JPH04332302A - Control method for heat collecting device - Google Patents

Abstract

PURPOSE:To maintain a boiler operation pattern in its optimum condition by setting the temperature of exhaust gas mixed after it has passed an air preheater and an exhaust gas cooler respectively as a minimum value and controlling the flow rate of circulation water in response to the concentration of SO2 with a flow rate control valve. CONSTITUTION:An exhaust gas-contained SO2 meter 12 is adapted to measure the exhaust gas of a boiler 1 which uses a blast furnace gas(BFG) as a fuel, which flows past a combustion air preheater 2 and an exhaust gas cooler 3 and is sucked and mixed by an induction fan 8. A circulation water low temperature side thermometer 11 is adapted to measure the temperature of circulation water of a BFG heater 9. The measurement data enters a control device 24 together with the signals of the SO2 meter, a BFG flow meter 22 and a circulation water flow meter 23. A flow rate control valve 18 of circulation water is adjusted so that the temperature of the exhaust gas may be minimized in conformity with the concentration of SO2. It is, therefore, possible to prevent the corrosion of an element at a low temperature portion, reduce the temperature of the exhaust gas and preserve high heat with efficiency.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling a forced circulation indirect low temperature heat recovery device.

0002

[Prior Art] Boilers for private power generation at steel plants contain blast furnace gas (Blast Fu), which is generated as a by-product gas from blast furnaces.
Some boilers use rnace gas (hereinafter abbreviated as BFG) as fuel;
In order to recover heat that cannot be recovered by the air preheater alone,
A low-temperature heat recovery device is installed.

[0003] As an example of such an exhaust gas heating type low-temperature heat recovery device, there is a conventional one constructed as shown in FIG. In this device, exhaust gas discharged from a boiler 1 is branched and supplied to an air preheater 2 and an exhaust gas cooler 3, which is a low-temperature heat recovery device. Here, the distribution of exhaust gas supply amount to these air preheater 2 and exhaust gas cooler 3 is as follows:
This is done by an air preheater inlet damper 4 and an exhaust gas cooler inlet damper 5. The exhaust gas supplied to the exhaust gas cooler 3 is cooled by circulating water circulated by the circulating water pump 7, and is returned to the outlet side of the air preheater 2 via the exhaust gas cooler outlet damper 6, where it is combined with the exhaust gas discharged from the air preheater 2. It joins at point A and is discharged through the induced draft fan 8.

On the other hand, BFG derived from a blast furnace is
The circulating water is heated in the heater 9 and supplied to the boiler 1 . At the same time, the air taken in by the forced draft fan 10 is heated by the air preheater 2 and then introduced into the boiler 1. In the figure, 11 indicates a circulating water low temperature side thermometer, and 12 indicates an SO2 concentration meter.

By the way, in order to optimize the operating condition of such a boiler, the exhaust gas temperature of the boiler 1, that is, the temperature after the exhaust gases exiting the air preheater 2 and the exhaust gas cooler 3 are combined, must be the lowest. It is necessary to set the circulating water flow rate as follows. Conventionally, fixed value control was used to keep the circulating water flow rate constant depending on the presence or absence of heavy oil co-firing.

[0006]

Problems to be Solved by the Invention In the fixed value control based on the presence or absence of heavy oil co-firing as described above, the boiler 1 cannot necessarily be operated with the exhaust gas temperature optimized.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of controlling a low-temperature heat recovery device that can always keep the boiler operating pattern in an optimal state.

[0008]

[Means for Solving the Problems] In order to solve the above-mentioned conventional problems, the present invention provides an air preheater that heats combustion air with a part of the exhaust gas of a boiler that uses blast furnace gas as fuel, and a A heat recovery device comprising: an exhaust gas cooler that heats circulating water with the remainder; and a blast furnace gas heater that heats the blast furnace gas supplied to the boiler with the heated circulating water, wherein the air preheater and the exhaust gas cooler The flow rate of the circulating water is controlled so that the temperature of the mixed exhaust gas becomes the lowest after passing through each, and the temperature of the circulating water is limited according to the SO2 concentration in the mixed exhaust gas. This invention proposes a method of controlling a heat recovery device characterized by adjustment using a valve.

[0009]

[Operation] The present invention is a low-temperature heat recovery device that distributes the exhaust gas of a boiler to which blast furnace gas generated from a blast furnace is supplied as fuel to an air preheater and an exhaust gas cooler, and uses circulating water as a medium for heat exchange with the blast furnace gas. In this control, we focused on the fact that the boiler exhaust gas temperature is a barometer for optimal boiler operation, and set the circulating water flow rate so that the boiler exhaust gas temperature is the lowest, and also set the acid dew point of the low temperature element of the low temperature heat recovery equipment. SO2 in exhaust gas to prevent corrosion
Control is performed to limit the concentration.

[0010] As a result, the circulating water flow rate is controlled so that the boiler exhaust gas temperature is the lowest, so that operation at a point where the boiler efficiency is high can be realized. In addition, S in the exhaust gas
Since a limit is set based on the O2 concentration, acid dew point corrosion of the low temperature section element can be prevented.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a system diagram showing an example of an apparatus for implementing the method of the present invention, and FIG. 2 is a control flow diagram.

The boiler efficiency is evaluated after the exhaust gas that has passed through the air preheater 2 and the exhaust gas cooler 3 is mixed (point A).
The lower the temperature, the higher the boiler efficiency.

[0013] Here, BFG has a low calorie content (approximately 800k
cal/Nm3), the amount input to the boiler is larger than other fuels (such as heavy oil), and the amount of exhaust gas produced is large. Therefore, heat exchange between exhaust gas and air in the air preheater 2 is strongly influenced by the BFG flow rate. Further, the exhaust gas cooler 3 exchanges heat with the BFG heater 9. Therefore, it is strongly influenced by the BFG flow rate. Like this B
In FG-fired boilers, changes in fuel amount have a stronger effect on boiler characteristics than in boilers that use other fuels (heavy oil, etc.), and the optimal circulating water flow rate at which the boiler outlet exhaust gas temperature is the lowest is determined by the BFG flow rate. Ru. Therefore, in this embodiment, the BFG flow rate is actually measured and determined by the flowmeter 22, the value is given to the function generator 13, and the output of this function generator 13 is given to the subtracter 15. In this function generator 13, a function as shown in FIG. 3, for example, is set.

Next, the actual value of the circulating water flow rate measured by the flowmeter 23 is applied to the subtracter 15 via the transfer relay 14, and compared with the output of the function generator 13, and the difference is applied to the proportional-integral calculator 16. .

On the other hand, the SO2 concentration in the exhaust gas measured by the SO2 concentration meter 12 is applied to the function generator 19 to determine the limit value for the current operating state. Further, the low temperature side temperature of the circulating water is actually measured by the thermometer 11 and compared with the above-mentioned limit value, which is the output of the function generator 19, by the subtracter 20 via the transfer relay 21. And adder 17
, the output of the proportional integral calculator 16 is converted to SO in the exhaust gas.
2. Correct by the output of the subtracter 20, which is a concentration-based limit value, and give a signal to the flow rate control valve 18.

According to such a configuration, the optimum circulating water flow rate corresponding to each operation pattern is determined by the function generator 13, and the limit value depending on the SO2 concentration in the exhaust gas is determined by the function generator 19. , optimal and reliable boiler operation can be achieved.

[0017]

[Effects of the Invention] According to the method of the present invention, the circulating water flow rate is set so that the boiler exhaust gas temperature is the lowest, and a restriction is added to prevent acid dew point corrosion of the low temperature section element of the exhaust gas cooler. Since the control is carried out, the circulating water flow rate is controlled so that corrosion of the low-temperature section element is always prevented and the exhaust gas temperature of the boiler is lowered, that is, the thermal efficiency is increased.

[Brief explanation of drawings]

FIG. 1 is a system diagram showing an example of an apparatus for carrying out the method of the present invention.

FIG. 2 is a diagram showing an example of a control flow for implementing the method of the present invention.

3 is a diagram showing an example of a function set in the function generator 13 in FIG. 2. FIG.

FIG. 4 is a system diagram showing an example of a conventional heat recovery device.

[Explanation of symbols]

1 Boiler 2 Air preheater 3 Exhaust gas cooler 4, 5, 6 Damper 7 Circulating water pump 8 Induced draft fan 9 BFG heater 10 Forced draft fan 11 Circulating water low temperature side thermometer 12
SO2 in exhaust gas Total 13 Function generator 14 Transfer relay 15
Subtractor 16 Proportional integral calculator 17 Adder 18 Flow control valve 19 Function generator 20 Subtractor 21 Transfer relays 22, 23
Flowmeter 24 Control device

Claims (1)

[Claims] Claim 1: An air preheater that heats combustion air with a part of the exhaust gas of a boiler that uses blast furnace gas as fuel; an exhaust gas cooler that heats circulating water with the remainder of the exhaust gas; In a heat recovery device equipped with a blast furnace gas heater that heats blast furnace gas with the heated circulating water, the temperature of the mixed exhaust gas is the lowest after passing through the air preheater and the exhaust gas cooler, respectively, and SO in the exhaust gas after the above mixing
2. A method for controlling a heat recovery device, characterized in that the flow rate of the circulating water is adjusted by a flow control valve so as to limit the temperature of the circulating water according to the concentration.