Gasification or combustion of coal and biomass is the most important form of power generation today. However, the use of coal/biomass at high temperatures has an inherent problem related to the ash generated. The formation of ash leads to a problematic phenomenon called slagging. Slagging is the accumulation of molten ash on the walls of the furnace, gasifier, or boiler and is detrimental as it reduces the heat transfer rate, and the combustion/gasification rate of unburnt carbon, causes mechanical failure, high-temperature corrosion and on occasions, superheater explosions. To improve the gasifier/ combustor facility, it is very important to understand the key ash properties, slag characteristics, viscosity and critical viscosity temperature. This paper reviews the content, compositions, and melting characteristics of ashes in differently ranked coal and biomass, and discusses the formation mechanism, characteristics, and structure of slag. In particular, this paper focuses on low-rank coal and biomass that have been receiving increased attention recently. Besides, it reviews the available methodologies and formulae for slag viscosity measurement/prediction and summarizes the current limitations and potential applications. Moreover, it discusses the slagging behavior of different ranks of coal and biomass by examining the applicability of the current viscosity measurement methods to these fuels, and the viscosity prediction models and factors that affect the slag viscosity. This review shows that the existing viscosity models and slagging indices can only satisfactorily predict the viscosity and slagging propensity of high-rank coals but cannot predict the slagging propensity and slag viscosity of low-rank coal, and especially biomass ashes, even if they are limited to a particular composition only. Thus, there is a critical need for the development of an index, or a model or even a measurement method, which can predict/measure the slagging propensity and slag viscosity correctly for all low-rank coal and biomass ashes.

煤炭和生物质的气化或燃烧是当今最重要的发电形式。然而，在高温下使用煤/生物质具有与产生的灰分有关的固有问题。灰分的形成导致称为成渣的问题现象。结渣是熔融灰烬在炉，气化炉或锅炉壁上的积聚，并且有害，因为它降低了传热速率，未燃烧碳的燃烧/气化速率导致机械故障，高温腐蚀，有时，过热器爆炸。为了改善气化炉/燃烧室的设施，了解关键的灰分特性，炉渣特性，粘度和临界粘度温度非常重要。本文对内容，成分，煤和生物质等级不同的烟灰的熔解和熔化特性，并讨论了熔渣的形成机理，特性和结构。尤其是，本文重点关注最近受到越来越多关注的低等级煤和生物质。此外，它回顾了用于炉渣粘度测量/预测的可用方法和公式，并总结了当前的局限性和潜在的应用。此外，通过检查当前粘度测量方法对这些燃料的适用性，以及粘度预测模型和影响炉渣粘度的因素，讨论了不同等级的煤和生物质的结渣行为。这项综述表明，现有的粘度模型和排渣指数只能令人满意地预测高等级煤的粘度和结渣倾向，而不能预测低等级煤（尤其是生物质灰烬）的结渣倾向和渣黏度，即使它们受到限制仅针对特定的成分。因此，迫切需要开发能够正确预测/测量所有低品位煤和生物质灰烬的结渣倾向和炉渣粘度的指标，模型，甚至测量方法。