Abstract The purpose of this paper is to review the present knowledge on ash formation, ash particle transport and deposition during solid fuel combustion, with emphasis on particle sticking and rebound behavior. A substantial part of the fuel can be inorganic, forming inorganic vapors and ash particles. The impaction of solid, molten or partially molten particles on surfaces is dependent on the particle and surface characteristics. For instance, a particulate deposit might capture incoming particles or be removed due to erosion, while a molten layer will collect all impacting particles, no matter if they are sticky or not. The main properties affecting the particle stickiness are the viscosity and surface tension for silicate-rich ashes. On the contrary, the stickiness of salt-rich ashes – typical for herbaceous biomass and wood- or waste-based fuels – is often described using the liquid melt fraction. Furthermore, the particle kinetic energy and the angle of impaction, are crucial parameters. If all kinetic energy is dissipated during the impact, the particle will remain on the surface. This review presents an overview of major ash forming elements found in biomass and coal, and discusses the heterogeneity of particles’ inorganic composition. Ash transport and deposition mechanisms as well as their mathematical description are given and discussed, together with composition- and temperature-depended models for the estimation of ash particle and deposit properties. These properties are essential in order to describe the particle sticking and rebound behavior. Ash particle sticking and rebound criteria can be divided into three main groups, based on either: (1) the particle melt fraction, (2) the particle viscosity, or (3) the energy dissipation of a particle, upon impaction. Sticking criteria are presented, their required parameters are discussed and typical particle and surface properties found in combustion systems, are summarized. Eight different sticking criteria are implemented in a computational fluid dynamics code and computations are compared against measurements from an entrained flow reactor. Uniform sized soda-lime glass particles are applied instead of inhomogeneous fly ash particles, since soda-lime glass is known to behave similar to coal fly ash. Best agreement for the deposition rates on a clean tube is achieved using a criterion based on the work of Srinivasachar et al. [1] . In this model, the sticking and rebound threshold, is a function of the particle kinetic energy, the angle of impaction, and, the particle viscosity. Particularly, the particle viscosity is confirmed as a key parameter for silicate-rich ashes. It should be calculated using temperature- and composition-dependent correlations, being aware that there is a significant scattering in the results from such models and that the models are often only valid in narrow compositional ranges, and cannot be used outside these. A mechanistic model is used to explain results from glass particle experiments and their dependence on the particle kinetic energy. Therefore, the impaction process is subdivided in four steps, and the energy dissipation of each step is calculated. These theoretical considerations show that the contact angle of a molten droplet with the substrate is of minor importance, and that the majority of depositing particles are dominated by the work of deformation against viscosity, rather than surface tension effects. This review underlines the importance of the particle viscosity, and its accurate prediction for silicate-rich ashes. The proposed criterion is able to predict the sticking of small, solid particles below 10 µm diameter, as it is often observed in literature. Also, it is crucial to consider the surface structure and stickiness, in order to predict deposition rates in solid fuel-fired systems. Biomass ashes and their stickiness are more difficult, due to a different ash particle chemistry, compared to coal ashes. Salt-rich particles and their stickiness are controlled by the amount of liquid phase. Here, a link between the viscosity and amount of liquid phase is a promising approach, and should be addressed in future work. Furthermore, the viscosity of different ash particles – silicate-, salt- or Ca-rich – should preferentially be modeled from the chemical and physical structure instead of an empirical fitting procedure between fuel chemistry and viscosity measurements.

中文翻译：

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煤和生物质燃烧系统中的灰分形成和沉积：灰分颗粒粘附和回弹行为领域的进展和挑战

摘要 本文的目的是回顾目前关于固体燃料燃烧过程中灰分形成、灰分输送和沉积的知识，重点是颗粒粘附和回弹行为。大部分燃料可以是无机的，形成无机蒸气和灰颗粒。固体、熔融或部分熔融颗粒对表面的冲击取决于颗粒和表面特性。例如，颗粒沉积物可能会捕获进入的颗粒或因侵蚀而被移除，而熔融层将收集所有撞击颗粒，无论它们是否具有粘性。影响颗粒粘性的主要特性是富含硅酸盐的灰分的粘度和表面张力。相反，富含盐分的灰烬的粘性——通常用于草本生物质和基于木材或废物的燃料——通常使用液体熔体部分来描述。此外，粒子动能和撞击角度是关键参数。如果在撞击过程中所有动能都消散了，则粒子将保留在表面上。本综述概述了生物质和煤中发现的主要灰分形成元素，并讨论了颗粒无机成分的异质性。给出并讨论了灰分输送和沉积机制及其数学描述，以及用于估计灰分颗粒和沉积物特性的组成和温度相关模型。这些属性对于描述颗粒粘附和回弹行为是必不可少的。灰分颗粒粘附和回弹标准可分为三个主要组，基于：(1) 颗粒熔体分数，(2) 颗粒粘度，或 (3) 颗粒在撞击时的能量耗散。提出了粘附标准，讨论了它们所需的参数，并总结了燃烧系统中发现的典型颗粒和表面特性。在计算流体动力学代码中实施了八种不同的粘附标准，并将计算与来自夹带流动反应器的测量值进行比较。使用均匀尺寸的钠钙玻璃颗粒代替不均匀的粉煤灰颗粒，因为已知钠钙玻璃的行为类似于粉煤灰。使用基于 Srinivasachar 等人工作的标准来实现清洁管上沉积速率的最佳一致性。[1] . 在该模型中，粘附和回弹阈值是颗粒动能、撞击角度和颗粒粘度的函数。特别是，颗粒粘度被确认为富硅酸盐灰分的关键参数。应该使用与温度和成分相关的相关性来计算它，要知道这些模型的结果存在显着的散射，并且这些模型通常只在狭窄的成分范围内有效，不能在这些范围之外使用。机械模型用于解释玻璃粒子实验的结果及其对粒子动能的依赖性。因此，将撞击过程细分为四个步骤，并计算每个步骤的能量耗散。这些理论考虑表明，熔滴与基材的接触角不太重要，并且大多数沉积颗粒受变形对粘度的影响，而不是表面张力效应。这篇综述强调了颗粒粘度的重要性，以及它对富含硅酸盐的灰烬的准确预测。建议的标准能够预测直径低于 10 µm 的小固体颗粒的粘附，正如文献中经常观察到的那样。此外，为了预测固体燃料燃烧系统中的沉积率，考虑表面结构和粘性也很重要。与煤灰相比，生物质灰及其粘性更加困难，因为灰颗粒化学性质不同。富盐颗粒及其粘性受液相量控制。在这里，粘度和液相量之间的联系是一种很有前景的方法，应该在未来的工作中加以解决。此外，不同灰分颗粒的粘度——硅酸盐、盐或钙——应该优先从化学和物理结构建模，而不是燃料化学和粘度测量之间的经验拟合程序。