Fluidized Bed Combustion and Gasification of Fossil and Renewable Slurry Fuels

This article provides a comprehensive review of the state of the art and more recent developments of the thermochemical treatments of slurry fuels in fluidized beds (FB). The review focuses on FB combustion and gasification of slurry fuels based on coal, biomass, sludge, and wastes from industry, agriculture, and the civil sector. The investigations at research and industrial levels over the last decades are presented and discussed, highlighting the adopted technological solutions, the results in terms of feasibility and efficiency, and the perspectives of future development. The different behavior between bubbling and circulating beds was addressed, in particular the optimal choice depending on the process (combustion/gasification/pyrolysis) and fuel properties (e.g., water content). Fundamental studies on interactions between the slurry fuels and the hot bed materials are also reviewed. The cumulative trend of reviewed investigations over the last decades depicts the abandonment of coal-based mixtures used in large plants, and the growing interest in the use of biomass-based slurries for small size application. In this respect, the shift from coal to biomass opens new challenges because of the different properties of biomass (density, fibrous structure, spontaneous degradation, hydrophilic behavior, etc.). Biomass-based slurries circumvent problems posed by using solid dry biomass, particularly in handling, storing, and feeding. Although slurry fuels represent a narrow sector, the results of the research investigations and the experience gained with coal can be exploited to contribute to the achievement of a circular approach based on renewable resources in the near future.