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Fuel from farm waste

Anand Karve

Appropriate Rural Technology Institute (ARTI), through their experiments, brought out many new insights in the technology of biogas production. While dispelling the myth that cowdung is essential for biogas production, ARTI’s experiments concluded that pure carbohydrates devoid of any nitrogen also yielded biogas in ample quantities. Agricultural waste which mainly consists of cellulose, hemicellulose and lignin was found as alternative resource in producing biogas.


Farm waste is an alternative resource for producing biogas

Our petroleum import during the fiscal year 2018-19 is estimated to be about 113 million tonnes at an estimated cost of about US$125 billion. These figures represent the largest single item of import into India and also the largest annual outflow of money from India for importing a single item. Petroleum derivatives like L.P.G., petrol, kerosene and diesel not only have a high calorific value, but are also suitable as fuels for internal combustion engines as these fuels burn without leaving any solid residue (like ash) behind. As far as the quantitative availability, calorific value and burning characteristics of petroleum fuels are concerned, it is just not possible to substitute them by any other fuel that is currently being produced in India.

Ethanol and biodiesel are being considered as possible alternatives for the present automotive fuels, but producing them on a large scale would deprive agriculture of land and water. The same agriculture also generates a lot of waste, which is a byproduct of agriculture. As a rule of thumb, almost 60% of the biomass produced by a crop plant represents agricultural waste. Taking into consideration the official figures of different agricultural commodities produced in India, it is estimated that agriculture in India generates annually about 800 million tonnes of waste biomass. Although generated in huge quantities, agriculture waste owing to its solid nature and its undesirable burning characteristics is not fit to be used as fuel in internal combustion engines. It can however be very easily converted into methane, an automotive fuel. This was demonstrated at the beginning of this century by Appropriate Rural Technology Institute (ARTI).

Biogas contains volumetrically about 60% methane (CH4) and about 40% carbon dioxide (CO2). On weight basis, methane constitutes about 35% and carbon dioxide about 65%. Till the time that ARTI started working on this process (see box1), biogas was being produced in India mainly from cattle dung and to some extent also from the residue of sugarcane molasses left behind after alcoholic fermentation. Both of them are effluents of an anaerobic fermentation reaction. Because such effluents are already depleted of oxygen, they are of no use to the anaerobic organisms as an oxygen source. Also, the yield of methane produced from such substances is low.

Methane has the same calorific value as the traditional automotive fuels and it burns cleanly, without leaving residues like ash or tar behind. This process also produces carbon dioxide, but it is quite easy to separate pure methane from biogas by passing the latter through any substance, like calcium hydroxide, which selectively absorbs carbon dioxide. After realization of these facts, the scientists of ARTI discarded cattle dung and the residues from alcoholic fermentation as the sources of methane.

ARTI scientists conducted experiments with oxygen rich organic substances called carbohydrates. In the course of these experiments, it turned out that carbohydrates, represented by organic acids, sugars as well as polysaccharides like starch and cellulose could be completely converted into biogas. It later turned out that any substance that was consumed by any animal could serve as feedstock in a biogas plant.

Agricultural waste consists mainly of cellulose, hemicelluloses and lignin. None of these substances is digested by humans. However, the cellulose and the hemicelluloses can be digested by animals and therefore they can be converted into biogas. As stated above, the amount of biogas produced by any material depends on the percentage of digestibility of that material. If it is assumed that about 50% of the agricultural waste is constituted by digestible matter like cellulose and hemicellulose, we get a theoretical figure of 400 million tonnes of biogas, or about 140 million tonnes of methane, which can completely replace all the automotive fuel currently being used in India.

Lignin, the material that cannot be digested by any organism under anaerobic conditions, comes out of the biogas plant as effluent slurry. But it is not a useless substance. Lignin is a plastic-like substance. It becomes soft at 200C. At that temperature, it can be molded into plywood-like boards as well as biodegradable eating utensils like cups, bowls, saucers, plates and spoons. Lignin can also be briquetted and used as fuel briquettes, it can be charred to produce biochar, or it can be applied to agricultural fields as organic manure.

The process designed by ARTI has been tried on an experimental basis on the farm of Mr. Santosh Gondhalekar at Pirangut near Pune. Santosh makes his own methane from agricultural waste, mainly rice straw and sells it as vehicular fuel. Converting the experimental initiative to a commercial venture calls for several  permissions and no-objection-certificates from various government bodies. Being a pioneer in the CNG business, Santosh had to face a lot of challenges in getting the necessary permissions. He is in the process of converting it into a commercial business.

Technology development, limitations and future

ARTI experiments proved that carbohydrates in any form could be converted to biogas

ARTI’s work on biogas led to many new insights into the biogas technology. It was earlier thought that cattle dung was essential for biogas production. But ARTI scientists showed that once the necessary bacteria have established themselves in the digester, there was no need for dung to be added to the digester with every batch of the feedstock. The ratio of carbon to nitrogen, technically called C/N ratio, was also considered formerly to be an important characteristic of the substrate. But this assumption was proved wrong by the finding that pure carbohydrates devoid of any nitrogen also yielded biogas in ample quantities. Another important finding of ARTI’s was anaerobic predigestion of agricultural waste. This process yielded organic acids, which can be very easily introduced into the main digester by simple gravity or by pumping the liquid from the predigester into the main digester. By this process, the non-digestible material is left behind in the predigester, from where it can safely be removed, without disturbing the main digester.

However, one big limitation of using agricultural waste as a feedstock for biogas is that this raw material occurs in a diffused manner, scattered all over the countryside. Collecting and transporting it to a central location poses difficulties. This would require the development of appropriate strategies, if generation of biogas from this feedstock is to be achieved on a commercially viable scale. One of the strategies on which ARTI is currently working is to convert the digestible fraction of agricultural waste into organic acids. If this reaction is conducted at a number of local centres, bringing the organic acids to a central biogas digester through pipelines would not pose much of a difficulty.

Dr. Anand Karve

Appropriate Rural Technology Institute,

Near Ganeshnagar, Algudewadi, Phaltan-Baramati Road,

Phaltan 415 523, Dist. Satara, Maharashtra

Email: adkarve@gmail.com