Article pubs.acs.org/EF
Cite This: Energy Fuels 2018, 32, 4538−4546
Effect of Ash Components on the Performance of Solid Oxide Electrolyte-Based Carbon Fuel Cells Kai Xu, Jizhou Dong, Hongyun Hu, Xianqing Zhu, and Hong Yao* State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, People’s Republic of China ABSTRACT: The effect of coal ash on the electrochemical performance of solid oxide electrolyte-based carbon fuel cells (SOCFCs) is investigated in this study. The polarization and durability performance of SO-CFCs fueled by lignite char, ash-free char, and ash-added char were measured and compared from 1023 to 1123 K. The influence of five different coal ashes and six typical inorganic species was evaluated in SO-CFCs during short-term operation. The anode morphologies were analyzed to clarify the cell degradation. The results indicate that lignite char shows a higher initial cell performance but a lower stability than ash-free char in SO-CFC. The higher initial cell performance is attributed to the superior CO2 gasification reactivity, while the lower stability is more likely caused by carbon and ash deposits. The effect of coal ash on the cell performance is strongly dependent upon the ash composition. Inorganic Ca, K, and Fe species in coal ash improve the initial cell performance by catalyzing char− CO2 gasification reactivity. Inorganic S and Fe species in coal ash tend to react with anode Ni particles and form large aggregations on the anode surface, leading to anode deactivation. In combination with inorganic Ca, Fe, and K, Si and Al compounds in ash could form irregular-shape deposits on the anode surface and clog up the anode pores. The ash with a high content of SiO2 and Al2O3 (93.39 wt % in total) shows no adverse effect on the cell performance. This study suggests that proper coal ash pretreatment should be conducted to improve the cell stability of coal-fueled SO-CFCs.
1. INTRODUCTION Coal is the second largest fossil resource after oil on the earth and plays a vital role in the world energy consumption. The coal consumption for China, the United States, and India, the top three coal-consuming countries, accounts for more than 70% of the world coal use. China’s share of world coal consumption is around 50% in 2016, and the vast coal reserves will ensure the dominance of coal in the coming decade.1 The current coal utilization technology (mainly coal-fired power generation) suffers from serious environment pollution and limited conversion efficiency. More efficient and cleaner coal utilization technologies are needed to mitigate potential environment and energy crises. A solid oxide electrolyte-based carbon fuel cell (SO-CFC) is an electrochemical technology, which can convert the chemical energy of solid carbon fuels into electricity directly in solid oxide fuel cells.2 The working principal is that oxygen (O2) obtains electrons and is reduced to oxygen ions (O2−) in the cathode via electrochemical reaction (reaction 1). O2− diffuses through the electrolyte to the anode. At the anode three-phase boundary, carbon reacts with O2− and releases electrons (reaction 2). Under an anode CO2-rich atmosphere, CO can be generated by C−CO2 gasification (reaction 3) and then electrooxidized to CO2 (reaction 4). The net reaction (reaction 5) for this cell, containing both the electrode reactions as well as C− CO2 gasification reaction, is the same as the carbon combustion reaction. SO-CFC allows for physical separation of the carbon stream completely from nitrogen as well as direct electrical output resulting from the electrochemical reactions. For the advantages of high efficiency, low emission, and low cost for CO2 sequestration, SO-CFC offers a promising alternative approach to generate electricity with direct utilization of coal,3,4 and hence, it was selected as one of the four advanced future © 2017 American Chemical Society
coal technologies in the 21st century by the International Energy Agency (IEA).5 cathode:
O2 + 4e− → 2O2 −
(1)
anode:
C + 2O2 − → CO2 + 4e−
(2)
anode:
C + CO2 → 2CO
(3)
anode:
CO + O2 − → CO2 + 2e−
(4)
C + O2 → CO2
(5)
net reaction:
In comparison to pure carbon materials, such as graphite and carbon black, coal is more complex and contains a considerable amount of organic and inorganic impurities, including sulfur, nitrogen, tars, halogens, and ash minerals. The role of these impurities is critical for the cell performance of coal-fueled SOCFCs.6,7 A clear understanding contributes to designing and developing low-cost fuel pre-processing techniques. On the basis of the proximate analysis, coal consists of moisture, volatile matter, fixed carbon, and ash. At the typical operation temperature of SO-CFCs (973−1123 K), raw air-dried coal will first experience thermal pyrolysis and release volatiles. It has been reported that the released volatiles participated in the anode electrochemical or chemical reactions and thus affected the cell performance.8−10 The influence of volatiles can be avoided by thermal treatment of raw coal before loading into Special Issue: 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 11, 2017 Revised: November 30, 2017 Published: December 4, 2017 4538
DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
Article
Energy & Fuels the anode. With the consumption of fixed carbon in coal char, the ash minerals will remain and accumulate on the anode surface. The effect of ash minerals on the cell performance is of great importance for direct coal utilization in SO-CFCs. Some investigations have been carried out to analyze the influence of the coal ash in SO-CFCs. For example, Ju et al.8 compared the electrochemical activity and durability performance of raw and ash-free coals in CFC. The polarization performance indicated that the existence of ashes did not compromise the electrocatalytic oxidation of coal fuels. While in the short term run test, the ash caused the poisoning of the anode catalyst. Jewulski et al.11 used lignite as fuel and compared the cell performance in the first and second batch. It was concluded that coal ash had a minor effect on the polarization performance but cut down the cell endurance in the short term run. Liu et al.12 investigated the cell polarization performance fueled by demineralized coal with varying ash contents (0, 3, 10, and 25%). It was suggested that the higher ash content in fuel contributed to lower cell performance. Rady et al.13 evaluated the electrochemical performance of SO-CFCs fueled by raw and acid-washed coal. Although they speculated that impurities, such as Si, Fe, and Mg, contributed to higher ohmic resistance, the superior performance was still achieved by the raw coal for the catalyzed Boudouard gasification by inherent inorganic species (Fe, Ca, and Mg). Besides, Fe, Ca, K, Mg, and Ni additives were widely reported to be good catalysts for carbon gasification and improve the cell performance significantly.14−16 The conclusions mentioned above provide some insights into the influence of coal ash on the performance of coal-fueled SOCFC. However, no systematic studies have been reported, especially during the short- or long-term durability operation. Also, it is still unknown how the ash species degraded the cell performance exactly. The aim of this study is to investigate the effect of coal ash species in Ni/YSZ anode SO-CFCs. The electrochemical performances of lignite char, ash-free char, and ash-added char were tested and compared at temperatures ranging from 1023 to 1123 K. Moreover, electrochemical behaviors of the lignite char loaded with six typical individual mineral species and five different coal ashes were evaluated. The scanning electron microscopy combined with energydispersive X-ray (SEM−EDX) analyses were conducted to examine the ash deposit on the anode surface.
instrument with a vertically movable furnace. A 100 mg sample (PCF or PCF−HF) was placed in a ceramic pan each time. When the furnace temperature was stable at 1123 K, it quickly moved upward until the ceramic pan was located in the constant temperature zone. In this case, the gasification reaction was considered to occur under isothermal conditions. The weight loss of the carbon fuels was recorded by TGA. By preparing mixed samples of 90 mg of PCF and 10 mg of mineral species (Fe2O3, CaO, MgO, or K2CO3), the catalytic effect of individual inorganic species on the char gasification was evaluated at the same conditions. Al2O3 and SiO2 were not investigated because they were widely acknowledged as inert material during coal−CO2 gasification. 2.2. Electrochemical Performance Testing Setup. The schematic of the SO-CFC system is shown in Figure 1. The anode-
Figure 1. Schematic diagram of the SO-CFC system. supported button SOFC is composed of four layers, namely, a Ni/ yttria-stabilized zirconia (Ni/YSZ) anode layer (400 μm), a YSZ electrolyte layer (15 μm), a GDC diffusion barrier layer (2−3 μm) and a LSCF/GDC cathode layer (25 μm). The diameters of the anode, electrolyte, and cathode are 20, 20, and 10 mm, respectively. The active reaction area (corresponding to the cathode area) is 0.785 cm2. The anode was adhered to one end of a ceramic tube by silver paste. Silver wires were connected to the electrodes and served as current collectors. The cathode was exposed to ambient static air. The ceramic tube was placed in a vertical tube furnace. Electrochemical performances, including polarization curve and durability performance, were measured by the electrochemical workstation. 2.3. Electrochemical Performance Testing Procedure. The experimental design can be divided into three parts according to the fuels feeding into the anode chamber. In the first part, the electrochemical performances of char (1.0 g of PCF), ash-free char (1.0 g of PCF−HF), and ash-added char (1.0 g of PCF with 0.1 g of ash, designated as PCF−ash) were investigated. The cell was heated from room temperature to 1023 K in 60 min, and then the anode was reduced by a 150 mL min−1 H2/Ar mixture (50.1 vol % H2) for 30 min. The anode was considered to be completely reduced once the open circuit voltage (OCV) reached approximately 1.0 V. The polarization curves were obtained by controlling the electrode potential automatically from OCV to 0 V at a scanning rate of 10 mV s−1. After the polarization test, the cell temperature was maintained at 1123 K and the short-term durability test was conducted by monitoring the current at a constant voltage load of 0.5 V. The second part was to evaluate the effect of different coal ashes on the cell performance of PCF-fueled cells. Four other coal ashes with different components were assessed in PCF-fueled SO-CFCs. A total of 0.1 g of coal ash was located between the anode surface and 1.0 g of PCF each time, providing more contact between the ash species and the anode surface. The anode was totally covered by coal ash, and PCF particles cannot contact the anode surface. The electrochemical performance was tested through the same procedure as described in the first part. In the third part, the effect of typical individual mineral species on the cell performance was examined. Six typical mineral species
2. EXPERIMENTAL SECTION 2.1. Preparation of Carbon Fuels and Ash Samples. A lignite char (PCF), which was derived from the pyrolysis of a raw lignite (74−106 um) at 1073 K for 3 h, was selected as the parent carbon sample. To remove the inorganic minerals in coal, PCF was pretreated with 4 N HF for 24 h and then 4 N HCl for 24 h. The acid-treated fuel was washed with distilled water and dried in air at 278 K for 24 h, designated as PCF−HF. Proximate and elemental analyses of the carbon fuels were conducted. X-ray photoelectric spectroscopy (XPS) analysis was conducted to determine the relative contents of C, O, and S on the particle surface of PCF and PCF−HF. X-ray diffraction (XRD) characterization was carried out to evaluate their crystalline nature using a Cu Kα X-ray source at a scanning rate of 2θ= 5−90° with a step size of 0.0131. The ashes of PCF and the other four lignite coals were prepared in a muffle furnace at 673 K and designated as ash, ash 2, ash 3, ash 4, and ash 5. The other four lignite coals were chosen according to their ash contents and components. These ash samples were analyzed via X-ray fluorescence (XRF) to determine elemental concentrations. The isothermal gasification experiments were carried out under CO2 flow (2.5 L min−1) in a thermogravimetric analysis (TGA) 4539
DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
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Energy & Fuels Table 1. Proximate and Ultimate Analyses of Lignite Char (PCF) and Ash-Free Char (PCF−HF) proximate analysis (wt %, ad)
a
ultimate analysis (wt %, daf)
material
moisture
ash
volatile matter
fixed carbon
C
H
Oa
N
S
PCF PCF−HF
3.82
26.18 2.64
2.63 8.45
71.19 85.09
66.40 80.62
1.32 1.68
30.07 16.04
0.56 0.67
1.01 0.99
By difference.
(K2CO3, Fe2O3, CaO, MgO, Al2O3, and SiO2) were selected to place between the anode surface and PCF layer each time. The weights of the mineral and PCF were 0.1 and 1.0 g, respectively. The electrochemical performance was tested at the same procedure as that in the first part. SEM−EDX analysis was used to investigate the morphology and elemental composition on the anode surface after the short-term durability test.
24° and 44°. The XRD analysis is in good agreement with the proximate analysis. The result of X-ray fluorescence analysis (XRF) of ashes is shown in Table 3. It can be seen that the main components of PCF ash are SiO2, Al2O3, Fe2O3, CaO, MgO, and SO3. Small amounts of K2O, MnO, P2O5, and TiO2 are also detected. Ash 2 mainly consists of SiO2 and Al2O3. Their total content is 93.39 wt %. Ash 3 contains considerable amounts of Fe2O3, MgO, CaO, and Na2O. For ash 4, 42.57 wt % CaO and 19.04 wt % Cl2O are detected. CaO and SO3 take account for 88.75 wt % in ash 5. The carbon conversion during CO2 gasification of PCF and PCF−HF is shown in Figure 3. Obviously, CO2 gasification reactivity of PCF−HF is lower than that of PCF, suggesting that the inherent mineral species in PCF have a catalytic effect on CO2 gasification of coal char. Besides, it can be seen that all four inorganic species (K2CO3, Fe2O3, CaO, and MgO) promote the CO2−char gasification rate. PCF mixed with K2CO3 shows much higher CO2 gasification reactivity than the others. The catalytic effect on CO2 gasification of PCF follows the order: K2CO3 > CaO > Fe2O3 > MgO. 3.2. Electrochemical Performance of Coal Char, AshFree Char, and Ash-Added Char. Figure 4 shows the polarization performance of SO-CFCs fueled by coal char (PCF), ash-free char (PCF−HF), and ash-added char (PCF− ash), respectively. With the cell temperature increasing from 1023 to 1123 K, the maximum current densities (MCDs) and OCVs of the three fuels all increase. The polarization curves of the three fuels are remarkably different, especially at 1123 K. In terms of MCDs, OCVs, and power density, PCF−ash shows the best polarization performance, while PCF−HF shows the worst. Figure 5a compares the maximum power densities (MPDs) of PCF, PCF−HF, and PCF−ash at 1023, 1073, and 1123 K. As seen, the MPDs of the three fuels are significantly improved from 1023 to 1123 K. At 1123 K, the MPDs of PCF, PCF−HF, and PCF−ash are 99.3, 82.7, and 109.0 mW cm−2, respectively. Accordingly, the MPDs of PCF are lower than that of PCF−ash but higher than that of PCF−HF, indicating that the ash species of PCF enhance the anode electrochemical reactions. It is widely acknowledged that the electrochemical oxidation of carbon is limited by the poor contact of the carbon particles with anode active sites.3,17,18 The cell performance is mainly governed by CO electro-oxidation combined with in situ C−CO2 gasification, especially in the anode-supported SOFCs.19−22 Although the distance between the PCF layer and anode surface in a PCF−ash-fueled cell is longer than that of a PCF-fueled cell, the electrochemical performance of PCF− ash is compensated by the ash-enhanced C−CO2 gasification. The polarization performance agrees well with the CO2 gasification reactivity of PCF and PCF−HF. Figure 5b shows the durability of the three samples at a constant load of 0.5 V at 1123 K. In the first 120 min of discharge, the current density experiences a rapid decline and then decreases generally at a relatively stable rate for the three samples. The durability curves of PCF and PCF−ash appear to
3. RESULTS AND DISCUSSION 3.1. Properties of Char Fuels. The proximate and elemental analyses of the parent lignite char (PCF) and the demineralized sample (PCF−HF) are shown in Table 1. The ash contents of PCF and PCF−HF are 26.18 and 2.64%, respectively, indicating that acid-washing pretreatment is an effective way to remove mineral matter in coal char. The volatile matter content of PCF is as low as 2.63%. After acidwashing pretreatment, it increases to 8.45% for PCF−HF. XPS analysis of PCF and PCF−HF is shown in Table 2. In Table 2. XPS Analysis (wt %) of Lignite Char (PCF) and Ash-Free Char (PCF−HF) element
C
O
N
S
PCF PCF−HF
79.91 94.81
19.68 4.15
0.11 0.21
0.30 0.83
comparison to PCF, PCF−HF has higher C and S contents and lower O content on the particle surface. The higher C content and lower O content are attributed to the removal of ash species, while the higher content of S suggests that acid treatment causes the transformation of S to the particle surface. The XRD patterns of the two samples are presented in Figure 2. PCF and PCF−HF both have a clear C(002) diffraction
Figure 2. XRD patterns of PCF and PCF−HF.
peak at around 2θ = 24° and another weak and broad C(100) diffraction peak at around 2θ = 44°, indicating that they contain a short-range, graphite-like structure. Obviously, the pattern of PCF is more complicated. The diffraction peaks corresponding to inorganic species, such as CaS and FeS, can be observed. For PCF−HF, the curve is smooth and no special peaks are observed, except for the diffraction peaks of graphite at 2θ = 4540
DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
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Energy & Fuels Table 3. Ash Composition Analysis (wt %) material ash ash ash ash ash a
2 3 4 5
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Cl2O
Na2O
TiO2
48.14 59.76 6.31 14.08 1.03
16.44 33.63 5.92 7.64 2.17
10.85 1.33 22.2 6.17 3.60
7.53 1.64 25.8 42.57 54.29
2.73 a 6.24 3.10 a
13.87 1.57 20.9 5.33 34.46
a a a 19.04 a
a a 6.28 a a
0.43 2.07 0.38 1.20 4.46
Not detected.
Figure 3. Char conversion curves under CO2 gasification: PCF, PCF− HF, and PCF with different inorganic additives.
overlap from 120 to 400 min. Apparently, the initial current densities of PCF and PCF−ash are higher than that of PCF− HF, which is due to the higher gasification rate of PCF and PCF−ash than PCF−HF. However, the current densities of PCF and PCF−ash degrade more rapidly than that of PCF− HF. After 300 min, PCF−HF generates a higher current density than PCF and PCF−ash, indicating that SO-CFC fueled by PCF−HF is more stable in the short term stability. Moreover, it can be observed that PCF−ash shows a slightly higher current degradation rate than PCF after 400 min. On the basis of the durability performance of the three samples, it is more likely to contribute to the rapid degradation of PCF to the ash species. To clarify this conjecture, the cell anode was analyzed by SEM−EDX. The anode surface morphologies are shown in Figure 6, and the EDX elemental information on the pointed areas in Figure 6 is shown in Table 4. Figure 6a shows the anode surface SEM image of PCF magnified 5000 times. Clearly, there are several spherical particles accumulated on the anode surface, which is detected by EDX to be S−Ni compounds. In the flat area (spectrum 1), a considerable amount of carbon is detected by the EDX analysis on the anode surface of the PCF-fueled cell, which can block the anode surface and increase the diffusion resistance of active gas species to the anode pores. No ash deposit was observed in the SEM image magnified 5000 times. However, in the view of 20 000 times (Figure 6b), some deposits with an irregular shape are clearly observed. The EDX data show that these deposits (spectra 2−4) contained considerable ash elements (Ca, Fe, Al, Si, O, Na, K, and Mg), which is strong evidence of ash poisoning on the anode surface. It can be deduced that the rapid degradation of PCF-fueled SO-CFC resulted from the ash deposit, carbon deposit, and S poisoning. Besides, the decrease of the char−CO2 gasification rate during the durability test could also cause the current degradation. For the PCF−HF-fueled cell (Figure 6c), only spherical particles but no irregular deposits can be observed on the anode surface. The EDX analysis shows that the spherical particles (spectra 5−7) mainly consist of Ni and S. Some spherical
Figure 4. Current density−voltage and current density−power density curves of (a) PCF, (b) PCF−HF, and (c) PCF−ash.
particles are much larger than that observed in PCF- and PCF− ash-fueled cells. In the flat area, no elemental carbon is detected by EDX (spectrum 8). From the amounts of spherical particles on the anode surface, it can be deduced that the cell fueled by PCF−HF suffers from more serious S poisoning than that of PCF, which is consistent with the XPS analysis (Table 2) of the two samples. For the PCF−ash-fueled cell (Figure 6d), more deposits are observed on the anode surface compared to that of the PCFfueled cell. Clearly, these deposits display different morphology features. The deposit with an irregular shape (spectrum 9) has high contents of Si (26.84%), O (67.09%), and minor contents of Al, Fe, and Ca. It can be deduced that this deposit is silicate 4541
DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
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Energy & Fuels
and contribute to poor discharge stability. As the SEM−EDX indicated, the cell fueled by PCF−HF suffers from more serious S poisoning than that of PCF and PCF−ash. It is no doubt that S poisoning would cause cell degradation of Ni/YSZ anode SOCFC. The higher degradation rate of PCF and PCF−ash suggests that ash and carbon deposits are more harmful to the cell stability than S poisoning during the short-term durability tests. 3.3. Electrochemical Performance of PCF Loaded with Different Coal Ashes. The other four coal ashes were also assessed in PCF-fueled SO-CFCs. Their electrochemical performances were measured and compared to that of PCF and PCF−ash. Figure 7a shows the MPDs derived from the polarization performances. As discussed in section 3.1, the MPDs are mainly determined by PCF gasification reactivity with CO2. The addition of PCF ash slightly improves the MPD because of the catalytic effect of Fe2O3 and CaO on gasification. As expected, the MPDs of PCF−ash 2 are similar to that of PCF because ash 2 mainly consists of SiO2 and Al2O3. PCF− ash 3 and PCF−ash 4 generate much higher MPDs than PCF, which resulted from the catalytic effect of Fe2O3, CaO, MgO, and Na2O on the C−CO2 gasification. Higher MPDs of PCF− ash 5 are attributed to the significant contents of CaO and Fe2O3 in ash 5 Figure 7b presents the durability curves of PCF loaded with the four ashes. The morphologies of the anode surfaces after durability tests are analyzed by SEM−EDX (as shown in Figure 8 and Table 5). It can be seen that the current density variation of PCF−ash 2 is almost the same as that of PCF, verifying that Si and Al have an insignificant effect on the cell performance, with minor contents of Fe and Ca in the ash. The current density of PCF−ash 3 experiences a rapid decrease from 240 to 280 min. The SEM image (Figure 8b) clearly shows that significant amounts of deposits are formed on the anode surface. EDX analysis reveals that these deposits are carbon
Figure 5. Comparison of (a) MPDs and (b) durability performance of PCF, PCF−HF, and PCF−ash.
compounds. Dudek et al.23 has reported that SiO2 may react with alkaline oxides in coal to form various kinds of glass and cause cell degradation. The deposit (spectrum 10) with a spherical shape is mainly the compounds of Ni (43.50%) and S (24.15%). The irregular-shaped deposits cover the anode pores and increase diffusion resistance of active gas species, while the spherical deposits deactivate the Ni particle and decrease anode reactive sites. Both of them cause the cell anode degradation
Figure 6. Anode surface images of (a) PCF magnified 5000 times, (b) PCF magnified 20 000 times, (c) PCF−HF magnified 5000 times, and (d) PCF−ash magnified 5000 times. 4542
DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
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Energy & Fuels Table 4. EDX Analysis (Atomic Percent) of the Designated Areas in Figure 6
a
atom
1
2
3
4
5
6
7
8
9
10
Ni Zr O C S Si Al Ca Fe Mg Na K
16.03 22.47 47.34 14.16 a a a a a a a a
6.12 16.38 37.91 32.67 a 1.80 2.47 2.30 0.16 a a 0.19
5.11 9.61 15.14 69.72 a a 0.24 a 0.18 a a a
6.56 8.68 36.85 39.74 a 2.84 2.16 2.02 0.18 0.39 0.44 0.14
36.54 11.37 32.24 a 19.75 a a a a a a a
54.37 3.94 20.24 a 21.46 a a a a a a a
60.94
22.2 24.44 53.36 a a a a a a a a a
0.93 3.25 67.09 a a 26.84 1.07 0.35 0.31 a a 0.16
43.50 3.15 24.69 a 24.15 3.13 1.03 a 0.35 a a a
8.75 a 30.31 a a a a a a a
Not detected.
with the degradation rates of the corresponding durability curves. These ash deposits display different morphology features, including spherical-shaped deposits, irregular-shaped deposits, and cluster deposits. They all deactivate the anode and result in cell degradation. 3.4. Electrochemical Performance of PCF Mixed with Individual Mineral Species. To examine the interaction between individual specific mineral species and the anode, 0.1 g of mineral species was placed between PCF (1.0 g) and the anode surface. The polarization performance was tested from 1023 to 1123 K, and the MPDs were calculated, as shown in Figure 9a. It can be seen that PCF−K2CO3 displayed much higher MPDs than the others. The MPDs of PCF−CaO and PCF−FeO are slightly better than that of PCF. While for PCF−MgO, PCF−Al2O3, and PCF−SiO2, the MPDs are quite close to that of PCF. Their polarization performances are consistent with the corresponding PCF−ash species gasification reactivity in Figure 3b. Ash species, such as K2CO3, CaO, and Fe2O3, improve the cell polarization performance by catalyzing C−CO2 gasification. Besides, it is also demonstrated that the anode reactions are CO electro-oxidation and C−CO 2 gasification because the direct electro-oxidation of carbon cannot occur as a result of no direct contact of PCF particles and the anode surface. After the polarization test, the shortterm stability test was conducted at a constant voltage load of 0.5 V. Figure 9b shows the durability tests of PCF in 330 min with individual inorganic species covering the anode surface. It can be clearly seen that PCF−K2CO3 produces a much higher initial current density. However, the current density declines rapidly to zero in less than 90 min. CaO and Fe2O3 additions in the PCF-fueled cell slightly promote the current density. The current density of PCF−Fe2O3 experiences accelerated degradation after 240 min. For PCF with MgO, Al2O3, and SiO2 additions, the discharge curves are similar to that of PCF and have almost the same degradation rate during the 330 min discharge. The anode morphologies and elemental characteristics after short-term tests were analyzed by SEM−EDX. For PCF−CaO-, PCF−MgO-, PCF−Al2O3-, and PCF−SiO2-fueled cells, the anode pores can be clearly observed and no special element (excluding O, C, Ni, and Zr) is detected, suggesting that these inorganic species (CaO, MgO, Al2O3, and SiO2) do not react with the anode material or deposit on the anode surface individually. The anode SEM−EDX analysis of PCF−K2CO3 and PCF−Fe2O3 is shown in Figure 10 and Table 6. The anode surface of PCF−K2CO3 (Figure 10a) is extremely different
Figure 7. Comparison of (a) MPDs and (b) durability performance of PCF with different ashes.
deposits and compounds of S and Fe species combined with Ni (spectra 1 and 2). For PCF−ash 4, the current density is much higher than that of other ashes but drops sharply in less than 120 min. The high initial current density is attributed to the catalytic effect of CaO on C−CO2 gasification. The sharp drop may be caused by the large aggregations, as shown in Figure 8c. The large aggregations contain ∼90% Ni and ∼10% Fe (spectra 3 and 4) and have a size of ∼5 μm, resulting in clogging up and reduction of triple-phase boundaries. PCF−ash 5 displays a similar current density with PCF in 0−270 min. After that, the degradation rate is accelerated but no rapid degradation is observed. The SEM image (Figure 8d) shows that some cluster deposits are formed on the anode surface. These deposits are consisted elements of S, Fe, Ca, Zr, and Ni (spectra 5 and 6). Besides, it can also be found that the amounts of deposits on the anode surfaces follow the order of PCF−ash 4 > PCF−ash 3 > PCF−ash 5 > PCF−ash 2, which is in good accordance 4543
DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
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Energy & Fuels
Figure 8. Anode surface images of (a) PCF−ash 2, (b) PCF−ash 3, (c) PCF−ash 4, and (d) PCF−ash 5.
Table 5. EDX Analysis (Atomic Percent) of the Designated Areas in Figure 8
a
atom
1
2
3
4
5
6
Ni Zr O C S Fe Ca
18.97 a 36.59 38.64 3.90 1.73 0.17
26.25 1.78 34.45 28.47 7.83 1.22 a
91.72 0.22 a a a 8.06 a
88.87 0.19 a a a 10.94 a
42.15 a 52.73 a 4.21 0.33 0.58
23.71 10.39 60.04 a 3.35 1.67 0.84
Not detected.
from that of the others. No micropores can be observed, but many aggregations are formed covering the anode surface. The EDX analysis (spectrum 1) shows that considerable amounts of Ni (9.92%) and K (11.65%) are detected on the anode surface. The absence of elemental Zr on the anode surface indicates that K2CO3 may react with Ni rather than ZrO2 and produce the aggregations. For PCF−Fe2O3 (Figure 10b), some spherical deposits are observed on the anode surface and detected by EDX to contain 72.32% Ni and 1.4% Fe (spectrum 3), which may be the cause of accelerated degradation of the durability curve after 240 min. In briefly, K2CO3 causes rapid degradation by destroying the anode structure. Fe2O3 addition gives rise to aggregation of Ni particles and deactivates the anode slightly. CaO improves the cell performance by catalyzing C−CO2 gasification. MgO, Al2O3, and SiO2 display insignificant adverse impact on the cell performance.
Figure 9. Comparison of (a) MPDs and (b) durability performance of PCF with different inorganic species additions.
catalytic effect of inherent inorganic species, and the lower stability is caused by ash and carbon deposits. It is found that different ash components result in deposits with different morphologies on the anode surface. The large spherical deposits are the aggregations of anode Ni bonded with Fe or S, while the deposits with an irregular shape are the silicate compounds containing considerable amounts of Si, Al, O, Fe, and Ca. The cluster deposits resulted from S, Fe, and Ca ash species. By loading individual specific mineral species between
4. CONCLUSION The effect of coal ash on the performance of lignite char-fueled SO-CFCs was investigated in this study. The results shows that the lignite char (PCF) displays higher initial electrochemical performance but more rapid cell degradation than the ash-free char (PCF−HF). The higher initial performance of PCF is due to the superior CO2 gasification reactivity, resulting from the 4544
DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
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ACKNOWLEDGMENTS
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REFERENCES
The authors thank the financial support from the International Science and Technology Cooperation Program of China (2015DFA60410) and the National Natural Science Foundation of China (51476065).
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Figure 10. Anode surface images of (a) PCF−K2CO3 and (b) PCF− Fe2O3 magnified 5000 times.
Table 6. EDX Analysis (Atomic Percent) of the Designated Areas in Figure 10
a
atom
1
2
3
O Ni Zr C Fe K S
65.10 9.92 a 10.73 2.48 11.65 0.12
41.79 15.53 19.22 23.02 0.43 a a
17.98 72.32 8.29 a 1.40 a a
Not detected.
PCF and the anode surface, it is suggested that CaO, Fe2O3, and K2CO3 promote the anode reactions and improve the initial cell performance by catalyzing char−CO2 gasification. However, K2CO3 causes severe anode corrosion, and Fe2O3 gives rise to the aggregation of anode Ni particles, both leading to cell degradation. SiO2, Al2O3, and MgO individually have an insignificant effect on the anode performance, neither reacting with the anode material nor depositing on the anode surface. To avoid anode degradation of coal char-fueled SO-CFCs, proper fuel pretreatment is needed to remove sulfur and specific ash impurities.
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AUTHOR INFORMATION
Corresponding Author
*Telephone/Fax: +86-27-87545526. E-mail:
[email protected]. cn. ORCID
Hong Yao: 0000-0002-2836-7803 Notes
The authors declare no competing financial interest. 4545
DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
Article
Energy & Fuels (22) Xu, K.; Hu, H.; Li, Z.; Zhu, X.; Liu, H.; Luo, G.; Li, X.; Yao, H. Investigation of the anode reactions in solid oxide electrolyte based carbon fuel cells. Int. J. Hydrogen Energy 2017, 42 (15), 10264−10274. (23) Dudek, M.; Skrzypkiewicz, M.; Moskała, N.; Grzywacz, P.; Sitarz, M.; Lubarska-Radziejewska, I. The impact of physicochemical properties of coal on direct carbon solid oxide fuel cells. Int. J. Hydrogen Energy 2016, 41 (41), 18872−18883.
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DOI: 10.1021/acs.energyfuels.7b03068 Energy Fuels 2018, 32, 4538−4546
