ex Situ Coal Char with CO2 in a Micro

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Isothermal Gasification of in Situ/ex Situ Coal Char with CO2 in a Micro Fluidized Bed Reaction Analyzer Fang Wang,† Xi Zeng,*,‡ Ruyi Shao,§ Yonggang Wang,† Jian Yu,‡ and Guangwen Xu*,‡ †

Downloaded by CENTRAL MICHIGAN UNIV on September 15, 2015 | http://pubs.acs.org Publication Date (Web): July 30, 2015 | doi: 10.1021/acs.energyfuels.5b00676

School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing 100083, People’s Republic of China ‡ State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot, Inner Mongolia 010051, People’s Republic of China ABSTRACT: The so-called micro fluidized bed reaction analyzer (MFBRA) was adopted to implement the isothermal gasification of in situ coal char with CO2 under minimized external diffusion inhibition. It was done by keeping the reaction atmosphere and temperature in the MFBRA for both coal pyrolysis and char gasification. This was further compared to the gasification of two other kinds of ex situ chars. While the gasification of ex situ char 1 referred to the coal pyrolysis in Ar and then CO2 gasification of the resulting hot char without thermal annealing (cooling) in the same MFBRA, that of the ex situ char 2 was by first a separate coal pyrolysis in Ar and then gasification of the char in the MFBRA after thermal annealing (cooling). Reaction characterization and kinetic parameters based on the measured time-series product gas composition of CO clarified that the in situ coal char had the highest gasification reactivity and the lowest activation energy, whereas the ex situ coal char 2 showed the lowest gasification reactivity and the highest activation energy. Comparing the gasification behavior of chars made with Yima (YM) bituminous coal and Xilinhaote (XLHT) lignite further demonstrated that the char of XLHT lignite had the higher reactivity and smaller activation energy, complying with the higher gasification activity of lignite char than that of bituminite char. influence the char gasification reactivity.15 Up to now, numerous literature studies have reported the effect of the preparation condition of char on its gasification reactivity.16 However, in an industrial gasifier, regardless of a fluidized bed, an entrained flow bed, or others, fuel particles undergo a continuous process from drying, pyrolysis, and gasification under nearly the same conditions. Despite this, limited by measurement principle and analyzer structure, most of the char gasification data in the literature cannot fully reflect the entire practical experience of fuel particles in a real gasifier. The condition of gasification is usually different from that of char preparation.17,18 This raises the necessity to study the gasification behavior and its kinetics of in situ char that is prepared in the same atmosphere and at the same temperature as gasification and has no thermal annealing (cooling) after pyrolysis. This study introduces the so-called micro fluidized bed reaction analyzer (MFBRA) as a new gas−solid reaction analyzer19−21 to prepare and gasify the preceding in situ char. Using a micro fluidized bed (MFB) reactor in inner diameters of 10−20 mm, the analyzer enables quick transfer of heat and mass at a differential reaction scale to realize rapid heating and, meanwhile, to minimize the (external) diffusion inhibition. The online jetting feed of a reactant sample in micrograms into the MFB at a preset temperature guarantees the applicability of MFBRA to quick reactions and thermally unstable materials. In

1. INTRODUCTION Gasification is the high-efficiency clean technology for converting carbonaceous materials into synthesis gas and fuel gas.1 In any commercial gasifier, it has to involve a complicated process interlinking many physiochemical interactions, such as drying, pyrolysis, combustion, char gasification, and homogeneous gas-phase reactions. Of them, char gasification is always the rate-determining step because of its low reaction rate.2−4 The clear and precise understanding of char gasification reactions, including char−CO2, char−steam, and char−O2 reactions, and their kinetics comprises the essential fundamental for optimal design and operation of actual gasifiers.5,6 Summarizing the extensive studies in the literature about char gasification, one may see that there are generally two groups of experimental methods. One is by pyrolysis of coal in a reactor inside an inert atmosphere and then gasification of the hot char through switching the inert atmosphere into a reactive atmosphere, such as from N2 into CO2.7−9 Another is by first preparing char in a separate reactor and then putting the cooled char sample into another reactor to perform the gasification experiment.10,11 The chars used in these two methods are defined as ex situ char 1 and ex situ char 2, respectively. The conditions of pyrolysis always strongly affect the physiochemical properties of the prepared char.12−14 The first method (ex situ char 1) uses thermogravimetric analysis (TGA) as the experimental apparatus, and its replacement of a gas atmosphere would seriously affect the reaction because of gas mixing and diffusion. For the ex situ char 2, the cooling and secondary heating to the char in the gasification must change the properties of the char sample and, in turn, strongly © 2015 American Chemical Society

Received: March 31, 2015 Revised: July 4, 2015 Published: July 30, 2015 4795

DOI: 10.1021/acs.energyfuels.5b00676 Energy Fuels 2015, 29, 4795−4802

Article

Energy & Fuels Table 1. Proximate and Ultimate Analyses of Coal Used in Experiments


proximate analysis (wt %, air dried)

ultimate analysis (wt %, dry and ash free)

coal

M

A

V

FC

C

H

S

O

N

YM XLHT

7.9 12.5

10.2 14.6

32.0 31.7

49.9 41.2

80.3 75.1

5.0 4.3

0.4 1.1

13.1 18.7

1.2 0.8

Figure 1. (a) Schematic diagram and (b) picture of the adopted MFBRA. Previous studies have shown that this flow rate minimized the effect of heat- and mass-transfer resistance.22 After the desired temperature and stable fluidization of quartz sand was reached by CO2, about 50 mg of coal reactant was instantaneously injected into the reaction zone of the MFB reactor to initiate the in situ char pyrolysis and, in turn, gasification reaction. As shown in Figure 2a, the ending point of devolatilization and also the starting point of gasification for the in situ char can be determined by combining the concentration curves of typical gas components (e.g., H2 and CH4) and the baseline of mass spectrometry (MS) (without reaction). This actually decoupled the coal fast pyrolysis and char gasification to take full advantage of the MFBRA, including instantaneous online sample loading, high heating rate, and real-time monitor of the product gas composition. Via thermogravimetric analysis (TGA), it is impossible to realize such a reaction decoupling and also the pyrolysis at a given temperature because its coal sample has to be loaded into the cell before heating the analyzer. The ending point of the char−CO2 gasification reaction in the MFBRA can be determined by combing the concentration curve of CO and the baseline of MS. All of the gas produced in the gasification reaction was collected using a few gas bags in time series to analyze the gas composition in a micro gas chromatograph (micro GC, Agilent 3000 A). The reaction characteristics were, in turn, analyzed in terms of product composition, gas concentration, mass balance, and kinetic parameters on the basis of the data measured by both MS and micro GC. To perform the gasification of ex situ char 1, the coal sample was first injected into the reaction zone of the MFB reactor to perform pyrolysis in an Ar atmosphere. Then, gasification of the generated char was started by switching the Ar stream to CO2 stream, as shown in Figure 2b. In testing the gasification of ex situ char 2, the char sample was prepared in advance by laboratory fluidized bed pyrolysis in highpurity Ar (99.999 vol %).24 The produced char, i.e., the ex situ char 2, was obtained by naturally cooling it to room temperature in an Ar atmosphere. The char was, in turn, injected into the reactor of the MFBRA to testing its CO2 gasification. Table 2 benchmarks the conditions for the three tests of char gasification. Only the first test is for the in situ char that was prepared and gasified under the same conditions, including atmosphere,

our previous study, the MFBRA has been successfully used to characterize a series of reactions, including biomass pyrolysis,19 graphite combustion,20,21 ex situ char gasification,22 and material preparation via calcination and reduction.21,23 However, the feasibility of MFBRA for in situ char gasification remains to be further verified. The important feature of this study is to implement a gasification test for in situ char with CO2 in the MFBRA under minimized limitations from heat and mass transfer. For comparison, the char−CO2 gasification behavior and kinetics are also studied for the ex situ coal chars 1 and 2 described above. Then, comprehensive analyses are performed to gain a better understanding of the char−CO2 reaction under conditions close to that occurring in commercial gasifiers.

2. EXPERIMENTAL SECTION 2.1. Experimental Approaches. Two kinds of coal with widely different physiochemical properties were adopted to make the char samples for gasification tests, which were YiMa (YM) bituminous coal and XiLinHaoTe (XLHT) lignite. Table 1 shows the results of their proximate and ultimate analyses. The volatile content is equivalent for both such coals, but the YM coal has the higher content of fixed carbon and element carbon. Prior to their use in tests, the coal particles were crushed and sieved to the desired sizes of 150−180 μm and dried in an air oven at 105 °C for 2 h. Panels a and b Figure 1 show a schematic diagram and a picture of the employed MFBRA, respectively. It mainly included a gas supply section, an electric heating furnace, a MFB reactor, an online sample feeding system, and a product purification and measurement system. The quartz MFB reactor had an inner diameter of 20 mm and two gas distributors, making it into a bottom gas preheating zone of 70 mm long, a middle reaction zone of 50 mm long, and a top section of 70 mm long for catching the solid particles escaping from the reaction zone. Quartz sand of 100−150 μm (about 3 g) was used as the fluidization medium. The carrier gas Ar (99.999 vol %) and gasification agent CO2 (99.999 vol %) were both fed at a flow rate of 1.0 L/min. 4796


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estimated by considering the main reaction of C + CO2 → 2CO, according to the following eqs 1−4. To understand the data analysis method better, Figure 3 shows the main parameters involved in the

Figure 3. Analysis approach adopted for the char−CO2 reaction in the MFBRA. equations and their definitions according to the MS data for the gasification of the ex situ char 2 (at 900 °C) made from YM coal.

wf = Figure 2. Approach to measure the gasification reaction with CO2 at 900 °C for (a) in situ YM coal char and (b) ex situ YM coal char 1 in the MFBRA.

wi =

Table 2. Comparison on Conditions for in Situ and ex Situ Char Gasification

in situ char ex situ char 1 ex situ char 2

pyrolysis atmosphere

gasification agent

char sample condition

CO2 Ar Ar

CO2 CO2 CO2

hot in situ hot ex situ cold ex situ

12LC̅CO(tf − t0) 22.4 S0 → ti S0 → t f

t

wf =

ti t0 − Imass ) dt 12LC̅ (t − t ) ∫0 i (Imass CO f 0 tf t t0 i 22.4 ∫0 (Imass − Imass) dt

S0 → tiwf w xi = i = = wf S0 → t f wf

R=−

(1)

(2)

t

ti t0 ) dt − Imass ∫0 i (Imass t

tf t0 − Imass ) dt ∫0 f (Imass

dx 1 dwi = i wf dt dt

× 100% (3)

(4)

In eqs 1−4, wi and wf represent the mass of carbon in produced CO from the start of reaction (at time t0) to arbitrary time ti and the reaction-ending time tf, respectively, S0 → ti and S0 → tf refer to the integration area between the curve of CO and the baseline of MS (without reaction) from time t0 to arbitrary time ti and reaction-ending t0 ti tf , Imass , and Imass are the intensity values in the time tf, respectively, Imass MS spectrum at reaction start (the intensity of the baseline) and reaction times ti and tf, respectively, L and C̅ CO denote the gas flow at the reactor outlet and average molar CO concentration obtained by analyzing the collected gas between t0 and tf using a GC, respectively. The differential equation for expressing the gas−solid gasification rate can be shown in eq 5. Taking the logarithm on both sides of this equation, we can obtain eq 6 to calculate the kinetic data, including activation energy E, as

temperature, and also reactor. The major difference between the ex situ chars 1 and 2 is that char 1 is not cooled to room temperature or does not experience a thermal annealing (cooling) after pyrolysis and both coal pyrolysis and char gasification occur in the same reactor. The ex situ char 2 is separately made using another reactor and is further cooled to room temperature, so that it is in fact a separately made cold char. The pyrolysis time in making the char sample should have a big influence on the properties of the resulting char. In this study, the pyrolysis time for the in situ char was automatically decided by reaction (see Figure 2a), while for the ex situ chars 1 and 2, the pyrolysis was kept to the reaction end, judged by monitoring the released gas product. Regardless of the in situ and ex situ char, in a gasification test, the char sample used was fully converted, which was determined according to the release curve of CO and also the baseline in process MS. 2.2. Analysis Approaches. To analyze the structure of char, the samples of in situ char and ex situ char 1 were also prepared in a fluidized bed reactor at 850 °C in atmospheres of CO2 and Ar, respectively. Different from natural cooling of char in nitrogen, after pyrolysis, the in situ char and ex situ char 1 were quickly discharged into a tank with liquid nitrogen. This led to rapid quench of the char samples to preserve the origin physiochemical properties of the char in possibly the biggest degree.15,25 The pore structure of the three kinds of char were measured using an automatic volumetric adsorption analyzer (Micromeritics ASAP 2020) adopting a N2 adsorption method at 77 K. For isothermal gasification of the in situ and ex situ chars in the MFBRA, the carbon conversion (x) and reaction rate (R) are

⎛ E ⎞ dx ⎟f (x) = k(T )f (x) = A exp⎜− ⎝ RT ⎠ dt

(5)

dx E =− + ln(A) + ln f (x) dt RT

(6)

ln

where k(T) is the reaction rate constant as a function of the temperature T and A is the frequency factor.

3. RESULTS AND DISCUSSION 3.1. Gasification Behavior of in Situ and ex Situ Chars. Panels a and b of Figure 4 show the relationship of carbon conversion (x) and reaction time (t) for gasification with CO2 of the in situ YM coal char and ex situ YM coal chars 1 and 2 at 4797


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Figure 4. Variation of char conversion with reaction time in the MFBRA for YM coal char.

Figure 5. Variation of char conversion with reaction time in the MFBRA for XLHT coal char.

temperatures of 800−950 °C. For each kind of YM coal char, the gasification reaction became quicker at a higher reaction temperature. For a given conversion, a shorter reaction time is thus needed for a higher reaction temperature. For example, gasifying the in situ YM coal char at 800 °C needed about 10 min for reaching the conversion of 0.5 but only 3 min at 950 °C. In comparison of three kinds of coal char, one can see that, for achieving a specified conversion, the reaction time needed for the in situ char was shortest, while it was longest for the ex situ char 2. These differences indicate the strong influence of atmosphere and annealing in making char on the gasification behavior of the resulting char. A similar phenomena and variation can also be observed for the XLHT lignite char, as shown in panels a and b of Figure 5. For example, at 850 °C, the reaction time needed for reaching 50% conversion of 0.5 was 1.26, 2.73, and 4.44 min for the in situ char, ex situ char 1, and ex situ char 2, respectively. Panels a and b of Figure 6 display the relationship of the reaction rate and carbon conversion for the in situ and ex situ YM coal chars at gasification temperatures of 800−950 °C. As a strong endothermic reaction, char gasification was found to be very sensitive to the reaction temperature. For each kind of coal char, the reaction rate increased quickly with raising the temperature. When the gasification reaction of in situ char is taken with CO2 at 800 and 900 °C as examples, at the conversion of 0.1, the reaction rate was 3 times higher for the higher temperature. In comparison of the three kinds of YM coal char at the same temperature and conversion, one can see that the reaction rate followed a decreasing order of in situ char > ex situ char 1 > ex situ char 2. The similar results were obtained also for the XLHT lignite char, as shown in panels a and b of Figure 7. In comparison of the same type of coal char

Figure 6. Variation of the reaction rate with conversion for char−CO2 gasification of YM coal char in the MFBRA.

for YM and XLHT coals in Figures 6 and 7, one can see that the XLHT coal char had the higher reaction rate and 4798


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Figure 8. Ratio of the reaction rate at conversions of 0 and 50% between in situ and ex situ chars for their CO2 gasification: (a) YM coal char and (b) XLHT coal char.

Figure 7. Variation of the reaction rate with conversion for char−CO2 gasification of XLHT coal char in the MFBRA.

gasification reactivity. Table 3 shows the pore structure of the tested YM chars. The in situ char had the most developed pore

gasification reaction activity, especially at relatively lower conversions. For example, at 900 °C and conversion of 0.1, the reaction rates of the in situ YM coal char and XLHT coal char were about 0.005 and 0.05 s−1, respectively. Moreover, the curve shape of the reaction rate versus conversion for the two types of coal char was also different, especially at temperatures above 850 °C. The decrease of the reaction rate with increasing conversion was relatively slower for YM coal char than for XLHT coal char at the lower conversions, such as below 0.5. To clarify the difference in the reaction rate among the tested coal chars, the relative activity index Rin situ/ex situ was estimated to represent the rate ratio between the in situ char and ex situ char 1 or between the in situ char and ex situ char 2 at a given conversion.25 Panels a and b of Figure 8 show the variation of this relative activity index with the temperature in the range of 800−950 °C at conversions of 0 (initial reaction) and 0.5 for the YM and XLHT coal chars, respectively. Both Rin situ char/ex situ char 1 and Rin situ char/ex situ char 2 gradually decreased with raising the temperature. The difference in the reaction rate between the in situ char and ex situ char 2 was more obvious. At 800 °C, the initial reactivity of the in situ char was 1.5 and 2.0 times higher than that of the ex situ char 1 but 2.1 and 4 times higher than that of the ex situ char 2 for YM and XLHT coals, respectively. Thus, the data about the in situ char reflect the reaction nature more in actual gasifiers and allow for more accurate industrial designs, while the MFBRA provides a possible instrument to obtain the kinetic data of in situ chars. The differences in the char gasification rate between in situ char and ex situ char further demonstrate that the pyrolysis atmosphere and annealing process strongly affect the char

Table 3. Pore Structure of in Situ and ex Situ Chars for YM Coal Char

a

char

SBET (m2 g−1)

Smicro (m2 g−1)

Vtotal (mL g−1)

Daa (nm)

in situ ex situ 1 ex situ 2

40.48 34.63 32.28

28.83 24.42 21.26

0.11 0.086 0.081

1.26 1.28 1.31

Da = average diameter of pores.

structure and the biggest total surface area, whereas the ex situ char 2 had the smallest total surface area and largest average pore diameter. Generally, the use of CO2 in a pyrolysis atmosphere evidently increases the total surface area, especially the area of mesopores, to form more active sites on the char surface (like activation by CO2).26,27 On the other hand, the carbon crystalline structure in char resulting from an active atmosphere is less ordered than that from an inert atmosphere.28,29 For the ex situ char 2, during its annealing, the pore structure and functional group on the char surface would become different from those of the ex situ char 1.30,31 All of these would be responsible for the highest gasification reactivity of the in situ char and lowest gasification reactivity of the ex situ char 2. Here, we did not show many morphological characteristics of the compared char samples because we had difficulty in obtaining a real in situ coal char sample. On the other hand, a previous study of ours has also clarified the different surface characteristics of in situ and ex situ chars.15 4799


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3.2. Reaction Kinetics of in Situ and ex Situ Chars. To describe the reaction behavior of char gasification, the volume reaction model (VRM) and shrinking core model (SCM) were used to fit the experimental data presented above. Equations 7 and 8 present the two models,32,33 respectively. ⎛ E ⎞ dx ⎟(1 − x) = A exp⎜ − or ⎝ RT ⎠ dt ⎛ E ⎞ ⎟t −ln(1 − x) = A exp⎜ − ⎝ RT ⎠

(7)

⎛ E ⎞ dx 2/3 ⎟(1 − x) = A exp⎜ − or ⎝ RT ⎠ dt ⎛ E ⎞ ⎟t 3[1 − (1 − x)1/3 ] = A exp⎜ − ⎝ RT ⎠

(8)

Panels a and b of Figure 9 compare the results fitting the experimental data for the in situ YM coal char by the VRM and

Figure 10. Arrhenius plot for CO2 gasification of YM coal char at different conversions: (a) in situ char, (b) ex situ char 1, and (c) ex situ char 2.

slope and intercept of each curve, the activation energies and pre-exponential factors for the in situ and ex situ chars can be calculated, and Table 4 summarizes the obtained data. Regardless of YM coal and XLHT coal, the in situ char gasification had the lowest activation energy, whereas the gasification of the ex situ char 2 had the largest activation energy. Moreover, for each kind of coal char, the kinetic data from YM coal char were higher than those from XLHT coal char, showing that lignite char has the higher gasification activity. These obvious differences in kinetic data further demonstrate the necessity and essential for using in situ char to perform the kinetic study. This is also helpful to the understanding of the char gasification behavior and to the design of an actual gasifier.

Figure 9. Comparison of experimental data and predictions by models of (a) VRM and (b) SCM.

SCM, respectively. From Figure 9a, one can see that, at low (below 0.1) and high (above 0.75) conversions, the deviation between the prediction by VRM and the experiment was large, especially at 800−850 °C. For the SCM shown in Figure 9b, the model prediction fits the experimental data well at conversions below 0.93 and the fitting coefficient was above 0.96 for all examined temperatures. For the ex situ YM coal chars 1 and 2 and also the three kinds of XILT coal chars, similar results were obtained in Figure 9. Consequently, the SCM was finally adopted to depict the char gasification reaction and also to estimate the kinetic parameters. The kinetic data of gasification by CO2 for the YM and XLHT coal chars can be obtained from linear fitting of ln K and 1/T, as shown in Figures 10 and 11. In all of the panels, the fitting lines were basically parallel for different conversions with the fitting degree above 0.95, further demonstrating the good fitness of the experimental data by the SCM. According to the

4. CONCLUSION A distinctive characteristic of this study is the implementation of gasification of in situ YM coal char and XLHT coal char with CO2 in the so-called MFBRA through clearly decoupling coal pyrolysis and char gasification. For comparison, the other two kinds of ex situ chars (named ex situ chars 1 and 2; please refer to the Experimental Section) commonly adopted are also tested in the MFBRA. With minimized limitations of heat and mass 4800


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transfer, the use of the MFBRA clarifies the gasification behavior and its corresponding reaction kinetics of in situ and ex situ chars. The results demonstrate that the in situ coal char had the highest reactivity and lowest activation energy for gasification, whereas the ex situ coal char 2 had the lowest reactivity and highest activation energy. In comparison to each kind of YM coal char, the XLHT coal char had the higher reactivity for CO2 gasification and, thus, the lower activation energy. These differences between the in situ coal char and ex situ coal chars fully show the great effect of the pyrolysis atmosphere and annealing (cooling) on the char gasification behavior and kinetics. The work also verified the feasibility of using the MFBRA to realize the reactions for in situ char.

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AUTHOR INFORMATION

Corresponding Authors Downloaded by CENTRAL MICHIGAN UNIV on September 15, 2015 | http://pubs.acs.org Publication Date (Web): July 30, 2015 | doi: 10.1021/acs.energyfuels.5b00676

*Telephone/Fax: +86-10-8254-4886. E-mail: [email protected]. *Telephone/Fax: +86-10-8254-4886. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Natural Science Foundation of China (21306209), the financial support from the National Basic Research Program of China (2011CB201304), the Strategic Priority Research Program of Chinese Academy of Sciences (CAS) on clean and high-efficiency utilization of lowrank coal (XDA07050400), and the National Instrumentation Grant Program (2011YQ120039).

■

Figure 11. Arrhenius plot for CO2 gasification of XLHT coal char at different conversions: (a) in situ char, (b) ex situ char 1, and (c) ex situ char 2.

Table 4. Kinetic Data of Measured Char−CO2 Gasification in This Study coal YM coal

XLHT coal

Ea (kJ/mol)

char in situ ex situ ex situ in situ ex situ ex situ

1 2 1 2

183.82 210.72 238.04 147.85 170.70 207.42

± ± ± ± ± ±

7 9 12 8 14 19

A (s−1) 1.26 1.96 3.49 1.73 1.18 2.09

× × × × × ×

R2 5

10 106 107 104 105 107

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>0.96 >0.95 >0.98 >0.99 >0.98 >0.99 4801


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ex Situ Coal Char with CO2 in a Micro

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