How much NO x can we form?

1. How much NOx can we form? HANDS ON • Equilibrium calculations tell us the eventual concentrations of species, for a given set of conditions • Example: • Start with pure air (N2 + O2) • Constrain temperature and pressure • Let P = 1 atm, vary T = 300 to 2300 K • How much NOx will be formed?

2. Example: Calculate Equilibrium NOx HANDS ON Temperature-dependence of NOx • Create a New project, e.g., “myNOxEquil” • Drag an Equilibrium icon onto the Diagram and Update Project • Create a new chemistry set • Set Working Directory to a new directory,e.g., “MyNOxEquil” • Create new Gas-phase chemistry file • Only need elements and species: N, O, NO, NO2, N2O, N2, O2 • Select the standard Thermodata file • Fill in panels describing equilibrium conditions • Hint: Air consists of about 79% N2 and 21% O2 • Hint: Vary temperature using Continuations • Create input files and Run • Plot the resulting NOx components vs. T

3. HANDS ON Example: Calculate Equilibrium NOx

4. Example: Calculate Equilibrium NOx HANDS ON • What is the dominant component of NOx • Does the answer change with Temperature? • How many ppm of NOx is formed at T=2300 K? • Typical NOx emissions regulation: “…combined-cycle combustion turbines firing natural gas and distillate oil must limit NOx emissions to 42 and 65 parts per million” • At what temperature does NO formation begin to become important?

5. Example: Calculate Equilibrium NOx HANDS ON Pressure-dependence of NOx • Create a New project to determine equilibrium conditions for varying pressure • Fix T = 2300 K • Hint: Air consists of about 79% N2 and 21% O2 • Vary P from 0.1 to 10 atm • Run • Plot the resulting NOx components vs. P How dependent is NO on Pressure?

6. Example: Use PFR to calculate the time to equilibrium HANDS ON • Plug flow reactor model shows balance between flow times and kinetics • Example: • Run air through a flow tube • Fix the temperature • Include N2/O2reaction kinetics T = 2300 K Air 300 cm3/s 1 atm 2.54 cm ! Air reactions extracted from GRI-Mech Version 3.0 ELEMENTS O H N AR END SPECIES O O2 N NO NO2 N2O N2 END REACTIONS 2O+M<=>O2+M 1.200E+17 -1.000 .00 N+NO<=>N2+O 2.700E+13 .000 355.00 N+O2<=>NO+O 9.000E+09 1.000 6500.00 N2O+O<=>N2+O2 1.400E+12 .000 10810.00 N2O+O<=>2NO 2.900E+13 .000 23150.00 N2O(+M)<=>N2+O(+M) 7.910E+10 .000 56020.00 LOW /6.370E+14 .000 56640.00/ NO+O+M<=>NO2+M 1.060E+20 -1.410 .00 NO2+O<=>NO+O2 3.900E+12 .000 -240.00 END 30 cm ...training\nox_emissions\plug_timescales_nOx\

7. Example: Use PFR to calculate the time to equilibrium HANDS ON • Create a New project, e.g., “myPFRNox” • Set the working dir to the training directory : • ...training\NOx_Emissions\Plug_Timescales_NOx • Browse and select the “Air_NOx.cks” chemistry set • Note: reaction kinetics now included in the “Air_chem.inp” file • Set up the plug-flow problem • (settings on previous slide) • Create input and Run • Plot residence time vs. distance • Plot NO vs. residence time How long does it take for NO to reach the equilibrium value?

8. t = 0.49 s x = 28 cm Example: Use PFR to calculate the time to equilibrium HANDS ON T = 2300 K • NO increases exponentially with temperature, as before • For longer channel, we get closer to equilibrium • Higher temperatures reach equilibrium faster Equilibrium = 15400 ppm T = 2000 K NO Time vs. Distance T = 1500 K

9. Summary: How do we reduce NOx? Calculations suggest: • Reduce time spent by gases at high temperatures (residence time) • Don’t let conditions approach equilibrium • Keep combustion temperatures low • Focus on NO; dominant NOx component • Reduce nitrogen-containing compounds? • Introduce additional chemistry so other species are formed?