Analysis Of Flow Field In The Back-end Flue And Research On Flow Measurement Method Of Coal-fired Power Plants

Under the background of carbon peak and carbon neutrality,China’s coal consumption will inevitably decrease.However,thermal power still accounts for a high proportion of China’s power generation equipment.Therefore,the large amount of gaseous pollutants produced by coal burning in thermal power plants is still the focus of China’s environmental protection agencies.Accurate measurement of flue gas emissions is the basis for the monitoring and control of pollutant emissions in coal-fired power plants.Only when the flue gas emission is measured correctly,the emission of pollutants can be effectively calculated.According to the layout rules of CEMS(Continuous Flue Gas Emission Monitoring System)for thermal power plants,flow measuring points are generally arranged in the back-end flue.The length of the straight pipe section of the back-end flue is too short.It is difficult for the flue gas field to develop fully inside it and there are secondary flow and recirculation areas,this in turn leads to large fluctuations or even no value for a long time in some measuring points in the actual measurement process,so the accuracy of online flow monitoring is low.The main purpose of this research is to explore the flow field distribution in the back-end flue of a coal-fired power plant,overcome the shortcomings of the existing flow measuring point layout scheme,and propose a stable measuring point arrangement scheme with higher accuracy and fewer points.The back-end flue from the induced draft fan to the flue-gas stack after desulfurization in a 600 MW thermal power plant is used as the research object.First,the flow in the flue under the condition of a single inlet velocity distribution is simulated.Based on the analysis of the simulation results,it is preliminarily determined that the velocity distribution on the cross section at 1m from the flue exit is relatively uniform,with less secondary eddy currents and less affected by the recirculation zone.At the same time,there is also a certain distance from the exit of the flue,which can be used as a suitable measuring point layout section.The appropriate measuring point layout section is verified and determined under a variety of inlet velocity distribution conditions.After the section is determined,the specific measuring point arrangement position on the section is studied.First,the velocity distribution cloud map and the velocity component distribution cloud map on the selected section under various inlet velocity distribution conditions are analyzed,and the flow measurement point should avoid the secondary flow intensity action zone and the recirculation zone.The measurement points are located on the left side of the flue,the non-central area in the middle and not too close to the wall.The method of numerical integration is used to calculate the height of the measuring points.Under the Gauss method and Chebyshev method,four height layout schemes are obtained respectively.The average velocity in the vertical direction of the selected section under various inlet velocity conditions is compared,and the average velocity at 0.3m and 2.5m from the left side of the flue has a good consistency.The undetermined coefficient method can be used to regress the average speed of these two positions to the true value.The regression overdetermined equation is solved and it is obtained that different coefficients need to be applied to the measurement results of 0.3m and 2.5m from the left side of the flue.The inlet flow rate remains unchanged,and the inlet velocity distribution change conditions are used to simulate the measurement scheme.The results show that the 8-point layout scheme using the Gaussian method can meet the measurement accuracy requirements.Based on the above theory and simulation research,1:20 model and air medium are used for experiment.In order to ensure the validity of the experiment,the experiment model is verified by simulation.The simulation results show that the error of the measuring point arrangement scheme does not exceed 2%.The air output is changed by adjusting the fan valve and an anemometer is used to measure the internal measuring points of the experimental model.According to the experiment,the maximum relative error of the measuring point layout proposed in this paper is 2.45%.The equal area method and log-linear method are inferior to the Gaussian 8-point measurement method.It is confirmed that some measuring points are affected by the recirculation zone and the secondary flow.In contrast,the Gaussian 8-point measurement scheme has higher accuracy and better stability.Reasonable measurement cross-sections and feasible measurement point layout methods are obtained through simulation research and experimental verification.The Gaussian 8-point measurement method is proven to achieve the accuracy and stability of the measurement.The cross-section layout and the number of measuring points of the CEMS measuring system are perfected by this conclusion.This conclusion provides theoretical support for the flow measurement of complex flows with large cross-sections.The correct measurement of total pollutant emissions is guided.