Membrane laminar wet electrostatic precipitator

Details Membrane laminar wet electrostatic precipitator Membrane laminar wet electrostatic precipitator

2003-05-09

2004-08-31

Chiesa, Richard L. (Department: 1724)

Gas separation: apparatus

Electric field separation apparatus

Electrode cleaner, apparatus part flusher, discharger, or...

C096S045000, C096S060000, C096S075000, C096S079000, C096S098000

Reexamination Certificate

active

06783575

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to electrostatic precipitators (ESPs) used to precipitate particulate matter from exhaust gases onto collection substrates by electrostatic charge, and more particularly to a laminar flow, wet membrane collecting electrode ESP.
2. Description of the Related Art
Industrial ESPs are used in coal-fired power plants, the cement industry, mineral ore processing and many other industries to remove particulate matter from a gas stream. ESPs are particularly well suited for high efficiency removal of very fine particles from a gas stream. Specially designed ESPs have attained particle collection efficiencies as high as 99.9%. However, conventional ESP collection efficiencies are at their lowest values for fine particle sizes between 0.1-1.0 &mgr;m. Additionally, conventional ESPs cannot address the problem of gaseous emissions or gas-to-particle conversion.
In 1997 the Environmental Protection Agency (EPA) proposed new air quality standards for fine particulate matter. The focus of the regulations is the emissions of fine particulate, i.e., particles below 2.5 &mgr;m in aerodynamic diameter (PM2.5). These fine particulates are a health danger, because the human body cannot prevent these small particles from entering the respiratory tract and lungs.
In a typical conventional ESP, vertical wire electrodes are placed in the midsection of a channel formed between vertical parallel collector substrates. The heavy, typically steel, plates are suspended from a support structure that is anchored to an external framework. Commonly, ten or more of the single precipitation channels constitute a field. Industrial precipitators have three or more fields in series. An example of such a structure is shown and described in U.S. Pat. Nos. 4,276,056, 4,321,067, 4,239,514, 4,058,377, and 4,035,886, which are incorporated herein by reference.
A DC voltage of about 50 kV is applied between the wire electrodes (discharging electrodes) and the grounded substrate collector plates (collecting electrodes), inducing a corona discharge between them. A small fraction of ions, which migrate from the wires toward the plates, attach to the dust particles in the exhaust gas flowing between the plates. These particles are then forced by the electric field to migrate toward, and collect on, the plates where a dust layer is formed.
In dry ESPs, the dust layer is periodically removed from dry ESPs by hammers imparting sharp blows to the edges of the plates, typically referred to as “rapping” the plates. When ESPs are rapped, the dust layer is supposed to drop vertically downward from the plates due to a shear force between the plate and the parallel dust layer. The compressive loading in this so-called normal-rapping mode generates fast propagating stress waves, along and across the plate, that are manifested in large lateral amplitude displacements of the plates in the direction perpendicular to the plane of the plate.
Pasic et al., in U.S. Pat. No. 6,231,643, which is incorporated herein by reference, first disclosed the principle of using a membrane as a collecting electrode in a dry or a wet ESP in order to avoid the large deflection of the electrode due to rapping. However, the turbulent flow of gases around the membrane electrodes prevented substantial collection of acid aerosols and fine particulate.
Control of fine particulate and acid aerosols are of vital importance to the burning of coal that is inherently high in sulfur. The higher the sulfur content, the higher the SO
3
content, and therefore, the more likely that sulfuric acid aerosol formation will occur, especially in units that use selective catalytic reduction (SCR) for NOx control. The resulting opacity from the acid aerosols has caused plants to reduce their output during these exceedances.
Current particulate control devices, such as precipitators and bag filters, have problems with collection of fine particulate and acid gases, which later form aerosols known as secondary PM 2.5. Effective collection of submicron particles with bag filters is inherently difficult and creates unacceptably large pressure drops across the filter. ESPs have a particularly difficult time collecting particles in the size range of 0.1-1.0 &mgr;m, because the two dominant modes of particle charging, field and diffusion, go through combined minimums in this size range, and because particle charge depends on the strength of the electric field. In dry precipitators corona current and electric field strength is suppressed as the electrically resistive ash layer builds on the collecting surfaces. This effect can even lead to formation of back corona in dry precipitators.
The control of NO
x
emissions using selective catalytic reduction (SCR) technology is likely to aggravate SO
3
emissions at existing coal-fired power plants. Several plants with SCRs have experienced catalytic oxidation of SO
2
to SO
3
. SO
3
vapor, in combination with water vapor, converts to gaseous sulfuric acid. When SO
3
vapor reaches saturation upon cooling or in contact with water, aerosols of fine sulfuric acid mist are formed. Most of these aerosols reside in a particle size range between 0.1 and 0.5 &mgr;m. At these sub-micron particle sizes the light scattering phenomenon is also at a maximum. This will result in a highly visible plume even for relatively small amounts of sulfuric acid aerosols. The resulting opacity can lead to temporary de-ratings of units, costing the plant potential sales.
A conventional ESP operates with turbulent flow in the gas channels. Because of the turbulent eddies, 100% collection efficiency is approached only asymptotically and cannot be attained no matter how large the precipitator. One theory that has been commercialized for dry precipitators to address their inherent problems with fine particulate collection is the use of laminar flow in precipitation. In laminar flow the flow streamlines are parallel and in the direction of flow, and therefore, there are no turbulent forces causing particles, especially fine particles, near the collecting surface to be blown back into the central flow region. Therefore, 100% collection efficiency is possible in laminar flow.
To create laminar flow, as is known, the Reynolds number (Re) must first be less than 2300 where
Re
=
V
gas
⁢
ρ
g
⁢
D
h
μ
g
where D
h
is the hydraulic diameter defined by
D
h
=
2
⁢
(
Δ
⁢
 
⁢
x
)
⁢
H
Δ
⁢
 
⁢
x
+
H
where &Dgr;
x
is plate spacing and H is the height of the collection electrode.
Reducing gas velocity to attain Re<2300 has been attainable since the first precipitator was built. However, laminar flow in ESPs is still prevented by the cross flow due to corona wind. The cross-flow caused by corona wind continuously disrupts the laminar flow conditions and creates a rebound effect from the solid collecting surfaces.
In 1998 Environmental Elements Corporation (EEC) overcame the problem of cross-flow caused by corona wind by using planar discharge electrodes with lower voltage, that are positioned much closer together than in conventional ESPs and have virtually no current flow. The idea behind a laminar flow precipitator is to vastly reduce the distance between the collection plates and as such, lower the Reynolds number below 2300, the generally accepted condition for transition to turbulent flow. Further, the plates must be smooth, as surface imperfections create disruptions of the boundary layer or induce turbulence outright. Both factors are employed to limit formation of turbulent flow.
The EEC device relies on upstream, turbulent flow electrostatic precipitator fields to remove 95+% of particulate in the gas stream and to charge all remaining particles before the particles reach the laminar region. However, the dry laminar precipitator in the EEC device fails to permanently collect particles. This is because, although the EEC device eliminates corona wind, it also eliminates the current flow that serves, in conventional ESPs, as the main adhesiv

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