Environmental Engineering Reference
In-Depth Information
the correct
settlement, according to DVGW G 685, meet all
the
requirements of calibration regulations at the customer's end.
16.4.1 Heating value adjustment by LPG
Where biogas is fed into the public gas supply network, heating value
adjustment by LPG is the standard method of achieving the network
compatibility required by DVGW G 685. The addition of LPG mainly raises
the heating value of the treated biogas to the heating value of the natural gas
distributed in the local network. In some cases, LPG and air must be added
systematically to adjust the network compatibility of the biogas because not
only is the heating value (in terms of the superior heating value at standard
temperature and pressure (STP)) important, but so are the Wobbe index
(STP) and the relative density (methane number) of the gas. TheWobbe index
is the quotient of the heating value and the square root of the relative density
(quotient of gas density and density of dry air at equal temperature and
pressure).
Conditioning commonly uses LPG composition of 95% propane and 5%
butane according to DIN 51622 (DIN, 1985). DIN 51622 defines different
classifications for LPG and specifies the permitted limits for sulfur and other
trace elements as well as the permitted fractions of alkenes (butene, propene
and their isomers). It is also possible to use other LPG compositions and
thereby keep within the limits set out in DVGW G 486 (DVGW, 1992) for
propane and butane concentrations with appropriately mixed LPG.
Generally, LPG mixtures with a butane content of up to 60% are permitted.
This may be considered for some individual cases (e.g. when feeding biogas
into a natural gas network with high heating values) because LPG cannot be
added in any quantity.
Apart from heating value, the following restrictions also need to be
observed.
.
For application in engines, the methane number (MN) is important. It is
defined by the composition of methane and hydrogen: 100% methane
corresponds to MN = 100. The MN defines the knocking stability;
MN = 70 is considered to be the lower limit for vehicle fuel.
.
The danger of condensation of higher hydrocarbons, for example due to
compression or expansion at natural gas filling or transfer stations,
needs to be considered.
Figure 16.2 illustrates the required LPG concentrations for obtaining typical
heating values within the gas network downstream of the mixing point as a
function of the upgrading level of biomethane. The following different
heating values were assumed for the German gas grid:
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