Nitrogen oxides (NOx) are inorganic gaseous compounds formed by the combination of oxygen and nitrogen. Their origin is typically found in various combustion processes that occur at high temperatures.
Nitric oxide and nitrogen dioxide are the two most dangerous nitrogen oxides, as they can be very harmful toxicologically. Nitrogen dioxide also has an unpleasant and very strong odor. Despite this, neither of them is flammable.
Nitrogen oxides can be generated in various industries and processes such as energy production, combustion of coal, oil, or natural gas, electroplating, metal engraving, or different types of welding.
The emission of nitrogen oxides is very dangerous to health, as it affects the respiratory systems of people and animals, potentially leading to respiratory and cardiovascular diseases due to its acidic nature. Furthermore, once emitted, they can give rise to other secondary pollutants, such as PAN (peroxyacetyl nitrate). The reactions produced in the atmosphere by these compounds are very complex, involving radicals such as OH, O3, NO, and others.
For all these reasons, it is very important that industries that generate NOx emissions as a result of their production processes take the necessary measures to limit the emission of these inorganic compounds. Air treatment technologies to control NOx emissions can be classified into two groups: those applied in combustion to reduce the formation of NOx, and those that involve the treatment of the effluent to remove NOx.
In the first case, we find different options such as low NOx production burners, gas recirculation, water or steam injection, etc. The problem with these solutions is that in many cases, the reduction of NOx achieved is not sufficient to comply with the existing strict regulations.
Therefore, it is much safer to opt for technologies aimed at the removal of NOx, which are based on the retention of NOx or their transformation into harmless compounds or elements. Among the various methods for treating combustion gases, catalytic technology has proven to be the most effective. Thus, the process of Selective Catalytic Reduction (SCR), which uses ammonia as a reducing agent, is currently the most widely used and developed technology in the world, as it allows for the effective, selective, and economical removal of NOx.
The SCR process is based on the reduction of NOx with NH3, in the presence of excess O2 and an appropriate catalyst, to transform into harmless substances such as water and nitrogen according to the following reactions. Ammonia in the form of liquid ammonium hydroxide is vaporized, diluted with air, and injected directly into the gas stream to be treated through a distributor.
However, the occurrence of undesirable secondary reactions is also possible, such as the formation of nitrous oxide, or molecular nitrogen and nitric oxide, when ammonia reacts with oxygen. In the case of fuels with a high sulfur content, SO2 is also produced during combustion, which can be catalytically oxidized to SO3. The oxidation of SO3 can react with water and unreacted ammonia to form sulfuric acid and ammonium sulfate.
Sulfate salts can deposit and accumulate on the catalyst, leading to its deactivation if the catalyst temperature is not sufficiently high, and the sulfuric acid formed can cause downstream corrosion problems in the plant. Therefore, depending on the required operating conditions, a highly selective DeNOx catalytic system must be available to reduce NOx with NH3 in the presence of O2, avoiding all undesirable secondary reactions.
Related reading: Removal of NOx
