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 degrees 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 degrees C) using fuels with different aromatic contents (i.e., straw, wood, and coal). Biomass and coal combustion at 300 degrees 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.

Automated summary

The study found that combustion of biomass and coal at low temperatures can produce significant elemental carbon (EC), possibly due to aromatics reducing EC nucleation barriers. The composition of fuels plays a crucial role in EC and polycyclic aromatic hydrocarbon (PAH) formation at different ignition temperatures.