Continuous Hydrogen Production Via The Steameiron Reaction By Chemical Looping In A Circulating fluidized-Bed Reactor

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 7 ( 2 0 1 2 ) 4 8 4 3 e4 8 5 4

Available online at www.sciencedirect.com

journal homepage: www.elsevier.com/locate/he

Continuous hydrogen production via the steameiron reaction
by chemical looping in a circulating fluidized-bed reactor
Magnus Ryde´n a,*, Mehdi Arjmand b
a

Department of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden
Department of Chemical and Biological Engineering, Division of Environmental Inorganic Chemistry, Chalmers University of Technology,
SE-412 96 Go¨teborg, Sweden
b

article info

abstract

Article history:

The steameiron reaction was examined in a two-compartment fluidized-bed reactor at

Received 24 October 2011

800e900  C and atmospheric pressure. In the fuel reactor compartment, freeze-granulated

Received in revised form

oxygen carrier particles consisting of Fe3O4 supported on inert MgAl2O4 were reduced to

24 November 2011

FeO with carbon monoxide or synthesis gas. The reduced particles were transferred to

Accepted 5 December 2011

a steam reactor compartment, where they were oxidized back to Fe3O4 by steam, while at

Available online 10 January 2012

the same time producing H2. The process was operated continuously and the particles were
transferred between the reactor compartments in a cyclic manner. In total, 12 h of

Keywords:

experiments were conducted of which 9 h involved H2 generation. The reactivity of the

Steameiron process

oxygen carrier particles with carbon monoxide and synthesis gas was high, providing gas

Chemical-looping combustion

concentrations reasonably close to thermodynamic equilibrium, especially at lower fuel

Chemical-looping reforming

flows. The amount of H2 produced in the steam reactor was found to correspond well with

Hydrogen

the amount of fuel oxidized in the fuel reactor, which suggests that all FeO that was formed

Iron oxide

were also re-oxidized. Despite reduction of the oxygen carrier to FeO, defluidization or
stops in the solid circulation were not experienced. Used oxygen carrier particles exhibited
decreased BET specific surface area, increased bulk density and decreased particle size
compared to fresh. This indicates that the particles were subject to densification during
operation, likely due to thermal sintering. However, stable operation, low attrition and
absence of defluidization were still achieved, which suggest that the overall behaviour of
the oxygen carrier particles were satisfactory.
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
reserved.

1.

Introduction

Hydrogen (H2) is an important feedstock with many
applications such as in the production of ammonia and
fertilizers, upgrading of fuels in the refining industry,
methanol synthesis, manufacturing of electronics and
metallurgic processes. There is also an increasing interest
in hydrogen as a future energy carrier, see for example the

review by Ogden [1]. The so-called hydrogen economy
would be favourable in a number of ways. When H2 is
burnt, the only product is water vapour (H2O). Therefore

vehicles using H2 as fuel rather than petroleum products
would neither emit greenhouse gases such as carbon
dioxide (CO2) and methane (CH4), nor other harmful
carbon based pollutants such as carbon monoxide (CO),
soot or particulate matter.

* Corresponding author. Tel.: þ46 31 7721457.
E-mail address: (M. Ryde´n).
0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ijhydene.2011.12.037


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Nomenclature
ar, AR
CLC
CnHm
F
fr, FR
Ln/min
m
n
M
NG

Air reactor
Chemical-looping combustion

Generic hydrocarbon fuel
Volumetric flow (Ln/min)
Fuel reactor
Normal litres per minute
Mass (g)
Number of moles
Molar mass
Natural gas

The dominating industrial route for generation of H2
currently is steam reforming of natural gas. However, in order
for H2 to be a feasible and environmentally benign energy
carrier it needs to be produced as cheap and efficient as
possible, and preferably with CO2 capture and sequestration
(CCS). For a general assessment of the different options for
which this could be achieved, see Mueller-Langer et al. [2] or
Cormos [3] for an overview of hydrogen fuelled power generation schemes with CCS.
H2 production via the steameiron reaction involves
oxidation of reduced iron oxides with steam. Although outdated, recent technological advances such as the development of chemical-looping combustion could possibly make
the steameiron process an attractive process yet again. Fully
developed, a combined process involving H2 production via
the steameiron reaction and regeneration of the iron oxide via
chemical-looping combustion would provide high purity H2
with inherent CO2 capture, without the need for wateregas
shift reactors, gas purification or other costly downstream
processing.

2.

Background

2.1.

Steameiron process

The steameiron process is one of the oldest methods for
industrial production of high purity H2. The process was
developed in the beginning of the 20th century by pioneers
such as Messerschmitt [4] and Lane [5], mainly for production
of H2 for airships and balloons. The conventional steameiron
process uses iron oxide to reduce steam to hydrogen. In the
first step of the process, iron oxide is reduced from hematite
(Fe2O3), to magnetite (Fe3O4), to wustite (FeO), and sometimes
all the way to metallic iron (Fe). Typically, gasified coal was
used to perform the reduction but a modern process could
use a wide range of fuels such as gasified biomass, natural
gas, petroleum products, industrial waste gas from blast
furnaces or refineries etc. With for instance CO as reducing
gas, the product is reduced iron oxide and CO2, see reactions
(1)e(3):

3Fe2O3(s) þ CO(g) / 2Fe3O4(s) þ CO2(g) DH900 C ¼ À35.3 kJ/mol
(1)

ox
PSD
s
SG
SIR
sr, SR

t
wt.%
DH
x
gCO2
u

Oxidized sample
Particle size distribution
Sample
Synthesis gas
Steameiron reaction
Steam reactor
Time (s, min)
Percentage by weight
Heat of reaction (kJ/mol)
Dry-gas concentration (%)
CO2 yield (%)
Mass-based degree of reduction

Fe3O4(s) þ CO(g) / 3FeO(s) þ CO2(g) DH900 C ¼ 10.1 kJ/mol (2)

FeO(s) þ CO(g) / Fe(s) þ CO2(g) DH900 C ¼ À16.3 kJ/mol

(3)

Naturally, the reduction could as well be performed with
H2, producing H2O as product, or with hydrocarbons
producing a mix of CO2 and H2O. Regardless of fuel choice,
reduction proceeds until the desired amount of FeO and Fe is

obtained, at which point the reducing gas is switched to
steam. In the second step of the process, H2 is produced by
oxidizing FeO and Fe with steam in accordance with reactions
(4) and (5):
Fe(s) þ H2O(g) / FeO(s) þ H2(g) DH900 C ¼ À16.8 kJ/mol

(4)

3FeO(s) þ H2O(g) / Fe3O4(s) þ H2(g) DH900 C ¼ À43.2 kJ/mol (5)
It is necessary to provide steam in excess, nevertheless
pure H2 is obtained when the product mixture is cooled down
and the steam is condensed to liquid water. Due to thermodynamic constraints, it is not possible to oxidize Fe3O4 to
Fe2O3 with steam. If desirable, this oxidation step has to be
performed with oxygen provided for example with air, see
reaction (6):
2Fe3O4(s) þ ½O2(g) / 3Fe2O3(s) DH900 C ¼ À232.2 kJ/mol

(6)

Reaction (6) is strongly exothermic. Since the reduction of
Fe3O4 to FeO, reaction (2) is endothermic when a hydrocarbon
containing gas is used, reaction (6) is necessary in this case in
order to obtain a continuous and thermally balanced process.
Reaction (6) is also needed in order to burn away sulphur and
coke, which otherwise may accumulate on the surface of the
iron oxide particles.
The steameiron process has not been used commercially for
several decades. Typically, the reactions outlined above were
performed batch-wise at temperatures in the range of
550e900  C. Each reaction step release or requires certain

amounts of heat and their reaction kinetics and thermodynamics are dependent upon temperature. Hence it is preferred
to conduct the different reactions at different temperature
levels, a fact that made the batch-wise steameiron process hard
to optimize and as a consequence not overwhelmingly efficient.


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The petrochemical revolution in the second half of the 20th
century resulted in the development of new methods for large
scale H2 production, such as steam reforming of natural gas,
which were seen as economically more attractive. In this era,
steameiron processes built up from fluidized beds were also
developed [6e9]. Although these processes did not emerge as
commercially viable alternatives, they did demonstrate that
continuous and efficient operation of the steameiron process
is possible.
In later years, the interest in the steameiron process had
grown. The increased consciousness about the connection
between emissions of CO2 and anthropogenic climate change
is one reason for this surge of interest. The steameiron
process could be configured so that pure CO2 is delivered in
a separate process stream, and thus seems as a convenient
method for H2 production with CO2 capture. Another reason
for the increased interest could be the fact that the dominant
technology used for H2 production today, which is steam
reforming of CH4, requires light hydrocarbons such as natural
gas as feedstock which makes the long-term prospects for

current technology somewhat uncertain. Further, new
insights in research areas such as catalysis, thermodynamics,
process integration and combustion in fluidized beds, as well
as the development of entirely new technologies such as
chemical-looping combustion, could help facilitate the realization of a new generation of steameiron process.
In recent years, several research groups have presented
interesting results concerning processes for use of the steameiron reaction for H2 production. Fan et al. [10] have suggested
various processes for conversion of coal to H2, some of which
involves the steameiron reaction. Chiesa et al. [11] have conducted a detailed process study, examining a three-reactor
chemical-looping process with H2 generation by the steameiron process. On the experimental and the material development side, batch experiments in fixed-bed reactor [12e15]
using both ordinary iron oxides and iron oxide supported on
magnesium, silica, chromium, titania and aluminium have
been conducted by several research groups. Lorente et al. [16]
also performed thermogravimetric analysis using iron ores.
Further, Bleeker et al. [17] used pyrolysis oil for reduction of
iron oxide in a batch fluidized bed, followed by H2 generation
by oxidation with steam. Yang et al. [18] presented similar
experiments, using char for direct reduction of the iron oxide.

2.2.

Chemical-looping combustion

Chemical-looping combustion (CLC) is an innovative method
for oxidation of fuels with inherent CO2 separation. In this
process, two separate reactor vessels are used with a solid
oxygen carrier performing the task of transporting oxygen
between the reactors as shown in Fig. 1.
In the fuel reactor (FR), the oxygen carrier is reduced by the
fuel, which in turn is oxidized to CO2 and H2O. In the air

reactor (AR), the reduced oxygen carrier is re-oxidized to its
initial state with O2 from air. Different kinds of oxygen carrier
materials can be used. The most commonly proposed are iron
oxide, manganese oxide, copper oxide and nickel oxide [19].
Reactions (7) and (8) describe the chemical-looping combustion of methane, using iron oxide as oxygen carrier, in the fuel
(FR) and the air reactor (AR), respectively:

CH4(g) þ 12Fe2O3(s) / 8Fe3O4(s) þ CO2(g) þ 2H2O(g)
DH900 C ¼ 184.0 kJ/mol

(7)

2O2(g) þ 8Fe3O4(s) / 12Fe2O3(s) DH900 C ¼ À986.5 kJ/mol

(8)

Since reactions (7) and (8) are conducted in a cyclic manner,
the sum of reactions is the combustion of the fuel with oxygen
as per reaction (9):
CH4(g) þ 2O2(g) / CO2(g) þ 2H2O(g) DH900 C ¼ À802.5 kJ/mol(9)
From reaction (9), it can be seen that the chemical-looping
combustion produces the same amount of heat as conventional combustion. The difference is that the reaction is
divided into two steps, thus obtaining two separate process
streams. Chemical-looping combustion has several attractive
features. Most importantly, the gas from the fuel reactor
consists essentially of CO2 and H2O. Hence cooling in
a condenser is all that is needed to obtain almost pure CO2,
which makes chemical-looping combustion an ideal technology for heat and power production with carbon
sequestration.
The general principles of chemical-looping combustion

were laid out as early as in the 1950’s by Lewis and Gilliland
[20], who suggested to carry out the reactions in fluidized-bed
reactors with particles of oxygen carrier particles as bed
material. This remains the favoured design and several
prototype reactors have been constructed using this principle.
For a straightforward explanation of the basic principles of
chemical-looping combustion, see Lyngfelt et al. [21]. In the

Depleted air
O2, N2

Products
CO2, H2O

Fe2O3(s)

AR

FR
Fe3O4(s)

Air
O2, N2

Fuel
CnHm, CO, H2

Fig. 1 e Schematic description of chemical-looping
combustion using iron oxide as oxygen carrier.

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past decade, research about chemical-looping combustion
has taken pace and providing a complete overview is not
within the scoop of this publication. Progress within the area
has been reviewed by Lyngfelt [19], Fang et al. [22], Hossain
and de Lasa [23] and Adanez et al. [24].

the volumetric increase involved in converting hydrocarbons
to H2, a pressurized system would clearly be favourable.
Otherwise the size of the plant would be very large and the
mechanical work needed for compression of the products
would reduce the overall process efficiency considerably.

2.3.

2.4.

H2 production via chemical-looping combustion

The chemical-looping concept has also been proposed as
a method to generate H2. Using the same general setup as in
Fig. 1 and by introducing the air to the air reactor in understoichiometric proportions, partial oxidation of the fuel can
be achieved, i.e. synthesis gas (CO and H2) is produced in the
fuel reactor rather than CO2 and H2O. The synthesis gas can
then be used for generation of H2, or other products. This
concept, typically referred to as chemical-looping autothermal reforming, has been demonstrated by Ryde´n et al.

[25e27] in a small circulating fluidized bed-reactor, by Kolbitsch et al. [28] in a dual circulating fluidized bed, and by Ortiz
et al. [29,30] in a pressurized semi-continuous fluidized bedreactor. Another approach would be to use steam reforming
and pressure swing adsorption for generation of H2, with
chemical-looping combustion used for production of heat by
combustion of the resulting waste gases, as has been
proposed by Ryde´n and Lyngfelt [31] and Ortiz et al. [32].
A third option would be to add a third reactor to the
chemical-looping combustion process, in this paper referred
to as a steam reactor (SR). In this reactor, reduced iron based
oxygen carrier would be oxidized with steam producing pure
H2 in a similar way as in the conventional steameiron process.
In fact, the resulting three-reactor process could be described
as a hybrid of chemical-looping combustion and the steameiron process, as is shown in Fig. 2.
In Fig. 2, reactions (1) and (2) take place in the fuel reactor
and reaction (5) is performed in the steam reactor, while
reaction (8) is carried out in the air reactor. The iron oxide
circulates continuously through the system providing stable
flows of solids and gases.
Similar processes to the one shown in Fig. 2 have been
suggested recently by several authors, see for example Fan
et al. [10], Chiesa et al. [11], Yang et al. [33] and Chen et al. [13].
The detailed process study by Chiesa et al. [11] shows that
a process as the one in Fig. 2 could achieve similar efficiency as
conventional processes for H2 production, without the need
for downstream separation systems, wateregas shift reactors,
cryogenic distillation of air or other expensive and energy
consuming equipment. Furthermore, high purity H2 would be
produced and CO2 for sequestration would be obtained simply
by cooling of the stream from the fuel reactor, condensing
steam to water. An analysis of oxygen carrier selection criteria

for such a three-reactor chemical-looping process can be
found in the work by Kang et al. [34].
Naturally, the concept described in Fig. 2 involves some
technical difficulties as well. The fuel reactor needs to be
arranged in counter-current fashion with Fe2O3 added from
the top and fuel added from the bottom; else the fuel
conversion will be limited by thermodynamic constraints, as
will be explained below. Counter-current flow could be achieved for example by using moving bed reactor, see Li et al.
[35], or possibly by using a staged fluidized bed. Further, due to

Aim of this study

The main objective of this study is to examine H2 generation
via the steameiron reaction in a continuously operating
reactor consisting of two interconnected fluidized beds. The
study aims to cover the current lack of experiments examining the steameiron reaction during such conditions.
A secondary objective is to examine whether continuous
operation with iron oxide reduced to FeO is feasible, since the
general experience from chemical-looping combustion
experiments with iron oxide as oxygen carrier is that there is
a strong correlation between reduction to FeO and defluidization of the particle bed, see Cho et al. [36] and Ryde´n et al.
[37,38].

3.

Experimental

3.1.

Manufacturing of oxygen carrier particles

Synthetic oxygen carrier particles consisting of 60 wt.% Fe2O3
supported on 40 wt.% MgAl2O4 manufactured by freezegranulation were used. MgAl2O4 has some attractive features
which suggest that it should be a good support material for
oxygen carriers for chemical looping applications, such as
high melting point and high thermal and chemical stability. In
a previous study by Mattisson et al. [39] concerning the

Fig. 2 e Schematic description of hydrogen generation via
the steameiron reaction configured in accordance to the
principles of chemical-looping combustion.


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development of iron oxide based oxygen carrier particles for
chemical-looping combustion in fluidized bed-reactors,
MgAl2O4 was identified as a suitable inert support material for
this application, exhibiting higher reactivity than iron oxide
supported on Al2O3, ZrO2 and TiO2.
The manufacturing procedure was as follows: A waterbased slurry of Fe2O3 and support powder (MgAl2O4) with
weight ratio of 60/40 along with small amount of dispersant
(acrylic acid) was prepared. The mixture was then ball milled
for 24 h. Subsequently polyvinyl alcohol was added as binder
prior to granulation. The slurry was pumped through a spray
nozzle and into liquid nitrogen to form spherical particles
upon instantaneous freezing. The particles were initially
calcined at 1100  C for 6 h at a ramp rate of 5  C/min. However,

this did not result in particles of the desired strength and
density, thus the particles were calcined for another 6 h at
1150  C. This resulted in particles with a bulk density of
approximately 1000 kg/m3, which deemed suitable for the
experiments. The particles were then sieved through stainless
steel screens to yield particles in the size range of 90e250 mm.

3.2.

Characterization of oxygen carrier particles

The oxygen carrier particles were analysed before and after
the experiments using powder X-ray diffraction (Siemens
D5000 Diffractometer) with CuKa radiations. The morphological investigation was carried out with an environmental
scanning electron microscope (ESEM) fitted with a field emission gun (FEI, Quanta 200). The BET surface area of the particles was evaluated with TriStar 3000 (Micromeritics). The
particle size distribution (PSD) before and after the experiments was determined using a light microscope (Nikon
SMZ800) and using ImageJ [40] software to measure the area of
an ellipse fitted to a large number of particles. The crushing
strength of the particles was measured as the strength needed
to fracture the particles ranging within 180e250 mm for an
average of 30 tests per sample using a digital force gauge
(Shimpo FGN-5). The crushing strength was found to be
approximately 0.6 N. In some cases particles with a crushing
strength below 1 N are considered too soft [41]. However, this
did not cause any problem such as defluidization in the
reactor, as determined by the pressure drop in the bed.
Nonetheless, for use in a full-scale plant, the crushing
strength may need to be increased further which could be
done by either increasing the sintering time or temperature.
Table 1 summarizes the physical properties of the oxygen

carrier used in this investigation.

3.3.

Two-compartment fluidized-bed reactor

The experiments were carried out in a small-scale laboratory
reactor constructed of 253 MA steel, which is a temperature,
creep and deformation resistant stainless steel with the
approximate composition 67.9% Fe, 21% Cr, 11% Ni and 0.1% C.
The reactor is similar to but not identical with a system
previously used for various chemical-looping experiments
[25,26,42e44]. A schematic description of the reactor is shown
in Fig. 3.
The reactor is designed for chemical-looping combustion
experiments using gaseous and liquid fuels, but steameiron

Table 1 e Properties of fresh oxygen carrier particles.
Oxygen carrier
Theoretical Fe2O3 content [wt.%]
Support phase
Size interval of particles [mm]
Bulk density [kg/m3]
BET specific surface area [m2/g]
Crystalline phase
Crushing strength [N]

Fe2O3/MgAl2O4
60
MgAl2O4 (S30CR, Baikowski)

90e250
1000
5.25
Fe2O3, MgAl2O4
0.6

reaction experiments could be conducted simply by replacing
air normally fed to the air reactor with steam. The reactor is
300 mm high. The fuel reactor is 25 mm  25 mm. The base of
the air reactor is 25 mm  42 mm, while the upper narrow part
is 25 mm  25 mm. Fuel and air enter the system through
separate wind boxes, located in the bottom of the reactor.
Porous quartz plates act as gas distributors.
In the air reactor the gas velocity is sufficiently high for
oxygen carrier particles to be thrown upwards. Above the
reactor there is a particle separation box in which the crosssection area is increased and gas velocity reduced so that
particles fall back into the reactor. A fraction of these particles
falls into the downcomer, entering a J-type loop-seal. From the
loop-seal, particles overflow into the fuel reactor via the
return orifice. The fuel reactor is a bubbling bed. In the bottom
particles return to the air reactor through a U-type slot and
thus a continuous circulation of oxygen carrier particles is
obtained. The downcomer and the slot are fluidized with inert
gas such as argon, which is added via thin pipes perforated by
small holes, rather than through porous plates.
In order to make it possible to reach and sustain a suitable
temperature, the reactor is placed inside an electrically heated
furnace. The furnace also makes it possible to conduct
continuous steameiron reaction experiments even if the net
reaction for the fuel reactor and the steam reactor is endothermic, thus omitting the air reactor from Fig. 2. The

temperature in each reactor section is measured with thermocouples located inside the particle beds, a few centimetres
above each bottom plate.
The reactor is operated roughly at atmospheric pressure.
However, a water-seal is located downstream the fuel reactor
which makes it possible to apply an overpressure of z250 Pa
to the fuel reactor, in order to inhibit leakage of air into the
fuel reactor. Along the reactor sections there are thirteen
separate pressure measuring taps. By measuring differential
pressures between these spots, it is possible to estimate where
particles are located in the system, and to detect abnormalities in the fluidization.
For gas analysis, roughly 0.50 Ln/min gas was extracted
downstream of the air reactor and fuel reactor respectively.
Each of these flows passed through separate particle filters,
coolers and water traps. Hence all measurements were made
on dry gas. CO2, CO and CH4 were measured using infrared
analysers while O2 was measured with paramagnetic sensors.
The gas from the air/steam reactor was also examined with
a gas chromatograph which measured H2 and N2, in addition
to the gas components mentioned above. The gas chromatograph provided measuring points every 3e4 min. Excess gas
that was not needed for analysis passed through a textile filter


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CO(g) þ H2O(g) 4 CO2(g) þ H2(g) DH900 C ¼ À33.1 kJ/mol

(10)

Synthesis gas and natural gas was used mainly since they
represent possible fuels for a real-world steameiron process.

3.5.

Assumptions and data evaluation

The conversion of fuel in the fuel reactor is expressed as
carbon dioxide yield gCO2:
gCO2 ¼

xCO2
xCO2 þ xCH4 þ xCO

(11)

Here xi denotes the composition of the respective gas, obtained from measured concentration in the gas analyser.
When CO is used as fuel, gCO2 provides and accurate description of the degree of fuel conversion and the combustion
efficiency. However, when synthesis gas or natural gas is used
as fuel, gCO2 only provides an adequate estimation since the
conversion of H2 to H2O in the fuel reactor may differ slightly
from the conversion of CO to CO2, due to differing thermodynamic properties of H2 and CO. However, at the temperature levels investigated in this work (800e950  C) the
difference should be rather small, as is shown in Fig. 4. Natural
gas contains small amounts of higher hydrocarbons such as
ethane and propane but those are expected to be much more
reactive with the oxygen carrier than methane and have not
been included in gCO2 .
The mass-based degree of reduction, u, can be used to
describe the reduction of the oxygen carrier particles and is
defined in expression (12):

u¼

m
mox

(12)

u describes the amount of oxygen that has been removed
from the oxygen carrier compared to the oxidized state and
can be calculated with a species balance as follows:
Fig. 3 e Schematic description of the two-compartment
fluidized-bed reactor.

in order to catch elutriated fines and particles, prior to release
in a chimney.
For the experiments presented in this paper, 300 g of the
particles were added to the reactor. This corresponds to a bed
height in the air and fuel reactor of roughly 10 cm, taken into
consideration that a considerable share of the particles was
located in the downcomer during operation.

3.4.

Fuel gases

Three different fuel gases were used for reduction of the
oxygen carrier namely pure carbon monoxide (CO), synthesis
gas (SG) consisting of 50% CO and 50% H2, and natural gas (NG)
with a composition equivalent to C1.14H4.25O0.01N0.005. Using
CO as fuel has the advantage that solving the species balance

for the reactor system becomes trivial. Hence most of the
figures included the experiments with carbon monoxide as
fuel. With CO as fuel the sum of reactions will be the wateregas shift reaction:

Fig. 4 e Equilibrium composition of the gas phase for
mixtures of CO/CO2 and H2/H2O in presence of Fe3O4eFeO,
as calculated using thermodynamic data from FactSage 6.1.


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Zt1
ui ¼ uiÀ1 À
t0

Á
n_ out MO À
4xCO2 þ 3xCO À xH2 dt
mox

4.2.

ui is the instantaneous conversion at time i, uiÀ1 is the
conversion in the preceding instant, t0 and t1 are the initial
and final time of measurement. MO is the molar mass of
oxygen, n_ in and n_ out are the dry molar flow rates of the gas at
inlet and outlet of the reactor, respectively.

4.

Results

4.1.

Chemical-looping combustion experiments

Steameiron reaction experiments

(13)

The aim of the chemical-looping combustion experiments
was to examine the reactivity of the oxygen carrier with
potential fuel gases, i.e. carbon monoxide, synthesis gas and
natural gas. The furnace was heated to a temperature slightly
above the desired fuel-reactor temperature, which in this case
was 900e950  C. During this period both reactor sections were
fluidized with air, while the particle seals were fluidized with
argon. When the desired temperature was reached, the air
stream that was introduced to the fuel reactor was replaced
initially with N2, and after a few minutes, by fuel. The reactor
was then operated for 1 h with more or less stable process
parameters.
Following that, the oxygen carrier particles were reoxidized according to the following procedure. First, the fuel
gas was replaced with an equally large flow of N2. Other
parameters were not changed. Thus it is believed that the
solids circulation should not have been affected since the gas
flows were essentially the same. Reduced particles present in
the fuel reactor were eventually transferred to the air reactor,

where they were oxidized with oxygen from air. Therefore the
time for the O2 concentration in the air reactor to reach
a stable value corresponds to the particles residence time in
the fuel reactor, a parameter that could be used to estimate
the solid circulation between the reactor sections. Once stable
O2 concentration in the air reactor was obtained, the inert
flows to the fuel reactor and particle seals were switched to air
to make sure that all active materials were properly oxidized
back to Fe2O3. A summary of conducted chemical-looping
combustion experiments can be found in Table 2.
The operation of the chemical-looping combustion experiments was satisfactory. Almost complete conversion of
carbon monoxide and synthesis gas to CO2 and H2O was obtained. As can be seen in Table 2, gCO2 was higher than 99.9%
for these fuels, leaving only traces of unconverted CO in the
products. For natural gas, gCO2 was limited to 75e90%, leaving
excess CH4, CO and H2 in the product gas. The results with
complete conversion of synthesis gas and much lower
conversion for other hydrocarbons is in accordance with
the expected behaviour of freeze-granulated iron oxide
particles [45].
Following the chemical-looping combustion experiments,
it was decided to use CO and synthesis gas as fuel for the
steameiron experiments. This was due to the high reactivity
and simple chemistry of these fuels, which facilitates
the interpretation of the experimental data, compared to
natural gas.

The steameiron reaction experiments were conducted with
the same experimental setup as the chemical-looping
combustion experiments. The only difference was that
instead of air, steam was added to the air reactor. This way

oxidation to Fe2O3 was not possible due to thermodynamic
constraints and FeO would eventually be generated in the fuel
reactor. The solids circulation between the reactor compartments were quite high, as will be explained in section 4.3
below. Hence any FeO produced in the fuel reactor would
quickly be transferred to the air reactor, where it would be
oxidized back to Fe3O4 by steam according to reaction (5),
producing H2. Formation of metallic Fe in the fuel reactor
seems unlikely, as will be further discussed in Section 4.3
below.
The steam flow added to the steam reactor was 4.0e5.0 Ln/
min and the conversion of steam to H2 in presence of FeO
could be expected to be 26e36% depending on temperature,
according to Fig. 4. Since the fuel flow was in the order of
1.0 Ln/min or lower, this means that steam was always added
in excess. Once the flow of reducing gas was removed from the
fuel reactor generation of H2 in the steam reactor ceased,
which indicates that FeO did not accumulate within the
reactor system during operation. Hence the H2 production
during stable operation should be determined simply by the
amount of FeO produced in the fuel reactor by reduction with
fuel. A summary of conducted steameiron experiments can
be found in Table 3.
The steameiron reaction experiments were initiated in the
same way as the chemical-looping combustion experiments,
but steam was added to the air reactor instead of air. Hence
the oxygen carrier would gradually become reduced to Fe3O4
according to reaction (1). At the point where Fe2O3 was no
longer present in the system, the fuel added to the fuel reactor
would become partially oxidized according to reaction (2). The
degree of oxidation of the fuel could be expected to be

confined by thermodynamics. Measured data for an experiment with CO as fuel is shown in Fig. 5.
In Fig. 5, it can be seen that there was almost complete
conversion of CO to CO2 as long as there was Fe2O3 present in
the oxygen carrier. This is in accordance with theory and with
the chemical-looping combustion experiments. Once Fe2O3
was depleted, the process shifted to a new equilibrium corresponding to oxidation of CO with Fe3O4. From this point
onwards, about 75% of the added CO appears to have been
converted to CO2.

Table 2 e Summary of conducted chemical-looping
combustion experiments. Ffuel,fr is the flow of fuel, Fair,ar
is the flow of air, Tfr is the fuel reactor temperature and
gCO2 is CO2 yield.
Fair,ar
Experiment Operation Ffuel,fr
(Ln/min) (Ln/min)
(min)
CLC CO
CLC SG
CLC NG

60
60
60

1.00e1.15
1.00
0.42

5.0

5.0
5.0

Tfr
( C)

gCO2
(%)

900
>99.94
900
>99.95
900e950 75e90


4850

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 7 ( 2 0 1 2 ) 4 8 4 3 e4 8 5 4

Table 3 e Summary of conducted steameiron reaction experiments. gCO2 ;fr and FH2 ;produced are representative average values
over the period.
Experiment
SIR
SIR
SIR
SIR
SIR

COI

COII
COIII
SGI
SGII

Operation
(min)

Tsr ( C)

Fsteam,sr
(Ln/min)

FN2 ;sr
(Ln/min)

Ffuel,fr
(Ln/min)

gCO2 ;fr (%)

FH2 ;produced
(Ln/min)

60
60
60
120
240

900
850
800
900
900

5.0
5.0
5.0
4.0e5.0
5.0

0.60
0.60
0.60
1.00
0.60

0.87
0.89
0.92
0.78e0.87
0.50e1.25

75.0
71.5
67.0
77.5e75.5
81.0e60.5

z0.53
z0.52
z0.49
z0.46e0.53
z0.33e0.58

Any FeO produced in the fuel reactor would eventually be
transferred via the solid circulation to the air reactor, where it
was oxidized by steam according to reaction (5), producing H2
in the process. Measured data for a one-hour experiment is
shown in Fig. 6.
In Fig. 6, it can be seen that H2 was not produced during the
initial few minutes, where Fe2O3 was still present. Once the
particles were deprived of Fe2O3, H2 was produced continuously, as could be expected. The presence of CO2 and CO is due
to gas leakage through the slot in the bottom of the reactor
which is discussed in Section 4.3 below. The balance is the Ar
gas which was used for fluidization of the particle seals.
The volumetric H2 production could be estimated by
comparison with the measured N2 concentration, which
origins is a trace gas flow of 0.60 Ln/min. The amount of H2

Fig. 5 e Dry-gas concentrations from the fuel reactor for
the initiation period of steameiron experiments with
0.87 Ln/min CO as fuel at 900  C (SIR COIII). The dotted line
at t z 10 min describes the theoretical point for complete
reduction of Fe2O3 to Fe3O4. Dilution with Ar is from the
fluidization gas added to the particle seals.

produced was found to vary with the amount of fuel added to
the fuel reactor, see Fig. 7.

Fig. 8 shows the H2 production as function of fuel flow. It
can be seen that at the lowest fuel flow (i.e. Fsg,fr ¼ 0.50 Ln/
min), the volumetric H2 generation corresponded well to the
theoretical maximum, which is defined as when the fuel
reacts with the oxygen carrier in the fuel reactor so that
thermodynamic equilibrium is achieved, and all the resulting
FeO is oxidized in the air reactor, producing H2 and Fe3O4.
It is evident from Figs. 7 and 8 that increasing the fuel flow
did increase H2 generation, but that the increase was not as
significant as expected. Furthermore, the fuel flows for the
experiments with CO (SIR COI-III) was specifically chosen so that
0.60 Ln/min H2 would be produced, if there was no gas leakage
between the reactors and thermodynamic equilibrium was
reached. In reality, the amounts of H2 generated was slightly
lower, i.e. 0.49e0.53 Ln/min, as can be seen in Table 3. Since no
accumulation of FeO in the system was noticed during operation, this suggests that the conversion of fuel in the fuel reactor
was to slow to achieve equilibrium when fuel flows higher than
0.50 Ln/min were used. This is also supported by examining the
CO2 yield, gCO2 , as a function of fuel flow as shown in Fig. 9.
In Fig. 9, it can be seen that the gCO2 is highly dependent on
the fuel flow. Higher flow results in lower conversion to CO2
and vice versa. This clearly indicates that the reaction in the
fuel reactor does not reach thermodynamic equilibrium.
Hence less FeO than expected is formed and less H2 than the
theoretical maximum is produced, as shown in Figs. 7 and 8.
It can be observed in Fig. 4 that the expected conversion of
CO to CO2 is approximately 70% at 900  C. However, Figs. 5 and
6 shows a conversion of about 75%, and in Fig. 9, over 80%
conversion is achieved for the lowest fuel flow. Thus the fuel
conversion is higher than what theoretically should be

possible. The likely explanation for this phenomenon is that
there was a small leakage of steam from the steam reactor
into the fuel reactor, for example via the particle seals. When
synthesis gas is used as fuel, steam is also formed in the
reaction with the oxygen carrier. The presence of steam in the
gas from the fuel reactor could be expected to result in higher
measured gCO2 because the gas from the fuel reactor has
a residence time of several seconds in the particle separation
box above the reactor, in which the temperature is about as
high as in the reactor itself. Hence steam may react with CO
forming additional CO2 via the wateregas shift, see reaction
(10). This means that the values of yCO2 ;fr presented in the
figures and in Table 3 are likely somewhat higher than what
they are in reality. In theory, a leakage as small as 3% of the
steam added to the steam reactor could explain such high
values of gCO2 .


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 7 ( 2 0 1 2 ) 4 8 4 3 e4 8 5 4

Fig. 6 e Dry-gas concentration from the steam reactor for
steameiron experiments with 0.87 Ln/min CO as fuel at
900  C (SIR COIII).

4.3.

Reactor performance

As have been explained above, inert argon was used as
fluidization gas in the downcomer and in the slot. The flow

rates 0.70 Ln/min to the downcomer and 0.20 Ln/min to the
slot. The argon was fairly evenly distributed between the air
and the fuel reactor, diluting the product gases somewhat.
This behaviour was expected.
No problems with defluidization or unwanted stops in the
particle circulation were experienced, despite reduction of the
oxygen carrier to FeO in the fuel reactor, which otherwise
have been reported to propagate defluidization [36e38]. Minor
irregularities such as uneven particle circulation were
observed when the fuel flow was reduced to 0.50 Ln/min,

Fig. 7 e Dry-gas concentration from the steam reactor as
function of fuel added to the fuel reactor for steameiron
experiments with 0.50e1.25 Ln/min synthesis gas as fuel at
900  C (SIR SGII).

4851

Fig. 8 e Comparison of the theoretical (—) and actual (A) H2
production in the steam reactor as function of fuel added to
the fuel reactor for steameiron experiments with
0.50e1.25 Ln/min synthesis gas as fuel at 900  C (SIR SGII).

which could be expected due to the resulting low gas velocity.
These irregularities did not lead to any practical problems.
The solids circulation was estimated to be approximately
3e4 g/s using the following procedure. Firstly, the amount of
particles present in the fuel reactor was estimated from the
particle density and reactor geometry, while the residence
time of particles in the fuel reactor was estimated as the time

to reach stable gas concentrations during reoxidation, see

Fig. 9 e CO2 yield gCO2 ;fr as function of fuel added to the fuel
reactor for steameiron experiments with 0.50e1.25 Ln/min
synthesis gas as fuel at 900  C (SIR SGII).


4852

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 7 ( 2 0 1 2 ) 4 8 4 3 e4 8 5 4

Fig. 10 e Particle size distribution for fresh and used
oxygen carrier.

roughly only 4e6% of the 4.0 wt.% oxygen available in reaction
(2) was utilized, which corresponds to operation at u between
0.980 and 0.978. Although the reduction of iron oxides do not
necessarily proceed sequentially and formation of small
amounts of metallic Fe in the fuel reactor can not be ruled out,
the state of the iron oxide in the fuel reactor should be
approximately 94e96 wt.% Fe3O4 and 4-6 wt.% FeO.
The gas leakage from the fuel reactor to the air reactor
could be estimated by measuring the CO2 and CO concentrations after the air reactor, as is shown in Fig. 6. The leakage
corresponded to 8e20% of the gas added to the fuel reactor,
which is in the same order of magnitude as for earlier
chemical-looping experiments in a similar two-compartment
reactor [25,26,42e44]. As stated above, gas leakage from the
steam reactor to the fuel reactors could have affected
measured CO2 yields to some extent, since CO and CO2 could
possibly react with steam above the particle beds via the

wateregas shift reaction. The effect should be relatively small
though, and should have no impact on the general conclusions of the study. As can be seen in Table 3, continuous
operation was possible and the volumetric H2 production
typically reached 80e85% of the theoretical value. For the
lowest fuel flow examined, the H2 production was very close
to the theoretical maximum, as is shown in Fig. 8 above.

4.4.
section 4.1 above. The solids circulation could then be calculated simply as the mass of the particles present in the fuel
reactor divided by the residence time. The result is a rather
rough estimation, but should be sufficient to conclude that the
solids circulation was more than sufficient for the experiments conducted.
The chemical-looping combustion experiments required
oxygen carrier particles corresponding to approximately
0.50 Ln/min O2 to be transferred to the fuel reactor via the
solids circulation. The amount of oxygen available in iron
oxide for reaction (1) is 3.3 wt.%, which for an oxygen carrier
with 60 wt.% active material is reduced to 2.0 wt.%. This
suggests that for the chemical-looping combustion experiments, only about 15e20% of the oxygen available in reaction
(1) was utilized, i.e. that the oxygen carrier was operated at u
between 1.000 and 0.996. For the steameiron reaction,
approximately 0.30 Ln/min O2 needed to be transferred. Thus

Effect on oxygen carrier particles

Following the experiments, the reactor was disassembled and
the used oxygen carrier recovered. Out of the 300 g oxygen
carrier added to the reactor, 285 g was found inside the reactor
system itself. 7 g of the material was found in the fuel reactor
windbox, having somehow passed through the fitting

between the porous plate and the reactor. These particles had
been extensively reduced during operation and had formed
soft agglomerations. 5 g of the material had been blown out of
the system and was collected in the filters downstream. The
blown out material was very fine, with 1 g < 45 mm and
3 g < 90 mm. 3 g of the material was missing and could have
been spilled during the disassembling of the reactor system,
or possibly stuck in one of the pressure measuring taps.
In general, the particles behaved satisfactory. The analysis
shows that the particle size had decreased somewhat
compared to the fresh material. Fig. 10 shows the particle size

Fig. 11 e ESEM images of (a) fresh and (b) used Fe2O3/MgAl2O4 oxygen carrier.


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 7 ( 2 0 1 2 ) 4 8 4 3 e4 8 5 4

distribution (PSD) for fresh and used oxygen carrier. The size
range for the fresh particles, as measured using light microscope, is slightly higher than the original sieving due to the
fact that the particles were not perfectly spherical. However,
the shift towards smaller sizes could clearly be observed for
the used particles.
Moreover, the bulk density of the used particles increased
to 1600 kg/m3 compared to 1000 kg/m3 for the fresh materials.
The BET specific surface area of the used particles also
decreased to 2.55 m2/g, compared to 5.25 m2/g for the fresh
oxygen carrier. These facts indicate that the particles became
densified during operation, possibly due to thermal sintering
at high temperature. The densification of the particles during
the initial oxidation and reduction cycles was expected and

had been accounted for in advance by adding a comparably
large volume of particles to the reactor. The formation of fines
was very low, considering that most oxygen carrier particles
tend to form dust during the initial hours of operation.
The X-ray diffraction spectra from used oxygen carrier did
not differ from fresh material. This indicates that the oxygen
carrier was fully oxidized without formation of any unwanted
ternary compound.
Fig. 11 shows fresh and post-experiments scanning electron microscope images of the particles. It can be observed
that the surface morphology of the oxygen carrier was not
affected by the redox operation.

5.

Conclusions

Freeze-granulated particles of 60 wt.% Fe2O3 and 40 wt.% inert
MgAl2O4 proved to perform well as oxygen carrier both for
chemical-looping combustion and for the steameiron reaction. As long as there was Fe2O3 present, carbon monoxide or
synthesis gas added to the fuel reactor was more or less
completely oxidized to CO2 and H2O. Once all Fe2O3 had been
reduced to Fe3O4, carbon monoxide or synthesis gas added to
the fuel reactor would become partially oxidized to a composition corresponding approximately to thermodynamic equilibrium at the relevant temperature, producing FeO in the
process. The fuel conversion was lower at high fuel flows
though, but this could be expected.
The reduced oxygen carrier was transferred to the steam
reactor where it was oxidized with steam forming Fe3O4 and
H2. The amount of H2 produced in the steam reactor was
found to correspond reasonably well with the amount of fuel
oxidized in the fuel reactor, which suggests that all FeO was

indeed oxidized in the air reactor. Continuous operation of the
process was achieved. No problems with defluidization or
unwanted stops in the particle circulation were experienced,
despite reduction of the oxygen carrier to FeO in the fuel
reactor. This suggests that the MgAl2O4 supported oxygen
carrier particles used have favourable properties, with respect
to fluidization behaviour. The oxygen carrier experienced
densification during operation, but otherwise behaved
satisfactory.
It is concluded that continuous operation of the
steameiron reaction in a fluidized-bed reactors is feasible.
Since it is established that chemical-looping combustion
using iron oxide as oxygen carrier is viable, it should be

4853

possible to realize a continuous three-reactor system as
proposed in Fig. 2. Due to the potentially favourable characteristics of such process, providing pure H2 and CO2 without
gas separation or wateregas shift reactor, this opportunity
should be further examined.

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