Purification of high alkali coal: Insights into syngas production and nitrogen conversion mechanism

The purification–combustion strategy has emerged as a promising pathway for addressing the dual challenge of clean and efficient utilization of high alkali coal (HAC), a fuel widely present in many resource-rich countries. This study investigates the migration and transformation mechanisms of carbon, hydrogen, and nitrogen during the high-temperature purification of HAC using a drop-tube furnace across a range of purification temperatures (T_p). Results indicate that an increase in T_p significantly enhances syngas (CO and H₂) production via the water–gas reaction, promoting the transition from heterogeneous to homogeneous fuel conversion. At 1300 °C, the cold gas efficiency reached 62.96%. The purification also refined particle characteristics, leading to reduced particle size, increased microporosity, and enhanced gas–solid reactivity. Raman spectroscopy revealed a decline in stable graphitic structures, accompanied by an increase in defect carbon frameworks. Furthermore, high temperatures accelerated hydrocarbon cracking and liquid–gas transformation, significantly improving fuel-C conversion. The transformation of fuel-N into gas-phase nitrogen peaked at 86.35% under strong reducing conditions, where NH₃ and HCN preferentially reacted to form N₂ rather than NO_x. Aromatic nitrogen structures in char became thermally stable above 1300 °C, predominantly in the form of N-Q. The conversion trends of different nitrogen species were temperature-dependent, with N-5 decomposing into N-6 and N-Q, while N-6 exhibited a progressive decline due to oxidation or volatilization. These findings offer new insights into fuel-N transformation behavior during purification, providing theoretical support for the optimization of T_p in purification–combustion systems. The outcomes are highly relevant to the global deployment of high-efficiency, low-emission coal utilization technologies in advanced energy systems.