ae a,

aCenter for Biorening and Department of Bioproducts and Biosystems Engineering, University of Minnesota, 1390 Eckles Ave., St. Paul, MN 55108, United Statesering, Fenchanguzhou U

ted pyryrolyzeimumum bio-bents fo

widely available, biomass is a potentially carbonneutral energysource (McKendry, 2002). Solid wastes such as crop residues andprocessing byproducts can be converted into solid, liquid, and gas-eous products through various thermochemical processes includ-ing pyrolysis (Bridgwater and Bridge, 1991; Huber et al., 2006).

ar from organicsman, 2004). Thebe as a sorm, and as

leum fuel substitutes in the long term (Bridgwater, 2008to obtain higher yields and better quality of bio-oil from pyhave been investigated by many groups. Extensive reviewsphysical (Fagerns, 1995) and chemical properties (Fagerns, 1995;Radlein, 1999) of pyrolysis bio-oils have been published.

Currently, uidized bed and xed bed (downdraft or updraft)are the dominant reactor types for biomass pyrolysis, in whichthe heating is provided by heated surfaces, sands, etc. (Czernikand Bridgwater, 2004; Meier and Faix, 1999; Mohan et al., 2006).Microwave irradiation is an alternative heating method.

Corresponding author at: Yangtze Scholar Distinguished Guest Professor,Nanchang University, and Professor, University of Minnesota. Tel.: +1 612 6251710; fax: +1 612 624 3005.

E-mail address: ruanx001@umn.edu (R. Ruan).

Bioresource Technology 156 (2014) 267274

Contents lists availab

Bioresource T

elsIn order to improve the global energy efciency, alternative en-ergy resources and technologies for sustainable development ofworlds economy are required. In addition to being abundant and

production of bio-oil, combustible gases and chin biomass (Bridgwater and Peacocke, 2000; Yacommercial uses of fast pyrolysis are thought tohigh valued, speciality chemicals in the short tehttp://dx.doi.org/10.1016/j.biortech.2014.01.0380960-8524/ 2014 Elsevier Ltd. All rights reserved.urce ofpetro-). Howrolysison theArticle history:Received 21 October 2013Received in revised form 9 January 2014Accepted 12 January 2014Available online 22 January 2014

Keywords:Fast pyrolysisMicrowaveBiomassBio-oilModel compounds

A novel concept of fast microwave assisted pyrolysis (fMAP) in the presence of microwave absorbentswas presented and examined. Wood sawdust and corn stover were pyrolyzed by means of microwaveheating and silicon carbide (SiC) as microwave absorbent. The bio-oil was characterized, and the effectsof temperature, feedstock loading, particle sizes, and vacuum degree were analyzed. For wood sawdust, atemperature of 480 C, 50 grit SiC, with 2 g/min of biomass feeding, were the optimal conditions, with amaximum bio-oil yield of 65 wt.%. For corn stover, temperatures ranging from 490 C to 560 C, biomassparticle sizes from 0.9 mm to 1.9 mm, and vacuum degree lower than 100 mmHg obtained a maximumbio-oil yield of 64 wt.%. This study shows that the use of microwave absorbents for fMAP is feasible and apromising technology to improve the practical values and commercial application outlook of microwavebased pyrolysis.

2014 Elsevier Ltd. All rights reserved.

1. Introduction Pyrolysis is a well-recognized thermochemical platform fora r t i c l e i n f o a b s t r a c tbGIMSCOP and Department of Chemical EnginecMOE Biomass Engineering Research Center, NadCollege of Biological Science and Technology, F

h i g h l i g h t s

A new concept of fast microwave assis Wood sawdust and corn stover were p Wood sawdust fMAP obtained the max Corn stover fMAP obtained the maxim The results show that the use of absorderal University of Rio Grande do Sul, Rua Engenheiro Luiz Englert, Prdio 12204, 90040-040 Porto Alegre, RS, BrazilUniversity, Nanchang, Jiangxi 370047, PR Chinaniversity, Fuzhou 350108, PR China

olysis using absorbents was developed.d using SiC as absorbent.bio-oil yield of 65 wt.%.oil yield of 64 wt.%.r fMAP of biomass is feasible.Yiqin Wan c, Yuhuan Liu c, Rongbi Zhu d, Xiangyang Lin d, Paul Chen a, Roger Ruan a,c,Fast microwave assisted pyrolysis of biomabsorbent

Fernanda Cabral Borges a,b, Zhenyi Du a, Qinglong Xi

journal homepage: www.ss using microwave

Jorge Otvio Trierweiler b, Yanling Cheng a,

le at ScienceDirect

echnology

evier .com/locate /bior tech

biomasses (Budarin et al., 2009; Domnguez et al., 2007; Huanget al., 2010; Miura et al., 2004; Wan et al., 2009; Wang et al.,

2.3. Experimental design

A 23 factorial central composite experimental design (CCD),with 3 repetitions at the central point, was used to estimate theeffects of the main variables. For each biomass, a total of 11 exper-iments were carried out. For wood sawdust experiments, the inde-pendent variables studied were: temperature (x1a, C), feedstockloading (x2a, g), and absorbent particle size (x3a, grit). In order toevaluate different effects, in addition to temperature, other inde-pendent variables were studied for corn stover experiments: tem-perature (x1b, C), biomass particle size (x2b, mm), and vacuumdegree (x3b, mmHg). The dependent output variables were theyields of bio-oil (y1, %), gas fraction (y2, %) and char (y3, %), themoisture content of the bio-oil (y4, %) and the yield of syngas (y5,

Tech2009; Yu et al., 2007).In recent work, Yin (2012a) discussed microwave-assisted

pyrolysis (MAP) of biomass and reviewed the researches anddevelopments efforts and their major ndings. Most of the MAPmethods developed previously used batch-type process. Also, sincethe biomass takes several minutes to be heated to the reactiontemperature, these processes fall into the intermediate rate pyroly-sis category. In fact, increasing the heating rate and consequen-tially reduce the reaction time, is one of the most challengingissues in order to obtain higher yields and quality of bio-oil.

Carbon materials (e.g., carbon, charcoal, activated carbon) areeasily heated by microwaves as they are, in general, very goodabsorbents of microwaves (Menndez et al., 2010). Recently, a no-vel concept of pyrolysis utilizing microwave absorbents is beingdeveloped, in which the use of these absorbents could signicantlyimprove the heating rate. With minimal energy input throughmicrowave irradiation, the temperature of the reactor is steadywhen the solid residue is dropped directly onto the heated absor-bent. In this concept, the biomass is heated due to two heatingmechanisms simultaneously used: the microwave irradiation andconduction due to the high temperature of the absorbents. Sincethe biomass is almost instantaneously heated (less than 1 s) tothe reaction temperature, it can be feed continuously or semi-con-tinuously to the reactor. These ndings suggest that MAP, whichwas in the intermediate rate pyrolysis category, can reach fastmicrowave assisted pyrolysis (fMAP) conditions with this newheating mechanism, thus achieving higher product yield and qual-ity at the same time.

A small bench scale of fMAP was developed in the authors labto process biomasses with semi-continuous biomass feeding, in thepresence of microwave absorbents. In this study, wood sawdustand corn stover were pyrolyzed using SiC as microwave absorbent.The preliminary data showed that the temperature of SiC samplescould reach up to 960 C in a small 750 Wmicrowave oven system.The objective of the study was to examine the effects of microwaveabsorbent on yields and properties of the bio-oil, char and gas. Theeffects of key process variables, such as temperature, feedstockloading, particle sizes of absorbent and biomass, and vacuum de-gree, on the bio-oil yield were also analyzed. Detailed physicaland chemical characterization of the bio-oil, char and gas producedwere carried out.

2. Methods

2.1. Materials

Wood sawdust and corn stover were used in the fMAP experi-ments. The wood sawdust was a residue from pine wood pelletingprocess, obtained from Spearsh Pellet Company LLC, and locatedin Spearsh, South Dakota. The corn stover was obtained from corncrop residue from St. Paul Campus, at the University of Minnesota,Integrating microwave heating into pyrolysis is a novel conceptwhich has been attracting increasing attention in recent years.

This method offers many advantages over traditional processes,including uniform internal heating of large biomass particles,instantaneous response for rapid start-up and shut down, no needfor agitation of uidization and hence fewer particles (ashes) in thebio-oil, no syngas dilution by the carrying gas, and easy-to-imple-ment technology. Furthermore, studies suggest that this is a highlyscalable technology suitable for distributed conversion of bulky

268 F.C. Borges et al. / BioresourceTwin Cities. Both biomasses samples were milled and dried at 80 Cfor 24 h. The main characteristics of the wood sawdust and cornstover are listed in Table 1.2.2. Microwave assisted fast pyrolysis

The fMAP experiments were carried out in a small benchscale using a microwave oven (MAX, from CEM Corporation),with a power of 750W at a frequency of 2450 MHz The experi-mental apparatus consists of: (1) feeder that allows semi-contin-uous biomass feeding; (2) quartz inlet connectors; (3) microwaveoven; (4) specially made quartz reactor with two necks; (5)absorbent bed; (6) thermocouple (K-type) to measure the cavitytemperature; (7) thermocouple (K-type) to measure the temper-ature of the absorbent bed and to control the heating; (8) quartzoutlet connectors; (9) liquid fraction collectors; (10) condensers;(11) connection for gas sampling and to a vacuum device todraw the volatiles out of the reactor to a series of refrigeratedwater cooled condensers. The vacuum was to adjust the gas/va-por residence time. For safety purpose, a microwave detector(MD-2000, Digital Readout) was used to monitor microwaveleakage.

Firstly, 500 g of SiC particles were put in the quartz reactor toform a layer of absorbent bed. Then, the reactor was placed inthe oven cavity. After connecting the inlet and outlet quartz tubes,the oven was turned on for the heating process. When the temper-atures of the absorbent bed reached a designed level, the biomasssamples were semi-continually dropped onto the hot SiC bed,while the microwaves oven cycles on and off every 15 seconds inorder to improve the biomass heating, and maintain the set tem-perature of the absorbent bed. Samples of the gas product werecollected during the process, while the liquid fraction and charsamples were collected at the end of the experiment. The solidand liquid fraction yields were calculated from the weight of eachfraction, while the gas yield was calculated by differences based onthe mass balance.Table 1Characteristics of wood sawdust and corn stover.

Wood sawdust Corn stover

Proximate analysis (wet basis, wt.%)Moisture content 5.15 5.27Ash content 0.11 2.06Volatiles 95 93

Elemental analysis (dry basis, wt.%)C 42.62 40.38H 5.47 5.16N 0.38 0.38Oa 51.43 52.01

a Calculated by difference, O (%) = 100CHNAsh.

nology 156 (2014) 267274%). Table 2 presents the code and values of the independentvariables. Statistica 8.0 (StatSoft. Inc.) was used to analyze theexperimental data statistically.

TechTable 2Code and values of independent variables.

Experiments Variables

Wood sawdust Temperature (x1a,C)Feedstock loading (x2a, g/min)Bed particle size (x3a, grit)

Corn stover Temperature (x1b, C)

F.C. Borges et al. / Bioresource2.4. Products analysis

The bio-oil properties were characterized using the methodol-ogy described in by Du et al. (2011), which is depicted as follows:

An RVA Super 4 Visco Analyzer (Newport Scientic Pty Ltd.,Australia) was used to analyze the viscosity of the bio-oil. An ele-mental analyzer (CE-440, Exerter Analytical Inc., USA) was usedto examine the elemental composition, and to calculate the higherheating value (HHV), according to Friedl et al. (2005). The compo-sitions of the liquid products were identied using an Agilent78905975C gas chromatography/mass spectrometer with an

Biomass particle size (x2b, mm)Vacuum degree (x3b, mmHg)

Fig. 1. Products yields from

Fig. 2. Products yields froLevels

1 0 1450 500 5501 3 54670 30 8

450 500 550

nology 156 (2014) 267274 269HP-5 MS capillary column. Helium was used as the carrier gas atthe ow rate of 1.2 ml/min. The injection size was 1 ll, with a splitratio of 1:10. The initial temperature of the oven was 40 C, it washeld for 3 min, and then it was increased to 290 C at the rate of5 C/min, and then held for 5 min, while the detector and injectorwere conserved at a constant temperature of 230 C and 250 C,respectively. The National Institute of Standards and Technology(NIST) mass spectral data library was used to compare their massspectra with those from identied compounds. The moisture con-tent was determined by a Karl-Fischer titrator. A Varian Micro-GCCP4900/thermal conductivity detector (TCD) with a 5A molecular

0.5 1 20 170 270

wood sawdust fMAP.

m corn stover fMAP.

Fig. 3. Bio-oil yield desirability surfaces and contours for (a) temperature feeding loading; (b) temperature SiC particles size; and (c) feeding loading SiC particle size.

270 F.C. Borges et al. / Bioresource Technology 156 (2014) 267274

Fig. 4. Bio-oil yield desirability surfaces and contours for (a) temperature biomass particle size; (b) temperature vacuum degree; and (c) biomass particle size vacuumdegree.

F.C. Borges et al. / Bioresource Technology 156 (2014) 267274 271

40 and 65 wt.% organic condensate, 1020% char, 1030% gas (Die-bold and Bridgwater, 2008).

ate sizes of SiC. While big sizes of absorbent particles reduce heat-

3.2.1. Physical properties and elemental analysis of bio-oil

TechIn the wood sawdust experiments, relatively low values ofmoisture content in the liquid phase were seen at the centralpoints, although the lowest value observed was 34.59% at the sec-ond experiment. For the corn stover experiments, the minimumvalue found was 48.17%, observed at the central point.

The main and interaction effects of the process variables on thefactional yields were statistically analyzed based on the data fromsieve column and a PPQ column was used to analyze the gaseousproducts. The temperatures of the injector and the detector wereconserved both at 110 C. The temperatures of the PPQ columnand the 5A molecular sieve were kept at 150 C and 80 C, respec-tively. In order to identify the elements present on char, besidesthe elemental analysis, a microwave digest and ICP-OES multi-element determination were performed using an ARL 3560.

3. Results and Discussion

3.1. Product fractional yields

The percentage yields of the wood sawdust and corn stoverfMAP products obtained from each experiment are presented inFigs. 1 and 2, respectively. The maximum yield of the bio-oil was65 wt.% for wood sawdust, and 64 wt.% for corn stover, both ob-served at the central points. These results are higher to those re-ported for MAP (Yin, 2012a). They are also comparable to resultsreported for fast pyrolysis processes, which generally are between

Table 3Characteristics of bio-oil from wood sawdust and corn stover fMAP.

Properties Wood sawdust Corn stover

Elemental composition (wt.%)C 24.86 13.00H 7.17 8.08N 0.35 0.53Oa 67.61 78.39

HHV (MJ/kg)b 20.38 20.39Density (kg/L, at 25 C) 1.06 1.02pH 2.07 2.64Viscosity (cP, at 40 C) 14 13

a Calculated by difference.b Calculated using the equation HHV (MJ/kg) = (3.55 C2 232 C

2230 H + 51.2 C H + 131 N + 20,600) 103.

272 F.C. Borges et al. / Bioresourcethe factorial experiments. For the wood sawdust, temperature hadthe most statistically signicant effect on the products yields. Theabsolute value of the standardized effect estimate was 10.2832,which exceeds the acceptable value of 4.3027 (based on a2-sided t-test, with 2 degrees of freedom and a signicance levelof 0.05). In addition, the interaction effect of the feedstock loadingand SiC particle size was also signicant. The absolute value of thestandardized effect estimate was 4.43078, which exceeds theacceptable value. For the corn stover, the interaction effect of bio-mass particle size and vacuum degree showed to be statisticallysignicant. The absolute value of the standardized effect estimatewas 6.96818. These results agree with those on fast pyrolysisfrom literature reports, which showed the essential features offast pyrolysis process: heat transfer rate, biomass particle size,temperature control and volatiles residence time (Bridgwater,2008).

Figs. 3 and 4 show the desirability surfaces and contours forliquid fraction yields, based on the experiments using wood saw-dust and corn stover, respectively. According to related literature,processes operated at lower temperatures generally tends toThe characteristics of the bio-oil obtained from wood sawdustand corn stover are shown in Table 3. The bio-oil obtained fromwood sawdust fMAP exhibited lower pH, density and carboncontent, but higher hydrogen, nitrogen and moisture content com-pared with the typical wood derived pyrolytic bio-oil (Bridgwater,2008). Despite the lower carbon content, fMAP oil had HHV valuesimilar to that of wood derived pyrolytic bio-oil likely because ofits higher hydrogen content. The corn stover derived oil had higherhydrogen and nitrogen contents than wood sawdust derived oilwhile its other characteristics, such as HHV, density, pH and viscos-ity are similar to those of wood sawdust derived oil. The HHV ofcorn stover derived oil is also comparable to the reported values(Mullen et al., 2010; Shah et al., 2012). However, the carbon con-tent and HHV of both corn stover and wood sawdust are still notcomparable to those of fossil oil.

3.2.2. GCMS characterization of bio-oilA semi-quantitative analysis was accomplished by calculating

the relative percentage of areas under the chromatographic peaks.The main chemicals of the bio-oil from wood sawdust and cornstover are shown in Table 4, with the respective percentages ofareas determined.

Compounds such as aliphatic hydrocarbons, aromatic hydro-carbons, nitrogenated compounds, phenols, polycyclic aromatichydrocarbons (PAHs), and others were identied. Among thesecompounds, hydrocarbons are valuable components in bio-oilfrom the point of view of fuel application. Specically, aromatichydrocarbons serve as important industrial chemicals and trans-portation fuel additives to increase the octane number. In thising exchange with the biomass, smaller particles, despite havinglarger surface area, tend to agglomerate and also reduce the ef-ciency of biomass heating. Fig. 3(c) also shows the best conditionincludes low biomass feeding rate and intermediate size of absor-bents. The temperature needed to increase the liquid fraction yieldis higher for corn stover than for wood sawdust. It can be seen inFig. 4(a) that temperatures ranging from 490 C to 560 C wereoptimal for particles from 0.9 mm to 1.9 mm. In all cases, applyinga low vacuum degree (

Corn stover

a (%

9597431

TechTable 4Relative proportion (area %) of the main compounds present in the bio-oil.

Wood sawdust

Chemicals Are

1 Acetic acid 5.2 2-Methoxy-4-vinylphenol 4.3 1,2-Benzenediol 3.4 Furfural 2.5 2-Propanone, 1-hydroxy- 2.6 Phenol, 2-methoxy-4-(1-propenyl)-, (E)- 2.7 Phenol, 4-methyl- 2.

F.C. Borges et al. / Bioresource3.3. Composition of gaseous products

The non-condensable gas collected is composed of H2, CO, CO2and gaseous hydrocarbons. The quantitative results of the fourmain components (H2, CO, CO2 and CH4) obtained at central pointsfor wood sawdust showed that the gas consists of mainly CO(13.49%) and CO2 (13.37%), with small amounts of CH4 (5.27%)and H2 (6.69%) present. The results for corn stover showed thatthe gas contains mainly CO (25.12%), CO2 (16.39%), and smallamounts of CH4 (7.31%) and H2 (2.92%). The highest concentrationof H2 + CO (syngas) found was 20.18% and 28.04%, for woodsawdust and corn stover fMAP gases, respectively. Although theconcentration of syngas in wood sawdust gas product is lower, it

quality fuel than that from corn stover.

8 Phenol, 2-methoxy- 2.09 Phenol 2.010 Naphthalene 1.911 Ethanone, 1-(4-hydroxy-3-methoxyphenyl)- 1.812 Phenol, 4-ethyl-2-methoxy- 1.813 Phenol, 2-methoxy-4-(1-propenyl)- 1.814 Phenol, 2-methoxy-4-methyl- 1.715 Indene 1.716 Benzeneacetic acid, 4-hydroxy-3-methoxy- 1.6 2-Cyclopenten-1-one, 2-hydroxy-3-methyl- 1.2717 2-Cyclopenten-1-one, 2-hydroxy- 1.518 4-Ethylcatechol 1.319 Homovanillyl alcohol 1.320 2-Furancarboxaldehyde, 5-methyl- 1.221 5-(Acetylaminomethyl)-4-amino-2-methylpyrimidine 1.222 Phenol, 2-methyl- 1.123 4-Hydroxy-2-methoxycinnamaldehyde 1.124 3-Allyl-6-methoxyphenol 1.025 2-Furancarboxaldehyde, 5-(hydroxymethyl)- 1.0

Cumulative area (%) 50.90

Table 5Elemental analysis of fMAP char.

Wood sawdust Corn stover

Elemental composition (wt.%)C 58.94 50.50H 3.28 2.09N 0.35 0.55

Elemental composition (ppm)Al 365.88 121.55B 3.198 3.861Ca 956.85 1517.7Cd 0.01 0.214Cr 1.3 0.357Cu 8.134 8.276Fe 421.81 71.148K 11,020 1554.6Mg 1096.5 505.87Mn 23.015 59.218Na 123.76

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.biortech.2014.01.038.

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Fast microwave assisted pyrolysis of biomass using microwave absorbent1 Introduction2 Methods2.1 Materials2.2 Microwave assisted fast pyrolysis2.3 Experimental design2.4 Products analysis

3 Results and Discussion3.1 Product fractional yields3.2 Properties of bio-oils3.2.1 Physical properties and elemental analysis of bio-oil3.2.2 GCMS characterization of bio-oil

3.3 Composition of gaseous products3.4 Composition of char

4 ConclusionsAcknowledgementsAppendix A Supplementary dataReferences