Coal combustion analysis using Rock-Eval: importance of S4-Tpeak

Abstract

The Rock-Eval technique has been conventionally used for source-rock analysis. In this work we document the importance of Rock-Eval S4 oxidation graphics and S4-Tpeak as indicators for coal reactivity and thermal maturity. Two non-coking coals (lower maturity), one coking coal (higher maturity), and one jhama (intrusion-induced metamorphosed coal), were collected and studied in terms of their reactivity and combustion properties. Our results indicate that Rock-Eval S4-Tpeak can be convincingly used to decipher the thermal maturity level of a coal sample. The two non-coking coals owing to their higher reactivity and corresponding lower activation energies, combusted at lower temperatures, almost entirely below 650 °C, and showed lower Rock-Eval S4-Tpeak. The coking coal sample on the other hand due to its higher thermal maturity level and lower reactivity, combusted at higher temperature, showing higher S4-Tpeak. While the S2 Tmax showed higher maturity for the coking coal than the jhama, the S4 oxidation graphics and S4-Tpeak clearly revealed higher thermal maturity of the jhama relative to the other samples. With increasing sample weights, the S4CO2 curves were observed to be broader, and consequently the S4-Tpeak was observed to be higher and erroneous, the errors being more for the more-mature coking coal and jhama. With lowering sample weights, the curves became tighter and the S4-Tpeak became lower and more precise. Parameters calculated using TG-DTG-DSC were observed to complement the data from Rock-Eval oxidation-stage, and revealed higher maturity, less reactivity, higher temperatures of ignition and burn out for the jhama, followed by the coking coal. Our results also indicate the suitability of applying Rock-Eval for combustion-profiling of coals, beyond source-rock characterization and CBM reservoir analysis.