The Decomposition of Surrogate Fuel ... - kinetics.nist.gov
Transcript of The Decomposition of Surrogate Fuel ... - kinetics.nist.gov
Physical and Chemical Properties Division
The Decomposition of Surrogate Fuel Molecules DuringCombustion
Wing Tsang, Jeffrey A Manion, Iftikhar Awan, W. Sean McGivern and James A. Walker National Institute of Standards and Technolgy
Gaithersburg, MD 20899
MULTI AGENCY COORDINATION COMMITTEEFOR COMBUSTION RESEARCH (MACCCR)
FUELS RESEARCH REVIEWGaithersburg MD8-10 September
Work carried out with the support of the AFOSR (Julian Tishkoff) and BES/DOE (Frank Tully)
Physical and Chemical Properties Division
RATIONALE
Advances in CFD codes have led to the possibility of simulating the behavior of real fuels in real devices
To enable this technology there is the need for an appropriate chemical kinetics database
Special problems with real fuels
size and types of molecules
mixtures
Leading to new design tools
Physical and Chemical Properties Division
FUEL PROBLEM
Real fuels are complex mixtures of hydrocarbons
What are the mixing rules for mixtures of varying compositions
particularly important with current interest on alternative fuels
More fundamentally based approach makes mixing rules more transparent
Much current interest in surrogate mixtures
Physical and Chemical Properties Division
TYPICAL SURROGATE JP-8 MIXTURE
Compounds Mole Percentage
methylcyclohexane 5%
cyclooctane 5%
Butylbenzene 5%
Tetralin 5%
Meta-xylene 5%
1,2,4,5-tetramethylbenzene 5%
1-methylnaphthalene 5%
Isooctane 5%
Decane 15%
Dodecane 20%
Tetradecabe 15%
Hexadecane 10%
Investigators are convinced that combustion properties can be captured with limited number of compounds representative of groups
in fuels
Physical and Chemical Properties Division
Mechanism For Fuel Breakdown During Combustion Fuel
Radical attack
Fuel Radical 1-olefin + smaller alkyl radical
Radical O2 attack
1-olefinyl H, methylEthylenePropene
Fuel –O2 allyl dienes
Olefin + HO2
QOOH β-bond scission
Cyclic Ether + OH O2
O2QOOH
Ketohydroperoxide +OH
Fuel =>
Fuel Radical =>
Smaller Radical + 1-olefin =>
1-olefinyl radical =>
olefins, diene and cyclic radicals.
Physical and Chemical Properties Division
PROBLEMS IN CURRENT DATABASES
Incomplete with respect to soot and cracking
•missing reactions
•wrong rate constants
Neglect of energy transfer effect
•Cannot describe reaction in Arrhenius form
•Rate constant may be pressure as well as temperature dependent
•Improper description of chemical activation processes
Competitive with oxidation
Formation of precursors to soot
Neglected in most databasesDetermine consequence
for multichannel reactions with low
thresholds
Most mechanisms (oxidation) are based on single component fuels.
Due to necessity for “tuning mechanisms” a mechanism for a mixture cannot be a
composite of single fuel data bases
Physical and Chemical Properties Division
Kinetics Modules in Databases
GRIMECH - methane (light hydrocarbon) combustion
Pyrolysis of fuels
Oxidation of larger fuels
Soot formation
Current NIST Experimental and Data
Evaluation ProgramTarget of most models
next target in NIST program
Widely used, problems with rich
mixtures
Many models, begin with unsaturates
7
Physical and Chemical Properties Division
SPECIAL ROLE OFUNIMOLECULAR REACTIONS
Only unimolecular and bimolecular reactions in database
Bimolecular reactions shuffle atoms
OH +RH = H2 O +R*
Rate constants are the same for similar groupings
Unimolecular reactions reduces size of molecule
Reduces fuel to small unsaturated and radicals found inexisting databases
Present work will focus on this category of processes
Physical and Chemical Properties Division
PREMISE OF PRESENT WORK
There are simple correlations between structure of fuel molecules and their rate constants for decomposition and isomerization
These correlations can be uncovered from an
examination of the literature
suitable experiments
theoretical treatments
FOR A HOMOLOGOUS SERIES DATA BASE FOR EVERY LARGE FUEL MOLECULE CONTAINS AS A SUBSET THE
DATABASE FOR SMALLER FUEL MOLECULES
Physical and Chemical Properties Division
DETERMINATION OF MECHANISMS AND RATE CONSTANTS
Need for clearly defined experimental systems
understand the role of interfering processes
Ideal conditions most easily realizable in shock tubes
well defined boundary conditions
very little possibility of surface reactions
short reaction times
Physical and Chemical Properties Division
E xpan sion fan
In ciden t sh ock
R eflec ted shock
D istan ce
Tim
e
In terface
L ow pressureH igh pressure
D iaphragm
Tim
e
P ressu re T em p eratu re
E xpan sion fan
P artic le pa th
S am pling port
D um p tank
2
1
3
4
5
Single Pulse Shock Tube and Associated Wave Processes
500 microsecs pulse reactor
Physical and Chemical Properties Division
RATE CONSTANTS FOR BOND BREAKING REACTIONS FROM FUEL MOLECULES
Fuel molecule in dilute solutions
Chemical inhibitor to capture active radicals
Internal standard to determine reaction conditions
Physical and Chemical Properties Division
Log A-factor15.6 15.8 16.0 16.2 16.4 16.6 16.8
Num
ber o
f com
poun
ds
0
2
4
6
8
10
12
14
16
18
C
alkanes
alkenes
EXPERIMENTAL A-FACTORS ON FUEL MOLECULE DECOMPOSITION FROM SINGLE PULSE SHOCK TUBE
STUDIES AT 1050 K
When combined with activation energies lead to present day accepted bond energies
Physical and Chemical Properties Division
CALCULATIONAL TRAIN
Molecular Properties (molecules and transition states)
Partition Functions
Thermodynamic properties
Equilibrium constants
Rate constants (equilibrium, thermal)
Pressure dependent rate constant
Chemical Activation processes
Physical and Chemical Properties Division
Use bond energies of 412 kJ/mol for secondary C-H bond, 420 kJ/mol for primary C-H bond and 50 kJ/mol for allyl resonance energy
ESTIMATION OF THERMODYNAMIC PROPERTIESALKANES
follow Pitzer on frequencies from addition of CH2 into alkane structure
rotational constants from standard bond lengths and angles
barriers to internal rotation 3.4 kcal
small adjustment of skeletal bends to reproduce entropies in standard tables
ALKYL RADICALS
follow Benson: remove three frequencies characteristic of H motion
barrier of internal rotations adjacent to radical site lowered to zero
Energies characteristic of primary and secondary C-H bond: difference of 2.5 kcal/mol
Physical and Chemical Properties DivisionP h ys ic a l a n d C h e m ic a l P ro p e rtie s D iv i s io n
TYPICAL WINDOW FOR MASTER EQUATION SOLVER (CHEMRATE)
Physical and Chemical Properties Division
RATE CONSTANTS FOR DODECANE DECOMPOSITION
1000/T0.5 0.6 0.7 0.8 0.9
Log
k
0
2
4
6
1000/T0.5 0.6 0.7 0.8 0.9
0
2
4
6
Log
k
per Cn-Cn bond for n=2 to 10
per C-C11 bond
100 BAR
10 BAR
1 BAR
100 BAR
10 BAR
1 BAR
HPHP
Physical and Chemical Properties Division
0.5 0.6 0.7 0.8 0.90
2
4
6
0.5 0.6 0.7 0.8 0.9Lo
g [K
]
0
2
4
6
1000/T1000/T
Log
[K]
100 bar
10 bar
1 bar
100 bar
10 bar
1 bar
hp hp
per Cn-Cn bond for n =2 to 5 per C-C6 Bond
RATE CONSTANTS FOR HEPTANE DECOMPOSITION
Physical and Chemical Properties Division
DETERMINING MECHANISMS AND RATES OF DECOMPOSITION OF FUEL RADICALS
Generate fuel radicals through decomposition of appropriate precursors; alkyl iodide, branched hydrocarbons
Carry out studies in single pulse shock tube
dilute mixtures
short residence times
presence of inhibitors to isolate reaction
obtain direct measure of branching ratios
thermal cracking patterns
Convert to high pressure rate expressions
Extension to cover all combustion conditions
Solution of the master equation to take into account energy transfer effects
Physical and Chemical Properties Division
ESTIMATING RATE EXPRESSIONS FOR UNIMOLECULAR REACTIONS OF FUEL RADICALS
Begin with experimental measurements
direct studies
rate constants for reverse reaction
chemical activation studies
Convert to high pressure rate expressions
Expend to cover all relevant pressures as well as temperatures
Many reactions, few reaction type s
beta bond scissions
localized
Isomerizations: Hydrogen transfer
involves entire molecule
Cyclizations
H*
*
*
Physical and Chemical Properties Division
RADICALS STUDIED (initial reactant)
C H3CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2CH2CH2*
CH2=CHCH2CH2CH2*
CH2=CHCHCH2CH2CH2*
CYCLOHEXYL
CYCLOPENTYL
CH3CH(CH3)CH2CH2CH2*
CH3CH(CH3)CH2CH2CH2CH2*
Includes all isomerization products
From 1-olefin fuels and products from alkyl radical decomposition
From cyclic fuels and 1- olefinyl radical isomerization
Effects of methyl substitution (Fischer-Tropsch fuels)
Physical and Chemical Properties Division
2 -C 6 H 1 3
1 - C 6 H 1 3
3 - C 6 H 1 3
C 3 H 6
C 3 H 7
C 2 H 4
1 -C 4 H 9
1 -C 5 H 1 0
C H 3
1 -C 4 H 8
C 2 H 5
C 2 H 4
C H 3
C 2 H 4
C 2 H 5
C 2 H 4
H
C 2 H 4
H
MECHANISMS AND BRANCHING RATIOS FOR THE DECOMPOSITION OF HEXYL RADICALS
Physical and Chemical Properties Division
1 2 3 5 4 3 2 1C C C C C C C C
*CH2(CH2 )6CH3 (1-octyl)
CH3CH*(CH2)5 CH3 (2-octyl) CH3CH2CH*(CH2)4CH3(3-octyl)
CH3(CH2)2CH*(CH2)3CH3 (4-octyl) CH3(CH2)2CH*(CH2)3CH3 (4-octyl)
1,2 (8) 1,3 (7)
1,5 (5) 1,4 (6)
2,3 (6)
2,5 (5) CH2=CH2 + 1-C6H11
1-C5H11 + CH2=CHCH3
CH3 + 1-C7H141-C4H8 + 1-C4H9
1-C6H12 + C2H51-C5H10+ 1-C3H7
MECHANISM AND BRANCHING RATIOS FOR THE DECOMPOSITION OF OCTYL RADICALS
4 isomers undergoing 6 beta bond scissions and 6 reversible isomerizations
1000/T1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14
Log
[ole
fin b
ranc
hing
ratio
]
-2.0
-1.8
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0C2H4
C3H6(square)
C7H14-1
C4H8-1
C6H12-1(diamond) C5H10-1(circle}
Physical and Chemical Properties Division
1000/T1.0 1.5 2.0 2.5 3.0
Log 1
0k(d
ec) s
-1
-4
-2
0
2
4
6
8
Jitariu et al
Knyazev and Slage (dotted lines)n-butyl decompCalc. from propyl +ethyl (dash line) Kerr and ParsonageCalc. from butyl +ethyl (solid line) Kerr and Parsonage
1000/T1.0 1.5 2.0 2.5 3.0
Log
k(is
om) s
-1
0
1
2
3
4
5
6
7
8
Dobe et al
Watkins
Shock tube results extrapolated from 900-1000K
Curran et al
shock tube results and tunneling with width of 1.15A(top) and 1.5 A (bottom) for Eckart barrier
SUMMARY OF LITERATURE DATA PERTINENT TO THE UNIMOLECULAR REACTIONS OF FUEL RADICALS
Beta bond scissions 1-5 H-atom transfer isomerization
Physical and Chemical Properties Division
DETERMINATION OF RATE EXPRESSIONS FROM THERMAL CRACKING PATTERNS
Begin with smallest fuel radicals – beta bond scissions from direct experimental results from decomposition and reverse addition process at
lower temperatures
Use data on isomerization involving smaller radicals as guide to prediction for same process involving larger radicals.
1-butyl does not isomerize
1-pentyl can only undergo 1-4 hydrogen transfer
1-hexyl can undergo 1-4, 1-5 isomerizations
1-heptyl can undergo 1-4,1-5,1-6,2-5 isomrizations
KEY CONCLUSIONS: SIMILAR TYPE OF ISOMERIZATIONS HAVE EQUIVALENT RATE EXPRESSIONS
ISOMERIZATIONS INVOLVING 6-MEMBER TRANSITION STATE ARE SO FAST THAT LARGER TRANSITION STATE BECOMES
UNIMPORTANT IN LARGE MOLECULES
Physical and Chemical Properties Division
Reaction Log A N Activation energyE/R
1-C7 H15 = C2 H4 + 1-C5 H11 11.90 .33 13694
2-C7 H15 = C3 H6 + 1-C4 H9 11.70 .56 14138
3-C7 H15 = 1-C4 H8 + 1-C3 H7 12.47 .31 14221
3-C7 H15 = 1-C6 H12 + CH3 11.04 .75 14797
4-C7 H15 = 1-C5 H10 + C2 H5 12.77 .31 14221
3-C7 H15 = 1-C7 H15 2.87 2.43 6441
4-C7 H15 = 1-C7 H15 2.10 2.88 9884
2-C7 H15 = 1-C7 H15 1.52 2.81 7561
3-C7 H15 = 2-C7 H15 2.30 2.83 9048
1-C7 H15 = 3-C7 H15 2.83 2.39 5237
1-C7 H15 = 4-C7 H15 2.07 2.85 8680
1-C7 H15 = 2-C7 H15 2.39 2.51 6292
2-C7 H15 = 3-C7 H15 1.39 3.09 9113
HIGH PRESSURE RATE EXPRESSIONS FOR UNIMOLECULAR REACTIONS INVOLVING HEPTYL
RADICALS
Physical and Chemical Properties Division
T [K]600 800 1000 1200 1400 1600 1800
Log
[kin
f/k]
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6starting radical1-heptyl:red2-heptyl:dark green3-hepty: blue4-heptyl: black
reaction1-heptyl =C2H4+nC5H7: solid 2-heptyl = C3H6+nC4H9 long dash 3-heptyl = 1-C4H8+nC3H7 short dash3-heptyl = 1-C6H12+CH3 dot 4-heptyl = 1-C5H10+C2H5dash dot dot dash
DEPARTURE FROM HIGH PRESSURE BEHAVIOR FOR BETA- BOND SCISSION REACTIONS OF HEPTYL RADICALS
Physical and Chemical Properties Division
Temp [K]600 800 1000 1200 1400 1600 1800
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Log
[ kim
f/k ]
1-C4H9
1-C5H11
1-C6H13
1-C7H15
1-C8H17
FALL-OFF BEHAVIOR FOR THE REACTION N-ALKYL = ETHYLENE + 1-(N-2)OLEFIN FOR N=4 TO 8 AT 1 BAR
Fall-off does not monotonically increase due to
low reaction threshold
Fall off effects decreases only slowly with molecular size
Fall off effects are insensitive to molecular size near high
pressure limit
Physical and Chemical Properties Division
SUMMARY
Unimolecular rate constants on the cracking of larger fuel compounds and radicals have been determined on the basis of past work and present experiments
It is now possible to describe the pyrolytic decomposition of fuel molecules individually or in mixtures
Form basis for studying mechanisms and rate constants for oxidation processes
Need for systematic exploration of chemically activated decomposition in oxidative and soot formation processes