Optimization plan for the desulfurization system using complexed iron with coke oven gas

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This post was last edited by Steven_p0kTk on 2024-10-29 at 14:18. Currently, there are several suppliers of complexed iron catalysts in China, and some of the existing coke oven gas desulfurization units are facing the replacement of PDS-type catalysts with complexed iron catalysts. The hydrogen sulfide removal from coke oven gas at a coking plant in Yunnan is carried out using an HPF desulfurization unit that relies on ammonia as the alkaline source. To improve desulfurization efficiency and reduce the discharge of desulfurization wastewater, the original PDS-type catalysts were replaced directly with complexed iron catalysts. However, even after their use, problems such as severe excess levels of H2S downstream of the tower and a continuous increase in the pressure drop across the desulfurization tower occurred. In March 2023, GLT complexed iron desulfurization catalysts were adopted; following appropriate technical modifications and adjustments, the desulfurization unit is now operating steadily. 1 Operation status of the desulfurization unit and analysis of problems: Initially, the catalyst used in the desulfurization unit was replaced from a PDS catalyst with a certain type of complexed iron catalyst. The two desulfurization towers were connected in series for desulfurization. After the unit operated for some time, the pressure drop across the towers reached 2.2–2.4 kPa. The H2S content in the coke oven gas after desulfurization was significantly above the allowable levels; according to on-site measurements, the hydrogen sulfide concentration in the gas exceeded 600 mg/Nm3. The desulfurization efficiency was only 85%–90%, which posed significant challenges for further removal of sulfur dioxide from the flue gas. On the other hand, when large amounts of liquid are discharged by the desulfurization unit, the total amount of the three salts in the desulfurization solution can reach 400 g/L; the heat exchangers for this dilute solution often get clogged, which sometimes forces a reduction in the volume of the circulating liquid ； Furthermore, there is little sulfur foam in the two spray regeneration tanks; the sulfur foam layer is thin, and the overflowing sulfur foam contains a large amount of liquid. The two plate and frame filter presses with a filtration area of 100 m2 produce only about 5 tons of sulfur paste per day, corresponding to roughly 2 plates, resulting in a sulfur recovery rate of only 50%–60%. Analysis shows that the following problems exist during the use of this complexed iron catalyst: (1) The complexed iron catalyst used has poor stability; the content of ineffective iron in the desulfurization solution is high while the content of effective iron is low, and there are iron hydroxide colloids present in the desulfurization solution. The complexed iron catalyst lacks stability and has a low effective iron content; moreover, the concentration of the catalyst in the desulfurization solution does not match the hydrogen sulfide load of the coke oven gas processed by the desulfurization unit. This not only reduces the desulfurization efficiency of the unit but also leads to an increase in the by-products present in the desulfurization solution. (2) In the two-stage series desulfurization process, regeneration in the first stage of desulfurization is severely insufficient; as a result, it is not possible to achieve a balance between the absorption load and the regeneration load required for iron complexation to remove hydrogen sulfide, which exacerbates the increase in the content of by-products in the desulfurization solution. (3) When the desulfurization tower is used as the second-stage desulfurization unit, the effective iron concentration of the iron catalyst in the desulfurization solution deviates significantly from the hydrogen sulfide load of the coke oven gas processed by the desulfurization unit; as a result, highly adhesive S8 is formed directly on the packing of the desulfurization tower, causing sulfur blockage in the packing. (4) The gas pre-cooling tower is old, resulting in a rapid increase in operating resistance and frequent need to flush the tower. This makes it impossible to effectively wash and cool the coke oven gas, leading to the presence of large amounts of impurities such as light tar in the coke oven gas that enters the desulfurization tower. As a consequence, the desulfurization liquid contains a high amount of tar, which in turn results in a less dense sulfur foam layer. 2 System Optimization Plan 2.1 Adjustment of the iron catalyst in the desulfurization solution After calculation, it is necessary to initially add a certain amount of GLT-structured iron catalyst to this desulfurization unit. On one hand, this is done to improve the stability of the iron catalyst present in the existing desulfurization solution and to eliminate the iron hydroxide colloids present in it; on the other hand, it is necessary to increase the concentration of catalyst in the desulfurization solution so as to reach the designed catalyst concentration, thereby meeting the H2S load required to be handled by the existing desulfurization unit. The initial addition of the GLT iron complexing catalyst is carried out in multiple batches; during this process, it is necessary to monitor the properties of the desulfurization solution promptly and observe any changes in the sulfur foam in the regeneration tank. 2.2 Adjustment of the ammonia water supplied to the washing, cooling, and desulfurization unit for the coal gas entering the tower: Given that the existing pre-cooling tower was quite old and thus provided poor washing and cooling performance for coke oven gas, a new pre-cooling tower was added later. The two towers operate in a manner of one being in use while the other is on standby, allowing for alternating use and ensuring optimal cooling, washing, and oil removal of the coke oven gas before it reaches the desulfurization tower. In addition, the ammonia water in the ammonia vaporization tower contains a certain amount of light tar substances; if it is added directly to the desulfurization unit, this will inevitably result in a high oil content in the desulfurization solution, leading to deterioration in the properties of the sulfur foam as well as an increase in the density of the desulfurization solution. Furthermore, there is also the issue of the mismatch between the properties of the complexed iron catalyst and the operating conditions of the desulfurization unit. Therefore, systematic optimization is necessary from both the perspective of the complexed iron catalyst and the operating conditions of the desulfurization unit. The ammonia water from the ammonia vaporization tower is diverted to the front-end circulating ammonia water tank; the ammonia required by the desulfurization unit can be provided by the ammonia contained in the gas stream. 2.3 Adjustment of the spray regeneration tank The adjustment of the spray regeneration tank involves two aspects: one is to increase the spray pressure in this tank to 0.41–0.45 MPa by adjusting the distribution of the circulating liquid volume between the two towers and regulating the air intake openings of the injectors in the spray regeneration tank. An increased spray pressure allows for more air to be drawn in, thereby meeting the requirements for regeneration during the iron complexation desulfurization process ； The second method is to use a separate compressed air pipe connected to the air intake of the injector in the spray regeneration tank, in order to increase the air intake volume of the injector and enhance the regeneration process. 2.4 Optimization of the desulfurization lean liquid pipeline: The desulfurization lean liquid is pressurized by a lean liquid pump and then flows through a DN350 main pipe for lean liquids; from there it enters the lean liquid heat exchanger via a DN250 pipeline. The pipeline exiting the heat exchanger is also DN250, before returning to the DN350 main pipe for lean liquids. If crystallization occurs and blocks the heat exchanger, it will inevitably lead to a reduction in the circulation volume of the desulfurization liquid, as well as in the spraying density. This not only results in an uneven distribution of the desulfurization liquid on the surface of the packing, thereby significantly reducing the removal of H2S from the coke oven gas inside the desulfurization tower, but it also means that the reduced spraying density over time reduces the scouring effect of the desulfurization liquid on the packing surface, which in turn increases the resistance within the desulfurization tower. To this end, DN350 pipes and valves are added before and after the solution inlet and outlet of the lean liquid heat exchanger to connect with the main lean liquid pipeline, thereby ensuring a stable circulation rate of the lean liquid entering the desulfurization tower and facilitating the maintenance of the heat exchanger. 3 Conclusions Through the analysis of the operating conditions and problems of the desulfurization unit, corresponding technical optimization and modification plans were proposed to address the existing issues after switching to GLT complexed iron catalysts. The results of practical operation show that: (1) When two desulfurization towers are connected in series, it is possible to keep the H2S content in the coke oven gas exiting the towers below 100 mg/Nm3. Meanwhile, by optimizing the pipelines for the desulfurization lean liquid to increase the volume of circulating liquid, as well as by adjusting the air intake pipes in the spray regeneration tank to boost the air intake volume, the desulfurization unit can operate using a single tower, thereby ensuring that the H2S content in the desulfurized coke oven gas remains below 100 mg/Nm3. (2) After the reaction characteristics of the GLT iron complex catalyst were adjusted to match the operating conditions of the desulfurization unit, the content of by-products in the desulfurization solution was brought under control and continued to decline; the total amount of by-products in the desulfurization solution dropped from 400 g/L to 360 g/L. (3) Within the nearly 7 months of operation after the desulfurization unit was upgraded, the resistance of the desulfurization tower remained stable. (4) The properties of sulfur foam are stable; the output of sulfur paste has increased from 5 t to over 9.5 t, resulting in a significant improvement in sulfur recovery rate.