1. Optimizing and reducing emissions requires understanding of kinetics
Outline
NOx emissions
Equilibrium
Kinetics
Temperature-dependence
Optimizing and reducing emissions requires understanding of kinetics

2. Sources of Nitrogen Oxides (NOx)
Most of NOx is in the form of NO (typically > 90%)
Nitrogen Dioxide
Nitric Oxide
Combustion Processes that contribute:
Oxidation of atmospheric nitrogen (Thermal NOx)
Prompt NO formation (From radicals)
Oxidation of nitrogen in fuel (Fuel-nitrogen)

3. 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?

4. Example: Calculate Equilibrium NOx
HANDS ON
Example: Calculate Equilibrium NOx
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 Thermo data 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

5. Example: Calculate Equilibrium NOx
HANDS ON
Example: Calculate Equilibrium NOx

6. Example: Calculate Equilibrium NOx
HANDS ON
Example: Calculate Equilibrium NOx
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?

7. Example: Calculate Equilibrium NOx
HANDS ON
Example: Calculate Equilibrium NOx
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?

8. Example: Use PFR to calculate the time to equilibrium
HANDS ON
Example: Use PFR to calculate the time to equilibrium
Plug flow reactor model shows balance between flow times and kinetics
Example:
Run air through a flow tube
Fix the temperature
Include N2/O2 reaction 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
REACTIONS
2O+M<=>O2+M E
N+NO<=>N2+O E
N+O2<=>NO+O E
N2O+O<=>N2+O E
N2O+O<=>2NO E
N2O(+M)<=>N2+O(+M) E
LOW /6.370E /
NO+O+M<=>NO2+M E
NO2+O<=>NO+O E
30 cm
...training\nox_emissions\plug_timescales_nOx\

9. Example: Use PFR to calculate the time to equilibrium
HANDS ON
Example: Use PFR to calculate the time to equilibrium
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?

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