A multi-component model for the vertical spindle mill

The coal fired power plants are one of the oldest, most common and wide spread generators of electricity. They utilise heat generated by burning coal, that is pulverised in different type of grinding machines selected based on the coal type that is being processed, to produce steam power. For bituminous coal, vertical spindle pulverisers are the preferred and most common equipment for pulverisation. In this thesis, the vertical spindle pulverisers were of interest and their operation is modelled. In order to achieve this goal, a sampling procedure was developed to collect samples from these pulverisers. A standard procedure for sample analyses was followed; and when required, new experimental and sampling tools were developed and manufactured. A multi-component data set of five industrial scale surveys was produced for the pulveriser operating at different coal and air flow conditions. The multi-component data are based on size and ash content as well as density for one test and the feed samples. The selected samples were classified into size-by-size density bins by float/sink density analysis. One of the general outcomes of using multi-component data was confirmation of the mineral matter accumulation in the mill showing its dependency on size. Moreover, the multi-component data were utilised to calculate the flow rates and unknown particle size distributions of the mill interior streams by mass balancing. Hence, the mill interior streams particle size distributions and flow rates, as well as size-by-size ash content distributions were obtained; thus the circulating loads that develop in the pulveriser were calculated. This information was used in constructing the ball feed stream via combining the reject streams with the coal fed to the pulveriser, which is the key stream to compute comminution information and model parameters. The mass balanced streams information were utilised to evaluate the classifications that occur in the pulveriser. The model parameters describing the classification curves that correspond to the two different zones in the pulveriser were found to be closely related to the operational conditions. A 3D multi-component breakage function was developed with the results obtained from compressive breakage tests. It is observed that the coal and mineral matter differ in breakage behaviour. This difference was incorporated into the 3D breakage model in order to be able to predict the breakage of these component particles and their composites. The mass balanced stream information together with the multi-component breakage function was utilised to successfully determine the rate of breakage of particles in the grinding zone. The model parameters describing the rate of breakage of the particles were found to be related to the operational variables. A demonstration of the developed approach was done using a simulator which linked comminution and classification models based on size and density components, to work in closed circuit operation. The classification function used in the overall simulation model was changed to an efficiency curve model based on settling velocities of particles to enable integration of both density and size of particles into the model; rather than employing empirical relations developed based only on particle size. Similarly, parameters for rate of breakage of particles have differed from that of parameters calculated during the computations for open circuit grinding conditions. The results obtained from the simulation studies were satisfactory in describing the existing conditions that occur in the pulveriser at all the sampled conditions. In conclusion, a mathematical model of an industrial scale ball-and-race pulveriser was developed by determining the effect of operational variables on the comminution and classification operations within the pulveriser.