Combustion Kinetics (3) - NCCRDnccrd.in/ICIWSIndia2015-assets/docs/Ranzi3.pdf · Combustion...

Winter Combustion School IIT Madras December 2015

Combustion Kinetics (3)a Complex Hydrocarbon Mixtures

(Automatic generation and Lumping procedures)

b Combustion of practical fuels (Surrogates and Renewable Fuels).

Eliseo Ranzi

Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”Politecnico di Milano


Outline 2

3.b Combustion of practical fuels (diesel, gasoline, jet fuels) Surrogates, Oxy/bio Alternative Fuels (Syngas/FT).

3.a Pyrolysis and combustion of complex hydrocarbon mixtures n-alkanes (n-heptane, n-decane, ..)Reaction classes and automatic generationLumping procedures and reduction techniques


3Detailed Oxidation Mechanism of n-pentane

E.Ranzi, T.Faravelli, P.Gaffuri, G.Pennati “Low Temperature combustion: Automatic generation of primary oxidation reactions and Lumping Procedures” Combust. Flame 102: 179-192. 1995.

Combustion of large molecules

Complex kinetic mechanisms.

Alkyl radicals decompose or forms Peroxy radicals

Succesive reactionsof Peroxy Radicals explainLow Temperature reactivity


4

J.F.Griffiths, J.A.Barnard ‘Flame and Combustion’ Blackie Academic London 1994

GC distribution of alkanes in a liquid fraction

Liquid fuels are Complex Hydrocarbon Mixtures

Liquid fuels are mostly constituted by complex mixtures of large

hydrocarbons derived from refinery

Typical composition of a kerosene


Primary Hydrocarbon Molecules 5

CnH2n+2

CnH2n

CnH2n-6

CnH2n

CnH2n-12


6Detailed Oxidation Mechanism of n-pentane

Pyrolysis Mechanism

Pyrolysis reactions hierarchically preceed oxidation reactions.

E.Ranzi, T.Faravelli, P.Gaffuri, G.Pennati “Low Temperature combustion: Automatic generation of primary oxidation reactions and Lumping Procedures” Combust. Flame 102: 179-192. 1995.

Combustion of large molecules

Complex kinetic mechanisms.


7High temperature Reactions of n-pentane

At High Temperatures, life time of alkyl radicals is lower than 10-6 -10-7 s.

Decomposition and dehydrogenation reactions of alkyl radicals

kDEC = 1013.5 * exp[(-32000 )/RT] [s-1]kDeHyd= 1014 * exp[(-40000 )/RT] [s-1]


High Temperature mechanism mainly involves interactions amongstsmall and stable radicals (H, CH3, C2H3, C3H3, …)and small stable species such as C2H4 and C2H2

as well as oxigenated species ( O2, O, OH, HO2, …..)

Alkyl-radicals

Alkanes

Alkenes

Small radicals

High Temperature Oxidation MechanismDecomposition of Large Molecules

High Temperature mechanism isnot very sensitive to the

structure of the hydrocarbon fuel

8

High temperature mechanism is simply constituted by pyrolysis reactions.

Only then, oxidation reactions of small olefins and radicals take place.


9High temperature Reactions of n-pentane

At High Temperatures, life time of alkyl radicals is lower than 10-6 -10-7 s.

Decomposition and dehydrogenation reactions of alkyl radicals

kDEC = 1013.5 * exp[(-32000 )/RT] [s-1]kDeHyd= 1014 * exp[(-40000 )/RT] [s-1]

High Temperature oxidation mechanism first involves chain initiation and H-abstraction reactions.

Then, alkyl radicals isomerize and decompose.

H-abstraction reactions form alkyl radicals


10

Activation energy E is the dissociation energy of the C-C bonds (BDE)

n-C4H10 ↔ C2H5 + C2H5

k= A * exp(-E/RT) [s-1]

Initiation Reactions

Radicals

Activation energy of Radical Recombination is ~0.

kInC4H10= Kref (Cs-Cs) ~ .5 1017× exp(-82000/RT) [1/s]


Bond Dissociation Energies 11

http://www.kshitij-iitjee.com/Thermodynamics

BDE depends on the type of C-atoms

Dehydrogenation are more difficult than pyrolysis reactions.


12

n-paraffins Cp-Cs 84. kcal/mol

Cs-Cs 82. kcal/mol

Bond Dissociation Energies

iso-paraffins Ct-Cs 80. kcal/mol

Cq-Cp 80. kcal/mol


Dissociation of Alkenesforms Allyl Resonantly Stabilized Radicals

13

olefins Callyl-Cp 72. kcal/mol

CH2=CHCH2CH3 CH2=CHCH2● + ●CH3

CH2=CH-CH2● ●CH2-CH=CH2

1-C4H8


H-abstraction reactions 14

The BDE of the C-H bonds depends on the type of the H atoms:- Primary C-CH3 (e.g. Ethane) BDE ~ 98 kcal/mole- Secondary C-CH2-C (e.g. Propane) BDE ~ 95 kcal/mole- Tertiary (C)3-CH (e.g. isobutane) BDE ~ 92 kcal/mole- CH4 BDE ~ 103 kcal/mole

Tertiary H atoms (BDE=92) are easier to be removed, with respect to secondary (BDE=95) and primary ones (BDE=98). The kinetic rates are very similar for all the alkyl radicals.

Only a few reference kinetic parameters allows to describe the primary reactions of all the hydrocarbons.

Rate constants for the abstraction of a single H-atomReference rate parameters [l/mole/s]

kH= 1010.25 exp(-10500/RT) kOH= 109.5 exp(-3500/RT) kCH3= 108.5 exp(-11500/RT)


15H-abstraction reactions

Similarly, allyl H atoms (BDE=86) are easily removed (dashed lines),while it is very difficult to remove vinyl H atoms (BDE=107) .

- Secondary (e.g. Propane) BDE ~ 95 kcal/mole- Vinyl (e.g. ethylene) BDE ~ 107 kcal/mole- Allyl (e.g. propylene) BDE ~ 86 kcal/mole

Rate constants for the abstraction of a single H-atom


H-abstraction of a single Primary H atomby a Primary Alkyl radical kref= 108.3*exp(-13500/RT) [l/mole/s]

H-Abstraction Reactions Reference Kinetic Parameters

R + RH + 6 primary H atoms

k=6 108.3*exp(-13500/RT) [l/mole/s]

R + RH + 4 secondary H atoms

Correction for H-sites - Sec./prim. exp (2300/RT) E= 13500-2300

k=4 108.3*exp(-11200/RT) [l/mole/s]

Correction for H-sites - Tert./prim. exp (4500/RT)

These corrections reflect the BDE of the different H-sites.

At 1000 K, secondary H atoms are removed ~3 time faster than primary H atoms

tertiary H atoms are removed ~ 9-10 time faster than primary H atoms


Selectivities of primary products from linear and branched alkanes

17

E.Ranzi, M.Dente, S.Pierucci, G.Biardi "Initial product distributions from pyrolisis of normal and branched paraffins" Ind.Eng.Chem. Fundam, 22, 132 (1983).

Chain propagation dominate over the chain initiation reactions

Chain initiation reactions form the radical pool

H-abstraction reactions rule the product distributions(reaction lenght of 10-20)


Chain Initiation Reactions4-methyl-heptane

18

+ CH3

+ CH3

+

+

kDECC4H10= KREF= .5 1017× exp(-82000/RT) [1/s]

k2= 2 x .5 1017× exp(-82000/RT) [1/s]Csec-Csec

k1= 2 x .5 1017× exp(-80500/RT) [1/s]Ctert-Csec

k3= 2 x . 5 1017× exp(-84500/RT) [1/s]Csec-CCH3

k4= .5 1017× exp(-83500/RT) [1/s]Ctert-CCH3


Selectivities of primary productsfrom linear 4-methyl-heptane

9 + 24 + 9


Selectivities of primary productsfrom linear 4-methyl-heptane

Including also isomerization reactions, it is possible (SSA) to derive:


21

21

Internal H-abstraction: 2-methyl-pentanelog A E

[s-1] [kcal/kmol]

10.2 14500

11.0 19800

(1-5) H transfer

H HH

(six membered ring intermediate)

Difference in activation energy reflects the strain of the five membered ring.Difference in frequency factor is due to the # rotors blocked in the transition phase.

Isomerization Reactions

Kossiakoff, A., & Rice, F. O. (1943). Thermal Decomposition of Hydrocarbons, Resonance Stabilization and Isomerization of Free Radicals. Journal of the American Chemical Society, 65(4), 590-595.

(1-4) H transfer

(five membered ring intermediate)H H

H


Decomposition and Isomerization Reactionsof Large Alkyl Radicals

kDEC = 1 1014 * exp(-30000/RT) [1/s]

kISOM=3 1010.2*exp(-14500/RT) [1/s]

H

kISOM

kDEC

H

At Temperatures higher than 1000 K decomposition prevails on isomerization reactions

Kinetic constants vs 1000/T [K]

kDEC

kISOM


23H-Abstraction Reactions on n-dodecane

The Six nC12H25 Radicalscan isomerize and/or decompose

High T oxidation mechanism (Pyrolysis) require to define the kinetic parameters of:

- Initiation reactions- H-abstraction- Isomerization- Decomposition Reactions


Reference Kinetic Parameters 24

E.Ranzi, M.Dente, S.Pierucci, G.Biardi "Initial product distributions from pyrolisisof normal and branched paraffins“ Ind. Eng. Chem. Fundam., 22, 132 (1983).

Reference Kinetic Parameters are known since several years.

Reference Kinetic Parameters mainly depends on

- the type of radicals - the type of H

Winter Combustion School IIT Madras December 2015Winter Combustion School IIT Madras December 2015

Intrinsic Reference Kinetic Parameters 25

H-Abstraction Reactions Primary H atom Secondary H atom Tertiary H atom Primary radical 108.0 exp (-13.5/RT) 108.0 exp (-11.2/RT) 108.0 exp (-9/RT) Secondary radical 108.0 exp (-14.5/RT) 108.0 exp (-12.2/RT) 108.0 exp (-10/RT) Tertiary radical 108.0 exp (-15/RT) 108.0 exp (-12.7/RT) 108.0 exp (-10.5/RT) Isomerization Reactions (Transfer of a Primary H-atom) 1-4 H Transfer 1-5 H Transfer 1-6 H Transfer Primary radical 1011.0 exp (-20.6/RT) 1010.2 exp (-14.5/RT) 109.7 exp (-14.5/RT) Alkyl Radical Decomposition Reactions to form Primary Radicals Primary radical Secondary radical Tertiary radical 1014.0 exp (-30/RT) 1014.0 exp (-31/RT) 1014.0 exp (-31.5/RT)

Corrections in Activation Energy to form: Methyl radical Secondary radical Tertiary radical

+ 2. - 2. - 3.

Pyrolysis reactions

Similar kinetic parameters can be found in:CK Westbrook et al. "A comprehensive detailed chemical kinetic reaction mechanism for combustion of n-alkane hydrocarbons from n-octane to n-hexadecane." Combustion and Flame 156.1 (2009): 181-199.

Dente, M., Bozzano, G., Faravelli, T., Marongiu, A., Pierucci, S., & Ranzi, E. (2007). Kinetic modelling of pyrolysis processes in gas and condensed phase. Advances in chemical engineering, 32, 51-166.


Automatic generation of Kinetic Scheme

AUTOMATIC GENERATION OF

Primary elementary reactions

Detailed Reaction Scheme

Classes of reactions

1. H abstraction Reactions2. isomerization Reactions R R’3. Decomposition of alkyl radicals

R → CnH2n+R’

Reference kinetic parameters•H-Abstraction Reactions (Primary H-Atoms)

log A E- Primary radical 8.3 13500 - Secondary radical 8.3 14500 - Tertiary radical 8.3 15000

•Isomerization Reactions(Primary on primary internal H-abstraction)

- (1-5) H Transfer 10.2 14500 - (1-4) H Transfer 11.0 19800•Decomposition Reactions

(to form Primary Radicals)- Primary radical 14 30000- Secondary radical 14 31000- Tertiary radical 14 32000


27Automatic Generation of Detailed Reaction Schemes

Primary propagation reactions of n-dodecane pyrolysis(Units are: m kmol s kcal.)

E. Ranzi, A. Frassoldati, S. Granata, and T. Faravelli ‘Wide-Range Kinetic Modeling Study of the Pyrolysis, Partial Oxidation, and Combustion of Heavy n-Alkanes’ Ind. Eng. Chem. Res. 2005, 44, 5170-5183

β-decomposition reactions

H-abstraction reactions

A E


28

Isomerization (H-transfer) reactions A E

Automatic Generation of Detailed Reaction Schemes

Primary propagation reactions of n-dodecane pyrolysis(Units are: m kmol s kcal.)

Dimension of these detailed kinetic schemes calls for simplifications.

It is not of great interest to generate detailed mechanisms with thousands of species and reactions.

A compromise has to be found between computation efforts and prediction accuracy.


Automatic generation of Lumped ReactionsClasses of reactions

1. H abstraction Reactions2. isomerization Reactions R R’3. Decomposition of alkyl radicals R → CnH2n+R’

Reference kinetic parameters•H-Abstraction Reactions (Primary H-Atoms)

log A E- Primary radical 8.3 13500 - Secondary radical 8.3 14500 - Tertiary radical 8.3 15000

•Isomerization Reactions(Primary on primary internal H-abstraction)

- (1-5) H Transfer 10.2 14500 - (1-4) H Transfer 11.0 19800•Decomposition Reactions

(to form Primary Radicals)- Primary radical 14 30000- Secondary radical 14 31000- Tertiary radical 14 32000

MAMA Program1-Generation of Primary Reactions

2- QSS Assumption for Large Alkyl Radicals

3- Generation of Lumped Reactions

It is convenient to directly link a post-processor to the kinetic generator with the purpose of lumping

intermediate and final products into a more limited number of lumped components.


Steady State Approximation (SSA) and ‘lumped reactions’ (at 1040K)

30

Intermediate alkyl radicals larger than C4 are linearly transformed into their final products.(μ-radicals QSSA: isomerization and decomposition)

β-decompositions and isomerizations of large alkyl radicals are lumped into a single equivalent reaction.

At high temperatures, interactions of large alkyl radicals with the reacting mixture

(Additions and H-Abstractions) are negligible

Large Alkyl Radicals (Rj ) , initially formed at rate Pj , are involved in

Decomposition ( kD) and Isomerization ( kI) Reactions.

Continuity equations of isomer radicals give rise to a system of linear equations:

Disappearance Rate = Formation Rate


3131H-abstraction reactions on n-octane

Once the distribution of octyl radical (and their decomposition products) is obtained, similar linear systems for hexyl and pentyl radicals are solved.

Thus, the products distribution of an equivalent or "lumped reaction" is obtained.

kIj,i kD

j

Steady-state approximation (SSA) of the 4 octyl radicals means that radical disappearance (kD+kI) must be equal to their formation(P). The following linear system is obtained:

is the initial formation, via H abstraction

kDj is the total rate constant of decomposition

kIj,i are the rate constant of isomerizations Rj•(8) ↔ Ri•(8)

Disappearance = Formation


Lumped Pyrolysis Mechanism of n-decaneIntermediate radicals (larger than C4) are transformed into their final products (QSSA).

The linear system of continuity equations (SSA) of the five

nC10H21 radicals gives the first decomposition path.

Successive decomposition reactions of large radicals are then analized


On the basis of detailed kinetics and SSA of large Alkyl Radicals, it is possible to generate ‘lumped reactions’

Reduction of # of species and # of reactions

Lumped Reactions of n-decane

R• + nC10H22 RH + {mixC10H21•}

{mixC10H21•} .0205 H• + .0803 CH3• + .2593 C2H5• + .4061 nC3H7• + .2339 1C4H9•+ .3785 C2H4 + .3127 C3H6 + .2114 1- C4H8 + .1870 1-C5H10 + .1815 1-C6H12

+ .1461 1-C7H14 +.1284 1-C8H16 + .0540 1-C9H18 + .0025 1-C10H20

+ .0006 2-C5H10 + .0012 C6H12s + .0013 C7H14s + .0005 C8H16s + .0100 C10H20s

These stoichiometries, i.e.thedecomposition products of large radicals, are evaluated at a given

temperature ( T=1040 K).

At low temperatures (T<900 K), alkyl radicals also add on oxygen to form peroxyl radicals, before decomposition.

Other reactions need to be included.

Large alkyl radicals are directly transformed into their decomposition/isomerization products, with a large

reduction of the total number of species.


‘Lumped Reactions’ are generated at a fixed Temperature (@ 1040K)

34

Intermediate radicals larger than C4 are transformed (QSSA - isomerized and decomposed) into their final products.Lumped H-abstraction Reactions on large molecules become:

[mixC12H25] .0226 H + .0735 CH3 + .2518 C2H5 + .4283 1C3H7 + .2238 1C4H9+ .4529 C2H4 + .2936 C3H6 + .1935 1C4H8 + .1857 1C5H10 + .00054 2C5H10

+ .2056 1C6H12 + .00091 2C6H12 + .00023 3C6H12 + .1352 1C7H14 + .00121 C7H14s + .1179 1C8H16 + .00088 C8H16s + .1057 1C9H18 + .00081 C9H18s + .1002 1C10H20+ .00042 C10H20s + .04506 1C11H22 + .00194 1C12H24 + .01005 C12H24s

R + nC12H26 RH + [mix C12H25]

Similarly, in pyrolysis conditions or at high temperatures, decyl radicals decomposes:

[mixC10H21] .0205 H• + .0803 CH3• + .2593 C2H5• + .4061 nC3H7• + .2339 1C4H9•+ .3785 C2H4 + .3127 C3H6 + .2114 1- C4H8 + .1870 1-C5H10 + .1815 1-C6H12 + .1461 1-C7H14 +.1284 1-C8H16 + .0540 1-C9H18 + .0025 1-C10H20 + .0006 2-C5H10 + .0012 C6H12s + .0013 C7H14s + .0005 C8H16s + .0100 C10H20s

Significant reduction of both species and reactions, but …… weak temperature dependence Acceptale deviations


Vertical lumping of homologous species 35

The similarity of n-decane and n-dodecane pyrolysis suggests some simplification.A limited number of reference components can represent intermediate species.This lumping technique can be applied to homologous species (e.g. alkanes, alkenes,..)

Thus, n-tri-decane (C13H28) can be considered as 75% n-dodecane and 25% of n-cetane.

C12H26

C13H28

C16H34

C14H30

C15H32

3/4

1/4

C5H10

C6H12

C7H14 1/21/2

Similarly, 1-hexene is equally splittedbetween 1-pentene and 1-heptene.

The net result is a significant reduction of the total number of species


STEAM CRACKING OF HYDROCARBONS

Operating Conditions

•Temperature 900-1150 K•Pressure: 1.5-2.5 bar•Contact time: 100-400 ms

Feedstocks

•Ethane and gases E/P/C4

•Naphthas (C4-C10)•Gasoils (up to C40s)

M.Dente, E.Ranzi "Mathematical modelling of pyrolisis reactions" in "Pyrolysis: Theory and Industrial Practice" Chap. 7 (L.F.Albright, B.L.Crines, W.H.Corcoran Eds), Academic Press (1983).

HydrocarbonsSteam

C2H4C3H6ButadieneBTX

Pyrolysis Coils inConventional Furnaces

36

The interest is an accurate prediction of alkene selectivities

(i.e. a correct characterization of the pyrolysis mechanism.)


Distribution of components CnH2n-zNumber of C atoms vs. Z (dehydrogenation degree)

SPYRO 2000

Z

C ATOMS0 10 20 30 40 50

0

10

20

30

40

50

60

0 10 20 30 40 500 10 20 30 40 500 10 20 30 40 500 10 20 30 40 500 10 20 30 40 500 10 20 30 40 50

7654321

N- and iso-Paraffins

Alkyl-Benzenes

Alkyl-PhenanthrenesAlkyl-Naphtalenes

H/C =1

H/C=0.5

37

Only 240 molecular and radical speciescharacterize the pyrolysis system.

Horizontal Lumping: Isomers are grouped in ‘equivalent’ components

Vertical Lumping: Homologous species are distributed between ‘equivalent’ components.

C22 is considered as 60% C20 and 40% of C25

SSA: lumped reactions of large alkyl radicals


38

Complexity of the Liquid Feedstocks: Naphthas, Kerosene, and Gasoils

Altgelt and Boduszynski 1994

Carbon Number

Boiling Temperature

[°C]

Number of Paraffin Isomers

Petroleum Fraction

8 126 18 Gasoline and Naphthas10 174 75 Kerosene12 216 355 Jet Fuels15 271 4347 Diesel Fuels20 344 3.66 105 Light Gasoil25 402 3.67 107 Gasoil30 449 4.11 109 Heavy Gasoil35 489 4.93 1011 Atmospheric Residue


Alkyl-radicals

Alkanes

Alkenes

Small radicals

High Temperature Oxidation MechanismDecomposition of Large Molecules

High Temperature mechanism mainly involves interactions amongstsmall and stable radicals (H, CH3, C2H3, C3H3, …)and small stable species such as C2H4 and C2H2

as well as oxigenated species ( O2, O, OH, HO2, …..)

High Temperature mechanism is not very sensitive to the structure of the hydrocarbon fuel

39


Low Temperature Oxidation Mechanism

Low Temperature oxidation mechanism requiresto define new reaction classes

40


Reaction Paths of nC7H16 oxidation 41

nC H 7 16 X +

R7OO

OOQ7OOH

1.00

0.79 0.04

0.17

0.79 0.19

0.26

0.03

Q7OOH

R7 -decomposition products

β

Coniugate olefins

nC H 7 16 X +

R7OO

OOQ7OOH

OQ7OOH

Eterocycles

branching products

1.00

0.98 0.02

0.00

0.98 0.01

0.14

0.03 0.80

0.75 0.05

2 + HO

Q7OOH

R7

0.63

0.31

0.03

0.03

T = 620 K Conversion 54.5 %

T = 820 K Conversion 77.6 %

OQ7OOH

-decomposition products

β


β

Coniugate olefins

Eterocycles


β

branching products

Ranzi, E., Gaffuri, P., Faravelli, T., & Dagaut, P. (1995). A wide-range modeling study of n-heptane oxidation. Comb. Flame, 103(1), 91-106.


Low and High Temperature ReactionsAt high temperatures, the alkyl radical R decomposes, producing olefin and smaller alkyl

radicals. H+O2 O + OH is the dominant chain branching reaction.

42

These are the New Reaction Classes and they need their

Reference Kinetic Parameters

At lower temperatures, O2 adds to the alkyl radicals:R + O2 ↔ RO2

The equilibrium constant is strongly temperature dependent and is in favor of RO2 at low T, shifting toward R + O2 as T increases. The “ceiling temperature” is the temperature above which this equilibrium favors the dissociation path.


Explosion Diagrams: C3H8/O2 mixture

43

SLOW COMBUSTION

DELAYED TWO STAGEIGNITION

PRESSURE (mmHg)0 200 400 600 800

EXPLOSION

High Temperature Mechanism

Low Temperature Mechanism

The Ceiling Temperature R + O2 ↔ RO2

rules the transition betweenLow and High T Mechanisms


Explosion DiagramnC4H10/O2

Low Temperature Mechanism

High Temperature Mechanism


45Simplified Scheme of n-alkane (nC10H22)Primary Oxidation Reactions

Alkyl radicals forms Peroxy radicals

Succesive reactions of these radicals explain

the system reactivity

Peroxy radicals isomerize to form Alkyl-hydroperoxy radicals


Reaction Classes 46High temperature mechanism

Reaction class 1: Unimolecular fuel decompositionReaction class 2: H-atom abstractionsReaction class 3: Alkyl radical decompositionReaction class 4: Alkyl radical+O2=olefin+HO2Reaction class 5: Alkyl radical isomerizationReaction class 6: H atom abstraction from olefinsReaction class 7: Addition of radical species to olefinsReaction class 8: Alkenyl radical decompositionReaction class 9: Olefin decomposition

Low temperature (high pressure) mechanismReaction class 10: Alkyl radical addition to O2Reaction class 11: R+R′O2=RO+R′OReaction class 12: Alkylperoxy radical isomerizationReaction class 13: RO2+HO2=ROOH+O2Reaction class 14: RO2+H2O2=ROOH+HO2Reaction class 15: RO2+CH3O2=RO+CH3O+O2Reaction class 16: RO2+R′O2=RO+R′O+O2Reaction class 17: RO2H=RO+OHReaction class 18: Alkoxy radical decompositionReaction class 19: QOOH decomposition and production of cyclic ethersReaction class 20: QOOH beta decomposition to produce olefin+HO2Reaction class 21: QOOH decomposition to small olefin, aldehyde and OHReaction class 22: Addition of QOOH to molecular oxygen O2Reaction class 23: O2QOOH isomerization to carbonylhydroperoxide + OHReaction class 24: Carbonylhydroperoxide decompositionReaction class 25: Reactions of cyclic ethers with OH and HO2

CK Westbrook et al. "A comprehensive detailed chemical kinetic reaction mechanism for combustion of n-alkane hydrocarbons from n-octane to n-hexadecane." Combustion and Flame 156.1 (2009): 181-199.


Intrinsic Reference kinetic parameters 47

H-Abstraction Reactions Primary H atom Secondary H atom Tertiary H atom Primary radical 108.0 exp (-13.5/RT) 108.0 exp (-11.2/RT) 108.0 exp (-9/RT) Secondary radical 108.0 exp (-14.5/RT) 108.0 exp (-12.2/RT) 108.0 exp (-10/RT) Tertiary radical 108.0 exp (-15/RT) 108.0 exp (-12.7/RT) 108.0 exp (-10.5/RT) Isomerization Reactions (Transfer of a Primary H-atom) 1-4 H Transfer 1-5 H Transfer 1-6 H Transfer Primary radical 1011.0 exp (-20.6/RT) 1010.2 exp (-14.5/RT) 109.7 exp (-14.5/RT) Alkyl Radical Decomposition Reactions to form Primary Radicals Primary radical Secondary radical Tertiary radical 1014.0 exp (-30/RT) 1014.0 exp (-31/RT) 1014.0 exp (-31.5/RT)

Corrections in Activation Energy to form: Methyl radical Secondary radical Tertiary radical

+ 2. - 2. - 3.

Pyrolysis reactions

H-Abstraction Reactions Primary H atom Secondary H atom Tertiary H atom Peroxyl radical 108.7 exp (-21.5/RT) 108.7 exp (-18.8/RT) 108.7 exp (-16.5/RT) Isomerization Reactions (Transfer of a Primary H-atom) 1-4 H Transfer 1-5 H Transfer 1-6 H Transfer Peroxyl radical 1011.8 exp (-29.1/RT) 1011.0 exp (-23.0/RT) 1010.6 exp (-23.0/RT) Hydroperoxy-Alkyl Radical Decomposition Reactions to form:

HO2• and Conjugate Alkenes Smaller Alkenes 1014.0 exp (-23/RT) 1013.2 exp (-22.5/RT)

to form Cyclic Ethers Xirans Oxetans Furans

1012.0 exp (-18/RT) 1011.2 exp (-17/RT) 1010.4 exp (-8.5/RT)

Oxidation reactions


48Automatic generation

H-Abstraction Reactions Primary H atom Secondary H atom Tertiary H atom Primary radical 108.0 exp (-13.5/RT) 108.0 exp (-11.2/RT) 108.0 exp (-9/RT) Secondary radical 108.0 exp (-14.5/RT) 108.0 exp (-12.2/RT) 108.0 exp (-10/RT) Tertiary radical 108.0 exp (-15/RT) 108.0 exp (-12.7/RT) 108.0 exp (-10.5/RT) Peroxyl radical 108.7 exp (-21.5/RT) 108.7 exp (-18.8/RT) 108.7 exp (-16.5/RT) Isomerization Reactions (Transfer of a Primary H-atom) 1-4 H Transfer 1-5 H Transfer 1-6 H Transfer Primary radicalb 1011.0 exp (-20.6/RT) 1010.2 exp (-14.5/RT) 109.7 exp (-14.5/RT) Peroxyl radical 1011.8 exp (-29.1/RT) 1011.0 exp (-23.0/RT) 1010.6 exp (-23.0/RT) Alkyl Radical Decomposition Reactions to form Primary Radicals Primary radical Secondary radical Tertiary radical 1014.0 exp (-30/RT) 1014.0 exp (-31/RT) 1014.0 exp (-31.5/RT) Hydroperoxy-Alkyl Radical Decomposition Reactions to form

HO2• and Conjugate Olefins Smaller Olefins 1014.0 exp (-23/RT) 1013.2 exp (-22.5/RT)

to form Cyclic Ethers Xirans Oxetans Furans

1012.0 exp (-18/RT) 1011.2 exp (-17/RT) 1010.4 exp (-8.5/RT) Corrections in Activation Energy to form:

Methyl radical Secondary radical Tertiary radical + 2. - 2. - 3.

Reference kinetic parameters

AUTOMATIC GENERATION OF PRIMARY OXIDATION REACTIONS

MAMOX Program

Classes of reactions1. Decomposition of alkyl radicals R → CnH2n+R’2. O2 addition to alkyl radicals R+O2 ROO3. Internal isomerization ROO QOOH4. O2 addition to hydroperoxyalkyl radicals

QOOH +O2 OOQOOH5. Decomposition of hydroperoxyalkyl peroxy radicals

OOQOOH OOQOOH + OH… … …

E. Ranzi, T. Faravelli, P. Gaffuri, E. Garavaglia, A. Goldaniga Ind. Eng. Chem. Res. 36, 3336-3344 (1997)


49

Combustion Reactions

Automatic Generation of Detailed Reaction SchemesPrimary propagation reactions of n-dodecane pyrolysis(Units are: m kmol s kcal).

R5a -Isomerization of ROO• to •QOOH radicals A ECOO*-C-C-C-C-C-C-C-C-C-C-C ==> COOH-*C-C-C-C-C-C-C-C-C-C-C 1.26E+12 26800COO*-C-C-C-C-C-C-C-C-C-C-C ==> COOH-C-*C-C-C-C-C-C-C-C-C-C 2.00E+11 20700COO*-C-C-C-C-C-C-C-C-C-C-C ==> COOH-C-C-*C-C-C-C-C-C-C-C-C 8.00E+10 20700C-COO*-C-C-C-C-C-C-C-C-C-C ==> *C-COOH-C-C-C-C-C-C-C-C-C-C 2.00E+12 29100C-COO*-C-C-C-C-C-C-C-C-C-C ==> C-COOH-*C-C-C-C-C-C-C-C-C-C 1.26E+12 26800C-COO*-C-C-C-C-C-C-C-C-C-C ==> C-COOH-C-*C-C-C-C-C-C-C-C-C 2.00E+11 20700C-COO*-C-C-C-C-C-C-C-C-C-C ==> C-COOH-C-C-*C-C-C-C-C-C-C-C 8.00E+10 20700C-C-COO*-C-C-C-C-C-C-C-C-C ==> C-*C-COOH-C-C-C-C-C-C-C-C-C 1.26E+12 26800C-C-COO*-C-C-C-C-C-C-C-C-C ==> C-C-COOH-*C-C-C-C-C-C-C-C-C 1.26E+12 26800C-C-COO*-C-C-C-C-C-C-C-C-C ==> *C-C-COOH-C-C-C-C-C-C-C-C-C 3.00E+11 23000C-C-COO*-C-C-C-C-C-C-C-C-C ==> C-C-COOH-C-*C-C-C-C-C-C-C-C 2.00E+11 20700C-C-COO*-C-C-C-C-C-C-C-C-C ==> C-C-COOH-C-C-*C-C-C-C-C-C-C 8.00E+10 20700C-C-C-COO*-C-C-C-C-C-C-C-C ==> C-C-*C-COOH-C-C-C-C-C-C-C-C 1.26E+12 26800C-C-C-COO*-C-C-C-C-C-C-C-C ==> C-C-C-COOH-*C-C-C-C-C-C-C-C 1.26E+12 26800C-C-C-COO*-C-C-C-C-C-C-C-C ==> C-*C-C-COOH-C-C-C-C-C-C-C-C 2.00E+11 20700C-C-C-COO*-C-C-C-C-C-C-C-C ==> C-C-C-COOH-C-*C-C-C-C-C-C-C 2.00E+11 20700………………….………………….


50

R5b -Isomerization of •QOOH to ROO• radicals A E

………………….………………….

COOH-*C-C-C-C-C-C-C-C-C-C-C ==> COO*-C-C-C-C-C-C-C-C-C-C-C 9.45E+10 19100COOH-C-*C-C-C-C-C-C-C-C-C-C ==> COO*-C-C-C-C-C-C-C-C-C-C-C 1.50E+10 13000COOH-C-C-*C-C-C-C-C-C-C-C-C ==> COO*-C-C-C-C-C-C-C-C-C-C-C 6.00E+09 13000*C-COOH-C-C-C-C-C-C-C-C-C-C ==> C-COO*-C-C-C-C-C-C-C-C-C-C 9.45E+10 18100C-COOH-*C-C-C-C-C-C-C-C-C-C ==> C-COO*-C-C-C-C-C-C-C-C-C-C 9.45E+10 19100C-COOH-C-*C-C-C-C-C-C-C-C-C ==> C-COO*-C-C-C-C-C-C-C-C-C-C 1.50E+10 13000C-COOH-C-C-*C-C-C-C-C-C-C-C ==> C-COO*-C-C-C-C-C-C-C-C-C-C 6.00E+09 13000*C-C-COOH-C-C-C-C-C-C-C-C-C ==> C-C-COO*-C-C-C-C-C-C-C-C-C 1.50E+10 12000C-*C-COOH-C-C-C-C-C-C-C-C-C ==> C-C-COO*-C-C-C-C-C-C-C-C-C 9.45E+10 19100C-C-COOH-*C-C-C-C-C-C-C-C-C ==> C-C-COO*-C-C-C-C-C-C-C-C-C 9.45E+10 19100C-C-COOH-C-*C-C-C-C-C-C-C-C ==> C-C-COO*-C-C-C-C-C-C-C-C-C 1.50E+10 13000C-C-COOH-C-C-*C-C-C-C-C-C-C ==> C-C-COO*-C-C-C-C-C-C-C-C-C 6.00E+09 13000*C-C-C-COOH-C-C-C-C-C-C-C-C ==> C-C-C-COO*-C-C-C-C-C-C-C-C 6.00E+09 12000C-*C-C-COOH-C-C-C-C-C-C-C-C ==> C-C-C-COO*-C-C-C-C-C-C-C-C 1.50E+10 13000C-C-*C-COOH-C-C-C-C-C-C-C-C ==> C-C-C-COO*-C-C-C-C-C-C-C-C 9.45E+10 19100

Combustion Reactions

Automatic Generation of Detailed Reaction SchemesPrimary propagation reactions of n-dodecane pyrolysis(Units are: m kmol s kcal).


51n-dodecane Primary Oxidation Reactions

Detailed Scheme

258 Primary reactions

72 Intermediate radicals

58 Primary products(retaining nC12 structure)

6 n-dodecenes16 O-cyclic-ethers6 hydroperoxides

30 keto-hydroperoxides

Low and High Temperature oxidation mechanisms are

conveniently simplified by grouping intermediate Species and Reactions.


Low Temperature Combustion

52

Lumping of Alkyl, Peroxy, Alkyl-hydroperoxy and

Peroxy-alkyl-hydroperoxy

Lumping of Alkenes, Cyclic ethers, Peroxides and Ketohydroperoxides

52


53Lumped Scheme ofn-alkane Primary Oxidation Reactions


54n-dodecane Primary Oxidation Reactions

Detailed Scheme




6 n-dodecenes16 O-cyclic-ethers6 hydroperoxides


Lumped Scheme

15 Primary lumped reactions


4 Primary lumped products

1 lumped n-dodecene1 lumped O-cyclic-ether1 lumped hydroperoxide1 lumped keto-hydroperoxides

Significant mechanism reductions refer to primary products (same structure of original fuel). Secondary reactions (primary reactions of primary products) can be better analysed.


55n-C10H22 Primary Oxidation Reactions

Detailed Scheme




5 n-decenes13 O-cyclic-ethers5 hydroperoxides


Lumped Scheme

15 Primary lumped reactions


4 Primary lumped products

1 lumped n-decene1 lumped O-cyclic-ether1 lumped hydroperoxide1 lumped keto-hydroperoxides

Kinetic Mechanisms always require a reasonable and well balanced presence of ‘primary’ and ‘secondary’ reactions.


Detailed Mechanisms of n-Alkane Oxidation 56



0

20

40

60

80

100

700 800 900 1000Temperature [K]

P=15 atmSele

ctiv

ity %

Objective function

BranchingConjugate alkenes from QOOH radicalsCyclic-EthersQOOH b-decompositionConjugate alkenes from alkyl radicalsAlkyl radical decomposition

Sele

ctiv

ity %

P=1 atm0

20

40

60

80

100Detailed mechanis

lumped mechanism

( )f

0 0

P T n 2

det,j lump,jj 1P T

Min S S dTdP=

−

∑∫ ∫0

0k ,Ek ,E

Least square method:

Rate constants of lumped reactions

Rate constants of lumped reactionsare obtained with an optimizationprocedure, by fitting detailed and

lumped initial selectivities, in a wide range of P and T conditions.


58Lumped Mechanisms of Heavy n-Alkane Oxidation


Low and High temperature primary mechanismof different n-alkanes heavier than n-heptane are always described with

4 lumped radicals (R, ROO, QOOH, and OOQOOH) and

15 similar reactions, with the same lumped kinetic parameters

The similarity of kinetic parameters and the similarity in reaction products justify the

‘vertical’ lumping and the choice of n-heptane, n-decane, n-dodecane and n-cetane.


Similarity of n-alkanes at HT and LT.Vertical Lumping

59

CO2

C2H4

CO

n-C7H16

n-C12H26

The laminar flame speeds of n-alkanes larger than C4 are very similar.

Predicted ignition delay times for stoichiometric n-alkanes/air at 13.5 bar (*)

(*) C.K. Westbrook, W.J. Pitz, O. Herbinet, H.J. Curran, E.J. Silke. "A comprehensive detailed chemical kinetic reaction mechanism for combustion of n-alkane hydrocarbons from n-octane to n-hexadecane." Combustion and Flame 156.1 (2009): 181-199.


Lumped mechanism of iso-octane60

Ranzi, E., Faravelli, T., Gaffuri, P., Sogaro, A., D'Anna, A., & Ciajolo, A. (1997). A wide-range modeling study of iso-octaneoxidation. Combustion and Flame, 108(1), 24-42.

Two different lumped alkyl-hydroperoxide radicals are assumed.

Only one promotes LT reactions


61

61Low Temperature Oxidation of Alkanes: Reference Kinetics ROO QOOH

•

H

O•O OHO

•O

HO

•OH

•O

1-5 H transfer of a primary H-atomKref= 1011 exp (-23000/RT) [s-1]

1-4 H transfer of a primary H-atomKref= 1011.8 exp (-29100/RT) [s-1]

Applying the rule to iso-octane and n-heptane…

Ea correction (secondary H-atom): -2300 cal/mol

Acorr: 2 H-atoms available for internal abstraction

Kcorr: 2 x 1011.8 exp (-26800/RT) [s-1]

Kcorr (700 K)≈104.0 [s-1]

•

Ea correction (secondary H-atom): -2300 cal/mol

Acorr: 4 H-atoms available for internal abstraction

Kcorr: 4 x 1011 exp (-20700/RT) [s-1]

Kcorr (700 K)≈105.4 [s-1]

ROO•=•QOOH Isomerizations of peroxy radicals explain the differentignition propensity of Primary Reference Fuels


Ignition Propensityof n-heptane and iso-octane

Fieweger et al, Combustion and Flame, 1997

% n-heptane

• Low temperature kineticsexplains the ignitionpropensity of hydrocarbonsat engine relevant conditions

• At low temperatures the ignition behavior is stronglyfuel dependent (linear or branched alkanes)

• High temperature kinetics isless sensitive to the fuelnature

Octane Number (ON) and Cetane Number (CN)

20


n-heptane-iso-octane mixtures (1,2,3)

Lille RCMIgnition delay times [ms]

0

20

40

60

80

100

600 700 800 900Temperature [K]

ON 100ON 95ON 90

Princeton PFRReleased heat (T(i) – T)

0

20

40

60

80

100

120

140

500 600 700 800 900Initial Temperature [K]

n-heptane (0 ON)62 ON PRF87 ON PRF

iso-octane (100 ON)

(1) Callahan C. V., Held T. J., Dryer F. L., Minetti R., Ribaucour M., Sochet L. R., Faravelli T., Gaffuri P. and Ranzi E., (1996) 26th Symposium (International) on combustion, The Combustion Institute, Pittsburgh, pp. 739-746

(2) Minetti R., Ribaucour M., Carlier M., Fittschen C and Sochet L. R., (1994). Combust. Flame 96:201

(3) Held T. J. and Dryer F. L., (1994) 25th Symposium (International) on combustion, The Combustion Institute, Pittsburgh, pp. 901-908


64Overall Oxidation Mechanism

Hierarchy and Modularityare the main features of Detailed Kinetic Schemes

• CH4 and gas mechanism (GRI)

• PRF (nC7-iC8) PRF: Gasoline Surrogate

•Diesel and Jet Fuels (S. Diego Surrogates)

•Alcohols (Propanols and Butanols)

•Biofuels – FAMECO

C3

CH4C2

nC7-iC8

H - O2 2

Bio Diesel Fuels Alcohols

Ranzi, E., Frassoldati, A., Grana, R., Cuoci, A., Faravelli, T., Kelley, A. P., & Law, C. K. (2012). Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels. Progress in Energy and Combustion Science, 38(4), 468-501.

Combustion of practical fuels (diesel, gasoline, jet fuels)

Complexity of the Liquid Fuels


Outline 65

3.b Combustion of practical fuels (diesel, gasoline, jet fuels)Complexity of the Liquid Fuels

Surrogate MixturesCombustion of oxy/bio alternative fuels (syngas/FT).

Reduced and Skeletal Kinetic ModelsCoupling of Detailed Kinetics and Complex Hydrodynamics

3.a Pyrolysis and combustion of complex hydrocarbon mixtures Reaction classes and automatic generationLumping procedures and reduction techniques


66

J.F.Griffiths, J.A.Barnard ‘Flame and Combustion’ Blackie Academic London 1994

GC distribution of alkanes in a liquid fraction

Liquid fuels are Complex Hydrocarbon Mixtures

Liquid fuels are mostly constituted by complex mixtures of large

hydrocarbons derived from refinery

Typical composition of a kerosene


67Crude Oil. Refinery Fractions

170 °C200 °C

350 °C

550 °C

John Jechura


68

John Jechura


Liquid Feedstocks- Boiling Temperatures 69

Speight, J. G. (Ed.). (1997). Petroleum chemistry and refining. CRC Press.

Liquid Fuels are constituted by Alkanes, Naphthenes (cyclo-alkanes) and Aromatics.

Kerosene (Jet Fuel) mainly contains C10-C12components, intermediate between

Naphthas and Gasoils,.


70Boiling Range of Oil Fractions


Complexity of the Liquid Feeds 71

71

Naphtha, Gasoil feeds are mostly constituted by complex mixtures of large hydrocarbons derived from the refinery

Gieleciak, R., and C. Fairbridge. (2013) "Detailedhydrocarbon Analysis of FACE Diesel Fuels Using Comprehensive 2D Gas Chromatography.". REPORT CDEV-2013-2065-RT

3D representation of a GCxGC-FID chromatogram


Bubble plot chromatogram with selected groups. 72

n-Alkanes iso-Alkanescyclo-Alkanes

IndansTetralins

Alkyl-benzenes

Alkyl-naphthalenes

Poly-Naphthenes



Bubble plot representation as a function of polarity vs. boiling point.

73



74Liquid fuels are complex mixtures

of large hydrocarbons derived from the refinery

Altgelt and Boduszynski 1994

The complexity of these mixtures calls for lumping and simplifications


75Size of Detailed Kinetic Mechanisms

T.F. Lu, C.K. Law ‘Toward accommodating realistic fuel chemistry in large-scale computations’Progress in Energy and Combustion Science 35 (2009) 192–215

75

Automatic Generation of kinetic mechanisms easily produces

Large Kinetic Models

Large Methyl esters:Rapeseed and soybean oil

Detailed kinetic mechanism consists 4800 species and ~20,000

reactionsC K Westbrook et al. Comb. Flame

158 (2011): 742-755.


Surrogates 76

Fuel surrogates are defined as physical or chemical surrogate depending on whether the surrogate mixture has the similar physical or chemical properties as the fuel to be studied.

Surrogates provide a cleaner basis for developing and testing models of the fuel properties in practical combustors.

Detailed reaction mechanisms for surrogates of gasoline, jet, and diesel fuels typically contain large numbers of species and reactions.

Gasoline surrogates include Primary Reference Fuels (n-heptane and iso-octane), but also aromatic species (toluene).


Regular-grade Unleaded Gasolines 77

Typical Distillation Profiles (ASTM D 86)Summer and Winter Gasolines

http://www.chevron.com/products/prodserv/fuels/bulletin/motorgas/1_driving-performance/pg2.asp

Summer ConventionalGasolines

WinterConventional Gasolines

200

20

160

120

80

40

0 40 60 800

100

T [°C]

Percent Evaporated

Real Fuels contain thousands of compounds greatly varying with feedstock origins, with seasons and with economic factors


78

260 325 400 464 530 T/KAromatics

n-Paraffins


Gasolines, PRF and Octane NumbersPrimary Reference Fuels define the

Octane Number

Surrogate mixtures of n-heptane (ON=0)

and iso-octane (ON=100)(2,2,4-trimethyl-octane)

A gasoline with an ON=92has the same knock as a mixture of 92% isooctaneand 8% n-heptane, under the standard test conditions.

http://www.chevron.com/products/prodserv/fuels/bulletin/motorgas/3_refining-testing/

Ternary Toluene Reference Fuels (TRFs) can better reproduce H/C

ratio and aromatic contents


Fuels for Advanced Combustion Engines (FACE) Gasolines

80

Anand, K., Ra, Y., Reitz, R. D., & Bunting, B. (2011). Surrogate model development for fuels for advanced combustion engines. E&F,25, 1474.A. Ahmed, G. Goteng, V.S. Shankar, K. Al-Qurashi, W.L. Roberts, S.M. Sarathy. Fuel 143 (2015) 290–300 .

Collaborative research program led by KAUST with LLNL, UConn, RPI, UC Berkeley...- Acquisition of 6 FACE fuels (A, C, F, G, I, J)- Testing in ST and RCM at different facilities- Compositional Analysis and Formulation of suitable surrogates- Kinetic analysis

Courtesy of Prof. Sarathi. KAUST


81


Gasoline Fuel Surrogate Palette 82

n-alkanesbranched alkanesCycloalkanes and alkenesaromatics

2,5-dimethylhexane

Iso-pentane

toluene

N-butane

cyclohexanecyclopentane

1-hexene

Courtesy of Prof. Mani Sarathi. KAUST


Variability of Jet Fuels 83

Colket, M., Edwards, T., Williams, S., Cernansky, N. P., Miller, D. L., Egolfopoulos, F., ... & Tsang, W. (2008). Identification of target validation data for development of surrogate jet fuels. In 46th AIAA, Aerospace Sciences Meeting and Exhibit, Reno, NV, Paper No. AIAA (Vol. 972).

AromaticsDensityFischer Tropsch


Fischer Tropsch Alternative Fuel 84

Corporan, E., DeWitt, M. J., Belovich, V., Pawlik, R., Lynch, A. C., Gord, J. R., & Meyer, T. R. (2007). Emissions characteristics of a turbine engineand research combustor burning a Fischer-Tropsch jet fuel. Energy & fuels, 21(5), 2615-2626.

Absence of Aromatics Very low PAH and soot emissions of

a Fischer-Tropsch jet fuel


Surrogate Mixture of JP8 85

Humer, S., Frassoldati, A., Granata, S., Faravelli, T., Ranzi, E., Seiser, R., & Seshadri, K. (2007). Experimental and kinetic modeling study of combustion of JP-8, its surrogates and reference components in laminar nonpremixed flows. Proc. Combustion Institute, 31(1), 393-400.


86Diesel - Surrogates


Chemical Kinetic Mechanisms 87

LLNL Lawrence Livermore Natl. Laboratories


Skeletal Kinetic Models of Different Surrogate Fuels 88

E. Ranzi, A. Frassoldati, A. Stagni, M. Pelucchi, A. Cuoci, T. Faravelli (2014) ‘Reduced Kinetic Schemes of Complex Reaction Systems: Fossil and Biomass-Derived Transportation Fuels’ Int J Chem Kinet 1–31.


89

Skeletal Kinetic Models of Different Surrogate Fuels

E. Ranzi, A. Frassoldati, A. Stagni, M. Pelucchi, A. Cuoci, T. Faravelli (2014) ‘Reduced Kinetic Schemes of Complex Reaction Systems: Fossil and Biomass-Derived Transportation Fuels’ Int J Chem Kinet 1–31.


90

sugarcane

Bio Fuels from Biomass Feedstocks

Biomass is biological material derived from living, or recently living organisms. It often refers to plant-based materials which are called lignocellulosic biomass.

grain (rice, wheat)corn

strawarundo donax mais stover saw dust

Second Generation Biofuels

First Generation Biofuelsenergy crops that potentially compete with food crops

Food prices will be affected due to increased production of energy crops that potentially compete with food crops for land use.


91Biomass Feedstock

Naik, S. N., Goud, V. V., Rout, P. K., & Dalai, A. K. (2010). Production of I and II generation biofuels: a comprehensive review. Renewable and Sustainable Energy Reviews, 14(2).

First Generation Bio-FuelsFood Competition

Second GenerationUnused resources

Third Generation

Algae are simple autotrophic organisms, ranging from unicellular

to multicellular forms. The lipid production rate in green microalgae is 10-30 times greater

than those of the best crop


Renewable Transportation Fuels: Ethanol and Biodiesel

Cellulosic and algal renewable fuels will need to emerge with economic advantage to accelerate alternative fuel usage, and in a manner that better addresses fuel distribution and storage.

Alternative feed stocks, composed of fully hydrogenated species similar to those found in fossil fuels, can overcome fuel distribution and storage problems.

Upgrading will require additional hydrogen, and methods for generating hydrogen without increasing carbon emissions are critical needs for the future.

92

Dryer, F. L. (2015). Chemical kinetic and combustion characteristics of transportation fuels. Proceedings of the Combustion Institute, 35(1), 117-144.

At present, there are only two renewable alternative fuels that are widely used for transportation: ethanol, and biodiesel. But, neither ethanol nor biodiesel can be distributed through pipeline systems distributing petroleum products.


Chemical Routes to Altervative Fuels 93

Today, biomass is the only available renewable source for producing liquid biofuels such as ethanol or biodiesel. These fuels can offer renewable alternatives to transportation fuels that presently are obtained almost exclusively from oil. There are two chemical routes for production of ethanol and diesel:

(1) Ethanol,through the fermentation of sugar

(2) Diesel, through the transesterification or the hydroprocessing of fatty acids

Ethanol, the most common biofuel, is produced by fermentation of annually grown crops (sugar cane, corn, grapes, etc.). In this process, starch or carbohydrates (sugars) are decomposed by microorganisms to produce ethanol. Ethanol can be produced from a wide variety of sugar or starch crops, including sugar beet and sugar cane and their byproducts, potatoes and corn surplus.


World Biofuels Production Trends 94

The ethanol market is more than four times larger than the global biodiesel market.

Markets for both are steadily increasing, not only in traditional markets such as the European Union, Brazil and the United States, but also in China, India and Argentina.


95Biological Conversion: BioethanolEthanol is produced by fermentation of biomass. Starch and Sugars are decomposed by microorganisms to produce ethanol.

To convert lignocellulosic biomass to biofuels, the polysaccharides must first be hydrolysed, or broken down, into simple

sugars using either acid or enzymes.

Ethanol can be produced from a variety of sugar or starch crops,

including sugar beet, sugar cane, potatoes and corn surplus.


Bio Ethanol as Bio-Gasoline and Additive

Ethanol has an ON of approximately 100 and strongly resists knocking behavior.

Many states in the United States now permit as much as 10% ethanol in ordinary gasoline as an antiknock additive.

In Brazil, where ethanol is produced in very large quantities from plentiful crops of sugar cane, automobile engines using 100% ethanol are used widely since many years.

96


Soot formation from Ethanol

0.00E+00

4.00E-03

8.00E-03

1.20E-02

1.60E-02

1.00E-05 1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00

Mas

s Fra

ctio

n

Time [s]

C2H5OH

C2H4

CO

C2H2

CH3CHO

Ethanol Pyrolysis at 1300 °C

1.00E-06

1.00E-05

1.00E-04

1.00E-03

1.00E-04 1.00E-03 1.00E-02 1.00E-01 1.00E+00

Mas

s Fra

ctio

n

Time [s]

C2H5OH

CH2CH2OH CH3CHOH CH3CH2O

CH3CHO

CH3CO

CH3

CO CH2O

C2H4

C2H3

C2H2

C3H4 C6H5C2H

CH4

Soot

100 C atoms

+C2H2

C6H5C6H6

26 204014

13

14

5612

26

20

10

1010

10

314 3

3

6

18

4 (+ 8)8 (+4)

4 (+12)

8

4 12 (+4)

4

Via C3H3

T. Faravelli SMARTCAT Meeting COST (2015)


Alcohols and AldehydesEffect of Oxygen Atom on C-H BDE (kcal/mole) (*)

95

10296

96

1-butanol

AlcoholsRO-H bond is ~104 kcal/mol (higher than C-H in methyl)C-H in α position is weaker than C-H in secondary sites

89

91butanal

AldehydesThe BDE of acylic H-atom in the -CHO group is ~89 kcal/mol(lower than a tertiary C-H in iso-butane)C-H in α position to the CHO groupis weaker than C-H in secondary sites

(*) NIST. Washington. 1970

Molecular dehydration reactions:

CH3-CH2-CH2-CH2-OH H2O + CH3-CH2-CH=CH2

k=5.0×1013exp[−68,600/(RT)] [s−1]


Butanol oxidation:Role of molecular dehydration reactions

C4H9OHC4H8 +H2O

99

Four-center molecular dehydration reactions

k=5.0×1013exp[−68600/(RT)] [s−1]


100Biodiesel: Trans-Esterification of Vegetable oils

acid catalyst

Vegetable oils Fatty acidsBiodiesel Heavy methyl esters (FAME)


Chemical Structures of FAME

Fatty Acid FAME Chemical Structure Chemical Formula

Decanoic [10:0] CH3(-CH2-)8CO-OCH3 C11H22O2Lauric [12:0] CH3(-CH2-)10CO-OCH3 C13H26O2Myristic [14:0] CH3(-CH2-)12CO-OCH3 C15H30O2Palmitic [16:0] CH3(-CH2-)14CO-OCH3 C17H34O2

Stearic [18:0] CH3(-CH2-)16CO-OCH3 C19H38O2Oleic [18:1] CH3(-CH2-)7CH=CH(-CH2-)7CO-OCH3 C19H36O2Linoleic [18:2] CH3(-CH2-)3(CH2-CH=CH)2(-CH2-)7CO-OCH3 C19H34O2Linolenic [18:3] CH3-(CH2-CH=CH)3(-CH2-)7CO-OCH3 C19H32O2

Arachidic [20:0] CH3(-CH2-)18CO-OCH3 C21H42O2Behenic [22:0] CH3(-CH2-)20CO-OCH3 C23H46O2Erucic [22:1] CH3(-CH2-)7CH=CH(-CH2-)11CO-OCH3 C23H44O2

101

O

OCH3CH3

“Detailed” kinetic scheme


Common Biodiesel Fuels- Reference Compositions

Fatty Acid Soybean Cottonseed Rapeseed Palm Lard Tallow CoconutLauric 0.1 0.1 0.1 0.1 0.1 0.1 53.1Myristic 0.1 0.7 0.1 1.0 1.5 3.1 21.9Palmitic 10.3 20.4 4.3 43.1 24.9 25.4 11.2Stearic 3.7 2.6 1.3 4.5 15.0 21.1 3.4Oleic 23.0 19.5 59.9 40.8 46.7 46.2 7.9Linoleic 54.1 56.0 21.1 10.2 11.3 3.2 2.5Linolenic 8.7 0.6 13.2 0.2 0.4 1.0 0.0

102

Saggese, C., Frassoldati, A., Cuoci, A., Faravelli, T., & Ranzi, E. (2013). A lumped approach to the kinetic modeling of pyrolysis and combustion of biodiesel fuels. Proc. Combustion Institute, 34(1), 427.

Rapeseed methyl esters (RME) in Europe

Soybean methyl ester (SME) in USA


Biodiesel characterization of model compounds

Biodiesel is composed by saturated and unsaturated heavy methyl esters.

R

O

CH3O

Methyl esters

O

OCH3CH3

O

OCH3CH3

O

OCH3CH3

O

OCH3CH3

The five major components are:

O

OCH3CH3

C.K. Westbrook, C.V. Naik, O. Herbinet, et al., Combust. Flame (2011)

“Detailed” kinetic scheme

methyl palmitate (MPA) – CH3-C16H31O2

methyl stearate (MSTEA) - CH3-C18H35O2

methyl oleate (MEOLE) - CH3-C18H33O2

methyl linoleate (MLINO) - CH3-C18H31O2

methyl linolenate (MLIN1) - CH3-C18H29O2


104

Lumped Mechanism of Methyl EstersStearate, Oleate, and Linoleate

Again, with the lumped approach, only a few new species allow to describe both the high and low temperature mechanism.


Oxidation of methyl-esters in a JSR 105

Mole fraction profiles of methyl esters and Oxygen(P = 1.05 atm, τ = 2 s, Esters = 4×10-4, Benzene= 5000, Oxygen 45000 ppm).

A. Rodriguez et al., (2015) Submitted to Combustion and Flame.


Waddington mechanism: methyl-oleate forming nonanal and 9-oxo,methyl nonanoate

106

OH addition

addition on O2

isomerization

OH and aldehyde formation


Relative reactivityof saturated and unsaturated methyl esters

107

A. Rodriguez et al., (2015) Submitted to Combustion and Flame.


Mechanism Dimensions

Adapted from: T.F. Lu, C.K. Law, Prog. Energy Comb. Sci., 35 (2009)

biodiesel (POLIMI)

biodiesel (LLNL)

Biodiesel + NOx + soot (POLIMI)

computational cost associated with detailed mechanisms is usually very high

need of reduction methods, numerical techniques and computational tools to make:

-use of large kinetic schemes computationally efficient

-easy integration in new and/or existing codes

Lumping and reduction methods can result in effective approaches to face the problem



Handling mechanisms

RANS LES DNS

Accuracy

Size of kinetic mechanisms

Computational cost

Detailed mechanisms: not directly applicable in large-scale computations

3 objectives:

Set up a robust and efficient framework for ad hoc mechanism reduction.

Address skeletal reduction to customtargets, beyond reactivity and ignition delay

Obtain the optimal trade-off between sizeand accuracy

Lumping and Skeletal Reduction: more compact mechanisms with the same accuracy

Ranzi E. et al. (2014) International Journal of Chemical KineticsSeveral time scales involved

Isothermal PFR C2H4/air @ 1800 K



110

CRECK Modeling Group at Politecnico di Milano

Thanks for the attention

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