NOx formationin ultra-low-NOx gas burners

Zoran M. Djurisic, Eric G. EddingsUniversity of Utah

Controlling mechanisms Thermal NOx (Zeldovich)

Direct N2 oxidation High temperature required (> 1800 K)

Prompt NOx (Fenimore) NN 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

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

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

Ultra-low NOx burners (contd.)

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

Forced Internal Recirculation (FIR) burner

Case study:NOx from steel-making by-product fuels

By-product fuels composition variability

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

Resulting NOx emissions variability

Predicted NO emissions for stoichiometric oxidationin plug-flow reactor at 1200 K and 1 atm

COG BFG

NOx formation pathway analysis

NO2

HNO

NH

N2O

NO

NNH

N2

O2H

O

O

O

O

H

H OH

OH H OOH

HCO

O2

H

O, HO2

Prompt NOx controlling reactions - summary

Methylidene is not to blame CH+N2 has 10000 times lower rate coefficient

than H + N2

Typical HC flame contains 105 times more H than CH

Initial step: N2 + H NNHNNH 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

U-NOx datacenter

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.

Minimizing NOx emissionsfrom hydrogen-containing fuels