Auto-ignition of Alcohol and Furan based Biofuels -

Modeling, Experiments and Theory

Ravi Fernandes

05.04.2016

Seite 2 von 18

Autoignition data and its reliability for model validation

• Crucial data in auto-ignition /kinetics (examples from alcohols and furans)

• Accuracy and reliability of auto-ignition data and their influence on kinetic models

• Improve the predictive capabilities of kinetic models through good data coupled with goodtheory and good mechanisms

• High pressure (up to 300 bar) combustion data lacking ! … existing models critically fail …

• Data evaluation from experts in their respective fields of expertise is key towards improving predictive models

• PTB is working towards a experimental user facility for chemical kinetics and setting up open databases (evaluated) for thermophysical quantities, kinetics and PI crossections

Shock tube facility

Driven section

11mDriver section

4.5 m

Double-diaphragm chamberPressure gauges

Technical data:

Total length: 15.5 m

Inner diameter: 142 mm

Maximum temperature: 200°C

Maximum working pressure: 1000 bar

Material: stainless steel

Driven volume:170 litres

Photomultiplier

Investigated fuel molecules:

Alcohols, Furans, Esters, Ethers

Oxidizer:

Technical Air (20.5% O2, 79.5% N2)

Conditions:

Φ = 1.0, 10 - 90 bar, 750 - 1700 K

Rapid Compression Machine (RCM) focus on the LTC regime at high pressures

Ignition measurements – validation of comprehensive kinetic models

- Low temperature (600 -1200 K) and high pressure (up to 100 bar)

- Pressure diagnostics, Emission based PMT measurements and GC-MS for chemical composition

- Also provides a validation regime (Temperature overlap) for comparison with Shock tubes

- Thin wire thermocouple measurements planned in addition to laser diagnostics

Usually pressure diagnostics are employed in Shock tubes and RCM to

investigate fuel auto-ignition delays and thereby validate kinetic models

0 2 4 6 8

0

20

40

60

80

100

120

140

160

0

20

40

60

80

100

120

Inte

nsity

[arb

.u]

Pre

ssu

re [

ba

r]

t [ms]

tign

RCMShock tube

Typical measuring times: 1-15 ms Typical measuring times: 100-200 ms

Seite 6 von 18

Sometimes reported ignition delay data can be misleading for modeling !

The ethanol case at high pressures

Reaction rate for C2H5OH + HO2 → H2O2 + R increased by 7 orders of magnitude !

CH

3C

HO

(OO

H)

The CH3CHOH + O2 potential energy surface

0 2 4 6 8 10 12 14 16 18 20 22 24 26

-40

-30

-20

-10

0

10

CH2CHOH + OH

CH

3C

HO

H+

O2

CH3CHOH(OO)

CH3CHO…HO2

CH3CHO + HO2

CH2CHOH(OOH)

CH2CHOH+ HO2

O

E/

kcal

mo

l-1

RQCISD(T)/cc-pVZ // B3LYP/6-311++G(d,p) (COCl)2 + hv(248 nm) 2 Cl + 2 CO

C2H5OH + Cl CH3CHOH + HCl

C2H5OH + Cl CH2CH2OH + HCl

Zador et al, Proc. Comb. Inst. 2008

Detailed chemical kinetic model alone cannot capture the experimental

results well for the ethanol ignition

1,0 1,2

10-1

100

101

Experiments

Aachen model (LTC+ HTC)ig

nitio

n d

ela

y [

ms]

1000/T [1/K]

ethanol = 1

Low temperature pre-ignition effects are strong for ethanol as compared

to n-butanol

n-butanol ethanol

P5 P1P3 P2P4

2

P5 P1P3 P2P4

1

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

3

4

5

6

1

2-6

P5 P1P3 P2P4

2

1

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

3

1

P5 P1P3 P2P4

4

6

5 6

Ethanol, Φ=1, T = 929 K Ethanol, Φ=1, T = 1135 K

Optical investigations for ethanol ignition indicate transition from mild

inhomogenous to strong ignition at low temperatures

Kinetic model combined with pre-ignition effects reconciles all data….

confirms the lack of NTC behavior in ethanol even at the highest pressures

1,0 1,2

10-1

100

101

ethanol = 1

Experiments

Aachen model (LTC+ HTC)

Kinetic model + pre-ignition effects

ign

itio

n d

ela

y [

ms]

1000/T [1/K]

Models can also be misleading

Model X Model Y

Reliable experimental data can be explained

by inaacurate models.

Fast growing number of different models (and their versions)

does not add to the level of confidence.

Slide Courtesy : Nils Hansen

Theory can also be misleading if not done right !?

Seite 14 von 18

IUPAC Task Group on Atmospheric Chemical Kinetic Data Evaluation

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Thanks to these contributors… but whats the future of evaluated databases ?

MET Approach

Modelling Experiments Theory

PTB is taking its first steps to improve the quality of data and assign uncertainty budgets to the data acquired

Furans and Alcohols represent the prime fuel class of the second generation biofuelsshow some interesting combustion chemistry and in some cases have shown „zero –soot“ potential in engine operation.

Advancing new engine concepts aim at High Efficiency and Clean Combustion (HECC)

• Broad range of experimental conditions needed to understand these complexities

Improved new standards (ISO) need to be in place (e.g. MN for Gas engines): EU 20/20/20

• Old standard is based on empirical correlations, 40 years old. Not harmonised

Conventional fuels /Blends still expected to go a long way during the transition period

- Limited understanding on the interaction of bioderived fuel molecules with conventional

fuels

Reliable data for model validation assisted by good theory !

RC

M

flo

w r

eac

tor

Ignition in the range 500 – 1000 K

Kinetics of peroxy radicals

FUEL

ALKYL

PEROXY

R H

R

R O 2

QO OH

O 2QOO H

R 'OOHOH R 'O OH

OHP roducts +

O 2 HO 2 Alkene+

S maller radical (R 1) Alkene

O2

(A1)

(A2)

+OH

Termination

Propagation

B ranching

HYDROPEROXY

KETOHYDROPEROXY

Changing pressure affects collisional stabilization

Isomerization produces chain branching species

sho

ck t

ub

e /

Co

mb

ust

ion

bo

mb H2O2

HO2

C-H

C-C

H + alkene

R’ + alkene

Branching

Diverse tools are needed to explore the chemistry of interest since ignition

is complex network of temperature and pressure dependent reactions

J. Miller, M. Pilling, J. Troe, Proc. Comb. Inst. 30, 2005, 43

Alcohols, Esters and the Furans show striking differences in their

ignition behavior

p = 20 bar p ~ 80 bar

Crucial reactions in Butanol auto-ignition ?

1. J. Zador, R.X. Fernandes, Y.Georgievski, G. Meloni, C.A.Taatjes, J. Miller . Proc. Combust.Inst. 2009, 32, 279

2. R.X. Fernandes, J. Zador, C.A.Taatjes, J. Miller . J. Phys Chem.A in preparation

Low temperature peroxy chemistry of butanol included

(analogous to the Sandia – Low T (400- 1000 K) ethanol model 1,2):

R + O2 RO2

RO2 QOOH

QOOH + O2 OH + OH + R’

R + HO2 OH + OH + aldehyde

RO2 + HO2 OH + O2 + R’

HO2 / H2O2 reactions

- n-butanol + HO2 → H2O2 + R (discrepancy of a factor of 20 in current models)

- high P limit k H2O2 decomposition adopted from a recent studies 3-5 (factor 2)

- n- butanol decomposition is pressure dependent (ab-intio based rates)

- n-butanol + OH reaction ( factor 2 higher; Vasu et al)

3. Hong, Z. K.; Farooq, A.; Barbour, E. A.; Davidson, D. F.; Hanson, R. K. Journal of Physical Chemistry A 2009, 113 (46), 12919

4. Selllevag, S. R.; Georgievskii, Y.; Miller, J. A. Journal of Physical Chemistry A 2009, 113 (16), 4457

5. Troe, J.; Ushakov, V. G. Physical Chemistry Chemical Physics 2008, 10 (26), 3915

Five primary radicals possible for n-butanol and their formation rates are very different

Higher accuracy needed for crucial n-butanol reactions– can theory help ?

n-butanol + HO2 → H2O2 + R

Reliable data assisted with good theory helps !

Oehlschlaeger et alHeufer et alHeufer et alThis workCurran Model

Experimental Data: Furan Fuels

0,6 0,8 1,0 1,2 1,4 1,6

10-1

100

101

102

2,5-DMF, Pexp

= 20 bar

2,5-DMF, Pexp

= 20 bar

2-MF, Pexp

= 20 bar

Furan, Pexp

= 20 bar

2-MTHF, Pexp

= 20 bar

2-MTHF, Pexp

= 20 bar

THF, Pexp

= 20 bar

THF, Pexp

= 20 bar

THF, simulation

t ign [m

s]

1000 / T [1/K]

open symbol : shock tube

filled symbol : RCM

Experimental results on ignition delays show significant differences in

Furan derivatives

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

1

P5 P1P3 P2P4

6

1

2

3

4

5

6

2-MTHF, Φ=1, T = 932 K

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

P5 P1P3 P2P4

1

P5 P1P3 P2P4

6

1

2 5

4

3 6

2,5-DMF, Φ=1, T = 959 K

2-MTHF and 2,5-DMF ignite quite differently at low temperatures

Zador, Taatjes, Fernandes, Progress in Energy and Combustion Science 2010

Reliable theoretical methods are employed to improve accuracy of

reaction rates crucial to Biofuel combustion

Geometry optimization/ frequency calculations with B3LYP/CBSB7

Theoretical Calculations performed for different functional groups, molecular classes guide in developing

NEW & VALIDATED RATE RULES FOR CRUCIAL REACTIONS

Sensitive reaction rates are theoretically determined through ab-initio kinetics

Increasing order of abstraction rates by OH from ring at CBS-QB3 level:

F < MF < DMF < THF < MTHF < DMTHF

Increasing order of abstraction rates by OH and HO2 from CH3 at CSB-QB3 level:

MTHF < DMTHF < MF < DMF (HO2 , OH)

Increasing order of abstraction rates by HO2 from ring at CSB-QB3 level:

F < MF < DMF < DMTHF < MTHF < THF

Calculated H atom abstraction reaction rates from furan, 2-MF and 2,5-DMF

by HO2

To improve the predictive capability of kinetic models, higher accuracy of

certain reaction rates is crucial

0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7

10-2

10-1

100

101

102

ethanol, Pexp

= 40 bar

ethanol, Pexp

= 40 bar

ethanol, simulation

iso-pentanol, Pexp

= 20 bar

n-butanol, Pexp

= 20 bar

n-butanol, Pexp

= 20 bar

n-butanol, simulation

butyl formate, Pexp

= 20 bar

butyl formate, Pexp

= 20 bar

butyl formate, simulation

THF, Pexp

= 20 bar

THF, Pexp

= 20 bar

THF, simulation

t ign [m

s]

1000 / T [1/K]

Unimolecular thermal dissociation of Fuel

H-abstraction from Fuel by HO2 and H

and H2/O2 Chemistry

H-abstraction from Fuel by OH

R + O2, RO2, QOOH Chemistry

Fuel + HO2

Future experiments in Auto-ignition need to focus on “real” fuel mixtures

1019

1020

1021

1022

0

20

40

60

80

100

p dependence of degree of complexation

Deg

ree o

f C

om

ple

xation /

%

[M] / molecule cm-3

A = CH3

CO2

N2

Ar

He

200 300 400 500 600

0

20

40

60

80

100

T dependence of degree of complexation

De

gre

e o

f co

mp

lexa

tio

n / %

T / K

A = CH3, M = Ar

10 bar 100 bar 1000 bar

Energy Transfer (ET) v/s Radical Complex Mechanism

log k

k TOTAL

kRC

kET

log [M]

2

*

2

*

2

*

2

AMA

AAA

AAA

0

0

ET c

k k Mk M F

k k M

222

0

1 MTK

MTKkMkMk

eq

eq

RCRC

RC

2

2 2

A M AM

AM A A M

AM AM A M

↔

3

0

5.1

0 /45

32

3

8

]][[

kT

kT

ε

MA

AMKeq

Radical Complex Mechanism

Energy Transfer- Mechanism

Earlier experiments (from Göttingen) indicate the role of an unusual

“Radical Complex Mechanism” being operative at very high pressures

1018

1019

1020

1021

1022

2

4

6

8

10

0.1 1 10 100 1000

300 K

k 1 / 1

0-1

1 c

m3 m

ole

cule

-1 s

-1

[M] / molecule cm-3

Ar

N2

CO2

He

SF6

CF3H

CF4

p (Ar) / bar

Luther et al. , PCCP, 6 (2004) 4133

1E18 1E19 1E20 1E21 1E22

2

4

6

8

10

12

14

k1 / 1

0-1

1 c

m3 m

ole

cule

-1 s

-1

[M] / molecule cm-3

[M] = CO2

[M] = Ar

[M] = He

C. Lee , PhD thesis, Göttingen

p-fluorobenzyl radicalsbenzyl radical recombination

1021

1022

10-12

10-11

kdiff

p (N2) at 300 K / bar

300 K N

2

Ar

~ 315 K; CO2

k1 /

c

m3 m

ole

cule

-1 s

-1

[M] / molecule cm-3

10 100 1000

Fernandes et al, JPC 2010

CH3 + O2 (+N2) CH3O2 (+N2)

Page 30

Molecular beam sampling

IonizationIonization chamber

1. Stage chamber

Burner chamber

Burner

Flame

Page 31

Multiple isomers at exact mass

Distinction by ionization energies and

photoionization efficiency curves

A total of 6 species can be

identified at m/z = 58,

glyoxal

n- and iso-butane

Identifying flame components

F.N. Egolfopolus, N. Hansen et al. PECS 2014

Page 32

Challenges:

Number of possible isomers increases with molecular size

Smaller differences in heats of formation and similar structural features result in almost

identical IE‘s and indistinguishable PIE curves

IE‘s and PIE curves may not be known and need to be measured/calculated

Identifying the Flame Components

F.N. Egolfopolus, N. Hansen et al. PECS 2014

Page 33

Status Quo

• Availability is scarce for higher mass species

• Calculations

– possible with reasonable accuracy

– no dedicated projects

• Measurements

– Beamtime use Efficiency

– Only most pressing project specific needs addressed

Page 34

Futher Considerations

• Conformers can have a wide range of Ionization Energies Moshammer et. al. JPCA

2015

– Whole PIE needed for identification

• PEPICO used in larger extent for flame analysis

– Reference PEPICO Spectra needed for full benefit

K. Moshammer, A. Jasper et al. JPCA 2014

Page 35

Status Quo

• Documentation and usability

– Scattered in supplements

– Assembled in “individual” “databases”

– Even IEs scattered over NIST Webbook and outdated

– NSRL snapshot literature content 2011

• Updated since then

• No cross validation

• Known linking errors

Page 36

Moving Forward

• Express Community Demand

• Identify greatest needs

• Identify and acquire funding sources in the metrology community

• Acquire dedicated Beamtime

• Validate

– Ring experiments (Beamline/Machine Characteristics)

– Standardize procedures

Page 37

Moving Forward

• Identify other Areas of need for Metrology support in the Combustion Community

Integrated approach of experiments & theory and modeling is the key and the advancements in our understanding of combustion – from fundamentals to applications – will play a critical role in meeting the challenges for Clean and efficient combustion of the 21st

century transportation fuels

Shock tube & Rapid Compression Machine are good experimental tools and will continue to contribute towards investigations in combustion. Data interpretation is however tricky !

„Raw data“ sharing will become essential to improve our understanding in combustion

chemistry

High pressure combustion (kinetics) will be important to improve our understanding on fuel

and pressure effects

Interactions of novel fuel molecules with conventional fuel molecules in blends will be

important and would need further studies

Improved diagnostics are needed to unravel the mechanisms for varied experimental conditions

PTB is working towards open science and open databases for kinetics, thermophysical quantities and PIMS

Uncertainty analysis and rate data evaluation will be the key towards our future projects

Outlook:

Acknowledgements

Physikalisch-Technische Bundesanstalt

Braunschweig und Berlin

Bundesallee 100

38116 Braunschweig

Ravi Fernandes

Thermophysical Quantities

Telefon: 0531 592-3300

E-Mail: ravi.fernandes@ptb.de

www.ptb.de

Stand: 02/16

Thank you !