The evolution law of nano-pore structure characteristics during the underground pyrolysis process of tar-rich coal

Abstract

The dynamic evolution of nanopore architectures critically governs the adsorption efficiency of hydrocarbons in tar-rich coal during subsurface pyrolysis, offering a pathway to optimize product yields. In this study, two types of Chinese coal were analyzed using the small-angle neutron scattering (SANS) instrument to monitor changes in nanopore structure throughout the underground pyrolysis process. The experiment uses the in-situ SANS instrument at the China Spallation Neutron Source (CSNS), with temperatures ranging from 30℃ to 800℃, under two different heating rates (5 and 20℃/min). These measurements were complemented by Brunauer-Emmett-Teller (BET) analysis, scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The experimental results show that the average nanopore size increases progressively with rising temperatures, particularly between 300°C and 500°C during the devolatilization stage, where it grows from 112.8 nm to 131.3 nm. Furthermore, at elevated temperatures, coal nanopores tend to become isotropic, regardless of their initial distribution. A slower heating rate results in higher overall scattering intensity and more pronounced nanopore development.