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Division 1.20.2 Nitrogen oxides (NOX)

United Kingdom

Original text. Later amendments are not shown.

TechniqueDescription
Combustion modifications
Staged combustion

  • Air staging — involves substoichiometric firing in a first step and the subsequent addition of the remaining air or oxygen into the furnace to complete combustion

  • Fuel staging — a low impulse primary flame is developed in the port neck; a secondary flame covers the root of the primary flame reducing its core temperature

Flue-gas recirculation

Reinjection of waste gas from the furnace into the flame to reduce the oxygen content and therefore the temperature of the flame.

Special burners using the internal recirculation of combustion gases to cool the root of the flames and reduce the oxygen content in the hottest part of the flames

Use of low-NOX burners (LNB)The technique (including ultra-low-NOX burners) is based on the principles of reducing peak flame temperatures, delaying but completing the combustion and increasing the heat transfer (increased emissivity of the flame). It may be associated with a modified design of the furnace combustion chamber. The design of ultra-low-NOX burners (ULNB) includes combustion staging (air/fuel) and flue-gas recirculation. Dry low-NOX burners (DLNB) are used for gas turbines
Optimisation of combustionBased on permanent monitoring of appropriate combustion parameters (e.g. O2, CO content, fuel to air (or oxygen) ratio, unburnt components), the technique uses control technology for achieving the best combustion conditions
Diluent injectionInert diluents, e.g. flue-gas, steam, water, nitrogen added to combustion equipment reduce the flame temperature and consequently the concentration of NOX in the flue-gases
Selective catalytic reduction (SCR)

The technique is based on the reduction of NOX to nitrogen in a catalytic bed by reaction with ammonia (in general aqueous solution) at an optimum operating temperature of around 300-450 °C.

One or two layers of catalyst may be applied. A higher NOX reduction is achieved with the use of higher amounts of catalyst (two layers)

Selective non-catalytic reduction (SNCR)

The technique is based on the reduction of NOX to nitrogen by reaction with ammonia or urea at a high temperature.

The operating temperature window must be maintained between 900 °C and 1 050 °C for optimal reaction

Low temperature NOX oxidationThe low temperature oxidation process injects ozone into a flue-gas stream at optimal temperatures below 150 °C, to oxidise insoluble NO and NO2 to highly soluble N2O5. The N2O5 is removed in a wet scrubber by forming dilute nitric acid waste water that can be used in plant processes or neutralised for release and may need additional nitrogen removal