Case Study on the Application of Dongshi DSH Desulfurization Catalyst in Biogas Desulfurization Systems

Thread Content

Biogas is a combustible gas produced by the fermentation of various organic substances under anaerobic conditions (reducing conditions), at suitable temperatures and pH levels, through the action of microorganisms. Hydrogen sulfide contained in biogas must be removed prior to use due to environmental and process requirements. At a bioenergy company in Xinjiang, as the production load increased, the hydrogen sulfide concentration also doubled, resulting in significantly higher levels of hydrogen sulfide in the output after desulfurization, which failed to meet the requirements of downstream customers. The company communicated with many relevant technicians and came up with a plan for equipment modification; however, time and financial constraints caused it great difficulties. After on-site inspection by Dongshi technicians, it was determined through analysis that Dongshi’s DSH high-sulfur-capacity salt-suppression catalyst can resolve this issue, thanks to its high sulfur capacity and excellent salt suppression properties. Furthermore, replacing the catalyst yielded excellent results: the hydrogen sulfide indicators remained within acceptable and stable limits, alkali consumption was reduced, and this was well-received by the client. Keywords: biogas, hydrogen sulfide, Dongshi brand DSH catalyst. 0. Introduction In China, biogas has broad application prospects; various geographical environments from the south to the north provide favorable conditions for the development of biogas energy. The “four-in-one” biogas ecological agriculture model, which combines plastic greenhouses, biogas digesters, livestock and poultry sheds, and toilets, has been promoted in northern China. In the central region, the “pig–biogas–fruit” ecological orchard model has been developed; this model integrates greenhouse vegetable cultivation with biogas as a key link. In the southern regions, models such as “pig–biogas–fruit”, “pig–biogas–fish”, and “cattle–biogas–grass” have been established. Additionally, some areas have adopted localized models like “breeding–biogas–planting (or breeding)”. All these models utilize biogas as a connecting element to achieve multi-level energy utilization and cyclic material use within an ecological agricultural framework. During the anaerobic digestion of organic waste to produce biogas, H2S is generated. H2S is a highly corrosive compound that can severely damage gas pipelines, instruments, and combustion equipment. Its combustion product, sulfur dioxide, is also a highly corrosive gas; when released into the atmosphere, it can lead to the formation of “acid rain”. H2S is also highly harmful to the human body; it irritates the mucous membranes and respiratory tract. An air concentration of 0.1% H2S can be fatal within seconds, which is why removing hydrogen sulfide is an essential process. 1. Biogas process flow and its main components
1.1 Biogas process flow
Biogas fermentation is an anaerobic fermentation technology. This technology primarily involves the pretreatment of raw materials, which are then fed into a fermenter. Fermentation takes place under suitable temperature and pH conditions. The resulting biogas, after undergoing desulfurization and decarburization processes, can be compressed to produce CNG; alternatively, it can be used directly as fuel following desulfurization. After fermentation, solid-liquid separation of the residues yields biogas slurry and biogas residue. The biogas slurry, when mixed with water, can be used for irrigation to enhance soil fertility and provide various nutrients for plants. The biogas residue can be processed into organic fertilizer to improve soil structure. 1.2 Main components of biogas: Generally speaking, the main component of biogas is methane. Biogas is composed of 50%–80% methane (CH4), 20%–40% carbon dioxide (CO2), 0%–5% nitrogen (N2), less than 1% hydrogen (H2), less than 0.4% oxygen (O2), and 0.1%–3% hydrogen sulfide (H2S), among other gases. Since biogas contains a small amount of hydrogen sulfide, it has a slight odor. Taking the actual application of our company in a bioenergy company under CGN in Xinjiang as an example, the hydrogen sulfide (H2S) content in that company’s biogas is 0.5–1.5%. 2. Challenges of biogas desulfurization systems: Due to the significant fluctuations in the biogas production process and in hydrogen sulfide levels, coupled with biogas’s inherent characteristics of high hydrogen sulfide and high carbon dioxide content, its desulfurization proves to be quite difficult. (1) In such a gas source environment with a high hydrogen sulfide content, traditional cobalt phthalocyanine catalysts exhibit deteriorating adaptability in terms of their catalytic oxidation capacity under fixed desulfurization process conditions, especially when the production load is high and the hydrogen sulfide level is also elevated—that is, when both indicators are at high levels. (2) A high carbon dioxide content, on the other hand, leads to an increase in the concentration of NaHCO3 in the solution, resulting in increased alkali consumption as well as the crystallization of NaHCO3 within the solution, which can cause blockages in the piping and the packing inside the tower. 3. Examples of the successful application of new DSH desulfurization catalysts in biogas desulfurization systems include the practical use of the Dongshi brand DSH high-sulfur-capacity salt-suppressing desulfurization catalyst at a bioenergy company owned by China General Nuclear Power in Xinjiang. In this project, the biogas flow rate was 15,000 Nm3/d, with a H2S content in the biogas ranging from 5,000 to 15,000 ppm. Two desulfurization towers operated in parallel; the inlet pressure was ≤2 Kpa, the resistance of the desulfurization towers was ≤1 Kpa, and the temperature of the inlet biogas was 28–36°C℃ ； The total solution volume is approximately 80 m3, and the designed circulation rate of the solution in the two towers is 70 m3/h; however, due to crystallization blockages in the pipelines, the actual circulation rate is currently 50 m3/h. Initially, the biogas production was around 8,000 Nm3/day, but increasing this production led to a significant rise in the hydrogen sulfide content in the biogas, causing the hydrogen sulfide level at the desulfurization outlet to often exceed the limit, reaching 600 ppm. It severely affects the usage needs of downstream customers; however, due to a lack of time for maintenance and additional investment in upgrades, it was decided to use the Toyo Shoki DSH catalyst to resolve this issue. (1) Without any modifications to the equipment, and with an increase in gas flow rate leading to a rise in hydrogen sulfide concentration, the desulfurization efficiency improved significantly, rising from 90.37% to 99.17%, while the average hydrogen sulfide level at the outlet decreased from 517 ppm to 53 ppm. Before application: The hydrogen sulfide levels at the outlet were significantly above the acceptable limits, affecting customer use; the average hydrogen sulfide level at the inlet was 5367 ppm, with a maximum of 6000 ppm, while the level at the outlet was 517 ppm. The desulfurization efficiency was 90.37%. After application: Exported hydrogen sulfide levels decreased significantly, meeting the requirements. The average imported hydrogen sulfide concentration was 6,416 ppm, with a maximum of 12,000 ppm. The average exported concentration was 53 ppm, and the desulfurization efficiency reached 99.17%. (2) Sulfur paste production increased by 71.42% ； Before application: On average, two filter plates worth of sulfur paste are removed per week ； After application: on average, one plate and frame is used per day for desulfurizing paste. (3) No waste liquid discharge, reducing desalinated water consumption ； Before application: The system’s regeneration effect was poor, which affected the absorption efficiency; 3 cubic meters of liquid had to be drained daily. After application: As the regeneration effect improves, the absorption efficiency increases significantly, liquid discharge is greatly reduced, thereby lowering the consumption of demineralized water. (4) Soda ash consumption decreased significantly, by 27.5%. Before application: Average daily consumption of soda ash was 120 kilograms ； After application: the average daily consumption is 85 kilograms, a 27.5% reduction compared to before. 4. In summary, the DSH high-sulfur-tolerance desalination catalyst developed independently by Dongshi offers significant advantages for biogas desulfurization; it enables the maintenance of stable hydrogen sulfide levels even in conditions of high carbon dioxide levels and reduced solution circulation volume ； It can also promptly float sulfur foam out of the system, thereby increasing the yield of sulfur paste. This prevents sulfur particles from remaining in the system and clogging the tower due to ineffective flotation. Additionally, it significantly reduces the formation of by-product salts and the volume of effluent discharged; consequently, it also decreases the consumption of soda ash. It fully demonstrates the catalyst’s excellent properties, such as high sulfur capacity, strong salt suppression ability, and great convenience in practical applications.