From localized flames to distributed reactions: Spatio-temporal diagnostics and regime identification of pulverized coal MILD combustion in O₂/CO₂ atmosphere

The transition from conventional flame-front combustion to distributed Moderate or Intense Low-oxygen Dilution (MILD) combustion of pulverized coal in O₂/CO₂ atmospheres is essential for clean and efficient coal utilization under carbon-capture conditions. However, the spatio-temporal dynamics of this transition remain poorly understood, mainly because the limited optical access of pilot-scale facilities prevents time-resolved, full-field diagnostics. In this work, a Hencken-type flat-flame burner with full optical access to the entire reaction zone was combined with 5 kHz high-speed CH* chemiluminescence imaging to investigate this transition under two co-flow oxygen levels (5% and 15% O₂ at 1800 K) across jet velocities of 20–100 m/s. The spatial evolution of the reaction zone was quantified using the coefficient of variation ( CV ) of the radial intensity profile. Pixel-resolved temporal distribution uniformity ( TDU ) analysis and its derived spatial uniformity index ( SUI ) further showed how localized fluctuation hotspots are progressively suppressed. Continuous wavelet transform revealed that the oscillation energy shifts from coherent low-frequency modes to broadband high-frequency fluctuations as the MILD regime is established. Combining these diagnostics, a two-dimensional classification framework on the CV – LFEP (low-frequency energy proportion) plane is proposed, which separates conventional, transitional, and MILD combustion regimes. This framework offers a quantitative, non-intrusive basis for regime identification in oxy-fuel MILD combustion systems.