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Analysis of Several Major Issues in Circulating Fluidized Bed Boilers 1 Introduction With its unique advantages such as a wide range of fuel compatibility, low-cost desulfurization, low pollutant emissions, high combustion efficiency, large load regulation capacity, and comprehensive utilization of ash and slag, the circulating fluidized bed boiler has become a promising direction in clean combustion technology, attracting attention from various countries and enabling its rapid development. Currently, a number of CFB boilers in China with a capacity of over 410 t/h are in commercial operation. Our Company’s Boiler No. 3 and No. 4 are the largest CFB boilers in operation in China, with a capacity of 465 t/h each. The combustion technology of circulating fluidized bed boilers is a new technique; it represents a combustion method that lies between the suspended combustion in coal powder boilers and the fixed-bed combustion in chain grate boilers, namely what is commonly referred to as semi-suspended combustion. During operation, a large amount of materials such as solid bed material containing fuel, fuel ash, limestone, and their reaction products continuously circulate at high temperatures within a closed loop of furnace – separator – return valve – furnace, carried by the flue gas. In addition to the external circulation of the bed material in this circuit, a large amount of the bed material undergoes internal circulation along the four walls of the furnace under the action of gravity, thereby enabling the fuel combustion, material circulation, and heat transfer processes in the fluidized bed boiler. 2 Introduction to the Boiler: The expansion project for the 2×135MW circulating fluidized bed thermal power units at Huadian Zibo Thermal Power Co., Ltd. utilizes HG465－13.7－L.PM7 type circulating fluidized bed boilers produced by Harbin Boiler Factory. These are boilers with primary intermediate reheating and ultra-high pressure natural circulation; they were designed by Harbin Boiler Factory based on the circulating fluidized bed boiler technology introduced from Alstom Company. Main design parameters: Superheated steam flow rate – 465 t/h; Superheated steam pressure – 13.7 MPa; Superheated steam temperature – 540°C; Feedwater temperature – 244°C. Reheated steam flow rate – 411 t/h; Reheated steam pressure – 4.06 MPa; Reheated steam temperature – 540°C; Reheated steam inlet temperature – 373.8°C. The overall layout of the boiler is as follows: The boiler utilizes circulating fluidized bed combustion technology and consists mainly of a furnace, a high-temperature insulated separator, a self-balancing “U”-shaped return valve, and tail convection flues. The heating surfaces of the combustion chamber are equipped with membrane-type water walls; double-sided water walls are arranged inside the combustion chamber, dividing the furnace into two combustion chambers. A screen-type secondary superheater and a screen-type hot-section reheater are placed at the upper part of the combustion chamber, while a tertiary superheater, a primary superheater, and a cold-section reheater are located in the tail flue. The flue is fitted with wall-enclosing superheaters that have a membrane-type structure. The water-cooled air distribution plate uses large-diameter bell-type air nozzles. Two cyclone separators are arranged between the combustion chamber and the tail convection flue, lined with insulating materials and wear-resistant refractory materials. A non-mechanical return valve is installed below the separator return leg; the return flow is of the self-balancing type, and the fluidization seal air is supplied separately by a high-pressure fluidization fan. Reliable anti-wear measures are taken for the parts of the boiler that are prone to wear; high-temperature and wear-resistant three-way non-metallic expansion joints are installed between the combustion chamber and the separator, as well as between the return valve and the cold slag holder. Additionally, such expansion joints are installed between the separator and the return valve, and between the tail flue. Four start-up burners are arranged under and above the bed. The boiler uses four points of coal feeding at the front wall, with primary air serving as the coal spreading air. Tapered valves are installed on both sides of the boiler for slag discharge; two air-water combined slag coolers are placed at the bottom of the boiler to keep the slag discharge temperature below 150°C. The bottom slag is then sent to a scraper-type slag remover and from there to the slag storage tank. 3 Main problems of the boilers and improvement measures: Our company’s Boiler No. 3 and Boiler No. 4 were put into trial production in August and December 2003, respectively. The operational performance since commissioning shows that the boiler has excellent performance, with an evaporation rate of over 480 t/h, exceeding the designed BMCR value; all major operating parameters meet the design specifications. However, during operation, some problems specific to CFB boilers have also emerged, such as: the particle size of the bed material making it difficult to discharge slag from the cold slag remover, frequent interruptions in coal supply due to blockages in the coal bins, severe wear on the water-cooled walls inside the furnace, and leaks in the dense phase region of the furnace. Next, we will analyze and discuss these aspects from the perspectives of design, manufacturing, installation, operation, and the performance of auxiliary equipment, and propose improvement measures. (1) Difficulties in discharging slag from the slag cooler: This furnace is equipped with two fluidized-bed type air-water combined slag coolers; since the capacity of these types of slag coolers can be designed to be high, they provide excellent cooling effects. Therefore, large CFB boilers both at home and abroad choose this type of equipment; however, it is sensitive to the particle size of the bottom slag. If a large proportion of coarse particles are present in the bottom slag, it deteriorates the fluidization quality inside the cold slag quencher, making it difficult for the ash and slag to be discharged through the overflow port. This can even lead to the bed being compressed and coking, as well as the blockage of the coarse slag discharge port. During the operation of the cold slag holder, the emergency slag discharge outlet is often used for discharging slag. Since the hot bottom slag is only air-cooled in one chamber, the slag discharge temperature is high, which frequently leads to failures in the subsequent slag removal equipment – mainly bearing seizure in the slag conveyors due to high temperatures, severe wear of the scrapers, and deformation of the internal components as a result of high temperatures. The main reason for the above phenomenon is that the particle size of the coal fed into the furnace is not up to standard; the manufacturer’s requirement for the particle size of the coal entering the furnace is that the maximum particle size, dmaax, should be ≤ 7 mm ； Median particle size d50=0.6mm ； Particles with a diameter of less than 0.2 mm account for ≯25%. If the coal preparation system (double rotary screen plus a primary crusher) can meet the above requirements, the slag discharge system will be able to operate properly. However, based on several months of operation, the poor quality of the coal supplied to the plant – with a high content of stones and gangue – combined with the low capacity of the crushers to break down those stones and severe wear of the hammers, which results in large gaps between the crushing elements, has led to a proportion of coal particles larger than 6 mm accounting for 10%–40% of the total amount of coal fed into the furnace; this is a significant deviation from the design specifications ； Furthermore, low-quality coal used for combustion has poor ash-forming characteristics such as thermal explosion and abrasion, which is the main reason for the large size of the ash. On-site observations show that there are many ash particles with a diameter of 10 mm to 40 mm, and large stones can sometimes reach 70 mm in size. The main measures to address difficulties in slag discharge are: first, controlling the quality of the coal supplied to the plant, minimizing the amount of stones mixed in, and adjusting the gap of the crusher in order to control the particle size distribution of the coal fed into the furnace. Secondly, control the air distribution among the three air chambers of the slag cooler to ensure that all three slag cooling chambers achieve a good state of fluidization ； It is very important to control the slag discharge method of the conical slag discharge valve as well as the operation mode of the large slag discharge valve in the slag cooler. Based on the bed pressure in the furnace, it is necessary to discharge slag in small amounts multiple times in a uniform manner, in order to avoid overpressuring one chamber of the slag cooler. Finally, it is also necessary to adapt to the reality of low-quality coal supplied to the plant by making some structural changes to the slag cooler, such as increasing the diameter of the coarse slag discharge valve to enhance the capacity for emergency slag discharge ； Reduce the height of the partition wall in the cold slag holder to improve the ability of bottom slag to overflow ； Plug doors are installed on the return air duct from the slag cooler to the furnace, in order to clean the slag cooler during boiler operation ； Improving the reliability of slag transfer equipment, etc. (2) Coal blockage in coal bins: Due to high moisture content in the coal fed into the furnace, an unreasonable structure of the coal bins, and poor lining inside these bins, coal discharge is hindered, leading to frequent blockages. The coal accumulation on the inner walls of the bins can reach up to 2 meters thick, which reduces their effective volume. As a result, the interval between coal deliveries to the bins becomes shorter, and in some cases the bins even run out of coal. If not handled properly, this can cause smoke to flow back, leading to damage to the belts of the coal feeders and severely affecting the safe operation of the boiler. The boiler was originally designed with four raw coal bins, each with a capacity of 165 cubic meters; they are made of steel plates and have a pyramidal shape, with an inner lining of 2 mm thick stainless steel plate. The lower part of the coal bin tapers into a square cylinder, equipped with an electric sliding door, and is connected to the coal hopper beneath the top and bottom structures; this hopper again tapers into a circular cylinder that leads to the inlet of the coal feeder. Four air cannons are installed on the walls of the coal hopper to loosen the accumulated coal. Based on the operation status, the coal bins are severely clogged with coal, and the effect of using air cannons to loosen the coal is minimal; continuous cleaning efforts prove ineffective as well. Only manual knocking is effective, and the problem becomes even worse when the moisture content of the coal fed into the furnace is high. Later, a loosening machine was installed on the inner walls surrounding the coal bin to loosen the coal adhering to those walls, but the effect was still not significant. Ultimately, we installed ultra-high molecular weight polyethylene sheets as lining inside the coal bin to modify the lower part of the coal bin; we moved the sliding door downward to the location where the feeder connects, removed the upper and lower coal hoppers associated with that sliding door, and installed a circular coal hopper under the steel coal hopper, so that the coal could flow smoothly to the bottom of the coal bin and then enter the feeder through the sliding door. After the modification, the problem of coal blockage in the coal bin was completely resolved. (3) Water wall wear issue: Our company’s CFB boilers use a soft landing structure along the upper edge of the dense phase zone in the furnace, along with tapered tubes and wear-resistant protective plates, to prevent water wall wear. After operation began, it was found that the water wall tubes and fins in this area were severely worn. We believe that the abnormal particle size distribution of the coal fed into the furnace resulted in a coarser bed material; coupled with high ash content, large amounts of gangue, and stones in the coal, a higher volume of primary air was used to achieve adequate fluidization, which in turn led to increased wear ； On the other hand, the assembly and welding quality here is poor, and the application of the wear-resistant castable also does not meet the requirements specified in the drawings. These two are the main causes of water wall wear. The former is resolved through operating methods, while the latter requires measures to be taken in terms of installation and construction. The detailed analysis is as follows: 1) Wear caused by uneven surfaces of the water wall is manifested in the following aspects: The installation technique of the anti-wear guards at the diameter-changing sections of the water wall is poor; there is a large gap between the anti-wear guards and the water wall tubes, resulting in a bulge. The accumulation of material against this bulge is the main cause of wear on the water wall and its fins. The uneven weld beads on the fins cause severe wear on those fins; once they are worn through, ash leaks out, which in turn erodes the water wall. There are several reasons for uneven weld waves. One is that at the transition sections of the membrane screens in the boiler plant’s water wall, the fins do not align properly above and below each other, resulting in protrusions ； Secondly, the weld bumps formed where the anti-wear plates are welded to the fins have not been polished, resulting in protrusions ； Third, the installation lifting holes on site were not polished smooth during the repair welding of the fins. Fourth, when installing the water wall panels, after separating the lower fins, the weld seams are uneven with protrusions. Inspection reveals that the fins and welds suffer mild wear in a gable-like pattern, while more severe wear occurs in a bulge-like pattern. The welds of the water wall tubes were not polished smoothly, resulting in abrasive wear on those tubes. There is widespread erosion of the water-cooled wall tubes in the upper part of the soft landing platform that lack anti-wear guards. 2) Wear caused by blockage of the internal material flow channels in the furnace. A large amount of material circulates internally within the furnace to facilitate heat exchange. When a large amount of material undergoes continuous internal circulation along the water wall, the amount of material flowing down along the fin grooves is much greater than that flowing down along the outer curved surface of the water wall tubes. The fins are like channels; if the “channels” get blocked, the “water” overflows from the sides. By the same principle, if the fins are blocked by casting materials, weld beads, etc., the material flows to one or both sides, thereby scouring the water wall tubes. It turns out that such a scouring speed is extremely fast. There are several situations: l The pouring process is not carried out properly, resulting in the edges of the pouring material ending up on the fins, which in turn blocks the normal flow of materials. For example, in areas such as the double-sided water wall and the water walls that pass through the front and rear walls, wherever there is casting material, wear of the water wall may occur at the edges. The pouring material soft landing platform developed slopes due to poor construction; as a result, the soft landing failed to provide cushioning or prevent the rebound of materials, leading to wear on the water wall tubes located near those slopes. The fin welding at the water wall tubes in the four corners of the furnace is poor, resulting in the failure to form proper flow channels; if large pieces of bed material get trapped in those corners, it will exacerbate the wear on the water wall. (4) Ash leakage in the boiler: For various reasons, ash leakage from the boiler itself is a serious issue, occurring mainly along the fins of the water-cooled wall in the dense phase zone, as well as in the sealing boxes of that zone. There are mainly the following situations: 1) The upper edge of the dense-phase zone, along the water-cooled wall fins, is severely worn out, wearing through the fins and causing material leakage. 2) Poor installation quality of the sealing box, resulting in severe dust leakage: The perforated plate at the upper part of the sealing box was not welded, especially on the inner side of the rigid beam in the dense phase area ； The adjacent sealing boxes are not separated, and uneven expansion pulls the sealing boxes apart ； The gap between the secondary air duct and the castable is large, allowing material to penetrate into the seal box and empty out its insulating castable, resulting in the seal box being damaged, deformed, and leaking ash. 3) The lower secondary air ducts suffer from low amounts of secondary air due to the coarse bed material and high bed pressure, which causes the lower secondary air inlets to be covered by ash, leading to duct deformation and damage as well as leakage of the bed material. 4) The welding at the coal feeding port is poor, and the lack of a sealing box on the outside results in ash leakage. 5) Cracks in the welds and ash leakage occur in components such as separators and return valves due to poor welding. 6) The moisture vent during the baking process was not welded, resulting in ash leakage. 7) During installation, the inner and outer sides of the furnace chamber measurement point were not welded, resulting in ash leakage at that point. In short, the problem of ash leakage should primarily be addressed from the aspects of installation quality and operational adjustment. (5) Several points should be noted during boiler installation: 1) The welds of the pressure-bearing components inside the furnace chamber, as well as the lifting holes and fins, must be polished smoothly after welding; there should be no misalignments, protrusions, or depressions, in order to prevent excessive wear on these pressure-bearing components due to the impact of materials during operation. 2) When installing the wear-resistant and fire-resistant materials inside the furnace, special care must be taken to ensure that the edges of these materials do not obstruct the internal circulation of materials, thereby preventing wear on the adjacent pressure-bearing pipes. 3) When installing wear-resistant and fire-resistant materials, it is essential to ensure the correct amount of water is used. Molding during construction is very important, as it ensures that the final product matches the design specifications after demolding. Expansion joints must be controlled carefully, and ventilation holes must be properly installed during furnace drying to ensure thorough moisture removal. 4) The most important thing when installing non-metallic expansion joints is to control the misalignment dimension to ensure sufficient expansion space. When selecting a product manufacturer, pay attention to whether its parameters meet the design requirements. The filling of the internal packing in the expansion joint and the external environment are very important; their requirements for operation must be met. Pay special attention to ensure that the expansion joint at the water-cooled air chamber is not damaged by leaking hot air, which could burn the non-metallic expansion joint. 5) Pay special attention to the welding of the sealing plates on the pressure-bearing pipes of the boiler, to prevent cracks in these sealing plates and the base material during operation due to external thermal stresses, which can result from poor welding quality or uneven expansion. 6) Attention is often not paid to the welding of the separator cylinder during construction; it is necessary to prevent cracks from occurring, and the welding must meet the required standards to ensure high strength. The return valve needs to be reinforced to prevent shaking during operation. 7) The installation positions of the air volume measurement elements and the test port holes used for air volume calibration must comply with relevant requirements to prevent errors in air volume calibration. 8) The elevation of the pressure measurement points and temperature measurement points in the furnace bed of the same layer must be consistent during installation, to ensure that no construction errors occur when installing the wear-resistant and fire-resistant materials. 9) The slag discharge opening in the furnace chamber must be smooth; the outer hole of the air cap at this location should be enlarged, and the lining material inside the cone valve must be smooth and seamless to prevent difficulties in slag discharge. 10) High-strength heat-resistant steel must be used for the lower secondary air ducts to prevent the ducts from warping due to heat during operation. 4 Conclusion In summary, the main problems encountered during the operation of large CFB boilers are poor coal feeding and frequent interruptions in coal supply ； It is difficult to discharge slag from the fengshui combined cold slag remover, and the slag discharge outlet may even get blocked ； Local wear of the water wall leads to ash leakage from the furnace wall; in severe cases, the tubes in the water wall burst ； The sealing structure is damaged, resulting in ash leakage near the dense-phase zone of the furnace body. The occurrence of these problems is due to reasons related to design, manufacturing, and installation, as well as to operating conditions and performance levels. Therefore, it is necessary to address the issue at its source, and comprehensive management from multiple aspects is required to achieve a fundamental solution.