Coal Comminution Characterization for Industrial Scale Circulating Fluidized Bed

Report, 2002

The management of the bed solids inventory and particle size distribution in fluidized bed combustors has long been recognized as a key factor for the efficient and trouble-free operation of industrial scale fluidized bed ombustors. From a coal particle to an ash particle getting out the circulating loop, the particle size reduction factor can be hundred. Detailed pilot scale test observations related to the size of circulating particles and ash particles, show very different behaviours for different coals. In order to describe those behaviours related to a large particle size spectrum, a general frame work is proposed based on several concepts. The key concept is represented by the Primary Ash Particle Size Distribution (PAPSD). According to this concept, Primary Ash Particles are liberated from the carbon matrix under the combined action of Combustion and Primary Attrition of the mother fuel particles. By Primary Attrition it is meant here a complex of particle comminution phenomena closely associated with the progress of reactions (devolatilization/char combustion), along pathways and with mechanisms that are extensively discussed by Chirone et al. (1991). Further (Secondary) attrition of ash particles can occur after char combustion is complete and is not associated with the parallel progress of chemical reactions but to mechanical attrition only. Those concepts are associated with coal characterization procedures in order to have information on the size reduction of a coal particle and the final size istribution of ash. The measurement obtained on a lab scale is validated by the analysis of some results of industrial-scale tests. In association with procedure definition, a qualitative approach allows prediction and inter-coal comparison. A modelisation work is also presented considering however that quantitative predictions of the particle size distribution inside a CFBC is still a far objective.