Effects of saturation medium and pressure on strength parameters of Latrobe Valley brown coal: Carbon dioxide, water and nitrogen saturations

Adsorption of carbon dioxide (CO2) into coal matrix causes significant change in its chemical and physical structure, resulting in negligible permeability values and overall strength reduction. The main objective of this study is to investigate the effects of water, nitrogen (N2) and CO2 saturations at different saturation pressures on the strength of brown coal using uniaxial experiments. A series of uniaxial experiments was conducted on 38 mm diameter by 76 mm height Latrobe Valley brown coal samples with different saturation media (water, N2, CO2) and pressures (1, 2 and 3 MPa). According to the test results, water and CO2 saturations cause the uniaxial compressive strength (UCS) of brown coal to be reduced by about 17% and 10% respectively. In contrast, N2 saturation causes it to increase by about 2%. Moreover, Young’s modulus of brown coal is reduced by about 8% and 16% due to water and CO2 saturations respectively, and is increased up to 5.5% due to N2 saturation. It can be concluded that CO2 and water saturations cause the strength of brown coal to be reduced while improving its toughness, and N2 saturation causes the strength of brown coal to increase while reducing its toughness. The fracture propagation pattern of each sample was then observed using advanced acoustic emission (AE). Findings indicate that CO2 saturation causes early crack initiation due to the CO2 adsorption-induced swelled layer and early crack damage and failure points due to lower surface energy. In contrast, N2 saturation causes delays in crack initiation, damage and failure due to the removal of both water and naturally available CO2 from the coal mass during the saturation.