UBC Theses and Dissertations
Biomass granular feeding for gasification and combustion Dai, Jianjun
Successful feeding is critical to biomass utilization processes, but is difficult due to the heterogeneity, peculiar physical characteristics and moisture content of the biomass particles. The objectives of this project were to define what limits screw feeding in terms of the mechanisms of blockage and to examine the effects of key properties like mean particle size, size distribution, shape, moisture content (10-60%), density and compressibility on screw feeding of biomass. Wood pellets, ground wood pellets, sawdust, hog fuel, ground hog fuel and wood shavings were used in a screw feeder/lock hopper system previously employed to feed biomass to a circulating fluidized-bed gasifier. Three hopper levels (0.3, 0.45, 0.6 m), five casing configurations (common straight, tapered and extended sections) and two screws with different configurations were investigated. Experimental results showed that large particles, wide size distributions, irregular shapes, rough particle surfaces, large bulk densities and high moisture contents, as well as higher hopper levels and special casing configurations, generally led to large torque requirements for screw feeding. The "choke section" and seal plug play important roles in determining torque requirements for biomass fuels. The unique characteristics of biomass and system requirements of biomass processes create special challenges for biomass feeding. A fundamental study on a Particulate Flow Loop was also conducted to investigate the probability of blockage/bridging as a function of particle size, shape, density, hardness, flexibility and compressibility. Experimental results showed that large particle size, irregular shape, and large ratio of particle to constriction dimension can all increase the blockage tendency. Reynolds number based on water mean velocity and hydraulic diameter of duct, constriction dimensions and shape, particle density, particle hardness, flexibility and compressibility are also important factors affecting blockage probability. The present study developed a new theoretical model with consideration of compression, aimed to understand the mechanism of biomass screw feeding and to predict torque requirements to turn the screw feeder instead of being blocked. Boundaries around the bulk material within a pocket were considered, and forces acting on these surfaces were analyzed. Two parameters are employed in this model to describe stress in screw pockets in the hopper and to analyze compression in the choke section. The model extends previous models by considering effects of all boundaries on torque, and allowing for compression in the choke section. The torque requirement is approximately proportional to the vertical stress exerted on the hopper outlet by the bulk material in the hopper and to the third power of the screw diameter based on the theoretical analysis. This indicates that large screws and high feed loads require large torque. The starting torque and volumetric efficiency of screw feeding with consideration of compression in the choke section were also estimated with reasonable success based on this model. Special casing configurations (e.g. tapered and extended sections) are also considered in the model, leading to better understanding of blockage in the choke section and approximate prediction of torque requirements for screw feeders of special casing configurations. The choke section length, screw and casing configurations were closely related to plug formation and plug sealing of the reactor, while also affecting the torque requirements. The new theoretical model successfully predicted torque requirements and efficiencies for both compressible and incompressible materials for certain screw configuration. The present experiments and model are expected to be very useful for biomass utilization.
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