Numerical Simulation Of The Combination Of Low Niteogen Oxides Combustion And SNCR Denitrification In A Down-fired Boiler

Currently,hard-to-burn coal such as anthracite and lean coal accounts for more than 40%of the power coal in China,which is characterized by difficult ignition,poor combustion stability and low burnout performance.Down-fired boilers（DFBs）with special chamber structure and air distribution are widely used to burn poor quality coal due to its high chamber temperature and long air/fuel stroke.However,in actual operation,one of the major problems of DFBs is that NOx emission is much higher than that of general coal-fired boilers,which can reach 1500～2000 mg/m³.To address this problem,a 660MW DFB is chosen as the research object in this paper,of which the whole process of low-NOx combustion and SNCR denitrification are simulated as a combination based on the actual operation data.Firstly,the mathematical models used to carry out numerical simulation are introduced,and sub-models such as realizableκ-εmodel,Lagrange stochastic trajectory model,two-competing-rate model,species transport model,diffusion/kinetics model,P-1 model and eddy-dissipation-concept model are used to describe the physical and chemical processes in combustion and SNCR denitrification.The computational domain of boiler is divided into structured grids and the boundary conditions of inlet,outlet,and wall are set.Then,simulation results of the original working condition show that NOx concentration and fly ash carbon content at furnace outlet under a full load are 1569mg/m³ and 4.15%,respectively,while the measured values obtained from engineering tests are 1652mg/m³ and 3.98%,with the relative errors controlled within 5%,which can prove a preferable reliability of selected model.Secondly,the combined model is used to simulate the technical transformation project of air staged low nitrogen combustion of boiler,where the effect of excessive air coefficientα,overfired air rateηand downdip angle of secondary air at F level on the flow field,temperature distribution and speices concentration are specifically analyzed.The study shows that asαincreases from 1.05 to1.25,the carbon content of fly ash at furnace outlet decreases by 2.18%and NOx concentration increases by 38.6%;asηincreases from 20%to 25%,the carbon content of fly ash at furnace outlet increases by 0.14%and NOx concentration decreases by 17.2%;as the downdip angle of secondary air at F level increases from 0°to 45°,the carbon content of fly ash at furnace outlet increases and then decreases while the NOx concentration tends to contrarily vary.When the downdip angle of secondary air at F level is 60°,the secondary air at F level will directly scour the wall of cold ash hopper,which is not conducive to the safe operation of boiler.Considering the combustion performance of pulverized coal,NOx emission and position of flame center in lower chamber,an excessive air coefficient of 1.15,a overfired air rate of 22%and a downdip angle of secondary air at F level of 30°are selected to obtain the optimized combustion where carbon content of fly ash and NOx concentration at furnace outlet are 4.31%and 896 mg/m³,respectively.Finally,the selective non-catalytic reduction（SNCR）denitrification system is investigated based on the results of combustion after low-NOx optimization.The optimal combination of spray guns is determined by evaluating the effect of guns at different furnace heights on NOx removal and NH₃slip.Accordingly,three optimization strategies are proposed for the atomization parameters of reductant:（i）applying diverse atomization diameters of spray droplets hierarchically owing to the diversity of evaporation time and evaporation distance at different furnace heights and guaranteeing a larger one in high temperature area;（ii）adjusting the atomization angle of sprayed droplets according to the uneven distribution of NO in the width direction of furnace chamber;（iii）modifying the flow rate of spray guns on the sidewall at a fixed total injection flow rate due to the whirlwind at arch nose where NO enriches.Through the above optimization steps,the simulation results indicate that it can improve NOx removal efficiency to 43.1%and reduce NH₃slip to 26.7 ppm;the ultimate NOx concentration at the furnace outlet is reduced to 509.6 mg/m³.Furthermore,both NOx removal efficiency and NH₃ slip of SNCR are positively correlated with normalized stoichiometric ratio（NSR）of NH₃/NO.Increasing NSR has little effect on improving NOx removal efficiency when it reaches1.5,while the amount of NH₃ leakage continues to increase.