Modeling And Simulation Of Circulating Fluidized Bed Combustors: Solid Segregation, Radiative Heat Transfer And Coal Combustion

Sustainable development needs efficient energy production and low environmental impact. At short and medium terms, low emission of coal combustion may have an important role in this content, particularly in China. Circulating fluidized bed combustors (CFBC) with some particular advantages in this respect provide the availability of reliable design method and control tools.In spite of the great industrial and academic interest in circulating fluidized bed combustors, an effective method to design boiler with various possible fuel for CFB boiler and to scale-up furnace is strongly needed.This dissertation aims to develop an overall mathematical model of circulating fluidized bed (CFB) combustors on up-to-date theories and experimental data of previous research work.Since solid particles exhibit a wide particle size distribution in CFB boilers, a hydrodynamic model based on the semi-empirical approach is developed to approximate the local particle size distribution in CFB. The core/annulus flow structure is applied in this model and the particles in the bed are discretized into several size groups. The model accounts for the disintegration and shrinking of coal particles during the combustion process of each group of particles. It shows that coarser particles are gathered near the walls and the average particle diameter decreases along the boiler height, and this trend is more significant in the splash region.A three-dimensional model is developed to predict the bed-to-wall radiative heat transfer coefficient in the upper dilute zone of CFB combustors. The radiative transfer equation is solved by the discrete ordinates method. The Mie scattering theory is applied to calculate the absorption and scattering efficiency factors of particles existing in CFB combustors. The model considers the influences of particle properties (including particle size distribution, particle optical constants and solid composition) on the radiative heat transfer coefficient. Simulation results show that the particle properties have significant influences on the bed-to-wall radiative heat transfer coefficient in CFB combustors. The cluster renewal model is applied to predict the convective heat transfer coefficient. Therefore, the total heat transfer coefficient is simulated.A coal combustion model is developed combined to the hydrodynamic model and heat transfer model. It considers the devolatilization, volatile combustion and char combustion. The main gas concentration profiles in the CFB combustor are simulated.