Coal Combustion-Related Acoustics: A Three-Stage Framework for Temperature Diagnosis During Oxidation and Conversion

Coal combustion-related acoustics comprise self-generated responses during coal oxidation and conversion and may enable non-contact temperature-related diagnosis. This review classifies these signals into three families: (i) thermally induced acoustic emission (AE), (ii) fluid–thermal–chemical coupled acoustic responses associated with volatile release and pressure disturbances, and (iii) low-frequency combustion sound during sustained flaming. Reported frequency and temperature domains overlap and vary with coal rank, packing, moisture, atmosphere, heating protocol and instrumentation; they are therefore conditional rather than universal thresholds. Active transmitted-wave studies in loose coal report temperature errors, whereas passive AE and combustion-sound studies mainly report correlations, classification accuracy or transition points, with continuous inversion errors rarely available. The three-stage framework links signal dominance, rather than fixed boundaries, to early warning, process-state discrimination and high-temperature event identification. Key challenges are multi-source interference, cross-condition calibration and uncertainty-aware inversion.