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# How much NO x can we form? - PowerPoint PPT Presentation

How much NO x 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 (N 2 + O 2 ) Constrain temperature and pressure Let P = 1 atm, vary T = 300 to 2300 K

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How much NOx can we form?

HANDS ON

• Equilibrium calculations tell us the eventual concentrations of species, for a given set of conditions

• Example:

• Constrain temperature and pressure

• Let P = 1 atm, vary T = 300 to 2300 K

• How much NOx will be formed?

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

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?

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?

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\

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?

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

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?