Chemical looping combustion of coke oven gas by using Fe₂O₃/CuO with MgAl₂O₄ as oxygen carrier

Abstract

Chemical looping combustion (CLC) is a new combustion technology that is clean and highly efficienct. The reactivity of four potential oxygen carrier particles composed of Fe₂O₃/CuO and MgAl₂O₄ have been investigated using thermogravimetric analysis (TGA) and a laboratory pressurized circulating fluidized bed system. According to the research, the following findings can be made: first, oxygen carrier particles consisting of 45% Fe₂O₃ and 15% CuO supported on 40% MgAl₂O₄ (F45C15M40) were found to be the best oxygen carrier for CLC of coke oven gas (COG). Secondly, results of multicycle reduction–oxidation tests in TGA showed that the reactivity of F45C15M40 oxygen carrier remained high and stable after 15 reduction–oxidation reaction cycles. Thirdly, the reaction temperature and cycle number showed a great effect on the reactivity of the oxygen carrier and this was attributed to the high chemical reaction rate of oxygen carrier and morphological change in CuO at high temperature. The pressurized CLC unit was in continuous operation with fuel for a duration of 15 h. The pressurized circulating fluidized bed system for CLC was found to work well with F45C15M40 oxygen carriers. It showed high reactivity in a fluidized bed reactor and gave a high conversion of the fuel, as well as reasonable crushing strength and resistance toward agglomeration and fragmentation. The maximum fuel conversion reached 92.33%.

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Broader context

The utilization of COG is limited and still at a very low level in China. Unless it is effectively and economically utilized, it not only causes the pollution of the environment but is also a large waste resources. In addition, combustion of fossil fuels releases a massive amount of carbon as carbon dioxide into the atmosphere. To address these issues, we present here a new method for the combustion of COG based on Fe₂O₃/CuO and MgAl₂O₄ as the oxygen carrier for chemical looping combustion (CLC). In CLC, the products CO₂ and H₂O are not diluted with N₂ from the combustion atmosphere, and pure CO₂ can be obtained without expending any extra energy needed for separation. Considering this unique characteristic, CLC appears to have the potential for delivering a very efficient and low cost technology, especially for increasing thermal efficiency in power generation stations and for CO₂ capture with minimum energy losses.

1. Introduction

The coking process generates a large amount of coke oven gas (COG), which is mainly made up of hydrogen and methane. China is a vast country abundant in COG resources. But in fact, the utilization of COG is limited and still at a very low level.¹ If not being used efficiently and economically, COG would cause serious pollution to the environment. Meanwhile, it is also a large waste of natural resources. In this paper, a new method for the combustion of COG based on the chemical looping concept is proposed by the authors to solve the above problems. Chemical looping combustion (CLC) has been suggested as an energetically efficient method for capture of CO₂ from the combustion of fuel.² The CLC process is accomplished by using two reactors and a circulating metal oxide, see Fig. 1. The oxygen carrier constantly circulates between the fuel reactor and air reactor. In the fuel reactor, it is reduced by the fuel, and is oxidized to CO₂ and H₂O through reaction (1). In the air reactor, it is oxidized to its initial state with O₂ through (2). Due to the fact that the fuel and air are separated in CLC, the combustion products of CO₂ and H₂O are not diluted with nitrogen, which means that by condensing the H₂O, it is possible to obtain almost pure CO₂ without expending any extra energy needed for separation. The combustion mode changes from the traditional gas–gas reaction to a gas–solid reaction through the use of COG in CLC. In this method of combustion the gas and solid have strong and complete contact with each other, therefore it is suitable for dealing with the low calorific value and complicated composition of COG.

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

M_xO_y−1 + 1/2O₂ → M_xO_y (2)

Fig. 1 Chemical looping combustion.

Oxygen carriers can carry oxygen and heat in CLC system, so it is a crucial element that has great importance in the whole process. Oxygen carriers are usually consist of active components and inert carriers. The oxygen carrier particles must be resistant to mechanical and chemical degradation by attrition, agglomeration, fragmentation, composition changes, etc. To date, results of numerous researches show that several metal/metal oxides have been investigated for potential use as oxygen carriers in CLC, e.g. Ni/NiO, Fe₂O₃/Fe₃O, Cu/Cu₂O and MnO/Mn₃O₄.^3–6 To increase the performance of these active materials, they should be supported by an inert material which provides a larger surface area for reaction and may act as a binder to increase their mechanical strength and hence their attrition resistance.³ These inert materials include SiO₂, Al₂O₃, ZrO₂, TiO₂, MgO, YSZ, NiAl₂O₄, MgAl₂O₄ and CoAl₂O₄.^7–10 Iron oxide has large natural reserves and is relatively inexpensive, both of which are very important in CLC for commercial applications in the future. In addition, iron oxide does not agglomerate, retains activity and remains durable through multiple oxidation–reduction cycles. However, the ability of carrying oxygen is poor and it has a low reaction speed compared with nickel and copper oxides, so the quantity needed is larger than for the other two oxygen carriers.¹¹ Copper oxide has higher reactivity and it is easier to make the fuel react completely to generate CO₂ and H₂O. Copper oxide is exothermic in oxidation–reduction, so it is not necessary to provide extra energy in the reduction reaction. However, its defect is that its high temperature resistant performance is not strong.^12,13 Spinel MgAl₂O₄ offers a desirable combination of properties such as high melting point, high resistance to chemical attack and high strength at elevated temperatures.¹⁴ Mattisson et al. developed iron oxides on six different inert materials and of different mass active oxide/inert support ratios. Of these, 60% Fe₂O₃ on 40% MgAl₂O₄ showed the highest reactivity in a fluidized bed reactor, as well as reasonable crushing strength and resistance toward agglomeration and fragmentation.¹¹ Johansson et al. examined and optimized different ratios of Fe₂O₃ to MgAl₂O₄ as well as different sintering temperatures for chemical looping combustion. They found 60% Fe₂O₃ on 40% MgAl₂O₄ sintered at 1100 °C was the most suitable oxygen carrier. The particle had a high reactivity and showed no tendency to agglomerate or break apart during the cyclic tests.¹⁵

Here, we propose a new oxygen carrier based on Fe₂O₃/CuO supported on MgAl₂O₄, the purpose of this work is to asses its feasibility as an oxygen carrier for CLC. Particles were prepared with different ratios of Fe₂O₃ to CuO. Multicycle redox tests were investigated using thermogravimetric analysis (TGA) and a hot laboratory-sized pressurized circulating fluidized bed, respectively. Additionally, the structural and mechanical properties of these samples were investigated by means of powder XRD, BET surface area measurements, scanning electron microscopy and energy dispersive X-ray spectrometry.

2. Experiments

2.1 Oxygen carrier preparation

Four different types of oxygen carriers were prepared by mechanical mixing. In this process, Al₂O₃ and MgO powder were mixed according to desired mass ratio and stirred uniformly. The mixture was then put into a muffle furnace to sinter at 1300 °C for 6 h using a heating rate of 10 °C min⁻¹ to get MgAl₂O₄. Then Fe₂O₃/CuO and MgAl₂O₄ powder were mixed with a mass ratio of 3 : 2, they were stirred uniformly and mixed with distilled water to obtain a paste. The paste was then extrusion moulded in a tablet machine and dried at 110 °C for 24 h in an oven and sintered at 900 °C for 3 h in muffle furnace. The oxygen carriers were then sieved to a size range of 100–200 μm. In this paper, each oxygen carrier sample will be presented in the form F45C15M40, which signifies in order of appearance, Fe₂O₃ (F), 45% of Fe₂O₃ (45); CuO (C), 15% of CuO (15); MgAl₂O₄, 40% of MgAl₂O₄ (40). Similarly, the four oxygen carriers are marked as F60M40, F45C15M40, F30C30M40 and C60M40, respectively.

The major components of COG in the experiment are shown in Table 1.

Table 1 Composition of COG/%(v)

Component	H2	CH4	CO	CO2	N2
Volume content (%)	59	28	7	3	3

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2.2 Experimental set-up and procedure

The experimental study was carried out by thermogravimetric analyzer (TGA) and laboratory-sized pressurized circulating fluidized bed system respectively. TGA experiments were conducted in WCT-2C, which was produced by Beijing Optics Instrument factory.

The CLC system was designed for a fuel power range of 3–10 kW_th for either syngas or natural gas as the fuel. Fig.2 shows the schematic diagram of this laboratory unit. The system consists of an air reactor (1) where the metal oxides are oxidized and led to a cyclone by a riser (4) where gas and solid particles are separated, exhaust gas is discharged from the cyclone exit, oxygen carriers particles fall into the solid reservoir (5) and then fall into the fuel reactor (2) via a solids valve (6). The air reactor was 0.5 m in length and 70 mm in diameter, the fuel reactor was 0.5 m in length and 102 mm in diameter. Two porous quartz plates, on which the oxygen carrier sample is applied, are located above the bottom of the fuel reactor and air reactor. During operation, the sample is fluidized by adding gas to the bottom of the two reactors, and the porous plate acts as gas distributor. In order to reach a suitable temperature, the two reactors are placed inside an electrical heater. Reactors temperatures are measured using thermocouples enclosed in quartz shells. The exhaust gas from the fuel reactor is led to a cooler, in which the water is removed. The pressure difference is measured along the two reactors using pressure transducers. The system maximum heat resistance temperature is 950 °C and maintains the pressure at 0.3 MPa during the whole experiment process.

Fig. 2 Schematic description of the CLC facility. 1 – air reactor; 2 – fuel reactor; 3 – loop seal; 4 – cyclone; 5 – solid reservoir; 6 – solids valve; 7 – filter; 8 – condensing equipment; 9 – sample point; 10 – back pressure valve; 11 – air; 12 – methane; 13 – nitrogen; 14 – air compressor; 15 – electrical heater

Before the system was filled with fuel, 2.3 kg and 2.4 kg oxygen carrier was added to the air reactor and fuel reactor, respectively. In the very beginning, N₂ was introduced for 100 s to avoid air and COG mixing during the shift between the reducing and oxidizing phase. After the remaining air was purged completely, we began to pump in the COG and air. The inlet superficial gas velocity into the reactor was 0.09–0.22 m s⁻¹ in the reduction phase and 0.55–1.25 m s⁻¹ in the oxidation phase. In this way, a continuous circulation of chemical looping combustion was obtained. After all conditions were continuous and steady for 30 min, we extracted the first gas sample from the sample point and kept doing the same every 5 min 9 subsequent times. The gas samples were examined by gas chromatography (HP6890), which is indispensable for measuring the concentration of CO₂, CO, CH₄ and H₂. At the end of the experiment, oxygen carrier samples which we got from the fuel and air reactor were characterized by SEM and compared with those before the experiment.

3. Results and analysis

3.1 Thermogravimetric analysis (TGA)

To evaluate the reaction performance and stability of the four different types of oxygen carriers, numerous reduction and oxidation cycles were conducted by TGA within the temperature range of 750–900 °C. The weight of oxygen carrier samples were 15 mg and the particle size range was 0.1–0.2 mm. Samples were heated in a quartz bowl to the reaction temperature with a nitrogen purge. COG was used for the reduction segment, and dry air was utilized for the oxidation segment. Reaction gas flow rates were set at 60 mL min⁻¹, reduction reaction time was 10 min and oxidation reaction time was 60 min for all experiments. To avoid the addition of air into the reduction gas, the system was flushed with nitrogen for 5 min before and after each reaction phase.

In order to characterize the redox properties of oxygen carrier, the fractional conversions (X) were calculated utilizing the TGA data. The fractional conversion (X) is defined as shown below.

Where X_red is the fractional reduction, X_ox is the fractional oxidation, m_ox is the weight of the fully oxidized sample, m is the instantaneous weight of the metal oxide, and m_red is the weight of the fully reduced sample.

3.1.1 Effect of active components. The oxygen carrier studied in this paper used two metal oxides as its active components. Owing to the reason that different metal oxides possess different reaction characteristics, therefore, as oxide carriers, they will produce different results. Based on this fact, it is easy to conclude that metal oxides have great effect on the reaction characteristic. Fig. 3 shows the effect of the active components on the reduction properties of oxygen carrier at 900 °C. As shown in the Figure, with reducing of Fe₂O₃ content and increasing of CuO content, the X_red displayed a sharp increase but the conversion time was significantly reduced.

Fig. 3 Effect of active components on properties of oxygen carriers.

The results of the XRD composition analysis of Fe₂O₃/MgAl₂O₄, Fe₂O₃–CuO/MgAl₂O₄, CuO/MgAl₂O₄ after reduction are shown in Table 2. It can be seen from the composition analysis results that Fe₃O₄ was not reduced to Fe completely at 900 °C but to the mixture containing a Fe₃O₄ phase, FeO phase and Fe phase. While after reduction, CuO was reduced to Cu and CuAl₂O₄, this is why pure CuO oxygen carrier conversion is not 1.

Table 2 XRD composition analysis of oxygen carrier after reduction

Sample	T/°C	Composition of fresh sample	Composition of reduced sample
a Indicates trace amount.
Fe2O3/MgAl2O4	900	Fe2O3, Fe3O4, MgAl2O4	Fe3O4, FeO, Fe, MgAl2O4
Fe2O3–CuO/MgAl2O4	900	Fe2O3, Fe3O4, CuO, Cu2O, CuAl2O4^a, MgAl2O4	Fe3O4, FeO, Fe, Cu, CuAl2O4^a, MgAl2O4
CuO/MgAl2O4	800	CuO, Cu2O, CuAl2O4, MgAl2O4	Cu, CuAl2O4^a, MgAl2O4

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3.1.2 Effect of cycle number. In this study, 15 cycles of TGA experimental data for the F45C15M40 oxygen carrier were utilized to analyze the effect of cycle numbers on redox reaction properties at a reaction temperature of 900 °C. Panels (a) and (b) in Fig. 4, show F45C15M40 oxygen carrier in the 1st, 5th, 10th and 15th cycles of reduction reaction and oxidation reaction, respectively. Fig. 4 (a) shows that with the increase of cycle number, the reduction properties increase monotonically. Fig. 4 (b) shows that the fractional conversion of oxygen carrier in the 1st cycle was lower than that in the 5th, and the fractional conversion of oxygen carrier increased only a little bit from the 5th to 15th cycles. This is explained by the fact that oxygen carrier particles produced granular cracks in multiple cycles, the pore volume and specific surface area increased, which increased the reaction properties of the oxygen carrier particles . In order to prove these results directly, specific surface area measurement techniques were used to characterize the oxygen carrier. The results are shown in Table 3. As can be seen in Table 3, with the increase of cycle number, specific surface area and pore volume were enlarged, but on a small scale and tended to slow down. These data coincide well with the phenomenon in Fig. 3.

Table 3 Oxygen carrier data

Oxygen carrier	Specific surface area/m^2 g^−1	Pore volume/cm^3 g^−1
Fresh OC	2.157	0.0037
5th	2.302	0.0043
15th	2.38	0.0045

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Fig. 4 Effect of cycle number on the redox reaction properties of F45C15M40 oxygen carrier (a) reduction, (b) oxidation.

3.1.3 Effect of temperature. The reaction properties of F45C15M40 at various temperature (T = 750 °C, 800 °C, 850 °C, 900 °C) are shown in Fig. 5. The reaction data indicates that the reaction properties of F45C15M40 were improved both in the reduction and oxidation reaction with the increasing reaction temperature. This is due to the reason that chemical reaction rate increases with the increasing temperature.

Fig. 5 Effect of reaction temperature on properties of F45C15M40 oxygen carrier (a) reduction, (b) oxidation.

3.1.4 Microanalysis. The scanning electron microscope (SEM) images of fresh F45C15M40 and F45C15M40 sample after 15 oxidation–reduction cycles in TGA at 900 °C are shown in Fig. 6. When the two images were compared, we found small but significant differences between them. The images of the fresh oxygen carrier samples have dense surfaces and the grains are irregular, the local environment has burn marks after the sintering process. After repeated oxidation–reduction cycles, samples had many morphological changes. Surface grains become loose and regular, pore volume increased. The increase in pore volume in the particles was consistent with the result of a slight increase in the oxidation–reduction rates, with the cyclic reactions, because the bigger pore was favourable for diffusion and penetration of COG and oxygen into the oxygen carrier particles. These also confirm the result that the oxygen carrier properties were enhanced after several cycles.

Fig. 6 SEM images of F45C15M40 oxygen carrier before and after 15 cycles of reaction.

3.1.5 Analysis of carbon deposition. Under certain conditions solid carbon deposition on the oxygen carrier particles may occur if a carbon-containing fuel is used. This could have an adverse effect on the CLC process. The carbon formed can be transported back to the air reactor causing CO₂ formation, resulting in lower separation efficiency of CO₂.¹⁶ In addition, carbon formation may have further blocked the reaction between the oxygen in the inner core of the particles and the gases.¹⁷ Jin et al.¹⁸ thought that solid carbon may form on the particles under certain conditions through methane decomposition:

CH₄ → C + 2H₂ (5)

Or through the Boudouard reaction:

CO → 0.5C + 0.5CO₂ (6)

Fig. 7 shows the TG curves of F45C15M40 oxygen carrier in COG flow and air flow at 900 °C. It is found that weight curve didn't show weight gain during the reduction completing stage or show weight lose during the initial stages of oxidation. The results indicate that there is no carbon deposition formed in the reduction and oxidation process. In addition, the used oxygen carrier was examined by us based on energy spectrum analysis, see Table 4. Table 4 shows that carbon didn't appear in the oxygen carrier, which suggests that the reaction between COG and the oxygen carrier have not formed a carbon deposition. There are two possible reasons: reaction (5) was restricted by H₂ in the COG; methanation reaction (7) produced water vapour which can restrict carbon deposition.

CO + 3H₂ → CH₄ + H₂O (7)

Table 4 Energy spectrum analysis of oxygen carrier

Element	O	Mg	Al	Fe	Cu
(keV)	0.525	1.253	1.486	6.398	8.040
Mass (%)	28.43	7.45	16.21	34.56	13.35
Error (%)	0.17	0.32	0.26	0.45	0.99

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Fig. 7 TG curves of F45C15M40 oxygen carrier in COG flow (a) and air flow (b) at 900 °C.

3.2 Hot laboratory CLC combustion system

A hot laboratory-sized pressurized circulating fluidized bed system was designed, built and operated for chemical looping combustion of COG. The pressurized CLC unit was in continuous operation with fuel for 15 h. In the CLC system, although the conversion to CO₂ was always high, combustion of COG was not complete. The combustion tests showed the presence of CO, H₂ and CH₄ in the exit gas stream from the fuel reactor. The conversion of combustible carbon and hydrogen is presented below as the fraction of total carbon or hydrogen leaving the fuel reactor. In addition, the relating H₂ : C is 3.026 : 1 in the COG and combustible carbon accounted for 92.11% of total carbon, we defined some important items as follows:

(1) combustible carbon conversion ω_C:

(2) combustible hydrogen conversion ω_H2:

(3) conversion of COG ω_coke:

Where f_i is the volume fraction of i in dry exit gas from the fuel reactor.

3.2.1 Effect of fuel gas velocity. Fig. 8 shows the average value of the gas components in combustion products under different gas velocities at 850 °C. As shown in the Figure, an increase in the fuel gas velocity gives an increase in the concentrations of CH₄, H₂ and CO, and a sharp decrease in the concentration of CO₂. This is expected, which can be explained by the following reasons. Firstly, because the amount of oxygen carrier in the fuel reactor and the oxygen supply into the fuel reactor were constant, but the flow of fuel increased. The circulation of the particles, and thereby oxygen supply, is mainly determined by the velocity in the air reactor, which was constant. Secondly, there is less efficient contact and shorter contact time between the oxygen carrier particles and the gas at higher velocities. As a result, the fuel gas did not complete the reaction before the gas flow was discharged from the reactor outlet.

Fig. 8 Effect of fuel gas velocity on gas composition of fuel reactor.

In order to describe the burning degree of fuel gas directly, we rearranged the data on the basis of relevant definition parameter. The effect of fuel gas velocity on conversion of COG is shown in Fig. 9. Clearly, the ω_C, ω_H2 and ω_coke decreased with increasing fuel gas velocity. The conversion of COG is about 91.43% at 9 cm s⁻¹, but this fell to 62.62% at 21 cm s⁻¹.

Fig. 9 Effect of fuel gas velocity on the conversion of COG.

3.2.2 Effect of the temperature. Fig. 10 shows the effect of fuel reactor temperature on the concentration of CH₄, H₂ and CO at the fuel reactor outlet, gas velocity is 12 cm s⁻¹. An increase in the fuel reactor temperature produces an increase in the concentration of CO₂. This effect was due to higher oxygen carrier reactivity as a consequence of the dependence of the kinetic constant with the temperature. Obviously, higher combustion efficiencies were obtained at higher temperatures as a consequence of the higher oxygen carrier reactivity. In addition, the study carried out about reactivity showed the different behavior obtained with the three fuel gases.¹⁹ The highest reaction rate for the fuel gas was always obtained with H₂. The reaction rate followed the order H₂>CO> CH₄. Therefore, the concentration of CH₄ is higher than H₂ and CO.

Fig. 10 Effect of fuel reactor temperature on the gas composition.

Fig. 11 shows the effect of the temperature on the conversion of COG. In the temperature range 750–900 °C, ω_c, ω_H2 and ω_coke increase gradually and the conversion of COG increases from 69.8% at 750 °C to 92.33% at 900 °C. COG is not fully converted into CO₂ and H₂O. Firstly, on any operating condition, unreacted combustible fuel gas is more or less taken by high speed air flow out of the reaction system, which will lead to fuel gas loss. Secondly, for the Cu and Fe based oxygen carriers, the reduction reaction with CH₄, CO and H₂ is controlled by chemical reaction resistance.^19,20

Fig. 11 Effect of fuel temperature on the conversion of COG.

3.2.3 Oxygen carrier particles. During the experiment, the oxygen carrier particles have been fluidized by recirculation in hot conditions for approximately 15 h. The used particles were collected and particle size analysis was carried out. Table 5 shows the particle size distribution both before and after the reaction. It is obvious that after a long circulation flow time and continuous reaction, the distribution of oxygen carrier particles went through some changes. The used particles have more small particles compared with the new ones. This can be very detrimental to the process of CLC. Small particles are more difficult to separate by cyclone and more likely to be taken out of the reactor by fluidized air, particularly under the conditions of high fluidization velocities in a full-scale fluidized bed chemical looping combustor.

Table 5 The particle size distribution both before and after the reaction

Particle size distribution (μm)	Percentage distribution before the reaction (%)	Percentage distribution after the reaction (%)
0–70	0	6.4
70–100	0	10.2
100–150	67.3	71.8
150–200	33.7	11.6

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Throughout the experiment, the gas composition had no significant changes according to the analysis of the gas from the fuel reactor outlet, which suggests that the particle has high durability and reactivity. Scanning electron microscope (SEM) images of the fresh and used particles are shown in Fig. 12. It can be seen that minor changes occurred on the particle surface. The used particles surface is rougher and particle size has decreased. The results of the specific surface area determination are shown in Table 6. The specific surface area and pore volume of the used particles has increased.

Table 6 Specific surface area determination

Oxygen carrier	Specific surface area/m^2 g^−1	Pore volume/cm^3 g^−1
Fresh	2.157	0.0037
Used	3.214	0.0054

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Fig. 12 SEM images of F45C15M40 oxygen carriers. (a) Fresh, and (b) used.

4. Agglomeration

Agglomeration of particles in fluidized beds is a potential risk for difficulties in operation and in the worst cases could cause process failure.²¹ It is easy to see agglomeration at high temperature in the copper oxide based oxygen carrier . But the F45C15M40 oxygen carrier didn't show any tendency of agglomeration at 900 °C after reduction-oxidation for 1.5 h, which was attributed to the high melting point of Fe₂O₃ and MgAl₂O₄. It can be seen that agglomeration does not seem to be a problem for these particles at the temperature used.

5. Conclusions

The properties of COG chemical looping combustion over four different oxygen carriers produced by mechanical mixing with Fe₂O₃/CuO and MgAl₂O₄ as the active materials had been investigated using thermogravimetric analysis (TGA) and a laboratory pressurized circulating fluidized bed system. The F45C15M40 oxygen carrier was found to be the best oxygen carrier for CLC of COG. Results of multicycle reduction–oxidation tests in TGA show that the activity of the F45C15M40 oxygen carrier remained high and stable during 15 oxidation–reduction reaction cycles. The reaction temperature and cycle number showed a great effect on the reaction properties of the oxygen carrier and this was attributed to the high chemical reaction rate of oxygen carrier and morphological change in CuO at high temperatures. The pressurized CLC unit was in continuous operation with fuel for 15 h. The pressurized circulating fluidized bed system for CLC was found to work well with F45C15M40 oxygen carriers. It showed high reactivity in a fluidized bed reactor and provided a high conversion of the fuel, as well as reasonable crushing strength and resistance toward agglomeration and fragmentation. The maximum fuel conversion it can reach is 92.33%. It must be noted that the fluidization velocities and experimental time are not comparable with that expected in a full-scale fluidized bed chemical looping combustion.

Acknowledgements

This work was supported by National Science and Technology Supporting Program of China (2006BAKA02B03) and the Program for New Century Excellent Talents in University of Chinese Education Ministry (NCET -07-0678).

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