Simulation Research Of Flow And Chemical Reaction In SCR De-NO_x System For Coal-Fired Boiler

Thermal power plants are one of the main sources of atmospheric pollutant NO_x,and the emission standards of NO_x are becoming stricter.Selective Catalytic Reduction?SCR?flue gas denitrification technology has become the primary choice for the control of NO_x emission in thermal power units because of its advantages of small impact on unit operation,high denitrification efficiency and no secondary pollution.The structure of SCR denitrification system in power plant boilers is complex.The velocity distribution and uniformity of flue gas and ammonia mixture have great influence on denitrification performance.In order to improve the operation performance of denitrification system and prevent the blockage and corrosion of fly ash,flue gas guide plate and rectifier grid are added to make the distribution of velocity and reactant concentration as uniform as possible.And the denitrification reaction in SCR denitrification system is the fundamental factor affecting the performance of denitrification.The operating parameters which have great influence on denitrification reaction mainly include flue gas temperature,flow rate,ammonia-nitrogen ratio and so on.In SCR denitrification system,there is a temperature window of denitrification reaction,which has the higher catalyst activity.The flue gas velocity directly affects the mixing degree of NH₃ and NO_x and the residence time of the flue gas in the catalyst.Reasonable flow rate can ensure the uniform mixing of NH₃ and NO_x and the full reaction.The denitrification efficiency generally increases with the increase of ammonia-nitrogen molar ratio,but when the ammonia-nitrogen ratio is greater than 1.1,the ammonia escape amount will increase sharply,and the side effects such as ammonia oxidation will also be aggravated.It is necessary to determine the optimum ammonia-nitrogen ratio.Therefore,based on the mechanism analysis of denitrification reaction,the flow field,concentration field and influencing factors of denitrification reaction were simulated to improve denitrification efficiency,reduce ammonia escape and realize"ultra-low"emission of nitrogen oxides.Taking the SCR denitrification system of 1000MW boiler in a power plant as the research object,the optimal layout of diversion plate and rectifier grille was found through numerical simulation to improve the uniformity of flow field and concentration field.The results show that six arc-straight guide plates are arranged at two corner in the inlet flue of SCR denitrification system.The rectifier grille with a spacing of 0.2m and a height of 0.6m above the catalyst has a good effect on optimizing the flow field and concentration field.The velocity deviation of downstream section of ammonia injection decreases from 26.78%to 11.83%before modification,the velocity deviation of the inlet section of the first catalyst layer decreases from 30.1%to 12.9%,and the concentration deviation of NH₃ decreases from 11.4%to 3.3%.The SCR denitrification reaction was simulated by the software CHEMKIN.The mechanism of selective catalytic reduction denitrification was studied by sensitivity analysis,reaction path analysis,and surface coverage analysis.The mechanism of CHEMKIN denitrification reaction was coupled with FLUENT numerical simulation,and the effects of temperature,ammonia-nitrogen ratio and space velocity on denitrification reaction were simulated and analyzed.The results show that the temperature window of denitrification reaction is 330⁴20?,the denitrification efficiency can be maximized by controlling the ammonia-nitrogen ratio between 1 and1.05,and the ammonia escape rate is small.The denitrification efficiency is higher at low space velocity,and when the space velocity is greater than 4500h^-1,the denitrification efficiency decreases with the increase of the space velocity.