Complexed iron desulfurization is gradually replacing PDS desulfurization – could this be the emerging trend?

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Currently, the main purification technologies used for desulfurizing and decyaniding coke oven gas in coking enterprises are the PDS method and the tannin method. For desulfurization absorption towers, spray towers and packed towers are primarily employed; a small number of enterprises have switched to counter-current reactor absorption towers and bubble columns (towers without internal mass transfer components). The complexed iron desulfurization technology has been widely applied in industries such as sulfur recovery from refinery off-gases, and the desulfurization and purification of natural gas and biogas; therefore, its process characteristics need not be described again. In traditional coke oven gas desulfurization and purification processes, a large amount of desulfurization by-products are generated (mainly thiosulfates, thiocyanates, and sulfates), there is high consumption of alkali, and a large volume of desulfurization waste liquid must be discharged daily to maintain the proper functioning of the desulfurization solution. The increasingly strict environmental regulations pose significant pressure on coking enterprises ； Some coking enterprises are attempting to directly add complexed iron desulfurization agents to their existing desulfurization systems in order to reduce the formation of by-products and wastewater discharge. Practice has proven that it also achieves good application results. However, due to the desulfurization mechanism of the iron-based complexing desulfurizer and certain characteristics of the catalyst itself, considering the composition of the gas source in existing coke oven gas as well as the current state of traditional desulfurization equipment, there may be some issues that need to be addressed with long-term use. These issues are as follows: First, coke oven gas contains hydrogen cyanide (HCN), which forms thiocyanates in alkaline desulfurization solutions. The iron-based complexing desulfurizer can only control the formation of sodium thiosulfate and sulfates, but it is unable to remove thiocyanates. Due to the high solubility of thiocyanates, their concentration in the system will increase over time, thereby having a negative impact on the desulfurization system. II. Complexed iron desulfurization technology: The oxidation desulfurization process is primarily carried out within the desulfurization tower, where the sulfur content in the desulfurization solution is high ； If the desulfurization tower is of the packed tower design, sulfur tends to adhere to the packing, and over time this may lead to an increase in resistance within the tower. III. The complexed iron desulfurization agents in the Lokeit series are somewhat corrosive; it is recommended to use equipment made of 316L or 304 stainless steel. Some of the coke oven gas desulfurization units in existing coking plants are equipped with carbon steel structures lined with anti-corrosion materials. If there are local damage issues to these anti-corrosion layers, long-term use of complexed iron desulfurization agents may lead to corrosion and leakage in the equipment. In response to these issues existing in the desulfurization systems for coke oven gas, the following recommendations are put forward regarding the use of complexed iron desulfurization technology: 1. Regularly monitor the HCN content in coke oven gas, and control its level at the coal source ； Since it is not possible to control the HCN content in coke oven gas at its source, a set of catalytic hydrolysis units for HCN and COS should be added before the coke oven gas enters the desulfurization device. These units convert HCN in the gas into ammonia and carbon monoxide, while converting COS into hydrogen sulfide and carbon dioxide. The resulting gases then enter the complexed iron desulfurization system, thereby addressing the issue of increasing cyanate levels in the desulfurization liquid system. 2. If a packed tower is still used in the desulfurization tower, due to the large circulation volume of the desulfurization liquid in the coke oven gas purification system, the catalyst supplier needs to modify the composition of the complexed iron desulfurizer so that the desulfurization liquid has a certain cleaning capability. The iron ion content and redox potential in the desulfurization liquid must be adjusted so that part of the desulfurization and oxidation reactions take place outside the desulfurization tower ； If a combined desulfurization system using bubble columns and power wave reactors is employed, the desulfurization and regeneration systems can be designed using the existing design principles of complexed iron processes. 3. Check the anti-corrosion properties and material composition of the equipment, pipelines, and pumps to determine whether they are suitable for use in the complex iron desulfurization process.