5. Methods for removing hydrogen sulphide from biogas
General
Of the many processes traditionally and presently employed, that
have been used for large-scale desulphurization of technical gases, only the
so-called "dry" process is suitable on a smaller scale for biogas plants. They
are acceptable from the point of view of technical complexity and maintenance
and the degree of purification is satisfactory.
The desulphurization of biogas is based on a chemical reaction
of H2S with a suitable substance.
The lime process
The oldest process is the desulphurization of gases with quick
lime, slaked lime in solid form or with slaked lime in liquid form. The process
using quick or slaked lime has not been applied on a large scale for a long
time. The large amounts of odourous residue that are produced cannot be
satisfactorily disposed of. The handling of large amounts of dissolved or
suspended slaked lime requires elaborate equipment.
Large concentrations of CO2 which are present in
biogas make the satisfactory removal of H2S difficult. The
CO2 also reacts with the quick and slaked lime and uses it up
quickly. The Ca(HCO3)2 formed reacts with
Ca(SH)2 which is formed by the reaction of H2S with
Ca(OH)2 thus resulting in the reoccurance of H2S. However,
a large scale biogas plant in Germany with the cogeneration of heat and power
has recently been constructed using a lime purifier. The results of long term
tests are not yet available.
In as far as enough lump, quick lime is
available in the countries concerned, this process could be considered for
desulphurization. The apparatus for utilizing quick lime corresponds in
construction and function to that used for the desulphurization with
iron-containing substances.
Ferrous materials
Ferrous materials in the form of natural soils or certain iron
ores are often employed to remove H2S.
Principle
The ferrous material is placed in a closed, gas tight container
(of steel, brickwork or concrete). The gas to be purified flows through the
ferrous absorbing agent from the bottom and leaves the container at the top,
freed from H2S.
Chemistry
The absorbing material must contain iron in the form of oxides,
hydrated oxides or hydroxides. These react as follows:
2 Fe(OH)3 + 3 H2S à
Fe2S3 + 6 H2O Fe(OH)2 +
H2S à FeS + 2H2O
This process terminates, of course, after some time. The greater
part of the iron is then present as a sulphide.
Regeneration
However, by treating the sulphidized absorbent with atmospheric
oxygen, the iron can be returned to the active oxide form required for the
purification of the gas:
2 Fe2S3 + 3 O2 + 6
H2O à 4 Fe(OH)3 +3 S2
2 FeS + O2
+ 2 H2O à 2 Fe(OH)2 + S2
The used absorbent can, therefore, be "regenerated". This
regeneration cannot be repeated indefinitely. After a certain time the absorbent
becomes coated with elementary sulphur and its pores become clogged. Purifying
absorbents in gasworks (coke plants) acquire a sulphur content of up to 25% of
their original weight, i.e. 40% sulphur by dry weight.
Process techniques
There are three different, dry desulphurizing processes
available.
Without regeneration
The purification chamber consists of a box or drum. The
absorbent is placed inside it on several, intermediate trays (sieve floors) to
ensure that the depth of the absorbent is not more than 20-30 cm. Otherwise the
absorbent would easily press together causing an increase in the resistance to
the gas flow.
The biogas is fed in at the bottom of the box, flows through the
absorbent and leaves the purification chamber at the top, freed from
H2S. When the absorbent becomes loaded with iron sulphides, the gas
leaving the chamber contains increasingly more H2S. The chamber is
then opened at the top and the trays with the spent absorbent are removed. Then
fresh absorbent is placed on the trays.
After the air in the purification chamber has again been
displaced with biogas, the gas connection to the user is re-opened.
The spent
absorbent is disposed of as described under the heading "Disposal of spent
absorbent".
With regeneration
The spent, sulphide containing absorbent can also be regenerated
by exposing it to oxygen. This can either be done by taking the used absorbent
out of the chamber and exposing it to the air, or inside the purification
chamber by simply sucking ambient air through it.
Since regeneration inside
the chamber requires precautions against the formation of unwanted and dangerous
air-gas mixtures and would require powerful fans, regeneration outside the
chamber is usually preferred. The absorbent that is to be regenerated, is spread
out on the ground in as thin a layer as possible. From time to time it is turned
over with a shovel. After a few days it is ready for use again.
This
regeneration process can be repeated up to ten times, after which the absorbent
is finally spent.
Simultaneous regeneration and loading
Simultaneous regeneration and loading of the absorbent is a
special case. Here a certain, small amount of air is added to the biogas. Then
sulphide formation and regeneration occur at the same time and place. As such,
the absorbent acts effectively as a catalyst.
Expensive gas-measuring and
mixing equipment is required for this process, however, so that it is not
suitable for small biogas
plants.
