Relationship between pyrolysis products and organic aerosols formed during coal combustion

Coal combustion is one of the major primary emission source of atmospheric organic aerosols, especially in developing countries. However, the formation mechanisms of organic aerosols during coal combustion have not been adequately studied. This study presents a detailed comparison of the chemical compositions between organic aerosol emissions from coal combustion and organic tars from coal pyrolysis, which is an early stage of coal combustion. Two coals, PRB coal and ILL#6 coal, were combusted in a laboratory drop-tube furnace coal combustor; and pyrolyzed in a flat-flame system, representative of a fast pyrolysis process. The compositions of organic constituents of the combustion aerosols and pyrolysis products were measured by an aerosol mass spectrometer (AMS) and a thermal desorption aerosol gas chromatography-mass spectrometer (TAG). The chemical composition of major organic species for both combustion aerosols and pyrolysis products are non-aromatic hydrocarbons, carboxylic acids and aromatic compounds. A list of specific organic compounds has been identified. The similarities of the chemical compositions strongly suggest that that the coal pyrolysis products are the precursors of the organic aerosols. In addition, more carboxylic acids/oxygenated organic compounds were found in the combustion aerosols, indicating that many pyrolysis products are oxidized before final emissions of organic aerosols. Thermal gravimetric analysis (TGA) was also conducted to study the pyrolysis process of the two coals. The activation energy distributions were calculated from their TGA results using a distributed activation energy model (DAEM).