Criteria and governing mechanisms for pulverized-coal MILD combustion revealed by RGB imaging pyrometry and CFD modeling

Pulverized-coal (PC) Moderate or Intense Low-oxygen Dilution (MILD) combustion offers a promising route for reducing pollutant emissions while maintaining stable combustion, but its formation mechanisms and quantitative identification criteria remain insufficiently understood for practical implementation. In this study, RGB imaging pyrometry and computational fluid dynamics (CFD) modeling were adopted to investigate PC MILD combustion at 1800 K, with jet velocities of 20–100 m/s and oxygen concentration of 5–30%. The measured flame temperature agrees reasonably with the simulations, which supports applicability of the CFD models and the 27S58R mechanism developed by Yuan et al., 2025. The combined measurements and simulations reveal the decoupling between hydrodynamic and chemical controls: jet momentum determines flow topology and particle dispersion, whereas oxygen availability regulates combustion pathways and intensity. Increasing jet momentum enhances hot-gas entrainment and turbulent mixing, which initially advances ignition but simultaneous strengthens dilution, suppressing localized high-temperature combustion and driving the flame toward a spatially distributed, temperature-uniform state, namely MILD regime. Based on this transition, an experimentally accessible flame-temperature uniformity criterion of σ < 0.115 is for high-volatile bituminous coals, together with a complementary CFD-based criterion of ∆ T max < 33%. The latter shows consistent behavior for different medium- to high-volatile bituminous coals, supporting its broader applicability. MILD combustion is identified at 5% O 2 and 80 m/s and over the range of 5–15% O 2 at 100 m/s. Within this regime, 10% O 2 and 100 m/s provides a favorable balance between burnout and NO emissions, achieving an approximately 36% reduction in NO while maintaining effective fuel conversion. These findings provide a temperature-based framework for identifying and optimizing PC MILD combustion. Novelty and significance statement: RGB imaging pyrometry provides the key temperature data for CFD model validation, while CFD modeling elucidates the underlying mechanisms governing stable ignition in pulverized-coal (PC) MILD combustion. Their combination reveals the distinct roles of hydrodynamic and chemical controls and enables the establishment of a quantitative and experimentally accessible criterion for PC MILD combustion based on flame temperature uniformity coefficient ( σ ) and maximum temperature variation ratio (Δ T max ). The proposed criterion provides a practical basis for the design and optimization of PC MILD combustion systems.