Chemical Looping Combustion (CLC) is a technique which utilises lattice oxygen contained in solid particles of an inorganic oxide (the ‘oxygen carrier’) to combust fuels, enabling the collection of products of combustion (CO₂ and H₂O) undiluted with nitrogen. It offers, therefore, an attractive method for capturing CO₂ for use or for sequestration in the Earth. This Dissertation concerns the combustion of biomass chars in a fluidised bed using Chemical Looping Combustion with Oxygen Uncoupling (CLOU), a variant of CLC in which the oxygen carriers can release gaseous oxygen. In CLOU, the gaseous O₂ released from the oxygen carriers reacts heterogeneously with the particles of char, producing a mixture of combustion products, CO and CO₂. The CO, in turn, can react either with the gaseous O₂ or heterogeneously with lattice oxygen in the carrier particles. The overall objective of the research described in this Dissertation has been to investigate how the presence of the CLOU particles affects the rate of combustion, and hence, the burnout time of biomass char, an important fuel for future power generation, but which has received scant attention in the literature. To scrutinise the influence of the CLOU particles, the ratio of CO to CO₂ when combusting biomass char had first to be established experimentally, because that ratio affects fundamentally the transport processes occurring in CLOU combustion, e.g. the external rate of transfer of O₂ to the surface of the char particle. However, information about CO to CO₂ ratios when combusting biomass chars is lacking in the published literature, with previous investigators assuming that ratios were similar to those produced by coal chars. Experimental measurements of the ratio of CO to CO₂ from various types of char were undertaken using a thermogravimetric method. The experiments were conducted at known temperatures between 973 and 1173 K, with a range of partial pressures of O₂ (pO₂) from 0.0057 and 0.023 bar. The CO to CO₂ ratios for chars derived from biomass were significantly different to those for chars derived from coal. Thus, models that utilise the values of CO to CO₂ ratio measured from coal chars will be erroneous if applied to predict the combustion performance of a biomass char. Further investigations substantiated this conclusion insofar as the burnout time predicted in particle-scale models using the ratio of CO to CO₂ of a coal char under-predicted the results from independent burnout experiments with single particles birch char (~ 4 mm dia.) when combusted in a laminar flow of oxidising gas. To evaluate the influence of the CLOU materials on the rate of combustion, particle-scale models have been developed, accounting for the reactions and transport phenomena occurring within, and around, the particle of char in a CLOU arrangement. The char studied here was of a size typically used for fluidised combustion such that external mass transfer could be a significant controlling factor (~ 1 mm dia.). Results from the models were compared with experiments performed by combusting biomass char in a bubbling fluidised bed (i.d. 30 mm) of a CLOU material (a Cu-based oxygen carrier) or inert silica sand. The experiments were undertaken with a partial pressure of oxygen, pO₂, close to the equilibrium pressure of O₂ of the Cu-based oxygen carrier. The results from the model agreed with the experimental observations and revealed that, when a CLOU material is present, there is a significant increase in the combustion rate of char compared to that in silica sand. The increase stems from the release of oxygen by the carrier within the external mass transfer film surrounding the char and was predicted by the theoretical work. The models were also used for investigating the performance of CLOU when the particles of char were combusted under the typical operating conditions used in industrial applications, e.g. where concentrations of CO₂ might be substantially higher, giving contributions from gasification. Finally, the combustion of biomass char in a non-stoichiometric perovskite CLOU material, SrFeO3-δ, was also investigated, both experimentally and theoretically. The performance was compared with combustion in Cu-based CLOU materials and with combustion in an oxygen carrier without CLOU properties, namely Fe₂O₃. Experiments were undertaken at the same operating temperature and pO₂ in the fluidising gas. Significant differences in the burnout time when using the different bed materials were observed and plausible explanations, including accounting for the difference in the oxygen availability of the oxygen carriers, were discussed.