International Journal of Hydrogen Energy 32 (2007) 212–223www.elsevier.com/locate/ijhydene

Thermodynamic analysis of hydrogen production via steam reformingof selected components of aqueous bio-oil fraction

Ekaterini Ch. Vagia, Angeliki A. Lemonidou∗

Department of Chemical Engineering, Aristotle University of Thessaloniki and CERTH/CPERI,P.O. Box 1517, University Campus, GR-54006 Thessaloniki, Greece

Available online 28 September 2006

Abstract

This work presents thermodynamics analysis of hydrogen production via steam reforming of bio-oil components. The model compounds,acetic acid, ethylene glycol and acetone, representatives of the major classes of components present in the aqueous fraction of bio-oil were usedfor the study. The equilibrium product compositions were investigated in a broad range of conditions like temperature (400–1300 K), steam tofuel ratio (1–9) and pressure (1–20 atm). Any of the three model compounds can be fully reformed even at low temperatures producing hydrogenwith maximum yield ranging from 80% to 90% at 900 K. Steam to fuel ratio positively affect the hydrogen content over the entire range oftemperature studied. Conversely, higher pressure decreases the hydrogen yield. The formation of solid carbon (graphite) does not constitute aproblem provided that reforming temperatures higher than 600 K and steam to fuel ratios higher than 4 for acetic acid and ethylene glycol and6 for acetone are to be used. Thermal decomposition of the bio-oil components is thermodynamically feasible, forming a mixture containingC(s), CH4, H2, CO, CO2, and H2O at various proportions depending on the specific nature of the compound and the temperature. Material andenergy balances of complete reforming system demonstrated that the production of 1 kmol/s hydrogen from bio-oil steam reforming requiresalmost the same amount of energy as with natural gas reforming.� 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Keywords: Hydrogen production; Steam reforming; Thermodynamic analysis; Bio-oil; Oxygenated hydrocarbons; Acetic acid; Ethylene glycol; Acetone;Methane

1. Introduction

Hydrogen and electricity seem to be the key solutions forthe 21st century, enabling clean efficient production of powerand heat from a range of primary energy sources. Today, hy-drogen is mainly produced from natural gas via steam methanereforming, a process suffering from several limitations likethe thermodynamic equilibrium limitations, high energy de-mand, catalyst deactivation due to carbon deposition andincreased CO2 emissions [1,2]. Considerable research effortshave been also directed to the production of hydrogen viapartial oxidation and CO2 reforming [3–7]. Since the above-mentioned processes rely on a non renewable fossil fuel, theyare not a viable long-term source of hydrogen. Petroleumand natural gas are expected to become scarce in the coming

∗ Corresponding author. Tel.: +30 2310 996273; fax: +30 2310 996184.E-mail address: alemonidou@cheng.auth.gr (A.A. Lemonidou).

0360-3199/$ - see front matter � 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.ijhydene.2006.08.021

decades. Renewable energy sources are clean and will not runout in the foreseeable future. Because of their consistent long-term availability, renewable energy resources are also inherentlymore stable in price than fossil fuels [8,9].

Hydrogen production from renewable sources such asbiomass, is gaining attention as a CO2 neutral energy supply.The rationale behind this approach is the fact that the CO2released into the atmosphere during thermochemical conver-sion of biomass is offset by the uptake of CO2 during biomassgrowth [10].

Biomass can be used to produce hydrogen or hydrogen-richgas via different technical pathways, i.e. anaerobic digestion,fermentation, metabolic processing, high-pressure supercriti-cal conversion, gasification and pyrolysis [10,11]. Comparedwith other pathways, gasification and pyrolysis appear techno-economically viable at the current stage. The combinationof fast pyrolysis of biomass followed by steam reform-ing of bio-oil produced, has appeared in literature [12–14]

E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223 213

as one of the most promising and economical viable methodfor hydrogen production. Bio-oil has a higher energy densitythan biomass, it can be readily stored, transported and canbe used either as a renewable liquid fuel or chemical produc-tion. Using procedures such as water addition, the bio-oil canbe separated into a water-monomer-rich phase that containsmostly carbohydrate-derived compounds and a hydrophobic-oligomer-phase composed mainly of lignin-derived oligomers[15,16]. The water-rich phase of the bio-oil containing mostlycarbohydrate-derived compounds consists of 20% organics and80% water [17–19]. Steam reforming of the aqueous phase ofbio-oil and its major components [20–25], is characterized withall the difficulties, typical for the well-developed methane steamreforming process. To these, the extremely heterogeneous com-position of the bio-oil and the thermal instability of the oxy-genated compounds should be added [24,26,27].

In the present work, the possibility of reforming the water-rich phase of bio-oil is explored thermodynamically. Extensiveliterature studies dealing with the thermodynamic analysis un-der reforming conditions for methane [28–31], higher hydrocar-bons [28,32,33], methanol [28], ethanol [28,34,35], dimethylether [28,36] have been published. These studies identified ther-modynamic favorable operating conditions at which the carboncontaining compounds are converted to hydrogen-rich streams.However, there is a lack of studies dealing with the system-atic thermodynamic analysis of steam reforming of the bio-oilcomponents.

Our interest focuses to the thermodynamic calculations of thereforming reaction of oxygenated hydrocarbons, componentsof the aqueous fraction of bio-oil. The Aspen plus 11.1 soft-ware is used for these thermodynamic calculations. The prod-uct distribution as a function of parameters like temperature,steam to fuel and pressure is investigated. The equilibrium com-positions are mapped for each condition and an optimal tem-perature, pressure and feed composition are determined. Thethermal energy requirements of reforming systems processingbio-oil components and methane are compared and evaluatedby performing material and energy balances.

2. Thermodynamic analysis

2.1. Methodology

Equilibrium compositions were calculated by the minimiza-tion of the Gibb’s free energy. Aspen Plus 11.1 software hasbeen used for the calculations. This code requires specificationof the system—at least the reactor—for the reaction calcula-tions. The RGibbs reactor has been selected for the calculationsusing the Peng–Robinson property method.

To simulate the reforming of bio-oil compounds, three com-ponents, representatives of the major classes with the highestcomposition of the mixture were selected. Acetic acid, ethyleneglycol and acetone from acids, aldehydes and ketones, respec-tively, were the pure model compounds examined separatelyat reforming conditions in the presence of steam. The physicalproperties of the three selected model compounds are presentedin Table 1. Aspen Plus code requires also definition of the

Table 1Physical properties of model compounds

Properties Acetic acid Acetone Ethyleneglycol

Molecular formula C2H4O2 C3H6O C2H6O2

Heat of combustion298 K (Kcal/mol) −209.4 −427.79 −281,9Liquid density (gr/ml), (water:1) 1.05 0.8 1.1Boiling point (K) 391 329 471Melting point (K) 289.7 178 260Flash point (K) 312 255 384

products. Hydrogen, carbon monoxide, carbon dioxide,methane and carbon (graphite) as well as the remaining fuel,and water were considered as the products of the reforming.Ethane, ethylene, acetylene and various oxygenated com-pounds were also included to the products pool, but calcula-tions showed that their concentrations in equilibrium streamwere negligible.

Apart from the specification of the reactants, products and in-let composition, other parameters like the inlet temperature andpressure of the reactants, the reaction temperature, the steamto fuel ratio and the pressure are necessary to be defined. Theconsumption of thermal energy is a key issue in the design ofa reforming system. It was found that in steam reforming re-action the input feed temperature of reactants does not affectthe thermodynamic results as long as the reactor temperatureis fixed at a certain value. This is because the temperature ofthe SR reactor is determined by the external heat transfer tothe reactor [37]. For the purpose of the thermodynamic calcu-lations, the reactor temperature of the SR reactor was given asan input parameter. While there is uncertainty in choosing asingle temperature to represent a real reformer with tempera-ture differences in the axial direction of the catalyst tubes andamong them, equilibrium analysis still provides a realistic esti-mate of the extent of reaction [33]. Reactor temperature variedfrom 400 to 1300 K, steam to fuel ratio (S/F ) from 1 to 9 andpressure from 1 to 20 atm. The results are presented as molefractions of the gaseous products on dry basis. The term carbonselectivity is used for the solid carbon C(s). Carbon selectivityis defined as the ratio of the carbon atoms appeared as solidunder reaction conditions to the number of carbon atoms in theoxygenated feed.

Equilibrium calculations performed, ignore kinetic aspectsof the reforming. Because of this, the results only locate re-gions where the proposed processes are likely to occur. Theyalso reveal areas of temperature and pressure where the pro-posed processes are unrealistic. Further refinement of optimaloperating conditions requires kinetic studies [28].

2.2. Chemical reaction analysis

Steam reforming of the bio-oil components with chemi-cal formula of CnHmOk can be described by the followingreaction (1):

CnHmOk + (n − k)H2O → nCO + (n + m/2 − k)H2, (1)

214 E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223

water gas shift reaction constitutes an integral part of the re-forming

nCO + nH2O ↔ nCO2 + nH2. (2)

Given that both reactions go to completion, the overall reactioncan be represented as follows:

CnHmOk + (2n − k)H2O → nCO2 + (2n + m/2 − k)H2. (3)

The overall reforming reactions of the model compounds are:Acetic acid:

C2H4O2 + 2H2O → 2CO2 + 4H2, (4)

�H298 K = 32.21 Kcal/mol, �G298 K = 10.18 Kcal/mol,

Acetone:

C3H6O + 5H2O → 3CO2 + 8H2, (5)

�H298 K = 58.62 Kcal/mol, �G298 K = 26.89 Kcal/mol,

Ethylene glycol:

C2H6O2 + 2H2O → 2CO2 + 5H2, (6)

�H298 K = 21.23 Kcal/mol, �G298 K = −7.12 Kcal/mol.

For the model compounds studied, the maximum stoichiometricyield is: acetic acid 2 mol H2/mol C, acetone 2.67 mol H2/molC and ethylene glycol 2.5 mol H2/mol C. However, the yield ofhydrogen is lower than the stoichiometric maximum becauseof the two main undesirable products, CO and CH4, which arealso formed via the WGS and the methanation reaction.

Bio-oil components are in general thermally unstable at thetypical temperatures of the reformer [24,26,27]. As a result,thermal decomposition (cracking) for most oxygenates can oc-cur forming mainly coke and a mixture of gases as describedin the following reaction:

CnHmOk→CxHyOz+gas(H2, CO, CO2, CH4 . . .)+coke. (5)

3. Results and discussion

3.1. Acetic acid steam reforming

Acetic acid, CH3COOH, is one of the major components ofbio-oil. According to literature, depending on the biomass na-ture, its concentration to bio-oil amounts up to 12 wt% [38].Equilibrium compositions of acetic acid steam reforming werecalculated at atmospheric pressure using as parameters the re-actor temperature 400–1300 K, and the steam to fuel ratio 1 to9, which corresponds to steam to carbon ratio (S/C) 0.5–4.5.The effect of pressure was also examined at optimum steam tofuel and temperature conditions and will be presented in Sec-tion 3.3. It is worthy to note that the conversion of acetic acid isnot limited by equilibrium, approaching 100% for all operatingconditions examined.

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400 500 600 700 800 900 1000 1100 1200 1300T (K)

Pro

duct

s m

ole

frac

tion

H2

CO

CH4 CO2

Fig. 1. Steam reforming of acetic acid—effect of temperature on equilibriumproduct composition, at S/F = 6.

3.1.1. Effect of temperatureThe equilibrium mixture formed from reforming of acetic

acid consists of hydrogen, carbon monoxide and dioxide,methane, unconverted steam and coke (carbon). Fig. 1 presentsthe equilibrium mole fraction of the gaseous products in drybasis as a function of temperature, at steam to fuel ratio equalto 6. At the lowest temperature used, 400 K, acetic acid seemsto be fully decomposed to an equimolar mixture of CO2 andCH4. As the temperature rises, hydrogen appears as a product atthe expense of methane. The methane mole fraction decreasesto zero at 900 K, a temperature at which hydrogen mole frac-tion attains maximum, demonstrating that H2 formation routeparallels with CH4 consumption. The hydrogen content is aweak function of temperature from temperatures greater than900 K. It decreases from 0.63 to 0.60 when the temperatureis increased from 900 to 1300 K. The decrease in hydrogencontent is accompanied by an increase in carbon monoxide,which is primarily due to water gas shift reaction equilibrium.

3.1.2. Effect of steam to fuel ratioSteam to fuel ratio plays an important role in the reforming

reaction. The effect of this parameter is complicated by thefact that its influence depends on the temperature. Fig. 2(a)–(d)depicts the mole fraction of the gaseous products as a functionof the S/F ratio and the temperature. Steam to fuel variationdoes not change the shape of the curve (curve: the hydrogenmole fraction as a function of temperature), which means thathydrogen production has a maximum at 900 K under any S/F

variation (Fig. 2a). Increasing S/F from 1 to 9 results in 20%increase in hydrogen molar fraction at 900 K. The maximumhydrogen content 0.644 is achieved at S/F = 9.

Steam to fuel ratio inversely affect the methane concentration(Fig. 2b). The highest is the S/F , the lowest is the methane molefraction at constant temperature. The effect of steam to fuel ratioto carbon monoxide production is quite interesting (Fig. 2c).Up to 600 K the production of carbon monoxide is negligible,while the effect of S/F ratio becomes important at temperatureshigher than 700 K. Higher concentrations of water in the feedmixture disfavor the formation of CO over the whole range of

E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223 215

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T (K)

H2

mol

e fr

actio

n

S / F = 1

S / F = 3

S / F = 6

S / F = 9

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CO

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4 m

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tion

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T (K)

CO

2 m

ole

frac

tion

(a) (b)

(c) (d)

Fig. 2. Steam reforming of acetic acid—effect of steam to fuel ratio on equilibrium: (a) hydrogen, (b) methane, (c) carbon monoxide, and (d) carbon dioxide,composition.

reaction temperature. Carbon dioxide mole fraction is affectedby the steam to fuel ratio in a different way (Fig. 2d). The weakfunction of steam to the CO2 concentration at temperatures lessthan 800 K might be due to the predominance of acetic aciddecarboxylation reaction to CO2 and CH4 (see Section 3.4).At higher temperatures where reforming and water gas shiftreactions prevail, the dependence of CO2 on the amount ofsteam is quite strong with the highest concentration obtainedat the highest S/F .

As mentioned to the previous section, equilibrium calcula-tions included solid carbon as a product. The production ofC(s) constitutes one of the major problems in reforming reac-tion of hydrocarbons [39]. Both S/F and temperature affectthe selectivity to carbon as clearly shown in Fig. 3. For carbonfree operation, temperatures higher than 1000 K are necessarywhen low S/F = 1 is used. The temperature limit for carbonfree operation shifts to significantly lower temperatures 600 Kat S/F = 3. Further increase of steam concentration to the re-actant mixture effectively suppresses carbon formation to zero.It is important to mention that depending on catalyst selectivity(hence kinetics), carbon formation may or may not be observedeven though thermodynamics predicts it.

0

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T (K)

Car

bon

sele

ctiv

ity

S / F = 1

S / F = 3

S / F = 6

S / F = 9

Fig. 3. Steam reforming of acetic acid—effect of steam to fuel ratio andtemperature on equilibrium carbon selectivity.

3.2. Acetone steam reforming

Acetone, C3H6O, is the model compound selected as arepresentative of the ketones present in bio-oil at appreciable

216 E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223

amounts up to 2.8% [38]. Equilibrium calculations were per-formed under the same operating conditions as for the aceticacid. Full conversion of the oxygenate is attained even at thelowest temperature examined.

3.2.1. Effect of temperatureFig. 4 depicts the mole fraction of the products formed from

acetone steam reforming as a function of temperature from 400to 1300 K. Steam to fuel ratio 9, in the same range S/C = 3 aswith the acetic acid was used. Methane prevails over CO2 at lowtemperature while gradual increase of temperature is accompa-nied by the rise in hydrogen content and the respective declinein methane. Maximum hydrogen content 0.68–0.69 is attainedat 900–1000 K while further increase of temperature slightlydecreases its concentration to the equilibrium mixture. Carbonmonoxide emerges as a product at higher than 700 K and itsconcentration monotonously increases up to 0.2 at 1300 K.

3.2.2. Effect of steam to fuel ratioThe thermodynamic results of acetone steam reforming at

different steam to fuel ratios 1 to 9 follow the curves presentedin Fig. 5(a)–(d). Hydrogen mole fraction generally increaseswith high steam to fuel ratios. The inconsistency in hydrogen

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H2

mol

e fr

actio

n

S / F = 1

S / F = 3

S / F = 6

S / F = 9

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CO

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CH

4 m

ole

frac

tion

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CO

2 m

ole

frac

tion

(a) (b)

(c) (d)

Fig. 5. Steam reforming of acetone—effect of steam to fuel ratio on equilibrium: (a) hydrogen, (b) methane, (c) carbon monoxide, and (d) carbon dioxide,composition.

content at low S/F ratio (higher H2 mole fraction at S/F = 1compared to that at S/F = 3) can be ascribed to the lean steamconditions. According to Eq. (3) the stoichiometric amount ofsteam necessary for reforming 1 mol of acetone is 5 mol.

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Pro

duct

s m

ole

frac

tion

H2

CO

CO2

CH4

Fig. 4. Steam reforming of acetone—effect of temperature on equilibriumproduct composition, at S/F = 9.

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0

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Car

bon

sele

ctiv

ity

S / F = 1

S / F = 3

S / F = 6

S / F = 9

Fig. 6. Steam reforming of acetone—effect of steam to fuel ratio and tem-perature on equilibrium carbon selectivity.

In agreement with the results obtained with acetic acid, highsteam to fuel ratio and temperature result in the consumptionof methane, which is completed at temperatures higher than1000 K. Lower concentration in carbon monoxide is attained atthe highest S/F ratio used.

Carbon dioxide concentration is a strong function of S/F ra-tio and the temperature. Its content passes through a maximumat relatively low temperatures for S/F less than 6 drifting tolower values at higher temperature levels.

Carbon selectivity results of acetone reforming are depictedin Fig. 6. At S/F =1 (H2O content less than the stoichiometric)and temperatures lower than 900 K more than 50% of the car-bon atoms of acetone are converted to carbon. Even at 1300 K,one over three-carbon atoms is not converted to gaseous re-forming products. The threshold for carbon free operation isset at temperatures higher than 1000 K for S/F = 3 droppingto 500 K for S/F = 6.

3.3. Ethylene glycol steam reforming

Ethylene glycol, (CH2OH)2 is an important constituent in theaqueous fraction of bio-oil with concentration up to 2%wt [38].Thermodynamic calculations using ethylene glycol as a modelcompound were performed under various conditions, tempera-ture (400–1300 K), and steam to fuel (1–9). The effect of pres-sure variation from 1 to 20 atm on equilibrium compositionwas also examined. Under all conditions examined, the equi-librium composition did not contain any appreciable amountof ethylene glycol confirming the easiness of the oxygenatedcomponents of the bio-oil to be fully converted.

3.3.1. Effect of temperatureFig. 7 shows the effect of temperature on the distribution of

the gaseous products (on dry basis) at specific steam to fuelratio equal to 6. At low temperature 400 K, CH4 and CO2 arethe only gaseous products formed via decomposition of thealcohol. High-temperatures favor the production of hydrogen

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Pro

duct

s m

ole

frac

tion

H2

CH4

CO2

CO

Fig. 7. Steam reforming of ethylene glycol—effect of temperature on equi-librium product composition, at S/F = 6.

and carbon monoxide, while methane and carbon dioxide molefraction decrease. It is evident from the graph that there is amaximum in hydrogen production at 900 K, a temperature thatcoincides with the full conversion of methane. Hydrogen molarfraction approaches 0.68 at 900 K while a slight decrease of itsconcentration appears at higher temperature. The trends for theCO and CO2 are similar to those of acetic acid.

3.3.2. Effect of steam to fuel ratioSteam to fuel ratio has a strong influence on the reforming

products distribution. Fig. 8(a)–(d) illustrates the products pro-files at various steam concentrations over the temperature range400–1300 K. Increase of S/F ratio favorably affects the pro-duction of hydrogen. The maximum mole fraction of hydrogenapproaches 0.69 at 900 K for the highest S/F = 9. The higheris the steam to fuel ratio, the lower is the content in methaneand carbon monoxide. Concerning the dependence of carbondioxide on S/F two visibly distinct zones can be identified. Attemperatures lower than 800 K the CO2 content is almost in-dependent of steam to fuel ratio, while at higher temperaturesis strongly related. Worthy to note that at S/F = 1, which ac-cording to Eq. (3) corresponds to steam content less than thestoichiometric one, the concentration of CO2 is much lowerprobably due to the high extent of decomposition.

Coking tendency of ethylene glycol is a function of bothparameters, temperature and S/F . The dependence of carbonformation on the S/F ratio is similar to that of acetic acid asshown in Fig. 9. Steam to fuel ratio higher than 3 ensure carbonfree operation.

3.3.3. Effect of pressureApart from reaction temperature and S/F ratio, the effect of

pressure on the distribution of the products formed via reform-ing of oxygenates was also investigated. Given that the differ-ences observed in equilibrium compositions among the threemodel compounds are rather minimal, the effect of pressurevariation is presented only for ethylene glycol. The equilibrium

218 E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223

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CO

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e fr

actio

n

S / F = 1

S / F = 3

S / F = 6

S / F = 9

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CO

2 m

ole

frac

tion

(a) (b)

(c) (d)

Fig. 8. Steam reforming of ethylene glycol—effect of steam to fuel ratio on equilibrium: (a) hydrogen, (b) methane, (c) carbon monoxide, and (d) carbondioxide, composition.

0

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Car

bon

sele

ctiv

ity

S / F = 1

S / F = 3

S / F = 6

S / F = 9

Fig. 9. Steam reforming of ethylene glycol—effect of steam to fuel ratio andtemperature on equilibrium carbon selectivity.

compositions at constant S/F =6 and various pressures are pre-sented in Fig. 10(a)–(d) as a function of temperature. Pressureaffects to a great extent the product distribution in the temper-

ature range between 600 and 1000 K. Hydrogen and methaneare the two products mostly affected by the pressure. Increasingthe pressure from 1 to 20 atm results in a decrease of hydrogenmolar fraction at 900 K from 0.68 to 0.49. Under the same con-ditions methane content increases from 0 to 0.2. The direct re-lation between hydrogen and methane demonstrates that steamreforming of methane is the main route of hydrogen produc-tion. At temperatures higher than 1100 K the effect of pressureis negligible, since at this level the distribution of the productsis mainly determined by water gas shift equilibrium, a reactionwith no volume variation. Increased pressure does not favor car-bon formation, which remains negligible under all conditionsexamined S/F = 6 and temperature 400–1300 K (not shown).

The simulation results reveal that pressure is one of the crit-ical factors, which affect the equilibrium state demonstratingthat it is desirable to keep the pressure of the reactor as low aspossible in order to maximize hydrogen production. However,to maintain a high degree of hydrogen production efficiency, incase that higher pressure is necessary for the target process ofhydrogen utilization, reaction temperatures higher than 1100 Kshould be applied.

E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223 219

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H2

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e fr

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P = 1atm

P = 5

P = 10

P = 15

P = 20

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CH

4 m

ole

frac

tion

(a) (b)

(c) (d)

Fig. 10. Steam reforming of ethylene glycol—effect of pressure on equilibrium: (a) hydrogen, (b) methane, (c) carbon monoxide, and (d) carbon dioxidecomposition, at S/F = 6.

3.4. Thermal decomposition of oxygenates

One of the major problems encountered in the processingof bio-oil components is their thermal instability. Oxygenates,CnHmOk , easily decompose at relatively low-temperaturesforming mainly solid carbonaceous deposits which cause se-vere plugging of the transfer lines and reactors [24,26,27].Even though decomposition of the components is kineticallycontrolled, it was considered essential to examine the thermo-dynamic results of models compounds decomposition in theabsence of water. The molar fractions of the equilibrium prod-ucts was calculated under the same temperature range from400 to 1300 K. The model compounds (acetic acid, ethyleneglycol and acetone) decompose to a mixture of H2, CO, CO2,CH4, H2O and solid carbon. Equilibrium composition doesnot contain any traces of the oxygenates in the whole span oftemperature in agreement to that obtained in steam reforming.

Water and solid carbon at high molar proportions togetherwith methane and carbon dioxide are present in equilibrium atlow temperature (Fig. 11(a)–(c)). Hydrogen and especially car-bon monoxide seem to be secondary products formed at highertemperatures. Their concentration steadily increases becom-

ing the dominant species above 1000 K at least for acetic acid(Fig. 11a) and ethylene glycol (Fig. 11c). At 900 K, the opti-mum temperature for maximum hydrogen efficiency in steamreforming, the highest hydrogen mole fraction produced byethylene glycol is 0.41. The respective values for the other twomodel compounds are 0.32 and 0.35 for acetic acid and ace-tone, respectively. These results demonstrate the significanceof the steam presence (maximum H2 mole fraction around 0.68for ethylene glycol reforming at 900 K and S/F = 6), whichactively contributes to the increase in hydrogen efficiency.

Carbon formation prevails in the thermal decomposition ofthe bio-oil components. Carbon concentration decreases withtemperature and approaches zero at relatively high tempera-tures. Carbon free operation is possible only above 1300 K withacetic acid and ethylene glycol, while with acetone such an op-tion is not possible. In particular, the equilibrium mixture con-tains carbon in molar quantity higher than 0.3 even at 1300 K.

3.5. Optimum conditions

The primary goal for reforming of bio-oil is to convert itscomponents to hydrogen-rich streams. Hydrogen produced by

220 E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223

0.0

0.1

0.2

0.3

0.4

0.5

0.6

400 500 600 700 800 900 1000 1100 1200 1300

T (K)

Pro

du

cts

mo

le f

ract

ion

H2

CO

CH4

CO2

Coke

H2O

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

400 500 600 700 800 900 1000 1100 1200 1300

T (K)

Pro

du

cts

mo

le f

ract

ion

H2

CO

CH4

CO2

Coke

H2O

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

400 500 600 700 800 900 1000 1100 1200 1300

T (K)

Pro

du

cts

mo

le f

ract

ion

Coke

H2

H2OCO

CH4

CO2

(a)

(b)

(c)

Fig. 11. Thermal decomposition of model compounds—effect of temperatureon equilibrium product composition: (a) acetic acid, (b) acetone, and (c)ethylene glycol.

reforming of the liquid bio-oil can be used in decentralizedenergy producing systems (fuel cells) [40]. High hydrogenreforming efficiency can be achieved using operating vari-ables, which ensure increased hydrogen content in equilibriummixture. The production of carbon monoxide is an inefficientby-product that impacts the overall size of the fuel proces-sor, especially the water gas shift reactors. Methane, eventhough does not have an impact on the performance of PEM

Table 2Optimum thermodynamic yields by steam reforming of the three modelcompounds of bio-oil

Operating variable Aceticacid SR

AcetoneSR

Ethyleneglycol SR

S/F ratio (S/C ratio) 6/1 (3/1) 9/1 (3/1) 6/1 (3/1)Reactor temperature (K) 900 900 900Reactor pressure (atm) 1 1 1Conversion (%) 100 100 100

Yield % (based on carbon)H2

a 84.76 79.46 84.44CO 24.01 32.23 27.34CO2 74.37 62.14 69.77CH4 1.62 5.63 2.89

aBased on stoichiometric hydrogen.

fuel cell is not considered as desirable product as it containshydrogen, decreasing thus the overall hydrogen yield.

Examination of the model compounds which represent thethree major classes of the bio-oil components, revealed that thedifferences in product distribution are rather minimal, render-ing possible the specification of optimum operating conditionsfor maximum hydrogen efficiency. Temperature is one of thecritical parameters in reforming. Maximum hydrogen contentis obtained at the temperature of 900 K common for the threemodel compounds. As steam is a co-reactant, its molar ratio tothe oxygenate is also of great significance. The higher is thisratio, the higher is the hydrogen yield. However, high excess ofsteam has a negative impact to the energy consumption and tothe size of the units. Reforming of a mixture containing steamand oxygenates, at ratios S/F = 6 for acetic acid and ethyleneglycol and S/F = 9 for acetone at temperature 900 K, satisfiesall the criteria for high hydrogen concentration followed withlow CO and CH4 and carbon free operation.

Pressure as expected has a negative effect on hydrogencontent. Maximum hydrogen efficiency can be achieved at at-mospheric pressure. However, if hydrogen downstream processnecessitates the use of higher pressures, reforming temperatureshigher than 1100 K should be used. At such high temperaturesthe penalty in hydrogen content is minimal, since the effect ofhigh pressure and temperature is insignificant (see Fig. 10).

The results presented up to now were based on mole fractionof the products. Table 2 tabulates the yields of the products forthe three model compounds at optimum conditions of temper-ature, steam to fuel and pressure. The thermodynamically pre-dicted hydrogen yield amounts to almost 85% for acetic acidand ethylene glycol and 80% for acetone.

3.6. Comparison between bio-oil and natural gas reformingin terms of energy consumption

The hydrogen content of biomass is relatively low (11.2%)[10] compared to almost 25% of natural gas. For this reason,producing hydrogen via the biomass pyrolysis/steam reform-ing process cannot compete on a cost basis with the well de-veloped commercial technology of steam reforming of natural

E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223 221

Table 3Material balance of the reforming scheme

RGIBBSPREHEATE

32

R-SI-GIB

4 6

8

7

5

COOLER

H2OMethane

H2O

Reformer Shift reactor

1

9H2O

SPLITTER

Acetic AcidEthylene Glycol

Acetone

or

Material streams 1 2 3 4 5 6 7 8 9

(a) Simulated bio-oilTemperature (K) 375.1 293.1 900.1 900.1 473.7 473.1 293.1 375.1 375.1Mole flow (kmol/s) 1.348 0.208 1.556 2.26 2.26 2.282 5.155 5.155 3.807

Mole flow (kmol/s)Acetic acid 0 0.139 0.139 0 0 0 0 0 0Ethylene glycol 0 0.034 0.034 0 0 0 0 0 0Acetone 0 0.034 0.034 0 0 0 0 0 0CH4 0 0 0 0.011 0.011 0 0 0 0H2O 1.348 0 1.348 0.971 0.971 0.832 5.155 5.155 3.807H2 0 0 0 0.839 0.839 1.000 0 0 0CO 0 0 0 0.12 0.12 0.002 0 0 0CO2 0 0 0 0.319 0.319 0.448 0 0 0C 0 0 0 0 0 0 0 0 0

(b) MethaneTemperature (K) 375.1 293.1 1073.2 1073.2 474 473.1 293.1 375.1 375.1Mole flow (kmol/s) 0.753 0.251 1.004 1.505 1.505 1.506 4.51 4.51 3.757Mole flow (kmol/s)CH4 0 0.251 0.251 0.001 0.001 0 0 0 0H2O 0.753 0 0.753 0.415 0.415 0.255 4.51 4.51 3.757H2 0 0 0 0.839 0.839 1.000 0 0 0CO 0 0 0 0.163 0.163 0.004 0 0 0CO2 0 0 0 0.088 0.088 0.247 0 0 0C 0 0 0 0 0 0 0 0 0

Numbers in bold show the important input and output component molar flows.

gas. However, an integrated process, in which the hydrophobicpart of the biomass is used to produce more valuable materialsor chemicals and the aqueous fractions are utilized to generatehydrogen can be an economically viable option [12,23].

It is out of the scope of this study to perform a completetechno-economic analysis of hydrogen production via bio-oilreforming. Though, it is useful and feasible to compare thecost of energy required for the production of a specific amountof hydrogen via steam reforming of bio-oil and natural gas.It is worthy to state that the model used to compare materialand energy balances for methane and bio-oil reforming is asimple one with basic thermal integration. The output flow rateof hydrogen is specified to 1 kmol/s in order to compare thetwo systems. The input conditions of water and oxygenates ormethane are set at 293 K and 1atm. The organic part of the bio-oil aqueous fraction was represented by a mixture comprising67% acetic acid, 16.5% ethylene glycol and 16.5% acetonemore closely simulating the actual composition of bio-oil. Theratio of steam to fuel entering the reformer is specified at 6.5for bio-oil and 3 for methane, which corresponds to steam tocarbon 3

1 for both systems. It was assumed that natural gasconsists of 100% methane. The reactor temperature was set at

900 K for bio-oil reforming a temperature at which, based onthe results obtained, maximum yield in hydrogen is attained.The commonly used temperature of 1073 K was selected forthe simulation of natural gas reforming, where the conversionof methane is almost complete.

In order to determine the amount of energy to generatea given amount of hydrogen, a common complete systemwas employed in the study. The configuration of the systemcomprises, a heater, a reforming reactor, a cooler and a shiftreactor and a splitter (see Table 3). Steam is generated in theheat exchanger which is used for the cooling of the reformereffluent. The configuration proposed does not comprise extrastream for steam addition to the shift reactor because the ini-tially added steam is high enough to ensure full conversion ofCO in the shifter [37]. The preheaters employed operate at 900and 1073 K for the heating up of the bio-oil and the naturalgas, respectively. It is obvious that this temperature is highenough for the compounds to react. To avoid these reactionsunder realistic conditions we should either preheat in lowertemperature or use separate heaters. In our case, we used oneheater for all the reactants to calculate the total amount ofrequired energy. Moreover, the preheat temperature was that

222 E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223

Table 4Comparison of material and energy balances of simulated bio-oil and naturalgas reforming

Bio-oil SR Natgas SR

Input (kmol/s)Simulated bio-oilCH3COOH 0.139 —(CH2OH)2 0.034 —CH3COCH3 0.034 —CH4 — 0.251H2O 1.348 0.753

Output (kmol/s)H2 1.000 1.000H2O 0.832 0.255CO 0.002 0.004CO2 0.448 0.247CH4 0.011 0.001

Conversion (%)Bio-oil 100CH4 99.96

Energy balance (MW)Preheater 49.336 31.029Reforming reactor 37.029 53.524Cooler −34.551 −30.228Shift reactor −2.826 −6.281Heat-transfer efficiency 0.80 0.80

Total net energy (MW) 48.988 48.044

of the reactor temperature to obtain the endothermicity of thereforming reaction. The material balances of the proposedconfigurations using the Aspen Plus code are presented inTable 3. Both the reformers and the shift reactors were modeledas RGibbs equilibrium models.

The bio-oil reformer effluent contains 37.12% hydrogen cor-responding to 84% yield, 5.3% CO and 14.1% CO2 on wetbasis. Methane concentration is limited to 0.49%. Before en-tering the shift reactor, which operates at 473 K, the syngasproduced is cooled down from 900 to 473 K passing througha cooler. The shifter effluent is enriched in hydrogen so as itsoverall yield amounts to 100%. In conclusion, processing of0.208 kmol/s of the oxygenate mixture results in a productionof 1 kmol of hydrogen. The concentration of the remaining COat the shifter exit amounts to 1379 ppm on dry basis.

The natural gas reformer exit stream contains 55.75% H2,10.83% CO and 5.8% CO2 on wet basis. The methane concen-tration does not overcome 0.066%. The conversion of the largeamount of CO is accomplished in the shift reactor operatingat 473 K. The overall hydrogen quantity 1 kmol/s, requires theprocessing of 0.251 kmol/s of CH4. The CO concentration indry gas at the exit of the shift reactor is 3197 ppm.

The comparison of the material and energy balances of thetwo reforming systems is presented in Table 4. The total en-ergy sums up the heat duties of each unit that comprise thereforming system. Even though, each unit has different heattransfer efficiency, the same heat transfer efficiency 0.80 wasadopted to simplify the calculations [37]. The methane flowrate required for the production of 1 kmol/s H2 is 0.251, while

0.208 kmol/s of the bio-oil are necessary to be processed withsteam which means that lower amount of bio-oil is needed forthe same amount of H2. Both processes are energy intensive asshown in Table 4. Of interest to point that for the productionof the same amount of hydrogen both process routes requirealmost the same amount of energy. Further optimization andheat integration are necessary to improve the heat balance. Asimple way to drastically reduce the heat duty is by adding thenecessary quantity of oxygen to reformer (autothermal reform-ing) so as to minimize the heat duty of the reforming reactor.Equilibrium calculations of autothermal reforming of bio-oilcomponents are in progress.

4. Conclusions

Steam reforming of the aqueous phase of bio-oil producedvia biomass flash pyrolysis is a potentially viable route forhydrogen production. A thermodynamic analysis using theASPEN plus software was conducted to specify the conditionsaffecting reforming of bio-oil aqueous fraction. Three modelcompounds characteristic of the major classes of the bio-oilcomponents were selected for the equilibrium calculations,acetic acid , ethylene glycol and acetone.

Bio-oil components are easily reformed even at low tem-peratures forming a mixture of hydrogen, carbon monoxide,carbon dioxide and methane with varying composition. Tem-perature, steam to fuel ratio and pressure are the operatingvariables which affect to a great extent the equilibrium com-position. Temperature increase favors the formation of hydro-gen up to 900 K where maximum concentration of hydrogenis attained. The amount of steam to the inlet mixture deter-mines to a great extent the hydrogen yield. The higher is theS/F ratio the higher is the hydrogen concentration. Best resultsconcerning hydrogen yield are attained at atmospheric pres-sure. Carbon free operation is possible at temperatures higherthan 600 K and S/F higher than 4 for acetic acid and ethy-lene glycol and higher than 6 for acetone. Methane is a ma-jor product at low temperatures minimizing at 900 K. Carbonmonoxide and carbon dioxide are also components of the equi-librium mixture with their concentrations determined by thewater gas shift equilibrium. The equilibrium composition underthe various operating conditions does not differ significantlyamong the three model compounds.

Simulations for a complete system including steam reformerand shift reactor at the optimum conditions (T = 900 K, atmo-spheric pressure and S/C = 3) revealed that 1 kmol/s hydro-gen can be produced by processing 0.208 kmol/s of a mixturecomprising of acetic acid, ethylene glycol and acetone at 4/1/1molar ratios. Preliminary calculations showed that bio-oil aque-ous fraction reforming is as energy intensive as material gasreforming.

Acknowledgments

Financial support was provided by the EPEAEK programmeof the Ministry of Education (Grant Pythagoras I).

E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212–223 223

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