Impact of potassium on bio-ash slagging and resultant slag flowing characteristics under mild reducing environment

This study aims to clarify the role of potassium (K) on the flowability of bio-slag in the reducing gas environment at 900-1400 °C. Through the use of an inclined plate method, a number of approaches were attempted to clarify the flow rate of bio-slags as a function of basic-to-acid ratio, as well as the comparison with a reference coal ash and synthetic ashes made by the blending of bio-ash with CaO, Al₂O₃ and K₂O. At the same base-to-acid ratio (B/A) and 1400 °C, coal slag has a flow rate that is nearly four times larger than that of bio-slag. This is due to the co-existence of abundant K₂O and Al₂O₃ in bio-slag that is in favour of the formation of solid KAlSi₂O₆. In contrast, the co-existence of abundant CaO and Al₂O₃ in coal slag is in favour of the formation of molten species. In addition, the inherent K in bio-ash underwent considerable mass loss due to both vaporisation and its interaction with the corundum plate. Upon the increase in the inclination angle of the plate, the extent of K penetrating within the corundum plate was mitigated considerably, which in turn minimised the plate corrosion. Simultaneously, the bio-slag formed readily spread out along the plate. The addition of extra K into bio-ash is beneficial in promoting the flow rate. However, even with an increased base-to-acid ratio to 1.2, the flow rate of bio-slag is still less than that of coal slag at the base-to-acid ratio of 0.72, due to the superiority of Ca²⁺ over K⁺ in promoting the bio-slag flow. For practical application, the use of CaO as a flux additive is beneficial in discharging the bio-slag out of the furnace. Moreover, the flow rates of bio-slag in the locations with low angles to the ground within the furnace should be monitored to minimise the accumulation of bio-slag as well as its corrosion against the refractory wall.