Plastics whose service life has ended are called Waste Plastics. There are several types of waste plastics including packaging waste, waste electrical and electronic equipment (WEEE), plastic waste from end-of-life vehicles etc. In the municipal solid waste (MSW), plastics present are designated as Plastic Solid Waste (PSW). Household items like packaging plastic films, packaging foam, disposable cups, plastic plates, cutlery etc. are one source of PSW in MSW. Mulch films, fertilizer bags, feed bags etc., used in agriculture are another source of PSW in MSW. It is also well-understood that PSW is almost always mixed with MSW. PSW is broadly classified into seven different classes of plastics:
- Polyethylene terephthalate (PET)
- High density polyethylene (HDPE)
- Polyvinyl chloride (PVC)
- Low density polyethylene (LDPE)
- Polypropylene (PP)
- Polystyrene (PS)
- Others (includes plastics like polymethylmethacrylate (PMMA) used in touch screens, protective coatings (e.g., epoxy coatings etc.), Acrylonitrile butadiene styrene (ABS) used to make hub caps etc.)
In order to recycle PSW, it needs to be efficiently separated from MSW and cleaned. After separation, PSW can be recycled mechanically or chemically. PSW can also be incinerated to recover energy but the gases released during incineration can have environmental effects if not treated properly before being discharged to the atmosphere. Therefore, mechanical and chemical recycling methods are among the most commercially and environmentally attractive methods of plastics waste recycling.
Mechanical recycling is suitable for thermoplastics and involves heating (melting) and conversion of cleaned plastic waste into granulates. These granulates are either mixed with virgin plastics with in the same production line to produce a certain product or sold to other companies who use them in their production process.
Chemical recycling actually means that the waste plastics are decomposed either into the monomers from which they were formed or into chemicals which can be used in different ways. For example, a waste polystyrene packaging can be chemically recycled into styrene monomer. This styrene monomer can be polymerized again to form polystyrene. Similarly, gasoline or diesel products can produced by chemical recycling of HDPE waste can or PP waste bottle caps. Both of these options are commercially attractive as they can help in solving the problem of managing plastic waste and in reducing our fossil-fuel demand for the production of monomers for different plastics. Recall that both styrene and ethylene are produced mainly from fossil-fuels.
Among chemical recycling methods, pyrolysis is one of the attractive options.
What is Plastic Pyrolysis?
The conversion of long polymer chains into small molecules in the absence of oxygen is called plastic pyrolysis. Intense heat (> 300 °C) is required for a certain time inside the chemical reactor to convert/break the polymer into smaller molecules. Cracking term is also sometimes used instead of pyrolysis.
The figure below is a schematic representation of pyrolysis of waste polyolefins (i.e., HDPE, LDPE and PP etc.). It can be seen that a number of commercially important chemicals can be obtained by chemically recycling waste polyolefins via the process of pyrolysis. Light olefins include ethylene, propylene and butenes which are the main monomers for HDPE, LDPE, PP etc., whereas the liquid oils produced can be used as lubricants. Similarly, waxes produced in such a process also find numerous applications in our daily life products. It is important to mention here that, although lab scale pyrolysis experiments with waste plastics have shown promising results, large scale pyrolysis of waste plastics is a challenging process.
The important factors to be considered in getting the desired product from the pyrolysis of waste plastics include feed conditions (e.g., type of plastics used, impurities and additives present in them etc.), reactor operating conditions (temperature, pressure, catalytic process, thermal process, reactor type etc.) and type of the downstream processing.
Note that pyrolysis is not only used for waste plastics. It is a well-established process for treating biomass and mixed feedstocks like biomass combined with waste plastics.
Advantages of Pyrolysis
- It is considered as a flexible process. If one wants to maximize the yield of a specific product, process parameters can be changed to do so.
- Quality of some of the obtained products allows them to be used directly for different purposes. For example, waxes produced from plastic pyrolysis can sometimes be co-fed with other waxes obtained from processes other than waste plastic pyrolysis; liquid oils produced can be fed to boilers, turbines, furnaces etc.
- In some cases, handling of waste plastics in pyrolysis process can be easier than that in mechanical recycling. Stringent purity requirements in the recycling processes make the sorting process more labour intensive than that in plastic pyrolysis.
Types of Pyrolysis Process
There are different classes of pyrolysis tested for the chemical recycling of waste plastics. This classification is sometimes based upon the time spent by the gaseous products (i.e., the volatiles) inside the reactor (i.e., the residence time) or upon the fact that whether a catalyst has been used or not in the reactor.
In this process, reactor temperatures can be between 170 to 600 °C with average volatiles residence time of several hundred seconds (e.g., 450 s). Long residence times and high temperatures lead to side reactions influencing the product composition. Consequently, this process is not recommended if one wants to recover monomers from the waste plastics.
When the average residence time of volatiles is around 0.5 to 10 s inside the pyrolysis reactor which is set to temperatures between 400 to 900 °C then the process is typically termed as a fast pyrolysis. Short residence times reduce the side reactions.
In this process, the average residence time of volatiles is less than 0.5 s in a reactor set to a temperature between 650 to 950 °C. Side reactions in gaseous products are suppressed due to short residence time.
Fast and flash pyrolysis are both recommended for monomer recovery from waste plastics due to the better control over the product composition allowed by the short residence time of volatiles in these processes.
As indicated by the name, this process uses only heat to break the waste plastics into smaller molecules. Reactor temperatures can be as high as 900 °C. Product distributions can be wide in some cases due to poor control over the heat and mass transfer inside the reactor. Remember that the polymers have high melt viscosity and low thermal conductivity leading to the problems of heat and mass transfer. This is an energy intensive process.
Pyrolysis of waste plastics in the presence of a catalyst is called catalytic pyrolysis. Similar to the other catalytic processes, catalysts lower the activation energy of the reactions responsible for plastic decomposition. Consequently, energy demand of catalytic pyrolysis is significantly lower than the thermal pyrolysis process (reactor temperatures are in the range of 350 °C to 500 °C). This process also allows better control over the product composition as the catalyst and low reaction temperatures help in reducing the side reactions.
Zeolites, AlCl3, silica-alumina, clays, ordered mesoporous aluminosilicates (MCM-41, Al-MCM-41 and Al-SBA-15) and FCC catalysts are the solid catalysts being tested extensively in the pyrolysis of waste plastics to obtain a variety of products.
Challenges Associated with Pyrolysis of Waste Plastics
Pyrolysis of plastics seems to be an attractive chemical recycling process for solving the problem of waste plastics. But there are many challenges involved in plastic pyrolysis which are hindering the large scale implementation of this process. These challenges include:
- Constant availability of good quality waste plastics as a feedstock for the reactor. Good quality may have several meanings here e.g., single plastic or a mixed plastic feed, good sorting of plastic waste, impurities present in the feed etc. Waste plastics are normally mixtures of different materials e.g., a food packaging sheet has several layers of different plastics and, therefore, it can be very difficult to separate such layers or to use that sheet directly into a reactor which is designed for one specific plastic feed. In 1996, BASF stopped the plan to install a pyrolysis plant where waste plastics were to be converted into commercially important chemicals (in accordance with the current concept of circular economy). It appears that the major contributor in this decision was the failure to secure a long term supply of good quality waste plastics feedstock.
- Large capital investments and the associated risks. Capital investments are high due to several reasons e.g., energy requirements can be high in plastic pyrolysis process due to high reactor temperatures and wide product distribution which means more purification units are required in the plant to get the desired purity of a specific product/chemical. These factors combined with the above-mentioned feed availability and quality problems cause the capital investments and associated risks to increase. This makes it difficult to attract investors in waste plastic pyrolysis.