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Nitrogen Oxides (NO x ). Chapter 12 Page 147-168. NO x emissions include:. Nitric oxide, NO, and Nitrogen dioxide, NO 2 , are normally categorized as NO x Nitrous oxide, N 2 O, is a green house gas (GHG) and receives special attention. Smog precursors:.

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Nitrogen oxides no x

Nitrogen Oxides (NOx)

Chapter 12

Page 147-168


No x emissions include
NOx emissions include:

  • Nitric oxide, NO, and Nitrogen dioxide, NO2, are normally categorized as NOx

  • Nitrous oxide, N2O, is a green house gas (GHG) and receives special attention


Smog precursors
Smog precursors:

  • NOx, SO2, particulate matter (PM2.5) and volatile organic compounds (VOC).


Developing NOx and SOx Emission Limits” – December 2002, Ontario’s Clean Air Plan for Industry

Broad base of sources with close to 50% from the Electricity sector in 1999


No x reaction mechanisms
NOx reaction mechanisms:

  • highly endothermic with Dhf = +90.4 kJ/mol

  • NO formation favoured by the high temperatures encountered in combustion processes


Zeldovich mechanism 1946
Zeldovich mechanism (1946):

k+1 = 1.8  108 exp{-38,370/T}

k-1 = 3.8  107 exp{-425/T}

k+2 = 1.8  104 T exp{-4680/T}

k-2 = 3.8  103 T exp{-20,820/T}

k+3 = 7.1  107 exp{-450/T}

k-3 = 1.7  108 exp{-24,560/T}


Rate-limiting step in the process

k+1 = 1.8  108 exp{-38,370/T}

k-1 = 3.8  107 exp{-425/T}

K+1 is highly temperature dependent


Combine zeldovich mechanism with
Combine Zeldovich mechanism with

To obtain

If the initial concentrations of [NO] and [OH] are low and only the forward reaction rates are significant

Modelling NOx emissions is difficult because of the competition for the [O] species in combustion processes


Prompt no mechanism 1971
“Prompt” NO mechanism (1971):

This scheme occurs at lower temperature, fuel-rich conditions and short residence times


Fuel no x
Fuel NOx

Organic, fuel bound nitrogen compounds in solid fuels

C-N bond is much weaker than the N-N bond increasing the likelihood of NOx formation



No x control strategies

Reduce peak temperatures hydrocarbon flames

Reduce residence time in peak temperature zones

Reduce O2 content in primary flame zone

Low excess air

Staged combustion

Flue gas recirculation

Reduce air preheat

Reduce firing rates

Water injection

NOx control strategies:

Combustion Modification

Modified Operating Conditions


Control strategies
Control strategies: hydrocarbon flames

  • Reburning – injection of hydrocarbon fuel downstream of the primary combustion zone to provide a fuel-rich region, converting NO to HCN.

  • Post-combustion treatment include selective catalytic reduction (SCR) with ammonia injection, or selective noncatalytic reduction (SNCR) with urea or ammonia-based chemical chemical injection to convert NOx to N2.


SCR process: hydrocarbon flames

4 NO + 4 NH3 + O2 4 N2 + 6 H2O

2 NO2 + 4 NH3 + O2 3 N2 + 6 H2O


SNCR process: hydrocarbon flames

4 NH3 + 6 NO  5 N2 + 6 H2O

CO(NH3)2 + 2 NO ½ O2 2 N2 + CO2 + 2 H2O


Low no x burners
Low NO hydrocarbon flamesX burners:

Dilute combustion technology


Industrial furnace combustion
Industrial furnace combustion: hydrocarbon flames

  • Natural gas is arguably “cleanest” fuel – perhaps not the cheapest.

  • Independent injection of fuel and oxidant streams (“non-premixed”). Industrial furnaces have multi-burner operation.

  • Traditional thinking has been that a rapid mixing of fuel and oxidant ensures best operation.

  • This approach gives high local temperatures in the flame zone with low HC but high NOx emissions.

  • Heat transfer to a load in the furnace (radiatively dominated) must be controlled by adjustment of burners.




Dilute oxygen combustion
Dilute oxygen combustion: oxidant

  • An extreme case of staged-combustion.

  • Fuel and oxidant streams supplied as separate injections to the furnace.

  • Initial mixing of fuel and oxidant with hot combustion products within the furnace (fuel-rich/fuel-lean jets).

  • Lower flame temperature (but same heat release) and more uniform furnace temperature (good heat transfer).

  • Low NOx emissions – “single digit ppm levels”


Strong jet weak jet aerodynamics
Strong-jet/Weak-jet Aerodynamics oxidant

  • Strong jet = oxidant

  • Weak jet = fuel



Cgri burner
CGRI burner oxidant



Queen s test facility
Queen’s test facility fuel and oxidant


Queen s test facility1
Queen’s test facility fuel and oxidant




Cgri burner performance 1100 o c
CGRI burner performance (1100 fuel and oxidantOC)


Oxygen enriched combustion
Oxygen-enriched combustion: fuel and oxidant

  • Oxidant stream supplied with high concentrations of oxygen.

  • Nitrogen “ballast” component in the oxidant stream is reduced – less energy requirements and less NOx reactant.

  • Conventional oxy-fuel combustion leads to high efficiency combustion but high temperatures and high NOx levels.

  • Higher efficiency combustion leads to lower fuel requirements and corresponding reduction in CO2 emissions.

  • Does this work with dilute oxygen combustion???



Firing rate as a function of oxygen-enrichment level required to maintain 1100oC furnace temperature


Is oxygen enrichment a nox reduction strategy
Is oxygen-enrichment a NOx reduction strategy? required to maintain 1100

  • NOx emissions are reduced at high oxygen-enrichment levels … but …

  • Only at quite significant enrichment levels, and

  • With no air infiltration (a source of N2).


Nox emissions as a function of furnace n 2 concentration
NOx emissions as a function of furnace N required to maintain 11002 concentration


Capabilities of oxygen enriched combustion
Capabilities of oxygen-enriched combustion: required to maintain 1100

  • Dilute oxygen combustion systems can work with oxygen-enriched combustion.

  • NOx emissions are comparable to air-oxidant operation and NOx reductions are limited by air infiltration.

  • NOx emissions also limited by N2 content of the fuel.

  • Primary benefit is energy conservation (reduced fuel consumption) and associated CO2 reduction.


Limitations of oxygen enrichment
Limitations of oxygen-enrichment: required to maintain 1100

  • This is not a totally new technology !!!

  • Cost of oxygen – high purity O2 is expensive, lower purity is more feasible in some situations.

  • Infrastructure costs – oxygen supply and handling.

  • Furnace modifications – burners, gas handling, etc.


Final examination

Final Examination

  • Tuesday, April 22, 1900h

  • 3rd Floor Ellis Hall

  • Open book, open notes

  • Red or gold calculator


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