9. Operation procedures for gas desulphurization
Filling
Remove the cover of the purification chamber. Put in the bottom
tray, which has spacers attached both below and above. Spread a layer of
absorbent 20 - 30 cm thick over it. The absorbent at the edge of the tray is
´'piled up" against the wall of the chamber.
The material should not be pressed and should be uniformly
distributed.
The second, intermediate tray is then placed on the spacers of
the bottom one and covered with absorbent. Then the rest of the intermediate
trays, which are covered in turn with absorbent, are placed in position.
Sealing
Put the cover on the purification chamber together with the seal
and screw it down tight.
Scavenging
Open both the feed valve in front of the chamber and the
scavenging vent valve. Leave the control valve to the user closed. Let a volume
of gas equivalent to three times the chamber volume escape from the scavenging
vent (caution danger of fire and explosion!). Then close the scavenging vent and
open the valve to the biogas user.
Emptying the chamber
When the H2S concentration of the purified biogas
begins to rise, the absorbent should be exchanged. See Section 5 for the
determination of the H2S of the gas.
Exchanging the absorbent
Close the control valves (feed and user valves) in front of and
behind the purification chamber. Remove the cover of the chamber. Caution -
danger of fire and explosion!
Remove the absorbent layer after layer from the
chamber. In some cases the individual intermediate trays can be removed together
with the entire layer of absorbent. Check the trays for damage and, if
necessary, repair or replace them. Then the purification chamber can be refilled
with fresh absorbent.
What to do with the spent absorbent?
The sulphur-loaded absorbent can either be regenerated by
exposure to the air (oxidation) or be discarded if there is an ample supply. See
the next section for details.
Regeneration
The absorbent is regenerated by spreading it on the ground in a
thin layer and turning it over periodically. It is oxidized again within a few
days and can be re-used.
After it has been utilized and regenerated several
times the absorbent finally becomes inactive and must be discarded. Spent
material highly concentrated with powdered sulphur reacts under certain
conditions with the oxygen in the air, to form sulphur dioxide - SO2
- which irritates breathing passages and the eyes, as well as being harmful to
the environment.
Self-ignition
It is only seldom that the heat of reaction is sufficient to
cause spontaneous combustion. The spent absorbent should however be disposed of
in a way to avoid these problems.
Disposal of spent absorbent
Discarded absorbent should thus be placed in pits and
immediately covered with earth. When this is done the soil bacteria transform
the sulphur and sulphides to relatively harmless sulphates.
It might also be
possible to mix the used absorbent with the digested sludge. In this way the
sulphur, which had been removed, could be fed back through the soil to the
plants, thus completing the natural cycle. A decision can only be made after
tests under local conditions.
Degree of desulphurization
Since the desulphurization capacity drops with continued use,
the efficiency is not constant during the service life of a charge of absorbent.
As it becomes loaded with sulphur, that is as the sulphur concentration
increases, the H2S content of the purified gas also increases. Nearly
complete desulphurization can only be achieved when the absorbent is regularly
exchanged and then discarded. The absorbent is not completely utilized with this
procedure and the time interval between refillings is considerably shortened.
Two chambers in series
Another way of producing a very low H2S concentration
is to use at least two purification chambers in series, that is one after the
other.
The first serves as a coarse purification chamber while the
second serves as a fine filler chamber. Even when the first chamber allows
considerable amounts of H2S to pass through, the second is capable of
reliably binding the remainder. The resulting purified biogas is almost
completely free from sulphur.
The absorbent is almost completely utilized. As soon as the
first chamber is unable to remove H2S, its absorbent is exchanged.
This chamber is now used as the fine filtering chamber. The second chamber which
was the fine chamber is now used as the first, coarse purification chamber. That
is, the order of the gas flow has been reversed.
Temperature during desulphurization
The temperature of the gas in the purification chamber should be
held as constant as possible to prevent the absorbent from drying out or
becoming moist. If necessary the purification chamber should be insulated. The
chemical reactions in the purification process operate best at temperatures
between 15-25 °C. A higher temperature is better than too low a
temperature.
Regeneration of the spent absorbent
(1)
Regeneration of the spent absorbent
(2)
Starting up the chamber after
refilling with regenerated absorbent (1)
Starting up the chamber after
refilling with regenerated absorbent
(2)
