Synergetic Catalysis of Calcium Oxide and Iron in Hydrogasification of

Dec 2, 2016 - Coal/char catalytic hydrogasification (CCHG) is a direct method to produce CH4 (synthetic natural gas, SNG). CaO has been studied as a p...
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Synergetic Catalysis of Calcium Oxide and Iron in Hydrogasification of Char Juantao Jiang, Zhenyu Liu, and Qingya Liu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China S Supporting Information *

ABSTRACT: Coal/char catalytic hydrogasification (CCHG) is a direct method to produce CH4 (synthetic natural gas, SNG). CaO has been studied as a promoter of transition metal compound catalysts for this process for decades. Our earlier work indicated that CaO alone has a high catalytic activity in CCHG as long as the temperature is higher than 750 °C and the Ca loading is higher than 0.42 mmol/(g of char). Our recent work and literature review indicate that the high activity of CaO may be related to the iron in ash of the char used, and the threshold loading of CaO may be attributed to the presence of sulfur in the char. This work aims to address these issues particularly on synergetic catalysis of Fe−CaO and transformation of Fe−CaO−S during the hydrogasification. All the hydrogasification experiments were performed in a fixed-bed reactor at 800 °C and under a H2 pressure of 1.5 MPa. Results indicate that both CaO alone and metallic Fe alone have little catalytic activity, but interaction of them yields a high activity. In this sense, CaO is not a catalyst promoter reported in the literature but a crucial catalytic component. The amount of Fe to fully excite the catalysis of 0.710 mmol of CaO is around 0.170 mmol, while the amount of CaO to fully excite the catalysis of 0.084 mmol of Fe is around 0.310 mmol. To yield a high catalytic activity, the optimum CaO/ Fe molar ratio is around 4.0. The synergetic catalysis of Fe−CaO transforms from the CaO-dominate sites to Fe-dominate sites during hydrogasification, possibly due to differences in diffusion of Fe and CaO.

1. INTRODUCTION Coal/char catalytic hydrogasification (CCHG) for synthetic natural gas (SNG) production is a viable way to alleviate the natural gas shortage in some countries. The catalysts commonly studied include alkali metal compounds such as K- and Nacompounds and transition metal compounds such as Ni-, Co-, and Fe-compounds.1−4 These catalysts need to be recycled because of their high price. However, the alkali metal compounds tend to interact with the ash components in coal/char to form water-insoluble compounds,5−7 and the transition metal compounds are prone to sulfur poisoning and sintering,8−10 which increase their recycle cost and hinder their application in practice. Calcium-based catalysts attracted the researchers’ attention because they are low in cost, abundant as a large industrial waste in calcium carbide chemical process, and environmentally benign, and therefore do not require recovery. Our recent experiments on catalytic hydrogasification of an acid-washed coal char indicated that CaO alone is much more active than the catalysts reported in the literature as long as the temperature is higher than 750 °C and the Ca loading is higher than 0.42 mmol/g.11 However, most of the literature reported that Ca-compounds had little catalytic activity in CCHG and at the best were promoters of transition metal compounds to increase their dispersion on the coal/char surface or to prevent them from sintering and poisoning.4,12−14 A further study, therefore, is needed to disclose the reason for the different observations. It is well-recognized that coal-derived char is complex in minerals’ composition and distribution, as well as in sulfur form. The acid treatment methods adopted commonly are not able to © XXXX American Chemical Society

remove all the minerals due to differences in leach ability of different minerals and confinement of minerals by carbon structure. The organic sulfur in coal is also resistant to the acid treatments. These are evidenced by the char used in our earlier study, which contains approximately 3.15 wt % ash and 0.37 wt % sulfur.11 It is therefore curious to know whether the residual ash and sulfur in char played a significant role in the catalytic activity of CaO. The main ash in coal-derived char includes quartz (SiO2), mullite (3Al2O3·2SiO2), magnetite (Fe3O4), hematite (Fe2O3), and Mg-compounds. Reaction of SiO2 and CaO occurs at about 1300 °C,15,16 and SiO2 is commonly recognized to be inert to the char hydrogasification. Effects of Mg- and Al-compounds on char hydrogasification cannot be found in the literature and are likely to be very low if there are any. Consequently the most possible compounds that would influence the catalytic activity of CaO are Fe-compounds and organic sulfur. Although CaO may react with Fe2O3 to yield CaFe2O3 (ferrite), the ferrite may be reduced to CaO and metallic Fe by H2 during hydrogasification.17 Therefore, the form of iron in the CCHG process may be metallic Fe. As mentioned above, some literature had reported a promoting effect of CaO on catalytic activity of metallic Fe and the catalytic activity of metallic Fe alone.13,18,19 Haga and Nishiyama13,19 reported that metallic Fe had little activity during hydrogasification of pitch coke, but Suzuki et al.,17 Ohtsuka et al.,8 Cyprès et al.,20 and Hüttinger and Krauss21 showed that metallic Fe was active. Received: August 12, 2016 Revised: November 6, 2016 Published: December 2, 2016 A

DOI: 10.1021/acs.energyfuels.6b02026 Energy Fuels XXXX, XXX, XXX−XXX

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2.3. Characterization of Char. The metal contents of ash in the chars were determined by dissolving the ground ash in a mixed acid of HCl, HF, and HNO3 and then analyzing the solution by inductively coupled plasma optical emission spectroscopy (ICP-OES; iCAP 6000 SERIES, Thermo Scientific, Waltham, MA, USA). Two replicates were analyzed for each sample. X-ray diffraction (XRD) was carried out on a Rigaku D/Max 2500 unit using Cu Kα1 (λ = 0.15406 nm) radiation operated at 40 kV and 200 mA. The samples were scanned over a 2θ range of 10−90° at a step of 0.02° and a retention time of 1 s for each step.

Based on the above analysis, the aim of this work is to identify the catalytic activities of CaO alone, Fe alone, and their mixture, as well as interaction of sulfur with CaO during hydrogasification of coal-derived char. Two char samples were prepared from the same coal but different batches of somewhat different ash contents to identify the effect of minerals on the catalytic activity of CaO. The purpose is to maintain the carbon structure of the char samples as similar as possible to minimize the effect of different carbon structure on the catalysis evaluation.

3. RESULTS AND DISCUSSION 3.1. Catalytic Hydrogasification and Properties of Char-1 and Char-2. Figure 1 shows vCH4 (a) and YCH4 (b)

2. EXPERIMENTAL SECTION 2.1. Material Preparation. The coal used was the same as that reported earlier,11 Yulin coal of China. Two batches of the same coal of somewhat different ash contents were subjected to the same treatments to obtain two chars. Both of the coal samples were sieved to 20−80 mesh in size and then treated by a HF + HCl solution in order to eliminate most of the minerals. The acid washing started with mixing 30 g of coal with 305 mL of HF (2.4 mol/L) and 255 mL of HCl (1.1 mol/L) at 60 °C for 24 h under stirring, followed by filtrating and washing the coal with deionized water to neutral, and with drying of the coal at 110 °C for 8 h in an air-blast drier. The acid treated coal was then pyrolyzed in a vertical quartz tube under a flow of N2 at 900 °C for 3 h. The two char samples were marked as char-1 and char-2. Pore properties analysis indicates that these chars are similar in the pore size distribution (see Figure S1 in the Supporting Information). The chars were sieved to 40−80 mesh for catalyst loading. A char of 0.5 g was mixed with Ca(OH)2, FeS2 (pyrite), CaS, or their mixture in a Wig-L-Bug for 0.5 min and then mixed with deionized water at a ratio of 0.75 g/(g of char), except the sample with CaS. The wet mixture was kept at ambient temperature for 5 h and then dried at 110 °C for 8 h in an air-blast drier. Metallic Fe was loaded onto the chars by pore volume impregnation with an aqueous Fe(NO3)3·9H2O solution at ambient temperature for 12 h, which was followed by drying at 50 °C for 5 h and 110 °C for 8 h in an air-blast drier and then reduction in H2 flow at 350 °C for 1 h as reported in the literature.18,19 The char loaded with both metallic Fe and Ca(OH)2 was prepared by mechanically mixing Fe/char with Ca(OH)2. Since Ca(OH)2 decomposes to CaO at temperature lower than that of the hydrogasification experiment, these samples are termed as CaO-xM/ char-1 or CaO-xM/char-2, where M is FeS2, CaS, or Fe, and x denotes the molar loading of M on the basis of char mass. The loading of Ca(OH)2 is 0.710 mmol/g unless otherwise specified. The properties of the chemicals used are shown in Table S1 in the Supporting Information. 2.2. Hydrogasification Experiment. The hydrogasification experiments were performed in a pressurized fixed-bed reactor (10 mm in diameter and 1000 mm in length) described in detail in our previous work.11 The char sample was loaded in the reactor by sandwiching it with quartz wool and then quartz sand. The reactor was purged with N2 at a flow of 50 mL/min to remove air and then pressurized to 3.0 MPa before being heated to 800 °C at a heating rate of 15 °C/min. At 800 °C, a flow of H2 (99.999%) at a rate of 50 mL/ min was introduced into the reactor to start the hydrogasification reaction. The composition of effluent gas was analyzed online with a gas chromatograph (GC, Agilent 7890B with a TCD) equipped with a 5A molecular sieve column coupled with a PQ column. The carrier gas was He, and the sampling interval was 5.5 min. The CH4 production rate (vCH4, mL/min) was quantified with N2 in the effluent as the internal standard. The CH4 yield (YCH4) was determined by eq 1, where m and wC are the weight of char and the weight percentage of carbon in the char on an air-dry basis, respectively.

Figure 1. Catalytic activity of CaO during hydrogasification of char-1 and char-2: (a) vCH4; (b) YCH4. Reaction conditions: T = 800 °C, Ptotal = 3.0 MPa, PH2 = 1.5 MPa, and Ca loading on chars = 0.710 mmol/g if used.

curves of char-1 and char-2 with and without CaO addition. It can be seen that the two chars present the same low vCH4 in the absence of CaO. Addition of CaO to the chars significantly increases their vCH4, but the vCH4 of CaO/char-1 is obviously higher than that of CaO/char-2. At t of 451 min, the YCH4 of CaO/char-1 and CaO/char-2 are 62.5% and 33.8%, respectively. Since these two chars were prepared from the same coal and were similar in carbon structure, the higher CaO activity in

t

YCH4 /% =

12∫ vCH4 dt 0

22.4 × 103mwC

× 100

(1) B

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of the Charsa proximate analysis (wt %, ad)

a

ultimate analysis (wt %, daf)

sample

M

A

VM

FC

C

H

Ob

N

St

Sorg

char-1 char-2

1.89 1.64

3.15 1.02

1.56 1.54

93.40 95.80

97.54 97.40

0.59 0.61

0.30 0.47

1.20 1.33

0.37 0.19

0.33 0.19

Key: ad, air-dry basis; daf, dry-ash-free basis; M, moisture; A, ash; VM, volatile matter; FC, fixed carbon. bBy difference.

char-1 than that in char-2 may be attributed to different interactions of ash and/or sulfur with CaO. Table 1 shows the results of proximate and ultimate analyses of char-1 and char-2. It is clear that the largest differences lie in ash and sulfur contents: char-1 contains 3.15 wt % ash and 0.37 wt % sulfur, while char-2 contains 1.02 wt % ash and 0.19 wt % sulfur. Analysis of the metal compositions of ash in chars indicates that char-1 contains more Si, Mg, Ca, and Fe than char-2 (see Table 2). As analyzed in the Introduction, the difference in iron content may be responsible for the different CaO activities in hydrogasification of char-1 and char-2. Table 2. Elemental Content of the Ashes in Char-1 and Char-2 elemental content (mg/(g of char), ad) sample

Na

Mg

Al

Si

K

Ca

Fe

char-1 char-2

0.16 0.16

0.22 0.13

0.66 0.63

19.14 14.71

0.03 0.03

3.69 1.24

4.07 1.33

Figure 2. Effect of FeS2 on the catalytic activity of CaO under the reaction conditions of Figure 1.

of CaO or a synergetic effect between FeS2 and CaO. It is surprising that the vCH4 decreases to the level of char-2 with increasing the FeS2 loading to 0.347 mmol/g (CaO-0.347FeS2/ char-2), indicating disappearance of the catalytic activity of CaO at the high FeS2 loading. To understand the effect of FeS2 loading on the activity of CaO in CCHG, transformation of Fe−Ca−S−O during the char hydrogasification are preliminarily analyzed by referencing the literature. It is well-recognized that the Fe-compounds commonly seen in coal/char can be readily reduced to metallic Fe in the presence of H2 at 800 °C.22,23,28 In addition to the transformation of FeS2 to Fe, CaO may transform to CaS via interaction with FeS2 or H2S produced from the reduction of FeS2 by H2. These behaviors are confirmed by the XRD analysis of hydrogasification residues shown in Figure 3. In the figure the diffraction peaks of CaF2 (2θ = 47.00°, 28.27°, and 55.76°) can be discerned in all the samples, which is attributed to the reaction of Ca-compounds in coal with HF during the acid treatment as reported in the literature.29,30 Diffraction peaks of Ca(OH)2 (2θ = 34.09°, 18.09°, and 47.12°) are observed in some samples due to the deliquescence of CaO during the grinding process. More importantly, it can be seen in the figure that the residue of CaO-0.084FeS2/char-2 shows obvious diffraction peaks of CaO (2θ = 37.35°, 53.85°, and 32.20°), CaS (2θ = 31.41°, 45.00°, and 55.88°), and α-Fe (2θ = 44.67°, 65.02°, and 82.33°), which is consistent with the references discussed above. With increasing FeS2 loading, the diffraction intensities of CaO decrease and those of CaS and Fe increase. At the FeS2 loading of 0.347 mmol/g (CaO-0.347FeS2/char-2), the diffraction peaks of CaO disappear and those of CaS increase to the maximum, suggesting complete conversion of CaO to CaS. This is consistent with the fact that the molar ratio of Ca/S in CaO-0.347FeS2/char-2 approximates to 1 (CaO = 0.710 mmol/g and S = 0.694 mmol/g). Diffraction peaks of other Fe-compounds except the metallic Fe cannot be

To identify the form of iron in the chars, the form of sulfur was analyzed. Results indicate that the organic sulfur constitutes 89.2% and 100% of total sulfur in char-1 and char-2, respectively, suggesting that the form of iron may not be Fe1−xS. However, the sulfur in the parent coals are mainly pyritic sulfur, 0.35 wt % (87.5% of total sulfur) for coal-1 and 0.24 wt % (75.0% of total sulfur) for coal-2. It is also interesting to note that the molar ratio of S/Fe in the chars is about 1.5− 2.5 (the calculation details are specified in the Supporting Information), around the ratio in FeS2. These suggest that the organic sulfur in the chars resulted possibly from transformation of pyritic sulfur in the parent coal during char preparation as commonly reported in coal pyrolysis studies.22−25 Transformation of sulfur from FeS2 to organic matrix would result in the appearance of elemental Fe in chars, similar to the reduction of Fe-compounds by C reported in the literature.26,27 The above analysis indicates transformation of iron and sulfur during char preparation. The threshold loading of CaO reported in our previous work11 suggests that CaO may also take part in the transformation during hydrogasification and interaction of CaO−Fe−S is far more complex than that in char preparation. To evaluate this interaction, Fe and S were introduced into the CaO/char in the form of FeS2. 3.2. Effect of FeS2 on the Catalytic Activity of CaO. Figure 2 shows the effect of FeS2 loading on the catalytic activity of CaO during hydrogasification of char-2, along with the vCH4 curves of char-2, CaO/char-2 and 0.170FeS2/char-2 for comparison. It can be seen that the vCH4 curve of 0.170FeS2/ char-2 overlaps with that of char-2, suggesting little catalytic activity of FeS2 itself. However, the vCH4 curves of CaO-xFeS2/ char-2 with FeS2 loading of 0.084−0.258 mmol/g are much higher than those of 0.170FeS2/char-2 and CaO/char-2, suggesting a promoting effect of FeS2 on the catalytic activity C

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It is noted that vCH4 of CaO-0.170Fe/char-2 in Figure 4 is far higher than those of the samples loaded with FeS2 and CaO in Figure 2. Since Fe itself has little catalytic activity, the high activity of CaO-0.170Fe/char-2 may be attributed either to CaO or to the combined effect of Fe−CaO. To differentiate the catalytic activity of CaO from Fe−CaO, an ash-free walnut shell char with very low Fe content (approximately 0.294 mg/g31,32) was prepared at 900 °C and loaded with CaO for hydrogasification; the results in Figure 5 indicate that CaO itself has little catalytic activity in char hydrogasification.

Figure 3. XRD analyses to the hydrogasification residues of CaOxFeS2/char-2.

discerned in all the samples, confirming the complete reduction of Fe−S compounds during the hydrogasification. Since the actual forms of iron and calcium in CaO0.347FeS2/char-2 are Fe and CaS, the low vCH4 of this sample suggests that Fe, CaS, and their mixture are not catalytically active in the char hydrogasification. To confirm this deduction, the catalytic activities of individual CaS and metallic Fe were tested at a CaS loading of 0.340 mmol/g and a Fe loading of 0.170 mmol/g (equivalent to 0.170 mmol/g FeS2). The results in Figure 4 indicate that both of them have little catalytic

Figure 5. Catalytic activity of CaO in the hydrogasification of walnut shell char under the reaction conditions of Figure 1.

The discussion so far confirms that neither metallic Fe nor CaO has catalytic activity for char hydrogasification at 800 °C and, therefore, the high activity of CaO-0.170Fe/char-2 in Figure 4 can only be attributed to the coexistence of Fe and CaO. In this sense, CaO should not be considered as a promoter of Fe but as a catalytic component, and similarly Fe should not be considered as a promoter of CaO but as a catalytic component too. Therefore, the catalytic activity of Fe reported in the literature in CCHG8,17,20,21 is possibly under the influence of CaO presented in the ash of the chars. 3.3. Effects of Fe Dispersion and Loading on the Catalytic Activity of Fe−CaO. It is noted that all the vCH4 curves of CaO-xFeS2/char-2 in Figure 2 are lower than that of CaO/char-1 in Figure 1a, although the Fe contents, including the inherent Fe in char, of CaO-xFeS2/char-2 (0.108−0.282 mmol/g) are higher than that of CaO/char-1 (0.073 mmol/g). Similarly, the vCH4 of CaO-0.170FeS2/char-2 in Figure 2 is much lower than that of CaO-0.170Fe/char-2 in Figure 4. These results suggest that other parameters besides Fe loading influence the catalytic activity of Fe−CaO, such as the dispersion/distribution and chemical form of Fe. Since the metallic Fe in CaO-0.170FeS 2/char-2 was obtained by reduction of mechanically mixed FeS2, most of the Fe distributed initially in the outer surface of the char. In contrast, the metallic Fe in CaO-0.170Fe/char-2 was obtained by pore volume impregnation with a Fe(NO3)3·9H2O solution followed by reduction with H2 at 350 °C, most of the Fe distributed initially inside the pores of the char. This difference in Fe loading method may result in different Fe distribution in the chars at hydrogasification conditions. However, the SEM-EDX mapping of the chars subjected to the hydrogasification shows

Figure 4. Effects of CaS and Fe on the catalytic activity of CaO under the reaction conditions of Figure 1.

activity (0.340CaS/char-2 and 0.170Fe/char-2). Clearly the inactivity of Fe in the char hydrogasification is adverse to the report in many literature studies.8,17,20,21 A possible explanation of the inactivity of Fe may be the reaction of Fe with silica to form iron silicates, but this may not occur under the conditions of this work because Suzuki et al. had proved that iron silicates are reduced to metallic Fe by H2 at 800 °C.17 Therefore, it is certain that metallic Fe should have little catalytic activity in char hydrogasification. D

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Energy & Fuels little difference in Fe distribution (Figure S2 in Supporting Information). XPS analysis indicates that the iron cannot be detected for the hydrogasification residue but can be detected for 0.170Fe/char-2 without being subjected to the hydrogasification (Figure S3 in Supporting Information), suggesting diffusion of Fe into the interior of char during the hydrogasification since the detection depth of XPS is less than 10 nm.17 If the metallic Fe does not diffuse into the char, they should enrich on the surface other than disappear from the surface with continuous consumption of carbon during the hydrogasification. The decreasing bulk density of the char during the hydrogasification reported in our previous work11 further supports the statement “diffusion of Fe into the bulk of char during hydrogasification”. Since the Fe-impregnated char yields a high vCH4, it was further used to study the effect of Fe loading in the range of 0.042−0.347 mmol/g, the same as those for FeS2 in Figure 2. It can be seen in Figure 6 that vCH4 increases with an increase in

Figure 7. Effect of CaO loading on the catalytic activity of Fe−CaO during char hydrogasification under the reaction conditions of Figure 1.

0.100 mmol/g, while peak-2 except that of 0.100CaO-0.084Fe/ char-2 changes little with varying CaO loading. To clearly identify the relations between the catalytic activity of Fe−CaO and the loadings of Fe and CaO, the vCH4 curve of char-2 is subtracted from the vCH4 curves in Figures 6 and 7, and the resulting curves are deconvoluted into three peaks. The vCH4 curves obtained in such a way are shown in Figures S5 and S6, and deconvolution results are listed in Tables S3 and S4 (Supporting Information, where VCH4 is the area of each peak in milliliters). It can be seen that peak-1 centers around 60 min and peak-2 centers around 90−100 min, while peak 3 centers around 170−180 min. The VCH4 of each peak vs the Fe loading are shown in Figure 8a. It indicates that VCH4 of peak-1 increases obviously with increasing Fe loading from 0.042 to 0.170 mmol/g to a steady value of approximately 88 mL. This suggests that the amount of Fe to fully excite the catalysis of 0.710 mmol of CaO is about 0.170 mmol. The VCH4 of peak-2 increases and that of peak-3 decreases with increasing Fe loading to steady values of 175 and 275 mL, respectively, also at the Fe loading of 0.170 mmol/g. The plots of VCH4 of each peak vs CaO loading are shown in Figure 8b. It indicates that the VCH4 of peak-1 is very low at CaO loadings of 0.100 and 0.164 mmol/g but sharply increases to a high and steady level of 83 mL at CaO loading of 0.310 mmol/g. The VCH4 of peak-2 and peak-3 are very low at a CaO loading of 0.100 mmol/g but reach high and constant values of 150 and 290 mL, respectively, at a CaO loading of 0.310 mmol/g. This suggests that the amount of CaO to fully excite the catalysis of 0.084 mmol of Fe is 0.310 mmol. The data presented so far indicate that to obtain a high synergetic catalysis of Fe−CaO, the loading of Fe is best to be higher than 0.170 mmol/g and the loading of CaO is best to be higher than 0.310 mmol/g. It is interesting to note that the CaO/Fe molar ratios that yield the best catalytic activity in the two cases are around 4, specifically 4.2 for Figure 8a and 3.7 for Figure 8b. The above discussion indicates that the active catalyst composition of Fe−CaO varies from CaO-dominate (peak-1) to Fe-dominate (peak-2) during the course of hydrogasification. This evolution suggests gradual change of local composition of

Figure 6. Effect of Fe loading on the catalytic activity of Fe−CaO during char hydrogasification under the reaction conditions of Figure 1. The CaO loading was 0.710 mmol/g.

Fe loading in the presence of CaO and the catalytic activity of Fe−CaO is remarkable at metallic Fe loadings of 0.084−0.347 mmol/g with the maximum vCH4 of 5.1 mL/min. The corresponding YCH4 is also high, reaching 77.9% at 451 min at the Fe loadings of 0.258 and 0.347 mmol/g (Figure S4 in the Supporting Information). Detailed observation in Figure 6 indicates that each of the vCH4 curves contains three peaks from 30 min to the end. The first peak (peak-1) centers around 50−65 min and the second peak (peak-2) centers around 75−90 min, while the third peak (peak-3) centers at time greater than 150 min. With an increase in Fe loading, peak-1 of all the curves, except that of CaO0.042Fe/char-2, changes little but peak-2 increases slightly. This phenomenon seems to suggest that CaO mainly influences the first peak with the help of Fe while the metallic Fe mainly influences the second peak with the help of CaO. To verify this deduction, the effect of CaO loading was also investigated, where the Fe loading was kept at 0.084 mmol/g. The results in Figure 7 again show that each of the vCH4 curves contains three peaks. Peak-1 decreases gradually with decreasing CaO loading, from 0.710 mmol/g to 0.164 mmol/g and even disappears at E

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(1) Neither Fe nor CaO itself has a catalytic activity at 800 °C. The synergetic interaction of Fe and CaO warrants a high catalytic activity. The sulfur in char or catalysts may convert CaO to CaS, and CaS or Fe−CaS has little catalytic activity. The inconsistent results on the catalytic activity of Fe in the literature may result from the presence or absence of CaO in the ash of char. (2) The dispersity of Fe significantly influences the synergetic catalysis: the Fe formed from reduction of mechanically mixed FeS2 is much less active than that of impregnated Fe(NO3)3. In the latter, the Fe amount to fully excite the catalysis of 0.710 mmol of CaO is 0.170 mmol while the CaO amount to fully excite the catalysis of 0.084 mmol of Fe is 0.310 mmol. The Fe−CaO catalysts with the best activity have a CaO/Fe molar ratio of around 4. (3) The synergetic catalysis of Fe−CaO transforms from the CaO-dominate sites to the Fe-dominate sites during the hydrogasification possibly due to the different diffusion behaviors of Fe and CaO, but this needs to be further investigated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02026. Characterizations of pore properties, SEM-EDX mapping and XPS, YCH4, calculation detail of the molar ratios of S/ Fe in chars, properties of chemicals, and curve deconvolution process and results (PDF)

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Figure 8. Relations between the VCH4 of different peaks and the loadings of Fe (a) and CaO (b).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-10-64421077. Fax: +86-10-64421077. ORCID

Fe−CaO due to their diffusion. The local composition possibly transforms from CaO centered sites surrounded by scattered Fe to Fe centered sites surrounded by scattered CaO. Diffusion of CaO has been proven in our previous work,11 but the transformation of active centers needs to be further investigated. Tracing back to our previous results regarding char-1, CaO showed little catalytic activity at the loading of 0.210 mmol/g but a high and similar catalytic activity at the loading of 0.420 mmol/g and higher.11 The content of inherent Fe in char-1 is about 0.073 mmol/g. The content of inherent S is about 0.109 mmol/g, which consumes the same amount of CaO to form CaS. Therefore, the CaO contents that were active for the hydrogasification were about 0.101 and 0.311 mmol/g when the apparent CaO loadings were 0.210 and 0.420 mmol/g, respectively. The former is insufficient to fully excite the Fe while the latter is sufficient. This explains the sudden occurrence of catalytic hydrogasification with increasing CaO loading from 0.210 to 0.420 mmol/g. Importantly, the latter case again indicates that the CaO/Fe molar ratio for the best catalytic activity is around 4, specifically 4.2.

Zhenyu Liu: 0000-0002-3525-273X Qingya Liu: 0000-0003-0354-9026 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Key Research and Development Program of China (Grant No. 2016YFB0600300).

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REFERENCES

(1) Skodras, G.; Panagiotidou, S.; Kokorotsikos, P.; Serafidou, M. Potassium catalyzed hydrogasification of low-rank coal for synthetic natural gas production. Open Chem. 2016, 14 (1), 92−109. (2) Sheth, A. C.; Sastry, C.; Yeboah, Y. D.; Xu, Y.; Agarwal, P. Catalytic gasification of coal using eutectic salts: reaction kinetics for hydrogasification using binary and ternary eutectic catalysts. Fuel 2004, 83 (4), 557−572. (3) Murakami, K.; Arai, M.; Shirai, M. Hydrogasification of Loy Yang brown coal by ion-exchanged nickel species. Energy Fuels 2000, 14 (6), 1240−1244. (4) Yuan, S.; Qu, X.; Zhang, R.; Bi, J. Effect of calcium additive on product yields in hydrogasification of nickel-loaded Chinese subbituminous coal. Fuel 2015, 147, 133−140. (5) Gallagher, J.; Euker, C. Catalytic coal gasification for SNG manufacture. Int. J. Energy Res. 1980, 4 (2), 137−147.

4. CONCLUSION This work advances the understanding on catalytic activity of CaO and Fe in hydrogasification of char to produce CH4. The main conclusions are as follows: F

DOI: 10.1021/acs.energyfuels.6b02026 Energy Fuels XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.energyfuels.6b02026 Energy Fuels XXXX, XXX, XXX−XXX