JPS5920954B2 - Waste heat recovery method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to waste heat recovery, and in particular to a method and apparatus for recovering heat from hot gases.

Heat exchange is an important aspect of essentially all processing operations, whether hot or cold processing conditions.

Usually, economics is to utilize heat transfer devices more effectively in the process stream.

Waste heat recovery generally refers to the recovery of geothermal heat for basic heat treatment.
For example, in a steam generator, there is usually a convection section in the device where the temperature level is insufficient for steam generation, but where sensible heat is available for heating work such as preheating of the water sent to the steam drum. .

For example, there are some process operations, such as cupola operations, in which heat may be recovered but not effectively recovered.

In a typical casting operation, coke, limestone, and metal parts such as pig iron and pig iron are introduced into a cupola through a charging door.

Cold injection air is introduced through the bottom slats and serves as a medium for burning the coke.

Furthermore, air is introduced through the charging door by means of an exhaust fan.

An afterburner located above the charging door provides an ignition source for carbon monoxide leaving the floor and provides heat to the cupola when it is not in production.

The air entering the cupola as injection air, charge door air and afterburner air is normally cold and heated to operating temperature by consuming fuel in the afterburner or by consuming coke in the lower part of the cupola.

Hot gases at temperatures between 98F) and 1204C (180C to 2200F) are withdrawn from the top of the cupola and passed through a generally vertically oriented water scrubber, where the gases are
Before being introduced into a solids collector, e.g. an electrostatic precipitator or a baghouse,
00°F).

Direct water cooling and cleaning generates large amounts of steam and increases the amount of gas to downstream equipment.

A small number of factories have heat transfer or storage heat recovery equipment installed.

The heat transfer type uses an expensive high-alloy heat exchanger to heat the injection air and cool the hot gas.

This type of heat exchanger is extremely expensive;
High metal temperature (981-1204℃) (1800℃~
This is due to the high alloy construction required to withstand temperatures (2200°F) and the large amount of heat transfer surface due to the poor heat transfer coefficient of hot gas to cold air.

The heat transfer system operates once per day with fluctuations ranging from ambient humidity to 1093°C (2000°F) during daily start and stop times.
Occurs 4 times (70!'~1093℃) (1306)
Large temperature fluctuations (from 2000°F to 2000°F) are common;
resulting in mechanical failure.

In a regenerative system, an expensive mesh or wheel rotates and is alternately heated by hot gas and cooled by cold air.

This type of heat exchanger is very large and causes significant maintenance and plant closures due to seal failure and corrosion when the cold air condenses moisture and sulfur dioxide from the hot gases.

These heat transfer type and heat storage type waste heat recovery devices operate at the operating temperature of the plant, that is, 982 to 1093 degrees Celsius.
It works effectively only when a large amount of injection air is required at (180θ to 20000F) (gas temperature).

During no load, the afterburner heats the cupola to approximately 704°C (1
300°F) and no injection air is required, a small amount of heat is recovered.

The unloaded time is 8 hours or at most 12 hours per day.

The corresponding melting time is only 8 or 4 hours, and the effective heat recovery time is 8 to 4 hours per day.

Generally, such devices have been limited to the heat recovery necessary to preheat the combustion air to save fuel.

Some processing operations require gas combustion auxiliary equipment because combustion of fuel oil produces a dirty, soot-stained exhaust gas that is unacceptable for some processing operations.

As briefly mentioned above, heat exchangers have been used for various waste heat treatments using all common heat transfer media.

U.S. Pat. No. 3,426,733 discloses the use of closed-loop devices to recover heat with heat transfer fluids such as eutectic salt mixtures, aromatic heat transfer oils, and tetrachlorobiphenyl compounds; Since it is a closed loop, it has its own problems.

U.S. Pat. No. 2,910,244 discloses a method and apparatus for carrying out endothermic chemical reactions using molten salt mixtures as intermediate heat transfer media.

The above and other objects of the present invention are as follows:
Achieved by heat exchange equipment that uses a eutectic salt melt, e.g., consisting of potassium and sodium salts, as an intermediate heat transfer medium for process operations such as the operation of cupolas producing waste gases or waste gases at temperatures between 0 theta and 2500 degrees Fahrenheit. be done.

In other embodiments, the objects of the invention are achieved using at least two heat exchange devices that utilize intermediate heat transfer media for processing operations.

With such a device, the heat exchanger can be fabricated using ordinary construction materials instead of expensive high-alloy construction materials;
Furthermore, heat can be recovered significantly more than when using a single intermediate heat transfer medium as described for the first embodiment.

For a clearer understanding of the invention, exemplary embodiments will now be described with reference to the accompanying drawings, each of which shows a schematic pig diagram.

In FIG. 1, a cylindrical cupola is indicated generally at 10, which includes a vessel 12 with a hemispherical top 14, a charging door 16, a tuyere 18, indicated generally at 20. Consists of a molten iron drawing assembly.

The container is equipped with a jet hot air line 22, a charging door hot air line 24, and an afterburner line 28, and the charging door is connected to line 2.
6 opens to the outside.

The upper portion of the vessel 12 is provided with a crossover duct 30 in fluid communication with a heat exchanger 32 of a heat recovery system, indicated generally at 34.

The heat recovery device 34 also includes a salt bath 36 and a heat use device 38.

It was made of a material that held the eutectic salt melt and did not react with the eutectic salt.

The salt tank 36 is in fluid communication with the suction side of a pump 42 attached to the tank 36, and its flow side is connected by a conduit 44 to the tube or shell side of the heat exchanger 32.

The liquid side outlet of heat exchanger 32 is in fluid communication with heat use device 38 by conduit 46, while device 38 is in fluid flow communication by conduit 48.

Salt bath 36 is provided with conduit 50 to take it out of service, as will be discussed in more detail below.

As previously mentioned, the heat-using devices include conduits 22, 24 and 28.
It is equipped with a heat exchanger that preheats the gas flowing through it, a steam generator that heats the space, or a steam turbine that generates electric or compressed gas refrigerant.

In operation, the heat recovery device 34 uses its intermediate thermal fluid to recover heat from the hot gas, store it during melting and no-load operation, and heat the injection air, burner air, and charging door air. It is used to utilize heat in various ways, such as by generating steam in the salt in a steam generating heat exchanger.

In winter, the base is set to the lowest setting to recover the maximum amount of heat and the steam produced is used for space heating of the factory or adjacent offices and residences.

In the summer, the base is set to its highest setting to preheat the injection air, burner air, and charging door air, and to generate electricity for standard steam turbine generators to power factory motors or air conditioning for the factory, adjacent offices, and residences. Drive the device.

One of the important features of the present invention is that the high temperature gas recovered from the cupola is
The melting action of the cupola stores heat in the heat transfer fluid system when the temperature is 704°C (1300°F) and rejects heat when the system is unloaded, the afterburner is activated and the hot gas is 704°C (1300°F). It is an ability.

A typical run is 30 minutes melt and 30 minutes dry run for a total of 16 hours each day.

The heat recovery device operates as a storage device with a base range of 204-538°C (400-1000°F).

The low temperature is set as the lowest stable temperature selected as the maximum allowable temperature of the heat transfer fluid.

Furthermore, by using a salt mixture, auxiliary combustion is performed using fuel oil, resulting in less gas and coke.

Another feature of the invention is the use of hot charge door air.

The charging door is usually an opening in the side of the cupola;
To facilitate operation, this cupola is always open to allow air to flow into the cupola.

Air is added to either side of the charge door at a point below the charge door or through one or more openings and heated by the recovery device.

Such hot air reduces the amount of cold air entering the charge door because it prevents smoke and gases generated in the lower region of the cupola from leaving the cupola through the charge door.

The rising smoke and gases are pushed or guided away from the charge door by the hot charge door air flowing horizontally into the cupola.

For example, the injection air is 566.3 m37 minutes (standard condition)
(20,000sefm) Charging door suction is 20,000
scfm Flue gas temperature is 982℃ (18000F
), a large cupola is assumed to operate for 6000 hours each year.

According to the heat recovery device of the present invention, the flue gas is
00°F) and using the recovered heat, 2.9
Annual savings of $1 million are realized by reducing gas and coke consumption at an average cost of $3 per million BtU.

Turning now to FIG. 2, there is shown a cylindrical cupola, indicated generally by the numeral 110, which includes a container 112 with a hemispherical top 114, a charging door 116, a siding 118,
and a molten iron drawing assembly indicated generally at 120.

The container 112 has a hot injection air line 122, a charging door air line 124, and a charging door vent line 12 that opens to the outside.
6 and an afterburner conduit 128.

The upper portion of vessel 112 is provided with a crossover duct 130 in fluid communication with primary and secondary heat exchangers 132, 134, respectively, of a heat recovery system, indicated generally at 136.

Heat recovery device 136 also includes a salt bath (not shown), where the molten salt comprises one of the intermediate heat transfer fluids.

The primary heat exchanger 132 is connected to conduits 140 and 142 and 1.
44, they are in fluid communication with the tubes or shell sides of heat exchangers 146, 148, respectively.

The primary heat transfer media side outlets of heat exchangers 146 and 148 are in fluid communication with primary heat exchanger 132 by conduits 150 and 152, respectively, and via conduit 154.

Secondary heat exchanger 134 is in fluid communication through conduit 156 and with the tube or shell sides of heat exchangers 162 and 164 through conduits 158 and 160, respectively.

The outlets of heat exchangers 162 and 164 are connected to conduits 166 and 16, respectively.
8, these conduits are combined into conduit 1
70 and returns to the secondary heat exchanger 134.

Conduit 180 containing the caloric thermal fluid is in fluid communication with heat exchangers 164 and 166 by conduit 182, with an outlet of heat exchanger 146 leading to conduit 184, which connects conduits 128, 12
4 and 122.

Conduit 186 containing another thermal fluid is in fluid communication with heat exchangers 162 and 148 by conduit 188, and heat exchanger 14
The outlet of 8 is in fluid communication with conduit 190.

The outlet of the secondary heat exchanger 134 is connected by conduit 192 to a wet scrubber 194 and by conduit 196 to a precipitator 198.
The air is vented to the atmosphere through an exhaust fan 100 and an exhaust fan 100.

The following table shows that the injection air is 226.527713/min (standard condition) (8,000sefm) and the charging door suction is 226.527713/min (standard condition).
．． 52m3/min (standard condition) (8,000sefm)
shows the cupola operating conditions for 6000 hours per year with a flue gas temperature of 9.82°C (18000F).

A heat recovery system according to a second embodiment of the invention cools the flue gas to 204°C (400°F) and uses the recovered heat to generate steam, further reducing gas and coke consumption. Achieved annual savings of $400,000.

The intermediate heat transfer media of the primary and secondary heat exchangers 132, 134 are a base compound and water, respectively.

The following table ■ shows the operating conditions of the cupola under no-load conditions.

Cupola I was operated in the same way with the intermediate heat transfer oil used in the primary and secondary heat exchangers and had the conditions shown in Table 1 below.

The following table ■ shows the no-load conditions.

Note that although the airflow temperature is different, the exhaust gas temperature and flow rate are the same, and the fuel conditions change depending on the difference.

Since the heat recovery device of the present invention significantly reduces the amount of gas,
Significantly improves the construction and maintenance of pollution control equipment associated with various processes (i.e., wet scrubbers, dust collectors, bugs, or mechanical dust collectors).

In existing cast cupolas with wet scrubbers, water consumption is significantly reduced when the flue gases are cooled before being quenched in the scrubber.

This reduction in water evaporation significantly reduces the amount and weight of saturated gas that the equipment fan must process.

Therefore, by heat recovery rather than direct spray water cooling, the gas can be moved from 982°C (1800°F) to 260°C (500°F).
) to reduce the flow rate by 31 percent.

The heat recovery device of the present invention has many benefits, including:

(1) The needle base device has a large heat capacity, so it accumulates a large amount of heat.
Let it be stored.

Reuse of recovered energy can be used to peak loads or satisfy other conditions that have different usage patterns than waste heat sources.

(2) Because the heat transfer coefficient between the exchanger and the molten salt is very large, the residual heat transfer rate is much higher than that of a gas-to-air heat exchanger.

The salt film transfer coefficient is approximately 50 times greater than the air film transfer coefficient of a gas-to-air heat exchanger.

The heat transfer surface area required is therefore approximately half that required for an equal power gas-to-air exchanger.

(3) Since the heat transfer coefficient is large as described above, the exchanger surface temperature is maintained within a few degrees of the molten salt temperature.

The high temperature causes the metal surfaces of the salt unit exchanger to be 500 degrees cooler than the metal surfaces of the gas-to-air exchanger.

This lower metal temperature contributes to economy of construction and reliability of operation.

Standard construction materials are used in salt unit exchangers instead of the high alloys required in gas-to-air exchangers.

(The four exchangers and near equality in basicity, combined with the large heat capacity of the circulating salt, provide an exchange surface that is independent of rapid fluctuations in flue gas temperature.

Therefore, salt unit exchangers are not subject to the large metal temperature fluctuations common to gas-to-air exchangers.

(5) The salt diluter provides considerable flexibility in choosing the method and rate of reuse of recovered heat.

This heat can be used for preheating air, generating steam, direct heating, etc.

Other waste heat recovery devices do not have this flexibility.

(6) Multiple cupola isometric waste heat sources in a large foundry are operated by a single-needle base circulation system, making the control, circulation and reuse system extremely economical.

(7) Molten salt is non-flammable and non-corrosive, and equipment using it operates at atmospheric pressure and position.

The salt is also thermally stable up to 538°C (1000QF).

(8) Salt dilution techniques (i.e., adding water to the salt at shutdown to preserve the fluidity of the molten salt used as a heat transfer medium;
Furthermore, by adopting a technology that condenses salt at the start of startup and returns it to a steady operating state, the problem of solidification of molten salt during startup and shutdown operations can be resolved.

Although the present invention has been described above with reference to the introduction of a heat recovery device in combination with a cupola, this device can also be used to operate units such as metal processing, chemical processing or refining processes, in particular to remove dust before it is released into the atmosphere. Useful for processes that generate hot, dirty gases containing fines that are separated within the equipment.

Before passing through the dust removal equipment, the high temperature dirty gas is heated to 204~2
Because cooling to 60 DEG C. (408-500 DEG F.) is required, the method and apparatus of the present invention is a particularly economical alternative to existing methods.

Furthermore, two or more heat exchangers are arranged in tandem, and intermediate heat transfer fluids with different temperatures are used, such as molten salt, oil and water, or molten salt, oil and oil.

The operating temperature of the heat transfer fluid is (temperature is typically 538°C, respectively).
Thermal stability of salts and oils (10000F) and 316°C (6000F) and vapor pressure of water (17.4kg/Cl1L)
(247 psia) and typically varies depending on the vapor pressure at 204°C (4000F).

[Brief explanation of the drawing]

1 and 2 are schematic system diagrams showing embodiments of the present invention, respectively. Explanation of symbols of main parts, 10,110...Cupola, 12,112...Container, 14,114...Semispherical upper lid, 16,116...Charging door, 18,118...
・Panel, 20,120... Molten iron drawn assembly, 30°130... Crossover duct, 32.132.134,14
6゜148.162,164...heat exchanger, 34,1
36... Heat recovery device, 36... Salt cypress, 38... Heat use device, 42... Pump, 110... Exhaust fan, 194... Wet scrubber, 198... Precipitator.

Claims (1)

[Scope of Claim] A method for recovering heat from exhaust gas having a temperature of 1260° C. to 1372° C., comprising: (a) passing said exhaust gas through a salt mixture in an indirect heat transfer manner; A method comprising: (1) passing the heated salt mixture of step (b) through a heat transfer fluid to cool said heated salt mixture; and returning the cooled salt mixture of step (b) to step (b). A method according to claim 1, wherein the exhaust gas is at a temperature of 982° C. to 1205° C. and is cooled to a temperature of about 204° C. to 260° C. 3. The exhaust gas is an exhaust gas generated from a cupola; b) passing said gas through the salt mixture in an indirect heat transfer manner; (0) passing said 7JD heated salt mixture in an indirect heat transfer manner through an air stream subsequently introduced into the cupola; The method according to claim 1 or 2, in particular, in which the cooled salt mixture from the box → is returned to the step (→). 4. The heated air stream of step (b) 5. A method according to claim 3 for providing the required quantities of injection air, charge door air and afterburner air. 6. The method of claim 3, wherein the salt mixture is withdrawn under heating from an afterburner air stream. 6. The method of claim 5, wherein said heated salt mixture is stored during said pause. 7. A method according to claim 3, wherein a diluent is added to the salt mixture during shutdown. 8. A method for recovering heat from humid exhaust gases of 260° C. to 1372° C. generated in unit operation. (→ passing said exhaust gas in an indirect heat transfer manner through an intermediate heat transfer medium salt in at least two successive heat exchange zones operating at different temperatures; (b) recovering heat from said intermediate transfer fluid at different temperature levels; Then, (・) cooled intermediate heat transfer medium salt is processed (
The only way to do this is to return to →. 9. According to claim 8, wherein a first intermediate heat transfer medium salt having a higher operating temperature is passed through a first heat transfer zone and a second intermediate heat transfer medium salt having a lower operating temperature is passed to the next heat transfer zone. Method described. 10. The method of claim 9, wherein the recovered heat is used to preheat air introduced into the unit operation. 11. The method of claim 10, wherein the preheated air stream provides the necessary injection air, charge door air and afterburner air in the cupola. 12 The exhaust gas has a temperature of 982°C to 1205°C,
12. The method of claim 11, wherein the method is cooled to a temperature of about 204<0>C to 260<0>C. 13. Claim 1, wherein the cupola is in a suspended state and the exhausted gas is withdrawn at a temperature of about 704° C. together with the air stream heating the afterburner air stream.
The method described in Section 2.