Primary fragmentation behavior of refuse derived fuel pellets during rapid pyrolysis

Abstract

Primary fragmentation during rapid pyrolysis is considered to be the critical size reduction mechanism as it determines the particle size distribution and char conversion rates in the fixed bed gasifier. This study aims to investigate the primary fragmentation behavior of Refuse Derived Fuel (RDF) pellets during a rapid pyrolysis process. RDF pellets consisted of different blending ratios of cardboard and polyethylene (PE) were produced by a single-pellet densification process. Pyrolysis tests at temperature ranges of 500–700 °C were performed for each pellet type, and the number and particle size of the fragmented particles were analyzed. The results demonstrate that the different composition of RDF causes different particle binding mechanisms that behave differently during pyrolysis. Densification of cardboard particles can maintain the structure of the pellet char at higher temperatures compared to PE, due to the more stable lignin binding mechanism. A modified Fragmentation Ratio (FR) is introduced to quantify the degree of fragmentation. It is shown that a raise of the concentration of PE from 25 to 75 wt% increases the FR value by up to 4.2 times, whereas an increase in pyrolysis temperature from 500 to 700 °C only slightly changes the FR values. It can be concluded that the type of materials in the RDF pellet has a more dominating effect on the fragmentation compared to the pyrolysis temperature. Further, the volatile matter content of pellets shows a linear correlation with the FR value, whereas no clear relation is found in the case of the pellet's mechanical strength.