Thermodynamic analysis of hydrogen production by steam and autothermal reforming of soybean waste frying oil

Energy Conversion and Management 70 (2013) 174–186

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Thermodynamic analysis of hydrogen production by steam and autothermal reforming of soybean waste frying oil Hajjaji Noureddine a,⇑, Faleh Nahla a, Khila Zouhour a,b, Pons Marie-Noëlle b a Unité de Recherche Catalyse et Matériaux pour l’Environnement et les Procédés URCMEP (UR11ES85), Ecole Nationale d’Ingénieurs de Gabès, Université de Gabès, Rue Omar Ibn Alkhattab, 6029 Gabès, Tunisia b Laboratoire Réactions et Génie des Procédés – CNRS, Université de Lorraine, 1 rue Grandville, BP 20451, 54001 Nancy Cedex, France

a r t i c l e

i n f o

Article history: Received 14 January 2013 Accepted 4 March 2013 Available online 2 April 2013 Keywords: Hydrogen Waste frying oil Steam reforming Autothermal reforming Thermoneutral

a b s t r a c t Hydrogen production via steam and autothermal reforming of soybean waste frying oils (WFOs) is thermodynamically investigated via the Gibbs free energy minimization method. The thermodynamic optimum conditions are determined to maximize hydrogen production while minimizing the methane and carbon monoxide contents and coke formation. Equilibrium calculations are performed at atmospheric pressure over a wide range of temperatures (400–1200 °C), steam-to-WFO ratios (S/C: 1–15) and oxygen-to-WFO ratios (O/C: 0.0–2.0). The baseline case used for the study considers soybean WFO after 8 h of use (WFO8). The influence of frying time on the performance of reforming reactors is also discussed. The results show that the optimum conditions for steam reforming can be achieved at reforming temperatures between 650 °C and 850 °C and at a steam to carbon molar (S/C) ratio of approximately 5. The recommended operation conditions for the SR of WFO8 are proposed to be T = 650 °C and S/C ratio = 5. Under these conditions, a hydrogen yield of 169.83 mol/kg WFO8 can be obtained with a CO concentration in the SG of 3.91% and trace CH4 (0.03%), without the risk of coke formation. Hydrogen production from autothermal systems can be optimized at temperatures of 600–800 °C, S/C ratios of 3–5, and O/C ratios of 0.0–0.5. Under these conditions, thermoneutrality is obtained with O/C ratios of 0.391–0.455. The recommended thermoneutral conditions are S/C = 5, T = 600 °C and O/C = 0.453. Under these conditions, 146.45 mol H2/kg WFO8 can be produced with only 2.89% CO and 0.06% CH4 in the synthesis gas. The effect of frying time of soybean WFO on hydrogen productivity is shown to be negligible. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Fossil fuel resources are continually depleted due to rapid increases in population and industrialization around the world. Therefore, the last several decades have seen an increase in the search for new energy sources and carriers to ensure that global energy needs can be met and to reduce dependence on fossil fuel resources. Hydrogen energy systems appear to be one of the most effective solutions and can play a significant role in achieving a better environment and improving sustainability [1,2]. Hydrogen has several advantages. First, hydrogen is the simplest and most abundant element in the universe. Second, hydrogen is equally distributed around the world, mostly bound with oxygen in water, and therefore, its supply is essentially limitless. Third, hydrogen has a high energy content and yields only water when burned [3]. Note that hydrogen is an energy carrier but not an energy ⇑ Corresponding author. Tel.: +216 24 04 04 00; fax: +216 75 29 00 41. E-mail address: [email protected] (H. Noureddine). 0196-8904/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2013.03.009

resource; thus, hydrogen must first be produced. All hydrogen production processes are based on the separation of hydrogen from hydrogen-containing compounds in fossil fuels, water, biomass, or other starting materials [4]. The increasing production of waste frying oil (WFO) from household and industrial sources is a growing problem in Tunisia and worldwide. WFO is an oil-based substance consisting of vegetable matter that has been used in the preparation of foods and is no longer suitable for human consumption [5]. Table 1 shows that more than 15 million tons of WFO annually are generated from a select group of countries. In some countries, such as France, WFO from the food industry and from restaurants is supposed to be collected for recycling. However, this residue is regularly poured down the drain, resulting in problems for wastewater treatment plants and a loss of energy. Since the bovine spongiform encephalopathy (BSE) crisis in the 1990s, the integration of WFO into animal food is restricted for fear of potential effects on human health [12]. Furthermore, as there is a good deal of evidence that highly oxidized fats (during frying, oils are exposed to high temperatures

H. Noureddine et al. / Energy Conversion and Management 70 (2013) 174–186 Table 1 Worldwide waste frying oil production [5]. Country

Quantity (million tons/year)

China [6] Malaysia [7] United States [8] Taiwan [9] Europe [10] Canada [10] Japan [11]

4.5 0.5 10.0 0.07 0.7–1.0 0.12 0.45–0.57

in the presence of atmospheric oxygen) may have carcinogenic properties, governments prohibit the use of waste cooking oil in animal food [13]. Consequently, the disposal of large amounts of WFO has become a problem in most countries: – WFO cannot be discharged into sewers or drains because this will lead to blockages, odor or vermin problems and may also pollute watercourses, causing problems for wildlife [14]. – WFO is also a prohibited substance and will cause many problems if it is dumped in municipal solid waste landfills and municipal sewage treatment plants [5]. Therefore, it is essential to find useful and eco-friendly applications for renewable, inexpensive WFO. A variety of technologies currently allow for the use of WFO as a raw material in industrial processes to obtain added value products such as soap, surfactants, lubricants, and more recently biodiesel. The production of biodiesel from virgin vegetable oils has recently been controversial because the use of fertile lands to produce biofuels reduces the land area available for food crops, contributing to an increase in the prices and scarcity of staple foods [15]. Moreover, virgin vegetable oils account for 70–95% of the total cost of biodiesel production [16,17]. Therefore, WFO is a good raw material because of its low cost, the valorization of the residue, and the environmental advantages gained by finding an appropriate use for WFO. In this context, many scientists have investigated biodiesel production from WFO. Chen et al. [18] investigated the yields of biodiesel made from waste cooking oil with a microwave heating system. They considered the effects of catalyst type, catalyst amount, reaction time, molar ratio of methanol to oil, and microwave power on biodiesel yield. They concluded that the optimal reaction conditions are 0.75 wt.% CH3ONa catalyst, a methanol-to-oil molar ratio of 6, a reaction time of 3 min, and a microwave power of 750 W. Lee et al. [19] optimized the supercritical transesterification reaction with waste canola oil to examine the effects of process variables such as reaction time and temperature, and the weight ratio of methanol to oil on the biodiesel yield. The effects of reaction conditions on the biodiesel yield were studied using a design of experiments procedure. The results showed that reaction time, temperature, and their interaction were the most significant factors affecting the yield. The highest biodiesel yield was achieved at 270 °C and 10 MPa with a methanol/oil weight ratio of 2 and a 45 min reaction time. The WFO-to-biodiesel system was further investigated by Uzun et al. [20], who studied the effects of various conditions of the alkali-catalyzed transesterification of WFO on the biodiesel yield. They concluded that the optimum conditions were 0.5 wt.% of NaOH for 30 min at 50 °C reaction temperature with a 7.5 methanol to oil ratio and purification with hot distilled water. Although numerous studies [21–24] have been conducted on WFO-to-biodiesel production systems in the open literature, very few papers and reports have been published on hydrogen production from WFO. Hence, there are some previous works dealing with hydrogen production through WFO, using the route of chemical

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looping reforming (CLR). CLR is an alternative route to autothermal reforming. The main advantage is that, with CLR route, the oxygen-carrier is an oxide, such as NiO. In this way, air is used instead of pure oxygen, and N2 never mixes with hydrogen. Pimenidou et al. [25] used CLR route for hydrogen production from WFO in a packed bed reactor. They concluded that the steam to carbon ratio of 4 and temperatures between 600 and 700 °C yielded the best results of the range of conditions tested. Repeated cycling revealed some output oscillations in reactant conversion and in the extent of Ni–NiO conversion, but did not exhibit deterioration by the 6th cycle. The selectivity of CO, CO2 and CH4 were remarkably constant over the performed cycles, resulting in a repeatable synthesis gas composition with H2 selectivity very close to the optimum. Hydrogen production via CLR route on a Ni–Al2O3 catalyst in the presence of dolomite for in situ CO2 sorption was also investigated by Pimenidou et al. [26]. They showed that initially, the dolomite carbonation was very efficient (100%), and 98% purity hydrogen was produced, but the carbonation decreased to around 56% with a purity of 95% respectively in the following cycles. Reduction of the nickel catalyst occurred alongside steam reforming, water gas shift and carbonation, with H2 produced continuously under fuel–steam feeds. This study presents a comprehensive thermodynamic investigation of WFO-based hydrogen production systems. Indeed, we believe that WFO is a promising renewable feedstock for hydrogen production because the oxygen content is low and the potential yield of hydrogen is high. Moreover, WFO has not been previously subjected to a thermodynamic equilibrium investigation because of its complex chemical structure and because its physical and chemical properties are not available. Thermodynamic equilibrium investigation is an important tool to determine the equilibrium product composition and identify thermodynamically favorable operating conditions. This analysis aids in reactor modeling, in examining kinetic schemes or reaction mechanisms, and in identifying rate-controlling processes [27]. This paper intends to identify thermodynamically favorable operating conditions at which WFO can be converted to hydrogen via the steam reforming (SR) and autothermal reforming (ATR) processes. The composition of the syngas (SG) is determined by simulation to minimize the Gibbs free energy using the Aspen Plus™ 10.2 software (Aspen Technology, Inc., Burlington, MA, USA). Moreover, this paper presents an original investigation considering the influence of frying time on the behavior of reforming reactors. Soybean WFO has been selected for the case study. Soybean oil represents 28% of the total vegetable oils produced in the world, after palm oil (34%) and before rapeseed oil (15%) [28]. However, its production is distributed unevenly around the world, with China, USA, Argentina and Brazil as the top producers (27%, 19%, 17% and 16%, respectively). Soybean oil is produced (4%) and consumed (3%) at low levels in Europe. In Tunisia, soybean oil represents approximately 80% of the imported vegetable oil and 65% of the vegetable oil consumed, and the remainder consists of olive oil, which is not used for frying.

2. Modeling and simulation methodology 2.1. WFO characterization Vegetable oils (VOs) and fats are primarily composed of triglycerides, with minor amounts of mono and diglycerides (Fig. 1) [29]. A triglyceride consists of a three-carbon glycerol head group conjugated to three fatty acid chains [30]. The fatty acids are described in terms of the number of carbon atoms in the chain and the number of double bonds, or the degree of unsaturation, in the chain. The aliphatic hydrocarbon backbone of fatty acids present in VO

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its minimum value [38,39]. The total Gibbs free energy (Gt) of a system is given by Eq. (1)

Gt ¼

Fig. 1. Structures of mono-, di- and triglycerides, where R1, R2 and R3 represent the fatty acids chains [29].

generally varies from 8 to 24 carbon atoms, with the majority possessing 16 or 18 carbon atoms [31]. These distributions are important because they define the applications for which the oils and fats are most suitable. Soybean oil consists mainly of palmitic (C16:0), stearic (C18:0), oleic (C18:1), linoleic (C18:2), and linolenic (C18:3) acids [32]. According to the notation used here, C18 represents a fatty acid with 18 carbons and no double bonds, while C18:1 describes an acid with 18 carbon atoms and one double bond. During the frying process, the VO is heated at temperatures of approximately 180 °C for relatively long periods of time in the presence of light and air [10]. This causes VO degradation processes that lead to changes in the physico-chemical properties of the oil, resulting in unsuitable oils after some time. Among the most common alterations are a change in color and increases in viscosity, specific heat, free acid content, and tendency to foam [10,33]. Table 2 shows the effect of frying time on soybean oil composition. Degradation due to oxidation and the polymerization of triglycerides occurs more in unsaturated fatty acids than in saturated fatty acids. The present work investigates a WFO collected after 8 h of use, namely WFO8. Other oils (WFO0: virgin soybean oil, WFO16 and WFO32) are considered in Section 4. During the thermodynamic investigation of WFO reforming (SR and ATR), the composition of the SG is determined using a simulation to minimize the Gibbs free energy using the Aspen Plus™ 10.2 software (Aspen Technology, Inc., Burlington, MA, USA). Not all triglyceride forms (such as tripalmitin, tristearin, trioleate, trilinolein and trilinolenin) are included in the Aspen Plus™ databank [35], so they needed to be added before the simulation could begin. This required various data for these compounds to be inserted into the Aspen Plus™ dialog box such as the normal boiling point, chemical structure, molecular weight, temperature-dependent vapor pressure and heat capacity, standard enthalpy and Gibbs free energy of formation. All data required for the regression of the property method are taken from sources in the open literature [31,36,37].

2.2. Minimization of the Gibbs free energy Minimization of the total Gibbs free energy is a suitable method to compute the equilibrium compositions of any reacting system. This is based on the fact that the reaction system is thermodynamically favored when its total Gibbs free energy, expressed as a function of temperature, pressure and component concentrations, is at

N N N N X X X X fi ni Gi ¼ ni li ¼ ni G0i þ RT ni ln 0 f i i¼1 i¼1 i¼1 i¼1

ð1Þ

where G0i is the standard Gibbs free energy, Gi is the partial molar Gibbs free energy of species i, li is the chemical potential, R is the molar gas constant, P is the system pressure, T is the temperature of the system, fi is the fugacity in the system, fi0 is the standard-state fugacity, and ni is the moles of species i. For reaction equilibria in the gas-phase, fi ¼ yi /i P;fi0 ¼ P0 and G0i ¼ DG0fi . The minimum Gibbs free energy of each gaseous species and that of the total system can be expressed by Eqs. (2) and (3), respectively, using Lagrange’s undetermined multiplier method.

DG0fi þ RT ln

yi /i P P

0

þ

X kk aik ¼ 0

ð2Þ

k

N X y / P X ni DG0fi þ RT ln i 0i þ kk aik P i¼1 k

! ¼0

ð3Þ

where P0 is the standard-state pressure of 101.3 kPa, DG0fi is the standard Gibbs function of formation of species i, /i is the fugacity coefficient of species i, yi is the gas phase mole fraction, aik is the number of atoms of the kth element present in each molecule of species i, kk is the Lagrange multiplier, and Ak is the total mass of kth element in the feed. With the constraint of the element balance equation: N X ni aik ¼ 0

ð4Þ

i¼1

When solid carbon (coke) is involved in the system, exploitation of the vapor–solid phase equilibrium is applied to the Gibbs-energy of carbon as shown in Eq. (5). Substituting Eq. (1) with Eq. (2) for gaseous species and with Eq. (5) for solid species gives the minimization function of Gibbs-energy as shown in Eq. (6):

GCðgÞ ¼ GCðsÞ ¼ GCðsÞ ffi DG0fCðsÞ ¼ 0 N X y / P X ni DG0fi þ RT ln i 0i þ kk aik P i¼1 k

ð5Þ !

  þ nc DG0fCðsÞ ¼ 0

ð6Þ

where GCðsÞ , GCðgÞ , GC(s), DG0fCðsÞ and nc are the partial molar Gibbs free energy of solid carbon, the partial molar Gibbs free energy of gaseous carbon, the molar Gibbs free energy of solid carbon, the standard Gibbs free energy of formation of solid carbon, and the moles of carbon, respectively. In this investigation, equilibrium compositions are calculated by the minimization of the Gibbs free energy with the aid of the Aspen Plus™ software. The R-Gibbs reactor [35] is selected for the calculations using the UNIF-LBY equation of state. This code requires the identification of the possible products that might be

Table 2 Effect of frying time on soybean oil composition [34]. Frying time (h) Formula Trilinolein C57H98O6 Trioleate C57H104O6 Tripalmitin C51H98O6 Trilinolenin C57H92O6 Tristearin C57H110O6 Average molecular composition

0 WFO0

8 WFO8

16 WFO16

32 WFO32

56 23 11 6 4 C56,34H99.5O6

54 23 12 6 5 C56,28H99.62O6

47.5 30 11.5 5 6 C56,31H100,22O6

39 36 13 4 8 C56,22H100,88O6

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generated as intermediates or products of side reactions [35]. The primary species involved in WFO are restricted to CO2, H2O, CO, H2, CH4, C(graphite), tripalmitin (C51H98O6), tristearin (C57H110O6), trioleate (C57H104O6), trilinolein (C57H98O6) and trilinolenin (C57H92O6). Indeed, in our previous study of beef tallow reforming [40], we showed that the formation of by-products such as alkanes containing two or more carbon atoms, alkenes, acids and various alcohols are negligible. 3. Hydrogen production by WFO reforming 3.1. Steam reforming (SR) SR is the most commonly used method of producing hydrogen in the chemical industry. In this process, the substrate is reacted with steam in the presence of a catalyst to produce SG [41]. The steam reforming process is highly endothermic. In general, the process can be depicted as follows:

SubstrateðWFOÞ þ Steam ! Synthesis gas ðCO2 ; CO;H2 O;CH4 ; H2 ; etc:Þ DH > 0

ð7Þ

Steam reforming of an oxygenated hydrocarbon such as WFO involves a complex reaction system with undesired reaction paths [42]. The key reactions involved in the reforming process are:

DH298K ¼ 10:02 MJ=mol

3.2. Autothermal reforming (ATR) The ATR system has been suggested as a method to overcome the difficulties of running an SR process. In particular, ATR overcomes the SR limitations of high temperature operations and fast dynamic responses. Additionally, an autothermal reformer can reduce the size, weight, start-up and shut-down times, and other dynamic response times [46,47]. ATR is a combination of partial oxidation (POX) and SR with the goal of conducting reforming in a single reactor. POX is an exothermic reaction (i.e., it produces heat), while the SR reaction is endothermic, and heat must be generated externally to drive the reforming process. Typically, ATR reactions are considered to be thermally self-sustaining and consequently do not produce or consume external thermal energy. The main chemical reaction for POX of WFO is given by Eq. (18).

C56:28 H99:62 O6 þ 25:14O2 $ 56:28CO þ 49:81H2 

DH298K ¼ 4:35MJ=mole

ð8Þ

C55:08 H103:24 O6 þ aO2 þ bH2 O $ cCO þ dCO2 þ eH2 þ f CH4 þ gC ð19Þ

Water gas shift (WGS):

CO þ H2 O $ CO2 þ H2



DH298K ¼ 41:17 kJ=mol

ð9Þ

Methanation: 

CO þ 3H2 $ CH4 þ H2 O DH298K ¼ 206:11 kJ=mol 

CO2 þ 4H2 $ CH4 þ 2H2 O DH298K ¼ 164:94 kJ=mol

ð10Þ ð11Þ

Methane CO2 reforming:

CO2 þ CH4 $ 2H2 þ 2CO DH298K ¼ 247:28 kJ=mol

ð12Þ

Carbon formation: 

2CO $ CO2 þ C DH298K ¼ 172:43 kJ=mol 

CH4 $ 2H2 þ C DH298K ¼ 74:85 kJ=mol 

CO þ H2 $ C þ H2 O DH298K ¼ 131:26 kJ=mol

ð13Þ ð14Þ ð15Þ

C56:28 H99:62 O6 þ 106:56H2 O $ 56:28CO2 þ 156:37H2 

DH298K ¼ 7:70 MJ=mol

ð16Þ

This overall reaction (Eq. (16)) is highly endothermic and requires a large amount of external heat. The equilibrium composition of the SG depends on the reformer temperature (T) and pressure (P), as well as the initial composition of the WFO-steam mixture expressed by the steam to carbon molar ratio (S/C). The S/C ratio is given by Eq. (17) [31].

¼

The stoichiometric coefficients (a–g) depend on the reformer temperature (T), pressure (P), S/C ratio and the oxygen to carbon molar ratio (O/C). The O/C ratio is given by Eq. (20) [31].

O=C ¼ ¼



S=C ¼

ð18Þ

The overall reaction of ATR can be expressed by Eq. (19).

C56;28 H99:62 O6 þ 50:28H2 O $ 56:28CO þ 100:09H2 

hydrogen content [43,44]. The increase in pressure shifts the equilibrium towards the reactants in the overall hydrogen forming reactions (Eq. (16)). Carbon monoxide methanation increases with an increase in pressure, which results in increased methane selectivity and negatively affects the hydrogen and carbon monoxide yields [45].

    moles of steam moles of steam = moles of WFO moles of WFO stoichiometric ðmoles of steamÞ 106:56  ðmoles of WFOÞ

ð17Þ

This investigation is performed at atmospheric pressure because previous investigations of SR of oxygenated hydrocarbons have already shown that higher pressures have a negative effect on the

   moles of oxygen moles of oxygen moles of WFO moles of WFO stoichiometric ðmoles of oxygenÞ 25:14  ðmoles of WFOÞ

ð20Þ

In our thermodynamic equilibrium investigation, pure oxygen is used as an oxidizing agent because using air as an oxygen-carrier strongly increases the nitrogen content in the reformer effluent. The dilution of H2 stream by the excessive N2 results in an increased anode over potential during the operation of a PEMFC. Thus, the use of air in ATR system is deleterious to the performance of PEMFC system, if the H2 produced is to be used in fuel cells [48]. However, it would be interesting to determine whether using air instead of pure oxygen could improve hydrogen production. It is well known that pure oxygen significantly increases production costs because its production consumes a significant amount of power. 4. Results and discussion 4.1. Coke formation Operating the reforming system (SR and ATR) in the coke-free regions can avoid coke formation. Coke formation during catalytic reforming can lead to the deactivation of catalysts, resulting in low durability and activity [49,50]. Therefore, coke formation is investigated to determine the coke-formed and coke-free regions. Fig. 2 illustrates coke formation (kg C/kg WFO8) as a function of reforming temperature for different S/C operating conditions. There is a high propensity for coke formation at low S/C ratios (0.25–0.75) for all of the reforming temperatures investigated. For example, at S/C = 0.25, coke can be formed at all temperatures

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considered (100–1000 °C). For a given S/C ratio, the temperature at which the first disappearance of carbon solid (TCD) was achieved is considered to be the carbon boundary. TCD decreases with increasing S/C ratio, as TCD drops from 850 °C to 700 °C when the S/C ratio is varied from 0.5 to 0.75. At a given temperature, an increase in the S/C ratio decreases the amount of coke formed; for S/C ratios higher than 1, no coke can be formed at temperatures above 350 °C. Otherwise, at low S/C ratios, the coke formation plot does not show monotone behavior but rather two regions with strong coke formation. This can be explained by the competition between the different coke formation reactions (Eqs. (13)–(15)). These reactions are in equilibrium, and because the formation of coke via exothermic reactions (Eqs. (13) and (15)) becomes less favored as the temperature increases, coke-formation via an endothermic reaction (Eq. (14)) becomes increasingly important. Therefore, the peak of coke formation (at approximately 550 °C) shown in Fig. 2 results from the competition between carbon deposition and carbon elimination across the whole temperature range. Fig. 3 confirms the above conclusions. For a given S/C ratio, this figure indicates the temperature at which coke can form and shows that the coke formation is avoided by increasing the reactor temperature and/or the S/C ratio. The regions above and below the boundary are the coke-free and coke-forming regions, respectively. The competition between the different coke formation reactions described above results in the peak of coking formation (at approximately 550 °C) and cause the coke-formed region to be wide at a temperature of approximately 550 °C. The coke formation behavior in the ATR of WFO8 is evaluated. Fig. 4 shows the temperature of the coke formation boundary for different O/C ratios over a range of different S/C ratios. For a given O/C ratio, coke formation is avoided by increasing the temperature and/or increasing the S/C ratio. As an example, for O/C = 1 and S/ C = 0.75, carbon formation is suppressed at temperatures above 400 °C, and for O/C = 1 and 400 °C, coke formation is avoided at S/C ratios above 0.75. In conclusion, high temperatures and a high S/C ratio inhibit coke formation. Coke formation can be avoided for S/C ratios higher than 1 and with temperatures above 300 °C.

Fig. 3. Coke formation boundary of SR of WFO8 as a function of S/C ratio and temperature at 1 atm.

Fig. 5a–d shows the influences of the S/C ratio and the reforming temperature on the equilibrium SG yields in the WFO8 steam reforming reactor.

Fig. 5a shows the amount of hydrogen produced (mole H2/kg WFO8) as a function of the S/C ratio and temperature. Relatively little hydrogen is produced at temperatures below 500 °C compared to the formation at 700 °C. Increasing temperature improves hydrogen production, which reaches a maximum and then decreases slightly. This behavior is the result of competition between the endothermic reaction (SR) and exothermic reactions (WGS and methanation). As the SR reaction (Eq. (16)) is endothermic, the equilibrium shifts toward the product side with increasing temperature, corresponding to an increase in hydrogen yield. The WGS (Eq. (9)) and methanation (Eqs. (10), (11)) reactions are exothermic, and the equilibrium shifts towards the reactant side with increasing temperature, resulting in the consumption of hydrogen. Fig. 5b–c shows that methane and carbon dioxide are the predominant products at low temperatures, with almost no carbon monoxide produced. At higher temperatures (T > 800 °C), the carbon monoxide content strongly increases, which can be attributed to the thermodynamics of the WGS (Eq. (9)) reaction (exothermic reaction disfavored at higher temperatures), whereas the methane content is lower. The increase in the carbon monoxide concentration requires a larger gas cleaning unit. In fact, the SG produced in the reformer is passed through a water gas shift (WGS) reactor

Fig. 2. Thermodynamically predicted coke formation in SR of WFO8 as a function of temperature for different S/C ratios at 1 atm.

Fig. 4. Coke formation boundary of ATR of WFO8 as a function of the O/C ratio for a range of different S/C ratios at 1 atm.

4.2. WFO8 steam reforming

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Fig. 5. Equilibrium synthesis gas yields in the WFO8 steam reforming reactor as a function of temperature and S/C ratio at 1 atm: (a) H2, (b) CO, (c) CH4, and (d) CO2.

Fig. 6. Effect of S/C ratio and temperature on heat consumption of WFO8 steam reforming reactor at 1 atm (a) energy demand in the reformer (b) H2 produced per amount of fuel burned.

in which CO is converted to CO2 and H2. The number of WGS stages required depends strongly on the CO content in the SG. Simultaneously aiming for a high H2 content and a low CO content is contradictory because the reforming temperature has to be high enough to obtain a reasonable H2 yield, whereas the

reforming temperature has to be as low as possible to minimize the CO content. Moreover, increasing the reformer temperature increases the heat required for the reforming reaction, which can affect the energetic efficiency of the process. On the other hand, high

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Table 3 Characteristics of a WFO8 steam reforming reactor under optimal conditions. Reactor conditions S/C

Synthesis gas composition% (dry basis) T (°C)

5 650 Hydrogen productivity

P (atm)

H2

CO

CO2

CH4

1

72.47 169.83 mol/kg WFO8

3.91

23.59

0.03

Fig. 7. Moles of H2 produced per kg of WFO8 in an ATR reactor as function of S/C and O/C ratios at 1 atm and (a) 400 °C, (b) 600 °C, (c) 800 °C and (d) 1000 °C.

temperatures can impose a risk to the reformer and catalytic reforming system. Taking all of these considerations into account, reforming temperatures ranging from 650 °C to 850 °C appear to be reasonable for WFO8 steam reforming. As observed in Fig. 5a, H2 productivity increases as the S/C ratio increases from 1 to 15. This behavior is consistent with Le Chatelier’s Principle, which states that if a dynamic equilibrium is disturbed by varying the operating conditions, the position of the equilibrium shifts to counteract the change. With an increase in S/C, the number of moles of water on the reactant side of Eqs. (8), (9) increases, and hence the equilibrium of SR and the water gas shift reaction (WGS) shifts towards the product side, resulting in increased hydrogen yield. The gain in hydrogen productivity is not significant for S/C ratios above 5, whereas the reaction system consumes excessive amounts of water and becomes highly endothermic. Fig. 6a gives the energy demand in the reformer (kJ/mole of hydrogen produced) as a function of the S/C ratio and the reforming temperature. The increases of the S/C ratio or the

reforming temperature improve the H2 productivity whereas the energy demand to generate 1 mol of hydrogen becomes too high. The energy requirement in reformer is generally provided by an auxiliary combustion reaction. Assuming that the fuel fed to the burner is methane (lower heating value, LHV = 50,020 kJ/kg), Fig. 6b gives the amount of H2 that would be produced per amount of fuel (LHV base). When the WFO8 steam reforming reactor operates at high temperature or high S/C ratio, the heat consumed by this reactor will be higher, and so will be the fuel burned for this purpose. This results in a low hydrogen production per mole of methane burned (purple part in Fig. 6b1). Fig. 5b shows that the CO content decreases slightly with an increasing S/C ratio because of an increase in the rate of WGS reaction with S/C. For hydrogen production, it is clear that CH4 is not a desirable product because the formation of CH4 competes with H2 production. Fortunately, at a given temperature, the methane con1 For interpretation of color in Figs. 6–9, the reader is referred to the web version of this article.

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181

Fig. 8. Moles of CO produced per kg of WFO8 in the ATR reactor as a function of the S/C and O/C ratios at 1 atm and (a) 400 °C, (b) 600 °C, (c) 800 °C and (d) 1000 °C.

tent decreases as the S/C ratio increases. This effect is mostly appreciable at low temperatures because the methane reforming reaction is endothermic, which is favored by increasing S/C ratios. In conclusion, favorable operating conditions for steam reforming of WFO8 can be ensured by the proper combination of reactor temperature and S/C ratio. SG compositions that maximize hydrogen production while minimizing the methane and carbon monoxide contents without forming coke can be achieved at reforming temperatures between 650 °C and 850 °C and an S/C ratio of approximately 5. Table 3 reports the SG composition at the optimum conditions for a WFO8 steam reforming reactor. Under thermodynamically optimal conditions, a hydrogen yield of 169.8 mol/kg WFO8 can be obtained without coke formation, an attractive value with the potential to stimulate experimental research. For S/C = 5, approximately 11 kg of steam is required to achieve the reforming reaction of 1 kg of WFO8. In an entire WFO8-to-hydrogen plant with heaters, steam generators, and other equipment, the overall energy balance could be highly endothermic. Therefore, the energy balance should be established in order to compute the energy consumption and then the energetic performance of such a process. 4.3. WFO8 autothermal reforming Figs. 7–9 illustrate the investigation of the WFO8 autothermal reforming reactor. Fig. 7 shows that the capacity for hydrogen production is improved at low O/C ratios, especially at high tempera-

tures and S/C ratios. This observation is in good agreement with the ATR of methane [51,52], propane [53,54], ethanol [55,56], glycerol [57,58] and vegetable oil [31]. The O/C ratio controls the amount of WFO8 retained for oxidation, and the high O/C ratio results in a low amount of WFO8 available for the reforming reaction (i.e., a decrease in hydrogen yield). Obviously, if a small amount of oxygen (low O/C ratio) is inadequate to fully convert the WFO8 and compensate for the endothermicity of the SR reaction, a strong excess of oxygen burns all chemicals species, yielding CO2 and H2O. For low O/C ratios and high temperatures, the autothermal reforming system becomes highly endothermic and an external heat supply is required, similar to the SR system. Consequently, the O/C ratio, S/C ratio and reforming temperature should be chosen appropriately to create a thermally self-sustaining system. This consideration will be discussed in Section 4.5. Moreover, the increase in the S/C ratio improves hydrogen production. This behavior is consistent with Le Chatelier’s Principle, as described above. The gain in hydrogen productivity is not significant for S/C ratios above 5. Fig. 8 shows the CO produced per kg of WFO8 in the ATR reactor as a function of S/C and O/C ratios at different temperatures. At low temperatures, the CO content is quite low and increases with increasing temperature. This is because low temperatures favor the exothermic WGS and methanation reactions, leading to low CO content. As mentioned in the WFO8 SR system, simultaneously obtaining high hydrogen content and low carbon monoxide

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Fig. 9. Moles of CH4 produced per kg of WFO8 in the ATR reactor as function of S/C and O/C ratios at 1 atm and (a) 400 °C, (b) 600 °C, (c) 800 °C and (d) 1000 °C.

content is contradictory. However, because the amount of hydrogen production can be reduced at low temperatures, moderately high temperatures are preferred, even though a small amount of CO will be formed. At a given temperature, when the amount of oxygen in the feed is increased, more CO is produced at low S/C values than at higher S/C values. In fact, the increase in the O/C ratio shifts the POX reaction (Eq. (18)) to the carbon monoxide formation side. The conversion of this CO via the WGS reaction (Eq. (9)) at high S/C ratios is more important than CO conversion at lower S/C values. The exothermic aspect of the methanation reaction (Eqs. (10), (11)) leads to a decrease in methane yield as the temperature increases. Fig. 9 shows that the maximum amount of methane produced decreases from 38 to 0.003 mol/kg WFO8 when the temperature is increased from 400 °C to 1000 °C. At a fixed temperature and S/C ratio, the methane yield continues to decrease as the O/C ratio increases. To conclude, the optimization of the WFO8 ATR system should achieve high hydrogen yield (green part in Fig. 7), moderate carbon monoxide yield (yellow part in Fig. 8), and trace amounts of methane (purple part in Fig. 9) and avoid coke formation. These figures show that hydrogen production via ATR of WFO8 can be optimized at temperatures of 600–800 °C, S/C ratios of 3–5, and O/C ratios of 0.0–0.5. The O/C ratio and reforming temperature should be chosen appropriately to create a thermally self-sustaining system. Further optimal conditions for hydrogen production will be noted in the thermoneutral condition discussion.

4.4. Thermoneutral conditions ATR is an excellent process because of its ability to achieve a thermoneutral or slightly exothermic reaction by adjusting the reactor parameters, which can reduce the need to heat the reactor. We described above that the main parameters controlling the thermal characteristics of the ATR reaction are the S/C ratio, the reforming temperature and the O/C ratio. For a self-sustaining system, these parameters should be chosen appropriately so that the reformer can be operated without using external energy for cooling or heating, increasing the value of the system when considering energy consumption [59]. The thermoneutral investigation is performed by considering WFO8, steam and oxygen as feeds that enter into the reactor at the temperature of the reactor. The operating temperature at which the external heat flow is equal to zero is also known as the adiabatic temperature. Fig. 10 shows the variation in the heat duty with O/C ratios at different S/C ratios and reforming temperatures. The O/C ratio has a very strong effect on the overall heat of the reaction. The increase in the O/C ratio can even transform the reaction from endothermic to exothermic. At 600 °C, O/C ratios of 0.448–0.455 are feasible to obtain thermoneutral conditions with S/C values between 3 and 5, whereas at 900 °C, the O/C ratio is 0.391–0.413. For all configurations considered, thermoneutrality is obtained with O/C ratios of 0.391–0.455. The recommended conditions, which maximize hydrogen production while minimizing the methane and carbon monoxide con-

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Fig. 10. Reactor heat duty as a function of S/C and O/C ratios at 1 atm and (a) 600 °C, (b) 700 °C, (c) 800 °C and (d) 900 °C.

Table 4 Characteristics of WFO8 ATR reactor under the optimal conditions. Reactor conditions S/C

Synthesis gas composition% (dry basis) T (°C)

5 600 Hydrogen productivity

O/C

P (atm)

H2

CO

CO2

CH4

0.453

1

69.42 146.45 mol/kg WFO8

2.89

27.62

0.06

Fig. 11. Effect of frying time on hydrogen yield for the SR system. Conditions: (a) temperature 650 °C and 1 atm, (b) S/C = 5 and 1 atm.

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Table 5 Characteristics of WFO steam reforming reactors for different frying times, T = 650 °C and P = 1 atm. Frying time (h)

Oil average molecular composition

H2 productivity (mol/kg WFO)

0 8 16 32

C56.34H99.5O6 C56.28H99.62O6 C56.31H100.22O6 C56.22H100.88O6

169.78 169.83 170.04 170.28

Synthesis gas composition% (dry basis) H2

CO

CH4

CO2

72.46 72.47 72.51 72.56

3.91 3.91 3.91 3.91

0.02 0.03 0.03 0.03

23.60 23.59 23.55 23.50

Fig. 12. Effect of frying time on hydrogen yield for the ATR system at different O/C ratios. Conditions: (a) 600 °C and 1 atm and (b) S/C = 5 and 1 atm.

Table 6 Characteristics of WFO autothermal reactors under thermoneutral conditions for different frying times, T = 600 °C and P = 1 atm. Frying time (h)

O/C ratio

H2 productivity (mol/kg WFO)

0 8 16 32

0.454 0.453 0.449 0.442

146.36 146.45 146.90 147.55

tents and avoiding coke formation at thermoneutral conditions, are S/C = 5, T = 600 °C and O/C = 0.453. Table 4 presents the characteristics of the WFO8 ATR reactor under the recommended conditions.

Synthesis gas composition% (dry basis) H2

CO

CH4

CO2

69.40 69.42 69.50 69.62

2.89 2.89 2.89 2.90

27.65 27.63 27.55 27.42

0.06 0.06 0.06 0.07

C56.34H99.5O6 (WFO0), C56.28H99.62O6, C56.31H100.22O6, and C56.22H100.88O6 (WFO32). Even if it slightly improves the hydrogen productivity, the effect of frying time on soybean oil SR can be considered negligible.

4.5. Influence of frying time on reforming reactor behavior During the frying process, the fat undergoes several physicochemical changes caused by heat, water and atmospheric oxygen. With prolonged heating during frying, the accumulation of deterioration products leads to organoleptic failures and a decrease in the nutritive value. The baseline case in this study considered soybean WFO after 8 h of use. This section examines the effect of frying time on reforming reactor behavior, considering the soybean WFO described in Table 2. 4.5.1. Influence of frying time on WFO steam reforming Fig. 11 shows the effects of the S/C ratio and the reforming temperature on hydrogen yield. The effect of frying time on hydrogen productivity is not significant. These curves are almost identical when the temperature or S/C ratios are varied. The table gives the characteristics of WFO SR reactors at the recommended conditions for different frying times. Table 5 shows that the hydrogen yield increases slightly with frying time. This change in hydrogen yield results because the hydrogen content in WFO molecules increases in the order of

4.5.2. Influence of frying time on WFO autothermal reforming The influence of frying time on WFO autothermal reforming is also investigated. Fig. 12 shows the effect of the reforming temperature and S/C ratio on hydrogen productivity at different O/C ratios. For a given O/C ratio, roughly similar curves are obtained. This confirms the results obtained for the SR system, indicating that frying time has a negligible effect on hydrogen productivity. Table 6 lists the characteristics of WFO ATR reactors under the recommended and thermoneutral conditions at different frying times. The effects of frying time on the ATR system can be interpreted similarly to the effects on the SR system. Even if WFO loses its nutritional value over a short frying time, soybean vegetable oil maintains its full capacity to produce hydrogen. This result encourages experimental research on hydrogen production by reforming cheap polluting waste oils. 5. Conclusion The thermodynamic equilibrium of soybean WFO steam and autothermal reforming is studied under atmospheric pressure via

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Gibbs free energy minimization. Based on the thermodynamic calculations, the following main conclusions can be made: – Favorable operating conditions for WFO8 SR can be ensured through proper selection of the reactor temperature and S/C ratio. The optimal conditions, determined based on maximizing hydrogen production while minimizing the methane and carbon monoxide contents and coke formation, can be achieved at reforming temperatures between 650 °C and 850 °C and an S/C ratio of approximately 5.

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