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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 Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, United Kingdomb BP Institute, Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, United Kingdomc 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 2011Received in revised form 11 July 2011Accepted 13 July 2011Available online 28 July 2011

Keywords:

PyrolysisMicrowave pyrolysisWaste oilPyrolysis oil

Fuel

a b s t r a c t

The pyrolysis of waste automotive engine oil was investigated using microwave energy as the heatsource, and the yield and characteristics of the pyrolysis oils (i.e. elemental analysis, hydrocarbon com-position, and potential fuel properties) are presented and discussed. The microwave-heated pyrolysisgenerated 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 compo-sition of the oils showed the formation of light aliphatic and aromatic hydrocarbons that could also beused as a chemical feedstock. The oil product showed significantly high recovery(90%) of the energy pres-ent in the waste oil, and is also relatively contaminant free with low levels of sulphur, oxygen, and toxicPAH compounds. The high yield of pyrolysis oil can be attributed to the unique heating mode and chem-ical 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 dem-onstrating that feed injection rate, flow rate of purge-gas, and heating source influence the concentrationand the molecular nature of the different hydrocarbons formed in the pyrolysis oils. The microwave-heated pyrolysis can be performed in a continuous operation, and the apparatus described which is fittedwith magnetrons capable of delivering 5 kW of microwave power is capable of treating waste oil at a feedrate of 5 kg/h with a positive energy ratio of 8 (energy content of hydrocarbon products/electrical energysupplied for microwave heating) and a net energy output of 179,390 kJ/h. Our results indicate that micro-wave-heated pyrolysis shows exceptional promise as a means for recycling and treating problematicwaste oil.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

The production of waste automotive engine oil is estimated at24 million tons each year throughout the world, posing a signifi-cant treatment and disposal problem for modern society. Thewaste oil, containing a mixture of aliphatic and aromatic hydrocar-bons, also represents a potential source of high-value fuel andchemical feedstock. The preferred disposal options in most coun-tries are incineration and combustion for energy recovery, and vac-uum distillation and hydro-treatment for re-refining the waste oil[1]. However, these disposal routes recover only the chemical valueof the waste and they are becoming increasingly impracticable as

concerns over environmental pollution, and the difficulties andadditional costs of sludge disposal[2,3]are recognised due to theundesirable contaminants present in waste oil [1].

Pyrolysis techniques have recently shown great promise as aneconomic and environmentally friendly disposal method for wasteoil [46] the waste material is thermally cracked and decom-posed in an inert atmosphere, with the resulting pyrolysis oilsand gases able to be used as a fuel or chemical feedstock, andthe char produced used as a substitute for activated carbon, thoughsuch practice is yet to become popular. The pyrolysis oil producedis of particular interest due to its easy storage and transportationas a liquid fuel or chemical feedstock. The oil can be catalyticallyupgraded to transport-grade fuels, or added to petroleum refineryfeedstocks for further processing[7]. Most of the literature reportsfocus on pyrolysis using conventional electric resistance and elec-tric arc heating [4,6,8,9]. However, microwave-heated pyrolysishas recently shown promise as a route to treating and recycling

of waste oil [7]; the advantages of microwave-heated pyrolysis

0016-2361/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.fuel.2011.07.027

Corresponding author at: Department of Chemical Engineering and Biotech-nology, University of Cambridge, New Museums Site, Pembroke Street, CambridgeCB2 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).

Fuel 92 (2012) 327339

Contents lists available at ScienceDirect

Fuel

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l


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have been elaborated in previous work[5]and will not be dupli-cated here. In this process, waste oil is mixed with highly micro-wave-absorbent material such as particulate carbon. As a resultof microwave heating, the oil is thermally cracked in the absenceof oxygen into shorter hydrocarbon chains. The resulting volatile

products are subsequently recondensed into pyrolysis oils of dif-ferent composition depending on the reaction conditions. The useof microwave radiation as a heat source is known to offer addi-tional advantages over traditional thermal heat sources [5,10],and the combination of carbon-based material and microwaveheating in pyrolysis processes is a fairly novel subject that is ofincreasing interest as reflected by the increasing amount of re-search reported in the literature [7,11,12]. Microwave radiationprovides a rapid and energy-efficient heating process comparedto conventional technologies. The diffuse nature of the electromag-netic field allows microwave heating to evenly heat many sub-stances in bulk, thereby offering an improved uniformity of heatdistribution, excellent heat transfer, and providing better controlover the heating process. Other advantages include higher power

densities and the ability to reach high temperatures at faster heat-ing rates, facilitating increased production speeds and decreasedproduction costs. Furthermore, energy is targeted only to micro-wave receptive materials and not to air or containers within theheating chamber. It can promote or accelerate certain chemicalreactions by selectively heating the reactants, leading to a moreuniform temperature profile and improved yield of desirable prod-ucts. The process is physically gentle, allowing for a wide variety ofapplications in diverse fields.

This study investigates the influence of process parameters(feed injection rate, purge gas flow, and heating source) on theyield and characteristics of the pyrolysis oils produced from micro-wave-heated pyrolysis of the waste oil, with a focus on their ele-mental and hydrocarbon composition, and potential fuelproperties. These evaluations are important to assess the technicalfeasibility and applicability of the pyrolysis process as a route toenergy recovery/feedstock recycling from waste oil. There havebeen no reports on the composition of the oil products resultingfrom microwave-heated pyrolysis of waste oil, and limited infor-mation is available concerning the influence of key process param-eters on the pyrolysis of waste oil, although a few studies havebeen performed on electric resistance heated pyrolysis of wasteoil[6,1315].

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 enginesrun on unleaded fuel. The hydrocarbon composition of the wasteoil, the particulate carbon (TIMREX FC250 Coke, TIMCAL Ltd., Bod-io, Switzerland) used as microwave absorbent to heat the waste oil,and their pre-experimental treatment have been reported in previ-ous works[5,7]. Before pyrolysis, the waste oil samples were fil-tered such that the size of any remaining particulates (i.e. metalparticles, carbon soots, and other impurities) were less than100lm; samples were examined for C, H, N, S, O content by ele-mental analysis; calorific value was determined by bombcalorimetry.

2.2. Experimental details

The experimental apparatus for this investigation has been de-scribed in detail in previous work [5]. The only change from thisdescription is the addition of a mixed-cellulose-ester membranefilter (0.45 lm ME25 filter, Schlecher & Schuell, Germany) (see(5) inFig. 1) to remove any metallic solid residues present in thepyrolysis volatiles before they passed through the condensationsystem. The refined experimental apparatus is shown inFig. 1.

Microwave-heated pyrolysis of waste oil was performed in abell-shaped quartz reactor (180 180 180 mm) (2) filled with1 kg of particulate carbon, which is stirred (3) and heated by a5 kWmicrowave oven (1) at a heating rate of about 60 C/min overa range of pyrolysis temperatures (250700 C), feed injectionrates (0.45 kg/h) and purge gas flows (0.10.75 L/min) to under-stand the influence of these process parameters on the final pyro-

lysis oils obtained; N2purge-gas was vented through the system tomaintain the apparatus in an inert nitrogen atmosphere, and theparticulate carbon, added initially into the reactor in one batch,was stirred to ensure a uniform temperature distribution through-out the reactor and to maximise heat transfer during pyrolysis. Thewaste oil is nearly transparent to microwaves due to its non-polar-ity nature, therefore it requires heating by contact with microwaveabsorbent materials (e.g. carbon-based materials such as particu-late 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) ateach target feeding rate over a period of 2 h as soon as the targetpyrolysis temperature was achieved; it was previously ascertainedthat the magnetron system of the microwave oven was able to gen-erate 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 ofhydrocarbon gases, liquids, and suspended solids existing in a va-pour phase), leave the reactor and pass through a condensationsystem (6, 7, 8, 9), and either condense into pyrolysis oil (10, 11)or are sampled as incondensable pyrolysis gases (13, 14) beforebeing vented from the system.

The amount of residue material not converted to gaseous or li-quid products was determined by measurement of the weightchange in the reactor and its contents before and after the reaction.The residue materials are likely to be chars produced from tertiarycracking reactions of the pyrolysis process [7]; these were particlesthat possessed a harder texture, darker colour, and most of whichhad a smaller size than the particulate carbon used in the bed of

the reactor. These chars, which mostly accumulated on the surfaceof the carbon bed, were separated from the carbon particles usingsieves (90 and 250lm) and mixed with the metallic residuesdeposited on the ME25-filter installed between the reactor andthe condensation system for later analysis. The yield of pyrolysisoil was determined by measuring the weight increase in the col-lecting vessels and filter. The pyrolysis oil was then transferredinto glass bottles until further analysis (see Section2.4). The gasyield was determined by mass balance and assumes that whatevermass of added sample that is not accounted for by the residue andpyrolysis oil measurements left the system in gaseous form. All thepyrolysis experiments were repeated several times and the data re-corded is the average of the results obtained from three valid re-peated runs performed under identical conditions. These runsshowed good reproducibility and precision with low standarddeviations shown in the product yield (15 wt.%). The virgin oil(FO), the waste oil, and the pyrolysis oils were examined for hydro-carbon composition by gas chromatography coupled with a massselective spectrometry detector (GCMS) and a flame ionisationdetector (GC-FID). These oil samples were also analysed by FourierTransform Infrared Spectroscopy (FTIR) and elemental analyzer toidentify 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 dis-tribution) were determined according to ASTM standard methods(see Section2.4).

2.3. Temperature measurement

The temperature of the carbon bed in the system was moni-tored using two thermocouples; one ducted into the middle layerof the carbon bed through the centre of the shaft that protrudesfrom the bottom of the stainless steel stirrer shaft, the other entersthe reaction chamber through a side port on the top of the reactorand is positioned at the top of the carbon bed. Both thermocouplesremain in direct contact with the carbon inside the reactor. In addi-tion, ferrite core thermocouple connectors and cable clamps wereused to reduce the electromagnetic interference caused by themicrowaves on the temperature measurement.

Accurate measurement of the evolution of the temperature ofthe carbon bed was difficult during the heating process firstly,there are inherent difficulties involved in measuring this parame-ter in microwave devices [16]; secondly, it should be noted that

the temperature is not uniform throughout the carbon bed duringthe initial heating to the target temperature; electrical arcing wasfound to occur for a relatively short period at the beginning of theheating process, but it stopped when the carbon bed had been

heated to the target temperature. A stirred bed reactor is used inthis study in which the physical movement and mixing of carbonparticles by the stirring system creates a uniform temperature dis-tribution, independent of the penetration depth of the microwavesinto the bed of particulate carbon. Provided the temperature is

kept consistent and uniform in this system, once the thermal equil-ibration and steady state temperature were reached, the tempera-ture shown by the thermocouples are assumed to give a reliablereading of the average temperature of the bulk carbon bed.

2.4. Analytical methods

Oil samples were analysed using a 6890/5973 GCMS instru-ment (Agilent Technologies, Palo Alto, CA), allowing the quantifica-tion of compounds by both species and size; compounds wereidentified using the NIST 2005 mass spectral library using similar-ity indices of >70%, or by comparison with published GCMS datafor similar products; the detailed description of this analyticalmethod have been reported in previous work [7] For polycyclic

aromatic hydrocarbons (PAHs), the GCMS instrument was pro-grammed into selected-ion-monitoring (SIM) mode and the peakswere compared to external PAH standards for quantification. Inaddition to GCMS analysis, the oils were analysed by the 6890gas chromatograph coupled with a flame ionisation detector, usinga 30 m HP-5 capillary column (5% phenyl methyl silicone, I.D.0.53 mm, film thickness 5 lm). The GC-FID oven was programmedfrom 40 C, held for 5 min, then ramped at 5 C/min to 280 C witha final holding time of 30 min. Quantification of compounds on theGC-FID was obtained by external standard method and relativeretention times once the component has been identified by theGCMS analysis. The data obtained from GC-FID were used to eval-uate the simulated distillation curves of the various oil samplesaccording to ASTM D2887-08[17].

Elemental analyses of oil samples were performed using a LECOCHNS-932 elemental analyzer (LECO Corporation, Michigan, USA).Samples were burned at 1000 C in a flowing stream of oxygen. Theproducts of combustion (CO2, H2O, N2 and SO2) then passedthrough the system with He as the carrier gas, and their contentof carbon, hydrogen, and sulphur were measured quantitativelyby selective IR absorption detectors, except for the nitrogen, whichwas measured by a thermal conductivity detector. Oxygen contentwas measured by pyrolysing a separate sample at 1300 C in a VTF-900 pyrolysis furnace (LECO Corporation, Michigan, USA). The oxy-gen released in the pyrolysis reaction then reacts with activatedcharcoal to form CO, which was converted to CO2 by passingthrough an oxidation tube with He as the carrier gas. The CO2gen-erated was then measured as above by an IR detector. The analysisof chemical functional group in oil samples was performed using a

Bruker Tensor 27 FTIR Spectrometer (Bruker Optics, Ettlingen, Ger-many) that produces IR spectra for each sample.

Experimental analysis of the fuel properties of the oil sampleswas 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 ofhydrocarbons in oil samples were determined using the 6890GC-FID instrument by a simulated distillation method accordingto ASTM D2887-08[17]and ASTM D3710-95(2009)[21]; the sam-ple was heated from ambient temperature to 550 C (300 C forASTM D3710-95) at a heating rate of 10 C/min with high purityHe gas vented through the apparatus at a flow rate of 0.1 L/min.Oil samples were also analysed for calorific value using a Parr6200 Isoperibol bomb calorimeter (Parr Instrument, Moline, Illi-

nois). Detailed analysis and sample preparation were performedaccording to ASTM Standard D4809-09a. [22] The solid content(ethanol-insoluble) was determined by filtering the pyrolysis oilthrough a 0.1-lm polycarbonate membrane filter (Milipore Co.,

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Billerica, MA) and measurement of the weight change before andafter filtration.

3. Results and discussion

In this study, different process parameters were studied todetermine the optimum conditions that provide the greatest yieldof pyrolysis products containing highly desired compounds (e.g.commercially valuable light hydrocarbons), and to investigate theeffects of these parameters on the compositions of pyrolysis prod-ucts, with a special emphasis on the pyrolysis oil. The main objec-tive was to convert the waste oil to petrochemical productssuitable for use as a fuel or raw chemical feedstock. This paper ex-plored the effects of varying the feed injection rate of waste oil, andthe purge gas flow rate. The investigation of pyrolysis products atdifferent temperatures have been reported in our previous works[5,7]. Pyrolysis at a constant temperature of 550 C was used asthroughout based on the optimum temperature obtained in theprevious work[7]that showed the greatest yield of valuable light

hydrocarbons and only low levels of residual metals in the pyroly-sis oil.

3.1. Product yield

Fig. 2outlines the effect of N2purge rate and waste oil feed rateon the fraction of waste oil converted to pyrolysis gases, pyrolysisoils, and char residues. Increasing the N2 purge rate from 100 to250 ml/min resulted in an increased yield of pyrolysis oil. The in-crease in waste oil feed rate demonstrated a minor influence onthe yield of pyrolysis oil, as the total increase in the yield of pyro-lysis oil resulting from the increase in waste oil feed rate from 0.4to 5 kg/h was only 4%. The slight upward trend in yield could bedue 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 theincrease in waste oil feed rate. When the hydrocarbons in waste oilundergo cracking into shorter molecules, which then vaporise aspyrolysis volatiles, the volume increase accompanying the phasechange from liquid to vapour creates an increase of pressure inthe reactor that drives the pyrolysis volatiles out of the reactor intothe product collection system. At higher waste oil feed rates, thisprocess is likely to occur more rapidly and more molecules arelikely to enter the vapour phase earlier, causing a more rapid flowof pyrolysis volatiles out of the reaction hot zone inside the reac-tor. It is also believed that the higher rates of N2purge gas resultedin a more rapid flow of pyrolysis volatiles out of the reactor due tothe higher pressure created in the reactor. The residence time of

pyrolysis products in the reactor thus becomes dependent on thesetwo process parameters, which leads to shorter residence times ofthe pyrolysis volatiles in the reactor at higher purge and feed rates.The decrease in residence time decreases the exposure of pyrolysisvolatiles (evolved from the waste oil compounds) to secondaryreactions (e.g. secondary thermal cracking to form pyrolysis gases,tertiary cracking and repolymerisation to form chars) and leads tohigher yield of pyrolysis oil and smaller yields of both pyrolysisgases and residues (likely to be char) observed under these condi-tions; similar findings were reported by other workers [23]. Atveryhigh N2 purge rates (750 ml/min), the yield of pyrolysis oil de-creases, suggesting that the installed condensation system is un-able to condense the pyrolysis volatiles at this considerablyfaster vapour flow rate, which leads to a higher corresponding

yield of pyrolysis gases. The slightly higher amounts of residuesobserved at higher feed rates (P2.5 kg/h) could be attributed tothe formation of residues such as char and residual unpyrolysedwaste oil, the amounts of which are likely to increase at higher feed

rates. The results show that extended heating of the generatedpyrolysis volatiles in the reactor could promote different productcompositions due to secondary reactions of the primary pyrolysisproduct; hence it was observed that some waste oil is consumed

in the production of pyrolysis gases and char in addition to pyroly-sis oil.

The waste oil feed rate, when compared with the mass of themicrowave absorbent (in this case particulate carbon), providesuseful information for chemical reactor design by assessing theweight hourly space velocity (WHSV) of the pyrolysis process.The WHSV, defined as the waste oil feed rate divided by the massof particulate carbon, was calculated based on the different wasteoil feed rates. The variation of the yield of pyrolysis oil with thisparameter is presented inFig. 3. The results show that the pyroly-sis reactor is able to process each hour a waste oil feed up to fivetimes the mass of the particulate carbon while maintaining a rela-tively high production of pyrolysis oil from the feed (8588 wt.%).The use of a microwave-heated bed of particulate carbon in this

set-up showed good heat transfer and cracking capacity to pyro-lyse the waste oil. The microwave energy is targeted to and heatsmainly the microwave-receptive particulate carbon which in turntransfers thermal energy to the waste oil added into the carbon

Fig. 2. Product yields (wt.%) as a function of N2purge rate (up) and waste oil feedrate (bottom). Process conditions: pyrolysis at a constant waste oil feed rate of0.4 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 ofvarying the waste oil feed rate, and all experiments were performed at a constanttemperature of 550 C.

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bed in order for pyrolysis to occur [5]. Overall, the waste oil feedrate was found to have little influence on the yields of pyrolysisproduct (Fig. 2), indicating the potential of this pyrolysis processin treating high volumes of waste oil while maintaining a relativelyhigh throughput of pyrolysis oil (Fig. 3).

The highest yield of pyrolysis oil (88 wt.%) was observed at250 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 be-tween sufficiently high purge and feed rates to sweep the evolvedpyrolysis volatiles out of the reactor, thus producing condensablevapours (pyrolysis oil) by avoiding the promotion of secondaryreactions that would result in increased formation of incondens-able gases and char residues, while not being so high that the con-densation system was unable to condense the pyrolysis volatiles. Itshould be noted that 5 kg/h is the highest waste oil feed rate thatcan be processed in this experimental device as it was ascertainedthat the 5 kW magnetron system of the microwave oven could nolonger generate sufficient heat to maintain the desired operatingtemperature (550 C) at flow rates of 6 kg/h and above. Our resultshave shown that a recovery of pyrolysis oil of approximately 8488% of the initial mass of waste oil is possible under theseconditions.

Fig. 4compares our results to those of waste oil pyrolysis pro-cesses heated by conventional electric heating either using wasteoil on its own or in the additional presence of coal or scrap tyres.The use of the microwave-heated bed of particulate carbon, com-pared to the other methods of operation, seemed to have a benefi-cial effect in cracking the waste oil to produce higher amounts ofcondensable products. This may be attributed to the different heatdistributions present during microwave-heated pyrolysis. The ap-plied microwave radiation heats mainly the carbon, creating alocalised reaction hot zone as opposed to conventional electricheating which is externally applied to the reactor and heats allthe substances in the reactor including the evolved pyrolysis vola-tiles 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 ofpyrolysis gases and a lower yield of pyrolysis oil. Similar differ-ences between conventional and microwave pyrolysis have also

been observed during the treatment of other types of waste[2426]. These results suggest that the pyrolysis of waste oil is influ-enced by the heating system (heating source, heating media) inaddition 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 occurwhen the operating temperature was above 400 C, where over50 wt.% of the product was a viscous oil mixture. The maximumconversion of waste oil was accomplished at 550 C, during whichP67 wt.% of the waste oil was transformed into pyrolysis oil(Fig. 2). The pyrolysis oil obtained at 550 C was a fairly pale yel-lowish-gold hydrocarbon liquid (Fig. 5) containing a small amountof dark solids. It is thought that these solids derived from the smallquantities of very fine carbon particles originally present in thepyrolysis reactor; these are likely to escape from the reactor andco-migrate with the pyrolysis oil. These particles can be removed

Fig. 3. The yields (wt.%) of pyrolysis oil in relation to weight hourly space velocity(h1).

Fig. 4. Comparison of oil yield in wasteoil pyrolysis processes heated with differentmedia (waste oil only [6,8,13,4446], coal[47,48], scrap tires[15]), and by differentheating system (microwave heating, conventional electric heating).

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 denseand viscous compared to the waste oil, indicating the cracking ofheavy hydrocarbon chains in the waste oil to lighter fragmentsby the microwave-heated pyrolysis.

3.3. Chemical composition

GCMS analysis revealed that both virgin and waste oils areformed from a mixture of low and high molecular weight aliphaticand aromatic hydrocarbons (Table 1). The majority of the hydro-carbon compounds were in the C21C45 range for virgin oil, butonly C11C40 were detected in waste oil (Table 1). This concurswith previous findings[5,7]that suggest the conversion of a frac-tion of heavier hydrocarbons (originally present in unused virginoil) to lighter hydrocarbons while acting as lubricant in engineoperation.

Quantitative analyses of the pyrolysis oils were undertaken andthe results are presented inTable 1. The compounds present weregrouped into different classes of organic compounds, i.e. naphth-

enes (cycloalkanes), alkanes, alkenes, aromatics, and unknowns(unidentified peaks). It should be noted that analysis of some ofthe oil samples showed over one hundred peaks, however onlythe compounds present at 0.5 wt.% and above are presented inthe results.

The study showed that the waste oil, containing C11C40hydro-carbons, was thermally cracked to oil products comprising mainly

of C5C30 hydrocarbons, and which were dominated by aliphatichydrocarbons (5071 wt.%) and significant amounts of aromatics(2342 wt.%). This indicates the occurrence of cracking of com-pounds to produce some aromatic structures, possibly derivedfrom cyclisation and aromatisation reactions that occurred during

pyrolysis. The aliphatic hydrocarbons were mostly alkanes (3147wt.%), and alkanes from pentane to decane were present at thehighest concentrations. Alkenes (923 wt.%), with carbon chainlengths ranging from C5 to C15, were also present, with hepteneto decene being the most abundant. These aliphatic hydrocarbons,particularly the C5C20 aliphatic fractions, represent a potentiallyhigh-value chemical feedstock or fuel source. The alkanes couldbe upgraded to produce transport-grade fuels, or gasified to com-mercially valuable gaseous products including hydrogen, whereasthe alkenes are highly desired feedstocks in petrochemical indus-try, especially in plastic manufacture.

The pyrolysis oils were found to contain many different lengthsof aliphatic chains, showing that waste oil was randomly crackedinto short fragments of different carbon chain lengths. The wide

distribution of aliphatic chains in the pyrolysis oils suggested thatthe thermal cracking of waste oil in this process predominantly fol-lows the free-radical-induced random scission mechanism [2730]. This mechanism may have led to the production of hydrocar-bon radicals that were stabilised by capturing the hydrogen atomsfrom nearby molecules, producing alkanes and alkenes via the freeradical 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

N2purge rate (ml/min)a WO feed rate (kg/h)a

100 250 750 0.4 1 2.5 5

Aliphatics

Alkanes 89.7 91 31.4 39.6 47 39.6 40.4 42 43.9Naphthenes (Cycloalkanes) 0.5 0.5 10.2 7.6 1.4 7.6 6.5 5.7 3.9Alkenesb 0.8 0.5 8.6 17.7 22.6 17.7 18.2 18.8 20.1

Total 91 92 50.2 64.9 71 64.9 65.1 66.5 67.9

Carbon components

C5C10 c 29 32 20.4 32 29.2 24.4 21.5

C11C15 3 13.2 15 14.3 15 14.8 14.9 13.4C16C20 7 7 13.3 18.2 13.3 15.2 17.9 18.8C21C30 60 57 1 4.6 18.1 4.6 5.9 9.3 14.2C31C40 29 25 C41C45 2

Aromatics

Benzene (C6H6) 11.8 6.6 2.1 6.6 4.5 4.1 3.5Toluene (C7H8 11.9 7.8 2.3 7.8 5.6 4.7 3.4

Xylene (C8H10) 9.4 7.1 3.6 7.1 6.2 5.4 4.7Alkylbenzenesd 0.5 1.1 8.4 8.2 14.6 8.2 12.8 13.1 13.9

Total 0.5 1.1 41.5 29.7 22.6 29.7 29.1 27.3 25.5

PAHs

Naphthalene 0.03 0.1 0.41 0.23 0.13 0.23 0.17 0.10 0.12Acenaphthene 0.09 0.03 0.02 0.03 0.02 0.02 0.02Acenaphthylene 0.08 0.04 0.02 0.04 0.02 0.02 0.02Phenanthrene 0.003 0.01 0.07 0.04 0.04 0.03 0.01 0.01Anthracene 0.16 0.11 0.03 0.11 0.06 0.05 0.03Pyrene 0.002 0.02 0.09 0.05 0.05

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 WOfeed rate of 0.4 kg/h was used when theeffects of varying theN2purgerate were studied, whereas a constant N2 purge rateof 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 550C.

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 al-kanes, naphthenes, and alkenes observed for each carbon numberacross all collected samples.

Valuable light aromatics such as BTX (the sum of benzene, tol-uene, and xylene) were found in significant quantities (833 wt.%)

in the pyrolysis oils. The aromatics were composed mainly of sin-gle ring alkyl aromatics, including benzene derivatives such as 1,3-diethylbenzene, 1-methyl-2-propylbenzene, and hexyl-benzene, aswell as benzene rings with short alkyl groups mainly toluene,ethyl-benzene, and xylene. The subsistent chains attached to thebenzene rings ranged from C1to C6groups and were mostly non-branched saturated compounds. Polycyclic aromatic compounds(PAHs) were observed in the pyrolysis oil but only in minor quan-tities (60.9 wt.%). Several sulphur and/or nitrogen-containing com-pounds such as thioureas, amines, and benzothiazoles were alsodetected in the pyrolysis oils, but the quantities were low (60.1wt.%) and were therefore not presented in the table.

In this study, the decrease in purge and feed rates was thoughtto lead to an increased residence time of the pyrolysis volatiles in

the reactor, resulting in an increase in aromatic content along witha decrease in aliphatic content in the pyrolysis oils, and the ali-phatic and aromatic content improved towards smaller hydrocar-bon chains. The amounts of alkanes and alkenes were reduced infavour of aromatic formation (including PAHs). This agrees withthe findings in previous works[5,30,31], which propose that an in-crease in residence time may promote secondary reactions ofhydrocarbons (e.g. cyclisation and aromatisation) in the pyrolysisvolatiles to form aromatics; the production of aromatics by thesesecondary reactions have also been reported in pyrolysis studiesof pure hydrocarbon compounds[32,33]. The aromatics are likelyto be formed via DielsAlder type secondary reactions, which in-volve dehydrogenation and cyclisation of alkenes (produced frompyrolysis cracking of waste oil) to form aromatic hydrocarbons.These secondary reactions are likely to occur in this microwave-heated pyrolysis in a manner similar to that reported in other pyro-lysis processes [25,34], since n-alkenes and dialkenes (e.g. 1.3-butadiene) were found in the pyrolysis oils (Table 1). Furthermore,the decreased concentrations of alkenes at low purge and feedrates (Table 1) suggest that it was the increased occurrence ofthese DielsAlder type secondary reactions that had combinedand converted the n-alkenes and dialkenes into aromatics, leadingto increased formation of benzene, toluene, and alkylbenzenes. Themain products formed by the DielsAlder type reactions in pyroly-sis cracking are reported to be benzene, toluene, and alkylbenzenes[27,33,34], and these compounds were detected in the pyrolysisoils obtained in this study, supporting the proposed reaction mech-anism for the formation of aromatics. Depending on the degree ofaromatisation, 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 detectionof PAHs in the pyrolysis oils (Table 1) supports the proposed occur-rence of condensation reactions for PAH formation.

The yield of pyrolysis oil was found to increase with increasingwaste oil feed rate, however, this reduced the yield of the desired

fraction of light aliphatic hydrocarbons (C5C20) in the pyrolysisoil. The pyrolysis oils formed under high feeding rate were foundto contain higher amounts of heavier hydrocarbon components(>C20). At higher feed rates, the increased flow of pyrolysis volatilesout from the reactor led to decreased participation of pyrolysis vol-atiles (evolved from the waste oil compounds) in secondary reac-tions (e.g. secondary thermal cracking) and helps to explain thegreater quantities of heavier hydrocarbon components in the pyro-lysis oil. Moreover, it is thought that most of the heavier compo-nents were obtained as a result of evaporation of somecomponents in the waste oil that escaped from the reactor withoutbeing cracked[9]. The main implication is that a high waste oilfeed rate does not necessarily represent an ideal condition to pro-duce valuable oil products with this microwave pyrolysis appara-

tus. The benefit of pyrolysing waste oil at high feed rates toincrease the yield of total oil product is negated by the significantincrease in the yield of undesirable heavier hydrocarbon compo-nents (>C20) in the oil product under these conditions. Overall,the results shows that the waste oil can be thermally crackedand condensed to pyrolysis oils comprising valuable light aliphaticand aromatic hydrocarbons, which could be treated and used aseither an energy source or valuable chemical feedstock.

3.4. Elemental composition

Table 2 shows the elemental composition of the waste oil beforepyrolysis treatment and the pyrolysis oils obtained at differentpurge and feed rates. Carbon and hydrogen represented the main

elements present in waste oil, whereas nitrogen, sulphur, and oxy-gen were detected in very low concentrations (62.1 wt.%). The car-bon and hydrogen are mainly from the base oils from which thelubricating oil is formulated, whereas nitrogen, sulphur and oxy-gen are likely to originate from the additives (e.g. antioxidants)present in the engine oil [2]. The low content of sulphur in thewaste oil suggests that sulphur originally present in engine oil islikely to have reacted with oxygen in the air to form sulphur oxi-des, which subsequently escape to the atmosphere during engineoperation.

The pyrolysis oils showed a much lower content of oxygen anda significantly higher H/O atomic ratio than the waste oil. The oilsalso showed a very low O/C atomic ratio, indicating the very lowlevel of oxygenation in the oils. The use of a bed of carbon as theheating medium in our set-up, which also provides a reducingchemical 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


100 250 750 0.4 1 2.5 5

C 82.7 81.7 88.6 85.6 85.7 85.6 85.1 84.9 84.6H 12.5 15.1 11.2 14.2 13.7 14.2 14.3 14.5 14.6N 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2S 2.3 0.8 0.02 0.02 0.03 0.02 0.02 0.03 0.03O 2.3 2.1 0.1 0.1 0.5 0.1 0.1 0.2 0.2H/C (mol/mol) 1.8 2.2 1.5 2.0 1.9 2.0 2.0 2.0 2.1

H/O (mol/mol) 86 114 1778 2254 435 2254 2270 1151 1159O/C (mol/mol) 0.02 0.02 0.001 0.001 0.004 0.001 0.001 0.002 0.002

a Process conditions: pyrolysis at a constant WOfeed rate of 0.4 kg/h was used when the effects of varying theN2 purgerate were studied, whereas a constant N2 purgerateof 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 550C.

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to decrease the formation of undesired oxidised species during thepyrolysis, and thus leading to the decreased oxygen content in thepyrolysis oil. This is desirable since oxidised species (e.g. acids andreactive peroxides) may catalyse undesired polymerisation reac-tions of unsaturated compounds in the pyrolysis oil during storage,

generating larger molecules (e.g. tar or sludge from polymerisationof olefins) that have poor mutual solubility with other compoundsin the oil, resulting in increased viscosity and low heating value ofthe oil. Any carbon that becomes oxidised as a result of redox reac-tions is of little concern as it will usually be converted to CO or CO2which 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 withthe oxygen present in the additives in waste oil [2]to form metaloxides that are retained within the carbon bed during pyrolysis;the presence of metals in the waste oil has been reported in previ-ous works[5]. The reduction in oxygen content is also likely to bedue to decarboxylation commonly occurring during thermal treat-ment processes; this agrees with the findings of Sinaget al.[6]intheir waste oil pyrolysis study. The deoxygenation of oxygenated

species (e.g. phenols originating from the antioxidant additives inthe engine oil) to thermally stable aromatic compounds[34]is an-other likely source of the reduction of oxygen content. The lowoxygen content in the pyrolysis oils represents a favourable featurein producing a potential fuel source with high calorific value.

The H/C atomic ratio is a good indicator of the existence ofhydrocarbons in the waste oil and the pyrolysis oils, and the vari-ations in the ratio indicate the different levels of saturation in theCAC bonds. A decrease in H/C ratio was observed for pyrolysis oilscompared to the waste oil, suggesting that dehydrogenation andaromatisation had occurred to some extent to form compounds

containing carbon double-bonds (e.g. alkenes, aromatics) with alower H/C ratio; the result is consistent with the hydrocarbon com-position and functional groups found by GCMS and FTIR analysis(see Sections3.3 and 3.5). The purge and feed rates seem to havehad only a minor influence on the elemental composition of the

pyrolysis oils; however, at lower purge rates, increased secondaryreactions as a result of the longer residence times of the pyrolysisvolatiles in the reactor are likely to increase the degree of randomscission thermal cracking and aromatisation as indicated by thelower H/C ratio in the oils obtained under these conditions.

The pyrolysis oils contain a lower sulphur content compared tothe waste oil; this suggests that sulphur, although present in verylow concentrations in waste oil, is likely to have reacted with oxy-gen during pyrolysis to form sulphur oxides. In addition, new sul-phur compounds may be formed during pyrolysis, e.g. metal ornon-volatile inorganic sulphides[15,35], which remain in the car-bon bed; these reactions lead to decreased sulphur content in thepyrolysis-oils. In all cases the sulphur content of the pyrolysis oilobtained (2030 ppm) was found to meet internationally pre-

scribed standards for unleaded petrol (sulphur: 6150 ppm or60.1 wt.%) and diesel fuels (sulphur: 6350 ppm or 60.1 wt.%),e.g. European Fuels Directive 98/70/EC and UK Motor Fuels Regula-tions 1999 [36], British Standard of Unleaded Petrol (BS EN228:2008 [37]), and British Standard of Diesel (BS EN590:2009+A1:2010 [38]). The use of fuel derived from theselow-sulphur pyrolysis oils can potentially lead to a reduction ofSOx emissions compared to fossil fuels (e.g. diesel). The resultsfrom elemental analysis show that the microwave-heatedpyrolysis generated a pyrolysis oil with a low sulphur and oxygencontent, 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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treating this kind of problematic waste oils. The low sulphur andoxygen content is beneficial to upgrading the pyrolysis oil to trans-port-grade fuels.

3.5. FTIR analysis

Fig. 6shows the infrared spectrum of the pyrolysis oil obtainedby FTIR analysis and the general classification of chemical com-pounds from the FTIR spectrum; the classification was definedbased on the degree of infrared absorption (or transmittance) de-tected at different frequencies (or wave number) over the infraredspectra obtained from the pyrolysis oils. The data presented showtypical results for the pyrolysis oils produced by microwave-heated pyrolysis of waste oil as there was little difference in theFTIR spectra obtained with the different process parameters (N2purge flow and waste oil feed rate), except for a small variationin the transmittance peak intensities detected at the different fre-quency ranges. The results indicate that most of the hydrocarbonsfound in the pyrolysis oils were alkanes, alkenes, and single-ring

aromatics.Interestingly, compounds with OAH and C@O stretching vibra-

tions, such as phenols, aldehydes, carboxylic acid, showed very lowpeak intensities, suggesting that these compounds were presentonly in minor quantities in the pyrolysis oil. The very low charac-teristic peaks of these compounds observed in the FTIR spectrumprovide useful information when assessing the extent of oxidationthat had occurred in the oils. Oil oxidation normally results in thesequential addition of oxygen to base oil molecules, causing theformation of oxygenated by-products with hydroxyl (OAH) andcarbonyl groups (C@O), e.g. aldehydes and carboxylic acids. Car-boxylic acids, in particular, are undesirable as they are the commoncause of acidic corrosion and sludge formation (as a result of poly-merisation in which carboxylic acids combine to form larger

molecular species), which can lead to increased oil viscosity andcausing problems such as filter plugging and system fouling. Over-all, the trace amount of oxygenated compounds (e.g. aldehydes,carboxylic acids) in the pyrolysis oils indicates that very little oiloxidation occurred in this pyrolysis process; this corroborates the

low oxygen contents found in the pyrolysis oil by elemental anal-ysis (seeTable 2), and the beneficial effects of the carbon bed (act-ing as a reducing reaction environment) in decreasing both the oiloxidation and the resulting formation of undesired oxidised spe-cies (see Section3.4).

3.6. Fuel properties

The oil products derived from microwave-heated pyrolysiswere examined for their properties as a fuel and these values werecompared to those of the virgin oil, the waste oil, and gasoline col-lected from a local petrol station (Table 3). The waste oil shows alower density but higher calorific value than the virgin oil. It isthought that some of the heavier hydrocarbons in virgin oil were

decomposed into lighter hydrocarbons in waste oil. The lower cal-orific value of the virgin oil is likely due to the presence of carbonandlong-chain carbon compounds of lower calorific value in the oilmatrix.

The densities and viscosities of the pyrolysis oils were found tobe lower than those of the waste oil due to the cracking of heavyhydrocarbons to lighter compounds. The densities of the pyrolysisoils (757773 kg/m3), except for that from the experiments con-ducted at a N2 purge rate of 750 ml/min, are quite close to thatfor gasoline, and it is also within the prescribed range of 720775 kg/m3 given in British Standard of Unleaded Petrol (BS EN228:2008 [37]). The flash points of the pyrolysis oils were all foundto 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 Gasoline Desired liquid fuelb


100 250 750 0.4 1 2.5 5

Density, q (kg/m3) 879 858 757 770 784 770 771 773 773 767 720850c

Flash point (C) 215 197


11/14


The low flash points suggest that the un-refined pyrolysis oils con-tain components that have a lower boiling point range than diesel.The pyrolysis oils possess slightly higher kinematic viscosities (67 mm2/s) than diesel (24 mm2/s) [40], but are considerably higherthan that of gasoline (0.7 mm2/s). Further treatment may be

needed to reduce the viscosity of the pyrolysis oil to a value com-parable to gasoline (


12/14


purge rates and waste oil feed rates. However, at lower N 2 purgeand waste oil feed rates, the pyrolysis oils were found to have high-er amounts of lighter fractions with low boiling points. Decreasingthese two process parameters results in an increase in formation oflight hydrocarbons, which are likely to derive from enhanced

cracking of heavier hydrocarbons (i.e. higher boiling point compo-nents) into smaller molecules due to longer residence times in-curred by the pyrolysis volatiles in the reactor.

Fig. 7also compares the boiling point distributions of hydrocar-bons in the pyrolysis oil to those of commercial liquid fuels. Dataare presented for pyrolysis oil obtained with 1 kg/h of waste oilfeed rate and 250 ml/min of N2purge rate as those conditions pro-duce the highest yield of pyrolysis oils that are closely matched tocommercial fuels, particularly gasoline (see Section 3.6). Nearly80% of the pyrolysis oil was found to fall below the boiling pointof diesel and kerosene. The recovered oil had a lower initial boilingpoint and contained lighter fractions than those of diesel; similarfindings were reported by other researchers working on otherstudies of waste oil pyrolysis[6,15].

For comparison purposes, the boiling point range obtained forthe pyrolysis oil was categorised into four petroleum fractions:gasoline, kerosene, diesel, and heavy oil (>C19); the petroleum frac-tions are defined based on the major carbon components and theircorresponding boiling point ranges detected in the various oils.Ta-ble 4compares the petroleum fractions in the pyrolysis oils pro-duced from microwave-heated pyrolysis to those that areobtained by pyrolysis using conventional electric heating. Themicrowave-heated pyrolysis method generates an oil productcomprising 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 thatthe waste oil, containing C11C40 hydrocarbons, was thermallycracked to mainly C4C19 hydrocarbons, which were then con-densed as pyrolysis oil, in which nearly 96% of the compoundswere within the C4C19range. It is likely that a significant portionof the pyrolysis oil was obtained through pyrolysis cracking ratherthan from the evaporation of the waste oil, since the pyrolysis oilcontains only 4% of long chain hydrocarbon components (>C19)typical of the compounds in the original waste oil (Table 4). Over-all, the pyrolysis oil demonstrates a boiling point range quite com-parable to commercial liquid fuels, particularly gasoline,suggesting that the oils could potentially be further processed totransport-grade fuels.

The pyrolysis oil produced from microwave-heated pyrolysisshowed higher amounts of light fractions (C4C19) compared withthose obtained from the oils pyrolysed by conventional electricheating (Table 4). In addition, the examination of the hydrocarbon

composition of the oils (Table 1) revealed that waste oil was ther-mally cracked to mainly 6C20hydrocarbons compared to the 6C35hydrocarbons produced in conventional electric-heated pyrolysis[42]. These results suggest that cracking reactions are enhancedin microwave-heated pyrolysis. The use of microwave heating as

a heat source shows improved efficiency in cracking the wasteoil to a desirable lighter fraction, comprising components (e.g.C4C12hydrocarbons) within the range of commercial liquid fuelswhilst also showing a significantly greater yield of pyrolysis oil(Fig 4, see Section3.1). Possible explanations accounting for thisdifference include the use of the microwave-heated carbon bedin our set-up (in which the added waste oil becomes totally im-mersed, providing excellent heat transfer, and also acts as a reduc-ing reaction environment), and the microwave heating processitself, which has been shown to produce different products fromconventional heating when all other factors are held equal[10,24,43]. Mechanisms underlying this difference include possi-bilities such as different heat distributions. Microwave heating vol-ume heats only the carbon, creating a localised reaction hot zone as

opposed to electric heating which is externally applied and heatsall substances in the reactor volume including the surroundinggas. Additionally, the creation of free elections on the surface ofthe carbon particles as a result of microwave-induction may influ-ence the reaction pathway. The proposed mechanisms are cur-rently under investigation to further verify the explanationspostulated above. Pyrolysis using conventional electric heating[6,15] was found to contain a higher amount (3453%) of heavyoil components (>C19), compared to only 4% observed in this study(Table 4). We postulate that a significant portion of the heavy oilfraction in pyrolysis oils is derived from distillation or evaporationthat occurs during the heating of waste oil. These processes trans-fer hydrocarbons from both the uncracked and less-cracked frac-tions of waste oil in the reactor to the condensation system andthus into the recovered pyrolysis oil. Evaporation has also been ob-served in several pyrolysis studies of waste oil[9,14].

3.7. Energy recovery

Table 5shows estimates of the energy recovery in the pyrolyticproducts compared with the electrical energy consumption in themicrowave pyrolysis process. These estimates provide a usefulmeasure of the energy efficiency of the process. It should be men-tioned that the calculations are limited by the followingassumptions:

1. Electrical consumption is based on the electrical power input(7.5 kW) during the pyrolysis treatment, which is estimated tobe approximately 1.5 times the nominal output of the 4 magne-

trons (5 kW) for the sum of the periods when they are switchedon during the application of waste oil. However, it should benoted that the 7.5 kW of electrical power input is an overesti-mate of the actual electrical consumption, considering thecrude set-up of the prototype reactor and the fact that theactual amount of absorbed power is not measured in this appli-cation. Inclusion of a device in the set-up to log the forward andreflected power would optimise both the absorbed power andthe control over the process, increasing the viability to scaleup the process.

2. Heat losses from the prototype reactor are substantial andwould not be representative of the losses that would occur atpilot or industrial scale. No attempt has been made to recoverenergy during the cooling of the products in the condensation

system nor to fully insulate the reaction vessel and associatedfittings.

3. Pilot or industrial scale operation would involve the continuousaddition of waste oil without the need to repetitively heat the

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 Gasoline Kerosene Diesel Heavyoil

C4C12 C11C15 C15C19 >C1940190 C

175246 C

251342 C

>342 C

Microwave-heated pyrolysis(this study)

69 16 15 4

Pyrolysis heated by electricfurnace[6]

2.538 1 714 53

Pyrolysis heated by electricoven[15]

40 18 13 34

a Thepetroleum fractionsare categorised based on themajorcarboncomponentsand their corresponding boiling point ranges detected in the various oils.

b The data from literature were estimated from the boiling point curve of wasteoil obtained in their waste oil pyrolysis studies.

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apparatus from roomtemperature. Calculations that include theelectrical energy used to heat the reactor to the desired processtemperature will seriously overestimate the actual electricalconsumption during pilot or industrial scale, hence, this energyexpenditure is excluded from the energy calculations shown inTable 5.

4. The calorific value of the non-condensable pyrolysis products isignored.

The pyrolysis oils showed significantly high recovery of the en-ergy content of the waste oil, ranging between 67% and 90%. Theelectrical energy (Epyrolysis) supplied to power the microwave-heated pyrolysis process ranged from 11% to 42% of the calorificvalue of the waste oil pyrolysed (Epyrolysis/EWO) over the range ofpurge and feed rates considered. The results also show that moreelectrical energy (Epyrolysis) was needed to pyrolyze the waste oilat higher feed rates, as a direct result of the need for more energyto heat higher quantities of waste oil to the operational tempera-ture and to supply the endothermic enthalpy to drive the higheramount of pyrolysis reactions. The higher relative electrical energyusage (Epyrolysis/EWO) observed at lower waste oil feed rates indi-cates that a the process is less energy efficient when pyrolysis isconducted at lower feed rates, probably as a result of a greaterinfluence 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 energywas needed to overcome the cooling effects that resulted fromthe higher N2purge rates.

Even given the limitations involved in estimating energy con-sumption in the laboratory scale equipment described above, it isclear that the process is capable of recovering a hydrocarbon prod-uct (pyrolysis oil) whose calorific value is many times greater thanthe amount of electrical energy used in the operation of the pro-cess, as indicated by energy ratios ranging between 2 and 8 (Ta-ble 5). This favourable situation would be even more apparentduring the operation of pilot or industrial scale equipment inwhich attempts to improve heat integration and recovery havebeen implemented. These results suggest that a reactor heatedby 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 contentequivalent to approximately 60 kW. The calorific value of thenon-condensable pyrolysis products has been ignored in this

assessment. Inclusion of the energy content of this stream wouldfurther increase the amount of energy that can be recovered fromwaste oil. This gaseous stream could be used for on-site generationof electrical energy to power the magnetron system.

However, it should be noted that the high energy recovery ra-tios observed in this study involve the assumption that the onlyenergy input of the process is the electrical energy used in thepyrolysis reactor. In practice lower energy ratios should be consid-ered in which higher energy inputs should be taken into account,including the energy needed for the collection and transport ofwaste oil to the processing plant, and for the refining of the pyro-lysis oils if they are needed to be further processed to produce acommercial gasoline fuel (e.g. the conversion of both the aromaticand the heavy hydrocarbon components into gasoline type compo-nents). Overall, the results show that the microwave-heated pyro-lysis method may be a viable means of recycling the waste oil intoa useful oil product, in addition to a disposal method for the wasteoil.

4. Conclusion

N2purge rate and waste oil feed rate, in addition to the use of amicrowave heated bed of particulate carbon, were found to haveeffects on the fraction of original waste oil converted to pyrolysisgases, pyrolysis oils, and char residues. These process parameters

also influenced the concentrations and molecular nature of the dif-ferent hydrocarbons formed in the pyrolysis oils. These results, incombination with results from previous work [5], demonstrate thatmicrowave-heated pyrolysis generated an 88 wt.% yield of gaso-line-like oil product that contains potentially valuable light ali-phatic and aromatic hydrocarbons. The oil product is alsorelatively contaminant free with low levels of sulphur, oxygen,and toxic PAH compounds, and is almost entirely free of metalsas reported in previous work [5], thereby reflecting its potentialas a green energy source or chemical feedstock. The pyrolysis oilcould potentially be treated and upgraded to transport grade fuels,or added to petroleum refinery as a chemical feedstock for furtherprocessing, although further studies are needed to confirm thesepossibilities. The microwave-heated pyrolysis can be performed

in a continuous operation to treat high volumes of waste oil whileshowing both a positive energy ratio and a high net energy output.It is clear that microwave-heated pyrolysis offers an exciting greenapproach to the treatment and recycling of automotive lubricating

Table 5

Energy recovery and consumption in microwave-heated pyrolysis of waste automotive engine oil (WO).

Process conditions EWOa (kJ/h) EPO

b (kJ/h) Erecoveryc (%) Epyrolysis

d (kJ/h) Epyrolysise/EWO(%) Eratio

f Ebalanceg (kJ/h)

WO feed rateh

0.4 kg/h 18,160 15,624 86 6480 36 2 9144

1.0 kg/h 45,400 39,525 87 12,960 29 3 26,5652.5 kg/h 113,500 100,190 88 20,250 18 5 79,9405.0 kg/h 227,000 205,040 90 25,650 11 8 179,390

N2 purge rateh

100 ml/min 18,160 12,758 70 4320 24 3 8438250 ml/min 18,160 15,624 86 6480 36 2 9144750 ml/min 18,160 12,248 67 7560 42 2 4688

a 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.b Estimated calorific value of thepyrolysis oil (PO) based on thedifferentCVs (Table 3) and wt.% yields (Fig. 2) of the pyrolysis oil, and the differentfeeding rates of the WO,

i.e. CV of pyrolysis oil *wt.% yield of pyrolysis oil *WO feed rate/100.c Energy recovery (%) in the pyrolysis oil was calculated based on the energy recovered from the waste oil, i.e. EPO * 100/EWO.d 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.e Amount of energy (fromEWO) consumed byEpyrolysis.f 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.g 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.h

Process conditions: Pyrolysis at a constant WOfeed rate of 0.4 kg/h was used when theeffects of varying the N2 purge rate were studied, whereas a constant N2 purge rateof 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 550C.

338 S.S. Lam et al./ Fuel 92 (2012) 327339


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oil. This unique combination of properties enables the potentialgeneration of products with significant commercial value out ofwhat would otherwise be a toxic and difficult to dispose of chem-ical liability.

Acknowledgements

Su Shiung Lam acknowledges the financial assistance by Public-Service-Department of Malaysia Government and UniversityMalaysia Terengganu.

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