Evaluation of the fusion and agglomeration properties of ashes from combustion of biomass, coal and their mixtures and the effects of K2CO3 additives

Abstract

This study investigated the ash fusion and agglomeration characteristics of a Chinese bituminous, “Neimongol” (NM) coal, soybean straw (SS) material and their blends, as well as the effects of high biomass blending ratio (20–80 wt%) and the addition of K2CO3 (0–20 wt%) on the ash agglomeration propensity of the mixtures in a muffle furnace at 815 °C. The ash fusion and agglomeration characteristics of the fuel blends and their modified behaviours based on the blending ratio and flux were examined using ash fusion temperature (AFT) analysis, X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The results indicated that, compared to Neimongol coal ash (NMA), the contents of SiO2 and Al2O3 in soybean straw ash (SSA) were lower, and the contents of basic oxides were higher, which was the main reason for the lower AFT of SSA. The AFT curves for the NM/SS blends showed a “bathtub” shaped trend with increasing SS mass ratios. For a given coal/biomass blend, XRD analysis indicated that for blends without added K2CO3, mineral phases were dominated by arcanite and quartz; with increasing SS mass ratios, the diffraction intensity of quartz decreased obviously. With increasing the K2CO3 additions, the content of potassium feldspars and sylvine in ash increased, and the quartz content decreased. The EDX analysis confirmed that the K2CO3 addition made it possible for kyanite and mullite to react with K-bearing minerals and form low melting point eutectics. The SEM images revealed that the fused particles in SSA were coated with KCl, whereas the NMA particles mostly contained aluminosilicate minerals. As the K2CO3 addition increased, the morphologies of the ashes from blends changed from slightly agglomerated to seriously slagged, and the molten and solidified zone on the slag surface featured high contents of sylvine, quartz, and potassium aluminosilicates.