Influence of calcium chloride on the fine particulate matter formation during coal pyrolysis

This study investigated the influence of CaCl2 on fine particulate matter (PM0.5) formation during coal pyrolysis. Two acid-washed coals were loaded with different amount of CaCl2 by physical impregnation. Pyrolysis experiments were conducted in a flat-flame burner at 1400 K, 1600 K, and 1800 K. The PM0.5 yield was evaluated through the size-segregated sampling with a low pressure impactor (DLPI+). Then, the transformation properties of CaCl2 was analyzed with X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP-OES). Finally, the soot properties were analyzed with transmission electron microscopy (TEM), Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). The results show that PM0.5 yield is affected by coal type, CaCl2 loading level, and pyrolysis temperature. Loading small amount of CaCl2 can reduce the PM0.5 yield through soot inhibition, while high loading of CaCl2 increases the PM0.5 yield by forming mineral particles, the lowest PM0.5 yield occurs at 1.5 % Ca content. The PM0.5 reduction rate of CaCl2 increases then decreases with temperature from 1400 K to 1800 K. On all tested conditions, the PM0.5 yield of low-rank Zhundong subbituminous coal was lower than Shenhua bituminous coal. The release rate of Ca into PM0.5 is positively related to pyrolysis temperature and CaCl2 loading level. Ca loading decreases the sizes of soot primary particles and agglomerates, and leads to smaller amorphous cores and more disordered graphitic shells in the soot primary particles. CaCl2 loading also causes the dealkylation and deoxygenation of soot. The influence of CaCl2 on soot formation can be summarized as the catalytic effect on soot precursor formation, the enhancement of soot nucleation, and the inhibition of soot coagulation.