Energetic and exergetic analysis of biomass gasification
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2015-03-23
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A crescente preocupação energética, económica e ambiental obriga a uma utilização eficiente dos recursos energéticos naturais. Recursos energéticos renováveis poderão ser um bom substituto aos combustíveis fósseis que estão a ser consumidos rapidamente.
Atualmente, biomassa e biocombustíveis estão a ser considerados devido às suas características não poluentes e às suas capacidades de fornecimento de energia. Uma das preocupações mais urgentes em relação ao uso da biomassa como energia renovável é o aumento da sua eficiência energética e principalmente a sua eficiência exergética. De forma a melhorar a eficiência exergética é necessário melhorar o processo de gasificação, criando modelos numéricos que otimizem o seu design e as suas condições operatórias. Seguindo este raciocínio, a presente tese realiza uma revisão exergética e de seguida um modelo numérico é apresentado para a otimização das condições operacionais de forma a melhorar a eficiência do processo de gasificação.
O primeiro capítulo desta tese apresenta uma revisão dos artigos existentes na análise de exergia da gasificação da biomassa. Muita atenção foi dada à produção de hidrogénio a partir do processo de gasificação e à tecnologia da gasificação de carvão integrada com o uso de uma célula de combustível (SOFC). Na maioria das análises exergéticas realizadas, a gasificação foi considerada como o processo termodinâmico principal.
Adicionalmente, os modelos exergoeconómicos permitem combinar economia com termodinâmica de forma a atingir uma avaliação conjunta. Uma das mais populares estratégias para desenvolver análises exergoeconomicas é o método SPECO.
O método SPECO apresenta de uma forma geral, sistemática e simples uma aproximação para o desenvolvimento das eficiências energéticas de um sistema térmico e os seus respetivos componentes.
A necessidade deste tipo de investigações é devida à falta de informação nas análises exergéticas da gasificação da biomassa.
De forma a otimizar as condições operatórias do processo de gasificação da biomassa foi desenvolvido um modelo numérico com base na estrutura CFD. Este modelo consiste num modelo computacional de 2 dimensões que descrevia o processo de gasificação da biomassa
num gasificador de leito fluidizado usando cascas de café com o auxílio do código comercial FLUENT. Ambas as fases, sólida e gasosa, foram descritas usando uma aproximação Euleriana-Euleriana havendo troca de massa, energia e de momento. Resultados do modelo numérico foram depois comparados com resultados experimentais. O estudo foi realizado numa central semi-industrial instalada no parque industrial de Portalegre baseada na tecnologia de leito fluidizado, com uma capacidade de processo de 70 Kg/h e operando à volta de 800 °C. Os testes foram realizados continuamente durante vários dias, usando diferentes resíduos de forma a otimizar a composição do syngas produzido. Resultados obtidos apresentam boa concordância com os experimentais.
The growing concern for energy, economy and environment calls for an efficient utilization of natural energy resources in developing useful work. Renewable energy sources can be a good substitute of the fossil fuels which are being terminated fast. Nowadays biomass and biofuels are considered because of their environment friendly characteristics and their ability of supplying much more energy. One of the most urgent concerns regarding the use of biomass as a renewable energy is getting a superior energetic and most importantly an exergetic efficiency. In order to improve the exergetic efficiency it is necessary to improve the gasification process, by developing numerical models that optimize the design and operation conditions. Accordingly, the present thesis shows an exergy review and later a numerical model is developed for operational condition optimization in order to make the gasification process more efficient. The first chapter of this thesis has reviewed the existent papers on the exergy analysis of biomass gasification. A much extended report was made on hydrogen production by the process of gasification and coal gasification technology integrated with a solid oxide fuel cell. In most of the exergy analysis that had been done, gasification was found to be the main thermochemical process. Additionally, the Exergoeconomical modeling allows combining economics with thermodynamically required energy inputs in order to achieve an integrated evaluation. One of the most popular strategies to develop the exergoeconomical analysis is the SPECO method. The SPECO method presents a general, systematic, simple and unambiguous approach for developing the exergetic efficiencies of a thermal system and its components. The need for such investigations is due to the lack of information in the exergy analysis on the biomass gasification. To optimize the operating conditions of a biomass gasification process it was developed a numerical model based on the CFD framework. This model was based on a two-dimensional computational model that described the biomass gasification process in a fluidized bed reactor using coffee husks within the commercial CFD code FLUENT. Both, gas phase and solid phase, were described using and Eulerian-Eulerian approach exchanging mass, energy and momentum. Results from the numerical model were later compared with experimental data. The study was conducted in a pilot thermal gasification plant, installed at Portalegre’s Industrial Park based on the fluidized bed technology, with a processing capacity of 70 kg/h, and operating at around 800 ºC. The gasification tests were performed continuously for several days, using different wastes in order to optimize the composition of produced syngas. Numerical results shows good agreement with experimental data.
The growing concern for energy, economy and environment calls for an efficient utilization of natural energy resources in developing useful work. Renewable energy sources can be a good substitute of the fossil fuels which are being terminated fast. Nowadays biomass and biofuels are considered because of their environment friendly characteristics and their ability of supplying much more energy. One of the most urgent concerns regarding the use of biomass as a renewable energy is getting a superior energetic and most importantly an exergetic efficiency. In order to improve the exergetic efficiency it is necessary to improve the gasification process, by developing numerical models that optimize the design and operation conditions. Accordingly, the present thesis shows an exergy review and later a numerical model is developed for operational condition optimization in order to make the gasification process more efficient. The first chapter of this thesis has reviewed the existent papers on the exergy analysis of biomass gasification. A much extended report was made on hydrogen production by the process of gasification and coal gasification technology integrated with a solid oxide fuel cell. In most of the exergy analysis that had been done, gasification was found to be the main thermochemical process. Additionally, the Exergoeconomical modeling allows combining economics with thermodynamically required energy inputs in order to achieve an integrated evaluation. One of the most popular strategies to develop the exergoeconomical analysis is the SPECO method. The SPECO method presents a general, systematic, simple and unambiguous approach for developing the exergetic efficiencies of a thermal system and its components. The need for such investigations is due to the lack of information in the exergy analysis on the biomass gasification. To optimize the operating conditions of a biomass gasification process it was developed a numerical model based on the CFD framework. This model was based on a two-dimensional computational model that described the biomass gasification process in a fluidized bed reactor using coffee husks within the commercial CFD code FLUENT. Both, gas phase and solid phase, were described using and Eulerian-Eulerian approach exchanging mass, energy and momentum. Results from the numerical model were later compared with experimental data. The study was conducted in a pilot thermal gasification plant, installed at Portalegre’s Industrial Park based on the fluidized bed technology, with a processing capacity of 70 kg/h, and operating at around 800 ºC. The gasification tests were performed continuously for several days, using different wastes in order to optimize the composition of produced syngas. Numerical results shows good agreement with experimental data.
Descrição
Dissertação de Mestrado em Engenharia Mecânica
Palavras-chave
Gasificação , Exergia , CFD , Gasificador semi-industrial