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Interactions between fuel ash and bed material, which lead to agglomeration, are impeding a widespread use of alternative biomass fuels for energy conversion in fluidized bed boilers.Reactions of certain ash components (alkaline metals in the case of biomass) with silicon of the quartz sand, commonly used as bed material, have been identified as main drivers for this phenomenon. Still, there is no consensus on the detailed mechanisms causing the attachment of ash on bed particles and the consequent growth of agglomerates. Only through a better understanding of these processes will it be possible to cast precise predictions and implement effective countermeasures for agglomeration issues. This paper reviews the current state of knowledge and delivers new insights into the mechanisms of coating induced agglomeration.In particular, the influence of heterogeneous reactions on coating formation and the metamorphism of the coatings during agglomeration are investigated. The results are derived from experiments performed on lab-scale reactors described in our earlier publications. In addition, coated particles and agglomerates from large scale plants, or such extracted during the lab scale experiments, were evaluated using SEM/EDX analyses. The findings are summarized in an updated description of the agglomeration process, eliminating heterogeneous chemical reactions from the major influencing factors and proposing a hypothesis for the behavior of the typical two-layer coatings during agglomeration.

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Power-to-Methane as one part of Power-to-Gas has been recognized globally as one of the key elements for the transition towards a sustainable energy system. While plants that produce methane catalytically have been in operation for a long time, biological methanation has just reached industrial pilot scale and near-term commercial application. The growing importance of the biological method is reflected by an increasing number of scientific articles describing novel approaches to improve this technology. However, these studies are difficult to compare because they lack a coherent nomenclature. In this article, we present a comprehensive set of parameters allowing the characterization and comparison of various biological methanation processes. To identify relevant parameters needed for a proper description of this technology, we summarized existing literature and defined system boundaries for Power-to-Methane process steps. On this basis, we derive system parameters providing information on the methanation system, its performance, the biology and cost aspects. As a result, three different standards are provided as a blueprint matrix for use in academia and industry applicable to both, biological and catalytic methanation. Hence, this review attempts to set the standards for a comprehensive description of biological and chemical methanation processes.