Note: Descriptions are shown in the official language in which they were submitted.
CA2 1 53841
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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. :~
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
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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,
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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
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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.
CA 2 1 538 4 1
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.
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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
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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
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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
C A 2 1 53 84 1
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
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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
11
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
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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.
14
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.
