Na

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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Steam Gasification of Low-Rank Coal with a Nanoscale Ca/Na Composite Catalyst Prepared by Ion Exchange Naoto Tsubouchi,*,† Yuuki Mochizuki,† Yuji Shinohara,† Yuu Hanaoka,‡ Takemitsu Kikuchi,‡ and Yasuo Ohtsuka‡ †

Center for Advanced Research of Energy and Materials, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577, Japan

‡

ABSTRACT: Ca2+ and Na+ cations were separately or consecutively ion-exchanged with an Indonesian low-rank coal using saturated aqueous solutions of Ca(OH)2 and/or soda ash at 20−40 °C without pH adjustment. This was done by adding aqueous solutions of the soda ash and/or calcium hydroxide to a dispersion of coal particles in deionized water, while monitoring the Na+ and Ca2+ concentrations and pH of the resulting mixture. A 3.5 mass % Ca2+-exchanged coal, a 1.6 mass % Na+exchanged coal and a 3.8 mass % Ca2+- and 1.0 mass % Na+-exchanged coal were obtained. The catalytic pyrolysis and subsequent gasification of these ion-exchanged specimens were performed in a fixed bed quartz reactor at 650, 700, or 750 °C. Xray diffraction analyses of the samples pyrolyzed at 700 °C found no evidence of Na or Ca species, indicating that these were present as nanoscale particles. The Ca2+/Na+-exchanged coal underwent steam gasification more readily than the other samples, with complete char conversion of this material following 1 h of processing at 700 °C. The use of a binary metal catalyst was also found to lower the required reaction temperature by more than 100 °C.

1. INTRODUCTION The improvement of coal utilization efficiency is one means of reducing CO2 emissions. Consequently, various techniques for this purpose have been developed, including supercritical thermal power generation, the integrated gasification combined cycle (IGCC), and polygeneration. The utilization of low-rank coal, a potentially plentiful resource, is also important to establishing a stable future supply. Despite this, at present, there is little economic incentive to burn low-rank coal because of the associated production costs. For this reason, the Japanese Strategic Technical Platform for Clean Coal Technology project (STEP CCT) includes the goal of achieving low-rankcoal gasification at or below 900 °C by using catalysts. To date, there have been a number of studies regarding the application of Ni or K carbonate catalysts to coal gasification.1−3 Tomita et al. studied Ni-supported Yallourn brown coal catalysts in conjunction with a fluidized-bed reactor at 600 °C. These catalysts exhibited high conversion ratios of at least 80 wt %.4 It has become apparent that the K carbonate catalysts have superior catalytic properties, and Exxon has developed a pilotscale coal gasification process using these materials.5,6 This system is based on a fluidized bed gasifier operating at 3 MPa and 700 °C, using steam or H2 for fluidization. The process, however, has been found to generate a methane-rich product, and to date, a commercially viable gasification catalyst has not been developed. Na and Ca composite catalysts were studied and have shown catalytic activity. McCormick et al. have shown that Illinois No. 6 coal with Ca or Na + Ca catalysts using low pH solution by impregnation have synergistic effects.7 Haga et al. found that some composite catalysts impregnated with Na + Ca, Na + Fe, Ca + Fe, and Na + Ca + Fe had high catalytic activity.8 However, these experiments adopted the catalytic preparation by an impregnation method and using pH-adjusted solution for © XXXX American Chemical Society

a long stretch of time. Therefore, due to the amounts of Na and Ca in these catalysts researchers were unable to distinguish a physical adsorption from a chemical adsorption. In this report, the Na+ and Ca2+ concentrations as well as the pH were monitored using continuous observation apparatus such as Na+selective, Ca2+-selective, and pH electrodes, respectively, and without pH-adjusted solution. In addition, the ion balances were determined based on analyses of the unreacted solutions and the wash solutions, using an ion electrode or inductively coupled plasma atomic emission spectroscopy (ICP-AES). Therefore, precise chemical adsorption amounts of Na+ and Ca2+ were determined. In the present work, the catalytic steam gasification of ionexchanged coals was reexamined. In a prior study by our group,9 a 2.9 mass % Na+-exchanged low-rank coal (Loy Yang coal) was completely gasified following a 1 h treatment at 700 °C, while subbituminous Adaro coal exhibited minimal gasification. The purpose of the research presented herein was therefore to assess the possibility of accelerating the gasification rate of Adaro coal by using various additives.

2. EXPERIMENTAL SECTION 2.1. Coal Samples. In the present work, an Indonesian subbituminous coal (Adaro coal), denoted AD, was used. The coal sample was air-dried at room temperature, crushed, and then sieved to obtain particles in the size range of 75−150 μm. Table 1 provides the results from the analysis of the coal, and shows that the sample had very low sulfur and ash levels. The concentrations of various metals in the coal are summarized in Table 2; these data indicate that Si, Al, Fe, and Ca were present at the highest concentrations. Received: October 3, 2017 Revised: December 5, 2017 Published: December 7, 2017 A

DOI: 10.1021/acs.energyfuels.7b02964 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

could not be precisely determined using a Ca ion conductivity meter because many Na+ and Ca2+ cations were already present in the solution to which the soda ash and calcium hydroxide were added. Therefore, the Ca2+ concentration following the ion-exchange reaction was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The dried samples prepared as above were used for subsequent gasification experiments. 2.3. Pyrolysis and Steam Gasification. Pyrolysis and subsequent gasification were conducted using a fixed bed quartz reactor at a constant temperature of either 650, 700, or 750 °C under ambient pressure. Approximately 500 mg of the test sample was heated at a rate of 300 °C/min to the desired temperature in a stream of high-purity He, then held at this temperature for 10 min to remove volatile matter, and finally exposed to 50 vol % H2O in He to gasify the resulting char in situ. Pyrolysis-only trials were also conducted in the same manner, using He as an inert atmosphere, to determine the mass losses associated with this process. Details of the apparatus and procedure have been described in previous reports. The extent of char conversion during steam gasification was estimated on a dry, ash-free, catalyst-free basis (dacf) from the change in sample weight before and after the reaction. The gases produced during this process were analyzed after the removal of H2O using two online micro gas chromatographs (GCs) with thermal conductivity detectors to determine the concentrations of H2, CO, CO2, and CH4. 2.4. Characterization. Na+ and Ca2+ ions were extracted from dried ion-exchanged coal samples with an aqueous solution of ammonium acetate,10,11 and the concentrations of these ions in the extraction phase were determined by ICP-AES. The CO2 evolution from the raw coal was determined using a GC in conjunction with temperature-programmed desorption (TPD), heating the sample at 3 °C/min in a stream of high-purity He, as in a previously published report.12 The concentration of free COOH groups in the sample was subsequently calculated by assuming that all the CO2 released by the coal originated from these groups, including free carboxyl groups, excepting groups in the forms of Na, K, and Ca salts. X-ray diffraction (XRD) analyses of selected char samples were also performed.

Table 1. Proximate and Elemental Analyses of the Adaro Coal Sample proximate analysis (mass %, dry) b

ash

VM

1.9

48.3

FC

ultimate analysis (mass %, dafa) b,c

49.8

C

H

N

S

Oc

70.2

5.2

0.64

0.13

23.8

daf = dry ash free. bVM = volatile matter; FC = fixed carbon. c Determined by difference. a

Table 2. Metal Concentrations in the Adaro Coal Sample metal

concna (mass %, dry)

Na Al Si K Ca Fe

0.007 0.17 0.50 0.015 0.17 0.22

a

Determined by ICP-AES after acid leaching of the ashes obtained by combustion at 815 °C.

2.2. Catalyst Materials and the Ion-Exchange Methodology. Calcium hydroxide (99.5% purity, Wako Pure Chemical Industries, Ltd.) and soda ash (Soda Ash Japan Co., Ltd., Green River, WY, USA) were used as precursors for the ion-exchanged Ca and Na catalysis. The particle sizes of soda ash were found to be in the range 150−650 μm, and both were used as received without any pretreatment. The soda ash contained greater than 99 mass % Na2CO3 and was nearly free of Cl. An aqueous solution of either the soda ash or the calcium hydroxide was added to a dispersion of coal particles in deionized water held at a constant temperature in the range 20−40 °C without the addition of any pH-adjusting reagents. The resulting mixture was stirred and the Na+ and Ca2+ concentrations as well as the pH were monitored using Na+-selective, Ca2+-selective, and pH electrodes, respectively. The initial cation concentrations were 2530 mg of Na+/L and 649 mg of Ca2+/L, unless otherwise stated. The amounts of Na+ and Ca2+ ions actually exchanged were calculated based on measured changes in concentration. After the completion of the ion-exchange reaction, the Na+- or Ca2+-exchanged coal was separated by filtration, washed repeatedly with high-purity water, and dried under vacuum at 60 °C. A composite Ca2+/Na+-exchanged catalyst was subsequently prepared. An aqueous solution of soda ash was first added to an aqueous dispersion of coal particles, after which the saturated calcium hydroxide solution was added. This process was adopted because a CaCO3 precipitation was observed when the soda ash and calcium hydroxide were directly mixed. The Ca2+ concentration in this solution

3. RESULTS AND DISCUSSION 3.1. Ion-Exchange Reactions between Soda Ash, Calcium Hydroxide, and Coal. Figure 1 plots the Ca2+ and Na+ concentrations and the pH in ion exchange solutions at 30 ± 0.1 °C as functions of time. It is evident that the Na+ concentration decreased more slowly when using the soda ash solution. As noted, mixing the soda ash with the calcium hydroxide generated a precipitate. Therefore, the calcium hydroxide solution was first added to the aqueous dispersion of coal particles, after which the soda ash was added. These data

Figure 1. Changes in (a) Ca2+ and Na+ concentrations and (b) pH in ion-exchange dispersions with stirring time. B


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Table 3. Comparison of pH Values, Ion-Exchange Amounts, and Ion Balances of Ion-Exchanged Catalysts Prepared from Different Water-Soluble Precursors exchange amt (mass %)

pH catalyst

precursor

before

after

Ca

Na

Na Ca Ca/Na

soda ash solution calcium hydroxide solution calcium hydroxide solution and soda ash solution

11.3 11.6 11.6

10.2 11.0 11.1

− 3.5 3.8

1.6 − 1.0

total exchange amt (mmol/g of coal) 0.70 0.88 1.4

balance Na

Ca

0.94 − 1.03

− 1.00 1.01

from different water-soluble precursors are summarized in Table 3. The exchange amount (mass %) was calculated using the following equation. exchange amount (mass %) = [change in concentration before and after reaction (mg/L) × solution quantity (L)] × [weight of coal after drying (mg)]−1 × 100

The quantity of Ca2+ cations exchanged when using the Ca(OH)2 solution was evidently greater than the amount of Na+ cations when employing the soda ash. These results indicate that the Ca2+ cations were exchanged with the H+ of both COOH and OH groups in the coal. The quantity of COOH groups necessary to exchange with all the available Ca2+ cations is 0.88 × 2 = 1.76 mmol/g of coal, which is greater than the quantity of COOH groups calculated on the basis of TPD data (1.2 mmol/g of coal). Accordingly, another H+ source must be available. When the soda ash was added to the solution which had been exchanged with Ca2+ cations, the pH was found to increase somewhat (Figure 1b), although the subsequent exchange rate was slower because the number of remaining functional groups was lower. From Table 3, the total amount of Ca2+ and Na+ ions exchanged with the coal was 1.4 mmol/g of coal, which represents approximately 1.6−2.0 times the amount exchanged when using solely Na or Ca. The ion balances were determined to be in the range of 0.94−1.03 based on analyses of the unreacted solutions and the wash solutions, using an ion electrode or ICP-AES. The investigation indicates that the total amount of loaded catalyst for the Ca/Na combined catalyst is almost doubled compared with Na or Ca catalyst alone which may have been caused by the degree of ionization for carboxyl groups and the metallic ions (Ca2+ and Na+). Therefore, it is possible that Na+ dissociates more easily than Ca2+ from the carboxyl group, hence a lot of Na+ dissociated from the carboxyl group with the equilibrium state as compared with Ca2+. It is considered that when Ca2+ and COOH were reacted followed by Na+ and COOH were reacted, Na+ was difficult to dissociate from COO− as compared with Ca2+. However, the determination of the degree of ionization is very difficult because that value’s change is large with pH and concentrations at that time. Further consideration will be needed to yield any findings about the degree of ionization for carboxyl groups and the metallic ions (Ca2+ and Na+). 3.2. Steam Gasification of Na+-, Ca2+-, and Ca2+/Na+Exchanged Coals. Figure 2 summarizes the variations in char conversion with reaction time during the steam gasification of raw coal and Na+-, Ca2+-, and Ca2+/Na+-exchanged coals at 700 °C. The raw coal conversion was approximately 20% after 2 h, while the conversion values of the Na+- and Ca2+-exchanged samples were 75 and 82% after 1 h, respectively. The conversion of the Ca2+/Na+-exchanged coal reached 70%

Figure 2. Char conversions over time during steam gasification of raw coal and Na+-, Ca2+-, and Ca2+/Na+-exchanged coals at 700 °C.

Figure 3. Char conversions of raw coal and Na+-, Ca2+-, and Ca2+/ Na+-exchanged coals after 1 h of gasification as functions of temperature.

also indicate a sudden drop in the Ca2+ concentration to approximately 10% of the initial concentration after 5 min (Figure 1a). As the plots show, the soda ash was added 10 min after the calcium hydroxide had been combined with the coal particles dispersion. Under these conditions, the Na+ concentration decreased more slowly, indicating that the reactive functional groups on the coal particles had already been exchanged to a significant extent. Figure 1b demonstrates that the pH of the combined soda ash/calcium hydroxide solution decreased as the Ca2+ and Na+ concentrations became lower. This result suggests that the H+ on the COOH groups and/or phenol OH groups in the coal were exchanged with Ca2+ and/or Na+. The ion exchange amounts, pH values, and ion balances of the ion-exchanged Na+, Ca2+, and Ca2+/Na+ catalysts prepared C


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Figure 4. (a) Carbon conversions as functions of time and (b) specific reaction rates as functions of carbon conversion for gasification of raw coal and Na+-, Ca2+-, and Ca2+/Na+-exchanged coals at 700 °C.

Figure 7. Gaseous product yields as functions of char conversion during gasification of raw coal and Ca2+-, Na+-, and Ca2+/Na+exchanged coals at 700 °C.

Figure 5. Carbon conversions as functions of time during gasification of raw coal and Ca2+/Na+-, Ca2+-, and Na+-exchanged coals at 650 °C.

reports that high gasification temperatures promote the reaction of Na catalysts with mineral materials in the coal13 and also induce volatilization of the Na,14 both of which reduce the catalytic activity. These data also show that the reaction temperature required for 60% conversion of the Ca2+/Na+exchanged coal was approximately 150 °C lower than that required for the raw coal. From examining the findings, it is the reason causing the significant catalytic difference of the combined catalyst that the total amount of loaded catalyst for the Ca/Na combined catalyst is almost doubled compared with Na or Ca catalyst alone. The investigation shows that Ca and Na might be exchanged at the center of coal. Hence, the Ca/Na combined catalyst showed higher activities than Na or Ca catalyst alone in the steam gasification. The relationships between gasification time and carbon conversion at 700 °C are shown in Figure 4a. The conversion of the Na+-exchanged coal increased over time in a linear manner, while the conversion of the Ca2+-exchanged coal began to gradually decrease. The conversion of the Ca2+/Na+exchanged coal in the early stage was almost equal to that of the Ca2+-exchanged coal but became greater after approximately 20 min. Previous studies have demonstrated that alkali metal catalysts can liquefy at the onset of the catalytic reaction, giving a comparatively constant reaction rate throughout the catalytic reaction.15 The catalysts in the present work appear to have exhibited similar behavior.

Figure 6. Specific reaction rates as functions of temperature at carbon conversions of (a) 15 and (b) 40%.

after 30 min, and this same sample was completely gasified after 1 h. Figure 3 plots the char conversions of raw coal and Na+-, Ca2+-, and Ca2+/Na+-exchanged coals after 1 h as functions of temperature, where each gas yield is expressed on a dry, ashfree, catalyst-free basis (dacf). Irrespective of the temperature, the char conversion was in the order Ca2+/Na+ > Ca2+ > Na+. While a conversion of 100% was obtained at 750 °C using the Na+-exchanged coal, only 700 °C was required for complete conversion of the Ca2+/Na+-exchanged coal. There have been D


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Figure 8. Gaseous product yields as functions of char conversion during gasification of raw coal and Ca2+-, Na+-, and Ca2+/Na+-exchanged coals at (a) 650 and (b) 700 °C.

Figure 9. XRD diffraction patterns of char samples obtained from Na+-, Ca2+-, and Ca2+/Na+-exchanged coals after pyrolysis at 700 °C.

Figure 10. XRD diffraction patterns obtained following pyrolysis of Ca2+/Na+-exchanged coal (0% conversion) and following gasification of the same sample for 30 min (69% conversion) and 1 h (100% conversion) at 700 °C.

Table 4. Calculated Amounts of Catalysts and Sizes of Crystallites in Char Samples Obtained from Na+-, Ca2+-, and Ca2+/Na+-Exchanged Coals after Pyrolysis at 700 °Ca calcd amt of catalyst (mass %)

a

McCormick et al. reported about normalized gasification rates (mg/(mginitial·min)) using Illinois No. 6 coal with Ca or Na + Ca catalysts.7 In their paper, the rate at 68% conversion was 2.1 × 10−3 mg/(mginitial·min) with 1.5 wt % (Na/Ca = 1.2) catalyzed CO2 gasification at 800 °C. The rate is incomparable with our research’s rate because of different reaction conditions of the data. For reference, the rate, which was calculated from Figure 2, was 10.7 × 10−3 mg/(mginitial·min) at the same conversion using the 3.8 mass % Ca2+-exchanged and 1.0 mass % Na+-exchanged coal steam gasification at 700 °C. Moreover, Haga et al. found that the composite catalyst impregnated with Na + Ca had high gasification rates (mg/(g·min)) at 20% conversion.8 The rate was 56.2 mg/(g·min) using 0.25% Na + 0.25% Ca on Blair Athol char with H2O−He gas at 800 °C. In our study, the rate, which was estimated from Figure 2, was 90.7 mg/(g·min) at the same conversion with the 3.8 mass % Ca2+-exchanged and 1.0 mass % Na+-exchanged coal steam gasification at 700 °C.

crystallite size (nm)

catalyst

CaO

Na2CO3

CaO

Na2CO3

Na Ca Ca/Na

− 9.1 9.8

6.9 − 4.2

− n.d. 9.2

n.d. − n.d.

n.d. = none detected.

The specific reaction rates of each specimen are plotted in Figure 4b. These values were calculated using the following equation. specific rate (h−1) =

carbon conversion per unit time (%/h) residual char (%)

The specific rate of the Ca2+/Na+-exchanged coal during the second half of the reaction is seen to have increased compared with that of the Ca2+-exchanged coal. E


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Na+-exchanged coal was the same as that of the Na+-exchanged coal at 750 °C, indicating that it is preferable to use Ca2+/Na+exchanged coal below 700 °C. The relationships between the gaseous product yields and char conversion during the gasification of the raw coal and Ca2+-, Na+-, and Ca2+/Na+-exchanged coals are shown in Figure 7 (at 700 °C) and Figure 8, parts a (650 °C) and b (700 °C). These yields were based on the unit mass of the char and were found to increase in the order CO < CO2 < H2, regardless of the coal type. In general, the gas yield is increased by the presence of a catalyst when employing fixed bed gasification. However, in this study, the product gas yield was little affected by using these catalysts. 3.3. Catalyst States after Pyrolysis and Gasification. Figure 9 presents the XRD diffraction patterns of the Na+-, Ca2+-, and Ca2+/Na+-exchanged samples after pyrolysis at 700 °C, while Table 4 shows the calculated amounts of each catalyst in the same samples. The catalyst quantities in the Na+- and Ca2+-exchanged chars were determined to be 6.9 and 9.1 mass %, respectively. However, peaks attributable to Na or Ca species could not be detected, suggesting that these species were present as nanoscale particles (less than 5 nm in size) and so could not be observed by XRD. The crystallite size of the CaO was approximately 9.2 nm in the Ca2+/Na+-exchanged char, while Na species could not be detected in the same sample. It is not clear why a CaO peak was generated solely by the Ca2+/Na+-exchanged char, but in any case it is evident that all three samples had highly dispersed catalysts. The XRD diffraction patterns obtained following the pyrolysis of the Ca2+/Na+-exchanged sample at 700 °C and the gasification of the same sample at the same temperature are provided in Figure 10. The patterns obtained after the pyrolysis of the Ca2+/Na+-exchanged sample (at 0% conversion) and after 1 h of gasification (100% conversion at 750 °C and 62% conversion at 650 °C) are shown in Figure 11. It is evident that the CaO peak width was increased as the reaction proceeded. In addition, at 100% conversion, a Na2CO3 peak appears in Figure 10. Following processing at 750 °C, the CaO crystallite size in the Ca2+/Na+-exchanged char was 18 nm (Figure 11). It is therefore apparent that the CaO particles were more highly aggregated following pyrolysis at 750 °C. Tables 5 and 6 summarize the calculated amounts of catalysts and the crystallite sizes in the Na+-, Ca2+-, and Ca2+/Na+exchanged chars following gasification at 700 °C. At 100% char conversion (700 °C), the Na2CO3 crystallite sizes were in the order Ca/Na < Na (Table 5), while at char conversions in the range 69−82%, the CaO crystallite sizes were in the order Ca/ Na > Ca at both 700 and 650 °C. In general, larger crystallite sizes will reduce the gasification activity of Ca-based catalysts.16 Therefore, the increased reaction rate observed for the Ca2+/ Na+-exchanged char during the second half of the reaction (Figures 4 and 5) is believed to result from the concentration of Na species on the char.

Figure 11. XRD diffraction patterns obtained following pyrolysis of Ca2+/Na+-exchanged coal (0% conversion) and following gasification of the same material for 1 h at 750 °C (100% conversion) and 650 °C (62% conversion).

Table 5. Calculated Amounts of Catalysts and Crystallite Sizes in Char Samples (Na+, Ca2+, and Ca2+/Na+ Exchanged) after Gasification at 700 °C calcd amt of catalyst (mass %) catalyst Na Ca Ca/Na


char conv (mass %)

CaO

Na2CO3

CaO

Na2CO3

72 100 82 69 100

− − 32 23 58

22 72 − 10 25

− − 20 26 71

25 34 − − 25

Table 6. Calculated Amounts of Catalysts and Crystallite Sizes in Char Samples (Na+, Ca2+, and Ca2+/Na+ Exchanged) after Gasification at 650 °C calcd amt of catalyst (mass %)


catalyst

char conv (mass %)

CaO

Na2CO3

CaO

Na2CO3

Na Ca Ca/Na

33 50 62

− 16.0 20.0

9.2 − 8.7

− 7.8 26.0

24.0 − −

Figure 5 presents the carbon conversions as functions of time during the gasification of the raw coal and Ca2+/Na+-, Ca2+-, and Na+-exchanged coals at 650 °C. The conversion at this temperature increased in the order Ca2+/Na+ > Ca2+ > Na+. The conversion of the Ca2+/Na+-exchanged coal was about 60% after 1 h of gasification, and became increasingly larger than that of the Ca2+-exchanged coal during the later stage of the reaction. This trend is the same as that observed in the gasification data acquired at 700 °C (Figure 4a). The relationships between the specific reaction rates and temperature are shown in Figure 6 (at carbon conversions of 15 (a) and 40% (b)). The activation energies of the raw coal and the Na+-exchanged coal were determined to be 140 and 120 kJ/ mol, respectively, and the respective frequency factors were 1.0 × 106 and 1.7 × 105 h−1. Adding Na+ cations to the raw coal thus reduced the activation energy by 20 kJ/mol while increasing the frequency factor. The specific rate of the Ca2+/

4. CONCLUSION Three catalysts (Na+, Ca2+, and Ca2+/Na+) were loaded onto low-rank-coal samples (at particle sizes in the range 0.075−0.15 nm) using an ion-exchange method, and the combined amount of Ca2+ and Na+ cations deposited was about 1.6−2.0 times that of the Na+ or Ca2+ deposited individually. At 700 °C, the Ca2+/ Na+ catalyst exhibited superior catalytic activity compared with Ca2+ and Na+ catalysts, such that the former char was F


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Energy & Fuels completely gasified after 1 h. At 650 and 700 °C, the specific reaction rate of the Ca2+/Na+ catalyst became greater than that of the Ca2+-exchanged material during the second reaction stage. The CaO crystallite sizes after gasification at 650 and 700 °C were found to be larger when Na+ was also present.

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(16) Ohtsuka, Y.; Asami, K. Highly active catalysts from inexpensive raw materials for coal gasification. Catal. Today 1997, 39, 111−125.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +81-11-706-6850. Fax: +81-11-726-0731. E-mail: [email protected]. ORCID

Naoto Tsubouchi: 0000-0001-8236-8252 Yuuki Mochizuki: 0000-0003-1977-413X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported in part by Japan Coal Energy Center (JCOAL) commissioned by the Strategic Technical Platform for Clean Coal Technology (STEP CCT) of the New Energy and Industrial Development Organization (NEDO) with the subsidy of the Ministry of Economy, Trade and Industry of Japan.

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REFERENCES

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