Potential use of natural rubber to produce liquid fuels using hydrous pyrolysis – a review

Abstract

Natural rubber is a tropical plantation crop that mainly consists of the hydrocarbon polyisoprene (cis-1,4-polyisoprene). Rubber can be converted through depolymerization into a liquid, which can be used as fuels or a chemical feedstock. This paper provides information on natural rubber and its alternative sources worldwide. Although we focus on hydrous pyrolysis, we also introduced various depolymerization processes, including pyrolysis, gasification, chemical degradation, catalytic cracking and hydrogenation. Hydrous pyrolysis improves the depolymerization process, such that raw materials can be fed without requiring a drying process. This process can also be conducted at low temperatures, and water can be solely used as the reaction medium and can be easily separated from the oil product. This paper also reviews the role of process parameters such as temperature, rubber to water ratio, reaction time, and type of gases on product quality and quantity. Moreover, this study highlights the environmental and economic feasibility of hydrous pyrolysis.

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Nabeel Ahmad

Nabeel Ahmad received his bachelor degree and master degree in Chemical Engineering at University of the Punjab, Lahore, Pakistan in 2012 and 2014 respectively. In October 2012, he joined Kansai Paints Private limited as a production executive. He has one year of industrial experience in the Paint manufacturing. Later on in January 2015, he joined COMSATS Institute of Information Technology, Lahore, Pakistan. Since that time till now he is working as a lecturer. Currently, he is on study leave and doing PhD in chemical engineering under the supervisions of Prof. Dr Wan Mohd Ashri Wan Daud and Dr Faisal Abnisa. His PhD topic is on the depolymerization of natural rubber to produce liquid fuels.

Faisal Abnisa

Faisal Abnisa is a Doctor in Chemical Engineering from University of Malaya, Malaysia. He has been actively involved in the research of pyrolysis of biomass since 2008 and has published extensively. Currently, he is working as a Postdoctoral Research Fellow in the Department of Chemical Engineering, University of Malaya. His research interests focus on the thermal conversion of biomass and the use of pyrolysis liquid for fuel and chemical products.

Wan Mohd Ashri Wan Daud

Professor Wan Mohd Ashri Wan Daud received his bachelor degree in Chemical Engineering at Leeds University, Leeds, UK in 1991 and his master's degree in Chemical Engineering at the University of Sheffield, Sheffield, UK in 1993. He received his PhD degree in Chemical Engineering at the University of Sheffield in 1996. After nine years as an academic and scientist at the Faculty of Engineering, in 2005, he became Professor of Chemical Engineering. Since 2005 until now, he has worked as a Professor of Chemical Engineering at the University of Malaya, Malaysia. His research fields include energy, biomass conversion to bio-fuel, catalysts synthesis, polymerization and separation processes, and hydrogen storage. He has more than 131 publications in Web Science journals.

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Introduction

The depletion of fossil fuels and their impact on global warming has become a well-known and dominant issue over the past decades; consequently, this issue has motivated researchers to create alternatives for fossil fuels.^4,5 The reserves of fossil fuels, such as petroleum, gas and coal, are available up to 2044, 2046, and 2112, respectively.⁶ Natural gas and crude oil are consumed by around 9413.69 million cubic meter per day and 77.83 million barrel per day worldwide.⁷ The high consumption as well as depletion of fossil fuels are predicted to result in excessive increase in their prices in the next decades.⁸ To address this issue, scholars have developed alternative fuels that are efficient, environment friendly, and economical.^9,10

Environmental aspect associated with using fossil fuels has been a major global issue for the past decades.¹¹ Burning of fossil fuels has substantially increased environmental problems because of emission of harmful pollutants, such as SO_x, NO_x, CO₂, and hydrocarbons, which cause ozone depletion, acid rain, and global warming.^12–16 The use of fossil fuels contribute to 62% of global CO₂ emissions.¹⁷ China and the USA are the major emitters (about 44% of the global emission) of greenhouse gases from fossil fuels.^18,19 According to the National Energy Information Center, the USA currently contributes approximately 19% of the worldwide CO₂ emissions.^20,21 By the year 2030, the CO₂ emission levels are estimated to increase to 40 billion Mg per year, which is an alarming situation.²² To combat these environmental issues, Barbose et al.²³ suggested that the use of fossil fuels should be reduced, and renewable fuels should be deployed as replacement for fossil fuels for environmental and social benefits.^23,24

Many alternative energy resources for fossil fuels are available worldwide. Fuels are employed to produce heat, mechanical work, and power generation for subsequent use in people's daily lives.²⁵ An ideal fuel should possess the following properties: high hydrocarbon content, high calorific value, low cost, low moisture content, controllable combustion, harmless combustion products, easily transported at low cost, and low storage cost;^26,27 a suitable alternative fuel should also fulfill the criteria of ideal fuel.^28,29 Various alternative renewable energy sources, such as biomass, bio-diesel, alcohol-fuels, ammonia, solar thermal power, hydropower, and wind power, can be used as substitute for fossil fuels.^13,30

Biomass is one of the available potential alternative renewable sources of energy and can be used as substitute to fossil fuels.³¹ Biomass is an organic material derived from living organisms, such as plants or animals.³² Biomass is available in bulk and can be found in agricultural residues (rice husk, pine cone, bagasse, wheat straw, etc.^33–37), wood residues (pine, weed, and fir sawdust³⁸), and in industrial and municipal solid wastes (palm shell, wheat straw, legume straw, walnut shell, and so on^31,39–43). Natural rubber is a biomass usually obtained from the rubber tree, guayule plant, Russian dandelion, and rubber rabbitbrush.^44–53

Natural rubber is one of the potential sources in meeting future energy needs. Natural rubber is a vital, strategic, and unique feedstock used in huge quantities worldwide.⁴⁴ Natural rubber is an important plantation crop in tropical Asia and is abundant in Malaysia, Indonesia, Thailand, and various central African countries.⁴⁵ The rubber tree grows optimally in warm, humid climate.⁴⁴ Natural rubber is a polymer of the organic compound isoprene (polyisoprene (C₅H₈)_n) with minor amounts of impurities, including other organic compounds and water.⁵⁴ The structure of natural rubber and its monomer is shown in Fig. 1.

Fig. 1 Structure of natural rubber and its monomer isoprene.⁵⁵ This figure has been reproduced from ref. 55 with permission from ELSEVIER.

In 1997, the global production of natural rubber was estimated about 5 million metric tons, which increases gradually on account of its high usage.⁵⁶ Due to its high availability, researchers utilize natural rubber as a useful feedstock for production of valuable chemical commodities (fuels, fertilizers, and chemicals) through depolymerization.⁵⁷

Depolymerization is the process of breaking down large organic polymeric molecules into small molecules or their respective monomers.⁵⁸ Depolymerization processes include thermal degradation (pyrolysis, hydrogenation, and gasification), photo-induced degradation, chemical degradation (solvolysis, hydrolysis, ozonolysis, and oxidation), and biological degradation.^59,60 Hydrous pyrolysis, is an appropriate technique for conversion of natural rubber into liquid fuels.^32,61 In hydrous pyrolysis, materials, such as plastic, rubber, or biomass are heated using water to generate useful chemical commodities, such as liquid fuels. The use of water in hydrous pyrolysis presents an advantage over other depolymerization techniques because water is abundant, cheap, and environment friendly. Moreover, separation of the produced liquid fuels from water is easier and more economical compared with other techniques.^62–64 Generally, the products of pyrolysis are char, oil, and gas. Hydrous pyrolysis is a three step process: (i) the material is chopped into small pieces; (ii) the material is then heated at temperatures ranging from 200 °C to 300 °C using water at high pressure; and (iii) the produced hydrocarbons are broken down into light hydrocarbons at about 500 °C.³² This process is commercially employed to convert agricultural organic waste into valuable chemical commodities, such as fuels, fertilizers, and other chemicals.³² Changing World Technologies and the Renewable Environment Solutions LLC (RES) established the first commercial plant of hydrous pyrolysis in Carthage.⁶⁵

This article presents a review of the depolymerization of natural rubber through hydrous pyrolysis. The influences of process parameters such as temperature, pressure, and water to material ratio on the product yield and composition are also discussed. Moreover, this paper provides important information related to natural rubber statistics and alternative sources.

Natural rubber and its sources

Natural rubber

Generally, natural rubber is produced from the rubber tree (Hevea brasiliensis), guayule plant, Russian dandelion, rubber rabbitbrush, fig tree, goldenrod, and sunflower. Natural rubber, also known as cis-1,4-polyisoprene and denoted as (C₅H₈)_n, is a hydrocarbon with a molecular weight ranging from 1 to 2.5 × 10⁶.^66,67

Rubber is recovered from liquid latex through coagulation and addition of acids, such as formic acid. The coagulum, a soft solid slab, is squeezed through a series of rollers to remove excess water and increase surface area. The obtained rubber sheets are dried using smoke ovens.^68,69

Production of natural rubber

According to 2015 statistics, the global production of natural rubber almost doubled from 2000 to 2014. The amount of rubber produced was 6.8 million metric tons in 2000 and 12 million metric tons in 2014.⁷⁰ The global consumption of natural rubber was about 7 million metric tons in 2000;⁷¹ consumption increased to 12.1 million metric tons in 2014.⁷⁰ The world production and consumption of natural rubber are shown in Fig. 2.

Fig. 2 Global natural rubber production and consumption trends from 2000 to 2014.⁷²

Thailand, Indonesia, and Malaysia are the major producers of natural rubber. The amount of natural rubber produced in Malaysia was 927 608 tons in 2000 and 668 613 tons in 2014.⁷⁰ Moreover, the amounts of natural rubber consumed in China, India, the United States, Thailand, and Malaysia were 864, 900, 380, 3863, and 826 thousand tons, respectively, in 2013.⁷³ According to the statistics of the Food and Agriculture Organization, the natural rubber production in Indonesia increased from 1792 to 3108 thousand tons from 2003 to 2013.⁷⁴

Sources of natural rubber

Natural rubber is a bio-polymer obtained as latex from different plants; H. brasiliensis, commonly known as the Hevea rubber tree, is the most significant commercial source of natural rubber.^51,75 The rubber tree grows optimally in warm, humid, even climate at 24–28 °C throughout the year, with humidity above 70%, and scattered well-distributed rainfall of 1800–2000 mm per year on well-drained soils⁴⁴ Several years are needed for a rubber tree fully mature and to be ready for extraction of natural rubber latex.^68,76 The H. brasiliensis is shown in Fig. 3.

Fig. 3 Natural rubber latex collected in a mug after skillful tapping of the bark of a H. brasiliensis tree. These figures has been reproduced from ref. 1–3 with permission from ELSEVIER.

In the wild, Hevea rubber tree can grow to a height of 100–130 ft and can live up to 100 years. However, in the semi-wild environment, the tree can only live up to 30 years because tapping decreases its productivity. Wild and semi-wild H. brasiliensis plantations are commonly found in South America (Brazil, Guiana Francesa, Suriname, Guiana, Venezuela, Colombia, Equador, Peru, and Bolivia, as shown in Fig. 4c), followed by South East Asian countries (Malaysia, Indonesia, Thailand, Vietnam, Sri Lanka, China, India, and Papua New Guinea, as shown in Fig. 4a) and African countries (Nigeria, Côte d'Ivoire, Cameroon, Liberia, Ghana, Republic of Congo, and Gabon, as shown in Fig. 4b).^46–48,51 Approximately 90% of the total natural rubber worldwide is obtained from H. brasiliensis.⁷⁷

Fig. 4 Natural rubber plantation in African, South East Asian and in South American countries.

The other crops that produce natural rubber include guayule plant, Russian dandelion, rubber rabbitbrush, fig tree, goldenrod, and sun flower. Rasutis et al.⁷⁸ reported that guayule (Parthenium argentatum Gray) is a dry, non-tropical, and low-input perennial plant native to Mexico and southern Texas; this plant has received significant research attention because of its potential as a substitute source of natural rubber. Natural rubber is harvested in parenchyma cells in the bark of guayule plant (Fig. 5a).⁷⁸ Guayule plant is a feasible alternative source to Hevea rubber tree because of the high quantity and quality of the produced natural rubber, which exhibits the same molecular weight as that of natural rubber from the Hevea rubber tree.⁴⁹ Guayule plant is the only non-tropical plantation crop used commercially to produce natural rubber in the early 20th century.⁴⁹ Guayule requires fewer nutrients and pesticides compared with other plantation crops.^50,79,80 In addition, the residual, non-latex guayule exhibits a potential to produce useful chemical commodities, such as bio-fuels, insulations, and paper pulps.^78,81

Fig. 5 (a) Guayule plant⁷⁸ and (b) Russian dandelion.⁴⁹ These figures have been adapted from ref. 49 and 78 with permission from ELSEVIER.

Russian dandelion is another alternative source of natural rubber discovered in Kazakhstan, Soviet Union in 1932.^52,75 Russian dandelion fully matures within 85 to 95 days. Rubber is collected in the roots and leaves of Russian dandelion.⁸² Van Beilen and Poirier⁵² reported that Russian dandelion produced 150–500 kg per ha per year natural rubber during World War II to fabricate make tires for the Soviet Union and Germany.⁵³ Russian dandelion is shown in Fig. 5b.

The other plant species used as a source of natural rubber include rubber rabbitbrush, fig tree, goldenrod, and sunflower. The available sources of natural rubber are shown in Table 1.

Table 1 Available sources of natural rubber

Rubber source	Rubber MW (kDa)	Production (tons per year)	Rubber contents (%)	Yield (kg per ha per year)	Comments	References
Rubber tree H. Brasiliensis	1310	9 000 000 (2005)	30 to 50 in latex, 2% of tree dry weight	500 to 3000	The usual maturity duration of the rubber tree is 6 years. The tree can live up to 100 years but is usually cut after 30 years because tapping decreases the productivity of latex. Latex re-growth takes a couple of days depending on the condition of the tree	92–95
Guayule shrub P. argentatum Gray	1280	10 000 (1910)	3 to 12 of plant	300 to 1000	Production time usually takes 2–5 years, and re-growth time is 12 to 18 months	47, 92, 96–99
Russian dandelion T. kok-saghyz	2180	3000 (1943)	0 to 15 of root	150 to 500	Russian dandelion is usually planted in the early spring and takes 85 to 95 days to fully mature	53, 92 and 100
Rubber rabbitbrush C. nauseosus	585	n.a	Less than 7 of plant	n.a	Rubber rabbit brush reaches maturity within 2 to 4 years and has a lifetime of 5 to 20 years. This plant produces seed at the age of 2 years or more	46, 96, 97 and 101
Goldenrod S. virgaurea minuta	160–240	n.a	5 to 12 of root dry weight	110 to 155	A low-quality rubber producer reported in a demonstration project in 1931	46, 96 and 97
Sunflower Helianthus sp.	279, 69	Research stage	0.1 to 1 of plant	n.a	This plant yields rubber with a low molecular weight	96, 97 and 102
Fig tree Ficus carica	190	n.a	4 in latex	n.a	The research and development of this plant is related to biochemistry	49, 92 and 103
Lettuce Lactuca serriola	1380	Research stage	1.6 to 2.2 in latex	n.a	The research and development of this plant is related to genetic engineering and characterization	46, 49 and 104

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Uses of natural rubber

Currently, thousands of commodities, such as tires, balloons, and boots, are fabricated using latex obtained from rubber trees.⁸³ Natural rubber exhibits distinct physical properties and function as a perfect insulator;⁸⁴ hence, natural rubber is used or cable insulation and production of scrap tires, automotive parts, and galoshes.^{56,83,85–87} Natural rubber is also used to develop heavy mega structures and vibration insulators.⁸⁸ Evans and Evans⁸⁹ reported that scrap tire contains 45–47 wt% of natural rubber along with carbon black filler, styrene–butadiene rubber, butadiene rubber, and other commodities.^56,89 Natural rubber is also applied in the food, cosmetics, packaging, paper, clothing, wall covering, and medical industries.⁹⁰ Moreover, natural rubber is used extensively for preparation of adhesives, thermoplastic polymers, binders, resins, paints, and varnishes.⁹¹ The uses of natural rubber in various sectors are summarized in Fig. 6.

Fig. 6 Uses of natural rubber.

Methods for conversion of natural rubber to fuel

Various techniques, such as pyrolysis, gasification, chemical degradation, catalytic cracking, and hydrogenation, are used to convert natural rubber to fuels and valuable chemicals.

Pyrolysis

Pyrolysis, also known as thermolysis, is the process of thermally breaking down organic materials into relatively smaller molecules at high temperatures of 400–600 °C. Pyrolysis is classified as a slow or a fast process based on heating rate. If the time required to heat the raw material to the pyrolysis temperature is longer than the characteristic pyrolysis reaction time, then the process is considered slow; otherwise, the process is considered fast.³² Pyrolysis can also be classified into hydropyrolysis, hydrous-pyrolysis, oxidative-pyrolysis, vacuum pyrolysis, and catalytic pyrolysis based on the type of the environment where the process is employed.⁸⁶ Pyrolysis is characterized based on operating parameters, such as reaction period, heating rate, temperature, pressure, and the nature of agents and catalyst used.^31,61,105 The products of pyrolysis are solid (char or carbon), liquid (tar, hydrocarbons, and water) and gas (CO₂, CO, H₂O, C₂H₂, C₂H₄, and C₂H₆).³²

Various investigations were conducted on the pyrolysis of natural rubber. Chen and Qian⁵⁶ performed pyrolysis of natural rubber (cis-1,4-polyisoprene) in an inert atmosphere to determine the effect of temperature on the composition and yield of the pyrolysis product, such as pyrolytic oil, residues, and gases; however, only the yield of the main component from pyrolytic oil has been reported. Studies also revealed that di-pentene is a main component of pyrolytic oil at temperatures below 431 °C. At ambient temperatures of – 330 °C, 331–390 °C, and 391–430 °C, the di-pentene yields were 53.57%, 29.03%, and 11.89%, respectively.⁵⁶ Cataldo¹⁰⁶ reported the detailed pyrolysis of synthetic and natural rubber (cis-1,4-polyisoprene) in a heated flask under direct flame at low pressure; the produced oil presented a yield of 44.5% and mainly consists of 90% di-pentene with small amount of isoprene (3.5%) and other commodities. Munger¹⁰⁷ outlined the production of gaseous and liquid fuels from small tire pieces at temperatures below 482 °C. The yields of products, such as gas, oil, and carbon black were 5% to 50%, 20% to 50%, and 30% to 50%, respectively. The gross calorific value of the oil product was estimated as 18 000 Btu per pound.¹⁰⁷ Groves et al.¹⁰⁸ studied the thermal degradation of natural rubber through pyrolysis to investigate the oil product obtained at 500 °C; the major products obtained include monomer, isoprene, dimmer, and di-pentene with other commodities in substantial concentrations.¹⁰⁸ Similar investigations on pyrolysis showed that di-pentene and isoprene are the major products of natural rubber pyrolysis.^109,110

The pyrolysis of natural rubber can be carried out at 330–400 °C to obtain liquid fuels. Heating rate is an important factor that should be considered because it positively influences the yield of the pyrolysis product. Heating rate can be determined by the type of pyrolysis process used.³²

Gasification

Gasification is a thermal process that converts organic- or fossil-based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide, and methane.¹¹¹ This process is achieved by subjecting the material to high temperatures (>700 °C) by using a controlled amount of oxygen and/or steam without performing combustion.^111,112 The resulting gas product is called syngas (synthesis gas or synthetic gas) or producer gas, which is a fuel with a heating value.^111–113 Furthermore, gasification stores energy into a chemical bond.¹¹³ Gasification is carried out in three types of reactors: (i) fixed-bed gasification, (ii) fluidized-bed gasification, and (iii) entrained-flow gasification. When choosing the appropriate gasification process, the different factors that should be considered include fuel reactivity, plant size, raw material used, and oxidant type (air, steam, or air/steam).^32,111,112

Thus far, gasification of 100% pure natural rubber has not been investigated. However, various studies reported that the gasification of tire rubber (consists of 40–52.2% natural rubber^64,89,114) in an oxidizing medium under low-temperature conditions produced gaseous products, oil, and solid residues.¹¹⁵ Ahmed and Gupta¹¹⁶ performed pyrolysis and steam gasification of rubber tires at 800 °C and 900 °C to produce syngas. A comparative study of gasification and pyrolysis was also conducted to determine the effect of temperature on gaseous product yield. Furthermore, the characteristics of syngas were investigated. At 800–900 °C, the yield of hydrogen from gasification is higher than that from pyrolysis.¹¹⁶ Rubber can be used as an additive for coal gasification. Straka et al.¹¹⁷ used a moving bed gasifier for co-gasification of rubber with brown coal on laboratory and industrial scales at 850 °C. The use of rubber particles (10–20 wt%) improved the calorific value of the final product (10.67–11.78 MJ m⁻³).¹¹⁷ Straka and Bučko¹¹⁸ performed oxygen–steam co-gasification of lignite with tire rubber through Lurgi gasification. The net heating value of the final gas product is higher for the mixture of lignite and rubber tyre 10–20 wt% compared with that obtained through gasification of lignite alone. The gross calorific values of the final product obtained through co-gasification of lignite/waste-tire and gasification of lignite alone were calculated as 12.77 and 12.40 MJ m⁻³, respectively. Moreover, the sulfur contents in the gas product are lower in co-gasification than those in gasification of lignite alone.¹¹⁸

Gasification of natural rubber can be conducted at 800 °C to 900 °C. Rubber can also be employed for coal co-gasification because it reduces sulfur content in the gas product and improves the calorific value.

Chemical degradation

Chemical degradation is the decomposition of polymeric materials into useful chemical commodities by using chemicals, such as acids, bases, and solvents.¹¹⁹ This process is due to many types of chemical reactions, which mostly result in the breakage of double bonds.¹²⁰ Chemical degradation processes include hydrolysis, ozonolysis, and solvolysis.¹²¹

Campistron et al.¹²² performed chemical degradation of natural rubber in a controlled manner by using m-chloroperbenzoic acid; the results showed that reaction time and periodic amount of acid can be used to control the degree of breakdown. Chaikumpollert et al.¹²³ examined the chemical degradation of natural rubber with potassium persulfate at 30 °C; the viscosity of natural rubber was observed to be a function of the amount of potassium persulfate used. FT-IR and ¹H NMR analyses were then performed to study the structure of natural rubber; the products obtained from oxidation degradation included carbonyl and formyl groups.¹²³ Nor and Ebdon¹²⁴ studied the chemical degradation of natural rubber in chloroform solution through ozonolysis. The molecular weight of natural rubber rapidly decreased upon addition of various oxygenated functional groups.¹²⁴ Similarly, Anachkov et al.¹²⁵ investigated the ozonolysis of cis-1,4-polyisoprene and trans-1,4-polyisoprene by using carbon tetrachloride solution. Analysis using IR-spectroscopy and ¹H NMR spectroscopy showed that the products included ozonides, aldehydes, and ketones.¹²⁵

Catalytic cracking

Catalytic cracking process is the process of breaking down polymeric organic materials by using a catalyst.⁵⁹ This process is faster than thermal degradation.⁵⁹ A wide range of catalysts (Friedel–Crafts catalyst, basic and acidic solids, and bifunctional solids) can be employed to promote the catalytic cracking of rubber and plastics materials.⁵⁹ Depending on the types of catalysts and operational conditions used, different mechanisms and approaches have been observed during the process. The product of catalytic cracking is of higher quality compared with those of thermal degradation.^32,59,61

Catalytic cracking exhibits potential in the preparation of high-value commodities from organic materials. Many studies were conducted to depolymerize rubber through pyrolysis, co-gasification, and hydrogenation; however, limited information is available regarding the depolymerization of rubber through catalytic cracking. Larsen¹²⁶ investigated the catalytic cracking of waste rubber (scrap tire) by using molten salts, which exhibit the properties of Lewis acids, such as ZnCl₂, SnCl₂, and SbI₃, at 380–500 °C. The yields of oil (38–78 wt%), gas (10–17 wt%), and solid residues (45–49 wt%) are similar to the product yield obtained from thermal decomposition.¹²⁶ Wingfield et al. (1984 & 1985) developed a catalytic cracking process for decomposition of plastic and rubber waste by using zinc and copper salts. The use of these salts as catalysts could decrease sulfur and nitrogen contents.¹²⁷ The use of a basic salt catalyst can also improve the yield of oil and gas products.¹²⁸ Hall et al.¹²⁹ performed the pyrolysis of latex gloves by using Y-zeolite as catalyst at 380 °C and 480 °C; the experiment resulted in high yields of valuable aromatic hydrocarbon compounds. The use of catalyst also increased the overall product yield. In the absence of a catalyst, the yield of pyrolytic oil increased from 57.9 wt% to 79.8 wt% at 380 °C to 480 °C. However, the use of catalyst reduced the oil yield but increased the yield of the gaseous product from 7.4% to 11.7%. High product yields were observed at high temperatures in the presence and absence of a catalyst.¹²⁹

Hydrogenation

Hydrogenation is a potential alternative for depolymerization of rubber and plastic polymeric materials. This process uses hydrogen mixed with a typical catalyst, such as Ni, Mo, Fe, and Pt, for breaking down double and triple bonds.¹³⁰ Hydrogenation reduces the number of saturated hydrocarbon compounds and promotes the removal of sulfur, chlorides, and nitrogen.⁶¹

Several studies were conducted on the hydrogenation of natural rubber. Bhattacharjee et al.¹³¹ studied the depolymerization of epoxidized natural rubber by using C₄H₆O₄Pd as catalyst. Combined with epoxy groups, the catalyst played a remarkable role in the breakage of carbon double bonds. Infrared and nuclear magnetic spectroscopy techniques were used to analyze the products.¹³¹ Mahittikul et al.¹³² examined the hydrogenation of natural rubber latex by using iridium catalyst ([Ir(cod)(PCy₃)(py)]PF₆). In mono-chlorobenzene, [Ir(cod)(PCy₃)(py)]PF₆ was found to be an effective catalyst for hydrogenation of natural rubber latex. Observations also showed that the use of sulfonic acid retarded the poisoning of the catalyst during hydrogenation of natural rubber latex.¹³²

Hydrous pyrolysis

Hydrous pyrolysis is a technique that converts organic materials (rubber, biomass, plastics, etc.) into liquid fuel. This process is conducted using water at high temperatures and high pressure conditions and induces the decomposition of long-chain polymers of carbon, hydrogen, and oxygen into small-chain petrochemicals (monomers).^32,133 Hydrous pyrolysis is performed in water at high temperatures (250–400 °C) and pressures (4–22 MPa). This process can also be conducted under self-generated pressure. This process is similar to other processes that uses hot water such as hydrothermal liquefaction, thermochemical conversion, and hydrothermal processing.^134–137 One of the most important advantages of this technique is that it can use a raw material with high moisture content without the need for pre-drying. The products of hydrous pyrolysis are oil, solid residue, and gases. The amount of liquid oil obtained is higher than that of solid residue and gaseous products. In addition, the oil obtained through hydrous pyrolysis exhibits similar properties to those of naturally occurring crude oil.¹³⁸

Various studies were conducted on the depolymerization of kraft lignin, polystyrene, polytrimethylene terephthalate, nylon-6, circuit board waste, shale, kerogen, and coal through hydrous pyrolysis.^139–146 Chen et al.¹⁴⁷ performed hydrous pyrolysis of nylon-6 using phosphotungstic heteropoly acid as catalyst at 280–330 °C. Under the optimum conditions, the main product of hydrous pyrolysis was found to be caprolactum, with a yield of 77.96 wt%, and a small amount of 6-aminocaproic acid and oligomers.¹⁴⁷ Miknis et al.¹⁴⁶ conducted hydrous pyrolysis of sub-bituminous rank coal in helium atmosphere at 290–360 °C and 20 psi. The liquid products were obtained in two separate phases, namely, oil and solid residues. Oil was obtained as floating liquid on the water surface and as absorbed oil on the coal surface. The floating material contained more than 75% expelled oil.¹⁴⁶ Similarly, Beltrame et al.¹⁴⁵ examined the hydrous pyrolysis of polystyrene. The process was carried out under in argon atmosphere at 300–350 °C and 18 MPa. Liquid oil was obtained as the main product, with an overall amount of 95%, and mainly consisted of toluene, cumene, and ethylbenzene.¹⁴⁵

Nguyen et al.¹³⁹ reported that the yield of char obtained from hydrous pyrolysis of kraft lignin at 350 °C and 25 MPa ranged from 17% to 20%. The gaseous products were not collected because no significant amount of gas was obtained in the sampling bags. Similarly, Li et al.¹⁴⁸ reported the production of gaseous products through hydrous pyrolysis of brown coal at 320 °C. The yield of the gaseous product was estimated to be 54.8 kg per ton coal, which tends to increase with increasing temperature.¹⁴⁸

The first full-scale commercial plant of hydrous pyrolysis was established by the Changing World Technologies and the Renewable Environment Solutions LLC (RES) in Carthage, Missouri for conversion of waste into useful chemical commodities (fertilizers, fuels, etc.). RES reported that any type of waste materials, such as plastic waste, sewage waste, and rubbers could be used as feedstock. This plant converts 200 tons of waste into 500 barrels of oil per day. RES also claimed the energy efficiency of the process to be as much as 85%, depending on the heating value of the product and the dry feedstock.⁶⁵

Use of natural rubber in hydrous pyrolysis

Few studies utilized 100% pure natural rubber for hydrous pyrolysis. However, information is available regarding depolymerization of rubber tire containing 40–52.2 wt% of rubber^64,89,114 through hydrous pyrolysis. Furthermore, various investigations were conducted on hydrous pyrolysis of rubber tire to assess the effect of different parameters for production of useful liquid products.^{12,62,64,149,150}

Many parameters affect hydrous pyrolysis; these parameters include particle size, reaction time, water to material ratio, heating rate, and operating atmosphere. Raw material quality also significantly influences the final product, overall, temperature was found to be the most effective parameter that significantly affects product quantity and quality. Studies on hydrous pyrolysis of rubber are shown in Table 2.

Table 2 Various reported investigations on hydrous pyrolysis of rubber

Ref.	Materials	Process parameters					Product analysis		Comments
		Type of reactor	Temp. (°C)	Press (MPa)	Water as an agent	Reaction time	Quantitative analysis	Qualitative analysis
149	Rubber tire	All experiments were conducted in a 180 mL Hastelloy autoclave	380 °C	27.6 MPa	Water at the super-critical condition	0.5–3 h	Oil was obtained as the main pyrolysis product. Gases were not collected. Solid residues were also obtained and identified as carbon black (about 30 wt%)	The NMR analysis of the products showed the presence of aromatic, olefinic, and aliphatic groups	In this work, super-critical H2O and CO2 were used to depolymerize tire and natural rubber in a controllable manner. Molecular weight analysis of the degraded material indicates that reaction time can be used to control the degree of breakdown; materials in the molecular weight range of 10^−3 to 10^−4 were obtained. In addition, both material composition and super-critical fluid affect the rate at which the material is depolymerized. Functional analysis of the processed material shows carbonyl and aromatic groups
	Natural rubber							Natural rubber produces homogeneous organic liquid. The carbon content of natural rubber is low (<1 wt%); consequently, no solid is produced
								Tire rubber product fraction includes the aqueous phase; if SCF water is used, organic fraction is present as a free layer or absorbed on carbon black (around 70 wt%)
62	Rubber tire	Hydrous pyrolysis was conducted using a batch reactor. The reactor was heated using a molten salt bath. The temperature was controlled using a thermocouple	200–430 °C	0–28 MPa	Water at a sub-critical and super-critical condition	20–120 min	Maximum liquid product yield was 52.7 wt% at 370 °C	Under the optimum conditions, the calorific value of oil, energy recovery efficiency, carbon content, and hydrogen content were reported to be 44.09–45.09 MJ kg^−1, 62.49%, 86.66 wt%, and 10.99 wt%, respectively	Hydrous pyrolysis of rubber tire was performed to produce liquid fuels. The effect of different parameters, such as temperature (200 °C to 430 °C), corresponding pressure (0 to 28 MPa), water to rubber tire ratio (0/3 to 12/3), time (20 to 120 min), and environment (air, CO2, CO, H2, and N2) was studied. Temperature significantly influenced the yield and characteristics of the end product
					H2O/rubber tire mass ratio is 0/3 to 12/3		At 390 °C, the lowest yield of char was achieved (about 38 wt%)
158	Rubber tire	Rubber tires and water were placed into a batch reactor and heated with the inlet opening. After water boiling and vapor was used, 30 min was needed to exclude air in the reactor. The reactor was sealed and heated electrically. After residence time, the reaction products were drained when the temperature and pressure reached the requirement	350–550 °C	13.9–25 MPa	Water at sub-critical and super-critical conditions	30 min	Oil, gas, and solid residues were obtained as the main products	Carbon contents were high in solid residues. The average value of carbon contents was estimated to be higher than 80 wt%	Pyrolysis efficiency of used rubber tires and the effect of temperature and pressure on sub-critical/super-critical water were studied. Products were analyzed by GS-Mass, FTIR, and elementary analyzer
							Oil was not collected because of some limitations	The hydrogen content was estimated to be 0.58–4.76 wt%
							Gas products consist of CO2, CO, H2, CH4, and other low-molecular-weight hydrocarbons. The amounts of CH4 and H2 are comparatively higher than the other components of the gas	At 500 °C and 22.2 MPa, the percentages of CH4, H2, and CO2 were approximately 38%, 38%, and 16%
150	Rubber tire	Thermal depolymerization was conducted using a U-shaped stainless steel batch reactor consisting of electrically heated fluidized sand bath, cold water bath, and thermocouple. The working volume of the reactor was 35.2 mL. About 1 g of the rubber sample was used with 0.1 g of catalyst and 6.5 mL of water. The experiment was conducted under different atmospheres, such as air and helium	400–450 °C	22.7–31.1 MPa	Water at super-critical conditions	15–30 min	The maximum oil yield and conversion of rubber tire were reported to be 44 wt% and 58%, respectively at 400 °C in helium atmosphere	—	Thermal degradation of rubber tire in a batch reactor using super-critical water was performed to investigate the effect of process variables on product yield and conversion. The temperature and gas atmosphere were the most significant factors affecting oil yield and conversion
					1 g of rubber:6.5 mL of water
64	Rubber tire	An autoclave with a capacity of 0.7 L was used to conduct hydrous pyrolysis of rubber tire. About 1 g of the sample was loaded in an autoclave	330–450 °C	5.2–23 MPa	Water at a sub-critical and super-critical conditions	5.5 h	The yield of the oil product and solid residues was estimated to be 47.6 and 42.9 wt%, respectively, at 380 °C and 22 MPa	Oil product consisting of about 85 wt% and 10 wt% of carbon and hydrogen, respectively	Autoclave as a batch reactor was used to perform the thermal degradation of rubber tire (containing 52.2 wt% natural rubber) by using super-critical water and n-pentane. Although the magnitude of pressure of super-critical water is higher than the super-critical pentane (critical pressure is 3.37 MPa), water at super-critical condition was as effective as super-critical pentane
								Solid residues consisted of 91.6 and 1.48 wt% carbon and hydrogen, respectively
12	Rubber tire	Hydrous pyrolysis experiments were conducted using 316 stainless steel vessels as a batch reactor. The reactor was capable of handling 4137 MPa pressure. The volume of the reactor is 480 ± 20 μL	150–400 °C	—	Exp Set 1	48 h	The product yields obtained in the presence and absence of oxalic acid were reported to be 47.53 wt% and 41.95 wt%, respectively, at 350 °C	Alkanes, methyl n-alkanoates, plasticizers, n-alkanoic acids, phenols, and polycyclic aromatic hydrocarbons were found to be the major products of hydrous pyrolysis	This research mainly aimed to study the effect of temperature and different experimental conditions in hydrous pyrolysis on the chemical composition of products from scrap tire
					Double distilled water
					30 mg of rubber tire and 480 mg of water
					Exp Set 2
					Double distilled water and oxalic acid
					30 mg of rubber tire and 480 mg of water with 4 mg of oxalic acid

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Effect of water

In hydrous pyrolysis of rubber tire, water is used in sub-critical and super-critical conditions for degradation various materials.⁶² If the temperature of the water is above the atmospheric boiling point (100 °C) but below it's the critical temperature (374 °C), the water is said to be in sub-critical condition. However, if the temperature and pressure of water is equal to or above 374 °C and 22.1 MPa, respectively, the water is said to be in super-critical condition.^151,152 Water under the sub-critical condition is used as solvent for polar and ionic compounds; conversely, water under super-critical condition exhibits the properties of non-polar solvents and is thus used to dissolve and degrade various non-polar compounds.¹⁵³ Super-critical water is more aggressive than sub-critical water because the water changes its structure and most of hydrogen bonds are broken under super-critical condition; this phenomenon shift the polar forces of attractions to dipole–dipole attractions, which subsequently decreases the dielectric constant of water.^154,155 Degradation of rubber tire can also be carried out using various compounds, such as pentane, carbon dioxide, and toluene. However, toluene and pentane are hazardous to the environment and can cause water pollution, landfill contamination, and serious harm to human life.^156,157 Hence, water is the most suitable solvent for degradation because it is not only environment friendly but also cheap and easily available.

Various investigations were conducted on the use of water under sub-critical and super-critical conditions for degradation of rubber tire.^{64,149,158–160} Under super-critical condition, water is miscible with non-polar rubber, provides the thermal energy for bond breaking, and acts as a source of heat transfer agent and hydrogen provider.⁶² Rubber is decomposed almost completely in super-critical water compared with that in sub-critical water, thereby suppressing the formation of char.¹⁵⁸

The amount of water influences the yield of the liquid product. Zhang et al.⁶² reported that the amount of liquid products increased with increasing water to rubber tire ratio during hydrous pyrolysis of rubber tire. The yield of the liquid products also increased from 46 to 53 wt% when water/rubber tire ratio was increased from 3/3 to 12/3 at 390 °C. Furthermore, the high amount of water increased water density and nucleophile concentration, thereby increasing the pressure of the reactor. This condition is unfavorable for gas formation.⁶²

Chen et al.¹⁴⁹ examined the thermal depolymerization of rubber tire and natural rubber at 380 °C and 27.6 MPa by using carbon dioxide and water under super-critical conditions. During thermal degradation using super-critical water and carbon dioxide, natural rubber yielded products with lower molecular weight than those obtained using rubber tire. These results showed that super-critical water renders a more aggressive environment for degradation of materials compared with super-critical carbon dioxide. The discrepancy in the results is due to the fact that water is a stronger nucleophilic compound than carbon dioxide.¹⁴⁹

Similarly, Funazukuri et al.⁶⁴ reported the pyrolysis of rubber tire using super-critical water and n-pentane, with a super-critical point of 196.7 °C and 3.37 MPa. Although the level of pressure of super-critical water is higher than that of super-critical pentane, both solvents were considered effective. The yields of liquid products obtained using super-critical water and pentane were found to be 47.6 and 48 wt%, respectively.⁶⁴

Effect of temperature

Various investigations were conducted to study the effect of temperature on hydrous pyrolysis products. Zhang et al.⁶² performed hydrous pyrolysis of rubber tire at 200 °C to 430 °C. The temperature highly influenced the yield of the product. The yield of the liquid product increased from 13.5 to 52.7 wt% as the temperature was increased from 200 °C to 370 °C. A further increase in temperature at 400 °C did not significantly increase the liquid product yield. However, in the yield of the liquid product decreased at temperatures higher than 400 °C. This decrease in yield could be attributed to the degradation of oil into lighter fractions and volatiles.⁶²

Park and Gloyna¹⁵⁰ performed thermal degradation of rubber tire in a batch reactor using super-critical water. The experimental results showed that the maximum yield of oil was attained at 400 °C. However, the oil yield decreased at 400 °C to 450 °C because of the conversion of oil into lighter hydrocarbons and volatiles during thermal degradation. At 450 °C, the conversion of rubber was found to be 5% to 6% higher than that achieved at 400 °C. The maximum oil yield and conversion of rubber tire were reported to be 44 wt% and 58%, respectively, at 400 °C.¹⁵⁰

Funazukuri et al.⁶⁴ used autoclave as a batch reactor for thermal degradation of rubber tire (containing 52.2 wt% natural rubber) using super-critical water. At 380 °C and 22 MPa, the yields of oil and solid products were 47.6 and 42.9 wt%, respectively.⁶⁴ Similarly, Rushdi et al.¹² conducted hydrous pyrolysis of rubber tire using stainless steel batch reactor at 150 °C to 400 °C. Two sets of experiments were conducted. One experiment set was conducted using rubber tire and water, whereas the other experiment used the same materials and oxalic acid. The product yield increased with increasing temperature for both experimental sets. The concentration of hydrocarbons also increased as the temperature was increased to 250 °C in the presence and absence of oxalic acid. At 350 °C, the yields of the liquid product in the presence and absence of oxalic acid were reported to be 47.53 wt% and 41.95 wt%, respectively. Furthermore, the oil product yield decreased at temperatures higher than 400 °C. This decrease could be due to the decomposition of oil products into lighter hydrocarbons and volatiles. However, the oil yield was high in the presence of oxalic acid (Fig. 7). The presence of oxalic acid provided excess hydrogen, which can enhance the reduction of rubber.¹²

Fig. 7 Effect of temperature on the yield of oil produced from rubber tire.¹²

Similarly, Yi et al.¹⁵⁸ conducted hydrous pyrolysis of rubber tire in a batch reactor at 350–550 °C and 13.9–25 MPa. Oil, gas, and solid residues were obtained as the main products. However, oil was not collected because of limitations in the system. Pyrolysis efficiency increased with increasing temperature and pressure. The pyrolysis efficiencies were 40% at 350 °C and 13.9 MPa and 71% at 550 °C and 25 MPa. The reaction completed at 550 °C and 25 MPa because of the super-critical nature of water.¹⁵⁸

Similarly, Zhang et al.⁶² also reported that the lowest yield of char (40 wt%) was obtained at 370 °C. The yield of gaseous products was low at temperatures above 300 °C. The energy recovery of the process increased with increasing temperature.⁶² Funazukuri et al.⁶⁴ reported that the yield of the solid product was 42.9 wt% at 380 °C and 22 MPa.

In hydrous pyrolysis of rubber tire, temperature is reported to be the most significant variable. At 370–400 °C, the yield of the liquid products was 44–53 wt%. However, the degradation of the liquid products was observed as the temperature exceeded 400 °C, thereby decreasing the product yield. The complete decomposition of rubber tire occurred at around 500 °C during hydrous pyrolysis.

Effect of gas atmosphere

Various gas atmospheres or environments could be used to carry out hydrous pyrolysis of rubber; these gas environments include helium, nitrogen, carbon monoxide, carbon dioxide, hydrogen, and air. Gas atmosphere minimally affect the yield of the final product.⁶² Zhang et al.⁶² conducted hydrous pyrolysis under different atmospheres, such as air, CO₂, CO, H₂, and N₂. The results showed that gas atmosphere did not affect or enhance the product yield. The liquid products obtained under CO and H₂ showed a higher proportion of light fractions compared with those produced under CO₂ or N₂.⁶²

Similarly, Park and Gloyna¹⁵⁰ performed hydrous pyrolysis of rubber tire in air and helium atmospheres. The gas atmosphere is the second most important parameter after temperature that affects the liquid product yield. However, the atmosphere showed minimal effect on oil yield. The use of helium, instead of air, was more effective for oil recovery. The maximum yield of the oil product was 44 wt% at 400 °C in helium atmosphere (inert atmosphere). The use of air as the atmosphere results in oxidative reaction, which increased the volume of effluent gas.¹⁵⁰

Effect of time

Various studies were conducted to determine the effect of reaction time on product quality during hydrous pyrolysis. Zhang et al.⁶² studied the effect of reaction time from 20–120 min at 390 °C. At 390 °C, the reaction time of 20 min was sufficient for degradation of rubber tire and the yield of the liquid product was 40 wt%. The liquid product yield increased to a maximum of 53 wt% when the reaction time was further increased to 60 min with slight increase in temperature.⁶² Chen et al.¹⁴⁹ observed that the degree of depolymerization can be controlled using reaction time and can be employed to control the molecular weight for formation of chemicals. The molecular weight decreased during time interval of 30–180 min.¹⁴⁹ Similarly, Funazukuri et al.⁶⁴ reported that the molecular weight decreased as a function of time at 380 °C and 22 MPa. Zhang et al.⁶² also reported that prolonged reaction led to low nitrogen and sulfur contents in the liquid product.

Characteristics of products

Various investigations were conducted to characterize products obtained during pyrolysis and hydrous pyrolysis of rubber. Zhou et al.⁶² reported that the calorific value of oil obtained under the optimum conditions is a 44.09–45.09 MJ kg⁻¹. The calorific value obtained is close to that of petroleum derived from crude oil, such as diesel (44.8 MJ kg⁻¹).¹⁶¹ The properties of liquid oil obtained from hydrous pyrolysis of rubber tire are comparable with other crude-oil-derived fractions, such as gasoline and diesel, as well as other solid and gaseous fuels (Table 3).

Table 3 Calorific value of various fuels

Fuel	Calorific value	Ref.
Oil from rubber tire	45.09 MJ kg^−1	62
High rank coal	36.0 MJ kg^−1	162
Biomass	17.072 MJ kg^−1	163
Diesel	44.5 MJ kg^−1	161
Natural gas	54 MJ kg^−1	164
Gasoline	46 MJ kg^−1	165

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According to Zhou et al.,⁶² elemental analysis of oil product obtained under optimum conditions contained carbon and hydrogen contents of 86.66 wt% and 10.99 wt%, respectively. Similarly, Funazukuri et al.⁶⁴ reported that oil products contained 85 wt% and 10 wt% carbon and hydrogen, respectively, at 380 °C and 22 MPa.

Funazukuri et al.⁶⁴ performed elemental analysis of solid residues; the results showed that the residues contained 91.8 wt% and 1.48 wt% carbon and hydrogen, respectively, at 380 °C and 22 MPa.⁶⁴ Moreover, Yi et al.¹⁵⁸ reported the presence of high carbon contents in solid residues. The average value of carbon contents was estimated to be higher than 80 wt%.

Yi et al.¹⁵⁸ reported that the gas product mostly consisted of CH₄, H₂, CO, CO₂, and hydrocarbons with low molecular mass. The concentrations of CH₄ and H₂ were high in the product. The yield of CH₄ increased at 400 °C to 550 °C. Furthermore, the yields of CH₄, H₄, and CO₂ were approximately 38%, 38%, and 16%, respectively, at 500 °C and 22.2 MPa; meanwhile, NO_x and SO_x were not detected in the gas products.¹⁵⁸

Cataldo¹⁰⁶ reported a detailed pyrolysis of synthetic and natural rubber (cis-1,4 polyisoprene). The yield of the oil product was 44.5%. The pyrolytic oil mainly consisted of 90% di-pentene with low amounts of isoprene (3.5%) and other commodities.¹⁰⁶ Groves et al.¹⁰⁸ studied the thermal degradation of natural rubber through pyrolysis to investigate the oil product derived at 500 °C. The major products obtained were monomer, isoprene, dimmer, and di-pentene with other commodities in substantial concentrations.¹⁰⁸ In a similar investigation on pyrolysis, di-pentene and isoprene were found to be the major products of natural rubber pyrolysis.^109,110 According to Chen and Qian,⁵⁶ the pyrolytic oil obtained from natural rubber (cis-1,4-polyisoprene) at temperatures below 431 °C mainly consisted of di-pentene. The yields of di-pentene at temperature ranges of ambient – 330 °C, 331–390 °C, and 391–430 °C were 53.57%, 29.03%, and 11.89% respectively. Roy et al.¹⁶⁶ performed pyrolysis of pure natural rubber and commercial rubber (consisting of 52% polyisoprene) at 500 °C with varying pressures from 8 kPa to 28 kPa. The oil yield for pure natural rubber was estimated to be 97.3 and 90.3 wt% at 0.8 and 28 kPa, respectively. For commercial rubber, the yields of pyrolytic oil were 62.6 and 61.7 wt% at 0.8 and 28 kPa, respectively. Characterization of naphtha obtained from pyrolysis showed high amounts of DL-limonene. The yield of DL-limonene from naphtha obtained from pure natural rubber was estimated to be 54.64 wt% at 0.8 kPa. For commercial rubber, the yield of DL-limonene was reported to be 31.40 and 31.22 wt% at 0.8 and 28 kPa, respectively. The total amounts of aromatics, olefins, and alkanes obtained from pure natural rubber were reported to be 4.24, 70.35, and 3.89 wt%, respectively at 0.8 kPa. Chen et al.¹⁴⁹ performed NMR analysis of liquid products and reported the presence of aromatic, olefinic, and aliphatic groups. Products with a molecular mass of 10³ to 10⁴ were obtained. The reported compositions of hydrocarbons, aromatic rings, and olefinic groups in the products were 81 wt%, 19 wt%, and 0.4 wt%, respectively, at 380 °C and 27.6 MPa.¹⁴⁹ Zhang et al.⁶² reported the composition of oil products at 250 °C, 350 °C, and 430 °C. Aromatics, saturated, and unsaturated hydrocarbons were 15.97 wt%, 14.08 wt%, and 7.10 wt%, respectively, at 350 °C. This difference in composition is due to changes in temperature. According to Zhang et al.,⁶² the composition of aromatics and unsaturated hydrocarbons increased to 50.49 and 45.68 wt%, respectively, at 430 °C. Rushdi et al.¹² reported that alkanes, methyl n-alkanoates, plasticizers, n-alkanoic acids, phenols, and polycyclic aromatic hydrocarbons were found to be the major products of hydrous pyrolysis. According to Rushdi et al.,¹² the polycyclic aromatic hydrocarbons in the absence of oxalic acid was 25.78% at 350 °C. The concentration of cyclic aromatic hydrocarbon increased with increasing temperature. In the presence of oxalic acid, the concentration increased from 0.001 mg g⁻¹ at 150 °C to 5.15 mg g⁻¹ at 400 °C; by contrast, in the absence of oxalic acid, the concentration increased from 0.002 mg g⁻¹ at 150 °C to 5.21 mg g⁻¹ at 400 °C.

Environmental and economic feasibility

Hydrous pyrolysis is a viable and environment friendly technique for conversion of wastes (such as scrap tires and organic wastes) into liquid fuels. Hydrous pyrolysis of rubber tire yields three main products, namely, liquid oil, gas, and solid residues. During hydrous pyrolysis of rubber tire, most sulfur and nitrogen contents are retained in the solid residue, thereby substantially reducing the emission of SO_x and NO_x in the gas products.^149,150,158 However, reduction of nitrogen and sulfur contents in solid residues must be further improved. Similarly for liquid products, minimal improvements are required for reduction of nitrogen and sulfur contents.⁶² Zhou et al. reported the sulfur contents in oil obtained were 1.07 to 2.28%. Similarly, Chen et al.¹⁴⁹ and Funazukuri et al.⁶⁴ reported 1 wt% and 2 wt% sulfur in the oil obtained from the hydrous pyrolysis of scrap tire respectively. For greenhouse gas emissions, Pilusa et al.¹⁶⁷ reported quantitative analysis of the flue gas produced by the combustion of tire derived oil. According to their studies, the burning of tire oil generated 0.23, 12.5, 0.003, and 0.01 (vol%) of SO_x, CO₂, NO_x and CO respectively.^167,168 Table 4 shows the greenhouse gases emissions from tire oil.

Table 4 Greenhouse gases emissions from tire derived oil^167,168

Gas components	Concentration (vol%)	Emission level (g kW^−1 h^−1)
Sulfur dioxide (SO2)	0.23	8
Nitrogen oxide (NOx)	0.003	0.08
Carbon dioxide (CO2)	12.5	296
Carbon monoxide (CO)	0.01	0.15
Hydrocarbons (HC)	0.02	0.18
Nitrogen (N2)	73.6	1110
Oxygen (O2)	3.5	60
Moisture (H2O)	10.1	98

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Hydrous pyrolysis of rubber tire produces high quality and quantity of oil products. Thus, this technique exhibits a potential for developments in the energy industry. Hydrous pyrolysis of rubber tire is a cost-effective technique used to convert rubber tire into fuel.¹² Generally, pyrolysis of rubber tire is conducted at temperatures higher than 400 °C, thereby increasing the operational cost of the process.¹⁶⁹ Therefore, studies have focused on optimizing pyrolysis by using water below 400 °C. Rushdi et al.¹² presented the economic viability of hydrous pyrolysis of rubber tire. According to their report, a yearly profit of $1.18–1.78M can be obtained through hydrous pyrolysis of 30 MT rubber tire per day. The process could be more profitable using solid residues as a fuel and product gases for waste heat recovery through process upgrading.

Another study conducted by California Integrated Waste Management Board reported that the gross revenue of $1.45 per tire rubber could be made through this process. This study also estimated the capital cost and operational cost for the plant. According to this report, capital cost of $257 per tons per annum and $238 per kilowatt were required for the plant, whereas the operational cost of the plant was calculated 4.5% of capital cost with heat recovery system.¹⁷⁰

Similarly, Huffman and Shah (1997)¹⁷¹ reported that 8.4 million barrel of oil per year could be produced by using the rubber tire of 2.8 million tons per year, generating the revenue of $168 million per year (including the revenue from by-products).

A techno-economic study conducted by Shelley and El-Halwagi¹⁷² showed the promising economic feasibility of pyrolysis for economic assessment of liquefaction of rubber tire. According to their study, investment decisions were made using return on investment (ROI) approach. ROI is defined as, a profitability measure that evaluates the performance of investment or a business by dividing the net profit by net worth.^173,174 ROI is one of the important tools used to measure the efficiency or consequences of investment. In many cases, it is also used to compare the efficiency of a number of different investments. In this approach, the investment was considered positive if the ROI was positive. The results showed the promising and advantageous economic feasibility with ROI of approximately 12%. In addition, the tipping fee obtained for raw material was found to play a key role in overall profitability. The high tipping fees received will be linearly subsidized to increase in ROI.

Conclusions

Natural rubber is one of the promising sources of alternative energy and can be employed to solve the problem of energy crisis. Studies in the literature showed numerous techniques are available for depolymerization of natural rubber; these techniques include pyrolysis, gasification, chemical degradation, catalytic cracking, hydrogenation, and hydrous pyrolysis. Hydrous pyrolysis of natural rubber is the most cost-effective process because it produces high liquid yield under low-temperature conditions. Hydrous pyrolysis of natural rubber is the most cost-effective process because it produces high liquid yield under low-temperature conditions. Most studies also focused on using rubber tires. A maximum liquid yield of approximately 44% was obtained under the optimum conditions of 400 °C, water/rubber ratio of 2 : 1, and reaction time of 60 min. This process can also use raw materials with high moisture contents without requiring a pre-drying process. Moreover, the use of water as a reaction medium in hydrous pyrolysis has gained considerable attention because it can easily separate water from oil products and is more economical compared with other organic solvents, such as pentane and toluene. For environmental concerns, during hydrous pyrolysis of rubber tire, NO_x and SO_x were not produced in the gas product. The use of pure natural rubber as a feedstock in hydrous pyrolysis is environment friendly because pure natural rubber does not contain sulfur. Furthermore, this process is an optional solution to contribute to fulfillment of energy requirements and reduce dependence on fossil fuels. Hydrous pyrolysis seems to be one of the promising techniques for production of alternative fuels and chemicals. However, limited information is available regarding hydrous pyrolysis of natural rubber. Therefore, further research is needed on hydrous pyrolysis of natural rubber to assess the effect of different parameters on product quality and quantity and determine environmental and economic feasibility.

Acknowledgements

The authors acknowledge the financial support provided by the University of Malaya through GSP-MOHE (MO008-2015).

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