Behaviors and mechanism of volatiles producing during coal pyrolysis with calcium oxide loading: Insights from model compounds analysis

Calcium is a highly effective catalyst for controlling the tar compounds produced during the pyrolysis of low-rank coal, forming valuable end products. However, the exact catalytic mechanism of calcium in coal pyrolysis remains unclear due to the complex nature of coal. In this study, the pyrolysis process and tar composition of lignite, both with and without the addition of calcium, were investigated across three distinct temperature intervals. The addition of calcium oxide increased the hydrocarbon content of tar by 16.39 % in the low-temperature range and 26.53 % in the intermediate-temperature range. Four model compounds containing different coal-related functional groups were selected to investigate the effects of calcium on pyrolysis performance and tar composition using thermogravimetric analysis, fixed bed reactor experiments, gas chromatography/mass spectrometry, in situ Fourier transform infrared spectroscopy, X-ray diffractometer, and X-ray photoelectron spectroscopy. The addition of calcium did not affect the thermal decomposition of polystyrene or polyethylene terephthalate; however, the primary pyrolysates underwent secondary reactions on the surface of calcium, which affected the composition of the liquid pyrolysis product. Meanwhile, calcium interacted with the carboxyl and phenolic hydroxyl groups in trimeric acid and phenolic resin to form carboxylates and phenates, which affected the thermal reaction and distribution of liquid products. The influence of calcium on the transformation mechanism of coal-related functional groups provides a theoretical basis for understanding the mechanism of calcium-catalyzed coal pyrolysis.