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Types of domestic biogas digesters With the development of biogas technology in China and the popularization of rural domestic biogas, according to the local use requirements and temperature and geological conditions, domestic biogas digesters include hydraulic tanks with fixed arch cover, large hydraulic tanks without cover, hydraulic tanks with hanging pipes, hydraulic tanks with tortuous water distribution, hydraulic tanks with backwater at the top, separate floating covers, semi-plastic tanks, all-plastic tanks and tank tanks. Although there are various forms, they are generally composed of four basic types of biogas digesters: hydraulic biogas digesters, floating hood biogas digesters, semi-plastic biogas digesters and tank biogas digesters. The biogas digesters supporting the four-in-one ecological greenhouse model are generally hydraulic biogas digesters, which have several different forms. 2. 1 hydraulic biogas digester with fixed arch cover There are three types of hydraulic biogas digesters with fixed arch cover: cylindrical (see figure 2. 1), spherical (see figure 2.2) and ellipsoid (see figure 2.3). The upper air chamber of the tank body is completely closed, and with the continuous production of biogas, the biogas pressure increases accordingly. This increased air pressure forces part of the feed liquid in the biogas digester to enter the hydraulic chamber communicated with the tank body, so that the liquid level in the hydraulic chamber rises. In this way, there is a water level difference between the water pressure room and the biogas digester. This water level difference is called "water pressure" (that is, the value displayed by the U-tube biogas pressure gauge). When using gas, turn on the biogas switch, and the biogas will be discharged under water pressure; When the biogas decreases, the feed liquid in the water pressure chamber flows back to the tank, which makes the water level difference decrease continuously, resulting in a corresponding decrease in biogas pressure. This kind of biogas digester, which uses part of the feed liquid to reciprocate in series, causes repeated changes in water pressure to store and discharge biogas, is called hydraulic biogas digester Hydraulic biogas digester is the earliest and largest pool type in China, which is improved on the basis of summarizing the pool construction of "three combinations", "round and shallow", "movable cover", "straight pipe feeding" and "middle layer discharging". "Three-in-one" means that the toilet, pigsty and biogas digester are integrated, and human and animal feces can be directly cleaned and fermented in the biogas digester. "Round, small and shallow" means that the pool is round, small in size and shallow in depth. The "movable cover" is a movable cover plate added to the top of the biogas digester. Hydraulic biogas digester has the following advantages: (1) The structure of biogas digester has good mechanical properties, which makes full use of the bearing capacity of soil, so it saves labor and materials, and the cost is relatively low. (2) It is suitable for filling a variety of fermentation raw materials, especially a large number of crop straws, which is very beneficial to the accumulation of fertilizers in rural areas. (3) In order to facilitate frequent feeding, toilets and pigsty can be built on the biogas digester, and feces can be cleaned into the pool at any time. (4) The periphery of the biogas digester is in contact with the soil, which has a certain effect on the insulation of the biogas digester. Hydraulic biogas digesters also have some shortcomings, mainly: (1) Because the air pressure changes repeatedly, it generally changes between 4 ~ 16 kPa (that is, 40 ~ 160 cm Hg). This has an adverse effect on the stability and strength of the pool and the combustion efficiency of lamps and cookers. (2) Because there is no stirring device, the scum in the tank is easy to crust and not easy to break, so the utilization rate of fermentation raw materials is not high, and the gas production rate per cubic meter of tank volume (that is, the gas production rate day and night) is low, and the general gas production rate is only about 0. 1.5m 3/m3/ day. (3) Because the diameter of the movable cover can not be increased, it is difficult to discharge the biogas digester with straw as the main fermentation raw material. Therefore, it is best to use unloading machinery when unloading. 2.2 The reformed hydraulic biogas digester (1) is a central hanging tube biogas digester (see Figure 2.4). The movable cover is changed into a steel mesh cement inlet and outlet hanging pipe, which has the function of one pipe with three functions (instead of the feed pipe, the discharge pipe and the movable cover), which simplifies the structure and reduces the construction cost of the biogas digester. Moreover, the arch cover of biogas digester is conducive to the improvement of its air tightness because the feed liquid is often in a wet state. But also convenient for discharging and manual stirring. However, fresh raw materials are often mixed with fermented old liquid, and the utilization rate of raw materials decreases. 2.3 Hydraulic biogas digester with bottom discharge without movable cover The hydraulic biogas digester with bottom discharge without movable cover is a variant of hydraulic biogas digester. This pool type cancels the movable cover of the hydraulic biogas digester, seals the arched cover of the biogas digester, and only leaves the air duct, which increases the volume of the hydraulic chamber and can avoid the problems caused by the loose sealing of the movable cover of the biogas digester. In the northern rural areas of China, the new biogas digesters supporting the "model" advocate this pool type. This book will introduce the construction technology of this biogas digester. Fig. 2. 10 shows the structure of bottom-drain hydraulic biogas digester without movable cover. The biogas digester is cylindrical with an inclined bottom. It consists of a fermentation chamber, an air storage chamber, a feed inlet, a discharge outlet, a water pressure chamber, an air duct and the like. (1) The feed inlet and feed pipe are located on the ground and underground of the pigsty respectively. Toilets, pig houses and collected human and animal excrement are injected into the fermentation chamber of biogas digester from the feed inlet through the feed pipe. (2) The discharge port and the water pressure room are arranged in the sunlight greenhouse connected with the pool body. Its purpose is to facilitate the application of biogas fertilizer in vegetable production, and at the same time, the discharge port releases carbon dioxide to the solar greenhouse at any time to promote the growth of vegetables. The lower end of the water pressure chamber is communicated with the fermentation chamber through a discharge channel. A cover plate should be set at the discharge port to prevent people and animals from entering the pool by mistake. (3) The bottom of the pool is pot-bottom. There is a U-shaped groove between the center of the pool bottom and the bottom of the hydraulic chamber, with a downward slope of 5%, which is convenient for bottom drainage. (4) Working principle ① When no gas is generated, the feed liquid in the feed pipe, fermentation chamber and hydraulic chamber is on the same horizontal plane. (2) When producing gas, the biogas produced by microbial fermentation and decomposition rises to the gas storage chamber. Because the gas storage chamber is airtight, methane accumulates continuously, resulting in pressure. When the biogas pressure exceeds atmospheric pressure, the materials in the biogas digester are pumped out, the water level in the feed pipe and the hydraulic chamber rises, and the water pressure in the fermentation chamber drops, resulting in a water level difference. Due to the water pressure, the biogas in the gas storage chamber keeps a certain pressure. (3) When gas is used, biogas is output from the air duct, and the water in the hydraulic chamber flows back to the fermentation chamber, that is, the water level in the hydraulic chamber drops and the water level in the fermentation chamber rises. Depending on the automatic rise and fall of the water level in the water pressure room, the biogas pressure in the gas storage room can be automatically adjusted to keep the firepower of the combustion equipment stable. (4) When the gas production is too small, if the biogas produced in the fermentation room can't keep up with the gas demand, the water level in the fermentation room will gradually be equal to that in the water pressure room, and finally the pressure difference will disappear and the biogas output will stop. 3. 1 design principle of biogas digester to build a biogas digester on the "model", we must first design it well. Summarizing the verification of scientific experiments and production practice for many years, the following principles must be adhered to in designing biogas digesters that match the model: (1) The principle of "four combinations" must be adhered to. "Four combinations" means that the biogas digester is connected with the barn, toilet and solar greenhouse, so that human and animal manure can continuously enter the biogas digester, ensuring normal and continuous gas production, facilitating manure management and improving environmental sanitation, and biogas slurry can be conveniently transported to vegetable fields in solar greenhouse for production. (2) Adhere to the principle of "round, small and shallow". "Round, small and shallow" means that the tank is mainly cylindrical, with a tank capacity of 6 ~ 12 cubic meters and a tank depth of about 2 meters. Circular biogas digesters have the following advantages: First, according to the geometric principle, the surface area of biogas digesters with the same volume is smaller than that of squares or rectangles, which saves materials. Second, airtight and firm. The internal structure of the circular tank is reasonable, and the tank wall has no right angle, which is easy to solve the sealing problem, and the stress around it is uniform and the tank body is firm. Thirdly, the temperature in northern China is low, and the circular pool is placed underground, which is conducive to thermal insulation and safe wintering in winter. Fourth, it is suitable for promotion. Whether in the south or the north, the construction of circular biogas digesters is conducive to ensuring the quality of pool construction, so as to build one, succeed one, use one, consolidate one and actively and steadily promote it. Small means that the volume of the main fuel tank cannot be too large. Shallow, in order to reduce the digging depth and avoid groundwater, at the same time, the surface area of fermentation broth is relatively enlarged, which is beneficial to the generation and discharge of gas. (3) Adhere to the principle of straight feeding, adding grates at the entrance and stamping at the exit. The purpose of straight tube feeding is to feed smoothly and facilitate mixing. Grate is added at the feed inlet to prevent pigs from falling into the feed pipe of biogas digester. The purpose of capping the discharge port is to keep the environment clean, eliminate the breeding grounds of mosquitoes and flies, and prevent people and animals from falling into the pool. 3.2 The design of biogas digesters is based on the design of biogas digesters that match the "model". The following factors must be considered when formulating the construction scheme of biogas digesters: (1) The soil for building biogas digesters should be considered in the selection of foundation. The selection of foundation is very important, which is related to the quality and life of biogas digesters and must be taken seriously. Because biogas digesters are underground buildings, it has a great relationship with the quality of soil. Different soils have different densities and solidity, so the allowable bearing capacity is different. And in the same place, the soil layer is different. If the soil layer is soft, sandy soil or decomposed soil with high groundwater level, the bearing capacity of the pool foundation is not large, and it is unbearable to build a pool here, which will inevitably lead to the settlement or uneven settlement of the pool body, resulting in the rupture and leakage of the pool body. Generally speaking, the allowable bearing capacity of natural soil layer is greater than 10 ton/m2, so it is no problem to build biogas digesters on such natural soil layer. Therefore, the foundation of the pond should be chosen where the soil is solid, the groundwater level is low, and there are no hidden dangers such as tunnels, cellars, seepage wells, springs and virtual soil at the bottom of the soil layer; Moreover, there should be a certain distance between the pond and the trees, bamboo forests or ponds to prevent the roots and bamboo roots from penetrating into the pond or affecting the pond body when the pond rises, resulting in water leakage and air leakage; In the arid area in the north, it should also be considered that the pool should be close to the water source and users. If the pool is far away from users, it is not only inconvenient to manage (such as adding water and feeding), but also the pipeline for transporting biogas is very long, which affects the pressure of biogas and has poor combustion effect. In addition, as far as possible, choose the leeward and sunny place to build the pool. (2) The design of the pool should consider the load. Determining the load is a very important link in the design of biogas digesters. The so-called load refers to the weight per unit area. If the load is determined to be too large, the structural section of the designed biogas digester is too large, resulting in too much material and waste; If it is determined that the load is too small and the design strength is insufficient, it is easy to cause the tank to rupture. Generally, the calculation standard of load is: the dead weight of the tank (calculated by the amount of concrete) is about 2.5 tons/m3, the vault is about 2 tons/m3, the volume of fermentation raw materials in the tank is about 1.2 tons/m3, and the pressure in the tank after biogas production is about 1 ton. In addition, the maximum pressure of people and animals that often appear at the top of the pool is about 1 ton. Therefore, the foundation and bearing capacity should not be less than 8 tons per square meter. (3) When designing biogas digesters, we should consider the rise-span ratio of the vault and the quality of the pool wall to build biogas digesters. Generally, brittle materials are used, which have good compressive properties and poor tensile properties. According to the calculation of the internal force of the truncated arch cover, when the rise-span ratio of the pool cover is 1∶5.35, it is the dividing line between the circumferential internal force and the tensile force of the pool cover. If it is larger than this dividing line, the pool cover may be broken if steel bars are not installed. Therefore, the ratio of rise to span (that is, the ratio of rise to diameter) is designed when cutting the arch cover of spherical pool. Ascension refers to the vertical distance from the arch foot to the vault) is generally between 1:4 ~ 1:6; The rise-span ratio is about 1∶8 (the specific ratio should also be determined according to the pool size, arch cover span and construction conditions, etc.). ). Pay attention to build by laying bricks or stones after the arch cover is built, and the pedal should be firm, so that it can bear the horizontal thrust of the self-weight of the arch cover, soil covering and other loads (such as barns and toilets, etc.). Generally speaking, a concrete arch cover with a diameter of 5m, a rise-span ratio of 1:5 and a thickness of 10cm has a maximum tensile force of about 65438. Secondly, the quality of the pool wall must be firm. The width of pool wall foundation (ring foundation) shall not be less than 40 cm (this is the smallest dimension in engineering structure), and the thickness of foundation shall not be less than 25 cm. Generally, the width-thickness ratio of foundation should be in the range of 1: (1.5 ~ 2). 3.3 Calculation of the volume of biogas digesters To build a biogas digester, the volume of the pool should be calculated in advance, that is, it is best to plan to build a large pool. In principle, the calculation amount should be determined according to the purpose and dosage. If the pool is too small, the gas production will be less, which can not meet the needs of production and life; If the pool is too large, the gas production rate is often low due to insufficient fermentation raw materials or management errors. At present, the biogas production rate of rural biogas digesters in China is generally unstable. The gas output per cubic meter of pool capacity can be about 0. 15 cubic meter in summer and 0. 1 cubic meter in winter. In general, a family of five in rural areas needs about 1.5 cubic meters of gas for cooking and boiling every day (each person actually needs about 0.2 cubic meters of gas for daily life, and the maximum is not more than 0.3 cubic meters). At the same time, production fertilizer should be considered. Therefore, it is more appropriate to build ponds in rural areas according to the average effective volume per person 1.5 ~ 2 cubic meters (effective volume generally refers to the total volume of fermentation rooms and gas storage boxes). According to this standard, the average effective volume of a pool is less for families with large population and more for families with small population; Generally, the temperature in the north is lower, but a little more, and the temperature in the south is higher, but a little less. For example, if a family of five builds a biogas digester with an area of 8 ~ 10 cubic meter, the biogas generated can basically meet the needs of a family to cook, boil water or light a lamp all year round. Even if the temperature is low in winter in the north and the gas production is reduced, it can still be used to cook two meals or boil water. Therefore, the average family raises 6 ~ 10 pigs, and the surface area of solar greenhouse is 100 ~ 150 m2, so it is appropriate to build 6, 8 and 10 m3 biogas digesters. For the four-in-one ecological greenhouse model built in the field, the general "model" area is about 667 square meters (1 mu). At this time, the number of pigs can be increased, and the capacity of biogas digesters is better, but this is not absolute, because if the capacity of biogas digesters can not meet the demand for cooking energy and fertilizer in vegetable production, energy and fertilizer can be supplemented from outside to make the "model" work normally. 3.4 About solar biogas digesters Solar biogas digesters mainly rely on collecting the heat of sunlight to improve the fermentation temperature of biogas digesters, so as to better realize gas production. A solar biogas digester with condensing convex lens is collected here, patent number: 20082005275 1.8, which is a new type of solar biogas digester. It comprises a fermented aggregate box, a compound convex lens, a protective cover, a solar heat collecting plate, a heat preservation container, an electrothermal converter, a temperature sensor, a heat preservation controller box, a rapid fermented aggregate box and a support, wherein the compound convex lens is composed of a plurality of convex lenses with curved surfaces as bottom surfaces, the focus of light gathered by the plurality of convex lenses on the compound convex lens is on the solar heat collecting plate, the solar heat collecting plate is located at the top of the heat preservation container, the heat preservation container is installed at the upper part of the rapid fermented aggregate box, and the rapid fermented aggregate box is provided with 6544. The utility model can gather solar energy heat in the center of the biogas digester, and provide and control the required or optimal survival temperature or reproduction temperature of methanogenic bacteria. And properly classify and treat the gas-producing raw materials to ensure the full utilization of organic waste and biogas digesters.