Biochar as a Fuel: 4. Emission Behavior and Characteristics of PM₁ and PM₁₀ from the Combustion of Pulverized Biochar in a Drop-Tube Furnace

Six biochar samples were produced from both slow and fast pyrolysis of mallee bark at 400–550 °C, respectively; such temperatures are typically used for biochar and/or bio-oil production in practice. Under the pyrolysis conditions, the biochar yields range from 26.7 to 37.0% and the majority (78.5–100.0%) of alkali and alkaline earth metallic (AAEM) species (mainly Na, K, Mg, and Ca) in biomass are retained in the biochars, while the retention of Cl in biochars is only 2.0–33.4%. The raw biomass and its derived biochar samples were then combusted in a laboratory-scale drop-tube furnace (DTF) at 1300 °C to investigate the emission behavior and characteristics of sub-micrometer particulate matter (PM 1 ) and PM with an aerodynamic diameter less than 10.0 μm (PM 10 ). The particle size distribution (PSD) of PM 10 from raw biomass combustion has a bimodal size distribution, while the PSDs of PM 10 from biochar combustion generally show a unimodal distribution. Although most inorganic species are retained in the biochar during pyrolysis, it is interesting to note that the combustion of biochars leads to a substantial reduction in the emission of PM 1 (and the mass of Na, K, and Cl in PM 1 ) that dominantly consists of PM with a size less than 0.1 μm (PM 0.1 ) in comparison to biomass combustion, apparently because of the removal of volatiles and Cl from the raw biomass during pyrolysis for biochar preparation. The results imply that the combustion of volatiles (including the released inorganic species), which is particularly important during biomass combustion, is mainly responsible for PM 1 emission. Meanwhile, considerable increases in the emission of coarser particulate matter with an aerodynamic diameter between 1 and 10 μm (PM 1−10 ) and the mass of Mg and Ca in PM 1−10 are also evident during biochar combustion, most likely as a result of more porous structure and increased ash loading of biochars.