Effect of Char Preparation Conditions on Gasification in a Carbon

Dec 23, 2016 - Pyrolysis conditions have a substantial impact on the properties of the resulting char and, consequently, on the kinetics of the gasifi...
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Effect of Char Preparation Conditions on Gasification in a Carbon Dioxide Atmosphere Grzegorz Czerski,* Katarzyna Zubek, Przemysław Grzywacz, and Stanisław Porada AGH University of Science and Technology, Faculty of Energy and Fuels, aleja Adama Mickiewicza 30, 30-059 Krakow, Poland ABSTRACT: Pyrolysis conditions have a substantial impact on the properties of the resulting char and, consequently, on the kinetics of the gasification process. The aim of this study was to determine the impact of coal pyrolysis on the gasification stage. By applying the thermogravimetric method, kinetic analyses of CO2 gasification of chars derived from Polish “Janina” coal were conducted. Gasification examinations were performed for chars prepared earlier in an argon atmosphere at various heating rates, which, after cooling, were subjected to CO2 gasification (indirect char gasification). Examinations were also carried out in the case of chars formed during heating of coal samples in a CO2 atmosphere (direct char gasification). Samples of chars were gasified in non-isothermal conditions of up to 1100 °C under 0.1 MPa pressure at various heating rates. The char gasification reaction order with CO2 was determined with the use of the Coats−Redfern method, and it can be assumed that it is a first-order reaction. The activation energy and pre-exponential factor were calculated using two first-order models: Coats−Redfern method and Senum−Yang method. The results were subsequently compared to kinetic parameters calculated on the basis of the modelfree isoconversional method combined with the model-dependent Coats−Redfern method. Despite the differences in values of kinetic parameters obtained from the use of a given model, all results confirmed that the method of char preparation has an influence on the gasification stage and direct char gasification is more favorable. Activation energy obtained from the use of models based on the first-order reaction ranged between 275 and 296 kJ/mol for direct gasification of chars, while the chars gasified indirectly between 307 and 342 kJ/mol, depending upon the heating rate that was used. The model-free isoconversional method confirmed these results. The values for chars gasified directly amounted to Ea = 257−277 kJ/mol, and the values for indirect char gasification obtained by the pyrolysis process at heating rates of 3, 10, and 20 K/min amounted to Ea = 280−291, 287−309, and 289−305 kJ/mol, respectively.

1. INTRODUCTION The search for new, efficient processes of electricity generation is nowadays one of the most important objectives of the energy industry. A huge demand for energy intensified by the needs of developing countries must be met. At the same time, social awareness is growing to acknowledge the fact that the coal combustion process, which is the source of approximately 40% of the world’s electricity generation, has ramifications in the form of acid rain, water contamination, or greenhouse effect, to name just a few.1,2 Therefore, efficient and environmentally friendly ways of coal processing are highly desirable. One of the promising technologies is coal gasification, whereby gas is produced, which may be applied in both energy and chemical industries.3−6 At a time when excessive carbon dioxide emissions are considered to be one of the causes of the greenhouse effect, it seems very reasonable to use this gas as a gasifying agent. This solution, besides lowering the relative CO2 emissions, allows for the reduction of the amount of coal and oxidant used in the process, making it more economical.7,8 These advantages have made the process of solid fuel gasification in a carbon dioxide atmosphere become the basis of numerous scientific studies.9−16 An analysis of the coal gasification reveals that it is a very complex process, which consists of two basic stages: pyrolysis with the formation of char and volatile products and gasification of the resulting char. Char gasification with CO2 as a gasifying agent is possible thanks to the Boudouard reaction (eq 1), in which CO, one of the main components of syngas, is obtained.17,18 © 2016 American Chemical Society

C + CO2 ⇔ 2CO

◦ ΔH298 = 172.5 kJ/mol

(1)

This stage is the slowest; therefore, the whole process depends upon the rate of char gasification. For this reason, in numerous scientific studies, previously obtained char was subjected to the gasification process,19−22 which became the basis for defining the impact of the temperature,23 pressure,24 or heating rate25 on the char gasification stage. In each case, the influence of these parameters on the process was observed, thereby reflecting a high dependence of the gasification stage upon predefined conditions. However, the pyrolysis step was omitted in these studies, which, although short, affected the properties of the resulting char and its reactivity. Reactivity, as the main feature of char that determines its usefulness in the gasification stage, largely depends upon the properties of the used coal. Reactivity should be high, which explains why low-rank coals and chars derived from these coals are better feedstock for the gasification process than high-rank coals and their chars.26 The quantity and composition of parent coal mineral matter are also important, as proven that some elements, such as sodium, potassium, or calcium, are catalytically active in the gasification process and compounds, such as SiO2 or Al2O3, show an inhibiting effect.27−30 Char reactivity largely depends also upon its properties, particularly upon porosity, which is affected by the porosity of parent coal and Received: August 24, 2016 Revised: November 25, 2016 Published: December 23, 2016 815

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Energy & Fuels conditions of the pyrolysis process.31−33 Thus, lignites, which are characterized by the highest porosity, seem to be the most appropriate raw materials to the process, in contrast to bituminous coals, which have the lowest porosity.34 During pyrolysis, however, the porosity and the specific surface area (thus, the reactivity) significantly increase.35 The conditions under which the pyrolysis process is carried out, such as the temperature, pressure, atmosphere, heating rate, or preparation of char through devolatilization of coal and rapid cooling of the obtained product prior to being subjected to the gasification process, also affect the properties of the char.36−40 As noticed, the char gasification, which is the slowest step limiting the whole process, is very complex. Despite the fact, a relatively short step of pyrolysis should not be underestimated.41 In view of this observation, the aim of the study was to analyze gasification in the atmosphere of carbon dioxide of char derived from “Janina” coal formed under various conditions. The aim of this study was to determine the impact of char preparation conditions on the gasification stage. Gasification examinations were performed for chars prepared in advance in an argon atmosphere at various heating rates, which, after cooling, were subjected to CO2 gasification (indirect char gasification) and for chars formed during heating of coal samples in a CO2 atmosphere (direct char gasification). On the basis of the obtained results, curves of carbon conversion degree were developed. The influence of the sample type and heating rate on the kinetics of carbon dioxide gasification were determined. The char gasification reaction order with CO2 was evaluated with the use of the Coats−Redfern method.42 The CO2 gasification reaction rate constants for each sample, and the kinetic parameters (pre-exponential factor and activation energy) were calculated using the Coats−Redfern method42 and the Senum−Yang method.43 These results were subsequently compared to kinetic parameters calculated on the basis of the model-free isoconversional method combined with the model-dependent Coats−Redfern method.44

ensure the closest mass of char, which will be gasified (about 30 mg). Measurements were carried out, each consisting of three steps: Stage I, stabilization: the initial conditions were stabilized; i.e., the pressure was at 0.1 MPa, and argon (pyrolysis) or carbon dioxide (gasification) flow was 200 mL/min. This stage lasted 30 min. Stage II, measurement: the temperature was ramped from ambient temperature to 900 °C (pyrolysis) or 1100 °C (gasification) at three different heating rates, 3, 10, and 20 K/min, and the flow of argon or CO2, supplied from the dosing system, was 200 mL/min. The length of this step was dependent upon the heating rate. The final temperature of gasification was designed to ensure the total conversion of the tested samples. Stage III, ending: the heating was turned off, and the temperature decreased in an uncontrolled manner. The flow of argon (pyrolysis) was raised to 400 mL/min to “freeze” the resulting char, and CO2 flow (gasification) was 150 mL/min. 2.2. Characteristics of the Material. The Polish coal from the “Janina” mine was used in this work. This coal as a result of its high reactivity compared to other hard coals can be successfully used in the gasification process. To characterize the coal, proximate and ultimate analyses and elemental composition of the ash were performed. The results are summarized in Table 1.

Table 1. Characteristics of the Coal parameter proximate analysis (wt %) moisture, Ma ash, Aa volatile matter, VMdaf ultimate analysis (wt %) Cdaf Hdaf Sat ash composition (%) SiO2 Al2O3 Fe2O3 MgO CaO K2O Na2O

2. MATERIALS AND METHODS 2.1. Equipment and Examination Methodology. To determine the kinetics of the gasification stage of chars derived from “Janina” coal in a CO2 atmosphere, non-isothermal measurements of up to 1100 °C under 0.1 MPa were carried out using the DynTHERM thermogravimetric analyzer by Rubotherm. A fully automated instrument is a combination of two basic systems: (i) system of the magnetic suspension balance and reactor, where basic measurements are taken, and (ii) a gas and/or vapor dosing system, supplying gases to the reaction zone. The measurements were divided into two parts: (1) the formation of chars in the pyrolysis process conducted separately under a argon atmosphere at various heating rates and (2) the process of CO2 gasification of chars formed during heating of coal (direct gasification) and previously obtained chars (indirect gasification) at various heating rates. During the thermogravimetric measurements, argon and carbon dioxide of high purity (99.998%) were used. The coal sample with a mass of 46 mg and particle size below 0.2 mm was placed in a titanium crucible, transferred to the reactor, and subjected to pyrolysis. The chars obtained in this way remained in an inert atmosphere for a few hours until the initial parameters of the apparatus were achieved and measurement of char gasification (indirect gasification) could be carried out. Moreover, samples of “Janina” coal were subjected to gasification (direct gasification) to compare both processes. A total of 46 mg of coal or 30 mg of char was gasified. The difference in the weight of samples was included to

Janina coal 8.7 14.0 46.1 77.8 3.9 1.0 59.3 23.2 8.5 1.2 2.2 2.9 2.7

The mineral matter and moisture are generally regarded as ballast. The main components of ash, i.e., SiO2 and Al2O3, inhibit the gasification reaction. On the other hand, certain components may act catalytically; therefore, the amount and, above all, the ash composition can have an impact on the course of this process.45 As shown in Table 1, besides inhibiting aluminum and silicon oxides in the ash from the “Janina” coal, there were also oxides of alkali metals, such as potassium, sodium, or calcium, which catalyze the gasification reactions. 2.3. Methodology of Kinetic Analysis. On the basis of the sample weight loss as a result of the increasing temperature, the order of the gasification reaction with CO2 was determined and then the kinetic parameters of the char gasification stage were calculated using three different approaches. 2.3.1. Determination of the Reaction Order by the Coats− Redfern Method. The reaction order was determined by employing a rate law written as follows: dα = k(1 − ∝ )n dt

(2)

where

α=1− 816

m m0

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Figure 1. Loss of the sample weight during coal pyrolysis and gasification processes at a heating rate of 3 K/min. In the above equations, α stands for the gasification reaction progress variable ranging from 0 to 1, m is the current weight of the sample (mg), while m0 is the original weight of the char sample (mg), t stands for the time (min), n is the reaction order, and k is the reaction rate constant (min−1). The rate constant can be represented by the Arrhenius equation

⎛ E ⎞ k = A exp⎜ − a ⎟ ⎝ RT ⎠

Therefore, these equations can be put in a linear form, and the value of n giving the best fit is identified by the correlation coefficient by means of linear regression. In this study, n equal to 0, 1/3, 1/2, 2/3, 1, and 2 was used, because it has been stated that these values have a chemical basis to be considered a reaction order.42 2.3.2. Determination of Kinetic Parameters. 2.3.2.1. First-Order Kinetic Model. On the basis of eq 7, A and Ea can be determined from the intercept and slope of a plot ln[ln(1/(1 − α))] − 2 ln T against 1/ T. 2.3.2.2. Senum−Yang Integral Method. The kinetics of the gasification process may be described by the first-order reaction model (eq 5) for n = 1, which after rearrangements takes the form

(4) −1

where A, Ea, T, and R are the pre-exponential factor (min ), the apparent activation energy (J/mol), the reaction temperature (K), and the molar gas constant (8.314 J mol−1 K−1), respectively. In non-isothermal experiments, the temperature changes as well as the time, and connection between them is provided by the heating rate β, which remains constant throughout the experiment and is the change in the temperature with the time, dT/dt. Combining eqs 2 and 4 and taking into account a constant heating rate provides ⎛ E ⎞ dα A = exp⎜− a ⎟dT ⎝ RT ⎠ β (1 − ∝ )n

⎛ E ⎞ dα A = (1 − α)exp⎜− a ⎟ ⎝ RT ⎠ β dT

The variation of the progress variable α with the temperature is obtained upon the integration of eq 8. Introducing a new variable x = Ea/R/T provides

− ln(1 − α) =

(5)

The left-hand side of eq 5 may be integrated for various values of n, while the right-hand side cannot be integrated directly. The solution is to give an approximation as a series and then truncate it after a small number of terms. After that, the result for n ≠ 1 is expressed as ln

E 1 − (1 − ∝ )1 − n 2RT ⎞ AR ⎛ = ln ⎜1 − ⎟− a 2 βEa ⎝ Ea ⎠ RT (1 − n)T

E 1 2RT ⎞ AR ⎛ − 2 ln T = ln ⎜1 − ⎟− a 1−α βEa ⎝ Ea ⎠ RT

AEa p(x) βR

(9)

While the kinetic parameters are obtained from experimental data, the p(x) integral is not calculated directly. Instead, some approximations are used such as the approximation developed by Senum and Yang.43

p(x) ≅ (6)

exp(− x) x 3 + 18x 2 + 86x + 96 x2 x 4 + 20x 3 + 120x 2 + 240x + 120

(10)

After this approximation is inserted into eq 9, where x = Ea/R/T, a nonlinear regression is required to obtain kinetic parameters. This method uses nonlinear regression proposed by Senum and Yang, which makes it more accurate over a wider range of thermogravimetric analysis (TGA) data and circumvents the inaccuracies related to the analytical approximation of the temperature integral.46

and for n = 1, the equation becomes

ln ln

(8)

(7)

For most of the reaction, Ea ≫ RT, which is why the term ln(AR/ βEa)(1 − (2RT/Ea)) in eqs 6 and 7 can be considered to be constant. 817

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Figure 2. Carbon conversion degree of chars during the gasification stage. 2.3.2.3. Model-Free Isoconversional Method Combined with the Model-Dependent Coats−Redfern Method. As stated before, under non-isothermal conditions, the temperature integral cannot be calculated directly. Thus, it is highly recommended to start the analysis by determining one element, such as the activation energy with the use of isoconversional methods, through a series of measurements taken at different heating rates. The integral isoconversional method is based on the Murray and White approximation of the temperature integral p(x), which leads to ln

E β AT = − a Eg (α) RT T2

occurring reactions. At the beginning of the pyrolysis step, a number of primary reactions (dehydration, dehydrogenation, decarboxylation, repolymerization, and recombination) occurred in parallel, causing decomposition of organic matter and char formation. Above the temperature of inflection, secondary reactions occurred consecutively, which had a significant impact on the properties of char. In primary and secondary pyrolysis, the loss in the sample weight was very similar to each other, in the range of about 13−14%. The greatest weight loss, nearly 50%, was observed in the char gasification stage, which started at about 760 °C and lasted until the conversion of the entire organic matter. In the other measurements of the coal gasification process, the temperature range in which the above steps occurred was shifted to higher temperatures with an increasing heating rate; nevertheless, the accompanying mass losses were very similar. The next step was to carry out the process of pyrolysis in an argon atmosphere at various heating rates (3, 10, and 20 K/ min) to obtain different chars. The process of coal pyrolysis at a heating rate of 3 K/min is shown in Figure 1. When the sample is heated in an inert atmosphere, the pyrolysis process occurred with a distinction between primary and secondary pyrolysis. While the temperature range and weight loss during primary pyrolysis were similar to those in the case of CO2 coal gasification, the secondary pyrolysis lasted much longer, almost to the end of the measurement at 900 °C. A longer duration of the secondary pyrolysis caused also a slightly greater weight loss of the sample in this step. This means that, during heating the coal sample in the atmosphere of CO2, the gasification stage, starting in the range of 760 °C, overlapped with the secondary pyrolysis step, preventing its observation. However, at the end of secondary pyrolysis, the changes in the weight of samples were so small that a successful assumption could be made that the stage of char gasification had already begun. With the increase in the heating rate during pyrolysis, the resulting curves were shifted toward higher temperatures; however, their nature was the same as presented in Figure 1. A similar dependence was observed when increasing the heating rate

(11) 2

Thus, Ea can be evaluated from the slop of a plot ln(β/T ) against 1/T, obtained from the curves at several heating rates. The pre-exponential factor can be determined from the application of the Coats−Redfern method, assuming that Ea and A are correlated through the following relation of compensation:

ln A = a + bEa

(12)

Correlation parameters were obtained from the plot ln[g(α)/T2] versus 1/T, where g(α) is the best fitting model, identified by the correlation coefficient. Thus, once the correlation relation was established and a model-free activation energy value was available, the value of the pre-exponential factor could be determined unambiguously.44

3. RESULTS AND DISCUSSION In the first stage of the research, “Janina” coal was subjected to the direct gasification process at various heating rates (3, 10, and 20 K/min). When the coal was heated to 1100 °C in a carbon dioxide atmosphere, the samples underwent characteristic changes. The process of non-isothermal coal gasification at a heating rate of 3 K/min is shown in Figure 1. In the initial stage of the process, the moisture from the sample was evaporated and gases adsorbed on the surface were removed. The pyrolysis step, i.e., thermal decomposition of the sample, accompanied by the evolution of pyrolysis gases and char formation, started at a temperature of 340 °C and lasted until about 760 °C. The nature of the curve in this temperature range was not uniform (in the vicinity of 460 °C, inflection of the curve took place), indicating a change in the type of 818

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Energy & Fuels Table 2. Coefficients of Determination Calculated for Various Reaction Orders R2 reaction order material chars gasified directly

chars gasified indirectly (pyrolysis at 3 K/min)

chars gasified indirectly (pyrolysis at 10 K/min)

chars gasified indirectly (pyrolysis at 20 K/min)

heating rate during gasification (K/min)

0

0.33

0.5

0.67

1

2

3 10 20 3 10 20 3 10 20 3 10 20

0.9596 0.9088 0.9040 0.9127 0.9363 0.9272 0.9100 0.9146 0.9277 0.9274 0.9271 0.9314

0.9760 0.9437 0.9455 0.9448 0.9628 0.9562 0.9411 0.9454 0.9558 0.9552 0.9540 0.9601

0.9826 0.9598 0.9643 0.9598 0.9745 0.9690 0.9535 0.9595 0.9683 0.9677 0.9661 0.9727

0.9878 0.9736 0.9801 0.9730 0.9842 0.9799 0.9641 0.9718 0.9790 0.9783 0.9766 0.9830

0.9926 0.9887 0.9943 0.9898 0.9946 0.9919 0.9829 0.9877 0.9921 0.9890 0.9898 0.9935

0.9568 0.9123 0.8948 0.9376 0.9421 0.9413 0.9765 0.9475 0.9565 0.9173 0.9552 0.9357

determined. The results for chars subjected to direct and indirect gasification for all measurements are summarized in Table 2. In all cases, the first-order reaction showed the best fit, as evidenced by the value of R2 closest to unity (not less than 0.9829). The results indicated that it can be concluded with a high probability that the gasification stage was the first-order reaction. The above results became the basis for further calculations of kinetic parameters based on the assumption that the gasification stage was the first-order reaction. The research was additionally developed using the Senum−Yang method, which also meets this assumption. Moreover, it circumvents the inaccuracies related to the analytical approximation of the temperature integral. The fit between carbon conversion degree calculated using the Senum−Yang method and the experimental data of direct and indirect char gasification has been shown in Figure 3. The fit between the Senum−Yang integral method and the experimental data in both of these cases was good throughout the gasification stage. A slightly better fit can be observed for direct gasification compared to indirect gasification. It is the best fitted model and experimental data that we receive for 3 K/min heating rate cases, which is caused by the largest number of measurement points. A very good fit of curves proved that the Senum−Yang method can be applied successfully to describe the char gasification stage in a CO2 atmosphere. An adequacy of the first-order reaction, determined by the Coats−Redfern method and confirmed by the high value of R2 determination coefficients, and adequacy of the Senum−Yang method, confirmed by a good fit between experimental data and model curves, contributed to the fact that kinetic parameters were calculated on the basis of such methods. The calculated kinetic parameters are summarized in Table 3. The values of kinetic parameters calculated on the basis of the proposed models for the direct char gasification process in a given heating rate were almost identical. It can also be observed that the increase in the heating rate during the measurement resulted in a slight increase in the values of the activation energy and a more significant increase in the values of the preexponential factor. The activation energy calculated at a heating rate of 3 K/min was approximately 275 kJ/mol, and the preexponential factor amounted to 1.8 × 1011. The increase in the heating rate to the level of 20 K/min resulted in an increase of

during the gasification of chars obtained in a separately conducted pyrolysis process. The main objective of this study was to determine the effect of the separately conducted pyrolysis process on the char gasification stage (indirect gasification). Curves illustrating the course of the direct and indirect gasification of chars at heating rates of 3, 10, and 20 K/min are shown in Figure 2. The heating rate in the pyrolysis process was the same as the heating rate at which they were subsequently gasified. As seen, with an increasing heating rate, all curves were shifted toward higher temperatures, without any appreciable distortion in the typical sigmoidal shape. The gasification reaction occurred in a temperature range from 750 to 950 °C in the case of char formed during the direct gasification process and at a heating rate of 3 K/min and from 850 to 1050 °C in the case of char formed during a separately conducted pyrolysis process and subjected to indirect gasification at a heating rate of 20 K/min. Furthermore, at a specific heating rate, the indirect gasification of chars was started at higher temperatures than gasification of chars obtained during heating coal samples in a carbon dioxide atmosphere. This phenomenon means that chars formed in the direct gasification process are more reactive. It is common knowledge that gasification of carbon with carbon dioxide occurs on surface carbon active sites that can detach an oxygen atom from a gaseous CO2 molecule.47,48 The number of carbon active sites is strongly dependent upon parameters such as porosity or surface area, because they are associated with edges or defects on the surface.49 Hence, higher reactive chars with a larger surface area have a higher quantity of active sites. The decrease in the reactivity of chars cooled prior to gasification as a result of lower porosity and, thus, a lower specific surface area was repeatedly confirmed in the literature.40,44,50,51 Furthermore, the higher reactivity of chars obtained during pyrolysis before direct gasification could also be caused by the Boudouard reaction. The inclusion of oxygen may have elevated the total surface area, leading to the formation of new pores or closing of pores opened by CO2.52 A lower reactivity of cooled chars (indirect gasification) should be reflected in a higher activation energy of the gasification stage. To calculate the kinetic parameters of the gasification stage, the reaction order was determined with the use of the Coats−Redfern method. The analysis of possible reaction orders covered only those values that are chemically justified, i.e., 0, 0.33, 0.5, 0.66, 1, and 2.42 On the basis of the R2 determination coefficient, the best fitting reaction order was 819

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Figure 3. Fit between carbon conversion degree calculated using the Senum−Yang method and experimental data of (a) direct gasification and (b) indirect char gasification.

the activation energy of nearly 20 kJ/mol and an increase of the pre-exponential factor by the order of magnitude. The kinetic parameters calculated on the basis of both methods for each indirect gasification measurement also showed similar values; however, the differences between the results were in most cases higher than those for the direct char gasification process. More significant differences appeared only in values of pre-exponential factors, calculated for the chars derived from the separately conducted pyrolysis process at a heating rate of 20 K/min and subjected to indirect gasification at heating rates of 3 and 20 K/min. In these two cases, the difference amounted to an order of magnitude and the higher values were obtained with the use of the Senum−Yang method. The heating rate during the indirect gasification of chars did not have such a clear impact on the values of the kinetic parameters. Although in the course of the indirect char gasification

performed at the highest heating rate of 20 K/min, both kinetic parameters reached the highest values, the lowest values were obtained during the gasification at a heating rate of 10 K/ min, contrary to expectations at the rate of 3 K/min. The heating rate during the gasification is important, even more important than this parameter during separately conducted pyrolysis. Literature reports indicate that the more violent devolatilization of the coal sample during the separately conducted pyrolysis process, the more reactive char is obtained.53,54 However, this dependency was not observed in this study. The values of activation energy, regardless of the heating rates during separate pyrolysis, were very similar for each indirect gasification process at specified heating rates and amounted to ∼316 kJ/mol at 3 K/min, ∼309 kJ/mol at 10 K/ min, and ∼339 kJ/mol at 20 K/min. The given values are averaged for the specific heating rate during indirect gas820

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Energy & Fuels Table 3. Calculated Kinetic Parameters of CO2 Gasification model Coats−Redfern material chars gasified directly

chars gasified indirectly (pyrolysis at 3 K/min)

chars gasified indirectly (pyrolysis at 10 K/min)

chars gasified indirectly (pyrolysis at 20 K/min)

−1

heating rate during gasification (K/min)

A (min )

Ea (kJ/mol)

3 10 20 3 10 20 3 10 20 3 10 20

× × × × × × × × × × × ×

275.2 280.1 295.7 316.9 307.1 338.4 311.7 307.7 342.6 316.2 313.0 331.9

1.8 2.5 1.6 8.0 2.1 6.4 4.3 2.3 1.1 7.2 3.4 3.6

11

10 1011 1012 1012 1012 1013 1012 1012 1014 1012 1012 1013

Senum−Yang A (min−1)

Ea (kJ/mol)

× × × × × × × × × × × ×

274.2 279.2 294.8 310.6 306.7 337.9 312.8 310.6 342.3 321.9 309.5 343.0

1.8 2.5 1.6 4.3 2.1 6.4 4.3 3.1 1.1 1.3 2.4 1.5

1011 1011 1012 1012 1012 1013 1012 1012 1014 1013 1012 1014

Figure 4. Carbon conversion degree of chars from the pyrolysis process at different heating rates during gasification at 3 K/min.

of char slightly increases as a result of the generation of active sites.52,54 Heating rates analyzed in this study were therefore insufficient to observe changes in the reactivity of the chars prepared during the separate pyrolysis process.55 The lower reactivity of chars prepared in the separate pyrolysis process was reflected in higher activation energies, which confirms the negative impact of such proceedings on the results of kinetic analysis of the gasification process. The results showed that, depending upon the heating rate during the gasification, averaged differences between the calculated activation energies for direct and indirect gasification processes amounted to ∼40 kJ/mol at 3 K/min, ∼25 kJ/mol at 10 K/ min, and ∼43 kJ/mol at 20 K/min. Prior preparation of char, therefore, had an impact on the value of the activation energy. When the gasification process was carried out using the previously prepared chars, a heating rate equal to 10 K/min seems to be the most suitable. However, the kinetic parameters calculated at this heating rate still revealed higher values in comparison to the parameters determined for the direct gasification of chars obtained in a CO2 atmosphere. The final stage of this research was to compare previously calculated kinetic parameters to the parameters obtained on the

ification. Larger variations occurred in the values of the preexponential factor. The differences between the values obtained for indirect gasification at a specified heating rate amounted to 1 order of magnitude. As it turned out, the heating rate during the separately conducted pyrolysis process in the range between 3 and 20 K/min did not significantly influence the progress of the indirect char gasification stage. This conclusion was also confirmed by Figure 4, which shows the carbon conversion degree of chars obtained under various heating rates during separate pyrolysis and subjected to indirect gasification at a heating rate of 3 K/min. The curves almost overlapped. It is not possible to observe a considerable influence of the heating rate during the separately conducted pyrolysis on the reactivity of the resulting chars. These results indicated that the process of pyrolysis conducted at heating rates between 3 and 20 K/min affected the morphology of the resulting char in a very similar way. Some literature reports state that slow heating during pyrolysis allows for volatile compounds to slowly escape from particles without the formation of new pores. In contrast, rapid heating stimulates the rapid formation of volatiles and, subsequently, creates pores in the char.52,53 As a result, the intrinsic reactivity 821

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Energy & Fuels Table 4. Kinetic Parameters Calculated from Isoconverional Method Combined with the Coats−Redfern Method direct gasification α (%)

A (min−1)

10 20 30 40 50 60 70 80 90

× × × × × × × × ×

2.4 5.2 1.0 1.3 1.7 1.8 1.7 1.5 2.4

10

10 1010 1011 1011 1011 1011 1011 1011 1011

indirect gasification (pyrolysis at 3 K/min)

Ea (kJ/mol)

A (min−1)

257.4 264.1 269.7 271.7 274.2 274.6 274.4 273.3 277.0

× × × × × × × × ×

1.2 2.1 2.6 2.6 2.7 2.9 3.2 3.2 4.9

11

10 1011 1011 1011 1011 1011 1011 1011 1011

indirect gasification (pyrolysis at 10 K/min)

Ea (kJ/mol)

A (min−1)

279.7 284.3 286.3 285.9 286.7 287.1 288.0 288.1 291.9

× × × × × × × × ×

basis of the model-free isoconversional method combined with the model-dependent Coats−Redfern method. The application of these methods requires the determination of the absolute temperature at which a fixed extent of gasification from several thermogravimetry (TG) curves is recorded at different heating rates.56 The kinetic parameters calculated these methods in the 10−90% conversion range are listed in Table 4. The values of the activation energy, obtained for each char based on data collected during non-isothermal gasification at three different heating rates, were lower than the values obtained for the same char at a single heating rate. For the char from the direct gasification process, Ea = 257−277 kJ/mol, and for indirect gasification of chars obtained by the pyrolysis process at heating rates of 3, 10, and 20 K/min, Ea = 280−291, 287−309, and 289−305 kJ/mol, respectively. Relatively high values of activation energy were obtained for all used methods, but they are comparable to the values presented in some literature reports for similar coals (i.e., low-rank Samhwa coal, 255 kJ/mol;57 YiMa Chinese bituminous coal, 287 kJ/mol at 2.5 K/min;58 and high-volatile Australian bituminous coal, 282 kJ/mol59). Pre-exponential factors calculated by these methods were also lower than the values obtained for a previous calculation at a single heating rate. In the case of char subjected to the direct gasification process, the values of the pre-exponential factor were lower around 1 order of magnitude, and for the chars gasified indirectly, the differences ranged from 1 to even 3 orders of magnitude. However, kinetic parameters calculated by this method confirmed that conducting a separate pyrolysis process and cooling the resulting char had a negative impact on the progress of subsequent gasification. The results listed in Table 3 indicate that the indirect gasification of chars required about 20−30 kJ/mol more energy for reaction to occur than the gasification of char obtained by heating directly in a CO2 atmosphere. The values of the pre-exponential factor were also higher. Moreover, the results obtained on the basis of this model confirmed that the heating rate during the separately conducted pyrolysis process, in the range between 3 and 20 K/ min, had no significant impact on the course of the gasification stage; kinetic parameters obtained for indirect gasification of chars were similar. A general trend of increasing kinetic parameters with the increase in conversion degree may be noted as a result of the change in the gasification control mechanism. At lower temperatures, weight change profiles are not always covered only by the gasification kinetics. The effect of chemisorption may play an important role because it needs less activation energy than the kinetics in the carbon gasification.56,60

3.9 7.5 1.0 8.5 7.0 7.8 9.7 3.1 3.1

11

10 1011 1012 1011 1011 1011 1011 1012 1012

indirect gasification (pyrolysis at 20 K/min)

Ea (kJ/mol)

A (min−1)

Ea (kJ/mol)

286.5 293.4 296.8 294.8 292.7 293.8 296.2 308.5 308.7

× × × × × × × × ×

288.8 292.7 295.2 293.9 294.0 294.2 295.0 295.2 304.9

3.3 5.1 5.8 5.8 5.9 6.1 5.3 6.7 2.0

11

10 1011 1011 1011 1011 1011 1011 1011 1012

Therefore, at the beginning of this step, gasification could be under the combined control of kinetics and chemisorption. With an increasing temperature, the effect of chemisorption is small enough so that it may be neglected and the process controlled only by kinetics.61 The declining reactivity with carbon conversion degree was, moreover, affected by growing mineral matter in the char.

4. CONCLUSION The process of coal gasification is currently one of the promising alternatives to conventional coal combustion; however, its complexity causes difficulties in terms of kinetic analysis. In this study, the kinetics of indirect gasification of char formed in a separate pyrolysis process was compared to the kinetics of direct gasification of chars formed during the heating of coal in a CO2 atmosphere. The obtained results showed that the char gasification stage was the first-order reaction, regardless of the manner in which they were formed. Moreover, the calculated kinetic parameters showed a negative effect of the separately conducted pyrolysis on the progress of the gasification stage. Values of activation energy calculated by Coats−Redfern and Senum−Yang methods were higher in the case of indirect gasification of chars by about 40 kJ/mol at a heating rate of 3 K/min, 25 kJ/mol at a heating rate of 10 K/ min, and 43 kJ/mol at a heating rate of 20 K/min. The isoconversional method also confirmed this finding: for chars gasified directly, Ea = 257−277 kJ/mol, and for indirect char gasification obtained by the pyrolysis process at heating rates of 3, 10, and 20 K/min, Ea = 280−291, 287−309, and 289−305 kJ/mol, respectively. Kinetic parameters were also affected by a heating rate during gasification. With an increasing heating rate, the activation energy and pre-exponential factor calculated for the direct gasification of chars also increased. In the case of indirect gasification, this dependence was not so clear. Moreover, the heating rate during separate pyrolysis in the range between 3 and 20 K/min did not affect the course of the gasification stage. As seen, the direct char gasification is a more efficient solution. During the kinetic analysis of the indirect gasification process, it must be taken into account that this process is not the same as direct gasification.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +48126172906. Fax: +48126172577. E-mail: [email protected]. ORCID

Grzegorz Czerski: 0000-0003-1318-4729 822

DOI: 10.1021/acs.energyfuels.6b02139 Energy Fuels 2017, 31, 815−823

Article

Energy & Fuels Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper was prepared as part of the statutory activity of the Faculty of Energy and Fuels at the AGH University of Science and Technology (11.11.210.213).

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DOI: 10.1021/acs.energyfuels.6b02139 Energy Fuels 2017, 31, 815−823