Pressure relief
The task of running a fixed-dome system can be made easier by installing a spring-loaded pressure
reducing valve that guarantees a constant (adjustable) supply pressure.
Water separation
If at all possible, the water trap should operate automatically. However since fixed-dome systems
need a high water seal, often amounting to more than 1 m WG, the use of condensate collector with
a manually operated drain valve is advisable.
Backflow prevention
As a rule, the water trap also functions as a flashback chamber. If deemed necessary, a gravel trap
can be installed for added safety.
5.5 Biogas utilization
5.5.1 Composition and properties of biogas
Biogas is a mixture of gases that is composed chiefly of:
- methane, CH4
- carbon dioxide, CO2
- other gases
- hydrogen H2
- hydrogen sulfide, H2S
40 - 70 vol. %
30-60 vol. %
1 - 5 vol.%, including
0-1 vol. %
0-3 vol. %
Like those of any gas, the characteristic values of biogas are pressure and temperature-dependent.
They are also affected by water vapor. The factors of main interest are:
- volumetric change as a function of temperature and pressure,
- change in value as a function of temperature, pressure and water-vapor con" tent, and
- change in water-vapor content as a function of temperature and pressure.
- Chapter 10.2 contains pertinent tables, formulae and nomograms for use in calculating
conditions of state.
5.5.2 Conditioning of biogas
While the biogas produced by the plant can normally be used as it is, i.e. without further
treatment/conditioning, various conditioning processes are described in this chapter to cover
possible eventualities.
Reducing the moisture content of the biogas, which is usually fully saturated with water vapor. This
involves cooling the gas, e.g. by routing it through an underground pipe, so that the excess water
vapor condenses out at the lower temperature. When the gas warms up again, its relative vapor
content decreases (cf. chapter 10.2 for calculations). The "drying" of biogas is especially useful in
connection with the use of dry gas meters, which otherwise would eventually fill up with condensed
water.
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