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 410

2000 h (on average every 465 h) are compulsory (Deublein and Steinhauser,
2008).
Diesel engines (compression ignition diesel engines) are among the most
efficient simple-cycle power generation options on the market. Diesel
engines work by direct injection, as combustion in a pre-combustion
chamber may develop hot spots, resulting in uncontrolled spark failures
with biogas. The quality of fuel injection significantly affects diesel engine
operating characteristics, fuel efficiency and emissions. Diesel engines are
operated at an air-fuel ratio ( ) of less than 1.9; efficiency is 15% more than

that of a four-stroke engine (Deublein and Steinhauser, 2008). However,
high temperatures of combustion can lead to an increase in NO x emissions,
while low temperatures of combustion can lead to incomplete combustion
and unburnt carbon in the exhaust gas. Depending on the engine and fuel
quality, diesel engines produce 5 to 20 times the NO x (on a ppmv basis) of a
lean-burn biogas engine (US EPA CHPP, 2008). However, diesel engines
produce significantly less CO than lean-burn gas engines (US EPA CHPP,
2008).
Diesel engines can sometimes knock because of a premature self-ignition
of the mixture with high methane content. This problem is exacerbated by
the presence of NH 3 in the biogas (Deublein and Steinhauser, 2008), but can
be prevented by varying the feed of ignition oil. For instance, when the
methane content in the biogas is low, more ignition oil must be added. In the
case of an adequate biogas quality, feeding of 10-18% ignition oil

λ

is

recommended (Deublein and Steinhauser, 2008).
Both mineral oil and vegetable oil can be used to reduce NO x emissions.
However, operation with mineral ignition oil requires special storage. If
vegetable oil and/or biodiesel (rapeseed fatty acid methyl ester (FAME)) is
used instead of mineral ignition oil, then a leaner air-fuel ratio (compared
with the diesel process) can be used. Advantages of renewable ignition oils
are lower CO emissions, sulphur-free exhaust and biodegradability. The
NO x emissions from diesel engines burning heavy oil are typically 25-30%
higher than those using distillate oil (US EPA CHPP, 2008). The common
NO x control techniques are delayed fuel injection, exhaust gas recirculation,
water injection, fuel-water emulsification, inlet air cooling, intake air
humidification, and compression ratio and/or turbocharger modifications
(US EPA CHPP, 2008). Furthermore, an increasing number of larger diesel
engines are equipped with selective catalytic reduction and oxidation
systems for post-combustion emissions reduction (US EPA CHPP, 2008).

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17.3.4 CHP plants
The use of reciprocating engines (spark plug or compression ignition) is
expected to grow in various distributed power generation applications.