The experimental evaluation of nitrogen transformation in South African coal chars and the concomitant release of nitrogenous species

Abstract

The effect of typical South African (SA) coal properties on nitrogen functional transformation and release were examined in the course of pyrolysis in a fluidised bed (FB) and drop-tube furnace (DTF). Chars were generated at temperatures ranging from 740 C to 980 C in the FB, and at 1000 C up to 1400 C in the DTF. X-ray photoelectron spectroscopy (XPS) analysis of the parent coals showed that pyrrolic nitrogen (N-5) was the dominant functionality, followed by pyridinic- (N-6), and quaternary nitrogen (N-Q), respectively. On the contrary, analysis of chars revealed that N-Q was the most prevalent N functional form, followed by N-6, N-5, and protonated-/oxidised pyridinic nitrogen (N-X), respectively. Chars generated from DTF, emanating from relatively high total reactive macerals, behaved differently at elevated temperatures. The nitrogen functionality of all coals and FB chars that underwent HCl/HF/HCl treatment seemed not to experience meaningful alteration. Remnants of acid treated DTF chars that were prepared from inertinite-rich coal, as well as possessing low total reactive macerals, also did not show apparent changes in nitrogen functionalities. Nevertheless, de-ashing of chars emanating from a vitrinite-rich coal, and also from a severely pyrolysed inertinite-rich coal possessing relatively high total reactive macerals, resulted in the emergence of additional moieties of nitrogen in the remnants. X-ray diffraction (XRD) results illustrated that aromaticity (fa) and average crystallite diameter (La) of chars simultaneously rose as the pyrolysis temperature increased. Nevertheless, increasing temperature resulted in apparent decrease in the fraction of amorphous carbon (XA) and the associated degree of disorder index (DOI). Examination of XPS and XRD results enabled the correlation of simultaneous transformations of nitrogen functional forms and condensed aromatic crystallites emanating from pyrolysis. Increase in N-Q exhibited a good relationship with fa, while the decrease in N-5 displayed a satisfactory correspondence with XA and DOI. The fractions of inherent coal nitrogen that were emitted from the volatile stream as NH3, HCN and tar-N were also evaluated. Tar-N emitted at 740—900 C primarily comprised the following functionalities in given order; N-5 > N-6 > N-Q. The N functional form distribution in tars produced at this temperature range was comparable to that of raw coals. Increasing temperature caused a simultaneous increase in NQ, decrease in N-5, as well as a subtle diminishing of N-6 in tars. XPS analysis of tar-N was limited to tars released from FB. Coals with a substantial composition of vitrinite, total reactive macerals and mineral matter, released a significant fraction of fuel nitrogen as volatile-N during FB and DTF pyrolysis. A greater portion of coal-N was emitted as NH3 than HCN during FB pyrolysis. However, DTF pyrolysis prompted conversion of an appreciable coal-N fraction into HCN rather than NH3. Coals with relatively high total reactive macerals displayed similar behaviour with regard to nitrogen transformations and release patterns. A combination of total mineral matter and maceral composition exhibited greater influence on the nitrogen product distribution.