An Improved Reaction Mechanism for Combustion of Core (C 0 -C 4 ) Fuels

Transcription

1 An Improved Reaction Mechanism for Combustion of Core (C 0 -C 4 ) Fuels Chitralkumar V. Naik*, Karthik V. Puduppakkam, Ellen Meeks July 13, 2011 The 7th International Conference on Chemical Kinetics

2 Outline Why core hydrocarbons? Issues with existing reaction mechanisms Modeling approach Extensive model validation Concluding remarks 2

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 Natural gas well, H 2, C 1 -C 4 hydrocarbons Selected oxygenates A coal-gas powered taxicab in England,

4 Core hydrocarbons chemistry is core to heavier liquid fuels C 2 Oxygenated species C 1 Hydrogen C 3 NO x chemistry C 4 Unsaturated species PAH precursors Liquid hydrocarbon combustion and emissions 4

5 Outline What are core hydrocarbons? Issues with existing reaction mechanisms Modeling approach Extensive model validation Concluding remarks 5

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 6

7 Core chemistry in large mechanisms may not be accurate or validated Flame speed (cm/s) Ethane/air, 298 K 1 atm 2 atm LLNL n-heptane mechanism overpredicts ethane flame speeds 20 5 atm Equivalence ratio Data from Jomaas et al Predictions for unsaturated components are worse 7

8 Often mechanisms are validated for one or two types of experiments Shock tube Laminar flames Stirred reactor RCM Flow reactor Opposed-flow reactor 8 Together they cover the conditions in practical applications (engines, turbines)

9 Legacy fuel models are not suitable for applications at high-p and low-t Natural Gas Components Composition Methane % Ethane, Propane, Butane 0-20 % CO % N 2, H 2 S 0-5 % Core hydrocarbons (C 0 -C 4 ) GRI-mech limited to methane Complex Negative Temperature Coefficient (NTC) behavior <1000 K Autoignition of fuels critical for engines and turbines 9

10 Existing core mechanisms may not be comprehensive Dependencies between various small species e. g. ethylene chemistry also involves C 4 species e.g. O 2 CH 4 CH 3 CH 3 O H CH 3 OH CH 3 CH 3 OCH 3 Missing components and inconsistencies may affect predictive nature Be careful in merging various mechanisms Need comprehensive, accurate, and validated core reaction mechanism 10

11 Outline What are core hydrocarbons? Issues with existing reaction mechanisms Modeling approach Extensive model validation Concluding remarks 11

12 The mechanism is generated in a systematic way Initial sub-mechanisms from publications Reactions Rate Parameters Thermo Transport - Add missing or necessary reactions - Enforce self-consistency - Update k and thermo - Optimize k within the uncertainty Comprehensive Accurate mechanism Validation 12

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 NO x, soot precursors up to benzene 13

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+O 2 = O+OH 3.55E E+04 [12] CO+OH = CO 2 +H 2.20E E+03 [12], A*1.24 HCO+M = H+CO+M 4.75E E+04 [11] a H+OH+M = H 2 O+M 4.50E E+00 [19] a C 3 H 5 -a+h(+m) = C 3 H 6 (+M) 2.00E E+00 [13] Low pressure limit: 1.33E E+03 Troe parameters: 0.02, 1.10E+03, 1.10E+03, 6.86E+03 CH 3 +CH 3 (+M) = C 2 H 6 (+M) 9.21E E+02 [12] a Low pressure limit: 1.14E E+03 Troe parameters: 0.405, 1.12E+03, 69.6, 1.00E+10 CH 3 +HO 2 = CH 3 O+OH 1.00E E+02 [12] CH 4 +H = CH 3 +H E E+03 [12] HO 2 +HO 2 = H 2 O 2 +O E E+04 [18] b 1.30E E+03 CH 4 +HO 2 = CH 3 +H 2 O E E+04 [12] Collision efficiencies: CH 4 2.0, CO 1.9, CO 2 3.8, C 2 H 6 3.0, H 2 O 6.0, H 2 2.0, Ar

15 Example of the CO+OH = CO 2 +H rate constants records in the NIST database 91 experimental data 20 theoretical calculations 7 review recommendations data fit k = 2e11 * exp(-300/rt) Recent data Agree within 25% Typical uncertainty ~ factor of 2

16 Outline What are core hydrocarbons? Issues with existing reaction mechanisms Modeling approach Extensive model validation Concluding remarks 16

17 Fundamental data are the best for reaction mechanism validation 11 saturated fuel components 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 17

18 Broad range of conditions covered Saturated fuels Experiment type Laminar flame speeds T (K) P (atm) Dilution of oxidizer %* 295 to to to to 15 Shock-tubes 650 to to 340 Pyrolysis, 0.3 to 3 0 to >98 Flow reactors 500 to to to to 98.8 Stirred reactors 700 to to to 97.9 Burner-stabilized flames to to 0.8 N 2 /O 2 : 2.2/1 for oxidizer Dilution with N 2, Ar, or H 2 O with air or O 2 as oxidizer. 18

19 Broad range of conditions covered for unsaturated components and blends Un-saturated fuels Dilution of T (K) P (atm) Experiment type oxidizer % Laminar flame speeds Shock-tubes Flow reactors Stirred reactors Fuel blends Dilution of T (K) P (atm) Experiment type oxidizer % Laminar flame speeds Shock-tubes Stirred reactors

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 20

21 Laminar flame speed (cm/s) Laminar flame speeds: Fuel, T, P, and effects captured Laminar flame speed (cm/s) Fuel/air Propane Jomaas et al., 1 atm Vagelopoulos et al., 1 atm Jomaas et al., 2 atm 30 Jomaas et al., 5 atm Model, 1 atm 20 P Model, 2 atm Model, 5 atm Equivalence ratio 85 Ethanol Gulder et al., 300K Egolfopoulos et al., 298K Ethylene 65 T in Egolfopoulos et al., 363K Egolfopoulos et al., 453K Equivalence ratio 21 Liao et al., 358K Model, 298K Model, 360K Model, 453K P

22 Laminar flame speed (cm/s) Laminar flame speeds: Diluents and composition effects captured N 2 Ar 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 H 2 with N 2 or Ar Equivalence ratio H 2 Syngas with 50/50 to 95/5 CO/H 2 22

23 Ignition delay time (s) Autoignition delay times: Effects of fuel, T, P, and 1.E-02 captured 1.E-03 1.E-04 H 2 Herzler et al., 1 atm Herzler et al., 4 atm Herzler et al., 16 atm Model, 1 atm P 1.E-05 1.E-06 P /T (1/K) Model, 4 atm Model, 16 atm O 2 /Ar 90/10 Methane/propane Propyne Fuel-rich O 2 /Ar 23

24 Autoignition delay times: Effects of loading and diluents captured Ignition time (s) Ethylene 1.E-02 1.E-03 n-butanol Model, 0.6% fuel Model, 3.5% fuel 1.E-04 1.E-05 loading Black et al., 3.5% fuel Black et al., 0.6% fuel 24 1.E /T (1/K) O 2 /Ar

25 Species profiles: Shock-tube Accurate predictions at very high pressures Mole fraction 3.0E E E-04 Uncertainty in 250 ppm ethane in Ar Sivaramakrishnan et al., ethane Sivaramakrishnan et al., ethylene 1.5E-04 Sivaramakrishnan et al., acetylene Model, ethane 1.0E E-05 Model, ethylene Model, acetylene : ms 0.0E Inlet temperature (K) Pyrolysis at 340 bar 25

26 Propyne mole fraction Species profiles: Flow-reactor Effect of on propyne oxidation captured Mole fraction 3.0E E E-03 Fuel-rich Davis et al., phi of 0.7 Davis et al., phi of 1 Davis et al., phi of K, 1 atm, 0.03% fuel 1.5E-03 Model, phi of E-03 Model, phi of 1 Model, phi of E E Time (sec) Products acetylene and allene predicted well Important PAH precursors 3.0E E E E E E E+00 = Time (sec) Davis et al., propyne Davis et al., allene Davis et al., acetylene Model, acetylene Model, allene Model, propyne 26

27 Species profiles: Stirred-reactor Products evolution in methanol captured Mole fraction 1.0E E ppm fuel, 800 ppm H 2 O 0.6, 1 s Dyma et al., CH3OH Model, CO 6.0E-03 Dyma et al., CO Model, CO 4.0E E-03 Dyma et al., CO2 Model, CO2 Dyma et al., CH2O Model, CH2O 0.0E Inlet temperature (K) Methanol oxidation at 10 atm in a stirred-reactor 27

28 NO x emissions from premixed flames: Pressure and effects captured Fuel-rich Methane with N 2 /O 2 in 2.2/1 ratio T flame : K High-pressure premixed flames Radiation heat losses included in CHEMKIN-PRO 28

29 Outline What are core hydrocarbons? The problem and our modeling approach Extensive model validation Concluding remarks 29

30 Accurate predictions need comprehensive core mechanism Sensitivity analysis for autoignition delay time 2CH3(+M) = C2H6(+M) 2HO2 = H2O2+O2 CH4+H = CH3+H2 H+O2(+M) = HO2(+M) HCO+HO2 = CH2O+O2 CH3+HO2 = CH4+O2 CH4+HO2 = CH3+H2O2 CH3+HO2 = CH3O+OH CH3O2+CH3 = 2CH3O CH3+O2 = CH2O+OH Methane/air, atm, 1400 K Temperature sensitivity coefficient Acetaldehyde Ethylene Ethane Methane Formaldehyde Ethanol 30

31 Comprehensiveness of the core mechanism even more important for blends 31

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 32

33 Summary Successful development of a detailed core (H 2, C 1 -C 4 ) reaction mechanism Comprehensive: 18 neat fuels and their 8+ blends Includes oxygenated fuels NO x, 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 33 * Results on saturated components to appear in J. Eng. Gas Turbines Power

34 Thank You 34