CA2153841C - Two stage downflow flue gas treatment condensing heat exchanger - Google Patents
Two stage downflow flue gas treatment condensing heat exchanger Download PDFInfo
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- CA2153841C CA2153841C CA002153841A CA2153841A CA2153841C CA 2153841 C CA2153841 C CA 2153841C CA 002153841 A CA002153841 A CA 002153841A CA 2153841 A CA2153841 A CA 2153841A CA 2153841 C CA2153841 C CA 2153841C
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- heat exchanger
- flue gas
- housing
- tubular heat
- stage
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Classifications
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D47/00—Separating dispersed particles from gases, air or vapours by liquid as separating agent
- B01D47/06—Spray cleaning
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/34—Chemical or biological purification of waste gases
- B01D53/74—General processes for purification of waste gases; Apparatus or devices specially adapted therefor
- B01D53/77—Liquid phase processes
- B01D53/78—Liquid phase processes with gas-liquid contact
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES
- F23J15/00—Arrangements of devices for treating smoke or fumes
- F23J15/02—Arrangements of devices for treating smoke or fumes of purifiers, e.g. for removing noxious material
- F23J15/04—Arrangements of devices for treating smoke or fumes of purifiers, e.g. for removing noxious material using washing fluids
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F23—COMBUSTION APPARATUS; COMBUSTION PROCESSES
- F23J—REMOVAL OR TREATMENT OF COMBUSTION PRODUCTS OR COMBUSTION RESIDUES; FLUES
- F23J15/00—Arrangements of devices for treating smoke or fumes
- F23J15/06—Arrangements of devices for treating smoke or fumes of coolers
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E20/00—Combustion technologies with mitigation potential
- Y02E20/30—Technologies for a more efficient combustion or heat usage
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- Engineering & Computer Science (AREA)
- Chemical & Material Sciences (AREA)
- Environmental & Geological Engineering (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Health & Medical Sciences (AREA)
- Biomedical Technology (AREA)
- Analytical Chemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Oil, Petroleum & Natural Gas (AREA)
- Treating Waste Gases (AREA)
Abstract
A two-stage downflow flue gas treatment condensing heat exchanger system allows for flue gas to be passed into a two-stage housing at an upper end of the housing. The flue gas is channeled through a first stage of the housing having a first condensing heat exchanger which cools the flue gas. The flue gas is then channeled through a second stage having a second condensing heat exchanger which is located directly beneath the first stage and the first condensing heat exchanger for further cooling the flue gas.
The flue gas travels in a downward direction only through the housing and exits the housing at the lower end of the housing beneath the second stage. A collection tank is located beneath the second stage of the housing for collecting liquids, condensate, particulate and reaction product.
The flue gas travels in a downward direction only through the housing and exits the housing at the lower end of the housing beneath the second stage. A collection tank is located beneath the second stage of the housing for collecting liquids, condensate, particulate and reaction product.
Description
-IWO STAGE DOWN~OW FLUE GAS TREATMENT
CONI)}3:NSING }IEAT EXC~IANGER
BACKGROUNI) OF T~ ~VENIION
1. F~ELD OF T~EINVENnlON
The present in~ention relates, in general, to the removal of con~m;n~nts from flue gas, and, in particular, to a new and useful method to recover useful heat while removing particulates (fly ash), sulfur oxides and/or other cont~m;n~nts contained in flue gases formed during the combustion of waste materials, coal and other fossil fuels, which are burned by electric power generating plants, waste-to-energy plants and other industrial processes through the use of a two-stage downflow flue gas treatment condensing heat exchanger. :~
CONI)}3:NSING }IEAT EXC~IANGER
BACKGROUNI) OF T~ ~VENIION
1. F~ELD OF T~EINVENnlON
The present in~ention relates, in general, to the removal of con~m;n~nts from flue gas, and, in particular, to a new and useful method to recover useful heat while removing particulates (fly ash), sulfur oxides and/or other cont~m;n~nts contained in flue gases formed during the combustion of waste materials, coal and other fossil fuels, which are burned by electric power generating plants, waste-to-energy plants and other industrial processes through the use of a two-stage downflow flue gas treatment condensing heat exchanger. :~
2. DESC~UP11ON OFl~lE RELATED ART
In the power generating field, there are several known 1~ devices and methods which relate to the integrated heat recovery and pollutant removal of particulates, sulfur oxides and/or cont~min~nts from a hot combustion exhaust gas for complying with federal and state emissions requirements.
C.~2153~41 One device which has been used is a condensing heat exchanger, as shown in Fig. 1, which recovers both sensible and latent heat from flue gas 11 in a single unit 10. The device allows for the gas 11 to pass down through a heat 5- exchanger 12 while water 14 passes upward in a serpentine path through the tubes of heat exchanger 12. Condensation occurs within the heat exchanger 12 as the gas temperature at the tube surface is brought below the dew point. The condensate falls as a constant rain over the tube array of heat exchanger 12 and is removed at the bottom by con~en~ate drain 16. Gas cl~An;ng may occur within the heat exchanger 12 as the particulates impact the tubes and gas con~pnqation occurs.
The heat ~xch~nger tubes and inside surfaces of the heat exchanger shell are made of corrosion resistant material or are covered with Teflo~ in order to protect them from corrosion when the flue gas temperature is brought below the acid dew point. Interconnections between the heat exchanger tubes are made outside of the tube sheet and are not exposed to the corrosive flue gas stream 11.
Another device used in this area is an integrated flue gas treatment (IFGT) con~en~ing heat ~hAnger 20, schematically shown in Fig. 2, which is a condensing heat exchanger designed to ~hAnce the removal of pollutants ~A2 15384 1 from flue gas stream 22. It is also made of corrosion resistant material or has all of the inside surfaces covered by Teflon.
There are four major sections of the IFGT 20: a first 5- heat exchanger stage 24, an interstage transition region 26, a second heat exchanger stage 28, and a mist el;m;n~tor 30. The major differences between the integrated flue gas treatment design of Fig. 2 and the con~entional co~Pn.~ing heat exchanger design of Fig. 1 are:
1. the integrated flue gas treatment design usés two heat exchanger ~tages 24 and 28 instead of one heat exchanger 12 (Fig. 1);
2. the interstage or transition region 26, located between heat exchanger stages 24 and 28, is used to direct the gas 22 to the second heat exchanger stage 28, and acts as a collection tank and allows for treatment of the gas 22 between the stages 24 and 28;
In the power generating field, there are several known 1~ devices and methods which relate to the integrated heat recovery and pollutant removal of particulates, sulfur oxides and/or cont~min~nts from a hot combustion exhaust gas for complying with federal and state emissions requirements.
C.~2153~41 One device which has been used is a condensing heat exchanger, as shown in Fig. 1, which recovers both sensible and latent heat from flue gas 11 in a single unit 10. The device allows for the gas 11 to pass down through a heat 5- exchanger 12 while water 14 passes upward in a serpentine path through the tubes of heat exchanger 12. Condensation occurs within the heat exchanger 12 as the gas temperature at the tube surface is brought below the dew point. The condensate falls as a constant rain over the tube array of heat exchanger 12 and is removed at the bottom by con~en~ate drain 16. Gas cl~An;ng may occur within the heat exchanger 12 as the particulates impact the tubes and gas con~pnqation occurs.
The heat ~xch~nger tubes and inside surfaces of the heat exchanger shell are made of corrosion resistant material or are covered with Teflo~ in order to protect them from corrosion when the flue gas temperature is brought below the acid dew point. Interconnections between the heat exchanger tubes are made outside of the tube sheet and are not exposed to the corrosive flue gas stream 11.
Another device used in this area is an integrated flue gas treatment (IFGT) con~en~ing heat ~hAnger 20, schematically shown in Fig. 2, which is a condensing heat exchanger designed to ~hAnce the removal of pollutants ~A2 15384 1 from flue gas stream 22. It is also made of corrosion resistant material or has all of the inside surfaces covered by Teflon.
There are four major sections of the IFGT 20: a first 5- heat exchanger stage 24, an interstage transition region 26, a second heat exchanger stage 28, and a mist el;m;n~tor 30. The major differences between the integrated flue gas treatment design of Fig. 2 and the con~entional co~Pn.~ing heat exchanger design of Fig. 1 are:
1. the integrated flue gas treatment design usés two heat exchanger ~tages 24 and 28 instead of one heat exchanger 12 (Fig. 1);
2. the interstage or transition region 26, located between heat exchanger stages 24 and 28, is used to direct the gas 22 to the second heat exchanger stage 28, and acts as a collection tank and allows for treatment of the gas 22 between the stages 24 and 28;
3. the gas flow in the second heat exchanger stage 28 is upward, rather than downward;
4. gas outlet 29 of the second heat exchanger stage is equipped with an alkali reagent spray system, generally designated 40, comprising reagent source 42 with a pump 44 for pumping reagent 42, ~A~ I 53~41 recirculated co~pn~ate~ and spent reagent to sprayers 46 and 48; and 5. the mist el; m; nAtor 30 is used to separate the water formed by condensation and sprays from the 5flue gas.
Most of the sensible heat is removed from the gas 22 in the first heat exchanger stage 24 of the IFGT 20. The transition region 26 can be equipped with a water or alkali spray system 48. The system 20 saturates the flue gas 22 with moisture before it enters the second heat exchanger stage 28 and also assists in removing sulfur pollutants from the gas 22.
The transition piece 26 is made of or lined with corrosion resistant fiberglass-reinforced plastic or other corrosion resistant material. Additionally, the second heat exchanger stage 28 is operated in the con~p~ing mode, removing:latent heat from the gas 22 along with pollutants.
Also, the top of the second heat exchanger qtage 28 is equipped with an alkali solution or qlurry spray device 46.
The gas 22 in this stage 28 is flowing upward while the droplets in the gas 22 faIl downward. This counter-current gas/droplet flow provides a scrubbing mech~n;sm that ~nh~nces particulate and pollutant capture. The condensed gases, particulates, and reacted alkali solution are ~A 2 1 5384 1 collected at the bottom of the transition section 26. The flue gas outlet 29 of the IFGT 20 is equipped with the mist el;~;n~tor 30 to reduce the chance of moisture carryover.
5 SUI\~MARY OF TEE INVENrIlON
The present invention is a two-stage downflow flue gas treatment condensing heat e~ch~nger system which utilizes a housing having an inlet at an upper end and a outlet at its lower end. Flue gas enters the housing at the inlet and travels downwardly through the housing and exits the housing at its lower end through the outlet. The housing has an upper stage beneath the inlet which contains a first con~n~ing heat ~x~h~nger which withdraws heat from the flue gas in order to cool the flue gas aæ the flue gas is ~h~nneled downwardly through the housing. A second stage located directly beneath the first stage contains a second condensing heat ~xch~nger which provides a further cooling of the flue gas by withdrawing more heat from the flue gas as the flue gas passes downwardly through the second qtage toward the outlet. A collection tank located at the lower end of the housing beneath the second heat exchanger collects condensate, liquid, particulate and reaction product.
A mist el;m;n~tor is located at the lower end of the housing beneath the collection tank for demisting the flue gas prior to its exit through the outlet. A reagent spray system is located at the second stage for spraying the flue gas with an alkaline reagent solution or slurry for removing cont~m;n~nts such as SO2 from the flue gas. A
spray wash system is located at the upper end of the housing for spraying clP~n;ng liquid down the housing for cl~n;~g both heat exchanger stages.
lo It i9 an object of the present invention to provide a system and method for treating a flue gas which utilizes two separate stages in a vertical relationship which c~nnels the flue gas in a downward direction only.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better underst~n~;ng of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embo~;ments of the invention are ill~strated.
~v A 2 ~ 53~3 4 1 BRIEF DESCRIPTION OF 1~; DRAWINGS
Fig. 1 is a srhPm~tic view illustrating a known con~enqing heat exchanger system;
Fig. 2 is a schP~tic view illustrating an integrated flue gas treatment system; and Fig. 3 is a schematic view illustrating a two-stage downflow flue gas treatment system according to the present invention.
0 DESCRIPTION OF ~; PR~ RED EMBODIMENTS
The present invention is a two-stage downflow flue gas treatment c~n~Pn~ing heat P~ch~nger system and method using co-current gas/droplet flow, as shown in Fig. 3. The purpose of the present invention is to provide improved lS heat recovery and pollutant Le...oval performance compared to the IFGT system shown in Fig. 2. -A preferred embo~;mPnt of the present invention, as best illustrated in Fig. 3, is a two-stage device, generally designated 50 having two con~Pn~ing heat exchanger stages 56 and 62 mounted vertically in series in whlch flue gas 52 enters at the top of the device 50 through inlet 51 and exits at the bottom of the unit 50 through outlet 68. A transition section 60 separates the -two heat exchanger sections 56 and 62. Transition section 54 communicates between inlet 51 and first heat exchanger 56. Structure 58 is used to support the first heat exchanger 56. Structure 64 supports the second heat 5 exchanger 62.
Most of the sensible heat is removed from the flue gas 52 in the first heat exchanger stage 56 and after being passed through the transition section 60, the flue gas 52 enters the second or lower heat ~ch~nger stage 62 where latent heat i8 remo~ed. Droplets are formed in both stages due to con~pn~tion~ The droplets fall downward due to the combined effects of gra~ity and the downward travel direction of the flue gas flow 52. The Qecond heat exchanger stage 62 can be smaller than the first atage 56 in order t-o maintain the optimum ~elocity around the tubes for the cooler gas.
A collection tank 66 iS pro~ided near the bottom of the second stage 62 to collect the water droplets, condensed gases, particulates, reaction products, and 20 alkali reagent. Additional collection me~h~n;.~ms can also be added in the region of the collection tank 66 to aid in the removal of particulates and pollutants from the flue gas stream 52.
The top of the second stage 62 iS optionally equipped 5 ~ 8 4 1 with an alkali reagent spray system 72 to provide ~nh~nced removal of sulfur oxides and other pollutants from gas 52.
The gas 52 leaves the second heat exchanger stage 62 and passes through a--mist el; m; n~tor which is not shown but is located in the region of outlet plenum 68. The liquid collected by the mist el;~;n~tor is fed back to the collection tank 66 through recycle or ~h~nn~ing mP~n~.
The two stage downflow flue gas treatment system 50 also includes a spray washing system 70 located at the top of the unit 50. Periodic wa.~;ng of the heat exch~nger tubes of heat exch~ngers 56 and 62 pre~ents potential plugging of the heat PxchAngers 56 and 62 and provides consistent thermal performance.
The major differences between the two-stage downflow flue gas treatment system 50 and the IFGT system 20 (Fig.
2~ are:
1. ~he flue gas flows in the downward direction in the two-stage downflow flue gas treatment system 50 unlike the multi-directional flow of the IFGT
system 20. In the IFGT system 20, the direction of gas flow in the qecond heat Pxch~nger 28 is upward.
2. In the IFGT system 20, the direction of flow for the particulates and droplets collected in the second stage 28 is opposite to the direction of the flue gas flow. For the two-stage downflow system SO, the direction of flow in the heat exchangers is always the same for the flue gas, droplets, and particles, i.e. downward.
3. In the IFGT system- 20, the particulates and droplets in the second stage 28 must be large enough to overcome the drag forces of the flue gas 22 before they reach the collection area 26.
This is not a requirement for the two-stage downflow design according to the present in~ention.
4. In the IFGT system 20, the transition section 26 acts as the collection tank. The transition section 26 is located between the first and second heat exchanger stages 24 and 28, upstream from the coolest regions of the heat P~ch~nger.
The direction of flue gas flow in the second stage 28 is away from the collection region 26.
For the two-stage downflow system 50 of the present in~ention, the collection tank 66 is downstream from the second heat exchanger stage 62. It is located downstream from the coolest regions of the heat exchanger 62 and the rA 21 5384 1 direction of the flue gas flow is toward the collection tank 66.
The two-stage downflow flue gas treatment system 50 is an improvement over the IFGT design 20. The advantages listed below compare the performance of the two-stage downflow flue gas treatment system 50 with a IFGT design 20.
The present invention has a smaller footprint than the standard IFGT condensing heat exchanger design, thus requiring less space f,or installation.
The present invention has a lower gas side pressure drop than comparable IFGT designs. The reason for this is that all of the flow is in the downward direction. The downflow (co-current droplet/gas flow) in the second heat exchanger stage 60 has a lower pressure drop than the gas upflow, droplet/particulate downflow condition (counter-current droplet/gas flow) encountered in the IFGT design.
The lower pressure drop will permit a smaller forced or induced draft fan to be used in retrofit applications and result in lower parasitic losses during operation.
The present i,,nvention has superior heat recovery performance when compared to IFGT designs. Testing performed on the condensing heat exchangers 56 and 62 of the present invention has demonstrated that the gas C~2153841 downflow design provides maximum heat recovery performance.
All of the heat recovered in the present invention is recovered under gas downflow conditions, while the second stage 28 of the IFGT deslgn 20 recovers heat under gas upflow conditions.
The present invention also has improved particle removal performance, especially for very small particulates. The upflow direction of the flue gas stream 22 in the second stage 28 of a standard IFGT 20 carries particles away from the collection tank 26. In the standard IFGT design 20, very small particles will not be removed unless they become large enough (through water condensation, etc.) to overcome the drag forces of the gas stream and can fall back through the heat exchanger 28 to the collection tank 26. For the present invention, however, the downflow direction of the flow stream 52 always directs the particulates toward the collection tank 64.
The present invention has improved condensable gas removal performance. Condensable gases, such as heavy metals and organic compounds, will form in very small droplets in the cooler regions of the heat exchanger. For the IFGT design 20, the coolest region of the heat exchanger 28 is downstream of the collection tank 26. For the same reasons as cited....
C~2153~41 above, many of the condensable gas droplets formed in the IFGT design 20 will be carried out with the gas stream 22 and can only be collected in the mist el;m;n~tor 30. For the present invention, however, the downflow direction of the flow stream 52 always directs the droplets toward the collection tank 64. In this case, the mist el;m;n~tor 66 captures those droplets that are not removed at the collection tank 64.
The single water spray system 70 in the present lo invention cleans the whole area of both heat exchangers 56 and 62 since the clPAn;ng water will flow through both heat exchangers 56 and 62. In the IFGT design 26, two separate spray clPAn;ng systems are required to achie~e the same result.
The loading on the mist el; m; n~tor 66 is less for the present invention because most of the mist will be removed in the collection tank 64. The small mist droplets will have a greater opportunity to form into larger droplets in the two-stage downflow design 50; and the momentum forces imparted to the droplets by the flue gas 52 is in the direction of the collection tank 64. For the IFGT design 20, most of the mist leaving the heat exchanger 28 will reach the mist el;m;nAtor 30; and when collected, must form droplets of sufficient size to be removed from the gas C A 2 ~ 5 384 1 stream 22 and drained to the collection tank 26.
Although not illustrated, the present invention may incorporate other features which were not described above.
The present in~ention may also include a third heat exchanger stage which could be added downstream of the second stage to improve the removal of condensing organics, heavy metals, and other condensible air pollutants from the flue gas. The third stage would operate indepPn~nt of the rest of the ~ystem and would not be used for heat recovery.
lo The third stage would have a closed cycle refrigerant loop, similar to a ~pht~m;difier, for the purpose of lowering the flue gas temperature further to remove the c~en~ible pollutants.
Also, the present invention can be tailored to incorporate multiple stages, rather than just the two stages described above. Each stage would be designed to optimize the removal of a particular pollutant of concern and would pretreat the flue gas for the next stage.
An additional transition section can also be added between the outlet of the second stage and the mist el;~in~tors to coalesce droplets and particulates and/or impart momentum to the droplets and particulates in order to increase separation performance before the exhaust gas enters the mist el;~;n~tors.
C~2 1 S3&4 ~
The present invention can be used to pre-treat a flue gas prior to entering a wet scrubber. Advantages of this use include: lowering the inlet flue gas temperature which will allow the wet scrubber to operate more efficiently for S2 removal; the two stage downflow unit can be used to subcool the flue gas to m~X;mi ze removal of particulates, HF, HCl, and con~Pn~able air toxics while the wet scrubber is used for S2 removal; a limestone based wet scrubber would produce high quality gypsum without the need for additional washing if the two stage downflow unit removed undesirable materials, such as chloride ions and inert particulates, during pretreatment of the flue ga~; and there would be less reagent lost in a sodium regenerable process if the two-stage downflow unit ~el-wved HF, S3~ N2~
and HCl during pre-treatment of the flue gas. This application would also reduce or el;m,n~te the need for a purge to remove inert materials from the process.
While specific embo~;m~nts of the invention have been shown and described in de-tail to illustrate the application of the principles of the in~ention, it will be lln~rstood that the in~ention may be embodied otherwise without departing from such principles.
Most of the sensible heat is removed from the gas 22 in the first heat exchanger stage 24 of the IFGT 20. The transition region 26 can be equipped with a water or alkali spray system 48. The system 20 saturates the flue gas 22 with moisture before it enters the second heat exchanger stage 28 and also assists in removing sulfur pollutants from the gas 22.
The transition piece 26 is made of or lined with corrosion resistant fiberglass-reinforced plastic or other corrosion resistant material. Additionally, the second heat exchanger stage 28 is operated in the con~p~ing mode, removing:latent heat from the gas 22 along with pollutants.
Also, the top of the second heat exchanger qtage 28 is equipped with an alkali solution or qlurry spray device 46.
The gas 22 in this stage 28 is flowing upward while the droplets in the gas 22 faIl downward. This counter-current gas/droplet flow provides a scrubbing mech~n;sm that ~nh~nces particulate and pollutant capture. The condensed gases, particulates, and reacted alkali solution are ~A 2 1 5384 1 collected at the bottom of the transition section 26. The flue gas outlet 29 of the IFGT 20 is equipped with the mist el;~;n~tor 30 to reduce the chance of moisture carryover.
5 SUI\~MARY OF TEE INVENrIlON
The present invention is a two-stage downflow flue gas treatment condensing heat e~ch~nger system which utilizes a housing having an inlet at an upper end and a outlet at its lower end. Flue gas enters the housing at the inlet and travels downwardly through the housing and exits the housing at its lower end through the outlet. The housing has an upper stage beneath the inlet which contains a first con~n~ing heat ~x~h~nger which withdraws heat from the flue gas in order to cool the flue gas aæ the flue gas is ~h~nneled downwardly through the housing. A second stage located directly beneath the first stage contains a second condensing heat ~xch~nger which provides a further cooling of the flue gas by withdrawing more heat from the flue gas as the flue gas passes downwardly through the second qtage toward the outlet. A collection tank located at the lower end of the housing beneath the second heat exchanger collects condensate, liquid, particulate and reaction product.
A mist el;m;n~tor is located at the lower end of the housing beneath the collection tank for demisting the flue gas prior to its exit through the outlet. A reagent spray system is located at the second stage for spraying the flue gas with an alkaline reagent solution or slurry for removing cont~m;n~nts such as SO2 from the flue gas. A
spray wash system is located at the upper end of the housing for spraying clP~n;ng liquid down the housing for cl~n;~g both heat exchanger stages.
lo It i9 an object of the present invention to provide a system and method for treating a flue gas which utilizes two separate stages in a vertical relationship which c~nnels the flue gas in a downward direction only.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better underst~n~;ng of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embo~;ments of the invention are ill~strated.
~v A 2 ~ 53~3 4 1 BRIEF DESCRIPTION OF 1~; DRAWINGS
Fig. 1 is a srhPm~tic view illustrating a known con~enqing heat exchanger system;
Fig. 2 is a schP~tic view illustrating an integrated flue gas treatment system; and Fig. 3 is a schematic view illustrating a two-stage downflow flue gas treatment system according to the present invention.
0 DESCRIPTION OF ~; PR~ RED EMBODIMENTS
The present invention is a two-stage downflow flue gas treatment c~n~Pn~ing heat P~ch~nger system and method using co-current gas/droplet flow, as shown in Fig. 3. The purpose of the present invention is to provide improved lS heat recovery and pollutant Le...oval performance compared to the IFGT system shown in Fig. 2. -A preferred embo~;mPnt of the present invention, as best illustrated in Fig. 3, is a two-stage device, generally designated 50 having two con~Pn~ing heat exchanger stages 56 and 62 mounted vertically in series in whlch flue gas 52 enters at the top of the device 50 through inlet 51 and exits at the bottom of the unit 50 through outlet 68. A transition section 60 separates the -two heat exchanger sections 56 and 62. Transition section 54 communicates between inlet 51 and first heat exchanger 56. Structure 58 is used to support the first heat exchanger 56. Structure 64 supports the second heat 5 exchanger 62.
Most of the sensible heat is removed from the flue gas 52 in the first heat exchanger stage 56 and after being passed through the transition section 60, the flue gas 52 enters the second or lower heat ~ch~nger stage 62 where latent heat i8 remo~ed. Droplets are formed in both stages due to con~pn~tion~ The droplets fall downward due to the combined effects of gra~ity and the downward travel direction of the flue gas flow 52. The Qecond heat exchanger stage 62 can be smaller than the first atage 56 in order t-o maintain the optimum ~elocity around the tubes for the cooler gas.
A collection tank 66 iS pro~ided near the bottom of the second stage 62 to collect the water droplets, condensed gases, particulates, reaction products, and 20 alkali reagent. Additional collection me~h~n;.~ms can also be added in the region of the collection tank 66 to aid in the removal of particulates and pollutants from the flue gas stream 52.
The top of the second stage 62 iS optionally equipped 5 ~ 8 4 1 with an alkali reagent spray system 72 to provide ~nh~nced removal of sulfur oxides and other pollutants from gas 52.
The gas 52 leaves the second heat exchanger stage 62 and passes through a--mist el; m; n~tor which is not shown but is located in the region of outlet plenum 68. The liquid collected by the mist el;~;n~tor is fed back to the collection tank 66 through recycle or ~h~nn~ing mP~n~.
The two stage downflow flue gas treatment system 50 also includes a spray washing system 70 located at the top of the unit 50. Periodic wa.~;ng of the heat exch~nger tubes of heat exch~ngers 56 and 62 pre~ents potential plugging of the heat PxchAngers 56 and 62 and provides consistent thermal performance.
The major differences between the two-stage downflow flue gas treatment system 50 and the IFGT system 20 (Fig.
2~ are:
1. ~he flue gas flows in the downward direction in the two-stage downflow flue gas treatment system 50 unlike the multi-directional flow of the IFGT
system 20. In the IFGT system 20, the direction of gas flow in the qecond heat Pxch~nger 28 is upward.
2. In the IFGT system 20, the direction of flow for the particulates and droplets collected in the second stage 28 is opposite to the direction of the flue gas flow. For the two-stage downflow system SO, the direction of flow in the heat exchangers is always the same for the flue gas, droplets, and particles, i.e. downward.
3. In the IFGT system- 20, the particulates and droplets in the second stage 28 must be large enough to overcome the drag forces of the flue gas 22 before they reach the collection area 26.
This is not a requirement for the two-stage downflow design according to the present in~ention.
4. In the IFGT system 20, the transition section 26 acts as the collection tank. The transition section 26 is located between the first and second heat exchanger stages 24 and 28, upstream from the coolest regions of the heat P~ch~nger.
The direction of flue gas flow in the second stage 28 is away from the collection region 26.
For the two-stage downflow system 50 of the present in~ention, the collection tank 66 is downstream from the second heat exchanger stage 62. It is located downstream from the coolest regions of the heat exchanger 62 and the rA 21 5384 1 direction of the flue gas flow is toward the collection tank 66.
The two-stage downflow flue gas treatment system 50 is an improvement over the IFGT design 20. The advantages listed below compare the performance of the two-stage downflow flue gas treatment system 50 with a IFGT design 20.
The present invention has a smaller footprint than the standard IFGT condensing heat exchanger design, thus requiring less space f,or installation.
The present invention has a lower gas side pressure drop than comparable IFGT designs. The reason for this is that all of the flow is in the downward direction. The downflow (co-current droplet/gas flow) in the second heat exchanger stage 60 has a lower pressure drop than the gas upflow, droplet/particulate downflow condition (counter-current droplet/gas flow) encountered in the IFGT design.
The lower pressure drop will permit a smaller forced or induced draft fan to be used in retrofit applications and result in lower parasitic losses during operation.
The present i,,nvention has superior heat recovery performance when compared to IFGT designs. Testing performed on the condensing heat exchangers 56 and 62 of the present invention has demonstrated that the gas C~2153841 downflow design provides maximum heat recovery performance.
All of the heat recovered in the present invention is recovered under gas downflow conditions, while the second stage 28 of the IFGT deslgn 20 recovers heat under gas upflow conditions.
The present invention also has improved particle removal performance, especially for very small particulates. The upflow direction of the flue gas stream 22 in the second stage 28 of a standard IFGT 20 carries particles away from the collection tank 26. In the standard IFGT design 20, very small particles will not be removed unless they become large enough (through water condensation, etc.) to overcome the drag forces of the gas stream and can fall back through the heat exchanger 28 to the collection tank 26. For the present invention, however, the downflow direction of the flow stream 52 always directs the particulates toward the collection tank 64.
The present invention has improved condensable gas removal performance. Condensable gases, such as heavy metals and organic compounds, will form in very small droplets in the cooler regions of the heat exchanger. For the IFGT design 20, the coolest region of the heat exchanger 28 is downstream of the collection tank 26. For the same reasons as cited....
C~2153~41 above, many of the condensable gas droplets formed in the IFGT design 20 will be carried out with the gas stream 22 and can only be collected in the mist el;m;n~tor 30. For the present invention, however, the downflow direction of the flow stream 52 always directs the droplets toward the collection tank 64. In this case, the mist el;m;n~tor 66 captures those droplets that are not removed at the collection tank 64.
The single water spray system 70 in the present lo invention cleans the whole area of both heat exchangers 56 and 62 since the clPAn;ng water will flow through both heat exchangers 56 and 62. In the IFGT design 26, two separate spray clPAn;ng systems are required to achie~e the same result.
The loading on the mist el; m; n~tor 66 is less for the present invention because most of the mist will be removed in the collection tank 64. The small mist droplets will have a greater opportunity to form into larger droplets in the two-stage downflow design 50; and the momentum forces imparted to the droplets by the flue gas 52 is in the direction of the collection tank 64. For the IFGT design 20, most of the mist leaving the heat exchanger 28 will reach the mist el;m;nAtor 30; and when collected, must form droplets of sufficient size to be removed from the gas C A 2 ~ 5 384 1 stream 22 and drained to the collection tank 26.
Although not illustrated, the present invention may incorporate other features which were not described above.
The present in~ention may also include a third heat exchanger stage which could be added downstream of the second stage to improve the removal of condensing organics, heavy metals, and other condensible air pollutants from the flue gas. The third stage would operate indepPn~nt of the rest of the ~ystem and would not be used for heat recovery.
lo The third stage would have a closed cycle refrigerant loop, similar to a ~pht~m;difier, for the purpose of lowering the flue gas temperature further to remove the c~en~ible pollutants.
Also, the present invention can be tailored to incorporate multiple stages, rather than just the two stages described above. Each stage would be designed to optimize the removal of a particular pollutant of concern and would pretreat the flue gas for the next stage.
An additional transition section can also be added between the outlet of the second stage and the mist el;~in~tors to coalesce droplets and particulates and/or impart momentum to the droplets and particulates in order to increase separation performance before the exhaust gas enters the mist el;~;n~tors.
C~2 1 S3&4 ~
The present invention can be used to pre-treat a flue gas prior to entering a wet scrubber. Advantages of this use include: lowering the inlet flue gas temperature which will allow the wet scrubber to operate more efficiently for S2 removal; the two stage downflow unit can be used to subcool the flue gas to m~X;mi ze removal of particulates, HF, HCl, and con~Pn~able air toxics while the wet scrubber is used for S2 removal; a limestone based wet scrubber would produce high quality gypsum without the need for additional washing if the two stage downflow unit removed undesirable materials, such as chloride ions and inert particulates, during pretreatment of the flue ga~; and there would be less reagent lost in a sodium regenerable process if the two-stage downflow unit ~el-wved HF, S3~ N2~
and HCl during pre-treatment of the flue gas. This application would also reduce or el;m,n~te the need for a purge to remove inert materials from the process.
While specific embo~;m~nts of the invention have been shown and described in de-tail to illustrate the application of the principles of the in~ention, it will be lln~rstood that the in~ention may be embodied otherwise without departing from such principles.
Claims (6)
1. A two stage downflow flue gas treatment system for treating a flue gas, comprising:
a housing having an inlet in an upper end and an outlet in a lower end, the flue gas entering the inlet and traveling downwardly through the housing and exiting through the outlet, the housing having a first portion which is larger than a second portion, the first portion being adjacent the upper end of the housing and the second portion being adjacent the lower end of the housing;
first tubular heat exchanger means positioned in the first portion of the housing for cooling the flue gas, the first tubular heat exchanger means including corrosion resistant tubes positioned horizontally in the housing;
second tubular heat exchanger means positioned in the housing beneath the first tubular heat exchanger means for further cooling the flue gas, the first and second tubular heat exchanger means being mounted vertically in series in the housing, the second tubular heat exchanger means including corrosion resistant tubes positioned horizontally in the housing, the second tubular heat exchanger means being smaller than the first tubular heat exchanger means;
Alkali reagent spray means positioned above the second tubular heat exchanger means for cleaning pollutants from the flue gas;
spray wash means located above the first tubular heat exchanger means for washing the first and second tubular heat exchanger means; and collection means in the lower end of the housing below the second tubular heat exchanger means for collecting liquids and particulate.
a housing having an inlet in an upper end and an outlet in a lower end, the flue gas entering the inlet and traveling downwardly through the housing and exiting through the outlet, the housing having a first portion which is larger than a second portion, the first portion being adjacent the upper end of the housing and the second portion being adjacent the lower end of the housing;
first tubular heat exchanger means positioned in the first portion of the housing for cooling the flue gas, the first tubular heat exchanger means including corrosion resistant tubes positioned horizontally in the housing;
second tubular heat exchanger means positioned in the housing beneath the first tubular heat exchanger means for further cooling the flue gas, the first and second tubular heat exchanger means being mounted vertically in series in the housing, the second tubular heat exchanger means including corrosion resistant tubes positioned horizontally in the housing, the second tubular heat exchanger means being smaller than the first tubular heat exchanger means;
Alkali reagent spray means positioned above the second tubular heat exchanger means for cleaning pollutants from the flue gas;
spray wash means located above the first tubular heat exchanger means for washing the first and second tubular heat exchanger means; and collection means in the lower end of the housing below the second tubular heat exchanger means for collecting liquids and particulate.
2. The system according to claim 1, including mist elimination means for removing mist from the flue gas located prior to the outlet of the housing.
3. The system according to claim 1, including a transition section between the first tubular heat exchanger means and the second tubular heat exchanger means.
4. The system according to claim 1, including a structure for supporting the first tubular heat exchanger means in the housing.
5. The system according to claim 1, including a structure for supporting the second tubular heat exchanger means in the housing.
6. The system according to claim 1, wherein the spray wash means is means for periodically washing the first and second tubular heat exchanger means.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002153841A CA2153841C (en) | 1995-07-13 | 1995-07-13 | Two stage downflow flue gas treatment condensing heat exchanger |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002153841A CA2153841C (en) | 1995-07-13 | 1995-07-13 | Two stage downflow flue gas treatment condensing heat exchanger |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2153841A1 CA2153841A1 (en) | 1997-01-14 |
| CA2153841C true CA2153841C (en) | 2000-02-15 |
Family
ID=4156221
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002153841A Expired - Fee Related CA2153841C (en) | 1995-07-13 | 1995-07-13 | Two stage downflow flue gas treatment condensing heat exchanger |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2153841C (en) |
-
1995
- 1995-07-13 CA CA002153841A patent/CA2153841C/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| CA2153841A1 (en) | 1997-01-14 |
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| EEER | Examination request | ||
| MKLA | Lapsed |