Fragmentation and agglomeration of biomass in fluidised bed pyrolysis and combustion

Abstract

The results of experiments on primary fragmentation of wood biomass particles during fast pyrolysis in an inert medium at different temperatures are presented. Growth of temperature and initial size in the range t = 800 – 950°C increases the probability and intensity of fragmentation in the process of rapid pyrolysis of wood particles. Oblong particles are more prone to fragmentation than quasi-circular (cubic) particles with the same equivalent diameter. The critical diameter of fragmentation for spruce wood particles at a process temperature of 950 °C is dv0 =18-19 mm at the ratio of longitudinal and transverse size h/a =1 and is in the range of 7-9 mm at h/a =8. Study of microstructure and chemical composition of coke particles (fragmentes) by microscope with SEM analysis showed existence of thin outer layer (0-15 μm) on the particle faces, containing increased quantity of mineral components. Microspheres with a size of 13-23 μm and increased content of sulfur were detected on the particle faces, resulting at 950°C. Coinciding of maximum and minimum of chlorine, sodium and potassium content in the outer layer denotes on existence of sodium and potassium chlorides on the surface of coke particles. Results of experimental studies of physical and chemical processes occurring between the fuel ash and the bed materials under prolonged temperature exposure were obtained. The aim of these studies was to determine the time and proportion of agglomerated (sintered) particles of several types of biomass depending on the bed temperature. The experiments were carried out in a furnace on biomass ash, sand and mixture of sand and ash. First experiments showed a clear dependence of the temperature of agglomeration beginning on the potassium concentration in the bed and gave the boundary values of the concentrations and temperatures in the bed.