4. Conclusions
Chemical oxidative absorption is a promising method for the
H2S removal in the biogas upgrading process, since the irreversible
oxidation reactions can guarantee constant and effective
H2S removal, even at relatively high and strongly fluctuating H2S
contents in the crude gas, while the CO2 absorption and thus the
consumption of base is kept low. Also, no harmful substances are
produced during oxidation, thus the remnant liquid can be fed back
into the fermenter and discharged with the digestate.
In the laboratory experiments, virtually complete H2S absorption
could be ensured at any time by maintaining the pH above
7.7 and the redox potential above 100 mV. Investigations on the
solution aging showed that the increasing content of HCO3
− ions,
which were formed by the previously absorbed CO2, provided a pH
buffer with increasing strength. Therefore, the aging of the solution
had a positive effect on H2S absorption and NaOH consumption.
However, at high ionic strength a decrease in absorption rates was
observed, which can be attributed to limitations in mass transfer.
The H2O2 consumption is directly proportional to the H2S content
in the crude gas, but it can also be negatively influenced by the
overdosage of H2O2. High concentrations of H2O2 present in the
solution lead to a higher specific uptake because the dissociated
H2S is oxidized all the way to SO4
2−, while for low concentrations of
H2O2 in the solution elemental sulfur is the final oxidation product.
In the experiments on the pilot plant, high separation efficiencies
(with an average of 97%) could be reached, despite
strongly fluctuating H2S contents in the crude gas. The average
H2O2 consumption equaled 4.8 molH2O2/molH2S, which is
higher than the values reported for the laboratory column
(1.2–2.9 molH2O2/molH2S). It is also higher than the maximum theoretical
consumption (4 molH2O2/molH2S). This overconsumption
can be attributed to the oxidation of other trace components that
are present in biogas, to the loss of the oxidizing agent during the
periodical exchange of the scrubbing solution, or to a decomposition
of H2O2. Future work includes the reduction of the specific
H2O2 consumption in the pilot plant through an optimized control
system as well as optimization efforts in process design.