JP7520587B2 - Once-through boiler operation control device, operation control method, and once-through boiler - Google Patents

Description

The present invention relates to an operation control device and an operation control method for a once-through boiler, and in particular to a once-through boiler that is composed of heat transfer tubes, forcibly circulates water from one end, repeatedly heats and evaporates it, and extracts superheated steam from the other end, and aims to improve the operability of load changes during once-through operation.

A once-through boiler has many heat transfer tubes arranged in the wall of a furnace, and water is supplied to the heat transfer tubes. A burner is used to generate a flame inside the furnace, and the water inside the heat transfer tubes is heated by the flame to generate steam. This superheated steam is then extracted and supplied to a steam turbine, for example, to generate electricity.

In such a once-through boiler (hereafter abbreviated as "boiler"), a given amount of water is usually supplied to the boiler's heat transfer tubes by a feedwater pump, and the entire amount of this water is heated in the boiler to become steam, which is then taken out as superheated steam (once-through operation). However, when the amount of heat in the boiler is low, such as immediately after the boiler is started up or during low load, the boiler is unable to turn all of the water into steam, and the circulating water separated by the steam separator is returned to the feedwater line (circulating operation). In this case, returning the circulating water to the feedwater line means that the heat contained in the water heated in the boiler is wasted, which is not thermally efficient. In addition, a separate pump is required to return the circulating water to the feedwater line, which increases equipment and power costs. For this reason, it is desirable to set the amount of water supply to correspond to the amount of heat generated by the boiler.

The inventors have therefore devised a method for controlling a once-through boiler, as described in Patent Document 1, which reduces the amount of circulating water, thereby improving thermal efficiency, reducing equipment costs and power costs, and maintaining the integrity of the furnace walls, by setting in advance an upper limit value for the quality at which the nucleate boiling state of the water in the heat transfer tubes can be maintained, setting a lower limit value for the amount of water supplied to the heat transfer tubes that corresponds to this upper limit value of the quality, and adjusting the amount of water supplied to the heat transfer tubes to this lower limit value when the boiler is started.

Patent No. 5766527

In recent years, with the introduction of renewable energy, there is a demand to operate once-through boilers at lower loads.

As shown in the conventional example in Figure 4, once-through boilers generally switch between once-through and circulating operation at a load of approximately 25-30% (switching load: corresponding to point Lc0 in Figure 4), but the once-through load at which stable continuous operation is possible is approximately 30% or more. The low-load side is in circulating operation, and the high-load side is in once-through operation.

When lowering the load from high load operation and passing the switching load, it is necessary to switch from once-through operation to circulation operation, but several steps are required for operation, which takes more time and effort than lowering the load to the switching load.

If the switching load, which is generally said to be 25 to 30%, could be further reduced, the load could be reduced more easily and in a shorter time, greatly improving the operability of the once-through boiler.

However, when reducing the load in once-through operation, if the flow rate is reduced too much compared to the thermal load, the heat transfer tubes may overheat (film boiling occurs) and be damaged, so it is important to control the flow rate to an appropriate level compared to the thermal load.

In Patent Document 1, the minimum water supply amount required to maintain the nucleate boiling state of water in the heat transfer tube is calculated for areas with large heat flux, mainly during boiler startup (circulation operation), and the soundness of the heat transfer tube is maintained. However, when the load changes during once-through operation, overheating is possible depending on the constraints, even in areas with small heat flux, so the soundness of the entire heat transfer tube must be evaluated.

The present invention aims to provide an operation control device and method that can reduce the amount of water feed when the load changes during once-through operation or lower the lower limit of the load band while maintaining the overall integrity of the heat transfer tubes of a once-through boiler.

To achieve the above object, the present invention provides an operation control device and method for a once-through boiler that performs once-through operation to supply water to the heat transfer tubes of the boiler and extract superheated steam generated by heat exchange with an internal heat source, and the operation control device divides the heat transfer tubes into multiple regions and applies different constraints to each region to calculate the minimum feedwater amount that can be maintained while the once-through operation is maintained, selects the minimum feedwater amount with the highest feedwater amount from the multiple calculated minimum feedwater amounts as the allowable feedwater amount, and adjusts the amount of water supplied to the heat transfer tubes during once-through operation so as to satisfy the allowable feedwater amount.

By dividing the heat transfer tube into multiple regions and calculating the minimum water supply amount that can maintain once-through operation in a different manner for each region, it is possible to correctly evaluate the health of the heat transfer tube for each region in an appropriate manner, even if the cause (phenomenon) of the heat transfer tube overheating changes for each region when the load changes. In addition, by adjusting the amount of water supply to the heat transfer tube during once-through operation at the allowable water supply amount, which is the highest minimum water supply amount for all regions, it is possible to reduce the amount of water supply while maintaining the health of the entire heat transfer tube.

The present invention also relates to an operation control device and method for a once-through boiler, in which the once-through boiler performs either a circulation operation in which circulating water separated by a steam-water separator connected downstream of the heat transfer tube is fed to the upstream of the heat transfer tube, or the once-through operation, and calculates a minimum load at which the minimum water feed rate that can be maintained for once-through operation cannot be calculated by applying different constraints to each region of the heat transfer tube, and selects the highest minimum load from among the multiple calculated minimum loads as a switchover load, and performs control for circulation operation below the switchover load, and performs control for once-through operation above the switchover load.

The minimum load at which the minimum water supply flow rate cannot be calculated is calculated using a different method for each region, and the highest minimum load among these, the switching load, is used to lower the switching load and expand the load range in which flow-through operation is possible.

The present invention also relates to a once-through boiler equipped with the above-mentioned operation control device.

According to the present invention, it is possible to reduce the amount of water fed during load changes in once-through operation or lower the lower limit of the load band while maintaining the overall integrity of the heat transfer tubes of the once-through boiler. Objectives, configurations, and effects other than those described above will be made clear in the following embodiments.

1 is a schematic diagram of an operation control device for a once-through boiler. FIG. 1 is a schematic diagram of a once-through boiler. 1 is a graph showing the height distribution of the metal temperature of a heat transfer tube. 1 is a graph showing a boiler feedwater amount versus a boiler load. 1 is a graph of steam mass velocity versus steam quality. FIG. 2 is a schematic diagram of a heat flux measuring sensor. 1 is a flowchart showing the flow of an operation control method when switching from once-through operation to circulation operation. FIG. 2 is a diagram illustrating a hardware configuration of an operation control device.

The preferred embodiment of the present invention will be described in detail below with reference to the attached drawings. The same components are given the same reference numerals throughout the drawings, and duplicate explanations will be omitted. Note that the present invention is not limited to this embodiment, and when there are multiple embodiments, it also includes configurations that combine the respective embodiments.

Figure 1 is a schematic diagram of an operation control device 70 for a once-through boiler 10 according to an embodiment of the present invention, and Figure 2 is a schematic diagram of the once-through boiler 10.

As shown in Figures 1 and 2, the once-through boiler 10 is a pulverized coal-fired boiler that uses, for example, pulverized coal made from crushed coal as pulverized fuel, burns the pulverized coal in a combustion burner, and is capable of recovering the heat generated by this combustion.

This once-through boiler 10 is a conventional boiler, and has a furnace 11 and a combustion device 12. The furnace 11 has a hollow rectangular cylinder shape and is installed vertically, and the combustion device 12 is provided at the bottom of the furnace wall that constitutes this furnace 11. The furnace 11 is sealed by the furnace wall formed by a number of heat transfer tubes 53 (not shown).

The combustion device 12 has multiple combustion burners 21, 22, 23, 24, and 25 attached to the furnace wall. In this embodiment, the combustion burners 21, 22, 23, 24, and 25 are arranged in five sets, i.e., five tiers, along the vertical direction, with four burners arranged at equal intervals along the circumferential direction as one set.

The combustion burners 21, 22, 23, 24, 25 are connected to the coal pulverizers (mills) 31, 32, 33, 34, 35 via the pulverized coal supply pipes 26, 27, 28, 29, 30. The coal pulverizers 31, 32, 33, 34, 35 are configured such that a grinding table is supported in a housing with a vertical axis of rotation (not shown) so that it can be driven and rotated, and a number of grinding rollers are supported facing the upper part of the grinding table so that they can rotate in conjunction with the rotation of the grinding table. Therefore, when coal is fed between the multiple grinding rollers and the grinding table, it is pulverized to a predetermined size here, and the pulverized coal classified by the conveying air (primary air) can be supplied to the combustion burners 21, 22, 23, 24, 25 from the pulverized coal supply pipes 26, 27, 28, 29, 30.

Furnace 11 is also provided with a wind box 36 at the mounting position of each combustion burner 21, 22, 23, 24, 25, with one end of an air duct 37 connected to this wind box 36, and a blower 38 attached to the other end of this air duct 37. Therefore, the combustion air (secondary air) sent by the blower 38 can be supplied from the air duct 37 to the wind box 36, and from this wind box 36 to each combustion burner 21, 22, 23, 24, 25.

Therefore, in the combustion device 12, each combustion burner 21, 22, 23, 24, and 25 can inject a pulverized fuel mixture made by mixing pulverized coal and primary air into the furnace 11, and can also inject secondary air into the furnace 11. A flame can be formed by igniting the pulverized fuel mixture with an ignition torch (not shown).

Generally, when the boiler is started, each combustion burner 21, 22, 23, 24, and 25 injects oil fuel into the furnace 11 to form a flame.

The furnace 11 is connected to a flue 40 at the top, and this flue 40 is equipped with superheaters 41, 42, reheaters 43, 44, and economizers 45, 46, 47 that function as convection heat transfer sections to recover heat from the exhaust gas, and heat is exchanged between the exhaust gas generated by combustion in the furnace 11 and water.

The flue (exhaust gas passage) 40 is connected to an exhaust gas pipe 48 downstream, through which the exhaust gas that has undergone heat exchange is discharged. An air heater 49 is provided between the exhaust gas pipe 48 and the air duct 37, and heat is exchanged between the air flowing through the air duct 37 and the exhaust gas flowing through the exhaust gas pipe 48, thereby raising the temperature of the combustion air supplied to the combustion burners 21, 22, 23, 24, and 25.

Although not shown, the exhaust gas pipe 48 is equipped with a denitration device, an electric dust collector, an induced draft fan, and a desulfurization device, and has a chimney at the downstream end.

Therefore, when the coal pulverizers 31, 32, 33, 34, and 35 are driven, the generated pulverized coal is supplied together with the transport air through the pulverized coal supply pipes 26, 27, 28, 29, and 30 to the combustion burners 21, 22, 23, 24, and 25. In addition, heated combustion air is supplied from the air duct 37 to each combustion burner 21, 22, 23, 24, and 25 through the wind box 36. Then, the combustion burners 21, 22, 23, 24, and 25 blow a pulverized fuel mixture of pulverized coal and transport air into the furnace 11 and also blow the combustion air into the furnace 11, which is ignited at this time to form a flame. In this furnace 11, the pulverized fuel mixture and combustion air are combusted to generate a flame. When a flame is generated in the lower part of the furnace 11, the combustion gas (exhaust gas) rises inside the furnace 11 and is discharged into the flue 40.

At this time, the water supplied from the feedwater pump 52 is preheated by the economizers 45, 46, and 47, and then supplied to the heat transfer tubes 53 that make up the furnace wall, where it is heated and turned into steam. The steam is then introduced into the superheaters 41 and 42, where it is superheated by the combustion gas. The superheated steam generated in the superheaters 41 and 42 is supplied to the power plant (for example, the steam turbine 56). In addition, the steam extracted during the expansion process in the steam turbine 56 is introduced into the reheaters 43 and 44, where it is superheated again and returned to the turbine.

Then, the exhaust gas that has passed through the economizers 45, 46, and 47 in the flue 40 is sent to the exhaust gas pipe 48, where a denitrification device (not shown) uses a catalyst to remove harmful substances such as NOx, an electric dust collector removes particulate matter, and a desulfurization device removes sulfur, before the gas is discharged into the atmosphere from the chimney.

Here, the flow of water and steam in the once-through boiler 10 will be described. As shown in FIG. 1, the feedwater line 51 is equipped with a feedwater pump 52, and the downstream part is connected to the economizer 45 (46, 47), which is connected to a heat transfer tube 53 constituting the furnace wall of the furnace 11. The downstream part of the heat transfer tube 53 is connected to a steam separator 54, which is connected to a steam turbine 56 and a turbine bypass valve 57 via a steam line 55. The steam turbine 56 is connected to a condenser 59 by a discharge line 58, and the condenser 59 is connected to the upstream part of the feedwater line 51. The steam separator 54 is connected to the feedwater line 51 downstream of the feedwater pump 52 by a recirculation line (recirculation path) 60. A drain tank 61 and a recirculation water pump 62 are attached to the recirculation line 60.

Therefore, when the feedwater pump 52 is driven, a predetermined amount of water is heated in the economizer 45 from the feedwater line 51 and then supplied to the heat transfer tube 53, where it is heated by heat exchange in the once-through boiler 10 to generate steam. This steam is separated into steam and moisture in the steam separator 54, and the superheated steam is supplied to the steam turbine 56 via the steam line 55, which drives the steam turbine 56 to generate electricity. The steam that has done work in the steam turbine 56 is then sent to the condenser 59 via the exhaust line 58, where it is cooled and condensed into water, which is returned to the feedwater line 51. Meanwhile, the moisture separated from the superheated steam in the steam separator 54 is temporarily stored in the drain tank 61 via the recirculation line 60, and is returned from this drain tank 61 to the feedwater line 51 by the recirculation water pump 62.

In the once-through boiler 10 configured in this manner, the heat transfer tube 53 is divided into multiple regions, and the minimum water feed rate at which once-through operation can be maintained is calculated for each region in a different manner. During once-through operation, the water feed rate to the heat transfer tube 53 is adjusted so that the allowable water feed rate, which is the highest of these minimum water feed rates, can be secured.

Furthermore, the minimum load at which the minimum water supply rate that can be maintained with once-through operation cannot be calculated is calculated using a different method for each area, and switching between once-through operation and circulation operation is performed at the switching load, which is the highest minimum load among these.

To perform these controls, a water supply amount sensor 65 that measures the amount of water supplied to the heat transfer tube 53, a water supply pressure sensor 67 that measures the water supply pressure, a heat flux sensor 68 that measures the heat flux of the furnace 11, and a control device (pump control unit) 70 that calculates the allowable water supply amount based on the water supply amount, water supply pressure, and heat flux, and adjusts the amount of water supplied to the heat transfer tube 53 by the water supply pump 52 so that this allowable water supply amount is achieved are provided. Here, the water supply pressure sensor 67 and the heat flux sensor 68 can be placed in any position, and for example, the water supply pressure sensor 67 may be placed in the steam-water separator 54. The heat flux sensor 68 may be placed in another position, and may not be placed if the heat flux distribution can be grasped by a calculated value as described below.

In addition, assuming circulation operation, the operation control device 70 is also able to take into account the amount of recirculated water from the recirculated water amount sensor 66, which measures the amount of recirculated water that is separated from the steam (wet steam) and returned to the water supply line 51.

The reason for these controls will be explained using Figure 3. Figure 3 is a graph of the height distribution of the metal temperature of the heat transfer tube 53.

To lower the lower limit of the load change band (switching load) during once-through operation, it is necessary to measure or calculate the heat flux on the heat transfer tubes 53 of the furnace wall and control it to the minimum allowable water supply amount (allowable water supply amount). The allowable water supply amount is the amount of water supply that prevents the temperature of the heat transfer tubes 53 from exceeding the allowable value. This allowable water supply amount changes depending on the heat flux and water supply pressure acting on the furnace wall, so it must be determined based on measurements and calculations during operation.

The allowable water supply amount is determined so as to simultaneously satisfy different constraints in the following two areas:

The first region is equal to the height range of the furnace 11 and has a large heat flux, and the constraint is to suppress film boiling that occurs. The second region is equal to the height range of the flue 40 at the top of the furnace 11 and has a high quality (dryness fraction, the proportion of steam in the fluid) during once-through operation, and the constraint is to keep the metal temperature after dryout that occurs in this second region below the allowable value. Dryout refers to when all the fluid inside the heat transfer tube 53 becomes steam.

The procedure for evaluating the first minimum water supply amount determined by the constraints of the first domain is as follows.
(1-1) Based on temperature data measured by the heat flux sensor 68 installed on the furnace wall, the heat flux distribution on the furnace wall surface is calculated.
(1-2) The feed water pressure is measured by the feed water pressure sensor 67 at an arbitrary location in the water system of the once-through boiler 10, and the fluid pressure in the furnace wall tube is calculated from that.
(1-3) A first minimum feedwater amount that does not cause film boiling is calculated from the heat flux calculated in (1-1) and the feedwater pressure calculated in (1-2). Alternatively, a database of minimum feedwater amounts according to heat flux and feedwater pressure is prepared in advance, and the first minimum feedwater amount is determined during operation according to the operating state.

The procedure for evaluating the second minimum water supply amount determined by the constraints in the second domain is as follows.
(2-1) Based on the temperature data measured by the heat flux sensor 68 installed on the furnace wall, the heat flux distribution on the furnace wall surface and the feed water pressure are calculated.
(2-2) From the calculated heat flux and water supply pressure, the metal temperature of the heat transfer tube 53 after drying out is calculated according to the water supply amount. Generally, as the water supply amount decreases, the metal temperature after drying out increases, so there is a lower limit to the water supply amount to keep the metal temperature below the allowable value. This is the second minimum water supply amount determined by the constraints of the second region.

By controlling the water supply rate to the furnace wall so that the larger of the first and second minimum water supply rates (allowable water supply rate) is ensured, as shown in the solid line in Figure 3, the metal temperature of the heat transfer tube 53 can be kept below the allowable value. This makes it possible to reduce the water supply rate while maintaining once-through operation, appropriately lowering the switching load, and minimizing the time and operations required to switch between circulation and once-through operation.

If the first minimum water supply amount cannot be secured, the metal temperature will rise sharply in the first region, exceeding the allowable value and causing overheating, as shown in Figure 3 (dashed line 1: when the constraints of the first region are not met). Similarly, if the second minimum water supply amount cannot be secured, the metal temperature will rise sharply in the second region, exceeding the allowable value and causing overheating, as shown in Figure 3 (dashed line 2: when the constraints of the second region are not met). The magnitude relationship between the first and second minimum water supply amounts can change depending on the constraints, so it is important to control the water supply amount so that both of these flow rates are satisfied.

The effects of these controls will be explained using Figure 4. Figure 4 is a graph of the boiler feedwater volume versus boiler load. The boiler feedwater volume is expressed as a ratio, with the feedwater volume at 100% load being taken as 100.

Conventionally, the switching load is set to 25-30% (usually 30% for stable operation), and the water supply volume remains constant while performing circulating operation in the load range below the switching load.

In this embodiment, by adjusting the amount of water supplied to the heat transfer tube 53 during once-through operation at the allowable water supply amount, it is possible to reduce the amount of water supplied while maintaining the integrity of the entire heat transfer tube, making once-through operation possible up to a switching load lower than before.

In this embodiment, the load when the first minimum water supply amount cannot be calculated in each of (1-3) and (2-2) (first switching load candidate) and the load when the second minimum water supply amount cannot be calculated (second switching load candidate) are calculated, and the larger load between the first switching load candidate and the second switching load candidate is determined as the switching load Lc1.

Using Figure 5, we will explain how to calculate the first minimum feedwater amount in (1-3). Figure 5 is a graph of steam mass velocity against steam quality (quality).

Figure 5 shows three lines showing the relationship between quality and mass velocity, but we will first explain using the central line L2. If the amount of water supplied to the heat transfer tube 53 in the furnace 11 decreases during once-through operation, the quality increases. If this quality exceeds a certain value (upper limit), the boiling state of the water flowing through the heat transfer tube 53 will transition from nucleate boiling to film boiling beyond the nucleate boiling limit (DNB) point (transition from the left to the right region of line L2), causing a decrease in the heat transfer coefficient inside the heat transfer tube 53 and a sudden rise in the metal temperature of this heat transfer tube 53, making it impossible to maintain the integrity of the furnace wall.

The relationship between quality and mass velocity changes depending on the heat flux and water supply pressure. For example, if the water supply pressure is constant, the quality increases as the heat flux decreases (the line changes from L1 to L3). Similarly, if the heat flux is constant, the quality increases as the water pressure decreases (the line changes from L1 to L3). In this way, the relationship between quality and mass velocity (the criterion for calculating the first minimum water supply amount that can maintain a nucleate boiling state) is affected by heat flux and water supply pressure. Therefore, measured or calculated values in the heat flux field and measured water supply pressure are required.

Although not shown in FIG. 5, the inner shape of the heat transfer tube 53 also has an effect. A spiral tube (a tube with spiral ribs) tends to provide better quality (the first minimum water supply amount can be reduced) than a smooth tube under the same heat flux and water supply pressure constraints. Therefore, a spiral tube is more suitable for this embodiment.

The measurement sensor that measures the heat flux distribution will be described using Figure 6. Figure 6 is a schematic diagram of the heat flux sensor 68.

The heat flux sensor 68 is composed of a pad thermocouple 81 installed on the outside of the furnace and a chordal thermocouple 80 installed on the opposing inside of the furnace. The heat flux is evaluated from the results of both temperature measurements.

The installation positions of the chordal thermocouple 80 and the pad thermocouple 81 may be the entire heat transfer tube so that the heat flux distribution in the entire boiler can be correctly evaluated. Alternatively, as shown in FIG. 1, they may be limited to the area in the furnace wall where the heat flux is large. Here, the area where the heat flux is large is selected from the vertical height range of the furnace 11, but the area where the heat flux is particularly large may be the height range from the lowest combustion burner 25 to the highest combustion burner 21, or to the after-air port (not shown) located above the combustion burner 25.

In addition, a heat flux meter may be used instead of the combination of a chordal thermocouple 80 and a pad thermocouple 81.

Figure 7 is a flowchart showing the flow of the operation control method for the once-through boiler 10 according to this embodiment.

When the once-through boiler 10 is in once-through operation (S01: Yes), the operation control device 70 acquires the sensor outputs used to calculate the first minimum feedwater amount w1 determined from the constraint conditions of the first domain and the second minimum feedwater amount w2 determined from the constraint conditions of the second domain, in this embodiment, the sensor outputs of the feedwater amount sensor 65 and the heat flux sensor 68 (S02).

Then, the operation control device 70 calculates the first minimum water supply amount w1 using the above-mentioned (1-1) to (1-3) (S03). During the calculation of (1-3), the operation control device 70 also calculates a load (first switching load candidate Lc11) at which the first minimum water supply amount w1 cannot be calculated.

The operation control device 70 further calculates the second minimum water supply amount w2 using the above-mentioned (2-1) to (2-2) (S04). During the calculation of (2-2), the operation control device 70 also calculates a load (second switching load candidate Lc12) at which the second minimum water supply amount w2 cannot be calculated. Steps S03 and S04 may be performed in the reverse order.

The operation control device 70 compares the first minimum water supply amount w1 and the second minimum water supply amount w2 in magnitude, and if the first minimum water supply amount w1 is equal to or greater than the second minimum water supply amount w2 (S05: Yes), it selects the first minimum water supply amount w1 as the allowable water supply amount and controls the water supply amount to the furnace wall so as to ensure the allowable water supply amount (first minimum water supply amount w1) (S06). The operation control device 70 calculates the increase/decrease in the water supply amount Δ = (first minimum water supply amount w1 - water supply amount indicated by the water supply amount sensor 65) and outputs a signal to the water supply pump 52 to control the increase/decrease in the water supply amount Δ.

On the other hand, if the first minimum water supply amount w1 is less than the second minimum water supply amount w2 (S05: No), the operation control device 70 sets the second minimum water supply amount w2 as the allowable water supply amount, and controls the water supply amount to the furnace wall so as to ensure the allowable water supply amount (second minimum water supply amount w2) (S07). The operation control device 70 calculates the increase/decrease in the water supply amount Δ = (second minimum water supply amount w2 - water supply amount indicated by the sensor of the water supply amount sensor 65), and outputs a signal to the water supply pump 52 to control the increase/decrease in the water supply amount Δ.

The operation control device 70 compares the magnitude relationship between the first switching load candidate Lc11 and the second switching load candidate Lc12. If the first switching load candidate Lc11 is equal to or greater than the second switching load candidate Lc12 (S08: Yes), the operation control device 70 selects the first switching load candidate Lc11 as the switching load Lc1 (S09). If the first switching load candidate Lc11 is less than the second switching load candidate Lc12 (S08: No), the operation control device 70 selects the second switching load candidate Lc12 as the switching load Lc1 (S10). The switching load Lc1 is preferably set to 10% or more and less than 30% of the boiler load of the once-through boiler 10. By setting it to 10% or more, troubles caused by once-through operation at low loads can be avoided, and by setting it to less than the conventional 30%, the operating efficiency can be improved compared to the conventional case.

If the boiler load Lc of the once-through boiler 10 is greater than the switching load Lc1 (S11: No), return to step S02 and continue once-through operation.

If the boiler load Lc of the once-through boiler 10 is equal to or less than the switching load Lc1 (S11: Yes), for example, when the boiler load Lc is lowered during once-through operation to reach the switching load Lc1, the operation control device 70 switches to circulation operation (S12). The operation control device 70 outputs a start signal to the recirculation water pump 62.

During circulation operation immediately after starting the once-through boiler 10 (S01: No), or after switching from once-through operation to circulation operation (S12), the operation control device 70 acquires the sensor output of the recirculated water volume sensor 66 (S13) and monitors the recirculated water volume (S14).

If the boiler load Lc is less than the switching load Lc1 (S15: No) and the boiler load is not 0 (S16: No), return to S13 and continue circulation operation.

In this embodiment, the switching load Lc1 (switching load in S15) used to determine whether to switch from circulation operation to once-through operation is the same as the switching load Lc1 (switching load in S11) used to determine whether to switch from once-through operation to circulation operation, but a different value may be used.

When the boiler load Lc becomes equal to or greater than the switching load Lc1 (S15: Yes), the operation control device 70 switches from circulation operation to once-through operation (S17). Then, the process proceeds to S02, where the allowable water supply amount during once-through operation is optimized.

When the boiler load Lc becomes 0 (S16: Yes), the once-through boiler 10 stops.

Figure 8 is a diagram showing the hardware configuration of the operation control device 70. The operation control device 70 includes a CPU (Central Processing Unit) 301, a RAM (Random Access Memory) 302, a ROM (Read Only Memory) 303, a HDD (Hard Disk Drive) 304, an input I/F 305, and an output I/F 306, which are connected to each other via a bus 307. Note that the hardware configuration of the operation control device 70 is not limited to the above, and may be configured by a combination of a control circuit and a storage device.

The configuration and effects of this embodiment can be summarized as follows:

1) The operation control device 70 of the once-through boiler 10 according to this embodiment is an operation control device 70 of the once-through boiler 10 that performs once-through operation to supply water to the heat transfer tubes 53 of the once-through boiler 10 and extract superheated steam generated by heat exchange with an internal heat source, divides the heat transfer tubes 53 into multiple regions, calculates the minimum water feed rate at which once-through operation can be maintained by applying different constraints to each region, selects the minimum water feed rate with the highest water feed rate from the multiple minimum water feed rates calculated as the allowable water feed rate, and adjusts the water feed rate to the heat transfer tubes 53 during once-through operation so as to satisfy the allowable water feed rate.

By dividing the heat transfer tube 53 into multiple regions and calculating the minimum water supply amount that allows once-through operation to be maintained in a different manner for each region, the health of the heat transfer tube 53 for each region can be correctly evaluated in an appropriate manner even if the cause (phenomenon) of the heat transfer tube 53 overheating during load change varies for each region. In addition, by adjusting the amount of water supply to the heat transfer tube 53 during once-through operation at the allowable water supply amount, which is the highest minimum water supply amount among the minimum water supply amounts for all regions, it is possible to reduce the amount of water supply while maintaining the health of the entire heat transfer tube.

2) The once-through boiler 10 performs either a circulation operation in which circulating water separated by a steam-water separator 54 connected downstream of the heat transfer tube 53 is supplied to the upstream of the heat transfer tube 53, or a once-through operation, and the operation control device 70 applies different constraints to each region of the heat transfer tube 53 to calculate a minimum load at which the minimum water supply amount that can be maintained by the once-through operation cannot be calculated, selects the highest minimum load from among the calculated minimum loads as a switching load, and performs control for the circulation operation below the switching load and controls for the once-through operation above the switching load, in the operation control device 70 for the once-through boiler 10 described in 1).

The minimum load at which the minimum water supply flow rate cannot be calculated is calculated using a different method for each region, and the highest minimum load among these, the switching load, is used to lower the switching load and expand the load range in which flow-through operation is possible.

3) The operation control device 70 for the once-through boiler 10 described in 1) includes at least two regions: a first region in which the heat flux inside the once-through boiler 10 is relatively high, and a second region in which the quality of the internal fluid of the heat transfer tube 53 is relatively high.

By targeting two different areas, one with high heat flux and one with high quality, the minimum water supply or switching load can be specifically evaluated.

4) In the first region, maintaining the nucleate boiling state of the internal fluid of the heat transfer tube 53 is set as a constraint, and a first minimum feedwater amount that satisfies the constraint is calculated, and in the second region, maintaining the metal temperature in the heat transfer tube 53 after dryout during the once-through operation at or below an allowable value is set as a constraint, and a second minimum feedwater amount that satisfies the constraint is calculated, in the operation control device 70 for the once-through boiler 10 described in 3).

The minimum water supply or switching load can be accurately evaluated in two different areas: an area with high heat flux and an area with high quality.

5) The switching load is set for switching from once-through operation to circulation operation at least when the load decreases, in the operation control device 70 for the once-through boiler 10 described in 2).

When the load decreases, the heat flux distribution tends to show complex behavior, and this embodiment is suitable because it can properly take this effect into account.

6) The operation control device 70 is an operation control device 70 for a once-through boiler 10 described in 1), in which the minimum water supply amount is calculated based on at least a measured or calculated value of the heat flux distribution of the heat transfer tube 53 and a measured value of the pressure of the internal fluid in the heat transfer tube 53.

The amount of heat absorbed by the furnace 11 varies depending on the type of coal and the degree of fouling of the furnace 11, but by calculating the minimum water feed rate based on the heat flux measurements and calculations as described above, an appropriate minimum water feed rate that reflects changes in fuel properties and changes in the boiler condition over time can be calculated, and once-through operation can always be performed with the optimal water feed rate.

7) The operation control device 70 for the once-through boiler 10 described in 6) is such that the measured value of the heat flux distribution of the heat transfer tube is measured only in an area where the heat flux is large.

The heat flux sensor 68 can be placed in a cost-effective area only.

8) The measurement value of the heat flux distribution of the heat transfer tube is measured using a thermocouple or a heat flux meter;
6) is an operation control device 70 for the once-through boiler 10 described in

Heat flux distribution can be measured accurately.

9) The shape of the heat transfer tubes 53 of the boiler is a rifled tube, and the operation control device 70 of the once-through boiler 10 described in 6) is the same as that of the control device 70 of the once-through boiler 10 described in 9).

The shape of the heat transfer tube 53 is rifled rather than smooth, which is advantageous for reducing the amount of water supplied and lowering the switching load.

10) The operation control device 70 for the once-through boiler 10 described in 2), in which the switching load is set in the range of 10% or more and less than 30%.

By setting it to less than 30%, the switching load can be reduced compared to conventional methods, and by setting it to 10% or more, unexpected problems can be avoided.

11) The operation control device 70 for the once-through boiler 10 described in 2) is configured such that the switching load is set to a different value when switching from circulation operation to once-through operation when the load increases and when switching from once-through operation to circulation operation when the load decreases.

Even with the same load, it is expected that the heat flux distribution and water supply pressure will be different when the load increases and when the load decreases, and by properly evaluating these effects, it is possible to provide switching loads that are appropriate for both patterns.

In addition, this also includes the operation control methods of the once-through boiler 10 corresponding to 1) to 11), and once-through boilers equipped with such operation control devices.

This invention can be widely applied to the operation control of once-through boilers, etc.

10: once-through boiler 11: furnace 12: combustion device 21, 22, 23, 24, 25: combustion burner 26, 27, 28, 29, 30: pulverized coal supply pipe 31, 32, 33, 34, 35: coal pulverizer 36: wind box 37: air duct 38: blower 40: flue 41, 42: superheater 43, 44: reheater 45, 46, 47: coal economizer 48: exhaust gas pipe 49: air heater 51: water supply line 52: water supply pump 53: heat transfer tube 54: steam separator 55: steam line 56: steam turbine 57: turbine bypass valve 58: discharge line 59: condenser 60: recirculation line 61: drain tank 62: recirculation water pump 65 : Water supply amount sensor 66 : Recirculating water amount sensor 67 : Water supply pressure sensor 68 : Heat flux sensor 70 : Operation control device 80 : Cordal thermocouple 81 : Pad thermocouple 301 : CPU
302: RAM
303: ROM
304: HDD
305: Input I/F
306: Output I/F
307: Bus

Claims (15)

An operation control device for a once-through boiler that performs a once-through operation in which water is supplied to a heat transfer tube of the boiler and superheated steam generated by heat exchange with an internal heat source is taken out,
The operation control device includes:
dividing the heat transfer tube into a plurality of regions, applying different constraint conditions to each of the regions to calculate a minimum feedwater amount at which the once-through operation can be maintained, selecting the minimum feedwater amount having the largest feedwater amount from the plurality of calculated minimum feedwater amounts as an allowable feedwater amount, and adjusting the feedwater amount to the heat transfer tube during the once-through operation so as to satisfy the allowable feedwater amount.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 1,
The once-through boiler performs either a circulation operation in which circulating water separated by a steam separator connected downstream of the heat transfer tube is supplied to the upstream of the heat transfer tube, or the once-through operation,
The operation control device applies different constraint conditions to each region of the heat transfer tube to calculate a minimum load at which the minimum feedwater amount that can be maintained in the once-through operation cannot be calculated, selects the highest minimum load from the calculated minimum loads as a switching load, and performs control for the circulation operation when the load is less than the switching load, and performs control for the once-through operation when the load is equal to or greater than the switching load.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 1,
The plurality of regions include at least two regions: a first region in which the heat flux inside the once-through boiler is relatively large; and a second region in which the quality of the internal fluid of the heat transfer tube is relatively high.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 3,
In the first region, maintaining a nucleate boiling state of the internal fluid of the heat transfer tube is set as a constraint condition, and a first minimum feedwater amount that satisfies the constraint condition is calculated;
In the second region, a constraint is set to maintain the metal temperature in the heat transfer tube after dryout during the once-through operation at or below an allowable value, and a second minimum feedwater amount that satisfies the constraint is calculated.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 2,
The switching load is set at least for switching from once-through operation to circulation operation when the load is reduced;
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 1,
The operation control device calculates the minimum water supply amount based on at least a measured value or a calculated value of a heat flux distribution of the heat transfer tube and a measured value of a pressure of an internal fluid in the heat transfer tube.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 6,
The measurement value of the heat flux distribution of the heat transfer tube is measured only in a region where the heat flux is large.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 6,
The measurement value of the heat flux distribution of the heat transfer tube is measured using a thermocouple or a heat flux meter.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 6,
The shape of the heat transfer tube is a rifled tube.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 2,
The switching load is set in a range of 10% to 30% of the load of the once-through boiler.
Operation control device for once-through boiler. The operation control device for a once-through boiler according to claim 2,
The switching load is set to a different value when switching from the circulation operation to the once-through operation at the time of load increase and when switching from the once-through operation to the circulation operation at the time of load decrease.
Operation control device for once-through boiler. A method for controlling an operation of a once-through boiler that performs a once-through operation in which water is supplied to a heat transfer tube of the boiler and superheated steam generated by heat exchange with an internal heat source is taken out, comprising the steps of:
dividing the heat transfer tube into a plurality of regions, applying different constraint conditions to each of the regions to calculate a minimum feedwater amount at which the once-through operation can be maintained, selecting the minimum feedwater amount having the largest feedwater amount from the plurality of calculated minimum feedwater amounts as an allowable feedwater amount, and adjusting the feedwater amount to the heat transfer tube during the once-through operation so as to satisfy the allowable feedwater amount.
A method for controlling the operation of a once-through boiler. The method for controlling operation of a once-through boiler according to claim 12,
The once-through boiler performs either a circulation operation in which circulating water separated by a steam separator connected downstream of the heat transfer tube is supplied to the upstream of the heat transfer tube, or the once-through operation,
calculating a minimum load at which a minimum feedwater amount that can be maintained in the once-through operation cannot be calculated by applying different constraint conditions to each region of the heat transfer tube, selecting the lowest load among the calculated minimum loads as a switching load, and performing control for the circulation operation when the load is less than the switching load, and performing control for the once-through operation when the load is equal to or greater than the switching load;
A method for controlling the operation of a once-through boiler. A once-through boiler that performs once-through operation to supply water to the heat transfer tubes of the boiler and extract superheated steam generated by heat exchange with an internal heat source,
The once-through boiler is provided with an operation control device that controls the amount of feedwater during the once-through operation,
The operation control device includes:
dividing the heat transfer tube into a plurality of regions, applying different constraint conditions to each of the regions to calculate a minimum feedwater amount at which the once-through operation can be maintained, selecting the minimum feedwater amount having the largest feedwater amount from the plurality of calculated minimum feedwater amounts as an allowable feedwater amount, and adjusting the feedwater amount to the heat transfer tube during the once-through operation so as to satisfy the allowable feedwater amount.
Once-through boiler. 15. The once-through boiler according to claim 14,
The once-through boiler performs either a circulation operation in which circulating water separated by a steam separator connected downstream of the heat transfer tube is supplied to the upstream of the heat transfer tube, or the once-through operation,
The operation control device applies different constraint conditions to each region of the heat transfer tube to calculate a minimum load at which the minimum feedwater amount that can be maintained in the once-through operation cannot be calculated, selects the highest minimum load from the calculated minimum loads as a switching load, and performs control for circulation operation when the load is less than the switching load, and performs control for the once-through operation when the load is equal to or greater than the switching load.
Once-through boiler.

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