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

Abstract

A new experimental facility was developed to study the reactive kinetics associated with plasma-assisted combustion (PAC). Experiments were performed in a nearly isothermal plasma flow reactor (PFR) using reactant mixtures diluted in an inert (i.e., Ar, He, or N2) to minimize temperature changes from chemical reactions. At the end of the isothermal reaction zone, the gas temperature was rapidly lowered to quench reactions. Gas composition was determined with several online ex situ techniques, including Fourier transform infrared (FTIR), non-dispersive infrared (NDIR), and by sample extraction/storage into a multi-position valve for subsequent analysis by offline gas chromatography (GC). Hydroxyl radical concentrations were measured in situ with laserinduced fluorescence (LIF) technique. Experiments were performed by fixing the flow rate or residence time in the reactor, and varying the temperature to achieve a reactivity map. Fuels studied were hydrogen, carbon monoxide, ethylene and C1-C7 alkane hydrocarbons, to examine pyrolysis and oxidation kinetics with and without the effects of a high-voltage nanosecond plasma discharge at atmospheric pressure from 400-1250 K. Experimental studies were complimented with detailed chemical kinetic modeling and sensitivity analyses to determine the dominant and rate-controlling mechanisms and to compare the thermal reactions with the plasma-assisted reactions. Hydrogen oxidation showed no thermal reaction until 865 K, where the second explosion limit occurs at atmospheric pressure, at which point the hydrogen was rapidly consumed within the residence time of the reactor. With the plasma discharge, reaction occurred at all temperatures and exhibited an autocatalytic acceleration from 400 K up to approximately 800 K where all the hydrogen was consumed. For ethylene, kinetic results with the discharge indicated that pyrolysis type reactions were nearly as important as oxidative reactions in consuming ethylene below 750 K. Above 750 K, the thermal reactions coupled to the plasma reactions provided further oxidation. Modeling analysis of plasma-assisted pyrolysis revealed that ethylene dissociation by electron impact resulted in the direct formation of acetylene and larger hydrocarbons by way of the vinyl radical. During plasma-assisted oxidation, direct dissociation and excitation of oxygen led to further fuel consumption, and enhanced low-temperature oxidative chemistry by effective production of methyl and formyl radicals. At the highest temperatures, the radical production by neutral thermal reactions became competitive and the effectiveness of the plasma discharge decreased. Under the effects of the plasma, alkane fuels exhibited significant reaction over the entire temperature range considered compared to that of the thermal reactions. At atmospheric pressure, propane and butane exhibited cool flame chemistry between 400-700 K, which normally occurs at higher pressures (P > 1 atm) for thermal conditions alone. This chemistry is characterized by alkylperoxy radical formation, isomerization to the hydroperoxyalkyl radical, and dissociation to form aldehydes and ketones, where intermediate temperature chemistry between 700-950 K is characterized by β-scission of the initial alkyl radical to form alkenes and smaller alkanes. The results demonstrate new insight into the kinetics governing PAC and provide a new experimental database to facilitate the development and validation of a PAC-specific kinetic mechanism.