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 409

ignite the compressed fuel-air mixture within the cylinder. This results in
low exhaust gas emissions and a long life of the spark plugs
(US EPA CHPP, 2008). Moreover, as the ignition is controlled by the
microprocessor, the engine can adjust to different kinds of gas and volatile
liquid fuel, ranging from propane to natural gas and gasoline or varying gas
quality. This is usually accomplished with a proper fuel system, engine
compression ratio and tuning. In the case of a biogas-fuelled engine, ignition
timing needs to be changed depending on the methane content in the biogas.
This is usually monitored through an on-line methane analyser or a
knocking monitor (Deublein and Steinhauser, 2008). Using high-energy
ignition technology, very lean fuel air mixtures can be burned in biogas
engines by lowering the peak temperature within the cylinders
(US EPA CHPP, 2008). This will result in reduced NO x emissions. The
lean-burn approach in reciprocating engines is analogous to dry low-NO x
combustors in gas turbines (US EPA CHPP, 2008).
The CO content in the exhaust gas is dependent on the air-fuel ratio.
According to European regulations, the CO content in the exhaust gas must
be less than 650mg/Nm 3 (Germany) or 3000mg/Nm 3 (Denmark). This is
usually achieved by cooling the exhaust gas to below 400 C in water-cooled

collectors as the oxidation of hydrocarbons to CO (a post-reaction in the tail
pipe) is slowed down at these temperatures (Deublein and Steinhauser,
2008). However, the lower temperature brings the efficiency down to 27-
35% before the turbocharger (Deublein and Steinhauser, 2008;
US EPA CHPP, 2008). Alternatively, a low CO concentration in the
exhaust gas can be achieved by using an oxidation catalyst mainly consisting
of activated charcoal. An activated charcoal filter located between the
suction pipe and the CHP unit will retain catalytic poisons such as siloxanes
and sulphur compounds. Thus, the engine efficiency is improved by 3% and
emissions of SO 2 (formed by reaction of sulphur and oxygen to SO 2 in the
engine) are prevented in the exhaust gas (Deublein and Steinhauser, 2008).
In order to prevent engine knock-out with varying gas qualities, a methane
content of at least 45% should be ensured (Deublein and Steinhauser, 2008).
All parts of the engine that come in contact with sulphur compounds can
be corroded and thus must be protected. To avoid corrosion, specially
designed bearings (sputter bearings) should be used instead of normal ball
bearings and rings. In addition, special lubricating oils are used, which are
low in ash and provide high alkalinity for long periods. These reduce the oil
change interval time and considerable wear of the cylinder heads due to the
sulphuric acid in the biogas. Finally, CHP units designed for biogas
utilisation are generally equipped with large lubricating oil tanks in order to
provide high capacity for impurities and the oil will last longer. Depending
on the sulphur content of the biogas, regular oil analyses at intervals of 160-

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