Effect of hydrothermal dewatering on the pyrolysis characteristics of Chinese low-rank coals

This paper describes the effect of the hydrothermal dewatering (HTD) on the pyrolysis characteristics of low-rank coals (LRCs). The effect of HTD on the characteristics of LRCs was explored based on scanning electronic microscopy and N2 adsorption analyses. The pyrolysis behaviors and gas products were determined using a TG-FTIR instrument. The results show that the crosslink structure and overall gel structure were broken. The pore structure expanded to the micropore region, and the surface area and total pore volume initially increased and then decreased as the treatment temperature increased. The TG-DTG results confirmed that the pyrolysis process moved towards the high-temperature and bituminous coal region. The characteristic parameters of pyrolysis indicated that the thermal stability of the coal structure was improved. According to the TG-FTIR results, the amount of released gas products from stable and saturated components (such as p-xylene, phenol and CH4) increased, whereas the amount of released gas products from unstable and unsaturated components (such as CO2, CO and formic acid) decreased. The unstable structure and components of the LRCs were decomposed and transformed, and a stable structure and phase were created. Moreover, pyrolysis activity declined, and thermal stability improved.