Experimental Study And Numerical Simulation Of Co-combustion Of Coal And Biomass In Fluidized Bed

Co-combustion of coal and biomass is a promising technology,is considered to be a method to reduce the coal consumption and to dispose the agricultural and forestry waste effectively.In this dissertation,co-pyrolysis of coal and biomass and Thermal gravimetric analysis of co-combustion are studied.Experimental study and numerical simulation of co-firing coal with biomass are carried out in a fluidized bed.The present work is devoted to exploring the interaction,the characteristics and pollutant emissions of co-combustion.The experiments laid a good foundation for engineering scale up.In this study,coal and sawdust were used as the research object,the contents of two components(coal and sawdust)were firstly optimized according to the Simplex lattice design method.Then,the fast pyrolysis of two components and the mixture of the two compounds were conducted in a thermogravimetric analyzer(The pyrolysis carrier gas is N2 and air respectively)to explore the pyrolysis characteristics of the mixtures and the synergistic relationship between the two components.Moreover,the prediction model of the kinetic parameters of the pyrolysis process was established and verified.The exploration results indicated that the pyrolysis of the mixture of coal and sawdust in the low temperature section(410?540?)and the high temperature section(560?660?)show different dynamics,and the dynamic regression equation is obtained.The regression prediction model of the pyrolysis process of three components had a high accuracyly and could predict the kinetic parameters of coal and biomass pyrolysis effectively.The combustion of coal,saw dust and their mixtures were carried out in a self-developed small scale fluidized bed.The characteristics of combustion and pollutant emission of co-combustion were investigated.Influences of bed temperature,mixing ratio and excess air ratio were analyzed on the pollutant emission.The results show that the carbon content of flying ash decreases with increasing of bed temperature significantly.The mixing of sawdust improves the combustion of coal,the carbon content of flying ash decreases and this trend is much more obvious with increasing the proportion of sawdust.The volume fraction of CO decreases with increasing the proportion of sawdust.The volume fraction of SO2 increases with increasing of bed temperature.Sawdust has a significant effect on reducing SO2 emission and the effect is more significant with increasing of bed temperature.Excess air ratio has a great influence on the combustion in fluidized bed,with the increase of excess air ratio,the emission of SO2 decreases.The volume fractions of NO increases with increasing of bed temperature.With increasing the proportion of sawdust,the less emission of NO is observed and the magnitude of the reduction gradually decreases.Excess air ratio has great impacts on the emission of NO.With the increase of excess air ratio,the volume fraction of NO increases,and the magnitude of the increase decreases.Within the bed temperature range of 750 ?-900 ? in fluidized bed,the suitable excess air ratio for combustion range between 1.2 and 1.3.In this paper,regarding a fluidized bed combustor as a research object,comprehensive models of co-combustion in combustor were established,coal combustion and co-combustion processes were simulated respectively.Under the Euler-Euler method,a model was established to describe the gas-solid multiphase flow.The constitutive equations for solid phase are closed by the Kinetic Theory of Granular Flow,The gas-solid two-phase heat transfer and the radiation model were coupled to the flow model and interphase heat transfer process in fluidized bed was considered.In the co-combustion processes,relevant reaction models were applied to describe the processes of devolatilization,char combustion,volatile combustion.Coupling with the reaction models and the flow model and heat transfer model,a comprehensive CFD model of co-combustion based on Euler-Euler framework were established.The model was employed to predict and analyse the processes of co-combustion coal with biomass.The experimental verification shows that experiment results agreed with simulations well,proving that the comprehensive model had a certain accuracy and reliability.The hydrodynamics and the distributions of temperature and the combustion characteristics were simulated and analyzed in detail.Effects of the co-combustion of mixing sawdust in coal on flow field,temperature field and combustion characteristics were discussed.The numerical simulation reveals that when coal combusted with biomass,the overall combustion is better.The overall temperature in the fluided bed is lower under co?combustion condition due to the lower heat value of biomass.The whole bed reaches a steady state,the upper zone of the bed is dilute and the lower zone is dense.Inhomogeneity is obviously observed in the concentration of solid phase along radial direction.The particle clusters form in the near-wall zone and fall along the wall,which presents a typical bubbling fluided state.With the increase of the bed temperature,the O2 and CO concentration at oulet decrease while the CO2 concentration at oulet increases.