Summary of Ammonia Synthesis Desulfurization and Purification in Our Company

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0 Preface: The synthetic ammonia production capacity of Jinchang Chemical Group’s Synthetic Ammonia Company is currently 125 kt/year, with supporting capacities for caustic soda at 180 kt/year and diammonium phosphate at 12 kt/year. The total volume of semi-water gas is approximately 55,000 m3/h (at standard conditions). The desulfurization unit employs a wet desulfurization process using soda ash together with the \"888\" desulfurizing agent. Since the ammonia synthesis capacity has been developed through various modifications, three desulfurization towers are currently in use in parallel. φ2200mm packed tower, H=33 910mm, gas flow rate 10000～13000m3/h (standard conditions) ； φ2400mm spiral plate tower, H=18140mm, gas flow rate 18000～20000m3/h (standard conditions) ； φ3000mm packed tower, H=26650mm, gas flow rate 20000～25000m3/h (standard conditions). Among them, the φ2200mm tower and the φ2400mm tower share one solution system; the regeneration tank has dimensions of φ5,018 mm × 5,550 mm, with a solution circulation rate of 340 m3/h, and spray regeneration is used ； The φ3000mm tower uses a separate solution system; the regeneration tank is φ6000mm×9700mm, with a solution circulation rate of 200 m3/h, and air bubbling is used for regeneration ； The two systems share one sedimentation tank, are regenerated using the “888” desulfurization agent, and are equipped with one set of φ600mm continuous sulfur melting equipment. 1 Operation status: Since the desulfurization system is designed for burning high-quality lump coal, the mass concentration of H2S in the semi-water gas is 500–600 mg/m3, while the mass concentration of H2S in the desulfurized gas can be controlled at 30–70 mg/m3. Starting in 2005, the company changed its raw coal source and began using locally available low-quality coal dust to produce coal briquettes for gas generation; due to the high sulfur content in this coal dust, the H2S level in the semi-water gas increased ； In the second half of 2006, high-sulfur coal with a sulfur content of 4% to 6% (by mass) was gradually introduced, causing the H2S concentration in the semi-water gas to exceed 1,000 mg/m3, with peak values reaching 1,700 mg/m3. The H2S mass concentration at the outlet of the desulfurization tower reaches 100–200 mg/m3, which severely affects the production in the ammonia synthesis and soda ash production units. 2 Solutions: In order to be able to burn a portion of high-sulfur coal within the capabilities of the existing equipment, starting in October 2006, the company adjusted the operating procedures of the desulfurization unit, attempted various technical solutions, and achieved certain results. (1) Due to the unstable amount of high-sulfur coal added, the amounts of soda ash and desulfurizing agents are adjusted according to changes in the H2S content in the semi-water gas, in order to standardize the addition of these agents; the data on the amounts added are shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload1/071026915498148.jpg (2) Using \"888\" desulfurizer as the main component, an appropriate amount of tannin is added. Initially, the amount of tannin added was 5–10 kg/d. Due to limited design capacity, incomplete regeneration, and too small sedimentation tanks, the foam could not be fully settled, resulting in high levels of suspended sulfur. When the mass concentration of H2S in the semi-water gas reaches 1,000 mg/m3 or higher, sulfur foam from the regeneration tank and the rich liquid tank overflows from the top ； Furthermore, the viscosity of the sulfur foam increases, making it difficult to precipitate, and the recovery rate of sulfur is low. To ensure the precipitation of sulfur foam, the amount of tannin added was gradually reduced, and it was finally adjusted to 2 kg/d ； When the mass concentration of H2S in semi-water gas is less than 500 mg/m3, no tannin is added to the system. After adding tannin, the most noticeable effect was an increase in the Na2CO3 level in the solution, which in turn improved the desulfurization efficiency. Especially when the mass concentration of H2S in semi-water gas reaches 1,000 mg/m3, the desulfurization outlet parameters can be kept within the specified limits; this facilitates the use of high-sulfur coal even when the desulfurization capacity is limited. (3) Strengthen solution management and ensure proper sulfur melting. Due to the small size of the sedimentation tank, frequent sulfur melting is not conducive to the precipitation of foam ； Repeated boiling of the solution also accelerates the formation of by-products. When the mass concentration of H2S in semi-water gas is greater than 1000 mg/m3, sulfur melting is carried out once per day during the daytime ； When the H2S mass concentration is less than 1,000 mg/m3 or lower, sulfur melting is carried out once every 2–3 days. Every time sulfur is melted, the foam in the sedimentation tank must be cleaned thoroughly. To prevent the high-temperature molten sulfur solution from entering the system directly, it is cooled and precipitated in a dedicated sedimentation tank before being recycled, which helps to reduce the formation of by-products. (4) To enable the combustion of more high-sulfur coal within the existing equipment capacity, and to address the issues of excessive sulfur foam and incomplete precipitation, the main measure is to improve the regeneration capacity of the Stage I desulfurization unit. To this end, the company plans to add one more regeneration tank and double the volume of the sedimentation tank. Furthermore, due to the high temperature of the gas in the company, the temperature of the desulfurization solution rises to 50–56°C during summer, which affects the desulfurization efficiency and leads to significant loss of the solution ； The fact that the solution temperature drops to around 40°C in winter is also a factor contributing to improved desulfurization efficiency. To ensure that the solution temperature remains within the specified limits during the summer, the company plans to focus on controlling the gas temperature this year, reducing it from a maximum of 55°C to below 40°C. This will help keep all process parameters in the desulfurization unit at their optimal levels, thereby increasing the production capacity of this unit. 3 Comparison before and after adding tannin: (1) By adjusting the amount of the additive and adding an appropriate amount of tannin, it is possible to remove sulfur from high-sulfur gas and maintain normal production. The desulfurization effect data before and after the addition of tannin are shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload1/071026916301575.jpg (2) By adjusting the amount of the reagent and adding tannin, the regeneration efficiency of the solution improves, while there is no significant change in the content of by-products. The data before and after adjusting the drug dosage are shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload1/071026917003933.jpg 4 Conclusion From the experimental results, adding an appropriate amount of tannin to the desulfurization solution in which “888” is used as the main desulfurizing agent helps to improve the regeneration of the solution and its desulfurization capacity. However, due to the limited capacity of the equipment, regeneration and precipitation are not complete; therefore, too much tannin cannot be added. When funds for technical upgrades are limited, using a combination of \"888\" and tannin-based desulfurization agents for desulfurization can increase the production capacity of equipment, save on investment costs, and reduce production expenses.