Numerical Investigation On Combustion Of Ultra Supercritical Double Reheat Boilers

The thermal power unit is still the major unit in Chinese electricity industry.In order to achieve the goals of high efficiency and low pollutant emission,the ultra-supercritical unit with high steam parameters is considered as the future mode of thermal power industry.However,the bottle-neck of ultra-supercritical unit development is material,which is still underdeveloped.Given a steam temperature,the double reheat ultrasupercritical unit technology is a practical scheme to improve efficiency and reduce emissions.The major difficulties in the double reheat technology are the design of complicated heating surfaces and the adjustment of heat load.So it is considerably necessary to numerically investigate the operating characteristic of double reheat boilers.In this paper,a CFD model is developed to numerically investigate combustion and heat transfer in two 1000MW-class ultra-supercritical double reheat boilers.The simulation domain is discrete by 3D unstructured body-fitted meshes.The number of grids is determined based on mesh independence analyze.A reasonable boundary condition of heat transfer is established based on the characteristics of heating surface of tower boilers.Eulerian-Lagrangian method is adopted to simulate the combustion process of pulverized coal in the furnace.The Realizable k-ε model is used to calculate the turbulence.The DO model is used to calculate the radiative heat transfer in the boiler.Presumed PDF model is used to calculate the turbulent chemical reactions.The velocity distribution,temperature distribution,component concentration and heat flux distribution in the boiler are obtained by CFD simulation.The heat loads of different heating surfaces fit with the designed values.The effects of activation energy and OFA ratio are analyzed base on the simulation results.The changes of temperature field,heat flux distribution and NOx fraction are obtained among these cases.It is shown that the pulverized coal combustion has a sensitive range on activation energy while using the kinetic-diffusion model.When the activation energy is in the sensitive range,the CFD results of temperature,heat flux and coal burnout rate change obviously with different activation energy.When the activation energy increase,the burnout time of particle increase,the burnout rate decrease,and the high temperature zone moves up to the OFA area.When the OFA rate increases from 0% to 20%,the NO emissions decreases.The higher OFA rate help to reduce the emissions of pollutants.At the same time,with the increase of the OFA rate,the heat flux in screen heating surfaces increases,including superheater,reheater,and economizer.Therefore,the reasonable OFA ratio should also consider the effect of heat flux distribution on boiler heat transfer process.