Layer Characteristics on Quartz and Feldspar Bed Particles in an Industrial-Scale Fast Pyrolysis Process of Woody Biomass

Although the layer formation process and characteristics of the bed particles utilized in fluidized bed combustion and gasification of different types of biomasses have been thoroughly discussed previously, a knowledge gap exists in the literature regarding the bed particle layer formation in industrial-scale fast pyrolysis process. In this work, bed particle layer formation and characteristics for quartz and feldspar particles utilized in three different industrial-scale fast pyrolysis plants of woody biomass (namely FPB1, FPB2, and FPB3) were studied, with estimated exposure times of 20–30, 14, and 10–13 days, respectively. Pure quartz served as the bed material in FPB1 whereas FPB2 and FPB3 utilized natural sand comprising both quartz and feldspar. The bed particle samples collected at their fresh state and at the end of each campaign were analyzed via scanning electron microscopy and energy dispersive spectroscopy to identify the bed particle layer properties for each bed material. Formation of a Ca-reaction layer (i.e., an inner layer), an ash deposition layer (i.e., an outer layer), and a K-reaction layer (i.e., an inner–inner layer) was identified on quartz bed particles taken from all plants. However, only one homogeneous layer was found on K-feldspar bed particles utilized in FPB2 and Na-feldspar particles taken from FPB3. No layer was observed on K-feldspar particles used in FPB3. The results revealed significant differences compared to layer characteristics on similar bed types utilized in the combustion and gasification of woody biomass. Overall, the bed particle layers found in all bed particle types were comparably thinner compared to those in combustion or gasification with similar exposure times. For quartz bed particles across all three plants, the inner layer was characterized by high concentrations of Si and Ca. However, the Ca content was relatively lower compared to that observed in combustion and gasification processes. The outer layer primarily consisted of Si, Ca, and Mg and had only partial coverage on the bed particle surface. K-feldspar bed particles used in FPB3 exhibited no layers, but a sporadic layer dominated by Si and Ca was identified on those utilized in FPB2. Similarly, Na-feldspar particles utilized in FPB3 exhibited partial coverage with a single layer rich in Si, Ca, and Al. Notably, no crack layers, as previously observed in quartz bed particles utilized in the combustion and gasification of woody biomass, were found in any of the examined bed particles. Despite these distinct characteristics, the distribution of bed particles on different surface morphologies resembled that in combustion and gasification of woody biomass for all bed types, as the layer was mainly found on the convex and flat areas and was thinner or missing in the concave areas of the bed particle surface.