Controlling Bed Solids Inventory and Particle Size Distribution duringFluidized Bed Combustion of Solid Fuels

摘要

The management of bed solids inventory and particle size distribution is critical to the efficient andreliable operation of fluidized bed combustors (FBC), as it affects bed hydrodynamics, heatextraction, cyclone performance, bottom vs fly ash balance, fouling, etc.. The population of bed ashmaterial establishing at steady state in a FBC is the result of concurring processes among whichparticle size reduction due to combustion and different modes of attrition plays a key role.A simplified theoretical framework is presented for the assessment of the inventory and particlesize distribution of ash material establishing in the bed during steady operation of a fluidized bedcombustor. Key properties are represented by the amount and the particle size distribution of thePrimary Ash Particles (PAPs) liberated by the fuel along burn-off. The propensity of the PrimaryAsh Particles to undergo further secondary attrition represents another important variable in theassessment procedure.The experimental procedure for the characterization of the amount and size distribution of PAPsliberated from a solid fuel is presented. Results of the application of the procedure to a variety offossil and biogenous fuels are reported. The impact of the PAPSD of the different fuels on thebuild-up of bed material during steady fluidized bed combustion is discussed.The study further addresses the assessment of the fractional ash reporting to fly ash during FBcombustion of a bituminous coal and a granulated sludge. These fuels have been selected onaccount of the different conversion patterns they feature during burn-off: the shrinkingparticle/constant density conversion pattern is exhibited by bituminous coal particles; the shrinkingcore/constant particle size is instead the typical conversion pattern for the granulated sludge. Therelative importance of the primary generation of ash material (PAPs) and of secondary attrition ofPAPs is discussed in the light of the relevant conversion pattern of the parent fuel.