Thermal Pretreatment ofLignocellulosic BiomassWei Yan, Tapas C. Acharjee, Charles J. Coronella, and Victor R. VasquezChemical and Metallurgical Engineering Department, University of Nevada, MS170 Reno, NV 89557; coronella@unr.edu(for correspondence)

Published online 5 August 2009 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ep.10385

Torrefaction is a process to convert diverse ligno-cellulosic biomass feedstocks into an energy dense ho-mogeneous solid, a pretreatment for subsequent ther-mochemical conversion. Loblolly pine was treated bywet torrefaction (hot compressed water, 200–2608C)and dry torrefaction (nitrogen, 250–3008C), withmass yield of solid product ranging between 57 and89%, and energy densification to 108–136% of theoriginal feedstock. The solid product has been charac-terized, including proximate analysis, fiber analysis,ultimate analysis, and equilibrium moisture. In bothdry and wet torrefaction, increasing temperatureresults in decreased mass yield and increased energydensification, and results in a solid with increasedcarbon content, decreased oxygen content, anddecreased volatiles. The biomass is transformed into afuel similar to a low-rank coal. Generally, the wet tor-refaction process produces a solid with greater energydensity than dry torrefaction, with the same massyield. The fiber analysis indicates that hemicelluloseis quickly removed during wet torrefaction, and thesolid product contains substantial quantities of aque-ous soluble compounds. The equilibrium moisturecontent of solids produced by both processes is some-what decreased from that of the biomass feedstock,indicating a hydrophobic solid suitable for storage andtransportation. � 2009 American Institute of ChemicalEngineers Environ Prog, 28: 435–440, 2009Keywords: biomass pretreatment, torrefaction, ligno-

cellulosic biomass, thermochemical conversion

INTRODUCTION

Thermochemical conversion is a promising routeto convert lignocellulosic biomass to fuel, chemicals,and renewable power [1, 2]. However, both pyrolysisand gasification of raw biomass feedstock have signif-icant, but distinct, technical barriers that must be

eliminated for widespread successful commercializa-tion [2, 3]. In both cases, feedstock handling iscomplex due to the diverse nature of important feed-stocks, including commercial timber, agricultural resi-dues as diverse as rice hulls, corn stover and wheatchaff, and energy crops such as switch grass. Sea-sonal availability and low forest density make feed-stock logistics complex and expensive. To solve theseproblems, there is a need for a process to homoge-nize the feedstocks, while simultaneously producinga stable, energy-dense, solid fuel. Two distinct ther-mal pretreatment technologies are investigated in thisstudy: wet torrefaction and dry torrefaction.

In the process we call wet torrefaction, alsoknown as hydrothermal pretreatment, biomass istreated in hot compressed water, resulting in threeproducts, including gases, aqueous chemicals, and asolid fuel [4–6]. The temperature is in the range of200–2608C, and the pressures are up to 700 psi. Thesolid product contains about 55–90% of the mass and80–95% of the fuel value of the original feedstock.The gas product is about 10% by mass of the feed-stock, and the aqueous chemicals, primarily sugars,make up the balance [7–9]. On the other hand, drytorrefaction, sometimes called low-temperature pyrol-ysis, is a process in which the biomass is heated inan inert gas environment at temperatures rangingfrom 200 to 3008C [3, 10–12]. The dry torrefactionreaction has two products, a solid and a gas. Thesolid has about 60–80% of the mass, and approxi-mately 70–90% of the fuel value of the original bio-mass [10–12].

Both torrefaction processes exhibit some commonfeatures. The solid product has a higher energy den-sity relative to the starting biomass feedstock in bothcases. Both solid products are easily friable and havean aroma similar to wood char. The reaction mecha-nisms of both torrefaction processes are poorly under-stood. The objective of this work is to investigate how� 2009 American Institute of Chemical Engineers

Environmental Progress & Sustainable Energy (Vol.28, No.3) DOI 10.1002/ep October 2009 435

both torrefaction processes affect product distributionand to assess the quality of solid products for thermo-chemical conversion. Characterization of solid prod-ucts is performed, which includes proximate analysis,ultimate analysis, higher heating value, fiber analysis,and equilibrium moisture content. A comparativestudy is made to elucidate the capability of both ther-mal pretreatment technologies to homogenize ligno-cellulosic biomass and produce the immediate solidfuel for thermochemical conversion.

MATERIALS AND METHODS

Biomass and ChemicalsLoblolly pine (Alabama, USA) was acquired as a

typical lignocellulosic biomass for torrefaction. Thesolutions and filter bags for fiber analysis were pur-chased from ANKOM Technology Inc (Macedon,NY).

Wet TorrefactionWet torrefaction of loblolly pine was performed in

a 100-mL Parr bench-top reactor (Moline, IL) at tem-peratures ranging from 200 to 2608C. The tempera-ture of the reactor was controlled using PID control-ler. The reactor pressure was not controlled but indi-cated by a pressure gauge and ranged from 200 to700 psi, approximately in accord with the watervapor pressure. For each run, a mixture of loblollypine and water (water:biomass 5 5:1, w/w) wasloaded into the reaction vessel. The biomass samplesize was about 2 g. Prior to the reaction, the biomasswas stirred manually to ensure complete wetting.Nitrogen was passed through the reactor for 10 minto purge oxygen out of the reactor. The reactor washeated up in 15–30 min and maintained at thedesired temperature for a desired period of time(�5 min). Then, the reactor was cooled to room tem-perature rapidly by immersion in an ice bath. The gassample was released. The liquid sample and solidsample were separated and collected for further anal-ysis.

Dry TorrefactionDry torrefaction of loblolly pine was carried out in

the 100-mL Parr reactor described above, but fittedwith a glass sample holder to separate the biomassfrom the hot reactor walls. The pretreatment was per-formed with a nitrogen flow rate of 10 mL (STP)/min.The glass sample loader with biomass particles(�5 g) were placed in the center of reactor. The tor-refaction temperature was between 250 and 3008Cand was controlled using a PID controller. The tem-perature program consists of a dynamic heating pe-riod following by an isothermal period. After adesired isothermal reaction period (�80 min), thesample was cooled to room temperature with contin-uous nitrogen flow through the reactor. The solidsample was collected for further analysis.

Analytical Methods

Proximate AnalysisSolid samples were analyzed with Perkin Elmer

TGA-7 Thermogravimetric Analyzer (Waltham, MA) todetermine the contents of moisture, volatile matter,and fixed carbon [13]. Thermogravimetric analysiswas carried out under a nitrogen purge at a constantrate of 50 mL/min to prevent oxidation of samples.Samples were first heated up to 1058C at the rate of108C/min, maintained at 1058C for 10 min, then jumpto 9008C at the rate of 508C/min and hold for 60 min.Mass evolved at 1058C is said to be moisture. Massevolved at temperatures between 105 and 9008Cbelongs to volatiles. All mass remaining after heatingto 9008C consists of fixed carbon and ash. Ash con-tent of the pretreated biomass was measured sepa-rately using a standard method for ash in biomass[14]. The solid sample was first dried at 1058C for 24h and brought to constant weight by igniting at 5758Cfor 5 h in a muffle furnace.

Ultimate AnalysisSolid samples were dried in an oven at 1058C for

24 h. Ultimate analysis of both biomass feedstock andpretreated biomass was performed with a FlashEA1112 (Pittsburgh, PA) elemental analyzer for fulldetermination of C, H, N, S, and O [15].

Higher Heating ValueThe higher heating value (HHV) of solid samples

was measured in a Parr 1241 adiabatic oxygen bombcalorimeter (Moline, IL) fitted with continuous tem-perature recording [16]. The sample (0.2–0.5 g) wasdried at 1058C in the oven for 24 h prior to analysis.

Fiber AnalysisA modified Van Soest method using the ANKOM

A200 Filter Bag Technique (FBT) was used to deter-mine the contents of hemicellulose, cellulose, lignin,and aqueous soluble compounds in solid samples[17, 18]. The solid samples are first crushed andsieved to the desired particle size (20–65 mesh) andwere dried at 1058C for 24 h prior to the analysis.The contents of hemicellulose, cellulose, lignin, andsolubles were calculated from the difference of neu-tral detergent fiber (NDF), acid detergent fiber (ADF),acid detergent liquid (ADL), and ash.

Equilibrium Moisture ContentPrevious literature has suggested that torrefaction

solid products are relatively hydrophobic [12]. Toevaluate that report, the equilibrium moisture content(EMC) was measured at 308C by the static desiccatortechnique. The solid sample is exposed to an envi-ronment with constant humidity and temperatureover a long period of time, until the moisture in thesolid reaches an equilibrium value. The humidity inthe chamber is maintained at a constant value bykeeping the air in equilibrium with an aqueous solu-tion that is saturated with a particular salt [19].Two salt solutions were used, with relative humidities11.3 and 83.6% for lithium chloride and potassium

436 October 2009 Environmental Progress & Sustainable Energy (Vol.28, No.3) DOI 10.1002/ep

chloride, respectively. The solid sample (�3 g) wasdried at 1058C for 24 h and then immediately trans-ferred into the chamber for 8–11 days to reach theequilibrium. Equilibrium was determined when thesample mass was constant for three consecutive days.

RESULTS AND DISCUSSION

Wet and dry torrefaction of loblolly pine were per-formed at various temperatures. All thermal pretreat-ments and analytical experiments were performed atleast twice, and mean values are reported. Mass yield,energy densification ratio, and energy yield are threeimportant measures in this study, which are definedas:

Mass yield ¼ Mass of dried pretreated solid

Mass of dried biomass3 100%

Energy densification ratio

¼ HHV of dried pretreated solid

HHV of dried biomass

Energy yield ¼ mass yield 3 energy densification ratio

As summarized in Table 1, the results show thatreaction temperature is a significant variable in bothpretreatment processes. In both cases, increasing tem-perature causes a reduction in both mass and energyyields, while energy densification increases. Over thetemperature ranges studied, with similar mass yields,

wet torrefaction yields a higher energy densification,and hence higher energy yield, than dry torrefaction.For example, wet torrefaction at 2608C has a massyield of 57.0% and an energy yield of 77.5%, whilethe mass yield of torrefaction at 3008C is 60.5% andthe energy yield is 73.2%. Apparently, wet torrefac-tion is more successful at energy densification.

Table 2 shows ultimate analysis of biomass feed-stock and thermal pretreated solids. Both torrefactionprocesses produce a solid fuel with increased carboncontent and decreased oxygen content. This is con-sistent with an increase in energy density of thermalpretreated solids. The change in carbon and oxygencontent is significant, but less dramatic in the case ofdry torrefaction. Wet torrefaction can significantlyincrease carbon content from 50 up to 72%, anddecrease oxygen content from 43 down to 23%.

Proximate analysis is commonly used to establishthe rank of coals [13] and other solid fuels for suit-ability of combustion, pyrolysis, and gasification.Table 3 shows the proximate analysis of loblolly pineand torrefied loblolly pine. The proximate analysis ofraw feedstock is similar to the proximate analysis ofbiomass wet torrefied at low temperatures (200 and2308C), and it is also similar to that of biomass drytorrefied at low temperatures (250 and 2758C). How-ever, when processed at higher temperatures (2608Cin the case of wet torrefaction, 3008C in the case ofdry torrefaction), the pretreated biomass exhibitsdecreased volatiles and increased fixed carbon con-tent. This result is consistent with trends reported inthe literature [20] correlating increased heating valuewith increasing fixed carbon content. The proximateand ultimate analyses suggest that torrefaction con-verts biomass from a low energy-density feedstock

Table 1. Thermal pretreatment of loblolly pine.

PretreatmentTemperature

(8C)Mass

yield (%)HHV(cal/g)

Energydensification ratio

Energyyield (%)

Wet torrefaction 200 88.7 5043.3 1.08 95.8230 70.6 5276.8 1.13 79.8260 57.0 6342.5 1.36 77.5

Dry torrefaction 250 83.8 5005.4 1.07 89.7275 74.2 5207.2 1.12 83.1300 60.5 5627.4 1.21 73.2

The HHV of dried loblolly pine is 4669.8 cal/g.

Table 2. Ultimate analysis of biomass feedstock and thermal pretreated solid products.

Pretreatment Temperature (8C) C (%) H (%) N (%) S (%) O (%)

Biomass — 50.25 5.97 0 0 43.34Wet torrefaction 200 54.72 6.03 0.14 0 39.11

230 56.05 5.94 0.09 0 37.92260 72.07 4.90 0.16 0 22.89

Dry torrefaction 250 50.73 6.21 0.12 0 42.94275 52.27 6.13 0.15 0 41.45300 54.81 5.94 0.14 0 39.11

Environmental Progress & Sustainable Energy (Vol.28, No.3) DOI 10.1002/ep October 2009 437

into a high energy-density feedstock, which mayhave favorable properties for application as a fuel forthermal conversion processes.

Fiber analysis is used to show the reactions of theprimary components of lignocellulosic biomass,which may help elucidate the reaction mechanism inboth pretreatments. Table 4a, as described above thedistribution of hemicellulose, cellulose, lignin, aque-ous soluble compounds, and ash of the biomassfeedstock and pretreated solids. If each fiber fractionis considered as a separate component, it is possibleto determine the conversion of each component bytorrefaction with the aid of the mass yield shown inTable 1. Table 4b, as described above the conversionof each of hemicellulose, cellulose, lignin, and solublecompounds using this approach. A negative conver-sion indicates that more of that compound is presentafter torrefaction than was present in the raw biomass.

Inspection of Table 4 indicates several noteworthytrends. First, with respect to wet torrefaction, thehemicellulose is nearly completely removed at 2008C,and it is completely reacted at 230 and 2608C. Hemi-cellulose is clearly quite reactive in hot water. Also

note that both cellulose and lignin are partiallyreacted by hot water, and the extent of reactionincreases with temperature. Most interesting is thefinding that aqueous soluble compounds are appa-rently produced by the wet torrefaction reaction. Thiscan be explained by considering the products of thedegradation of hemicellulose and cellulose, assumedto consist primarily of sugars. The remaining solidproduct is very porous, and water soluble productsare apparently deposited onto the pores of the solid,leaving behind significant quantities of these com-pounds. Apparently, the solvents used in the fiberanalysis are strong enough to remove these com-pounds. A similar conclusion was reached in a priorstudy [6].

Next consider the fiber analysis of solid productproduced by dry torrefaction, also shown in Table 4.Hemicellulose and aqueous soluble compounds arequite reactive, and are reacted at rates increasing withtemperature. Cellulose exhibits similar, albeit slower,behavior. Dry torrefaction apparently produces com-pounds that behave in a manner identical to that oflignin in the NDF/ADF/ADL fiber analysis, especially

Table 3. Proximate analysis of biomass feedstock and thermal pretreated solid products.

Pretreatment Temperature (8C) Moisture (%) Volatiles (%) Fixed carbon (%) Ash (%)

Biomass — 3.6 83.7 12.3 0.4Wet torrefaction 200 1.3 87.1 12.4 0.5

230 1.3 83.8 15.8 0.4260 1.3 73.2 26.3 0.5

Dry torrefaction 250 0 87.7 11.8 0.5275 0 83.0 16.4 0.6300 0 82.3 17.0 0.7

Table 4. Fiber analysis of biomass feedstock and thermal pretreated solid products; conversion of individualfiber components, relative to initial starting feedstock.

PretreatmentTemperature

(8C)Hemicellulose

(%)Cellulose

(%)Lignin(%)

Aqueoussolubles (%)

Ash(%)

A. Fiber analysis of biomass feedstock and thermal pretreated solid productsBiomass — 11.9 54.0 25.0 8.7 0.4Wet torrefaction 200 0.4 47.4 27.8 24.3 0.5

230 0 44.1 30.2 25.3 0.4260 0 33.9 33.8 31.8 0.5

Dry torrefaction 250 9.2 51.0 34.1 5.2 0.5275 6.5 52.7 36.7 3.5 0.6300 2.3 32.7 62.1 2.2 0.7

B. Conversion of individual fiber components, relative to initial starting feedstockWet torrefaction 200 97.0 22.1 1.4 2147.7

230 100.0 42.3 14.7 2105.3260 100.0 64.2 22.9 2108.3

Dry torrefaction 250 35.2 20.9 214.3 49.9275 59.5 27.6 28.9 70.1300 88.3 63.4 250.3 84.7

A negative conversion indicates a fiber compound that was apparently produced by torrefaction.

438 October 2009 Environmental Progress & Sustainable Energy (Vol.28, No.3) DOI 10.1002/ep

at 3008C. This was unexpected, but might beexplained in consideration of the chemistry of thetorrefaction reaction. Previous work [21] describedlow temperature pyrolysis of cellulose as a scissionreaction with significant aromatization and cross-link-ing, producing a solid which has characteristics simi-lar to that of lignin. At 250 and 2758C, generation oflignin-like compounds was quite limited, but at3008C, significant lignin was generated; this is seen asconsistent with the aromatization reactions describedabove.

Equilibrium moisture content (EMC) can be usedas an indicator of hydrophobicity of a solid. Forlignocellulosic biomass, moisture can be absorbedinto the cell walls and hydrogen-bonded to thehydroxyl groups of the cell wall components [22].Breakdown of these hydroxyl groups with thermalpretreatments results in lower EMC and the solid issaid to become more hydrophobic. A solid withreduced EMC can be stored stably over time, withlow risk of biological deterioration. Also, transporta-tion of stored hydrophobic solids is less expensive,because there will less moisture to transport alongwith the biomass. Table 5 clearly shows that pre-treated solids become more hydrophobic in compari-son with biomass feedstock. The EMC of pretreatedbiomass decreases with increasing pretreatment tem-perature, and wet torrefaction produces a solid morehydrophobic than that produced by dry torrefaction.At RH 5 84%, EMC of torrefied biomass decreasesfrom 16% for raw biomass to 5% (wet process) andto 9% (dry process).

CONCLUSION

Wet and dry torrefaction are promising methods toconvert diverse lignocellulosic biomass feedstocks toa homogeneous solid fuel for subsequent thermo-chemical conversion. In both thermal pretreatments,reaction temperature significantly affects the productdistribution and energy densification, and characterof solid product. The energy density of pretreatedloblolly pine can be increased by 7–36%, dependingon the pretreatment conditions. Torrefied biomasshas increased fixed carbon (proximate analysis) andatomic carbon (ultimate analysis), indicating that the

biomass has been transformed into a fuel with prop-erties resembling low rank coal. The reduced EMCindicates that the pretreated solid is more hydropho-bic and can be easily stored to accommodate sea-sonal availability. Although both thermal pretreat-ments show some common features, the reactionmechanism is quite different as reflected by the fiberanalysis. In wet torrefaction, hemicellulose is very re-active even at 2008C, while both cellulose and lignincan be dissolved partially at higher temperatures.Both cellulose and hemicellulose are somewhat reac-tive in dry torrefaction, producing a solid that resistsdissolution, similar to lignin. The solid product pro-duced by dry torrefaction is not at all soluble by fiberanalysis measurements.

This study reports analysis on initial scopingexperiments; there is no attempt to optimize the con-ditions of torrefaction. In particular, the optimal reac-tion time and temperature have not been identified,nor have reaction kinetics. Pretreated biomass pre-sumably will be pelletized for feeding to a thermo-chemical conversion process, and there is no attempthere to describe the solid characteristics for requiredhandling (friability, morphology, etc.). To establishtecho-economic feasibility, mass and energy balanceof both torrefaction processes and the subsequentthermal conversion process (e.g. gasification or fastpyrolysis) must be investigated as well. We anticipatefurther reports from our lab and from other labs onthese issues.

ACKNOWLEDGMENTS

We gratefully acknowledge financial support fromthe US Department of Energy (Award Number: DE-FG36-01GO11082). The authors acknowledge mean-ingful conversations with project partners, includingLarry Felix of the Gas Technology Institute (GTI),Kent Hoekman of the Desert Research Institute (DRI),and Craig Einfeldt of Changing World Technologies(CWT). The authors gratefully acknowledge assis-tance of Jeremey Riggle of DRI for his help in con-ducting the ultimate analyses, and assistance of JasonHastings at UNR who conducted most of the torrefac-tion experiments.

Table 5. Equilibrium moisture content of biomass feedstock and thermal pretreated solid products.

PretreatmentTemperature

(8C)EMC (%),

RH 5 11.3%EMC (%),

RH 5 83.6%

Biomass — 3.5 15.6Wet torrefaction 200 1.8 12.8

230 0.9 8.2260 0.4 5.3

Dry torrefaction 250 2.3 10.4275 2.2 8.7300 2.3 8.7

RH, relative humidity. All EMC measurements were done at 308C.

Environmental Progress & Sustainable Energy (Vol.28, No.3) DOI 10.1002/ep October 2009 439

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