Gas separation: processes – Electric or electrostatic field – And nonelectrical separation of fluid mixture
Reexamination Certificate
2000-07-28
2003-02-04
Chiesa, Richard L. (Department: 1724)
Gas separation: processes
Electric or electrostatic field
And nonelectrical separation of fluid mixture
C055S341100, C096S055000, C110S216000
Reexamination Certificate
active
06514315
ABSTRACT:
BRIEF DESCRIPTION OF THE INVENTION
This invention relates generally to the control of pollutants during a combustion process. More particularly, this invention relates to a technique of using high permeability filter bags in a barrier filter, such as a bag house.
BACKGROUND OF THE INVENTION
FIG. 1
illustrates a flue gas treatment system
10
constructed in accordance with the prior art. The system
10
is described in U.S. Pat. No. 5,158,580, which is assigned to the assignee of the present invention, and which is incorporated by reference herein. The flue gas treatment system
10
treats a flue gas exiting a boiler
12
. The boiler
12
is of the type used in a utility fossil-fuel-fired power plant. Fuel supply
18
may be, for example, coal, oil, refuse derived fuel (RDF) or municipal solid waster (MSW). Boiler
12
also receives air
20
through inlet duct
22
. Boiler
12
functions to combust the fuel
14
with air
20
to form flue gas
24
, which exits boiler
12
by means of outlet duct
26
. Boiler
12
also has a water inlet pipe
28
and a steam outlet pipe
30
for removing heat in the form of steam from boiler
12
generated by the combustion of fuel
14
with air
20
.
Flue gas
24
is comprised of components of air and the products of combustion in gaseous form which include: water vapor, carbon dioxide, halides, volatile organic compounds, trace metals vapors, and sulfur and nitrogen oxides and the components of air such as oxygen and nitrogen. Flue gas
24
also contains particulate comprising unburned and partially combusted fuel which includes inorganic oxides of the fuel (known as flyash), carbon particles, trace metals, and agglomerates. Flue gas
24
may also contain particulate generated by the addition of removal agents
19
for sulfur oxide and other gas phase contaminates, such as halides and trace metal vapors. The removal agents
19
may be added into duct
21
, duct
26
, or into reactor vessel
17
by way of duct
23
. Ducts
21
,
23
and
26
may also convey solid materials if required for the selected removal agents
19
for the respective duct. Examples of sulfur oxide and other gas phase contaminate removal agents
19
include calcium carbonates, oxides and hydroxides, and sodium carbonates and bicarbonates. The particles or particulate in flue gas
24
can vary considerably in size, shape, concentration and chemical composition.
Flue gas
24
passes through duct
26
through reactor vessel
17
and through duct
27
as flue gas
25
to an inlet of electrostatic precipitator
34
, which functions to charge and collect particles on electrodes within the electrostatic precipitator
34
. Reactor vessel
17
may facilitate the chemical reaction of removal agents
19
with flue gas
24
to provide treated flue gas
25
. Electrostatic precipitator
34
may remove, for example, from 90-99.9% of the particles and/or particulate. Therefore, flue gas
24
exits electrostatic precipitator
34
as treated flue gas
36
entering outlet duct
38
. Treated flue gas
36
has roughly from 0.1-10% of the particulate or particles contained in the original flue gas
24
and also contains a certain amount of electrostatic charge which was transferred to it from the electrostatic precipitator
34
. These particles were not collected within the electrostatic precipitator, but exited at outlet duct
38
.
The particle concentration in the flue gas
36
exiting the electrostatic precipitator
34
is reduced significantly by the precipitator and contains a residual charge imparted by the precipitator. Particulate
36
leaving the electrostatic precipitator may lose charge if there is a long path between the electrostatic precipitator
34
and the barrier filter
44
. To prevent this problem, a pre-charging unit
40
may be used. Gas
36
enters pre-charging unit
40
through inlet duct
38
. The pre-charging unit
40
operates to charge particulate and then deliver it to a barrier filter
44
.
Examples of acceptable barrier filters
44
include baghouses of the pulse-jet type, reverse flow, or shake-deflate type for periodically removing the dust cake accumulated on the surface of the bag filter. Prior art barrier filters are formed with bags having a nominal air permeability of 25 to 50 acfm/sq ft (actual cubic feet per minute of air flow per square foot of filter surface area at a pressure drop of one half inch water).
The flue gas
48
exiting barrier filter
44
passes through outlet duct
50
, through fan
52
and through duct
54
to the inlet of smoke stack
46
. Flue gas
48
exits smoke stack
46
as gas
58
, which in turn mixes with the ambient air or atmosphere.
Fan
52
functions to overcome the additional pressure drop required to draw flue gas
48
across the barrier filter
44
. Fan
52
also functions to draw flue gases
36
and
24
from electrostatic precipitator
34
and boiler
12
respectively. Fan
52
further functions to move flue gas
48
through duct
54
and out of smoke stack
46
as flue gas
58
.
One problem associated with the system
10
is that the bags used in the barrier filter
44
have a relatively short life. Chemical degradation of the bag material and high pressure drop problems shorten bag life significantly. The causes for the degradation of the bag material are frequently unknown. For example, the causes for the degradation of bag material formed of RYTON (sold by American Fiber & Yarn) in the flue gas are not fully understood. New bag materials, such as TEFLON and glass, appear to provide improved resistance to chemical degradation. However, felted fabrics made of TEFLON are very expensive and both high efficiency TEFLON and glass filter felts still have high pressure drop problems.
The high pressure drop problem can be traced to the relatively poor efficiency of the existing ESP 34, which leads to high dust loading in the baghouse (i.e., barrier filter
44
) and to the high air-to-cloth ratio associated with the baghouse. Thus, the high efficiency conventional felted RYTON filter needs to be cleaned frequently, leading to eventual bag blinding (i.e., penetration of dust into the felted fabric).
In view of the foregoing, it would be highly desirable to provide an improved barrier filter for use in a flue gas treatment system.
SUMMARY OF THE INVENTION
The invention includes a method of treating flue gas. Particulate is removed from the flue gas with a first particulate removal technique to produce treated flue gas. The treated flue gas is then applied to a high permeability barrier filter to remove additional particulate from the treated flue gas. The high permeability filter includes high permeability filter bags with air permeability greater than 75 actual cubic feet per minute of air flow per square foot of filter surface area at a pressure drop of one half inch water.
The invention also includes a flue gas treatment apparatus. The flue gas treatment apparatus includes a combustor to generate a flue gas. An electrostatic precipitator is connected to the combustor to remove particulate from the flue gas. A barrier filter is connected to the electrostatic precipitator to remove particulate from the flue gas. The barrier filter includes high permeability filter bags with air permeability greater than 75 actual cubic feet per minute of air flow per square foot of filter surface area at a pressure drop of one half inch water.
The fabric filter of the invention is easier to clean, provides reduced pressure drop, and is not subject to blinding. The filter fabric of the invention can also resist chemical degradation. The invention can be exploited in a variety of polishing baghouse operations where a baghouse is placed after a primary particulate collector to capture residual flyash.
REFERENCES:
patent: 1853393 (1932-04-01), Anderson
patent: 3915676 (1975-10-01), Reed et al.
patent: 3966435 (1976-06-01), Penney
patent: 4147522 (1979-04-01), Gonas et al.
patent: 4354858 (1982-10-01), Kumar et al.
patent: 4357151 (1982-11-01), Helfritch et al.
patent: 4411674 (1983-10-01), Forgac
patent: 5024681 (1991-06-01), Chang
patent: 51585
Chiesa Richard L.
Electric Power Research Institute Inc.
Pennie & Edmonds LLP
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