Experimental analysis and kinetic modelling for steam gasification of the Turkish lignites

Abstract

Keywords: Random Pore Model; Coal Gasification; Elevated Temperature; Porosity

Introduction

Turkey has a considerable amount of coal reserves of which lignites constitutes a major portion with a reserve of 8.05 x 106 tones and mainly utilized conventionally for power production and heating purposes [1]. The studies in gasification processes are based on the coal activation energy, the rate of conversion, the amount of obtained gas, the gas compound and especially the pore lengths, the pore volume and the surface area of the coal [2-6]. These processes basically occur in two stages: The first stage is the devolatilization of coal to char, which classically occurs in the temperature range 150–800 °C. The second stage is the gasification of the char with an oxidizing agent, such as steam, carbon dioxide, oxygen or a mixture of these gases at higher temperatures. The pyrolysis stage is shown major changing of the textural and molecular structure. So, pore and surface structures change very much. The second stage is relatively slow except plasma, some of the fluidized bed and free falling flow processes [1]. The data of gasification that used in this study are for the relatively slow process. Either the gasification process and its type or pyrolysis process and its type characteristics produce different significant parameters. In order to design the dimensions of a reactor, we use conversion type and kinetic parameters. A large number of studies on the reactivity and kinetics of char have been conducted. De Micco et al. included the effect of pyrolysis temperature in the gasification reactions in their studies [7]. Liu et al. analyzed the chars at elevated temperatures, and showed all chars tendency to increase reaction rate with elevated temperatures [8, 9]. Matsumoto et al. studied on steam and oxygen gasification of biomass chars [10]. Naturally, the atmosphere of the pyrolysis and gasification affects the structural parameters, which are the reason for change in the conversion rate. Everson et al. studied on CO2 gasification and properties of high ash coal-char particles derived from inertinite-rich coal [11]. Kajitani et al. studied on rate analysis of coal char gasification. In two of their samples, structural parameters were equal to 1 that means surface reactions during gasification were relatively very fast; and the other samples’ structural parameters increased from 10 to 26 with the elevated temperature [12]. Various mathematical models have been developed in order to express gasification processes [13]. Levenspiel developed a volumetric pore model, in which it was assumed that the reaction occurred at all pore volumes, and also he developed shrinking pore model, in which the reaction occurred from the outside to the inside of pore and was assumed that the structure shrank during the reaction [13-15]. Bhatia et al. developed random pore model which varied depending on the structural parameters, ensuring suitability for lignite with low reactivity in terms of getting into the impact of structural features of coal, and these reactions were basically classified as assuming the occurrence from outside to inside and from inside to outside [16-20]. In this study, the structural parameters of the gasification of the Zonguldak and Beypazarı lignites were determined. The gasification results were taken from Zonguldak and Beypazarı lignites with steam gasification This paper was recommended for publication in revised form by Regional Editor Bekir Yilbas 1 Department of Mechatronics Engineering, Yildiz Technical University, Istanbul, TURKEY *E-mail address: oguney@yildiz.edu.tr, koyun@yildiz.edu.tr Orcid id: 0000-0003-3107-9199, 0000-0002-5527-4151 Manuscript Received 29 May 2018, Accepted 13 September 2018

Journal of Thermal Engineering, Research Article, Vol. 6, No. 3, pp. 204-213 April, 2020

after pyrolysis of them and some of the pore characteristics of Zonguldak coal taken from an experimental study of Balci and Durusoy [21-23]. Particularly, measurements of pore characteristics were made in mercury porosimeter of Beypazarı lignite. The model parameters of Zonguldak and Beypazarı lignites have been developed using Bhatia and Perlmutter’s random pore model. Suitable kinetic parameters were determined by fitting the gasification model to experimental data. The R2 values that used to evaluate the quality of the linear fit are higher than 0.95 which can be interpreted as very good fit. The activation energy and collision factor required for the gasification was determined to examine the structural and kinetic behaviors of coal during gasification. Theoretical and Experimental Study The general assumption of Bhatia, Perlmutter and Balci in the modeling of solid-gas reaction is that the form of reaction develops in the inner surface layer of the pores [19-23]. Models are basically divided into two classes that the reaction on the surface of non-porous particles and on the pore surfaces in the solid. 1

The porosity ε0 is directly determined usually at the beginning of gasification. S0 and L0 are determined by measuring the absorption or mercury porosimetry. v(r) can be calculated from the pore volume distribution of the mercury pore volume value corresponding to each pressure on the surface measured by the mercury porosimetry. On the actual surface reaction rate is proportional to the total surface area. If we desire to express the rate of the process by means of conversion term, we can write: 𝑚

Change of surface area at any time during the reaction can be expressed as follows [20]: 𝑆

The particle diameters of lignites and surface area are highly large which means σ is infinite, so the Eq. 4 becomes: 𝑆 𝑆0

Total conversion expression that depends structural parameter is: 3

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Figure 1. Experimental setup schematic Experiments are performed in a quartz tubular reactor which is placed into a tubular electrical furnace. The furnace is adjustable to the desired isothermal temperature by using a temperature controller. The sample is placed into a platinum mesh sample holder, then it is hung into the hot reactor where the reactant gas passes. Nitrogen is used to stop the gasification reaction and to cool the reacted sample. Experimental setup diagram can be shown in Fig. 1. The coal samples used in the experiments are brought from Zonguldak and Beypazarı lignites. Some parameters and elemental analyses of the samples are given in Table 1 and ash analyses of the samples are given in Table 2. The pore volume distribution in terms of the pore diameter was first found by the Gaussian distribution function. The surface area, pore length, pore diameter and density used to produce model parameters for conversion temperatures. Table 1. The proximate and elemental analyses of the samples Proximate Analysis Zonguldak Coal Beypazarı Coal Moisture 0.46 15.20 Volatile Matter 30.20 27.58 Fixed Carbon 55.96 23.82 Ash 13.38 33.40 Ultimate Analysis Carbon 61.94 34.18 Hydrogen 3.69 2.26 Nitrogen 18.85 1.60 Sulfur 0.46 1.78 Oxygen 4.68 21.73 Ash 13.38 38.47 Table 2. Composition of the Ash Compound Zonguldak Coal Beypazarı Coal SiO2 48.55 44.8 Al2O3 + TiO2 29.8 13.5 Fe2O3 6.59 11.8 CaO 5.41 9.8 Na2 + K2O 3.61 6.6 MgO 1.87 6.5

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Pore analyses of the samples and structural analyses of the samples are given in Tables 3 and 4.

Coal Sample Beypazarı Zonguldak @800 °C Zonguldak @900 °C Zonguldak @1000 °C Zonguldak @1100 °C

Coal Sample Beypazarı Zonguldak @800 °C Zonguldak @900 °C Zonguldak @1000 °C

Table 3. Pore analyses of the samples Pore volume / cm3 g-1 > 50μm 50 – 1μm < 1μm 0.028 0.0517 0.1589 0 0.1905 0.1171 0 0.1417 0.1059 0 0.1085 0.0783 0 0.0869 0.0661 Table 4. Structural Analyses of the samples e0 L0 . 1012 S0 dpore.avg cm cm-3 cm2 g-1 μm 0.313 0.7308 109899 60.2 0.323 27.988 16300 2.39 0.296 27.988 12700 2.34 0.257 28.019 12625 2.51

Coal gasification is a heterogeneous reaction. Porosity and pore surface area are very important parameters for the characterization of the reaction. Depending on increasing pyrolysis temperature and pyrolysis time, pore size decreases that reactivity of coal also simultaneously decreases. The highest porosity occurs from 750 to 850 °C. The reactivity is more dependent upon the heat treatment conditions than the type of coal at high temperature over 700 °C.

Results And Discussion

The data are taken from studies of Balcı and Durusoy, and also measured characteristics in this study are combined. The kinetic characteristics are extracted via using model. The results obtained from the conversion value of the gasification process using 50% conversion time in Eq. 3; after that τ and ψ values are calculated. The same procedure performed for the other temperatures drawn Arrhenius curve, k 0 and E values are determined. τ and ψ parameters obtained from experimental data are given in Table 5. Table 5. Experimental τ and ψ parameters Coal Type ψ τ Beypazarı @700 °C 3.73 2.214 x 10-4 Beypazarı @800 °C 4.21 2.851 x 10-4 Beypazarı @900 °C 9.57 3.378 x 10-4 Beypazarı @1000 °C 11.5 7.001 x 10-4 Zonguldak @800 °C 1.41 4.321 x 10-4 Zonguldak @900 °C 1.01 2.308 x 10-4 Zonguldak @1000 °C 0.861 4.511 x 10-4 As shown in Fig. 2 and 3, the experimental data have been successfully adopted into the model. Conversion curves drawn by using ψ parameters obtained from experimental results and calculated from Eq. 6 by using ψ parameters given in Table 4 are shown together in Figs. 2 and 3. σ is infinite (σ→∞), because the particle diameters of lignites and surface area are highly large. For that reason, σ and τ parameters have been neglected used in Eq. 6.

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Figure 2. Carbon Conversion – Time figure for Beypazarı Lignite 1,2 1

Figure 3. Carbon Conversion – Time figure for Zonguldak Lignite Accordingly, the dimensionless time for 50% conversion for using ψ values particle parameter σ is given in Fig. 4. As shown in Fig. 4, there is no significant change in conversion for σ>100 condition. In fact, it can be concluded that the outer surface area of the particles is negligible compared with the internal surface area. As also seen in Fig. 4, there is an adverse effect that the reaction time in sufficiently small particles is independent from internal structure for ψ<100. In the transition region the inner surface area and outer surface area are comparable in terms of impact. In cases where the outer surface is very small, σ parameter is ignored because it is very high, so that the conversion curve can be drawn only by using ψ parameters. Here, the rate (slope) usually decreases with time. In cases where the ψ parameter is very small, pore walls are closed and the total surface area is reduced as the reaction proceeds. With the high ψ values, this event is delayed because of relatively high conversion curves.

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Figure 4. Required time for %50 conversion for different ψ parameters With respect to the surface area change during the reactions; as determined from Eq. 3 and using ψ values in Table 5, the change in pore surface area during the conversion obtained from experimental data are given in Figs. 5 and 6. 1,8 1,6 1,4

Figure 5. Change of pore surface area for Beypazarı lignite Micco et al. studied the gasification reactions by including the effect of pyrolysis temperature [7]. Although their analyzed samples are very similar to Zonguldak char, ψ parameters showed differences because of the different ash structure and pore surface volumes [7]. Matsumoto and his colleagues studied on steam and oxygen gasification of biomass chars, and their samples are very similar to ours, moreover, structural parameters and conversion rates are very close when comparing experimental results with ours [10]. Bhatia and Perlmutter studied the behavior of pores during the gasification of coal and characterized the dependency of the porosity of the coal structure on the structural parameter values that vary the change rate of reaction [19, 20].

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Figure 6. Change of pore surface area for Zonguldak lignite The overall effect during the reaction for high ψ values; as seen from Figs. 5 and 6 is carried out in the direction of growth of the pores and reaction surfaces. Using Eq. 6 for σ→∞, we write;

Using Eq. 8, the maximum conversion rate takes place around 0 < Xm < 0.393 for 2 < ψ < ∞. The highest change values are given in Table 4. As seen in Figs. 5 and 6, in advanced stages of conversion, the rate goes through the highest point for X = 0.36 and 0.30 values, respectively. Here, the role of the diffusion event is clearly defined and the kinetically controlled conversion stage in the pore generation area was studied. The compliance at high temperatures that we observed at the conversion curve strengthened but small deviations in the model seemed to start at low temperatures. These deviations can also be caused by the diffusion event being more active at low temperatures for small co-particle diameter. Conversion curves which are drawn according to the ψ parameters obtained for both types of lignites are 1-9% higher than the experimental results. In this case, also examining the impact of the diffusion for conversion would be useful to perform additional tests for different particle diameters at the same temperature for the same coal. The model provides a better fit for Beypazari lignite than for Zonguldak lignite. Reaction rate values have been identified between 0.55-5.5 x 10-4 s-1 in the literature for various types of coal which have suddenly been hydropyrolysed. The parameters for both lignites are given in Table 6. Table 6. Surface reaction rates and its parameters for particle pore model Coal Type ks k0 E m Beypazarı @700 °C

Conclusion

In this study, the structural parameters of the gasification of the Zonguldak and Beypazarı lignites were determined. The coefficient of determination R2 was used to evaluate the quality of the linear fit, and the results show that all R2 values are higher than 0.95 which can be interpreted as very good fit. The model parameters of Zonguldak and Beypazarı lignites have been developed using the random pore model. The activation energy and collision factor required for the gasification was determined to examine structural and kinetic behaviors of coal during gasification as well. Kinetic studies have been conducted for the gasification process, particularly the classical models like particle and shrinking core ones, which cannot accurately determine the gasification of low valance lignites' activation energy and frequency of collisions due to determining the inaccurate reaction rate, because the reaction rates decrease depending on the reaction progress according to the models except for the random pore model. However, looking at the experimental results, the reaction rate remained at the highest point even though the reaction proceeds in time during gasification of the low-valance lignites. Due to this reason, Bhatia and Perlmutter developed a model that highlighted pore structure features. In their model, the coal structure defined by a particular structure parameter containing particle sizes and the pore structure parameters incorporating the features, such as coal porosity, reaction surface area and pore lengths. Because of the deviation of the coal beyond the 60% conversion on the conversion-time curves obtained from the gasification experiments using this model, a new model can be developed including again structural variables as a future work.

Nomenclature

E Activation energy ks Reaction rate constant L0 Pore length per unit volume mc Mass rs Reaction rate S0 Initial surface area R Ideal gas constant v(r) Pore volume distribution X Fractional conversion of carbon ε0 Initial porosity σ Layer thickness in solid τ Dimensionless time ψ Structural parameter T Reaction temperature

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