How to choose for biogas desulfurization

For large-scale and high-concentration desulfurization, the wet method is used; conversely, the dry method is employed. Those who have the means and are not averse to extra effort opt for the biological method

Additionally, the desulfurization accuracy of the wet method is not high; it can only achieve a level of a few hundred ppm

It’s very insightful: for large-scale and high-concentration desulfurization, the wet method is used; conversely, the dry method is employed. Those who have the means and are not averse to extra effort opt for the biological method. Both wet and dry methods can achieve levels below 30 ppm; biological desulfurization does not have such precision.

However, biological treatment produces cleaner results, while both wet and dry methods have issues with secondary pollution. This is especially true for the dry method: the used desulfurization agents can only be landfilled, and when they are removed, a significant amount of sulfur reverts to a gaseous state and escapes

What is the adaptability of raw materials for biological desulfurization? It seems that the cost of microbial strains remains a significant issue

The removal accuracy of dry and wet methods is not limited to these values; especially with the dry method, it’s possible to achieve 1 PPM without any problem. The key factors here are sulfur capacity and the replacement cycle. Could you tell me what level of accuracy can be achieved in biological desulfurization, according to your knowledge?

I agree with your view. In biogas systems, sulfur exists in various forms; aside from hydrogen sulfide, there are also various organic sulfur compounds. Does biological desulfurization exhibit corresponding selectivity?

Wet flue gas desulfurization can be classified into three methods: physical absorption, chemical absorption, and oxidation. The chemical absorption method uses substances such as sodium carbonate, ammonia, and alcoholamine solutions to absorb hydrogen sulfide ； The method of using cold methanol to absorb hydrogen sulfide is a physical absorption method ； The method that uses an alkaline solution as an absorbent and adds an oxygen carrier as a catalyst to absorb hydrogen sulfide and oxidize it into elemental sulfur is known as the oxidation method. This method relies on solution absorption and features oxidation and regeneration; therefore, this desulfurization method is also called wet oxidation. Among the above three desulfurization methods, physical absorption and chemical absorption pose the problem of hydrogen sulfide reprocessing. Currently, the wet oxidation method is mature and reliable, with a desulfurization efficiency of over 99.5%. The domestic wet oxidation methods are mainly represented by the modified ADA method, tannin method, PDS method, etc. (1) Modified ADA method: Also known as the modified anthraquinone disulfonic acid method, it is a mature oxidation desulfurization technique with a desulfurization efficiency of over 99.5%. Its main reactions are as follows: The reaction in the desulfurization tower: Hydrogen sulfide is absorbed by a dilute alkaline solution with a pH of 8.5–9.2 to form hydrosulfides. Na2CO3 + H2S → NaHS + NaHCO3. Hydrogen sulfide reacts with sodium metavanadate to produce elemental sulfur: 2NaHS + 4NaVO3 + H2O → Na2V4O9 + 4NaOH + 2S. The oxidized form of ADA is used to oxidize pyrovanadate sodium to yield sodium metavanadate: Na2V4O9 + 2ADA (oxidized form) + 2NaOH + H2O → 4NaVO3 + 2ADA (reduced form). In the regeneration process, the reduced form of ADA is oxidized by oxygen in the air to its oxidized form, after which the solution is pumped back into the absorption tower for reuse. 2ADA (reduced form) + O2 → 2ADA (oxidized form) + 2H2O. In the modified ADA solution, sodium carbonate (Na2CO3) acts as an absorbent; ADA serves as the oxygen carrier for sulfur deposition, while vanadate acts as a catalyst for this sulfur deposition process. The improved ADA method is a desulfurization technique with mature technology, high degree of process standardization, stable solution properties, and favorable technical and economic indicators. This method also has the advantages of a high sulfur recovery rate, high purity of the recovered sulfur, no toxic effects on humans and organisms, and no corrosive effect on carbon steel. Its drawback is that sulfur buildup can easily occur in the packing of the desulfurization tower. (2) Tannin method: Tannin is a polyphenol (tannin) substance that can replace ADA as an oxygen carrier, and it is inexpensive. The absorption efficiency of this method is similar to that of ADA, and it has advantages such as low likelihood of clogging the desulfurization tower packing, abundant tannin resources, and low cost. Its drawback is that both preparing the desulfurization solution and adding tannin to the system require a heating and melting process. (3) PDS method: PDS is a mixture of phthalocyanine sulfonate compounds, which possesses high catalytic activity. PDS desulfurization shares the same production process as various other wet oxidation desulfurization methods; therefore, switching from the ADA method or the tannin method to PDS desulfurization does not require any changes to the existing process – simply PDS is used in place of ADA or tannin in the desulfurization solution. PDS has good catalytic activity, requires a small amount, and results in low consumption. During solution regeneration, the sulfur crystal particles that produce sulfur foam are large and easy to separate; therefore, the suspended sulfur content in the desulfurization solution is low. This method not only removes inorganic sulfur but also exhibits excellent efficiency in removing organic sulfur. PDS has no toxic effects on the human body, and the desulfurization liquid causes little corrosion to equipment. Due to the extremely low PDS content in the desulfurization solution and its tendency to be carried away by sulfur foam, adding it all at once would result in PDS being removed from the system by the foam before it can participate in the reaction, leading to significant losses. Therefore, it is necessary to add it in a continuous 24-hour drip process. Additionally, the most recent representatives of desulfurization agents are SY-7 and 888; they are catalysts developed to address the shortcomings of PDS, and they feature good catalytic activity, low usage requirements, and reduced consumption.

Dry desulfurization is a method that uses solid absorbents or adsorbents to remove hydrogen sulfide or organic sulfur. Dry desulfurization has the advantages of a short process, simple equipment structure, high gas purification efficiency, and stable operation. However, this method typically uses a fixed-bed reactor, requires regular replacement of the desulfurization agent, and cannot operate continuously. Due to the limitation of the sulfur capacity of desulfurizers (the maximum amount of sulfur that can be removed per unit mass of desulfurizer), dry desulfurization is generally used in cases with low sulfur content. There are many methods for dry desulfurization, and the main ones currently in use include the iron oxide method, activated carbon method, zinc oxide method, and hydrolysis of organic sulfur (COS, CS2) methods. The advantage of wet sodium carbonate is its ability to withstand fluctuations in gas volume and hydrogen sulfide levels, while the advantage of dry iron oxide desulfurization is its high precision in removing sulfur. The gas first undergoes preliminary rough desulfurization via wet desulfurization to remove most of the hydrogen sulfide from biogas. The small remaining amount of hydrogen sulfide that is not absorbed by wet flue gas desulfurization is absorbed by the iron oxide desulfurization agent used in dry flue gas desulfurization; as a result of the combination of wet and dry desulfurization methods, the hydrogen sulfide concentration is reduced to <8 mg/Nm3. Wet sodium carbonate desulfurization employs a process of recycling the desulfurization liquid, resulting in no polluted wastewater being discharged. Moreover, the sulfur foam filtered out during wet desulfurization, as well as the desulfurizing agent that has absorbed hydrogen sulfide in dry desulfurization, can be collected and sold to sulfuric acid plants. The chemical raw materials and equipment used in this process are all non-toxic and harmless, easy to operate, and pose no safety risks. Compared with sodium hydroxide desulfurization and pure dry desulfurization, it has advantages such as high desulfurization efficiency and low operating costs. Building on the mature tannin-based desulfurization technique used for gas desulfurization in the past, this process has undergone numerous improvements and now reaches the advanced level of the international industry. Wet sodium carbonate desulfurization achieves stable efficiency, while dry desulfurization offers high precision. By adopting a combined wet-dry desulfurization method, the advantages of both approaches are utilized to optimize both desulfurization precision and operating costs.

The biological treatment processes that I am aware of include those in which absorption and reaction take place in one reactor, and those in which absorption and biological reaction occur in two separate reactors. I am more familiar with the latter approach; it seems to offer reasonable levels of investment cost and operational simplicity. I’m not sure if this refers to the first type of process.