Fuel Processing Technology 106 (2013) 695–703

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Fuel Processing Technology

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Hydrogen production from yellow glycerol via catalytic oxidative steam reforming

Krongthong Kamonsuangkasem a, Supaporn Therdthianwong a,⁎, Apichai Therdthianwong b

a Department of Chemical Engineering, Faculty of Engineering, King Mongkut's University of Technology Thonburi, 126 Pracha Uthit Rd., Bang Mod, Thung Khru, Bangkok, 10140 Thailandb Fuel Cells and Hydrogen Research and Engineering Center, CES, Pilot Plant Development and Training Institute, King Mongkut's University of Technology Thonburi, 126 Pracha Uthit Rd.,Bang Mod, Thung Khru, Bangkok, 10140 Thailand

⁎ Corresponding author. Tel.: +66 2 4709222x403.E-mail address: supaporn.the@kmutt.ac.th (S. Therd

0378-3820/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.fuproc.2012.10.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 January 2012Received in revised form 19 September 2012Accepted 7 October 2012Available online 31 October 2012

Keywords:Yellow glycerolOxidative steam reformingHydrogen productionNi/CeZrO2/Al2O3

In this research study, a low-impurity product from glycerol purification unit of a biodiesel manufacturing calledyellow glycerol, was shown to be an alternative raw material for H2 generation via oxidative steam reformingreaction over Ni/CeZrO2/Al2O3 catalyst. The effects of operating parameters, such as water-to-yellow glycerolratio between 3 and 9, reaction temperature in the range of 550–650 °C, and oxygen-to-yellow glycerol ratiofrom 0.25 to 0.75, were explored. Partial oxidation, steam reforming and decomposition of glycerol incorporatedwith water gas shift reaction were the main reactions involved producing H2, CO and CO2 as the main gas prod-ucts. The suitable operating condition found was at water-to-yellow glycerol ratio of 9, oxygen-to-yellowglycerol ratio of 0.5 and 650 °C where nearly complete conversion of yellow glycerol was achieved with maxi-mized hydrogen selectivity and yield at 69% and 67%, respectively. This promoted catalyst exhibited superior ac-tivity to the unpromotedNi/Al2O3 in enhancing themain reactions producing hydrogenwhile strongly inhibitingcoke deposition. Compared to pure glycerol, the reforming of yellow glycerol byproduct produced relatively highH2 yield, 4 mol H2/mol yellow glycerol vs. 4.9 mol H2/mol pure glycerol demonstrating that yellowglycerol is anattractive raw material for hydrogen production via the catalytic oxidative steam reforming.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The increasing prices of crude oil and the widespread concern forglobal warming have influenced the expansion of the worldwide pro-duction of biodiesel. Hence, glycerol, a major by-product from biodieselplant, would be abundantly available as the demand has not been pro-portionally growing. As a consequence, considerable efforts have beenproposed to utilize the surplus glycerol. One of the technologies thathas a great development and can utilize glycerol is fuel cell. It needshydrogen fuel which can be produced from several hydrocarbonsources, such as petroleum products, methane, methanol, ethanol, etc.Glycerol is, thus, an attractive hydrocarbon source for hydrogenproduction.

Although several processes can be applied for hydrogen productionfrom glycerol, the thermo-chemical approach is considered to be themost effective method. This included pyrolysis, steam reforming andautothermal or oxidative steam reforming. Pyrolysis of glycerol hasbeen studied with the purpose of producing hydrogen or syngas eitherin absence or presence of catalysts [1–3]. However in this process,methane and small hydrocarbonmolecules including charwere typicallygenerated when the reaction temperature was not sufficiently high.Many research groups have paid attention to the catalytic steamreforming of glycerol on different types of catalysts to generate hydrogen

thianwong).

rights reserved.

to lessen the unwanted by-products. These catalysts include nickel-based catalysts [4–7], modified nickel-based catalysts [8,9] and othernoble metal-based catalysts such as Ru, Rh, Ir and Pt [10–13]. CO andCO2 are the main gas products from these steam reforming processes.It has been proposed to integrate the principle reformer unit withother units such as a water gas shift reactor [14] or a CO2 adsorbingbed [15–17] to further reduce gases of carbon oxides.

In addition to the experimental work, some researchers have alsoperformed thermodynamic analyses of glycerol steam reforming pro-cesses and provided very useful information regarding to the effects ofoperating conditions and the most favorable conditions [16–20]. Forexample, steam to glycerol ratio in the range of 9–12, 600–700 °C reac-tion temperature and low oxygen to glycerol ratio favor the productionof hydrogen.

Autothermal reaction or oxidative steam reforming (OSR), a com-bined partial oxidation and steam reforming process, is an efficientprocess for generating H2 from glycerol. However, it has been givenless interest than steam reforming process. This process has certainadvantages over steam reforming in terms of energy saving andcoke reduction. Previous research was emphasized in both thermody-namic calculation and experiment. Based on equilibrium calculation,the effects of operating parameters (steam to carbon ratio, oxygento carbon ratio and temperature) under thermoneutral condition onthe product gas selectivity was well understood [18,19,21]. A few ex-perimental works were carried out at several operating conditions(oxygen to glycerol ratio (O2/G), steam to glycerol ratio (S/G) andtemperatures) and only over noble metal-contained catalysts such

Table 1Composition analysis of yellow glycerol.

Chemical composition Proximate analysis Ultimate analysis

(%) (%dry basis) (% dry basis)

Glycerin 94 Ash 0.04 C 36.673-methoxy-1,2 propanediol 6 Volatile matter 99.92 H 9.69

Fixed carbon 0.04 N 2.18O (by diff) 51.42

696 K. Kamonsuangkasem et al. / Fuel Processing Technology 106 (2013) 695–703

as Pd, Rh, Pt and Ir [19,21–23] giving a wide range of hydrogenselectivity.

Though nickel-based catalysts on a variety of supports have beenextensively studied in steam reforming of glycerol as aforementioned,they have not been investigated in the oxidative steam reforming.Furthermore, most of previous research emphasized on reforming ofpurified glycerol byproduct, while different contaminated glyceroltypes, mainly depending on biodiesel production process, are gener-ally produced. Utilizing them as feedstocks for hydrogen productionwould make the cost of hydrogen produced be considerably less ex-pensive and more economic since purification is not needed. Hence,yellow glycerol, a byproduct from glycerol purification unit, wasused as the feedstock in this study. The cost of hydrogen producedfrom this raw material combined with the cost of the process couldthen be considerably inexpensive. The promoted non noble metal cat-alyst, Ni/CeZrO2/Al2O3, was chosen to study the oxidative steamreforming of yellow glycerol for hydrogen production as it hasshown a great ability in ethanol oxidative steam reforming for hydro-gen production in our previous work [24]. The effects of operating con-dition, i.e. steam-to-yellow glycerol ratio (S/YG), oxygen-to-yellowglycerol ratio (O2/YG), and reaction temperature, on glycerol conver-sion and hydrogen selectivity were carried out. Additionally, the ac-tivity of this promoted catalyst was compared with the unpromotednickel catalyst.

2. Experimental procedure

2.1. Catalyst preparation

The 15%Ni/Al2O3 promoted by 8%CeO2-ZrO2 catalyst used in thisstudy was first prepared by co-impregnating ceria and zirconia pre-cursors onto a commercial Al2O3 support (JRC-ALO2, a product ofJapan Chemical) which has a surface area of 189 m2 g−1. All precur-sors used were in nitrate form, i.e. Ce(NO3)3.6H2O (supplied byAcros Organics) and ZrO(NO3)2.xH2O (supplied by Sigma Aldrich).The calculated amount of Al2O3 powder was added into the preparedaqueous solution of cerium and zirconium nitrates. The mixture wasmixed and stirred for overnight. The promoted support mixture(8%CeO2-ZrO2/Al2O3) was then boiled to remove water. The powderpaste obtained was then dried at 120 °C for 3 h and subsequentlycalcined at 500 °C for 3 h. Following the same procedure and condi-tions previously mentioned, 15 wt.% nickel was then deposited ontothe support by using an aqueous solution of Ni(NO3)2.6H2O (suppliedby Carlo Erba). Similarly, the unpromoted nickel catalyst, 15%Ni/Al2O3, tested in comparison with the promoted catalyst was prepared.The detailed preparation procedure can also be seen in our previouswork [24]. The 8%CeO2-ZrO2/Al2O3, 15%Ni/8%CeO2-ZrO2/Al2O3 and15%Ni/Al2O3 catalysts obtained and reduced prior to activity testwere designated as “CeZrAl”, “NiCeZrAl” and “NiAl”, respectively.

2.2. Catalyst characterization

The surface area of the catalysts was measured using Autosorb I(Quantachrome Co. Ltd.) by the Brunauer–Emmett–Teller (BET)method of N2 physisorption at liquid nitrogen temperature (77 K)while pore size distribution of the catalysts was determined by theBarret–Joyner–Hallender (BJH) method applied to the desorptionbranch of nitrogen isotherm.

Nickel dispersion on the catalysts was measured using a pulse H2-chemisorption technique in Chembet 2000 apparatus (Quantachrome)at 303 K. Prior to themeasurement, the catalystwas reduced byflowingH2 at 500 °C for 3 h and subsequently flushed under He until the tem-perature decreased to room temperature. The amount of hydrogenuptake was determined by periodically injecting 99.99%H2 into thecatalyst. A chemisorption stoichiometry H:Ni of 1:1 and a spherical

geometry of Ni with cross sectional area of 6.49×10−20 m2 Ni-atom−1

were assumed [25].X-ray diffraction spectra were obtained from Phillips PW 1830

X-ray diffractometer equipped with CuKα source (λ=0.1538 nm)in the 2 Theta range of 10–90°. The XRD patterns obtained werefurther analyzed to determine the mean nickel metal crystallite diam-eter using Scherer's equation.

Temperature programmed reduction (TPR) tests of the fresh andreduced catalysts were carried out in Chembet 2000 apparatus(Quantachrome) with H2/N2 ratio of 1/9 and the heating rate of10 °C min−1 from room temperature up to 900 °C. The amount ofhydrogen consumed was recorded.

The temperature programmed oxidation analyses of used catalystswere performed using a thermogravimetric analyzer (Mettler ToledoTGA/SDTA 851e) to determine the amount of carbon deposited on thecatalysts. The weight change for a ca. 10 mg sample was recordedafter heating it from room temperature to 110 °C and maintainingthis temperature for 10 min before further heating the sample to900 °C at a rate of 10 °C min−1 under a 60 ml min−1

flow of O2.The amount of coke formed on the catalysts after the reaction ispresented in terms of coke yield (mmolC gcat−1 h−1).

The morphology of the spent catalysts and the carbon depositedwere examined by transmission electron microscopy (TEM, JEOLJEM-2010).

2.3. Analysis of yellow glycerol

Yellow glycerol, a byproduct from glycerol purification process,was obtained from a biodiesel manufacturer in Thailand. Substancesin the yellow glycerol were identified with gas chromatograph-massspectrometer (GC–MS, HP6890). A HP-5MS column with a dimensionof 30 m×0.25 mm×0.25 μm was used with He carrier gas. Tempera-ture program was started at 35 °C (held for 3 min) and ramped to100 °C at 2 °C min−1 (held for 1 min), and from 100 °C to 260 °C at10 °C min−1 (held for 10 min). It was also analyzed for its composi-tion by using the proximate and the ultimate analyses following thestandard test method of ASTM D3172-3175 and D5291. The analysisresult indicates that the major substances of yellow glycerol are 94%glycerol and 6% 3-methoxy 1,2 propanediol (C4H10O3) with trace of1,2 propanediol (C3H8O2) as shown in Table 1. The main constituentsare volatile matter (99.92% dry basis) with very small content of ash(0.04%) and fixed carbon (0.04%). The ultimate analysis of yellowglycerol showed that carbon, hydrogen, and oxygen account for36.67, 9.69 and 51.42% respectively of the mass. Based on thesevalues, the calculated atomic ratio of C:H:O of yellow glycerol isequal to 3.0:9.5:3.2 and it was used to calculate all gas product yieldsand selectivities throughout the course of experiments.

2.4. Catalytic reforming of yellow glycerol

The catalytic reforming of glycerol was conducted at an atmo-spheric pressure in a fixed bed reactor made from a 316-stainlesssteel tube with an inner diameter of 1.27 cm. The catalyst samplewas kept in place using quartz wool. The reactor set-up was similarto that was used in our previous work [24]. In the catalytic reformingtest, the aqueous solution of glycerol was injected to the system by a

0 10 20 30 40 50 60 70 80 90 100

Inte

nsit

y (a

.u.)

2-Theta

NiO Ni γ-Al2O3 Ce0.75Zr0.25O2

(a)

(b)

(c)

(d)

Fig. 1. XRD patterns of (a) fresh NiAl, (b) fresh NiCeZrAl, (c) reduced NiAl and (d)reduced NiCeZrAl.

697K. Kamonsuangkasem et al. / Fuel Processing Technology 106 (2013) 695–703

HPLC Pump (Eldex). The feed solution and oxygen were separatelypassed through a vaporizer controlled at 300 °C±10 °C to ensurecomplete evaporation of yellow glycerol before being fed into the re-actor controlled at a desired temperature. Helium (99.99%) was usedas a carrier gas. The outlet stream was cooled in a condenser to con-dense liquid effluent while the gas product was collected in a glasssampling tube. The gas sample was subsequently injected into a gaschromatograph (GC, Shimadzu 14B) coupled with a thermal conduc-tivity detector (TCD) and a flame ionized detector (FID). A Porapak Qcolumn was used to analyze CO2, CH4, C2H2, C2H4 and C2H6 levels, anda Molecular Sieve-13X was used to determine H2, CO, CH4, and O2

levels. The gas compositions from both TCD and FID were relatedthrough CH4. Each experiment was conducted for 6 h and the gasproduct was hourly measured for the flow rate and collected for com-position analysis. Then the calculated glycerol conversion, productyield and product selectivity were averaged.

Prior to performing the catalytic reaction, the catalystwas reduced insitu at 500 °C with H2 flowing at a rate of 30 cm3 min−1 for 3 h. In all ofthe experiments, a gas hourly space velocity (GHSV) ca. 16,000 L−1

was used. Three operating parameters were studied: steam to yellowglycerol ratios (S/YG) at 3, 6 and 9, reaction temperatures at 550, 600and 650 °C, and O2 to yellow glycerol ratios (O2/YG) at 0.25, 0.50and 0.75. The effect of S/YG was tested at 550 °C and O2/YG of 0.5,whereas the influence of reaction temperature was studied at S/YG of9 and O2/YG of 0.5, and the effect of O2/YG was studied at the reactiontemperature of 600 °C and S/YG of 9. To distinguish the effect of pro-moter on the activity of the catalyst, the reaction over the unpromotedNi/Al2O3 catalyst was performed and compared with the promotedcatalyst.

Since yellow glycerol mainly comprises glycerol, the species par-ticipated in the initial reactions was balanced and written based onglycerol. The gas product evaluated in terms of glycerol conversion,product selectivity and product yield are shown as follows:

Glycerol conversion %ð Þ ¼ molof totalcarbonatomsin gas productmol of carbon atoms in f eed

� 100

For hydrogen gas:

H2 yield %ð Þ ¼ molof H2 ingasproduct6�molof yellowglycerol in f eed

� 100

Selectivity of H2 %ð Þ ¼ mol of H2in gas productmol of C atom in gas product

� 1RR

� 100

where RR=H2/CO2 stoichiometric ratio and it is equal to 6/3 based onoxidative steam reforming of glycerol as written in Eq. (1).

C3H8O3 þ 2H2Oþ 12O2→3CO2 þ 6H2 ΔH298 ¼ 122:86 kJ=molð Þ ð1Þ

For CO, CO2 and CH4:

Yield of species i %ð Þ ¼ molof species i in gasproductmol of carbon atoms in f eed

� 100

Selectivity of species i %ð Þ ¼ molof species i in gas productmol of C atom in gas product

� 100

3. Results and discussion

3.1. Characterization of catalyst before reaction

The XRD patterns of the fresh and reduced NiCeZrAl and NiAl cat-alysts are shown in Fig. 1. It was found that the characteristic peaks ofγ-Al2O3 support exhibited at 19.41°, 37.67°, 45.82°, and 66.82° in all

catalysts. The diffraction peaks at 37.24°, 43.28°, 62.85°, 75.40°, and79.37° identified as NiO phase were observed in both fresh catalysts.Considering the promoted Ni/Al2O3 catalyst (NiCeZrAl), there wereno peaks corresponding to ZrO2 phase. However, the observed 2θpeaks corresponding to CeO2 were slightly shifted to higher valueswith the broader peak because of the replacement of the smaller zir-conium ion (Zr4+) by the cerium ion (Ce4+) resulting in smaller crys-tal size of CeO2 [25]. It forms uniformly distributed Ce0.75Zr0.25O2

phase shown at 28.88°, 33.48°, 48.06°, and 57.00°. After reduction,the nickel species presenting on both supports was in the form of Nimetal, and its crystal size calculated based on the peak correspondingto (200) plane for NiCeZrAl was smaller (6.4 nm) than that for NiAl(7.7 nm) as shown in Table 2. It indicates that the ceria-zirconiasolid solution enhanced dispersion of the nickel species on the sup-port which can be confirmed by the increase of Ni dispersion from0.27% to 2.29% after adding the promoter as shown in Table 2.

The BET isotherms and BJH pore size distributions of the un-promoted (NiAl) and promoted (NiCeZrAl) catalysts after calcinationdetermined by nitrogen adsorption–desorption isotherm measure-ment are shown in Fig. 2. Both catalysts presented Type-IV isothermwith H2-type hysteresis loops indicating the existence of wide-mouth mesopores on their surfaces. The broad pore size distributionof the catalysts presents in the range of 2–20 nm. The detailed phys-ical properties of the catalysts are summarized in Table 2. As can beseen, the incorporation of ceria-zirconia promoter into Al2O3 supportdid not considerably affect the pore volume and the average pore sizediameter of the catalysts. Nevertheless, the ceria-zirconia promotercould partially cover some of the pores of the Al2O3 support resultingin a slight reduction of the BET surface area from 187 to 174 m2.g−1

when the promoter was present in the support.Fig. 3 shows the H2-TPR profiles of NiAl, NiCeZrAl and CeZrAl. For

NiAl, a broad reduction peak starting at 450 °C with a maximum at575 °C, followed by a small reduction peak with a maximum atabout 800 °C were detected. The first peak was ascribed to the reduc-tion of NiO surface species and the second peak was assigned to thereduction of NiAl2O4 species [26,27]. For CeZrAl, the reduction peaksat 685 °C and 857 °C indicate a strong interaction betweenceria-zirconia solid solution and Al2O3 support [28]. The TPR curveof NiCeZrAl was similar to that of NiAl but with two distinctions.The first one was the broader reduction peak starting towards lowertemperature (350 °C) which indicates the reduction of NiO speciesstrongly interacting with ceria-zirconia solid solution [26]. Thesecond observation was the smaller 800 °C peak area of NiCeZrAlthan that of NiAl catalyst suggesting the inhibition of NiAl2O4 forma-tion by ceria-zirconia [26,27]. The latter result revealed that the

Table 2Properties of the catalysts.

Catalyst BET SA Pore volume Pore diameter Ni dispersion Ni crystal size

(m2/g) (cm3/g) (nm) (%) (nm)

NiAl 187 0.60 12.81 0.27 7.7NiCeZrAl 174 0.58 13.40 2.29 6.4

0 200 400 600 800 1000

H2

cons

umpt

ion

(a.u

.)

Temperature ( C)

(b) NiCeZrAl

(c) CeZrAl

(e) Reduced NiCeZrAl

(a) NiAl

(d) Reduced NiAl

Fig. 3. TPR of a) NiAl, b) NiCeZrAl, c) CeZrAl, d) reduced NiAl and e) reduced NiCeZrAl.

698 K. Kamonsuangkasem et al. / Fuel Processing Technology 106 (2013) 695–703

amount of NiAl2O4 in NiCeZrAl was lower than that in NiAl. In otherwords, the active nickel species in NiCeZrAl catalyst was either higheror dispersed more than that in NiAl.

The TPR analysis was also performed on the NiAl and NiCeZrAl re-duced at 500 °C under H2 flow rate of 30 ml/min for 3 h to estimatefor the percentage of nickel species that can be reduced. The H2 con-sumptions of both reduced catalysts were extremely low and that ofthe “reduced NiCeZrAl” was lower than that of the “reduced NiAl”. Itindicates that the nickel species in the catalysts were close to com-plete reduction, and the reduction of nickel species in the “reducedNiCeZrAl” was greater than that in the “reduced NiAl”. By calculatingthe area under the peaks of the fresh and the reduced catalysts, thepercentage of the nickel reduced in the “reduced NiCeZrAl”was largerthan that in the “reduced NiAl” (98% vs. 86%). It is inferred that theceria-zirconia enhanced the reduction of the nickel species to activenickel species. The TPR analysis results were well in agreement withthe H2 chemisorption analysis showing greater and better dispersionof active nickel species in NiCeZrAl than in NiAl.

3.2. Effect of steam-to-yellow glycerol ratio

The effects of operating conditions on the product gas yield after thereaction are summarized in Table 3. Fig. 4 depicts the effects of S/YG onglycerol conversion and selectivity to gas products obtained from thecatalytic oxidative steam reforming of yellow glycerol over NiCeZrAlcatalyst at 550 °C and O2/YG of 0.5. Although the reaction temperaturewas relatively low, the experiment had achieved high glycerol conver-sion in the gas phase (52–70%) revealing that the promoted catalystwas sufficiently active for the reaction.

The gas products obtained were typically a reformed gas mixtureobtained from reforming oxygenate hydrocarbons but with differentcompositions. The gas mainly contains H2, CO and CO2 at the selectivityof 49.5–61.4%, 43.7–57.2% and 33.8–50.7%, respectively. The yields ofthe other gas products changed with S/YG are in a similar trend as theselectivities. High yields of carbon oxide gases (CO and CO2) wereobtained from the partial oxidation, reforming and decomposition ofglycerol alongwithwater-gas-shift reaction, because of the high oxygen

Fig. 2. Nitrogen adsorption–desorption isotherms and pore size distribution ofNiCeZrAl and NiAl catalysts.

to carbon atoms ratio in the glycerolmolecule. These principal reactionsinvolved follow Eqs. (2)–(5), respectively.

C3H8O3 þ12O2→2COþ CO2 þ 4H2 ΔH298 ¼ −36:67kJ=molð Þ ð2Þ

C3H8O3 þ H2O→CO2 þ 2COþ 5H2 ΔH298 ¼ 205:16 kJ=molð Þ ð3Þ

C3H8O3→3COþ 4H2 ΔH298 ¼ 246:3kJ=molð Þ ð4Þ

COþ H2O→CO2 þ H2 ΔH298 ¼ 40:16 kJ=molð Þ ð5Þ

In addition to the main gas products, methane and trace of C2 hy-drocarbon gases, i.e. CH4 yield of 0.08-0.12 with traces of C2H4 andC2H6 were detected. Methanation was the possible main route formethane production (Eqs. (6) and (7)).

COþ 3H2→CH4 þ H2O ΔH298 ¼ −206:11 kJ=molð Þ ð6Þ

COþ 2H2→CH4 þ12O2 ΔH298 ¼ −247:28kJ=molð Þ ð7Þ

The glycerol conversion and selectivities of H2 and CO2 increasedas S/YG increased because the increase of water added into the feedwas largely utilized in reforming of glycerol and hydrocarbon mole-cules, and water-gas-shift reactions. This latter reaction is prevalentas confirmed by the increase in CO2 selectivity and decrease of CO se-lectivity, thus improving the yields of H2 and CO2 but lowering COyield. This result is in accordance with the thermodynamic calculatedvalue shown that steam to glycerol should be larger or equal to 9 forhigh hydrogen generation [18,19].

Another important factor for the reaction was coke formation onthe catalyst surface as it could diminish the catalyst activity by eithercovering the active nickel species or blocking the pores of the catalyst.Carbon or coke can be generated according to the following reactions:

2CO→Cþ CO2 ΔH298 ¼ −172:45kJ=molð Þ ð8Þ

CH4→Cþ 2H2 ΔH298 ¼ 17:9kJ=molð Þ ð9Þ

H2 þ CO→CþH2O ΔH298 ¼ −130:77kJ=molð Þ ð10Þ

Here coke formation was measured from TGA spectra (not shownhere) and is presented as coke yield in Table 3. It can be seen that thecoke yield decreases from 4.4 to 2.9 mmolC gcat−1 h−1 as S/YG in-creases from 3:1 to 9:1. This is because carbon monoxide and/ormethane was converted to H2 and CO2 via water-gas-shift reaction

C Ni γ-Al2O3 CeZrO2(i) S/G=9**, 650 °C, O2/G=0.50

Table 3Product gas and coke yields, and nickel particle sizes of spent NiCeZrAl catalysts.

Condition Product gas yield (%) H2 yield Coke yield Ni size

No. S/YG T (°C) O2/YG H2 CO CO2 CH4 C2 (mol/mol YG) (mmolC gcat−1 h−1) (nm)

a 3 550 0.5 25.0 29.2 19.8 2.8 0.34 1.5 4.4 10.9b 6 550 0.5 38.7 40.4 23.3 4.0 1.13 2.3 4.3 9.5c 9 550 0.5 39.3 28.9 33.2 3.1 0.31 2.4 2.9 9.1d 9 600 0.5 49.3 36.4 40.4 4.7 0.44 3.0 2.9 10.0e 9 650 0.5 67.1 44.5 49.7 5.4 0.32 4.0 2.2 13.1f 9 600 0.25 40.3 27.1 24.9 3.2 0.25 2.4 2.9 9.0g 9 600 0.75 27.6 49.0 25.0 5.1 1.83 1.7 3.8 12.1h 9a 650 0.5 42.1 47.4 28.1 5.4 1.1 2.5 3.2 12.8i S/G=9b 650 0.5 80.9 25.9 70.2 3.9 0.22 4.9 2.3 13.0

a Catalyst used was NiAl.b Feed was pure glycerol. S/G is steam to pure glycerol ratio.

699K. Kamonsuangkasem et al. / Fuel Processing Technology 106 (2013) 695–703

and steam reforming, respectively, instead of undergoing Boudouardreaction forming coke when excess water was introduced to thesystem.

Considering the phase of the catalyst after the reaction for all con-dition effects using XRD as depicted in Fig. 5, the phases of all compo-nents in the freshly reduced catalyst remained, except for the crystalsize of the nickel particles and the appearance of the peak at 26.2°corresponding to a carbon species. After exposure to the reaction en-vironment, the nickel particles were sintered giving a larger size thanthat of the freshly reduced catalyst. Feeding more water into the reac-tion could lessen this phenomenon as it can be seen that the nickelcrystal size at S/YG of 9:1 (9.1 nm) is smaller than that at 3:1(10.9 nm). It shows that more water fed to the reactor could helpremoving heat from the catalyst surface as well as reacting with thecarbon deposited on the catalyst, thereby inhibited sintering of nickelcrystals and coke formation, respectively. At this low reaction tem-perature (550 °C), sintering of nickel and coke deposit should havebeen small. However at low S/YG ratios (3–6), the coke yield wassubstantially high suggesting that coke formation would be themajor cause of the catalyst deactivation.

3.3. Effect of reaction temperature

The effects of temperature on oxidative steam reforming of yellowglycerol were examined at 550 °C, 600 °C, and 650 °C at S/YG of 9 andO2/YG of 0.5. The glycerol conversion and gas selectivity results, aswell as yield of product gas are displayed in Fig. 6 and Table 3, respec-tively. As expected, increasing temperature led to an increase in glyc-erol conversion and gas product yields. A high reaction temperature

0

20

40

60

80

100

120

0

20

40

60

80

100

3:1 6:1 9:1

Gly

cero

l con

vers

ion

in g

as p

hase

(%

)

Sele

ctiv

ity

(%)

S/YG

H2 CO2CO CH4Conversion

Fig. 4. Influences of water-to-yellow glycerol ratio on glycerol conversion and productselectivity from oxidative steam reforming of yellow glycerol operated at O2/YG of 0.5and 550 °C over NiCeZeAl catalyst.

enhanced both the reforming reaction and the decomposition ofhydrocarbons producing more H2, CO and CO2, as seen in the increasein their gas yields. The H2 yield was enhanced from 39.3% to 67.1%, H2

yield in terms of mol H2/mol of YG from 2.4 to 4.0, and H2 selectivityfrom 61% to 69% with an almost complete conversion when the reac-tion temperature was raised from 550 °C to 650 °C.

The overall oxidative steam reforming of glycerol is an endother-mic reaction (at low O2/YG of 0.5) as presented in Eq. (1): therefore,a thermodynamically high temperature was favored for H2 produc-tion. However, CO and CO2 selectivities were insignificantly affectedby the increase of the temperature due to a combination of reformingand partial oxidation reactions. For CH4 and C2 products, they tenta-tively increased with the increase of temperature since the crackingis favorable at high temperature.

The coke formation on the catalysts decreased when the reactiontemperature was increased from 2.9 mmolC gcat−1 h−1 at 600 °Cto 2.2 mmolC gcat−1 h−1 at 650 °C as shown in Table 3. This resultis in correspondence with the research analyzing thermodynamicequilibrium in autothermal reforming showing that high tem-peratures favorably inhibit carbon deposition [18]. Similar tothe effect of water-to-glycerol ratio, the spent catalysts showed theXRD peaks representing γ-Al2O3, nickel, Ce0.75Zr0.25O2 and carbon,but the change of Ni crystal sizes was different as seen in Fig. 5and Table 3. The increment in reaction temperature from 550 °C to600 °C and 650 °C led to sintering of nickel crystals, and hence,increasing the Ni crystal sizes from 9.1 nm to 10.0 nm and 13.1 nm,

0 10 20 30 40 50 60 70 80 90 100

Inte

nsit

y (a

.u.)

2-Theta

(a) S/YG=3, 550°C, O2/YG=0.50

(b) S/YG=6, 550 °C, O2/YG=0.50

(c) S/YG=9, 550°C, O2/YG=0.50

(d) S/YG=9, 600 °C, O2/YG=0.50

(e) S/YG=9, 650°C, O2/YG=0.50

(f) S/YG=9, 600°C, O2/YG=0.25

(g) S/YG=9, 600°C, O2/YG=0.75

(h) S/YG=9*, 650 °C, O2/YG=0.50

Fig. 5. XRD patterns of used (a)–(g) NiCeZrAl and (h) NiAl catalysts after oxidativereforming of yellow glycerol at various conditions, and (i) NiCeZrAl after oxidativereforming of pure glycerol at 650 °C, S:G=9:1 and O2:G=0.5. Note that the operatingconditions correspond to the conditions labeled in Table 3.

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cero

l con

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ion

in g

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hase

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)

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H2 CO2CO CH4Conversion

Fig. 6. Influences of reaction temperature on glycerol conversion and selectivity ofproducts after oxidative steam reforming of yellow glycerol conducted at S/YG of 9and O2/YG of 0.5 over NiCeZrAl catalyst.

700 K. Kamonsuangkasem et al. / Fuel Processing Technology 106 (2013) 695–703

respectively, but decreasing the coke yield. A remarkable increase inH2 yield with the least coke yield in the study of this effect showingthat coke formation was not the severe incident for catalyst deactiva-tion. On the other hand, at high temperature (650 °C), the nickel crys-tal size was considerably large indicating that sintering was a seriouscause of catalyst deactivation.

3.4. Effect of oxygen-to-yellow glycerol ratio

The effects of O2 to yellow glycerol ratio on yellow glycerol conver-sion and product gas selectivity, and product yields at S/YG of 9 and600 °C are displayed in Fig. 7 and Table 3, respectively. Glycerol conver-sion was improved from 56% to 82% when the O2/YG was increasedfrom 0.25 to 0.50, and not significantly changed when oxygen was fur-ther added to the O2/YG value of 0.75. Since the reactionwas conductedat a moderate temperature (600 °C), the oxygenate hydrocarbon didnot completely convert into product gas. The higher yields of H2, COand CO2 obtained at O2/YG of 0.5 are attributed to the escalation of par-tial oxidation of glycerol in addition to a typical steam reforming anddecomposition of glycerol. However, a further increase of O2/YG from0.5 to 0.75 declined the selectivities and yields of H2 and CO2, but raisedCO yield and selectivity. This can most likely be ascribed to the promo-tion of partial oxidation of the glycerol to produce more CO than CO2

(see Eq. (2)), and to the oxidation reaction of H2 with O2 to formwater as is evidenced by the increase of water content in the liquid ef-fluent stream.

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0.25:1 0.50:1 0.75:1

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O2 /YG

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Fig. 7. Influences of oxygen-to-yellowglycerol ratio on glycerol conversion and selectivityof product gas from oxidative steam reforming of yellow glycerol operated at S/YG of 9and 600 °C over NiCeZrAl catalyst.

The change of H2 selectivity and CO selectivity toward theoxygen-to-yellow glycerol ratio observed here is in accordance withthe thermodynamic equilibrium calculated by Yang et al. [19], whohad also explained that the water-gas shift reaction had been limitedby the abundance of oxygen in the system coming from the glycerolmolecules and the oxygen feed, so the CO yield and selectivity,increased with the oxygen increment.

The influence of the oxygen-to-yellow glycerol ratio on carbonformation reveals that the coke yield was the same for the O2/YG at0.25 and 0.5, but surprisingly increased when oxygen was added upto the O2/YG of 0.75. The explanation for this could be given thatthe water-gas-shift reaction had been inhibited by the high oxygencontent in the feed as previously explained. Meanwhile a number ofCO molecules in the system can further hydrogenated followingEq. (9) producing more coke. This was confirmed by the great reduc-tion of H2 yield and selectivity.

For the nickel crystal sizes of the spent catalysts, they were mar-ginally increased from 9.0 to 12.1 nm with the increase ofoxygen-to-yellow glycerol ratio, indicating a greater sintering degreeof the metal particles for high O2/YG. More oxygen in the systemcould have increased the temperature of the catalyst surface via exo-thermic partial oxidation and combustion reactions, which is favor-able for particle sintering.

It can be concluded from the H2 yields that the optimum O2/YG foroxidative steam reforming of yellow glycerol was at 0.5. This value isclose to the thermoneutral condition (O2:glycerol of 0.36:1) ofautothermal reforming of glycerol, which was obtained from thethermodynamic prediction of water-to-glycerol ratio in the range of9 to 12 and 627 °C [18].

As it can be seen, there was an optimum O2/YG (0.5) for a highcatalytic activity accompanied by the increases of nickel crystal sizeand coke yield. Both phenomena were more seriously occurring at ahigher O2/YG (0.75). Hence both nickel sintering and coke formationcould be the possible major causes of the catalyst deactivation.

From the catalytic activity test, the yellow glycerol was notcompletely reformed to gas products at the operating conditionsstudied. It was partially converted to liquid product which were ob-served but not analyzed in this work. Based on the mechanisms ofglycerol reforming proposed by many researchers [29–31], the possi-ble condensable products for this reaction system were acetone, ace-tal, acetaldehyde, 1-propenal and formic acid.

Table 4 compares the results from oxidative steam reforming ofpure glycerol performed over a few noble metal catalysts [19,21,23]with that of yellow glycerol over the NiCeZrAl catalyst prepared inthis work at similar operating conditions. The hydrogen yield andselectivity were calculated according to Eq. (1) based on the oxidativesteam reforming reaction of glycerol (RR=6/3). It can be seen thatthe NiCeZrAl catalyst is an excellent catalyst exhibiting the highcatalytic activity and giving high yield and selectivity to hydrogenwhich are higher or comparable to those of the noble metal basedcatalysts.

Table 4Comparison between experimental OSRG results over different catalysts.

Catalyst H2 selectivity (%) Condition Source

Rh-Ce/Al2O3-SiO2 ~46 S/G=3, O2/G=1.5,T=600 °C

[21]

Pt/Al2O3-SiO2 ~14 S/G=9, O2/G=0.5,T=700 °C

[21]

Pd/Cu/Ni/K ~54 S/G=9, O2/G=0.45,T=700 °C

[23]

Ir/La2O3 ~40 S/G=6, O2/G=1.5,T=650 °C

[19]

Ni/Ce0.75Zr0.25O2/Al2O3 ~69 S/YG=9, O2/YG=0.5,T=650 °C

This work

701K. Kamonsuangkasem et al. / Fuel Processing Technology 106 (2013) 695–703

3.5. Comparison of catalytic activity between the promoted and unpromotedcatalysts

The activity of the promoted nickel alumina catalyst, NiCeZrAl,and unpromoted catalyst, NiAl, in oxidative steam reforming ofyellow glycerol at 650 °C, S/YG of 9:1, and O2/YG of 0.5:1 were com-pared. Glycerol conversion obtained over NiAl (87%) was lower thanthat over NiCeZrAl (~99.6%). The product gas yields and selectivitiesover both catalysts are displayed in Table 3 and Fig. 8(a), respectively.The H2 and CO2 yields of the promoted catalyst were much higherthan those of the unpromoted catalyst while the yields of CO, CH4

and C2 gases (C2H4, C2H6) were smaller. It was also important tonote that the NiCeZrAl produced more gas product with lower COand C2 gas selectivities but higher H2 and CO2 selectivities than theNiAl did. This indicated that the promoted catalyst was more activefor glycerol reforming reaction than the unpromoted catalyst. Theseresults are corresponding to the higher nickel dispersion in the pro-moted catalyst due to the presence of Ce0.75Zr0.25O2 in NiCeZrAlcatalyst.

The ceria-zirconia solid solution has remarkable oxygen storage ca-pacity that could provide efficient oxidation of CO, thereby lowering CO,CH4 and C2 selectivities [32–34]. Another important role of ceria-zirconia promoter was the coke formation inhibitor. The TGA curvesof NiAl and NiCeZrAl catalysts exhibiting two peaks at 465 °C and580 °C shown in Fig. 8b revealed the presence of “hard coke” species[35]. The lower temperature peak of carbon could be associated to fila-mentous coke whereas the second peak could be ascribed to graphitic

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(a)

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(b)

Fig. 8. (a) Gas selectivity and (b) TGA spectra of fresh and spent catalysts from oxida-tive steam reforming of yellow glycerol tested at 650 °C and O2/YG of 0.5 over NiCeZrAland NiAl catalysts.

carbon [36,37]. The TEM images of the spent catalysts illustrated inFig. 9 are corresponding to the TGA analyses showing both carbon fila-ments and graphitic flake-like carbons. The XRD characteristic peak ofgraphite appears for both NiAl and NiCeZrAl catalysts as shown inFig. 5 confirming the formation of graphitic carbon on the catalysts. Inaddition, both the TGA and TEM results revealed greater filamentouscarbons in the used NiAl than that of used NiCeZrAl. As previouslymen-tioned regarding to the excellent OSC of ceria-zirconia, the oxidation ofthe hydrocarbon species produced, such as CH4 and C2 species [33,34],was promoted, thereby diminishing the filamentous carbon depositedon the NiCeZrAl catalyst. Consequently, the lower coke yield overNiCeZrAl than that over NiAl (2.2 vs. 3.2 mmolC gcat−1 h−1) wasobserved.

3.6. Comparison between the OSR of yellow and pure glycerol

The oxidative steam reforming was also conducted with pure glyc-erol over the same catalyst type and condition at 650 °C, S/YG of 9:1,and O2/YG of 0.5:1. Pure glycerol was completely converted to productgases, H2, CO2, CO and CH4 with trace of C2 species, similar to yellowglycerol. From Fig. 10 and Table 3, higher selectivity and yield ofH2 and CO2 but lower CO yield and selectivity from the catalyticoxidative reforming of pure glycerol than those from yellow glycerolwere obtained. Theoretically, in oxidative reforming 3-methoxy, 1,2propanediol containing in yellow glycerol to H2 and CO2, more waterand/or oxygen is required than pure glycerol as seen in Eq. (11). Inother words, with the same amount of water and oxygen, more CObut less H2 and CO2 can be produced from yellow glycerol than frompure glycerol (See Eq. (12)).

C4H10O3 þ 3H2Oþ O2→4CO2 þ 8H2 ð11Þ

C4H10O3 þ 2H2Oþ 12O2→2COþ 2CO2 þ 7H2 ð12Þ

The yield of hydrogen defined as mol of H2 to mol of glycerol feedobtained from this work is 4.9 which is close to the equilibrium valueof ~5.1 predicted thermodynamically at similar operating condition(oxygen-to-glycerol ratio of 0.5, water-to-glycerol ratio of 9 and reac-tion temperature of 627 °C) [18].

Considering the spent catalysts after the reaction from the XRDpatterns, the same peaks corresponding to Ni, Ce0.75Zr0.25O2, Al2O3

and carbon phases as those found in pure glycerol are appearedwithout any other metal or metal compound species as shown inFig. 5. In addition, the Ni crystal size and coke formation yield forboth pure and yellow glycerol feed were similar. Consequently, abyproduct from biodiesel separation unit like yellow glycerol is an ex-cellent alternative source for hydrogen or syngas production via oxi-dative steam reforming over NiCeZrAl catalyst.

4. Conclusion

In this work, the oxidative catalytic steam reforming over Ni/CeZrO2/Al2O3 catalyst is shown to be a promising process for con-verting a byproduct from biodiesel manufacturing, such as yellowglycerol, to hydrogen. The reaction was carried out in the reactiontemperature range of 550–650 °C with the water-to-yellow glycerolratio of 3–9, and with the oxygen-to-yellow glycerol ratio of 0.25–0.75at the constant GHSV of 16,000 h−1. It was illustrated that the majorgas product consisted of H2, CO and CO2, with low content of CH4,and traces of C2H4 and C2H6. Themechanisms involvedwere decompo-sition, steam reforming and partial oxidation of glycerol includingwater gas shift reaction. Increasing the reaction temperature andwater-to-glycerol ratio enhanced both glycerol decomposition andsteam reforming of hydrocarbon giving higher reactant conversion,and yields and selectivities of H2 and other product gases.

Fig. 9. TEM images of (a) NiCeZrAl and (b) NiAl catalysts after testing oxidative steam reforming of yellow glycerol at 650 °C, S/YG of 9, and O2/YG of 0.5.

702 K. Kamonsuangkasem et al. / Fuel Processing Technology 106 (2013) 695–703

For the effect of oxygen-to-yellow glycerol ratio, increasing theoxygen generally improved glycerol conversion but significantlydiminished hydrogen selectivity due to the hydrogen reacting withoxygen to form water. However, in the high oxygen content environ-ment, more CO product than CO2 was generated, and this CO wasfurther hydrogenated producing more carbon on the catalyst. Con-sequently, the suitable conditions for oxidative steam reformingof yellow glycerol was found at water-to-yellow glycerol ratio of9, oxygen-to-yellow glycerol ratio of 0.5 and reaction temperature of650 °C which is similar to the thermodynamic calculation values. Atthis best condition, hydrogen selectivity obtained from yellow glycerolwas appreciably high compared to previous work, though it is some-what lower than that from pure glycerol due to the presence of 3methoxy, 1, 2 propanediol. This promoted catalyst, Ni/CeZrO2/Al2O3,exhibited the excellent activity in oxidizing and reforming reactions ofyellow glycerol to gas products giving high hydrogen selectivity whilesuppressing coke formation.

Acknowledgments

The authors would like to thank the National Research Council ofThailand (NRCT), and the Higher Education Research Promotion andthe National Research University (NRU) Project of Thailand, Office ofthe Higher Education Commission for their financial supports of thisresearch.

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Fig. 10. Selectivity to gas products from oxidative steam reforming of pure glycerol andyellow glycerol tested at 650 °C and O2/YG of 0.5 over NiCeZrAl catalyst.

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