What is the best process for desulfurizing biogas?

Biogas is a renewable energy source that is gaining increasing attention in light of the current energy shortage. Typically, the gas composition of biogas consists of methane (60%–70%) and carbon dioxide (30%–40%), with small amounts of gases such as hydrogen sulfide also present. Hydrogen sulfide is a highly toxic and harmful gas that has a strong corrosive effect on pipes, burners, instruments, and equipment ； After combustion, hydrogen sulfide turns into sulfur dioxide, which pollutes the environment and affects human health. China’s environmental protection standards stipulate strictly that when using biogas as an energy source, the hydrogen sulfide content in the biogas gas must not exceed 20 mg/m3. The mass concentration of hydrogen sulfide in biogas is generally 1–12 g/m3, which is far higher than the limits specified by China’s environmental protection standards. Therefore, the removal of hydrogen sulfide becomes an essential step in the utilization of biogas. 　　1 Traditional desulfurization methods 1.1 Wet desulfurization Wet desulfurization involves using a specific solvent to come into countercurrent contact with gas in order to remove hydrogen sulfide from it; the solvent is then regenerated and reused for absorption. Based on different absorption mechanisms, they are further divided into chemical absorption, physical absorption, physicochemical absorption, and wet oxidation. The wet desulfurization process is complex and requires significant investment; it is suitable for applications with large gas handling volumes and high hydrogen sulfide concentrations. Among them, the methods commonly used for biogas desulfurization include naphthoquinone absorption and ammonia water methods. 　　1.1.1 Naphthoquinone absorption method: The absorption solution is alkaline and capable of absorbing acidic gases; moreover, due to the buffering effect of its weak acidity, the pH value does not change rapidly when acidic gases are absorbed, thereby ensuring the stability of system operation. Furthermore, sodium carbonate solution absorbs H2S more quickly than CO2. Since these two acidic gases coexist in biogas, it is possible to selectively absorb H2S to some extent. This method has been successfully used to remove large amounts of CO2 from gases, and it can also be employed to remove acidic gases from natural gas and biogas containing CO2 and hydrogen sulfide. 　　The chemical reaction equation for the absorption of H2S by this solution is Na2CO3 + H2S = NaHCO3 + NaHS. Xu Ning, Zhu Yanmei, and others conducted improved research on naphthoquinone absorption solutions based on the composition of biogas, and determined an appropriate formula for using such solutions in the absorption of hydrogen sulfide from biogas, achieving a desulfurization rate of 99%–99.5%. 　　The main advantage of this method is its simple and inexpensive equipment ； The main drawback is that part of the sodium carbonate turns into bicarbonate, reducing absorption efficiency, while another part turns into sulfate and is consumed. 　　1.1.2 Ammonia solution method Hydrogen sulfide is an acidic gas; when it is absorbed by the basic ammonia solution, a neutralization reaction occurs, namely H2S+NH4OH=NH4HS+H2O. The first step is a physical dissolution process, during which hydrogen sulfide in the gas dissolves into the ammonia solution ； The second step is the chemical absorption process, during which the dissolved hydrogen sulfide and ammonium hydroxide undergo a neutralization reaction. The regeneration method involves blowing air into a solution containing ammonium hydrosulfide to carry out the reverse of the absorption reaction, thereby releasing hydrogen sulfide gas. The purified ammonium hydroxide solution is further used for absorption after being replenished with fresh ammonia ； The hydrogen sulfide generated during regeneration must be further processed to prevent environmental pollution. In the case of liquid-phase catalytic desulfurization using ammonia, the oxidation of hydroquinone by the solution causes sulfur compounds to be converted into elemental sulfur and separated, while the solution is regenerated at the same time. The resulting sulfur particles are relatively fine, making them difficult to filter and recover; they also have a strong adhesion to the packing and the wall of the tower, which can lead to sulfur blockages inside the tower and disrupt production. 　　The ammonia method uses ammonia water as an absorbent; it causes significant corrosion to equipment and pollutes the environment. However, it has economic advantages when treating coke oven gas, as it can utilize the alkali sources produced within the coking plant. The Chengdu Biogas Science Research Institute of the Ministry of Agriculture is researching techniques for removing CO2 and H2S from biogas using ammonia water, achieving an average removal rate of 99.9% for H2S. 　　1.2 Dry desulfurization Dry desulfurization is commonly used for the treatment of gases with low sulfur content. Common methods include membrane separation, molecular sieve separation, pressure swing adsorption (PSA), non-regenerable fixed-bed adsorption, and low-temperature separation. The desulfurization of biogas commonly employs the non-renewable fixed-bed adsorption method. There are various types of such non-renewable fixed-bed adsorption methods, which can be roughly classified into iron-based, zinc-based, activated carbon, activated alumina, and silica gel categories; they are often used in the precise desulfurization of gases with low sulfur content. 　　Iron oxide is an ancient desulfurizing agent, commonly known as sponge iron. Its desulfurization mechanism involves first reacting with H2S to form iron sulfide, and then oxidizing the iron sulfide into iron oxide. It can remove H2S at the 10-6 level from gases, and various improved methods have been developed for industrial use. 　　The dry method has disadvantages such as discontinuous operation, high labor intensity for replacing the desulfurization agent, and large floor space required for the installation. To this end, Niu Kesheng and Sun Yansheng continuously and quantitatively supplied air through an automatic control system while desulfurizing, thereby achieving the continuous reduction and regeneration of the desulfurizer. 　　At present, desulfurization technologies in China are fairly mature, with a variety of desulfurization methods and processes available. However, they all have the following drawbacks: dry desulfurization has low efficiency, it is difficult to regenerate the desulfurizing agents, and their sulfur capacity is relatively low; therefore, these methods are mainly suitable for precise desulfurization ； Wet desulfurization has a large processing capacity and high desulfurization efficiency, and it can operate continuously; however, the investment and operating costs are also high, which is difficult for ordinary users to bear when it comes to utilizing biogas. With increasingly strict environmental regulations, the development of efficient, low-cost, resource-based technologies that avoid secondary pollution has become the mainstream trend in the advancement of desulfurization technologies. Some new desulfurization methods (such as microbial decomposition, ozone oxidation, sulfur dioxide method, electrochemical method, and microwave method) are attracting increasing attention. Among them, biological desulfurization features low pollution, low energy consumption, and high efficiency, making it a focus of research. 　　2 Biological desulfurization Biological desulfurization involves using the metabolic activities of microorganisms to convert sulfides in biogas into elemental sulfur or sulfates. The process can be divided into 3 stages: the first is the dissolution process of H2S gas, that is, the transfer from the gas phase to the liquid phase ； Second, the dissolved H2S is absorbed by microorganisms and transferred into their cells ； Thirdly, H2S that enters microbial cells is broken down, transformed, and utilized by the microbes as a nutrient, thereby achieving the removal of H2S. 　　Based on the type of microbial activity, there are three types of microorganisms capable of converting sulfides, namely photosynthetic bacteria, denitrifying bacteria, and colorless sulfur bacteria. Among them, denitrifying bacteria require nitrates in the process of oxidizing sulfides, which thus imposes certain limitations on the application of this technology. 　　2.1 Photosynthetic bacteria Photosynthetic bacteria, also known as sulfur-oxidizing bacteria, are phototrophic bacteria that can use light energy for growth; they are widely distributed in natural environments. They play an important role in the cycling of elements such as carbon, nitrogen, and sulfur in nature, and can obtain energy from light. Photosynthetic bacteria use bacterial chlorophyll to capture light energy, with molecular hydrogen, reduced sulfides, or organic compounds serving as external electron donors, rather than water molecules. Therefore, bacterial photosynthesis differs from that of plants and algae; it is an anaerobic process that does not produce oxygen. During photosynthesis, different photosynthetic bacteria use various sulfur compounds (such as sulfides, elemental sulfur, sulfites, and thiosulfides) as electron donors to assimilate CO2. Some photosynthetic bacteria can also utilize thiol compounds. When using sulfides, photosynthetic bacteria first oxidize sulfides to elemental sulfur, which is then deposited either inside the cell (as in purple sulfur bacteria) or outside the cell (as in certain species of purple extrachromatic sulfur bacteria, green sulfur bacteria, and purple non-sulfur bacteria). Once the sulfides are exhausted, these bacteria further oxidize the elemental sulfur deposited inside and outside the cell into sulfate. 　　There are many types of photosynthetic bacteria, but only certain strains such as purple sulfur bacteria and green sulfur bacteria are capable of metabolizing sulfides. Only a very small number of purple non-sulfur bacteria can tolerate and utilize high concentrations of sulfides. 　　Under anaerobic and illuminated conditions, photosynthetic bacteria use H2S as a hydrogen donor to reduce CO2 and synthesize bacterial cells, while H2S is oxidized to S0 or further oxidized to sulfuric acid. The desulfurization reaction of photosynthetic bacteria can be expressed as follows: In bacteria belonging to the green and purple sulfur families, sulfides are first oxidized to S0. The oxidation reaction S2- → S0 is a non-enzymatic process that occurs very easily. While carrying out the H2S→S0 reaction, purple sulfur bacteria also carry out the S0→SO42- reaction. The latter reacts slowly, and the sulfur particles produced during the reaction accumulate within the cells; whereas green sulfur bacteria can only carry out the H2S→S0 reaction in the presence of H2S. The resulting sulfur particles adhere to the outside of the cells, causing the wastewater to become turbid and reducing its light transmittance, thereby affecting the desulfurization efficiency. Furthermore, photosynthetic bacteria require a low treatment load and a long hydraulic retention time, as well as stringent conditions such as light and anaerobic environment; thus, little progress has been made in this area, and it remains in the exploratory stage of batch tests or laboratory-scale trials. The main tasks in the future should be the development of photosynthetic desulfurization reactors, improving the efficiency of this process, controlling the light intensity, and addressing issues related to the separation of elemental sulfur. 　　2.2 Colorless sulfur bacteria Colorless sulfur bacteria include strictly chemoautotrophs and chemoheterotrophs, as well as some transitional types. This category of bacteria is merely a physiological term, not a taxonomic one, and includes species such as Thiobacillus beijerinckii and the genus Thiobacillus. They are capable of oxidizing hydrogen sulfide, elemental sulfur, thiosulfates, and tetrasulfates to form sulfuric acid, obtaining energy in the process. 　　China has conducted extensive research on colorless sulfur bacteria; for example, Fang Shi and others studied the use of biological tower aeration to remove H2S from biogas, achieving a maximum desulfurization rate of 98.6%. Wang Aijie et al. used Thiobacillus denitratans to simultaneously remove nitrogen and sulfur from wastewater, indicating that the sulfur-to-nitrogen ratio and sulfide concentration are the key factors in this simultaneous denitrification and desulfurization technique. Maintaining these ratios at 5:3 and the sulfide concentration below 300 mg/L respectively can achieve good desulfurization and denitrification results, with the conversion rate of elemental sulfur reaching up to 94% ； Li Yaxin et al. conducted experiments on the removal of hydrogen sulfide using colorless sulfur bacteria, and recovered the elemental sulfur produced as a result of desulfurization. It was found that the dissolved oxygen level in the reactor was linearly related to the volumetric load of sulfides in the influent water, while the pH value of the effluent water and the increase in this pH value were linearly related to the recovery rate of sulfur. Among the microbial groups of colorless sulfur bacteria, not all sulfur bacteria can be used for sulfide oxidation. Since some sulfur bacteria accumulate the produced sulfur within their cells, in addition, the growth of unwanted microorganisms can cause sludge bulking in the reactor, posing difficulties in the separation of elemental sulfur. If separation is not carried out in a timely manner, further oxidation will occur, which affects the desulfurization efficiency; therefore, during the operation of the desulfurization unit, it is also necessary to strictly control the reaction conditions in order to prevent the excessive growth of such microorganisms. 　　3 Conclusions and Perspectives Traditional methods for desulfurizing biogas have their limitations, whereas microbial desulfurization features mild conditions, low energy consumption, minimal investment requirements, and reduced waste emissions; it is particularly suitable for treating biogas with moderate to low sulfur content, and thus has gradually become a new focus of research in the field of desulfurization. 　　In terms of desulfurization equipment and processes, biological desulfurization is currently mainly at the laboratory research stage; to achieve large-scale application, further efforts are needed in terms of desulfurizing microorganisms, bioreactors, and desulfurization processes. Conduct detailed physiological and biochemical studies on the currently discovered sulfide-oxidizing bacteria, in order to identify efficient strains that can be used for the biological removal of H2S ； Utilize modern biotechnological tools to isolate and identify genes associated with sulfur oxidation or functional genes, in order to develop new high-efficiency bacterial strains ； By integrating biotechnology and chemical engineering technologies, efficient biological reactors with continuous flow operation and stable desulfurization performance are developed, and the process is optimized to provide technical support for further industrialization.