Advanced Chemical Analysis of Combustion Enhancement in Compressed Natural Gas-Air Flames using Low-Emission Systems

Compressed Natural Gas (CNG), predominantly composed of methane (CH₄) with minor constituents of higher hydrocarbons and inert gases, is renowned for its clean-burning properties due to its favorable hydrogen-to-carbon ratio. However, under ultra-lean conditions, achieving stable and efficient combustion is chemically challenging because of limited radical formation and suboptimal reaction kinetics. This study presents an in-depth chemical analysis of CNG-Air combustion using an innovative dual-stream burner design. The system integrates an annular pre-mixed CNG-Air stream with a centrally injected diffusion stream of pure CNG, a configuration that alters the local chemical environment and reaction pathways. By varying the diffusion ratio and adjusting the fuel flow rates at different overall equivalence ratios, the study revealed notable chemical and operational enhancements. At a 15% diffusion ratio, the upper limit of the equivalence ratio increased to φ = 1.328 by achieving an increase of 11% compared to a configuration without central diffusion. Conversely, a 5% diffusion ratio resulted in a dramatic decrease to φ = 0.039 and a percentage decrease up to 91.5% in the lower equivalence limit. These chemical modifications enhance the lower limits due to the presence of sufficient oxygen for improved chemical reactions, effectively broadening the operability range of ultra-lean combustors. Computational results indicate that the minimum axial temperature and minimum CO₂ concentration were achieved at a 15% diffusion ratio with a recorded decrease of 21.86% and 26.59%, respectively, compared to a 0% diffusion ratio. This is explained by the fact that at greater equivalence ratio values, there is less oxygen available for combustion. However, at a 5% diffusion ratio, the minimum axial temperature and minimum CO₂ concentration were recorded a decrease of 14.75% and 16.89%, respectively, compared to a 0% diffusion ratio. Environmentally, these improvements lead to significantly lower fuel consumption, lower maximum temperature and reduced emissions of greenhouse gases and nitrogen oxides, paving the way for cleaner and more sustainable combustion technologies.