A novel measurement method for ash deposition based on coplanar capacitance principle: Theoretical, numerical and experimental studies

Monitoring for the fouling and slagging is critical to ensure the stable operation of gasification system. This paper proposes a novel measurement method for the ash deposition based on coplanar capacitance principle. A theoretical analysis of electrostatic field is conducted to obtain the correlation between the coplanar capacitance value and the deposition thickness. A finite element simulation is carried out to discuss the electric field characteristics of ash depositing on the coplanar capacitor. It shows that the capacitance value initially increases and tends to be constant with the increase of deposition thickness. The lower distance between positive and negative electrodes is favorable to the capacitance value (signal strength) but unfavorable to the maximum measurable thickness. Furthermore, a cold state experiment is conducted to verify the correctnesses of the theoretical and numerical models, and a hot state experiment is carried out to study the capacitance characteristics of sensor in the high temperature range of 700 °C–1200 °C. Results show that the permittivity of the material is lower under the higher test frequency, which leads to the lower capacitance value of sensor. The complex impedance curve of ash is conformed to IBLC dielectric model, this explains that the sensor capacitance increases with the test temperature increasing caused by the increase of permittivity due to the enlargement of ash grain. The change of Si/Al content has little effect on the dielectric properties of ash, however, the higher Fe2O3 content or the lower CaO content results into the lower ash fusion point, larger grain size, and thence higher permittivity and sensor capacitance. The maximum measurable thicknesses of the sensors in the cold state and hot state are 11.2 mm and 10.0 mm, respectively, which are independent with the test frequency, test temperature, and ash component.