Biogas cleaning - The Biogas: Science, Production and Applications - page 332

Chemical removal

To prevent the presence of hydrogen sulphide in the produced biogas, iron
ions (Fe 2+ or Fe 3+ ) in the form of FeCl 2 , FeCl 3 or FeSO 4 can be added to
the digester. This produces insoluble iron sulphide (FeS), which will
precipitate in the digester. The iron sulphide will leave the digester together
with the digestate. The reactions involved are:

Fe 2 þ þ S

2

! FeS ½ 14 : 1

Fe 3 þ þ

2

2 3 S ! 2 FeS þ S ½ 14 2

:

This is done in sewage sludge treatment plants for simultaneous removal of
phosphate. Hydrogen sulphide can also be removed by this principle after
the digester, in a reaction with iron oxide coated materials (e.g. wood chips)
to form iron sulphide. The process operates with two columns. Iron oxide is
regenerated by heating and/or exposure to oxygen. Eventually, the material
becomes saturated with sulphur and cannot be regenerated.

14.3.3 Other impurities

Oxygen and nitrogen can be removed by adsorption using activated carbon,
molecular sieves or membranes; they are also removed, to some extent, in
some desulphurisation processes and some upgrading processes. However,
these gases are difficult to remove and their presence should be avoided if
utilisation of the gas limits the levels of oxygen and nitrogen (e.g. if the gas is
injected to the gas grid or used as vehicle fuel).
Ammonia in raw biogas is usually removed when the gas is dried, since
ammonia will dissolve in water. Ammonia is also removed simultaneously
when the gas is upgraded and a separate cleaning step is often not necessary.
Siloxanes can be removed by absorption with organic solvents, strong
acids or strong bases, by adsorption with silica gel or activated carbon, or in
a cryogenic process (Ryckebosch et al., 2011). Some particles are removed
when the gas is dried or can be removed by filters or cyclones. Halogenated
hydrocarbons can be removed by adsorption with activated carbon. They
are also removed in many of the upgrading technologies.

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14.4 Biogas cleaning in combination with upgrading

When biogas is upgraded, carbon dioxide is removed from the methane in
order to increase the volumetric energy content in the gas. If the biogas is
going to be upgraded this affects the set-up of the cleaning step (Fig. 14.1).
Some impurities are removed in the upgrading step, but this is dependent on