Bipolar charge distribution of a mixture of particles with different electrostatic characteristics in gas–solids pipe flow

Electrostatic charge distribution of fine particles in gas–solids pipe flow has been studied experimentally. The powders used were fly ash, 4.2 μm count median diameter (CMD), and alumina powder, 3.9 μm CMD. These powders were fully dispersed and pneumatically transported in a spiral stainless steel pipe, 6 mm in inner diameter. The average air velocity was in the range from 20 to 39 m/s, and the mass flow ratio of particles to air was 5×10−4. The particles were charged by contact with the inner wall of the pipe. The charge and the diameter of individual particles were measured with an electrical-single particle aerodynamic relaxation time (E-SPART) analyzer. The experimental results showed that the charge on the particles increased with pipe length and reached an equilibrium (maximum) value. The fly ash was generally charged negatively, and the alumina powder was charged positively. However, the value of the charge of each powder was widely distributed having charge of opposite sign. Furthermore, a bipolar charge distribution was measured using a mixture of fly ash and alumina particles, showing that the charge distribution in a dilute phase transport system could be explained by the linear additivity of the charge distribution of each powder.