The development of nitrogen functionality in model chars during gasification in CO2 and O2

The development of the nitrogen functionality of model chars as a function of burn-off for gasification in CO2 or in O2 has been studied by X-ray photoelectron spectroscopy (XPS). The type of carbon precursor (sucrose or phenolformaldehyde resin) and of nitrogen precursor (uracil, aniline or 3-hydroxypyridine), to synthesise the nitrogen doped model chars, did not have an influence on this development. The high-temperature chars (1373 K) exhibit N-functionalities attributed to pyridinic nitrogen, pyridones, and oxidic nitrogen species at the edges of the graphene structures and quaternary nitrogen incorporated in the graphene structure. With increasing burn-off levels, nitrogen accumulates in the char, especially during O2 gasification. A gradual transition from quaternary nitrogen to pyridone and pyridinic nitrogen is observed, due to the removal of surrounding carbon. In O2 this phenomenon is more pronounced than in CO2 gasification, and more pyridone is formed due to its association with carbon-oxygen functionalities. A schematic model is presented that accounts for the development of the nitrogen functionalities and the nitrogen retention.