04/19/2012 | Autor / Editor: Eddie McGee * / Marcel Dröttboom
Feeding biomass materials into pyrolysis and gasifier processes to produce energy can be a difficult task, because they are often poor flowing and may contain foreign matter. The challenge is to feed this material at a consistent rate into a combustion chamber.
Biomass energy is derived from a wide variety of fuels: municipal solid waste (MSW), construction and demolition materials (CDM). Wood feedstocks can be quite variable from chipped virgin wood, wooden pellets, broken up pallets even chopped up door frames. As a result the biomass materials can be poor flowing and may contain foreign matter. The challenge for biomass energy companies is to feed this material at a consistent rate into a combustion or decomposition chamber, at an elevated temperature, while preventing air ingress. Plug screw feeder technology is proving to be particularly effective in meeting these conditions for continuous plant operation.
Like any industrial process involving bulk materials, the ease with which materials are processed is dependent upon their flow characteristics. Biomass processing is no different. Even though the range of biomass materials creates an uncontrolled variability of feed stocks, it is important to understand the flow characteristics for the biomass materials with respect to foreign bodies, size of constituent components such as lumps, and size of biomass pieces, and design biomass processing plant equipment accordingly, in order to ensure continuous operation. Assessing the flow characteristics of biomass fuels is dependent upon successful measuring the material's bulk density in loose and compacted conditions (taking into account variation with stress), shear strength and wall friction affecting hopper flow, and internal friction and permeability which influence screw feeder plug formation and effectiveness as a barrier to air ingress.
In addition, the materials will have variable moisture content, density, elasticity. These factors, and the interlocking nature of these materials, mean that the classical Jenike approach to assessing flow characteristics is often unsuitable.
A better approach to determine the flow characteristics of biomass materials is to use multi-axis Spider Diagrams. These allow an assessment of biomass flow behaviour based on multi-attribute of flow sensitivity, with wall friction, stress-sensitivity of density and interlocking characteristics. Interpreting the spider diagram correctly is critical to the design of hopper shape, wall slope, outlet size and screw feeder.
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