Effect of hematite addition to CaSO₄ oxygen carrier in chemical looping combustion of coal char

Abstract

Chemical looping combustion (CLC) is a very promising technology, which combines the potential to reduce costs and the energy penalty significantly for CO₂ capture. For CaSO₄ oxygen carriers, their low reactivity and the evolution of sulfur species limit their practical application in CLC. In this study, CaSO₄ oxygen carriers decorated with hematite were prepared by a mechanical blending method with natural anhydrite as active support and hematite as additive. Experiments on the gasification and CLC of coal char in a steam medium were conducted in a laboratory-scale fluidized-bed reactor at atmospheric pressure. The effects of the reaction temperature, iron–sulfur ratio and cycle numbers on the performance of CaSO₄/Fe₂O₃ oxygen carriers were investigated in terms of carbon conversion and CO₂ yield as well as the rate of evolution of SO₂. Temperature and hematite favored an enhancement of the carbon conversion and CO₂ yield. The evolution of SO₂ increased with a rise in reaction temperature, but the peak time was delayed, which was ascribed to the sulfur-suppressing effect of hematite. The iron–sulfur ratio had slight influence on the product gas concentrations and there was no obvious decrease in the rate of evolution of SO₂ when the iron–sulfur ratio reached 0.13. Redox cycling tests showed that the cumulative rate of evolution of SO₂ in reduction/oxidation decreased first and increased subsequently with the increase of cycle number. Related chemical equations involving an oxygen transfer mechanism of iron-catalyzed reduction were proposed.

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1. Introduction

It is well known that fossil fuel consumption is the major source of anthropogenic CO₂ emissions. There are a number of techniques that can be used to separate CO₂ from combustion, but most need a large amount of energy for separation and compression.^1–3 Chemical looping combustion (CLC) is a new combustion technology where CO₂ is separated from flue gases without such an energy-consuming gas separation process.^4–6

CLC involves the use of an oxygen carrier, which transfers oxygen from the air to the fuel, and typically employs a dual fluidized-bed system: an air reactor (AR) and a fuel reactor (FR).⁷ In the FR, the fuel (natural gas, refinery gas, and synthesis gas from coal gasification) is oxidized by the oxygen carrier, thus producing an outlet gas of concentrated CO₂ and steam. After condensation, almost pure CO₂ can be obtained without any loss of energy during separation. The reduced oxygen carrier is transferred to the AR where it is oxidized, while emitting a large amount of heat and producing high-temperature gases consisting of N₂ and residual O₂. The regenerated oxygen carrier is recirculated to the FR for a new cycle, thereby avoiding direct contact between the fuel and the air. The reduction is either endothermic or slightly exothermic, depending on the type of oxygen carrier and fuel. The total amount of heat evolved from the two reactions is equivalent to that from normal combustion of the same fuel.^8–10

At present, many researchers pay more attention to the utilization of solid fuels (such as coal and biomass) in the CLC process^11,12 because solid fuels are more abundant and less expensive than gaseous fuels. There are two routes to realize CLC with solid fuels. Solid fuels can be used in CLC if they are first gasified by a separate gasification process and then oxidized in the FR. The disadvantages of the first method are the difficulties associated with gasification and the need for an energy-intensive air separation unit. The second method is to introduce solid fuels directly into the FR.⁸ Solid fuels should be gasified first by steam or CO₂ to produce syngas and the oxygen carrier subsequently reacts with syngas to produce CO₂ and H₂O. Despite its many technical obstacles, the second route is becoming more and more accepted.^13–16

The oxygen carrier should satisfy several requirements: low cost, high oxygen transfer capacity and selectivity toward CO₂ and H₂O, high reactivity during cycling tests, high mechanical strength, high resistance to agglomeration and sintering, and is environment-friendly.¹⁷ Recently, a CaSO₄ oxygen carrier has attracted increasing attention as a potential oxygen carrier for CLC, because of its high oxygen transport capacity and low cost. Alstom Power Co. Ltd.¹⁸ developed a limestone-based chemical looping process and a plant facility at a scale of 3 MW_th fuel power using CaSO₄ as an oxygen carrier is in the debugging and operating stage. Tian and Guo¹⁹ investigated the influence of the partial pressure of CO on the reduction behavior. Wang and Anthony²⁰ proposed solid-fuel gasification combined with CLC with a CaSO₄ oxygen carrier for clean combustion. A research group led by Shen and Xiao at the Southeast University in China also investigated the feasibility of CaSO₄ as an oxygen carrier,²¹ a reactivity test with gaseous and solid fuels,^22,23 and the reduction kinetics.^24,25 The thermodynamic and experimental results demonstrated that a CaSO₄ oxygen carrier may be an alternative oxygen carrier with high oxygen capacity. The main reactions occur in the FR (R1)–(R3) and the AR (R4) and are listed as follows:

CaSO₄ + 4H₂ → CaS + 4H₂O (R1)

CaSO₄ + 4CO → CaS + 4CO₂ (R2)

CaSO₄ + CH₄ → CaS + CO₂ + 2H₂O (R3)

CaS + 2O₂ → CaSO₄ (R4)

Currently, there are challenges that need to be overcome before the practical use of a CaSO₄ oxygen carrier in a CLC system. One challenge is the low reaction rate of CaSO₄ with fuels, particularly solid fuels, e.g. coal, which means the CLC system needs to be operated at high temperature, which will give rise to sintering and deactivation of the CaSO₄ oxygen carrier. Another challenge is the release of sulfur (mainly in the form of SO₂), which confirms that the potential release of sulfur results in a decline in the reactivity of CaSO₄ during the reduction/oxidation process. As demonstrated in many investigations,^19–23 the emission of sulfur that is involved with CaSO₄ and CaS varies with the operating conditions. In this study, the formation of SO₂ occurs mainly from the following side reactions during reduction (R5)–(R8) and oxidation (R7)–(R9).

CaSO₄ + H₂ → CaO + H₂O + SO₂ (R5)

CaSO₄ + CO → CaO + CO₂ + SO₂ (R6)

CaSO₄ → CaO + SO₂ + 1/2O₂ (R7)

3CaSO₄ + CaS → 4CaO + 4SO₂ (R8)

CaS + 3/2O₂ → CaO + SO₂ (R9)

Although sulfur-containing gases (such as SO₂ and H₂S) can have a positive effect on fuel gasification to some extent,²⁶ they will not only corrode the furnace and pipelines of a CLC installation but will also present difficulties for CO₂ capture. As a consequence, measures to increase the reactivity of the CaSO₄ oxygen carrier and minimize the release of sulfur species should be taken. Zheng et al.²² investigated the effects of temperature and gasification intermediates on the reduction and release of sulfur of a CaSO₄ oxygen carrier with bituminous Shenhua coal. Liu et al.²⁷ employed different commercial powders and sols as binders to increase the mechanical strength of CaSO₄ and the results of thermogravimetric analysis showed that the addition of SiO₂ could increase the reduction rate of CaSO₄ with gaseous fuels. Zhang et al.²⁸ studied the reactivity of CaSO₄ and the retention of sulfur with CaO and CaCO₃ as desulfurizers, and desulfurization tests indicated that CaO provided higher desulfurization efficiency at a higher temperature, higher pressure, and higher Ca/S ratio than CaCO₃. Song and Zhang^29,30 investigated the influence of Fe₂O₃ on a CaSO₄ oxygen carrier. Zhang²⁹ thought that the presence of Fe₂O₃ could maybe resist sintering and agglomeration on the surface of Fe₂O₃/CaSO₄ particles; however, Song³⁰ proposed that it was not the desulfurization capacity of iron oxide that lowered the emission of sulfur species but the suppression of side reactions. In our research team, using a Taguchi robust design method, binder-supported CaSO₄ oxygen carriers prepared from calcined CaSO₄, highly sticky pseudo-boehmite (SB powder), acetic acid and water were investigated by intuitive analysis with crushing strength and conversion as target functions and optimal extrusion conditions were determined.³¹ In addition, there are very few studies that report the reactivity enhancement and the reduction of the release of sulfur of CaSO₄ oxygen carriers, particularly in coal-fueled CLC systems.

Many research studies^32–34 have reported on the use of iron catalysts for the gasification of solid fuels, because iron is inexpensive but is one of the most promising catalysts. Asamil et al.³² investigated the gasification of brown coal and char with CO₂ using iron catalysts precipitated from an aqueous solution of FeCl₃. Comparison of the initial rates of uncatalyzed and catalyzed gasification revealed that the addition of iron could lower the reaction temperature by 120 °C and result in complete gasification within a short reaction time. Furthermore, previous studies³⁴ indicated that iron catalysts provide higher carbon conversion and hydrogen production than alkali metal-based catalysts.

In the field of catalytic reduction of CaSO₄ to CaS, Fe₂O₃ was proved to be an efficient catalyst.^35,36 In our previous study,³⁵ CaSO₄ decorated with Fe₂O₃ was tested in a fixed-bed reactor and the result showed that Fe₂O₃ greatly improved the reactivity of CaSO₄, and CaS was the only reduction product. Li et al.³⁶ investigated the catalytic effect of various iron catalysts on the reduction of CaSO₄ to CaS and discovered that Fe₂O₃ could effectively promote the reduction reaction.

It is well known that mixed complex oxygen carriers may sometimes provide better properties than those of individual oxygen carriers.^37–40 Based on the obvious catalysis of Fe₂O₃, the combination of active CaSO₄ with Fe₂O₃ as additive may create a synergistic effect to increase the reactivity of CaSO₄ and reduce the emission of sulfur-containing gases, which is similar to the synergistic effect that is created by NiO/Al₂O₃,³⁹ CoO/NiO³⁸ and CuO/Fe₂O₃.^39,40 In this study, experiments on the gasification and CLC of char in a steam medium in a batch fluidized-bed reactor were conducted for investigating the reactivity of CaSO₄ decorated with hematite. The effects of the reaction temperature, iron–sulfur ratio (i.e. Fe₂O₃ loading content) and cycle numbers on carbon conversion, CO₂ yield, and the rate of evolution of SO₂ as well as the synergistic effect were investigated. Furthermore, to determine the reaction mechanism, fresh and reacted oxygen carriers were also characterized by X-ray diffraction (XRD), field emission scanning electron microscopy equipped with an energy-dispersive spectrometer (FESEM-EDS), and pore structure analysis.

2. Experimental section

2.1. Materials

The particles of CaSO₄ oxygen carrier that were used were produced from natural anhydrite ore, which were further crushed by a pulverizer and sieved to a size range of 180–250 μm. The natural anhydrite ore was composed largely of CaSO₄ and a small proportion of other impurities, as shown in Table 1. The apparent density and bulk density of the particles were 2950 and 1510 kg m⁻³, respectively.

Table 1 Composition of natural anhydrite (wt%)

Chemical composition	Value
CaSO4	95.02
CaO	1.25
SiO2	0.65
MgO	0.46
Al2O3	0.25
TiO2	0.05
Fe2O3	0.02
Crystallization water	2.30

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Mechanically mixed samples were prepared by mixing anhydrite particles with particles of hematite. High-quality hematite from Australia was used as an additive and the related composition is given in Table 2. Due to the density of hematite particles being about twice that of anhydrite, hematite with a particle diameter of 125–180 μm was chosen in order to avoid serious stratification phenomena in a fluidized bed. Coal char selected as the solid fuel sample was prepared in an electric muffle furnace by calcining bituminous Shenfu coal. It was also sieved to a particle diameter of 400–500 μm. Proximate and ultimate analyses of Shenfu coal and coal char are summarized in Table 3. Coal ash was composed of 43.98% CaO, 15.68% SiO₂, 14.82% Fe₂O₃, 10.42% SO₃, 9.46% Al₂O₃ and some other minor phases through by X-ray fluorescence (XRF) analysis.

Table 2 Composition of hematite (wt%)

Chemical composition	Value
Fe2O3	87.40
SiO2	6.52
Al2O3	5.39
TiO2	0.23
MgO	0.16
Other	0.30

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Table 3 Proximate and ultimate analyses of Shenfu coal and coal char

Parameter	Coal	Char
Proximate analysis (wt%, as-received basis)
Moisture	7.34	5.12
Volatiles	35.58	4.32
Ash	4.65	10.32
Fixed carbon	52.43	80.24
Ultimate analysis (wt%, dry basis)
C	72.41	84.98
H	4.84	0.88
O	21.54	14.32
N	0.84	0.38
S	0.37	0.32

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2.2. Experimental setup and procedure

Fig. 1 shows the experimental setup used for testing oxygen carriers. It consisted of a gas feed, a fluidized reactor, a tube furnace, a cooler and a gas analysis system. A porous distributor plate was located in the central stainless-steel tube (I.D. = 40 mm, length = 900 mm). The reactor was heated in an electric furnace, and the furnace temperature was controlled by a K-type thermocouple between the reactor tube and the heater, while the reaction temperature was monitored by another K-type thermocouple inside the oxygen carrier particles. The reactor had two connected pressure taps in order to measure the differential pressure in the bed and monitor the fluidization state. In the tests, coal particles were fed into the fluidized bed with the help of a fuel chute. The upper part of the chute had a valve system that created a reservoir in which the coal particles were placed and later pressurized with N₂ to ensure rapid feeding of the coal particles.

Fig. 1 Schematic of the batch fluidized-bed reactor system.

The flow rates of the fluidizing gas and gasification agent were measured by mass flow controllers. The steam generator was composed of a TBP-5002 constant-flow-type pump and a cast-steel heater, and the mass flow of steam was controlled precisely by adjusting the amount of deionized water. The product gases from the reactor flowed through a cooler filled with CaCl₂ desiccant, which could condense steam without the adsorption of acid or neutral gas, and then were sent to the gas analyzers. Three gas analyzers continuously measured the gas composition at each time. Concentrations of CO₂, CO and SO₂ gas as well as the gas flow at the outlet were measured by a GA-21 Plus flue gas analyzer, and then the product gases were divided into two steams by a three-way valve. One gas stream was tested using a GASboard-3100 coal gas analyzer to determine the concentration of dry H₂; the other gas stream was tested using a Geotech Biogas Check analyzer to measure the concentrations of CH₄ and O₂.

2.3. Experimental procedure

The experiments were performed in a laboratory fluidized-bed reactor under atmospheric pressure using individual CaSO₄ or a mixture of CaSO₄/Fe₂O₃ as an oxygen carrier. In each run, 100 g of quartz sand with a size range of 1500–2000 μm was added above the porous plate while preheating and uniformly distributing the reaction gas through the bed. Then, a sample of 50 g CaSO₄ or CaSO₄ as well as a certain amount of hematite was placed on the quartz bed with the internal thermocouple located in the middle of the layer. During the preheating period, the reactor was purged with a steam/N₂ gas flow. After the temperature reached the desired temperature and became stable, 1 g coal char was quickly introduced into the bed under the pressurizing effect of an instantaneous addition of nitrogen and then the reduction process began. When the reduction was finished, the gas flow of steam was turned off and the system was purged with a N₂ flow until the product gases were cleared away, after which N₂ was replaced by an oxidizing gas for the subsequent oxidation period. A mixture of O₂ and N₂ (10% O₂/N₂) was introduced during oxidation until the outlet O₂ concentration reached the initial value, and then the gas was switched to N₂ for the next cycling test. A lower O₂ concentration was used instead of air to avoid a large increase in temperature because of the heat generated from intense exothermic oxidation, because there was no installation to cool the reactor in the present setup.

When all tests were finished, the furnace was shut down. A sample was cooled in a N₂ flow to ambient temperature and collected for further analysis. The N₂ concentration was not displayed in this study and the specific experimental conditions are shown in Table 4.

Table 4 Experimental conditions

Oxygen carrier	CaSO4 or CaSO4/Fe2O3
Pressure (atm)	1
Reaction temperature (°C)	850, 900, 925, 950, 975
Particle size (μm)	CaSO4: 180–250
	Fe2O3: 125–180
Particle mass (g)	CaSO4: 50
	Fe2O3: 3.1, 6.5, 10.4, 14.0
Steam gas flow (g min^−1)	1.2
N2 gas flow in reduction (ml min^−1)	300
Reduction time (min)	30, 50
Oxidation gas	O2/N2 = 10/90%
Oxidation gas flow (ml min^−1)	1150
Oxidation time (min)	50
Sweeping gas flow (ml min^−1)	1000

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The compositions of fresh and reduced samples were determined by the XRD technique (X'pert Pro, Holland) using copper Kα radiation over a 2θ range of 15°–85°. The surface morphologies and elemental distributions of samples were measured by FESEM-EDS in a microscope system (Sirion 200, Holland). The pore structure properties of samples were measured by nitrogen adsorption/desorption isotherms at 77 K with a Micromeritics ASAP 2020 instrument. The surface area and pore volume were calculated from the Brunauer–Emmett–Teller (BET) equation and Barrett–Joyner–Halenda (BJH) method, respectively.

2.4. Data evaluation

In order to quantitatively describe the relationship between the gas composition and time, the rate of evolution of the gaseous product C_i(t) is the molar ratio of gaseous product species i per unit time to the total amount of carbon introduced into the FR and is calculated as follows:

The cumulative amount of gaseous product X_i(t) is an integral result of the evolution rate and is defined as follows:

The carbon conversion X_C is the ratio of carbon consumed to carbon introduced into the FR and is defined in the following equation:

The calculation of the carbon conversion X_C, which is an indirect measurement, is based on the main carbon-containing products (CO₂, CO and CH₄), so it ignores other carbon-containing species such as tar, hydrocarbons and residual carbon in ash. For this reason, the carbon conversion X_C that is given in the present study has to be understood and treated only as an apparent value. Moreover, on the basis of the conservation of mass, the carbon residue R_C could be calculated as follows:

R_C = 1 − X_C (4)

However, the measurement of carbon residue is inevitably interfered with the reduced oxygen carrier. Therefore, the value of R_C after an individual char gasification step is used instead. Moreover, the selectivity for the formation of CO₂ Y_CO2(t) means the fraction of CO₂ in the carbon-containing species (CO₂, CO and CH₄) leaving the FR. This evaluation index gives a measure of the degree of completion of the combustion process in the FR and is defined as follows:

The CO₂ yield η_CO2 is the ratio of carbon converted to CO₂ to carbon introduced into the FR. It is another way of determining the degree of completion of the combustion process from aspects of the products and is calculated as follows:

For the purpose of convenience, the iron–sulfur ratio can be expressed as the amount of hematite that was added divided by the amount of CaSO₄, which is written as Fe/S. Furthermore, in order to obtain reliable data, some repeated experiments were conducted.

3. Results and discussion

3.1. Performance of CaSO₄ oxygen carrier with coal char

The masses of coal char and CaSO₄ were 1.0 and 50 g, respectively. With a reaction time of 50 min, the influence of the temperature (900, 925, 950 and 975 °C) on the product gas concentrations was investigated. Profiles of the rate of evolution of product gas as a function of time at typical reaction temperatures of 900 and 975 °C are shown in Fig. 2. The peak times of product gases and variation trends were in line with experimental results from char gasification, whereas the peak value of the rate of evolution of CO₂ was higher than that in char gasification experiments. This showed that char was gasified by steam, and simultaneously the gasification products were oxidized by the CaSO₄ oxygen carrier particles to CO₂ and steam. The concentration of CO₂ was 40.56% at 900 °C and increased to 68.13% at 975 °C, whereas the concentration of H₂ was reduced to 20.16% from 50.96%. On the one hand, this is an important factor that influenced the reactivity of the char gasification and CaSO₄ reduction reactions. On the other hand, the existence of a large amount of H₂ and CO indicates that the reaction rate of the CaSO₄ oxygen carrier with the products of char gasification was lower. Therefore, further work is needed to enhance the reactivity with the aim that CaSO₄ is utilized in a large-scale CLC system.

Fig. 2 Rates of evolution of product gases and gas concentration profiles during the reduction of CaSO₄ with coal char in steam medium: (a) 900 °C and (b) 975 °C.

3.2. Performance of CaSO₄/Fe₂O₃ oxygen carrier with coal char

3.2.1 Effect of reaction temperature. The effect of the reaction temperature and reaction time on the reduction reaction of a CaSO₄ oxygen carrier decorated with hematite was investigated at temperatures of 850, 900, 925, 950 and 975 °C. The mass of hematite was 6.5 g, and the reaction time was 30 min. The other parameters such as char mass, CaSO₄ mass and steam gas flow were kept constant (Table 4).

Fig. 3 displays the rates of evolution of product gases as a function of time during the reduction of CaSO₄/Fe₂O₃ with coal char at 850 and 950 °C. By comparison of Fig. 3a and b, the peak evolution of product gases appeared at the beginning and the rate of evolution of CO₂ increased as the reaction temperature increased. Moreover, as soon as the temperature was above 950 °C, the results showed that char gasification was the controlling step in the reduction of CaSO₄/Fe₂O₃ with coal char and the reduction of CaSO₄ occurred at a sufficient rate to oxidize most of the gasification products to CO₂ and steam, which was consistent with the conclusions of other researchers.^22,30 By comparison with Fig. 2, the reactivity of a CaSO₄ oxygen carrier modified by hematite was significantly increased. When the reaction temperature was below 900 °C, the rate of reduction of CaSO₄ by gasification products was lower than that for char gasification. This led to high concentrations of H₂ and CO in the flue gas (Fig. 4). As the reaction temperature increased, the chemical reaction rate constants of both char gasification reaction and CaSO₄ reduction, particularly the latter, increased sharply. This difference in growth rates increased the partial pressure of the gasification medium (CO₂ and H₂O), which might accelerate the char gasification process.

Fig. 3 Rates of evolution of product gases as a function of time during the reduction of CaSO₄/Fe₂O₃ with coal char in steam medium: (a) 850 °C and (b) 950 °C.

Fig. 4 Concentrations of product gas as a function of temperature during the reduction of CaSO₄/Fe₂O₃ with coal char in steam medium.

As shown in Fig. 5, the rate of evolution of H₂ first increased to a peak within 5 min and decreased markedly with the reaction temperature, which was also verified in the relevant literature.^22,41 Particularly at temperatures of 925–975 °C, the rate of evolution of H₂ was very low and H₂ was carried away by the flow of flue gas due to a shorter contact time between the char gasification products and CaSO₄. With the end of the char gasification reaction, the rate of evolution of H₂ was reduced to zero. As the complicated reactions proceeded, the surface of CaSO₄ particles would be gradually covered by generated CaS and small amounts of CaO, and the gaseous products of char gasification first got through the CaS/CaO product layer and then reacted with the CaSO₄ oxygen carrier. Thus, the reaction could be divided into two steps: an earlier stage controlled by chemical resistance and a later stage controlled by chemical resistance and diffusion resistance, and the diffusion resistance increased.

Fig. 5 The rate of evolution of H₂ as a function of time during the reduction of CaSO₄/Fe₂O₃ with coal char in steam medium.

The rate of evolution of CO as a function of time during the reduction of CaSO₄/Fe₂O₃ with char is shown in Fig. 6. At lower temperatures (850–900 °C), the time to reach a peak was 4–5 min; however, at higher temperatures (925–975 °C), the time was only 3 min. Because of the lower reactivity of the CaSO₄ oxygen carrier at 850 °C, part of the unreacted CO was carried away by the flow of flue gas. Within the temperature range of 900–975 °C, the reduction in C_CO(t) increased with the increase of temperature and C_CO(t) decreased to zero in the 15^th min. Compared to Fig. 5, C_CO(t) and C_H2(t) were basically the same at temperatures of 900–975 °C. Considering that the rate of evolution of H₂ was far greater than that of CO during char gasification, the reactivity of H₂ with the CaSO₄ oxygen carrier was considerably higher than that of CO. This was in accordance with previous thermodynamic analysis and experimental studies.^22,42

Fig. 6 The rate of evolution of CO as a function of time during the reduction of CaSO₄/Fe₂O₃ with coal char in steam medium.

The rate of evolution of CO₂ as a function of time during the reduction of CaSO₄/Fe₂O₃ with char is shown in Fig. 7a. At lower temperatures (850–900 °C), the time to reach a peak was 9–10 min; however, at higher temperatures (925–975 °C), the time was at least 1–2 min less, which indicates that an increase in temperature was beneficial for the formation of CO₂. The cumulative rate of evolution of CO₂ (i.e. X_CO2(t)) as a function of time is presented in Fig. 7b. Equilibrium would be reached when dX_CO2(t)/dt was reduced to zero. As shown in Fig. 7b, the higher the temperature, the shorter was the time to reach equilibrium. The reason for this was that with regard to the chemical kinetics, a rise in the temperature led to an increase in the chemical reaction rate constants of char gasification and reduction.

Fig. 7 The rate of evolution (a) and cumulative rate of evolution (b) of CO₂ as a function of time during the reduction of CaSO₄/Fe₂O₃ with coal char in steam medium.

The release of sulfur from coal/char was considerably lower in comparison with that from side reactions.²² In this study, the emission of SO₂ was dominant, of which the cumulative amount was nearly 20 times that of H₂S. Moreover, H₂S was unstable and could be oxidized to SO₂ by the remaining CaSO₄. Thus, the rate of evolution of H₂S is not displayed. As indicated in Fig. 8a, the curves of SO₂ evolution have single-peak characteristics and the peak time was delayed with a rise in the reaction temperature, which was completely the opposite of the results for an individual CaSO₄ oxygen carrier. Therefore, it is reasonable to infer that the sulfur-suppressing effect of hematite, which was confirmed by Song et al.,³⁰ resulted in a delay in the peak time of SO₂ release. From Fig. 8b, X_SO2(t) increased with a rise in temperature and X_SO2(t) at 975 °C was an order of magnitude higher than at 850 °C. Elevated temperatures not only increased the reaction rate of the principal reaction, but also increased that of side reactions.

Fig. 8 The rate of evolution (a) and the cumulative rate of evolution (b) of SO₂ as a function of time during the reduction of CaSO₄/Fe₂O₃ with coal char in steam medium.

Fig. 9 illustrates the carbon conversion, the carbon residue, selectivity for the formation of CO₂ and CO₂ yield versus temperature. Among these, the carbon conversion, selectivity for the formation of CO₂ and CO₂ yield increased with an increase of temperature in the temperature range from 850 to 950 °C. The growth rates of the selectivity for the formation of CO₂ and CO₂ yield were significantly greater than that of carbon conversion. As the temperature increased successively, it promoted char gasification, but inhibited the reduction of CaSO₄, because sintering and agglomeration of the oxygen carrier at 975 °C led to a decline in the reactivity of the CaSO₄ oxygen carrier, which has been demonstrated by our and other research groups.^21,31 Although the carbon conversion, selectivity for the formation of CO₂ and CO₂ yield at 975 °C were still higher than at 950 °C, the growth rate was very low. The main reasons for this growth arise from two factors: one is an excess of the CaSO₄ oxygen carrier during the first reduction, so that sintered CaSO₄ could be ignored, and the other is that more CO₂ was generated from the exothermic water–gas shift reaction (R10).

Fig. 9 Effect of reaction temperature on carbon conversion, carbon residue, selectivity for formation of CO₂ and CO₂ yield.

3.2.2 Effect of iron–sulfur ratio. The iron–sulfur ratio was changed from 0.062 to 0.280 (i.e., the corresponding mass of hematite was increased from 3.1 to 14.0 g) to investigate the effect on the combined process of char gasification and CaSO₄ reduction by gasification gases in the FR. The other operating conditions were kept constant: reaction temperature (950 °C), mass of char (1.0 g), mass of CaSO₄ oxygen carrier (50 g), flow rate of steam (1.2 g min⁻¹), and reaction time (30 min). Fig. 10 shows variations in product gas concentrations at four different Fe/S ratios. The CO₂ concentration increased steadily, which was ascribed to the greater amount of hematite, whereas the CO, H₂ and CH₄ concentrations were stable or displayed a slight decline. Therefore, there was slight influence on the product gas concentrations due to the four Fe/S ratios, which was in accordance with the conclusions of Li et al.³⁶

Fig. 10 Effect of iron–sulfur ratio on product gas concentrations.

The effect of the Fe/S ratio on the rate of evolution and cumulative rate of evolution of SO₂, as shown in Fig. 11, shows that the rate of evolution of SO₂ gradually reduced with an increase in Fe/S ratio, and the range of ratios with the biggest decline was 0.062–0.13. However, within the range of 0.13–0.28, the drop was very limited. Hematite was first reduced by gasification gases (H₂, CO and CH₄) and then the formed iron-based oxides catalyzed the reduction of CaSO₄ to CaS, which promoted the formation of CO₂. Correspondingly, the side reaction of CaSO₄ to give SO₂ may be suppressed. However, when the mass of the catalyst reached the maximum value for a monolayer dispersion on the surface of CaSO₄, further increases in the rate of formation of CO₂ and decreases in that of SO₂ were limited to a certain extent. Therefore, there was no obvious influence on the suppression of the release of sulfur from the CaSO₄ oxygen carrier when the Fe/S ratio reached 0.13. Considering the variations in product gas concentrations, especially that of SO₂, the reasonable Fe/S ratio was 0.13.

Fig. 11 Rate of evolution (a) and cumulative rate of evolution (b) of SO₂ as a function of the Fe/S ratio during the reduction of CaSO₄/Fe₂O₃ with coal char in steam medium.

3.2.3 Effect of cycle number. Five reduction/oxidation cycling tests were carried out when the Fe/S ratio was 0.13. The concentrations of flue gas as a function of the cycle number are shown in Fig. 12. As the number of cycles increased, the CO₂ concentration diminished gradually, and accordingly the CO concentration increased and its growth rate was significant. It is evident that the reactivity of CaSO₄ after reduction decreased gradually in the cycling test, which was inconsistent with the results for a Ni-based oxygen carrier.⁴¹ Possible reasons for the decrease are as follows: first, the release of sulfur species during the cycling test led to an irreversible loss of oxygen atoms in the CaSO₄ oxygen carrier and a decrease in oxygen transport capacity; second, due to the attachment of ash in char onto the outside surface of CaSO₄, a dense oxidation film was formed, which resulted in an increase in the gas diffusion resistance for gasification products; last, it is also entirely possible that the Ca-based oxygen carrier was sintered and fractured because of the higher temperature and fluidizing velocity during the long cycling tests.

Fig. 12 Effect of cycle number on the product gas concentrations.

After a 5-cycle test, the total concentration of combustible gases reached about 20%. One possibility to address this incomplete gas conversion would be to separate combustible gases from CO₂ in connection with the liquefaction of CO₂. However, suitable processes and costs for achieving this are not well established. Another method of “oxygen polishing” was proposed by Professor Lyngfelt,⁴³ which meant that the combustible gases that remained in the gas from the FR were oxidized immediately after the cyclone by adding a stream of oxygen.

Fig. 13 illustrates the cumulative rates of evolution of SO₂ as a function of the cycle number during the 5-cycle test. The cumulative rates of evolution of SO₂ in reduction were significantly lower than those in oxidation. Although the O₂ concentration in the oxidizing gas was artificially reduced from 21% in air to 10%, the exothermic nature of the oxidation reaction still led to the release of heat which aggravated sintering of the oxygen carrier and promoted the side reactions. On the other hand, the cumulative rates of evolution of SO₂ in reduction or oxidation decreased first and increased subsequently with an increase in the cycle number. Combined with the experimental conclusions in Section 3.2.1, the decrease in the cumulative rates of evolution of SO₂ was ascribed to the presence of hematite and a detailed analysis is carried out in the following section with the characterization of oxygen carriers.

Fig. 13 Cumulative rates of evolution of SO₂ as a function of the cycle number.

3.3. Characterization analysis

3.3.1 Phase characterization. The results of XRD analysis of fresh anhydrite and reacted CaSO₄ decorated with 6.5 g hematite at different reaction temperatures are shown in Fig. 14. The presence of anhydrite (CaSO₄) as the main crystalline phase in the fresh natural anhydrite was clearly demonstrated. Supposing that all the char reacted with the CaSO₄ oxygen carrier, the mass of CaSO₄ that was involved in the reaction only occupied about 11% of the whole CaSO₄ oxygen carrier. Thus, the peak intensity of CaS was significantly lower that that of CaSO₄. In addition, considering that CaS in the sample could be easily oxidized by air at room temperature, the peak intensity of CaS was generally low in XRD patterns. Therefore, the ratio of the peak intensity of CaS to that of CaSO₄ could not be used to determine the extent of the reduction of CaSO₄, but the ratio of the peak intensity of CaO to that of CaSO₄ could be used to determine the extent of the side reactions and the ratios with increasing temperature were 0.0192, 0.0204, 0.0347, and 0.0381. Hence, the side products (CaO and SO₂) increased gradually with an increase in temperature, which was in line with the analysis of SO₂ concentration in Section 3.2.1. Within the temperature range of 900–950 °C, only Fe₃O₄ was detected after the reduction, which proved that all the hematite was involved in the reduction reaction. However, the existence of Ca₂Fe₂O₅ at 975 °C suggested that a solid–solid reaction occurred between hematite and CaO. Although Ca₂Fe₂O₅ as an inert support could increase the specific surface area and mechanical strength of oxygen carrier particles, oxygen transfer or catalysis by Fe₂O₃ was gradually reduced or even disappeared. In other words, 975 °C was not appropriate as a reduction temperature for char-fueled CLC with a CaSO₄/Fe₂O₃ oxygen carrier.

Fig. 14 XRD patterns of Ca-based oxygen carriers: (a) anhydrite, (b)–(e) CaSO₄ with 6.5 g hematite at different reaction temperatures.

The results of XRD analysis of the reduction products of CaSO₄ decorated with different loading contents of Fe₂O₃ and the oxidation products of CaSO₄ decorated with 6.5 g hematite after a 5-cycle test at 950 °C are shown in Fig. 15. From parts (a) to (c) of Fig. 15, the influence of the Fe/S ratio on the reduction products was not significant and hematite was completely reduced to Fe₃O₄. After the 5-cycle test, hematite could still exist in the oxygen carrier as Fe₂O₃. As a consequence, hematite could continuously and effectively catalyze the reduction of a CaSO₄ oxygen carrier at 950 °C.

Fig. 15 XRD patterns of Ca-based oxygen carrier at 950 °C: (a) CaSO₄ with 3.1 g hematite, (b) CaSO₄ with 10.4 g hematite, (c) CaSO₄ with 14.0 g hematite, and (d) CaSO₄ with 6.5 g hematite after a 5-cycle test.

3.3.2 Surface morphology. According to variations in the surface microscopic morphology of the oxygen carrier before and after the reaction, such as grain size, cracking and fracture of oxygen carrier particles, changes in the reactivity of the oxygen carrier can be explained to some extent. In this section, the morphological features of oxygen carrier particles were analyzed using FESEM before and after the reaction. Parts (a) and (b) of Fig. 16 show the surface morphology of fresh anhydrite and fresh hematite, respectively. The fresh anhydrite was compact and impervious, whereas the fresh hematite had a more developed pore structure than anhydrite, which was conductive to the diffusion of gasification product gases. After the first reduction at 950 °C, as shown in part (c) of Fig. 16, the surface of CaSO₄ oxygen carrier particles was rough and composed of comparably sized grains, which were not evenly distributed and moved closer together. Moreover, a small amount of grain aggregates was found on the surface of particles, which was in agreement with a previous study.²¹ The crystal grains seemed to be formed as a consequence of the reduction reaction, where part of CaSO₄ was reduced to CaS and the molar volume decreased from 46.0 to 28.9 cm³ mol⁻¹. Therefore, it can be concluded that CaS would exist in considerably the same form of evenly distributed grains and unreacted CaSO₄ in a state of aggregation.²³ The left-hand figure is an electron backscatter diffraction pattern (EBSP) in part (d) of Fig. 16, and metal compounds (Fe₂O₃ or Fe₃O₄) and Ca-based compounds (CaSO₄, CaS or CaO) can be clearly distinguished via EBSP. Therefore, the shiny particle in the middle of the left-hand figure is Fe₃O₄. The appearance of the surface in part (d) of Fig. 16 is basically similar to that shown in part (c) of Fig. 16 with porosity and interstices. However, a difference from the particle with hematite after the first reduction is that the number of aggregates has reduced, grains are evenly distributed and small grains of a size around 2–3 μm have increased. Fig. 16e also shows that there was no sintering between oxygen carrier particles and the oxygen carrier retained its beehive shape after the 5-cycle test. The majority of grains of a size around 1–2 μm were uniformly distributed on the surface of CaSO₄ oxygen carrier particles, which was advantageous for the introduction of reactive gases into the internal spaces of particles. The other interesting feature to note is that a small number of particles became slightly larger in a compact agglomerated state. It seems that grains on the surface of the oxygen carrier may be sintered after more cycling tests, which would result in a decrease in the surface area and reactivity of the CaSO₄ oxygen carrier.

Fig. 16 FESEM micrographs of fresh and used oxygen carriers: (a) fresh natural anhydrite ore; (b) fresh hematite; (c) CaSO₄ after the first reduction at 950 °C; (d) CaSO₄/Fe₂O₃ after the first reduction at 950 °C; (e) CaSO₄/Fe₂O₃ after the 5-cycle test at 950 °C.

3.3.3 BET analysis. The BET surface area, pore structure and pore size distribution of fresh and used oxygen carrier particles are listed in Table 5. The BET surface area of fresh anhydrite was only 0.2573 m² g⁻¹, which is closely related to the structure of anhydrite as shown in Fig. 16a. The surface area and total pore volume of individual CaSO₄ oxygen carriers rapidly increased after the first reduction at 950 °C, whereas the average pore diameter decreased. The increase in BET surface area might be ascribed to the release of CO₂ and steam gases leaving the solid particles, which can be obviously observed in part (c) of Fig. 16. Due to the small amount of reacted CaSO₄ and the inhomogeneous mixture of CaSO₄ and hematite, the surface area, average pore diameter and total pore volume of CaSO₄ with 6.5 g hematite were near to those without hematite after the first reduction. However, the surface area of CaSO₄ with hematite increased to 3.1466 m² g⁻¹ and the total pore volume reached 0.01728 cm³ g⁻¹ after the 5-cycle test, which is confirmed by part (e) of Fig. 16; moreover, the average pore diameter decreased sharply. Because of the larger contact area between hematite and the Ca-based oxygen carrier in fluidization and the cycling test, hematite not only played the role of an oxygen carrier, but also had a large influence on the reactivity of the CaSO₄ oxygen carrier. From the point of view of the whole subject area, it is very interesting that the structural characteristics of the CaSO₄/Fe₂O₃ oxygen carrier underwent a transition into those of fresh hematite after the 5-cycle test and the related mechanism is still not known.

Table 5 Surface properties of fresh and used oxygen carrier particles

Species	BET surface area (m^2 g^−1)	Total pore volume (cm^3 g^−1)	Average pore diameter (nm)
Fresh anhydrite	0.2573	0.001475	45.1998
Fresh hematite	5.0380	0.025860	18.7569
CaSO4-950 °C reduction	0.6996	0.004887	40.6276
CaSO4/Fe2O3-950 °C reduction	0.7090	0.004085	44.3986
CaSO4/Fe2O3-950 °C 5 cycles	3.1466	0.017280	22.7888

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3.4. Catalytic mechanism analysis

3.4.1 Catalysis in gasification of char. Researchers have achieved a consensus that alkali metals, alkaline-earth metals and transition metals (such as Fe and Ni) are the major source of active catalysts for the steam gasification of coal/char.^32–34 During the steam gasification of coal/char, the gasification rate is proportional to the number of active sites at the chemical reaction boundary and the active surface area. The addition of catalysts is effective in increasing the number of active sites and active surface area, so the gasification rate is significantly increased.³² Dispersion of an iron catalyst can determine its activity during char gasification, and more highly dispersed iron is more active; however, when the mass of a catalyst reaches the maximum value for a monolayer dispersion on the surface of coal/char, the diffusion of gasification agent gases and escape of gasification product gases are suppressed, and further increases in the gasification rate are limited to a certain extent. Therefore, a suitable added mass of catalyst did exist.³⁴ When Fe₂O₃ was added to char, H₂ and CO₂ concentrations increased in the gasification product gases and the CO concentration decreased. The decrease in CO concentration could be ascribed to the catalytic impact of Fe₂O₃ on the water gas shift reaction (R10). Although the effect of hematite on char gasification was not investigated in this work, hematite could improve the gasification rate of char and a related schematic diagram is shown in Fig. 17a.

Fig. 17 Schematic diagram of the mechanism of the catalytic synergistic effect: (a) catalytic gasification, (b) reduction of Fe₂O₃, (c) catalytic reduction of CaSO₄, and (d) deactivation of catalyst.

3.4.2 Catalytic mechanism in reduction. Under the influence of a higher fluidization velocity as well as collisions between particles and the reactor wall, it is possible to infer that some grains broke off the hematite and were attached to CaSO₄ particles. Distributions of Ca and Fe on the surface of the CaSO₄/Fe₂O₃ oxygen carrier after the 5-cycle test are shown in Fig. 18, which further confirms the above-mentioned inference. As shown in Fig. 17b, because hematite has a larger surface area and pore volume and the reactivity of hematite was proved to be better than that of the CaSO₄ oxygen carrier,³¹ gasification gases (such as H₂ and CO) were easily oxidized to form CO₂ and H₂O by hematite. In addition, hematite was reduced to form Fe₃O₄, which was identified by XRD analysis. Furthermore, Fe-based oxide as a catalyst could reduce the activation energy of the reduction reaction between the CaSO₄ oxygen carrier and reducing gases and improve the reduction reaction rate. It is likely that the iron-catalyzed reduction proceeded via an oxygen transfer mechanism involving a redox cycle of iron oxides (R10)–(R12), as shown in Fig. 17c. Ultimately, with the continuation of reduction and the increase of the cycle number, the probability of direct contact between CaSO₄ and Fe-based oxide was reduced until the generated CaS layer completely hindered direct contact between Fe-based oxide and the CaSO₄ oxygen carrier, which might result in the deactivation of the iron catalyst, as shown in Fig. 17d. Then, the reducing gas needed to diffuse through the CaS layer and react with the CaSO₄ oxygen carrier and therefore the reaction rate of CaSO₄ reduction was gradually lowered. Moreover, a previous study³⁴ indicated that Fe₂O₃ might react with the side product CaO to form Ca₂Fe₂O₅. In addition, the result of a CO₂ sorption test illustrated that the release of free Fe₂O₃ rarely occurred, as shown in the following equation (R13). To summarize, Fe-based oxide has a synergistic effect on the CaSO₄ oxygen carrier with solid fuels, which could simultaneously catalyze both the steam gasification and the CaSO₄ reduction reaction and suppress side reactions. Further study needs to be carried out to clarify the mechanism of this synergistic effect.

8Fe₃O₄ + CaSO₄ → 12Fe₂O₃ + CaS (R10)

3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂ (R11)

3Fe₂O₃ + H₂ → 2Fe₃O₄ + H₂O (R12)

Ca₂Fe₂O₅ + 2CO₂ → Fe₂O₃ + 2CaCO₃ (R13)

Fig. 18 Distributions of elemental Ca and Fe on the surface of the CaSO₄/Fe₂O₃ oxygen carrier after the 5-cycle test: (a) surface morphology of particles, (b) Ca, and (c) Fe.

4. Conclusions

In this study, experiments on the gasification and CLC of char in a steam medium in a laboratory-scale fluidized-bed reactor were conducted for investigating the reactivity of CaSO₄ decorated with hematite. The effects of the reaction temperature, iron–sulfur ratio and cycle numbers on carbon conversion, CO₂ yield, and the rate of evolution of SO₂ as well as the surface morphology of the oxygen carrier were discussed. The main results are summarized as follows:

(1) A rise in the reaction temperature and the addition of hematite led to an increase in carbon conversion and CO₂ yield. The peak values of SO₂ evolution increased with a rise in the reaction temperature, but the peak time was delayed, which was ascribed to the sulfur-suppressing effect of hematite. Carbon conversion, selectivity for the formation of CO₂ and CO₂ yield also increased with temperature, but the iron–sulfur ratio had slight influence on the product gas concentrations. Although the rate of evolution of SO₂ was reduced gradually with an increase in the Fe/S ratio, there was no obvious decrease in the rate of evolution of SO₂ when the Fe/S ratio exceeded 0.13.

(2) Five reduction/oxidation cycling tests were carried out, and the results showed that the reactivity of CaSO₄ decreased gradually in the cycling test. The cumulative rates of evolution of SO₂ in reduction/oxidation decreased first and increased subsequently with an increase in the cycle number. Moreover, the cumulative rate of evolution of SO₂ in reduction was significantly lower than that in oxidation.

(3) XRD analysis revealed that hematite can still exist in the oxygen carrier as Fe₂O₃ and not Ca₂Fe₂O₅ after the 5-cycle test. FESEM analysis demonstrated that there was no sintering between oxygen carrier particles and the oxygen carrier retained its beehive shape after the 5-cycle test. The majority of grains of a size around 1–2 μm were uniformly distributed on the surface of CaSO₄ oxygen carrier particles. BET analysis suggests that the structural characteristics of the CaSO₄/Fe₂O₃ oxygen carrier underwent a transition into those of fresh hematite after the 5-cycle test.

(4) According to the analyses mentioned above, Fe-based oxide had a synergistic effect on the CaSO₄ oxygen carrier with solid fuels. The catalytic reduction mechanism of hematite was discussed and related chemical equations that involved an oxygen transfer mechanism of iron-catalyzed reduction were proposed.

Nomenclature

Ci(t)	Rate of evolution of gaseous product i (min^−1)
n	Time elapsed since the reaction started in the FR (s)
Ni,out(t)	Molar amount of product i produced within the time interval Δt (mol)
NC,fuel	Total molar amount of carbon introduced into the FR (mol)
RC	Carbon residue
t	Time variable (s)
Xi(t)	Cumulative amount of gaseous product i (min^−1)
XC	Carbon conversion
YCO2(t)	Selectivity for formation of CO2

Greek letters

ηCO2

CO2 yield

Acknowledgements

This work was supported by the National Natural Science Foundation of China (no. 51206102), the Foundation for Outstanding Young Scientists in Shandong Province (no. BS2014NJ013) and the Youth Science Fund Project of Shandong Academy of Sciences (No. 2014N014). The authors are also grateful to the analytical and testing centers of Shandong Province and Huazhong University of Science and Technology for XRD, FESEM-EDS and BET measurements.

References

1. N. N. Hlaing, S. Sreekantan, R. Othman, S. Y. Pung, H. Hinode, W. Kurniawan, A. A. Thant, A. R. Mohamed and C. Salime, Sol-gel hydrothermal synthesis of microstructured CaO-based adsorbents for CO₂ capture, RSC Adv., 2015, 5, 6051–6060.
2. S. M. Hong, S. H. Kim, B. G. Jeong, S. M. Jo and K. B. Lee, Development of porous carbon nanofibers from electrospun polyvinylidene fluoride for CO₂ capture, RSC Adv., 2014, 103, 58956–58963.
3. S. C. Tian, J. G. Jiang, K. M. Li, F. Yan and X. J. Chen, Performance of steel slag in carbonation-calcination looping for CO₂ capture from industrial flue gas, RSC Adv., 2014, 14, 6858–6862.
4. M. C. Stern and T. A. Hatton, Bench-scale demonstration of CO₂ capture with electrochemically -mediated amine regeneration, RSC Adv., 2014, 4, 5906–5914.
5. M. Sayyah, B. R. Ito, M. Rostam-Abadi, Y. Q. Lu and K. S. Suslick, CaO-based sorbents for CO₂ capture prepared by ultrasonic spray pyrolysis, RSC Adv., 2013, 3, 19872–19875.
6. R. Sathre and E. Masanet, Prospective life-cycle modeling of a carbon capture and storage system using metal-organic frameworks for CO₂ capture, RSC Adv., 2013, 3, 4964–4975.
7. N. Ding, W. R. Wang, Y. Zheng, C. Cong, P. F. Fu and C. G. Zheng, Development and testing of an interconnected fluidized-bed system for chemical looping combustion, Chem. Eng. Technol., 2012, 35, 532–538.
8. Y. Cao and W. P. Pan, Investigation of chemical looping combustion by solid fuels. 1. Process analysis, Energy Fuels, 2006, 20, 1836–1844.
9. S. A. Scott, J. S. Dennis, A. N. Hayhurst and T. Brown, In situ gasification of a solid fuel and CO₂ separation using chemical looping, AIChE J., 2006, 52, 3325–3328.
10. L. F. de Diego, M. Ortiz, J. Adánez, F. García-Labiano, A. Abad and P. Gayán, Synthesis gas generation by chemical-looping reforming in a batch fluidized bed reactor using Ni-based oxygen carriers, Chem. Eng. J., 2008, 144, 289–298.
11. A. Lyngfelt, Chemical looping combustion of solid fuels – Status of development, Appl. Energy, 2014, 113, 1869–1873.
12. J. Adánez, A. Abad, F. García-Labiano, P. Gayán and L. F. de Diego, Progress in chemical looping combustion and reforming technologies, Prog. Energy Combust. Sci., 2012, 38, 215–282.
13. P. Markström, C. Linderholm and A. Lyngfelt, Chemical looping combustion of solid fuels – Design and operation of a 100 kW unit with bituminous coal, Int. J. Greenhouse Gas Control, 2013, 15, 150–162.
14. T. Mendiara, L. F. de Diego, F. García-Labiano, P. Gayán, A. Abad and J. Adánez, On the use of a highly reactive ore in chemical looping combustion of different coals, Fuel, 2014, 126, 239–249.
15. R. Xiao, Q. L. Song, M. Song, Z. J. Lu, S. Zhang and L. H. Shen, Pressurized chemical looping combustion of coal with an iron ore-based oxygen carrier, Combust. Flame, 2010, 157, 1140–1153.
16. J. H. Bao, Z. S. Li and N. S. Cai, Experiments of char particle segregation effect on the gas conversion behavior in the fuel reactor for chemical looping combustion, Appl. Energy, 2014, 113, 1874–1882.
17. M. M. Hossain and H. I. de Lasa, Chemical-looping combustion (CLC) for inherent CO₂ separations – A review, Chem. Eng. Sci., 2008, 65, 4433–4451.
18. H. E. Andrus, J. H. Chiu, P. R. Thibeault and A. Brautsch, in The 34^th International Technical Conference on Clean Coal and Fuel Systems, Clearwater, 2009.
19. H. J. Tian and Q. J. Guo, Investigation into the behavior of reductive decomposition of calcium sulfate by carbon monoxide in chemical looping combustion, Ind. Eng. Chem. Res., 2009, 48, 5624–5632.
20. J. Wang and E. J. Anthony, Clean combustion of solid fuels, Appl. Energy, 2008, 85, 73–79.
21. Q. L. Song, R. Xiao, Z. Y. Deng, L. H. Shen, J. Xiao and M. Y. Zhang, Effect of temperature on reduction of CaSO₄ oxygen carrier in chemical-looping combustion of simulated coal gas in a fluidized bed reactor, Ind. Eng. Chem. Res., 2008, 47, 8148–8159.
22. M. Zheng, L. H. Shen and J. Xiao, Reduction of CaSO₄ oxygen carrier with coal in chemical-looping combustion: Effects of temperature and gasification intermediate, Int. J. Greenhouse Gas Control, 2010, 4, 716–728.
23. Q. L. Song, R. Xiao, Z. Y. Deng, W. G. Zheng, L. H. Shen and J. Xiao, Multicycle study on chemical-looping combustion of simulated coal gas with a CaSO₄ oxygen carrier in a fluidized bed reactor, Energy Fuels, 2008, 22, 3661–3672.
24. M. Zheng, L. H. Shen, X. Q. Feng and J. Xiao, Kinetic model for parallel reactions of CaSO₄ with CO in chemical-looping combustion, Ind. Eng. Chem. Res., 2011, 50, 5414–5427.
25. X. Rui and Q. L. Song, Characterization and kinetics of reduction of CaSO₄ with carbon monoxide for chemical-looping combustion, Combust. Flame, 2011, 158, 2524–2539.
26. R. K. Lyon and J. A. Cole, Unmixed combustion: An alternative to fire, Combust. Flame, 2000, 121, 249–261.
27. S. M. Liu, D. H. Lee, M. Liu, L. L. Li and R. Yan, Selection and application of binders for CaSO₄ oxygen carrier in chemical looping combustion, Energy Fuels, 2010, 24, 6675–6681.
28. S. Zhang, R. Xiao, Y. C. Yang and L. Y. Chen, CO₂ capture and desulfurization in chemical looping combustion of coal with a CaSO₄ oxygen carrier, Chem. Eng. Technol., 2013, 36, 1469–1478.
29. S. Zhang, R. Xiao, J. Liu and S. Bhattacharya, Performance of Fe₂O₃/CaSO₄ composite oxygen carrier on inhibition of sulfur release in calcium-based chemical looping combustion, Int. J. Greenhouse Gas Control, 2010, 4, 716–728.
30. T. Song, M. Zheng, L. H. Shen, T. Zhang, X. Niu and J. Xiao, Mechanism investigation of enhancing reaction performance with CaSO₄/Fe₂O₃ oxygen carrier in chemical looping combustion of coal, Ind. Eng. Chem. Res., 2013, 52, 4059–4071.
31. N. Ding, Y. Zheng, C. Luo, Q. L. Wu, P. F. Fu and C. G. Zheng, Development and performance of binder-supported CaSO₄ oxygen carriers for chemical looping combustion, Chem. Eng. J., 2011, 171, 1018–1026.
32. K. Asamil, R. Sears and E. Furirnsly, Gasification of brown coal and char with carbon dioxide in the presence of finely dispersed iron catalysts, Fuel Process. Technol., 1996, 12, 139–151.
33. G. Domazetis, M. Raoarun, B. D. James and J. Liesegang, Molecular modeling and experimental studies on steam gasification of low-rank coals catalysed by iron species, Appl. Catal., A, 2008, 340, 105–118.
34. B. S. Huang, H. Y. Chen, K. H. Chuang, R. X. Yang and M. Y. Wey, Hydrogen production by biomass gasification in a fluidized-bed reactor promoted by an Fe/CaO catalyst, Int. J. Hydrogen Energy, 2012, 37, 6511–6518.
35. N. Ding, Y. Zheng, C. Luo, Q. L. Wu, P. F. Fu and C. G. Zheng, Investigation into compound CaSO₄ oxygen carrier for chemical looping combustion, J. Fuel Chem. Technol., 2011, 39, 161–168.
36. H. J. Li and Y. H. Zhuang, Catalytic reduction of calcium sulfate to calcium sulfide by carbon monoxide, Ind. Eng. Chem. Res., 1999, 38, 3333–3337.
37. C. Dueso, A. Abad, F. García-Labiano, L. F. de Diego, P. Gayán, J. Adánez and A. Lyngfelt, Reactivity of a NiO/Al₂O₃ oxygen carrier prepared by impregnation for chemical looping combustion, Fuel, 2010, 89, 3399–3409.
38. H. G. Jin, T. Okamoto and M. Ishida, Development of a novel chemical looping combustion: Synthesis of a looping material with a double metal oxide of CoO-NiO, Energy Fuels, 1998, 12, 1272–1277.
39. B. W. Wang, R. Yan, H. B. Zhao, Y. Zheng, Z. H. Liu and C. G. Zheng, Investigation of chemical looping combustion of coal with CuFe₂O₄ oxygen carrier, Energy Fuels, 2011, 25, 3344–3354.
40. B. Moghtaderi and H. Song, Reduction properties of physically mixed metallic oxide oxygen carriers in chemical looping combustion, Energy Fuels, 2010, 24, 5359–5368.
41. Z. P. Gao, L. H. Shen, J. Xiao, C. J. Qing and Q. L. Song, Use of coal as fuel for chemical looping combustion with Ni-based oxygen carrier, Ind. Eng. Chem. Res., 2008, 47, 9279–9287.
42. L. H. Shen, M. Zheng, J. Xiao and R. Xiao, A mechanistic investigation of a calcium-based oxygen carrier for chemical looping combustion, Combust. Flame, 2008, 154, 489–506.
43. N. Berguerand and A. Lyngfelt, The use of petroleum coke as fuel in a 10kW_th chemical looping combustor, Int. J. Greenhouse Gas Control, 2008, 2, 169–179.

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