JP2000205556A - Operating method of once-through boiler - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the cost by controlling the excess air rate in the subcritical pressure region of a partial load according to a program preset individually for each kind of fuel coal to be used thereby reducing the superheater spray capacity facility while ensuring an appropriate degree of superheat at the superheater inlet. SOLUTION: Coals are subjected to coal properties analysis and fixed carbon, volatile component and ash content in industrial analysis are evaluated before being classified into groups of low, intermediate and high fuel ratio coals. Each selected group is then corrected with respect to an excess air ratio curve for each fuel ratio and furnace heat absorption is controlled y switching the setting of excess air ratio curve for a load according to a program when coal of each selected group is combusted. According to the method, fluctuation in the heat absorption on the furnace wall of a boiler due to combustion of coal can be suppressed through excess air rate control at the rate of spray quantity of each load.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of operating a once-through boiler apparatus, and more particularly to a method for controlling a fluctuation range of a heat absorption amount of a boiler furnace wall in a sub-critical pressure region of a partial load to reduce costs. The present invention relates to a preferred method of operating a coal-fired variable pressure once-through boiler.

[0002]

2. Description of the Related Art A system diagram of a variable-pressure once-through boiler is shown in FIG.
In FIG. 5, the boiler furnace wall 4 is formed of a membrane wall in which water tubes are continuously welded in a plate shape, and forms a heat transfer surface. Water supplied to the boiler furnace wall 4 is first sent from the water supply pump 1 to the feed water heater 2 and heated, and then sent to the boiler furnace wall 4 through the economizer 3. The feedwater rises while being heated by the boiler furnace wall 4, and finally generates steam. The generated steam is sent to the high-pressure turbine 8 through the steam separator 13, the primary superheater 5, the superheater overheat reducer 14a, the secondary superheater 6, the superheater overheat reducer 14b, and the tertiary superheater 7. The steam that has worked in the high-pressure turbine 8 is systematically configured to send the steam to the medium-pressure turbine 11 through the primary reheater 9, the reheater overheat reducer 15, and the secondary reheater 10. The steam discharged from the intermediate pressure turbine 11 is recovered in a condenser 12 for heat and is circulated to a water supply system.

Further, the steam-water mixed fluid flowing out of the boiler furnace wall 4 is separated by a steam-water separator 13 so that the water is not circulated to the primary superheater 5 which is a flow-through area, but is circulated to the economizer 3 side. A switching valve 16 is provided. Further, a part of the feed water heated by the feed water heater 2 and supplied to the boiler furnace wall 4 is extracted from the inlet of the economizer 3 through a branch pipe provided with the superheater injection flow control valve 17 and the superheater. It is supplied to the overheat reducers 14a and 14b. Further, a part of the supply water having a low pressure from the middle stage of the water supply pump 1 can be extracted and supplied to the reheater overheat reducer 15 from a branch pipe provided with the reheater injection flow control valve 18.

FIG. 3 shows an enthalpy-pressure diagram of each heat exchanger in the above-mentioned coal-fired variable pressure once-through boiler. The horizontal axis of FIG. 3 shows the outlet pressure of the internal fluid, and the vertical axis shows the outlet enthalpy of the internal fluid. Further, the thin line in the horizontal axis direction in FIG. 3 is an isotherm of the internal fluid, and the region where the temperature rises sharply inside the mountain shape on the left side is a steaming generation region.

As shown in FIG. 3, in the supercritical pressure region, the mixed fluid of steam and water contains only steam at the outlet of the furnace. However, in the subcritical region, there is a region where steam and water can exist in a mixture near the furnace outlet.

When the once-through boiler is operated in the subcritical pressure region as described above, the water is separated so that the steam-water mixed fluid flowing out of the boiler furnace wall 4 does not flow to the superheater 5 which is the once-through region, and the water is separated. It is necessary to perform an operation of circulating to the charcoal device 3 side (circulation operation) and a case of switching the switching valve 16 after the steam-water mixed fluid flowing out of the boiler furnace 4 changes to only steam and flowing only the once-through region. . In order to smoothly switch between the circulation and the once-through flow in the subcritical pressure region, the superheat degree S (see FIG. 3) of the steam at the superheater inlet, that is, the superheat temperature with respect to the saturation temperature of the fluid at the superheater inlet is determined. It is important to secure them properly.

However, since the heat absorption rate in the furnace greatly changes depending on the type of coal, it is difficult to always maintain this degree of superheat at an appropriate value. Therefore, the conventional coal-fired variable-pressure once-through boiler apparatus, as shown in FIG. 5, is provided with water before being heated by the water wall constituting the furnace wall as shown in FIG. Is sprayed by a superheater / superheater / lower heater 14a, 14b to change the flow rate of the gas-water mixed fluid passing through the furnace wall surface, thereby ensuring an appropriate degree of superheat of the steam temperature at the inlet of the superheater. The temperature of the steam passing through the superheaters 5 and 6 is adjusted by the spray.

The MCR (Maximum) shown in FIG.
mContinuous Ratio: maximum continuous load), the degree of vacuum in the condenser is reduced in summer and the efficiency of the turbines 8 and 11 is reduced.
It has about 7% to 8% as a margin above the% load, and refers to the load including the margin. 1 for normal operation
It is sufficient to consider the load at 00%, and therefore, it is ignored in the present invention.

[0009]

As shown in FIG. 4, the conventional boiler apparatus operates at a predetermined excess air ratio at each load regardless of the type of coal (discrimination based on the fuel ratio or the like). As a result, large fluctuations in furnace heat absorption due to differences in coal type (discrimination based on fuel ratio, etc.) could not be suppressed by any method other than the superheater spray.

Therefore, in order to keep the degree of superheat of the steam within a certain range for stable operation, the superheater and superheater 14
It is necessary to greatly change the spray amount in a and 14b, and there is a problem that the superheater spray equipment capacity increases and leads to an increase in cost. An object of the present invention is to reduce the cost by reducing the superheater spray capacity equipment while ensuring an appropriate degree of superheat at the superheater inlet.

[0011]

The object of the present invention can be solved by controlling not only the superheater spray but also the excess air ratio to suppress the fluctuation range of the heat absorption of the boiler furnace wall. The present invention is a method of operating a once-through boiler apparatus in which the excess air ratio in the subcritical pressure region of a partial load is controlled according to a preset program for each coal type of fuel coal used.

When a large number of coal types are used as fuel, the number of available coal types is divided into available groups based on the fuel ratio which is a function of the fixed carbon content and the volatile content in the fuel coal used. Set the excess air ratio to Then, for example, the used fuel coal is converted into a fuel ratio (fixed carbon content /
Volatile content), it is divided into three groups: low fuel ratio coal, medium fuel ratio coal, and high fuel ratio coal. The fuel-to-fuel ratio operates the once-through boiler device by lowering the excess air ratio. In this way, the size of the superheater spray capacity equipment can be reduced, and the cost can be reduced.

Here, the individual program setting means, for example, as shown in FIG. 1, depending on the properties of the coal to be burned in the boiler, the coal is, for example, a low fuel specific coal, a medium fuel specific coal, and a high fuel specific coal. The group is divided into groups, and the curve of the excess air ratio with respect to the load is programmed for each group, and the curve of the excess air ratio is varied depending on the type of coal (fuel ratio, etc.) to control the excess air ratio for each load. It is.

[0014]

The heat transfer from the combustion gas to the fluid flowing in the boiler furnace wall is mainly governed by radiant heat transfer. The combustion gas emissivity, which is a main parameter of radiant heat transfer, also depends on the ambient temperature, but the concentration per unit volume of radiant gas such as CO ₂ and H ₂ O, and radiant solid particles such as soot and fly ash. Also depends.

Since the concentration of the radiating solid particles per unit volume can be changed by the excess air ratio, the amount of heat absorbed by the boiler furnace wall is controlled by changing the combustion gas mass ratio based on the excess air ratio. be able to.

For example, as shown in the cross-sectional view of the boiler furnace in FIG. 2, coal having a high volatile content in the burner portion 21, that is, coal having a low fuel ratio generates a large amount of soot of radiating solid particles. The rate is high, and the amount of heat absorbed in the furnace 20 increases. Therefore, in such a case, by increasing the excess air ratio, the concentration of the radiant gas and the radiant solid particles is reduced, and the emissivity of the combustion gas is reduced.
The heat absorption of the heat from the combustion gas to the fluid in the inside is suppressed.

As described above, due to the change in the excess air ratio,
A large fluctuation range of the heat absorption amount in the boiler furnace wall due to the difference in the coal type (fuel ratio, etc.) can be reduced by setting the excess air ratio for each coal type (fuel ratio, etc.) in an individual program. The ash content is used to bias the excess air ratio obtained from the fuel ratio.

[0018]

Embodiments of the present invention will be described below. Grouping of coal necessary for explanation of excess air ratio control to reduce the fluctuation range of heat absorption in boiler furnace wall, setting of excess air ratio for difference in coal type (fuel ratio etc.), excess air ratio coal type The individual program control for each (fuel ratio, etc.) will be described. Coal grouping First, before burning the target coal, the coal is analyzed for its coal properties to evaluate the fixed carbon, volatile and ash content in industrial analysis. Then, the target coal is classified into groups such as low-fuel ratio coal, medium-fuel ratio coal, and high-fuel ratio coal by a function of the fuel ratio (fixed carbon content / volatile content). Setting of excess air ratio for difference in coal type (fuel ratio etc.) One of the excess air ratio curves for each fuel ratio shown in FIG. 1 (a) is selected for each of the groups selected above.
Here, as shown in FIG. 1B, when the ash content is high, the correction is performed on the selected excess air rate curve. When burning coal for each group selected in individual program control for each coal type (fuel ratio, etc.), the furnace heat absorption is controlled by switching the excess air ratio curve setting for the load by a program. Thus, as shown in Table 1 by the ratio of the spray amount at each load, the fluctuation range of the heat absorption amount of the boiler furnace wall due to coal combustion could be reduced by the air excess ratio control of the present invention.

[0019]

[Table 1] The numerical values in Table 1 above represent the ratio of the spray amount based on the high calorific value coal that maximizes the spray amount. As a result, the amount of the superheater spray for securing the degree of superheat can be reduced to three stages of the superheater spray, which is common knowledge in the conventional large-capacity coal-fired variable-pressure once-through boiler, and the cost can be reduced.

[0020]

According to the present invention, the fluctuation range of the heat absorption amount of the large boiler furnace wall due to coal combustion can be reduced by controlling the excess air ratio. As a result, the amount of the superheater spray for securing the degree of superheat could be reduced, and the cost was reduced.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example of setting an excess air ratio with respect to a load according to an embodiment of the present invention.

FIG. 2 is a view showing heat transfer characteristics in a furnace when burning low-fuel-ratio coal types.

FIG. 3 is a diagram showing an enthalpy-pressure diagram of each heat exchanger in the variable-pressure once-through boiler.

FIG. 4 is a diagram showing a setting example of an excess air ratio with respect to a load in the related art.

FIG. 5 is a system diagram of a variable-pressure once-through boiler.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 feed water pump 2 feed water heater 3 economizer 4 boiler furnace wall 5 primary superheater 6 secondary superheater 7 tertiary superheater 8 high pressure turbine 9 primary reheater 10 secondary reheater 11 medium pressure turbine 12 condenser 13 Steam separator 14 Superheater overheat reducer 15 Reheater overheat reducer 16 Switching valve 17 Superheater injection flow control valve 18 Reheater injection flow control valve

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Shigeki Morita 6-9 Takara-cho, Kure-shi, Hiroshima Babcock Hitachi Kure Factory F-term (reference) 3K003 AA03 AC01 CA05 DA05 3K046 AA02 AB01 BA01 EA05 FA06 3K091 BB02 CC13 CC24 DD02 3L021 AA04 BA01 CA10 DA25 DA26 EA01 FA13

Claims (4)

[Claims] 1. A method of operating a once-through boiler apparatus, wherein an excess air ratio in a subcritical region of a partial load is controlled in accordance with a preset program for each type of coal used as fuel. 2. The once-through flow according to claim 1, wherein the used fuel coal is divided into groups by a fuel ratio which is a function of a fixed carbon content and a volatile content, and an excess air ratio is set for each group. Operating method of boiler device. 3. The method according to claim 2, wherein the excess air ratio during combustion for each coal type is individually programmed by a function based on the fixed carbon content, volatile content, and ash content in the fuel used. The method for operating the once-through boiler device according to claim 1. 4. The fuel used is divided into at least two or more groups according to the size of the fuel ratio, and coal having a low fuel ratio increases the excess air ratio, and coal having a high fuel ratio decreases the excess air ratio. The method for operating the once-through boiler device according to claim 2.