Chemical kinetics and instability in non-equilibrium reactive plasmas

Abstract

There is a significant interest in utilizing non-equilibrium reactive plasmas powered by renewable electricity for clean energy conversion and chemical manufacturing. However, such plasma has many excited species and is highly non-equilibrium. Thechemical kinetics mostly remain unknown due to lack of time-resolved in situ quantitative diagnostics. In addition, the reactive plasmas are vulnerable to perturbations and generate local “hot spots”, which will degrade the efficiency, chemical reactivity and selectivity for plasma applications. The instability and pattern formation require further theoretical analysis and modeling efforts. To address the above challenges, in this dissertation, first, advanced time-resolved in situ laser diagnostics including direct absorption spectroscopy and Faraday rotation spectroscopy were developed and applied in a photolysis flow reactor to investigate the kinetics of excited oxygen atom O(1D) and hydroperoxy radical HO2. This new experimental apparatus is capable of fast (microsecond time resolution) and sensitive (down to ppm level) measurements for many plasma-generated active species and enables quantitative determination of the reaction rates and branching ratios of reaction channels for elementary reactions. Second, the kinetics of low temperature pyrolysis and oxidation of large alkane and zero-carbon fuels including n-dodecane and ammonia in a repetitively-pulsed nanosecond dielectric barrier discharge were experimentally and numerically explored. The results revealed that the nanosecond plasma discharge could dramatically accelerate fuel pyrolysis and oxidation at low temperatures. Moreover, a strong NO kinetic effect on plasma-assisted low-temperature combustion is identified via NOx coupling with fuel chemistry. Finally, a new instability, the plasma thermal-chemical instability, governed by the coupling between plasma dynamics and chemical kinetics, was developed using stability analysis and chemical mode analysis. The numerical modeling and experimental measurements revealed that heat release, species generation and transport were critical for the thermal-chemical instability, accelerating the transition from homogeneous plasmas to filamentary structures. The research in this dissertation presents key insight into elementary kinetics in plasma and combustion chemistry. Further, the systematic investigation of the thermal-chemical mechanism provides guidance for the active control of plasma instability. The overall presented work brings broader impact for advanced engines, sustainable chemical conversion, and the fuel leakage effect on atmospheric chemistry.