Effects of temperature and molecular mass on the nitrogen functionality of tars produced under high heating rate conditions

A German bituminous coal (Göttelborn) was pyrolysed at high heating rate in a fluidised-bed reactor. The reactor featured a relatively long freeboard heated to similar temperatures to the sand bed, where the nascent volatiles are further thermally cracked. Tars from the pyrolysis of the bituminous coal were separated into fractions with a preparative size exclusion chromatography (SEC) system. Nitrogen functionalities of the separated tar fractions and the raw coal were determined by X-ray photoelectron spectroscopy (XPS). The nitrogen functionality of the tars was found to change both with pyrolysis temperature and molecular mass. There seemed to be a relative enrichment of pyrrolic nitrogen in the higher molecular mass SEC fractions of tars produced at 600-800°C with respect to other forms of nitrogen in coal. The nitrile (-CN) group, not present in the raw coal and the tars produced at 600 and 700°C, appeared in the SEC fractions of the tar produced at 800°C. The temperature at which the -CN group is first observed coincides with the temperature at which nitrogen-containing model compounds are reported to begin to decompose. The XPS data indicated conversion of pyridinic nitrogen to nitrile nitrogen at temperatures in the range from 600-800°C. Comparison of the pyrolysis of coals and model compounds suggests that the presence of H and other reactive species in the reacting solid coal and/or char enhances the decomposition of N-containing heteroaromatic ring systems, resulting in the release of nitrogen at temperatures lower than 750-800°C. The presence of some substituents, such as phenolic groups, may also lower the thermal stability of these N-containing heteroaromatic ring systems.