Because the solubility of carbon dioxide and hydrogen sulfide in water is greater than that of methane, water washing can not only remove carbon dioxide, but also remove hydrogen sulfide. This absorption process is a pure physical reaction. Generally, biogas enters from the bottom of the absorption column after compression, and water enters from the top for reverse flow absorption. The water absorbing carbon dioxide and hydrogen sulfide can be recycled and recycled. It can be regenerated by decompression or air absorption in the absorption column. When the concentration of hydrogen sulfide in the water is relatively high, air blowing is generally not recommended, because the water is quickly polluted by sulfur. If there is waste water available, regeneration of water is not recommended.
Polyethylene glycol washing process
Polyethylene glycol (selexol) washing is a physical absorption process just like water washing. The main component is dimethyl polyethylene glycol. As in water, the solubility of carbon dioxide and hydrogen sulfide in selexol is higher than that of methane. The difference is that the amount of selexol needed is less than that of water, which is more economical and energy-saving. In addition, water and halogenated hydrocarbon (components of landfill gas) can also be removed by selexol washing. Selexol can be regenerated and reused, and water vapor or inert gas (purified biogas and natural gas) can be used to blow off elemental sulfur in selexol, but air is not recommended.
Membrane separation process
There are two main methods of membrane separation: one is the high-pressure gas separation with gas phase on both sides of the membrane; the other is the low-pressure gas-liquid absorption separation of molecules passing through the membrane by liquid absorption diffusion.
1) High pressure gas phase separation
The biogas compressed to 36 × 105Pa first passes through the activated carbon bed to remove the halogenated hydrocarbon and hydrogen sulfide, and then flows into the filter bed and heater. The membrane is made of cellulose acetate, which can be used to separate polar molecules such as carbon dioxide, water vapor and residual hydrogen sulfide. It has certain selectivity, that is, it absorbs hydrogen sulfide and carbon dioxide in different areas, but it can not separate nitrogen in methane. Experience has shown that the membrane can last for three years, after one and a half years of use, the permeability of the membrane will be reduced by 30% due to shrinkage.
2) Gas liquid absorption membrane separation
The gas-liquid absorption membrane technology has been used in biogas purification in recent years. Its essence is that hydrogen sulfide and carbon dioxide molecules in biogas are absorbed and removed in the liquid phase through a porous sleeping membrane. The hydrogen sulfide and carbon dioxide molecules in the gas entering from the bottom can diffuse through the membrane, and then be absorbed by the liquid flowing in the opposite direction. The absorption membrane works at a standard atmospheric pressure. From 25 ℃ to 35 ℃, the concentration of hydrogen sulfide in biogas can be effectively reduced from 2% to less than 250PPM, and NaOH solution can be used as absorbent in liquid phase. The NaOH solution absorbed hydrogen sulfide can be used in water treatment to remove heavy metals. Carbon dioxide can be removed by amine solution as liquid phase. Amine solution can be regenerated by heating. The pure carbon dioxide released can be used in industry.