Production of biochar and bioenergy from rice husk: Influence of feedstock drying on particulate matter and the associated polycyclic aromatic hydrocarbon emissions

This study investigates the effect of feedstock drying on the emissions of polycyclic aromatic hydrocarbons (PAHs) associated with the particulate matter (PM) produced during the co-generation of biochar and bioenergy. Raw pyrolysis volatile mixtures were generated from the pyrolysis of rice husk at 400, 500, 600, 700, and 800 °C using a laboratory-scale continuous pyrolysis-combustion system and combusted at 850 °C. PM samples from the combustion were collected and analysed for 15 priority PAH species with a gas chromatography/mass spectrometer (GC/MS). It was found that the utilization of the as received (AR) rice husk resulted in significantly greater energy-based yields of PM10 (1.2 times at 400 °C and 1.6 times at 800 °C) than the dried rice husk. The majority of the increase was of the PM2.1-10 size fraction. The PM-bound PAH concentration was found to be 2.1 and 2.8 times higher for the AR rice husk at 400 and 800 °C, respectively. This resulted in a significant increase in the energy-based yield of PAHs over the entire volatile production temperature range for the AR rice husk. Nevertheless, the majority of the PM-bound PAH species generated from the AR rice husk consisted of 2 and 3 ring PAHs (naphthalene, acenapthylene, and acenaphthene) with relatively low toxicity. The concentration of 4, 5, and 6 ring PAHs was generally lower than that generated from the dried rice husk. This resulted in the benzo(a)pyrene-equivalent toxicity of the PM generated from the AR rice husk being lower than the dried counterpart.