Emission characteristics and formation pathways of carbonyl compounds from the combustion of biomass and their cellulose, hemicellulose, and lignin at different temperatures and oxygen concentrations

Abstract

Carbonyl compounds are important precursors of free radicals, ozone (O3), and secondary organic aerosols (SOA) in the atmosphere. Biomass burning is an important source of carbonyl compounds. To explore the formation pathway of carbonyl compounds from biomass burning and the effects of fuel composition and burning conditions, this study performed tube-furnace combustion experiments using three biomass types (rice straw, pine, and poplar) and their three component extracts (cellulose, hemicellulose, and lignin) at seven ignition temperatures (300–900 °C, 100 °C gradient) and two oxygen concentrations (21% and 10.5%). The results showed that the average emission factors of carbonyl compounds (EFCC) from rice straw, pine, and poplar were 2.28 ± 1.72, 3.09 ± 2.98, and 2.90 ± 2.78 g/kg, respectively. EFCC were significantly higher (5.37 ± 0.25 g/kg) in lower temperature (300–500 °C) stage than in higher temperature (0.55 ± 0.03 g/kg). Furthermore, oxygen reduction promoted the emission of carbonyl compounds, increasing the EFCC by 21.6% on average. The average EFCC for the three components of biomass decreased in the order of cellulose (3.78 ± 3.90 g/kg), hemicellulose (2.90 ± 2.60 g/kg) and lignin (1.98 ± 1.72 g/kg), and their CCs profiles were also different. Cellulose and hemicellulose produced more formaldehyde and acetaldehyde than lignin, contributing 71.1 ± 12.6%, 69.0 ± 4.4% and 62.4 ± 15.5% in CCs, respectively. Acetone was mainly produced by the combustion of hemicellulose and lignin. Aromatic aldehydes were mainly produced by lignin burning, and also were significantly affected by temperature, the proportion under higher temperature is 2.7 times that under lower temperature. The EFCC values by weighted average calculation from the three components were good matched with those of parent fuels (r = 0.98), while the fitting results for those species such as aromatic aldehydes were relatively weak (r = 0.66), showing that there are different formation pathways and sensitivity to fuel composition and burning condition among different CCs.