- Title
- Transport characteristics of binary mixture of particles in chemical looping combustion applications
- Creator
- Alghamdi, Yusif Adel
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- This thesis is a combined experimental and theoretical study of the transport characteristics of binary mixture of particles in a cold flow model of a chemical looping combustor (CFM–CLC). The main objectives of the study were (i) to gain a better understanding of mechanisms influencing the mixing and segregation of binary mixture of particles circulating between two interconnected fluidised beds, (ii) to provide an accurate description of the particles transport in such systems and (iii) to verify the hydrodynamic similarity between 10 kWth and 200 kWth scaled-up CFM–CLC units and propose an experimental scale-up relationship that directly takes into account the variation in the system solid volume fraction between the two scaled units. This work was motivated by the desire to improve the overall performance of CLC-based combustion processes which use binary mixture of particles of oxygen carriers (i.e metal oxides). The findings of this work could prove beneficial in achieving process intensification in CLC-based technologies for other industrial applications, including oxygen production, reforming for hydrogen production and ventilation air methane abatement. It is worth mentioning that the scope of the study was limited to the cold flow model of chemical looping combustion system. Experiments were performed to obtain an insight into the role, interplay and relative importance of operating parameters such as the particle size and density, mixture composition, gas fluidisation superficial velocity, and solids inventory in the fluidisation of binary mixtures of particles in the riser, air and fuel reactors of a chemical looping combustor. These results were compared with those obtained using single species bed materials. In general, pressure profiles similar to those of CLC systems with single species bed materials were obtained, and the solids holdup and circulation rate for binary mixtures of particles were found to be greater than those of CLC systems with a single species particle system. The experimental results for binary mixtures of particles showed that at a given solids inventory, the CLC operation is strongly dependent on the gas superficial velocity in the air reactor, the riser and the ratio of the particles’ terminal velocities. The results indicate that the air reactor and the riser, but not the fuel reactor, are the main CLC components influencing the global solid circulation throughout the CLC system. In the fuel reactor, for mixtures of species with low to high terminal velocity ratios of greater than 0.7, minimum particle segregation was achieved by maintaining the ratio of superficial gas velocity to the mixture minimum fluidisation velocity above 5. For the tested conditions, component segregation between the two reactors was avoided by maintaining the ratio of the riser superficial velocity to the terminal velocity of the species with a high terminal velocity between 1.25 and 2. These findings were utilised to develop operating maps for the CLC system; specifically, the mixing and segregation regimes of the fuel reactor and the component segregation between the two reactors were mapped. For the theoretical part of the project, a kinematic model was developed based on the volumetric solids flux balance along the height of the riser to predict the solid circulation rate (SCR) in the system and describe the solids holdup profile of the air reactor and riser. The theoretical description takes into account the axial solids holdup profile as well as the lateral solids flux profile within the riser, and hence the riser reduced solids flux. The proposed model was validated against the experimental data obtained in this work and those reported in the literature. A significant improvement in prediction accuracy of SCR (up to 60% with a residual value of ±15%) was achieved over predictions obtained based on the conventional method of using the riser overall pressure drop. The model could be used to gain an insight into the bulk characteristics of the system in terms of the SCR and solids holdup profile as a function of particle properties and operating parameters. As part of this study, a solids holdup correlation for a particles riser flow with Redp = 3.7−366 was developed to predict the solids holdup at the top section of the riser. The developed empirical correlation takes into account the important pertinent operating parameters, and is considered as a function of (i) the SCR, (ii) the gas density and velocity, (iii) the particle properties and (iv) the riser inner diameter. The relevant dimensionless values, namely, the dimensionless SCR, the ratio of the solid-gas density, the riser-particle diameter ratio, Froude number, the ratio of the riser height to inner diameters and the ratio of the gas velocity to the terminal velocity of the particles, were used in the developed correlation. The empirical correlation also takes into account the effect of the reduced solids flux. The correlation showed reasonable accuracy when compared with the existing empirical correlations, in which 90% of the predicted data points lie within ±5% to ±25% deviation (with an average around ±15%) of the experimental data. A computational fluid dynamics and discrete element model (CFD–DEM) at the infancy stage of its development was also used to gain an insight into the driving mechanisms for mixing and segregation of binary mixtures with different sizes and densities as well as the local behaviour of the two-phase flow in a bubbling fluidised bed under conditions pertinent to the fuel reactor of a chemical looping combustor. The results showed that good mixing at high particle size ratios of the binary mixture would invariably involve higher gas velocities, and hence, maintaining the size ratio as low as possible appears to be the best method of improving the mixing at lower gas velocities. A reasonable agreement between the experimental data and the model predictions was obtained. In addition, it is believed that the transition occurred from segregation to mixing when the bulk density of species 1 (heavier and smaller) became nearly equal to the actual density of species 2 (lighter and larger). For the investigated cases, the transition regime started when superficial gas velocity was about 2 times the minimum fluidization of the binary mixture (uumf(mixture)). While the end of the transition regime (i.e. at well mixed binary mixture system), the velocity was almost 1.5-2 times the superficial gas velocity at which the transition regime started. It seems that the particle with the higher density but smaller size influences the mixing and segregation behaviour in a binary mixture that differs in size and density. Finally, a 200 kWth CFM–CLC scaled-up unit of a 10 kWth unit (the first of its kind in Australia) was designed, fabricated and commissioned. The hydrodynamic similarity of these two units in terms of pressure drop profile and SCR were investigated. The results showed that Glicksman’s scaling law was successfully implemented. The scaling factor calculated by using the ratio of the gas velocity relation with the square root of the height ratio of the air reactor between the two units was 2.4, which was found to be well in agreement with the experimentally obtained factor of 2.56. Therefore, by using this factor and keeping the system solid volume fraction between the two units equalling 1, hydrodynamic similarity between the two units was demonstrated for the same SCR. However, a modification on the dimensionless fluidisation velocity was proposed by including the system solid volume fraction term in the relation. The new modified relation takes into account any changes in the ratio of фsys,(L)/фsys(S) between the two units. The empirical correlation was validated against the experimental results, which showed a deviation of less than ±6% between the predicted and the actual experimental results.
- Subject
- chemical looping combustion; binary mixture of particles; solid circulation rate; solids holdup; solids holdup profile; cold flow model; interconnected circulating fluidized bed
- Identifier
- http://hdl.handle.net/1959.13/1312781
- Identifier
- uon:22461
- Rights
- Copyright 2016 Yusif Adel Alghamdi
- Language
- eng
- Full Text
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