Biogas Plant Construction Manual
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A biogas plant is an anaerobic digester of organic material for the purposes of treating waste and concurrently generating biogas fuel. The treated waste is a nutrient-rich, nitrogen-rich fertilizer while the biogas is mostly methane gas with inert gases including carbon dioxide and nitrogen. Biogas plants are a preferred alternative to burning dried animal dung as a fuel and can be used for the treatment of human waste. Other feedstock which can be used includes plant material, non-meat or grease food-wastes, and most types of animal dung. Over a million biogas plants have been constructed in the developing world for treatment of organic wastes, alternative energy supply to direct burning in the home, and overall improvement of human health and the environment. Many factors for selection of feedstock and site location must be researched before deciding to install a biogas plant.
Successful construction of the biogas plant requires a proper design and adherence to follow correct construction methods. The success or failure of any biogas plant primarily depends upon the quality of construction work. The following instructions are based on the step-by-step instructions from the Government of Nepal Biogas Support Program Gobar Gas and Agricultural Equipment Development Company of Nepal has developed the design for model 2047 biogas plant. This biogas plant model has become prolific across Asia and is known as a fixed-dome plant. The advantages of the fixed dome plant include the simplicity of design, few moving parts, low cost to construct and low maintenance. The disadvantages when compared to a floating-dome digester are primarily the inability to store gas for use on demand; gas from the fixed dome digester must be used as generated or expelled to avoid damaging the digester.
Determining Plant Size
This manual includes design and construction material quantities for the Gobar biogas plant models of 4, 6, 8, 10, 15 and 20 cubic meters capacity. Design and size of a plant other than mentioned above is feasible and a skilled engineer should be consulted for deviations from the provided designs. The biogas plant size is dependent on the average daily feed stock and expected hydraulic retention time of the material in the biogas system. Generally, 24 kilograms of feedstock complimented with 24 liters of water per day with a hydraulic retention time of 35 days will require a 4-cubic meter plant. Table 2.1 below gives some relevant data about the six different sizes of biogas plants presented in this manual.
Selection of Construction Materials
If the materials used in the plant construction such as cement, sand, aggregate etc. are not of good quality, the quality of the plant will be poor even if the design and workmanship are excellent. A brief description regarding the specifications for some of the construction materials is provided below to assist with selection of the best quality materials. The list of construction materials is given in Table 3.1.
The cement to use in the plant construction must be of high quality Portland cement from a brand with a known reputation. It must be fresh, without lumps and stored in a dry place. Bags of cement should never be stacked directly on the floor or against the walls to protect the cement from absorbing moisture before use.
Sand for construction purpose must be clean. Dirty sand has a very negative effect on the strength of the structure. If the sand contains 3% or more impurities by volume, it must be washed. The quantity of impurities especially mud in the sand can be determined by a simple test using a bottle and clean water. For the test, the bottle is half-filled with sand, filled with clean water, and then stirred vigorously. Allow the bottle to sit stationary to allow the sand to settle. The particles of sand will settle first while mud particles will settle last. After 20-25 minutes, compare the thickness of the mud layer to the sand inside the bottle are; the percent of mud should be less than 3% of the overall volume. Course and granular sand can be used for concrete work however fine sand is necessary for plastering work.
Gravel size should not be too big or too small. Individual gravel diameter should not be greater than 25% of the thickness of concrete product where it is used. As the slabs and the top of the dome are not greater than 8 cm (3") thick, gravel should not be larger than 2 cm (0.75") in size. Furthermore, the gravel must be clean. If it is dirty, it should be washed with clean water.
Water is mainly used for preparing the mortar for masonry, concrete and plastering work. It is also used to soak bricks/stones before using them. Water is also used for washing sand and aggregates. It is advised not to use water from ponds and irrigation canals for these purposes, as it is usually too dirty. Dirty water has an adverse effect the strength of the structure; hence, water to be used must be clean.
Construction of Inlet Tank
The Inlet Tank is constructed to mix feedstock and water. This can be constructed with or without a mixing device. Installation of a mixing device is preferable because not only it makes plant operation easier but it also improves the quality of mix. When a mixer is installed, it has to be firmly attached to the structure, easy to operate, effective in the mixing process and the steel parts in contact with the feedstock should be galvanized. The top of the structure should not be more than one meter above ground level and both inside and outside of the tank must be covered with a smooth layer of plaster (Mix: 1:3 cement, sand). The finished bottom of the Inlet Tank must be at least 5 cm above the Outlet Chamber overflow level. The position of the inlet pipe must be such that a pole or rod can be inserted through it to the digester vessel without obstructions. This will allow the operator to clear blockages in the inlet pipe. For the same reason the inlet pipe must be without bends. Even if a mixing device is not installed, the inlet pit should be round in shape as this is a more economical use of material and easier for hand mixing.
Lay-out of Pipeline
The gas pipe conveying the gas from the plant to point of user is vulnerable to damages by people, domestic animals and rodents. Therefore, only light quality galvanized iron pipe should be used which must be, where possible, buried 30 cm (1 foot) below ground level. Fittings in the pipeline must be sealed with zinc putty, Teflon tape or jute and paint. Any other sealing agent, like grease, paint only, soap etc. must not be used. The use of fittings, especially unions, should be kept to a minimum to reduce the risk of leakage. No fittings should be placed between the main gas valve and the dome gas pipe. The pipe size, inside diameter should be between .6 and 1 Cm. Pipe size is determined by the size of the digester, (amount of gas produced) and amount of gas required in the house. (Are the stove, heater lights going to be used simultaneously?)