Characteristics of lignite char derived from oxy-pyrolysis

Lignite chars were prepared in a fixed bed reactor under various oxygen concentrations (0-15 vol%) and pyrolysis temperatures (873 K-1173 K). Characterization techniques such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and BET were applied to reveal the impacts of temperature and oxygen concentration on char structure evolution during pyrolysis. Combustion performances of the prepared chars were examined on a thermal gravimetric analyzer, using ignition and burnout temperatures as indexes of char reactivity. On a whole, oxy-pyrolysis char is more reactive than the char derived under inert (N2) atmosphere. Raman analysis indicates that the presence of oxygen during pyrolysis can reduce the extent of char graphitization at high temperatures and favors the removal of oxygen containing groups and aliphatic structures. Pore volume and specific surface area can be improved due to the presence of oxygen in pyrolysis. There is a critical oxygen concentration at high pyrolysis temperatures. Releasing behaviors of sulfur and nitrogen, and their speciation and distributions on the prepared chars were also discussed.