Alterations in the textural structural properties of coals exposed to flue gas for long-term carbon sequestration

Abstract

Significant progress has recently been made in carbon sequestration in unmineable coal seams, including the coal physicochemical properties and environmental parameters that may affect the sequestration process. However, the effect of prolonged exposure to a flue gas on the pore structural changes of coals have not yet been considered. This study presents textural properties of two coals from South African coalfields probed using low-pressure gas adsorption (LPGA) of carbon dioxide (CO2) and nitrogen (N2) after flue gas exposure. The coals were obtained from Somkhele and Ermelo coalfields and were subjected to a flue gas synthesized to resemble that of a coal-fired power plant comprising 82% N2, 12% CO2, 5.5% O2, 0.38% SO2, and 0.12% NO2 on short-term (120 h) and long-term (2 232 h) basis. The coal samples were subjected to a flue gas at a high-pressure of 9.0 MPa and a temperature of 60 °C using a volumetric sorption system. After flue gas exposure, the surface area of the coals reduced by a maximum of 39% mainly owing to the reduced active sites. The micropore size distribution pattern for the coals maintained the original shape after being exposed to the flue gas with the pores distributed within a size range of 0.30 and 0.50 nm, showing the existence of narrow micropores in the coals; however, the highest peak value reduced by up to 17.7% relative to untreated coals. The mesopore size distribution acquired from the flue gas treated coals ranges from 1.6 nm to 2.4 nm, validating the existence of wide mesopores yielding an increase in the differential pore volume capacity relative to untreated coals.