Thermal behaviour and kinetic study of co-pyrolysis of microalgae with different plastics

The coexistence of plastics and microalgae in the ocean has brought great challenges to the environment. Therefore, co-pyrolysis of microalgae Dunaliella salina (DS) and typical plastics (polypropylene (PP), polystyrene (PS), polyethylene terephthalate (PET), and polyvinyl chloride (PVC)) were investigated using thermogravimetric analyzer with Fourier transform infrared spectrometer. The results showed that the coating effect of the molten plastics promoted the pyrolysis of DS. The solid residue amounts of DS-PP, DS-PS, and DS-PET blends were reduced by 1.55 wt%, 1.39 wt%, 1.69 wt%, respectively, as a result of the hydrogenation reaction between the unsaturated products generated by plastics and biochar. While for DS-PVC, attributed to the physical and chemical effects during the co-pyrolysis process, the solid residue was increased by 1.36 wt%. For the other three blends, the solid residues were reduced due to the hydrogenation reaction between the unsaturated products generated by plastics and biochar. FTIR analysis of gaseous products indicated the total CO₂ production increased significantly for DS-PET. Besides, the alkyls generated by DS reacted with HCl during DS-PVC co-pyrolysis, the resulting products were then fixed in biochar. Kinetic results suggested that due to the co-pyrolysis with DS, the activation energies of PP, PS, and PET were reduced by 1/2, 1/3, and 3/4, respectively, and this value for PVC in its second stage was reduced by 1/4. Our results indicated the advantage to co-pyrolyze the microalgae and marine plastics.