Numerical Study of Critical Recirculation Ratio for Achieving Unconditional MILD Combustion

Abstract

Moderate or Intense Low Oxygen Dilution (MILD) combustion is a regime in which fuels burn in a distributed reaction zone generating ultra-low emissions and no visible flame front. Also known as flameless combustion, it can be achieved by recirculating flue gases into the reaction zone. A recirculation factor based on the mass recirculated is used to characterize a flameless regime. However, the original definition of MILD is based on the temperature levels of a reactor. This work connects both criteria by introducing a parameter called recirculation ratio derived theoretically from a mass balance. The ratio was numerically calculated by simulating a network of perfectly stirred reactors and performing an energy balance using open-source software. This methodology was validated against experimental and simulated data from the literature. Simulations were carried out for methane, ethane, propane, and hydrogen under adiabatic conditions and equivalence ratios from 0.6 to 1. Results indicate that a critical recirculation ratio is required for establishing unconditional MILD combustion, which changes with fuel type and equivalence ratio. In all cases studied, the critical ratio diminishes as the equivalence ratio is reduced. Hydrogen and methane require the highest and lowest critical ratio, respectively. Results suggest that externally diluting hydrogen with carbon dioxide could potentially reduce recirculation requirements for MILD combustion.