10. Summary
Hydrogen sulphide is a natural component of biogas. Its
concentration depends on the sulphur content of the raw material being digested
and lies in the range 1,500 ppm 5,000 ppm or 0,15-0.5 vol. % or 2.1-7 g
H2S/m³. Hydrogen sulphide is responsible for corrosion of
various parts of the plant equipment.
The use of biogas in an internal combustion engine causes
uncontrollable corrosion problems. Engine components are very susceptible to
corrosion by H2S and its reaction product SO2.
The experience of various manufacturers and users, as well as
the (older) literature indicate that with the sulphur concentrations mentioned,
the service life of the engines can be reduced by up to 15% of their normal
value. Not only are the resulting repair and maintenance costs increased but
also the costs for service materials such as spark plugs and lubricating oils.
Even with special oils the oil change interval drops up to one-fifth of that
under normal conditions. The oil becomes acidic from sulphur dioxide and thus
loses its lubricating properties.
Acidic exhaust gases corrode the exhaust systems very severely.
Desulphurizing biogas with acceptable investment and operating
costs is only possible employing the dry desulphurization method.
Iron-containing materials of specified compositions are utilized as absorbing
agents for H2S. Alongside the traditional, commercially available
absorbents, certain substitutes can be used. Various tropical and subtropical
soils contain sufficient iron in a suitable form. They must be prepared, in
order to obtain the proper purifying characteristics. The material must be
loose, porous, moist and granular.
The raw soil has to be ground and mixed with a filler and water
to obtain a homogeneous texture. The finished absorbent is placed on
gas-permeable trays in a purification chamber. The raw biogas is fed in at the
bottom and the desulphurized or partially desulphurized gas is extracted from
the upper part of the chamber.
Eventually the absorbing agent is saturated with sulphur and can
be regenerated either inside the chamber or outside through natural ventilation
with air (oxygen). The absorbent material can, therefore, be re-used several
times.
Using two or more purification chambers connected in series
ensures a continual production of purified gas and allows a good capacity
utilization.
The spent absorbent can be disposed of safely by burying it.
Various factors must be considered when dimensioning the purification chambers.
A certain maximum flowspeed should not be exceeded The gas volume to be purified
per unit time determines the cross section of the purification chamber. The
chamber volume and, hence, the amount of absorbent determine the operating time
for the purification process up to regeneration or exhange of the absorbent.
A calculation procedure simplifies the dimensioning of the
desulphurization
unit.
