Waste plastic to pyrolytic oil and its utilization in CI engine: Performance analysis and combustion characteristics

Fuel xxx (xxxx) xxxx

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

Waste plastic to pyrolytic oil and its utilization in CI engine: Performance analysis and combustion characteristics ⁎

R.K. Singha, Biswajit Rujb, , A.K. Sadhukhana, P. Guptaa, V.P. Tiggac a

Department of Chemical Engineering, National Institute of Technology, Durgapur 713209, West Bengal, India Environmental Engineering Group, CSIR-Central Mechanical Engineering Research Institute, Durgapur 713209, West Bengal, India c Energy Research and Technology Group, CSIR-Central Mechanical Engineering Research Institute, Durgapur 713209, West Bengal, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: Plastic waste Pyrolytic oil Diesel engine Blending Performance Emission

For application of pure plastic pyrolytic oil (PPO) several modifications in the engine is required which rejects the utilization of the existing engines while a blend of conventional fuel and PPO can be used with a slight change in engine without having a high impact on engine performance and hence the blends are preferred over the utilization of PPO as crude oil for diesel engines. In this study, the non-catalytic pyrolysis of mixed plastic waste at a temperature of 450 °C is done to obtain high-grade pyrolytic oil having a composition similar to petroleum fuels such as gasoline and diesel. Physical properties of the PPO were analysed, and the compound analysis was done with GC–MS. Further FTIR of PPO and diesel were analysed and compared. Five different ratios of 10, 20, 30, 40 and 50% PPO with diesel in blends were utilized as a fuel in a diesel engine to determine the engine performance and characteristics. The higher presence of PPO in blend increases the brake thermal efficiency (BTE) and reduces specific fuel consumption (SFC) with an increase in load as reported. The presence of PPO results in high heat release and delayed ignition resulting in high in-cylinder pressure. Further high amount of oxygenated compounds in PPO helps in reducing the emission from the combustion. The utilization of PPO with diesel upto 50% in the blend can be used in diesel engines with a slight increase in emission of CO at higher loads.

1. Introduction Recovery of energy from wastes in the form of fuel oil and gas has attracted worldwide attention towards fulfilling environmental and energy security. This has encouraged researchers to search for low-cost alternate fuels with properties comparable to that of petroleum-based fuels (such as alcohol, biodiesel, etc.) for use in the IC engines [1–3] in an environmentally friendly manner without sacrificing its performance [4–6]. Discarded plastics, containing a large number of hydrocarbons with high calorific value, are good sources of such alternate fuels due to their abundant availability and environmental concern for their disposal [6]. It was reported that 5.6 million ton per annum of waste plastics are discarded to landfill or burned in the open in India [7], while only a small fraction was utilized for energy generation via processes like incineration. Both these processes are environmentally detrimental. The waste plastics can be converted to gasoline and diesel like fuel oils via pyrolysis without impacting the ecology [8]. Though the conversion of individual waste plastics into fuel oil provides highquality fuel oil, their separation is not economic. To overcome this

⁎

problem, mixed plastic wastes as received may be converted to fuel oil called plastic pyrolytic oil (PPO) [6] through pyrolysis which degrades the long-chain hydrocarbons into shorter hydrocarbons [9]. This results in economic formation of diesel and gasoline-range hydrocarbon mixture with a high recovery. The single cylinder diesel engine using waste plastic oil showed a stable performance with thermal brake efficiency similar to that of diesel [3,9–12] though CO emission from an engine fuelled by waste plastic oil was found to be higher than that with conventional diesel [10]. Tamilkolundu & Murugesan [12] suggested that a liquid fuel from plastic waste would be a better alternative as its calorific value is comparable to that of diesel, around 40 MJ/kg. The physical properties of PPO were found to be excellent due to absence of water content; near neutral pH and low viscosity [13]. Small nitrogen and sulfur contents in PPO help reduce the emission of NOx and SOx in the exhaust when used in blends with diesel and gasoline [6,8]. Different researchers have used PPO obtained from individual plastic waste such as polyethylene (PE) [14], polypropene (PP) [15,16], polystyrene (PS) [16], and mixed plastic waste [9,10,12]. However, in all these cases reported, PPO

Corresponding author. E-mail address: [email protected] (B. Ruj).

https://doi.org/10.1016/j.fuel.2019.116539 Received 12 May 2019; Received in revised form 24 October 2019; Accepted 29 October 2019 0016-2361/ © 2019 Published by Elsevier Ltd.

Please cite this article as: R.K. Singh, et al., Fuel, https://doi.org/10.1016/j.fuel.2019.116539

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

Fig. 1. Schematic diagram of the laboratory-scale pyrolysis setup.

solids was done using an ultimate analyzer (make: Thermofisher; model: Flash 2000 CHNS) using the pre-obtained standard (ASTM D3176-15) [19]. For proximate analysis (ASTM D3172-13) [20] of solids, moisture was analysed by using hot air oven maintained at a temp of 110 °C for 4 h, for ash content the sample crucible was kept open in muffle furnace for 1 h at a temperature of 850 °C while volatiles were determined by keeping the sample crucible in closed lid position for 5 min at same temperature. The fixed carbon was determined by difference. The diesel oil used in the blend was commercial diesel fuel which is used locally (Indian oil corporation Ltd, Durgapur) has been collected for our baseline study. Further, it was noted that the PPO used in this study is crude plastic waste oil with no distillation while it was filtered through a whatmann’s 51 filter paper for the removal of suspended particles.

produced was upgraded before using it in the fuel testing engine setup. The processing temperature plays an important role in the production of PPO during pyrolysis. High temperature produces heavier hydrocarbons while a low temperature produces lighter hydrocarbons with decreased yield [9,17]. Pyrolysis temperature was also found to have a high impact on the liquid yield and its properties with hardly any effect observed on the gaseous and solid fractions. A low temperature (500 °C) results in low viscosity while a high temperature (> 700 °C) may result in increased viscosity with a decrease in yield [6]. At temperature 500–600 °C, the viscosity increases as rapid cracking results in long chain hydrocarbons which form wax in the oil phase product while the mass yield increases at higher temperature [13]. Also, the high temperature favours the formation of more aromatics in the liquid fuel due to secondary reactions [18]. The PPO obtained from the non-catalytic pyrolysis process contains a higher amount of heavy hydrocarbons while the use of catalyst further degrades it to lighter hydrocarbons [6,14]. Use of crude PPO in an existing engine will require several modifications while a blend of conventional fuel and PPO may be used with a small change in the engine without any impact on its performance. In this study, the crude PPO obtained from the non-catalytic pyrolysis of mixed plastic wastes with properties more similar to that of gasoline was used in blends with diesel at different ratios without subjecting it to any pre-treatment as done by others. The obtained PPO properties were analyzed by GC–MS and FTIR to determine their composition and were compared with diesel. The combustion characteristics were analyzed in a diesel engine fuel testing setup.

2.2. Preparation of waste plastic oil & its characterisation The waste plastic was pyrolysed in an electrically heated, semibatch pyrolysis unit with a handling capacity of 1ltr waste for the production of PPO. The process temperature was kept at 450 °C in isothermal condition for 30 min with a heating rate of 20 °C/min from room temperature. The pyrolysis condition such as temperature were based on the TGA analysis representing no degradation before 350 °C and after 550 °C and the heating rate was considered based on the optimum time for the reaction to take place. The reaction temperature of 500 °C produces pyrolytic oil with no wax component and highest pyrolytic oil yield [9,18]. The schematic of the experimental setup for production of PPO is shown in Fig. 1. As soon as the degradation temperature of the material reaches, the material starts to degrade and the volatiles formed were passed through a condensing section where it condenses in the form of liquid oil called as PPO while the non-condensable gases were further passed through an ice bath section where its temperature decreases and was collected. Further to keep the pressure low in the reactor system the gases were released at defined intervals. The physical properties of the PPO obtained such as density (ASTM D1298) [21], kinematic viscosity (ASTM D445) [22], flash and fire point (ASTM D92) [23], pour point (ASTM D97) [24], cloud point (ASTM D2500) [25], ash content (ASTM D482) [26], Sulphur content (ASTM D129) [27], carbon residue (ASTM D189) [28], calorific value (ASTM D240) [29], cetane number (ASTM D613) [30], etc were analysed based on the ASTM standards which were also reported by other researchers [31–33]. The waste was characterized based on their component composition and was reported in Table 1. Further, the GC–MS analysis of the PPO was done to determine the components and

2. Material and methods 2.1. Raw material and its characterisation Mixed plastic waste collected from different location of Durgapur was taken as our feed material for the production of plastic waste oil. For obtaining a homogenous mixture of waste, the waste was collected from five different locations in a batch of 1 kg each which was then again mixed, and the sample was taken. Primarily, the waste was sun dried for moisture removal, shredded for size reduction and sieved for removal of an inert material like sand. The mixed plastics were categorized based on their identification, and their composition was reported as it contains 58.6% PE (high-density (HDPE) and low-density (LDPE)), 26.9% of PP, 8.8% of PS, 5.6% of poly-ethene terephthalate (PET) and 0.1% of other plastics such as thermoset plastics. During the collection/separation, polyvinyl chloride (PVC) was excluded from the mix as it deteriorates the plastic oil quality. The ultimate analysis of 2

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

Table 1 Proximate and ultimate analysis of mixed plastic waste.

Table 2 Specification of a diesel engine experimental setup for fuel testing.

Ultimate analysis (ASTM D3176-15) [19]

Percentage

Proximate analysis (ASTM D3172-13) [20]

Percentage

Carbon Hydrogen Nitrogen Sulphur

79.77 15.47 2.76 0.04

Moisture Volatiles Fixed carbon Ash

0.6 87.7 8.4 3.3

carbon fraction. Also, FTIR analysis of the PPO and diesel oil was analysed, compared and reported.

Parameters

Specifications

Engine Type Engine model Rated power BHP Compression ratio Injection pressure Bore/stroke Start of Fuel injection Engine rated speed

Single cylinder, 4 S, Diesel Engine Kirlosker, model: TV1 5.2 kW 7 BHP 17.5:1 215 bars 87.5/110 23° BTDC 1500 rpm

100:1. The heating starts from 40 °C to 300 °C with a heating rate of 5 °C/min. The compounds were identified using NIST 17 library and those having similarity > 89% were only reported. Further, the PPO oil and diesel oils were analysed through an FTIR analyzer (make: PerkinElmer; model: SPECTRUM II). The exhaust gas analysis was performed using online exhaust gas analyzer.

2.3. Experimental setup for IC engine For the testing of fuel blends, the schematic of engine testing setup is shown in Fig. 2. The specifications of the same were reported in Table 2. Before starting the experimentations, the engine was fully warmed up for testing. The engine performance tests were conducted with an eddy current dynamometer; water cooled with loading unit’s research engine setup. The tests are performed with a constant speed of 1750 rpm with varying load from zero to 15 kg for diesel and different PPO blends. The experimental data was collected twice for reproducibility and found within ± 4.8%.The parameters like SFC, BTE, volumetric efficiency, and exhaust temperature were reported with variation in load at different blend ratios. The cylinder pressure and heat release with respect to crank were analysed and reported. All the values/calculations reported were obtained from engine performance analysis software (Engine Soft) interfaced with test engine setup.

3. Result and discussion 3.1. Plastic waste oil characterization 3.1.1. Physical properties of crude PPO The physical properties of the PPO obtained shows a lower density of 0.734 g/cm3 compared to diesel whereas lies in the range of gasoline (Table 3). The density of PPO increases with an increase in process temperature while a decrease in volume at a lower temperature was reported by several researchers [17,34,35]. The pour and cloud point are lower than the values of diesel which shows that it can be used in cold climate regions. The flash and fire make lower than diesel grade fuel while similar to the gasoline fraction. Similar results were also reported by other researchers [14,17,31]. The obtained viscosity of the PPO is higher than the diesel grade fuel whereas distillation can reduce the value and is reported in the literature [9–12]. The sulfur and nitrogen contents were negligible due to the lack of these components in the raw material [10–14]. The boiling point ranges from 58 °C to 278 °C while there is a minimal amount of condensate was observed. These

2.4. Analytical instrument The physical properties of the PPO obtained were analysed as per the ASTM standards [21–30]. Further, the component analysis of PPO was done through a GC–MS (Make: Agilent Technologies India Pvt. Ltd.; Model: GC-7890B; MS-5977A). A non-polar HP-5MS column was used for the component analysis of PPO. The column length is 30 m; inner diameter is 0.25 mm with 0.25 μm mixed coating of 5% di-phenyl and 95% dimethylpolysiloxane. The sample injection split ratio was kept at

Fig. 2. Schematic of a diesel engine experimental setup for fuel testing. 3

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

Table 3 Physical characterization of crude PPO and comparison with commercial diesel fuel. Properties 3

Density(kg/m ) Ash content (wt%) Calorific value (KJ/kg) Kinematic viscosity @40 °C (cSt) Cetane number Flash point (oC) Fire point (oC) Carbon residue (wt%) Sulphur content (%) Pour point (oC) Cloud point (oC) Aromatic content (%)

Testing methods

PPO

Diesel

ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM ASTM –

734 ± 2.4 < 1.01 ± 0.003 41,254 ± 6.4 2.9 ± 0.1 49 ± 1 46 ± 1 51 ± 1 0.01 ± 0.002 < 0.001 ± 0.0001 −3 ± 1 −27 ± 1 32 ± 1.8

820–850 0.045 42,000 3.05 55 50 56 0.20% < 0.035 3–15 – 11–15

D1298 [21] D482 [26] D240 [29] D445 [22] D613 [30] D92 [23] D92 [23] D189 [28] D129 [27] D97 [24] D2500 [25]

–CH3 group at 1375 cm−1 and 1450 cm−1 [37]. The peak at 1650 cm−1 represents the C]C stretching vibrations suggesting the presence of alkenes in the sample. The presences of phenols, ether, alcohols were represented by the peak obtained at 1150 cm−1 which is minimal/absent in diesel fuel [38]. The peaks at 880 cm−1 and 960 cm−1 represents the presence of aromatics and CO– stretching group which is also absent in diesel fuel oil providing lack of aromatics and oxygenated compounds in the fuel oil [33,39]. Overall, the presence of all these functional groups and compounds are verified with the GC–MS results reported in Section 3.1.2 and hence are in good agreement. The functional groups were identified with the standard data and presented in Table 6.

condensed particles can be removed via simple centrifugal treatment. Overall the properties of the obtained oil fits the values in the range of diesel to gasoline and hence can be used as an individual or in blends with diesel. The properties such as density, viscosity, aromatic of the obtained PPO are more similar to gasoline fraction and shows a higher similarity (Table 3) [6,12]. 3.1.2. GC–MS analysis of crude PPO The crude PPO was investigated by GC–MS which identified the majority of the compounds and were reported. Table 4 describes the carbon number distribution and Table 5 represent the identification and distribution of the compounds. It was observed that a high amount of alkane and alkenes were observed with a high concentration of aromatic compounds. Based on the carbon number distribution it was observed that approximately 35.41% oil consists of C6-C9 compounds while the concentration of compounds having carbon number rage of C10-C14 consists of approximately 48.40% and similar was also reported in the literature [10]. The PPO contains a low amount of heavier hydrocarbons (> C20) as reported resulting in wax free liquid while similar results were obtained by Kumar et al. [14] while analyzing HDPE pyrolysis oil. Oxygenated compounds (17.54%) were also observed in the PPO sample such as acetyl cyclopentanone, 1-cyclopentyl ethanone, cyclohexanedione, etc. The presence of trialkyl silane was also observed due to the presence of silica/sand in the waste material during degradation. The compounds reported are similar to diesel and gasoline fraction and can be used as an alternative or in blends [14].

3.2. Performance analysis of CI engine 3.2.1. Load vs. break thermal efficiency The variation in brake thermal power for diesel and PPO blends at different loads is presented in Fig. 4. The results show minimal variation in the BTE when PPO is mixed with various proportions. Increase in engine load, the heat generated in the cylinder increases which in result provides increased thermal efficiency. Also, increased temperature in exhaust also results in lower BTE due to the lower conversion of heat into energy [10,11]. It was observed that with an increase in PPO fraction in blends reduces the BTE with a very slight variation at the reduced loads while the increase in load represents significant variation in BTE (Fig. 4). At full load the BTE for diesel is 30.01% while for blends is 29.88%, 31.61%, 31.29%, 31.27%, and 31.22% for 10%, 20%, 30%, 40% and 50% respectively. Presence of higher amount of lighter hydrocarbons (gasoline range) in PPO results in excellent atomization and speedy combustion which also helps in improved BTE at higher loads and at higher mixed fractions [14]. High aromatic content is the main reason for the reduction in BTE as the bonds require higher energy to break which results in delayed combustion and high heat loss [34]. Although the obtained PPO has a lower viscosity which helps in the formation of small droplets during atomization for proper combustion when sparked/combusted (Table 3). The diesel can be blended up to 50% with PPO without having any significant variation in BTE at higher loads. Increased load results in higher conversion and less heat loss which results in increased BTE. The presence of a higher amount of oxygenated compounds in PPO produces more heat of combustion during the process which in turn increases the BTE as reported in Fig. 4 [3,10–12].

3.1.3. FTIR analysis of crude PPO The obtained crude PPO mainly consists of alkanes and alkenes with a high amount of aromatics as observed in Fig. 3. The figure represents the comparative FTIR analysis of crude PPO obtained from mixed plastic waste and diesel. The peak at 725 cm−1 represents the proportional length of the hydrocarbons as well as it represents the presence of methylene –(CH2)– rocking band [36]. The peaks between 3000 and 3100 cm−1 represent the presence of alkene substitution ((CH)C]CH2) functional group which is absent in case of diesel oil [31]. Further, the high-intensity peaks between 2800 and 3000 cm−1 represent a higher concentration of alkanes and alkenes [36]. Presence of alkanes is also represented by the peaks between 1320 and 1480 cm−1 formed due to symmetric and asymmetric deformation of Table 4 Carbon number distribution for PPO via GC–MS. Carbon number distribution

Mass Percentage (%)

C6-C9 C10-C14 C15-C20 > C20

35.41 ± 0.54 48.40 ± 0.8 13.21 ± 0.04 1.83 ± 0.03

3.2.2. Load vs. specific fuel consumption With an increase in the load, the SFC for different blends is represented in Fig. 5. It was observed that with an increase in load the SFC reduces while with blends the SFC reduces comparatively. For diesel, the SFC value is 7.47 kg/kWh at zero loads while at full load the value reduces to 0.274 kg/kWh. In the case of blends, 20% blend shows 4

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

Table 5 Compound identified via GC–MS analysis of the crude PPO obtained at 450 °C. Compound Name

Area %

Molecular formulae

Compound Name

Area %

Molecular formulae

2-Methoxycarbonylspiro [2.3]hexane Cyclohexane 2-Pentene, 2-methylEthanone, 1-cyclopentyl2-Acetylcyclopentanone Heptane, 4-methyl2-Propen-1-one, 1-(2,2-dimethylcyclopropyl)6,6-Dimethylhepta-2,4-diene 1,6-Heptadiene, 3,5-dimethyl1,1,4-Trimethylcyclohexane Cyclohexane, 1,3,5-trimethyl 2,4-Dimethyl-1-heptene Cyclopentane, 1,1,3,4-tetramethyl-, transCyclopentane, 1,1,3,4-tetramethyl-, cisCyclohexane, 1,1,2-trimethylCyclohexane, 1,2,4-trimethylHeptane, 2,4-dimethyl4-Isopropyl-1,3-cyclohexanedione Triallylsilane Cyclopentane, 1,2,3,4,5-pentamethyl2-sec-Butyl-3-methyl-1-pentene Cyclohexane, 1,1,3,5-tetramethyl-, cisCyclohexane, 1-ethyl-2,3-dimethyl3-Hexene, 2,2,5,5-tetramethyl-, (Z)Decane Cyclodecanone Ketone, 2,2-dimethylcyclohexyl methyl Bicyclo[2.2.1]heptane-2,5-dione, 1,7,7-trimethylCyclohexane, 2,4-diethyl-1-methylCyclohexane, 1,2-diethyl-3-methylCyclohexane, 1-ethyl-2-propylCyclopentane, 1-butyl-2-ethylCyclohexane, 1-ethyl-2-propylCyclohexane, 1,2-diethyl-3-methylCyclopropane, 1-heptyl-2-methyl1-Undecene 5-Undecene, (E)-

0.8827 2.3446 3.2507 1.9404 5.3754 1.5565 0.9665 1.1142 0.3895 0.2406 1.0583 3.7785 0.814 0.6147 0.2967 2.0391 0.2406 3.2647 1.3541 1.74 2.6094 0.2826 2.049 0.4147 0.3175 4.7282 0.2989 2.187 0.2601 0.6678 1.6367 0.3537 0.7775 1.9606 0.4456 0.2695 0.3011

C6H10 C6H12 C6H12 C7H12O C7H10O2 C8H18 C8H14o C9H16 C9H16 C9H18 C9H18 C9H18 C9H18 C9H18 C9H18 C9H18 C9H20 C9H14O2 C9H15Si C10H20 C10H20 C10H20 C10H20 C10H20 C10H22 C10H18O C10H18O C10H14O2 C11H22 C11H22 C11H22 C11H22 C11H22 C11H22 C11H22 C11H22 C11H22

Undecane 4,8-Dimethylnona-3,8-dien-2-one 3-Decene, 2,2-dimethyl-, (E)2-Undecene, 4-methylCyclopropane, 1-methyl-2-octylCyclohexane, (1,2-dimethylbutyl)3-Heptene, 2,2,3,5,6-pentamethylOctane, 2,3,6,7-tetramethylDodecane 2,6,8-Trimethyl-4-nonyl acetate 1-Tridecene Tridecane 7-Tetradecene 2-Tetradecene, (E)Dodecane, 4,6-dimethyl2,3-Dimethyldodecane Dodecane, 4,6-dimethylTetradecane Pentadecane 1,1′-Bicyclohexyl, 2-(1-methylethyl)-, transOxalic acid, allyl decyl ester Hexadecane 2-Propylhept-3-enoic acid, phenylthio ester 3-Heptadecene, (Z)Heptadecane 5-Octadecene, (E)1-Octadecene 3-Octadecene, (E)Octadecane Z-5-Nonadecene Nonadecane Hexadecane, 2,6,10,14 tetramethylEicosane 5-Methyl-Z-5-docosene 2-Methyl-2-docosene Carbonic acid, eicosyl vinyl ester

0.3095 2.2286 2.3806 2.6799 0.4634 0.3047 1.8124 0.7859 0.9752 3.7869 0.8648 0.8074 0.6155 1.1481 0.5668 0.6535 0.7381 0.66 0.798 0.6369 0.4373 1.2032 1.188 0.7207 0.775 0.7865 0.8673 0.5402 2.1879 0.6574 0.9601 0.3308 0.6803 0.3769 0.2516 1.0089

C11H24 C11H18O C12H24 C12H24 C12H24 C12H24 C12H24 C12H26 C12H26 C12H24O C13H26 C13H28 C14H28 C14H28 C14H30 C14H30 C14H30 C14H30 C15H32 C15H18 C15H26O4 C16H34 C16H22OS C17H34 C17H36 C18H36 C18H36 C18H36 C18H38 C19H38 C19H40 C20H42 C20H42 C23H46 C23H46O C23H44O3

Table 6 A comparative FTIR analysis of oils for functional group identification. Wavenumber (cm−1)

Functional group

Assignment

3075 2925 2857 1650 1450 1375 1150 960 880

CH2 Alkene CH2 Branched Alkane CH2 Branched Alkane C]C Alkene rCH2 Alkane –CH3 Alkane C-O Alcohol ]C–H Alkene C–H Alkene

725 699

C–H Alkane C–H Alkane

Alkene (CH)CH]CH2 Methylene C–H asym. stretch Methylene C–H sym. stretch Alkenyl C]C stretch Methylene C–H bend Methyl C–H sym. stretch Tertiary alcohol, C-O stretch Trans –C–H out-of-plane bend Vinylidene C–H out-of-plane bend Methylene –(CH2)n- rocking Cis –C–H out-of-plane bend

and more gasoline fraction compounds which also helps in improved SFC [10]. At full load, all the values for blends are lower than diesel whereas at lower loads the values are higher as reported (Fig. 5). Further presence of oxygenated compounds in PPO also helps in complete and early combustion which is also confirmed by increased BTE for blends (Fig. 5) [34].

Fig. 3. A comparative FTIR analysis of PPO and commercial diesel.

the minimum value of 0.270 kg/kWh at full load whereas the value for 50% blend is 0.272 kg/kWh. At higher engine speed the fuel combustion is improved due to improved mixing of fuel and air while at higher loads the improved fuel atomization, better mixing, and high-in-cylinder temperature also promote the combustion process providing low specific fuel consumption [11]. In case of the blends, the reduced SFC has mostly observed due to lower heating value, and higher viscosity of the fuel whereas the existing fuel has a lower viscosity and heating value similar to diesel

3.2.3. Load vs. volume efficiency With an increase in load, the volume efficiency decreases for diesel fuel from 80.09% at zero load to 78.23% at full load. The addition of PPO with diesel increases fuel consumption with a decrease in volume efficiency as reported in Fig. 6. Increase in load results in increased cut off ratio which results in reduced volume efficiency. At low blends of 10 and 20% at full load, the 5

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

Fig. 7. Load vs. exhaust temperature. Fig. 4. Load vs. brake thermal efficiency.

volume efficiency is equivalent to diesel fuel whereas with an increase in ratio to 50% a decrease in volume efficiency to 77.71% is reported. High consumption of fuel is observed due to the lower calorific value of PPO to generate a high amount of heat and pressure to run the engine which in turn reduces the volume efficiency for blends as well as for high load. As the load increases, the air-fuel ratio increases which in turn increase the fuel injection volume. Increased fuel consumption and less oxygen in combustion cylinder during combustion results in incomplete combustion till the piston has left the TDC resulting in low volumetric efficiency. High consumption of fuel is observed due to the low calorific value of PPO to generate a high amount of heat and pressure to run the engine which in turn reduces the volume efficiency for blends at all ratios as well as for high load (Fig. 6) [9,12,35]. 3.2.4. Load vs. Exhaust temperature Fig. 7 represents the exhaust temperature variation at different loads for different blends. For 100% diesel the exhaust temperature changes from 117.36 °C at zero load to 347.68 °C at full load while for 50% blend, the initial temperature was 117.47 °C at zero load which increase upto 357.40 °C at full load. It is evident that the increase in PPO in blend increases the exhaust temperature as depicted in Fig. 7. Lower calorific value and delayed combustion in the cylinder results in low heat transfer which results in higher exhaust temperature [9,12]. At a blend of 20%, the exhaust temperature is maximum whereas with further increase in PPO proportion the temperature decreases. The exhaust temperature at no load was 121.78 °C which increase upto a value of 370.29 °C at full load. At lower loads (0.2–7 kg) the variation is within 2–3 °C with an increase in PPO blend ratio (Fig. 7). Further at higher loads the exhaust temperature first increases upto 20% blend and then start reducing with further increase in load. At full load the exhaust temperature was 347.68 °C, 347.88 °C, 370.29 °C, 362.00 °C, 354.49 °C and 357.4 °C for 100% diesel, 10% PPO, 20% PPO, 30% PPO, 40% PPO and 50% PPO respectively. In literature, a higher exhaust temperature was reported when compared to the obtained results [3,9–12].

Fig. 5. Load vs. specific fuel consumption.

3.3. Combustion characteristics 3.3.1. Cylinder pressure vs. crank angle It was observed that the cylinder pressure is the highest (67.87 bar) at 4°CA in case of 50% PPO blend whereas the diesel fuel produces a lower cylinder pressure (62.52 bar) at 1°CA at full load represented in Fig. 8. The peak pressure for all blends is delayed and occurred after TDC providing that less fuel available for auto-ignition which results in delayed combustion and the diffusion combustion phase is prominent

Fig. 6. Load vs. volumetric efficiency.

6

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

in ignition results in increased heat release rate. High fuel accumulation and low cetane value increase the ignition delay while rapid combustion results in increased heat release rate. The maximum heat release rate for diesel is reported as 115.41 J/°CA at 2°CA. increase of PPO in blends produces a maximum value of heating rate as 136, 141.33, 144.96, 147.59, and 149.18 for 10%, 20%, 30%, 40%, and 50% respectively (Fig. 9). Kumar & Prabhu [40] reported that short ignition delay, high oxygen content in biodiesel helps in enhanced combustion producing high in-cylinder temperatures. In case of PPO blends the heat release mainly occurred in the premixed combustion. Delayed ignition helps in obtaining high heat release during the premixed combustion. The high air–fuel ratio also results in a high heat release rate in the case of PPO blends. Also, high in-cylinder temperature helps in increased heat release rate [34]. 3.3.3. Exhaust emission analysis During the combustion of fuel in the presence of air, the oxides of nitrogen emission are highly anticipated and mainly NO at high temperatures [42]. During the combustion of fuel, the predominant factors are the air-fuel ratio, in-cylinder temperature and residence time for completing the reaction [43]. Fig. 10a illustrates the emission of oxides of nitrogen with an increase in load for diesel and PPO blends. During combustion of fuel and air mixture in the cylinder, the oxidation of nitrogen present in air results in the formation of oxides of nitrogen in the form of nitric oxide (NO) and nitrogen dioxide (NO2). These oxides of nitrogen have a high impact on the combustion characteristics of the diesel engine. At lower load, the high presence of air to fuel ratio produces more oxides of nitrogen while with an increase in load decreases air-fuel ratio resulting in less oxygen to react and forms fewer oxides at higher loads as reported in Fig. 10a. In case of diesel fuel, the NOX formation reduces with increase in load from 473.68 ppm at no load to 46.05 ppm at full load (Fig. 10a). In the case of PPO blends similar trend of decrease in NOX with an increase in load is reported with increased values. For 10%, 20%, 30%, 40%, and 50% PPO in blend increases the NOX emission due to higher oxygenated compounds. The presence of oxygenated compounds resulted in better combustion and increased the formation of NOX in the exhaust (Fig. 10a) [10,44]. Devaraj et al. [11] has also reported the use of DEE in PPO reduces the combustion time by increasing the cetane number which helps in reduced production of oxides of nitrogen. Delayed combustion and high injection time prior to ignition can also be another reason for the increased level of NOX [17,44]. Also high heat release during combustion, high temperature and high pressure in cylinder for blends results in the significant formation of NOX [45]. Incomplete fuel combustion, over lean mixture formation and low in-cylinder temperature, are the major causes for the emission of unburnt hydrocarbons (Fig. 10b) [42,46]. Fuel viscosity, volatility and cold region of the combustion chamber also result in the emission of unburnt hydrocarbons (UHC). High viscosity leads to large droplet formation, and reduced vapor pressure causes incomplete combustion and increases the UHC emission. At lower loads the high air to fuel ratio and low-temperature results in the easy escape of the UHC into the exhaust [10,14]. Increase in load reduces the air–fuel ratio which helps in high in-cylinder pressure and in-cylinder temperature resulting in reduced combustion inefficiency. The variation of UHC emission with increasing load for diesel and PPO blends is represented in Fig. 10b. For diesel fuel, the UHC decreases from 59.37 ppm at zero load to 31.25 ppm at full load. For PPO with a blend ratio of 10, 20, 30, 40, and 50 shows a value of 60.10, 61.43, 62.33, 63.70, and 64.47 ppm at zero load upto a lower value of 34.06, 35.22, 36.51, 37.01 and 38.22 ppm at full load. The increase in PPO% fraction in the blend, the emission increases due to incomplete combustion and un-reacted hydrocarbons. There are two main reasons reported for higher UHC emission with PPO blends. First, the fuel (blended PPO) does not propagate more rooted into the combustion chamber resulting in the congregation along the cylinder wall

Fig. 8. Cylinder pressure vs. crank angle at full load (15 kg).

[14,40]. Also, low cetane value delayed the combustion of fuel which further on combustion releases high heat with increased cylinder pressure. The near value of viscosity of diesel and PPO helps in the controlled injection of ignition fuel in the cylinder. Proper mixing of fuel and air helps in better combustion with increased cylinder pressure as can be seen from Fig. 4 for all ratios. It can be said that the high amount of PPO in mix causes high viscosity resulting in high fuel injection while delayed combustion results in sudden expansion causing high cylinder pressure [12,15,16].

3.3.2. Heat release rate vs. crank angle Fig. 9 represents the comparative heat release rate for diesel and PPO blends at full load. Premixed combustion phase and diffusion combustion phase are classified for the characterization of heat release curve. Delayed heat release for all PPO blends was reported when compared to diesel fuel also the heat release duration is also higher in the case of blends [41]. In the initial phase, the drop in heat release at −7°CA is due to the ignition of a fuel-air mixture prepared during the delayed period. The PPO blends show a higher drop as a small variation for diesel is observed (Fig. 9) in the initial phase of the ignition. It was reported that during the compression process injection of fuel in the higher amount due to short ignition delay caused premixed combustion resulting in high heat release rate. Mani et al. [17] reported that delay

Fig. 9. Heat release rate vs. crank angle at full load (15 kg). 7

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

Fig. 10. Exhaust emission analysis for diesel and PPO blends at increasing loads: a) NOX emission; b) Unburnt hydrocarbons (UHC); c) CO emission; and d) CO2 emission.

the increase in the emission was observed. At full load the CO emission is 0.81%vol, 0.81%vol, 0.83%vol, 0.85%vol, and 0.85%vol at 10%, 20%, 30%, 40%, and 50% PPO fraction respectively. PPO contains a high amount of oxygenated compounds which results in increases CO emission. At reduced load, the CO emission decreased due to proper combustion whereas a further increase in load reduces the combustion efficiency resulting in increased CO emission [14]. Kalagaris et al. [16] also reported that low cetane number and higher aromatic content of blends causes longer ignition delay and short combustion time as shown in cylinder pressure and heat release figures which are in context to the obtained results [15,16]. Complete oxidation of fuel results in high heat release and high CO2 formation. From the environmental aspect, the CO2 emission has to be reduced which adds to the greenhouse effect [16,18]. Fig. 10d represents the emission of CO2 with variation in load for diesel and PPO blends at different ratios. It was observed that the CO2 emission is lower in case of diesel oil due to inefficient combustion while blends produced an increased CO2 emission due to increased combustion. At full load, the CO2 emission for 50% PPO was 2.79%vol while at zero load is 8.38%vol. At lower loads, high availability of oxygen causes complete combustion resulting in high emission while at increased load low oxygen content causes inefficient combustion resulting in low CO2 emission. Incomplete combustion is also caused due to delayed burning

as well as in the crevice and left un-burnt. The second major reason is the non-reactive unsaturated hydrocarbons which do not break during the combustion process and gets emitted with exhaust [40,44–46]. Another reason for the high UHC emissions with blends is due to high aromatic contents which reduce the combustion duration and increases the ignition delay period [9,16]. He reported that higher the PPO fraction higher will be the UHC emission due to increased aromatic content in fuel [15,16]. Combustion of hydrocarbons produces CO as an intermediate compound which forms due to incomplete combustion. Air fuel ratio relative to stoichiometric proportions is the major component in the production of CO. Rich combustion invariably produces CO, and emissions increase nearly linearly with the deviation from the stoichiometry [47]. The results show that the PPO blended fuel produces more CO as compared to diesel fuel. The reason for the increase in CO concentration is the high fuel consumption at higher loads. Also, it was reported that the increased CO emission is due to low in-cylinder temperature during the combustion [10]. Fig. 10c represents the variation in CO emission with variation in PPO blends at different loads. The result signifies that at higher load and higher blend ration the CO emission is higher. It was observed that the diesel produces a maximum of 0.76%vol at full load whereas a minimum value of 0.67%vol is obtained at a load of 3Kg. For an increased proportion of PPO in the blend, 8

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

and short combustion period as reported by Mani et al. [10]. The increased CO2 emission with PPO blends is mainly due to high availability of oxygenated compounds in PPO which during combustion results in improved combustion at higher loads and produces high CO2 emission compared to diesel fuel.

Calorim 2019;136:281–93. https://doi.org/10.1007/s10973-018-7933-0. [4] Gardy J, Rehan M, Hassanpour A, Lai X, Nizami A-S. Advances in nano-catalysts based biodiesel production from non-food feedstocks. J Environ Manage 2019;249:109316https://doi.org/10.1016/j.jenvman.2019.109316. [5] Anup TJ, Watwe V. Waste plastic pyrolysis oil as alternative for SI and CI engines. Int J Innovative Res Sci, Eng Technol 2014;3:14680–7. [6] Dayana S, Sharuddin A, Abnisa F, Mohd W, Wan A. A review on pyrolysis of plastic wastes. Energy Convers Manage 2016;115:308–26. https://doi.org/10.1016/j. enconman.2016.02.037. [7] Bhawan P, Nagar EA. Website Material on Plastic Waste Management. Central Pollution Control Board n.d.: 22. [8] Singh RK, Ruj B. Plasticwaste management and disposal techniques – Indian scenario. Int J Plastics Technol 2015;19. https://doi.org/10.1007/s12588-015-9120-5. [9] Singh RK, Ruj B, Sadhukhan AK, Gupta P. Impact of fast and slow pyrolysis on the degradation of mixed plastic waste: product yield analysis and their characterization. J Energy Inst 2019:1–11. https://doi.org/10.1016/j.joei.2019.01.009. [10] Mani M, Subash C, Nagarajan G. Performance, emission and combustion characteristics of a DI diesel engine using waste plastic oil. Appl Therm Eng 2009;29:2738–44. https://doi.org/10.1016/j.applthermaleng.2009.01.007. [11] Devaraj J, Robinson Y, Ganapathi P. Experimental investigation of performance, emission and combustion characteristics of waste plastic pyrolysis oil blended with diethyl ether used as fuel for diesel engine. Energy 2015;85:304–9. https://doi.org/ 10.1016/j.energy.2015.03.075. [12] Tamilkolundu S, Murugesan C. The Evaluation of blend of Waste Plastic Oil- Diesel fuel for use as alternate fuel for transportation. 2nd International Conference on Chemical, Ecology and Environmental Sciences. 2012. p. 66–70. [13] Singh RK, Ruj B. Time and temperature depended fuel gas generation from pyrolysis of real world municipal plastic waste. Fuel 2016;174:164–71. https://doi.org/10. 1016/j.fuel.2016.01.049. [14] Kumar S, Prakash R, Murugan S, Singh RK. Performance and emission analysis of blends of waste plastic oil obtained by catalytic pyrolysis of waste HDPE with diesel in a CI engine. Energy Convers Manage 2013. https://doi.org/10.1016/j.enconman. 2013.05.028. [15] Kalargaris I, Tian G, Gu S. Experimental characterisation of a diesel engine running on polypropylene oils produced at different pyrolysis temperatures. Fuel 2018;211:797–803. https://doi.org/10.1016/j.fuel.2017.09.101. [16] Kalargaris I, Tian G, Gu S. The utilisation of oils produced from plastic waste at different pyrolysis temperatures in a DI diesel engine. Energy 2017;131:179–85. https://doi.org/10.1016/j.energy.2017.05.024. [17] Mani M, Nagarajan G, Sampath S. Characterisation and effect of using waste plastic oil and diesel fuel blends in compression ignition engine. Energy 2011;36:212–9. https://doi.org/10.1016/j.energy.2010.10.049. [18] Singh RK, Ruj B, Sadhukhan AK, Gupta P. Thermal degradation of waste plastics under non-sweeping atmosphere : Part 1: Effect of temperature, product optimization, and degradation mechanism. J Environ Manage 2019;239:395–406. https:// doi.org/10.1016/j.jenvman.2019.03.067. [19] ASTM D3176-15. Standard Practice for Ultimate Analysis of Coal and Coke. American Society for Testing and Materials (ASTM); 2015. [20] ASTM D3172-13. Standard Practice for Proximate Analysis of Coal and Coke. American Society for Testing and Materials (ASTM); 2013. [21] ASTM D1298. Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method. American Society for Testing and Materials (ASTM); 2017. [22] ASTM D445. Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. American Society for Testing and Materials (ASTM); 2019. [23] ASTM D92. Standard Test Method for Flash and Fire Points by Cleveland Open Cup Tester. American Society for Testing and Materials (ASTM); 2018. [24] ASTM D97. Standard Test Method for Pour Point of Petroleum Products. American Society for Testing and Materials (ASTM); 2017. [25] ASTM D2500-17a. Standard Test Method for Cloud Point of Petroleum Products and Liquid fuels. American Society for Testing and Materials (ASTM); 2017. [26] ASTM D482. Standard Test Method for Ash from Petroleum Products. American Society for Testing and Materials (ASTM); 2013. [27] ASTM D129. Standard Test Method for Sulfur in Petroleum Products. American Society for Testing and Materials (ASTM); 2018. [28] ASTM D189. Standard Test Method for Conradson Carbon Residue of Petroleum Products. American Society for Testing and Materials (ASTM); 2014. [29] ASTM D240. Standard Test Method for Heat of Combustion of Liquid Hydrocarbon Fuels by Bomb Calorimeter. American Society for Testing and Materials (ASTM); 2017. [30] ASTM D613. Standard Test Method for Cetane Number of Diesel Fuel Oil. American Society for Testing and Materials (ASTM); 2018. [31] Karisathan Sundararajan N, Ramachandran Bhagavathi A. Experimental investigation on thermocatalytic pyrolysis of HDPE plastic waste and the effects of its liquid yield over the performance, emission, and combustion characteristics of CI engine. Energy Fuels 2016;30:5379–90. https://doi.org/10.1021/acs.energyfuels.6b00407. [32] Khan MZH, Sultana M, Al-Mamun MR, Hasan MR. Pyrolytic waste plastic oil and its diesel blend: fuel characterization. J. Environ. Public Health 2016;2016. https:// doi.org/10.1155/2016/7869080. [33] Venkatesan H, Sivamani S, Bhutoria K, Vora HH. Experimental study on combustion and performance characteristics in a DI CI engine fuelled with blends of waste plastic oil. Alexandria Eng J 2018;57:2257–63. https://doi.org/10.1016/j.aej.2017. 09.001. [34] Senthilkumar P, Sankaranarayanan G. Effect of Jatropha methyl ester on waste plastic oil fueled DI diesel engine. J Energy Inst 2016;89:504–12. https://doi.org/ 10.1016/j.joei.2015.07.006.

4. Conclusion The utilization of plastic waste for the conversion into fuel oil via pyrolysis is available method with the high amount and superior quality. The utilization of crude PPO with diesel in different proportions was tested in a diesel engine, and the results are reported as: 1. Pyrolysis of mixed plastic waste at 450 °C produces a high-quality fuel having physical properties similar to conventional fuels like diesel and petrol represents an excellent alternative to be used as fuel in diesel engines. 2. Addition of crude PPO upto 50% with diesel shows a slight increase in BTE when compared to diesel fuel and hence provides increased efficiency while the SFC characteristics show a decrease in value when compared to diesel and hence shows good fuel characteristics. 3. The presence of crude PPO in diesel blend upto 50% decreases the volume efficiency with increase in exhaust temperature. The low calorific value of PPO results in high fuel consumption whereas high oxygenated compounds cause elevated exhaust temperature while no rigorous variation in values is observed and hence can be utilized in diesel engines. 4. Delayed ignition, high fuel consumption, and low viscosity resulted in shorter combustion duration producing high heat release rate and increased in-cylinder pressure and temperature. 5. The utilization of crude PPO with diesel in different blend ratios shows an increase in exhaust emission while the variation is comparatively low when compared with 100% diesel. At high load, low oxygen content, delayed ignition, short combustion duration results in increased emission values whereas shows a good combustion characteristic for utilization as fuel in a diesel engine. The utilization of crude PPO with diesel blends upto 50% can be utilized in diesel engines with a minor loss in efficiency and with a small increase in exhaust emission when compared to 100% diesel fuel characteristics. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements Authors are thankful to Director, CSIR-CMERI, Durgapur, and Director-NIT, Durgapur for their support to carry out this research work. The authors acknowledge Dr. Lal Gopal Das of CSIR-CMERI, Durgapur for allowing to conduct experiments on Fuel Testing Setup. The authors acknowledge the use of the facility created by the FIST program of DST, Government of India for the analysis of samples. References [1] Gardy J, Nourafkan E, Osatiashtiani A, Lee AF, Wilson K, Hassanpour A, et al. A core-shell SO4/Mg-Al-Fe3O4 catalyst for biodiesel production. Appl Catal B 2019;259:118093https://doi.org/10.1016/j.apcatb.2019.118093. [2] Miandad R, Barakat MA, Rehan M, Aburiazaiza AS, Gardy J, Nizami AS. Effect of advanced catalysts on tire waste pyrolysis oil. Process Saf Environ Prot 2018;116:542–52. https://doi.org/10.1016/j.psep.2018.03.024. [3] Bharathiraja M, Venkatachalam R, Senthilmurugan V. Performance, emission, energy and exergy analyses of gasoline fumigated DI diesel engine. J Therm Anal

9

Fuel xxx (xxxx) xxxx

R.K. Singh, et al.

[35] Williams PT, Slaney E. Analysis of products from the pyrolysis and liquefaction of single plastics and waste plastic mixtures. Resour Conserv Recycl 2007;51:754–69. https://doi.org/10.1016/j.resconrec.2006.12.002. [36] Singh RK, Ruj B, Sadhukhan AK, Gupta P. A TG-FTIR investigation on the co-pyrolysis of the waste HDPE, PP, PS and PET under high heating conditions. J Energy Inst 2019. https://doi.org/10.1016/J.JOEI.2019.09.003. [37] Ayodhya AS, Lamani VT, Thirumoorthy M, Kumar GN. NOx reduction studies on a diesel engine operating on waste plastic oil blend using selective catalytic reduction technique. J Energy Inst 2019;92:341–50. https://doi.org/10.1016/j.joei.2018.01. 002. [38] Rehan M, Miandad R, Barakat MA, Ismail IMI, Almeelbi T, Gardy J, et al. Effect of zeolite catalysts on pyrolysis liquid oil. Int Biodeterior Biodegrad 2017;119:162–75. https://doi.org/10.1016/j.ibiod.2016.11.015. [39] Muhammad C, Onwudili JA, Williams PT. Thermal degradation of real-world waste plastics and simulated mixed plastics in a two-stage pyrolysis-catalysis reactor for fuel production. Energy Fuels 2015;29:2601–9. https://doi.org/10.1021/ ef502749h. [40] Senthil R, Kumar MP. Experimental investigation of a di diesel engine using tyre pyrolysis oil-diesel blends as a biodiesel. Linternational J Mech Eng Technol 2014;5:74–90. [41] Thiyagarajan S, Sonthalia A, Edwin Geo V, Ashok B, Nanthagopal K, Karthickeyan V, et al. Effect of electromagnet-based fuel-reforming system on high-viscous and

[42] [43]

[44]

[45]

[46]

[47]

10

low-viscous biofuel fueled in heavy-duty CI engine. J Therm Anal Calorim 2019;0123456789. https://doi.org/10.1007/s10973-019-08123-w. Heywood JB. Internal Combustion Engine Fundamentals by. John B. Heywood.pdf; 1988. Çelikten I. An experimental investigation of the effect of the injection pressure on engine performance and exhaust emission in indirect injection diesel engines. Appl Therm Eng 2003;23:2051–60. https://doi.org/10.1016/S1359-4311(03)00171-6. Ilkiliç C. The emission characteristics of a CI engine fueled with a biofuel blend. Energy Sources Part A 2012;34:1901–12. https://doi.org/10.1080/ 15567031003773270. Ren Y, Huang Z, Jiang D, Liu L, Zeng K, Liu B, et al. Combustion characteristics of a compression-ignition engine fuelled with diesel-dimethoxy methane blends under various fuel injection advance angles. Appl Therm Eng 2006;26:327–37. https:// doi.org/10.1016/j.applthermaleng.2005.07.009. Imtenan S, Masjuki HH, Varman M, Rizwanul Fattah IM, Sajjad H, Arbab MI. Effect of n-butanol and diethyl ether as oxygenated additives on combustion-emissionperformance characteristics of a multiple cylinder diesel engine fuelled with dieseljatropha biodiesel blend. Energy Convers Manage 2015;94:84–94. https://doi.org/ 10.1016/j.enconman.2015.01.047. Kumaravel ST, Murugesan A, Kumaravel A. Tyre pyrolysis oil as an alternative fuel for diesel engines – a review. Renew Sustain Energy Rev 2016;60:1678–85. https:// doi.org/10.1016/j.rser.2016.03.035.