Permeability variations of lignite and bituminous coals under elevated pyrolysis temperatures (35–600 °C): An experimental study

Abstract

Studies on permeability under pyrolysis temperatures (PTs) can provide instructions for syngas production of underground coal gasification (UCG). Permeability variations of lignite, high volatile bituminous coal (HVBC) and medium volatile bituminous coal (MVBC) under elevated PTs (35°C–600 °C) were studied. Structure and weightlessness as well as maturity of coal detected by methods of micro structure observation, nuclear magnetic resonance, thermogravimetric analysis, and liquid yield measurement, were investigated and clarified to probe the factors influencing permeability variation. The results showed: (1) permeability of lignite and HVBC as well as MVBC increased during 35–200 °C. Lignite permeability decreased during 200–600 °C while HVBC and MVBC permeability decreased during 200–400 °C and then increased during 400–600 °C; (2) for coals with different maturities, pyrolysis reactive strength controlled the permeability. Lignite permeability were always the greatest followed by HVBC, and MVBC permeability were always the lowest, at PTs of 125–500 °C; (3) for an individual coal, permeability variation was controlled by the relative strength between positive factors (water and volatiles’ discharge, stable char framework, increased inertoid content, and high porosity of char) and negative factors (blocking of penetration channels by tars or char fines or a mix of these two, closure of macro-pores and fractures, and unstable char framework).