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Catalytic effect of ion-exchanged calcium on steam gasification of low-rank coal with a circulating fluidized bed reactor
Fuel 234 (2018) 406–413
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Full Length Article
Catalytic effect of ion-exchanged calcium on steam gasification of low-rank coal with a circulating fluidized bed reactor
T
⁎
Naoto Tsubouchia, , Yuuki Mochizukia, Yuji Shinoharaa, Yuu Hanaokab, Yasuo Ohtsukab, Koji Kuramotoc, Koichi Matsuokac a
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 c Research Institute of Energy Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8569, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Sub-bituminous coal Ion-exchange Steam gasification Circulating dual bubbling fluidized bed (CDBFB) Fixed bed (FB)
Calcium ion exchange was performed in Indonesian low-rank coal using saturated aqueous solutions of Ca(OH)2 at 30.0 ± 0.1 °C without pH adjustment. The Ca2+ concentration and pH were monitored during ion exchange. A coal product was obtained with 3.2 mass% Ca2+. Gasification of the ion-exchanged samples was conducted in a circulating dual bubbling fluidized bed (CDBFB) reactor at 700 °C, 800 °C, and 900 °C. Coal carbon conversion into carbonaceous gases (CO, CO2, CH4, and C2 hydrocarbons) using the Ca2+-exchanged coal achieved 78% at 900 °C in the CDBFB. The Ca catalyst led to the greatest rate enhancement at 800 °C, resulting in a twofold increase in the gas product yield compared to that without Ca. A comparison of carbon conversion between the CDBFB and fixed-bed reactors revealed that the conversion value using the CDBFB at 700 °C was very small. Regarding an examination of this influence in detail, it seems that the existing H2 adhered primarily to the active sites of the char, and the use of Ca2+-exchanged coal led to a reverse shift reaction.
1. Introduction Improving the efficiency of coal is a high priority for reducing CO2 output. Systems that have performed well include ultra-supercritical pulverized coal combustion, integrated coal gasification combined cycle (IGCC), integrated gasification fuel cells (IGFC), and polygeneration. Moreover, the utilization of low-rank coal, a potentially plentiful resource, is also important to establish a stable future energy supply. Despite this, at present, there is little economic incentive to burn low-rank coal because of the associated production costs. Consequently, in the publication Strategic Technical Platform for Clean Coal Technology (STEP CCT), the New Energy and Industrial Technology Development Organization (NEDO) in Japan describes the development of the IGCC system using low-rank coal gasification at or below 900 °C in the presence of catalysts. In addition, advanced IGCC and advanced IGFC systems with exergy recuperation was proposed in order to increase the thermal efficiency of IGCC systems [1–7]. In regards to these systems, steam recovers the exhaust heat from a solid oxide fuel cell or
a gas turbine. Hot exhaust gas cools greatly in an entrained-flow bed reactor [8]. Therefore, our group adopted a circulating dual bubbling fluidized bed (CDBFB) reactor as it induces a heat insulation effect is obtained. This causes the exhaust gas in the system to maintain its temperature when the CDBFB reactor is used. As for the strong volatile–char interaction and reduction of tar emissions, a downer pyrolyzer has a major influence on subsequent steam gasification using a novel triple-bed combined circulating fluidized bed (TBCFB) reactor [1,9,10]. For instance, steam gasification of Adaro coal (particle size, dp = 0.5–1.0 mm) char at 700–900 °C was studied using a lab-scale TBCFB [9,10]. Coal pyrolysis has been performed previously in a drop-tube reactor (DTR) [11–16] and a downer reactor [17–19]. The reactivity of Loy Yang coal chars and those blended with coal at 50%–85% ratios was investigated using a DTR at 900 and 950 °C [15]. The reactivities of Na and Ca in the raw coal were investigated using the same DTR at 850–900 °C [20]. However, catalytic steam gasification of Ca2+-exchanged low-rank coal using the CDBFB and the TBCFB reactor has not
Abbreviations: CDBFB, circulating dual bubbling fluidized bed; FB, fixed bed; IGCC, integrated coal gasification combined cycle; IGFC, integrated gasification fuel cells; STEP CCT, Strategic Technical Platform for Clean Coal Technology; NEDO, the New Energy and Industrial Technology Development Organization; TBCFB, triplebed combined circulating fluidized bed; DTR, drop-tube reactor; XRD, X-ray diffraction ⁎ Corresponding author. E-mail address: [email protected] (N. Tsubouchi). https://doi.org/10.1016/j.fuel.2018.07.046 Received 2 April 2018; Received in revised form 9 July 2018; Accepted 10 July 2018 0016-2361/ © 2018 Published by Elsevier Ltd.
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been studied. As regards a catalyst with steam gasification of coal, achieving lowtemperature gasification requires application of a catalyst. Our previous results show that catalytic steam gasification of low-rank coal with ionexchanged Ca prepared from saturated Ca(OH)2 solution was an excellent catalyst for use in a fixed-bed reactor [21]. Therefore, we expected this material to show similar performance in a CDBFB reactor. The present study thus examines the effect of Ca catalyst on steam gasification of a sub-bituminous coal in a CDBFB system. If the inexpensive catalyst can decrease the gasification temperature and increase the gasification rate, it may present several advantages, such as the circulation of large amounts of coal particles and enhanced-exergy recuperation in the CDBFB system.
Table 2 Metal concentrations in the Adaro coal sample. Metal concentrationa (mass%, dry) Na
Al
Si
K
Ca
Fe
0.010
0.18
0.41
0.014
0.18
0.24
a Determined by inductively coupled plasma emission spectroscopy (ICP-ES) after acid leaching of ash obtained by pyrolysis at 815 °C.
Table 3 Ca2+ ion-exchange reaction conditions. Coal (g)
2. Experimental 2.1. Coal samples
60.00
Saturated calcium hydroxide solution Ca(OH)2 (g)
Deionized water (ml)
Ca2+ (mg/ L)
Reaction temperature (°C)
3.6
3000
649
30.0 ± 0.1
Indonesian sub-bituminous coal (Adaro) was used, denoted AD, because this coal was one of standard samples in the STEP CCT project. The coal sample was air-dried at room temperature, crushed, and sieved to obtain particles with a size range of 150–250 µm. Table 1 shows the results from compositional analyses. Table 2 summarizes the metal analysis (Na, Al, Si, K, Ca, and Fe) of the coal and shows that the main constituent elements were Si, Al, Fe, and Ca. 2.2. Catalyst materials and ion-exchange methodology Saturated Ca(OH)2 (Wako Pure Chemical Industries. Ltd.) was used as the precursor for the ion-exchanged Ca catalyst. The size fraction of coal samples as received was in the range of 150–250 μm. The samples were used without any pretreatment. As in our previous study [21], a saturated Ca(OH)2 solution was added to a suspension of coal particles and deionized water, which was held at constant temperature (30.0 ± 0.1 °C) without pH adjustment. The Ca2+ concentration and pH of the mixture during stirring were monitored using Ca2+-selective and pH electrodes (Table 3). The Ca2+exchange reaction used 60 g of coal per reaction for a total of 5 exchange reactions. The initial cation concentration was 649 mg Ca/L (Ca2+ ions). The amount of Ca2+ ions actually exchanged was calculated using the following equation:
Exchange amount (mass %) change in concentration before and after the reaction (mg =
/L) × the solution quantity (L) weight of coal after drying (mg)
× 100 Fig. 1. Schematic drawing of the circulating dual bubbling fluidized bed reactor used in this study.
(1) After completion of the ion-exchange reaction, the Ca2+-exchanged coal was separated by filtration, washed with high-purity water, dried under vacuum at 60 °C, and then classified according to particle size (150–250 µm). The recovery ratio of Ca2+-exchanged coal to total coal amount (60 g × 5 times = 300 g) was 72%–75%. The total amount of obtained Ca2+-exchanged coal was about 220 g.
2.3. Circulating dual bubbling fluidized bed reactor and product gas analysis A CDBFB reactor was used for our experiments [22]. Fig. 1 shows a schematic drawing of the CDBFB reactor. The size and reaction temperature of the apparatus are shown in Table 4, and the gasification conditions are shown in Table 5. The coal feed rate was 1.0 g/min. The samples were supplied steam (H2O/coal = 1.4–1.6 kg/kg, 20 vol% H2O/N2) and silica sand heating-medium particles (150–250 µm; 95 mass% SiO2) into the top of the fluidized bed in the gasifier with a screw feeder in the CDBFB. Our previous steam gasification results using woody biomass using a CDBFB reactor showed that the increase of carbon conversion with S/C was not observed at higher S/C (1 < S/ C < 3, i.e., 12–36 vol% steam concentration) [22]. Accordingly, 20 vol % steam concentration (S/C = 1.5) was adopted. The formed gas and tar captured by the siliceous and unconverted char were transported
Table 1 Ultimate and proximate analyses of Adaro. Ultimate analysis (mass%, dafa)
Proximate analysis (mass%, dry)
C
H
N
S
Ob
Ash
VMc
FCb,c
67.8
5.1
0.44
0.14
26.5
2.5
46.7
50.8
a b c
daf = dry ash free. Determined by difference. VM = volatile matter; FC = fixed carbon. 407
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Table 4 Size of the circulating dual bubbling fluidized bed reactor and reaction temperature. Gasifier
Combustor
Riser
Inner diameter (mm)
Length (mm)
Inner diameter (mm)
Length (mm)
Temperature (°C)
Inner diameter (mm)
Length (mm)
Temperature (°C)
80
270
80
150
950
18
1800
> 300
upward through the riser. The product gas was separated with a cyclone separator and was analyzed using high-speed gas chromatography and detector tubes (Table 6). The char and (coke-deposited) silica sand were transported to the combustor by a loop seal. The regenerated silica sand was transported back to the gasifier by a downer and a pneumatic Lvalve. Large carbon conversion differences were found between pyrolysis and steam gasification. As a first step, a simple comparative analysis was performed with two samples (row coal and Ca2+-exchanged coal) in order to determine the catalytic activities.
Table 6 Gas analysis conditions. Analysis method
Detected gas
High-speed gas chromatography Detector tube
H2, CO, CO2, CH4, C2H4, C2H6 C6H6, C7H8, C8H10
demonstrate that ion exchange proceeded and suggest that H+ on the COOH groups and/or phenol OH groups in the coal were exchanged with Ca2+.
2.4. Fixed-bed gasification reactor and product analysis
3.2. Pyrolysis and steam gasification
To compare carbon conversion between the CDBFB reactor and fixed-bed (FB) reactor, catalytic pyrolysis and gasification of Ca2+-exchanged coal was performed in an FB quartz reactor at 700 °C. 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. It was then held at this temperature for 10 min to remove volatile matter, and it was finally exposed to 50 vol% H2O in He to gasify the resulting char in situ for 1 h. The CDBFB conversion ratio was less than that obtained using the FB, and it appeared that coexisting H2 and CO adhered primarily to the active sites of the char. To examine this influence in detail, the test sample was reacted with steam using steam mixing fuel gas (H2O/H2/ CO/CO2/He = 50 vol%/28 vol%/7.0 vol%/13 vol%/2 vol%) instead of He at 700 °C and 900 °C. The gases produced during this process were analyzed after removal of H2O using two online micro gas chromatographs (GCs) with thermal conductivity detectors to determine the concentrations of H2, CO, CO2, and CH4 [23].
Fig. 3 shows gas yield plots for raw coal and Ca2+-exchanged coal as a function of reaction time at 900 °C. The holding time ranged from 20 to 60 min (solid residence time was 20 min) because the steady state was only achieved 20 min after the samples were placed in the reactor, resulting in stable yield of product gases after 60 min. In addition, the reaction remained stable for up to 75 min, and the most stable value (60 min) was adopted in this study. Stability of the reaction system was confirmed in the preliminary study. Circulation of the silica sand bed material was significantly dependent of aeration of L valve. The circulation could be controlled by appropriate control of the aeration. The main product gases were H2, CO, and CO2. However, pyrolysis and steam gasification occurred in the reaction system simultaneously, yielding small amounts of CH4, C2H4, and C2H6 from pyrolysis. Fig. 4 shows carbon conversion to gas from raw coal and Ca2+-exchanged coal as a function of temperature (700 °C, 800 °C, and 900 °C). Coal carbon conversion was calculated using gas chromatography measurement of CO, CO2, CH4, and C2 hydrocarbons with the following equation (2):
2.5. Characterization
Coal carbon conversion (%) = Selected char samples were analyzed using X-ray diffraction (XRD).
CO+CO2 + CH 4 + C2 hydorocarbon (mol) Coal carbon content (mol) (2)
× 100 3. Results and discussion
Coal carbon conversion to gas was approximately 15% at 700 °C and 56% at 900 °C. In contrast, Ca2+-exchanged coal carbon conversion was approximately 28% at 700 °C and 78% at 900 °C. In this experiment, Ca2+-exchanged carbon conversion was 78% at 900 °C. Thus, carbon conversion could be improved if the steam/coal ratio was increased. Pyrolysis and steam gasification were performed in the pressurized fluidized bed gasification system using 4.4 mass% Ca2+-exchanged coal according to Takarada et al. [24]. The results indicate that increasing the steam/coal ratio from 2.5 to 5.0 promoted gas production from coal in the coal tar and increased the coal conversion. By contrast, our previous results addressing steam gasification
3.1. Catalyst material and ion-exchange The ion exchange method applied to AD coal with 150–250 µm particle size resulted in a product with 3.2 mass% Ca. Fig. 2 shows the Ca2+ concentration (Fig. 2a) and pH (Fig. 2b) of the ion exchange solutions as a function of reaction time. The Ca2+ concentration underwent a sudden increase upon addition of the Ca(OH)2 solution to the coal (Fig. 2a), which then decreased and stabilized at 100 mg/L after 15 min, while pH decreased with time (Fig. 2b). These results Table 5 Gasification conditions and temperature. Gasification temperature (°C)
700–900 a
Medium particles Composition
Size (mm)
Surface area (m2/g)
SiO2 > 95 mass %
0.15–0.25
1.0
Solid residence time. 408
Coal feed rate (g/min)
Steam feed rate S/C (kg/kg)
Residence timea (min)
1.0
1.4–1.6
20
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(a) 700
35
(b) 13
34 33
500
Temperature, ºC Temperature,
400
12
30
pH pH
300
31
29 28
200
Temperature,
32 pH
Ca2+ concentration, mg/L
600
27 100 0
26 0
5
10
11 0
15
Time, min
5
Time, min
10
25 15
Fig. 2. (a) Calcium ion concentration and (b) pH of ion-exchange solutions as a function of reaction time.
using woody biomass with a CDBFB reactor showed that carbon conversion did not increase at higher S/C (1 < S/C < 3, i.e., 12–36 vol% steam concentration) [22]. Further consideration is required to deduce any findings regarding the steam/coal ratio using Ca2+-exchanged coal in a CDBFB reactor.
3.3. Product gas composition The composition of product gases using raw coal and Ca2+-exchanged coal is shown in Table 7. The total amount of gases, which included H2, CO, CO2, CH4, and C2H4 increased with temperature regardless of the presence of a catalyst. The Ca catalyst led to the greatest rate enhancement at 800 °C, resulting in a twofold increase in the gas product yield compared to that without Ca. For Ca2+-exchanged coal, CH4 and C2H4 production must occur due to the decomposition of coal tar or due to steam reformation after decomposition into H2 and CO. A previous report indicated that the amount of coal tar produced by pyrolysis was small for Ca2+ supported on demineralized charcoal [25]. Therefore, future investigations should examine coal-tar decomposition on Ca2+-exchanged coal. The relationship between carbon conversion and product gas yield is shown in Fig. 5. The fraction of hydrocarbons decreased in the presence of the Ca catalyst (dotted circles in Fig. 5), suggesting that
Fig. 4. Coal carbon conversion to gas from raw coal and Ca2+-exchanged coal after 20 min as a function of temperature (700 °C, 800 °C, and 900 °C).
Fig. 3. Gas yield from (a) raw coal and (b) Ca2+-exchanged coal as a function of reaction time at 900 °C. 409
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samples prepared at 800 °C and 900 °C in the CDBFB. The XRD diffraction patterns of the Ca2+-exchanged samples at 700 °C and 800 °C didn’t contain peaks due to the presence of the Ca species. Only small amounts of Ca were present in the samples, and separation of the silica sand and the Ca2+-exchanged coal was difficult. The XRD diffraction patterns for the Ca2+-exchanged samples at 900 °C produced peaks attributed to Ca species, obviously due to the presence of Ca. After successful coal gasification at high temperature (900 °C), the concentration of Ca seems to increase due to coal loss. The CaO crystalline size was approximately 38 nm in the Ca2+-exchanged sample prepared at 900 °C. This value was larger than that of 20 nm obtained using FB gasification at 700 °C, which was presented previously [23]. The low reaction temperature improved the maximum catalytic activity because Ca particle aggregation accelerates at high temperatures. Therefore, the Ca2+-exchanged coal carbon conversion had the greatest rate enhancement at 800 °C (Fig. 4).
Table 7 Temperature dependence of product gas yield (CDBFB). Catalyst
Temperature (°C)
Gas yield (mol/100 mol C in coal) Total
H2
CO
CO2
CH4
C2H4
C2H6
None
700 800 900
28 74 130
13 41 73
4.4 10.0 26.0
7.4 17 24
2.7 4.1 4.9
0.90 1.5 1.9
0.19 0.17 0.05
Ca
700 800 900
73 150 173
44 86 96
7.4 30.0 44.0
19 29 26
2.1 4.3 4.9
0.66 1.6 1.9
0.16 0.05 0.20
3.5. Comparison of carbon conversion between the circulating dual bubbling fluidized bed and fixed-bed reactors Catalytic steam gasification was performed on the ion-exchanged coal in a FB quartz reactor at 700 °C in order to predict a carbon conversion value with CDBFB reactor using data from the FB. Fig. 8 shows carbon conversion to CO and CO2 after 1 h with the CDBFB and FB reactors. Carbon conversion during steam gasification was calculated using gas chromatography using Eq. (2). Carbon conversion at 700 °C in the CDBFB was much lower than that in the FB, and a temperature of 100 °C was required to realize carbon conversion equal to that from the FB. It is still unknown why carbon conversion in the CDBFB at 800 °C and the value in the FB at 700 °C were nearly equal. The actual gasification time with the CDBFB was 20 min (solid residence time was 20 min); whereas, the reaction time using the FB was 1 h. Different gasification times seem to influence carbon conversion in each reactor. As a result, we could not predict a carbon conversion value for the CDBFB reactor using data from the FB reactor.
Fig. 5. Relationship between carbon conversion and product gas yield.
H2
Raw
3.6. Influence of fuel gas in fixed-bed gasification
CO
900 ºC
CO2
Ca
The conversion ratio using CDBFB was less than that obtained using FB. It was previously reported that H2 and/or CO interfere with the active site [26]. Therefore, this influence was investigated. To examine this influence in detail, steam was reacted using steam mixing fuel gas (H2O/H2/CO/CO2/He = 50 vol%/28 vol%/7.0 vol%/13 vol%/2 vol%) instead of He at 700 °C or 900 °C (Fig. 9). Carbon conversion during steam gasification was calculated using gas chromatography data via Eq. (2). Conversion of raw coal and Ca2+-exchanged coal at 700 °C were small using fuel gas. Upon raising the temperature to 900 °C, no effect of Ca-catalytic activity was found. Table 8 shows the gas concentration in the CDBFB, which was used to calculate gas yield. The H2 concentration using Ca2+-exchanged coal at 900 °C was 13 vol%, which was the greatest concentration. The influence of evaporation velocity appeared to be large. Fig. 10 shows gas concentration as a function of reaction time at 700 °C using FB gasification under a fuel gas atmosphere. The values in Fig. 10 were calculated using a dry gas standard (H2/CO/CO2/ He = 56 vol%/14 vol%/26 vol%/4 vol%). For Ca2+-exchanged coal, CO concentration increased while the concentrations of H2 and CO2 decreased. Therefore, the following reverse shift reaction occurred alongside steam gasification under these conditions:
CH4 C2H4
Raw
C2H6
800 ºC
Ca
Raw 700 ºC
Ca 0
10 20 30 40 50 60 70 80 90 100 Composition, mol%
Fig. 6. Composition of formation gases at 700 °C, 800 °C, and 900 °C in the circulating dual bubbling fluidized bed.
hydrocarbon reforming occurred. The composition of product gases at 700 °C, 800 °C, and 900 °C using CDBFB is shown in Fig. 6. At the same temperature, it was found that the Ca2+-exchanged coal has a smaller proportion of hydrocarbons compared to raw coal. In addition, it was found that the H2/CO ratio becomes larger compared with other temperatures when the Ca catalyst is present at 700 °C.
3.4. States of the catalysts Fig. 7 presents XRD diffraction patterns for the Ca
2+
C+ H2 O→ CO + H2
(5)
CO2 + H2 → CO + H2 O
(6)
Furthermore, the catalytic activity of Ca by the formation of a
-exchanged 410
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Diffraction intensity
CaO SiO2 CaCO3
900
800
10
20
30
40
50
60
2 (Cu K ), degrees Fig. 7. XRD diffraction patterns of Ca2+-exchanged samples at 800 °C and 900 °C in the circulating dual bubbling fluidized bed.
100 CDBFB,
FB; Ca2+-exchanged coal
CDBFB,
FB; Raw coal
Coal carbon conversion to gas, %
Coal carbon conversion to gas, %
100
80
60
40
20
0 650
700
750
800
850
Temperature, ºC
900
Fig. 8. Comparison of carbon conversion using the circulating dual bubbling fluidized bed and the fixed-bed after 1 h.
H2O/Fuel (FB),
CDBFB; Ca2+-exchanged coal
H2O/Fuel (FB),
CDBFB; Row coal
80
60
40
20
0 650
950
H2O/He (FB), H2O/He (FB),
700
750
800
850
Temperature, ºC
900
950
Fig. 9. Temperature dependence of carbon conversion during exposure to fuel gas and H2O/He in the circulating dual bubbling fluidized bed and the fixedbed reactor.
surface peroxide was explained by Sears et al. [27]:
CaO + CO2 → CaO·[O] + CO
(7)
CaO·[O] + C→ CaO + C[O]
(8)
C[O] → CO
(9)
Table 8 Product gas concentration (CDBFB). Catalyst
Temperature (°C)
Gas concentrationa (vol%) H2
(CaO·[O] and C[O]; Reactive intermediate.) The above catalytic activity of Ca in reactions (7)–(9) seems to have occurred in addition to the steam gasification reaction. Fig. 11 shows the gas concentration as a function of reaction time for FB gasification at 900 °C during exposure to fuel gas using raw coal and Ca2+-exchanged coal. A decrease in H2 and CO2 concentrations and an increase in CO concentration were promoted compared to the results obtained at 700 °C. Kyotani et al. examined the inhibitory reaction of H2 with CO2 gasification on the Ca2+-exchanged coal char [26]. The results indicate that adherence of H2 to the boundary-surface neighborhood of carbon and the Ca catalyst hinders gasification by the Ca catalyst, which appeared to be related to the reverse shift reaction in this study. Thus, the reverse shift reaction had a large influence when the reaction temperature was high.
CO
CO2
CH4
C2H4
C2H6
None
700 800 900
1.6 5.0 8.9
0.54 1.3 3.2
0.9 2.1 3.0
0.32 0.49 0.60
0.11 0.18 0.23
0.02 0.02 0.01
Ca
700 800 900
5.9 11 13
1.0 4.0 5.9
2.5 3.9 3.5
0.29 0.57 0.65
0.09 0.21 0.25
0.02 0.01 0.03
a
H2O = 20 vol% and N2 = balance.
The relationship between H2/CO ratio and steam conversion in the CDBFB is shown in Fig. 12. Steam conversion using Ca2+-exchanged coal was maximized at 900 °C. The quantitative relationship between the H2/CO ratio and the Ca2+-exchanged coal showed the opposite relationship as that for the raw coal at 700 °C to 800 °C. Therefore, the 411
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Fig. 10. Gas concentration as a function of reaction time for fixed-bed gasification at 700 °C under fuel gas using (a) raw coal and (b) Ca2+-exchanged coal.
investigation suggests that the use of Ca2+-exchanged coal led to a reverse shift reaction and steam gasification in the 800–900 °C temperature range. The H2/CO ratios of raw coal and Ca2+-exchanged coal decreased simultaneously from 800 °C to 900 °C. When the temperature reached 900 °C, the Ca crystalline size increased and the catalytic activity decreased. Thus, it was considered that the H2/CO ratio from the Ca2+-exchanged coal was reduced in the same way as in raw coals.
A comparison of carbon conversion using the CDBFB with that using the FB showed that carbon conversion using the CDBFB at 700 °C was very low. Regarding an examination of this influence in detail, the reason for this was that the existing H2 adhered primarily to the active sites of the char, and the use of Ca2+-exchanged coal led to a reverse shift reaction. Since it is well known that low-rank coals such as AD coal have similar surface functionalities and ash compositions, it is possible that a Ca catalyst will show high conversion performance during gasification of such coals. This point should be examined in detail in future studies.
4. Conclusions Calcium ion catalysts were loaded onto low-rank coal samples (150–250 µm particle size) using an ion exchange method to obtain a coal with 3.2 mass% Ca. Subsequent gasification of the ion-exchanged samples was performed in the CDBFB at 700 °C, 800 °C, and 900 °C. As the temperature increased, carbon conversion to CO, CO2, and C1–C2 hydrocarbons increased, reaching 80% at 900 °C. The Ca catalyst resulted in the greatest rate enhancement at 800 °C, for which the amount of product gas was twofold greater than that without Ca.
5. Notes The authors declare no competing financial interest. Acknowledgments This work was supported in part by Japan Coal Energy Center
Fig. 11. Gas concentration as a function of reaction time for fixed-bed gasification at 900 °C under fuel gas using (a) raw coal and (b) Ca2+-exchanged coal. 412
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Fig. 12. Relationship between H2/CO ratio and steam conversion in the circulating dual bubbling fluidized bed.
(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. References [1] Hayashi J-I, Hosokai S, Sonoyama N. Trans IChemE Part B Process Saf Environ Prot 2006;86:409–19.
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Full Length Article
Catalytic effect of ion-exchanged calcium on steam gasification of low-rank coal with a circulating fluidized bed reactor
T
⁎
Naoto Tsubouchia, , Yuuki Mochizukia, Yuji Shinoharaa, Yuu Hanaokab, Yasuo Ohtsukab, Koji Kuramotoc, Koichi Matsuokac a
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 c Research Institute of Energy Frontier, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8569, Japan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Sub-bituminous coal Ion-exchange Steam gasification Circulating dual bubbling fluidized bed (CDBFB) Fixed bed (FB)
Calcium ion exchange was performed in Indonesian low-rank coal using saturated aqueous solutions of Ca(OH)2 at 30.0 ± 0.1 °C without pH adjustment. The Ca2+ concentration and pH were monitored during ion exchange. A coal product was obtained with 3.2 mass% Ca2+. Gasification of the ion-exchanged samples was conducted in a circulating dual bubbling fluidized bed (CDBFB) reactor at 700 °C, 800 °C, and 900 °C. Coal carbon conversion into carbonaceous gases (CO, CO2, CH4, and C2 hydrocarbons) using the Ca2+-exchanged coal achieved 78% at 900 °C in the CDBFB. The Ca catalyst led to the greatest rate enhancement at 800 °C, resulting in a twofold increase in the gas product yield compared to that without Ca. A comparison of carbon conversion between the CDBFB and fixed-bed reactors revealed that the conversion value using the CDBFB at 700 °C was very small. Regarding an examination of this influence in detail, it seems that the existing H2 adhered primarily to the active sites of the char, and the use of Ca2+-exchanged coal led to a reverse shift reaction.
1. Introduction Improving the efficiency of coal is a high priority for reducing CO2 output. Systems that have performed well include ultra-supercritical pulverized coal combustion, integrated coal gasification combined cycle (IGCC), integrated gasification fuel cells (IGFC), and polygeneration. Moreover, the utilization of low-rank coal, a potentially plentiful resource, is also important to establish a stable future energy supply. Despite this, at present, there is little economic incentive to burn low-rank coal because of the associated production costs. Consequently, in the publication Strategic Technical Platform for Clean Coal Technology (STEP CCT), the New Energy and Industrial Technology Development Organization (NEDO) in Japan describes the development of the IGCC system using low-rank coal gasification at or below 900 °C in the presence of catalysts. In addition, advanced IGCC and advanced IGFC systems with exergy recuperation was proposed in order to increase the thermal efficiency of IGCC systems [1–7]. In regards to these systems, steam recovers the exhaust heat from a solid oxide fuel cell or
a gas turbine. Hot exhaust gas cools greatly in an entrained-flow bed reactor [8]. Therefore, our group adopted a circulating dual bubbling fluidized bed (CDBFB) reactor as it induces a heat insulation effect is obtained. This causes the exhaust gas in the system to maintain its temperature when the CDBFB reactor is used. As for the strong volatile–char interaction and reduction of tar emissions, a downer pyrolyzer has a major influence on subsequent steam gasification using a novel triple-bed combined circulating fluidized bed (TBCFB) reactor [1,9,10]. For instance, steam gasification of Adaro coal (particle size, dp = 0.5–1.0 mm) char at 700–900 °C was studied using a lab-scale TBCFB [9,10]. Coal pyrolysis has been performed previously in a drop-tube reactor (DTR) [11–16] and a downer reactor [17–19]. The reactivity of Loy Yang coal chars and those blended with coal at 50%–85% ratios was investigated using a DTR at 900 and 950 °C [15]. The reactivities of Na and Ca in the raw coal were investigated using the same DTR at 850–900 °C [20]. However, catalytic steam gasification of Ca2+-exchanged low-rank coal using the CDBFB and the TBCFB reactor has not
Abbreviations: CDBFB, circulating dual bubbling fluidized bed; FB, fixed bed; IGCC, integrated coal gasification combined cycle; IGFC, integrated gasification fuel cells; STEP CCT, Strategic Technical Platform for Clean Coal Technology; NEDO, the New Energy and Industrial Technology Development Organization; TBCFB, triplebed combined circulating fluidized bed; DTR, drop-tube reactor; XRD, X-ray diffraction ⁎ Corresponding author. E-mail address: [email protected] (N. Tsubouchi). https://doi.org/10.1016/j.fuel.2018.07.046 Received 2 April 2018; Received in revised form 9 July 2018; Accepted 10 July 2018 0016-2361/ © 2018 Published by Elsevier Ltd.
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been studied. As regards a catalyst with steam gasification of coal, achieving lowtemperature gasification requires application of a catalyst. Our previous results show that catalytic steam gasification of low-rank coal with ionexchanged Ca prepared from saturated Ca(OH)2 solution was an excellent catalyst for use in a fixed-bed reactor [21]. Therefore, we expected this material to show similar performance in a CDBFB reactor. The present study thus examines the effect of Ca catalyst on steam gasification of a sub-bituminous coal in a CDBFB system. If the inexpensive catalyst can decrease the gasification temperature and increase the gasification rate, it may present several advantages, such as the circulation of large amounts of coal particles and enhanced-exergy recuperation in the CDBFB system.
Table 2 Metal concentrations in the Adaro coal sample. Metal concentrationa (mass%, dry) Na
Al
Si
K
Ca
Fe
0.010
0.18
0.41
0.014
0.18
0.24
a Determined by inductively coupled plasma emission spectroscopy (ICP-ES) after acid leaching of ash obtained by pyrolysis at 815 °C.
Table 3 Ca2+ ion-exchange reaction conditions. Coal (g)
2. Experimental 2.1. Coal samples
60.00
Saturated calcium hydroxide solution Ca(OH)2 (g)
Deionized water (ml)
Ca2+ (mg/ L)
Reaction temperature (°C)
3.6
3000
649
30.0 ± 0.1
Indonesian sub-bituminous coal (Adaro) was used, denoted AD, because this coal was one of standard samples in the STEP CCT project. The coal sample was air-dried at room temperature, crushed, and sieved to obtain particles with a size range of 150–250 µm. Table 1 shows the results from compositional analyses. Table 2 summarizes the metal analysis (Na, Al, Si, K, Ca, and Fe) of the coal and shows that the main constituent elements were Si, Al, Fe, and Ca. 2.2. Catalyst materials and ion-exchange methodology Saturated Ca(OH)2 (Wako Pure Chemical Industries. Ltd.) was used as the precursor for the ion-exchanged Ca catalyst. The size fraction of coal samples as received was in the range of 150–250 μm. The samples were used without any pretreatment. As in our previous study [21], a saturated Ca(OH)2 solution was added to a suspension of coal particles and deionized water, which was held at constant temperature (30.0 ± 0.1 °C) without pH adjustment. The Ca2+ concentration and pH of the mixture during stirring were monitored using Ca2+-selective and pH electrodes (Table 3). The Ca2+exchange reaction used 60 g of coal per reaction for a total of 5 exchange reactions. The initial cation concentration was 649 mg Ca/L (Ca2+ ions). The amount of Ca2+ ions actually exchanged was calculated using the following equation:
Exchange amount (mass %) change in concentration before and after the reaction (mg =
/L) × the solution quantity (L) weight of coal after drying (mg)
× 100 Fig. 1. Schematic drawing of the circulating dual bubbling fluidized bed reactor used in this study.
(1) After completion of the ion-exchange reaction, the Ca2+-exchanged coal was separated by filtration, washed with high-purity water, dried under vacuum at 60 °C, and then classified according to particle size (150–250 µm). The recovery ratio of Ca2+-exchanged coal to total coal amount (60 g × 5 times = 300 g) was 72%–75%. The total amount of obtained Ca2+-exchanged coal was about 220 g.
2.3. Circulating dual bubbling fluidized bed reactor and product gas analysis A CDBFB reactor was used for our experiments [22]. Fig. 1 shows a schematic drawing of the CDBFB reactor. The size and reaction temperature of the apparatus are shown in Table 4, and the gasification conditions are shown in Table 5. The coal feed rate was 1.0 g/min. The samples were supplied steam (H2O/coal = 1.4–1.6 kg/kg, 20 vol% H2O/N2) and silica sand heating-medium particles (150–250 µm; 95 mass% SiO2) into the top of the fluidized bed in the gasifier with a screw feeder in the CDBFB. Our previous steam gasification results using woody biomass using a CDBFB reactor showed that the increase of carbon conversion with S/C was not observed at higher S/C (1 < S/ C < 3, i.e., 12–36 vol% steam concentration) [22]. Accordingly, 20 vol % steam concentration (S/C = 1.5) was adopted. The formed gas and tar captured by the siliceous and unconverted char were transported
Table 1 Ultimate and proximate analyses of Adaro. Ultimate analysis (mass%, dafa)
Proximate analysis (mass%, dry)
C
H
N
S
Ob
Ash
VMc
FCb,c
67.8
5.1
0.44
0.14
26.5
2.5
46.7
50.8
a b c
daf = dry ash free. Determined by difference. VM = volatile matter; FC = fixed carbon. 407
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Table 4 Size of the circulating dual bubbling fluidized bed reactor and reaction temperature. Gasifier
Combustor
Riser
Inner diameter (mm)
Length (mm)
Inner diameter (mm)
Length (mm)
Temperature (°C)
Inner diameter (mm)
Length (mm)
Temperature (°C)
80
270
80
150
950
18
1800
> 300
upward through the riser. The product gas was separated with a cyclone separator and was analyzed using high-speed gas chromatography and detector tubes (Table 6). The char and (coke-deposited) silica sand were transported to the combustor by a loop seal. The regenerated silica sand was transported back to the gasifier by a downer and a pneumatic Lvalve. Large carbon conversion differences were found between pyrolysis and steam gasification. As a first step, a simple comparative analysis was performed with two samples (row coal and Ca2+-exchanged coal) in order to determine the catalytic activities.
Table 6 Gas analysis conditions. Analysis method
Detected gas
High-speed gas chromatography Detector tube
H2, CO, CO2, CH4, C2H4, C2H6 C6H6, C7H8, C8H10
demonstrate that ion exchange proceeded and suggest that H+ on the COOH groups and/or phenol OH groups in the coal were exchanged with Ca2+.
2.4. Fixed-bed gasification reactor and product analysis
3.2. Pyrolysis and steam gasification
To compare carbon conversion between the CDBFB reactor and fixed-bed (FB) reactor, catalytic pyrolysis and gasification of Ca2+-exchanged coal was performed in an FB quartz reactor at 700 °C. 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. It was then held at this temperature for 10 min to remove volatile matter, and it was finally exposed to 50 vol% H2O in He to gasify the resulting char in situ for 1 h. The CDBFB conversion ratio was less than that obtained using the FB, and it appeared that coexisting H2 and CO adhered primarily to the active sites of the char. To examine this influence in detail, the test sample was reacted with steam using steam mixing fuel gas (H2O/H2/ CO/CO2/He = 50 vol%/28 vol%/7.0 vol%/13 vol%/2 vol%) instead of He at 700 °C and 900 °C. The gases produced during this process were analyzed after removal of H2O using two online micro gas chromatographs (GCs) with thermal conductivity detectors to determine the concentrations of H2, CO, CO2, and CH4 [23].
Fig. 3 shows gas yield plots for raw coal and Ca2+-exchanged coal as a function of reaction time at 900 °C. The holding time ranged from 20 to 60 min (solid residence time was 20 min) because the steady state was only achieved 20 min after the samples were placed in the reactor, resulting in stable yield of product gases after 60 min. In addition, the reaction remained stable for up to 75 min, and the most stable value (60 min) was adopted in this study. Stability of the reaction system was confirmed in the preliminary study. Circulation of the silica sand bed material was significantly dependent of aeration of L valve. The circulation could be controlled by appropriate control of the aeration. The main product gases were H2, CO, and CO2. However, pyrolysis and steam gasification occurred in the reaction system simultaneously, yielding small amounts of CH4, C2H4, and C2H6 from pyrolysis. Fig. 4 shows carbon conversion to gas from raw coal and Ca2+-exchanged coal as a function of temperature (700 °C, 800 °C, and 900 °C). Coal carbon conversion was calculated using gas chromatography measurement of CO, CO2, CH4, and C2 hydrocarbons with the following equation (2):
2.5. Characterization
Coal carbon conversion (%) = Selected char samples were analyzed using X-ray diffraction (XRD).
CO+CO2 + CH 4 + C2 hydorocarbon (mol) Coal carbon content (mol) (2)
× 100 3. Results and discussion
Coal carbon conversion to gas was approximately 15% at 700 °C and 56% at 900 °C. In contrast, Ca2+-exchanged coal carbon conversion was approximately 28% at 700 °C and 78% at 900 °C. In this experiment, Ca2+-exchanged carbon conversion was 78% at 900 °C. Thus, carbon conversion could be improved if the steam/coal ratio was increased. Pyrolysis and steam gasification were performed in the pressurized fluidized bed gasification system using 4.4 mass% Ca2+-exchanged coal according to Takarada et al. [24]. The results indicate that increasing the steam/coal ratio from 2.5 to 5.0 promoted gas production from coal in the coal tar and increased the coal conversion. By contrast, our previous results addressing steam gasification
3.1. Catalyst material and ion-exchange The ion exchange method applied to AD coal with 150–250 µm particle size resulted in a product with 3.2 mass% Ca. Fig. 2 shows the Ca2+ concentration (Fig. 2a) and pH (Fig. 2b) of the ion exchange solutions as a function of reaction time. The Ca2+ concentration underwent a sudden increase upon addition of the Ca(OH)2 solution to the coal (Fig. 2a), which then decreased and stabilized at 100 mg/L after 15 min, while pH decreased with time (Fig. 2b). These results Table 5 Gasification conditions and temperature. Gasification temperature (°C)
700–900 a
Medium particles Composition
Size (mm)
Surface area (m2/g)
SiO2 > 95 mass %
0.15–0.25
1.0
Solid residence time. 408
Coal feed rate (g/min)
Steam feed rate S/C (kg/kg)
Residence timea (min)
1.0
1.4–1.6
20
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N. Tsubouchi et al.
(a) 700
35
(b) 13
34 33
500
Temperature, ºC Temperature,
400
12
30
pH pH
300
31
29 28
200
Temperature,
32 pH
Ca2+ concentration, mg/L
600
27 100 0
26 0
5
10
11 0
15
Time, min
5
Time, min
10
25 15
Fig. 2. (a) Calcium ion concentration and (b) pH of ion-exchange solutions as a function of reaction time.
using woody biomass with a CDBFB reactor showed that carbon conversion did not increase at higher S/C (1 < S/C < 3, i.e., 12–36 vol% steam concentration) [22]. Further consideration is required to deduce any findings regarding the steam/coal ratio using Ca2+-exchanged coal in a CDBFB reactor.
3.3. Product gas composition The composition of product gases using raw coal and Ca2+-exchanged coal is shown in Table 7. The total amount of gases, which included H2, CO, CO2, CH4, and C2H4 increased with temperature regardless of the presence of a catalyst. The Ca catalyst led to the greatest rate enhancement at 800 °C, resulting in a twofold increase in the gas product yield compared to that without Ca. For Ca2+-exchanged coal, CH4 and C2H4 production must occur due to the decomposition of coal tar or due to steam reformation after decomposition into H2 and CO. A previous report indicated that the amount of coal tar produced by pyrolysis was small for Ca2+ supported on demineralized charcoal [25]. Therefore, future investigations should examine coal-tar decomposition on Ca2+-exchanged coal. The relationship between carbon conversion and product gas yield is shown in Fig. 5. The fraction of hydrocarbons decreased in the presence of the Ca catalyst (dotted circles in Fig. 5), suggesting that
Fig. 4. Coal carbon conversion to gas from raw coal and Ca2+-exchanged coal after 20 min as a function of temperature (700 °C, 800 °C, and 900 °C).
Fig. 3. Gas yield from (a) raw coal and (b) Ca2+-exchanged coal as a function of reaction time at 900 °C. 409
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samples prepared at 800 °C and 900 °C in the CDBFB. The XRD diffraction patterns of the Ca2+-exchanged samples at 700 °C and 800 °C didn’t contain peaks due to the presence of the Ca species. Only small amounts of Ca were present in the samples, and separation of the silica sand and the Ca2+-exchanged coal was difficult. The XRD diffraction patterns for the Ca2+-exchanged samples at 900 °C produced peaks attributed to Ca species, obviously due to the presence of Ca. After successful coal gasification at high temperature (900 °C), the concentration of Ca seems to increase due to coal loss. The CaO crystalline size was approximately 38 nm in the Ca2+-exchanged sample prepared at 900 °C. This value was larger than that of 20 nm obtained using FB gasification at 700 °C, which was presented previously [23]. The low reaction temperature improved the maximum catalytic activity because Ca particle aggregation accelerates at high temperatures. Therefore, the Ca2+-exchanged coal carbon conversion had the greatest rate enhancement at 800 °C (Fig. 4).
Table 7 Temperature dependence of product gas yield (CDBFB). Catalyst
Temperature (°C)
Gas yield (mol/100 mol C in coal) Total
H2
CO
CO2
CH4
C2H4
C2H6
None
700 800 900
28 74 130
13 41 73
4.4 10.0 26.0
7.4 17 24
2.7 4.1 4.9
0.90 1.5 1.9
0.19 0.17 0.05
Ca
700 800 900
73 150 173
44 86 96
7.4 30.0 44.0
19 29 26
2.1 4.3 4.9
0.66 1.6 1.9
0.16 0.05 0.20
3.5. Comparison of carbon conversion between the circulating dual bubbling fluidized bed and fixed-bed reactors Catalytic steam gasification was performed on the ion-exchanged coal in a FB quartz reactor at 700 °C in order to predict a carbon conversion value with CDBFB reactor using data from the FB. Fig. 8 shows carbon conversion to CO and CO2 after 1 h with the CDBFB and FB reactors. Carbon conversion during steam gasification was calculated using gas chromatography using Eq. (2). Carbon conversion at 700 °C in the CDBFB was much lower than that in the FB, and a temperature of 100 °C was required to realize carbon conversion equal to that from the FB. It is still unknown why carbon conversion in the CDBFB at 800 °C and the value in the FB at 700 °C were nearly equal. The actual gasification time with the CDBFB was 20 min (solid residence time was 20 min); whereas, the reaction time using the FB was 1 h. Different gasification times seem to influence carbon conversion in each reactor. As a result, we could not predict a carbon conversion value for the CDBFB reactor using data from the FB reactor.
Fig. 5. Relationship between carbon conversion and product gas yield.
H2
Raw
3.6. Influence of fuel gas in fixed-bed gasification
CO
900 ºC
CO2
Ca
The conversion ratio using CDBFB was less than that obtained using FB. It was previously reported that H2 and/or CO interfere with the active site [26]. Therefore, this influence was investigated. To examine this influence in detail, steam was reacted using steam mixing fuel gas (H2O/H2/CO/CO2/He = 50 vol%/28 vol%/7.0 vol%/13 vol%/2 vol%) instead of He at 700 °C or 900 °C (Fig. 9). Carbon conversion during steam gasification was calculated using gas chromatography data via Eq. (2). Conversion of raw coal and Ca2+-exchanged coal at 700 °C were small using fuel gas. Upon raising the temperature to 900 °C, no effect of Ca-catalytic activity was found. Table 8 shows the gas concentration in the CDBFB, which was used to calculate gas yield. The H2 concentration using Ca2+-exchanged coal at 900 °C was 13 vol%, which was the greatest concentration. The influence of evaporation velocity appeared to be large. Fig. 10 shows gas concentration as a function of reaction time at 700 °C using FB gasification under a fuel gas atmosphere. The values in Fig. 10 were calculated using a dry gas standard (H2/CO/CO2/ He = 56 vol%/14 vol%/26 vol%/4 vol%). For Ca2+-exchanged coal, CO concentration increased while the concentrations of H2 and CO2 decreased. Therefore, the following reverse shift reaction occurred alongside steam gasification under these conditions:
CH4 C2H4
Raw
C2H6
800 ºC
Ca
Raw 700 ºC
Ca 0
10 20 30 40 50 60 70 80 90 100 Composition, mol%
Fig. 6. Composition of formation gases at 700 °C, 800 °C, and 900 °C in the circulating dual bubbling fluidized bed.
hydrocarbon reforming occurred. The composition of product gases at 700 °C, 800 °C, and 900 °C using CDBFB is shown in Fig. 6. At the same temperature, it was found that the Ca2+-exchanged coal has a smaller proportion of hydrocarbons compared to raw coal. In addition, it was found that the H2/CO ratio becomes larger compared with other temperatures when the Ca catalyst is present at 700 °C.
3.4. States of the catalysts Fig. 7 presents XRD diffraction patterns for the Ca
2+
C+ H2 O→ CO + H2
(5)
CO2 + H2 → CO + H2 O
(6)
Furthermore, the catalytic activity of Ca by the formation of a
-exchanged 410
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Diffraction intensity
CaO SiO2 CaCO3
900
800
10
20
30
40
50
60
2 (Cu K ), degrees Fig. 7. XRD diffraction patterns of Ca2+-exchanged samples at 800 °C and 900 °C in the circulating dual bubbling fluidized bed.
100 CDBFB,
FB; Ca2+-exchanged coal
CDBFB,
FB; Raw coal
Coal carbon conversion to gas, %
Coal carbon conversion to gas, %
100
80
60
40
20
0 650
700
750
800
850
Temperature, ºC
900
Fig. 8. Comparison of carbon conversion using the circulating dual bubbling fluidized bed and the fixed-bed after 1 h.
H2O/Fuel (FB),
CDBFB; Ca2+-exchanged coal
H2O/Fuel (FB),
CDBFB; Row coal
80
60
40
20
0 650
950
H2O/He (FB), H2O/He (FB),
700
750
800
850
Temperature, ºC
900
950
Fig. 9. Temperature dependence of carbon conversion during exposure to fuel gas and H2O/He in the circulating dual bubbling fluidized bed and the fixedbed reactor.
surface peroxide was explained by Sears et al. [27]:
CaO + CO2 → CaO·[O] + CO
(7)
CaO·[O] + C→ CaO + C[O]
(8)
C[O] → CO
(9)
Table 8 Product gas concentration (CDBFB). Catalyst
Temperature (°C)
Gas concentrationa (vol%) H2
(CaO·[O] and C[O]; Reactive intermediate.) The above catalytic activity of Ca in reactions (7)–(9) seems to have occurred in addition to the steam gasification reaction. Fig. 11 shows the gas concentration as a function of reaction time for FB gasification at 900 °C during exposure to fuel gas using raw coal and Ca2+-exchanged coal. A decrease in H2 and CO2 concentrations and an increase in CO concentration were promoted compared to the results obtained at 700 °C. Kyotani et al. examined the inhibitory reaction of H2 with CO2 gasification on the Ca2+-exchanged coal char [26]. The results indicate that adherence of H2 to the boundary-surface neighborhood of carbon and the Ca catalyst hinders gasification by the Ca catalyst, which appeared to be related to the reverse shift reaction in this study. Thus, the reverse shift reaction had a large influence when the reaction temperature was high.
CO
CO2
CH4
C2H4
C2H6
None
700 800 900
1.6 5.0 8.9
0.54 1.3 3.2
0.9 2.1 3.0
0.32 0.49 0.60
0.11 0.18 0.23
0.02 0.02 0.01
Ca
700 800 900
5.9 11 13
1.0 4.0 5.9
2.5 3.9 3.5
0.29 0.57 0.65
0.09 0.21 0.25
0.02 0.01 0.03
a
H2O = 20 vol% and N2 = balance.
The relationship between H2/CO ratio and steam conversion in the CDBFB is shown in Fig. 12. Steam conversion using Ca2+-exchanged coal was maximized at 900 °C. The quantitative relationship between the H2/CO ratio and the Ca2+-exchanged coal showed the opposite relationship as that for the raw coal at 700 °C to 800 °C. Therefore, the 411
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Fig. 10. Gas concentration as a function of reaction time for fixed-bed gasification at 700 °C under fuel gas using (a) raw coal and (b) Ca2+-exchanged coal.
investigation suggests that the use of Ca2+-exchanged coal led to a reverse shift reaction and steam gasification in the 800–900 °C temperature range. The H2/CO ratios of raw coal and Ca2+-exchanged coal decreased simultaneously from 800 °C to 900 °C. When the temperature reached 900 °C, the Ca crystalline size increased and the catalytic activity decreased. Thus, it was considered that the H2/CO ratio from the Ca2+-exchanged coal was reduced in the same way as in raw coals.
A comparison of carbon conversion using the CDBFB with that using the FB showed that carbon conversion using the CDBFB at 700 °C was very low. Regarding an examination of this influence in detail, the reason for this was that the existing H2 adhered primarily to the active sites of the char, and the use of Ca2+-exchanged coal led to a reverse shift reaction. Since it is well known that low-rank coals such as AD coal have similar surface functionalities and ash compositions, it is possible that a Ca catalyst will show high conversion performance during gasification of such coals. This point should be examined in detail in future studies.
4. Conclusions Calcium ion catalysts were loaded onto low-rank coal samples (150–250 µm particle size) using an ion exchange method to obtain a coal with 3.2 mass% Ca. Subsequent gasification of the ion-exchanged samples was performed in the CDBFB at 700 °C, 800 °C, and 900 °C. As the temperature increased, carbon conversion to CO, CO2, and C1–C2 hydrocarbons increased, reaching 80% at 900 °C. The Ca catalyst resulted in the greatest rate enhancement at 800 °C, for which the amount of product gas was twofold greater than that without Ca.
5. Notes The authors declare no competing financial interest. Acknowledgments This work was supported in part by Japan Coal Energy Center
Fig. 11. Gas concentration as a function of reaction time for fixed-bed gasification at 900 °C under fuel gas using (a) raw coal and (b) Ca2+-exchanged coal. 412
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