Abstract Municipal solid waste (MSW) combustion in CFB boiler is a promising waste-to-energy technology. However, inorganic minerals in MSW can be transformed into particulate matters (PM) through a series of ash partitioning mechanisms. These generated PM not only can deposit on heat transfer surface and significantly reduce the boiler efficiency, but also might be released to the atmosphere and cause serious health issues for human beings. This work investigated the particulate matter formation mechanism in a full-scale CFB boiler, which incinerates shoe manufacturing waste (SMW) with 576 t/d treatment capability. During the experimental work, PM samples near superheaters were directly sampled through a water-cooled isokinetic probe. Particle size distribution showed that there exists an accumulation mode in submicron particle region. The compositional analysis indicated that ultrafine particles with particle size smaller than 0.1 μm are mainly composed of alkali chlorides. The supermicron particles are mostly composed of calcium. Furthermore, heavy metal contents (Pb and Cr) are fairly enriched in ultrafine particles. Overall, this work sheds the light on the ash partitioning behavior during SMW combustion in circulating fluidized bed, which can provide some insights on the design and optimization of waste-to-energy CFB boiler.