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Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading
Journal of the Energy Institute xxx (2018) 1e9
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Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading Xiangzhou Yuan a, b, *, Ki Bong Lee a, Hyung-Taek Kim b a b
Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea Department of Energy Systems Research, Ajou University, 206 Worldcup-ro, Woncheon-dong, Youngtong-gu, Suwon 16499, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2017 Received in revised form 14 June 2018 Accepted 18 June 2018 Available online xxx
Catalytic steam gasification is considered one of promising technologies for converting solid carbonaceous feedstocks into hydrogen-rich syngas, which is an important source of hydrogen for various industrial sectors. The K2CO3-catalyzed steam gasification of low rank coals (LRCs) was conducted in a fixed bed reactor for elucidating the effects of gasifying temperature and catalyst loading amount on hydrogen yield. Hydrogen-rich syngas can be obtained at gasifying temperature of 800 C and loading amount of 10 wt% K2CO3. The loading amount of 10 wt% K2CO3 was the saturation point and provided a good gasification reactivity in catalytic steam gasification of three LRCs. The experimental data of these three LRCs were well described by the random pore model (RPM). The RPM fitted the experimental data at 800 C better than the experimental data obtained at 700 C and 600 C. Reactivity index (R0.5), activation energy (Ea) and reaction rate constant (k) were also used to predict the characteristics of the K2CO3catalyzed steam gasification process. Catalytic steam gasification utilizing the mixture of three LRCs as a feedstock was also investigated and displayed XC of 86.22% and 0.95 mol mol1-C, indicating a good feasibility and potential industrial applications. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Catalytic steam gasification Low rank coal Potassium carbonate H2-rich syngas Random pore model
1. Introduction Catalytic gasification is currently receiving increasing attention and considered one of the promising alternatives for improving the energy efficiency and economics of conversion technology. Various methods are being studied in order to develop more efficient and environmentally friendly coal conversion technologies [1,2]. One such method is the addition of various alkali and alkaline earth metal species (AAEMs), which is often termed catalytic gasification. Catalytic coal gasification has advantages over un-catalyzed coal gasification of having a low-temperature operation and high-efficient throughout. As early as 1867, du Montay and Marechal patented the catalytic gasification of carbonaceous materials with AAEMs [3]. Since then, a great deal of literature has testified that AAEMs, especially in the form of carbonate, are the superior catalysts for the gasification reactions [4e6]; K, Na, and Ca are considered to be the most effective AAEMs. Kapeijn et al. revealed the catalytic activities of all Group Ia metal carbonates in the following order: Cs > Rb > K > Na > Li when the activated carbon was gasified at the same carbon/metal atomic ratio [7]. Therefore, because K2CO3 can effectively promote steam gasification and decrease the operation temperature, it is the catalyst of choice among alkali salts for use in the catalytic coal gasification process [8e10]. Karimi et al. confirmed that K2CO3 has a better catalytic activity than Na2CO3 for the steam gasification of bitumen coke at a fixed molar loading [11]. In addition, K2CO3 is able to significantly increase gasification reaction rates and has good mobility to disperse throughout the coal sample under gasification operating conditions [12e14]. Currently, the kinetic advantages of using a catalyst for coal gasification are considered to be the most important for commercializing the catalytic coal gasification process. The kinetics of char gasification have been the continual subject of a large number of studies [15e17], as it is of crucial importance in determining the rate and process of coal gasification. Therefore, a kinetic study on the catalytic gasification of low rank coal (LRC) is urgently needed for the following four reasons. Firstly, LRC, which contains more AAEMs than high rank coal, shows a higher gasification reactivity which is mainly enhanced by the AAEMs [18,19]. Secondly, it is generally expected that LRC will play a significant role as an energy source, mainly because it is the most abundant and least expensive fossil fuel available. Thirdly, until now, none of * Corresponding author. Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea. E-mail address: [email protected] (X. Yuan). https://doi.org/10.1016/j.joei.2018.06.011 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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X. Yuan et al. / Journal of the Energy Institute xxx (2018) 1e9
the catalytic steam gasification technologies has been utilized commercially, especially for the catalytic steam gasification of LRC. Finally, a study on the catalyst recovery process of gasified residue collected from a K2CO3-catalyzed steam gasification process showed that the potassium catalyst recovery efficiency reached about 90% by utilizing a combined water and limewater washing method [13,20]. In this research, three Indonesian LRCs were selected for the experimental and kinetic investigations on K2CO3-catalyzed steam gasification at different gasification temperatures (600 C, 700 C, and 800 C) and catalyst loading amounts (5 wt%, 10 wt%, and 20 wt%). The gasification reactivity (R0.5) and random pore model (RPM) were adopted to describe the gasification characteristics. The activation energy (Ea) and reaction rate constant (k) were utilized for evaluating and optimizing the K2CO3-catalyzed steam gasification process of LRCs. And the mixture of three LRCs was also used for figuring out the gasification characteristics. 2. Experimental 2.1. Coal sample and catalyst loading method Three Indonesian LRCs referred to as Lanna, KPU, and IBC coals were utilized. These three coals were first placed in a drying furnace for 24 h at 105 C in order to remove the maximum amount of moisture in coal samples; the samples were then crushed and pulverized in order to obtain a fraction with a particle size of 150e300 mm. The proximate and ultimate analyses and higher heating value of these three coals are presented in Table 1. Potassium carbonate (99.5%, powder, DAEJUNG CHEMICALS &METALS CO., LTD) was selected and used in the thermal system as a catalyst to improve the yields of syngas production from the coal steam gasification process. A physical mixing method was adopted for catalyst loading because K2CO3 has good mobility to disperse throughout the coal sample under gasification operating conditions [21]. 2.2. Experimental apparatus and operation The catalytic steam gasification of LRCs was conducted in a lab-scale tubular fixed bed reactor (see in Fig. 1). The tubular fixed bed reactor was 22.5 mm in diameter (ID) and 200 mm in height, constructed of stainless steel with a temperature controller. The experiments were conducted at ambient atmospheric pressure over the gasification temperature range of 600e800 C. Details of the experimental process are given in Ref. [22]: First, the reactor was heated using an electric furnace to a predetermined temperature under a steam of nitrogen (99.9 vol %, Han-il gas Inc.) with a flow rate of 1 L min1 controlled through a flow meter. Secondly, the prepared samples supported on a mesh wire were gasified at 600 C, 700 C, and 800 C for 1 h by introducing steam and N2 with flow rates of 500 L min1 and 1 L min1, respectively. The syngas produced from the fixed bed reactor was firstly cooled down by a cooling system (Lab. Companion RW-0525G), and then was cleaned by a filter device, and finally the major product gases were quantitatively and continuously determined online using a gas analyzer system (MRU analyzer, temporal resolution (s)). 2.3. Data processing methods In this study, the carbon conversion (XC) is calculated using Eq. (1) as follows:
XC ¼
molCO þ molCO2 þ molCH4 12 100 masscoal pctcarbon
(1)
where molCO, molCO2, and molCH4 represent the number of moles of CO, CO2, and CH4, respectively; masscoal represents the weight of the coal sample and; pctcarbon represents the mass percentage of carbon from the ultimate analyses of Lanna, KPU, and IBC coals. To quantify the gasification reactivity, the reactivity index R0.5 was used to denote the rate of gasification, which was reported by Takarade et al. as the reciprocal of the steam gasification time with 50% carbon conversion multiplied by 0.5 [23,24]. R0.5 is calculated using the following equation:
R0:5 ¼
0:5
t0:5
min1
(2)
where t0.5 is the time required from the starting of steam gasification to reach a 50% carbon conversion. When the final carbon conversion is lower than 50%, the reactivity is calculated by the reciprocal of the gasification time multiplied by the corresponding carbon conversion (XC). Many different models have been proposed to describe coal char gasification reactions with steam. The change in the apparent gasification reaction rate can be expressed as Eq. (3) [25]. Table 1 Proximate and ultimate analyses and higher heating value of the coal samples. Sample y
Proximate Analysis (wt%, db )
V. M. F. C. Ash C H O* N S
Ultimate Analysis (wt%, dafz)
HHV (kcal kg1) Note: yDry basis (db); zdry ash free basis (daf);
*
Lanna
KPU
IBC
45.74 50.05 4.21 69.70 2.11 22.34 1.50 1.35 6908
49.94 49.47 3.59 75.90 5.73 17.21 1.30 0.26 6758
51.52 45.81 2.67 70.73 4.68 23.32 1.07 0.20 6078
calculated by difference.
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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Fig. 1. Schematic of the experimental process: 1- carrier-gas, 2- steam generator, 3- preheater, 4- gasification reactor, 5- electric furnace, 6- structure of fixed bed reactor, 7- filter, 8gas analyzer system, 9- data recorder (PC).
dXC ¼ kðTÞf ðXC Þ dt
(3)
where k is the rate constant, dependent on temperature (T) when the partial pressure in the gas phase remains constant and f(XC) describes the changes in the physical or chemical properties of the sample as the gasification proceeds and corresponds to the chosen nth-order expressions, which is changed depending on which kinetic model and assumptions are used. The kinetic constant is a function of temperature and can be expressed by using the Arrhenius equation in Eq. (4).
Ea kðTÞ ¼ A exp RT
(4)
where A and Ea are the pre-exponential factor and activation energy (kJ mol1), respectively; R is the ideal gas constant (8.314 J mol1 K1); and T is the absolute temperature (K). Different models incorporating structural effects in addition to the intrinsic kinetics have been evaluated for predicting overall reaction rates, which include the volumetric model (VM), shrinking core model (SCM), and random pore model (RPM). The RPM is used in this study and has been used by many investigators with success and has been improved to account for non-uniform pore size distributions, as presented for the first time by Bhatia and Perlmutter [26]. The RPM model considers the overlapping of pore surfaces, which reduces the area available for reaction and applied to coal gasification reaction [26,27]. This model is able to predict the maximum reactivity as the reaction proceeds, as it considers the competing effects of pore growth during the initial stages of gasification and the destruction of the pores due to the coalescence of neighboring pores during the reaction. This model gained popularity due to its capacity to reproduce a maximum reaction rate [28,29]. The basic equation for this model is:
dXC ¼ kRPM ð1 XC Þ dt
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 Jlnð1 XC Þ
where XC is the carbon conversion, t represents the gasification time, kRPM denotes the reaction rate constant (min1), and j is a parameter associated with the internal surface structure of non-converted char. 2.4. Other analyses Proximate analysis was performed using an STA-MS (NETZSCH) and ultimate analysis was conducted on a CHN-2000, S-144DR (LECO Corp.). The higher heating value of the coal sample was detected by a Parr 6300 calorimeter (U.S.A.). X-ray fluorescence (XRF) spectrometry was conducted on three different raw LRCs using a ZSX Primus (Rigaku Japan). The morphology of catalyst and mixtures of both LRCs and catalyst were examined by scanning electron microscopy (SEM) conducted on a Hitachi S-4300 apparatus. X-ray diffraction (XRD) experiments were performed on an X-ray diffractometer with small angle X-ray scattering (Rigaku D/Max-3A) and the samples were scanned in 4 min1 over an angular range 5e80 using 100 mA and 40 kV Cu Ka radiation. 3. Results and discussion Table 1 shows the results of the proximate and ultimate analyses and higher heating values (HHV) of the three LRCs (Lanna, KPU, and IBC coals). As illustrated in Table 1, these three coals are similar in their elemental composition, holding characteristics of high volatile matter Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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content, low ash content, and low sulfur content. Ash analysis on these LRCs was presented in Table 2. SEM analyses of K2CO3 catalyst and mixtures of each coal and 10 wt% K2CO3 were carried out for confirming well mixed LRCs with catalyst. It can be clearly seen from Fig. 2 that these three kinds of coal were thoroughly mixed with K2CO3 catalyst by physical mixing method, and the textural structure of both coal sample and K2CO3 catalyst were not affected. 3.1. Effect of gasification temperature and catalyst loading amount Because gasification temperature and catalyst loading amount are crucial factors in the process of catalytic steam gasification [22], the catalytic steam gasification of Lanna coal was conducted at three different gasification temperatures and three different catalyst loading amounts. For comparison, the non-catalytic steam gasification of Lanna coal was also investigated. It can be seen from Fig. 3 that 1) with a fixed catalyst loading amount, the XC increased as gasification temperature increased from 600 to 800 C. Because both the carbon steam reaction (C þ H2 O/CO þ H2 ; H0 ¼ þ 131:3 kJ mol1 ) and the Boudouard reaction (C þ CO2 /2CO; H 0 ¼ þ 162:4 kJ mol1 ) are endothermic reactions, the carbon contained in the coal sample can be more steadily transformed to CO and CO2 as the gasification temperature was increased, which directly increases XC; 2) with a fixed gasification temperature, the XC increased as the catalyst loading amount increased from 0 to 20 wt%. It can be seen that K2CO3 can effectively promote the gasification reactions, which is consistent with other studies [22,30]. The generated syngas composition is not only one of the indicators of the overall performance of the gasification process, but also affects the potential uses of the syngas. In this research, H2, CO, CO2, and CH4 were investigated for evaluating the performance of the K2CO3catalyzed steam gasification process. Other hydrocarbons were not studied in detail because of the low yields (<1% of total gaseous products) obtained in this research. Fig. 4 presents that as increasing the gasification temperature, the molar yields of H2, CO, and CO2 (normalized per mole carbon in Lanna coal) showed a dramatically increasing trend. At 800 C, the 20 wt% K2CO3 loading amount increased the molar yields of H2 and CO to 1.04 and 0.30 mol mol1-C from 0.31 to 0.07 mol mol1-C obtained by non-catalytic steam gasification process. However, the 10 wt% K2CO3 loading amount can also increase the molar yields of H2 and CO to 1.02 and 0.28 mol mol1-C, respectively. Therefore, the 10 wt% K2CO3 loading amount was the saturation capacity for conducting the catalytic steam gasification of Lanna coal at a gasification temperature of 800 C. 3.2. Effect of utilization of different LRCs Fig. 5 shows the syngas compositions after 1 h catalytic steam gasification process by changing LRCs. In Fig. 5, the molar yields of syngas (H2 and CO) from Lanna and IBC coals were kept at similar value and only the molar yields of syngas from KPU coal displayed a little difference, when increasing the catalyst loading amount from 10 to 20 wt%. When integrating the molar yields of syngas from these three Table 2 XRF analysis on Lanna, KPU, and IBC coal.
*
Samples
Mg
Al
Si
P
S
K
Ca
Fe
Mn
Others
Lanna KPU IBC
0.18 19.40 25.79
1.21 14.58 4.36
2.17 9.94 5.27
1.62 e e
16.40 10.37 10.58
1.38 0.93 0.35
6.65 35.72 30.35
68.90 7.19 22.39
0.45 0.02 0.25
1.04 1.84 0.66
All based on weight percentage (wt%).
Fig. 2. SEM analysis on a) K2CO3 catalyst, b) Lanna coal with 10 wt% K2CO3, c) KPU coal with 10 wt% K2CO3, and d) IBC coal with 10 wt% K2CO3.
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
X. Yuan et al. / Journal of the Energy Institute xxx (2018) 1e9
Fig. 3. Carbon conversions of Lanna coal by changing catalyst loading amounts and gasification temperatures within 1 h.
Fig. 4. Syngas compositions of Lanna coal at different temperatures within 1 h: a) H2, b) CO, c) CO2, and d) CH4.
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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X. Yuan et al. / Journal of the Energy Institute xxx (2018) 1e9
Fig. 5. Syngas compositions at gasification temperature of 800 C within 1 h: a) Lanna coal, b) KPU coal, and c) IBC coal.
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coals, it can be concluded that the K2CO3 loading amount of 10 wt% and gasification temperature of 800 C were the optimal operating parameters for producing H2-rich syngas in catalytic steam gasification of these two LRCs. On the other hand, Fig. 5 also shows that when loading the K2CO3 catalyst or not, it can be seen that the significant difference of CH4 and H2 yields occurred, the reason for this phenomena mainly caused by the methane decomposition (CH4 þ H2 O/CO þ 3H2 ; H 0 ¼ 206 kJ mol1 ) which could be enhanced by loading K2CO3 catalyst [22]. The gasification reactivity was studied by using Eq. (2). Table 3 shows the reactivity index R0.5 with different catalyst loading amounts and gasification temperatures. It can be seen that the R0.5 showed an increasing trend when only the gasification temperature increased and other parameters were maintained as constants. At 800 C, the R0.5 increased dramatically as increasing the K2CO3 loading amount from 10 to 20 wt%, compared to the R0.5 attained from the non-catalytic steam gasification process (Raw coal); this means that as the amount of the loaded K2CO3 increased, the gasification reactivity obtained by these three LRCs also increased. However, for these three LRCs, no significant difference was observed in R0.5 when loading 10 wt% catalyst or 20 wt% catalyst at each gasification temperature. Therefore, it can be concluded that for these three LRCs, the 10 wt% K2CO3 loading amount was the saturation point and provided equivalent gasification reactivity. Based on Tables 1 and 2, the higher Ca content in KPU and IBC coals than that of Lanna at the same quantity of each coal, and the synergistic effect between Ca and K was verified in gasification process [31]. Thus, the R0.5 obtained from KPU and IBC coals presented higher values than R0.5 obtained from Lanna coal at the same gasification temperature.
3.3. Kinetic study by changing gasification temperature and coal sample Based on the fitted data for the K2CO3-catalyzed steam gasification of three LRCs using the RPM model, the reaction rate constants at the three different gasification temperatures were calculated. It can be seen from Table 4 that the kRPM increased as the gasification temperature was increased and obviously increased when loading 10 wt% K2CO3 at each gasification temperature, compared with the kRPM obtained by the non-catalytic steam gasification process. In addition, both the activation energy Ea and the pre-exponential factor A were obtained from the Arrhenius equation (Eq. (4)). Therefore, the ln k(T)-1/T plot for these two different types of LRCs are shown in Fig. 6, and the fitting results for A and Ea are listed in Table 5. The results show that compared with the Ea obtained from the non-catalytic steam gasification, the Ea can be reduced by 33.70 kJ mol1 for Lanna coal, 14.21 kJ mol1 for KPU coal, and 18.66 kJ mol1 for IBC coal, when loading with 10 wt% K2CO3. In addition, after conducting the catalytic steam gasification at 800 C, these three gasified residues were collected and then analyzed by XRD method. Fig. 7 presents that KHCO3, K2SO4 and KAlSiO4 mainly existed in these three gasified residues, which well correlates with several previous studies [13,20,32e34]. For lowering the catalyst investment of catalytic steam gasification, the potassium catalyst should be recovered by our previous studies [13,20], indicating that potassium catalyzed steam gasification equipped with catalyst recovery technology will play a significant role in the near future.
3.4. Performance evaluation with the mixture of LRCs These three LRCs were mixed together at mass ratio of 1:1:1, termed as LKI sample. This LKI sample was utilized for conducting K2CO3catalyzed steam gasification at 800 with 10 wt% catalyst loading amount. The XC reached 86.22% and the syngas yields were obtained as 0.33 mol mol1-C of CO, 0.51 mol mol1-C of CO2, 0.95 mol mol1-C H2, and 0.03 mol mol1-C of CH4. Therefore, these three LRCs can be mixed for conducting catalytic steam gasification with K2CO3 catalyst loading, which is more feasible and applicable for industrial applications.
Table 3 Reactivity index R0.5 (*103 min1) calculation at different operating conditions. Temp.
Lanna
( C)
Raw Coal
10% K
20% K
KPU Raw Coal
10% K
20% K
IBC Raw Coal
10% K
20% K
600 700 800
1.83 3.83 6.33
2.33 6.97 13.72
2.83 7.44 14.61
2.41 4.87 10.54
3.23 8.11 18.30
4.16 9.43 19.24
3.17 5.27 11.90
4.33 9.26 19.10
5.00 9.80 18.67
Table 4 Reaction rate constant (k) calculations by RPM at different operating conditions. Temp.
RPM
Lanna
( C)
(*104 s1)
Raw Coal
10% K
Raw Coal
10% K
Raw Coal
10% K
600
k R2 k R2 k R2
0.10 0.979 0.55 0.997 1.25 0.999
0.48 0.989 1.35 0.999 2.56 0.999
0.28 0.996 0.91 0.997 2.59 0.998
0.56 0.999 1.12 0.992 3.67 0.997
0.27 0.996 0.85 0.999 2.14 0.999
0.79 0.998 1.78 0.999 3.89 0.998
700 800
KPU
IBC
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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Fig. 6. Arrhenius plots with Lanna coal (square symbol), KPU coal (diamond symbol), and IBC coal (circle symbol).
Table 5 Activation energy (Ea) and pre-exponential (A) factors by RPM. Parameter
1
A (s ) Ea (kJ mol1) R2
Lanna
KPU
IBC
Raw Coal
10% K
Raw Coal
10% K
Raw Coal
10% K
0.41 99.18 0.961
9.44 65.48 0.989
1.07 86.50 0.999
4.14 72.29 0.913
0.39 80.61 0.999
1.80 61.95 0.995
Fig. 7. XRD analyses of K2CO3 and gasified residues, where 1, 2, and 3 represent KHCO3, K2SO4, and KAlSiO4, respectively.
4. Conclusions In this research, both experimental and kinetic investigations on K2CO3-catalyzed steam gasification of three LRCs were conducted by changing gasifying temperature and catalyst loading amount, in order to optimize this entire process for hydrogen-rich syngas production. After conducting experiments of K2CO3-catalyzed steam gasification, the H2-rich syngas could be obtained at gasifying temperature of 800 C with loading amount of 10 wt% K2CO3. The 10 wt% K2CO3 loading amount was proved as the saturation point for this K2CO3-catalyzed steam gasification of LRCs in a fixed bed reactor and provided equivalent gasification reactivity when compared with those using 20 wt% K2CO3 loading amount. For the kinetic study, the RPM was used to predict the characteristics of K2CO3-catalyzed steam gasification process,
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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including activation energy (Ea) and reaction rate constant (k). The RPM model well fitted the experimental data at 800 C better than those at 700 C and 600 C, since it revealed the highest R2 (0.999) value at 800 C. The mixture sample (LKI) of three LRCs was also investigated as a feedstock in catalytic steam gasification process, implying that it is more feasible and applicable in industrial applications. Acknowledgement This work was supported by the “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and was granted financial resource from the Ministry of Trade, Industry & Energy, South Korea. (Project No: 2015-4010-200820). References [1] M. Shahnaz, S. Yusup, A. Inayat, D.O. Patric, A. Pratama, Application of response surface methodology to investigate the effect of different variables on conversion of palm kernel shell in steam gasification using coal bottom ash, Appl. Energy 184 (2016) 1306e1315. [2] L. 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Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading Xiangzhou Yuan a, b, *, Ki Bong Lee a, Hyung-Taek Kim b a b
Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea Department of Energy Systems Research, Ajou University, 206 Worldcup-ro, Woncheon-dong, Youngtong-gu, Suwon 16499, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2017 Received in revised form 14 June 2018 Accepted 18 June 2018 Available online xxx
Catalytic steam gasification is considered one of promising technologies for converting solid carbonaceous feedstocks into hydrogen-rich syngas, which is an important source of hydrogen for various industrial sectors. The K2CO3-catalyzed steam gasification of low rank coals (LRCs) was conducted in a fixed bed reactor for elucidating the effects of gasifying temperature and catalyst loading amount on hydrogen yield. Hydrogen-rich syngas can be obtained at gasifying temperature of 800 C and loading amount of 10 wt% K2CO3. The loading amount of 10 wt% K2CO3 was the saturation point and provided a good gasification reactivity in catalytic steam gasification of three LRCs. The experimental data of these three LRCs were well described by the random pore model (RPM). The RPM fitted the experimental data at 800 C better than the experimental data obtained at 700 C and 600 C. Reactivity index (R0.5), activation energy (Ea) and reaction rate constant (k) were also used to predict the characteristics of the K2CO3catalyzed steam gasification process. Catalytic steam gasification utilizing the mixture of three LRCs as a feedstock was also investigated and displayed XC of 86.22% and 0.95 mol mol1-C, indicating a good feasibility and potential industrial applications. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: Catalytic steam gasification Low rank coal Potassium carbonate H2-rich syngas Random pore model
1. Introduction Catalytic gasification is currently receiving increasing attention and considered one of the promising alternatives for improving the energy efficiency and economics of conversion technology. Various methods are being studied in order to develop more efficient and environmentally friendly coal conversion technologies [1,2]. One such method is the addition of various alkali and alkaline earth metal species (AAEMs), which is often termed catalytic gasification. Catalytic coal gasification has advantages over un-catalyzed coal gasification of having a low-temperature operation and high-efficient throughout. As early as 1867, du Montay and Marechal patented the catalytic gasification of carbonaceous materials with AAEMs [3]. Since then, a great deal of literature has testified that AAEMs, especially in the form of carbonate, are the superior catalysts for the gasification reactions [4e6]; K, Na, and Ca are considered to be the most effective AAEMs. Kapeijn et al. revealed the catalytic activities of all Group Ia metal carbonates in the following order: Cs > Rb > K > Na > Li when the activated carbon was gasified at the same carbon/metal atomic ratio [7]. Therefore, because K2CO3 can effectively promote steam gasification and decrease the operation temperature, it is the catalyst of choice among alkali salts for use in the catalytic coal gasification process [8e10]. Karimi et al. confirmed that K2CO3 has a better catalytic activity than Na2CO3 for the steam gasification of bitumen coke at a fixed molar loading [11]. In addition, K2CO3 is able to significantly increase gasification reaction rates and has good mobility to disperse throughout the coal sample under gasification operating conditions [12e14]. Currently, the kinetic advantages of using a catalyst for coal gasification are considered to be the most important for commercializing the catalytic coal gasification process. The kinetics of char gasification have been the continual subject of a large number of studies [15e17], as it is of crucial importance in determining the rate and process of coal gasification. Therefore, a kinetic study on the catalytic gasification of low rank coal (LRC) is urgently needed for the following four reasons. Firstly, LRC, which contains more AAEMs than high rank coal, shows a higher gasification reactivity which is mainly enhanced by the AAEMs [18,19]. Secondly, it is generally expected that LRC will play a significant role as an energy source, mainly because it is the most abundant and least expensive fossil fuel available. Thirdly, until now, none of * Corresponding author. Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea. E-mail address: [email protected] (X. Yuan). https://doi.org/10.1016/j.joei.2018.06.011 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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the catalytic steam gasification technologies has been utilized commercially, especially for the catalytic steam gasification of LRC. Finally, a study on the catalyst recovery process of gasified residue collected from a K2CO3-catalyzed steam gasification process showed that the potassium catalyst recovery efficiency reached about 90% by utilizing a combined water and limewater washing method [13,20]. In this research, three Indonesian LRCs were selected for the experimental and kinetic investigations on K2CO3-catalyzed steam gasification at different gasification temperatures (600 C, 700 C, and 800 C) and catalyst loading amounts (5 wt%, 10 wt%, and 20 wt%). The gasification reactivity (R0.5) and random pore model (RPM) were adopted to describe the gasification characteristics. The activation energy (Ea) and reaction rate constant (k) were utilized for evaluating and optimizing the K2CO3-catalyzed steam gasification process of LRCs. And the mixture of three LRCs was also used for figuring out the gasification characteristics. 2. Experimental 2.1. Coal sample and catalyst loading method Three Indonesian LRCs referred to as Lanna, KPU, and IBC coals were utilized. These three coals were first placed in a drying furnace for 24 h at 105 C in order to remove the maximum amount of moisture in coal samples; the samples were then crushed and pulverized in order to obtain a fraction with a particle size of 150e300 mm. The proximate and ultimate analyses and higher heating value of these three coals are presented in Table 1. Potassium carbonate (99.5%, powder, DAEJUNG CHEMICALS &METALS CO., LTD) was selected and used in the thermal system as a catalyst to improve the yields of syngas production from the coal steam gasification process. A physical mixing method was adopted for catalyst loading because K2CO3 has good mobility to disperse throughout the coal sample under gasification operating conditions [21]. 2.2. Experimental apparatus and operation The catalytic steam gasification of LRCs was conducted in a lab-scale tubular fixed bed reactor (see in Fig. 1). The tubular fixed bed reactor was 22.5 mm in diameter (ID) and 200 mm in height, constructed of stainless steel with a temperature controller. The experiments were conducted at ambient atmospheric pressure over the gasification temperature range of 600e800 C. Details of the experimental process are given in Ref. [22]: First, the reactor was heated using an electric furnace to a predetermined temperature under a steam of nitrogen (99.9 vol %, Han-il gas Inc.) with a flow rate of 1 L min1 controlled through a flow meter. Secondly, the prepared samples supported on a mesh wire were gasified at 600 C, 700 C, and 800 C for 1 h by introducing steam and N2 with flow rates of 500 L min1 and 1 L min1, respectively. The syngas produced from the fixed bed reactor was firstly cooled down by a cooling system (Lab. Companion RW-0525G), and then was cleaned by a filter device, and finally the major product gases were quantitatively and continuously determined online using a gas analyzer system (MRU analyzer, temporal resolution (s)). 2.3. Data processing methods In this study, the carbon conversion (XC) is calculated using Eq. (1) as follows:
XC ¼
molCO þ molCO2 þ molCH4 12 100 masscoal pctcarbon
(1)
where molCO, molCO2, and molCH4 represent the number of moles of CO, CO2, and CH4, respectively; masscoal represents the weight of the coal sample and; pctcarbon represents the mass percentage of carbon from the ultimate analyses of Lanna, KPU, and IBC coals. To quantify the gasification reactivity, the reactivity index R0.5 was used to denote the rate of gasification, which was reported by Takarade et al. as the reciprocal of the steam gasification time with 50% carbon conversion multiplied by 0.5 [23,24]. R0.5 is calculated using the following equation:
R0:5 ¼
0:5
t0:5
min1
(2)
where t0.5 is the time required from the starting of steam gasification to reach a 50% carbon conversion. When the final carbon conversion is lower than 50%, the reactivity is calculated by the reciprocal of the gasification time multiplied by the corresponding carbon conversion (XC). Many different models have been proposed to describe coal char gasification reactions with steam. The change in the apparent gasification reaction rate can be expressed as Eq. (3) [25]. Table 1 Proximate and ultimate analyses and higher heating value of the coal samples. Sample y
Proximate Analysis (wt%, db )
V. M. F. C. Ash C H O* N S
Ultimate Analysis (wt%, dafz)
HHV (kcal kg1) Note: yDry basis (db); zdry ash free basis (daf);
*
Lanna
KPU
IBC
45.74 50.05 4.21 69.70 2.11 22.34 1.50 1.35 6908
49.94 49.47 3.59 75.90 5.73 17.21 1.30 0.26 6758
51.52 45.81 2.67 70.73 4.68 23.32 1.07 0.20 6078
calculated by difference.
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3
Fig. 1. Schematic of the experimental process: 1- carrier-gas, 2- steam generator, 3- preheater, 4- gasification reactor, 5- electric furnace, 6- structure of fixed bed reactor, 7- filter, 8gas analyzer system, 9- data recorder (PC).
dXC ¼ kðTÞf ðXC Þ dt
(3)
where k is the rate constant, dependent on temperature (T) when the partial pressure in the gas phase remains constant and f(XC) describes the changes in the physical or chemical properties of the sample as the gasification proceeds and corresponds to the chosen nth-order expressions, which is changed depending on which kinetic model and assumptions are used. The kinetic constant is a function of temperature and can be expressed by using the Arrhenius equation in Eq. (4).
Ea kðTÞ ¼ A exp RT
(4)
where A and Ea are the pre-exponential factor and activation energy (kJ mol1), respectively; R is the ideal gas constant (8.314 J mol1 K1); and T is the absolute temperature (K). Different models incorporating structural effects in addition to the intrinsic kinetics have been evaluated for predicting overall reaction rates, which include the volumetric model (VM), shrinking core model (SCM), and random pore model (RPM). The RPM is used in this study and has been used by many investigators with success and has been improved to account for non-uniform pore size distributions, as presented for the first time by Bhatia and Perlmutter [26]. The RPM model considers the overlapping of pore surfaces, which reduces the area available for reaction and applied to coal gasification reaction [26,27]. This model is able to predict the maximum reactivity as the reaction proceeds, as it considers the competing effects of pore growth during the initial stages of gasification and the destruction of the pores due to the coalescence of neighboring pores during the reaction. This model gained popularity due to its capacity to reproduce a maximum reaction rate [28,29]. The basic equation for this model is:
dXC ¼ kRPM ð1 XC Þ dt
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 Jlnð1 XC Þ
where XC is the carbon conversion, t represents the gasification time, kRPM denotes the reaction rate constant (min1), and j is a parameter associated with the internal surface structure of non-converted char. 2.4. Other analyses Proximate analysis was performed using an STA-MS (NETZSCH) and ultimate analysis was conducted on a CHN-2000, S-144DR (LECO Corp.). The higher heating value of the coal sample was detected by a Parr 6300 calorimeter (U.S.A.). X-ray fluorescence (XRF) spectrometry was conducted on three different raw LRCs using a ZSX Primus (Rigaku Japan). The morphology of catalyst and mixtures of both LRCs and catalyst were examined by scanning electron microscopy (SEM) conducted on a Hitachi S-4300 apparatus. X-ray diffraction (XRD) experiments were performed on an X-ray diffractometer with small angle X-ray scattering (Rigaku D/Max-3A) and the samples were scanned in 4 min1 over an angular range 5e80 using 100 mA and 40 kV Cu Ka radiation. 3. Results and discussion Table 1 shows the results of the proximate and ultimate analyses and higher heating values (HHV) of the three LRCs (Lanna, KPU, and IBC coals). As illustrated in Table 1, these three coals are similar in their elemental composition, holding characteristics of high volatile matter Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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content, low ash content, and low sulfur content. Ash analysis on these LRCs was presented in Table 2. SEM analyses of K2CO3 catalyst and mixtures of each coal and 10 wt% K2CO3 were carried out for confirming well mixed LRCs with catalyst. It can be clearly seen from Fig. 2 that these three kinds of coal were thoroughly mixed with K2CO3 catalyst by physical mixing method, and the textural structure of both coal sample and K2CO3 catalyst were not affected. 3.1. Effect of gasification temperature and catalyst loading amount Because gasification temperature and catalyst loading amount are crucial factors in the process of catalytic steam gasification [22], the catalytic steam gasification of Lanna coal was conducted at three different gasification temperatures and three different catalyst loading amounts. For comparison, the non-catalytic steam gasification of Lanna coal was also investigated. It can be seen from Fig. 3 that 1) with a fixed catalyst loading amount, the XC increased as gasification temperature increased from 600 to 800 C. Because both the carbon steam reaction (C þ H2 O/CO þ H2 ; H0 ¼ þ 131:3 kJ mol1 ) and the Boudouard reaction (C þ CO2 /2CO; H 0 ¼ þ 162:4 kJ mol1 ) are endothermic reactions, the carbon contained in the coal sample can be more steadily transformed to CO and CO2 as the gasification temperature was increased, which directly increases XC; 2) with a fixed gasification temperature, the XC increased as the catalyst loading amount increased from 0 to 20 wt%. It can be seen that K2CO3 can effectively promote the gasification reactions, which is consistent with other studies [22,30]. The generated syngas composition is not only one of the indicators of the overall performance of the gasification process, but also affects the potential uses of the syngas. In this research, H2, CO, CO2, and CH4 were investigated for evaluating the performance of the K2CO3catalyzed steam gasification process. Other hydrocarbons were not studied in detail because of the low yields (<1% of total gaseous products) obtained in this research. Fig. 4 presents that as increasing the gasification temperature, the molar yields of H2, CO, and CO2 (normalized per mole carbon in Lanna coal) showed a dramatically increasing trend. At 800 C, the 20 wt% K2CO3 loading amount increased the molar yields of H2 and CO to 1.04 and 0.30 mol mol1-C from 0.31 to 0.07 mol mol1-C obtained by non-catalytic steam gasification process. However, the 10 wt% K2CO3 loading amount can also increase the molar yields of H2 and CO to 1.02 and 0.28 mol mol1-C, respectively. Therefore, the 10 wt% K2CO3 loading amount was the saturation capacity for conducting the catalytic steam gasification of Lanna coal at a gasification temperature of 800 C. 3.2. Effect of utilization of different LRCs Fig. 5 shows the syngas compositions after 1 h catalytic steam gasification process by changing LRCs. In Fig. 5, the molar yields of syngas (H2 and CO) from Lanna and IBC coals were kept at similar value and only the molar yields of syngas from KPU coal displayed a little difference, when increasing the catalyst loading amount from 10 to 20 wt%. When integrating the molar yields of syngas from these three Table 2 XRF analysis on Lanna, KPU, and IBC coal.
*
Samples
Mg
Al
Si
P
S
K
Ca
Fe
Mn
Others
Lanna KPU IBC
0.18 19.40 25.79
1.21 14.58 4.36
2.17 9.94 5.27
1.62 e e
16.40 10.37 10.58
1.38 0.93 0.35
6.65 35.72 30.35
68.90 7.19 22.39
0.45 0.02 0.25
1.04 1.84 0.66
All based on weight percentage (wt%).
Fig. 2. SEM analysis on a) K2CO3 catalyst, b) Lanna coal with 10 wt% K2CO3, c) KPU coal with 10 wt% K2CO3, and d) IBC coal with 10 wt% K2CO3.
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X. Yuan et al. / Journal of the Energy Institute xxx (2018) 1e9
Fig. 3. Carbon conversions of Lanna coal by changing catalyst loading amounts and gasification temperatures within 1 h.
Fig. 4. Syngas compositions of Lanna coal at different temperatures within 1 h: a) H2, b) CO, c) CO2, and d) CH4.
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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6
X. Yuan et al. / Journal of the Energy Institute xxx (2018) 1e9
Fig. 5. Syngas compositions at gasification temperature of 800 C within 1 h: a) Lanna coal, b) KPU coal, and c) IBC coal.
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coals, it can be concluded that the K2CO3 loading amount of 10 wt% and gasification temperature of 800 C were the optimal operating parameters for producing H2-rich syngas in catalytic steam gasification of these two LRCs. On the other hand, Fig. 5 also shows that when loading the K2CO3 catalyst or not, it can be seen that the significant difference of CH4 and H2 yields occurred, the reason for this phenomena mainly caused by the methane decomposition (CH4 þ H2 O/CO þ 3H2 ; H 0 ¼ 206 kJ mol1 ) which could be enhanced by loading K2CO3 catalyst [22]. The gasification reactivity was studied by using Eq. (2). Table 3 shows the reactivity index R0.5 with different catalyst loading amounts and gasification temperatures. It can be seen that the R0.5 showed an increasing trend when only the gasification temperature increased and other parameters were maintained as constants. At 800 C, the R0.5 increased dramatically as increasing the K2CO3 loading amount from 10 to 20 wt%, compared to the R0.5 attained from the non-catalytic steam gasification process (Raw coal); this means that as the amount of the loaded K2CO3 increased, the gasification reactivity obtained by these three LRCs also increased. However, for these three LRCs, no significant difference was observed in R0.5 when loading 10 wt% catalyst or 20 wt% catalyst at each gasification temperature. Therefore, it can be concluded that for these three LRCs, the 10 wt% K2CO3 loading amount was the saturation point and provided equivalent gasification reactivity. Based on Tables 1 and 2, the higher Ca content in KPU and IBC coals than that of Lanna at the same quantity of each coal, and the synergistic effect between Ca and K was verified in gasification process [31]. Thus, the R0.5 obtained from KPU and IBC coals presented higher values than R0.5 obtained from Lanna coal at the same gasification temperature.
3.3. Kinetic study by changing gasification temperature and coal sample Based on the fitted data for the K2CO3-catalyzed steam gasification of three LRCs using the RPM model, the reaction rate constants at the three different gasification temperatures were calculated. It can be seen from Table 4 that the kRPM increased as the gasification temperature was increased and obviously increased when loading 10 wt% K2CO3 at each gasification temperature, compared with the kRPM obtained by the non-catalytic steam gasification process. In addition, both the activation energy Ea and the pre-exponential factor A were obtained from the Arrhenius equation (Eq. (4)). Therefore, the ln k(T)-1/T plot for these two different types of LRCs are shown in Fig. 6, and the fitting results for A and Ea are listed in Table 5. The results show that compared with the Ea obtained from the non-catalytic steam gasification, the Ea can be reduced by 33.70 kJ mol1 for Lanna coal, 14.21 kJ mol1 for KPU coal, and 18.66 kJ mol1 for IBC coal, when loading with 10 wt% K2CO3. In addition, after conducting the catalytic steam gasification at 800 C, these three gasified residues were collected and then analyzed by XRD method. Fig. 7 presents that KHCO3, K2SO4 and KAlSiO4 mainly existed in these three gasified residues, which well correlates with several previous studies [13,20,32e34]. For lowering the catalyst investment of catalytic steam gasification, the potassium catalyst should be recovered by our previous studies [13,20], indicating that potassium catalyzed steam gasification equipped with catalyst recovery technology will play a significant role in the near future.
3.4. Performance evaluation with the mixture of LRCs These three LRCs were mixed together at mass ratio of 1:1:1, termed as LKI sample. This LKI sample was utilized for conducting K2CO3catalyzed steam gasification at 800 with 10 wt% catalyst loading amount. The XC reached 86.22% and the syngas yields were obtained as 0.33 mol mol1-C of CO, 0.51 mol mol1-C of CO2, 0.95 mol mol1-C H2, and 0.03 mol mol1-C of CH4. Therefore, these three LRCs can be mixed for conducting catalytic steam gasification with K2CO3 catalyst loading, which is more feasible and applicable for industrial applications.
Table 3 Reactivity index R0.5 (*103 min1) calculation at different operating conditions. Temp.
Lanna
( C)
Raw Coal
10% K
20% K
KPU Raw Coal
10% K
20% K
IBC Raw Coal
10% K
20% K
600 700 800
1.83 3.83 6.33
2.33 6.97 13.72
2.83 7.44 14.61
2.41 4.87 10.54
3.23 8.11 18.30
4.16 9.43 19.24
3.17 5.27 11.90
4.33 9.26 19.10
5.00 9.80 18.67
Table 4 Reaction rate constant (k) calculations by RPM at different operating conditions. Temp.
RPM
Lanna
( C)
(*104 s1)
Raw Coal
10% K
Raw Coal
10% K
Raw Coal
10% K
600
k R2 k R2 k R2
0.10 0.979 0.55 0.997 1.25 0.999
0.48 0.989 1.35 0.999 2.56 0.999
0.28 0.996 0.91 0.997 2.59 0.998
0.56 0.999 1.12 0.992 3.67 0.997
0.27 0.996 0.85 0.999 2.14 0.999
0.79 0.998 1.78 0.999 3.89 0.998
700 800
KPU
IBC
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Fig. 6. Arrhenius plots with Lanna coal (square symbol), KPU coal (diamond symbol), and IBC coal (circle symbol).
Table 5 Activation energy (Ea) and pre-exponential (A) factors by RPM. Parameter
1
A (s ) Ea (kJ mol1) R2
Lanna
KPU
IBC
Raw Coal
10% K
Raw Coal
10% K
Raw Coal
10% K
0.41 99.18 0.961
9.44 65.48 0.989
1.07 86.50 0.999
4.14 72.29 0.913
0.39 80.61 0.999
1.80 61.95 0.995
Fig. 7. XRD analyses of K2CO3 and gasified residues, where 1, 2, and 3 represent KHCO3, K2SO4, and KAlSiO4, respectively.
4. Conclusions In this research, both experimental and kinetic investigations on K2CO3-catalyzed steam gasification of three LRCs were conducted by changing gasifying temperature and catalyst loading amount, in order to optimize this entire process for hydrogen-rich syngas production. After conducting experiments of K2CO3-catalyzed steam gasification, the H2-rich syngas could be obtained at gasifying temperature of 800 C with loading amount of 10 wt% K2CO3. The 10 wt% K2CO3 loading amount was proved as the saturation point for this K2CO3-catalyzed steam gasification of LRCs in a fixed bed reactor and provided equivalent gasification reactivity when compared with those using 20 wt% K2CO3 loading amount. For the kinetic study, the RPM was used to predict the characteristics of K2CO3-catalyzed steam gasification process,
Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011
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including activation energy (Ea) and reaction rate constant (k). The RPM model well fitted the experimental data at 800 C better than those at 700 C and 600 C, since it revealed the highest R2 (0.999) value at 800 C. The mixture sample (LKI) of three LRCs was also investigated as a feedstock in catalytic steam gasification process, implying that it is more feasible and applicable in industrial applications. Acknowledgement This work was supported by the “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), and was granted financial resource from the Ministry of Trade, Industry & Energy, South Korea. (Project No: 2015-4010-200820). References [1] M. Shahnaz, S. Yusup, A. Inayat, D.O. Patric, A. Pratama, Application of response surface methodology to investigate the effect of different variables on conversion of palm kernel shell in steam gasification using coal bottom ash, Appl. Energy 184 (2016) 1306e1315. [2] L. 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Please cite this article in press as: X. Yuan, et al., Investigation of Indonesian low rank coals gasification in a fixed bed reactor with K2CO3 catalyst loading, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.011