Commissioning of large-scale circulating fluidized bed boilers

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Abstract: Based on the principles and structure of circulating fluidized bed boilers, this paper presents the commissioning experience of the SG475/13.7-M567 type circulating fluidized bed boiler at Jingdezhen Power Plant, analyzes the operating characteristics of this boiler, and provides references for relevant personnel. Keywords: circulating fluidized bed boiler ； Debugging ； Cold-state test ； Oven drying ； Slag removal 0 Introduction: The boiler of the 150MW unit at Jingdezhen Power Plant is a SG475/13.7-M567 type circulating fluidized bed boiler, manufactured by Shanghai Boiler Factory using technology imported from Alstom in the United States. The unit began its 72+24 hour trial operation on December 3, 2004; it passed this trial period on December 7, 2004, was handed over for commercial operation, and is currently one of the large-scale circulating fluidized bed boilers in operation in China. During the 72+24-hour trial operation period, all parameters of the boiler met the requirements specified in the \"New Commissioning Regulations\"; oil supply was interrupted, electric dust removal was in operation, the boiler’s combustion was stable, and there were no severe cases of coking. During the entire 72+24 hour trial operation period, the average load was 141.8 MW, with an average load factor of 94.5%. 1 General Overview of the Equipment 1.1 Main Design Features and Process Flow of the Boiler The boiler furnace is a combustion chamber with a depth of 7683.4 mm, a width of 13373.1 mm, and a height of 34290 mm. At the upper part of the furnace, there are 4 water-cooled screens as well as 16 superheating screens in two stages, according to the design specifications. The bed material, including fuel, is aerated by the primary airflow; this turbulence transports the solids (fuel, limestone, and ash particles) throughout the entire height of the combustion chamber. Primary air is introduced from the air chamber at the bottom of the furnace, and flows through the \"T\"-shaped air nozzles on the air distribution plate to fluidize the bed material. Secondary air is supplied into the furnace in two layers at a certain height above the air distribution plate in the furnace chamber, thereby achieving staged combustion. The heat generated by fuel combustion is transferred to the water-cooled walls of the furnace, as well as to the water-cooled and superheating surfaces located inside the furnace. Under BMCR operating conditions, the combustion temperature in the furnace is 870–880°C, with no visible flame. At this temperature, high desulfurization efficiency and low NOX emissions can be achieved. The hot flue gas, carrying most of the solids, fuel, and ash, exits from the top of the combustion chamber and is drawn at high speed into two insulated cyclone separators with an inner diameter of 7925 mm, which are located at the furnace outlet. Inside the separator, centrifugal force is used to separate solids from the gas, and the separated materials are returned to the furnace through a return conveyor installed below the separator for cyclic combustion, thereby improving the efficiency of the boiler. To meet the requirements for furnace heat transfer and circulation rate, it is necessary to establish an adequate amount of circulating material. In the circulation loop composed of a combustion chamber, a separator, and a return conveyor, limestone and SO2 in the flue gas are thoroughly mixed, thereby achieving a high desulfurization efficiency. Primary air accounts for about 50-55% of the total air volume in the boiler, while secondary air accounts for 35-40%. By supplying air in a staged manner through primary and secondary air, NOX emissions can be further reduced and the combustion conditions inside the boiler can be improved. The flue gas separated by the separator enters the rear convection flue duct. This duct features a balanced dual-flue design: all the reheater heating surfaces are located on one side, while the high-temperature superheater and high-temperature economizer heating surfaces are situated on the other side. After passing through the regulating dampers of these dual flues, the flue gas proceeds to the low-temperature economizer and the air preheater. Finally, it enters the electrostatic precipitator and is discharged through the chimney via an exhaust fan. One fluidized bed slag cooler is installed at each lower side of the furnace. During operation, the high-temperature hot slag discharged into the slag coolers is cooled to below 200°C by the convection tube bundles located within them before being discharged. The cooling medium in these convection tube bundles is the cooling water from the heat recovery system, allowing most of the thermal energy contained in the ash and slag to be recovered by the system. 1.2 Main parameters of the boiler: Superheated steam flow rate: 475 t/h; Superheated steam outlet pressure: 13.9 MPa; Superheated steam outlet temperature: 540°C. Reheated steam flow rate: 403.47 t/h; Reheated steam inlet pressure: 2.662 MPa; Reheated steam outlet pressure: 2.527 MPa; Reheated steam inlet temperature: 314°C; Reheated steam outlet temperature: 540°C. Feedwater temperature: 247°C. 2. Cold-state tests: 2.1 During the cold-state tests, wind speed measurements were taken at the upper edge of the furnace’s air distribution caps (primary air volume: 180,000 Nm3/h); the measurement data are shown in Table 1. Wind speed measurement results in the fluidization zone (m/s): Front wall at the expansion end (near the control room): Fixed end – 2.5, 1.9, 2.2; 0.8, 2.1, 2.2, 2.6; 1.8, 1.1, 1.2, 1.9; 1.0, 0.9, 2.2, 2.2; 2.5, 1.1, 1.9. Back wall (near the electrostatic precipitator): Based on the test data, when the primary air volume is 180,000 Nm3/h, the lowest wind speed at the cross-section of the air distribution nozzles is 0.9 m/s, which is greater than 0.7 m/s (the minimum wind speed required for fluidization) ； However, it was found that the wind speed in the middle was higher than that around it. When checking the fluidization quality during the tests on the bed material properties, it was also observed that the fluidization quality around the area was worse than in the middle, which could easily lead to bed material leaking into the primary air chamber. It is recommended to install sand discharge pipes at the bottom of the primary air chamber (which have already been installed). 2.2 Test on the resistance characteristics of the air distribution plate: Start the induced draft fan and the primary fan to set the primary air flow at its minimum value; then vary the air flow rate and record the corresponding resistance of the air distribution plate at each flow rate. Determine the relationship between the air flow rate and the resistance of the air distribution plate, and plot the curve showing this relationship, as shown in Figure 1. 2.3 Measurement of flue gas distribution uniformity at the furnace outlet: The flue gas velocity at the inlet of the cyclone separator at the furnace outlet was measured under conditions where the primary fluidization air volume was 167,699 Nm3/h and 241,684 Nm3/h, respectively. At the operating condition of 167699 Nm3/h, the results are: 2.63 m/s on the left side and 2.51 m/s on the right side; the deviation in flue gas flow rate is 4.78% ； At the operating condition of 241684 Nm3/h, the results were: 3.07 m/s on the left side and 2.98 m/s on the right side; the deviation in flue gas flow rate was 3.02%, indicating that the flue gas flow rates on both sides were relatively uniform. 2.4 The second phase of the cold-state test (the bed material properties test section) involved conducting bed material property tests at three bed height levels of 700, 800, and 900 mm in order to determine the minimum fluidization air volume; the results are shown in Figure 2. As can be seen from Figure 2, the critical fluidization air volume for the material is around 120,000 Nm3/h. The air volume displayed on the operator station’s CRT represents the total primary air volume, which includes both the fluidization air volume and the air volume used for feeding coal. Therefore, during normal operation, the minimum primary air volume should be maintained at above 170,000 Nm3/h to ensure adequate fluidization of the bed material and high material circulation. If the bed height is above 1100 mm before cold start, the fluidization air volume and primary air volume also need to be increased accordingly. 2.5 Test on air distribution uniformity: This test is carried out concurrently with the test on the resistance characteristics of the material layer. At different heights of the material layer, when the bed material is approaching a state of fluidization, observation through the 8-meter-high access door reveals that there are no areas of poor fluidization across the entire bed surface. After shutting down all the fans, an inspection of the furnace interior shows that the entire bed surface is relatively smooth, with no areas at the edges or corners where fluidization is inadequate; however, the quality of fluidization in the peripheral areas is worse than in the central area. As the height of the material layer increases, this phenomenon becomes more severe, which can easily lead to the material leaking from the air nozzles into the primary air chamber. 2.6 Characteristic testing of the return valve: Through adjustments to the air volume and on-site observation, it was confirmed that the return valve can perform material return in a stable and smooth manner; after the fan stops operating, the material layer formed by the return valve remains very even, with uniform air distribution. 3 Boiler Drying 3.1 Purpose of Drying The inner surfaces of the furnace and flue ducts in a circulating fluidized bed boiler are lined with a large number of wear-resistant and fire-resistant bricks, as well as fire-resistant insulating bricks and various wear-resistant, fire-resistant and insulating casting materials. After natural drying in air (usually for more than three days), these anti-wear materials still contain a certain amount of free moisture within the entire refractory material. If the material is put into operation without being dried in a furnace, the moisture within it evaporates when heated, causing the volume to expand and generating pressure. This leads to cracks, deformation, and damage to the refractory material; in severe cases, the wear-resistant material may even fall off. Therefore, before the boiler is put into formal operation, it needs to be dried using a controlled heating method; meanwhile, this drying process also accelerates the physicochemical changes in the furnace wall materials, helping to stabilize their properties so that they can function reliably at high temperatures over extended periods of time. 3.2 Furnace heating range: Wear-resistant and refractory materials are mainly installed in the furnace air distribution plates, the conical section of the furnace, the water-cooled screens at the upper part of the furnace, as well as the lower surfaces and wall-penetrating parts of the screen-type superheater, the cyclone separator (including the inlet and outlet flues), the return conveyor, the cold slag holder, and other similar components. 3.3 Oven drying methods: The main oven drying methods currently used in China are wood-based drying and hot flue gas drying. The traditional wood-burning drying method is not only time-consuming and energy-intensive but also requires a great deal of labor and resources. Moreover, the presence of an open flame causes significant damage to the wear-resistant layer and boiler components, and it also makes it easier for dead zones to form within the boiler. Hot flue gas is used to dry the furnace with a furnace dryer, allowing for accurate monitoring and control of the temperature during the drying process, thereby ensuring that the refractory materials meet the required performance standards. Taking advantage of the benefits of heating with hot flue gas, hot flue gas is used for boiler drying this time. The furnace drying process is carried out in two stages. In the first stage, a drying machine provided by Yixing Fangyuan Furnace Drying Co., Ltd. is used for drying at low temperatures; in the second stage, pipe punching is carried out simultaneously, during which the drying at medium temperatures is completed. For baking in various sections, the monitoring points include the smoke temperature at the burner outlet, the smoke temperature in the water-cooled air chamber, the temperature of the thermocouple on the bed, the inlet and outlet temperatures of the cyclone separator, the temperature of the return valve, and the temperature in each compartment of the slag cooler. For the installation of drainage holes: sufficient holes for draining water and venting steam should be provided in the slag cooler, the conical part of the cyclone separator, below the return valve, and at the inlet and outlet of the cyclone separator; additionally, the gaps around the pouring ports can also be used as such drainage and venting holes. The vent holes are created by cutting rectangular slots measuring 10mm × 150mm in length. Arrangement of furnace drying machines: Based on the structural design of this boiler, a total of 16 furnace drying machines are installed, including 6 in the furnace chamber, 2 in the slag cooler, 2 at the inlet of the cyclone separator, 2 at the outlet of the cyclone separator, 2 more in the cyclone separators themselves, and 2 for the inclined legs of the return valve. 3.4 Effect of furnace drying The furnace drying process was essentially completed from August 22 to September 3, 2004, over a period of 13 days. By comparing the drying curve established in this study with the actual temperature changes, it was found that the rate of temperature increase and decrease was slower than intended, and the constant-temperature periods were relatively long, which is beneficial for the curing of the refractory materials. The main reasons for the extended furnace drying time are as follows: First, during the installation of the refractory materials, it was the rainy season and the deadline was tight; work had to be carried out while it was raining, resulting in a high level of moisture in the castables. Moreover, lightweight castables have a strong water absorption capacity ； Second, the diesel used for this furnace drying contained many impurities, which caused blockages in the oil pipes and nozzles, resulting in unstable combustion of the furnace drying machine. However, in terms of curve control and furnace drying effect, this drying process was quite successful. The moisture content of the materials in various sections after furnace drying is shown in Table 2. Table 2: Moisture content of materials after drying process
Serial No. | Location | Moisture content (%) | Remarks
1 | Ash cooler (left) | 1.52 | Since the flue gas outlet of the separator is at the end of the drying flue gas and also at the inlet of the temporary chimney, the drying temperature here must remain lower than that in other areas; high-temperature drying at this location is carried out during the boiler startup phase.
2 | Ash cooler (right) | 1.48 |
3 | Flue gas outlet of separator (left) | 6.94 |
4 | Flue gas outlet of separator (right) | 6.08 |
5 | Return conveyor (left) | 3.15 |
6 | Return conveyor (right) | 3.68 |
7 | Flue gas inlet of separator (left) | 1.58 |
8 | Flue gas inlet of separator (right) | 1.76 |

4. Commissioning of the coal feeding system
1. Conduct a thorough inspection of the coal feeding system before startup. The coal should meet the following conditions: no MFT signal, bed temperature > 600°C, and the furnace should be ready. 2. For the first coal feeding, a \"pulsating\" feeding method is adopted. 3. Open the electric gate valve at the coal feeder outlet. 4. Start feeder #2 (or #3) and adjust its speed to the lowest level. 5. Open the door of the rod inserted under the coal bin to control a smaller coal feeding rate. 6. 90 seconds after coal is fed into the furnace, suspend coal feeding for 90 seconds. 7. Determine whether the coal has been successfully ignited based on the increase in bed temperature, changes in oxygen levels in the furnace, and the glowing phenomenon resulting from the combustion of coal particles inside the furnace ； If ignition is unsuccessful, do not attempt to start it repeatedly to prevent a large accumulation of coal in the furnace. 8. During operation, the temperature at the coal feed port must be closely monitored; if flue gas is detected flowing back into the coal feeder, the flow rates and pressures of the coal spreading air and the coal feeding seal air should be adjusted promptly. 9. Repeat steps 6 and 7 twice; once the same results are obtained, confirm that the coal has caught fire and is burning properly before continuing to feed coal. Adjust the amounts of primary and secondary air as needed, based on the boiler’s coal feeding rate and bed temperature, and gradually increase the coal feeding rate as well as activate any backup coal feeders in accordance with the load requirements. According to the manufacturer’s design requirements, coal can be fed into the furnace when the bed temperature is >600°C. In actual commissioning, since the combustion method of CFB boilers differs from that of conventional pulverized coal boilers, the timing of fuel feeding is closely related to the ignition temperature of the coal type. Considering the actual type of coal used in our plant, coal feeding was attempted at a bed temperature of 550–580°C. After starting the coal feeding, waiting for 5–10 minutes, and then assessing changes in the boiler’s oxygen level, furnace temperature, as well as the glowing behavior of the coal particles inside the furnace, it was determined that the coal was burning properly. This process was repeated three times; once it was confirmed that the coal was successfully ignited, continuous coal feeding could begin, along with adjustments to the combustion process, in order to prevent competition between coal and oil for oxygen during combustion and thus avoid unstable burning conditions. When the furnace temperature reaches 760°C, all oil burners are shut down depending on the coal quality, and the bed oil burner is put into standby mode. Coal quality at the first coal feeding 5 Commissioning of the slag removal system 5.1 Introduction to the slag removal system Two slag discharge ports are located at the air distribution plates on the front wall of the furnace; two conical control valves for slag inlet are used to regulate the discharge of ash and slag based on the slag discharge pressure. The slag discharge pressure is the differential pressure between the inlet air pressure of the hot primary air chamber at the bottom of the furnace and the outlet air pressure of the furnace chamber; when this differential pressure exceeds 13.5 kPa (a provisional value, with the exact figure to be determined based on the coal quality), the conical valve is activated to discharge the slag. During the commissioning period, the slag inlet mechanical control valve can be operated manually. Slag discharge can be carried out on both the left and right sides simultaneously, or on just one side. Key design and operation data: Rated slag flow rate: 12,871 kg/h per unit; Inlet temperature of slag: 892°C; Outlet temperature of slag: 150°C; Cooling water flow rate: 80,000 kg/h per unit; Temperature rise of cooling water: 35°C; Fluidization air volume: 251,164 Nm3/h. 5.2 Preparations before operating the slag cooler: 1) After the slag cooler is installed, its internal refractory lining must be baked as required. 2) All thermal measurement and control instruments (temperature, pressure, flow rate, damper opening) indicate correctly, and the operation control is reliable. 3) The cooling water system should be flushed before startup, and the flushing requirements shall follow those for the feed water prior to the boiler itself. 4) The slag cooler is unobstructed, with no debris present. 5.3 Operation of the slag coolers: At the beginning of operation of the boiler, the quality of the coal was poor, with a lower heating value of between 11,000 and 12,000 kJ/kg and an ash content of over 45%. When the load increased to around 150 MW, the coal feed rate reached about 140 t/h, which corresponded to a slag production rate of around 32 t/h – exceeding the designed capacity of the two slag coolers (10 t/h each) ； The cold slag fan is designed with an outlet head of 42.6 kPa and a current of 61 A; in actual operation, the outlet pressure is 33 kPa and the current is 60.5 A. To increase the fan outlet pressure, the bypass valve between the cold slag air and the secondary air was tried to be closed, resulting in overcurrent in the fan. Even after cleaning the inlet filter of the cold slag fan as recommended by the manufacturer, the situation did not improve; this caused significant difficulties in fluidizing the slag inside the cold slag holder, which in turn led to problems in discharging the slag from the cold slag holder. Based on the conditions of the equipment and operational experience, the slag discharge method for burning low-quality coal has been established as follows: 1) The boiler bed pressure is maintained between 12 kPa and 14.5 kPa; slag discharge begins when the bed pressure exceeds 13.5 kPa. 2) When the boiler bed pressure exceeds 17.5 kPa and shows a tendency to rise further due to poor slag discharge, request the shift supervisor to reduce the load; stop operating the coal feeder when the bed pressure reaches 18 kPa. 3) When discharging slag, make sure to maintain the current of the cold slag fan at around 61A; it is strictly prohibited to operate the cold slag fan under overload conditions. 4) The slag cooler should operate at a low flow rate on a continuous basis, with large particles of cold slag being removed from it at regular intervals; initially, this should be done every half hour until only red slag remains. 5) When slag discharge is not timely during full-load operation, the operators should determine the height of the material layer based on the pressure in the air chamber of the cold slag holder. If the internal height of the material layer is found to be too high, the ACV valve should be reduced in size or closed, and at the same time the bottom slag discharge door should be opened; it should then be closed once the pressure returns to normal. 6) When discharging cold slag or slag from the bottom, it is crucial to avoid allowing air to enter, as this can degrade the fluidization quality. If air does enter, the air valves should be closed, and they should be reopened only after the slag has reached a certain height. 7) If a blockage is detected in the first chamber of the cold slag holder, report it to the shift supervisor promptly and close the conical valve on that side. Please arrange for someone to remove the debris; operation can be resumed only after the blockage has been cleared. 8) Keep the manual slag discharge door of the cold slag holder at its current opening; do not open it fully. 6 Major issues during debugging 6.1 During the trial operation, slag leakage was detected in the primary air chamber; therefore, 6 slag discharge pipes were added below the primary air chamber. Based on preliminary analysis, the slag leakage in the primary air chamber is caused by insufficient resistance of the air distribution plate, as well as defects in the air nozzles and the system itself. Following discussions among the power plant and relevant research institutions, it has been recommended to replace the type and quantity of air nozzles in the primary air chamber during the major maintenance period, in order to eliminate the slag leakage issue. 6.2 Since the coal market is currently a seller’s market, the quality of coal supplied to power plants has been affected to some extent. At the beginning of operation of the boiler, the quality of the coal was poor: its lower calorific value ranged from 11,000 kJ/kg to 12,000 kJ/kg, and its ash content was above 45%. When the load increased to around 150 MW, the coal feed rate reached approximately 140 t/h (the designed full-load coal feed rate for the boiler is 80 t/h). Coal ash makes up a large proportion of coal used for combustion. Due to the hardness protection provided by the crushing machines, this large amount of coal ash cannot be properly crushed, which results in an inconsistent fineness of the coal fed into the furnace. The particle diameter exceeds the designated value of 8 mm by a significant margin; in fact, the particle diameter is around 20 mm. The use of low calorific value and high ash content led to a significant increase in the coal feed rate during boiler operation, resulting in a large amount of slag being generated – up to around 32 t/h at its peak – which is higher than the boiler’s designed slag output of 10 t/h. The output of the cold slagger is no longer sufficient to cope with the current coal quality; neither its cooling capacity nor its fluidization condition allows the boiler to discharge slag smoothly. It is recommended to replace the slag cooler with one that has a greater slag removal capacity and better fluidization performance in order to meet the requirements of the boiler when operating under load. 6.3 During the boiler startup process, the temperature at the outlet of the low-temperature superheater was too high. At 70 MW, which corresponds to about 50% of the boiler’s load, the drum pressure was 9.5 MPa, while the steam temperature at the outlet of the low-temperature superheater reached 523°C. During operation, it was observed that the lower the load, the higher the outlet temperature; during loads ranging from 30 to 60 MW, local temperatures as high as 530°C were recorded. After adjustments, the temperature remained around 500–520°C (with the high-temperature superheater in operation). However, when the boiler operates stably within a load range of 100–150 MW, the outlet temperature at the low-temperature superheater screen remains around 460°C, which is about 20°C higher than the designed outlet temperature of 443°C at that point. Following discussions among the designers from the electrical research institute, the power plant, and the boiler factory, it was initially determined that the issue was caused by an excessive number of heating surfaces in the cold section of the superheater; the boiler factory’s designers have proposed a modification plan. 7 Conclusion After implementing the appropriate temporary measures, the 150MW unit at Jingdezhen Power Plant successfully passed the 72+24 hour test on December 7. Throughout the entire commissioning period, systems such as the coal conveying system, coal feeding system, slag removal system, ash removal system, electrostatic precipitator, soot blowing system, feedwater cooling system, flue gas and air system, and steam-water system were put into operation one after another. The boiler operated well with no signs of coking, and both the main and auxiliary equipment functioned properly, enabling the unit to meet full-load operating requirements. The thermal expansion system and the support/suspension system of the boiler exhibit uniform and good expansion, with no signs of jamming or collision. The main technical parameters of the boiler, such as the main steam temperature, steam pressure, feedwater and flue gas temperatures, as well as the temperatures of the primary and secondary air, all meet the design specifications. The oxygen level at the furnace exit is maintained at around 3–4%, and the temperature throughout the furnace is fairly uniform, ranging from 880 to 910°C. This post was last edited by *anpangpang on 2008-8-1 14:07]