Investigation on ash deposition formation during co-firing of coal with wheat straw

Co-firing with coal is a promising technology for alleviating the ash-related issues during biomass combustion. This paper examines the effects of co-firing coal and straw on deposition characteristics under various biomass fractions and illustrates the mechanism of deposition formation during co-firing. Deposition experiments were conducted on a drop tube furnace with an air-cooled probe simulating the surface of the superheater. To clarify the detailed formation process of deposition, the micromorphology of deposits collected at different depths along the perpendicular direction to the deposition probe, as well as the elemental composition and mineralogy were characterized by SEM/EDS, XRF and XRD. Results showed that co-firing with coal reduced the serious slagging observed during biomass combustion, whereas resulted in higher deposition efficiency than that occurred in unblended coal/biomass combustion. As biomass fractions increased, the deposition efficiency exhibited a nonmonotonic downward trend, whereas the sintering degree intensified remarkably. The formation of Ca sulfates and Fe-AAEMs aluminosilicates during co-firing led to the enrichment of Ca, S, Al, and Fe in deposits, and thus promoted melt-induced agglomeration. Viscous particles and adhesive films rich in alkali sulfates adhered closely to the probe surface were believed to be the initial layer, which triggered the deposition formation during cofiring. The inner layer composed of low-melting particles accelerated the formation and growth of deposition. Meanwhile, the fusion characteristics of coarse particles in the outer layer dominated the sintering degree. Cofiring with coal greatly alleviated the slagging tendency compared with biomass combustion, but we should be aware of the negative glue effect of AAEMs sulfates.