Ramachandra. T.V., Vijaya Prasad. B.K. and Samapika Padhy Centre for Ecological Sciences, Indian Institute of Science, Bangalore 560 012, India

Alternatives: Techno Economic Analyses of Bio Energy Systems

The fundamental forms of bio energy use are:

1. The traditional domestic uses: for household cooking, lighting  and water heating (for bath). The efficiency of conversion of the biomass to useful energy is between 5% and 15%.
2. The rural industrial use in agro processing, bricks and tiles, pig iron  where the biomass is considered as free energy source. There is generally little incentive to use the biomass efficiently so conversion of feedstock to useful energy commonly occurs at an efficiency of 15% or less.
3. Biological conversion including anaerobic digestion for biogas production and fermentation for alcohol.

The overall efficiency of biomass utilisation depends on the moisture content of the fuel and type of stoves used. Freshly cut wood contains about 25-60% moisture. The removal of a kilo gram of water from wood involves an expenditure of about 620 - 670 kcals. It is noticed that a reduction of 25% moisture in fuelwood would cause a saving of nearly 15% of the fuel wood.  It is observed that dried wood with moisture content of 8% releases heat too fast and the whole log tends to burn bringing the flame out of the stove.

1.    Fuel Efficient Stoves:

Most commonly used stoves in most of households for cooking are either mud stoves or three stone stoves also referred as traditional stoves (TC'S). The efficiency of these stoves are less than 10%. Applying the principles of combustion and heat transfer, fuel efficient wood and other biomass burning designed by ASTRA (Lokras, 1992)  also called as Astra stoves or Improved cook stoves (IC's). In Astra stoves complete combustion of fuel wood with as little excess air as practicable to generate the highest temperature of flue gases. In IC's combustion of fuelwood is carried out over a grate in an enclosed fuel box with ports of suitable size for entry of air. The grate helps in entry of air below the fuel bed to burn the char as well as for separation of ash from fuel. Air required for burning the volatile matter released as consequence of heating the fuel (also referred as secondary air), enters through a port at a level slightly above the grate. Heat gets transferred to pans by the mechanism of conduction convection and radiation. Fuel efficiency studies conducted in 82 households of the cluster of villages in Sirsimakki microcatchment of Sirsi Taluk [Ramachandra, T.V., and Shastri, C.M., 1995] have shown that the fuel need for cooking is about 1.92(avg)±1.02(Sd) kg per person per day for cooking in traditional stoves while in IC's about 1.1(avg)±0.78(Sd) kg per person per day. Which, means that there is a saving of about 42% in the quantity of fuel used by switching over to IC's from TC's.

2.    Energy Plantations:

Technically speaking, energy plantations mean growing select species of trees and shrubs that are harvestable in a comparably shorter time and are specific means for fuel. The fuelwood may be used either directly into wood burning stoves, boilers  or processed into Methanol, Ethanol and Producer Gas. These plantations help to provide wood either for purposes of cooking in homes and for industrial use so as to satisfy local energy needs in a decentralised manner (Vimal and Tyagi, 1984). The energy plantations provide almost inexhaustible renewable sources (has total time constant of 3-8 years only for each cycle) of energy that is essentially local and independent of unreliable and finite sources of fuel. The attractive features of wood are (a) its heat content is similar to that of Indian coal, (b) wood has low  sulphur that is not likely to pollute the atmosphere, (c) ash obtainable from burning is a valuable fertiliser, (d) Utilisation of erosion prone lands for raising wood plantations help to reduce wind and water erosion, by that minimising hazards from floods, siltation, loss of nitrogen and minerals from soil. (e) helps in rural employment generation: It is estimated that an acre of energy plantation provides the job for at least three persons regularly. Selection of multipurpose species provides number of by products like oils, organic compounds, fruits, edible leaves, forage for livestock etc. Data collected from Forest Department (plantations in Bagepalli, Srinivasapur, Chintamani Taluks under Social Forestry Programme) reveals that annual woody biomass available is in the range 11.9 to 21 tonnes/ha/yr. An energy plantation at the Hosalli village, Tumkur District to support wood gasifier plant has annual yield of 6t/ha/yr. In each taluk about ten percent of lands are unsuitable for agriculture. These wastelands available in each taluk could be used for  energy plantations with species acceptable to local people.

3.    Biogas Technology:

Biogas is a product of anaerobic fermentation of organic matters, and consists of about 60-70% Methane, 30-40% Carbon-di-oxide etc. The input materials for biogas digesters are the wastes that are found locally such as animal dung, agricultural residues, and leaf litter from forests. The residues are introduced into a closed digester, where, without the presence of free oxygen, the responsible micro organisms work successively to convert complex organic matter in to CH₄, CO₂, H₂, H₂S, etc. The optimum conditions for biogas production are: temperature 30-35°C, Ph 6.8-7.5, Carbon/Nitrogen Ratio 20-30, solid contents 7-9%, retention time 20-40 days.      Among these parameters, temperature is the most difficult or costly to control. The gas formation virtually stops when the temperature drops below 10°C. Retention time decides the rate at which the waste is digested. The longer the time, the larger the volume of gas produced from a given amount of waste and vice versa. Thus if the available amount of input materials is limited, a bigger digester can be adopted to more fully exploit the gas potential; and where the waste is abundant, the waste can be fed at a higher loading rate into a small digester to maximise the gas production per unit volume of the digester. The optimum retention time depends on the temperature. In practice, a longer retention time is usually adopted to cope with cool seasons. There are various designs of biogas digesters such as:

1. Floating gas holder type designed by Kadhi and Village Industries Commission (Directorate of Gobar Gas Scheme, 1979).
2. Optimised design developed by Application of Science and Technology to Rural Areas (ASTRA) at our institute (Subramanian, 1984).
3. Fixed dome type designed by University of Agricultural Sciences - Bhagyalaxmi design.
4. Raitabandu Biogas Plant - designed by a farmer from Sagar Taluk, Shimoga District to suit the needs of the malnad region.

Biogas Usage: Biogas can be used for many purposes, mainly for cooking and lighting in rural area. Biogas can be burned with a gas mantle or can be converted to electricity using a dual mode engine. The per capita requirement of gas for cooking is in the range 0.34-0.43 m³/day (efficiency of a standard burner is about 60%). The gas requirement to generate one unit of electricity (kWh) is about 0.54 m³. The calorific value of m³ of gas is about 4713 kcals. As indicated, biogas can meet the cooking requirement of at least 20% of the total District population. Villagewise biogas availability and demand is computed for the following four cases.:

Case I:      taking dung yield for cattle as 3 kg/animal/day, for buffalo 12 kg/animal/day. Per capita requirement of biogas as 0.34 m³/day. 

Case II:     taking dung yield for cattle as 3 kg/animal/day, for buffalo 12 kg/animal/day. Per capita requirement of biogas as 0.43 m³/day.

Case III:    taking dung yield for cattle as 7.5 kg/animal/day, for buffalo 15 kg/animal/day. Per capita requirement of biogas as 0.34 m³/day.

Case IV:    taking dung yield for cattle as 7.5 kg/animal/day, for buffalo 15 kg/animal/day. Per capita requirement of biogas as 0.43 m³/day.

Biogas potential assessment for these four cases were carried out for each taluk. Results are listed in Table 2.

Table 2 shows that Biogas can meet 15-30% populations domestic energy requirement in seven taluks as per case I. In Case II, Biogas is sufficient to meet 15-30%, 20-45% and 45-60% of  populations domestic energy requirement in five, five and one taluks of Kolar district respectively.

Community biogas plants are the best solution to meet the domestic energy requirements. In order to select villages suitable for this purpose, similar analyses were carried out villagewise and results are listed in Table 3a- 3d.

This shows that in 99 villages of Kolar taluk, biogas can meet energy demand of  15% of population. In 112 villages biogas potential is sufficient to meet 15-30% of population's energy requirements.  30-45%  of population  energy requirements is met in 86 villages by switching over to biogas.  About 65 villages have potential which can meet more than 45% of population requirements.

 

 

 

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