Review of Biomass Agglomeration for Fluidized-Bed Gasification or Combustion Processes with a Focus on the Effect of Alkali Salts

Agglomeration inhibits the smooth operation of fluidized-bed reactors by forming relatively large agglomerates to interrupt fluidization during gasification or combustion processes. Generally, biomass tends to induce agglomeration because inorganic elements from biomass together with or without inorganic elements from bed materials can generate molten compounds to adhere bed materials together to form agglomerates. Alkali metals, which mainly are present in biomass as different types of alkali salts (carbonates, sulfates, phosphates, or chlorides), are generally responsible for biomass agglomeration. Although there are several reviews about biomass agglomeration for fluidized-bed reactors, how different types of alkali salts influence biomass agglomeration has seldom been summarized. In addition, the interaction between alkali salts with organic compounds in biomass can influence biomass agglomeration by altering the occurrence mode of alkali salts. Therefore, this review highlights the interaction between different types of alkali salts in biomass with either the bed material or the organic compound in biomass, the contribution of gas–solid interactions between alkali salts with bed materials to bed agglomeration, and how reaction atmosphere affects the behavior of alkali salts for bed agglomeration. In addition, the role of calcium (Ca) and phosphorus (P) and their interactions play in biomass agglomeration, which is seldom overviewed in other reviews, are also explored. The investigation of these aspects can provide a deep and systematic understanding of biomass agglomeration. To give a background knowledge of biomass agglomeration, several well-established agglomeration mechanisms, including the "coating-induced" and "melting-induced" mechanisms, the "burning char" mechanism, and other mechanisms are presented. The process of the time-dependent coating layer formation for silica sand is provided and compared with other bed materials. This review contributes to the utilization of different types of biomass species in fluidized-bed gasification or combustion processes.