Reasons for the excessive alkali consumption in traditional wet-process technologies in coal chemical industry

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Due to factors such as tight coal supplies and enterprises’ efforts to save energy and reduce consumption, high-sulfur coal is being used extensively in production. There are fewer and fewer enterprises whose coal gas contains less than 1 g/Nm3 of H2S; many enterprises are burning high-sulfur coal with a H2S content of 2 g/Nm3 or even higher. Furthermore, as the production scale of enterprises grows larger, the amount of hydrogen sulfide that can be removed by the desulfurization systems per unit of time also increases. Under such circumstances, traditional desulfurization systems exhibit numerous problems, particularly a decline in the system’s desulfurization efficiency, an accelerated increase in the amount of by-products in the solution, increased alkali consumption, and a significant drop in sulfur recovery rates. As a result, the consumption of auxiliary materials for desulfurization far exceeds the company’s budgeted figures, leading to increased production costs for the enterprise. The formation of by-products in wet oxidation desulfurization is inevitable; in other words, it is an inherent phenomenon, and no desulfurizing agent can eliminate or prevent the generation of these by-products. So, how exactly are by-products formed? What factors are related to their creation? I. Causes of side reactions: Carbon dioxide in the feed gas is an acidic gas that can react with sodium carbonate to produce the following reaction: Na2CO3 + CO2 + H2O = 2NaHCO3. At the beginning of desulfurization operations, the solution consists entirely of Na2CO3; as the absorption of CO2 proceeds, the amount of NaHCO3 in the solution gradually increases. When the amount of CO2 absorbed is balanced by the amount of CO2 desorbed during regeneration, the concentrations of Na2CO3 and NaHCO3 in the liquid phase remain constant. In conventional atmospheric-pressure desulfurization, when the CO2 content in the feed gas is 8% to 6% and the total alkalinity of the solution is 0.4 N (equivalent to 21.2 g/L based on Na2CO3), the concentration of Na2CO3 is around 5–6 g/L, while that of NaHCO3 is around 25 g/L. During the pressurized desulfurization process (such as variable desulfurization), due to the high operating pressure, the CO2 concentration is also high, which results in a significant decrease in the amount of Na2CO3 in the liquid phase; it generally accounts for only 5% to 10% of the total alkalinity, with the majority being NaHCO3, thereby affecting the absorption of hydrogen sulfide. Compared with traditional wet oxidation methods, the GLT iron-chelation technology features a high sulfur capacity and strong selective oxidation capability; as a result, the circulation volume is much lower than that of conventional PDS. This leads to less CO2 absorption through gas-liquid contact, and the system temperature is generally maintained around 50°C. At higher temperatures, the solubility of CO2 is low, so the process is not affected by CO2, resulting in lower alkali consumption compared to traditional wet desulfurization technologies. When hydrogen sulfide comes into contact with oxygen, thiosulfate is formed: 2HS- + 2O2 = S2O32- + H2O. In traditional wet oxidation methods, this reaction takes place primarily in the regeneration tank, where there is an abundance of air and a high concentration of dissolved oxygen in the liquid phase. When the production load is high and the regeneration effect is poor, the potential of the lean solution becomes low; as a result, the hydrogen sulfide absorbed cannot be completely oxidized to elemental sulfur in the reaction tank, and a considerable amount of hydrogen sulfide is oxidized by air to thiosulfate. During high-temperature sulfur melting, sulfur reacts rapidly with alkalis and other substances, resulting in the formation of a large amount of by-products, among which sodium thiosulfate and sulfite by-reactions are the most common. S0+ 6OH-= 2S2-+ S2O32-+ 3H2O. 4Na2S2O3= 3Na2SO4+ Na2S + 4S. Traditional wet desulfurization processes involve many such reactions; the resulting clear solution contains by-products that must be removed, leading to significant losses of chemicals. The GLT iron-chelation technology employs a unique washing and filtration method – in the sulfur paste produced, the amount of chemicals used is low, almost no by-products are generated during sulfur melting, and the clear solution can be recovered to reduce consumption. II. Effects and hazards of side reactions: Since the absorption of H2S is carried out through Na2CO3, if the concentration of Na2CO3 in the solution is too low, it will affect the desulfurization efficiency. Moreover, a decrease in the pH of the solution leads to poor oxygen absorption by the regeneration solution, which is detrimental to the regeneration process ； And excessive pH values exacerbate side reactions. If side reactions are severe, alkali consumption increases dramatically; sometimes, even with large amounts of alkali added, it is difficult to keep the alkalinity within the specified range, which directly affects absorption and regeneration, leading to production disruptions and increased production costs. When the total amount of by-products in the solution is very high, the viscosity and specific gravity of the solution increase, leading to higher power consumption. This also affects mass and heat transfer, thereby hindering absorption and regeneration. When the concentration of sodium sulfate reaches a certain level, the corrosion of the equipment caused by the solution intensifies. Due to its low solubility, sodium sulfate tends to form crystals in cold weather, which can block the pipes and equipment affected by corrosion.