Nitrogen Oxides (NO x )

1. Nitrogen Oxides (NOx) Chapter 12 Page 147-168

2. 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

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

4. “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

5. NOx reaction mechanisms: • highly endothermic with Dhf = +90.4 kJ/mol • NO formation favoured by the high temperatures encountered in combustion processes

6. 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}

7. 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

8. 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

9. “Prompt” NO mechanism (1971): This scheme occurs at lower temperature, fuel-rich conditions and short residence times

10. 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

11. Example of proposed reaction pathway for fuel-rich hydrocarbon flames

12. 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 NOx control strategies: Combustion Modification Modified Operating Conditions

13. 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.

14. SCR process: 4 NO + 4 NH3 + O2 4 N2 + 6 H2O 2 NO2 + 4 NH3 + O2 3 N2 + 6 H2O

15. SNCR process: 4 NH3 + 6 NO  5 N2 + 6 H2O CO(NH3)2 + 2 NO ½ O2 2 N2 + CO2 + 2 H2O

16. Low NOX burners: Dilute combustion technology

17. 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.

18. 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

19. A conventional burner

20. 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”

21. Strong-jet/Weak-jet Aerodynamics • Strong jet = oxidant • Weak jet = fuel

22. Strong-jet/Weak-jet aerodynamics

23. CGRI burner

24. 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

25. Queen’s test facility

26. Queen’s test facility

27. CGRI burner in operation at 1100OC

28. CFD rendering of the fuel flow pattern

29. CGRI burner performance (1100OC)

30. 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???

31. NOx emissions as a function of oxygen enrichment

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

33. 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).

34. NOx emissions as a function of furnace N2 concentration

35. 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.

36. 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.

37. CHEE 481 Tutorial Session • Saturday, April 19, 0900h • Dupuis Hall 217 Final Examination • Tuesday, April 22, 1900h • 3rd Floor Ellis Hall • Open book, open notes • Red or gold calculator