The importance of “equilibrium” in the desulfurization of coke oven gas using complexed iron

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In chemical production, emphasis is placed on safe, stable, and long-term operation. Each process works in coordination with and influences one another, making stability and balance particularly important. Coke oven gas has a complex composition and contains many harmful impurities, which significantly affect the recovery of chemical products; therefore, it must be purified. The hydrogen sulfide removal process is the most important step in gas purification; the vast majority of harmful impurities are removed during this process. Therefore, the desulfurization process is of particular importance. Whether it is the HPF technology or the complexed iron technology, achieving a \"balance\" is the key to the stable operation of this process. In the wet oxidation method for desulfurization of coke oven gas, regardless of the catalyst used, the following equilibria must be maintained: ① Sulfur balance: The amount of sulfur entering and leaving the system must be equal; it must not accumulate within the system, otherwise serious problems such as tower blockage may occur. This is the most fundamental requirement of this process, and it is closely related to the absorption of hydrogen sulfide, the separation of sulfur foam, and the treatment of sulfur foam. ② Water balance: As the name implies, the amount of water entering and leaving the system, as well as that generated by reactions, should be in equilibrium; otherwise, it can easily lead to overflow issues. The structure of the pre-cooling tower, the method of treating sulfur foam, make-up water, and the temperatures of the gas and liquid phases all affect the system’s water balance. ③ Salt balance: The accumulation of salt directly affects the mass transfer during desulfurization absorption, thereby influencing the desulfurization efficiency. The stability of the catalyst, the pH value/alkalinity of the desulfurization solution, the HCN content in the gas, the concentration of the catalyst, the amount of liquid discharged, the method used for treating sulfur foam, and the temperature control of the desulfurization solution—all of these factors can affect the salt balance. ④ Impurity balance: The accumulation of impurities can lead to issues such as a high specific gravity and difficulty in floating foam. Impurity levels in the gas (coal ash, tar, benzene, toluene, etc.), light tar in the additional excess ammonia water, wastewater from the sump return system, and sulfur foam treatment methods all determine the impurity content in the system. ⑤ Alkaline balance: Desulfurization takes place in a weakly alkaline environment; the higher the alkalinity, the better the desulfurization effect in theory, but side reactions are also more likely to occur, so the alkalinity must be controlled properly. The type of alkali source, the temperature of the desulfurization liquid, the treatment method for sulfur foam, the inlet hydrogen sulfide concentration, and the liquid discharge volume all have a direct relationship with alkali balance. ⑥ Heat balance: The inlet and outlet temperatures of gas, the treatment method for sulfur foam, the heat exchange area of heat exchangers, the conditions of cooling or heating media, climatic conditions, etc., all affect the system’s heat balance. Furthermore, thermal balance also directly affects water balance, alkali balance, salt balance, etc. The balance among these various aspects is interrelated and interdependent; they cannot be controlled separately. A systematic approach is necessary, taking into account everything from the project design stage through to its later operation, in order to achieve a stable state. The coke oven gas desulfurization system is not a rudimentary system; careful attention is required in its control and operation, with a balanced approach being applied flexibly to daily system management.