- Email: [email protected]
CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal
Applied Energy xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal Haiming Gu ⇑, Laihong Shen ⇑, Zhaoping Zhong, Xin Niu, Weidong Liu, Huijun Ge, Shouxi Jiang, Lulu Wang 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 Iron ore as oxygen carrier was modified with CaO by two different methods. Fuel conversion was enhanced with the CaO-modified iron ore. Formation of Ca2Al2SiO7 in Cement/CaO-combined iron ore prevented sintering.
a r t i c l e
i n f o
Article history: Received 2 December 2014 Received in revised form 4 June 2015 Accepted 14 June 2015 Available online xxxx Keywords: Oxygen carrier Chemical looping combustion Iron ore Coal
a b s t r a c t Chemical looping combustion (CLC) of solid fuels is considered as a potential technology to reduce the energy penalty and the cost for CO2 capture. However, the low efficiency of carbon conversion and gasification products conversion is a big concern for the in-situ gasification chemical looping combustion (IG-CLC) process with the low-cost natural iron ore as an oxygen carrier. This paper evaluates the enhanced fuel conversion with a new CaO-modified iron ore as oxygen carrier during the continuous coal CLC in a 1 kWth reactor. Both CaO-mixed iron ore and cement/CaO-combined iron ore were tested. The effect of oxygen carrier on the gaseous products was evaluated in the fuel reactor temperature range of 880–980 °C. The samples of oxygen carrier were characterized using BET, SEM–EDX and XRD. The results indicate that compared with the pure iron ore oxygen carrier, the utilization of both CaO-mixed iron ore and the cement/CaO-combined iron ore could efficiently enhance coal conversion and gaseous conversion in the fuel reactor. However, when the CaO-mixed iron ore was used, some CaO powder due to the attrition during continuous operation adhered to the particle surface of iron ore. The Ca-containing compounds, i.e., potential eutectic of a low melting point caused sintering on the particle surface of iron ore. As a result, it caused the reactivity deterioration of oxygen carrier at a high fuel reactor temperature. To avoid the occurrence of sintering on the particle surface of the oxygen carrier, an improvement of cement/CaO-combined iron ore as oxygen carrier was proposed. A stable structure of Ca2Al2SiO7 was formed during the calcination process of the cement/CaO-combined iron ore. The sintering resistance of the iron ore was improved and the oxygen carrier after experiments maintained the porous structure. Reactivity deterioration of the combined oxygen carrier did not occur during the 5 h continuous operation even at the fuel reactor temperature of 980 °C. Overall, the Cement/CaO-combined iron ore is a competitive oxygen carrier in the coal CLC process. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Chemical looping combustion (CLC) is an emerging promising combustion technology with inherent separation of CO2 without extra energy penalty [1]. Basically, the CLC process contains two reactions in cycle, i.e., the oxidation and the reduction of oxygen carrier, as is shown in Fig. 1. The reduced oxygen carrier is first ⇑ Corresponding authors. Tel.: +86 25 83795598; fax: +86 25 83793452.
oxidized in the air reactor to obtain molecular oxygen from the air and then, the oxidized oxygen carrier is reduced in the fuel reactor to provide lattice oxygen for the fuel. Through the repeated oxidation and reduction of oxygen carrier, the oxygen is transformed from air to fuel avoiding direct contact between them, and the flue gas from the fuel reactor is not mixed with N2. Therefore, the flue gas from the fuel reactor after water removal is, ideally, pure CO2. By this means, CO2 is inherently captured during the fuel conversion without energy penalty.
E-mail addresses: [email protected] (H. Gu), [email protected] (L. Shen). http://dx.doi.org/10.1016/j.apenergy.2015.06.023 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
2
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
N2+O2
H2O+CO2 CO2
Oxygen carrier oxidation
Oxygen
Oxygen carrier reduction
H2O
MexOy
Air
Condensation
MexOy-1
Fuel
Fig. 1. Schematic of chemical looping combustion.
For the solid fuel CLC process, the most common approach is the in-situ gasification chemical looping combustion (IG-CLC) [2]. In this approach, reaction patterns in the fuel reactor involve the fuel pyrolysis/gasification and the subsequent oxygen carrier reduction by the gaseous products, as is shown in Fig. 2. The feasibility of the IG-CLC technology has been successfully demonstrated in continuous reactors scaled from 500 W to 100 kWth with different solid fuels [2]. However, it is important to note that loss of oxygen carrier due to the ash removal process is inevitable in the IG-CLC process. The addition of oxygen carrier is then necessary to maintain the normal operation of the CLC process, and it would increase the capital cost. Therefore, increasing investigations have recently focused on the inexpensive Fe-based oxygen carrier, including synthetic materials [3–5], natural iron ores [6–8] and industry byproducts [9–11]. Unfortunately, these Fe-based oxygen carriers often exhibited a relatively poor performance during the IG-CLC process, i.e., low efficiency of coal conversion and gas conversion [12]. Therefore, larger bed inventory or reactivity improvement of the Fe-based oxygen carrier is necessary to realize high fuel conversion efficiency. To promote the solid fuel conversion, some investigations were conducted to modify the Fe-based oxygen carriers with common gasification catalysts. One way is to mechanically add NiO [13] or CaO [7,14,15] into the Fe-based oxygen carrier. These mixtures as oxygen carrier created a synergistic effect on enhancing the fuel conversion due to the inevitable contact among char, catalysts and oxygen carrier. The other way is to develop a combined Fe-based oxygen carrier with carbonates [16,17] and cement [18,19]. Commonly, these combined oxygen carriers involve the creation of new compounds, which promote the reaction between gaseous products and oxygen carrier [19–21]. The enhanced gas– solid reaction could further promote the char gasification due to
H2OǃCO2
Oxidized Oxygen Carrier
Oxygen carrier reduction
Reduced Oxygen Carrier
the reaction equilibrium. Among these additives, CaO seems to have a broader application prospect than other materials, e.g., NiO, K2CO3 and Na2CO3 due to its low cost. The CaO-modified oxygen carrier includes a mixture of lime and ilmenite and cement-combined oxygen carrier. For the mixed oxygen carrier, the CaO or lime may face with some potential problems at a high temperature, e.g., attrition and sintering, which could cause the reactivity deterioration of oxygen carrier during long-time operation. Addition of aluminate cement as a binder can improve the mechanical strength, chemical properties and sintering resistance of the particles [22–24]. Then, the cement-combined oxygen carrier was developed and the new stable formation of Ca2Al2SiO7 was formed with enhanced sintering resistance. However, the performance of the combined oxygen carrier has not been tested in a continuous reactor, which was the most likely CLC reactor [25]. Accordingly, the present investigation aims at evaluating the performance of CaO-modified Australia iron ore with different modification methods in a 1 kWth interconnected fluidized bed. A series of characterization analysis were conducted to explore the behavior of the CaO-modified iron ore, including X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDX). 2. Experimental 2.1. Fuel In the present investigation, Shenhua coal from China was selected as a feed stock. The proximate analysis and ultimate analysis are listed in Table 1. The coal was crushed into small size and double-sieved into the size range of 0.1–0.3 mm. 2.2. Preparation of oxygen carrier According to the X-ray Fluorescence (XRF) analysis results in Table 2, the raw iron ore after calcination at 950 °C mainly consists of 83.08% Fe2O3, 6.8% Al2O3 and 9.13% SiO2. The chemical compositions of the involved raw materials, i.e., calcined dolomite and cement are also displayed in Table 2. The dolomite after calcination was used as the CaO decoration. Two different methods were employed to add CaO into iron ore, i.e., mechanically mixed CaO/iron ore and cement/CaO-combined iron ore, noted as CIO and CCIO, respectively. For the CIO, the iron ore and the dolomite were initially calcined at 950 and 900 °C, respectively. Afterwards, the particles were sieved to 0.1–0.3 mm and mixed at the iron ore/dolomite mass ratio of 4/1. The CCIO involved the utilization of a high alumina cement to combine the iron ore and dolomite. Dolomite was added to make use of the excess Al2O3 and SiO2 in cement and iron ore to maximize the fraction of Ca2Al2SiO7 in the oxygen carrier. The particles of these raw materials were first crushed to the size range below 0.03 mm. The fine powder was mixed with the mass ratio of iron ore/cement/dolomite at 7/1/2 in a dry condition. Afterwards, some water was added to make a wet mixture, and it was then fed into a two-gear-roller granulator to extrude to form particles, as is displayed in Fig. 3. The combined oxygen carrier was then calcined
Syngas Solid Fuel
Gasification
Fly Ash
H2OǃCO2 Fig. 2. Fuel conversion path in the fuel reactor in the IG-CLC process.
Table 1 The proximate analysis and ultimate analysis of the bituminous coal. Proximate analysis Moisture (wt.%, ad) Volatile (wt.%, ad) Fixed carbon (wt.%, ad) Ash (wt.%, ad) Lower heating value (MJ/kg)
Ultimate analysis 6 35.1 54.1 4.8 27.1
C (wt.%, ad) H (wt.%, ad) O (wt.%, ad) N (wt.%, ad) S (wt.%, ad)
69.6 4.3 13.1 1 1.2
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
3
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx Table 2 The composition of iron ore and the modification material (wt%). Materials
Fe2O3
Al2O3
SiO2
TiO2
P2O5
CaO
K2O
MgO
MnO
Others
Iron ore Cement Dolomite
83.1 1.8 0.1
6.8 51.1 0.1
9.1 7.8 0.8
0.1 2.5 –
0.2 – –
0.1 33.1 30.5
– – –
0.2 1.5 23.9
0.3 – –
0.1 2.2 44.6(LOI)
LOI: loss on incineration.
Fig. 3. Cement/CaO combined iron ore oxygen carrier prepared by extrusion method: (a) Twin-screw extrusion granulation equipment, (b) The CCIO before crush and (c) The CCIO after crush and sieve.
in a muffle oven at 950 °C in air atmosphere for 3 h. Finally, the oxygen carrier was also sieved to 0.1–0.3 mm for use. 2.3. Reactor system and test condition The coal CLC were conducted in a 1 kWth interconnected fluidized-bed reactor, as is shown in Fig. 4. Briefly, it consists of a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor and an outer loop-seal. This system has been described in detail in previous work [26].
To conveniently compare the performance of different bed materials, the experiments were conducted with the same parameters, as are listed in Table 3. For each case, gas sampling began after 30 min of stable operation. The gaseous products from both reactors after water removal were sampled by gas bags for offline analysis. The concentrations of CO2, CH4, CO, O2 and H2 were measured by an Emerson gas analyzer including NGA 2000 and Hydros 100. 2.4. Data evaluation With the known experiment parameters and the measured gas concentrations Wi,FR and Wj,FR, i.e., the gas concentration of i (i = CO, CO2,CH4, H2 and NO) in the fuel reactor and the gas concentration of j (j = CO2, O2 and NO) in the air reactor, the gas flow could be obtained according to Eqs. (1)–(5). Total gas flow from the fuel reactor:
N2 + O2
Oven
F out;FR ¼
F N2 ;FR;in 1 W CO;FR W CO2 ;FR W CH4 ;FR W H2 ;FR
ð1Þ
Carbonaceous gas flow from the fuel reactor:
F C;FR ¼ F out;FR ðW CO;FR þ W CO2 ;FR þ W CH4 ;FR Þ CO 2 + H 2 O Air reactor
CO2 flow from the fuel reactor:
F CO2 ;FR ¼ F out;FR W CO2 ;FR Loopseal
ð2Þ
ð3Þ
Total gas flow from the fuel reactor:
Fuel reactor
F out;AR ¼
F N2 ;AR;in 1 W O2 ;AR W CO2 ;AR
ð4Þ
Carbonaceous gas flow from the air reactor: Coal/ biomass
N2
Air
Steam
Fig. 4. Configuration of the 1 kWth interconnected fluidized bed CLC reactor.
F C;AR ¼ F out;AR W CO2 ;AR
ð5Þ
Table 4 shows the technology assessment indexes to characterize the performance of the employed oxygen carrier in the continuous coal CLC. Coal gasification efficiency in the fuel reactor gGas is defined to illustrate the conversion extent of coal into carbonaceous gases. Gaseous product conversion in the fuel reactor f i (i = CO, CO2 and CH4) is used to evaluate the reactivity of oxygen
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
4
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
Table 3 Experimental design and operating parameters for the continuous coal CLC. Experiment parameters
Value
Coal feeding rate mcoal (g/h) Input N2 flow of air reactor F N2 ;AR;in (m3/h) Input O2 flow of air reactor F O2 ;AR;in (m3/h) Input H2O flow of loopseal F H2 O;LS (g/h) Input H2O flow of fuel reactor F H2 O;FR;in (g/h) Input N2 flow of fuel reactor F N2 ;FR;in (m3/h) Fuel reactor temperature T FR (°C)
100 0.66 0.18 180 150 0.27 880–980
Table 4 The main technology assessment indexes and calculations. Technology assessment
Calculation equations
Coal gasification efficiency
12F C;FR gGas ¼ 22:4m coal M C;coal
Gas conversion efficiency in fuel reactor Extra oxygen demand
f i ¼ W CO;FR þW COi;FR;FR þW CH
(6)
W
0:5W
(7)
4 ;FR
2
þ2W CH4 ;FR þ0:5W H2 ;FR þW CH4 ;FR þW H2 ;FR 2 ;FR
XOD ¼ W CO;FRCO;FR þW CO F
CO2 capture efficiency
gC ¼
F C;FR F C;FR þF C;AR
3. Results and discussions 3.1. The performance of the Iron ore as oxygen carrier During the coal CLC process, the coal particles and the gasification medium were added from the bottom of the fuel reactor, and the coal particles were heated to the reactor temperature. Afterwards, the coal underwent pyrolysis/gasification, which could be described as (R1)–(R4). The main intermediates, i.e., H2 and CO and char were oxidized into CO2 and H2O by the oxygen carrier in the fuel reactor, according to (R5)–(R8). The enhanced coal pyrolysis/gasification at the elevated temperature led to these reactions to proceed in the positive direction. Therefore, with the temperature increasing from 880 °C to 980 °C, the concentrations of CO, CO2 and H2 from the fuel reactor kept increasing trends with
C þ H2 O ! CO þ H2
ðR2Þ
C þ CO2 ! 2CO
ðR3Þ
CO þ H2 O ! CO2 þ H2
ðR4Þ
3Fe2 O3 þ CO ! 2Fe3 O4 þ CO2
ðR5Þ
3Fe2 O3 þ H2 ! 2Fe3 O4 þ H2 O
ðR6Þ
12Fe2 O3 þ CH4 ! 8Fe3 O4 þ CO2 þ 2H2 O
ðR7Þ
6Fe2 O3 þ C ! 4Fe3 O4 þ CO2
ðR8Þ
Fig. 6 exhibits the fuel conversion proportions in both reactors during the gasification process using sand and iron ore as bed materials, respectively. For both processes, the coal conversion in the air reactor exhibited a decreasing trend with the coal conversion in the fuel reactor increasing. The endothermic coal gasification was enhanced at the elevated temperature in both processes. During the CLC process, most of the gasification products were consumed by the oxygen carrier, enhancing the coal gasification due to the reaction equilibrium. Simultaneously, the enhanced coal conversion in the fuel reactor led to less char escaping into the air reactor and lower fuel loss from the system. Although, the unconverted combustible gases, i.e., H2, CO and CH4 in the fuel reactor would, to some extent, play an inhibition role in the fuel conversion. Some fine char along with ash was still elutriated out from the fuel reactor and therefore, the coal conversion could not reach 100%. Therefore, an improvement on gas conversion in the fuel reactor would further enhance the coal gasification and reduce the fuel loss. Aiming at enhancing the fuel conversion and gas conversion in the fuel reactor, the following investigation focused on the performance of CaO-modified iron ore as an oxygen carrier for coal CLC.
10
25
Gas concentration (%)
Gas concentration (%)
ðR1Þ
(10)
carrier with gaseous products in the fuel reactor. f CO2 ¼ 100% means that the gaseous product could be completely oxidized into CO2 by the oxygen carrier. However, in the downstream flow from the fuel reactor, some extra oxygen is still necessary to convert the inevitable combustible gases, and the amount of oxygen carrier is defined as oxygen demand XOD. CO2 capture efficiency gCO2 and carbon capture efficiency are also defined to evaluate the CLC system.
CO2
20 15 10
CO
5 0
Coal ! Volatile þ Char
(8) (9)
2 ;FR gCO2 ¼ F C;FRCOþF C;AR
Carbon capture efficiency
temperature while CH4 concentration maintained a relatively low value, as is shown in Fig. 5. At 900 °C, the concentrations of CO2, CO, H2 and CH4 were 21.19%, 3.27%, 0.7% and 1.03%, and then they became 24.09%, 5.9%, 2.4% and 0.67% at 980 °C. Also, some char after gasification would inevitably circulate with the oxygen carrier into the air reactor producing CO2. CO2 concentration in the air reactor was determined by the amount of residual char after coal gasification in the fuel reactor. Therefore, a higher fuel conversion at the elevated temperature means a lower CO2 concentration in the air reactor.
CH4
H2
8
O2
6 4
CO 2
2 0
880
900
920
940
960
980
880
900
920
940
960
980
Fuel reactor temperature ( C)
Fuel reactor temperature ( C)
(a) Fuel reactor
(b) Air reactor
Fig. 5. The effect of fuel reactor temperature on gas concentration during coal CLC process.
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
Sand bed material Iron ore bed material 60
°
°
980 C
880 C
°
880°C
105
980 C
50
90
40
75 60
30
45 20 30 10
15
0
40
50
60
70
80
90
Total coal conversion (%)
Coal conversion in air reactor (%)
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
100
Coal conversion in fuel reactor (%) Fig. 6. Coal conversion efficiency during gasification with sand and iron ore as bed materials, respectively.
3.2. CaO-mixed Iron ore as oxygen carrier 3.2.1. Gas composition In this section, the mixture of CaO (dolomite) and iron ore was used as an oxygen carrier for coal CLC. In the fuel reactor, the CaO catalyzed the water gas shift reaction, resulting into a larger H2 yield than CO yield. It was accepted that the reaction reactivity of H2–Fe2O3 was higher than that of CO–Fe2O3, which also accounts for a lower H2 concentration than CO concentration with the fuel reactor temperature between 880 and 950 °C, as is shown in Fig. 7. It could enhance the coal gasification according to the reaction equilibrium. As a result, CO2 concentration rapidly increased from 26.44% to 29.01%, while CO concentration slowly increased from 0.96% to 1.54% with a negligible H2 concentration. Unfortunately, with the fuel reactor temperature further increasing from 950 °C to 980 °C, CO2 concentration rapidly decreased to 22.9%, while the concentrations of CO and H2 increased to 5.33% and 7.8%, respectively. It could be mainly ascribed to the reactivity deterioration of oxygen carrier at a higher temperature caused by the CaO addition, as is confirmed by the following SEM–EDX and BET analysis. Otherwise, the H2 concentration would be lower than CO concentration because the former has a higher reactivity with iron ore. 3.2.2. SEM–EDX of the CIO To explore the failure mechanism of the CIO at a high temperature, SEM analysis was conducted to characterize the morphological features of reduced oxygen carrier. In this section, the CIO
Gas concentration (%)
CO2
20 15 10
H2 CO
5
CH4
0 880
900
920
particles refer to the reduced iron ore particles, which have been separated from dolomite particles according to different colours. The SEM images in Fig. 8 clearly display the differences between the used IO and CIO particles. For the reduced IO in Fig. 8(a), the particle surface was uniformly covered with small grains in the size range of 0–0.3 lm without grain sintering. However, significant differences in morphological feature can be observed on the particle surface of the used CIO. According to Fig. 8(b), the particle surface was also coated with dense materials, and it indicates that sintering occurred on the particle surface. The sintering on the particles was unfavourable for the diffusion of gaseous products into the reactive site of oxygen carrier. It caused a reactivity deterioration of oxygen carrier at a higher temperature, as is shown in Fig. 7. To explain the reasons for the occurrence of sintering on the particle surface of the CIO, additional EDX analysis was conducted and the results are shown in Fig. 9. A weak Ca intensity was observed on the used IO particles surface, and it could be ascribed to the transport of Ca from coal ash to the oxygen carrier particle surface. According to the quantitative analysis, the molar content of Ca on different areas of the IO particle surface was lower than 0.6%. On the contrary, according to Fig. 9(b), a much stronger Ca intensity was observed on the reduced CIO particle surface. The quantitative analysis of the Ca contents on the areas of A and B marked in Fig. 8 were 12.83% and 5.61%, respectively. It indicates that Ca content in the area coated by dense materials was also relatively higher. Therefore, the sintering could be mainly attributed to the fine powder of Ca-containing materials from the added dolomite. The sintering could be attributed to the formation of CaO or potential eutectic of a low melting point. On the one hand, whilst with a high the melting point, CaO could cause particles sintering when the temperature is above 900 °C [27]. Borgwardt [28] also reported that sintering on the CaO surface was enhanced during the long-time operation in the presence of CO2 and H2O, which was similar to the CLC atmosphere. On the other hand, the CaOFe2O3 could be formed via the reaction of CaO + Fe2O3 ? CaOFe2O3. The relatively low melting point of calcium ferrite may cause sintering on the particle surface. However, this should be taken with caution because the intensity reflection of calcium ferrite was not observed according to the XRD pattern. As a result of the long-time operation of continuous coal CLC, a dense material was formed on the particle surface. It was unfavorable for the diffusion of gaseous products into the active center of oxygen carrier during the coal CLC. Therefore, the concentrations of CO and H2 sharply increased at 980 °C, as is shown in Fig. 7. 3.3. Cement/CaO-combined iron ore as oxygen carrier In this section, a cement/CaO combined iron ore was prepared to improve the sintering resistance of the oxygen carrier. A total of 5 h experiments using the CCIO as oxygen carrier were conducted with the temperature increasing from 880 °C to 980 °C.
30 25
5
940
960
980
Fuel reactor temperature (oC) Fig. 7. The effect of the fuel reactor temperature on the gas composition of the fuel reactor with CIO as oxygen carrier.
3.3.1. Phase pattern Fig. 10 displays the XRD pattern of CCIO both before and after experiments. During the calcination process of CCIO at 950 °C, Ca2Al2SiO7 and CaAl2SiO6 were formed according to reactions among CaO, Al2O3 and SiO2, which could be described by (R9) and (R10). The formation of Ca2Al2SiO7 and CaAl2SiO6 depended on the ratio of reactants, and the excessive CaO addition favoured the formation of Ca2Al2SiO7. Although the elevated temperature is unfavourable for these reactions, these compounds could be easily formed at a high temperature. For example, the log(K) for (R9) is larger than 3 even at 1200 °C. The existence of Ca2Al2SiO7 in the reduced CCIO also indicates that Ca2Al2SiO7 was relatively stable. Furthermore, the formations of Ca2Al2SiO7 and CaAl2SiO6 seemed
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
6
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
Fig. 8. The morphological features of the oxygen carrier of: (a) reduced iron ore and (b) reduced CaO-mixed iron ore.
(b) CaO-mixed iron ore
(a) Iron ore
Fig. 9. The EDX spectra of the reduced oxygen carrier particles.
5000 4000
D
After reaction
Intensity (CPS)
2000
D AB
1000
6000
D-Fe3O4 E-CaAl 2SiO6
A
3000
0 7500
A-Fe2O3 B-SiO2 C-Ca 2 Al2SiO7
DC
Before reaction
A
4500 3000
A
D DCA
AD ACD
0 10
20
A
C
30
CaO þ Al2 O3 þ SiO2 ! CaAl2 SiO6
D A
D
2CaO þ Al2 O3 þ SiO2 ! Ca2 Al2 SiO7
ðR9Þ ðR10Þ
A
A B CEE CE
1500
to improve the sintering resistance which was confirmed by the SEM images and the BET analysis, as is discussed in Section 3.3.2. The similar result were also reported by Song et al. [19].
40
A
C 50
AA AB 60
A A AA 70
80
2θ (°) Fig. 10. The XRD patterns of the used cement/CaO combined iron ore.
3.3.2. Microstructure feature Fig. 11 shows the SEM images of CCIO both before and after experiments. These images exhibit a relatively rougher particle surface of CCIO, and the flat surface turned into a porous structure covered with small grains. Although the oxygen carrier of CCIO also faced with the risk of sintering, the new formation of Ca2Al2SiO7 and CaAl2SiO6 supplied a relatively stable supporting skeleton for Ca atom or potential eutectic of a low melting point, efficiently
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
7
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
Fig. 11. SEM images of the CCIO: (a) before reaction and (b) after reaction.
3.3.3. The reactivity of the CCIO during continuous coal CLC Fig. 12 shows the effect of the fuel reactor temperature on the gas compositions in the fuel reactor when CCIO was used. Significant changes in the gas concentrations in the fuel reactor could be observed when compared with the case using IO and
Table 5 BET analysis results for the oxygen carriers. Oxygen carrier
The specific surface area (m2/g)
Fresh IO Used IO Fresh CCIO Used CCIO Used CIO
1.6 0.8 1.3 0.7 0.3
35
Gas concentration (%)
preventing the occurrence of sintering. Therefore, during the whole continuous operation, the CCIO particle maintained the porous surface covered with grains in the size range of 0–0.2 lm without visible sintering. The porous structure of the oxygen carrier was favorable for the reaction between gaseous products and the oxygen carrier. BET analysis was conducted to explore the different mechanism of the CaO-modified iron ore with two preparing methods. Table 5 shows the BET datas of the oxygen carrier before and after reaction at 985 °C. The decrease in the BET surface during the coal CLC process occurred on all these oxygen carriers. Due to the preparation process of oxygen carrier, the BET surface of the fresh CCIO is somewhat lower than the fresh IO. It indicates that the enhanced performance of the CCIO should be explained by the reactivity rather than the improved BET. Although, compared with CIO, the surface area of the used CCIO was improved due to the enhanced sintering resistance. It favored the diffusion of gaseous products into the active site of the oxygen carrier and thus, enhancing the reactivity of the oxygen carrier even at 980 °C, as is discussed in Section 3.3.3. It indicates that the utilization of CCIO was a better choice than CIO for the coal CLC process.
CO2
30
CO CO2 CH4 H2
25 20 15 10 5 0 880
900
920
940
960
980
Fuel reactor temperature ( C) Fig. 12. The effect of the fuel reactor temperature on the gas compositions in the fuel reactor with CCIO as oxygen carrier.
CIO. With the temperature increasing from 880 °C to 980 °C, CO2 concentration increased from 26.09% to 32.33%, while CH4 concentration decreased from 0.76% to 0.36%. The CO concentration maintained between 0.42% and 0.5%. H2 concentration was negligible during the whole temperature range of 880–980 °C. It was similar to the case using CIO during the fuel reactor temperature range of 880–930 °C. However, the difference is that the reactivity deterioration of CCIO was not observed even at 980 °C. The similar effect of the CCIO on the gas compositions of the fuel reactor was also observed with anthracite as a solid fuel which, however, was not shown in this research. Table 6 exhibits the main technology assessment indexes to evaluate the continuous operation using these three oxygen carriers. With the CIO as an oxygen carrier, CaO in the dolomite functioned as a catalyst for the water gas shift reaction, which could promote the conversion of CO and H2O into CO2 and H2. The reactivity of H2 and Fe2O3 was higher than that of CO and Fe2O3. As a result, the gaseous product conversion was enhanced, promoting the coal gasification due to the reaction equilibrium. Both CO2
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
8
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
Table 6 The main technology assessment indexes during the continuous coal CLC. Indexes
fCO (%) f CO2 ð%Þ f CH4 (%) gC (%) gCO2 ð%Þ XOD (%) gGas ð%Þ
IO
CIO
CCIO
980 °C
930 °C
880 °C
980 °C
930 °C
880 °C
980 °C
930 °C
880 °C
19.2 78.6 2.2 96.9 76.1 17.9 88.7
17.1 79.4 3.5 87.5 69.4 18.6 78.8
12.8 83.1 4 74.5 62 15.9 66.9
16.7 80.9 2.4 98.2 79.5 26.6 88.8
4.9 92.5 2.6 94.1 87.1 7.6 88.5
3.4 93.5 3.1 85.8 80.2 7.9 76.4
1.4 97.5 1.1 98.7 96.3 2.9 96.1
1.6 96.4 1.9 94.5 91.2 4.7 85.4
1.5 95.7 2.8 85.8 82.1 6.3 72.6
fraction and CO2 capture efficiency were improved, while the extra oxygen demand XOD was decreased. However, the reactivity of the oxygen carrier deteriorated at a higher temperature due to the occurrence of sintering on the particle surface. Compared with IO, the utilization of iron ore/Ca2Al2SiO7 could also enhance the reaction between iron ore and H2 and CO, promoting the oxygen carrier conversion rate [19] i.e., (R5) and (R6). Furthermore, according to Table 6, the CH4 fraction also decreased when the CCIO was used. It indicates that the CCIO as an oxygen carrier could enhance the reduction reactivity of iron ore. Therefore, the conversion of combustible gases into CO2 and H2O was enhanced, reducing the extra oxygen demand in the fuel reactor and increasing the CO2 fraction and CO2 capture efficiency. Furthermore, due to the enhanced sintering resistance of CCIO because of the formation of Ca2Si2Al2O7, the reactivity deterioration was never observed during the whole temperature range. Overall, the CCIO is a competitive oxygen carrier for the coal CLC process.
4. Conclusions In this study, the iron ore as oxygen carrier was modified with dolomite by two different preparation methods. The performance of the modified iron ore was evaluated during the coal CLC in a 1 kWth continuous reactor, including gas concentration, gasification efficiency, gas conversion and CO2 capture efficiency. SEM, EDX and XRD were employed to characterize the oxygen carrier both pre-test and pro-test. The following conclusions may be drawn: During the coal CLC using an Australia iron ore, the oxygen carrier exhibited a relatively low reactivity in converting the coal and the gasification products to CO2, especially at a low temperature. The utilization of the dolomite-modified iron ore could enhance the conversion of coal and the gasification products into CO2 and H2O. However, the mechanically mixed oxygen carrier would cause the sintering on the particle surface of iron ore due to the poor sintering resistance of CaO in CLC atmosphere or potential CaOFe2O3 of a low melting point. It caused reactivity deterioration at a temperature above 950 °C. As for the cement/dolomite-combined iron ore, stable compound of Ca2Si2AlO7 was formed during the calcination process of the oxygen carrier. It helped to improve the sintering resistance of oxygen carrier, and the oxygen carrier maintained the porous structure. A stable enhanced reactivity was obtained during the coal CLC process even at a high temperature. Overall, the cement/dolomite-combined iron ore is a competitive oxygen carrier for the coal CLC process. Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant Nos. 51406035, 51276037, 51476029), China Postdoctoral Science Foundation (2014M551489), The Fundamental Research Funds for the
Central Universities, Jiangsu Planned Projects for Postdoctoral Research Funds. References [1] Richter H, Knoche K. Reversibility of combustion process. Symp Ser 1983;235(1):71–86. [2] Adánez J, Abad A, García-Labiano F, Gayán P, Luis F. Progress in chemicallooping combustion and reforming technologies. Prog Energy Combust 2012;38(2):215–82. [3] Leion H, Mattisson T, Lyngfelt A. Solid fuels in chemical-looping combustion. Int J Greenh Gas Con 2014;2(2):180–93. [4] Cabello A, Dueso C, García-Labiano F, Gayán P, Abad A, de Diego LF, et al. Performance of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier with CH4 and H2S in a 500 Wth CLC unit. Fuel 2014;121:117–25. [5] Wang C, Cui H, Di H, Guo Q, Huang F. Experimental investigation on multicycle performance of coal/straw chemical loop combustion with a-Fe2O3 as oxygen carrier. Energ Fuel 2014;28(6):4162–6. [6] Leion H, Jerndal E, Steenari BM, Hermansson S, Israelsson M, Jansson E, et al. Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers. Fuel 2009;88(10):1945–54. [7] Cuadrat A, Linderholm C, Abad A, Lyngfelt A, Adánez J. Influence of limestone addition in a 10 kWth chemical-looping combustion unit operated with petcoke. Energ Fuel 2011;25(10):4818–28. [8] Cuadrat A, Abad A, Adánez J, De Diego LF, García-Labiano F, Gayán P. Behavior of ilmenite as oxygen carrier in chemical-looping combustion. Fuel Process Technol 2012;94(1):101–12. [9] Zhang S, Saha C, Yang Y, Bhattacharya S, Xiao R. Use of Fe2O3-containing industrial wastes as the oxygen carrier for chemical-looping combustion of coal: effects of pressure and cycles. Energ Fuel 2011;25(10):4357–66. [10] Mendiara T, de Diego LF, García-Labiano F, Gayán P, Abad A, Adánez J. Behaviour of a bauxite waste material as oxygen carrier in a 500Wth CLC unit with coal. Int J Greenh Gas Con 2013;17:170–82. [11] Mendiara T, García-Labiano F, Gayán P, Abad A, de Diego LF, Adánez J. Evaluation of the use of different coals in Chemical Looping Combustion using a bauxite waste as oxygen carrier. Fuel 2013;106:814–26. [12] Lyngfelt A. Chemical-looping combustion of solid fuels–status of development. Appl Energy 2014;113:1869–73. [13] Song T, Wu J, Zhang H, Shen L. Characterization of an Australia hematite oxygen carrier in chemical looping combustion with coal. Int J Greenh Gas Con 2012;11:326–36. [14] Teyssié G, Leion H, Schwebel GL, Lyngfelt A, Mattisson T. Influence of lime addition to ilmenite in chemical-looping combustion (CLC) with solid fuels. Energy Fuel 2011;25(8):3843–53. [15] Guo Q, Cheng Y, Liu Y, Jia W, Ryu HJ. Coal chemical looping gasification for syngas generation using an iron-based oxygen carrier. Ind Eng Chem Res 2013;53(1):78–86. [16] Yu Z, Li C, Fang Y, Huang J, Wang Z. Reduction rate enhancements for coal direct chemical looping combustion with an iron oxide oxygen carrier. Energ Fuel 2012;26(4):2505–11. [17] Gu H, Shen L, Xiao J, Zhang S, Song T, Chen D. Iron ore as oxygen carrier improved with potassium for chemical looping combustion of anthracite coal. Combust Flame 2012;159(7):2480–90. [18] Song T, Shen L, Guo W, Chen D, Xiao J. Enhanced reaction performance with hematite/Ca2Al2SiO7 oxygen carrier in chemical looping combustion of coal. Ind Eng Chem Res 2013;52(28):9573–85. [19] Song T, Shen L, Zhang S, Chen D, Xiao J. Performance of hematite/Ca2Al2SiO7 oxygen carrier in chemical looping combustion of coal. Ind Eng Chem Res 2013;52(22):7350–61. [20] Bao J, Li Z, Sun H, Cai N. Continuous test of ilmenite-based oxygen carriers for chemical looping combustion in a dual fluidized bed reactor system. Ind Eng Chem Res 2013;52(42):14817–27. [21] Bao J, Li Z, Cai N. Promoting the reduction reactivity of ilmenite by introducing foreign ions in chemical looping combustion. Ind Eng Chem Res 2013;52(18):6119–28. [22] Chen H, Zhao C, Yang Y, Zhang P. CO2 capture and attrition performance of CaO pellets with aluminate cement under pressurized carbonation. Appl Energy 2012;91(1):334–40.
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx [23] Manovic V, Anthony EJ. CaO-based pellets supported by calcium aluminate cements for high-temperature CO2 capture. Environ Sci Technol 2009;43(18): 7117–22. [24] Xu L, Wang J, Li Z, Cai N. Experimental study of cement-supported CuO oxygen carriers in chemical looping with oxygen uncoupling (CLOU). Energy Fuel 2013;27(3):1522–30. [25] Lyngfelt A, Leckner B, Mattisson T. A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem Eng Sci 2001;56(10):3101–13.
9
[26] Shen L, Wu J, Gao Z, Xiao J. Characterization of chemical looping combustion of coal in a 1 kWth reactor with a nickel-based oxygen carrier. Combust Flame 2010;157(5):934–42. [27] Blamey J, Anthony EJ, Wang J, Fennell PS. The calcium looping cycle for largescale CO2 capture. Prog Energy Combust 2010;36(2):260–79. [28] Borgwardt RH. Calcium oxide sintering in atmospheres containing water and carbon dioxide. Ind Eng Chem Res 1989;28(4):493–500.
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
Contents lists available at ScienceDirect
Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal Haiming Gu ⇑, Laihong Shen ⇑, Zhaoping Zhong, Xin Niu, Weidong Liu, Huijun Ge, Shouxi Jiang, Lulu Wang 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 Iron ore as oxygen carrier was modified with CaO by two different methods. Fuel conversion was enhanced with the CaO-modified iron ore. Formation of Ca2Al2SiO7 in Cement/CaO-combined iron ore prevented sintering.
a r t i c l e
i n f o
Article history: Received 2 December 2014 Received in revised form 4 June 2015 Accepted 14 June 2015 Available online xxxx Keywords: Oxygen carrier Chemical looping combustion Iron ore Coal
a b s t r a c t Chemical looping combustion (CLC) of solid fuels is considered as a potential technology to reduce the energy penalty and the cost for CO2 capture. However, the low efficiency of carbon conversion and gasification products conversion is a big concern for the in-situ gasification chemical looping combustion (IG-CLC) process with the low-cost natural iron ore as an oxygen carrier. This paper evaluates the enhanced fuel conversion with a new CaO-modified iron ore as oxygen carrier during the continuous coal CLC in a 1 kWth reactor. Both CaO-mixed iron ore and cement/CaO-combined iron ore were tested. The effect of oxygen carrier on the gaseous products was evaluated in the fuel reactor temperature range of 880–980 °C. The samples of oxygen carrier were characterized using BET, SEM–EDX and XRD. The results indicate that compared with the pure iron ore oxygen carrier, the utilization of both CaO-mixed iron ore and the cement/CaO-combined iron ore could efficiently enhance coal conversion and gaseous conversion in the fuel reactor. However, when the CaO-mixed iron ore was used, some CaO powder due to the attrition during continuous operation adhered to the particle surface of iron ore. The Ca-containing compounds, i.e., potential eutectic of a low melting point caused sintering on the particle surface of iron ore. As a result, it caused the reactivity deterioration of oxygen carrier at a high fuel reactor temperature. To avoid the occurrence of sintering on the particle surface of the oxygen carrier, an improvement of cement/CaO-combined iron ore as oxygen carrier was proposed. A stable structure of Ca2Al2SiO7 was formed during the calcination process of the cement/CaO-combined iron ore. The sintering resistance of the iron ore was improved and the oxygen carrier after experiments maintained the porous structure. Reactivity deterioration of the combined oxygen carrier did not occur during the 5 h continuous operation even at the fuel reactor temperature of 980 °C. Overall, the Cement/CaO-combined iron ore is a competitive oxygen carrier in the coal CLC process. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Chemical looping combustion (CLC) is an emerging promising combustion technology with inherent separation of CO2 without extra energy penalty [1]. Basically, the CLC process contains two reactions in cycle, i.e., the oxidation and the reduction of oxygen carrier, as is shown in Fig. 1. The reduced oxygen carrier is first ⇑ Corresponding authors. Tel.: +86 25 83795598; fax: +86 25 83793452.
oxidized in the air reactor to obtain molecular oxygen from the air and then, the oxidized oxygen carrier is reduced in the fuel reactor to provide lattice oxygen for the fuel. Through the repeated oxidation and reduction of oxygen carrier, the oxygen is transformed from air to fuel avoiding direct contact between them, and the flue gas from the fuel reactor is not mixed with N2. Therefore, the flue gas from the fuel reactor after water removal is, ideally, pure CO2. By this means, CO2 is inherently captured during the fuel conversion without energy penalty.
E-mail addresses: [email protected] (H. Gu), [email protected] (L. Shen). http://dx.doi.org/10.1016/j.apenergy.2015.06.023 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
2
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
N2+O2
H2O+CO2 CO2
Oxygen carrier oxidation
Oxygen
Oxygen carrier reduction
H2O
MexOy
Air
Condensation
MexOy-1
Fuel
Fig. 1. Schematic of chemical looping combustion.
For the solid fuel CLC process, the most common approach is the in-situ gasification chemical looping combustion (IG-CLC) [2]. In this approach, reaction patterns in the fuel reactor involve the fuel pyrolysis/gasification and the subsequent oxygen carrier reduction by the gaseous products, as is shown in Fig. 2. The feasibility of the IG-CLC technology has been successfully demonstrated in continuous reactors scaled from 500 W to 100 kWth with different solid fuels [2]. However, it is important to note that loss of oxygen carrier due to the ash removal process is inevitable in the IG-CLC process. The addition of oxygen carrier is then necessary to maintain the normal operation of the CLC process, and it would increase the capital cost. Therefore, increasing investigations have recently focused on the inexpensive Fe-based oxygen carrier, including synthetic materials [3–5], natural iron ores [6–8] and industry byproducts [9–11]. Unfortunately, these Fe-based oxygen carriers often exhibited a relatively poor performance during the IG-CLC process, i.e., low efficiency of coal conversion and gas conversion [12]. Therefore, larger bed inventory or reactivity improvement of the Fe-based oxygen carrier is necessary to realize high fuel conversion efficiency. To promote the solid fuel conversion, some investigations were conducted to modify the Fe-based oxygen carriers with common gasification catalysts. One way is to mechanically add NiO [13] or CaO [7,14,15] into the Fe-based oxygen carrier. These mixtures as oxygen carrier created a synergistic effect on enhancing the fuel conversion due to the inevitable contact among char, catalysts and oxygen carrier. The other way is to develop a combined Fe-based oxygen carrier with carbonates [16,17] and cement [18,19]. Commonly, these combined oxygen carriers involve the creation of new compounds, which promote the reaction between gaseous products and oxygen carrier [19–21]. The enhanced gas– solid reaction could further promote the char gasification due to
H2OǃCO2
Oxidized Oxygen Carrier
Oxygen carrier reduction
Reduced Oxygen Carrier
the reaction equilibrium. Among these additives, CaO seems to have a broader application prospect than other materials, e.g., NiO, K2CO3 and Na2CO3 due to its low cost. The CaO-modified oxygen carrier includes a mixture of lime and ilmenite and cement-combined oxygen carrier. For the mixed oxygen carrier, the CaO or lime may face with some potential problems at a high temperature, e.g., attrition and sintering, which could cause the reactivity deterioration of oxygen carrier during long-time operation. Addition of aluminate cement as a binder can improve the mechanical strength, chemical properties and sintering resistance of the particles [22–24]. Then, the cement-combined oxygen carrier was developed and the new stable formation of Ca2Al2SiO7 was formed with enhanced sintering resistance. However, the performance of the combined oxygen carrier has not been tested in a continuous reactor, which was the most likely CLC reactor [25]. Accordingly, the present investigation aims at evaluating the performance of CaO-modified Australia iron ore with different modification methods in a 1 kWth interconnected fluidized bed. A series of characterization analysis were conducted to explore the behavior of the CaO-modified iron ore, including X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDX). 2. Experimental 2.1. Fuel In the present investigation, Shenhua coal from China was selected as a feed stock. The proximate analysis and ultimate analysis are listed in Table 1. The coal was crushed into small size and double-sieved into the size range of 0.1–0.3 mm. 2.2. Preparation of oxygen carrier According to the X-ray Fluorescence (XRF) analysis results in Table 2, the raw iron ore after calcination at 950 °C mainly consists of 83.08% Fe2O3, 6.8% Al2O3 and 9.13% SiO2. The chemical compositions of the involved raw materials, i.e., calcined dolomite and cement are also displayed in Table 2. The dolomite after calcination was used as the CaO decoration. Two different methods were employed to add CaO into iron ore, i.e., mechanically mixed CaO/iron ore and cement/CaO-combined iron ore, noted as CIO and CCIO, respectively. For the CIO, the iron ore and the dolomite were initially calcined at 950 and 900 °C, respectively. Afterwards, the particles were sieved to 0.1–0.3 mm and mixed at the iron ore/dolomite mass ratio of 4/1. The CCIO involved the utilization of a high alumina cement to combine the iron ore and dolomite. Dolomite was added to make use of the excess Al2O3 and SiO2 in cement and iron ore to maximize the fraction of Ca2Al2SiO7 in the oxygen carrier. The particles of these raw materials were first crushed to the size range below 0.03 mm. The fine powder was mixed with the mass ratio of iron ore/cement/dolomite at 7/1/2 in a dry condition. Afterwards, some water was added to make a wet mixture, and it was then fed into a two-gear-roller granulator to extrude to form particles, as is displayed in Fig. 3. The combined oxygen carrier was then calcined
Syngas Solid Fuel
Gasification
Fly Ash
H2OǃCO2 Fig. 2. Fuel conversion path in the fuel reactor in the IG-CLC process.
Table 1 The proximate analysis and ultimate analysis of the bituminous coal. Proximate analysis Moisture (wt.%, ad) Volatile (wt.%, ad) Fixed carbon (wt.%, ad) Ash (wt.%, ad) Lower heating value (MJ/kg)
Ultimate analysis 6 35.1 54.1 4.8 27.1
C (wt.%, ad) H (wt.%, ad) O (wt.%, ad) N (wt.%, ad) S (wt.%, ad)
69.6 4.3 13.1 1 1.2
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
3
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx Table 2 The composition of iron ore and the modification material (wt%). Materials
Fe2O3
Al2O3
SiO2
TiO2
P2O5
CaO
K2O
MgO
MnO
Others
Iron ore Cement Dolomite
83.1 1.8 0.1
6.8 51.1 0.1
9.1 7.8 0.8
0.1 2.5 –
0.2 – –
0.1 33.1 30.5
– – –
0.2 1.5 23.9
0.3 – –
0.1 2.2 44.6(LOI)
LOI: loss on incineration.
Fig. 3. Cement/CaO combined iron ore oxygen carrier prepared by extrusion method: (a) Twin-screw extrusion granulation equipment, (b) The CCIO before crush and (c) The CCIO after crush and sieve.
in a muffle oven at 950 °C in air atmosphere for 3 h. Finally, the oxygen carrier was also sieved to 0.1–0.3 mm for use. 2.3. Reactor system and test condition The coal CLC were conducted in a 1 kWth interconnected fluidized-bed reactor, as is shown in Fig. 4. Briefly, it consists of a fast fluidized bed as an air reactor, a cyclone, a spout-fluid bed as a fuel reactor and an outer loop-seal. This system has been described in detail in previous work [26].
To conveniently compare the performance of different bed materials, the experiments were conducted with the same parameters, as are listed in Table 3. For each case, gas sampling began after 30 min of stable operation. The gaseous products from both reactors after water removal were sampled by gas bags for offline analysis. The concentrations of CO2, CH4, CO, O2 and H2 were measured by an Emerson gas analyzer including NGA 2000 and Hydros 100. 2.4. Data evaluation With the known experiment parameters and the measured gas concentrations Wi,FR and Wj,FR, i.e., the gas concentration of i (i = CO, CO2,CH4, H2 and NO) in the fuel reactor and the gas concentration of j (j = CO2, O2 and NO) in the air reactor, the gas flow could be obtained according to Eqs. (1)–(5). Total gas flow from the fuel reactor:
N2 + O2
Oven
F out;FR ¼
F N2 ;FR;in 1 W CO;FR W CO2 ;FR W CH4 ;FR W H2 ;FR
ð1Þ
Carbonaceous gas flow from the fuel reactor:
F C;FR ¼ F out;FR ðW CO;FR þ W CO2 ;FR þ W CH4 ;FR Þ CO 2 + H 2 O Air reactor
CO2 flow from the fuel reactor:
F CO2 ;FR ¼ F out;FR W CO2 ;FR Loopseal
ð2Þ
ð3Þ
Total gas flow from the fuel reactor:
Fuel reactor
F out;AR ¼
F N2 ;AR;in 1 W O2 ;AR W CO2 ;AR
ð4Þ
Carbonaceous gas flow from the air reactor: Coal/ biomass
N2
Air
Steam
Fig. 4. Configuration of the 1 kWth interconnected fluidized bed CLC reactor.
F C;AR ¼ F out;AR W CO2 ;AR
ð5Þ
Table 4 shows the technology assessment indexes to characterize the performance of the employed oxygen carrier in the continuous coal CLC. Coal gasification efficiency in the fuel reactor gGas is defined to illustrate the conversion extent of coal into carbonaceous gases. Gaseous product conversion in the fuel reactor f i (i = CO, CO2 and CH4) is used to evaluate the reactivity of oxygen
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
4
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
Table 3 Experimental design and operating parameters for the continuous coal CLC. Experiment parameters
Value
Coal feeding rate mcoal (g/h) Input N2 flow of air reactor F N2 ;AR;in (m3/h) Input O2 flow of air reactor F O2 ;AR;in (m3/h) Input H2O flow of loopseal F H2 O;LS (g/h) Input H2O flow of fuel reactor F H2 O;FR;in (g/h) Input N2 flow of fuel reactor F N2 ;FR;in (m3/h) Fuel reactor temperature T FR (°C)
100 0.66 0.18 180 150 0.27 880–980
Table 4 The main technology assessment indexes and calculations. Technology assessment
Calculation equations
Coal gasification efficiency
12F C;FR gGas ¼ 22:4m coal M C;coal
Gas conversion efficiency in fuel reactor Extra oxygen demand
f i ¼ W CO;FR þW COi;FR;FR þW CH
(6)
W
0:5W
(7)
4 ;FR
2
þ2W CH4 ;FR þ0:5W H2 ;FR þW CH4 ;FR þW H2 ;FR 2 ;FR
XOD ¼ W CO;FRCO;FR þW CO F
CO2 capture efficiency
gC ¼
F C;FR F C;FR þF C;AR
3. Results and discussions 3.1. The performance of the Iron ore as oxygen carrier During the coal CLC process, the coal particles and the gasification medium were added from the bottom of the fuel reactor, and the coal particles were heated to the reactor temperature. Afterwards, the coal underwent pyrolysis/gasification, which could be described as (R1)–(R4). The main intermediates, i.e., H2 and CO and char were oxidized into CO2 and H2O by the oxygen carrier in the fuel reactor, according to (R5)–(R8). The enhanced coal pyrolysis/gasification at the elevated temperature led to these reactions to proceed in the positive direction. Therefore, with the temperature increasing from 880 °C to 980 °C, the concentrations of CO, CO2 and H2 from the fuel reactor kept increasing trends with
C þ H2 O ! CO þ H2
ðR2Þ
C þ CO2 ! 2CO
ðR3Þ
CO þ H2 O ! CO2 þ H2
ðR4Þ
3Fe2 O3 þ CO ! 2Fe3 O4 þ CO2
ðR5Þ
3Fe2 O3 þ H2 ! 2Fe3 O4 þ H2 O
ðR6Þ
12Fe2 O3 þ CH4 ! 8Fe3 O4 þ CO2 þ 2H2 O
ðR7Þ
6Fe2 O3 þ C ! 4Fe3 O4 þ CO2
ðR8Þ
Fig. 6 exhibits the fuel conversion proportions in both reactors during the gasification process using sand and iron ore as bed materials, respectively. For both processes, the coal conversion in the air reactor exhibited a decreasing trend with the coal conversion in the fuel reactor increasing. The endothermic coal gasification was enhanced at the elevated temperature in both processes. During the CLC process, most of the gasification products were consumed by the oxygen carrier, enhancing the coal gasification due to the reaction equilibrium. Simultaneously, the enhanced coal conversion in the fuel reactor led to less char escaping into the air reactor and lower fuel loss from the system. Although, the unconverted combustible gases, i.e., H2, CO and CH4 in the fuel reactor would, to some extent, play an inhibition role in the fuel conversion. Some fine char along with ash was still elutriated out from the fuel reactor and therefore, the coal conversion could not reach 100%. Therefore, an improvement on gas conversion in the fuel reactor would further enhance the coal gasification and reduce the fuel loss. Aiming at enhancing the fuel conversion and gas conversion in the fuel reactor, the following investigation focused on the performance of CaO-modified iron ore as an oxygen carrier for coal CLC.
10
25
Gas concentration (%)
Gas concentration (%)
ðR1Þ
(10)
carrier with gaseous products in the fuel reactor. f CO2 ¼ 100% means that the gaseous product could be completely oxidized into CO2 by the oxygen carrier. However, in the downstream flow from the fuel reactor, some extra oxygen is still necessary to convert the inevitable combustible gases, and the amount of oxygen carrier is defined as oxygen demand XOD. CO2 capture efficiency gCO2 and carbon capture efficiency are also defined to evaluate the CLC system.
CO2
20 15 10
CO
5 0
Coal ! Volatile þ Char
(8) (9)
2 ;FR gCO2 ¼ F C;FRCOþF C;AR
Carbon capture efficiency
temperature while CH4 concentration maintained a relatively low value, as is shown in Fig. 5. At 900 °C, the concentrations of CO2, CO, H2 and CH4 were 21.19%, 3.27%, 0.7% and 1.03%, and then they became 24.09%, 5.9%, 2.4% and 0.67% at 980 °C. Also, some char after gasification would inevitably circulate with the oxygen carrier into the air reactor producing CO2. CO2 concentration in the air reactor was determined by the amount of residual char after coal gasification in the fuel reactor. Therefore, a higher fuel conversion at the elevated temperature means a lower CO2 concentration in the air reactor.
CH4
H2
8
O2
6 4
CO 2
2 0
880
900
920
940
960
980
880
900
920
940
960
980
Fuel reactor temperature ( C)
Fuel reactor temperature ( C)
(a) Fuel reactor
(b) Air reactor
Fig. 5. The effect of fuel reactor temperature on gas concentration during coal CLC process.
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
Sand bed material Iron ore bed material 60
°
°
980 C
880 C
°
880°C
105
980 C
50
90
40
75 60
30
45 20 30 10
15
0
40
50
60
70
80
90
Total coal conversion (%)
Coal conversion in air reactor (%)
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
100
Coal conversion in fuel reactor (%) Fig. 6. Coal conversion efficiency during gasification with sand and iron ore as bed materials, respectively.
3.2. CaO-mixed Iron ore as oxygen carrier 3.2.1. Gas composition In this section, the mixture of CaO (dolomite) and iron ore was used as an oxygen carrier for coal CLC. In the fuel reactor, the CaO catalyzed the water gas shift reaction, resulting into a larger H2 yield than CO yield. It was accepted that the reaction reactivity of H2–Fe2O3 was higher than that of CO–Fe2O3, which also accounts for a lower H2 concentration than CO concentration with the fuel reactor temperature between 880 and 950 °C, as is shown in Fig. 7. It could enhance the coal gasification according to the reaction equilibrium. As a result, CO2 concentration rapidly increased from 26.44% to 29.01%, while CO concentration slowly increased from 0.96% to 1.54% with a negligible H2 concentration. Unfortunately, with the fuel reactor temperature further increasing from 950 °C to 980 °C, CO2 concentration rapidly decreased to 22.9%, while the concentrations of CO and H2 increased to 5.33% and 7.8%, respectively. It could be mainly ascribed to the reactivity deterioration of oxygen carrier at a higher temperature caused by the CaO addition, as is confirmed by the following SEM–EDX and BET analysis. Otherwise, the H2 concentration would be lower than CO concentration because the former has a higher reactivity with iron ore. 3.2.2. SEM–EDX of the CIO To explore the failure mechanism of the CIO at a high temperature, SEM analysis was conducted to characterize the morphological features of reduced oxygen carrier. In this section, the CIO
Gas concentration (%)
CO2
20 15 10
H2 CO
5
CH4
0 880
900
920
particles refer to the reduced iron ore particles, which have been separated from dolomite particles according to different colours. The SEM images in Fig. 8 clearly display the differences between the used IO and CIO particles. For the reduced IO in Fig. 8(a), the particle surface was uniformly covered with small grains in the size range of 0–0.3 lm without grain sintering. However, significant differences in morphological feature can be observed on the particle surface of the used CIO. According to Fig. 8(b), the particle surface was also coated with dense materials, and it indicates that sintering occurred on the particle surface. The sintering on the particles was unfavourable for the diffusion of gaseous products into the reactive site of oxygen carrier. It caused a reactivity deterioration of oxygen carrier at a higher temperature, as is shown in Fig. 7. To explain the reasons for the occurrence of sintering on the particle surface of the CIO, additional EDX analysis was conducted and the results are shown in Fig. 9. A weak Ca intensity was observed on the used IO particles surface, and it could be ascribed to the transport of Ca from coal ash to the oxygen carrier particle surface. According to the quantitative analysis, the molar content of Ca on different areas of the IO particle surface was lower than 0.6%. On the contrary, according to Fig. 9(b), a much stronger Ca intensity was observed on the reduced CIO particle surface. The quantitative analysis of the Ca contents on the areas of A and B marked in Fig. 8 were 12.83% and 5.61%, respectively. It indicates that Ca content in the area coated by dense materials was also relatively higher. Therefore, the sintering could be mainly attributed to the fine powder of Ca-containing materials from the added dolomite. The sintering could be attributed to the formation of CaO or potential eutectic of a low melting point. On the one hand, whilst with a high the melting point, CaO could cause particles sintering when the temperature is above 900 °C [27]. Borgwardt [28] also reported that sintering on the CaO surface was enhanced during the long-time operation in the presence of CO2 and H2O, which was similar to the CLC atmosphere. On the other hand, the CaOFe2O3 could be formed via the reaction of CaO + Fe2O3 ? CaOFe2O3. The relatively low melting point of calcium ferrite may cause sintering on the particle surface. However, this should be taken with caution because the intensity reflection of calcium ferrite was not observed according to the XRD pattern. As a result of the long-time operation of continuous coal CLC, a dense material was formed on the particle surface. It was unfavorable for the diffusion of gaseous products into the active center of oxygen carrier during the coal CLC. Therefore, the concentrations of CO and H2 sharply increased at 980 °C, as is shown in Fig. 7. 3.3. Cement/CaO-combined iron ore as oxygen carrier In this section, a cement/CaO combined iron ore was prepared to improve the sintering resistance of the oxygen carrier. A total of 5 h experiments using the CCIO as oxygen carrier were conducted with the temperature increasing from 880 °C to 980 °C.
30 25
5
940
960
980
Fuel reactor temperature (oC) Fig. 7. The effect of the fuel reactor temperature on the gas composition of the fuel reactor with CIO as oxygen carrier.
3.3.1. Phase pattern Fig. 10 displays the XRD pattern of CCIO both before and after experiments. During the calcination process of CCIO at 950 °C, Ca2Al2SiO7 and CaAl2SiO6 were formed according to reactions among CaO, Al2O3 and SiO2, which could be described by (R9) and (R10). The formation of Ca2Al2SiO7 and CaAl2SiO6 depended on the ratio of reactants, and the excessive CaO addition favoured the formation of Ca2Al2SiO7. Although the elevated temperature is unfavourable for these reactions, these compounds could be easily formed at a high temperature. For example, the log(K) for (R9) is larger than 3 even at 1200 °C. The existence of Ca2Al2SiO7 in the reduced CCIO also indicates that Ca2Al2SiO7 was relatively stable. Furthermore, the formations of Ca2Al2SiO7 and CaAl2SiO6 seemed
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
6
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
Fig. 8. The morphological features of the oxygen carrier of: (a) reduced iron ore and (b) reduced CaO-mixed iron ore.
(b) CaO-mixed iron ore
(a) Iron ore
Fig. 9. The EDX spectra of the reduced oxygen carrier particles.
5000 4000
D
After reaction
Intensity (CPS)
2000
D AB
1000
6000
D-Fe3O4 E-CaAl 2SiO6
A
3000
0 7500
A-Fe2O3 B-SiO2 C-Ca 2 Al2SiO7
DC
Before reaction
A
4500 3000
A
D DCA
AD ACD
0 10
20
A
C
30
CaO þ Al2 O3 þ SiO2 ! CaAl2 SiO6
D A
D
2CaO þ Al2 O3 þ SiO2 ! Ca2 Al2 SiO7
ðR9Þ ðR10Þ
A
A B CEE CE
1500
to improve the sintering resistance which was confirmed by the SEM images and the BET analysis, as is discussed in Section 3.3.2. The similar result were also reported by Song et al. [19].
40
A
C 50
AA AB 60
A A AA 70
80
2θ (°) Fig. 10. The XRD patterns of the used cement/CaO combined iron ore.
3.3.2. Microstructure feature Fig. 11 shows the SEM images of CCIO both before and after experiments. These images exhibit a relatively rougher particle surface of CCIO, and the flat surface turned into a porous structure covered with small grains. Although the oxygen carrier of CCIO also faced with the risk of sintering, the new formation of Ca2Al2SiO7 and CaAl2SiO6 supplied a relatively stable supporting skeleton for Ca atom or potential eutectic of a low melting point, efficiently
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
7
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
Fig. 11. SEM images of the CCIO: (a) before reaction and (b) after reaction.
3.3.3. The reactivity of the CCIO during continuous coal CLC Fig. 12 shows the effect of the fuel reactor temperature on the gas compositions in the fuel reactor when CCIO was used. Significant changes in the gas concentrations in the fuel reactor could be observed when compared with the case using IO and
Table 5 BET analysis results for the oxygen carriers. Oxygen carrier
The specific surface area (m2/g)
Fresh IO Used IO Fresh CCIO Used CCIO Used CIO
1.6 0.8 1.3 0.7 0.3
35
Gas concentration (%)
preventing the occurrence of sintering. Therefore, during the whole continuous operation, the CCIO particle maintained the porous surface covered with grains in the size range of 0–0.2 lm without visible sintering. The porous structure of the oxygen carrier was favorable for the reaction between gaseous products and the oxygen carrier. BET analysis was conducted to explore the different mechanism of the CaO-modified iron ore with two preparing methods. Table 5 shows the BET datas of the oxygen carrier before and after reaction at 985 °C. The decrease in the BET surface during the coal CLC process occurred on all these oxygen carriers. Due to the preparation process of oxygen carrier, the BET surface of the fresh CCIO is somewhat lower than the fresh IO. It indicates that the enhanced performance of the CCIO should be explained by the reactivity rather than the improved BET. Although, compared with CIO, the surface area of the used CCIO was improved due to the enhanced sintering resistance. It favored the diffusion of gaseous products into the active site of the oxygen carrier and thus, enhancing the reactivity of the oxygen carrier even at 980 °C, as is discussed in Section 3.3.3. It indicates that the utilization of CCIO was a better choice than CIO for the coal CLC process.
CO2
30
CO CO2 CH4 H2
25 20 15 10 5 0 880
900
920
940
960
980
Fuel reactor temperature ( C) Fig. 12. The effect of the fuel reactor temperature on the gas compositions in the fuel reactor with CCIO as oxygen carrier.
CIO. With the temperature increasing from 880 °C to 980 °C, CO2 concentration increased from 26.09% to 32.33%, while CH4 concentration decreased from 0.76% to 0.36%. The CO concentration maintained between 0.42% and 0.5%. H2 concentration was negligible during the whole temperature range of 880–980 °C. It was similar to the case using CIO during the fuel reactor temperature range of 880–930 °C. However, the difference is that the reactivity deterioration of CCIO was not observed even at 980 °C. The similar effect of the CCIO on the gas compositions of the fuel reactor was also observed with anthracite as a solid fuel which, however, was not shown in this research. Table 6 exhibits the main technology assessment indexes to evaluate the continuous operation using these three oxygen carriers. With the CIO as an oxygen carrier, CaO in the dolomite functioned as a catalyst for the water gas shift reaction, which could promote the conversion of CO and H2O into CO2 and H2. The reactivity of H2 and Fe2O3 was higher than that of CO and Fe2O3. As a result, the gaseous product conversion was enhanced, promoting the coal gasification due to the reaction equilibrium. Both CO2
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
8
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx
Table 6 The main technology assessment indexes during the continuous coal CLC. Indexes
fCO (%) f CO2 ð%Þ f CH4 (%) gC (%) gCO2 ð%Þ XOD (%) gGas ð%Þ
IO
CIO
CCIO
980 °C
930 °C
880 °C
980 °C
930 °C
880 °C
980 °C
930 °C
880 °C
19.2 78.6 2.2 96.9 76.1 17.9 88.7
17.1 79.4 3.5 87.5 69.4 18.6 78.8
12.8 83.1 4 74.5 62 15.9 66.9
16.7 80.9 2.4 98.2 79.5 26.6 88.8
4.9 92.5 2.6 94.1 87.1 7.6 88.5
3.4 93.5 3.1 85.8 80.2 7.9 76.4
1.4 97.5 1.1 98.7 96.3 2.9 96.1
1.6 96.4 1.9 94.5 91.2 4.7 85.4
1.5 95.7 2.8 85.8 82.1 6.3 72.6
fraction and CO2 capture efficiency were improved, while the extra oxygen demand XOD was decreased. However, the reactivity of the oxygen carrier deteriorated at a higher temperature due to the occurrence of sintering on the particle surface. Compared with IO, the utilization of iron ore/Ca2Al2SiO7 could also enhance the reaction between iron ore and H2 and CO, promoting the oxygen carrier conversion rate [19] i.e., (R5) and (R6). Furthermore, according to Table 6, the CH4 fraction also decreased when the CCIO was used. It indicates that the CCIO as an oxygen carrier could enhance the reduction reactivity of iron ore. Therefore, the conversion of combustible gases into CO2 and H2O was enhanced, reducing the extra oxygen demand in the fuel reactor and increasing the CO2 fraction and CO2 capture efficiency. Furthermore, due to the enhanced sintering resistance of CCIO because of the formation of Ca2Si2Al2O7, the reactivity deterioration was never observed during the whole temperature range. Overall, the CCIO is a competitive oxygen carrier for the coal CLC process.
4. Conclusions In this study, the iron ore as oxygen carrier was modified with dolomite by two different preparation methods. The performance of the modified iron ore was evaluated during the coal CLC in a 1 kWth continuous reactor, including gas concentration, gasification efficiency, gas conversion and CO2 capture efficiency. SEM, EDX and XRD were employed to characterize the oxygen carrier both pre-test and pro-test. The following conclusions may be drawn: During the coal CLC using an Australia iron ore, the oxygen carrier exhibited a relatively low reactivity in converting the coal and the gasification products to CO2, especially at a low temperature. The utilization of the dolomite-modified iron ore could enhance the conversion of coal and the gasification products into CO2 and H2O. However, the mechanically mixed oxygen carrier would cause the sintering on the particle surface of iron ore due to the poor sintering resistance of CaO in CLC atmosphere or potential CaOFe2O3 of a low melting point. It caused reactivity deterioration at a temperature above 950 °C. As for the cement/dolomite-combined iron ore, stable compound of Ca2Si2AlO7 was formed during the calcination process of the oxygen carrier. It helped to improve the sintering resistance of oxygen carrier, and the oxygen carrier maintained the porous structure. A stable enhanced reactivity was obtained during the coal CLC process even at a high temperature. Overall, the cement/dolomite-combined iron ore is a competitive oxygen carrier for the coal CLC process. Acknowledgements This research was supported by the National Natural Science Foundation of China (Grant Nos. 51406035, 51276037, 51476029), China Postdoctoral Science Foundation (2014M551489), The Fundamental Research Funds for the
Central Universities, Jiangsu Planned Projects for Postdoctoral Research Funds. References [1] Richter H, Knoche K. Reversibility of combustion process. Symp Ser 1983;235(1):71–86. [2] Adánez J, Abad A, García-Labiano F, Gayán P, Luis F. Progress in chemicallooping combustion and reforming technologies. Prog Energy Combust 2012;38(2):215–82. [3] Leion H, Mattisson T, Lyngfelt A. Solid fuels in chemical-looping combustion. Int J Greenh Gas Con 2014;2(2):180–93. [4] Cabello A, Dueso C, García-Labiano F, Gayán P, Abad A, de Diego LF, et al. Performance of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier with CH4 and H2S in a 500 Wth CLC unit. Fuel 2014;121:117–25. [5] Wang C, Cui H, Di H, Guo Q, Huang F. Experimental investigation on multicycle performance of coal/straw chemical loop combustion with a-Fe2O3 as oxygen carrier. Energ Fuel 2014;28(6):4162–6. [6] Leion H, Jerndal E, Steenari BM, Hermansson S, Israelsson M, Jansson E, et al. Solid fuels in chemical-looping combustion using oxide scale and unprocessed iron ore as oxygen carriers. Fuel 2009;88(10):1945–54. [7] Cuadrat A, Linderholm C, Abad A, Lyngfelt A, Adánez J. Influence of limestone addition in a 10 kWth chemical-looping combustion unit operated with petcoke. Energ Fuel 2011;25(10):4818–28. [8] Cuadrat A, Abad A, Adánez J, De Diego LF, García-Labiano F, Gayán P. Behavior of ilmenite as oxygen carrier in chemical-looping combustion. Fuel Process Technol 2012;94(1):101–12. [9] Zhang S, Saha C, Yang Y, Bhattacharya S, Xiao R. Use of Fe2O3-containing industrial wastes as the oxygen carrier for chemical-looping combustion of coal: effects of pressure and cycles. Energ Fuel 2011;25(10):4357–66. [10] Mendiara T, de Diego LF, García-Labiano F, Gayán P, Abad A, Adánez J. Behaviour of a bauxite waste material as oxygen carrier in a 500Wth CLC unit with coal. Int J Greenh Gas Con 2013;17:170–82. [11] Mendiara T, García-Labiano F, Gayán P, Abad A, de Diego LF, Adánez J. Evaluation of the use of different coals in Chemical Looping Combustion using a bauxite waste as oxygen carrier. Fuel 2013;106:814–26. [12] Lyngfelt A. Chemical-looping combustion of solid fuels–status of development. Appl Energy 2014;113:1869–73. [13] Song T, Wu J, Zhang H, Shen L. Characterization of an Australia hematite oxygen carrier in chemical looping combustion with coal. Int J Greenh Gas Con 2012;11:326–36. [14] Teyssié G, Leion H, Schwebel GL, Lyngfelt A, Mattisson T. Influence of lime addition to ilmenite in chemical-looping combustion (CLC) with solid fuels. Energy Fuel 2011;25(8):3843–53. [15] Guo Q, Cheng Y, Liu Y, Jia W, Ryu HJ. Coal chemical looping gasification for syngas generation using an iron-based oxygen carrier. Ind Eng Chem Res 2013;53(1):78–86. [16] Yu Z, Li C, Fang Y, Huang J, Wang Z. Reduction rate enhancements for coal direct chemical looping combustion with an iron oxide oxygen carrier. Energ Fuel 2012;26(4):2505–11. [17] Gu H, Shen L, Xiao J, Zhang S, Song T, Chen D. Iron ore as oxygen carrier improved with potassium for chemical looping combustion of anthracite coal. Combust Flame 2012;159(7):2480–90. [18] Song T, Shen L, Guo W, Chen D, Xiao J. Enhanced reaction performance with hematite/Ca2Al2SiO7 oxygen carrier in chemical looping combustion of coal. Ind Eng Chem Res 2013;52(28):9573–85. [19] Song T, Shen L, Zhang S, Chen D, Xiao J. Performance of hematite/Ca2Al2SiO7 oxygen carrier in chemical looping combustion of coal. Ind Eng Chem Res 2013;52(22):7350–61. [20] Bao J, Li Z, Sun H, Cai N. Continuous test of ilmenite-based oxygen carriers for chemical looping combustion in a dual fluidized bed reactor system. Ind Eng Chem Res 2013;52(42):14817–27. [21] Bao J, Li Z, Cai N. Promoting the reduction reactivity of ilmenite by introducing foreign ions in chemical looping combustion. Ind Eng Chem Res 2013;52(18):6119–28. [22] Chen H, Zhao C, Yang Y, Zhang P. CO2 capture and attrition performance of CaO pellets with aluminate cement under pressurized carbonation. Appl Energy 2012;91(1):334–40.
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023
H. Gu et al. / Applied Energy xxx (2015) xxx–xxx [23] Manovic V, Anthony EJ. CaO-based pellets supported by calcium aluminate cements for high-temperature CO2 capture. Environ Sci Technol 2009;43(18): 7117–22. [24] Xu L, Wang J, Li Z, Cai N. Experimental study of cement-supported CuO oxygen carriers in chemical looping with oxygen uncoupling (CLOU). Energy Fuel 2013;27(3):1522–30. [25] Lyngfelt A, Leckner B, Mattisson T. A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem Eng Sci 2001;56(10):3101–13.
9
[26] Shen L, Wu J, Gao Z, Xiao J. Characterization of chemical looping combustion of coal in a 1 kWth reactor with a nickel-based oxygen carrier. Combust Flame 2010;157(5):934–42. [27] Blamey J, Anthony EJ, Wang J, Fennell PS. The calcium looping cycle for largescale CO2 capture. Prog Energy Combust 2010;36(2):260–79. [28] Borgwardt RH. Calcium oxide sintering in atmospheres containing water and carbon dioxide. Ind Eng Chem Res 1989;28(4):493–500.
Please cite this article in press as: Gu H et al. Cement/CaO-modified iron ore as oxygen carrier for chemical looping combustion of coal. Appl Energy (2015), http://dx.doi.org/10.1016/j.apenergy.2015.06.023