JPS5892725A - Exhaust heat collection in hot-air oven - Google Patents

Abstract

PURPOSE:To provide the exhaust heat collection in hot-air oven the collection efficiency of which is very high by a method wherein a metal temperature is controlled within a sulphuric acid corrosion restriction zone. CONSTITUTION:If the metal temperature of a meter 14 exceeds a preset value, the opening degree of a flow rate adjustor valve 13 becomes small to reduce a bypass flow rate. As a result of this, a preheated fluid flow rate through a heat exchanger 2 becomes relatively large and the amount of collected heat from an exhaust gas 4a increases. Accordingly, the temperature of the exhaust gas 4b at an outlet of the heat exchanger 2 drops down, therefore the temperature of a low temperature side metal 11 in an exhaust gas passage 2A of the heat exchanger 2 becomes the preset value. When the detected metal temperature of the meter 14 goes down lower than the preset value, the temperature of the low temperature side metal 11 is controlled automatically to the preset value by the above-mentioned operations in the reversed order. Thus, the sulphuric acid corrosion of the metal in the exhaust gas passage 2A of the metallic heat exchanger 2 can be prevented effectively.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a method for recovering exhaust heat from a hot-blast stove, which uses a metal heat exchanger to preheat combustion air and/or combustion fuel gas in a hot-blast stove with exhaust gas from the hot-blast stove. Heat exhaust port IIl!
2, within the limits of metal heat exchanger protection, and provide a method for reducing fuel usage.

Conventionally, as shown in FIG. 1, a plate-shaped metal heat exchanger 2, for example, is attached to a hot air stove l, and in this heat exchanger, exhaust gas supplied from the hot air stove l through a pipe 3 is The preheated fluid 7 is preheated by exchanging heat with the combustion air and/or combustion fuel gas (hereinafter referred to as preheated fluid) 7a of the hot blast stove l supplied from the processor 5 through the piping t.
B is supplied to a burner (not shown) of a hot air stove L through a pipe L to obtain an energy saving effect of saving fuel gas. Note that the exhaust gas pb whose temperature has been lowered in the heat exchanger λ passes through the pipe 9 and is dissipated into the atmosphere from the chimney lθ.

2 and JB indicate the exhaust gas passage and the preheated fluid passage of the heat exchanger.

In the hot blast stove l equipped with the heat exchanger A, the temperature of the exhaust gas +a at the outlet of the hot blast stove l changes due to operational fluctuations such as the amount of air blown from the hot blast furnace to the blast furnace (not shown) and fluctuations in the air temperature. is well known. For example, the dome temperature lψoo'c in a particular hot air stove l.

During steady operation at a blowing temperature of 300″C, hot blast furnace exhaust gas e
The average value of a is 2°C; for example, if the air temperature is 1
When the temperature decreases to 100° C. and the air flow rate decreases by 7θ, the average value of the exhaust gas pa is /71r″C, and there is a change in the average humidity of the exhaust gas of r+″C.

When the operating rate of the hot blast stove 1 decreases and the temperature of the exhaust gas aa of the hot blast stove 1 decreases, the temperature of the exhaust gas IIb discharged from the heat exchanger 1 also decreases. Then, when the temperature of the exhaust gas tIb drops extremely, the low temperature side metal of the exhaust gas passage JA of the heat exchanger 2
The temperature of the exhaust gas IIa decreases and SO3 in the exhaust gas IIa
Combined with H2O inside, dew condenses on the surface of the metal/l in the form of sulfuric acid, and the heat exchanger λ is corroded by sulfuric acid. If the heat exchanger λ is not replaced before the hot blast stove reaches the end of its service life. It will stop happening.

Therefore, conventionally, when the operating rate (exhaust gas temperature) of the hot blast stove is at its lowest, the temperature of the metal 1 is set at least to a temperature range (sulfuric acid corrosion suppression zone) in which sulfuric acid is deposited but does not corrode with sulfuric acid within the life of the hot blast stove. The capacity (heat transfer area) of the heat exchanger 2 is determined based on the conditions to be maintained, and exhaust heat recovery is performed.

However, during the steady operation period, which occupies most of the operating period of the hot blast stove, the exhaust gas temperature is high, so that the metal/1rysvib' degree of the heat exchanger 2 is maintained in a temperature range in which sulfuric acid does not precipitate (sulfuric acid corrosion prevention range). However, the temperature of the exhaust gas at the outlet of the heat exchanger 2 is high, and sufficient exhaust heat recovery is not performed.

In order to sufficiently recover waste heat, the inventor has determined that the operating rate of the hot blast furnace (
The idea was to determine the heat exchanger capacity and recover exhaust heat under the condition that the metal temperature falls within the sulfuric acid corrosion suppression range when the exhaust gas temperature is at or near the maximum value. However, in this case, it has been found that when the operating rate (exhaust gas temperature) of the hot air stove reaches its lowest value, the metal temperature falls into the sulfuric acid corrosion acceleration range, resulting in a problem that the life of the heat exchanger is shortened.

The present invention advantageously solves the above-mentioned problems and provides a method for recovering exhaust heat from a hot-blast stove, which sufficiently recovers exhaust heat, and the gist thereof is as follows.

In an exhaust heat recovery method for a hot-blast stove, in which heat exchange is performed between the flue gas of the hot-blast stove whose temperature fluctuates and the combustion air and/or combustion fuel gas of the hot-blast stove using a metal heat exchanger, the heat exchange is performed using the heat exchange method described above. The temperature of the exhaust gas is lowered until the metal temperature on the low temperature side of the exhaust gas passage of the device reaches the sulfuric acid corrosion acceleration range, and a bypass pipe for the fluid to be preheated is installed between the outlet piping of the L heat exchanger for the fluid to be preheated. A circulation piping is provided, the temperature of the metal on the low temperature side of the exhaust gas passage of the heat exchanger is detected, and the bypass flow rate and/or circulation flow rate of the fluid to be preheated is controlled to control the metal temperature to a sulfuric acid corrosion suppression range. The feature lies in the exhaust heat recovery method of hot air stoves.

The above sulfuric acid corrosion acceleration region and sulfuric acid corrosion inhibition region are 8 in exhaust gas.
It is determined by 0311 degrees, moisture, the metal material of the metal heat exchanger, etc. For example, in Figure 2, the SO3 concentration is /pp
This shows the relationship between metal surface temperature and corrosion rate when the metal material is sulfuric acid-resistant steel in an exhaust gas with a moisture content of about lθ%. ~/10″C is the sulfuric acid corrosion inhibition range, and 7
Below 0°C is the sulfuric acid corrosion acceleration range. Also, the temperature of the metal on the low temperature side of the exhaust gas passage of the heat exchanger varies depending on the type of heat exchanger, for example, in the case of a plate type heat exchanger or a heat medium type heat exchanger, the temperature of the metal on the low temperature side of the exhaust gas passage is 7'7.

Detection is performed by direct measurement using a Shida thermocouple or the like, and in the case of a rotary heat exchanger, by indirect measurement estimated from the temperature and flow rate of the fluid to be preheated and the exhaust gas.

Further, the low-temperature side metal temperature in the present invention refers to the lowest temperature among the metal surface temperatures that directly contact exhaust gas in a metal heat exchanger.

The present invention will be explained in detail below with reference to the drawings.

In FIG. 3, /2 indicates the outlet piping t of the preheated fluid of the heat exchanger λ (material: sulfuric acid resistant II) in FIG.

1 is a bypass pipe connected between 1 and 1, and 13 is a flow rate control valve disposed in the bypass pipe 12. l'l is a temperature measuring device, such as a thermocouple, for detecting the metal temperature on the low temperature side metal // of the exhaust gas passage 2A of the heat exchanger, and lS is a measuring device/41.
This is a temperature controller that controls the opening degree of the flow rate regulating valve 13 based on the actual temperature from the sulfuric acid corrosion suppression range and the set temperature preset from the sulfuric acid corrosion suppression range shown in FIG. In this embodiment, the set temperature is set to, for example, 75''C.

The low-temperature side metal of the exhaust gas passage 2A of the heat exchanger configured as above// (D) Providing a control device that automatically controls the temperature to the set temperature G, - 1 measuring device/4'
When the metal temperature becomes higher than the preset level L, the opening degree of the flow rate control valve 13 becomes smaller, the bypass flow rate is reduced, and the flow rate of the preheated fluid passing through the heat exchanger 2 increases, so that the flow rate of the preheated fluid increases more than the exhaust gas +a. The amount of recovered heat increases, the temperature of the exhaust gas pb at the outlet of the heat exchanger 2 decreases, and the temperature of the low-temperature side metal // of the two exhaust gas passages of the two heat exchangers reaches the set temperature. - When the detected metal temperature of the blade side regulator/l becomes lower than the above set temperature, the opening degree of the flow rate control valve 13 increases, the bypass flow rate increases, and the relative flow rate of the preheated fluid passing through the heat exchanger 2 decreases, the amount of heat recovered from the exhaust gas IIa decreases, and the temperature of the exhaust gas Ilb at the exit of the heat exchanger 2 increases.
The temperature of the low-temperature side metal of the two exhaust gas passages is automatically controlled to a set temperature of 4. As a result, the metal heat exchanger λ
Sulfuric acid corrosion of the two metals in the exhaust gas passage is effectively prevented.

Fig. q shows another actual IM embodiment of the method of the present invention, in which a circulation pipe 16 is provided between the preheated fluid of the heat exchanger λ and the outlet pipe 6°, and this pipe 1 is connected to the heat exchanger. Outlet piping of the container g
A blower 17 is provided so that the fluid to be preheated can be fed back to the artificial piping 6, and a flow rate control valve 13 is provided at the outlet piping of the blower 77, and a temperature measuring device llS temperature controller/ S operates the flow rate control valve 13, in detail, operates the circulation flow rate of the fluid to be preheated, adjusts the inlet temperature of the fluid to be preheated to the heat exchanger, and controls the flow rate of the two exhaust gas flow paths of the heat exchanger. The temperature of the low temperature side metal // is automatically controlled to the set temperature.

Figure 5 shows an embodiment of the method of the present invention when a heat medium type heat exchanger is used as the metal heat exchanger to preheat the combustion air and combustion fuel gas of a hot air stove. hot stove l
11.1 is a heat exchange chamber between the exhaust gas and the heating medium provided in the exhaust gas pipe 19; 24I is a heat exchange chamber between air and a heat medium provided in the air pipe 'B', 24I is a heat medium circulation pipe, and B is a heat medium circulation pump. S is a thermocouple that detects the temperature of the low-temperature side metal in the exhaust gas passage of the heat exchanger, S is a bypass pipe installed between the exit pipes of the heat exchange room N, and I is a bypass pipe installed between the exhaust pipes of the heat exchange room n.

The flow rate control valve installed on the piping knife compares the actual temperature of the thermocouple with the set temperature and adjusts the opening degree of the flow rate control valve J to control the low temperature side metal temperature to the set value. It is a controller.

As described above, a bypass piping and/or circulation piping for the preheated fluid is provided between the outlet piping of the metal heat exchanger for the preheated fluid, and the temperature of the metal on the low temperature side of the exhaust gas passage of the heat exchanger is detected. Although the metal temperature is controlled to a set temperature by manipulating the bypass flow rate and/or circulation flow rate of the preheating fluid, the sulfuric acid corrosion rate in the heat exchanger can be reduced even if the hot blast furnace exhaust gas temperature fluctuates. As a result, there is no need to replace the heat exchanger due to sulfuric acid corrosion within the life of the hot blast stove.

In addition, even if the hot air stove exhaust gas temperature fluctuates, it is possible to reduce the sulfuric acid corrosion rate of the heat exchanger. The capacity of the heat exchanger can be determined under the condition of maximizing the amount of recovery, and exhaust heat can be recovered, and the performance can be achieved by performing the maximum heat recovery. As a result, maximum fuel gas savings can be achieved.

For example, in the exhaust heat recovery device shown in Figure S, the minimum temperature of the hot air stove in °C1/71"C1 is the set temperature of the metal temperature control device 75
``C1 At this time, the exhaust gas temperature at the heat exchanger outlet / 20''C, in the conventional exhaust heat recovery method that does not use a metal temperature control device, the minimum exhaust gas temperature is 17g'' to prevent sulfuric acid corrosion.
At C, the capacity of the heat exchanger (heat transfer area of the exhaust gas passage) is determined so that the amount of heat exchanger x exhaust gas fB degrees/20"C (metal temperature 75"C), so the capacity is jooo1
112, and when the hot stove exhaust gas temperature is at its maximum of 2°C, the outlet exhaust gas temperature of the heat exchanger is 17/''C. Even though it is equipped, the exhaust gas temperature is 262°
At the maximum value of C', the exhaust gas temperature at the outlet of the heat exchanger is /2
The capacity of the heat exchanger can be determined so that the temperature is 0''C (metal temperature 7s''C), and the capacity is /6000112. Therefore, in the method of the present invention, compared to the conventional method, the amount of heat is 1. s can be recovered by 6 times, 56
You can save fuel gas by %.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram of the conventional exhaust heat recovery method for a hot-blast stove. Figure 1 is an explanation of the relationship between the metal temperature of the heat exchanger and the corrosion rate. FIG. 2 is an explanatory diagram of an exhaust heat recovery method. 1...Hot stove, 2...Heat exchanger, 3. t, r, q・
...Piping, +a, +b...exhaust gas,! , /7
...Blower-17a. 7b...Fluid to be preheated, 10...Chimney, ll...Low temperature side metal, 1"/J s E "" bypass piping, 1
3,1...flow control valve, /41-1 temperature measuring device, /j
, ! ...Temperature controller, 16...Circulation piping, /I
, 27 , 22... Heat exchange chamber, /D... Exhaust gas piping, J... Fuel gas piping, n... Air piping, 24
I...heat medium circulation piping, j...pump, B...thermocouple. Patent applicant: Nippon Steel, Hayashi Shiki Kaisha

Claims (1)

[Claims] In a hot-blast stove exhaust heat recovery method in which the flue gas of the hot-blast stove whose temperature fluctuates and the combustion air and/or combustion fuel gas of the hot-blast stove are not exchanged with a metal heat exchanger, heat exchange is performed with the flue gas of the heat exchanger. The temperature of the exhaust gas is lowered until the metal temperature on the low temperature side of the passage reaches the sulfuric acid corrosion acceleration range, and bypass piping and/or circulation piping for the preheated fluid is provided between the outlet piping of the heat exchanger for the fluid to be preheated. is installed to detect the temperature of the metal on the low temperature side of the exhaust gas passage of the heat exchanger,
Method 0 for recovering exhaust heat 1' of a hot blast stove, characterized by controlling the metal temperature to a sulfuric acid corrosion suppression range by controlling the bypass flow rate and/or circulation flow rate of the fluid to be preheated.