The tracer gas dilution method can address the issue of significant measurement errors in flue gas flow caused by the complex flow field in large-diameter stacks of power plants. The method is traceable and operates on a measurement principle different from the conventional velocity-area method, making it a promising candidate for on-site calibration of flow measurements. To this end, this paper employs numerical simulation to analyze the feasibility and accuracy of the tracer gas dilution method for measuring flue gas flow in power plant stacks. On this basis, it studies the influence of the tracer gas dilution ratio and injection cross-section on measurement results. In addition, different tracer gas sampling schemes were designed to evaluate the stability of the measurements. The results demonstrate that, at a height of approximately 9D (where D is the stack diameter), the tracer gas achieves full mixing with the flue gas; both excessively high and low tracer gas dilution ratios can negatively affect the mixing efficiency; injecting the tracer gas at the flue section can effectively reduce flow measurement errors. Under 80% load rate, when the tracer gas is injected into the stack, the measurement errors vary considerably across different sampling schemes. However, the three-point sampling method demonstrates a stable and accurate performance, with measurement errors of only -3.59%, -0.69%, and -1.05% at the 3D, 8D, and 12D cross-sections, respectively. When the tracer gas is injected into the horizontal flue, the flow measurement errors for all sampling schemes remain within ±10%. Specifically, with three-point sampling, the errors at the 3D, 8D, and 12D cross-sections are 0.98%, -0.52%, and 0.21%, respectively—all within ±1%. These results demonstrate the feasibility and accuracy of the tracer gas dilution method for flue gas flow measurement in large-diameter stacks.

To improve the uniformity of NO_x mass concentration distribution at the outlet of SCR (selective catalytic reduction) denitration reactor of a 660 MW unit, this paper proposed an optimal ammonia injection strategy based on the kinetic model with the objective of minimizing the relative standard deviation of NO_x mass concentration distribution at the outlet of SCR. The on-site measured data was used as the inlet boundary condition, and CFD numerical simulation calculations were conducted by coupling turbulent flow, component transport, and chemical reactions in the reactor. The ammonia flow characteristics was visually analyzed, and the flow influence coefficient of the ammonia injection grid partition/nozzle was defined. The optimal controllable ammonia injection flow of different partitions or nozzles of the ammonia injection grid can be obtained by directly solving the optimization matrix equation, and combining the mathematical relationship between the outlet NO_x concentration distribution and the inlet NH₃ concentration, which is based on the SCR reaction kinetics model. The simulation results show that under the uniform ammonia injection mode, the mixing matching degree of ammonia nitrogen concentration in different zones is not high, the relative standard deviation of NO_x mass concentration at the outlet of SCR reactor reaches 40.1%, and the uniformity of NO_x mass concentration distribution at the outlet is poor. After the optimization of ammonia injection according to the method proposed in the paper, the relative standard deviation of NO_x mass concentration at the outlet decreases to 6.8%, and the uniformity of the outlet is greatly improved. This can not only realize the pressure line emission of NO_x to meet environmental requirements but also avoid areas with high and low denitrification efficiency. The flow field was visualized through numerical simulation in the article, and ammonia injection optimization was carried out by quantitatively solving the ammonia injection volume of each valve. This can provide theoretical reference for actual ammonia injection optimization debugging in power plants and reduce the blindness of adjustment.