The Oxidation of Fuel Radicals - NIST...The Oxidation of Fuel Radicals Wing Tsang, Iftikhar Awan and...
Transcript of The Oxidation of Fuel Radicals - NIST...The Oxidation of Fuel Radicals Wing Tsang, Iftikhar Awan and...
The Oxidation of Fuel Radicals
Wing Tsang, Iftikhar Awan and Jeffrey A. ManionNational Institute of Standards and Technology
Gaithersburg, MD 20899
Multi Agency Coordination Committee for Combustion Research [MACCCR]
Fuel Summit; Princeton, New JerseySeptember 20, 2010
Supported by the AFOSR (Julian Tishkoff)
AIM OF WORK
Develop a quantitative understanding of the processes responsible for the degradation of real fuels during combustion
By building a database of fundamental and transferable information base of rate expressions and thermodynamic and physical properties of
reactants and intermediates
Providing information necessary for the simulation of combustion processes and hence serve as a complement to physical testing
APPROACH
Combine existing literature (experiments and theory) with experiments in single pulse shock tube
Highlights importance of fundamental approach
permits interpolation and extrapolation
use of data in mixtures
calibration of theory
Focus on the decomposition and isomerization reactions of fuel and fuel radicals characteristic of real fuel mixtures
pyrolysis (C,H)
oxidation (C, H, O)
PREMISE OF 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
Kinetics Modules in Databases
GRIMECH - methane (light hydrocarbon) combustion
Pyrolysis of fuels
Oxidation of larger fuels
Soot formation
Recent NIST experimental and data
evaluation programTarget of most models Focus of current
NIST work
Widely used Problems with rich
mixtures
Many models, begin with unsaturates
7
SPECIAL ROLE OF UNIMOLECULAR REACTIONS
Bimolecular reactions shuffle atomsOH + RH = H2O + R*
Rate constants are the same for similar groupings
Only unimolecular and bimolecular reactions in simulation database
Unimolecular reactions reduces size of molecule
Reduces fuel to small unsaturates and radical found in existing databases
Unimolecular rate expressions are fundamental properties of any polyatomic molecule and hence should be calculable
How accurate are the calculations?
GENERIC TYPES OF UNIMOLECULAR REACTIONS
Reaction from Boltzmann Distribution
limiting high pressure rate constant, no pressure dependence
High pressure rate expresion; All experimental accessible properties of transition state
Reaction from truncated Boltzmann Distribution: reaction from higher energy levels faster than equilibration by collisions
Pressure dependence of rate constants “fall-off”
Reaction from arbitrary initial distribution function: Chemical activation
Branching ratios for reactions and stabilization: pressure dependence
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
Detection of practically all stable products
(particularly suitable for large molecules)
SINGLE PULSE SHOCK TUBE AND ASSOCIATED WAVE PROCESSES
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
PAST WORK: 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
RADICALS STUDIED
C H3CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2CH2CH2*
CH2=CHCH2CH2CH2*
CH2=CHCHCH2CH2CH2*
CYCLOHEXYL
CYCLOPENTYL
CH3CH(CH3)CH2CH2CH2*
CH3CH(CH3)CH2CH2CH2CH2*
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}
OXIDATION OF FUEL RADICALS: PAST WORKNo direct studies leading to high pressure unimolecular rate expressions
Products from flame and cool flames sampling
Oxygenated organics
Unsaturated organics
Cyclic ethers
Thermal rate constants used in models; no consideration of chemical activation processes
OH and HO2 from smaller fuel molecules and modeling
Sandia (Taatjes)
Ab-initio calculations
Green et al
Merle et al
Bozzellie et al
Sandia (Klippenstein)
Rate Constants from Chemical Activation Processes
Radical + O2 , <=> RadicalOO* => Variety of products
RadicalOO
M
STRATEGY FOR STUDYING RADICAL OXIDATION
Generate radicals as in pyrolysis experiments
Dilute concentrations of radicals
Vast excess of chemical inhibitor
Add sufficient amount of oxygen molecules to change reaction direction from pyrolysis to oxidation
Determine cracking patterns as function of oxygen concentration
Reproduce oxidative cracking pattern through solution of the master equation for chemical activation process
SOURCES OF n-BUTYL RADICALS
n-butyl-I = n-butyl + I
but potential interfering process n-butyl-I = a-butene + HI
general source of radicals for pyrolysis experiments
custom synthesis
N-pentyl-O-NO = n-pentoxy + NO
n-pentoxy = n-butyl + H2CO
potential interfering process: isomerization and decomposition processes of pentoxy
home synthesis
CH3CH2CH2CH2 + O2 CH3CH2CH2CH2OO CH3CH2CH=CH2+H
CH2CH2CH2CH2OOH CH3CHCH2CH2OOH
oHO + C2H4 + CH2CH2OOH C2H4 + HO2
C3H6+ CH2OOH
CH2O + O
Oxidative Decomposition of n-Butyl Radical
CH3CH2CH2CH2ICH3CH2CH=CH2 +HI CH3CH2CH2CH2
CH3CH2 + C2H4
C2H4 + H
Pyrolytic Decomposition of n-Butyl Radical (through n-Butyl Iodide)
8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 0 1 0 5 0 1 1 0 0
LOG
[MO
L-FR
ACTI
ON
]
-1 .0
-0 .5
0 .0
0 .5
1 .0
8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 0 1 0 5 0 1 1 0 0
-3 .0
-2 .5
-2 .0
-1 .5
-1 .0
-0 .5
8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 0 1 0 5 0 1 1 0 0
-2 .5
-2 .0
-1 .5
-1 .0
-0 .5
0 .0
1 50 to rr O % O 21 50 to rr 1 .54 % O 2 1 50 to rr 4 .07 % O 21 50 to rr 9 .21 % O 23 00 to rr 1 0 .3 5% O 26 00 to rr 1 0 .1 5% O 2
8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 0 1 0 5 0 1 1 0 0
-3 .0
-2 .5
-2 .0
-1 .5
-1 .0
8 0 0 8 5 0 9 0 0 9 5 0 1 0 0 0 1 0 5 0 1 1 0 0-3 .5
-3 .0
-2 .5-2 .0
-1 .5
-1 .0
-0 .5
LOG
[MO
L-FR
ACTI
ON
]
LOG
[MO
L-FR
ACTI
ON
] LO
G [M
OL-
FRAC
TIO
N]
E th yle n e
P ro p e n e
1 -B u te n e
B u tan a l
T H F
T em p [K } T e m p [K }
T e m p [K } T e m p [K }
T e m p [K }
PRODUCT YIELDS FROM THE DECOMPOSITION OF 100 PPM IODOBUTANE IN 1% MESITYLENE
1 0 0 0 /T0 .9 0 0 .9 5 1 .0 0 1 .0 5 1 .1 0 1 .1 5 1 .2 0
(pro
duct
s+re
acta
nt] fi
nal/r
eact
ant in
ital
0 .7
0 .8
0 .9
1 .0
1 .1
n o o x y g e n 1 5 0 to rr in it ia l p re s s u re9 .1 5 % o x y g e n 1 5 0 to rr in it ia l p re s s u re1 0 .1 5 % o x y g e n 6 0 0 to rr in it ia l p re s s u re1 0 .3 5 % o x y g e n 3 0 0 to rr in it ia l p re s s u re
a ll m ix tu re s c o n ta in in g 1 0 0 p p m n -b u ty l- I a n d 1 % m e s ity le n e
MASS BALANCE
850 900 950 1000 1050
Log
[mol
e-fr
actio
n]
-1 .0
-0.5
0.0
0.5
1.0
850 900 950 1000 1050
-2.0
-1.5
-1.0
-0.5
0.0
850 900 950 1000 1050-2.5
-2.0
-1.5
-1.0
-0.5
850 900 950 1000 1050
-2.5
-2.0
-1.5
-1.0
850 900 950 1000 1050
-3.0
-2.5
-2.0
-1.5
-1.0
150 torr 0% O 2150 torr 1 .25% O 2150 torr 3 .13% O 2150 torr 6 .05% O 2150 torr 10.1% O 2150 torr 22.6% O 2
Ethylene
Log
[mol
e-fr
actio
n]Lo
g [m
ole-
fract
ion]
Log
[mol
e-fr
actio
n]Lo
g [m
ole-
frac
tion]
Tem p [K ]
Propene
1-B utene
B utanal
TH F
Tem p [K ] Tem p [K ]
PRODUCT DISTRIBUTION FROM THE DECOMPOSITION OF N-PENTYL NITRITE 100 ppm IN 1% MESITYLENE
ETHYLENE/THF RATIOS AS A FUNCTION OF TEMPERATURE AND OXYGEN CONCENTRATION
1/O2 [ mol/cc]0 1e+5 2e+5 3e+5 4e+5 5e+5
C2H
4/TH
F
0
100
200
300
400
500
1050
1020
990
960
930900
870
Ethylene/THF Yields as a Function of 1/O2: Extrapolation to Infinite Oxygen Concentration
Log [C2H4/(2xTHF)] = 7.35 - 6000/T =1.1 at 930 K
1000 /T0 .90 0 .95 1 .00 1 .05 1 .10 1 .15 1 .20
Log[
C3H
6/TH
F]
0 .0
0 .5
1 .0
1 .5
2 .0
150 , 1 .54%
150 .4 .04%
150 , 9 .15%300 ,10 .35% 600 .10 .15%
PROPENE/THF RATIOS AS A FUNCTION OF TEMPERATURE AND OXYGEN CONCENTRATION
Log [C3H6/THF] = 2.92 -2120/T= .64 (930K)
THEORETICAL BASIS FOR PRESENT WORK
Need for fundamental molecular properties of molecules, intermediates and transition states
None of the required information is available from experiments
Reliance on ab initio calculations
Bozzilli
Hadad
Green
Uncertainties
< 1.5 kcals/mol for molecules using isodesmic reactions
no tests for transition states
MASTER EQUATION SOLVER: PROGRAM TO DETERMINE MOLECULAR DISTRIBUTION FUNCTION
RELEVANT RATE CONSTANTS RESPONSIBLE FOR PRODUCT YIELDS
1-Butene propene ethylene
Log [time.s]-12 -10 -8 -6 -4
Log
[bra
nchi
ng ra
tio]
-6
-5
-4
-3
-2
-1
0n-butyl + O2
Ethylene + CH2CH2OOH
Propene + CH2OOH
1-Butene + OOH
THF + OH
1-Butanal + OH
C4H9OO
1-C4H9OO
2-C4H9OO
3-C4H9OO
CALCULATED BRANCHING RATIOS FOR THE CHEMICALLY ACTIVATED DECOMPOSITION OF PROPYLPEROXY
RADICALS: 900 K, 3 BAR
Log [time,s]-12 -10 -8 -6 -4
Log
[bra
nchi
ng ra
tio]
-5
-4
-3
-2
-1
0nC4H9 + O2
Ethylene +CH2CH2OOHPropene +CH2OOH
1-Butene +HO2
OH + THF
1-Butanal +OH
BRANCHING RATIOS WITH SOME RATE EXPRESSIONS ADJUSTED TO FIT OBSERVATIONS 900 k 3 BAR
SUMMARY
Single pulse shock tube has been used to study the oxidation of n-butyl radical
In contrast to the pyrolytic situation many new channels are opened
The necessity of treating the reactions as chemical activation processes
The problems involved in converting results to fundamental unimolecular high pressure rate expressions are desc ribed
Pyrolysis decomposition extends to regions of high oxygen concentration and lower temperatures