Gas separation: processes – Liquid contacting – With cooling
Reexamination Certificate
1995-12-01
2001-08-14
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Liquid contacting
With cooling
C095S235000
Reexamination Certificate
active
06273940
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to the decontamination of flue gas and, in particular, to a new and useful method to recover (fly ash), sulfur oxides and/or other contaminants contained in flue gases formed during the combustion of waste materials, coal, oil and other fossil fuels, which are burned by electric power generating plants, process steam production plants, waste-to-energy plants and other industrial processes.
2. Description of the Related Art
There are several systems relating to integrated heat recovery and the removal of particulates, sulfur oxides/acid gases and contaminants from a hot combustion exhaust gas in order to comply with federal and state requirements.
One system, which is shown in
FIG. 1
, is a condensing heat exchanger, generally designated
10
, which recovers both sensible and latent heat from flue gas
11
in a single unit. The arrangement allows for the gas
11
to pass down through heat exchanger
12
while water
14
passes upward in a serpentine path through tubes
13
. 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 and is removed at the bottom at outlet
16
. Gas cleaning can occur within the heat exchanger
12
by the mechanisms of absorption, condensation and impaction as the gas
11
is cooled below the dew point.
The heat exchanger tubes
13
(
FIGS. 2
b
and
2
c
) and inside surfaces of heat exchanger shell
15
are made of corrosion resistant material or are covered with a fluoroplastic such as fluorinated ethylene propylene (FEP), tetrafluoroethylene (TFE) or polytetrafluoroethylene (PTFE) like Teflon
17
, a registered trademark of Du Pont Corp., to protect them from corrosion when the flue gas temperature is brought below the acid dew point. Interconnections between the heat exchanger tubes
13
are made outside the tube sheet
15
through holes
19
which are sealed by Teflon seal
18
and are not exposed to the corrosive flue gas stream
11
. The modular design of this heat exchanger is shown in
FIG. 2
a.
Another system used in this area is an integrated flue gas treatment (IFGT) condensing heat exchanger, generally designated
20
, which is schematically shown in FIG.
3
. Condensing heat exchanger unit
20
is designed to enhance the removal of pollutants, particulate, sulfur oxides/acid gases and other contaminants from flue gas stream
22
. It is also made of corrosion resistant material or has all of the inside surfaces covered by Teflon, or like material.
There are four major sections of the IFGT
20
: a first heat exchanger stage
24
, an interstage transition region
26
, a second heat exchanger stage
28
, and a mist eliminator
30
. The major differences between the integrated flue gas treatment design of FIG.
2
and the conventional condensing heat exchanger design of
FIG. 1
are:
1. the integrated flue gas treatment design uses two heat exchanger stages
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
to sprayers
46
; and
5. the mist eliminator
30
is used to separate the water formed by condensation and sprays from the flue 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 particulate, sulfur pollutants, acid gases and other contaminants from the gas
22
.
The transition piece
26
is made of corrosion resistant material like fiberglass-reinforced plastic. Additionally, the second heat exchanger stage
28
is operated in the condensing mode, removing latent heat from the gas
22
along with pollutants. Also, the top of the second heat exchanger stage
28
is equipped with an alkali solution spray device
46
. The gas
22
in this stage
28
is flowing upward while the droplets in the gas
22
fall downward. This counter-current gas/droplet flow provides a scrubbing mechanism that enhances particulate and pollutant capture. The captured particulate, sulfur oxides/acid gases and contaminants that are contained in the falling condensate/reacted alkali droplets flow downward and are collected at the bottom of the transition section
26
. The flue gas outlet
29
of the IFGT
20
is equipped with the mist eliminator
30
to reduce the chance of moisture carryover.
Other treatment methods include wet chemical absorption processes, i.e. the use of wet scrubbers such as the unit
50
shown in
FIG. 4
, and in particular those applications where the hot gas
22
is typically washed in an upflow gas-liquid contact device
52
(i.e. spray tower) with an aqueous alkaline solution or slurry by sprayers
54
in order to remove sulfur oxides and/or other contaminants from the gas
22
.
Wet chemical absorption systems installed by electric power generating plants typically utilize calcium, magnesium or sodium based process chemistries, with or without the use of additives, for flue gas desulfurization.
In addition, one known wet scrubbing system is described in U.S. Pat. No. 4,263,021 utilizes a gas-liquid contact system for obtaining counter-current gas-liquid contact between a flue gas containing sulfur dioxide and an aqueous slurry solution. This system is currently referred to as a tray or gas distribution device.
Other wet scrubber devices utilize various types of packing inside the spray tower to improve gas-liquid distribution which works well with clear solution chemistry processes, but are prone to gas channeling and pluggage in slurry services.
Most of the wet scrubbers use mist eliminators, normally 2-3 stages, such as mist eliminators
56
,
58
as shown in
FIG. 4
, in order to remove entrained water droplets from the scrubbed gas.
SUMMARY OF THE INVENTION
The present invention is a system for eliminating mist from a flue gas while providing air toxic control in a wet scrubber reactor. The system comprises a wet scrubber housing having an inlet at one end of the housing for the entry of the flue gas and an outlet at an opposite end of the housing for the exit of the flue gas. Sprayers are located in the housing for spraying the flue gas with a cleaning liquid in order to remove contaminants from the flue gas. At least one heat exchanger is located in the housing above the sprayers in order to remove entrained contaminants from the flue gas by cooling the flue gas. The flue gas is channeled from the inlet past the sprayers and heat exchanger prior to exiting the housing through the outlet. A mist eliminator is also provided for eliminating mist from the flue gas.
The present invention also comprises a method for carrying out the flue gas treatment described above.
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 understanding 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 embodiments of the invention are illustrated.
REFERENCES:
patent: 1861158 (1932-05-01), Hilger
patent: 1927869 (1933-09-01), Downs
patent: 3473298 (1969-10-01), Berman
patent: 3522000 (1970-07-01), Kinney
patent: 3854909 (1974-12-01), Ho
Bhat Pervaje A.
Bielawski Gregory T.
Johnson Dennis W.
Myers Robert B.
Edwards Robert J.
Kalka Daniel S.
Marich Eric
Spitzer Robert H.
The Babcock & Wilcox Company
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