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Application of particle-scale modelling in combustion of char particle in an inert and active fluidised bed

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This research concerns chemical looping combustion of biomass char in a fluidised bed of material capable of oxygen uncoupling (CLOU). An analytical solution for CLOU was developed, accounting for the chemical reactions within a mass-transfer boundary layer of a finite thickness, 𝛿, surrounding the char particle. The reactions considered in this model were: (1) combustion of char with oxygen to CO and CO2, (2) homogenous reaction of CO with O2, (3) oxygen release from CLOU particles. The thickness of the boundary layer, 𝛿, was evaluated based on the experimental conditions. The combustion of the char particle was modelled as a shrinking particle. Results from the model were compared with experiments performed by combusting char from birch-wood and also activated carbon in a fluidised bed (i.d. 30 mm) of an active oxygen carrier (CuO supported on mayenite) or inert silica sand. Both types of experiments were carried out with a partial pressure of oxygen, pO2, close to the equilibrium pressure of O2 of CuO at 1173 K, i.e. ~0.016 bar of O2. Despite the same operating conditions for the inert and active beds, the CLOU reaction from the active CuO particles resulted in a significant increase in the combustion rate. At 1173 K, the burnout time of particles of birch char and of activated carbon in the active CLOU bed was, respectively, around five times and two times faster than with silica sand. The results from the analytical model agreed with the experimental observations from both beds. In addition to the analytical solution, a model was developed, in which a char particle and its surroundings were discretised in 1D to solve the mass balance equations numerically and assess the validity of the assumptions made in the analytical solution, such as (i) the applicability of the Fick’s law for mass transport, (ii) the negligible effect of specific diffusivity coefficients for all gases. To establish the range of conditions in which the analytical solution is valid, various case studies were simulated. For example, char combustion and gasification in a bed of CuO-particles fluidised by CO2-rich gas. Hence, besides the validation of the analytical solution, the results provide insight into the importance of char gasification with CO2 on the overall particle conversion in CLOU at the industrial scale.



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24th Fluidised Bed Conversion Conference 2022

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