TECHNOLOGIES
Compared to pyrolysis, gasification requires
fewer process steps. Pre-treatment of the waste (including water removal) is followed by the gasification step and then a cleaning stage to remove contaminants such as ammonia, H2 metals, NOx
S, alkali and tars. Like pyrolysis, it is an energy
Above: Plastic Energy has developed a pyrolysis-based process for recycling mixed polyolefins
the yields of each component can be controlled to some extent by adjusting temperature, pressure, and residence time, as well as through the use of particular catalysts and thermal profiles. Pyrolysis takes place in the absence of oxygen,
so the process is only really suitable for polymers with a limited oxygen content, such as PE, PP and PS. Polymers containing high levels of oxygen or halogens — particularly PVC and compounds containing brominated flame retardants — must be sorted and removed from the waste input stream. Oxygen and halogen concerns aside, pyrolysis can handle mixed polymer waste streams that would be highly challenging for mechanical or depolymeri- sation chemical recycling methods. However, it is an energy intensive process and the quality and mix of output materials is still dependent to some extent on the input stream. Much of the gas and liquid output from pyrolysis operations is likely to be burnt as fuel, either to provide energy for the process itself or because it is not of sufficient purity to be used as a chemical feedstock. Many of the companies active in this area prioritise conversion to fuel products but, under some regulatory and accreditation regimes, this is not recognised as recycling. Key players in the development of pyrolysis technologies include Luxembourg-headquar- tered Clariter, Plastic Energy in the UK, Quanta- fuel in Norway, Alterra Energy, Encina and Nexus Fuels in the US, and GreenMantra Technologies in Canada.
Gasification is also a thermal cracking process
but it differs from pyrolysis in that it is performed in the presence of a limited but controlled amount of oxygen. It can process almost any organic material — including plastic waste and biomass — and unlike pyrolysis can, in theory at least, accept polymers containing oxygen or halogens. The end result is predominantly syngas that, depending on its compo- sition and purity, can provide production feedstocks.
16
intensive process involving temperatures of 900°C or more and a significant part of the conversion output is used for energy. Gasification technologies are under development for plastic waste applications by, among others, Enerkem in the Netherlands, and Eastman in the US (which calls this carbon renewal technology to differentiate it from another process it uses for polyester recycling). Dissolution differs from depolymerisation,
pyrolysis and gasification in that the plastic waste is not chemically converted to a new form but is dissolved in a carefully-selected solvent that allows fillers, pigments and other contaminants — potentially including secondary polymers — to be separated out. Proponents of dissolution emphasise that the polymer undergoes a physical, rather than chemical, change and use terms such as solvent-based purification. However, it is clearly not a mechanical recycling process and, if for no other reason than its emergence as a recycling contender at the same time as chemical-based technologies, is usually considered as a chemical recycling process. The key to success in dissolution is the selection of a solvent that recovers only the target polymer. This means it is best suited for use with relatively homog- enous waste streams. A number of pilot projects are already well advanced — Purecycle Technologies in the US, for instance, is targeting PP with a technology licensed from P&G while Canada’s Polystyvert is focusing its efforts on PS. Germany’s APK is exploring technology to recover LDPE and PA from multi-layer films. Trinseo has inaugurated an R&D pilot dissolu- tion plant for recycling PC at its site at Terneuzen in the Netherlands with a focus on high purity PC resin. In theory, dissolution exposes the polymer to less thermal and physical stress during the recovery process than conventional mechanical recycling. However, as the recovered polymer is likely to require compounding or pelletising to make it suitable for further use, that gain may be mitigated. In addition, the cost of the numerous processing steps involved— pre-treatment, dissolution, filtration, precipitation, solvent removal and reformulation — is likely to make dissolution most attractive for processing of largely material waste streams with a relatively high level of contaminants that would be difficult to remove mechanically otherwise.
Chemical Recycling – Global Insight 2024
IMAGE: PLASTIC ENERGY
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64
