ReviewA review on pyrolysis of plastic wastes
Introduction
Plastic plays a vital role in enhancing the standard lives of human being for more than 50 years. It is a key of innovation of many products in various sectors such as construction, healthcare, electronic, automotive, packaging and others. The demand of commodity plastics has been increased due to the rapid growth of the world population. The global production of plastic has reached about 299 million tons in 2013 and has increased by 4% over 2012 [1]. The continuous rising of plastic demand led to the growing in waste accumulation every year. It was reported that 33 million tons of plastic waste are generated in the US based on 2013 statistic [2]. As in Europe, 25 million tons of plastic ended up in waste stream during the year of 2012 [1]. Based on the statistic established in Europe, about 38% of the plastic waste still went to the landfill, 26% were recycled while 36% were utilized for energy recovery [1]. This shows that the percentage of plastic waste ended up in the landfill still very high that it occupied a huge space. Plastics may take up to billions of years to degrade naturally. They degrade gradually since the molecular bonds containing hydrogen, carbon and few other elements such as nitrogen, chlorine and others that make plastic very durable. The continuous disposal of plastic in the landfill would definitely cause serious environmental problem.
In order to reduce plastic disposal to the landfill, recycling method is considered as another alternative to manage plastic waste. Back to the statistic mentioned above, the percentage of recycling still at the lowest. Recycling plastic has proven difficult and it can be costly because of the constraints on water contamination and inadequate separation prior to recycle that is labor intensive [3]. Separation is needed since plastics are made of different resin compound, transparency and color. Normally, pigmented or dyed plastics have lower market value. Clearly transparent plastics are often desirable by the manufacturers since they can be dyed to transform into new products, thus have greater flexibility [4]. With the stringent requirement to get high value product, recycling plastic becomes quite challenging nowadays.
Although plastic recycling able to reduce some amount of plastic waste, the more reliable and sustainable method has been established. Since high demand of plastics have been received each year, the reduction of fossil fuel such as coal, gas and especially petroleum that made up plastic itself has gained great interest of many researchers to discover and develop potential energy resources due to the rising in energy demand. Some of the new energy resources that have been explored include solar energy, wind power, geothermal and hydropower technology. Recently, the energy conversion from waste has been an intelligent way to fully utilize the waste to meet the increased energy demand. The conversion of plastics to valuable energy is possible as they are derived from petrochemical source, essentially having high calorific value. Hence, pyrolysis is one of the routes to waste minimization that has been gaining interest recently.
Pyrolysis is the process of thermally degrading long chain polymer molecules into smaller, less complex molecules through heat and pressure. The process requires intense heat with shorter duration and in absence of oxygen. The three major products that are produced during pyrolysis are oil, gas and char which are valuable for industries especially production and refineries. Pyrolysis was chosen by many researchers since the process able to produce high amount of liquid oil up to 80 wt% at moderate temperature around 500 °C [5]. In addition, pyrolysis is also very flexible since the process parameters can be manipulated to optimize the product yield based on preferences. The liquid oil produced can be used in multiple applications such as furnaces, boilers, turbines and diesel engines without the needs of upgrading or treatment [6]. Unlike recycling, pyrolysis does not cause water contamination and is considered as green technology when even the pyrolysis by-product which is gaseous has substantial calorific value that it can be reused to compensate the overall energy requirement of the pyrolysis plant [7]. The process handling is also much easier and flexible than the common recycling method since it does not need an intense sorting process, thus less labor intensive.
Many research papers have been published regarding the potential of various types of plastics in pyrolysis processes for liquid production. It should be noted that the product yield and quality heavily depends on the set up parameters. Therefore, this review focused on different type of plastic pyrolysis that has been explored together with the main affecting parameters in plastic pyrolysis process that need an attention in order to maximize liquid oil production and enhance the oil quality. The main parameters include temperature, type of reactors, residence time, pressure, different catalysts usage and type of fluidizing gas with its flow rate. Additionally, some relevant discussion regarding the optimization of liquid oil yield was also presented in this paper.
Section snippets
Pyrolysis of plastics
Fundamentally, different types of plastics have different compositions that normally reported in terms of their proximate analysis. Proximate analysis can be defined as a technique to measure the chemical properties of the plastic compound based on four particular elements which are moisture content, fixed carbon, volatile matter and ash content [8]. Volatile matter and ash content are the major factors that influence the liquid oil yield in pyrolysis process. High volatile matter favored the
Process parameters condition
Parameters play major role in optimizing the product yield and composition in any processes. In plastic pyrolysis, the key process parameters may influence the production of final end products such as liquid oil, gaseous and char. Those important parameters may be summarized as temperature, type of reactors, pressure, residence time, catalysts, type of fluidizing gas and its rate. The desired product can be achieved by controlling the parameters at different settings. In-depth discussions of
Physical properties
Table 6 summarized the fuel properties of the liquid oil produced in pyrolysis process. The experimental calorific value of HDPE, PP and LDPE are all above 40 MJ/kg and were considered high for energy utilization. According to Ahmad et al. [23], the calculated calorific value for both HDPE and PP were above 45 MJ/kg, and thus very closer to the commercial fuel grade criteria of gasoline and diesel. The calorific value of PS was commonly lower than the polyolefin plastic due to the existence of
By-products of the plastic pyrolysis
Pyrolysis of plastics also produces char and gas as by-products. The proportion of by-product in pyrolysis strongly depends on several parameters such as temperature, heating rate, pressure and residence time. Some information about the by-products generated is discussed below.
Discussion on plastic pyrolysis scenarios
This review showed that many researches have been done to study the potential of plastic pyrolysis process in order to produce valuable products such as liquid oil and the results were convincing. This technique offers several advantages such as enhancing the waste management system, reducing the reliability to fossil fuels, increasing energy sources and also prevents the contamination to the environment. The technique can be executed at different parameters that resulted in different liquid
Conclusion
This review has provided concise summary of plastic pyrolysis for each type and a discussion of the main affecting parameters to optimize liquid oil yield. Based on the studies on literatures, pyrolysis process was chosen by most researchers because of its potential to convert the most energy from plastic waste to valuable liquid oil, gaseous and char. Therefore, it is the best alternative for plastic waste conversion and also economical in terms of operation. The flexibility that it provides
Acknowledgement
The authors would like to thank the University of Malaya for fully funding the work described in this publication through the PPP project number PG066-2015B. A special thanks is also extended to the Universiti Teknologi Mara (UiTM) and Ministry of Higher Education (MOHE) for providing financial support under the Young Lecturer's Scheme (TPM).
References (130)
Review of fast pyrolysis of biomass and product upgrading
Biomass Bioenergy
(2012)- et al.
A review on co-pyrolysis of biomass: an optional technique to obtain a high-grade pyrolysis oil
Energy Convers Manage
(2014) - et al.
Thermogravimetry as a tool to classify waste components to be used for energy generation
J Anal Appl Pyrol
(2004) - et al.
Catalytic efficiency of some novel nanostructured heterogeneous solid catalysts in pyrolysis of HDPE
Polym Degrad Stab
(2013) - et al.
Study on pyrolysis characteristics of refuse plastic fuel using lab-scale tube furnace and thermogravimetric analysis reactor
J Anal Appl Pyrol
(2012) - et al.
Thermal degradation behaviors of polyethylene and polypropylene. Part I: pyrolysis kinetics and mechanisms
Energy Convers Manage
(2010) - et al.
Pyrolysis of a fraction of waste polypropylene and polyethylene for the recovery of BTX aromatics using a fluidized bed reactor
Fuel Process Technol
(2010) - et al.
Thermal and kinetic behaviors of biomass and plastic wastes in co-pyrolysis
Energy Convers Manage
(2013) - et al.
Thermal and catalytic pyrolysis of polyethylene over HZSM5 and HUSY zeolites in a batch reactor under dynamic conditions
Appl Catal B Environ
(2009) - et al.
Dechlorination of fuels in pyrolysis of PVC containing plastic wastes
Fuel Process Technol
(2011)
Feedstock recycling of polyethylene in a two-step thermo-catalytic reaction system
J Anal Appl Pyrol
Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: effects of temperature and residence time
J Anal Appl Pyrol
Degradation of polyethylene and polypropylene into fuel oil by using solid acid and non-solid acid catalysts
J Anal Appl Pyrol
Pyrolysis of municipal plastic wastes for recovery of gasoline-range hydrocarbons
J Anal Appl Pyrol
Thermal degradation of mixed plastic waste to aromatics and gas
Polym Degrad Stab
Pyrolysis of polyolefins for increasing the yield of monomers’ recovery
Waste Manage
Kinetic studies of co-pyrolysis of rubber seed shell with high density polyethylene
Energy Convers Manage
Study of the catalytic pyrolysis behaviour of polyethylene–polypropylene mixtures
J Anal Appl Pyrol
Evolution of products during the degradation of polyethylene in a batch reactor
J Anal Appl Pyrol
Catalytic degradation of waste high-density polyethylene into fuel products using BaCO3 as a catalyst
Fuel Process Technol
Thermal and thermo-catalytic degradation of high-density polyethylene waste
J Anal Appl Pyrol
Thermo-catalytic pyrolysis of polystyrene in the presence of zinc bulk catalysts
J Taiwan Inst Chem Eng
Catalytic cracking of HDPE over hybrid zeolitic–mesoporous materials
J Anal Appl Pyrol
Pyrolysis characteristics of polystyrene and polypropylene in a stirred batch reactor
Chem Eng J
Tertiary recycling of polypropylene by catalytic cracking in a semibatch stirred reactor: use of spent equilibrium FCC commercial catalyst
Appl Catal B Environ
Pyrolysis of plastic packaging waste: a comparison of plastic residuals from material recovery facilities with simulated plastic waste
Waste Manage
Characteristics of liquid product from the pyrolysis of waste plastic mixture at low and high temperatures: influence of lapse time of reaction
Waste Manage
Catalytic pyrolysis of LDPE leads to valuable resource recovery and reduction of waste problems
Energy Convers Manage
Evaluation of pyrolysis process parameters on polypropylene degradation products
J Anal Appl Pyrol
Investigation of catalytic degradation of high-density polyethylene by hydrocarbon group type analysis
J Anal Appl Pyrol
Evaluation of pyrolysis product of virgin high density polyethylene degradation using different process parameters in a stirred reactor
Fuel Process Technol
Composition of aromatic products in the catalytic degradation of the mixture of waste polystyrene and high-density polyethylene using spent FCC catalyst
Polym Degrad Stab
Influence of process conditions on syngas production from the thermal processing of waste high density polyethylene
J Anal Appl Pyrol
Stability of ZSM-11 and BETA zeolites during the catalytic cracking of low-density polyethylene
J Anal Appl Pyrol
A kinetic approach to the temperature-programmed pyrolysis of low-and high-density polyethylene in a fixed bed reactor: determination of kinetic parameters for the evolution of n-paraffins and 1-olefins
Fuel
Pyrolysis of mixed plastics into aromatics
J Anal Appl Pyrol
Fluidised bed pyrolysis of polypropylene over cracking catalysts for producing hydrocarbons
Polym Degrad Stab
Study of the polymer–catalyst contact effectivity and the heating rate influence on the HDPE pyrolysis
J Anal Appl Pyrol
Catalytic degradation of high density polyethylene over mesoporous and microporous catalysts in a fluidised-bed reactor
Polym Degrad Stab
Catalytic degradation of high density polyethylene over nanocrystalline HZSM-5 zeolite
Polym Degrad Stab
Case studies—problem solving in fluidized bed waste fuel incineration
Energy Convers Manage
Catalytic degradation of high density polyethylene and polypropylene into liquid fuel in a powder-particle fluidized bed
Polym Degrad Stab
Catalytic pyrolysis of HDPE in continuous mode over zeolite catalysts in a conical spouted bed reactor
J Anal Appl Pyrol
Influence of FCC catalyst steaming on HDPE pyrolysis product distribution
J Anal Appl Pyrol
Characterization of the waxes obtained by the pyrolysis of polyolefin plastics in a conical spouted bed reactor
J Anal Appl Pyrol
Catalytic pyrolysis of high density polyethylene in a conical spouted bed reactor
J Anal Appl Pyrol
Efficient disposal of waste polyolefins through microwave assisted pyrolysis
Fuel
Effect of pressure on thermal degradation of polyethylene
J Anal Appl Pyrol
Fluidized bed thermal degradation products of HDPE in an inert atmosphere and in air–nitrogen mixtures
J Anal Appl Pyrol
Selective catalytic degradation of polyolefins
Prog Polym Sci
Cited by (1329)
Optimizing refuse-derived fuel production from scheduled wastes through Aspen plus simulation
2024, Environmental ResearchSorting plastics waste for a circular economy: Perspectives for lanthanide luminescent markers
2024, Resources, Conservation and RecyclingRobust downstream technologies in polystyrene waste pyrolysis: Design and prospective life-cycle assessment of pyrolysis oil reintegration pathways
2024, Resources, Conservation and Recycling