Fluidization of biomass: a correlation to assess the minimum fluidization velocity considering the influence of the sphericity factor

This paper deals with the influence of different definitions for evaluating the sphericity factor on the prediction of minimum fluidization velocity (U (mf)) of different agro-industrial/forestry biomass residues and sand. Three biowastes (sawdust, grape marc, and grape stalk) and sand were characterized by sieving, and sphericity was calculated using images obtained by scanning electron microscopy (SEM). Two particle size populations (with mean diameters and density corresponding to B and D Geldart groups) were adopted for each biomass. As for sand, particles with a mean diameter of 0.33 mm were used. Tests were carried out in a lab-scale fluidized bed. Bed pressure drop was measured as a function of the superficial velocity of air at two different initial bed heights, for each type of biomass and mixtures with sand. Ratios of 0.50 and 0.75 (v/v) were used for mixtures. For all cases, U (mf) values were calculated experimentally and were compared against predicted values obtained from Ergun's equation and other correlations. It was found that the methodology adopted for determining sphericity significantly influences the obtained value of U (mf). The lowest relative error between predicted and experimental U (mf) values was obtained using Riley's sphericity method. Finally, a new correlation for U (mf) was proposed.