Selecting and Testing of Cement-Bonded Magnetite and Chalcopyrite as Oxygen Carrier for Chemical-Looping Combustion

Abstract

Combining iron and copper ores can generate an oxygen carrier that has a synergic effect of high temperature resistance and high reactivity. In this work, typical cements available in the market were studied as binders to bind magnetite and chalcopyrite to develop a suitable oxygen carrier for chemical-looping combustion (CLC). A first selection step suggested that an aluminate cement, namely CA70, could favor the generation of oxygen carrier particles having good crushing strength, good particle yield, and high reactivity. The CA70-bonded oxygen carrier was then subjected to cyclic tests with CH₄, CO, and H₂ in reduction and in air oxidation at temperatures of 850, 900, and 950 °C with gas concentrations of 5, 10, 15, and 20% in a batch-fluidized bed reactor. The increase in temperature promoted the fuel conversion. At 950 °C, the conversions of CH₄ and CO reached up to 80.4% and 99.2%, respectively. During more than 30 cycles, the oxygen carrier kept a similar reactivity to the fresh carrier and maintained its composition and physical properties. The oxygen transport capacity was maintained at 21–23%, and the phases were CuO, Fe₂O₃, Al₂O₃, and minor CaS. In the used sample, some grains were observed, but the morphology was not greatly changed. Agglomeration was absent during all the cycles, except for the deep reduction with H₂.

1. Introduction

Chemical-looping combustion (CLC) provides a novel carbon capture technology, which has the property of inherently separating CO₂ from combustion air and, thus, requiring low energy for CO₂ capture [1]. In CLC, the fuel combustion is split into a two-step reaction in two reactors, i.e., the fuel reactor and the air reactor, as described in Figure 1. In the fuel reactor, the oxygen carrier (MO_x, transition metal oxides) provides the oxygen for fuel combustion through reaction (1), where the fuel (C_nH_2m) is oxidized to H₂O and CO₂. The reduced oxygen carrier (MO_x−1) is then conveyed to the air reactor, where the oxygen carrier reacts with air oxygen through reaction (2) to regenerate to MO_x for the next cycle. Ideally, there are only H₂O and CO₂, where the latter can be easily captured through simple condensation; thus, intensive CO₂ separation is avoided in CLC.

The fuel reactor oxygen carrier reduction with fuel is as follows:

(2n + m)MO_x + C_nH_2m→(2n + m) MO_x−1 + mH₂O + nCO₂

(1)

The air reactor oxygen carrier regeneration is as follows:

(2n + m)MO_x−1 + (n + m/2)O₂→(2n + m)MO_x

(2)

The oxygen carrier is a cornerstone in CLC, as it functions to transport oxygen for fuel combustion [2]. The oxygen carrier should have properties of good reactivity, high oxygen transport capacity, environmentally benign, no toxicity, and a long lifetime [3]. In the case of large-scale, solid-fuel CLC, the ash residue from combustion needs to be removed from the system to avoid accumulation [4]. Together with the ash, loss of the oxygen carrier is inevitable. In this context, low-cost natural ores and byproducts from industrial process are preferred [5,6,7,8,9,10,11]. Several such materials have been demonstrated in CLC, which is equal to more than 2200 h of operations in continuous CLC systems having a thermal power of 0.5–1 MW_th [12,13]. Among them, copper ore showed high reactivity [14] and had potential for gas-phase O₂ release, which is a property called chemical-looping with oxygen uncoupling (CLOU) [15,16,17]. Actually, a study showed the property of gaseous oxygen release and solid fuel combustion with a copper ore in CLOU mode [14]. Various operation conditions were explored in this study, and slight agglomeration of the particles was noticed [14]. Studies with copper ore as an oxygen carrier investigating its agglomeration behavior were summarized in a review work [8], but recently, few reports can be found using copper ore as an oxygen carrier. A reason might be the limited accessibility of copper ore since it is less available and more expensive than iron ore and manganese ore [18,19,20]. On the contrary, iron ore is more available, more abundant, high-temperature resistant, and less expensive. However, it has lower reactivity than copper ore [21] since iron-based materials cannot release gaseous oxygen, and the CLC process must happen in a gas–solid reaction fashion [22]. Some methods, e.g., doping with foreign ions, were attempted to improve the performance of iron-based materials [23]. Another option is the combination of copper ore and iron ore to form a new oxygen carrier, which can benefit the synergic effect of copper and iron [21,24]. In this case, some new attempts have been carried out to combine copper ore and iron ore and redmud using a cement binder through simple mechanical mixing [25,26] or an extrusion–spheronization method [27]. In these works, aluminosilicate cements have been used to test their suitability as binders for oxygen carrier preparation from natural ores. However, in the market, there is a wide range of different types of cement, which are potential binders and need to be studied. Since these cements have different compositions, properties, and features, as well as costs, it can be an opportunity to find a less expensive and more suitable cement binder through a thorough exploration of a wide range of cements.

In this work, nine cement samples from three types are employed as binders to synthesize a magnetite-chalcopyrite oxygen carrier. The selection of the mechanical properties is based on 1400 cylindrical samples and 36 cubic samples obtained through a mold synthesis method. From this preliminary selection, an aluminate cement, called CA70, is selected as the best candidate for the bonded oxygen carrier preparation. The CA70-bonded oxygen carrier is then subjected to cyclic tests with CH₄, CO, and H₂ in a batch-fluidized bed reactor operating under various gas concentrations and temperatures. The characterization of the fresh and used oxygen carrier is performed through techniques such as XRD, SEM, XPS, and ICP-OES to evaluate the evolution of the physical and chemical properties of the oxygen carrier before and after the CLC cycles.

2. Experimental Section

2.1. Characterization Techniques

Various characterizations were employed to study the physical and chemical properties of the samples involved in the current work. X-ray diffraction (XRD) analysis was used to study the crystal phases of fresh and used samples. This analysis was carried out with a D8ADVANCE polycrystalline X-ray diffractometer, and the scanning range was 2θ = 5–85° with a step size 0.02°. The equipment was operated under Cu kα radiation (λ = 1.5418 Å), a voltage of 40 kV, and a current of 40 mA. Particle surface and morphology of the samples were analyzed through a scanning electron microscopy (SEM) technique with an Apreo C HiVac scanning electron microscope. Element contents (Fe, Cu, Al, Mg, Ca, and S) in the samples were determined through an inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis with an ElementAl analysis Agilent725 instrument. Surface ions (Fe²⁺, Fe³⁺) of the samples were analyzed with a VG multilab 2000 X-ray photoelectron spectroscopy (XPS) analyzer. The particle size distribution of the oxygen carrier samples was detected with a Winner2008A laser particle size analyzer. Crushing and cutting strength tests was performed using a Food Technology TMS-Pro strength tester, while the crushing strength of the particles was determined by fracturing 20 randomly selected particles with a Nidec-Shimpo FGJ-5 dynamometer. The bulk density of the particles was measured by weighing a known volume of sample following a WHO standard method [28,29]. The reactivity and oxygen transport capacity of the oxygen carriers were analyzed with a thermogravimetric analyzer (TGA) at 950 °C in a reducing atmosphere of 15% CO with 20% CO₂ or 15% H₂ balanced with N₂, as well as in an oxidizing atmosphere of 100% air.

2.2. The Magnetite, Chalcopyrite, and Cements

A magnetite iron ore (Ekou, Shanxi Province) and a chalcopyrite copper ore (Zhongtiaoshan, Shanxi Province), which both were crushed and sieved to a size smaller than 106 μm, were used as the active parts of the oxygen carrier in the current work. The compositions of these ores are shown in

Table 1. Phase composition of the magnetite and chalcopyrite.

Name	Type	Main XRD Phase
Magnetite	Fe-ore	Fe3O4
Chalcopyrite	Cu-ore	CuFeS2, FeS2, Cu4Fe5S8

, where it is seen that the magnetite had mainly Fe₃O₄, and the chalcopyrite had CuFeS₂, FeS₂, and Cu₄Fe₅S₈ as the major phases. After high-temperature treatment, the chalcopyrite could be oxidized to mainly CuO. The CuO and Fe₃O₄ were both active components for the oxygen carrier in the CLC process. In fact, iron ore was tested and demonstrated previously as an oxygen carrier [30]. In addition, chalcopyrite was tested with an anthracite coal in a batch-fluidized bed reactor [14]. The magnetite showed a medium reactivity and high resistance against agglomeration, whereas the chalcopyrite exhibited high reactivity but tended to agglomerate when the temperature was more than 950 °C. The combination of these ores can generate a bimetallic oxygen carrier and lead to a synergy effect, as well [21,26].

Cement, a common construction material, was proved as a good candidate for binding copper and iron ores to form an oxygen carrier for the CLC process [25]. However, in that work, only one cement was tested, but there are many types of commercial cement in the market. In this work, 9 cement samples from 3 categories (Portland, aluminate, and sulfoaluminate cement), which are available at a commercial scale, were tested as binders for synthesizing the magnetite-and-chalcopyrite oxygen carrier. As seen in

Table 2. Cements and compositions.

Category and Type		Main XRD Phase
Portland	PO32.5R	Ca2SiO4, Ca3SiO5
	PO42.5	Ca3SiO5, SiO2
	PO52.5	Ca3SiO5, SiO2
Aluminate	CA50-A600	CaAl2O4, Ca2Al2SiO7, CaAl4O7
	CA50-A700	CaAl2O4, Ca2Al2SiO7, CaAl4O7
	CA50-A900	CaAl2O4, Ca2Al2SiO7, CaAl4O7
	CA70	CaAl4O7, CaAl2O4
	CA80	Al2O3, CaAl2O4, CaAl4O7
Sulfoaluminate	SA42.5	Ca2SiO4, CaSO4, Ca4Al6SO16

, the Portland cement (PO32.5R, PO42.5, and PO52.5) had XRD phases from the Ca-Si-O system, the aluminate cement (CA50–A600, CA50–A700, CA50–A900, CA70, and CA80) compositions were mainly based on the Ca-Al-O system, and the sulfoaluminate cement (SA42.5) had Ca₂SiO₄, CaSO₄, and Ca₄Al₆SO₁₆ as the major phases.

2.3. Cement-Bonded Magnetite-Chalcopyrite Samples

2.3.1. Cylinder Samples for Preliminary Selection

Cement hardening is a crucial step for combining magnetite and chalcopyrite ores, as this step involves a hydration reaction that is usually quite slow and can be time-consuming for large-scale oxygen carrier manufacture. Cylinder-shaped samples were prepared to study the effect of aging time and to optimize the hydration reaction. The nine cements shown in Table 2 were used as binders for the preparation of magnetite-chalcopyrite samples through mechanical mixing and a mold method. In the mixture, the mass ratio chalcopyrite/magnetite was fixed to 2/8, according to previous works [21,25], while the proportion of cement was set to 20%. Water was added during the mixing, and its amount corresponded to a water/cement ratio of 1.65/1. After thorough stirring of the mixture of cement, water, magnetite, and chalcopyrite, the obtained slurry was poured into a cylindrical mold with length geometry of “diameter × height” of “Φ × H = 4 × 4 mm” to prepare cylinder samples, as shown in Figure 2. The mixture was placed in the mold and dried at room temperature under a natural atmosphere for 10 periods (0.5–12 days), where the 12th day corresponded to a hydration time for complete hardening of the cement, and the rest periods were used to find optimal hydration and aging time for the oxygen carrier preparation.

2.3.2. Cubic Samples for Secondary Selection

The preliminary selection based on cylindrical samples suggested that CA70- and SA42.5-bonded materials had the highest mechanical performances before calcination (detailed below in Section 4.1.1); meanwhile, PO32.5R was the most common cement and had the lowest price in the market. Therefore, these three cements were further considered in this section for the secondary selection, where the high-temperature calcination performance was mainly focused. Since a larger amount of particles was required in this step of selection, a cubic mold with a bigger volume (with “length × width × height” of “30 × 30 × 30 mm”, see Figure 3) was used for sample preparation. In total, eighteen sets of samples were prepared with a period of 0.5–6 days of natural drying (hydration reaction).

2.4. The CA70-Cement-Bonded Oxygen Carrier

Based on the preliminary and secondary selections, the CA70 cement was considered as the best binder for the magnetite-chalcopyrite oxygen carrier preparation, which is detailed below in Section 4.1.1. Thus, the copper ore weight percentage was set to 20 wt.%, while the iron ore amount was 80 wt.%, and the weight percentage of cement was 20 wt.% of the total solids mass. After being fully uniformed under continuous stirring, the mixture was poured into the cubic mold and naturally dried for two days (48 h). Different from the cylindrical samples, the cubic blocks were dried and placed in a muffle furnace to calcine at 500 °C for 5 h, and then 1000 °C for 10 h, to study the effect of calcination. After cooling down to room temperature, the calcined cubic samples were crushed and sieved to 106–300 μm particles, which were used as oxygen carriers in the current work. The main properties of the oxygen carrier are presented in

Table 3. Characterizations and properties of the CA70-bonded oxygen carrier.

XRD Phase	Fe2O3, CuO,
	Ca2Al2SiO7, CaSO4
Fe2O3, CuO (wt.%)	42, 2.73
Roc (wt.%)	23
Particle size (μm)	106–300
Bulk density (g/cm3)	1.65
Crushing strength (N)	2.27

, where the XRD phase, oxide content, oxygen transport capacity, bulk density, and crushing strength are shown. After calcination, the sulfur that originated from the chalcopyrite was not completely discarded, as CaS was detected. The crushing strength was 2.27 N, which is sufficient for use in fluidization systems [31].

2.5. Batch-Fluidized Bed Experiments

The reduction–oxidation cycle in the CLC process was simulated in a batch-fluidized bed reactor by switching the reacting environment between reduction (CH₄, CO, and H₂) and oxidation (air) under various temperatures (850, 900, and 950 °C) and gas concentrations (5, 10, 15, and 20%). A schematic description of the reactor is displayed in Figure 4. The reactor was made from high-temperature stainless steel and heated by a surrounding high-temperature furnace. Fluidizing, reacting gases and water steam entered the reactor from the bottom. After reacting with and passing through the bed material, the gas stream left from the reactor top, where low-granulometry particles were filtered with two identical filters. Afterward, the gas stream was cooled down by a cooler and then led to a gas analyzer to determine the gas concentration or passed through the stack before being emitted to the atmosphere. The gas analyzer (Gasboard-3100, Hubei Ruiyi Automatic Control System Co., Ltd., Wuhan, China) could measure the concentrations of CO, CO₂, CH₄, H₂, and O₂, and these concentrations were collected by a data logger and registered to a computer at a rate of one point per second. The performance of the oxygen carrier was studied by the reactions with CH₄, CO, and H₂ in the reduction step, while the gas concentration was varied from 5, 10, 15, and 20%. The lowest concentration (5%) was not used with CO and H₂ because of their high reactivity, which could lead to full conversion of these gases and was, thus, not convenient for data processing, as detailed below in Section 3. In the case of CH₄ and H₂, water steam was mixed with up to 50% H₂O with the fuels, whereas in the case of CO reduction, 20% CO₂ was mixed. The H₂O and CO₂ were used to either avoid carbon deposition from CH₄ and CO or to prevent deep reduction of the oxygen carrier in the cases of agglomeration and defluidization [32]. The total gas flow rate was kept at 200 L/h for both reduction and oxidation. An amount of 300 g of the CA70-bonded magnetite-chalcopyrite oxygen carrier was used as the bed. Three temperatures (850, 900, and 950 °C) were used to study the effect of temperature. In most of the experiments, ten cycles were performed under each reacting condition, and in total, more than 30 cycles were iterated.

3. Data Evaluation

In the reduction with CH₄ and CO, the gas conversion to CO₂ was calculated and used to evaluate the gas reactivity with the oxygen carrier. In the H₂ experiments, all the H₂ was converted to steam, which however, was not measured; thus, the H₂ conversion was not calculated. The gas conversion for CH₄ and CO was calculated with the following formulas:

$$X_{\left({\mathrm{CH}}_4\right)}=\frac{C_{{\mathrm{CO}}_{2,\mathrm{out}}}}{C_{{\mathrm{CO}}_{2,\mathrm{out}}}+C_{\mathrm{CO},\mathrm{out}}+C_{\mathrm{C}{\mathrm{H}}_{4,\mathrm{out}}}}$$

(3)

$$X_{\left(\mathrm{CO}\right)}=\frac{C_{{\mathrm{CO}}_{2,\mathrm{out}}}}{C_{{\mathrm{CO}}_{2,\mathrm{out}}}+C_{\mathrm{CO},\mathrm{out}}}$$

(4)

where X(i) is the conversion of gas i (i = CH₄ or CO), and C_i,out is the gas concentration of i (CH₄, CO, or CO₂) measured with the gas analyzer.

The oxygen carrier reactivity was evaluated using a mass-based conversion of the bed, which was calculated based on the measured gas concentration and oxygen balance during the reactions. The mass-based conversion during the CH₄, CO, and H₂ experiments was calculated through the equations below:

$${\omega }_{{\mathrm{CH}}_4}=1-{{\displaystyle \int }}_{t_0}^t\frac{n_{\mathrm{out}}{M}_{\mathrm{O}}}{m_{\mathrm{ox}}}\left(4C_{{\mathrm{CO}}_{2,\mathrm{out}}}+3C_{\mathrm{CO},\mathrm{out}}-C_{{\mathrm{H}}_{2,\mathrm{out}}}\right)dt$$

(5)

$${\omega }_{\mathrm{CO}}=1-{{\displaystyle \int }}_{t_0}^t\frac{n_{\mathrm{out}}{M}_{\mathrm{O}}}{m_{\mathrm{ox}}}{C}_{{\mathrm{CO}}_{2,\mathrm{out}}}dt$$

(6)

$${\omega }_{{\mathrm{H}}_2}=1-{{\displaystyle \int }}_{t_0}^t\frac{M_{\mathrm{O}}}{m_{\mathrm{ox}}}(n_{\mathrm{in}}{C}_{{\mathrm{H}}_2,\mathrm{in}}-n_{\mathrm{out}}{C}_{{\mathrm{H}}_2,\mathrm{out}})dt$$

(7)

$$n_{\mathrm{out}}=\frac{n_{{\mathrm{N}}_{2,\mathrm{in}}}}{1-{{\displaystyle \sum }}_i{C}_i}$$

(8)

$$n_i=n_{\mathrm{out}}·C_i$$

(9)

where ω_i is the mass-based conversion during the reaction between the oxygen carrier and gas i (i = CO₂, CO, CH₄, H₂ or O₂), n_in is the molar flow rate of the inlet gas, n_out is the molar flow rate of the dry gas leaving the reactor, m_ox is the mass of the oxygen carrier in the bed, and M_O is the molar mass of an oxygen atom.

4. Results and Discussion

4.1. Screening of the Cement Binder

4.1.1. Crushing Strength of the Cylindrical Samples

Four cylindrical samples were obtained each day (see Figure 2), and these samples were all subjected to crushing strength tests; thus, the average value of the four cylinders was used as the crushing strength for evaluation. Figure 5 shows the crushing strength of the nine cement-bonded materials as a function of aging time. It is seen that, with the increase in drying time (or aging time), the crushing strength of the samples showed an upward trend. In particular, among all the cements, the crushing strength of the cylinders bonded by CA70 and SA42.5 increased the most rapidly as a function of the aging time studied. The crushing strengths of these two samples already became higher than 10 N after two days of aging. Following 4 days of aging, the crushing strengths of the cylinders approached stable values of around 40 N and 15 N for CA70 and SA42.5, respectively. The big variation in the crushing strength might be a result of the wide range of netting force from the different components formed during the aging period with the cements. In addition, the PO32.5R cement is the most widely used in engineering and industry, as well as having the lowest market price. Therefore, these three cements, CA70, SA42.5, and PO32.5R, were all used in the secondary selection based on cubic samples, as discussed in the next section.

4.1.2. Cement Selection Based on Cubic Samples

Cutting and Crushing Strength of the Cubic Samples

The cubic samples obtained through the method described in Section 2.3.2 were calcined (500 °C for 5 h; then, 1000 °C for 10 h), cooled down, and then subjected to a cutting strength test. The cutting strength test was performed on a complete cubic sample and could reflect the binding strength between the raw fine particles; thus, it was considered as an indicator for the preliminary selection of the cement binders. After the cutting strength test, the samples were broken into two half-cubes, which were then used to test the crushing strength. Therefore, a total of 36 strength tests were conducted for 18 groups of samples, (three cements by six aging periods). The cutting strengths and crushing strengths are shown in

Table 4. Cutting strengths and crushing strengths of the cubic samples using various cements as binders.

Dry Time (Day)		0.5	1	2	3	4	5
	Cement Type
Cutting strength (N)	PO32.5R	654.2	691.2	349.1	762.6	>980	235
	CA70	>980	>980	>980	>980	>980	>980
	SA42.5	355.8	405.7	157.6	460	153.3	128.2
Crushing strength (N)	PO32.5R	>980	>980	>424.7	>980	>980	>980
	CA70	>980	>980	>980	>980	>980	>980
	SA42.5	696.7	713	>980	791	364.7	396.8

. As compared to the crushing strength, most of the samples showed smaller cutting strength values. Among the three cements, the crushing strengths and cutting strengths of the CA70-bonded samples were the highest, and the values were all beyond the upper limit of the dynamometer, i.e., greater than 980 N. However, the samples bonded with the PO32.5R and SA42.5 cements had relatively smaller cutting strengths and crushing strengths.

Particle Yield and Particle Crushing Strength

The crushed cubic samples from the above cutting and crushing strength tests were then smashed with an electric crusher using the same procedure and then sieved with stainless steel sieves to 106-300 µm. During the crushing, the gap between the crusher plates was fixed, and the cubic samples were passed through the crusher only once to try to maintain the same procedure for all the samples. Figure 6a shows the mass-based yield of particles with sizes from 106–300 µm. The yield increased as a function of aging time before 50 h and then became stable afterwards. It is clear that the CA70-bonded sample had the highest yield, i.e., around 16–20 wt.%, after around 48 h of aging and calcination, while PO32.5 and SA42.5 shared a similar yield of around 10 wt.%. Figure 6b displays the crushing strength of the particles, and there was not a clear tendency of crushing strength variation over aging time. However, the crushing strength of the CA70-bonded particles peaked around 1.2 N after 48 h of aging and calcination at 1000 °C.

4.1.3. Reactivity Comparison in TGA

The CA70, SA42.5, and PO32.5R cement-bonded oxygen carrier particles were also subjected to reactivity tests with CO in TGA. Figure 7 shows the weight loss of these oxygen carriers during reduction in TGA. All these samples had a final weight loss of around 87%, which suggested very similar oxygen transport capacities of the oxygen carriers. On the contrary, the reactivity with CO was different among the CA70-, SA42.5-, and PO32.5R-bonded oxygen carriers. The CA70-bonded sample had the highest reactivity, i.e., showing the steepest slope of the curve between 100–200 s. Therefore, the CA70-bonded particles were selected for further reactivity tests with various gases in the batch reactor.

4.2. Performance of Oxygen Carrier in the Batch Reactor

4.2.1. Reaction Progress with CH₄, CO, and H₂

Figure 8 shows an example of the reaction progress of the CA70-bonded oxygen carrier with CH₄, CO, and H₂ at 85 °C. As the reduction with CH₄ started, the concentration of CH₄ increased rapidly to a peak around 20%, which indicated the slippage of CH₄, probably through gas bypass in the bubbles rising from the dense zone of the fluidized bed [33,34]. Afterwards, this decreased gradually to about 5% from 200–600 s, while the CO₂ was relatively stable at around 30%. In the cases of CO and H₂, in the initial stages of reduction, i.e., 0–200 s and 0–300 s, respectively, the fuel components were completely converted. Therefore, the CO and H₂ were more reactive than CH₄ with the same bed amount. Thus, the rate of oxygen carrier conversion, i.e., the slope of the blue triangle labelled ω-t curve, with H₂ was significantly higher than CO and CH₄. However, after reacting with H₂, the bed agglomerated due to the high extent of the reduction. The tests with H₂ at other temperatures were cancelled, and only data for the CO and CH₄ experiments are shown and discussed below.

4.2.2. Influence of Temperature

Three temperatures (850, 900, and 950 °C) were used to study the influence on the combustion performance, as seen in Figure 9. Although the CH₄, CO, and H₂ were all tested at these three temperatures, the H₂ experiments are not shown here, as agglomeration occurred, as stated above in Section 4.2.1. In the case of the CH₄ experiments, the CH₄ concentration showed an overall upward trend at the three temperatures. The difference was that the CH₄ concentration rose rapidly at 850 °C, and this indicated that the reactivity was not enough to completely convert the CH₄ at this temperature. This was similar for the reaction at 900 °C; however, when the temperature was raised to 950 °C, the CH₄ increased much more slowly, displaying a much better conversion of CH₄. With the progress of the reaction, the CO₂ concentration in the outlet gas was kept above 8%, which was close to the theoretical value of 10% for complete CH₄ conversion. Meanwhile, the CH₄ concentration was kept below 2%. In the CO experiments, reduction with a gas mixture composed of 20% CO, 20% CO₂, and 60% N₂ was employed for evaluation. As seen in Figure 9b, when the temperature was 850 °C, the CO concentration was initially zero, and it started to rise at 100 s. At 900 °C, the time for CO concentration rising was 180 s, which was 80 s slower than that at 850 °C. When the temperature was further increased to 950 °C, it was hard to observe residual CO in the whole reduction stage, and the concentration of CO₂ was almost unchanged after reaching the peak value. Therefore, the temperature increase improved the reaction rate and enhanced CO conversion. The positive effect of temperature is also seen in Figure 9c with fuel conversion. Here, it can also be found that the CO had a much better conversion than CH₄ at all three temperatures.

4.2.3. Influence of Fuel Concentration

Figure 10 shows the effect of fuel concentration at 950 °C using CH₄ and CO as the reducing gases. In the case of the CH₄ fuel, the concentration of CH₄ was varied at 5, 10, and 20%. It can be seen from Figure 10a that the concentration of CO₂ in the whole reduction stage with 5% CH₄ was relatively stable, and the whole process had a very high methane conversion. However, the CO₂ concentration in the cases of 10% and 20% CH₄ decreased gradually with the reduction stage, and the CH₄ concentration at the reactor outlet increased gradually from 0 to 12% for 10% CH₄ and from 0 to 20% for 20% CH₄. However, the fuel conversion showed a slightly different trend, as the CH₄ concentration was varied. The level of 5% CH₄ had the highest fuel gas conversion, with 20% CH₄ having the middle gas conversion amount, whereas 10% CH₄ had the lowest conversion. In Figure 10b, the uses of 10, 15, and 20% CO as fuel also showed an effect on the gas distribution. The higher CO the concentration, the higher the CO₂ detected and the more residual CO in the reactor exit. However, in general, the fuel conversion was always equal to 1, especially when the oxygen carrier conversion value was higher than 0.92 when more oxygen was available in the oxygen carrier. The effect of fuel concentration on the gas conversion was a result of the fuel-to-oxygen carrier ratio. In general, the higher the fuel concentration, the lower the ratio and, thus, less fuel is converted. Therefore, the increase in gas concentration is usually accompanied with less fuel conversion. The 10% CH₄ was an exception, which could be attributed to artificial errors during the experiments.

4.2.4. Cyclic Stability of the CA70-Bonded Oxygen Carrier

The stability of the oxygen carrier reactivity was compared with the rate of oxygen carrier conversion in the cycles, as seen in Figure 11. In all the cycles shown in the figure, the reactivity of the oxygen carrier was quite similar to the first cycle, where the oxygen carrier was considered fresh. In particular, the CH₄ cycle had an oxygen carrier conversion rate of 1.0–1.3 × 10⁻⁴ 1/s, excepting cycle 5 where the rate was higher than the other cycles. This could be considered as an artificial error, considering that the other cycles had a very similar rate of conversion. The stability of the oxygen carrier reactivity was also found in the cycles with CO, as seen in Figure 11b, where the reactivity was around 1.3 × 10⁻⁴ 1/s for all the cycles shown.

4.3. Thermogravimetric Analysis for Fresh and Used Oxygen Carrier

Before the cycles in TGA started, i.e., temperature-ramping period, it was noted that the weight of the oxygen carrier decreased slowly when the temperature was increased to 950 °C. This could be attributed to the oxygen uncoupling behavior of CuO in the oxygen carrier, i.e., the gaseous O₂ release at high temperatures under an N₂ environment. However, the intensity of the O₂ release was not high, which meant that oxygen uncoupling was not relevant for this material. Typical TGA curves for the reaction between H₂ and the CA-70-bonded oxygen carrier before and after the batch reactor experiments are shown in Figure 12. With the increase in cycle number, the reduction extent of the fresh oxygen carrier tended to decrease, which meant there might be some structure reconstruction, a normal phenomenon observed with a synthetic Cu-based oxygen carrier [16]. This could be a result of Cu–Al interactions during the tests. For the used oxygen carrier samples, the degradation of the oxygen transport capacity was not obvious. Despite the degradation of the oxygen transport capacity, the reactivity of the oxygen carrier was not changed, as the slopes observed from the TGA curves in the reduction periods were similar. In the final cycle, it can be seen that both the fresh oxygen carrier and the used oxygen carrier kept high oxygen transport capacities, i.e., 23% and 21%, respectively.

4.4. XRD and XPS Analyses

The crystal phase compositions of the fresh and used oxygen carriers were identified using an XRD technique and are shown in Figure 13. The active components of both the fresh and used samples were mainly Fe₂O₃ and CuO, while the inert components were Al₂O₃ and SiO₂. A small fraction of CaS was also detected, and this component was also active in the CLC process through the redox pair CaSO₄/CaS; however, due to its low amount, it was considered not relevant for CLC in the current work. Further comparison of the phases suggested that the crystal phase compositions of fresh and used oxygen carriers had changes in the SiO₂. The used oxygen carrier had a higher intensity of SiO₂ compared to the fresh sample, and this could be partly related to the possible structure rearrangement of the material after cycles in the reactor.

The trivalent and divalent iron contents of the fresh and used oxygen carriers were obtained through XPS characterization and are compared in Figure 13, while the copper ions were not measured. However, there was little change in both of the irons before and after the reaction, which also reflected the stability of the oxygen carrier composition and was, thus, probably one reason for the stable reactivity of the oxygen carrier.

4.5. Evolution of Elements and Morphology

Figure 14 shows the contents of Ca, Fe, Cu, Mg, S, and Si in the fresh and used oxygen carriers determined with an ICP-OES technique. In the fresh sample, the iron content was around 42%, and the copper was around 3%. In the case of the used oxygen carrier after 30 cycles, the content of Cu stayed at the same level as that in the fresh sample, whereas the Fe increased to around 54%. This might be a result of the losses in oxygen or sulfur during the cycles and, as indicated in Figure 12, the change in the oxygen transport capacity from 23% to 21%.

Figure 15 displays the morphologies of the fresh and used oxygen carriers. The surfaces of fresh oxygen carriers were fluffy, and this was maintained for the used particles. A difference observed between the fresh and used particles was that, on the used particles, some grains could be found, i.e., the micrometer-scale, spheric-shaped sites on the particle’s surface shown in Figure 15b. This might suggest a migration of the component from the oxygen carrier bulk to the surface during the cycles.

4.6. Particle Size Analysis

The particle size distributions of the fresh and used samples are shown in Figure 16. The particle size of the fresh oxygen carrier was basically distributed in the range of 106–300 μm, and its volume fraction decreased with the increase in particle size. After the cycles in the batch reactor, the particle size of the oxygen carrier was relatively more uniform, but parts smaller than 106 μm and larger than 300 μm increased slightly, and the average particle size of the used sample was increased from 184.1 μm for fresh material to 204.4 μm for the used material. This might be caused by the attrition that generally happens in a fluidization system. In this case, the small particles generated due to attrition were elutriated, and the remaining particles were bigger. After a size distribution test, the remaining particles in the bed had a slightly higher average particle size than the measured average before the particle size distribution test.

5. Conclusions

Out of the nine cements, the aluminate CA70 cement was selected as the best binder for binding magnetite and chalcopyrite to generate oxygen carrier particles that had high physical properties and reasonable reactivity. During more than 30 cyclic tests with CH₄, CO, and H₂, the CA70-bonded oxygen carrier maintained stable physical and chemical properties. The CA70-bonded oxygen carrier had an oxygen transport capacity of 21–23%, although it was slightly decreased after the cycles. The increase in temperature from 850 to 950 °C improved the reactivity between the oxygen carrier and the fuel gases. H₂ was always completely converted under the conditions studied in this work and could lead to agglomerations due to deep reduction in the bed. The conversion of CH₄ peaked at 80.4% in the cast with 950 °C and 5% CH₄. Almost all the CO was converted during the whole reduction stage. No carbon deposition was found during the cycles. Although some migrated elements were found forming grains, particle agglomeration was not observed with CH₄ and CO, where deep reduction of the oxygen carrier was not reached.