•Exhaust gas recirculation
To reduce maximum temperature of the system, whilst keeping the oxygen concentration low. This can be applied to both stationary combustors and reciprocating engines.
•Direct heat removal from the flame front to give a low flame temperature (<1200ºC). This is only really applicable to gas-fired systems.
- •Air staging. This is when a fuel rich first stage is produced running with normally
- only 70-80% of the required stoichiometric air. The resulting reducing atmosphere reduces any NOX to N2.
- Some heat removal is normally made at this point. The rest of the combustion air is then added either in 1 or 2 stages to give complete fuel burnout. This is widely applied using a stratified charge in reciprocating engines, as well as stationary combustion systems.
•Fuel staging. This is when the fuel is added sequentially into the system with some cooling between the stages so as to lower overall maximum temperatures.
•Reburn. This is when the fuel is added at the end of a combustion process to form a fuel rich zone where NOX can be reduced to N2. This is being used as a retrofit application in many power stations. Extra air must be added at some time to burn the extra fuel.
•Minimisation of residence time in high temperature regions. The formation rate of NOX is very dependent on time and its formation can be reduced by lowering the overall system residence time, i.e. not giving sufficient time for the NOX forming reactions to take place.