Key Techniques Research For High-temperature Flue Gas Velocity Measurement System

With the rapid development of Chinese national economy,the installed capacity of coalfired power plants has been increasing,which has directly led to the emission of large amounts of sulfur dioxide,nitrogen oxides,and other air pollutants.It has also brought with it environmental problems such as acid rain and haze.The accurate monitoring and fine control of important parameters in the operation of boilers have put forward higher requirements.The real-time measurement of the inlet flue gas flow rate of the SCR system provides important guidance for the ammonia injection process,which plays an important role in increasing the efficiency of denitration and reducing the emission of NOx.Precise measurement of tail gas emissions from power plants is of great significance for real-time monitoring and effective control of pollutant emissions.For the measurement of gas flow velocity in power plant,the hot-wire anemometer,optical method and acoustic method are easily disturbed by dust and noise.And the pitot tubes needs to be calibrated corrected regularly owing to the effect of the fluid density.The electrostatic velocity measurement method has achieved good application in the measurement of primary wind speed in power plant as one of the low cost,stable and reliable measurement methods.However,because of the larger structure of the probe,it has not been popularized in other applications.This paper aims to solve problems in the measurement of the flue gas velocity in the power plant by improving the existing measurement method and the structure of the probe based on the basic principle of electrostatic velocity measurement.At present,the following problems need to be solved in the measurement of high temperature flue gas velocity.The main issues include:(1)The size of the cross section of the power plant flue is large.With few straight pipe segments and uneven flow field distribution,single-point velocity measurement methods such as Pitot tube can not meet the measurement requirements.At present,the commonly used multi-point matrix measurement device takes the flow of the flue gas in the pressure tube during actual measurement,which does not meet the pressure required when the pitot tube velocity is measured.This leads to lack of sufficient theoretical basis for measurement results.(2)There is a lot of noise,vibration and other disturbances in the environment of the power plant.The temperature of the SCR inlet flue gas is up to 350° C,which puts forward higher requirements on the high temperature performance of velocity probes.More demanding,there are few related reports on the real-time measurement method of the SCR inlet flue gas flow rate.(3)The traditional static velocity measurement method is actually measuring the velocity of charged particles in the airflow,but the concentration of particles in flue gas st the tail of the power plant is at a very low level,which results in very few charged particles.Therefore,the conventional electrostatic velocity measurement method is not suitable in the tail of the power plant.In response to the above problems,the following research work was carried out in this paper:1.In view of the problem that the existing intercorrelation calculation process is timeconsuming and two signal amplitude are greatly different,the signal preprocessing method of amplitude correction is proposed to improve the measurement accuracy,and a cross correlation fast peak finding algorithm is proposed to achieve real-time acquisition of transit time and velocity.Through the analysis of measured inductive signals,the effects of induced signal amplitude and noise interference on the transit time calculation accuracy were studied.The signal preprocessing method using FFT filtering and amplitude correction was proposed to improve the measurement accuracy.Compared with the direct correlation peak seeking algorithm,the single-cycle average time taken to obtain the transit time via the fast cross-correlation peak finding algorithm based on reduced sampling is reduced by more than 50 times under the same velocity measurement precision.2.The flow velocity distribution of the flue gas in the SCR inlet of the power plant is not uniform,so the local average velocity of the wide area is not significant to the guidance of the ammonia injection process.Aiming at this problem,the design method of electrostatic probe,which is matrix,miniaturization,high temperature resistance and wear resistance,is put forward to measure the velocity distribution of high temperature flue gas.The influence of different electrode width and spacing on the measurement results is compared by experiments,and the optimum electrode width and spacing of the miniaturized probe are determined.The use of martensitic stainless steel and ceramic structures enhances the adaptability of velocity probes to high temperature and high ash environments.On this basis,the signal acquisition and processing module is designed to realize the real-time,accurate and continuous measurement of flue gas velocity.The multi-velocity synchronous measurement system based on ARM and LabVIEW was developed in combination with an electrostatic velocity probe array.The field measurement signal was used to simulate the synchronous measurement of the flow rate of the SCR inlet multi-channel flue gas,indicating that the completion of the system can provide effective flow velocity distribution information for the operation adjustment of the SCR system.3.In view of the low concentration of charged particles in the low ash flue gas of electrostatic precipitator and the problem of insufficient SNR in traditional electrostatic speed measurement,a new method of gas velocity measurement based on ion tracing is proposed.High voltage ionization is used to produce tracer ions,which solves the problem of insufficient signal to noise ratio when the charged particles are scarce.Moreover,the charged ions of gas have better follower than the traditional charged particles.The structure parameters of discharge electrode and induction electrode were determined by studying the high voltage ion source and the inductive electrode of the ion velocity probe.The method is tested in a DC wind tunnel,and the relative error of measurement is less than 3.21%.In the flow measurement of high humidity and low ash gas,the relative error between the measured value and the measured value of the pitot tube is less than 2.17%,and the change trend of the measured gas velocity is basically consistent with the trend of the change of the boiler operating load.