INVESTIGATION OF THE CLASSIFICATION OPERATION IN A COAL PULVERISING VERTICAL SPINDLE MILL

摘要

Coal pulverisation in power plants is a major electrical energy consumer. In 2006 more than 41 perrncent of world electricity generation was from coal where pulverisers use 0.5 to 1 per cent of grossrngeneration. Therefore, pulverisers are an important target for optimisation not only for reducingrncosts of power generation but also for reduction of greenhouse gas emissions.rnA better understanding of the operation of pulverisers is an essential requirement for theirrnoptimisation. However, little information is available, other than operating experience, on thernperformance of pulverisers and how these react to changes in operational variables. This studyrninvestigates the effect of changes in coal and airflow feed rates on the classification performancernthat occurs in an industrial scale ball-and-race vertical spindle mill in which the pulverisation andrnclassification processes are linked together due to the separator classifier being an integral componentrnwithin the pulveriser.rnIn order to decouple the breakage and classification processes, samples were cut from upwardrnflowing streams through sampling ports drilled on the pulveriser body at different locations. Specificrnsampling tools were developed. Samples collected from the pulveriser were size and density fractionedrnto distinguish the differences in behaviour of the components, namely coal and mineral matter. Therncoal flow rates on dry basis within the mill and around the separator classifier were calculated byrnmass balancing the multi-component data.rnThe difference in classification behaviour of components during pneumatic transport was confirmed.rnThe response of the circulating load in the mill (～600 to ～1100 per cent) as well as the dependencyrnof fineness of the streams to the operating variables was demonstrated.rnThe approach taken in this study has proved that development of a classification model for a verticalrnspindle mill pulverising non-homogeneous materials, eg coal, requires treating these materials asrnmulti-component/composite substances.