Biomass, as a renewable energy resource, can be utilized to generate chemicals, heat, and electricity. Compared with biomass combustion, biomass gasification is more eco-friendly because it generates less amount of green gas (CO2) and other polluting gases (NOx and SO2). This research is focused on biomass gasification using a circulating fluidized bed. In the gasifier, fully fluidized biomass particles react with water vapor and air to generate syngas (CO and H2). A comprehensive model, consisting of three modules, hydrodynamics, mass transfer and energy transfer modules, is built to simulate this process using ANSYS Fluent software and C programming language. In the hydrodynamics module, the k-epsilon turbulence equations are coupled with the fluctuating energy equation to simulate gas-particle interaction in the turbulent flows occurring in the riser. In the mass transfer and energy transfer modules, heat transfer and mass transfer in turbulent flows are simulated to solve for the profiles of temperature and species concentration in the gasifier. The impacts of thermal radiation, water gas shift reaction (WGS), equivalence ratio (ER), and char combustion product distribution coefficient are also investigated to gain deeper understanding of biomass gasification process.

A cold flow model circulating fluidized bed reactor was designed and built to determine any correlation that may exist between the percentage of fine bed material in the overall reactor inventory, and both the solids circulation rate as well as the riser axial particle distribution. It was determined that for Geldart group B particles (sand), there may be a direct relationship between an increase in the percentage of fine particles and an increase in the solids circulation rate for a given riser superficial velocity. There may also be a direct relationship between the percentage of fine particles and an increase in the overall solids concentration in the upper zones of the riser for a given riser superficial velocity. It is theorized that these effects are due to a reduction in the overall mean particle size of any particle clusters formed due to the increase of the percentage of fines.

Submitted by Maicon Juliano Schmidt (maicons) on 2015-04-15T20:36:34Z No. of bitstreams: 1 Fabiano Anderson Pedroso.pdf: 4283933 bytes, checksum: 9e8c395fe9319e367efba6745553280e (MD5) / Made available in DSpace on 2015-04-15T20:36:34Z (GMT). No. of bitstreams: 1 Fabiano Anderson Pedroso.pdf: 4283933 bytes, checksum: 9e8c395fe9319e367efba6745553280e (MD5) Previous issue date: 2013-01-31 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / PROSUP - Programa de Suporte à Pós-Gradução de Instituições de Ensino Particulares / A combustão em Leito Fluidizado (LF) é caracterizada por sua capacidade de redução das emissões de poluentes em relação aos métodos tradicionais de queima do combustível pulverizado. No Brasil, há um potencial de geração de energia em LF pela combustão do carvão mineral, dada a quantidade de reservas disponíveis, das quais mais de 99% se concentram nos estados do Rio Grande do Sul e de Santa Catarina. Dentre as tecnologias disponíveis para combustão e gaseificação em LF, destaca-se a de Leito Fluidizado Circulante (LFC), devido ao potencial de uso em gaseificação integrada a um ciclo combinado de conversão de energia (IGCC). No projeto, dimensionamento e operação de LFs, o entendimento do escoamento multifásico gás-sólido é de suma importância. As leis de escalonamento de Glicksman fornecem as regras necessárias para construção de leitos em escala com similaridade fluidodinâmica, permitindo reproduzir em escala piloto ou mesmo de laboratório a fluidodinâmica de um leito em escala industrial. Aliado a isso, a Dinâmica dos Fluidos Computacional (CFD) vem se estabelecendo como uma poderosa ferramenta para a simulação dos processos em LFC. Portanto, o objetivo deste trabalho é desenvolver um modelo computacional para a simulação da fluidodinâmica de LFCs utilizando o código livre MFIX e aplicar esse modelo para validação das leis de escala através da modelagem numérica de um LFC em escala real com validação experimental e um leito em escala reduzida de acordo com o conjunto simplificado. Para isso, foram desenvolvidos um leito em completa correspondência com o conjunto simplificado, seis leitos escalonados com alteração de parâmetros operacionais e um leito escalonado pelo conjunto completo das leis de escalonamento de Glicksman. O modelo computacional é baseado na abordagem Euler-Granular, em que as fases gás e sólido são consideradas como meios contínuos interpenetrantes. A fase sólida é modelada como um fluido cujo tensor tensão é construído de modo a descrever o escoamento da fase particulada conforme a teoria cinética dos escoamentos granulares (KTGF). O modelo físico aproximado para validação da simulação para a escala real foi o Terceiro Desafio promovido em parceria pela NETL e PSRI. Entre as escalas, foram comparados os perfis horizontal e vertical de fração volumétrica de gás; perfis horizontais de velocidade vertical adimensional dos sólidos e fluxo mássico adimensional de sólidos; perfil vertical de perda de carga adimensional e a evolução temporal da fração volumétrica de gás média. No presente estudo, os resultados permitiram verificar que, na modelagem numérica de uma escala reduzida a partir do conjunto simplificado das leis de escalonamento de Glicksman, a média do Erro Relativo Médio (ERM) ponderado sobre todos os perfis analisados apresentou um valor de 14,2% em relação a escala real, aceitável para esse tipo de sistema. Também se verificou que a diminuição do diâmetro das partículas em não conformidade com as leis de escala, em alguns dos perfis analisados, implicou em uma redução do ERM em comparação com aquele obtido pelos resultados do conjunto simplificado, devido a maior aproximação do diâmetro da partícula do valor determinado pelo conjunto completo. Em relação à comparação dos resultados obtidos pelo conjunto simplificado e completo das leis de escalonamento de Glicksman, confirma-se o esperado - uma maior correspondência para o leito escalonado pelo conjunto completo, com destaque à correta previsão do perfil horizontal do fluxo mássico adimensional de sólidos, não previsto pelo conjunto simplificado. Dessa forma, considera-se que o conjunto simplificado das leis de escala de Glicksman, dentro de suas limitações intrínsecas, fornece uma boa aproximação para o escalonamento de LFCs através da simulação numérica Euler-Granular. / Fluidized bed (FB) combustion has as main feature its capacity to reduce the release of pollutants in relation to conventional burning methods of pulverized fuel. Brazil has a potential in energy generation with FB through the combustion of coal, given the number of available reservoirs, of which 99% are located in the Southern states of Rio Grande do Sul and Santa Catarina. Among the available technologies for combustion and gasification on FB, we can highlight that of Circulating Fluidized Beds (CFB), given its use in Integrated Gasification Combined Cycles (IGCC). In the project, design and operation of FBs, the understanding of the gas-solid multiphase flow is highly important. Glicksman’s scaling laws provide the guidance needed for building beds in scale with fluid dynamics similarity, allowing the reproduction in pilot or even laboratory level of the fluid dynamics of a bed in industrial level. Along with that, Computational Fluid Dynamics (CFD) has established itself as a powerful tool in the simulation of CFB processes. Therefore, the aim of this paper is to develop a computational model for the simulation of CFBs fluid dynamics, using the MFIX code and to apply this model to the validation of scaling rules through the numerical modeling of a CFB in real scale with experimental validation and a bed in reduced scale according to a reduced set. For that to happen, a bed in fully correspondence wtih the reduced set, six scaled beds with alterations in their operational parameters, and a bed scaled by the full-set of Glicksman’s scaling laws have been developed. The computational model is based on the Euler-Granular Approach, in which the solid and gas phases are considered as interpenetrating continua. The solid phase is modeled as a fluid whose tensors are built in order to describe the flow of the granular phase according to the kinetic theory of granular flows (KTGF). The approximate physical model for the validation of this simulation to real scale was the Third Challenge held by NETL and PSRI. A comparison was made among the scales, one of the horizontal and vertical profiles of gas volume fraction; horizontal of vertical dimensionless speed of solids and dimensionless mass flux of solids; vertical of dimensionless pressure drop and the temporal evolution of the average gas volume fraction. In this study, the results allowed to verify that, in the numeral modeling of a reduced scale based on the reduced set of Glicksman’s scaling laws, the average of Relative Error (RE) considered over all the analyzed profiles showed a 14.2% value in relation to the real scale, which is acceptable for this kind of system. It has also been verified that the reduction in diameter of particles which were not suitable with the scaling laws, in some of the analyzed profiles, resulted in a reduction of RE when compared to that obtained through the results of the reduced set, due to a larger approximation of the particles diameter to the value determined by the full-set. Regarding the comparison of the results obtained through the reduced and full-set of Glicksman’s scaling laws, the most expected was confirmed – a larger matching for the bed scaled through the full-set, highlighting the correct prediction of the horizontal profile of the dimensionless mass flux of solids, which was not predicted by the reduced set. Thus, the reduced set of Glicksman’s scaling laws provides, within its inherent limitations, a good approximation for the scaling of CFBs through the Euler-Granular numerical simulation.

Many particulate processes are preferably implemented in circulating fluidized beds (CFB) over traditional low-velocity fluidization to take advantage of the many benefits of circulating systems. Fluid catalytic cracking (FCC) is one of the most successfully applied processes in CFB technology, with more than 350 FCC units in operation worldwide. Despite its extensive use, an understanding of the complex behaviour of these units is incomplete.<p> A theoretical and experimental evaluation of the fluidization behaviour was conducted in the CFB riser, standpipe, and stripper. Initially, an extension of the existing CFB in the Fluidization Laboratory of Saskatchewan was designed. The experimental program conducted in this study included an examination of the solids flow behaviour in the riser, interstitial gas velocity in the downcomer, and stripping efficiency measurements. The hydrodynamic behaviour of the stripper was modeled using Multiphase Flow with Interphase eXchanges (MFIX) CFD code.<p> The solids flow behaviour in the bottom zone of a high-density riser was investigated by measuring the local upwards and downwards solids flux. Solids circulation rates between 125 and 243 kg/(m2⋅s) were evaluated at a constant riser superficial gas velocity of 5.3 m/s. The effect of the riser superficial gas velocity of the local upflow at the riser centerline was also conducted at a solids circulation rate of 187 kg/(m2⋅s). The results show that there is little variation in the local net solids flux at radial locations between 0.00 ¡Ü r/R ¡Ü 0.87. The results indicate that a sharp regime change from a typical parabolic solids flux profile to this more radially uniform solids flux profile occurs at a gas velocity between 4.8 and 4.9 m/s.<p> To quantify stripping efficiency, the underflow of an injected tracer into the standpipe must be known. Quantification of the underflow into the standpipe requires knowledge of two main variables: the interstitial gas velocity and the tracer gas concentration profiles in the standpipe. Stripping efficiency was determined for stripper solids circulation rates of 44, 60, and 74 kg/(m2⋅s) and gas velocities of 0.1, 0.2, and 0.3 m/s. For most conditions studied, the interstitial gas velocity profile was found to be flat for both fluidized and packed bed flow. The stripping efficiency was found to be sensitive to the operating conditions. The highest efficiency is attained at low solids circulation rates and high stripping gas velocities.<p> In the numeric study, stripper hydrodynamics were examined for similar operating conditions as those used in the experimental program. Due to an improved radial distribution of gas and decreasing bubble rise velocity, mass transfer is deemed most intense as bubbles crest above the baffles into the interspace between disc and donut baffles. Stripping efficiency is thought to improve with increasing gas velocity due to an increased bubbling frequency. Stripping efficiency is thought to decrease with increasing solids circulation rates due to a lower emulsion-cloud gas interchange coefficient and a decreased residence time of the emulsion in the stripper.

Many particulate processes are preferably implemented in circulating fluidized beds (CFB) over traditional low-velocity fluidization to take advantage of the many benefits of circulating systems. Fluid catalytic cracking (FCC) is one of the most successfully applied processes in CFB technology, with more than 350 FCC units in operation worldwide. Despite its extensive use, an understanding of the complex behaviour of these units is incomplete.<p> A theoretical and experimental evaluation of the fluidization behaviour was conducted in the CFB riser, standpipe, and stripper. Initially, an extension of the existing CFB in the Fluidization Laboratory of Saskatchewan was designed. The experimental program conducted in this study included an examination of the solids flow behaviour in the riser, interstitial gas velocity in the downcomer, and stripping efficiency measurements. The hydrodynamic behaviour of the stripper was modeled using Multiphase Flow with Interphase eXchanges (MFIX) CFD code.<p> The solids flow behaviour in the bottom zone of a high-density riser was investigated by measuring the local upwards and downwards solids flux. Solids circulation rates between 125 and 243 kg/(m2⋅s) were evaluated at a constant riser superficial gas velocity of 5.3 m/s. The effect of the riser superficial gas velocity of the local upflow at the riser centerline was also conducted at a solids circulation rate of 187 kg/(m2⋅s). The results show that there is little variation in the local net solids flux at radial locations between 0.00 ¡Ü r/R ¡Ü 0.87. The results indicate that a sharp regime change from a typical parabolic solids flux profile to this more radially uniform solids flux profile occurs at a gas velocity between 4.8 and 4.9 m/s.<p> To quantify stripping efficiency, the underflow of an injected tracer into the standpipe must be known. Quantification of the underflow into the standpipe requires knowledge of two main variables: the interstitial gas velocity and the tracer gas concentration profiles in the standpipe. Stripping efficiency was determined for stripper solids circulation rates of 44, 60, and 74 kg/(m2⋅s) and gas velocities of 0.1, 0.2, and 0.3 m/s. For most conditions studied, the interstitial gas velocity profile was found to be flat for both fluidized and packed bed flow. The stripping efficiency was found to be sensitive to the operating conditions. The highest efficiency is attained at low solids circulation rates and high stripping gas velocities.<p> In the numeric study, stripper hydrodynamics were examined for similar operating conditions as those used in the experimental program. Due to an improved radial distribution of gas and decreasing bubble rise velocity, mass transfer is deemed most intense as bubbles crest above the baffles into the interspace between disc and donut baffles. Stripping efficiency is thought to improve with increasing gas velocity due to an increased bubbling frequency. Stripping efficiency is thought to decrease with increasing solids circulation rates due to a lower emulsion-cloud gas interchange coefficient and a decreased residence time of the emulsion in the stripper.

A dynamic mathematical model for simulation of atmospheric circulating fluidized bed combustors has been developed on the basis of first principles and empirical correlations. The model accounts for dense and dilute zone hydrodynamics, volatiles release and combustion, char particles combustion and their size distribution, and heat transfer from/to gas, particles, waterwalls and refractory. Inputs to the model include configuration and dimensions of the combustor and its internals, air and coal flows, coal analysis, all solid and gas properties, inlet temperatures of air, cooling water, and feed solids, size distribution of feed solids / whereas outputs include transient values of combustor temperatures, gas concentrations, char and inert hold-ups and their size distributions. The solution procedure employs method of lines approach for the governing non-linear partial differential equations and combined bisection and secant rule for non-linear algebraic equations. The initial conditions required for the model are provided from the simultaneous solution of governing equations of dynamic model with all temporal derivatives set to zero. By setting all temporal derivatives to zero, model can also be utilized for steady state performance prediction. In order to assess the validity and predictive accuracy of the model, it was applied to the prediction of the steady state behavior of Technical University of Nova Scotia 0.3 MWt CFBC Test Rig and predictions were compared with measurements taken on the same rig. Comparison of model predictions at steady state conditions revealed that the predictions of the model are physically correct and agree well with the measurements and the model is successful in qualitatively and quantitatively simulating the processes taking place in a circulating fluidized bed combustor.

A comprehensive model, previously developed for a rectangular parallelepiped shaped 0.3 MWt circulating fluidized bed combustor (CFBC) fired with high calorific value coal burning in sand and validated against experimental data is adapted to cylindrical configuration and is extended to incorporate NOx formation and reduction reactions and pressure drops around cyclone, downcomer and loop seal. Its predictive accuracy is tested by applying it to the simulation of Middle East Technical University (METU) 150 kWt CFBC burning low calorific value indigenous lignite with high Volatile Matter/Fixed Carbon (VM/FC) ratio in its own ash and comparing its predictions with measurements. Favorable comparisons are obtained between the predicted and measured temperatures, pressure profiles and emissions of gaseous species. Results reveal that predictive accuracy in pressure profile strongly depends on the correlation utilized for entrainment in dilute zone and that accuracy in NO emission requires data on partitioning of coal nitrogen into char-N and volatile-N and is affected significantly by dilute zone oxygen content.

No description available.

Orientador: Arai Augusta Bernardez Pecora / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecanica / Made available in DSpace on 2018-08-11T13:29:17Z (GMT). No. of bitstreams: 1 Hory_RogerioIshikawa_D.pdf: 3549862 bytes, checksum: 727c6ab5dce7454b6af9546842232b2e (MD5) Previous issue date: 2007 / Resumo: Este trabalho apresenta o estudo experimental do processo de combustão de carvão mineral brasileiro em reator de Leito Fluidizado Circulante (LFC) com o objetivo de avaliar as taxas de emissões de di óxido de enxofre (S02) e eficiência de conversão de carbono no processo de combustão. Para diminuir a emissão do S02 nos gases de exaustão, foi adicionado, ao carvão, quantidades de calcário dolomítico (dolomita), de modo que a relação molar entre o cálcio (Ca) presente no calcário e o enxofre (S) presente no carvão variou entre 0,0 e 2,0. A variação na relação molar Ca/S, como é denominada na literatura, foi um dos dois fatores avaliados neste trabalho. O outro fator envolvido nos testes foi o excesso de ar utilizado para combustão do carvão. Trabalhou-se com excesso de ar na faixa de 20 a 30%. Foram realizados 11 ensaios seguindo um planejamento experimental estatístico do tipo CCD (Composite Central Design) e mais 02 ensaios complementares. A relação molar Ca/S de 0,6 apresentou emissão zero de S02 e propôs-se utilizar uma nova relação molar: (Ca+Mg)/S para retratar processos de sorção com dolomitas. Para emissão zero de S02, uma relação molar (Ca+Mg)/S de 1,8 já é suficiente. Foram obtidas eficiências de conversão do carbono entre 86,0 e 93,0% para combustão do carvão / Abstract: This work presents an experimental study with Brazilian mineral coal combustion in a Circulating Fluidized Bed (CFB) reactor with the objective to evaluate the sulphur dioxide emissions (S02) and the carbon conversion efficiency in the combustion processo Dolomite was added to the coal to minimize the S02 emissions during combustion. The quantity of dolomite added was calculated based on the molar relation of calcium (Ca) present in the dolomite and sulphur (S) present in the coal. Ca/S molar relation had a variation between 0,0 and 2,0. Two factors were evaluated in this work: Ca/S relation and excess of air during combustion which had a variation between 20 and 30%. Eleven (11) experimental tests were evaluated following a statistical experimental design called CCD (Composite Central Design) and 02 (two) more complementary tests. Emissions of S02 with zero value were obtained for a Ca/S of 0,6. It was also proposed in this work a new relation for sorption of S02 with dolomite: (Ca+Mg)/S relation. Tests showed that a relation of 1.8 for (Ca+Mg)/S is sufficient for zero emissions of S02. Carbon conversion efficiency between 86.0 and 93.0 was also obtained during coal combustion / Doutorado / Termica e Fluidos / Mestre em Engenharia Mecânica

Reatores de leito fluidizado circulante são intensamente utilizados em aplicações de larga escala como craqueamento catalítico de petróleo e combustão de carvão. Projeto e desenvolvimento nestas áreas são fortemente baseados em plantas de demonstração, a custos elevadíssimos. Nesse contexto, tratamentos utilizando mecânica dos fluidos computacional assumem considerável relevância. Os complexos padrões de escoamento gás-sólido que se desenvolvem nos reatores de leito fluidizado circulante determinam taxas de reação e exigem, portanto, descrições hidrodinâmicas rigorosas. Os modelos Eulerianos do contínuo ou de dois fluidos são correntemente considerados a escolha mais prática na busca destas descrições. Estas formulações são baseadas na aplicação da hipótese de meio contínuo tanto para fases macroscopicamente contínuas quanto dispersas. Neste trabalho, desenvolve-se simulações numéricas para descrever processos hidrodinâmicos e reativos em reatores de leito fluidizado circulante aplicando modelagem de dois fluidos. Considera-se situações típicas de combustão em leito fluidizado circulante de carvão mineral. Como processo reativo considera-se a absorção de dióxido de enxofre por calcário. Realiza-se análises hidrodinâmicas, e de efeitos hidrodinâmicos sobre a reação de interesse. Realiza-se simulações de regime permanente e de regime transiente. Mostra-se que as simulações de regime permanente permitem análises qualitativas do processo, e provêem condições iniciais para simulações transientes diretamente no regime de escoamento estatisticamente permanente. Em relação à hidrodinâmica, conclui-se que os modelos de dois fluidos correntes são ainda bastante crus. São claramente necessárias melhores descrições reológicas e relações constitutivas sub-grade mais acuradas. Em relação à reação química, conclui-se que predições reativas acuradas somente poderão ser obtidas se descrições hidrodinâmicas rigorosas forem combinadas com descrições reativas igualmente rigorosas. / Circulating fluidized bed reactors are widely used in large scale applications such as catalytic cracking of petrol and coal combustion. Development and design in those areas are strongly based on demonstration plants, at extremely high costs. In this context, treatments applying computational fluid mechanics assume considerable relevance. The complex gas-solid flow patterns which develop inside the circulating fluidized bed reactors determine reaction rates, so that rigorous hydrodynamic descriptions are required. The continuum Eulerian or two-fluid models are currently considered the more practical choice for providing such descriptions. Those formulations are based on the application of the continuum hypothesis for both macroscopically continuous and dispersed phases. In this work numerical simulation is performed to describe both hydrodynamics and reactive processes in circulating fluidized beds applying two-fluid modeling. Typical situations of circulating fluidized bed coal combustion are considered. The reactive process considered is the absorption of sulfur dioxide by limestone. Analyses are performed of hydrodynamics, and regarding hydrodynamic effects over the concerning reaction. Both steady state and transient simulations are performed. It is shown that steady state simulations allow qualitative analyses, and do provide initial conditions for transient runs straightly inside the statistical steady state flow regime. Concerning hydrodynamics, it is concluded that the current two-fluid models are still very crude. Clearly, better rheological descriptions are required alongside with more accurate sub-grid constitutive relations. Regarding chemical reaction, it is concluded that accurate reactive predictions shall only be found if rigorous hydrodynamic descriptions are combined with equally rigorous reaction descriptions.