Structural characteristics of pores and fractures during lignite pyrolysis obtained from X-ray computed tomography

Abstract

In-situ mining of lignite requires dehydration, pyrolysis, gasification, and other stages. The injected fluid, dehydrated water, and pyrolysis products are transported in the developing pores and fractures. The internal structure and properties of lignite change significantly under the joint action of temperature and fluid pressure. In this study, X-ray computed tomography (X-CT) was used to scan lignite samples in the temperature range of 25°C–450 °C. Grayscale images and three-dimensional reconstruction images of the internal structure were obtained to investigate the evolution of the internal pore structure during lignite pyrolysis. It is found that the porosity of lignite increased as the temperature rose from 25 °C to 250 °C. The porosity was 6.54% at 250 °C. At 350 °C, the porosity decreased to 2.45% due to channel blockage and softening of the coal. At 450 °C, the pyrolysis of the lignite organic matter resulted in numerous large and interconnected honeycomb pore clusters. At this temperature, the porosity was 16.02%. X-CT and nuclear magnetic resonance enabled detailed and quantitative characterization of the internal structure of lignite. The research results provide theoretical and technical information on the evolution of migration channels in lignite for the potential improvement of in-situ pyrolysis and gasification efficiency of in-situ lignite mining.