Environmental Engineering Reference
In-Depth Information
Quality demands on biogas for transport fuel or grid
For biogas to be used as transport fuel, it is advantageous to use biogas with
a high energy content; cleaned and upgraded biogas is thus preferred.
However, lower concentrations of methane could work from a technical
point of view. Water in the gas can cause problems; for example, it may
freeze and clog the system when the pressure drops. Carbon dioxide can
have a corrosive effect, but only in combination with water; the same is true
for hydrogen sulphide. Different standards are used for biogas used as
vehicle fuel. Sweden has a standard specifically for biogas to be used as
vehicle fuel (SIS, 1999), which regulates the content of methane, hydrogen
sulphide and water. Other standards have been published in the USA (SAE
lnternational, 1994), Switzerland (ISO, 2006) and Germany (DIN, 2008); the
UNECE Vehicle Regulations also apply (UNECE, 1958). Standards are
currently being developed by the European standards work group CEN/TC
408 - Biomethane for use in transport and injection in the natural gas grid.
If the biogas is going to be injected to the gas grid it has to fulfil the gas
quality demands for the grid, independent of the final use of the gas. This
means that if the biogas is going to be used in a boiler, but transported to
the boiler via the gas grid, it has to fulfil the quality demands for the gas
grid, which are in general higher than those for the boiler itself. More details
on biogas upgrading are given in Chapter 15 of this topic.
14.3 Biogas cleaning techniques
This section reviews different cleaning techniques for the most common
impurities found in raw biogas. The removal of carbon dioxide is covered in
Chapter 15.
14.3.1 Water
Water can be removed by changing the parameters that affect the solubility
of water in the gas, i.e. pressure and temperature. Water vapour will
condense if the pressure is increased or if the temperature is lowered, since
the concentration corresponding to saturation in the gas will then decrease.
Thus, technologies to remove water from biogas are compression and/or
cooling. Water can also be removed by absorption and adsorption. Other
impurities that can dissolve in water, like particles and siloxanes, will be
removed together with the water and this has to be considered when
designing systems for disposal of the water.
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