Release of nitrogenous volatile species from South African bituminous coals during pyrolysis

Abstract

The influence of typical South African coal attributes on the release of nitrogen into the volatile stream during pyrolysis was studied by utilizing three bituminous coals. The majority of South African coals are characterized by high mineral matter and are rich in inertinite maceral. Pyrolysis was conducted in a bench-scale fluidized bed (FB) at 740–980 °C, and also in a drop-tube furnace (DTF) at 1000–1400 °C. Levels of nitrogenous species in the volatile stream in the form of NH3, HCN, and tar-N were determined. Nitrogen functional forms of tars released at low temperatures were predominantly distinguished by high levels of pyrrolic nitrogen, followed by pyridinic and quaternary nitrogen, respectively. Tars liberated at 740 °C possessed similar nitrogen functional form attributes as those of parent coals. However, an increase in pyrolysis temperature caused a gradual increase in quaternary nitrogen as well as a concurrent decrease in pyrrolic nitrogen and a concomitant subtle decrease in pyridinic nitrogen. The analysis of nitrogen in tars was only confined to tars extracted from the FB. Vitrinite-rich and/or high mineral matter coal released high yields of nitrogenous species into the volatile stream at low FB temperatures. A large amount of NH3 was released relative to HCN under FB pyrolysis conditions. However, more HCN was released than NH3 during DTF pyrolysis. Two coals, one characterized by high mineral matter and being rich in vitrinite, and the other distinguished by relatively low mineral matter and being rich in inertinite, behaved similarly by reaching respective peak amounts of NH3 yields at 820 °C under FB pyrolysis conditions. On the contrary, an opposite profile displaying a slump at 820 °C was observed for HCN yields from the two respective coals. The third coal, a high mineral matter and inertinite-rich coal, released high NH3 yields and simultaneously the least HCN yields at 740 °C. Under DTF experimental conditions, both NH3 and HCN steadily increased with temperature in all coals. The low mineral matter and inertinite-rich coal released high yields of total volatile-N from 1000 to 1270 °C, only to be surpassed by the vitrinite-rich/high mineral matter coal at 1400 °C. The inertinite-rich/high mineral matter coal released the least throughout the entire DTF temperature range. The total mineral matter content of the coals played a significant role toward the nitrogen product distribution. On the other hand, the total reactive macerals also influenced the emission of volatile species at 1130–1400 °C DTF temperature range. The yields and composition of the released nitrogenous species have been attributed to a combination of mineral matter content, petrographic properties of the parent coals, and the utilized conditions. Pyrolysis temperature, coal particle size, and residence time also play a significant role toward the yields and composition of the released nitrogenous species