A comparative study of experimental maturation of peat, brown coal and subbituminous coal: Implications for coalification

Laboratory experiments were performed on a peat, a brown coal, and a subbituminous coal using closed-system pyrolysis. The samples were heated isothermally for 48 h from 150 to 500 °C. All three samples had similar pathways of thermal evolution, which can be divided into four phases based on vitrinite reflectance (Rm). Phase 1 (< 250 °C or < 0.6% Rm) was characterized by a rapid increase in CO2, phase 2 (200–350 °C or, 0.6% < Rm < 1.1%) by simultaneous generation of oil and gas, and phase 3 (350–400 °C, 1.1% < Rm < 1.3%) and phase 4 (> 400 °C, Rm > 1.3%) by production of methane. Phase 3 and phase 4 also caused the condensation of aromatic rings, which resulted in a rapid increase in Rm in the solid residue and decrease in the potential of hydrocarbon generation. The yield of gas, however, varied from sample to sample. The amount and composition of bitumen generated also differed significantly among these samples.The results indicated that coalification under these experimental conditions appeared to have similar evolutionary pathways regardless of the original stage; the quantity and the composition of products, however may vary during the coalification process. In this study, the difference mainly occurred during the process from peat to high volatile bituminous coal, which generated a large amount of CO2 and bitumen that contained over 95% polar compounds. This study also demonstrated that early-stage coalification occurred mainly as a loss of oxygen in organic matter. Results of this comparative study enhanced our understanding of the mechanisms of coalification.