|M.Sc Student||Gendel Youri|
|Subject||Investigation of a Chemical-Biological Process for H2S|
Removal from Biogas Emissions
|Department||Department of Agricultural Engineering||Supervisor||Professor Ori Lahav|
|Full Thesis text|
The study was concerned with the development of a treatment process for minimizing H2S(g) emissions from Animal Feeding Operations (AFOs). The suggested process consists of three units: In the first unit H2S(g) is absorbed into an acidic (pH<2) ferric iron solution and oxidized by free ferric iron to S0 in a bubble column reactor, while in parallel, ferric iron is reduced to ferrous iron. In the second unit ferrous iron is bio-oxidized back to ferric iron by Acidithiobacillus ferrooxidans bacteria. In the third unit S0 is separated from solution in a gravity settler.
The work focused on three sub-processes related to the overall method: the kinetics of H2S absorption into a ferric solution at low pH, the kinetics of Fe2+ oxidation by Acidithiobacillus ferrooxidans and the factors that affect ferric iron precipitation (hypothesized to be the main obstacle for a continuous operation of the proposed process) under the chosen operational conditions.
H2S removal efficiency was found to be higher at higher ferric concentrations, higher H2S(g) concentrations and lower treated air flow rates. H2S(g) removal efficiency of >95% was recorded with a ferric iron concentration of 9 g/l, which is the most commonly used concentration in Acidithiobacillus ferrooxidans related studies.
The second part of the work focused on the kinetics of ferrous oxidation by Acidithiobacillus ferrooxidans. A new, general laboratory technique was developed for determining the kinetic equation and the extant kinetic coefficients (KS, Kp and mmax) for these bacteria.
The third part of the work focused on the phenomenon of iron oxide precipitation. Theoretical chemical equilibrium calculations performed using the MINEQL+ computer program showed that iron precipitation was significant in all possible feeding media, at the relevant pH range (1.6-2.2). The results of the precipitation study showed that the precipitation rate of the principle iron compounds formed (jarosites), was slower at lower pH values, but that other suggestions to remedy this problem, appearing in the literature, are not substantiated.
Continuous culture experiments were conducted in a packed-bed reactor to investigate the long term effect of pH on jarosite formation and ferrous oxidation. It was found that at lower pH (1.5) jarosite accumulation was slower and that the performance of the Acidithiobacillus ferrooxidans at this pH was sufficient for successive operation of the proposed process.