Effect of water-leaching on the fine particle formation during biomass combustion

While biomass is considered as carbon-neutral, the incomplete combustion and the high content of water-soluble alkali minerals in the fuel (e.g., potassium) result in emission of ultrafine particles, including soot, organic species, and minerals. In this study, the effect of water-leaching on the ignition and fine particle formation during straw combustion was studied in a flat-flame burner, entrained flow reactor, and thermogravimetric analyzer (TGA). A high-speed video camera and high-resolution electrical low pressure impactor (HR-ELPI) were employed to capture the ignition and fine particle formation along the height of the flat-flame burner reactor. The fine particles from the entrained flow reactor were sampled by a Dekati low pressure impactor (DLPI) and analyzed by scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDS). The experimental results show that water-leaching removes most of the potassium and all of the chlorine in the ash. After water-leaching, delays are observed for both devolatilization and char-oxidation, indicating the strong catalytic effect that water-soluble minerals have on biomass burning. In the flat-flame burner reactor, a tri-modal particle size distributions (PSDs) of PM_1.0 is observed at the early stage of biomass burning (< 110 ms), exhibiting peaks at about 20, 370, and 900 nm. Along the reactor height, the PM_1.0 concentrations reach a maximum at 4-6 cm (27∼55 ms) and then decrease. The reducing ratio of water-leaching on PM_1.0 exceeds 72% at 1000°C and exceeds 90% at 1300°C. This indicates that at the early stage of biomass burning, water-soluble minerals are dominant in PM_1.0. The PM_2.5 from the entrained flow reactor (∼3 s) exhibits bimodal PSDs, and the smaller mode is mainly due to condensation and growth of potassium chlorides and sulfates, while the larger mode also contains Si, P, Mg and Fe. Similar to the results in the flat-flame burner, water-leaching reduces the PM_2.5 emission by 82-98%. The PM_2.5 morphologies show that the drastic reduction in potassium by water-leaching inhibits the growth of potassium crystals, and the smaller mode peaks move from 100-300 nm to ∼57 nm.