The effect of acid demineralising bituminous coals and de-ashing the respective chars on nitrogen functional forms

Abstract

An opportunity presented itself to compare changes in nitrogen functional forms brought by the acid treatment of South African bituminous coals and their respective chars. X-ray photoelectron spectroscopy (XPS) was used to determine functional forms of the raw coals, acid-treated coals, respective chars prepared at 740 and 980°C in a bench-scale fluidised-bed (FB), and at 1000 and 1400°C in a drop-tube furnace (DTF), as well as their corresponding de-ashed remnants. The XPS N 1s spectra for the raw coals were typically similar to previous widely reported bituminous coals, of which pyrrolic nitrogen was the predominant form of organically bound nitrogen, followed by pyridinic and quaternary nitrogen. In pyrolysed chars, quaternary nitrogen was the dominant form followed by pyridinic, pyrrolic and protonated-/oxidised heterocyclic nitrogen forms respectively. Nonetheless, XPS N 1s analysis for DTF severely pyrolysed chars (1000 and 1400°C) prepared from high ash and vitrinite-rich coal, and also a char (1400°C) from a relatively low ash and inertinite-rich coal, gave a spectra with only two sub-peaks corresponding to quaternary and pyridinic nitrogen. It seems that the HCl/HF/HCl sequential demineralising/de-ashing process had no effect on the nitrogen functional forms of raw coals and the entire chars prepared from the FB. De-ashing of DTF severely pyrolysed chars emanating from high ash and inertinite-rich coal exhibited no marked change to the nitrogen functional forms. However, acid treatment of DTF chars derived from a high ash and vitrinite-rich coal, a char from relatively low ash and inertinite-rich coal, which initially contained pyridinic and quaternary nitrogen resulted in additional nitrogen moieties of pyrrolic and protonated/oxidised nitrogen.