Thread Content
Discussion on the use and improvement of air-water combined slag coolers and L-valves: As the capacity of circulating fluidized bed boilers increases, air-water combined slag coolers have essentially become the flagship products of major boiler manufacturers. Our company currently has two 220t/h circulating fluidized bed boilers manufactured by Harbin Boiler Group. Each boiler is equipped with two ash discharge systems, located on either side of the furnace, with the ash discharge outlets situated at the bottom of the side walls. The system uses an L-valve and a combined air-water slag cooler; the L-valve is controlled to keep the bed pressure in the furnace within a specified range, while air and water cooling coils are used to cool the ash and slag to around 150°C, after which it is discharged through the normal slag discharge port onto the slag transfer hopper. 1. Structure. 1.1 Structure of the L valve: The L valve gets its name from its shape, which resembles the uppercase letter “L”. Its vertical section is approximately 2.2 m in height, while the horizontal section is about 2.8 m long. Three air nozzles for loosening the material are installed near the bottom of the vertical section, and 10 air nozzles (air caps) for fluidization are arranged along the horizontal section at intervals of 20 mm. The air supply comes from a compressed air system, and its pressure is maintained at around 0.3 MPa using a pressure reducing valve. The flow rate of the air used for fluidization is controlled by an electric control valve, with a manual main valve located in front of it. Additionally, a purge air nozzle is installed at the corner of the L-valve; it draws compressed air without pressure reduction and is controlled manually. The inner diameter of valve L is 250 mm, with a fire-resistant and wear-resistant cast material 238 mm thick on its inner wall. 1.2. Structure of the slag cooler: The slag cooler is essentially a small bubbling fluidized bed in rectangular shape, with slag inlet and normal slag discharge ports at its two ends. Its interior is divided into three air chambers; each chamber is equipped with an air distribution plate and an air blower. The air distribution plate in the first chamber has 28 small bell-shaped air nozzles, while the second and third chambers each have 56 such small bell-shaped air nozzles, all of which are of the same size. The slag inlet is located at the upper part of the chamber and is connected to the L-valve; there is a manhole below the slag inlet ； The normal slag discharge port is connected to the outlet of the three-chamber unit, and the inlet of the return air duct is also connected here. An electric air lock is installed on the slag discharge pipe to control the amount of slag discharged. The three wind chambers have a total of five temperature measurement points: the point in Chamber 1 is located below the manhole, the points in Chamber 2 are installed on the back wall, one at the top and one at the bottom; Chamber 3 has the same arrangement as Chamber 2. The inner wall of the slag cooler is lined with 200 mm thick refractory and wear-resistant cast material. Inside the slag cooler, there is an air-cooled partition wall 1500 mm high between chamber two and chamber three; this wall is at the same height as the normal slag discharge outlet. It divides the slag cooler into two compartments – chamber one and two form one compartment, while chamber three forms another. Each compartment is equipped with an emergency slag discharge pipe, which is used primarily for discharging large quantities of slag. Water-cooled tube bundles are also installed in chambers two and three inside the slag cooler, in order to accelerate the cooling rate of the ash and slag. Wind and fine gray particles enter the furnace from the side of the furnace through the return air ducts. 2. Working principle. 2.1. Working principle of the L valve: It was originally designed to automatically and continuously discharge slag using the bed pressure signal; when the bed pressure rises to a certain level and there is an excess of bed material, the flow control valve of the fluidizing air for the L valve opens automatically, adjusting the air flow rate to such a level that the ash and slag in the horizontal section of the L valve are in a suspended fluidized state. The pressure difference between the vertical and horizontal sections of the L valve is then used to send the ash and slag into the cold slag holder. When the bed pressure drops, the L-valve fluidization air control valve is closed, and the ash falls inside the valve, ceasing to flow. However, due to various reasons (such as substandard coal particle size, excessive fluctuations in bed pressure signals, and air leakage caused by improperly sealed control valves), continuous slag discharge is not possible, and only intermittent slag discharge can be used. That is, when the bed pressure increases, the L-valve flue gas adjustment valve is manually opened to adjust the air flow rate; the ash then flows into the cold slag holder. When the bed pressure drops to a certain level or the pressure difference in the cold slag holder becomes too large, the L-valve flue gas adjustment valve is closed, thereby ending the ash discharge process. 2.2. Working principle of the slag cooler: There are no design issues with this type of slag cooler; its design is reasonable. Problems usually arise from improper use. The slag cooler is designed for continuous slag discharge and cannot operate with a variable ash level; in other words, the water-cooled tubes are covered with slag before starting the furnace, in order to prevent the slag at nearly 800°C during slag discharge from directly scouring the water-cooled tubes and causing them to overheat and get damaged. Each slag cooler is supplied with 8,500 NM3/h of air; the air flow ratio among chambers 1, 2, and 3 is 3:3:4 or 3:2.8:4.2. The cooling air enters through the air boxes located beneath each chamber, and then reaches the chambers via distribution plates and air nozzles, thereby fluidizing and cooling the ash and slag. Together with the water-cooled tubes, this mechanism helps to reduce the temperature of the ash and slag, which is why it is called a combined air-water slag cooler. The ash and slag discharged from valve L into the cold slag chamber first mix with the ash and slag already present there; they are cooled by air in one chamber, before entering another chamber where they are cooled simultaneously by air and water-cooled tubes. Due to the barrier provided by the air-cooling partition, larger pieces of slag remain in this chamber and are discharged onto the slag transport conveyor via the emergency slag discharge outlet located at the bottom of this chamber (when the accumulation reaches a certain level) ； The fine-grained slag will pass over the air-cooling partition and flow into the third chamber (the second storage chamber), where it continues to be cooled simultaneously by air and water-cooling tubes. A portion of the slag that enters this third chamber is also discharged onto the slag transfer hopper through the emergency slag discharge outlet located below the chamber (this is not used very often) ； The fine-grained ash overflows from the three chambers and is evenly discharged onto the ash conveying hopper through an electric air lock. If the L valve discharges slag continuously, the operation process of the aforementioned slag cooler will be continuous. For various reasons, continuous slag discharge is not possible; only intermittent slag discharge can be used. This involves first emptying the slag in the cold slag holder, then discharging the slag into the cold slag holder through valve L. After it has cooled slightly, someone is sent to open the emergency slag discharge pipes located beneath the two chambers in order to empty the slag completely, before proceeding with the next discharge cycle. Under such operating conditions, one of the chambers is most prone to being crushed. In actual operation, there is no anemometer; only the pressure difference can be measured. Although there is a certain relationship between the pressure difference and air volume, it does not reflect the actual air volume. Because Room 1 has 28 air caps, while Rooms 2 and 3 each have 56 air caps. How can the slag discharge volume be monitored in the case of intermittent slag discharge? This mainly involves monitoring the pressure difference in the cold slag holder; a positive signal is taken from the blower box beneath each air chamber, while a negative signal is taken from the upper part of the cold slag holder. By feeding these two signals into a pressure difference transmitter, we can obtain the pressure difference – there will be three such differences for the three air chambers. The amount of slag discharged is primarily determined by observing these three pressure differences; generally, the pressure difference increases by about 3–5 KPa with each discharge of slag. When using the intermittent slag discharge method, it should be done in small amounts but frequently, that is, the slag should be discharged every 5–15 minutes (when the quality of the coal is poor); the pressure difference in the cold slag chamber increases by about 1–2 KPa. After discharging the slag 3–4 times, the slag disposal worker should be notified to remove all the slag through the emergency slag discharge pipe. If the quality of the coal is good, more flexibility can be applied in this regard. 3. There are problems. 3.1. During the hot-state commissioning of the L valve, it became clogged on several occasions, preventing slag discharge and resulting in shutdown of the furnace. Summary of reasons: ① The feed particle size was not up to standard; many large pieces of coal gangue and stones were stuck inside valve L ； ②The flow control valve for the fluidizing air in valve L does not close properly, resulting in air leakage (20–100 NM3/h). When slag is not discharged, this air that leaks into valve L reacts with the ash at a high temperature of around 800°C (which contains a small amount of combustible materials), forming large pieces of low-temperature slag that get stuck inside valve L. The problem now is that during the first few discharges after starting the furnace, the discharge does not proceed smoothly, and purge air is required for purging. 3.2. During the hot-state commissioning of the cold slag holder, large pieces of low-temperature slag often accumulate inside it, preventing the slag from being discharged. At the same time, the ash and slag in the furnace cannot be sent into the cold slag holder through the L valve, which results in high bed pressure and several shutdowns of the furnace. Analyzing the reasons: ① The air volume distribution among the three air chambers is unreasonable (due to the absence of air flow meters), and the total air volume is also low; ② The amount of slag discharged each time is too large, which causes the air chambers to be under excessive pressure and prevents fluidization, resulting in low-temperature slag; ③ When using the emergency slag discharge pipe, the discharge valve is opened too wide, causing ash and slag to fill the slag channel and block the slag transport chain buckets, preventing them from functioning. Also, during the trial operation of Reactor No. 1, for some reason, the height of the air-cooled partition wall and the normal slag discharge port in the right-side cold slag holder was reduced to 300 mm; the water-cooled tube bundles were removed. Additionally, the electric air lockers (used for continuous slag discharge) on the normal slag discharge pipes, as well as the electric doors for discharging slag from the lower sections of Chambers 1 and 2 (in both the left and right cold slag holders), were all removed, leaving only manual doors. The right-side slag cooler has been turned into a large, empty box; since there are no water-cooling tubes involved in the cooling process, the cooling efficiency is greatly reduced. It takes several hours for each batch of slag to cool down, and large pieces of slag often form inside it – these are also low-temperature slags. It was not possible to use it properly for several months after that; only then was the water-cooling tube bundle in the right-side slag cooler restored. However, slag deposits still formed frequently, just as before, with large deposits reaching diameters of 600mm to 700mm. These deposits mainly accumulated in Chamber 1, as there were no water-cooling tube bundles in that chamber ； The small diameters also range from 100 mm to 200 mm, filling every corner of the two- and three-chambered areas. (As shown in the upper image on the next page, this is the condition after the water-cooling tube bundle was restored, less than two months later – the water-cooling tube bundle had become severely deformed.) The cold slag holder on the left side performed somewhat better during the trial operation period (as shown in the image; it had been in use for about 10 months with only slight deformation). The height of the air-cooling partition wall and the normal slag discharge outlet was reduced from 1500 mm to 1100 mm, but since these changes were minor, they did not significantly affect the functionality of the cold slag holder. Its performance remained fairly normal during subsequent operations; occasionally large slag pieces were found, but this occurred when the air flow rate was low while the slag discharge volume was high. Once, on the night before operation, a unit used the left-side slag cooler to discharge slag; within 8 minutes, slag at a pressure difference of 11 KPa was discharged. Ash and slag at around 800°C directly washed against the water-cooled tubes, causing them to be damaged and leak within 1 minute after slag discharge stopped. This led to an increase in bed pressure, forcing the shutdown of the furnace. Since then, the water-cooled tubes of these four slag coolers have leaked on **five occasions**. But a few months later, for some reason, the left cold slag holder was modified again, with the height of the air-cooling partition wall and the normal slag discharge opening reduced to 500 mm. This modification caused large amounts of slag to form in the left cold slag holder, just as it did in the right cold slag holder. After 48 hours of operation, the bed pressure increased because neither the cold slaggers on the left nor those on the right could discharge slag, forcing a shutdown of the furnace. After opening the manhole, a large number of large slag pieces were removed from inside the cold slaggers on both the left and right sides. An analysis of the slag discharge based on the historical data shows that a reaction of large-scale slag formation occurred during the third slag discharge after the furnace was started. Later, during the tuning of Reactor No. 2, the height of the air-cooling partition walls and the normal slag discharge ports in the cold slaggers on both the left and right sides was reduced to 630 mm. During normal operation, the same problem occurred – large amounts of slag accumulated inside the cold slagger on the right side of Reactor No. 1 (as observed through the instruments). After a few days of operation, the reactor had to be shut down because the high bed pressure prevented the cold slagger from discharging the slag. After shutting down the furnace, the manhole of the cold slag holder was opened for inspection; the slag inside was piled up to a high level, even higher than the manhole itself. During cleaning, large pieces of slag came out one after another, and even the fine slag could not be discharged through the slag discharge port. When slag blocks the cold slag holder, one must use a steel rod to push upward from the emergency slag discharge port until it is cleared. 4. Improvements. 4.1. Reducing large-grained feed: Since there is only one stage of crushing in the fuel transportation system, and this situation cannot be changed for the time being, grids with a spacing of 20 mm have been installed at the entrance of each coal bin. This is also a compromise solution: it prevents excessively large amounts of feed from entering the coal bin, while at the same time avoiding the accumulation of wet coal on the grates, which would prevent fuel from being fed into the bin. Because in practical operation, it has been found that as long as the maximum coal feed particle diameter is no greater than 20 mm, there are no issues with slag discharge using the L valve. But the best approach is to add an additional screening stage and another crushing stage in the coal conveying system, so that the particle size of the coal fed can be fully controlled within the range required by the boiler (≤6 mm). 4.2. Improvement of the L valve: It was found in subsequent practice that after stopping slag discharge, closing the manual main valve for the fluidizing air of the L valve basically eliminated slag formation inside the valve. Therefore, after stopping slag discharge, someone must close the manual main valve for the fluidizing air at valve L to prevent air from leaking in. To reduce the physical strain on workers and eliminate potential safety hazards, an electromagnetic quick-close valve can be added after the manual main valve for the fluidizing air in the L valve. This valve can be controlled by the furnace operator via the DCS; during slag discharge, the electromagnetic valve is opened first, followed by the electric control valve ； When the slag has been discharged, first close the electric adjustment valve, and then close the solenoid valve; this will also prevent air from entering valve L. Additionally, an electromagnetic valve should be installed at the manual door for the purge air; if slag removal is difficult after starting the furnace, the operator can control this electromagnetic valve via the DCS to carry out purging. This helps ensure safer operation and reduces the physical strain associated with manual slag removal. 4.3. Improvements to the slag cooler: First, restore the height of the air-cooling partition inside the slag cooler as well as that of the normal slag discharge opening, and reinstall the electric air lock that was removed during the renovation on the normal slag discharge pipe (this is the most important aspect) ； Secondly, adjust the air volume in the three air chambers appropriately and add air flow meters to meet the design requirements ； Finally, before starting the furnace each time, the ash should be used to cover the water-cooled tube bundle. Since there are no air flow meters in the three air chambers of the slag cooler (there is a total air flow meter at the outlet of the fluidization fan for the slag cooler), it is possible to determine the air flow rate only by measuring the pressure difference. During normal operation, I believe that the pressure difference in chamber 1 is much higher than that in chambers 2 and 3, as their flow cross-sections differ (chamber 1 has 28 air nozzles, while chambers 2 and 3 each have 56 air nozzles). To achieve the same air flow rate, it is necessary to increase the pressure difference in chamber 1; this pressure difference is nearly twice that in chamber 2. When there is no ash or slag inside the slag cooler, the pressure difference in chamber 1 should be set around 15 KPa, while that in chamber 2 should be around 8 KPa, and that in chamber 3 around 10 KPa. But we have someone here who doesn’t think so; instead, he believes that the pressure difference is equivalent to the air volume, and that the pressure difference in one room should be the same as that in rooms two and three. He suggests setting the pressure differences in all three rooms to be equal. As a result, after the No. 1 furnace operated for a few days using the left cold slag extractor for slag discharge, the thermometer in Chamber 1 stopped changing, indicating that Chamber 1 was crushed by the slag. When we arrived at work, we noticed this issue and conducted an analysis; we increased the pressure difference in Room 1 to a level higher than that in Rooms 2 and 3. When slag was discharged, the thermometer in Room 1 changed again, indicating that the slag accumulated in Room 1 had been blown away. However, we still observed that the temperature changed abnormally, dropping very rapidly, which may suggest the presence of large clumps of slag. Inspections after shutdown revealed a small amount of large slag pieces in one chamber. Subsequent observations showed that the chamber whose temperature dropped particularly fast or particularly slowly after slag removal was the one with a problem (this was the case when the height of the air-cooled partition wall was 1100 mm). 4.4. Addition of an automatic slag discharge program: When slag discharge is required or the bed pressure rises, the furnace operator opens the slag discharge interface on the DCS and clicks the “Start Slag Discharge” button; the program will then automatically activate the L-valve flue gas solenoid valve (which needs to be added) as well as the electric control valve for the flue gas flow, adjusting them to the pre-set wind flow rate in order to carry out slag discharge. When the bed pressure drops to a certain level or reaches the specified value, click the “Slag Discharge Completed” button; the program will then automatically close the L-valve fluidization air electric control valve and the L-valve fluidization air solenoid valve, thereby completing this round of slag discharge. It is important to strictly control the amount of slag discharged, prefering small amounts discharged multiple times. The ash and slag that enter the cold slag chamber pass through chambers one, two, and three in sequence, and are ultimately discharged onto the slag conveying hopper via the electric air lock on the normal slag discharge pipe, allowing the cold slag chamber to operate in a continuous slag discharge mode. The advantage of this method is that the pressure in the water-cooled air chamber and the bed pressure remain basically stable, as do the boiler load, bed temperature, pressure in the reversing chamber, as well as the flow rates of the primary and secondary air. This effectively prevents large fluctuations in the boiler’s operating parameters. It also reduces the number of times the furnace has to be shut down due to an inability to discharge slag, ensuring higher equipment availability and improving economic efficiency. 5. Conclusion. Although there are various problems associated with the use of the fengshui combined cold slagger and L valve, through analysis and research, some targeted improvement suggestions have been put forward. Given that the actual conditions vary from plant to plant, a case-by-case approach should be adopted. Some of them have already been put into use in our plant, while more still need to be improved (such as the height of the air-cooling partition inside the slag cooler and the normal slag discharge opening, as well as the electric air lock that was removed during the modifications to the normal slag discharge pipe). However, I believe that this combined air-water slag cooling system together with the L-valve will become increasingly effective over time. The images in this article are from the Safety Supervision Department.