Ignition of a circulating fluidized bed boiler

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I found some information on the ignition of fluidized beds and am sharing it here for everyone’s learning*. Ignition of circulating fluidized bed boilers 　　 【Abstract】 The ignition of circulating fluidized bed boilers is a challenging issue in their operation. For those who have never worked with circulating fluidized bed boilers or bubbling bed boilers, it is easy to cause coking of the bed material or extinguishment of the fire before they master the proper techniques for ignition. This article provides a detailed overview of the three main ignition methods for fluidized bed combustion, as well as the preparatory work such as cold tests prior to ignition. 　　 【Keywords】 Circulating fluidized bed boiler; Cold-state test; Ignition process 1. Ignition process and method The ignition of a circulating fluidized bed boiler involves heating the bed material in the combustion chamber to a certain temperature by some means, and then introducing air to bring the material in the bed into a fluidized state, until the fuel supplied continuously by the coal feeder can burn stably. The ignition of circulating fluidized bed boilers differs from that of other types of boilers. The ignition process has always been a challenging issue in the operation of such boilers. Especially for those who have no experience with circulating fluidized bed or bubbling bed boilers, failing to master the proper ignition methods can easily lead to coking of the bed material or shutdown of the boiler, which not only hinders the timely and proper startup of the boiler but also results in waste of manpower and resources. 　　 The ignition methods for circulating fluidized bed boilers are mainly divided into: fixed-bed ignition ； Fluidized ignition using an oil gun on the bed surface ； There are four types of ignition methods: premix chamber fluidized fuel ignition and hot air fluidized ignition. A comparison of their advantages and disadvantages is shown in Table 1. The first three ignition methods are more commonly used, and they will be described in detail later. 　　2. Cold-state performance tests After installation or major repair, before ignition, a cold-state test should be conducted on the combustion system of the circulating fluidized bed boiler, including the air supply system, air distribution devices, bed thickness, and fly ash circulation system. Its purpose is to: (1) determine whether the air volume and air pressure of the blower are sufficient to meet the requirements of fluidized combustion. 　　 (2) Measure the air distribution plate resistance and the bed resistance. 　　(3) Check the fluidization quality throughout the bed; if any dead zones exist during cold-state fluidization, they should be eliminated. 　　(4) Measure the layer thickness, air supply volume, and resistance characteristic curves to determine the critical fluidization air volume at cold conditions; this value is used to guide the adjustments during the ignition process, and it also provides a parameter basis for operation at high temperatures. 　　(5) Check the operational performance of the fly ash system. 　　2.1 Check of the uniformity of fluidization in the bed material During testing, slag particles with a size of 3 mm or less are spread across the bed surface; the layer thickness is approximately 300–500 mm, sufficient to ensure fluidization. The uniformity of fluidization can be checked using two methods. One method is to turn on the exhaust fan and the blower, slowly adjust the air supply valve to gradually increase the airflow until the entire material layer becomes fluidized. Then, stop the air supply suddenly and check whether the surface of the material layer is flat. If it is flat, this indicates even air distribution; if the surface is uneven, with higher areas indicating low airflow and lower areas indicating high airflow, the experiment should be stopped to identify the cause and take corrective action promptly ； Another method is to, once the material layer is fluidized, use a longer fire rake to move it back and forth continuously within the bed. If the resistance felt is low and uniform, it indicates that the material layer is well fluidized; otherwise, the air distribution is uneven or the air nozzles are blocked. Areas with low resistance are well fluidized, while areas with high resistance may have dead zones. 　　By checking the uniformity of bed fluidization, it is also possible to determine the minimum bed thickness required for the fluidization state. This data is crucial for fluidized bed ignition; if the layer is too thin, it is difficult to achieve a stable fluidized state, and the boiler cannot be ignited or operated. If the layer of material is too thick, it will prolong the ignition time and result in an increased amount of fuel needed for ignition. 　　Even gas distribution is a necessary condition for ignition in a fluidized bed, for stable combustion at low loads, and for preventing particle stratification and bed coking. 　　2.2 Measurement of the air distribution plate resistance The air distribution plate resistance refers to the pressure drop of air passing through the air distribution plate when no substrate is placed on it. To allow air to pass through the distribution plate as required by the design and to create a stable fluidized bed layer, it is necessary for the distribution plate to have a certain level of resistance. The resistance of the air distribution plate consists of the local resistance at the inlet of the air chamber, the resistance in the air cap channel, and the local resistance at the small holes in the air cap. Under normal conditions, the local resistance at these small holes is the greatest among the three; the sum of the resistances of the other two components accounts for only a fraction of one-tenth of the total resistance of the air distribution plate. Therefore, the resistance ΔΡ of the air distribution plate can be calculated using equation 1: ΔΡ = ξ (Pa) (1), where μ represents the wind speed at the small holes, in m/s ； 　　　ξ—wind cap drag coefficient ； 　　　ρ—is the gas density, kg/m3. 　　During the measurement, first close all furnace doors and seal all slag discharge pipes and ash discharge pipes tightly. After starting the blower and exhaust fan, gradually open the large air doors, increasing the air volume slowly and evenly, and adjust the exhaust flow accordingly to bring the negative pressure in the furnace chamber to zero. For each air supply volume, the pressure in the air chamber read from the static pressure gauge of that chamber at that time represents the resistance of the air distribution plate. Keep increasing it until the maximum airflow is reached; each time a reading is taken, record the values of both the airflow and the static pressure in the air chamber. Then, starting from the maximum air volume, the air volume is gradually reduced, and the values of the air volume and the static pressure in the air chamber are recorded at each step, until all the dampers are closed. The average values of the test data for the upward and downward flows are plotted to form a curve showing the relationship between distributor plate resistance and air volume, as shown in Figure 1, for estimating the layer thickness during operation. 　　2.3 Determination of the layer resistance The layer resistance is determined by placing a layer of material of a certain thickness on the air distribution plate, and then measuring the static pressure in the air chamber at different air flow rates, in a manner similar to that used for determining the resistance of the air distribution plate. Whenever the thickness of the material layer is changed, the measurement of the relationship between air volume and static pressure in the air chamber must be repeated. The static pressure in the air chamber equals the sum of the resistance posed by the distribution plate and that posed by the material layer; that is:
Material layer resistance = Static pressure in the air chamber – Resistance of the distribution plate.
The values of these three quantities in the above equation correspond to those at the same air volume. 　　Based on the results obtained from these two tests, it is possible to determine the relationship between the layer resistance and air volume at different layer thicknesses, and this relationship can also be plotted as a curve showing layer resistance versus air volume. Numerous statistical data show that the resistance of a fluidized bed is approximately equal to the difference between the weight of the bed material per unit area of the distributor plate and the buoyant force of the fluid. That is, ΔP = hfg(ρp – ρf)(1 – ε) (2), where: ΔΡ is the resistance of the fluidized bed layer, in Pa ； 　　　G — mass of the material in the fluidized bed, kg; g — acceleration due to gravity, m/s2 ； 　　　hf — height of the fluidized bed layer, m ； 　　　Fb — area of the fluidized bed layer, m2 ； 　　　ρp, ρf — true density of the material and density of air, in kg/m3; ε — average void fraction of the fluidized bed layer. 　　Since ρp > ρf, the effect of ρf can be ignored in calculations; therefore, △Ρ = hfgρp(1 – ε). Through further simplification based on experiments, it is expressed using the bulk density of the solid bed material before fluidization: △Ρ=Ahgρd g (3), where hg is the height of the stationary material layer, in meters ； 　　ρd—bulk density of the material layer, kg/m3 ； 　 A—The proportionality coefficient determined by the coal type, as shown in Table 2. 　　When the thickness of the stationary material layer hg exceeds 0.3 m, the calculated results are very close to the experimental data. As can be seen from Equation 3, the resistance of the material layer is proportional to the thickness of the stationary material layer; the thicker the layer, the greater the resistance. For simplicity, Table 3 can be used to estimate the layer thickness based on the layer resistance. 　　2.4 Determining the critical fluidization air volume The critical fluidization air volume is the lower limit of air volume that governs the operation of a circulating fluidized bed boiler at low loads; below this value, coking may occur. The minimum operating air volume is generally related to the particle size and density of the bed material, as well as the pore space in the bed layer; it is determined through cold-state tests. When determining the resistance of the material layer, for each thickness of the material layer, the critical fluidization air volume for that layer must be determined based on the critical fluidization conditions within the furnace; the highest value among these values serves as the minimum air volume required during operation at high temperatures. Generally speaking, the cold no-load surface velocity of a circulating fluidized bed boiler should not be less than 0.7 m/s. In actual operation, it is difficult to measure the layer resistance directly; generally, the total resistance (the sum of the distributor plate resistance and the layer resistance) or the static pressure in the air chamber is used to monitor the operation. 　　Determining the critical fluidization air volume is crucial for the ignition of circulating fluidized bed boilers. After the fixed-bed ignition bed phase is completed and the blowers and induced draft fans are started for ignition, if the amount of primary air is adjusted too much, resulting in intense fluidization, it is very likely that the boiler will go out within a few minutes. If the air volume is too low, fluidization is poor, which can lead to coking. For fluidized bed oil ignition under the bed, if the air volume is too high, the bed material heats up slowly, heat loss is severe, and the ignition time is prolonged. If the air volume is too low, the bed material cannot be properly fluidized, which leads to an accumulation of large amounts of hot flue gas in the air chamber – a situation that is very dangerous; in severe cases, it can cause an explosion in the air chamber. This is why some circulating fluidized bed boilers that use fluidized oil ignition beneath the bed are equipped with explosion vents in their air chambers. Therefore, the critical fluidization air volume is an important parameter when adjusting the ignition operation. 　　 3. Checks and preparations before ignition 　　(1) Inspect the combustion and circulation systems, such as the burner chamber air distribution plates and separators, to ensure they are clean; the air nozzles are in good condition, and the ventilation holes are unobstructed. The slag discharge pipe, ash discharge pipe, and return valve show no signs of blockage and operate smoothly when closed. 　　(2) There should be no peeling or damage to the insulation and fire-resistant layer of the boiler itself; all manholes and observation holes must be closed and properly sealed. 　　(3) Check whether the air regulation valves of the drum induced draft fan, as well as the air supply valves in the air chamber and for oil ignition, are functioning properly; the switches should operate smoothly and give accurate indications. 　　(4) Check that auxiliary systems such as coal bins, feeders, and dust collectors are operating properly. 　(5) The oil ignition system’s air compressor (for air atomization), oil pump, pipes and valves, as well as the igniter were thoroughly inspected, and all functions worked properly upon testing. 　　(6) Check whether the anchor bolts of the induced draft fan, blower, and secondary fan are loose. Check whether the fan’s cooling water and oil levels are normal; the fan should rotate smoothly, with no frictional noises inside it. 　(7) Check that the pipes and valves in the steam and water system are in good condition, and that the switches operate smoothly. 　　(8) Check that all gauges such as pressure gauges, temperature gauges, and flow meters are in good condition and functioning properly, with correct readings. 　　(9) Prepare a certain amount of ignition charge with a particle size of 0–3 mm. For fixed-bed ignition, a certain amount of bituminous coal and firewood also need to be prepared. 　　(10) Confirm that the boiler drum water level or circulation water volume is normal. 　　4. Fixed-bed ignition: In this ignition method, the material at the bottom is first heated while remaining in a fixed, stationary state. When the temperature reaches 400–500°C, the blower is turned on to supply air gradually; during this process, ignition coal is added. The combustion of this coal is used to continue heating the material, until the coal supplied by the coal feeder can catch fire and burn. Heating the charge with solid fuel for ignition is a relatively simple method that does not require specialized ignition equipment. The steps for ignition are as follows: (1) Lay a base material with a particle size of 0–3 mm on the bed to a thickness of approximately 300–400 mm, or use the thinnest thickness that ensures uniform fluidization, as determined through tests on fluidization efficiency; this helps to reduce the ignition time and save fuel used for ignition. The carbon content in the base material should not exceed 3%. 　　(2) Place the prepared wood chips on top of the bedding material in the furnace and light them; then add the screened lump coal (about 50 mm in size) and level it out. The thickness of the wood chips and lump coal should be maintained at around 150–200 mm. This process is called a hotbed. 　　(3) The incubation time is generally 3 to 5 hours; during this period, the exhaust fan damper can be opened or the exhaust fan can be turned on for a short time depending on the burning conditions inside the furnace. The bedding process essentially involves heating the bottom material and the furnace chamber. If the time is too short, the bottom material cannot be heated properly; if the time is too long, the wood and coal pieces may catch fire. Either situation is not conducive to starting the fire, so it is necessary to adjust the timing according to the actual conditions. 　　(4) After the bed is finished, use a fire hook to check for any large pieces of coal that have not burned completely; if any are found, remove them and level out the coals on the surface of the bed. This operational process is very important; sometimes, local low-temperature coking occurs during the ignition of the boiler, and this is due to those unburned coal chunks that, after the bed material begins to fluidize, sink to the bottom layer and stick close to the air cap, resulting from sufficient oxygen supply and intense combustion. 　　(5) Start the induced draft fan and blower; use the air volume determined through cold-state tests to bring the material layer to a state of micro-fluidization as quickly as possible. At the same time, introduce ignition bituminous coal into the furnace, maintaining a slight negative pressure inside it. This is a process in which the red charcoal fire generated by upper combustion is used to gradually heat the entire material layer and ignite the bituminous coal; it generally takes 5 to 8 minutes. At first, the red flames in the furnace disappear and the light fades; after a few minutes, bright sparks can be seen moving within the furnace, and their number gradually increases. This indicates that the smaller particles of the bituminous coal have caught fire. At this point, it is necessary to increase the air supply slightly, and red flames and flames will appear on the surface of the material layer. The flames gradually change from dark to a dark-red color. Continuing to add more bituminous coal and increasing the air supply further will cause the flames in the furnace to shift from dark red to red, becoming increasingly brighter. This signals that the bed temperature has reached 600–700°C. 　　(6) Once the bed temperature rises to 700°C, a small amount more bituminous coal can be added, but the bed temperature should rise steadily and gradually. When it reaches 800°C, the furnace door can be closed, and the coal feeder can be activated to supply normal amounts of fuel; at the same time, the air flow rate should be increased to bring the material layer into a normal fluidized state. Thereafter, the temperature rise is controlled by adjusting the speed of the coal feeder until the normal operating temperature of 850–950°C is reached, at which point the ignition and startup process is complete. It should be noted here that increasing the air volume means matching an increase in both guidance and blowing. 　　(7) Throughout the coal feeding and air supply process, controlling the air volume is key to ignition; it is necessary to adjust the air flow based on the condition of the fire, ensuring that any changes in air volume are made promptly and accurately. If it is found that the air flow is too high and there is a risk of fire, the air supply should be reduced or stopped immediately. Once the bituminous coal on the surface of the material layer begins to burn, the air supply can be increased slightly, and a small amount of bituminous coal dust should be scattered over the area where flames are present to raise the temperature of the material layer again. However, it is necessary to constantly check with a furnace hook whether coking has formed at the bottom of the material layer; if there are coked masses, they should be removed promptly. To prevent low-temperature coking and high-temperature coking during ignition, bituminous coal should be added in small amounts, frequently, and evenly. After introducing air for fluidization, the bed material should be stirred regularly using a furnace hook to keep the bed temperature as uniform as possible, allowing for a steady and gradual rise in temperature. Fixed-bed ignition requires a high level of experience from the operator. 　　5. Premix chamber fluidized oil ignition (oil ignition under the bed) – Oil ignition under the bed is a type of fluidized ignition, with the entire startup process taking place in a fluidized state. Its basic principle is that the fuel is atomized and completely burned in the pre-combustion chamber; the resulting high-temperature flue gases and flames (1500°C) mix evenly with the cool air supplied by the blower to form hot flue gases at around 850°C, which then enter the bed through the air chambers and air nozzles to heat the bed material. This ignition method does not cause coking at low or high temperatures. 　　Lighting oil generally uses light diesel, and currently there are two methods of atomization: mechanical atomization and pressure air atomization. Lighting can also be achieved either by using torches or by automatic ignition with high-energy igniters. The ignition procedure is as follows: (1) Lay a base material of a certain particle size and thickness on the bed (the same as for fixed-bed ignition). 　　(2) Start the air compressor (for air atomization) and the oil pump, and adjust the air pressure and flow rate sowie the ignition oil pressure and flow rate to normal values for ignition. 　　(3) Before its first use, the oil gun should undergo a misting test. This is done by removing the oil gun from the pre-ignition chamber and inserting it into a container; then the misting air valve and the oil gun valve are opened to observe the misting effect of the oil gun. The air pressure and flow rate at the time of the best misting effect, as well as the pressure and flow rate of the fuel used for ignition, are recorded to serve as reference parameters for ignition. 　　(4) Start the induced draft fan and blower, close the air supply dampers, light the oil gun, then open the air supply valve and adjust the air supply volume to bring the bed material to a critical fluidization state as quickly as possible. This is extremely important from a safety perspective for oil ignition under the bed, as it prevents the accumulation and expansion of hot smoke in a closed air chamber. 　　(5) By adjusting the oil pressure and fuel injection volume of the oil gun, as well as the volume and ratio of the combustion air and mixing air in the air ducts of the hot flue gas generator, it is possible to control the temperature and volume of the hot flue gas. To improve the thermal efficiency of the hot flue gas and reduce fuel consumption, the amount of hot flue gas used for ignition should be sufficient to keep the bed material in a fluidized state; a too high fluidization speed should be avoided. 　　(6) To prevent the air cap from being damaged, it is essential to control the temperature of the hot flue gas; this temperature must not exceed 900°C. The thermocouple used to measure the temperature of the flue gas at the time of ignition should be inserted into the air chamber by a distance of more than 800–1000 mm, in order to accurately reflect the temperature of the hot flue gas. 　　(7) The rate of temperature rise during startup should be controlled, taking into account both the thermal expansion requirements of the refractory materials and the safety aspects related to water circulation. In particular, during the initial stages of cold startup, the bed temperature must be strictly controlled, with the rate of increase not exceeding 10°C/min. The time required for cold startup varies depending on the boiler capacity; the larger the capacity, the longer the startup time. For a 130t/h boiler, this time is approximately 2–3 hours. Warm-up startup is rapid, taking 20–40 minutes. 　　(8) During cold start-up, the temperature of the bed material rises slowly from room temperature to 300–400°C. As the temperature continues to increase, a large amount of volatiles in the coal are released, causing the bed temperature to rise rapidly at 450–600°C. This temperature range depends on the type of coal being used. When this phenomenon occurs (it is necessary to use a direct-reading digital thermometer in the combustion chamber to accurately monitor the bed temperature), it is possible to start adding a small amount of coal to the combustion chamber while reducing the amount of fuel oil injected. Once the bed temperature reaches 650–700°C, the fuel oil injectors can be turned off, and normal coal feeding can proceed. 　　(9) When burning bituminous coal, in order to reduce fuel consumption and shorten startup time, the startup fuel should also be bituminous coal. Extensive practice has shown that adding bituminous coal with a carbon content of no more than 10% to the starting charge is very effective in reducing fuel consumption and shortening ignition time. 　　The oil ignition method under the bed has advantages such as low fuel consumption, fast startup, high success rate, good environmental hygiene, and reduced labor intensity for workers. Ignition under the bed can also be carried out using heavy oil or gas fuel, with the method being the same as that for lighting with light diesel as described above. 　　6. Fluidized ignition using an oil gun on the bed surface (oil ignition on the bed) Similar to oil ignition under the bed, the entire startup process takes place in a fluidized state. It is easier to operate than fixed-bed ignition, and it is not as dangerous as oil ignition under the bed. The disadvantages are higher fuel consumption during ignition, a slower rate of temperature rise, and low thermal efficiency of the fuel. At the same time, due to the uneven heating by the oil gun, the temperature of the bed material is uneven during ignition, and poor control can easily lead to localized overheating. The steps for ignition are as follows: When igniting, first light the oil gun, then start the drum and induced draft fans, and adjust the air supply volume so that the bed material is in a critical fluidized state. In a fluidized state, the high-temperature flue gas and flames generated by the oil gun are used to heat the bed material; once the temperature of the bed material reaches 400°C, ignition coal can be added. At this temperature, the volatiles in the introductory coal evaporate and burn, and the heat generated thereby further heats the material at the bottom of the bed; as a result, the bed temperature rises steadily. Once the ignition temperature of the coal is reached, the coal feeder can be activated to supply more coal. After the bed temperature rises to 700°C, turn off the oil gun, adjust the ratio of air to coal, and bring it into normal operation.