Biogas desulfurization – odor control
H2S (hydrogen sulfide, sulphide)
H2S (hydrogen sulfide, sulphide) is a toxic, as well as a flammable gas with the typical rotten egg smell. H2S is the result of the decomposition process of many sulfurous organic substances, such as proteins. Hydrogen sulfide also also occurs in natural gas, as well as in intestinal gasses (including livestock manure). The typical rotten egg smell is only perceived in very low concentrations, but in case of prolonged exposure or in case of higher concentrations the intensity of the sensation is receding. Because of the toxicity of this substance, even breathing low concentrations of hydrogen sulfide for a longer period will be dangerous, thus the importance of an efficient separation of this element from industrial emissions. Even a tiny quantity of this gas can cause odour nuisance (the odor treshold for this gas is 0,0005 PPM!). The odour treshold in olfactometry is the lowest concentration leading to odour nuisance.
Biogas desulfurization : separation of H2S from biogas by means of biological air scrubbers
Gas scrubbers or air scrubbers are used to absorb the H2S from the biogas. This kind of installation consists of one or more gas scrubbers (scrubbing stages). Inside the air scrubber, the biogas is brought in contact with water from the aeration tank, which is circulated once over the air scrubbers and reconducted into the aeration tank. The absorbed H2S will be oxidated in the aeration tank. For this biological air scrubbing technique, the gas scrubbers are preferrably installed in the direct neighbourhood of the aeration tanks (see installation overview pictures above).
Biogas desulfurization : odour removal efficiency of biological air scrubbers
With this method the total removal efficiency is situated around 95%. This means that there won’t be any odour nuisance left for the surroundings, fact that can easily be confirmed by basic olfactometry techniques.
Biogas desulfurization : advantages of biological air scrubbers for the removal of H2S
One of the most important advantages of this odor removal technique is the very low operating cost (no continuous supply of chemicals needed).
H2S separation with chemical gas scrubbers
In certain cases, where biological air scrubbing methods will not be applicable for the separation of hydrogen sulfide, a one or multistage gas scrubber installation might be an appropriate solution. In this case, the gas will be scrubbed by adding chemicals to the water. For an efficient separation of H2S out of industrial emissions, several stages (gas scrubbers) will ne necessary, in practice, to obtain the required result. A typical multistage gas scrubbing installation will be a first gas scrubber operated with cheaper chemicals to separate most of the hydrogen sulfide and a second gas scrubber operated with more expensive specialty chemicals to eliminate the last odor particles.
Odor removal efficiency of chemical gas scrubbers for the separation of H2S
A correct dosage and use of the correct chemicals will allow to reduce the olfactive nuisance for the neighbourhood to an imperceptible level. It goes without saying that basic olfactometry techniques will allow to bring evidence.