Plastic carbonization is a technology that converts plastic waste into valuable carbon materials through thermal decomposition in a low-oxygen environment. The process may involve heating, pressurization, and chemical treatment, producing carbon residue and gaseous by-products.
Overview Carbonization process Generally, the carbonization process is described in three overlapping stages as the temperature increases.. The boundaries between the stages are defined by temperature, but the exact values of these temperatures depend on the characteristics of the raw materials and other conditions beyond temperature. The detailed mechanisms of such carbonization reactions were studied in the 1960s.
Carbonization under pressure conditions
Pressure conditions are essential in plastic carbonization.. When carbonization of low-density polyethylene is carried out at atmospheric pressure, little carbonized material is obtained, but direct pressurized carbonization in a closed system yields a residue with a 45% carbon yield.
Direct pressurized carbonization When carbonization material is heated in a pressure-resistant sealed container such as an autoclave, the internal pressure gradually increases as gas molecules are released due to the thermal decomposition of the material. The material is then heated and carbonized while maintaining the pressurized condition. In one study, at high pressures of 300 bar (30 MPA) or higher (without a catalyst), mixing plastic with hydrocarbon components and functional components in the appropriate ratio led to the formation of fine spherical carbon residue particles. Here, the hydrocarbon component is a hydrocarbon plastic consisting only of carbon and hydrogen,Literatures often refer to "oxygen-free plastics" that is not accurate, because plastics containing heteroatoms other than oxygen, such as polyvinyl chloride and polyacrylonitrile, are also chemically oxygen-free. Therefore in this article, the term "hydrocarbon plastics" is used consistently. and the functional component is a heteroatom-containing plastic that includes atoms other than carbon (heteroatoms: oxygen, nitrogen, chlorine, etc.). For example, when 50% by weight PET (functional component) was added to polyethylene (hydrocarbon component), it was effective in forming spherical carbon particles. Examples of hydrocarbon plastics, such as polyethylene and polystyrene, have also been carbonized under pressure without a catalyst.
For heteroatom-containing plastics, oxygen-free conditions are generally used. Inert gases such as nitrogen or argon are introduced into a tubular furnace for carbonization. By varying the temperature, the characteristics of the resulting carbonized material, such as the carbon form, composition, and surface chemical functional groups, can be controlled. For instance, carbonized material obtained at temperatures around 500 °C retains a relatively high number of surface groups, making it potentially suitable as an adsorbent for pollutants. In contrast, at temperatures exceeding 700 °C, the oxygen-containing functional groups of the carbonized material decrease, and its porosity and conductivity improve.
Hydrothermal carbonization Hydrothermal carbonization (HTC) is a type of pressurized carbonization in which heating is performed with water, and pressure is applied with steam. Hydrothermal technology refers to techniques that apply physical and chemical reactions involving high-temperature, high-pressure water or aqueous solutions, and is generally classified into the following three types based on the operational temperature range: Hydrothermal Carbonization (HTC) is typically carried out at relatively low to medium pressures of 180–250 °C and 10–40 bar. It often takes several hours to complete, with carbonized products being the main product formed. Hydrothermal Liquefaction (HTL) is usually conducted at high pressures of 250–500 °C and 50–200 bar or more. It takes a few minutes to a few hours, and when using biomass as raw material, liquid bio-oil is the main product. Hydrothermal Gasification (HTG) is typically performed at high pressures of 500–800 °C and 50–200 bar or more. It only takes a few minutes to complete, with synthetic gas (syngas) being the primary product.
Hydrothermal carbonization is further divided into low-temperature and high-temperature carbonization based on the operational temperature, with 250 °C as the dividing line. In low-temperature hydrothermal carbonization, carbonized material with hydrogen atoms remaining (hydrocar) is produced. As the reaction temperature increases in high-temperature hydrothermal carbonization, products such as graphite, activated carbon, and carbon nanotubes, which are high-purity carbonized materials, are generated. Low-temperature hydrothermal carbonization can be used for the dehydrochlorination treatment of polyvinyl chloride (PVC), which is important for the subsequent formation of aromatic structures in the carbonized products.
Hydrothermal carbonization can be applied to plastics and also to urban waste, including food and paper.
Improving carbonization efficiency through pretreatment To obtain quality carbonized products from plastics, the formation of high-order cross-linked structures between plastic polymer chains in the intermediate products during heating is essential. Otherwise, the carbon-carbon chains of the polymers will independently break, leading to the formation and volatilization of low-molecular hydrocarbons (such as propylene, methane, benzene), and only a small amount of carbon residue will be obtained. This is particularly notable in hydrocarbon plastics made of only carbon and hydrogen, such as polyethylene, polypropylene, and polystyrene, which have very low carbonization residue yields unless modified.
Methods of oxidation and chemical modification include heating the polymer chains in an oxygen atmosphere using a catalyst during the pre-carbonization stage, which oxidizes some (but not all) of the carbon atoms in the polymer chains or using sulfuric acid to introduce sulfonic acid groups.
Carbonization catalysts and additives When plastics are thermally decomposed in the presence of a catalyst, the reaction efficiency improves, enabling carbonization at lower temperatures and the production of high-purity carbon nanomaterials. Catalysts often play a dual role: initially promoting polymer decomposition and later catalyzing the formation of carbon materials.
Common catalysts include zeolites and metal oxides (such as iron oxide, nickel oxide, titanium oxide, etc.). For example, zeolites provide an acidic reaction environment that promotes the cleavage of plastic polymer bonds and aromatization. Transition metal catalysts such as nickel and iron promote dehydrogenation reactions and induce the growth of carbon nanotubes (CNTs) and graphene. The use of a binary catalyst, such as iron/alumina, has been reported to efficiently generate CNTs from polyethylene thermal decomposition. Calcium oxide can be used as an additive in polyvinyl chloride (PVC) carbonization treatments to absorb and recover harmful hydrogen chloride gas from PVC as calcium chloride.
Microwave heating Microwave heating, commonly known from household microwave ovens, can also be applied to the thermal decomposition and carbonization of organic materials, contributing to effective plastic waste utilization and reducing environmental impact. Microwave heating of plastics for carbonization is a process that achieves efficient decomposition and carbonization through selective heating (dielectric heating) using electromagnetic waves, with the theoretical basis being the absorption of microwave energy and the dielectric properties of the material, similar to how a microwave oven works. Commonly used microwave frequencies are 915 MHz and 2,450 MHz (the same as in household microwave ovens).
Plastics themselves generally do not absorb microwaves well, but additives (such as metal oxides) can enhance absorption, allowing for rapid temperature increases. For example, by using an inexpensive iron-based catalyst as a microwave sensor, commercially available ground plastics can be decomposed in just 30–90 seconds, with a high yield of hydrogen gas (55.6 millimoles or 0.112 grams per gram) and multi-layer carbon nanotube carbon residues obtained.
Flash Joule Heating Developed in 2020, Flash Joule Heating is a rapid and efficient carbonization method that can convert carbon sources, such as coal, petroleum coke, biochar, carbon black, food waste, rubber tires, and mixed plastic waste, into quality graphene on a gram-scale by subjecting them to Joule heating (temperature > 2,427 °C) for a short time (about 100 milliseconds). This process does not require high-temperature furnaces, solvents, or reactive gases. When carbon-rich raw materials such as carbon black, smokeless coal, or calcined coke (though not plastic) are used, the yield of graphene ranges from 80–90%, with its carbon purity exceeding 99% without the need for further purification. The electrical energy used during this process is as low as approximately 7.2 kilojoules per gram.
As a development of this technology, a method was also developed in 2023 to recover hydrogen atoms generated as byproducts during the graphene conversion of plastics, capturing them as hydrogen gas. Unlike conventional hydrogen production methods such as steam methane reforming, this procedure does not produce carbon dioxide or carbon monoxide. It can generate hydrogen gas with a purity of up to 94% at a high mass yield. Even when selling graphene product at just 5% of the 2023 market price, the cost of hydrogen gas production may be recovered, with a life cycle assessment showing that carbon dioxide emissions are reduced by 39–84% compared to other hydrogen production methods.
Gasification Though not a form of carbonization, plastic thermal decomposition can generate gaseous substances, which can then be used as an energy recovery method. The recovery of hydrogen gas, as mentioned in the Flash Joule Heating section, is one such example. While not involving plastics, hydrothermal gasification of biomass is suitable for hydrogen production, with a maximum hydrogen yield of 45 grams per kilogram of biomass.
Types of plastics and carbonization behaviour
Organic molecules composed solely of carbon and hydrogen are typically less reactive and, without forming cross-linked structures between the polymer chains, tend to break down into low-molecular-weight compounds during carbonization. In contrast, when heteroatoms like oxygen are substituted into the polymer chains, the bonds between carbon atoms and these heteroatoms can serve as a foundation for forming cross-linked structures. These cross-linking structures play a critical role in preventing the polymer chains from breaking down into small molecules during thermal decomposition, which is result in carbonization residue.
Hydrocarbon plastics For hydrocarbon plastics such as polyethylene and polystyrene, as mentioned above, direct thermal decomposition results in the fragmentation and vaporization of small molecular hydrocarbons, with little carbonization residue obtained.
Oxygen treatment