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This review presents recent advances in research on fluidized bed combustion of solid biomass. While the main focus is on thermochemical processes occurring in fluidized beds, applicable investigations on biomass combustion from other research areas are also described as they often provide fundamental insight about the processing of biomass, applicable also in the context of fluidized beds. This review aims to summarize lessons learned and indicate some remaining gaps in the existing body of knowledge. The review discusses combustion in separate stages, such as drying, pyrolysis, and homo-and heterogeneous combustion, and characterizes how fluidized beds affect combustion, discussing heat and mass transfer, material segregation, and bed agglomeration. Finally, advances in new research trends and the prospects for technology development are considered, highlighting the chemical-looping technology with its inherent potential for carbon capture, scale-up of advanced computational models, and progress in spectroscopic and tomographic studies applied to combustion.

This research focuses on the combustion of biomass char in fluidized beds of various particulate solids, which, under the conditions of the reaction, were either inert or capable of supplying oxygen to reactions. The latter were termed oxygen carriers. The solids used were SiO2, as an inert material, and three oxygen carriers: (1) Fe2O3 prepared from a natural pyrite ore, (2) CuO supported on mayenite, and (3) SrFeO3−δ strontium ferrite perovskite. Combustion experiments were undertaken by introducing a sample of partially devolatilized biomass (commercial “biochar”) to a hot bubbling bed (inner diameter of 30 mm), fluidized by a mixture of oxygen and nitrogen, then analyzing the composition of the off-gas and the burnout time of the char sample. In the temperature range investigated in this work (1023–1168 K), CuO and SrFeO3−δ but not Fe2O3 thermally decomposed, releasing gaseous O2 [so-called “chemical looping oxygen uncoupling” (CLOU)]. Hence, to make the combustion conditions comparable to various oxygen carriers, all experiments were performed using a fluidizing gas with a fixed partial pressure of O2 (pO2) of ∼0.015 bar. Despite the same nominal pO2, the occurrence of the oxygen uncoupling reaction increased the total net amount of O2(g) available in the process, affecting external mass transfer of O2 to the char particle and accelerating its rate of combustion. The time needed to totally combust 0.1 g of biochar particles in different beds at 1168 K followed the trend CuO < SrFeO3−δ < Fe2O3 ≈ silica sand. The difference in the performance of CuO and SrFeO3−δ was ascribed to the lower oxygen availability via CLOU in perovskite compared to copper oxide. Interestingly, combustion in the bed of Fe2O3 particles took a similar amount of time as combustion in the inert bed of SiO2, despite iron oxide playing an active role in the process. The finding is explained by Fe2O3 reacting with CO produced from incomplete char combustion, which results in the reduced oxide competing with char for O2(g) and effectively decreasing the local pO2.

A novel chemical looping (CL) process was demonstrated to produce acetaldehyde (AA) via oxidative dehydrogenation (ODH) of ethanol. Here, the ODH of ethanol takes place in the absence of a gaseous oxygen stream; instead, oxygen is supplied from a metal oxide, an active support for an ODH catalyst. The support material reduces as the reaction takes place and needs to be regenerated in air in a separate step, resulting in a CL process. Here, strontium ferrite perovskite (SrFeO 3−δ ) was used as the active support, with both silver and copper as the ODH catalysts. The performance of Ag/SrFeO 3−δ and Cu/SrFeO 3−δ was investigated in a packed bed reactor, operated at temperatures from 200 to 270 °C and a gas hourly space velocity of 9600 h −1 . The CL capability to produce AA was then compared to the performance of bare SrFeO 3−δ (no catalysts) and materials comprising a catalyst on an inert support, Cu or Ag on Al 2 O 3 . The Ag/Al 2 O 3 catalyst was completely inactive in the absence of air, confirming that oxygen supplied from the support is required to oxidize ethanol to AA and water, while Cu/Al 2 O 3 gradually got covered in coke, indicating cracking of ethanol. The bare SrFeO 3−δ achieved a similar selectivity to AA as Ag/SrFeO 3−δ but at a greatly reduced activity. For the best performing catalyst, Ag/SrFeO 3−δ , the obtained selectivity to AA reached 92−98% at yields of up to 70%, comparable to the incumbent Veba-Chemie process for ethanol ODH, but at around 250 °C lower temperature. The CL-ODH setup was operated at high effective production times (i.e., the time spent producing AA to the time spent regenerating SrFeO 3−δ ). In the investigated configuration with 2 g of the CLC catalyst and 200 mL/min feed flowrate ∼5.8 vol % ethanol, only three reactors would be required for the pseudo-continuous production of AA via CL-ODH.

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