Characteristics of particulate emissions from coal char combustion: Char fragmentation and ash coalescence behaviors

Abstract

Air pollution from fossil fuel combustion motivated the extensive and in-depth investigation on particulate matter (PM) to meet the stringent emission norms. The relationship between char fragmentation/mineral coalescence and PM formation is elucidated in this paper. To separately investigate their contributions to PM formation, the effects of particle bursting during devolatilization and fragmentation of excluded minerals have been stripped off. We prepared size-graded and density-fractionated chars with varying ash content levels and conducted combustion experiments in a drop-tube furnace at 1300/1600/1800 K. The mass particle size distributions (PSDs) of collected ash were measured by an Electrical Low Pressure Impactor (for PM10) and a laser particle-size analyzer (for PM10+). The micro- morphologies and elemental compositions analyses were also shown for particles of different sizes. The results indicate that char fragmentation significantly increases the number of ash particles in fine-fragmentation mode (FFM), and the ash peak in the coarse mode comes from the char particles that show fewer fragmentation behaviors. Dense char structure disadvantages char fragmentation but enhances coalescence of included minerals. The distribution of fly ash at 1600/1800 K exhibits bi-modal with two peaks around 10 and 105 μm, while the distribution at 1300 K/50%O2 exhibits uni-modal with one peak. This can be well explained by the percolation theory that the char combustion in the kinetically-limited regime is dominated by uniform fragmentation, while particles at 1600/1800 K mainly undergo perimeter fragmentation. The effect of ash content is more related to the mineral grains distributions within the char particles.