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Gan Changhai (Tianjin No. 2 Gas Factory); Tian Zuolin (Changchun Changle New Environmental Protection Materials Manufacturing Co., Ltd.). The coke oven gas desulfurization unit at Tianjin No. 2 Gas Factory was originally designed to use the FR (Formax-Lodacos) method for gas desulfurization, with ammonia in the gas serving as the base and picric acid as the catalyst. The gas from the condensation and blower process enters desulfurization towers A and B respectively, and the gas from these two towers is combined and sent to the ammonium sulfate production process. Inside the desulfurization tower, the gas comes into contact from bottom to top with the oxalic acid aqueous solution sprayed from the top of the tower; this solution absorbs H2S and HCN from the gas. The reaction mechanisms are as follows: NH4OH + H2S → NH4HS + H2O; NH4OH + HCN → NH4CN + H2O. The liquid rich in H2S and HCN is pumped by a circulation pump to the bottom of the regeneration tower, where it mixes with compressed air before entering the regeneration tower, where redox reactions take place under the catalysis of oxalic acid. NH4HS + R-NO + H2O → NH4OH + S↓ + R-NHOH; R-NHOH + 1/2O2 → R-NO + H2O. NH3 + H2S + xS → (NH4)2Sx + 1; (NH4)2Sx + 1 + NH4CN → (NH4)2Sx + NH4SCN. The regenerated sulfur foam flows into the buffer tank through the overflow pipe of the regeneration tower, while the regenerated desulfurization liquid is pumped back to the top of the desulfurization tower using a circulation pump for spraying. The liquid in the buffer tank is defoamed using a pumping regeneration tower; the sulfur foam is sent to a centrifuge for separation. The resulting sulfur slurry is packaged, while the filtrate is sent to a filtrate tank and pumped back to the buffer tank. When the concentration of by-products in the system reaches 200 g/L, it is sent to a waste liquid tank for storage. 1 Problems exist: The desulfurization process using picric acid as a catalyst is effective, but since picric acid is a hazardous chemical that is flammable and explosive, it poses significant risks during transportation, storage, addition to the reaction mixture, and the maintenance of related equipment, thereby adding complications and difficulties to the operation process. It is also highly sensitive to the levels of tar and dust in the gas; fluctuations in these levels have a significant impact on its operation, which requires a high collection efficiency from the electrostatic tar catcher. Furthermore, the concentration of by-products increases rapidly, reaching 200 g/L in about 2 months on average; it is therefore necessary to use waste liquid discharge methods to reduce the concentration of by-products in the desulfurization solution. Therefore, it is necessary to find catalysts with high desulfurization efficiency and slow increase in by-product salt concentration to replace the picric acid catalyst. 2 Applications of ZL catalyst 2.1 Selection of ZL catalyst After research, we decided to use the ZL catalyst as a substitute for picric acid. The choice was based on the following properties of ZL catalysts: it can adsorb and activate dissolved oxygen in alkaline solutions to form highly active large ions ； When encountering sulfur-containing compounds such as H2S, they are adsorbed onto the highly active surfaces of large ions, causing the sulfur in these compounds to be oxidized into elemental sulfur or polysulfides ； Elemental sulfur or polysulfides desorb from the ZL surface and leave ； ZL is regenerated by regaining oxygen. Furthermore, under the action of the ZL catalyst, NH4SCN can also undergo the following transformation reactions: SH + NH4SCN → NH2–C=CH → NH2–C–NH2; SH + OH → NH2–C=NH + ON– → NH2–C=NH + HS–; OH + 2H2O → NH2–C=NH → NH2–C–NH2 → (NH4)2CO3. During oxidation regeneration, the ZL catalyst can catalyze the oxidation of HS–: HS– + 1/2O2 → OH– + S↓. It can be seen that by using the ZL catalyst in the desulfurization process, side reactions can be controlled more effectively, thereby reducing the rate of formation of by-products. Under the same process conditions, compared with other catalysts, the ZL catalyst features larger sulfur foam particles that are easy to separate and do not clog the equipment; it also has advantages such as lower usage amounts and reduced operating costs. Table 1 Process parameters of the ZL catalyst: Gas temperature, °C: 35; Absorption liquid temperature, °C: 35; Circulation rate of desulfurization liquid, m3/h: 500; Amount of air used for regeneration, m3/h: 600; Amount of solution used for defoaming, m3/h: 30; Gas holdup in the regeneration tower, %: 10; ZL concentration in the desulfurization liquid, ppm: 10–20; ZL concentration in the regeneration liquid, ppm: 30–40; Consumption of ZL catalyst, kg/month: 70; Concentration of by-products in the solution, g/L: <200. 2.2 Usage of the ZL catalyst: (1) Adjustment and control. Since our factory began using ZL catalysts in 2004, in order to make the bagging of sulfur more convenient, we replaced the centrifuge with a plate and frame filter press; no changes were made to the main processing procedures. Apart from adjusting the amount of ZL added based on the hydrogen sulfide content in the gas coming out of the desulfurization tower, no other adjustments were made to the process parameters. The process control parameters are shown in Table 1. (2) Usage effect. After using the ZL catalyst, the desulfurization efficiency shows no significant difference compared to that of picric acid, but it has clear advantages in other aspects. Firstly, the amount of catalyst used is reduced, and its application is simplified, which eliminates the risks associated with picric acid during transportation, storage, application, and maintenance. Just by reducing catalyst consumption, 280,000 yuan can be saved each year. Secondly, the growth rate of by-products slowed down significantly, and the discharge cycle of desulfurization wastewater was extended from 1–2 months to 3–4 months. In particular, tar and dust content in the gas have almost no effect on it, completely resolving the issue of disruptions in the desulfurization system caused by improper operation of the electrostatic tar catcher. 3 Conclusion The ZL desulfurization catalyst features high desulfurization efficiency, a slow increase in the concentration of by-products, minimal impact from tar and dust in the gas, high safety, and low cost; it is a successful alternative to picric acid in the FR process for gas desulfurization.