Plastics are synthetic organic polymers, which have been serving humans in day-to-day life, because of their large range of physical and chemical attributes like strength, durability, lightweight, flexibility, resistance to the extremity (thermal, electrical, and chemical), and their ability to mold into different shapes. The global production of plastics (polymer resins and fibers) in the year 2015 was estimated as 381 million metric tons (MT), increased by 75% since the year 2000. The per capita consumption of plastics in India is around 11 kg/year (2016) whereas the average global consumption is about 28 kg/year. The largest share of plastics consumption is in packaging applications (44%). The presence of plastic waste in municipal solid waste (MSW) has been increased from less than 1% in 1960 to more than 10% in recent years. Plastic waste generation leads to huge accumulation, instead of decay in the landfill and in the various natural habitats like rivers and oceans. Thermal treatments like combustion or incineration can eliminate plastic waste permanently but such processes generate harmful emissions to the environment. Recycling (mechanical/chemical) is a possible path for plastic waste disposal. However, most of the recycling processes are costly, energy-intensive, and end up producing low-grade products. Pyrolysis is a very viable and sustainable waste management process in the treatment of municipal solid waste containing carbonaceous materials like plastics and biomass.
The process of pyrolysis involves the degradation of complex molecules like polymers (plastics) into short-chain, less complex molecules by the application of heat and/or pressure under inert conditions. Most plastics pyrolysis plants utilize high temperatures (700 C) to moderate temperatures (500 C) in the presence of a suitable catalyst. The high-temperature pyrolysis process is generally performed in fluidized bed type reactors for a very short reaction time (fast pyrolysis) which produces an excess amount of gaseous products. On the other hand, the liquid product obtained under such conditions requires post-process up-gradation (distillation) to be utilized as diesel, petrol, and fuel oil alternative To increase the production of high-quality liquid products (fuel), the concept of slow pyrolysis can be applied. The advantage of slow pyrolysis over fast pyrolysis is manifold, particularly the long duration in slow pyrolysis leads to superior (controlled) heat transfer, better control over flow rates of inlet and outlet (controlled product collection), and high liquid yield. Detailed understanding of slow pyrolysis including the effect of long duration is still lacking in terms of product distribution to target value-added production, and limited reports are available. carried out low-temperature pyrolysis of low-density polyethylene (LDPE) and polystyrene (PS) and their mixture in the range of 350–500 C for 1 h and 2 h durations. LDPE degradation started at 400 C and the highest yield of oil was reported at 425 C with a slight alteration in the oil compositions due to the long duration of pyrolysis. Similarly, carried out the pyrolysis of polypropylene (PP) and polystyrene (PS) at two isothermal conditions; 350 and 420 C for two durations, 180 min, and 18 h. They observed an increase in the conversion with reaction time up to a certain extent (90 min) and further perceived no significant change.
In the current study, the three most common plastics comprised of low and high-density polyethylene (LDPE and HDPE) and polypropylene (PP) abundantly used in packaging applications were chosen for slow dynamic (heating rate of 1 C min 1) pyrolysis studies. The plastic derive oil (PDO) obtained at various intervals during the slow pyrolysis were identified and quantified by various analytical techniques and the gases were analyzed and quantified using gas chromatography (GC). before pyrolysis. The proximate analysis and calorific values of virgin and waste samples (dry) are reported in Table 1.
The liquid hydrocarbon standards (ASTM D5307 internal standards from Supelco) and gas sampling bags (Tedlar bags) were procured from Sigma Aldrich. Gaseous hydrocarbon standard (mixture) and the standard permanent gas mixture (CO2, CO, and H2) were procured from Chemtron scientific, India, and Centurion, India respectively. Dichloromethane (HPLC grade) and Deuterated chloroform (CdCl3 99.8%) were obtained from Merck (India). Packaging plastic waste consisting of low and high-density polyethylene and polypropylene was pyrolyzed in a lab-scale semi-batch reactor at a very slow dynamic condition (1 C min1). Gaseous and liquid products were collected at regular intervals starting from their inception during the degradation process. Detailed analysis was carried out to estimate the properties of plastic derive oil (PDO) obtained at different stages of the pyrolysis process. The pyrolysis temperature has a significant effect on the product compositions. The paraffin concentration increases with increasing pyrolysis temperatures. On the other hand, increased pyrolysis temperature decreases olefin concentration. Olefinic content in the PDO was found comparatively higher when PP was in the feed. The presence of polypropylene in the feed caused the production of PDOs with branch-chain hydrocarbon components with a high isoparaffin index and research octane number (RON). The PDOs obtained (for all feed studied) at the early stages of the degradation process have light hydrocarbon liquid fractions belonging to light and middle distillates of petroleum (C6 – C20). The yield of both light and middle fractions decreased as the pyrolysis reactor temperature reached the maximum value (400 C). Gas evolution pattern depends on both pyrolysis temperature and the feed composition. Propylene was found more dominating among other major components of gases like methane, ethane, ethylene, propane, n-butane, 1-butene, isobutylene, n-pentane, etc.
Conclusion
Non-isothermal slow pyrolysis of polyolefins (LDPE, HDPE, and PP) provides a detailed understanding of the influence of reaction temperature on the product distribution. The utilization of slow pyrolysis as an alternative approach to producing targeted value-added products (gasoline/diesel) from plastic waste is significant considering the plastic disposal and management issues. NMR analysis of the PDO obtained from PP has shown encouraging prospects of producing gasoline blend components with a very high RON value (92). High values of the H/C ratio of the PDOs ensure clean burning if used as a liquid fuel. PDO samples obtained at low temperatures have a higher percentage of lighter fractions (C6 – C11). The evolution of hydrocarbon gas also varies as the process progresses, due to the effect of temperature on gas formation.
The overall gas yield (<C6) decreases with the initiation of gas condensation. Propylene concentration was found to be higher compared to other gases. The gas evolved during pyrolysis will play a significant role in the implementation of the process as most of these gases have high calorific value and can be exploited as a fuel source for the process. However, more in-depth research is necessary for terms of large-scale implementation. This study provides valuable insights, which may help design and optimize pyrolysis process plants to convert packaging plastic waste into valuable fuel.
