Comparison of Catalysts Based on Individual Alkali and Alkaline Earth

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Comparison of Catalysts Based on Individual Alkali and Alkaline Earth Metals with Their Composites Used for Steam Gasification of Coal Katarzyna Zubek,* Grzegorz Czerski, and Stanisław Porada AGH University of Science and Technology, Faculty of Energy and Fuels, al. Adama Mickiewicza 30, 30-059 Krakow, Poland ABSTRACT: One-component catalysts based on alkali and alkaline earth metals (sodium, potassium, and calcium) as well as their composites were applied to the surface of coal samples used in the gasification process. The aim of this work was to compare the impact of these catalysts on the steam gasification of coal by analyzing the results of catalytic and noncatalytic measurements. The use of composites was intended to check whether it is possible to accomplish the synergistic effects (an effect arising between two or more substances interacting together to produce an effect greater than the sum of their individual effects) and overcome the shortcomings of individual metal catalysts. Measurements of steam gasification were conducted by a thermovolumetric method under isothermal conditions at an elevated pressure of 1 × 106 Pa and at four temperatures ranging from 1073 to 1273 K. On the basis of the obtained results, curves of the formation rate of gasification products were developed and yields of main products (hydrogen and carbon monoxide) were evaluated. The influence of the temperature and type of catalyst on the kinetics of H2 and CO formation was determined, and kinetic parameters (activation energy and pre-exponential factor) were calculated on the basis of three models (isoconversional method, grain model, and random pore model). The obtained results showed the effectiveness of the catalysts tested, especially at low temperatures (1073−1173 K). In this temperature range, the one-component catalyst based on Na (3 wt %) was the most effective. Other single-component catalysts (3 wt % K and 3 wt % Ca) were less catalytically active at 1073 K than composite catalysts consisting of 1 wt % Na and 1 wt % K or 1 wt % Na, 1 wt % K, and 1 wt % Ca, although the former composite contained a smaller amount of catalytically active material. These results indicate that at low temperatures the type of catalyst is more important than the quantity. However, the addition of a catalyst, regardless of type, caused a decrease in the activation energy of CO and H2 formation reactions by nearly half in comparison with that seen for the noncatalytic process. catalyst. Scientific works in this field made it possible to distinguish some mechanisms that may be classified as oxidation-transfer theories and electron-transfer theories.13−16 However, despite the extensive research, there have rarely been reports comparing changes in morphology during the in situ coal−ash transition of the catalytic processes that could help in determining the mechanism of reaction for a specific catalyst. For that reason, an analysis of each particular case is needed to obtain unequivocal results. Three categories of catalysts, including alkali metals, alkaline earth metals, and transition metals, are intensively examined because of their efficiency, availability, and low cost.17,18 The AAEM species shows high mobility within the char, which means that in many cases alkali and alkaline earth metals are the most active catalysts.19 An interesting and less common approach is using composites of metals mentioned above as catalysts for the coal gasification process to achieve the synergistic effects and overcome the shortcomings of individual metal catalysts.20,21 The superiority of properly prepared composites over the single-metal catalysts was observed by, among others, Meng et al.22 They found that composite catalysts such as Li2CO3 with Na2CO3 and Li2CO3

1. INTRODUCTION Gasification is a competitive technology for converting carbonaceous materials, including coal, biomass, char, or waste, into syngas or fuel gas.1 Because of the reduced environmental impact and the higher efficiency in comparison with those of traditional combustion, coal gasification has recently received more attention.2,3 The efficiency of this endothermic process depends on various aspects, especially on the operating temperature of gasifiers.4,5 The high temperature is favorable for carbon conversion, but it may bring problems associated with ash melting and high costs of gasification units.6,7 At lower temperatures, such as the temperatures at which fluidized bed reactors operate, a problem of a low fuel conversion may appear. To maintain a high efficiency at low temperatures, the improvement of the process through the addition of catalysts is analyzed.8 Catalysts have the capacity to lower the gasification temperature, increase conversion rates, and enhance the production of the desired gases by changing the selectivity of the process.9,10 However, the exact manner in which individual catalysts influence the process depends on many factors, such as the type of coal, the composition of ash, process conditions, etc. Interaction of a catalyst with coal that is conditioned by aggregation states of catalysts on the coal surface and its porous structure is also an important aspect. This means that the choice of the appropriate catalyst may be problematic.11,12 Understanding the mechanism of the catalytic gasification process could facilitate the selection of a suitable © XXXX American Chemical Society

Special Issue: SEED17 Received: November 15, 2017 Revised: December 22, 2017 Published: December 24, 2017 A

DOI: 10.1021/acs.energyfuels.7b03562 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Characteristics of “Janina” Coal and Ash Compositiona

with K2CO3 show a catalytic effect stronger than that of each of these catalysts separately, and a synergistic effect between Li and K or Na carbonates appears above 1300 K. Zhang et al.23 also proved that coal samples containing two metals, calcium and sodium, exhibit reactivity that is higher than that of either Ca- or Na-loaded coal. Jiang et al.24 investigated the effect of calcium additives on potassium deactivation that is one of the shortcomings of a single-K catalyst. The obtained results indicate that each additive acted as a deterrent to potassium’s deactivation, thus promoting catalytic gasification. Similar results were obtained by Hu et al.,25 which suggests that composite catalysts may be an interesting alternative to the commonly used single-component catalysts. For the reasons given above, the objective of this research was to assess the effect of the addition of various catalysts (one-, two-, and three-component catalysts) on the steam gasification of bituminous coal. For this purpose, the coal samples (with and without catalysts) were subjected to gasification using unique laboratory equipment based on a thermovolumetric method. On the basis of obtained results, the rates of formation of gasification products such as CO, H2, CH4, and CO2 as well as their percentage shares in the resulting gas were determined. Further analysis was conducted with respect to the main components of the resulting gas, carbon monoxide and hydrogen. The kinetics of CO and H2 formation reactions as well as the yields of these gaseous products were determined at various temperatures. Finally, the kinetic parameters of the formation of individual products (CO and H2) were calculated by using the isoconversional method, random pore model (RPM), and grain model (GM). As a result of this work, a suitable catalyst (composition and amount) under given temperature conditions was determined.

value proximate analysis (wt %) moisture, Mad ash, Aad volatile matter, VMdaf fixed carbon, FC ultimate analysis (wt %) carbon, Cdaf hydrogen, Hdaf sulfur, Sdaft nitrogen, Ndaf oxygen,b Odaf ash composition (wt %) SiO2 Al2O3 Fe2O3 MgO CaO K2O Na2O a b

8.7 14.0 46.1 41.7 77.8 3.9 1.3 1.1 15.9 59.4 23.3 8.2 1.2 2.2 2.9 2.8

Abbreviations: daf, dry and ash free state; ad, air-dried state; t, total. Calculated by difference.

Table 2. Coal Samples (CS) Prepared for Gasification type and amount (wt %) of catalyst no catalyst one-component catalyst

2. MATERIAL AND METHODOLOGY 2.1. Characteristics of the Material. Typically, studies of steam gasification are based on measurements using char, with omission of the pyrolysis step, while the authors believe that they should be performed for coals.26,27 That is why the “Janina” coal that can be used for the gasification process in fluidized bed reactors was selected as the material for the research. The results of proximate and ultimate analysis of the raw coal samples as well as the composition of ash from that coal are summarized in Table 1. The selected coal is considered as a relatively reactive feedstock. In the case of gasification, the ash content is especially important because its main components, SiO2 and Al2O3, inhibit the process. The ash from “Janina” coal admittedly contains oxides that catalyze gasification, such as sodium, potassium, or calcium oxides; however, they are in the minority. To increase the number of catalytically active species, samples with additional cations of alkali and alkaline earth metals, introduced by the wet impregnation method, were prepared. The obtained coal samples may be divided into those containing 3 wt % one-component catalysts and those with composite catalysts: (i) 2 wt % two-component catalysts (1 wt % of each component) and (ii) 3 wt % three-component catalyst (1 wt % of each component) (as shown in Table 2). In the case of two-component composite catalysts, the total amount of catalytically active material was 2 wt %, thus less than for the other catalysts. 2.2. Methodology of Measurements. The examinations were performed by the thermovolumetric method based on an analysis of the resulting gas. The experimental methodology developed by the authors allows the investigation of the pressurized gasification process of coal, taking into account the pyrolysis stage. The measurements allow us to determine the composition of the resulting gas and yields of the gaseous products, developing kinetic curves of the formation of the gasification products as well as determining kinetic parameters of the gasification reactions. Moreover, this methodology allows us to

composite catalyst

sample

K

Na

Ca

CS-0 CS-1-Na CS-1-K CS-1-Ca CS-2-K/Na CS-2-K/Ca CS-2-Na/Ca CS-3-K/Na/Ca

− − 3 − 1 1 − 1

− 3 − − 1 − 1 1

− − − 3 − 1 1 1

assess the effectiveness and selectivity of catalysts used in the steam gasification process. The schematic diagram of the laboratory equipment on which the measurements were taken is shown in Figure 1. The equipment consists of several basic systems: a high-pressure reactor with a heating system, a system for feeding the reactor with the gasifying agent (steam), the carrier gas (argon), and coal, a system for collecting and purifying the resulting gas, and the gas analysis system. After the conditions of measurement is stabilized, the coal sample is introduced onto the grate of retort by a piston feeder. The heating of the retort with the sample is performed in an electric oven. The temperature of the coal sample is measured by the sensor of a thermocouple, which additionally sends impulses to the controllerprogrammer maintaining the required temperature of the sample. The system for feeding steam and argon to the reaction zone is composed of a micro pump dosing water, a steam generator, compressed gas cylinders with argon, a set of valves, and a flow rate regulator. The resulting gas is cooled, then cleared, and dried on the filter. After decompression, the contents of carbon monoxide, carbon dioxide, and methane are continuously controlled with an automatic analyzer. In addition, the content of hydrogen is analyzed using gas chromatographs equipped with a thermal conductivity detector (TCD). Isothermal measurements were taken at four temperatures [1073, 1173, 1223, and 1273 K (800, 900, 950, and 1000 °C, respectively)] under an elevated pressure of 1 × 106 Pa. The mass of the sample fed into the retort was 1 × 10−3 kg with a particle size of CS2-K/Ca ≈ CS-1-K ≈ CS-1-Ca > CS-2-Na/Ca > coal without a catalyst. Among the coal samples with one-component catalysts, gasification with Na was definitely the most effective while gasification with K or Ca lasted much longer. Comparing the gasification of coal with one-component catalysts based on K and Ca, one can observe that curves of CO formation were similar, while some differences appeared for curves of H2 formation. We may conclude that potassium- and calciumbased catalysts can be used alternately. The impact of twocomponent catalysts was more varied; i.e., gasification of CS-2K/Na was the most effective (9.6 × 103 s) followed by CS-2-K/ Ca (10.8 × 103 s) and then CS-2-Na/Ca (12.0 × 103 s). These results suggest that two-component composites containing calcium (especially in combination with Na) are less effective than the composite composed of K and Na (CS-2-K/Na). Comparing the two-component composites to one-component catalysts, we found that gasification of CS-2-K/Na was more effective while gasification of CS-2-K/Ca proceeded like gasification of CS-1-K and CS-1-Ca, even though the latter (samples with one-component catalysts) contained 3 wt % catalytically active material. These results confirmed that the E

DOI: 10.1021/acs.energyfuels.7b03562 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 4. Yields of H2 and CO during coal gasification at various temperatures.

An increase in temperature caused acceleration of CO formation in each case (Table 3), but this effect was particularly evident during noncatalytic gasification. At 1073 K, all additives catalyzed the formation of CO. At 1173 K ,the catalytic effect was observed mainly during gasification of coal samples containing 3 wt % catalysts, i.e., CS-1 and CS-3. The twocomponent catalysts had either a slight positive or even a negative impact on the process. At the next temperature, 1223 K, only one catalyst, CS-1-Na, catalyzed CO formation, and at 1273 K, no catalytic effect was observed. The negative impact of a high temperature on catalytic gasification could be caused by melting of compounds formed by interaction between catalyst and ash components or aggregation/sintering of catalysts, resulting in mass-transfer resistance.31,32 The values of k as well as τ0.5 of CO formation confirmed that from the coal samples with one-component catalysts the gasification of CS-1-Na was the most effective at all temperatures, and the influence of the other two catalysts (Ca and K) on CO

smaller amount of composite catalyst, however properly selected, can catalyze the process more effectively than an even larger amount of a one-component catalyst. Gasification of coal with a three-component catalyst (CS-3-K/Na/Ca) lasted 9.6 × 103 s, indicating that supplementation of the most effective two-component catalyst (CS-2-K/Na) with 1 wt % calcium did not affect further enhancement of the catalytic effect. Previous analyses were performed on the basis of measurement curves. However, for quantitative analysis of the gasification process, it is better to use numerical parameters such as the half-time of the reaction (τ0.5) and the reaction rate constant (k). These parameters for CO and H2 formation reactions at all temperatures are listed in Tables 3 and 4. In each case, the k values (calculated in the range from 0 to 80% of the reaction progress) from RPM were lower than analogues values determined on the basis of the GM. F

DOI: 10.1021/acs.energyfuels.7b03562 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels formation was very similar. Among the two-component catalysts, CS-2-K/Na was the most effective additive for CO formation at all temperatures, followed by CS-2-K/Ca and finally CS-2-Na/Ca. The last of the analyzed composites, threecomponent CS-3-K/Na/Ca, catalyzed CO formation at temperatures of 1073−1173 K. At higher temperatures, the use of this catalyst was unfounded. The H2 formation reaction rate constants and half-times of this reaction at various temperatures are summarized in Table 4. The values of kH2 were lower than the values of kCO, suggesting that formation of hydrogen proceeded slower than formation of carbon monoxide. The increase in temperature accelerated H2 formation but to a lesser extent than it did CO formation; thus, the presence of catalysts affected the hydrogen formation reaction also at higher temperatures. At 1073 K, all catalysts showed catalytic activity toward formation of H2. At 1173 K, this effect was mainly observed during gasification of CS-1 followed by CS-3 and one coal sample with twocomponent catalyst CS-2-K/Na. The same catalysts accelerated H2 formation at 1223 K. At the highest temperature, the acceleration of hydrogen formation, caused by the presence of a catalyst, was observed only during gasification of CS-1-Na. The activity of the individual catalytic additives within a certain group of catalysts at all temperatures was as follows: Na > K ≈ Ca for CS-1, and K/Na > K/Ca > Na/Ca for CS-2. The catalytic effect of the three-component catalyst (CS-3-K/Na/ Ca) was observed during gasification in the range of 1073− 1223 K. In summary, at 1073 K, all catalysts accelerated H2 and CO formation. As the temperature increased, the influence of catalysts was weaker. At 1173 K, catalytic activity was observed during gasification of CS-1 (Na > K ≈ Ca), CS-3-K/Na/Ca, and CS-2-K/Na. These catalysts also accelerated H2 formation at 1223 K; however, CO formation at this temperature was catalyzed by only CS-1-Na. Moreover, the one-component catalyst based on sodium was the only additive that affected the process at 1273 K (catalyzed H2 formation) and can be considered as the most effective catalyst. Thus, no synergistic effect of composite catalysts has been found at high temperatures. It can be also stated that the catalytic activity of calcium was greater when it was used as a one-component catalyst (CS-1-Ca) than in combination with other catalytically active materials. 3.3. Yields of Gaseous Products. The progress of formation reactions of main components allows us to determine the course of the process, but information about the yields of these gaseous products should also be provided. The yields of H2 and CO at various temperatures are presented in Figure 4. In addition, the percentages of individual gas components are listed in Table 5. During the noncatalytic gasification of coal, hydrogen was the main product, and its yield and percentage in the resulting gas were almost 2 times higher than those of carbon monoxide. The addition of a catalyst, regardless of its type, resulted in an increase in the amount of main gases (H2 and CO) formed in the process, and hydrogen remained the main component of the resulting gas. The highest yields and percentages of H2 were obtained by using a one-component catalyst based on Na and by using composites CS-2-Na/Ca, CS-2-K/Na, and CS-3-K/ Na/Ca, irrespective of the operating temperature. These results suggest that the presence of sodium cations results in increased yields of this gas. The effect of the other catalysts on the

Table 5. Percentages of Individual Gaseous Components in the Resulting Gas share (vol %) sample CS-0

CS-1-Na

CS-1-K

CS-1-Ca

CS-2-K/Na

CS-2-K/Ca

CS-2-Na/Ca

CS-3-K/Na/Ca

product

1073 K

1073 K

1223 K

1273 K

CO H2 CO2 CH4 CO H2 CO2 CH4 CO H2 CO2 CH4 CO H2 CO2 CH4 CO H2 CO2 CH4 CO H2 CO2 CH4 CO H2 CO2 CH4 CO H2 CO2 CH4

21.3 57.6 19.5 1.6 25.1 59.9 13.9 1.1 30.2 57.2 11.6 1.0 25.8 57.0 15.5 1.7 25.7 58.6 14.3 1.3 26.1 56.8 15.8 0.7 25.5 58.3 15.5 0.7 26.2 59.2 13.9 0.7

24.3 58.3 15.7 1.7 28.3 58.1 12.5 1.1 32.1 57.3 9.1 1.5 31.0 58.0 9.7 1.3 27.4 58.5 13.4 0.7 30.0 56.8 12.3 0.9 26.1 58.6 14.6 0.7 27.7 58.9 12.8 0.6

28.7 58.0 13.2 1.1 30.1 59.9 8.7 1.3 33.2 57.0 8.6 1.2 32.6 56.5 9.0 1.9 29.3 58.2 11.9 0.6 31.1 56.6 11.9 0.4 26.8 58.3 14.4 0.5 28.9 58.9 11.4 0.8

34.1 56.5 8.2 1.2 31.5 59.3 7.9 1.3 33.7 57.0 8.1 1.2 33.7 56.5 8.3 1.5 30.5 58.6 10.2 0.7 31.3 56.8 11.2 0.6 27.2 58.5 13.9 0.4 30.1 58.8 10.3 0.6

amount of H2 formed during gasification was very similar. Moreover, there was no unequivocal impact of temperature on hydrogen yield. The increase in temperature caused, however, an increase in the yield and percentage of carbon monoxide in the resulting gas. At 1073 K, the highest yield as well as percentage of CO was obtained by using a one-component catalyst based on potassium (CS-1-K). The amount and percentage of CO formed during gasification with other analyzed catalysts were very similar. At 1173 K, the highest and comparable yields of carbon monoxide were obtained during gasification in the presence of 3 wt % catalytically active material (whether in the form of one-component catalysts or a three-component composite) and in the presence of composite CS-2-K/Ca. As the temperature increased to 1223 K, this tendency was maintained. The percentage of carbon dioxide, which was generated during pyrolysis as well as during gasification (as a result of secondary reactions), was connected with CO formation. An increase in the yield of CO with an increase in temperature as well as in case of some measurements with a catalyst resulted in a decrease in the percentage of CO2. The percentage of methane in the resulting gas remained at a similar level for all measurements (formation of CH4 took place only during the pyrolysis stage). 3.4. Kinetic Parameters. The final step of the work was to calculate the kinetic parameters of CO and H2 formation. The G

DOI: 10.1021/acs.energyfuels.7b03562 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 6. Kinetic Parameters of CO and H2 Formation isoconversional method 3

RPM 3

sample

Ea(CO)/Ea(H2) (×10 J/mol)

Ea(CO)/Ea(H2) (×10 J/mol)

CS-0 CS-1-Na CS-1-K CS-1-Ca CS-2-K/Na CS-2-K/Ca CS-2-Na/Ca CS-3-K/Na/Ca

78.5/71.6 40.3/35.8 42.7/43.1 42.1/50.1 42.0/41.6 43.1/48.1 42.9/42.3 38.9/39.2

83.3/72.8 44.2/37.9 45.6/45.9 47.4/46.6 42.1/44.8 44.9/46.6 44.3/44.0 41.3/43.3

GM −1

A(CO)/A(H2) (×10

−1

s )

Ea(CO)/Ea(H2) (×10 J/mol)

A(CO)/A(H2) (×10−1 s−1)

83.8/72.8 44.4/37.9 45.4/45.5 47.6/45.7 42.8/45.0 45.5/46.3 44.7/43.3 41.8/43.1

13.5/3.60 0.35/0.15 0.32/0.27 0.38/0.27 0.22/0.25 0.27/0.25 0.20/0.17 0.20/0.22

10.9/3.08 0.28/0.13 0.27/0.23 0.32/0.25 0.17/0.22 0.2/0.22 0.17/0.15 0.17/0.18

3

models used were rather low, but similar values may be found in the literature.33−35

isoconversional method was used as the reference method; moreover, two models describing the heterogeneous gas−solid reaction, i.e., RPM and GM, were used to calculate values of activation energy (Ea) and pre-exponential factor A. The results are summarized in Table 6. The values of activation energy for specific measurement were similar regardless of the method used; however, Ea values calculated on the basis of the RPM or GM were slightly lower than those calculated by using the reference method. The activation energy of CO formation during noncatalytic gasification ranged from 78.5 × 103 to 83.8 × 103 J/mol, depending on the model used. The addition of a catalyst, regardless of its type, caused a significant (almost double) decrease in this value, which proved that all the tested materials can be considered as effective catalysts for the gasification process. The values of the second kinetic parameter, the preexponential factor, were also significantly reduced as a result of the addition of a catalyst, from 1.09 s−1 for the RPM and 1.35 s−1 for the GM in the case of noncatalytic gasification to 0.017− 0.038 s−1 during gasification with catalysts. The Ea value of hydrogen formation from noncatalytic gasification was lower than the Ea of CO formation and amounted to ∼72.0 × 103 J/ mol. The addition of a catalyst resulted a decrease in the activation energy in a varied range, depending on the catalysts used. The lowest values of activation energy were obtained when sodium was the component of the catalyst (especially in the case of CS-1-Na), which confirmed that this element is the most effective catalyst for hydrogen formation. Moreover, twocomponent composites CS-2-K/Na and CS-2-Na/Ca were more catalytically active than most of the one-component catalysts containing 3 wt % active material (except CS-1-Na). Besides, the value of the activation energy calculated for gasification of CS-3-K/Na/Ca was only slightly lower than the Ea for CS-2-K/Na, which suggests that addition of 1 wt % calcium to the latter catalyst affects the reaction of H2 to a very small extent. The effect of catalysts on the pre-exponential factor was less unequivocal; the A values ranged from ∼0.36 s−1 (noncatalytic process) to 0.013−0.027 s−1 (catalytic measurements). In summary, the greatest reduction in kinetic parameters was obtained during gasification of CS-1-Na followed by samples with composite catalysts containing sodium cations: CS-3-K/ Na/Ca > CS-2-K/Na > CS-2-Na/Ca. The rest of the samples behaved as follows: CS-1-K > CS-2-K/Ca > CS-1-Ca. Because the Ea values of CO formation are very similar, this order is primarily dependent on the Ea of hydrogen formation. However, it should be mentioned that the use of these catalysts is mainly effective when the operating temperatures are relatively low. The values calculated on the basis of the

4. CONCLUSION The obtained results confirmed the effectiveness of catalysts based on alkali and alkaline earths metals in steam gasification of coal, especially at temperatures of 1073−1173 K. As the temperature of the process increased, the rate of catalytic gasification also increased but to a lesser extent than the rate of noncatalytic gasification. As a result, at 1273 K, the use of catalysts was unfounded. The negative impact of a high temperature on catalytic gasification was especially visible during the use of composites; thus, no synergetic effect was observed at high temperatures. However, the catalytic activity at 1073−1173 K is a very desirable effect as it allows for efficient gasification in a fluidized bed reactor, operating at relatively low temperatures. The one-component catalyst with Na turned out to be the most catalytically active. Nevertheless, at low temperatures, good results were also obtained in the case of using composites, in particular consisting of K and Na (CS-2K/Na). At 1073 K, this two-component catalyst was more effective than the one-component catalysts containing 3 wt % K or Ca, although the amount of catalytically active material was smaller (2 wt %). Moreover, the results show that the presence of catalysts containing sodium resulted in higher hydrogen yields as well as the greatest reduction in the activation energy of H2 formation among all tested catalysts. On the other hand, potassium cations had the greatest effect on the CO yields, especially at low temperatures. The obtained kinetic parameters show that catalysts tested in this work affect CO formation in a very similar way, so all of them may be considered as effective catalysts of this reaction. Thus, the 3 wt % one-component catalyst based on sodium seems to be the most effective additive for the gasification process, but utilization of a smaller amount of two-component composites (especially CS-2-K/Na) may be an interesting alternative when the process is conducted at low temperatures.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +48696716509. Fax: +48126172577. E-mail: [email protected]. ORCID

Katarzyna Zubek: 0000-0003-1899-0279 Grzegorz Czerski: 0000-0003-1318-4729 Notes

The authors declare no competing financial interest. H

DOI: 10.1021/acs.energyfuels.7b03562 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

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ACKNOWLEDGMENTS This paper was prepared as part of the Dean Grant 15.11.210.405. REFERENCES

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DOI: 10.1021/acs.energyfuels.7b03562 Energy Fuels XXXX, XXX, XXX−XXX