Drying of black liquor in spouted bed of inert particles

This work is aimed at developing a low-cost technique to produce powdery fuel from wood and nonwood black liquors. This consists in injecting the liquor into a bed of inert polypropylene particles spouted by hot gas. The liquor coats these particles with a thin film, which is then dried and reduced to powder during particle circulation inside the bed. The powder is carried away by an exhaust gas and separated in a cyclone. Attrition between particles, resulted from particle circulation, induces high shear rates in the thin liquor film reducing its viscosity and making possible to process it. If particle circulation is optimized, the drying temperature can be well controlled to avoid agglomeration and adhesion to the dryer wall. A methodology developed for operating and controlling such variables in a semi-pilot unit is discussed to make feasible the continuous production of the powdery fuel. Using Eucalyptus and Bamboo liquors and the experimental factorial design, the effects of the drying operational variables on powder quality and efficiency are analyzed to optimize this process. Results demonstrate that the powder produced is a particulate fuel with the net calorific value varying from 13,579 (Eucalyptus) to 14,673 (Bamboo) kJ/kg, at the residual moisture content from 9.5% (Eucalyptus) to 6.3% (Bamboo). Although the powder production efficiency can reach high values (>60%), the thermal performance of the dryer given in terms of the energy factor is unsatisfactory so possible modifications to the dryer are pointed out.