Oxy-Combustion Fundamentals for Direct Fired CyclesPete Strakey, NETL

UTSR MeetingNov 2, 2017Pittsburgh, PA

2

Effects of pressure and diluents on flames. Identification of target conditions. Overview of characteristic time and length scales. CFD simulations of turbulent time and length scales. Chemical kinetic mechanisms. Carbon monoxide formation. Thermo-acoustic instabilities.

Outline

3

Effect of Pressure on Laminar Flame Speed

Laminar Flame SpeedCantera, GRI 3.0Phi=1, T=300K

P (atm)0.1 1 10 100 1000

SL (m

/s)

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

CH4+Air, Tad=2226KCH4+(.31O2/.69CO2), Tad=2230K

RR= Reaction RateD= Molecular Diffusivity 1

=

Cantera with GRI 3.0 used to calculate premixed laminar flame speed.

Flame speed with 31%O2/69%CO2 lower than air mainly due to lower diffusivity.

= k/CP

Laminar Flame Thickness

4

Temperature profiles through flame region. CO2 dilution scaled to provide same equilibrium flame temperature as air flames.

Cantera Premixed Laminar Flame Profiles

Premixed CH4/AirPhi=1 GRI 3.0

Axial Position (m)0.0600 0.0602 0.0604 0.0606 0.0608 0.0610

T (K

)

0

500

1000

1500

2000

2500

1 atm10 atm100 atm300 atm

Premixed CH4+(.31O2 .69CO2)Phi=1 GRI 3.0

Axial Position (m)0.0030 0.0032 0.0034 0.0036 0.0038 0.0040

T (K

)

0

500

1000

1500

2000

2500

1 atm10 atm100 atm300 atm

Flames get much thinner at high pressure.

5

OH profiles through flame region. CO2 dilution scaled to provide same equilibrium flame temperature as air flames.

Cantera Premixed Laminar Flame Profiles

Premixed CH4/AirPhi=1 GRI 3.0

Axial Position (m)0.0600 0.0602 0.0604 0.0606 0.0608 0.0610

OH

(mol

e fr

actio

n)

0.000

0.002

0.004

0.006

0.008

0.0101 atm10 atm100 atm300 atm

Premixed CH4+(.31O2 .69CO2)Phi=1 GRI 3.0

Axial Position (m)0.0030 0.0032 0.0034 0.0036 0.0038 0.0040

T (K

)

0.000

0.002

0.004

0.006

0.008

0.0101 atm10 atm100 atm300 atm

Radical concentrations reduced at high pressure due to 3-body recombination reactions.

6

Cantera Non-Premixed Laminar Flame Profiles

Opposed Diffusion CH4/AirPhi=1 GRI 3.0

Axial Position (m)0.004 0.006 0.008 0.010 0.012 0.014

T (K

)

0

500

1000

1500

2000

2500

1 atm10 atm100 atm300 atm

Opposed Diffusion CH4+(.31O2 .69CO2)Phi=1 GRI 3.0

Axial Position (m)0.004 0.006 0.008 0.010 0.012 0.014

T (K

)

0

500

1000

1500

2000

2500

1 atm10 atm100 atm300 atm

Temperature profiles through flame region. CO2 dilution scaled to provide same equilibrium flame temperature as air flames.

Opposed Diffusion CH4+(.31O2 .69CO2)Phi=1 GRI 3.0

Axial Position (m)0.004 0.006 0.008 0.010 0.012 0.014

Stra

in R

ate

(1/s

)

0

10

20

30

40

50

60

1 atm10 atm100 atm300 atm

Sep= 2cmVF=VO=27 cm/s

7

Cantera Non-Premixed Laminar Flame Profiles OH profiles through flame region. CO2 dilution scaled to provide same equilibrium flame temperature as air flames.

Opposed Diffusion CH4/AirPhi=1 GRI 3.0

Axial Position (m)0.004 0.006 0.008 0.010 0.012 0.014

OH

(mol

e fr

actio

n)

0.000

0.002

0.004

0.006

0.008

1 atm10 atm100 atm300 atm

Opposed Diffusion CH4+(.31O2 .69CO2)Phi=1 GRI 3.0

Axial Position (m)0.004 0.006 0.008 0.010 0.012 0.014

OH

(mol

e fr

actio

n)

0.000

0.002

0.004

0.006

0.008

1 atm10 atm100 atm300 atm

8

AramcoMech 2.0 used for ignition delay time.

Extinction Strain Rate & Ignition Delay Time

Extinction Strain RateCantera, GRI 3.0Phi=1, T=985K

P (atm)0.1 1 10 100 1000

Stra

in R

ate

(1/s

)

0.02.0e+44.0e+46.0e+48.0e+41.0e+51.2e+51.4e+51.6e+51.8e+52.0e+5

CH4+(.31O2/.69CO2), Tad=2230K

Ignition Delay TimeCantera, AramcoMech2.0

Phi=1, T=985K

P (atm)0.1 1 10 100 1000

t ign (

s)

0.0001

0.001

0.01

0.1

1

10

31%O2 / 69%CO29%O2 / 91%CO2

9R.J. Allam et. al., ASME GT2014-26952, 2014

Allam Cycle Goal is to estimate some characteristic combustion scales for high pressure oxy-fuel

flames for direct-fired sCO2 cycles. Target is the Allam cycle conditions (O2 15% to 30% molar concentration)*.

* Allam, et al., Energy Procedia 37, 2013, 1135-1149

10

Borghi Diagram indicates regime of combustion (wrinkled flames, corrugated flames, stirred reactor, etc.

Borghi Combustion Diagram

Gas TurbinesIC Engines

:

C. Sorusbay, Turbulent Premixed Combustion in Engines, Istanbul Technical University

P=300 bar50 MW Thermal InputPhi=1.0

combustor

O2 (4.2 kg/s, T=1015K)

CO2 (14.0 kg/s, T=1015K)

CH4 (1 kg/s, T=496 K)

CO2 (43.6 kg/s, T=1015K)

11

Included here for completeness

Characteristic Scales and Dimensionless Numbers

=

=2

2Karlovitz Number (chemical time / Kolmogorov time)

=

Laminar flame thickness (thermal diffusivity / laminar flame speed)

=3

1/4

Kolmogorov length scale (kinematic viscosity / turbulent dissipation rate)

= 0.21/2

Integral length scale (turbulent kinetic energy / turbulent dissipation rate)

=23

Turbulent fluctuating velocity

K>1 means the smallest eddies can enter and thicken the flame front

D =

= .247

2

Damkohler Number (turbulent time / chemical time) Da>>1 means the chemistry is fast

compared to turbulent mixing

12

50 MW Conceptual CombustorSSME Preburner type combustor 21 coaxial injectors, 4M Cells

D=0.125 mFuel in

Oxidizer in

Purge inP=300 bar50 MW Thermal Input

O2 (4.2 kg/s, T=1015K)CO2 (12.6 kg/s, T=1015K)

CH4 (1 kg/s, T=496 K)

CO2 (43.6 kg/s, T=1015K)

13

Turbulent Time and Length ScalesTwo Limiting Cases:1) 25% of CO2 by mass mixed in with O2 (XO2=0.30, =1.0)2) Fully mixed (100% of CO2 mixed in with O2) (XO2=0.09, =1.0)

25% of CO2 with O2 (XO2=0.30)75% of CO2 through purgelT=1.9 mmU=7.5 m/s

100% of CO2 with O2 (XO2=0.09)lT=2.0 mmU=23.8 m/s

T (K)

Steady RANS k-eDRM19 reduced CH4 mechanismNo Combustion model

14

Borghi Diagram for Oxy-Combustion Two cases shown for 300 bar oxy-combustion define a range of

conditions spanning the thickened, corrugated flame regime and stirred reactor.

Gas TurbinesIC Engines

300 bar sCO2 (.31O2+.69CO2)SL = 0.58 m/s (TF=2690K)L = 0.67 mKa=0.7Tign=9.2e-4 s

300 bar sCO2 (.09O2+.91CO2)SL = 0.05 m/s (TF=1610K)L = 6.60 mKa=361Tign=2.5e-3 s

Significantly outside the range of gas turbine and IC engine operation.

Requires assessment of appropriate turbulent combustion models.

15

Potential Range of Operating Conditions

30%O2/70%CO2U = 30 m/sR = 12.7 msT = 2.6e-4 slT = 1.9 mmK=3.4e-6 slK = 1.6 m = 3.9e-7 m2/sSL = 0.58 m/sign = 0.92 ms

9%O2/91%CO2U = 50 m/sR = 7.6 msT = 9.3e-5 slT = 2.0 mmK=4.1e-7 slK = 0.36 m = 3.3e-7 m2/sSL = 0.05 m/sign = 2.5 ms

30% O2 case looks like a conventional turbulent flame. 9% O2 case looks like autoignition.

T (K)

Large Eddy Simulations

16

No detailed mechanisms validated at sCO2 conditions. Best available is likely Aramco Mech (U. Galway). Validated with flame-speed up to 60 bar and ignition delay to 260 bar. Likely better than GRI 3.0.

Huge mechanism, 103 species, 480 reactions after reduction to C2 and smaller.

Need for compact skeletal mechanisms amenable to CFD modeling (10-30 species maximum).

Chemical Kinetic Mechanisms

Need flame speed, species profiles and induction time data for direct-fired conditions!

17

Methane oxidation kinetics very different at 300 bar.

More reaction paths play an important role, therefor larger skeletal mechanisms needed to adequately represent kinetics.

Mechanism ReductionReaction Path Analysis, 0-D ReactorT=985K, CH4=.12989, O2=.27351, CO2=.5966

1 bar 300 bar

18

Mechanism Reduction

Flame Speed SensitivityAramcoMech 2.0 300 barT=985, CH4:.12989, O2: .27351, CO2:.5966

Sensitivity

0.0 0.1 0.2 0.3 0.4 0.5

Rea

ctio

n #

0

1

2

3

4

5

6

7

8

9

10

11

H2O2 + OH = H2O + HO2

CH3OH +M = CH3 + OH + M

CH3 + OH = CH2OH + H

CH4 + HO2 = CH3 + H2O2

CH2O + O2 = HCO + HO2

CH3 + HO2 = CH4 + O2

CH3 + O2 = CH2O + OH

HO2 + OH = H2O + O2

CH3 + HO2 = CH3O + OH

H + O2 = O + OH

Combination of reaction path analysis, flame-speed sensitivity and ignition delay time sensitivity.

Optimized for Allam cycle combustor conditions 300 bar Tpreheat ~ 1000K Oxidizer: 25% O2 + 75% CO2

Flame speed sensitivity at 300 bar

Several skeletal mechanisms developed with 33, 29, 26 and 17 species.

19

Ignition Delay TimeCantera

Phi=1, T=985K

P (atm)0.1 1 10 100 1000

t ign (

s)

0.0001

0.001

0.01

0.1

1

10

AramcoMech2.0Aramco-33GRI3.0Aramco29Aramco17

Performance comparison of various skeletal mechanisms. Flame speed and ignition delay improve with the inclusion of more species and reactions.

33 and 29 species able to predict flame-speed and ignition delay very well. 17 species able to predict flame-speed to within ~40% error.

Mechanism Reduction

Laminar Flame SpeedP=60 Bar, T=298K, CH4 + (.15O2/He)

Phi0.8 0.9 1.0 1.1 1.2 1.3 1.4

S L (m

/s)

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24 Experimental DataAramcoMech2.0GRI3.0Aramco33Aramco26Aramco29Aramco17

20

Mechanism ReductionLaminar Flame Profiles

P=300 bar, T=995KCH4=.12989, O2=.27351, CO2=.5966

pos (m)2e-5 3e-5 4e-5 5e-5 6e-5

CO

mol

e fr

actio

n0.00

0.02

0.04

0.06

0.08

AramcoMech2.0Aramco-33GRI 3.0Aramco29Aramco17

Performance comparison of various skeletal mechanisms.

All do very well for CO production profiles.

CO prediction important for accurate cycle efficiency calculations.

21

Estimation of CO ProductionLarge Eddy Simulation, Dynamic Smagorinksy17-species skeletal mechanismFDF Combustion Model, Modified Curl, CM=2

Time-Averaged CO mass fraction

Instantaneous CO mass fraction30%O2/70%CO2, =1.09%O2/91%CO2, =1.0

XC0=.026XCO=.00098

Equilibrium CalcsFlame (30% O2) : XCO=.024Comb Exit: XCO = 1.2e-5

22

CO concentration is well above equilibrium at turbine inlet conditions. CO oxidation reactions are slow relative to residence time in turbine. CO is effectively frozen at flame conditions.

Fate of CO in TurbineRANS, SST-kOmega, DRM19, Vblade=150 m/s, Pr=2.85

XCO

P (Pa)

T (K)Equilibrium Calcs

Flame: XCO=.024 (30% O2, =1.0)Comb Exit: XCO = 1.2e-5

Simulation ResultsInlet: XCO=.026Outlet: XCO=.023

Temperature

Pressure

XCO

23

Effect of CO on sCO2 Cycle Efficiency

Source: NETL SEA (Chuck White)

Preliminary cycle calculations indicate as much as a 1.6 percentage point decrease in cycle thermal efficiency per mole percentage of CO in the combustor exhaust.

Drop off in cycle and process efficiency is due to an increase in compression power as a result of CO in working fluid.

24

Modeling Thermo-Acoustic Instabilities

Radial mode thermoacoustic instability @ 3kHz.Peak-to-Peak pressure oscillation 60% of mean combustor pressure.

Pressure Field (Pa)

Slice at Z=2 cm

Aramco17 (17 species, 45 reactions) 50 mW (1 kg/s CH4 flow). Oxidizer 25%O2 + 75%CO2 by mass. 3M cells, LES model

OpenFOAM LES, pressure-based solver

25

Oxy-combustion at 300 bar is somewhat uncharted territory. Conditions more representative of rocket engines. Limited data available.

Need for validated detailed chemical kinetic mechanisms as well as reduced mechanisms.

Must take care in selecting appropriate combustion models (fast mixing, flamelet, EDC, PDF, etc).

CO levels must be kept low (XCO < .0015) for maximum cycle efficiency. Thermo-acoustic stability must be assessed.

Modeling can help. Passive devises (baffles, Helmholtz resonators) may be necessary.

Concluding Remarks

Oxy-Combustion Fundamentals for Direct Fired CyclesPete Strakey, NETLOutlineEffect of Pressure on Laminar Flame SpeedCantera Premixed Laminar Flame ProfilesCantera Premixed Laminar Flame ProfilesCantera Non-Premixed Laminar Flame ProfilesCantera Non-Premixed Laminar Flame ProfilesExtinction Strain Rate & Ignition Delay TimeAllam CycleBorghi Combustion DiagramCharacteristic Scales and Dimensionless Numbers50 MW Conceptual CombustorTurbulent Time and Length ScalesBorghi Diagram for Oxy-CombustionPotential Range of Operating ConditionsChemical Kinetic MechanismsMechanism ReductionMechanism ReductionMechanism ReductionMechanism ReductionEstimation of CO ProductionFate of CO in TurbineEffect of CO on sCO2 Cycle EfficiencyModeling Thermo-Acoustic InstabilitiesConcluding Remarks