Environmental Engineering Reference
In-Depth Information
If higher pressure differentials are required, the compressor needs to be
cooled. Depending on the end pressure, flow rate and initial pressure, one-
stage or two-stage compressors can be used. Critical parameters for the
selection of a compressor are volume flow and pressure after compression
(network pressure). Because only low volume flows are involved, screw or
piston compressors are most often used. Screw compressors require less
maintenance and are less susceptible to technical trouble than piston
compressors, but they have distinctly higher capital expenditure and
operating costs. The higher purchase price and operating costs of a screw
compressor must be weighed carefully against
the benefit of higher
availability.
If the biogas is fed into a high-pressure transport network, two-stage
compression is usually necessary. In this case, a screw compressor can serve
as the first compression stage and the second stage is provided by a piston
compressor.
16.3.3 Pressure regulating and metering devices
Pressure metering and regulating devices are included in the technical
equipment for pressure regulation and gas volume metering. Gas pressure
regulating facilities regulate and limit the fluctuating inlet pressure and
expand the gas to a uniform, lower delivery pressure. Usually, gas pressure
regulating facilities are only needed when biomethane is fed into a low-
pressure natural gas grid. Gas meters register the gas flow in the operating
state. Together with pressure and temperature probes and determination of
gas composition, the gas volume can be converted to a standard state
according to calibration standards (Nm 3 or m 3 STP). Depending on the
required safety, accuracy and cost effectiveness, displacement meters or flow
meters are used.
￿ ￿ ￿ ￿ ￿ ￿
16.3.4 Determination of gas composition
Constant monitoring of the gas composition, particularly the combustion
characteristics of the treated biogas, is important for complying with the
requirements on gas quality (as stated in DVGW G 260 and DVGW G 262)
and the custody transfer metering for accounting purposes according to
DVGW G 685 (DVGW, 2008e). Readings of the gas composition (CH 4 ,
CO 2 ,O 2 ,N 2 ,H 2 and other trace substances) allow defining combustion
characteristics such as superior heating value, inferior heating value, Wobbe
index and standard density (see glossary in Section 16.10). Water and
hydrocarbon dew points can also be determined according to DVGW G 488
(DVGW, 1999).
Continuous monitoring of gas composition ensures that the quality of the
Search WWH ::




Custom Search