CA1108852A

CA1108852A - Waste heat recovery process

Info

Publication number: CA1108852A
Application number: CA296,831A
Authority: CA
Inventors: Alfred Bruhn; William Bernard
Original assignee: American Hydrotherm Corp
Current assignee: American Hydrotherm Corp
Priority date: 1977-02-14
Filing date: 1978-02-13
Publication date: 1981-09-15
Also published as: JPS53122609A; JPS5920954B2; DE2805840C2; DE2805840A1; GB1585748A

Abstract

Abstract of the Disclosure There is disclosed a process and apparatus for the recovery of heat from exhaust or waste gases having a temperature of from about 500 to about 2500°F generated in a process operation utilizing a molten substance; such as an eutectic salt mixture, as an intermediate heat transfer medium and to use all or a portion of such recovered heat in such process operation. The apparatus which is provided for recovering heat from the exhaust gases of a cupola includes a first heat exchange means for passing the exhaust gases in indirect heat transfer relationship to a heat transfer media to thereby cool the exhaust gases and heat the media with a second heat exchanger means provided for passing the heated media in indirect heat transfer relationship to a fluid to be heated with a storage tank for receiving the media from the second heat exchanger means with a pump means for passing the media through the heat exchanger means.

Description

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 or low temperature processing con-ditions. Economics normally dictate effective utilization of heat transfer equiprnent with respect to processing streams. Waste heat recovery generally relates to the recovery of heat over and above basic heat re-quirements, e. g. in steam generation equipment9 there are normally a convection section disposed in the equipment whereat the temperature level i9 in6uEEicient Eor steam generation but at a level where sensible heat is available Eor 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 cloor into a cupola, Cold blast air is introduced through tuyeres in the bottom to provide the cor~bustion medium for the coke~ Adclitional air is induced through the charge door by an exhaust Ean. Afterburners located above the charge cloor provide a source of ignition ~or 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, char.ge door air, and afterburner air is normally cold ancl is heated to .~

c~

operating temperature by consuming fuel at the afterburners or b~ con-suming coke in the lower portion of the cupola~
Hot gases at a temperature of from about 1800 to about 22~10F are withdrawn from the top of the cupola and are generally passed to vertically disposed water scrubber wherein the gas is cooled to a temperature of from 400 to 500F 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 sma]l number of plants in the form of either a recuperative or regenexative types of heat r ?covery systems. With a recuperative type, expensive higl1 alloy heat exchange~r 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 (1800F to 220Ei) and the large amount of heat transfer surface as a re-sult of the poor heat transfer coefficient of hot gas to cool air. The re-cuperative type is subject to mechanical failures due to the frequent wide swings in termp~ratures from 1300-2000F which can occur as much as l'L times a day with swings rangin~ from ambient to 2000F occuring with the daily startup and 5hutdown 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 condensec moisture and sulfur dioxide from the hot gas.
Both the recuperative and regenerative type of waste heat recovery systems effectively Çunction only when the plant is at operating temperatures, i.e, 1800 to 2000F (gas temperature) and large amounts of blast air are needed, During idle time, when the after-burners are holding the cupola at around 1300F and no blast air is requir~
ed, negligible heat is recovered. Idel time can amount to 8 hours per day or as much as 12 hours per day. Corresponding melting time would onLy be 3 hours or aS hours with effective heat recovery time of 8 or 4 hours per day. Generally, such systems were limited to recovery heat necessary ~or preheating combustion air to reduce Euel requirerrlents.
Some process operations require g~as 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 hereinabove indicated, heat exchangers have been used for various waste heat duty us;ng the conventional heat trans~
fer mediums. In U. S. Patent No. 3, 426, 733, reference is made to the use of close looped systems for heat recovery utilizing heat ~ransfer ~luids, such as eutectic salt mixtures, aromatic heat transfer oils, tetrachlorobiphenyl compunds, ànd the like, however, indicating that such systems had inherent difficulties because such systems were closed loops. In U.S. Patent No. 2,910J244, there is disclosed a process and apparatus for effecting an endothermic che)nical reaction utilizing a mol-ten salt mixture as an intermediate heat transfer medium.

These and other objects of the present invention are achieved by a heat exchange systern utilizing a molten suhstance, such as an eutectic salt system as an intermediate heat transfer mediurn for a process operation, such as the operation of a cupola in which there is produced an exhaust or waste gas at temperatures of from 500 to 2500F.
In another embodiment, the objects of the present invention are achieved using at least two heat exchange recovery systems utilizing in~
termediate heat transfer mediums for a process operation. By using such system, the heat exchanger unit or units may be fabricated using conventional materials of construction vice more expernsive, high alloys materials of construction, additionally, heat may be recovered at levels substantially higher than with the use o~ a single intermediate heat transEer mediumt such as discribed with reference to the first embodiment .
The invention will be rn~re clearly understoocl by reference to the following description of exemplary embodiments thereof in conjunction with the accompanying drawings, illustrating respective schematic flow diagram thereof .
Referring to Flgure 1, there is illustrated a cylindrical shaped cupola, generally indicated as 10, comprised of a vessel 12 provided with an upper hemispherically 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

2~ 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 equiprnent 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 s ide `.f~

of heat exchanger 32, The outlet from the l;cluid side of heat e~changer 32 is in fluid communication by conduit a~6 with the heat user system 38 whLch in turn is in fluid 'l~w communication by conduit 48. The salt tank 36 is provided with a conduit 50 for shutdown operation, as rnore fully hereinafte3 S described. As hereinabove indicated, the heat user equipment includes ga heat exchangers for preheating the gases flowing in Lines 22, 24 and 28J
steam generating equipment for space heating duty or steam turbine utilizc for the generation of electricity or compressing gaseous refrigerants.
In operation, the heat recovery system 34J with its inter-mediate 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, ~nd charging door air; and generating steam in a salt to steam generation heat exchanger.
In the winter, the salt temperature is set at a minimum to recover the maximum amount of heat with generated steam being used for space heating in the plant or adjacent offices and residence. In the summer the salt temperature is set at a ma~imum 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 condictioning equip-ment for the plant, adjacent offices ancl for residence,s.
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 2000F and to reject heat when the system is idling, the afterburners are on, and the hot gas is at 1300F. 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 systernis operated as a storage system whereby the bulk salt temperatur e ranges from 400 to 1000F, The lower temperature is determined by the lowest safe temperature selected as the maximum allowable tempexature for the heat transrer 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 s ide of the cupola which, for ease of operation, is always open and permits col 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 chargin 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 smo~e and gas generated in the lower section of the cupo from leaving the cupola through the charging door. The vertically r ising smok and gas woulcl be pushecl or inducet] away from the charging door by the hot charging door air whicll would be directed hori~ontally 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 1800F stack gas temperature for 6000 hours per year. A heat recovery system of the present invention installed to cool the stack gas to 500F 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, thexe 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 indicat ed 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 l28. The upper portion of the vessel 112 is provided with a cross over duct 130 in fluid com-munication with a primary and secondary heat exchangers 132 and 13a~, re-spectively, of the heat recovery system generally indicated as 136.

The heat recovery system 134 Ir~Ly also inclu~le a salt tank (not sha~n), should molten salt constitute one of the intermediate heat transfer flu~ds. me pr~r~y heat exch~nger 132 is in rlu~d carllrn~cation k~ a condu~t 140 and wiffi conduits 142 and 144 with the tube or shell side of heat exchangers 146 and 1489 respectively. The outlet from the prirnary 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 162and 164, respectively. The outlet from the heat exchangers 162 and 164 are in Eluid communication by conduits 166 and 168, respectively, which combine in confiuit 170 for return flow to the secondary heat exchanger 134. A conduit 180 containing a fluid to be heated is in tluid flow communication with ex-changers 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 witll exchangers 162 and 148 by conduit 188J with theoutlet from heat exchanger 148 being in fluid flow cornmunication with a conduit 190.
The outlet from the secondary heat exchanger 134 is passed by conduit 192 ~o a wet scrubber 194 and vented to the atmosphere by line 196 via precipitator 198 and exhaust fan 100.
The following Table I sets forth condi~ions of cupola operatin at 8, 000 scfm blast air; 8, 000 scfm charge door indraft, and at an 1800F
stack gas temperature Eor 6000 hours per year. A heat recovery system of the second embodiment of the present invention installed to cool the stack gas to 400F. 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.

~d~

The inter mediate 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 3721,000 line 156 400 78, 700 line 170 300 78,700 line 122 750 36J 624 line 124 750 18,312 line 128 750 2, 812 (air) ~ r~

The following Table II sets forth operation condi-tions of such a cupola in an idling mode.
TABLE II
Conduits ~F Flow Rate ~/hr -122 _ 0 124 450F 18,312 128 450 10,163 (air) 130 1300 47,807 1~0 566 37~,000 154 S0~ 372/000 156 331 78,700 170 . 300 .78,700 The copula I is similarly operated with an intermediate heat transfer oil used in the primar~ and.:secondary exchan~
gers has the conditions set forth in -the following Table III:
TABLE III
Conduit F Flow Rate ~/hr ..... __ .. _ . __.
122 600F 36,624 12~ 600 18,312 128 6002,312 (air) 130 1800 75,791 140 700 349,000 154 600 349,000 156 400 13~,000 170 300 134,000 An idling mode conditions are set forth in the following Table lV:
TABLE IV
Conduit FFlow Ra-te #/hr ~ _ ... ~ .. _ _ .
122 _ 0 12~ 400 18,312 128 40010,163 (air) 140 442 432,000 154 400 432,000 156 331 142,000 170 300 142,000 l[t 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 reduct;on in gas volumne.
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 volumne and weight of saturated gas which t~e system fan must hanclle. Thus, there is 31% reduction in volurrln flow by cooling the gas from k800F. to 500~F,, by heat recovery instead of direct spray water cooling.
The heat recovery system of the present invention has many advantages:
l. The high heat capacity of a salt storage system permits accumulation and storage of large amounts of heat. Reuse of the recoverecl energy can be scheduled to level peak loads or meet other requir`ements 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 there-~ore about one-half of that required for a gas-to-air exchanger of equal duty.

3. The high heat transfer coefficient descr;bed above mai n-tains the exchanger surface temperature within a relatively few degrees of the molten salt temperature~ In high tem-perature application, the metal surfaces of the salt system exchanger may be 500 degrees cooler than the metal sur faces of a gas-to-air exchanger. This lower metal tem-perature contributes to economy of ~esign and to depend-ability of operation. Standard materials of con3truction can be uscd for a salt system exchangers instead of the high all~ys required for a gas-to-air e~changers.
~. The near equality of e~changer and salt temperatures coupled with the high heat capacity Oe the circulating salt mal~es the exchanger surface relatively independent of rapid fluctuations in stack gas temperature. The salt system ex-changer 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 re-zo covered heat. The heat can be used for process air preheat-ing, for steam generàtionJ for direct process heating, etc.
Other waste heat recovery systems do not possess such flex-ib il ity.
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 controlJ circulating, ancl 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 1000F.
8. Utilizing salt d;lution techniques (i. e. water concen-tration or dilution cluring operation shutdown or start-up) elim;nate freeze-up problems during such start-up and shut-down operations.
While the present invention has ~een 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 ~00-500t~', the process and apparatus of the present invention provides a particularly economically att ractive alternate to presently practical techniques Additonally, more than two heat e~changers may be clisposed in tandum 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 stabil;ty properties for salt and oil (normally 1000F and 600F, 2~ respectively) ancl the vapor pressure for water (normally ~00F at 2~7 psia vapor pressure).

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for effecting the operation of a cupola cycling between a melt mode and idling mode wherein exhaust gases are withdrawn at a temperature of from 500 to 2500°F., the improvement comprising:
a) passing said exhaust gases in indirect heat transfer relationship to an intermediate heat transfer medium;
b) passing at least a portion of said heated intermediate heat transfer medium in indirect heat transfer relationship to an air stream to preheat said air stream for subsequent introduction into said cupola, said preheated air stream providing combustion air, blast air, and charge door air requirements of said cupola;
c) passing the heat transfer medium of step b) to a storage zone; and d) withdrawing and passing the heat transfer medium from said storage zone as heat transfer medium of step a), said heat transfer medium being raised in temperature during said melt mode and being lowered in temperature during said idling mode.

2. The process of claim 1 for effecting the operation of a cupola wherein said heated air stream of step (b) provides the blast air, charge door air and afterburner air requirements of said cupola.

3. The process of claim 1 for effecting the operation of a cupola wherein said exhaust gas is at a temperature of from 1800 to 2200°F
and is cooled to a temperature of from about 400 to 500°F.

4. The process of claim 1 for effecting the operation of a cupola wherein said cupola is placed in an standby mode and said exhausted gases are withdrawn at a temperature of about 1300°F with said air stream heating an afterburner air stream.

5. The process of Claim 4 wherein said generated steam provides heat for space heating requirements.

6. The process of Claim 4 wherein said generated steam provides the steam requirements for a steam turbine.

7 . An apparatus for recovering heat from an exhaust gas at a temperature of from 500 to 2500°F recovered during the operation of a cupola which comprises:
a first heat exchanger means for passing said exhaust gas in indirect heat transfer relationship to a salt mixture to thereby cool said exhaust gas and heat said salt mixture;
a second heat exchanger means for passing said heated salt mixture in indirect heat transfer relationship to a fluid to be heated;
a storage tank for receiving the salt mixture from said second heat exchanger means; and pump means for passing said salt mixture through said heat exchanger means.

8. The apparatus of Claim 7 wherein said second heat exchanger means includes conduit means for heating a fluid to be passed to said cupola during the operation thereof.

9. The apparatus of Claim 7 wherein said storage tank includes conduit means for introducing a diluent during shut-down operation of said cupola.

10. The apparatus of Claim 9 wherein said storage tank includes conduit means for withdrawing diluent during start-up operation of said cupola.

11. In a process for recovering heat from an exhaust gas having a temperature of from 500 to 2500°F generated in a unit operation effecting an exothermic reaction and cycling between an operational mode and an idling mode, the improvement comprising:
a) passing said exhaust gas during said operational mode indirect heat transfer relationship to at least two inter-mediate heat transfer media in at least two successive heat exchange zones operating at different temperature levels;
b) recovering heat from said intermediate heat transfer media at different temperature levels:
c) passing one intermediate heat transfer media through a first heat transfer zone;
d) passing another intermediate heat transfer medium having a lower operational temperature level through a succeed-ing heat transfer zone;
e) passing an air stream sequentially through said succeeding heat transfer zone and said first heat transfer zone to preheat said air stream;

f) introducing said preheated air into said unit operation;
g) passing said intermediate heat transfer media to respective storage zones thereby -to use said intermediate heat transfer media as a source of heat during said idling mode;
and h) passing to step (a) said intermediate heat transfer media from said storage zones.

12. The process for effecting the operation of a cupola as defined in Claim 11 wherein said preheated air stream provides afterburner air requirements during said idling mode.

13 . The process of Claim 11 wherein one of said intermediate heat transfer media is a molten salt and a diluent is added to said molten salt in said storage zone during shut-down operation.