Laminar Burning Velocity, Adiabatic Flame Temperature, and Pollutants of Biogas/Air Mixture at Various CO_2 Concentrations and Plasma Assist

摘要

Mixtures of CH_4 and CO_2, known as biogas, are considered as renewable gas with low heat energy, which makes it extremely difficult to use as fuel in conventional natural gas equipment. To better understand the behavior of biogas mixtures and their properties, numerical results of the impact of carbon dioxide on laminar burning velocity and flame temperature of methane (CH_4)-carbon dioxide (CO_2)-air combustion were done. One of the most promising methods to enhance combustion is the use of a plasma source to assist unstable flames. For this purpose, experimental results to investigate the effect of microsecond pulsed plasma on non-premixed biogas flames were conducted. In the first section, laminar burning velocities and adiabatic flame temperature have been numerically calculated by one-dimensional steady code using several chemical kinetic mechanisms. Numerical results were compared with several experimental results from the literature. The laminar burning velocity and the adiabatic temperature of biogas/air combustion were calculated with CO_2 concentration ranging from 0% to 50% in the mixture. The equivalence ratio was varied from 0.4 to 1.4, the initial pressure was set at 1 bar, and the temperature was set to 300 K. The impact of CO_2 addition on the CO and NO_x emissions at equivalence ratios from lean to rich (0.4-1.4) was investigated. It was found that increasing CO_2 in the biogas lowers NO_x and increases CO. In the second section, the effects of microsecond pulsed plasma on non-premixed swirling biogas/air flames were experimentally investigated. The plasma-assisted combustion was carried out in a coaxial burner. It consists of two concentric tubes; the central tube delivers the biogas flow to the injector and the annular tube contains a swirler and supplies the airflow. To create plasma in the stabilization zone of the flame, a rod-ring microsecond plasma configuration was used. OH* chemiluminescence measurements are used to describe the structure and stability of the flame. Results showed that plasma generated by microsecond high-voltage (HV) pulses can improve flame stability. An analysis of the optical emission spectra (OES) showed that plasma generates chemically active species such as excited N_2*, CH*, OH* molecules, and H* atoms, leading to improved flame stability. The dependence of their intensities on both the pulse repetition frequency (PRF) and the input electrical voltage was investigated. The gas analysis of the exhaust gas was carried out with and without plasma. The exhaust gas concentration measurements revealed that the rod-ring pulsed plasma reduces CO with a low impact on the NOx level.