Transformation of nitrogen functional forms and the accompanying chemical-structural properties emanating from pyrolysis of bituminous coals

Abstract

Characterisation of simultaneous changes in nitrogen functionalities and condensed aromatic crystallites during pyrolysis of bituminous coals was conducted. X-ray photoelectron spectroscopy (XPS) was utilised to determine nitrogen functional forms in three South African bituminous coals and the subsequent transformation in respective chars. Corresponding structural properties of coal and char were deduced through X-ray diffraction (XRD) analysis. Carbon structural properties in parent coals were also determined by solid state 13C nuclear magnetic resonance (ss NMR). The chars were prepared by pyrolysis at 740–980 °C in a bench-scale fluidised-bed (FB) and at 1000–1400 °C in a drop-tube furnace (DTF). The changes in XPS N 1s spectra of the coals through the respective chars were used to determine the nitrogen functionality transformations. Deconvolution of the XPS N 1s spectra revealed that pyrrolic nitrogen decreased with increasing pyrolysis temperature while quaternary nitrogen increased appreciably. Simultaneously, information deduced from XRD spectra showed that aromaticity (fa) and average crystallite diameter (La) increased with severity of pyrolysis temperature in all the chars, while the fraction of amorphous carbon (XA) and degree of disorder index (DOI) decreased significantly. Chars derived from the vitrinite-rich (also high in total reactive macerals) coal were more susceptible to thermal treatment with regard to nitrogen functional forms and other carbon crystallite transformations; high temperature chars only contained pyridinic and quaternary nitrogen, and exhibited a significant increase in crystallite height (Lc) and the average number of aromatic carbons (Nave). Aromaticity of coals determined from ss 13C NMR and XRD corresponded. Comparison of structural changes brought by pyrolysis, as measured by XPS and XRD, showed that a good correlation existed between increasing quaternary nitrogen and fa. In a marked contrast, the diminishing of pyrrolic nitrogen displayed a good efficacy with DOI and XA. The reported concomitant transformations of nitrogen functional forms with char morphological changes are considered precursors to nitrogen release. This will inform future detailed studies on the conversion of coal nitrogen in solid fuel fired systems, such as in the applications of low-NOx burner technologies towards the release and reduction of nitrogen oxides in pulverised coal combustion.