Process for the removal of nitrogen oxide from flue gases

Nitrogen oxide is removed from flue gases of combustion plants or reaction gases by a process wherein the nitrogen oxide contained in the stream of gas is converted into a stochiometric composition of N.sub.2 O.sub.3 and removed from the gas stream.

This invention relates to a process for the removal of nitrogen oxides from 
flue gases of combustion plants or reaction gases. 
BACKGROUND OF THE INVENTION 
It is known to remove the nitrogen oxides from exhaust gases by means of 
catalysts suitable for this purpose together with a supply of ammonia. The 
exhaust gas containing nitrogen oxide is converted into nitrogen in the 
process so that the NO originally present is converted into an inert 
substance. 
EP 0 174 907 A2 discloses a process in which both nitrogen oxide and 
sulphur dioxide can be removed from exhaust gases. In this process, a 
solution of nitric acid is added to the exhaust gas in a so called 
denitrating zone. This procedure adjusts the NO/NO.sub.2 ratio to such a 
value that the dinitrogen trioxide formed can easily be concentrated as 
nitrosyl sulphuric acid in several successive sulphuric acid absorption 
stages. From the nitrosyl sulfuric acid a dilute nitric acid and a 
nitrous-free sulphuric acid are prepared in a separate step of the process 
at an elevated temperature and with the addition of air, and this nitric 
acid and sulphuric acid are used again in the denitrating zone and the 
sulphuric acid absorption stages. This process requires about 7 to 8 
absorption towers for concentrating the dinitrogen trioxide as nitrosyl 
sulphuric acid and therefore involves considerable technical expenditure. 
DD 212 495 A1 describes a process for the removal of nitrogen oxides from 
exhaust gases by successive washing with water and with dilute nitric acid 
combined with absorption in concentrated sulphuric acid. This process in 
in principle similar to the one described above but is carried out by a 
modified procedure. 
Although substantial removal of nitrogen oxides from exhaust gases can be 
achieved by the processes described above, it is not possible to evaluate 
the nitrogen oxide content of the exhaust gas directly without 
intermediate stages of working up for the recovery of trivalent, 
tetravalent or pentavalent nitrogen oxides. 
It is therefore an object of the present invention to provide a process 
which does not have the disadvantages of the processes described above.

DETAILED DESCRIPTION OF THE INVENTION 
It has been discovered that when nitrogen oxide-containing flue gases are 
contacted with a sulphuric acid solution containing from 0.1 to 5% by 
weight of nitric acid, the nitrogen oxide in the flue gas is converted 
into a stochiometric composition of NO.sub.2 and can readily be removed as 
such from the gas stream. 
The present invention thus relates to a process for the removal of nitrogen 
oxide components from flue gases of combustion plants or reaction gases, 
in which the nitrogen oxide content present in the stream of the gas is 
first converted into a stochiometric composition of N.sub.2 O.sub.3. 
The process is preferably carried out in such a manner that the reaction of 
the nitrogen oxide takes place as a result of contact with a sulphuric 
acid of medium concentration containing from 0.1 to 5% by weight, 
preferably from 0.2 to 1.5% by weight of nitric acid. 
This sulphuric acid solution used according to the invention contains 
nitric acid as oxidizing agent. The proportion of nitric acid in this 
solution may in particular be from 0.1 to 5% by weight but will, of 
course, depend on the nitrogen oxide content of the exhaust gas. In 
practice, the proportion of nitric acid would be from 0.20 to 1.5% by 
weight. 
Due to the low proportion of nitric acid, the vapour pressure of nitric 
acid in the sulphuric acid is also kept very low so that any nitrate 
present in the gas is negligible. 
The sulphuric acid concentrations in the solution used may vary over a wide 
range, namely from 5 to 96% by weight H.sub.2 SO.sub.4. 
In practice, however, the concentration would vary from 30 to 70% by weight 
of H.sub.2 SO.sub.4, preferably from 55 to 65% by weight. 
In order to ensure almost complete conversion of the NO.sub.x content, the 
sulphuric acid concentration should be adjusted so that even nitrogen 
oxides containing nitrogen in oxidation stage 3 or 4 will not be absorbed. 
In a preferred embodiment of the process according to the invention, the 
sulphuric acid of medium concentration is at a temperature of from 
25.degree. C. to 80.degree. C., preferably from 50.degree. C. to 
65.degree. C. 
The temperatues employed according to the invention ensure that the 
oxidized exhaust gas will be easily and completely driven out of these 
solutions so that no trivalent or tetravalent nitrogen oxides are left in 
the sulphuric acid of medium concentration. 
The oxidation of the nitrogen oxide substantially reduces the sulphuric 
acid concentration due to the water of reaction being dissolved. At the 
same time, the exhaust gas to be purified may also contain a small 
quantity of water vapor depending on its temperature, and this will also 
lower the concentration of sulphuric acid. 
In view of these circumstances, it is an important advantage of this 
temperature control that it enables the sulphuric acid concentration to be 
adjusted to the required level. An elevated temperature will keep the 
water balance in the solution constant and hence also the sulphuric acid 
concentration so that the above mentioned advantages are preserved. 
If the concentraion rises to an undesirable extent, the sulphuric acid 
concentration is adjusted according to the invention by the addition of 
water and/or dilute nitric acid solution. If the concentration falls too 
low, a more highly concentrated sulphuric acid is added. A dilute nitric 
acid solution may also be added as this will both adjust the sulphuric 
acid concentration and control the nitric acid concentration more 
accurately. 
The combined use of nitric acid and sulphuric acid according to the 
invention has altogether several advantages. The inclusion of sulphuric 
acid in the circulating liquid prevents an increase in the concentration 
of nitric acid during the oxidation of the nitrogen oxides. 
Moreover, when this procedure is adopted, the total quantity of nitric acid 
in the sulphuric acid solution may be kept very low so that no 
overoxidation of NO to NO.sub.2 occurs. 
In a preferred embodiment of the process according to the invention, the 
N.sub.2 O.sub.3 may be converted into nitrogen by a reaction with 
equivalent quantities of ammonia, optionally in the presence of a suitable 
catalyst. It may also be advantageous to react the N.sub.2 O.sub.3 with an 
alkaline component to convert it into the corresponding nitrite. The 
alkaline component may be used as a solution or suspension or in the solid 
state. 
The process according to the invention is described below by way of 
examples which should not, however, be regarded as a limitation. 
EXAMPLE 1 
An exhaust gas containing NO and having the analytical composition of I is 
successively brought into contact with a sulphuric acid solution 
containing nitric acid (0.34% HNO.sub.3) and a 15% by weight sodium 
hydroxide solution. The two solutions are carried through separate 
absorption towers. The newly formed exhaust gas has the analytical 
composition II, corresponding to a degree of efficiency of 96.7%. 
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Exhaust 
Exhaust 
Gas I Gas II 
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NO Vol.-% 0.34 0.011 
N.sub.2 Vol.-% 92.66 93.18 
O.sub.2 Vol.-% 3.6 3.6 
H.sub.2 O Vol.-% 3.40 3.20 
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EXAMPLE 2 
3421 parts by weight of an exhaust gas containing NO and having the 
analytical composition III is brought into contact with a sulphuric acid 
solution IV in countercurrent therewith at a temperature of 60.degree. C. 
The oxidized exhaust gas is absorbed in countercurrent in 2338 parts by 
weight of a 15% by weight sodium hydroxide solution to form a sodium 
nitrite solution V and a purified exhaust gas. The exhaust gas obtained 
now contains 190 ppm of NO. 
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Exhaust Gas III Sulphuric Acid Solution IV 
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NO: 11 parts by H.sub.2 SO.sub.4 : 60.9% by weight 
weight 
N.sub.2 : 3208 parts by 
HNO.sub.3 : 0.37% by weight 
weight 
O.sub.2 : 154 parts by 
weight 
H.sub.2 O: 48 parts by 
weight 
Sodium Nitrite Solution V 
NaNO.sub.2 : 12.4 g/l 
NaNO.sub.3 : 0.13% by weight 
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