Doctor of Philosophy
Chemical and Biochemical Engineering
de Lasa, Hugo
Serrano Rosales, Benito
Universidad Autonoma de Zacatecas
Given the worldwide concerns regarding fossil fuel availability, global warming, water, and air pollution and economy needs, there is an urgent requirement to develop and implement alternative energies. Biomass gasification can consume vegetal waste to produce syngas, which is a mix of carbon monoxide, hydrogen, and carbon dioxide. When biomass gasification is carried out in a fluidized bed, the process is highly efficient.
The experimental study of the present Ph.D. dissertation took place in a Plexiglas 0.44 m diameter unit, equipped with a CREC Optiprobe system and a video camera. The unit was filled with 200-900 µm sand particles and biomass pellets measuring 2.7 cm in length and 0.8 cm in diameter. This was used to take experimental measurements of bubbles size, velocity, and shape.
Single bubbles injected in a sand bed at minimum fluidization with and without biomass were first considered. Later, bubbles evolving in a sand bed, operating in the dense phase bubble regime were studied. On this basis, the capabilities of a developed geometrical model to predict the BAC (Bubble Axial Chord) and the BACs correlated with the BRVs (Bubble Rising Velocity) were examined. The bubble dynamics model considered includes an adjustable bubble wake parameter, with predictions providing the bubble chord, the bubble frontal-radius, and the bubble rising velocity. Following this step, the effects of biomass pellet addition on the BAC, the BRV, and the bubble assigned shape were evaluated.
Computational Fluid Dynamics Multiphase Particle-in-Cell (CFD-MPPIC) simulation using a CPFD Barracuda VR® v17.4.1 software was considered, and this for the various cases studied. The proposed CFD-MPPIC model involved the following: a) an equivalent 1 cm3 biomass pellets, b) an adjusted particle drag model, c) a bubble-wall interaction, d) a selection of computational cells, e) a Multiphase Particle-in-Cell population sizing, and f) a cell distribution implemented throughout the column.
Experimental results in the Plexiglas 0.44 m diameter unit, validated the proposed CFD-MPPIC model with good predictions of both BACs and BRVs. This was the case for all conditions studied, with the only exception of bubbling beds with added biomass, where calculated BRVs were found for some operating conditions as less trustable.
Summary for Lay Audience
The world urgently needs to address pollution, fuel depletion, and energy accessibility using innovative ways to transform energy resources. Biomass is present in every plant dead or alive in many locations around the world and can be used as a source of renewable energy. Agricultural waste, for example, is an issue for most farmers since it increases the cost of production, given its disposal fees. Biomass gasification in fluidized beds is a technology that can help to mitigate agricultural waste toxicity. It can convert it into synthesis gas (syngas), which is a valuable product that can be used to produce chemicals or be employed as a fuel due to its heating value. Typically for biomass gasification in a fluidized bed, a combined flow of air and stream is circulated upwards in the form of bubbles. This contributes to both gasification and sand mixing, with gasification taking place at a close to a constant temperature. Designing, scaling-up, and operating these units can be facilitated by using fiber optic sensors as well as by performing numerical calculations based on fundamental physicochemical principles. Using this approach, the present Ph.D. dissertation studies bubble dynamics in a sand fluidized bed prototype with and without biomass pellets. On this basis, both the size and velocity of the formed bubbles are measured experimentally and predicted with the proposed mathematical model. As a result, the outcome of this project provides a better understanding of bubble phenomena in sand fluidized bed gasifiers, as well as contributes to the establishment of numerical tools for designing, predicting, and controlling the operation of this kind of unit.
Torres Brauer, Nicolas, "Bubble Dynamics in a Sand Fluidized Bed in the Presence of Biomass Pellets" (2020). Electronic Thesis and Dissertation Repository. 7045.
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