Problems and Countermeasures of Gas Desulfurization Devices Using Vacuum Sodium Carbonate Method

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In order to remove harmful substances such as hydrogen sulfide from coke oven gas, thereby purifying it and reducing environmental pollution, Anyang Iron and Steel Group Company and the Japan Coal Comprehensive Utilization Center jointly established a demonstration project for the desulfurization of coke oven gas at the Anyang Iron and Steel Coke Plant. 1 Process flow: The gas desulfurization unit consists of the vacuum sodium carbonate desulfurization process (VASC) and the Claus process for sulfur recovery (SCL). The VASC process uses sodium carbonate as a base source to remove H2S and HCN from coke oven gas in a desulfurization tower; the resulting solution is then sent to a regeneration tower where acidic gases such as H2S and HCN are separated out, with the sodium carbonate solution being reused. After being cooled by a condenser and dehydrated by a separator, the acidic gases are pumped into the SCL process using a vacuum pump. In the SCL process, acidic gases are burned in a Claus furnace; 1/3 of the hydrogen sulfide is converted into SO2 to form process gas, which is then reacted in a catalyzed multi-stage converter to produce elemental sulfur. The regeneration tower is heated by ammonia water circulated from the coke oven, and the oil-containing condensate from the vacuum pump is regularly sent to a mechanical clarifier. 2 Operating conditions and existing problems (1) Desulfurization efficiency and sulfur quality. The capacity of this device for treating coke oven gas is around 70,000 m3/h. To evaluate the performance of the device, China and Japan jointly conducted two 24-hour continuous tests in January and April 2003 to measure the sulfur content in the gas exiting the desulfurization tower, the purity of the sulfur product, and the key operational parameters of the system. The hydrogen sulfide content in the gas after the desulfurization tower is on average 320 mg/m3, and the purity of sulfur is 99.7%, both of which meet the design specifications (H2S ≤ 500 mg/m3 and sulfur purity ≥ 99.5%). The hydrogen cyanide content in the clean gas can be reduced to 170 mg/m3. (2) There are problems. Since the commissioning of the desulfurization unit, some problems have occurred; those related to the VASC process were resolved satisfactorily by the end of 2004. However, components in the SCL process such as the acidic gas preheater, hydrogen cyanide decomposer and its outlet pipes, as well as the acidic gas burner, often become clogged, severely affecting normal production. 3 Blockage Analysis: In January and March 2005, during system maintenance, samples of the blockages in the acidic gas preheater, the TiO2 catalyst layers in the hydrogen cyanide decomposer, and the front part of the acidic gas burners were taken. Table 1 shows the analysis results of these blockage samples conducted by Nippon Steel Corporation. Table 1: Appearance and Composition of the Blockages. Sampling Location, Sampling Date, Appearance, Mass Fraction of Soluble Components, %, Main Components. Acid Gas Preheater Inlet: March 2005, black powder, 23.0%; naphthalene, acenaphthene, C10H8, C12H10. Hydrogen cyanide decomposer catalyst layer: January 2005, black solid, 17.6%; ammonium thiocyanate and a small amount of sulfur. Acid gas burner inlet: March 2005, black tar, 83.8%; 1,3,5-triaminotriazine, ammonium thiocyanate, thiazole, thiazidine. Front part of the acid gas burner: March 2005, black solid, 2.7%; solid composed of carbon and nitrogen elements. 4. Measures to Prevent Blockages: (1) Improve the operation of the primary and secondary cooling systems as well as the naphthalene removal and electrostatic tar collectors, in order to reduce the content of impurities in the gas. Taking advantage of the shutdown for maintenance of the gas system, we installed labyrinth-type pipe mist catchers on the main gas pipeline after benzene washing in August 2005. In September 2005, to reduce the clogging of the acidic gas burners, mist collectors were installed on the acidic gas pipeline downstream of the hydrogen cyanide decomposer. After these two mist catchers were put into operation, they effectively reduced the amount of debris generated and extended the production cycle. (2) Prevent blockage of the acidic gas preheater. Firstly, it is necessary to regularly discharge the deposits at the bottom of the acid gas preheater, and at the same time switch between System A and System B on a monthly basis while carrying out thorough cleaning. (3) Prevent clogging of the hydrogen cyanide decomposer. The main cause of blockage in the hydrogen cyanide decomposer is ammonium thiocyanate formed inside it. The melting point of ammonium thiocyanate is 149°C, and it decomposes into NH3, CS2, and H2S at 200–300°C. Therefore, by keeping the internal temperature of the decomposer above 200°C, blockage caused by ammonium thiocyanate can be effectively prevented. Currently, the inlet temperature of the hydrogen cyanide decomposer is controlled at around 190°C, but the temperature of the internal catalyst layer is only 140–150°C. To ensure that the internal temperature of the hydrogen cyanide decomposer remains above 200°C, the following measures must be taken. First, strengthen the insulation of the inlet pipeline of the decomposer ； Second, maintain an appropriate flow rate of acidic gases. The designed flow rate for the acidic gas is 420 m3/h; when the flow rate of the acidic gas falls below 60% of the designed value, the temperature of the acidic gas decreases due to heat dissipation in the pipes. (4) Prevent blockage by sulfur and sulfides. When analyzing the clog sample, a small amount of sulfur and hexasulfur carbon was found. If the amount of air introduced for the oxidation of HCN is excessive, some hydrogen sulfide will be oxidized to sulfur dioxide, and sulfur and sulfides will be produced during the Claus reaction. Therefore, the hydrogen cyanide decomposer must be operated at an appropriate air ratio to ensure that the HCN concentration at its outlet does not exceed 1%. (5) Prevent blockage at the burner inlet and front end due to acidic gases. Firstly, it is necessary to prevent ammonium thiocyanate at the outlet of the hydrogen cyanide decomposer from flowing into the acidic gas burner and causing blockages ； Secondly, it is necessary to enhance the thermal insulation of the pipeline from the outlet of the hydrogen cyanide decomposer to the acid gas burner ； Third, strengthen the drainage operations of the new mist collectors installed at the inlet of the acidic gas burners. 5 Implementation Results: By adopting the aforementioned improvement measures, initial results were achieved; the operating cycle of the desulfurization unit was extended compared to before. Since this process is the first of its kind in China, and also the first to adopt this configuration pattern in the world, some problems and phenomena have not yet been fully understood or grasped, requiring further extensive research.