Causes of tower blockage in wet flue gas desulfurization systems and solutions

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The last edit to this post was made by 955559 on 2018-9-18 at 14:23. 1. Hazards of tower blockage in wet flue gas desulfurization systems 1.1 High power consumption: The direct consequence of tower blockage is an increase in the pressure difference across the entire desulfurization system. In the case of our plant’s semi-desulfurization unit, the pressure difference was only 25 mmHg at the start of operation; it rose to 130–140 mmHg when the tower became blocked. The current drawn by the Roots pump increased by nearly 10 A, reaching the upper limit of the motor’s capacity. This results in an additional power consumption of 360 kW per hour, which is equivalent to having one more Roots pump in operation. Additionally, after blocking the tower, to maintain the H2S levels in the desulfurization system, it is common practice for many companies to increase the circulation rate, which in turn increases electricity consumption. In the cost structure of ammonia synthesis, electricity consumption accounts for a proportion second only to coal consumption, which represents a significant disadvantage for the nitrogen fertilizer industry, which is operating in an environment focused on energy conservation and reduction. 1.2 Low desulfurization efficiency: The tower clogging process results in a layer of sulfur paste forming on the surface of the packing, which inevitably reduces the effectiveness of the packing’s distribution function. This leads to uneven flow of the liquid, a decrease in the gas-liquid contact area, and a shorter contact time, all of which affect the desulfurization efficiency. 1.3 Equipment maintenance work has increased, with frequent repairs required. Desulfurization maintenance is already a task that is both time-consuming and physically demanding; there is already a high rate of staff turnover in this field. When the tower gets blocked, it causes pump failure, and the packing used for sealing can easily be damaged. In severe cases, it is necessary to close all the inlet and outlet valves before any new packing can be installed. Furthermore, in order to ensure desulfurization efficiency, increasing the circulation rate can also cause severe erosion and corrosion of the outlet valve elements and pipes, which will **reduce the pump’s service life**. In some ammonia synthesis plants, large pressure differences lead to system shutdowns for maintenance, which occur once every two to three months on average, **impacting the coordinated operation of the system. 1.4 The high resistance of the gas flow means that production levels cannot be increased; blockages in the tower inevitably lead to poor flow of gas through the purification section, which affects the compression capacity of the compressors and ultimately impacts the overall ammonia production. Today, ammonia synthesis plants have narrow profit margins; they rely mostly on production volume to maintain normal operations. At the same time, achieving high production levels is also a way to reduce costs. 1.5 High consumption of auxiliary materials leads to tower blockage, which inevitably increases the gas velocity and makes it easy for the desulfurization liquid to be carried out of the tower ； Also, after blocking the towers, many manufacturers have to resort to increasing the composition of the desulfurization solution in order to maintain the H2S level in the subsequent processing stages; this leads to increased consumption of auxiliary materials. 1.6 Safety hazards exist: If the desulfurization tower becomes clogged, the pressure at the outlet of the Roots pump will continue to rise, which can easily lead to the shutdown of the pump. If the outlet is not closed in time, the pump may start running in reverse, and in severe cases, this could even result in the pump being destroyed. Furthermore, frequent pump adjustments and repairs inevitably increase safety risks and affect safe production. 2 Reasons for tower blockage in the wet flue gas desulfurization system (1) Control of indicators in the desulfurization system. (2) Heat in the liquid phase when the temperature is low. This item is listed separately because it is a common practice among nitrogen fertilizer manufacturers at present; perhaps it is the only option available, or maybe they have not yet realized the harms of doing so. (3) Selection of internals for the desulfurization tower. (4) The appropriate desulfurizing agent was not selected. (5) The regeneration effect is poor, and the sulfur content in the lean solution is high. (6) Quality of sulfur recovery. (7) Dust removal efficiency of the previous section. 3 Countermeasures for Tower Blockage in Wet Flue Gas Desulfurization Systems 3.1 Strict control of various process parameters Temperature is a key factor for the proper operation of the desulfurization system; low temperatures result in high liquid viscosity, leading to poor desulfurization efficiency, while high temperatures cause an increase in the formation of by-products, which also hinders the absorption of H2S. Experiments have shown that as long as the temperature of the desulfurization solution is above 45°C, the formation rates of Na2S2O3 and Na2SO4 increase sharply. Furthermore, when the regeneration temperature is too high, excessive foam forms in the regeneration tank, making it difficult to aggregate and float sulfur particles, which results in a gradual increase in suspended sulfur in the lean solution. The desulfurization temperature should generally be maintained between 38°C and 42°C; it should not be lower than 35°C, nor higher than 45°C. pH value is also an important factor in chemical reactions; the pH of the regeneration solution must be strictly controlled in the desulfurization process, generally keeping it between 8.2 and 8.8. In production, efforts should be made to avoid a pH level higher than 9.0. When the pH value of the solution is greater than 9.2, the formation rate of by-products also increases linearly. The main components of the desulfurization solution need to be adjusted in a timely manner according to the production process, while strictly controlling the levels of suspended sulfur and by-products in the solution. 3.2 Reasonable methods for raising temperature: Many companies, especially those in the northern regions, use steam heaters to heat the desulfurization liquid in order to raise its temperature during the cold winters; steam is used directly to heat this liquid. Some other companies add steam coils at the bottom of the regeneration tank or introduce steam directly. Our factory used to do the same thing; in 2010, we installed a steam jacket on the gas inlet pipe of the desulfurization tower to heat the gas with steam, and the result was good. If the heat supply wasn’t sufficient, steam could be directly introduced into the gas pipe to raise its temperature. 3.3 Using a spray air tower in place of a packed tower: http://5b0988e595225.cdn.sohucs.com/images/20180918/14cb7f577d714b82bcf0d6c0e1d52a53.png In the environmental-friendly wet oxidation method for desulfurization, traditional packed towers are still commonly used as absorption towers; only a distributor is installed at the top of the tower. The desulfurization liquid flows from top to bottom through the packing layer. Since the height of such towers can be several dozen meters, combined with the resistance posed by the coal gas, flow deviation can easily occur over time. Today, many nitrogen fertilizer manufacturers have implemented technical upgrades by adding redistributors above each layer of packing, in order to reduce liquid skewing, prevent the formation of dry zones, and effectively alleviate tower clogging. Currently, with the continuous advancement of industrial technology, problems such as poor atomization performance of atomizing nozzles have been resolved. Thanks to its advantages such as simple structure, high utilization rate of the tower, efficient mass transfer between the gas and liquid phases, and low operating costs, the empty-tower spraying technology is gradually coming into focus. It has shown excellent performance in several affiliated companies, with virtually zero tower pressure difference, thereby completely preventing tower blockages. 3.4 Selection of high-quality catalysts: The choice of desulfurization catalysts fundamentally determines key parameters such as desulfurization efficiency, auxiliary material consumption, and desulfurization costs. There are currently many types of wet desulfurization agents available in China; among those in the cobalt phthalocyanine series, we have 888, T90-2, RTS, PDS-600, and TTS, etc. In addition, there are iron-based catalysts such as DDS and NDC. There is no fixed standard for selecting desulfurization agents, but it must be determined based on the existing process flow and equipment configuration, as well as the H2S content in the gas and the desulfurization targets to be achieved. For example, the level of H2S in half-imported gas at our company is 2500–3500 mg/m3. Since the shift reaction uses a full low-temperature shift process, it is required that the H2S level in the desulfurization system be around 100 mg/m3. We use a combination of tannin-based catalysts and 888 as catalysts, which is also a commonly accepted optimal combination in the industry. However, there is a problem now: due to the safety concerns associated with V2O5, the scale of manufacturers producing it is constantly shrinking, and prices are rising. It has also caused a lot of trouble during use now. Some places have already issued regulations banning the use of V2O5. Therefore, the author remains optimistic about cobalt phthalocyanine-based desulfurization catalysts; their advantages include strong oxygen-carrying capacity, low viscosity, which facilitates tower flushing, and the ability to remove some organic sulfur compounds. 3.5 Poor regeneration of the regeneration equipment means that NaHS, which is formed when Na2CO3 absorbs H2S inside the tower, is not fully oxidized to elemental sulfur; instead, it ends up in the foam tank as a result of flotation and is then carried into the desulfurization tower. It is only in the upper section of the tower that the oxidation reaction takes place, resulting in the formation of elemental sulfur that adheres to the surface of the packing. This is the main cause of blockages in the upper section of the desulfurization tower. The spray regeneration tank must be designed in strict accordance with the requirements for synthetic ammonia production, and it is divided into three sections: an inner reaction tank, an intermediate lean liquid ring tank, and an outer foam ring tank. The distance between the top of the inner cylinder and the sulfur foam overflow weir should be 700 mm, while the distance from the tail pipe to the bottom of the tank is recommended to be 900 mm. Notch patterns can be cut on the end of the tail pipe to improve the distribution effect. It is recommended to weld the tail pipe and the deck plate together, as we found during maintenance that the area where they meet is severely worn. The nozzle is best made of stainless steel, as this is also a part that is prone to corrosion. It is best to hire a professional installation team for installing the ejector to ensure vertical alignment and concentricity; otherwise, it will affect the air extraction volume. During production, the thickness of the foam layer must be strictly controlled through regenerative pressure and level regulators, in order to prevent large fluctuations and surges in the liquid level, ensure normal overflow of the sulfur foam, and at the same time avoid excessive liquid in the foam. It is common to check whether the nozzles and throats are clogged or scaled, based on the wind speed in the intake chamber or the occurrence of backflow. 3.6 Improving the sulfur recovery process: The quality of sulfur recovery directly reflects the amount of H2S that has been removed. Even if the desulfurization system is operating perfectly, if sulfur cannot be recovered, it must remain inside the tower, posing a risk of blocking the tower. Also, if the sulfur melting backflow returns to the system, it is an important factor contributing to high levels of by-products in the salt. Many companies use continuous sulfur melting due to environmental and cost pressures; the liquid resulting from sulfur melting is usually cooled before being returned to the system. Our factory recovers 30 cubic meters of such liquid per shift. The steam temperature entering the sulfur melting tank can reach 190°C, and the temperature of the liquid returning from the tank is at least 85°C. Such high temperatures inevitably lead to the formation of many by-products salts. The common practice nowadays is to introduce the returned liquid into a sedimentation tank, where it settles and cools before being fed into the lean liquid tank; some companies also use cooling fans. However, this approach is only effective in lowering the temperature of the returned liquid, while it has little effect on the cooling and sedimentation of by-products. The author conducted experiments in which a portion of the returned liquid was left to settle at room temperature for 1 month, and the content of by-products remained almost unchanged. We are currently carrying out technical upgrades: the sulfur foam produced by flotation in the regeneration tank is first fed into a filter to remove most of the clear liquid, after which the concentrated sulfur paste is fed into the sulfur melting vessel. This **reduces the amount of molten sulfur that returns, thereby minimizing the formation of by-products. It would be more beneficial for the proper operation of the desulfurization system if raw sulfur could be sold directly or used in downstream products. 3.7 Ensuring gas purity: It is well known that the gases entering the desulfurization system have a complex composition, containing numerous impurities and contaminants. Once these gases enter the desulfurization tower, it is difficult to remove them; they mix with the sulfur paste and can cause blockages in the tower. Typical packed towers are divided into 3 sections; if the pressure difference across the packing in the lowest section is high, it is most likely due to poor dust removal performance. Ensure the proper operation of the dust remover at the outlet of the gasification furnace, and remove ash on schedule. It is advisable to introduce micro-vortices into the gas generation circulating water. To operate an electrostatic dust removal device properly, it is necessary not only to maintain the secondary voltage and current of the electrostatic filter, but also to perform regular thermal cleaning of the filter, in order to minimize errors in the readings of the secondary voltage and current meters and thus avoid misjudgments regarding the efficiency of dust removal. Manage the circulating water in the scrubber towers and cooling and cleaning towers properly; companies that have the means should replace it frequently, ideally by carrying out both addition and discharge processes continuously. Years of on-site management have made the author deeply realize that desulfurization is a job that requires integrity. After the process and equipment have been selected, the day-to-day management of minor details is also key to carrying out desulfurization effectively. Employees responsible for desulfurization not only need to have high professional skills, enabling them to analyze and solve problems, but also must possess a strong sense of responsibility. They need to have a thorough understanding of the desulfurization process at all times, be able to identify issues that arise during production promptly, and take effective actions right away. What has been mentioned above represents some of the insights gained by technical personnel working on the production front lines. It focuses on issues related to tower blockages, and relevant information has been compiled in the hope of being useful to engineers and technical staff who are working hard in the field of desulfurization