Separating Energy and Recurring-Cost Parity for Post-Consumer Polyolefins: Mechanical Recycling and Pyrolysis of HDPE and PP

Selecting a recycling route for post-consumer polyolefins requires distinguishing energy savings from purchased-input cost reductions, yet the two are often assumed to coincide. Here, mechanical recycling and pyrolysis of high-density polyethylene (HDPE) and polypropylene (PP) were compared to test whether equal energy demand implies equal operating cost. The functional unit (FU) covered pretreatment and conversion of waste supplying 1 kg of clean, dry polymer. Calculated extrusion electricity and an engineering-based pyrolysis inventory supported energy and recurring-cost models. Across the three cases, base cumulative energy demand was 12.8–14.2 MJ/FU and recurring costs 0.030–0.033 USD/FU. Direct-energy parity occurred at extrusion electricity of 0.7766 kWh/kg, but cost parity at base prices occurred at 0.8431 kWh/kg. Between these values, mechanical recycling used more energy but cost less, because avoided purge nitrogen outweighed its additional electricity and cooling-water costs. HDPE (0.838 kWh/kg) fell within this interval, whereas for PP (0.711 kWh/kg) mechanical recycling was favored on both energy and cost. Jointly varying conversion electricity, nitrogen consumption and prices reversed the cost orderings for both polymers within the tested ranges. Lower energy demand therefore does not guarantee lower operating cost; routes should be compared on both bases. These results concern processing only, not product equivalence or plant profitability.