Characteristics of lump lignite pyrolysis and the influence of temperature on lignite swelling in underground coal gasification

• Pyrolysis of large lignite particles (50 mm) was investigated in the context of UCG.
• The mechanism of high temperature-induced lignite swelling was analyzed.
• The differences in gas composition between lump and particle lignite were analyzed.
• The structure of surface char was different from that of interior char.

Underground coal gasification (UCG) allows in situ conversion of unmineable coal deposits into a combustible gas. UCG involves a number of steps, including combustion, oxidation–reduction (REDOX) reactions, pyrolysis, and drying, within the gasification channel at high temperatures. Pyrolysis, in particular, plays a vital role in UCG. We herein investigated the pyrolysis behavior of lump lignite from Inner Mongolia, China, and investigated the influence of pyrolysis on the swelling behavior of the lump lignite. The samples (50 × 40  × 40 mm3) were studied at a heating rate of 3 °C/min. The resulting chars were analyzed by scanning electron microscopy and Fourier transform infrared spectroscopy. In addition, the composition of the pyrolysis gas released from lump lignite was compared with that released from particle lignite (particle diameter = 1 mm). The results showed that obvious swelling between 100 and 600 °C, followed by subtle swelling events between 600 and 900 °C. The concentrations of H2 and CO gases in lump lignite were higher than those in particle lignite when exposed to elevated temperatures, and the main functional groups disappeared at high temperatures due to thermal cracking. Consequently, pyrolysis occurring in the gasification channel converted the coal around the channel into a highly porous char whose permeability aided the transport of gasification agents and product gases in coal seam.