Effect of mechanical thermal expression drying technology on lignite structure

Mechanical thermal expression (MTE) is a developing nonevaporative lignite dewatering technology. It has been proved to be effective to dewater high moisture content in low-rank coals via the application of mechanical force and thermal energy at elevated temperatures. In this paper, the dewatering behavior of the Xiaolongtang lignite in Yunnan province, China during the MTE process was studied with three process parameters: time, temperature, and pressure. Meanwhile, the mechanism was also explored of how variations in temperature and pressure during the MTE process affect the surface oxygen functional groups and pore structure, which was mainly conducted by means of Fourier transform-infrared spectrometer (FTIR) and mercury intrusion porosimetry (MIP). Increases in MTE temperature and pressure resulted in significant reductions in residual moisture content and moisture-holding capacity, along with the increase in fixed carbon content and content reductions of other elements, especially oxygen content, this could be largely attributed to the destruction of the surface oxygen functional groups and porosity in the lignite. Technologically, the optimal conditions for temperature and pressure are 150–220°C and 6–10 MPa. The residual moisture content of the lignite treated by MTE at 200°C, 10 MPa is lower than 8%; the dewatering rate reaches over 76% with the calorific value being approximately 22 MJ/kg. Carboxyl and hydroxyl groups break down at drying temperatures above 120°C and constant applied pressure 10 MPa; with the pore volume significantly reduced, only few pores (diameter < 1 µm) remain at 30 MPa and 150°C (as well as above). The reduction in residual moisture content, surface oxygen functional groups, and pore volume under increasingly severe MTE conditions are suggestive of the structure changes that accompany increased coalification (rank) within the lignitic range.