1

Chemical kinetic modeling development and validation experiments for direct fired sCO2

combustor PI: Subith Vasu1

Co-PIs: Jayanta Kapat1, Artem Masunov1, Scott Martin2, Ron Hanson3

1University of Central Florida, Orlando, FL2Embry-Riddle University, Daytona Beach, FL

3 Stanford University, Stanford, CA

DE-FE0025260PM: Dr. Seth Lawson

Duration 3 years: 10/1/2015-9/30/2018UTSR Meeting, Daytona Beach, FL, 11/1/2018

Distribution A: Approved for public release

Contact: subith@ucf.edu

Direct-fired sCO2 combustionIdeal Oxy-fuel combustionCH4 + O2 + CO2 → CO2 + H2O

CO2 is directly-fired based onthe Brayton Cycle ConceptTypically highest pressuresranges from 200 - 300 bar athigh pressures to just abovethe critical point

Figure adapted from Strakey, 2014, sCO2 symposium Figure adapted from Delimont, 2016, sCO2 symposium

• Existing state‐of‐the‐art, such as GRI‐3.0 Mechanism, has only been validated for pressures up to 10 atm

• Mechanisms have not been developed for CO2 diluted mixtures

• Updated/new mechanism will allow for accurate combustor modeling with multi‐step combustion using a validated mechanism

• Current CFD combustion models do not consider non‐ideal effects 

• Thermodynamics and kinetics are currently unknown!!

• Fundamental work can shed light into this challenge

Knowledge gaps

Effects of Increasing Pressure. Equilibrium calculation for CH4/O2/CO2 at φ = 1. Figure adapted from Strakey, 2014, sCO2 symposium

State-of-the-knowledgeState‐of‐the‐art models differ in their predictions even at atmospheric pressure with high CO2

• GRI 3.0 is still a widely used mechanism created 15 years ago• Aramco Mech 2.0 is a recent well‐validated mechanism 

Combustion chemistry snapshot

ReactionMechanismDevelopment

1. DecompositionPathways

2. IntermediateSpecies Sub‐Mechanisms

3. Full Mechanisms

4. Reduced Mechanisms

5. Validation

MODEL

KineticTargets(Shock Tubes, FlamesReactors)

1. Ignition Time Measurements 

2. SpeciesTime‐Histories

3. Direct Rate Measurements

4. Flame speed

5. Ignition energy, pressure rise

EXPERIMENTSFuel +O2+CO2

InitialDecomposition

Products

Intermediate Species

H-Abstraction& Oxidation

Products

Ignition

CO, CO2, H2O

OH, CH3, C2H4, C2H2, H2, CO, etc.

time=0

time= µs

time= ms

time= s

Combustion chemistry/kinetics are different at high pressure1 bar 300 bar

(Strakey, SCO2 symposium 2018, NETL)

Project/Task Summary

Task 1: Project Management

Tasks 2&3: Acquire kinetics and ignition data in highly CO2 diluted mixtures with shock tube experiments

Task 4: Refine and validate a chemical kinetic mechanism for Supercritical Carbon Dioxide (sCO2) Mixtures

Task 5: Develop a CFD Code that utilizes mechanism for sCO2 combustors

Overall Progress from this project in 3 years Oxy-Combustion/SCO2

- 20 Journal Papers

- 3 in J. Engineering for Gas Turbines and Power (ASME)

- 4 in J. Energy Resources Technology (ASME)

- 2 in Combustion and Flame

- 7 in J. Physical Chemistry A

- 2 in Energy & Fuels

- 1 Int J Chemical Kinetics

- 1 Proc. of Combust. Inst.

- > 30 conference papers at ASME Turbo Expo, sCO2 symposium, AIAA Meetings, Combustion Institute Meetings

- 4 additional journal papers currently in review

Sample Experiments Results

Shock tube operation: Pre-shock filling

High P Low P

Driver section

(high-pressure)

Driven section

(low-pressure)

Optical diagnostics for

absorption,

emission

DiaphragmPressure

transducers

4 1

Shock tubes are ideal for studying combustion chemistry Step change in T, P and well-defined time zero Simple fuel loadingAccurate mixtures and pre-shock conditions

Laser absorption spectroscopy

Beer - Lambert law

Spectral absorption coefficientkλ= S(T)ꞏ(T,P,X)

S(T): line strength, (T,P,X): line shape

Io

IPhoto detector

Laser source

L

I/Io = exp (-k Ptotal Xspecies L)32600 32610 32620 32630 32640

0

20

40

60

80

k [a

tm-1

cm-1

] [cm-1]

= 306.47nm

Ignition Results: Without CO2• Comparison of measured and simulated methane concentration for– Stoichiometric ignition of 3.5% CH4 in Argon, 1600K

0 500 1000 1500 20000.00

0.05

0.10

0.15

X CH

4

Time [sec]

XCH4 (Measured) XCH4 (Aramco) XCH4 (GRI)

0.0

0.5

1.0

1.5

2.0

2.5 CH* Pressure

P [a

tm] /

CH

*With out CO2 addition there is pressure rise after ignition

• Ignition delay times measured from the arrival of reflected shockwave to rise of the pressure trace

• Arrival of shockwave determined as midpoint of the second pressure rise (rise due to reflected shock)

• Rise of OH Emissions measured asthe intersection between the baseline and the tangent line drawn from maximum rise of OH

Ignition Results: 60% CO2 addition• Comparison of measured and simulated methane concentration for– Stoichiometric ignition of 3.5% CH4 in Argon diluted with 30% CO2, 1600K

0 100 200 300 400 5000.00

0.05

0.10

0.15

X CH

4

Time [sec]

XCH4 (Measured) XCH4 (Aramco) XCH4 (GRI)

0.0

0.5

1.0

1.5

2.0 CH* Pressure

P [a

tm] /

CH

*

With CO2 addition there is no pressure rise after ignitionTrue observation for all mixtures?

Ignition Results in CH4 Under SCO2 Conditions: 77.5% CO2 addition

With CO2 addition there is some pressure rise (7.5% fuel) after ignition but not as bad as the ones without CO2

0 100 200 300 400 5000

5

10

15

20

Time (s)

Pres

sure

Sig

nal (

volts

)

7.5% CH4/ 15% O2 /CO2

1277 K, 111.8 atmign = 367 s

Non-reactive Pressure

Pressure

OH* Emission

0.0

0.1

0.2

0.3

0.4

0.5

OH

* Em

issi

on

Ignition Results in CH4/O2 Under SCO2Conditions: 85% CO2 addition

With CO2 addition there is no pressure rise after ignition (4% fuel)

0 500 10000

5

10

15

Time (s)

Pres

sure

Sig

nal (

volts

)

3.91% CH4/ 9.92% O2 /CO2

1352 K, 75.1 atmign = 541 s

Pressure

OH* Emission

0.00

0.01

0.02

OH

* Em

issi

on

Low CH4 Loading: 3.9% CH4/O2/CO2 data: =0.7885% CO2 addition

• Small scatter in IDT data

• ARAMCO and FFCM in excellent agreement with IDT up to 80 atm

• But two models differ significantly at high pressure (260 atm)

Next: How do the models compare with higher fuel loading?

0.7 0.8 0.9100

1000

100001429 1250 1111

Temperature (K)

Igni

tion

Del

ay T

ime

(ms)

1000/T (1/K)

Modeling:Solid: ARAMCO Ver. 2 (2013)Dash: FFCM (2018)

80 atm 260 atm

39 atm

0.7 0.8 0.9 1.010

100

1000

100001429 1250 1111 1000

Temperature (K)

Igni

tion

Del

ay T

ime

(s)

1000/T (1/K)

Modeling:Solid: ARAMCO Ver. 2 (2013)Dash: FFCM (2018)

110 atm

33 atm

High CH4 Loading 7.5% CH4/O2/CO2 data: =1.085% CO2 addition

• Good agreement of models with data at 33 atm

• Continue good agreement up to 110 atm

• But models diverge at low T

CO2-diluted Ignition Experiments are challenging: High-Speed Imaging for Accurate Ignition Determination

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Reflected Shock Bifurcation• Bifurcation is a result of differences in the energy level between the

boundary layer and core flow. Occurs with diatomic and polyatomicmolecules in the driven section

• Bifurcation induces inhomogeneities 100μ

s15

0μs

200μ

s

Reflected shock Endwall

4 cm

Adapted from Kline et al. ISSW 1992Penyazkov et al., 2016 SW

Adapted from Yoo, Hanson, et al. 2010

Methane/O2/CO2 Ignition Imaging Results CO2=89.5%

Experiments with CO2 addition is not trivial in shock tubes Boundary layer effects, shock bifurcation

Solution Use multiple diagnostics to study ignition process

Methane/O2/CO2 Ignition: High-Speed Imaging Results CO2=0 %

--------------Ignition sequence imaged via high-speed (63, 000 fps)-------------

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Methane/O2/CO2 Ignition Imaging ResultsCO2=85%

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Methane/O2/CO2 Ignition Results With Imaging

(a) Mixture 1: XCH4 = 0.035, XO2 = 0.07, XCO2 = 0.00, XAr = 0.895. No CO2

(b) Mixture 2: XCH4 = 0.035, XO2 = 0.07, XCO2 = 0.60, XAr = 0.295. 60% CO2

Difference in various definitions increase with addition of CO2However, the data can be compared to modeling predictions for various definitions

Select Syngas/O2/CO2 Ignition Delay TimesNote: Syngas fuel is a mixture of CO and H2

-200 0 2000.0

0.2

0.4

0.6

0.8

1.0

1.2

T = 1203 KP = 120 atmTpeak= 76.5 usTslope= 62 us

Nor

mal

ized

Em

issi

on

Time [s]

Normalized OH* Emission Measured Pressure

Mixture:CO= 16.653%H2= 6.969%O2= 11.887%CO2=64.491%

0

20

40

60

80

100

120

140

160

180

P [a

tm]

Ignition Results in syngas Under SCO2 Conditions: 65% CO2 addition

With CO2 addition there is no significant pressure rise after ignition

Syngas /O2/CO2 Ignition Delay Time Measurements: Comparisons with Modeling

CO2=85%

12 Literature kinetic mechanisms tested

All mechanisms overpredict dataat high pressure !

0.78 0.79 0.80 0.81 0.82 0.83 0.84 0.85 0.86

100

1000

10000

100000Pavg= 40.374 atmConstant U,P = 0.11= 1.0XFuel= 0.10XCO2= 0.85

1000/T (K-1)

Aramco CRECK Davis FFCM GRI Kermones Li-2007 Li-2015 NUIG San Diego USC Varga Measured Data

Reaction Mechanism for sCO2

NSTC

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Chemical Mechanism Development Summary

NSTC

• Combustion kinetics model refinement/development

• Existing kinetic models are only valid at low pressures < 50 atm

• We used multi-scale simulations to extend their validity to mixtures up to 300 bar by:

1. Quantum Mechanic simulations of the activation enthalpies in gas vs. CO2 environment

2. Molecular Dynamic simulations of reaction processes

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Sensitivity Analysis

OH Sensitivity: Methane mixture 300 bar

Rxn#5: O2+H = O+OHRxn#26: H2O2+OH = H2O+HO2

Rxn#30: OH+HO2 = H2O+O2Rxn#31: OH+HO2 = H2O+O2

Rxn#32: 2HO2 = H2O2+O2Rxn#34: H+O2(+M) = HO2(+M)

Rxn#36: CO+OH = CO2+HRxn#46: CH4+OH = CH3+H2O

Rxn#47: CH4+HO2 = CH3+H2O2Rxn#49: CH3+HO2 = CH4+O2Rxn#90: CH3+O2 = CH3O+O

Rxn#91: CH3+O2 = CH2O+OHRxn#99: CH3+HO2 = CH3O+OH

Rxn#105: CH3O2+CH3 = 2CH3ORxn#153: CH2O+O2 = HCO+HO2Rxn#156: CH2O+OH = HCO+H2O

Rxn#157: CH2O+HO2 = HCO+H2O2Rxn#158: CH2O+CH3 = HCO+CH4

Rxn#194: 2CH3(+M) = C2H6(+M)Rxn#248: C2H4+OH = C2H3+H2O

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0

Normalized OH Sensitivity

List

of R

eact

ions

NSTC

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CH Sensitivity: Methane mixture at 300 barRxn#3: H2+OH = H+H2ORxn#4: 2O(+M) = O2(+M)

Rxn#5: O2+H = O+OHRxn#26: H2O2+OH = H2O+HO2

Rxn#30: OH+HO2 = H2O+O2Rxn#31: OH+HO2 = H2O+O2

Rxn#34: H+O2(+M) = HO2(+M)Rxn#36: CO+OH = CO2+HRxn#37: CO+OH = CO2+H

Rxn#43: CH3+H(+M) = CH4(+M)Rxn#44: CH4+H = CH3+H2

Rxn#46: CH4+OH = CH3+H2ORxn#47: CH4+HO2 = CH3+H2O2

Rxn#48: CH4+CH3O2 = CH3+CH3O2HRxn#49: CH3+HO2 = CH4+O2

Rxn#89: CH3+O2(+M) = CH3O2(+M)Rxn#90: CH3+O2 = CH3O+O

Rxn#91: CH3+O2 = CH2O+OHRxn#140: CH3O+O2 = CH2O+HO2

Rxn#174: CH3O(+M) = CH2O+H(+M)-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1

Normalized CH Sensitivity

List

of R

eact

ions

Sensitivity Analysis

NSTC

31 31

The important elementary steps in high-pressure C0-C4 fuel combustion

NSTCNaik, C. V.; Puduppakkam, K. V.; Meeks, E. J Eng Gas Turbines Power 2012, 134, 021504.

32

What is really happening during a reaction? Transition State Theory: 3-D potential energy surface

A+BC→ X≠ → AB+C

A+BCAB+C

X≠

X≠

Need collisions

NSTC

Figure adapted from Pilling and Seakins

NSTC

Gas-Phase Heat release Reaction CO+OH→CO2+H

Energy

Pathways shown for gas phase

NSTC

Effects of supercritical solvent within framework of Transition State Theory

NSTC

The supercritical solvent can modify predictions of this model in three ways: • changing the ability to reach the equilibrium by the

reactants and/or TS • shifting this equilibrium, and • changing probability of TS to convert to the products

NSTC

35

Elementary Reaction CO+OH→CO2+H (results with covalent CO2 addition as a spectator: new mechanism discovered !!)

NSTC

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Elementary Reaction CO+OH→CO2+H (results with spectator CO2 molecule)

NSTC

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Elementary Reaction of CO+OH→CO2+H (results with 1 additional spectator CO2)

Energy

• Pathways shown for CO2 autocatalytic effect published in J. Phys Chem A- Masunov & Vasu (2016)- above work

• CO2 opens up new pathways and accelerates heat release• Similar catalytic effects by CO2 seen in other reactions (HO2+HO2 → H2O2 + O2) but not in

H2CO + HO2 → HCO + H2O2

NSTC

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Molecular Dynamic Study: CO+OH→CO2+H (results)

OVERALL REACTION: ∙OH + CO CO2 + H∙ (R1, k1)

Actually goes through these 3 reactions including HOCO intermediate

∙OH + CO HOCO∙ (R2, k2)

HOCO∙ ∙OH + CO (R2r, k-2)

HOCO∙ CO2 +H∙ (R3, k3)

32

321 kk

kkk

NSTC

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Molecular Dynamic Study: CO+OH→CO2+H (results)

NSTC

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Molecular Dynamic Study: CO+OH→CO2+H (results)

NSTC

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Molecular Dynamic Study: CO+OH→CO2+H (results)

NSTC

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Molecular Dynamic Study: CO+OH→CO2+H (results)

∙OH + CO CO2 + H∙

(R1, k1)

NSTC

43

Molecular Dynamic Study: CO+OH→CO2+H (results)

• CO2 molecules are among the most efficient to accelerate heat release reaction with pressure

• mixed quantum mechanics/molecular mechanics (QM/MM) theory level and molecular dynamics (MD) approach

NSTC

44

Task 5: CFD development and implementation in OpenFOAM• Real Gas Equations of State for sCO2

• Thermal properties for sCO2 combustor

• CFD simulation status

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CFD Modeling DevelopmentA CFD code is being developed using an existing open source CFD program called

OpenFOAM, with the incorporation of a thermo-physical library and chemical kineticsmechanism that are applicable in the super critical regime. The resulting code will be ableto simulate reacting and non-reacting CO2 flow through a large range of thermodynamicconditions, as experienced in a theoretical super critical engine cycle.

This will entail 4 steps;1. Incorporate real gas equation of state.2. Incorporate a super critical thermodynamic library.3. Incorporate detailed sCO2 kinetic mechanism.4. Incorporate the non-premixed CMC turbulent combustion model.

Once the CFD code is validated for supercritical CO2 flows, sensitivity and design studieswill be performed on concept burners.

sCO2 CFD Modeling

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Objective

Objective: To identify necessary thermal and transport models for sCO2combustor simulations and to calculate elementary thermal properties for sCO2 combustors.

• To find best equation of state for sCO2 combustor modeling.

• To develop a model for ‘compressibility factor’.

• To find best viscosity and thermal conductivity models.

• To quantify accurate elementary properties for sCO2 combustors.

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The comparison of EOS for sCO2 and sO2:

• PRS is accurate for sCO2 and SRK is accurate for sO2.• The EOS has to be validate for mixtures of combustion.

48

Comparison of supercritical mixture viscosity mixture models:

• No reference data to compare.• Models predict within 10% of each other. • Preferred model is identified based on the computational time.

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Comparison of supercritical mixture viscosity models:

Computational time (s)

0 5 10 15 20 25 30 35

Mix

ing

rule

Weighted Average

Wilkes Mixing rule

Lucas Mixing rule

Chung Mixing rule

Chung Mixing rule with Kij

Time

• Times are calculated for 100,000 cells.• Lucas et al., method is more suitable for detailed kinetic mechanisms and

LES or DNS simulations.

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Thermal conductivity Mixture model

Chung et. al., Stiel and Thodos

Tim

e (s

)

0

10

20

30

40

50

Computational time

Comparison of supercritical mixture thermal conductivity models:

• Computational time calculated for 100,000 cells.• Stiel and Thodos is the least expensive computationally and suitable for LES

or DNS.

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• Aramco 2.0 , CHEMKIN-RG and PCMC are coupled to investigate thermal properties of the sCO2 mixture.

• Soave-Redlich-Kwong EOS is identified as the better accurate EOS for modelling sCO2 combustion density.

Property Conclusions

• Several preliminary thermal properties of sCO2 combustor are quantified. A model is suggested for ‘Z’ in sCO2 combustor.

• Lucas et. al., method is identified as the accurate and computationally advantageous Viscosity correlation for sCO2 combustion.

• An accurate and inexpensive method is proposed by using Stieland Thodos thermal conductivity correlation.

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Comparing conventional and supercritical combustion:

Simulated DLR-Jet

90m/s-300bar-CO2

90m/s-1bar-Air

OH

OH

• Simulation is performed with the premixed CMC in the OpenFOAM RANS CFD code.

• The current PCMC-OpenFOAM model is capable of using large mechanisms. The current simulation uses 493 species and 2,714 reactions.

• Stoichiometric CH4/O2 with 95% by mass CO2.

90m/s-300bar-CO2

90m/s-1bar-Air

Chemical Source Term (1/s)

Chemical Source Term (1/s)

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Design strategy identified for future CFD simulation of dilution zone:

• Stoichiometric CH4/O2 with 95% CO2by mass. Half of the CO2 injected inthe PSR and the remainder added to thePFRs.

• Real gas code with detailed Aramcomechanism

Effect of mixing strategy on PFR emissionsMixing strategies in Dilution zone

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Lean operating strategy identified

• Lean burn sCO2 reduces exit CO and shortens the reactor length.

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Acknowledgement: DE‐FE0025260 (Dr. Seth Lawson as program manager)

Thank you, Questions?

Vasu Lab 2016