In this article we will discuss about the procedure for constructing movable type and fixed dome type biogas plants.
Procedure for Constructing Movable Drum Type Plant:
Biogas plant should be constructed in consonance with the technical specifications as any imperfection will result in low gas yield, reduce plant life, and may even require far more work to remedy defects. The choice of a suitable base is a key factor for the final quality of the pit. Before construction a study should be made with regard to soil conditions and the underground water level.
For constructing a movable drum type of biogas plant, generally the following steps need be followed sequentially:
1. Collection of Material:
Collect all the materials required, including bricks, stones, cement, steel, mortar etc. Care should be taken to store cement in a dry place. Ensure availability of labour input required including the self-help to be provided by the family members of the plant owner.
2. Marking of Digester Pit:
Mark the outline of the pit on the ground with the help of a chord and marker.
3. Excavation of Digester Pit:
Dig the digester pit within the marked ring. Sometimes it may happen that sides may collapse if the soil is not firm. To avoid collapsing, it is customary to dig a 30-50 cm deep ring outside the edge of the pit to be filled with concrete comprising stones, sand and cement in the ratio 10: 3: 1.
4. Pouring the Foundation Slab:
The pit should be straight and the floor horizontal. Before concreting, loose sand and loam is required to be removed. If possible, line with a plastic sheet. The concrete is needed to be poured in a moist condition comprising a mix of 30 mm size gravel or chippings, clean sand and cement in the ratio of 6: 3: 1. Carry out necessary stamping to eliminate trapped air which helps to increase strength and save cement. Mark the inside face of the pit-wall on the base slab with a cord which should be tied at the centre.
5. Pit-Wall Brickwork:
The upper edge of the first layer should be laid horizontally. Surface irregularities should be filled up with extra mortar. Mortar should be sufficiently smooth and sticky. Mixing ratio although dependent on sand quality is generally taken as 1 part of cement to 6 part of sand, or lime, cement and sand mixed in proportions of 1: 1: 12. Bricks or stones to be used in construction are required to be dipped in water before use.
6. Back-Filling of Pit-Wall:
Every morning earth or sand should be filled into the space behind the section of the wall built the previous day followed by proper ramming. The partition wall should be built when the outer pit wall has reached the height of the projecting ledge. Install the central guide frame for the gasholder with metal cross in the ledge ring after ensuring its perpendicularity.
The ring ledge consists of stones or bricks laid at right angle to the rest of the wall. Complete the rest of the pit masonry work. Cut the outlet pipe to the desired length and insert it into the wall of the composting pit. The inlet pipe should be approximately 50 cm higher than the outlet pipe. Build the mixing tank. It is to be ensured that connections to toilets on one hand should not be lower than the mixing chamber and on the other hand should be higher than the outlet pipe.
7. Plastering of Surfaces:
Round off all edges and corners. Apply plastering material in two layers comprising 1 portion of cement with 3 portions of sand and two parts of water. The floor of the mixing chamber should be made with correct slope. Apply rust-proofing paint to the guide frame and its metal cross. Wherever possible, bitumen coating should be applied to all the plastered surfaces.
8. Construction of the Gasholder:
Independent of pit-construction, construction of gasholder can simultaneously start in a workshop. Sheet metal of which gasholder is made should not be less than 2 mm thick and weld seams must be gastight. The holder is cylindrical. The outer wall is 3.14 times the diameter in length. The height of the outer wall is the same as the overall height of the holder.
The cover of the gasholder is slightly conical. It is formed by marking out and cutting the sheet metal in steps namely, by marking out a width of radius 1.5 cm greater than the radius of the gasholder; measuring and marking a length of 9.5 cm on the circumference of the circle; cutting out a wedge, cutting out a ring for the guide tube plate (10 cm radius); welding together the cut edges of the wedge; and welding the cover to the outer panel.
This is to be followed by application of rust-resistant paint to the inside and outside surfaces of the gasholder. Before applying paint, existing patches of rust must be removed with a wire brush. This is to be followed by application of bitumen coating.
Following operations also need to be carried out:
Grease the guide-frame sufficiently. Mount the gasholder after closing the main gas tap of the gasholder, if leaking, it should be repaired. Open the main gas tap to exhaust air from the gasholder. Gas tap should remain open till the gas has started coming out.
Normally gas pipes vary from 2.5 to 5 cm in diameter. Gas pipes can be either buried in ground or made to lie above ground. However, it is preferred to keep them underground for keeping them cool. At the lowest point of gas pine there should be provision for trapping and removing moisture condensate.
For constructing a fixed dome type biogas plant, generally the following steps need be followed sequentially.
1. Collection of Material:
Collect all the materials required including bricks, stones, cement etc. Care should be observed to store cement in a dry place. Ensure availability of labour input required including the self-help to be provided by the family members of the plant owner.
2. Marking of Digester Pit:
Mark the outline of the digester pit the inlet and outlet tanks on the ground.
3. Excavation of Digester Pit:
Dig the digester pit and inlet and outlet tanks according to the marked profile. Sometimes it may happen that sides may collapse if the soil is not firm. To prevent collapsing it is sometimes advisable to taper off the digester walls. It is also possible to excavate the inlet and outlet after the digester pit has been lined. In rocky ground or when the ground-water level is high, the base of the biogas plant can be raised accordingly. For achieving desired insulation, soil around the digester pit should be appropriately packed.
4. Pouring the Foundation Slab:
The floor can be either of concrete or brickwork and is generally 20 to 40 cm thick. Before concreting loose sand and loam is required to be removed. The floor should be provided with necessary slope as per the design.
5. Digester Pit Brickwork:
Brickwork should be performed accurately with adequate filling in the joints. For this purpose, bricks, concrete block, or stones can be used. Thickness of digester walls generally varies from 15 to 30 cm. The floor of the digester pit should be strong enough to bear the entire water pressure.
6. Construction of Inlet and Outlet Tanks:
Generally pipes of diameter 25 to 40 cm are appropriate for connecting the inlet and outlet tanks to the digester pit. While making inlet and outlet of brickwork, connecting part to the pit should be most carefully made and if need be, it can be reinforced with concrete. Inlet and outlet pipes are normally inclined at an angle of 50 to 60 degrees.
7. Back-Filling of Digester Pit Wall:
Every morning earth or sand should be filled into the space behind the section of the wall built the previous day followed by proper ramming.
8. Building of Dome:
Dome-building is a difficult phase in construction of fixed dome type biogas plant as it requires considerable skill and experience. One easy construction method is to use a full wooden shuttering. With necessary skill, a mason can construct a spherical roof without shuttering.
Care should be taken to ensure that transition between the dome and vertical wall is adequately reinforced and supported against the surrounding soil. Back-filling is generally recommended to be done every day with the completion of 3 to 4 brick-courses.
9. Plastering of Surfaces:
Round off all edges and corners. Apply plastering material in two layers comprising 1 portion of cement with 3 portions of sand and two parts of water. After plastering seating coats need to be applied.
Plastering and application of special seating paints are required to make the structure gas and water tight. The plastering need to be carried out in 2-3 successive layers to a total of 2 to 4 cm thickness. Plastered surfaces should be smooth and even and finally given a coating of pure cement slurry.
If cement is not available, mortar can be used in its place which can be categorised as masonry mortar and plastering mortar. Both masonry and plastering mortar can have varying compositions.
Following are the standard mortar mixes normally used in China:
i. Masonry Mortar:
3-component mortar 2/2/8 (lime/washed sand/alumina), 4- component mortar 1/2/2/4 (lime/fine ash/alumina/washed sand), 4-component mortar 1/8/8/42 (cement/lime/fine ash/washed sand) or 4-component mortar 0.1/1/2/6 (cement/lime/alumina/washed sand).
ii. Plastering Mortar:
For the first layer ‘A’ 1/2/3 (lime/fine ash/medium sand), for the second layer ‘B’ 1/1/2 (lime/fine ash/medium sand) with a thickness of less than 1 cm, for the third layer ‘C’ 1/1/1/2 (cement/lime/fine ash/fine sand) with a thickness of 0.5 cm. If in the third layer cement is not to be used, 1/1/1 (lime/fine ash/fine sand) with a thickness of 0.5 cm can be used instead.
A week after the plastering is done, three layers of bitumen or three coats of a synthetic sealant to provide insulating cover are applied.
iii. Assembly of Hatch Cover:
The hatch cover must be cast in concrete with a step all round. Water is digester slurry keeps the seal moist which makes it unusable after some time. As a result seal is required to be replaced after some time.
For breaking the fibrous material a stirrer passes through gas dome. Lower end is pivoted onto the floor of digester. It is sealed by an outer tube which extends down below the minimum slurry level so that gas cannot escape. Upper part of this tube is anchored and sealed in the fixed dome brickwork.
In China for constructing these plants following materials are commonly used:
i. Lime Clay:
This is commonly used building material in China which after mixing with water in right proportions (21-24 per cent) and requisite curing imparts needed compactness and hardness. Clay easily reacts with lime to form hydrate calcium silicate and hydrate calcium aluminate. Water-hardening capability of lime-clay essentially depends on quality and mixing proportions of lime and clay used, compactness, curing conditions and water content.
Moisture content in lime-clay lubricates its grains and makes it plastic. In lime-clay proportions of lime and clay vary from 1: 9 to 1: 19 by weight which after curing in water for 5 to 7 days imparts in needed hydro-stability (i.e., it does not give in or collapse in water). Addition of 20 to 30 per cent of coal slag helps to counter shrinking characteristics of lime clay. Moist climate is considered more conducive for efficient curing than arid-climate.
ii. Lime Concrete:
It is another building material commonly used in China. It consists of mixture of lime, sand and gravel in proportion of 1: 3: 6 by volume in water.
iii. Low-Strength Concrete:
This is obtained by mixing concrete with a compressive strength of 30 kg/cm2 in lime and clay. Concrete mixture is prepared with cement-to-concrete ratio within a desired range of 1.25 to 4 and with some slaked lime powder added to it which enables low quality cement to be used without any loss of strength.
Other additives include coal ash, lime kiln slags and clay. Efficiency of various additives used to lower cement grade is represented by a factor called β. Compressive strength of concrete in kg/cm2 and achievable after a curing period of 28 days is given by-
Strength of concrete essentially depends on properties of individual constituents used, water to lime ratio, degree of compactness and size of sand. For retaining properties in low-strength concrete, additive proportions should not exceed 40 per cent limit.
In China concrete is also prepared in clinker-free-cement which is made of lime, gypsum and Pozzolanic (Cinder) material. Concrete prepared from this cement is called clinker-free-concrete. Coal slags, like-kiln slags, crushed bricks and tiles from part of Pozzolanic (Cinder) materials.
In this concrete water, clinker-free cement, fine-sand and gravel are mixed in ratio of 0.67: 1: 1.85: 4.07 by weight. Clinker-free-concrete acquires strength with the passage of time and is known to be stronger in moist than arid climate.
Besides use of lime-clay, lime-concrete, low-strength concrete, clinker- free concrete for plant construction, digesters in China are also made of bricks, stones, rocks etc., as is done in India, with their specific choice being governed by local availability of a particular item.
Based on Chinese experience for constructing 6 to 8 m3 capacity plants, 500 kg of cement, 1200 kg of gravel, 1200 kg of sand, and 50 bricks weighing 25 kg are consumed. If bricks and cement are used for dome construction; 5 per cent lime 95 per cent clay is used for digester construction, then roughly 200 kg of cement, 300 kg of lime and 500 bricks are needed.
i. Hydraulic Testing for Water and Gas Tightness:
Before the plant is put to operation, it is normally advisable to carry out hydraulic testing under water pressure for maintaining the desired level of water and gas-tightness.
ii. Testing for Water Leakage:
For carrying out this test, digester is required to be half-filled with eater and water level noted each day for a period of 6 to 7 days. Significant drop in water level is an indication of water leakage.
iii. Testing for Gas Leakage:
For carrying out this test, all digester outlets are required to be closed to render it gas-tight. This is followed by complete filling in of digester with water to its full capacity and noting down water level each day for a period of 6 to 7 days. If there occurs more than 5 to 10 per cent drop in water level, it is an indication that gas dome, seating is not perfect and needs to be corrected.
Safety pressure gauge helps to detect gas leakage from digester, estimate the gas availability within the digester, and releases extra un-utilised gas automatically formed within the digester. The gauge is made of two 1-m long glass tubes (internal diameter- 0.5 cm) and one of rubber, in U-shape, and the glass tubes are filled with coloured water.
Tube A is connected with the gas pipe attached to the digester through rubber tube and tube B is connected to a 100-ml (or larger) bottle. A 2-holed rubber stopper is inserted in the mouth of the bottle, and two glass tubes, one short and one long are placed through the holes of the stopper, the short tube is connected to tube B by a rubber hose, and the bottle is inverted and attached near the safety gauge.
When the pressure on the water column is high from the biogas plant, the coloured water is forced into the bottle through tube C and any extra biogas is released through tube D. When the pressure inside the tank is reduced, the coloured water in the bottle flows back into the U-shaped tube through tube C. This mechanism enables the water in the gauge to adjust automatically to changes in the digester’s internal pressure.