Effect of Ca3(PO4)2 additive on the slagging behavior during the cofiring of high-sodium coal and iron-rich coal

The study aims to understand the potential of Ca-3(PO4)(2) for relieving the slagging propensity of Ca-, Na- and Ferich Zhundong coal blends. Firstly, the effect of varying proportion of Ca-3(PO4)(2) on ash fusion temperatures of coal blends was studied. The experiment about cofiring coal blends with optimal blending ratio of Ca-3(PO4)(2) was then conducted in a drop tubular furnace. The chemical composition, mineral phase and micro-morphology of slags or deposits collected on an un-cooling probe at different temperatures (1200, 1000, 800 and 600 degrees C) were characterized. The results show that the deformation and softening temperatures of coal blends with 3% Ca-3(PO4)(2) can be increased by 200 degrees C and 150 degrees C, respectively. Without additive, the slags at 1200 degrees C appeared to be molten at a large proportion, which are mainly comprised of Fe2O3 and Si-Al-Ca mineral phases. While for coal blends with 3% Ca-3(PO4)(2), the particles adhered to the probe at 1200 degrees C presented to be granular and the melting proportion sharply decreased. At 1000 degrees C the deposits became loose and easy to be removed off. This is mainly due to the fact that Ca-3(PO4)(2) combining with Na- or Ca-containing components can transform into Ca-5(PO4)(2)SiO4, Ca-7(PO4)(2)Si2O8 and Na3Ca6(PO4)(5) with high melting point.