the oxidation of fuel radicals
TRANSCRIPT
Physical and Chemical Properties Division
The Oxidation of Fuel Radicals
Wing TsangNational Institute of Standards and Technology
Gaithersburg, MD 20899
Multi Agency Coordination Committee for Combustion Research [MACCCR]
Fuel Research ReviewLos Angeles, California
September 16, 2009
* + O2
O-O*ISOMERIZATION
PATHWAYS
Physical and Chemical Properties Division
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
Physical and Chemical Properties Division
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
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
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
RADICALS STUDIED
C H3CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2CH2*
CH3CH2CH2CH2CH2CH2CH2CH2*
CH2=CHCH2CH2CH2*
CH2=CHCHCH2CH2CH2*
CYCLOHEXYL
CYCLOPENTYL
CH3CH(CH3)CH2CH2CH2*
CH3CH(CH3)CH2CH2CH2CH2*
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
..
. .
.
+
+ .+
.+CH3.
+.
.
+ .
+
+CH3
CH3
.
i-C4H8
2-C6H12-cis
2-C6H12-trans
3-CH3-C4H7-1
k1
k2
k3
k4k5
k6
k7k8
k9
-k9
k10
-k10
k11-k11k12-k12
k13
-k13
1-C6H12
5MeH-6 5MeH-1 5MeH-4
5MeH-2 5MeH-5.
Figure 1. Alkene yields from the decomposition of 5-methylhexylradicals. Symbols are experimental data: ◆ = ethene; ○ = isobutene; ■ =propene; □ = 3-methylbutene; △ = cis-2-hexene; ▲ = trans-2-hexene; ◇ = 1-hexene. The values for 1-hexene have been divided by two for clarity. Thelines represent fits from the RRKM/Master Equation model described inthe text.
C2H4iC4H8
C3H6
3CH3C4H7
c--C6H12-2
t-C6H12-2
C6H12-1
MECHANISM AND BRANCHING RATIOS FOR REACTIONS INITIATED WITH 5-METHYL HEXYL RADICALS
Physical and Chemical Properties Division
MASTER EQUATION SOLVER: PROGRAM TO DETERMINE MOLECULAR DISTRIBUTION FUNCTION
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
A : R eactio n L o g A n E /R L o g (k [1 0 0 0 ])1 1 -o c tyl = e thene + 1 -hex yl 1 1 .9 6 .3 1 1 3 7 08 6 .9 32 2 -o c tyl = p ro p ene + 1 -p en ty l 1 0 .7 8 .8 4 1 4 0 01 7 .2 43 3 -o c tyl = 1 -b u tene + 1 -b u ty l 1 3 .3 1 .0 4 1 4 3 40 7 .2 14 3 -o c tyl = 1 -hep tene + m eth yl 9 .9 8 1 .0 8 1 4 7 90 6 .7 85 4 -o c tyl= 1 -p en tene + 1 -p ro p yl 1 1 .7 4 .5 5 1 4 1 34 7 .2 66 4 -o c tyl = 1 -hexe ne + eth yl 9 .8 3 1 .1 1 1 3 6 00 7 .2 57 1 -o c tyl = 4 -o c ty l (1 -4 -H tra ns) .7 1 3 .2 3 8 4 7 9 6 .7 28 1 -o c tyl = 4 -o c ty l (1 -5 -H tra ns) 1 .3 6 2 .8 2 5 4 1 3 7 .4 69 1 -o c tyl = 3 -o c ty l .4 7 3 .0 8 5 5 4 4 7 .2 91 0 4 -o c tyl = 1 o c tyl 1 -4 -H tran s -.3 8 3 .5 7 9 5 3 2 6 .2 01 1 4 -o c tyl = 1 -o c ty l 1 -5 -H trans .2 7 3 .1 6 6 4 6 6 6 .9 41 2 3 -o c tyl = 1 -o c ty l .5 2 3 .1 1 6 5 7 9 6 .9 91 3 2 -o c tyl= 3 -o c ty l .2 7 3 .2 7 6 6 4 2 7 .2 01 4 2 -o c tyl = 4 -o cty l .1 5 3 .3 2 8 1 2 5 6 .6 01 5 3 -o c tyl = 2 -o c ty l 1 .3 3 2 .9 6 6 6 2 5 7 .3 41 6 4 -o c tyl = 2 -o c ty l .0 7 1 3 .3 2 8 1 2 8 6 .5 2C o m p o u nd E q uilib riu m co nstan t o f fo rm atio n lo g [k p]O ctyl-1 -3 8 .9 3 1 -1 .0 31 x1 0 3 /T + 4 .9 0 4 x1 0 6/T 2-2 .6 01 x1 0 9/T 3+ 4 .47 3 x1 0 11/T 4
O ctyl-2 -3 9 .1 9 6 -1 .9 41 x1 0 2 /T + 4 .7 3 6 x1 0 6/T 2-2 .5 54 x1 0 9/T 3+ 4 .40 3 x1 0 11/T 4
O ctyl-3 -3 9 .0 9 8 -5 .6 33 x1 0 2/T + 4 .91 2 x1 0 6/T 2-2 .6 08 x1 0 9/T 3+ 4 .48 5 x1 0 1 1/T 4
HIGH PRESSURE RATE EXPRESSIONS FOR OCTYL RADICAL DECOMPOSITION AND ISOMERIZATION
Physical and Chemical Properties Division
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)
Physical and Chemical Properties Division
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
Physical and Chemical Properties Division
1000/T0.95 1.00 1.05 1.10 1.15 1.20
Log[
prod
uct b
ranc
hing
ratio
]
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
SUMMARY OF EXPERIMENTAL RESULTS
cyan =ethylene
Red = 1-butene
Blue = propene
Green = tetrahydrofuran
Circle =150, 9.15%
Square = 150, 4.04 %
Triangle = 150, 1.51%
White (blue) = 300, 10.35%
Hexagon = 600, 10.15%
n-Butyl Iodide (100 ppm) decomposition with various concentrations of oxygen
Physical and Chemical Properties Division
Rate Constants from Chemical Activation Processes
Radical + O2 , <=> RadicalOO* => Variety of products
RadicalOO
M
Physical and Chemical Properties Division
500 K, 0.1 bar
Log[time,s]-10 -8 -6 -4 -2 0
Log
[bra
nchi
ng ra
tio, p
rodu
cts]
-8
-6
-4
-2
0
500 K, 100 bar
Log [time,s]-10 -8 -6 -4 -2 0
Log
[bra
nchi
ng ra
tio, p
rodu
cts]
-8
-6
-4
-2
0
CH3CH2CH2OO *=>CH3CH2CH2*+O2
CH3CH2CH2OO*
*CH2CH2CH2OOH =>Oxetane + OH
CH3CH*CH2OOH => propene + HO2
*CH2CH2CH2OOH => Ethylene + *CH2OOH
CH3CH*CH2OOH => methyloxirane + OH
CH3CH2CH2OO * => propanal + OH
CH3CH2CH2OO => propene + HO2
CH3CH2CH2OO*
CH3CH2CH2OO *=>CH3CH2CH2*+O2
CH3CH2CH2OO => propene + HO2
*CH2CH2CH2OOH =>Oxetane + OH
*CH2CH2CH2OOH => Ethylene + *CH2OOH
CH3CH*CH2OOH => methyloxirane + OH
CH3CH*CH2OOH => propene + HO2
CH3CH2CH2OO * => propanal + OH
*CH2CH2CH2OOH
*CH2CH2CH2OOH
CH3CH*CH2OOH
CH3CH*CH2OOH
Branching Ratios for Products from n-Propyl Peroxy Radical Decomposition
Physical and Chemical Properties Division
TREATMENT OF DATA
Product (ethylene. Propene, 1-butene) = kuni,BI(butyl-I) + kuni,P(butyl) + kox.P(butyl) x O2
tetrahydrofuran (THF) = kox,T(butyl) x O2
Product/THF = ( kuni,BI(butyl-I) + kuni,P(butyl))/kox,T(butyl) x O2
+
kox.P/ kox,T
Physical and Chemical Properties Division
CH3CH2CH2CH2 + O2 CH3CH2CH2CH2OO CH3CH2CH=CH2+HO2
CH2CH2CH2CH2OOH CH3CHCH2CH2OOH
oHO + C2H4 + CH2CH2OOH C2H4 + HO2
C3H6+ CH2OOH
CH2O + OH
Oxidative Decomposition of n-Butyl Radical
CH3CH2CH2CH2ICH3CH2CH=CH2 +HI CH3CH2CH2CH2
CH3CH2 + C2H4
C2H4 + H
Pyrolytic Decomposition of n-Butyl Radical (through n-Butyl Iodide)
Physical and Chemical Properties Division
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
Physical and Chemical Properties Division
ETHYLENE/THF RATIOS AS A FUNCTION OF TEMPERATURE AND OXYGEN CONCENTRATION
Physical and Chemical Properties Division
1000/T0.90 0.95 1.00 1.05 1.10 1.15 1.20
Log
[1-C
4H8/
THF
]
1.0
1.5
2.0
2.5
300,10.35%
600,10.15%
150,9.16%
150, 1.54%
150,4.04%
1-BUTENE/THF RATIOS AS A FUNCTION OF TEMPERATURE AND OXYGEN CONCENTRATION
Physical and Chemical Properties Division
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)
Physical and Chemical Properties Division
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
Physical and Chemical Properties Division
1/O 2 [m ol/cc]0 1e+5 2e+5 3e+5 4e+5 5e+5
C 3
H6/
THF
0
10
20
30
40
50
60
1050
1020
990
960
930
900
870
Log [1-C4H8/THF] = 4.717 -3422/T
1-BUTENE/THF Yields as a Function of 1/O2: Extrapolation to Infinite Oxygen Concentration
Physical and Chemical Properties Division
1000/T [K]0.8 0.9 1.0 1.1 1.2 1.3 1.4
Log
[ole
fin/T
HF]
-1
0
1
2 C2H4
1-C4H8C3H6
OLEFIN VS TETRAHYDROFRAN YIELDS
Physical and Chemical Properties Division
MECHANISTIC INFERENCES
Ethylene and propene are formed from beta bond scissions after H-transfer isomerization
1-Butene is formed prior to isomerization
No formation of larger aldehydes
Contribution from oxirane (3) and oxetane (4) channels are neglible
Physical and Chemical Properties Division
TASKS AND PROBLEMS
Molecular properties of oxygenates: hydroperoxides and peroxy radicals depend on ab initio calculations
Fit results through solution of master equation using CHEMRATE
Expand data to cover all relevant combustion conditions differentiate between chemical activation and thermal processing
Larger radicals: n-pentyl
Lower temperatures: smaller oxygen loading
Use nitrites
Physical and Chemical Properties Division
SUMMARY
A new method for determining the mechanisms and rate constants for the oxidation of alkyl radicals has been developed
Results have been demonstrated for n-butyl radicals generated from n-butyl-iodide pyrolysis in varying concentrations of oxygen and excesses
of a chemical inhibitor
Ethylene is the main product, smaller amounts of 1-butene, propene and THF are also detected
Results for limiting oxygen concentrations have been determined