Method for removing nitrogen oxides from gases

Method for removing nitrogen oxides from gases in which, at first, the molar ratio between NO and NO.sub.2 is set to 1 in said gas and subsequently the nitrogen oxides are absorbed in a hydrous ammonia solution, whereby ammonium nitrate is formed. In order to achieve the desired molar ratio in a simple manner, it is intended for setting the molar ratio between NO and NO.sub.2, to inject NO.sub.2 into the gas in a controlled manner, said NO.sub.2 having been formed by the reaction between ammonium nitrate with a strong acid.

The invention refers to a method for removing nitrogen oxides from gases in 
which, at first, the molar ratio of NO to NO.sub.2 in such gas is set to 1 
and that subsequently the nitrogen oxides are absorbed in a hydrous 
ammonia solution, whereby ammonium nitrate is formed. 
From the U.S. Pat. No. 3,453,071 a method is known for removing nitrogen 
oxides from flue gases, said method consisting in introducing the gas to 
be purified into a hydrous solution of ammonia and ammonium nitrate at a 
temperature of about 20.degree. C. to 30.degree. C. In such a solution the 
nitrogen oxides react with ammonia to form ammonium nitrite which 
subsequently oxidizes to ammonium nitrate. Such a reaction requires the 
nitrogen monooxide and nitrogen dioxide to be present in a molar ratio of 
1. The U.S. Pat. No. 3,453,071 further states that such a desired molar 
ratio is to be achieved by various means, such as by a selective supply of 
the nitrogen oxide, said nitrogen oxide being present in a deficit, or by 
oxidation of nitrogen monooxide to nitrogen dioxide or, vice versa, by 
reducing nitrogen dioxide to nitrogen monooxide. 
In general practice, however, flue gases resulting from burning processes 
usually contain a relatively large proportion of nitrogen monooxide, 
whereas, in comparison thereto, the proportion of nitrogen dioxide is low. 
In order to achieve the desired molar ratio of 1 it would be necessary to 
either introduce additional nitrogen dioxide into the flue gases or 
oxidize part of the nitrogen monooxide to form nitrogen dioxide. Due to 
the large amounts of nitrogen dioxide which would be necessary therefor, 
this method has proved not to be feasible in general practice. Therefore 
ozone is introduced into the flue gases in plants presently in operation, 
whereby the reactive oxygen of said ozone oxidizes part of the molecules 
of nitrogen monooxide to form nitrogen dioxide. The desired molar ratio of 
nitrogen monooxide to nitrogen dioxide is set by precisely regulating the 
supply of ozone. The production of ozone, however, is technically complex 
and requires considerable amounts of energy. 
It is the object of the present invention to avoid such disadvantages and 
to create a method which enables simple and energetically advantageous 
setting of the molar ratio between nitrogen monooxide and nitrogen 
dioxide. 
In accordance with the invention, the molar ratio between NO and NO.sub.2 
is adjusted by injecting NO.sub.2 into the gas in a controlled manner, 
said NO.sub.2 being derived from the reaction between ammonium nitrate and 
a strong acid. 
The nitrogen dioxide which is necessary for the method is produced in a 
simple manner directly from the final product of the method, i.e. ammonium 
nitrate. This method is simple and may be performed in one step in the 
reactor. 
It is particularly preferable if hydrochloric acid is used as strong acid. 
In such a case the yield will amount to nearly 100 percent. 
In accordance with a further preferable variation of this method, sulphuric 
acid is used as strong acid. The advantage here lies in the fact that the 
final product of the method is ammonium sulphate which constitutes a 
valuable raw material for further processes.

EXAMPLE 1 
In a packed column the gas to be purified is allowed to flow from below in 
an upward direction. A desulphurization step by means of lime milk is 
performed in said packed column, said step being well known and not 
requiring further description. After said desulphurization step, NO.sub.2 
is injected into said column in order to produce a molar ratio of 1 
between NO and NO.sub.2. The gas treated in such a manner is led by means 
of a counterflow to a hydrous solution of ammonia or, possibly, ammonium 
nitrate. The following chemical reaction takes place in the interior of 
said column: 
EQU NO+NO.sub.2 +2 NH.sub.3 +H.sub.2 O.fwdarw.2 NH.sub.4 NO.sub.2 (1) 
The ammonium nitrite received in such a manner is further oxidized to form 
ammonium nitrate: 
EQU 2 NH.sub.4 NO.sub.2 +O.sub.2 .fwdarw.2 NH.sub.4 NO.sub.3 (2) 
The NO.sub.2 necessary for the method is obtained by splitting the ammonium 
nitrate by means of hydrochloric acid. The main reaction taking place may 
be described as follows: 
EQU 4 NH.sub.4 NO.sub.3 +2 HCl+1/2 O.sub.2 .fwdarw.2 NH.sub.4 Cl+2 NH.sub.4 
NO.sub.3 +2 NO.sub.2 +H.sub.2 O (3) 
This reaction is performed in a Teflon coated generator made from titanium. 
The resting period in the reactor is approx. 10 minutes. 
For the purpose of starting up the plant, ozone is injected into the column 
as hitherto known. As soon as the first ammonium nitrate is present and 
nitrogen dioxide has been sufficiently formed, the ozone injection is 
switched to NO.sub.2 injection. In the case that there are several 
reaction towers, only one tower requires an ozone device, whereas all 
other towers may be started up one after the other with the NO.sub.2 of 
the respective preceding device. 
In a concretely performed project the purification of 2.17.times.10.sup.6 
Nm.sup.3 flue gas, said gas being loaded with 500 mg/Nm.sup.3 NO. Other 
nitrogen oxides were only present in negligible amounts. 5204 kg/h of a 50 
percent solution of ammonium nitrate were produced at a flow rate of 1085 
kg/h NO. In order to produce the required amount of NO.sub.2, 1170 kg/h 
HCl were fed. 
EXAMPLE 2 
In a packed column the gas to be purified is allowed to flow from below in 
an upward direction. A desulphurization step by means of lime milk is 
performed in said packed column, said step being well known and not 
requiring further description. After said desulphurization step NO.sub.2 
is injected into said column in order to produce a molar ratio of 1 
between NO and NO.sub.2. The gas treated in such a manner is led by means 
of a counterflow to a hydrous solution of ammonia or, possibly, ammonium 
nitrate. The following chemical reaction takes place in the interior of 
said column: 
EQU NO+NO.sub.2 +2 NH.sub.3 +H.sub.2 O.fwdarw.2 NH.sub.4 NO.sub.2 (4) 
The ammonium nitrite received in such a manner is further oxidized to form 
ammonium nitrate: 
EQU 2 NH.sub.4 NO.sub.2 +O.sub.2 .fwdarw.2 NH.sub.4 NO.sub.3 (5) 
The NO.sub.2 which is necessary for the method is obtained by splittinq 
ammonium nitrate by means of sulphuric acid. The main chemical reaction 
taking place here can be described as follows: 
EQU 2 NH.sub.4 NO.sub.3 +H.sub.2 SO.sub.4 .fwdarw.2 HNO.sub.3 +(NH.sub.4).sub.2 
SO.sub.4 (6) 
EQU 2 NH.sub.4 NO.sub.2 +H.sub.2 SO.sub.4 .fwdarw.2 HNO.sub.2 +(NH.sub.4).sub.2 
SO.sub.4 (7) 
EQU HNO.sub.3 +HNO.sub.2 .fwdarw.2 NO.sub.2 +2 H.sub.2 O (8) 
This reaction is performed in a reactor made from austenitic steel which 
has an enamel coating inside. The resting period in the reactor is approx. 
10 minutes. 
For the purpose of starting up the plant, ammonium nitrate is fed into the 
reactor from a buffer store. The nitrogen dioxide which results therefrom 
is fed into the column. During further operation the necessary ammonium 
nitrate, said nitrate being mixed with ammonium nitrite, is taken from the 
column. 
In the present example 1253 kg NO.sub.2 were formed by using 4358 kg of a 
50 percent solution of ammonium nitrate/ammonium nitrite and 3338 kg of 40 
percent sulphuric acid. This resulted in 1798 kg of ammonium sulphate. 
The inventive method enables an extremely economical disposal of flue gases 
loaded with nitrogen oxides.