Article pubs.acs.org/EF
Experimental Investigation on the Multi-cycle Performance of Coal/ Straw Chemical Loop Combustion with α‑Fe2O3 as the Oxygen Carrier Cuiping Wang,*,† Hairong Cui,† Haisheng Di,† Qingjie Guo,‡ and Fei Huang§ †
Electrical and Mechanical Engineering College, College of Software Technology, Qingdao University, Qingdao, Shandong 266071, People’s Republic of China ‡ Chemical Engineering College, Qingdao University of Science and Technology, Qingdao, Shandong 266070, People’s Republic of China § SINOPEC Safety Engineering Institute, Qingdao, Shandong 266070, People’s Republic of China ABSTRACT: On the basis of the thermogravimetric analysis and scanning electron microscopy techniques, the multi-cycle chemical loop combustion (CLC) of blending fuels of coal/straw was investigated through serious experiments with α-Fe2O3 as the oxygen carrier. The results show that the α-Fe2O3 reduction reactivity is improved by blending straw into coal for more alkaline metal ash product and less ash quantity accumulation. The blended straw improves the CLC persistence, including the conversion rates of the sample combustion reaction and oxygen carrier regeneration reaction degrees, as compared to the coal fuel. It is of great significance to improve the sustainable performance of the α-Fe2O3 oxygen carrier by blending straw into coal. have been concerned with this,6,7 few conclusions were drawn in detail. Metal oxides of Fe, Ni, Co, Cu, Mn, and Cd have been discussed as the OC in the literature for CLC of gaseous fuels, such as natural gas and CH4.8,9 The function of OCs is to transfer the oxygen from its matrix to the fuel but also to withstand the repeated cyclic looping operation and the interaction with coal; therefore, the characteristics of high mechanical strength and low agglomeration are needed for OCs.10,11 α-Fe2O3 is of economical strength and environmental benignity. The performance of α-Fe2O3 as the OC in CLC of the blends of coal and straw will be investigated in detail.
1. INTRODUCTION The large consumption of fossil fuels, especially coal, has been recognized as one of the main anthropogenic sources of CO2 emission into the atmosphere, which exerts great concerns, such as a detrimental greenhouse effect and global warming. The chemical looping combustion (CLC) process is suggested among the best alternatives to reduce the economic cost of CO2 capture and is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO2.1,2 CLC has been widely studied, and some important progress has been made for combustion of natural gases as fuel and metallic oxides as the oxygen carrier (OC).3 Ferric oxide is a cheap and high-performance OC.4 Not so many studies have been conducted directly using solid fuels, such as coal and biomass,4−6 for the difficulties to increase the solid−solid reaction rate between the OC and solid fuel. Coal and biomass fuels are more abundant and cheap and geographically well spread all over the world in comparison to natural gas and oil. Therefore, it is important to develop and improve CLC technologies for easier and more efficient capture of CO2 from solid fuel combustion. As the co-firing, the combustion might be improved by the biomass blending with coal. The CLC of coal by blending biomass might have improvement in the reaction rate, which will be tested in this paper. For the continuous operation of solid fuel CLC, there was a key cause to influence the reaction rate, which was the ash induced by fuel. It was difficult to separate the ash clearly from the circulating materials6 for the similar density or diameter; therefore, the ash accumulation and crush occurred during the CLC cycles until the fine ash flew with the flue gas into another electrostatic precipitator. The accumulated ash had sophisticated influence on the OC performance; although some studies © 2014 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Coal and Straw. The mixed coal powder was collected from the Qingdao Heat and Power Plant and of bituminous coal properties. The straw was selected from rural north China, Shandong province. The coal powder was about 70 μm in diameter, and the straw samples were pulverized and sieved to 100−150 μm size. Then, the samples were dried in an oven for 5 h at 105 °C and used in the following tests. The proximate and ultimate analyses of dried coal and straw were presented in Table 1 on an air-dried basis (ad) and an as-received basis (ar), respectively. The properties of ashes of coal, straw, and their blending fuels after fired in air were investigated in detail and presented in a prior work.12 2.2. OC. The OC used in this study was analytically pure α-Fe2O3 powder. The average size of the OC particle was 75 μm. Before the test, the α-Fe2O3 powder was heated at 500 °C in a muffle furnace for 2 h to remove most of the crystallization water. The scanning electron microscopy (SEM) image of fresh α-Fe2O3 particles is shown in Figure 1. Received: February 11, 2014 Revised: May 6, 2014 Published: May 6, 2014 4162
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Table 1. Proximate and Ultimate Analyses of Coal and Straw proximate analysis (wt %) straw coal
ultimate analysis (wt %)
Mad
Vad
Aad
FCad
Car
Har
Oar
Nar
Sar
4.44 1.74
72.86 28.11
6.62 22.62
16.08 47.53
38.6 54.6
3.6 2.9
50 16.3
1.4 1.6
0.4 0.6
Table 2. Mass Content of Coal, Straw, and Fe2O3 in the Blending Fuel Sample sample sample sample sample
coal (mg)
straw (mg)
Fe2O3 (mg)
15 9 6 0
0 6 9 15
600 576 505 504
characterization, and the BET specific surface area was measured with the gas analyzer of Tristar 3000S. The CLC characteristics of the OC (α-Fe2O3) with coal/straw were investigated using TGA. A pure N2 atmosphere was used during the fuel oxidation stage. The gas flow rate was 50 mL/min. To keep the instrument stable, the sample was first kept in the crucible for 5 min at ambient temperature and a N2 atmosphere. Second, the temperature in each cycle was heated from ambient to 900 °C at 30 K/min rate and kept for 30 min at the final temperature to ensure the sufficient conversion of the solid fuel and OC. Following the fuel oxidation stage, air was introduced to TGA in place of N2. The reduction OC was regenerated at 900 °C for 10 min. At last, the heating process was over, and the crucible was cooled to ambient temperature. A CLC cycle was finished. Five-cycle TGA tests were conducted in succession to evaluate the reduction and reoxidation performances of the OC under the condition of ash accumulation. Ash accumulated in the TGA crucibles cycle by cycle. When a cycle was finished and the TGA crucibles were cooled to ambient temperature, the OC/ash was mixed with the same amount of fuel used in the previous cycles and then the next cycle started.
Figure 1. SEM image of fresh Fe2O3 powders. As seen from Figure 1, the Fe2O3 particles were small and very loose, with a lot of pores and a coarse surface. The Brunauer− Emmett−Teller (BET) specific surface area of the fresh Fe2O3 powders was about 4.9729 m2/g. 2.3. Blending Ratios of OC/Fuel in Each Sample. The conversion rate of coal during CLC was determined by the availability of the oxygen transfer performance of the OC. Although the coal was pyrolyzed and the volatile gas (CO, H2, or H2S) reacted with the OC first, the pyrolysis was strengthened by the oxidation of the volatile gas and then the OC could completely react with gas and fixed carbon components. The supplied OC amount had a key influence on the operation of the CLC system economically. According to the coal/ straw mass balance during the combustion reaction, the theoretical amount of the α-Fe2O3 OC was calculated. From the proximate and ultimate analysis results, the content of different atoms (including C, H, O, N, and S) contained in the coal or straw samples was used to determine the relative chemical formula as in eqs 1 and 2. Equation 1 was the reduced reaction between coal and the α-Fe2O3 OC, and eq 2 was the reduced reaction between straw and the Fe2O3 OC.
3. RESULT AND DISCUSSION The Fe2O3 OC performance was discussed from the microstructure change, performance persistence, and reaction rate change, with the cycles increasing. 3.1. Microstructure Analysis. The morphology of reaction residues was characterized using SEM. The results are shown in panels a−d of Figure 2 for each fuel sample. From the white and wee ash powders adhered on the OC particles and the cluster particle size, the ash sintering degree was clearer from sample 1, sample 2, sample 3, to sample 4, in turn. The image of Figure 2a indicated that the particle after five cycles (fuel combustion and OC oxidation regeneration) seemed that most agglomeration occurred in comparison to the fresh OC particle (Figure 1), the OC particle was less porous, the particles were not loose as before, and the particle specific surface area was decreased to 0.83 m2/g. As shown in Figure 2b, after the reaction of sample 2, the particles were more porous than the particle in Figure 2a but with bigger cluster powders on particles. Its specific surface area was 0.86 m2/g. For the reaction of sample 3 in Figure 2c, the fiber shape ash particle was clearer. In Figure 2d of the reaction of sample 4, the α-Fe2O3 particle was more porous, with a BET specific surface area of 1.43 m2/g. From Figures 1 and 2, the solid residue of the sample with a high proportion of coal was badly agglomerated but with a high proportion of straw being less agglomerated. The agglomer-
C4.55H 2.9O2.456N0.1875S0.01875 + 24.4Fe2O3 = 16.26Fe3O4 + 4.55CO2 + 1.95H 2O + 0.0928N2 + 0.01875SO2
1 2 3 4
(1)
C3.38H 7.6O3.125N0.1S0.0125 + 22.38Fe2O3 = 14.92Fe3O4 + 3.38CO2 + 3.8H 2O + 0.05N2 + 0.0125SO2 (2) From eqs 1 and 2, the theoretical stoichiometric Fe2O3 OC ratios needed for the full conversion of coal and straw were 24.4:1 and 22.4:1. For the fuel of different blending ratios of coal/straw, the OC amount has to be weighted as calculated according to the two ratios. The applied mass content in blends of coal with straw and the OC was presented in Table 2. All four blending fuels were of the same weight and then mixed with the designed little redundant OC (stoichiometric coefficient is 1.0). The ash produced in the thermogravimetric analysis (TGA) crucible was reserved to mix into the fuel for the next cycle. 2.4. Experimental Procedure. The CLC experiments were carried out in a thermogravimetric analyzer (NETZSCH STA 449C) apparatus, whose resolution was 0.1 μg and whose crucible was large, of 12 mL, to reduce the error from the blending. The S3400 scanning electron microscope was used for the particle microstructure 4163
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Figure 2. SEM images of (a) sample 1, (b) sample 2, (c) sample 3, and (d) sample 4.
Figure 3. Five fuel oxidization cycles of four samples in a N2 environment.
before the redox test. Besides the releasing of little left crystallization water in the fresh fuel and OC, some volatile gas revealing or incomplete oxidation with the OC caused more weight loss, as seen in Figure 4 during the first cycle, and the releasing of carbon monoxide, vapor, and carbon dioxide was all more than that in the next cycles. X showed a decreasing trend in consecutive cycles. The OC particles gave off oxygen more easily for combustion at the first and second cycles because of the fresh porous particle structure. However, in the next cycles, the continuous ash accumulation in the TGA crucible might restrict the contact between fresh coal and the reoxidized metal oxide OC. There was no influence from crystallization water in the OC but little crystallization water in fresh fuel. X in the next cycles were all less than 1. From the third to the fifth cycle, about 5% reduction occurred for the combustion percentage compared to its prior cycle.
ations were the sintering and cluster ashes on particles to cause the specific surface area to decrease. 3.2. Sustainable Performance Analysis of the Fe2O3 OC. 3.2.1. Analysis of the Combustion Percentage. The definition of the combustion percentage was as follows in eq 3: combustion percentage (%) = X actual weight loss of the sample in TGA = × 100% theoretical weight loss of the sample
(3)
Five cycles of the TGA redox test of four sample combustion in N2 and OC oxidation in an air environment were carried out in TGA, and the weight loss process in the combustion stage was plotted in Figure 3. The combustion percent (X) was at a maximum at the first cycle, with the weight loss even to 150% to the theoretical loss. The reason could be generated that part of crystallization water in the fuel and OC did not separate out 4164
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OC, it was useful to reduce the ash interference to the OC to a certain extent by blending straw into coal in the CLC system. 3.2.2. Analysis of the Oxidation Process. The following eq 4 was used here to define the percentage of OC oxidation regeneration. percent of OC regeneration (%) = Y actual weight gain of the sample = × 100% theoretical weight gain of the sample
(4)
Figure 4 showed the maximum OC oxidation regeneration reactivity of each sample from cycle 2 to cycle 5 during OC regeneration (oxidation reaction). The OC regeneration percentage in each cycle was confirmatory-affected by the ash content in four fuels. The OC blended with coal was regenerated only 37.5% of its original OC ability before cycle 2; therefore, the fuel combustion in cycle 2 was deficient of the oxygen supply. In addition, because the OC regenerated about 0.375−0.5 of the stoichiometric coefficient in cycle 2 in Figure 4, the fuel oxidization in cycle 3 was about 60% in Figure 3, more than the regenerated OC portion. The excessive weight loss reaction could be deduced from the vapor, and some volatile gas (SO2 and CO) released without the reaction with the OC at a lower temperature period, as shown in Figure 5. Only when the temperature in TGA was higher, the volatile and pyrolyzed gases could react with the OC and release much CO2; therefore, the OC did not reduce or accordingly regenerate to the theoretical value. In late cycles, the fuel combustion weight loss rate X was about 60% and changed little, while the Y decreased steadily. For example, Y of sample 4 from 48% of cycle 2 decreased to 41.5% of cycle 5. Therefore, the OC reduction degree X and OC oxidization degree Y in successive cycles were consistent. As for the OC regeneration, it showed an increasing trend from sample 1 to sample 4 in each cycle in Figure 4. The regenerative reactivity between reduction of the OC and oxygen was improved by blending straw into coal. 3.3. Analysis of the Reaction Rate Change. Figure 6 showed dX/dt curves of samples 1−4 during five cycles. dX/dt curves showed that the reaction rate changed with the cycle development. There were five peaks in the first cycle in whole, while there were four peaks from cycle 2 to cycle 5. The third peak (the corresponding time is 26 min, and the corresponding temperature is 650 °C) of the first cycle represented the influence of crystallization water, as mentioned before. For the second cycle, as an example, the first peak represented the
Figure 4. Maximum OC oxidation reactivity of each sample from cycles 2 to 5 during OC oxidation.
Figure 5. Gas emissions of sample 2 firing in cycle 1 by Fourier transform infrared spectroscopy (FTIR).
In Figure 3, the combustion percent of sample 1 to sample 4 in each same cycle showed an increasing trend. Sample 4 showed the maximum X for pure straw, and the ash content of proximate analysis in Table 1 was the least. The pre-study works showed that blends with coal and straw were able to inhibit ash production with lower coal ratios to straw and increased ash quantity with higher coal blending ratios during straw/coal co-combustion.12 For more alkaline ash composition product from straw and less ash quantity accumulation on the
Figure 6. dX/dt curves of samples 1−4 during five cycles. 4165
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(9) Saha, C.; Bhattacharya, S. Comparison of CuO and NiO as oxygen carrier in chemical looping combustion of a Victorian brown coal. Hydrogen Energy 2011, 36, 12048−12057. (10) Aurora, R.; Kunlei, L.; Jim, N.; Darrell, T. Oxygen carriers for chemical looping combustion of solid fuels. Fuel 2009, 88, 876−884. (11) Ranjani, S.; Hanjing, T.; George, R.; Simonyi, T.; James, P. Chemical-looping combustion of coal with metal oxide oxgen carriers. Energy Fuels 2009, 23, 3885−3892. (12) Wang, C.; Wu, Y.; Liu, Q.; Yang, H. Study of the characteristics of the ashing process of straw/coal combustion. Fuel 2011, 90, 2939− 2944.
precipitation of water in new coal and straw, the second peak indicated the precipitation of volatile gas and the reaction between volatile gas and the OC, the third peak showed the reaction between coke and the OC, and the fourth peak represented the reoxidation of the OC. From the reoxidation process of the OC, the reaction rate showed a decreasing trend from cycle 1 to cycle 5 in each sample while an increasing trend from sample 1 to sample 4 in the same cycle and sample 4 showed the maximum reaction rate in each cycle compared to the other samples. Therefore, the reaction rate was improved by blending straw into coal.
4. CONCLUSION The performance of multi-cycle CLC of blends of coal and straw with α-Fe2O3 as the OC was assessed through five alternating cycles of reduction and oxidation processes in a N2 environment using TGA. The primary conclusions were concluded in this paper: (1) The α-Fe2O3 OC is less agglomerated with ash particles when blending straw into coal compared to the pure coal in CLC. (2) With the ash accumulating cycle by cycle, the ash influences clearly on the reaction between the OC and solid fuel (coal and straw) and the reaction between the reduced OC and oxygen. The combustion percentage and percentage of OC regeneration are improved by blending straw compared to coal, while the reaction rate of OC regeneration is improved too. (3) With the straw blending ratio increasing, the ash suppression on the CLC rate is to be weakened.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financed by the National High-Tech Research and Development Program of China (863 Program, Grant 2008AA05Z311) and the Natural Science Foundation of Shandong Province (ZR2010EM004).
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REFERENCES
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