Method for corona discharge enhanced flue gas clean-up

A process for removing pollutants including sulfur dioxide and/or nitrogen oxides from effluent gas which relies on the use of corona discharge to enhance the efficiency of the process. In one embodiment, corona discharge is utilized in a conventional spray dryer. In another it is combined with an ammonia injection technique. In yet another embodiment corona discharge treatment is followed by exposing the acidic mist byproduct leaving the discharge treatment chamber to a neutralizing reagent directed into the path of the effluent. The reagent may be provided with an electrical charge opposite to that on the acidic mist byproduct emerging from the corona reaction chamber to enhance neutralization.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to an improved process for removing 
pollutants, especially sulphur dioxide and nitrogen oxides, from effluent 
gases resulting from various chemical processes and by the combustion of 
carbonaceous fuels. This invention relates to treating effluent gases so 
that these pollutants may be changed into particle form or mist form, 
thereby enabling collection of the particles or mist pollutants by 
conventional collecting means such as precipitators, filters and the like. 
Specifically, the invention relates to the combination of corona discharge 
treatment with previous processes known to provide for the removal of 
sulphur dioxide and nitrogen oxide from such fuels. 
(2) Description of the Prior Art 
The need to deal with the deleterious effects of emissions generated from 
various sources such as industrial processes and energy production of 
various kinds is well known. The impact of such emissions on human health 
has recently been the subject of intense study and public debate, and 
legislative action to mandate safer emissions has already been taken and 
will probably be enlarged or extended in the future. The "acid rain" 
controversy has, of late, focussed the public's attention on the effects 
of such emissions on the natural environment and promises to provide a 
continuing debate over the causes of water pollution and appropriate 
legislative action that should be taken to curtail such pollution. The 
main emissions identified as having such adverse effects are various forms 
of nitrogen oxides (NO.sub.x) and sulphur dioxide (SO.sub.2). Other 
byproducts of the energy creation process such as particulates are of less 
concern since known (albeit expensive) methods are available for their 
removal. 
Each of the above problems forces the public to choose between the present 
and relatively cheap methods for producing energy and chemicals on the one 
hand, and increasing the cost of such production and processes by either 
changing to a different energy production process or altering current 
processes by the addition of equipment designed to remove such pollutants. 
In response to the above, several methods and apparatus have been developed 
to minimize the sulphur dioxide pollution caused by the burning of carbon 
fuels. Typically, such methods and apparatus are costly and have large 
operating and maintenance problems. 
The prior art has included various methods for removing the above noted 
contaminants from the energy production process. One such technique is 
referred to as the spray drying process. In the spray drying process, a 
mixture of water and alkali reagent such as lime is injected into the 
stream of effluent gases in a reaction chamber resulting in the formation 
of solid products from the reaction between the reagent and the 
pollutants. From the chamber the effluent gases pass into a particulate 
collection means such as a fabric or bag filter or an electrostatic 
precipitator whereat the solid particles of the reaction between the 
reagent and the pollutants are removed. The purified effluent gases are 
passed into a stack from which they are discharged after passing through 
the particulate collection means. 
In order to operate the spray drying process most efficiently, large 
amounts of alkali reagent are needed, but such large amounts of reagent 
require correspondingly large amounts of water to operate properly. For 
this reason methods for increasing the efficiency of the spray drying 
process which do not require the use of additional amounts of reagent are 
desirable. 
Another type of prior art pollution control system utilizes radiation in 
various forms to improve the efficiency of the process. Such systems use 
electron beam or ultraviolet light to oxidize the nitrogen oxides and 
sulphur dioxides in the effluent gases. The ionization caused by the 
electron beam irradiation converts the sulphur dioxide and nitrogen oxides 
to acid mist at low temperatures and/or solid particles at high 
temperatures (in the presence of ammonia) which latter byproducts are 
removed in a conventional manner for later disposition. In the electron 
beam method, costly and elaborate shielding measures must be employed. 
OBJECTS AND SUMMARY OF THE INVENTION 
A primary object therefore is to provide any improved method and apparatus 
for removing pollutants from effluent gases before releasing them into the 
atmosphere. 
Another object of the invention is to provide an improved technique for 
removing sulphur dioxide and nitrogen oxides from effluent gases resulting 
from the burning of carbonaceous or other fuels or from other commercial 
or industrial processes. 
A further object of the invention is to improve efficiency and usefulness 
of the spray drying process applicable to high sulphur content coals. 
A further object of the present invention is to combine the use of a corona 
discharge treatment of the effluent gases with the conventional spray 
drying process so as to increase the efficiency and effectiveness of the 
spray drying process. 
Yet another object of the invention is to provide an air pollution control 
system having a spray dryer which incorporates a corona discharge device 
in the same reaction chamber to thereby improve the efficiency and 
compactness of the treatment process. 
An additional object is to improve the neutralization of acidic mist 
byproducts produced by a corona discharge enhanced reaction by subjecting 
such byproducts to the action of liquid or dry neutralizing reagent after 
they exit from the corona discharge reaction chamber. 
An additional object is to improve the efficiency of the process for 
neutralizing acidic byproducts produced in a corona discharge reaction 
chamber by precharging the alkali reagent with a polarity opposite to that 
of the acidic mist byproducts in order to promote a more rapid 
interaction. 
The above and other objects of the invention are accomplished by delivering 
effluent gases into a spray dryer type reaction chamber while concurrently 
subjecting the effluent gases in the chamber to a corona discharge thereby 
improving the compactness and efficiency of the spray drying treatment 
process. 
A further embodiment of the invention comprises a process including the 
steps of first injecting ammonia into the contaminated effluent flue gases 
followed by injection of the resulting gas mixture into a corona discharge 
chamber. The effluent gas emanating from the corona discharge reaction 
chamber are in the form of particulates which are collected by 
conventional means prior to emitting the cleaned flue gases to the 
atmosphere. 
Yet another embodiment of the invention provides for the use of a corona 
discharge reaction chamber in which the sulphur dioxide contaminants are 
converted into an electrically charged acidic mist which is then subjected 
to a neutralization treatment step at which the alkali reagent, in either 
liquid or powder form, is first charged oppositely to the mist then made 
to combine with the mist emanating from the corona discharge reactor. The 
neutralizing reagent may be electrically charged prior to intermingling 
with the acidic mist, with a polarity opposite to that of the mist to 
further improve the neutralization process. The latter arrangement permits 
more efficient neutralization of the acidic mist prior to collection and 
disposal.

DESCRIPTION OF THE PREFERRED METHOD AND EMBODIMENT 
Referring to FIG. 1, there is shown a generalized layout in block form of a 
typical treatment system according to the invention. As shown in FIG. 1, 
contaminated flue gases are first fed into a corona reactor 11 wherein the 
flue gases are exposed to reactive species generated by the corona 
reactor. The reactive species generated within the corona reactor lead to 
the oxidation of the NO.sub.x /SO.sub.x contaminants into nitric and 
sulphuric acids. The nitric and sulphuric acids are then fed to a 
neutralization reactor 12 in which the nitric and sulphuric acids are 
neutralized by the addition of suitable bases, for example, ammonia or 
lime. From the neutralization reactor, the effluent gases pass into a 
particulate collection means 13 such as a fabric filter or electrostatic 
precipitator, whereat the solid products emanating from the alkaline 
neutralization reactor are removed. As illustrated in FIG. 1, the purified 
effluent gases passing out of the collection means 13 are discharged into 
the atmosphere through any appropriate means such as a stack. Although in 
accordance with the general layout of FIG. 1, the corona reactor and 
alkaline neutralization reactor are shown as separate chambers, it is 
possible that the reactions brought about within each of the above-noted 
reactors may be carried out in a single reactor by injecting the 
combination of ingredients thereinto. 
One such arrangement may be seen in diagrammatic form in FIG. 2. Referring 
to FIG. 2, there is shown an arrangement which combines the aforediscussed 
spray drying process with corona treatment to perform the cleanup of 
contaminated gases in an efficient manner. In FIG. 2, combustion gas 
containing the SO.sub.x and NO.sub.x is fed from the boiler or some such 
other typical carbon fuel combustion apparatus into a conventional spray 
drying reactor 20. As is conventional in the spray drying process, a 
reagent such as lime, limestone, sodium compounds, magnesium compounds or 
mixtures thereof is combined with water to form a slurry by means of a 
mixing means 24. This slurry is then sprayed or directed by means of a 
directing means 29 in the reactor in a manner such that the contaminated 
flue gases flowing into reactor 20 are made to intermingle with or contact 
the slurry mixture. The sulphur dioxide and nitrous oxides in the 
contaminated flue gases react with the reagent slurry in a conventional 
manner as in prior art processes. The reactions that take place in such 
chamber are well known and may be briefly described immediately below as 
follows: 
##STR1## 
However, unlike the prior art processes, the reaction taking place in the 
spray drying reactor 20 is also influenced by a corona discharge 
environment produced concurrently within the reactor chamber. The corona 
discharge atmosphere is developed in any conventional manner by either a 
single pair of electrodes or various pairs of electrodes of conventional 
configuration. The overall aim is to envelop with corona a substantial 
portion of the reaction volume between the contaminated gases and the 
slurry to thereby render more efficient the conversion of the contaminants 
and flue gases to removable form. The corona discharge could, of course, 
be produced by DC or AC electrical voltages from a source 28 applied 
across appropriate electrodes 22 positioned within or about the path of 
the flue gases. The geometries of these gaps and placement of electrodes 
would, of course, be optimized for the specific geometry of the reaction 
chamber in which they were to be employed. Either uniform or nonuniform 
electric field configurations could be employed to optimize the efficiency 
of the conversion process. As with other prior art conventional 
techniques, the corona discharge would generate reactive species, both 
neutral and ionic, which lead to the formation of nitric and sulphuric 
acids which are then neutralized by the base present in the reactor 
slurry. Some of the more important of such reactive species referred to 
above are: 
NEUTRAL 
SO.sub.3, HSO.sub.3, O, OH, HO.sub.2, NO.sub.2, O.sub.3 
IONIC 
SO.sub.3.sup.-, HSO.sub.3.sup.-, O.sup.-, OH.sup.-, HO.sub.2.sup.-, 
NO.sub.2.sup.- 
It is the reactive species which are, in large measure, responsible for the 
improved efficiency of the process by combining with or enhancing 
reactions of the pollutants to safer or removable forms. 
There are several advantages possessed by the use of corona discharge in 
the reaction chamber as shown in FIG. 2 as contrasted to prior art 
electron beam or light induced reactions. Firstly, corona discharge 
chemistry is energetically more efficient because energy put into a 
molecule is less likely to be squandered in light emission as compared to 
the use of radiation to bring about a similar result. This is due to the 
fact that in discharges most of the reactive species are excited to 
optically forbidden levels; thus they survive long enough without losing 
energy by luminescence so that they can enter into chemically significant 
interactions. Secondly, the corona discharge technique is not limited by 
the very stringent radiation protection requirements usually required in 
some of the more conventional techniques. Thirdly, the equipment for 
producing the corona discharge is relatively inexpensive and the process 
itself is quite well understood since it is used in a large number of 
other applications. The equipment conventionally used in such other 
applications would, for the most part, be applicable to use in the 
environment of the invention. 
Another embodiment of the invention is shown in FIG. 3 in which a corona 
discharge reactor is used to provide oxidation of sulphur dioxide and 
nitrogen oxides in an otherwise conventional ammonia injection process. 
Referring to FIG. 3, the contaminated flue gas entering the treatment 
system is first injected with ammonia at an inlet 39. Water also may be 
injected at this stage in order to cool the incoming gas and to bring the 
concentration of water in the gas to be treated to within an acceptable 
range for the ensuing reaction. The cooled mixture is then fed to a corona 
discharge reactor 40 wherein the contaminated flue gas and ammonia mixture 
is brought into contact with the corona discharge in the reactor 40. The 
reactive species produced by the corona discharge in the process of FIG. 3 
comprise the same species identified herein before with respect to the 
process of FIG. 2 but, in addition, include NH.sub.2, NH.sub.3.sup.+, and 
NH.sub.4.sup.+. The corona discharge is produced in a manner similar to 
that discussed above in connection with FIG. 2 by providing suitable 
electrodes 42 which are energized from a high voltage power supply 44. The 
configuration of electrodes 42 is such that the reactive species generated 
by the corona discharge are brought into good contact with the incoming 
ammonia and contaminated flue gas mixture. The reactive species generated 
by the corona discharge then act within the reactor 40 to oxidize the 
SO.sub.x and NO.sub.x to sulphuric and nitric acids. Subsequent 
neutralization reactions take place within the reactor 40 between the 
acids, ammonia and water vapor to form ammonium sulphate and ammonium 
sulphate nitrate. A generalized description of the reaction according to 
the embodiment of FIG. 3 is as generally diagrammed below. 
##STR2## 
The gas emanating from the reactor 40 is then subjected to a particulate 
collection process in which the solid particles are collected for 
disposition. The remaining clean flue gases are discharged to the 
atmosphere in a conventional manner. 
FIG. 4 shows yet another embodiment of the invention which uses corona 
discharge not only to generate a reactive species for oxidizing sulfur 
dioxide in flue gas but also selectively charging an alkali reagent for 
more efficient neutralization of the acid generated in the above reaction. 
Referring to FIG. 4, contaminated flue gases entering the system may first 
be enriched by the addition of water and oxygen, if needed, and then 
passed through a corona discharge reactor 52. As noted before with respect 
to the other embodiments, the corona discharge is produced by DC or AC 
electrical energy with conventional means, the electrode geometry having 
been optimized for maximum reaction in the reactor chamber 52. When a 
mixture of O.sub.2, H.sub.2 O, and SO.sub.2 is passed through an 
electrical dischage as shown in FIG. 4, various active species are 
generated leading to the oxidation of SO.sub.2 to SO.sub.3 or HSO.sub.3, 
similar to the previously described embodiments. These species then 
proceed to react with water to give sulphuric acid which rapidly forms 
condensation nuclei. Depending on the relative humidity, the nuclei will 
grow into droplets forming an acid mist 53. The acidic mist 53 exiting 
from the discharge reactor has an electrical charge, the sign of which is 
dependent on the nature of the discharge in the reactor 52. The acidic 
mist 53 is brought into contact with an alkaline powder such as 
Ca(OH).sub.2 or Na.sub.2 CO.sub.3 which is injected into the airstream via 
inlet means 55. Appropriately placed corona electrodes 54 impart an 
electrical charge to the alkaline powder entering into contact with the 
mist 53. The effect of such charging of the alkaline powder results in a 
more efficient neutralization of the acidic mist 53. The charging 
electrodes 54 may take on a variety of configurations and shapes to 
accomplish the above-noted charging of particles of the alkali reagent 
powder and conventional techniques and equipment for providing such charge 
are readily available. By virtue of the arrangement of FIG. 4, the 
negatively charged acidic mist emanating from the reactor 52 is 
neutralized and passed to a particulate collection means 60 whereat the 
solid particles are removed from the airstream. The clean gases are then 
allowed to exit to the atmosphere in a conventional manner. It should be 
understood that in connection with the embodiment of FIG. 4, the alkaline 
material fed through the inlet 55 may be either in wet or dry form. 
The foregoing describes apparatus and methods for cleaning up contaminated 
flue gas by the use of corona discharge in combination with several 
conventionally used process steps. The corona discharge reaction provides 
a means for improving the efficiency and cost of these prior art methods. 
While only certain preferred features of the invention have been shown by 
way of illustration, many modifications and changes will occur to those 
skilled in the art. It is to be understood that the appended claims are 
intended to cover all such modifications and changes as fall within the 
true spirit and scope of the invention.