Hydrogen sulfide in biogas; occurence, hazards and removal techniques
High hydrogen sulfide content in biogas is not accepted. If the target of the anaerobic digestion process is to produce high-quality biogas then efficient removal of sulfide from both aqueous and gaseous phases is extremely important.
In general terms, biogas is composed of approximately 60–70% (v/v) methane (CH4), 30–40% (v/v) carbon dioxide (CO2), 1–2% (v/v) nitrogen (N2), 1000–3000 ppmv H2S, 20-30 ppmv of VFAs and 10– 30 ppmv of ammonia (NH3). The exact biogas composition depends strongly on the organic substrate used during the anaerobic process. For example, biogas generated from the anaerobic digestion of paper waste has higher percentage of methane than the one produced from cow dung. Similarly, slaughterhouse wastes and veterinary sanitation wastes which may contain up to 10-13 % wt. of organically bonded nitrogen, they convert it during anaerobic digestion mostly to ammonia.
Even though various sulfur-containing impurities may be present in raw biogas, hydrogen sulfide (H2S) is the most commonly reported reduced sulfur compound and represents content up to 2% (v/v).
However, as mentioned above, this H2S concentration can be higher when a rich protein/sulfate-rich feedstock is used. Some sulfate-rich substrates include stillage, macroalgae and wastewater from the paper and food industry.
Hydrogen sulfide is generally formed by bacteria that have the ability to reduce sulfate present in the digester to hydrogen sulfide. These bacteria are present in the digester and compete with methane-forming bacteria (methanogens) for the same substrate but, instead of forming biogas, hydrogen sulfide is formed. It is difficult to mitigate the simultaneous development of sulfate reducing bacteria but even if biogas contains relatively high amounts of hydrogen sulfide, the overall rate of methane production is not directly affected.
This, of course, does not mean that the high biogas hydrogen sulfide content can be accepted. On the contrary, if the target of the anaerobic digestion process is to produce high-quality biogas then efficient removal of sulfide from both aqueous and gaseous phases is extremely important.
Hydrogen sulfide in the fermentation gas affects significantly the lifetime of biogas transportation piping network along with all other installations for the utilization of biogas. H2S is extremely reactive and forms sulfuric acid in presence of moisture. The sulfuric acid formed is very aggressive to ferrous equipment and can corrode pipelines, gas storage tanks, and gas utilization equipment. The presence of hydrogen sulfide in biogas is the most common reason to shorten usable life of many of the components in the digester biogas handling system. Another aspect which makes hydrogen sulfide clearly unwanted is that when H2S containing biogas is burned, it is converted into sulfur oxides, which on the one hand corrode metallic components and on the other hand acidify the engine oil, e.g., of the engine in the CHP. Also the combustion of biogas containing hydrogen sulfide will lead to emissions of sulfuric acid formed during combustion.
But these are not the only problems associated with H2S. Regarding health and safety issues in biogas plants, hydrogen sulfide can be lethal at concentrations above 700 ppmv and even though it has the awful smell of rotten eggs, after a short exposure, the nose becomes numb to odor, which can lead to the dangerously mistaken conclusion that the hazard has diminished or disappeared. Therefore, appropriate gas detection instruments are necessary so as to protect from even the slightest indication of H2S gas. Calibrated detection instruments are of highest importance since hydrogen sulfide is also highly flammable and explosive and can cause possible life-threatening situations if not properly handled. In addition, hydrogen sulfide gas burns and produces other toxic vapors and gases, such as sulfur dioxide. The explosive range of hydrogen sulfide in air is 4.5 to 45.5% v/v.
In order to prevent damage of the CHP and other equipment, e.g., heat exchangers and catalysts, hydrogen sulfide must be removed from the biogas. There are various techniques that can be applied.
Hydrogen sulfide can be removed either during the anaerobic digestion process in the digester itself or after the digester, using three kinds of methods; chemical, physical or biological methods.
Biological treatments are cost effective and environmentally friendly processes. For example, biological oxidation/biofiltration is one of the most promising clean technologies for reducing emissions of malodorous gases and other pollutants into the atmosphere. This technology has been proven to effectively control reduced sulfur compounds in diluted gas streams. Biological desulfurization processes are effective at concentrations up to 3000 mg Nm3. In the biological desulfurization process, the hydrogen sulfide is absorbed in water and then decreased biologically. Microorganisms of the species Thiobacillus and Sulfolobus, which are omnipresent and therefore do not have to be inoculated especially, degrade the hydrogen sulfide. Biological oxidation has the benefit of being a very economical alternative compared to other techniques.
Regarding the chemical alternatives to remove hydrogen sulfide from biogas, one commonly used method is biogas chemical absorption by selective amines, such as diglycolamine. Another popular chemical method applied is the addition in the tank of Fe 2+ ions in the form of iron (II) chloride (FeCl2) or Fe 3+ ions in the forms of iron(III) chloride or iron(II) sulfate. That leads to the precipitation of stable iron (II) sulfide and sulfur, which remain in the residue and thus is practically removed from digester gas. For sulfide precipitation, only an additional mixing tank and a dosing pump are necessary. The process is applied predominantly in sewage water treatment plants. It is referred that when a dosage of 3–5 g FeCl2/m3 is added to waste water, the content of hydrogen sulfide can be lowered at best to less than 150 mg/Nm3.
As far as the physical removal methods are referred adsorption on activated charcoal and zeolites are the most famous techniques. The adsorption is a clean process without any by-products. Its main drawback is its operational cost since H2S adsorption on activated charcoal is irreversible, thus requiring significant amounts of adsorbent. Another physical method applied is the compressed gas scrubbing, where the good solubility of H2S in water makes it possible to clean the biogas by means of scrubbing. In order to minimize the scrubber volume, the biogas should be compressed before processing it. The major disadvantage of scrubbing is the fact that it is very energy intensive and requires about 10% of the electric current generated from the biogas.
Generally, before selecting the right desulfurization alternative, various parameters must be taken into serious consideration, such as biogas H2S concentration, the mass flow of sulfur, and also the possibilities for disposal of the residues from the gas cleaning.
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