Layer formation on quartz particles during fluidized bed combustion/gasification of woody biomass

Sammanfattning: The formation of sticky layers on bed particles has been considered as a prerequisite for bed agglomeration in fluidized bed combustion and gasification of woody fuels. In addition, the layer formed makes the bed particle less resistant against fragmentation. The fragments from quartz particles can result in deposition. The present investigation was undertaken to determine the layer formation process on quartz bed particles during combustion of wood-derived fuels and to determine the effect of quartz particle layers on deposit build-up in full-scale dual fluidized bed (olivine) steam gasification of logging residues. Bed material samples were collected from three different combustion appliances: bench-scale bubbling fluidized bed, full-scale bubbling fluidized bed and full-scale circulating fluidized bed. These were collected at different sampling times from start-up with fresh bed material. In addition, samples of deposits, bed material, coarse ash, fine ash, and fly coke from a dual fluidized bed gasification process were also collected. Scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD) were used to explore layer morphology, chemical composition and to gain information about crystalline phases of the layers. Significant differences in layer morphology and composition were found for quartz bed particles with different ages. For bed samples with operational durations of less than 1 day, only one thin Ca-, Si-, O- and K-rich homogeneous quartz bed particle layer with a relatively high K/Ca molar ratio was found. For quartz bed particles with an age of around 1 day to 2 weeks, an outer more particlerich coating layer was also found. During the initial days of this period, the layer growth rate was high but decreased over time, with decreasing K/Ca and increasing Ca/Si molar ratios in the inner bed particle layer. For bed particles with ages between 2 to 3 weeks, a much lower layer growth rate was observed. At the same time, the Ca/Si molar ratio reached high values and the K concentration remained at a very low level. In addition to these layer formation processes mentioned, an innerinner-/ crack layer was also formed simultaneously in the circulating fluidized bed (CFB) quartz bed particles along with the inner bed particle layer. By combining the experimental results on layer characteristics for samples with durations from 4 h to 23 days, with phase diagrams, thermochemical equilibrium calculations and a diffusion model, a mechanism of quartz bed particle layer formation was proposed. For younger bed particles (< around 1 day), the layer growth process is accelerated due to a high diffusion of calcium in a K-rich silicate melt. But with continuous addition of calcium into the layer, the amount of melt decreases and crystalline Ca-silicates starts to form. Ca2SiO4 is the dominating crystalline phase in the inner layer, while the formation of CaSiO3 and Ca3SiO5 are favored for younger and older bed particles, respectively. The decreasing amount of melt and formation of crystalline phases resulted in low diffusion rates of calcium in the inner layer and the layer growth process becomes diffusioncontrolled after around 1 day. The observation of formation of Ca3SiO5 in a thick bed layer after around two weeks may indicate substantially higher diffusion resistance and lower layer growth rate. The practical implication of the results from this work is that a low bed material renewal rate during wood-derived fired bubbling fluidized bed boilers using natural sand is recommended. The formation of high-melting Ca-silicates for older quartz bed particles protects the bed particle layer surface from further attack by potassium, leading to reduced agglomeration tendency. In dual fluidized (olivine) bed steam gasification, impurities, mainly composed of quartz particles brought into the fluidized bed with the feedstock, play a critical role for deposit formation in the post-combustion zone. Interaction between biomass ash and the quartz particles leads to formation of sticky potassium-rich silicate layers. Recirculation of coarse ash back into the combustion zone therefore leads to the enrichment of critical fragments. Improving the management of inorganic streams and controlling of temperature levels are therefore essential in operating with woody biomass fuels containing impurities (i.e. sand minerals).