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Show Breakage and Mass Transport of Coal Particles in Ring-roller Mills K.Shoj*i", N.Meguri, K.Sato PAPER NO.12 Kure Research Laboratory, Babcock-Hitachi K.K. H.Kanemoto, T.Shima and T.Hasegawa Kure Works, Babcock-Hitachi K.K. ABSTRACT A mathematical model was developed for a ring-roller mill based on specific rates of breakage, primary breakage distributions and primary and secondary classification parameters. The model parameters were evaluated by laboratory and pilot-scale tests. At first, the specific rates of breakage and breakage distribution parameters of various types of coals were measured in a batch roller mill. Secondly, the primary classification of particles falling back onto the grinding table due to gravity and the secondary classification due to the built-on classifier were studied in model classifiers and empirical operating-design equations were then developed. Lastly, steady-state continuous grinding tests were carried out in pilot-scale mills to correlate variations of scale factors with make-up feed rates, mill geometry and operating conditions. Furthermore, empirical correlations on mass transport through the mills were also developed. The validity of the model was verified by steady-state data obtained in pilot- and industrial-scale mills. The effect of various operating condi tions on the mill performance was first investigated by model simulations. The unsteady-state behaviors of a mill were then predicted by the model. I. INTRODUCTION Ring-roller mills are the most widely used types of grinding mills in preparing pulverized coal for firing power plant boiler furnaces. Figure I shows a ring-roller mill. Raw coal is introduced into the center of a rotating table through a central coal chute and is thrown outwards toward the edge of the table by centrifugal action. It is then rolled over by'three heavily-loaded rollers running against the ring. The ground coal passes over the rim of the grinding ring and is swept up by hot air flowing up around the rim where it is introduced into the mill through the throat ring. The larger particles in the upflowing air stream 1 Pulverized coal Central chute Rotating classifier--+-- Throat ring Grinding table Fig.l Gear box An industrial ringroller mill |