NOx formation in ultralowNOx gas burners

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NOx formationin ultra-low-NOx gas burners

• Zoran M. Djurisic, Eric G. Eddings
• University of Utah

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Controlling mechanisms

• Thermal NOx (Zeldovich)
• Direct N2 oxidation
• High temperature required (gt 1800 K)
• Prompt NOx (Fenimore)
• N?N bond scission by flame radicals
• Occurs only in flame fronts
• N2O Pathway
• Through N2 O M? N2O M
• Relevant under elevated pressures
• Fuel NOx
• NO formation from N-containing fuel fragments
  (CN, NH)
• Relevant if fuel contains chemically-bound
  nitrogen

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NOx control strategies

• Flame control
• Temperature
• Stoichiometry
• Species dilution and scavenging
• Post-flame control
• Post-flame NOx reduction by
• Reburning
• Non-catalytic selective reduction
• Catalytic selective reduction

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Low-NOx burners

• NOx-control strategies by burner design
• Staging
• Swirling
• Recirculation
• These techniques effectively control
• Flame core stoichiometry
• Peak flame temperature
• Ultra-low NOx target sub-10 ppm
• NOx emission levels comparable to selective
  catalytic reduction technology (SCR) at
  significantly lower cost

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Ultra-low NOx burners (contd.)
Forced Internal Recirculation (FIR) burner

• Commercial ultra-low NOx burner (9 vppm)
• Forced Internal Recirculation
• Flame temperature 1200 - 1400 K.

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Case studyNOx from steel-making by-product fuels
By-product fuels composition variability

• Potential NOx formation mechanisms
• Thermal NOx
• Prompt NOx
• Fuel NOx
• N2O path

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Resulting NOx emissions variability
COG
BFG
Predicted NO emissions for stoichiometric
oxidationin plug-flow reactor at 1200 K and 1 atm
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NOx formation pathway analysis
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Prompt NOx controlling reactions - summary

• Methylidene is not to blame
• CHN2 has 10000 times lower rate coefficientthan
  H N2
• Typical HC flame contains 105 times more H than
  CH
• Initial step N2 H ? NNH
• NNH oxidation to NO is relatively fast and easy
• Competing process any H scavenging process
• CH4 H ? CH3 H2
• C2H6 H ? C2H5 H2
• C2H5 H ? C2H4 H2

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U-NOx datacenter
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Acknowledgements

• We gratefully acknowledge funding for this work
  provided by the Gas Technology Institute through
  a grant with the U.S. Department of Energy.
• Additional funding was provided by Reaction
  Engineering International and the University of
  Utah Research Fund.

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Minimizing NOx emissionsfrom hydrogen-containing
fuels