presentazione di powerpoint - nccrd.innccrd.in/iciwsindia2015-assets/docs/ranzi2.pdf · c2h6. c3h8....

Winter Combustion School IIT Madras December 2015

Combustion KineticsLow and high temperature combustionIdeal ReactorsPAH and Soot

Eliseo Ranzi

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


Outline 2

2.a Chemical Kinetics:Combustion of methane.Low and high temperature combustion of hydrocarbons.

2.b Reactions in Ideal Reactors with negligible molecular transport: plug flow and stirred reactors, shock tubes and rapid compression machines. Premixed and diffusion flames.

2.c PAH and Soot formation mechanisms.

Winter Combustion School IIT Madras December 2015 3

C 2H 6

C 2H 5

C 2H 4

C 2H 3

C 2H 2

Aromatics

Soot

Pyrolysis

O2

CHi

CH3OOH

CH3OH

CH2OH

O2

OH

Oxidation

CH3OO

CH3

CH3O

CH2O

HCO

CO

CO2

CH4

NOx

Detailed Kinetics of Methane CombustionCH4+ 2 O2 CO2 + 2 H2O

It is important to include all the relevant reactions and the proper relative

selectivity of parallel reaction paths.


Detailed Oxidation Mechanism of n-pentane

4

4

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.


High temperature Reactions of n-pentane

5

5

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 MechanismDecomposition of Large Molecules

6

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

6


High Temperature Combustion Processes

Combustion reactions proceed via a chain radical mechanism.

Reaction Classes.

Chain Initiation Reactions (Formation of radicals from stable species):C3H8 C2H5 + CH3

Chain Propagation Reactions (propagate the reaction chain):OH + C3H8 H2O + C3H7( nC3H7 CH3 + C2H4 and iC3H7 H + C3H6 )

Chain Termination Reactions (radical recombination):

CH3 + CH3 C2H6

A very sensitive high temperature combustion reaction is the

Chain Branching Reaction (increase the # of radicals).H + O2 O + OH


At High Temperatures,H radical strongly influences the combustion rate

All the reactions favouring the H radical production increase the combustion rates: e.g., dehydrogenation reaction of ethyl radical (C2H5):

C2H5 C2H4 + H

For this reason methane and ethane show the lowest and the highestcombustion rate, respectively.

High Temperature Combustion Processes

On the contrary, CH3 radical recombination to form ethane:CH3 + CH3 C2H6

reduces the radical concentration and the reaction rate.

n-butyl radicals form both the radicals CH3 and C2H5 :1 C4H9 C2H4 + C2H5

n C4H10 2 C4H9 C3H6 +CH3


Ignition delay time

9

Typical Shock Tube experimental record

The ignition delay time is the time period between the start of injection and the first identifiable pressure increase, i.e. the time required to burn the fuel.


Ignition delay times [µs] vs Temperature [K]

1

10

100

1000

5 6 7 810000/T [1/K]

CH4

C2H6

C3H8C4H10

High Temperature Combustion ProcessesCH4 and C2H6 show the lowest and the highest combustion rate.

Propane and butane have intermediate and similar reactivity.

CH4 ignites slowest because methyl radicals lead to chain termination. Ethane is most

reactive because every ethyl radical, resulting from H-atom abstraction, produces H atoms, thus promoting: H+O2 O + OH

1540 K

6 µs

300 µs


Laminar Flame speed of CH4

Maximum flame speedat rich condition

Relevant effect of the initial temperature

36 cm/s

58 cm/s2

00

LTs sT

≈ ⋅


Laminar Flame speeds of small hydrocarbons: CH4, C2H6, C3H8 and nC4H10

Again, Ethane and Methane show the highest and lowest flame speeds.E. Ranzi, A. Frassoldati, R. Grana, A. Cuoci, T. Faravelli, A.P. Kelley, C.K. Law. Progress in Energy and Combustion Science 38 (2012) 468-501

44 cm/s36 cm/s

40 cm/s40 cm/s


13

Laminar air-stoichiometric flames of small hydrocarbons (T0=298 K, P=1 atm).1- Predicted profiles of relevant radicals: H, CH3

Stoichiometric Flames of small hydrocarbons Hydrogen and Methyl radical concentration


14Laminar Flame speeds of small hydrocarbons

Sensitivity coefficients of laminar flame speed on reaction rate coefficients, for small alkanes/air flames at Φ=1, T=298 K and atmospheric pressure.

Allyl radicals reduce the system reactivity

H+O2 is always the most sensitive reaction.Recombinations show a negative coefficients.

Methyl substitutions reduce flamespeeds not only of alkanes but alsoof oxygenated fuels, mainly due torecombination reactions, .


Similarity of the structures of air-stoichiometricn-heptane and n-dodecane flames.

15

CO2

C2H4

CO

n-C7H16

n-C12H26

The laminar flame speeds of all the n-alkanes larger than C3-C4, as well as their flame structures, are very similar.


Methane Pyrolysis and Oxidation Reactions

CHi

O2

OH

Oxidation

CH3OOH

CH3OH

CH2OH

CH3OO

CH3

CH3O

CH2O

HCO

CO

CO2

C 2H 6

C 2H 5

C 2H 4

C 2H 3

C 2H 2

Aromatics

Soot

Pyrolysis

O2

CH4

NOx


C2H4

C2H6

0

0.001

0.002

0.003

0 2e-06 4e-06 6e-06 8e-06 1e-05

C2H2

CO2

H2

O2

CH4

0

0.05

0.1

0.15

0.2

0.25

0 2e-06 4e-06 6e-06 8e-06 1e-05

H2O

CO

Stoichiometric Mixture -- 1 atm -- 1550 K

Main Species Profiles as a function of Time [s]• Fast conversion of CH4 to CO in about 10-5 s.

• Slow final oxidation of CO to CO2

Methane/Air Combustion


NOx FormationNitric oxides are mainly formed from atmospheric N2 through three mechanisms:

Thermal, Prompt, and via N2O. The Thermal or Zel’dovich mechanism (1946) consists of three major reactions:

O· + N2 ↔ NO + N· k1f = 2·1014 exp(-75250/RT)N· + O2 ↔ NO + O· k2f = 6.4·109 exp(-6000/RT)N· + OH· ↔ NO + H· k3f = 3.8·1013

The concentration of O· and OH· radicals are ruled by combustion mechanism. Rate of NO formation is:

[ ] [ ][ ] [ ][ ] [ ][ ]OHNkONkNOkdtNOd

32221 ++=

The State State Approximation for N radicals gives:

By substituting [N], NO formation rate becomes:

[ ] [ ][ ] [ ] [ ] [ ]{ } 032221 ≅+−= OHkOkNNOkdtNd [ ] [ ][ ]

[ ] [ ]OHkOkNOkN322

21

+=

[ ] [ ][ ] [ ][ ] [ ][ ] [ ][ ]2132221 2 NOkOHNkONkNOkdtNOd

=++=

The first reaction is the rate controlling step: it requires the breaking of the tight N2bond and is favoured at high temperatures. The [O] concentration is obtained by usingthe partial equilibrium assumption for O2 ↔ 2 O


Prompt NOx. Fenimore (1971)

Under practical conditions, often the amount of NO formed was higher than Thermal NOx. The prompt NO mechanism involve the initial reaction of N2 with CH and CH2, producing NCN, HCN (hydrogen cyanide) and the H and NH radicals:

19

CH + N2 ↔ HCN + NCH2 + N2 ↔ HCN + NH

Important in lean combustion in gas turbines (Correa, 1992)At low temperature and high pressures, a contribution in fuel-lean mixtures is due to:

O + N2 + M → N2O + MAt high Temperatures, N2O is removed by: H + N2O → N2 + OH

O + N2O → N2 + O2 → NO + NOThe lifetime of N2O is less than 10 ms at 1500 K, then the mechanism is active only atlow-T.

N2O Mechanism

HCN + O ↔ NCO + H NCO + H ↔ NH + COR + HCN ↔ RH + CN R + NH ↔ RH + NO + CN ↔ N + CO

The HCN and NH formed undergo further reactions forming N (Bowman 1973):

N is then oxidized with the previous thermal NOx reactions.


Prompt and Fuel NOxReburning Bowman [1973]

20

Major reaction steps in prompt NO formation.Reaction path diagram also illustrates the conversion of fuel nitrogen in flames, and reburning process.


Air Staging – Overfire Air - Low NOx Burner21

http://www.powermag.com/pollution-control-low-nox-combustion-retrofit-options/

Low-NOx Staged Burner of pulverized CoalStaged Combustion

Thermal Power Plant


Flameless Combustion 22

Absence of a flame front.Pre-heated air (high efficiency)hot gas recirculationLower peak temperatures


Combustion of Methane

2000 K C/O = 1/4 (stechiometrico)

Φ=1Effect of stoichiometryImportance of a ‘micro’ mixing

2000 K C/O = 1

Φ=4


Oxygen CombustionProducts

Fuel

C

FUEL

AIR

Flame Front

Premixed Flame

Temperature and Concentration Profiles in

Fuel

Oxygen

AirAir

Fuel

Flame Front

Diffusive Flame

CombustionProducts

C

PyrolysisRich conditions

High T


0

0.002

0.004

0.006

0 0.05 .1 .15 .2

C2H6

C2H4

C2H2

T=1800 KΦ=1

0

0.05

0.1

0.15

0.2

0 0.05 .1 .15 .2

CH4

O2 H2O

CO

H2

CO2

T=1800 KΦ=1


T=1800 K e Φ=1

Mole fractions vs. contact time (ms)


0

0.05

0.1

0.15

CH4

O2 H2O

CO

H2

CO2

0 0.05 .1 .15 .2

T=1800 KΦ=2

T=1800 KΦ=2

0

0.005

0.01

0.015

C2H6

C2H4C2H2

0 0.05 .1 .15 .2


T=1800 K e Φ=2



0

0.1

0.2

0.3

0 .1 .2 .3 .4

CH4

O2 H2O

CO

H2

CO2

T=1800 KΦ=4

0

0.01

0.02

0.03

0.04

0 0.1 0.2 0.3 0.4

C2H6

C2H4

C2H2

T=1800 KΦ=4


T=1800 K e Φ=4


CH4 C2H4 C2H2

CH3 + C2H4 C3H7

C3H7 C3H6 C3H4

(Methyl-acetylene and Propadiene)


Propargyl Radicalsand First Aromatic Ring Formation

28

The dehydrogenation reaction of allene and methyl acetylene forms propargyl radicals:C3H4s C3H3+H

The very stable propargyl radicals can recombine to form C6H6 components:

Miller, J. A., & Klippenstein, S. J. (2003). The recombination of propargyl radicals and other reactions on a C6H6 potential. The Journal of Physical Chemistry A, 107(39), 7783-7799.

1,5-hexadiyne

Benzene

2 C3H3 C6H6

Fulvene


29Propargyl Radicalsand First Aromatic Ring Formation

Miller, J. A., & Klippenstein, S. J. (2003). The recombination of propargyl radicals and other reactions on a C6H6 potential. The Journal of Physical Chemistry A, 107(39), 7783-7799.

2 C3H3 C6H6


0

100

200

300

400

0 .5 1. 1.5 2.

Mole (ppm) vs. contact time (ms)


T=1800 K - Φ=4Formation of Benzene and Aromatics


Benzene and Pyrene (ppm) vs. Contact times (s)

0

200

400

600

0 0.005 0.01 0.015 0.02

1500 K

1600 K

1700 K

1800 K1900 K

0

2

4

6

8

10

12

0 0.005 0.01 0.015 0.02

1500 K

1600 K

1700 K

1800 K

1900 K

Methane

Combustion

Ф=4


Successive Growth of Benzene and PAH species

+ + 2 H

+ 2 H+

Cyclopentadienyl Radicals (2)

(1) Frenklach M. and Wang H., (1990). 23rd Symposium on combustion, The Combustion Institute, Pittsburgh, p. 1105(2) Miller J. A. and Melius C. F., (1992). Combust. Flame, 114:192

- H

- H

- H

+ H- H2

+ H

+ C2H2 C2H C2H

+ C2H2

+ C2H2

HACA (1) HACA Mechanism:H abstraction

C (C2H2) addition

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

Soot Formation in Combustion 33

CH CH3

CH2CH2

CHCH

CH2

CH

CH2CH

O2

COCO2

CH2

CH2

0 ms

1 ms

10 ms

100 ms

CH4

H. Bockhorn ‘Soot Formation in Combustion’ Springer-Verlag Berlin Heidelberg 1994

NucleationSoot Inception

Surface growth

Coagulation

100 nm

1 nm

Formation andGrowth of PAH

ParticleAggregation


34

Low and High Temperature Reactions


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.

35

At lower temperatures, alkyl radicals add to O2 forming Peroxy Radicals :R + O2 ↔ RO2

The equilibrium constant is strongly temperature dependent and is in favor of RO2at low T, shifting toward R + O2 as T increases.

The Ceiling Temperature is the temperature above which this equilibrium favors the dissociation path.

O•O H OOH•

Keto-/Carbonyl-hydroperoxides fastly decompose to form two radicals:

•OH +O

HOO

O

•O •OH +O

• + CH3CHO


Explosion Diagrams: C3H8/O2 mixture

36

SLOW COMBUSTION

DELAYED TWO STAGEIGNITION

PRESSURE (mmHg)0 200 400 600 800

EXPLOSION

The Ceiling Temperature R + O2 ↔ RO2

rules the transition betweenLow and High T Mechanisms

Low Temperature Mechanism

Peroxy radicals

High Temperature MechanismAlkyl radicals


Low Temperature MechanismInternal Combustion Engine (4 strokes)


Oxidation of alkanes 38

nC7H16

+ O2

R7OO

nC7H15

Q7OOH

+ O2

OOQ7OOH

DegenerateBranching Path

•

• • CH3• •

••β-Decomposition

Products

+ XH•

X•H

• OO•

O•

O H HOO•

HOO•

HOO

•OO

OQ7OOH + OH• HOO

O•

OH

O

HOO•

HOO

HOO

+ •OH


39

+ O2

OH• + Cyclic Ethers

OH• + •RCHO + CnH2n

HO2• + nC7H14

Oxidation of alkanes

nC7H16

+ O2

R7OO

nC7H15

Q7OOH

+ O2

OOQ7OOH


β-Decomposition Products

OQ7OOH + OH•


40

Low Temperature Mechanism

High Temperature Mechanism(Eapp ≅ 30000 cal/mol)

+ O2

OH• + Cyclic Ethers

OH• + •RCHO + CnH2n

HO2• + nC7H14

β-Decomposition Products

NTC

nC7H16

nC7H15

+ O2

R7OO

Q7OOH

+ O2

OOQ7OOH


OQ7OOH + OH•

Oxidation of n-heptane in an Isothermal PFR

Chain Initiation Reactions [s-1]nC7H16 Products

C7H14O3 Products

1710 exp( 80000 )Fuelk RT−

1610 exp( 43000 )khpk RT−

conv

ersio

n

Reactor Temperature

Isothermal PFR

9 1010 10khp fuelk k ÷

at 800-900 K


Ceiling temperatureTransition between Low and High T mechanisms

41

Alkyl radical addition to O2 and peroxy radicals decomposition rule this transition:

kadd= 109 [l/mol/s]kdec= 1013 exp (-28000/RT) [1/s]R● + O2 ROO●

Competitive pathways: at high temperatures alkyl radicals are favored over the peroxy radicals, or pyrolysis is favored over oxidation.

Ceiling Temperature is the transition temperature from one mechanism to the other

At the equilibrium the addition (forward) and the decomposition (reverse) reaction rates are equal:

[ ][ ][ ]

[ ][ ] [ ]

2

2

13

92

1

2800010 exp( )

10

decdec

add add

dec

addO

k ROOrr k O R

R k RTPROO k O x

RT

•= ≅

•

−•= ≅

•

Benson, S. W. (1965). Effects of Resonance and Structure on the Thermochemistry of Organic Peroxy Radicals and the Kinetics of Combustion Reactions. Journal of the American Chemical Society, 87(5), 972-979


Pressure effect on Ceiling Temperature. 42

2

13

9

2800010 exp( )

10O

RT RTPx

−≅

[ ][ ] [ ]

2

13

92

2800010 exp( )

10dec

addO

R k RTPROO k O x

RT

−•= ≅

•

0.01

0.1

1

10

100

1000

800 900 1000 1100 1200 1300

Tceiling [K]

Pres

ure

[atm

]

Ceiling temperature increases with pressure (NTC region moves toward higher temperatures) : Higher oxygen concentrations favor addition reaction to form peroxy radical

Benson, S. W. (1965). Effects of Resonance and Structure on the Thermochemistry of Organic Peroxy Radicals and the Kinetics of Combustion Reactions. Journal of the American Chemical Society, 87(5), 972-979


43

N-Heptane OxidationLow and High Temperature Ignitions

E. Ranzi, P. Gaffuri, T. Faravelli, P. Dagaut ‘A Wide-Range Modeling Study of n-Heptane Oxidation’ (1995) Combust. Flame 103: 91-106

0.01

0.1

1

10

100

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

igni

tion

time

[ms]

1000/T [K]

P = 6.5 barP = 13.5 barP = 42 bar

Pressure effect on the NTC region

Pressure effect on Ceiling Temperature.

Low T mechanism

High T mechanism


Propane Oxidation Mechanism 44

Ranzi, E., Cavallotti, C., Cuoci, A., Frassoldati, A., Pelucchi, M., & Faravelli, T. (2015). New reaction classes in the kinetic modeling of lowtemperature oxidation of n-alkanes. Combustion and flame, 162(5), 1679-1691.

Alkyl radicals

Peroxy radicals

Alkyl-hydro-Peroxy radicals

Carbonyl-hydro-Peroxides


45Explosion diagram of C3H8/O2 systemin a batch reactor with heat exchange

45

500

600

700

800

0 0.2 0.4 0.6P atm

SLOW COMBUSTION

IGNITION

COOL FLAMES

500

600

700

800

A

time

T

∆T>1000 K

B – D: Hot IgnitionB

D

time

T∆T=100-200 K

C : Cool Flames

C

U∆T


46Two Stage Ignitions, Cool Flames, NTC and Low T Behaviour

Gaffuri, P., Faravelli, T., Ranzi, E., Cernansky, N. P., Miller, D., d'Anna, A., & Ciajolo, A. (1997). Comprehensive kinetic model for the low temperature oxidation of hydrocarbons. AIChE journal, 43(5), 1278-1286.


47Oscillatory Cool flames in JSR experimentsi-C8H18/air mixture in a JSR at 7 bar.

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


Outline 48

2.a Chemical Kinetics:Combustion of methane.Low and high temperature combustion of hydrocarbons.

2.b Reactions in Ideal Reactors with negligible molecular transport: plug flow and stirred reactors, shock tubes and rapid compression machines. Premixed and diffusion flames.

2.c PAH and soot formation.


Reaction mechanisms are developed and validated with experiments in which fluid mixing is (almost) completely suppressed.

Reactors of this type include:- shock tubes, - turbulent flow reactors,- rapid compression machines.

Adapted from H. Wang , Princeton Summer School 2012

Degree of Mixing in Research Reactor

The degree and type of fluid mixing is an important feature in reacting flow simulation.

The mixing can occur among the reactants or between reactants and products.

Perfectly StirredReactor


Ideal Chemical Reactors

Adapted from A. Cuoci, A. Frassoldati. COST- Milan Summer School 2013


Rapid Compression Machine Shock Tube Jet Stirred Reactor

Experimental reactors and devices contribute to mechanism validation

Adapted from H. Curran, COST- Milan Summer School 2013

Flat Flame Burner

Counterflow burner Isolated Droplet in μg

Flow reactor


Premixed Laminar Flames

Adapted from A. Cuoci, A. Frassoldati. COST- Milan Summer School 2013


Shock Tube Device 53

The available reaction time is limited to 0.1-5 ms by the arrival of the reflected waves.

Experimental facilities: Stanford University (CA); Galway University (Ireland);Rensselaer Polytechnic, Troy (NY) and UIC University of Illinois at Chicago (USA);KAUST, Saudi Arabia; IVG Duisburg and RWTH Aachen, Germany…. and others

A diaphragm initially separates a high and a low pressure region. Bursting the diaphragm, a shock wave is generated and propagates. It acts as a piston. The reactants at low T/p are instantly heated and pressurized to high values.


Shock Tube Device as a Batch Reactor

The set of equations describing the mass, velocity and temperature profiles downstream of the shock (derived from conservation laws) are similar to the ones of an adiabatic batch reactor. Diffusion, and viscous effects are neglected.

Typical ignition times are on the order of 0.1-5. ms

54

Shock Tube

Shock attenuation and boundary layer interactions become important at longer times.


Typical results of Shock TubeIgnition Propensity of n-heptane and iso-octane

Fieweger et al, Combustion and Flame, 1997

% n-heptane

20

• At low temperatures the ignition behavior is stronglyfuel dependent

• High temperature kinetics isless sensitive to the fuelstructure

Octane Number (ON)Mix of Reference Fuels

Ignition delay times0.1-5. ms


Ignition delay times of PRF

iso-octane

n-heptane

High Temperature Mechanism Low

Temperature Mechanism


n-C12H26 oxidation ( 1410 K, and 2.3 atm)

57

Davidson, D. F., Hong, Z., Pilla, G. L., Farooq, A., Cook, R. D., & Hanson, R. K. (2011). Multi-species time-history measurementsduring n-dodecane oxidation behind reflected shock waves. Proc. Comb. Institute, 33(1), 151-157.

Multi-species time-history

Typical results of Shock Tube


58

http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2011/adv_combustion/ace054_gupta_2011_o.pdf

The fuel-air mixture is initially contained in a chamber with one or two pistons forming the end walls. At t = 0, the pistons are driven into the chamber and the reactants are rapidly heated and compressed to high temperature and pressure.

Rapid Compression Machine (as Batch Reactor)


This technique allows to determine the ignition delay times (10-300 ms), and the temporal variation of the ‘uniform’ mixture, temperature, and composition.

The system is supposed to reach the ideal conditions of a batch reactor.

Rapid Compression Machine (as Batch Reactor)

Galway University


60

Rapid Compression Machine

Creviced Piston

The formation of a vortex on the piston face disrupts the uniformity inside the RCM.

Sung, C. J., & Curran, H. J. (2014). Using rapid compression machines for chemical kinetics studies. Progress in Energy and Combustion Science, 44, 1-18.


Creviced Piston in RCM 61

Experimental facilitiesNUI. Galway University (Ireland); MIT and Argonne Natl Lab. (USA); University of Leeds, UK; CNRS Lille (France); University of Stuttgart and Munich (Germany); KAUST, Saudi Arabia; Tokyo and Gifu University (Japan); and … many others.


Are Shock Tube and RCM Ideal Batch Reactors?

62

Ignition delay time data at high pressure and low temperatures are of interest mainly for the validation of kinetic mechanisms at practical engine conditions.

Propane ignition delay time measurements with ST and RCM at 20-40 atm


RCM

ST

RCM and Shock Tube (ST) data can disagree at high pressures and T < 1100 K (and high τ). Ignition delay times obtained from ST are faster than those from RCM

(and model predictions).Moreover, data from different RCMs can also disagree for different ‘facility effects’.


Non-reactive experiments, where N2 replaces O2, characterize the heat loss during compression.

Heat loss effects can be modelled as change in volume. A volume profile is deduced from the pressure profile assuming isentropic behaviour:

( )2 2

1 1

Tp Vp V

γ

=

Rapid Compression MachineIdeality and ‘Facility effects’

The RCM determines the ignition delay times, and can be simulated as a batch reactor at constant volume…..but…

Adapted from H. Curran, COST- Milan Summer School 2013


RCM: variable volume simulations 64

butyl-benzene/air at Ф=1, compressed gas T=893 K, P=10 atm.RCM simulation using OpenSMOKE++ and the kinetic mechanism of Nakamura et al. [2014]. Effect of variable volume simulations.

Cuoci, A., Frassoldati, A., Faravelli, T., & Ranzi, E. (2015). OpenSMOKE++: An object-oriented framework for the numerical modeling of reactive systems with detailed kinetic mechanisms. Computer Physics Communications, 192, 237-264.

The ignition delay time changes from

20 to 60 ms, acconting for the

‘facility effect’V/Vo=f(t)


65Shock Tube Device

0 1000 2000 3000 4000

0

40

80

τign

1003 K29.9 atmC3H8-Air φ = 0.5

Sign

al (A

U)

Time (µs)

Pressure, atm CH* Emission

Standard Ignition

0 2000 4000 6000 8000 10000

0

20

40

60

80

100

τign

τ1

827 K19.7 atmCH4/C2H6-Air φ = 0.5

Sign

al (A

U)Time (µs)

Pressure, atm CH* Emission

Early Pressure Rise

ST simulations assuming a dP/dt of 2-7% -/ms agrees

better at lower temperatures.


N2

Ar

66Shock Tube Device (simulation using dP/dt)

Propane ignition delay time measurements at low temperatures and high pressures for fuel in ‘air’ mixtures at Ф=0.5, PC=30 atm, - in N2 and Ar, and comparison with a traditional constant energy and volume (or density) simulation and with a simulation using dP/dt =7% [-/ms].


At temperatures below 1100 K, the predicted ignition delay times using an adiabatic, constant volume simulation are slower than experiments. Simulations assuming a dP/dt of 7%/ms agrees better at the lower temperatures.


The reactants steadily burn and the products exit from the reactor chamber, at controlled flow rates, i.e. residence times.

Concentrations of the products, ignition and extinction of the reacting mixture are determined, as a function of Temperature, pressure and residence time.

Jet-Stirred Reactors (JSR) and Perfectly-Stirred Reactors (PSR)

Jet Stirred Reactor (CNRS Orléans)

A fused silica jet-stirred reactor (~30 cm3):4 injectors of 1 mm i.d.The fuels and O2 are diluted with N2. Controlled Flow rates res. Time.On line GC with FID/MS

Dagaut et al., Proc. Comb. Inst. 2013

Further Experimental Devices:LRGP (ENSIC)-Lorraine University (France)University of Science and Technology, Hefei (China)CNR-IRC Naples (italy)

…and many others

Reactants are injected into a spherical chamber with a high injection velocity in order to reach an instantaneous mixing and uniform conditions within the reactor.


JSR CNRS OrleansOxidation of butanol− n-heptane mixtures

68

Dagaut, P., & Togbe ́, C. (2009). Experimental and modeling study of the kinetics of oxidation of butanol− n-heptane mixtures in a jet-stirred reactor. Energy & Fuels, 23(7), 3527-3535.


Turbulent Flow Reactor

Fuel injectionOperating Conditions


70

http://www.princeton.edu/~combust/MURI/papers/Dryer_presentation.pdf

Ideal Plug Flow Reactor Assumption


71Initial Mixing effect on the overall reaction time

tot PSR PFRτ τ τ= +

2276 ppm CH4, 3.69% O2, 4% H2O in N2T=1165 K

At different 𝜏𝜏𝑃𝑃𝑃𝑃𝑃𝑃, the CO profiles, can be overlayedidentically by time shifting.

Dryer, F. L., Haas, F. M., Santner, J., Farouk, T. I., & Chaos, M. (2014). Interpreting chemical kinetics from complex reaction–advection–diffusion systems: Modeling of flow reactors and related experiments. Progress in Energy and Combustion Science, 44, 19-39.

The overall residence time in the PFR isreduced to account for a residence time inside the mixing zone (PSR):

In absence of reactions in the mixing zone a simple time shifting is

sufficient to compare model predictions and experiments.


Reactions in the mixing zoneOxidation of t-butanol (Fuel/O2/N2 2500/15,000/982,500 ppm).

t-C4H9OH iC4H8 + H2O

72

Lefkowitz, J. K., Heyne, J. S., Won, S. H., Dooley, S., Kim, H. H., Haas, F. M., ... & Ju, Y. (2012). A chemical kinetic study of t-butanol in a flow reactor and a counterflow diffusion flame. Comb. Flame, 159(3), 968-978.

Experimental data are symbols and lines are Grana et al. [2011] kinetic model computations.

Fuel concentration is initially larger than that in the test. It is necessary to modify the initial conditions.

12.5 atmτ= 1.8 s

Line represents the end of the mixer region, and horizontal lines shows that there is no conversion after the mixing region.

Similar reactivity, before the complete mixing, is also observed for different oxygenated fuels.

Pyrolysis Experimentt-butanol/N2=2500/997,500 ppm

Concentrations vs τ in the flow reactor.


University of Ghent (Belgium)The reactor is a 6-mm internal diameter tube ~1. 5 m long.Van Geem et al., Comb. Sci. Technology, 184:7-8, 942-955

While highly turbulent reactors (Reynolds >> 2100) are typical cases where PFR is a good approximation, PFR assumption is not any more valid for laminar flow reactors since the radial velocity profile induces large variations of composition and temperature.

University of Zaragoza (Spain)It has a reaction zone of 8.7 mm diameter and 200 mm in length. Alzueta, Oliva and Glarborg, Int. J. Chem. Kinetics 30(9) 1999

Technical University of Lingby (Denmark)The reactor is a 12-mm internal diameter tube of 2 m.Rasmussen, Glarborg et al. (2004) Comb.Flame, 136, 91-128.

Laminar Flow Reactors:


Laminar Flow Reactors 74

αβ

χ ββ

∞ −

= − ∫Ae d3

1

1 2 00

α β= ⋅ =tk t et

with:A B Conversion:

An approximated solution is:

Aris-Taylor dispersion coefficient⋅

= +⋅

*Lam

Lam

R vD DD

2 2

48

Hilder, M.H. Trans. Ichem E 59 p143(1979)

τχ τ ττ τ

+= + + ⋅=

.. ( . )( )

kA k k e 0 5

0

0 25 1 0 252

The PFR assumption gives large errors, mainly at high conversions.

A. Frassoldati. COST- Milan Summer School 2013

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3

Adim

ensio

nal c

once

ntra

tion

Da=kτ

PFR

PSR

Laminar flow without diffusion

PFR with axial diffusion

Series of 3 PSRs

PFR

PSR


A. Frassoldati. COST- Milan Summer School 2013

Examples of Laminar Flames and Devices


The opposed-flow geometry is an attractive experimental configuration, because the flames are flat, allowing for autoignition and extinction esperiments, as well as detailed study of the flame chemistry and structure.

Mathematically the 2D flow can be reduced to 1D. it leads to a significant simplification.

Kinetic studies can be performed using detailed mechanisms in a reasonable CPU time.

Counterflow Diffusion Flames

T. Bieleveld et al. / Proceedings of the Combustion Institute 32 (2009) 493–500


F.N. Egolfopoulos, N. Hansen, Y. Ju, K. Kohse-Höinghaus, C.K. Law, F. Qi ‘Advances and challenges in laminar flame experiments and implications for combustion chemistry’ Progress in Energy and Combustion Science, 43, 2014, 36-67

bar

Laminar flame studies at various pressures. Experimental approaches and measurements


78

Ignition delay time

Concentration–time profile

Laminar burning velocity

Rapid compression machine (RCM)

Shock tube

Flow reactor

Jet-stirred reactor (JSR)

Bunsen burner (flame cone method)Counterflow twin-flame configuration

Heat flux burner

Adiabatic system with the volume as a function of time

Adiabatic system withconstant volume

Steady, laminar, one-dimensional premixed flame

Perfectly stirred reactor(spatially homogeneous mixture)

SENKIN

PSR

PREMIXOpenSMOKE

MEASUREMENT FACILITY MODELING APPROACH SOLVER

Ideal Experimental Systems and Simulation framework

Courtesy of Carsten Olm (2015) Eotvos University. Budapest. Hungary


Outline 79

Chemical Kinetics:Combustion of methane.Low and high temperature combustion of

hydrocarbons.

Reactions in ideal reactors with negligible molecular transport: plug flow and stirred reactors, shock tubes and rapid compression machines. Premixed and diffusion flames.

PAH and Soot formation.


PAH and Soot formation 80

S. Mary MagdalenGeorge de La Tour’s (1593-1652)

“You would hardly think that all these substances which fly about London, in the forms of soots and blacks, are the very beauty of the flames…”

Michael Faraday, 1861 ‘The Chemical History of the Candle’


Soot emissions Combustion is the main source of PAH and Soot or Particulate Matter (PM). On-road and non-road diesel engines are leading emitters (about 70%) in Europe,

North America, and Latin America. Soot forms at high temperatures and rich conditions (it also lowers the efficiency).

Global BC Emissions in 2000by region and source

T.C. Bond, E. Bhardwaj, R. Dong, et al., Global Biogeochem. Cy. 21 (2007).

BC emissions (Gg)T.C. Bond, S.J. Doherty, D.W. Fahey, et al., J. Geophys.

Res-Atmos. 118 (2013) 5380-5552.

81


PM 10 (coarse particles) 10 μm

PM 1.0 (fine particles) 1 μm

PM 0.1 (ultrafine particles) 100 nm

Particle dimension is the critical issue

Oberdorster G. et al (2004): “Translocation of inhaled ultrafine particles to the brain”, Inhalation Toxicology, 16(6-7), 437.

Soot Impact on Human Health


1-80 nm

80 - 1000 nm

1000 - 10000 nm

Numberconcentration

Mass concentrazione

80

60

100

20

40

0

%Fine and ultrafine particles in PM10

Ultrafine Particles are more dangerous than larger ones [1]. Ultrafine Particles are more numerous and with larger surface areas [2] .

[1] Oberdorster G. et al., Inhalation Toxicology, (2004) 16 (6-7) : 437[2] Woo et al., Aerosol Sci. Technol., (2001) 34: 75-87


Elemental Carbon (EC) and Organic Carbon (OC)rises when particle dimensions decrease.

Chemical Composition of Atmospheric Aerosol

National Ambient Air Quality Standards for Particulate Matter: Policy Assessment of Scientific and Technical Information OAQPS Staff Paper EPA-452/R-05-005 June 2005

Senfield & Pandis, (1998)

Multi-modal distribution of atmospheric aerosol


Soot Formation in Combustion 85

CH CH3

CH2CH2

CHCH

CH2

CH

CH2CH

O2

COCO2

CH2

CH2

0 ms

1 ms

10 ms

100 ms

CH4

H. Bockhorn ‘Soot Formation in Combustion’ Springer-Verlag Berlin Heidelberg 1994

NucleationSoot Inception

Surface growth

Coagulation

100 nm

1 nm


ParticleAggregation


Growing Mechanisms of PAH

- H

- H

- H+ H- H2

+ H

+ C2H2 C2H C2H

+ C2H2

+ C2H2

HACA (1)

+ + 2 H

+ 2 H+

Resonantly Stabilized Radicals (2)

(1) Frenklach M. and Wang H., (1990). 23rd Symposium on combustion, The Combustion Institute, Pittsburgh, p. 1105(2) Miller J. A. and Melius C. F., (1992). Combust. Flame, 114:192


0.0

0.2

0.4

0.6

0.8

1.0

1.2

1 2 3 4 5 6log MW

H/C

CHCH

CH

CHPAHs

Soot Molecular Weight and H/C ratio

aromers tar

Soot

A.D’Anna (2004) Università ‘Federico ii’ Napoli adapted from:

Growing Mechanisms of PAH and Soot


Soot: Primary particles ~ 20nm, and aggregate ~ 500nm

(Laminar Diffusion Flame, Dobbins RA & Megaridis)

A.Sarofim. ‘The Dark and Light Sides of Soot: Impacts on Human Health, Luminous Radiation, and Global Climate’ NIST (2003)

Soot Microstructure and SEM imagine of a Soot Aggregate

Soot Microstructure.

Soot: Primary particle ~ 20nm

Basic Structural Unit

~ 0.35nm between the different layers


Sooting Processin Homogeneous Combustion

1 ms

10 ms

100 ms

C H2CH

CH2

CH2

CH CH3

CH2CH2

CHCH

CH2

CH

O2

COCO2

CH4


1 nm

Combustion

Particle inception

Surface Growth

Coagulation

100 nm


90

C2H4

C2H3

C2H2

CH2CHO

CH2COHCCO

pC3H4

C3H3 C6H6

C6H5C6H5O

C6H5C2H

C6H4C2H C10H7 C12H8 C14H10 C16H10 SOOT

PAH Formation in Ethylene Flames

Then HACA mechanism (C2H2) is the main responsible

for Soot formation.

HACA mechanism

Benzene and acetylene are the main intermediates in the first PAH formation.


Overall Oxidation Mechanism 91

Hierarchy and Modularityare the main features of Detailed Kinetic Schemes

• GRI scheme for Gases

• PRF (nC7-iC8) and additivesfor Gasolines

•Alcohols•Diesel and Jet Fuels•Biofuels – FAME – FAEE •HT C2H2 and Benzene Kinetics

CO

C3

CH4

C2

nC7-iC8

H - O2 2

Diesel Fuels

Alcohols

The soot kinetic model is based on a discrete sectional approach with an

extensive use of lumping rules.

#Species #Reactions

104 ~8400

SOOT


92Soot kinetic model: pseudo-species

First soot particle: BIN5 with 320 carbon atoms and equivalent spherical diameter of ~2 nm.

Primary particle: BIN12 with an equivalent spherical diameter of ~10 nm (dp).

Spherical shape (ρsoot = 1.5 g/cm3) up to BIN12 and then aggregates of Np primary particles.

Heavy PAHs (after pyrene) and soot particles are divided in classes of pseudo-species (BINs) and their thermodynamics is estimated using the Benson’s group additivity method.

C. Saggese, S. Ferrario, J. Camacho, A. Cuoci, A. Frassoldati, E. Ranzi, H. Wang, T Faravelli, Combust. Flame (2015)

Molecular mass increase from a class to the next one First heavy PAH: with 20 carbon atoms as

corannulene.

H/C ratio

Homann & Wagner, Proc. Combust. Inst. 1967, 371-379.


93Soot kinetic model: reaction classes

Particles kinetic can be defined in analogy with the gas phase chemistry following aerosol

dynamic principles.

6 reaction classes are defined:

HACA mechanism Soot inception Surface growth Dehydrogenation Particle coalescence and aggregation Oxidation

C. Saggese, S. Ferrario, J. Camacho, A. Cuoci, A. Frassoldati, E. Ranzi, H. Wang, T Faravelli, Combust. Flame (2015)


94Soot kinetic model: reaction classesParticle coalescence and aggregation

Particle aggregation occurs between BINs from BIN13 to BIN20

A. D’Anna; M. Sirignano; J. Kent, Combustion and Flame 157 (2010) 2106-2115


Comparison with experimental data: particle size distribution functions (PSDF)

95

Discrete sectional method is employed to solve the time evolution of the particle size distribution function of the ethylene flame

J. Camacho; C. Liu; C. Gu; H. Lin; Z. Huang; Q. Tang; X. You; C. Saggese; Y. Li; H. Jung; L. Deng; I. Wlokas; H. Wang, Combust. Flame (2015) C. Saggese, S. Ferrario, J. Camacho, A. Cuoci, A. Frassoldati, E. Ranzi, H. Wang, T Faravelli, Combust. Flame (2015)


96

presentazione di powerpoint - nccrd.innccrd.in/iciwsindia2015-assets/docs/ranzi2.pdf · c2h6. c3h8....

Documents