The Oxidation of Fuel Radicals

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


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


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


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


SINGLE PULSE SHOCK TUBE AND ASSOCIATED WAVE PROCESSES


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}


..

. .

.

+

+ .+

.+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


MASTER EQUATION SOLVER: PROGRAM TO DETERMINE MOLECULAR DISTRIBUTION FUNCTION


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


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


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)


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


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


Rate Constants from Chemical Activation Processes

Radical + O2 , <=> RadicalOO* => Variety of products

RadicalOO

M


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


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


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)


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


ETHYLENE/THF RATIOS AS A FUNCTION OF TEMPERATURE AND OXYGEN CONCENTRATION


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


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)


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


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


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


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


NEXT STEPS

Derive high pressure rate expressions


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


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


ACKNOWLEDGEMENT

This work was carried out by the Resl Fuels Group at NIST

Iftikhar Awan, SeanMcGivern, Jaffrey Manion and Wing Tsang

Partial support is provided by the

AFOSR (Julian Tishkoff)

BES/DOE (Frank Tully and Wade Sisk )

Documents

The Oxidation of Fuel Radicals