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Development and performance of binder-supported CaSO4 oxygen carriers for chemical looping combustion
Chemical Engineering Journal 171 (2011) 1018–1026
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Development and performance of binder-supported CaSO4 oxygen carriers for chemical looping combustion Ning Ding, Ying Zheng ∗ , Cong Luo, Qi-long Wu, Pei-fang Fu, Chu-guang Zheng State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 2 January 2011 Received in revised form 1 April 2011 Accepted 24 April 2011 Keywords: Binder-supported CaSO4 Chemical looping combustion Taguchi robust design Fixed bed
a b s t r a c t Chemical-looping combustion (CLC) has been recognized as an energy-efficient method for CO2 capture. For calcium sulfate (CaSO4 ) oxygen carrier, the rapid fall in the mechanical strength and reactivity limits its application in CLC. In this study, Taguchi robust design method with L9 orthogonal array was implemented to optimize extrusion condition for the preparation of binder-supported CaSO4 oxygen carriers. The orthogonal experiment results showed the addition of SB powder enhanced both mechanical strength and conversion of CaSO4 , and the optimal extrusion condition was 30 g CaSO4 , 12 g SB powder, 2.5 mL acetic acid, and 15 mL water. The favorable oxygen carriers were comprehensively studied in a fixed bed reactor. The results of reduction test showed the mass-based reaction rates of the binder-supported CaSO4 were significantly increased, because the fresh samples had the higher surface area which was evidenced by pore structure analysis. Moreover, the favorable performance of binder-supported CaSO4 was explained by formation of CaAl2 O4 which had excellent thermal stability and provided a stable nanosized framework between the crystal grains observed by field emission scanning electron microscope (FESEM). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fossil fuel combustion is the main contributor to increase atmospheric CO2 concentration, an important greenhouse gas that leads to global climate change [1]. Due to their low cost, availability, existing reliable technology for energy production, and energy density, fossil fuels currently supply over 85% of the energy needs [2,3]. Among the different CO2 capture technologies that are under development, chemical-looping combustion (CLC) has been recognized as a promising technology with efficient use of energy and inherent separation of CO2 . CLC is an entirely new combustion technology which involves the use of an oxygen carrier that transfers oxygen from air to the fuel, avoiding the direct contact between fuel and air. CLC system is composed of an air reactor and a fuel reactor. In the fuel reactor, the fuel (natural gas, refinery gas, etc.) is oxidized to CO2 and H2 O by an oxygen carrier. The reduced oxygen carrier is transferred to the air reactor where it is oxidized. The oxygen carrier regenerated is ready to initiate a new cycle. The flue gas from the air reactor is mainly N2 and residual O2 ; and the exit gas from the fuel reactor contains CO2 and water. After the water condensation, almost pure CO2 is obtained without any loss of energy during separation. Based on the CLC system with a Ca-based oxygen carrier, the
∗ Corresponding author. Tel.: +86 27 63120550; fax: +86 27 87545526. E-mail address: [email protected] (Y. Zheng). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.04.054
related reactions in the fuel reactor and the air reactor are shown as follows: CaSO4 + CH4 → CaS + CO2 + 2H2 O H298K = 159.6 kJ/mol CaS + 2O2 → CaSO4
H298K
= −962.4 kJ/mol
(1) (2)
The development of an oxygen carrier is a key issue in the application of CLC. The majority of early studies on oxygen carriers were concentrated on metal oxides, such as Ni-based [4–8], Febased [9–12], Co-based [10,11], Cu-based [13,14], Mn-based [9–11] oxides, and the bimetallic oxides [9,15]. Even though metal oxides exhibited high reactivity and stability, they would be limited in future because of the high cost, low oxygen ratios, bad environmental sound and sulfur-poisoning. Recently calcium sulfate (CaSO4 ) is believed as a novel oxygen carrier for its advantages [16–18]. First, CaSO4 has an oxygen transport capacity larger than the metal oxide oxygen carriers. The oxygen ratio represents the maximum mass fraction of the oxygen transferred by the oxygen carrier from the air to the fuel. The theoretical value of RO for CaSO4 –CaS is 0.4706 while the values are 0.2212, 0.2011 and 0.1001 for NiO–Ni, CuO–Cu and Fe2 O3 –FeO, respectively. Second, anhydrite, of which the major composition is CaSO4 , is a stable ore distributed widely in nature. Therefore, the cost of CaSO4 is very cheap. Finally, CaSO4 is much more environmentally sound as an oxygen carrier. Consequently CaSO4 is becoming a promising oxygen carrier for the application of CLC. Still, there are thermodynamic limitations for Ca-based oxygen
N. Ding et al. / Chemical Engineering Journal 171 (2011) 1018–1026
Nomenclature Ij Kj m mox mox1 mred MO n˙ out RO RO1 dX dt
Xr yi,out Z
level j corresponding conversion average in the corresponding column level j corresponding crushing strength average in the corresponding column mass of oxygen carrier (kg) mass of the fully oxidized oxygen carrier (kg) mass of calcium sulfate in the sample (kg) mass of the fully reduced oxygen carrier (kg) atomic weight of oxygen (kg/mol) molar flow of the gas exiting the reactor after condensation (mol/s) oxygen ratio of the oxygen carrier, defined as RO = (mox − mred )/mox oxygen ratio of calcium sulfate t time (s) the reduction rate of oxygen carrier (1/s) oxygen carrier conversion during the reduction outlet molar fraction of the gas species i exiting the reactor after condensation extreme difference, defined as ZO = max {O1 , O2 , O3 } − min {O1 , O2 , O3 }
Greek letters X change of the conversion of oxygen carrier ω change of the mass conversion of oxygen carrier mass based conversion of oxygen carrier ω
carrier that could cause sulfur release, incomplete conversion of fuel gas and poor mechanical properties as reported [18,19]. The sulfur release is a challenge for the practical use of CaSO4 as an oxygen carrier. By now, Song et al. [20] proposed a reasonable method to capture the sulfur by adding a small amount of fresh limestone into the CLC system. In addition, Wang and Anthony [21] suggested the sulfur released from the fuel, such as petroleum coke and high-sulfur coal, may be captured with CO2 together for sequestration and plays a positive role to inhibit the formation of CaO. Song et al. [22] also found the mass-based reaction rate of natural anhydrite ore was only one-four that of the Fe2 O3 -based oxygen carrier. More work is needed to reduce the sulfur release and improve the reaction rate with the purpose that CaSO4 is utilized into large-scale CLC system. CLC is designed as a cyclic process for CO2 capture (reduction) and regenerated (oxidation) numerous times. Therefore, mechanical properties of an oxygen carrier are vital for practical use. Although the correlation between crushing strength and attrition is not clear, it is unlikely that an oxygen carrier with very low crushing strength will be feasible [23]. Many investigations showed that inert materials used as binders can sustain the chemical stability and structural integrity of metal oxide oxygen carriers [9,23]. Besides, inert materials also can improve the reactivity of metal oxides. For example, using Al2 O3 as an inert support, Fe-based oxygen carrier demonstrated a high reactivity during the reduction [9]. SiO2 , Al2 O3 , TiO2 , ZrO2 , MgO, yttria-stabilized zirconium (YSZ), NiAl2 O4 , etc. have been widely applied as supports for improving the performance of metal oxide oxygen carriers [24]. Apart from Liu et al. [25] studied SiO2 -supported CaSO4 oxygen carrier through thermogravimetric analysis (TGA), few work is given to the effects of binders on CaSO4 as an oxygen carrier. There are several factors that are important to the extrusion of binder-supported CaSO4 process, such as the dosage of binder, peptizer, and water. The interrelationships between the above factors are complex, and the analysis of this process to optimize the fac-
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Table 1 The SB powder characteristics. Model no. Bulk density (kg/m3 ) BET surface area (m2 /g) Ignition loss (%) Chemical composition (%) Al2 O3 SiO2 MgO Fe2 O3 Na2 O
HV-D-03 1200 240 28.97 70.03 0.41 0.26 0.18 0.15
tors is a time and labor-consuming work. Hence, the analyses using conventional experimental methods are inefficient. Taguchi robust design is one of the important statistical methods that use the Taguchi factor design methodology and it allows the effects of many factors with two or more levels on a response, to be studied in a relatively small number of runs [26]. The orthogonal array facilitates the analysis of the design. If used properly, orthogonal design may provide a powerful and efficient method to find an optimal combination of factor levels that may achieve optimum [27]. Thus, orthogonal arrays were used in this experimental design in order to explore an optimal extrusion condition, and series of experiments were conducted. The symbol La is used to represent the orthogonal array, where ‘L’ means Latin square and ‘a’ is the number of experimental runs. Our work takes the first initiative to apply Taguchi robust method to optimization of oxygen carrier extrusion condition. And the favorable binder-supported CaSO4 oxygen carriers were investigated in a fixed bed reactor. The product gas concentrations, oxygen carrier conversion, and release of sulfur during the reduction were also studied. The fresh and reduced oxygen carriers were also characterized through X-ray diffraction (XRD) analysis, field emission scanning electron microscope (FESEM), pore structure analysis.
2. Experimental 2.1. Starting materials In this study, analytical grade dihydrate calcium sulfate (CaSO4 ·2H2 O, Shanghai Aladdin Reagent Co., Ltd.), acetic acid, high sticky pseudo-boehmite (SB powder, Shandong Aluminums Co., Ltd.) and distilled water were used for all runs. The main properties are shown in Table 1. In the extrusion process, the calcined CaSO4 was used as active material of oxygen carrier. The particle size distribution of SB powder and CaSO4 is shown in Fig. 1, and the average particle sizes are 46.49 m and 9.30 m, respectively. High-purity methane (Wuhan Iron and Steel Gases Co., Ltd., 99.99% CH4 ) was used as the reducing gas and high-purity nitrogen (Wuhan Iron and Steel Gases Co., Ltd., 99.99% N2 ) was used as the inert component in the gas mixtures.
2.2. Orthogonal array and experimental factors Taguchi’s orthogonal array table was used by choosing three factors that could affect the extrusion process. Table 2 shows the factors and levels used in this experiment. The numbers 1, 2 and 3 in Table 2 indicate the first, second, and third levels of a factor, respectively. The orthogonal array of L9 type is represented in Table 3. No. 0 is a blank test and No. 10 is a verification test, respectively. Four three-level factors can be positioned in a L9 orthogonal array table.
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N. Ding et al. / Chemical Engineering Journal 171 (2011) 1018–1026 Table 3 Experimental measured values for the crushing strength and conversion of the binder-supported CaSO4 oxygen carriers. No.
Fig. 1. The particle sizes distribution of the SB powder and CaSO4 .
0 1 2 3 4 5 6 7 8 9 10 K1 K2 K3 ZK I1 II2 III3 ZI
Factors
Experimental results
A
B
C
Crushing strength/ standard deviation (N)
Conversion (–)
0 6.0 6.0 6.0 9.0 9.0 9.0 12.0 12.0 12.0 12.0 29.39 36.91 43.06 13.67 0.682 0.633 0.758 0.125
16.5 16.5 15.0 18.0 16.5 15.0 18.0 16.5 15.0 18.0 15.0 48.75 29.94 30.67 18.81 0.714 0.670 0.689 0.044
0 1.5 2.5 3.5 2.5 3.5 1.5 3.5 1.5 2.5 2.5 29.44 49.79 30.13 20.35 0.677 0.743 0.653 0.090
5.15/1.65 16.73/5.02 42.63/14.07 28.80/10.08 55.26/18.70 43.76/15.75 11.72/3.52 17.83/4.99 59.86/21.55 51.49/15.96 68.63/24.02
0.552 0.632 0.728 0.687 0.708 0.603 0.587 0.669 0.812 0.793 0.828
2.3. Preparation of the binder-supported oxygen carriers Oxygen carriers were prepared by a mixed sintering method and the specific process was performed in the following: Firstly, 30 g CaSO4 , SB powder, and acetic acid were mixed together in a glass beaker according to the desired concentration from Table 3. Water was slowly added to minimize the effects of heat release due to the exothermic hydration process. The mixture was lightly Table 2 Levels and factors of orthogonal design. Levels
Low (1) Medium (2) High (3)
Factors A: SB powder (g)
B: water (mL)
C: acetic acid (mL)
6.0 9.0 12.0
15.0 16.5 18.0
1.5 2.5 3.5
stirred by a glass rod until that a gel was formed, similar to mortar. Secondly, the gel was extruded through an extrusion apparatus to obtain cylindrical materials of 2 mm × 3 mm. Finally, the resulting samples were dried at ambient temperature for 4 h, then at 120 ◦ C for 4 h and sintered at 500 ◦ C for 2 h in a muffle furnace. The composition of the fresh and reduced samples was determined by XRD technique using copper K˛ radiation over a 2 range of 15–85◦ . The surface morphology of samples was measured by FESEM in a microscope system (Sirion 200, Holland). The pore structure properties of the fresh and reduced samples were measured by nitrogen adsorption/desorption isotherms at 77 K with a Micromeritics instrument ASAP 2020. The surface area and pore volume were calculated from the Brunauer–Emmett–Teller (BET) equations and Barrett–Joyner–Halenda (BJH) method, respectively.
Fig. 2. The fixed bed experimental setup.
N. Ding et al. / Chemical Engineering Journal 171 (2011) 1018–1026 Table 4 The technical parameter of gas analyzers.
2.6. Data evaluation
Gas component
Sensors
Full scale
Accuracy
Uncertainty
CO2 CO SO2 H2 S CH4 H2
Infrared sensor Electrochemical sensor Electrochemical sensor Electrochemical sensor Infrared sensor Thermal conductivity sensor
100% 20,000 ppm 5000 ppm 2000 ppm 25% 40%
<0.03% <5 ppm <5 ppm <5 ppm <1% <2%
±0.03% ±0.0005% ±0.0005% ±0.0005% ±0.25% ±0.8%
2.4. Crushing strength and reduction reactivity of the binder-supported oxygen carriers
The conversion of oxygen carrier or degree of oxidation (X) is defined as: X=
m − mred mox − mred
The mechanical strength, determined using a Shimpo FHJ-5 crushing strength apparatus, was taken as the average value of 20 measurements of the force needed to fracture a particle. The fixed bed reactor system consisted of a gas feeding, quartz reactor, tube furnace, cooler and gas analysis system. The schematic diagram of the system is shown in Fig. 2. A porous distributor plate located in the middle quartz tube (I.D. = 20 mm, length = 1000 mm). The reactor was heated in an electric furnace and the furnace temperature was controlled by a K-type thermocouple between the tube and the heater while the reaction temperature was monitored by another K-type thermocouple inside the oxygen carrier particles. The flow rates of CH4 and N2 were measured by mass flow controllers (MFC, Beijing Sevenstar Huachuang Electronics Co., Ltd.). The product gas from the reactor flowed to a cooler filled CaCl2 desiccant which could condense the steam without the adsorption of acid or neutral gas, then was diluted by the other inert gas N2 to meet the requirement of flow, and finally was sent to the gas analyzers. The concentrations of CH4 and H2 were measured by a GASBOARD-3100 coal gas analyzer, and the concentrations of CO2 , SO2 , H2 S and CO were measured by a GA-21 Plus flue gas analyzer. The N2 concentration was not displayed in this study, and the detailed specification and accuracy of gas analyzers are presented in Table 4.
(3)
The conversion of the CaSO4 oxygen carrier as a function of time during the reduction period can be calculated assuming the mass balance of oxygen from the oxygen carrier to the product gas, using the relationship: Xr,i = Xr,i−1
−
ti
ti−1
MO n˙ out (4yCO2 ,out + 3yCO,out − yH2 ,out + 2ySO2 ,out )dt mox1 RO1
dX MO n˙ out = (4yCO2 ,out + 3yCO,out − yH2 ,out + 2ySO2 ,out ) mox1 RO1 dt
(4)
(5)
In order to facilitate a comparison between the different oxygen carriers, a mass-based conversion is often applied as follows: ω=
m = 1 + RO (X − 1) mox
(6)
The mass-based reduction rate is calculated as follows: dω dX = RO dt dt
(7)
The oxygen ratio is dependent on the active material used as oxygen carrier as well as the amount of inert in the particles [28]. Such as the binder-supported CaSO4 containing the active material 80 wt% CaSO4 , RO is equal to 0.3765. 3. Results and discussion 3.1. Determination of optimal extrusion condition using Taguchi robust design
2.5. Experimental procedure The experiment was conducted in a horizontal fixed bed reactor under the atmospheric pressure. In each run, 10 g of the sample was placed on the distributor. During the preheating period, the reactor system was purged with nitrogen gas flow. When the reaction temperature was reached 950 ◦ C and became stable, the reducing gas (200 mL/min) was continuously introduced, and the total reduction time was 60 min. When the reduction was finished, the system was again purged with nitrogen until the product gas was cleared away. The sample was cooled in the nitrogen flow to ambient temperature and collected for further analysis. The specific experimental condition is shown in Table 5.
Table 5 Experimental condition. Oxygen carrier Pressure Particle size Sample mass Reaction temperature Reducing gas Reducing gas flow Reduction time Inert gas Inert gas flow
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Binder-supported CaSO4 1 atm 2 mm × 3 mm 10 g 950 ◦ C 50% CH4 /N2 200 mL/min 60 min N2 1300 mL/min
Taguchi robust design method was used to identify the optimal extrusion condition and to select the factors having the most principal influence on the performance of the binder-supported CaSO4 . The structure of Taguchi’s orthogonal robust design and the results of measurement are shown in Table 3. The optimal extrusion condition of binder-supported CaSO4 oxygen carrier was determined by intuitive analysis with the crushing strength and conversion at 30 min as target functions. According to the extreme difference in crushing strength (ZK ), the importance of factors can be ranked as acetic acid > the ratio of water > SB powder. According to the extreme difference in conversion (ZI ), the importance of factors can be ranked as SB powder > acetic acid > the ratio of water. Based on the maximum crushing strength and conversion, the optimum formulation was A3 B1 C1 (sample 8). The maximum crushing strength was 59.86 N, and the maximum conversion was 0.812. Fig. 3 shows the relationship between levels of factors and average values of crushing strength or conversion. The optimum formulation from Fig. 3 was A3 B1 C2 (sample 10), not A3 B1 C1 obtained from Table 3. Consequently, a verification experiment was conducted to check the result of optimal formulation of A3 B1 C2 . The experimental results are also summarized in Table 3. The optimal formulation resulted in the maximum crushing strength of 68.63 N and conversion of 0.828, which were higher than that of formulation of A3 B1 C1 . Therefore, the optimal extrusion condition was: 30 g CaSO4 , 12 g SB powder, 2.5 mL acetic acid, and 15 mL water.
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Fig. 3. Response curves of orthogonal design, SB powder (A), water (B) and acetic acid (C).
3.2. Reduction reactivity of the binder-supported CaSO4 oxygen carriers
carrier during reduction was investigated through the release of by-products CO and H2 .
3.2.1. Gas concentration profiles The product gas concentration profiles as a function of reaction time at 950 ◦ C for the reduction test are presented in Fig. 4. As shown in Fig. 4, the CO2 concentrations of samples 8 and 10 first increased to peaks at 15 min and 10 min, respectively, and decreased gradually. These variation curves are similar to the results in the literature where iron oxide served as an oxygen carrier [23]. As the reaction proceeded, the unreacted CH4 was found at about 4 min due to the short residence time and high concentration of CH4 . And its concentrations exceeded CO2 at 18 min for the first time. Generally, the formation of CO and H2 yields to the thermodynamic limitations of CaSO4 which is the case with the Nibased oxygen carrier. Also trace amounts of CO which was released along with CH4 showed a slow growth trend, while H2 was released at around 23 min. However, when most of CaSO4 was consumed, the significant carbon deposition was observed. The main reactions for carbon formation involve the methane decomposition (Eq. (8)), gasification with H2 O (Eq. (9)) and Boudouard reaction (Eq. (10)) [17,29]. In this study, the carbon deposition on CaSO4 oxygen
CH4 → C + 2H2
H298K = 73.8 kJ/mol
= 73.8 kJ/mol C + 2CO2 → 2CO H298K
C + H2 O → CO + H2
H298K = 172.1 kJ/mol
(8) (9) (10)
Interestingly, the concentrations of CO and H2 slightly decreased at the later period, the reason may come from the following aspects: On the one hand, the carbon formation on CaSO4 particles took up the most gas–solid reaction surface, which may inhibit the decomposition of CH4 (Eq. (8)). On the other hand, the decrease of CO2 and H2 O resulted in the decline of the reactions (Eqs. (9) and (10)). In comparison to sample 8, the amounts of CO and H2 of sample 10 were relatively low. In a word, CO2 was the main product gas during the earlier period, but the carbon deposition was serious during the later period. Therefore, the formation of CO, H2 and carbon could decrease with an excess oxygen carrier and short reduction time. Carbon deposition was not the key issue in this study. It is generally accepted that the carbon deposition does not appear to be a major problem if the oxidation reaction is carried out at a high enough temperature.
Fig. 4. Product gas concentrations and dry gas flow during reduction as a function of time. (a) Sample 8, and (b) sample 10.
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Fig. 5. (a) Oxygen carrier conversion (X) as a function of time, and (b) mass-based reaction rates (dω/dt) as a function of mass conversion (ω) for the reduction period.
The dry gas flow is also presented in Fig. 4. The gas flow was calculated relative to the gas analysis from the balance of nitrogen gas diluted in the outlet of reactor before the gas analyzers. The flow reduction was mainly a result of condensation of steam, and the variation of gas flow rate reflected the transient changes in gas concentration.
3.2.2. Oxygen carrier conversion and reaction rate The conversion of samples 8 and 10 during the reduction test is shown in Fig. 5(a). The decrease of conversion indicates the increase of extent of reduction and that more oxygen in the oxygen carrier was transferred to the fuel gas or sulfur release [19]. The conversion of samples 8 and 10 decreased to 0.258 and 0.189 after 30 min, respectively. And the oxygen carriers can be completely reduced after 60 min. Fig. 5(b) shows the massbased reaction rates of samples 8 and 10 as a function of the mass conversion (ω). The maximum change of mass conversion (ω) of metal oxide oxygen carriers is about 0.2, however, the mass conversion of CaSO4 decreased from 1.0 to 0.531 that was no better than theoretical value (0.529). The experimental results had proven the high oxygen transport capacity. At ω over 0.72, the mass-based reaction rate (dω/dt) of sample 10 was obviously greater than that of sample 8, which approached to half of dω/dt of the Fe2 O3 -based oxygen carrier in the literature [24]. But, comparison to natural anhydrite ore [20], the reactivity of the binder-supported CaSO4 had greatly improved for the addition of SB powder and acetic acid. So the binder-supported
Fig. 6. Outlet concentrations of SO2 and H2 S as a function of time during reduction.
CaSO4 oxygen carrier is promising for the practical application of CLC. 3.2.3. Sulfur release The sulfur release is a key problem in the CLC system using a CaSO4 oxygen carrier. As illustrated in many investigations [20,30–34], the emission of sulfur involves with CaSO4 and CaS is
Fig. 7. XRD of the binder-supported CaSO4 oxygen carriers. (a) Sample 6 and (b) sample 10.
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Fig. 8. FESEM micrographs of the fresh and reduced CaSO4 oxygen carriers. (a) Fresh sample 6, 1 m, (b) reduced sample 6, 1 m, (c) fresh sample 10, 1 m, (d) reduced sample 10, 1 m.
varied with the operating conditions. In this study, SO2 and H2 S formation are mainly due to the competing side reactions (Eqs. (11) and (12)) and the solid–solid reaction (Eq. (13)) during the reduction: 4CaSO4 +CH4 → 4CaO+CO2 +4SO2 +2H2 O H298K = 1220.8 kJ/mol (11) CaSO4 + CH4 → CaO + CO2 + H2 S + H2 O H298K = 221.1 kJ/mol
(12) 3CaSO4 + CaS → 4CaO + 4SO2
H298K = 1061.3 kJ/mol
(13)
The variation of SO2 and H2 S in the dry gas during reduction test is displayed in Fig. 6. The curves of SO2 and H2 S showed a single peak characteristic, and reached their peaks at 10 min and 15 min, respectively. Interestingly, the release peak time of SO2 and H2 S was consistent with that of CO2 . This indicates that the competing side reductions occurred at the early period and then the target reduction became dominant. On the whole, the maximum and total of H2 S was much lower than that of SO2 , because the solid–solid reaction may be enhanced owing to the formation of the outer product layer of CaS. However, sulfur loss rates of samples 8 and 10 were just 0.14% and 0.18%, respectively. The emission of sulfur in this study is only for the reduction of the fresh oxygen carriers, further work on formation of sulfur in oxidation process
and cyclic test in CLC system is needed. As suggested by Wang and Anthony [21], to obtain more product of CaS, a certain concentration of SO2 from fuel is forced into the reactors. In this experiment, SO2 was not fed because this study focused on the reactivity of the binder-supported CaSO4 oxygen carrier particles with CH4 . 3.3. Characterization analysis 3.3.1. Phase characterization Fig. 7 shows the XRD patterns for the reduced samples 6 and 10, and the number marked above the peak is the relative intensity. It is evident that CaS was the dominant reduced product at reducing condition. The CaSO4 diffraction peak was not found, which was in accordance with the results of conversion. Fig. 7 shows a new phase formation (CaAl2 O4 ) after reduction, which indicates that Al2 O3 reacted with product CaO. The CaAl2 O4 diffraction peaks of sample 10 were higher than those of sample 6, and a part of Al2 O3 did not react with CaO in the sample 6. That is also a reason that the crushing strength of sample 10 was greater than that of sample 6. And the existing of CaO would lead to decrease of the reaction rate. 3.3.2. Surface morphology The particles of samples 6 and 10 were examined using FESEM before and after reduction. Fig. 8(a) shows the surface of sample 6 was dense and impervious which was similar to analytical grade
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Fig. 9. BJH desorption pore volume distribution changes of fresh and reduced CaSO4 oxygen carriers.
CaSO4 . Interestingly, the plate-like particles were observed from the fresh sample 10 in Fig. 8(c). As shown in Fig. 8(b) and (d), the surfaces of the reduced CaSO4 oxygen carriers appeared much rougher and more porous. In particular, it was shown that samples provided a stable nanosized framework between the crystal grains, which might retard the sintering of active CaSO4 sites [35]. The material of framework may be the main alumina compound, which had excellent thermal stability and was also found in synthetic sorbents. Nevertheless, one difference between two samples was that the crystal grains size of sample 10 was much smaller than that of sample 6. Therefore, sample 10 may have higher surface area, which would be verified by pore structure analysis. 3.3.3. Pore structure analysis The BET surface area, average pore diameter and BJH pore volume distribution of fresh and reduced samples are illustrated in Table 6 and Fig. 9. The surface area of the fresh samples 6 and 10 was about 8 times higher than analytical grade CaSO4 (9.1667 m2 /g), which could be attributed to the addition of SB powder and acetic acid. A higher surface area of the binder-supported CaSO4 oxygen carrier could give rise to a higher reactivity. Possibly due to sintering and carbon deposition, the surface area of the reduced samples had Table 6 BET surface area and average pore diameter of fresh and reduced CaSO4 oxygen carriers. Sample
Fresh samples 2
Sample 6 Sample 10
Reduced samples
SBET (m /g)
Average pore diameter (nm)
SBET (m2 /g)
Average pore diameter (nm)
71.1776 78.8670
7.6005 8.1906
10.8930 15.1320
18.7727 20.7283
a pronounced decrease, while the average pore diameter increased. But the favorable porous structure of sample 10 was more beneficial to oxidizing reaction than sample 6, which can be obviously observed in Fig. 8(d). Further research is needed to clarify if there will be a relationship between mechanical strength and porosity. 4. Conclusions In this study, using Taguchi robust design method with L9 orthogonal array, the binder-supported CaSO4 oxygen carriers prepared from the calcined CaSO4 , SB powder, acetic acid and water were investigated by intuitive analysis with the crushing strength and conversion as target functions and the favorable oxygen carriers were comprehensively studied in a fixed bed reactor. The orthogonal experiment results showed the addition of SB powder enhanced both mechanical strength and conversion of CaSO4 , and the optimal extrusion condition was: 30 g CaSO4 , 12 g SB powder, 2.5 mL acetic acid, and 15 mL water. The results of reduction test showed CO2 were the main product gas during the earlier period. CaSO4 could be completely reduced which was proved by conversion and XRD analysis, and the release of H2 S and SO2 during the reduction was found to be responsible for the decrease of the reactivity of oxygen carriers. The mass-based reaction rates of the binder-supported CaSO4 were significantly enhanced, because the fresh samples had the higher surface area evidenced by pore structure analysis. Moreover, the favorable performance of the binder-supported CaSO4 was explained by formation of CaAl2 O4 which had excellent thermal stability and provided a stable nanosized framework between the crystal grains observed by FESEM. Based on the results, the optimal extrusion condition is recommended and the favorable binder-supported CaSO4 oxygen carrier
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could have great potential to be used in practical application of CLC system. Acknowledgments The authors acknowledge the financial supports of the Special Funds for National Basic Research Program of China (No. 2011CB707301), the National Natural Science Foundation of China (Nos. 50936001, 51021065). The authors are also grateful to the analytical and testing center of Huazhong University of Science and Technology for the XRD, FESEM measurements. References [1] Climate Change 2007: Mitigation of Climate Change. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, 2008. [2] J.D. Figueroa, T. Fout, S. Plasynski, H. McIlvried, R.D. Srivastava, Advances in CO2 capture technology – the U.S. department of energy’s carbon sequestration program, Int. J. Greenhouse Gas Control 2 (1) (2008) 9–20. [3] P. Riemer, Greenhouse gas mitigation technologies, an overview of the CO2 capture, storage and future activities of the IEA greenhouse gas R&D programme, Energy Convers. Manage. 37 (6–8) (1996) 665–670. [4] L.F. de Diego, M. Ortiz, J. Adánez, F. García-Labiano, A. Abad, P. Gayán, Synthesis gas generation by chemical-looping reforming in a batch fluidized bed reactor using Ni-based oxygen carriers, Chem. Eng. J. 144 (2) (2008) 289–298. [5] M. Ishida, H.G. Jin, T. Okamoto, A fundamental study of a new kind of medium material for chemical-looping combustion, Energy Fuels 10 (4) (1996) 958–963. [6] J. Adánez, C. Dueso, L.F. de Diego, F. García-Labiano, P. Gayán, A. Abad, Methane combustion in a 500 Wth chemical-looping combustion system using an impregnated Ni-based oxygen carrier, Energy Fuels 23 (1) (2009) 130–142. [7] H.J. Ryu, D.H. Bae, G.T. Jin, Effect of temperature on reduction reactivity of oxygen carrier particles in a fixed bed chemical-looping combustor, Korean J. Chem. Eng. 20 (5) (2003) 960–966. [8] L.H. Shen, J.H. Wu, Z.P. Gao, J. Xiao, Characterization of chemical looping combustion of coal in a 1 kWth reactor with a nickel-based oxygen carrier, Combust. Flame 157 (5) (2010) 934–942. [9] J. Adánez, L.F. de Diego, F. García-Labiano, P. Gayán, A. Abad, J.M. Palacios, Selection of oxygen carriers for chemical-looping combustion, Energy Fuels 18 (2) (2004) 371–377. [10] M.M. Hossain, H.I. de Lasa, Reduction and oxidation kinetics of Co–Ni/Al2 O3 oxygen carrier involved in a chemical-looping combustion cycles, Chem. Eng. Sci. 65 (1) (2010) 98–106. [11] Y. Cao, W.P. Pan, Investigation of chemical looping combustion by solid fuels. 1. Process analysis, Energy Fuels 20 (5) (2006) 1836–1844. [12] H. Leion, T. Mattisson, A. Lyngfelt, Solid fuels in chemical-looping combustion, Int. J. Greenhouse Gas Control 2 (2) (2008) 180–193. [13] S.A. Scott, J.S. Dennis, A.N. Hayhurst, T. Brown, In situ gasification of a solid fuel and CO2 separation using chemical looping, AIChE J. 52 (9) (2006) 3325–3328. [14] 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. [15] H Jin, T. Okamoto, M. Ishida, Development of a novel chemical-looping combustion: synthesis of a looping material with a double metal oxide of CoO–NiO, Energy Fuels 12 (6) (1998) 1272–1277. [16] H.J. Tian, Q.J. Guo, J. Chang, Investigation into decomposition behavior of CaSO4 in chemical-looping combustion, Energy Fuels 22 (6) (2008) 3915–3921.
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Development and performance of binder-supported CaSO4 oxygen carriers for chemical looping combustion Ning Ding, Ying Zheng ∗ , Cong Luo, Qi-long Wu, Pei-fang Fu, Chu-guang Zheng State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 2 January 2011 Received in revised form 1 April 2011 Accepted 24 April 2011 Keywords: Binder-supported CaSO4 Chemical looping combustion Taguchi robust design Fixed bed
a b s t r a c t Chemical-looping combustion (CLC) has been recognized as an energy-efficient method for CO2 capture. For calcium sulfate (CaSO4 ) oxygen carrier, the rapid fall in the mechanical strength and reactivity limits its application in CLC. In this study, Taguchi robust design method with L9 orthogonal array was implemented to optimize extrusion condition for the preparation of binder-supported CaSO4 oxygen carriers. The orthogonal experiment results showed the addition of SB powder enhanced both mechanical strength and conversion of CaSO4 , and the optimal extrusion condition was 30 g CaSO4 , 12 g SB powder, 2.5 mL acetic acid, and 15 mL water. The favorable oxygen carriers were comprehensively studied in a fixed bed reactor. The results of reduction test showed the mass-based reaction rates of the binder-supported CaSO4 were significantly increased, because the fresh samples had the higher surface area which was evidenced by pore structure analysis. Moreover, the favorable performance of binder-supported CaSO4 was explained by formation of CaAl2 O4 which had excellent thermal stability and provided a stable nanosized framework between the crystal grains observed by field emission scanning electron microscope (FESEM). © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fossil fuel combustion is the main contributor to increase atmospheric CO2 concentration, an important greenhouse gas that leads to global climate change [1]. Due to their low cost, availability, existing reliable technology for energy production, and energy density, fossil fuels currently supply over 85% of the energy needs [2,3]. Among the different CO2 capture technologies that are under development, chemical-looping combustion (CLC) has been recognized as a promising technology with efficient use of energy and inherent separation of CO2 . CLC is an entirely new combustion technology which involves the use of an oxygen carrier that transfers oxygen from air to the fuel, avoiding the direct contact between fuel and air. CLC system is composed of an air reactor and a fuel reactor. In the fuel reactor, the fuel (natural gas, refinery gas, etc.) is oxidized to CO2 and H2 O by an oxygen carrier. The reduced oxygen carrier is transferred to the air reactor where it is oxidized. The oxygen carrier regenerated is ready to initiate a new cycle. The flue gas from the air reactor is mainly N2 and residual O2 ; and the exit gas from the fuel reactor contains CO2 and water. After the water condensation, almost pure CO2 is obtained without any loss of energy during separation. Based on the CLC system with a Ca-based oxygen carrier, the
∗ Corresponding author. Tel.: +86 27 63120550; fax: +86 27 87545526. E-mail address: [email protected] (Y. Zheng). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2011.04.054
related reactions in the fuel reactor and the air reactor are shown as follows: CaSO4 + CH4 → CaS + CO2 + 2H2 O H298K = 159.6 kJ/mol CaS + 2O2 → CaSO4
H298K
= −962.4 kJ/mol
(1) (2)
The development of an oxygen carrier is a key issue in the application of CLC. The majority of early studies on oxygen carriers were concentrated on metal oxides, such as Ni-based [4–8], Febased [9–12], Co-based [10,11], Cu-based [13,14], Mn-based [9–11] oxides, and the bimetallic oxides [9,15]. Even though metal oxides exhibited high reactivity and stability, they would be limited in future because of the high cost, low oxygen ratios, bad environmental sound and sulfur-poisoning. Recently calcium sulfate (CaSO4 ) is believed as a novel oxygen carrier for its advantages [16–18]. First, CaSO4 has an oxygen transport capacity larger than the metal oxide oxygen carriers. The oxygen ratio represents the maximum mass fraction of the oxygen transferred by the oxygen carrier from the air to the fuel. The theoretical value of RO for CaSO4 –CaS is 0.4706 while the values are 0.2212, 0.2011 and 0.1001 for NiO–Ni, CuO–Cu and Fe2 O3 –FeO, respectively. Second, anhydrite, of which the major composition is CaSO4 , is a stable ore distributed widely in nature. Therefore, the cost of CaSO4 is very cheap. Finally, CaSO4 is much more environmentally sound as an oxygen carrier. Consequently CaSO4 is becoming a promising oxygen carrier for the application of CLC. Still, there are thermodynamic limitations for Ca-based oxygen
N. Ding et al. / Chemical Engineering Journal 171 (2011) 1018–1026
Nomenclature Ij Kj m mox mox1 mred MO n˙ out RO RO1 dX dt
Xr yi,out Z
level j corresponding conversion average in the corresponding column level j corresponding crushing strength average in the corresponding column mass of oxygen carrier (kg) mass of the fully oxidized oxygen carrier (kg) mass of calcium sulfate in the sample (kg) mass of the fully reduced oxygen carrier (kg) atomic weight of oxygen (kg/mol) molar flow of the gas exiting the reactor after condensation (mol/s) oxygen ratio of the oxygen carrier, defined as RO = (mox − mred )/mox oxygen ratio of calcium sulfate t time (s) the reduction rate of oxygen carrier (1/s) oxygen carrier conversion during the reduction outlet molar fraction of the gas species i exiting the reactor after condensation extreme difference, defined as ZO = max {O1 , O2 , O3 } − min {O1 , O2 , O3 }
Greek letters X change of the conversion of oxygen carrier ω change of the mass conversion of oxygen carrier mass based conversion of oxygen carrier ω
carrier that could cause sulfur release, incomplete conversion of fuel gas and poor mechanical properties as reported [18,19]. The sulfur release is a challenge for the practical use of CaSO4 as an oxygen carrier. By now, Song et al. [20] proposed a reasonable method to capture the sulfur by adding a small amount of fresh limestone into the CLC system. In addition, Wang and Anthony [21] suggested the sulfur released from the fuel, such as petroleum coke and high-sulfur coal, may be captured with CO2 together for sequestration and plays a positive role to inhibit the formation of CaO. Song et al. [22] also found the mass-based reaction rate of natural anhydrite ore was only one-four that of the Fe2 O3 -based oxygen carrier. More work is needed to reduce the sulfur release and improve the reaction rate with the purpose that CaSO4 is utilized into large-scale CLC system. CLC is designed as a cyclic process for CO2 capture (reduction) and regenerated (oxidation) numerous times. Therefore, mechanical properties of an oxygen carrier are vital for practical use. Although the correlation between crushing strength and attrition is not clear, it is unlikely that an oxygen carrier with very low crushing strength will be feasible [23]. Many investigations showed that inert materials used as binders can sustain the chemical stability and structural integrity of metal oxide oxygen carriers [9,23]. Besides, inert materials also can improve the reactivity of metal oxides. For example, using Al2 O3 as an inert support, Fe-based oxygen carrier demonstrated a high reactivity during the reduction [9]. SiO2 , Al2 O3 , TiO2 , ZrO2 , MgO, yttria-stabilized zirconium (YSZ), NiAl2 O4 , etc. have been widely applied as supports for improving the performance of metal oxide oxygen carriers [24]. Apart from Liu et al. [25] studied SiO2 -supported CaSO4 oxygen carrier through thermogravimetric analysis (TGA), few work is given to the effects of binders on CaSO4 as an oxygen carrier. There are several factors that are important to the extrusion of binder-supported CaSO4 process, such as the dosage of binder, peptizer, and water. The interrelationships between the above factors are complex, and the analysis of this process to optimize the fac-
1019
Table 1 The SB powder characteristics. Model no. Bulk density (kg/m3 ) BET surface area (m2 /g) Ignition loss (%) Chemical composition (%) Al2 O3 SiO2 MgO Fe2 O3 Na2 O
HV-D-03 1200 240 28.97 70.03 0.41 0.26 0.18 0.15
tors is a time and labor-consuming work. Hence, the analyses using conventional experimental methods are inefficient. Taguchi robust design is one of the important statistical methods that use the Taguchi factor design methodology and it allows the effects of many factors with two or more levels on a response, to be studied in a relatively small number of runs [26]. The orthogonal array facilitates the analysis of the design. If used properly, orthogonal design may provide a powerful and efficient method to find an optimal combination of factor levels that may achieve optimum [27]. Thus, orthogonal arrays were used in this experimental design in order to explore an optimal extrusion condition, and series of experiments were conducted. The symbol La is used to represent the orthogonal array, where ‘L’ means Latin square and ‘a’ is the number of experimental runs. Our work takes the first initiative to apply Taguchi robust method to optimization of oxygen carrier extrusion condition. And the favorable binder-supported CaSO4 oxygen carriers were investigated in a fixed bed reactor. The product gas concentrations, oxygen carrier conversion, and release of sulfur during the reduction were also studied. The fresh and reduced oxygen carriers were also characterized through X-ray diffraction (XRD) analysis, field emission scanning electron microscope (FESEM), pore structure analysis.
2. Experimental 2.1. Starting materials In this study, analytical grade dihydrate calcium sulfate (CaSO4 ·2H2 O, Shanghai Aladdin Reagent Co., Ltd.), acetic acid, high sticky pseudo-boehmite (SB powder, Shandong Aluminums Co., Ltd.) and distilled water were used for all runs. The main properties are shown in Table 1. In the extrusion process, the calcined CaSO4 was used as active material of oxygen carrier. The particle size distribution of SB powder and CaSO4 is shown in Fig. 1, and the average particle sizes are 46.49 m and 9.30 m, respectively. High-purity methane (Wuhan Iron and Steel Gases Co., Ltd., 99.99% CH4 ) was used as the reducing gas and high-purity nitrogen (Wuhan Iron and Steel Gases Co., Ltd., 99.99% N2 ) was used as the inert component in the gas mixtures.
2.2. Orthogonal array and experimental factors Taguchi’s orthogonal array table was used by choosing three factors that could affect the extrusion process. Table 2 shows the factors and levels used in this experiment. The numbers 1, 2 and 3 in Table 2 indicate the first, second, and third levels of a factor, respectively. The orthogonal array of L9 type is represented in Table 3. No. 0 is a blank test and No. 10 is a verification test, respectively. Four three-level factors can be positioned in a L9 orthogonal array table.
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N. Ding et al. / Chemical Engineering Journal 171 (2011) 1018–1026 Table 3 Experimental measured values for the crushing strength and conversion of the binder-supported CaSO4 oxygen carriers. No.
Fig. 1. The particle sizes distribution of the SB powder and CaSO4 .
0 1 2 3 4 5 6 7 8 9 10 K1 K2 K3 ZK I1 II2 III3 ZI
Factors
Experimental results
A
B
C
Crushing strength/ standard deviation (N)
Conversion (–)
0 6.0 6.0 6.0 9.0 9.0 9.0 12.0 12.0 12.0 12.0 29.39 36.91 43.06 13.67 0.682 0.633 0.758 0.125
16.5 16.5 15.0 18.0 16.5 15.0 18.0 16.5 15.0 18.0 15.0 48.75 29.94 30.67 18.81 0.714 0.670 0.689 0.044
0 1.5 2.5 3.5 2.5 3.5 1.5 3.5 1.5 2.5 2.5 29.44 49.79 30.13 20.35 0.677 0.743 0.653 0.090
5.15/1.65 16.73/5.02 42.63/14.07 28.80/10.08 55.26/18.70 43.76/15.75 11.72/3.52 17.83/4.99 59.86/21.55 51.49/15.96 68.63/24.02
0.552 0.632 0.728 0.687 0.708 0.603 0.587 0.669 0.812 0.793 0.828
2.3. Preparation of the binder-supported oxygen carriers Oxygen carriers were prepared by a mixed sintering method and the specific process was performed in the following: Firstly, 30 g CaSO4 , SB powder, and acetic acid were mixed together in a glass beaker according to the desired concentration from Table 3. Water was slowly added to minimize the effects of heat release due to the exothermic hydration process. The mixture was lightly Table 2 Levels and factors of orthogonal design. Levels
Low (1) Medium (2) High (3)
Factors A: SB powder (g)
B: water (mL)
C: acetic acid (mL)
6.0 9.0 12.0
15.0 16.5 18.0
1.5 2.5 3.5
stirred by a glass rod until that a gel was formed, similar to mortar. Secondly, the gel was extruded through an extrusion apparatus to obtain cylindrical materials of 2 mm × 3 mm. Finally, the resulting samples were dried at ambient temperature for 4 h, then at 120 ◦ C for 4 h and sintered at 500 ◦ C for 2 h in a muffle furnace. The composition of the fresh and reduced samples was determined by XRD technique using copper K˛ radiation over a 2 range of 15–85◦ . The surface morphology of samples was measured by FESEM in a microscope system (Sirion 200, Holland). The pore structure properties of the fresh and reduced samples were measured by nitrogen adsorption/desorption isotherms at 77 K with a Micromeritics instrument ASAP 2020. The surface area and pore volume were calculated from the Brunauer–Emmett–Teller (BET) equations and Barrett–Joyner–Halenda (BJH) method, respectively.
Fig. 2. The fixed bed experimental setup.
N. Ding et al. / Chemical Engineering Journal 171 (2011) 1018–1026 Table 4 The technical parameter of gas analyzers.
2.6. Data evaluation
Gas component
Sensors
Full scale
Accuracy
Uncertainty
CO2 CO SO2 H2 S CH4 H2
Infrared sensor Electrochemical sensor Electrochemical sensor Electrochemical sensor Infrared sensor Thermal conductivity sensor
100% 20,000 ppm 5000 ppm 2000 ppm 25% 40%
<0.03% <5 ppm <5 ppm <5 ppm <1% <2%
±0.03% ±0.0005% ±0.0005% ±0.0005% ±0.25% ±0.8%
2.4. Crushing strength and reduction reactivity of the binder-supported oxygen carriers
The conversion of oxygen carrier or degree of oxidation (X) is defined as: X=
m − mred mox − mred
The mechanical strength, determined using a Shimpo FHJ-5 crushing strength apparatus, was taken as the average value of 20 measurements of the force needed to fracture a particle. The fixed bed reactor system consisted of a gas feeding, quartz reactor, tube furnace, cooler and gas analysis system. The schematic diagram of the system is shown in Fig. 2. A porous distributor plate located in the middle quartz tube (I.D. = 20 mm, length = 1000 mm). The reactor was heated in an electric furnace and the furnace temperature was controlled by a K-type thermocouple between the tube and the heater while the reaction temperature was monitored by another K-type thermocouple inside the oxygen carrier particles. The flow rates of CH4 and N2 were measured by mass flow controllers (MFC, Beijing Sevenstar Huachuang Electronics Co., Ltd.). The product gas from the reactor flowed to a cooler filled CaCl2 desiccant which could condense the steam without the adsorption of acid or neutral gas, then was diluted by the other inert gas N2 to meet the requirement of flow, and finally was sent to the gas analyzers. The concentrations of CH4 and H2 were measured by a GASBOARD-3100 coal gas analyzer, and the concentrations of CO2 , SO2 , H2 S and CO were measured by a GA-21 Plus flue gas analyzer. The N2 concentration was not displayed in this study, and the detailed specification and accuracy of gas analyzers are presented in Table 4.
(3)
The conversion of the CaSO4 oxygen carrier as a function of time during the reduction period can be calculated assuming the mass balance of oxygen from the oxygen carrier to the product gas, using the relationship: Xr,i = Xr,i−1
−
ti
ti−1
MO n˙ out (4yCO2 ,out + 3yCO,out − yH2 ,out + 2ySO2 ,out )dt mox1 RO1
dX MO n˙ out = (4yCO2 ,out + 3yCO,out − yH2 ,out + 2ySO2 ,out ) mox1 RO1 dt
(4)
(5)
In order to facilitate a comparison between the different oxygen carriers, a mass-based conversion is often applied as follows: ω=
m = 1 + RO (X − 1) mox
(6)
The mass-based reduction rate is calculated as follows: dω dX = RO dt dt
(7)
The oxygen ratio is dependent on the active material used as oxygen carrier as well as the amount of inert in the particles [28]. Such as the binder-supported CaSO4 containing the active material 80 wt% CaSO4 , RO is equal to 0.3765. 3. Results and discussion 3.1. Determination of optimal extrusion condition using Taguchi robust design
2.5. Experimental procedure The experiment was conducted in a horizontal fixed bed reactor under the atmospheric pressure. In each run, 10 g of the sample was placed on the distributor. During the preheating period, the reactor system was purged with nitrogen gas flow. When the reaction temperature was reached 950 ◦ C and became stable, the reducing gas (200 mL/min) was continuously introduced, and the total reduction time was 60 min. When the reduction was finished, the system was again purged with nitrogen until the product gas was cleared away. The sample was cooled in the nitrogen flow to ambient temperature and collected for further analysis. The specific experimental condition is shown in Table 5.
Table 5 Experimental condition. Oxygen carrier Pressure Particle size Sample mass Reaction temperature Reducing gas Reducing gas flow Reduction time Inert gas Inert gas flow
1021
Binder-supported CaSO4 1 atm 2 mm × 3 mm 10 g 950 ◦ C 50% CH4 /N2 200 mL/min 60 min N2 1300 mL/min
Taguchi robust design method was used to identify the optimal extrusion condition and to select the factors having the most principal influence on the performance of the binder-supported CaSO4 . The structure of Taguchi’s orthogonal robust design and the results of measurement are shown in Table 3. The optimal extrusion condition of binder-supported CaSO4 oxygen carrier was determined by intuitive analysis with the crushing strength and conversion at 30 min as target functions. According to the extreme difference in crushing strength (ZK ), the importance of factors can be ranked as acetic acid > the ratio of water > SB powder. According to the extreme difference in conversion (ZI ), the importance of factors can be ranked as SB powder > acetic acid > the ratio of water. Based on the maximum crushing strength and conversion, the optimum formulation was A3 B1 C1 (sample 8). The maximum crushing strength was 59.86 N, and the maximum conversion was 0.812. Fig. 3 shows the relationship between levels of factors and average values of crushing strength or conversion. The optimum formulation from Fig. 3 was A3 B1 C2 (sample 10), not A3 B1 C1 obtained from Table 3. Consequently, a verification experiment was conducted to check the result of optimal formulation of A3 B1 C2 . The experimental results are also summarized in Table 3. The optimal formulation resulted in the maximum crushing strength of 68.63 N and conversion of 0.828, which were higher than that of formulation of A3 B1 C1 . Therefore, the optimal extrusion condition was: 30 g CaSO4 , 12 g SB powder, 2.5 mL acetic acid, and 15 mL water.
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Fig. 3. Response curves of orthogonal design, SB powder (A), water (B) and acetic acid (C).
3.2. Reduction reactivity of the binder-supported CaSO4 oxygen carriers
carrier during reduction was investigated through the release of by-products CO and H2 .
3.2.1. Gas concentration profiles The product gas concentration profiles as a function of reaction time at 950 ◦ C for the reduction test are presented in Fig. 4. As shown in Fig. 4, the CO2 concentrations of samples 8 and 10 first increased to peaks at 15 min and 10 min, respectively, and decreased gradually. These variation curves are similar to the results in the literature where iron oxide served as an oxygen carrier [23]. As the reaction proceeded, the unreacted CH4 was found at about 4 min due to the short residence time and high concentration of CH4 . And its concentrations exceeded CO2 at 18 min for the first time. Generally, the formation of CO and H2 yields to the thermodynamic limitations of CaSO4 which is the case with the Nibased oxygen carrier. Also trace amounts of CO which was released along with CH4 showed a slow growth trend, while H2 was released at around 23 min. However, when most of CaSO4 was consumed, the significant carbon deposition was observed. The main reactions for carbon formation involve the methane decomposition (Eq. (8)), gasification with H2 O (Eq. (9)) and Boudouard reaction (Eq. (10)) [17,29]. In this study, the carbon deposition on CaSO4 oxygen
CH4 → C + 2H2
H298K = 73.8 kJ/mol
= 73.8 kJ/mol C + 2CO2 → 2CO H298K
C + H2 O → CO + H2
H298K = 172.1 kJ/mol
(8) (9) (10)
Interestingly, the concentrations of CO and H2 slightly decreased at the later period, the reason may come from the following aspects: On the one hand, the carbon formation on CaSO4 particles took up the most gas–solid reaction surface, which may inhibit the decomposition of CH4 (Eq. (8)). On the other hand, the decrease of CO2 and H2 O resulted in the decline of the reactions (Eqs. (9) and (10)). In comparison to sample 8, the amounts of CO and H2 of sample 10 were relatively low. In a word, CO2 was the main product gas during the earlier period, but the carbon deposition was serious during the later period. Therefore, the formation of CO, H2 and carbon could decrease with an excess oxygen carrier and short reduction time. Carbon deposition was not the key issue in this study. It is generally accepted that the carbon deposition does not appear to be a major problem if the oxidation reaction is carried out at a high enough temperature.
Fig. 4. Product gas concentrations and dry gas flow during reduction as a function of time. (a) Sample 8, and (b) sample 10.
N. Ding et al. / Chemical Engineering Journal 171 (2011) 1018–1026
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Fig. 5. (a) Oxygen carrier conversion (X) as a function of time, and (b) mass-based reaction rates (dω/dt) as a function of mass conversion (ω) for the reduction period.
The dry gas flow is also presented in Fig. 4. The gas flow was calculated relative to the gas analysis from the balance of nitrogen gas diluted in the outlet of reactor before the gas analyzers. The flow reduction was mainly a result of condensation of steam, and the variation of gas flow rate reflected the transient changes in gas concentration.
3.2.2. Oxygen carrier conversion and reaction rate The conversion of samples 8 and 10 during the reduction test is shown in Fig. 5(a). The decrease of conversion indicates the increase of extent of reduction and that more oxygen in the oxygen carrier was transferred to the fuel gas or sulfur release [19]. The conversion of samples 8 and 10 decreased to 0.258 and 0.189 after 30 min, respectively. And the oxygen carriers can be completely reduced after 60 min. Fig. 5(b) shows the massbased reaction rates of samples 8 and 10 as a function of the mass conversion (ω). The maximum change of mass conversion (ω) of metal oxide oxygen carriers is about 0.2, however, the mass conversion of CaSO4 decreased from 1.0 to 0.531 that was no better than theoretical value (0.529). The experimental results had proven the high oxygen transport capacity. At ω over 0.72, the mass-based reaction rate (dω/dt) of sample 10 was obviously greater than that of sample 8, which approached to half of dω/dt of the Fe2 O3 -based oxygen carrier in the literature [24]. But, comparison to natural anhydrite ore [20], the reactivity of the binder-supported CaSO4 had greatly improved for the addition of SB powder and acetic acid. So the binder-supported
Fig. 6. Outlet concentrations of SO2 and H2 S as a function of time during reduction.
CaSO4 oxygen carrier is promising for the practical application of CLC. 3.2.3. Sulfur release The sulfur release is a key problem in the CLC system using a CaSO4 oxygen carrier. As illustrated in many investigations [20,30–34], the emission of sulfur involves with CaSO4 and CaS is
Fig. 7. XRD of the binder-supported CaSO4 oxygen carriers. (a) Sample 6 and (b) sample 10.
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Fig. 8. FESEM micrographs of the fresh and reduced CaSO4 oxygen carriers. (a) Fresh sample 6, 1 m, (b) reduced sample 6, 1 m, (c) fresh sample 10, 1 m, (d) reduced sample 10, 1 m.
varied with the operating conditions. In this study, SO2 and H2 S formation are mainly due to the competing side reactions (Eqs. (11) and (12)) and the solid–solid reaction (Eq. (13)) during the reduction: 4CaSO4 +CH4 → 4CaO+CO2 +4SO2 +2H2 O H298K = 1220.8 kJ/mol (11) CaSO4 + CH4 → CaO + CO2 + H2 S + H2 O H298K = 221.1 kJ/mol
(12) 3CaSO4 + CaS → 4CaO + 4SO2
H298K = 1061.3 kJ/mol
(13)
The variation of SO2 and H2 S in the dry gas during reduction test is displayed in Fig. 6. The curves of SO2 and H2 S showed a single peak characteristic, and reached their peaks at 10 min and 15 min, respectively. Interestingly, the release peak time of SO2 and H2 S was consistent with that of CO2 . This indicates that the competing side reductions occurred at the early period and then the target reduction became dominant. On the whole, the maximum and total of H2 S was much lower than that of SO2 , because the solid–solid reaction may be enhanced owing to the formation of the outer product layer of CaS. However, sulfur loss rates of samples 8 and 10 were just 0.14% and 0.18%, respectively. The emission of sulfur in this study is only for the reduction of the fresh oxygen carriers, further work on formation of sulfur in oxidation process
and cyclic test in CLC system is needed. As suggested by Wang and Anthony [21], to obtain more product of CaS, a certain concentration of SO2 from fuel is forced into the reactors. In this experiment, SO2 was not fed because this study focused on the reactivity of the binder-supported CaSO4 oxygen carrier particles with CH4 . 3.3. Characterization analysis 3.3.1. Phase characterization Fig. 7 shows the XRD patterns for the reduced samples 6 and 10, and the number marked above the peak is the relative intensity. It is evident that CaS was the dominant reduced product at reducing condition. The CaSO4 diffraction peak was not found, which was in accordance with the results of conversion. Fig. 7 shows a new phase formation (CaAl2 O4 ) after reduction, which indicates that Al2 O3 reacted with product CaO. The CaAl2 O4 diffraction peaks of sample 10 were higher than those of sample 6, and a part of Al2 O3 did not react with CaO in the sample 6. That is also a reason that the crushing strength of sample 10 was greater than that of sample 6. And the existing of CaO would lead to decrease of the reaction rate. 3.3.2. Surface morphology The particles of samples 6 and 10 were examined using FESEM before and after reduction. Fig. 8(a) shows the surface of sample 6 was dense and impervious which was similar to analytical grade
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Fig. 9. BJH desorption pore volume distribution changes of fresh and reduced CaSO4 oxygen carriers.
CaSO4 . Interestingly, the plate-like particles were observed from the fresh sample 10 in Fig. 8(c). As shown in Fig. 8(b) and (d), the surfaces of the reduced CaSO4 oxygen carriers appeared much rougher and more porous. In particular, it was shown that samples provided a stable nanosized framework between the crystal grains, which might retard the sintering of active CaSO4 sites [35]. The material of framework may be the main alumina compound, which had excellent thermal stability and was also found in synthetic sorbents. Nevertheless, one difference between two samples was that the crystal grains size of sample 10 was much smaller than that of sample 6. Therefore, sample 10 may have higher surface area, which would be verified by pore structure analysis. 3.3.3. Pore structure analysis The BET surface area, average pore diameter and BJH pore volume distribution of fresh and reduced samples are illustrated in Table 6 and Fig. 9. The surface area of the fresh samples 6 and 10 was about 8 times higher than analytical grade CaSO4 (9.1667 m2 /g), which could be attributed to the addition of SB powder and acetic acid. A higher surface area of the binder-supported CaSO4 oxygen carrier could give rise to a higher reactivity. Possibly due to sintering and carbon deposition, the surface area of the reduced samples had Table 6 BET surface area and average pore diameter of fresh and reduced CaSO4 oxygen carriers. Sample
Fresh samples 2
Sample 6 Sample 10
Reduced samples
SBET (m /g)
Average pore diameter (nm)
SBET (m2 /g)
Average pore diameter (nm)
71.1776 78.8670
7.6005 8.1906
10.8930 15.1320
18.7727 20.7283
a pronounced decrease, while the average pore diameter increased. But the favorable porous structure of sample 10 was more beneficial to oxidizing reaction than sample 6, which can be obviously observed in Fig. 8(d). Further research is needed to clarify if there will be a relationship between mechanical strength and porosity. 4. Conclusions In this study, using Taguchi robust design method with L9 orthogonal array, the binder-supported CaSO4 oxygen carriers prepared from the calcined CaSO4 , SB powder, acetic acid and water were investigated by intuitive analysis with the crushing strength and conversion as target functions and the favorable oxygen carriers were comprehensively studied in a fixed bed reactor. The orthogonal experiment results showed the addition of SB powder enhanced both mechanical strength and conversion of CaSO4 , and the optimal extrusion condition was: 30 g CaSO4 , 12 g SB powder, 2.5 mL acetic acid, and 15 mL water. The results of reduction test showed CO2 were the main product gas during the earlier period. CaSO4 could be completely reduced which was proved by conversion and XRD analysis, and the release of H2 S and SO2 during the reduction was found to be responsible for the decrease of the reactivity of oxygen carriers. The mass-based reaction rates of the binder-supported CaSO4 were significantly enhanced, because the fresh samples had the higher surface area evidenced by pore structure analysis. Moreover, the favorable performance of the binder-supported CaSO4 was explained by formation of CaAl2 O4 which had excellent thermal stability and provided a stable nanosized framework between the crystal grains observed by FESEM. Based on the results, the optimal extrusion condition is recommended and the favorable binder-supported CaSO4 oxygen carrier
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