## 2 CFD Simulation and FMT Validation  
### Facility Optimization  
The schematic geometry of a tower type SCR-deNO\({}_{x}\) facility for a 1000 MW coal-fired power plant is shown in Figure 1. The power plant is located in Tuandong Town, Neixiang County, Nanyang City, Henan Province of China. It belongs to the State Development and Investment Corp. The overall height of the SCR-deNO\({}_{x}\) facility is about 100 m and the flow rate of the flue gas is about 1030 kg/s with a temperature of 647.15 K under 100% BMCR condition. The flue gas inlet size is about 21 m \(\times\) 6 m and the sectional area of the catalyst layer is approximately 430 m\({}^{2}\). Figure 1: The schematic geometry of a tower type SCR-deNO\({}_{x}\) facility for a 1000 MW coal-fired power plant. AIG: Ammonia Injection Grid. The flue gas leaves the economizer and passes the first turning. Then it mixes with NH\({}_{3}\) that is injected by the AIG. After that, the flue gas goes through the hybrid grid and the expansion duct. Finally, it enters the SCR reactors, which contain two catalyst layers. The mixing distance for flue gas and NH\({}_{3}\) is approximately 25 m from the AIG to the catalyst entrance. The relative standard deviation (RSD) at the entrance of catalyst layers is used to evaluate the uniformity of both gas velocity and NH\({}_{3}\) concentration distribution, which is defined as [24; 25]:  
\[\text{RSD}=\frac{\sqrt{\sum_{i=1}^{n}(x_{i}-\overline{x})^{2}}}{\left(n-1 \right)^{1/2}\cdot\overline{x}} \tag{3}\]  
where \(x_{i}\) is the \(i\)-th measured value of the variable, \(\overline{x}\) is its average value, and \(n\) is the total sampling number. Usually, the RSD is set to less than 15% for the flue gas velocity distribution and less than 5% for the NH\({}_{3}\) concentration distribution. In the facility design stage, pressure loss is also a very important issue because it is directly related to energy consumption. Correspondingly, a total pressure loss of less than 400 Pa is required excluding the catalysts' pressure loss. In addition, with the promotion of China's high-quality development, the consumption of steel plate also needs to be considered. For this tower type SCR-deNO\({}_{x}\) facility, optimal design is required with spatial constraints. No baffles are allowed to be installed inside the first turning because it is too close to the economizer. Therefore, it is more difficult to achieve the requirements of RSD mentioned previously and to control the pressure loss. A variety of regulation measures should be taken during the design stage of the SCR-deNO\({}_{x}\) facility. Arc-shaped baffles with small sizes are used due to their good performance in regulating the flow field while reducing the total pressure loss. Meanwhile, attempts also are made to regulate the flow field at the AIG as a potential solution to spatial constraints. Installing the baffles at the upstream side of the facility would reduce the difficulty in regulating the downstream flow field, especially for the expansion duct. Correspondingly, a non-uniformed NH\({}_{3}\) injection strategy was adopted to increase the uniformity of NH\({}_{3}\) concentration distribution. Considering the large sectional area and insufficient turbulence, it was necessary to use the non-uniformity NH\({}_{3}\) injection strategy to enhance the mixing performance. In this case, the AIG had 21 independent NH\({}_{3}\) injection areas (3 \(\times\) 7) and the NH\({}_{3}\) injection velocity of each area was set to be proportional to the gas velocity based on the gas velocity distribution before the AIG. With the flow field regulated at the AIG, it was much easier to determine the injection velocities. The optimization process was carried out based on CFD simulation and FMT validation. Firstly, the CFD simulation of the tower type SCR-deNO\({}_{x}\) facility with no baffles geometry was performed as an indication of the flow field. Secondly, on the basis of the preliminary simulation, the baffles geometry was designed.