Pyrolysis of Plastic Waste: Char and Gas Production
Plastic waste fractions constitute a significant portion of inorganic fractions of municipal solid waste (MSW), moreover, plastic waste accounts for roughly 10–13% of the total MSW throughout the world. The plastic waste is composed mainly (∼60 wt %) of polyolefins, including polyethylene (HDPE, LDPE and LLDPE) and polypropylene (PP), the remaining fraction being polyethylene terephthalate (PET), polyvinyl chloride (PVC), polystyrene (PS), and other polymers. The largest share of plastic waste is packaging such as bags, trays for packing and storing food, thermal dishes, cups and bottles for beverages.
Pyrolysis is a promising waste-to-energy technology for the sustainable management of plastic waste along with the production of liquid oil as a source of energy and char and gases as value-added products. Pyrolysis of plastic waste takes place at temperatures ranging from 200 ◦C to 1300 ◦C depending on pyrolysis methods and the type of plastic used. Temperature is one of the most significant operating parameters in the pyrolysis process of plastics since it controls the cracking reaction of the polymer chain. Different plastics have different degradation temperatures depending on their chemical structure. The thermal degradation of common plastics such as HDPE, LDPE, PP, PET and PS starts at 350◦C; however, in the case of PVC, it starts at a lower temperature of 220◦C.
During the pyrolysis process, the plastic wastes are heated and melted at a high temperature, and thus their macromolecules are broken down into fragments and small molecules, mainly aliphatic and aromatic hydrocarbons. Finally, the products are separated into oil, gas, and char (solid residue). The percentage yields of pyrolysis products can be impacted by several parameters, including temperature, residence time, pressure, heating rate, feedstock composition, catalyst, and reactor type.
Char formation:
Generally, the char formation in the pyrolysis process is maximized by the slow heating rate and long residence time, and its formation is normally low in the fast pyrolysis process. Pyrolysis of waste plastics such as PE and PP at high temperatures produces a very low amount of char. For example, the pyrolysis of PE and PP at 668°C and 746°C produces 2–4 wt% and 0.7-2 wt% of char, respectively. The char product is mainly composed of a carbon-rich matrix that contains almost all the inorganic compounds previously present in the raw waste plastics. Therefore, the char produced contains inorganic compounds, and such type of char is not ideal to be used as a fuel, but it can be used as a building material and for road surfacing. Other potential applications include the use of the pyrolysis char as a feedstock in the preparation of activated carbon and as a solid fuel for boilers. In some cases, the char is treated to improve these surface properties and increase its adsorbent capacity. The well-treated and upgraded chars are used as adsorbents in wastewater treatments for the removal of pesticides, organic dyes and heavy metals.
Previous studies have reported that the calorific value of char produced from the pyrolysis of HDPE is about 18.84 MJ/kg. While, the char produced from PS waste has a HHV (higher heating value) of 36.29 MJ/kg, so it can be used as an energy source.
Gas Formation:
Pyrolysis of the waste plastics at higher temperatures and long residence times are the best conditions to maximize gas production in the process. Overall, the amount of gases produced from the pyrolysis of polyolefins and PS plastics is quite low in the range of 5–20 wt%, and it strongly depends on the temperature, catalyst, and type of plastic used. The pyrolysis of PET and PVC plastics produced large volumes of gases as compared with PE, PP, PS, and PC.
Several studies have reported that the yield of the gas produced from the pyrolysis of PET and PVC waste plastics is much higher than the liquid yield. Since PET needs very little energy to be converted into other chemical structures, more gas can be produced. Conversely, a de-hydro chlorination stage may occur during the pyrolysis of PVC, and this causes a large amount of gas to be released rather than the liquid product.
The main gases produced during the pyrolysis process of HDPE, LDPE, PP, and PS mostly consist of hydrocarbon gases such as methane, ethylene, butadiene, propane, propene, n-butane, and other miscellaneous hydrocarbons that can be used as a fuel gas due to their high calorific value.
On the other hand, the pyrolysis of PET yields toxic gases such as CO2 and CO, in addition to the formation of hydrocarbon gases. Carbon dioxide and carbon monoxide are produced from PET due to the oxygen present in its molecular structure; the highest content of CO2 from the pyrolysis of PET is reported to be 49.79%. The PVC pyrolysis releases HCl gas, which causes metal corrosion and other environmental impacts.
The gas obtained from the pyrolysis of PE or PP has been reported to have high calorific values of 42–50 MJ/kg. Thus, the pyrolysis gas has a high potential to be used as a heating source in industrial plants. Moreover, ethylene and propene gases can be used as a chemical feedstock for the production of polyolefins if separated from other mixed gases. The gases can also be used in gas turbines to generate electricity and in direct-fired boilers without the need for further treatment.