Flowsheet Analysis of Valourising Mixed Lignocellulose and Plastic Wastes via Fast Pyrolysis at a Paper Mill

Purpose A valourising process annexed to a recycling paper mill for converting three plastic-fibre rejects (PFRs) into oil, char and utility energy through pyrolysis provides renewable energy and prevents waste disposal. The economic and technical responses of the process are measured by varying the conversion temperature, feedstock characteristics and energy recovery approach, in order to optimise the processs. Method Previously measured responses of valourising PFRs were simulated in AspenPlus® according to variations in the fast-pyrolysis temperatures (350–550 °C); the contents of lignocellulosic fibres and plastics; and methods for adding value to excess process heat as steam and/or electricity. Economic models scaled at 540 kg/h were derived to determine the required minimum fuel selling price (MFSP) of the oil product. A base case for heat recovery assumes both steam and electricity. Results For a PFR stream composing mainly lignocellulose, the MFSP of oils ranged from 341 US$/GJ for pyrolysis at 350 °C to 65 US$/GJ for pyrolysis at 550 °C, while the process at 450 °C was most energy efficient. When only recovering steam from waste energy, the MFSPs were reduced by 6%. Pyrolysis of reject streams with increased plastics contents reduced the MFSP of the oil to 37 US$/GJ, due to the higher calorific value of the products. Increasing the feed rate of PFRs to 2400 kg/h could potentially reduce the MFSP to a market-competitive price of 15.3 US$/GJ, at a scale equivalent to a power-plant of 5MWe. Conclusion PFRs containing higher proportions of plastics are preferred for valourisation through pyrolysis at 550 °C, while waste energy is best recovered as steam. Graphic Abstract