Pyrolysis behavior and kinetics of the trapped small molecular phase in a lignite

[Display omitted] •Thorough extraction prior to leaching is effective to capture trapped small molecular phase.•Normal heptane eluate contributes to forming of aliphatic gases.•Arenes and oxygenates produced mainly from toluene and ethyl acetate eluates.•Kinetic parameters from model-free and distributed activation energy methods are consistent.•Kinetic compensation effect is observed for raw coal and the four eluates. Trapped small molecular phase is an important part in coal and strongly affects reactivity and volatile release during pyrolysis. In this work, it was separated by normal heptane leaching of extracts from thorough extraction of a lignite and was further fractionated into four sub-fractions by column chromatography. Chemical properties of each sub-fraction were evaluated first and then pyrolysis behavior, gaseous product evolution and kinetics of the raw coal and its four sub-fractions were systematically investigated using a thermogravimetric analyzer coupled with on-line Fourier Transform Infrared spectrometer. Determination results corresponding to the total yield and chemical properties of the sub-fractions proved the effectiveness of the separation procedure. Normal heptane eluate presented one mass loss stage and decomposition of aliphatic moieties in it mainly contributed to formation of methane, ethylene, and other high carbon number aliphatics. Toluene eluate exhibited one broad mass loss stage and produced largest quantities of light arenes and ether bond containing species, while two apparent mass losses occurred in pyrolysis profile of ethyl acetate eluate, during which ketonic bond containing species generated at the first pyrolysis stage. The formation of coke during pyrolysis was mainly caused by the presence of methanol eluate. Kinetic results revealed that distributed activation energy model was suitable for describing the pyrolysis process of raw coal and its four sub-fractions. Trapped small molecular phase could be easily pyrolyzed and the average bond energy of the components was in the order of raw coal≫ethyl acetate eluate>normal heptane eluate>toluene eluate>methanol eluate. In addition, the kinetic compensation effects were observed for almost the whole pyrolysis processes of both the raw coal and the four eluates.