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Many reaction mechanisms have been developed over the past few decades to predict flame characteristics. A detailed reaction mechanism can predict flame characteristics well, but at a high computational cost. The reason for reducing reaction mechanisms is to reduce the computational time needed to simulate a problem. The focus of this work is on the validity of reduced methane-air combustion mechanisms, particularly pertaining to satisfying the entropy inequality. While much of this work involves a two-step reaction mechanism developed by Dr. Charles Westbrook and Dr. Frederick Dryer, some consideration is given to the four-step and three-step mechanisms of Dr. Norbert Peters. These mechanisms are used to simulate the Flame A experiment from Sandia National Laboratories. The two-step mechanism of Westbrook and Dryer is found to generate results that violate the entropy inequality. Modifications are made to the two-step mechanism simulation in an effort to reduce these violations. Two new mechanisms, Mech 1 and Mech 2, are developed from the original two-step reaction mechanism by modifying the empirical data constants in the Arrhenius reaction form. The reaction exponents are set to the stoichiometric coefficients of the reaction, and the concentrations computed from a one-dimensional flame simulation are matched by changing the Arrhenius parameters. The new mechanisms match experimental data more closely than the original two-step mechanism and result in a significant reduction in entropy inequality violations. The solution from Mech 1 had only 9 cells that violated the entropy inequality, while the original two-step mechanism of Westbrook and Dryer had 22,016 cells that violated the entropy inequality. The solution from Mech 2 did not have entropy inequality violations. The method used herein for developing the new mechanisms can be applied to more complex reaction mechanisms.

Orientador: Heloise de Oliveira Pastore / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-12T11:21:11Z (GMT). No. of bitstreams: 1 Strauss_Mathias_M.pdf: 12828792 bytes, checksum: 704e6ce09b644ccacab0bb51fa7baf10 (MD5) Previous issue date: 2008 / Resumo: A busca por fontes de energia mais limpas do que aquelas provenientes do petróleo tem sido a razão de estudo de diversos grupos de pesquisa. É também com este intuito que a síntese de novos catalisadores microporosos com metais de transição estruturais vem sendo estudada. Neste trabalho é apresentada a síntese de catalisadores originais para a combustão catalítica de metano denominados [M]-CAL-2 sendo estes materiais microporosos do tipo MeAPSO com estrutura chabasita análoga ao SAPO-34. Os catalisadores foram preparados utilizando diferentes metais de transição (Co, Mn e Fe) e em diferentes razões molares durante a preparação do gel de síntese. São apresentadas, nesta dissertação, as caracterizações estruturais, a composição, os estudos espectroscópicos e os testes catalíticos da combustão de CH4 para os sólidos obtidos. / Abstract: The search for cleaner energy sources than oil is the reason for studies conducted by several groups around the world. It is also with this intention that the synthesis of new microporous catalyst with structural transition metals is explored. In this work is presented the synthesis of new methane combustion catalysts called [M]-CAL-2, they are MeAPSO-type microporous materials with chabazite like structure analogous to the SAPO-34. The catalysts were prepared using different transition metals (Co, Mn and Fe) in different molar ratios in the synthesis gel. The morphology and composition, the spectroscopic studies and the catalytic tests for the CH4 combustion reaction are presented for the obtained solids. / Mestrado / Quimica Inorganica / Mestre em Química

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Doctor of Philosophy (PhD) / Ultra-lean methane combustion in porous burners is investigated by means of a pilot-scale demonstration of the technology supported by a computational fluid dynamics (CFD) modelling study. The suitability of porous burners as a lean-burn technology for the mitigation of methane emissions is also evaluated. Methane constitutes 14.3% of total global anthropogenic greenhouse gas emissions. The mitigation of these emissions could have a significant near-term effect on slowing global warming, and recovering and burning the methane would allow a wasted energy resource to be exploited. The typically low and fluctuating energy content of the emission streams makes combustion difficult; however porous burners—an advanced combustion technology capable of burning low-calorific value fuels below the conventional flammability limit—are a possible mitigation solution. A pilot-scale porous burner is designed expressly for the purpose of ultra-lean methane combustion. The burner comprises a cylindrical combustion chamber filled with a porous bed of alumina saddles, combined with an arrangement of heat exchanger tubes for preheating the incoming methane/air mixture. A CFD model is developed to aid in the design process. Results illustrating the operating range and behaviour of the burner are presented. Running on natural gas, the stable lean flammability limit of the system is 2.3 vol%, a considerable extension of the conventional lean limit of 4.3 vol%; operating in the transient combustion regime allows the lean limit to be reduced further still, to 1.1 vol%. The heat exchanger arrangement is found to be effective; preheat temperatures of up to 800K are recorded. Emissions of carbon monoxide and unburned hydrocarbons are negligible. The process appears stable to fluctuations in fuel concentration and flow rate, typically taking several hours to react to any changes. A CFD model of the porous burner is developed based on the commercial CFD code ANSYS CFX 12.0. The burner is modelled as a single 1-dimensional porous domain. Pressure loss due to the presence of the porous solid is accounted for using an isotropic loss model. Separate energy equations for the gas and solid phases are applied. Models for conductive heat transfer within the solid phase, and for convective heat transport between the gas and solid phases, are added. Combustion is modelled using a finite rate chemistry model; a skeletal mechanism for ultra-lean methane combustion is developed and incorporated into the model to describe the combustion reaction. Results from the model are presented and validated against experimental data; the model correctly predicts the main features of burner behaviour. Porous burners are found to show potential as a methane mitigation technology.

Doctor of Philosophy (PhD) / Ultra-lean methane combustion in porous burners is investigated by means of a pilot-scale demonstration of the technology supported by a computational fluid dynamics (CFD) modelling study. The suitability of porous burners as a lean-burn technology for the mitigation of methane emissions is also evaluated. Methane constitutes 14.3% of total global anthropogenic greenhouse gas emissions. The mitigation of these emissions could have a significant near-term effect on slowing global warming, and recovering and burning the methane would allow a wasted energy resource to be exploited. The typically low and fluctuating energy content of the emission streams makes combustion difficult; however porous burners—an advanced combustion technology capable of burning low-calorific value fuels below the conventional flammability limit—are a possible mitigation solution. A pilot-scale porous burner is designed expressly for the purpose of ultra-lean methane combustion. The burner comprises a cylindrical combustion chamber filled with a porous bed of alumina saddles, combined with an arrangement of heat exchanger tubes for preheating the incoming methane/air mixture. A CFD model is developed to aid in the design process. Results illustrating the operating range and behaviour of the burner are presented. Running on natural gas, the stable lean flammability limit of the system is 2.3 vol%, a considerable extension of the conventional lean limit of 4.3 vol%; operating in the transient combustion regime allows the lean limit to be reduced further still, to 1.1 vol%. The heat exchanger arrangement is found to be effective; preheat temperatures of up to 800K are recorded. Emissions of carbon monoxide and unburned hydrocarbons are negligible. The process appears stable to fluctuations in fuel concentration and flow rate, typically taking several hours to react to any changes. A CFD model of the porous burner is developed based on the commercial CFD code ANSYS CFX 12.0. The burner is modelled as a single 1-dimensional porous domain. Pressure loss due to the presence of the porous solid is accounted for using an isotropic loss model. Separate energy equations for the gas and solid phases are applied. Models for conductive heat transfer within the solid phase, and for convective heat transport between the gas and solid phases, are added. Combustion is modelled using a finite rate chemistry model; a skeletal mechanism for ultra-lean methane combustion is developed and incorporated into the model to describe the combustion reaction. Results from the model are presented and validated against experimental data; the model correctly predicts the main features of burner behaviour. Porous burners are found to show potential as a methane mitigation technology.

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<p>Current industrial practice in the steel Industry involves the use of natural gas with high methane content as a primary energy source. Natural combustion produces greenhouse gases, and with the continued focus on managing and reducing harmful emissions from industrial processes, there is a need for research into alternative sources of energy. Among several alternatives that have been studied is hydrogen: a non-carbon-based fuel. This work uses a coupled computational fluid dynamics (CFD)-finite element analysis (FEA) combustion model to investigate hydrogen utilization as a fuel in a reheat furnace and how it impacts the quality of the steel produced by understanding the three dimensional (3D) flow behavior, furnace temperature profile, thermal stress distribution, heat flux, formation of iron oxides, emission gases and mode of heat transfer onto the steel slabs. The modeling process integrates the five different zones of a pusher type reheating furnace (top and bottom) and modeled using Ansys Fluent 2020R1 and Ansys Workbench 2022R1. Changes in these parameters are determined by comparison to a baseline case that uses methane as fuel and maintaining the same heat input in terms of chemical energy into the furnace. Global mechanism was used for hydrogen and two step mechanism was used for methane combustion. Results revealed a 2.6% increase in average temperature to 1478K across the furnace for hydrogen which resulted in 6.45% increase in maximum heat flux into the slabs. Similar flue gas flow patterns were seen for both cases and heat transfer mode from the combustion gases to the slabs was primarily by radiation (~97%) for both methane and hydrogen. 11.5% increase in iron oxide formation on the slab was recorded for the hydrogen case, however, the bulk of the iron oxide formed was more of wüstites which are the easiest form of iron oxide to descale. However, elevated nitrogen oxide (NOx) levels were recorded for hydrogen combustion which led to further study into NOx mitigation techniques. Application of the staged combustion method using hydrogen fuel showed potentials for NOx reduction. The use of regenerative burners further conserved exergy losses in hydrogen fuel application. Insignificant deviation from base case thermal stress distribution and zero carbon emission from the hydrogen case indicates the usability of hydrogen as an alternative fuel in reheating furnace operations. </p>

The effect of temporal variations in equivalence ratio on the NO_x emissions of a premixed methane-air flame was measured in a burner. The NO_x emissions are compared among steady flames with spatially uniform equivalence ratio distributions, steady flames with spatially nonuniform equivalence ratio distributions, and unsteady flames with temporal equivalence ratio fluctuations. Time-varying equivalence ratio was measured optically, time-varying temperatures were measured with thermocouples, and mean NO_x emissions were measured by probe sampling and a chemiluminescent analyzer. These measurements quantify the effect of temporal unsteadiness and spatial nonuniformity of equivalence ratio on NO_x emissions. For lean flames, both spatial nonuniformities and temporal fluctuations in equivalence ratio contribute to an increase in NO_x emissions with respect to steady uniform flames at the same mean flame temperatures. For lean flames, higher amplitude temperature fluctuations result in larger increases in NO_x with respect to steady flames. The dissertation also describes the optical technique for nonintrusive temporal measurements of equivalence ratio fluctuations and techniques for thermocouple compensation at frequencies up to 10 Hz. / Ph. D.

O presente trabalho simula numericamente os efeitos na transferência de calor radiativa a partir da injeção de fuligem no interior de uma câmara cilíndrica que opera com chama turbulenta. São resolvidas equações de conservação de massa, de energia, de quantidade de movimento, da variância da flutuação de temperatura, de espécies químicas gasosas e de fuligem para um problema físico conhecido, partindo da combustão da mistura de metano e ar dentro de uma câmara com dimensões e condições de contorno já exploradas em outros trabalhos a fim de possibilitar comparações de resultados. Para a turbulência é utilizado o modelo k - E padrão. Na modelagem das interações turbulência-radiação, é considerada a correlação combinada entre coeficiente de absorção e temperatura e a autocorrelação de temperatura. Foram adotados os modelos de Eddy Break-Up – Arrhenius para a combustão, utilizando a reação de combustão em duas etapas. O método de ordenadas discretas, considerando a soma-ponderada-de-gases-cinza (WSGG – do inglês: weighted-sum-of-graygases) é utilizado para calcular o termo fonte de calor radiativo. A dependência espectral das propriedades radiativas do meio participante foi modelada pelo método WSGG, que permite a solução de problemas com concentração variável das espécies participantes com alto nível de confiabilidade. A simulação da injeção de fuligem foi realizada alterando as condições de contorno do problema, resolvendo-se os cálculos de forma acoplada. Os resultados obtidos após a injeção externa da fuligem foram comparados com simulações que apenas consideravam a formação natural e a posterior oxidação das partículas. Foram analisados os campos do termo fonte de calor radiante em toda a câmara, que mostraram aumento sensível da radiação após a injeção ser considerada. Comparou-se também o fluxo de calor que atinge as paredes da câmara, como principal análise do presente trabalho, indicando que mesmo injetando pequenas quantidades de fuligem, há um aumento no fluxo de calor. O campo de temperatura não apresentou alterações consideráveis, apenas reduzindo-se a temperatura máxima no interior da câmara. De uma forma geral, o efeito da fuligem é mais significativo nas regiões de alta temperatura. / This work simulates numerically the effects on radiative heat transfer after the soot injection into a cylindrical combustion chamber that operates with turbulent flames. A known physical problem of burning methane with air inside a chamber is considered. The dimensions and boundary conditions were already considered in other papers to enable comparisons between the results. Conservation equations for mass, momentum, gaseous chemical species and soot, energy, and temperature variance equations, are solved. The turbulence is modeled by standard k -E model. Consideration of TRI (Turbulence-Radiation Interactions) effects is made through a methodology that considers both cross-correlation between absorption coefficient and temperature self correlation. The combustion model is Eddy Break-Up – Arrhenius, with two steps for the combustion reaction. The radiative heat source term is calculated with the discrete ordinates method, considering the weighted-sum-of-gray-gases model (WSGG). The spectral dependence of the participant media radiative properties was modeled by WSGG method, which allows the solution of problems with varying concentration of the participating species with high level of reliability. The simulation of soot injection was performed by changing the contour conditions of the problem. The calculation was solved in a coupled way. The results obtained after foreign soot injection were compared with simulations which only considered the natural formation and subsequent oxidation of the particles. The fields of the radiative heat source term showed significant increase of radiation after the soot injection was considered. The radiative heat flow that reaches the chamber walls is compared between the cases, as one of the main analysis of this work. It indicates that even when small amounts of soot injection are considered, there is an increase in the radiative heat flow to the walls. The temperature behavior showed no significant change, except on reducing the maximum temperature within the chamber. In general, the effects on the radiative heat transfer after the soot injection are greater in the high temperature areas.