Sorption Enhanced Steam Reforming of Glycerol: Use of La-Modified

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Sorption enhanced steam reforming of glycerol: use of La-modified Ni/Al2O3 as catalyst Nuria Sánchez, Jose María Encinar, and J.F. Gonzalez Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b04084 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on March 24, 2016

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Industrial & Engineering Chemistry Research

Sorption enhanced steam reforming of glycerol: use of La-modified Ni/Al2O3 as catalyst Nuria Sánchez†*, J. María Encinar†, J. Félix González‡.

†Department of Chemical Engineering and Physical Chemistry. University of Extremadura, Avenida de Elvas s/n, 06006 Badajoz, Spain. ‡Department of Applied Physics. University of Extremadura. Avda. Elvas s/n, 06006 Badajoz, Spain. KEYWORDS. Glycerol, steam reforming, carbon dioxide, calcium oxide, dolomite, magnesium oxide.

ABSTRACT. Glycerol is being produced on a massive scale as by-product of biodiesel. In this work, glycerol steam reforming was improved by CO2 abatement to obtain high-value gases. CaO and activated dolomite were used as sorbents and these results were compared to reactions using Al2O3 and MgO. The effect of packing material was evaluated at 600, 700, 800 and 850 ºC. Hydrogen production was promoted either in presence of a sorbent at 600 ºC or at higher temperatures with any of the packing material. Sorbents also affected Water-Gas shift reaction, decreasing CO concentration. Simultaneous use of Ni/La2O3/Al2O3 and CaO improved gas production and hydrogen selectivity. H2 percentage in the produced gas was close to 70 % and

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around 5 moles of H2 were produced per mole of glycerol. Catalyst and sorbent were used for several reaction cycles.

INTRODUCTION Nowadays, petroleum depletion and environmental contamination make imperative the use of renewable energy resources. Biodiesel is one of these resources since the use of this fuel considerably reduces carbon dioxide emissions, regarding diesel emissions.1 Glycerol is the main by-product of biodiesel production, for this reason, huge amounts of this compound is currently produced and they must be transformed into value-added product.2 Steam reforming (SR) is one of the main available processes to turn glycerol into a valuable gaseous phase, mostly composed by H2, CO and CO2. The gas could be used as fuel gas or syngas to produce hydrogen since the global demand for this compound is expected to increase greatly in the future.3 The overall reaction in the glycerol steam reforming can be expressed as equation (1); this is a combination of glycerol decomposition reaction (2) and water-gas shift reaction (WGS) (3). In addition, other reactions such as the formation of methane from CO and CO2 or coke formation could take place:4 C3H8O3 +3H2O C3H8O3

7H2 + 3CO2 4H2 + 3CO

∆Hº298K = 127.6 kJ·mol-1

(1)

∆Hº298K = 251.2 kJ·mol-1

(2)

CO + H2O

CO2 + H2

∆Hº298K = -41.2 kJ·mol-1

(3)

CO + 3H2

CH4 + H2O

∆Hº298K = -206.1 kJ·mol-1

(4)

CO2 + 4H2

CH4 + 2H2O

∆Hº298K = -164.9 kJ·mol-1

(5)

CO2 + CH4

H2 + 2CO

∆Hº298K = 247.3 kJ·mol-1

(6)


2

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CO + H2

C(s) + H2O

∆Hº298K = -131.3 kJ·mol-1

(7)

CH4

C(s) + 2H2

∆Hº298K = 74.9 kJ·mol-1

(8)

2CO

C(s) + CO2

∆Hº298K = -172.4 kJ·mol-1

(9)

A limitation of the glycerol reforming is the formation of large quantities of CO2 that contribute to greenhouse effect. This compound could be removed from the exhaust gases by means of washing with amines, but the process involves additional costs and technological difficulties. The use of a new method, sorption enhanced steam reforming (SE-SR), allows the capture of CO2 during the steam reforming reaction. Thus, H2 production would be increased by the promotion of the conversion of CO to CO2 by the WGS reaction. So, the steam reforming reaction could be carried out at lower temperatures reaching similar results. After the reforming, the sorbent would be regenerated increasing the temperature and the released CO2 could be captured and stored.5, 6 The sorbents are metallic oxides that react with CO2 to form carbonates. Between these sorbents CaO and dolomite, that are abundant and cheap, are widely-used. Carbon dioxide abatement would take place by means of the reaction (10), which has reversible character and its direction depends on CO2 partial pressure and temperature. After many cycles of carbonation-decomposition, the sorbent usually loses activity.7

CaO (s) + CO2 (g)

CaCO3 (s)

(10)

Glycerol steam reforming could be carried out in presence or absence of catalyst. The most used catalysts are composed of transition metals as active phase, supported on metallic oxides that confer suitable mechanical and thermic resistance. Furthermore, they can be modified with additives or promoters that improve the activity of the catalyst or prevent their deactivation.8


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Nickel is the most common active phase because it has shown good results in steam reforming reactions and it has a reasonable price compared to other noble metals that can be used for the same reaction.9 The properties of the catalyst support are related to the mechanical resistance, the stability, the reactivity and the effectiveness of the catalyst, so it has to be carefully chosen. Alumina is a widely-used support due to its mechanical and chemical resistance under the typical reaction conditions of SR. However, acid supports as alumina promote the dehydration of glycerol, causing the formation of unsaturated hydrocarbons, as ethylene, that are coke precursors.10 In order to optimize the properties of the supports, in many cases additives or promoters are added with good results.11 In this way, La2O3 incorporation allowed to increase the activity and the stability of the catalyst Ni/Al2O3. The catalyst Ni/La2O3/Al2O3 has showed very good results in the SR of glycerol and other compounds.12, 13 Considering the previous background, a study of the SR of glycerol in presence of a CO2 sorbent was carried out. Calcium oxide and dolomite were used as sorbents and these results were compared to reactions with magnesium oxide and alumina. In addition, Ni/La2O3/Al2O3 was used as catalyst to test the simultaneous effect of the catalyst and the sorbent in successive cycles of reaction. The experimental set-up can be considered an intermedia stage between bench scale and pilot plant.

EXPERIMENTAL MATERIALS. Glycerol (PRS) was purchased from Panreac. Magnesium oxide (PRS, 0.150.20 mm of average particle diameter, 49 m2·g-1 BET surface) was purchased from Panreac. Calcium oxide (97 wt %, 0.15-0.20 mm of average particle diameter, 4 m2·g-1 BET surface) was supplied by Riedel-de Haën. Dolomite (~54 % CaCO3, ~45 % MgCO3, 0.15-0.20 mm of average


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particle diameter, 7 m2·g-1 BET surface) was supplied by Pedro Diaz Carbonatos S.L. (Gilena, Spain). α-Al2O3 spheres (18-19 mm of particle diameter, 1 m2·g-1 BET surface) were purchased from Ceremex. For the catalyst: nickel (II) nitrate hexahydrate (Ni(NO3)2·6H2O) and lanthanum nitrate hexahydrate (La(NO3)3·6H2O) were purchased from Panreac and Merck, respectively, whereas γ-Al2O3 rings (2 mm I.D., 5 mm O.D., 3-5 mm length, 159 m2·g-1 BET surface) were donated by Saint-Gobain NorPro. CATALIST PREPARATION. Catalyst was prepared on γ-Al2O3 rings, which were calcined at 750 ºC for 3 h. An aqueous solution of La(NO3)3·6H2O was impregnated by incipient wetness technique. After impregnation, the catalyst was dried by microwave radiation for 30 min. This drying technique would allow to reduce drying times and more uniform distribution of the metal on the support.14 Finally, the solid was calcined in a furnace at 650 ºC for 6 h (5 ºC·min-1). Then, an aqueous solution of Ni(NO3)2·6H2O was impregnated, it was dried by microwave irradiation and calcined at 500 ºC for 4 h (5 ºC·min-1). La2O3 and NiO on the catalyst showed weight percentage of 5 and 16 %, respectively. This method of catalyst preparation has been previously used and suitable results were obtained in steam reforming reaction.15, 16 EQUIPMENT AND METHODS. The experimental set-up consists of two reactors in series, placed into both ceramic ovens. Reactors have temperature and N2 flow control systems. A highpressure pump was used to supply the water-glycerol mixture. A condensation system, a volumetric flowmeter and a gas sample collecting system complete the installation. The produced syngas was analyzed by gas chromatography with a detector of thermal conductivity (TCD), supplied with a packed column Carboxen 1000 60/80 mesh of 15 ft of length and 1/8 in of ID. Argon was used as carrier gas and column temperature was varied from 100 to 200 ºC during the analysis. In Figure 1 is shown a general diagram of the experimental installation.


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Reactors (both made of stainless steel 316) present an internal diameter of 5.2 cm and 30 cm of length. Each reactor has a basket, which was filled as mentioned below. Three types of reactions were carried out. In the first reactions, the basket of the first reactor was filled with calcium oxide, dolomite or magnesium oxide (0.75 moles) plus spheres made of α-Al2O3. These reactions were used to discuss the first section of Results and Discussion. When dolomite was used, it was activated by heating under inert atmosphere (250 mL·min-1 STP N2) at 900 ºC, for 6 h, before the reaction. Then, for the second reactions, Ni/La2O3/Al2O3 was used as catalyst. It was placed in the first reactor and 0.75 mole of CaO in the second reactor; both baskets were also filled with αAl2O3 spheres. Five grams of catalyst were used and they were reduced in situ by H2/N2 flow, before the reaction. Thirdly, in the last kind of reactions, only catalyst was used. In this case, the catalyst was also reduced before the reaction. For all reactions, the glycerol-water mixture flow was 1.15 g·min-1, with a steam to carbon feed ratio of 5.7. Nitrogen was used as carrier gas and 1.3 molar ratio of nitrogen to glycerol was used.

A

B

D

E M

C

F H L G K I

J


6

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Figure 1. Experimental set-up. A, B, C: glycerol-water, H2 and N2 feeding system, respectively. D: gas flowmeter. E: temperature control system. F, G: 1st and 2nd reactor, respectively. H: basket. I: electronic furnace (heating system). J: liquid sample collecting and condensation system. K: gas sample collecting system. L: total gas flowmeter (gas bubble flowmeter). M: gas outlet. DATA ANALYSIS. The distribution of products indicated as H2, CO, CO2, CH4, C2H4, C2H6% was calculated as: produced moles of H2, CO, CO2, CH4, C2H4, C2H6, respectively, divided by total moles of gas phase × 100. The hydrogen production was indicated as H2/glyc. and it was calculated based on the following equation: H2

=

H 2 moles in gas products

glyc. glycerol moles in the feedstock

(11)

where 7 would be the maximum number of H2 moles that could be produced per glycerol mole, according to equation (1).

RESULTS AND DISCUSSION EFFECT OF CARBON DIOXIDE ABATEMENT IN GLYCEROL STEAM REFORMING. Calcium oxide and activated dolomite were used as sorbents and the results were compared to reactions with magnesium oxide and alumina. These reactions were carried out without catalyst at temperatures between 600 and 850 ºC. The influence of the temperature on the H2/glyc. molar ratio was shown in Figure 2. At 600 ºC, the lowest hydrogen production was obtained when Al2O3 without sorbent was used. In the presence of MgO the hydrogen production was close to the production obtained with Al2O3. However, the use of dolomite and CaO improved the results.


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At this temperature MgO is a stable compound because its carbonation is produced when the temperature is lower than its calcination temperature (~385 ºC). On the other hand, gas-solid carbonation of CaO will be observed up to ~900 ºC, beyond this temperature thermodynamic favors the CaO state.17 Therefore, the substantial carbonation conversion of CaO and activated dolomite at 600 ºC led to 5.2 and 2.7 times higher hydrogen production, respectively, than that obtained with alumina. 4

3 H2/glyc.

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CaO Dolomite MgO Al2O3

2

1

0 600

700

850

Temperature (ºC)

Figure 2. H2/glyc. molar ratio as a function of the temperature. When temperature was increased, two contrary effects were seen. Firstly, the hydrogen production increased because of SR global reaction is endothermic. On the other hand, the position of the equilibrium CaCO3-CaO was moved to CaO and the carbon dioxide sorption decreased. In this way, the highest H2 production was achieved at 850 ºC, due to the effect of the temperature. Two extra reactions with CaO were carried out because this sorbent was the most effective and another reaction was done in presence of Al2O3. The first reaction with CaO was carried out at 500 ºC, and 0.18 moles of H2 were produced per mole of glycerol. The gas flow was continuously decreasing and after 2.5 h of reaction, the gas flow rate had decreased 26 %. The other reactions were carried out at 800 ºC and a H2/glyc. ratio of 2.41 was achieved in presence of CaO, against 1.72 achieved in presence of Al2O3. So, these results seem that CaO


8

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remains some CO2 sorption capacity at 800 ºC. In order to reach the optimal conditions, it would be necessary to evaluate energy consumption, H2 production and released CO2. However, the maximum adsorption effect was seen at 600 ºC, similar to the results obtained by other authors in a thermodynamic analysis using CaO.4, 18 Whereas, in experimental studies of methane SE-SR the optimal temperature was established at 725 ºC.4, 19 The gas composition of the produced gas at 600 ºC is shown in Figure 3a. When the experiment with Al2O3 is taken as a reference, the presence of MgO caused a rise in H2 and CO2 concentration and a drop in CO and hydrocarbons percentages. The particle size of magnesium oxide was smaller than that of alumina, the reduction in the particle diameter of the packing material offers resistance to the flow of the reactants and favors its uniform distribution; therefore, the smaller particle size would increase the contact between the reactant and promote additional reaction such as WGS.20, 21 The presence of CaO and activated dolomite led to higher H2 concentration and less CO and hydrocarbon concentration because of the influence of the removal of CO2 in the system.7


9


70

60

a Gas composition (%)

Gas composition (%)

60 50 40 30 20 10 0

b

50 40 30 20 10 0

Al2O3

MgO

Dolomite

CaO

70

Al2O3

MgO

Dolomite

70

c

CaO

d

60 Gas composition (%)

60 Gas composition (%)

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50 40 30 20 10

50 40 30 20 10

0

0 Al2O3

MgO

H2

2 CO

Dolomite

CaO

CO CH4

Al2O3

4 CO2

C2H4

CaO

C2H6

Figure 3. Gas composition as a function of the CO2 sorbent at (a) 600 ºC, (b) 700 ºC, (c) 850 ºC and (d) 800 ºC. Regarding gas composition at 700 ºC (Figure 3b), when Al2O3 and MgO were used, hydrogen and carbon monoxide percentage were similar. This composition seems to be the result of the promotion of the glycerol decomposition reaction (reaction 2). Under these conditions, the increase of temperature from 600 to 700 ºC led to a bit higher hydrogen flow rate, as shown in Figure 2. However WGS reaction seemed not to be promoted since H2 and CO concentrations were almost the same. This could be due to the exothermicity of the reaction (reaction 3) or the lack of catalyst and sorbent which could favor this reaction even at 700 ºC. Dolomite and CaO still showed sorption capacity as long as the gas composition was more similar to that at 600 ºC.


10

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On the other hand, the results obtained at 850 ºC (Figure 3c) are not related to the capacity of CO2 removal of the packing materials, because at this temperature the equilibrium CaCO3-CaO is mainly moved to CaO, so CO2 sorption is few significant.17 The composition of the gas produced in presence of CaO, dolomite and MgO was similar and could be influenced by the particle size (0.15-0.20 mm diameter) of these materials against 18-19 mm of diameter of Al2O3. As aforementioned, smaller particles of packing material could improve the contact between the reagents, achieving higher conversion in the steam reforming reaction with higher H2 and lower CO compositions as shown in Figure 3c.20, 21 At 800 ºC (Figure 3d), this fact together with a possible sorption effect of CaO led to a more important development of glycerol steam reforming, that is higher H2 concentration and lower CO percentage, when CaO was used in comparison with the results with Al2O3. The highest effect of carbon dioxide abatement was seen at 600 ºC when CaO was used. Under these conditions, the molar ratio of hydrogen to glycerol increased five times in relation to the experiment without sorbent. Calcium oxide had the lowest surface area (4 m2·g-1), whereas additional surface area would allow CO2 to be removed more easily. However its higher stoichiometric sorption capacity (17.96 mol·kg-1) in contrast to dolomite (10.96 mol·kg-1) led to the highest CO2 sorption.2, 22 EFFECT OF CARBON DIOXIDE ABATEMENT IN CATALYTIC GLYCEROL STEAM REFORMING. As far as CaO showed the greatest effect as CO2 sorbent, new reactions were carried out where this compound was simultaneously used with a catalyst, Ni/La2O3/Al2O3. According to previous experiments carried out by our research group, this catalyst was active in glycerol steam reforming and the most important effect was seen at 700 ºC. On the other hand, the highest effectiveness of CaO as sorbent was obtained at 600 ºC, as seen in this paper.


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Ni/La2O3/Al2O3 and CaO were placed in the first and the second reactor, respectively, working at 700 and 600 ºC, respectively. In this way, the steam reforming reactions were promoted in the first reactor, and the removal of CO2 in the second reactor led to an additional development of these reactions, including WGS reaction.23 Catalyst and sorbent were subjected to five consecutive reaction cycles. The obtained results are shown in Figure 4, where the cycles are separated by discontinuous vertical lines. In addition, three extra reactions with five consecutive cycles were carried out using only catalyst, without sorbent. The results of these reactions were also collected in Figure 4, and in Table 1. After each cycle, a flow of H2/N2 (50 %) was used, at 700 ºC, to reduce any nickel particle which could have been oxidized by the contact with the steam. Only for the reactions when sorbent was used, the reactor with CaO was warmed up to 900 ºC to release a part of the stored CO2. The evolution of the hydrogen production over the time is shown in Figure 4; solid symbols represent the experiment with catalyst and sorbent, and white symbols show the experiment just with catalyst. As can be seen, the hydrogen production using sorbent (first reactor at 700 ºC and second reactor at 600 ºC) was similar to the production obtained at 800 ºC when only the catalyst was used. The average hydrogen to glycerol molar ratio was 5 for both reactions. According to a bibliographic thermodynamic study, at 700 ºC, this ratio should be around 5.9 without sorbent and around 6.6 if 80 % CO2 were removed by sorption.18 At 700 ºC, the hydrogen to glycerol molar ratio increased by 0.5 because of the incorporation of the sorbent; nevertheless, it was not enough to reach the values of the thermodynamic study, although a significant reduction of the reaction temperature was allowed. When only the catalyst was used, the hydrogen production was almost constant during all cycles of reaction. However, with catalyst plus sorbent, this production was higher at the


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beginning of the third, fourth and fifth cycle and it was decreasing up to a value close to that in the first and second cycles.

6 5

700ºC Ni/La/Al2O3 800ºC Ni/La/Al2O3

4 H2/glyc.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


900ºC Ni/La/Al2O3 700ºC Ni/La/Al2O3 + 600ºC CaO

3 2 1 0 0

250

500

750

1000

1250

1500

Time (min)

Figure 4. H2/glyc. molar ratio over several cycles of reutilization. Gas composition of the reaction with sorbent plus catalyst is shown in Figure 5. Average gas compositions of the reactions just with catalyst were collected in Table 1. The highest hydrogen percentage without sorbent was 65.2 % when the reaction was carried out at 900 ºC in two serial reactors. However, hydrogen percentage with catalyst in the first reactor at 700 ºC and sorbent in the second reactor at 600 ºC was always higher than 67 %. In all cases, low hydrocarbon composition was obtained, although, according to thermodynamic studies at this temperature, hydrocarbons would not be present.18 When sorbent was used, CO and CO2 contents were higher and lower, respectively, than in the gas obtained without sorbent. Gas composition confirmed CO2 sorption, increasing H2 production and decreasing CO2 content. By using this process, greenhouse emissions could also be reduced.5 Total sorption of this gas was not obtained; CO2 concentration was not lower than 15 % in any case. In previous works micro-reactors were mostly used and very low feed flow rates are usually utilized.10, 12, 24 However, in this case, the


13


reactor was bigger and high feed flow rate was used to achieve high gas production and a more efficient process. These facts and the formation of a layer of CaCO3 over CaO particles, which hinders the diffusion and sorption of higher quantities of CO2,25-27 could have caused the incomplete CO2 sorption. On the other hand, there was a continuous increase of CO2 concentration and decrease of H2 after the second cycle of reaction. This could show the decrease of the activity of the sorbent. As seen in previous works, sorbent can reach an interval during which the sorption reaction efficiency starts decreasing because of their saturation.7, 28 During the last reaction cycles, the sorbent lost its sorption capacity relatively fast. This fact was usually explained by other authors because of the sintering of the material.29-32 However, the CaO used in this work had low surface area, so this effect could be more limited and the sintering of this material would probably lead to smaller changes in its activity than that in other sorbents. In conclusion, the combination of the catalyst with a CO2 sorbent improved hydrogen production; it was 12 % higher than the results without sorbent. In addition, after five cycles of reuse, the catalyst showed similar activity to the initial one and the hydrogen concentration in the produced gas was close to 70 %, although CaO seemed to show loss of sorption capacity. 70 Gas composition (%)

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60

H2 CO CH4 CO2 C2H4 C2H6

50 40 30 20 10 0 0

250

500

750

1000

1250

1500

Time (min)

Figure 5. Gas composition over several cycles of reutilization.


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Table 1. Average gas composition for the reactions with catalyst and without sorbent at several temperatures.

Reaction temperature, ºC

Average gas composition, % H2

CO

CH4

CO2

C 2H 4

C 2H 6

700

62.5

9.7

2.1

24.4

1.0

0.3

800

63.9

9.9

2.2

23.3

0.6

0.1

900

65.2

10.7

1.7

22.2

0.1

0.1

CONCLUSIONS Glycerol produced as biodiesel by-product was tested as suitable biomass to produce syngas or a hydrogen-rich gas by steam reforming. An intermediate scale between bench scale and pilot plant was used. Calcium oxide and activated dolomite showed high capacity to capture CO2 during the reaction. This capacity was maximum at 600 ºC, whereas magnesium oxide did not show sorbent skills. At higher temperatures, the effect of CO2 abatement decreased progressively, although a rise in the temperature affected the steam reforming positively. Simultaneous use of Ni/La2O3/Al2O3 and CaO improved the results. Hydrogen percentage close to 70 % was reached and more than 5 mole of hydrogen per mole of glycerol were achieved. After two reutilization cycles, the sorbent sorption capacity was affected, although it was still high.

AUTHOR INFORMATION Corresponding Author


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*Nuria Sánchez, Avda. Elvas s/n, 06006 Badajoz (Spain) tlf: +34 924289672, e-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Conflict of Interest Disclosure The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to acknowledge “MICINN” from Spain and “Gobierno de Extremadura” for the financial support by means of Projects ENE2009-13881, PRI09B102 and “ayuda a grupos GR15034”, respectively. Nuria Sánchez would gladly like to acknowledge Ministry of Education from Spain for FPU grant received. Authors also acknowledge the SAIUEX service of the University of Extremadura for the characterization analysis and SaintGobain NorPro for donating the support of the catalyst. REFERENCES (1) Balat, M.; Balat, H. Progress in biodiesel processing. Appl. Energ. 2010, 87, 1815-1835. (2) Dou, B.; Song, Y.; Wang, C.; Chen, H.; Xu, Y. Hydrogen production from catalytic steam reforming of biodiesel byproduct glycerol: Issues and challenges. Renew. Sust. Energ. Rev. 2014, 30, 950-960.


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For Table of Content Only

CATALYTIC STEAM REFORMING C3H8O3 + 3H2O

Ni/La2O3/Al2O3

7H2 + 3CO2

CO2 SORPTION CaO (s) + CO2

CaCO3 (s)


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Sorption Enhanced Steam Reforming of Glycerol: Use of La-Modified

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