Utilize este identificador para referenciar este registo: http://hdl.handle.net/10400.9/428
Título: Estudo da formação e destruição de óxidos de azoto em chamas de hidrogénio e /ou hidrocarbonetos com ou sem presença de compostos azotados
Autor: Azevedo, Pedro
Palavras-chave: Combustão
Dinâmica de fluidos computacional
Cinética química
Óxidos de azoto
Chamas laminares
Combustion
Computational fluid dynamics
Nitrogen oxides
Laminar flames
Data de Defesa: 2007
Editora: Faculdade de Engenharia da Universidade do Porto
Resumo: In the nineties, the World Health Organization updated their “Air Quality Guidelines for Europe” (WHO, 2000) with the objective to provide detailed information on atmospheric pollutants and their adverse effects on human health. Consequence of exposure to compounds as the nitrogen oxides, volatile organic compounds (VOC), peroxyacetylnitrates (PAN) and aldehydes, upon reacting with solar light, produce the photochemical smog along with the formation the tropospheric ozone. Among other undesirable consequences, the nitrogen oxides act as main harmful precursors for the formation of the photochemical smog (Broekhuizen, 2002). This work aims at studying the formation of nitrogen oxides and their subsequent destruction in hydrogen and/or hydrocarbon flames with or without seeding with nitrogen compounds, hence producing conditions for a reduced kinetic mechanism that can be used in CFD software. This research work was carried out involving both experimental and numerical studies. The experimental work studied the influence of the parameters for hydrocarbons combustion and NOx formation and destruction. The parameters varied were the temperature, equivalence ratio, fuel composition and the presence of nitrogen compounds artificially added. The main reactants used were hydrogen, methane, oxygen and argon and there was seeding of flames with ammonia or nitrogen oxide, varying between 500 and 1000 ppmv. The experimental runs were carried out using a test bench specially built for this work and used laminar flames with equivalence ratios ranging from 0.8 to 1.2. The results were obtained through flue gas monitoring inside the flame and this was achieved by placing a probe with extraction of gases at several axial distances ranging from 10 to 30 mm above the burner. Temperature and velocity of burnt gases were calculated. The concentration of some combustion products, namely oxygen, hydrogen, carbon oxides was determined on a gas chromatograph, the nitrogen oxides were measured using an on line gas analyzer with an electrochemical cell and the levels of both hydrogen cyanide and ammonia were quantified with selective electrodes in aqueous solution. The use of detailed kinetic mechanisms for a 3 D reactive computational fluid dynamics flow as the one representing a flame is very difficult. So a reduced kinetic mechanism was employed and estimation methods were utilised for the calculation of physical properties of chemical species. The reduced kinetic mechanism was compared with three more detailed mechanisms and one reduced kinetic mechanism in one-dimensional chemical kinetic package, Chemkin, employing reaction parameters like flue gas composition distribution, temperature and velocity profiles along burner axial distance and residence time. The results were found to be in good agreement. The reduced kinetic mechanism was then integrated in the computational fluid dynamics package, Fluent, together with physical properties of chemical species for a reactive flow in 3 D. The numerical calculations were then compared with experimental data and there was a good agreement with respect to the overall combustion mechanism of methane. However, the results for nitrogen oxides, though demonstrating the same tendency as experimental work, suggest that some improvements in the model are needed in order to obtain more satisfactory correlation. Based on the validation results, a reduced kinetic mechanism involving 50 reactions integrating nitrogen chemistry was developed that could represent H2 and/or CH4 flames with nitrogen chemistry. The extension of this work to more complex hydrocarbons could be achieved without significantly increasing the chemical reactions or species quantity involved.
Na década de 90, a Organização Mundial de Saúde actualizou as suas “Orientações da qualidade do ar para Europa” (WHO, 2000), para fornecer informação detalhada sobre os efeitos adversos à saúde humana, decorrentes da exposição aos diferentes poluentes atmosféricos. Determinados compostos, como o dos óxidos de azoto, os compostos orgânicos voláteis, os nitratos de peroxiacetilo e os aldeídos, quando reagem com a luz solar produzem o designado smog fotoquímico, cuja principal consequência é a presença de ozono na troposfera. Entre outras consequências nefastas, os óxidos de azoto encontram se entre os principais agentes nocivos que constituem o smog fotoquímico (Broekhuizen, 2002). O presente trabalho visa o estudo da formação e destruição de óxidos de azoto em chamas de hidrogénio e/ou hidrocarbonetos com ou sem presença de compostos azotados, através da definição de um mecanismo cinético reduzido, para utilização em aplicações de CFD. Desta forma, esta investigação foi realizada através de um trabalho multidisciplinar com componentes experimental e numérica. O trabalho laboratorial incidiu no estudo de parâmetros que influenciam a combustão de hidrocarbonetos e a formação e destruição de NOx, através da análise dos efeitos da razão de equivalência, composição do combustível e presença com ou sem adição de compostos azotados. As espécies consideradas na alimentação do sistema foram o metano, o hidrogénio, o oxigénio e o árgon e as dopagens, quando existentes, foram efectuadas com amónia e monóxido de azoto, a variar de 500 a 1000 ppmv. Estes ensaios foram realizados numa instalação montada, especificamente, para este trabalho, e abrangeram chamas laminares com razões de equivalência a variar de 0,8 a 1,2. Os resultados foram obtidos através da monitorização dos gases em chama, o que se conseguiu através da extracção de amostras a diferentes distâncias axiais do centro do queimador, a variarem de 10 a 30 mm, e do cálculo quer da temperatura quer da velocidade dos gases queimados. Os produtos da combustão como oxigénio, hidrogénio, monóxido e dióxido de carbono, metano e outros hidrocarbonetos foram medidos por cromatografia em fase gasosa, os óxidos de azoto foram quantificados num analisador de gases por célula electroquímica e o cianeto de hidrogénio e a amónia foram medidos em solução aquosa por eléctrodos selectivos. A dificuldade em utilizar mecanismos cinéticos detalhados em aplicações tridimensionais de escoamento com reactividade conduziu a componente numérica deste trabalho a incluir não só a proposta de um mecanismo cinético reduzido e respectiva validação, como também a determinação das propriedades físicas das espécies químicas envolvidas, através de métodos de estimativa. O mecanismo cinético reduzido foi testado contra três mecanismos cinéticos detalhados e um mecanismo reduzido, por meio da aplicação unidimensional Chemkin, através de parâmetros como a distribuição da composição dos gases, perfis de temperatura e de velocidade em função da distância axial ao queimador e do tempo de residência. Nos ensaios efectuados, o mecanismo proposto apresentou boa concordância com os mecanismos detalhados e melhores resultados que o mecanismo reduzido testado. O mecanismo cinético reduzido proposto foi implementado, tal como as propriedades físicas das suas espécies químicas, na aplicação de dinâmica de fluidos computacional, Fluent. Nesta aplicação, o escoamento reactivo foi estudado com morfologia tridimensional e validado em comparação com os ensaios experimentais com resultados bastante bons no que respeita à oxidação do metano e com tendências interessantes quanto aos óxidos de azoto. O mecanismo necessita de ser melhorado de forma a representar quantitativamente a formação/destruição dos óxidos de azoto. Como resultado, foi obtido um mecanismo cinético reduzido, com menos de 50 reacções químicas, capaz de representar a combustão de hidrogénio e/ou metano com inclusão da química do azoto e a extensão deste trabalho a outros hidrocarbonetos não deverá aumentar significativamente o número de reacções nem de espécies químicas. In the nineties, the World Health Organization updated their “Air Quality Guidelines for Europe” (WHO, 2000) with the objective to provide detailed information on atmospheric pollutants and their adverse effects on human health. Consequence of exposure to compounds as the nitrogen oxides, volatile organic compounds (VOC), peroxyacetylnitrates (PAN) and aldehydes, upon reacting with solar light, produce the photochemical smog along with the formation the tropospheric ozone. Among other undesirable consequences, the nitrogen oxides act as main harmful precursors for the formation of the photochemical smog (Broekhuizen, 2002). This work aims at studying the formation of nitrogen oxides and their subsequent destruction in hydrogen and/or hydrocarbon flames with or without seeding with nitrogen compounds, hence producing conditions for a reduced kinetic mechanism that can be used in CFD software. This research work was carried out involving both experimental and numerical studies. The experimental work studied the influence of the parameters for hydrocarbons combustion and NOx formation and destruction. The parameters varied were the temperature, equivalence ratio, fuel composition and the presence of nitrogen compounds artificially added. The main reactants used were hydrogen, methane, oxygen and argon and there was seeding of flames with ammonia or nitrogen oxide, varying between 500 and 1000 ppmv. The experimental runs were carried out using a test bench specially built for this work and used laminar flames with equivalence ratios ranging from 0.8 to 1.2. The results were obtained through flue gas monitoring inside the flame and this was achieved by placing a probe with extraction of gases at several axial distances ranging from 10 to 30 mm above the burner. Temperature and velocity of burnt gases were calculated. The concentration of some combustion products, namely oxygen, hydrogen, carbon oxides was determined on a gas chromatograph, the nitrogen oxides were measured using an on line gas analyzer with an electrochemical cell and the levels of both hydrogen cyanide and ammonia were quantified with selective electrodes in aqueous solution. The use of detailed kinetic mechanisms for a 3 D reactive computational fluid dynamics flow as the one representing a flame is very difficult. So a reduced kinetic mechanism was employed and estimation methods were utilised for the calculation of physical properties of chemical species. The reduced kinetic mechanism was compared with three more detailed mechanisms and one reduced kinetic mechanism in one-dimensional chemical kinetic package, Chemkin, employing reaction parameters like flue gas composition distribution, temperature and velocity profiles along burner axial distance and residence time. The results were found to be in good agreement. The reduced kinetic mechanism was then integrated in the computational fluid dynamics package, Fluent, together with physical properties of chemical species for a reactive flow in 3 D. The numerical calculations were then compared with experimental data and there was a good agreement with respect to the overall combustion mechanism of methane. However, the results for nitrogen oxides, though demonstrating the same tendency as experimental work, suggest that some improvements in the model are needed in order to obtain more satisfactory correlation. Based on the validation results, a reduced kinetic mechanism involving 50 reactions integrating nitrogen chemistry was developed that could represent H2 and/or CH4 flames with nitrogen chemistry. The extension of this work to more complex hydrocarbons could be achieved without significantly increasing the chemical reactions or species quantity involved.
URI: http://hdl.handle.net/10400.9/428
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