In-situ FTIR study of reaction mechanism and chemical kinetics of a Xundian lignite during non-isothermal low temperature pyrolysis

• Pyrolysis of functional groups in lignite coal was studied by in-situ FTIR.
• Kinetic characteristics of each stage of functional groups were presented.
• The decomposition of CO played an important role during lignite pyrolysis.
• The hydrogen bonds in lignite were mainly related to certain kinds of CO and CO.

Investigating the real time changes of functional groups during lignite pyrolysis can deepen understanding of reaction mechanism of lignite under oxygen-free condition, and help upgrading its quality and enhancing the conversion efficiency. In this study, non-isothermal low temperature pyrolysis of lignite using in-situ Fourier Transform Infrared Spectroscopy (FTIR) was carried out to investigate the evolution of six most abundant functional groups in lignite. The evolution of these functional groups was divided into several stages with different mechanisms and kinetic characteristics. The decomposition of CO associated with hydroxyls by hydrogen bonds (HBs) played an important role in the pyrolysis process below 400 °C. The activation energies for the pyrolysis of lignite at various stages were mostly lower than 70 kJ/mol, and demonstrated the important role of HBs in the low temperature pyrolysis. Additionally, the HBs were mainly related to the aromatic phenol, aromatic ether and phenoxy for CO, and quinone and carboxyl for CO. At higher temperature stage (above 300 °C), the oxygen contained functional groups, including carboxyl, carbonyl and especially ether, began to decompose, resulting in greater loss of coal materials. At the temperature up to 400 °C, the CC bonds began to break apart, which caused the primary decomposition of aliphatic groups and the increasing intensity of aromatic CH.