Plasma-assisted combustion in lean, high-pressure, preheated air-methane mixtures

We combine a simplified physical model with a detailed plasma-chemical reaction mechanism to analyze the use of plasmas to improve flame stability in a gas turbine used for electric power generation. For this application the combustion occurs in a lean mixture of air and methane at high pressure (18.6 atm) and at ``preheat'' temperature 700~K, and the flame zone is both recirculating and turbulent. The system is modeled as a sequence of reactors: a pulsed uniform plasma (Boltzmann), an afterglow region (plug-flow), a flame region (perfectly-stirred), and a downstream region (plug-flow). The plasma-chemical reaction mechanism includes electron-impact on the feedstock species, relaxation in the afterglow to neutral molecules and radicals, and methane combustion chemistry (GRI-Mech 3.0), with extensions to properly describe low-temperature combustion 700--1000~K [M Deminsky et al, Chem Phys \textbf{32}, 1 (2013)]. We find that plasma treatment of the incoming air-fuel mixture can improve the stability of lean flames, expressed as a reduction in the adiabatic flame temperature at lean blow-out, but that the plasma also generates oxides of nitrogen at the preheat temperature through the reactions $e +$ N$_{2} \to $ N $+$ N and N $+$ O$_{2} \to $ NO $+$ O. We find that flame stability is improved with less undesirable NOx formation when the plasma reduced-electric-field $E$/$N$ is smaller.