A systematic study on the fine particular matter emissions from coal combustion of four kinds of coals of different ranks (lignite, sub-bituminous, bituminous, coking coal), including three typical Canadian coals and a Chinese coal have been presented. The results show that a tri-modal mass size distribution (coarse, fine and ultrafine mode) was commonly identified, and it was confirmed by size-segregated elemental distributions. The tri-modal distribution is of temperature and coal ranks independence. Coarse particles (Da similar to 5 mu m) presented as round-shaped sphere with smooth surface. Both round-shaped particles and irregular-shaped particles were observed in fine particles (Da similar to 0.5 mu m). Clusters of nano-particles (Da similar to 0.05 mu m) were identified in the ultrafine mode. The concentration of fine particles (Da < 1 mu m) generated from the TECK coal is the highest (about 150 mg/Nm(3)), while fine particles (Da < 10 mu m) generated from Boundary coal (about 580 mg/Nm(3)) is highest among the four coals. Higher volatile content in coals gives priority for enhancing the formation of fine particles. Excluded minerals would mostly fragmented to form ash residual with irregular shapes, while part of them would experience coalescence individually to form round-shaped particles with size larger than 10 mu m. Included minerals adhered firmly on the surface of char particles contribute greater in the irregular-shaped particles in fine mode, while the shedding included minerals from char particles should take the major responsibility for the round shaped particles in fine mode. (C) 2015 Elsevier Ltd. All rights reserved.