An Improved Reaction Mechanism for Combustion of Core (C0...

An Improved Reaction

Mechanism for Combustion of

Core (C0-C4) Fuels

Chitralkumar V. Naik*,

Karthik V. Puduppakkam, Ellen Meeks

July 13, 2011

The 7th International Conference on Chemical Kinetics

ICCK 2011 2

Outline

● Why core hydrocarbons?

● Issues with existing reaction mechanisms

● Modeling approach

● Extensive model validation

● Concluding remarks

ICCK 2011 3

Core hydrocarbons include all gaseous fuels

● Natural gas

– 425,000 wells in the US

– 105 trillion cubic feet worldwide

● Syngas, coal gas

– From gasification of coal, biomass

– In-situ coal gasification

● Biofuels

– Ethanol, butanol

● H2, C1-C4 hydrocarbons

– Selected oxygenates A coal-gas powered taxicab in England, 1920.

Natural gas well, 1907.

ICCK 2011 4

Core hydrocarbons chemistry is core to

heavier liquid fuels

Hydrogen C1

C2

C3

C4

Liquid hydrocarbon combustion and emissions

Oxygenated species

NOx chemistry

Unsaturated species

PAH precursors

ICCK 2011 5

Outline

● What are core hydrocarbons?





ICCK 2011 6

Typical issues with many existing reaction

mechanisms

1. Applicability

– Not comprehensive enough

2. Accuracy

– Not accurate enough for broad range of conditions

3. Extensive validation

– Limited validation range and types of experiments

ICCK 2011 7

Core chemistry in large mechanisms may

not be accurate or validated

Data from Jomaas et al.

2005 10

20

30

40

50

0.6 0.9 1.2 1.5

Fla

me

sp

ee

d (c

m/s

)

Equivalence ratio

Ethane/air, 298 K

1 atm

2 atm

5 atm

● Predictions for unsaturated components are worse

● LLNL n-heptane

mechanism

overpredicts

ethane flame

speeds

ICCK 2011 8

Often mechanisms are validated for one or

two types of experiments

Flow reactor

Shock tube

Stirred reactor

Opposed-flow reactor

Laminar flames

RCM

● Together they cover the conditions in practical

applications (engines, turbines)

ICCK 2011 9

Legacy fuel models are not suitable for

applications at high-P and low-T

Natural Gas Components Composition

Methane 70-90 %

Ethane, Propane, Butane 0-20 %

CO2 0-8 %

N2, H2S 0-5 %

Core hydrocarbons (C0-C4)

– GRI-mech limited to methane

Complex Negative Temperature Coefficient (NTC)

behavior <1000 K

– Autoignition of fuels critical for engines and turbines

ICCK 2011 10

Existing core mechanisms may not be

comprehensive

● Dependencies between various small species

– e. g. ethylene chemistry also involves C4 species

– e.g.

● Missing components and inconsistencies

may affect predictive nature

– Be careful in merging various mechanisms

CH4 CH3 CH3O O2

H

CH3 CH3OCH3

CH3OH

Need comprehensive, accurate, and validated

core reaction mechanism

ICCK 2011 11

Outline






ICCK 2011 12

Comprehensive

mechanism

The mechanism is generated in a systematic

way

Validation

Reactions

Rate Parameters

Thermo

Transport

Initial sub-mechanisms

from publications

- Add missing or necessary

reactions

- Enforce self-consistency

- Update k and thermo

- Optimize k within the

uncertainty

Accurate

mechanism

ICCK 2011 13

RD2010 core mechanism is comprehensive

● Starting component sub-mechanisms from

various sources

– Merged to a consistent base

● Included low- and high-temperature pathways

● Updated rate constants based on recent

studies

– Pressure-dependent rate constants for important

reaction systems

● 18 fuel components

– 1161 species and 5622 elementary reactions

– Saturated, un-saturated, oxygenates

– NOx, soot precursors up to benzene

ICCK 2011 14

The core mechanism is optimized for

accuracy

● Optimized rate constants for less than 5%

of the reactions

– Within the expected uncertainty

Reaction A n Ea Ref.

H+O2 = O+OH 3.55E+15 -0.406 1.66E+04 [12]

CO+OH = CO2+H 2.20E+05 1.89 -1.16E+03 [12], A*1.24

HCO+M = H+CO+M 4.75E+11 0.7 1.49E+04 [11]a

H+OH+M = H2O+M 4.50E+22 -2 0.00E+00 [19]a

C3H5-a+H(+M) = C3H6(+M) 2.00E+14 0 0.00E+00 [13]

Low pressure limit: 1.33E+60 -12 5.97E+03

Troe parameters: 0.02, 1.10E+03, 1.10E+03, 6.86E+03

CH3+CH3(+M) = C2H6(+M) 9.21E+16 -1.17 6.36E+02 [12]a

Low pressure limit: 1.14E+36 -5.246 1.71E+03

Troe parameters: 0.405, 1.12E+03, 69.6, 1.00E+10

CH3+HO2 = CH3O+OH 1.00E+12 0.269 -6.88E+02 [12]

CH4+H = CH3+H2 6.14E+05 2.5 9.59E+03 [12]

HO2+HO2 = H2O2+O2 4.20E+14 0 1.20E+04 [18]b

1.30E+11 0 -1.63E+03

CH4+HO2 = CH3+H2O2 1.13E+01 3.74 2.10E+04 [12]

Collision efficiencies: CH4 2.0, CO 1.9, CO2 3.8, C2H6 3.0, H2O 6.0, H2 2.0, Ar 0.7

ICCK 2011 15

Example of the CO+OH = CO2+H rate

constants

● 118 records in the NIST

database

– 91 experimental data

– 20 theoretical calculations

– 7 review recommendations

– 1981-2007 data fit

k = 2e11 * exp(-300/RT)

● Recent data

– Agree within 25%

● Typical uncertainty ~

factor of 2

ICCK 2011 16

Outline






ICCK 2011 17

Fundamental data are the best for reaction

mechanism validation

● Reduced impact of transport ➔

More focus on chemistry

Neat fuels

Laminar

flame speed

Shock

tube

Flow

reactor

Stirred

reactors

Burner

flames

Hydrogen √* √

Formaldehyde √

Methane √ √ √

Methanol √

Ethane √ √

DME √ √ √

Ethanol √

Propane √

n-Butane √ √

iso-Butane √

n-Butanol √ √ √

NOx √ √ 11

sa

tura

ted

fu

el

co

mp

on

en

ts

ICCK 2011 18

Broad range of conditions covered

Experiment type T (K) P (atm)

Dilution of

oxidizer %*

Laminar flame

speeds 295 to 453 1 to 5 0.6 to 1.6 0 to 15

Shock-tubes 650 to 1800 1 to 340 Pyrolysis,

0.3 to 3 0 to >98

Flow reactors 500 to 948 1.5 to 12.5 0.005 to 1.19 97.6 to 98.8

Stirred reactors 700 to 1100 1-10 0.1 to 1 97.1 to 97.9

Burner-stabilized

flames 300 1 to 14.6 0.6 to 0.8

N2/O2 : 2.2/1 for

oxidizer

● Dilution with N2, Ar, or H2O with air or O2 as oxidizer.

● Saturated fuels

ICCK 2011 19

Broad range of conditions covered for un-

saturated components and blends

● Un-saturated fuels


Dilution of

oxidizer %

Laminar flame

speeds 298 1-5 0.6-2 0-16.5

Shock-tubes 1050-2250 0.85-9 0.5-2 0-97

Flow reactors 700-1350 1 0.05-1.4 97.9-99.7

Stirred reactors 700-1100 1-10 0.1-1.5 96-99.5


Dilution of

oxidizer %

Laminar flame

speeds 298 1-2 0.5-4.5 0-16.5

Shock-tubes 1020-1750 1-256 0.5-3 92-99.9

Stirred reactors 950-1450 1-10 0.3-2 66.1-98.5

● Fuel blends

ICCK 2011 20

Over 90+ comparison plots for validation

● A comparison plot contains one or more

data sets

● Overview of comparisons

– Grouped by experiment types

● Lines – predictions

● Symbols – published experimental data

ICCK 2011 21

Laminar flame speeds: Fuel, T, P, and effects captured

5

25

45

65

85

0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Equivalence ratio

La

min

ar

fla

me

sp

ee

d (

cm

/s) Gulder et al., 300K

Egolfopoulos et al., 298K



Liao et al., 358K

Model, 298K

Model, 360K

Model, 453K

Ethanol

10

20

30

40

50

0.6 0.8 1 1.2 1.4 1.6Equivalence ratio

La

min

ar

fla

me

sp

ee

d (

cm

/s)

Jomaas et al., 1 atm

Vagelopoulos et al., 1 atm



Model, 1 atm

Model, 2 atm

Model, 5 atm

Propane

P

Ethylene

Fuel/air

P

Tin

ICCK 2011 22

Laminar flame speeds:

Diluents and composition effects captured

● H2 with N2 or Ar

0

50

100

150

200

250

300

350

400

0 1 2 3 4 5Equivalence ratio

La

min

ar

fla

me

sp

ee

d (

cm

/s)

Aung et al.

Dowdy et al.

Tse et al.

Kwon et al.

Vagelopoulos et al.

Verhelst et al.

Model

Kwon et al., 79/21 Ar/O2

Model, 79/21 Ar/O2

● Syngas with 50/50

to 95/5 CO/H2

H2

Ar

N2

ICCK 2011 23

Autoignition delay times: Effects of fuel, T, P, and captured

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0.5 0.7 0.9 1.11000/T (1/K)

Ign

itio

n d

ela

y t

ime

(s

) Herzler et al., 1 atm

Herzler et al., 4 atm

Herzler et al., 16 atm

Model, 1 atm

Model, 4 atm

Model, 16 atm

H2

Propyne

90/10

Methane/propane

P

P

Fuel-rich

O2/Ar

O2/Ar

ICCK 2011 24

Autoignition delay times:

Effects of loading and diluents captured

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

0.5 0.6 0.7 0.8 0.9

1000/T (1/K)

Ign

itio

n t

ime

(s

)

Model, 0.6% fuel

Model, 3.5% fuel

Black et al., 3.5% fuel

Black et al., 0.6% fuel

n-Butanol

Ethylene

loading

O2/Ar

ICCK 2011 25

Species profiles: Shock-tube

Accurate predictions at very high pressures

● Pyrolysis at 340 bar

0.0E+00

5.0E-05

1.0E-04

1.5E-04

2.0E-04

2.5E-04

3.0E-04

1000 1100 1200 1300 1400 1500Inlet temperature (K)

Mo

le f

racti

on

Sivaramakrishnan et al., ethane

Sivaramakrishnan et al., ethylene

Sivaramakrishnan et al., acetylene

Model, ethane

Model, ethylene

Model, acetylene

250 ppm ethane in Ar

Uncertainty in

: 1.25 - 1.6 ms

ICCK 2011 26

Species profiles: Flow-reactor Effect of on propyne oxidation captured

● Products acetylene

and allene predicted

well

– Important PAH

precursors

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

0.00 0.05 0.10

Time (sec)

Pro

py

ne

mo

le f

rac

tio

n Davis et al., phi of 0.7

Davis et al., phi of 1

Davis et al., phi of 1.4

Model, phi of 0.7

Model, phi of 1

Model, phi of 1.4

0.0E+00

5.0E-04

1.0E-03

1.5E-03

2.0E-03

2.5E-03

3.0E-03

0.00 0.05 0.10

Time (sec)

Mo

le f

rac

tio

n

Davis et al., propyne

Davis et al., allene

Davis et al., acetylene

Model, acetylene

Model, allene

Model, propyne

● 1170 K, 1 atm,

0.03% fuel Fuel-rich

= 1

ICCK 2011 27

Species profiles: Stirred-reactor

Products evolution in methanol captured

● Methanol oxidation at 10 atm in a stirred-reactor

0.0E+00

2.0E-03

4.0E-03

6.0E-03

8.0E-03

1.0E-02

700 800 900 1000 1100Inlet temperature (K)

Mo

le f

rac

tio

n

Dyma et al., CH3OH

Model, CO

Dyma et al., CO

Model, CO

Dyma et al., CO2

Model, CO2

Dyma et al., CH2O

Model, CH2O

0.6, 1 s 8000 ppm fuel, 800 ppm H2O

ICCK 2011 28

NOx emissions from premixed flames: Pressure and effects captured

● High-pressure premixed flames

– Radiation heat losses included in CHEMKIN-PRO

Methane with

N2/O2 in 2.2/1 ratio

Tflame: 1750-1900 K

Fuel-rich

ICCK 2011 29

Outline


● The problem and our modeling approach



ICCK 2011 30

Accurate predictions need comprehensive

core mechanism

● Sensitivity analysis for autoignition delay time

-0.15 -0.1 -0.05 0 0.05 0.1

CH3+O2 = CH2O+OH

CH3O2+CH3 = 2CH3O

CH3+HO2 = CH3O+OH

CH4+HO2 = CH3+H2O2

CH3+HO2 = CH4+O2

HCO+HO2 = CH2O+O2

H+O2(+M) = HO2(+M)

CH4+H = CH3+H2

2HO2 = H2O2+O2

2CH3(+M) = C2H6(+M)

Temperature sensitivity coefficient

Methane/air, 3

140 atm, 1400 K

Methane Ethane Formaldehyde

Ethanol Ethylene Acetaldehyde

ICCK 2011 31

Comprehensiveness of the core mechanism

even more important for blends

ICCK 2011 32

Chemistry of unsaturated fuels is more

involved than their saturated counterparts

● Allene oxidation in a stirred-reactor

– 1000 K, 10 atm, 1.5 s, and phi of 1.5

● Reactions of C1 and C2 most sensitive

● Reactions of C4 and C5 also important

– C5H6+H <= C2H2+C3H5-a

ICCK 2011 33

Summary

● Successful development of a detailed core

(H2, C1-C4) reaction mechanism

– Comprehensive: 18 neat fuels and their 8+ blends

Includes oxygenated fuels

NOx, and PAH precursors

● Successful validation using broad range of

conditions and experiments

– Over 90+ comparison plots using 30 years of the data

from the publications

* Results on saturated components to appear in

J. Eng. Gas Turbines Power

ICCK 2011 34

Thank You

An Improved Reaction Mechanism for Combustion of Core (C0...

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