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 399

CH 4 (55-65%) in biogas (Rasi et al., 2007; Petersson and Wellinger, 2009).
This calorific value is typically between 50 and 70% of the commercial
quality natural gas caloric value of 31.6-39MJ/m 3 , which depends on source
and composition (Petersson and Wellinger, 2009).
Biogas has numerous end-use applications compared with other renew-
able energy resources. Traditionally, biogas has been used as fuel for boilers
to produce heat or for cogeneration of heat and electrical power in
combined heat and power (CHP) generation plants. In CHP plants,
electricity is generated by burning fuel (natural gas or biogas) and then a
heat recovery unit is used to capture heat from the combustion system's
exhaust stream. This heat can be converted into useful thermal energy,
usually in the form of steam or hot water. These CHP systems are normally
provided with a four-stroke engine or a diesel engine. Biogas can also be
used in a boiler to produce steam for driving engines or turbines; examples
include the organic rankine cycle (ORC), the Cheng cycle, the steam turbine,
the steam piston engine and/or the steam screw engine. Other alternatives
that can use biogas as fuel are Stirling engines or gas turbines, micro gas
turbines, high- and low-temperature fuel cells and a combination of a high-
temperature fuel cell with a gas turbine. However, biogas cleaning (see
Chapter 14) is a prerequisite to meet the gas quality requirements of the
equipment used (Fig. 17.1). The various utilisations of biogas in stationary
applications are

. the production of heat and/or steam in a boiler (the lowest value chain

utilisation)

. electricity production in CHP production with waste heat recovery

. as an industrial energy source for heat, steam and/or electricity and

cooling in a micro turbine

. electricity generation by direct use of methane or reforming to hydrogen

in fuel cells.

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17.2 Biogas and biomethane combustion issues

17.2.1 Heat and electric power generation from biogas

Biogas properties will have a significant impact on the selection of
technology for conversion to heat and/or electricity. In general, the biogas
composition and production rate are influenced by the type of digestion
process and feedstock used. Biogas composition and energy content will also
affect the choice of equipment for biogas utilisation. For instance, de-rating
is noticed in boilers due to limited volumetric throughput of the fuel into the
combustion system. Moreover, boilers do not need a high-quality biogas.
The maintenance cost for reciprocating engines increases with increasing