Optimization Study On Ammonia Injection And Mixing Device Of SCR System

NOx emissions of coal fired power plant are strictly limited by new emission standard of air pollutants for thermal power plants. Selective catalytic reduction (SCR) has been the inevitable choice for plants to control NOx due to its mature technology and high deNOx efficiency. The uniformity of the ammonia-nitrogen ratio is a key factor affecting deNOx efficiency of SCR system. In our nation, reserved mixing length is usually short because of space and load limitations in retrofit projects. So the optimization research on ammonia injection and mixing devices, especially developing ammonia injection and high-efficiency mixing devices, is of great significance. Based on the flue characteristic, the thesis puts forward a new design of ammonia injection and mixing unit, optimizes its structural parameters and verifies its applicability.The optimization study about ammonia injection and mixing devices includes three parts. Firstly, the flow field downstream a mixing element is analyzed with numerical simulation. The results show turbulent kinetic energy and turbulent energy dissipation rate reach its maximum at the end of longitudinally symmetrical cross-section of V-shaped element, and the location can be used for injecting ammonia. Secondly, testing system is set up according to the simplification of SCR flue duct, and the influence of structural parameters on mixing performance of ammonia injection and mixing devices is researched with numerical model which has been verified by the testing data. It is found that the amount of ammonia injection and mixing unit increases, and thus the dimensions of mixing elements decreases, initial standard deviation coefficient of concentration is lowered, vortex mixing is weakened, and the distance required for mixing shortens at first and then extends. The fitting result indicates mixing length is a quadratic function of the number of ammonia injection and mixing units. The vortex mixing intensity is stronger and the distance of action is longer with aligned arrangements of mixing elements than that with staggered arrangements. The cross-section coverage ratio of V-mixing elements expands, mixing length reduces, pressure drop rises, and mixer efficiency is the highest and its value is0.957when the coverage ratio is0.25. The angle of V-shaped element increase, energy consumption linearly goes up. When the angle is equal to90°, mixer efficiency is larger than others. The concentration distribution of testing system with trapezium or rectangle blades is more homogeneous than that with triangle or ellipse blade. At last, the mixing performance of ammonia injection and V-shaped mixing devices in SCR system of certain600MW coal-fired plant is tested and compared with the combination of certain ammonia injection grid and swirl mixer. The results show the standard deviation coefficient of concentration at the entrance of catalyst is lower than5%when ammonia injection and V-shaped mixing devices is adopted.Furthermore, its comprehensive performance can match with the combination scheme.