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

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

Thermodynamic analysis of hydrogen production via steam reforming of 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 used for the study. The equilibrium product compositions were investigated in a broad range of conditions like temperature (400–1300 K), steam to fuel ratio (1–9) and pressure (1–20 atm). Any of the three model compounds can be fully reformed even at low temperatures producing hydrogen with maximum yield ranging from 80% to 90% at 900 K. Steam to fuel ratio positively affect the hydrogen content over the entire range of temperature studied. Conversely, higher pressure decreases the hydrogen yield. The formation of solid carbon (graphite) does not constitute a problem provided that reforming temperatures higher than 600 K and steam to fuel ratios higher than 4 for acetic acid and ethylene glycol and 6 for acetone are to be used. Thermal decomposition of the bio-oil components is thermodynamically feasible, forming a mixture containing C(s) , CH4 , H2 , CO, CO2 , and H2 O at various proportions depending on the specific nature of the compound and the temperature. Material and energy balances of complete reforming system demonstrated that the production of 1 kmol/s hydrogen from bio-oil steam reforming requires almost the same amount of energy as with natural gas reforming. 䉷 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 for the 21st century, enabling clean efficient production of power and heat from a range of primary energy sources. Today, hydrogen is mainly produced from natural gas via steam methane reforming, a process suffering from several limitations like the thermodynamic equilibrium limitations, high energy demand, catalyst deactivation due to carbon deposition and increased CO2 emissions [1,2]. Considerable research efforts have been also directed to the production of hydrogen via partial oxidation and CO2 reforming [3–7]. Since the abovementioned processes rely on a non renewable fossil fuel, they are not a viable long-term source of hydrogen. Petroleum and natural gas are expected to become scarce in the coming ∗ Corresponding author. Tel.: +30 2310 996273; fax: +30 2310 996184.

E-mail address: [email protected] (A.A. Lemonidou).

decades. Renewable energy sources are clean and will not run out in the foreseeable future. Because of their consistent longterm availability, renewable energy resources are also inherently more stable in price than fossil fuels [8,9]. Hydrogen production from renewable sources such as biomass, is gaining attention as a CO2 neutral energy supply. The rationale behind this approach is the fact that the CO2 released into the atmosphere during thermochemical conversion of biomass is offset by the uptake of CO2 during biomass growth [10]. Biomass can be used to produce hydrogen or hydrogen-rich gas via different technical pathways, i.e. anaerobic digestion, fermentation, metabolic processing, high-pressure supercritical conversion, gasification and pyrolysis [10,11]. Compared with other pathways, gasification and pyrolysis appear technoeconomically viable at the current stage. The combination of fast pyrolysis of biomass followed by steam reforming of bio-oil produced, has appeared in literature [12–14]

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

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

as one of the most promising and economical viable method for hydrogen production. Bio-oil has a higher energy density than biomass, it can be readily stored, transported and can be used either as a renewable liquid fuel or chemical production. Using procedures such as water addition, the bio-oil can be separated into a water-monomer-rich phase that contains mostly carbohydrate-derived compounds and a hydrophobicoligomer-phase composed mainly of lignin-derived oligomers [15,16]. The water-rich phase of the bio-oil containing mostly carbohydrate-derived compounds consists of 20% organics and 80% water [17–19]. Steam reforming of the aqueous phase of bio-oil and its major components [20–25], is characterized with all the difficulties, typical for the well-developed methane steam reforming process. To these, the extremely heterogeneous composition of the bio-oil and the thermal instability of the oxygenated compounds should be added [24,26,27]. In the present work, the possibility of reforming the waterrich phase of bio-oil is explored thermodynamically. Extensive literature studies dealing with the thermodynamic analysis under reforming conditions for methane [28–31], higher hydrocarbons [28,32,33], methanol [28], ethanol [28,34,35], dimethyl ether [28,36] have been published. These studies identified thermodynamic favorable operating conditions at which the carbon containing compounds are converted to hydrogen-rich streams. However, there is a lack of studies dealing with the systematic thermodynamic analysis of steam reforming of the bio-oil components. Our interest focuses to the thermodynamic calculations of the reforming reaction of oxygenated hydrocarbons, components of the aqueous fraction of bio-oil. The Aspen plus 11.1 software is used for these thermodynamic calculations. The product distribution as a function of parameters like temperature, steam to fuel and pressure is investigated. The equilibrium compositions are mapped for each condition and an optimal temperature, pressure and feed composition are determined. The thermal energy requirements of reforming systems processing bio-oil components and methane are compared and evaluated by performing material and energy balances. 2. Thermodynamic analysis 2.1. Methodology Equilibrium compositions were calculated by the minimization of the Gibb’s free energy. Aspen Plus 11.1 software has been used for the calculations. This code requires specification of the system—at least the reactor—for the reaction calculations. The RGibbs reactor has been selected for the calculations using the Peng–Robinson property method. To simulate the reforming of bio-oil compounds, three components, representatives of the major classes with the highest composition of the mixture were selected. Acetic acid, ethylene glycol and acetone from acids, aldehydes and ketones, respectively, were the pure model compounds examined separately at reforming conditions in the presence of steam. The physical properties of the three selected model compounds are presented in Table 1. Aspen Plus code requires also definition of the

213

Table 1 Physical properties of model compounds Properties

Acetic acid

Acetone

Ethylene glycol

Molecular formula Heat of combustion298 K (Kcal/mol) Liquid density (gr/ml), (water:1) Boiling point (K) Melting point (K) Flash point (K)

C 2 H4 O2 −209.4 1.05 391 289.7 312

C3 H 6 O −427.79 0.8 329 178 255

C2 H6 O2 −281,9 1.1 471 260 384

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 compounds were also included to the products pool, but calculations showed that their concentrations in equilibrium stream were negligible. Apart from the specification of the reactants, products and inlet composition, other parameters like the inlet temperature and pressure of the reactants, the reaction temperature, the steam to fuel ratio and the pressure are necessary to be defined. The consumption of thermal energy is a key issue in the design of a reforming system. It was found that in steam reforming reaction the input feed temperature of reactants does not affect the thermodynamic results as long as the reactor temperature is fixed at a certain value. This is because the temperature of the SR reactor is determined by the external heat transfer to the reactor [37]. For the purpose of the thermodynamic calculations, the reactor temperature of the SR reactor was given as an input parameter. While there is uncertainty in choosing a single temperature to represent a real reformer with temperature differences in the axial direction of the catalyst tubes and among them, equilibrium analysis still provides a realistic estimate of the extent of reaction [33]. Reactor temperature varied from 400 to 1300 K, steam to fuel ratio (S/F ) from 1 to 9 and pressure from 1 to 20 atm. The results are presented as mole fractions of the gaseous products on dry basis. The term carbon selectivity is used for the solid carbon C(s) . Carbon selectivity is defined as the ratio of the carbon atoms appeared as solid under reaction conditions to the number of carbon atoms in the oxygenated feed. Equilibrium calculations performed, ignore kinetic aspects of the reforming. Because of this, the results only locate regions where the proposed processes are likely to occur. They also reveal areas of temperature and pressure where the proposed processes are unrealistic. Further refinement of optimal operating conditions requires kinetic studies [28]. 2.2. Chemical reaction analysis Steam reforming of the bio-oil components with chemical formula of Cn Hm Ok can be described by the following reaction (1): Cn Hm Ok + (n − k)H2 O → nCO + (n + m/2 − k)H2 ,

(1)

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water gas shift reaction constitutes an integral part of the reforming

0.7 H2

nCO + nH2 O ↔ nCO2 + nH2 .

(2)

Given that both reactions go to completion, the overall reaction can be represented as follows: Cn Hm Ok + (2n − k)H2 O → nCO2 + (2n + m/2 − k)H2 . (3) The overall reforming reactions of the model compounds are: Acetic acid: C2 H4 O2 + 2H2 O → 2CO2 + 4H2 , H298 K = 32.21 Kcal/mol,

(4)

G298 K = 10.18 Kcal/mol,

H298 K = 58.62 Kcal/mol,

(5)

G298 K = 26.89 Kcal/mol,

Ethylene glycol: C2 H6 O2 + 2H2 O → 2CO2 + 5H2 , H298 K = 21.23 Kcal/mol,

(6)

G298 K = −7.12 Kcal/mol.

For the model compounds studied, the maximum stoichiometric yield is: acetic acid 2 mol H2 /mol C, acetone 2.67 mol H2 /mol C and ethylene glycol 2.5 mol H2 /mol C. However, the yield of hydrogen is lower than the stoichiometric maximum because of the two main undesirable products, CO and CH4 , which are also formed via the WGS and the methanation reaction. Bio-oil components are in general thermally unstable at the typical temperatures of the reformer [24,26,27]. As a result, thermal decomposition (cracking) for most oxygenates can occur forming mainly coke and a mixture of gases as described in the following reaction: Cn Hm Ok →Cx Hy Oz +gas(H2 , CO, CO2 , CH4 . . .)+coke.

0.5 0.4 0.3

CH4

CO2

0.2 0.1 0.0 400

CO

500

600

700

800 900 1000 1100 1200 1300 T (K)

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

Acetone: C3 H6 O + 5H2 O → 3CO2 + 8H2 ,

Products mole fraction

0.6

(5)

3. Results and discussion 3.1. Acetic acid steam reforming Acetic acid, CH3 COOH, is one of the major components of bio-oil. According to literature, depending on the biomass nature, its concentration to bio-oil amounts up to 12 wt% [38]. Equilibrium compositions of acetic acid steam reforming were calculated at atmospheric pressure using as parameters the reactor temperature 400–1300 K, and the steam to fuel ratio 1 to 9, which corresponds to steam to carbon ratio (S/C) 0.5–4.5. The effect of pressure was also examined at optimum steam to fuel and temperature conditions and will be presented in Section 3.3. It is worthy to note that the conversion of acetic acid is not limited by equilibrium, approaching 100% for all operating conditions examined.

3.1.1. Effect of temperature The equilibrium mixture formed from reforming of acetic acid consists of hydrogen, carbon monoxide and dioxide, methane, unconverted steam and coke (carbon). Fig. 1 presents the equilibrium mole fraction of the gaseous products in dry basis as a function of temperature, at steam to fuel ratio equal to 6. At the lowest temperature used, 400 K, acetic acid seems to be fully decomposed to an equimolar mixture of CO2 and CH4 . As the temperature rises, hydrogen appears as a product at the expense of methane. The methane mole fraction decreases to zero at 900 K, a temperature at which hydrogen mole fraction attains maximum, demonstrating that H2 formation route parallels with CH4 consumption. The hydrogen content is a weak function of temperature from temperatures greater than 900 K. It decreases from 0.63 to 0.60 when the temperature is increased from 900 to 1300 K. The decrease in hydrogen content 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 ratio Steam to fuel ratio plays an important role in the reforming reaction. The effect of this parameter is complicated by the fact that its influence depends on the temperature. Fig. 2(a)–(d) depicts the mole fraction of the gaseous products as a function of the S/F ratio and the temperature. Steam to fuel variation does not change the shape of the curve (curve: the hydrogen mole fraction as a function of temperature), which means that hydrogen 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 maximum hydrogen 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 mole fraction at constant temperature. The effect of steam to fuel ratio to 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 temperatures higher than 700 K. Higher concentrations of water in the feed mixture disfavor the formation of CO over the whole range of

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0.7

215

1 0.9

0.6

0.4 0.3

S/F=1 S/F=3

0.2 0.1

CH4 mole fraction

H2 mole fraction

0.8 0.5

0.7 0.6 0.5 0.4 0.3

S/F=6

0.2

S/F=9

0.1

0.0 400

500

600

700

800

T (K)

(a)

0 400

900 1000 1100 1200 1300

500

600

700

(b)

0.4

800 900 1000 1100 1200 1300 T (K)

0.5

0.4 CO2 mole fraction

CO mole fraction

0.3

0.2

0.1

0 400 (c)

0.3

0.2

0.1

500

600

700

800

0 400

900 1000 1100 1200 1300

T (K)

500

600

700

(d)

800 900 1000 1100 1200 1300 T (K)

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.

0.6 S/F=1 0.5 Carbon selectivity

reaction temperature. Carbon dioxide mole fraction is affected by the steam to fuel ratio in a different way (Fig. 2d). The weak function of steam to the CO2 concentration at temperatures less than 800 K might be due to the predominance of acetic acid decarboxylation reaction to CO2 and CH4 (see Section 3.4). At higher temperatures where reforming and water gas shift reactions prevail, the dependence of CO2 on the amount of steam is quite strong with the highest concentration obtained at the highest S/F . As mentioned to the previous section, equilibrium calculations included solid carbon as a product. The production of C(s) constitutes one of the major problems in reforming reaction of hydrocarbons [39]. Both S/F and temperature affect the selectivity to carbon as clearly shown in Fig. 3. For carbon free operation, temperatures higher than 1000 K are necessary when low S/F = 1 is used. The temperature limit for carbon free operation shifts to significantly lower temperatures 600 K at S/F = 3. Further increase of steam concentration to the reactant 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 observed even though thermodynamics predicts it.

S/F=3 S/F=6

0.4

S/F=9

0.3 0.2 0.1 0 400

500

600

700

800

900 1000 1100 1200 1300

T (K) Fig. 3. Steam reforming of acetic acid—effect of steam to fuel ratio and temperature on equilibrium carbon selectivity.

3.2. Acetone steam reforming Acetone, C3 H6 O, is the model compound selected as a representative of the ketones present in bio-oil at appreciable

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

amounts up to 2.8% [38]. Equilibrium calculations were performed under the same operating conditions as for the acetic acid. Full conversion of the oxygenate is attained even at the lowest temperature examined. 3.2.1. Effect of temperature Fig. 4 depicts the mole fraction of the products formed from acetone steam reforming as a function of temperature from 400 to 1300 K. Steam to fuel ratio 9, in the same range S/C = 3 as with the acetic acid was used. Methane prevails over CO2 at low temperature while gradual increase of temperature is accompanied by the rise in hydrogen content and the respective decline in methane. Maximum hydrogen content 0.68–0.69 is attained at 900–1000 K while further increase of temperature slightly decreases its concentration to the equilibrium mixture. Carbon monoxide emerges as a product at higher than 700 K and its concentration monotonously increases up to 0.2 at 1300 K. 3.2.2. Effect of steam to fuel ratio The thermodynamic results of acetone steam reforming at different steam to fuel ratios 1 to 9 follow the curves presented in Fig. 5(a)–(d). Hydrogen mole fraction generally increases with high steam to fuel ratios. The inconsistency in hydrogen

content at low S/F ratio (higher H2 mole fraction at S/F = 1 compared to that at S/F = 3) can be ascribed to the lean steam conditions. According to Eq. (3) the stoichiometric amount of steam necessary for reforming 1 mol of acetone is 5 mol. 0.7 CH4

0.6 Products mole fraction

216

H2

0.5 0.4

CO2

0.3 CO 0.2 0.1 0 400

500

600

700

800 900 1000 1100 1200 1300 T (K)

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

1

0.7

0.9

0.6

0.4 0.3

S/F=1 S/F=3

0.2

CH4 mole fraction

H2 mole fraction

0.8 0.5

500

600

700

(a)

800 900 T (K)

0.5 0.4 0.3

0.1

S/F=9 0 400

0.6

0.2

S/F=6

0.1

0.7

0 400

1000 1100 1200 1300

500

600

700

800 900 T (K)

1000 1100 1200 1300

500

600

700

800 900 T (K)

1000 1100 1200 1300

(b)

0.4

0.5

0.4 CO2 mole fraction

CO mole fraction

0.3

0.2

0.3

0.2

0.1 0.1

0 400 (c)

500

600

700

800 900 T (K)

0 400

1000 1100 1200 1300 (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.

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

0.6

0.7

0.5

0.6

217

0.4 0.3 0.2

S/F=1 S/F=3

0.1

S/F=6 S/F=9

0 400

500

600

700

800 900 1000 1100 1200 1300 T (K)

Products mole fraction

Carbon selectivity

CH4 H2

0.5 0.4

CO2

0.3 0.2 CO 0.1 0.0 400

500

600

700

800 900 1000 1100 1200 1300 T (K)

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

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

In agreement with the results obtained with acetic acid, high steam to fuel ratio and temperature result in the consumption of methane, which is completed at temperatures higher than 1000 K. Lower concentration in carbon monoxide is attained at the highest S/F ratio used. Carbon dioxide concentration is a strong function of S/F ratio and the temperature. Its content passes through a maximum at relatively low temperatures for S/F less than 6 drifting to lower values at higher temperature levels. Carbon selectivity results of acetone reforming are depicted in Fig. 6. At S/F =1 (H2 O content less than the stoichiometric) and temperatures lower than 900 K more than 50% of the carbon atoms of acetone are converted to carbon. Even at 1300 K, one over three-carbon atoms is not converted to gaseous reforming products. The threshold for carbon free operation is set at temperatures higher than 1000 K for S/F = 3 dropping to 500 K for S/F = 6.

and carbon monoxide, while methane and carbon dioxide mole fraction decrease. It is evident from the graph that there is a maximum in hydrogen production at 900 K, a temperature that coincides with the full conversion of methane. Hydrogen molar fraction approaches 0.68 at 900 K while a slight decrease of its concentration appears at higher temperature. The trends for the CO and CO2 are similar to those of acetic acid.

3.3. Ethylene glycol steam reforming Ethylene glycol, (CH2 OH)2 is an important constituent in the aqueous fraction of bio-oil with concentration up to 2%wt [38]. Thermodynamic calculations using ethylene glycol as a model compound were performed under various conditions, temperature (400–1300 K), and steam to fuel (1–9). The effect of pressure variation from 1 to 20 atm on equilibrium composition was also examined. Under all conditions examined, the equilibrium composition did not contain any appreciable amount of ethylene glycol confirming the easiness of the oxygenated components of the bio-oil to be fully converted. 3.3.1. Effect of temperature Fig. 7 shows the effect of temperature on the distribution of the gaseous products (on dry basis) at specific steam to fuel ratio equal to 6. At low temperature 400 K, CH4 and CO2 are the only gaseous products formed via decomposition of the alcohol. High-temperatures favor the production of hydrogen

3.3.2. Effect of steam to fuel ratio Steam to fuel ratio has a strong influence on the reforming products distribution. Fig. 8(a)–(d) illustrates the products profiles at various steam concentrations over the temperature range 400–1300 K. Increase of S/F ratio favorably affects the production of hydrogen. The maximum mole fraction of hydrogen approaches 0.69 at 900 K for the highest S/F = 9. The higher is the steam to fuel ratio, the lower is the content in methane and carbon monoxide. Concerning the dependence of carbon dioxide on S/F two visibly distinct zones can be identified. At temperatures lower than 800 K the CO2 content is almost independent of steam to fuel ratio, while at higher temperatures is strongly related. Worthy to note that at S/F = 1, which according to Eq. (3) corresponds to steam content less than the stoichiometric one, the concentration of CO2 is much lower probably due to the high extent of decomposition. Coking tendency of ethylene glycol is a function of both parameters, temperature and S/F . The dependence of carbon formation on the S/F ratio is similar to that of acetic acid as shown in Fig. 9. Steam to fuel ratio higher than 3 ensure carbon free operation. 3.3.3. Effect of pressure Apart from reaction temperature and S/F ratio, the effect of pressure on the distribution of the products formed via reforming of oxygenates was also investigated. Given that the differences observed in equilibrium compositions among the three model compounds are rather minimal, the effect of pressure variation is presented only for ethylene glycol. The equilibrium

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

0.7

1

0.6

0.9

0.4 S/F=1

0.3

S/F=3

0.2

CH4 mole fraction

H2 mole fraction

0.8 0.5

S/F=6 0.1 0.0 400

600

700

0.5 0.4 0.3

0.1 0 400

1000 1100 1200 1300

0.4

0.4

0.3

0.3

0.2

0.1

0 400

500

600

700

800 900 1000 1100 1200 1300 T (K)

500

600

700

800 900 T (K)

(b)

CO2 mole fraction

CO mole fraction

(a)

800 900 T (K)

0.6

0.2

S/F=9 500

0.7

0.2

0.1

500

600

700

(c)

800 900 1000 1100 1200 1300 T (K)

0 400 (d)

1000 1100 1200 1300

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

0.6 S/F=1 Carbon selectivity

0.5

S/F=3 S/F=6

0.4

S/F=9

0.3 0.2 0.1 0 400

500

600

700

800 900 1000 1100 1200 1300 T (K)

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

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

ature range between 600 and 1000 K. Hydrogen and methane are the two products mostly affected by the pressure. Increasing the pressure from 1 to 20 atm results in a decrease of hydrogen molar fraction at 900 K from 0.68 to 0.49. Under the same conditions methane content increases from 0 to 0.2. The direct relation between hydrogen and methane demonstrates that steam reforming of methane is the main route of hydrogen production. At temperatures higher than 1100 K the effect of pressure is negligible, since at this level the distribution of the products is mainly determined by water gas shift equilibrium, a reaction with no volume variation. Increased pressure does not favor carbon formation, which remains negligible under all conditions examined S/F = 6 and temperature 400–1300 K (not shown). The simulation results reveal that pressure is one of the critical factors, which affect the equilibrium state demonstrating that it is desirable to keep the pressure of the reactor as low as possible in order to maximize hydrogen production. However, to maintain a high degree of hydrogen production efficiency, in case that higher pressure is necessary for the target process of hydrogen utilization, reaction temperatures higher than 1100 K should be applied.

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

219

1

0.7

0.9 0.6

0.4 P = 1atm

0.3

P=5 0.2

CH4 mole fraction

H2 mole fraction

0.8 0.5

P = 10

500

600

700

(a)

800 900 T (K)

0.4 0.3

0 400

1000 1100 1200 1300

500

600

700

800 900 T (K)

500

600

700

800 900 1000 1100 1200 1300 T (K)

(b)

0.4

1000 1100 1200 1300

0.5

0.4 CO2 mole fraction

0.3 CO mole fraction

0.5

0.1

P = 20 0.0 400

0.6

0.2

P = 15

0.1

0.7

0.2

0.3

0.2

0.1 0.1

0 400 (c)

500

600

700

800 900 T (K)

0.0 400

1000 1100 1200 1300 (d)

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

3.4. Thermal decomposition of oxygenates One of the major problems encountered in the processing of bio-oil components is their thermal instability. Oxygenates, Cn Hm Ok , easily decompose at relatively low-temperatures forming mainly solid carbonaceous deposits which cause severe plugging of the transfer lines and reactors [24,26,27]. Even though decomposition of the components is kinetically controlled, it was considered essential to examine the thermodynamic results of models compounds decomposition in the absence of water. The molar fractions of the equilibrium products was calculated under the same temperature range from 400 to 1300 K. The model compounds (acetic acid, ethylene glycol and acetone) decompose to a mixture of H2 , CO, CO2 , CH4 , H2 O and solid carbon. Equilibrium composition does not contain any traces of the oxygenates in the whole span of temperature in agreement to that obtained in steam reforming. Water and solid carbon at high molar proportions together with methane and carbon dioxide are present in equilibrium at low temperature (Fig. 11(a)–(c)). Hydrogen and especially carbon monoxide seem to be secondary products formed at higher temperatures. 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 optimum temperature for maximum hydrogen efficiency in steam reforming, the highest hydrogen mole fraction produced by ethylene glycol is 0.41. The respective values for the other two model compounds are 0.32 and 0.35 for acetic acid and acetone, respectively. These results demonstrate the significance of the steam presence (maximum H2 mole fraction around 0.68 for ethylene glycol reforming at 900 K and S/F = 6), which actively contributes to the increase in hydrogen efficiency. Carbon formation prevails in the thermal decomposition of the bio-oil components. Carbon concentration decreases with temperature and approaches zero at relatively high temperatures. Carbon free operation is possible only above 1300 K with acetic acid and ethylene glycol, while with acetone such an option is not possible. In particular, the equilibrium mixture contains 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 its components to hydrogen-rich streams. Hydrogen produced by

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E.Ch. Vagia, A.A. Lemonidou / International Journal of Hydrogen Energy 32 (2007) 212 – 223 Table 2 Optimum thermodynamic yields by steam reforming of the three model compounds of bio-oil

Products mole fraction

0.7 0.6 H2

0.5 Coke

0.4 0.3 CO2

0.1

CH4

0.0 400

500

600

700

800

(a)

Acetic acid SR

Acetone SR

Ethylene glycol SR

S/F ratio (S/C ratio) Reactor temperature (K) Reactor pressure (atm) Conversion (%)

6/1 (3/1) 900 1 100

9/1 (3/1) 900 1 100

6/1 (3/1) 900 1 100

Yield % (based on carbon) H2 a CO CO2 CH4

84.76 24.01 74.37 1.62

79.46 32.23 62.14 5.63

84.44 27.34 69.77 2.89

CO

H2O

0.2

Operating variable

900 1000 1100 1200 1300

T (K)

a Based

on stoichiometric hydrogen.

Products mole fraction

0.6 Coke

0.5

H2

0.4 CH4

0.3 0.2

CO H2O

0.1 CO2 0.0 400

500

600

700

800

900 1000 1100 1200 1300

T (K)

(b) 0.7

Products mole fraction

0.6

H2

0.5

H2O Coke

0.3 CH4

0.2 0.1

CO2

0.0 400 (c)

CO

0.4

500

600

700

800

900 1000 1100 1200 1300

T (K)

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

reforming of the liquid bio-oil can be used in decentralized energy producing systems (fuel cells) [40]. High hydrogen reforming efficiency can be achieved using operating variables, which ensure increased hydrogen content in equilibrium mixture. The production of carbon monoxide is an inefficient by-product that impacts the overall size of the fuel processor, especially the water gas shift reactors. Methane, even though does not have an impact on the performance of PEM

fuel cell is not considered as desirable product as it contains hydrogen, decreasing thus the overall hydrogen yield. Examination of the model compounds which represent the three major classes of the bio-oil components, revealed that the differences in product distribution are rather minimal, rendering possible the specification of optimum operating conditions for maximum hydrogen efficiency. Temperature is one of the critical parameters in reforming. Maximum hydrogen content is obtained at the temperature of 900 K common for the three model compounds. As steam is a co-reactant, its molar ratio to the oxygenate is also of great significance. The higher is this ratio, the higher is the hydrogen yield. However, high excess of steam has a negative impact to the energy consumption and to the size of the units. Reforming of a mixture containing steam and oxygenates, at ratios S/F = 6 for acetic acid and ethylene glycol and S/F = 9 for acetone at temperature 900 K, satisfies all the criteria for high hydrogen concentration followed with low CO and CH4 and carbon free operation. Pressure as expected has a negative effect on hydrogen content. Maximum hydrogen efficiency can be achieved at atmospheric pressure. However, if hydrogen downstream process necessitates the use of higher pressures, reforming temperatures higher than 1100 K should be used. At such high temperatures the penalty in hydrogen content is minimal, since the effect of high pressure and temperature is insignificant (see Fig. 10). The results presented up to now were based on mole fraction of the products. Table 2 tabulates the yields of the products for the three model compounds at optimum conditions of temperature, steam to fuel and pressure. The thermodynamically predicted hydrogen yield amounts to almost 85% for acetic acid and ethylene glycol and 80% for acetone. 3.6. Comparison between bio-oil and natural gas reforming in 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 reforming process cannot compete on a cost basis with the well developed commercial technology of steam reforming of natural

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

221

Table 3 Material balance of the reforming scheme

7

Acetic Acid Ethylene Glycol Acetone or

R-SI-GIB

RGIBBS

Shift reactor

Reformer

PREHEATE

COOLER 4

2

5

6

3

Methane

9

H2O

H2O

8

H2O 1

SPLITTER

Material streams

1

2

3

4

5

6

7

8

9

(a) Simulated bio-oil Temperature (K) Mole flow (kmol/s)

375.1 1.348

293.1 0.208

900.1 1.556

900.1 2.26

473.7 2.26

473.1 2.282

293.1 5.155

375.1 5.155

375.1 3.807

Mole flow (kmol/s) Acetic acid Ethylene glycol Acetone CH4 H2 O H2 CO CO2 C

0 0 0 0 1.348 0 0 0 0

0.139 0.034 0.034 0 0 0 0 0 0

0.139 0.034 0.034 0 1.348 0 0 0 0

0 0 0 0.011 0.971 0.839 0.12 0.319 0

0 0 0 0.011 0.971 0.839 0.12 0.319 0

0 0 0 0 0.832 1.000 0.002 0.448 0

0 0 0 0 5.155 0 0 0 0

0 0 0 0 5.155 0 0 0 0

0 0 0 0 3.807 0 0 0 0

375.1 0.753

293.1 0.251

1073.2 1.004

1073.2 1.505

474 1.505

473.1 1.506

293.1 4.51

375.1 4.51

375.1 3.757

0 0.753 0 0 0 0

0.251 0 0 0 0 0

0.251 0.753 0 0 0 0

0.001 0.415 0.839 0.163 0.088 0

0.001 0.415 0.839 0.163 0.088 0

0 0.255 1.000 0.004 0.247 0

0 4.51 0 0 0 0

0 4.51 0 0 0 0

0 3.757 0 0 0 0

(b) Methane Temperature (K) Mole flow (kmol/s) Mole flow (kmol/s) CH4 H2 O H2 CO CO2 C

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

gas. However, an integrated process, in which the hydrophobic part of the biomass is used to produce more valuable materials or chemicals and the aqueous fractions are utilized to generate hydrogen can be an economically viable option [12,23]. It is out of the scope of this study to perform a complete techno-economic analysis of hydrogen production via bio-oil reforming. Though, it is useful and feasible to compare the cost of energy required for the production of a specific amount of hydrogen via steam reforming of bio-oil and natural gas. It is worthy to state that the model used to compare material and energy balances for methane and bio-oil reforming is a simple one with basic thermal integration. The output flow rate of hydrogen is specified to 1 kmol/s in order to compare the two systems. The input conditions of water and oxygenates or methane are set at 293 K and 1atm. The organic part of the biooil aqueous fraction was represented by a mixture comprising 67% acetic acid, 16.5% ethylene glycol and 16.5% acetone more closely simulating the actual composition of bio-oil. The ratio of steam to fuel entering the reformer is specified at 6.5 for bio-oil and 3 for methane, which corresponds to steam to carbon 31 for both systems. It was assumed that natural gas consists of 100% methane. The reactor temperature was set at

900 K for bio-oil reforming a temperature at which, based on the results obtained, maximum yield in hydrogen is attained. The commonly used temperature of 1073 K was selected for the simulation of natural gas reforming, where the conversion of methane is almost complete. In order to determine the amount of energy to generate a given amount of hydrogen, a common complete system was employed in the study. The configuration of the system comprises, a heater, a reforming reactor, a cooler and a shift reactor and a splitter (see Table 3). Steam is generated in the heat exchanger which is used for the cooling of the reformer effluent. The configuration proposed does not comprise extra stream for steam addition to the shift reactor because the initially added steam is high enough to ensure full conversion of CO in the shifter [37]. The preheaters employed operate at 900 and 1073 K for the heating up of the bio-oil and the natural gas, respectively. It is obvious that this temperature is high enough for the compounds to react. To avoid these reactions under realistic conditions we should either preheat in lower temperature or use separate heaters. In our case, we used one heater for all the reactants to calculate the total amount of required energy. Moreover, the preheat temperature was that

222

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

Table 4 Comparison of material and energy balances of simulated bio-oil and natural gas reforming Bio-oil SR

Natgas SR

Input (kmol/s) Simulated bio-oil CH3 COOH (CH2 OH)2 CH3 COCH3 CH4 H2 O

0.139 0.034 0.034 — 1.348

— — — 0.251 0.753

Output (kmol/s) H2 H2 O CO CO2 CH4

1.000 0.832 0.002 0.448 0.011

1.000 0.255 0.004 0.247 0.001

Conversion (%) Bio-oil CH4

100 99.96

Energy balance (MW) Preheater Reforming reactor Cooler Shift reactor Heat-transfer efficiency

49.336 37.029 −34.551 −2.826 0.80

31.029 53.524 −30.228 −6.281 0.80

Total net energy (MW)

48.988

48.044

of the reactor temperature to obtain the endothermicity of the reforming reaction. The material balances of the proposed configurations using the Aspen Plus code are presented in Table 3. Both the reformers and the shift reactors were modeled as RGibbs equilibrium models. The bio-oil reformer effluent contains 37.12% hydrogen corresponding to 84% yield, 5.3% CO and 14.1% CO2 on wet basis. Methane concentration is limited to 0.49%. Before entering the shift reactor, which operates at 473 K, the syngas produced is cooled down from 900 to 473 K passing through a cooler. The shifter effluent is enriched in hydrogen so as its overall yield amounts to 100%. In conclusion, processing of 0.208 kmol/s of the oxygenate mixture results in a production of 1 kmol of hydrogen. The concentration of the remaining CO at 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 concentration does not overcome 0.066%. The conversion of the large amount of CO is accomplished in the shift reactor operating at 473 K. The overall hydrogen quantity 1 kmol/s, requires the processing of 0.251 kmol/s of CH4 . The CO concentration in dry gas at the exit of the shift reactor is 3197 ppm. The comparison of the material and energy balances of the two reforming systems is presented in Table 4. The total energy sums up the heat duties of each unit that comprise the reforming system. Even though, each unit has different heat transfer efficiency, the same heat transfer efficiency 0.80 was adopted to simplify the calculations [37]. The methane flow rate 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 with steam which means that lower amount of bio-oil is needed for the same amount of H2 . Both processes are energy intensive as shown in Table 4. Of interest to point that for the production of the same amount of hydrogen both process routes require almost the same amount of energy. Further optimization and heat integration are necessary to improve the heat balance. A simple way to drastically reduce the heat duty is by adding the necessary quantity of oxygen to reformer (autothermal reforming) so as to minimize the heat duty of the reforming reactor. Equilibrium calculations of autothermal reforming of bio-oil components are in progress. 4. Conclusions Steam reforming of the aqueous phase of bio-oil produced via biomass flash pyrolysis is a potentially viable route for hydrogen production. A thermodynamic analysis using the ASPEN plus software was conducted to specify the conditions affecting reforming of bio-oil aqueous fraction. Three model compounds characteristic of the major classes of the bio-oil components were selected for the equilibrium calculations, acetic acid , ethylene glycol and acetone. Bio-oil components are easily reformed even at low temperatures forming a mixture of hydrogen, carbon monoxide, carbon dioxide and methane with varying composition. Temperature, steam to fuel ratio and pressure are the operating variables which affect to a great extent the equilibrium composition. Temperature increase favors the formation of hydrogen up to 900 K where maximum concentration of hydrogen is attained. The amount of steam to the inlet mixture determines to a great extent the hydrogen yield. The higher is the S/F ratio the higher is the hydrogen concentration. Best results concerning hydrogen yield are attained at atmospheric pressure. Carbon free operation is possible at temperatures higher than 600 K and S/F higher than 4 for acetic acid and ethylene glycol and higher than 6 for acetone. Methane is a major product at low temperatures minimizing at 900 K. Carbon monoxide and carbon dioxide are also components of the equilibrium mixture with their concentrations determined by the water gas shift equilibrium. The equilibrium composition under the various operating conditions does not differ significantly among the three model compounds. Simulations for a complete system including steam reformer and shift reactor at the optimum conditions (T = 900 K, atmospheric pressure and S/C = 3) revealed that 1 kmol/s hydrogen can be produced by processing 0.208 kmol/s of a mixture comprising of acetic acid, ethylene glycol and acetone at 4/1/1 molar ratios. Preliminary calculations showed that bio-oil aqueous fraction reforming is as energy intensive as material gas reforming. Acknowledgments Financial support was provided by the EPEAEK programme of the Ministry of Education (Grant Pythagoras I).

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