Oxides of Nitrogen
Oxides of Nitrogen (NₓOᵧ) consist of a group of gases made of Nitrogen and Oxygen consisting predominantly of Nitric Oxide (NO), Nitrogen Dioxide (NO₂) and Nitrous Oxide (N₂O). When oxides of nitrogen are referred to as NOₓ, this relates to nitric oxide and nitrogen dioxide only..
Nitrogen oxides are produced predominantly through the process of combustion at high temperatures in air, often called Thermal NOₓ. This can either be from fuel combustion industrially from coal fire power plants or electric power boilers or through vehicle engine emissions. As well as being produced naturally through lightning strikes..
Oxides of nitrogen can also be produced by the oxidation of nitrogen in the fuel, often called Fuel NOₓ, and also through the rapid reaction of atmospheric nitrogen and oxygen containing free radicals, often called Prompt NOₓ .
Nitric oxide is considered harmful in association with the formation of Nitrogen Dioxide and also the formation of acid rain through reactions of NO and HO₂ to then reactions with OH.
Ozone layer depletion occurs from the reaction of nitric oxide with stratospheric ozone (O₃) to form oxygen (O₂) and nitrogen dioxide.
Nitrogen Dioxide is highly toxic, causing inflammation to airways and increase susceptibility to respiratory, lung and heart conditions. It also can lead to biodiversity changes in the nitrogen cycle impacting soil chemistry and sensitive habitats.
Both nitric oxide and nitrogen dioxide are a major factor in the formation of photochemical smog. The nitrogen oxides react with sunlight and volatile organic compounds producing the brown discoloured haze in often seen in urban areas.
Controlling combustion parameters such as air-to-fuel ratio, residence time and turbulence can signifcantly reduce NOₓ emissions. This is due to the subsequent reduction in flame temperature and the removal of hot spots in the combustion chamber. Exhaust gas recirculation (EGR) works similarly due to the higher heat capacity of flue gas compaired with primary combustion air.
When controlling combustion parameters does not reduce nitrogen oxide emissions sufficiently post combustion treatment of flue gases may be necessary. Oxides of nitrogen can be removed by injection of ammonia (NH₃) into the hot flue gas, this reacts to produce nitrogen (N₂) and water (H₂O). When a catalyst is utilised in the abatement it is known as selective catalytic reduction (SCR) and conversely selective non-catalytic reduction (SNCR) when catalysts are not utilised.
Alkali Environmental perform periodic measurements of Oxides of Nitrogen (NO and NO₂) in ducted emissions from point sources to the standard reference method BS EN 14792. The reported results carry UKAS and MCERTS accreditation where the method can be performed to the standard.
These measurements are taken using an on-line combustion gas analyser where instantaneous and logged results can be taken. Often reported results are corrected for oxygen concentration to avoid the affect of dilution air on the result.
Some examples of common carbon monoxide emission limits:
100mg/m³ – Thermal Oxidisers
600mg/m³ at 6% O₂ – Waste Wood Combustion (EPTN 5/1(18))
500mg/m³ at 5% O₂ – Landfill Gas / Biogas Engines (LFTGN08, post-2005)
650mg/m³ at 5% O₂ – Landfill Gas / Biogas Engines (LFTGN08, pre-2006)