Biogas Desulfurization - H2S Removal From Biogas Origin, Effect, And Treatments

<p style="text-align: justify;"><span data-preserver-spaces="true">The turn of biogas into biomethane for injection into natural gas networks, as well as the alternative of "in situ electricity generation", requires the rapid separation of hydrogen sulfide from the biogas process. For this, there are two kinds of removal methods: origin/source or in biogas line (external and inside).</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">The removal of hydrogen sulfide (H2S) in the biogas stream becomes a necessity not only to keep the pipes, machines, and equipment involved in its transport, use, and application in good condition but also, above all, to preserve the life of all personnel in charge of the operation and maintenance of the biogas production facility, as well as avoiding toxic gas emissions into the atmosphere.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">Gas phase</span><span data-preserver-spaces="true">: Hydrogen sulfide (H2S)&nbsp;</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">Liquid phase</span><span data-preserver-spaces="true">: Sulfide acid (H2S)&nbsp;</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">Detection</span><strong><span data-preserver-spaces="true">:</span></strong><span data-preserver-spaces="true"> Strong rotten egg odor</span></p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 14pt;" data-preserver-spaces="true">Development</span></h2><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Origin</span></strong></p><p style="text-align: justify;"><span data-preserver-spaces="true">Hydrogen sulfide (H2S) is present in most rotting or biological degradation processes of organic matter when this process is carried out in the absence of oxygen. Under anaerobic conditions, sulfur-reducing bacteria acquire oxygen from sulfate (SO4), converting it into sulfur (S2-), which will give rise to H2S and hence the presence of H2S in the biogas.&nbsp;&nbsp;</span></p><p style="text-align: justify;">&nbsp;</p><p style="padding-left: 80px; text-align: justify;">&nbsp;<img src="" width="665" height="402"></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; Figure 1</span></strong><span data-preserver-spaces="true">. H2S production mechanism</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">The hydrogen sulfide concentration in the gas is a function of the digester feed substrate and the inorganic sulfate content. Wastes with a high protein content with sulfur-based amino acids (methionine and cysteine) can significantly influence the levels of hydrogen sulfide in the biogas and the presence of sulfates in the waters or liquids to be treated. H2S concentrations, therefore, vary according to the substrate to be treated, and its production is linked to the organic load that said substrate has.</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Figure 1.1. s</span></strong><span data-preserver-spaces="true">hows how H2S is formed and the desulfurization process in origin by air injection. Mainly manifestations of sulfur in biogas reactors. The hydrogen sulfide formation is carried out by anaerobic bacteria.</span></p><p style="padding-left: 200px; text-align: justify;"><img src=""></p><p style="text-align: justify; padding-left: 120px;"><strong><span data-preserver-spaces="true">Figure 1.1.&nbsp;</span></strong><span data-preserver-spaces="true">H2S production and desulfurization process by air injection&nbsp;&nbsp;</span></p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 14pt;"><strong><span data-preserver-spaces="true">Effects&nbsp;</span></strong></span></h2><p style="text-align: justify;"><span data-preserver-spaces="true">The removal/reduction of hydrogen sulfide (H2S) is necessary due to its contaminating effect at different levels, among which they can be mentioned.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">On the environment:</span><strong><span data-preserver-spaces="true">&nbsp;</span></strong><span data-preserver-spaces="true">Acid rain production.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">On equipment, machines, and ducts:</span><span data-preserver-spaces="true">&nbsp;Corrosion and minimization of their useful life.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">Corrosion caused by hydrogen sulfide in the Wastewater Treatment (WWTP) Plants is a well-studied and documented problem. This corrosion starts when bacteria oxidize H2S to sulfuric acid on pipe surfaces, machines, or concrete walls. Sulfuric acid corrodes the concrete (such is the case of the concrete base in which the biogas storage gasometers are installed), after which the steel framework is exposed and begins to corrode and lose its ability to resist different efforts.</span></p><p style="padding-left: 160px; text-align: justify;"><img src=""></p><p style="text-align: justify; padding-left: 200px;"><strong><span data-preserver-spaces="true">Figure 2. </span></strong><span data-preserver-spaces="true">Effect of H2S on the engine and devices CHP system</span></p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 14pt;"><strong><span data-preserver-spaces="true">About people (Staff)</span></strong></span></h2><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Table N&ordm; 1</span></strong><span data-preserver-spaces="true">&nbsp;shows the effects of the H2S concentration level on the human being who oversees the operation of the biogas facilities or any plant and site.</span></p><p style="text-align: justify; padding-left: 80px;"><span data-preserver-spaces="true">&nbsp; <img src="" width="623" height="348"></span></p><p style="text-align: justify;"><span data-preserver-spaces="true">For this reason, one of the most important aspects of the biogas facility is the H2S removal before its storage, use, or application due to the damage it produces in different areas: people, equipment, and the environment.</span></p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 14pt;"><span data-preserver-spaces="true">Treatments. (H2S removal)</span></span></h2><p style="text-align: justify;"><span data-preserver-spaces="true">Currently, many processes are available on the market for removing hydrogen sulfide (H2S) from biogas, each with its advantages and limitations. Some are limited by the H2S, CO2, or gas stream level. Others do not remove mercaptans. Others are not suitable due to their complexity of operation, initial capital investment, and/or high maintenance/operating costs. </span><span data-preserver-spaces="true">See figure 3</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">In the case of WWTPs (Wastewater Treatment Plants), there are two points or parts within the facility where this desulfurization process can be carried out.</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">In origin/source, that is, in the reactor or the sludge to be digested</span></strong></p><p style="text-align: justify;"><span data-preserver-spaces="true">In the case of the digester, we can speak of the injection of air, the injection of oxygen in the dome, or the addition of iron hydroxide or ferric chloride. In the case of sludge, thermal hydrolysis or the addition of iron hydroxide in the sludge flow entering the reactor is of interest (pretreatments).</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">In line</span></strong></p><p style="text-align: justify;"><span data-preserver-spaces="true">The biogas that comes out of the digester is a dirty gas full of impurities, which is required before a cleaning. This cleaning aims to remove foams, particles, condensates, and sediments. This kind of cleaning is known as coarse removal and is useful for defining the biogas flow.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">After this, it is advisable to carry out this H2S removal operation before entering the gasholder; in this way, it is avoided. On the one hand, the storage of toxic gas and on the other to keep the useful life of this equipment due to the gasometer membrane losing its properties more prematurely, and the concrete base could suffer corrosion damage.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">The in-line desulfurization processes can be broadly classified as dry and wet. These, in turn, depending on whether there is a chemical reaction, can be divided with or without a chemical reaction.</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Dry desulfurization Process</span></strong></p><ul style="text-align: justify;"><li style="text-align: justify;"><span data-preserver-spaces="true">Impregnated activated carbon (NaOH, KI, etc.)</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Catalytic activated carbon</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Molecular sieves</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Iron oxide</span></li></ul><p style="text-align: justify;"><span data-preserver-spaces="true">a) Iron filings</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">b) Steel Woolf</span></p><ul style="text-align: justify;"><li style="text-align: justify;"><span data-preserver-spaces="true">Wood pellet impregnated with iron oxide</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Iron oxide pellet</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Separation by membranes</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Regenerative adsorption</span></li></ul><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 14pt;"><strong><span data-preserver-spaces="true">Wet Desulfurization Process</span></strong></span></h2><ul style="text-align: justify;"><li style="text-align: justify;"><span data-preserver-spaces="true">Washing with sodium hydroxide (NaOH)</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">In one stage and in two stages</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Washing with iron compounds</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Washing with amines</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Biological desulfurization</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Washing with high-pressure water and low temperatures</span></li><li style="text-align: justify;"><span data-preserver-spaces="true">Biochemical desulfurization</span></li></ul><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify; padding-left: 80px;"><span data-preserver-spaces="true"><img src="" width="633" height="460"> &nbsp; &nbsp;&nbsp;</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">In the&nbsp;biogas desulfurization industry, as can be seen, there are numerous techniques. However, there are three of them that predominate in the market when treating large flows at a high concentration of H2S, which are: chemical scrubbing, biological scrubbing, and biochemical scrubbing, this last one many occasions combine with a dry and polishing stage.&nbsp;</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">Each of them is applied under specific flow conditions, H2S concentration, and gas stream conditions to be treated. However, when it tries to high concentrations of H2S, that is, above &gt; 5,000 ppm, chemical or biochemical processes are the ones that report the best results.</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Figure 3 </span></strong><span data-preserver-spaces="true">shows the different desulfurization processes applied to biogas cleaning according to the place where they are carried out and the technique/method used.</span></p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 14pt;" data-preserver-spaces="true">Comparative Between Processes</span></h2><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Wet techniques</span></strong></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Table N&ordm; 2</span></strong><span data-preserver-spaces="true"> shows some of the pros and cons&nbsp;between Biological and Biochemical techniques used for gas desulfurization.&nbsp;&nbsp;</span></p><p style="text-align: justify; padding-left: 80px;"><span data-preserver-spaces="true"><img src="" width="637" height="527"></span></p><p style="text-align: justify;"><span data-preserver-spaces="true">The current trend in the use of biogas is mainly aimed at producing biomethane as a substitute for Natural Gas (NG). When the objective of its use is directed to this application, that is, the production of biomethane, the biological process of desulfurization can be ruled out because, with the injection of air required for this process, N2 appears in the biogas stream. This N2 reduces the quality of the biomethane and is costly to separate from the own biomethane after.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">Regardless of the process chosen for desulfurizing the biogas, these require a subsequent stage of drying and biogas polishing that guarantees the quality of the biogas as a raw material for use in energy production or to produce new products such as biomethane.</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Figure 4 </span></strong><span data-preserver-spaces="true">shows the biochemical scrubber combined with a polishing stage that guarantees the stability of the desulfurization process and the suitable quality of the biogas for its use.</span></p><p style="padding-left: 120px; text-align: justify;"><img src="" width="588" height="333"></p><p style="text-align: justify; padding-left: 120px;"><strong><span data-preserver-spaces="true">Figure 4</span></strong><span data-preserver-spaces="true">. Biochemical scrubber combined with drying and polishing stage of biogas.&nbsp;</span></p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;"><span style="font-size: 10pt;"><em>This article was contributed by our expert&nbsp;<a href="" target="_blank" rel="noopener">Joaqu&iacute;n Reina Hern&aacute;ndez</a></em></span></p><p style="text-align: justify;">&nbsp;</p><h3 style="text-align: justify;">&nbsp;</h3><h3 style="text-align: justify;"><span style="font-size: 18pt;">Frequently Asked Questions Answered by Joaqu&iacute;n Reina Hern&aacute;ndez</span></h3><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 12pt;" data-preserver-spaces="true">1. What are the long-term sustainability considerations associated with biogas desulfurization technologies?&nbsp;</span></h2><p style="text-align: justify;"><span data-preserver-spaces="true">In this case, the technology must be environmentally and productively sustainable.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">By productively sustainable process, we understand that desulfurization technology minimizes the use of natural resources, the generation of toxic materials, waste, and polluting emissions, while environmentally sustainable implies a balance between techno-economic development and the protection of the environment surrounding said technology.</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">With desulfurization technology, on the one hand, operating costs are minimized, both in the energy production system and the production systems of other products such as Biomethane, by minimizing changes in inputs, parts, and devices of the technologies involved, which guarantees the sustainability from the point of view process. At the same time, the emission of polluting gases, such as acid gases, into the atmosphere is avoided, guaranteeing environmental sustainability.</span></p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span style="font-size: 12pt;" data-preserver-spaces="true">2. What factors influence the selection of a particular biogas desulfurization treatment method?</span></h2><p style="text-align: justify;"><span data-preserver-spaces="true">We can comment that several factors can determine the use of one method or another of biogas desulfurization, among them:</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">The costs of the technology to be applied</span></strong></p><p style="text-align: justify;"><span data-preserver-spaces="true">Mainly the operating costs of said method. In this case, the option that provides the lowest production cost will always be sought.</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">The robustness and sensitivity of the method to be applied</span></strong></p><p style="text-align: justify;"><span data-preserver-spaces="true">A robust method that adapts to any change that the biogas may undergo in terms of flow and concentration of H2S is much more appropriate than one that does not have these characteristics.</span></p><p style="text-align: justify;"><strong><span data-preserver-spaces="true">Future use or application of biogas</span></strong></p><p style="text-align: justify;"><span data-preserver-spaces="true">If the objective of biogas is its use as raw material to produce Biomethane or other products where the minimum degree of contamination is required, a method must be chosen that does not contaminate the biogas in compounds that are difficult to remove, such as O2 and N2.</span></p><p style="text-align: justify;">&nbsp;</p><h2 style="text-align: justify;"><span data-preserver-spaces="true"><span style="font-size: 12pt;">3. What future trends do you foresee in developing and adopting gas technologies for biogas desulfurization?</span>&nbsp;</span></h2><p style="text-align: justify;"><span data-preserver-spaces="true">Until now, two technologies compete in the biogas desulfurization market when it comes to high biogas flows and H2S concentrations. Biology and biochemistry have been significantly improved to date, although biochemical technology has gained greater application due to its main characteristic of adaptability to any change in operating conditions: flow and concentration of H2S.&nbsp;</span></p><p style="text-align: justify;"><span data-preserver-spaces="true">Although to make both technologies more effective, it is necessary to consecutively apply technology for refining the treated biogas to guarantee high-quality biogas.</span></p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;">&nbsp;</p><p style="text-align: justify;">&nbsp;</p>
KR Expert - Joaquín Reina Hernández

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