STUDY ON KINETICS OF BIOMASS PYROLYSIS IN THE FIXED BED. 3. WAYS OF INTENSIFYING PELLETS HEATING IN THE RETORT

With the use of the constructed model [1, 2], numerical studies of various methods of biomass heating intensification in a fixed bed were carried to choose a pyrolyzer design. Biomass particles heating in seven pyrolyzer design variants were analyzed. The optimal design of the pyrolyzer was found, which allows achieving the minimum time (50−52 s) for heating the pellets in the fixed bed to the minimum permissible temperature of 656−694 °С, at which almost complete conversion of the tar into gas and carbon residue occurs. The heating rate of the fixed bed is high and equal to 788–801 °С/min which characterizes the process of fast pyrolysis. This design consists of two axisymmetric concentric cylinders with two external sources of heat energy: the outer surface electric heating of the cylinder with a diameter of D = 100 mm and electric heating of the pipe inner surface (or heating by combustion products passing through the inner pipe) with a diameter of d = 50 mm. It was found that the ratio D/d significantly affects the bed heating rate. With a 5 times decrease in the inner pipe diameter, the bed heating rate decreases by 3.5 times and is 229 °C/min, which characterizes the slow pyrolysis. When adding a third source of thermal energy — pyrolysis gas with a temperature of 800 °С, the pellets heating rate at D/d = 2 increases by 24 %. The dependences on time of heat flow and temperature, necessary for regulating the enclosure and inner pipe walls temperatures in the range of 800–850 °С were obtained. Bibl. 31, Fig. 9, Tab. 2.