Control method for low load operation of a opposed firing boiler
By adjusting the pulverizing and combustion system parameters of the counter-firing boiler, the problems of water-cooled wall overheating and uneven combustion during deep peak shaving and low-load operation were solved, improving the unit's safety and economy.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- GUODIAN NANJING ELECTRIC POWER TEST RES CO LTD
- Filing Date
- 2023-08-28
- Publication Date
- 2026-05-29
Smart Images
Figure CN117308134B_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of coal-fired power generation technology, and more specifically, to a control method for low-load operation of a counter-firing boiler. Background Technology
[0002] The key technology of ultracritical (supercritical) boilers lies in the water-cooled walls. Compared to natural circulation boilers, the temperature of the working fluid inside the water-cooled wall tubes changes with the heat absorbed under supercritical pressure. During deep peak shaving and low-load operation, the feedwater flow rate and pressure inside the water-cooled wall tubes decrease, and the under-enthalpy of the working fluid at the inlet of the heating surface increases. The reduced feedwater flow rate means increased unevenness in the flow distribution within the water-cooled wall tubes; the reduced pressure means increased volume change in the steam-to-water ratio within the water-cooled wall tubes; and the increased under-enthalpy of the working fluid changes the resistance ratio of the evaporator and economizer. These factors combined bring the hydrodynamic stability of the water-cooled wall to a critical point of instability under low-load conditions, making it highly susceptible to instability under external disturbances, leading to overheating in localized areas of the water-cooled wall. Furthermore, the mass flow rate of pulverized coal at the pulverizer outlet generally varies, resulting in differences in the load of different burners. For tangential combustion boilers, due to intense mixing within the furnace, the difference in burner load has little impact on combustion. In contrast, the burners of a counter-firing pulverized coal boiler are concentrated on the front and rear walls, resulting in poor lateral mixing within the furnace. Deviations in pulverized coal flow rates among burners in the same layer can lead to uneven combustion. Under high loads, with multiple burners in operation, the inter-layer deviations compensate for each other, preventing significant differences.
[0003] Under low load conditions, especially during deep peak shaving, the main problems of counter-fired boilers are as follows: (1) During deep peak shaving, the number of coal mills in operation is relatively small, and the burner heat load deviation has a significant impact on the uniformity of combustion in the boiler. In addition, the stability of the hydrodynamics in the water-cooled wall tubes is also poor under low load. The interaction between the two leads to a very prominent problem of high temperature in the water-cooled wall under low load during deep peak shaving. (2) The boiler reheater mainly relies on convection heat exchange. Under low load during deep peak shaving, due to the low amount of coal fed into the furnace and the small amount of flue gas, the heat exchange between the reheater and the flue gas is insufficient, resulting in a low reheat steam temperature under low load, which affects the economic efficiency of the unit operation. (3) During deep peak shaving, the number of coal mills in operation is relatively small. If the coal mill encounters a sudden situation and the coal is suddenly cut off, there will be a risk of boiler fire extinguishing, which will affect the safety of the unit operation.
[0004] Currently, two methods are used to address the problem of localized overheating in water-cooled walls. One method involves optimizing the boiler pulverizing system and conducting optimization tests on the combustion system. Pulverizing system optimization mainly involves leveling the air velocity in the pulverized coal tubes and finding the optimal pulverized coal fineness. Combustion system optimization tests mainly determine the optimal operating oxygen content, optimal air distribution method, and optimal primary air velocity. The ultimate goal is to reduce uneven combustion in the furnace under low loads, ensuring safe and economical boiler operation. This method can alleviate the problem of uneven combustion to some extent, but it cannot completely solve it. For example, uneven pulverized coal concentration in the pulverized coal tubes is a major cause of the deviation. The other method involves equipment modification. Equipment modification mainly involves adding an online pulverized coal concentration regulating device, which can adjust the pulverized coal concentration in the primary pulverized coal tubes in real time. Combined with the aforementioned optimization of the pulverizing system and combustion system, this can alleviate the problem of uneven combustion. However, due to the long operating time, most of the burner tie rods in the opposed combustion boiler are in a stuck state. Although the pulverized coal concentration can be adjusted online, it is difficult to adjust the corresponding airflow of the burner. This limits the maximum effectiveness of the online pulverized coal concentration regulating device. At the same time, sudden coal cut-off under low loads can affect the safe operation of the unit and cause the reheater temperature to be too low under low loads.
[0005] With the innovative development of technology, a new type of coal feeding system has emerged in the past two years – the rapid raw coal switching system. This system eliminates the gate between the raw coal bunker and the coal feeder, and divides the boiler's raw coal bunker into two smaller compartments. These two compartments store commonly used coal and peak-shaving coal, respectively. Peak-shaving coal is the most suitable coal type for low-load operation. Each compartment has an independent hydraulic gate. By controlling the opening and closing of these hydraulic gates, the type of coal entering the feeder from each compartment can be quickly switched in real time. Under normal load, the gate on the peak-shaving coal side is closed, and the gate on the commonly used coal side is opened, using the commonly used coal to reduce fuel costs. When deep peak shaving is required, the gate on the peak-shaving coal side is opened, and the gate on the commonly used coal side is closed, using the peak-shaving coal to improve the boiler's stable combustion capability under low load. Based on these advantages, this system has quickly been adopted by multiple power plants, and its market prospects are promising, with its application expected to become increasingly widespread. At the same time, the system also provides a possibility for solving the problems of large combustion deviation in low-load boilers during deep adjustment, boiler combustion safety, and low reheat steam temperature.
[0006] However, the application of this system is currently in a bottleneck period, and the problems are as follows: (1) At present, coal type switching is all done manually. If the coal type deviation is large and the load is low during the peak adjustment process, the flame center may rise due to the change in coal quality after the coal type switching. If the combustion parameters such as oxygen content are not adjusted in time, the wall temperature may be too high or fluctuate greatly, and even lead to the risk of oxide scale falling off the high-temperature heating surface of the furnace wall, affecting the safety of unit operation. (2) When the coal type is switched, the coal quality changes greatly and the parameters of the coal mill cannot be adjusted in time. This may result in large changes in the mill outlet temperature and the coal powder fineness not meeting the standards, which seriously affects the safety and economy of unit operation. (3) In order to prevent coal blockage at the gate, the installation position of the new gate is raised. According to the shape of the cone below the original coal bunker, this will increase the coal flow area at the gate. The coal flow area of the new single compartment gate is about 1.2 times the coal flow area after the original gate is opened before the coal bunker is separated, so as to ensure that there is no coal blockage. This upward movement of the gate increases the coal storage space below it, significantly increasing the time required for the newly switched coal to flow from the gate, be ground into pulverized coal, and be injected into the furnace. Simultaneously, after the coal type switch, the original coal type remains inside the mill, ultimately leading to a longer time required for the newly switched coal to flow from the gate, be ground into pulverized coal, be injected into the furnace, and be completely burned. During this period, the proportion of different coal types entering the furnace constantly changes, making the control of the corresponding pulverizing system parameters and the overall combustion system parameters difficult and lacking reference data.
[0007] Therefore, there is an urgent need for a method that can solve at least one of the above problems. Summary of the Invention
[0008] The purpose of this invention is to provide a control method for low-load operation of a counter-firing boiler, which solves the problem of local overheating of the water-cooled wall in the counter-firing boiler under deep peak shaving conditions during low-load operation in the prior art.
[0009] To achieve the above objectives, the present invention provides a control method for low-load operation of a counter-firing boiler, applied to a coal-fired power generation unit. The control method for low-load operation of the counter-firing boiler includes:
[0010] Determine the coal type switching load and switch the coal type input to the boiler from the coal mill according to the coal type switching load;
[0011] Determine the changes in key operating parameters of the pulverizing system and the combustion system after switching coal types;
[0012] The transition curves of different key operating parameters of the pulverizing system are obtained based on the changes in the key operating parameters of the pulverizing system, and the transition curves of different key operating parameters of the combustion system are obtained based on the changes in the key operating parameters of the combustion system.
[0013] The key operating parameters of the pulverizing system are adjusted according to the transition curves of the key operating parameters of different pulverizing systems, and the key operating parameters of the combustion system are adjusted according to the transition curves of the key operating parameters of different combustion systems.
[0014] Specifically, determining the coal type switching load includes:
[0015] The coal type switching load is determined based on the load corresponding to the minimum output of the coal type before the coal type is switched at the coal mill under low load conditions.
[0016] Specifically, the key operating parameters of the pulverizing system include: the coal mill outlet air-coal mixing temperature T and the coal mill dynamic separator speed W.
[0017] Specifically, the coal mill outlet air-coal mixing temperature T is calculated as follows:
[0018] T = (82 - V) daf )*5 / 3±5;
[0019] Among them, V daf It is a dry, ash-free volatile matter component of coal.
[0020] Specifically, determining the rotational speed W of the coal mill dynamic separator includes:
[0021] The speed W of the dynamic separator of the coal mill is determined based on the speed of the dynamic separator of the coal mill for the coal type before the grinding switch and the speed of the dynamic separator of the coal mill for the coal type after the grinding switch.
[0022] Specifically, the transition curves of the key operating parameters of the pulverizing system include: the transition curve of the coal mill outlet air-coal mixing temperature and the transition curve of the coal mill dynamic separator speed;
[0023] Transition curves for different key operating parameters of the pulverizing system are obtained based on the changes in these parameters, including:
[0024] The transition curve of the coal mill outlet air-coal mixing temperature T is obtained based on the change value of the coal mill outlet air-coal mixing temperature T within a specified time period t.
[0025] The transition curve of the dynamic separator speed of the coal mill is obtained by measuring the change value of the speed W of the dynamic separator of the coal mill within a specified time period t.
[0026] Specifically, the key operating parameters of the pulverizing system are adjusted according to the transition curves of different key operating parameters of the pulverizing system, including:
[0027] The coal mill outlet air-coal mixing temperature is adjusted according to the transition curve of the coal mill outlet air-coal mixing temperature.
[0028] The speed of the dynamic separator of the coal mill is adjusted according to the speed transition curve of the dynamic separator of the coal mill.
[0029] Specifically, the key operating parameters of the combustion system include: boiler operating oxygen content, primary air velocity supplied to the boiler, opening degree of secondary air damper, and opening degree of burnout air damper.
[0030] Specifically, the transition curves of the key operating parameters of the combustion system include: the operating oxygen content transition curve, the primary air velocity transition curve, the secondary air damper opening transition curve, and the burnout air damper opening transition curve.
[0031] Based on the changes in the key operating parameters of the combustion system, transition curves for different key operating parameters of the combustion system are obtained, including:
[0032] The operating oxygen content transition curve is obtained based on the change in the boiler operating oxygen content within a specified time period t.
[0033] The primary air velocity transition curve is obtained based on the change value of the primary air velocity fed into the boiler within a specified time period t.
[0034] The transition curve of the secondary air damper opening is obtained based on the change value of the secondary air damper opening within a specified time period t.
[0035] The transition curve of the burnout air damper opening is obtained by measuring the change in the opening of the burnout air damper within a specified time period t.
[0036] Specifically, the key operating parameters of the combustion system are adjusted according to the transition curves of different key operating parameters of the combustion system, including:
[0037] Adjust the boiler's operating oxygen level according to the operating oxygen level transition curve;
[0038] Adjust the primary wind speed according to the primary wind speed transition curve;
[0039] Adjust the opening of the secondary air damper according to the secondary air damper opening transition curve;
[0040] Adjust the opening of the burnout air damper according to the burnout air damper opening transition curve.
[0041] The present invention provides a control method for low-load operation of a counter-firing boiler. When the boiler is operating at low load, the coal type switching load is first determined. The coal mill switches the coal type fed into the boiler according to the coal type switching load. Then, the changes in key operating parameters of the pulverizing system and the combustion system after the coal type switch are obtained. Transition curves for different key operating parameters of the pulverizing system and the combustion system are obtained based on the changes in the key operating parameters of the pulverizing system. The key operating parameters of the pulverizing system are then adjusted according to the transition curves of the key operating parameters of the pulverizing system, and the key operating parameters of the combustion system are adjusted according to the transition curves of the key operating parameters of the combustion system. By adjusting the operating parameters, the key operating parameters of the pulverizing system and the combustion system were improved, increasing the amount of flue gas generated under low load conditions. This, in turn, increased the convective heat exchange between the flue gas and the reheater in the coal-fired power plant, raising the temperature of the reheat steam. Simultaneously, by switching coal types, the number of operating coal mills was not reduced, lowering the risk of boiler flameout due to sudden coal shortages and improving the safety of unit operation. Because the number of operating coal mills remained unchanged, the amount of flue gas generated by boiler combustion increased, resulting in better flame coverage in the boiler furnace. This significantly reduced the problem of occasional uneven combustion under low load conditions and solved the problem of localized overheating of the water-cooled walls in existing counter-firing boilers during low-load operation under deep peak shaving conditions.
[0042] Other features and advantages of the embodiments of the present invention will be described in detail in the following detailed description section. Attached Figure Description
[0043] The accompanying drawings are provided to further illustrate embodiments of the present invention and form part of the specification. They are used together with the following detailed description to explain the embodiments of the present invention, but do not constitute a limitation thereof. In the drawings:
[0044] Figure 1 This is a flowchart of a control method for low-load operation of a counter-firing boiler provided in an embodiment of the present invention;
[0045] Figure 2 This is a schematic diagram of the coal bunker structure in a coal-fired power generation unit where the control method for low-load operation of the opposed-firing combustion boiler provided in this embodiment of the invention is applied.
[0046] Figure 3 This is the transition curve of the coal mill outlet air-coal mixing temperature T within a specified time period t in the control method for low-load operation of a counter-firing boiler provided by the present invention.
[0047] Figure 4This is a graph showing the oxygen distribution at the economizer outlet in a coal-fired power generation unit where the control method for low-load operation of the counter-firing boiler provided by this invention is applied.
[0048] Figure 5 This is a curve showing the wall temperature distribution of the front water-cooled wall in a coal-fired power generation unit, which is an application of the control method for low-load operation of a counter-firing boiler provided by this invention.
[0049] Explanation of reference numerals in the attached figures
[0050] 1-Raw coal bunker; 2-Coal bunker partition; 3-Hydraulic plug-in door; 4-Coal feeder; 5-Anti-blocking device. Detailed Implementation
[0051] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the scope of the present invention.
[0052] Figure 1 This is a flowchart of a control method for low-load operation of a counter-firing boiler; Figure 2 This is a schematic diagram of the coal bunker structure in a coal-fired power generation unit where the control method for low-load operation of a counter-firing boiler is applied. Figure 3 This is the transition curve of the coal mill outlet air-coal mixing temperature T within a specified time period t in the control method for low-load operation of a counter-firing boiler. Figure 4 This is a graph showing the distribution of oxygen at the economizer outlet in a coal-fired power generation unit, which illustrates the control method for low-load operation of a counter-firing boiler. Figure 5 This is a curve showing the wall temperature distribution of the front water-cooled wall in a coal-fired power generation unit, which is an example of the control method for low-load operation of a counter-firing boiler.
[0053] like Figures 1-5 As shown, this invention provides a control method for low-load operation of a counter-firing boiler, applied to a coal-fired power generation unit. The control method for low-load operation of the counter-firing boiler includes:
[0054] Determine the coal type switching load and switch the coal type input to the boiler from the coal mill according to the coal type switching load;
[0055] Determine the changes in key operating parameters of the pulverizing system and the combustion system after switching coal types;
[0056] Different transition curves of key operating parameters of the pulverizing system are obtained based on the changes in key operating parameters of the pulverizing system, and different transition curves of key operating parameters of the combustion system are obtained based on the changes in key operating parameters of the combustion system.
[0057] Adjust the operating parameters of the pulverizing system according to the transition curves of the key operating parameters of different pulverizing systems, and adjust the operating parameters of the combustion system according to the transition curves of the key operating parameters of different combustion systems.
[0058] The present invention provides a control method for low-load operation of a counter-firing boiler. First, the coal type switching load of the coal-fired power generation unit under low-load conditions is determined. Based on the coal type switching load, the coal type input to the boiler of the coal-fired power generation unit is switched. After switching the coal type, the changes in key operating parameters of the boiler pulverizing system and the combustion system are detected and obtained. Different transition curves for the key operating parameters of the pulverizing system and the combustion system are generated based on these changes. Then, the key operating parameters of the pulverizing system are adjusted according to the obtained transition curves, and the key operating parameters of the combustion system are adjusted accordingly. The transition curve adjusts the key operating parameters of the combustion system. After the key operating parameters of the pulverizing system and the combustion system are adjusted, the amount of flue gas generated by combustion under low load conditions is increased due to the coal type switching. This results in a corresponding increase in the convective heat exchange between the flue gas and the reheater of the coal-fired unit, raising the temperature of the reheat steam. At the same time, by switching coal types, there is no need to reduce the number of coal mills in operation, reducing the risk of boiler flameout due to sudden coal shortages in the operating mills and improving the safety of unit operation. Since the number of operating coal mills is not reduced, the flue gas volume is large, and the furnace flame coverage is good, which greatly reduces the problem of boiler uneven burning under low load. This solves the problem of local overheating of the water-cooled wall in the existing technology of opposed-flow combustion boilers under deep peak shaving conditions during low load operation.
[0059] Key operating parameters of the pulverizing system include: the coal mill outlet air-coal mixing temperature T and the coal mill dynamic separator speed W.
[0060] The transition curves of key operating parameters of the pulverizing system include: the transition curve of the coal mill outlet air-coal mixing temperature and the transition curve of the coal mill dynamic separator speed;
[0061] In one embodiment, a 600MW coal-fired power generating unit is provided. The boiler of the coal-fired power generating unit is a supercritical parameter variable pressure operation coal-fired once-through boiler, model HG-1913 / 25.4-YM3. The boiler adopts a positive pressure direct-fired pulverizing system, and each boiler is equipped with 6 coal mills. The combustion system adopts a front and rear wall opposed combustion method, with 3 layers of burners on each front and rear wall, and 5 burners arranged on each side layer. Specifically, the coal mills on the front wall, from bottom to top, correspond to the burners F, C, and D of each layer of burners as F mill, C mill, and D mill, respectively. The coal mills on the rear wall, from bottom to top, correspond to the burners A, E, and B of each layer of burners as A mill, E mill, and B mill, respectively. Each side layer of burners is equipped with one ZGM113N type coal mill. The boiler mainly uses mixed coal from Kazakhstan and supplemented by Indonesian coal. Table 1 shows the parameter values for mixed coal from Kazakhstan and Indonesian coal. Currently, due to the high cost of transporting Indonesian coal, it is necessary to meet the demand of using more mixed coal from Kazakhstan and less Indonesian coal. However, considering the impact on safe combustion of the unit, it is proposed to burn high-calorific-value coal at high loads and low-calorific-value coal at low loads. Therefore, under low load conditions, the type of coal used is switched to solve the problem of uneven combustion in the furnace caused by the small number of coal mills in operation, which leads to local overheating of the water-cooled wall.
[0062] Table 1:
[0063]
[0064]
[0065] like Figure 1As shown, the coal type switching load is determined based on the load corresponding to the lowest output of the coal type before the coal type switching of the coal mill under low load conditions. A coal bunker 1 is set up corresponding to each coal mill in the coal-fired power generation unit. A partition 2 is installed in the coal bunker 1, dividing it into two storage spaces. These spaces store mixed coal and Indonesian coal, respectively. The outlets of the two storage spaces are connected to the inlet of the coal feeder 4 of the coal-fired power generation unit. Hydraulic gates 3 are installed at the outlets of the two storage spaces. The hydraulic gates 3 control the connection or disconnection between the outlets of the two storage spaces and the inlet of the coal feeder 4. When the hydraulic gates 3 are open, the coal stored in the storage space can enter the coal feeder 4; when the hydraulic gates 3 are closed, the connection between the outlet of the storage space and the inlet of the coal feeder 4 is cut off. Coal type switching is completed by controlling the opening and closing of the corresponding hydraulic gates 3. An anti-blocking device 5 is installed above each hydraulic gate 3 to prevent coal blockage at the outlet of the storage space. During the operation of coal-fired power generating units, the load above 300MW uses mixed coal. The number of coal mills operating for a load of 300MW is 4. When the load decreases from 300MW to a low load of 240MW, the output of the ZGM113N coal mill decreases to about 25t / h. At this time, the coal mills vibrate under the low load condition. All operating coal mills are still grinding mixed coal. When the coal mills vibrate, they are at their lowest output. The coal type switching load is determined based on the lowest output of all operating coal mills grinding mixed coal. The load corresponding to 240MW is considered to be the coal type switching load. When a coal mill vibrates, the standard procedure is to shut down one mill, leaving only three mills running to resolve the vibration. However, shutting down one mill can cause uneven burning within the furnace, ultimately leading to localized overheating of the water-cooled walls. Therefore, to avoid uneven burning while simultaneously addressing the mill vibration issue, the type of coal fed into the boiler is switched. The previously used mixed coal is replaced with Indonesian coal. The hydraulic gate 3 at the outlet of the storage space containing the Indonesian coal in coal bunker 1 is opened, allowing the Indonesian coal to enter the feeder. Simultaneously, the hydraulic gate 3 at the outlet of the storage space containing the mixed coal is closed, allowing the Indonesian coal to enter the feeder. After coal is pulverized by a coal mill, it is fed into the furnace for combustion. After switching to Indonesian coal, the calorific value of the coal decreases, the amount of coal burned in the boiler increases, and the output of the coal mill will increase. The number of coal mills in operation will not decrease further, thus avoiding coal mill vibration. At the same time, due to the operation of the multi-layer burners in the combustion system, the deviation between layers is compensated for, which greatly reduces the probability of uneven burning in the furnace and can alleviate the problem of local overheating of the water-cooled wall. In addition, by switching coal types, there is no need to reduce the number of coal mills in operation, which reduces the risk of boiler flameout due to sudden coal supply interruption of the operating mills and improves the safety of unit operation.
[0066] After switching coal types, because the mixed coal previously fed into the pulverizer has not been completely consumed, Indonesian coal will not be immediately fed into the furnace for combustion. Before the Indonesian coal enters the boiler furnace for combustion, there is still some unconsumed mixed coal. Therefore, during the combustion process, the ratio of mixed coal and Indonesian coal fed into the furnace is constantly changing, making it difficult to control the key operating parameters of the pulverizing system and the key operating parameters of the combustion system of the coal-fired power generation unit. The key operating parameters of the pulverizing system include: the pulverizer outlet air-coal mixing temperature T and the pulverizer dynamic separator speed W. The pulverizer outlet air-coal mixing temperature T is calculated as follows: T = (82 - V) daf )*5 / 3±5, where, V daf The volatile matter content is based on the dry ash-free basis of the coal. The transition curve of the pulverized coal outlet air-air mixing temperature T was obtained by calculating the change in the ratio of blended coal and Indonesian coal at the pulverizer outlet within a specified time period t after the coal type switch. The start point of the specified time period t is calculated from the start of the coal type switch, and the end point is calculated when the pulverizer has completely ground the Indonesian coal. After that, the pulverized coal outlet air-air mixing temperature T is more stable when all Indonesian coal is burned.
[0067] When the coal-fired power generating unit is operating at a load of 240MW, all four coal mills (mills A, B, C, and F) are running, burning a mixture of coal and gas. The process involves T = (82 - V) daf The mixing temperature T at the outlet of the coal mill is calculated as 5 / 3±5 when burning mixed coal. T is between 70℃ and 80℃, preferably between 70℃ and 75℃. After switching coal types, the coal type will gradually transition from mixed coal to Indonesian coal. During this period, when the coal mill is grinding Indonesian coal, the mixing temperature T at the outlet of the coal mill is <70℃. Due to the influence of high moisture content and drying capacity of the coal, the mixing temperature T at the outlet of the coal mill is preferably 60℃.
[0068] When the coal-fired power generating unit is operating at a load of 240MW, all four coal mills (Mill A, Mill B, Mill C, and Mill F) are running, burning blended coal, and the coal powder fineness is measured by R... 90 =0.5nV daf The calculated coal powder fineness for burning Shen blended coal is 18.5%. Table 2 is a comparison table of the rotation speed of the dynamic separator and the coal powder fineness for burning Shen blended coal. The calculated coal powder fineness for burning Shen blended coal is 18.5%, and the corresponding rotation speed W of the dynamic separator is 45% according to Table 2.
[0069] Table 2:
[0070] Rotation speed of dynamic separator ％ 40 45 50 <![CDATA[Coal fineness R 200 > ％ 1.46 1.16 1.32 <![CDATA[Coal fineness R 90 > ％ 20.51 19.23 17.46
[0071] After the coal type is switched, there is still unconsumed mixed coal in the coal mill. The change value of the speed W of the dynamic separator of the coal mill within a specified time period t is based on the combustion ratio of mixed coal and Indonesian coal.
[0072] The dynamic separator rotation speed W1 during the grinding of blended coal is calculated as follows:
[0073] Calculate the fineness R of pulverized coal 90 =0.5nV daf Among them, V daf The volatile matter content is based on the dry, ash-free basis of the coal, and n is the coal powder uniformity index. Based on grinding experience, the fineness R of Indonesian coal powder is... 90 The rotational speed W2 of the dynamic separator is between 30% and 40% when burning Indonesian coal, depending on the fineness of the coal powder. Preferably, the rotational speed W2 of the dynamic separator is 40% when burning Indonesian coal.
[0074] The speed W of the dynamic separator of the coal mill is determined based on the speed of the dynamic separator of the coal mill for the coal type before the grinding switch and the speed of the dynamic separator of the coal mill for the coal type after the grinding switch.
[0075] The speed of the dynamic separator of the coal mill, W, is determined based on the speed of W1 of the dynamic separator of the coal mill when grinding mixed coal and the speed of W2 of the dynamic separator of the coal mill when grinding Indonesian coal.
[0076] The transition curve of the dynamic separator speed of the coal mill is obtained by measuring the change value of the speed W of the dynamic separator of the coal mill within a specified time period t after the coal type is switched.
[0077] Due to the significant difference in moisture content between Indonesian coal and blended coal, the outlet air-coal mixing temperature of the coal mill varies greatly under the same operating conditions (i.e., the same inlet air volume and temperature). Taking the aforementioned coal-fired power plant as an example, when grinding only blended coal, the outlet air-coal mixing temperature is 72.5℃. When the coal type is switched to Indonesian coal, and the inlet air volume and temperature of the coal mill are not adjusted, the outlet air-coal mixing temperature changes rapidly after approximately 60 seconds, and then stabilizes at 54℃ after 480 seconds. Based on the outlet temperature change curve, an actual outlet temperature transition curve is plotted. This curve represents the transition temperature before adjusting the outlet air-coal mixing temperature, while the actual outlet temperature transition curve represents the transition temperature after adjusting the outlet air-coal mixing temperature.
[0078] After obtaining the transition curves of key operating parameters for different pulverizing systems, the key operating parameters of the pulverizing system are adjusted according to these transition curves, including:
[0079] The coal mill outlet air-coal mixing temperature is adjusted according to the transition curve of the coal mill outlet air-coal mixing temperature; and the speed of the coal mill dynamic separator is adjusted according to the transition curve of the coal mill dynamic separator speed.
[0080] The opening of the hot and cold air dampers of the coal mill is logically controlled based on the transition curve of the coal mill outlet air-coal mixing temperature, thereby adjusting the coal mill outlet air-coal mixing temperature. Meanwhile, the speed of the dynamic separator of the coal mill is automatically adjusted according to the transition curve of the speed of the dynamic separator of the coal mill.
[0081] To prevent localized overheating of the water-cooled wall, key operating parameters of the combustion system are adjusted. These key operating parameters include: boiler oxygen supply, primary air velocity supplied to the boiler, secondary air damper opening, and burnout air damper opening.
[0082] The transition curves for the key operating parameters of the combustion system include: the oxygen content transition curve, the primary air velocity transition curve, the secondary air damper opening transition curve, and the burnout air damper opening transition curve.
[0083] Based on the changes in the key operating parameters of the combustion system, transition curves for different key operating parameters of the combustion system are obtained, including:
[0084] The operating oxygen content transition curve is obtained based on the change in the boiler operating oxygen content within a specified time period t.
[0085] The primary air velocity transition curve is obtained based on the change value of the primary air velocity fed into the boiler within a specified time period t.
[0086] The transition curve of the secondary air damper opening is obtained based on the change value of the secondary air damper opening within a specified time period t.
[0087] The transition curve of the burnout air damper opening is obtained by measuring the change in the opening of the burnout air damper within a specified time period t.
[0088] Regarding boiler operating oxygen content, when using blended coal and the coal-fired power unit is operating at a 240MW load, all four coal mills (A, B, C, and F) are running. Maintaining the pulverizing system and secondary air distribution method unchanged, the boiler operating oxygen content is varied to analyze its impact on boiler combustion efficiency, economizer outlet nitrogen oxide emission concentration, and reheat steam temperature. The experiment adjusted the average economizer outlet oxygen content to 5.5% and 6.1%, corresponding to boiler thermal efficiencies of 93.13% and 93.02%, average reheat steam temperatures of 565.3℃ and 565.6℃, and economizer outlet nitrogen oxide emission concentrations of 251 mg / m³ and 283 mg / m³, respectively. The changes in economizer outlet oxygen content and water-cooled wall temperature were relatively small. Considering all factors, it is recommended that the average economizer outlet oxygen content be 5.5% when using blended coal.
[0089] When using Indonesian coal, the pulverizing system and secondary air distribution method are kept unchanged. The boiler operating oxygen content is varied to analyze the impact of changes in boiler operating oxygen content on boiler combustion efficiency, economizer outlet nitrogen oxide emission concentration, and reheat steam temperature. The experiment recommends controlling the economizer outlet oxygen content at 5.5%. However, due to changes in the calorific value and quantity of coal after coal type switching, the total air volume required for combustion must also change. When switching from mixed coal to Indonesian coal, the total air volume needs to be increased in advance. Based on the fact that the combustion ratio of mixed coal and Indonesian coal changes continuously after coal type switching, the boiler operating oxygen content changes with the combustion ratio of mixed coal and Indonesian coal. The change in boiler operating oxygen content over a specified time period t is used to obtain the operating oxygen content transition curve.
[0090] Regarding primary air velocity, when using mixed coal and the coal-fired power unit is operating at a 240MW load, all four coal mills (Mills A, B, C, and F) are running. Maintaining constant boiler oxygen levels and secondary air distribution, the primary air velocity at the mill outlet pulverized coal tubes is varied to analyze its impact on boiler combustion efficiency, economizer outlet nitrogen oxide emission concentration, and reheat steam temperature. The experiment adjusted the average primary air velocity to 19 m / s and 21 m / s. The corresponding boiler thermal efficiencies were 93.13% and 93.06%, the average reheat steam temperatures were 565.3℃ and 565.7℃, and the economizer outlet nitrogen oxide emission concentrations were 251 mg / m³ and 279 mg / m³, respectively. The economizer outlet oxygen levels and water-cooled wall temperatures showed relatively small changes. Considering all factors, a primary air velocity of 19 m / s is recommended when using mixed coal.
[0091] When using Indonesian coal, determining the primary air velocity fed into the boiler involves maintaining the boiler's operating oxygen level and secondary air distribution method constant, while changing the primary air velocity in the pulverized coal outlet pipe. The impact of primary air velocity changes on boiler combustion efficiency, economizer outlet nitrogen oxide emission concentration, and reheat steam temperature is analyzed. Due to the high moisture and volatile matter content of this coal, the primary air velocity needs to be increased. The average pulverized coal outlet pipe velocity is controlled between 21 m / s and 23 m / s, with 21 m / s recommended from experimental values. After switching coal types, the combustion ratio of the blended coal and Indonesian coal continuously changes. A primary air velocity transition curve is obtained based on the change in primary air velocity in the pulverized coal outlet pipe over a specified time period t.
[0092] Regarding the opening of the secondary air dampers, when using mixed coal and the coal-fired power unit is operating at a 240MW load, all four coal mills (A, B, C, and F) are running. Maintaining constant oxygen levels and primary air velocity, the opening of the secondary air dampers is varied to analyze the impact of these changes on boiler combustion efficiency, economizer outlet nitrogen oxide emission concentration, and reheat steam temperature. The secondary air damper openings were adjusted in the following conditions: Condition 1: 60% opening for the secondary air box dampers corresponding to mills A and F; 40% opening for mills B and C; and 20% opening for the secondary air box dampers corresponding to the other non-operating mills. Condition 2: 40% opening for the secondary air box dampers corresponding to mills A and F; 30% opening for mills B and C; and 20% opening for the secondary air box dampers corresponding to the other non-operating mills. Condition 3: [The text abruptly ends here, likely due to an incomplete sentence or missing information.] With the secondary air damper opening at 70%, the opening of the secondary air box dampers for mills B and C is 60%, and the opening of the secondary air box dampers for other non-operating mills is 20%. Under these conditions, the boiler thermal efficiencies for conditions one through three are 93.13%, 93.04%, and 93.12%, respectively, with corresponding average reheat steam temperatures of 565.3℃, 565.5℃, and 564.9℃, and economizer outlet nitrogen oxide emission concentrations of 251 mg / m³, 247 mg / m³, and 277 mg / m³, respectively. The economizer outlet oxygen content and water-cooled wall temperature deviations are relatively small. Considering all factors, it is recommended that when using a blended coal type, the secondary air damper openings are as follows: 60% for mills A and F, 40% for mills B and C, and 20% for other non-operating mills.
[0093] When burning Indonesian coal, due to the high proportion of primary air and low proportion of secondary air, the corresponding secondary air damper opening is lower than when burning mixed coal. When the coal-fired power unit is operating at a 240MW load, all four coal mills (A, B, C, and F) are running. Maintaining constant oxygen levels and primary air velocity, the secondary air damper opening is varied to analyze its impact on boiler combustion efficiency, economizer outlet nitrogen oxide emission concentration, and reheat steam temperature. The secondary air damper opening is adjusted as follows: 50% for mills A and F, 30% for mills B and C, and 20% for other non-operating mills. After switching coal types, the combustion ratio of mixed coal and Indonesian coal continuously changes. The change in secondary air damper opening over a specified time period t is used to obtain the secondary air damper opening transition curve.
[0094] Regarding the opening of the burnout air damper, when using mixed coal and the coal-fired power unit is operating at a load of 240MW, all four coal mills (A, B, C, and F) are running. The oxygen content and secondary air distribution method are kept constant. The opening of the burnout air damper is changed to analyze the impact on boiler combustion efficiency, economizer outlet nitrogen oxide emission concentration, and reheat steam temperature. The burnout air damper opening was adjusted to 35%, 20%, and 50%. The corresponding boiler thermal efficiencies were 93.13%, 93.14%, and 93.08%, respectively; the corresponding average reheat steam temperatures were 565.3℃, 564.8℃, and 565.5℃, respectively; and the economizer outlet nitrogen oxide emission concentrations were 251 mg / m³, 271 mg / m³, and 244 mg / m³, respectively. The economizer outlet oxygen content and water-cooled wall temperature deviations are relatively small. Taking all factors into consideration, it is recommended that the burnout air damper opening be controlled at 35% when using mixed coal.
[0095] When using Indonesian coal, due to its high volatile matter and low nitrogen oxide emissions, the coal-fired power unit was operating at a 240MW load. All four mills (A, B, C, and F) were running, maintaining constant oxygen levels and secondary air distribution. The burnout air damper opening was varied to analyze its impact on boiler combustion efficiency, economizer outlet nitrogen oxide emission concentration, and reheat steam temperature. Because of the high volatile matter and low nitrogen oxide emissions of this coal, the experiment recommended controlling the burnout air damper opening at 20%. After coal type switching, the combustion ratio of the blended coal and Indonesian coal changed. The burnout air damper opening was varied over a specified time period t to obtain a transition curve.
[0096] Adjust the key operating parameters of the combustion system according to the transition curves of different key operating parameters of the combustion system, including:
[0097] Adjust the boiler's operating oxygen level according to the operating oxygen level transition curve;
[0098] Adjust the primary wind speed according to the primary wind speed transition curve;
[0099] Adjust the opening of the secondary air damper according to the secondary air damper opening transition curve;
[0100] Adjust the opening of the burnout air damper according to the burnout air damper opening transition curve.
[0101] After obtaining the transition curves of different key operating parameters of the combustion system, the opening of the blower blades is adjusted according to the oxygen content transition curve, thereby achieving the purpose of adjusting the oxygen content of the boiler.
[0102] The primary wind speed is adjusted by adjusting the opening of the primary wind turbine blades according to the primary wind speed transition curve. The corresponding average wind speed of the coal mill pulverizer pipe changes accordingly. Based on experience, it is ideal to control the average wind speed of the coal mill pulverizer pipe between 21 m / s and 23 m / s. The experimental value recommended is 21 m / s.
[0103] The opening of the secondary air damper corresponding to the coal mill is adjusted according to the secondary air damper opening transition curve. The opening of the secondary air box damper corresponding to mill A and mill F is adjusted to 50%, the opening of the secondary air box damper corresponding to mill B and mill C is adjusted to 30%, and the opening of the secondary air box damper corresponding to other non-operational coal mills is adjusted to 20%. The air distribution method of the secondary air is also adjusted by adjusting the opening of the secondary air damper.
[0104] The opening of the burnout air damper is adjusted according to the transition curve of the burnout air damper opening. Since this type of coal has high volatile matter and low nitrogen oxide emissions, the recommended burnout air damper opening is controlled at 20% when determining this type of coal. By controlling the opening of the burnout air damper, the air volume of burnout air sent into the combustion system is adjusted.
[0105] After switching coal types, the temperature T of the coal mill outlet air-coal mixture becomes more sensitive. The trends of the subsequent transition curves of the coal mill dynamic separator speed, operating oxygen content, primary air velocity, secondary air damper opening, and burnout air damper opening are all similar to the trend of the coal mill outlet air-coal mixture temperature transition curve.
[0106] According to the control method of low-load operation of the counter-firing boiler, the coal type switching process is simple. It is compared with two operating conditions. In operating condition 1, under the coal type switching load of 240MW, all four coal mills are running and burning Indonesian coal. In operating condition 2, instead of switching the coal type, the number of coal mills is reduced from four to three, and mixed coal is used. Table 3 is the combustion deviation index table for operating conditions 1 and 2.
[0107] Table 3:
[0108] project unit Operating Condition 1 Operating Condition 2 grinding group method / ABCF ACF Final stage superheater outlet steam temperature A / B ℃ 570.4 / 569.9 571.0 / 570.3 Final stage reheater outlet steam temperature A / B ℃ 566.7 / 564.4 567.4 / 559.5 Primary superheated desuperheating water flow rate A / B t / h 6.23 / 7.84 12.88 / 0.3 Secondary superheated desuperheating water flow rate A / B t / h 6.98 / 2.70 10.05 / 3.52 Total amount of superheated desuperheating water t / h 23.70 26.8 Reheater desuperheating water flow rate A / B t / h 0 / 0 2.1 / 0 Total amount of reheat cooling water t / h 0 0
[0109] From Table 3, Figure 3 and Figure 4 As is known, after switching coal types, when the four coal mills are running, due to the lower calorific value of the coal, the larger coal quantity, the larger flue gas volume, and the better flame filling in the furnace, the deviation of the steam temperature at the outlet of the final reheater is significantly smaller than when the three coal mills are running, and the average steam temperature is higher. When the four coal mills are running, the oxygen distribution at the economizer outlet is significantly more uniform than when the three coal mills are running. When the four coal mills are running, the deviation between the highest and lowest wall temperatures of the vertical water-cooled wall of the boiler front wall is 10.1℃, while when the three coal mills are running, the deviation is 17.1℃. Compared with the three coal mills, the deviation between the maximum and minimum wall temperatures of the vertical water-cooled wall of the boiler front wall is smaller when the four coal mills are running, and the distribution is more uniform, which can better avoid local overheating of the water-cooled wall.
[0110] The present invention provides a control method for low-load operation of a counter-firing boiler. When the boiler is operating at low load, the coal type switching load is first determined. The coal mill switches the coal type fed into the boiler according to the coal type switching load. Then, the changes in key operating parameters of the pulverizing system and the combustion system after the coal type switch are obtained. Transition curves for different key operating parameters of the pulverizing system and the combustion system are obtained based on the changes in the key operating parameters of the pulverizing system. The key operating parameters of the pulverizing system are then adjusted according to the transition curves of the key operating parameters of the pulverizing system, and the key operating parameters of the combustion system are adjusted according to the transition curves of the key operating parameters of the combustion system. By adjusting the operating parameters, the key operating parameters of the pulverizing system and the combustion system were improved, increasing the amount of flue gas generated under low load conditions. This, in turn, increased the convective heat exchange between the flue gas and the reheater in the coal-fired power plant, raising the temperature of the reheat steam. Simultaneously, by switching coal types, the number of operating coal mills was not reduced, lowering the risk of boiler flameout due to sudden coal shortages and improving the safety of unit operation. Because the number of operating coal mills remained unchanged, the amount of flue gas generated by boiler combustion increased, resulting in better flame coverage in the boiler furnace. This significantly reduced the problem of occasional uneven combustion under low load conditions and solved the problem of localized overheating of the water-cooled walls in existing counter-firing boilers during low-load operation under deep peak shaving conditions.
[0111] The optional embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the embodiments of the present invention are not limited to the specific details in the above embodiments. Within the scope of the technical concept of the embodiments of the present invention, various simple modifications can be made to the technical solutions of the embodiments of the present invention, and these simple modifications all fall within the protection scope of the embodiments of the present invention.
[0112] It should also be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the embodiments of the present invention will not describe the various possible combinations separately.
[0113] Furthermore, various different implementations of the present invention can be combined arbitrarily, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed in the present invention.
Claims
1. A control method for low-load operation of a counter-firing boiler, applied to a coal-fired power generation unit, characterized in that, The control method for low-load operation of the opposed-firing boiler includes: Determine the coal type switching load and switch the coal type of the coal mill to the boiler according to the coal type switching load. The coal type switching load is determined according to the load corresponding to the coal type of the coal mill that is put into operation under low load conditions when the coal type before the grinding switch is at its lowest output. The changes in key operating parameters of the pulverizing system and the combustion system after switching coal types were determined. The key operating parameters of the pulverizing system include: the coal mill outlet air-coal mixing temperature T and the coal mill dynamic separator speed W; the key operating parameters of the combustion system include: boiler operating oxygen content, primary air velocity fed into the boiler, secondary air damper opening, and burnout air damper opening. The transition curves of different key operating parameters of the pulverizing system are obtained based on the changes in the key operating parameters of the pulverizing system, and the transition curves of different key operating parameters of the combustion system are obtained based on the changes in the key operating parameters of the combustion system. The key operating parameters of the pulverizing system are adjusted according to the transition curves of the key operating parameters of different pulverizing systems, and the key operating parameters of the combustion system are adjusted according to the transition curves of the key operating parameters of different combustion systems.
2. The control method for low-load operation of a counter-firing boiler according to claim 1, characterized in that, The coal mill outlet air-coal mixing temperature T is calculated as follows: T=(82-V daf ) 5 / 3±5; Among them, V daf It is a dry, ash-free volatile matter-based coal; the coal types include mixed coal and Indonesian coal.
3. The control method for low-load operation of a counter-firing boiler according to claim 1, characterized in that, Determining the rotational speed W of the dynamic separator in the coal mill includes: The speed W of the dynamic separator of the coal mill is determined based on the speed of the dynamic separator of the coal mill for the coal type before the grinding switch and the speed of the dynamic separator of the coal mill for the coal type after the grinding switch.
4. The control method for low-load operation of a counter-firing boiler according to claim 1, characterized in that, The transition curves of the key operating parameters of the pulverizing system include: the transition curve of the coal mill outlet air-coal mixing temperature and the transition curve of the coal mill dynamic separator speed. Transition curves for different key operating parameters of the pulverizing system are obtained based on the changes in these parameters, including: The transition curve of the coal mill outlet air-coal mixing temperature T is obtained based on the change value of the coal mill outlet air-coal mixing temperature T within a specified time period t. The transition curve of the dynamic separator speed of the coal mill is obtained by measuring the change value of the speed W of the dynamic separator of the coal mill within a specified time period t.
5. The control method for low-load operation of a counter-firing boiler according to claim 4, characterized in that, Adjust the key operating parameters of the pulverizing system according to the transition curves of different key operating parameters, including: The coal mill outlet air-coal mixing temperature is adjusted according to the transition curve of the coal mill outlet air-coal mixing temperature. The speed of the dynamic separator of the coal mill is adjusted according to the speed transition curve of the dynamic separator of the coal mill.
6. The control method for low-load operation of a counter-firing boiler according to claim 4, characterized in that, The transition curves for the key operating parameters of the combustion system include: the oxygen content transition curve, the primary air velocity transition curve, the secondary air damper opening transition curve, and the burnout air damper opening transition curve. Based on the changes in the key operating parameters of the combustion system, transition curves for different key operating parameters of the combustion system are obtained, including: The operating oxygen content transition curve is obtained based on the change in the boiler operating oxygen content within a specified time period t. The primary air velocity transition curve is obtained based on the change value of the primary air velocity fed into the boiler within a specified time period t. The transition curve of the secondary air damper opening is obtained based on the change value of the secondary air damper opening within a specified time period t; The transition curve of the burnout air damper opening is obtained based on the change value of the burnout air damper opening within a specified time period t.
7. The control method for low-load operation of a counter-firing boiler according to claim 6, characterized in that, Adjust the key operating parameters of the combustion system according to the transition curves of different key operating parameters of the combustion system, including: Adjust the boiler's operating oxygen level according to the operating oxygen level transition curve; Adjust the primary wind speed according to the primary wind speed transition curve; Adjust the opening of the secondary air damper according to the secondary air damper opening transition curve; Adjust the opening of the burnout air damper according to the burnout air damper opening transition curve.