Non-equilibrium plasma for ignition and combustion enhancement

Abstract

This review is intended to give an overview of recent results on plasma-assisted ignition and combustion. The influence of electrical discharges on combustion processes is introduced, and the excited species and radicals present in large quantity in the plasma are detailed. A description of theoretical problems related to modeling plasma-assisted combustion is given, elucidating the role of excited states in enhancing the reaction rates. Then some experiments are reported, focused on fundamental aspects on the effects of high pressure discharges in improving the ignition of combustion. To conclude, some applications for plasma-assisted combustion and detonation are presented, activated by different kinds of discharges.

Combustion, Chemistry, and Carbon Neutrality

“…570,573 With appropriate control over such mechanisms, e.g., by tailoring the electron energy, reaction processes can be altered or enhanced in a desired way, for example, to reduce pollutant emissions at high combustion efficiency. [570][571][572]574 Control over the deposition of energy into the target gas, i.e., regarding vibrational or electronic excitation, dissociation, or ionization is possible over short time scales before relaxation, with very short (nanosecond) repetitively pulsed discharges. 570,573 Ignition delay times can be reduced and reactivity and burning velocities increased using appropriate plasma−combustion system designs, potentially from combining thermal and chemical effects.…”

“…Details of the coupling between the combustion process and the plasma must be understood on a fundamental basis to feasibly employ such new reaction strategies. Several recent reviews present excellent insights into plasma-assisted combustion, plasma-chemistry interaction, and plasma-supported control strategies, describing respective types of plasmas and instrumentation as well as the status of experimental analysis, reaction mechanism development, and numerical simulation. − …”

“…Thermal plasmas provide fast heating of the combustible gas mixture, and although they can be useful for ignition, they need substantial electric energy input and will produce high (thermal) NO concentrations . Nonequilibrium plasmas in comparison are characterized by high electron temperatures, and through efficient electron impact, excitation, dissociation, and ionization processes can produce chemically activated species that may not be accessible thermally. , With appropriate control over such mechanisms, e.g., by tailoring the electron energy, reaction processes can be altered or enhanced in a desired way, for example, to reduce pollutant emissions at high combustion efficiency. − , Control over the deposition of energy into the target gas, i.e., regarding vibrational or electronic excitation, dissociation, or ionization is possible over short time scales before relaxation, with very short (nanosecond) repetitively pulsed discharges. , Ignition delay times can be reduced and reactivity and burning velocities increased using appropriate plasma–combustion system designs, potentially from combining thermal and chemical effects. , As pointed out by Ju and Sun, fundamental-level experiments under well-controlled conditions are needed to improve the knowledge on the related processes and their dependence on numerous influence parameters on both, the plasma and the molecular (fuel) reaction side, and on the interdependences between such influences . In addition to the concentrations of the main and intermediate combustion species, including excited and charged particles, the electric field and electron density distribution should be determined as a function of the fuel–oxidizer–bath gas system and of the relevant discharge parameters, on the time scales of the related processes. ,, …”

Experimental study of the discharge characteristics of a 3D vortex gliding arc plasmatron

“…Gliding arc plasma has been widely used in many fields, such as material preparation, waste gas treatment, CO 2 conversion, and plasma-assisted ignition due to its high electron density (10 12 -10 14 cm −3 ), high temperature (1,000-4,000 K), and high chemical selectivity. [1][2][3][4][5][6][7] The traditional gliding arc operates between a pair of blade electrodes. The arc is ignited at the smallest gap between the electrodes and is then forced to move and elongate by air flow until it is extinguished when the energy cannot sustain the arc length.…”

Electron Impact Ionization of Metastable States of Diatomic Molecules

“…The kinetics of nonequilibrium, low-temperature plasmas is driven by the presence of radicals and excited species that can be regarded as reactivity enhancers, activating channels otherwise inaccessible and modifying the route to products. Mechanisms activated by excited species can significantly affect the efficiency of plasma technologies impacting different fields of applications, i.e., CO 2 plasma reduction for environment [1], plasmaassisted combustion [2], plasma medicine, and agriculture [3,4].…”