Characterization of flame propagation and burning rates of various mixtures of biogas and nitrous oxide

Abstract

Biogas is a potential alternative energy source with low environmental impact. However, the practical applications of biogas are relatively limited due to the existence of carbon dioxide (CO2) which acts as a diluent that decreases the calorific value and the burning rate of biogas. Nitrous oxide (N2O) is known to be a powerful oxidizing agent for propulsion applications which can enhance the combustion rate of biogas, however, there is a lack of studies that investigate the fundamental characteristics of biogas-N2O combustion. The aim of this study is to gain insight into the fundamental combustion characteristics of biogas-N2O mixtures in terms of laminar burning velocity and flame stability. In the present work, spherically expanding flames following central ignition at constant volume combustion chamber (CVCC) were employed to investigate laminar burning velocity (LBV), hydrodynamic instability, and diffusive-thermal instability of biogas-N2O mixtures at wide equivalence ratio;?, from 0.6 to 1.4, at 303K and atmospheric pressure. Two mechanisms were used in CHEMKIN-PRO 17 software in order to estimate the predicted combustion characteristics of biogas-N2O mixtures. The results indicate that the decline in LBVs was prominent in the fuel-rich mixtures than in the fuel-lean mixtures with CO2 dilution. It is found that the influence of curvature on the flame front is weakened at the fuel lean-to-stoichiometric mixture due to the decrease in the flame thickness. Therefore, flame instability tends to increase at the lean-to-stoichiometric region. The increase in CO2 in biogas by 10%, increases the Lewis number (Le) value by 3.6% to 4.6%. The diffusive-thermal instability was dominant for all biogas-N2O mixtures, as Le was less than unity throughout the entire equivalence ratio range. The thermal reaction of N2O decomposition is the most significant reaction in biogas-N2O combustion at lean mixtures of ? = 0.6 and ? = 0.8. The LBVs of biogas-N2O mixture revealed a considerable enhancement at the lean equivalence ratio of 0.8 compared to the other biogas-air mixtures with H2 addition. The effect of nitrous oxide as an oxidizer on biogas detonation characteristics is studied numerically using Chemical Equilibrium Applications (CEA) and compared with other oxidizers. Mixtures with N2O oxidant revealed 32% and 16% higher detonation pressure and detonation Mach number, respectively, at ? = 0.6, compared to that of mixtures with pure oxygen oxidant. Overall, employing N2O oxidant has enhanced the fundamental combustion characteristics of biogas significantly, which may lead to the use of biogas as a clean fuel in commercial applications that demand high combustion rates, such as power generation and aerospace.