Estimation of synergetic effects of CO2 in high ash coal-char steam gasification

Applied Thermal Engineering 110 (2017) 991–998

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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng

Research Paper

Estimation of synergetic effects of CO2 in high ash coal-char steam gasification K. Jayaraman a,⇑, I. Gökalp a, S. Jeyakumar b a b

Institut de Combustion Aerothermique Reactivite et Environnement-ICARE, Centre National de la Recherche Scientifique, 45071 Orleans Cedex 2, France Mechanical Engineering, Kalasalingam University, India

h i g h l i g h t s
Pyrolysis heating rate of coal affects the char structure.
Steam and CO2 gasification rate varied based on char production methods.
Gasification rate determination for different conversion levels
Synergetic phenomenon between steam and CO2 in char gasification.
Kinetic constants are differed based on particle size and pyrolysis rates.

a r t i c l e

i n f o

Article history: Received 21 June 2016 Revised 2 September 2016 Accepted 4 September 2016 Available online 7 September 2016 Keywords: High ash coal Char gasification in steam and CO2 Synergetic effect Kinetic models

a b s t r a c t Char particles of high ash Turkish coal with different sizes are produced in a thermo-gravimetric apparatus furnace using three heating rates. The effect of coal particle size on char production under various heating rates is evaluated. The gasification experiments of the produced char in steam and blended (steam + CO2) ambiences were performed over the temperature range of 850–950 °C under ambient pressure conditions. It is observed that the changes in char structure porosity for various heating rates affect its gasification rates. Also, char gasification is affected by the parametric conditions such as reaction temperature, char production method, and particle size in addition to chemical composition and physical structure of char. The maximum reaction rate is shifted to higher conversion levels when the chars are produced from high heating rate pyrolysis conditions. In the argon, steam and CO2 blended ambience, the substitution of CO2 for argon improves the char gasification rate. There is no CO2 inhibition effect observed in the char-steam gasification, whereas some synergetic effects are anticipated. Three kinetic models are used to describe the char gasification kinetics: volumetric model, grain model, and random pore model. The variation of activation energy during gasification is based on the char generation method, particle size and reactant concentrations. The activation energy for steam gasification is 156–173 kJ/mol, whereas in the steam blended with CO2 gasification values are in between 162 and 196 kJ/mol for 3 mm particles. Besides, these values are considerably low for 800 lm particles. Similar trends are observed from Arrhenius constant values for both sized particles. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Coal is considered as an important source of energy, not only for power generation, but also for the fuels (e.g., coal-to-syngas) and chemicals production; numerous studies are ongoing to improve its gasification efficiency. Coal has undergone a variety of physical and chemical changes towards for reaching the thermal decomposition temperature. Thermal decomposition and gasification of coal ⇑ Corresponding author at: Veltech University, Chennai, India. E-mail address: [email protected] (K. Jayaraman). http://dx.doi.org/10.1016/j.applthermaleng.2016.09.011 1359-4311/Ó 2016 Elsevier Ltd. All rights reserved.

has been widely studied by many investigators [1–8]. Coal pyrolysis conditions can alter the resulting char structure, particle size, and density, thereby its reactivity. The thermogravimetric (TG) study of coal is a renowned method to understand the changes in the structural features of coal during thermal decomposition and its oxidation and gasification process [9–14]. Kristiansen [15] investigated for optimizing the design parameters during the coal gasification process under CO2 and steam ambience. Several researchers [16–19] have reported the effect of coal rank on gasification using steam and CO2 and its kinetics parameter estimation. Li et al. [20] examined the fast pyrolysis

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K. Jayaraman et al. / Applied Thermal Engineering 110 (2017) 991–998

features of various originated coals. Van Heek and Muhlen [21] reported the properties effect of coal and char on the gasification phenomena. Chen et al. [22] examined the effect of fast pyrolysis char production using lignite and its gasification reactivity using steam and CO2. Recently, Aranda et al. [23] have compared the high ash coal gasification rate and its kinetics in TG study and fluidized bed conditions under steam and CO2 ambience. They have reported the gasification rate variations which are majorly instigated by coal particle shape and size, heating characteristics during thermal decomposition, reactant’s inhibition and synergetic issues, and other fluid-dynamic effects. The mechanisms of carbon/char–CO2 and carbon/char–H2O reactions have been extensively reported, by both experimental methods [11,13,24–26] and through computational chemistry process [27,28]. There is a debate on the interactions between CO2 and H2O during the char gasification in this oxidant mixture. Several researchers [11,29–31] have reported that char–CO2 and char–H2O reactions occurred separately on discrete active sites on the char. Alternatively, Roberts and Harris [25] stated that the char–CO2 and char–H2O gasification reactions are not independent and the possibility of enhancing effect. Supplement the carbon dioxide along with the steam slows down the char-gasification reaction [20], stating that CO2 and H2O share same active sites on the char surface [25,32,33]. The present study aims to further elucidate the above phenomenon using high ash char particles. The char gasification rate can be computed using several models such as the volumetric model, the grain model and the random pore model [25,31,34–38]. A detailed understanding and comprehensive study of char reactivity in steam and CO2 and the corresponding gasification kinetics is considered as vital parameter for the mathematical and process modeling of practical gasifier systems. Besides, coal properties also usually varied based on its origination, kinetic parameter estimation for char gasification characteristics is usually coal-specific. In our on-going research, we are investigating pyrolysis, combustion and gasification reactions of Turkish and Indian high ash coals [4,11,13,14]. The main objective of the present study is to investigate the gasification kinetics of chars produced from Turkish high ash coals using thermogravimetric (TG) analysis at different heating rates. The effect of parent coal size variation and the appropriate kinetic models to determine the char reactivity variation under steam and blended mixture gasification and its potential synergetic effects

during gasification reactions are investigated. The kinetic parameters (e.g., activation energy and the Arrhenius constant) are estimated using the specified three kinetic models. 2. Experimental details High ash Turkish coal from Saray region in the Western Turkey (Thrace bassinà with average particle sizes of 800 lm and 3 mm) are used for this study. A NETZSCH STA 429 thermo gravimetry analyser (TGA) with platinum furnace is used to prepare the char particles through pyrolysis. Three heating rates, namely 100 K/min, 500 K/min and 800 K/min are employed to produce char particles in an argon ambience. The experimental setup used for the gasification studies was specified in detail elsewhere [9,11,13,14] and also shown in Fig. 1. A water vapour (steam) generation system is connected with the TG apparatus. The steam generator and the transfer lines are maintained at the temperature of 180 °C and 150 °C respectively. The generated char particles has undergone the dynamic heating of 40 K/min in an argon ambience to the preset temperature and further gasified in steam and blended (steam + CO2) ambience under isothermal regime for a specified time. The TG system tracks the record of sample mass loss using a highly sensitive analytical balance with a resolution of 103 mg. The experimental error limits are estimated, obtaining an accuracy for all studied samples of ±0.5% in weight loss measurements and ±2 °C in temperature measurements. The ultimate and proximate analyses of the Turkish coal are tabulated (Table 1). 3. Kinetic models The TG experimental results, obtained as mass loss versus time data, are expressed as conversion level (X) versus time profiles (on ash-free basis), defined as

X¼

mo  m mo  mash

ð1Þ

m denotes the instantaneous mass of the sample, m0, initial mass; and mash is the remaining mass, which corresponds to the ash content. The reaction rate is calculated as a differential of the conver. sion degree versus time, denoted as dX dt The apparent reaction rate is calculated from the following relation,

TGA - Furnace Steam generator MS

Temperature controllers

Fig. 1. Experimental set-up (steam generator-TGA-MS).

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K. Jayaraman et al. / Applied Thermal Engineering 110 (2017) 991–998 Table 1 Proximate and ultimate analyses of Turkish high ash coal (as received). Proximate analysis

Ultimate analysis

Heating value

Ash

V.M

Fixed Carbon

C

H

N

S

HCV (MJ/kg)

32.33

36.4

21.7

54.34

3.74

1.57

3.74

19.85


 E kðTÞ ¼ A exp  RT

ð3Þ

where A and E denotes the pre-exponential factor and the activation energy, respectively, whereas T represents the absolute temperature. Three kinetic models are considered to derive the reactivity of the studied chars: namely, the volumetric model (VM), the grain model (GM) and the random pore model (RPM). Kinetic models present discrete formulations for the expression of f(X). The VM brings the heterogeneous gas–solid reactions of coal-char gasification into an homogeneous reaction by assuming uniform gas diffusion over the entire particle surface [32]. The reaction rate expression for the kinetic constant as follows,

dX ¼ kvm ð1  XÞ; dt

integrated form;

 lnð1  XÞ ¼ Kvm t

ð4Þ

The GM or shrinking core model is developed by Szekely and Evans [39] which is considered that a porous particle holds of an assembly of identical nonporous spherical grains and the reaction proceeds from the surface of these grains. The shrinking core reaction is applicable to each of these grains during the gasification. The overall reaction rate of RPM is expressed in this model as [37]: 3

X ¼ 1  ð1  kGM tÞ ; after differentiation and integration; 1=3

Þ ¼ kGM t

ð5Þ

The RPM model considers the overlapping of pore surface area which can be applied to gasification reactions [40]. The basic equation for this model is:

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi dX ¼ kRPM ð1  XÞ 1  wlnð1  XÞ dt

ð6Þ

This model can be able to assess the maximum reactivity and considers the competing effects of pore growth during the earlier stages of gasification, and the pores collapse due to the coalescence of neighbouring pores as the gasification proceeds. This model consider two parameters, the reaction rate constant, kRPM, and w, the parameter represents the pore structure of the unreacted sample (X = 0). The structural parameter is estimated by means of maximal conversion degree of the char, Xmax 11,13,41], from the maximum reaction rate.For the evaluation of the rate constant, the integrated form of relation (6) gives

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ð 1  wlnð1  XÞ  1Þ ¼ kRPM t w 4. Results and discussion In this part, initially the results of char production under various heating rates are presented and subsequently the char gasification rates in single and mixed atmospheres containing

Mass loss( %)

where k is the rate constant, depending on temperature T; f(X), describes the changes in the physical or chemical properties of the sample as the gasification proceeds and corresponds to chosen nth-order expressions. The kinetic constant k depends on the temperature, based on the Arrhenius relationship

½1  ð1  XÞ

800 mic 100 K/min 3 mm 100 K/min Temp 100 K/min

ð2Þ 105 100 95 90 85 80 75 70 65 60 55

0

2

4

800 mic 500 K/min 3 mm 500 K/min Temp 500 K/min

6

8

10

800 mic 800 K/min 3 mm 800 K/min Temp 800 K/min

12

14

1000 900 800 700 600 500 400 300 200 100 0

Temperature, °C

dX ¼ kðTÞfðXÞ dt

Time (min) Fig. 2. Char generation in argon ambience.

H2O + Argon or/and CO2 are demonstrated. The influence on pyrolysis heating rate during char gasification will be examined for two char particle sizes. Primarily, the effect of CO2 addition in char + steam gasification reaction mechanisms and its synergetic effect have been analyzed. 4.1. Char production 4.1.1. Thermogravimetric analysis of coal pyrolysis under various heating rates Pyrolysis characterization of Turkish coal particles have been estimated in three dynamic heating of 100 K/min, 500 K/min and 800 K/min using thermogravimetric analysis in an argon ambience, as shown in Fig. 2. In order to remove volatiles, the particle is maintained at constant temperature for 5 min immediately after it reaches the temperature of 950 °C. The mass loss data present that the devolatilisation (or char generation) essentially based on the heating rate. For instance @ 800 K/min, the maximum devolatilisation occurs within a minute when compared to more than 10 min @ 100 K/min. Even at higher heating rates, particle sizes do not influence the pyrolysis process and the rate of volatilization is virtually constant. The final char content variation is mainly due to the presence of ash and volatile content over the tested particle sizes; smaller particles have comparatively higher char and ash content [11]. 4.2. Char gasification using steam and CO2 The gasification experiments were conducted at three ambient conditions, (i) Partial pressure of steam is 0.8 (Argon – 100 ml); (ii) Partial pressure of steam is 0.7 and CO2 partial pressure of 0.1 (Argon – 75 ml + CO2 – 25 ml) (iii) Steam partial pressure of 0.6 and CO2 partial pressure of 0.2 (Argon – 75 ml + CO2 – 50 ml). Argon gas is compensated the remaining partial pressure by carrying the steam which is also used as protective gas for the TG system. The char-gasification experiments are carried out in three isothermal conditions of 850, 900 and 950 °C, hence these conditions are very similar to fluidized bed gasifier operations for high ash coal applications.

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K. Jayaraman et al. / Applied Thermal Engineering 110 (2017) 991–998

4.2.1. Effect of heating rate Fig. 3a and b show the char conversion during gasification of 3 mm char particles produced from different pyrolysis rates. The present test is to examine the pathways for char-CO2 (Boudouard reaction: C + CO2 M 2CO) and char-H2O gasification rate. The char generated from elevated heating rates shows enhanced gasification rates in majority of the cases over the three tested temperatures, which has been pointed in [20,21]. Chen et al. [22] and Wu et al. [40] also reported the additional gasification rate of the coal-char which is produced from fast pyrolysis, caused due to the difference in the external surface area. Recently authors also emphasized the similar behaviour for Indian origin coal [4,13]. The external surface area mainly controls the gasification reactivity of carbonaceous materials. High heating rate pyrolysed chars already have their pores open and shows the relatively more surface area, whereas those generated from low heating rate have less-developed reactive surface and comparatively narrower porous network [42]. We have also analyzed the char structure using SEM (Scanning Electron Microscopy) micrographs and BET (Brunauer–Emmett–T eller) surface analysis [13]. During pyrolysis process, the coal

experience internal structural modifications and the phenomena such as pore enlargement, coalescence or blocking. In addition to that, the active sites located in microspores participate in the gasification reaction. Product gas diffuses into the porous structure of the coal-char. This further leads to change in the number of carbon-active sites present for the gasifying agents. Alternatively, the 800 lm char shows the gasification dependency on pyrolysis heating rate only at 850 °C; as the temperature rises, this effect becomes minimal as seen in Fig. 4a and b. This is confirmed from the reaction rate curves (Fig. 5a and b), as the heating rate effect on reaction rate is relatively low for 800 lm particles when compared to 3 mm char, which is well demonstrated by the present authors [11]. 4.2.2. Effect of temperature and CO2 partial pressure on char-H2O gasification It is evident that the reaction rate mainly depends on the course of temperature i.e., reaction rate is significant at higher temperature levels. The results presented in Figs. 3–5 with steam and CO2 ambience show that the conversion level steeply increases

1 0.9

900 °C

950 °C

Conversion level ( X)

0.8 0.7 0.6

850 °C

0.5

100 K/min 850 C 100 ml Ar 100 K/min 900 C 100 ml Ar 100 K/min 950 C 100 ml Ar 500 K/min 850 C 100 ml Ar 500 K/min 900 C 100 ml Ar 500 K/min 950 C 100 ml Ar 100 K/min 850 C 75 ml Ar + 25 ml CO2 100 K/min 900 C 75 ml Ar + 25 ml CO2 100 K/min 950 C 75 ml Ar + 25 ml CO2 500 K/min 850 C 75 ml Ar + 25 ml CO2 500 K/min 900 C 75 ml Ar + 25 ml CO2 500 K/min 950 C 75 ml Ar + 25 ml CO2

0.4 0.3 0.2 0.1 0

0

5

10

15

20

25

30

35

40

Time (min)

(a) Argon and CO2 1

950 °C

0.9

Conversion level (X)

0.8 0.7

900 °C 0.6

850 °C

0.5

100 k/min 850 C 75 ml Ar + 50 ml CO2 100 K/min 900 C 75 ml Ar + 50 ml CO2 100 K/min 950 C 75 ml Ar + 50 ml CO2 800 k/min 850 C 75 ml Ar + 50 ml CO2 800 k/min 900 C 75 ml Ar + 50 ml CO2 800 k/ min 950 C 75 ml Ar + 50 ml CO2 100 K/min 850 C 75 ml Ar + 25 ml CO2 100 K/min 900 C 75 ml Ar + 25 ml CO2 100 K/min 950 C 75 ml Ar + 25 ml CO2 800 K/min 850 C 75 ml Ar + 25 ml CO2 800 K/min 900 C 75 ml Ar + 25 ml CO2 800 K/min 950 C 75 ml Ar + 25 ml CO2

0.4 0.3 0.2 0.1 0

0

5

10

15

20

Time (min)

25

30

35

40

(b) CO2 at different concentrations Fig. 3. Comparison of char conversion produced at various heating rates in steam and CO2 at various CO2 concentrations of 3 mm particles.

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K. Jayaraman et al. / Applied Thermal Engineering 110 (2017) 991–998

1

950 °C

0.9 0.8

900 °C

Conversion level (X)

0.7 100 K/min 850 C 100 ml Ar 100 K/min 900 C 100 ml Ar 100 K/min 950 C 100 ml Ar 500 K/min 850 C 100 ml Ar 500 K/min 900 C 100 ml Ar 500 K/min 950 C 100 ml Ar 800 K/min 850 C 100 ml Ar 800 K/min 900 C 100 ml Ar 800 K/min 950 C 100 ml Ar 100 K/min 850 C 75 ml Ar + 25 ml CO2 100 K/min 900 C 75 ml Ar + 25 ml CO2 100 K/min 950 C 75 ml Ar + 25 ml CO2 500 K/min 850 C 75 ml Ar + 25 ml CO2 500 K/min 900 C 75 ml Ar + 25 ml CO2 500 K/min 950 C 75 ml Ar + 25 ml CO2 800 K/min 850 C 75 ml Ar + 25 ml CO2 800 K/min 900 C 75 ml Ar + 25 ml CO2 800 K/min 950 C 75 ml Ar + 25 ml CO2

0.6 0.5 0.4

850 °C

0.3 0.2 0.1 0

0

5

10

15

20

25

30

35

40

Time (min)

(a) Argon and CO2 1

950 °C

0.9

Conversion level (X)

0.8

900 °C

0.7

850 °C

0.6

500 k/min 850 C 75 ml Ar + 50 ml CO2 500 K/min 900 C 75 ml Ar + 50 ml CO2 500 K/min 950 C 75 ml Ar + 50 ml CO2 800 K/min 850 C 75 ml Ar + 50 ml CO2 800 K/min 900 C 75 ml Ar + 50 ml CO2 800 K/min 950 C 75 ml Ar + 50 ml CO2 500 K/min 850 C 75 ml Ar + 25 ml CO2 500 K/min 900 C 75 ml Ar + 25 ml CO2 500 K/min 950 C 75 ml Ar + 25 ml CO2 800 K/min 850 C 75 ml Ar + 25 ml CO2 800 K/min 900 C 75 ml Ar + 25 ml CO2 800 K/min 950 C 75 ml Ar + 25 ml CO2

0.5 0.4 0.3 0.2 0.1 0

0

5

10

15

20

25

30

35

40

Time (min)

(b) CO2 at different concentrations Fig. 4. Comparison of char conversion produced at various heating rates in steam and CO2 at various CO2 concentrations of 800 lm particles.

once the gasification temperature is increased from 850 °C to 950 °C. In the initial stages, the reaction rate is sharply increased and reaches the maximum value over the conversion range of about 30–70%, as depicted in Fig. 4. It is can be seen that the maximum gasification rate is shifted to elevated conversion level while the chars are produced under high heating rates pyrolysis. Besides, the maximum reaction rate occurred at the conversion level of 0.35 and 0.55 for the temperatures at 900 and 950 °C respectively. The early sharp increment in conversion level is directly linked to the rapid growth of the surface area, which continues untill all the pores to be collapsed. The gasification reactivity is an indirect indication for the modifications in structure of char during the advance of reaction. As reported by Komarova et al. [43], the evolvement of char surface area during lower temperature is slightly different from those occurred at elevated temperatures. With the purpose to examine the consequence of introduction of CO2 as a co-reactant alongside steam, the gasification tests are conducted in an argon ambience with CO2 at partial pressure of 0.1 and under CO2 free conditions. It is observed that with the replacement of the argon flow of 25 ml with CO2, the gasification

rate is considerably improved at each temperature conditions for 3 mm particles (Fig. 3a). However, this effect is insignificant for 800 lm particles (Fig. 3a). Hence, over the tested particles and conditions, the gasification rate of steam and CO2 with char is controlled by the available carbon active sites rather than the reactant flow conditions. Further, the gasification experiments are done in CO2 at partial pressures of 0.1 and 0.2. The literature mentions two possible surface reaction mechanisms that can take place [22,23,29–32]. One phenomenon demonstrates that C–H2O and C–CO2 reactions happen in common active sites [27,35] whereas the other one considers that CO2 and steam reactions occurred independently on separate active sites [11,29–32]. Some researchers [29,44] pointed out that even the reactions occurred at different active sites, possibility of an active cooperation among the reactants towards the accessibility of reactive surfaces, hence the contribution between the gases to improve the char reactivity towards the creation of extra porosity or by the retention of catalytic mineral species inside the char structure. By comparing the conversion levels for steam partial pressures of 0.6 and 0.7 and the corresponding CO2 partial pressure of 0.2 and 0.1

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K. Jayaraman et al. / Applied Thermal Engineering 110 (2017) 991–998

3 mm 100 K/min 900 C - Ar 100 ml 3 mm 500 K/min 900 C - Ar 100 ml 800 mic 100 K/min 900 C- Ar 100 ml 800 mic 500 K/min 900 C- Ar 100 ml

0.18

3 mm 100 K/min 950 C - Ar 100 ml 3 mm 500 K/min 950 C - Ar 100 ml 800 mic 100 K/min 950 C - Ar 100 ml 800 mic 500 K/min 950 C - Ar 100 ml

Reacon rate (1/min)

0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Conversion level (X)

(a) Steam (steam - 0.8, argon – 0.2) 3 mm 100 K/min 900 C - CO2 25 ml + Ar 75 ml

3 mm 100 K/min 950 C - CO2 25 ml + Ar 75 ml

3 mm 500 K/min 900 C - CO2 75 ml + Ar 25 ml

3 mm 500 K/min 950 C - CO2 75 ml + Ar 25 ml

800 mic 100 K/min 900 C- CO2 75 ml + Ar 25 ml

800 mic 100 K/min 950 C - CO2 75 ml + Ar 25 ml

800 mic 500 K/min 900 C- CO2 75 ml + Ar 25 ml

800 mic 500 K/min 950 C - CO2 75 ml + Ar 25 ml

0.2

Reacon rate (1/min)

0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Conversion level (X)

(b) Steam, argon and CO2 blend (steam - 0.7, CO2 – 0.1, argon – 0.1) Fig. 5. Reaction rate vs. char conversion in different mixture conditions.

(Figs. 3b and 4b), it is observed that the gasification rate is almost the same. This phenomenon occurs for both 3 mm and 800 lm particles. As reported in several researches [11,29–31,44], the present study shows that the introduction of CO2 would not actively supress the steam–char gasification reactions and also do not compete for reactive sites. In addition to that there is a possibility of synergetic effect between steam and CO2 to enhance the gasification reaction of the high ash coal-char particles.

1/T (K-1) 0.0008 -1

0.00085

100 K/min VM 500 K/min VM 800 K/min VM 100 K/min GM 500 K/min GM 800 K/min GM 100 K/min RPM 500 K/min RPM 800 K/min RPM

-1.5

-2

y(0.1) = -21528x + 16.17 R² = 0.9987

ln Ki

4.3. Determination of kinetic parameters for the H2O–CO2-char gasification using different kinetic models

y(0.2) = -20744x + 15.664 R² = 0.9989

-2.5

In order to establish the applicability over the selected kinetic models and envisage the kinetic behaviour of the tested char samples, the TGA experimental data are used to fit the models. In general, the kinetic parameters are determined over the conversion level from 10% to 50% of char reactivity which is usually adopted in several investigations [11,13,29]. The rate constants are calculated from TG data for three temperatures. The Arrhenius plots for the 3 mm chars are shown in Fig. 6. The correlation coefficient

0.0009

y (0.7)= -21600x + 16.008 R² = 0.999

-3

y (0.8)= -20785x + 15.474 R² = 0.9991 y (0.9)= -22957x + 17.348 R² = 0.9991

y (0.3)= -22943x + 17.562 R² = 0.9991 y (0.4)= -21541x + 16.082 R² = 0.9989 y(0.5) = -20758x + 15.575 R² = 0.999 y (0.6)= -22926x + 17.447 R² = 0.9991

-3.5 Fig. 6. Arrhenius relationships of 3 mm char in blended ambience.

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K. Jayaraman et al. / Applied Thermal Engineering 110 (2017) 991–998 Table 2 Comparison of the activation energy values (kJ/mol) of 800 lm and 3 mm chars gasified at 850, 900 and 950 °C under Argon, CO2 and steam ambience. Heating rate

100 ml Argon (steam - 0.8)

K/min

VM

75 ml Argon + 25 ml CO2 (steam -0.7 & CO2-0.1)

GM

75 ml Argon + 50 ml CO2 (steam -0.6 & CO2-0.2)

RPM

VM

GM

RPM

VM

GM

RPM

179.1 172.6 190.6

179.6 172.8 190.9

173.9 169.1 196.7

164.2 169.1 196.7

162.3 167.6 194.7

156.7 111.8 169.1

156.6 112.1 169.0

133.4 127.4 149.9

133.5 127.6 150.1

131.3 126.2 147.1

100 500 800

161.2 172.9 170.6

158.8 172.9 170.9

156.4 173.6 171.2

3 mm 179.0 172.7 190.7

100 500 800

155.3 132.3 152.1

154.6 131.8 152.1

154.5 131.6 152.2

800 lm 156.7 111.5 169.4

Table 3 Comparison of the pre-exponential factor (min1) of 800 lm and 3 mm chars gasified at 850, 900 and 950 °C in Argon, CO2 and steam ambience. Heating rate

3–100 ml Argon

3–75 ml Argon + 25 ml CO2

800 mic – 100 Argon

K/min

VM

GM

RPM

VM

GM

RPM

VM

GM

RPM

VM

GM

RPM

100 500 800

1.66E+06 6.26E+06 4.93E+06

1.20E+06 5.72E+06 4.63E+06

8.22E+05 5.17E+06 4.20E+06

1.05E+07 6.32E+06 4.23E+07

9.63E+06 5.78E+06 3.78E+07

8.89E+06 5.23E+06 3.39E+07

6.93E+05 8.93E+04 5.68E+05

5.40E+05 7.31E+04 4.89E+05

4.89E+05 6.62E+04 4.42E+05

8.19E+05 9.90E+03 3.27E+06

7.68E+05 9.32E+03 2.93E+06

6.53E+05 8.52E+03 2.57E+06

of the equation was found to be more than 90% for all the cases, which gave a higher confidence to use the correlation thus utilized. A noticeable variation in the slopes of the Arrhenius plot intend in the char gasification denotes the dependency on pyrolysis heating rates. Tables 2 and 3 summarize the kinetic parameters (the activation energy, E and pre-exponential factor, A) estimated from the TG data using the models for 800 lm and 3 mm samples. The activation energy of 3 mm particles varies from 156 to 173 kJ mol1 and 162–196 kJ mol1 in steam and blended ambience respectively. These values are in range of 111–169 kJ mol1 for 800 lm particles. These values are in good agreement with the recently reported studies using different reactant concentration and origins of coal [11,13,22,23,41]. Also, the pre-exponential factors are in the range from 8.22  105 to 6.26  106 in steam ambience, 8.52  103 to 4.23  107 in blended ambience. These values are in accordance with the same order as those found in literature [13,22,41,42,45]. In general, the RPM model shows slightly lower values of activation energy when compared to other two models. The variation of the activation energies with the char heating rate are nearly consistent irrespective of the particle sizes in blended ambience. Besides, it is significant that the gasification activation energy of 3 mm char is high when compared to 800 lm char particles. This may be caused from the higher reactivity potential of 800 lm char due to its higher specific surface area. 5. Conclusions In order to optimize the coal-char gasification processes and to facilitate the efficient use of high-ash coals for energy production, this research work further elucidates the conversion and kinetics of the Turkish high-ash coal in CO2 and steam gasification conditions. Mainly the synergetic phenomenon between steam and CO2 during gasification is investigated and following are the major conclusions.
Char particles of two sizes were produced from a high ash Turkish coal using an atmospheric pressure thermo-gravimetric apparatus with three pyrolysis heating rates (100 K/min, 500 K/min and 800 K/min). It is affirmed that the particle size (for particles between 0.8 and 3 mm) dependency during the pyrolysis process result is ruled out.

800 mic–75 ml Argon + 25 ml CO2


Char gasification rates investigation in steam and with different partial pressures of steam and CO2 blended ambience shows that the pyrolysis heating rate has a considerable impact on the gasification reactivity of the char.
The chars generated under high heating rates present enhanced gasification rates in most of the cases. This factor can be elucidated through the alteration of the char pore structure and the accessibility of more active sites to have reaction with the gasification agent when the char particles are produced from high heating rates.
Char particle size influences the gasification rate. Hence, diffusion restrictions and heat transfer limitations cannot be neglected for this type of high ash coal. The maximum gasification rate is shifted towards elevated conversion levels when the chars are produced at high heating rates and higher temperatures.
The introduction of CO2 would not inhibit the steam - char gasification reactions and also the possibility of synergetic effect between steam and CO2 to enhance the gasification reaction.
The kinetic parameters (activation energy and pre-exponential factor) for char gasification are mainly based on the char production method and its sizes. The activation energy for steam gasification is 156–173 kJ/mol, whereas in the steam blended with CO2 ambience gasification values are in between 162 and 196 kJ/mol for 3 mm particles.
These findings reveals that more studies can be carried out with various rank coals to estimate the char reactivity based on char generation methods without using the kinetic behaviour during the gasification process.

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