Abstract
The application of plastics in various sectors led to its increased production globally and this demand, in turn, caused an overflow of plastic waste in landfills, illegal dumping in the sea, and environmental pollution. To overcome this issue, several alternatives for managing plastic wastes have been developed and among them, reuse, recycling, and energy recovery methods are highly acknowledged methods. Nonetheless, recycling methods come with certain disadvantages like mixing and segregation of wastes, high labour costs associated with segregation and processing, by-product disposal, and its usage. Researchers have shifted their focus to energy recovery systems because of these drawbacks. Extensive research in this area led to the development of converting waste plastics into liquid fuel through the process called pyrolysis. The pyrolysis process can thermally degrade plastics in the absence of oxygenproducing oil and monomers. The temperature has the most impact on the pyrolysis process and depending on the types of plastic wastes, the pyrolysis temperature varies between 300 – 800 oC. The oil yield due to the variation in temperature varies between 45 – 95 wt.% and the calorific value of the oil has been observed to be in the range of 9679 – 11428.5 kCal/kg, which is similar to the other commercial fuels. Also, the review indicates that it is possible to extract up to 84% of fuel from 1-kg plastic at 360 oC. As a result, following refining/blending with conventional fuels, pyrolysis oil can be utilised as an alternate source of energy and transportation fuel. Apart from the temperature, the other influencing factors include, the reactor design and its size, pressure, heating rate, residence time and feedstock composition. The pyrolysis process was examined in terms of plastic types and primary process factors that impacted the end result, such as oil, gaseous, and char. Temperatures, reactor types, residence duration, pressure, catalysts, and other critical factors were examined in this work. Furthermore, the study examines technological problems and current advances.
Highlights
The application of plastics in various sectors led to its increased production globally and this demand, in turn, caused an overflow of plastic waste in landfills, illegal dumping in the sea, and environmental pollution
Research indicates that plastics such as Polyethylene terephthalate (PET) and PVC produced little liquid yield and some plastics like PVC will never be used in the pyrolysis process as they produce harmful gases with harmful substances as by-products [4]
It was proved that Zeolite helps in reducing impurities and enhances the efficiency of liquid yield.A study on the use of bentonite clay in the form of pelletized catalyst for the pyrolysis of plastic wastes such as PS, PP, Low-density polyethylene (LDPE), and High-density polyethylene (HDPE) was taken up by Supattra Budsaereechai et al [33]; observations revealed that the liquid oil produced, had a higher calorific value than compared with the experiments that used catalyst.Maximum production of gas was reported in the case of pyrolysis of PP with an alumina-loaded catalyst by Obali et al [34]
Summary
Pyrolysis oil as observed from various literature is made up of various hydrocarbons, many of which are partially oxygenated with the presence of water, ash, and charcoal. The toxicity of oil and its unstableness in air possess challenges for having to be used directly into the engine. The main goal of the pyrolysis of plastic waste oil is to use it in engines directly, but the existing technology does not allow it to do so. The presence of charcoal, water and ash particles needs to be removed before blending it along with conventional fuels and or to replace the existing conventional fuels, raw material availability, viscosity, combustion behaviour, wax formation, the choice of reactors, etc., are other challenges that are discussed below
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