EVOLVING PORE STRUCTURES OF LIGNITE DURING PYROLYSIS OBSERVED BY COMPUTED TOMOGRAPHY

Lignite is a low-rank coal with high moisture, low calorific value, and high volatiles content. To prevent spontaneous lignite combustion and to reduce long-distance transportation costs, we proposed an industrial solution for lignite dehydration modified qualitatively by in situ heat injection. We studied the evolution of lignite pore microstructures by microcomputed tomography up to 600°C. From room temperature to 600°C, pore-structure evolution of lignite occurred in three phases. At room temperature to 200°C (phase I), the numbers of pores and fissures and the specific surface area increased rapidly. At the end of this phase, the porosity increased to 37.9%, the percolation probability exceeded the percolation threshold of 31.17%, and seepage began. During phase II (200°C-500°C), pyrolysis increased the numbers of new pores and fissures significantly. The original and newly developed pores and fissures become interconnected to form larger pores and pore groups. The specific surface area and porosity increased slowly. During phase III, from 500°C to 600°C, the porosity increased rapidly to 47.8%. The ratio of the pore morphology along the long and short axis varied from 2.63 to 3.