For centuries, oil has been part and parcel of the energy sector for producing heat and electrical energy at domestic and industrial scale because of its high energy density. The advantage of the oil is its simplicity, easy availability throughout the world and flexibility. It can be used for multiple applications ranging from domestic cooking and heating to industrial applications like powering engines, furnace and boilers, and even generators at accessible and remote locations.
This oil can be either synthetically produced from crude oil via fractional distillation or extracted from plant seeds and tree barks. Earlier, oil was sourced from almost all the available natural biomass; and was used for a very long time owing to their sufficient availability. However, with the rising population count and demand for food, only non-edible biomasses were used for extracting oil. This step addressed the “food vs fuel” concern and assured that no food source was used for fuel production. Later on, this was eased with the invention of crude oil, where liquid petro fuels like petrol and diesel and Liquefied petroleum gas (LPG) slowly started taking over the world energy production market; and made a huge impact globally. But, with concerns over climate change and the greenhouse effect, the trend for fossil fuels has now slowly started to fade, which opened an opportunity for the oils from biomasses to make their resurgence. Oil from edible feedstocks is currently widely used for edible and medical purposes; whereas, oil from non-edible biomasses is widely used for medicinal, industrial and energy applications.
Moreover, petrol and diesel fuels are limited to supplying power to engines and generators for transportation and power generation applications. In recent times, the conversion of non-edible oils and biomass to biofuel is gaining momentum. One such technique used for extracting oil from biomass is pyrolysis, which is used for extracting oil from biomass that has complex macromolecules and requires high temperature for their thermal degradation and converts into oil.
What is Pyrolysis?
Pyrolysis can be defined as a thermal decomposition process of the biomass in the absence of air. In fact, this pyrolysis reaction also occurs at the initiation stage of the combustion and gasification reaction, wherein the combustible products start breaking down at their molecular level. In general, pyrolysis of any biomass gives the following products: solid bio-char, liquid bio-oil and gases like methane (CH4), hydrogen (H2), carbon monoxide (CO), and carbon dioxide (CO2). The formation of these products depends on the operating temperature inside the reactor and the heating rate.
The solid biochar is produced upon maintaining temperatures below 450 C, and the yield of bio-oil and gases increases with the increase in temperature. Similarly, slow pyrolysis tends to favour char production and fast pyrolysis favours liquid and gas production.
Pyrolysis has many advantages, such as small compact sized reactors, simple operation, and its ability to produce high energy density products from biomass, making it highly suitable for remote production processes. Additionally, the bio-oil obtained can be directly used for producing heat and electricity. The bio-oil has a high calorific value and can be used for co-firing in furnaces or boilers. Numerous feedstocks have been used to extract oil through pyrolysis, including lignocellulosic biomass and agro residues like wood wastes, rice husk & straw, empty fruit bunch, and even municipal solid wastes (Reddy and Vinu, 2018). In recent times, waste plastics and rubber tyres have also been used as feedstock for pyrolysis and extract oil from them ( Vijayakumar and Sebastian, 2018; Alsaleh and Sattler, 2014). Indeed, these wastes have been identified as a replacement for organic biomass and can be used for producing saturated and aromatic hydrocarbons. Even though this technique serves as an effective solution to address the problems related to the disposal of waste plastics and rubber tyres, one must consider the increased risk associated with it. The potential threats include the release of dangerous emission gases like Hydrogen sulphide (H2S), methane (CH4), and carbon monoxide (CO); release of heat to the surrounding; and safety concerns associated with it. For this purpose, one has to understand the science involved with the pyrolysis of plastics and rubber tyres; and gain knowledge regarding the products developed during the process operated at different temperatures.
A Scientific View On Pyrolysis of Plastics and Rubber Tyres:
Plastics require a very long time to undergo degradation; and are affected by numerous factors like sunlight, ambient air and moisture content, thermal stress induced by weather, and degradation due to chemical and biological substances. In addition, oxygen leads to photo-oxidative degradation, making them brittle and crack over time (Shah et al., 2008). It is impossible to convert these macromolecules into small chained monomers/polymers using ordinary heating or any simple chemical treatment because of its durable molecular structure. Thus, pyrolysis is chosen as the effective means for degrading these polymeric chains into short-chained chemical compounds, which are then suitable for combustion applications. In the same manner, even rubbers require prolonged time for degrading; and are also affected by the factors which affect the plastics. Also, they require very high temperatures for their polymeric units to get converted into smaller monomeric or polymeric units in the form of bio-oil or other compounds. A wide variety of plastics have been used in pyrolysis for producing bio-oil, and not all plastics give rise to the same desired products. Moreover, these products vary with the operating temperature and depend entirely on the nature of the plastic used. Table 1 summarises the products obtained during the pyrolysis of different types of plastics.
Table 1: Chemical compounds obtained during the pyrolysis of different types of plastics (Qureshi et al., 2020)
Similarly, rubber tyres are manufactured using multiple chemical compounds to ensure their longevity even in the harshest conditions. Yet, the most commonly used chemical compounds include rubber (60–65 wt. %), carbon black (25–35 wt. %), fillers (3 wt. %), and accelerators. Here, this rubber comprises natural and synthetic blend, where natural rubber is extracted from heave tree, and synthetic rubber is derived from petroleum-based products. Also, carbon black strengthens it, while aromatic, naphthenic, and paraffinic organic chemical compounds are used as fillers to make it soft and workable (Martínez et al., 2013). Table 2 summarises the products produced during pyrolysis of different types of tyre materials.
Table 2: products produced during pyrolysis of different types of tyre materials (Alsaleh and Sattler, 2014)
From table 1 and 2, it is evident that pyrolysis of both plastics and rubber tyres yields a higher number of hydrocarbons like polyaromatic hydrocarbons (PAHs), polyaromatic nitrogen hydrocarbon (PANH), and polyaromatic sulphur hydrocarbons (PASHs); with their carbon chain length ranging between 5 and 20. On the other hand, aliphatic hydrocarbons were reported as straight-chain alkanes with carbon chain length ranging between 6 and 37. Most commonly occurring aromatic HCs include benzene, indene, limonene, styrene, toluene and xylenes. Moreover, upon distilling the pyrolysis oil, light fraction yielded BTEX and styrene, while heavy fractions yielded chemical compounds similar to asphalt. Also, high sulphur content is reported for pyrolysis oil at processing temperatures near 400OC and 650OC. However, intermediate temperatures reported minimal sulphur content (Williams, 2013). Likewise, major gas constituents include hydrogen (H2), methane (CH4), ethane (C2H6), ethene (C2H4), propane (C3H8), propene (C3H6), butane (C4H10), butene (C4H8), butadiene (C4H6), carbon dioxide (CO2), carbon monoxide (CO), and hydrogen sulfide (H2S) (Kaminsky et al., 2009).
One can appreciate the benefits reaped from the pyrolysis of plastic and rubber tyres, which produced varieties of hydrocarbons that can be used for fuelling engines. In fact, this technique redemonstrates the fractional distillation of crude oil, except it is replaced with plastic or rubber wastes. Chemical compounds produced during this process can be processed according to their requirement and are almost the same as the products developed during fractional distillation of crude oil. However, not all products are desired and environmental friendly; instead have the ability to produce high toxic emission which can be harmful to both living beings and environment. Moreover, the amount of heat consumed and liberated during the process is very high and can add up more heat to the surroundings. On the whole, pyrolysis can be an effective technique for producing high-quality chemical compounds that are highly combustible but is a time consuming and resource-consuming process, which makes the readers decide themselves regarding how environmentally friendly these process are upon using these products.
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Martínez, J. D., Puy, N., Murillo, R., García, T., Navarro, M. V., & Mastral, A. M. (2013). Waste tyre pyrolysis–A review. Renewable and Sustainable Energy Reviews, 23, 179-213.
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