Table of Contents
Summary
Due to the increase in demand and increased rates for power and energy resources, engineers are trying different methods to produce fuel from hydrocarbons. Though most of the efforts are emphasized on biofuel, a small portion of efforts are also focused on producing fuel from waste plastics. Plastic is used in abundant quantities all over the world and its disposal is a huge problem since it does not decompose and cannot be recycled in several cases. Moreover, it produces harmful emissions and heavy metals that are injurious to health during the process of recycling. Thus, converting plastic into fuel is an attractive option that could resolve the issue of plastic recycling and fuel production. Different methods are used to produce fuel from waste plastics. The most commonly used method is pyrolysis or thermal degradation. The converted fuel has several benefits and is used in various applications such as jet fuel, transportation, and power generation. Moreover, this green energy-based alternative got immense potential in various energy sectors.
1 INTRODUCTION
The current pace of commercial development requires the production of alternative fuels from renewable sources or recycling waste to compensate for the increased fuel requirements. There are several replacements for fossil fuels like biomass, hydropower, and wind power. Likewise, an appropriate administration approach to this waste is an additional vital feature. Improvement and innovation have led to an enormous rise in the manufacturing of all types of materials that ultimately produce waste. Plastics had been one of these materials due to their extensive applications, flexibility in handling and comparatively small price [1].
The accumulation of waste plastic is increasing at a distressing level, with the rise of the human populace, fast financial evolution, incessant expansion, and variations in ways of living. Moreover, the small useful life of day-to-day plastic quickens the accumulation of plastic left-over on a regular base. Worldwide plastic manufacturing is estimated at 300 million tons every year and is unceasingly growing each year. Plastics are manufactured from petrochemical hydrocarbons with fillers like flame-retardants, fillers, and antioxidants that prevent degradation via milder routes. The reprocessing of left-over plastics in most underdeveloped countries is carried out as open or landfilling dumping. The dumping of plastic left over in landfills offer habitation for bugs and rats that might produce diverse kinds of infections. Moreover, the price of conveyance, workforce and upkeep might intensify the price of reprocessing ventures. Moreover, owing to fast suburbanization, the area accessible for landfilling, particularly in metropolises, is decreasing [2].
The waste plastics can be reprocessed via de-polymerization, thermal degradation, and catalytic cracking to produce various high-valued oils like petroleum, paraffin oil, diesel, lube oil, and many others. Transforming this waste into petroleum possesses prodigious assurance for both the ecological and financial circumstances. This is because considerable capital and energy are required to safely process the plastic litter [3].
Figure 1. WPF obtained from mixed-plastic waste [4]
1.1 PLASTICS
As an ephemeral overview of plastics, plastics can be defined as synthetic organic macromolecules manufactured by the process of polymerization. They are usually of large molar weight and are often added with fillers to increase stability and reliability. They are broadly classified into two kinds:
- Thermosetting plastics
- Thermoplastics plastics
Plastics can be categorized in numerous methods based on their chemical arrangement, production procedure, concreteness, and other kinds of stuff. To assist in reprocessing waste plastics, the Organization of Plastic Manufacturing demarcated an encryption scheme that divides the plastics into seven primary sets based on their chemical nature and uses [5].
1.1.1 Types of Plastics
There are primarily seven kinds of plastics that are described here [6-9]:
- Polyethylene Terephthalate: It is a thermoplastic polymer resin that belongs to the polyester group. It could be recycled.
- High-density Polyethylene also called HDPE: Is a thermoplastic polymer that is prepared from the monomer ethylene. It could also be recycled.
- PVC or Polyvinyl Chloride: It is a rock-solid plastic that is prepared from the monomers of vinyl chloride. It could also be recycled.
- Low-density Polyethylene or LDPE: LDPE is a thermoplastic that is polymerized from the monomers of ethylene by a high-pressure procedure through a free radical polymerizing process.
- Polypropylene also called PP is a thermoplastic polymer that is prepared through the process of chain growing polymerization from the monomers of PP. It cannot be recycled.
- Styrofoam or Polystyrene is an artificial fragrant hydrocarbon polymer that is prepared from the monomers of styrene. It cannot be recycled.
- Other plastics are Polycarbonate, polylactide, acrylic, nylon, styrene, and fiber sheets: they cannot be recycled. The different types of plastics along with their properties and uses are given in figure 2.
The common plastic waste found in landfill sites is a combination of the above-mentioned polymers. Among these, only high-density polyethylene and polyethylene terephthalate are usually recycled.
Figure 2. Types of plastics [10]
1.1.2 Plastic Waste & Its Influence
The collection of used plastics that are discarded without any systematic procedure is called plastic left-over or waste. It normally includes routine plastics, for instance, single-use containers, packaging material, and similar plastics. This kind of waste also differs from small plastics – minor elements of less than 5 mm of plastic distributed in the atmosphere – to huge parts or products [11].
Waste plastic has an important influence on soils, waters, animals, and humans. About 50% of plastics are not recyclable and a minor fraction of these plastics are reused. Henceforth, they can stay undamaged in the environment for considerable periods unless discarded properly [12].
- Influence on terrestrial land: When the chlorine-containing plastics, are landfilled in the earth, then it not alone disturbs the earth, but likewise the nearby water foundations and ecological balance since it discharges injurious chemical substances.
- Influence on waters: The studies have estimated that about 165 million tons of plastic litter are discarded in the biosphere’s water. These plastics disturb sea life, and eventually, the human generation since they consume seafood infected with poisonous chemicals and compounds. Taking the seafood that comprises these pollutants could source a rise in cancer, immunity illnesses, and natal imperfections [12].
1.1.3 Plastic Waste & Reprocessing
Waste plastics are considered an asset for making alternative fuels due to their abundant availability and growing accessibility in developed or underdeveloped societies. The selection of reprocessing techniques of waste plastics into useful oil is dependent on the type and variety of waste feedstock. Generally, the conversion of waste plastic into petroleum needs feedstock that is harmless and can be safely processed without damaging the processing equipment.
For example, fillers like flare retardants comprising antimony and bromine compounds or plastics having nitrogen, halogens, sulfur, or any similar compounds can cause a prospective hazard to people and the atmosphere. The type of plastics and their chemical structures usually determines the reprocessing method required to convert them to fuels. Moreover, their chemical composition also defines the key processing parameters such as the heating temperature, the quality of the fuel produced, emission gases such as NOx and SOx, and the leftover residue and ash [13].
In most cases, the waste plastic is first sorted before recycling. For example, some types of packaging materials used to keep food fresh usually constitute multilayers of different plastics. Processing such type of plastic waste can produce more byproducts leading to a WPF of inferior quality. Efforts are in progress in this regard to develop chemical solvents that can preprocess and simply the management of mixed plastic waste [14, 15].
2 Production and Characterization Techniques of Waste Plastic Fuel
Considering the abundance of waste plastics, their removal generates huge difficulties for the atmosphere. Though, thermal procedures can be used to change plastics into organic oils like gasoline, diesel, and jet fuel that have limitless usages in aerospace, transportation and power production. The techniques and methods to reprocess waste plastics are discussed in the subsequent sections.
2.1 Pyrolysis
Pyrolysis or thermal degradation is a procedure in which a substance is degraded in the absence of oxygen gas. In this method, waste plastics are thermally degraded at a temperature range of 370 °C to 420 °C without air in a cylinder-shaped reactor and the pyrolytic vapors are processed to produce a variety of gaseous and liquid hydrocarbons having linear or cyclic structures [16].
The benefits of the pyrolysis procedure are:
- The constituents of the waste plastics are considerably reduced (< 50–90 %) to useful products
- A variety of gaseous and liquid fuels could be produced depending on the type of plastic
- Storable and transferrable petrol or liquid fuel stock is achieved
- A convenient method of reusing waste resources such as public plastic waste or sewerage mud
- The cost of processing is low
There are diverse kinds of pyrolysis procedures termed as slow, fast, and flash pyrolysis. In all types of these processes a combination of products such as solid residues, waxes or liquid oil, and gas is produced depending upon the process conditions [17-19].
Traditional pyrolysis also called slow pyrolysis is usually carried out between 300 °C to 500 °C at a heating rate of 1 to 10 °C per minute. It produces substantial amounts of lighter hydrocarbons and charcoal. This is because a slower heating rate facilitates complete degradation of the feedstock and the formation of solid residue.
Fast pyrolysis is carried out in a temperature range of 500 °C to 700 °C with a heating rate of around 1000 °C per minute. Here, the residence time of vapors is merely a few seconds and this facilitates the formation of liquid products with high conversions (~90%). This process is often used for the pyrolysis of polyolefins.
Flash pyrolysis is often used for higher concentrations of oils and similar lighter components. However, in the case of waste plastics, higher concentrations of gaseous hydrocarbons are obtained along with similar lighter products. The process is usually carried out at temperatures higher than 700 °C with an exceptionally high heating rate and the residence time of vapors limited to a few milliseconds.
Figure 3. Pyrolysis of waste plastic to fuel [20]
2.2 Catalytic Pyrolysis
In this technique, a catalyst is used to carry out the thermal degradation procedure. The use of a catalyst enables pyrolysis to occur under milder conditions in a short duration of time. Likewise, catalysts play a critical role in improving the efficiency of the process, selective production of hydrocarbons, and reducing the number of byproducts. It also enables the formation of the straight chain as well as branched hydrocarbons enabling the formation of oil with a high RON value. A variety of catalysts can be used for this process such as the MCM-41, ZSM-5, Y-zeolite, and FCC [2, 21].
2.3 Gasification
The plastic waste can be heated with air, oxygen or steam at a temperature ranging from 500°C to 1300°C producing gaseous fuels such as hydrogen, methane, carbon dioxide, and carbon monoxide. Among these, the synthesis gas or syngas, a combination of carbon monoxide and hydrogen, is an important product. It can either be used directly as a fuel or it can be further processed into useful hydrocarbon compounds. Keeping in view the end products, a gasification method is completely different from the previously discussed pyrolysis techniques. The purpose of pyrolysis is to fully consume the feedstock producing a variety of solid, liquid or gaseous products. Whereas, the key aim of the gasification process is to produce a feedstock that can be further processed into useful hydrocarbons [22-24]. One of the key advantages of gasification is the usability of a feedstock comprising any type of plastic along with other waste. Moreover, a combination of feedstocks comprising waste plastic and biomass can also be used. However, the gasification of waste plastics results in larger amounts of tar and a highly efficient gas cleaning system is required before its further processing [25-27].
2.4 Limitations of Waste Plastic Fuel Production Methods
The processing of waste plastics constitutes an important role in decreasing waste-connected ecological contamination and GHG releases. Additionally, it produces considerable financial benefits and could aid in achieving a circular economy in any country. There are numerous areas in pyrolysis-centered plants that need improvements to ensure cleaner processing of mixed plastics into useable fuels. In terms of emissions, the WPFs are cleaner as compared to fossil petroleum and a few other oils. However, considerable processing stages are required to obtain an acceptable quality of these fuels. This implies that thermal degradation fuel might not be much better than conservative oil or other natural oils about GHG releases, however, thorough investigation research on mass and power stability through the entire procedure’s restrictions is required to approve this. To overpower these procedure energy necessities, innovative types of machinery could be established utilizing the incorporation of reusable energy sources like solar or hydra with pyrolysis-built plants, to attain utmost financial and ecological assistance [28].
The processing of WPF is a key challenge for pyrolysis-based refineries. Its processing is not very effective, particularly in developing countries. The vapors formed by pyrolyzing some plastics like polyvinyl chloride are highly poisonous, and consequently, modern post-processing facilities are required to attain the ultimate ecological goal. Specifically, the WPF fuel obtained from several plastic categories must be processed considerably before usage. Though large proportions of the pyrolysis fuel are regular petrol or diesel fractions, some hydrocarbons like benzene, toluene, and styrene can be distilled and sold to other industries. Nevertheless, a good proportion of these hydrocarbons are well-known poisonous chemicals and can pose severe issues to human well-being and the environment [29].
3 Waste Plastics Fuels vs Traditional Oils
With increasing environmental awareness and ecological concerns, the usage of substitute fuels has become inevitably vital. The WPFs are certainly a potential substitute in which waste administration and power production must be discussed. The following table provides a comparison between the properties of traditional fuel and WPF.
Table 1. Properties of WPF vs regular diesel fuel [30-32]
3.1 Engine Efficiency and Emission Characteristics
The usage of WPFs in the place of petrol in automobile engines can not only decrease the dependency on conventional fuels but also influence the waste administration strategies executed throughout the world. Recently, an investigation was made by Kumar et. al. by using mixtures of WPFs in the ratio of 25%, 50%, 75%, 90%, and 100%. It was found that the brake thermal efficacy of the engine decreased by 2.5 % under minimum load and 6.75 % at full load in comparison to diesel [33]. This is because the viscosity of WPFs is considerably higher as compared to conventional diesel. The greater viscosity leads to a delay in the ignition which results in higher in-cylinder compression pressure. This will require a thicker engine block and adds more load to the cooling system.
In another research study, the ignition and emission characteristics of a diesel engine were studied by using a mix of conventional diesel and WPF. The results showed an increment in NOx discharge with increasing WPF composition in the fuel mixture. As compared to conventional diesel, the oxygen contents of WPF are much higher and this leads to a high temperature in the combustion chamber, eventually resulting in high proportions of NOx. These problems can be largely addressed by adding additives such as diethyl ether and alcohol to such fuel mixtures. This not only improves the viscosity of the WPF mixture but also improves its ignition characteristics [34].
Adding alcohol to plastic pyrolysis oils is a workable measure to decrease the harmful emissions of conventional engines. In this regard, butanol was blended with WPF in different proportions and the mixed fuel was tested on a regular direct-injection 4-cylinder diesel engine. This fuel mixture was also compared with another WPF blend having diethyl ether and castor oil in several proportions. The fuel blends are listed as follows,
- WPF: 84 %, Butanol: 16 %
- WPF: 84 %, Diethyl Ether: 16 %
- WPF: 83.5 %, Butanol: 11.5 %, Castor Oil: 5 %
- WPF: 83.5 %, Diethyl Ether: 11.5 %, Castor Oil: 5 %
At low loads, the emissions of hydrocarbons and smoke were much higher with diethyl ether as compared to the butanol blend. Whereas, under higher loads, vice versa was observed with similar emissions of nitrogen and carbon oxides. The addition of castor oil exhibited a considerable reduction in the above-mentioned emissions under both loads. However, this reduction was muchly noticeable with diethyl ether than with the butanol blend [35].
Similarly, another research study compared the performance of a larger direct injection diesel engine by using blends of pyrolysis oils with conventional diesel. The pyrolysis oils were obtained at different pyrolysis temperatures ranging from 700°C to 900°C. The feedstock was composed of a combination of waste plastics and the main components were polyesters and styrene butadiene. The synthesized WPF was also blended with commercial diesel with 25% and 75% proportions. The experimental setup is shown in the figure 4. The diesel engine was an AKSA A4CRX46T1 and the experimental tests were performed at a rated speed of 1500 rpm under various loads ranging from 75% to 100% of the rated power [36].
Figure 4. A schematic of the experimental setup [36]
For the last many years, compression ignition engines have been the preferable choice when it comes to automobiles, transportation machinery and electricity production. However, their widescale use is producing severe ecological issues and limiting the fossil fuel reserves. As discussed in this section, several research studies focused on the production of various WPFs from different feedstocks and methods. These WPFs were compared with conventional gasoline or diesel in different blends on regular compression ignition engines. All of these investigations came to a unanimous recommendation of using WPFs in blends having a major proportion of conventional fuels [34, 39-41].
4 Prospects & Market Analysis of WPF
At present, the reduction and reprocessing of waste plastics is a key area of research. It is considered to have a huge potential as fuel for future applications. However, as per the estimates, 300 million tons of waste plastic are produced every year and only around 27 million tons of this waste is recycled into useful products. This is mainly due to the high costs and limited efficiencies associated with the available commercial-scale technologies. In this regard, there had been an increasing realization of substitute approaches for waste plastics reprocessing. One of them is the plastic to petroleum ventures that have gained considerable attention in power plants of developing countries [42, 43]. In a similar effort, experts at Swansea College have developed a method to transform waste plastics into hydrogen energy for hydrogen-powered vehicles. Remarkably, any type of plastic can be conveniently used as a feedstock without any preliminary treatment or cleaning. In this process, the feedstock is exposed to solar radiation in a solution added with light-absorbing additives. The process, however, is still under development in the lab and might take considerable time to commercialization [44].
On a global scale, the market size of oils produced from mixed waste plastics is expected to grow at a cumulative annual growth rate of nearly 4% between 2020-2030. The major players in this market are Plastic2Oil Inc., Alterra Energy, Agilyx, Inc., Nexus Fuels, Brightmark LLC., etc. Among the different processes available at the moment, flash pyrolysis is expected to be the mainstream method for making WPF during the forecasted period. This is because flash pyrolysis offers 70% conversion of waste feedstock into useful bio-oil with low content of impurities such as water [4]. A number of factors are expected to contribute to this market growth such as,
- Global interest in the enhancement of energy security
- Regulations limiting the accumulation of waste plastics in the landfill sites
- Investment opportunities in conventional or modern pyrolysis plants
In developing countries, the management of plastic waste is often poor and this is expected to limit their contribution to the global reprocessing market. The key challenge to overcome in this regard is the development of infrastructure for the collection and management of waste. Other challenges include enforcement of governmental waste-management policies, more emphasis on environmental protection, and translation of recycling into economic benefits [30, 45, 46].
5 Conclusion
The wide-scale application of single-use plastics is enhancing at a fast pace because of the fast financial evolution, incessant human expansion, and variations in way of living. This, along with other plastic wastes, has sharply increased the accumulation of plastic waste in the natural ecosystem. As a result, the development of methods and technologies is needed to effectively reprocess and manage plastic waste. The waste-to-energy approach in this regard has captured widescale attention and efforts are in progress to produce alternative fuels from this waste. Different methods of converting waste plastics to fuels include pyrolysis, catalytic degradation, and gasification. It is determined that fuel obtained from waste plastic is certainly a prospective substitute in which the waste administration alongside power production can be synchronized. More and more companies are now limiting the use of single-use plastics and finding new, eco-friendly ways to effectively reuse plastic waste.
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