Slow Pyrolysis Products Derived from Extrudates Produced from Discard Coal Fines and Recycled Plastics as Binders

The pyrolysis products derived from both coal and plastics have been extensively evaluated in literature; however, their copyrolysis product yields together with the characterization of the generated products have received less attention. Most studies use high heating rates with small particle sizes of the mechanically mixed blends. This study aims to improve the understanding of the slow copyrolysis behavior of extrudates produced from coal fines from the South African Highveld coalfield combined with recycled waste low-density polyethylene (LDPE) and polypropylene (PP). The fraction of plastic was varied between 10 and 100 wt % during extrusion. The extrudates (10 mm diameter) underwent slow pyrolysis in a modified Fischer assay setup by increasing the temperature (5 °C/min) under a nitrogen atmosphere to 520, 720, and 920 °C. The pyrolysis products (char, condensable products, water, and gas) were collected and characterized. The coal fines produced up to 83% char, whereas the plastics produced over 90% condensable products. The slow heating rate and the small reactor volume favored the production of condensable products, which increased with plastic concentration. The extrudates produced char and condensable product yields that fit the additive model of the raw materials. Statistical equations were developed to predict the pyrolysis yields and characteristics as functions of coal and plastic composition. The equations accurately predict the char yield, condensable product yield, fixed carbon content, ash content, sulfur content of chars, and carbon and hydrogen contents of chars and condensable products. Gas chromatography-mass spectrometry results indicate that the extrudates' condensable products consist mainly of alkanes and alkenes, similar to the plastics. Furthermore, the condensable products derived from PP and coal extrudates have fewer components with lower boiling points compared with the raw materials.