Analysis of Central Air Injection for Enhancing Biogas Combustion Efficiency in a Reactive Flow System

Abstract

This study presents a comprehensive Computational Fluid Dynamics (CFD) analysis of a reactive flow system to investigate impact of central air injection on the combustion efficiency and emission characteristics of a biogas. Owing to its high carbon dioxide content (CO2) content, biogas typically exhibits flame instability and low calorific value. To mitigate these challenges, a central secondary air injection strategy is proposed to modify the shear layer dynamics and enhance turbulence-chemistry interactions. Numerical simulations were executed by solving the Reynolds--Averaged Navier--Stokes (RANS) equations coupled with the Realizable k-? turbulence model. The complex chemistry of the methane-carbon dioxide mixture was resolved using the Eddy Dissipation Concept (EDC) integrated with a reduced chemical kinetic mechanism. The results reveal that the aerodynamic structure induced by the central air jet establishes a stable Central Toroidal Recirculation Zone (CTRZ), which acts as a thermal anchor, significantly stabilizing the flame root and preventing its lift-off. Optimization of the central air injection velocity has led to a more uniform temperature distribution and to a remarkable decrease in unburned hydrocarbons (UHC) and Carbon Monoxide (CO) emissions at the exhaust. However, an upper threshold for the injection velocity was identified, beyond which localized thermal Nitrogen Oxide (NOx) generation increases due to intensive thermal damping from the excess cold core air. These findings offer practical guidelines for the aerodynamic optimization and design of eco-friendly, low-calorific biofuel burners.