Microwave assisted co-pyrolysis of biomasses with polypropylene and polystyrene for high quality bio-oil production

In this study, co-pyrolysis of five lignocellulosic biomass residues, viz. groundnut shell (G), bagasse (B), rice husk (RH), Prosopis juliflora (PJF) and mixed wood sawdust (MWSD), with two synthetic plastics, polypropylene (PP) and polystyrene (PS), was conducted to improve the quality of bio-oil. Equal composition of biomass and plastic was pyrolyzed at 450 W microwave power using graphite susceptor, and the yields of bio-oil, char and gas fractions were determined. The bio-oil yields from PS-biomass blends varied in the range of 51–60 wt%, with maximum yield obtained from PS-MWSD, while that from PP-biomass blends were 25–41 wt% with maximum yield obtained from PP-RH. The higher heating value of co-pyrolysis bio-oil was high (38–42 MJ kg−1) as compared to bio-oil obtained from only biomass pyrolysis (20–30 MJ kg−1). Energy yield of co-pyrolysis bio-oil was significantly higher with PS mixtures (73–81%) than PP mixtures (32–62%). Importantly, the bio-oil, char and gas yields, and organic composition of bio-oil were non-additive with respect to that obtained from pyrolysis of the individual feedstocks. The presence of PS boosted the production of aromatic hydrocarbons (48–54 wt%), while PP promoted aliphatic hydrocarbons (19–33 wt%) in co-pyrolysis bio-oil. Specifically, high selectivity towards alkanes in bio-oil was obtained from PP-RH mixture. Co-pyrolysis bio-oil contained less moisture with lower acidity compared to pyrolysis bio-oil. RH-PS and RH-PP mixtures were shown to be the better biomass-plastic combinations from the viewpoint of recovering high energy in bio-oil. The microwave co-pyrolysis process efficiency was also high (63–68%) and promising.