Effect of the Coal Fly Ash Blending Ratio on Biomass Slagging Structure Modification and Alkali Metal Migration

The analysis of ash deposition samples by electron microscopy (EM), scanning electron microscopy (SEM)/energy dispersive scanning (EDS), and X-ray diffraction (XRD) was carried out at three sampling points of a one-dimensional furnace (sampling temperatures were TL5 = 960 °C, TL6 = 855 °C, and TL7 = 820 °C, respectively). The improvement of ash deposition characteristics of biomass combustion with the different addition ratios of coal fly ash (CFA) (Mr = 1:1, 1.5:1 or 2:1) and the heterogeneous removal of gas phase alkali metal heterogeneous were studied. The results showed that under the condition of Mr = 1:1, the ash sample presented a three-dimensional porous loose structure like "coral reef" with the rice husk ash as the skeleton and the combustion products of CFA as the bonding point. The spherical and filamentous crystals mainly composed of SiO2, Al2O3, and silicates were distributed in the slag, and the surfaces of these crystals were accompanied by the eutectic composite salt of K–Al–Si with a high melting point, reflecting the heterogeneous curing effect of CFA on the gas phase migration of the alkali metal (K). With the decreasing addition ratio of CFA (Mr = 1.5:1 or 2:1), the spherical and filamentous crystals were increased in the slag. MgO, Fe2O3, and Al2O3 may be involved in the formation of high-temperature refractory Mg-Fe-aluminate, Ca- and Mg-silicate, which were the precursors of the rigid slag. The alkali metals mostly presented in the form of K2O and KCl in the gas–solid conversion under the temperature effect, serving as binders for bridging, agglomeration, and slagging of the above precursors. In addition, the kalium enrichment was accompanied by Al and S in the CFA, indicating that the Al- and S-active sites have obvious adsorption to kalium.