Fuel 88 (2009) 2207–2215

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Fuel

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Energy-dense liquid fuel intermediates by pyrolysis of guayule(Parthenium argentatum) shrub and bagasse

Akwasi A. Boateng a,*, Charles A. Mullen a, Neil M. Goldberg a, Kevin B. Hicks a, Colleen M. McMahan b,Maureen C. Whalen b, Katrina Cornish c

a Eastern Regional Research Center, Agricultural Research Service, US Department of Agriculture, 600 E. Mermaid Lane, Wyndmoor, PA 19038, United Statesb Western Regional Research Center, Agricultural Research Service, US Department of Agriculture, 800 Buchanan Street Albany, CA 94710, United Statesc Yulex Corporation, 37860 W. Smith-Enke Road, Maricopa, AZ 85238, United States

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 September 2008Received in revised form 1 May 2009Accepted 26 May 2009Available online 12 June 2009

Keywords:PyrolysisBio-oilGuayuleBiofuels and bioenergy

0016-2361/$ - see front matter Published by Elsevierdoi:10.1016/j.fuel.2009.05.023

* Corresponding author. Tel.: +1 215 233 6493; faxE-mail address: akwasi.boateng@ars.usda.gov (A.A

Guayule is a perennial shrub grown in the southwestern United States that is used to produce high qual-ity, natural rubber latex. However, only about 10% of the plant material is used for latex production; theremaining biomass, called bagasse, can be used for renewable fuel production. Fast pyrolysis of guayule,both whole shrub and bagasse was performed. From both feedstocks a very viscous, high energy content(�30 MJ/kg) pyrolysis liquid (bio-oil) was produced in yields averaging over 60% without any catalyst.The properties and compositions of the bio-oils were found to be similar in the two feedstocks. Co-prod-ucts, charcoal (20–30 wt%) and non-condensable gas (5–15%), were also dense and had a high energycontent. Of the two feedstocks, the whole shrub yielded higher quantities of charcoal that also had ahigher energy content than the charcoal produced from bagasse. As a result, the energy recovery, esti-mated as the percentage of the energy products, to energy input into the reactor was lower (60%) for gua-yule bagasse than for the whole shrub (73%). This notwithstanding, the bagasse is a more attractivefeedstock for thermochemical conversion, not only because it is a residue from a primary process (latexextraction) that is on-site, but also because it has a high energy content. Moreover, it produces high qual-ity pyrolysis products. Co-production of latex rubber from the whole shrub and renewable fuels from theresidual bagasse by pyrolysis should improve the already positive economics of the guayule latex rubberindustry.

Published by Elsevier Ltd.

1. Introduction

Guayule (Parthenium argentatum), is a hardwood desert shrubnative to the Chihuahuan desert of Texas and North Central Mexicothat biosynthesizes high quality natural rubber. Recent commer-cial success in producing guayule latex has resulted in guayule’sre-introduction as a new crop in the United States. Prior efforts(pre 1990s) to commercialize guayule had been abandoned be-cause production of solid/bulk rubber from guayule was not eco-nomically competitive with imported Hevea latex [1]. Latex use iswidespread in medical and personal hygiene products; however,products containing Hevea latex can cause life-threatening allergicreactions. The relativity recent discovery that guayule latex is safefor people with Type I latex allergy [2–4] sparked new interest inguayule production and research and added value. Because onlyabout 10% of the guayule biomass is used for latex, sustainablecommercialization of guayule would be further improved with in-

Ltd.

: +1 215 233 6406.. Boateng).

come derived from co-products [1]. With the recent interest inrenewable sources of energy, fuel produced from conversion ofthe residual biomass after latex extraction could be an importantco-product.

Latex is extracted from guayule shrubs by an alkaline aqueousextraction process, throughout which the extracted rubber ismaintained as a stable emulsion [3,4]. In this process, extractionof rubber from guayule shrubs (usually harvested just above theground) leaves a finely-divided, solid biomass residue (guayule ba-gasse), made up mostly of cellulose, hemicellulose, lignin, and re-sin. Resin, a complex mixture including terpenes, sesquiterpenes,and fatty acid compounds, can be extracted from the bagasse orthe whole plant [5], and may be used as a plastic binder, adhesive,or wood treatment for insect control [1]. The bagasse, with or with-out resin, can be used in plastic composite materials [6] and for soilamendment. It has also been proposed as a source of bioenergy.

Because of the drive to reduce the use of fossil fuels, the bioen-ergy proposition has sparked great interest. Previously, guayulehas been used as an energy source in the form of fireplace logs[7] and pelletized fuel by blending with cotton gin trash [1]. How-ever, few reports on conversion of guayule to liquid fuels exist with

2208 A.A. Boateng et al. / Fuel 88 (2009) 2207–2215

current conversion technologies. Guayule resin has been converteddirectly into liquid hydrocarbons [8]. Ethanol has been produced inlow yield by fermentation of the guayule cellulosic material [1].One thermochemical conversion process reported thus far wasdeveloped by Kuester [9] and involved steam gasification of gua-yule followed by liquefaction of the gas to produce diesel fuel usingthe Fischer–Tropsch synthesis. Although a patent was issued forthis development, no commercial process has been realized todate, most likely because of the prohibitive cost associated withscaling up the process.

Another thermochemical conversion process with a smallerphysical footprint, and which can be easily adapted to a smallerscale, is fast pyrolysis. Fast pyrolysis involves the rapid heating oforganic material in the absence of air, and has been applied to var-ious biomass feedstocks for the production of pyrolysis liquids(bio-oil) (for recent reviews see: [10,11]). Pyrolysis co-products,such as non-condensable gases (NCG: CO2, CO, H2, and light hydro-carbons) and biocharcoal, also are produced. The NCG can providethe energy needs for the endothermic pyrolysis reaction and thebiocharcoal can be used as a soil amendment. The bio-oil itself isa complex mixture of hundreds of oxygenated organic compounds.Raw bio-oil, produced from non-catalytic pyrolysis of wood, gener-ally has an energy content of about 50% that of diesel oil; it can beburned as boiler fuel or upgraded, by either catalytic cracking orcatalytic hydro-deoxygenation, to hydrocarbon fuels [12]. To ourknowledge, no report exists on the production of pyrolysis liquidsfrom guayule shrub or its residues, despite being a potentially richsource of hydrocarbon compounds from the rubber and resin com-ponents. Pyrolysis of guayule bagasse is an attractive proposition,not only because of the need to develop sources of renewable fuels,but also the need to add value to a material that is considered‘‘waste” after the extraction of the rubber. We herein report the re-sults of fast pyrolysis of guayule bagasse and whole raw plantmaterial carried out in a fluidized bed reactor to produce bio-oil.Thermochemical analysis of the biomass feedstock, its conversionefficiencies, and product analysis suggest that an energy-densebio-oil is produced from both guayule feedstocks up to about a60% yield. Accompanying the bio-oil are charcoal (30% yield) and7.8–15.3% non-condensable syngas (NCG). Although the bio-oilsproduced from the two feedstocks had similar energy contentand properties, the charcoal and NCG from the bagasse had higherenergy contents than those from the whole shrub. Based on thehigh energy content of bagasse pyrolysis products, it would beworthwhile to evaluate the economics of co-locating a bioenergyrefinery that uses the fast pyrolysis process with the latex extrac-tion refinery.

2. Methods and materials

2.1. Biomass (guayule)

Guayule samples tested included whole shrub and bagasse, aresidue after latex extraction. These were provided by Yulex Cor-poration (Maricopa, AZ). Whole guayule shrubs (mature, AZ-2 vari-ety) were harvested above the ground from fields near Maricopa,AZ, in December, 2007. Ten kilograms each of shrub and bagassematerial was collected during a commercial latex production runof several thousand pounds of shrubs. The process first removesthe leaf portion of the material (up to 30–70 wt% depending onseason) and then extracts latex rubber (5–10 dry wt%), resultingin a finely-divided, dry solid. Because the shrubs are defoliatedprior to latex extraction, the bagasse is comprised mainly of a mix-ture of wood and bark, plus the resin contained in those plantparts. The shrub sample was collected from harvested feedstockfollowing shipping but prior to defoliation and latex extraction.Both samples were dried at 60 �C for 24 h and ground in a Wiley

mill using a 2-mm screen (#10 US Mesh). Proximate and ultimateanalysis (ASTM D-5291) and X-ray fluorescence (XRF) mineralanalysis on the ash content (ASTM D-4326) were carried out atWyoming Analytical Laboratories, Inc. (Laramie, WY). Analysis forcell wall carbohydrates (cellulose, hemicellulose) and lignin wereperformed at the ARS-USDA Plant Science Research Unit (St. Paul,MN) using methods described by Dien et al. [13].

2.2. Pilot-scale pyrolysis

A pyrolysis liquid fuel intermediate was produced by carryingout pyrolysis in a bubbling fluidized bed of quartz sand at tem-peratures in the 450–550 �C range. The apparatus, described pre-viously in detail by Boateng et al. [14,15], comprises a 7.62 cm (3-in) diameter fluidized reactor section, 2 cyclones in series for gasclean-up followed by a series of 4 condensing canisters cooled bya dry-ice/water bath and completed with a bank of three electro-static precipitators that collects the bulk of the pyrolysis oil pro-duced. The functions of each of the components described havebeen detailed by Boateng et al. [14]. Temperature and pressuremeasurements were carried out using thermocouples and pres-sure transducers and logged using a Labview data acquisitionand control system (National Instruments, Austin, TX). Mass bal-ance was carried out by gravimetric analysis of the feedstockmaterial used versus the materials produced including bio-oiland charcoal. The uncondensed gas composition was analyzedby gas chromatography (GC) (Agilent MicroGC 3000A). Gas quan-tity was determined by the difference between the fluidizing gasmeasured by a mass flow meter (Alicat Scientific, Tucson, AZ) andthe effluent gas measured with a bulk gas flow meter (Metris).Energy flow in and out of the reactor was determined by thestreams’ enthalpy and energy balance using the mass rates andtemperatures, as well as the energy content measured for thebio-oil and charcoal products. Two independent pyrolysis runswere performed for the bagasse (12/4/07 and 12/19/07) andshrub (12/10/07 and 12/13/07) within two weeks of receivingthe materials.

2.3. Product analysis

Bio-oil water content was determined by Karl–Fischer (K–F)titration using 3:1 methanol:chloroform as solvent. Water insolu-bles (consisting of pyrolytic lignin and extractives) were deter-mined by mixing warm water and bio-oil in a 2:1 ratio, filtrationof the sample through a 0.45 lm PTFE filter, followed by determi-nation of the water content of the aqueous layer by K–F. Water sol-ubles and insolubles were then calculated following Meier [16].GC/MS analysis of bio-oil was performed on an Agilent 6890N GCequipped with an Agilent 5973 mass selective detector (MSD).The GC column used was a DB-1701, 60 m � 0.25 mm, 0.25 lmfilm thickness. The oven was programmed to hold at 45 �C for4 min, ramp at 3 �C/min to 280 �C and hold there for 20 min. Theinjector temperature was 250 �C, and the injector split ratio setto 30:1. The flow rate was 1 mL/min of the He carrier gas. Thebio-oil samples were prepared as 6 ± 1% solutions in acetone whichwere filtered through a 0.45 lm PTFE filters prior to injection. Forquantification, response factors relative to the internal standard,fluroanthene, were determined for each quantified component fol-lowing Oasmaa and Meier [17].

HPLC analysis of the bio-oils was performed on a Waters BreezeHPLC system using a refractive index detector, set at 30 �C. The mo-bile phase was 0.007 N H3PO4. The column used was an AminexHPX-87H, 300 � 7.8 mm (Bio-Rad, Inc.), and was heated to 30 �C.The pump was programmed at a flow rate 0.6 mL/min and n-pro-panol was used as the internal standard for quantification [17].Proximate and ultimate analyses of the biomass feedstock and

Table 2Ash analysis for guayule biomass (XRF, wt% of ash).

Bagasse Whole

Silicon dioxide (wt%) 14.38 11.03Aluminum oxide (wt%) 3.34 2.12Iron oxide (wt%) 1.31 1.13Calcium oxide (wt%) 52.63 34.38Magnesium oxide (wt%) 6.26 5.14Sodium oxide (wt%) 0.39 9.37Potassium oxide (wt%) 6.52 19.17Titanium dioxide (wt%) 0.16 0.12Manganese dioxide (wt%) 0.11 0.09Phosphorus pentoxide (wt%) 4.00 13.26Strontium oxide (wt%) 0.40 0.40Barium oxide (wt%) 0.06 0.06Sulfur trioxide (wt%) 10.44 3.73

A.A. Boateng et al. / Fuel 88 (2009) 2207–2215 2209

bio-oil product were performed by Galbraith Laboratories, Inc.(Knoxville, TN).

3. Results and discussion

3.1. Feedstock analysis

Proximate analysis (Table 1) shows that the whole guayuleshrub contained �12.5% ash, whereas bagasse contained only�3% ash. This difference is likely attributable to mineral accumula-tion in leaves which were present in the whole shrub but not partof the bagasse sample, and also washing out of some inorganiccomponents out during aqueous phase processing of the bagasse.Because shrubs are defoliated prior to latex extraction, bagasse ismainly a mixture of the woody and bark fractions from the stem,plus any resin in those fractions. Organic nitrogen in the form ofplant protein is mostly removed from the shrub during defoliationand the aqueous extraction process steps. Bagasse had less nitro-gen and sulfur, at 0.7% and 0.2%, respectively, than whole shrub,at 1.6% and 0.7%, respectively. The same analysis also indicatedthat the bagasse used in this study had higher carbon (57.0%,DAF) and higher hydrogen (6.65% DAF) contents, but lower oxygen(35.57%, DAF) than the whole shrub with corresponding elementalcontent of 54.0% C, 6.0% H, 37.61% O, DAF. Both guayule feedstockscontained more carbon than typical biomass which normally fallsin the 45–50% range [14,15]. XRF analysis of the ash from bothfeedstocks (Table 2) indicated that the most abundant mineral inthe samples was calcium, with a larger concentration found inthe ash of the bagasse sample. Other major mineral elements foundincluded silicon, potassium, phosphorous, sodium and magnesium.Potassium, phosphorus and sodium were all found in much higherconcentrations in the ash of the whole shrub than in the ash of thebagasse, suggesting they were washed away in solution duringaqueous phase processing.

Analysis of cell wall carbohydrates, lignin, and protein for thetwo guayule feedstocks is presented in Table 3. Overall cell wallcontent was 807 g/kg for the bagasse and 548 g/kg for the whole

Table 1Analysis of guayule samples used for pyrolysis.

As received

BagasseProximate (wt%)

Moisture 3.14Ash 3.05Volatile matter 79.05Fixed carbon 14.76

Ultimate (wt%)C 53.46H 6.15N 0.63S 0.19O 33.38HHV (kJ/kg) 21,680

WholeProximate (wt%)

Moisture 2.63Ash 12.46Volatile matter 72.68Fixed carbon 12.23

Ultimate (wt%)C 45.85H 5.09N 1.34S 0.60O 32.03HHV (kJ/kg) 17,847

shrub material. The bagasse was richer in cellulose, hemicelluloseand lignin, while the whole shrub was richer in protein. This isagain attributed to the presence of leaves in the whole shrub sam-ple but not in the bagasse. The entire compositional structure ulti-mately results in the bagasse feedstock having higher energycontent, 24,177 kJ/kg, than the whole shrub at 21,018 kJ/kg on adry ash free basis prior to pyrolysis. It is noteworthy that these val-ues are significantly higher than the energy content of biomassmaterials such as wood and grasses, which typically range between15,000 and 19,000 kJ/kg [10,14,15]. With a heating value similar tothat of some coal [18], guayule bagasse has good potential as acombustion fuel ‘‘as is” on a dry basis, presumably with resinand any residual rubber contributing hydrocarbon compounds aspart of the energy source.

3.2. Product yields and energy distribution

As mentioned earlier, the condensation system used to collectpyrolysis oil comprised a series of condensers, followed by a bankof electrostatic precipitators (ESP) capable of fractionating thebio-oil vapors into fractions with varying water content. Unlike

Dry basis Dry, ash free

3.1581.61 84.2615.24 15.74

55.19 56.996.35 6.560.65 0.670.20 0.2134.36 35.5722,385 24,177

12.8074.64 85.6012.56 14.40

47.09 54.005.23 6.001.38 1.580.62 0.7132.88 37.7118,329 21,018

Table 3Chemical composition of guayule samples used in pyrolysis study.

(%, dry matter)

Total cell wall Cellulosea Hemicelluloseb Lignin Crude proteinc

Bagasse 80.7 23.0 25.8 30.0 4.2Whole 54.8 14.8 18.9 18.7 9.9

a Cellulose as cell wall glucose.b Hemicellulose as sum of cell wall xylose, arabinose, mannose and uronic acids.c Determined by %N � 6.25.

Table 4Process conditions for the experimental runs.

Guayule Bagasse Bagasse Whole Whole

Pyrolysis run date 12/4/2007 12/19/2007 12/10/2007 12/13/2007Operational conditionsBiomass (g) 1648 1997 2332 2552Feed rate (g/h) 336.3 443.7 514.8 639.6Bed temperature (�C) 478.9 471.9 473.8 463.0Condenser #1 temperature (�C) 240.7 227.5 245.9 214.2Condenser #3 temperature (�C) 89.0 65.2 79.6 86.5Condenser #4 temperature (�C) 61.1 45.7 53.2 52.9ESP temperature (�C) 45.8 37.1 39.2 39.6Heat rate (�C/s) 4190 4129 4146 4051Total quench rate (�C/s) 54.1 54.4 54.3 52.9

Table 5Pyrolysis product distributions.

Bagasse (%) Whole (%)

Bio-oila 64.1 59.1Charcoal 20.6 33.2NCGb 15.3 7.8

a Includes coke and unrecovered liquid product trapped in the system.b NCG = non-condensable gas.

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other biomass feedstocks the condensation chemistry of the pyro-lyzed guayule vapors in this system had a higher selectivity tocoke formation. The pathway to early coke formation is unknown,but one possible reason for it in the guayule feedstocks is thepresence of low-melting compounds such as terpenes found inthe resin. Preliminary studies show that the resin component ofthe guayule biomass melts at 60 �C. Early coke formation encoun-tered in the first condensers can result in equipment foulingproblems and presents a potential industrial challenge. However,no coke formation was experienced at the ESP where the bulk ofthe bio-oil was recovered. ESP recovery constituted about 23.4%of the feedstock for the bagasse and 20.5% for the whole shrub(data not shown). Besides coking, the low-melting point of thesamples and the high viscosity of the bio-oil produced thereofpresented unusual operational challenges. Therefore, the screwfeeder was chilled below this temperature (60 �C) to mitigatefeeding problems. The bio-oil discharged at the ESP was unusu-ally viscous compared to pyrolysis liquids derived from otherplant-based feedstocks such as herbaceous grasses and legumes[14,15] perhaps associated with the rheology of the resin com-pounds. The extremely high viscosity required modifying the pro-cess conditions to include higher condenser and ESP temperaturesfor each run (Table 4) to ensure flowability. The drawback tohigher temperatures is a reduced quench rate and, in turn, in-creased coking, both of which contribute to lower product yield.When the bio-oil and coked product recovered in the condensersand that remaining in the system piping are counted as bio-oilproduct, the overall bio-oil yield was estimated at 64.1% for thebagasse and 59.1% for the whole shrub (Table 5).

Along with the bio-oil, 20.6 and 33.2 wt% of charcoal were,respectively, produced from the bagasse and the whole shrub.The higher ash content of the whole shrub contributed to highercharcoal yield for the bagasse, granted that the char constitutescarbon and ash. Conversely, the lower ash content of the bagassecharcoal constitutes a higher fixed carbon content and, in turn, ahigher energy content (vida infra). The third product besides bio-oil and charcoal, the NCG, constituted 15.3 and 7.8 wt% for the ba-gasse and whole shrub, respectively.

The energy contents of the bio-oils collected at the ESP and thecondensers were very similar for both feedstocks (Table 6) with

average high heating values (HHV) of 30,508 kJ/kg for the bagasseand 30,428 kJ/kg for the whole shrub on a wet weight basis. De-spite the low water content (�2%, vida infra), these bio-oils stillhave exceptionally high energy contents compared to other ligno-cellulosic feedstocks on a dry weight basis, an unprecedented 75%of the gross heating value of diesel fuel for non-catalytic pyrolysis[10,11,14,15]. Several attributes may contribute to the high en-ergy-dense bio-oil from this plant: (i) high carbon content bio-oils,in turn, attributable to higher carbon precursor feedstocks, (ii) highlignin content resulting in higher conversion to more aromatic,pyrolytic lignin compounds such as phenols, and perhaps mostimportantly, (iii) conversion of the inherent rubber and resin com-pounds in the feedstock into higher molecular weight hydrocar-bons in the bio-oil. Islam et al. [19] attributed higher C/H ratiosin pyrolysis oil produced from used tires to diesel-like hydrocarbonstructures including limonene (1-methyl 4-(1-methylethenyl)cyclohexene), a compound that was detected in the bio-oil fromguayule (Tables 7 and 8). The pyrolysis co-products also had highenergy content. The gross heating value of the charcoal co-prod-ucts (Table 9) averaged 24,438 kJ/kg and 15,130 kJ/kg for the ba-gasse and whole shrub, respectively. These, compare with21,680 kJ/kg and 17,847 kJ/kg for the parent biomass feedstocknoted earlier. The overall energy balance and efficiency of recoveryis depicted in Table 10. The energy efficiency, i.e., amount of energythat is recovered in the products as a percentage of the energy in-put (biomass and heat), can be evaluated on a product basis. Forthe guayule bagasse, about 45% of the energy input was recoveredin the bio-oil compared to about 56% from whole shrub. When theenergy in the charcoal and NCG is accounted for as potential usefulenergy, about 60% of the energy is recovered from bagasse

Table 6Elemental analysis and physical properties of bio-oil.a

Bagasse Whole

Elemental analysis (wt%, dry basis)C 69.97 69.93H 7.96 8.54N 0.82 2.92S 0.07 0.20Cl 0.021 0.0288O 21.38 19.31Ash 0.09 0.08

HHV (kJ/kg, wet basis) 30,508 30,428Density (g/mL) 1.1382 1.0904Flashpoint (�C) >120 >120Viscosity @ 60 �C (cSt) 510.3 2472.7

a Average of the two bio-oils produced for each feedstock.

A.A. Boateng et al. / Fuel 88 (2009) 2207–2215 2211

compared to about 73% from whole shrub, considered statisticallyequivalent within overall system errors (2–11%).

3.3. Bio-oil composition and properties

Only slight differences in bio-oil yield, energy content, etc.between the two feedstocks were observed. These results werevalidated by the compositional analysis, which also revealedsmall differences in the bio-oil. Elemental analysis (Table 6)shows similar carbon (�70%) and hydrogen (8–8.5%) contentsof the liquid products, hence similar C/H ratios, thereby resultingin comparable HHV. However, the carbon content of the bio-oilswas unusually high compared to that of the parent biomass (Ta-ble 1), indicating a higher carbon conversion in the liquid frac-tion than is typical for wood and grasses [11,14]. Normally theelemental composition of bio-oils produced from non-catalyticpyrolysis has closely followed that of the feedstock [11]. Thistrend is different in guayule. The oxygen content of bio-oil pro-duced from both feedstocks was lower than the parent biomass,making the product a more upgradable fuel intermediate thanthat produced from other plant-based feedstocks. Oxygen re-moval by hydro-deoxygenation is one process path to upgradingpyrolysis liquids to hydrocarbon fuels. Therefore, guayule has anadvantage over other feedstocks with respect to conversion togasoline/diesel fuel intermediates. Nitrogen content of the pyro-lysis bio-oil derived from the whole shrub (Table 6) was foundto be about 3.5 times greater than bagasse (2.92% vs. 0.82%),mirroring the levels of nitrogen found in the feedstock. This isprobably derived from the higher protein content in the wholeshrub compared to the bagasse. The high nitrogen places thebio-oil from the whole shrub at a disadvantage for combustionfuel because it will contribute to fuel NOX emission. A similarscenario prevails with respect to sulfur and as such, SO2

emissions.As discussed elsewhere [11,14], pyrolysis liquids consist of

water soluble and water insoluble fractions. Quantification ofthe water content of the guayule bio-oils collected at the ESP(Table 11) demonstrates that it was extremely low in both feed-stocks at 1.4% and 2.1% from the bagasse and whole shrub,respectively. Water solubles, largely derived from the celluloseand hemicellulose components of the feedstock, averaged about45%, and water insolubles, which include pyrolytic lignin andhydrocarbon soluble extractables, averaged about 54% for thebio-oils from both feedstock. These characteristics make thebio-oil from guayule of higher quality for fuel than that pro-duced from wood or grasses [14,15]. Several factors may contrib-ute to these characteristics, including high lignin content, as wellas resin and residual rubber. The resin and rubber decompositionproducts may persist in the bio-oil as evidenced by the extre-

mely high viscosity of the bio-oils. Limonene, a terpene constit-uent of guayule resin earlier identified by Scora and Kumamoto[20], was found in the guayule feedstocks and in the bio-oilsproduced from them (0.13 wt% in the bio-oil from bagasse,0.08 wt% in bio-oil from the whole plant) by GC/MS analysis (Ta-bles 7 and 8). Limonene has been identified as a component oftire pyrolysis oils derived from natural rubber and significantlycontributes to higher hydrocarbon fractions and higher calorificvalue fuel [21,22]. Despite the low percentage of limonene inthe guayule bio-oils, its presence suggests that other resin andrubber hydrocarbon decomposition products, perhaps of highermolecular weight than detectable by GC, are present and con-tribute to the unique properties of these bio-oils. Interestingly,bio-oil produced from the whole shrub (from which no rubberhas been extracted) was about 4.8 times more viscous than thatproduced from the bagasse (Table 6), strongly suggesting thatrubber makes a significant contribution to the high viscosity ofthe pyrolysis liquids. Due to the high resin content of leaves[23], the whole shrub would most likely also have more resin,which also would contribute to the higher viscosity of shrubbio-oil. How the resin and rubber decomposition pathway prod-ucts specifically contribute to viscosity increases is unclear.

Among the few differences in bio-oil composition in the twosamples of guayule were products derived from the polysaccha-rides. The bagasse bio-oil contained less acetic acid (about1.3 wt% compared to 3.1 wt%) and higher levels of levoglucosan(1.4 wt% vs. 0.8 wt%) and acetol (0.7 wt% vs. <0.1 wt%) than thatfrom the whole shrub. Pathways to production of compoundsother than levoglucosan, a major product of cellulose pyrolysis,can be catalyzed by alkali metals in the ash [24,25]. The bagassecontains more cellulose than the whole shrub; a possible explana-tion of why a higher fraction of levoglucosan was found. Irrespec-tive of the reason for the differences in levoglucosan fractions inthe two samples, the levels found in the bio-oils in both of theguayule feedstocks are still lower than what is typically foundin pyrolysis oils from wood [26]. Most of the lignin-derived com-pounds quantified were found in higher quantities in the bagasse-derived bio-oil, reflecting the higher lignin concentration of thatfeedstock (Table 3). Among these, phenol was the most abundantphenolic compound in the whole shrub-derived bio-oil, while iso-eugenol was the most abundant phenol compound in the bio-oilproduced from the bagasse feedstock (Table 8). Again, these dif-ferences may be attributable to the different ash content, as onestudy found a positive correlation between alkali metals and phe-nol formation over more substituted phenolic lignin monomers[25].

3.4. Co-products of pyrolysis liquid production

Biocharcoal has received particular attention recently, havingbeen proposed as a potential carbon-sequestering, soil-amendingco-product of the pyrolysis process [27,28]. Additionally charcoalcan have value as a solid fuel or as a metal adsorption agent.Pyrolysis conditions can be adjusted to maximize char yieldand produce high surface area charcoals used in metal adsorp-tion; however, the mode of fast pyrolysis used here maximizesbio-oil yield and the charcoal produced tends to posses low sur-face area [29,30]. The bio-oil production reported herein wasaccompanied by charcoal production of 20.6 wt% of the feedstockfor the bagasse and 33.2 wt% for the whole shrub (Table 5). Thecharcoal products from the two feedstocks differed in chemicalcomposition significantly (Table 9). The higher ash whole shrubfeedstock led to a higher ash-containing charcoal averaging42.5% compared to 24.9% in the bagasse charcoal. On a dry,ash free basis (DAF) the bagasse charcoal at 33,325 kJ/kg had ahigher energy content than that produced from the whole shrub

Table 7Compounds detected in GC/MS of pyrolysis oil from guayule.

RT (min) Compound Grouping Bagasse Whole

9.5 Acetic acid Acids X X10.9 Acetol Oxygenates X X11.2 Toluene Aromatic X X14 Propionic acid Acids X15.8 p-Xylene Aromatic X X17.8 Styrene? (cyclooctatetraene) Aromatic X X18.6 Furfural Furan X X19.2 4-Hydroxy-4-methyl-2-pentanone Oxygenates X X20.6 1,2,4-Trimethylbenzene Aromatic X X20.9 Furfuryl alcohol Furan X X21.4 2-Methyl-2-cyclopenten-1-one Ketone X X22.8 Limonene Terpene X X26.2 2,3-Dimethyl-2-cyclopenten-1-one Ketone X X26.5 Indene Aromatic X X26.5 3-Methyl-2-cyclopenten-1-one Ketone X X26.7 Butyrolactone Ester X X29.3 2-Hydroxy-3-methyl-2-cyclopenten-1-one Ketone X X31.0 Phenol Phenol X X31.6 Methyl Indene Aromatic X X31.8 Guaiacol Phenol X X32.2 3-Ethyl-2-cyclopenten-1-one Ketone X33.4 o-Cresol Phenol X X33.7 3-Ethyl-2-hydroxy-2-cyclopentene-1-one Ketone X X34.5 Naphthalene Aromatic X X35.0 p-Cresol Phenol X X35.1 m-Cresol Phenol X X36.4 Benzylnitrile Nitrogen X X36.6 2-Methoxy-4-methyl phenol Phenol X X36.9 2-Ethylphenol Phenol X37.0 Dimethyl Indene Aromatic X X37.2 2,4-Dimethyl phenol Phenol X X38.9 3,5-Dimethylphenol Phenol X X39.1 4-Ethyl phenol Phenol X X39.2 3-Ethylphenol Phenol X X39.3 Tetradecene Alkene X39.6 Methyl naphthalene Aromatic X X40.4 4-Ethyl-2-methoxyphenol Phenol X40.5 Pyridinol Nitrogen X40.9 1-Ethyl-4-methoxy benzene Oxygenates X41.0 4-Ethyl-3-methyl phenol Phenol X X41.3 4-(1-Methylethyl)phenol Phenol41.3 2,3-Anhydro-D-mannosan Sugar X41.7 2,3-Dihydroxy-1H-Inden-1-one Ketone X X42.0 1,4:3,6-Dianhydro-a-D-glucopyranose Sugar X X43.0 2-Methoxy-4-vinyl phenol Phenol X X43.7 Ethyl naphthalene Aromatic X X44.0 Eugenol Phenol X X44.2 Dimethyl naphthalene Aromatic X X44.7 2-Allyl phenol Phenol X X45.2 2,6-Dimethoxyphenol (syringol) Phenol X X45.4 Indole Nitrogen X48.4 Isoeugenol Phenol X X49.7 Hydroqunione Alcohol X51.3 2-Me-1,4-benzenediol Phenol X53.1 1,2,3,4,4a,5,6,7-Octahydro-a,a-tetramethyl-2-naphthalenemethanol Alcohol X X58.1 Levoglucosan Sugar X X58.7 2,6-Dimethoxy-4-(2-propenyl)phenol Phenol X X59.8 4-Hydroxy-dimethoxy-benzaldehyde Oxygenates X

X = detected.

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(26,749 kJ/kg) owing to the higher carbon (84.1%, DAF) and low-er oxygen (11.6%, DAF) content of the bagasse charcoal versus (C74.4%, O 18.6%) the whole shrub charcoal. Nitrogen and sulfurwere lower for the bagasse charcoal (1.2% and 0.2%, respectively)than the whole shrub charcoal, 2.2% and 2.0%, respectively, mak-ing the latter charcoal potentially less greenhouse gas (NOX andSOX) emitting during combustion.

Given that NCG is a potential source of fuel to provide heat forthe endothermic pyrolysis reaction, its fuel quality is equallyimportant. The compositions of the NCG on a dry volume basisare presented in Fig. 1. NCG from the guayule bagasse represents

a better synthesis gas, having higher proportions of combustiblegases such as hydrogen, carbon monoxide and methane than thatproduced from the whole shrub, which typically contained morecarbon dioxide. Although there is the likelihood that the NCG con-tains water that was not condensed with the bio-oils, no measure-ment was made to quantify it and is therefore unknown.

All things considered, guayule bagasse is an attractive bioener-gy feedstock for pyrolysis. Importantly, it is an industrial by-prod-uct that is generated at site and therefore requires notransportation and/or no major pre-processing, a major cost inbiomass conversion. Also, unlike energy crops such as switch

Table 8Quantification of some bio-oil components (wt%).

Quantification method Compound Group Bagasse Whole Typical wooda

Cellulose/hemicellulose derived compounds (wt%)HPLC Acetic acid Acids 1.29 3.09 0.5-12GC Furfural Furan 0.03GC Furfural alcohol Furan 0.09 0.07 0.1-1.1GC 2-Methyl-2-cyclopenten-1-one Ketone 0.06 0.08 0.1-5.2GC 3-Methyl-2-cyclopenten-1-one Ketone 0.09 0.08GC 4-Hydroxy-4-methyl-2-pentantone Oxygenates 0.08 0.07HPLC Acetol Oxygenates 0.73 0.02 0.7-7.4HPLC Levoglucosan Sugar 1.37 0.80 4.8-5.4

Lignin derived compounds (wt%)GC Guaiacol Phenol 0.39 0.30 0.1-1.1GC 2-Methoxy-4-methyl phenol Phenol 0.15 0.13 0.1-1.9GC Isoeugenol Phenol 0.73 0.39 0.1-7.2GC Phenol Phenol 0.65 1.30 0.1-3.8GC o-Cresol Phenol 0.21 0.21 0.1-0.6GC p-Cresol Phenol 0.19 0.41 0.1-0.5GC m-Cresol Phenol 0.27 0.21 0.1-0.3GC 2,4-Dimethylphenol Phenol 0.13 0.14GC 3,5-Dimethylphenol Phenol 0.07 0.08GC 4-Ethyl phenol Phenol 0.11 0.23GC 3-Ethyl phenol Phenol 0.04 0.04

Protein derived compounds (wt%)GC Benzyl nitrile Nitrogen Trace 0.06GC Indole Nitrogen 0.01

Resin derived compounds (wt%)GC Limonene Terpene 0.13 0.08

a From Diebold et al. [25].

Table 9Analysis of charcoal co-product.a

As received Dry basis Dry, ash free

BagasseProximate (wt%)

Moisture 1.82Ash 24.87 25.33Volatile matter 23.46 53.87 32.03Fixed carbon 49.88 50.81 67.98

Ultimate (wt%)C 61.73 62.88 84.08H 2.13 2.17 2.90N 0.90 0.91 1.22S 0.15 0.15 0.20O 8.42 8.57 11.61HHV (kJ/kg) 24,438 24,892 33,325

WholeProximate (wt%)

Moisture 0.94Ash 42.5 42.91Volatile matter 32.68 32.99 44.72Fixed carbon 23.88 24.11 55.29

Ultimate (wt%)C 42.07 42.47 74.39H 1.62 1.63 2.86N 1.25 1.26 2.20S 1.125 1.14 1.99O 10.505 10.61 18.58HHV (kJ/kg) 15,130 15,274 26,749

a Average of the two charcoal samples produced for each feedstock.

Table 10Pyrolysis product energy efficiency.

Bagasse Whole

Bio-oila 44.97 56.17Bio-oil + char 58.07 70.84Bio-oil + char + NCGb 60.45 72.56

a Includes coke and unrecovered liquid product trapped in the system.b NCG = non-condensable gas.

A.A. Boateng et al. / Fuel 88 (2009) 2207–2215 2213

grass and Miscanthus, the crop production costs are born by thevalue of the latex primary product. Because the bagasse materialis derived mainly from guayule stems, the bio-oil and charcoalproduced have lower nitrogen and ash contents than whole shrubmaking it process- and environmentally-friendly for combustionapplications. Additionally, the charcoal and NCG co-products arehigh in energy.

Table 11ESP bio-oil water solubility profile.

Bagasse Whole

Water 1.44 2.11Water insolubles 53.57 54.25Water solubles 44.99 43.64

Bagasse, dry basis

CO49%

H211%

CH415% CO2

25%

Whole, dry basis

CO243%

CO35%

H210%

CH412%

Fig. 1. Composition of non-condensable gases.

2214 A.A. Boateng et al. / Fuel 88 (2009) 2207–2215

4. Conclusions

The fast pyrolysis of guayule was performed on two types ofsamples at 500 �C in a fluidized bed reactor. The first sample wasguayule bagasse, a residue after latex extraction, and the secondwas the whole shrub material. The yields and properties of thebio-oil produced were similar for the two high energy feedstocks.Bio-oil yield was about 60% for each feedstock, with energy contentof approximately 30,000 kJ/kg, 75% that of heavy fuel oil and muchhigher than typical biomass pyrolysis oils (�20,000 kJ/kg). Highgross heating value charcoal co-products were produced in about20 wt% yield from the bagasse and about 30 wt% yield from thewhole shrub.

Because an energy dense bio-oil can be produced as a high en-ergy fuel intermediate from guayule bagassse which presently is alow value co-product of guayule latex production, co-locating afast pyrolysis based bio-refinery with a rubber production facilitymay provide economic benefits. Compared with other previously

developed thermochemical conversion methods for guayule, i.e.,steam gasification followed by fuel production Fisher–Tropsch syn-thesis, fast pyrolysis has the advantage of being amenable to smal-ler scale systems that fit the current latex industry refinery size,and therefore increase the economic advantages of that methodof biofuels production. Further study on the economics of co-locat-ing a fast pyrolysis facility at a rubber production site would berequired.

Acknowledgments

The authors thank Rich Cook, Mike Mandel, Johnny Morphew,Kenneth Schaffer, Eric Tilman, Kenneth Todd, and Jhanel Wilsonof USDA-ARS, ERRC for technical support. We would also like tothank Hans-Joachim Jung (USDA-ARS, St. Paul, MN) for providingthe cell wall chemical analysis, and Ray McCoy, Yulex Corporation,for providing guayule samples. We further appreciate the useful in-sight and discussions with Terry Coffelt, Francis Nakayama (USDA-ARS, USALARC) and Kevin Holtman (USDA-ARS WRRC).

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