Method for removing NO.sub.x from a gas stream while producing an alkaline earth nitrate fertilizer composition

A method of producing an alkaline earth nitrate fertilizer composition where a gas stream containing nitrogen oxides is treated to convert the nitrogen oxides to nitric acid and the gas stream then contacted with an aqueous medium containing an alkaline earth compound which reacts with the nitric acid to form an aqueous effluent solution of an alkaline earth nitrate. A portion of the aqueous effluent solution is used to form additional aqueous medium containing an alkaline earth compound for further contact with the gas stream, such that the concentration of alkaline earth nitrate in the aqueous effluent is increased.

CROSS-REFERENCE TO RELATED APPLICATIONS 
The present application is related to application Ser. No. 08/706,657, 
filed Sep. 6, 1996, in the names of M. Babu and J. College, and entitled 
"Process For Removing SO.sub.2 and NO.sub.x From A Gaseous Stream", 
pending, and to application Ser. No. 08/801,770, filed Feb. 18, 1997, also 
in the names of M. Babu and J. College and entitled "Aqueous Alkaline 
Earth Nitrate Fertilizer Composition and Process For Making The Same", 
pending, both applications being assigned to the assignee of the present 
application and both applications being incorporated by reference herein. 
FIELD OF THE INVENTION 
The present invention relates to a method for removing nitrogen oxides from 
a gas stream, such as a hot combustion gas stream, while producing calcium 
nitrate or magnesium nitrate in a concentrated aqueous solution. 
BACKGROUND OF THE INVENTION 
The removal of nitrogen oxides, as well as removal of sulfur dioxides, from 
hot combustion gas streams is becoming more necessary due to the need to 
reduce contaminants in the atmosphere which cause acid rain conditions. 
Various processes have been proposed to remove nitrogen oxides from hot 
combustion gas streams. One such process involves the use of a coronal 
discharge system as described in U.S. Pat. No. 5,458,748, U.S. Pat. No. 
5,240,575, and U.S. Pat. No. 5,147,516 to convert NO.sub.x in a gas stream 
to nitric acid. Another such process involves the use of an electron beam 
to remove NO.sub.x, as well as sulfur dioxide, by conversion of the 
NO.sub.x to nitric acid. 
In the related applications, hereinbefore identified, Ser. No. 08/706,657, 
filed Sep. 6, 1996 and Ser. No. 08/801,770, filed Feb. 18, 1997, processes 
are described which remove NO.sub.x from a gas stream with the attendant 
production of nitric acid, and the removal of nitric acid from the gas 
stream by reaction with an alkaline earth compound, such as calcium 
carbonate, calcium hydroxide or magnesium hydroxide, in an aqueous medium, 
to produce calcium nitrate or magnesium nitrate in an aqueous solution 
that is usable as a fertilizer composition. The fertilizer compositions so 
produced are aqueous solutions of an alkaline earth nitrate which, if they 
are to be concentrated, so as to increase the percent by weight of 
alkaline earth nitrate in water, would involve the cost of such 
concentration, e.g. provision of heat to remove water from the solution. 
It is an object of the present invention to provide for the removal of 
nitric acid from a gas stream resulting from electron beam or coronal 
discharge treatment systems and formation of alkaline earth nitrate 
solutions therefrom, where the solutions are provided in a more 
concentrated form. 
SUMMARY OF THE INVENTION 
The present method enables the production of an alkaline earth nitrate 
fertilizer composition, such as a concentrated aqueous solution of calcium 
nitrate or magnesium nitrate, upon removal of nitric acid from a gas 
stream. 
A gas stream, containing nitrogen oxides, is treated to convert the 
nitrogen oxides to nitric acid, preferably by contact with a coronal 
discharge or electron beam irradiation, and is then contacted with an 
aqueous medium containing an alkaline earth compound, such as calcium 
hydroxide, calcium carbonate, dolomite or magnesium hydroxide, which will 
react with nitric acid to form an alkaline earth nitrate. After contact of 
the gas stream containing nitric acid with the aqueous medium containing 
the alkaline earth compound, the clean gas is discharged and an aqueous 
effluent containing the alkaline earth nitrate in solution is removed from 
the wet scrubbing unit. 
In accordance with the present method, a portion of the removed aqueous 
effluent containing alkaline earth nitrate in solution is used to form 
additional aqueous media containing an alkaline earth compound for further 
contact with a gas stream in the wet scrubbing unit. 
A portion of the removed aqueous effluent solution containing alkaline 
earth nitrate is used to form additional aqueous medium containing an 
alkaline earth compound for further contact with the gas stream, such that 
the concentration of alkaline earth nitrate in the aqueous effluent is 
increased.

DETAILED DESCRIPTION 
The present method is an improved method of removing NO.sub.x from a gas 
stream where a concentrated aqueous solution of an alkaline earth nitrate 
is produced as a by-product. 
In accordance with the present method, a gas stream, such as a hot 
combustion gas stream resulting from the combustion of coal, which 
contains nitrogen oxides (NO.sub.x) is treated to convert the NO.sub.x to 
nitric acid. 
One embodiment of such a treatment involves the contact of a gas stream 
containing NO.sub.x, which has been substantially saturated with water, 
with a coronal discharge in a coronal discharge unit. The exposure of the 
humidified gas stream to a coronal discharge effects a reaction of 
NO.sub.x compounds to produce nitric acid which is entrained in the gas 
stream. The formation of nitric acid by use of a coronal discharge is 
believed to follow the following reaction sequence: 
Production of oxidizing species by coronal discharge: 
(1) O.sub.2, H.sub.2 O.fwdarw.OH, O, HO.sub.2 
Production of nitric acid: 
##STR1## 
While various coronal discharge systems may be used in the present process, 
an especially useful system is that described in U.S. Pat. No. 5,458,748, 
the contents of said patent being incorporated by reference herein. The 
system described in that patent uses a sulfur tolerant, high water vapor 
tolerant, packing free coronal catalyst, and optionally suggests the use 
of downstream scrubbers of wet or dry configuration to absorb particular 
NO.sub.x reduction products NO.sub.2 and HNO.sub.3, upon exposure to an 
arresting agent, including basic, caustic, or alkali materials such as CaO 
or NH.sub.3. Other coronal discharge systems for converting NO.sub.x to 
nitric acid are described in U.S. Pat. Nos. 5,147,516 and 5,240,575, the 
contents of both of which are incorporated by reference herein. 
Another embodiment of a treatment of a gas stream containing NO.sub.x, 
which has been substantially saturated with water, to produce nitric acid 
from the NO.sub.x uses an electron beam. Such treatment is known in the 
art and involves electron beam irradiation of the gas, the electron beam 
being a kind of ionizing radiation and a flow of electron accelerated by 
high voltage. In the electron beam process, the high-energy electrons hit 
the gases breaking them into ions and radicals, the reaction products 
having a high oxidation potential, with the radicals reacting with 
NO.sub.x, in the presence of water, to produce nitric acid in the gas 
stream. 
Referring now to the drawing which schematically illustrates the present 
method, a gas stream containing nitrogen oxides from line 1 is treated in 
a reactor 2 to convert the nitrogen oxides to nitric acid. The gas stream, 
now containing nitric acid, is passed through line 3 to a wet scrubbing 
unit 4. An aqueous medium containing an alkaline earth compound that will 
react with nitric acid to form an alkaline earth nitrate is formed in tank 
5, such as a mixing tank, by addition of an alkaline earth compound from a 
source 6, through line 7 to tank 5 and admixture with water from a source 
(not shown) through line 8 to the tank 5. The aqueous medium containing an 
alkaline earth compound is then passed through line 9 to a recycle tank 10 
associated with the wet scrubbing unit 4. Aqueous medium containing an 
alkaline earth compound from recycle tank 10 is passed through line 11 to 
pump 12 and then through line 13 to liquid injectors 14 located in the wet 
scrubbing unit 4. The aqueous medium containing an alkaline earth 
compound, injected through injectors 14 contacts the gas stream fed to the 
wet scrubbing unit 4 through line 3 and the alkaline earth compound reacts 
with nitric acid present in the gas stream to form an alkaline earth 
nitrate which dissolves in the aqueous medium. The clean gas is discharged 
from the wet scrubbing unit 4 through line 15, while the aqueous medium, 
now containing alkaline earth nitrate in solution, returns through line 16 
to the recycle tank 10. 
An aqueous effluent containing an alkaline earth nitrate in solution is 
removed through valve 17 and discharged through line 18 for collection in 
a collection tank 19. In accordance with the present method, at least a 
portion of the removed aqueous effluent containing an alkaline earth 
nitrate is removed through valve 20 and charged through line 21 to the 
tank 5 for use in forming the aqueous medium containing the alkaline earth 
compound, in lieu of at least a portion of water from line 8. 
Slaking tests were conducted to show the affect of calcium nitrate addition 
to slaking water for lime slaking. Four tests were made using a lime 
having a composition of: 
CaO=88-96.5 weight percent 
MgO=2-5.5 weight percent 
SiO.sub.2 =0.1-3 weight percent 
Two tests used water without addition of calcium nitrate and two tests were 
run using water containing 40 weight percent calcium nitrate. The results 
of the tests are plotted in FIGS. 2 and 3. FIG. 2 shows an initial 
temperature of the slaking water of about 25.degree. C. while FIG. 3 shows 
an initial temperature of the slaking water of about 45.degree. C. The 
results show clearly that with nitrate ions present, the slaking rates of 
the lime were faster at both temperatures. With an increase in the slaking 
rate provided according to the present method, a smaller slaking device 
can be used which would reduce the cost of equipment necessary.