Emission characteristics and formation mechanisms of PM_2.5 and gases from different geological maturities coals combustion

To evaluate the influence of geological maturity on coal combustion pollutant emissions, six chunk coals and corresponding briquettes (volatile matter [VM] content from 8.0% to 39.9%) were selected for simulated residential combustion in a carefully designed chamber, and the formation mechanism involved was investigated. VM was revealed to have a nonlinear effect on the emission factors (EFs) of particulate matter with an aerodynamic diameter of <2.5 μm (PM_2.5), polycyclic aromatic hydrocarbons (PAHs), elemental carbon (EC), and organic carbon (OC), with the highest EFs observed in VM of 15.8% and 26.8%. Coal tar yield and air-supply from VM combustion in its initial stage was found to be a key factor influencing PM_2.5 and PAH emissions when compared with the char combustion and burnout stages. The highest PM_2.5 and PAH EFs were observed in coal with 15.8% VM. Additionally, significant amounts of hydrogen cyanide (HCN) were detected at stage 1 and were released as volatile-N through the pyrolysis of coal-N. N₂O and NO_x were also emitted through homogeneous gas-phase HCN reactions. However, only a small proportion of NO_x was produced, indicating that the contribution of VM to HCN and N₂O emissions is greater than that of NO_x. In the char reaction stage (stage 2), char-N reacted with O₂ to form large amounts of NO, and the reaction length of the entire coal combustion process exhibited a positive relationship with coal maturity (correlation coefficient 0.69, p < 0.05), suggesting that the contribution of NO from char-N to overall NO emissions increased with the coal's geological maturity due to the increase in coal-N retention in char. Furthermore, coal briquetting was found to decrease the EFs of pollutants, which may be explained by an increase in MCE (modified combustion efficiency) and the conversion from coal tar to carbon and hydrogen. This study elucidated the effects of geological maturity on pollutant emissions from coal combustion and examined relevant underlying mechanism. These data can serve as the foundation for a clean coal project in China or other counties concerned with coal combustion emissions.