Article pubs.acs.org/EF
Gasification of Indonesian Sub-bituminous Coal with Different Gasifying Agents Using Ca and K Catalysts Shumin Fan,* Xiangzhou Yuan, Joo-Chang Park, Li-Hua Xu, Tae-Jin Kang, and Hyung-Taek Kim* Department of Energy Systems Research, Ajou University, Woncheon-Dong, Youngtong-Gu, Suwon 443-749, Republic of Korea S Supporting Information *
ABSTRACT: Syngas, liquid fuels, and chemicals could be obtained through coal gasification. The catalytic effect of K2CO3 and CaCO3 on gasification with H2O/CO2 mixtures was studied in this research. The syngas production and char reactivity were compared with different gasifying atmospheres by using two catalysts. The H2/CO mole ratio was between 0.5 and 3 by varying the H2O/CO2 ratios, which could be used for the chemical synthesis. Higher carbon conversion during char gasification was achieved with the mixed atmosphere (H2O/CO2 = 80/20) than with pure steam and CO2 atmospheres by using both catalysts. Further, the coal sample was investigated by Brunauer−Emmett−Teller measurements and emission scanning electron microscopy, and the higher surface area of coal particles and highly dispersed catalyst may be responsible for the observed higher reactivity with mixed gasifying agents.
1. INTRODUCTION Coal is a major source of energy. Coal gasification is “clean coal” technology to convert solid fuel into gaseous fuel.1 Coal gasification can generate energy in the form of synthesis gas (syngas), including H2, CO, CO2, and CH4. Syngas is a versatile feedstock for a number of high value products, including Fischer−Tropsch (F-T) hydrocarbons2,3 and hydrogen,4,5 or applied to power generation.6 Gasification, which is endothermic, requires a temperature higher than 1000 °C to reach the required rate for commercial application. Catalytic gasification of coal could reach acceptable reaction rates even at low temperature. Alkali and alkaline earth metals (AAEM) have been proved effective for gasification.7−10 The most significant catalysts for gasification are K and Ca species.11 Kopyscinski et al. used K2CO3 as catalyst for gasification. The reaction rate increased 20 times, and the activation energy decreased 100 kJ mol−1 compared with that of gasification without catalyst.8 Murakami et al. used CaCO3 as catalyst during steam gasification and found that the catalyst was finely dispersed and effectively improved the carbon conversion even at low catalyst loadings.12 The contrast of catalytic effect of K and Ca was obtained during CO2 gasification of biomass.11 The difference was investigated at lower temperatures; the activity of Ca catalytic gasification initially kept ahead, but decreased earlier than that of K. The two catalysts showed similar gasification reactivity at 850 °C. In this study, K2CO3 and CaCO3 were used as catalysts to improve the reactivity of low rank coal gasification. Numerous studies have been performed to study the gasification with H2O/CO2 mixtures that reached fundamentally different conclusions about surface reaction mechanisms. The active sites have a great influence on char gasification because the gas molecules could only be adsorbed on active sites to start the gasification reaction.13 Guizani et al. used a macro-TGA reactor to study gasification with H2O/CO2 mixtures, and the reactivity of gasification with H2O/CO2 mixtures was the sum of the reactivity using pure H2O and © XXXX American Chemical Society
CO2, which confirmed that H2O and CO2 react on separate active sites.14 Chen et al. studied the gasification with H2O/ CO2 mixtures and found that the char gasification rate with the mixtures showed competitive advantages compared with each individual reaction. However, it is lower than the sum of them. It indicated that H2O and CO2 should partly react on the same active sites.13 Besides these two types of results, the synergy effect between H2O and CO2 was also found to enhance the gasification reactivity with H2O/CO2 mixtures. Bai et al. investigated gasification using H2O/CO2 mixtures, and found that the mixed gasifying agent improved the char reactivity, higher than the sum of the individual reactivity using either H2O or CO2 below 900 °C. The high catalytic activity of Ca in minerals was responsible for the higher gasification rate in mixed H2O/CO2.15 Wang et al. studied the role of minerals that existed in coal during gasification in H2O/CO2 mixtures and concluded that the calcite present in coal caused the synergistic effect with H2O/CO2 mixtures.16 Therefore, it is better to study further the effect of catalysts for gasification with H2O/CO2 mixtures. In this study, the catalytic effect of K2CO3 and CaCO3 on gasification of low rank coal with H2O/CO2 mixtures was investigated. The mole ratio of H2/CO was adjusted to different values at low temperatures, which could be used as a feedstock for F-T synthesis. Furthermore, the char structure and char reactivity were studied by BET, SEM, and XRD analyses, and the catalysts were characterized to provide additional understanding about their behavior. The objective of this research is to study the gasification of coal with H2O/ CO2 mixtures and determine the role of the catalyst. Received: June 1, 2016 Revised: October 24, 2016 Published: October 25, 2016 A
DOI: 10.1021/acs.energyfuels.6b01329 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Proximate and Ultimate Analyses of KPU Coal proximate analysis (as-received basis, wt %)
ultimate analysis (dry, ash-free basis, wt %)
sample
moisture
ash
volatile matter
fixed carbon
C
H
N
S
O
KPU
16.7
3.0
39.1
41.2
75.9
5.3
1.3
0.3
17.2
2.3. Structure Analysis of Samples. Surface structure parameters of samples were carried out using Brunauer−Emmett−Teller (BET) analysis and Barrett−Joyner−Halenda (BJH) analysis methods. Physical adsorption of N2 isotherms at 77 K was measured with a Micromeritics Tristar 3000 instrument; the samples were pretreated at 100 °C for 8 h. The sample morphologies of samples were analyzed with a JEOL JSM-6700F field emission scanning electron microscope (SEM) with a Schottky field emission gun at an accelerating voltage of 20 kV. X-ray diffraction (XRD) analysis of the samples was performed on a Rigaku high-power X-ray diffractometer using the Cu Kα1 line (1.5406 Å) obtained at 40 kV/40 mA, with 2θ ranging from 10° to 80° with 0.020° steps and a dwell time of 0.05 s/step.
2. EXPERIMENTAL SECTION 2.1. Samples. Indonesian sub-bituminous KPU coal was utilized. The raw coal was ground and meshed to 300−500 μm. Proximate analysis and ultimate analysis were conducted on a TGA-701 Thermogravimeter (LECO Co., USA) and a chn-2000 Elemental Analyzer (LECO Co., USA). The properties of KPU are summarized in Table 1. The impregnation method was used for catalyst loading. The K2CO3 added was intended to be about 10 wt %. A 10 g portion of K2CO3 was added to 100 g of KPU coal to prepare the mixture. After adding water, the paste was dried at 105 °C for 24 h. Pure CaCO3 was loaded with KPU by the same method with the weight fraction of 5 wt %. The catalyst loading was determined by comparing the gasification reactivity and economics. 10 wt % of K2CO3 and 5 wt % of CaCO3 were used for the comparison of gasification with two catalysts, and the details are shown in Figure S1 in the Supporting Information. 2.2. Gasification. The fixed-bed reactor used in this research is shown in Figure 1, which is 22.5 mm in diameter (ID) and 200 mm in
3. RESULTS AND DISCUSSION 3.1. Catalytic Effect on Syngas with Different Gasifying Agents. The composition of syngas is closely related to its potential uses. Figure 2 shows the syngas molar yields during gasification with two catalysts at 800 °C. The steam gasification with CaCO3 had a higher syngas yield of CO, CO2, and H2 (the major product) than that with K2CO3, with 1.42 mol-/mol-C (58 vol %) H2 and 0.42 mol-/mol-C (17 vol %) CO. However, the gasification with K2CO3 catalyst produced a higher amount of CO (the major product) than that with CaCO3 during CO2 gasification. The CO production with K2CO3 catalyst during CO2 gasification was 1.19 mol-/mol-C (87 vol % without CO2), a 59% increase compared with gasification with CaCO3. For gasification with the mixed atmosphere (H2O/CO2 = 80/20), the production of CO and H2 during gasification with K2CO3 is both higher than that with CaCO3. During K2CO3 catalytic gasification with H2O/CO2 = 80/20, the H2 and CO production was 0.91 and 0.9 mol-/mol-C, 8% and 29% increase compared with gasification with CaCO3. Regarding the production of CO and H2, the K2CO3 catalyst performed better than CaCO3 during gasification with the mixed atmosphere (H2O/CO2 = 80/20). It could be seen that both K2CO3 and CaCO3 are effective for coal char gasification with different gasifying atmospheres for producing a higher amount of syngas than that of raw coal gasification without catalyst. Table 2 shows the H2/CO mole ratio during gasification with different gasifying atmospheres. It could be seen that the H2/ CO mole ratio decreased with the increase of temperature, which is closely related to the reaction rate and equilibrium during gasification. When steam is used as the gasifying agent, H2 is produced from steam char gasification and water gas shift (WGS) reaction. The WGS reaction is exothermic, while the Boudouard reaction (the resultant is CO) is endothermic. As a result, the growth of H2 production becomes slower with the increase of temperature, while the growth of CO production is accelerated, resulting in the decrease of the H2/CO mole ratio. At 800 °C with two kinds of catalyst, the H2/CO mole ratio was almost equal to 0.5 when H2O/CO2 = 50/50, while the ratio increased to 1 when H2O/CO2 = 80/20, which could be used for the F-T process. The F-T synthesis process is an indirect method of producing liquids/chemicals. On the basis of the target product, different H2/CO mole ratios are required, for example, H2/CO = 1 for DME production and H2/CO = 2 for methanol production.17 Different molar ratios of H2/CO were required according to the desired product or processing
Figure 1. Schematic diagram of the catalytic steam gasification process. height. First, the temperature was increased to 800 °C from room temperature under a nitrogen atmosphere (99.9 vol %, Han-il Gas Inc.; flow rate: 1 L/min). Subsequently, 2.000 g of KPU coal loaded with catalyst was pyrolyzed at 800 °C for 30 min. The resultant char after pyrolysis was gasified by introducing a mixture of steam/CO2 (200 mL/min). The steam and CO2 was mixed to different ratios: 100% steam, 80% steam + 20% CO2 (H2O/CO2 = 80/20), 50% steam + 50% CO2 (H2O/CO2 = 50/50), and 100% CO2 (on volume basis). Steam in the out-going gas was removed through the cooling system, followed by a filter device for cleaning. The dried gas, including H2, CO, CO2, and CH4, was analyzed by a nondispersive infrared sensor (NDIR, A&D 9000 Series). B
DOI: 10.1021/acs.energyfuels.6b01329 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 2. Syngas molar yields of gasification with different catalysts at 800 °C.
Table 2. H2/CO Mole Ratio during Gasification with Different Gasifying Atmospheres K2CO3
CaCO3
gasification atmosphere
steam
H2O/CO2 = 80/20
H2O/CO2 = 50/50
CO2
steam
H2O/CO2 = 80/20
H2O/CO2 = 50/50
CO2
600 °C 700 °C 800 °C
10.9 5.5 3.9
1.5 1.2 1.0
1.1 0.7 0.5
0.7 0.2 0.1
9.1 4.7 3.4
1.5 1.3 1.2
1.2 0.75 0.6
0.7 0.3 0.2
technology.18 The syngas composition is affected by the raw material, gasifier type, and running conditions (temperature, pressure, catalyst, and gasifying agent).19 To adjust the H2/CO ratio, steam and CO2 were used. Figure 3 shows the production of CO and H2 during gasification with H2O/CO2 mixtures. It is clear that H2/CO was adjusted to 1 and 2 for F-T synthesis by changing the steam/CO2 ratio during char gasification. When the H2/CO ratio of the syngas is not the stoichiometry for the downstream fuel synthesis process, different technologies are needed to adjust the ratio, such as membrane separators, pressure swing adsorption (PSA), and downstream shift reactors.18 With a membrane separator, H2 could be removed selectively through the membrane by permeation, decreasing the H2/CO ratio in the retentate. The polyimide hollow fiber membrane could be used for syngas ratio adjustment, with high performance of separating H2 from
CO.20 PSA technology could be used for hydrogen purification and H2/CO ratio adjustment. Ribeiro et al. used PSA technology to adjust the syngas from biomass gasification for direct methanol synthesis, with simultaneously captures of CO2 for future sequestration.19 The CO-shift reactor could be used to increase the H2 composition. Corella et al. used the CO-shift reactor after biomass gasification, and the H2 content could be increased from 0.8 to 14 vol %.21 Typically, the syngas produced from steam gasification is too rich in hydrogen (H2/ CO = 3.9 with K2CO3) to meet the stoichiometric ratio, while syngas is H2 deficient from gasification with H2O/CO2 = 50/50 (H2/CO = 0.5 with K2CO3). By using the adjustment techniques, syngas is a versatile feedstock for the downstream fuel process. The H2/CO mole ratio was between 0.1 and 3.9 during K2CO3 catalytic gasification with different gasifying agents, while the H2/CO mole ratio was between 0.2 and 3.4 C
DOI: 10.1021/acs.energyfuels.6b01329 Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3. Adjustment of H2/CO mole ratio by varying H2O/CO2 during gasification: (a) H2/CO = 1; (b) H2/CO = 2.
with CaCO3 catalyst. Taking advantage of these adjustment techniques, the H2/CO mole ratio could be adjusted for the typical downstream process for hydrogen or liquid F-T. 3.2. Char Structure and Char Reactivity. The carbon conversion of gasification was calculated by using eq 1
XC =
m 0 − mt m0 − mash
(1)
where m0 is the mass of char in the beginning, mt is the mass of char at time t, and mash is the mass of ash. Figure 4 shows the carbon conversion of gasification with different gasifying agents at 800 °C. It could be seen that higher carbon conversion during char gasification was clearly achieved with the mixed atmosphere (H2O/CO2 = 80/20) than with pure steam and CO2 atmospheres by using both catalysts. During steam gasification, CaCO3 achieved higher carbon conversion, whereas, during CO2 gasification, K2CO3 achieved higher carbon conversion. During gasification with the mixed atmosphere, K2CO3 performed a little better than CaCO3. BET analysis was performed to investigate the structure variations during gasification. Table 3 shows the results of characterization of the porous structure of the char samples after gasification for 20 min. The surface areas of samples
Figure 4. Carbon conversion of gasification with different catalysts using steam/CO2 mixtures at 800 °C.
subjected to various atmospheres with the two catalysts both followed the descending sequence: mixed atmosphere (H2O/ CO2 = 80/20) > steam > CO2, indicating that gasification D
DOI: 10.1021/acs.energyfuels.6b01329 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 3. BET Specific Surface Area of Char at Gasification Time of 20 min K2CO3 gasification atmosphere 2
specific surface (m /g) total pore volume (cm3/g) average pore size (nm)
CaCO3
steam
H2O/CO2 = 80/20
CO2
steam
H2O/CO2 = 80/20
CO2
195 0.38 7.94
256 0.42 6.53
129 0.31 8.11
230 0.39 7.69
239 0.41 7.61
77 0.22 8.36
under the mixed atmosphere generated the largest specific surface areas. Higher surface area of the char promotes the probability of interaction of “active sites” and gasification agent, resulting in higher reactivity.22 Adschiri et al.23 measured the gasification reactivity surface area of samples and found that the apparent reaction rate of char was closely related to BET specific surface areas. SEM photographs of residue after gasification at 800 °C were analyzed (Figures 5 and 6). The images show a very clear
Figure 6. SEM photographs of residue after gasification at 800 °C with CaCO3: (a, b) steam gasification; (c, d) CO2 gasification; (e, f) H2O/ CO2 = 80/20.
lower reactivity during CO2 gasification. During gasification with H2O/CO2 = 80/20, the images of particles were like a coral-reef pattern with K2CO3, and the catalyst distributed evenly through the surface of particles. Compared with the dense structure of coal, the coral-reef like pattern structure has a higher surface area.24 By using CaCO3 during gasification with H2O/CO2 = 80/20, the catalyst also distributed equally, and there were some convex balls on the surface of particles, which may increase the surface area and lead to higher reactivity. Thus, the higher reactivity with mixed gasifying agents is probably caused by the higher surface area of the coal particles and highly dispersed catalyst, resulting in the higher carbon conversion with the mixed gasifying agents. The analysis of SEM photographs was exactly consistent with the results of carbon conversion in Figure 4. XRD results for char samples after a gasification time of 20 min are shown in Figure 7. During gasification with K2CO3, the peaks correspond to K2CO3, KHCO3, KO2, and KO3 (Figure 7a). During steam gasification, K2CO3 kept its form, whereas, during CO2 gasification, K2CO3 was mainly transformed to KHCO3. However, K2CO3 was mainly transformed to KO2 and
Figure 5. SEM photographs of residue after gasification at 800 °C with K2CO3: (a, b) steam gasification; (c, d) CO2 gasification; (e, f) H2O/ CO2 = 80/20.
difference with different gasifying agents. During steam gasification, expanded layers and large cracks were formed with K2CO3. The agglomeration could be found on coal particles because of the reaction between potassium and mineral matter in coal. During CaCO3 catalytic steam gasification, the catalyst was finely dispersed over the surface of particles, which may be the reason that CaCO3 performed better than K2CO3 during steam gasification. During CO2 gasification, the coal structure showed a few cracks with a little roughness using K2CO3. However, a large area of agglomeration covered on the particles using CaCO3, leading to the E
DOI: 10.1021/acs.energyfuels.6b01329 Energy Fuels XXXX, XXX, XXX−XXX
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(1) The steam gasification with CaCO3 had higher syngas yield of CO and H2 (the major product) than that with K2CO3; the CO2 gasification with K2CO3 catalyst produced a higher amount of CO (the major product) than that with CaCO3. During gasification with the mixed atmosphere, the K2CO3 catalyst performed better than CaCO3 with a higher amount of CO and H2. (2) Syngas of gasification with the two catalysts was adjusted to 1 and 2 by varying the steam/CO2 ratio of the gasifying agent at 800 °C, which could be used in the F-T synthesis. (3) Higher carbon conversion during char gasification was clearly achieved with the mixed atmosphere (H2O/CO2 = 80/20) than with pure steam and CO2 atmospheres by using both catalysts. During gasification with H2O/CO2 = 80/20, the two catalysts were finely dispersed on the surface of particles, and the char sample showed more stereoscopic structure than that with pure steam and CO2, which provide higher surface area for gasification.
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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.6b01329. The syngas production with different catalyst loadings during steam gasification at 800°C (PDF)
Figure 7. XRD analysis of char after gasification time of 20 min at 800 °C with different gasifying agents.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.-T.K.). *E-mail:
[email protected] (S.F.).
KO3 during gasification with the mixed gasifying agents. Mckee found that carbonates, oxides, and hydroxides of alkali metal are all most effective catalysts; other existing formations could transform to these species with operating conditions.25 The carbonate could provide surface bound oxygen to react with carbon. Potassium−carbon intermediates might also be oxidized during gasification.26 The crystalline structure of potassium intermediates is smaller than 5 nm, which cannot be analyzed with the XRD technique. During gasification with CaCO3, the peaks correspond to CaO and CaCO3. During steam or CO2 gasification, the diffraction peak attributable to CaO appeared (Figure 6). However, the catalyst existed as CaO and CaCO3 during gasification with the mixed gasifying agents. The decomposition of CaCO3 to CaO takes place at 650−750 °C;21,27 as a result, the diffraction intensity of CaO was found during steam or CO2 gasification. CaCO3 only existed during gasification with the mixed gasifying agents. The existing form of catalyst during gasification with the mixed gasifying agents was different from the gasification with pure steam or CO2, which may be related to the role of catalyst with different gasifying agents. The effect of Ca catalyst is caused by the enhancement of the dissociation of O-containing gas and oxygen spillover. Ohtsuka27 et al. found that calcium could increase the number of reactive sites for coal gasification, thus increasing the gasification reactivity.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Human Resources Program in Energy Technology of the Korea Institute of Energy Technology Evaluation and Planning (KETEP). Financial support was granted by the Ministry of Trade, Industry & Energy, Republic of Korea (Project No: 2015 4010 200820).
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REFERENCES
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4. CONCLUSIONS K2CO3 and CaCO3 were used as catalysts for gasification of Indonesian sub-bituminous KPU coal with H2O/CO2 mixtures. The main conclusions are summarized from this study: F
DOI: 10.1021/acs.energyfuels.6b01329 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.6b01329 Energy Fuels XXXX, XXX, XXX−XXX
