Among the most harmful chemical compounds for human health, as well as for the environment, are nitrogen oxides (NOx. They are responsible for the formation of smog, which subsequently causes respiratory, circulatory, and nervous system diseases, and contributes to allergies and cancerous lesions. NOx is also a source of carcinogenic compounds in the soil, and their oxidation contributes to acid rain. Their formation occurs during the combustion of liquid, solid and gaseous fuels, and their mixtures. Power plants and means of transport for people and goods are mainly responsible for emissions.

Increased emissions of nitrogen oxides are caused by several factors: poor quality of the raw material being combusted, increased combustion temperature, and the amount of excess air. Methods for reducing emissions of these chemicals are divided into primary and secondary.

Primary methodsThese ways of reducing the release of NOx into the atmosphere require relatively little investment, while their effectiveness is lower than secondary methods. These measures include:

• air grading
• fuel graduation
• reducing air heating
• use of low-emission burners
• recirculation of flue gases

Secondary methods
There are two secondary methods to prevent the release of NOx into the atmosphere:

• selective catalytic reduction (SCR), where a catalyst converts nitrogen oxides into nitrogen and water vapour under the influence of ammonia injected into the exhaust gas;
• selective non-catalytic reduction (SNCR), where the reagent (urea or ammonia) is injected into the combustion chamber in the high-temperature region (typically 850-1100 degrees Celsius). As a result of this action, nitrogen oxide is reduced to molecular nitrogen and water vapour.

To meet the increasingly stringent standards imposed by the European Union, a hybrid combination of these two secondary methods is also gaining popularity. NOx emissions are then reduced in two stages. Secondary methods enable nitrogen oxides to be effectively eliminated from the exhaust gas, but they are quite costly to implement.