Analysis of a cause for the increase in hydrogen sulfide after the gas passes through the ammonium sulfate saturator in the positive-pressure desulfurization process for coke oven gas

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In the coal desulfurization process for coking furnaces, the positive-pressure ammonia method is used the most often. Replacing the existing PDS catalyst with the complexed iron desulfurization technology in HPF desulfurization enables the hydrogen sulfide level at the outlet of the desulfurization system to be kept at a very low level. In some units, the hydrogen sulfide level in the desulfurized gas is low, but it rises abnormally after passing through the ammonium sulfate saturator. In extreme cases, the hydrogen sulfide level at the outlet of the desulfurization system is kept near 0 mg/Nm3; however, after passing through the ammonium sulfate saturator, the hydrogen sulfide content in the gas still exceeds 200 mg/Nm3. Is this caused by the desulfurization system itself, by the organic sulfur present in the gas, or are there other factors at play? This causes some difficulties in production. In the ammonia vaporization section of coke oven gas gasification, the concentrated ammonia solution produced as a result of ammonia vaporization is generally sent to the ammonium sulfate production process or to the circulating ammonia solution tank at the earlier stage. The fate of the gaseous components that are not condensed during ammonia vaporization varies; some coking plants feed these gaseous components together with the concentrated ammonia into the saturator used in ammonium sulfate production, from where they are then carried along with the gas to the subsequent stages of the process. The non-condensable gases resulting from ammonia evaporation in this section contain high concentrations of hydrogen sulfide; analyses have shown that the hydrogen sulfide content in such gases from some coking plants can reach up to 8000 mg/Nm3, and this figure can exceed 10000 mg/Nm3 when high-sulfur coal is used in production. A coking plant in Yunnan encountered similar problems while replacing its existing PDS catalyst with a chelated iron-based desulfurization method; the hydrogen sulfide level at the outlet of the coke oven gas desulfurization system was reduced to nearly 0 mg/Nm3, but after the gas passed through the ammonium sulfate saturator, the hydrogen sulfide concentration increased to 300 mg/Nm3. After a brief shutdown of the ammonium sulfate process and diverting the ammonia vapor phase out of the ammonium sulfate system, the hydrogen sulfide concentration in the gas at the outlet of the ammonium sulfate saturator was found to be nearly 0 mg/Nm3 ； Restart the ammonium sulfide system and maintain the removal of ammonia vapor from the system; upon testing, the hydrogen sulfide level in the gas at the saturator outlet remains close to 0 mg/Nm3 ； Finally, after introducing the vapor-phase non-condensable gases from ammonia evaporation into the ammonium sulfate system, the hydrogen sulfide content in the gas at the outlet of the saturator was measured again and found to be 300 mg/Nm3. This indicates that the direct cause of the increased hydrogen sulfide level in the gas after passing through the ammonium sulfate system is the hydrogen sulfide present in those vapor-phase non-condensable gases, and it has nothing to do with the desulfurization process itself. The treatment of this ammonia-vapor non-condensable gas requires caution, as it contains not only high concentrations of hydrogen sulfide but also large amounts of hydrogen cyanide gas. Currently, other common causes of abnormally high hydrogen sulfide levels include liquid carried in the gas stream to the saturator, which leads to a reaction between ammonium polysulfide and acidic fluids, releasing hydrogen sulfide, and so on.