- Email: [email protected]
The investigations of hematite-CuO oxygen carrier in chemical looping combustion
Chemical Engineering Journal 317 (2017) 132–142
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
The investigations of hematite-CuO oxygen carrier in chemical looping combustion Shouxi Jiang, Laihong Shen ⇑, Jian Wu, Jingchun Yan, Tao Song Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China
h i g h l i g h t s Hematite-CuO interaction for the direct mixing method of Fe-Cu bimetallic oxygen carrier. Effect of Cu in oxygen carrier on reactivity and stability in structure. Cyclic characteristics of oxygen carriers in fluidized reactor and Cu loss with cycle. The distribution of Cu and exist form in the oxygen carriers.
a r t i c l e
i n f o
Article history: Received 1 December 2016 Received in revised form 20 January 2017 Accepted 21 January 2017 Available online 23 January 2017 Keywords: Chemical looping combustion Bimetallic oxygen carrier Hematite Copper
a b s t r a c t Cu-Fe bimetallic oxygen carrier is a promising candidate for chemical looping combustion, but the cost of manufacture is a significant limit for its application. In this study, the bimetallic oxygen carrier was prepared using hematite and CuO simply through direct mixing method with low cost. The cyclic combustion performance and effects of temperature were investigated in a batch fluidized bed. Cycle experiments with gas fuel indicated that the bimetallic oxygen carrier possessed increasing reactivity with cycle, which could be ascribed to improved porous structure. However, some agglomeration occurred when the content of Cu reached 20% in weight. Besides, the cyclic operation with coal showed that the bimetallic oxygen carriers with Cu ratios of 5% and 10% possessed good long term operation characteristics and the oxygen carrier with 10% Cu performed better. The conversion of coal increased with the reaction temperature and higher temperature, above 900 °C, was needed for fast conversion of coal. Additionally, the manufactured oxygen carriers were characterized. In the bimetallic oxygen carriers, Cu distributed uniformly on the surface of particle but the content of Cu decreased with cycles caused by attrition. Besides, the bimetallic oxygen carrier possessed a stable chemical composition. Overall, the Cu-Fe bimetallic oxygen carrier prepared by direct mixing method is a competitive oxygen carrier, which deserves more attention. Ó 2017 Elsevier B.V. All rights reserved.
0. Introduction With the increasing pressure of global warming caused by the anthropogenic emission of greenhouse gases (mainly CO2 from fossil fuel combustion), the measures for reducing the emission of CO2 must be taken immediately to inhibit the increase of temperature in global with allowing utilization of fossil fuels [1]. It is generally accepted that carbon capture and store (CCS) technologies are unavoidable and feasible way of diminishing the emission of CO2 from fossil fuels combustion. However, the cost of the existed CCS technologies is high. Chemical looping combustion (CLC) technology becomes more popular with inherent separating of CO2 as ⇑ Corresponding author. E-mail address: [email protected] (L. Shen). http://dx.doi.org/10.1016/j.cej.2017.01.091 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
one of the most promising technology among technologies currently developed for CCS [2–5]. The schematic illustration of CLC process is shown in Fig. 1. CLC is usually composed of two fluidized bed reactors: an air reactor (AR) and a fuel reactor (FR). The oxygen carrier particles (usually metal oxides) are used to circulate between the two reactors to carry the lattice oxygen and heat from the AR to FR for the combustion of fuel in the FR averting the direct blending of fuel and air. In the FR, the solid oxygen carrier is reduced by the fuel and then the reduced oxygen carrier is oxidized to regenerate in the AR completing a cycle. Therefore, the fuel reacts with the oxygen carrier particles only producing CO2 and vapor in the FR, theoretically. Then, after condensing the steam, the generated CO2 can be captured and stored easily with low cost. As a consequence, the CLC process does not require extra CO2 separation process reducing the cost of capturing CO2.
133
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
Fig. 1. Schematic illustration of CLC.
According to above, it can be known that the successful operation of a CLC system depends on cyclic reactions of oxygen carrier in both AR and FR. As a result, finding a suitable oxygen carrier is significant for the practice of CLC. The properties of oxygen carriers are important and the following properties are desired: high reactivity to both reduction and oxidation, high resistance to both attrition and thermal sintering, and environment benign with low cost. The metal oxides such as the oxides of Fe, Cu and Ni have been investigated as candidates of oxygen carrier, and some inert materials such as Al2O3, MgAl2O4 are used as support of manufacturing the oxygen carrier to increase the reactivity, life time and resistance to sintering [6–9]. The oxidation and reduction reactivity of Ni based oxygen carrier are high, and the results of chemical looping combustion for coal on an interconnected reactor show that the concentration of CO2 in the exhaust gas of FR can reach to above 95% in volume, but Ni is expensive and toxic [10]. Cu-based oxygen carriers are also not suggested for its low melting point even though the reactivity of Cu-based oxygen carrier is high. Although oxides of Fe are relatively low in reactivity, the reactivity of these is enough to be feasible for a suggested CLC system [11]. In addition, the oxides of Fe are abundant and environmental benign. So the iron oxides remain the most favorable. Since oxygen carrier particles will be carried out from reactor by the flue gas during the operation, the low-cost oxygen carriers are the trend [12]. The industrial by-products were tested as oxygen carrier candidate to reduce the cost of the process [13]. The ash from sewage sludge was effectively used as low-cost oxygen carrier with appreciable reaction reactivity with high resistance to attrition and thermal sintering [14]. Recently, natural iron ore has received many investigations as oxygen carriers for its abundance, low cost and environmental benign, and the results show that natural iron ore is a good selection for oxygen carrier [15–21]. Although the iron ore oxygen carrier possesses some advantages, its reactivity is not high enough to obtain a desired high conversion of solid fuel [20,22–24]. So some solutions are needed to be implemented to promote the reactivity of natural iron ore. It was investigated that the metals, Na+, K+, Ca2+, introduced to decorate the oxygen carrier could increase the reactivity of iron based oxygen carrier, but those introduced metals would cause sintering of oxygen carrier directly and volatilize during operation leading to possible ash related problems such as fouling [12,25,26]. So other methods to increase the reactivity of oxygen carrier are needed to seek. Bimetallic oxygen carriers attract attentions with some advantages overmatching single metal oxygen carrier, and Fe based oxygen carriers can be
improved by introducing other metal oxides with comparatively high reactivity such as Ni, Mn and Cu based metal oxides [27–30]. One interesting among these is the Cu-Fe bimetallic oxygen carrier. It was found that a synergistic effect between Cu and Fe oxides in improving oxygen release was observed during CLC process and the reactivity of iron oxides was enhanced [31,32]. Cu also can provide the new active sites and modify the electronic properties of iron based oxygen carriers [33,34]. Besides, Cu as a kind of oxygen carrier with relatively low cost can release gas phase O2 as the followed reaction (R1: 4CuO = 2Cu2O + O2) and can be applied to chemical-looping with oxygen uncoupling (CLOU) [35]. More importantly, it was reported that the Cu in the Cu-Fe oxygen carrier obtained a better stability in physical [31]. The investigations of Wang et al. indicated that the oxygen carriers prepared by mechanical mixing with CuO/Fe2O3 showed high reactivity with high resistance to agglomeration [36]. The limit in the application of Cu-containing oxygen carrier caused by agglomeration and sintering can be mitigated by the bimetallic oxygen carrier of Fe and Cu. Therefore, bimetallic Cu-Fe oxygen carriers composed by iron ore and Cu can be a good option and significant relevant for CLC. Recent paper showed that hematite-CuO oxygen carrier prepared by wet-impregnation has been investigated and this oxygen carrier obtains a good performance [37]. But the wet-impregnation is relatively complicated process with high cost and brings some environment problem such as the emission of NOx during calcination. So it is important to seek a better method of preparing hematite-CuO oxygen carrier with low cost and environment benign. Siriwardane et al. found that the direct mixing method was a better preparation method of Cu-Fe oxygen carriers because this method was simple and the corresponding sample showed good performance on thermogravimetric analyzer (TGA) [31,38]. The present work investigated the characteristics of the hematiteCuO with different contents of Cu prepared by mechanical mixing method on TGA and a batch fluidized bed. The cyclic performances of oxygen carriers were studied using gas fuel and solid fuel, respectively, and the effect of temperature was also investigated. In addition, the oxygen carrier after long term operation on the batch fluidized bed using coal as fuel was characterized and the loss of Cu was discussed briefly. 1. Experiment 1.1. Fuel The coal used in this work was a kind of bituminous coal produced from China. The coal was crushed and sieved to particles in a size range of 0.3–0.4 mm. The proximate analysis of this coal is shown in Table 1. 1.2. Oxygen carrier In this work, the utilized natural hematite was provided by Nanjing Steel Manufacturing Company in China. The natural Table 1 The proximate analysis of coal (wt%). M
V
FC
A
4.11
31.68
53.57
10.64
Table 2 The elemental ratios of Cu to Fe for the three oxygen carriers (wt%).
Cu/Fe
HC5
HC10
HC20
0.0707
0.1451
0.2720
134
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
hematite is from the Rio Tinto Company of Australia. The hematite particles (0.2–0.3 mm) were calcinated in a muffle oven at 950 °C for 3 h and utilized to prepare the bimetallic oxygen carrier. The bimetallic oxygen carrier hematite-CuO was prepared by direct mixing method: the powder of industrial-grade CuO (98%)
was mixed with the calcinated hematite particles thoroughly by a blender (CH20, Changzhou Lidu Drying Equipment Co., Ltd, China) at the room temperature (about 30 °C) in the weight ratio of 5:95, 10:90 and 20:80, which were labelled as HC5, HC10 and HC20, respectively. And then, the deionized water was added into
Fig. 2. The XRD results of HC5, HC10 and HC20 oxygen carriers.
135
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
the mixture to obtain a paste. At last, after 24 hour’s standing at room temperature for drying, the mixture was calcinated at 900 °C for 8 h in a muffle oven. The prepared oxygen carrier hematite-CuO was sieved to yield in a size range of 0.2–0.3 mm. According to the X-ray fluorescence (XRF) analysis results of these manufactured oxygen carriers, the ratios of Cu to Fe in weight for HC5, HC10 and HC20 are showed in Table 2. The ratios of Cu to Fe increase with the set weight percentage of Cu in oxygen carrier proportionally indicating that Cu can be loaded on the hematite particles for three hematite-CuO oxygen carriers efficiently by this method. During the calcination process, CuO reacted with a fraction of Fe2O3 in hematite forming CuFe2O4 above the temperature of 488 °C and the reactions could be described by R2 (R2: CuO + Fe2O3 = CuFe2O4) based on the thermodynamic calculations with Factsage software [31]. This is identified by the X-ray Diffraction (XRD) results shown in Fig. 2. Since the percentage of Fe2O3 is much higher than that of CuO, most of the iron in the hematite is still in the form of Fe2O3. In addition, no CuO is detected. This means that all the Cu in the hematite-CuO oxygen carriers exists in the form of CuFe2O4. 1.3. Thermogravimetric analyzer (TGA) Taking the three prepared oxygen carriers as examples, experiments on the performance in reactivity of the oxygen carriers were conducted on TGA. About 50 mg of oxygen carrier was loaded into quartz crucible with 5 mm diameter and 4 mm deep. The sample was heated to designed temperature at a heating rate of 20 °C/ min in air atmosphere, and, the inlet gas was switched to N2 (100 ml/min) in about 5 min. At last, after 5 min, the intake flow was changed to the mixture of N2 and H2 (100 ml/min and 100 ml/min) for about 35 min. 1.4. Batch fluidized bed The cyclic experiments were conducted on a batch fluidized bed reactor as shown in Fig. 3. This system consists of three parts: inlet flow, reactor and flue gas analyzer. The gas flow is controlled by mass flow controller. The reactor is straight iron tube with 32 mm inner diameter and 600 mm height. In the tube reactor, porous distributor plate is located at 300 mm from the bottom.
The reactor is heated by electric heating furnace and the temperature in reactor is detected by thermocouple. The outlet gas was sampled by gas bags for offline analysis per 1 or 2 min and the compositions of the flue gas are analyzed by a NGA 2000 type gas analyzer (Emerson Company USA). In each test for gas fuel, a sample of around 30 g oxygen carrier was placed on the distributor plate. When the temperature of reactor reached designed value and remained stable in N2 + O2 (2 L/ min + 100 ml/min) atmosphere, the inlet gas was switched to the mixture of CO and N2 (100 ml/min and 2 L/min) for 20 min completing the reduction process after purging 10 min using pure N2. During the process of oxidization, the mixture of O2 (100 ml/min) and N2 (2 L/min) was used as oxidizing gas. The N2 purge stage (2 mL/min) between the reduction and oxidation process maintained for 10 min. In the experiments for solid fuel, the steam is used as gasification agent. The deionized water was introduced by a macropump and then heated to 150 °C by steam generator. In each case for coal, a sample of around 30 g oxygen carrier was placed on the distributor plate. When the temperature of reactor reached designed value and remained stable in N2 and steam (2 L/min and 0.4 g/min) atmosphere. After that, the fuel (0.5 g) was added into reactor from the top of reactor. Simultaneously, the reactions of the fuel with oxygen carrier started and gas sampling began. The reduction duration was set to 50 min. The mixture of O2 (100 ml/ min) and N2 (2 L/min) was used in oxidization process. The N2 purge stage (2 mL/min) between the reduction and oxidation process maintained for 10 min. 1.5. Data The weight-loss ratio, Xw:t , represents the fractional reduction and it is define as followed,
Xw:t ¼
mt m0 mf m0
ð1Þ
where mt is the instantaneous weight of oxygen carrier, mf is the weight of oxygen carrier sample after reaction and m0 is the weight of fresh oxygen carrier sample. This parameter shows the weight loss ratio of oxygen carrier meaning the reactivity of oxygen carrier.
Valve 2 Thermocouple and Temperature Controller T H2 O
CO
N2
Reservoir Valve 1
Air Filter
Pump Steam Generator
Emerson
Vent
CO: 0-100% CO2: 0-100% CH4: 0-10% O2: 0-25% H2: 0-50%
Mass Flow Controller
Vent Thermocouple T Inlet Flow
Fig. 3. Schematic diagram of batch fluidized bed reactor.
Drier
136
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
The accumulative CO molar, Sam,CO.t, is defined as the sum of molar value of the CO detected during the first t minutes as followed (t is lower than 20).
Sac:CO:t ¼
X
q X CO dt ð1 X CO X CO2 Þ 22:4
ð2Þ
In this equal, q, Xi (i = CO, CO2) and dt is the gas flow rate of N2, the volume concentration of gas i and the period of obtaining each gas sample during reduction process, respectively. This parameter indicates the amount of oxygen released by the oxygen carrier to CO in the t minutes. The lower the value of the accumulative CO amount reaches, the more the oxygen is released to CO by the oxygen carrier and the higher reactivity the oxygen carrier exhibits during the reduction time. 2. Results and discussion 2.1. Performance of oxygen carrier on TGA Fig. 4 shows the results of experiments on TGA. Even though the CuO in the oxygen carrier of hematite-CuO existed in the form of CuFe2O4, weight loss also appeared in N2 atmosphere. This exhibits that the CuFe2O4 in the bimetallic metal oxygen carriers also possesses the ability of release gas phase O2 similar to the single metal oxide of CuO, which is in line with the results of Siriwardane [31]. After that, the weight loss could be ascribed to the reduction of oxygen carrier caused by the H2 at 900 °C. According to the results on the TGA, it is easier to conclude that the reactivity of oxygen carrier increases with the Cu content.
Fig. 4. The experiment results on TGA.
1.5
HC5
2.2. The cyclic characteristics of oxygen carrier To evaluate the cyclic characteristics of the oxygen carriers, cycle experiments were conducted on the batch fluidized bed using CO and coal as fuel, respectively, at 900 °C. In addition, the influence of temperature was also investigated with using coal as fuel. 2.2.1. The cycle characteristics of oxygen carriers with CO The Fig. 5 shows the concentration of CO in the flue gas for oxygen carriers HC5, HC10 and HC20 versus reduction time among different cycles (cycles, 1, 5, 8). The profiles corresponding to different cycles are similar in shape, and the concentration of CO increases with time for each oxygen carrier. When the flow gas of CO was introduced to the reactor, it was oxidized into CO2 by the oxygen carrier immediately but incompletely. So the CO existed in the exhaust gas during the reduction period. With the proceeding of the experiment, the oxygen carrier was reduced gradually and the reactivity of oxygen carrier for CO decreased [21,31]. As a result, the concentrations of CO increased with time. For the HC5 and HC10, the CO concentrations corresponding cycle 5 and cycle 8 are almost same and lower than that corresponding cycle 1 in the first t minutes (t = 8 for HC5 and t = 9 for HC10). In addition, the CO concentration corresponding cycle 8 begin to be lower than that corresponding cycle 5 in few minutes after the tth minute for HC5 and HC10. For the oxygen carrier of HC20, the CO concentrations for cycle 5 and cycle 8 are nearly same during all the reduction time and still lower than that for cycle 1 during first minutes. The lower the CO concentration reaches at the same time, the higher reactivity the oxygen carrier presents. For the three oxygen carriers, the oxygen carriers after cyclic operation exhibit a higher reactivity than the fresh oxygen carrier during first minutes. The increase in reactivity of oxygen carrier can be ascribed to the increase of micro pores caused by the migration of Cu ion facilitating the gas diffusion [31,36,37]. The Fig. 6 shows the accumulative molars of CO versus the reduction time among different cycles for oxygen carriers HC5, HC10, HC20. The profiles with semblable forms for different cycles exhibit an increase with reduction time for every oxygen carrier. For the three oxygen carriers, the accumulative molars of CO corresponding cycle 5 and cycle 8 are same approximately and lower than that corresponding cycle 1 during the first 10 min. For HC5 and HC10, the value of the accumulative molars of CO at each minute decreases with cycle in the 20 min. But for HC20, after about 12 min, the value of the accumulative molars of CO corresponding cycle 5 is the highest and that corresponding cycle 1 is the lowest at every time point. The decrease of accumulative molars of CO means the increase of oxygen released by the oxygen carrier during reduction time and reactivity of the oxygen carrier. For HC5 and HC10, the decrease of accumulative molars of CO with cycles can
HC10
2.0
HC20
1
cycle 1 cycle 5 cycle 8
0
The concentration of CO (%)
The concentration of CO(%)
The concentration of CO (%)
2
1.0
0.5
cycle 1 cycle 5 cycle 8
0.0
5
10
Time (min)
15
20
1.5
1.0
cycle 1 cycle 5 cycle 8
0.5
0.0
5
10
15
20
Time (Min)
Fig. 5. The CO concentration versus reduction time among cycles for oxygen carriers.
5
10
Time (min)
15
20
137
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142 0.025
0.022
HC10
HC20
0.020
0.015
0.010
cycle 1 cycle 5 cycle 8
0.005
0.020
0.018
Accumulative CO amount (mol)
Accumulative CO amount (mol)
0.020
Accumulative CO amount (mol)
0.025
0.024
HC5
0.016 0.014 0.012 0.010 0.008
cycle 1 cycle 5 cycle 8
0.006 0.004
0.000
10
15
0.010
cycle 1 cycle 5 cycle 8
0.005
0.002
0.000 5
0.015
20
0.000
5
10
Time (min)
15
20
5
Time (min)
10
15
20
Time (min)
Fig. 6. The accumulative CO molars versus reduction time among cycles for oxygen carriers.
Fig. 7. The images of HC20 (A, fresh HC20, B, reacted HC20 using CO as fuel after 8 cycles).
2COðgÞ ¼ C þ CO2 ðgÞ
ðR3Þ
When the gas flow of CO was introduced to reactor, CO reached to the surface of oxygen carrier particle reacting with the metal oxides on the surface of particles firstly, and then diffused to internal reducing the metal oxides in the interior of oxygen carrier. So the Fe2O3 on the surface is much easier to be reduced to Fe facilitating the generating the deposition of carbon than that in the internal of oxygen carrier. So the carbon main deposits on the surface of oxygen carrier particle in the form of Fe3C [41]. While the reduced oxygen carrier was oxidized by oxygen gas, the carbon deposited on the oxygen carrier was oxidized to CO2. Fig. 8 shows the generation of CO2 during oxidation period for HC10. The concentration of CO2 produced during oxidization process decreases with reaction time and cycles. The decrease of CO2 produced during oxidization period with cycles may be caused by increasing of micro pores facilitating the diffusion of gas, for which more CO reacts with internal metal oxides of particle decreasing the generation of Fe on the surface.
cycle 1 cycle 5 cycle 8
0.08
CO2 concentration
be explained by the increase of reactivity of oxygen carrier. In view of the accumulative molars of CO, HC10 is better than HC5 but HC20 is worst among the oxygen carriers. The higher of accumulative molars of CO for HC20 was caused by the agglomeration shown in Fig. 7. No apparent agglomeration was observed in the oxygen carrier for HC5 and HC10 after cyclic operation on the small scale reactors. HC5 and HC10 can be candidate for oxygen carrier that can operate on interconnected facility or industrial unit and HC10 performances better. Furthermore, the carbon deposition was investigated briefly in this section. The carbon deposition will lead to the decrease of reactivity and degradation of oxygen transfer capacity [39]. Besides, the deposited carbon will be oxidized to CO2 in AR and this will decrease the carbon capture efficiency for chemical looping combustion. Therefore, the carbon formation is important for oxygen carrier. The carbon deposition formed through the way of the reaction, R3 and the reaction is slow but the reaction will be catalyzed readily to occur with the presence of Fe [40].
0.06
0.04
0.02
0.00 2
4
6
8
10
12
Time (min) Fig. 8. The CO2 production during the oxidization process among cycles for HC10.
2.2.2. The cycle characteristics of oxygen carriers with coal Due to the low cost and the abundance of coal, it will be an advantage to apply chemical looping combustion to coal. Based on the results of above section, some experiments were conducted using coal as fuel on the batch fluidized bed using HC10 and HC5 as oxygen carrier at 900 °C. The loss of Cu by abrasion is a problem during reaction [37], so a long time operation (about 800 min) was conducted to study the loss of Cu briefly. Figs. 9 and 10 show the volume fractions of produced carbonaceous gaseous species (CO, CO2 and CH4) in the flue gas during the experiment for cycle 1, cycle 5 and cycle 8 with using HC10 and HC5 as oxygen carriers, respectively. The profiles corresponding to the different carbonaceous gas are similar. The maximum values for CO, CO2 and CH4 are obtained at the first minute and then decreased sharply versus reaction time. The high concentration of the produced carbonaceous gas during the first minutes is mainly caused by the release of volatile with the high generation
138
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142 2.0
CO CO2
1.5
The concentration of gas (%)
The concentration of gas (%)
2.0
cycle 5
cycle 1
CH4
1.0
0.5
0.0
cycle 8
CO CO2 CH4
1.5
The concentration of gas (%)
2.0
1.0
0.5
10
20
30
40
50
CH 4
1.5
1.0
0.5
0.0
0.0 0
CO CO2
0
10
20
Time (min)
30
40
50
0
10
20
Time (min)
30
40
50
Time (min)
Fig. 9. The carbonaceous gas concentration with using HC10 as oxygen carrier among different cycles. 2.0
2.0
CH4
1.5
2.0
1.0
0.5
0.0
cycle 8
CO CO2
cycle 5
CH4
1.5
The concentration of gas (%)
CO CO2
The concentration of gas (%)
The concentration of gas (%)
cycle 1
1.0
0.5
0.0 0
10
20
30
40
50
CO CO 2 CH 4
1.5
1.0
0.5
0.0 0
10
20
30
40
50
0
10
20
Time (min)
Time (min)
30
40
50
Time (min)
Fig. 10. The carbonaceous gas concentration with using HC5 as oxygen carrier among different cycles.
rate. From the results exhibited in Fig. 11, it can be known that there is still some residue carbon after reactions proceed 50 min, and the amount of residue carbon for HC10 is less than that for HC5. This means that HC10 is superior to HC5 in converting the coal into gas. The concentrations of carbonaceous gas corresponding to cycle 1, cycle 5 and cycle 8 are similar without too much change indicating that the oxygen carrier possessed the good long-term reactivity. In addition, no agglomeration appeared during this long term operation. It can be concluded that both the HC10 and HC5 are candidate for applying into coal combustion as oxygen carrier. Temperature has a significant influence on the reactivity of oxygen carriers and gasification of the coal. So the effects of temperature were investigated on the batch fluidized reactor using HC10 as oxygen carrier. The effect of temperature on the CO2 concentration in the exhaust gas of the reactor is shown in Fig. 12a. The CO2 concentrations increase with the increasing of reaction temperature. During the reaction process, the gasification of coal and the reduction of the oxygen carrier occurred. Gasification consists of two
0.40
HC5 HC10
0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
2
4
6
Time
8
10
min)
Fig. 11. The concentration of CO2 produced during oxidization period.
a
800 oC 830 oC 860 oC 900 oC
b
1.5
The CO2 concentration (%)
1.5
The CO2 concentration (%)
The concentration of CO2 (%)
0.35
1.0
0.5
900oC
1.0
0.5
0.0
0.0 0
10
20
30
Time (min)
40
50
0
10
20
30
40
Time (min)
Fig. 12. a, The CO2 concentration for HC10 at different temperatures, b, the CO2 concentration without oxygen carrier at 900 °C.
50
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
Fig. 13. The SEM images of oxygen carriers (a, fresh HC5, b, reacted HC5, c, fresh HC10, d, reacted HC10).
Fig. 14. The EDX analysis results of oxygen carriers (a, the fresh HC5, b, the reacted HC5 with coal, c, the fresh HC10, d, the reacted HC10 with coal).
139
140
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
successive processes: pyrolysis and char gasification (R4)–(R5) [21].
Pyrolysis : coal ! char þ volatile
ðR4Þ
Char gasification : C þ H2 O ! CO þ H2
ðR5Þ
Since the R4 and R5 are endothermic reactions, the increasing of temperature accelerates the gasification of coal producing more carbonaceous gas and the generated gas can be oxidized by the oxygen carrier into CO2, which mainly the reason of the increasing of CO2 with temperature. According to Fig. 12b, it can be known that the existence of oxygen carrier has a positive effect in generation of CO2. As a result, the increasing reactivity of oxygen carrier with the increase of temperature is the other reason of the increasing of CO2. It can be seen clearly that the CO2 concentrations for the 800 °C, 830 °C and 860 °C are same after three minutes and much lower than that for 900 °C. Therefore, higher temperature, above 900 °C, is needed to obtain a desired conversion rate and conversion efficiency for solid fuel of coal.
ages of Cu for reacted HC5 and HC10 are lower than corresponding that for fresh HC5 and HC10, respectively. After reaction, the ratio of Cu to Fe on the surface of HC5 becomes to 0.094 from 0.106 and that of HC10 changes to 0.370 from 0.522 indicating some decrease of Cu content on the oxygen carrier particle surface after operation. This change can be explained by the loss of Cu caused by surface abrasion and migration into internal of particles from surface during cycle experiments [37]. In addition, XRF analysis is also applied into the HC5 and HC10 after one cyclical experiment (about 100 min) and 8 cyclical experiments (about 800 min), respectively, and the results are shown in Table 4 as the ratio of copper to iron in weight. The decrease of Cu to Fe in ratio is caused by the loss of Cu. Abrasion can be the main
3. Characteristics of oxygen carrier The oxygen carriers were characterized by scanning electron microscope (SEM), Energy Dispersive X-ray Detector (EDX), XRF and XRD to understand the effect of Cu ions on the hematite and existence status of the Cu oxygen carrier after long term operation. The images of SEM for fresh and reacted oxygen carriers are shown in Fig. 13. The reacted oxygen carriers are HC5 and HC10 after eight cyclical experiments (800 min) using coal as fuel at 900 °C. The magnification of 10,000 was used to analyze the surface characteristics for oxygen carriers. The surface micro-pores decrease with increasing of Cu loading, so the fresh HC10 has a denser surface than fresh HC5, which is observed in the Fig. 13 [42]. The difference between fresh and reacted oxygen carriers is shown clearly. For the fresh oxygen carriers of HC5 and HC10, the micro-pores are rare and the surfaces are dense. However, morphological changes appear on the surface of reacted oxygen carrier particles after long term operation. More pores are observed on the reacted oxygen carrier, which can be ascribed to the migration of Cu during reaction period and explain the increase of reactivity of oxygen carriers with cyclic operation in the Section 2.2.1. In addition, no obvious agglomeration phenomenon caused by introducing iron of Cu is detected on the surface of fresh and reacted oxygen carriers for HC5 and HC10. Fig. 14 shows the EDX analyses results for fresh and reacted oxygen carriers. And these results in the form of elemental compositions expressed by weight percentage are shown in Table 3. It is clear that the surface of fresh and reacted oxygen carriers for HC5 and HC10 are mainly composed of Fe, Cu, Al, Si and O. The percent-
Fig. 15. The XRD analysis of reacted oxygen carriers.
Table 3 Elemental compositions on the surface of fresh and reacted oxygen carrier (wt%).
Fresh HC5 Reacted HC5 Fresh HC10 Reacted HC10
Fe
Cu
Si
Al
O
Cu/Fe
60.31 63.87 39.66 48.93
6.41 6.01 20.70 18.10
2.04 1.31 0.95 1.38
1.08 1.14 0.55 0.41
29.83 27.18 37.23 30.73
0.106 0.094 0.522 0.370
Table 4 The elemental ratio of Cu to Fe for HC5 and HC10 (wt%).
Cu/Fe
HC5 HC10
Fresh
Cycle 1
Cycle 8
0.0707 0.1451
0.0688 0.1211
0.0634 0.1117
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
141
Fig. 16. Element mapping of oxygen carriers. Red: Fe; green: Cu. (a, fresh HC5, b, reacted HC5 with coal, c, fresh HC10, d, reacted HC10 with coal.
reason of the Cu loss. According to the results, it can be known that the Cu is disappearing with cycles and the loss proportion of Cu is decreasing with cycles. The phase analysis is significant to understand the role of Cu in the oxygen carrier and the side reactions during the CLC process using hematite-CuO as oxygen carrier. The existence form of Cu in the reacted oxygen carrier was identified by XRD and the XRD analysis results of reacted HC5 and HC10 are shown in Fig. 15. According to the XRD results in Section 1.2, CuO loaded on the oxygen carrier reacted with Fe2O3 generating CuFe2O4. After long time operation, all loaded Cu is still in the form of CuFe2O4. There was no change in existence substance between the reacted oxygen carriers and fresh oxygen carriers, and no side reactions occurred. The conclusion of the hematite-CuO possessing a stable chemical phase characteristic can be obtained. The distribution of Cu is related to the sintering and agglomeration of oxygen carriers at high temperature and it is determined by the preparing method [33]. Fig. 16 shows the element mapping of the fresh oxygen carriers and the reacted oxygen carriers. It is clear that the distribution of element Cu is comparatively uniform for fresh and reacted oxygen carriers (HC5 and HC10) retarding the sintering of Cu at high temperature and improving the physical stability of CuO in the bimetallic oxygen carriers [33]. It also can be concluded that this method of preparing hematite-CuO oxygen carrier is acceptable. 4. Conclusions In this study, the bimetallic oxygen carrier of Fe-Cu prepared by direct mixing method was investigated on the TGA and a batch fluidized bed. According the results, these following conclusions may be drawn: The CuO can be loaded on the hematite efficiently in the form of CuFe2O4 through direct mixing method. The TGA results show that the oxygen carrier can release oxygen during N2 atmosphere and the reactivity of oxygen carriers increases with the content of Cu. During cyclic operation in the batch fluidized bed using CO as fuel, some agglomeration issues occur for HC20. There is no apparent agglomeration and sintering appeared on HC5 and HC10 fueled
by gas of CO and coal. In addition, both HC5 and HC10 show increasing reactivity with cycles caused by forming of micropores during reaction shown SEM images. Long time circulation indicates that HC5 and HC10 possess good long term operation characteristics. HC5 and HC10 can be excellent candidates for the CLC combustion of coal. The loss of Cu caused by attrition in the oxygen carrier is observed after operation according to EDX and XRF results. After one cycle, the loss rate becomes much low. Besides, the XRD results show that the Cu in the oxygen carrier also exists in the form of CuFe2O4 after long time operation with coal as fuel. No side reactions occur during the reaction process. The element mappings show that distribution of element Cu is much uniform for fresh and reacted oxygen carriers (HC5 and HC10) retarding the sintering and facilitating a stable structure. These indicate that the preparing method can be excellent candidate of manufacturing the bimetallic oxygen carrier of Fe-Cu. But this needs further investigations on the interconnected reactor. Acknowledgements We gratefully acknowledge the support of this research work by the National Natural Science Foundation of China (Grant Nos. 51476029, 51276037, 51561125001 and 51406035). References [1] R.S. Haszeldine, Carbon capture and storage: how green can black be?, Science 325 (5948) (2009) 1647–1652 [2] A. Pettersson, L.E. Åmand, B.M. Steenari, Leaching of ashes from co-combustion of sewage sludge and wood—Part I: recovery of phosphorus, Biomass Bioenergy 32 (3) (2008) 224–235. [3] M.B. Folgueras, R.M. Dı´az, J. Xiberta, Sulphur retention during co-combustion of coal and sewage sludge, Fuel 83 (10) (2004) 1315–1322. [4] C. Storm, H. Rüdiger, H. Spliethoff, Co-pyrolysis of Coal/Biomass and Coal/ Sewage Sludge Mixtures/ASME 1998 International Gas Turbine and Aeroengine Congress and Exhibition, American Society of Mechanical Engineers, 1998, V003T05A006-V003T05A006. [5] Y. Cao, W.P. Pan, Investigation of chemical looping combustion by solid fuels. 1. Process analysis, Energy Fuels 20 (5) (2006) 1836–1844. [6] J. Adánez, L.F. de Diego, F. García-Labiano, Selection of oxygen carriers for chemical-looping combustion, Energy Fuels 18 (2) (2004) 371–377.
142
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
[7] Y. Cao, B. Casenas, W.P. Pan, Investigation of chemical looping combustion by solid fuels. 2. redox reaction kinetics and product characterization with coal, biomass, and solid waste as solid fuels and CuO as an oxygen carrier, Energy Fuels 20 (5) (2006) 1845–1854. [8] T. Mattisson, M. Johansson, A. Lyngfelt, Multicycle reduction and oxidation of different types of iron oxide particles application to chemical-looping combustion, Energy Fuels 18 (3) (2004) 628–637. [9] F. Luis, F. Garcı, P. Gayán, et al., Operation of a 10kWth chemical-looping combustor during 200 h with a CuO–Al2O3 oxygen carrier, Fuel 86 (7) (2007) 1036–1045. [10] L. Shen, J. Wu, J. Xiao, Experiments on chemical looping combustion of coal with a NiO based oxygen carrier, Combust. Flame 156 (3) (2009) 721–728. [11] P. Cho, T. Mattisson, A. Lyngfelt, Comparison of iron-, nickel-, copper-and manganese-based oxygen carriers for chemical-looping combustion, Fuel 83 (9) (2004) 1215–1225. [12] J. Bao, Z. Li, N. Cai, Promoting the reduction reactivity of ilmenite by introducing foreign ions in chemical looping combustion, Ind. Eng. Chem. Res. 52 (18) (2013) 6119–6128. [13] P. Moldenhauer, M. Rydén, A. Lyngfelt, Testing of minerals and industrial byproducts as oxygen carriers for chemical-looping combustion in a circulating fluidized-bed 300 W laboratory reactor, Fuel 93 (2012) 351–363. [14] E. Ksepko, Sewage sludge ash as an alternative low-cost oxygen carrier for chemical looping combustion, J. Therm. Anal. Calorim. 116 (3) (2014) 1395– 1407. [15] H. Leion, E. Jerndal, B.M. Steenari, et al., Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers, Fuel 88 (10) (2009) 1945–1954. [16] T. Mattisson, A. Lyngfelt, P. Cho, The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2, Fuel 80 (13) (2001) 1953–1962. [17] T. Song, T. Shen, L. Shen, Evaluation of hematite oxygen carrier in chemicallooping combustion of coal, Fuel 104 (2013) 244–252. [18] H. Leion, E. Jerndal, B.M. Steenari, Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers, Fuel 88 (10) (2009) 1945–1954. [19] H. Leion, T. Mattisson, A. Lyngfelt, Use of ores and industrial products as oxygen carriers in chemical-looping combustion, Energy Fuels 23 (4) (2009) 2307–2315. [20] H. Gu, L. Shen, J. Xiao, Chemical looping combustion of biomass/coal with natural iron ore as oxygen carrier in a continuous reactor, Energy Fuels 25 (1) (2010) 446–455. [21] S. Jiang, L. Shen, X. Niu, et al., Chemical looping co-combustion of sewage sludge and zhundong coal with natural hematite as the oxygen carrier, Energy Fuels 30 (3) (2016) 1720–1729. [22] M.M. Azis, E. Jerndal, H. Leion, et al., On the evaluation of synthetic and natural ilmenite using syngas as fuel in chemical-looping combustion (CLC), Chem. Eng. Res. Des. 88 (11) (2010) 1505–1514. [23] H. Ge, L. Shen, H. Gu, et al., Combustion performance and sodium transformation of high-sodium ZhunDong coal during chemical looping combustion with hematite as oxygen carrier, Fuel 159 (2015) 107–117. [24] X. Niu, L. Shen, H. Gu, et al., Characteristics of hematite and fly ash during chemical looping combustion of sewage sludge, Chem. Eng. J. 268 (2015) 236– 244.
[25] Z. Yu, C. Li, Y. Fang, et al., Reduction rate enhancements for coal direct chemical looping combustion with an iron oxide oxygen carrier, Energy Fuels 26 (4) (2012) 2505–2511. [26] H. Gu, L. Shen, J. Xiao, et al., Iron ore as oxygen carrier improved with potassium for chemical looping combustion of anthracite coal, Combust. Flame 159 (7) (2012) 2480–2490. [27] S. Bhavsar, G. Veser, Bimetallic Fe–Ni oxygen carriers for chemical looping combustion, Ind. Eng. Chem. Res. 52 (44) (2013) 15342–15352. [28] F. He, N. Galinsky, F. Li, Chemical looping gasification of solid fuels using bimetallic oxygen carrier particles–feasibility assessment and process simulations, Int. J. Hydrogen Energy 38 (19) (2013) 7839–7854. [29] A. Lambert, C. Delquié, I. Clémeneçon, et al., Synthesis and characterization of bimetallic Fe/Mn oxides for chemical looping combustion, Energy Procedia 1 (1) (2009) 375–381. [30] S. Bhavsar, B. Tackett, G. Veser, Evaluation of iron-and manganese-based mono-and mixed-metallic oxygen carriers for chemical looping combustion, Fuel 136 (2014) 268–279. [31] R. Siriwardane, H. Tian, T. Simonyi, et al., Synergetic effects of mixed copper– iron oxides oxygen carriers in chemical looping combustion, Fuel 108 (2013) 319–333. [32] T. Mattisson, A. Järdnäs, A. Lyngfelt, Reactivity of some metal oxides supported on alumina with alternating methane and oxygen application for chemicallooping combustion, Energy Fuels 17 (3) (2003) 643–651. [33] L. Zhang, X. Wang, J.M.M. Millet, et al., Investigation of highly active Fe-Al-Cu catalysts for water-gas shift reaction, Appl. Catal. A 351 (1) (2008) 1–8. [34] C. Rhodes, G.J. Hutchings, Studies of the role of the copper promoter in the iron oxide/chromia high temperature water gas shift catalyst, Phys. Chem. Chem. Phys. 5 (12) (2003) 2719–2723. [35] H. Leion, T. Mattisson, A. Lyngfelt, Using chemical-looping with oxygen uncoupling (CLOU) for combustion of six different solid fuels, Energy Procedia 1 (1) (2009) 447–453. [36] S. Wang, G. Wang, F. Jiang, et al., Chemical looping combustion of coke oven gas by using Fe2O3/CuO with MgAl2O4 as oxygen carrier, Energy Environ. Sci. 3 (9) (2010) 1353–1360. [37] W. Yang, H. Zhao, J. Ma, et al., Copper-decorated hematite as an oxygen carrier for in situ gasification chemical looping combustion of coal, Energy Fuels 28 (6) (2014) 3970–3981. [38] R.V. Siriwardane, E. Ksepko, H. Tian, et al., Interaction of iron–copper mixed metal oxide oxygen carriers with simulated synthesis gas derived from steam gasification of coal, Appl. Energy 107 (2013) 111–123. [39] M.K. Chandel, A. Hoteit, A. Delebarre, Experimental investigation of some metal oxides for chemical looping combustion in a fluidized bed reactor, Fuel 88 (5) (2009) 898–908. [40] B. Wang, R. Yan, D.H. Lee, et al., Thermodynamic investigation of carbon deposition and sulfur evolution in chemical looping combustion with syngas, Energy Fuels 22 (2) (2008) 1012–1020. [41] M. Ishida, H. Jin, T. Okamoto, Kinetic behavior of solid particle in chemicallooping combustion: suppressing carbon deposition in reduction, Energy Fuels 12 (2) (1998) 223–229. [42] L. Dong, Z. Liu, Y. Hu, et al., Dispersion and reduction behavior of CuO/a-Fe2O3 systems, J. Chem. Soc. Faraday Trans. 94 (19) (1998) 3033–3038.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
The investigations of hematite-CuO oxygen carrier in chemical looping combustion Shouxi Jiang, Laihong Shen ⇑, Jian Wu, Jingchun Yan, Tao Song Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China
h i g h l i g h t s Hematite-CuO interaction for the direct mixing method of Fe-Cu bimetallic oxygen carrier. Effect of Cu in oxygen carrier on reactivity and stability in structure. Cyclic characteristics of oxygen carriers in fluidized reactor and Cu loss with cycle. The distribution of Cu and exist form in the oxygen carriers.
a r t i c l e
i n f o
Article history: Received 1 December 2016 Received in revised form 20 January 2017 Accepted 21 January 2017 Available online 23 January 2017 Keywords: Chemical looping combustion Bimetallic oxygen carrier Hematite Copper
a b s t r a c t Cu-Fe bimetallic oxygen carrier is a promising candidate for chemical looping combustion, but the cost of manufacture is a significant limit for its application. In this study, the bimetallic oxygen carrier was prepared using hematite and CuO simply through direct mixing method with low cost. The cyclic combustion performance and effects of temperature were investigated in a batch fluidized bed. Cycle experiments with gas fuel indicated that the bimetallic oxygen carrier possessed increasing reactivity with cycle, which could be ascribed to improved porous structure. However, some agglomeration occurred when the content of Cu reached 20% in weight. Besides, the cyclic operation with coal showed that the bimetallic oxygen carriers with Cu ratios of 5% and 10% possessed good long term operation characteristics and the oxygen carrier with 10% Cu performed better. The conversion of coal increased with the reaction temperature and higher temperature, above 900 °C, was needed for fast conversion of coal. Additionally, the manufactured oxygen carriers were characterized. In the bimetallic oxygen carriers, Cu distributed uniformly on the surface of particle but the content of Cu decreased with cycles caused by attrition. Besides, the bimetallic oxygen carrier possessed a stable chemical composition. Overall, the Cu-Fe bimetallic oxygen carrier prepared by direct mixing method is a competitive oxygen carrier, which deserves more attention. Ó 2017 Elsevier B.V. All rights reserved.
0. Introduction With the increasing pressure of global warming caused by the anthropogenic emission of greenhouse gases (mainly CO2 from fossil fuel combustion), the measures for reducing the emission of CO2 must be taken immediately to inhibit the increase of temperature in global with allowing utilization of fossil fuels [1]. It is generally accepted that carbon capture and store (CCS) technologies are unavoidable and feasible way of diminishing the emission of CO2 from fossil fuels combustion. However, the cost of the existed CCS technologies is high. Chemical looping combustion (CLC) technology becomes more popular with inherent separating of CO2 as ⇑ Corresponding author. E-mail address: [email protected] (L. Shen). http://dx.doi.org/10.1016/j.cej.2017.01.091 1385-8947/Ó 2017 Elsevier B.V. All rights reserved.
one of the most promising technology among technologies currently developed for CCS [2–5]. The schematic illustration of CLC process is shown in Fig. 1. CLC is usually composed of two fluidized bed reactors: an air reactor (AR) and a fuel reactor (FR). The oxygen carrier particles (usually metal oxides) are used to circulate between the two reactors to carry the lattice oxygen and heat from the AR to FR for the combustion of fuel in the FR averting the direct blending of fuel and air. In the FR, the solid oxygen carrier is reduced by the fuel and then the reduced oxygen carrier is oxidized to regenerate in the AR completing a cycle. Therefore, the fuel reacts with the oxygen carrier particles only producing CO2 and vapor in the FR, theoretically. Then, after condensing the steam, the generated CO2 can be captured and stored easily with low cost. As a consequence, the CLC process does not require extra CO2 separation process reducing the cost of capturing CO2.
133
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
Fig. 1. Schematic illustration of CLC.
According to above, it can be known that the successful operation of a CLC system depends on cyclic reactions of oxygen carrier in both AR and FR. As a result, finding a suitable oxygen carrier is significant for the practice of CLC. The properties of oxygen carriers are important and the following properties are desired: high reactivity to both reduction and oxidation, high resistance to both attrition and thermal sintering, and environment benign with low cost. The metal oxides such as the oxides of Fe, Cu and Ni have been investigated as candidates of oxygen carrier, and some inert materials such as Al2O3, MgAl2O4 are used as support of manufacturing the oxygen carrier to increase the reactivity, life time and resistance to sintering [6–9]. The oxidation and reduction reactivity of Ni based oxygen carrier are high, and the results of chemical looping combustion for coal on an interconnected reactor show that the concentration of CO2 in the exhaust gas of FR can reach to above 95% in volume, but Ni is expensive and toxic [10]. Cu-based oxygen carriers are also not suggested for its low melting point even though the reactivity of Cu-based oxygen carrier is high. Although oxides of Fe are relatively low in reactivity, the reactivity of these is enough to be feasible for a suggested CLC system [11]. In addition, the oxides of Fe are abundant and environmental benign. So the iron oxides remain the most favorable. Since oxygen carrier particles will be carried out from reactor by the flue gas during the operation, the low-cost oxygen carriers are the trend [12]. The industrial by-products were tested as oxygen carrier candidate to reduce the cost of the process [13]. The ash from sewage sludge was effectively used as low-cost oxygen carrier with appreciable reaction reactivity with high resistance to attrition and thermal sintering [14]. Recently, natural iron ore has received many investigations as oxygen carriers for its abundance, low cost and environmental benign, and the results show that natural iron ore is a good selection for oxygen carrier [15–21]. Although the iron ore oxygen carrier possesses some advantages, its reactivity is not high enough to obtain a desired high conversion of solid fuel [20,22–24]. So some solutions are needed to be implemented to promote the reactivity of natural iron ore. It was investigated that the metals, Na+, K+, Ca2+, introduced to decorate the oxygen carrier could increase the reactivity of iron based oxygen carrier, but those introduced metals would cause sintering of oxygen carrier directly and volatilize during operation leading to possible ash related problems such as fouling [12,25,26]. So other methods to increase the reactivity of oxygen carrier are needed to seek. Bimetallic oxygen carriers attract attentions with some advantages overmatching single metal oxygen carrier, and Fe based oxygen carriers can be
improved by introducing other metal oxides with comparatively high reactivity such as Ni, Mn and Cu based metal oxides [27–30]. One interesting among these is the Cu-Fe bimetallic oxygen carrier. It was found that a synergistic effect between Cu and Fe oxides in improving oxygen release was observed during CLC process and the reactivity of iron oxides was enhanced [31,32]. Cu also can provide the new active sites and modify the electronic properties of iron based oxygen carriers [33,34]. Besides, Cu as a kind of oxygen carrier with relatively low cost can release gas phase O2 as the followed reaction (R1: 4CuO = 2Cu2O + O2) and can be applied to chemical-looping with oxygen uncoupling (CLOU) [35]. More importantly, it was reported that the Cu in the Cu-Fe oxygen carrier obtained a better stability in physical [31]. The investigations of Wang et al. indicated that the oxygen carriers prepared by mechanical mixing with CuO/Fe2O3 showed high reactivity with high resistance to agglomeration [36]. The limit in the application of Cu-containing oxygen carrier caused by agglomeration and sintering can be mitigated by the bimetallic oxygen carrier of Fe and Cu. Therefore, bimetallic Cu-Fe oxygen carriers composed by iron ore and Cu can be a good option and significant relevant for CLC. Recent paper showed that hematite-CuO oxygen carrier prepared by wet-impregnation has been investigated and this oxygen carrier obtains a good performance [37]. But the wet-impregnation is relatively complicated process with high cost and brings some environment problem such as the emission of NOx during calcination. So it is important to seek a better method of preparing hematite-CuO oxygen carrier with low cost and environment benign. Siriwardane et al. found that the direct mixing method was a better preparation method of Cu-Fe oxygen carriers because this method was simple and the corresponding sample showed good performance on thermogravimetric analyzer (TGA) [31,38]. The present work investigated the characteristics of the hematiteCuO with different contents of Cu prepared by mechanical mixing method on TGA and a batch fluidized bed. The cyclic performances of oxygen carriers were studied using gas fuel and solid fuel, respectively, and the effect of temperature was also investigated. In addition, the oxygen carrier after long term operation on the batch fluidized bed using coal as fuel was characterized and the loss of Cu was discussed briefly. 1. Experiment 1.1. Fuel The coal used in this work was a kind of bituminous coal produced from China. The coal was crushed and sieved to particles in a size range of 0.3–0.4 mm. The proximate analysis of this coal is shown in Table 1. 1.2. Oxygen carrier In this work, the utilized natural hematite was provided by Nanjing Steel Manufacturing Company in China. The natural Table 1 The proximate analysis of coal (wt%). M
V
FC
A
4.11
31.68
53.57
10.64
Table 2 The elemental ratios of Cu to Fe for the three oxygen carriers (wt%).
Cu/Fe
HC5
HC10
HC20
0.0707
0.1451
0.2720
134
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
hematite is from the Rio Tinto Company of Australia. The hematite particles (0.2–0.3 mm) were calcinated in a muffle oven at 950 °C for 3 h and utilized to prepare the bimetallic oxygen carrier. The bimetallic oxygen carrier hematite-CuO was prepared by direct mixing method: the powder of industrial-grade CuO (98%)
was mixed with the calcinated hematite particles thoroughly by a blender (CH20, Changzhou Lidu Drying Equipment Co., Ltd, China) at the room temperature (about 30 °C) in the weight ratio of 5:95, 10:90 and 20:80, which were labelled as HC5, HC10 and HC20, respectively. And then, the deionized water was added into
Fig. 2. The XRD results of HC5, HC10 and HC20 oxygen carriers.
135
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
the mixture to obtain a paste. At last, after 24 hour’s standing at room temperature for drying, the mixture was calcinated at 900 °C for 8 h in a muffle oven. The prepared oxygen carrier hematite-CuO was sieved to yield in a size range of 0.2–0.3 mm. According to the X-ray fluorescence (XRF) analysis results of these manufactured oxygen carriers, the ratios of Cu to Fe in weight for HC5, HC10 and HC20 are showed in Table 2. The ratios of Cu to Fe increase with the set weight percentage of Cu in oxygen carrier proportionally indicating that Cu can be loaded on the hematite particles for three hematite-CuO oxygen carriers efficiently by this method. During the calcination process, CuO reacted with a fraction of Fe2O3 in hematite forming CuFe2O4 above the temperature of 488 °C and the reactions could be described by R2 (R2: CuO + Fe2O3 = CuFe2O4) based on the thermodynamic calculations with Factsage software [31]. This is identified by the X-ray Diffraction (XRD) results shown in Fig. 2. Since the percentage of Fe2O3 is much higher than that of CuO, most of the iron in the hematite is still in the form of Fe2O3. In addition, no CuO is detected. This means that all the Cu in the hematite-CuO oxygen carriers exists in the form of CuFe2O4. 1.3. Thermogravimetric analyzer (TGA) Taking the three prepared oxygen carriers as examples, experiments on the performance in reactivity of the oxygen carriers were conducted on TGA. About 50 mg of oxygen carrier was loaded into quartz crucible with 5 mm diameter and 4 mm deep. The sample was heated to designed temperature at a heating rate of 20 °C/ min in air atmosphere, and, the inlet gas was switched to N2 (100 ml/min) in about 5 min. At last, after 5 min, the intake flow was changed to the mixture of N2 and H2 (100 ml/min and 100 ml/min) for about 35 min. 1.4. Batch fluidized bed The cyclic experiments were conducted on a batch fluidized bed reactor as shown in Fig. 3. This system consists of three parts: inlet flow, reactor and flue gas analyzer. The gas flow is controlled by mass flow controller. The reactor is straight iron tube with 32 mm inner diameter and 600 mm height. In the tube reactor, porous distributor plate is located at 300 mm from the bottom.
The reactor is heated by electric heating furnace and the temperature in reactor is detected by thermocouple. The outlet gas was sampled by gas bags for offline analysis per 1 or 2 min and the compositions of the flue gas are analyzed by a NGA 2000 type gas analyzer (Emerson Company USA). In each test for gas fuel, a sample of around 30 g oxygen carrier was placed on the distributor plate. When the temperature of reactor reached designed value and remained stable in N2 + O2 (2 L/ min + 100 ml/min) atmosphere, the inlet gas was switched to the mixture of CO and N2 (100 ml/min and 2 L/min) for 20 min completing the reduction process after purging 10 min using pure N2. During the process of oxidization, the mixture of O2 (100 ml/min) and N2 (2 L/min) was used as oxidizing gas. The N2 purge stage (2 mL/min) between the reduction and oxidation process maintained for 10 min. In the experiments for solid fuel, the steam is used as gasification agent. The deionized water was introduced by a macropump and then heated to 150 °C by steam generator. In each case for coal, a sample of around 30 g oxygen carrier was placed on the distributor plate. When the temperature of reactor reached designed value and remained stable in N2 and steam (2 L/min and 0.4 g/min) atmosphere. After that, the fuel (0.5 g) was added into reactor from the top of reactor. Simultaneously, the reactions of the fuel with oxygen carrier started and gas sampling began. The reduction duration was set to 50 min. The mixture of O2 (100 ml/ min) and N2 (2 L/min) was used in oxidization process. The N2 purge stage (2 mL/min) between the reduction and oxidation process maintained for 10 min. 1.5. Data The weight-loss ratio, Xw:t , represents the fractional reduction and it is define as followed,
Xw:t ¼
mt m0 mf m0
ð1Þ
where mt is the instantaneous weight of oxygen carrier, mf is the weight of oxygen carrier sample after reaction and m0 is the weight of fresh oxygen carrier sample. This parameter shows the weight loss ratio of oxygen carrier meaning the reactivity of oxygen carrier.
Valve 2 Thermocouple and Temperature Controller T H2 O
CO
N2
Reservoir Valve 1
Air Filter
Pump Steam Generator
Emerson
Vent
CO: 0-100% CO2: 0-100% CH4: 0-10% O2: 0-25% H2: 0-50%
Mass Flow Controller
Vent Thermocouple T Inlet Flow
Fig. 3. Schematic diagram of batch fluidized bed reactor.
Drier
136
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
The accumulative CO molar, Sam,CO.t, is defined as the sum of molar value of the CO detected during the first t minutes as followed (t is lower than 20).
Sac:CO:t ¼
X
q X CO dt ð1 X CO X CO2 Þ 22:4
ð2Þ
In this equal, q, Xi (i = CO, CO2) and dt is the gas flow rate of N2, the volume concentration of gas i and the period of obtaining each gas sample during reduction process, respectively. This parameter indicates the amount of oxygen released by the oxygen carrier to CO in the t minutes. The lower the value of the accumulative CO amount reaches, the more the oxygen is released to CO by the oxygen carrier and the higher reactivity the oxygen carrier exhibits during the reduction time. 2. Results and discussion 2.1. Performance of oxygen carrier on TGA Fig. 4 shows the results of experiments on TGA. Even though the CuO in the oxygen carrier of hematite-CuO existed in the form of CuFe2O4, weight loss also appeared in N2 atmosphere. This exhibits that the CuFe2O4 in the bimetallic metal oxygen carriers also possesses the ability of release gas phase O2 similar to the single metal oxide of CuO, which is in line with the results of Siriwardane [31]. After that, the weight loss could be ascribed to the reduction of oxygen carrier caused by the H2 at 900 °C. According to the results on the TGA, it is easier to conclude that the reactivity of oxygen carrier increases with the Cu content.
Fig. 4. The experiment results on TGA.
1.5
HC5
2.2. The cyclic characteristics of oxygen carrier To evaluate the cyclic characteristics of the oxygen carriers, cycle experiments were conducted on the batch fluidized bed using CO and coal as fuel, respectively, at 900 °C. In addition, the influence of temperature was also investigated with using coal as fuel. 2.2.1. The cycle characteristics of oxygen carriers with CO The Fig. 5 shows the concentration of CO in the flue gas for oxygen carriers HC5, HC10 and HC20 versus reduction time among different cycles (cycles, 1, 5, 8). The profiles corresponding to different cycles are similar in shape, and the concentration of CO increases with time for each oxygen carrier. When the flow gas of CO was introduced to the reactor, it was oxidized into CO2 by the oxygen carrier immediately but incompletely. So the CO existed in the exhaust gas during the reduction period. With the proceeding of the experiment, the oxygen carrier was reduced gradually and the reactivity of oxygen carrier for CO decreased [21,31]. As a result, the concentrations of CO increased with time. For the HC5 and HC10, the CO concentrations corresponding cycle 5 and cycle 8 are almost same and lower than that corresponding cycle 1 in the first t minutes (t = 8 for HC5 and t = 9 for HC10). In addition, the CO concentration corresponding cycle 8 begin to be lower than that corresponding cycle 5 in few minutes after the tth minute for HC5 and HC10. For the oxygen carrier of HC20, the CO concentrations for cycle 5 and cycle 8 are nearly same during all the reduction time and still lower than that for cycle 1 during first minutes. The lower the CO concentration reaches at the same time, the higher reactivity the oxygen carrier presents. For the three oxygen carriers, the oxygen carriers after cyclic operation exhibit a higher reactivity than the fresh oxygen carrier during first minutes. The increase in reactivity of oxygen carrier can be ascribed to the increase of micro pores caused by the migration of Cu ion facilitating the gas diffusion [31,36,37]. The Fig. 6 shows the accumulative molars of CO versus the reduction time among different cycles for oxygen carriers HC5, HC10, HC20. The profiles with semblable forms for different cycles exhibit an increase with reduction time for every oxygen carrier. For the three oxygen carriers, the accumulative molars of CO corresponding cycle 5 and cycle 8 are same approximately and lower than that corresponding cycle 1 during the first 10 min. For HC5 and HC10, the value of the accumulative molars of CO at each minute decreases with cycle in the 20 min. But for HC20, after about 12 min, the value of the accumulative molars of CO corresponding cycle 5 is the highest and that corresponding cycle 1 is the lowest at every time point. The decrease of accumulative molars of CO means the increase of oxygen released by the oxygen carrier during reduction time and reactivity of the oxygen carrier. For HC5 and HC10, the decrease of accumulative molars of CO with cycles can
HC10
2.0
HC20
1
cycle 1 cycle 5 cycle 8
0
The concentration of CO (%)
The concentration of CO(%)
The concentration of CO (%)
2
1.0
0.5
cycle 1 cycle 5 cycle 8
0.0
5
10
Time (min)
15
20
1.5
1.0
cycle 1 cycle 5 cycle 8
0.5
0.0
5
10
15
20
Time (Min)
Fig. 5. The CO concentration versus reduction time among cycles for oxygen carriers.
5
10
Time (min)
15
20
137
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142 0.025
0.022
HC10
HC20
0.020
0.015
0.010
cycle 1 cycle 5 cycle 8
0.005
0.020
0.018
Accumulative CO amount (mol)
Accumulative CO amount (mol)
0.020
Accumulative CO amount (mol)
0.025
0.024
HC5
0.016 0.014 0.012 0.010 0.008
cycle 1 cycle 5 cycle 8
0.006 0.004
0.000
10
15
0.010
cycle 1 cycle 5 cycle 8
0.005
0.002
0.000 5
0.015
20
0.000
5
10
Time (min)
15
20
5
Time (min)
10
15
20
Time (min)
Fig. 6. The accumulative CO molars versus reduction time among cycles for oxygen carriers.
Fig. 7. The images of HC20 (A, fresh HC20, B, reacted HC20 using CO as fuel after 8 cycles).
2COðgÞ ¼ C þ CO2 ðgÞ
ðR3Þ
When the gas flow of CO was introduced to reactor, CO reached to the surface of oxygen carrier particle reacting with the metal oxides on the surface of particles firstly, and then diffused to internal reducing the metal oxides in the interior of oxygen carrier. So the Fe2O3 on the surface is much easier to be reduced to Fe facilitating the generating the deposition of carbon than that in the internal of oxygen carrier. So the carbon main deposits on the surface of oxygen carrier particle in the form of Fe3C [41]. While the reduced oxygen carrier was oxidized by oxygen gas, the carbon deposited on the oxygen carrier was oxidized to CO2. Fig. 8 shows the generation of CO2 during oxidation period for HC10. The concentration of CO2 produced during oxidization process decreases with reaction time and cycles. The decrease of CO2 produced during oxidization period with cycles may be caused by increasing of micro pores facilitating the diffusion of gas, for which more CO reacts with internal metal oxides of particle decreasing the generation of Fe on the surface.
cycle 1 cycle 5 cycle 8
0.08
CO2 concentration
be explained by the increase of reactivity of oxygen carrier. In view of the accumulative molars of CO, HC10 is better than HC5 but HC20 is worst among the oxygen carriers. The higher of accumulative molars of CO for HC20 was caused by the agglomeration shown in Fig. 7. No apparent agglomeration was observed in the oxygen carrier for HC5 and HC10 after cyclic operation on the small scale reactors. HC5 and HC10 can be candidate for oxygen carrier that can operate on interconnected facility or industrial unit and HC10 performances better. Furthermore, the carbon deposition was investigated briefly in this section. The carbon deposition will lead to the decrease of reactivity and degradation of oxygen transfer capacity [39]. Besides, the deposited carbon will be oxidized to CO2 in AR and this will decrease the carbon capture efficiency for chemical looping combustion. Therefore, the carbon formation is important for oxygen carrier. The carbon deposition formed through the way of the reaction, R3 and the reaction is slow but the reaction will be catalyzed readily to occur with the presence of Fe [40].
0.06
0.04
0.02
0.00 2
4
6
8
10
12
Time (min) Fig. 8. The CO2 production during the oxidization process among cycles for HC10.
2.2.2. The cycle characteristics of oxygen carriers with coal Due to the low cost and the abundance of coal, it will be an advantage to apply chemical looping combustion to coal. Based on the results of above section, some experiments were conducted using coal as fuel on the batch fluidized bed using HC10 and HC5 as oxygen carrier at 900 °C. The loss of Cu by abrasion is a problem during reaction [37], so a long time operation (about 800 min) was conducted to study the loss of Cu briefly. Figs. 9 and 10 show the volume fractions of produced carbonaceous gaseous species (CO, CO2 and CH4) in the flue gas during the experiment for cycle 1, cycle 5 and cycle 8 with using HC10 and HC5 as oxygen carriers, respectively. The profiles corresponding to the different carbonaceous gas are similar. The maximum values for CO, CO2 and CH4 are obtained at the first minute and then decreased sharply versus reaction time. The high concentration of the produced carbonaceous gas during the first minutes is mainly caused by the release of volatile with the high generation
138
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142 2.0
CO CO2
1.5
The concentration of gas (%)
The concentration of gas (%)
2.0
cycle 5
cycle 1
CH4
1.0
0.5
0.0
cycle 8
CO CO2 CH4
1.5
The concentration of gas (%)
2.0
1.0
0.5
10
20
30
40
50
CH 4
1.5
1.0
0.5
0.0
0.0 0
CO CO2
0
10
20
Time (min)
30
40
50
0
10
20
Time (min)
30
40
50
Time (min)
Fig. 9. The carbonaceous gas concentration with using HC10 as oxygen carrier among different cycles. 2.0
2.0
CH4
1.5
2.0
1.0
0.5
0.0
cycle 8
CO CO2
cycle 5
CH4
1.5
The concentration of gas (%)
CO CO2
The concentration of gas (%)
The concentration of gas (%)
cycle 1
1.0
0.5
0.0 0
10
20
30
40
50
CO CO 2 CH 4
1.5
1.0
0.5
0.0 0
10
20
30
40
50
0
10
20
Time (min)
Time (min)
30
40
50
Time (min)
Fig. 10. The carbonaceous gas concentration with using HC5 as oxygen carrier among different cycles.
rate. From the results exhibited in Fig. 11, it can be known that there is still some residue carbon after reactions proceed 50 min, and the amount of residue carbon for HC10 is less than that for HC5. This means that HC10 is superior to HC5 in converting the coal into gas. The concentrations of carbonaceous gas corresponding to cycle 1, cycle 5 and cycle 8 are similar without too much change indicating that the oxygen carrier possessed the good long-term reactivity. In addition, no agglomeration appeared during this long term operation. It can be concluded that both the HC10 and HC5 are candidate for applying into coal combustion as oxygen carrier. Temperature has a significant influence on the reactivity of oxygen carriers and gasification of the coal. So the effects of temperature were investigated on the batch fluidized reactor using HC10 as oxygen carrier. The effect of temperature on the CO2 concentration in the exhaust gas of the reactor is shown in Fig. 12a. The CO2 concentrations increase with the increasing of reaction temperature. During the reaction process, the gasification of coal and the reduction of the oxygen carrier occurred. Gasification consists of two
0.40
HC5 HC10
0.30 0.25 0.20 0.15 0.10 0.05 0.00 0
2
4
6
Time
8
10
min)
Fig. 11. The concentration of CO2 produced during oxidization period.
a
800 oC 830 oC 860 oC 900 oC
b
1.5
The CO2 concentration (%)
1.5
The CO2 concentration (%)
The concentration of CO2 (%)
0.35
1.0
0.5
900oC
1.0
0.5
0.0
0.0 0
10
20
30
Time (min)
40
50
0
10
20
30
40
Time (min)
Fig. 12. a, The CO2 concentration for HC10 at different temperatures, b, the CO2 concentration without oxygen carrier at 900 °C.
50
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
Fig. 13. The SEM images of oxygen carriers (a, fresh HC5, b, reacted HC5, c, fresh HC10, d, reacted HC10).
Fig. 14. The EDX analysis results of oxygen carriers (a, the fresh HC5, b, the reacted HC5 with coal, c, the fresh HC10, d, the reacted HC10 with coal).
139
140
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
successive processes: pyrolysis and char gasification (R4)–(R5) [21].
Pyrolysis : coal ! char þ volatile
ðR4Þ
Char gasification : C þ H2 O ! CO þ H2
ðR5Þ
Since the R4 and R5 are endothermic reactions, the increasing of temperature accelerates the gasification of coal producing more carbonaceous gas and the generated gas can be oxidized by the oxygen carrier into CO2, which mainly the reason of the increasing of CO2 with temperature. According to Fig. 12b, it can be known that the existence of oxygen carrier has a positive effect in generation of CO2. As a result, the increasing reactivity of oxygen carrier with the increase of temperature is the other reason of the increasing of CO2. It can be seen clearly that the CO2 concentrations for the 800 °C, 830 °C and 860 °C are same after three minutes and much lower than that for 900 °C. Therefore, higher temperature, above 900 °C, is needed to obtain a desired conversion rate and conversion efficiency for solid fuel of coal.
ages of Cu for reacted HC5 and HC10 are lower than corresponding that for fresh HC5 and HC10, respectively. After reaction, the ratio of Cu to Fe on the surface of HC5 becomes to 0.094 from 0.106 and that of HC10 changes to 0.370 from 0.522 indicating some decrease of Cu content on the oxygen carrier particle surface after operation. This change can be explained by the loss of Cu caused by surface abrasion and migration into internal of particles from surface during cycle experiments [37]. In addition, XRF analysis is also applied into the HC5 and HC10 after one cyclical experiment (about 100 min) and 8 cyclical experiments (about 800 min), respectively, and the results are shown in Table 4 as the ratio of copper to iron in weight. The decrease of Cu to Fe in ratio is caused by the loss of Cu. Abrasion can be the main
3. Characteristics of oxygen carrier The oxygen carriers were characterized by scanning electron microscope (SEM), Energy Dispersive X-ray Detector (EDX), XRF and XRD to understand the effect of Cu ions on the hematite and existence status of the Cu oxygen carrier after long term operation. The images of SEM for fresh and reacted oxygen carriers are shown in Fig. 13. The reacted oxygen carriers are HC5 and HC10 after eight cyclical experiments (800 min) using coal as fuel at 900 °C. The magnification of 10,000 was used to analyze the surface characteristics for oxygen carriers. The surface micro-pores decrease with increasing of Cu loading, so the fresh HC10 has a denser surface than fresh HC5, which is observed in the Fig. 13 [42]. The difference between fresh and reacted oxygen carriers is shown clearly. For the fresh oxygen carriers of HC5 and HC10, the micro-pores are rare and the surfaces are dense. However, morphological changes appear on the surface of reacted oxygen carrier particles after long term operation. More pores are observed on the reacted oxygen carrier, which can be ascribed to the migration of Cu during reaction period and explain the increase of reactivity of oxygen carriers with cyclic operation in the Section 2.2.1. In addition, no obvious agglomeration phenomenon caused by introducing iron of Cu is detected on the surface of fresh and reacted oxygen carriers for HC5 and HC10. Fig. 14 shows the EDX analyses results for fresh and reacted oxygen carriers. And these results in the form of elemental compositions expressed by weight percentage are shown in Table 3. It is clear that the surface of fresh and reacted oxygen carriers for HC5 and HC10 are mainly composed of Fe, Cu, Al, Si and O. The percent-
Fig. 15. The XRD analysis of reacted oxygen carriers.
Table 3 Elemental compositions on the surface of fresh and reacted oxygen carrier (wt%).
Fresh HC5 Reacted HC5 Fresh HC10 Reacted HC10
Fe
Cu
Si
Al
O
Cu/Fe
60.31 63.87 39.66 48.93
6.41 6.01 20.70 18.10
2.04 1.31 0.95 1.38
1.08 1.14 0.55 0.41
29.83 27.18 37.23 30.73
0.106 0.094 0.522 0.370
Table 4 The elemental ratio of Cu to Fe for HC5 and HC10 (wt%).
Cu/Fe
HC5 HC10
Fresh
Cycle 1
Cycle 8
0.0707 0.1451
0.0688 0.1211
0.0634 0.1117
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
141
Fig. 16. Element mapping of oxygen carriers. Red: Fe; green: Cu. (a, fresh HC5, b, reacted HC5 with coal, c, fresh HC10, d, reacted HC10 with coal.
reason of the Cu loss. According to the results, it can be known that the Cu is disappearing with cycles and the loss proportion of Cu is decreasing with cycles. The phase analysis is significant to understand the role of Cu in the oxygen carrier and the side reactions during the CLC process using hematite-CuO as oxygen carrier. The existence form of Cu in the reacted oxygen carrier was identified by XRD and the XRD analysis results of reacted HC5 and HC10 are shown in Fig. 15. According to the XRD results in Section 1.2, CuO loaded on the oxygen carrier reacted with Fe2O3 generating CuFe2O4. After long time operation, all loaded Cu is still in the form of CuFe2O4. There was no change in existence substance between the reacted oxygen carriers and fresh oxygen carriers, and no side reactions occurred. The conclusion of the hematite-CuO possessing a stable chemical phase characteristic can be obtained. The distribution of Cu is related to the sintering and agglomeration of oxygen carriers at high temperature and it is determined by the preparing method [33]. Fig. 16 shows the element mapping of the fresh oxygen carriers and the reacted oxygen carriers. It is clear that the distribution of element Cu is comparatively uniform for fresh and reacted oxygen carriers (HC5 and HC10) retarding the sintering of Cu at high temperature and improving the physical stability of CuO in the bimetallic oxygen carriers [33]. It also can be concluded that this method of preparing hematite-CuO oxygen carrier is acceptable. 4. Conclusions In this study, the bimetallic oxygen carrier of Fe-Cu prepared by direct mixing method was investigated on the TGA and a batch fluidized bed. According the results, these following conclusions may be drawn: The CuO can be loaded on the hematite efficiently in the form of CuFe2O4 through direct mixing method. The TGA results show that the oxygen carrier can release oxygen during N2 atmosphere and the reactivity of oxygen carriers increases with the content of Cu. During cyclic operation in the batch fluidized bed using CO as fuel, some agglomeration issues occur for HC20. There is no apparent agglomeration and sintering appeared on HC5 and HC10 fueled
by gas of CO and coal. In addition, both HC5 and HC10 show increasing reactivity with cycles caused by forming of micropores during reaction shown SEM images. Long time circulation indicates that HC5 and HC10 possess good long term operation characteristics. HC5 and HC10 can be excellent candidates for the CLC combustion of coal. The loss of Cu caused by attrition in the oxygen carrier is observed after operation according to EDX and XRF results. After one cycle, the loss rate becomes much low. Besides, the XRD results show that the Cu in the oxygen carrier also exists in the form of CuFe2O4 after long time operation with coal as fuel. No side reactions occur during the reaction process. The element mappings show that distribution of element Cu is much uniform for fresh and reacted oxygen carriers (HC5 and HC10) retarding the sintering and facilitating a stable structure. These indicate that the preparing method can be excellent candidate of manufacturing the bimetallic oxygen carrier of Fe-Cu. But this needs further investigations on the interconnected reactor. Acknowledgements We gratefully acknowledge the support of this research work by the National Natural Science Foundation of China (Grant Nos. 51476029, 51276037, 51561125001 and 51406035). References [1] R.S. Haszeldine, Carbon capture and storage: how green can black be?, Science 325 (5948) (2009) 1647–1652 [2] A. Pettersson, L.E. Åmand, B.M. Steenari, Leaching of ashes from co-combustion of sewage sludge and wood—Part I: recovery of phosphorus, Biomass Bioenergy 32 (3) (2008) 224–235. [3] M.B. Folgueras, R.M. Dı´az, J. Xiberta, Sulphur retention during co-combustion of coal and sewage sludge, Fuel 83 (10) (2004) 1315–1322. [4] C. Storm, H. Rüdiger, H. Spliethoff, Co-pyrolysis of Coal/Biomass and Coal/ Sewage Sludge Mixtures/ASME 1998 International Gas Turbine and Aeroengine Congress and Exhibition, American Society of Mechanical Engineers, 1998, V003T05A006-V003T05A006. [5] Y. Cao, W.P. Pan, Investigation of chemical looping combustion by solid fuels. 1. Process analysis, Energy Fuels 20 (5) (2006) 1836–1844. [6] J. Adánez, L.F. de Diego, F. García-Labiano, Selection of oxygen carriers for chemical-looping combustion, Energy Fuels 18 (2) (2004) 371–377.
142
S. Jiang et al. / Chemical Engineering Journal 317 (2017) 132–142
[7] Y. Cao, B. Casenas, W.P. Pan, Investigation of chemical looping combustion by solid fuels. 2. redox reaction kinetics and product characterization with coal, biomass, and solid waste as solid fuels and CuO as an oxygen carrier, Energy Fuels 20 (5) (2006) 1845–1854. [8] T. Mattisson, M. Johansson, A. Lyngfelt, Multicycle reduction and oxidation of different types of iron oxide particles application to chemical-looping combustion, Energy Fuels 18 (3) (2004) 628–637. [9] F. Luis, F. Garcı, P. Gayán, et al., Operation of a 10kWth chemical-looping combustor during 200 h with a CuO–Al2O3 oxygen carrier, Fuel 86 (7) (2007) 1036–1045. [10] L. Shen, J. Wu, J. Xiao, Experiments on chemical looping combustion of coal with a NiO based oxygen carrier, Combust. Flame 156 (3) (2009) 721–728. [11] P. Cho, T. Mattisson, A. Lyngfelt, Comparison of iron-, nickel-, copper-and manganese-based oxygen carriers for chemical-looping combustion, Fuel 83 (9) (2004) 1215–1225. [12] J. Bao, Z. Li, N. Cai, Promoting the reduction reactivity of ilmenite by introducing foreign ions in chemical looping combustion, Ind. Eng. Chem. Res. 52 (18) (2013) 6119–6128. [13] P. Moldenhauer, M. Rydén, A. Lyngfelt, Testing of minerals and industrial byproducts as oxygen carriers for chemical-looping combustion in a circulating fluidized-bed 300 W laboratory reactor, Fuel 93 (2012) 351–363. [14] E. Ksepko, Sewage sludge ash as an alternative low-cost oxygen carrier for chemical looping combustion, J. Therm. Anal. Calorim. 116 (3) (2014) 1395– 1407. [15] H. Leion, E. Jerndal, B.M. Steenari, et al., Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers, Fuel 88 (10) (2009) 1945–1954. [16] T. Mattisson, A. Lyngfelt, P. Cho, The use of iron oxide as an oxygen carrier in chemical-looping combustion of methane with inherent separation of CO2, Fuel 80 (13) (2001) 1953–1962. [17] T. Song, T. Shen, L. Shen, Evaluation of hematite oxygen carrier in chemicallooping combustion of coal, Fuel 104 (2013) 244–252. [18] H. Leion, E. Jerndal, B.M. Steenari, Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers, Fuel 88 (10) (2009) 1945–1954. [19] H. Leion, T. Mattisson, A. Lyngfelt, Use of ores and industrial products as oxygen carriers in chemical-looping combustion, Energy Fuels 23 (4) (2009) 2307–2315. [20] H. Gu, L. Shen, J. Xiao, Chemical looping combustion of biomass/coal with natural iron ore as oxygen carrier in a continuous reactor, Energy Fuels 25 (1) (2010) 446–455. [21] S. Jiang, L. Shen, X. Niu, et al., Chemical looping co-combustion of sewage sludge and zhundong coal with natural hematite as the oxygen carrier, Energy Fuels 30 (3) (2016) 1720–1729. [22] M.M. Azis, E. Jerndal, H. Leion, et al., On the evaluation of synthetic and natural ilmenite using syngas as fuel in chemical-looping combustion (CLC), Chem. Eng. Res. Des. 88 (11) (2010) 1505–1514. [23] H. Ge, L. Shen, H. Gu, et al., Combustion performance and sodium transformation of high-sodium ZhunDong coal during chemical looping combustion with hematite as oxygen carrier, Fuel 159 (2015) 107–117. [24] X. Niu, L. Shen, H. Gu, et al., Characteristics of hematite and fly ash during chemical looping combustion of sewage sludge, Chem. Eng. J. 268 (2015) 236– 244.
[25] Z. Yu, C. Li, Y. Fang, et al., Reduction rate enhancements for coal direct chemical looping combustion with an iron oxide oxygen carrier, Energy Fuels 26 (4) (2012) 2505–2511. [26] H. Gu, L. Shen, J. Xiao, et al., Iron ore as oxygen carrier improved with potassium for chemical looping combustion of anthracite coal, Combust. Flame 159 (7) (2012) 2480–2490. [27] S. Bhavsar, G. Veser, Bimetallic Fe–Ni oxygen carriers for chemical looping combustion, Ind. Eng. Chem. Res. 52 (44) (2013) 15342–15352. [28] F. He, N. Galinsky, F. Li, Chemical looping gasification of solid fuels using bimetallic oxygen carrier particles–feasibility assessment and process simulations, Int. J. Hydrogen Energy 38 (19) (2013) 7839–7854. [29] A. Lambert, C. Delquié, I. Clémeneçon, et al., Synthesis and characterization of bimetallic Fe/Mn oxides for chemical looping combustion, Energy Procedia 1 (1) (2009) 375–381. [30] S. Bhavsar, B. Tackett, G. Veser, Evaluation of iron-and manganese-based mono-and mixed-metallic oxygen carriers for chemical looping combustion, Fuel 136 (2014) 268–279. [31] R. Siriwardane, H. Tian, T. Simonyi, et al., Synergetic effects of mixed copper– iron oxides oxygen carriers in chemical looping combustion, Fuel 108 (2013) 319–333. [32] T. Mattisson, A. Järdnäs, A. Lyngfelt, Reactivity of some metal oxides supported on alumina with alternating methane and oxygen application for chemicallooping combustion, Energy Fuels 17 (3) (2003) 643–651. [33] L. Zhang, X. Wang, J.M.M. Millet, et al., Investigation of highly active Fe-Al-Cu catalysts for water-gas shift reaction, Appl. Catal. A 351 (1) (2008) 1–8. [34] C. Rhodes, G.J. Hutchings, Studies of the role of the copper promoter in the iron oxide/chromia high temperature water gas shift catalyst, Phys. Chem. Chem. Phys. 5 (12) (2003) 2719–2723. [35] H. Leion, T. Mattisson, A. Lyngfelt, Using chemical-looping with oxygen uncoupling (CLOU) for combustion of six different solid fuels, Energy Procedia 1 (1) (2009) 447–453. [36] S. Wang, G. Wang, F. Jiang, et al., Chemical looping combustion of coke oven gas by using Fe2O3/CuO with MgAl2O4 as oxygen carrier, Energy Environ. Sci. 3 (9) (2010) 1353–1360. [37] W. Yang, H. Zhao, J. Ma, et al., Copper-decorated hematite as an oxygen carrier for in situ gasification chemical looping combustion of coal, Energy Fuels 28 (6) (2014) 3970–3981. [38] R.V. Siriwardane, E. Ksepko, H. Tian, et al., Interaction of iron–copper mixed metal oxide oxygen carriers with simulated synthesis gas derived from steam gasification of coal, Appl. Energy 107 (2013) 111–123. [39] M.K. Chandel, A. Hoteit, A. Delebarre, Experimental investigation of some metal oxides for chemical looping combustion in a fluidized bed reactor, Fuel 88 (5) (2009) 898–908. [40] B. Wang, R. Yan, D.H. Lee, et al., Thermodynamic investigation of carbon deposition and sulfur evolution in chemical looping combustion with syngas, Energy Fuels 22 (2) (2008) 1012–1020. [41] M. Ishida, H. Jin, T. Okamoto, Kinetic behavior of solid particle in chemicallooping combustion: suppressing carbon deposition in reduction, Energy Fuels 12 (2) (1998) 223–229. [42] L. Dong, Z. Liu, Y. Hu, et al., Dispersion and reduction behavior of CuO/a-Fe2O3 systems, J. Chem. Soc. Faraday Trans. 94 (19) (1998) 3033–3038.