Impacts of oxidizer concentration and fuel composition on near-source aerosol emissions from lignocellulosic biomass and constituent burning

Abstract

Biomass burning (BB), a multi-step process including drying, devolatilization, and oxidation of volatiles and char, is a globally occurring phenomenon that is understood to produce significant quantities of aerosols that have a broad range of local and global effects on humans and the environment. Quantities and properties of BB-derived aerosols are difficult to predict due to the complex nature of BB. Near-source conditions, such as oxygen availability and fuel composition, have been identified as influential factors in the properties of aerosol emissions. This work examined the total and size-resolved number and mass aerosol emission factors from dry, pulverized lignocellulosic biomass and its major constituents under laboratory burning conditions to understand the influence of oxygen level and fuel composition on near-source aerosol production. Lignocellulosic biomass and major constituents, including hemicellulose (xylan), cellulose and lignin, were pyrolyzed and combusted in inert and oxidative environments at varying oxygen levels, and the aerosol particle emissions were characterized in terms of size and quantity at a fixed dilution temperature. Fuel mass and fuel heating rate were varied to assess the sensitivity of results to these factors. A previously developed summative model to predict near-source BB aerosol emissions was also tested. Results showed that the total number and mass aerosol emissions decreased with increased oxygen levels. Nucleation mode particles dominated the number emissions in both inert and oxidative environments, but larger aerosol sizes were observed in the oxidative environment. Aerosol particle coagulation and growth were observed at larger fuel masses, indicated by a significant decrease in the total number emission factors in both inert and oxidative environments, while the total mass emission factor only slightly decreased and increased in inert and oxidative environments, respectively. The size of aerosols formed was found to positively correlate with absolute fuel consumption rate, and the effects of oxidation through combustion chemistry and thermal feedback were explored. Good agreement between the simulated and measured number emission factors for pinewood was observed in both the inert and oxidative reaction environments over the range of tested fuel masses. Simulated and measured mass emission factors showed good agreement in the oxidative environment and poorer agreement in the inert environment. Reasons for this discrepancy were explored and the importance of constituent surrogate selection is highlighted.