Combustion chemistryChemkin exercise

This exercise

• In this exercise you will use the Chemkin software to calculatemethane combustion in a plug-flow reactor

• Considering the fundamental flame types, the plug-flow reactorcalculation can be said to correspond to the premixed flamedemonstrated in the lectures using the Bunsen burner

• You should note how quickly methane reacts (expressed as time, as well as distance along plug flow reactor) in a premixed gas mixture in which the reaction / conversion rate is determined by chemicalkinetics. This information you will compare to a CFD calculation to be completed later.

Preparations

• Download the Åbo Akademi chemical kinetics mechanism files(aaumech_rev.inp and aautherm_rev.dat), and place them in your workingdirectory (pchome recommended, desktop may not work properly)In this example, a folder named”Combustion_Chemistry_Chemkin”has been created on pchome.This will be the working directory

• Note: the Chemkin inp filemay be recognized as Aspen Plusfile, ignore this

Start Chemkin 19.2

• Note: Chemkin 17.2 may alsobe available on the computer;however, use Chemkin 19.2

Start Chemkin

• Click ”cancel” in case windows pop up for ”Product ImprovementProgram” or firewall settings; you will not have administrator rights to ok eg firewall settings.

• In case the Chemkin prompts you to ok preferences or settings, do this

Close the default Project

Default Close all projects You will make your own project

Make own project1.

2. Give project name and click OK3. Save projectinto your working directory(see next slide)

You can give the projecta name, or keep theproposed one

Then click ”Save”

Set-up plug-flow reactor

1. Drag and dropInlet, outlet, and plug flow reactor icons 2. Connect icons: draw lines with left

mouse button held down

3. Click ”Update Project”

Read in and pre-process chemistry files

1. Double-click ”Pre-Processing”

2. Browse to and select your working directory

Your working directoryshould show up as option(requires you havesaved your projectin this folder)

1. Click ”New Chemistry Set”

2. Select the files for mechanism (.inp) andthermodynamic properties (.dat)3. Give the ”short name”4. Press ”SaveAs”

5. Choose yourworking directoryandpress ”Save”

Your view/list of directories may be different.In any case, select your working directory

6. Press”Run Pre-Processor”,then close”Pre-Processing (Combustion_Chemistry)” window

Close window

1. Double click ”PFR (C1)”to expand tree.

C1_PFR: here you definereactor properties (T,p,…)

C1_Inlet1: here you defineinput species etc

Calculation using constant temperature

Set velocity

Input species, type species nameand give amount, press ”Add”

Close window

Methane combustion in air at λ = 1.21 CH4 + 2.4 O2 + (2.4/0.21*0.79) N2

1. Double click ”Run Calculations”and press ”Begin”

2. After calculation is finishedpress ”Next Step…”

3. Here you can select what toinclude in post-processing.Keep default values and press”Process Solution Data”

4. Select default and press”Next Step”

5. Select X variable (distance or residence time) andY variables (CH4, CO, CO2, H2, H2O, O2), and press ”Create Plot”

Hold down the ”Ctrl”key to selectseveral Y variables

You can zoom in on the plot by holding downthe left mouse button and drawing arectangle from top-left to bottom-right

Note the time and distance it takes for a gas(methane) to react at a temperature relevantfor combustion systems (1000 °C).

After doing this, you can experiment withcalculations (see next slides for suggestions)

Select to plot all species in logY-linear x plot

Note how there are many species involved, but most of these are presentat very low concentrations (1E-6 is 1 ppm)

Test how fast/slow the combustion is dependingon temperature (eg 600 °C, 800 °C, 1200 ° C)

Set reactor temperature Calculate Plot results(CH4, CO, CO2, H2, H2O, O2

as function of residence time)

Adiabatic flame temperatureSelect ”Solve Gas Energy Equation”Now, instead of maintaining temperatureconstant, it will vary depending onendothermic / exothermic reactions(in the case of CH4 combustion exothermic).As defailt there will no heat transfer withsurroundings, meaning an adiabatic process.

The input ”Temperature” is now the starting temperature

You can test calculating CH4 combustion at different air factors,and see how the final / adiabatic temperature depends on air factor