(1) H2O removal
H2O removal is due to the fact that H2S will dissolve and corrode the pipeline if water accumulates in the air duct. In addition, when methane is stored under pressure, the water in the methane will condense and freeze the air storage tank. The form of moisture contained in biogas from fermentation unit is saturated steam, which is generally removed by cold separation method. The temperature of the mixed gas is changed by adjusting the pressure, so that water vapor is condensed into liquid water from a gaseous state and then removed from biogas. This method is economical and simple, and is adopted by most biogas projects.
There are two ways of methane cooling: natural cooling and mechanical dehydration. The cooling temperature should also consider the water content requirements of different desulfurizers in the next desulfurization process, and appropriate preliminary cooling should be carried out according to the reasonable water content of the desulfurizer.
In the process of biogas transportation, there is still some water to be separated out. In order to prevent the precipitated water from corroding or blocking the pipeline, a water collector is often installed at the lowest part of the pipeline, and the water in the water collector is regularly drained.
(2) H2S removal
H2S removal is to avoid H2S corrosion of equipment, H2S poisoning and greater harm if H2S is oxidized to SO2 or SO3 during biogas combustion.
The eight H2S removal methods summarized can be divided into physical purification, chemical purification and biological absorption. Now, the methods commonly used in the three mechanisms are analyzed as follows.
(1) activated carbon adsorption process. H2S can be removed by activated carbon soaked with KI in pressure swing adsorption system. In this process, H2S is converted into elemental S and H2O, and elemental S is absorbed by activated carbon. Under the condition of continuous operation, the system should include two adsorption devices. If the concentration of H2S on activated carbon exceeds 3ppm, regeneration is required.
(2) iron oxide absorption process. The H2S in biogas is absorbed by ferric oxide. The desulfurization principle of this process is as follows:
Fe2O3? H2O+H2→Fe2S3? H2O+3H2O
(1) this reaction is exothermic, the minimum temperature is 12℃, and the optimal reaction temperature is 25℃ ~ 50℃. Therefore, in actual engineering operation, heating is required to reach the optimal reaction temperature. In addition, H2S removal is carried out by dissociating into H+ and S2+ in the water film on the surface of active iron oxide, and then carrying out chemical reaction. Because this reaction requires a certain amount of water, the moisture content of the desulfurizing agent is generally controlled at 10% ~ 15%. Therefore, the temperature of biogas in the previous dehydration process cannot be too low and the moisture cannot be removed too much, but the condensed water generated should also be avoided to make spherical iron oxide stick together, reducing the reaction surface and thus affecting the desulfurization effect.
Iron sulfide produced in the desulfurization process of this method can be oxidized and regenerated by air to produce iron oxide and elemental S .. The reaction principle of this process is as follows.
(2) As can be seen from the reaction formula, a large amount of heat will be released during regeneration, so spontaneous combustion often occurs. In order to prevent the desulfurizer from spontaneous combustion, it is generally necessary to spray a small amount of water on the discharged waste desulfurizer. In addition, after repeated use for many times, the surface of iron oxide will be covered with a layer of elemental sulfur, so it is necessary to replace iron oxide. Usually one unit is equipped with two reaction beds, one for desulfurization and the other for regeneration.
In the current biogas project of livestock and poultry farms, iron oxide absorption process is called dry desulfurization. In addition, there is a common wet desulfurization method. The desulfurizer for wet desulfurization is mostly sodium carbonate solution with concentration of 2% ~ 3%. The desulfurizer of both methods can be regenerated.
(3) biological desulfurization method. The method uses colorless sulfur bacteria, such as thiobacillus thiooxidans and thiobacillus ferrooxidans, to oxidize H2S into elemental S or H2SO3 under certain temperature, humidity and micro-oxygen conditions. The desulfurization mechanism is as follows: ①H2S gas dissolves, i.e. changes from gas phase to liquid phase; (2) Dissolved H2S is absorbed by microorganisms and transferred to microorganisms; (3) H2S entering microbial cells is decomposed, transformed and utilized by microorganisms as nutrients, thus achieving the purpose of H2S removal. The reaction equation is as follows:
Most sulfur oxidizing bacteria belong to the genus Thiobacillus. Thiobacillus can be found everywhere, and inoculation is not required for H2S removal. In addition, most thiobacilli are autotrophic, and the surface of anaerobic digests can provide them with a micro aerobic environment and necessary nutrition for growth. Therefore, it is simple to apply biological desulfurization by directly introducing a certain amount of O2 or air into the anaerobic digestion tank or gas storage tank and keeping it for a certain period of time. Proper temperature, reaction time and air volume can reduce H2S content in biogas to 50ppm.
In actual application, the amount of O2 or air introduced into the biogas is determined according to the H2S content in the biogas, which is generally 2% ~ 6% air. For methane with different methane content, the explosion range in the air is different, so certain safety measures must be taken to prevent methane from introducing excessive air to cause explosion. At the same time, biological desulfurization also has the disadvantages that the process is not easy to control, the removed elemental S is easy to block the filler and damage the aeration, etc.