The mechanical behaviour of coal relating to CO2 sequestration of Tanjung Enim, Indonesia

Abstract

Acoustic emission (AE) is a well-established method for geophysical calculations. Nowadays, it is widely used for damage evaluation. For a better understanding of the damage mechanics of materials such as rocks, the sonic wave that has led to AE, has been used to monitor stresses, which induce crack closure, crack initiation and crack damage. In the present study, an AE system was used to study the behaviour of discontinuities on some Tanjung Enim brown coal samples, which were subjected to uniaxial compression. As such, several samples were vacuumed in a container filled with 100% carbon dioxide (CO2) after the adsorption of high saturated CO2 at a certain pressure for 84 h prior to the testing. Meanwhile, some samples were vacuumed unsaturated. In order to determine whether CO2 had any adverse effect on the strength of coal, the comparison was performed between the unsaturated and saturated samples. Moreover, strain gauges were installed on the samples and the measured axial and volumetric strains were studied in conjunction with the AE counts. By virtue of this, the AE method was reliable to detect the onset of pre-crack initiation and the crack damage stress threshold of the black coal samples. Crack initiation and crack closure of the samples subjected to stress corresponding to the percentage of the peak strength when compared to the unsaturated and saturated samples. Nevertheless, crack damage occurred at a higher percentage of peak strength for in unsaturated condition compared to CO2 saturated samples. The results indicated that sorption of CO2 can cause the reduction in strength of the brown coal samples when tested under the uniaxial compression. As the coal samples were highly heteregeneous, more tests are required to confirm whether the adsorption of CO2 will cause strength reduction in coal. This will eventually identify the actual underlying CO2 sorption mechanisms.