste products and saving the environment, the Governments have to subsidise the price.
(Dr Mitra is a former Professor of Soil Science and Agricultural Chemistry, References cited may be accessed through internet. Email: gnmitra@gmail.com)