Pyrolysis of Plastic Waste: Oil Production

In general, pyrolysis converts mixed plastics into gas, liquid oil and solid residue (char). The liquid can be further refined for fuel or new plastics production. A higher heating rate, a lower pyrolysis temperature, and a shorter residence time maximize the yield of liquid products from the pyrolysis of polymer feedstock. Many observations have verified that the production of liquid fuels from plastic waste via pyrolysis is promising and commercially viable. Overall, the physical properties of plastic pyrolysis oil are very close to the properties of commercial gasoline and diesel. Therefore, pyrolysis oil has a very high potential to be used as a new energy resource.

Liquid oil from the pyrolysis of plastic waste may contain some impurities such as sulfur, chlorine, solid residue, moisture, and acids. The presence of these substances not only reduces the quality of liquid oil but also limits its commercial applications; therefore, liquid oil requires further processing. The procedure for improving the quality of liquid oil depends on the application being targeted. There are two ways for liquid oil upgrading. One of them is to mix the pyrolysis oil with conventional diesel to provide certain fuel properties. The other is refining. After upgrading the pyrolysis oil into fuel and removal of impurities, it can be used in modified diesel engines as transport fuel and for the generation of heat and electricity.

Several studies have reported that the most effective temperature to optimize the liquid oil yield in plastic pyrolysis is in the range of 500-550 ^◦C. However, with the usage of a catalyst in the pyrolysis of plastic waste, the optimum temperature can be lowered down to 450^◦C and a higher liquid yield is obtained. In most plastics, the usage of the catalyst in the process might improve the liquid oil yield, but PS is exceptional. This is because PS decomposes very well without the need for any catalysts to speed up the reaction and produces liquid oil by 97 wt%. Therefore, PS is the best plastic for the pyrolysis process since it produces the highest amount of liquid oil among all the plastics. Except PE, all types of plastic waste are degraded into liquid oil at a maximum pyrolysis temperature of 450 ^◦C. The pyrolysis of PE without a catalyst converts it into wax instead of liquid oil due to its long-carbon chain structure. The use of catalysts is very helpful to avoid wax formation during the pyrolysis process because catalysts accelerate chemical reactions.

The produced liquid oils from different types of plastic waste have different physical and chemical properties such as viscosity, density, flash point, and pour point. According to previous studies, the experimental calorific values of the liquid oil produced from the pyrolysis of PP, HDPE, and LDPE are all above 40 MJ/kg; these values may be considered sufficient to be used as fuel. The calorific value of PS was commonly lower than the polyolefin plastic due to the presence of an aromatic ring in the chemical structure that has less combustion energy as compared with aliphatic hydrocarbons. Overall, PET and PVC have the lowest calorific values (below 30 MJ/kg) due to the presence of benzoic acid in PET and chlorinated compounds in PVC.

Pyrolysis Oil of Mixed Waste Plastics:

According to current literature, the main components of liquid oil produced from the pyrolysis of mixed waste plastics are unsaturated hydrocarbons and aromatics. The composition of the pyrolysis oil depends not only on the process conditions, but also on the type of waste plastic. A higher yield of aromatic hydrocarbons is obtained when PS and PET are pyrolyzed. On the other hand, PP, HDPE and LDPE produce oil that is rich in paraffins, olefins and waxes based on aliphatic hydrocarbons. Wax is an intermediate product, mainly composed of long-chain hydrocarbons having carbon numbers greater than C₂₀, and further cracking is applied to convert it into liquid oils.

The chemical composition of plastic waste-derived oils can determine if the liquid oil can be used as gasoline, diesel, other fuels, or as chemicals. For example, with the goal of producing naphtha as a potential feedstock for virgin plastic production, more paraffins and less aromatics are desirable in naphtha. Branched hydrocarbons are essential components to offer improved physical properties, such as lower freezing point and better cold flow, for use as transportation fuels.

Based on our knowledge, a pyrolysis oil sample produced from plastic waste should be distilled into naphtha-range hydrocarbons (the boiling temperature ˂ 170 ^◦C), kerosene-range hydrocarbons (170 ^◦C ˂ the boiling temperature ˂ 280 ^◦C), diesel-range hydrocarbons (280 ^◦C ˂ the boiling temperature ˂400 ^◦C), and wax (the boiling temperature ˃ 400 ^◦C) fractions first.