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Fuel 92 (2012) 327–339
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Microwave-heated pyrolysis of waste automotive engine oil: Influence of operation parameters on the yield, composition, and fuel properties of pyrolysis oil Su Shiung Lam a,c,⇑, Alan D. Russell a, Chern Leing Lee b, Howard A. Chase a a b c
Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, United Kingdom BP Institute, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, United Kingdom Department of Engineering Science, University Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia
a r t i c l e
i n f o
Article history: Received 17 May 2011 Received in revised form 11 July 2011 Accepted 13 July 2011 Available online 28 July 2011 Keywords: Pyrolysis Microwave pyrolysis Waste oil Pyrolysis oil Fuel
a b s t r a c t The pyrolysis of waste automotive engine oil was investigated using microwave energy as the heat source, and the yield and characteristics of the pyrolysis oils (i.e. elemental analysis, hydrocarbon composition, and potential fuel properties) are presented and discussed. The microwave-heated pyrolysis generated an 88 wt.% yield of condensable pyrolysis oil with fuel properties (e.g. density, calorific value) comparable to traditional liquid transportation fuels derived from fossil fuel. Examination of the composition of the oils showed the formation of light aliphatic and aromatic hydrocarbons that could also be used as a chemical feedstock. The oil product showed significantly high recovery (90%) of the energy present in the waste oil, and is also relatively contaminant free with low levels of sulphur, oxygen, and toxic PAH compounds. The high yield of pyrolysis oil can be attributed to the unique heating mode and chemical environment present during microwave-heated pyrolysis. This study extends existing findings on the effects of pyrolysis process conditions on the overall yield and formation of the recovered oils, by demonstrating that feed injection rate, flow rate of purge-gas, and heating source influence the concentration and the molecular nature of the different hydrocarbons formed in the pyrolysis oils. The microwaveheated pyrolysis can be performed in a continuous operation, and the apparatus described which is fitted with magnetrons capable of delivering 5 kW of microwave power is capable of treating waste oil at a feed rate of 5 kg/h with a positive energy ratio of 8 (energy content of hydrocarbon products/electrical energy supplied for microwave heating) and a net energy output of 179,390 kJ/h. Our results indicate that microwave-heated pyrolysis shows exceptional promise as a means for recycling and treating problematic waste oil. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The production of waste automotive engine oil is estimated at 24 million tons each year throughout the world, posing a significant treatment and disposal problem for modern society. The waste oil, containing a mixture of aliphatic and aromatic hydrocarbons, also represents a potential source of high-value fuel and chemical feedstock. The preferred disposal options in most countries are incineration and combustion for energy recovery, and vacuum distillation and hydro-treatment for re-refining the waste oil [1]. However, these disposal routes recover only the chemical value of the waste and they are becoming increasingly impracticable as ⇑ Corresponding author at: Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, United Kingdom. Tel.: +44 (0)1223330132; fax: +44 (0)1223334796. E-mail addresses:
[email protected],
[email protected] (S.S. Lam), ar508@ cam.ac.uk (A.D. Russell),
[email protected] (C.L. Lee),
[email protected] (H.A. Chase). 0016-2361/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2011.07.027
concerns over environmental pollution, and the difficulties and additional costs of sludge disposal [2,3] are recognised due to the undesirable contaminants present in waste oil [1]. Pyrolysis techniques have recently shown great promise as an economic and environmentally friendly disposal method for waste oil [4–6] – the waste material is thermally cracked and decomposed in an inert atmosphere, with the resulting pyrolysis oils and gases able to be used as a fuel or chemical feedstock, and the char produced used as a substitute for activated carbon, though such practice is yet to become popular. The pyrolysis oil produced is of particular interest due to its easy storage and transportation as a liquid fuel or chemical feedstock. The oil can be catalytically upgraded to transport-grade fuels, or added to petroleum refinery feedstocks for further processing [7]. Most of the literature reports focus on pyrolysis using conventional electric resistance and electric arc heating [4,6,8,9]. However, microwave-heated pyrolysis has recently shown promise as a route to treating and recycling of waste oil [7]; the advantages of microwave-heated pyrolysis
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have been elaborated in previous work [5] and will not be duplicated here. In this process, waste oil is mixed with highly microwave-absorbent material such as particulate carbon. As a result of microwave heating, the oil is thermally cracked in the absence of oxygen into shorter hydrocarbon chains. The resulting volatile products are subsequently recondensed into pyrolysis oils of different composition depending on the reaction conditions. The use of microwave radiation as a heat source is known to offer additional advantages over traditional thermal heat sources [5,10], and the combination of carbon-based material and microwave heating in pyrolysis processes is a fairly novel subject that is of increasing interest as reflected by the increasing amount of research reported in the literature [7,11,12]. Microwave radiation provides a rapid and energy-efficient heating process compared to conventional technologies. The diffuse nature of the electromagnetic field allows microwave heating to evenly heat many substances in bulk, thereby offering an improved uniformity of heat distribution, excellent heat transfer, and providing better control over the heating process. Other advantages include higher power densities and the ability to reach high temperatures at faster heating rates, facilitating increased production speeds and decreased production costs. Furthermore, energy is targeted only to microwave receptive materials and not to air or containers within the heating chamber. It can promote or accelerate certain chemical reactions by selectively heating the reactants, leading to a more uniform temperature profile and improved yield of desirable products. The process is physically gentle, allowing for a wide variety of applications in diverse fields. This study investigates the influence of process parameters (feed injection rate, purge gas flow, and heating source) on the yield and characteristics of the pyrolysis oils produced from microwave-heated pyrolysis of the waste oil, with a focus on their elemental and hydrocarbon composition, and potential fuel properties. These evaluations are important to assess the technical feasibility and applicability of the pyrolysis process as a route to energy recovery/feedstock recycling from waste oil. There have been no reports on the composition of the oil products resulting from microwave-heated pyrolysis of waste oil, and limited information is available concerning the influence of key process parameters on the pyrolysis of waste oil, although a few studies have been performed on electric resistance heated pyrolysis of waste oil [6,13–15].
2. Experimental section 2.1. Materials Shell 10 w-40 motor oil was used throughout the experiments. The waste oil was sampled from the crankcase of diesel engines run on unleaded fuel. The hydrocarbon composition of the waste oil, the particulate carbon (TIMREX FC250 Coke, TIMCAL Ltd., Bodio, Switzerland) used as microwave absorbent to heat the waste oil, and their pre-experimental treatment have been reported in previous works [5,7]. Before pyrolysis, the waste oil samples were filtered such that the size of any remaining particulates (i.e. metal particles, carbon soots, and other impurities) were less than 100 lm; samples were examined for C, H, N, S, O content by elemental analysis; calorific value was determined by bomb calorimetry. 2.2. Experimental details The experimental apparatus for this investigation has been described in detail in previous work [5]. The only change from this description is the addition of a mixed-cellulose-ester membrane filter (0.45 lm ME25 filter, Schlecher & Schuell, Germany) (see (5) in Fig. 1) to remove any metallic solid residues present in the pyrolysis volatiles before they passed through the condensation system. The refined experimental apparatus is shown in Fig. 1. Microwave-heated pyrolysis of waste oil was performed in a bell-shaped quartz reactor (180 180 180 mm) (2) filled with 1 kg of particulate carbon, which is stirred (3) and heated by a 5 kW microwave oven (1) at a heating rate of about 60 °C/min over a range of pyrolysis temperatures (250–700 °C), feed injection rates (0.4–5 kg/h) and purge gas flows (0.1–0.75 L/min) to understand the influence of these process parameters on the final pyrolysis oils obtained; N2 purge-gas was vented through the system to maintain the apparatus in an inert nitrogen atmosphere, and the particulate carbon, added initially into the reactor in one batch, was stirred to ensure a uniform temperature distribution throughout the reactor and to maximise heat transfer during pyrolysis. The waste oil is nearly transparent to microwaves due to its non-polarity nature, therefore it requires heating by contact with microwave absorbent materials (e.g. carbon-based materials such as particulate carbon) in order to achieve pyrolytic thermal cracking. The
Fig. 1. Schematic layout of microwave-heated pyrolysis system.
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sample of waste oil was continuously added to the reactor (4) at each target feeding rate over a period of 2 h as soon as the target pyrolysis temperature was achieved; it was previously ascertained that the magnetron system of the microwave oven was able to generate sufficient heat to maintain the desired target temperature at all these flow rates. Products generated in the pyrolysis reaction, termed generally as pyrolysis volatiles (consisting of a mixture of hydrocarbon gases, liquids, and suspended solids existing in a vapour phase), leave the reactor and pass through a condensation system (6, 7, 8, 9), and either condense into pyrolysis oil (10, 11) or are sampled as incondensable pyrolysis gases (13, 14) before being vented from the system. The amount of residue material not converted to gaseous or liquid products was determined by measurement of the weight change in the reactor and its contents before and after the reaction. The residue materials are likely to be chars produced from tertiary cracking reactions of the pyrolysis process [7]; these were particles that possessed a harder texture, darker colour, and most of which had a smaller size than the particulate carbon used in the bed of the reactor. These chars, which mostly accumulated on the surface of the carbon bed, were separated from the carbon particles using sieves (90 and 250 lm) and mixed with the metallic residues deposited on the ME25-filter installed between the reactor and the condensation system for later analysis. The yield of pyrolysis oil was determined by measuring the weight increase in the collecting vessels and filter. The pyrolysis oil was then transferred into glass bottles until further analysis (see Section 2.4). The gas yield was determined by mass balance and assumes that whatever mass of added sample that is not accounted for by the residue and pyrolysis oil measurements left the system in gaseous form. All the pyrolysis experiments were repeated several times and the data recorded is the average of the results obtained from three valid repeated runs performed under identical conditions. These runs showed good reproducibility and precision with low standard deviations shown in the product yield (±1–5 wt.%). The virgin oil (FO), the waste oil, and the pyrolysis oils were examined for hydrocarbon composition by gas chromatography coupled with a mass selective spectrometry detector (GC–MS) and a flame ionisation detector (GC-FID). These oil samples were also analysed by Fourier Transform Infrared Spectroscopy (FTIR) and elemental analyzer to identify their chemical functional group and elemental content (C, H, N, S, and O). In addition, the fuel properties of these oil samples (i.e. calorific value, density, flash point, viscosity, boiling point distribution) were determined according to ASTM standard methods (see Section 2.4). 2.3. Temperature measurement The temperature of the carbon bed in the system was monitored using two thermocouples; one ducted into the middle layer of the carbon bed through the centre of the shaft that protrudes from the bottom of the stainless steel stirrer shaft, the other enters the reaction chamber through a side port on the top of the reactor and is positioned at the top of the carbon bed. Both thermocouples remain in direct contact with the carbon inside the reactor. In addition, ferrite core thermocouple connectors and cable clamps were used to reduce the electromagnetic interference caused by the microwaves on the temperature measurement. Accurate measurement of the evolution of the temperature of the carbon bed was difficult during the heating process – firstly, there are inherent difficulties involved in measuring this parameter in microwave devices [16]; secondly, it should be noted that the temperature is not uniform throughout the carbon bed during the initial heating to the target temperature; electrical arcing was found to occur for a relatively short period at the beginning of the heating process, but it stopped when the carbon bed had been
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heated to the target temperature. A stirred bed reactor is used in this study in which the physical movement and mixing of carbon particles by the stirring system creates a uniform temperature distribution, independent of the penetration depth of the microwaves into the bed of particulate carbon. Provided the temperature is kept consistent and uniform in this system, once the thermal equilibration and steady state temperature were reached, the temperature shown by the thermocouples are assumed to give a reliable reading of the average temperature of the bulk carbon bed. 2.4. Analytical methods Oil samples were analysed using a 6890/5973 GC–MS instrument (Agilent Technologies, Palo Alto, CA), allowing the quantification of compounds by both species and size; compounds were identified using the NIST 2005 mass spectral library using similarity indices of >70%, or by comparison with published GC–MS data for similar products; the detailed description of this analytical method have been reported in previous work [7] For polycyclic aromatic hydrocarbons (PAHs), the GC–MS instrument was programmed into selected-ion-monitoring (SIM) mode and the peaks were compared to external PAH standards for quantification. In addition to GC–MS analysis, the oils were analysed by the 6890 gas chromatograph coupled with a flame ionisation detector, using a 30 m HP-5 capillary column (5% phenyl methyl silicone, I.D. 0.53 mm, film thickness 5 lm). The GC-FID oven was programmed from 40 °C, held for 5 min, then ramped at 5 °C/min to 280 °C with a final holding time of 30 min. Quantification of compounds on the GC-FID was obtained by external standard method and relative retention times once the component has been identified by the GC–MS analysis. The data obtained from GC-FID were used to evaluate the simulated distillation curves of the various oil samples according to ASTM D2887-08 [17]. Elemental analyses of oil samples were performed using a LECO CHNS-932 elemental analyzer (LECO Corporation, Michigan, USA). Samples were burned at 1000 °C in a flowing stream of oxygen. The products of combustion (CO2, H2O, N2 and SO2) then passed through the system with He as the carrier gas, and their content of carbon, hydrogen, and sulphur were measured quantitatively by selective IR absorption detectors, except for the nitrogen, which was measured by a thermal conductivity detector. Oxygen content was measured by pyrolysing a separate sample at 1300 °C in a VTF900 pyrolysis furnace (LECO Corporation, Michigan, USA). The oxygen released in the pyrolysis reaction then reacts with activated charcoal to form CO, which was converted to CO2 by passing through an oxidation tube with He as the carrier gas. The CO2 generated was then measured as above by an IR detector. The analysis of chemical functional group in oil samples was performed using a Bruker Tensor 27 FTIR Spectrometer (Bruker Optics, Ettlingen, Germany) that produces IR spectra for each sample. Experimental analysis of the fuel properties of the oil samples was performed according to the following ASTM methods: D1298-99(2005) [18] for density, D1310-01 [19] for flash point, and D445-10 [20] for viscosity. The boiling point distribution of hydrocarbons in oil samples were determined using the 6890 GC-FID instrument by a simulated distillation method according to ASTM D2887-08 [17] and ASTM D3710-95(2009) [21]; the sample was heated from ambient temperature to 550 °C (300 °C for ASTM D3710-95) at a heating rate of 10 °C/min with high purity He gas vented through the apparatus at a flow rate of 0.1 L/min. Oil samples were also analysed for calorific value using a Parr 6200 Isoperibol bomb calorimeter (Parr Instrument, Moline, Illinois). Detailed analysis and sample preparation were performed according to ASTM Standard D4809-09a. [22] The solid content (ethanol-insoluble) was determined by filtering the pyrolysis oil through a 0.1-lm polycarbonate membrane filter (Milipore Co.,
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Billerica, MA) and measurement of the weight change before and after filtration. 3. Results and discussion In this study, different process parameters were studied to determine the optimum conditions that provide the greatest yield of pyrolysis products containing highly desired compounds (e.g. commercially valuable light hydrocarbons), and to investigate the effects of these parameters on the compositions of pyrolysis products, with a special emphasis on the pyrolysis oil. The main objective was to convert the waste oil to petrochemical products suitable for use as a fuel or raw chemical feedstock. This paper explored the effects of varying the feed injection rate of waste oil, and the purge gas flow rate. The investigation of pyrolysis products at different temperatures have been reported in our previous works [5,7]. Pyrolysis at a constant temperature of 550 °C was used as throughout based on the optimum temperature obtained in the previous work [7] that showed the greatest yield of valuable light hydrocarbons and only low levels of residual metals in the pyrolysis oil. 3.1. Product yield Fig. 2 outlines the effect of N2 purge rate and waste oil feed rate on the fraction of waste oil converted to pyrolysis gases, pyrolysis oils, and char residues. Increasing the N2 purge rate from 100 to 250 ml/min resulted in an increased yield of pyrolysis oil. The increase in waste oil feed rate demonstrated a minor influence on the yield of pyrolysis oil, as the total increase in the yield of pyrolysis oil resulting from the increase in waste oil feed rate from 0.4 to 5 kg/h was only 4%. The slight upward trend in yield could be due to experimental error as a maximum error rate of 5 wt.% was observed in the values of the product yields (see Section 2.2), but it is thought that this could be a real trend expected from the increase in waste oil feed rate. When the hydrocarbons in waste oil undergo cracking into shorter molecules, which then vaporise as pyrolysis volatiles, the volume increase accompanying the phase change from liquid to vapour creates an increase of pressure in the reactor that drives the pyrolysis volatiles out of the reactor into the product collection system. At higher waste oil feed rates, this process is likely to occur more rapidly and more molecules are likely to enter the vapour phase earlier, causing a more rapid flow of pyrolysis volatiles out of the reaction ‘hot zone’ inside the reactor. It is also believed that the higher rates of N2 purge gas resulted in a more rapid flow of pyrolysis volatiles out of the reactor due to the higher pressure created in the reactor. The residence time of pyrolysis products in the reactor thus becomes dependent on these two process parameters, which leads to shorter residence times of the pyrolysis volatiles in the reactor at higher purge and feed rates. The decrease in residence time decreases the exposure of pyrolysis volatiles (evolved from the waste oil compounds) to secondary reactions (e.g. secondary thermal cracking to form pyrolysis gases, tertiary cracking and repolymerisation to form chars) and leads to higher yield of pyrolysis oil and smaller yields of both pyrolysis gases and residues (likely to be char) observed under these conditions; similar findings were reported by other workers [23]. At very high N2 purge rates (750 ml/min), the yield of pyrolysis oil decreases, suggesting that the installed condensation system is unable to condense the pyrolysis volatiles at this considerably faster vapour flow rate, which leads to a higher corresponding yield of pyrolysis gases. The slightly higher amounts of residues observed at higher feed rates (P2.5 kg/h) could be attributed to the formation of residues such as char and residual unpyrolysed waste oil, the amounts of which are likely to increase at higher feed
Fig. 2. Product yields (wt.%) as a function of N2 purge rate (up) and waste oil feed rate (bottom). Process conditions: pyrolysis at a constant waste oil feed rate of 0.4 kg/h was used when the effects of varying the N2 purge rate were studied, whereas a constant N2 purge rate of 250 ml/min was used to study the effects of varying the waste oil feed rate, and all experiments were performed at a constant temperature of 550 °C.
rates. The results show that extended heating of the generated pyrolysis volatiles in the reactor could promote different product compositions due to secondary reactions of the primary pyrolysis product; hence it was observed that some waste oil is consumed in the production of pyrolysis gases and char in addition to pyrolysis oil. The waste oil feed rate, when compared with the mass of the microwave absorbent (in this case particulate carbon), provides useful information for chemical reactor design by assessing the weight hourly space velocity (WHSV) of the pyrolysis process. The WHSV, defined as the waste oil feed rate divided by the mass of particulate carbon, was calculated based on the different waste oil feed rates. The variation of the yield of pyrolysis oil with this parameter is presented in Fig. 3. The results show that the pyrolysis reactor is able to process each hour a waste oil feed up to five times the mass of the particulate carbon while maintaining a relatively high production of pyrolysis oil from the feed (85–88 wt.%). The use of a microwave-heated bed of particulate carbon in this set-up showed good heat transfer and cracking capacity to pyrolyse the waste oil. The microwave energy is targeted to and heats mainly the microwave-receptive particulate carbon which in turn transfers thermal energy to the waste oil added into the carbon
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Fig. 3. The yields (wt.%) of pyrolysis oil in relation to weight hourly space velocity (h 1).
bed in order for pyrolysis to occur [5]. Overall, the waste oil feed rate was found to have little influence on the yields of pyrolysis product (Fig. 2), indicating the potential of this pyrolysis process in treating high volumes of waste oil while maintaining a relatively high throughput of pyrolysis oil (Fig. 3). The highest yield of pyrolysis oil (88 wt.%) was observed at 250 ml/min of N2 purge rate, and 5 kg/h of waste oil feed rate (Fig. 2). It is assumed that this represents the optimum balance between sufficiently high purge and feed rates to sweep the evolved pyrolysis volatiles out of the reactor, thus producing condensable vapours (pyrolysis oil) by avoiding the promotion of secondary reactions that would result in increased formation of incondensable gases and char residues, while not being so high that the condensation system was unable to condense the pyrolysis volatiles. It should be noted that 5 kg/h is the highest waste oil feed rate that can be processed in this experimental device as it was ascertained that the 5 kW magnetron system of the microwave oven could no longer generate sufficient heat to maintain the desired operating temperature (550 °C) at flow rates of 6 kg/h and above. Our results have shown that a recovery of pyrolysis oil of approximately 84– 88% of the initial mass of waste oil is possible under these conditions. Fig. 4 compares our results to those of waste oil pyrolysis processes heated by conventional electric heating either using waste oil on its own or in the additional presence of coal or scrap tyres. The use of the microwave-heated bed of particulate carbon, compared to the other methods of operation, seemed to have a beneficial effect in cracking the waste oil to produce higher amounts of condensable products. This may be attributed to the different heat distributions present during microwave-heated pyrolysis. The applied microwave radiation heats mainly the carbon, creating a localised reaction ‘hot zone’ as opposed to conventional electric heating which is externally applied to the reactor and heats all the substances in the reactor including the evolved pyrolysis volatiles and gases. In conventional heating, the pyrolysis volatiles, being in a larger reaction ‘hot zone’ than occurs during microwave heating, are likely to undergo increased secondary reactions (i.e. secondary thermal cracking) and this leads to a higher yield of pyrolysis gases and a lower yield of pyrolysis oil. Similar differences between conventional and microwave pyrolysis have also
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Fig. 4. Comparison of oil yield in waste oil pyrolysis processes heated with different media (waste oil only [6,8,13,44–46], coal [47,48], scrap tires [15]), and by different heating system (microwave heating, conventional electric heating).
been observed during the treatment of other types of waste [24– 26]. These results suggest that the pyrolysis of waste oil is influenced by the heating system (heating source, heating media) in addition to the other factors commonly reported in literature (e.g. feed composition and flow rate, catalyst, temperature, pressure). 3.2. Visual inspection of pyrolysis operation The conversion of waste oil to pyrolysis oil started to occur when the operating temperature was above 400 °C, where over 50 wt.% of the product was a viscous oil mixture. The maximum conversion of waste oil was accomplished at 550 °C, during which P67 wt.% of the waste oil was transformed into pyrolysis oil (Fig. 2). The pyrolysis oil obtained at 550 °C was a fairly pale yellowish-gold hydrocarbon liquid (Fig. 5) containing a small amount of dark solids. It is thought that these solids derived from the small quantities of very fine carbon particles originally present in the pyrolysis reactor; these are likely to escape from the reactor and co-migrate with the pyrolysis oil. These particles can be removed
Fig. 5. Pyrolysis oil from microwave pyrolysis of waste oil.
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by filtration. The pyrolysis oil was observed to be much less dense and viscous compared to the waste oil, indicating the cracking of heavy hydrocarbon chains in the waste oil to lighter fragments by the microwave-heated pyrolysis. 3.3. Chemical composition GC–MS analysis revealed that both virgin and waste oils are formed from a mixture of low and high molecular weight aliphatic and aromatic hydrocarbons (Table 1). The majority of the hydrocarbon compounds were in the C21–C45 range for virgin oil, but only C11–C40 were detected in waste oil (Table 1). This concurs with previous findings [5,7] that suggest the conversion of a fraction of heavier hydrocarbons (originally present in unused virgin oil) to lighter hydrocarbons while acting as lubricant in engine operation. Quantitative analyses of the pyrolysis oils were undertaken and the results are presented in Table 1. The compounds present were grouped into different classes of organic compounds, i.e. naphthenes (cycloalkanes), alkanes, alkenes, aromatics, and unknowns (unidentified peaks). It should be noted that analysis of some of the oil samples showed over one hundred peaks, however only the compounds present at 0.5 wt.% and above are presented in the results. The study showed that the waste oil, containing C11–C40 hydrocarbons, was thermally cracked to oil products comprising mainly
of C5–C30 hydrocarbons, and which were dominated by aliphatic hydrocarbons (50–71 wt.%) and significant amounts of aromatics (23–42 wt.%). This indicates the occurrence of cracking of compounds to produce some aromatic structures, possibly derived from cyclisation and aromatisation reactions that occurred during pyrolysis. The aliphatic hydrocarbons were mostly alkanes (31–47 wt.%), and alkanes from pentane to decane were present at the highest concentrations. Alkenes (9–23 wt.%), with carbon chain lengths ranging from C5 to C15, were also present, with heptene to decene being the most abundant. These aliphatic hydrocarbons, particularly the C5–C20 aliphatic fractions, represent a potentially high-value chemical feedstock or fuel source. The alkanes could be upgraded to produce transport-grade fuels, or gasified to commercially valuable gaseous products including hydrogen, whereas the alkenes are highly desired feedstocks in petrochemical industry, especially in plastic manufacture. The pyrolysis oils were found to contain many different lengths of aliphatic chains, showing that waste oil was randomly cracked into short fragments of different carbon chain lengths. The wide distribution of aliphatic chains in the pyrolysis oils suggested that the thermal cracking of waste oil in this process predominantly follows the free-radical-induced random scission mechanism [27– 30]. This mechanism may have led to the production of hydrocarbon radicals that were stabilised by capturing the hydrogen atoms from nearby molecules, producing alkanes and alkenes via the free radical and b-scission reactions. Heavy n-alkanes and alkenes were
Table 1 Main chemical components (wt.%) of the virgin oil (FO), the waste automotive engine oil (WO), and the pyrolysis oils produced under various conditions. FO
WO
Pyrolysis oil N2 purge rate (ml/min)a
WO feed rate (kg/h)a
100
250
750
0.4
1
2.5
5
91 0.5 0.5
31.4 10.2 8.6
39.6 7.6 17.7
47 1.4 22.6
39.6 7.6 17.7
40.4 6.5 18.2
42 5.7 18.8
43.9 3.9 20.1
91
92
50.2
64.9
71
64.9
65.1
66.5
67.9
–c – – 60 29 2
– 3 7 57 25 –
29 13.2 7 1 – –
32 15 13.3 4.6 – –
20.4 14.3 18.2 18.1 – –
32 15 13.3 4.6 – –
29.2 14.8 15.2 5.9 – –
24.4 14.9 17.9 9.3 – –
21.5 13.4 18.8 14.2 – –
Aromatics Benzene (C6H6) Toluene (C7H8 Xylene (C8H10) Alkylbenzenesd
– – – 0.5
– – – 1.1
11.8 11.9 9.4 8.4
6.6 7.8 7.1 8.2
2.1 2.3 3.6 14.6
6.6 7.8 7.1 8.2
4.5 5.6 6.2 12.8
4.1 4.7 5.4 13.1
3.5 3.4 4.7 13.9
Total
0.5
1.1
41.5
29.7
22.6
29.7
29.1
27.3
25.5
PAHs Naphthalene Acenaphthene Acenaphthylene Phenanthrene Anthracene Pyrene
0.03 – – 0.003 – 0.002
0.1 – – 0.01 – 0.02
0.41 0.09 0.08 0.07 0.16 0.09
0.23 0.03 0.04 0.04 0.11 0.05
0.13 0.02 0.02 – 0.03 –
0.23 0.03 0.04 0.04 0.11 0.05
0.17 0.02 0.02 0.03 0.06 –
0.10 0.02 0.02 0.01 0.05 –
0.12 0.02 0.02 0.01 0.03 –
Aliphatics Alkanes Naphthenes (Cycloalkanes) Alkenesb
89.7 0.5 0.8
Total Carbon components C5–C10 C11–C15 C16–C20 C21–C30 C31–C40 C41–C45
Total
0.035
0.13
0.9
0.5
0.2
0.5
0.3
0.2
0.2
Otherse
8.5
6.8
7.4
4.9
5.8
4.9
5.5
6.0
6.4
a Process conditions: pyrolysis at a constant WO feed rate of 0.4 kg/h was used when the effects of varying the N2 purge rate were studied, whereas a constant N2 purge rate of 250 ml/min was used to study the effects of varying the WO feed rate, and all experiments were performed at a constant temperature of 550 °C. b Alkenes: n-alkenes, dialkenes. c Not detectable. d Alkylbenzenes: ethylbenzene (C8H10), allylbenzene (C9H10), propylbenzene (C9H12), trimethylbenzene (C9H12), 1,3-Diethylbenzene (C10H14), 1-methyl-2-propylbenzene (C10H14), and hexyl-benzene (C12H18). e Unknown compounds due to unidentified peaks.
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then cracked to form lighter compounds. This accounts for the alkanes, naphthenes, and alkenes observed for each carbon number across all collected samples. Valuable light aromatics such as BTX (the sum of benzene, toluene, and xylene) were found in significant quantities (8–33 wt.%) in the pyrolysis oils. The aromatics were composed mainly of single ring alkyl aromatics, including benzene derivatives such as 1,3diethylbenzene, 1-methyl-2-propylbenzene, and hexyl-benzene, as well as benzene rings with short alkyl groups – mainly toluene, ethyl-benzene, and xylene. The subsistent chains attached to the benzene rings ranged from C1 to C6 groups and were mostly nonbranched saturated compounds. Polycyclic aromatic compounds (PAHs) were observed in the pyrolysis oil but only in minor quantities (60.9 wt.%). Several sulphur and/or nitrogen-containing compounds such as thioureas, amines, and benzothiazoles were also detected in the pyrolysis oils, but the quantities were low (60.1 wt.%) and were therefore not presented in the table. In this study, the decrease in purge and feed rates was thought to lead to an increased residence time of the pyrolysis volatiles in the reactor, resulting in an increase in aromatic content along with a decrease in aliphatic content in the pyrolysis oils, and the aliphatic and aromatic content improved towards smaller hydrocarbon chains. The amounts of alkanes and alkenes were reduced in favour of aromatic formation (including PAHs). This agrees with the findings in previous works [5,30,31], which propose that an increase in residence time may promote secondary reactions of hydrocarbons (e.g. cyclisation and aromatisation) in the pyrolysis volatiles to form aromatics; the production of aromatics by these secondary reactions have also been reported in pyrolysis studies of pure hydrocarbon compounds [32,33]. The aromatics are likely to be formed via Diels–Alder type secondary reactions, which involve dehydrogenation and cyclisation of alkenes (produced from pyrolysis cracking of waste oil) to form aromatic hydrocarbons. These secondary reactions are likely to occur in this microwaveheated pyrolysis in a manner similar to that reported in other pyrolysis processes [25,34], since n-alkenes and dialkenes (e.g. 1.3butadiene) were found in the pyrolysis oils (Table 1). Furthermore, the decreased concentrations of alkenes at low purge and feed rates (Table 1) suggest that it was the increased occurrence of these Diels–Alder type secondary reactions that had combined and converted the n-alkenes and dialkenes into aromatics, leading to increased formation of benzene, toluene, and alkylbenzenes. The main products formed by the Diels–Alder type reactions in pyrolysis cracking are reported to be benzene, toluene, and alkylbenzenes [27,33,34], and these compounds were detected in the pyrolysis oils obtained in this study, supporting the proposed reaction mechanism for the formation of aromatics. Depending on the degree of aromatisation, condensation reactions of the ring structure of the aromatic compounds may occur subsequently and result in
formation of heavier aromatics such as PAHs [25,34]; the detection of PAHs in the pyrolysis oils (Table 1) supports the proposed occurrence of condensation reactions for PAH formation. The yield of pyrolysis oil was found to increase with increasing waste oil feed rate, however, this reduced the yield of the desired fraction of light aliphatic hydrocarbons (C5–C20) in the pyrolysis oil. The pyrolysis oils formed under high feeding rate were found to contain higher amounts of heavier hydrocarbon components (>C20). At higher feed rates, the increased flow of pyrolysis volatiles out from the reactor led to decreased participation of pyrolysis volatiles (evolved from the waste oil compounds) in secondary reactions (e.g. secondary thermal cracking) and helps to explain the greater quantities of heavier hydrocarbon components in the pyrolysis oil. Moreover, it is thought that most of the heavier components were obtained as a result of evaporation of some components in the waste oil that escaped from the reactor without being cracked [9]. The main implication is that a high waste oil feed rate does not necessarily represent an ideal condition to produce valuable oil products with this microwave pyrolysis apparatus. The benefit of pyrolysing waste oil at high feed rates to increase the yield of total oil product is negated by the significant increase in the yield of undesirable heavier hydrocarbon components (>C20) in the oil product under these conditions. Overall, the results shows that the waste oil can be thermally cracked and condensed to pyrolysis oils comprising valuable light aliphatic and aromatic hydrocarbons, which could be treated and used as either an energy source or valuable chemical feedstock. 3.4. Elemental composition Table 2 shows the elemental composition of the waste oil before pyrolysis treatment and the pyrolysis oils obtained at different purge and feed rates. Carbon and hydrogen represented the main elements present in waste oil, whereas nitrogen, sulphur, and oxygen were detected in very low concentrations (62.1 wt.%). The carbon and hydrogen are mainly from the base oils from which the lubricating oil is formulated, whereas nitrogen, sulphur and oxygen are likely to originate from the additives (e.g. antioxidants) present in the engine oil [2]. The low content of sulphur in the waste oil suggests that sulphur originally present in engine oil is likely to have reacted with oxygen in the air to form sulphur oxides, which subsequently escape to the atmosphere during engine operation. The pyrolysis oils showed a much lower content of oxygen and a significantly higher H/O atomic ratio than the waste oil. The oils also showed a very low O/C atomic ratio, indicating the very low level of oxygenation in the oils. The use of a bed of carbon as the heating medium in our set-up, which also provides a reducing chemical environment at the operational temperatures, appears
Table 2 Elemental analysis (wt.%) of the virgin oil (FO), the waste automotive engine oil (WO), and the pyrolysis oils. FO
WO
Pyrolysis oil N2 purge rate (ml/min)a
C H N S O H/C (mol/mol) H/O (mol/mol) O/C (mol/mol)
82.7 12.5 0.1 2.3 2.3 1.8 86 0.02
81.7 15.1 0.1 0.8 2.1 2.2 114 0.02
WO feed rate (kg/h)a
100
250
750
0.4
1
2.5
5
88.6 11.2 0.1 0.02 0.1 1.5 1778 0.001
85.6 14.2 0.1 0.02 0.1 2.0 2254 0.001
85.7 13.7 0.1 0.03 0.5 1.9 435 0.004
85.6 14.2 0.1 0.02 0.1 2.0 2254 0.001
85.1 14.3 0.1 0.02 0.1 2.0 2270 0.001
84.9 14.5 0.1 0.03 0.2 2.0 1151 0.002
84.6 14.6 0.2 0.03 0.2 2.1 1159 0.002
a Process conditions: pyrolysis at a constant WO feed rate of 0.4 kg/h was used when the effects of varying the N2 purge rate were studied, whereas a constant N2 purge rate of 250 ml/min was used to study the effects of varying the WO feed rate, and all experiments were performed at a constant temperature of 550 °C.
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to decrease the formation of undesired oxidised species during the pyrolysis, and thus leading to the decreased oxygen content in the pyrolysis oil. This is desirable since oxidised species (e.g. acids and reactive peroxides) may catalyse undesired polymerisation reactions of unsaturated compounds in the pyrolysis oil during storage, generating larger molecules (e.g. tar or sludge from polymerisation of olefins) that have poor mutual solubility with other compounds in the oil, resulting in increased viscosity and low heating value of the oil. Any carbon that becomes oxidised as a result of redox reactions is of little concern as it will usually be converted to CO or CO2 which then leaves the system in the gas phase. On the other hand, it is likely that some metals in the waste oil may have reacted with the oxygen present in the additives in waste oil [2] to form metal oxides that are retained within the carbon bed during pyrolysis; the presence of metals in the waste oil has been reported in previous works [5]. The reduction in oxygen content is also likely to be due to decarboxylation commonly occurring during thermal treatment processes; this agrees with the findings of Sinag˘ et al. [6] in their waste oil pyrolysis study. The deoxygenation of oxygenated species (e.g. phenols originating from the antioxidant additives in the engine oil) to thermally stable aromatic compounds [34] is another likely source of the reduction of oxygen content. The low oxygen content in the pyrolysis oils represents a favourable feature in producing a potential fuel source with high calorific value. The H/C atomic ratio is a good indicator of the existence of hydrocarbons in the waste oil and the pyrolysis oils, and the variations in the ratio indicate the different levels of saturation in the CAC bonds. A decrease in H/C ratio was observed for pyrolysis oils compared to the waste oil, suggesting that dehydrogenation and aromatisation had occurred to some extent to form compounds
containing carbon double-bonds (e.g. alkenes, aromatics) with a lower H/C ratio; the result is consistent with the hydrocarbon composition and functional groups found by GC–MS and FTIR analysis (see Sections 3.3 and 3.5). The purge and feed rates seem to have had only a minor influence on the elemental composition of the pyrolysis oils; however, at lower purge rates, increased secondary reactions as a result of the longer residence times of the pyrolysis volatiles in the reactor are likely to increase the degree of random scission thermal cracking and aromatisation as indicated by the lower H/C ratio in the oils obtained under these conditions. The pyrolysis oils contain a lower sulphur content compared to the waste oil; this suggests that sulphur, although present in very low concentrations in waste oil, is likely to have reacted with oxygen during pyrolysis to form sulphur oxides. In addition, new sulphur compounds may be formed during pyrolysis, e.g. metal or non-volatile inorganic sulphides [15,35], which remain in the carbon bed; these reactions lead to decreased sulphur content in the pyrolysis-oils. In all cases the sulphur content of the pyrolysis oil obtained (20–30 ppm) was found to meet internationally prescribed standards for unleaded petrol (sulphur: 6150 ppm or 60.1 wt.%) and diesel fuels (sulphur: 6350 ppm or 60.1 wt.%), e.g. European Fuels Directive 98/70/EC and UK Motor Fuels Regulations 1999 [36], British Standard of Unleaded Petrol (BS EN 228:2008 [37]), and British Standard of Diesel (BS EN 590:2009+A1:2010 [38]). The use of fuel derived from these low-sulphur pyrolysis oils can potentially lead to a reduction of SOx emissions compared to fossil fuels (e.g. diesel). The results from elemental analysis show that the microwave-heated pyrolysis generated a pyrolysis oil with a low sulphur and oxygen content, further indicating the potential of this pyrolysis process in
Fig. 6. FTIR spectrum (above) and the functional groups (bottom) detected in pyrolysis oil.
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molecular species), which can lead to increased oil viscosity and causing problems such as filter plugging and system fouling. Overall, the trace amount of oxygenated compounds (e.g. aldehydes, carboxylic acids) in the pyrolysis oils indicates that very little oil oxidation occurred in this pyrolysis process; this corroborates the low oxygen contents found in the pyrolysis oil by elemental analysis (see Table 2), and the beneficial effects of the carbon bed (acting as a reducing reaction environment) in decreasing both the oil oxidation and the resulting formation of undesired oxidised species (see Section 3.4).
treating this kind of problematic waste oils. The low sulphur and oxygen content is beneficial to upgrading the pyrolysis oil to transport-grade fuels. 3.5. FTIR analysis Fig. 6 shows the infrared spectrum of the pyrolysis oil obtained by FTIR analysis and the general classification of chemical compounds from the FTIR spectrum; the classification was defined based on the degree of infrared absorption (or transmittance) detected at different frequencies (or wave number) over the infrared spectra obtained from the pyrolysis oils. The data presented show typical results for the pyrolysis oils produced by microwaveheated pyrolysis of waste oil as there was little difference in the FTIR spectra obtained with the different process parameters (N2 purge flow and waste oil feed rate), except for a small variation in the transmittance peak intensities detected at the different frequency ranges. The results indicate that most of the hydrocarbons found in the pyrolysis oils were alkanes, alkenes, and single-ring aromatics. Interestingly, compounds with OAH and C@O stretching vibrations, such as phenols, aldehydes, carboxylic acid, showed very low peak intensities, suggesting that these compounds were present only in minor quantities in the pyrolysis oil. The very low characteristic peaks of these compounds observed in the FTIR spectrum provide useful information when assessing the extent of oxidation that had occurred in the oils. Oil oxidation normally results in the sequential addition of oxygen to base oil molecules, causing the formation of oxygenated by-products with hydroxyl (OAH) and carbonyl groups (C@O), e.g. aldehydes and carboxylic acids. Carboxylic acids, in particular, are undesirable as they are the common cause of acidic corrosion and sludge formation (as a result of polymerisation in which carboxylic acids combine to form larger
3.6. Fuel properties The oil products derived from microwave-heated pyrolysis were examined for their properties as a fuel and these values were compared to those of the virgin oil, the waste oil, and gasoline collected from a local petrol station (Table 3). The waste oil shows a lower density but higher calorific value than the virgin oil. It is thought that some of the heavier hydrocarbons in virgin oil were decomposed into lighter hydrocarbons in waste oil. The lower calorific value of the virgin oil is likely due to the presence of carbon and long-chain carbon compounds of lower calorific value in the oil matrix. The densities and viscosities of the pyrolysis oils were found to be lower than those of the waste oil due to the cracking of heavy hydrocarbons to lighter compounds. The densities of the pyrolysis oils (757–773 kg/m3), except for that from the experiments conducted at a N2 purge rate of 750 ml/min, are quite close to that for gasoline, and it is also within the prescribed range of 720– 775 kg/m3 given in British Standard of Unleaded Petrol (BS EN 228:2008 [37]). The flash points of the pyrolysis oils were all found to be lower than that of diesel but higher than that of gasoline; similar findings have been reported by other workers [13,15,39].
Table 3 Fuel properties of the virgin oil (FO), the waste automotive engine oil (WO) and the pyrolysis oils. FO
WO
Pyrolysis oil N2 purge rate (ml/min)
Density, q (kg/m3) Flash point (°C) Kinematic viscosity at 40 °C (mm2/s) CV (MJ/kg)f Sulphur (wt.%) Oxygen (ppm) Carbon components
879 215 85 43.6 2.3 2.3 C21–C45
858 197 52 45.4 0.8 2.1 C11–C40
Solid content (wt.%) Energy density (%)l
<0.1 –
<3k –
Viscosity of pyrolysis oil over time (mm2/s)m a
b c d e f g h i j k l m
a
Gasoline
Desired liquid fuelb
767 40 0.7 46.1 0.1 0 C5–C10
720–850c <55d 1–4e 36–48g 60.1h 62.5i C5–C20j
<0.001 –
– –
a
WO feed rate (kg/h)
100
250
750
0.4
1
2.5
5
757 <19 6.6 44.3 0.02 0.1 C5–C30 (1 wt.% C21–C30) <0.001 100
770 <19 6.2 46.5 0.02 0.1 C5–C30 (5 wt.% C21–C30) <0.001 100
784 <19 6.1 45.7 0.03 0.5 C5–C30 (18 wt.% C21–C30) <0.001 100
770 <19 6.2 46.5 0.02 0.1 C5–C30 (5 wt.% C21–C30) <0.001 100
771 <18 6.3 46.5 0.02 0.1 C5–C30 (6 wt.% C21–C30) <0.001 99
773 <18 6.5 46.6 0.03 0.2 C5–C30 (9 wt.% C21–C30) <0.001 97
773 <18 6.9 46.6 0.03 0.2 C5–C30 (14 wt.% C21–C30) <0.001 93
3 months
6 months
9 months
6.4
6.5
6.5
Process conditions: Pyrolysis at a constant WO feed rate of 0.4 kg/h was used when the effects of varying the N2 purge rate were studied, whereas a constant N2 purge rate of 250 ml/min was used to study the effects of varying the WO feed rate, and all experiments were performed at a constant temperature of 550 °C. Exhibiting desirable properties comparable to commercial liquid fuels derived from fossil fuel (e.g. diesel, kerosene, gasoline). The density for gasoline: 720–775 kg/m3 [37,39,49], diesel: 820–860 kg/m3 [39]. The flash point for gasoline: 43 °C [13]; diesel:>55 °C [39,45]. The viscosity for unleaded gasoline: 0.7 mm2/s; diesel: 2–4 mm2/s [40]. CV – Calorific value, also regarded as higher heating value (HHV) in literature. The calorific value (MJ/kg) of fossil fuels reported in literature: 43–46 for diesel [6,39,46], 43–48 for gasoline [13,39], 36–47 for kerosene [42,50]. The sulphur content for motor fuels reported in internationally prescribed standards:60.1 wt.% [36–38]. The oxygen content for gasoline and diesel: 0 wt.% [51], reformulated gasoline (RFG): 2 wt.% [52]. The carbon components of fossil fuels: C5–C20 (see Table 4). Before filtration. Energy density (%) in the pyrolysis oil was calculated based on the density and calorific value of gasoline, i.e. qPO * CVPO * 100/qgasoline * CVgasoline; PO – pyrolysis oil. Data are presented for pyrolysis oil obtained with 1 kg/h of WO feed rate and 250 ml/min of N2 purge rate due to its closest gasoline-like composition and fuel properties.
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The low flash points suggest that the un-refined pyrolysis oils contain components that have a lower boiling point range than diesel. The pyrolysis oils possess slightly higher kinematic viscosities (6– 7 mm2/s) than diesel (2–4 mm2/s) [40], but are considerably higher than that of gasoline (0.7 mm2/s). Further treatment may be needed to reduce the viscosity of the pyrolysis oil to a value comparable to gasoline (<1 mm2/s) since a lower viscosity is desirable and represents a favourable feature when it comes to handling and transportation. Fig. 7 demonstrates that nearly 10% of the pyrolysis oil shows a higher boiling range than that of diesel. This indicates that the pyrolysis oil contains certain heavier hydrocarbon fractions (i.e. higher boiling point components) that those of diesel. These heavier fragments of long chain hydrocarbon components may have contributed to the observation that the pyrolysis oil has a higher viscosity than diesel. The pyrolysis oils contain minor quantities of particulates (<0.001 wt.%) present in the micrometer range (62 lm in size as determined by Scanning-Electron-Microscopy). The particulates are likely to derive from the very fine carbon particles originally present in the pyrolysis reactor and chars generated during the pyrolysis that escaped through the 0.45 lm ME25-filter fitted after
Fig. 7. Simulated distillation plot showing the boiling point distribution for pyrolysis oil obtained in relation to different N2 purge rate (ml/min) and waste oil feed rate (kg/h) (above) and compared to commercial liquid fuels (bottom).
the reactor. These particulates are likely to arise from agglomeration of these smaller particles, which can be removed by filtration. The calorific value of a fuel is another important fuel property that indicates the gross energy output of combustion in the engine chamber. The calorific value of the pyrolysis oils (46–47 MJ/kg), except for the experiments conducted at 100 ml/min of N2 purge rate, was higher than those of waste oil and close to those of traditional liquid fuels derived from fossil fuel (43–48 MJ/kg), indicating that pyrolysis oils with high combustion energy can be obtained by microwave-heated pyrolysis. In particular, the pyrolysis oil obtained with 1 kg/h of waste oil feed rate and 250 ml/min of N2 purge rate shows the closest calorific value to that of gasoline, representing an ideal process condition to obtain gasoline-like fuel. The decrease in oxygen content compared to the original waste oil (Table 2, see Section 3.4) is likely to contribute to the increase in the calorific value of the pyrolysis oils. In addition, a decrease in H/C ratio (Table 2) was observed for pyrolysis oils compared to the waste oil, suggesting that more carbon–carbon bonds were formed in the pyrolysis oil, thus producing more energy during combustion and showing a higher calorific value. The lower calorific value of the waste oil is likely due to the presence of carbon and long-chain carbon compounds of lower calorific value in the oil matrix. The volumetric energy density of the pyrolysis oils was found to be 93–100% of that of gasoline. Overall, the process parameters (N2 purge rate and waste oil feed rate) seemed to have little influence on the fuel properties of the pyrolysis oils, suggesting that the microwave-heated pyrolysis may be a convenient way of recycling high volume of waste oil into a useful oil product. Taking into account results from previous work [5], for the particular configuration of the microwave pyrolysis system used in our apparatus, a pyrolysis temperature of 550 °C coupled with 1 kg/h of waste oil feed rate and 250 ml/min of N2 purge rate provides the optimum balance between high process yield while producing an oil product with fuel properties comparable to gasoline, thereby reflecting its potential to generate a valuable liquid fuel. In addition, the pyrolysis oil produced from these process conditions was checked for variations in viscosity over a time period of 9 months, and showed negligible change over this period and thus the pyrolysis oils exhibit good stability characteristics (Table 3). 3.6.1. Boiling point distributions To further examine the fuel properties of the pyrolysis oils, ASTM D2887-08 and ASTM D3710-95(2009) standard simulated distillations were performed on the pyrolysis oils and some of the commercial liquid fuels derived from fossil fuel (i.e. diesel, kerosene, and gasoline); ASTM D3710-95(2009) was used specifically for the analysis of samples having a low boiling point range (i.e. gasoline). The boiling point ranges obtained for the various oils were then plotted against the cumulative volume fractions of the components of the oil, and these are presented in Fig. 7. Fig. 7 outlines the effect of N2 purge rate and waste oil feed rate on the boiling point distribution of the pyrolysis oils; the boiling point curves plotted show the simulated distillation curves of the various oils. The pyrolysis oils show a wide and much lower boiling point range than that of waste oil, indicating the high extent of thermal cracking incurred by the waste oil in microwave-heated pyrolysis (see Section 3.3). The wide boiling point range is desirable as vaporisation at a narrow boiling point band could cause immediate fuel burning in the engine combustion chamber, leading to high rates of pressure and temperature increase that could damage the engine components. The wide and gradually increasing boiling point curve shown by the pyrolysis oil promotes partial vaporisation and combustion of fuels which are important for optimal performance and safe operation in the engine [41]. There was very little difference between the oils produced with different N2
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purge rates and waste oil feed rates. However, at lower N2 purge and waste oil feed rates, the pyrolysis oils were found to have higher amounts of lighter fractions with low boiling points. Decreasing these two process parameters results in an increase in formation of light hydrocarbons, which are likely to derive from enhanced cracking of heavier hydrocarbons (i.e. higher boiling point components) into smaller molecules due to longer residence times incurred by the pyrolysis volatiles in the reactor. Fig. 7 also compares the boiling point distributions of hydrocarbons in the pyrolysis oil to those of commercial liquid fuels. Data are presented for pyrolysis oil obtained with 1 kg/h of waste oil feed rate and 250 ml/min of N2 purge rate as those conditions produce the highest yield of pyrolysis oils that are closely matched to commercial fuels, particularly gasoline (see Section 3.6). Nearly 80% of the pyrolysis oil was found to fall below the boiling point of diesel and kerosene. The recovered oil had a lower initial boiling point and contained lighter fractions than those of diesel; similar findings were reported by other researchers working on other studies of waste oil pyrolysis [6,15]. For comparison purposes, the boiling point range obtained for the pyrolysis oil was categorised into four petroleum fractions: gasoline, kerosene, diesel, and heavy oil (>C19); the petroleum fractions are defined based on the major carbon components and their corresponding boiling point ranges detected in the various oils. Table 4 compares the petroleum fractions in the pyrolysis oils produced from microwave-heated pyrolysis to those that are obtained by pyrolysis using conventional electric heating. The microwave-heated pyrolysis method generates an oil product comprising mainly of compounds that are similar to gasoline (70%) and some heavier components found in the kerosene (16%) and diesel (17%). In addition, the results also reveal that the waste oil, containing C11–C40 hydrocarbons, was thermally cracked to mainly C4–C19 hydrocarbons, which were then condensed as pyrolysis oil, in which nearly 96% of the compounds were within the C4–C19 range. It is likely that a significant portion of the pyrolysis oil was obtained through pyrolysis cracking rather than from the evaporation of the waste oil, since the pyrolysis oil contains only 4% of long chain hydrocarbon components (>C19) typical of the compounds in the original waste oil (Table 4). Overall, the pyrolysis oil demonstrates a boiling point range quite comparable to commercial liquid fuels, particularly gasoline, suggesting that the oils could potentially be further processed to transport-grade fuels. The pyrolysis oil produced from microwave-heated pyrolysis showed higher amounts of light fractions (C4–C19) compared with those obtained from the oils pyrolysed by conventional electric heating (Table 4). In addition, the examination of the hydrocarbon
Table 4 Comparison of petroleum fraction (%)a in the pyrolysis oil between pyrolysis driven by microwave heating and conventional electric-resistance heatingb. Type of waste oil pyrolysis
Microwave-heated pyrolysis (this study) Pyrolysis heated by electric furnace [6] Pyrolysis heated by electric oven [15]
Gasoline
Kerosene
Diesel
C4–C12 40– 190 °C
C11–C15 175– 246 °C
C15–C19 251– 342 °C
69
16
15
2.5–38 40
1 18
Heavy oil >C19 >342 °C 4
7–14
53
13
34
a The petroleum fractions are categorised based on the major carbon components and their corresponding boiling point ranges detected in the various oils. b The data from literature were estimated from the boiling point curve of waste oil obtained in their waste oil pyrolysis studies.
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composition of the oils (Table 1) revealed that waste oil was thermally cracked to mainly 6C20 hydrocarbons compared to the 6C35 hydrocarbons produced in conventional electric-heated pyrolysis [42]. These results suggest that cracking reactions are enhanced in microwave-heated pyrolysis. The use of microwave heating as a heat source shows improved efficiency in cracking the waste oil to a desirable lighter fraction, comprising components (e.g. C4–C12 hydrocarbons) within the range of commercial liquid fuels whilst also showing a significantly greater yield of pyrolysis oil (Fig 4, see Section 3.1). Possible explanations accounting for this difference include the use of the microwave-heated carbon bed in our set-up (in which the added waste oil becomes totally immersed, providing excellent heat transfer, and also acts as a reducing reaction environment), and the microwave heating process itself, which has been shown to produce different products from conventional heating when all other factors are held equal [10,24,43]. Mechanisms underlying this difference include possibilities such as different heat distributions. Microwave heating volume heats only the carbon, creating a localised reaction hot zone as opposed to electric heating which is externally applied and heats all substances in the reactor volume including the surrounding gas. Additionally, the creation of free elections on the surface of the carbon particles as a result of microwave-induction may influence the reaction pathway. The proposed mechanisms are currently under investigation to further verify the explanations postulated above. Pyrolysis using conventional electric heating [6,15] was found to contain a higher amount (34–53%) of heavy oil components (>C19), compared to only 4% observed in this study (Table 4). We postulate that a significant portion of the heavy oil fraction in pyrolysis oils is derived from distillation or evaporation that occurs during the heating of waste oil. These processes transfer hydrocarbons from both the uncracked and ‘less-cracked’ fractions of waste oil in the reactor to the condensation system and thus into the recovered pyrolysis oil. Evaporation has also been observed in several pyrolysis studies of waste oil [9,14]. 3.7. Energy recovery Table 5 shows estimates of the energy recovery in the pyrolytic products compared with the electrical energy consumption in the microwave pyrolysis process. These estimates provide a useful measure of the energy efficiency of the process. It should be mentioned that the calculations are limited by the following assumptions: 1. Electrical consumption is based on the electrical power input (7.5 kW) during the pyrolysis treatment, which is estimated to be approximately 1.5 times the nominal output of the 4 magnetrons (5 kW) for the sum of the periods when they are switched on during the application of waste oil. However, it should be noted that the 7.5 kW of electrical power input is an overestimate of the actual electrical consumption, considering the crude set-up of the prototype reactor and the fact that the actual amount of absorbed power is not measured in this application. Inclusion of a device in the set-up to log the forward and reflected power would optimise both the absorbed power and the control over the process, increasing the viability to scale up the process. 2. Heat losses from the prototype reactor are substantial and would not be representative of the losses that would occur at pilot or industrial scale. No attempt has been made to recover energy during the cooling of the products in the condensation system nor to fully insulate the reaction vessel and associated fittings. 3. Pilot or industrial scale operation would involve the continuous addition of waste oil without the need to repetitively heat the
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Table 5 Energy recovery and consumption in microwave-heated pyrolysis of waste automotive engine oil (WO). Process conditions WO feed rate 0.4 kg/h 1.0 kg/h 2.5 kg/h 5.0 kg/h
N2 purge rateh 100 ml/min 250 ml/min 750 ml/min a b
c d
e f g h
EWOa (kJ/h)
EPOb (kJ/h)
Erecoveryc (%)
Epyrolysisd (kJ/h)
Epyrolysise/EWO (%)
Eratiof
Ebalanceg (kJ/h)
18,160 45,400 113,500 227,000
15,624 39,525 100,190 205,040
86 87 88 90
6480 12,960 20,250 25,650
36 29 18 11
2 3 5 8
9144 26,565 79,940 179,390
18,160 18,160 18,160
12,758 15,624 12,248
70 86 67
4320 6480 7560
24 36 42
3 2 2
8438 9144 4688
h
Estimated calorific value of the waste oil based on the CV and the different feeding rates of the WO (Table 4), i.e. CV of WO * WO feed rate. Estimated calorific value of the pyrolysis oil (PO) based on the different CVs (Table 3) and wt.% yields (Fig. 2) of the pyrolysis oil, and the different feeding rates of the WO, i.e. CV of pyrolysis oil * wt.% yield of pyrolysis oil * WO feed rate/100. Energy recovery (%) in the pyrolysis oil was calculated based on the energy recovered from the waste oil, i.e. EPO * 100/EWO. Electrical energy consumed during the pyrolysis treatment of the added WO sample (estimated during the time the WO sample started to be/was added to the reactor until feeding was stopped after 2 h of operation); electrical power input – 7.5 kW, i.e. 1.5 * 5 kW of nominal output of 4 magnetrons. Amount of energy (from EWO) consumed by Epyrolysis. Energy ratio, defined as the energy content of the pyrolysis oil divided by the electrical energy input needed to operate the system, i.e. EPO/Epyrolysis. Energy balance, defined as the energy content of the pyrolysis oil minus the electrical energy input needed to operate the system, i.e. EPO Epyrolysis. Process conditions: Pyrolysis at a constant WO feed rate of 0.4 kg/h was used when the effects of varying the N2 purge rate were studied, whereas a constant N2 purge rate of 250 ml/min was used to study the effects of varying the WO feed rate, and all experiments were performed at a constant temperature of 550 °C.
apparatus from room temperature. Calculations that include the electrical energy used to heat the reactor to the desired process temperature will seriously overestimate the actual electrical consumption during pilot or industrial scale, hence, this energy expenditure is excluded from the energy calculations shown in Table 5. 4. The calorific value of the non-condensable pyrolysis products is ignored. The pyrolysis oils showed significantly high recovery of the energy content of the waste oil, ranging between 67% and 90%. The electrical energy (Epyrolysis) supplied to power the microwaveheated pyrolysis process ranged from 11% to 42% of the calorific value of the waste oil pyrolysed (Epyrolysis/EWO) over the range of purge and feed rates considered. The results also show that more electrical energy (Epyrolysis) was needed to pyrolyze the waste oil at higher feed rates, as a direct result of the need for more energy to heat higher quantities of waste oil to the operational temperature and to supply the endothermic enthalpy to drive the higher amount of pyrolysis reactions. The higher relative electrical energy usage (Epyrolysis/EWO) observed at lower waste oil feed rates indicates that a the process is less energy efficient when pyrolysis is conducted at lower feed rates, probably as a result of a greater influence of heat loss from the reactor under these conditions. On the other hand, the higher energy consumption observed at higher N2 purge rates suggested that additional electrical energy was needed to overcome the cooling effects that resulted from the higher N2 purge rates. Even given the limitations involved in estimating energy consumption in the laboratory scale equipment described above, it is clear that the process is capable of recovering a hydrocarbon product (pyrolysis oil) whose calorific value is many times greater than the amount of electrical energy used in the operation of the process, as indicated by energy ratios ranging between 2 and 8 (Table 5). This favourable situation would be even more apparent during the operation of pilot or industrial scale equipment in which attempts to improve heat integration and recovery have been implemented. These results suggest that a reactor heated by a magnetron system with a total electrical power input of 7.5 kW is capable of processing waste oil at a flow rate of 5 kg/h, producing liquid pyrolysis products with an energy content equivalent to approximately 60 kW. The calorific value of the non-condensable pyrolysis products has been ignored in this
assessment. Inclusion of the energy content of this stream would further increase the amount of energy that can be recovered from waste oil. This gaseous stream could be used for on-site generation of electrical energy to power the magnetron system. However, it should be noted that the high energy recovery ratios observed in this study involve the assumption that the only energy input of the process is the electrical energy used in the pyrolysis reactor. In practice lower energy ratios should be considered in which higher energy inputs should be taken into account, including the energy needed for the collection and transport of waste oil to the processing plant, and for the refining of the pyrolysis oils if they are needed to be further processed to produce a commercial gasoline fuel (e.g. the conversion of both the aromatic and the heavy hydrocarbon components into gasoline type components). Overall, the results show that the microwave-heated pyrolysis method may be a viable means of recycling the waste oil into a useful oil product, in addition to a disposal method for the waste oil. 4. Conclusion N2 purge rate and waste oil feed rate, in addition to the use of a microwave heated bed of particulate carbon, were found to have effects on the fraction of original waste oil converted to pyrolysis gases, pyrolysis oils, and char residues. These process parameters also influenced the concentrations and molecular nature of the different hydrocarbons formed in the pyrolysis oils. These results, in combination with results from previous work [5], demonstrate that microwave-heated pyrolysis generated an 88 wt.% yield of gasoline-like oil product that contains potentially valuable light aliphatic and aromatic hydrocarbons. The oil product is also relatively contaminant free with low levels of sulphur, oxygen, and toxic PAH compounds, and is almost entirely free of metals as reported in previous work [5], thereby reflecting its potential as a green energy source or chemical feedstock. The pyrolysis oil could potentially be treated and upgraded to transport grade fuels, or added to petroleum refinery as a chemical feedstock for further processing, although further studies are needed to confirm these possibilities. The microwave-heated pyrolysis can be performed in a continuous operation to treat high volumes of waste oil while showing both a positive energy ratio and a high net energy output. It is clear that microwave-heated pyrolysis offers an exciting green approach to the treatment and recycling of automotive lubricating
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