Dong Jianxun and others studied the influence of ammonia concentration distribution on denitrification efficiency and ammonia escape through numerical simulation. Tang optimized the flow field of SCR reactor by numerical simulation. NISCHT et al. used numerical simulation and cold experiment methods to analyze the effects of inlet flue gas velocity, temperature and ammonia-nitrogen ratio on the flow field and pressure field in the back of SCR reactor. Liu Xiaobo and others established the corresponding mathematical model of SCR reactor, and optimized the flow field by adding deflector in numerical simulation, and verified the rationality of the optimization scheme. Zhu Tianyu and others used computational fluid dynamics to optimize the velocity, pressure and concentration distribution of flue gas in SCR reactor. Jiang et al. numerically simulated the flow field and concentration field of SCR reactor, and reduced the deviation between flue gas velocity and ammonia concentration by optimizing the design of guide plate. SCR technology has become the first choice for denitration of sintering flue gas because of its high denitration efficiency.
However, due to the low temperature, high dust content, complex composition and high humidity of sintering flue gas in iron and steel plants, the SCR technology in coal-fired power plants must be optimized according to the flue gas situation and flue shape. Among them, the most important problem to be solved is that the sintering flue gas temperature is low and the optimal reaction temperature range of SCR is not reached. In industry, sintering flue gas is usually reheated to raise the flue gas temperature to the catalyst reaction temperature range.
Direct introduction of coke oven flue gas into sintering flue gas will lead to uneven distribution of sintering flue gas temperature and flow rate, thus affecting the denitrification efficiency of SCR. Zhou et al. solved the problem of sintering flue gas heat supplement by turbulent heat mixing method of high temperature coke oven flue gas transverse jet. In this paper, FLUENT software is used to optimize the SCR denitration system of sintering flue gas in a steel plant, and the optimization scheme is verified in engineering.
1 numerical simulation method
1. 1 calculation model
The flue gas SCR denitration system added to the 600m2 sintering machine in a steel plant, the sintering flue gas enters the SCR reactor after passing through the injection device, deflector, rectifier and other equipment, and the nitrogen oxides are reduced to N2 by NH3 on the catalyst surface, and then reach the flue gas reheater [16]. See figure 1 for the reheating process of sintering flue gas.
Figure 1 Reheat process of sintering flue gas
The following indicators are required:
1) Under the design condition, the pressure drop of the whole sintering flue gas reheating system is not greater than1600 Pa;
2) The flow field distribution at the catalyst inlet of the first layer meets the following requirements:
A) The standard deviation coefficient of velocity is not greater than15%;
B) The angle of the flue gas incident on the catalyst layer (the angle with the vertical direction) is at most 65438 00;
C)c) The standard deviation coefficient of the molar ratio of nitrogen oxides to NH3 is not more than 5%.
The standard deviation coefficient is the percentage of the standard deviation of each part of the SCR reactor and the average value of some parameters.
1.2 geometric model
The simulation scope of computational fluid dynamics is the flue and internal structure between the inlet and outlet of flue gas reheater, including flue gas injection device, spoiler, deflector and rectifier of high temperature coke oven. The geometric model of the sintering flue gas reheat denitration system is shown in Figure 2.
In the numerical simulation, the grid division of the sintering flue gas reheat denitration system is shown in Figure 3.
Fig. 3 grid division of sintering flue gas reheat denitration system
The whole site adopts structured grid and part of unstructured grid. Modeling and gridding of 3D geometric figures in GAMBIT software. Because of the irregular geometric structure of the whole denitration device, it is necessary to generate grids in blocks to improve the calculation efficiency and accuracy.
1.3 mathematical model and boundary conditions
The numerical simulation of the flow field in the whole denitration device is realized by using the flow field simulation software FLUENT. Standardk-ε model is used to simulate turbulence. The component migration model is selected to calculate the migration and diffusion of flue gas components and ammonia in flue gas. Porous media model is used to simulate the flow resistance and pressure drop characteristics of rectifier and catalytic layer. For the rectifier, a large resistance coefficient is set in the direction perpendicular to the grid, and for the catalyst layer, the isotropic porous medium model is used to simplify it.
See table 1 for flue gas parameters. Sintering flue gas inlet and high temperature coke oven flue gas inlet are set as mass flow boundary conditions. The ammonia injection inlet temperature is 3 13K, the ammonia flow rate is 1 12.5kg/h, and the dilution air flow rate is 3500m3/h. ..
Table 1 flue gas parameters
2 Numerical simulation results and analysis
2. 1 simulation of reheating flow field of sintering flue gas
See Figure 4 for the temperature distribution of flue gas injection outlet of high temperature coke oven.
Fig. 4 Temperature distribution at flue gas outlet of high temperature coke oven
As can be seen from Figure 4, the temperature of sintering flue gas rises after the introduction of high-temperature coke oven flue gas, and after a certain distance, the temperature of high-temperature coke oven flue gas tends to be consistent with that of sintering flue gas.
Fig. 5 Temperature distribution of flue gas at the inlet of ammonia injection grid
See Figure 5 for the temperature distribution of flue gas at the entrance of ammonia spraying grid. It can be considered that the temperature distribution of flue gas is relatively uniform when it enters the entrance of ammonia spraying grid. After adding the sintering flue gas reheating system, the average temperature at the inlet of ammonia injection grid increased from 523K to 553K, which increased by 30K, providing a suitable temperature for SCR reaction in catalyst layer.
2.2 Flow field simulation without deflector
See Figure 6 for the velocity distribution of the longitudinal section of the sintering flue gas reheat denitration device without deflector.
Fig. 6 Velocity distribution of flue gas in longitudinal section without deflector.
As can be seen from Figure 6, when there is no deflector, there will be obvious uneven velocity distribution in the flue elbow area. The velocity distribution at the catalyst inlet of the first layer of SCR reactor without deflector is shown in Figure 7a, and the velocity standard deviation coefficient is less than 10%, which meets the technical requirements. The velocity vector distribution is shown in Figure 7b, and the maximum deviation angle of inlet velocity is 12.0, which exceeds the technical requirement of 10. The temperature distribution is shown in figure 7c, and the temperature deviation is 9.5K, which meets the technical requirements. The volume fraction distribution of ammonia is shown in fig. 7d. The uneven distribution of ammonia and high local concentration will cause local catalyst poisoning. The calculated standard deviation coefficient of the molar ratio of ammonia to nitrogen oxides is 6.28%, which does not meet the requirement of less than 5%, which will seriously affect the denitrification effect of SCR. The total pressure drop of the system is 1020Pa, which meets the technical requirements.
Fig. 7 Velocity distribution (a), velocity vector distribution (b), temperature distribution (c) and ammonia volume fraction distribution (d) at the inlet of the first layer catalyst without baffles.
2.3 flow field simulation after optimizing the deflector
The optimized deflector consists of two bent plates at the flue elbow and three short plates at the entrance of the reactor main body. Bending plate can effectively reduce the vertical component of rising flue gas, make it flow along bending plate, and reduce the occurrence of flue gas reflux; The short board can guide the flue gas above the spoiler and reduce the uneven speed.
See Figure 8 for the flue gas velocity distribution in the longitudinal section after optimization of the deflector. As can be seen from Figure 8, after setting the deflector, the uniformity of flue gas velocity at the flue elbow is obviously improved, and no flue gas backflow occurs.
Fig. 8 Flue gas velocity distribution in longitudinal section after optimizing deflector.
The velocity distribution at the catalyst inlet of the first layer of SCR reactor after optimizing the deflector is shown in Figure 9a, and the velocity standard deviation coefficient is less than 10%, which meets the technical requirements. The velocity vector distribution is shown in Figure 9b, and the maximum deviation angle of inlet velocity is only 8.2, which meets the technical requirements. The temperature distribution is shown in Figure 9c, and the temperature deviation is 7.8K, which meets the technical requirements. The volume fraction distribution of ammonia is shown in fig. 9d. Compared with the case without baffle, the uniformity is obviously improved, and the standard deviation coefficient of the molar ratio of ammonia to nitrogen oxides is reduced to 4. 1 1%, which meets the technical requirements. The total pressure drop of the system is 1039Pa, which meets the technical requirements.
Fig. 9 Velocity distribution (a), velocity vector distribution (b), temperature distribution (c) and ammonia volume fraction distribution (d) at the inlet of the first layer catalyst after optimizing the deflector.
3 engineering verification
Based on the optimization results of numerical simulation, the flow field of SCR system of sintering unit in a steel plant is optimized. The standard values and measured values of each index are shown in Table 2. As can be seen from Table 2, the measured values of all indicators have reached the standard values.
Table 2 Standard values and measured values of various indicators
4 conclusion
A) The SCR denitration system of sintering flue gas in a steel plant is optimized by numerical simulation. After the high temperature coke oven flue gas was injected into the sintering flue gas, the average temperature at the entrance of the ammonia injection grid increased from 523K to 553K, which increased by 30K, providing a suitable temperature for the SCR reaction of the catalyst layer.
B) When there is no deflector in the flue of the sintering flue gas reheat denitration device, the maximum deviation angle of the velocity at the catalyst inlet of the first layer of SCR reactor is 12.0, which exceeds the technical requirement of 10, and the standard deviation coefficient of the molar ratio of ammonia to nitrogen oxides is large, which is 6.28%, which does not meet the technical requirement of less than 5%. By adding guide plates at the flue elbow and the reactor inlet, the recirculation zone in the flue is effectively eliminated. The maximum deflection angle of the inlet velocity of the first layer catalyst is only 8.2, and the standard deviation coefficient of the molar ratio of ammonia to nitrogen oxides is reduced to 4. 1 1%, which all meet the technical requirements.
The denitrification rate of SCR reactor is 82.6%, which meets the standard requirements. The numerical simulation results provide a guarantee for the efficient operation of SCR denitration device for sintering flue gas.
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