Analysis and Countermeasures on Coking Causes of 440t/h Circulating Fluidized Bed Boiler
Thread Content
1. Overview In the past 9 years, more than one hundred domestic 410-450t/h high temperature and high pressure or 440-480t/h ultra-high pressure reheat CFB boilers have been ordered, installed and put into operation in small quantities in nearly 60 power plants. Large-scale CFB boilers are power station boilers that have only been developed in recent years. Its design and operation need to be continuously accumulated and improved, and some problems will inevitably occur during operation. Through a survey of more than ten large-scale 440t/h CFB boilers in my country, we found that compared with conventional pulverized coal boilers, coking in CFB boilers is the most common and serious problem. Improper handling will inevitably seriously affect the safe and economical operation of CFB boilers, and also affect the further development and application of CFB boilers. Therefore, analyzing the causes of coking in circulating fluidized bed boilers and proposing solutions will continuously improve the stable operation level of large CFB boilers. 2. Coking phenomenon 2.1 The main coking phenomena include: ⑴ CRT shows that bed temperature and bed pressure are extremely uneven, combustion is extremely unstable, and relevant parameters fluctuate greatly and have large deviations. Several bed temperature measurement points have large deviations (differences greater than 150°C) and jump significantly. ； The bed pressure values on both sides have a large deviation, sometimes reaching about 3kPa. ⑵ In the early stage of coking (local), the differential pressure of the material layer decreases. When coking is severe, the differential pressure of the material layer increases sharply. ⑶ Oxygen drops rapidly, almost to zero. ⑷The furnace negative pressure increases, the primary air volume, and the air pressure in the air chamber fluctuate greatly. ⑸ Load, pressure and steam temperature all drop. ⑹ The slag discharge is not smooth, the bed slag discharge pipe is blocked, single or multiple slag discharge ports cannot discharge slag, or there are loose and porous sintered cokes (localized coking) in the slag discharge. ； ⑺ Observe from the fire hole that there are white sparks in the fluidized bed, slag blocks can be seen, and the bed materials are moving abnormally in the furnace. ； ⑻ The differential pressure of the material layer suddenly increased (up to about 10KPa), and then dropped quickly after a short period of time (it was judged that the castable material in the furnace collapsed in a large area). 2.2 When the overall temperature of the bed is lower than the ash deformation temperature and is caused by local over-temperature or low-temperature sintering, it is called low-temperature coking. The low-temperature coke block is loose with many embedded unsintered particles. The coking phenomenon formed when the overall temperature level of the bed is high and fluidization is normal is called high-temperature coking. The high-temperature coke block is basically molten on the surface, and turns dark brown with a small amount of pores after cooling. The slowly growing coke that appears under normal conditions such as bed temperature, bed pressure and fluidization during operation is called progressive coking. This kind of coking is difficult to detect. Coking in the furnace is caused by high-temperature coking, low-temperature coking, progressive coking, long oil-coal mixed combustion time, and abnormal fluidization. No matter what the cause is, once the slag blocks exist in the bed material and as time goes by, the coke blocks will snowball and get bigger and bigger, making fluidization more difficult. 3. Analysis of coking causes 3.1 High bed temperature and poor fluidization conditions in the furnace are the two main reasons for coking. Coking may occur during ignition or during normal operating adjustments, and there are many reasons. ； It will not only appear in the bed during the startup process or pressure fire, but may also appear outside the furnace, such as in the return valve of the cyclone separator. It is more likely to occur when the alkali metal potassium and sodium content in the ash is high. Failure of the return valve to return material, collapse of the casting material in the furnace, too small ignition (fluidization) air volume under the bed, and too thin material layer can cause coking in the boiler. When the carbon content in the bed material is too high, high-temperature coking may occur if the air volume or return material volume cannot be adjusted in a timely manner to stabilize the bed temperature. Both high-temperature coking and low-temperature coking often occur during the ignition process, and will grow rapidly once they occur. Since sintering is an automatically intensified process, the coke block tends to grow faster and faster. Poor fluidization of the bed material causes accumulation, uneven coal feeding, uneven coal spreading, and insufficient combustion, etc., which may cause localized coking. 3.2 The main causes of progressive coking are: ⑴ The manufacturing and installation quality of the air distribution system is poor. ⑵ The particle size of the coal is too large, and there are even large pieces in the coal. ⑶ Improper control of operating parameters, etc. At the initial stage of operation of a new unit, check whether the wind cap and the small holes in the wind cap are misinstalled or blocked, and whether the corners and top corners of the partition wall and refractory layer in the furnace are properly designed.
3.3 Analysis of possible causes of coking during production operation: ⑴ The melting point of coal and bed material is too low, which can cause coking when the bed temperature is low. ⑵ The fluidizing air volume is low and the fluidization is poor all the time. The primary air volume is too small, lower than the critical fluidizing air volume, and the material fluidization is not good. The air pressure at the bottom of the furnace is too low, and the resistance of the air distribution plate is low (generally the resistance of the air distribution plate should be 25-30% of the resistance of the entire material layer). The air distribution is uneven, resulting in poor fluidization in the furnace and local blow-through in the bed. The air supply in other parts is insufficient, the bed temperature is high, and the materials are bonded, thus forming coke blocks. ⑶ The wind cap is damaged, resulting in uneven air distribution on the air distribution plate and partial material layer failure to fluidize. ⑷ Impact of returning materials. The return air is too small, causing the return device to return abnormal material, or the return device is suddenly blocked due to the collapse of the refractory material, or the return material cannot be returned to the furnace normally due to the material difference and high circulation ash leakage, causing the bed temperature to be too high and coking. If the pressure and steam temperature are maintained by adding coal, the bed temperature will rise sharply due to the dual effects of the returned material not returning to the furnace and adding coal, causing coking on the bed. During the furnace start-up process, if the fluidizing fan is put in after coal is put in, when the returned material suddenly returns to the furnace bed, the bed temperature will drop sharply, with a drop of more than 200°C. At this time, the coal particles in the hearth slow down or even stop burning due to the drop in bed temperature. At this time, if the operation is incorrect and the coal is added instead of stopping the coal in order to raise the bed temperature, it will cause the bed temperature to further drop and the coal to continue to accumulate in the furnace. When coal is stopped after realizing that the bed temperature cannot rise again, a large amount of combustible matter has accumulated in the furnace. When the fuel oil raises the bed temperature to the ignition point of the coal particles, the large amount of combustible matter accumulated in the furnace will burn rapidly, causing the bed temperature to be out of control and causing coking. ⑸ The bed temperature measuring device is faulty and the bed temperature meter is inaccurate, causing the operator to misjudge or be helpless when the bed temperature is too high at a single point. ⑹ The operator's lax monitoring of bed temperature resulted in over-temperature. According to some literature, the actual temperature of the particles is 150 to 200°C higher than the temperature measured at the bed temperature measuring point. It can be seen that although the temperature reflected by the bed temperature measuring point is not high, the actual temperature has reached more than 1000°C. Some particles become sticky, form coke, and gradually grow. When a combustion failure occurs, the bed temperature of the circulating fluidized bed boiler changes very quickly. Since there are a lot of materials in the furnace and the heat capacity is large, if the bed temperature cannot be controlled in time, coking will easily occur. ⑺ Improper operation during pressure fire allows cold air to enter the furnace. ⑻ The boiler has been overloaded for a long time or the load has increased too quickly, resulting in improper operation. ⑼ The material layer is too thin or too thick when starting the furnace. This will cause part of the bed to be blown out and short-circuit the flue gas, while the other part is prone to coking due to poor fluidization. ； If the material layer is too thick and the resistance of the material layer is too great, it will cause poor fluidization of the bed material and cause coking. There is less bed material in the furnace, and the amount of ash that can be carried away by the flue gas and separated by the separator and fall on the return leg is also small. There is never enough material level in the return valve, and a normal return cycle cannot be formed. Because there is too little bed material in the furnace, normal internal circulation cannot be formed in the furnace. If it is misjudged at this time that the poor fluidization is caused by low wind, so when there is not much bed material and the particles are large, the air volume should still be increased to make the air volume * * Exceeding the air volume required for normal operation also further aggravates the loss of bed material, easily forming an empty bed, and the furnace can only be shut down immediately. ⑽ The castables in the furnace collapsed over a large area, resulting in poor local fluidization, overheating and coking. ⑾ When starting up the furnace and adding coal, it is very easy to cause the coal falling point to fail to fluidize normally and heat up rapidly. The bed temperature at non-coal falling points will drop rapidly, and the bed temperature deviation at different parts can reach more than 300-400°C. Under this condition, if the furnace is continued to be forcibly started, it will easily cause coking. The amount of coal added to the furnace and the timing of coal feeding are improperly controlled. When the bed temperature is low or the coal quality is poor, the coal put into the bed does not catch fire or is difficult to burn completely, resulting in a large accumulation of combustible matter in the furnace. When the fuel heats up to a certain high value, the coal particles in the furnace ignite and burn, and the bed temperature further rises. The increase in bed temperature further accelerates the combustion of coal particles, causing the bed temperature to rise rapidly and uncontrollably, leading to coking. ⑿ During operation, due to abnormal operation of the coal feeder, the coal supply measurement was inaccurate and excessive coal was supplied, resulting in local over-temperature of the bed. ⒀ J valve fan failure causes coking after the boiler MFT. ⒁ The coal entering the plant contains gangue, and the secondary crusher in the coal conveying system cannot completely crush the gangue in the coal during operation, causing large pieces of gangue to be deposited in the bed, affecting fluidization and combustion, and causing coking in the furnace (which is not conducive to slag discharge). ⒂ Before the boiler is started, the fluidizing air nozzle is too clogged or there are wear-resistant materials and other debris left in the furnace. When the burner is put into operation, the air distribution is seriously out of balance or the combustion power is too high. ⒃ During the shutdown process, the fuel is not completely burned, and tar is precipitated, causing low-temperature coking. ⒄ During the operation of the boiler, the proportion of air and coal was improper for a long time and excessive coal was supplied. 4. Technical measures to prevent coking 4.1 It is necessary to ensure good and stable coal quality into the furnace, especially the particle size, fineness, gangue, melting point and other indicators must be strictly controlled. 4.2 Before ignition, a fluidization test must be carried out carefully, and the fluidization condition and thickness of the base material must be observed on-site to ensure it is qualified. A good aerodynamic field in the furnace can effectively control the secondary combustion of the cyclone separator and avoid over-temperature coking of the combustion chamber, cyclone separator and return device. Increasing the coal sowing air pressure and appropriately reducing coal feeding on both sides during low load conditions can basically avoid low-temperature coking in the furnace. 4.3 When the return material system is put in, check whether the return material is flowing smoothly to prevent coking caused by return material failure. 4.4 Speed up the startup speed to avoid coking. The start-up time of CFB boilers should be shortened as much as possible, otherwise the mixed burning time of oil and coal will be too long, and coking will easily occur if the adjustment is not done properly. Especially in the initial stage of mixed burning of coal and oil, a large amount of coal cannot be completely burned in the furnace, and it is easy to stick together with unburned oil to form local high-temperature coking. In the early stage of ignition, when the bed temperature reaches the coal feeding temperature, coal should be fed immediately, and the oil should be cut off immediately after combustion is stable. This includes cutting off the oil in a timely manner during accident handling, so as to shorten the kerosene mixed combustion time and prevent coking.
4.5 When a large amount of coal is initially added, the bed temperature will rise. When starting the furnace, the long time of jogging coal will cause the accumulation of combustibles and cause deflagration. During the ignition and operation of anthracite coal, great attention should be paid to the accumulation of combustible components to avoid deflagration. When you first start adding coal, do not add it too fast or too hard, and follow the principle of a small amount of interruption. First, a single coal feeder is jogged to feed a small amount of coal, and it is confirmed that the oxygen level in the furnace has dropped and the bed temperature has risen before the jogging coal feeding time can be extended again and the coal feeding amount can be increased gradually. Before 730°C, it is best to use inching coal feeding, and continuous coal feeding is prohibited. The timing of coal feeding can be based on changes in oxygen content. Before 800℃, the coal input amount must not exceed 10t/h. 4.6 Strictly control the bed temperature. The bed temperature measurement adopts a unique vertical and even distribution method on the bed surface, which can detect local over-temperature coking in time. During operation, the thermocouple measuring points evenly arranged on the air distribution board are monitored to take early measures for abnormal working conditions. ； When it is found that the bed temperature is too high, measures should be taken immediately to increase the primary air volume or reduce the fuel to lower the bed temperature. According to the rise in bed temperature, fine-tune the air volume and coal supply amount in a timely manner to maintain good fluidization, and control the bed temperature not to rise too fast to avoid large changes in bed temperature, which may cause a vicious cycle. Taking into account the impact on coking and Nox control, the bed temperature should generally be controlled between 850-950°C, and the maximum temperature should not exceed 1000°C. The main control means is to adjust the air-coal ratio and the amount of returned material. It should be noted that if the wave bed temperature changes due to reasons such as coarsening of coal particles or deterioration of coal quality, the primary air volume should be appropriately increased to fluidize the bed and stabilize the bed temperature, otherwise large particle deposition and bed stratification are likely to occur, resulting in local or overall over-temperature coking. If the bed temperature is extremely unbalanced at several points or extremely high at certain points, this is a very dangerous working condition and should be dealt with promptly. Bed temperature control should follow the principle of keeping it high, not low. Foreign research reports and domestic operating experience have proven that the coking temperature in the fluidized bed is much lower than that in the pulverized coal furnace. Generally, coking begins when the temperature in the fluidized bed is 150-250°C lower than the ash softening temperature. It is recommended to control the local bed temperature not to be higher than 950-1000°C. 4.7 Control the bed pressure. When the bed pressure is too high, the slag should be discharged immediately and the output of the unit should be reduced to keep the bed pressure within the design value range (7-10kPa). Control the differential pressure of the material layer during operation to control the thickness of the material layer. 4.8 The quality and construction quality of qualified castables and refractory and wear-resistant materials in the furnace should be ensured to prevent coking caused by collapse of castables and other materials. 4.9 When the furnace is started, the return leg is prone to clogging due to the low temperature of the return material and poor fluidity. It is recommended that changes in temperature and pressure of the return leg should be closely observed when starting the furnace. If the temperature remains unchanged, compressed air should be used for purging and fluidization. When purging, care should be taken to prevent a large amount of material in the return leg from suddenly returning to the furnace and affecting combustion. 4.10 After replacing the wind cap of the boiler, the resistance characteristics of the air distribution plate need to be re-measured and the operating personnel should be informed of the changes in these characteristics in a timely manner. Before starting, a critical fluidization air volume test should be done. On the one hand, it is checked whether the wind cap is blocked. On the other hand, this air volume is used to guide the operation adjustment during operation. During normal operation, the fluidization must be ensured and the primary air volume cannot be less than this air volume. 4.11 Appropriately increase the primary air volume and air pressure, and increase the air chamber air pressure to above 8Kpa, which is the guarantee for good fluidization and stable operation of the 440t/h CFB boiler. In order to ensure safe and stable operation, the uniformity of air distribution should be ensured during the ignition process, and attention should be paid to timely discharge of slag at the end of the ignition process. Progressive coking during operation can also be avoided after mastering the operating skills and controlling the size of the particles entering the furnace. The best way to avoid low-temperature coking is to ensure good fluidization in prone areas, and that the particles are mixed quickly and evenly or in a normal fluidized state. This way, the temperature is uniform and coking can be prevented. 4.12 Strictly implement the operating procedures of each manufacturer to ensure the safe operation of the recycled Roots blower equipment. Avoid slagging due to local dead zones in the return valve. The inflation volume of the return valve should be strictly controlled within 1% of the total air volume of the boiler to prevent unburned carbon particles from re-igniting in local areas and avoid slagging in the return valve. 4.13 Prevent coking at the return port of the dense phase area of the boiler that uses rear wall coal feeding. CFB boilers that use a back wall return valve to feed coal are prone to over-temperature coking at the return port during the ignition debugging stage. The reason is: The amount of feedback during the ignition stage is small, and the feed coal cannot be quickly brought into the furnace by the feedback, and accumulates at the return port, causing excessive local combustion and over-temperature coking. ； The amount of return material is small, causing the flue gas to flow back toward the return port, and a vortex is formed at the return port. ； The volatile components burn here and cause over-temperature coking. The method of transformation is to install a high-speed cold air duct on the return inclined leg toward its outlet. On the one hand, this air blows volatile components into the furnace, destroys the swirl flow of the return port, and prevents combustion. ； On the one hand, it plays the role of sowing coal wind. In this way, the problem of over-temperature coking at the return port can be basically solved. 4.14 Modify the fire holes on both sides of the fluidized bed and the manholes on both sides of the water-cooled air chamber so that operators can clearly see the fluidization of the bed material and the accumulation of leakage slag from the air cap in the water-cooled air chamber during operation. 4.15 In design: The manufacturer adopts advanced foreign technology to predict the thermal performance of the boiler to ensure the uniformity of the temperature field along the furnace section and along the furnace height direction. Select appropriate air distribution plates and bed resistance during design to basically ensure uniform bed fluidization during boiler operation, avoid the deposition of large particles on the air distribution plates, and basically ensure uniform air distribution, good fluidization quality, and no dead zones in the bed. A pneumatic coal seeding device in front of the furnace is used to make the coal feed into the furnace evenly to avoid local over-temperature and coking caused by local coal-rich areas deflagrating when exposed to oxygen during operation. The furnace adopts a fluidization process of thickening at the bottom and dilution at the top. The secondary air adjustment margin is designed to be large. Through the adjustment of the primary and secondary air, the purpose of quickly adjusting the bed temperature can be achieved and the bed temperature can be controlled within the allowable range. 5. Conclusion Coking in CFB boilers has subjective and objective reasons in design, manufacturing and operation. Design and manufacturing units should also conduct quality return visits, summarize experiences, and strive to continuously improve the design, solve structural hazards, and optimize the overall design. As an operator, you should strive to improve the theoretical level of large-scale CFB boiler technology, and at the same time learn from the operating experience of similar units, analyze the causes of coking, implement various technical measures to prevent coking, and continuously accumulate operating experience in practice. In this way, coking in CFB boilers can still be controlled and prevented.