Biogas dehumidification is a key step in biogas purification. There are many ways to dehumidify biogas, including heating dehumidification, condensation dehumidification, adsorption dehumidification, absorption dehumidification and membrane dehumidification.
In the above methods, heating and dehumidification is to reduce the relative humidity by heating the gas, and control the temperature to achieve the control of the relative humidity. For the heating of biogas, the relative humidity of biogas decreases with the increase of temperature. Although this method can not really reduce the content of water vapor in biogas, that is, the moisture content of biogas is unchanged, it can effectively avoid the formation of liquid water.
Condensation dehumidification is to use refrigerant provided by refrigeration system to reduce the temperature of biogas below its dew point, so that the steam in biogas condenses out, and the condensation heat is taken away by refrigeration equipment, so as to reduce the moisture content of biogas. Cooling dehumidification is a method of removing water vapor from wet methane by cooling it below dew point temperature through low temperature surface. Because it is cooled by a refrigerator, it is also called freezing dehumidification. It is a commonly used dehumidification method at present. The energy consumption of cooling and dehumidification is high because of the need for reheating.
Adsorption dehumidification refers to the use of adsorbents (such as silica gel, aluminium gel, molecular sieve, etc.) to adsorb water molecules POLARLY and reduce the moisture content of biogas to a certain level. Adsorption is a complex material transfer process, which is divided into chemical adsorption and physical adsorption. Physical adsorption means that the adsorption force between adsorbed substances and adsorbents is generated by van der Waals force, and the molecules of adsorbed substances are adsorbed to the surface of adsorbents or between molecular lattices. No chemical reaction occurs during the adsorption process, and the adsorption is reversible. Chemical adsorption means that the adsorption force between adsorbed substances and adsorbents is generated by chemical bonding force, and then adsorbed. The chemical properties of adsorbents have changed and the adsorption is irreversible. Physical adsorption is the main adsorbent in adsorption dehumidification, which is suitable for biogas dehumidification with low relative humidity, and can make dew point below - 40 C. Therefore, this method is mostly used in dehumidification and drying treatment.
In practical application, fixed bed dehumidification is often used, that is, when wet methane flow is accumulated by solid adsorbent, water vapor is adsorbed on solid adsorbent by capillary action, water content in methane is reduced, so as to achieve the purpose of dehumidification. The humidity of biogas can be reduced very low by choosing proper adsorbent and operation process. The adsorbent is recycled after regeneration. This method is used in the case of low humidity and high dehumidification requirement. Solid adsorbent dehumidification is not affected by dew point temperature and achieves very low humidity. Under low temperature and humidity conditions, the dehumidification efficiency is significantly higher than other dehumidification methods, and it has simple structure, convenient maintenance and reliable operation.
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