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Bioenergy for India: prospects, problems and tasks
Articles
Bioenergy for India: prospects, problems and tasks K.S. Jagadish Department of Civil Engineering, Indian Institute of Science, Bangalore-560 012, India
Biomass energy has always been considered an important element in the energy planning of a tropical country such as India. However, the attention of energy planners is focussed mainly on firewood and agro residues as sources of energy. In this paper the total annual biomass growth in India, including leaves, grasses and weeds, has been examined. The implication of the large livestock population for the availability of leafy biomass as a source of energy has been explored. The low conversion efficiency of the traditional use of bullock power has been pointed out. The improvements possible with a modern use of fodder biomass through biogas production are indicated. The strategy for a more efficient use of biomass resources has been discussed.
T
he use of bioenergy by converting biomass into high-quality fuels has been examined. The relevance of gaseous fuels such as biogas and producer gas is discussed. The potential of the available biomass in India for conversion into liquid fuels such as ethanol and methanol is explored. It has been shown that the available biomass is more suited to the production of methanol rather than ethanol. 1. Introduction
A discussion on renewable energy invariably involves consideration of solar, wind, hydro and biomass energy. In one sense, all the four types of energy are basically derived from solar energy. However, the practical implications of the four energies can be highly varied depending on the contexts. There are limitations on the quality of solar energy that can be delivered at acceptable costs. Wind and hydro energies are often location-specific and available during certain seasons. In contrast, biomass energy is available in a majority of locations that are not desertified. It is amenable to management and augmentation. It can be mobilized to produce a high-quality energy like electricity at costs that are not exorbitant. It can also be used as a source of low-grade heat. The diversity of sources of biomass is indeed quite high. Wood, agro-residues, leaf biomass and urban and rural wastes qualify as sources of biomass energy. In many countries, the quantum of biomass energy available is not small, although it is often used only as a source of heat. 2. Biomass availability in India The growth of biomass takes place wherever soils have adequate nutrients and moisture. Biomass occurs naturally in forests, wastelands and common lands and is artificially grown in agricultural lands through soil and water management. Table 1 shows a typical calculation of the annual availability of biomass in India from agriculture, taking 1995-96 as the reference year. Edible produce 28
Energy for Sustainable Development
such as cereals, pulses, and oilseeds has not been considered here since it is not available as a source of energy other than as food. This calculation can be easily made on the basis of annual agricultural statistics. In this table a distinction has been made between biomass in the form of leaves (good nitrogen content) and residues (low nitrogen content). The leafy biomass can also be used either for fodder or for manure. It can also be used for biogas production through anaerobic fermentation. Residues are such material as rice-husk, bagasse and groundnut husk which can be used mainly as fuel. The leafy biomass, especially resulting from cereal production, is often used as cattle fodder and may not be available for other applications such as manure or energy resource. The dung produced by livestock, which is a small fraction of the biomass they consume, is often valuable either as manure or as fuel. The growth of biomass takes place naturally in forests, wastelands and common lands. An analysis of the total biomass growing in India per annum from various sources is presented in Table 2. The biomass from forests may be estimated on the basis of growth rates of climax forests and the leaf-litter production as measured in various studies. The net primary productivity of woody biomass in mature forests in India is placed at 138.0 million tonnes (Mt) by Ravindranath and Hall [1995] for an area under forest of 63.5 million hectares (Mha). The leaf-litter production for the same area has been estimated by Bhat [1990] around 5.84 t/ha by averaging four different studies. Accordingly, the forest leaf-litter is around 371.0 Mt. The social forestry programmes led to an afforestation of about 15.0 Mha between 1975 and 1990, according to the Planning Commission [1992]. 2.0 Mha were reported to have been added during 1991 and 1992, according to Ravindranath and Hall [1995]. Out of this 17.0 Mha, one can estimate a woody biomass production of 85.0 Mt and a leaf-litter production of 51.0 Mt. A grass production of 51.0 Mt can also be estimated. Weeds in agricultural lands
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Table 1. Annual biomass production from agriculture in India, 1995-96 Crop
Area (Mha)
Production (Mt)
Straw/grain ratio
Leafy biomass (Mt)
Fuel (Mt)
Rice
42.91
79.62
1.4
111.47
23.48
Wheat
25.12
62.62
1.3
81.41
-
Jowar
11.44
9.55
2.01
19.20
-
Bajra
9.38
5.39
2.71
14.61
-
Maize
6.01
9.44
0.68
6.42
2.08
Ragi
-
2.33
1.96
4.57
-
Small millet
-
1.22
1.4
1.71
-
23.92
13.19
2.1
27.70
-
Sugarcane
4.14
283.0
-
80.42
87.26
Groundnut
7.7
7.81
1.0
7.81
2.12
Oilseeds
18.65
14.62
3.0
43.80
-
Mulberry
0.328
-
-
-
3.28
Cotton
9.06
-
-
-
26.93
Coconut
1.69
-
-
-
16.90
0.738
7.7
-
-
1.93
Potato
0.94
15.25
0.4
6.10
-
Castor
-
0.225
-
1.35
4.05
Pulses
Jute
Banana
0.064
4.5
-
5.40
-
Cassava
-
5.37
-
0.44
3.26
412.41
171.29
Table 2. Total biomass growth per annum in India
Forest Social forestry
Woody biomass (Mt)
Leafy biomass (Mt)
138.0
371.0
Other biomass (fuel) (Mt) -
85.0
102.0
-
Agriculture
-
412.4
171.3
Weeds
-
80.0
-
Village forests
27.0
27.0
-
Wastelands (Prosopis)
43.0
14.3
-
-
180.0
-
14.4
-
-
307.4 Mt
1186.7 Mt
171.3 Mt
Other wastelands Roadside trees Total Grand total
1665.4 Mt
also lead to about 80.0 Mt, assuming a pessimistic estimate of 0.5 t/ha. It is also useful to estimate the biomass in village forests. In a village-level study Ravindranath et al. [1991] report a standing biomass of about 0.83 t per capita. Extending this to the total rural population of India of about 650.0 million, there is a total standing biomass quantity of 540.0 Mt. Assuming an annual growth of 5 %, the wood growth per annum is 27.0 Mt. A leaf-litter-cumgrass growth of 27.0 Mt may also be estimated. It has been observed that wastelands in India have a significant coverage of an exotic tree species known as Prosopis juliflora. This has the property of rapid regeneration in alluvial and black soils. The four southern states Energy for Sustainable Development
of the country (Kerala, Tamil Nadu, Karnataka and Andhra Pradesh) have a wasteland area of 28.6 Mha and assuming 10 % of this to be covered by Prosopis, whose yield is estimated to be 15.0 t/ha per year, a woody biomass growth of 43.0 Mt can be considered. Again, assuming 5.0 t/ha of leaf-litter and grass, annual leafy biomass yield of 14.3 Mt may be estimated. Taking the estimate of wastelands of India at 90.0 Mha, we may assume a pessimistic figure of 2.0 t/ha of grass-cum-leaf biomass. This could add up to 180.0 Mt. An estimate of roadside tree growth of 14.4 Mt was also made by the author. The total biomass growth per annum may now be estimated at 1665.4 Mt, out of which woody biomass is l
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Figure 1. Livestock population in India
307.4 Mt, leafy biomass is 1186.7 Mt and residues are 171.3 Mt. It must be noted that leafy biomass is the main component of annual biomass growth. Vergara and Pimentel [1978] estimated the annual Indian biomass growth at 1339.0 Mt. The current estimate appears reasonably close. It is now clear that the biomass energy available annually is indeed significant. This quantum of biomass may be compared with the fossil fuel energy consumed in India. For instance in 2000-2001, the total commercial energy consumed in India was about 15,030 PJ (513.0 Mt of coal equivalent @ 29.3 GJ/t coal equivalent), according to GOI [2002]. Even if we assume that the biomass has 40 % less calorific value than coal equivalent (i.e., 1 t biomass contains 17.6 PJ), the total biomass energy growth is easily double the fossil fuel energy consumption. It must, however, be pointed out that not all the biomass energy may be available and what is available may have other current uses. There is a need to examine the current biomass utilization patterns and the various barriers to the expansion of biomass use. 3. Livestock, agriculture and biomass An earlier study by Gadgil et al. [1989] showed that the demand for fodder for livestock in India in 2001 would be around 660.0 Mt per annum. This resource should come from the ‘‘leafy biomass’’ listed in Table 2. Although the total leafy biomass growth is around 1186.7 Mt, it must be noted that the leafy biomass from forests (371.0 Mt) is often not available for general use. This means that there is only a surplus leafy biomass of about 150.0 Mt per annum, which could be considered for other uses. The livestock in India is thus the number one energy 30
Energy for Sustainable Development
guzzler of the economy and this distinction does not go to any other sector of the commercial economy. It is useful to examine the efficiencies of utilization of biomass by animals. Figure 1 shows the changes in the distribution of livestock population in India since 1965. The secular variations in the population of different livestock species over a three-decade period clearly indicate the changing pattern of biomass availability. It is easily seen that the bullock population has been practically stagnant and is in fact showing a slight reduction after 1990. The population of sheep has also been stagnant since 1982 while the population of cows is rising rather slowly. All these categories of livestock depend solely on grass biomass and the stagnant bullock and sheep population is a clear indicator of the shortage of grass fodder. Buffaloes are less choosy and more economical to maintain, and hence the buffalo population, which used to be half the cow population, is almost approaching the latter. It is also clear that cows are relatively more profitable to maintain than bullocks and sheep because of the income from milk. The grass fodder that is available is thus more profitably allocated to cows rather than bullocks. The bullocks are the mainstay of cereal production in agriculture, the economics of which has become very uncertain. The goat population, on the other hand, has been growing rapidly since the mid-seventies since goats depend on leaves of all kinds of plants and trees which are generally not consumed by other livestock. The relative scarcity of grass fodder in comparison with other leafy biomass is thus apparent. 1. Biomass energy conversion: fodder -- bullock It is now useful to examine the efficiency of conversion l
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Table 3. Land-use pattern and soil nutrient status Type of land use
Organic matter (%)
Nitrogen (%)
Phosphorus (%)
Dry land
0.80
0.06
0.064
Wasteland
0.75
0.05
0.12
Eucalyptus monoculture
1.43
0.07
0.045
Teak forest
1.74
0.09
0.062
Arjuna forest
2.06
0.12
0.139
Mixed forest
3.14
0.13
0.135
of fodder biomass into mechanical energy by bullocks. The annual consumption of grass fodder by a bullock can be estimated at 1.82 t (dry). This has a calorific value of 25.5 GJ. The mechanical energy delivered by the bullock, which works about 5 hours per day for 72 days in a year, can be estimated to be 0.48 GJ. The annual dung production by the bullock has energy content of about 4.1 GJ. Thus, the efficiency of bullock use is: Energy efficiency = 0.48/(25.5-4.1) = 2.25 % There are thus two problems in the use of bullocks in the economy. Firstly, the bullock is an inefficient converter of biomass energy to mechanical energy. Secondly, the bullock works only 20 % of the time. The 75 million bullocks in India guzzle an enormous amount of energy to work only 20 % of the time. We may regard the bullock population as an ‘‘engine of the economy’’ which idles throughout the year, wasting the energy consumed and giving back very little to the economy. It is small wonder, then, that the bullock population is not increasing. 2. Biomass energy conversion: fodder -- biogas -- engine It is possible to produce biogas from fodder grasses without going through the bullock route. Thus, the quantity of 1.82 t of fodder can give an annual biogas yield of 547.5 m3. Using this gas in an engine we can get 2.46 GJ of delivered energy. The wastes from the biogas plant will have an energy content of 6.82 GJ. Thus the energy efficiency of conversion through the biogas route is: Energy efficiency = 2.46/(25.5-6.82) = 13.2 % There are two advantages here. The energy utilizable is 6-fold higher. The biogas engine can be switched off at will, unlike the ‘‘bullock engine’’. The traditional dependence on livestock for energy is no longer viable, especially when fodder is scarce. It made sense when the country had abundant common land with no opportunity cost for grass. In today’s context it is better to depend on ‘‘bacterial’’ energy conversion than on energy conversion through large mammals. This route also generates significant quantities of manure for agriculture. 3. Biomass energy conversion: wood -- producer gas -engine It is also possible to use woody biomass through conversion into producer gas. Here, 1.82 t of wood has an Energy for Sustainable Development
energy content of 30.64 GJ. When converted to producer gas electricity the delivered energy is 5.24 GJ. Energy efficiency = 5.24/30.6 = 17.1 % The wood ash obtained as a by-product has valuable inorganic nutrients for agriculture such as calcium and potassium. It is now clear that the ‘‘modern’’ use of biomass through biogas and producer gas makes available a larger quantity of high-quality energy, besides producing large amounts of nutrients for sustainable agriculture. While it may not be possible to get rid of the huge bullock population of India all of a sudden, a slow, inexorable movement towards replacing bullocks by biogas/producer gas is possible and needs to be facilitated. The implications of the huge and burgeoning livestock population of India on the agricultural ecology of India need to be examined. The rural economy is critically dependent on livestock. This is especially true of sheep and goats, which are the mainstay of the landless and marginal farmers. Here is a situation where the herbivore population is going berserk since the poor are dependent on it. The consequences of excessive grazing by all the livestock are a slow but inexorable depletion of soil nutrients. Table 3 shows a typical pattern of soil nutrient distribution in different land-use patterns. The low nutrient status of dry lands and wastelands vis-à-vis lands with tree cover is now obvious. Village forests with mixed-species trees will now be a very important source of soil nutrients besides being a reservoir of biomass energy. Lands with grazing pressures immediately get degraded. There is thus a case for limiting the growth of livestock to improve soil nutrients. There is another problem that is not often appreciated with large livestock. There is in general a natural regeneration of trees and shrubs, which provide valuable biomass energy inputs at practically no cost. This process is severely inhibited due to grazing pressures. The livestock, besides depleting soil nutrients, effectively prevent green cover from establishing itself naturally. Thus, the rural economy, which is critically dependent on livestock for income generation, has to pay a very high ecological price by losing soil quality and green cover. Alternative approaches to limit the livestock population without jeopardizing the rural economy are urgently needed. A more efficient programme of cattle management using stall-feeding and better inputs can perhaps sustain milk production at current levels using a substantially l
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Figure 2. Land - bio-energy - water nexus
lower number of cows. Development of small rural enterprises to wean the rural landless away from sheep- and goat-rearing is also another option. This is not to suggest that the goat and sheep population should become extinct. It is desirable to reduce the sheep and goat population so that there is a balance between plant diversity and animal diversity. 4. Tasks for a new biomass-based rural transformation The foregoing discussion highlights the abundance of biomass energy in India. However, current utilization patterns in rural areas, whether in domestic cooking or maintenance of livestock, are highly inefficient. Cooking using firewood or agro-residues generally takes place at efficiencies around 10.0 %. The abysmally low conversion 32
Energy for Sustainable Development
efficiency of bullock energy has been pointed out above. A new approach to biomass utilization will have to be three-pronged: • biomass conservation • biomass generation • efficient conversion of biomass energy 4.1. Biomass conservation A significant amount of woody biomass is used in India for thermal energy applications such as cooking, jaggerymaking, brick-making and lime-burning. Many of these operations are inefficient. The Centre for Application of Science and Technology to Rural Areas (ASTRA), Indian Institute of Science, Bangalore, worked on many of these technologies and came up with improved devices that save biomass energy to a significant extent. However, dissemination of these technologies in the rural areas has not been l
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easy. There is a need for a long-term strategy to involve the rural population in a participatory technological change. A large number of professional groups have to emerge to work as stakeholders in this programme. To a certain extent, the technology of stabilised mud blocks developed to reduce brick-burning energy has been reasonably successfully disseminated, with about 9000 buildings using the technique across the country [Jagadish and Nanjunda Rao, 2002]. 4.2. Biomass generation Although India’s biomass energy endowment is not small, current practices of agriculture and silviculture are operating well below their true potential. India has the second largest area of arable land in the world (160 Mha). China, with only around 90 Mha, produces double the amount of food grain India produces. India can definitely double its food grain production. This will incidentally push the biomass availability to around 800 Mt from the current 421 Mt. India also has about 90 Mha of wasteland whose productivity is very low. A mixed-species forest, according to the studies of Shailaja et al. [1994] at ASTRA, can easily produce more than 500 Mt of biomass per annum. It is certainly not over-optimistic to expect an aggregate production of 2500 Mt per annum. This will easily provide a threefold increase in the energy supplied to the economy along with copious quantities of manure for sustainable agriculture. 4.3. Efficient conversion of biomass energy: bio-fuels 4.3.1. Gaseous fuels There are two distinct possibilities here. All the leafy biomass with a certain nitrogen content can be advantageously processed in biogas digesters to produce biogas. The work of Jagadish et al. [1998] in the development of a plugflow biomass digester has demonstrated the technical feasibility of the concept. In this process about 50 % to 60 % of the biomass can be digested and converted to biogas. This methane-rich biogas can be used for cooking or for running engines for mechanical energy or electricity generation. The rest of the biomass comes out as a waste, which has immense value as manure since it contains humus, nitrogen and several other inorganic nutrients. The quantum of this manure is not small, unlike in livestock utilization. Livestock use up and store biomass for their own metabolism while giving back small quantities of manure. However, in bacterial conversion, the bacteria quickly die after their job is done and offer the dead bacterial biomass as manure. The work of Mukunda at el. [1993] has shown the feasibility of using wood chips to generate producer gas. This has the potential of meeting the electricity requirements of rural India. The author believes that charcoal also offers a similar possibility of energy generation, provided charcoal is produced in an environmentally sound manner. The use of biomass energy has interesting possibilities of integrating energy generation, water management and fertilizer production in a mutually linked symbiotic operation. Figure 2 shows a schematic diagram integrating the various processes in a closed-loop network. Such an integration of processes is not possible in the open-ended use of fossil fuel energy. The biomass energy provides a Energy for Sustainable Development
balanced use of resources leading to symbiotic (non-exploitative) interaction of different segments of the economy. Developing countries in the South have a great potential to realize this dream. It is hoped that the Third World will come together for such an exciting venture. 4.3.2. Liquid fuels: ethanol and methanol Production of fuels such as ethanol and methanol from biomass is a distinct possibility to meet the transport needs of a country such as India. The experience of Brazil, discussed by Goldemberg et al. [1993], confirms the potential of ethanol production from sugarcane. In the Indian context ethanol can also be obtained as a by-product of the sugar industry, using molasses as the feedstock. However, the current production of ethanol from molasses is utilized either as industrial alcohol or as potable alcohol, leaving precious little for use as automobile fuel. Direct use of sugarcane to produce ethanol instead of sugar is probably the simplest way to obtain a liquid fuel. Very often sugar production in India is about 10 to 15 % more than the consumption. It is hence reasonable to assume that 10 % of the sugarcane grown can be diverted to ethanol production. According to the reference annual publication India 2002 [GOI, 2002], India produced 294.0 Mt of sugarcane in the year 2000--01. If 10 % of this is used for ethanol about 2.0 billion litres (Gl) of alcohol can be produced, according to the conversion factor quoted by Moreira and Serra [1990]. The ethanol so produced is adequate to replace petrol in automobiles to the extent of about 20 %. Ethanol can also be produced by hydrolysis of starch from cassava or maize. However, since cassava and maize are useful food crops in India it is unlikely that they can be spared for alcohol production. These facts make it clear that one cannot expect products of arable land to be used in a significant way for fuel production in view of the unfavourable land/man ratio. Cellulosic materials can also be used for ethanol production, but the yield of ethanol is as low as 160 l/t of wood, making the process rather uneconomical [Moreira and Serra, 1990]. It is hence clear that the options for producing ethanol in the Indian context are rather limited. It is only feasible to replace a part of the petrol in automobiles by ethanol produced from sugarcane. In contrast to ethanol, methanol can be produced from a wide variety of biomass sources. The usual route is to produce syngas from woody biomass or charcoal and then recombine carbon monoxide and hydrogen over a catalyst to produce methanol. According to Wyman et al. [1993] about 2.3 t of biomass is needed to produce 1 t of methanol. Several sources of biomass can be considered for the production of methanol. Wood from Prosopis juliflora, an exotic species which has become the dominant tree in many dry tracts of India, is an obvious first choice because of its productivity and concentration in specific locations. Cotton, mulberry and cassava stalks, which provide about 32 Mt of biomass per annum, are another option. Assuming a pessimistic estimate of 30 Mt of Prosopis wood per annum and 30 Mt of cotton and other stalks, India can produce about 26 Mt of methanol per annum. This is indeed quite significant in view of the current annual consumption of about 40 Mt of diesel (mostly used l
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for transport). With improvements in afforestation in wastelands and wood conservation in domestic cooking, 50 Mt of methanol production per annum is not unrealistic. It now appears that methanol will meet the renewable energy strategy for transport in India rather than ethanol. It goes without saying that more intensive R&D to use methanol either as a partial replacement for diesel or in pure methanol engines is needed to realize the goal of renewable energy-based transport. It is also encouraging to note that methanol has the possibility of being used in fuel cells as a source of hydrogen. Availability of methanol can lead to a smooth transition to clean and efficient transport based on fuel cells. 5. Conclusions The energy in the biomass that grows annually in India is easily twice the fossil fuel energy consumed in the country. Since all the biomass that grows is not readily available for use as energy, the efficiencies of the current biomass utilization must be examined. The livestock in India are the energy guzzler of the economy. The energy conversion efficiency of the bullock is only 2.25 %, indicating an enormous wastage of a major source of energy. The same biomass when used for producing biogas and then using it to run an engine will deliver a six-fold greater amount of energy. The livestock are also responsible for loss of green cover, loss of nutrients in soil and erosion of soil. Better management of forest and wastelands and a more efficient and hence reduced livestock population are essential for a better deployment of bioenergy resources. A three-pronged approach, looking at biomass conservation, biomass generation and efficient conversion of biomass to bio-fuels, is needed to improve the availability of useful energy in India. Gaseous fuels such as biogas and producer gas and liquid fuels such as ethanol and
methanol have the potential to meet a large portion of our energy budget. In liquid fuels methanol appears to be more feasible than ethanol. Acknowledgement This paper is based on an earlier paper submitted to the Sustainable Development Conference, Sustainable Development Policy Institute, Islamabad, 31 Oct-2 Nov, 2002. References Bhat, D.M., 1990. Private communications. Gadgil, M., Sinha, M., and Pillai, J., 1989. India: a Biomass Budget, Centre of Ecological Sciences, Indian Institute of Science, May. Government of India (GOI), 2002. India 2002, Publications Division, Government of India, New Delhi. Goldemberg, J., Monaco, L.C., and Macedo, I.C., 1993. ‘‘The Brazilian fuel-alcohol program’’, in Johansson, T.B., Kelly, H., Reddy, A.K.N., and Williams, R.H., (ed.), Renewable Energy: Sources for Fuel and Electricity, Island Press, Washington. Jagadish, K.S., Chanakya, H.N., Rajabapaiah, P., and Anand, V., 1998. ‘‘Plug-flow digestor for biogas generation from leaf biomass’’, Biomass and Bio-energy, 14, 5/6, pp. 415-423. Jagadish, K.S., and Nanjunda Rao, K.S., (eds.), National Workshop on Alternative Building Methods, Proc., Dept. of Civil Engg., Indian Institute of Science, Bangalore. Moreira, J.R., and Serra, G.E., 1990. Alternative Fuels: a Brazilian Outlook, Alternative Liquid Fuels, Wiley Eastern Ltd., IDRC, Ottawa, and UN University, Tokyo. Mukunda, H.S., Dasappa, and Shrinivasa, U., 1993. ‘‘Open top wood gasifier’’, in Johansson, T.B., Kelly, H., Reddy, A.K.N., and Williams, R.H., (ed.), Renewable Energy: Sources for Fuel and Electricity, Island Press, Washington, pp. 699-728. Planning Commission, 1992. Report of the Eighth Five-Year Plan, Government of India, New Delhi. Ravindranath, N.H., and Hall, D.O., 1995. Biomass, Energy and Environment, Oxford University Press, Oxford. Ravindranath, N.H., Nayak, M.M., Hiriyur, R.S., and Dinesh, C.R., 1991. ‘‘The status and use of tree biomass in a semi-arid village ecosystem’’, Biomass and Bio-energy, Vol. 1, No. 11, pp. 9-16. Shailaja, R., Ravindranath, N.H., Somashekar, H.I., and Jagadish, K.S., 1994. ‘‘Biomass generation in mixed tree plantations’’, Energy for Sustainable Development, Vol. I, No. 3, September, pp. 51-55. Vergara, W., and Pimentel, D., 1978. ‘‘Fuels from biomass -- comparative study of the potential in five countries: US, Brazil, India, Sudan and Sweden’’, in Auer, P., (ed.), Advances in Energy Systems and Technology, Academic Press, New York. Wyman, C.E., Bain, R.L., Hinman, N.D., and Stevens D.J., 1993. ‘‘Ethanol and methanol from cellulosic biomass’’, in Johansson, T.B., Kelly, H., Reddy, A.K.N., and Williams, R.H., (ed.), Renewable Energy: Sources for Fuels and Electicity, Island Press, Washington.
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Bioenergy for India: prospects, problems and tasks K.S. Jagadish Department of Civil Engineering, Indian Institute of Science, Bangalore-560 012, India
Biomass energy has always been considered an important element in the energy planning of a tropical country such as India. However, the attention of energy planners is focussed mainly on firewood and agro residues as sources of energy. In this paper the total annual biomass growth in India, including leaves, grasses and weeds, has been examined. The implication of the large livestock population for the availability of leafy biomass as a source of energy has been explored. The low conversion efficiency of the traditional use of bullock power has been pointed out. The improvements possible with a modern use of fodder biomass through biogas production are indicated. The strategy for a more efficient use of biomass resources has been discussed.
T
he use of bioenergy by converting biomass into high-quality fuels has been examined. The relevance of gaseous fuels such as biogas and producer gas is discussed. The potential of the available biomass in India for conversion into liquid fuels such as ethanol and methanol is explored. It has been shown that the available biomass is more suited to the production of methanol rather than ethanol. 1. Introduction
A discussion on renewable energy invariably involves consideration of solar, wind, hydro and biomass energy. In one sense, all the four types of energy are basically derived from solar energy. However, the practical implications of the four energies can be highly varied depending on the contexts. There are limitations on the quality of solar energy that can be delivered at acceptable costs. Wind and hydro energies are often location-specific and available during certain seasons. In contrast, biomass energy is available in a majority of locations that are not desertified. It is amenable to management and augmentation. It can be mobilized to produce a high-quality energy like electricity at costs that are not exorbitant. It can also be used as a source of low-grade heat. The diversity of sources of biomass is indeed quite high. Wood, agro-residues, leaf biomass and urban and rural wastes qualify as sources of biomass energy. In many countries, the quantum of biomass energy available is not small, although it is often used only as a source of heat. 2. Biomass availability in India The growth of biomass takes place wherever soils have adequate nutrients and moisture. Biomass occurs naturally in forests, wastelands and common lands and is artificially grown in agricultural lands through soil and water management. Table 1 shows a typical calculation of the annual availability of biomass in India from agriculture, taking 1995-96 as the reference year. Edible produce 28
Energy for Sustainable Development
such as cereals, pulses, and oilseeds has not been considered here since it is not available as a source of energy other than as food. This calculation can be easily made on the basis of annual agricultural statistics. In this table a distinction has been made between biomass in the form of leaves (good nitrogen content) and residues (low nitrogen content). The leafy biomass can also be used either for fodder or for manure. It can also be used for biogas production through anaerobic fermentation. Residues are such material as rice-husk, bagasse and groundnut husk which can be used mainly as fuel. The leafy biomass, especially resulting from cereal production, is often used as cattle fodder and may not be available for other applications such as manure or energy resource. The dung produced by livestock, which is a small fraction of the biomass they consume, is often valuable either as manure or as fuel. The growth of biomass takes place naturally in forests, wastelands and common lands. An analysis of the total biomass growing in India per annum from various sources is presented in Table 2. The biomass from forests may be estimated on the basis of growth rates of climax forests and the leaf-litter production as measured in various studies. The net primary productivity of woody biomass in mature forests in India is placed at 138.0 million tonnes (Mt) by Ravindranath and Hall [1995] for an area under forest of 63.5 million hectares (Mha). The leaf-litter production for the same area has been estimated by Bhat [1990] around 5.84 t/ha by averaging four different studies. Accordingly, the forest leaf-litter is around 371.0 Mt. The social forestry programmes led to an afforestation of about 15.0 Mha between 1975 and 1990, according to the Planning Commission [1992]. 2.0 Mha were reported to have been added during 1991 and 1992, according to Ravindranath and Hall [1995]. Out of this 17.0 Mha, one can estimate a woody biomass production of 85.0 Mt and a leaf-litter production of 51.0 Mt. A grass production of 51.0 Mt can also be estimated. Weeds in agricultural lands
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Table 1. Annual biomass production from agriculture in India, 1995-96 Crop
Area (Mha)
Production (Mt)
Straw/grain ratio
Leafy biomass (Mt)
Fuel (Mt)
Rice
42.91
79.62
1.4
111.47
23.48
Wheat
25.12
62.62
1.3
81.41
-
Jowar
11.44
9.55
2.01
19.20
-
Bajra
9.38
5.39
2.71
14.61
-
Maize
6.01
9.44
0.68
6.42
2.08
Ragi
-
2.33
1.96
4.57
-
Small millet
-
1.22
1.4
1.71
-
23.92
13.19
2.1
27.70
-
Sugarcane
4.14
283.0
-
80.42
87.26
Groundnut
7.7
7.81
1.0
7.81
2.12
Oilseeds
18.65
14.62
3.0
43.80
-
Mulberry
0.328
-
-
-
3.28
Cotton
9.06
-
-
-
26.93
Coconut
1.69
-
-
-
16.90
0.738
7.7
-
-
1.93
Potato
0.94
15.25
0.4
6.10
-
Castor
-
0.225
-
1.35
4.05
Pulses
Jute
Banana
0.064
4.5
-
5.40
-
Cassava
-
5.37
-
0.44
3.26
412.41
171.29
Table 2. Total biomass growth per annum in India
Forest Social forestry
Woody biomass (Mt)
Leafy biomass (Mt)
138.0
371.0
Other biomass (fuel) (Mt) -
85.0
102.0
-
Agriculture
-
412.4
171.3
Weeds
-
80.0
-
Village forests
27.0
27.0
-
Wastelands (Prosopis)
43.0
14.3
-
-
180.0
-
14.4
-
-
307.4 Mt
1186.7 Mt
171.3 Mt
Other wastelands Roadside trees Total Grand total
1665.4 Mt
also lead to about 80.0 Mt, assuming a pessimistic estimate of 0.5 t/ha. It is also useful to estimate the biomass in village forests. In a village-level study Ravindranath et al. [1991] report a standing biomass of about 0.83 t per capita. Extending this to the total rural population of India of about 650.0 million, there is a total standing biomass quantity of 540.0 Mt. Assuming an annual growth of 5 %, the wood growth per annum is 27.0 Mt. A leaf-litter-cumgrass growth of 27.0 Mt may also be estimated. It has been observed that wastelands in India have a significant coverage of an exotic tree species known as Prosopis juliflora. This has the property of rapid regeneration in alluvial and black soils. The four southern states Energy for Sustainable Development
of the country (Kerala, Tamil Nadu, Karnataka and Andhra Pradesh) have a wasteland area of 28.6 Mha and assuming 10 % of this to be covered by Prosopis, whose yield is estimated to be 15.0 t/ha per year, a woody biomass growth of 43.0 Mt can be considered. Again, assuming 5.0 t/ha of leaf-litter and grass, annual leafy biomass yield of 14.3 Mt may be estimated. Taking the estimate of wastelands of India at 90.0 Mha, we may assume a pessimistic figure of 2.0 t/ha of grass-cum-leaf biomass. This could add up to 180.0 Mt. An estimate of roadside tree growth of 14.4 Mt was also made by the author. The total biomass growth per annum may now be estimated at 1665.4 Mt, out of which woody biomass is l
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Figure 1. Livestock population in India
307.4 Mt, leafy biomass is 1186.7 Mt and residues are 171.3 Mt. It must be noted that leafy biomass is the main component of annual biomass growth. Vergara and Pimentel [1978] estimated the annual Indian biomass growth at 1339.0 Mt. The current estimate appears reasonably close. It is now clear that the biomass energy available annually is indeed significant. This quantum of biomass may be compared with the fossil fuel energy consumed in India. For instance in 2000-2001, the total commercial energy consumed in India was about 15,030 PJ (513.0 Mt of coal equivalent @ 29.3 GJ/t coal equivalent), according to GOI [2002]. Even if we assume that the biomass has 40 % less calorific value than coal equivalent (i.e., 1 t biomass contains 17.6 PJ), the total biomass energy growth is easily double the fossil fuel energy consumption. It must, however, be pointed out that not all the biomass energy may be available and what is available may have other current uses. There is a need to examine the current biomass utilization patterns and the various barriers to the expansion of biomass use. 3. Livestock, agriculture and biomass An earlier study by Gadgil et al. [1989] showed that the demand for fodder for livestock in India in 2001 would be around 660.0 Mt per annum. This resource should come from the ‘‘leafy biomass’’ listed in Table 2. Although the total leafy biomass growth is around 1186.7 Mt, it must be noted that the leafy biomass from forests (371.0 Mt) is often not available for general use. This means that there is only a surplus leafy biomass of about 150.0 Mt per annum, which could be considered for other uses. The livestock in India is thus the number one energy 30
Energy for Sustainable Development
guzzler of the economy and this distinction does not go to any other sector of the commercial economy. It is useful to examine the efficiencies of utilization of biomass by animals. Figure 1 shows the changes in the distribution of livestock population in India since 1965. The secular variations in the population of different livestock species over a three-decade period clearly indicate the changing pattern of biomass availability. It is easily seen that the bullock population has been practically stagnant and is in fact showing a slight reduction after 1990. The population of sheep has also been stagnant since 1982 while the population of cows is rising rather slowly. All these categories of livestock depend solely on grass biomass and the stagnant bullock and sheep population is a clear indicator of the shortage of grass fodder. Buffaloes are less choosy and more economical to maintain, and hence the buffalo population, which used to be half the cow population, is almost approaching the latter. It is also clear that cows are relatively more profitable to maintain than bullocks and sheep because of the income from milk. The grass fodder that is available is thus more profitably allocated to cows rather than bullocks. The bullocks are the mainstay of cereal production in agriculture, the economics of which has become very uncertain. The goat population, on the other hand, has been growing rapidly since the mid-seventies since goats depend on leaves of all kinds of plants and trees which are generally not consumed by other livestock. The relative scarcity of grass fodder in comparison with other leafy biomass is thus apparent. 1. Biomass energy conversion: fodder -- bullock It is now useful to examine the efficiency of conversion l
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Table 3. Land-use pattern and soil nutrient status Type of land use
Organic matter (%)
Nitrogen (%)
Phosphorus (%)
Dry land
0.80
0.06
0.064
Wasteland
0.75
0.05
0.12
Eucalyptus monoculture
1.43
0.07
0.045
Teak forest
1.74
0.09
0.062
Arjuna forest
2.06
0.12
0.139
Mixed forest
3.14
0.13
0.135
of fodder biomass into mechanical energy by bullocks. The annual consumption of grass fodder by a bullock can be estimated at 1.82 t (dry). This has a calorific value of 25.5 GJ. The mechanical energy delivered by the bullock, which works about 5 hours per day for 72 days in a year, can be estimated to be 0.48 GJ. The annual dung production by the bullock has energy content of about 4.1 GJ. Thus, the efficiency of bullock use is: Energy efficiency = 0.48/(25.5-4.1) = 2.25 % There are thus two problems in the use of bullocks in the economy. Firstly, the bullock is an inefficient converter of biomass energy to mechanical energy. Secondly, the bullock works only 20 % of the time. The 75 million bullocks in India guzzle an enormous amount of energy to work only 20 % of the time. We may regard the bullock population as an ‘‘engine of the economy’’ which idles throughout the year, wasting the energy consumed and giving back very little to the economy. It is small wonder, then, that the bullock population is not increasing. 2. Biomass energy conversion: fodder -- biogas -- engine It is possible to produce biogas from fodder grasses without going through the bullock route. Thus, the quantity of 1.82 t of fodder can give an annual biogas yield of 547.5 m3. Using this gas in an engine we can get 2.46 GJ of delivered energy. The wastes from the biogas plant will have an energy content of 6.82 GJ. Thus the energy efficiency of conversion through the biogas route is: Energy efficiency = 2.46/(25.5-6.82) = 13.2 % There are two advantages here. The energy utilizable is 6-fold higher. The biogas engine can be switched off at will, unlike the ‘‘bullock engine’’. The traditional dependence on livestock for energy is no longer viable, especially when fodder is scarce. It made sense when the country had abundant common land with no opportunity cost for grass. In today’s context it is better to depend on ‘‘bacterial’’ energy conversion than on energy conversion through large mammals. This route also generates significant quantities of manure for agriculture. 3. Biomass energy conversion: wood -- producer gas -engine It is also possible to use woody biomass through conversion into producer gas. Here, 1.82 t of wood has an Energy for Sustainable Development
energy content of 30.64 GJ. When converted to producer gas electricity the delivered energy is 5.24 GJ. Energy efficiency = 5.24/30.6 = 17.1 % The wood ash obtained as a by-product has valuable inorganic nutrients for agriculture such as calcium and potassium. It is now clear that the ‘‘modern’’ use of biomass through biogas and producer gas makes available a larger quantity of high-quality energy, besides producing large amounts of nutrients for sustainable agriculture. While it may not be possible to get rid of the huge bullock population of India all of a sudden, a slow, inexorable movement towards replacing bullocks by biogas/producer gas is possible and needs to be facilitated. The implications of the huge and burgeoning livestock population of India on the agricultural ecology of India need to be examined. The rural economy is critically dependent on livestock. This is especially true of sheep and goats, which are the mainstay of the landless and marginal farmers. Here is a situation where the herbivore population is going berserk since the poor are dependent on it. The consequences of excessive grazing by all the livestock are a slow but inexorable depletion of soil nutrients. Table 3 shows a typical pattern of soil nutrient distribution in different land-use patterns. The low nutrient status of dry lands and wastelands vis-à-vis lands with tree cover is now obvious. Village forests with mixed-species trees will now be a very important source of soil nutrients besides being a reservoir of biomass energy. Lands with grazing pressures immediately get degraded. There is thus a case for limiting the growth of livestock to improve soil nutrients. There is another problem that is not often appreciated with large livestock. There is in general a natural regeneration of trees and shrubs, which provide valuable biomass energy inputs at practically no cost. This process is severely inhibited due to grazing pressures. The livestock, besides depleting soil nutrients, effectively prevent green cover from establishing itself naturally. Thus, the rural economy, which is critically dependent on livestock for income generation, has to pay a very high ecological price by losing soil quality and green cover. Alternative approaches to limit the livestock population without jeopardizing the rural economy are urgently needed. A more efficient programme of cattle management using stall-feeding and better inputs can perhaps sustain milk production at current levels using a substantially l
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Figure 2. Land - bio-energy - water nexus
lower number of cows. Development of small rural enterprises to wean the rural landless away from sheep- and goat-rearing is also another option. This is not to suggest that the goat and sheep population should become extinct. It is desirable to reduce the sheep and goat population so that there is a balance between plant diversity and animal diversity. 4. Tasks for a new biomass-based rural transformation The foregoing discussion highlights the abundance of biomass energy in India. However, current utilization patterns in rural areas, whether in domestic cooking or maintenance of livestock, are highly inefficient. Cooking using firewood or agro-residues generally takes place at efficiencies around 10.0 %. The abysmally low conversion 32
Energy for Sustainable Development
efficiency of bullock energy has been pointed out above. A new approach to biomass utilization will have to be three-pronged: • biomass conservation • biomass generation • efficient conversion of biomass energy 4.1. Biomass conservation A significant amount of woody biomass is used in India for thermal energy applications such as cooking, jaggerymaking, brick-making and lime-burning. Many of these operations are inefficient. The Centre for Application of Science and Technology to Rural Areas (ASTRA), Indian Institute of Science, Bangalore, worked on many of these technologies and came up with improved devices that save biomass energy to a significant extent. However, dissemination of these technologies in the rural areas has not been l
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easy. There is a need for a long-term strategy to involve the rural population in a participatory technological change. A large number of professional groups have to emerge to work as stakeholders in this programme. To a certain extent, the technology of stabilised mud blocks developed to reduce brick-burning energy has been reasonably successfully disseminated, with about 9000 buildings using the technique across the country [Jagadish and Nanjunda Rao, 2002]. 4.2. Biomass generation Although India’s biomass energy endowment is not small, current practices of agriculture and silviculture are operating well below their true potential. India has the second largest area of arable land in the world (160 Mha). China, with only around 90 Mha, produces double the amount of food grain India produces. India can definitely double its food grain production. This will incidentally push the biomass availability to around 800 Mt from the current 421 Mt. India also has about 90 Mha of wasteland whose productivity is very low. A mixed-species forest, according to the studies of Shailaja et al. [1994] at ASTRA, can easily produce more than 500 Mt of biomass per annum. It is certainly not over-optimistic to expect an aggregate production of 2500 Mt per annum. This will easily provide a threefold increase in the energy supplied to the economy along with copious quantities of manure for sustainable agriculture. 4.3. Efficient conversion of biomass energy: bio-fuels 4.3.1. Gaseous fuels There are two distinct possibilities here. All the leafy biomass with a certain nitrogen content can be advantageously processed in biogas digesters to produce biogas. The work of Jagadish et al. [1998] in the development of a plugflow biomass digester has demonstrated the technical feasibility of the concept. In this process about 50 % to 60 % of the biomass can be digested and converted to biogas. This methane-rich biogas can be used for cooking or for running engines for mechanical energy or electricity generation. The rest of the biomass comes out as a waste, which has immense value as manure since it contains humus, nitrogen and several other inorganic nutrients. The quantum of this manure is not small, unlike in livestock utilization. Livestock use up and store biomass for their own metabolism while giving back small quantities of manure. However, in bacterial conversion, the bacteria quickly die after their job is done and offer the dead bacterial biomass as manure. The work of Mukunda at el. [1993] has shown the feasibility of using wood chips to generate producer gas. This has the potential of meeting the electricity requirements of rural India. The author believes that charcoal also offers a similar possibility of energy generation, provided charcoal is produced in an environmentally sound manner. The use of biomass energy has interesting possibilities of integrating energy generation, water management and fertilizer production in a mutually linked symbiotic operation. Figure 2 shows a schematic diagram integrating the various processes in a closed-loop network. Such an integration of processes is not possible in the open-ended use of fossil fuel energy. The biomass energy provides a Energy for Sustainable Development
balanced use of resources leading to symbiotic (non-exploitative) interaction of different segments of the economy. Developing countries in the South have a great potential to realize this dream. It is hoped that the Third World will come together for such an exciting venture. 4.3.2. Liquid fuels: ethanol and methanol Production of fuels such as ethanol and methanol from biomass is a distinct possibility to meet the transport needs of a country such as India. The experience of Brazil, discussed by Goldemberg et al. [1993], confirms the potential of ethanol production from sugarcane. In the Indian context ethanol can also be obtained as a by-product of the sugar industry, using molasses as the feedstock. However, the current production of ethanol from molasses is utilized either as industrial alcohol or as potable alcohol, leaving precious little for use as automobile fuel. Direct use of sugarcane to produce ethanol instead of sugar is probably the simplest way to obtain a liquid fuel. Very often sugar production in India is about 10 to 15 % more than the consumption. It is hence reasonable to assume that 10 % of the sugarcane grown can be diverted to ethanol production. According to the reference annual publication India 2002 [GOI, 2002], India produced 294.0 Mt of sugarcane in the year 2000--01. If 10 % of this is used for ethanol about 2.0 billion litres (Gl) of alcohol can be produced, according to the conversion factor quoted by Moreira and Serra [1990]. The ethanol so produced is adequate to replace petrol in automobiles to the extent of about 20 %. Ethanol can also be produced by hydrolysis of starch from cassava or maize. However, since cassava and maize are useful food crops in India it is unlikely that they can be spared for alcohol production. These facts make it clear that one cannot expect products of arable land to be used in a significant way for fuel production in view of the unfavourable land/man ratio. Cellulosic materials can also be used for ethanol production, but the yield of ethanol is as low as 160 l/t of wood, making the process rather uneconomical [Moreira and Serra, 1990]. It is hence clear that the options for producing ethanol in the Indian context are rather limited. It is only feasible to replace a part of the petrol in automobiles by ethanol produced from sugarcane. In contrast to ethanol, methanol can be produced from a wide variety of biomass sources. The usual route is to produce syngas from woody biomass or charcoal and then recombine carbon monoxide and hydrogen over a catalyst to produce methanol. According to Wyman et al. [1993] about 2.3 t of biomass is needed to produce 1 t of methanol. Several sources of biomass can be considered for the production of methanol. Wood from Prosopis juliflora, an exotic species which has become the dominant tree in many dry tracts of India, is an obvious first choice because of its productivity and concentration in specific locations. Cotton, mulberry and cassava stalks, which provide about 32 Mt of biomass per annum, are another option. Assuming a pessimistic estimate of 30 Mt of Prosopis wood per annum and 30 Mt of cotton and other stalks, India can produce about 26 Mt of methanol per annum. This is indeed quite significant in view of the current annual consumption of about 40 Mt of diesel (mostly used l
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for transport). With improvements in afforestation in wastelands and wood conservation in domestic cooking, 50 Mt of methanol production per annum is not unrealistic. It now appears that methanol will meet the renewable energy strategy for transport in India rather than ethanol. It goes without saying that more intensive R&D to use methanol either as a partial replacement for diesel or in pure methanol engines is needed to realize the goal of renewable energy-based transport. It is also encouraging to note that methanol has the possibility of being used in fuel cells as a source of hydrogen. Availability of methanol can lead to a smooth transition to clean and efficient transport based on fuel cells. 5. Conclusions The energy in the biomass that grows annually in India is easily twice the fossil fuel energy consumed in the country. Since all the biomass that grows is not readily available for use as energy, the efficiencies of the current biomass utilization must be examined. The livestock in India are the energy guzzler of the economy. The energy conversion efficiency of the bullock is only 2.25 %, indicating an enormous wastage of a major source of energy. The same biomass when used for producing biogas and then using it to run an engine will deliver a six-fold greater amount of energy. The livestock are also responsible for loss of green cover, loss of nutrients in soil and erosion of soil. Better management of forest and wastelands and a more efficient and hence reduced livestock population are essential for a better deployment of bioenergy resources. A three-pronged approach, looking at biomass conservation, biomass generation and efficient conversion of biomass to bio-fuels, is needed to improve the availability of useful energy in India. Gaseous fuels such as biogas and producer gas and liquid fuels such as ethanol and
methanol have the potential to meet a large portion of our energy budget. In liquid fuels methanol appears to be more feasible than ethanol. Acknowledgement This paper is based on an earlier paper submitted to the Sustainable Development Conference, Sustainable Development Policy Institute, Islamabad, 31 Oct-2 Nov, 2002. References Bhat, D.M., 1990. Private communications. Gadgil, M., Sinha, M., and Pillai, J., 1989. India: a Biomass Budget, Centre of Ecological Sciences, Indian Institute of Science, May. Government of India (GOI), 2002. India 2002, Publications Division, Government of India, New Delhi. Goldemberg, J., Monaco, L.C., and Macedo, I.C., 1993. ‘‘The Brazilian fuel-alcohol program’’, in Johansson, T.B., Kelly, H., Reddy, A.K.N., and Williams, R.H., (ed.), Renewable Energy: Sources for Fuel and Electricity, Island Press, Washington. Jagadish, K.S., Chanakya, H.N., Rajabapaiah, P., and Anand, V., 1998. ‘‘Plug-flow digestor for biogas generation from leaf biomass’’, Biomass and Bio-energy, 14, 5/6, pp. 415-423. Jagadish, K.S., and Nanjunda Rao, K.S., (eds.), National Workshop on Alternative Building Methods, Proc., Dept. of Civil Engg., Indian Institute of Science, Bangalore. Moreira, J.R., and Serra, G.E., 1990. Alternative Fuels: a Brazilian Outlook, Alternative Liquid Fuels, Wiley Eastern Ltd., IDRC, Ottawa, and UN University, Tokyo. Mukunda, H.S., Dasappa, and Shrinivasa, U., 1993. ‘‘Open top wood gasifier’’, in Johansson, T.B., Kelly, H., Reddy, A.K.N., and Williams, R.H., (ed.), Renewable Energy: Sources for Fuel and Electricity, Island Press, Washington, pp. 699-728. Planning Commission, 1992. Report of the Eighth Five-Year Plan, Government of India, New Delhi. Ravindranath, N.H., and Hall, D.O., 1995. Biomass, Energy and Environment, Oxford University Press, Oxford. Ravindranath, N.H., Nayak, M.M., Hiriyur, R.S., and Dinesh, C.R., 1991. ‘‘The status and use of tree biomass in a semi-arid village ecosystem’’, Biomass and Bio-energy, Vol. 1, No. 11, pp. 9-16. Shailaja, R., Ravindranath, N.H., Somashekar, H.I., and Jagadish, K.S., 1994. ‘‘Biomass generation in mixed tree plantations’’, Energy for Sustainable Development, Vol. I, No. 3, September, pp. 51-55. Vergara, W., and Pimentel, D., 1978. ‘‘Fuels from biomass -- comparative study of the potential in five countries: US, Brazil, India, Sudan and Sweden’’, in Auer, P., (ed.), Advances in Energy Systems and Technology, Academic Press, New York. Wyman, C.E., Bain, R.L., Hinman, N.D., and Stevens D.J., 1993. ‘‘Ethanol and methanol from cellulosic biomass’’, in Johansson, T.B., Kelly, H., Reddy, A.K.N., and Williams, R.H., (ed.), Renewable Energy: Sources for Fuels and Electicity, Island Press, Washington.
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