The purpose of biogas desulfurization is to avoid hydrogen sulfide corrosion of equipment, hydrogen sulfide poisoning, and to prevent hydrogen sulfide from being oxidized into sulfur dioxide or sulfur trioxides when biogas is burned. The removal method is as follows:
Sulfur in biogas can be removed by microorganisms. Most of the sulfur oxidizing bacteria belong to the genus Thiobacillus, and most of them are autotrophic, that is, they can use carbon dioxide in biogas to meet their c nutrition needs, the main product is elemental sulfur, but also part of sulfate, which will cause corrosion when sulfuric acid is formed in the solution. According to different content of hydrogen sulfide in biogas, 2% - 6% air can be introduced into biogas to meet the needs of biooxidation of sulfide.
The most direct and simple method is to directly inject a certain amount of oxygen or air into the anaerobic digestion tank or air storage tank and keep it for a certain period of time. Because Thiobacillus can be seen everywhere, it does not need to be inoculated. The surface of digestive products can provide them with a micro aerobic environment and necessary nutrients for their growth, and will form a layer of yellow sulfur on the colony. Appropriate temperature, reaction time and air quantity can reduce hydrogen sulfide to 50ppm.
For different methane content, the explosion range of biogas in the air is 6% - 12%. Therefore, certain safety measures must be taken to avoid excessive air in the biogas.
In the production of biogas in large-scale anaerobic digesters, water washing and biological desulfurization are often combined to remove hydrogen sulfide. Waste water or supernatant in digestion tank can be used to enter from the top of the filter bed, biogas from the bottom, and 4% - 6% air can be introduced into the biogas before entering the filter bed. The filter bed provides a sufficient contact surface for the water to absorb hydrogen sulfide and the growth of desulfurization microorganisms. In Denmark, several industrial wastewater treatment plants and many farms are using this process to purify biogas.
Process of adding ferric chloride to digested sludge
Adding ferric chloride directly into the digestion sludge, ferric chloride will react with hydrogen sulfide to form iron sulfide particles. This method can greatly reduce the production of hydrogen sulfide, but it can not be reduced to the level required by natural gas or automobile fuel, and further treatment is needed. The investment cost of this removal process is less. Only a tank and a quantitative pump are needed. The main cost is produced by ferric chloride.
Iron oxide absorption process
Hydrogen sulfide is easy to react with iron hydroxide and iron oxide to form iron sulfide, which is a slight endothermic reaction. The minimum temperature is required to be 12 ℃, and the optimum reaction temperature requires a certain amount of water, so biogas cannot be too dry, but condensate should be avoided to make the spherical iron chloride stick together and reduce the reaction surface.
The iron sulfide produced can be oxidized and regenerated by air to form iron oxide or iron hydroxide and sulfur. During the regeneration process, a lot of heat will be released, so spontaneous combustion often occurs. Since the surface of iron oxide will be covered with a layer of sulfur, it is necessary to replace iron oxide and iron hydroxide after repeated use for many times. Generally, one unit is equipped with two reaction beds. When one is in the process of desulfurization, the other can be regenerated.
Absorption process of iron oxide wood chips
Covering a layer of iron oxide on the wood chip has a larger specific surface area and a lower density than the same amount of iron oxide. About 100g of iron oxide wood chip can absorb 20g of hydrogen sulfide. This method is very popular in the United States, the price is relatively low, but it must be noted that the temperature of regeneration iron oxide filter cannot be too high.
Red mud particle absorption process
Red mud, a waste produced in alumina production, can be made into granules to absorb hydrogen sulfide. It has a larger specific surface area, but its density is much larger than that of iron oxide wood chips. This method is used in most sewage treatment plants in Germany and Switzerland.
Activated carbon adsorption process
In PSA system, hydrogen sulfide can be removed by activated carbon soaked in potassium iodide. Hydrogen sulfide is converted into elemental sulfur and water, and sulfur is absorbed by activated carbon. The optimum conditions for this reaction are: pressure 7-8 × 105Pa, temperature 50 ℃ - 70 ℃. In the process of compressed gas, it is easy to make the temperature above 50 ℃. Generally, the gas residence time is 4-8 hours. Under the condition of continuous operation, the system shall include two adsorption devices. If the concentration of hydrogen sulfide is above 3ppm, regeneration is required.
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