An improved system and method for separating combustible organic particles from noncombustible inorganic particles in coal, in preparation for combustion. The coal is size-reduced and size-graded to small pieces which are then supplied to input ends of water-immersed descending slides having ultrasonic transducers for vibratory separation of inorganic and organic particles. The slides have different longitudinal lengths with angles of declination configured to achieve time-differential exposure to the ultrasonic vibratory energy, with smaller coal pieces being subjected to shorter time ultrasonic vibratory exposure. In one preferred form, longitudinally spaced turbidity sensors along the slide provide signals used to control selected ultrasonic transducers upon substantially complete cleaning of the coal pieces.