Environmentally Persistent Free Radical Emissions from Indoor Burning of Pelletized Biofuels

Pelletization of biomass fuels has been promoted as an effective alternative to mitigate particulate matter (PM) emissions from the residential burning of raw biomass materials; however, environmentally persistent free radicals (EPFRs), a class of harmful components in PM, from the biomass pellet burning have been rarely studied yet. Here, laboratory-based combustion experiments were conducted to characterize EPFRs for different pellets burned in cooking and heating stoves and compared with those for the corresponding uncompressed biofuels. Emission factors (EFs) of EPFRs for biomass pellets ranged from 2.97 × 10¹⁷ to 2.33 × 10¹⁹ spins/kg, following a log-normal distribution, with a geometric mean of 4.21 × 10¹⁸ spins/kg. These EPFRs were carbon-centered free radicals adjacent to oxygen atoms. Emissions varied largely across different fuel-stove combinations, with the combustion efficiency and combustion temperature as key influencing factors explaining 49% of the variations in EFPR EFs. Compared to raw fuels, pelletized fuels showed 50-80% lower EPFR EFs and 40-70% lower EPFRs per PM but more different EPFR types, and there was no significant change in the degree of oxidation of the EPFRs. The burning of pellets made from crop residues in a clean cookstove can reduce nearly 90% EFPRs from the raw biomass burning, which is different from the reduction degree in PM mass. This study provides valuable data in promoting the understanding of EPFR formation and deployment of biomass pellets in the protection of air quality and human health.