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

Continuous hydrogen production via the steameiron reactionby chemical looping in a circulating fluidized-bed reactor aDepartment of Energy and Environment, Division of Energy Technology,

Continuous hydrogen production via the steameiron reaction

by chemical looping in a circulating fluidized-bed reactor

aDepartment of Energy and Environment, Division of Energy Technology, Chalmers University of Technology, SE-412 96 Go¨teborg, Sweden

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

a r t i c l e i n f o

Article history:

Received 24 October 2011

Received in revised form

24 November 2011

Accepted 5 December 2011

Available online 10 January 2012

Keywords:

Steameiron process

Chemical-looping combustion

Chemical-looping reforming

Hydrogen

Iron oxide

a b s t r a c t The steameiron reaction was examined in a two-compartment fluidized-bed reactor at 800e900
C and atmospheric pressure In the fuel reactor compartment, freeze-granulated oxygen carrier particles consisting of Fe3O4supported on inert MgAl2O4were reduced to FeO with carbon monoxide or synthesis gas The reduced particles were transferred to

a steam reactor compartment, where they were oxidized back to Fe3O4by steam, while at 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 experiments were conducted of which 9 h involved H2generation The reactivity of the oxygen carrier particles with carbon monoxide and synthesis gas was high, providing gas concentrations reasonably close to thermodynamic equilibrium, especially at lower fuel flows The amount of H2produced in the steam reactor was found to correspond well with the amount of fuel oxidized in the fuel reactor, which suggests that all FeO that was formed 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

Copyrightª 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:magnus.ryden@chalmers.se(M Ryde´n)

Available online at www.sciencedirect.com

journal hom epa ge: www.elsev ier.com/locate/he

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

The dominating industrial route for generation of H2

currently is steam reforming of natural gas However, in order

for H2to be a feasible and environmentally benign energy

carrier it needs to be produced as cheap and efficient as

possible, and preferably with CO2capture 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

gener-ation schemes with CCS

H2 production via the steameiron reaction involves

oxidation of reduced iron oxides with steam Although

out-dated, recent technological advances such as the

develop-ment of chemical-looping combustion could possibly make

the steameiron process an attractive process yet again Fully

developed, a combined process involving H2production via

the steameiron reaction and regeneration of the iron oxide via

chemical-looping combustion would provide high purity H2

with inherent CO2capture, 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 H2for 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
C¼ 35.3 kJ/mol

(1)

Fe3O4(s)þ CO(g) / 3FeO(s) þ CO2(g)DH900
C¼ 10.1 kJ/mol (2)

FeO(s)þ CO(g) / Fe(s) þ CO2(g)DH900
C¼ 16.3 kJ/mol (3)

Naturally, the reduction could as well be performed with

H2, producing H2O as product, or with hydrocarbons producing a mix of CO2and 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, H2is 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
C¼ 16.8 kJ/mol (4)

3FeO(s)þ H2O(g)/ Fe3O4(s)þ H2(g)DH900
C¼ 43.2 kJ/mol(5)

It is necessary to provide steam in excess, nevertheless pure H2is obtained when the product mixture is cooled down and the steam is condensed to liquid water Due to thermo-dynamic constraints, it is not possible to oxidize Fe3O4 to

Fe2O3with 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
C¼ 232.2 kJ/mol (6) Reaction(6)is strongly exothermic Since the reduction of

Fe3O4to 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 thermody-namics 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

Nomenclature

ar, AR Air reactor

CLC Chemical-looping combustion

CnHm Generic hydrocarbon fuel

F Volumetric flow (Ln/min)

fr, FR Fuel reactor

Ln/min Normal litres per minute

PSD Particle size distribution

SG Synthesis gas SIR Steameiron reaction

sr, SR Steam reactor

wt.% Percentage by weight

DH Heat of reaction (kJ/mol)

x Dry-gas concentration (%)

gCO2 CO2yield (%)

u Mass-based degree of reduction

The petrochemical revolution in the second half of the 20th

century resulted in the development of new methods for large

scale H2production, 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 CO2and anthropogenic climate change

is one reason for this surge of interest The steameiron

process could be configured so that pure CO2is delivered in

a separate process stream, and thus seems as a convenient

method for H2production with CO2capture 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

reali-zation of a new generation of steameiron process

In recent years, several research groups have presented

interesting results concerning processes for use of the

steam-eiron reaction for H2production 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

con-ducted a detailed process study, examining a three-reactor

chemical-looping process with H2 generation by the

steam-eiron process On the experimental and the material

devel-opment 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 H2generation

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 CO2separation 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 inFig 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 O2from 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

combus-tion 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
C¼ 184.0 kJ/mol (7) 2O2(g)þ 8Fe3O4(s)/ 12Fe2O3(s)DH900
C¼ 986.5 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
C¼ 802.5 kJ/mol(9) From reaction(9), it can be seen that the chemical-looping combustion produces the same amount of heat as conven-tional 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 tech-nology 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

Air

O2, N2

Fuel

CnHm, CO, H2

Depleted air

O2, N2

Products

CO2, H2O

Fe2O3(s)

Fe3O4(s)

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

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]

2.3 H2production 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 1and by introducing the air to the air reactor in

under-stoichiometric proportions, partial oxidation of the fuel can

be achieved, i.e synthesis gas (CO and H2) is produced in the

fuel reactor rather than CO2and H2O The synthesis gas can

then be used for generation of H2, or other products This

concept, typically referred to as chemical-looping

auto-thermal reforming, has been demonstrated by Ryde´n et al

[25e27]in a small circulating fluidized bed-reactor, by

Kol-bitsch et al.[28]in a dual circulating fluidized bed, and by Ortiz

et al.[29,30]in a pressurized semi-continuous fluidized

bed-reactor 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

H2in 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

steam-eiron process, as is shown inFig 2

InFig 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 inFig 2could achieve similar efficiency as

conventional processes for H2production, without the need

for downstream separation systems, wateregas shift reactors,

cryogenic distillation of air or other expensive and energy

consuming equipment Furthermore, high purity H2would be

produced and CO2for 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 2involves some

technical difficulties as well The fuel reactor needs to be

arranged in counter-current fashion with Fe2O3added 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

ach-ieved for example by using moving bed reactor, see Li et al

[35], or possibly by using a staged fluidized bed Further, due to

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 2.4 Aim of this study

The main objective of this study is to examine H2generation 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 exam-ining 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 defluid-ization 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 freeze-granulation were used MgAl2O4has 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 2e Schematic description of hydrogen generation via the steameiron reaction configured in accordance to the principles of chemical-looping combustion

development of iron oxide based oxygen carrier particles for

chemical-looping combustion in fluidized bed-reactors,

MgAl2O4was identified as a suitable inert support material for

this application, exhibiting higher reactivity than iron oxide

supported on Al2O3, ZrO2and TiO2

The manufacturing procedure was as follows: A

water-based 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

morpho-logical investigation was carried out with an environmental

scanning electron microscope (ESEM) fitted with a field

emis-sion gun (FEI, Quanta 200) The BET surface area of the

parti-cles was evaluated with TriStar 3000 (Micromeritics) The

particle size distribution (PSD) before and after the

experi-ments 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 1summarizes 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

inFig 3

The reactor is designed for chemical-looping combustion

experiments using gaseous and liquid fuels, but steameiron

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 cross-section 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 endo-thermic, thus omitting the air reactor from Fig 2 The temperature in each reactor section is measured with ther-mocouples 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 ofz250 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 abnormali-ties 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 CH4were measured using infrared analysers while O2was measured with paramagnetic sensors The gas from the air/steam reactor was also examined with

a gas chromatograph which measured H2and N2, in addition

to the gas components mentioned above The gas chromato-graph provided measuring points every 3e4 min Excess gas that was not needed for analysis passed through a textile filter

Table 1e Properties of fresh oxygen carrier particles

Theoretical Fe2O3content [wt.%] 60

Size interval of particles [mm] 90e250

BET specific surface area [m2/g] 5.25

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

water-egas shift reaction:

CO(g)þ H2O(g)4 CO2(g)þ H2(g)DH900
C¼ 33.1 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:

gCO2¼ xCO 2

xCO 2þ xCH 4þ xCO

(11) Here xidenotes the composition of the respective gas, ob-tained from measured concentration in the gas analyser When CO is used as fuel, gCO2provides and accurate descrip-tion of the degree of fuel conversion and the combusdescrip-tion efficiency However, when synthesis gas or natural gas is used

as fuel, gCO2 only provides an adequate estimation since the conversion of H2to H2O in the fuel reactor may differ slightly from the conversion of CO to CO2, due to differing thermo-dynamic properties of H2and CO However, at the tempera-ture levels investigated in this work (800e950
C) the difference should be rather small, as is shown inFig 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):

u ¼mm ox

(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 4e Equilibrium composition of the gas phase for mixtures of CO/CO2and H2/H2O in presence of Fe3O4eFeO,

as calculated using thermodynamic data from FactSage 6.1 Fig 3e Schematic description of the two-compartment

fluidized-bed reactor

ui¼ ui1Z

t 1

t 0

_noutMO

mox

 4xCO 2þ 3xCO xH 2



uiis the instantaneous conversion at time i, ui1 is the

conversion in the preceding instant, t0and t1are the initial

and final time of measurement MO is the molar mass of

oxygen, _ninand _noutare the dry molar flow rates of the gas at

inlet and outlet of the reactor, respectively

4 Results

4.1 Chemical-looping combustion experiments

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

re-oxidized 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

O2concentration 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 inTable 2

The operation of the chemical-looping combustion

exper-iments was satisfactory Almost complete conversion of

carbon monoxide and synthesis gas to CO2and H2O was

ob-tained As can be seen inTable 2, gCO2was higher than 99.9%

for these fuels, leaving only traces of unconverted CO in the

products For natural gas, gCO2was limited to 75e90%, leaving

excess CH4, CO and H2in 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

4.2 Steameiron reaction experiments 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 Fe2O3was not possible due to thermodynamic constraints and FeO would eventually be generated in the fuel reactor The solids circulation between the reactor compart-ments 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 H2in 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 inTable 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 Fe2O3was 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 experi-ment with CO as fuel is shown inFig 5

InFig 5, it can be seen that there was almost complete conversion of CO to CO2as long as there was Fe2O3present 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 cor-responding to oxidation of CO with Fe3O4 From this point onwards, about 75% of the added CO appears to have been converted to CO2

Table 2e Summary of conducted chemical-looping combustion experiments Ffuel,fris the flow of fuel, Fair,ar

is the flow of air, Tfris the fuel reactor temperature and

gCO2is CO2yield

Experiment Operation

(min)

Ffuel,fr (Ln/min)

Fair,ar (Ln/min)

Tfr (
C)

gCO2 (%)

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 inFig 6

InFig 6, it can be seen that H2was not produced during the

initial few minutes, where Fe2O3was still present Once the

particles were deprived of Fe2O3, H2was produced

continu-ously, as could be expected The presence of CO2and CO is due

to gas leakage through the slot in the bottom of the reactor

which is discussed in Section4.3below 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

produced was found to vary with the amount of fuel added to the fuel reactor, seeFig 7

Fig 8shows the H2production as function of fuel flow It can be seen that at the lowest fuel flow (i.e Fsg,fr¼ 0.50 Ln/ min), the volumetric H2generation 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 H2and Fe3O4

It is evident fromFigs 7 and 8that increasing the fuel flow did increase H2generation, 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 H2would be produced, if there was no gas leakage between the reactors and thermodynamic equilibrium was reached In reality, the amounts of H2generated was slightly lower, i.e 0.49e0.53 Ln/min, as can be seen inTable 3 Since no accumulation of FeO in the system was noticed during opera-tion, 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

CO2yield, gCO2, as a function of fuel flow as shown inFig 9

InFig 9, it can be seen that the gCO2is 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 H2than the theoretical maximum is produced, as shown inFigs 7 and 8

It can be observed inFig 4that the expected conversion of

CO to CO2is approximately 70% at 900
C However,Figs 5 and

6shows a conversion of about 75%, and inFig 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 CO2via the wateregas shift, see reaction (10) This means that the values of yCO2;fr presented in the figures and inTable 3are 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 g

Fig 5e 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

att z 10 min describes the theoretical point for complete

reduction of Fe2O3to Fe3O4 Dilution with Ar is from the

fluidization gas added to the particle seals

Table 3e Summary of conducted steameiron reaction experiments gCO 2 ;frandFH 2 ;producedare representative average values over the period

(min)

Tsr(
C) Fsteam,sr

(Ln/min)

FN 2 ;sr

(Ln/min)

Ffuel,fr (Ln/min)

(Ln/min)

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,

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 8e 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)

Fig 9e CO2yield gCO2;fras 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 SG )

Fig 7e 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 SG )

Fig 6e Dry-gas concentration from the steam reactor for

steameiron experiments with 0.87 Ln/min CO as fuel at

900
C (SIR COIII)

section4.1above The solids circulation could then be

calcu-lated 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

experi-ments conducted

The chemical-looping combustion experiments required

oxygen carrier particles corresponding to approximately

0.50 Ln/min O2to 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

experi-ments, 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 O2needed to be transferred Thus

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 CO2and CO concentra-tions after the air reactor, as is shown inFig 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 CO2yields to some extent, since CO and CO2could 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 conclu-sions 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 H2production was very close

to the theoretical maximum, as is shown inFig 8above 4.4 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 10shows the particle size

Fig 10e Particle size distribution for fresh and used

oxygen carrier

Fig 11e ESEM images of (a) fresh and (b) used FeO/MgAlO oxygen carrier