| Oxy-combustion technology is one of the most promising carbon-reduction technologies,which significantly reduces the costs of carbon capture.In this technology,pure O2 diluted with flue gas is used as the oxidizer.The products of oxy-combustion are mainly composed of CO2 and H20.CO2 can be easily separated and recycled from the combustion products to reduce CO2 emission.Because of the high CO2 concentration,the flame characteristics of fuel at O2/CO2 atmosphere are different from those in air condition.The studies of the flame characteristics of oxy-fuels are very important for the application of oxy-combustion.Now,the flame characteristics and basic characteristic combustion parameters of oxy-gas fuel mixtures are not clear.It is necessary to investigate the flame characteristics of gas fuel mixtures at O2/CO2 atmosphere.In this thesis,the experimental research and numerical simulation are performed to study the lamniar and turbulent combustion characteristics of oxy-methane mixtures,which provide the basic combustion characteristic parameters for the application of oxy-combustion.The laminar flame speeds of CH4/O2/CO2 mixtures were measured with a Bunsen burner in the condition of various equivalence ratios,oxygen concentrations,initial temperatures and dilutions.The kinetic simulations were performed with the PREMIX codes of the Chemkin package.Results showed that the laminar flame speeds of premixed oxy-methane flame reached a maximum value at the stoichiometric ratio and fell off on both sides.The laminar flame speeds increased as the O2 concentration increased and the maximum of flame speeds were 11.1 cm/s,15.7 cm/s,20.0 cm/s,26.1 cm/s and 29.6 cm/s corresponding to O2 concentration 25%,29%,31%,33%and 35%.The elevated temperature increased the laminar flame speeds and the elevated pressure decreased the laminar flame speeds of gas mixtures.The high CO2 concentration decreased the combustion reaction rate and flame temperature,which decreased the laminar flame speeds of CH4 at O2/CO2 atmosphere.The thermal,chemical and radiative effects of CO2 on laminar flame speeds were studied with the kinetic simulation.The thermal effect of CO2 was the major factor to decrease the laminar flame speeds of gas mixtures;the chemical effect of CO2 was the secondly factor;the radiative effect of CO2 was the thirdly factor.The effects of oxygen concentration and dilution gases on the flammability limits of oxy-methane mixtures were studied with a cylinder reactor.Results showed that the high CO2 concentration reduced the flammable range of CH4 at O2/CO2 atmosphere.It was found that the flammability limits of CH4/02/CO2 mainly depended on the thermal property of CO2 through the energy balance analysis.Based on the thermal theory,a revised model was deduced to predict the flammability limits of CH4 at O2/CO2 atmosphere.In order to reduce the computational cost,a minimal skeletal mechanism involving 20 species and 56 reactions was generated for methane at O2/CO2 atmosphere from the GRI 3.0 mechanism.The directed relation graph aided the sensitivity analysis method was used at first and then the principle component analysis method was used for the further reduction.Results showed that the minimal skeletal mechanism demonstrated to well reproduce the predictions from the detail mechanism and reduced the computational cost significantly.A partially premixed combustion model was constructed using the FGM method which coupled with the detailed mechanism.The effects of the oxygen concentrations,equivalence ratios and radiative heat loss on the turbulent flame characteristics of CH4/O2/CO2 were studied using this model.Results showed that the flame temperature,gas velocity and turbulent kinetic energy increased with the elevated oxygen concentrations.The flame temperature,gas velocity and turbulent kinetic energy of gas mixtures increased with the increase of equivalence ratios when the equivalence ratios were smaller than 1.0,but changed little when the the equivalence ratios were bigger than 1.0.The turbulent flame characteristic of CH4/O2/CO2 mixture was hardly changed with the effect of the radiative heat loss.A prediction model of the turbulent flame speed was developed with the FGM coupled G equation method in the large eddy simulation approach.The turbulent flame speeds of CH4/O2/CO2 mixtures were calculated with this model in the condition of the various oxygen concentrations and equivalence ratios.Results showed that the turbulent flame speeds increased with the increase of oxygen concentration.The turbulent flame speeds of gas mixtures reached a maximum value near the stoichiometric ratio and fell off on both sides.The elevated turbulent flame speeds reduced the flame wrinkles. |
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