Biodiesel Kinetics and Flame Chemistry
Yiguang Ju, Princeton University
On behalf of CEFRC: Biodiesel Thrust and Flame Chemistry Working Group Sept. 17-20, 2012, MACCCR
Synergistic •High-pressure flames •Turbulent flames •Flame chemistry •Laminar flame speeds, •Droplet processes •Biodiesel kinetics 2D Research extinction and ignition, •Model reduction & pollutants Structure multi-scale modeling
•Flow reactor experiments • DNS of HCCI/SACI combustion C. K. Law •Reactivity and species history • DNS data for model validation •H , CO, small HC chemistry • 2 High pressure turbulence/ •DME, small oxygenates chemistry interaction F. N. Egolfopoulos Y. Ju
J. H. Chen F. L. Dryer
•LES/PDF/ISAT turbulent •Mechanisms of butanols combustion •Automatic mechanism •Turbulence/chemistry S. B. Pope generation Three Thrusts W. H. Green Unite the Team •Low-T combustion engines H. Wang R. D. Reitz •HCCI and RCCI CFD modeling •Soot •Interface with DERC •Small HC chemistry consortium Alcohols Biodiesel •Transport properties •UQ methods E. A. Carter N. Hansen Foundation Fuels •Flame species by C. J. Sung •Ab initio methods D. G. Truhlar Synchrotron MS •Thermochemical kinetics •Burner-stabilized flames •Ester chemistry S. J. Klippenstein R. K. Hanson •Rapid compression machine •High-pressure ignition •Potential energy surfaces •Thermometry and species •Rate constants with tunneling •Shock tube/Laser •Potential energy surfaces diagnostics •Reaction rate constants •Ignition/Species histories •High pressure theories •Rate constants Motivation O
Biodiesel: R2 R1 O • Produced from vegetable oils, animal fats, & waste materials • Energy density much higher than ethanol • 28 billion gallons of biodiesel produced in 2010 worldwide
• Large molecules: C16-C18 with ester functional group • Different combustion chemistry/emissions from hydrocarbons • Large disparities in alkyl chain length and structures
Biodiesel Trans-esterification Sooting Propensity of Diesel Surrogate and Large Ester Flames
Diesel Biodiesel (Law, Princeton)
Diesel surrogate: 70% n-C10H22 + 30% 1-methyl naphthalene Dagaut and coworkers (2010) Scientific Questions?
How to address the knowledge gaps in kinetics of large, oxygenated fuel molecules? How can we use quantum chemistry and kinetic experiments to provide a better, predictive model? How to address the transport and chemistry interaction in flames? Research Objectives
Advance the understanding of combustion kinetics of methyl esters Develop a validated kinetic methyl ester kinetic mechanism to model oxidation with quantum chemistry calculations Advance understanding of chemistry/transport interaction 1. Biodiesel Kinetics: Hypothesis
O O O O
O O O O Methyl Formate Methyl Acetate MethylMethyl PropanoatePopanoate Methyl Butanoate O
O Methyl Decanoate
Similarity between Small/Large Esters?
Biodesel Methyl Butanoate Decomposition (C4+1) = + Alkane methyl stearate (C18+1) (C14) 1A. Small Methyl Ester Pyrolysis in Shock Tube Stanford University
1666K 1428K 1250K 1.0 2% Methyl Ester/Argon 0.8 1.5 atm, Yield at 1 ms
0.6
MB MP
0.4
Fractional Yield Yield Fractional
2 CO 0.2 MA
0.0 0.60 0.65 0.70 0.75 0.80 1000/T [K-1] The reactivity is strongly affected by the alkyl chain length 1B. Comparison of Premixed Flame Speeds
of Small Methyl-Esters/Air (C1-C4: 1 atm)
•Methyl formate has the highest reactivity Egolfopoulos et al. •Methyl propanoate is the second 1C. Comparison of Extinction Limits of
Methyl Esters (C1-C10)
Extinction limit vs. Transport weighted enthalpy (TWE) 500
Tf = 500 K, Tox = 298 K
400
[1/s] Methyl Formate E
a Methyl Ethanoate
300 Methyl Propanoate Methyl Butanoate Methyl Pentanoate 200 Methyl Hexanoate Methyl Octanoate Methyl Decanoate
100 Extinction strain rate strain Extinction
0 0.5 1 1.5 2 Transport-Weighted Enthalpy [cal/cm3]
•Uniqueness of small methyl esters: methyl formate & methyl propanoate •Similarity of large methyl esters 1D: BDEs (D298 ) (kcal/mol) in Biodiesel Methyl Butanoate (MB)
MRSDCI /cc-pV∞Z // B3LYP Oyeyemi, V. B.; Pavone, M.; Keith, J. A.; Carter, E. A. CBS-QB3-Isodesmic* in preparation, (2012). * Osmont et al. J. Phys. Chem. A, 111, 3727 (2007) H O H H H
98.0 MB 92.9 96.8 98.6 98.9 94.2 98.7 101.1 H C O C C C C H 101.2 95.4 83.1 85.8 101.3 93.5 84.4 89.1
H H H H Seshadri et al. : 80.8 kcal/mol, 2009
• Weakest bonds: dissociated radicals are resonance stabilized.
• C-C bonds are weaker than C-H bonds: alkyl fragments allow more structural relaxation than H. 1E. Kinetic Mechanism Development
(Ester-MECH: C2-C11 methyl esters)
H2/O 2 . MB: Ester functional group
Dooley et al., 2008
. C1-C7: n-heptane model C -C 1 7 Curran et al., 2008, 2010
. MB Extension H2/O2: PU hydrogen model
C8-C10 Dievart et al., 34th Symposium on Combustion on Comb., 2012
Dievart et al. Combustion and Flame, 2012, Vol.159 , pp. 1795-1803. 1F. Model Validation: Ignition Delay Time
Ignition delays from Hanson’s group (Aerosol Shock Tube, very lean mixtures, diluted in argon, ~7.5 atm) Present model in good agreement (35%), whereas literature models overestimate MD oxidation rate (50 to 80%)
Present model, Seshadri et al’s model: Metathesis reactions: 95% Fuel Decomposition: 5%
Seshadri et al’s model: Metathesis reactions: 55% Fuel Decomposition: 45%
Bond dissociation energy affects strongly fuel decomposition pathway Model validation: JSR & Flame speeds
methyl decanoate
• high temperature kinetics • speciation profiles, flame speeds
70 3000 25000
2500 CO 60
20000
and and
] -1
2000 CO
15000 50
2 Mole Fraction Mole [ppm]
1500
Mole Fraction [ppm] Fraction Mole
4 10000
H 40
2 1000 C
5000 Experimental data 500
30 Reduced model (529 species) Flame speed [cm.s speed Flame MD and MD Reduced model (228 species) 0 0 Seshadri et al.'s model [14] 500 600 700 800 900 1000 1100 Luo et al's model [20] 20 Temperature [K] 0.6 0.8 1.0 1.2 1.4 1.6
Jet-Stirred reactor (Glaude et al., Equivalence Ratio C&F 157, 2010) Laminar Flame Speeds P = 1atm, τ = 1.5 s (Wang et al., C&F 158, 2011) P = 1atm, T = 403 K
Glaude et al., CF, 2010. 14 Model comparison in diffusion flame: MD Model validation: Diffusion flame extinction
500
]
1 -
[s 400
E Methyl pentanoate Methyl hexanoate Methyl octanoate 300 Methyl decanoate Methyl formate Methyl ethanoate 200 Methyl propanoate Methyl butanoate
100 C2-C11 Extinction Strain rate, a rate, Strain Extinction esters
0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35
Fuel Mole Fraction, Xf
Dievart et al., 34th symposium on combustion, 2012 Model validation: Species time history
Methyl formate 3.0E-12
T = 1333 K 2.5E-12 P = 0.51 atm
Xfuel = 239.1 ppm
3] 2.0E-12 -
1.5E-12
1.0E-12
[H] , [mol.cm , [H] 5.0E-13 17
0.0E+00 0.E+00 5.E-04 1.E-03 2.E-03 2.E-03 Time [s] H abstraction reactions by OH and H: Methyl Formate Large deviations between the rate constants calculated by the Carter’s group (J. Phys. Chem. A, 2012) and the previous estimates or calculations.
CH3OCHO + OH = CH3OCO + H2O CH3OCHO + H = CH3OCO + H2
1.0E+13 1.0E+13
]
]
1
1
-
-
.s
.s
3 - 3 1.0E+12
1.0E+12 -
1.0E+11 1.0E+11 Ting Tan (Carter) Ting Tan (Carter) Good and Francisco Good and Francisco Current Model 1.0E+10 1.0E+10 Current Model Akih-Kumgeh
Szilagyi et al. (2004) Peukert et al. (Argonne)
Rate Rate constant[mol cm Rate constant [mol cm [molconstant Rate Peukert et al. 1.0E+09 1.0E+09 300 800 1300 1800 300 500 700 900 1100 1300 1500 1700 1900 Temperature [K] Temperature [K] Good and Francisco, J. Phys. Chem. A, 2002, Vol. 106, pp. 1733-1738 Peukert et al., Combustion and Flame, 2012, Vol. 159, pp. 2312-2323 Akih-Kumgeh and Bergthorson, Comb. Flame, 2011, Vol. 158, pp. 1037-1058 Szilagyi et al., J. Phys. Chem, 2004, Vol. 118, pp. 479-492 Methyl-Ester Radical Decomposition Reactions (Collaborative work : Carter, Klippenstein and Ju)
Decomposition of small methyl ester radicals such as CH3OCO (and C2H5OCO) are key reactions. Literature: only high pressure limit rate constant with low level PES is available (e.g. BH&HLYP/CC-PVTZ). Present method: MRACPF/CBS//CASPT2/CC-PVTZ method on PES and VARIFLEX for pressure dependence
1.0E+13
CH3OCO = CH3 + CO2
] 1
- 1.0E+11
1.0E+09
HPL 1.0E+07 100 atm
Rate constant [s constant Rate 10 atm 1.0E+05 1 atm HPL Huynh and Violi 1.0E+03 0.3 0.8 1.3 1.8 1000/T [K]
L.K. Huynh, A. Violi. J. Org. Chem. 72 (2008) 94-101. Collaborative structure of the Biodiesel
Yang, Raghu, Ju, Carter Klippenstein Hanson group
• Thermochemistry •Rate constants •Rate constants MX+ OH H, Cp, S CH3OCO X=F,A,P,B •Rate constants C2H5OCO MF, ME, MP… MF+X , ME+X, MP+X… Decomposition •Speciation time (OH, H, CH3, HO2) history
Ester-MECH
C2-C11 Esters Egolfopoulos, Ju,
Law Dryer, Hansen and Ju
•Flame speeds •Speciation experiments Sung and Hanson •Flame structure (Flow tube, flames) •Extinction •Ignition delay •Emissions (Shock tube, RCM)
20 Summary: OH + Methyl Esters Products
1429K 1250K 1111K 1000K 909K 833K
Methyl Ester + OH = Products
]
-1 s
-1 1E13
mol
3
[cm
MButanoate MPropanoate
MFormate Methyl Ester + OH Methyl
k MAcetate 1E12 Lines: Modified SAR (SAR x 0.75)
0.7 0.8 0.9 1.0 1.1 1.2 1000/T [1/K]
• Data agree within 25% with Structure Activity Relationship (SAR) estimated rate constants ( the same rate used in the current model).
21 Methyl Formate Decomposition Kinetics
Summary Arrhenius Plot k1: MF → CO + CH3OH 22
Wide T range
Low data scatter
Repeatable
±25% Advanced diagnostics- high pressure reactors at low and intermediate temperatures MBMS/mid-IR with flow reactor/jet stirred reactor
120 2500
100 In air, 60 Torr, 293 K CH4 2000
80 O O [ppm]
H2O 1500 2 60 1000 40
20 500 Measured H Measured Measured CH4Measured [ppm] 0 0 0 500 1000 1500 Calibration mole fraction [ppm] Multipath-IR H2O2 Measurements, DME/O2/He JSR (2 sec, 1 atm (0.02/0.1/0.88)
4000 3500 MBMS Modeling 3000 2500
Herriott cell 2000
reflections 1500
concentration (ppm) 2
O 1000
2 H
Mixing 500 H2O2 0 500 550 600 650 700 750 Temperature (K)
Preheated air Preheated HO ? High pressure, high temperature chamber temperature high pressure, High 2 Fuel 2. Flame Chemistry: Kinetic &Transport Interaction
•Interaction of Transport and Chemistry on Flame Extinction
•Low Temperature Ignition and New Flame Regimes 2A. Diffusion Flame Extinction Limits: From Methyl Formate to Methyl Decanoate
450
Tf = 500 K, Tox = 298 K
[1/s] E a 350 Methyl Formate Methyl Ethanoate Methyl Propanoate Methyl Butanoate 250 Methyl Pentanoate Methyl Hexanoate Methyl Octanoate
150 Methyl Decanoate Extinction strain rate strain Extinction
50 0.05 0.09 0.13 0.17 0.21 0.25 0.29 ΔH MW Fuel mole fraction, Xf comb (kcal/mol) (g/mol) How to separate chemistry from MB -651.6 102.14 transport and fuel heating value? MD -1533.3 186.29 A generic correlation for extinction limit: Transport weighted Enthalpy & radical index Theoretical analysis of Extinction Damkohler number 3 2 1 2YO, 1 Tf T Le3 P( , Le , Le ) L ( , Le ) exp a Da e Y2 FFFOFF T T T T E F, f a f
Extinction Strain Rate
1 YF ,QF ae *Ri M / M Cp (Tf T ) F Fuel chemistry Transport Heat release/heat loss Radical production rate Transport weighted Enthalpy *Radical index Won et al. CNF 159 (2012) Reactivity Scaling of Small/Large Methyl Esters:
From Methyl Formate (C1) to Methyl Decanoate (C10) Extinction limit vs. Transport weighted enthalpy (TWE) flux
500
Tf = 500 K, Tox = 298 K,
400
[1/s] Methyl Formate E
a Methyl Ethanoate 300 Methyl Propanoate Methyl Butanoate Methyl Pentanoate 200 Methyl Hexanoate Methyl Octanoate
100 Methyl Decanoate Extinction strain rate strain Extinction
0 0.5 1 1.5 2 Transport-Weighted Enthalpy [cal/cm3] •Uniqueness of small methyl ester •Similarity of large methyl ester Impact of alkyl chain length on methyl ester reactivity Methyl Formate, R0C Methyl Acetate, R1C
35% 18% 47% 47% +R/-RH +R/-RH +R/-RH
42% +R/-RH CH3OH 5% + CO 9% 95%
38% 62%
81% CH2CO + CH3O CH2O + CH3CO
+H CH3O CH3 56% + + 35% -H CH2O CO CO2 + CH3 + CO 88% 12% HCCO HCO +OH H + CO +M +O2 HO2 + CO +O CO + CO
Higher reactivity Lower reactivity H abstraction reactions, Diévart et al, 2012 Fuel, CH3OCO, and CH3OC(O)CH2 to presented on Monday at 34th decomposition reaction rates Symposium Extinction Limit: n-Alkanes, iso-Alkanes, Aromatics 500 n-decane Tf = 500 K and To = 300 K n-nonane n-alkanes n-heptane JETA POSF 4658
[1/s] 400
E Princeton Surrogate a iso-octane nPB 300 toluene aromatics 124TMB 135TMB
200
100
Extinction strain ratestrainExtinction How to separate chemistry from transport? What is the ranking high temperature reactivity? 0 0 0.05 0.1 0.15 0.2
Fuel mole fraction Xf A General Correlation of Hydrocarbon Fuel Extinction vs. TWE and Radical Index
500 n-decane n-nonane R² = 0.97 n-heptane
[1/s] 400
iso-octane E
a n-propyl benzene toluene 300 1,2,4-trimethly benzene 1,3,5-trimethly benzene
200
100 Extinction strain ratestrainExtinction
Tf = 500 K and To = 300 K 0 0.5 1 1.5 2
-1/2 303 Ri[Fuel]Hc(MWfuel/MWnitrogen) [cal/cm ] Radical Index for Screening of Alternative Fuels
• Extinction limits of diffusion flames for pure fuel samples have been completely measured and compared by using TWE
– Heat of combustion, Hc has been re-estimated based on H/C ratio correlation. – Re-evaluation of Hc might be necessary. • High temperature reactivity based on Radical index – SPK HRJ camelina HRJ Tallow > JP8 IPK (~iso-octane) – Similar order to DCN measurements, IPK must be heavily isomerized.
450 Extinction of diffusion flame in counterflow configuration Radical Tf = 500 K and Tair = 300 K @ 1 atm Fuel DCN
] 400
1 Index -
350 JP8 POSF 6169 0.78 47.3 300
250 SHELL SPK POSF 5729 0.85 58.4
200 JP8 POSF 6169 SHELL SPK POSF 5729 HRJ Camelina POSF 7720 0.82 58.9 HRJ Camelina POSF 7720 150 HRJ Tallow POSF 6308 SASOL IPK POSF 7629 Extinction strain rate [s ratestrainExtinction 100 n-alkane HRJ Tallow POSF 6308 0.8 58.1 iso-octane 50 0.5 1 1.5 2 2.5 SASOL IPK POSF 7629 0.76 31.3 Transport-weighted enthalpy [cal/cm3] -0.5 [fuel]Hc(MWf/MWn)
Won et al. CNF 159 (2012) 2B. Effects of Transport on Low Temperature Ignition in Non-premixed Counterflow Flames Law’s group
Heptane/air tNTC > tconv • NTC behavior extensively observed for flames homogeneous systems • Corresponding non-monotonic behavior signaling NTC chemistry in steady state strained has not been well studied in flows (e.g. counterflow), Seshadri et al., CF 2009. No NTC at 1 atm, 200/s • Reason: Reduced residence time => higher ignition temperature => shifting away tNTC ~ tconv ? from NTC temperature regime
• Explore possible existence of NTC behavior for flows – with low strain rates NTC temp. ↑ as pressure ↑ – at high pressures n-Heptane vs. Air in Counterflow Ignition
Decrease k
Single 2nd ignition, ignition High-T chemistry
High-T Chemistry
1st ignition, Low-T chemistry
Decrease P Increase P
Increase k Single ignition 2nd ign, Low-T chemistry high-T
1st ign, low-T chemistry Unsteady Flow Perturbation on Low Temperature Ignition in Diffusion Flame
6.17 ms at 74 Hz
850 K 30 atm 100 s-1 Rise from 72 to 73 Hz
• No effect on initial RO2 formation,
Reaction 2: RO2 = R’O2H • H2O2 decomposition is delayed by Reaction 3: H2O2 + M = 2OH + M heat loss at high strain rate.
Shan et al., 2012 Multi Flame Regimes in HCCI Ignition n-Heptane: Flame Initiation by a Spark at 40 atm, T=700 K
Movie 1.0 LTI at wall
0.8 Low temperature ignition
0.6 Hot ignition
S =27.5 cm/s Low temperature flame fdominated 0.4 double flame (decoupled) Sf=25.6 m/s
Transition Sf=15.3 cm/s 0.2 Single high temperature High temperature flame flame front dominated double flame (coupled) 0.0 0.000 0.005 0.010 0.015
Location of maximumheat release (cm) Time (s) Ju et al., 33rd symposium on Comb., 2011 Conclusions
Combustion properties, species, and kinetic data methyl esters are experimentally measured by a collective effort.
An updated methyl ester (C2-C11) kinetic mechanism is developed and partially validated.
Large uncertainties in elementary rate constant and species time history.
Flame theory to correlate flame extinction with TWE and radical index. Uniqueness and similarity of high temperature reactivity of methyl esters are demonstrated.
Significant impacts of low temperature ignition on ignition and flame propagation are demonstrated. New flame regimes are identified. Acknowledgement:
Pascale Dievart Sanghee Won Xueliang Yang
Funding support: DOE-BES CEFRC