Numerical Simulation on the Combustion Process in the Furnace of a 1000MW Lignite-Fired USC Boiler

The reserves of lignite in China are abundant, which are mostly used for coal power generation. Recently, as a mature, advanced and efficient power generation technique in the world, the Ultra Supercritical (USC) technique has become a necessary choice for China in the area of clean coal power generation. Lignite is a kind of low-grade coal with high moisture, ash levels and low heat value, which has high requirement for the combustion technique. In China no lignite-fired units rated at 1000MW have been in service so far. So in this paper, the numerical simulation studied on the combustion process in the furnace of a 1000 MW lignite-fired ultra supercritical unit to be built provides reference for the development of combustion system in 1000MW lignite-fired USC boilers of independent intellectual property rights.In this thesis a numerical simulation was done to investigate the pulverized coal combustion process in the first 1000 MW dual circle tangential firing single chamber lignite-fired USC boiler in China, employing the Computational Fluid Dynamics (CFD) code PHOENICS. In the numerical calculation of furnace process, standard k-εtwo-equation model was used for the simulation of gas phase turbulence flow, two fluid model based on Inter-phase Slip Algorithm for gas-solid two-phase flow, six-flux model for radiation, diffusion model for moisture evaporation, single-step reaction model for devolatilization, EBU-Arrhenius model for the burning of volatile and diffusion/kinetics model for the char combustion. The simulation of NOx emission is done as a post-processing procedure. The Finite Difference Method was used to disperse the differential equation. SIMPLEST arithmetic was used to solve the governing equations in the heterogeneous stagger structured grid system under the orthogonal coordinates.In the paper, the influences of over-fire air (OFA) ratio, OFA position, OFA velocity, primary air ratio, primary air velocity and secondary air distribution on the flow field, temperature field, char burnout and NOx emission were studied. And the results indicate that increase of OFA ratio or rising of OFA position reduces the NOx emission but is disadvantageous for char burnout. With the rising of OFA velocity char content at the outlet falls, however, the windage of gas temperature at the outlet increases. Moreover, the char burnout is more complete when the primary air ratio increases, at the same time, the possibility for the slagging of the water-cooled wall is larger and NOx emission increases. Raising the primary air velocity enhances the mix of coal flow, but the ignition of injection itself is defered. Considering the slagging of hot corner, char burnout and NOx emission, the obverse pagoda type air distribution is preferable.This thesis shows that numerical simulation of furnace process in pulverized coal fired boiler with proper mathematical models and calculation method is viable. The numerical simulation of temperature field and the concentration distributions of the combustion species in the furnace are reasonable and they can reveal the actual flow, heat transfer, combustion process and NO emission in boiler furnaces.