Simulation Optimization Of Flow Field And Reactants Blend On Denitration Reactors

Nitrogen oxides(NOx) is an important atmospheric pollutant. More than 90% of it comes from fossil fuel combustion.70% among them comes from coal combustion. Nitrogen oxides directly or indirectly do serious harm to people, plant, building and environment. With the fleet development of our country's economy, the exhaust amount of NOx will continue to increase. On the other hand, the series standards and power industry environment protection put more request to thermal power plant and give an important place to thermal power plant emission harness. Developing high-efficient NOX control technology is a urgent affairs.The selective catalytic reduction(SCR) technology has been used widely in Europe and Japan but has little use in China which has the advantage of the high efficiency and reliability, no byproducts, simple device, and so on. Research is now focus on how to improve efficiency, extend the catalyst life. We can simulate SCR DeNOx system to provide important parameters for SCR system design and improvement of flow duct, guide vanes and the injection system.The microcosmic reaction process was studied by using gaseous diffusion and mass transfer principle, the chemical reaction model of reducing NOx was constructed. The ways and means of improving SCR effect was studied theoretically. Take one 660MW unit of our country as an example, by comparison of different schemes, the optimal flow field and AIG layout scheme was established.By analysis of chemical reaction, microcosmic process and arrangement of SCR system, a denitration velocity model was established. Following can be concluded: Numbers of reactions involve in denitration process, but reaction:NH3+4NO+2O2= 4N2+6H2O is the dominant one among all reactions with common flue gas ingredient and common reaction condition. SCR denitration microcosmic process include 3 stages:they are outside mass transfer, inside mass transfer and face chemical reaction.On certain condition the reaction velocity is proportional to the reaction material concentration in flue gas and the micropore cross-sectional area.The SCR syetem denitration efficiency and NH3 slip is contrary.By simulation the influence of catalyst and AIG to inlet duct flow field, following can be gained:The arrangement of catalyst can increase the system flow resistance, but it can't improve the upstream inlet duct Y-direction velocity distribution. AIG can improve inlet duct Y-direction velocity distribution, improve the AIG upstream velocity uniformity, but it increase the system flow resistance at the same time.The arrangement of guide plates may improve flow field, it increases the system flow resistance at the same time. Properly arrangement of guide plates may improve flow field uniformity to CV≤15%, at the same time the NH3/NOx mixing is effective. Local deflection of flue can cause high velocity erosion which can be avoided by using of anti-grind shield plate.By simulation of SCR reactor flow field after inlet duct optimization and the simulation of the effect of guide plate and commutated gridiron, following can be gained:The SCR reactor flow field uniformity can't come true only through inlet optimization, the velocity increases gradually from left to right. CV≤15% is hardly an realistic object. The commutated gridiron may improve flow field at some extent which is related with the gridiron space between and height. The space between should be proper. The flow field uniformity increases with the gridiron height with certain space between, but system resistance become larger. When guide plates were installed with commutated gridiron at the same time, the phenomena of velocity increasing gradually from left to right due to centrifugal effect can be removed at some degree to improve the flow field uniformity, CV≤15% can be realized.By simulation of the SCR reactor concentration of NH3 of different AIG schemes, following can be gained:The effect of single layer nozzle is better than double layer of it when the nozzle number is certain, but the flow resistance is larger than that of the latter, but the fomer has no this phenomena. The direction of spout tube has no influence on mixing effect for single layer nozzle arrangement. X direction is better than Z direction when there are a few nozzles. With increase of spout tube number, the disturbance difference become smaller and smaller, the mixing result difference become smaller and smaller also. The direction of spout tube has little influence to flow resistance also. The resistance of X direction is smaller than Z direction, but the difference become smaller and smaller with the increase of the number of spout tubes.No matter single layer or double layer, the mixing effect become better and better with the increase of the number of nozzles.By analysis of the SCR system of different work condition, following can be gained:The mean velocity of inlet duct and reactor and system resistance change with the load changing, but the velocity uniformity before AIG and the 1st catalyst change little, all CV≤15%, that means the SCR system arrangement designed from BMCR condition can suit for changeable load for velocity uniformity.When different coal is used, excess air factor, theoretical and real combustion air amount and NOx in the flue gas are all different. But the mass concentration of NH3 in flue gas remain invariant and equal to it of BMCR condition,Cρ≤5%. That means the AIG scheme for BMCR condition is suitable for other conditions to NH3/NOx uniformity.