The release of nitrogen during the combustion of macerals, microlithotypes and their chars

Abstract

This paper describes an investigation of the variation of nitrogen content in a coal and its macerals and lithotypes and the release of nitrogen during combustion. Maceral concentrates were obtained by density-gradient centrifugation. Nitrogen release during the combustion of chars prepared from these materials was studied using both isothermal and temperature-programmed conditions in 20 vol.% oxygen-argon in a thermogravimetric analyser-mass spectrometer, whereas the combustion of the whole coal, macerals and lithotypes was studied only under temperature-programmed conditions. Significant differences were observed. The nitrogen contents of the macerals and their chars decreased in the order: vitrinite > semifusinite > fusinite. However, the conversion of coal and char nitrogen to NO showed the opposite trend: fusinite > semifusinite > vitrinite. Therefore although fusinite has less than half the nitrogen content of vitrinite, the conversion of the nitrogen to NO is approximately twice as great. Thus in comparison with vitrinite there appears to be a compensation effect between the quantity of nitrogen present in fusinite and its conversion to NO during combustion. The results are discussed in relation to the reactivity of the char and the reduction of the primary combustion product NO on the char.