Mechanism of Quartz Bed Particle Layer Formation in Fluidized Bed Combustion of Wood-Derived Fuels

Abstract

Agglomeration is among one of the major problems in the operation of fluidized bed boilers. The formation of bed particle layers is thought to play an important role on the occurrence of agglomeration in wood-fired fluidized (quartz) beds. In spite of frequent experimental reports on the quartz bed particle layer characteristics, the underlying bed layer formation process has not yet been presented. By combining our previously 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. However, with continuous addition of calcium into the layer, the amount of melt decreases and crystalline Ca−silicates starts to form. Ca 2 SiO 4 is the dominating crystalline phase in the inner layer, while the formation of CaSiO 3 and possibly Ca 3 SiO 5 are favored for younger and older bed particles, respectively. The decreasing amount of melt and formation of crystalline phases result in low diffusion rates of calcium in the inner layer and the layer growth process becomes diffusion controlled after around 1 day.

Impact of residual fuel ash layers on the catalytic activation of K-feldspar regarding the water–gas shift reaction

“…A detailed assessment of the ash transformation reactions in thermochemical conversion of biomass was published by Boström et al [13]. This ash layer formation has been studied for different types of bed materials, such as olivine [14], quartz [15], and K-feldspar [16,17], describing the mechanisms underlying the formation of those layers. The mechanisms differentiate significantly from another in the initiating steps of the layer formation.…”

Phase equilibria and liquid phase behavior of the K2O-CaO-SiO2 system for entrained flow biomass gasification

“…Failure to understand the behavior of the system can cause serious catastrophic blockage of the reactor [10,11]. In fluidized bed combustion of biomass, the formation of K-Ca-silicates layers on the sand particles can lead to agglomeration of the bed [12]. The system is also important component in the potassium silicate fertilizer because the slow rate release of potassium ion to the soil can minimize the carry-over of the chemicals from the farming field to the surrounding environment [13].…”

Layer formation on K-feldspar in fluidized bed combustion and gasification of bark and chicken manure

“…This interaction can be e.g. alkali-silicate formation, as observed for quartz where K reacts with silicon to form K-silicates [32]. Furthermore, inner layer formation can be based on a substitution reaction, as observed for olivine where Ca 2+ ions substitute Fe 2+ and Mg 2+ ions in the crystal structure of olivine.…”

“…Frequently proposed mechanisms for layer formation in the literature [32,33] typically refer to inner layer formation since outer layers are merely an accumulation of ash components and therefore highly dependent on the feedstock ash content itself.…”