Environmental Engineering Reference
In-Depth Information
biomethane is maintained and, if the biomethane is conditioned with LPG
or a mixture of air, the conditioning facility is also monitored. Generally,
two measuring methods have become standard: process gas chromato-
graphy and combustion calorimetry.
Process gas chromatographs can determine the composition of the treated
biogas or the conditioned biomethane in a discontinuous mode, and can
also calculate the heating value and the standard density of the gas. A
chromatograph can serve several measuring points. In most cases, separate
(paramagnetic or electrochemical) detection of oxygen and hydrogen is
required for monitoring the limits of these substances because their allowed
concentration is low (e.g. in Germany 1% (vol.) oxygen and 0.2% (vol.)
hydrogen). Process gas chromatographs are not sensitive enough for these
low concentrations. One drawback, particularly when several measuring
points must be served, is that gas analyses can only be performed
discontinuously at intervals of 3 to 6 minutes.
To determine the heating value using a combustion calorimeter, a defined
gas volume is combusted completely. The heat released by the combustion
provides a measure of the heating value of the gas measured. Gas
composition cannot be determined using a calorimeter. Exact control of
the conditioning facility (admixture of LPG) is often ensured by
nondispersive infrared sensors (NDIRs), which are low-cost and supply
continuous measuring signals for CH
4
or CO
2
. Minor constituents of the
gas, such as H
2
S, hydrogen or ammonia, can be measured by electro-
chemical sensors or by gas sampling and subsequent offline analysis.
In addition to gas chromatography, the density of the gas can also be
determined by buoyancy or pressure swing adsorption methods; CO
2
may
be determined using NDIRs. The dew point of water is determined by
hygroscopic, optical or physical methods. Other parameters, such as the
Wobbe index, need to be calculated.
16.3.5 Conditioning facility
Adjusting the heating value of the biogas to H-gas quality (H-gas is gas with
a high heating value) with LPG is currently the standard method. On the
other hand, if the biogas is fed into an L-gas network (L-gas is gas with a
low heating value) air must be added to reduce the heating value. LPG
conditioning facilities consist of a mixer (feeder), metering and regulating
equipment and LPG storage tank. Air conditioning facilities need a gas
mixer, an air compressor and suitable metering and regulating equipment.
The method of CO
2
separation is critical to the amount of LPG that must be
admixed and therefore to the design of the conditioning facility. If CO
2
is
separated efficiently, the cost of conditioning gas with LPG is low and
therefore of economic benefit. Generally, close cooperation between the
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