Non-Equilibrium Plasma-Assisted Flow Reactor Studies of Highly Diluted Reactive Mixtures

Non-Equilibrium Plasma-Assisted Flow Reactor Studies of Highly Diluted Reactive Mixtures R.A. Yetter, N. Tsolas, K. Togai Department of Mechanical and Nuclear Engineering The Pennsylvania State University University Park, PA, 1681 53 rd AIAA Aerospace Sciences Meeting, AFOSR MURI: Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion 5-9 th January 15, Kissimmee, FL

Objectives Development and validation of detailed lowtemperature plasma fuel oxidation and ignition mechanisms, starting with simple fuels and proceeding to surrogate fuels Identification of specific processes critical to the enhancement of basic combustion phenomena by nonequilibrium plasmas, in particular processes involving radical and/or excited metastable species.

Approach Flow reactor experiments between pressures from.1 and 3 bar and a temperature range of ambient to 1 K (quartz chamber). Perform dilute hydrocarbon oxidation experiments in excess nitrogen, argon, helium or oxygen to minimize the temperature rise of reaction and stretch the reaction spatially over a significant length. Perturb the reaction at different extents of reaction with a spatially defined plasma discharge. Measure temperature and product species by sample extraction and GC/FTIR analysis. In-situ measurements of radical species (OH LIF) Perform kinetic modeling of the reaction kinetics with sensitivity analysis.

PSU Experiments and Model Development Experiments: Plasma flow reactor experiment (T = 3 15 K) performed Outlet Inlet under highly diluted conditions using Ar/N/O/He Outlet mixtures to decouple interactions between plasma and thermal effects. Experiments performed to study the oxidative and pyrolysis enhancement Heating Zone = ft. characteristics of various fuels (H, CO, CH4, C1-C7 normal alkanes, specifically cool flame chemistry, and effects of NOx), and to elucidate its influence on the thermal chemistry. Ex-situ product species analysis using various diagnostic detectors (i.e., NDIR, FTIR, GC TCD and FID), capturing intermediate/product species as a result of perturbed in. reactions. In-situ measurements of radical species (OH LIF), in conjunction with team collaborators from OSU. Kinetic Modeling: CH4 specific plasma-assisted chemical mechanism analysis in collaboration with OSU. Newly developed model to predict plasma flow reactor experiments developed coupling CHEMKIN, SENKIN, and ZDPlasKin to perform sensitivity analysis. Development of new H to C1 mechanism with team collaborators. Rigorous sensitivity analysis performed on H/O/Ar mixtures predicts experimental results well, while elucidating kinetic features attributed to the observed enhancement in H-oxidation.

Plasma Flow Reactor 53 rd AIAA Aerospace Sciences Meeting, AFOSR MURI: Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion 5-9 th January 15, Kissimmee, FL

Plasma Discharge Assembly Cross-Section View (Front) Actual Image of Assembly in. Cross-Section View (Isometric)

Plasma Flow Reactor Assembly Actual Image Teflon End Caps w/ Electrode Feedthroughs The plasma discharge assembly is housed in a quartz tube (58mm I.D.) with Teflon end caps Actual flow reactor length is 154 mm (6 in.) in. The copper electrodes are connected to a ceramic insulated terminal feedthroughs Actual Image Discharge Assembly (Side View)

PFR Experimental Facility Heating is achieved using a three-zone tube furnace with a length 69 mm (4 in.) Product species analysis performed using ex-situ online NDIR, FTIR (not shown) and GC (both online and offline) in. Actual Image Tube Furnace w/ Plasma Flow Reactor Outlet Inlet

PFR Facility In Situ Measurments PFR facility has capability to perform in situ measurements, first experiments conducted on OH including OH thermometry for dilute H-oxidation studies using LIF in. ICCD camera mounted on a custom-built translational stage to obtain temporal/spatial measurements along the length of the reaction zone

Operating Conditions - Temperature Flow Direction Heating Ramp Inlet Reaction Zone (Isothermal) Cooling Ramp 14 Plasma Discharge 1 1153 ± 1 K 156 ± 1 K 1 8 6 959 ± 1 K 86 ± 1 K 765 ± 1 K 668 ± 1 K Sampling Location 4 L isothermal 4 6 8 1 1 Reactor Length [cm] Reactor Temperature Profile @ 1 LPM of Ar The measured temperature profiles are used to establish an empirical correlation with the tube furnace temperature over a range of 35 K < T iso < 15 K. The correlation is used for kinetic modeling. Initial experiments verify that heating from the plasma discharge is negligible in the diluent flow. In Ar/O flow, ~ 1 K increase in gas temperature observed at the maximum voltage setting for a given reactor temperature of 814K.

H Plasma Assisted Oxidation 53 rd AIAA Aerospace Sciences Meeting, AFOSR MURI: Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion 5-9 th January 15, Kissimmee, FL

H Oxidation. P=1 atm, Q=1 LPM, ppm H /3ppm O /balance Ar, 1kV, v=1 khz Thermal Reaction Plasma Assisted Reaction Species Concentration [ppm] 4 35 3 5 15 1 5 O H H O (1) OH LIF @ T = 918 K Species Concentration [ppm] 4 35 3 5 15 1 5 O H H O () OH LIF @ T = 668 K 4 5 6 7 8 9 1 11 4 5 6 7 8 9 1 11 Under thermal conditions reaction is non-existent at T < 86 K. Abrupt reaction occurs above 86 K, consuming all the initial fuel loading, consistent with second explosion limit @ 1 atm. Under the effects of the plasma reaction initiated at T = 47 K, with effectiveness increasing with temperature. By 84 K all initial fuel loading consumed, 4 K below onset of thermal effects.

OH Radical. P=1 atm, Q=1 LPM, ppm H /3ppm O /balance Ar, 1kV, v=1 khz (1) Thermal Reaction () Plasma Assisted Reaction Exponential growth/decay suggests chainbranched thermal reaction. Exothermicity of reaction results in 6 K rise. ICCD imaging and OH thermometry indicate plasma uniformity and temperature. Plasma is void of significant filamentation, and the induced OH profiles have little dependence on plasma gas heating. With plasma, OH formation entirely confined and sustained in the plasma region. OH attains rapid growth, linearly increases and decays rapidly upon exiting suggesting a chain-propagating reaction.

Plasma OH Radical Production. Effect of v. P=1 atm, Q=1 LPM, ppm H/3ppm O / balance Ar, 1kV, v = 1, 3, 5 khz OH Concentration [ppm] Fixed Delay @ 15us, T =668 K 6. 5 khz 5. 3 khz 4. 1 khz 3.. 1. OH Concentration [ppm] Fixed Location @ x = 45 mm, T =668 K 5. 5 khz 4. 3. 3 khz. 1 khz 1.. 8 16 4 3 4 48 56 Axial Distance [mm]. 4 6 8 1 Delay After High-Voltage Pulse [us] Increasing plasma perturbations, increases the OH production rate. The temporal effect in the OH consumption increases proportionally to the plasma perturbation time scale.

D Kinetics Model with Sensitivity Analysis SENKIN (CHEMKIN) Calculate k(t) Time integration Isothermal region modeled as constant T and P process Temperature ramps currently not included Plug flow assumption k n = AT exp N t i = ω i ( E RT ) a ZDPlasKin, Boltzmann equation solver (University of Toulouse, LAPLACE, 8) Calculate rates for electron-impact reactions Electron impact cross sections are fed Calculate v drift Circuit module Calculate E/N [Td = 1-1 Vm ] Electric charge conservation equation dvgap( t) l + d 1 dt εl Assumed homogeneity across discharge planes 1 = J = en dvapp( t) ld J dt ε ε E Vgap( t L = ) N N PAC H Chemical Reaction Mechanism (39 species, 418 reactions) Electron-impact reactions (Van Gaens and Bogaerts, 13 and Kossyi et al., 199) Excitation, Deexcitation, Dissociation, Ionization, Recombination Electronically excited and ionized species (Van Gaens and Bogaerts, 13, Kossyi et al., 199 and Popov, 11) Neutral, ground state species H /O Mechanism (Burke et al, 11), O 3 Chemistry (Bromly et al., 1996) p e v drift p ( t)

Fraction of electron energy losses into different degrees of freedom as a function of E/N. ppm H, 3 ppm O and balance Ar, T = 668 K. 1 O (vib) H (vib) Ar(el) Energy Loss Fraction.1 H (tr+rot) O (dis) Ar(ion) Ar(tr) O (el) H (dis) O (tr+rot).1 1-1 -1 1 1 1 1 1 3 E/N [Td]

Single discharge pulse. ppm H, 3 ppm O and balance Ar, T = 668 K, P = 1 atm Evolutions of reduced electric field and species Normalized sensitivity coefficients for the H O mole fraction Reduced Electric Field [Td] 15 E/N 1 5 1 4 1 3 1 H 1 1 O O HO H 1 Ar *m 1-8 1-7 1-6 1-5 1-4 1-3 1-1 -1 Time [s] OH 1-1 1-1 -3 Mole Fraction [ppm] Normalized H O Sensitivity Coefficient 1.5 1.5 -.5-1 e - +O <=>O - +O e - +Ar<=>Ar *m +e - e - +H O<=>O - +H HO +H<=>OH e - +O +M<=>O - O+H <=>H+OH +M e - +O <=>O - +O H +OH<=>H O+H HO +H<=>H +O HO +OH<=>H O+O HO +O<=>O +OH e - +Ar=>Ar + +e - -1.5 1-8 1-7 1-6 1-5 1-4 1-3 1-1 -1 Time [s] H+O (+M)<=>HO (+M)

1 khz Discharge Repetition Rate. 1kV, ppm H, 3 ppm O and balance Ar, T = 668 K, P = 1 atm Mole Fraction [ppm] Evolutions of major and minor species 3 5 O 15 H 1 5 H O.1..3.4.5.6 Mole Fraction [ppm] 4 3 Time [s] 1 H OH 4 6 8 1 Time [ms] O Normalized H O Sensitivity Coefficient Normalized sensitivity coefficients for the H O mole fraction.8.6.4. -. -.4 -.6 e - +Ar<=>Ar *m +e - HO +H<=>OH H +OH<=>H O+H O+H <=>H+OH e - +Ar<=>Ar ** +e - H+O <=>O+OH HO +H<=>H +O HO +O<=>O +OH -.8 1 3 4 5 6 7 8 Time [ms] e - +Ar=>Ar + +e - HO +OH<=>H O+O H+O (+M)<=>HO (+M)

Time evolution of the reaction flux of H O and OH in the plasma region (1 khz). 1kV, ppm H, 3 ppm O and balance Ar, T = 668 K, p = 1 atm. H O Production OH Production Rate [mol/(g s)] Rate [mol/(g s)].x1-4 1.5x1-4 1.x1-4 H +OH<=>H O+H 5.x1-5. 1 3 4 5 6 7 8.x1-4 1.5x1-4 1.x1-4 5.x1-5. -5.x1-5 -1.x1-4 -1.5x1-4 -.x1-4 Time [ms] HO +H<=>OH HO +O<=>O +OH O+H <=>H+OH HO +OH<=>H O+O H +OH<=>H O+H 1 3 4 5 6 7 8 Time [ms] Plasma drives reaction through dissociation H Ar *m H O (+M) HO H, O OH H O H O Ar *m O H H + OH Recycle H Reactions of electronically-excited O or O not important compared to above scheme.

C 1 -C 7 Alkanes Plasma Assisted Oxidation 53 rd AIAA Aerospace Sciences Meeting, AFOSR MURI: Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion 5-9 th January 15, Kissimmee, FL

C 1 -C 7 Alkane Fuel Consumption Normalized Fuel Concentration [CxHy]/[CxHy]i 1..8.6.4. Experimental Conditions: P = 1 atm, Q = 1 LPM, 16ppm CxHy/3ppm O / Ar balance Plasma Conditions: 1 kv, v = 1 khz 1 Thermal Reaction CH 4 C H 6 C 3 H 8 C 4 H 1 C 7 H 16 4 6 8 1 1 Normalized Fuel Concentration [CxHy]/[CxHy]i 1. 1.8.6.4. Plasma Assisted Reaction CH 4 C H 6 C 3 H 8 C 4 H 1 C 7 H 16 4 6 8 1 1 Without the plasma, onset of fuel consumption occurs for T > 9 K. With the plasma, onset of fuel consumption occurs at T = 4 K for all the fuels. The plasma also lowers the temperature for complete consumption of all fuels (the temperature for onset of high temperature ignition). For C3 and larger fuels, a transition in reaction rate is observed between 6 and 7 K.

Major Intermediate Species Generation Experimental Conditions: P = 1 atm, Q = 1 LPM, 16ppm CxHy/3ppm O / Ar balance Plasma Conditions: 1 kv, v = 1 khz 16 CO CO CH4 4 CO Concentration [ppm] 14 1 1 8 6 4 CH 4 C H 6 C 3 H 8 C H 4 1 C 7 H 16 CO Concentration [ppm] 15 1 5 CH 4 C H 6 C 3 H 8 C 4 H 1 C H 7 16 C H 4 Concentration [ppm] 35 3 5 15 1 5 CH 4 C H 6 C 3 H 8 C 4 H 1 C H 7 16 4 6 8 1 1 4 6 8 1 1 4 6 8 1 1 14 Before the onset of thermal effects, the plasma has a consistent effect on the formation CO; once the thermal effect is initiated both the plasma and thermal reactions contribute to forming and consuming CO Below 96K, very little heat producing CO is formed. With the exception of CH4, all fuels appear to form similar amounts of CH4 from 4K until the temperature where CH4 consumption exceeds its formation.

Kinetic Scheme of Primary Oxidation Reactions of n-alkanes X + RH R + O ROO + O β-decomposition products HO + Conjugate alkenes QOOH + O OOQOOH OH + Cyclic ethers OH + R CHO + CnHn OQ -H OOH + OH Branching Products

Thermal Oxidation of C 3 H 8 D. N. Koert, D. L. Miller, and N. P. Cernansky, Experimental Studies of Propane Oxidation through the Negative Temperature Coefficient Region at 1 and 15 Atmospheres, Combustion and Flame, 96, 34-49, 1994. Normalized Fuel Concentration [C3H8]/[C3H8]i 1.8.6.4. 6 65 7 75 8 85 Species Concentration [mols/m 3 ].4.3..1 CO C3H6 CHO CH3CHO CH4 C4H1 1 atm 1 atm 6 65 7 75 8 85 Species Concentration [mol/m 3 ].5..15.1.5 CO CH3OH CH5CHO CH3COCH3 Experimental Conditions: P = 1 atm, Q = 1 LPM, 533ppm C3H8/3ppm O / Ar balance 6 65 7 75 8 85 1 atm Species Concetration [ppm] 15 1 5 C 3 H 8 C H 4 CO CO 1 atm Species Concetration [ppm] 14 1 1 8 6 4 CH 4 C H 6 C 3 H 6 C 3 H 4 C 4 H 8 C 4 H 6 1 atm 4 6 8 1 1 8 9 1 11 1 13

C 3 H 8 Plasma Oxidation Species Concentration [ppm] Experimental Conditions: P = 1 atm, Q = 1 LPM, 533ppm C3H8/3ppm O / Ar by balance Plasma Conditions: 1 kv, v = 1 khz 16 14 1 1 8 6 4 1 8 6 4 Total Carbon C3H8 x 3 CO CO CH4 x 3 CH4 CH6 C3H6 CH3CHO CH CH3CHCHO C4H1 CH3COCH3 C4H8 4 5 6 7 8 9 1 15 11 115 1 15 Plasma assisted reactions initially enhance radical reservoirs (i.e., C3H7, O-atoms and H-atoms) by collisions quenching with excited species (Ar*) or electron-impact For T < 7 K, RO chemistry likely dominating (C3H7+O C3H7OO) leading to internal isomerization (C3H6OOH) and chain-propagation, forming OH via acetone and propanal. Further isomerization into stable ketohydroperoxide (OOC3H5OOH) acts as degenerate branching agent, which can decompose into oxygenated species (i.e., acetaldehyde) and x OH. For T > 7 K, alkene+ HO chemistry favored, signifying decrease in chain propagating/branching chemistry, increase in C3H6, CH4 and CH4.

C 3 H 8 Plasma Oxidation Scheme PLASMA REACTIONS (INITIATION) AR + e AR* + e Plasma acts as intiation step, alleviating pressure dependence of RO kinetics AR* + O AR + O + O AR* + C3H8 AR + C3H7 + H Branching HO +RH HO + C3H6 C-H Split H + C3H6 C3H7 +O C3H7OO C3H6OOH +O OOC3H6OOH OC3H5OOH + OH +O C-C Split Branching R + CH4 OH + CH3CHCHO, CH3COCH3 (Branching) OH + HCO + CH3CHO (Branching) HIGH-TEMPERATURE LOW-TEMPERATURE Enhancment O-atoms and H-atoms leads to early triggering of high-temperature chemistry

C 4 H 1 Plasma Oxidation Experimental Conditions: P = 1 atm, Q = 1 LPM, 4ppm C4H1/3ppm O / balance Ar Plasma Conditions: 1 kv, v = 1 khz Species Concentration [ppm] 18 16 14 1 1 8 6 4 1 1 8 6 4 5 Total Carbon CH3CHCOCH3 CH3CHCHO C3H8 C4H1 x 3 CH4 x 3 CO CO CH6 CH4 C3H6 CH CH3CHO C4H8 C5H1 C6H1 4 5 6 7 8 9 1 15 11 115 1 15 Normalized Fuel Consumption [C4H1]/[C4H1]i Species Concentration [ppm] 1.8.6.4. 15 1 5 4 6 8 1 1 C Total Thermal Reaction C 4 H 1 C H 4 4 6 8 1 1 5 atm Anand, A., MS Thesis, Drexel University, Philadelphia (1994). 1 atm CO CO

Summary Alkane (C1-C7) oxidation has been studied with and without plasma enhancement: Fuel consumption across T = 4 15 K has been significantly enhanced for all fuels, the plasma also lowers the temperature for complete fuel consumption (the temperature for onset of high temperature ignition) Experimental results indicate an extension of RO low temperature (cool flame) chemistry to lower pressures (identification/quantification of select species will be improved i.e., aldehydes) H oxidation has been studied with and without plasma enhancement: H and O consumption measured ex-situ GC analysis, modifications to the current facility have been made to perform in-situ OH LIF measurements in conjunction with OSU group Zero-D plasma kinetic model has been developed based on SENKIN to derive detailed sensitivity analysis Qualitative comparisons of model and experiment indicate current plasma assisted combustion models for hydrogen oxidation perform well. Experiment and model analysis of low temperature CH4/N/O/Ar mixtures suggest intermediate formation of nitromethane. Formation of such nitro and nitrate compounds at low temperatures may be a means to enhance ignition.

Future Directions Effect of Fuel Molecular Structure on Low Temperature Plasma Assisted Combustion Branched versus Straight Chain (high versus low octane number) Biofuels (CC double bonds) Methyl palmitate (C16:) O O Methyl stearate (C18:) Methyl oleate (C18:1) Methyl linoleate (C18:) Methyl linolenate (C18:3) O O O O O O O increasing cetane number Possible plasma effects: (i) Change the behavior of a low cetane number fuel to a high cetane number fuel in real time. (ii) Effect ignition criteria to control instabilities. Low Temperature High Pressure PAC Model Development for Fuels C and Larger Sensitivity analysis designed experiments to emphasize specific reactions Increased number of measured scalars for greater model constraint O

Thank You Acknowledgments The authors gratefully acknowledge support of this research by the AFOSR MURI program "Fundamental Mechanisms, Predictive Modeling and Novel Aerospace Applications of Plasma Assisted Combustion" with Dr. Julian Tishkoff and Dr. Chiping Li as technical monitors. The authors would like to thank collaborating members from the Ohio State University, Dr.'s Zhiyao Yin, Kraig Frederickson, Munetake Nishihara, Keisuke Takashima, Igor Adamovich, and Walter Lempert.

C H 4 Plasma Assisted Pyrolysis/Oxidation 53 rd AIAA Aerospace Sciences Meeting, AFOSR MURI: Fundamental Mechanisms, Predictive Modeling, and Novel Aerospace Applications of Plasma Assisted Combustion 5-9 th January 15, Kissimmee, FL

CH4 Pyrolysis and Oxidation Experiments Pyrolysis Species Mole Fraction [ppm] 1 8 6 4 C H C H 4 Thermal J.G. Lopez et al. 4 6 8 1 1 14 Species Mole Fraction [ppm] 1 C Total x 8 6 C H 4 4 C H 4 6 8 1 1 14 Plasma Assisted Species Mole Fraction [ppm] 4 35 3 5 15 1 5 C C H 6 3 H 8 CH 4 C H 4 1 C 4 H 8 C 3 H 4 C 3 H 6 4 6 8 1 1 14 Oxidation Species Mole Fraction [ppm] 15 1 C H 4 CO CO J.G. Lopez et al. 5 4 6 8 1 1 14 Species Mole Fraction [ppm] 16 1 C Total CO 8 C H 4 4 CO 4 6 8 1 1 14 Species Mole Fraction [ppm] 7 6 C H CH 4 5 4 3 CH 3 CHO C H 6 C 3 H 8 1 C 3 H 6 4 6 8 1 1 14 Experimental Conditions: P = 1 atm, Q = 1 LPM, 8ppm CH4/3ppm O / Ar by balance Plasma Conditions: 1 kv, v = 1 khz

Modeling Conditions: Temp Profiles 1 1 8 6 4 Heating Region 156 K 863 K 668 K Isothermal Region..4.6.8 1 1. Reactor Length [m] Cooling Region Effect on the chemical kinetics due to the heating and cooling ramps as a result of heat transfer from the reactor walls is considered, where the reactor inlet and exit temperatures are approximately the same Joule heating as a result of the plasma discharge is negligible, ~7 K rise at 1 kv with 3 ppm O loading, for T = 156 K Temperature rises ~ 5K from reaction with 8ppm CH4/ 3 ppm O at T=156 K

Modeling Conditions: Energy Coupling Using the OSU plane-to-plane nsec pulse discharge model the coupled energy in the PSU experimental discharge can be approximated For given reactor operating conditions (P= 1 atm, T = 35 K 15 K) the breakdown voltage is predicted based on the applied field to determine the effective E/N during the pulse 8 1 14 E/N [Td] 7 6 5 4 3 Coupled+Stored Coupled.8.6.4 Energy [mj] Predicted E/N [Td] 1 1 8 1. 6 5 1 15 5 3 35 4 Time [ns] OSU Nanosecond Pulse Discharge Model Prediction 4 4 6 8 1 1 14 Computed E/N based on Reactor Conditions E/N and coupled energy are provided as inputs into the plasma chemistry model, which assumes constant energy loading per molecule independent of temperature and pressure

Modeling Results for a Single Experiment Modeling Conditions: P = 1 atm, Q = 1 SLPM, T = 863 K, 8ppm CH4 / 3ppm O / Balance in Ar Plasma Conditions:.43 mev/molecule/pulse Species Mole Fraction [ppm] 9 8 7 6 5 4 3 8 6 4 CO C H 4 CO CH CH4 CH6 C3H8 CHO..4.6.8 1 1. Species Mole Fraction [ppm] 1 4 1 O O(a 1 ) 1 1-1 -4 1-6 1-8 1-1 1-1 1-14 O( 3 P) O(b 1 Σ) O3 Ar* O( 1 D)..4.6.8 1 1. Reactor Length [m] Reactor Length [m] Major Species + Minor Species Oxygen + Excited Species Most of the reaction is isolated in the plasma region, with some further reaction occurring in the isothermal region Experimental results correspond to species concentration at the exit of the reactor

C H 4 Plasma Pyrolysis 4 35 CH6 4 35 C3H8 Data Points: Experiments, Solid Lines: GRI Mech., Dashed Lines: USC Mech Experimental Conditions: P = 1 atm, Q = 1 SLPM, 8ppm CH4 / Balance in Ar Modeling Conditions:.43 mev/molecule/pulse 8 CH6 [ppm] 3 5 15 1 5 C3H8 [ppm] 3 5 15 1 5 Species Concentration [ppm] 7 6 5 4 3 1 CH4 CH 4 6 8 1 1 Comparison of thermal mechanisms indicates that GRI is able to predict experiments relativley well for minor species upto C3H8. USC mech poorly predicts plasma pyrolysis of CH4, though it has the ablity to account for chemistry upto C4H1 C4H1 [ppm] C3H4 [ppm] CH4 [ppm] 5 15 1 5 7 6 5 4 3 1 4 35 3 5 15 1 4 6 8 1 1 CH4 4 6 8 1 1 C3H4 4 6 8 1 1 C4H1 C3H6 [ppm] C4H8 [ppm] 14 1 1 8 6 4 1 8 6 4 4 6 8 1 1 C3H6 4 6 8 1 1 C4H1 4 6 8 1 1 5 4 6 8 1 1

C H 4 Plasma Oxidation Data Points: Experiments, Solid Lines: GRI Mech., Dashed Lines: USC Mech Experimental Conditions: P = 1 atm, Q = 1 SLPM, 8ppm CH4 / 3ppm O/ Balance in Ar Modeling Conditions:.43 mev/molecule/pulse Species Concentration [ppm] 18 15 1 9 6 3 CH4 CO 4 6 8 1 1 CO Signignificant improvement in minor species prediction with USC mech for oxidation experiments over GRI, with major species imporovment for T > 8 K. CH, reactions severely underpredicted in both mechansims, indicating it formation is directly attributed to plasma specific reaction. CH [ppm] CH6 [ppm] C3H6 [ppm] 7 6 5 4 3 1 5 4 3 1 7 6 5 4 3 1 t y e e as a O dat o CH 4 6 8 1 1 t y e e as a O dat o 6 / 9/ CH6 4 6 8 1 1 t y e e as a O dat o 6 / 9/ C3H6 4 6 8 1 1 CH3CHO [ppm] C3H8 [ppm] CH4 [ppm] 8 7 6 5 4 3 1 3 5 15 1 5 5 4 3 1 t y e e as a O dat o 6 / 9/ CH4 4 6 8 1 1 t y e e as a O dat o 6 / 9/ C3H8 4 6 8 1 1 t y e e as a O dat o 6 / 9/ CH3CHO 4 6 8 1 1

Production of O(D) O(D) is formed mostly via dissociation of O ( O+O(D)) by collisions with e of Ar* A similar process occurs to produce two ground-state O-atoims Electron impact excitation of ground state O into O(D) is considered, but not important due to low O atom concentration at time of discharge Consumption of O(D) When excess Ar present, the main consumption path of O(D) is collisional deexcitation to the ground state A possible competitor is O(D) + H OH + H. At 668 K, k = 8.58 1 13 cm 3 /(mol-s) while collional deexcitation with Ar is 7.9 1 1 cm 3 /(mol-s). Thus, a mixture with Ar:H = 11:1 would make rates comparable

Fuel Consumption Experimental Conditions: P = 1 atm, Q = 1 LPM, 16ppm CxHy/3ppm O / Ar by balance Plasma Conditions: 1 kv, v = 1 khz Normalized Fuel Concentration [CxHy]/[CxHy]i 1. 1.8.6.4. Thermal Reaction CH 4 C H 6 C H 3 8 C H 4 1 C H 7 16 4 6 8 1 1 Without the plasma onset of fuel consumption occurs for T > 9 K. With the plasma, onset of fuel consumption occurs at T= 4 K. The plasma lowers the temperature for complete consumption of all fuels (the temperature for the onset of high temperature ignition). For C3 and larger fuels, a transition in reaction rate is observed between 6 7 K. 1. Plasma Assisted Reaction Normalized Fuel Concentration [CxHy]/[CxHy]i 1.8.6.4. CH 4 C H 6 C H 3 8 C H 4 1 C H 7 16 4 6 8 1 1

Major Species Generation Experimental Conditions: P = 1 atm, Q = 1 LPM, 16ppm CxHy/3ppm O / Ar by balance Plasma Conditions: 1 kv, v = 1 khz 16 4 CO Concentration [ppm] 14 1 1 8 6 4 CH 4 C H 6 C H 3 8 C H 4 1 C H 7 16 CO CO Concentration [ppm] 15 1 5 CH 4 C H 6 C H 3 8 C H 4 1 C H 7 16 CO C H 4 Concentration [ppm] 35 3 5 15 1 5 CH 4 C H 6 C H 3 8 C H 4 1 C H 7 16 CH4 4 6 8 1 1 4 6 8 1 1 4 6 8 1 1 14 Before the onset of the thermal effects, the plasma has a consistent effect on the formation CO; once the thermal effect is initiated both the plasma and thermal reactions contribute to forming and consuming CO. Below 96 K, very little heat producing CO is formed. With the exception of CH4, all fuels appear to form similar amounts of CH4 from 4 K until the temperture where the CH4 consumption exceeds its formation.

C 3 H 8 Plasma Oxidation Experimental Conditions: P = 1 atm, Q = 1 LPM, 533ppm C3H8/3ppm O / Ar balance Plasma Conditions: 1 kv, v = 1 khz Species Concentration [ppm] Species Concentration [ppm] 15 1 5 1 8 6 4 Major Species C 3 H 8 C H 4 4 6 8 1 1 CH 4 C H 6 C 3 H 6 C 4 H 1 C 4 H 8 Minor Species 4 6 8 1 1 Signal Area [pa*s] 6 5 4 3 1 CO NA.1-15.46 min NA. - 19.748 min NA.3 -.11 min CH3CHO, acetaldehyde (& possibly some CHO) CH3CHCHO, propanal 4 6 8 1 1 CH3COCH3, acetone CO

Thermal Oxidation of C4H1 Anand, A., An Experimental Study of n-butane Oxidation at Elevated Pressures in the Low and Intermediate Temperature Regimes, MS Thesis, Drexel University, Philadelphia (1994). Normalized Fuel Consumption [C4H1]/[C4H1]i 1.8.6.4. 4 6 8 1 1 5 atm Experimental Conditions: P = 1 atm, Q = 1 LPM, 4ppm C4H1/3ppm O / Ar balance 1 Species Concentration [ppm] C Total 15 CO 1 5 C 4 H 1 C H 4 CO 4 6 8 1 1 Species Concentration [ppm] 1 C H C 3 H X 5 C H 8 3 6 CH 4 8 C 3 H 4 X 5 C 4 H X 5 6 6 C H 6 4 C H X 5 4 8 8 9 1 11 1 13

Species Concentration [ppm] C 4 H 1 Plasma Oxidation Experimental Conditions: P = 1 atm, Q = 1 LPM, 4ppm C4H1/3ppm O / Ar balance 1 8 6 4 Species Concentration [ppm] 16 14 1 1 8 6 4 Plasma Conditions: 1 kv, v = 1 khz C 4 H 1 C H 4 4 6 8 1 1 Minor Species CH C H 4 3 6 4 6 8 1 1 Major Species C H 6 C H Species Concentration [ppm]x5 1 8 6 4 Minor Species C 6 H 1 C 4 H 8 C 3 H 8 x 5 cis-c 4 H 8 C 5 H 1 C 4 H 6 CO 4 6 8 1 1 Signal Area [pa*s] 6 5 4 3 1 CO CH3CHO NA.1-15.416 min NA. - 19.768 min NA.3-3.498 min NA.4 -.88 min C4H8O, MEK, butanone C3H6O, propanal 4 6 8 1 1 C4H8O, butanal