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Received for review October 16, 1981 Accepted M a y 14,1982
EXPERIMENTAL TECHNIQUES
Low Rate Entrainment Feeder for Fine Solids Donald S. Scott’ and Jan Plskorz Department of Chemical Engineering, Unlversi?v of Waterloo, Waterloo, Ontario, Canada
A mechanically stirred entralnment type feeder for fine solids, particularly useful for finely divided fibrous biomass, has been developed w h i i will give rates constant to f5% for 1 h or m e . It has been used to feed coal, sawdust, and ground bark in sizes below 600 Mm at rates as low as 6 g/h. Gas to solids weight ratios obtained were from about 3:l to 13. The effect on feed rates of most of the operating and geometric parameters was investigated at low feed rates. A mechanism for control of the feed rate was tested and found to be satisfactory.
Introduction When operating bench scale reactors during studies of conversion of biomass or coal to other products, if one wishes to do this on a continuous basis, a feeder capable of low constant rates of supply is required. In general, “bench-scale”is usually assumed to mean feed rates of the order of less than 1kg/h and most frequently, rates of 100 g/h or less are preferred. There are many advantages of low-rate low-capacity operation, such as rapid approach to steady state allowing short run times, small scale facilities, and moderate operating expenses. Hence, for exploratory or screening work, low continuous constant solid feed rates are a necessary feature for small scale work. Feeders based on fluidized bed techniques capable of giving solid feed rates of 1-5 g/h in one design, and up to 100 g/h in other designs, have been described by Hamor and Smith (1971) for use with fine dry coal. The type giving the lowest feed rate (Etype)also yields a very dilute feed gas with a large gas/solid weight ratio, that is, about 10-50A. The Hamor and Smith type feeders are intended to be suspended from a recording balance to give a constant measure of weight loss. Our objective in this program was to construct and operate a bench scale flash pyrolysis unit based on a fluidized sand bed as the heating medium, patterned after that described by Tyler (1979) (and used successfully and extensively in his studies of flash pyrolysis). It was the intention of this study to apply this mini-fluidized bed flash pyrolysis technique to both biomass and to Canadian coals. Hence, a low rate feeder was required capable of delivering a constant small feed rate of both pulverized biomass particles (initially wood) or fine coal. Rates less than 100 g/h were desired. Development of a Biomass Feeder Initially, a Hamor and Smith E-type feeder was con0196-4313/82/1021-0319$01.25/0
structed with modifications as described by Tyler (1979). This feeder proved to be completely incapable of handling fine sawdust (-100 mesh) because the fibrous nature of the material, as with most biomass, prevented any reasonable type of fluidization. The biomass particles channelled readily, developed high static charges, and could not be made to entrain smoothly in the jet. Numerous modifications were tried, but without significant success. Hence, a new concept was sought. Most finely divided, reasonably dry biomass is of such a low density that it can be readily stirred mechanically, even in fairly large volumes. This is true of all pulverized solids, although denser solids present more difficulties. After some development, the feeder shown in Figure 1was designed. The feed hopper is a cylinder with a screw top to facilitate loading. The stirrer shaft enters through a stuffing box and has a bottom bearing for support and alignment. The stirrer motor is a constant torque Servodyne with variable speed drive. The stirrer itself has pairs of paddles of a figure-eight shape, each pair set at 90° to the others with an opening in the blade to reduce resistance. Located just below one of the lower sets of paddles is a horizontal entrainment tube which intersects the cylindrical body of the feeder on a chord located at the half-radial position. The tube, either 0.318 or 0.635 cm
