Controlling bed solids inventory and particle size distribution during fluidized bed combustion of solid fuels

The management of bed solids inventory and particle size distribution is critical to the efficient and reliable operation of fluidized bed combustors (FBC), as it affects bed hydrodynamics, heat extraction, cyclone performance, bottom vs fly ash balance, fouling, etc. The population of bed ash material establishing at steady state in a FBC is the result of concurring processes among which particle 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 particle size distribution of ash material establishing in the bed during steady operation of a fluidized bed combustor. Key properties are represented by the amount and the particle size distribution of the Primary Ash Particles (PAPs) liberated by the fuel along burn-off. The propensity of the Primary Ash Particles to undergo further secondary attrition represents another important variable in the assessment procedure. The experimental procedure for the characterization of the amount and size distribution of PAPs liberated from a solid fuel is presented. Results of the application of the procedure to a variety of fossil and biogenous fuels are reported. The impact of the PAPSD of the different fuels on the build-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 FB combustion of a bituminous coal and a granulated sludge. These fuels have been selected on account of the different conversion patterns they feature during burn-off: the shrinking particle/constant density conversion pattern is exhibited by bituminous coal particles; the shrinking core/constant particle size is instead the typical conversion pattern for the granulated sludge. The relative importance of the primary generation of ash material (PAPs) and of secondary attrition of PAPs is discussed in the light of the relevant conversion pattern of the parent fuel.