The German Federal Environmental Agency (UBA) has published its assessment of three chemical recycling technologies: pyrolysis, liquefaction, and gasification.
The report was compiled by researchers at RWTH Aachen University and Hochschule Merseburg, on behalf of the UBA.
The 223-page document presents the results of comparing the three kinds of thermo-chemical technologies for yield, greenhouse gas (GHG) emissions, and energy use, with mechanical recycling and energy recovery.
The team studied pyrolysis and liquefaction based on results from operating pilot industrial plants in Germany and comparative laboratory tests. For gasification, the researchers used historical data and results of model calculations given the lack of current practical data and the prohibitive cost of lab experiments.
Results show a mixed picture: chemical recycling performs considerably worse than mechanical recycling in energy efficiency and GHG emissions; however, it outperforms incineration in reducing GHG emissions and energy consumption, although only slightly for the latter.
Feedstocks
The scientists first tested different feedstocks for the pyrolysis and liquefaction experiments. These included PE, PP, with contents of PS, without additives (control case); mixed plastic waste; domestic and business waste including biodegradable material; and plastic waste from electrical and electronic equipment.
Results show that the only feedstock suitable for chemical recycling is mixed plastic waste.
“All other waste fractions contain too many contraries, which lead to increased coke formation and significantly lower oil yields,” the scientists concluded.
For all feedstocks, the team found that the resulting pyrolysis oil contained harmful components ‘far above the recommended values for use in a stream cracker’. They concluded that purification is ‘absolutely essential’ if sufficient dilution with virgin oil is not used.
Yields
The academics tested the technologies for yield of both primary products (oils, waxes, gases) and so-called High Value Chemicals (HVCs) (ethylene, propylene, butene, butadiene, aromatics) for re-polymerisation.
Pyrolysis and liquefaction showed similar yields. The hydrocarbon yield of pyrolysis was around 70% and 80% for liquefaction. The HVC yield for pyrolysis was 45% and 50% for liquefaction.
For gasification, the scientists used historical data from a large-scale gasification plant operated in Germany between 1997 and 2007 (SVZ Schwarze Pumpe - Lower Lusatia). That plant produced around 850 kg of synthesis gas from gasification of 1 Mg of plastic waste with the addition of about 500 kg of oxygen. With the calculated gas composition from the model concept, 780 kg of methanol can be produced from the synthesis gas and around 300 kg of HVC chemicals can be produced further downstream, a relatively low yield in comparison with the other two technologies
Environmental assessment
Pyrolysis and liquefaction showed similar energy demands (5–6.5 kWh per kilogram of HVC) and GHG emissions (1.5–2 kg CO2 equivalent per kilogram of HVC). These metrics are comparable to the production of HVCs from fossil-based naphtha.
Gasification, while less energy-intensive (3 kWh/kg HVC), generates higher GHG emissions (~4 kg CO2-EQ/kg HVC) due to carbon losses during the process.
It is clear, the scientists concluded, ‘that chemical recycling processes are significantly disadvantageous to mechanical recycling in terms of energy consumption and greenhouse gas emissions released’.
“As the calculations were based on favourable framework conditions for chemical recycling, these results should be robust and even more evident in practice,” they added, referring to the fact that they defined the production of by-products (coke, gas) as an additional benefit of chemical recycling in their comparison calculations.
“With regard to the energy recovery of plastic waste in waste incineration plants…, the chemical recycling routes show clear advantages in terms of the greenhouse gas emissions saved. The slight advantage in energy consumption identified in the analyses is due to the favourable assumptions made in the modelling and must first be realised in practice in optimised technical plants.”
The scientists concluded that chemical recycling has a complementary role to play to mechanical recycling. Strategic deployment in chemical parks or refinery sites, where by-products like waxes and gases can also be used, can enhance its efficiency. The use of renewable energy or hydrogen to power the processes could also reduce their environmental impact.