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 404

favourable (Allen et al., 1997). The concentration of total chloride and total
fluoride in biogas can range from 118 to 735mg/m 3 and 25.9 to 256mg/m 3
respectively (Allen et al., 1997; Jaffrin et al., 2003).

Siloxanes

Siloxanes are volatile silicones bonded by organic radicals and are generally
present in landfill and sewage gas (see Dewil et al., 2006). Siloxanes
(C 2 H 6 OSi) mainly originate from silicon-containing compounds widely used
in various industrial materials or consumer products such as shampoo,
detergents and cosmetics. During biogas combustion, siloxanes are
converted to microcrystalline silicon dioxide (SiO 2 ), a residue with chemical
and physical properties similar to glass. These crystalline deposits have a
surface thickness of several millimeters. Extensive engine erosion or
blockage can occur due to the deposition of siloxanes on engine valves,
cylinder walls and liners. In gas engines, siloxane deposits usually form on
nozzles and blades, causing erosion of the turbine blades and subsequently
lowering the operating efficiency (Tower, 2003). To prevent premature
engine failure due to silicate deposition, manufacturers of gas engines
introduced a limit value of 1mg/l of silicon in the oil of gas engines
(Prabucki et al., 2001) or a maximum total siloxane concentration of 5mg/
Nm 3 of biogas (Environment Agency, 2004). Silicate deposition can also
result in poor heat transfer in heat exchangers and can cause abrasions to
the blades in turbines. The silicon limit imposed in micro turbines is less
than 10 ppb (Accettola et al., 2008). In addition, the glassy residues
inactivate the catalyst of the emission control system and thus reduce the
removal efficiency of combustion products (CO and NO x ). A similar process
occurs in fuel cells, resulting in a lower voltage and power density for a given
current density (Accettola et al., 2008).
Siloxanes can be removed from biogas by means of adsorption onto
activated carbon, activated alumina or silica gel. A significant amount of
siloxanes (90-95%) can be removed along with the moisture when the gas is
dried, for example at

￿ ￿ ￿ ￿ ￿ ￿

C (low-temperature drying system). Activated
carbon or graphite media scrubbers followed by a gas dryer would offer a
cost-effective means of removing siloxanes. Chilling the gas to 5

23 8

Cwitha
gas dryer would remove some of the water and approximately 30-40 % of
the siloxanes (de Hullu et al., 2008). The remaining siloxanes can be
removed using a scrubber system.

8

Ammonia

Ammonia is formed during the anaerobic degradation of feedstocks rich in
protein. Ammonia concentration in biogas is generally very low, not