Modification Of Operating Conditions
- The operating conditions can be modified to achieve significant reductions in the rate of thermal NOx production. the various methods are:
- Low-excess firing
- Off-stoichiometric combustion (staged combustion)
- Flue gas recirculation
- Reduced air preheat
- Reduced firing rates
- Water Injection
Tail-end Control Processes
- Combustion modification and modification of operating conditions provide significant reductions in NOx, but not enough to meet regulations.
- For further reduction in emissions, tail-end control equipment is required.
- Some of the control processes are:
- Selective Catalytic Reduction
- Selective Non-catalytic Reduction
- Electron Beam Radiation
- Staged Combustion
Catalytic converters
- Two-way catalytic converters
- A two-way catalytic converter has two tasks:
- Oxidation of carbon monoxide to carbon dioxide:
- 2CO + O2 → 2CO2
- Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water:
- 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O
- This type of catalytic converter is widely used on diesel engines to reduce hydrocarbon and carbon monoxide emissions. They were also used on spark ignition (gasoline) engines in USA market automobiles up until 1981, when they were replaced by three-way converters due to regulatory changes requiring reductions on NOx emissions.
- There are two types of catalytic converter: 3 way and 2 way
- Three-way catalytic converters
- A three-way catalytic converter has three tasks:
- Reduction of nitrogen oxides to nitrogen and oxygen:
- 2NOx → xO2 + N2
- Oxidation of carbon monoxide to carbon dioxide:
- 2CO + O2 → 2CO2
- Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water:
- 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O
- These three reactions occur most efficiently when the engine is running slightly above the stoichiometric point. This is between 14.8 and 14.9 parts air to 1 part fuel, by weight, for gasoline (the ratio for LPG, natural gas and ethanol fuels is slightly different, requiring modified fuel system settings when using those fuels)