Large-eddy Simulation Of Pulverized-coal Pyrolysis,Combustion,and Alkali Metal Release And Reacting Dynamics

The energy structure of China is mainly based on coal.The combustion and utilization of coal resources make great contribution to the steady and rapid development of China's enconomy.However.at the same time the pollutants generated during coal combustion process,such as SO₂ and NO_x.cause serious environmental issues.In addition,the alkali metal species released during coal combustion can accelerate the fouling,slagging and corrosion of heat transfer surfaces in coal-fired boilers,and therefore severely affect the safe operation of the boilers.Hence,there is an urgent need to develop efficient&clean coal combustion and utilization technologies,such as the coal poly-generation technology.Development of efficient&clean coal combustion and utilization technologies requires a deep understanding of the pyrolysis,combustion and pollutant release and reaction process in pulverized-coal turbulent reaction.By using a combined experimental and numerical research method,the present work investigated several key issues in the research of efficient&clean coal combustion and utilization,i.e.,pyrolysis characteristics of coal,pyrolysis/combustion characteristics of turbulent pulverized-coal flow and reacting characteristics of alkali metal species in turbulent pulverized-coal flame.First,in order to further understand the coal pyrolysis process,the pyrolysis characteristics of two Chinese coals?Zhundong brown coal and Datong bituminous coal?,two biomass materials?wheat husk and corn stalk?,and their blends were systematically investigated.A self-designed single-particle reactor system was used to measure the time history of the mass loss and temperature rise of samples simultaneously,with a significantly higher heating rate than TGA?thermo-gravimetric analysis?.The effects of pyrolysis temperature,particle diameter,and fuel type on the pyrolysis characteristics of coal/biomass and the effects of blend mixing condition on the pyrolysis characteristics of coal-biomass blends were studied.A numerical method coupling the chemical percolation devolatilization?CPD?model with a particle energy equation was employed to analyze the pyrolysis process.By comparing the experimental measurements and simulation results,the pyrolysis characteristics of coal particles under various conditions can then be better revealed.Then,the pyrolysis characteristics of pulverized-coal particles in a hot turbulent nitrogen jet were investigated using large-eddy simulation?LES?.In the present study,both an advanced pyrolysis model,the CPD model,and a simplified pyrolysis model,the single first-order reaction model?SFOM?were incorporated into LES and the simulation results of the two methods were compared.In addition,the effects of important parameters,including the particle diameter,coal type,coal-feeding rate,carrier-phase velocity,and pyrolysis temperature,on the pulverized-coal pyrolysis process were investigated through parametric studies.Next,the investigation of turbulent pulverized-coal jet was extended from pyrolysis conditions to combustion conditions.Two different types of pulverized-coal jet flame,i.e.,flame A?meathane-piloted?and flame B?ignited by a preheated gas flow?were investigated using LES with advanced pyrolysis and volatile combustion models.The CPD model,partially stirred reactor?PaSR?model,and kinetic/diffusion surface reaction model were used for pyrolysis,volatile combustion,and char combustion,which are the three main stages of coal combustion,respectively.To further explore the release and reacting characteristics of alkali metal species in pulverized-coal combustion,the present work proposed an approach to modeling alkali metal reacting dynamic in turbulent pulverized-coal combustion using tabulated sodium chemistry.The chemistry table was built from a series of zero-dimensional simulations of chemical trajectories of sodium species based on a detailed sodium chemistry mechanism.Three parameters,i.e.,the equivalence ratio,the mass fraction of the sodium element and the gas-phase temperature,were employed to define the initial conditions of the chemical trajectories.The three parameters,along with the progress variable that represents the progress of sodium reactions,were then the four coordinates of the sodium chemistry table.A validation study had been performed to compare the predictions on sodium species evolutions in zero-dimensional simulations using the chemistry table against directly using the detailed sodium mechanism under various initial conditions,and their agreement was always good.The sodium chemistry table was then coupled to LES of a pulverized-coal jet flame ignited by a preheated gas flow,in order to predict the sodium reacting dynamics in the pulverized-coal flame.Finally,the parallel efficiency of LESsCOAL,which is a LES solver for pulverized-coal combustion developed in the present work,on massively parallel supercomputers was investigated and optimized,especially for the particle module and radiation module of the solver.After the optimization.LESsCOAL was able to achieve a good scaling performance when using up to 3,000 cores on the UK national supercomputer ARCHER,which provides us the possibility to perform large-scale pulverized-coal combustion simulations in the future,such as in an actual industrial coal burner.