Emission profiles of volatile organic compounds from various geological maturity coal and its clean coal briquetting in China

Coal is a dominant energy source used in household cooking and warming in rural China. Its combustions generate a large quantity of gaseous and particulate phase's pollutants. In between, the volatile organic compounds (VOCs) is a critical class of chemical that shows strong environmental and health impacts. In this study, the VOC emission characteristics, influencing factors and briquette technology that affected the combustions of different maturity coals were investigated. Emission factors (EFs) of targeted non-methane VOCs (EF_NMVOCs) for different coal samples ranged from 233 ± 89.8 to 1690 ± 722 mg/kg. The EF_NMVOCs decreased with volatile content (VC). Besides, EF_NMVOCs for the briquette coals showed 5.31–51.5% lower than that of the raw coals except the anthracite. Alkanes and alkenes are the two most dominated chemical groups, accounting for >50% of the total NMVOCs emissions. Coefficient of divergence (CD) values showed that a significant difference (>0.6) between the emissions of high-volatile and low-volatile raw coals, representing that the coal content should be sub-classified for either source apportionment or emission profile database development. Correlation analysis reported that VC is the most significant factor that correlated with EF_NMVOCs (R = 0.78), while modified combustion efficiency (MCE) negatively correlated (R = −0.78). The calculations of ozone formation potentials (OFP) and secondary organic aerosol potentials (SOAP) deducted that alkenes and aromatics are the top two contributors to the formations of these secondary pollutants. In addition, the briquette technology has been proved to reduce both OFP and SOAP from coal combustions. Our results perceptibly support that the use of low VC coals could efficiently reduce the emissions of VOC and the formations of more environmental and health harm substances, and the application of briquetting technology also has a certain effect especially in bituminous coals. The key findings provide feasible solutions to improve regional air quality regarding residential coal combustion in northwest China.