GB1585748A

GB1585748A - Waste heat recovery process

Info

Publication number: GB1585748A
Application number: GB5708/78A
Authority: GB
Original assignee: American Hydrotherm Corp
Current assignee: American Hydrotherm Corp
Priority date: 1977-02-14
Filing date: 1978-02-13
Publication date: 1981-03-11
Also published as: DE2805840C2; JPS5920954B2; JPS53122609A; CA1108852A; DE2805840A1

Description

(54) WASTE HEAT RECOVERY PROCESS (71) We, AMERICAN HYDROTHERM CORPORATION, a cprporation organised and existing under the laws of the State of New York, United States of America, of 470 Park Avenue South, New York, New York, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to waste heat recovery, and more particularly to a process and apparatus for the recovery of heat from high temperature gases.

Heat exchange is an important aspect of essentially all process operations whether at high orlow temperature processing conditions. Economics normally dictate effective utilization of heat transfer equipment with respect to processing streams. Waste heat recovery generally relates to the recovery of heat over and above basic heat requirements, e.g. in steam generation equipment, there are normally a convection section disposed in the equipment whereat the temperature level is insufficient for steam generation but at a level where sensible heat is available for heating duty, such as preheating water to be passed to a steam drum. There are some processing operations where heat is available for recovery, but is not effectively recovered, if at all, e.g., the operation of a cupola.

In a typical foundry operation. coke. limestone and a metallic portion, such as pig and scrap iron are introduced through a charge door into a cupola. Cold blast air is introduced through tuyeres in the bottom to provide the combustion medium for the coke. Additional air is induced through the charge door by an exhaust fan. Afterburners located above the charge door provide a source of ignition for carbon monoxide leaving the bed and for providing heat for the cupola when the cupola is not in production. Air entering the cupola in the form of blast air. charge door air, and afterburner air is normally cold and is heated to operating temperature by consuming fuel at the afterburners or by consuming coke in the lower portion of the cupola.

Hot gases at a tem-perature of from about 1800 to about 2200"F are withdrawn from the top of the cupola and are generally passed to a vertically disposed water scrubber wherein the gas is cooled to a temperature of from 400" to 500"F prior to introduction into a solids collector. e.g. an electrostatic precipitator or bag house. With direct water cooling and scrubbing. a large quantity of steam is produced which increases the volume of gas through the downstream equipment.

Heat recovery systems have been installed in a small number of plants in the form of either recuperative or regenerative types of heat recovery systems. With a recuperative type.

expensive high alloy heat exchanger is employed to cool the hot gas by heating the blast air.

This type of heat exchanger is very expensive due to the high alloy construction needed to withstand the high metal temperatures (1800"F to 22000F) and the large amount of heat transfer surface required as a result of the poor heat transfer coefficient of hot gas to cool air.

The recuperative type is subject to mechanical failures due to the frequent wide swings in temperatures from 1300"-2000"F which can occur as much as 14 times a day with swings ranging from ambient to 2000"F occurring with the daily startup and shutdown routine.

In the regenerative type. an expensive mesh wheel rotates and is alternately heated by hot gas and cooled by cool air. This type of heat exchanger is very large and is the source of much maintenance and plant shutdowns due to seal failures and corrosion where cold air condenses moisture and sulfur dioxide from the hot gas.

Both the recuperative and regenerative type of waste heat recovery systems effectively function only when the plant is at operating temperatures. i.e.. 1800 to 2000"F (gas temperature) and large amounts of blast air are needed. During idle time, when the afterburners are holding the cupola at around 1300"F and no blast air is required, negligible heat is recovered. Idle time can amount to 8 hours per day or as much as 12 hours per day.

Corresponding melting time would only be 8 hours or 4 hours with effective heat recovery time of 8 or 4 hours per day. Generally. such systems were limited to recovery heat necessary for preheating combustion air to reduce fuel requirements. Some process operations require gas fired auxiliary equipment since fuel oil firing produced a dirty or sooty exhaust gas which could not be tolerated by the process operation.

As briefly hereinbefore indicated, heat exchangers have been used for various waste heat duty using the conventional heat transfer mediums. In U.S. Patent No. 3,426,733, reference is made to the use of close looped systems for heat recovetlUtilizing heat transfer. Media such as eutectic salt mixtures. aromatic heat transfer oils, tetrachlorobiphenyl compounds. and the like, however. indicating that such systems had inherent difficulties because such systems were closed loops. In U.S. Patent No. 2.910,244. there is disclosed a process and apparatus for effecting an endothermic chemical reaction utilizing a molten salt mixture as an intermediate heat transfer medium.

The present invention provides a process for recovering heat from a unit process cycling between an operational mode and an idling mode wherein there is produced an exhaust gas having a temperature of from 500 to 2500"F comprising: a) passing said exhaust gases in indirect heat transfer relationship to a fluid heat transfer medium to heat said heat transfer medium; b) passing said heated heat transfer medium in indirect heat transfer relationship to a heat transfer fluid to be heated to thereby cool said heat transfer medium; c) passing the heat transfer medium of step (b) to a storage zone whereby the temperature of said heat transfer medium in said storage zone is raised during said operational mode; d) withdrawing and passing heat transfer medium from said storage zone as the heat transfer medium of step a); and e) recovering heat from said heat transfer medium in said storage zone during said idling mode whereby the temperature of heat transfer medium in said storage zone is lowered during said idling mode.

In another embodiment. at least two heat exchange recovery systems utilizing heat transfer mediums for a process operation are used. By using such system, the heat exchanger unit or units may be fabricated using conventional materials. Additionally. heat may be recovered at levels substantially higher than with the use of a single heat transfer medium.

The invention will be more clearly understood by reference to the following description of exemplary embodiments thereof in conjunction with the accompanying drawings. illustrating respective schematic flow diagrams thereof. wherein Figure 1 relates to an embodiment using a single heat exchanger. and Figure 2 uses two heat exchangers.

Referring to Figure 1. there is illustrated a cylindrical shaped cupola. generally indicated as 10. comprised of a vessel 12 provided with an upper hemispherical cover 14. a charge door 16. a tuyere 18. and a molten iron draw-off assembly. generally indicated as 20. The vessel 12 is provided with hot blast air line 22, hot charge door air line 24. charge door open to the outside by line 26 and an afterburner line 28. The upper portion of the vessel 12 is provided with a cross over duct 30 in fluid communication with a heat exchanger 32 of the heat recovery system. generally indicated as 34.

The heat recovery system 34 also includes a salt tank 36 and heat user equipment 38. The salt tank 36 is in fluid communication with the suction side of a pump 42 mounted on the tank 36 with the downstream side thereof being in fluid communication by conduit 44 with the tube or shell side of heat exchanger 32. The outlet from the liquid side of heat exchanger 32 is in fluid communication by conduit 48. The salt tank 36 is provided with a conduit 50 for shutdown operation. as more fully hereinafter described.As hereinabove indicated, the heat user equipment includes gas heat exchangers for preheating the gases flowing in lines 22. 24 and 28. steam generating equipment for-space heating duty or steam turbine utilization for the generation of electricity or compressing gaseous refrigerants.

In operation. the heat recovery system 34. with its intermediate heat fluids is used to recover heat from the hot gas. store the heat during the cyclic operation of melting and idling.

and utilize the heat in a variety of ways including heating the blast air. burner air. and charging door air; and generating steam in a salt-to-steam generation heat exchanger.

In the winter. the salt temperature is adjusted to be a minimum and thereby to recover the maximum amount of heat - the generated steam being used for space heating in the plant or adjacent offices and residence. In the summer the salt temperature is arranged to be a maximum for preheating blast air. burner air. and charging door air. and for generating electricity in a standard steam turbine-electric generator for driving plant motors or air conditioning equipment for the plant, adjacent offices and for residences.

An important feature of the present invention is the ability to store heat in the heat transfer fluid system from a melting operation of the cupola when the hot gas withdrawn therefrom is at 1800 to 20000F and to reject heat when the system is idling, the afterburners are on, and the hot gas is at 13000F. A typical operation consists of melting for 30 minutes and idling for 30 minutes for a total of 16 hours per day. The heat recovery system is operated as a storage system whereby the bulk salt temperature ranges from 400 to 10000F. The lower temperature is determined by the lowest safe temperature selected as the maximum allowable temperature for the heat transfer fluid. It will be appreciated that using a salt mixture permits auxiliary firing with fuel oil reducing gas and coke requirements.

Another feature of the present invention is the use of hot charging door air. The charging door is normally an opening in the side of the cupola which, for ease of operation, is always open and permits cold air to enter the cupola. It is proposed to add air, heated by the recovery system, at a point below the charging door or on either side of the charging door through one or more openings. Such hot air would reduce the amount of cold air which would have to enter the charging door since the hot air would prevent the smoke and gas generated in the lower section of the cupola from leaving the cupola through the charging door.The vertically rising smoke and gas would be pushed or induced away from the charging door by the hot charging door air which would be directed horizontallv into the cupola For example, assuming a large cupola operating at 20,000 scfm blast air; 20,000 scfm charge door indraft, and at an 18000F stack gas temperature for 6000 hours per year. A heat recovery system of the present invention installed to cool the stack gas to 5000F with recovered heat being used to reduce consumption of gas and coke having an average cost of $3 per million Btu, an annual saving would be realized of over $1,000,000.

Referring to Figure 2, there is illustrated a cylindrical shape cupola, generally indicated as 110, comprised of a vessel 112 provided with an upper hemispherically cover 114, a charge door 116, and tuyere 118, and a molten iron draw-off assembly, generally indicated as 120.

The vessel 112 is provided with hot blast air line 122, charge door air line 124, charge door draft line 126 open to the outside and an after burner line 128. The upper portion of the vessel 112 is provided with a cross over duct 130 in fluid communication with a primary and secondary heat exchangers 132 and 134, respectively, of the heat recovery system generally indicated as 136.

The heat recovery system 136 also includes a storage zone (not shown) which may comprise a salt tank such as described above should molten salt constitute one of the heat transfer media. The primary heat exchanger 132 is in fluid communication by a conduit 140 and with conduits 142 and 144 with the tube or shell side of heat exchangers 146 and 148, respectively. The outlet from the primary heat transfer medium side of heat exchangers 146 and 148 are in fluid communication by conduits 150 and 152, respectively, with conduit 154 and the primary heat exchanger 132. The secondary heat exchanger 134 is in fluid communication by a conduit 156 and with conduits 158 and 160 with the tube or shell side of heat exchangers 162 and 164, respectively.The outlet from the heat exchangers 162 and 164 are in fluid communication by conduits 166 and 168, respectively, which combine in conduit 170 for return flow to the secondary heat exchanger 134. A conduit 180 containing a fluid to be heated is in fluid flow communication with exchangers 164 and 146 by conduit 182, with the outlet from heat exchanger 146 being conduit 184 which is divided into conduits 128, 124 and 122. A conduit 186 containing another fluid to be heated is in fluid flow communication with exchangers 162 and 148 by conduit 188, with the outlet from heat exchanger 148 being in fluid flow communication with a conduit 190.

The outlet from the secondary heat exchanger 134 is passed by conduit 192 to a wet scribber 194 and vented to the atmosphere by line 196 via precipitator 198 and exhaust fan 100.

The following Table I sets forth conditions of cupola operating at 8,000 scfm blast air; 8,000 scfm charge door indraft, and at an 1800"F stack gas temperature for 6000 hours per year. A heat recovery system of the second embodiment of the present invention installed to cool the stack gas to 400"F. with recovered heat being used to produce steam and to reduce consumption of gas and coke would realize an annual saving of over $400,000. The intermediate heat transfer medium in the primary and secondary heat transfer vessels 132 and 134 is a salt mixture and water, respectively.

TABLE I Conduits "F Flow Rate #/hr.

line 130 1800 75,791 line 140 850 372,000 line 154 700 372,000 line 156 400 78,700 line 170 300 78,700 line 122 750 36,624 line 124 750 18,312 line 128 750 2,812(air) The following Table II sets forth operating conditions of such a cupola in an idling mode.

TABLE II Conduits "F Flow Rate #/.hr 122 - 0 124 450"F 18,312 128 450 10,163 (air) 130 1300 47,807 140 566 372,000 154 500 372.000 156 331 78.700 170 300 78.700 The cupola I is similarly operated with an intermediate heat transfer oil used in the primary and secondary exchangers has the conditions set forth in the following Table III: TABLE III Conduit "F Flow Rate &num;/hr 122 600"F 36,624 124 600 18.312 128 600 2.312(air) 130 1800 75,791 140 700 349.000 154 600 349.000 156 400 134.000 170 300 134,000 An idling mode conditions are set forth in the following Table IV:: TABLE IV Conduit "F Flow Rate &num;/hr 122 - 0 124 400 18.312 128 400 10.163 (air) 140 442 432.000 154 400 432.000 156 331 142.000 170 300 142.000 It is noted that the temperature of the air streams are different whereas the exhaust gas temperature and flow are the same -- the difference being varying fuel requirements.

The heat recovery system of the present invention greatly improves the design. operation and maintenance of pollution control system (i.e. wet scrubber. electrostatic precipitator. bag house or mechanical collector) associated with various processes. since there is realized a substantial reduction in gas volume.

Installation in an existing foundry cupola having a wet scrubber system. the sensible cooling of the stack gas prior to quenching in the scrubber substantially reduces water consumption. This reduction in water evaporation greatly reduces the volume and weight of saturated gas which the system fan must handle. Thus. there is 31% reduction in volume flow by cooling the gas from k800 F. to 500"F.. by heat recovery instead of direct spray water cooling.

The heat recovery system of the present invention has many advantages: 1. The high heat capacity of a salt storage system permits accumulation and storage of large amounts of heat. Reuse of the recovered energy can be scheduled to level peak loads or meet other requirements having usage patterns different from those of the waste heat source.

2. The extremely high coefficient of heat transfer between the exchanger and the molten salt results in an overall heat transfer rate much greater than that of a gas-to-air exchanger system. The salt film transfer coefficient is about 50 times higher than the air film transfer coefficient in gas-to-air heat exchangers. The heat transfer surface area required is therefore about one-half of that required for a gas-to-air exchanger of equal duty.

3. The high heat transfer coefficient described above maintains the exchanger surface temperature within a relatively few degrees of the molten salt temperature. In high temperature application the metal surfaces of the salt system exchanger may be 500 degrees cooler than the metal surfaces of a gas-to-air exchanger. This lower metal temperature contributes to economy of design and to dependability of operation. Standard materials of construction can be used for a salt system exchanger instead of the high alloys required for a gas-to-air exchangers.

4. The near equality of exchanger and salt temperatures coupled with the high heat capacity of the circulating salt makes the exchanger surface relatively independent of rapid fluctuations in stack gas temperature. The salt system exchanger is therefore not subjected to the damaging metal temperature fluctuations common to gas-to-air exchangers.

5. The salt dilution system offers considerable flexibility of choice regarding the manner and rate of re-use of the recovered heat. The heat can be used for process air preheating, for steam generation, for direct process heating, etc. Other waste heat recovery systems do not possess such flexibility.

6. Multiple waste heat sources, such as a number of cupolas in a large foundry, can be served by a single salt storage and circulating system resulting in substantial economies in the control, circulating, and re-use systems.

7. Molten salt is non-flammable and non-corrosive, and systems employing same may operate at atmospheric pressure plus static level. Salt is also thermally stable to 1000 F.

8. Utilizing salt dilution techniques (i.e. water concentration or dilution during operation shutdown or start-up) eliminate freeze-up problems during such start-up and shut-down operations.

While the present invention has been discussed with reference to the incorporation of a heat recovery system in combination with a cupola, it will be understood that such system may be used with any metallurgical, chemical or refinery process and particularly useful with processes which produce hot, dirty gas containing fines which have to be separated in dust removal equipment before being exhausted to the atmosphere. Since prior to passage through dust removal equipment, the hot, dirty gas must be cooled to 400-5000F, the process and apparatus of the present invention provides a particularly economically atttractive alternate to presently practical techniques.

Additionally, more than two heat exchangers may be disposed in series in the exhaust gas line and respectively utilizing intermediate heat transfer fluids at different temperature levels, e.g., molten salt, oil and water, or molten salt, oil and oil, etc. The operating temperature of the heat transfer fluids, are dependent on the thermal stability properties for salt and oil (normally 1000"F and 600"F, respectively) and the vapour pressure for water (normally 4000F at 247 psia vapour pressure).

WHAT WE CLAIM IS: 1. A process for recovering heat from a unit process cycling between an operational mode and an idling mode wherein there is produced an exhaust gas having a temperature of from 500 to 25000F comprising: a) passing said exhaust gases in indirect heat transfer relationship to a fluid heat transfer medium to heat said heat transfer medium; b) passing said heated heat transfer medium in indirect heat transfer relationship to a heat transfer fluid to be heated to thereby cool said heat transfer medium; c) passing the heat transfer medium of step b) to a storage zone whereby the temperature of said heat transfer medium in said storage zone is raised during said opera tional mode; d) withdrawing and passing heat transfer medium from said storage zone as the heat transfer medium of step a); and e) recovering heat from said heat transfer medium in said storage zone during said idling mode whereby the temperature of heat transfer medium in said storage zone is lowered during said idling mode.

2. A process according to Claim 1 wherein said exhaust gas is at a temperature of from 1800 to 2200"F and is cooled to a temperature of from 400 to 500"F.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. cooling. The heat recovery system of the present invention has many advantages: 1. The high heat capacity of a salt storage system permits accumulation and storage of large amounts of heat. Reuse of the recovered energy can be scheduled to level peak loads or meet other requirements having usage patterns different from those of the waste heat source. 2. The extremely high coefficient of heat transfer between the exchanger and the molten salt results in an overall heat transfer rate much greater than that of a gas-to-air exchanger system. The salt film transfer coefficient is about 50 times higher than the air film transfer coefficient in gas-to-air heat exchangers. The heat transfer surface area required is therefore about one-half of that required for a gas-to-air exchanger of equal duty. 3. The high heat transfer coefficient described above maintains the exchanger surface temperature within a relatively few degrees of the molten salt temperature. In high temperature application the metal surfaces of the salt system exchanger may be 500 degrees cooler than the metal surfaces of a gas-to-air exchanger. This lower metal temperature contributes to economy of design and to dependability of operation. Standard materials of construction can be used for a salt system exchanger instead of the high alloys required for a gas-to-air exchangers. 4. The near equality of exchanger and salt temperatures coupled with the high heat capacity of the circulating salt makes the exchanger surface relatively independent of rapid fluctuations in stack gas temperature. The salt system exchanger is therefore not subjected to the damaging metal temperature fluctuations common to gas-to-air exchangers. 5. The salt dilution system offers considerable flexibility of choice regarding the manner and rate of re-use of the recovered heat. The heat can be used for process air preheating, for steam generation, for direct process heating, etc. Other waste heat recovery systems do not possess such flexibility. 6. Multiple waste heat sources, such as a number of cupolas in a large foundry, can be served by a single salt storage and circulating system resulting in substantial economies in the control, circulating, and re-use systems. 7. Molten salt is non-flammable and non-corrosive, and systems employing same may operate at atmospheric pressure plus static level. Salt is also thermally stable to 1000 F. 8. Utilizing salt dilution techniques (i.e. water concentration or dilution during operation shutdown or start-up) eliminate freeze-up problems during such start-up and shut-down operations. While the present invention has been discussed with reference to the incorporation of a heat recovery system in combination with a cupola, it will be understood that such system may be used with any metallurgical, chemical or refinery process and particularly useful with processes which produce hot, dirty gas containing fines which have to be separated in dust removal equipment before being exhausted to the atmosphere. Since prior to passage through dust removal equipment, the hot, dirty gas must be cooled to 400-5000F, the process and apparatus of the present invention provides a particularly economically atttractive alternate to presently practical techniques. Additionally, more than two heat exchangers may be disposed in series in the exhaust gas line and respectively utilizing intermediate heat transfer fluids at different temperature levels, e.g., molten salt, oil and water, or molten salt, oil and oil, etc. The operating temperature of the heat transfer fluids, are dependent on the thermal stability properties for salt and oil (normally 1000"F and 600"F, respectively) and the vapour pressure for water (normally 4000F at 247 psia vapour pressure). WHAT WE CLAIM IS:

1. A process for recovering heat from a unit process cycling between an operational mode and an idling mode wherein there is produced an exhaust gas having a temperature of from 500 to 25000F comprising: a) passing said exhaust gases in indirect heat transfer relationship to a fluid heat transfer medium to heat said heat transfer medium; b) passing said heated heat transfer medium in indirect heat transfer relationship to a heat transfer fluid to be heated to thereby cool said heat transfer medium; c) passing the heat transfer medium of step b) to a storage zone whereby the temperature of said heat transfer medium in said storage zone is raised during said opera tional mode; d) withdrawing and passing heat transfer medium from said storage zone as the heat transfer medium of step a); and e) recovering heat from said heat transfer medium in said storage zone during said idling mode whereby the temperature of heat transfer medium in said storage zone is lowered during said idling mode.

3. A process according to Claim 1 or 2, wherein the heat transfer fluid is air, and said heat

transfer medium is a milten salt mixture.

4. A process according to Claim 3, wherein said operational mode is a melt mode of a cupola and wherein said heated air stream provides blast air and charge door air requirements of said cupola during said melt mode.

5. The process as defined in Claim 3, wherein said idling mode is an idling mode of a cupola and wherein said heated air stream provides after-burner air requirements of said cupola during said idling mode.

6. A process as defined in Claim 4 or 5 wherein a diluent is added to said heat transfer medium during shut-down operation.

7. A process according to Claim 1, wherein a) said exhaust gas is passed in indirect heat transfer relationship to heat transfer media in at least two successive heat exchange zones operating at different temperature levels; b) heat is recovered from said transfer media at different temperature levels; and c) thus cooled heat transfer medium is returned to step (a).

8. A process according to Claim 7 wherein a first heat transfer medium having a higher operational temperature level is passed through a first heat transfer zone and a second heat transfer medium having a lower operational temperature level is passed through a succeeding heat transfer zone.

9. A process according to Claim 7 or 8, wherein said recovered heat is utilized to preheat air prior to introduction into said unit process.

10. A process according to Claim 9 adapted for effecting the operation of a cupola.

wherein said pre-heated air stream provides the blast air, charge door air and afterburner air requirements of said cupola.

11. A process according to any of Claims 7 to 10 wherein said exhaust gas is at a temperature of from 1800 to 2200"F and is cooled to a temperature of from 400 to 5000F.

12. A process for effecting the operation of a cupola according to Claim 10 or 11, wherein said cupola is placed in a standby mode and said exhausted gases are withdrawn at a temperature of substantially 1300"F; said air stream heating an afterburner air stream.

13. A process for recovering heat substantially as herein described with reference to either of the drawings.

14. A process of operating a cupola substantially as herein described with reference to either of the drawings.