Investigation of flow behaviour in biomass gasifier using Electrical capacitance Tomography (ECT) and pressure sensors

Abstract

The particles in a biomass gasifier are mainly a mixture of wood chips, char particles and bed material. The charwood particles have a significant lower density than the bed material, and also a wider range of particle sizes and larger mean particle diameter. The difference in particle properties may cause segregation and thereby influence on the fluidization properties and the flow behavior in the bed. The aim of this work is to study the fluidization properties in cold fluidized bed with different mixtures of particles. ZrO and plastic particles with a density ratio of 6, are used in the experiments to simulate the bed material and the char-wood particles in a biomass gasifier. Experiments are performed in cylindrical beds with uniform air distribution. The fluidization properties are studied using pressure sensors and Electrical Capacitance Tomography (ECT). The ECT system is a non-intrusive measurement system and is a suitable method for monitoring the internal flow behavior of fluidized bed. The experimental results show that the minimum fluidization velocities very much depend on the particle composition in the bed. The fluidization velocity reaches a maximum when 20% plastic particles are added to the ZrO powder and decreases again when the fraction of plastic beads are further increased. The theoretical minimum fluidization velocities for the different mixtures agree well with the experimental data. The standard deviations of the pressure and the ECT measurements showed that the fluidization velocities are higher in the lower part than in the higher part of the bed. This observation is more significant in the mixtures with high fraction of plastic particles. This indicates that the plastic particles moves upward in the bed and that mainly ZrO particles are present in the lower part of the bed. This is also visually observed during the experiments. Segregation can give low degree of particle motion in parts of the bed. Investigation of fluidization behavior of the different mixtures in this study may be useful as an initial step of analyzing the complex system of a bubbling fluidized bed gasifier