Several issues considered in the design of 480t/h CFB

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Abstract: This paper introduces the analysis and selection carried out by Wuxi Huaguang Boiler Co., Ltd. in the design of 480t/h circulating fluidized bed boilers, focusing on certain specific structures, particularly those key components that affect the stable operation of the boilers. Keywords: slag cooler, coal feeding, soot blower, air cap, refractory and wear-resistant materials. Several issues considered in the design of a 480t/h CFB boiler – Su Xiaoping (Wuxi Huaguang Boiler Co., Ltd., 214028). Circulating fluidized bed boilers have advantages such as low pollution emissions, strong peak-shaving capacity, and wide adaptability to different types of coal, and they have seen rapid development in China. The 480t/h ultra-high-pressure circulating fluidized bed boiler with once-through reheat, developed by our company in collaboration with the Institute of Engineering Thermophysics of the Chinese Academy of Sciences, was put into commercial operation at Inner Mongolia Huadian Wuda Thermal Power Co., Ltd. at the end of March 2005. The boiler operates stably at full capacity, achieving the expected results; it is a large-scale circulating fluidized bed boiler with independent intellectual property rights in China. During the development of this product, the designers carried out extensive preliminary work. Below are some of the structural issues that were given particular attention at that time, provided for reference. 1 Selection of slag discharge and cold slagging methods: For large-scale circulating fluidized bed boilers, slag discharge is generally achieved through side slag discharge combined with a fluidized bed slag cooler that uses air and water for cooling; this technology originates from abroad. Most foreign boilers use commercial coal, which has low ash content. To maintain a sufficient amount of circulating material, the fine particles that are discharged along with the large slag must be sent back into the furnace using air separation devices, thereby serving to replenish the circulating material. The slag with a higher density and larger particles cools down during the fluidization process caused by the air flow; this is the purpose behind the design of fully air-cooled fluidized bed slag coolers. This technology is well utilized abroad. When introducing this technology to China, it was observed that most domestic boilers used low-quality coal, which produced large amounts of ash and slag; air cooling alone was not sufficient to recover the heat from these residues. Therefore, water-cooled serpentine tubes or water-cooled wall surfaces were added to the slag cooler to further reduce the temperature of the slag. This is how the multi-chamber fluidized bed slag cooler with combined air and water cooling came into being. In fact, domestic power plant operators have a low opinion of this slag discharge method, and many causes of abnormal shutdowns are attributed to it. Firstly, the control of slag discharge is achieved using a conical valve; however, domestic technology cannot meet the requirements for this type of valve, so it needs to be imported, as shown in Figure A. If an inclined chute at an angle to the horizontal plane is used as the slag discharge pipe, and a directional air cap is placed at the bottom of this pipe, the amount of slag discharged can be controlled by adjusting the intensity of the air flow, as shown in Figure B. This approach replaces the conventional inlet cone valve; it’s a clever idea. However, given the varying sources of coal used in power plants, as well as differences in the physical properties such as density and viscosity of the coal ash, these factors can affect the flow characteristics of the slag, thereby leading to difficulties in controlling the amount of slag discharged. Secondly, China uses low-quality coal for power generation mixtures. Low-quality coal is characterized by high ash content, high specific gravity, and the presence of coal gangue; generally, the higher the ash content, the greater the hardness of the coal, which obviously places higher demands on crushing equipment. If the crushing equipment does not meet the requirements, the large particles with high hardness cannot be crushed; as a result, the particle size of the gangue remains almost unchanged, with no degradation or wear. These particles settle at the bottom of the furnace and are then discharged through the slag discharge port to a fluidized bed slag cooler equipped with both air and water cooling. This inevitably leads to problems with poor fluidization. The measures taken during operation consist only of reducing the load and continuously discharging the red slag through the emergency slag discharge pipe. If the large particles manage to block the slag cooler, it will be necessary to shut down the furnace for repairs. If the flow rate of the fluidizing air is increased, on one hand, there is concern regarding wear on the heated surfaces, and on the other hand, the air flow rate is limited by the fluidizing fan. The problem isn’t just that. As mentioned earlier, low-quality coal makes up a large proportion of the coal used for generating power in our country, and its ash content is high. By relying on the ash content inherent in the coal itself and using efficient separators, it is entirely possible to meet the boiler’s requirements regarding the amount of circulating material; there is no need to send the fine ash separated by wind back into the boiler. Moreover, the wind used to transport this fine ash can enter the boiler at a low speed, representing an unorganized form of air supply (as high speeds of dust-laden air can cause wear problems). With the excess air coefficient set at the furnace outlet, or in other words, after the oxygen level during operation is set, the presence of this airflow means that other organized secondary air flows must be reduced accordingly; obviously, this is detrimental to achieving organized combustion in the boiler. Therefore, the sorting function of the cold slag cooler is not only unnecessary but also detrimental when burning low-quality coal. If the purpose is merely slag cooling and large amounts of slag can be removed without affecting the operation of the boiler, then using a bottom slag discharge system combined with a water-cooled drum slag cooler, as shown in Figure C, will yield much better results than a fluidized bed slag cooler. The control of the slag discharge volume in the drum slag cooler is achieved by regulating the rotation speed of the drum. When the drum does not rotate, the slag forms a certain angle of accumulation within it and then stays in place. To discharge the slag, the drum is made to rotate, and the slag discharge volume changes accordingly with the drum’s rotation speed, allowing for easy adjustment and thus solving the problem of controlling the slag discharge volume effectively. By connecting the pressure difference signal from the bed layer to the variable-frequency motor of the drum slag cooler, automatic control of slag discharge can also be achieved, which is very popular among operators. There is a layer between the outer shell and the inner wall of the drum slag cooler, which is used to pass water through in order to cool the slag. The cooling water is usually taken from chemical water or condensed water; after absorbing the heat released by the slag, it is pumped to the low-temperature heater via a cold water pump to recover the residual heat. To improve heat exchange efficiency, some manufacturers install various internal components within the drum that increase the heating surface area and prolong the residence time of the slag. A drum slag cooler capable of cooling 10 tons of slag per hour can have a diameter of no more than 1.5 meters and a length of no more than 2.3 meters. Roller-type slag coolers have relatively lenient requirements regarding the particle size of the slag, and they possess strong slag discharge capabilities. In improved roller-type slag coolers, as long as the slag can flow down through the slag discharge pipe, it can be removed from the cooler. Additionally, the slag moves forward in a rolling motion within the slag cooler, resulting in minimal wear on the equipment. The motor power of each slag cooler is also very low, saving factory electricity compared to the fluidization fans in fluidized-bed slag coolers. The so-called bottom slag discharge means locating the slag discharge outlet at the bottom of the furnace chamber. We chose to arrange five slag discharge ports along the symmetrical center line of the water-cooled air distribution plate at the bottom of the furnace, in the direction of the boiler’s width; these ports are positioned exactly between the gaps occupied by the six coal feeders. Five water-cooled jacketed slag discharge pipes pass through the water-cooled air chamber from the slag discharge port; one of them serves as an emergency slag discharge pipe, while the other four are connected to the drum slag cooler. The drum slag cooler is specified with a slag discharge capacity of over 200%, and a shut-off valve for isolation is installed between the slag discharge pipes and the drum slag cooler. Obviously, the distance from various points on the air distribution plate to the bottom slag discharge ports is much shorter than the distance to the side slag discharge ports, and the force required for slag discharge is also greater in this case; as a result, the operation of the boiler is not affected by slag discharge issues. After careful consideration, we have chosen a bottom slag discharge system combined with a water-cooled drum-type slag cooler for slag discharge and cooling in the 480t/h CFB design; this is also the first time such a system has been used in the design of boilers of this capacity. Based on the actual operational results, this slag discharge and cooling method is indeed much better than the fluidized bed slag cooler that uses a combination of air and water cooling. As of the time of submission, we learned that some 135MW units in Hebei and Shandong provinces have replaced their slag coolers with newer models. Practice has shown that when foreign technologies are applied in China, they need to be adapted to China’s national conditions; failure to carry out further technical development and ignoring the particularities of Chinese coal can lead to problems. Based on the operational experience with 480t/h circulating fluidized bed boilers, the author believes that for 300MW units, installing 4 water-cooled drum slag coolers at the bottom of each of the two fork-shaped furnaces to replace the fluidized bed slag coolers will **improve reliability**. 2 Selection of coal feeding method: There are two common coal feeding methods for circulating fluidized bed boilers. One is rear-wall coal feeding, in which coal enters the furnace through the rear-wall return valve ； Another method is coal feeding from the front wall, with the coal entering the furnace through coal drop pipes on the front wall. In the coal feeding method using the back wall, the designers believe that by placing the coal feeding point on the return valve, it is possible to preheat the coal in advance using the circulating material. Additionally, the energy generated as the circulating material moves downward in the return leg can be utilized to spread the coal rapidly throughout the furnace, thereby reducing the need for multiple coal feeding points. Theoretically, this method of feeding coal from the back wall return valve has its advantages. However, during our investigations we found that the plant operators do not like this coal feeding method. They consider the coal conveying system that uses return valves for feeding to be complex. The large coal storage bins are located in front of the boiler; coal from these bins is delivered to the return valves on the inside of columns K2-K3 of the boiler, via paths that involve many turns and long distances. This results in limited flexibility in adjustment and an increased risk of mechanical failures. If there is a problem with the coal conveying device on one side, normal coal feeding becomes impossible, which not only affects the load but also leads to a significant increase in temperature variations on the smoke side inside the furnace. In severe cases, this can directly affect the steam temperature as well. Furthermore, from a structural design perspective, in CFB boilers that use high-temperature separators, the entire circulation system operates in a high-temperature environment. The section of the return material system located below the return material box outlet and up to the return material inlet in the furnace is under positive pressure during operation. Placing the coal feeding point in this area results in complex design requirements for the connections; moreover, the sealing devices and rotary feed valves are exposed to harsh operating conditions, which constitutes a weak point in terms of operational reliability. After weighing the pros and cons, it can be seen that the disadvantages associated with feeding coal through the return valve – due to the complexity of the coal delivery system and its tendency to malfunction – far outweigh the advantages. With the aim of improving the reliability of the coal delivery system, our design for coal supply involves arranging six coal feeding points along the 15-meter-wide front wall of the boiler. The coal stored in the silos is delivered to these feeding points via six weighted, sealed belt conveyors, from where it enters the furnace through the coal drop pipes on the front wall. This approach offers a short distance for the coal to travel and a simple structure; of course, it is also possible to use four feeding points on the front wall. Given that the designed coal type has a relatively high volatility content, coal with high volatility results in higher temperatures in the lower part of the furnace under the same operating conditions. To prevent the formation of local hot spots in the furnace due to this high volatility, multiple feeding points are used; this is beneficial for creating a uniform temperature distribution within the furnace as well as for ensuring even coal supply throughout it. Even if two feeders, or three feeders located on different sides of the furnace, fail simultaneously, it does not affect operation. Therefore, six feeding points were chosen along the front wall, with the coal supply amount being designed with a 200% surplus. The actual operating conditions show that the design objectives have been achieved. 3 Selection of soot blowers: In early circulating fluidized bed boilers, due to the low efficiency of the separators, wear of the rear heating surfaces was a common problem. With the improvement in separator technology, and the use of efficient separators with a D50 value of around 25 μm, a new issue arose in these rear heating surfaces: ash accumulation. There are three common soot blowing methods. One is traditional steam soot blowing; no one doubts the soot blowing capacity of steam soot blowers, but concerns exist regarding mechanical failures that may occur during operation, as well as the high maintenance requirements. Additionally, there are dead zones in the area where soot blowing takes place – the ash on the leeward side cannot be removed. The second type is acoustic soot blowing, which utilizes the principle of resonance to loosen the accumulated soot, after which it is carried out of the flue along with the flue gas. This type of soot blower can be further divided into three categories: resonant chamber type, swirl flute type, and reed type. Opinions on the soot-blowing performance of acoustic soot blowers vary; some people praise them while others find them ineffective. The third type of soot blower is the shock wave soot blower, which utilizes a mixture of acetylene and air in a specific ratio; through an igniter, the **generated combined energy is used for soot removal. This approach takes advantage of both the cleaning effect of hot air currents and the cleaning effect of shock waves resulting from sudden pressure increases, as well as the cleaning effects of sound waves and vibration waves. Some people are concerned that this type of soot blowing might cause damage to the membrane walls or protective furnace linings. In reality, since the action occurs in an instant, lasting only between 1/1000 second and 5/1000 second, the maximum energy generated is only 1/20 to 1/5 of the furnace’s explosion-proof capacity, so it can be used with confidence. The real challenge with shock wave soot blowers lies in the distribution and control of flow rate; the quality of these aspects determines the effectiveness of soot blowing. The structure of the entire system, including the soot blower itself, affects its durability, while the ability to detect faults ensures the stability of the soot blowing system. For the Uda project, we opted for shock wave soot blowers, with 44 soot blowing points installed on the rear heating surface. The results in actual use were good: each soot blowing operation reduced the flue gas temperature by more than 10°C. One boiler consumes three bottles of acetylene per day; at a local price of 70 yuan per bottle, the cost of acetylene amounts to 210 yuan per day. 4 Selection of air nozzles: There are two common types of air nozzles used in circulating fluidized bed boilers. One type is the mushroom-shaped air nozzle, which has holes evenly distributed around its perimeter ； Another type belongs to the category of directional wind caps; as the name suggests, they have an opening on only one side, and must be installed to point in a specified direction. Both types of air nozzles can achieve an excellent level of fluidization of the bed material. However, both types share one common drawback: during operation, the bed material can easily flow back into the air chamber through the openings in the nozzles due to fluctuations in bed pressure. When this accumulation reaches a certain level, it affects the air intake into the air chamber, thereby impacting the operation of the boiler. The air cap we have chosen this time is called an embedded counterflow column-type air cap, commonly known as a bell-shaped air cap. It is an improved version of the mushroom-shaped air cap. The original mushroom-shaped air cap had a hollow interior, with air coming from the air chamber and being ejected in all directions through small holes evenly distributed around the circumference of the air cap. The improvement involved adding a central tube inside the hollow mushroom-shaped air cap; the air from the air chamber first rose up through this central tube, then turned back after hitting the inner wall at the top of the mushroom-shaped part. The air then flowed downward along the annular gap between the central tube and the air cap, before being ejected in all directions through the holes located at the bottom of the air cap. In essence, this adds a kind of labyrinth inside the air cap, and the results are very good. After the boiler operated continuously for over 3 months, an inspection of the air inlet chamber showed that only minor amounts of slag leakage occurred in certain areas at the bottom of the chamber, indicating that the leakage prevention effect was significant. At the same time, we also improved the shape of the air cap’s tip. The original tip was hemispherical, and it experienced the greatest impact and wear due to the force exerted by the bed material at an angle of 45° upward. The improved tip has a bamboo-joint shape; during operation, the recesses above these joints first get filled with ash particles, serving as a cushion. This cushioning effect reduces the impact force exerted by the fluidized bed material on the top of the air cap, thereby extending its service life. 5 Selection of fire-resistant and wear-resistant materials: The fire-resistant and wear-resistant materials used in circulating fluidized bed boilers are crucial for the reliable operation of these boilers. With the continuous improvement in the performance of unshaped fire-resistant materials, as well as their advantages in terms of integration with the boiler structure, there has been a shift in circulating fluidized bed boilers from using brick-lined structures to employing unshaped fire-resistant materials. Its performance is reflected in various physical and chemical parameters such as volume density, refractoriness, compressive strength at room temperature, flexural strength at room temperature, linear expansion rate, thermal shock resistance, wear resistance, and chemical composition. When selecting materials, it is essential to take into account the operating characteristics of circulating fluidized bed boilers. It is necessary to emphasize the performance criteria of materials during design, but pursuing excessively high standards can have the opposite effect. Many workers involved in the maintenance of circulating fluidized bed boiler furnace walls in power plants feel that the refractory and wear-resistant materials used have sufficient wear resistance but insufficient toughness; the wear-resistant layers tend to crack, and these cracks are wide, which is due to excessively high values of certain parameters in the materials. We hope that such materials will exhibit high wear resistance and toughness within the temperature range of 800–1000°C, with minimal linear strain variation. Additionally, it is very important to choose manufacturers who can produce high-quality refractory and wear-resistant materials. We recommend that suppliers of fire-resistant and wear-resistant materials be the same ones who carry out their installation, thereby linking product quality with installation quality. Boiler factories and power plants should avoid purchasing fire-resistant and wear-resistant materials at low prices. At present, the vast majority of manufacturers in China that produce such materials operate on a small scale and lack strong research and development capabilities. In the fierce market competition, many manufacturers rely on low prices to secure orders, and this price competition leads to a decline in quality. We also hope that capable suppliers of fire-resistant and wear-resistant materials will participate in the design of boiler furnace walls. By leveraging the expertise of these professional suppliers, the structure of the furnace walls can be made more rational, allowing the excellent properties of the materials to be fully utilized, thereby ensuring the long-term reliable operation of the boiler unit. 6 Conclusion: The successful operation of a boiler depends on its overall design and layout, while its reliability relies on the design of various individual components, especially those that are in motion during operation. As the boiler has been in operation for longer periods, some new problems have gradually emerged. For example, the ash accumulation problem in the flue ducts at the inlet and outlet of the separator ； Due to the cumulative errors in the manufacturing and installation of the membrane-type water wall, the gaps at the corners of the furnace are uneven. As a result, the sealing flat strips welded on-site vary in width and position, which leads to an accumulation or acceleration of particles falling downward in certain areas of those corners, thereby causing wear at the corners of the furnace’s water wall ； Deformation and bending problems of screen reheaters ； The fan head pressure is significantly higher than the wind pressure used in operation, and there are issues with the ratio between primary and secondary air not matching the actual operating conditions ； Issues such as a simple bed charging device are considered to reduce the labor intensity of loading the bed material. Some problems can be solved quickly, while others are not easy to handle. It is necessary to gradually explore the characteristics of circulating fluidized bed boilers, gain a thorough understanding of their underlying principles, and accumulate experience to make improvements in newly designed boilers. We believe that as our knowledge of large-scale circulating fluidized bed boilers deepens, the safety and reliability of their operation will surely improve significantly. Guided by the principle of serving users, our company will actively engage in communication with the operation and management personnel of power plants, striving continuously to ensure the reliable operation of large-scale circulating fluidized bed boilers in our country and to improve their technical performance. References: Bao Shaolin, Research Report on Slag Coolers (Internal Version); Lu Junfu et al., On the coal feeding issues in circulating fluidized bed boilers. Collection of Technical Papers on CFB Unit Technology