Nitrogen Oxides (NOx)

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Nitrogen Oxides (NOx)

• Chapter 12
• Page 147-168

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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

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Smog precursors

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

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Developing NOx and SOx Emission Limits
December 2002, Ontarios Clean Air Plan for
Industry
Broad base of sources with close to 50 from the
Electricity sector in 1999
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NOx reaction mechanisms

• highly endothermic with Dhf 90.4 kJ/mol
• NO formation favoured by the high temperatures
  encountered in combustion processes

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Zeldovich mechanism (1946)
k1 1.8 ? 108 exp-38,370/T k-1 3.8 ? 107
exp-425/T k2 1.8 ? 104 T exp-4680/T k-2
3.8 ? 103 T exp-20,820/T k3 7.1 ? 107
exp-450/T k-3 1.7 ? 108 exp-24,560/T
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Rate-limiting step in the process ?
k1 1.8 ? 108 exp-38,370/T k-1 3.8 ? 107
exp-425/T
K1 is highly temperature dependent
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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
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Prompt NO mechanism (1971)
This scheme occurs at lower temperature,
fuel-rich conditions and short residence times
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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
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Example of proposed reaction pathway for
fuel-rich hydrocarbon flames
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NOx control strategies
Combustion Modification
Modified Operating Conditions

• Reduce peak temperatures
• 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

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Control strategies

• 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.

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SCR process
4 NO 4 NH3 O2 ? 4 N2 6 H2O 2
NO2 4 NH3 O2 ? 3 N2 6 H2O
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SNCR process
4 NH3 6 NO ? 5 N2 6 H2O CO(NH3)2
2 NO ½ O2 ? 2 N2 CO2 2 H2O
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Low NOX burners
Dilute combustion technology
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Industrial furnace combustion

• 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.

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• High intensity combustion with rapid mixing of
  fuel and oxidant
• High temperature flame zones with low HC but
  high NOx
• Furnace efficiency increased by preheating the
  oxidant stream

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A conventional burner
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Dilute oxygen combustion

• 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

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Strong-jet/Weak-jet Aerodynamics

• Strong jet oxidant
• Weak jet fuel

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Strong-jet/Weak-jet aerodynamics
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CGRI burner
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• Dilute oxygen combustion operation with staged
  mixing of fuel and oxidant
• No visible flame (flameless combustion)
• More uniform temperature throughout flame and
  furnace
• Low HC and NOx emissions

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Queens test facility
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Queens test facility
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CGRI burner in operation at 1100OC
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CFD rendering of the fuel flow pattern
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CGRI burner performance (1100OC)
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Oxygen-enriched combustion

• 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???

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NOx emissions as a function of oxygen enrichment
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Firing rate as a function of oxygen-enrichment
level required to maintain 1100oC furnace
temperature
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Is oxygen-enrichment a NOx reduction strategy?

• 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).

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NOx emissions as a function of furnace N2
concentration
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Capabilities of oxygen-enriched combustion

• 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.

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Limitations of oxygen-enrichment

• 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.

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Final Examination

• CHEE 481 Tutorial Session
• Saturday, April 19, 0900h
• Dupuis Hall 217

• Tuesday, April 22, 1900h
• 3rd Floor Ellis Hall
• Open book, open notes
• Red or gold calculator