NiMgOAl2O3andNiMgO Catalyzed SiC Foam Absorbers for High Temperature Solar Reforming of Methane
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Transcript of NiMgOAl2O3andNiMgO Catalyzed SiC Foam Absorbers for High Temperature Solar Reforming of Methane
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Ni/MgOeAl2O3 and NieMgeO catalyzed SiC foam absorbers
for high temperature solar reforming of methane
Nobuyuki Gokon a,*, Yuhei Yamawaki b, Daisuke Nakazawa b, Tatsuya Kodama b
a Center for Transdisciplinary Research & Department of Chemistry and Chemical Engineering, Niigata University, 8050 Ikarashi 2-nocho,
Nishi-ku, Niigata 950-2181, Japanb Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Nishi-ku, Niigata 950-2181, Japan
a r t i c l e i n f o
Article history:
Received 15 February 2010
Received in revised form
5 April 2010
Accepted 8 April 2010
Available online 3 June 2010
Keywords:
Solar heat
Hydrogen production
Reforming
Thermochemical processNi catalyst
Reticulated ceramic foam
a b s t r a c t
Ni catalyst supported on MgOeAl2O3 (Ni/MgOeAl2O3) prepared from hydrotalcite, and
NieMgeO catalyst are studied in regard to their activity in the CO2 reforming of methane at
high temperatures in order to develop a catalytically activated foam receivereabsorber for
use in solar reforming. First, the activity of their powder catalysts is examined. Ni/MgO-
eAl2O3 powder catalyst exhibits a remarkable degree of high activity and thermal stability
as compared with NieMgeO powder catalyst. Secondly, a new type of catalytically acti-
vated ceramic foam absorber e Ni/MgOeAl2O3/SiC e and NieMgeO catalyzed SiC foam
absorber are prepared and their activity is evaluated using a laboratory-scale recei-
verereactor with a transparent quartz window and a sun-simulator. The present Ni-based
catalytic absorbers are more cost effective than conventional Rh/g-Al2O3 catalyzed
alumina and SiC foam absorbers and the alternative Ru/g-Al2O3 catalyzed SiC foam
absorbers. Ni/MgOeAl2O3 catalyzed SiC foam absorber, in particular, exhibits superiorreforming performance that provides results comparable to that of Rh/g-Al2O3 catalyzed
alumina foam absorber under a high flux condition or at high temperatures above 1000 C.
Ni/MgOeAl2O3 catalyzed SiC foam absorber will be desirable for use in solar recei-
verereactor systems to convert concentrated high solar fluxes to chemical fuels via
endothermic natural-gas reforming at high temperatures.
2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
1. Introduction
Currently, the countries that have abundant natural gasreservoirs close to solar sites favorably set in a sun-belt
region that receives more than 1500 kWh/m2 a year enjoy
a great potential to generate hydrogen using renewable
energy sources. This can provide a convenient alternative to
natural gas power plants, whose main advantage is signifi-
cantly reduced CO2 emissions. The thermochemical conver-
sion of concentrated solar heat to chemical fuels has the
advantage of producing energy carriers for the storage and
transportation of solar energy from the sun-belt to remote
population centers [1e4]. The direct thermochemical
conversion of solar radiation energy is characterized by anideal high efficiency; its thermodynamic limit for enthalpy
storage is close to 100%. From the perspective of the chemical
pathway for this process, the solar reforming of natural gas
has been investigated as one of the most promising solar
thermochemical processes [5e23]. The following endo-
thermic reformings of natural gas are the basis for upgrading
the calorific value of the hydrocarbons which produce
syngas:
* Corresponding author. Tel./fax: 81 25 262 6820.E-mail address: [email protected] (N. Gokon).
A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 7 4 4 1 e7 4 5 3
0360-3199/$ e see front matter 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2010.04.040
mailto:[email protected]://www.sciencedirect.com/http://www.elsevier.com/locate/hehttp://www.sciencedirect.com/http://www.elsevier.com/locate/hemailto:[email protected] -
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CH4 H2O(g) CO 3H2 DH298 K 206 kJ mol1, (1)
CH4 CO2 2CO 2H2 DH298 K 247 kJ mol1. (2)
In order to produce more hydrogen, solar-processed syngas
(CO and H2) is shifted via a water-shift reaction, in which
water catalytically reacts with CO in the following reaction:
CO H2O(g) CO2 H2 DH298 K 41 kJ mol1 (3)
The carbon dioxide product can then be separated, for
example, via a membrane, and sequestrated.In comparison to
the conventional reforming of natural gas, the solar reforming
of natural gas offers distinct advantages: (1) the utilization of
solar energy as a replacement for fossil fuels to obtain the
necessary process heat reduces fossil fuel consumption, (2)
concentrated solar radiation input theoretically upgrades the
reactants energy content by 22e28%, and (3) environmentally
hazardous pollutants and CO2 emissions are eliminated orreduced.
A project was proposed to develop a solar methanol
production system in the sun-belt [4]. In this project, the plan
is for methanol or DME to be produced from natural gas
(methane) and coal via methane reforming and coal gasifica-
tion using solar heat as the process heat. The liquid fuel that is
produced can then be transported overseas to Japan by
a modified oil tanker [4].
Concentrated solar radiation has the specific properties of
high density, heterogeneous distribution of thermal flux, and
frequent thermal transients due to the fluctuating insolation.
A solar-specific reactor for reforming was proposed on the
basis of a direct absorption concept in which the receiver andreformer comprise the same unit. This concept was realized in
a number of solar reforming systems such as the directly
irradiated volumetric reactors developed by the German
Aerospace Research Center (DLR) in Germany, Sandia
National Laboratories in the United States, and the Weizmann
Institute of Science (WIS) in Israel [7,9e11,13,16,17]. In this
concept, the concentrated solar radiation passes through
a transparent window and is absorbed by an absorber of
catalytically active, reticulated ceramic foam which is
mounted behind the window. In these volumetric reactors,
reticulated ceramic foam made of alumina and SiC, which
combines high gas permeability and turbulence of flow with
a geometry suitable for the effective and uniform absorptionof solar radiation, is considered preferable to conventional
honeycomb structures. The solar reforming of methane with
CO2 using the volumetric reactor system was first demon-
strated in DLR and Sandia National Laboratories in 1990 [7]. A
200e300 kW volumetric reactor was also demonstrated at the
WIS for the solar CO2 reforming of methane [17]. A prototype
was realized and tested as part of the SOLASYS (Solar
Upgrading of Fossil Fuels) project. The receiver has 400 kW of
thermal power absorbed by the gas and an operation pressure
of 10 bar. The project demonstrated the ability of the volu-
metric reactor receiver to generate processheat that could run
the chemical reaction at a temperature of about 900 C. The
syngas that was produced was used in a small gas turbine for
power generation. Further work in the ongoing SOLREF (Solar
Steam Reforming of Methane Rich Gas for Synthesis Gas
Production) aims to achieve a temperature higher than 900 C
in order to improve theefficiencyof the process and to make it
possible to couple the reactor with gas treatment for the
production of pure hydrogen.
The Commonwealth Science and Industrial Research
Organisation (CSIRO) in Australia operated a solar reformingprogram that used a solar dish concentrator from 1997 to 2002
[19]. Two reformer designs were tested: a multistraight tube
reformer consisting of six straight tubes connected in parallel,
and a single coiled reformer tube [19]. This successful opera-
tion demonstrated the production of proton exchange
membrane (PEM) fuel cell-quality H2. In 2004, CSIRO began
a new program to commercialize this technology by building
a single, small tower module of 500 kW solar capacity that
would demonstrate larger scale reforming [20,21]. A smaller
reformer has also been in operation for steam reforming
under solar irradiation [20,21].
Highly active catalysts based on metal Rh are frequently
used for the solar CO2 reforming of methane [5,8]. Rh/g-Al2O3-loaded alumina and SiC foam absorbers were extensively
tested for the solar reforming of methane by using solar
concentrating systemscapable of utilizing up to 100e300 kWthof solar power [7,9e11,13,17]. Ru catalysts have also been
proposed and demonstrated as highlyactivealternativesto Rh
catalysts for use in methane reforming. g-Al2O3-supported Ru
catalyst was examined for the CO2 reforming of methane at
a temperature of 550 C [24]. Berman and Epstein examined
the RueCe catalyst to improve the activity and thermal
stability of Ru/g-Al2O3 catalysts for the solar CO2 reforming of
methane [18]. Ru/(a-Al2O3MnOx) catalysts were studied for
use in solar reforming at temperatures of 500e900 C and
a total pressure of 1e7 atm [25].The present study introduces a new type of ceramic foam
absorber that is coated with cost effective and high-temper-
ature stable Ni/MgOeAl2O3 and NieMgeO catalysts. These
new products were prepared and their activity was tested
using a laboratory-scale receiverereactor with a transparent
quartz window and a sun-simulator.
2. Concept for development of new SiC foamabsorber activated by Ni-based catalyst
Ni-based catalysts are generally considered economically
suitable for solar reforming processes that producea synthetic gas and hydrogen. However, Ni-based catalysts
often cause carbon deposition on the surface of the catalyst
during the processes, resulting in a deactivation of the cata-
lyst. Many recent reports in the literature have indicated that
the activity and stability of Ni-based catalysts are greatly
improved by the use of rare-earth-based promoters or alka-
line-earth metal ions [26e37]. An Ni catalyst containing
MgOeCaO showed excellent activity and stability for the CO2reforming of methane [28]. A NickeleMagnesia solid solution,
Ni0.03Mg0.97O, provided ultrastable activity without carbon
deposition that sustained its desirable activity in the CO2reforming of methane for the long time of 100 days [34]. Stable
and cost effective NieMgeO was proposed as a catalyst for
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solar reforming and a solar receivereabsorber that used
alumina ceramic foam was prepared. The activity of the
absorber was studied for solar CO2 reforming in a laboratory-
scale volumetric receiverereactor under direct irradiation by
a sun-simulator [36]. The SiC foam, which was reported to be
more resistant to thermal shock at high temperatures in the
large-scale solar test of the Rh/g-Al2O3/SiC absorber [17], was
used instead of alumina foam as a matrix for the preparationof a catalytically activated Ru/NieMgeO absorber. The
reforming performance of the Ru/NieMgeO/SiC absorber was
investigated using a laboratory-scale volumetric recei-
verereactor and a sun-simulator [37]. However, as described
above, other active and thermally stable Ni-based absorbers
that do not use noble metals such as Pt, Rh and Ru will be
desirable for solar reforming at high temperatures.
Mixed oxides resulting from the thermal treatment of
hydrotalcites present several interesting properties [38].
Hydrotalcite-like compounds are layered double hydroxides.
The general formula of these compounds can be represented
as [M1 x(II) Mx
(III)(OH)2]x [Ax/n
n]mH2O where M(II) and M(III) are
divalent and trivalent cations and A is the compensationanion. This structure can accommodate a wide variety of the
different M(II) and M(III) metals, which in turn lend it different
properties. The thermal decomposition of these materials by
calcination results in the formation of homogeneous mixed
oxides with a high thermal stability and a larger surface area.
In addition, well-dispersed metallic particles are usually
obtained after a reduction treatment [38]. The preparation of
heterogeneous catalysts derived from MgeAl hydrotalcite-like
compounds containing Ni at the Mg sites as precursors, and
their utilization for the steam and dry reforming has been
reported [39e46]. Shishido et al. [40] tested Ni/MgeAl oxide
catalysts derived from MgeAl hydrotalcite precursor for the
CO2 reforming of methane under CH4/CO2 1:1 in a conven-tional flow reactor with a fixed bed quartz tubular reactor at
temperatures of 800 C at atmospheric pressure and a Gas
Hourly Space Velocity (GHSV) 51 Ndm3 gcat1 h1. It was found
that the Ni/MgeAl oxide catalysts exhibit higher activity than
those prepared by the conventional impregnation method,
such as Ni/a-Al2O3 and Ni/MgO. Tsyganok et al. [45] focused on
the deposition of coke onto the Ni catalyst surface, the spatial
distribution of supported Ni after the CO2 reforming of
methane, and the thermal stability of Ni catalyst supported on
MgeAl mixed oxide. Roh et al. [44] examined the combined
steam and CO2 reforming of methane using (H2O CO2)/CH4of 1.2 at 800 C under atmospheric pressure at
a GHSV 265 Ndm3 gcat1 h1. Ni/MgOeAl2O3 prepared from
hydrotalcite material revealed a high degree of activity and
stability in comparison to Ni/MgO, Ni/ZrO2, Ni/CeO2 and Ni/a-
Al2O3. Koo et al. [46] studied Ni catalyst supported MgOeAl2O3prepared from hydrotalcite material for the combined steam
and dry reforming of methane. Ni/MgOeAl2O3 prepared fromhydrotalcite material exhibited remarkable coke resistance,
but commercial Ni/MgeAl2O4 catalyst showed considerable
coke deposition during the combined reforming.
In the present work, Ni/MgOeAl2O3 catalyst derived from
hydrotalcite (Mg6Al2(OH)16CO34H2O) and NieMgeO catalyst,
which are much cheaper than Rh and Ru noble metal cata-
lysts, are first investigated in terms of their activity and the
thermal stability of the CO2 reforming of methane. Second,
a new type of catalytically activated ceramic foam absorberse
Ni/MgOeAl2O3 and NieMgeO catalyzed SiC foam absorbers e
were prepared and evaluated in terms of their activity and
thermal stability using a laboratory-scale receiverereactor
with a transparent quartz window under direct light irradia-tion by a sun-simulator.
3. Experimental procedure
3.1. Materials
The chemicals of hydrotalcite (Mg6Al2(OH)16CO34H2O) and Ni
(NO3)2$6H2O (purity 99.9%) were purchased from Wako Pure
Chemical Industries Ltd. MgO powder (Grade 500A, purchased
from Ube Material Industries Ltd.) was also used for prepara-
tion of the catalyst. SiC ceramic foam disks purchased from
Krosaki Harima Co. were used as a matrix for the preparationof foam absorbers. The disks had diameters of 20 and 30 mm,
a thickness of 10 mm, and cell size of 13 cpi (cells per linear
inch). Table 1 shows the foam absorbers prepared for the
present study.
3.2. Preparation of Ni/MgOeAl2O3 and NieMgeO
powder catalyst
For the preparation of Ni/MgOeAl2O3 catalyst, hydrotalcite
(Mg6Al2(OH)16CO3$4H2O) was used as a precursor. MgOeAl2O3
Table 1 e Catalytically activated foam absorbers used in this study.
Catalyst Foam matrix Porosity (cpi) Dimensions (mm)a Loading (wt%)
MgOb Nic NieMgeOb Ni/MgOeAl2O3b
Ni/MgO SiC 13 20 10 2e4 11
30 10 4 11
NieMgeO SiC 13 20 10 2e4
30 10 4
Ni/MgOeAl2O3 SiC 13 20 10 1e4
30 10 4
a Diameter and thickness.
b With respect to the mass of the foam matrix.
c With respect to the mass of the support.
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support derived from hydrotalcite was prepared by pre-calci-
nation at various temperatures in the range of 800e1200 C for
6 h in air at a flow rate of 0.3 dm3 min1 in a normal state. The
calcined powder was suspended in a nickel nitrate solution
and was then left to evaporate until it was dry. The loading
amountof Nion the support was fixedat 12wt% [46]. Next, the
dried powder was calcined at 600e1000 C for 6 h in air at
a flow rate of0.3Ndm3 min1 and then reducedby a mixed gasof 50%Ar and 50% H2 at aflowrate of0.1 Ndm
3 min1 at 800 C
for 1 h.
NieMgeO catalyst was prepared as follows. The powder of
the MgO support was suspended in a nickel nitrate solution
that was then left to evaporate until it was dry. The dried
powder was then ground in a mortar and then calcined at
750e1300 C for 6 h in air (at a flow rate of 0.3 Ndm3 min1).
The loadingamount of Ni on theMgO support was fixed at 11%
by weight [37]. The calcined powder was reduced at 800 C for
1 h in a mixed gas of 50% Ar and 50% H2 at a flow rate of
0.1 Ndm3 min1.
The BET surface areas of the catalysts used were measured
by nitrogen adsorption (Shimadzu, Micromeritics Flow Sorb II2300) at 77 K. The catalyst powders were analyzed using XRD
with CuKa
radiation (MAC Science, MX-Labo) for identification
of the formed phases.
3.3. Preparation of Ni/MgOeAl2O3 and NieMgeO
catalyzed SiC foam absorber
The Ni/MgOeAl2O3 catalyzed SiC foam absorber was prepared
as follows. The SiC ceramic foam disks used in the experi-
ments were initially coated with Ni/MgOeAl2O3 particles by
the wash-coat method described below. The coating of the
foam disks was performed by soaking them in a well-stirred
aqueous slurry of fine Ni/MgOeAl2O3 particles. The Ni/MgO-eAl2O3-coated foam was dried at 100 C for 24 h then calcined
at 1000 C f o r 2 h i n a n N2 stream at a flow rate of
0.4 Ndm3 min1. It was necessary to keep the density of the
slurry (10e20 g dm3) sufficiently low in order to prevent the
pores of the foam structure from clogging. These Ni/MgO-
eAl2O3 coating processes were repeated until the loading
reached 1e4 wt%, with the Ni/MgOeAl2O3 loadings estimated
from the difference in their weight before and after the
coating process.
For the preparation of the NieMgeO catalyzed SiC foam
absorber, the SiC foam disk was initially coated with
NieMgeO particles by the wash-coat method described below.
The coating of the foam disks was performed by soaking themin a well-stirred aqueous slurry of fine NieMgeO particles.
The NieMgeO-coated foam was subsequently dried at 100 C
for 24 h then calcinedat 700 C for 2 h in an N2 streamat a flow
rate of 0.4 Ndm3 min1. These NieMgeO coating processes
were repeated until the loading reached 2e4 wt%, with the
NieMgeO loadings estimated from their difference in weight
before and after the coating process.
For comparison, a SiC foam absorber activated with Ni/
MgO was also prepared using the same SiC foam disk,
according to the procedure used in a previous work [37]. The
SiC foam disk was first wash-coated with MgO slurry solution,
then dried at room temperature overnight, and then calcined
at 1000 C for 1 h in air. This MgO coating processwas repeated
until the MgO coating reached the desired degree of MgO
loading on the foam. The MgO loadings were calculated from
the masses of coated and uncoated foams. After the MgO
coating process, an Ni2 was applied at 11 wt% Ni with respect
to the mass of the magnesia support. An Ni(NO3)2 ethanol
solution was added dropwise to the MgO-coated ceramic foam
disk, then allowed to dry at room temperature overnight, and
thencalcined at 1000 C in air for 3 h. Hereinafter, theSiC foamabsorber activated with Ni/MgO is referred to in this paper as
Ni/MgO/SiC foam absorber.
The absorbers of the Ni/MgOeAl2O3e, NieMgeOe, and Ni/
MgO-catalyzedSiCfoamthatwerepreparedarelistedin Table1.
3.4. Activity testing of Ni/MgOeAl2O3 and NieMgeO
powder catalyst
The experimental setup is illustrated in Fig. 1(a). The powder
catalyst (0.0725 g) was packed in the reactor of a transparent
quartz tube with an inner diameter of 23 mm. The thickness of
the catalystbed was set to1 mm. The powdercatalystwas put on
a floccose quartz and sandwiched between SiC foam for fixationof the powder catalyst inside the quartz tube while the alumina
tubeswere insertedfor thepreheatingof thereactant gasmixture
beneath the lower SiC foam. The quartz tube reactor was placed
in an electric furnace and insulated with refractory bricks. A
CH4eCO2 gas mixture (pCH4 50% andpCO2 50%) was fed into
the reactor ata flow rateof 0.2 N dm3 min1 at Gas Hourly Space
Velocity(GHSV)165Ndm3 gcat1 h1 at1atm.Thepowdercatalyst
in the reactor was then heated at 900 C in order to carry out the
methane reforming. The temperature of the catalyst bed was
measured using a K-type thermocouple placed at the center of
the catalyst bed that was in contact with it and that was packed
inside the reactor. The effluent gases were analyzed by gas
chromatography equipment (Shimadzu, GC-8A) with a thermalconductivity detector (TCD) to determine the gas composition.
Ni/MgOeAl2O3 powder catalyst was tested at a flow rate of
1.4 N dm3 min1 (Gas Hourly Space Velocity
(GHSV) 1155 Ndm3 gcat1 h1), which was 7 times higher than
that which was used testing the NieMgeO powder catalyst.
Because it displayed extremely high activity, the methane
conversion reached 100% under all testing conditions at a flow
rate of 0.2 Ndm3 min1.
3.5. Activity testing of catalytically activated SiC foam
absorbers by Xe-light irradiation
A double-walled quartz reactor was used for the catalyticactivity tests of the prepared absorbers. The experimental
setup is illustrated in Fig. 1(b). The inner diameter of the
outer quartz reactor was 39 mm, while the inner quartz tube
had an inner diameter of 31 mm, and the thickness of both
quartz tubes was about 2 mm. The absorber disk was
attached to a porous quartz plate inside the inner tube and
the reactor was insulated with refractory bricks. The reaction
feed gas was fed into the outer annulus and was allowed to
flow through the foam absorber disk into the inner tube of
the reactor. A 50% CH4/50% CO2 gas mixture was fed into the
reactor at a flow rate of 1.34 dm3 min1 (GHSV of 25,000 h1)
for the foam with a diameter of 20 mm and 1.50 dm 3 min1
(GHSV of 25,000 h1) for the foam with a diameter of 30 mm,
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after which the absorber was irradiated with solar-simulated
Xe light in order to carry out the CO2 reforming of methane. A
Xe-arc lamp house set (CINEMECCANICA, 5 kW ZX8000H,
Milan, Italy) was used to simulate concentrated solar radia-
tion. The energy flux distribution of the simulated incident
light on the front surface of the absorber was measured
previously using a heat flux transducer with a sapphire
window attachment (Medherm, 64-1000-21). The tempera-
ture of the absorber was measured by a K-type thermocouple
shielded with a glossy metal cover at the center of the front
surface of the absorber. The steam in the effluent gas mixture
from the reactor was condensed in a cooling trap connectedto the outlet of the reactor, after which the composition of
the gas mixture was analyzed using a gas chromatograph
(Shimadzu, GC-8A) equipped with a TCD detector.
3.6. Calculation of efficiencies
The methane conversion (X ) was estimated from the
following equation:
X yCO yH2
4yCH4 yCO yH2(4)
where YCH4 , yCO, and YH2 are the respective mole fractions of
CH4, CO, and H2 in the effluent gas [36,37].
Next, the conversion from light to chemical energy, i.e., the
chemical storage efficiency (hchem), was estimated as follows.
First, the overall reaction occurring in the gas phase was
determined from the experimental data. The CO2 reforming of
methane, as described in Eq. (2), is frequently associated with
a reverse wateregas shift reaction
H2 CO2 CO H2O(g) H298 K 41 kJ mol1 (5)
and the H2/CO ratio in the product gas becomes lower than
the stoichiometric ratio of 1. In this case, the overall reaction
in the gas phase is written as:
CH4 (1 x)CO2 (2 x)CO (2 x)H2 xH2O(g) H298 K
(overall) 247 41x kJ mol1 (6)
where x indicates the contribution of the reverse wateregas
shift reaction. The value of x was determined from the
experimental H2/CO ratio in the product gas. Thus, the power
stored as chemical enthalpy by the overall reaction,
Wchem(kW), can be experimentally estimated from:
Wchem fCH4$X$DH298Koverall (7)
where fCH4 (mol s1) indicates the molar flow rate of methane
into the reactor inlet. The chemical storage efficiency, hchem,of the incident light with respect to the chemical enthalpy is
defined as:
hchem Wchem=Winc (8)
where Winc (kW) is the total power of the incident light on the
absorber.
The chemically absorbed power density, Pd (kW m2), of
the absorber was defined by the power absorbed in the
chemical reaction, Wchem, divided by the irradiated surface
area, S, of the absorber.
The power of the reformed gas (CO and H 2), HHVreformed gas(kW), was estimated from the following equation:
HHVreformed gas Fout$PCO$DHCO Fout$PH2$DHH2 (9)
where Fout, PCO, PH2 , DHCO, and DHH2 are the molar flow rate of
the outlet gas (mol s1), the partial pressure of CO and of H 2(%), and the combustion heat of CO and of H 2 (kJ mol
1),
respectively. The combustion heat of CO and H 2 are written as
follows:
CO 1/2O2 (g) CO2 DHCO 282.98 kJ mol1, (10)
H2 1=2O2 H2Ol DHH2 285:83 kJ mol1 (11)
The molar flow rate of the outlet gas,Fout, is expressed using
the molar feed rate of CH4,fCH4 , and CO2, fCO2 , (mol s1),
elpuocomrehT
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Fig. 1 e Schematics of the experimental setup for testing the activity of (a) powder catalyst in a tubular quartz reactor and (b)
absorber by solar-simulated Xe-light irradiation.
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methane conversion, X, and H2/CO ratio in the outlet gas as
follows:
Fout fCH4 fCO2 2X$fCH4
2 1 H2=CO
1 H2=CO
!$X$fCH4 (12)
4. Results and discussion
4.1. Activity of Ni/MgOeAl2O3 and NieMgeO powder
catalyst
The XRD patterns of the original hydrotalcite and the hydro-
talcite that was pre-calcined at various temperatures ranging
from 800to 1200 C are shown in Fig. 2(a). Hydrotalcite powder
was transformed after pre-calcination at 800 C into an
MgAlO4 (MgOeAl2O3)-like phase with broad peaks. After pre-
calcination at 1000 C, obvious MgOeAl2O3 peaks appeared in
the pattern. In addition, the intensity of the MgOeAl2O3 peaks
increased due to crystalline growth at increasing pre-calci-
nation temperature ranging from 800 to 1200 C. Fig. 2(b)
shows XRD patterns of Ni-loaded MgOeAl2O3 (Ni/MgOeAl2O3)
powder catalyst calcined at various temperatures ranging
from 600 to 1000 C. The loading amount of Ni catalyst on the
MgOeAl2O3 support was fixed at 12 wt%. Strong peaks due to
metallic Ni were not observed, but a very weak peak was
found at a diffraction angle 2q of around 50 in all the XRD
patterns. The results indicate that well-distributed fine Ni
particles are loaded on the MgOeAl2O3, although MgOeAl2O3powder after Ni impregnation was calcined at high tempera-
tures in the rangeof 600e1000 C. Fig. 3 shows XRD patterns of
Ni/MgOeAl2O3 and NieMgeO powder catalysts before use in
activity tests. As seen in Fig. 3(a), the peak intensities due to
metallic Ni for Ni/MgOeAl2O3 catalyst were not enhanced
after H2 reduction at 800 C. Thus, the Ni/MgOeAl2O3 powder
catalyst for use in the activity test was prepared at a pre-
calcination temperature of 1000 C forhydrotalciteregent, and
was subsequently treated at 800 C after Ni impregnation by
the wash-coating method. Ni/MgOeAl2O3/SiC foam absorber
was also prepared using the powder catalyst. On the other
hand, for NieMgeO catalyst, the peaks due to metallic Ni were
not observed in the XRD pattern (Fig. 3(b)). The high
0
2000
4000
6000
8000
10000
80706050403020
eticlatordyhlanigirO
C008
C0001
C0011
C0021
C006
C007
C009
C0001
2 ( K-uC
seerged/)
Intensity/cps
Intensity/cps
2 ( K-uC
seerged/)
C008
0
1000
2000
3000
4000
5000
6000
80706050403020
lAgM 2O4
gM 6 lA 2 )HO( 61 OC 3 H4 2O
iN
lAgM 2O4
a
b
Fig. 2 e XRD patterns of (a) original and pre-calcined hydrotalcite at various temperatures of 800e1200 C and (b) Ni-loaded
MgOe
Al2O3 (Ni/MgOe
Al2O3) powder catalyst pre-calcined at various temperatures of 600e
1200 C.
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temperature calcinations of 750e1300 C in the preparation
make it possible that the Ni ions were incorporated into the
lattice of the MgO support to form an NieMgeO solid solution.
The BET surface areas of the NieMgeO powder catalyst that
was calcined at 750e1300 C are listed in Table 2. The surface
areas of the NieMgeO catalyst were at a maximum value at
1200 C calcination. XRD patterns of the NieMgeO catalyst
calcined at 750e1300 C were observed in addition to the
measurement of BET surface area. The XRD peaks due to
NieMgeO particles broadened after the calcination at 1200 C
in comparison to the other calcination temperatures. This
result means that the calcination at 1200 C caused a forma-
tion of fine particles of an NieMgeO solid solution.
The catalytic activities of the NieMgeO powders calcined
at various temperatures ranging from 750 to 1300 C were
examined at a GHSV of 165 Ndm3 gcat1 h1 in the time span
from 60 to 130 min. The activity tests were performed under
homogeneous heating with an electric furnace. The time
variations of methane conversion for the NieMgeO powders
calcined at 750e1300 C are presented in Fig. 4. The NieMgeO
catalyst after calcination at 1200 C exhibited the greatest
activity among those calcined at temperatures of 750e1300 C.
In addition, the NieMgeO catalyst with H2 reduction (solid
circles) showed activity that was substantially higher than
that without the H2 reduction (open circles). Fig. 4 also
represents time variations of methane conversion for Ni/
MgOeAl2O
3powder catalyst during 70 min of reforming (open
squares). The Ni/MgOeAl2O3 powder catalyst sustained
a 100% methane conversion at a GHSV of 165 Ndm3 gcat1 h1
through the time period of reforming, and showed a higher
degree of methane conversion than NieMgeO powder
catalysts.
In order to optimize a preparation of Ni/MgOeAl2O3powder catalyst and evaluate the superiority of its catalytic
activity to NieMgeO powder catalyst, the catalytic activity of
Ni/MgOeAl2O3 powder was examined using MgOeAl2O3powder pre-calcined at various temperatures ranging from
800 to1200 C. The activity tests were performed usinga GHSV
of 1155 Ndm3 gcat1 h1, which is 7 times higher than was used
in the reforming test at a GHSV of 165 Ndm3 gcat1 h1, as shown
in Fig. 4. Fig. 5(a) shows the average methane conversion
during the time period of reforming and the BET surface area
of Ni/MgOeAl2O3 powder catalyst after the activity test.
Nevertheless, methane conversion and the BET surface area
reached their highest values of 72% and 60 m2 g1, respec-
tively, at a calcination temperature of 1000 C. Next, the
catalytic activity of Ni/MgOeAl2O3 powder after Ni
0
1000
2000
3000
4000
5000
6000
7000
0
200
400
600
800
1000
1200
1400
1600
1800
80706050403020
2 ( K-uC seerged/)
80706050403020
2 ( K-uC seerged/)
Intensity/cps
Intensity/cps
OgMiN
lAgM O
lA-OgM/iN2O
3 O-gM-iNa b
Fig. 3 e XRD patterns of (a) Ni/MgOeAl2O3 and (b) NieMgeO powder catalysts before activity testing. The Ni/MgOeAl2O3powder catalyst was prepared using the calcined temperature of 1000 C for original hydrotalcite and at 800 C for Ni
loading. The NieMgeO powder catalyst was prepared using calcination temperature of 1200 C.
Table 2 e BET surface area of NieMgeO powder catalystafter H2 reduction.
Calcination temperature (C) 750 1000 1100 1200 1300
BET surface area (m2/g) 32.9 14.4 12.4 20.5 0.2
Methaneco
nversion/%
Time / min
C057
C0001
C0011
C0021
Htuohtiw(C0021 2 )noitcuder
C0031
0
20
40
60
80
100
140120100806040200
lA-OgM/iN 2O3 )C0001(
Fig. 4 e Time variations of methane conversions for the Ni/
MgOeAl2O3 and NieMgeO powder catalysts at GHSV of
165 Ndm3 gcatL1 hL1.
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impregnation (Ni 12 wt%) calcined at various temperatures
ranging from 600 to 1000 C was examined. Fig. 5(b) shows the
time variations of methane conversion for the Ni/MgOeAl2O3powder catalyst. After Ni impregnation, the maximum
methane conversion was obtained at a calcination tempera-
ture of 800 C. Therefore, hydrotalcite was calcined at 1000 C,
and the obtained mixed oxide of MgOeAl2O3 was subse-
quently treated for Ni impregnation at 800 C. The resulting
Ni/MgOeAl2O3 powder catalyst was used for irradiation
testing of the Ni/MgOeAl2O3/SiC foam absorber.
4.2. Activity of absorbers by solar-simulated Xe-light
irradiation
Photographs of the original SiC foam (non-coated) and three
kinds of catalytically activated SiC foam absorbers before the
0
20
40
60
80
1200110010008000 0
20 20
40 40
60 60
80 80
C/erutarepmetnoitaniclaC
BETsurfacearea/(m2/g)
Averagemethaneconversion/%
6040200
Methaneconversion/%
Time / min
C006C007
C008
C009
C0001
a b
Fig. 5 e Reforming performances for Ni/MgOeAl2O3 powder catalyst at GHSV of 1155 Ndm3 gcatL1 hL1. (a) Average methane
conversion during reforming time of period and BET surface area after the activity test. The hydrotalcite calcined at various
temperatures of 800e1200 C was used for a preparation of the catalyst. (b) Time variations of methane conversion. The
powder catalysts after Ni loading were calcined at various temperatures of 600e1000 C.
Fig. 6 e Photographs of (a) original SiC foam and (bed) the prepared catalytically activated foam absorbers. (b) Ni/MgO, (c)
NieMgeO and (d) Ni/MgOeAl2O3-activated SiC foam absorbers before activity testing.
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Xe-light irradiation test are shown in Fig. 6. The color of the
original SiC foam was gray, and this did not change after
application of the Ni/MgO, NieMgeO, and Ni/MgOeAl2O3powder catalysts. In addition, many obvious open pores on
the prepared SiC foam absorbers were observed that were not
clogged by agglomeration of their powder catalysts. Thus,
their powder catalysts were successfully well-distributed and
loaded onto the SiC foam.
Fig. 7 shows the energy flux distribution of the incident
solar-simulated Xe light on the irradiated surface of the
absorbers. The total power input (Winc) and average flux
density of the incident Xe light input into absorbers that haddifferent diameters of 2 and 3 cm are shown in Table 3. The
total power input (Winc) of the incident Xe light into the
absorbers was 0.120e0.204 kW, and the peak flux density of
the incident Xe light on the exposed surface of the absorbers
was 547 kW m2. The average flux density (r) on the exposed
surface of the absorbers was 289e383 kW m2, which is
defined as r Winc/S, where S is the surface area of the irra-
diated absorbers.
The catalytic activity of the Ni/MgOeAl2O3/SiC, NieMgeO/
SiC, and Ni/MgO/SiC foam absorbers was examined at a GHSV
of 25,000 h1(1.34 Ndm3 min1) during Xe-light irradiation for
60 min. The irradiation tests were performed under the above-
mentioned irradiation condition (Fig. 7). Fig. 8(a) shows the
time variations of methane conversion for the Ni/MgOeAl2O3/
SiC foam absorber with a diameter of 2 cm. The loading
amount of Ni/MgOeAl2O3 catalyst varied from 1 to 4 wt% withrespectto the mass of theSiC foam matrix. As seen in Fig. 8(a),
the methane conversion gradually increased with increasing
irradiation time for all the foam absorbers, and almost pla-
teaued at an irradiation time of 60 min. The 2 wt% Ni/MgO-
eAl2O3/SiC foam absorber provided the highest degree of
methane conversion among them, which was twice as high as
that of the NieMgeO/SiC foam absorber, as described below.
Time variations of methane conversion for the Ni/MgO/SiC
prepared by the previous method [37] and the NieMgeO/SiC
foam absorbers prepared by the presentstudy were examined,
and a comparison on the basis of their activity is presented in
Fig. 8(b). According to the previous method that employed two
wash-coatings for MgO loading and subsequent Ni loading,the methane conversion initially increased but then gradually
decreased with increasing irradiation time. In addition, the
methane conversion was hardly enhanced at all, although the
loading amount of Ni/MgO catalyst was increased. On the
contrary, in the case of the present study that used a wash-
coating of prepared NieMgeO catalyst, the methane conver-
sion increased with an increasing loading amount of
NieMgeO catalyst, and deactivation was not observed for the
foam absorbers. It can thus be expected that much further
loading of NieMgeO catalyst would enhance methane
conversion.
Fig. 9 shows time variations of temperature and methane
conversion for the Ni/MgOeAl2O3/SiC and NieMgeO/SiC foamabsorbers when SiC foam with a diameter of 3 cm was used as
a foam matrix. The testing of catalytic activity involved
examination at a GHSV of 25,000 h1 (1.34 Ndm3 min1) during
Xe-light irradiation of 60 min. The temperature was measured
in the center of the irradiated surface of the foam absorber.
Both foam absorbers displayed nearly constant temperatures
of 1070e1080 C for the NieMgeO/SiC foam and 1000e1010 C
for the Ni/MgOeAl2O3/SiC foam during the time period of
0
100
200
300
400
500
600
700
20151050-5-10-15-20
)mm(sixa-X
Fluxdens
ity/kW-m-2
maofmm02
maofmm03
Fig. 7 e Energy flux distribution of incident Xe light on the
irradiated surface of the absorber. The total power and flux
density for the Xe-light irradiation condition are shown in
Table 3.
Table 3e
Results for CO2 reforming with catalytically activated foam absorbers used in this study under the irradiation byhigh flux Xe light.
Foamabsorber
Dimensions(mm)
Total powerinput(kW)
Average fluxdensity
(kW m2)a
Peak fluxdensity
(kW m2)b
GHSV(104 h1)
Temperature(C)c
Power ofreformedgas (kW)
Maximummethane
conversion(%)
Ni/MgO-
Al2O3/SiC
20 10 0.120 383 547 2.5 995e1033 0.21 37
30 10 0.204 289 547 2.5 1002e1008 0.44 35
NieMgeO/
SiC
20 10 0.120 383 547 2.5 1030e1084 0.11 19
30 10 0.204 289 547 2.5 1067e1085 0.30 23
a Mean flux density of solar-simulated Xe light for irradiation.
b Peak (or central) flux density of solar-simulated Xe light for irradiation.
c Temperature at the center of the irradiated surface of the absorber.
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irradiation. It should be noted that the foam temperature for
the Ni/MgOeAl2O3/SiC foam was lower than for the NieMgeO/
SiC foam under the same irradiation condition. The resulting
methane conversion was 1.5 times higher for the Ni/MgO-
eAl2O3/SiC foam than for the NieMgeO/SiC foam during the
2-h period of reforming. These results indicate that Ni/MgO-
eAl2O3/SiC foam exhibits stable and high catalytic activity
during direct irradiation with concentrated Xe light
throughout an entire period of at least 2 h. Thus, the Ni/
MgOeAl2O3-activated SiC foam absorber is preferable for use
at a relatively high average flux. In the next step of this study,the irradiation time period is much longer, and the thermal
endurance, deactivation, and local-overheating of the foam
absorber is necessary to be studied for long-term light irradi-
ation at the level of 100 h.
The power stored as chemical enthalpy by the overall
reaction (Wchem) is plotted against the irradiation time period
for the SiC foam absorbers activated with Ni/MgOeAl2O3 and
NieMgeO catalysts. The results are shown in Fig. 10. The
Wchem values for the Ni/MgOeAl2O3/SiC foam absorber were
1.8 timesgreater in the 2-cm foam and 2 times greater in the 3-
cm foam than those for the NieMgeO/SiC foam absorber.
Thus, the higher catalytic activity of the Ni/MgOeAl2O3/SiC
foam absorber makes it preferable for use at relatively high
values of average flux (r 289e383 kW m2).
Fig. 11 shows a comparison of the chemically absorbed
power density(Pd) of the SiC foam absorbers activated with Ni/
MgOeAl2O3 and NieMgeO catalysts. The Pd value was plotted
against the irradiation time period. The CH4eCO2 mixture was
passed through the absorbers at a GHSV of 25,000 h1
. The Pdvalues reached 82e97 kW m2 for the NieMgeO/SiC foam
absorbers and 145e151 kW m2 for the Ni/MgOeAl2O3/SiC
foam absorbers. The Pd values for the NieMgeO/SiC foam
absorbers were 1.8 times greater in the 2-cm foam and 1.5
times in the 3-cm foam than those for the NieMgeO/SiC foam
absorber. In order to evaluate the catalytic activity of the
present foam absorbers, the Pd values and the light-to-
chemical energy conversion efficiency, hchem, were compared
0 0
10
10
20
20
30
3040
6050403020100
lA-OgM/iN2O
3
Methaneconversion/%
Time / min
6050403020100
Time / min
%tw1
%tw2
%tw4
%tw3
Methaneconversion/%
)ydutssuoiverp(CiS/OgM/iN
)ydutstneserp(CiS/O-gM-iN
O-gM-iN
%tw2
%tw2
%tw4%tw4
%tw3 %tw3
a b
Fig. 8 e Time variations of methane conversion for (a) the Ni/MgOeAl2O3/SiC foam absorber and (b) the NieMgeO/SiC foam
absorber with diameter 20 mm. The loading amount for the powder catalysts varied in 1e4 wt% and 2e4 wt% with respect to
the mass of SiC foam matrix.
800
900
1000
1100
1200
1201008060402000
10
20
30
40
50
120100806040200
erutarepmeT noisrevnocenahteM
lA-OgM/iN 2O3 CiS/
CiS/O-gM-iN
CiS/O-gM-iN
Time / min Time / min
Temperature/C
Methaneconversion/% lA-OgM/iN 2O3 CiS/
a b
Fig. 9 e Time variations of (a) temperature and (b) methane conversion for the Ni/MgOeAl2O3/SiC and the NieMgeO/SiC
foam absorbers with diameter of 30 mm.
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with those reported elsewhere. The results for the Pd and
hchem values were plotted together with those from thereferences shown in Fig. 12. The result by Buck et al. [7] was
reported in regard to the CAESAR project that was selected as
a referencefor evaluation of the present results. In most of the
literature, the solar demonstration of catalytically activated
ceramic absorbers was carried out in much larger volumetric
reactors and at higher solar input levels. Thus, their Pd values
and hchem values cannot be directly compared with the current
results. However, according to the report [7], in the CAESAR
project, an Rh/g-Al2O3-activated ceramic absorber with
a diameter of 64 cm was mounted in a large-scale volumetric
receiverereactor and tested with a higher solar input level of
74e115 kW. Both uniform and non-uniform ceramic absorbers
were tested using the absorber design that modeleda flat platefoam disk. The results obtained by the uniform absorber were
selected and used for comparison in the present paper. As
seen in Fig. 12(a), the Pd values for the Ni/MgOeAl2O3/SiC
activated ceramic absorber were comparable with those of the
CAESER absorber. On the other hand, those of the NieMgeO/
SiC activated ceramic absorber were relatively lower than
those of the CAESER absorber at a high average flux density ofirradiation. However, the Pd value could be significantly
improved by the Ru/NieMgeO/SiC activated ceramic absorber
inthecasethatanNieMgeO/SiC absorber loads Ru catalyst on
the surface [37]. The chemical storage efficiencies, hchem, for
the Ni/MgOeAl2O3/SiC and NieMgeO/SiC activated ceramic
absorbers were compared with those of the CAESER absorber.
As seen in Fig. 12(b), the hchem values for both absorbers
increased with an increase in the foam size of the absorber. In
addition, the hchem values were 50% for the Ni/MgOeAl2O3/SiC
and 34% for the NieMgeO/SiC activated ceramic absorber
with the larger foam absorber (Table 3). The hchem value of 50%
is comparable with those reported for the CAESER absorber.
The present Ni-based catalytic absorbers are more costeffective than conventional Rh/g-Al2O3 and Ru/g-Al2O3 cata-
lyzed SiC foam absorbers. The Ni/MgOeAl2O3 catalyzed SiC
foam absorber, in particular, will be desirable for use in solar
receiverereactor systems for the conversion of concentrated
high solar fluxes to chemical fuels via endothermic natural-
gas reforming at high temperatures.
0
0.02
0.04
0.06
0.08
0.1
0.12
150100500
)mc3(CiS/O-gM-iN
)mc2(CiS/O-gM-iN
lA-OgM/iN O )mc2(CiS/
lA-OgM/iN O )mc3(CiS/
nim/emiT
Powerstoredasa
chemicalenthalpybythe
overallreaction(W
)/kW
Fig. 10 e Chemically absorbed power (Wchem) as a function
of irradiation time for the Ni/MgOeAl2O3/SiC and the
NieMgeO/SiC foam absorbers.
0
50
100
150
200
150100500PowerdensityabsorbedInthechemical
reaction(P)/kWm
nim/emiT
)mc3(CiS/O-gM-iN
)mc2(CiS/O-gM-iN
lA-OgM/iN O )mc2(CiS/lA-OgM/iN O )mc3(CiS/
Fig. 11 e Power density absorbed in the chemical reaction
(Pd) as a function of irradiation time for the Ni/MgOeAl2O3/
SiC and the NieMgeO/SiC foam absorbers. The CH4eCO2mixture was passed through the absorbers at a GHSV of
25,000 hL1.
0
50
100
150
200
200
250
250 450 450300 300400 400350 350
foytisnedxulfegarevA mWk/noitaidarri
Powerdensityabsorb
edinthe
chemicalreaction(P)/kWm-2
0
20
40
60
80
250200
ytisnedxulfegarevA mWk/noitaidarrifo
Light-to-chemicale
nergy
conversionefficien
cy/%
lA-OgM/iN(ydutstneserP O )CiS/
)CiS/O-gM-iN(ydutstneserP
/hR(RASEAC lA- O )animulA/
CiS/OgM-iN/uR
lA-OgM/iN(ydutstneserP O )CiS/
)CiS/O-gM-iN(ydutstneserP
/hR(RASEAC lA- O )animulA/
CiS/OgM-iN/uR
a b
Fig. 12 e Comparison of (a) chemically absorbed power densities of Pd and (b) chemical storage efficiencies ofhchem.
Symbols: Solid squares and circles are respectively for the Ni/MgOeAl2O3/SiC foam absorber and the NieMgeO/SiC foam
absorber in the present study. Asterisk is for Ru/NieMgeO/SiC foam absorber in the previous study [37]. Triangles are for the
Rh/g-Al2O3-activated alumina foam absorber in the CAESAR project, estimated from data in [7].
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5. Summary
For the absorption and chemical conversion of high energy
fluxes into fuels, Ni/MgOeAl2O3 and NieMgeO activated SiC
absorbers were tested by the application of concentrated Xe-
light radiation (around 280 and 380 kW m2 of average flux
density) and examined as to their reforming performance. TheNi/MgOeAl2O3 catalyzed SiC foam absorber is comparable in
its chemically absorbed power densities (Pd) and chemical
storage efficiencies (hchem) to that of the conventional Rh/g-
Al2O3. Furthermore, the new absorber is more cost effective
than the Ru/NieMgeO catalyzed SiC absorber reported in the
previous paper. Ni/MgOeAl2O3 catalyzed SiC foam absorber is
found to be a promising solar absorber for a volumetric
receiverereactor or reformer.
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