This paper presents a novel approach for on-line fuel identification using digital signal processing (DSP) and fuzzy inference techniques. A flame detector containing three photodiodes is used to derive multiple signals covering a wide spectrum of the flame from infrared to ultraviolet through visible band. Advanced digital signal processing and fuzzy inference techniques are deployed to identify the dynamic "fingerprints" of the flame both in time and frequency domains and ultimately the type of coal being burnt. A series of experiments was carried out using a 0.5-MWth combustion test facility operated by RWE Innogy plc, U.K. The results obtained demonstrate that this approach can be used to identify the type of coal being burnt under steady combustion conditions.