Numerical Study on the Autoignition of Biogas in Moderate or Intense Low Oxygen Dilution Nonpremixed Combustion Systems

The ignition delay of biogas in mixing layers is investigated using a one-dimensional combustion model, with its application in Moderate or Intense Low oxygen Dilution (MILD) combustion being the focus. The current study reveals the key aspects of the ignition of biogas in a nonpremixed, igniting mixing layer with a hot oxidizer of low oxygen content. The observed characteristics are contrasted against the existing studies on ignition in homogeneous mixtures under similar conditions. Biogas is considered here as a mixture of CH₄ with variable amounts CO₂. The influence of reactive, thermal, and transport properties of CO₂ on the ignition is evaluated using artificial species to mimic the respective characteristics of CO₂. While the ignition delay in homogeneous mixtures shows a strong dependence on CO₂ content in the fuel, the ignition delay predictions from one-dimensional mixing layers show no significant influence of CO₂ levels in biogas. In addition, the influence of oxidizer composition and temperature on ignition delay is determined for CO₂ levels ranging from 0% to 90%. A sensitivity analysis of chemical reactions on the ignition delay shows a negligible effect of CO₂ concentration in biogas. The current study emphasizes the role of oxidizer composition and temperature on the ignition characteristics of a MILD biogas flame