Unraveling the flame morphology and evolution in pulverized coal MILD combustion transition: experiments and theoretical analysis

Moderate or Intense Low-oxygen Dilution (MILD) combustion of pulverized coal is a key technology for achieving efficient, low-emission coal utilization, critical to the transition to low-carbon energy systems. However, previous studies have mostly focused on pilot-scale investigation, with limited attention to elucidating the fundamental characteristics during MILD combustion transition. This study employs a Hencken-type flat-flame burner under typical high-temperature, low-oxygen conditions (1800 K, 10% O₂) to systematically investigate the effects of a wide range of jet velocities (20–100 m/s) on flame morphology, steady-state temperature, and spatial uniformity in O₂/N₂ and O₂/CO₂ atmospheres. Using RGB pyrometry, this study achieves, for the first time, experimental measurements of global transient particle surface temperature distributions in pulverized coal MILD combustion across the wide jet velocity range. Results demonstrate that increasing jet velocity enhances particle dispersion, reduces ignition delay distance, and promotes reaction zone uniformity, with the O₂/CO₂ atmosphere exhibiting superior uniformity due to higher turbulence intensity and the physicochemical properties of CO₂. Furthermore, a dimensionless spatial coefficient of temperature deviation (CTD) is introduced to quantify spatial temperature uniformity, for identifying the MILD combustion state of pulverized coal. This study provides theoretical and experimental foundation for understanding MILD combustion characteristics, particularly the global particle surface temperature data, that are crucial to both subsequent numerical simulation studies and experiments.