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Several special factors affecting steam temperature: 1. Influence of the combustion fraction. Due to the complexity of combustion, flow, and heat transfer processes, there are many influencing factors; therefore, it is currently not possible to determine precisely how much fuel is burned in the dense-phase region and how much in the dilute-phase region. In other words, the combustion fraction is an unknown value influenced by various factors. In the existing designs of circulating fluidized bed boilers, the combustion fraction δ in the dense phase region is largely determined empirically. In actual operation, δ is related to factors such as the type of coal, the particle size distribution of the coal, the amount of air supplied, the furnace temperature, the coal feeding method, the efficiency of the regenerator, the circulation ratio, and the level of operational expertise. Therefore, there is inevitably a difference between the δ value selected during design and the δ value under actual operating conditions. In actual operation, due to the high content of fine particles in the coal and a large volume of air supplied, the combustion fraction δ in the dense phase region decreases, while the combustion fraction in the dilute phase region increases. As a result, the flue gas temperature at the exit of the dilute phase region rises, and the enthalpy of the steam exiting that area also increases. The calculation results show that when the combustion fraction in the dense phase region drops from 0.7 to 0.5, the temperature adjustment enthalpy increases by 86.6 KJ/KG (see Figure 1), which is equivalent to an increase in steam temperature of about 35°C. It is evident that changes in δ have a significant impact on steam temperature. 2. Influence of particle concentration and size in flue gas: In convective heating surfaces, the heating surface receives heat both through convection and radiation; changes in the concentration and size of fly ash lead to changes in radiant heat, which in turn causes changes in steam temperature. The concentration in conventional boilers remains relatively constant during operation, and the radiative heat transfer of particles is low (for layer-fired boilers). However, in circulating fluidized bed boilers, due to the high ash concentration in the flue gas, the proportion of heat transfer by radiation in the total heat transfer is relatively large; therefore, changes in ash concentration have a significant impact on the heat transfer rate. There are many factors during the operation of circulating fluidized bed boilers that can cause substantial changes in ash concentration, such as the particle size of the coal, the efficiency of the separator, and the amount of air supplied. The higher the circulation ratio, the greater the influence of these factors. So it must be taken into consideration. Figures 2 and 3 show the curves of temperature-regulating enthalpy as a function of fly ash concentration and fly ash particle size. As can be seen from the graph, if the fly ash concentration increases from 0.06 KG/M3 to 0.16 KG/M3, the temperature rise associated with this increase is equivalent to an increase in steam temperature of about 30°C. The fly ash particle size is 0.2 MM. A decrease in Ai to 0.1 MM increases the energy by 25 KJ/KG, which is equivalent to an increase in steam temperature of about 10°C. Furthermore, due to the high concentration and large particle size of fly ash, the heat transfer by contact when particles collide with the wall surface increases. It will also have a certain impact on the steam temperature, but this aspect of contact heat transfer is not taken into account in the current calculations; further research is needed on this topic. 3. Influence of fuel particle size: The coal used in fluidized bed boilers in China is generally of a coarsely screened type, with required natural crushing particle sizes of 0–8 MM or 0–10 MM. However, in actual operation, the particle size of the coal cannot meet these requirements due to the reasons listed below. (1) The coal production system is not appropriate; the raw coal is crushed after being screened first, resulting in an excessive amount of fine coal powder. (2) The screen does not operate properly; after running for a while, especially when the coal is wet, the sieve pores become partially blocked, resulting in the coal particles becoming increasingly fine. (3) There is no magnetizing device in the coal conveying system, or it is not functioning properly, allowing nails, iron pieces, etc. to enter the fluidized bed. (4) The crusher does not operate properly; for example, after running for a while, its crushing efficiency declines and the amount of coarse coal particles increases. If the coal is not screened after being crushed, large particles of coal will end up entering the furnace bed in large quantities. (5) Damage to the sieve and its meshing parts causes the sieve holes to enlarge, which in turn allows a large amount of coarse coal particles to enter the bed. In short, the coal production system is key to ensuring the supply of coal with the appropriate particle size. The particle size affects the air supply volume, combustion fraction, fly ash concentration, and fly ash particle size. This, in turn, affects the variation in steam temperature. For example, when coal used for combustion has a particle size greater than 8–10 mm (with the largest particles even reaching 50 MM) and a high proportion of fine particles, operating at the designed air flow rate may lead to the deposition of these coarse particles, thereby causing accidents (this is one of the main reasons why fluidized-bed boilers in China cannot operate stably over long periods). To keep these coarse particles in a fluidized state, it is necessary to increase the air supply volume; this results in an increased rate of particle suspension, a decrease in the combustion fraction δ in the dense phase region, and an increase in the combustion fraction in the dilute phase region. At the same time, increasing the air supply volume also increases the smoke volume in the superheater area, both of which contribute to an increase in steam temperature. In severe cases, some of the fine coal particles may burn in the superheater area, further raising the steam temperature. 4. Influence of load (air supply volume): In boilers with medium to high parameters, the superheaters are generally arranged as two stages of pure convective superheaters, high and low. The steam temperature in such a purely convective superheater varies with changes in load; as the load increases, combustion, air flow, flue gas volume, and flue gas velocity all increase, resulting in a rise in steam temperature. In a fluidized bed boiler, coal particles burn in a fluidized state; therefore, the amount of air supply must meet the requirements of both combustion and material fluidization. At around the rated load, the amount of air required for combustion is roughly the same as that needed for proper fluidization. However, when the load decreases (and to ensure the safety of the water circulation, it is better not to stop the operation of the boiler), in order to maintain proper fluidization in the dense-phase region, the amount of air cannot be reduced too much as the fuel supply decreases. At this point, the air required for fluidization exceeds that required for combustion; therefore, at low loads, the volume of flue gas passing through the heat exchanger is relatively large. As a result, the steam temperature drops less as the load decreases compared to conventional boilers. The temperature adjustment enthalpy of medium-pressure boilers is around 80 KJ/KG; when the load is below 70% of the rated value, the steam temperature fails to reach the designed level. However, in low-ratio circulating fluidized bed boilers, the temperature adjustment enthalpy allows for a larger range of load variations. 5. The impact of temperature regulation methods on steam temperature: In parameter-controlled circulating fluidized bed boilers, the methods for temperature regulation involve using surface-type coolers to reduce the temperature, or employing feedwater injection for cooling, or using self-cooled condensate for this purpose. As can be seen from the examples given, when several factors affect the steam temperature at the same time, the use of surface-type coolers results in an increase in steam temperature of over 100°C. If water injection is used for cooling between the different stages, the amount of water injected affects the volume of steam flowing through the low-temperature superheater; when an increase in temperature requires more water to be injected, the volume of steam flowing through the low-temperature superheater decreases, leading to a higher steam temperature at its outlet. Therefore, the use of water for cooling causes an even greater increase in steam temperature. The amount of self-cooling condensate depends on the pressure difference in the superheater; a higher flow rate through the superheater and a greater pressure difference result in more water being sprayed. At low load levels, the amount of self-cooling condensate decreases. If the steam temperature rises at such times, enough water needs to be sprayed for cooling, and in such cases it may seem that there isn’t sufficient water available for spraying. If the heat absorption rate of the low-temperature superheater is higher than designed, this contradiction will become more pronounced. 6. Influence of the aerodynamic field in the furnace on steam temperature: Similar to conventional boilers, the aerodynamic field in the furnace also affects the steam temperature in the superheater. In circulating fluidized bed boilers, there are mainly three reasons that cause uneven distribution of flue gas along the width of the boiler: (1) The cyclone separators in such boilers are usually located between the secondary superheaters or behind them, with the flue gas being directed to these cyclone separators from both sides of the horizontal flue in the superheater area. This arrangement inevitably leads to uneven smoke flow in the width direction of the furnace, with more flow on the two sides and less in the middle, thereby causing thermal variations between the various superheater tubes. (2) In circulating fluidized bed boilers, secondary air is commonly arranged in the dilute phase zone, either at the four tangential corners or in a counterflow arrangement. If the installation or flow rate of the secondary air is improper, it can lead to uneven distribution of flue gas across the width of the boiler, resulting in temperature differences between various tubes. (3) For low-ratio circulating fluidized bed boilers with medium to high parameters, there are generally several beds; most circulating fluidized bed boilers in the 35T/H–75T/H range have 2 or 6 beds. When operating at low load conditions, if some of these beds are shut down, it can still lead to temperature differences among the tubes in the superheater. Significant temperature differences may cause certain tubes in the superheater to overheat. In summary, there are many special factors in circulating fluidized bed boilers that affect the superheater steam temperature, and these factors influence one another. In actual operation, several factors usually come into play at the same time. When several factors act together, the steam temperature can vary significantly. For example, in the calculation of the superheater in a high-pressure 75 T/H low-ratio circulating fluidized bed boiler, the effects of fly ash concentration, combustion fraction, and fly ash particle size were taken into account. Specifically, the fly ash concentration U varied from 0.06 KG/M3 to 0.16 KG/M3, the combustion fraction δ decreased from 0.7 to 0.6, and the fly ash particle size DFH reduced from 0.2 MM to 0.1 MM. The calculation results showed that the temperature adjustment enthalpy Ai increased from 66 KJ/KG to 326.6 KJ/KG – an increase of 5 times compared to the design value, which corresponded to an increase in the superheater temperature of approximately 103°C. Even at a 60% low load, the same calculations indicated that the temperature adjustment enthalpy increased from 9.5 KJ/KG to 183 KJ/KG, meaning an increase of nearly 20 times, equivalent to an increase in the steam temperature of almost 700°C. Therefore, great attention should be paid to steam temperature control in circulating fluidized bed boilers, with careful consideration given during both design and operation. III. Key Design Points and Precautions for Superheaters As mentioned above, the superheated steam temperature in circulating fluidized bed boilers is influenced by various factors, and the degree of variation can be significant. To ensure the safety of the superheater and to meet the design parameters, based on our practical experience, the following points are highlighted as matters that should be given attention to during design and operation: 1. Appropriately adjust the heat transfer coefficient. Currently, there is a widespread issue where the actual measured superheater temperature is higher than the designed value. This is mainly due to inaccurate calculations of the superheater’s heat transfer coefficient, with the designed coefficient being too low, as well as an excessive number of heating surfaces. Therefore, the normal heat transfer coefficient should be adjusted when designing the superheater. 2. Controlling the particle size of coal: As mentioned above, the particle size of coal affects factors such as air volume, deflection ratio, fly ash concentration and particle size, combustion share, and separator efficiency. Therefore, ensuring an appropriate particle size for the coal is crucial for controlling the superheated steam temperature. If a power plant uses mixed coal, although the average particle size of the coal is only 1.04 MM, it still contains larger coal particles. Combined with other factors such as operation at high air flow rates, this results in an **increase in the amount of water used for temperature reduction**. 3. There should be means to adjust the combustion share. Changes in the combustion share within the dense phase region have a significant impact on the boiler load, the flue gas temperature in the dense phase region, and the flue gas temperature at the outlet of the dilute phase region; as a result, they also have a considerable effect on the steam temperature. However, in design, the combustion share is a value estimated based on practical experience; due to the many and complex influencing factors, its value can only be approximate. Therefore, certain measures should be considered during design to adjust the combustion fraction. If negative pressure coal feeding is used and a coal distribution nozzle is installed at the coal drop point, with a control valve fitted on this nozzle, it is possible to adjust the proportion of combustion within a certain range by changing the amount of air supplied for coal distribution as well as the angle of the air inlet, depending on the operating conditions. This helps to keep the combustion proportions as close as possible to the design values, thereby avoiding significant changes in steam temperature resulting from such adjustments. 4. Use of a low recirculation ratio: In the design of circulating fluidized bed boilers, the efficiency of the separator is closely related to the recirculation ratio. The separator efficiency determines the recycling ratio. When the cycle ratio is low, the effect of changes in separator efficiency on the cycle ratio is minimal. When the separator efficiency is high, even a slight change in its efficiency can lead to a significant change in the circulation ratio. As shown in Table 1, the circulation ratio of low-ratio circulating fluidized bed boilers is generally between 3 and 10 (corresponding to a separator efficiency of 70–90), which can cause the fly ash concentration to change by several times. In high-rate circulating fluidized bed boilers, changes in separator efficiency have a greater impact on fly ash concentration. Therefore, using a low magnification cycle makes it easy to control the steam temperature. Table 1: Separator efficiency – 70, 75, 80, 85, 90, 96, 98, 99, 99.6; Cycle ratio – 2.33, 3, 4, 5.66, 9, 24, 49, 99, 249. Additionally, the separator efficiency selected during design should be close to that observed during actual operation; if it is significantly lower, it can lead to an increase in the temperature of the superheated steam. In fact, the efficiency of the separator is affected by various factors such as load and air supply volume, making it difficult to achieve the same value as the designed one. In low-ratio circulating fluidized bed boilers, the impact of this discrepancy on steam temperature is less severe than in high-ratio circulating fluidized bed boilers. 5. Avoid operating at high air flow rates. In fluidized bed boilers, since the particle size of the raw coal often exceeds the specified range of 8–10 MM, high air flow rates are used to prevent coarse coal particles from settling at the bottom and causing accidents. Not only are the dampers kept fully open at rated load, but they are not reduced even at lower loads. This mode of operation not only leads to changes in smoke volume and temperature, but it also increases the amount of ash suspension, reduces the combustion proportion in the dense phase region, and raises the fly ash concentration, all of which affect the increase in steam temperature. Therefore, the operating air volume during operation should be appropriate to avoid a decrease in boiler efficiency and an increase in steam temperature. 6. Improve temperature regulation capability: The temperature reduction capacity of the desuperheater in circulating fluidized bed boilers should be greater than that in conventional boilers, and the value selected for the superheater’s temperature reduction enthalpy should be lower than the values recommended in the design, in order to enhance the ability to cope with the influence of various factors on steam temperature. 7. Regulation of primary and secondary air: By using the same fan for both primary and secondary air, it is possible to adjust the ratio between them over a wide range. When the load decreases, in order to prevent coarse particles from settling at the bottom and to maintain good fluidization, it is possible not to reduce or only slightly reduce the amount of primary air, while appropriately reducing the amount of secondary air. This approach helps to keep the total amount of air required for fluidization and combustion roughly constant, thus avoiding operation with excessive air flow. The secondary air flow rate should not be too low, as this will affect the combustion efficiency in the area supplied by the secondary air. Attention should also be paid to the quality of installation of the secondary air ducts, and cold-state testing should be carried out. During operation, the flue gas temperatures on both sides of the boiler should be monitored and adjusted accordingly; in short, it is necessary to ensure optimal operating conditions for the secondary air in order to maintain a uniform flow pattern inside the furnace. IV. Conclusions 1. There are many factors that affect the steam temperature characteristics of circulating fluidized bed boilers, and these factors are interrelated; they can cause significant variations in steam temperature and may also lead to an elevated steam temperature at the outlet of the low-temperature superheater. Temperature control in conventional boilers is complex. 2. The particle size of raw coal has a significant impact on the superheated steam temperature. Attention should be paid to and the coal supply system improved to ensure the supply of qualified coal particles. 3. The steam temperature in low-ratio circulating fluidized bed boilers is easier to control than that in high-ratio circulating fluidized bed boilers. 4. Under the condition of maintaining the steam temperature at the rated value, the load regulation range of circulating fluidized bed boilers is wider than that of conventional boilers.