Grain size and the miller's power demand in peat milling: Dissertation

The aim of this doctoral dissertation was to increase the average grain size of the milled layer from the present 5 mm up to 15 mm without an increase in the power demand of the miller by studying the present blade models already used in peat production and by developing new ones. If the target of the research can be achieved, the drying of peat will be improved by at least 20 % and hence the hectare yield will be increased and the production costs reduced. The research was carried out in a laboratory by using a milling machine simulator. The miller types investigated were rotating pin and screw millers already used in peat production, and new plate and granulating millers. The experiments were carried out using medium and well humified Carex and Carex/Sphagnum peats. The research plans were made using test planning software, and the results have been analysed by a multivariate method. Models were compiled for four miller and peat types, and they show the effects of the milled layer and driving parameters on the average grain size and on the power demand of the miller. The average grain size is increased with nearly all the miller and peat types as the milling depth, milling moisture content and the driving speed of the miller areincreased, and as the amount of the residual peat and the rotation speed of the miller drum are decreased. The power demand of the miller is increased as the driving speed, the rotation speed of the miller drum, the amount of residual peat and the milling depth are increased. It was not possible to reach the target fully. The best results were obtained using plate and screw millers for all four peat types. The average 14 mm grain size was obtained in the case of medium humified Carex and Carex/Sphagnum peats, as well as with well-humified Carex peats while using the plate miller. The average grain size of over 15 mm was obtained in the case of well and medium humified Carex peats while using the screw miller. The powe r demand of the plate miller is lowest, being about the same as that of the pin miller, but the power demand of the screw miller is higher than that of the plate and pin millers. On the basis of the average grain size and the power and maintenance demands of the miller, the plate miller is the best. The obtainable average grain size of the plate miller can be increased and the grain size distribution and power demand decreased by reducing both the diameter and the rotation speed of the miller drum. The plate miller can also be developed further in order to obtain the grain size target with all peat types.