Influence of anions on black carbon generation and formation mechanism of magnetic black carbon during coal combustion

Abstract

Coal used in thermal power plants frequently contains various anions, and to investigate whether these anions influence the formation of black carbon (BC) from incomplete coal combustion, we conducted simulated combustion experiments by adding iron salts containing common coal-derived anions to purified coal. Comparative analysis of combustion products revealed that acidic anions, particularly sulfate ions, released from iron salts during combustion tend to form acidic gases that corrode BC surfaces, thereby generating more porous and morphologically complex BC structures while simultaneously suppressing BC yield. In contrast, basic anions such as hydroxide ions exhibited a modest promoting effect on BC production. Quantum chemical calculations and literature review confirmed that various anions, when thermally released in gaseous form, significantly affect both the morphological characteristics and yield of BC. Furthermore, the same anion demonstrated varying effects depending on the properties of its carrier matrix. Through combined quantum chemical calculations and synchrotron radiation analysis, we elucidated the formation mechanism of magnetic black carbon (MBC): iron salts mixed with coal first release their anions upon heating while being oxidised to Fe₂O₃; subsequently, after substantial BC generation at high temperatures, Fe₂O₃ chemically adsorbs onto BC through Fe–C bond formation; finally, under sustained high temperatures, Fe₂O₃ reduces to Fe₃O₄ to form MBC. This comprehensive investigation provides fundamental insights into the anion-mediated regulation of BC properties during coal combustion.