Removal of water
Biogas must have a relative humidity of less than 60% to prevent the formation of condensate in the transporting pipelines. This condensate, particularly in combination with other impurities, could corrode the pipe walls. Often the biogas has to be dried before further cleaning, e.g., by means of downstream absorption plants.
In order to be allowed to feed biogas into the natural gas network, a fine drying process is categorically required according to many European regulations.
Biogas can be dried by compression and/or cooling of the gas, by adsorption at activated charcoal or silica-gel, or by absorption, mostly in glycol solutions.
After compression to pressures up to 12 bar, which is necessary for many biogas decontamination procedures, the biogas leaves predried, when the condensate is removed from the compressor.
By lowering the dewpoint to 5 °C, biogas with a relative humidity of 9 < 60% at normal temperature is obtained and corrosion can be prevented. In 60% of all agricultural biogas plants with power generators, the biogas is dehumidified by cooling it in a biogas pipe at least 50 m long enbedded in the soil. This pipe is inclined at ca. 1° to the horizontal and equipped with a condensate trap at its lowest point, this being protected against freezing and installed so as to be easily accessible.
Landfill gases are sometimes cooled down to 2°C or even to -18 °C by a refrigerating machine in order to lower the dewpoint to 0.5-1 °C. After separation of the condensate, the landfill gas is heated to ambient temperature.
For drying of biogas in an adsorption process, SiO 2 is mostly used, although activated charcoal or molecular sieves are used also. For continuous running, a minimum of two adsorbers are necessary. While one is being loaded at a pressure of 6-10 bar, the other is desorbed with hot air at 120-150°C. Vacuum or recirculation of a part of the still hot dry biogas also results in sufficient desorption.
For the absorption process, glycol or triethyleneglycol are appropriate as absorbents. They scrub out not only water vapor but also long- chain hydrocarbon gases out of the biogas. Two absorbers are normally used in industrial scale installations.
For fine drying, only adsorption onto activated charcoal is a suitable procedure. Activated charcoal separates not only water but also other trace impurities from the biogas.
Adsorption is preferably applied to small to medium flow rates of biogas (<100000m3h-1). Absorption is better suited to large flow rates. Both procedures are very costly and are only adopted when it is essential for the utilization of the biogas.
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