There are three types of hydraulic biogas digesters with fixed arch cover: cylindrical, spherical and ellipsoidal. 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 liquid level in the water pressure room and the liquid level in the biogas digester, which 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.

Improved hydraulic biogas digester

(1) The movable cover of the central hanging tube biogas digester is changed into a steel wire mesh cement feeding and discharging hanging tube, which makes it have the function of one tube with three functions (instead of the feeding tube, the discharging tube and the movable cover), which simplifies the structure, reduces the construction cost of the biogas digester, and makes the arch cover of the biogas digester always wet because of the feed liquid, which is beneficial to the improvement of its air tightness. 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.

Hydraulic biogas digester with bottom discharge and no movable cover

Hydraulic biogas digester with bottom discharge and no 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. The structure of bottom-draining hydraulic biogas digester without movable cover is shown in the figure. 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.