Fuel Aromaticity Promotes Low-Temperature Nucleation Processes of Elemental Carbon from Biomass and Coal Combustion

Significant elemental carbon (EC) emissions from low-temperature solid fuel combustion cannot be explained by classical mechanisms ascribing EC to higher-temperature condensation (> 850 °C). The importance of fuel composition in promoting EC nucleation was investigated by studying EC and polycyclic aromatic hydrocarbon (PAH) formation at multiple-ignition temperatures (300-900 °C) using fuels with different aromatic contents (i.e., straw, wood, and coal). Biomass and coal combustion at 300 °C can produce substantial EC containing a large amount of soot-EC, a known high-temperature condensation product, possibly because aromatics reduce EC nucleation barriers, corresponding to the increasing ratios of soot-EC to char-EC from straw to coal (1.22 to 3.61). High- to low-molecular-weight PAH ratios in biomass combustion were four times lower than those in coal combustion, resulting in different EC formation atmospheres. Specifically, 31.4% of PAHs from biomass combustion were indene, compared to only 0.24% for coal, indicating that resonance-stabilized hydrocarbon-radical chain reactions dominated EC nucleation in biomass combustion. Five- to six-membered PAH ratios were always higher than one in biomass combustion but increased significantly from 0.5 to 2 with increasing temperature in coal combustion, indicating that PAHs generated through aromatic decomposition in coal could form EC through van-der-Waals forces and phenyl addition/cyclization-based covalent bonding at low and high temperatures, respectively.