4. Determination of the hydrogen sulphide content of biogas
The H2S content of the purified gas can be measured
to check the effectivity of the desulphurization process.
Laboratory method
In the laboratory the H2S content of gases is usually
measured iodometrically using cadmium acetate. However, the necessary techniques
are too involved for an application here.
Lead acetate method
A simple way of determining the presence of H2S in
biogas is a test with lead acetate paper. A piece of paper soaked with lead
acetate solution is held in the gas stream for a short time. The presence of
H2S colours the strip black. The difficulty with this method is its
high sensitivity which means that even a very small amount of H2S can
be detected. A small amount of H2S, however, is not an indication of
greatly reduced efficiency of the desulphurization. Simple desulphurization
plants may still posses an adequate purifying performance.
Detection with iodine solution
Another simple method for detecting H2S is with an
alcoholic solution of iodine, such as often available in first aid kits. A small
amount of biogas is carefully introduced into the iodine solution. If
H2S is present the reddish brown solution will decolour. The
formation of elementary sulphur causes a milky turbidity.
The test-tube method
The test-tube method is a very exact and simple method of
determining the H2S concentration in biogas. Suitable tubes are
available for measuring the concentration in both raw and purified gas. The gas
detector apparatus (ca. 450,- DM) and the individual test tubes (ca. 5,- DM
each) are relatively expensive. Also, the test tubes can only be preserved for a
limited time. This method is only expedient in the regional biogas extension
service or similar advisory services. This apparatus could then be used to
provide empirical field values for individual plants. The intervals for
recharging the purifying agent can then be laid down.
As yet there is no simple, cheap, test method available. For
this reason a close control of the desulphurization plant is strongly
recommended.
