The purpose of a biological desulfurization system is to reduce the hydrogen sulfide content in biogas. Hydrogen sulfide is converted into sulfur dioxide during combustion, causing environmental pollution. The air pollution control standards clearly specify the limit content value of pollutants emitted into the atmosphere: hydrogen sulfide 4mg/m?, Sulfur dioxide 15mg/m? Generally speaking, when emitting 1500ppm of hydrogen sulfide, it is equivalent to producing 4250mg/m according to the German TA Luft standard? Sulfur dioxide should be desulfurized first before biogas energy utilization.
Generally, conventional biogas boilers and biogas generators require an inlet H2S concentration of 200ppm (or 300mg/m?). The function of a biological desulfurization system is to reduce the hydrogen sulfide content in biogas to 300mg/m? (200ppm) to avoid the corrosive effect of hydrogen sulfide on subsequent biogas utilization equipment (such as generators, boilers, etc.). The relative humidity of the purified biogas through the desulfurization tower is 100% (absolute humidity is about 40g water/m?), and the temperature ranges from 29 to 38 ℃. Therefore, for the biogas after biological desulfurization, appropriate condensation should be considered to collect and discharge condensate water, or separate cold drying and pressurization treatment should be set up.
A major characteristic of desulfurization systems is their specificity - that is, they only remove hydrogen sulfide from biogas without removing other components. During the operation of biological desulfurization, the carbon dioxide content in biogas also decreases as a result. The improvement of biogas quality leads to a corresponding reduction in carbon dioxide emissions into the atmosphere. During the desulfurization process, hydrogen sulfide is separated by utilizing the differences in solubility and degradability of different gas components. For example, methane, as the main component of biogas, has a Henry's coefficient (solubility of gas in water) 80 times lower than hydrogen sulfide at 25 ℃.
Desulfurization bacteria use carbon dioxide as a carbon source, which leads to an appropriate reduction in the carbon dioxide content in biogas. However, during the desulfurization process, oxygen needs to be supplemented, and the methane in biogas decreases with the addition of nitrogen in the air. A rough estimate can be made based on a 10% decrease in methane concentration and a 10% increase in total gas volume.
The working mechanism of biological desulfurization is described as follows: a certain amount of air is introduced into biogas containing hydrogen sulfide, and a large amount of biological fillers are installed in the reactor, providing sufficient space for bacterial reproduction. The tower body adopts a drip filtration form, and the system water and nutrient solution (NPK) continuously circulate for drip filtration, keeping the filling material moist and supplementing the nutrients required for bacterial growth and reproduction. Exclusive sulfur bacteria and sulfur bacteria absorb hydrogen sulfide during metabolism, convert it into elemental sulfur, and further oxidize it to sulfuric acid.
The chemical reaction formula is as follows:
H2S + 2O2 → H2SO4
H2S+2O2 → H2SO4
2 H2S + O2 → 2 S + 2 H2O
2 H2S+O2 → 2 S+2 H2O
S + H2O + 1.5 O2 → H2SO4
S+H2O+1.5 O2 → H2SO4
The dilute sulfuric acid generated by the reaction is buffered and neutralized by the nutrient solution, and is discharged from the system together with the nutrient solution. This process repeats itself.