Low NOx Combustion Retrofit For600MW Supercritical Boilers

This thesis aims at resolving the problem of higher-than-standard NOx emission fortwo600MW supercritical boilers (NO.3and NO.4). Based on literature research andprevious engineering experience, aretrofitschemecombining low NOx burner (LNB) andover fire air (OFA) was proposed.The associated problems of the proposed scheme wereinvestigated mainly through numerical simulation, experiments and industrial turning.The the retrofit project of the two boilers was conducted and the prospective goals wereachieved.Literature reviewshowed that the staging of secondary wind, separation the con-centrated and diluted coal streams in the primary air duct and the nozzle structure arethe key factors for LNB design, and the ratio and location of OFA affect both combus-tion efficiency and NOx emission. Therefor, during the process of LNB design, the re-lated factors are selectively analysed.This thesis designs a new LNB for#No.4boiler, in which combustion air was di-vided by four kinds of air streams: the centralair, the primary air, inner secondary airand out secondary air. The secondary air is swirl, while the primary air was not. Thespecific structure for separating the concentrated and diluted coal streamswas installedin the primary air duct to form circularly concentrated distribution of pulverized coalparticles outside the burner exit. The expansion outlets were installed at the duct exits ofthe burner to enlarge the central recirculation zone. According to the results of gasphase simulation and gas-solid two-phase simulation, the central recirculation zone wasobvious and pulverized coal concentration effect wasstrong. The new LNB was used inretrofit of#No.4boiler, and OFA nozzleswere also enlarged. Air flow field tests in-dicted that the new LNB had good swirling performance. After the hot-state adjustment,performance of#No.4boiler was tested. The results indicated that the operational safety,combustion stability and low NOx emissionwere achievedafter the retrofit. However,some problems, such as increase of loss of flying ash and high temperature corrosion,were found.Then, in the retrofit of#No.3boiler, improvements were applied to the LNB design, including the structure for separating the concentrated and diluted coal sterams in theprimary air duct. Pulverized coal concentration effect was strengthened.The structure ofnewly designed LNB, especially the optimal vane angle of the inner and outter second-ary airwere determined. In addition, the influences of some operational parameters, suchas ratio of the inner and outter secondary air, velocity of centralair, OFA ratio on theflow fieldwere numericalstudied. The hot simulations of the improved burner within thetwo layer OFA furnaceon the combustion and NOx emission were also conducted. Theimproved LNB was used in the retrofit of#No.3boiler. After hot adjustment, perfor-mance of#No.3was tested. The results showed that the retrofitof#No.3boilerwas bet-ter than that of#No.4boiler. Namely,the operational safety, flame stability, economic-performanceof#No.3boiler were further improved. The NOx emission of#No.3boilerwas controlled under300mg/m3.