THE RELEASE OF NITROGEN DURING THE COMBUSTION OF COAL CHARS - THE ROLE OF VOLATILE MATTER AND SURFACE-AREA

The effect of pyrolysis conditions on the release of nitrogen during c har combustion was investigated. A fluidized bed pyrolysis unit was us ed to produce several chars at different temperatures and pressures fr om a Spanish coal. In this way, a number of chars differing in volatil e matter and specific surface area were obtained. Gas evolution profil es of CO, CO2, NO and N-2 from temperature-programmed char combustion were recorded by a thermogravimetric analyser linked to a quadrupole m ass spectrometer. The NO evolution profiles show two zones whose inten sity depends on the volatile matter and specific surface of the chars. These zones of NO release are attributed to two different nitrogen so urces. The fraction of the total nitrogen char released as NO depends on the volatile matter and the specific surface of the chars. The high er the volatile matter and the lower the surface area, the higher the NO/N ratio, i.e. the fraction of the char nitrogen that is released as NO. The nitrogen released as NO + N-2 accounts for nearly all the nit rogen in chars of low surface area and high volatile matter. This is n ot the case for the chars of high specific surface and low volatile ma tter, showing that other nitrogen species, probably N2O, are formed du ring oxidation of such chars. The results are discussed on the basis o f the mechanisms proposed in the literature for NO release that involv e primary NO formation and subsequent reduction to N-2 and N2O by CO a nd char carbon. (C) 1996 Elsevier Science Ltd.