Pore structure evolution during lignite pyrolysis based on nuclear magnetic resonance

Increasing attention is being paid to the clean and efficient mining and utilization of coal resources. Lignite is a major component of coal; however, its high moisture and volatile contents and low calorific value cause it to have low mining and utilization efficiencies. During the dehydration, upgrading and in-situ heat injection or underground coal gasification of lignite, its pore structure plays an important role in the pyrolysis and product transfer processes. Therefore, it is necessary to systematically study the high-temperature pore structure of lignite. The evolution of pore fissures in gas-pyrolyzed lignite was studied by nuclear magnetic resonance combined with a gas heating reactor and test device. We found that the total connectivity, porosity and permeability of lignite increased between temperatures of 25 °C and 250 °C, then decreased at 350 °C and increased again at 450 °C. At 450 °C, the porosity, effective porosity and permeability of lignite were 18.32%, 30.22% and 856.98 md, respectively. The evolution in the pore structure parameters of lignite with temperature was observed to determine the changes in seepage channels that occur during the processes of lignite mining and utilization.