Discussion on Common Issues in Desulfurization and Sulfur Recovery of Coke Oven Gas

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The wet flue gas desulfurization unit consists of three main components: desulfurization, regeneration, and sulfur recovery. Sulfur recovery is the key to determining whether the entire desulfurization system can operate stably in the long term. In the desulfurization of coke oven gas, due to the scale of operations involved, the sulfur production per unit installation is generally higher than that in wet desulfurization processes used in other industries. Packing towers are typically used for this purpose. If there are issues with sulfur recovery, sulfur accumulates within the system, which can quickly lead to blockages in the towers and other problems that disrupt normal production. Therefore, for the desulfurization of coke oven gas, sulfur recovery is particularly important. Below, the common problems in the desulfurization and sulfur recovery process of coke oven gas are discussed in light of the design of coking desulfurization units and practical operation experience. The three main components of the coke oven gas desulfurization and sulfur recovery system are: the foam tank, the foam pump, and the sulfur separation equipment; we will analyze these three components. 1. Sulfur separation equipment: The sulfur separation equipment is the final device in sulfur recovery; it can even be considered the ultimate device in the desulfurization system. Common sulfur foam treatment devices include plate and frame filter presses, centrifuges, and sulfur melting tanks. Similar to centrifuges, plate and frame filter presses also use physical methods to separate the water from sulfur foam, resulting in sulfur paste or sulfur sludge that still contains a certain amount of moisture. The plate and frame structure remains relatively stable; the output per cycle for a single unit can be calculated based on the parameters of that structure. Additionally, the operation time required for each cycle (feeding time and unloading time) can also be determined using the foam concentration and the output of the plate and frame unit. As a result, the maximum daily production capacity of a single plate and frame unit is fixed. It is possible to determine how many such filter presses are needed, or what size filter presses should be used, based on the production demands of the facility. A coking plant with a capacity of 2.2 million tons per year was originally equipped with two 100 m2 diaphragm filter presses. The maximum daily amount of sulfur that could be removed was 13 tons (in pure form); converted to sulfur paste, this equaled approximately 20 tons. Initially, PDS was used for sulfur removal, with a sulfur selection efficiency of around 70%, resulting in approximately 14 tons of sulfur paste per day. The two 100 m2 filter presses operated intermittently, 6 to 7 times per day, with each operation lasting about 2 hours – which was sufficient to handle the volume of sulfur to be removed. After switching to complexed iron for desulfurization, the sulfur selectivity is high, and the output of sulfur paste amounts to nearly 20 tons per day. This requires operating the plate and frame filter 10 times a day, for at least 20 hours each time – with hardly any margin for extra time. In such a situation, it is necessary to increase the number of filters in order to reduce the average time per operation, or to use larger plate and frame units in order to increase the sulfur yield per operation. The centrifuge operates continuously and discharges material continuously; its processing capacity is limited by the size of the equipment as well as the concentration and viscosity of the sulfur foam. For the same device, its efficiency is higher when the foam concentration is high, while it is lower when the foam viscosity is high. Since the sulfur foam concentration during the desulfurization of coke oven gas using complexed iron is lower than that in PDS-based desulfurization, and its viscosity is often higher, this factor needs to be taken into consideration during equipment design and selection. Additionally, centrifuges need to take failure factors into account, so at least one spare unit is required. A sulfur melting tank utilizes the change in the state of sulfur when heated, as well as the density difference between sulfur and the desulfurization solution, to separate sulfur from the desulfurization liquid, thereby producing molten sulfur with almost no water content. The production capacity of a sulfur melting tank is generally limited by the size of the installation, the concentration of sulfur foam, salt content, and steam temperature. Additionally, when evaluating the production capacity of such a tank, it is also necessary to take into account its service life, that is, the issue of reduced internal heat transfer efficiency. A coking plant with a capacity of 1.3 million tons uses sulfur melting vessels for continuous sulfur melting; the sulfur production rate is around 10 tons per day. Four DN1000 continuous sulfur melting vessels are installed, and this setup meets the requirements as per the original design. After the desulfurization process was modified, the sulfur foam became weaker, which reduced its concentration; as a result, the 4 sulfur melting tanks were not sufficient for production. Therefore, more sulfur melting tanks were added, until a total of 8 tanks were in use. The steam consumption was very high, yet this still did not suffice to handle the entire amount of sulfur produced, causing the suspended sulfur content in the desulfurization solution to reach up to 50 g/L. This led to the gradual blockage of the desulfurization towers. This situation indicates that the efficiency of the sulfur melting tank is extremely low; it is necessary to consider ways to improve sulfur melting efficiency rather than simply adding more such tanks. Improving efficiency can be achieved by increasing the concentration of sulfur foam, reducing the salt content in the sulfur foam, raising the temperature of the sulfur melting steam, and enhancing the mass transfer within the sulfur melting tank. 2. Foam tank: The foam tank connects the regeneration tower (tank) to the sulfur separation equipment, serving to provide foam buffering. It is necessary to consider whether its buffering capacity is sufficient. In a coking desulfurization process involving 2.2 million tons of material, filter presses are used to handle sulfur foam. In the initial phase, the feeding resistance of the filter press is low, so the feeding rate is high; however, as time goes on, this feeding resistance increases, causing the feeding rate to slow down and the processing capacity to decline gradually. Meanwhile, the amount of foam that overflows from the regeneration tank remains constant. When the filter press’s ability to handle foam decreases, more foam accumulates in the intermediate foam tank, leading to an accelerated rise in the liquid level in that tank. Each filter press in this plant takes 3 hours to feed, and about 1 hour for blowing and unloading. Based on this, the tank used to hold foam needs to have a capacity that allows it to store foam for at least 1.5 hours. Based on a current foam overflow rate of 30 m3/h, the effective volume of the foam tank needs to be 45 m3. The total volume of the existing foam intermediate tank is 23 m3; considering that in actual operation a foam tank can only reach 80% of its capacity, the actual available space is only 18 m3. Therefore, the foam cushioning time is far from sufficient; this also leads to frequent overflow from the foam tank in this device, as well as high operational pressure for the workers. In this case, either the foam tank is increased in size or the sulfur separation process is enhanced to improve efficiency. Another example is a coking desulfurization project with a capacity of 1.3 million tons; in this case, slab-based treatment of sulfur foam was also used, with two vertical foam tanks equipped with agitators, each having dimensions of φ3.8*5.46. The buffering time for the foam was over 4 hours, and there was virtually no occurrence of foam overflowing from the tanks. 3. Foam pump: A foam pump is a device that transports foam from the foam tank to the sulfur separation equipment; it is necessary to consider whether its flow rate, head, and pipeline configuration are suitable for the foam production volume and the requirements of the foam separation equipment. An 1.8 million-ton coking plant is equipped with two foam pumps with a flow rate of 25 m3/h and a head of 40 m. The sulfur-foam separation device used is a plate and frame filter press; however, due to low pressure, the required pressure is not achieved once the plates are filled (the normal operating pressure for feeding into such filters is 0.6–0.8 Mpa). As a result, the actual density of the filter cake is insufficient, and the amount of product produced per cycle by the filter press is inadequate, which in turn reduces its efficiency. In another coking plant, the foam pump is located about 100 meters away from the plate and frame pipeline, with a height difference of around 20 meters. The foam pump in question has a flow rate of 40 m3/h and a head of 60 m; these values may seem high, but in practice, due to the distance and height difference, the efficiency of the foam pump is low. This in turn reduces the efficiency of the plate and frame system, resulting in delayed foam treatment. 4. Conclusion: In the sulfur recovery process, the three main components – sulfur foam separation equipment, foam tanks, and foam pumps – are key factors that affect the efficiency of sulfur recovery. Since the sulfur recovery process in turn influences desulfurization, it is necessary to conduct thorough evaluations during the design and selection of sulfur recovery equipment, as well as during daily production management and operation, in order to eliminate any bottlenecks.