OXIDES OF NITROGEN FOR IMPROVING ELECTROSTATIC PRECIPITATION OF DUST PARTICLES

OXYDES D'AZOTE FAVORISANT LA PRECIPITATION DES POUSSIERES PAR VOIE ELECTROSTATIQUE

English Abstract

ABSTRACT OF THE DISCLOSURE
A method is disclosed of increasing the space charge in a stream
of particle-laden gas formed by the burning of coal, which particles are
collected by an electrostatic precipitator. The method comprises treating
the gas-containing particles prior to contact with the electrostatic pre-
cipitator at a temperature not greater than about 1800°F with a space charge
increasing amount of an oxide of nitrogen and thereafter passing the gas to
the electrostatic precipitator.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of increasing the space charge in a stream of particle-
laden gas formed by the burning of coal, which particles are collected by an
electrostatic precipitator, which method comprises treating the stream of
particle-laden gas prior to contact with the electrostatic precipitator at a
temperature not greater than about 1800°F with a space charge increasing
amount of an oxide of nitrogen or a nitrogen compound which decomposes under
the conditions of use to provide a space charge increasing amount of an oxide
of nitrogen, and then passing the gas to the electrostatic precipitator.
2. The method of Claim 1 wherein the oxide of nitrogen is NO2.
3. The method of Claim 1 wherein from 0.1 to 6 weight percent of oxide
of nitrogen is used, based on the weight of the particles present in the gas
stream.
4. The method of Claim 1 wherein from 0.5 to 3 weight percent of oxide
of nitrogen is used, based on the weight of the particles present in the gas
stream.
5. The method of Claim 1 wherein the stream of particle-laden gas is
treated at a temperature of above about 600°F.
6. The method of Claim 1 wherein the stream of particle-laden gas is
treated at a temperature of above about 250°F.
7. The method of Claim 1 wherein an oxide of nitrogen is used.
8. The method of Claim 1 wherein a nitrogen compound is used.

9. The method of Claim 8 wherein the nitrogen compound is chosen from
nitric acid and nitrate salts.

Description

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This invention relates to a method of increasing the space charge
in a stream of particle-laden gas.
A conventional way of separating dust particles from a gas stream
in which the particles are entrained is by the use of an electrostatic pre-
cipitator. This apparatus utilizes the corona discharge effect, i.e., the
ionization of the particles by passing them through an ionization field es-
tablished by a plurality of discharge electrode wires suspended in a parallel
plane with a grounded collecting electrode plate. The ionized particles are
attracted to the collector plate from which they may be removed by vibrating
or rapping the plate. Examples of this type precipitator are found in
Cummings United States Patent ~o. 3,109,720 and Pennington United States
Patent No. 3,030,753.
Efficient capture of dust particles by a precipitator depends
chiefly upon precipitator size and the precipitation rate parameter. The
precipitation rate parameter (otherwise known as "migration velocity") is
defined by:
_ k ~ E ~:1 d
~, ..,
where k = constant
~ = permittivity
~c = charging field
= collecting field
= particle diameter
~ gas viscosity
The migration velocity is seen to depend linearly on both the charging and
collecting field. The collecting field is most affected by space charge since
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its polarity (+) attracts the negative (space) charges. This change in space
charge density (an increase at the collection plate) causes a larger E2 and
thus a larger precipitation rate parameter.
These effects are illustrated graphically in the attached Figures
wherein:
Figure 1 represents the magnitude of the electric field between the
corona wire and the collection plate without a space charge increase; and
Figure 2 represents the magnitude of the electric field between the
corona wire and the collection plate with a space charge increase.
The change in space charge is manifested by an increase in secondary
voltage, lower current, and higher collection rates for a given power input.
This is routinely observed in full size and pilot precipitators. We have
demonstrated in a pilot precipitator that the introduction of an oxide of
nitrogen does increase space charge and manifest these results.
It has now been found that the electrostatic removal of particles
entrained in a gas stream can be improved by the addition to such gas stream
of preselected amounts of an oxide of nitrogen. Accordingly the invention
provides a method of increasing the space charge in a stream of particle-laden
gas formed by the burning of coal, which particles are collected by an electro-
static precipitator, which method comprises treating the stream of particle-
laden gas prior to contact with the electrostatic precipitator at a temperature
not greater than about 1800F with a space charge increasing amount of an oxide
of nitrogen or a nitrogen compound which decomposes under the conditions of use
to provide a space charge increasing amount of an oxide of nitrogen, and then
passing the gas to the electrostatic precipitator.
The amount of the oxide of nitrogen that is effective in decreasing
the resistivity of the dust particles may vary. Generally it is used in an
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amount ranging from 0.1 up to about 6 weight percent based on the weight of
the particles presen~ in the gas stream. In a preferred embodiment, the
dosage ranges between 0.5 - 3% by weight.
Most large coal-fired boilers are composed of a number of regions
These regions, starting with the combustion flame and ending with the
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electrostatic precipitator which, in most instances, is located prior to the
exhaust gas stac~, form a series of progressively cooler gas temperature
zones. For purpose of simplification, these zones and their respective tem-
peratures are set forth below in a simplified manner:
Location Temperature
In the flame 2500 - 3500F.
In the furnace-radiant section 2000 - 2500F.
After super heater 1000 - 1600F.
After economizer 500 - 750F.
After air heater 250 - 350F.
Up the stack 250 - 350F.
The oxide of nitrogen is added to the gas stream at a temperature
in excess of about 250F., but, preferably, at a temperature greater than
600F. In certain instances, they can be employed at temperatures as high
as 1800F.
The oxides of nitrogen that may be used in the practice of the
invention may be selected from the following oxides: N20, N0 or (N0)2,
N203, N02 or (N02)2, or N205. Of these oxides, N02 is preferred. While
the oxides may be used as such, the invention also contemplates using com-
pounds which, under conditions of use, decompose to 1 or more of the oxidesof nitrogen mentioned above. Specifically it is possible to use nitric acid
or nitrate salts such as manganese nitrate which decomposes to form N02.
Example
Using nitric acid as a source of N02, tests were conducted in a
laboratory electrostatic precipitator. These tests showed an approximate 45
reduction in emissions when HN03 was decomposed in flue gas at 400F., at a
rate of 0.57% by wt. fly ash. By this application, the applied voltage im-
mediately jumped 2Kv while corona current decreased 45~a.
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