Analysis of the causes of coking in the cold slag holder

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Analysis of the causes of coking in the slag coolers: Boilers #8 and #9 at Datang Baoding Thermal Power Plant are two DG450/9.81-1 type CFB boilers, which utilize multi-chamber fluidized bed air-water cooled selective ash discharge slag coolers. Each furnace is equipped with four of them, arranged on both sides of the furnace chamber. Each slag cooler is divided into four small chambers, equipped with an inlet for slag, a slag discharge pipe, and two air outlets. Along the direction of the slag, the four small chambers of the slag cooler serve as the selection chamber for the first stage and the three cooling chambers for the subsequent stages. Each bin has its own independent air distribution system; the fluidization air comes from the hot and cold air of the primary air, while the air supplied to the slag inlet pipe originates from the fan associated with the J valve. In addition, the air for the slag inlet also comes from compressed air provided by the air compressor. The cold slag holder is equipped with an automatic **system, with four nozzles in each small compartment. Since the unit was handed over for operation, the boiler slag cooler has experienced difficulties in slag discharge or blockages on multiple occasions due to coking. Based on the operation over the past year, a brief analysis of coking in the slag cooler is provided. Starting with the mechanism of coking, it can be divided into high-temperature coking and low-temperature coking. High-temperature coking occurs when the temperature is too high during operation, causing the bed material to burn intensely and the temperature to rise rapidly; high-temperature coking takes place once the temperature exceeds the melting point of the ash. Low-temperature coking occurs because poor fluidization causes local areas to reach the ignition temperature; although fluidization is poor, the air volume at this time is sufficient to cause rapid combustion, resulting in the temperature of the material in those areas exceeding the ash melting point. If not addressed promptly, coking will occur. 1 Analysis of the causes of coking in the coolers of Furnaces #8 and #9: (1) When combustion inside the furnace is poor, the carbon content in the slag discharged increases. The slag continues to burn in the selection chamber, causing the bed temperature there to rise sharply. When this temperature exceeds the ash melting point, coking occurs. On November 14, 2002, the temperature in the selection chamber of the front slag cooler for Furnace #8 reached a maximum of 1024.28°C. On November 20, Furnace #8 was shut down for repairs; upon opening the selection chamber of its front slag cooler, it was found to be filled with high-temperature coke fragments. On December 9, 2002, the highest bed temperature in the selector chamber of the secondary slag cooler in Furnace #8 reached 1010.22°C. Subsequently, the cold slag holder failed and came out of operation. On December 10, the furnace was shut down for repairs; upon opening the cold slag chamber, it was found to be once again covered with high-temperature cinders. (2) When there is a large amount of material in the furnace, the bed pressure and pressure difference across the bed are high, while the wind pressure in the air chamber is high and the flowing air volume is relatively low. This leads to poor fluidization within the furnace; the dense phase region cannot be maintained in a reducing atmosphere, and it is not possible to ensure an adequate level of combustion in this region. The material burns under oxygen-deficient conditions, resulting in low combustion efficiency. Larger particles tend to settle, increasing the carbon content in the slag. It is also difficult to control the discharge of slag – large amounts of slag accumulate at the entrance of the slag inlet pipe as well as inside the pipe. Sometimes, the air used to feed the slag is not sufficient to push the slag into the slag cooler, forcing the use of compressed air for this purpose. Once slag starts flowing into the slag cooler, a significant amount of slag continues to enter it even after the compressed air supply is turned off. This results in a large accumulation of slag in the various chambers of the slag cooler; since the slag cannot be removed promptly, the carbon contained in the slag continues to burn, causing coking in the slag cooler. Another reason is that the pressure in the furnace bed layer is relatively high, typically ranging from 7.5 to 10 KPa, whereas the bed pressure in the chamber of the slag cooler is usually only 2 to 6 KPa. There is thus a significant pressure difference between the two; sometimes, even without the use of air to feed the slag, the slag will enter the slag cooler on its own due to this pressure difference. The amount of slag fed in cannot be controlled freely, and a stable bed pressure cannot be established in each chamber; as a result, the temperature in those chambers as well as the temperature at which the slag is discharged increase. This can lead to continued combustion of the slag in certain areas, with the temperature rising above the ash melting point and causing coking. On November 3, 2003, Furnace #9 had low combustion efficiency and the slag contained a high level of carbon. At 0:07, the slag discharge temperature from the secondary cooler was high and there was severe self-flowing of slag. Around 0:15, the bed temperature in the third cooling chamber reached a maximum of 1038.68°C; after slag discharge was stopped, the temperature gradually decreased. Upon restarting, it was found that the third cooling chamber was blocked and could not discharge slag. On November 8, the furnace was shut down for repairs; upon opening the third cooling chamber of the secondary slag cooler, a large amount of coke lumps were found. From November 25 to December 7, the slag cooler experienced multiple coking incidents; Boiler #8 had to have its slag removed several times during operation, and it was shut down four times for such purposes. (3) The lining material in the furnace and the return conveyor falls off, and there are many stones in the fuel; some of these stones are difficult to break down by the coal crusher. There is a large amount of flaky stones in the coal fed into the furnace, and these factors can easily cause blockages in the slag cooler. In some chambers, the amount of slag is high and its fluidization is poor, resulting in hot flue gas and fine ash returning from the slag cooler into the furnace, which raises the temperature of certain materials to their ignition point. When sufficient oxygen is present, intense combustion occurs. Low-temperature coking occurs when the temperature exceeds the ash fusion point. This situation is more common in Boilers #8 and #9. The furnace was shut down multiple times due to difficulties in slag discharge, and upon opening the cold slag holder, coked lumps in the form of caps were found in some chambers. (4) The cold slag holder in our plant originally had a total of five rows of air caps, arranged in a targeted pattern. Later, for various reasons, the two rows of air vents on each side were blocked off. As a result, the fluidization quality on both sides of the slag cooler is poor, and fluidization dead zones may occur; slag accumulates on these two sides. Since the temperature in the chamber is relatively high, usually around 750°C, if the carbon content in the slag is high and there is an appropriate amount of air flow, the slag will continue to burn, leading to coking on both sides of the slag cooler. The reduction in the wind cap also has a certain impact on fluidization, and it is one of the reasons for the self-flow in the slag cooler. 2　Solutions (1) Ensure proper combustion. Ensuring optimal combustion conditions is a prerequisite for preventing coking in the cold slag holder. After some experimentation, a proper ratio was determined for the primary air and secondary air: the amount of primary air meets the requirements of combustion in the dense-phase zone, keeping this zone in a reducing atmosphere, while the secondary air ensures complete combustion of the fuel. When burning low-quality coal, a higher primary air ratio is used ； When burning fuels with high volatility, a lower primary air ratio is used. At the same time, strive to maintain a high bed temperature, organize the combustion proportion of fuel in the dense-phase and dilute-phase zones appropriately, and reduce the carbon content in the ash. (2) Maintain a reasonable bed pressure. The level of bed pressure indicates the amount of material in the furnace, and circulating fluidized bed boilers have a high capacity for load regulation. Based on the characteristics of a fluidized bed, bed pressure and boiler load are correlated. After adjustment, the bed pressure was reduced from the original 8–10 KPa to the current 4–6 KPa. The fluidization inside the furnace is good; the difference between the bed pressure and the bed pressure in the slag cooler has decreased, eliminating the self-flow phenomenon in the slag cooler. (3) Improve the adjustment of the coal crusher: To ensure a more appropriate particle size of the coal being burned, the adjustment of the crusher’s fly hammer should be done every 5 days instead of every 15 days as before; adjustments can also be made at any time based on the actual particle distribution of the coal. Larger coal cinders and rocks have significantly decreased, resulting in smoother slag discharge. (4) Good control of the bed pressure and temperature in each chamber of the slag cooler. Since the phenomenon of natural convection is eliminated, the amount of slag fed into the slag cooler can also be well controlled. A certain bed pressure is maintained in each chamber, generally around 5 Kpa, keeping it at the same level as the bed pressure inside the furnace. This allows for free control of the amount of slag fed in, preventing excessive accumulation of ash and slag in the chambers and avoiding their re-ignition. As a result, the temperature in each chamber can be controlled, thereby preventing coking in the cold slag holder. (5) Improve the structure of the slag cooler. After more than a year of operation, it was found that blockages in the slag cooler often occur at its slag discharge port. After research, a slag removal hole was created on the outer wall of the slag outlet of the cold slag holder. When large stones, casting materials, coke lumps, and other debris block the slag outlet of the cold slag holder, this hole can be opened to remove the slag. Since the slag removal hole was put into operation, it has achieved very good results. 3 Conclusion Whether it is high-temperature coking or low-temperature coking in the slag cooler, it is caused by low fuel combustion efficiency and a high carbon content in the slag; the carbon in the slag undergoes secondary combustion within the slag cooler, leading to coking. Therefore, to prevent coking in the cold slag holder, it is first necessary to improve combustion efficiency and optimize air distribution, particularly by making proper use of fluidization air and secondary air, in order to reduce the carbon content in the slag. Next, it is necessary to strictly control the bed temperature in each chamber of the cold slagger, keeping it within the designed range; if needed, emergency measures can be employed to regulate the amount of slag discharged, ensuring uniform slag discharge. A certain bed pressure should be maintained in each chamber, as well as a consistent height for the fluidized bed layer, so that the slag can be cooled thoroughly. This can prevent coking.