Co-firing raw and torrefied rice husk with a high-Na/Ca/Cl coal: Impacts on fine particulates emission and elemental partitioning

Abstract While co-firing Si-rich biomass showed the potential for mitigating ash-related issues in low-rank coal combustion, the underlying mechanisms need further investigation. This work aims to examine the impacts of co-firing rice husk and its torrefied product with a high-Na/Ca/Cl low-rank coal on PM10 (particle size below 10 μm) emission and elemental partitioning. Combustion tests were performed on a drop tube furnace (DTF) at 1300 °C. The bulk ashes and PM10 were collected by fiberglass filters and Dekati low pressure impactor respectively. The ash samples were carefully characterized by X-ray diffraction (XRD), scanning electron microscope equipped with energy dispersive spectrometer (SEM-EDS), ion chromatography (IC) and inductively coupled plasma mass spectrometry (ICP-MS). It is found that co-firing raw or torrefied rice husk reduces emissions of PM1 (particle size below 1 μm) together with the Na and Cl in it, mainly due to the capture of Na by coarse Si-rich particles from rice husk. The emissions of PM1-10 (particle size between 1 and 10 μm) are elevated after co-firing. This is because the formation of Ca-silicates is facilitated instead of K-silicates, thus weaken the agglomeration between the coarse particulates with melting surface from rice husk. Compared to raw rice husk, its torrefied product leads to a lower reduction ratio regarding PM1 from co-firing. Combined the results in this and previous investigations, for raw/torrefied rice husk and Xinjiang low-rank coals, the PM1 yields from both of the individual fuels and the blends almost linearly correlated with the mole ratio of (Na + K + Cl + 2S)/(Si + Al) in the ash.