Catalytic performance of Na/Ca - based ﬂ uxes for coal char gasi ﬁ cation

: Flux, able to improve the ash fusibility, usually contains catalytic metal compounds. Unfortunately, the quantitative analysis of synergistic catalysis e ﬀ ects between di ﬀ erent ﬂ uxing agents on coal gasi ﬁ cation has not been investigated thoroughly. In this study, the e ﬀ ect of the kinds and content of Na/Ca - based ﬂ uxes on char gasi ﬁ cation was investigated in thermogravimetry analyzer ( TGA ) . The synergistic catalysis e ﬀ ects and mechanism between the two kinds of ﬂ uxes were also studied. Finally, based on the TGA tests, the kinetics models for char gasi ﬁ cation with ﬂ ux addition were developed. The results showed that all the four Na - based ﬂ uxes could increase the char reactivity. Na 2 CO 3 , which a ﬀ orded the best activity, could increase the reactivity by 5.3 times when the content was 5%. The ﬁ ve Ca - based ﬂ uxes had a weaker catalysis e ﬀ ect compared with Na - based ﬂ uxes. CaCl 2 , exhibiting the best activity among the ﬁ ve Ca - based ﬂ uxes, could increase the reactivity by 2.3 times when the content was 5%. For composite ﬂ uxes, Na 2 CO 3 – CaO and Na 2 CO 3 – CaCO 3 had a remarkable synergistic e ﬀ ect, whereas others had less e ﬀ ect. Na 2 CO 3 could inhibit the aggregation of Ca, which might cause the synergistic e ﬀ ects between Na and Ca ﬂ uxes. The random pore model was more suitable to describe the catalytic gasi ﬁ cation process.


Introduction
Coal leads a dominant position in the energy consumption structure of China, and this situation is hard to change in a short period of time [1][2][3]. Coal gasification, the core of the clean and efficient utilization of coal, is the important process unit of technology such as integrated gasification combined cycle system, coal-derived chemicals production, and fuel cell [4,5]. It is also one of the key technologies of the clean energy industry due to its high thermal efficiency and near zero emission [6].
Entrained flow bed, which embodies many features such as elevated operation temperature, high operation pressure, and sufficient material mixing, is the mainstream of the development of coal gasification [7,8]. Theoretically, entrained flow bed can apply to nearly all types of coal [9]. However, this technology has certain requirements on the coal due to the problems of engineering. Ash fusibility is one of the engineering problems needing major consideration [10]. The ash fusion temperature (AFT) of the coal for coal-water slurry and pulverized coal gasifier is typically less than 1,400°C owing to the slag-tapping of the gasifier, and coal with high AFT can hardly meet this requirement [6,11]. However, the average ash content of coal in China is high (about 27-28%), and the reserves of coal with AFT higher than 1,400°C account for 57% of the China coal retained reserve [12,13]. Therefore, resolving the problem of the gasification of high AFT is of great value to make comprehensive use of the coal and adjust energy structure.
Flux can improve the ash fusibility remarkably [14][15][16]. Küçükbayrak et al. [17] concluded that acidic oxides such as Al 2 O 3 and SiO 2 could form polymer in elevated temperature, which could increase the fusion temperature. The basic oxide could inhibit the formation of polymer, which could decrease the AFT [18]. Ca-based flux is the commonly used flux because of its remarkable ability to decrease the AFT and its low cost. Dai et al. [19] found that AFT decreased and then increased with the CaO content. CaO can not only decrease the AFT but also improve the viscosity-temperature characteristics. MgO can also decrease the AFT significantly because Mg formed solid composites with Si, Fe, Ca, Mg, and Al [20]. Several studies showed that there was a synergistic effect between different fluxes. Wu et al. [15] found that MgO and CaO could decrease the AFT, and AFT decreased with MgO or CaO content. In addition, a synergistic effect of MgO and CaO was observed. Zhang et al. [16] also confirmed the synergistic effect between CaO and MgO.
Most fluxing agents contain alkali metal and alkaline-earth metal elements that could also catalyze coal gasification. Na-based and Ca-based compounds are the typical catalysts been studied. Their catalytic mechanisms have been widely discussed [21][22][23]. Synergistic effects of catalysts containing different elements have also been investigated [23][24][25][26]. Gao et al. [25] conducted experiments to investigate the effect of Na/Ca compounds (calcium acetate and sodium acetate as precursors) on coal gasification performance and coal char characteristics. The results showed a combined effect toward the gasification reactivity, and Na compounds could inhibit the agglomeration of CaO. Mei et al. [24] found that Ca additive could inhibit the reaction between Na and Si, Al minerals, and, therefore, Na is kept active during catalytic gasification. Bai et al. [26] also found that Ca coexist with Na has a better catalytic effect compared to Na and Ca alone.
Although synergistic effects between Na and Ca compounds on coal gasification were studied, the quantitative analysis of synergistic effects between different Na and Ca compounds on coal gasification has not been investigated thoroughly. In this study, typical Na-based and Ca-based flux agents were selected. The effect of the kinds and content of flux on coal char gasification was investigated. The synergistic catalysis effects between Na-based and Ca-based flux were also analyzed quantitatively. Finally, based on the TGA tests and kinetics analysis, kinetics models for coal char gasification with flux addition were developed. The results should be helpful for the design of flux with fine catalytic effect on coal gasification.

Materials
In this study, typical Huainan coal (ZJX) having high AFT was selected. According to the national standard GB/T19494-2-2004, the selected coal sample was dried in air, crushed, sieved, and ground to below 74 µm. The proximate and ultimate analysis of coal samples were conducted according to the national standards GB/T212-2008 and GB/T476-2001, respectively. The results of the proximate and ultimate analysis are shown in Table 1.
The preparation of the coal char sample used in the thermogravimetric experiment was conducted as follows: (1) to eliminate the influence of ash on the coal char gasification, acid deashing was performed to remove the ash from the coal, and the detailed procedure is shown in Figure A1 (in Appendix) (2) The deashed coal was then put in a tube furnace (a schematic of the device is shown in Figure 1, where it was heated from room temperature to 900°C in a nitrogen (99.999%) atmosphere at a heating rate of 10°C·min −1 , and the final temperature was held for 1 h. Devolatilization under this condition could ensure eliminating the impact of volatility and moisture in the coal. The proximate and ultimate analysis of demineralized coal char are shown in Table 1.
(3) The flux (Na 2 CO 3 , CaCO 3 , CaO, NaCl, CaCl 2 , Na 2 SO 4 , CaSO 4 , Na 2 SiO 3 , or CaSiO 3 ) with mass fractions of 1, 2, 5, and 10% was added to the deashed coal char. The soluble additives (Na 2 CO 3 , NaCl, CaCl 2 , Na 2 SO 4 , and Na 2 SiO 3 ) were loaded by isometric solution impregnation method, and the insoluble components (CaCO 3 , CaO, CaSO 4 , and CaSiO 3 ) were loaded by mechanical stirring method. The composite flux was added according to its water solubility; that is, the water-soluble flux was first loaded by solution impregnation method, and the insoluble flux was loaded by mechanical stirring method after drying. After the flux loading, the samples were dried at 105°C for 12 h. To verify the accuracy of the concentrations of flux in prepared samples, the concentrations in partial samples (shown in Table A1 in Appendix) were detected by digestion combined with inductively coupled plasma optical emission spectrometry. The details of the digestion are shown in an earlier study [27], and the accuracy was controlled within ±5%. The source and grade of chemicals in this study are shown in Table 2.

Method and apparatus
The CO 2 gasification reactivity of coal char was determined using a NETZSCH (Germany) STA 449 F3 thermogravimetric analyzer. A preliminary experiment was conducted before the actual experiment to select the optimal particle size, gas flow rate, and sample quality to eliminate the influence of internal and external diffusion on the reaction. The experimental process was as follows: a certain mass of sample was weighed into a corundum crucible. The sample was heated to the target temperature at a heating rate of 20°C·min −1 in a nitrogen atmosphere, the gas was then switched to CO 2 , and the sample was kept warm for 90 min.
The carbon conversion rate was calculated according to the following equation: where X is the carbon conversion rate (%), m 0 is the initial mass of coal char (mg), m t is the mass of coal char at the reaction time t (mg), and m end is the mass of coal char after the reaction (mg).
To quantify the gasification reaction activity of coal char with different flux addition, the gasification reactivity index (R 0.5 ) was introduced. High R 0.5 values indicate a high coal gasification reaction activity. The R 0.5 value can be calculated as follows [28]: where R 0.5 is expressed in min −1 and τ 0.5 is the gasification time required to reach 50% of carbon conversion rate (min).
In this study, the R 0.5 values were in the range 0.005-0.040. To facilitate data processing and comparison, the obtained R 0.5 values were multiplied by 10 3 . In addition, all the tests were repeated three times, and the reported gasification indexes were average values. The relative error is less than 5%.
The distribution of Na and Ca elements was analyzed by a TESCAN (China) VEGA3 scanning electron microscope (SEM) coupled with a BRUKER (Germany) energy dispersive spectrometer.

Kinetic model
Different kinetic models such as random pore model (RPM), homogeneous model (HM), unification theory model, and shrinking core model (SCM) can be applied for investigating the kinetics of coal gasification [29][30][31][32].  To find the model that best fits the coal gasification process, HM, RPM, and SCM were selected based on earlier studies to fit the gasification reaction data.

HM [30]
The HM is the simplest and most commonly used coal gasification model. This model only considers the relationship between the conversion rate and the gasification time and not the influence of the gas-solid reaction surface area and active sites on the carbon conversion rate during the reaction process. HM assumes that the internal and external diffusion is so fast that its influence can be eliminated, and that the entire reaction process occurs inside. The coal particles maintain the same size during the gasification process, but the density changes uniformly. The integral rate expression is as follows [33,34]: where x is the carbon conversion rate, k is the gasification reaction rate constant (s −1 ), and t is the gasification reaction time (s).

RPM
RPM assumes that the coal char particles have a large number of cylinders with different pore diameters. The gas reactant reacts with the inner wall of the cylindrical hole of the coal char particles, and the connected pores are cross-linked during the reaction. All the products from the reaction process precipitate in the form of gas. The RPM ignores the influence of gas diffusion and is represented by the following expression [33]: where x is the carbon conversion rate, k is the gasification reaction rate constant (s −1 ), φ is the sample particle structure parameter, and t is the gasification reaction time (s).
where S 0 , L 0 , and ε 0 are the specific surface area in the initial reaction, the total volume pore length, and the initial porosity of the sample, respectively.

SCM
The SCM assumes that the gas-solid two-phase reaction only occurs on the outer surface of the solid particle. As the reaction proceeds, the reaction surface gradually migrates from the outside to the inside of the particle, and the size of the unreacted core gradually decreases until its complete disappearance. When the gasification process is controlled by the chemical reaction, the reaction formula of the SCM is as follows [35]: where x is the carbon conversion rate, k is the gasification reaction rate constant (s −1 ), and t is the gasification reaction time (s).

Results and discussion
3.1 Influence of Na-based flux on the reactivity of coal char gasification To investigate the influence of different Na-based fluxes on the coal char gasification characteristics, the deashed coal char was loaded with 5% Na 2 CO 3 , NaCl, Na 2 SO 4 , and Na 2 SiO 3 , and an isothermal CO 2 gasification experiment was performed at 1,000°C. The experimental results are shown in Figure 2, from which it can be extracted that the addition of Na-based flux can significantly improve the gasification reaction activity. Figure 2a shows that under a gasification time of 30 min, the carbon conversion rate was the lowest (20%) for deashed coal char without flux and the highest (95%) after adding Na 2 CO 3 . Meanwhile, the addition of Na 2 SO 4 , NaCl, and Na 2 SiO 3 afforded a carbon conversion rate of 91%, 69%, and 56%, respectively. Therefore, the activity of the four Na-based fluxes followed the order: Na 2 CO 3 > Na 2 SO 4 > NaCl > Na 2 SiO 3 . According to Figure 2, R 0.5 increased with the addition of the Na-based flux components from the lowest value of only 5.89 for the deashed coal char to 37.32 after loading Na 2 CO 3 , which represents an increase of 5.3 times, 36.09 after loading Na 2 SO 4 (5.1 times increase), 24.05 after loading NaCl (3.1 times increase), and 19.17 with Na 2 SiO 3 (2.3 times increase). These results confirm the trend for the catalytic activity obtained from the results of the carbon conversion rate. Figure 3 shows the influence of the Na-based flux content on R 0.5 . As shown in Figure 3a, R 0.5 initially increased with increasing the Na 2 CO 3 loading capacity up to 5%, reaching the maximum value of 37.2, which is 5.9 times higher than that obtained in the absence of flux. Then, upon further increasing the Na 2 CO 3 content, R 0.5 showed a downward trend. This suggests that the catalytic ability of Na 2 CO 3 is not always proportional to the loading capacity and exhibits a saturation point above which the catalytic ability decreases. As shown in Figure 3b, increasing the NaCl amount from 1% to 2% did not increase the R 0.5 value significantly. In contrast, the addition of NaCl up to 5% caused a sharp increase of R 0.5 to 24.05, which is 3.1 times higher than that of the sample without NaCl. Figure 3c displays the relationship between the Na 2 SO 4 content and R 0.5 . When the Na 2 SO 4 loading capacity was 1%, 2%, and 5%, R 0.5 was 11.62, 17.30, and 36.09, respectively, which are 1.9, 2.9, and 5.1 times higher than the values obtained without flux addition. The relationship between the Na 2 SiO 3 loading capacity and R 0.5 is shown in Figure 3d, which reveals that R 0.5 increased smoothly with the increase of the Na 2 SiO 3 loading  capacity. Thus, under a loading capacity of 1%, 2%, and 5%, R 0.5 was 11.36, 16.20, and 19.17, respectively, which represents an increase of 0.9, 1.8, and 2.3 times compared with the values obtained without catalyst addition. In summary, the 5% Na 2 CO 3 flux afforded the highest coal char reactivity. Mei et al. [24] also found that Na 2 CO 3 had the best catalytic effect on coal gasification compared to other Na-based compound such as NaAlO 2 and sodium aluminum silicate.

Influence of Ca-based flux on the coal char gasification reactivity
To investigate the influence of the Ca-based flux on the coal char gasification characteristics, deashed coal char was loaded with 5% CaO, CaCO 3 , CaCl 2 , CaSiO 3 , and CaSO 4 , and an isothermal CO 2 gasification experiment was conducted at 1,000°C. As shown in Figure 4, CaCl 2 and CaSO 4 had the best and the worst catalytic effect, respectively, and CaO, CaCO 3 , and CaSiO 3 exhibited similar catalytic ability between that of CaCl 2 and CaSO 4 . Overall, the Ca-based flux showed weaker catalytic ability than the Na-based flux. According to these results, the activity of the selected Ca-based fluxes followed the order: The influence of the Ca-based flux on R 0.5 is shown in Figure 4b. The R 0.5 value of the sample with CaCl 2 was 19.17, which is 2.3 times higher than that obtained without flux, and the reaction activity was the highest. The samples loaded with CaO, CaCO 3 , and CaSiO 3 exhibited similar R 0.5 values of 12.73, 13.61, and 12.81, respectively, which indicates that these three fluxes had similar catalytic ability. The R 0.5 of the sample loaded with CaSO 4 was 6.71, which is only 0.82 higher than that of the sample without catalyst, indicating an extremely weak catalytic capacity. Again, the R 0.5 value and the carbon conversion rate followed a consistent trend. Figure 5 shows the influence of the Ca-based flux loading capacity on the R 0.5 of coal char. R 0.5 increased from 5.89 to 12.50, which represents an increase of 1.1 times, with the increase of the CaO loading capacity up to 2%. Above this point, the increasing trend weakened. The observed weak catalytic effect of CaO may be related to the loading method. Some studies have shown that CaO loaded via the solution impregnation method has a better catalytic effect than that loaded using the mechanical mixing method. In the former method, CaO reacts with deionized water to form a Ca(OH) 2 emulsion; therefore, part of the Ca exists in the form of ions, which are easier to disperse evenly on the coal surface [36]. Murakami et al. [21] demonstrated that the activity of CaCO 3 was as high as catalyst using an solution impregnation method such as Ca(OH) 2 aqueous. This might be due to using steam as the gasification agent.
As shown in Figure 5b, the catalytic ability of CaCO 3 was similar to that of CaO. R 0.5 increased first and then decreased with the increase of CaCO 3 , reaching its maximum value at 5%. At this point, the reactivity index was determined to be 13.61, which is 1.3 times higher than that of the sample without flux. Upon further increasing the CaCO 3 amount, the catalytic ability was reduced. This is consistent with the results reported by Yu et al. [37], according to which an excess of catalyst would block the coal char gaps, affecting the transfer of the gasification agent. Figure 5c shows that R 0.5 also increased gradually with the increase of the CaCl 2 amount. This increase was relatively rapid below an amount of 2%, at which the R 0.5 value was 17.14 (1.9 times higher than that of the sample without catalyst). Above 2% of CaCl 2 amount, R 0.5 increased more slowly, reaching 19.17 (2.3 times higher than that in the absence of catalyst) at an amount of 5%. Figure 5d reveals the poor catalytic effect of CaSO 4 . No significant change in R 0.5 compared with the sample without catalyst was observed when adding 1% and 5% CaSO 4 , and an amount of 2% afforded a slight increase in R 0.5 (9.31, 0.6 times increase). As can be extracted from Figure 5e, when the CaSiO 3 loading capacity was 1%, 2%, and 5%, the R 0.5 value was 9.68, 9.70, and 12.81, respectively. With the increase of CaSiO 3 , R 0.5 first increased and then remained almost constant, to finally increase again. The addition of 1% CaSiO 3 enhanced the coal char gasification activity significantly, which was basically unchanged from 1% to 2%, and then improved again for 5% CaSiO 3 .

Influence of Na/Ca composite flux on the coal char gasification reactivity
To study the synergy between the Na and Ca fluxes, Na 2 CO 3 , which had the best effect among the single-component fluxes, was combined with other Ca-based catalysts for catalytic gasification. A CO 2 gasification experiment was performed at 1,000°C using the following samples: 2.5% Na 2 CO 3 /2.5% CaO, 2.5% Na 2 CO 3 /2.5% CaCO 3 , 2.5% Na 2 CO 3 /2.5% CaSiO 3 , 2.5% Na 2 CO 3 /2.5% CaSO 4 , 2.5% Na 2 CO 3 /2.5% CaCl 2 , and 2% Na 2 CO 3 . The experimental results are depicted in Figure 6. Figure 6a shows that the sample with the composite flux was the first to be gasified completely within 60 min, followed by the sample loaded with Na 2 CO 3 alone, which took 75 min to be gasified. For the sample without flux, the carbon conversion rate was only 50% after 85 min of reaction. Figure 6b shows the effect of the Na/Ca synergistic catalysis effect on R 0.5 . The sample without flux had the lowest R 0.5 , followed by the sample with Na 2 CO 3 alone. The Na/Ca composite flux afforded a higher R 0.5 value than that obtained with Na 2 CO 3 alone and without catalyst, indicating its higher activity. The sample containing Na 2 CO 3 /CaCO 3 exhibited the highest R 0.5 of 28.06, which is 3.8 times higher than that of the sample without catalyst and 0.6 times higher than that obtained with single Na 2 CO 3 . The sample with Na 2 CO 3 /CaO afforded an R 0.5 of 26.45, which is 3.5 times higher than that of the sample without catalyst and 0.5 times higher than that with single Na 2 CO 3 . The samples containing Na 2 CO 3 /CaSiO 3 , Na 2 CO 3 /CaSO 4 , and Na 2 CO 3 /CaCl 2 showed similar R 0.5 values of 20-22.5, which were about 2.5-2.8 times higher than that of the sample without catalyst and 0.2-0.3 times higher than that obtained with single Na 2 CO 3 .
To further explore the synergy between Na and Ca, the synergistic effects were quantified by calculating the theoretical reactivity index (R theory ) as follows: where X 1 and X 2 are the ratio coefficients of Na and Ca in the composite additives, R Na is the reactivity index when 5% Na additives are added, and R Ca is the reactivity index when 5% Ca flux is added. The as-calculated theoretical reactivity indexes were compared with the experimental values to confirm the existence of a synergistic effect between the Na/Ca catalysts, which occurs when the experimental value exceeds the theoretical value. The results are shown in Table 3.
Among the five compound additives, only 2.5% Na 2 CO 3 /2.5% CaO and 2.5% Na 2 CO 3 /2.5% CaCO 3 afforded experimental reactivity indexes greater than the theoretical values, which suggests the occurrence of synergistic effect only when Na 2 CO 3 was compounded with CaO or CaCO 3 . Moreover, the best synergistic effect was obtained with the combination of Na 2 CO 3 and CaCO 3 .
To investigate the influence of the ratio of the Na/Ca flux on the catalytic gasification of coal char, Na 2 CO 3 , and  CaO were compounded in different proportions. Samples containing 1.5% Na 2 CO 3 /3.5% CaO, 2.5% Na 2 CO 3 /2.5% CaO, and 3.5% Na 2 CO 3 /1.5% CaO were compared in a CO 2 gasification experiment at 1,000°C with those having exclusively Na 2 CO 3 or CaO. The experimental results are shown in Figure 7. Among the Na/Ca composite fluxes, the sample with the highest Na 2 CO 3 content afforded the best catalytic effect. Figure 7b shows the effect of the Na/Ca ratio on R 0.5 . The carbon conversion rate increased with the Na/Ca ratio. The sample with 1.5% Na 2 CO 3 /3.5% CaO exhibited significantly higher gasification activity than the sample with CaO alone, and the gasification reaction index increased by 0.67 times. The gasification reaction index of the samples with 2.5% Na 2 CO 3 /2.5% CaO and 3.5% Na 2 CO 3 /1.5% CaO increased by 1.17 and 1.46 times, respectively, compared with that of the sample with CaO alone. Figure 8 shows the gasification reaction of the sample with 2.5% Na 2 CO 3 /2.5% CaCO 3 at three different temperatures, that is, 900°C, 1,000°C, and 1,100°C. The gasification activity was found to increase significantly with the temperature. Thus, the reaction required 60 min to reach completion at 900°C, 40 min at 1,000°C, and only 25 min at 1,100°C. Similarly, R 0.5 increased with the gasification temperature, being 0.40 and 1.12 times higher at 1,000°C and 1,100°C, respectively, than at 900°C.
To investigate the mechanism of synergistic catalysis effects, the Na and Ca elemental mapping of char residues after TGA tests under 1,000°C were collected on SEM-EDX. As shown in Figure 9a and b, Na exhibited better dispersity, whereas Ca showed aggregation to some extent. This might be the reason for the better catalysis effect of Na 2 CO 3 . As shown in Figure 9c, Na 2 CO 3 could inhibit the aggregation of Ca and increase the dispersity of Ca, which might cause the synergistic catalysis effects between Na 2 CO 3 and CaCO 3 . Gao et al. [25] also found that Na-based catalyst could inhibit the agglomeration of CaO due to its fine mobility and the formation of Na-Ca compound.

Kinetics of char gasification with flux addition
The kinetics of the gasification reaction of the sample with 2.5% Na 2 CO 3 /2.5% CaCO 3 were investigated at 900°C, 1,000°C, and 1,100°C, and the most suitable kinetic model to describe the process in the presence of flux was determined.
The three models mentioned in Section 2.3 were used to fit the gasification reaction data, and the linear regression fitting curves of the samples at different   : Na/Ca element mapping analysis of char residue of gasification: (a) residues of char with 5% Na 2 CO 3 addition, (b) residues of char with 5% CaCO 3 addition, and (c) residues of char with 2.5% Na 2 CO 3 and 2.5% CaCO 3 addition. temperatures were obtained as shown in Figure 10. Fitting parameters of different kinetic models are shown in Table 4.
It can be seen from the fitting data summarized in Table 4 that the fitting coefficients of the three models were all more than 0.8, indicating that to a certain extent, the three models can be applied to the coal catalytic gasification process. The SCM and the RPM afforded high fitting coefficients more than 0.97, indicating that these two models are more suitable to describe the process of coal catalytic gasification. As the HM does not consider the influence of surface area and active sites of the gas-solid reaction on the carbon conversion rate during the reaction, the corresponding fitting coefficient was relatively low.
The kinetic parameters were calculated using the Arrhenius formula: where E is the apparent activation energy of the reaction (kJ·mol −1 ), T is the reaction temperature (K), A is the preexponential factor (min −1 ), and R is the gas reaction constant (8.314 J·mol −1 ·K −1 ).
By taking the logarithm on both sides of the Arrhenius formula, the following formula is obtained as follows: By calculating the thermogravimetric data at different temperatures and plotting ln k vs 1/T, the A and E values can be determined from the intercept and the slope of the resulting straight line, respectively. The fitting results of the three models are shown in Figure 11. Similarly, the gasification experimental data of the samples without flux, with 5% Na 2 CO 3 , and with 5% CaCO 3 were fitted at 900°C, 1,000°C, 1,100°C to obtain the kinetic parameters.
The results are shown in Table 5.
As can be extracted from Table 5, the fitting coefficient of the HM was low. In contrast with the fitting coefficient of the noncatalytic gasification, which was higher than 0.96, the fitting coefficients of the three catalytic gasification samples were lower than 0.94, the fitting coefficients of the RPM were all more than 0.98, and the fitting coefficients of the SCM were between 0.92   and 0.98. These results indicate that the RPM afforded the best fit and the HM the worst. According to the activation energy data, the noncatalytic gasification exhibited high reaction activation energy. The activation energy calculated using the RPM was the highest, 236.34 kJ·mol −1 , and that obtained with the HM was the lowest, 210.54 kJ·mol −1 . After the catalyst addition, the activation energy of the gasification reaction was significantly reduced. When 5% Na 2 CO 3 , 5% CaCO 3, and 2.5% Na 2 CO 3 /2.5% CaCO 3 were added, the activation energy for the catalytic gasification calculated using the RPM decreased to 133.21, 188.49, and 139.96 kJ·mol −1 , respectively, which represents a reduction of 44%, 20%, and 41%. The reduction in activation energy followed the order: 5% Na 2 CO 3 > 2.5% Na 2 CO 3 /2.5% CaCO 3 > 5% CaCO 3 .
Coal char gasification experiments were carried out in thermogravimetry in this study. The gasification process in thermogravimetry is different from entrained flow bed especially in mass transfer and heat transfer. Different diffusion rates of flux can influence the synergistic catalysis effects between different fluxing agents, whereas the heat transfer rate also has impact on the gasification rate. To further investigate the effect of heat and mass transfer on synergistic catalysis effects, dropper furnace may be more applicable. In addition, cross-sectional analysis of residues in different retention time needs to be paid more attention.

Conclusion
The catalytic performance of Na/Ca-based flux for coal gasification was systematically studied in thermogravimetric analyzer. The synergistic catalysis effects between Na-based and Ca-based flux were also studied. Based on the TGA tests and kinetics analysis, kinetics model for coal char gasification with flux addition was developed. The main conclusions are as follows: 1. The four Na-based flux, Na 2 CO 3 , Na 2 SO 4 , NaCl, and Na 2 SiO 3 , could increase the reactivity of coal char, and the strength of catalysis effect order was Na 2 CO 3 > Na 2 SO 4 > NaCl > Na 2 SiO 3 . 2. Ca-based flux had weaker catalysis effect compared with Na-based flux, and the strength of catalysis effect order of Ca-based flux was CaCl 2 > CaO ≈ CaCO 3 ≈ CaSiO 3 > CaSO 4 . 3. For composite flux, Na 2 CO 3 -CaO and Na 2 CO 3 -CaCO 3 had remarkable synergistic catalysis effect, whereas Na 2 CO 3 -CaSiO 3 , Na 2 CO 3 -CaCl 2 , and Na 2 CO 3 -CaSO 4 had no synergistic effect.

The RPM with the model function
ln 1 1 had better fitting effect for char gasification with flux addition.
Funding information: This research program was financially supported by Natural Science Research Project of Colleges and Universities in Anhui Province (no. KJ2021A0432), the Institute of Energy, Hefei Comprehensive National Science Center (no. 19KZS203), University-level key projects of Anhui University of science and technology (no. xjzd2020-07), the University Synergy Innovation Program of Anhui Province (no. GXXT-2020-006), and the Scientific Research Foundation for the Introduction of Talent, Anhui University of Science and Technology (no. 13190288).