Agro Waste Potential

The agricultural waste is a low-density biomass, scattered all over the country. Also, it is available in a wide variety of forms having a wide variety of physical and chemical properties. As a result, in spite of its tremendous potential as a renewable source of energy, it has remained more or less neglected by the energy planners as well as technocrats. Also, almost all attempts at finding economically feasible ways of using biomass as a source of energy on a wide scale have proved unsuccessful and unsustainable. We however, believe that it is techno economically feasible to use biomass as a source of energy.

 

An excellent example of the right approach to use of biomass energy is a chain of technologies developed and successfully commercialized by Appropriate Rural Technology Institute.

 

Every year, farmers in Maharashtra state alone are simply burning off millions of tons of sugarcane trash (dried leaves of sugarcane left in the field after harvesting of the cane). In 2005, the government undertook a project that attempted to explore means of converting this biomass into a value added fuel, namely char briquettes. Under the project an oven-and-retort type charring kiln was developed. It converted sugarcane trash into powdery char. The charring kiln can be easily dismantled and transferred from one location to other, thereby eliminating the need to transport large quantities of loose biomass. Operated as a continuous batch process, it consumes about 250 kg of trash to generate about 50 kg of char powder every day. Three unskilled labourers can operate two kilns simultaneously to produce 100 kg char powder per day. The powder can then be briquetted by using a briquetting machine. The production cost of the briquettes is about Rs. 8 per kg.

 

Kerosene, which is the preferred cooking fuel for of the urban poor, is getting costlier and costlier as the government is gradually withdrawing the subsidy on it. We felt that the char briquettes made from agricultural waste could be a suitable low cost alternative fuel for the urban poor. However, switching from one form of fuel to other also requires a switch over from one type of cooking stove to other. It was, therefore, necessary to develop a cooking stove that was designed to suit the combustion characteristics of the char briquettes. Therefore Sarai cooker was developed. The cooker combines the principle of a hotbox, with the principle of a fuel-efficient stove. The result is so energy efficient that it requires just about 100 gm of char briquettes to cook vegetables, rice and dal for a family of five, and another 50 gm or so for roasting rotis on the charcoal burning stove which is part of the cooker assembly. Even if the briquettes are available in the urban market at the rate of Rs.10 per kg, the Sarai system requires fuel worth just about Re.1 per meal. No other fuel-stove system has such a low operating cost.

 

The concept has taken off very well among urban as well as rural households in rice eating localities in Maharashtra. The reason for it is its practical feasibility which is as follow:

1. Conventional thinking had always focused on producing tons of briquettes in a centrally located factory. This involves collection and transportation of widely scattered and low density raw material, and the transportation cost itself renders the entire project impractical. Our approach of decentralized production of char, and transportation of the char to a centrally located briquetting facility makes more economic sense.

 

2. The Sarai cooker is assembled using components already available in the utensils market. Thus, the production of the cooker does not involve any dedicated machinery or infrastructure. As a result, the cooker can be produced at a relatively low cost of about Rs.350-500 (depending mainly on the cost of stainless steel) making it affordable for the target users and profitable for the producer and retailers.

 

3. Because of the efficient design of the stove, the quantity of fuel required per meal is very less, keeping the cost of fuel per meal cooked at the lowest possible level for the consumer. This allows the per kg cost of the briquettes to be high enough to provide sufficient net income to the char producers, the briquetters, as well as the retailers. This example clearly demonstrates that it is possible to find ways of using biomass energy in techno economically feasible ways. Considering the huge amount of agrowaste produced annually in India, it can be easily seen that the chain of technologies described here can have a tremendous positive impact on the rural economy of the country.

WEALTH FROM AGRO WASTE

Agricultural out put from India has seen phenomenal growth. Factors, which contributed to the development, were research in seeds, access to water and power, effective pesticides, communication and improvement in storage facility. With the growth in production of agricultural output agro-waste production is also increased. However, local thermal energy which are needs of rural sector were mainly managed through cow dung, wood, kerosene and lately LPG. Agro mass had few takers. In fact, storage of agro-mass posed problems such as security risk due to fire hazard, growth of pests, blockage of covered space etc. Simplest solution, which is even practiced today, is spread the waste and burn it. On the other hand, in urban sector industrial growth is pushing the energy needs to hilt. Queues at petrol pump and chimneys spewing, fossil fuel burnt CO2 are the concerns. Indian government had recognized and efforts to use biomass were started in 1980’s. Some of the difficulties, which were noted in use of biomass, were:

 

       Low bulk density and tendency to scatter around.

       Moisture content.

       Transportation costs.

       Drying and briquetting

 

Fiscal and economical incentives were announced and that led to installation of various briquetting plants. Early birds however, did not have the beginner’s advantage. On the contrary arm twisting techniques of urban buyers, poor credit facility, enforced most of the entrepreneurs to shut the shop. Today however, due to movement called clean development mechanism, renewed vigour is felt in this area.

 

Briquetting process

The briquetting process is pretty simple. The machinery involved is a simple reciprocating machine that compresses the bio-mass to one fifth volume. The mechanical pressure exerted on the biomass is equivalent to 1350 kg/cm2. Due to this high-pressure mechanical operation, there is heat generation, which evaporates the entrapped moisture and gives a polished finishing. The compacted biomass is extruded through a die, which also decided the diameter of briquette. Some raw materials are wet and do not form good solid compact briquette. There is a separate dryer provided in the system for making the material suitable for briquetting. Lubricating oil cooling system and electrical system are additionally required for briquetting plant. Power requirement for briquetting plant is proportional to tonnage. A broad requirement of 60 Units per ton of briquette can be considered as benchmark.

 

Raw Material

Though biomass is a very popularly talked about term, the real meaning of it is not clearly understood. Particularly, in the context of briquette manufacturing, all bio degradable agro wastes can be easily briquetted. The briquettable bio mass need not be always raw material in the same form. High moisture content bio mass needs be solar dried or dried by fuel firing before briquetting. Some of the raw materials are not easily briquetted. It is necessary to add agro or synthetic binders. Briquettes made out of such binders are not of desired quality, as the binders do not mix thoroughly and uniformly.

 

Business effectiveness

Manufacturing of briquette can be classified as high volume low cost business. Approximate breakup 45% raw material, 15% raw material transportation, 15% Power, 10% Investment (full production), 9% Other expenses and 6% Profit. Thus, it is essential to procure raw material in time. The rural sector demands immediate cash payments. Therefore, liquidity by prompt recovery of dues is necessary. Business is quite sensitive to cost of transport fuel. Convenient Transporters who are having two-way business offer low rates. Planning of loading unloading manpower, knowledge of status of inventory at user end can affect profitability substantially.

BIOMETHANATION PLANT

A reviewed interest in renewable energy and related conversion technologies is emerging again. Although the eventual depletion of fossil fuels lurks in the background as a long-term incentive for the development of sustainable energy forms, more urgent incentives to re-emphasize renewable energy are related to global environmental quality. The first concern to emerge was release of toxic compounds and oxides of nitrogen and sulphur resulting from combustion of fossil fuels. These air pollutants contribute globally to health and environment problems, the most common of which is referred to as acid rain. The greatest threat is that of global warming related to an increased concentration of carbon dioxide and other upper atmospheric pollutants resulting from anthropogenic activities.

 

Use of renewable biomass (including energy crops and organic wastes) as an energy resource is not only greener with respect to most pollutants, but it’s use represents a closed balanced carbon cycle with respect to atmospheric carbon dioxide. It could also mitigate atmospheric carbon dioxide levels through replacement of fossil fuels. A third concern is the recognized need for effective methods of treatments and disposal of large quantities of municipal, industrial and agricultural organic wastes. These wastes may not only represent a threat to environmental quality, but also represents a significant renewable energy resource.

 

Why methane?

Biomass may be converted to a variety of energy forms including heat, steam, electricity, hydrogen, ethanol, methanol and methane. Selection of product for conversion is dependent upon a number of factors including need for direct heat or steam, conversion efficiencies, conversion and use of hardware and environmental impact of conversion process, waste stream and product use. Compared to other fossil fuels methane produces few atmospheric pollutants and generates less carbon dioxide per unit energy because methane is comparatively a clean fuel. The trend is towards its increased use for appliances, vehicles, industrial applications and power generation. Ethanol is becoming a popular biomass – derived fuel.

 

Conversion processes

Methane can be produced from biomass by either thermal gasification or biological classification. Economic application of thermal processes is limited to feeds with either low water content or those having the potential to be mechanically dewatered inexpensively. Feedstocks containing 15% of total solids require all of the feed energy for water removal. Thermal processes for methane production also are only economic at large scales and generate a mixture of gaseous products that must be upgraded to methane. The product gas is composed primarily of methane and carbon dioxide with traces of hydrogen sulphide and water vapour. The major limitation of biological gasification is that conversion is usually incomplete, often leaving as much as 50% of the organic matter unconverted.

 

Principles of anaerobic digestion

It is the application of biological methanogenesis, which is anaerobic process responsible for degradation of much of the carbonaceous matter in natural environment, where organic accumulation results in depletion of oxygen for aerobic metabolism. This process, which is carried out by a consortium of different organisms is found in numerous environments, including sediments, flooded soils and land fills.

 

In generalized scheme for anaerobic digestion feedstock is harvested, shredded and placed into a reactor which has an active inoculum of microorganisms required for methane fermentation. A conventional reactor is mixed, fed once or more per day, heated to a temperature of 350C and operated at a hydraulic retention time of 20 – 30 days and loading rate of 1.7 kg VS m3d-1. Under these conditions about 60% reduction in organic matter is achieved corresponding to a methane yield of 0.24 m3 per kg VS added. The biogas composition is typically 60% methane and 40% carbon dioxide with traces of hydrogen sulphide and water vapour. The conventional design is being replaced by more innovative designs influenced primarily by feed suspended solids content.

The objective of these designs is to increase solids and microorganism retention, decrease reactor size and reduce process energy requirements. Improved designs have increased possible loading rates 20 fold, reduced residence times and improved process stability.

  

Renewable methane from biomass

Resource potential estimates for terrestrial biomass is estimated to be 22 EJ while for feed stocks like grass, wood, seaweed it is 7 EJ.The potential for marine biomass is huge at greater than 100 EJ per year.

 

As biomethanogenesis decomposes organic matter with production of useful energy products, anaerobic digestion of organic matter is receiving increased attention. Solid and agricultural wastes release undesired methane into the atmosphere due to anaerobic digestion in landfills, lagoons or stockpiles. Treatment and recovery of this gas in reactors would reduce this source of atmospheric methane. An attractive option for treatment of the organic fraction of these wastes is to separately treat organic fraction by composting and applying the stabilized residues in land as a soil amendment. The residues would reduce water needs and prevent erosion.

 

As population increases and technology development begin to result in significant resource depletion and environmental deterioration, we must take a universal view on the ground rules for sustaining our species in a manner that is compatible with preservation of biosphere. This will require production of feed, food and energy by technologies that are indefinitely sustainable and which have minimal environmental impacts. This will involve a major shift to renewable resources of energy, sustainable agricultural practices for production of food, feed and energy and recycle of all non- renewable resources. Derivation of methane from energy crops and organic wastes has an important role towards achieving this objective.

BIOMASS AS ENERGY SOURCE -II

Another form of standardised fuel which is already being used in the rural areas is that of biogenous methane. The biological process of methane production results in a mixture of methane and carbon dioxide, which is called biogas. Burnt in a properly designed burner, biogas produces a blue flame, which is absolutely clean. This technology is at least 150 years old.

 

Traditionally, cattle dung is used as feedstock for producing biogas, and therefore it is also called gobar gas in India. During the last 50 years, the Government of India has made great efforts to popularise the gobar gas technology, but the present figures indicate that there are only about 2.5 million working domestic biogas plants in India, covering hardly 1.8% of the rural households. The failure of the gobar gas technology in India was due to the fact that it is not a very user-friendly technology. It requires dung from at least 6 to 8 heads of cattle. In order that the dung be easily available, the cattle must be penned and not allowed to roam.

 

The present technology also requires the dung to be mixed with equal volume of water to form a slurry. Villagers do not have tap water in their houses. Therefore, the water has to be fetched by the women from a source that is often far away from the house. The water is generally carried in pots balanced on their heads. Fetching water for the household is itself quite a strenuous task. Fetching daily additional 40 to 50 litres of water for the biogas plant only adds to the women’s burden, which they generally resent. The drudgery doesn’t just stop at fetching dung and water. Disposal of daily about 80 litres of spent slurry is also often a problem.

 

The new proposed system produced a more user-friendly biogas system based on starchy or sugary feedstock. Just 2 kg of sugar yield as much biogas as 40 kg of dung, and while dung requires a retention period of about 40 days, sugar yields the gas within just a single day. Starch also works equally well as feedstock. Our novel biogas system operates on waste starchy or sugary material such as leftover food, oilcake of non-edible oilseeds, fruits, tubers, rhizomes or grain that cannot be marketed due to poor quality, or non-edible material like rhizomes of banana, fruits of wild ficus etc.

 

A biogas plant based on this technology is quite small, having a capacity of just 1000 litres, and its cost is also much less, only about Rs.6000. About 200 of such gas plants are already installed, in various parts of Maharashtra, and this number is going to increase to 2000 in the next year.

 

Biogas can also be used as fuel in internal combustion engines. The CNG technology that is currently available in India can be used in both ways as bigas and an automotive fuel. Wood gas is the third alternative representing standardized fuel made from biomass. This technology does not lend itself well to being used in domestic cookstoves, but larger stoves, used in bakeries, langars or restaurants can be based on it.

However, wood gas is currently being used as fuel in internal combustion engines for generating electricity. Many such units are being operated all over the country. Biogas based electricity generation should be seriously considered by our planners and administrators as a means of supplying electricity to villages.

 

The electricity demand of a village is not very high. Supply of such small amount of electricity from a central generating facility is generally very costly because of the capital expense of the conduction system. There are also losses and theft of electricity when it is transmitted over such long distances. The village level generators should be operated by the villagers themselves. They can then generate electricity as and when they want and also use it for whatever purpose they want.

 

This discussion would not be completed without mentioning biodiesel and alcohol. Biodiesel is made from vegetable oil. In the Western countries, edible oil like soybean oil or rapeseed oil are used as a source of biodiesel. Our country, currently imports almost 50% of its total demand of edible oil. Under such circumstances, using edible oils for biodiesel is out of question.

 

Among our indigenous plant species, castor and rice are the only sources of oil that are produced by farmers. Castor oil, having special chemical composition, is not only being used by industries but it is also exported, while rice bran oil is used almost entirely by the organised soap industry. The remaining non-edible oils, being produced from seeds of various uncultivated tree species, play only a minor role in our economy. Being uncultivated, their supply is unreliable and therefore one cannot base a major industry like biodiesel on them. Currently India requires annually about 50 million tonnes of diesel. Substituing just 5% of this by biodiesel would require 2.5 million tonnes of vegetable oil. Considering average yield of 500 kg oil per hectare, one would require an area of 5 million hectares under oilseed production. I quote these figures only to bring into focus the magnitude of this endeavour. There is talk of introducing Jatropha curcas as a new oil bearing plant. It is claimed that  Jatropha requires very little water.

 

It is clear  that all plant species, irrespective of whether they are drought tolerant or not, require monthly about 200 mm water, if they are to give a good yield. Tolerance to drought means only that the plant can survive under conditions of drought and that it does not die under drought. It does not mean that it would give high yield under such conditions. It has been shown that even Jatropha needs about 800 to 900 mm of water to become economically viable. If a farmer has at his disposal this much water, he would rather grow a cash crop like cotton, groundnut, soybean or onion, than a low yielding plant like Jatropha.

 

The situation of alcohol is similar to that of biodiesel. Currently, alcohol is made from

molasses, a free by product of the sugar industry. As the cost of sugarcane, its harvest, transport, and processing are borne by sugar, the present cost of alcohol is low. But if crops like sugarcane, sugar beet or sweet sorghum are grown exclusively for alcohol production, the above mentioned costs would have to be borne by alcohol, which then would not be so cheap. Also the area required to be planted to produce alcohol would be of the same magnitude as that required by biodiesel.

 

Production of biomass in any form requires the use of land, and it would require the

involvement of rural people to do it. Chemical fertilizers, an important input required in agriculture, need a large quantities of fossil fuel in their production.

 

This concept is based on the assumption that soil micro-organisms degrade the soil minerals to provide the green plants with all the mineral nutrients that they need. If the soil micro-organisms are adequately fed with organic matter, there is theoretically no need to apply chemical fertilizers to the soil. Traditional agricultural scientists recommend the application of organic matter in the form of compost. However, the nutritional value of composted organic matter is so low, that one has to apply 20 to 50 tonnes of compost per hectare. In practical terms, it means that one has to use the biomass produced in about 10 hectares for providing organic matter to one hectare.

 

Research has shown that if organic matter having high nutritive value, like sugar, starch, protein etc. is used as manure, application of just 10 to 25 kg per hectare of it is enough to produce high crop yield without using any other form of chemical or organic nutrients. This new discovery would reduce the cost of agriculture substantially and would also reduce the cost of producing biomass.

BIOMASS AS ENERGY SOURCE -I

Biomass is plentifully available in the rural regions. It is already being used by the rural people as a major source of energy, mainly in cooking food, which constitutes almost 50% of the total energy consumption. Assuming that there are about 140 million households in rural India, and assuming that each family uses annually about 3 tonnes of biomass as fuel, one comes to the figure of about 400 million tonnes of biomass utilised annually only for domestic cooking.

 

Engineers and energy scientists generally think only of the calorific value of fuels and of fuel use efficiency. But there is also a third dimension to fuel use, and that is the pollution arising due to burning of biomass. As cooking is done within the confines of a house, the pollution caused by cooking fires is generally not taken very seriously.

 

But according to statistics published by the World Health Organisation, annually about 500,000 women and children die prematurely in India due to air pollution caused by cooking fires in rural households. Considering the fact that almost 70% of our population is rural, giving the rural women a cleanly burning biofuel is a major task, which is unfortunately not tackled by any of our major research centres.

 

There are many options for providing a clean and economical burning biofuel. The biomass that is currently available to villagers is free of cost.

 

One way of tackling this problem is to redesign the cooking devices in such a way that they burn the biomass more cleanly, so that the pollution caused by them is reduced. This is achieved by providing the fuel with sufficient air, so that it burns completely, reducing automatically the carbon monoxide and the particular matter in the fuel gases. Another strategy is to design a stove in such a way that waste of heat is avoided and a major part of the heat generated by the burning biomass is transferred to the pot. This results in higher fuel use efficiency, requiring the user to burn less fuel. Pollution is naturally reduced if the amount of fuel is reduced. Both the strategies are combined in modern improved cook stoves.

 

However, in practical terms, both the strategies often fail, because the fuel that is used in the laboratory while designing the stove differs from the fuel that the rural housewife actually uses. In a laboratory experiment, one normally uses good quality firewood, that has been properly dried and cut into pieces of adequate size. In contrast to this, the fuel used by the rural housewife consists of stalks of plants like cotton, maize, safflower, arhar, or of bushes growing in the vicinity, maize cobs, dung cakes, rhizomes of sugarcane, etc.

 

The traditional cookstove is designed to burn such material and therefore, the housewife often finds that the improved cookstove emits more smoke and soot than her traditional stove, comparatively. Standardisation of fuel is, therefore, another strategy that is considered in the context of using biomass as cooking fuel. The easiest way of standardising woody biomass is to cut it into  uniform, small pieces called chips. Highly efficient and non-polluting stoves can be designed to burn these chips, but unfortunately not much effort has been made in this direction in India.

 

The second and traditional method of converting a non-standard fuel into standard one is to char it into charcoal. It is the volatile matter in biomass that gives rise to the particulate matter in the flue gases. In the process of charring, the volatiles are removed from the biomass to leave only the carbon and non-combustible matter behind. Therefore, when charcoal burns, it burns cleanly, without producing any smoke or soot. However, the traditional method of producing charcoal is itself highly polluting, because the volatiles are released into the atmosphere in this process. Sophisticated technologies are now available for charring, in which the volatiles are burned in the process of charring itself, to produce the heat required in the process.

 

Agricultural waste is an ideal source of charcoal. When one harvests any crop, one generally harvests only grain, fruits, pods, tubers or rhizomes. This constitutes only about 30 to 40% of the total biomass. This means that about 60 to 70% of the total agricultural biomass, or almost 600 million tonnes, is the waste biomass produced annually in India. A small part of it is used as fodder for cattle, but the rest is just wasted.

 

The standardised Sarai cooker, a stove-and-cooker system, can cook the meal for five persons, using just 100 g of our char briquettes. About 15,000 households in Maharashtra are already using the Sarai cooker.