In-situ synchrotron IR study on surface functional group evolution of Victorian and Thailand low-rank coals during pyrolysis

•Evolution of surface functional groups studied for five low-rank coals and chars.•Four Victorian coals −three low-ash (2-6%) brown coals and one high ash (25%) lignite.•In-situ synchrotron IR (to 550 °C) and thermogravimetry (to 900 °C) used.•Less functional groups released from high ash coal compared to the low-ash ones.•Significant number of functional groups remained beyond 550 °C for victorian coals.

In-situ synchrotron FTIR studies were conducted using four Victorian brown coals and one Thailand lignite to examine the evolution of functional groups from the surface of the sample during pyrolysis from room temperature to 550 °C. TGA data showed that the pyrolysis of all coals is comprised of a drying stage (30-200 °C) and devolatilization stage (200-900 °C). FTIR data showed that compared to Victorian brown coals, less surface functional groups were found in Thailand lignite. The loss of functional groups from Thailand lignite was observed during drying, and the devolatilization consisted of two stages. For Victorian brown coals, the concentration of oxygen contained hydroxyl and carboxyl groups decreased during the drying stage, due to the removal of water, and breakdown of weakly bonded alkene and alkyne. When temperature gradually increased during devolatilization stage, more functional groups were released. However, by 550 °C, the groups at the wavenumber of 1700-1340 cm−1 (carbonyl, carboxylate, aromatic ring, CH3 and CH2 groups) remained in the sample. Based on the spectra taken of two chars from the same brown coals, these groups were not completely removed until 1000 °C.