Experimental study on fixed-bed combustion and agglomeration of sawdust-polyethylene mixtures

Fixed-bed conversion of polymer-containing waste is one of the most common ways of their disposal. One of the problems with such conversion is the cross-sectional non-uniformity of the bed, the formation of agglomerates, and burnouts. The agglomeration process essentially depends on the weight fraction of melting components (plastic materials). Usually, the non-uniformity of the bed is associated with the random nature of the packing (fluctuations of local concentrations and percolation properties). In the present work, we investigate the agglomeration process caused by non-uniform conditions of heat transfer. To this end, combustion of model waste (mixtures of sawdust and polyethylene with a content of the latter of 20-80) is experimentally studied in a laboratory setup with a bed height of about 10 cm. Due to the heat supply through the wall, airflow, non-uniformities developed, associated with different gas permeability in different areas of the bed. The transition from intense thermal decomposition to smoldering is achieved in 5-15 min (the higher the polyethylene content, the shorter the transition time). When the polyethylene content is high, the permeable area decreases so that the air velocity becomes high enough to entrain sawdust particles. As the polymer decomposes, tarry substances are released and form dense deposits in the gas cooling and cleaning system.