Research On Monitoring Technology Of Coal Gasification Waste Boiler Ash Deposition Based On Coplanar Capacitance Principle

Coal gasification technology is one of the core technologies for efficient and clean utilization of coal.The waste heat boiler process is the main method for achieving high-temperature synthesis gas sensible heat recovery in coal gasification devices.A large amount of slag particles carried by high-temperature syngas is likely to cause ash accumulation and slagging in the waste heat boiler,and it may cause slag blocking accidents in severe cases.It is urgent to develop a monitoring technology that can accurately measure the thickness of the ash residue in the waste heat boiler in high temperature environment.Based on this,this paper innovatively puts forward the monitoring method of ash deposition thickness based on the principle of coplanar capacitance.Through a combination of theoretical analysis,numerical simulation and experimental testing,the electric field distribution characteristics,sensor performance and the effect of parameters of the ring coplanar capacitor with ash deposition are studied.Firstly,the measurement principle of ring coplanar capacitance sensor is deduced based on Gauss theorem,loop theorem and image method.The theoretical relationship between the measured capacitance and the deposition thickness is obtained.The principle and feasibility of coplanar capacitance used in the measurement of ash deposition thickness are proved.On this basis,the electric field characteristics of the fouling medium deposited on the coplanar capacitor are numerically simulated by ANSYS Maxwell commercial software.The effects of electrode structure parameters on the capacitance,maximum thickness and sensitivity of the sensor are analyzed.The results show that with the medium thickness increasing,the capacitance firstly increases then stabilizes and the sensitivity sharply decreases;the sensor with the larger positive radius or the smaller negative radius behaves better sensor performance;the results show that the inter electrode shielding and edge shielding have little effect on the electric field distribution characteristics,while the bottom shielding can significantly improve the test capacitance,but has little effect on the measurement sensitivity.Then,based on the results of numerical simulation,PCB ring coplanar capacitance sensor is designed and fabricated.A cold experimental platform for monitoring ash deposition thickness with coplanar capacitance sensor is built.The experimental studies are carried out with fly ash and sucrose as cold state simulation media.The results show that the capacitance increases first and then gently with the increasing of deposition thickness,and the effect of test frequency on fly ash is greater than that of sucrose.Taking sucrose as an example,the effect of ambient temperature on the sensing capacitance is studied.The results show that the capacitance value increases with the increasing of temperature before sucrose melting（30-120℃）,tends to be flat with the increasing of temperature during sucrose melting（120-150℃）,and decreases rapidly after sucrose melting（>160℃）.Combined with the relationship between dielectric polarization and dielectric properties,the reason is the gradual ordering of microcrystalline structure.At the same time,the experimental data are fitted,and the fitting relationship between capacitance value and deposition thickness and test frequency is obtained.Based on the cold state experiment,the coplanar capacitance sensor with high temperature resistance is fabricated and the hot state experiment platform of ash deposition thickness is built.Fly ash and slag are used as simulated media to carry out experiments at high temperature.The results show that the sensing capacitance increases slowly with the increasing of ambient temperature in the range of 0-500℃,and increases sharply with the increasing of ambient temperature in the range of 500-800℃.The reason is that in the high temperature range,the degree of conversion from amorphous carbon to carbon microcrystals increases,so that the polarity of the medium increases and the capacitance value increases significantly.By comparing the capacitance value of fly ash and slag,it is found that the capacitance value of slag is greater than that of fly ash at the same thickness.This is due to the fact that a large amount of Si O₂ in fly ash is transformed into Si C at high temperature after high temperature melting,and the order of ash structure is enhanced,which makes the dielectric properties parameters increase.