Pyrolysis as a value added method for plastic waste management: A review on converting LDPE and HDPE waste into fuel

doi:10.21203/rs.3.rs-1693804/v1

The global demand for plastic is increasing year by year due to its indispensable uses and excellent properties. The rate of post-consumer plastic waste discarded into the environment is also increasing in parallel with the demand for plastics. The amount of plastic waste generated varies based on its uses such as household, industrial, commercial, hospital, and others. Plastic wastes persist for many years due to their slow deterioration and cause severe environmental problems. Therefore, there is a growing focus worldwide on plastic waste disposal methods to overcome adverse environmental impacts. As the plastics are petroleum-based materials, the pyrolysis of plastics to fuel oil, gases, and char, has a great concern than the other plastic waste management methods of recycling and landfilling. A yield of 70-80 wt.% of liquid fuel from a pyrolysis waste has been reported elsewhere which emerges the importance and aptness of this method in plastic waste management. This paper review the existing literature on pyrolysis processes developed for HDPE and LDPE wastes globally and their governing factors of heating rate, temperature, processing time, polymer to catalysts ratio, type of the catalysts, and type of the reactor that influenced the yields of fuel oil and gases

Catalysts

fuel

plastics

pyrolysis

waste management

Plastic plays an important role by enhancing the human lifestyle in various sectors such as construction, automotive, healthcare, electronics, and packaging due to its excellent properties like lightweight, high strength, durability, non-corrosive and economic feasibility (Table 1) (Anuar Sharuddin et al. 2016). Plastics can be classified into several groups based on their chemical structure, synthesis process, and their properties. To assist the recycling of waste plastics, the Society of Plastic Industry(SPI) has defined a resin identification code system. Therein, the plastics have been divided into seven groups based on types of plastics used as raw materials for manufacturing articles (ASTM International 2013), and those groups are given code numbers from 1 to 7 (Table 1) (Okunola A et al. 2019). The global production of plastic has been estimated to reach 335 million tons by 2020 (Lee et al. 2020). As the consumption of plastic is increasing worldwide, plastic waste has become a major component in municipal solid waste. Municipal plastic waste (MPW) is a crucial alarming problem in many urban areas in Sri Lanka as in many other countries. Currently around 500,000 metric tonnes of primary forms and products imports into Sri Lanka according to the records of Sri Lanka Customs. Figure 1 shows the amounts of various types of plastics imported to Sri Lanka from 2016 to 2020. Accordingly, High-density polyethylene (HDPE), Polyvinyl chloride (PVC), Low-density polyethylene (LDPE), and Polypropylene (PP) are the most common plastic types imported into Sri Lanka. All other plastic types imported were around 100,000 metric tons in the last five years. The considerable decline in the quantity of plastics imported in 2019 and 2020 could be due to the downfall of the trade under the COVID-19 pandemic outbreak.

Before 2009, the key problems of municipal solid waste management in Sri Lanka were disposal, storage, processing, and transportation of solid waste. As a result of the considerable efforts made by the government to preserve the public sanitation environment, nowadays, the key concerns are the implementation of adequate intermediate treatment and final disposal methods (Fernando 2019). The identified reasons for rapid waste generation in Sri Lanka were urbanization, population evolution, migration of people to urban areas from remote areas, and modifications in lifestyles (Arachchige et al. 2019). When it is considered about the plastic waste, a national survey conducted in Sri Lanka by the Japan International Cooperation Agency(JICA) in 2016 shows that the amount of plastic waste is about 10% and 5% of urban and rural total solid waste, respectively (Karunarathna et al. 2020). The amount of soft plastics is higher than that of the hard plastics present in both urban and rural solid waste streams. The soft plastic waste commonly contains single used post-consumer packaging waste such as polyethylene bags, shopping bags, and lunch sheets.

1.1 The adverse effect of mismanagement of plastic waste

Mismanagement of the plastic waste adversely affects the natural environment. The most common plastic waste disposing methods practiced in Sri Lanka are open dumping to empty lands, collecting for recycling, and burning in the open fire (Maheshi et al. 2015). When plastic waste is discarded into open dumps, mostly the light-weight materials can spread over the open dumping sites into other lands resulting in an unpleasant environment in the cities. Animals near the open dump areas especially wild animals happen to eat those plastic waste with food waste and are susceptible to a painful death. On the other hand, plastic waste clogging in drainage systems in urban areas causes flooding even in light precipitates. Also, the hollow plastic articles act as water containers and after precipitation they create breeding sites for mosquitos and spread epidemic diseases such as Dengue in the tropical region in the world.

Furthermore, when plastic waste is dumped in open lands or landfilling, the hazardous chemicals embedded may leach out into soil contaminating ground and surface water. The leachates may contain toxic chemicals including Bisphenol A (BPA), phthalates, and chlorinated organic compounds released during the degradation of plastic materials (Okunola A et al. 2019; Asakura et al. 2004). In addition to the degradation, the toxic chemicals in additives used to enhance the properties of the plastics, such as alkylphenol additives and phthalate plasticizers, heavy metals in pigments(eg: Pb, Zn, Cu, Co, Cr, and Cd) can migrate to the soil after disposal of plastic waste in open dumps and landfills (Rafey and Siddiqui 2021; Campanale et al. 2020; Teuten et al. 2009). The migration of the additives in the plastic mainly depends on the degree of the crystallinity of the plastic (Hansen et al. 2013) and interaction of the additives with the polymer (Bejgarn et al. 2015). Moreover, the open burning of plastic waste can emit hazardous pollutants such as dioxins, polychlorinated biphenyls (PCBs), brominated compounds, furans, and heavy metals(eg: Cu, Cr, Co, Pb, and Hg), causing severe damage to the respiratory system in both humans and animals (Okunola A et al. 2019; Verma et al. 2016; Alam et al. 2019; Filella and Turner 2018). Further, these heavy metals and chemical compounds may destroy the helpful bacteria in the soil leading to infertility of the soil.

Plastic waste put down in open dumps, rivers, and waterways harm the marine organisms and the habitats of animals and eventually end up in the ocean (Okunola A et al. 2019). Respectively, Sri Lanka has been ranked as the fifth among 20 countries that released plastic waste into the ocean (Jambeck et al. 2015; Jang et al. 2018). Scientists have estimated that the weight of plastic waste in the ocean would be increased more than the weight of the live fish in the ocean by 2050 (Sarah Kaplan 2016). More than 260 species of marine organisms were found to be ingested or entangled in plastic debris and ending their lives in fatalities (Okunola A et al. 2019; Gregory et al. 2013; Purba et al. 2019). This plastic waste in the ocean can be categorized into different sizes such as macroplastics (˃ 200 mm), mesoplastics (5-200 mm), microplastics(1 µm-5 mm), and nanoplastics (< 1 µm) (Worm et al. 2017). Microplastic has been identified as the major pollutant from these four types. Microplatics may be created in the production process or may be formed after the degradation of plastics (Alomar et al. 2016). The microplastics may infiltrate through living tissues in the food chain and can cause severe health problems (Okunola A et al. 2019; Jambeck et al. 2015).

1.2 Better municipal plastic waste management strategies in developing countries

Due to enormous problems created by the plastic waste, many countries worldwide are struggling to find solutions for its management. Particularly, developing countries that have not implemented a feasible plastic waste management system are facing critical environmental and social problems. The studies carried out in India (Rafey and Siddiqui 2021), Bangladesh (Masud et al. 2017), Malaysia (Chen et al. 2021), Vietnam (Salhofer et al. 2021), and Thailand (Wichai-utcha and Chavalparit 2019) have been reported about the efforts taken to address the issues of the rapid accumulation and mismanagement of plastic waste in their countries. Therein, the reasons identified for the improper plastic waste management in developing countries were lack of capital investment and infrastructure, migration of population to urban areas, dearth of awareness in the society, lack of adequate technical instruments, dearth of strict restrictions on plastic waste disposal, low rate of recycling, lack of separation of household plastic fwaste, and practice of improper disposal methods (Fig. 2) (Rafey and Siddiqui 2021; Purba et al. 2019; Masud et al. 2017; Hossain et al. 2020; Padgelwar et al. 2021; Evode et al. 2021). As a positive approach to plastic waste management, China has banned imports of plastic waste from western countries (Brooks et al. 2018; Vollmer et al. 2020; Marks 2019). This action has been intensified the impetus of developed countries that have sufficient infrastructure, to reconsider plastic usage and to implement recycling programs without sending their plastic waste to other countries. Consequently, many developed countries such as the United Kingdom, Canada, United Sates, Japan, Ireland, and Taiwan have introduced bans on single-use plastic bags and bottles or collected a tax from customers or retailers to promote environmentally friendly alternatives for single-use plastics (Wichai-utcha and Chavalparit 2019; Palugaswewa 2018). National action plans have been launched in Indonesia to attempt a 70% reduction in marine plastic waste by the year 2025 (Purba et al. 2019). Moreover, manufacturers are forced by the rules and the restrictions to make plastic products that can be recycled (Wichai-utcha and Chavalparit 2019). However, economically developing countries have been challenged by plastic waste management issues due to the lack of sustainable methods of reducing or recovering plastic waste. The fish-bone diagram (Fig. 2) shows the above discussed common causes affecting the overall improper plastic waste management in Sri Lanka mainly under the characteristics of management, method, material, machine, community, and nature.

This Fish-bone diagram can be used to identify the origins related to improper waste management in developing countries. Most common plastic waste sources can be identified as general household plastic waste, industrial plastic waste, commercial plastic waste, and hospital plastic waste. Mismanagement of these wastes causes natural environment pollution and numerous other problems as mentioned before. The existing management methods including open dumping systems, poor planning and maintenance of landfills, and burning of plastic waste in the open air, which has not been properly addressed, have posed a great threat to the environment. Improper management of plastic waste, i.e., lack of policies, absence of source separation system, no substitutions to plastics, no particular authority for plastic waste collection, absence of proper recycling system, and low enforcement has triggered the arise of plastic waste in the country. In addition, dearth of machinery, technology and collection means has caused infrequent collection of plastic waste in the country. On the other hand, community support is minimal due to unawareness and ignorance. Especially majority is not aware of deleterious impact caused by the misusage of plastics and still moving away from the consumption of traditional environmentally friendly materials.

In order to mitigate the plastic pollution, proper management strategies such as reuse, recycling, and energy recovery methods have to be adapted (Ayeleru et al. 2020; Budsaereechai et al. 2019). When considering the recycling process, plastic waste can be categorized into two types such as mono-stream plastic waste (ex: post-industrial waste; runners from injection molding, waste from production changeovers, fall-out products, cuttings, and trimmings), and complex-stream plastic waste (post-consumer waste; mixed plastic of unknown composition, contaminated fractions with organic or non-organic materials) (Ragaert et al. 2017). Typically, the post-industrial plastic waste undergoes close-loop recycling which reuses the waste to produce the same product. The post-consumer plastic waste undergoes open-loop recycling process converting them to different product than the one they were originally recovered from (Ragaert et al. 2017; Al-Salem et al. 2009). Therefore, the latter type of recycling processes are expensive due to the multiple steps involved therein, including waste identification and separation, shredding, cleaning, melting, and pelletizing as shown in Fig. 3 (Masud et al. 2017; Ragaert et al. 2017; Klaimy et al. 2020). Ultimately, the recycled plastics also ended up in open dumps as a waste.

Plastic waste recycling processes can be identified under four major categories such as re-extrusion(primary), mechanical(secondary), chemical(tertiary), and energy recovery(quaternary) processes (Al-Salem et al. 2009; Schyns and Shaver 2021). The major limitation in mechanical recycling is the separation of the complex stream plastic waste into their particular categories. The “wet separation” is a common method used in plastic separation, which has been implemented in different modes such as sink-float separation (Bauer et al. 2018), froth flotation (Wang et al. 2015), hydrocycloning (Serranti and Bonifazi 2019). The separation of filler-containing and hollow waste plastic products, and composites are difficult using wet separation techniques (Vollmer et al. 2020). Also, additional energy is required for the drying process prior to the extrusion after the wet separation is another drawback (Arachchige et al. 2019). The “density separation” is not applicable for many plastics due almost nearer densities of different types (ρHDPE = 0.941, ρ_LDPE = 0.915–0.925, ρ_LLDPE = 0.91–0.94, ρ_PP = 0.90–0.94, ρ_PET = 1.35–1.40, ρ_PVC = 1.34–1.43 g/cc) (Al-Salem et al. 2009; Schyns and Shaver 2021). These shortcomings can be minimized during the incineration process, as even composite plastic waste can be used in the incineration process as no detailed source separation is required.

In the incineration process, plastic waste is burned at high temperatures (˃ 1000°C) (Dodbiba and Fujita 2004) in the oxygen environment and the releasing energy could be recovered as heat and transformed into steam and electricity (Gradus et al. 2017). However, the associated cost related to the investment, maintenance, and reducing environmental impacts (CO₂ emission, release of dioxins, other polychlorinated biphenyls, and furans) is high in the incineration process (Gradus et al. 2017; Hopewell et al. 2009). Process handling in incineration is more difficult than the pyrolysis process.

Among the recycling methods available, pyrolysis is the most effective and sustainable method for plastic waste management because of its viability of converting plastics to fuel oil (gasoline, kerosene, diesel, furnace oil), char, and gases. These end products can be used as value-added products (Verma et al. 2016; Budsaereechai et al. 2019). In the pyrolysis process, long-chain hydrocarbons are degraded into small chain hydrocarbons or less complex molecules upon heating in an oxygen-free environment (Ragaert et al. 2017; Sharuddin et al. 2018; Panda et al. 2010). Many research articles claimed that the pyrolysis of plastics produces a high amount of fuel oil (up to 80 wt.%) at moderate temperatures around 500°C (Anuar Sharuddin et al. 2016; Sharuddin et al. 2018; Wróblewska and Rydzkowski 2020; Eze et al. 2021). Considering the effectiveness of the pyrolysis process and its adaptability to the local context, this review discusses the pyrolysis of High-density polyethylene (HDPE) and Low-density polyethylene (LDPE), the most abundant waste plastics in the environment. Further, herein, the effects of process control parameters such as applied temperature & pressure, type of reactor, residence time, types of catalysts, and the type of fluidizing gas and its flow rate on the pyrolysis process are discussed.

The HDPE and LDPE are thermoplastic materials of the polyolefin family, which are of petrochemical origin. Polyethylene is the most common and well-known plastic material used to manufacture many products. The properties of HDPE and LDPE are tabulated in Table 2 (Sam et al. 2014; Kazemi Najafi 2013; Mendes et al. 2011; Kwon et al. 2002). HDPE is a linear polymer with high degree of crystallinity. It is widely used to manufacture containers/bottles for detergent, milk, oil, shampoo, conditioner, and bleaches (Adrados et al. 2012). LDPE has a low degree of crystallinity due to its branched structure. The branches make it more flexible than HDPE (Salih et al. 2013). Hence, LDPE can be applied for a wide range of general products in the packaging industry such as plastic bags, wrapping foils for packaging, trash bags, etc. (Anuar Sharuddin et al. 2016). Therefore, the amount of waste LDPE present in municipal solid waste is very high with compared to that of other plastics.

Table 2

Properties of LDPE and HDPE
Property	LDPE	HDPE
Chemical structure	More branching	Less branching, more linear
Density	0.91–0.94 g/cm³	0.95–0.97 g/cm³
Flexibility	More flexible due to low degree of crystallinity (50–60%)	More tough and rigid due to high degree of crystallinity (> 90%)
Melting point	101–115°C	135–145°C
Chemical resistance	Resistant to most alcohols, acids, and alkalis; low resistance to oxidizing agents and selected hydrocarbons	Superior resistance to solvents, alcohols, acids, and alkalis; low resistance to most hydrocarbons
Strength	Relatively increased impact strength in cold conditions	High tensile and specific strength
Transparency	High, due to amorphous condition	Low, due to increased level of crystallinity
Tensile strength at 20 °C	6–17 MPa	14–32 MPa

Many researches on the pyrolysis of HDPE have been conducted during the last decade (Budsaereechai et al. 2019; Adeniyi et al. 2019; Shukla et al. 2016). Budsaereechai et al. (2019) investigated the pyrolysis of HDPE using a bench-scale fixed-bed batch reactor. The pyrolysis was performed at 500°C under an optimum heating rate of 10°C/min in the presence of a nitrogen medium. And also, highly mesoporous bentonite clay catalysts with BET surface area of 47 m²/g were used in this research. The results revealed that the highest total conversion occurred yielding 88.9 wt.% fuel oil at 3:20 of catalyst to polymer ratio.

Adeniyi et al. (2019) have investigated, thermal cracking pyrolysis of HDPE in a simple batch reactor at 425°C under the heating rate of 7°C/min. This research has resulted in a high liquid yield (51.84%), a high char yield (45.33%), and a low gaseous yield (2.83%). It also reported that the possibility of cracking the resulted char into the liquid fuel with further heating above 550°C. Although the designing and process controlling of a simple batch reactor are easier than those of a fixed-bed batch reactor, the simple batch reactor could not be used for high-scale conversations. This is due to the high operation cost associated with batch-wise operations and the tendency of high coke formation in the batch process (Anuar Sharuddin et al. 2016).

Al-Salem (2019) has studied the thermal pyrolysis of HDPE in a novel fixed bed reactor system to produce gasoline range hydrocarbons. The pyrolysis trials were conducted at different temperatures between 500°C to 800°C while purging (20 ml/min) nitrogen carrier gas. Therein, the optimum liquid yield obtained was 70 wt.% at 550°C temperature. The novelty of this fixed bed reactor was the presence of two gas/liquid separators (GLS) used in the system. This pyrolysis system consisted of a collection hopper with nut and bolt at the bottom of the reactor for collecting ash/char. The resulting liquid yield of this study is moderately higher than that of the typical pyrolysis systems which employ fixed bed reactors (Adeniyi et al. 2019; Marcilla et al. 2009a; Patil et al. 2018; Mastral et al. 2006; Kumar et al. 2013). This high liquid yield was credited to the design of the reactor which has three heating elements that maintaining uniform temperature along with the profile of the reactor.

Further, Elordi et al. (2012) studied the pyrolysis of HDPE in a conical spouted bed reactor (Fig. 4) at the temperature of 500°C using two types of HZSM-5 zeolite catalysts. One of the catalysts composed of SiO₂/Al₂O₃ with a ratio of 80 and the other has been composed of SiO₂/Al₂O₃ with a ratio of 30. When the catalyst with SiO₂/Al₂O₃ ratio of 80 was used, the system has produced a 59.8 wt.% yield of olefin (C₂-C₄), a 25.4 wt.% yield of non-aromatic C₅-C₁₁ fraction, a 5.6 wt.% yield of light alkane fraction, and a 6.7 wt.% yield of monoaromatics. At the same time, the system has produced a 57.0 wt.% yield of olefin (C₂-C₄), a 15.5 wt.% yield of non-aromatic C₅-C₁₁ fraction, a 14.8 wt.% yield of light alkane fraction, and a 10.9 wt.% yield of monoaromatics with the catalyst that SiO₂/Al₂O₃ ratio of 30. The coke formation on the catalyst was found to be low when the SiO₂/Al₂O₃ ratio of the catalyst is high. It has been demonstrated that conical spouted bed reactor has special properties for avoiding the agglomeration of particles when they collide, as it facilitates the cyclic movement to the catalytic particles. Generally, the feed particle size of the conical spouted bed reactor is 14 mm (Srifa et al. 2019). The main difference between this reactor and the fluidized bed reactor is the conical spouted bed reactor allows for a continuous operation with higher plastic flows into the reactor unit offering a higher versatility. The schematic diagram of the conical spouted bed reactor is shown in Fig. 4. The main drawbacks in the conical spouted bed reactor are the difficulties in catalyst feeding and product collection (solid and liquid).

According to the literature, the majority of the laboratory scale plastic pyrolysis processes have been carried out in batch reactors, semi-batch reactors, or continuous flow reactors such as fixed bed, fluidized bed, and conical spouted bed reactors. There are some advantages and disadvantages in using each type of reactor in the plastic pyrolysis process. Table 3 summarizes a comparison between the uses of the batch reactor and the fluidized bed reactor in pyrolysis (Anuar Sharuddin et al. 2016; Scheirs and Kaminsky 2006; Kaminsky 2021).

Table 3

Advantages and limitations of the batch reactor and fluidized-bed reactor in the plastic pyrolysis process
Batch Reactor	Fluidized-bed reactor
Parameters can be easily controlled in the thermal pyrolysis process.	Difficult to control the parameters in the thermal pyrolysis process.
Offer a high liquid yield in the thermal pyrolysis process.	Offer a high gaseous yield in the thermal pyrolysis process.
Not suitable for the catalytic pyrolysis process due to the high tendency of coke formation on the catalyst surface.	Best reactor for the catalytic pyrolysis process due to the ability of reuse the catalyst many times without the forming of coke on the catalyst surface.
Operational cost is high for large-scale production due to frequent repeated recharging and restarting.	Operational cost is low due to low repeated feedstock recharging and no need of frequent restarting.
Suitable for lab-scale experiments.	Suitable for pilot scale operations.

Kun Wan and coworkers (2020) have studied the catalytic pyrolysis of LDPE at 500°C using biomass derivated active carbon as the catalyst and obtained jet-fuel range alkanes (C₈-C₁₆) and aromatics (< C₁₆). Herein, 75.3 wt.% of liquid yield, 23.4 wt.% of gaseous yield, and 1.3 wt.% of char yield have been obtained. This study further revealed that the activated carbon with strong acidity and high catalytic temperature parameters are beneficial towards the generation of aromatics, while activated carbon with weak acidity and low catalytic temperature parameters are favorable towards the generation of alkanes.

Fan et al. (2017) and Zhang et al. (2015) have studied the pyrolysis of LDPE using a microwave-assisted system. In this system, LDPE waste has been thermally cracked at 500°C of temperature using SiC as the microwave absorbent to enhance the heating of LDPE powder. The resulting vapor has been sent through an ex-situ catalytic system composed of a quartz column. This column was filled with the catalyst which is sandwiched between a layer of quartz wool and a polyporous (pore size was 90–150 mm) fritted disc. The quartz wool and fritted disc help to secure the catalyst powder in place and facilitate the uniform passage of the resultant vapors through the column. A heating tape was used to heat the catalytic bed and the temperature of the reaction was measured using a K-type thermocouple. By using microwave-assisted thermal cracking without using a catalyst Fan and coworkers and Zhang and coworkers have obtained 38.5 wt.% of liquid yield and 32.58 wt.% of a liquid yield, respectively (Fan et al. 2017; Zhang et al. 2015). In microwave-assisted thermal cracking followed by catalytic cracking, Fan and coworkers have used MgO as the catalyst whereas Zhang and coworkers have used ZSM-5 catalyst. In that effort, both research groups have been able to upgrade the liquid yield up to around 45 wt.%.

Bagri and Williams (2002) have investigated the polyethylene pyrolysis in a fixed-bed reactor at 500°C with a heating rate of 10°C/min. The pyrolysis was carried out for 20 min using nitrogen as fluidizing gas. Without employing the catalyst this has resulted in a high liquid yield of 95 wt.% with low gaseous yield and negligible char formation in the thermal pyrolysis process. When Y-zeolite was used as the catalyst in the process, this liquid yield has reduced to 85 wt.% and the gaseous yield has increased to 10 wt.%.

In another study of LDPE pyrolysis, Marcilla et al. (2009b) have observed that the thermal degradation of LDPE occurred between the temperature of 469–494°C, whereas in the HDPE thermal degradation occurred between the temperature of 490–510°C. Furthermore, Onwudili et al. (2009) have observed that the LDPE thermally decomposes into oil at the temperature of 425°C. A brown waxy material formed at a temperature below 410°C indicating the incomplete combustion of the material. They concluded that the most optimum temperature to obtain the highest liquid yield from LDPE was 425°C.

The chemical composition of the HDPE and LDPE plastic obtained through proximate analysis studies has shown in Table 4 (Anuar Sharuddin et al. 2016; Abnisa et al. 2014). According to the proximate analysis, the volatile matter of the plastic is high while the ash content and the moisture content are low.

Table 4

Proximate analysis of HDPE and LDPE plastics in various studies
Type of plastic	Moisture content wt.%	Fixed carbon wt.%	Volatile matter wt.%	Ash content wt.%	References
HDPE	0.00 0.00	0.01 0.03	99.81 98.57	0.18 1.40	Vijayakumar and Sebastian (2018); Aboulkas et al. (2010) Heikkinen et al. (2004)
LDPE	0.30 -	0.00 -	99.70 99.60	0.00 0.40	Aboulkas et al. (2010) Anuar Sharuddin et al. (2016)

This shows that HDPE and LDPE have high potential to transform into liquid oil and gaseous fuel products in the plastic pyrolysis process than that of the biomaterials which contain high moisture content in the composition.

The most effective temperature range to optimize the liquid oil yield would be 500–550°C for the thermal pyrolysis process of HDPE and LDPE as shown in Table 5. However, in the presence of catalysts, the optimum temperature for this pyrolysis could be lowered to 450°C with a further increase in the liquid yield.

In thermal pyrolysis, LDPE can offer a high liquid yield of around 93.1 wt.%, whereas HDPE can offer around 84.7 wt.%. However, with the addition of catalysts such as Fluid Catalytic Cracking (FCC) catalyst at the right operating temperature, the liquid yield of the HDPE could be further maximized to above 90 wt.%.

Table 5

Summary of the process parameters of HDPE and LDPE in the pyrolysis studies (Vijayakumar and Sebastian 2018)
Type of plastic	Reactor	Process parameters				Yield			Other details	References
Type of plastic	Reactor	Temperature (°C)	Pressure (atm)	Heating rate (°C/min)	Duration (min)	Liquid fraction Wt.%	Gaseous fraction Wt.%	Char Wt.%	Other details	References
HDPE	Batch	350	-	20	30	80.88	17.24	1.88		Ahmad et al. (2015)
HDPE	Semi batch	400	1	7	-	82	16	2	Stirring rate 200 rpm FCC catalyst 10 wt.%	Anuar Sharuddin et al. (2016)
HDPE	Batch	450	-	-	60	74.5	5.8	19.7		Miskolczi (2014)
HDPE	Semi batch	450	1	25	-	91.2	4.1	4.7	Stirring rate 50 rpm FCC catalyst 20 wt.%	Abbas-Abadi et al. (2013)
HDPE	Fluidized bed	500	-	-	60	85	10	5	Silica alumina catalysts	Luo et al. (2000)
HDPE	Batch	550	-	5	-	84.7	16.3	0		Marcilla et al. (2009a)
HDPE	Fluidized bed	650	-	-	20–25	68.5	31.5	0		Rezvanipour et al. (2014)
LDPE	Batch	425	-	10	60	89.5	10	0.5		Escola et al. (2011)
LDPE	Batch	430	-	3	-	75.6	8.2	7.5	Also yield wax = 8.7wt.%	Onwudili et al. (2009)
LDPE	-	500	1	6	-	80.41	19.43	0.16		Choi et al. (2010)
LDPE	Fixed bed	500	-	10	20	95	5	0		Fakhrhoseini and Dastanian (2013)
LDPE	Batch	550	-	5	-	93.1	14.6	0		Lee et al. (2003)
LDPE	Fluidized bed	600	1	-	-	51.0	24.2	0	Also yield wax = 24.8 wt.%	Williams and Williams (1999)

These fractions are defined based on the number of the carbon in the carbon chain such as Gaseous fraction; C₁-C₅, Liquid fraction; C₆-C₂₈, Char fraction; >C₃₀ (Wang et al. 2016).

Catalysts are widely used in the pyrolysis process of HDPE and LDPE to reduce the reaction temperature, improve the yield of volatile content and provide selectivity in the resulting hydrocarbon ranges (Bagri and Williams 2002; García et al. 2005). It is very beneficial in producing gasoline and diesel range fuel oil fractions from volatile products selectively (Anuar Sharuddin et al. 2016; Miandad et al. 2019). Catalysts reduce the activation energy of pyrolysis reactions and increase the speed of reactions. Thereby, catalysts are very useful in minimizing energy usage in the pyrolysis process (Elordi et al. 2009).

Catalysts can be classified into two groups as homogeneous catalysts and heterogeneous catalysts. In the case of homogeneous catalysts, the catalyst takes the same single phase as the reaction components do. Heterogeneous catalysts do not take the same phase as the reaction components do (Anuar Sharuddin et al. 2016; Miandad et al. 2019). According to the literature, heterogeneous catalysts are the most commonly used catalysts in the pyrolysis process because of the easiness of separation from the liquid product, low cost, and reusable property of the catalysts (Budsaereechai et al. 2019). Heterogeneous catalysts can be classified into several types including nanocrystalline zeolites, conventional acid solids, mesostructured catalysts, metal-supported on carbon, and basic oxides (Anuar Sharuddin et al. 2016; Elordi et al. 2009).

5.1 Silica-alumina catalysts used in the pyrolysis of HDPE and LDPE

The silica-alumina catalysts are acid catalysts that contain amorphous structures with Bronsted acid sites and Lewis acid sites (Anuar Sharuddin et al. 2016; Busca 2020). It does not contain a stable crystalline structure with compared to zeolite catalysts. Generally, the total pore volume by the amorphous silica-alumina catalysts is higher than those of crystalline zeolite catalysts (Klaimy et al. 2020). This is because the silica-alumina catalyst has larger pore sizes whereas the crystalline zeolite catalyst has micropore sizes (Pourjafar et al. 2018; Ishihara et al. 2010).

Klaimy et al. (Klaimy et al. 2020) have investigated the effect of the acidity, pore size distribution, and specific surface area of silica-alumina catalysts on the cracking of LDPE in a pilot reactor at 450°C of temperature. Four different catalysts including silicate-1(Si-MFI), ZSM-5(Si/Al-MFI), amorphous SiO₂ and amorphous silica-alumina(Si/Al) have been used in this cracking process. Table 6 illustrates the textural properties and the acidity of these catalysts.

Table 6

Textural properties and acidity of SiO₂, Si/Al, Si-MFI and Si/Al-MFI catalysts [a]BET surface area; [b]microporous surface area; [c]external surface area; [d]total pore volume; [e]micro pore volume; [f]quantity of NH₃ desorbed (Klaimy et al. 2020)
Catalyst	S_BET^[a] m²/g	S_µ^[b] m²/g	S_ext^[c] m²/g	V_p^[d] cm³/g	V_µ^[e] cm³/g	Acidity^[f] mmol/g
Catalyst	S_BET^[a] m²/g	S_µ^[b] m²/g	S_ext^[c] m²/g	V_p^[d] cm³/g	V_µ^[e] cm³/g	Total	Weak	Strong
SiO₂	240	18	222	0.69	0.01	0.045	0.030	0.015
Si/Al	161	20	140	0.48	0.01	0.209	0.071 0.138	-
Si-MFI	402	344	58	0.19	0.14	0.017	-	0.017
Si/Al-MFI	387	380	7	0.17	0.15	0.639	0.193	0.445

According to Table 6, the amorphous silica-based catalysts have weak acid sites and exhibit a low acidity resulted by the presence of silanol groups on their external structure. A higher total acidity in silica-alumina catalysts(Si/Al and Si/Al-MFI) than in silica catalysts due to the presence of aluminum in the catalytic structure which can produce Bronsted and Lewis acid sites. In addition, the crystalline silica-alumina catalyst(Si/Al-MFI) contains a higher density of both strong and weak acid sites.

When the silica-alumina catalysts(Si/Al and Si/Al-MFI) having high acidity were employed in pyrolysis, that have produced high gas and liquid yields without the formation of char residue. In contrast, the low acid activity catalysts(SiO₂ and Si-MFI) have produced low gas and liquid yields with wax and char residue (Klaimy et al. 2020; Elordi et al. 2009; Ma et al. 2013). When the Si/Al-MFI was used, it has produced 65 − 64 wt.% and 35–36 wt.% yields of gas and liquid oil fraction, respectively. It has also been reported that the Si/Al-MFI zeolite catalyst increases the formation of aromatics and cycled hydrocarbons whereas the mesoporous silica-alumina catalyst increases the formation of olefins in the liquid fraction (Klaimy et al. 2020).

In an another study, pyrolysis of HDPE over mesoporous silica, silica-alumina(SA-1 and SA-2), and ZSM-5 zeolite catalysts have been investigated by Sakata et al. (Sakata et al. 1997) in a semi-batch reactor at 430°C. Properties of the catalysts and the yields of the resultant products from both thermal and catalytic pyrolysis processes have shown in Table 7.

Table 7

Properties of the catalysts and the yields of the resultant products in the pyrolysis process (Sakata et al. 1997)
Property	Thermal	SA-1	SA-2	ZSM-5	Silica
Surface area (m²/g)	-	420	270	360	900
SiO₂/Al₂O₃ ratio	-	4.99	0.267	75.9	-
Yield
Liquid oil wt.%	63.3	67.8	74.3	49.8	71.1
Gas wt.%	9.6	23.7	13.4	44.3	11.0
Char wt.%	21.1	8.5	12.3	5.8	17.9

It has been reported that the acidity of these four catalysts is in the order of SA-1˃ZSM-5˃SA-2˃˃silica = 0. According to Table 7, the ZSM-5 catalyst which contains both weak and strong acid sites has been produced a higher amount of gaseous product (44.3 wt.%) when compared to the silica-alumina catalysts (SA-1, 23.7 wt.%; SA-2, 13.4 wt.%) that contain only weak acid sites. The moderately acidic SA-2 catalyst has been produced a higher yield of liquid oil than those of SA-1 and ZSM-5 catalysts. It has observed a slightly high coke formation in both silica-alumina catalysts than that of the ZSM-5 catalyst.

5.2 Zeolite catalysts used in the pyrolysis of HDPE and LDPE

Mainly Zeolite catalysts have been used in the plastic pyrolysis process due to their desirable properties in the selective recovery of gasoline and diesel range hydrocarbons as final products. They are aluminosilicate crystalline materials having a pore structure and ion exchange capabilities (Marcilla et al. 2009a; Elordi et al. 2012). The ratio of SiO₂/Al₂O₃ determines the zeolite type and its reactivity. The structure of the zeolite is formed by a three-dimensional framework where oxygen atoms link the tetrahedral sides with aluminum and silicon atoms (Anuar Sharuddin et al. 2016; Verdoliva et al. 2019; Nwankwor et al. 2021).

Many zeolites have been synthetically manufactured by chemical processes to obtain a uniform chemical composition(high purity), uniform pore size, and better ion-exchange abilities (Xu et al. 2010). These synthetic zeolites contain higher pore size and higher acidity than those of natural zeolites according to the purpose of commercial applications (Flanagan and Crangle 2017). The main types of synthetic zeolites are zeolites A, X, Y, ZSM-5, omega, and beta (Xu et al. 2010; Kulprathipanja 2010). HUSY zeolite has been modified from the zeolite Y to obtain an ultrastable form of zeolites (Ma et al. 2013; Xu et al. 2010). The pore size and Si/Al ratio of various types of synthetic zeolites have been summarized in Tables 8 and 9.

Table 8

The pore diameter of various types of synthetic zeolites (Petrov and Michalev 2012)
Type of the zeolite	Pore diameter (nm)	Examples
Small pore zeolites	0.30–0.45	Zeolite A
Medium pore zeolites	0.45–0.60	ZSM-5
Large pore zeolites	0.60–0.80	Zeolite X, Zeolite Y

Table 9

The Si/Al ratio of various types of synthetic zeolites (Xu et al. 2010; Kulprathipanja 2010)
Si/Al ratio	Examples of the zeolite
1-1.5	Zeolite A, X
2–5	Zeolite Y, L, omega
10–100	ZSM-5, beta

Framework diagrams of Zeolite types have shown in Fig. 5 according to the database of the International Zeolite Association(IZA) structure commission.

Elordi et al. (2012) have investigated the effect of the acidity of HZSM-5 zeolite catalysts on the cracking of HDPE in a conical spouted bed reactor at 500°C. Two different Zeolite catalysts with SiO₂/Al₂O₃ ratio of 30 and 80 and having different acid strength have been used in this cracking process. It has been observed that the increase in SiO₂/Al₂O₃ ratio decreases the total acidity thus decreases the acid strength of these catalysts. As a result of this, when the zeolite with SiO₂/Al₂O₃ ratio of 80 was used it has been able to obtain a high yield of C₂–C₄ olefins and non-aromatic C₅–C₁₁ fraction with a low yield of aromatic components and C₁–C₄ paraffin. In addition, using the same catalyst it has been able to obtain 59.8 wt.% and 32.1 wt.% yields of C₂–C₄ olefins and gasoline fraction (C₅–C₁₁), respectively. However, the development of the coke was increased as the SiO₂/Al₂O₃ ratio of the zeolite was increased.

The catalytic pyrolysis of HDPE and LDPE over HZSM-5 and HUSY zeolite catalysts have been investigated by Marcilla et al. (2009a) in a batch reactor. The resultant products of this thermal cracking and the catalytic cracking processes can be summarized as shown in Table 10.

Table 10

Yield of the different fractions obtained during the polyethylene pyrolysis process using two types of zeolite catalysts (Marcilla et al. 2009a)
Yield (mg/ 100 mg of polyethylene)	LDPE Thermal cracking	HDPE Thermal cracking	LDPE – HZSM5	HDPE –HZSM5	LDPE - HUSY	HDPE - HUSY
Gases	14.6	16.3	70.7	72.6	34.5	39.5
Liquid/waxes	93.1	84.3	18.3	17.3	61.6	41.0
Coke	-	-	0.5	0.7	1.9	1.9

In this study, the gas yield has been drastically increased when the HZSM-5 catalyst is used. In addition, the liquid/wax yield has been significantly decreased when the HZSM-5 catalyst is used. In the case of the HUSY catalyst, the resulting gas yield has been moderately increased while the liquid/wax yield has been moderately decreased when compared to the thermal cracking process. Also a slight formation of coke was observed in both of these catalytic cracking processes of LDPE and HDPE.

According to the literature, it has been confirmed that polyethylene molecules are able to diffuse into the narrow pores of the HZSM-5 catalyst unlike the other polymers such as polypropylene (PP) (Zhou et al. 2004). In another study, Marcilla et al. (Marcilla et al. 2004) have suggested that the branches or chain ends of polyethylene may penetrate the pores of HZSM-5 zeolite and contact with the acid sites located there, thus increase the catalytic reactivity. HZSM-5 has both strong and weak acid sites, whereas HUSY only has weak acid sites. When compared to weak acid sites, strong acid sites have a higher ability to degrade or crack the heavier hydrocarbon chains in the polymers (Marcilla et al. 2009a).

It has been observed a slight formation of coke with HUSY catalyst than that of HZSM-5 catalyst. The surface area and the pore volume are higher in HUSY when compared to the HZSM-5 as shown in Table 11. In addition, the HUSY catalyst contains only weak acid sites and the catalytic cracking process is not effective as in the HZSM-5 catalyst which contains both strong and weak acid sites. Therefore, polymer chains that are not degraded completely can be deposited as the coke yield.

Table 11

Properties of the catalysts (Marcilla et al. 2009a)
Property	HZSM-5	HUSY
Particle size (µm)	3	1
BET surface area (m²/g)	341	614
Micropore volume (cm³/g)	0.16	0.29

Miskolczi et al. (2016) have studied the catalytic effect of activated carbon, HZSM-5, and MCM-41 separately and their mixtures on the pyrolysis of a mixture of waste HDPE and LDPE. This pyrolysis has been carried out in a batch reactor at 530–540°C of temperature. In the absence of the catalysts, it has been able to obtain 42.7 wt.% and 5.1 wt.% yields of pyrolysis oil and gaseous products, respectively. When activated carbon catalyst was used the resulting pyrolysis oil and gas yields have been increased only to 49.2 wt.% and 7.2 wt.%.respectively. This shows that activated carbon is not a highly effective catalyst for this pyrolysis process. However, the sulfur content of pyrolysis oil has been significantly decreased in this process.

In the presence of MCM-41 catalyst, It has been able to obtain 63.9 wt.% and about 10 wt.% yields of pyrolysis oil and gaseous fraction, respectively while HZSM-5 catalyst yielded 61.4 wt.% and 21.1 wt.% of pyrolysis oil and gaseous fraction, respectively. The reason for the increment of the gaseous yield could be the differences in pore sizes and the acid strength of these two catalysts (Table 12). As the Si/Al ratio of MCM-41 is higher than that of HZSM-5 (40 and 25, respectively), the acidity of the MCM-41 is lower than that of the HZSM-5(Miskolczi et al. 2016). Also, the MCM-41catalyst contains a structure with a higher average pore size than the HZSM-5. Therefore, the MCM-41 is more catalytically active and increases the pyrolysis oil yield. As the pore size of HZSM-5 is smaller, it can increase the yield of the gaseous product in the plastic pyrolysis process (Miskolczi et al. 2016).

Table 12

Properties of the catalysts (Miskolczi et al. 2016)
Property	Activated carbon	MCM-41	HZSM-5
Si/Al ratio	-	40	25
Acidity, (NH₃/g)	-	0.15	0.60
BET area (m²/g)	859	824	298

5.3 FCC catalyst used in the pyrolysis of HDPE and LDPE

The FCC catalyst has three main parts including zeolite crystals and a non-zeolite acid matrix made of silica-alumina with a binder in the catalyst structure (Anuar Sharuddin et al. 2016; Lin et al. 2010). In the case of the plastic pyrolysis process, both fresh FCC catalyst and deactivated FCC catalyst named as spent or equilibrium catalyst has been used for the catalytic cracking process.

Achilias et al. (2007) carried out catalytic pyrolysis of both virgin and waste HDPE and LDPE in a fixed bed reactor with FCC catalyst at 450°C. The product yields obtained herein are shown in Table 13.

Table 13

Product yield from the catalytic pyrolysis of HDPE and LDPE (Achilias et al. 2007)
Plastic type	Gas wt.%	Liquid oil wt.%	Residue wt.%
Virgin LDPE	0.5	46.6	52.9
Virgin HDPE	0.5	38.5	61.0
Waste LDPE	8.5	72.1	19.4
Waste HDPE	3.3	44.2	52.5

In the above study, it has been observed that the liquid oil fraction of waste LDPE mainly contains gasoline range(C₇-C₁₂) hydrocarbons with iso-alkanes or iso-alkenes. In addition, the liquid oil fraction from waste HDPE mainly contains a high carbon range of hydrocarbons with normal alkenes.

In an another study, Marcilla et al. (2005) have studied the thermal behavior of different HDPE mixtures under three different catalysts including HZSM-5, FCC catalyst, and HUSY. The properties of these three catalysts are shown in Table 14.

Table 14

Properties of the catalysts used in pyrolysis of HDPE (Marcilla et al. 2005)
Property	HZSM-5	FCC	HUSY
Composition	SiO₂/Al₂O₃: 30 (molar basis)	Al₂O₃: 49 (wt.%) NaO: 0.25 (wt.%) Re₂O₃: 2.7 (wt.%) ZSM-5: 18 (wt.%)	SiO₂/Al₂O₃: 6 (molar basis)
BET area (m²/g)	420	135	790

In this study, it has been observed the effect on pyrolysis temperature of HDPE (uncatalyzed- 470°C), by the presence of the 3 catalysts of HZSM-5(374°C), HUSY(382°C), and FCC (415°C).

Olazar et al. (2009) studied the effect of fresh FCC and equilibrium FCC catalysts on the pyrolysis of HDPE in a conical spouted bed reactor at 500°C. After, pyrolyzing HDPE under fresh FCC and obtaining the resultant yield, the catalyst was deactivated by steaming treatments to obtain the equilibrium FCC catalyst. In the case of fresh FCC catalyst, it has been able to obtain 52 wt.%, 35 wt.%, and 13 wt.% yields of gases(C₁-C₄), light oil fraction(C₅-C₉), and diesel fraction (C₁₀⁺ hydrocarbons), respectively. In addition, the resultant diesel fraction has been increased to 40 wt.% and 69 wt.% with the equilibrium catalysts obtained through mild and severe steaming treatments, respectively. However, the gaseous yields have been decreased when using the equilibrium FCC catalysts(mild steam: 22 wt.%; severe steam: 8 wt.%) due to the reduction of acid sites in the FCC catalysts due to steaming treatments.

5.4 Other catalysts used in the pyrolysis of HDPE and LDPE

Several researchers have investigated the catalytic properties of TiO₂ in the plastic pyrolysis. TiO₂, has been identified as a suitable heterogeneous catalyst in pyrolysis due to its porous surface and product selectivity, high thermal stability and mechanical strength (Eschemann et al. 2014). The lewis acidity, as well as non-toxicity of TiO₂, were reported to be very useful for hydrocarbon cracking in plastic pyrolysis (Nwankwor et al. 2021).

Nwankwor et al. (2021) have studied the synthesis of gasoline range fuels by the catalytic cracking of waste plastics using TiO₂ and zeolite catalysts. This experiment has demonstrated that the liquid products of the LDPE pyrolysis process were mainly aliphatic hydrocarbons in the gasoline range. The use of zeolite catalyst in pyrolysis of LDPE has produced a higher amount of liquid products 44.2 wt.% than the TiO₂ catalyst (27.2 wt.%). In another study, TiO₂ has added as a nano additive(particle size, 30–40 nm) to liquid oil obtained after the pyrolysis of a mixture of plastic waste to investigate its ability to change the properties of liquid oil (Bharathy et al. 2019). By using this TiO₂ and liquid oil mixture in a diesel engine, it has been observed that the amount of hydrocarbons and CO pollutants in the emission can be minimized.

Panda et al. (2019) have used sulfated zirconium hydroxide to pyrolyze HDPE and LDPE in a batch reactor at 500°C to obtain gasoline, kerosene, and diesel range hydrocarbons(C₁₀-C₂₄) in the resultant liquid oil yield. The sulfated zirconium hydroxide catalyst contains both Bronsted and Lewis acid sites and can be easily synthesized for low cost for commercial use. It has been reported that the liquid oil yields from the pyrolysis process of HDPE and LDPE were 79.5 wt.% and 82 wt.%, respectively.

Kunwar et al. (2016) have carried out the pyrolysis of waste HDPE by employing MgCO₃ as the basic catalyst for the catalytic cracking at 450°C. It has been observed that a slight reduction in process temperature (by 10°C) with the use of MgCO₃ catalyst than that of the thermal pyrolysis process. In the presence of MgCO₃ catalyst, it has been able to obtain 80 wt.% and 18 wt.% yields of liquid oil and gaseous products, respectively. In addition, the resultant liquid oil yield was found to contain a higher amount of diesel range hydrocarbons. In the thermal pyrolysis process, 86.2 wt.% of the wax product was yielded indicating ineffective decomposition.

The extensive review of the literature reveals that the use of suitable catalysts enhanced the production of desirable yield (liquid oil yield or gaseous yield) in the resultant product at a lower temperature compared to the thermal pyrolysis process. At around 500°C of temperature, it has been observed that the HZSM-5 catalyst has selectivity toward the gaseous products whereas HUSY, MCM-41, silica-alumina, and other catalysts have the selectivity toward the liquid oil products in plastic pyrolysis (Miandad et al. 2019; Achilias et al. 2007; Li et al. 2016). Table 15 summarize several HDPE and LDPE pyrolysis experiments performed under different catalysts and temperatures.

Table 15

Summary of the yield obtained in thermal and catalytic pyrolyzing of HDPE and LDPE
Plastic-type	Catalytic pyrolysis					Polymer to catalyst ratio	Thermal pyrolysis				References
Plastic-type	Temperature °C	Catalyst	Liquid Wt.%	Char Wt.%	Gas Wt.%	Polymer to catalyst ratio	Temperature °C	Liquid Wt.%	Char Wt.%	Gas Wt.%	References
HDPE	430	Y-zeolite	75	6	19	10:1	460	86.2 wax	1	13	Kunwar et al. (2016)
HDPE	450	MgCO₃	80	2	18	10:1	460	86.2 wax	1	13	Kunwar et al. (2016)
HDPE	500	HZSM-5 (30)*	78.65	1.85	19.5	10:3	-	-	-	-	Elordi et al. (2012)
HDPE	500	HZSM-5 (80)*	72.98	1.02	26.0	10:3	-	-	-	-	Elordi et al. (2012)
HDPE/ LDPE	530	Activated carbon	49.2	-	7.2	25:1	540	42.7	-	5.1	Miskolczi et al. (2016)
		MCM-41	63.9	-	10
		HZSM-5	61.4	-	21.1
HDPE	450	FCC	91.2	4.1	4.7	10:1	450	85 wax	-	-	Olazar et al. (2009)
HDPE	500	Sulfated zirconium hydroxide	79.5	-	-	10:1	-	-	-	-	Panda et al. (2019)
LDPE	500	Sulfated zirconium hydroxide	82	-	-	10:1	-	-	-	-	Panda et al. (2019)
LDPE	99–198	TiO₂	27.2	-	-	10:4	82–140	28.5	-	-	Nwankwor et al. (2021)
*SiO₂/Al₂O₃ ratio

The amount of non-condensable gases in the pyrolysis mainly depends on the operation temperature, the residence time, and the catalytic behavior. In the incineration or gasification processes, the product formation occurs at high temperatures in the presence of oxygen and generates harmful compounds to the environment. Whereas in the pyrolysis thermal cracking process occurs at low temperatures (400–900°C) in the absence of oxygen, it prevents the formation of dioxins and reduces the formation of carbon monoxide(CO) and carbon dioxide(CO₂) gases (Singh and Ruj 2016; Miskolczi et al. 2009).

A very few studies had focused on the gas emissions in the pyrolysis processes of municipal plastic waste whereas many researchers have studied the gas emission from the pyrolysis process of virgin plastics and the mixtures of HDPE, LDPE, PP, and PS. It has been reported that the waste HDPE and LDPE have produced more gaseous components in the carbon range of C₃ and C₄, such as methane, ethane, ethene, n-butene in the pyrolysis at 500°C. In addition, it has been produced H₂, CO, and CO₂ gases during the pyrolysis of waste HDPE and LDPE (Singh and Ruj 2016).

N. Miskolczi et al. (2009) have analyzed the gaseous yield of the waste HDPE pyrolysis process at 520°C in the presence and absence of ZSM-5 catalyst. They have obtained 5.1 wt.% and 12.2 wt.% yields of gases with and without the catalyst, respectively. The composition of the gaseous product in this process is shown in Table 16.

Table 16

Composition of the gaseous product in the waste HDPE pyrolysis process using ZSM-5 catalyst (Miskolczi et al. 2009)
Gaseous product	HDPE without a catalyst	HDPE + ZSM-5
Methane	3.1	2.5
Ethene	30.6	26.1
Ethane	21.4	19.5
Propane	8.9	8.9
Propene	4.3	6.8
Butene	17.1	14.6
Butane	14.6	10.9
iso-Butane	0.0	10.7

According to Table 16, the formation of alkenes is slightly higher than the formation of alkanes in both thermal and catalytic pyrolysis processes.

It is important to conduct awareness among communities and to introduce a proper waste separation method to the country to develop energy recovery practices with plastic waste. There are some issues in the mixed plastic waste pyrolysis process including the difficulties in identifying the composition of the mixed waste, composite, laminated materials, and metal parts in the plastics. It is important to identify the waste composition of the mixed plastic waste because PVC and PET plastics emit hydrogen chloride and dioxin during the pyrolysis process which can cause corrosion of the reaction bed. The spectrum analysis method such as FTIR, Raman, and Near-IR spectrum analysis can be used to identify the composition of the mixed plastic waste. Whereas the different sorting techniques such as manual sorting, sorting by density, air classifications, electrostatic separations and sensor-based sorting techniques can be used to separate plastic types (Sutar 2015). The novel trend of plastic waste separation from municipal solid waste is the automated sorting technique (Gundupalli et al. 2017).

The influence of the catalysts is a governing factor in the plastic pyrolysis process when considering resultant product selectivity and reduction of the temperature. The high cost of the catalysts in the catalytic pyrolysis process is a critical issue when considering the implementation of pyrolysis technology in the plastic waste management system. Natural origin minerals such as Bentonite clay, Mordenite, Clinoptilolite, Red mud, Shewedaung clay, and Mabisan clay that are highly available in the country can be used as catalysts to reduce the cost of the pyrolysis process. The zeolite and different derivatives of zeolites have been used by many researchers because of their abundance. In addition, the repeated catalysts regeneration method has been used to improve the catalysts' lifetime and reduce the operation cost of the plastic catalytic cracking process.

When the objective is to maximize the liquid oil fraction in the pyrolysis process, the resultant gaseous product becomes an issue. These resultant gas components have a high calorific value and can be used as a heating source for the pyrolysis process again. Also, without further treatments, the resultant gases can be used in gas turbines to generate electricity and can be fired to operate boilers. The ethane and propene formed in the pyrolysis of plastics can be used as feedstocks in the chemical industry after the separation using a proper separating technique (Honus et al. 2018). In addition to that, the emission of hazardous gases in the plastic pyrolysis process can be avoided by using gas filters (Padmaja 2016). Filters with a porous material can trap gases or may convert them to other products by chemical reactions that occurred inside the pores of the filters.

Pyrolysis of plastic waste is a better solution for the plastic waste problem due to the potential of converting waste into valuable byproducts. This review has provided a summary of plastic pyrolysis of LDPE and HDPE with a discussion of the main affecting parameters that affect the fuel oil, char, and gaseous products. The plastic pyrolysis process can be carried out in both thermal and catalytic processes. The operating parameters such as temperature and reaction time can be reduced using appropriate catalysts that increased the desired product yields. In addition to that, there are some difficulties of feedstock selection of the plastic pyrolysis due to the low segregation methods of municipal waste in Sri Lanka. Although the effect of environmental pollution is lower in plastic pyrolysis than that of landfilling and incineration, it should be concerned about the filtering or trapping of gaseous products in the plastic waste pyrolysis process without releasing them directly to the environment. However, with the pyrolysis of plastic waste into valuable fuel oil, it can be reduced the high demand for fossil fuels to a certain extent.

Acknowledgment

The authors are grateful for Accelerating Higher Education Expansion and Development (AHEAD) Development Oriented Research (DOR) projects for financial support and the Sri Lanka Customs and Central Environmental Authority of Sri Lanka for provision of data on plastic imports.

Funding

This work was supported by the Accelerating Higher Education Expansion and Development (AHEAD) Development Oriented Research (DOR) Grant.

Competing interests

The authors have no relevant financial or non-financial interest to disclose.

Author Contributions

Writing – original draft preparation: [Panakaduwa Gamage Imesha Uthpalani]; Literature search and data analysis: [Panakaduwa Gamage Imesha Uthpalani]; Writing – review, editing and critically revised: [Jagath Kumara Premachandra]; Writing – review, editing and critically revised: [Deeyagahage Sujeewa Mallika De Silva]; Writing – review and editing: [Vithanage Primali Anuruddhika Weerasinghe].

Data Availablity

The datasets analysed during the current study are available from the corresponding author on reasonable request.

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Table 1 is available in supplementary file.

Table1Thecommonapplicationsofvarioustypesofplastics.docx

Pyrolysis as a value added method for plastic waste management: A review on converting LDPE and HDPE waste into fuel

Status:

Journal Publication

Version 1

Abstract

Figures

1. Introduction

1.1 The adverse effect of mismanagement of plastic waste

1.2 Better municipal plastic waste management strategies in developing countries

2. Pyrolysis Of Hdpe

3. Pyrolysis Of Ldpe

4. Comparison Between The Pyrolysis Processes Of Hdpe And Ldpe

5. Effect Of Catalysts On Pyrolysis Of Hdpe And Ldpe

5.1 Silica-alumina catalysts used in the pyrolysis of HDPE and LDPE

5.2 Zeolite catalysts used in the pyrolysis of HDPE and LDPE

5.3 FCC catalyst used in the pyrolysis of HDPE and LDPE

5.4 Other catalysts used in the pyrolysis of HDPE and LDPE

6. Emissions In The Pyrolysis Process

7. Suggestions For Future Work: Research Areas To Complete

8. Conclusion

Declarations

References

Table

Supplementary Files

Status:

Journal Publication

Version 1