Accident Handling 12: What are the measures to prevent coking of the bed material in circulating fluidized bed boilers?

1 Ensure good fluidization conditions to prevent bed material deposition. 1.1 Ensure the proper operation of the fuel preparation system, with the coal particle size meeting the design requirements. 1.2 Strictly control the differential pressure of the material layer to ensure uniform slag discharge. Manual slag removal should be carried out promptly, with small amounts of slag removed frequently; if there are lumps in the slag discharged, it should be reported to the furnace operator. Once slag removal is complete, the slag discharge door must be closed tightly. 1.3 Carefully monitor the temperature difference between the bottom and middle parts of the bed; if this difference exceeds the normal range, it indicates abnormal fluidization, with deposits or slag formation in the lower area. In such cases, the primary air supply can be increased for a short period to disperse the coke deposits, and the cold slag pipe can be used to remove the slag ； If it cannot be removed, the furnace should be stopped immediately for maintenance. 1.4 During operation at low load, if a sudden drop in bed temperature is observed, apart from coal supply interruption, it is likely due to bed material deposition. Increasing the coal supply in such a situation will only exacerbate this deposition, worsening the fluidization quality of the fluidized bed and leading to localized coking. When it is determined that bed material deposition has occurred, the cold slag discharge pipe should be opened to remove the slag; once the bed temperature returns to normal, operation at a higher load should be adjusted accordingly. 2　Strictly control the coal feed rate during ignition. During the ignition process, a small amount of coal can be added once the bed temperature reaches above 500°C in order to raise it further. If too much coal is added, incomplete combustion of the coal particles increases the carbon content throughout the bed. Once the air flow rate is increased, intense combustion occurs, causing the bed temperature to rise rapidly – sometimes even exceeding the softening temperature of the ash – resulting in overheating and coking of the entire bed. When the bed temperature exceeds 1050°C, and despite measures such as reducing coal usage and increasing air supply, the bed temperature continues to rise; in such cases, the furnace must be stopped immediately. It is generally necessary to wait until the bed temperature drops below 800°C before restarting it. 3. Strict control of bed temperature during variable load operation During variable load operation, the bed temperature must be kept within the allowable range. When increasing the load, air should be added first followed by coal; when reducing the load, coal should be reduced first followed by air. Burn adjustment should be carried out using a method of small amounts applied multiple times to avoid large fluctuations in bed temperature. 4　Proper operation during fire shutdown 　During fire shutdown, first stop feeding coal, then run the system for a few minutes before stopping the fans. During this period, it is essential to keep all furnace doors, as well as all air intake and slag discharge doors, tightly closed. 5　Carefully adjust the primary and secondary air. For high-temperature separators, ensure that the oxygen content remains at least 3%–5% at all times; this helps to reduce the combustible content in the fly ash and prevents secondary combustion within the separator and the return mechanism, which could lead to overheating. During operation, it is necessary to regularly check the return material condition and monitor whether the temperature of the return material bed is normal. If it exceeds the normal value by a large margin, secondary combustion may have occurred. At this time, the return air volume should be increased, and the ash discharge valve of the return bed should be opened to discharge the ash. If the temperature is much lower than the normal value, it indicates that the return conveyor is blocked; in such a case, the ash discharge valve should be opened to release the ash, and at the same time, the amount of air used for conveying the material back should be increased. If the fault still cannot be resolved, it is necessary to shut down the system for maintenance. 6 During boiler startup, the return device should be filled with ash. It must be filled with ash before it can be put into use during boiler startup; otherwise, air will flow in reverse. At the beginning of ignition, do not supply return air; once the fine ash in the bed fills the return air device, then return air should be turned on (usually half an hour after ignition) to ensure that there is material in the bed, otherwise it will be difficult to control the bed temperature. 7　Install furnace differential pressure device and return material booster fan: To monitor whether the return material mechanism is working properly and to prevent coking or blockages, a furnace differential pressure device and a return material booster fan can be installed. Furnace differential pressure refers to the pressure difference between the upper surface of the combustion chamber and the furnace outlet, and it is a parameter used to monitor whether the return conveyor is functioning properly. Generally, the furnace differential pressure is controlled at above 0.5 kPa. If the furnace differential pressure suddenly drops during operation, it indicates that material circulation has stopped and the return conveyor is blocked. As long as the furnace differential pressure is carefully monitored during operation, coking in the return material mechanism can be prevented. 　　To increase the pressure head of the return air and ensure good fluidization and movement of the return material bed, it is very useful to install a return air pressure booster fan when the primary air pressure is insufficient to meet the requirements for returning materials, as this helps to prevent coking in the return material device. 8　Changing the caking properties of coal 　 Properly mixing the coal to be fed into the furnace and altering its caking properties holds significant practical value in preventing coking in circulating fluidized bed boilers. 9 Thorough preparation before startup Before each start-up of the boiler, the air caps and air chambers should be carefully inspected, and any debris removed. During startup, a cold-state fluidization test should be conducted to ensure that the air distribution within the bed is even and that fluidization occurs properly.

The main measures to prevent coking of the bed material in circulating fluidized bed boilers are: controlling the particle size of the coal fed into the boiler to be below 10 mm； Strictly control the coal feed rate during ignition ； When increasing or decreasing the load, it is essential to first supply air and then add coal when increasing the load ； When reducing load, reduce coal consumption first and then air volume ； When adjusting the combustion, it is necessary to use a method of making small adjustments frequently in order to avoid large fluctuations in bed temperature ； Regularly check the coal feeding condition of the coal feeder, observe the color of the flame in the furnace, and ensure that the return conveyor is functioning properly ； During slag discharge, adjust the amount discharged to be less and more frequently based on the differential pressure across the material layer. Boiler operators should closely monitor on-site to check whether there are any cinder clumps in the discharged slag; after the discharge is complete, conduct a thorough inspection, and only leave the site once it is confirmed that the slag discharge door is properly closed. Measures to prevent coking at low temperatures include: improving the adjustment of the return feeder, increasing the amount of ash returned, adjusting the volume of primary fluidization air to ensure as uniform fluidization as possible of the bed layer; the coal supply from the coal feeder should be increased in a consistent manner, and slag removal should be intensified if necessary to remove any slag masses that have formed. When the bed temperature becomes uniform across all areas, it can be considered that the problem has been resolved.