Investigation on the mechanism of ash deposition formation from mineral components and characteristics of ash deposition on boiler heating surface during co-firing of coal and biomass

Abstract

The power generation by co-firing of coal and biomass is the most economical and promising technology for existing coal-fired power plants to achieve reducing emission of CO₂ and large-scale efficient utilization of biomass. The mechanism of ash deposition formation from mineral components and the characteristics of ash deposition on boiler heating surface during co-firing of coal and biomass were investigated in this paper. A water-cooled probe was used to simulate the heating surface of high-temperature super-heater in the furnace, and the experiments on ash deposition were carried out in a one-dimensional settling furnace. The micro-morphology, element composition and phase composition of the deposited ash on the surface of the water-cooled probe under different conditions were characterized by means of SEM, EDS, XRD and other methods. The results show that the migration and transformation pathways of alkali metal K and Cl elements in the mineral components can significantly affect the behavior of ash deposition formation. The meltbility of ash particles and the viscosity of deposit sediment are two important factors determining the degree of ash deposition. With the blending ratio of biomass in coal increasing, the content of K₂O and Cl in the deposited ash increases rapidly, conversely, the content of SiO₂ and Al₂O₃ in the deposited ash shows rapidly decreasing trend. Meanwhile, based on the micro-morphology features, the particles of the deposited ash gradually become smaller, the structure of the deposited ash becomes denser, and the agglomeration and melting phenomenon in the deposited ash is more obvious. The alkali metal K and Cl in the biomass and S, Si, Al and other components in the coal will have a synergistic effect during co-firing of coal and biomass. There are significant effects of fuel property, the blending ratio of biomass in coal and the temperature in furnace on the characteristics of ash deposition. With the continuous addition of biomass in coal, and the rising temperatures, it will produce KAl₂(AlSi₃O₁₀)(OH)₂, K(AlSi₃O₈), Ca(A1₂Si₂O₈), Mg₃Al₂(SiO₄)₃ and other substances with low melting point as well as CaSO₄ considered as the binder between ash particles. The overall melting point of the deposited ash is reduced, the viscosity is enhanced, and the degree of the ash deposition is aggravated.