Oxygen carriers for chemical-looping combustion

Oxygen carriers for chemical-looping combustion

11

A. Lyngfelt Chalmers University of Technology, Gothenburg, Sweden

11.1

Introduction

Oxygen-carrier materials constitute the cornerstone of chemical-looping combustion (CLC). The oxygen-carrier material transfers the oxygen from the air to the fuel and is vital for the function of the chemical-looping process, just like the haemoglobin in the blood is necessary to transfer oxygen from the air to the parts of the body where nutrients are burnt. Section 11.2 will give an overview of oxygen-carrier materials and their desired properties. Section 11.3 will give a more detailed overview of manufactured materials and operational experiences; and similarly, Section 11.4 will cover ores and waste materials. The chapter ends by giving some concluding remarks, some ideas for future trends and sources for further information.

11.2

Range of oxygen-carrier materials

11.2.1 Characteristics desired/required Important criteria for an oxygen carrier to be used in a fluidized-bed reactor system are the following: • • • • •

High reactivity with fuel and oxygen, and ability to convert the fuel fully to CO2 and H2O Low fragmentation and attrition, as well as low tendency for agglomeration Low cost Low risk for health and safety Sufficient oxygen transfer capacity

As a background for a discussion on the progress in oxygen-carrier material development, it is relevant to recall that CLC is a very novel technology and a fundamentally new principle of combustion, and that oxygen-carrier materials are the fundamental basis to make this process work. Well over 1000 materials have been tested, most of these in fixed beds. Furthermore, many hundred materials have been tested under laboratory fluidized conditions, often complemented with, for example, crushing strength tests. Significantly fewer materials, however, have been successfully tested

Calcium and Chemical Looping Technology for Power Generation and Carbon Dioxide (CO2) Capture http://dx.doi.org/10.1016/B978-0-85709-243-4.00011-2 Copyright © 2015 Elsevier Ltd. All rights reserved.

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in long-term operation. Laboratory tests rather give an indication of whether a material is likely not to work, but little evidence whether it will actually survive in real operation. It is far easier to find reactive materials that perform well in the laboratory than to find materials with high endurance in actual operation. There is, unfortunately, no easy way to get around the problem that costly and time-consuming operation in reasonably large units, i.e. with relevant velocities, is needed to obtain any reliable data. The search for relevant oxygen carriers is quite complex. Whether a given material is likely to fulfil the rather diverse criteria such as reactivity, chemical and mechanical integrity, reasonable cost, and low risk of agglomeration is not always easy to foresee. Low-cost natural materials are especially difficult as they are often inhomogeneous and chemically impure, and thus much more complex. Industrial practices or experiences relevant to oxygen carriers are lacking, and from a research point of view the search for oxygen carriers in the last 10e15 years resembles the discovery of a new continent, i.e. a vast number of possibilities and many possible directions to go in unknown terrain. The work on oxygen-carrier qualification raises the general issue how oxygencarrier development should best be organized to be successful. A possible structure for going from new material to operational tests where lifetimes can be established is outlined in Figure 11.1. It contains three basic elements: • • •

Lab testing, e.g. in fluidized reactor, and characterizations including crushing strength test and/or attrition testing (1e3) Testing in operation in small unit (4e5) Testing in larger unit with more relevant velocities (6e7)

In fact, a similar approach has been taken in the development of Ca-looping CO2 carriers (see Chapters 4 and 5). Elements in the scheme can of course be substituted, removed or added: for instance fixed-bed tests, e.g. thermogravimetric analysis (TGA), could be used before 3a. The advantage of TGA is of course less work effort per sample, whereas the drawback is less information. Nevertheless, it is inevitable that testing of larger numbers of materials requires a sequential procedure going from testing with low effort per material, followed by a selection of materials for testing in real operation. A general problem with such a development scheme is the long time needed in practice to go through all the various steps in sequence, both the testing and the manufacture or acquisition of materials. In order for a development scheme to be meaningful there must be feedback and an iterative procedure, in order to learn from the results of the material testing, which may significantly prolong the procedure. The worst case is obviously negative feedback coming very late in the scheme. It would simplify the development if good materials could be safely identified early on in the process, but although crushing strength testing and attrition testing are helpful, they are far from conclusive. A recent work comparing the actual lifetime of 25 materials in CLC operation with crushing strength tests and attrition tests shows a clear correlation (Rydén, Moldenhauer, et al., 2014). Nevertheless, some materials with high crushing strength performed poorly, and some materials with low attrition did not survive long in actual operation.

Oxygen carriers for chemical-looping combustion

223

No

No 1. Formulation of composition and production method

2. Preparation of first batch (20-50 g)

Is oxygen carrier promising?

Is oxygen carrier still promising?

Yes

Yes

4. Preparation of second batch (1 kg)

6. Preparation of third batch (25+ kg)

3a. Reactivity tests in batch reactor (15g) 3b. Characterization (XRD, BET, SEM etc) 3c. Crushing strength test 3d. Jet cup test

5. Reactivity and durability test in 300 Wth rector (250-400 g)

7. Reactivity and durability test in 10 kWth reactor (13-20 kg)

Equipment size, research effort, time, cost Figure 11.1 Example of general material development scheme. From Rydén, M., Moldenhauer, P., Lindqvist, S., Mattisson, T., & Lyngfelt, A. (2014). Measuring attrition resistance of oxygen carrier particles for chemical-looping combustion with the jet cup method. Powder Technology, 256, 75e86.

The execution of a comprehensive procedure as outlined in Figure 11.1 requires several years and large resources. A more realistic approach is to see the development of oxygen-carrier particles as a challenge where the efforts of several research groups and/or consecutive projects lead to the realization and validation of an oxygen carrier. To exemplify this, previous success stories of oxygen-carrier development can be considered. The three materials discussed below can be regarded as reasonably well established oxygen-carrier materials: •

•

•

NiO supported on alumina was first studied in the laboratory in the 1990s (Ishida, Jin, & Okamoto, 1998) and later such materials were investigated at Chalmers. Then, it was further studied in EU-project GRACE and successfully used in the operation of CLC in 2003. However, both the production process and raw materials used were very expensive. In the later EU-project CLC Gas Power, commercially viable raw materials and production methods were used. These materials were then validated in long-term operation (Linderholm, Mattisson, & Lyngfelt, 2009). Ilmenite was first studied in the EU-project GRACE for natural gas in 2003, but was discarded, with results never published, due to low reactivity towards methane. In a later project, it was found to be quite reactive towards CO and H2 and likely suitable for solid fuels. Further demonstration of the viability of ilmenite has been given in several later projects in the last few years. Calcium manganites were first studied by Sintef, Norway, in the ENCAP project in 2004. The first demonstration in continuous operation at the 300-W scale was in another project

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in 2009, and 10 kW demonstration of similar materials with more relevant velocities and adequate mass balances was made in 2012e2013, but still using pure and expensive raw materials. Current EU project SUCCESS 2013e2017 is investigating the scaling-up of material production using low-cost raw materials, and it is hoped that positive results will come as the project proceeds.

These examples have in common that the period from first testing until being well established is typically 10 years. Generally, it is less difficult to work with materials manufactured from easily accessible, more or less pure chemicals. Then, systematic studies of various compositions and manufacturing options can be performed. On the other hand, verification with chemically less pure raw materials of reasonable cost is needed. Based on the experiences it would be recommended to consider the following aspects for the early stages, i.e. 3ae3d, in a development scheme: • • • • • • • • •

Crushing strength Attrition index Reactivity with CO, H2 and CH4 Reactivity with oxygen Ability to release oxygen (for chemical looping with oxygen uncoupling [CLOU] materials) Expected cost of material Health, safety and environment aspects Risk of agglomeration Qualitative material assessment from characterization tests, e.g. results from powder X-ray diffraction, scanning electron microscopy, surface area, pore size distribution, densitometry, etc.

To be weighed into the selection is previous experiences with similar materials and the potential gain; e.g. for a material that releases oxygen and/or has high reactivity, a higher risk could be motivated.

11.2.2

Overview

The first phase of oxygen-carrier development focused mainly on the oxides of the four metals: Ni, Fe, Mn and Cu, and most investigations were performed in fixed beds, using TGA. The major focus was also on high reactivity towards methane. Normally, the active metal oxides studied were combined with an inert material, such as Al2O3. There were some studies of nonsupported materials, such as iron ore (Mattisson, Lyngfelt, & Cho, 2001). Although such material may have low costs, reactivity experiments simulating CLC performed on natural ores or unsupported metal oxides suggested fast degeneration or low reactivity of these materials towards methane (de Diego et al., 2004; Ishida & Jin, 1994; Lee et al., 2005; Mattisson & Lyngfelt, 2001). The use of inert material may increase the porosity and reactivity of the particles, help to maintain the structure and possibly also increase the ionic conductivity of the particles. Even though the ratio of free oxygen in a particle decreases with the addition of inert material, the reactivity with the fuel and oxygen can still be higher (Ishida & Jin, 1994).

Oxygen carriers for chemical-looping combustion

Table 11.1

Maximum CO conversion to CO2 at equilibrium Temperature,
C

Fe2O3/Fe3O4

Mn3O4/MnO

CuO/Cu

NiO/Ni

CoO/Co

225

CO conversion

800

1.0000

1000

1.0000

800

1.0000

1000

0.9999

800

1.0000

1000

1.0000

800

0.9949

1000

0.9883

800

0.9691

1000

0.9299

The ability of the oxygen carrier to convert a fuel gas fully to CO2 and H2O has been investigated thermodynamically, and the metal oxide systems of NiO/Ni, Mn3O4/MnO, Fe2O3/Fe3O4, Cu2O/Cu and CoO/Co were found to be feasible to use as oxygen carriers (Jerndal, Mattisson, & Lyngfelt, 2006). The maximum conversion of CO to CO2 is seen in Table 11.1. For H2 the maximum conversion is fairly similar to CO. The conversion of methane in itself is always complete, but in the case of NiO and CoO it results in the formation of CO and H2 at concentrations controlled by thermodynamics. For CoO/Co the thermodynamics are not so favourable, with maximum 93% conversion at 1000
C; moreover, this oxygen carrier is expensive and has health and safety risks. The oxides of copper, iron, manganese and nickel have advantages and disadvantages, as can be seen in Table 11.2. Note for instance that the most reactive materials, i.e. nickel oxides, are unfortunately also the most expensive. For NiO there are also health, safety and environmental aspects to be considered. Furthermore, NiO has a thermodynamic restriction; it cannot convert fuels fully to CO2 and H2O. The maximum conversion is 99%e99.5%, depending on temperature. All of the oxides have a more or less exothermic reaction in both reactors, if the fuel is H2 or CO, but with methane the reaction is endothermic for all the oxides except CuO. This is clearly an advantage for CuO, since it reduces the particle circulation needed to maintain fuel-reactor temperature. On the other hand, Cu has the disadvantage of a low melting temperature. As will be discussed in the following section on CLOU materials, CuO/Cu2O can be used with release of gaseous oxygen in the CLOU process. The oxygen carriers must also be able to transfer a sufficient amount of oxygen to the fuel. The oxygen transfer capacity, R0, i.e. the maximum mass fraction of oxygen that can be transferred, is also shown in Table 11.2. The R0 data in Table 11.2 are for pure systems, and will thus be reduced if diluted with support materials. For the case

226

Qualitative estimation of pros and cons for the active oxides Fe2O3/Fe3O4

Mn3O4/MnO

CuO/Cu

NiO/Ni

Comments

R0

0.03

0.07

0.20

0.21

Oxygen ratio

Reactivity towards methane

Moderate

þFairly high

þHigh

þVery high

Reactivity towards CO

þFairly high

þHigh

þHigh

þHigh

Cost

þLow

þFairly low

High

Very high

HSE risks

þLow

þLow

þFairly low

High Not full conversion

Thermodynamics

<99.5% conv. for NiO

Reaction enthalpy with CH4

þExoth.

CuO exothermic with CH4

Melting point

Low

1085
C for Cu

CLOU

þYes

CuO/Cu2O

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Table 11.2

Oxygen carriers for chemical-looping combustion

227

where the reaction in the fuel reactor is endothermic, the needed circulation is normally set not by the oxygen transfer capacity but by the heat balance, in order to reach sufficient fuel-reactor temperature (Jerndal et al., 2006; Lyngfelt, Leckner, & Mattisson, 2001). The oxygen carrier needs to react at a sufficient rate. The amount of oxygen carrier needed in the reactors is related to the reactivity of the oxygen carrier. Thus, a lower rate would necessitate a larger solids inventory or, worse, incomplete fuel conversion. There are a few works (Adanez et al., 2004; Johansson, 2007; Johansson, Mattisson, & Lyngfelt, 2006a) that have compared a larger number of different oxygen carriers. These generally indicate a large difference in reactivity between the different oxides, following the ranking shown in Table 11.2. However, most of the reactivity comparisons are focused on CH4. In general the reactivity is significantly higher using H2 and CO compared to CH4, and may be quite high for cheaper materials showing low reactivity with CH4. Reactivity may increase significantly with reaction temperature, especially in the case of low-reactivity materials and CH4. On the other hand, the reactivity of iron and manganese materials with CO and H2 may be high also at lower temperatures; the same applies to nickel materials with CH4. Normally, operation with oxygen carriers has used fuel-reactor temperatures in the range of 800e1000
C, although for instance, Ni materials have been operated at temperatures down to 600
C. The upper and lower limits of operational temperature are highly dependent on the oxygen-carrier properties and on the fuel. Thus, risk of agglomeration at high temperature and low reactivity at low temperature may be important limits, but higher temperature could also be limited by reactor wall materials or the ash melting temperature for fuels with ash. For Fe, Ni and Mn materials the reaction in the fuel reactor is endothermic when using hydrocarbon fuels; and in order to compensate for this, the air reactor would have an operational temperature that is typically 50
C higher than the fuel reactor. Studies of the effect of pressure are rare. However, an investigation of the effect of pressure in the range 1e30 bar, involving the reaction of copper, iron and nickel materials with H2, CO and O2, showed that increased pressure lowered the conversion rates considerably (Garcia-Labiano, Ad
anez, de Diego, Gayan, & Abad, 2006). Clearly, there are important advantages and disadvantages with all of the different oxide systems and the choice of oxygen carrier will be dependent on the application. Generally, oxygen carriers with low cost have an advantage for ash-containing solid fuels, whereas more expensive and reactive oxygen carriers could be suitable for gaseous fuels. The development in the latest years has been more diversified, with more work on combined metal oxides, mixed oxides, calcium sulphate/sulphide and low-cost materials. More work is also related to solid fuels and to materials releasing oxygen, i.e. CLOU materials (see Section 11.2.3). Combined metal oxides here refer to materials where two or more oxides are combined chemically, constituting new oxides, for example NiFeAlO4 or materials with perovskite structure like La1x SrxFe1yCoyO3d. Also of interest are combined Mn oxides with partial CLOU properties, i.e. with the ability to release some oxygen.

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This includes Mn combined with Ca, Mg, Ni and Fe. Ilmenite, FeTiO3, is a naturally occurring combined oxide commonly used with solid fuels, which is also a low-cost material. Contrary to combined oxides, the mixed-oxides concept does not involve the creation of new compounds with new properties; instead, it builds on synergies of physical mixing of oxygen-carrier materials with different properties. Low-cost materials have been investigated mainly for use with solid fuels; these studies include iron ore, manganese ore, ilmenite and industrial waste materials. Most of the studies have used ilmenite, being a cheap ore and having a reasonably high reactivity towards syngas and showing good fluidization behaviour. Another possible issue is deactivation by reaction with sulphur. A thermodynamic analysis indicates that sulphur poisoning would not be expected for Fe, Mn or Cu, but would be more likely with Ni, depending on the temperature and concentration of sulphur compounds (Jerndal et al., 2006). This is discussed further in Section 11.3.2. Finally, it should be said that significant experience has been accumulated in the last decade with a number of materials that have been in actual operation in more than 20 CLC units of sizes 0.3 kWe3 MW. Total operation reported is more than 6700 h.

11.2.3

Chemical looping with oxygen uncoupling materials

Chemical looping with oxygen uncoupling (CLOU) is closely related to chemicallooping combustion but differs from CLC through the spontaneous release of oxygen in the fuel reactor; see Figure 11.2.

N2, O2

Air reactor

CO2, H2O MeO (+ Me)

CuO => Cu2O + ½O2

Cu2O + ½O2 => CuO Me (+ MeO)

Air

Fuel reactor

C + O2 => CO2

Fuel

Figure 11.2 Chemical looping with oxygen uncoupling (CLOU) using CuO/Cu2O. The fuel is carbon to illustrate the principle. Lyngfelt, A. (2013). Chemical looping combustion, Chapter 20. In F. Scala (Ed.), Fluidized-bed technologies for near-zero emission combustion and gasification (pp. 895e930). Woodhead Publishing Limited. http://dx.doi.org/10.1533/9780857098801.4.895.

Oxygen carriers for chemical-looping combustion

229

Thus, instead of the fuel gas reacting directly with the oxide, the oxidation of the fuel involves two distinct steps, first the release of gaseous oxygen in Eqn (11.1) and then the combustion of the fuel by the oxygen release, as exemplified by Eqn (11.2a) and (11.2b) 1 2CuO/Cu2 O þ O2 2

(11.1)

O2 þ C/CO2

(11.2a)

2O2 þ CH4 /CO2 þ 2H2 O

(11.2b)

The CLOU process must have an oxygen carrier that has the ability to react with the oxygen in the combustion air in the air reactor but that decomposes to a reduced metal oxide and gas-phase oxygen in the fuel reactor. Three metal oxide systems with suitable thermodynamic properties have been identified: Mn2O3/Mn3O4, CuO/Cu2O and Co3O4/CoO (Mattisson, Lyngfelt, & Leion, 2009). Co3O4/CoO has the disadvantage of an overall endothermic reaction in the fuel reactor, as well as high costs and risks with respect to health and safety. The equilibrium oxygen concentration for CuO/Cu2O is close to 5% at a temperature of around 950
C. In a combustion process most of the oxygen in the combustion air needs to be consumed in order to avoid large flows and thermal losses with exiting flue gas. This means that the O2 concentration would need to be reduced to 5% or lower in the air reactor. Consequently the temperature of the air reactor needs to be below 950
C. CLOU using CuO has been shown to work, first in laboratory batch fluidized-bed tests with CuO and solid fuel (Leion, Mattisson, & Lyngfelt, 2008; Mattisson, Leion, & Lyngfelt, 2009) and later in continuous operation with solid fuel (Abad et al., 2012); see Section 11.3.3. The equilibrium concentration for Mn2O3/Mn3O4 is 5% at a temperature of around 800
C. Thus, for the Mn system we would need to be below around 800
C in the air reactor. It is not unlikely that the reactions at these temperatures are too slow. Although attempts have been made, no successful work is known where Mn2O3/Mn3O4 has been used as a CLOU material. As previously mentioned, an additional option could be combined manganese oxides, having lower-equilibrium oxygen partial pressures than the pure manganese system, and thus being able to oxidize at higher temperatures. Many of these materials can only release limited amounts of the oxygen in this way, but this could still be quite beneficial for the conversion of both solid and gaseous fuels. These materials will be further discussed in Section 11.3.7. Although this mechanism is clearly useful for any fuel, the advantages with CLOU are quite obvious for solid fuels. In normal CLC of solid fuels there is an intermediate gasification step of the char with steam or carbon dioxide to form reactive gaseous compounds, which then react with the oxygen-carrier particles. The gasification of char with H2O and CO2 is slow. This slow gasification is avoided in CLOU, since there is no intermediate gasification step needed and the char reacts directly with gas-phase oxygen.

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Also, for gaseous fuels the CLOU process could give significant improvement as the direct contact between reacting gas and oxygen carrier is not necessary. This could make it easier to reach full conversion, that is, to compensate for inadequate contact between gas (bubble phase) and particles (dense phase). It should be noted that with gaseous fuels the oxygen carrier could also react directly with the gas in parallel to oxygen release, and it may be difficult to clearly distinguish between the two mechanisms.

11.2.4

Performance versus costs

Because of the uncertainties in the lifetime of oxygen carriers in actual operation, the cost of the oxygen carrier is very important. Clearly, low-cost natural minerals or waste materials are an advantage. An analysis of the effect of cost of materials is made below. While the prices for ores, metals and oxides vary considerably between the years, current prices are in the range of 150e400 V/tonne for manganese ore, around 5400 V/tonne for copper and 13,000 V/tonne for nickel. Costs for large-scale material production, spray-drying for example, are uncertain but are likely within the range 500e5000 V/tonne, i.e. excluding raw materials. If we assume a copper price of 6000 V/tonne, a material with 40% CuO, and a cost of production including support material of 1000 V/tonne, the oxygen carrier cost would be 3000 V/tonne. Similarly, if we assume a cost of manganese oxide/ore of 400 V/kg, a cost of lime of 100 V/tonne and a production cost of 750 V/tonne, the cost of calcium manganite would be around 1000 V/tonne. The cost of oxygen carrier will add to the CO2 capture cost. This added cost for the oxygen carrier can be expressed as cost per tonne of CO2 captured and is given by CCCOC ¼

COC $SI SE$s

(11.3)

where CCCOC is the cost of CO2 capture caused by the oxygen carrier in V/tonne CO2 captured, COC is an estimated cost of oxygen carrier in V/tonne, SI is the solids inventory in tonne/MWth, SE is the specific emission in tonne CO2/MWhth and s is the average lifetime of the oxygen carrier. Table 11.3 gives an indication of the lifetimes that should be targeted for different materials. Here, the lifetime, s, was adjusted to give a cost of CO2 capture of around 1 V/tonne of CO2. This number of 1 V/tonne is taken as a case where cost of oxygen carrier is very small, and should be compared to total costs of conventional CO2 capture technologies of typically 50 V/tonne. Consequently, the lifetimes shown in Table 11.2 are indicative of lifetimes where oxygen carrier cost is small. Thus, with a low-cost oxygen carrier like ilmenite a lifetime of a few hundred hours is sufficient to make the oxygen carrier cost small. On the other hand, with more expensive copper materials, 10 times longer lifetime would be needed to reach similar low costs. There is obviously a trade-off where more expensive materials could be motivated if higher performance can bring down process costs correspondingly. Thus, as an

Oxygen carriers for chemical-looping combustion

231

Table 11.3 Examples of CO2 capture costs related to oxygen-carrier materials Ilmenite

Manganese ore

Calcium manganite

Copper

Nickel

SE, tonne/MWhtha

0.334

0.334

0.334

0.334

0.198

SI, tonne/MWhth

1

1

1

0.3

0.5

s(h)

300

1000

3000

3000

20 000

COC(V/tonne oxygen carrier)

100

350

1000

3000

8000

CCCOC(V/tonne CO2)

1.00

1.05

1.00

0.9

1.01

a

The specific emission used here for nickel is that of natural gas and for the other materials is that of coal. The emission is lower for natural gas, which gives higher specific cost, but on the other hand there is less CO2 that needs to be captured.

example, the use of CLOU copper materials with solid fuels would be able to accomplish full oxidation of the fuel, which should not be possible with ilmenite. If, for instance, the added costs for oxygen polishing would be 5e10 V/tonne, this could motivate using a copper material even if the added cost would be 5e10 V/tonne, which would correspond to a lifetime of only 300e600 h. As we do not have a full understanding either of how process costs are affected by oxygen carrier performance or of the actual lifetime, it is too early to make any safe conclusion on whether high- or low-cost materials are the most relevant. Examples of actual estimations of lifetimes for oxygen-carrier materials in Table 11.2, based on fines produced in actual operation, are for (1) ilmenite, 600e700 h (Linderholm, Knutsson, Schmitz, Markstr€om, & Lyngfelt, 2014); (2) calcium manganite, 12,000 h (K€allén, Rydén, Dueso, Mattisson, & Lyngfelt, 2013); (3) copper, 500e2700 h (Gay
an et al., 2011); and (4) nickel, 33,000 h (Linderholm et al., 2009). Although these numbers, as well as the other numbers used in Table 11.3, are uncertain, they suggest that it might be possible to reach a cost of CO2 capture caused by the oxygen carrier, CCCOC, that could be as low as 1 V/tonne CO2 captured.

11.3

Manufactured oxygen carriers

11.3.1 Methods of preparation There are a large number of ways of producing particles of adequate size. Here the discussion is focused on particle sizes in a range suitable for fluidized beds, i.e. typically around 100e300 mm. The production often starts with powders of active oxygen carrier and support material and normally involves several steps, e.g. mixing,

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grinding, granulation, calcination, sieving. Based on the principle used for producing particles of relevant size, we have three main routes: • •

•

Granulation technologies. Here, particles of the size desired are formed directly from a fine powder. The most common is spray-drying. Impregnation technologies. Here, a porous particle consisting of only support is first manufactured. The metal ion of the active oxide is then added to the particles in a water solution, whereupon the particles are dried and calcined. The calcination step converts the salt, normally a nitrate, to the desired oxide. Crushing technologies. These involve the production of larger particles or cakes, which are then calcined and crushed to the desired size. Particles or cakes are typically produced from dissolved raw materials, i.e. by wet-chemical methods, or from fine particles made into paste and extruded to form, for instance, cylindrically shaped materials.

A number of methods have been used to manufacture the materials; many of these are doubtless more suitable for making small amounts for laboratory testing but would not be realistic to use for production of thousands of tonnes. Below, a few technologies often used in connection with chemical-looping oxygen carriers are mentioned. Spray-drying is a granulation technology, where a fine powder is immersed in water together with organic binders and dispersants. The suspension is injected in a large spray-dryer via a nozzle designed to produce droplets of adequate size. The spraydryer is heated and needs to be sufficiently large to enable drying of the droplets before they reach the walls. The drying of the droplets results in reasonably spherical particles, although sometimes in the shape of doughnuts. The particles are calcined at a temperature giving sufficient strength of the material, and during calcination binders/dispersants are also burnt off. Spray-drying is a widely used technology for producing large amounts of materials. A typical commercial spray-dryer can produce tonnes of material per hour. Freeze-granulation has similarities with spray-drying, but the droplets are sprayed into liquid nitrogen. The frozen water in the particles is sublimated in a freeze-dryer and the particles are subsequently calcined. The method is not believed to be suitable for large-scale production. Impregnation is a commonly used method for producing catalysts. The obvious advantage is that the supporting particle can be either found from a number of already commercially available materials or tailor-made to fit the desired properties of the material. The cost is generally higher as it involves both the production of the supporting particles and the subsequent impregnation and calcination. Normally the available pore volume limits the amount of active oxygen carrier that can be added. There are a number of wet-chemical methods to produce solid materials that are chemically very homogeneous, like co-precipitation (Imtiaz, Broda, & M€uller, 2014) and solegel (Mei, Zhao, Ma, & Zheng, 2013). By these methods cakes can be produced that are crushed to give the right particle size. Wet-chemical methods can also be used to produce fine powders used in granulation. Extrusion is a method whereby powders are mixed with water to a paste of suitable viscosity and then extruded to produce cylindrical particles that are sintered and crushed to desired size (Adanez, et al., 2004).

Oxygen carriers for chemical-looping combustion

233

Normally a support material is used, e.g. Al2O3, TiO2, MgAl2O4, SiO2, ZrO2, which constitutes typically 40%e60% of the material.

11.3.2 Nickel-based materials The oxidized and reduced forms are NiO and Ni. Nickel oxide materials were early identified as being the oxygen carrier most reactive with methane, and have consequently been the most studied materials. The high reactivity towards methane is likely associated with the metallic nickel, the reduced form, being a strong reforming catalyst. Thus, nickel catalyzes the reaction CH4 þ H2 O/CO þ 3H2

(11.4)

This reaction breaks up the methane molecule, giving more reactive H2 and CO. The importance of this catalytic effect has been indicated in laboratory batch tests, where there is normally a short initial period of lower conversion, which is believed to be caused by an initial absence of Ni (Jerndal, Mattisson, Thijs, Snijkers, & Lyngfelt, 2010). This may also have implications in operation with nickel materials. Thus, in operation of a 10 kW unit it was found that increased circulation had a negative effect on methane conversion (Lyngfelt & Thunman, 2005). The explanation is that increased circulation resulted in less reduced Ni being present, thus less methane reforming (Linderholm, Abad, Mattisson, & Lyngfelt, 2008). The high reactivity with methane appears to also apply to other light hydrocarbons (Ad
anez, Garc
a-Labiano, et al., 2009). Laboratory data indicate that ideally, i.e. neglecting the by-pass in bubbles in fluidized beds, fuel reactor solids inventories as small as 10e20 kg/MW should be sufficient to reach full gas conversion (Abad, Ad
anez, et al., 2007; Mattisson, Jerndal, Linderholm, & Lyngfelt, 2011). In practice, however, pilot operation suggests that several hundred kg per MW is needed (Mattisson et al., 2011). From the literature, it is evident that a large number of nickel materials have been manufactured using various production methods, and generally it appears next to impossible to fail in making a reactive nickel material. With respect to the mechanical stability of nickel materials, both failures and successes have been noted. Operation with some materials has shown very low loss of fines. Thus, the first successful operation with such material showed a loss of material less than 45 mm of 0.0023%/h, corresponding to a lifetime of 40,000 h (Lyngfelt, Kronberger, Adanez, Morin, & Hurst, 2004). The earlier phases of research used nickel materials that were chemically pure and, for large-scale application, unrealistically expensive. This also applied to the production technologies. In order for chemical-looping systems to be commercially viable, it is important that the raw materials can be obtained in large quantities at a reasonable cost. Therefore a study was made to verify that commercially available raw or semifinished materials can be used to produce high-performing oxygen carriers of NiO/NiAl2O4. Moreover, this study involved a production method well suited for large-scale particle production, i.e. spray-drying. The study showed both

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that nickel raw materials at reasonable prices were commercially available and that they could be used together with a commercially relevant production technology (Jerndal et al., 2010; Mattisson, Adanez, et al., 2009). Further, the materials produced had adequate reactivity and were highly attrition resistant, with low loss of fines validated during 1000 h of operation, corresponding to a lifetime of 33,000 h (Linderholm et al., 2009). Many studies have used Al2O3 as support material with NiO in excess. In these materials the excess NiO has reacted with the alumina support, forming NiAl2O4, which had previously been viewed as basically inert. However, studies where NiO has been impregnated on Al2O3 support have in some cases led to loss of active NiO. This has been studied in some detail, indicating that (1) NiAl2O4 is in itself an oxygen carrier, albeit with a reactivity more than one order of magnitude lower than NiO; (2) the reduced oxygen carrier upon oxidation forms a mixture of NiO and NiAl2O4; and (3) the risk of NiO reacting with the support material is highly dependent on the alumina support used (Dueso et al., 2010; Gay
an et al., 2008). Even if commercial materials can be used, it is still an issue with nickel materials that the world market price of nickel is substantial, which is a consequence of the fact that nickel ores only contain a few percent quantity of nickel. Thus, nickel is distinctly more expensive than copper and much more expensive than manganese and iron materials. Nevertheless, for use with e.g. gaseous fuels, containing no ash, and assuming a lifetime of several thousand hours, it could still be realistic to use nickel materials, cf. Table 11.3. Another issue with nickel materials is health, safety and environmental (HSE) aspects. Work with such materials involves significant health risks and, consequently, also restrictions. Safe handling of such materials will also add to the costs. At least in Europe, with coming tighter restrictions, it would likely be difficult or even impossible to introduce a new process where nickel materials are used in large fluidized beds. Nickel materials are also sensitive to sulphur poisoning, as indicated by thermodynamic calculations, and expected negative effects of using fuel with sulphur together with nickel oxide carriers have been clearly confirmed in operation (Díaz-Castro, Mayer, Pr€ oll, & Hofbauer, 2012; Forero et al., 2010; Shen, Gao, Wu, & Xiao, 2010). Due to thermodynamic constraints, nickel materials are only able to reach around 99%e99.5% conversion of methane. Thus, to avoid minor amounts of H2 and CO in the effluent gas, an oxygen polishing step would be required. Costs and HSE concerns, as well as progress with other materials and a greater focus on solid fuels, have led to a reduced interest in nickel materials. Nickel materials have also been used in the ‘mixed-oxides’ concept. This refers to the physical mixing of two oxides where a synergy is provided by both being present. One option is to mix nickel material with a cheap oxygen carrier, e.g. an iron or manganese material. The idea is to use the reforming capability of the nickel material together with the good reactivity of low-cost materials with CO and H2. The first study using this idea found that small amounts of nickel added to an iron material could double the CO2 gas conversion (Johansson, Mattisson, & Lyngfelt, 2006b). A later study using ilmenite with some nickel material in continuous operation has also shown

Oxygen carriers for chemical-looping combustion

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a distinct improvement (Rydén, Johansson, Cleverstam, Lyngfelt, & Mattisson, 2010). Also, synergies have been obtained by mixing two nickel materials, one being more reactive and the other having better reforming properties (Linderholm, Jerndal, Mattisson, & Lyngfelt, 2010). In total, 2800 h of operation with nickel-based material have been reported from 12 different pilots: • • • • • •

• • • • • •

A 10-kW unit at Chalmers (Linderholm et al., 2008; Linderholm et al., 2009; Lyngfelt et al., 2004; Lyngfelt & Thunman, 2005) A 50-kW unit at Korea Institute of Energy Research (Ryu, Jin, & Yi, 2004) A 0.3-kW unit at Chalmers (Johansson, Mattisson, Lyngfelt, & Thunman, 2006c,d; Rydén, Lyngfelt, & Mattisson, 2006) (Linderholm et al., 2010; Rydén, Johansson, Lyngfelt, & Mattisson, 2009; Rydén, Lyngfelt, & Mattisson, 2008) A 1-kW unit at Korea Advanced Institute of Science and Technology (Son & Kim, 2006) A 0.5-kW unit at CSIC (Ad
anez, Duesco, et al., 2009a,b; Ad
anez, Garc
a-Labiano, et al., 2009; de Diego et al., 2009a,b; Dueso et al., 2009; García-Labiano et al., 2009; Gay
an et al., 2013) A 140-kW unit at Vienna University of Technology (Bolhar-Nordenkampf, Pr€ oll, Kolbitsch, & Hofbauer, 2009a,b; Kolbitsch, Bolhar-Nordenkampf, Pr€ oll, & Hofbauer, 2009; Kolbitsch, Proll, Bolhar-Nordenkampf, & Hofbauer, 2009a,b,c; Kolbitsch, Bolhar-Nordenkampf, Pr€ oll, & Hofbauer, 2010; Pr€oll, Bolhar-Nordenkampf, Kolbitsch, & Hofbauer, 2010; Pr€ oll, Kolbitsch, Bolhar-Nordenkampf, & Hofbauer, 2008; Pr€oll, Kolbitsch, Bolhar-Nordenkampf, & Hofbauer, 2009; Pr€oll, Kolbitsch, Bolhar-Nordenkampf, & Hofbauer, 2011) (Díaz-Castro et al., 2012) A 15-kW unit at Alstom (Mattisson, Adanez, et al., 2009) A 10-kW solid fuel unit at South-East University, Nanjing (Shen, Wu, Gao, & Xiao, 2009; Shen, Wu, & Xiao, 2009; Wu, Shen, Xiao, Wang, & Hao, 2009) A second 50-kW unit at Korea Institute of Energy Research (Ryu, Jo, Park, Bae, & Kim, 2010) A 1-kW unit for solid fuels at South-East University, Nanjing (Shen et al., 2010; Song, Shen, et al., 2012) A 10-kW unit for both gaseous and solid fuels at IFP, Lyon (Rifflart, Hoteit, Yazdanpanah, Pelletant, & Surla, 2011) A 0.3-kW unit modified for use with liquid fuel at Chalmers (Moldenhauer, Rydén, Mattisson, & Lyngfelt, 2012b)

11.3.3 Copper-based materials The oxidized form is CuO and the fully reduced form is Cu. However, copper can be used as a CLOU material, with the reduced form Cu2O. However, this was not really considered or realized in the earlier phases of studying copper materials. This is perhaps not so surprising, as the earlier studies were done at temperatures of 800e850
C, where the CLOU effect is small or moderate. Low temperatures were used firstly to avoid agglomerations, which were common in laboratory testing at higher temperatures involving reduction all the way to Cu, which has a low melting temperature, 1079
C (Cho, Mattisson, & Lyngfelt, 2004). Low temperatures were also reasonable because Cu materials are quite reactive also at these temperatures. Early and successful operation of copper materials for 120 h in a 10-kW unit was reported with full methane conversion, using a temperature of 800
C (Ad
anez et al., 2006).

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After the potential advantages with CLOU were realized, most studies have used higher temperatures, and agglomerations have not been noted. This is likely because complete reduction of the oxygen carrier all the way to Cu has been avoided. Both CuO and Cu2O have considerably higher melting temperatures as compared to Cu. Operation with methane using higher temperatures has shown excellent gas conversion. Also, operation with solid fuels has shown full gas conversion, indicating the important advantages of CLOU with solid fuels (Abad et al., 2012), where full gas conversion is otherwise not really possible. An advantage with copper materials is that the reactions in the fuel reactor are exothermic. Thus, it is possible to use lower material circulation as compared to the other oxygen-carrier materials, where the circulation needs to be sufficient to avoid large temperature differences between air and fuel reactor. Similar to NiO, CuO may react with Al2O3 support, forming CuAl2O4 or CuAlO2. CuAl2O4 is highly reactive as an oxygen carrier and seems to have a low tendency for agglomeration, but the drawback is that the formation of copper aluminates means that the CLOU property is lost (Arjmand, Azad, Leion, Mattisson, & Lyngfelt, 2012; Forero et al., 2011; Gay
an et al., 2011). Although the cost of copper is clearly lower than that of nickel, copper materials are nevertheless still expensive, as copper ores only contain a few percent of copper. Although copper materials have been used in a number of operational studies, there are still some uncertainties regarding material lifetime. Several copper materials studied have shown problems with dust formation, both spray-dried materials (Ad
anezRubio et al., 2012; Rydén, Jing, et al., 2014) and impregnated materials (Gay
an et al., 2011). In the latter study, however, an impregnated material with a little addition of NiO showed low attrition during 67 h of operation. In view of the cost of copper materials and the lifetimes thus needed, it would be necessary to verify longer operation in units with higher velocity. With this said, it is clear that copper materials are excellent materials, with a possibly high price/lifetime ratio as the only major concern. Provided that high lifetime can be attained, it is clearly an excellent oxygen carrier for gaseous or other low-ash fuels. For the use with ash-containing solid fuels, the loss of material with ash could be a show-stopper, unless separation of oxygen carrier and ash can be realized. In total, 627 h of operation with copper-based material have been reported from six different pilots: • • • • • •

A 10-kW unit at CSIC (Adanez et al., 2006; de Diego et al., 2007) A 0.5-kW unit at CSIC (Forero et al., 2009; Forero et al., 2011; Gay
an et al., 2011; Gay
an et al., 2010) A 0.5e1.5 kW unit for solid fuels at CSIC (Abad et al., 2012; Ad
anez-Rubio et al., 2013; Ad
anez-Rubio, Abad, Gay
an, de Diego, et al., 2014; Ad
anez-Rubio, Abad, Gay
an, García-Labiano, et al., 2014; Ad
anez-Rubio et al., 2012) A 0.3-kW unit modified for liquid fuels (Moldenhauer, Rydén, Mattisson, & Lyngfelt, 2012a) A 140-kW unit at Vienna University of Technology (Penthor et al., 2014) A 0.3-kW unit at Chalmers (Rydén, Jing, et al., 2014)

Oxygen carriers for chemical-looping combustion

237

11.3.4 Manganese-based materials The oxidized form is Mn3O4 and the reduced form is MnO. MnO cannot be further reduced under any sensible reaction conditions (Jerndal et al., 2006). Thus, in contrast to the other oxygen carriers the metallic form will never occur. Manganese materials have been identified as possible CLOU materials based on thermodynamic considerations. The oxidized and reduced forms would then be Mn2O3 and Mn3O4. However, the air reactor would need to be at a temperature lower than 800
C to be able to oxidize this material at an outlet oxygen concentration of around 5%. In practice, it has not been possible to accomplish the oxidation to Mn2O3 at such temperatures. On the other hand, Mn2O3/Mn3O4 could potentially be an excellent CLOU material under pressurized conditions and at higher temperature. Thus, with a pressure of e.g. 10 bar the temperature of the air reactor could be in the range of 900e950
C, where kinetics for oxidation are likely to be more favourable. This has, however, never been tested. On the other hand, if Mn is combined with other oxides it forms new oxides with CLOU properties, as is discussed below under combined oxides. Despite the fairly high reactivity and the moderate cost, manganese materials have generally received little attention, and Mn is less studied than Ni, Cu and Fe. Thus, only a few manufactured manganese materials have been used in operation. Operation with manganese materials has shown very high reactivity with CO and H2, as well as fairly high reactivity with methane. Manganese materials also appear to be the least likely to form agglomerations, such as has been seen at times with iron, copper and nickel materials, which is possibly associated with the fact that metallic Mn never forms. Operational data are also available for combined manganese oxides as well as for manganese ores (see subsequent sections). In contrast to nickel and copper ores, ores with high content of manganese are abundant, which also makes manganese materials much cheaper. Nevertheless, manganese ores are somewhat more expensive than iron ores, which is probably associated with the much smaller production. Although the global manganese ore production is large, it is nevertheless only around 1% of the iron ore production. In total, 91 h of operation with manufactured manganese-based material have been reported from two different pilots: • •

A 0.3-kW unit at Chalmers (Abad, Mattisson, Lyngfelt, & Rydén, 2006; Rydén, Lyngfelt, & Mattisson, 2011b) A 0.3-kW unit modified for use with liquid fuel at Chalmers (Moldenhauer et al., 2012a)

11.3.5 Iron-based materials The oxidized form is Fe2O3, whereas the reduced form is Fe3O4. Lower oxidation states can form, such as FeO or even Fe, but reduction to these lower states is not thermodynamically possible under conditions of full fuel conversion. This does not exclude formation locally where fuel concentration is high. In processes for direct hydrogen production, these lower forms are desired and can be accomplished by designing a fuel reactor where the fuel and oxygen carrier are in counter-current (Mizia et al., 2009).

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Manufactured iron materials have generally shown rather poor reactivity towards methane, whereas the reactivity towards syngas has been a great deal better (Mattisson et al., 2007). However, an impregnated iron material has also shown high reactivity with methane (Gay
an et al., 2012). In total, 1077 h of operation with manufactured iron-based material have been reported from eight different pilots: • • • • • • •

A 10-kW unit at Chalmers (Lyngfelt & Thunman, 2005) A 1-kW unit at Korea Advanced Institute of Science and Technology (Son & Kim, 2006) A 0.3-kW unit at Chalmers (Abad, Mattisson, Lyngfelt, & Johansson, 2007) A 10-kW unit for solid fuels at South-East University, Nanjing (Shen, Wu, Xiao, Song, & Xiao, 2009) A 0.5-kW unit at CSIC (Cabello, Duesco, et al., 2014; Gay
an et al., 2012; Pans et al., 2013) Two units for solid fuels at Ohio State University of 2.5 and 25 kW (Bayham et al., 2013; Kim et al., 2013; Sridhar et al., 2012; Tong et al., 2014; Tong, Sridhar, et al., 2013; Tong, Zeng, Kathe, Sridhar, & Fan, 2013) A 140-kW unit at Vienna University of Technology (Mattisson et al., 2014)

11.3.6

Cobalt-based materials

In view of the obvious drawbacks of this material previously mentioned, it is not likely that it will be used for large-scale operation. In total, around 180 h of operation with manufactured cobalt-based material have been reported from two different pilots: • •

A 50-kW unit at Korea Institute of Energy Research (Ryu, Jin, Bae, & Yi, 2004) Another 50-kW unit at Korea Institute of Energy Research with a different design (Ryu et al., 2010)

11.3.7

Combined oxide materials

Combined metal oxides, i.e. where two or more oxides are combined not only physically but chemically, constituting new oxides, include for example Cu0.95Fe1.05 AlO4, Co0.5Ni0.5FeAlO4, CoFeAlO4, CuFeGaO4 and NiFeAlO4 (Lambert, Briault, & Comte, 2011). Some of these materials have perovskite structure, e.g. La1x SrxFe1yCoyO3d and Sr(Mn1xNix)O3 (Ksepko, Talik, & Figa, 2008; Ryden et al., 2008). Combined Mn oxides may exhibit CLOU properties, i.e. the ability to release oxygen. Such materials include Mn combined with Ca, Fe, Si, Mg, Cu and Ni (Rydén, Leion, Mattisson, & Lyngfelt, 2012; Shulman, Cleverstam, Mattisson, & Lyngfelt, 2009, 2011). A combination of Mn and Fe was found to release large quantities of oxygen rapidly (Azimi, Rydén, Leion, Mattisson, & Lyngfelt, 2013). Combined manganese oxides tested in actual operation include MneFe (Rydén et al., 2011b), MneFe on Ti (Rydén et al., 2013) and FeeMneSi (K€allén, Hallberg, Rydén, Mattisson, & Lyngfelt, 2014). Generally, these materials show both oxygen release and good gas conversion, but unfortunately the materials have also shown dust formation. Calcium manganites, however, have been operated with very low formation of fines (Cabello, Abad, et al., 2014; K€allén et al., 2013; Rydén, Lyngfelt, & Mattisson,

Oxygen carriers for chemical-looping combustion

239

2011a). Although these materials have a lower direct reactivity towards methane than nickel materials, they seem to be able to perform equally as well, or even better, in pilot operation. The reason is likely that the release of oxygen makes it possible to convert methane that is not in direct contact with the oxygen carrier. Thus, the by-pass of gas in fluidized beds should have less effect on a CLOU material. Another reason is of course that nickel materials are thermodynamically restricted to 99%e99.5% gas conversion, whereas pilot operation with calcium manganite has reached full conversion (K€allén et al., 2013). If temperature and circulation are sufficient, operation with calcium manganite also gives an excess of oxygen. Except for the pressurized operation with a combined iron-copper oxide (Wang, Wang, Jiang, Luo, & Li, 2010), the operation reported uses combined manganese materials. In total, 545 h of operation with combined oxide material have been accomplished in six different pilots: • • • • • •

A 10-kW pressurized unit at Xi’an Jiaotong University (Wang et al., 2010) A 0.3-kW unit at Chalmers (Hallberg et al., 2014; K€allén et al., 2014; Rydén et al., 2013; Rydén et al., 2011a) A 10-kW unit at Chalmers (Hallberg, K€allén, Mattisson, Rydén, & Lyngfelt, 2014; K€allén et al., 2013) A 10-kW unit for solid fuels at Chalmers (Schmitz, Linderholm, & Lyngfelt, 2014a, 2014b) A 0.5-kW unit at CSIC (Cabello, Abad, et al., 2014) A 140-kW unit at Vienna University of Technology (Mattisson et al., 2014)

11.3.8 Mixed-oxide materials Mixed-oxide materials refers to the physical mixing, in contrast to the chemically combined oxide materials described above. By physical mixing of different materials, synergies can be obtained. Examples have been noted above, e.g. mixing nickel materials with different properties and mixing low-cost oxygen carriers with some nickel oxide. Other synergies reported are addition of limestone to ilmenite in solid-fuel CLC (Cuadrat, Linderholm, Abad, Lyngfelt, & Ad
anez, 2011; Linderholm, Lyngfelt, & Dueso, 2013; Teyssié, Leion, Schwebel, Lyngfelt, & Mattisson, 2011). Operation of mixed-oxide materials has been included with the material that has the highest concentration. For the rare cases where similar amounts of two materials have been used, it has been included with the most reactive of the two.

11.4

Ores and waste materials

11.4.1 Iron-based low-cost materials Early studies of iron ores showed low reactivity towards methane (Mattisson et al., 2001), whereas later studies with syngas have shown reasonably high reactivity (Leion, Mattisson, & Lyngfelt, 2009). The very low price of iron ores, in combination with decent reactivity towards syngas, makes iron ores quite interesting for use with

240

Calcium and Chemical-Looping Technology for Power Generation and CO2 Capture

solid fuels. Successful operation with iron ore and solid fuels has been reported from several studies (Gu, Shen, Xiao, Zhang, & Song, 2011; Mendiara, de Deigo, et al., 2014; Xiao, Song, Zhang, Zheng, & Yang, 2010). Also, operation with iron-based waste materials has been reported (Moldenhauer, Rydén, & Lyngfelt, 2012; Ortiz et al., 2011). In total, 404 h of operation with low-cost iron oxide material have been reported from four different pilots: • • • •

A 1-kW unit for solid fuels at South-East University, Nanjing (Gu et al., 2011; Wu, Shen, Hao, & Gu, 2010) (Chen et al., 2012; Song et al., 2013; Song, Wu, Zhang, & Shen, 2012) A 0.5-kW unit at CSIC (Ortiz et al., 2011) A 0.3-kW unit at Chalmers (Moldenhauer et al., 2012) A 0.5e1.5 kW unit for solid fuel at CSIC (Mendiara, Abad, et al., 2013; Mendiara, de Diego, et al., 2013; Mendiara, de Deigo, et al., 2014)

11.4.2

Ilmenite

Ilmenite is a combined oxide naturally occurring in the form FeTiO3, which is also the reduced form in CLC. The oxidized form is Fe2TiO5 þ TiO2. It has also been shown that there is a migration of Fe to the surface; thus in practice ilmenite is in part an iron oxide material (Adanez et al., 2010). A significant number of studies have used ilmenite, mostly Norwegian ilmenite. However, there are several possible sources for ilmenite. The important advantage of ilmenite is the low price in combination with having a reasonably high reactivity towards syngas and showing good fluidization behaviour. Estimations of the lifetime of ilmenite are around 700 h, although no real long-term operation has been accomplished. It would be relevant to say that ilmenite at present represents state of the art for solid fuels. In total, 810 h of operation with ilmenite ore have been reported from eight different pilots: • • • • • • • •

A 10-kW unit for solid fuels at Chalmers (Berguerand & Lyngfelt, 2008a,b; Berguerand & Lyngfelt, 2009a,b; Cuadrat, Linderholm, et al., 2011; Linderholm, Lyngfelt, Cuadrat, & Jerndal, 2012) A 140-kW unit at Vienna University of Technology (Kolbitsch et al., 2010; Kolbitsch et al., 2009c; Pr€oll, Mayer, et al., 2009) A 0.3-kW unit at Chalmers (Moldenhauer et al., 2012; Rydén et al., 2010) A 10-kW unit at University of Stuttgart (Bidwe et al., 2011) A 0.5e1.5 kW unit for solid fuels at CSIC (Cuadrat et al., 2011a,b; Cuadrat et al., 2012a,b; Mendiara, Izquierdo, et al., 2014) A 100-kW unit for solid fuels at Chalmers (Linderholm, Schmitz, Knutsson, K€allén, & Lyngfelt, 2014; Markstr€om, Linderholm, & Lyngfelt, 2012; Markstr€ om, Lyngfelt, & Linderholm, 2012; Markstr€om, Linderholm, & Lyngfelt, 2013a,b) A 25-kW unit for solid fuels at University of Hamburg (Thon, Kramp, Hartge, Heinrich, & Werther, 2014) A 0.3-kW unit modified for use with liquid fuels at Chalmers (Moldenhauer, Rydén, Mattisson, Younes, & Lyngfelt, 2014)

Oxygen carriers for chemical-looping combustion

241

In addition, a 1-MW chemical-looping pilot has been built and operated with ilmenite in partial CLC mode (Str€ ohle, Orth, & Epple, 2014), i.e. with support of air to maintain fuel-reactor temperature.

11.4.3 Manganese-based low-cost materials Although manganese ores are not as cheap as iron ores, they are still low cost. Manganese ores are available in several oxidation states, and often the manganese combines with other elements to form a number of different minerals. As Si and Fe are normally present in manganese ores, these could also potentially have CLOU properties. Laboratory testing (Arjmand, Leion, Mattisson, & Lyngfelt, 2014) as well as operation (Rydén, Lyngfelt, & Mattisson, 2011c) have verified that several ores have some limited CLOU properties. Operation with manganese ore shows that gas conversion can be significantly improved as compared to ilmenite. The drawback so far is that manganese ores tested often show dust formation at rates indicating insufficient lifetimes, e.g. (Linderholm et al., 2012; Rydén et al., 2011c). On the other hand, a manganese ore used by IFP in 100 h of operation showing no signs of fines formation is reported (Sozinho, Pelletant, Gauthier, & Stainton, 2012). A patent application for heat treatment of manganese ore has been made by IFP (Rifflart, Stainton, Perreault, & Patience, 2012), where the elemental analysis of an ore containing mainly MnO2 is given. As there are a variety of manganese ores with different compositions, it is not unlikely that materials with good reactivity, partial CLOU properties and sufficient lifetime should be possible to find. Another option could be mixing Mn ore with ilmenite. Preliminary results from a 100-kW unit at Chalmers indicate a quite significant improvement of gas conversion from adding 25% of manganese ore (Linderholm et al., 2014). In total, 148 h of operation with manganese ore have been reported from three different pilots: • • •

A 0.3-kW unit at Chalmers (Rydén et al., 2011b) A 10-kW unit for gaseous and solid fuels at IFP, Lyon (Sozinho et al., 2012) A 10-kW unit for solid fuels at Chalmers (Linderholm et al., 2014; Linderholm et al., 2012; Linderholm et al., 2013)

11.4.4 Other low-cost materials Limestone is a cheap and abundant material that can be sulphated to form CaSO4. CaSO4/CaS has been studied as a low-cost oxygen carrier for solid fuels (Deng, Xiao, Jin, & Song, 2009; Shen, Zheng, Xiao, & Xiao, 2008; Song, Xiao, Deng, Shen, et al., 2008; Song, Xiao, Deng, & Zhang, 2008; Song, Xiao, Deng, & Zheng, 2008; Tian & Guo, 2009; Tian, Guo, & Chang, 2008; Xiao et al., 2009). It has a uniquely high oxygen transfer capacity, 47%, but it has a thermodynamic constraint and cannot convert CO and H2 more than 98%e99%. There is also a risk of sulphur being lost, converting the oxygen carrier to CaO (Deng et al., 2009; Song, Xiao,

242

Calcium and Chemical-Looping Technology for Power Generation and CO2 Capture

Deng, Zheng, et al., 2008; Teyssié et al., 2011). Loss of sulphur is difficult to predict, as it takes place in the shifts between oxidizing and reducing conditions and will be very dependent on the process conditions, including temperature, fuel sulphur content and extent of fuel conversion. Release of sulphur from CaSO4 has previously been studied in relation to SO2 capture in fluidized beds, e.g. (Fern
andez, Lyngfelt, & Steenari, 2000; Hansen, Dam-Johansen, & Østergaard, 1993; Lyngfelt & Leckner, 1989). CaSO4/CaS has been used in a 3-MW chemical-looping pilot with more than 75 h of autothermal operation reported (Abdulally et al., 2014).

11.5

Concluding remarks

For the upscaling and commercialization of CLC, the availability of validated oxygen carriers is essential. This means finding the best materials based on cost and performance, and the need to validate these materials in long-term testing under conditions reasonably similar to large-scale facilities. There are many aspects to consider when choosing the best oxygen-carrier material, and it is too early to say where the optimal trade-off is when it comes to important aspects like price, reactivity and estimated lifetime. Rather, the development should focus on providing a portfolio of materials that can be suitable for different applications of chemical-looping technologies, or under different conditions. Economic optimizations, commercial experiences of the technology, technology developments or other changes in conditions may shift the emphasis on what is actually the best particle properties in relation to expected lifetime, reactivity, price, toxicity and suitable temperature range. A number of interesting materials with highly varying properties have been tested in actual operation, with promising results. Thus, there is already a portfolio of viable materials, and it can be expected that further development will give additional validation of both known and presently untested materials.

11.6

Future trends

Future development of chemical-looping oxygen carriers can be expected to involve more efforts related to: • • • •

Pilot operation and assessment of lifetime Assuring commercial availability at reasonable prices Oxygen-carrier materials with full or partial CLOU properties Low-cost oxygen-carrier materials relevant for solid fuels

The future development of oxygen-carrier materials will be highly dependent on where the focus will be in the scaling-up of chemical-looping technologies. Thus, the optimal materials for fluidized-bed systems are likely different from those of

Oxygen carriers for chemical-looping combustion

243

moving-bed systems or fixed-bed systems. The same applies to pressurized versus atmospheric systems and to the various chemical-looping processes for hydrogen production.

11.7

Sources of further Information and advice

A number of reviews related to oxygen carriers in CLC have previously been published. A review by Hossain and de Lasa (2008) is mostly focused on oxygen-carrier materials, and a review by Lyngfelt, Johansson, and Mattisson (2008) includes 600 oxygen-carrier materials and an update of this study includes another 300 materials (Lyngfelt & Mattisson, 2011). A very comprehensive review covering most aspects of chemical-looping technologies has also been published (Ad
anez, Abad, Garcia-Labiano, Gayan, & de Diego, 2012). Material overviews can also be found in PhD theses (e.g. Arjmand, 2014; Dueso, 2010; Jerndal, 2010; Johansson, 2007).

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an, P., & Celaya, J. (2007). Mapping of the range of operational conditions for Cu-, Fe-, and Ni-based oxygen carriers in chemical-looping combustion. Chemical Engineering Science, 62(1e2), 533e549. Abad, A., Ad
anez-Rubio, I., Gay
an, P., García-Labiano, F., de Diego, L. F., & Ad
anez, J. (2012). Demonstration of chemical-looping with oxygen uncoupling (CLOU) process in a 1.5 kWth continuously operating unit using a Cu-based oxygen-carrier. International Journal of Greenhouse Gas Control, 6, 189e200. Abad, A., Mattisson, T., Lyngfelt, A., & Johansson, M. (2007). The use of iron oxide as oxygen carrier in a chemical-looping reactor. Fuel, 86(7e8), 1021e1035. Abad, A., Mattisson, T., Lyngfelt, A., & Rydén, M. (2006). Chemical-looping combustion in a 300 W continuously operating reactor system using a manganese-based oxygen carrier. Fuel, 85(9), 1174e1185. Abdulally, I., Beal, C., Andrus, H., Epple, B., Lyngfelt, A., & White, B. (2014). Alstom’s chemical looping technology, program update. In Paper presented at the 11th annual conference on carbon capture utilization & sequestration Pittsburgh, Pennsylvania. Ad
anez, J., Abad, A., Garcia-Labiano, F., Gayan, P., & de Diego, L. (2012). Progress in chemical-looping combustion and reforming technologies. Progress in Energy and Combustion Science, 38(2), 215e282. Adanez, J., Cuadrat, A., Abad, A., Gayan, P., De Diego, L., & Garcia-Labiano, F. (2010). Ilmenite activation during consecutive redox cycles in chemical-looping combustion. Energy and Fuels, 24, 1402e1413. Adanez, J., de Diego, L. F., Garcia-Labiano, F., Gayan, P., Abad, A., & Palacios, J. M. (2004). Selection of oxygen carriers for chemical-looping combustion. Energy and Fuels, 18(2), 371e377.

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anez, J., Dueso, C., De Diego, L. F., García-Labiano, F., Gay
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