Generation of heat and power from biogas for stationary applications: boilers, gas engines and turbines, combined heat and power (CHP) plants and fuel cells - The Biogas: Science, Production and Applications - page 403

most of the CO 2 has to be removed. This process is normally designated as
'biogas upgrading to biomethane'. Different biogas upgrading technologies
for CO 2 removal include absorption in water, physical and chemical
absorption, adsorption with pressure, membrane and cryogenic processes
(see Chapter 15).

Water

Biogas is normally saturated with water vapour as the AD process generally
takes place at 35

C and under wet conditions with feedstocks
containing more than 70% moisture. Besides reducing the calorific value,
high moisture content in biogas can cause problems as it condensates in gas
pipelines, engines, etc. In addition, the condensed water can react, for
example with H 2 S to form ionic hydrogen and/or H 2 SO 4 , which can cause
severe steel corrosion. Often, the biogas has to be dried before further
cleaning, by means of downstream absorption plants for example. By
lowering the dew point to 5

8 Cor55 8

C, biogas with a relative humidity of less than
60% at normal temperature is obtained and corrosion can be prevented
(Deublein and Steinhauser, 2008).
Water vapour in biogas can be removed through suitable pipework or
freeze drying to condense and remove as water. Biogas piping (minimum of
50m length) is laid with a minimum 1% slope (10.4mm/m) toward the
collection point and the gas flow rate should not exceed 3.7m 3 /s through the
pipe (Vandeweyer et al., 2008). The dryer is typically made of stainless steel
or other corrosion-resistant material to minimise corrosion from H 2 S.
Corrosion from condensing acid can also be minimised by removing H 2 S
from the biogas prior to drying. Alternatively, water vapour can be removed
by adsorption on silica gel and Al 2 O 3 at low dew points or by adsorption
with glycol and hygroscopic salts at elevated temperatures and by using
molecular sieves.

8

￿ ￿ ￿ ￿ ￿ ￿

Halogenated compounds

Halogenated compounds are substances containing one or more halogen
atoms (fluorine, chlorine, bromine or iodine). Examples of such compounds
in biogas are carbon tetrachloride, chlorobenzene, chloroform and
trifluoromethane. These are often present in landfill gas, but rarely in
biogases from the digestion of sewage sludge or organic waste. Halogens are
oxidised during the combustion process. The combustion products are
corrosive, especially in the presence of water, and can cause corrosion in
downstream pipes and applications. They can also initiate the formation of
dioxines and furans (polychlorinated dibenzo-p-dioxins and polychlorinated
dibenzofurans) if the combustion conditions (temperature and time) are