1. NOx formation in ultra-low-NOx gas burners Zoran M. Djurisic, Eric G. Eddings University of Utah

2. Controlling mechanisms  Thermal NOx (Zeldovich) Direct N 2 oxidation High temperature required (> 1800 K)  Prompt NOx (Fenimore) NN bond scission by flame radicals Occurs only in flame fronts  N 2 O Pathway Through N 2 + O + M N 2 O + M Relevant under elevated pressures  Fuel NOx NO formation from N-containing fuel fragments (CN, NH) Relevant if fuel contains chemically-bound nitrogen

3. 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

4. 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

5. 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

6. Case study: NOx from steel-making by-product fuels By-product fuels composition variability Potential NOx formation mechanisms: Thermal NOx Prompt NOx Fuel NOx N 2 O path

7. Resulting NOx emissions variability Predicted NO emissions for stoichiometric oxidation in plug-flow reactor at 1200 K and 1 atm COGBFG

8. NOx formation pathway analysis NO 2 HNO NH N2ON2O NO NNH N2N2 O2O2 H O O O O H H OH OH H O OH HCO O2O2 H O, HO 2

9. Prompt NOx controlling reactions - summary  Methylidene is not to blame CH+N 2 has 10000 times lower rate coefficient than H + N 2 Typical HC flame contains 10 5 times more H than CH  Initial step: N 2 + H  NNH NNH oxidation to NO is relatively fast and easy  Competing process: any H scavenging process CH 4 + H  CH 3 + H 2 C 2 H 6 + H  C 2 H 5 + H 2 C 2 H 5 + H  C 2 H 4 + H 2

10. U-NOx datacenter

12. 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.

13. Minimizing NOx emissions from hydrogen-containing fuels