Do you ever wonder how much (or how little) of the plastic waste (packaging, containers, and milk cartons) you put in your recycling bin is actually recycled? The garbage truck transports it to a waste management facility for sorting and recycling. The 'good' plastic is recycled and the rest-which is either too dirty, mixed, or damaged for recycling-is incinerated.
In Denmark alone, around 370,000 tonnes of plastic waste is incinerated every year. According to the Danish Ministry of the Environment, plastic waste incineration makes up a significant portion of Denmark's fossil-fuel carbon emissions. In other words, there is a huge green potential in developing new methods for recycling even more plastic waste than we do now.
If we can also reduce the amount of non-renewable plastic waste, we can reduce the need for the non-renewable raw materials needed for plastic production-oil and natural gas.
A group of DTU researchers have therefore investigated new possibilities for recycling our plastic waste in collaboration with Roskilde University and a number of industry partners. Their research project RePlastic has shown that a valuable oil can be produced from otherwise useless plastic waste through pyrolysis.
"I'm surprised at the great potential of pyrolysis technology for the most impure and mixed plastic fractions. This process can handle the plastic we have no other uses for. This enables us to bring end-of-life plastic back into the cycle and make it useful again," says Anders Egede Daugaard, Associate Professor at DTU Chemical Engineering and head of the RePlastic project.
Plastic is not just plastic
To fully understand Anders Egede Daugaard's enthusiasm, you need to understand the challenges of recycling and sorting plastic waste into different categories and fractions. The current number of different plastic types with different properties is incredibly high-just take a look at your own plastic waste, where you will find hard, soft, ductile, coloured, and transparent plastics.
Plastic waste is generally divided into two categories: industrial and household. Industrial plastic waste is usually more uniform as it often consists of only one type of plastic, where both the additives and manufacturing process are known. Household waste, on the other hand, is more often a mixture of different types and grades of plastic. The plastic is then sorted into different fractions depending on properties and quality.
Because the chemical additives vary according to the properties of each plastic product, our plastic waste needs to be sorted before it can be recycled in a mechanical process that granulates, heats, and remoulds it into new plastic products. You cannot make new quality plastic from mixed plastic types because the melting point and additives differ and are often completely unknown.
In the RePlastic project, Associate Professor at DTU Anders Egede Daugaard and his team have assessed the potential of several plastic materials from the least valuable plastic fractions in our plastic waste. These fractions are where the majority of our household plastic waste ends up, along with industrial plastic waste that has already been recycled six or seven times and is therefore too worn out to be mechanically recycled again.
Pyrolysis creates new possibilities
The RePlastic project focused on using pyrolysis for chemical recycling. During the process, plastic waste is heated to high temperatures in a nitrogen-filled furnace, triggering a splitting of the chemical components of the plastic materials. Because there is no oxygen in the furnace, the plastic does not burn, but gasification occurs. The gas is then condensed into so-called pyrolysis oil, which can be used as an additive in fuel or new plastic products.
In the laboratories at DTU Chemical Engineering, researchers have been studying which plastic fractions can potentially be used for pyrolysis and how pure the plastic needs to be. The assessment of the purity required for the pyrolysis oil and its applications has been a key focal point for the project partners, as it is crucial to whether or not the technology can be commercialized.
The initial conclusion was that obtaining usable pyrolysis oil required a very clean system. This meant that the plastic waste had to be thoroughly sorted and cleaned before going in the pyrolysis furnace, and that the resulting pyrolysis oil subsequently had to be distilled and purified.
However, the RePlastic project shows that this is not actually necessary. The pyrolysis technology can handle the impurities in our mixed and dirty plastic waste.
