Figure 7. Extinction strain rate measurements of t-butanol, iso-butene, acetone, and methane compared with the Grana et al. model [3].

A Chemical Kinetic Study of the Alternative Transportation Fuel, tertiary-Butanol

J. K. Lefkowitz*, J.S. Heyne, Y. Ju, F.L. DryerDepartment of Mechanical and Aerospace Engineering, Princeton University, USA 

*jlefkowi@princeton.edu

Princeton University

Mechanical & AerospaceEngineering Department

The First International Workshop on Flame ChemistryJuly 28-29, 2012Warsaw, Poland

Experiment: Counterflow Diffusion Flame

VPFR Results

Experiment: Variable-Pressure Flow Reactor

Motivation The butanol isomers are second generation biofuels under

active investigation by the Combustion Energy Frontier Research Center (CEFRC).

Butanols can be synthesized from lignocellulosic biomass. Butanols have greater energy density, absorb less water, and

evaporate less than ethanol. t-Butanol is already an EPA High-Production Volume (HPV)

chemical, with more than 1 million lbs. produced every year.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 17

12

17

22

27

32

37

t-butanol

n-butanol

iso-butanol

sec-butanol

Volume Fraction Alcohol, mL/mL

DC

N

Background

In blends with n-heptane, Haas et al. [2] found t-butanol to react faster than the other isomers when in blends of less than 80% alcohol, see Figure 3.

They concluded that t-butanol does not participate in the ignition chemistry of n-heptane to the same extent as the other isomers, thus detracting the least from the ignition quality of the alkane.

Figure 3. Derived cetane number (DCN) as a function of blending ratio of the butanol isomers in n-heptane [2].

0.5 0.6 0.7 0.810

100

1000

t-butanol sec-butanol iso-butanol n-butanol

Ign

itio

n D

elay

Tim

e,

s

1000/T, K-1

Figure 2. Ignition delay times as a function of inverse temperature for all the butanol isomers at Φ= 1, 93% Ar.gon , at 1 bar [1].

The ignition delay times of the butanol isomers were measured by Moss et al. [1], and t-butanol was found to have the slowest chemistry, see Figure 2.

They determined that the long ignition delay time was due to the comparatively high activation energy for the decomposition reactions of t-butanol as well as the slow reactions of its primary intermediate, iso-butene.

Flux Analysis

Figure 3. Flux analysis for t-butanol using the Grana et al. model [3]. Blue numbers indicate flow reactor simulations at 775 K, 12.5 atm, and φ =1.0; red numbers indicate diffusion flame simulations at Xf = 0.3, a = 100s-1.

The symmetry of t-butanol allows for only two stable intermediate species: acetone and iso-butene.

At low temperatures, t-butanol was predicted to decompose primarily via hydrogen abstraction/alkyl beta-scission reactions to mostly acetone and methyl radicals.

At high temperatures, the primary decomposition route shifted drastically to the dehydration reaction to form iso-butene and water.

Figure 1. Isomers of butanol. Beginning from the left is n-butanol, sec-butanol, iso-butanol, and t-butanol.

Electric ResistanceHeater

Evaporator

Oxidizer Inlet

Slide Table

Fuel Injector and Mixer/Diffuser

Optical Access Ports

Sample Probe

Wall Heaters

Experimental approach:• Species distributions as a function of initial conditions at a constant residence time.• Near-adiabatic or isothermal (high dilution) operation.• Conditions run for this study

=> P = 12.5 atm=> 500 K < T < 1000 K=> τres = 1.8 s

Figure 4. Princeton Variable-Pressure Flow Reactor (VPFR)Reactor Duct Material: Fused Silica Reactor Section Dia.: 10 cm Pressure: 0.2 - 20 atm Mass Flow rate: 10 - 30 grams/secTemperature: 500- 1200 K Residence Time: 0.015 - 5 sec

650 700 750 800 850 900 950

0

500

1000

1500

Tsam

ple / K

(CH3)3COH C

3H

6

C4H

8 C

2H

4

CH3COCH

3 CH

4

H2O

Mola

r C

once

ntr

atio

n / p

pm

T / K

0

5000

10000

15000

O2 CO

H2O CO

2

700

800

900

1000

1100

1200

Tsample

Figure 5. Flow reactor oxidation of t-butanol/O2/N2 2500/15000/982500 ppm at 12.5 atm and a residence time of 1.8 seconds. Experimental data are symbols and lines are Grana et al. [3] kinetic model computations.

Onset of gas phase chemistry at approximately 775 K. Acetone and methane are found to be the most abundant

intermediate species, indicates that hydrogen abstraction/alkyl radical beta-scission reactions dominate t-butanol decomposition at low temperatures.

Model predicts experiments well in terms of overall species concentrations. However, it is too fast between 800-900 K.

Experimental approach:• Measure chemical species within flames

=> Provide detailed mechanistic information on flame oxidation.• Measure extinction strain rate

=> Fuel reactivity.=> Diffusive-kinetic coupling.

• Conditions run for this study=> P = 1 atm=> Tfuel = 400 K, Toxi = 300 K=> 100 < a < 500 s-1

Heated N2

N2

Laser sheet for LIF

Air

Fuel

PG

Positioning Stage

TC TC

Temperature Controller

PGTC

Mixing Chamber Heater

PG Pressure GaugeTC Thermocouple

Needle Valve

On/Off Valve

Three-way Valve

VacuumMultiple

Port Valve

PG

N2 Bypass

N2 Source

Figure 5. Schematic of counterflow burner at Princeton University

Counterflow Results: Speciation

iso-Butene is found to be the most abundant intermediate species, indicates that dehydration reactions dominate t-butanol decomposition at high temperatures.

Model predicts experiments well in terms of overall species peak locations. However, iso-butene is over-predicted and all profiles are narrower than the measurements.

1 2 3 4 5 6

0

2000

4000

6000

8000

10000

12000

14000 t-butanol/20 iso-butene methane C

3H

6O

Spec

ies

conce

ntr

atio

n, ppm

Distance from the burner, mm

1 2 3 4 5 6

0

2000

4000

6000

8000

10000

12000

14000 acetylene ethylene ethane propylene

Distance from the burner, mm

Counterflow Results: Extinction

5 10 15 200

200

400

600

acetone iso-butene methane t-butanol

Grana model Wang model

Transport-weighted Enthalpy, / J cm-3

Ext

inct

ion

stra

in r

ate,

aE

/ s

-1

0.1 0.2 0.3 0.40

200

400

600

Extinction s

train

rate

, aE

/ s

-1

acetone iso-butene methane t-butanol

Grana model Wang model

Fuel Mole Fraction, XF

t-butanol is found to be less reactive than its primary intermediates, iso-butene and acetone.

Reasons are the large production of water early in the decomposition process, the endothermicity of all the initial decomposition reactions, and the large concentration of methyl radicals (which react slower than other radicals).

Model predicts acetone and methane well, but over-predicts t-butanol and iso-butene. t-Butanol over-prediction is likely due to problems with the iso-butene sub-mechanism.

Figure 6. Speciation profile of the t-butanol diffusion flame, Xf = 0.28, a = 100s-1, compared with the Grana et al. model [3].

[1] J. T. Moss, A. M. Berkowitz, M. A. Oehlschlaeger, J. Biet, V. Warth, P. Glaude, and F. Battin-Leclerc, J. Phys. Chem. A 112 (2008) 10843-10855.

[2] F. M. Haas, M. Chaos, and F. L. Dryer, Combustion and Flame 156 (2009) 2346-2350.

[3] R. Grana, A. Frassoldati, T. Faravelli, U. Niemann, E. Ranzi, R. Seiser, R. Cattolica, and K. Seshadri, Combustion and Flame 157 (2010) 2137-2154.

[4] J. Lefkowitz, J. Heyne, S.H. Won, S. Dooley, H.H. Kim,

F.M. Haas, S. Jahangirian, F.L. Dryer, Y. Ju, 49th AIAA Aerospace Sciences Meeting (2011).

[5] J.K. Lefkowitz, J.S. Heyne, S.H. Won, S. Dooley, H.H. Kim, F.M. Haas, S. Jahangirian, F.L. Dryer, Y. Ju, 49th AIAA Aerospace Sciences Meeting (2011).

[6] J.S. Heyne, J.K. Lefkowitz, S. Dooley, S.H. Won, H.H. Kim, F.M. Haas, S. Jahangirian, Y. Ju, F.L. Dryer, 7th US National Technical Meeting of the Combustion Institute (2011).

References

This material is based upon work supported by the Combustion Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001198.

Acknowledgments