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Chemical engineering and reactor design of a fluidised bed gasifier

Al-Farraji, Abbas 2017. Chemical engineering and reactor design of a fluidised bed gasifier. PhD Thesis, Cardiff University.
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The design, modelling and optimisation of biofuel thermochemical processes are mainly based on the knowledge of reliable chemical kinetics. The determination of reaction kinetics of biomass at high heating rate still highly depends on the extrapolation of results from kinetic data determined at a comparatively low heating rate. To provide more comprehensive kinetic data for gas-solid reactions under isothermal conditions, a thermogravimetric fluidized bed reactor (TGFBR) was designed. Using this novel fluidised bed, gravimetric measurements and high heating rate, the thermal conversion of biomass was investigated. Using a thermogravimetric analyser (TGA) as a fixed bed and the TGFBR as a fluidized bed, the pyrolysis kinetics of olive kernels was studied. The pyrolysis in the TGFBR was analysed using the isothermal kinetic approach and it was theorised that the pyrolysis decomposition reaction occurred by two mechansims. Dependent on the temperature, the resultant activation energy was 67.4 kJ/mole at <500 °C and 60.8 kJ/mole at >500 °C. For comparsion, the TGA gave a higher activation energy of 74.4 kJ/mole due to external particle diffusion. To study the impact of torrefaction on gasification performance, gasification experiments were performed on “as received olive kernels” (AROK) and “as received torrefied olive kernels” (ARTOK) in the TGFBR. The effect of equivalence ratio (ER) (0.15-0.35) and bed temperature (550-750°C) on gasification performance was investigated. Based on thermogravimetric measurements using a mass balance model, the activation energy of AROK was found to be 84 kJ/mole, whereas ARTOK was found to be 106 kJ/mole. The results suggest that diffusion controls the reaction of AROK, while oxidation controls the reaction of torrefied biomass. The pyrolysis of date palm stones was also studied in the TGFBR, and the kinetic expression was determined using a model fitting method. The most probable reaction mechanism for the thermal decomposition of palm stones was three-dimensional diffusion. The activation energy for experiments between 350°C and 600°C for date palm stones was 27.67 kJ/mole. Furthermore, the gasification of date palm stones was investigated at ER (0.15-0.35) and a temperature range of 600-750°C in 50°C increments. Based on the energy yield (7 MJ/kg), the results suggest that the optimum conditions were at T=750°C and ER=0.2. Overall, the result reveals that the TGFBR, in comparison with TGA, would be a viable reactor that enables kinetic analysis of gas-solid reactions under isothermal conditions, benefiting from its features. The parameters obtained from the kinetic study of TGFBR are essential in the scale-up design of useful conversion technologies such as gasification. Also, the pre-treatment of biomass via torrefaction is a promising route to improve gas production in a bubbling fluidised bed gasifier.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Gasification; Olive Kernels; Date Stones; Torrefied; Bubble Fluidized Bed; Kinetics.
Date of First Compliant Deposit: 26 September 2017
Last Modified: 29 Apr 2021 15:33

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