Bio-Oil Production from Prosopis julif lora via Microwave PyrolysisDadi V. Suriapparao,† N. Pradeep,† and R. Vinu*,†,‡

†Department of Chemical Engineering and ‡National Centre for Combustion Research and Development, Indian Institute ofTechnology Madras, Chennai 600036, India

*S Supporting Information

ABSTRACT: Microwave pyrolysis is an efficient technique to valorize the abundantly available Prosopis julif lora (PJF) biomassinto fuel intermediates. In this study, the effects of microwave power, susceptor, PJF particle size, PJF to susceptor mass ratio, andinitial mass of PJF on bio-oil, gas, and char yields, composition of bio-oil, and energy recovery in bio-oil and char were evaluated.Five different susceptors, namely, graphite, char, aluminum, silicon carbide, and fly ash, an industry waste, were utilized. A highbio-oil yield of 40 wt % with a heating value of 26 MJ kg−1 was achieved with fly ash at a microwave power of 560 W, PJF particlesize of 2−4 mm, and PJF (50 g)/fly ash composition of 100:1 (wt/wt). The bio-oil contained a mixture of phenolic compounds,aromatic hydrocarbons, cyclopentanones, carboxylic acids, ketones, and furan derivatives. Nearly 51% deoxygenation of PJF wasachieved with an atomic O/C ratio of 0.24 in bio-oil. This work demonstrates that the yield and quality of bio-oil are dependenton key parameters such as microwave power, biomass particle size/composition, and type of susceptor.

1. INTRODUCTION

Lignocellulosic biomass is one of the potential renewablesources of energy to cater to our sharply increasing energyneeds. The valorization of abundantly available biomass is anattractive and viable pathway for energy production through acarbon neutral cycle.1 Pyrolysis is a prominent and cost-effective platform for thermochemical conversion of biomass tobio-oil, char, and syngas.2 Among the pyrolysis products, bio-oilcan be upgraded to transportation fuels via hydrodeoxygena-tion. The fast pyrolysis technique is proven to yield highamounts of bio-oil. Fluidized bed reactors are highly suitable toachieve fast pyrolysis conditions due to their ease of operation,shorter particle residence time, and rapid heat transfer.3

Recently, microwave pyrolysis has attracted the attention ofthe research community due to its selective, rapid, and uniformvolumetric heating mechanism.4 Importantly, rapid agitationand size control of biomass particles are not essential unlikefluidized bed pyrolysis.Microwave pyrolysis of various abundantly available biomass

feedstocks including douglas fir,5 pine,6,7 oil palm,8,9 ricestraw,10−12 wheat straw,13,14 bagasse,15,16 coffee hulls,17 cornstover,6,18 algae,19,20 and sewage sludge21,22 is reported in theliterature. Microwave absorbing materials, also known assusceptors, are necessary to convert incident microwave energyinto heat energy owing to a low dielectric loss factor ofbiomasses. Carbonaceous materials such as graphite, activatedcarbon and biochar,23 ceramics like silicon carbide,6 and metals,metal oxides, and metal hydroxides15 have been reported to actas good susceptors. Besides heat transfer, metal oxides areknown to induce catalytic activity and increase reaction rateswhile simultaneously reducing pyrolysis temperature.15 Pro-duction of high yields of bio-oil from biomass via microwavepyrolysis is a challenging task. Owing to the generation ofmicroplasma spots throughout the reaction mixture, very slowor fast heating rates lead to the formation of biochar andgaseous products. Chemical nature, quantity, size and shape ofthe susceptor are important factors that control heating rate,

pyrolysis temperature, and product distribution. Therefore, asystematic and comprehensive study involving multipleparameters such as biomass to susceptor composition, biomassparticle size, microwave power, catalysts, and gas atmospheresare critical to improve the yield of bio-oil via microwavepyrolysis. Borges et al.6 and Lei et al.24 adopted the centralcomposite design of experiments to evaluate the effects oftemperature, reaction time, biomass particle size, and loadingon volatile yield. However, a comprehensive evaluation ofproduct quality in terms of composition of the bio-oil underdifferent operating conditions is lacking.The current study focuses on upgrading the lignocellulosic

biomass variety Prosopis julif lora (P. Julif lora, PJF). It is a highlyinvasive nitrogen fixing species that can grow in arid andsemiarid regions even under harsh environmental conditionssuch as saline soils. It consumes less water and utilizes relativelyhigher amounts of CO2 from the atmosphere, which makes itan attractive carbon neutral and energy rich source compared toother lignocellulosic biomasses. Traditionally harvested as a fuelplant for domestic use, it is now widely used as a fuel for smallscale electricity generation by cofiring with coal in the state ofTamil Nadu, India.25 P. julif lora is also known to invademillions of hectares of rangeland in South Africa, East Africa,Australia, South America, and other parts of Asia.26 Betterrecovery of energy and resources from P. julif lora is possible viamicrowave pyrolysis rather than direct combustion. To the bestof our knowledge, this is the first study to report bio-oilproduction from P. julif lora via microwave pyrolysis.In this study, microwave pyrolysis of P. julif lora is conducted

under a wide range of reaction conditions to maximize the bio-oil yield and improve its quality. The effects of variousparameters such as (i) microwave power (280−700 W), (ii)PJF particle size (<0.25 to 4 mm), (iii) PJF to susceptor ratio

Received: February 13, 2015Revised: March 23, 2015

Article

pubs.acs.org/EF

© XXXX American Chemical Society A DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

(5:1 to 1000:1 (wt/wt)), (iv) different susceptors, and (v)initial mass of PJF (5−50 g) on the yield, quality, andcomposition of bio-oil were investigated. Susceptors belongingto various categories such as metals (aluminum), ceramics(SiC), carbonaceous materials (graphite, char), and industrialwaste (fly ash) were utilized. The bio-oil was characterized byheating value and detailed chemical composition measure-ments.

2. EXPERIMENTAL SECTION2.1. Materials and Methods. P. julif lora branches from IIT

Madras campus were cut and air-dried for 5 days. Further, they werecrushed, ground, and sieved to get five different size fractions, viz. 2−4mm, 1.4−2 mm, 0.5−1.4 mm, 0.25−0.5 mm, and <0.25 mm.Snapshots of the different particles are depicted in Figure S1 (inSupporting Information). The susceptors, namely, aluminum (99%purity), graphite (98 wt % carbon), and silicon carbide (SiC, 99.5%purity), were procured from S.D. Fine Chem Pvt. Ltd., India. Fly ash,obtained from the exit of a boiler stack, was procured from a localsugar manufacturing company. PJF char was obtained from microwavepyrolysis experiments. All susceptors were ground and sieved into fineparticles of size ∼50 μm.PJF feedstock of different particle sizes was characterized for

moisture, volatile matter, fixed carbon, and ash using proximateanalysis, C, H, N, S, and O composition by elemental analysis, andhigher heating value (HHV) using a bomb calorimeter. Proximateanalysis was performed according to the ASTM E1131-08 method27 ina thermogravimetric analyzer (SDT-Q600, TA Instruments). Ele-mental analysis was performed in Elementar Vario EL III elementalanalyzer, and HHV was determined in an IKA 2000 bomb calorimeterby using ca. 200 mg of samples. The basic characteristics of thefeedstock are reported in Table 1. It is clear from the proximateanalysis that 63−68 wt % of volatile matter can be theoreticallyconverted to bio-oil. The fixed carbon can be associated with lignincontent in PJF. As ash is a highly crystalline inorganic substance, it iseasily crushed and ground during the size reduction operation.Therefore, it settles to the bottom most tray during sieving. Similarvariation in ash composition with particle size is reported for Canadianmixed wood saw dust28 and poplar wood.29 Saravanakumar et al.30

evaluated the cellulose, hemicellulose, lignin, and ash content in PJF tobe 61.65%, 16.14%, 17.11%, and 5.2%, respectively. The fixed carbonand ash content corresponding to small PJF particles are comparablewith literature data.29 The high oxygen content is indicative of lowenergy content of PJF and the highly polar nature of the bio-oil. Fromthe HHVs in Table 1, it can be concluded that nearly 3-fold energydensification is required to convert raw PJF into transportation gradeliquid fuels. The elemental composition of fly ash was evaluated usingIS:1727-1967 and IS:9749/2007 test methods.31,32 The fly ashcontained SiO2 (26.97%), Fe2O3 (4.77%), CaO (17.85%), MgO(2.0%), Al2O3 (10.34%), Na2O (1.05%), K2O (8.4%), and TiO2(0.7%) with a loss on ignition of 14.71%. The composition of PJFchar was C (76.44%), H (1.96%), N (0.69%), and O (20.91%).2.2. Microwave Reactor Setup. Pyrolysis experiments were

conducted in a domestic multimode on−off microwave oven (LGMS2043DW) that operates at different power levels such as 280, 420,560, and 700 W with a frequency of 2450 MHz. A schematic of the

experimental setup is shown in Figure 1. Reactions were conducted ina 250 mL quartz round bottomed flask, and the pyrolysis vapors were

taken through a borosilicate adaptor to a double stage condensationsystem. The quartz flask was placed inside a cylindrical glass vesselwith glass wool insulation to avoid conductive heat losses to ovencavity. The condensers and the 100 mL collection flask weremaintained at a low temperature (10 °C) by continuous circulationof chilled water. Teflon tapes were used to seal all the joints in order toprevent the leakage of microwaves and the escaping of vapors to theatmosphere. A home-built chromel−alumel thermocouple (Figure S2in Supporting Information) was used to monitor the off-time reactiontemperature with an accuracy of ±2 °C. Thermocouple-basedtemperature measurement of the reaction mixture inside themicrowave oven is reported in the literature.6,33−36 The thermocoupleutilized in this work consisted of four layers of insulation includingalumina beads, Al foil, glass tube, and Al foil to minimize microwaveinterferences with temperature sensor, and arcing and tripping ofmicrowave oven. The ungrounded configuration ensured good electricand microwave insulation. Therefore, the transmitted temperature isonly that of the reaction mixture and not of the thermocouple. Duringexperiments, the thermocouple was inserted into the quartz reactorand positioned such that it measured the temperature of the reactionmixture accurately. A microwave receiver was used externally to avoidfluctuations in the digital temperature monitoring system. Thetemperature profile recorded during boiling of distilled water usingthe modified thermocouple is shown in Figure S3 (in SupportingInformation). This shows that the temperature recorded is that of thewater inside the reaction vessel.

2.3. Pyrolysis Experiments. Before conducting every experiment,the feedstock was air-dried at 110 °C until a constant sample mass wasattained. This ensured the nearly complete removal of physicallybound moisture from biomass. In order to ensure that the biomass andsusceptor are not segregated within the reaction vessel, dried PJF and

Table 1. Proximate Analysis, Elemental Analysis and Heating Values of P. julif lora Biomass of Different Particle Sizes

proximate analysis (wt %) elemental analysis (wt %)

size range (mm) moisture volatile matter fixed carbon ash C H N S Oa HHV (MJ kg−1)

2−4 12.0 62.7 24.8 1.5 44.97 7.06 1.89 0.09 45.99 17.391.4−2 10.8 65.4 22.8 1.0 44.51 6.88 1.87 0.09 46.65 17.010.5−1.4 9.4 67.9 19.1 3.6 43.87 6.65 1.39 0.08 48.01 17.430.25−0.5 12.0 66.6 16.8 4.6 43.85 6.57 1.44 0.18 47.96 16.76<0.25 12.5 63.7 18.3 5.5 43.01 6.27 1.70 0.38 48.64 16.51

aObtained by difference.

Figure 1. Schematic of microwave pyrolysis experimental setup.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

B

susceptor were thoroughly blended in required mass ratios and takenin the quartz flask. Before starting every experiment, the entire systemwas purged with N2 at a flow rate of 1 LSTP min

−1 for 10 min to ensurean inert environment inside the reactor. The desired microwave powerand time were set in the control panel of microwave oven, and thetemperature was recorded every 30 s. In order to determine thetemperature corresponding to completion of pyrolysis, experimentswere initially performed until 500 and 600 °C at 560 W power and PJF(20 g)/fly ash ratio of 100:1 (wt/wt). From entries E3 and E20 inTable 2, it can be ascertained that pyrolysis is complete before 500 °C.The pyrolysis residence time varied from 6 to 15 min based on thereaction conditions (Figure 2). The masses of char and bio-oil weremeasured gravimetrically at the end of each experiment, and the massof noncondensable gases was calculated by mass balance. The bio-oil,gas, and char yields on a dry basis were calculated as (mass of product)× 100/(initial mass of dried PJF). The organic phase of bio-oil wasseparated from aqueous phase by washing the condensers, elbow, andliquid collection flask with dichloromethane. A majority of theexperiments were nonconsecutively repeated three times, and thestandard deviation in bio-oil, gas, and char yields was within 5%.2.4. Product Characterization. A two-dimensional gas chromato-

graph−mass spectrometer (2D-GC/MS; Agilent 7890-5975C) wasused to characterize the composition of the bio-oils. The systemconsisted of a DB-5MS (30 m length × 0.25 mm i.d. × 0.25 μm filmthickness) column connected in series with HP-INNOWAX (5 mlength × 0.25 mm i.d. × 0.15 μm film thickness) column using a flowmodulator. Helium (99.9995% purity) was used as the carrier gas atflow rates of 0.8 mL min−1 and 30 mL min−1 through the first andsecond columns, respectively. The column oven was initiallymaintained at 45 °C for 1 min, then heated to 240 °C at a rate of 5°C min−1, and finally maintained at 240 °C for 15 min. MS ion sourceand detector temperatures were set at 320 and 150 °C, respectively.The mass range scanned was from 35 to 450 g mol−1. Bio-oil wasdiluted with dichloromethane at 1:100 (v/v), and 0.2 μL of dilutedsamples was injected. The injector temperature was set at 275 °C, andthe split ratio was 10:1. The compounds were identified by comparingthe mass spectra with NIST and biofuel libraries, and all the identifiedcompounds had a very high match factor (>85%). Area percentages ofthe peaks were utilized to evaluate the percent yield of variouscomponents in bio-oil in a semiquantitative way. Figure 3 depicts the

2D-GC/MS total ion chromatogram of bio-oil obtained at 560 W. Thepresence of two columns aided in better separation of oxygenated

Table 2. Master Table of Various Experimental Conditions, Corresponding Solid, Bio-Oil and Gas Yields, and Heating Valuesof Bio-Oil and Char

expt.code susceptor

PJF: susceptorratio (wt/wt)

microwavepower (W)

PJF particlesize (mm)

mass ofPJF (g)

average heatingrate (°C min−1)

solidyield(wt %)

oil yield(wt %)

gas yield(wt %)

char HHV(MJ kg−1)

oil HHV(MJ kg−1)

E1 flyash 100:1 280 2−4 20 19.31 28.25 29.84 41.91 28.59 23.62E2 flyash 100:1 420 2−4 20 38.24 25.61 35.41 38.98 27.73 23.25E3 flyash 100:1 560 2−4 20 52.56 25.95 36.74 37.31 28.00 26.16E4 flyash 100:1 700 2−4 20 76.00 24.40 35.64 39.96 27.54 25.10E5 flyash 5:1 560 2−4 20 60.75 22.65 34.55 42.80 27.43 21.14E6 flyash 400:1 560 2−4 20 73.23 23.44 38.66 37.90 28.93 22.56E7 flyash 1000:1 560 2−4 20 65.33 25.41 34.48 40.11 28.79 23.58E8 aluminum 100:1 560 2−4 20 60.53 25.84 36.83 37.33 30.12 27.32E9 graphite 100:1 560 2−4 20 30.32 28.96 31.25 39.79 28.98 21.79E10 char 100:1 560 2−4 20 64.27 24.60 32.95 42.45 29.52 24.47E11 SiC 100:1 560 2−4 20 70.86 25.67 26.37 47.96 28.82 29.22E12 flyash 100:1 560 2−4 5 68.29 26.67 9.60 63.73 26.67 28.21E13 flyash 100:1 560 2−4 10 67.43 23.45 26.97 49.58 28.55 25.24E14 flyash 100:1 560 2−4 30 66.14 23.83 38.50 37.67 28.34 22.66E15 flyash 100:1 560 2−4 50 52.78 25.07 39.87 35.06 27.64 25.61E16 flyash 100:1 560 <0.25 20 35.19 28.28 27.55 44.17 21.45 31.38E17 flyash 100:1 560 0.25−0.5 20 74.15 28.08 25.47 46.45 24.77 29.82E18 flyash 100:1 560 0.5−1.4 20 74.77 25.93 24.70 49.37 26.92 19.82E19 flyash 100:1 560 1.4−2 20 37.52 23.64 32.83 43.53 26.99 26.61E20a flyash 100:1 560 2−4 20 52.13 25.72 36.72 37.56 28.12 26.07

aExperiment conducted up to pyrolysis temperature of 600 °C. All other experiments were conducted up to 500 °C.

Figure 2. Reaction temperature profiles during microwave pyrolysisunder different conditions. (a) Effect of microwave power (experi-ments E1−E4), (b) effect of different susceptors (experiments E3,E8−E11), (c) effect of PJF to fly ash ratio (wt/wt) (experiments E3,E5−E7), (d) effect of PJF particle size (experiments E3, E16−E19),(e) effect of initial mass of PJF (experiments E3, E12−E15).

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

C

compounds, and hence, a comprehensive analysis of bio-oilcomposition was possible. The HHVs of bio-oil and char weremeasured using a bomb calorimeter.Noncondensable gases were analyzed in a GC equipped with flame

ionization and thermal conductivity detectors (FID/TCD) (Perki-nElmer Clarus 580). Elite plot-Q column (30 m length × 0.32 mmi.d.) was used to analyze the hydrocarbons in the gas fraction usingFID, and a Molsieve 13X column (2 m length × 1/8″ dia., 80/100mesh) was used to analyze H2, O2, and CO2 using TCD. Column ovenwas maintained at an isothermal temperature of 150 °C for 20 min,and injector temperature was set at 150 °C for both FID and TCDanalyses. FID and TCD temperatures were 250 and 110 °C,respectively. The flow rate of carrier gas, N2, was 2 and 30 mLmin−1 for FID and TCD analyses, respectively. 200 μL of gaseoussample was injected with a split ratio of 10:1. The flame in FID wassustained by continuous supply of H2 (45 mL min−1), air (450 mLmin−1), and N2 (30 mL min−1). Standard gases were used to identifythe various components in gas fraction.

3. RESULTS AND DISCUSSION

A thorough investigation of the effects of different operatingparameters such as microwave power, susceptor type, PJFparticle size, PJF/susceptor ratio, and initial mass of PJF onproduct yields and characteristics was carried out. Table 2depicts the various experimental conditions and the corre-sponding char, bio-oil and noncondensable gas yields, andheating values of char and bio-oil. Tables S1 and S2 (inSupporting Information) provide the yields and selectivities ofthe product groups in bio-oil obtained at different conditions,respectively. Tables S3−S21 (in Supporting Information)depict the composition of the bio-oils in terms of the yield ofindividual organic compounds. The compounds obtained frombio-oil were classified into seven main categories, viz. guaiacols,syringols, simple phenolics, aromatic hydrocarbons, C2−C5ketones, acids and alcohols, furan derivatives, and cyclo-pentanones. The yields and selectivities of the above productgroups were evaluated, and the variation was correlated withoperating conditions. Phenolic compounds originate fromlignin in PJF, while the formation of carboxylic acids, ketones

and furans can be attributed to both cellulose and hemicellulosepyrolysis. The origin of cyclopentanones is predominantly dueto cracking of hemicellulose. Aromatic hydrocarbons areformed usually via secondary pyrolysis of the above primarypyrolysates. Some of the major organics that are produced atyields >1 wt % are guaiacol, alkyl guaiacols, vinyl guaiacol,isoeugenol, syringol, methoxy eugenol, phenol, cresols,xylenols, benzene, toluene, xylene (BTX), naphthalenes,furfural, acetic acid, propanoic acid, propanones, butanones,and pentanones. The percent energy recovered in bio-oil andchar were evaluated using the formula %yieldproduct ×(HHVproduct)/(HHVraw PJF).

3.1. Effect of Microwave Power. Magnetron of adomestic microwave oven can produce only a constantmicrowave power output. The total duration for which themagnetron produces microwaves is called on-time. In amicrowave oven, the power, which is simply the time averageover a minute, is controlled by varying the on-time. Since tinymicroplasma spots are generated only when microwaves areincident on the susceptor surface, the microwave powerinfluences the duration of exposure to microplasma spots,and hence, the thermal energy accumulation within the reactionmixture. This in turn affects heating rates, product yields, andcomposition of bio-oil. In order to understand these effects,experiments were conducted by taking 20 g of PJF of size 2 mmat a PJF to fly ash (wt/wt) ratio of 100:1 at different microwavepowers ranging from 280 to 700 W. The corresponding bio-oil,char, gas yields, and average heating rates are reported in Table2 (E1−E4) and depicted in Figure S4 (see SupportingInformation). As evidenced in Figure 2a, the temperatureincrease is steep with average heating rates increasing in therange of 19.3−76.0 °C min−1 with microwave power. This is inagreement with an earlier study on microwave pyrolysis of ricestraw.12 The yield ratio (wt/wt) of bio-oil to char varies withmicrowave power in the order: 1.46 (700 W) > 1.42 (560 W) >1.38 (420 W) > 1.06 (280 W), whereas the yield ratio (wt/wt)of gas to char varies in the order: 1.64 (700 W) > 1.52 (420 W)

Figure 3. 2D-GC/MS total ion chromatogram of bio-oil obtained at a microwave power of 560 W and PJF (50 g, 2−4 mm)/fly ash ratio of 100:1wt/wt.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

D

> 1.48 (280 W) > 1.44 (560 W). Clearly, high microwavepower of 700 W favors biomass conversion to oil andnoncondensable gases. The absolute yields of bio-oil are similarat 420, 560, and 700 W (c.a. 35.93 ± 0.7 wt %). A similarobservation is also reported in the case of microwave pyrolysisof wheat straw wherein the bio-oil yield was ca. 31 wt % from400 to 600 W.37

High yield of bio-oil is not the only marker to qualify aparticular microwave power as optimum. The qualities of bio-oil in terms of heating value and composition are alsoimportant. The energy recovered in bio-oil follows the order:55.27% (560 W) > 51.44% (700 W) > 47.34% (420 W) >40.53% (280 W), while that recovered in char follows thetrend: 46.44% (280 W) > 41.78% (560 W) > 40.84% (420 W)> 38.64% (700 W). Huang et al.12 also observed a decreasingtrend of energy recovered in char with increasing microwavepower. At low microwave powers, most of the energy isrecovered in char, and at high powers most of the energy isrecovered in noncondensable gases. Optimum energy recoveryin bio-oil occurs at an intermediate microwave power of 560 W.Figure 4 depicts the variation of selectivities of major

compounds in bio-oil and noncondensable gases with micro-

wave power. It is clear that the selectivities of total phenolics,i.e., guaiacols + syringols + simple phenols, and aromaticcompounds increase with microwave power. This is accom-panied by a concomitant decrease in the production of C3−C5acids/ketones/alcohols, furan derivatives, and cyclopentanones.Importantly, the yields of acetic acid and furfural decrease withmicrowave power. As the average heating rate of the sampleincreases with microwave power proportionately, the describedvariations can also be correlated with heating rate. This suggeststhat high microwave powers favor more efficient cracking oflignin into phenolic derivatives, which gets further converted toaromatic compounds via liberation of noncondensable gasesfrom the propyl subunits of lignin. This can be verified by theincrease in production of C1−C3 hydrocarbons like methane,ethane, ethylene, propane and propylene, and hydrogen. It isknown that the yields of noncondensable gases comprising CO,H2, and light hydrocarbons increase with microwave power,which is consistent with our study.12,37,38 The evolution ofthese gases and CO2 can be related to secondary decom-

position of carbonyl compounds and carboxylic acids. Thevariation in yields of cyclopentanones and furan derivatives withmicrowave power is not significant, which means that most ofcellulose and hemicellulose fractions are being converted intogaseous compounds even at low powers without much variationin condensable compounds present in bio-oil.

3.2. Effect of Juliflora Particle Size. It is evident fromproximate and elemental analysis data (Table 1) that differentparticle sizes have different chemical composition especially interms of ash content due to a difference in the grindabilityindex of compounds present in PJF. The smaller size fractionscontain high ash content thereby giving a low heating value.Particle size also affects the surface area available for chemicalinteraction and heat transfer rates. Therefore, it is reasonable toexpect that biomass particle size controls product yields anddistribution in a significant manner. In order to obtain acomprehensive understanding about the effect of particle size,pyrolysis of PJF of five different particle sizes in the range of<0.25−4 mm was conducted at 560 W and a PJF to fly ash (wt/wt) ratio of 100:1. The reaction conditions, yields of productfractions, and average heating rates are reported in Table 2 (E3,E16−E19), and the variation of product yields is depicted inFigure S5 (see Supporting Information).It is evident from the temperature profiles (Figure 2d) that

average heating rates (°C min−1) for different particle sizesfollow the trend: 74.77 (0.5−1.4 mm) ≈ 74.15 (0.25−0.5 mm)> 52.56 (2−4 mm) > 37.52 (1.4−2 mm) ≈ 35.2 (<0.25 mm).The wide and irregular variation in average heating rates can beattributed to various complex heat transfer phenomenaoccurring in the reaction mixture controlled by the PJF particlesize. Owing to larger size, 1.4−4 mm particles exhibit moderateto low heating rates, while the smaller particles exhibit highheating rates owing to high external surface area and bettercontact with the susceptors. The anomalous behavior observedin the case of <0.25 mm particles may be due to the highamount of ash that resists heat transfer. The presence of ashand fixed carbon in biomass significantly influences the heatingrate in the initial time periods. It is evident from Table 1 thatthe largest particles contain a high amount of fixed carbon andlow ash, while the smallest particles contain high ash and lowfixed carbon. Therefore, these variations in inherent composi-tion of biomass also add to the effect of externally addedsusceptor in altering the heating rates. This also justifies the useof low amounts of susceptor to start-off the pyrolyticdecomposition reactions. The yield ratio (wt/wt) of bio-oil tochar varies with particle size as 1.42 (2−4 mm) > 1.39 > (1.4−2mm) > 0.97 (<0.25 mm) ≈ 0.95 (0.5−1.4 mm) ≈ 0.91 (0.25−0.5 mm), while the yield ratio (wt/wt) of gas to char varies inbetween 1.9 (0.5−1.4 mm) and 1.44 (2−4 mm). It is clear fromthe product yield ratios in our study that PJF particles in thesize range of 2−4 mm result in good yields of bio-oil. Lei et al.24

also observed a highest bio-oil yield of 34 wt % duringmicrowave pyrolysis of 4 mm corn stover particles. As bulkenergy transfer is involved in the presence of microwaves,pyrolysis occurs in the whole of the particle, unlike conven-tional pyrolysis wherein heat transfer occurs only from thesurface to the core of the particle. In this regard, the fixedcarbon intrinsically present in biomass aids in volumetricheating, while the externally added susceptors aid in surface tocore heating. Moreover, higher external surface area of smallerbiomass particles leads to better cracking compared to largerparticles. High extent of cracking leads to the formation of lightgases, while controlled cracking leads to the formation of bio-

Figure 4. Variation of selectivities of key components in bio-oil andgaseous fractions with microwave power.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

E

oil. For this reason, high yield of gaseous products are obtainedwith smaller particles, while larger particles yield bio-oil.The heating value (MJ kg−1) of bio-oil shows significant

variation with particle size in the following order: 31.38 (<0.25mm) > 29.82 (0.25−0.5 mm) > 26.61 (1.4−2 mm) ≈ 26.16(2−4 mm) > 19.82 (0.5−1.4 mm), while the HHV (MJ kg1−)of char follows an opposite trend: 28.00 (2−4 mm) > 26.99(1.4−2 mm) ≈ 26.92 (0.5−1.4 mm) > 24.77 (0.25−0.5 mm) >21.45 (<0.25 mm). The catalytic role played by ash is animportant factor to be considered in assessing the productyields and composition. It is clear from Table 1 that ash contentincreases with decreasing particle size, which might promotesecondary condensation reactions leading to the formation ofchar precursors with a concomitant release of noncondensablegases. High amounts of ash present in the char lead to a lowheating value in char product. Percentage energy recovery inchar follows the order: 41.78% (2−4 mm) ≈ 41.48% (0.25−0.5mm) > 40.05% (0.5−1.4 mm) > 37.49% (1.4−2 mm) ≈36.74% (<0.25 mm). Percentage energy recovery in oil varieswith particle size as 55.27% (2−4 mm) > 52.36% (<0.25 mm)> 51.36% (1.4−2 mm) > 45.32% (0.25−0.5 mm) > 28.09%(0.5−1.4 mm). Both bio-oil and char obtained with 2−4 mmsize range possess highest energy recovery, and therefore,lowest energy recovered in the form of noncondensable gases.This also implies low grinding costs if the process were to bescaled up.In order to understand the observed trends in energy

recovery and the effect of particle size, the selectivities andyields of major products were investigated (Figure 5). It is clear

that the selectivity of aromatic hydrocarbons decreased with anincrease in average particle size from 27.47% for <0.25 mmparticles to 3.66% for 2−4 mm particles, suggesting thatintermolecular condensation reactions like Diels−Alder andradical recombination reactions of low molecular weight C2−C5 organics are promoted by the ash particles. This is justifiedby the low selectivity (ca. 7%) of acids/ketones/alcohols for<0.25 mm PJF particles, which increased to 13.4 ± 0.63% and24% for 0.25−2 mm and 2−4 mm particles, respectively. This

also supports the variation of HHV of bio-oil with particle size.The selectivity of cyclopentanones increased by 30%, while thatof furans was doubled in moving from smallest (<0.25 mm) tolargest (2−4 mm) particle size range, indicating a higherproportion of cellulose and hemicellulose being converted intocondensable products. The intrinsic variation of cellulose andhemicellulose content with particle size can also lead to theobserved effect. Jacob et al.29 reported that bigger particles ofpoplar wood contained a slightly higher proportion of celluloseand hemicellulose than smaller ones (<200 μm) and bark.Importantly, the yield of total phenolics increased above anaverage particle size of 1 mm and is similar for 1.4−2 mm and2−4 mm particles. This supports the argument that, owing tolower ash content in bigger particles, the primary phenoliccompounds were not converted to benzene derivatives.Moreover, owing to the high content of fixed carbon in biggerparticles (Table 1), it is possible that these acted as self-susceptors thereby leading to the effective conversion of ligninto phenolic compounds. Hydrogen and C1−C3 hydrocarbonswere the dominant noncondensable gases produced withselectivities in the range of 31−48% and 31−42%, respectively,inclusive of all particle sizes.

3.3. Effect of Different Susceptors. The chemical natureof the susceptor affects both the ability of the susceptor toconvert microwave energy to heat energy as well as its catalyticeffect, which eventually affects the chemical composition of theproduct. Susceptors belonging to different materials such ascarbonaceous materials (char and graphite), metal (aluminum),ceramic (SiC), and industrial waste (fly ash) were used. Theeffect of these susceptors on the yield of products and theircomposition was investigated at 560 W microwave power, PJF(20 g) to susceptor ratio of 100:1 (wt/wt), and PJF particle sizeof 2−4 mm. The bio-oil, char, gas yields, and average heatingrates are reported in Table 2 (E3, E8−E11), and the variationsare depicted in Figure S6 (see Supporting Information).It is evident from the temperature profiles depicted in Figure

2b that the average heating rates (°C min−1) achieved withdifferent susceptors follow the order: SiC (70.86) > char(64.27) > aluminum (60.53) > fly ash (52.56) > graphite(30.32). It is also evident that raw PJF biomass without anysusceptor absorbs microwaves owing to the presence of fixedcarbon and ash. However, because of the absence of susceptorsthe final temperature attained at the end of 15 min is only 270°C. This emphasizes the importance of even minute quantitiesof susceptors to initiate the pyrolysis reactions and sustainthem. Moreover, this also shows the possibility of altering theheating rates using susceptors. Because of the formation oflocalized microplasma spots, the region around microplasma onthe surface of the susceptor shoots up to very hightemperatures.9 This leads to more energy efficient flashpyrolysis conditions. The formation of microplasma spots wasobserved in an independent experiment with bare graphite andactivated carbon subjected to microwaves. Menendez et al.39

observed two types of microplasma, viz. ball lightning plasmaand arc discharge plasma during microwave heating ofcarbonaceous materials. Importantly, the tiny micro flasheswere observed just after 1−2 s of microwave irradiation.39 Theearly evolution of gases such as CO and CH4 at low bulktemperatures (<150 °C) was also evidenced during theexperiments. These observations confirm the formation oflocal microplasma spots. During microwave off-time interval,microplasma spots disappear, and hence, the thermal energy isdissipated throughout the reaction mixture. While the average

Figure 5. Variation of (a) selectivities of different product groups inbio-oil, (b) total phenolics yield, and (c) selectivities of non-condensable gases, with average particle size of PJF.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

F

bulk temperature and the heating rates achieved are similar tothat of slow pyrolysis, the effect of microplasma makes it akin tolocalized fast and flash pyrolysis. The yield ratio (wt/wt) of bio-oil to char varies in the order: aluminum (1.43) ≈ fly ash (1.42)> char (1.34) > graphite (1.08) > SiC (1.03), suggesting thataluminum and fly ash are good susceptors to obtain high bio-oilyield while minimizing char yields. On the other hand, theproduct yield ratios (wt/wt) of gas to char are in the order ofSiC (1.87) > char (1.73) > aluminum ∼ fly ash (1.44) >graphite (1.37), which is similar to the trend observed foraverage heating rates. This shows that the production ofgaseous products can be correlated with average heating rate,while the bio-oil formation is linked to various parameters suchas particle size, microwave power, and susceptor type, andparameter optimization is essential.The heating values of bio-oil with different susceptors varied

in the range of 22−29 MJ kg−1. The highest value was achievedwith SiC followed by aluminum, fly ash, char, and graphite. Thepercent energy recovered in bio-oil follows the trend:aluminum (57.86%) ≈ fly ash (55.27%) > char (46.36%) ≈SiC (44.30%) > graphite (39.16%). Energy recovered in char isin the range of 41.76−48.26%; the highest value was noticedwith graphite followed by aluminum (44.76%) and char, SiC,and fly ash (all 42%). Even though a slightly higher energyrecovery in bio-oil was obtained with aluminum, fly ash waschosen to be the best susceptor for obtaining a high yield ofquality bio-oil by merit of being an easily available industrialwaste. The high activity obtained with fly ash can be attributedto the presence of oxides like SiO2, Al2O3, Fe2O3, CaO, andMgO that are known to catalyze the cracking of biomass andhydrocarbons. For example, alumina imparts acidity and acts asa solid acid catalyst in cracking reactions. The formation ofalumina via reaction of aluminum with traces of oxygenentrapped in the reaction mixture is possible at hightemperatures,40 which justifies the high activity of aluminumpowder as a susceptor for bio-oil formation. Moreover,aluminum powder reflects microwaves, and hence, causeshomogeneous cracking of the reaction mixture.Susceptors have a significant effect on the selectivity of

different organic products found in bio-oil. The chemicalcomposition of bio-oils obtained with different susceptors isshown in Figure 6. The absolute yield of total phenolics isunaffected by the choice of susceptors (15.91 ± 0.91 wt %);however, the trend in selectivity is different: SiC (55.44%) >graphite (52.45%) > char (46.47%) ≈ aluminum (45.58%) ≈fly ash (44.7%). The use of SiC was found to result in highselectivity of phenolic compounds (c.a. 57.76% for corn stoverand 61.5% for saw dust) in an earlier study.6 Fly ash selectivelyfavors the formation of acetic acid (3.18 wt %) and furfural(2.76 wt %), whereas SiC gives low yields of these lowmolecular weight organics (0.23 and 0.87 wt %). The selectivityof linear ketones, acids, and alcohols is in the range of 7.36−23.5% with maximum selectivity attained using fly ash followedby aluminum powder and char. Similarly, high selectivity offuran derivatives is attained with fly ash (ca. 16%). Importantly,very low yield (1.24 wt %) and selectivity (3.38%) of aromaticsobserved with fly ash suggest that compared to othersusceptors, the secondary cracking and condensation reactionsare minimized by using fly ash. The selectivity of cyclo-pentanones is independent of the type of susceptor used,suggesting that the hemicellulose component in PJF readilyundergoes cracking irrespective of susceptor used, and thechemical nature of the susceptor has a negligible effect on such

reactions. The selectivities of light gases for different susceptorsare depicted in Figure 7. It is clear that the evolution of

hydrogen follows the trend: graphite, char (41.86 ± 0.83%) >fly ash, aluminum (34.21%) > SiC (25.81%). In an earlier study,hydrogen evolution was found to be low with SiC.6 Theselectivity of C1−C3 hydrocarbons with different susceptorsalso follows a similar trend. High production of CO2, C2, andC3 hydrocarbons with fly ash suggests that decarboxylation oflinear low molecular weight acids is a plausible pathway.

3.4. Effect of Juliflora to Fly Ash Ratio. The compositionof PJF to fly ash can affect both microwave energy utilizationand catalytic activity. The efficiency of cracking depends on thedensity of tiny microplasma spots, which in turn depends onthe number of fly ash particles present in unit volume of thereaction mixture. In order to assess the effect of susceptorquantity on product yields and composition, experiments wereperformed at four different PJF to fly ash ratios (wt/wt), viz.5:1, 100:1, 400:1, and 1000:1 at 560 W with 20 g of 2−4 mmPJF particles. It can be observed from Figure 2c that the ratio of

Figure 6. Composition of bio-oil obtained with different susceptors(PJF mass = 20 g, PJF to susceptor ratio (wt/wt) = 100:1, PJF particlesize = 2−4 mm, microwave power = 560 W).

Figure 7. Selectivity of light gases with susceptors at 560 W and PJF(20 g)/susceptor ratio of 100:1 (wt/wt). Other gases include C4, C5hydrocarbons, and unidentified oxygenates.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

G

PJF/fly ash affects the temperature profiles, and hence, theaverage heating rates. The bio-oil, char, and gas yields withaverage heating rates are reported in Table 2 (E3, E5−E7). Theoverall product yields are depicted in Figure S7 (see SupportingInformation).It is evident from the bio-oil yields that low quantity of

susceptor is sufficient to promote and sustain pyrolysis. Thiscan be justified by the role of susceptor as only an initiator ofmicrowave energy conversion to heat, which is then sustainedby the formation of char and decomposition of ligninintrinsically present in biomass. A high yield ratio (wt/wt) ofbio-oil to char was observed with 400:1 composition (1.65),followed by 1.42 for 100:1 composition. Nevertheless, theheating value of bio-oil obtained with 100:1 composition washigher than that obtained with 400:1 composition. On thecontrary, a low PJF to fly ash ratio of 5:1 favored a high yield ofnoncondensable gases due to a greater extent of cracking ofprimary volatiles by the char formed and the susceptorsthemselves. The energy recovered in bio-oil followed the order:55.27% (100:1) > 50.15% (400:1) > 46.75% (1000:1) >42.00% (5:1). The HHV and energy recovered in char weresimilar for all PJF/fly ash compositions.The yields of various bio-oil components obtained with

different PJF/flyash ratios are shown in Figure 8. The selectivity

of total phenolic compounds lies in the range 45−55% with themaximum occurring for PJF to fly ash ratio (wt/wt) of 400:1and a minimum value for 100:1 ratio. Contrastingly, theselectivities of acids/ketones/alcohols and furans are maximumat 100:1 and having values of 23.5% and 15.8%, respectively.The production of aromatic compounds is maximum at a 5:1PJF/fly ash ratio with 14% selectivity, suggesting that a largeamount of susceptor only leads to inter- and intramolecularcondensation of low molecular weight organics. Thecomposition of bio-oil does not show significant variationsuggesting that at such low ratios, the PJF to susceptor ratio has

a negligible effect on pyrolysis; it mainly controls the heatingrate, and hence, affects the relative yields of char, bio-oil, andnoncondensable gases.

3.5. Effect of Initial Mass of Juliflora. In order to evaluatethe feasibility of scale-up of microwave pyrolysis process, PJFpyrolysis was conducted at different initial masses of feedstarting from 5 to 50 g at the established optimal conditions of560 W power and PJF (2−4 mm) to a fly ash ratio of 100:1(wt/wt). The bio-oil, char, and gas yields and correspondingaverage heating rates are reported in Table 2 (E3, E12−E15),and the overall product yields are depicted in Figure S8 (seeSupporting Information).It is evident from the temperature profiles depicted in Figure

2e that the average heating rate varied from 68.3 °C min−1 at 5g to 52.8 °C min−1 at 50 g. Even though there is an increase inquantity of both biomass and susceptor at a constant ratio of100:1, the effect of increase in mass on oil yield is significant inthe range of small masses (5, 10, and 20 g), while for initialmasses greater than 20 g, the oil yield tends to saturate at ca. 40wt % at 50 g (Figure 9). This corresponds to a steady increase

in the yield ratio (wt/wt) of bio-oil to char from 0.36 to 1.62.The yield ratio (wt/wt) of gas to char varies in the range of 2.39at 5 g to 1.40 at 50 g. Therefore, the high initial mass of PJFfavors bio-oil generation with low gas yields. The aboveobservations indicate that the number of microplasma spotsgenerated per unit volume may be a critical factor influencingthe heating rates achieved and the product yields. With a bulkdensity of 0.366 g cc−1 for 2−4 mm PJF particles, the volumeoccupied by 5 and 50 g of biomass are 13.66 cc and 136.6 cc,respectively. This shows that in the case of low initial mass ofthe sample, more microplasma spots per unit volume could begenerated, which eventually lead to excessive cracking ofbiomass, and hence, high gas yields. On the other hand, with ahigh initial mass of sample, lesser number of microplasma spotswill be generated per unit volume of the sample, which resultsin controlled cracking. Therefore, an optimal generation ofmicroplasma spots is the key to control the severity of pyrolysisduring microwave processing.

Figure 8. Composition of bio-oil obtained at different PJF to fly ashratios (wt/wt) (PJF mass = 20 g, PJF particle size = 2−4 mm,microwave power = 560 W).

Figure 9. Variation of wt % yields of different product groups in bio-oilwith initial mass of PJF. Other conditions include PJF to fly ash ratio(wt/wt) of 100:1, microwave power of 560 W, and PJF particle size of2−4 mm.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

H

The bio-oil yield obtained in microwave pyrolysis iscomparable with that of conventional pyrolysis in fluidizedbed and free-fall fast pyrolysis reactors. In the literature, 30−60wt % of bio-oil production is reported via catalytic fast pyrolysisof different types of biomass feedstocks at different operatingconditions such as temperature, particle size, and type ofcatalyst.41−45 Zhang et al.42 obtained nearly 50 wt % of bio-oilfrom corn cob biomass in a fluidized bed reactor with FCCcatalysts. Pattiya and co-workers43,44 observed ca. 50 and 60 wt% of bio-oil production from sugar cane tops and cassava stalksin free fall and fluidized bed reactors, respectively. Therefore,the obtained bio-oil yields from PJF via microwave pyrolysis arecomparable with the literature. Nevertheless, huge potentialexists in improving the bio-oil yields by making suitable processmodifications.A significant increase in energy recovery in bio-oil is observed

with the initial mass of PJF. The trend can be described asfollows: 58.72% (50 g) > 55.27% (20 g) > 50.16% (30 g) >39.14% (10 g) > 15.57% (5 g). No significant change in energyrecovery in char can be observed (39.94 ± 1.36%). Thevariations in composition of bio-oil components with differentinitial masses are shown in Figure 9. An increase in yield ofguaiacols, simple phenols, syringols, and hence, the totalphenolics with initial mass of PJF is evident. The selectivity oftotal phenolics is also high (64%) for 50 g of initial PJF. Theyield and selectivity of linear acids/ketones/alcohols and furansinitially increased with mass up to 20 g and then decreased. Theselectivities of aromatics, furans, and cyclopentanones gotstabilized at 7.7%, 9.7%, and 7.4%, respectively, for large initialmasses of PJF. These observations strongly suggest that scalingup of the process is possible to obtain high yield of bio-oil withhigh energy recovery.In order to obtain an overall understanding of the competing

reactions during microwave pyrolysis, the selectivities of acids/ketones/alcohols and furans + cyclopentanones are plottedagainst the selectivity of total phenolics + aromatics, as depictedin Figure 10. The portrayed data are inclusive of all theexperimental conditions reported in this study. Phenolics andaromatics are produced via lignin decomposition, and anincrease in selectivity of this group correlates well with adecrease in selectivity of furans + cyclopentanones and acids/ketones/alcohols. As furans and cyclopentanones are predom-inantly derived as primary pyrolysates from carbohydrates, theobserved correlation suggests that under microwave heating,the decomposition of lignin promotes the secondary cracking offurans and cyclopentanones to form noncondensable gases.C2−C4 organics like acetic acid, propionic acid, and propanoneare formed via pyrolysis of both carbohydrates and lignin.Importantly, acetic acid is a major pyrolysate obtained fromlignin and produced only in minor quantities from hemi-cellulose and cellulose pyrolysis.46−48 The observed trendssuggest that under mild microwave heating conditions, such aslow power levels, C2−C3 oxygenates are preferentiallyliberated from lignin without the cleavage of interlinkingcovalent bonds (like α-O-4, β-O-4, β-5, 4-O-5, and 5-5) inlignin, while at all other conditions, the primary oxygenatesliberated from lignin are further decomposed to light gases.An overall assessment of yields and heating values of biochars

reveals that char production is insensitive to reaction conditionswith an average yield of 25.5 ± 1.78 wt % and heating value of27.7 ± 1.87 MJ kg−1. Therefore, the effect of microwaveoperating conditions is only to alter the relative yields of bio-oiland gaseous fractions. In order to understand the extent of

valorization and deoxygenation of raw PJF into bio-oil and char,C, H, and O content in bio-oils and char are evaluated. TableS22 (in Supporting Information) depicts the C, H, and Ocontent in bio-oils calculated using GC/MS composition data.The O/C and H/C ratios of bio-oils lie in the range of 0.17−0.33 and 1.18−1.29, respectively. The atomic H/C and atomicO/C ratios of raw PJF, char, and bio-oil are depicted in theform of van Krevelen diagram in Figure 11 and compared with

coals of different ranks, ethanol, diesel, kerosene, and gasoline.At optimal conditions, the percentage oxygen removal achievedfor the conversion of raw PJF into char and bio-oil were 55%and 51%, respectively. Interestingly, the oxygen removal was ashigh as 65% for PJF particles of smallest size (<0.25 mm) with ahigh ash content, although the oil yield was low at thiscondition. The extent of deoxygenation achieved in this work is

Figure 10. Variation of selectivities of acids/ketones/alcohols andfurans + cyclopentanones versus total phenolics + aromatics. The datacorrespond to all the microwave pyrolysis experiments reported in thisstudy.

Figure 11. Van Krevelen diagram for coals, petroleum-derived liquidfuels, P. julif lora, P. julif lora-derived bio-oil and biochar.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

I

also comparable with that achieved in conventional fluidizedbed and free-fall fast pyrolysis reactors. Nearly 61%, 56%, 45%,and 45% deoxygenation was earlier reported for corn cob,cassava stalk, cassava rhizome, and sugar cane tops,respectively.42−44 Thus, microwave pyrolysis is shown to be apotential technique for bio-oil production from P. julif lora.Some of the other advantages of microwave pyrolysis such asuniform temperature distribution within the sample, shortreaction times, minimal crushing and agitation of the feedstock,and better control of heating rate and products using susceptorssuggest that this may emerge as a preferred technique forbiofuel production.

4. CONCLUSIONSThis study has demonstrated that fly ash, an industrial waste, isan efficient susceptor for bio-oil production from P. julif lora, afast growing and invasive plant species. At an optimalmicrowave power of 560 W, a maximum of 40 wt % bio-oilis obtained below 500 °C from large (2−4 mm) particles using1/100th by mass of fly ash. This corresponds to 59% energyrecovery along with 51% deoxygenation in bio-oil. The reactionconditions greatly influence the yields and selectivities of bio-oilcomponents. The particle size of P. julif lora significantly affectsthe bio-oil yield and quality owing to differences in initialcomposition and heating rate. Importantly, the susceptors,besides altering the heating rate and the yield of bio-oil, act ascatalysts and change the quality of bio-oil in terms ofcomposition of individual constituents and heating value. Theextent of lignin decomposition is found to determine theformation and decomposition of C2−C5 oxygenates, andevolution of noncondensable gases.

■ ASSOCIATED CONTENT*S Supporting InformationFigure S1. Snapshots of P. julif lora biomass of different particlesizes. Figure S2. Schematic diagram of modified temperaturesensor. Figure S3. Temperature profile measured using themodified temperature sensor during microwave heating of 200mL of distilled water. Yields of char, bio-oil and non-condensable gases for different microwave powers (FigureS4), different PJF particle sizes (Figure S5), different susceptors(Figure S6), different PJF to fly ash ratios (Figure S7), differentinitial masses of PJF feed (Figure S8). Table S1. Compositionof bio-oil (in wt % yield) obtained from microwave pyrolysis ofP. julif lora under different experimental conditions. Table S2.Selectivities of bio-oil components under different experimentalconditions. Yield of various compounds present in bio-oilobtained from experiment E1−19 (Tables S3−S21). Table S22.Estimated elemental composition of bio-oil calculated fromdata presented in Tables S3−S21. This material is available freeof charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*Phone: +91-44-22574187. E-mail: vinu@iitm.ac.in.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSR.V. thanks Indian Institute of Technology Madras for a newfaculty seed grant, and Chevron Inc. for project funding via analumni grant. The authors thank National Center for Catalysis

Research for providing GC-FID/TCD for gas analysis. TheNational Centre for Combustion Research and Development issponsored by Department of Science and Technology, India.

■ REFERENCES(1) McKendry, P. Energy production from biomass (part 2):conversion technologies. Bioresour. Technol. 2002, 83, 47−54.(2) Yaman, S. Pyrolysis of biomass to produce fuels and chemicals.Energy Convers. Manag. 2004, 45, 651−671.(3) Bridgwater, A. V. Review of fast pyrolysis of biomass and productupgrading. Biomass Bioenergy 2012, 38, 68−94.(4) Miura, M.; Kaga, H.; Sakurai, A.; Kakuchi, T.; Takahashi, K.Rapid pyrolysis of wood block by microwave heating. J. Anal. Appl.Pyrol. 2004, 71, 187−199.(5) Ren, S.; Lei, H.; Wang, L.; Chen, Q. B. S.; Wu, J.; Julson, J.; Ruan,R. Bio-fuel production and kinetics analysis for microwave pyrolysis ofDouglas fir sawdust pellet. J. Anal. Appl. Pyrol. 2012, 94, 163−169.(6) Borges, F. C.; Du, Z.; Xie, Q.; Trierweiler, J. O.; Cheng, Y.; Wan,Y.; Liu, Y.; Zhu, R.; Lin, X.; Chen, P.; Ruan, R. Fast microwave assistedpyrolysis of biomass using microwave absorbent. Bioresour. Technol.2014, 156, 267−274.(7) Chen, M. Q.; Wang, J.; Zhang, M. X.; Chen, M. G.; Zhu, W. F.;Min, F. F.; Tan, Z. C. Catalytic effects of eight inorganic additives onpyrolysis of pine wood sawdust by microwave heating. J. Anal. Appl.Pyrol. 2008, 82, 145−150.(8) Salema, A. A.; Ani, F. N. Microwave induced pyrolysis of oil palmbiomass. Bioresour. Technol. 2011, 102, 3388−3395.(9) Salema, A. A.; Ani, F. N. Pyrolysis of oil palm empty fruit bunchbiomass pellets using multimode microwave irradiation. Bioresour.Technol. 2012, 125, 102−107.(10) Huang, Y. F.; Kuan, W. H.; Lo, S. L.; Lin, C. F. Total recovery ofresources and energy from rice straw using microwave-inducedpyrolysis. Bioresour. Technol. 2008, 99, 8252−8258.(11) Huang, Y. F.; Kuan, W. H.; Lo, S. L.; Lin, C. F. Hydrogen-richfuel gas from rice straw via microwave-induced pyrolysis. Bioresour.Technol. 2010, 101, 1968−1973.(12) Huang, Y. F.; Chiueh, P. T.; Kuan, W. H.; Lo, S. L. Microwavepyrolysis of rice straw: Products, mechanism, and kinetics. Bioresour.Technol. 2013, 140, 620−624.(13) Zhao, X.; Wang, W.; Liu, H.; Mao, Y.; Ma, C.; Song, Z.Temperature rise and weight loss characteristics of wheat straw undermicrowave heating. J. Anal. Appl. Pyrol. 2014, 107, 59−66.(14) Budarin, V. L.; Clark, J. H.; Lanigan, B. A.; Shuttleworth, P.;Breeden, S. W.; Wilson, A. J.; Macquarrie, D. J.; Milkowski, K.; Jones,J.; Bridgeman, T.; Ross, A. The preparation of high-grade bio-oilsthrough the controlled, low temperature microwave activation ofwheat straw. Bioresour. Technol. 2009, 100, 6064−6068.(15) Kuan, W. H.; Huang, Y. F.; Chang, C. C.; Lo, S. L. Catalyticpyrolysis of sugarcane bagasse by using microwave heating. Bioresour.Technol. 2013, 146, 324−329.(16) Chen, W. H.; Tu, Y. J.; Sheen, H. K. Disruption of sugarcanebagasse lignocellulosic structure by means of dilute sulfuric acid pre-treatment with microwave-assisted heating. Appl. Energy 2011, 88,2726−2734.(17) Domínguez, A.; Menendez, J. A.; Fernandez, Y.; Pís, J. J.;Nabais, J. M. V.; Carrott, P. J. M.; Carrott, M. M. L. R. Conventionaland microwave induced pyrolysis of coffee hulls for the production of ahydrogen rich fuel gas. J. Anal. Appl. Pyrol. 2007, 79, 128−135.(18) Huang, Y. F.; Kuan, W. H.; Chang, C. C.; Tzou, Y. M. Catalyticand atmospheric effects on microwave pyrolysis of corn stover.Bioresour. Technol. 2013, 131, 274−280.(19) Benereso, D.; Bermudez, J. M.; Arenillas, A.; Menendez, J. A.Microwave pyrolysis of microalgae for high syngas production.Bioresour. Technol. 2013, 144, 240−246.(20) Du, Z.; Li, Y.; Wang, X.; Wan, Y.; Chen, Q.; Wang, C.; Lin, X.;Liu, Y.; Chen, P.; Ruan, R. Microwave-assisted pyrolysis of microalgaefor bio-fuel production. Bioresour. Technol. 2013, 102, 4890−4896.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

J

(21) Domínguez, A.; Menendez, J. A.; Inguanzo, M.; Pís, J. J.Investigations into the characteristics of oils produced from microwavepyrolysis of sewage sludge. Fuel Process. Technol. 2005, 86, 1007−1020.(22) Domínguez, A.; Menendez, J. A.; Inguanzo, M.; Pís, J. J.Production of bio-fuels by high temperature pyrolysis of sewage sludgeusing conventional and microwave heating. Bioresour. Technol. 2006,97, 1185−1193.(23) Menendez, J. A.; Arenillas, A.; Fidalgo, B.; Fernandez, Y.;Zubizarreta, L.; Calvo, E. G.; Bermudez, J. M. Microwave heatingprocesses involving carbon materials. Fuel Process. Technol. 2010, 91,1−8.(24) Lei, H.; Ren, S.; Julson, J. The effects of reaction temperatureand time and particle size of corn stover on microwave pyrolysis.Energy Fuels 2009, 23, 3254−3261.(25) Sato, T. Beyond water-intensive agriculture: Expansion ofProsopis Julif lora and its growing economic use in Tamil Nadu, India.Land Use Policy. 2013, 35, 283−292.(26) Invasive species compendium, http://www.cabi.org/isc/datasheet/43942#20057048817 (accessed January 7,2015).(27) ASTM E1131-08(2014). Standard test method for composi-tional analysis by thermogravimetry. http://www.astm.org/Standards/E1131.htm (accessed January 2015).(28) Aqsha, A.; Mahinpey, N.; Mani, T.; Salak, F.; Murugan, P. Studyof sawdust pyrolysis and its devolatilization kinetics. Can. J. Chem. Eng.2011, 89, 1451−1457.(29) Jacob, S.; Perez, D. D. S.; Dupont, C.; Commandre, J.-M.;Broust, F.; Carriau, A.; Sacco, D. Short rotation forestry feedstock:Influence of particle size segregation on biomass properties. Fuel 2013,111, 820−828.(30) Saravanakumar, S. S.; Kumaravel, A.; Nagarajan, T.; Sudhakar,P.; Baskaran, R. Characterization of a novel natural cellulosic fiberfrom Prosopis juliflora bark. Carbohydr. Polym. 2013, 92, 1928−1933.(31) IS: 1727−1967. Methods of test for pozzolanic materials.https://law.resource.org/pub/in/bis/S03/is.1727.1967.pdf (accessedJanuary 2015).(32) IS 9749:2007. Potash feldspar and soda feldspar for glass andceramic industry - specification. https://law.resource.org/pub/in/bis/S02/is.9749.2007.pdf (accessed January 2015).(33) Chandrasekharan, S.; Basak, T.; Srinivasan, R. Microwaveheating characteristics of graphite based powder mixtures. Int.Commun. Heat Mass Transfer 2013, 48, 22−27.(34) Wang, N.; Yu, J.; Tahmasebi, A.; Han, Y.; Lucas, J.; Wall, T.;Jiang, Y. Experimental study on microwave pyrolysis of an Indonesianlow-rank coal. Energy Fuels 2014, 28, 254−263.(35) Wang, X.; Morrison, W.; Du, Z.; Wan, Y.; Lin, X.; Chen, P.;Ruan, R. Biomass temperature profile development and itsimplications under the microwave-assisted pyrolysis condition. Appl.Energy 2012, 99, 386−392.(36) Motasemi, F.; Afzal, M. T. A review on the microwave-assistedpyrolysis technique. Renewable Sustainable Energy Rev. 2013, 28, 317−330.(37) Zhao, X.; Wang, M.; Liu, H.; Zhao, C.; Ma, C.; Song, Z. Effectof temperature and additives on the yields of products and microwavepyrolysis behaviors of wheat straw. J. Anal. Appl. Pyrol. 2013, 100, 49−55.(38) Zhao, X.; Wang, W.; Liu, H.; Ma, C.; Song, Z. Microwavepyrolysis of wheat straw: Product distribution and generationmechanism. Bioresour. Technol. 2014, 158, 278−285.(39) Menendez, J. A.; Juarez-Perez, E. J.; Ruisanchez, E.; Bermudez, J.M.; Arenillas, A. Ball lightning plasma and plasma arc formation duringthe microwave heating of carbons. Carbon 2011, 49, 346−351.(40) Hussain, Z.; Khan, K. M.; Perveen, S.; Hussain, K.; Voelter, W.The conversion of waste polystyrene into useful hydrocarbons viamicrowave-metal interaction pyrolysis. Fuel Proc. Technol. 2010, 94,145−150.(41) Liu, C.; Wang, H.; Karim, A. M.; Sun, J.; Wang, Y. Catalytic fastpyrolysis of lignocellulosic biomass. Chem. Soc. Rev. 2014, 43, 7594−7623.

(42) Zhang, H.; Xiao, R.; Wang, D.; Zhong, Z.; Song, M.; Pan, Q.;He, G. Catalytic fast pyrolysis of biomass in a fluidized bed with freshand spent fluidized catalytic cracking (FCC) catalysts. Energy Fuels2009, 23, 6199−6206.(43) Pattiya, A.; Suttibak, S. Production of bio-oil via fast pyrolysis ofagricultural residues from cassava plantations in a fluidised-bed reactorwith a hot vapour filtration unit. J. Anal. Appl. Pyrol. 2012, 95, 227−235.(44) Pattiya, A.; Sukkasi, S.; Goodwin, V. Fast pyrolysis of sugarcaneand cassava residues in a free-fall reactor. Energy 2012, 44, 1067−1077.(45) Collard, F.-X.; Blin, J.; Bensakhira, A.; Valette, J. Influence ofimpregnated metal on the pyrolysis conversion of biomassconstituents. J. Anal. Appl. Pyrol. 2012, 95, 213−226.(46) Patwardhan, P. R.; Satrio, J. A.; Brown, R. C.; Shanks, B. H.Product distribution from fast pyrolysis of glucose-based carbohy-drates. J. Anal. Appl. Pyrol. 2009, 86, 323−330.(47) Patwardhan, P. R.; Brown, R. C.; Shanks, B. H. Understandingthe fast pyrolysis of lignin. ChemSusChem 2011, 4, 1629−1636.(48) Patwardhan, P. R.; Brown, R. C.; Shanks, B. H. Productdistribution from the fast pyrolysis of hemicellulose. ChemSusChem2011, 4, 636−643.

Energy & Fuels Article

DOI: 10.1021/acs.energyfuels.5b00357Energy Fuels XXXX, XXX, XXX−XXX

K