Could that expert please share some information on 440-ton circulating fluidized bed systems?

Circulating fluidized bed boilers have advantages such as high efficiency and low pollution, and they have seen rapid development at home and abroad over the past decade or so. At present, many circulating fluidized bed boilers designed and manufactured in China have been put into operation, but there are still numerous issues. The following is a brief analysis of the 480-ton CFB boiler produced by Dongfang Boiler Factory: Manufacturer: Dongfang Boiler (Group) Co., Ltd. Boiler model: DG-480/13.73-Ⅱ11, ultra-high pressure, circulating fluidized bed type. Configuration: single drum natural circulation, CFB boiler, single furnace, once reheating, balanced ventilation, steam-cooled cyclone separators, tightly enclosed layout, fully steel furnace frame suspension system, solid slag discharge. Boiler capacity and parameters: Superheated steam. Maximum continuous evaporation rate (B-MCR): 480 t/h. Steam pressure: 13.73 Mpa(g). Superheater outlet temperature: 540°C. Reheated steam flow rate: 395.9 t/h. Steam pressure at reheater inlet/outlet: 2.86/2.68 MPa. Steam temperature at reheater inlet/outlet: 316/540°C. Feedwater temperature: 247°C. Type of air preheater: horizontal tube air preheater. Inlet air temperature: 20°C. Temperature of primary air: 257°C. Temperature of secondary air: 257°C. Flue gas temperature: 130°C. Boiler efficiency (calculated based on lower calorific value): 86.5%. Desulfurization efficiency: 80%. Calcium-sulfur ratio: 2.2. Limestone consumption: 1.85 t/h. The boiler consists mainly of a membrane-type water-cooled wall furnace, two steam-cooled cyclone separators, and a tail shaft covered by steam-cooled walls. The furnace is equipped with screen-type heating surfaces: six sets of screen-type superheater tubes, four sets of screen-type reheater tubes, and one water-cooled partition wall. The boiler is equipped with six coal feeding ports, all of which are located in front of the furnace and are evenly arranged along the width direction in the constricted section beneath the front wall water wall. The bottom of the furnace chamber is a water-cooled air chamber formed by bent water-cooling wall tubes; at the lower part of this water-cooled air chamber are located the ignition air ducts. Inside these ignition air ducts there are two under-bed start burners, and the boiler is started using this under-bed start method. Problems during operation: 1. Slag discharge issue. This boiler is equipped with a wind-water combined slag cooler, but its performance has not been satisfactory since it was put into use; the main issue is the difficulty in discharging slag. The high temperature of the slag causes the chain bucket to trip, and in severe cases, it even leads to deformation and damage. Theoretically, achieving a balance between the bed pressure in the furnace and that in the slag cooler is supposed to be accomplished by adjusting the rotation speed of the slag discharge valve in order to control slag discharge. However, due to the large amount of slag, this approach is completely inadequate to meet the actual requirements for slag discharge. The main reason is that the slag from this boiler is extremely fine – 80% of it consists of fine ash. When the air flow in the cold slag chamber is high, a large amount of this ash returns to the furnace; if the air flow is low, overheating occurs, which results in a consistently high bed pressure in the furnace, forcing repeated shutdowns of the boiler. 2. Coal clogging issue: Since the coal bunker of this boiler has not been modified either, it suffers from frequent coal clogging. The main reasons are the lack of polyethylene sheets installed on the inner walls of the coal bin, the fact that the bin walls have not been modified to have a hyperbolic shape (resulting in rough walls and a large slope), as well as the humidity and particle size of the coal. 3. Issue of pressure differences between the two sides: Since the water-cooled walls of the boiler’s partition wall extend into the furnace bed, the furnace is essentially divided into two sections. The balance holes are too small, and the rear wall is only 760 mm thick; as a result, pressure differences always occur between the two sides, sometimes reaching 4 kPa (9 kPa on side A and 4 kPa on side B), which affects the bed temperature on both sides as well as the flue gas temperature at the exits of the furnace. At times, the temperature difference between the flue gases exiting the furnace on both sides can reach 200°C, making it impossible to control the steam temperature and leading to the shutdown of the turbine. 4. The design of the cooling water is insufficient: at a load of 70 MW, the temperature still rises to 560 degrees even with all the cooling water valves opened. Primarily, it is the screen-type superheater installed in the furnace, whose inlet temperature far exceeds the designed value of 510 degrees, reaching 550 degrees. 5. Air preheater mixing issue: Due to the high pressure of the primary air in fluidized-bed boilers, in Dongguo’s tubular air preheaters, the primary air and secondary air are separated by only one layer of plate; as a result, there is a difference in pressure on both sides. Under the effects of vibrations caused by high air flow rates, it is very easy for the welds of this separating plate to crack, leading to mixing of the primary and secondary airs. The boiler developed air mixing issues after just 4 months of operation, and the volume of secondary air also increased. 6. Dust problem: This is perhaps one of the common issues associated with circulating fluidized bed boilers. Since the area below the burner plate in the furnace is a region of positive pressure, with a bed pressure of 5–8 kPa, the continuous circulation of material means that any leakage of air will result in the emission of hot ash, which not only affects the environment but also poses a risk to the health of the operators. Since the exhaust vents of this boiler were not sealed in a timely manner after drying, especially in the furnace area, and due to simultaneous operations at that time, insulation was applied right after drying. Although there were barriers made of cast material in the lower part, ash leakage still occurred from the exhaust vents located near the water-cooling fin plates; the main source of ash leakage was the cold slag chamber, with ash being blown out through the chain buckets and rotary slag discharge valves. The rotary slag discharge valves lost their sealing properties after operating for a short time, allowing cold slag air to escape and carry along large amounts of fine ash, resulting in a highly dusty environment in the boiler room. To address the above issues, the factory carefully organized relevant technical personnel to conduct multiple inspections and implemented a number of feasible measures, which increased the operating cycle of this boiler from 10 days to 65 days. It is also determined to continue making improvements in order to achieve a cycle length of 200 days. The following main measures were taken: 1: A stepped closely-pored plate air-cooled slag cooler was used. The previous water cooling system and the air distribution plates of the old slag cooler were removed, and dense-hole plates were used for air distribution; the area of these air distribution plates was reduced. fire resistance bricks were used to construct a trapezoidal structure above the air distribution plates, with a larger upper part and a smaller lower part, thereby improving the cooling capacity for hot slag. After this modification, slag discharge improved – not only was the amount of slag discharged larger, but the temperature of the slag was also below 150 degrees. To date, there have been no shutdowns of the furnace due to slag discharge issues. Additionally, control over the water used to suppress dust from coal was improved; vibrators were added, and polyethylene sheets were installed on the walls of the coal bunker. As a result, the frequency of coal supply interruptions has decreased significantly, and better procedures have been developed for dealing with such interruptions, with little impact on the operational load. 3: To address the issue of overheating, we tried to increase the main steam pressure; as a result, the pressure of the cooling water also increased, and overheating does not occur when it is above 11 MP. On the other hand, when the steam temperature is excessively high, intermittent closing of the main feedwater valve is used to increase the feedwater pressure as a solution; this issue can now be basically controlled. 4: Regarding the issue of bed pressure on both sides, we have been exploring solutions as well. The balance of coal supply to both sides has a significant impact on bed pressure; moreover, appropriately increasing the amount of primary air and keeping the bed pressure around 6 kPa can lead to a noticeable improvement. Try to avoid unilateral slag discharge; appropriately adjusting the opening of the mainstreaming damper can yield significant results. 5 Taking advantage of the shutdown period, the areas where ash was leaking were thoroughly sealed, and chain hoppers were installed on the negative pressure duct at the rear of the boiler. The negative pressure inside the furnace was increased to a range of –127 to –245 Pa, which has significantly improved the conditions in the boiler room. 6: A thorough inspection was carried out for air leaks in the furnace and the air preheater; the cracked partitions in the air preheater were reinforced, and since then no more air mixing has occurred. From design to installation, the relevant supervisors paid special attention to the wear of CFB boilers; during the design phase, a tubular arrangement was adopted between the furnace lining material and the water wall, and no significant wear was observed during shutdown inspections. Numerous investigations were also carried out regarding the issue of backflow; during installation, the lower part of the J-valve was raised, which reduced the height of the partition wall in the J-valve from 1.5 meters to 0.5 meters. After it went into operation, backflow problems occurred only once or twice, and they weren’t severe – simply blowing it with compressed air was sufficient to resolve them. Since its commissioning, the coal-feeding booster fan has not been used; the coal-feeding air is supplied directly from the hot primary air via a bypass, and practice has proven this to be feasible. After several attempts at ignition, we tried to regulate the process without using an ignition fan, by relying instead on the primary air flow; this approach was sufficient to meet the requirements of the oil gun, resulting in good combustion. The rate of change in bed temperature could be kept at 100 degrees per hour. This method avoided damage to the expansion joints in the main air ducts that occurred when an ignition fan was used. The bed temperature could reach 520 degrees, the temperature required for coal feeding. Ignition was successfully achieved through pulse coal feeding, and approximately 18 tons of diesel were used, achieving the desired results – both fuel savings and easier adjustment were achieved. A topic worth discussing: Since the fuel for this boiler is designed to consist of bituminous coal mixed with 30% gangue, and two-stage crushing is used, nearly 20% of the coal could have been used in its original particle size. However, no screening is employed in this boiler, and all the coal goes through the two-stage crushing process, which results in the fine particles becoming too fine ； When gangue is added, due to its high hardness and difficulty in being crushed, the particle sizes resulting can reach 20 mm, which leads to the particles becoming excessively coarse ； At the same time, due to the high exothermic intensity of the coal used in this boiler, over 60% of the material produced after combustion has a particle size of 0.3–1 mm. The particle size of the gangue does not decrease significantly after combustion; only its color turns white. Finding a proper and reasonable solution to this particle size distribution with larger sizes at both ends and a smaller size in the middle requires the enthusiastic assistance of relevant experts and leaders, as well as valuable suggestions from everyone! ! ! In summary: While efforts are being made across the country to improve the operation techniques of CFB boilers, this boiler is also making active efforts under the special attention of its leaders, through continuous exploration. Practice has shown that our efforts have yielded results. We hope to receive strong support from the CFB cooperation network in the days to come, and believe that in the near future we will be able to make progress together with other organizations that are performing even better! !