III Recovering plastic waste
Material recovery: recycling
The name of the game. Or is it?
- polyvinyl chloride (PVC);
- polyethylene (PE) and its high density (HDPE) or low density (LDPE) variants;
- polystyrene (PS);
- polyethylene terephthalate (PET);
- polypropylene (PP).
Plastics are collected in most places as a mixture of recyclable waste. This waste is firstly sent to a sorting centre where recyclables are sorted, recyclable plastics are separated from the rest and then sorted by families. They are then sent to a recycling factory. There, these plastics are crushed into small pieces and washed, in order to obtain granules that can be moulded again to become new items.
Although the amount of recycled plastics in Europe increased from 4.7 MT to 7.5 MT between 2006 and 2013 – at a rate of almost 8% per year – recycled plastic only represent about 13% of the 59 MT currently produced each year in Europe.
If we don’t change our habits, the global yearly plastic production should reach 1,124 MT by 2050; considering the current scope of the marine pollution, the Ellen MacArthur Foundation forecasts there will then be more plastic than fish in the ocean, in weight ratio.
Focus on packaging
- 2% is recycled in closed-loop: recycled into a similar or same-value product;
- 8% is recycled in cascade, that is in other plastics of lower value;
- 4% is process loss and cannot be recovered.
Energetic recovery: incineration
An alternative to landfill for non-recyclable waste
Energetic recovery consists of generating energy from wastes by burning them. This is the method of choice to dispose of complex plastic mixtures, like Tetrapak containers in which multiple layers of different type of plastics alternating with metallic films are glued together. For energetic recovery, plastics are not separated from household wastes and are incinerated together. This concerns about 14% of all plastic waste.
Comparative LHV of plastics and usual sources of energies (sources: Determining accurate heating value of non-recycled plastics, Demetra et al., 2016; Well-to-Wheel Studies, Heating Values, and the Energy Conservation Principle, Bossel, 2003).
The energetic recovery process is straightforward, as seen below: the waste is burnt in a furnace, producing smoke with temperatures ranging from 850°C to 1000°C. This smoke is used to heat water, turning it into steam, which is either:
- directly used to heat nearby houses, with an energetic yield of 70 to 80%. 1 ton of waste generates roughly 1,500 kW.h. For comparison, you’ll need 18,000 kW.h to heat an average house in France.
- converted to electricity, by using a turbine, with an energetic yield of 20 to 25%, that is 300 to 400 kW.h per ton of refuse, which roughly enough to power the lights of an average house for a year.
In addition to generating energy, incineration significantly reduces the volume of waste and make them more compact, less likely to be lost and pollute the environment. This does not mean that there’s no leakage: during combustion, the polymers are broken into smaller molecules, forming gases and particles. A great variety of molecules are found in incinerators fumes, including very toxic ones like dioxins. Using a filtering step is thus necessary to remove these toxic compounds from the fumes before they are let out.
The remains are ashes and mud, also potentially toxic and requiring a specific treatment before being used in asphalts, cements, concrete, etc. or being dumped in landfills.
Are incinerators overshadowing recycling?
Although energetic recovery enables us to produce energy that is sometimes considered as clean (even though not everybody agrees with this definition), it is legitimate to question the usage of this technique as it creates a competition with recycling. Indeed, recycling plastics cost less energy than incinerating them, as it removes the energetic costs of extracting new raw material from the plastics lifecycle.
Environmental agencies aiming a “zero waste” objective often take a stance against incinerators, considered as an obstacle to recycling. Cases of illegal diversion of refuse from recycling to incineration for economic reasons have recently made the news giving some weight to their arguments. However, it should be possible to use both solutions complementarity, as they don’t address the same issues.
A not-so-clean energy
This is not the only reason why people are debating energetic recovery. Even filtered, gas emissions from incinerators are problematic, and in particular CO2. Indeed, incinerating non-recyclable plastics generates more CO2 than if the same amount was put into landfill. The only way to obtain a negative CO2 balance during plastic incineration is to ensure a very high energetic yield, in which case the emissions would be lower than if we produced the same amount of energy with your typical fossil fuel. This requires expensive optimisations of incineration processes, which are more often than not, not performed.
CO2 is not the only residue emitted during plastic incineration. Studies have highlighted that heavy metals, PAH (Polycyclic Aromatic Hydrocarbons) and persistent free radicals – all known for their toxicity – were found in plastic fumes as well as in plastic ashes. While in theory they are filtered out, in practice the risks of soil and air contamination – with dire consequences on nearby inhabitants’ health – are still high in countries with weak environmental regulations, particularly the developing countries.
Chemical recovery: depolymerisation