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Show are separated from the stream and fall back to the grinding table while the smaller particles are conveyed to the secondary classifier( primary classification). The pulverized coal/air flow is then swirled by means of the rotating classifier blades fixed on the rotor. The centrifugal force acting ,on coal particles sweeps coarser particles back to the grinding table via the marginal zone next to the mill housing wall(secondary classification). Having passed through the classifier, finer particles are transported to burners via several individual fuel pipes. The mode of operation of the mills depends on the actual boilers. The design and operation of coal pulverizers are mainly based on empirical rules and accumulated experience. However, recent trends in Japanese utili ty boilers are to burn a variety of imported coals and to operate the boilers at various loads with rapid changes. Under these circumstances, the performance of a boiler can be strongly affected by the grinding mills. In particular, the transient behavior of the mills affects the boiler. Nevertheless, the boiler must meet strict environmental emission control regulations. To meet these boiler requirements, therefore, the uni t operation of grinding by ring-roller mills must be developed on a more elaborate basis. With this in mind, research and development work has been in progress at Babcock-Hitachi K.K. Austin e t al~-3) analyzed a ball-and-race milling system and developed a mathematical model for steady state milling based on specific rates of breakage and primary fragment distributions. The model includes primary and secondary classifications of particles returning to the grinding zone. Meguri et al~) made improvements on the model, by adding the operating-design equations on classifiers. Robinson5 proposed a model of the transient operation of a bowl mill based on conservation of coal mass in seven internal regions of a pulverizer and in 15 particle-size groups. Empirical parameters were evaluated by comparing the model's prediction to steadystate measurements acquired on an industrial pulverizer. The model was then used to predict the transient behavior of the pulverizer. The objectives of this paper are to present a mathematical model to describe both steady- and unsteady-state behaviors of the ringroller mill and to show some of the factors affecting the mill performance. 2.General Formulation of the Model The mill circuit to be analyzed is shown in Figure 2 where G(t) is the raw coal feed rate to the circuit and gik(t) is the weight fraction of the feed which is size interval i and grindability class k. Similarly, Fi'(t), F~'(t), F(t), T'(t), T(t) and Q(t) are coal flow rates at each stream point and f lk ' (t), wlk ( t), Pike t), t '( t), t ik ( t) and qik( t) are weight fractions of coal particles of stze interval ,i and grindability of k. Furthermore, Wet) denotes the hold-up in the mill and c i and Sl are fractions of particles of size interval i which are returned to the grinding zone from the primary and secondary classification zones, respectively. The following assumptions were made to develop a mathematical model. (I)Coal particles break according to the first-order breakage law, 2 |