Numerial Studies On Pressurized Oxy-fuel Coal Particle Ignition And Volatile Combustion

As one of the most common fossil fuels,coal will still play an important role in Chinese power generations for a long time.Coal-fired power plants consumed most of the coal resources in China,and this means power plants are the most important source of CO₂ emissions.The development of CO2 capture technologies in coal-fired power plants has become an important strategic research project in China.Among various carbon capture technologies,oxy-fuel combustion is one of the most promising technology to be commercially applied and large-scale promoted because of its technical feasibility and economic advantages.Pressurized oxy-fuel combustion is proposed for the improvement of traditional oxy-fuel combustion under atmospheric pressure.Under elevated pressures,the latent heat of vapor in the flue gas can be recovered and the initial cost of power plant construction can be reduced.Under elevated pressures and O₂/CO₂ atmospheres,the combustion characteristics of coal particles are basically different from those under atmospheric pressure and air condition.As one of the most important process of coal particle combustion,devolatilization and volatile combustion play a key role in the flame stability,pollutants formation and flame extinguishing.Investigation on the devolatilization,ignition and volatile combustion is significant for the development of clean and efficient utilization technology of coal.However,the devolatilization process occurs rapidly and it is difficult to measure.The developments of numerical models provide effective methods to investigating this process.In the previous studies,the volatile composition of coal particles was usually assumed to be a mixture of common flammable gases and hydrocarbons,and showed good agreement.Based on the assumption,the homogeneous reactions of hydrocarbons in different conditions were studied.A three-dimensional comprehensive model was built for the auto ignition and volatile combustion process by coupling the 1-D coal particle model and CFD frameworks,and quantitative analysis on the influence of atmosphere and pressure were made.The results obtained are summarized as follow:1.Based on the assumption of volatile composition,a methane jet flame,Sandia Flame D,was selected for the validation of numerical model and quantitative analysis.The effect of pressure elevations and atmosphere changes on homogeneous reactions was studied by the simulation of different oxidizer composition and ambient pressures.The tabulated chemistry model for gas-phase turbulent combustion was built and validated.The simulation results showed that under the same ambient pressure,the O₂/CO₂ atmosphere made the jet flame shorter and smaller in its geometry,and produced more CO and H₂O in the combustion area which fuel was rich.Moreover,the effect of pressure elevation was studied.Results showed that under elevated pressure,the changes caused by O₂/CO₂ atmosphere on the distribution of important radicals were similar to those under atmospheric pressure.With the pressure elevation,the reaction rate increased and made the flame shorter.High reaction rate also led to a promotion in flame peak temperature.The changes of species distribution caused by pressure were obviously different in different regions.In fuel-rich regions,the pressure elevation mainly caused the faster transfer from fuel to intermediate products,such as CO and other radicals.But in fuel-lean regions,the pressure elevation accelerated the consumption of intermediates,leading to low mass fraction of CO.2.The drying,devolatilization and volatile combustion processes of coal particles were numerically studied.A three-dimensional comprehensive model for the simulation of unsteady coal particle ignition process was built.The model combined a detailed 1-D drying and devolatilization model with 3-D CFD frameworks to simulate the volatile mass transfer and ignition caused by volatile homogeneous reactions.The ignition processes observed in experiments were reproduced for both air and O₂/CO₂ atmosphere based on the comprehensive model,and some characteristics which are hard to get in experiments are obtained in the simulation.Compared to the experimental data,the numerical results showed that the model was reliable and was able to study this process in different conditions.Results in different atmosphere showed that in the O₂/CO₂ atmosphere with 21%O₂ in volume,the ignition delay of coal particle was evidently longer than that in the air,and the gas-phase peak temperature is lower because of the higher heat capacity of CO₂.While in the O₂/CO₂ atmosphere with 30%O₂,the ignition delay and temperature were closed to the air-fired.The unsteady ignition characteristics of coal particles under pressurized condition were predicted.Lower gas velocity under elevated pressure and faster reaction rate caused more local heat accumulation,and as a result the local gas temperature increased faster.Thus,the ignition delay of coal particle was shorter and peak gas temperature was higher with the pressure elevations.3.Based on the tabulated chemistry model,the complex process of solving chemical reactions and species transport equations in the 3-D comprehensive model is greatly simplified.In the tabulated chemistry model,some control variables were introduced to the equation system to represent the flame characteristics.A new 3-D comprehensive model for coal particle ignition was built.More transport equations of control variables needed to be solved instead of solving reactions,and as a result the computational cost of new model with tabulated chemistry were reduced markedly.Numerical results of two new models,with SFM and FGM respectively,were compared to the traditional one which solved all reactions and species.Results showed that FGM model was more reliable in the simulation of ignition because of the progress variable which represented the extents of reactions.While the SFM model based on steady non-premixed flame was not able to reproduce the typically unsteady process.SFM model also overestimated the peak temperature of volatile combustion for the same reason,but FGM model simulated the gas temperature well.The comparison of important species and radicals distribution also showed that FGM model had great advantage on SFM when applied to simulate the ignition process.