The release of nitrogen during the combustion of coal chars: the role of volatile matter and surface area

Abstract

The effect of pyrolysis conditions on the release of nitrogen during char combustion was investigated. A fluidized bed pyrolysis unit was used to produce several chars at different temperatures and pressures from a Spanish coal. In this way, a number of chars differing in volatile matter and specific surface area were obtained. Gas evolution profiles of CO, CO 2, NO and N 2 from temperature-programmed char combustion were recorded by a thermogravimetric analyser linked to a quadrupole mass spectrometer. The NO evolution profiles show two zones whose intensity depends on the volatile matter and specific surface of the chars. These zones of NO release are attributed to two different nitrogen sources. The fraction of the total nitrogen char released as NO depends on the volatile matter and the specific surface of the chars. The higher 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 nitrogen in chars of low surface area and high volatile matter. This is not the case for the chars of high specific surface and low volatile matter, showing that other nitrogen species, probably N 2O, are formed during oxidation of such chars. The results are discussed on the basis of the mechanisms proposed in the literature for NO release that involve primary NO formation and subsequent reduction to N 2 and N 2O by CO and char carbon.