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The fertilizer production capacity of Jiangyan Fertilizer Company has now reached 180,000 tons per year of synthetic ammonia and 300,000 tons per year of urea. In recent years, amid fierce competition in the fertilizer industry and especially due to the increasing strain on coal supplies and rising coal prices, companies have, in an effort to control production costs, begun using more high-sulfur coal. This has led to a significant increase in the sulfur compound content in semi-water gas, posing serious risks to subsequent production processes; therefore, these compounds must be removed, which in turn increases the load on the treatment systems and turns it into a bottleneck for production. The author provides the following analysis and summary regarding the application of the catalytic-tannin method for desulfurization in the semi-continuous process section of Jiangyan Fertilizer Company. 1 Selection of the semi-desulfurization process: The company requires that the H2S content after semi-desulfurization be less than 0.03 g/m3. Before 1999, the semi-desulfurization process used an ammonia-water catalytic method for desulfurization; however, with subsequent technological upgrades and expansions, as well as the increasing use of high-sulfur coal, the load on the semi-desulfurization unit far exceeded its original design capacity. In addition, the ammonia-water catalytic desulfurization method also has many problems: ① Ammonia is lost in large amounts during regeneration due to air stripping ； ②The capacity is low, at only 0.1 g/L ； ③Large circulation volume; the solution circulation rate is 800 m3/h, with high power consumption ； ④Since a certain ammonia concentration (20–30) must be maintained during the desulfurization process, ammonia needs to be continuously added to the solution in order to ensure effective desulfurization and compensate for the ammonia lost during the regeneration process; as a result, high consumption and elevated operating costs arise. When ammonia water catalysis is used for desulfurization, the H2S content after partial desulfurization remains high for an extended period, reaching 0.06–0.086/m3 – which is 2 to 3 times higher than the allowable operating limits. As a result, sulfur blockages in the packing of the decarburization section and instability in the copper washing process occur on multiple occasions; this increases copper consumption and leads to liquid carryover in the copper washing section, causing severe damage to the synthetic catalysts. Therefore, technical upgrades to the semi-remelting process are imperative. In 2000, the company upgraded the semi-derusting section by adopting a desulfurization process with higher efficiency – tannin desulfurization. Under normal conditions, when the H2S content in the semi-removed inlet gas is less than 3.0 g/m3 and the solution circulation rate is 650 m3/h, the desulfurization efficiency can exceed 98%; the H2S content after semi-removal is between 0.006 and 0.008 g/m3, which meets the requirements set for operational parameters. However, when burning high-sulfur coal, the H2S content in the gas after semi-removal can sometimes reach 10–20 g/m3, resulting in a slightly high H2S level after semi-removal, at around 0.03–0.05 g/m3. Especially when high-sulfur coal is used for a long time, the semi-removal efficiency declines significantly; under poor operating conditions, the H2S content in the gas after desulfurization can reach as high as 0.08 g/m3. The system must operate at reduced capacity at this time; otherwise, it will affect the decarburization and copper cleaning processes. Through various analyses and experiments, it has been proven that using a single desulfurization agent is difficult to meet the current production requirements. In 2001, the company decided to adopt a catalytic-tannin desulfurization process that combined the Type 888 desulfurization catalyst produced by the Experimental Chemical Plant of Northeast Normal University with tannin desulfurizers in the semi-dry process in order to improve the desulfurization efficiency. Five years of operational experience with this single-catalyst desulfurization process have shown that, compared to desulfurization using tannin alone, its sulfur capacity is nearly twice as high; the circulation rate is 85% of that in the traditional method. It requires less input, produces larger sulfur particles that are easy to separate by flotation, results in low levels of suspended sulfur, has a low solution viscosity, reducing the risk of tower blockages, and allows for longer production cycles. When high-sulfur coal is used, the removal rate of inorganic sulfur reaches 99.9%, and the efficiency of removing organic sulfur is as high as 85%; whereas without the addition of catalyst type 888, the removal rate of organic sulfur is only 50%. A comparison of the conditions before and after using the Type 888 desulfurization catalyst is shown in Table 1. http://www.nmtech.com.cn/jishuwang/upload1/0707121456519475.jpg 2 Process Flow: Our company currently has 2 semi-dehydration units, all of which use the same process flow. Semi-water gas with a temperature of 48°C and an H2S content of 0.6–1.5 g/m3, originating from the gas holder, is cooled to 37°C in scrubber tower A; dust and tar are removed from it, after which it is pressurized to 0.05 MPa by a Roots blower. Its temperature rises to 62°C before it enters scrubber tower B, where it is cooled again to 38°C. After desulfurization, the semi-water gas has an H2S content of less than 0.005 g/m3. It then enters the bottom of the desulfurization tower, where it comes into countercurrent contact with the desulfurization liquid sprayed from the top of the tower to absorb the H2S present therein. The semi-water gas exits the top of the desulfurization tower, passes through a carbon filter to remove some impurities from the gas, and then goes through an electrostatic tar remover to further eliminate impurities such as tar. Finally, it is cooled once more to 32°C in scrubber tower C before reaching the inlet of the first stage of the hydrogen and nitrogen compressor. The rich liquid that has absorbed H2S has a temperature of 44°C; it exits from the bottom of the desulfurization tower and is pressurized to 0.05 MPa by a regeneration pump before being sent to the injector at the top of the regeneration tank. In the injector, the desulfurization liquid passes rapidly through nozzles, creating a local vacuum that draws in air. Air is thoroughly mixed with the rich liquid for oxidative regeneration; the two-phase fluid exiting from the nozzle tailpipe enters the lower part of the regeneration tank and rises upward, continuing the regeneration reaction while floating sulfur bubbles to the surface. In the regeneration tank, the desulfurization liquid is regenerated at a temperature of 40–43°C for a residence time of 15–18 minutes. After regeneration, the suspended sulfur content in the desulfurization liquid is less than 0.2 g/L, and its temperature is 38°C. It then flows into the lean liquid tank via a regulator, and from there it is pumped into the desulfurization tower for cyclic absorption and reuse. The sulfur foam floated up in the regeneration tank enters the foam pool through an overflow weir, and is then pumped into the sulfur melting vessel by a foam pump. The sulfur melting vessel is indirectly heated by steam, and sulfur paste is produced through high-temperature melting. Reaction mechanism of type 3 888 desulfurization catalyst: The type 888 desulfurization catalyst is composed of a trinuclear xanthophyll sulfonate metal-organic polymer complex, which possesses strong oxygen-absorbing capacity. It can not only activate the absorbed oxygen but also adsorb H2S, HS–, and Sx2–, and undergo redox reactions with the activated oxygen that has been absorbed to release sulfur. Since the generated elemental sulfur separates from the Type 888 desulfurization catalyst and combines with tiny sulfur particles in the regeneration tank to grow larger, it can be easily floated and separated out. After absorbing oxygen to become activated and releasing oxidized H2S, the Type 888 desulfurization catalyst can continue to absorb oxygen for activation. Its structure remains stable throughout its role as a catalyst, allowing it to repeat the processes of oxygen absorption, oxygen activation, oxygen release, and further oxygen absorption in a high-frequency catalytic cycle; as a result, it possesses strong desulfurization catalytic capabilities even at low concentrations. 3.1 Absorption reaction (1) H2S removal reaction: http://www.nmtech.com.cn/jishuwang/upload1/0707121457492094.jpg 3.2 Regeneration reaction: http://www.nmtech.com.cn/jishuwang/upload1/0707121458275629.jpg 3.3 Side reactions: When the gas contains CO2, O2, and HCN, the following side reactions are likely to occur: http://www.nmtech.com.cn/jishuwang/upload1/0707121459009517.jpg http://www.nmtech.com.cn/jishuwang/upload1/0707121508246348.jpg 4 Main equipment of the semi-desulfurization system: Both Unit 1 and Unit 2 of the desulfurization system use the same equipment configuration; the main equipment parameters are shown in Table 2. http://www.nmtech.com.cn/jishuwang/upload1/0707121509156803.jpg 5 Main operational parameters The main operational parameters of the desulfurization system are shown in Table 3. http://www.nmtech.com.cn/jishuwang/upload1/0707121510022838.jpg 6 Advantages of the catalytic-tannin-based desulfurization process: (1) It can achieve a removal rate of 99.9% for inorganic sulfur in high-sulfur coal, while the removal rate of organic sulfur is as high as 85%. (2) The suspended sulfur content in the desulfurization liquid is below 0.2 g/L; it has low viscosity, making tower clogging less likely, resulting in low system resistance and an extended production cycle. (3) The circulation volume of the desulfurization liquid is reduced to only 85% of the original amount. The sulfur capacity increased by nearly 2 times after using the Type 888 desulfurization catalyst. The sulfur particles obtained through regeneration by flotation are large, making them easy to separate and recover; as a result, the sulfur recovery rate improved, and the purity of the product reached 99%. (4) The Type 888 desulfurization catalyst exhibits a low-concentration effect. (5) The mass transfer coefficient during the absorption process is high, resulting in a significant advantage in semi-deactivation efficiency. (6) The desulfurization solution contains only one type of catalyst, with no need to add co-catalysts; it belongs to the mono-catalytic method. This results in few influencing factors on the desulfurization process (unlike DDS desulfurization, where the addition of multiple components leads to complex effects on desulfurization). Moreover, the pre-activation process is simple and requires little time. (7) Tannin can react with Fe to form a protective film on the surface of equipment, thereby providing anti-corrosion protection. 7 Factors Affecting Desulfurization Efficiency (1) Total alkali (expressed as Na2CO3): From the perspective of the H2S removal reaction, the higher the total alkali content, the greater the mass transfer coefficient, which facilitates absorption and increases the sulfur capacity. However, an excessive amount of total alkali leads to more side reactions and the formation of precipitates that can cause tower blockages. It also increases alkali consumption, resulting in waste and higher operating costs. The total alkali level is generally best kept between 0.5 and 0.7 N. (2) In the alum (based on NaVO3) solution, NaVO3 is generally prepared by adding crude V2O5; the reaction equation is V2O5 + 2NaCO3 + H2O = 2NaVO3 + 2NaHCO3. If the concentration of NaVO3 in the solution is too high, the resistance in the desulfurization tower is prone to fluctuations ； If it is too low, the reaction speed is slow and the degree of semi-degradation is poor. NaVO3 is generally preferably controlled at 0.5–1.0 g/L. (3) Tannin concentration: If the tannin content is too low, the utilization efficiency of alum decreases, the sulfur capacity drops, and there is an excess of by-product formation ； Excess tannin content increases the viscosity of the solution, reduces the absorption and mass transfer coefficients, results in poor semi-removal efficiency, and also causes waste, thereby increasing operating costs. The tannin content is generally preferably kept at 1.5–3.0 g/L, with a ratio of gum to alum of 1.5–2.0. The tannin must be fully matured; otherwise, the desulfurization solution tends to foam and become sticky, which can lead to blockages and affect the efficiency of the semi-desulfurization process. Aging conditions: Tannin is heated with steam in an alkaline solution (pH > 9.0), with air introduced for oxidation and agitation ； Oxidize at a temperature of 85–90°C for 1–2 hours until the nitration value stabilizes around a certain value, and then add it to the system for use. (4) Type 888 catalyst: The amount of Type 888 catalyst added is generally kept between 10×10‑6 and 20×10‑6; too low an amount results in poor activity of the desulfurization solution and low desulfurization efficiency ； Too high a level leads to excessive oxidation, an increase in by-products, and abnormal sulfur deposition. The Type 888 desulfurization catalyst should be added evenly to the lean liquid tank. (5) Temperature: The temperature of the absorption solution should be <40°C, while the regeneration temperature is 35–42°C. At excessively high temperatures, the solubility of H2S in the desulfurization solution decreases, resulting in a lower desulfurization efficiency. Meanwhile, the regeneration process is hindered by the reduced solubility of oxygen, which makes it difficult to regenerate the rich liquid ； At too low temperatures (mainly in winter), the solution becomes viscous, and the salts present in it tend to crystallize and block the tower, resulting in increased resistance as well as poor regeneration efficiency. (6) The appropriate liquid-to-gas ratio for the desulfurization liquid circulation volume should be determined based on the actual production load; generally, the spray density should not be less than 45 m3/m2·h. (7) Regeneration time: To ensure effective regeneration of the rich liquid, the residence time of the solution in the regeneration tank must be controlled between 12 and 20 minutes. If the time is too short, regeneration is incomplete; if it is too long, excessive oxidation occurs, which is not conducive to sulfur precipitation. (8) Air absorption volume: An appropriate amount of air self-absorbed by the ejector is beneficial for solution regeneration and CO2 stripping. The suction volume is too high, causing excessive agitation of the solution in the regeneration tank; as a result, elemental sulfur does not float easily ； Too low a vacuum level is not conducive to regeneration. The vacuum volume is controlled by the inlet pressure of the ejector, generally ranging from 0.45 to 0.5 MPa. (9) pH value: From the perspective of the H2S absorption reaction, a higher pH value is more favorable for absorption; however, this also increases the absorption rate of CO2 and raises the viscosity of the solution ； When pH > 9.0, side reactions increase. Therefore, it is appropriate to maintain the pH value between 8.4 and 8.6. (10) Quality of sulfur recovery liquid: Our company uses a continuous sulfur melting process to produce molten sulfur from sulfur foam. During the sulfur melting process, the sulfur recovery liquid reaches a temperature of 135°C, resulting in complex reactions and products. If the separated high-temperature clear liquid is not recycled, it is both a waste and causes environmental pollution ； In the case of a direct recovery system, the presence of suspended particles and the by-products formed due to high temperatures affect the quality of the desulfurization solution; moreover, it can clog the packing in the desulfurization tower, increasing the resistance within the tower and shortening the production cycle. With an emphasis on environmental protection, cost savings, and energy conservation, the company employs two-stage precipitation and cooling for the high-temperature liquid resulting from sulfur recovery; this process causes suspended impurities and by-products to precipitate and flow into a sump, where they are activated using air. Once the temperature drops below 40°C, the liquid is pumped back into the lean liquid tank using an submersible pump, thus enabling its recycling for reuse. 8 Conclusion (1) The catalytic-tannin method for desulfurization boasts advantages such as a simple process structure, mature technology, easy adjustment and control of production operations, corrosion protection for equipment, low risk of tower blockage, low system resistance, and high desulfurization efficiency. These features facilitate stable production in enterprises, help save energy and reduce costs, and improve economic benefits; therefore, it should be widely adopted in the semi-desulfurization systems of nitrogen fertilizer manufacturers. (2) Desulfurization, regeneration, and sulfur recovery are closely related, influencing and restricting each other; none of these steps can be ignored. (3) In the catalytic-tannin-based desulfurization process, it is essential to strictly control aspects such as the curing of tannin, the composition of the desulfurization solution, the regeneration of the concentrated solution, and the quality of sulfur recovery from residual liquids. Only by doing so can the Type 888 desulfurization catalyst exert its optimal catalytic effect, thereby improving the purity of the output, extending the production cycle, and laying a foundation for stable and high-level production in the system.