Modeling CO₂ Chemical Effects on CO Formation in Oxy-Fuel Diffusion Flames Using Detailed, Quasi-Global, and Global Reaction Mechanisms

Abstract

Interest in oxy-fuel combustion as one of the leading carbon capture technologies has grown significantly in the past two decades. Experimental studies have shown higher CO concentration in oxy-fuel diffusion flames than in traditional air-fuel flames of both gaseous and solid fuels. The higher CO concentration changes the flame profiles, and it may have impacts on pollutants formation. This article presents a numerical study regarding the chemical effects of CO₂ on CO formation in the flame region, and their modeling approaches in CFD simulations. Equilibrium calculation confirms higher CO concentration associated with fuel-rich stoichiometry in CO₂ diluted combustion environment. One-dimensional counter-flow diffusion flame simulation using detailed reaction mechanisms reveals that the reaction enhances CO formation in the presence of high CO₂ concentration, leading to a significantly higher CO concentration under oxy-fuel combustion conditions. High CO₂ concentration also impacts the reaction via OH radical and results in lower H₂ and higher H₂O concentrations in the flame profile. Computational fluid dynamics (CFD) simulations of a swirling diffusion flame under air-fired and oxy-fuel conditions were conducted using the eddy dissipation model and the eddy dissipation concept model with quasi-global and global kinetic mechanisms. Results show that reasonable CO predictions can only be obtained using finite-rate approach with appropriate mechanisms considering the CO₂ chemical effects. The Westbrook–Dryer two-step mechanism consistently underestimates the CO concentrations. In contrast, the Westbrook–Dryer multiple-step mechanism captures the chemical effects of CO₂, and improves the predictions.

Keywords:

• Carbon monoxide
• Computational fluid dynamics
• Oxy-fuel combustion
• Reaction mechanism