| Accurate parameter measurement of solid flow in pneumatic pipelines is critical to improving production efficiency and reducing pollutant emissions.However,as the nature of gas-solid two-phase flow is complex and the pneumatic conveying conditions in power plants are dilute,the flow parameters measurement of pneumatically conveyed solids is recognized as a long-standing problem in both academia and industry.Solid flow measurement using acoustic emission(AE)method is based on the principle of particle collision.Since the AE signal contains various information about the solid flow,it is difficult to extract a single flow parameter from the AE signal.The electrostatic method works on the principle of triboelectric electrification of particles,then the flow conditions of solid flow in the partial and full cross-section of the pipe can be detected using the specially designed electrodes.The relative relationship between the signals from different electrodes is less dependent upon the complex and changeable particle charging factors,which can provide supplementary information for AE signals.Therefore,by combining the individual advantages of AE and electrostatic sensors,a novel method for flow parameter measurement of pneumatically conveyed solids through AE detection and electrostatic sensing is developed.By means of theoretical analysis,system design and experimental studies,the working mechanism of AE and electrostatic sensors and solid flow characteristics are deeply studied.Measurement methods are proposed for common regular solids and highly irregular biomass solids with complex physical properties to realize the measurement of the key parameters of solid flow,namely solid velocity,mass flow rate,particle size distributiona,etc.With the advantages of high sensitivity,low manufacturing and maintenance costs,and suitablity for operation in harsh industrial environments,the instrumentation system combining AE and electrostatic sensors provides a new technical solution to the online flow parameter measurement of solids in pneumatic pipelines.The research work that has been conducted includes the following:(1)According to the sensing mechanism of AE and electrostatic sensors and the flow characteristics of solid flow in a pneumatic pipeline,an integrated sensing head and a corresponding multi-channel signal conditioning circuit are developed to realize the complementation of the two types of sensors.The integrated sensing head consists of an AE sensor with a waveguide and three arc-shaped electrostatic sensor arrays which are uniformly embedded in the pipe wall.A pneumatic conveying test rig is built for evaluating the performance of the instrument system under various test conditions and installation positions.(2)A novel method by combining AE and electrostatic sensors is proposed to achieve the mass flow rate measurement of pneumatically conveyed regular solids.The process of particle collision and AE signal generation is analyzed from the perspective of energy conversion.Subsequently,an analytical model that describing the relationship between the AE signal energy,mass flow rate and impact velocity of pneumatically conveyed solids is established.The solid velocity is determined by multi-channel correlation of the electrostatic signals from the arc-shaped electrodes.The mass flow rate of solid flow is calculated through the analytical model with the obtained AE signal energy and solid impact velocity.Experimental results show that for the mass flow rate measurement of regular solids in the vertical section of a pipe,the sensing head can be installed at any orientation away from the elbow with a relative error within ±6.52%;for the measurement of regular solids in the horizontal section of a pipe,the sensing head should be installed with the AE waveguide in the horizontal orientation,while the relative error of measurement results is within ±7.21%。(3)A novel particle sizing method for regular solids by combining AE and electrostatic sensors is proposed.The method is based on Hertz impact theory describing the relationship between the peak voltage of AE signal generated from a single particle impact,particle size and impact velocity.A peak detection algorithm based on threshold setting and local energy is used to extract the AE peak voltage,while Rosin-Rammler prediction is introduced to reduce the interference of overlapping impacts and airflow noise on detected peak distribution.The effects of different size ranges of solids and the installation positions of the sensing head on the instrumentation system performance are studied through a series of experiments by varying mass flow rates and impact velocities.The measured solid size distribution has a certain difference with the varying flow conditions,while the size of solids and installation position of the sensing head also affect the particle size measurement results.However,all in all,under different flow conditions,the standard deviation of the measured size distribution in each size segment is smaller than 2.73%,while the correlation coefficients between the measured and reference distribution is greater than 0.89.Therefore,the measured results achieve good repeatability and accuracy.(4)Flow parameters measurement of irregular biomass solids through AE detection and electrostatic sensing is proposed.Due to the complex physical and flow characteristics of biomass solids,it is difficult to achieve accurate measurement of biomass flow parameters.To tackle this challenge,the applicability of the methods for the measurement of regular solid velocity and mass flow rate to biomass solid flow is firstly analysed.The test solids are crushed wheat straw,saw dust and corncob,which are commonly used in biomass power plants.Experimental results demonstrate that the mass flow rate measurement method is effective for the biomass solid flow measurement with a relative error within 18%.The measured solid velocity of biomass solids is consistent with the conveying air velocity.The conveying capacity of air flow depends on factors such as the size and shape of the biomass solids.Secondly,in consideration of inelastic and irregular characteristics of biomass solids,the particle sizing method is improved.In the improved method,the inelastic impact model is used to describe the particle collision process,while the relationship between the shape characteristics of irregular solid and its impact size is analyzed.Experimental results demonstrate that the equivalent impact size obtained by the measurement of a large number of biomass solids can characterize the size of biomass solids.The measured particle size distribution has a certain difference under varying flow conditions,while the measurement results are repeatable with standard deviations of each size segment lower than 2.51%.The relative error between the measured average size and the reference value is within ±6.45%.Finally,based on the theoretical relationship between the vibration displacement spectrum and material properties,AE signals generated by different biomass solids are analyzed in the time-frequency domain.The variance contribution rate of wavelet packet coefficient is proposed for association analysis of different biomass types.Experimental results indicate that the variance contribution rates of wavelet packet coefficients of AE signals can effectively distinguish the different types of biomass,without being affected by varying flow conditions and the differences in signal amplitudes under specific conditions. |
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