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In this article we will discuss about the movable drum type and fixed drum type biogas plant designs that are commonly used in India.
1. Movable Drum Type Biogas Plants:
India can be rightly claimed as birth place of movable drum type biogas plants, where number of designs have been developed. It basically comprises an underground brick masonry digester connected with an inlet and outlet and covered by a movable steel gasholder for gas collection.
Gasholder moves up and down guided by a central guide pipe depending upon accumulation and discharge of gas. Movable gasholder made of mild steel alone accounts for some 40 per cent of the total plant cost and accordingly these plants are much more expensive than fixed dome type.
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Maintenance costs of these plants are also high in view of much the need to paint gasholder every year to prevent corrosion. This plant helps achieve more consistent gas pressure which can be adjusted by regulating weight. Based on series of chronological developments in India, IARI and KVIC developed two plant designs which have come to be known as the IARI and the KVIC models (Figs. 4.2 and 4.3).
Cross-sectional view of floating drum type plant developed and widely used in Nepal is shown in Fig. 4.4 one distinctive feature of which is a central guide-pipe. Based on average conditions with pig manure as feed it is possible to get yield of 0.4 to 0.55 m3 gas/day/m3 of digester volume of a movable drum type plant whereas with cattle dung as feed it is possible to obtain 0.3 to 0.4 m3 of gas yield, in the same units of measurement.
Complete details of movable drum type biogas plant with facility of counterweights to build adequate gas pressure approved vide IS: 9478-1980 are given in Fig. 4.7. Sketch of gasholder and recommended method of joining it to the pipeline leading to the biogas appliances are shown in Fig. 4.6 respectively.
The digester is to be made of RCC foundation with its minimum thickness as 150 mm for plants with capacities upto and including 10 m3 and 230 mm for plants with capacities more than 10 m3. The digester wall is made of brick with top edge having a slope to a depth of 150 mm on the outer-side (Fig. 4.7).
The digester wall includes an annular projecting support for gasholder. Stone, iron or RCC beam is used for fixing a detachable central guide pipe above the partition wall. Two pulleys with hanging counterweights are provided on both sides of the gasholder to build adequate gas pressure. Gasholder is made of GI sheet or welded steel construction. Both inlet and outlet tanks need be made of bricks which can have any shape. The bottom of the inlet should always be above the level of the digester.
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The inlet and outlet tanks should be connected to the digester with an asbestos cement pipe conforming to IS : 6908-1975. For plant capacities up to and including 15 m3 recommended diameter of pipeline for connecting digester to inlet and outlet is 100 and 150 mm respectively as per IS: 9478-1980. Gas outlet pipe leading to the biogas appliances is taken from the gasholder top.
2. Fixed Dome Type Biogas Plants:
In fixed dome type biogas plant which was first developed in China, gas is stored in upper part of the digester. In this design there is no separate gasholder and upper, portion of the digester pit itself acts as gasholder. Displaced level of slurry provides requisite pressure for release of gas for its subsequent use. The first pilot plant as per this design was developed by the Gobar Gas Research Station, Ajitmal, Etawah in 1977.
Most of the fixed-dome type biogas plants in China have their digester capacity between 6 to 8 m3 and a small proportion have their capacity upto 12 m3. Community plants-have their capacity between 50 to 100 m3 depending upon waste availability and gas requirement. The plant works on both animal wastes and plant waste materials and is convenient to work with in both continuous and batch mode. Normally animal wastes are fed daily whereas crop residues are fed in batches.
As the plant does not involve any steel parts and it can be built with local materials, its construction costs are low and operation costs virtually nil. Being underground, space needs are also minimal. Materials like lime-clay, lime-concrete, concrete, bricks, stones etc., can be used for plant construction. It, however, requires considerable skills in dome construction. Construction materials required are cheap and easily available.
In China a number of organisations have done pioneering work in developing and modifying fixed dome plants which include- National Biogas Office, Beijing; Science and Technology Association, Shanghai; Biogas Research Institute, Chengdu; Agricultural University, Hangchow; South-West Institute for Agriculture, Chengdu, and the Chengdu Institute of Biology. In Chinese designs it is usual to provide a manhole cover at the top.
In earlier designs the plant depth varied from 3 to 4 m3 but present trends limit it to within 2 m. In equivalent capacity moving drum type plant corresponding height varies from 3.5 to 4 m. Inlet and outlets are connected at the mid-point of the digester. Upper part of digester above liquid surface provides storage space for biogas. When gas is produced, level of digester liquid drops whereas that in outlet rises with height difference between the two varying according to gas pressure.
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This difference in height helps to regulate gas pressure within digester over a wide range. This self-regulating mechanism for gas pressure is the unique feature of fixed dome type plant. For obtaining higher gas yield per unit digester volume, it is desirable to keep free surface area of digester liquid larger in relation to height.
In Sichuan Province of China where there is a heavy concentration of biogas plants three designs are mainly followed which are commonly known as ‘tank cast-in-place’, ‘tank cut-in-place’ and masonry tank.
For tanks ‘cast- in-place’ suitable concrete mixtures are poured to cast walls, dome and base whereas ‘cut-in-place’ tanks are dug out directly from suitable hard clay soils with minimum of structure work. For masonry tanks, bricks, stones, precast cement structures etc., are used.
Based on Chinese design, Gobar Gas Research Station, Ajitmal (Etawah) developed the first ever fixed dome biogas plant in India in 1977 which has come to be known as the Janata Plant. The plant design has, however, undergone several modifications since then.
A cross-sectional view of Janata biogas plant involves an underground cylindrical digester and a hemispherical dome but without manhole cover. The dome is fitted with a GI pipe through which the biogas is taken out. It is made of bricks, cement and concrete. Following construction, digester walls and dome are cured for several days for imparting requisite strength to the plant. A comprehensive estimate of material requirement for constructing different capacity Janata plants.
Scientists at the Tata Energy Research Institute, New Delhi, following some modifications; have developed three alternative versions of Janata biogas plant which are commonly referred to as the TERI mark I, II and III. All the three TERI designs have a hydraulic retention time of 50 days.
The cost of Mark I for capacities of 1 -6 m3 is higher than that of Janata plants by 1 to 27 per cent. For capacities of 1-3 m3, Mark II models are 9 to 22 per cent costlier than the Janata models. The cost of the Mark III model is lower than that of Mark I and II for all capacities and comparable with the cost of the Janata model of capacity below 3 m3.