JPH0452407A - Method of controlling reheated steam temperature - Google Patents

Abstract

PURPOSE:To lower GR ratio and lower the rate of unburned fuel and perform a high efficiency operation by taking a value which allows under-shooting transiently for the recirculation combustion exhaust gas ratio and GR ratio while the load is rising and regulating the degree of opening of a recirculation damper so as to lower the ratios from the values for load increase during the boiler operation at the time other than load increase. CONSTITUTION:When GR ratio is lowered during the load is steady, the gas flow rate is lowered more on the side of a reheater 9 compared with the side of a superheater and fuel economizer row 10. As a result when the GR ratio is lowered with the load being steady, the gas flow rate on the side of the superheater 10 is lowered compared with the time of raising load and the gas flow rate on the side of the reheater 9 is increased in order to correct the control to raise the temperature of reheated steam. Also in order to make deviation of the temperature zero the control by parallel dampers 11 and 12 is carried out. Namely the damper 11 on the side of the reheater 9 in the first stage is operated in response to the signal of reheated steam temperature deviation, and the damper 12 on the side of the superheater and economizer row 10 is operated by the inverse operation signal by a function generator 38. With this arrangement it is possible to lower the GR ratio to the required lowest limit.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a boiler that uses both an exhaust gas recirculation method and a parallel damper method for reheating steam temperature control, and particularly relates to a boiler that uses both an exhaust gas recirculation method and a parallel damper method to control the temperature of reheated steam. This invention relates to a boiler reheat steam temperature control method that enables highly efficient operation by minimizing the necessary minimum temperature.

[Prior art] In a boiler that uses both the flue gas recirculation method and the parallel damper method for reheating steam temperature control, as shown in FIG. It is recirculated via the regulating damper 3 to the furnace bottom 4.
It is mixed in the furnace 7 with the combustion gas that is introduced from the furnace 7 and is combusted in the burner 5 and the two-stage combustion air port 6. The mixed combustion gas passes through a superheater and reheater row 8, a first stage reheater 9 which is divided into two liquid paths, and a first stage superheater and economizer row 10, and then is discharged. . On the downstream side of the reheater 9, superheater, and economizer row 10, there are dampers 11 and 12 for adjusting the flow rate of exhaust gas, respectively.
is provided.

The principles of reheated steam temperature control using the combustion exhaust gas recirculation method and the parallel damper method are as shown below.

(a) Exhaust gas recirculation method The recirculation exhaust gas ratio (hereinafter referred to as GR ratio) is defined as follows.

Changes in the GR ratio have the following effects on the reheat steam temperature.

(1) As the GR ratio increases (decreases), the gas temperature in the furnace decreases (increases), the gas residence time in the furnace decreases (extends), and the amount of heat absorption in the furnace decreases (increases).

(2) As the GR ratio increases (decreases), the gas flow rate outside the reheater steam pipe increases (decreases), and the reheater steam pipe heat transfer rate increases (decreases).

(3) Due to (1) and (2) above, the GR ratio increases (
(decrease), the reheat steam temperature increases (decrease).

FIG. 5 shows an example of static characteristics of the GR ratio. The GR ratio has the characteristic that the higher the boiler load, the lower it is, but even at maximum load,
A certain ratio is guaranteed. This is mainly because the undershoot width is provided in consideration of the transient undershoot when the load increases as shown in FIG.

In other words, when the load increases, the reheat steam temperature tends to rise excessively due to overfiring (overshoot of fuel and air), so in order to correct this, the GR
There is an undershoot in the ratio. In the basic system diagram for reheating steam temperature control shown in FIG. 7, this undershoot is performed by the load change correction circuit 33-.

(b) Parallel damper method Increase (decrease) the flow rate of the gas passing through the first stage reheater 9 side (Figure 4), and increase (decrease) the flow rate of the gas passing through the first stage superheater and economizer row 10 side (Figure 4). By decreasing (increasing) the amount, heat absorption on the first stage reheater 9 side increases (decreases), and the reheat steam temperature increases (decreases).

In addition, in the reheat steam temperature control method, by changing the GR ratio control program at boiler startup and during normal operation, it is possible to control the reheat steam temperature using the GR ratio even when the boiler load at boiler startup is small. It has been known. (Special Publication No. 62-35561).

[Problems to be Solved by the Invention] The reheat steam temperature control method using both the exhaust gas recirculation method and the parallel damper method takes into account the GR ratio undershoot when the load increases, so etc., the GR ratio is excessive. An excessive GR ratio causes the following efficiency decline.

(1) As mentioned above, the recirculated exhaust gas has the function of lowering the furnace gas temperature, but the decrease in the furnace gas temperature becomes a factor that lowers the combustion efficiency. Especially in coal combustion, G
The decrease in combustion efficiency due to the increase in R ratio is significant. Figure 8 is GR
The effect of the ratio on the unburned rate of pulverized coal is shown. As the GR ratio increases, the unburned rate of fuel increases.

(2) Excessive GR ratio leads to waste of power of exhaust gas recirculation fan (GRF).

Furthermore, since the invention described in Japanese Patent Publication No. 62-35561 enables reheated steam temperature control using the GR ratio when the load increases, it is necessary to prepare a special control program when the load increases.

Therefore, an object of the present invention is to reduce the excessive GR during the load stabilization.
The objective is to reduce the ratio, lower the unburned rate of fuel, and enable highly efficient operation.

Another object of the present invention is to enable reheat steam temperature control using the GR ratio, including when the load increases, without using a special control program, and to reduce the GR ratio when the load is stable. The purpose is to enable thermal steam temperature control.

[Means to solve the problem] The above object of the present invention is achieved by the following configuration.

That is, a reheater that generates reheat steam, a superheater that generates main steam, a reheater damper and a superheater damper that distribute and control the flow rate of flue gas passing through these reheaters and superheaters, and a boiler. In a reheat steam temperature control method for a boiler equipped with a recirculation damper that controls the recirculation flow rate of combustion exhaust gas that is recirculated to the main body side, it is the ratio of the amount of recirculation of combustion exhaust gas to the total amount of combustion exhaust gas. The recirculated flue gas ratio is set to a value that allows transient undershoot when the load increases, and when the boiler is operating other than when the load increases, the damper opening of the recirculation damper is adjusted so that the recirculated flue gas ratio is lower than when the load increases. A reheat steam temperature control method characterized by adjusting a reheater damper and a superheater damper in accordance with or in parallel with the recirculated combustion exhaust gas ratio during boiler operation other than when the load increases. The above-mentioned reheat steam temperature control method adjusts the opening degree of each, or the setting of the low recirculation exhaust gas ratio except when the load increases is based on the recirculation combustion exhaust gas ratio setting on the two sides depending on the fuel properties and operating conditions. The reheat steam temperature control method is selected from a program.

[Effect] The recirculation combustion exhaust gas ratio is set to a value large enough to allow transient undershoot when the load increases, and when the boiler is operating other than when the load increases, the opening degree of the recirculation damper is adjusted to reduce the lower than the value.

During boiler operation other than when the load increases, a reheater damper and/or a superheater damper is used to change the amount of flue gas passing through the reheater and superheater in accordance with or in parallel with the recirculated flue gas ratio. The reheating steam temperature can be controlled by adjusting the opening degree of the reheating steam.

The setting of the low recirculation exhaust gas ratio at times other than when the load increases can be selected from the recirculation combustion exhaust gas ratio setting program on the two sides, for example, by manual operation, depending on the fuel properties and operating conditions.

[Example] Embodiments of the present invention will be described with reference to the drawings.

Note that this control device is applied to the boiler shown in FIG.

At this time, when the GR ratio is lowered during load stabilization, the gas flow rate decreases more on the reheater 9 side than on the superheater and economizer array 10 side. Therefore, when the GR ratio is lowered when the load is stable, compared to when the load increases, as shown in Figure 1,
By reducing the gas flow rate on the superheater 10 side and increasing the gas flow rate on the reheater 9 side, corrective control is performed to increase the reheat steam temperature.

FIG. 2 shows an example of a basic system diagram of a control device embodying the above concept.
The subtractor 24 compares the reheat steam temperature with the reheat steam temperature set value by . That is, the damper 11 on the first stage reheater 9 side operates according to the reheat steam temperature deviation signal, and the superheater and economizer array 1oy! I! The damper 12 is a function generator 38
It operates according to the reverse operation signal.

The exhaust gas recirculation amount control system using the exhaust gas circulation control damper 3 (FIG. 4) is program controlled, and has three types of function generators 29 to 31 as setting programs.

The selection is made using the selection button 32 which has three types of buttons. The function generator 29 performs normal GR ratio setting according to the prior art, and the function generators 3o and 31 perform two types of settings in which the GR ratio is reduced.

The two types of settings for the low GR ratio are designed so that the settings can be changed depending on the fuel properties and operating conditions.

In the exhaust gas recirculation control system as well, correction is performed using the reheated steam temperature deviation signal, and a low GR ratio for which correction gain settings are given by the numerical value generator 25 can be selected under the following conditions.

(1) When the load is steady (2) When the load drops (as shown in Figure 6, the GR ratio will overshoot, so there is no problem even if the GR ratio is lowered a little.) (3) The undershoot of the GR ratio is small. By selecting a low GR ratio setting during gradual load changes, the amount of recirculated exhaust gas is reduced. As a result, the reheated steam temperature tends to decrease, but as explained earlier using FIG. 1, the reheated steam temperature is maintained at a predetermined value.

Other examples of the present invention include the following.

(1) When the load change conditions are standardized, the GR ratio setting at the time of load increase and the GR ratio setting for other conditions may be automatically performed based on the conditions.

(2) The power supply command is simple as shown in Figure 3, and there is enough time from the power supply command until the load is actually changed to allow for a change in the actual damper opening along with a change in the GR ratio setting. In some cases, the GR ratio setting may be automatically selected based on the power supply command.

(3) GR ratio setting function generators 29 to 3 shown in Fig. 2
The number can be increased or decreased as desired depending on the operating conditions.

[Effects of the Invention] According to the present invention, the GR ratio can be reduced to the necessary minimum through relatively simple control, which is effective in increasing boiler efficiency by improving combustion efficiency and reducing the power of the exhaust gas circulation fan (GRF). .

[Brief explanation of the drawing]

Figure 1 is a conceptual diagram of the present invention, Figure 2 is a basic control system diagram of the present invention, Figure 3 is a diagram showing an example of a simple load change pattern, and Figure 4 is an exhaust gas recirculation system and a parallel damper system. Fig. 5 is a diagram showing an example of GR ratio characteristics based on the conventional technology, Fig. 6 is a diagram showing an example of GR ratio behavior when the load changes, and Fig. 7 is a diagram showing an example of GR ratio behavior when the load changes. FIG. 8, a basic control system diagram according to the prior art, is a diagram showing an example of the influence of the GR ratio on the unburned rate. 3... Exhaust gas circulation amount adjustment damper, 9... First stage reheater, 10... Superheater, economizer row, 11... Reheater side parallel damper, 12... Superheater side parallel damper w
Figure c1 Applicant Babcock Hitachi Co., Ltd. Agent Patent Attorney Takayoshi Matsunaga 1 person Boiler load → Figure Figure Figure Boiler load □ Figure Time (h) Figure Time (h) Figure Time Figure GR ratio →

Claims (3)

[Claims] (1) A reheater that generates reheat steam, a superheater that generates main steam, a reheater damper and a superheater damper that distribute and control the flow rate of combustion exhaust gas to these reheaters and superheaters,
In a reheat steam temperature control method for a boiler equipped with a recirculation damper that controls the recirculation flow rate of flue gas recirculated to the boiler body, the ratio of the amount of recirculated flue gas to the total amount of flue gas is A certain recirculated flue gas ratio is set to a value that allows transient undershoot when the load increases, and when the boiler is operating other than when the load increases, the damper of the recirculation damper is opened so that the recirculated flue gas ratio is lower than when the load increases. A reheat steam temperature control method characterized by adjusting the temperature. (2) When the boiler is operated other than when the load increases, the opening degrees of the reheater damper and the superheater damper are respectively adjusted in accordance with or in parallel with the recirculated combustion exhaust gas ratio. 1. The reheat steam temperature control method according to 1. (3) The setting of the low recirculation exhaust gas ratio at times other than when the load increases is selected from two or more recirculation combustion exhaust gas ratio setting programs depending on fuel properties and operating conditions. reheat steam temperature control method.