CO 2 Gasification Reactivity of Chars Derived From Zhundong Coal

Coal gasification with carbon dioxide is a process for generating clean gaseous fuels and relieving greenhouse effect. Zhundong coal has high alkali and alkali earth metals (AAEMs) content, medium volatile and low ash in nature. Isothermal CO2 gasification of char derived from Zhundong coal (R-char) and char from acid washing R-char (AR-char) are performed in thermo-gravimetric analyzer (TGA). The effect of AAEMs is investigated on the gasification behavior in the range of temperatures 1073 K to 1273 K. The carbon conversion increases rapidly with increasing reaction temperature and CO2 concentration. R-char has high gasification rate and carbon conversion compared with AR-char. The accuracy of the free-model approach for calculating activation energy at different conversions is validated by compared with different kinetic models (volume reaction model, distributed activation energy model). Moreover, R-char gasification with CO2 shows a compensation effect as the Arrhenius parameters (EA and k0) increase or decrease simultaneously.


Introduction
The greenhouse effect primarily due to CO2 emissions, which has become one of important issues in related to energy, environment and economy [1].The statistics indicated the combustion of fossil fuels was the primary anthropogenic cause of CO2 emissions [2].Therefore, it is urgent to develop low-carbon approaches for clean coal utilization.Gasification is generally considered as an efficient process for coal conversion, especially, CO2 gasification has been regarded as a potential technology to directly utilize greenhouse gas [3].However, the gasification reaction with CO2 is relatively slow by compared with other gasification agents (i.e.O2, H2O).Previous investigators have concluded that high reactive coals are particularly suitable for CO2 gasification [3][4][5].Zhundong coal, with an estimated reserve of 3.9 Gt, is a super large energy source in China [6].It has high reactivity, medium volatile and low ash.Furthermore, it has high alkali and alkali earth metals (AAEMs) which play a significant role on gasification process [7][8][9].
The catalytic effect of AAEMs on the reactivity of CO2 gasification has been investigated [10][11][12][13].Li and co-workers [10,11] explored the volatilization and catalytic effects of AAEMs on the pyrolysis and gasification of Victorian brown coal.They concluded that NaCl in the brown coal was mainly released as Na and Cl separately rather than as NaCl molecules, and the volatile-char interactions were affected by the valence of the AAEMs in the char.Ding et al. [12] considered the catalytic ability and concentration of AAEMs were positively correlated below a certain concentration.Walk et al. [13] identified that catalytic ability of AAEMs on CO2 gasification decreased in the order of Na > K > Ca from anthracite to lignite, and pointed the catalytic ability was dependent on the concentration, chemical forms, and dispersion of AAEMs in coal.Meanwhile, several other studies focused on the transformation of Na, Cl and silica [10,[14][15][16].Kosminski et al. [15,16] examined the transformation of Na during the gasification of South Australian lignite.Their results indicated that release of Na and Cl was disproportionate to NaCl in coal.Kosminski et al. [17] also investigated the reaction between Na and silica or kaolin, and found liquid sodium silicates were major reaction products of Na and silica.
It is necessary to understand the reaction kinetics under controlled laboratory conditions to support large scale gasification trials, which makes the process may easily be modelled from the desired reaction conditions when the fundamental parameters are obtained.As for CO2 gasification of coal, the high char reactivity was essential for achieving high conversion levels as the gasification rate was limiting step in comparison to devolatilization of coal [5,18].
What is more, the kinetic modeling of char was crucial in determining the rate of whole coal gasification [3].The common kinetic models, such as volumetric reaction model (VRM) [19] and distributed activation energy model (DAEM) [20], have been proposed to widely predict reaction of coal gasification [21,22].Activation energy (EA) is an important parameter to evaluate reaction in kinetic analysis.A new method, free-model approach (FMA) as an alternative approach can estimate EA without assuming a kinetic model [23].However, the EA obtained from the free-model approach maybe lead to some unexpected results for CO2 gasification.
The objective of this study is to determine the CO2 gasification performance of chars derived from Zhundong coal, by considering the effect of reaction temperature and residence time, CO2 concentration and AAMEs content.Two kinetic models, i.e., VRM and DAEM, are used to investigate the gasification reactivity of char.The prediction ability of activation energy for the free-model approach is evaluated by comparing with the above two kinetic model during CO2 gasification.In addition, the relation of EA and k0 is explored during char gasification.

Char preparation
Coal used in this study is from the east of the Junggar Basin in Xinjiang, China.Before experiments, all samples are dried in vacuum oven at 378 K for 2 h.To eliminate the effect of intra pore diffusion on the gasification rate, coal samples are ground and sieved with particle size less than 90 μm [19,24].The proximate and ultimate analyses of Zhundong coal are listed in Table 1.
The chars are obtained by devolatilized the coal samples in the tubular reactor at the temperature 1073 K, which is equipped with the intake part, sampling part and temperature control part.A detailed description of the test unit is shown in Figure 1.Before experiments, a sweep N2 gas flow of 400 mL/min was used to remove the air in the device for 30 min.Then, the device is heated at a rate of 10 K/min.Once the temperature is reached 1073 K, coal samples are loaded into the reactor rapidly.Then the samples are maintained for 30 min at the temperature under a continuous nitrogen atmosphere, and then the samples are pushed to the cooling zone promptly by sample push rod.After cooling to room temperature, the char sample named as R-char is taken out, a part of which is stored in a desiccator, whilst the other is stirred in 0.1 mol/L sulphuric acid for about 24 h under nitrogen atmosphere before washing with distilled water.After drying this kind of char is referred to as AR-char, which is also stored in the desiccator.Prior to testing, all the char samples are dried at 378 K for more than 24 h.The composition of ash is determined by X-ray fluorescence (Shimadzu XRF-1800).
According to AS 1038.14.1-2003 (R2013), low temperature ash of R-char and AR-char samples are prepared in a muffle furnace at 773 K.The primary compositions of ash are listed in Table 2.It is clear that the ash from R-char is dominated by basic oxides (CaO, MgO, Na2O, Fe2O3, and K2O) with a minor fraction of acidic oxides (SiO2, Al2O3 and TiO2).Thus, it can be obtained the data of 3.15 by the ratio of basic oxides to acid oxides.In contrast, the AR-char ash has a low B/A ratio approximately 0.15, containing high acidic oxides including SiO2 and Al2O.
In addition, A scanning electron microscopy (Tescan Vega 3) coupled with energy dispersive X-ray spectroscopy are used to observe the morphology of the samples, and to obtain semi-quantitative elemental analysis of the char particles.In this study, carbon conversion rate for gasification, x, is: where m0 is the initial mass of the sample, mt is the mass at a particular time, and mash is the mass of the ash.
Gasification rate is defined as differential of carbon conversion to gasification time, the gasification rate, r, is expressed as: where, x is the carbon conversion rate, t refers to time.

Kinetic models and free-model approach
There are many kinetic models for description of CO2 gasification.Two common models [19,20] are adopted in this study.
The first model is volumetric reaction model (VRM), which is an ideal model for describing gas-solid reaction.The reaction is considered to take place everywhere in the volume of the particle, as below: where, kVRM is the rate coefficient, t is time, Xc is the carbon conversion rate.
The second model is distributed activation energy model (DAEM).This model assumes that the char gasification reaction consists of many parallel first order reactions, and integrated activation energy of char gasification presents distribution function [20].Under the condition of isothermal DAEM is given by: where, X is the carbon conversion rate, E is activation energy, R is molar gas constant and k0 refers to the pre-exponential factor, min -1 .
Making a curve (lnt~1/T) can get straight slope and intercept at the same conversion rate on isothermal gasification experiments, which can obtain average E and k0 refer to carbon conversion rate in the process of gasification temperature range.
The free-model approach was recently proposed to obtain the activation energy when gasification temperature was below 1273 K [22,23].The method can be used to obtain the parameters of the Arrhenius equation for a set of isothermal experiments, and it can apply to scaling industrial processes or testing the consistency of a particular kinetic mode [22].The activation energy can be calculated by this method, based on a deduction from the Arrhenius equation and a general rate law, without transformation of variables or assumption of a particular kinetic model.The advantage of this method is required only one regression, rather than the n+1 (n is the number of isothermal experiments) when a kinetic model is used.The plot of the logarithm of residence time, ln (time), for a particular conversion versus the reciprocal of temperature, 1/T, follows a linear trend in the same way.

Effect of temperature on gasification
The processed conversion results for samples are presented in  The reaction rate of R-char is faster than that of AR-char at the same time, especially at low temperatures.This is largely due to the alkali and alkali earth metals play an important role in gasification at low temperature zone.The effect of alkali and alkali earth metals on gasification rate decreases with increasing reaction temperature.
According to molecular collision theory [25], changes in the number of reactive collisions are taking place during the reaction, and the amount of active sites is also alterant.The rate of reaction is proportional to the number of active sites.The more active sites are, the easier gasification reactions happen.It is well known that the amount of reactive collisions increases with temperature.Therefore, it can be obtained that carbon conversion rises at high temperatures.

Effect of CO2 concentration
The effect of CO2 concentration on the carbon conversion for AR-char and R-char gasification is examined at the temperature of 1173 K.As presented in Figure 4, the conversion rate increases with an increase of CO2 concentration at their corresponding time.Furthermore, the reaction time of R-char (Figure 4a) and AR-char (Figure 4b) are significantly reduced with the concentration of gasification agent increasing from 25% v/v up to 75% v/v.When operated at the same condition, the conversion rate of R-char is higher than that of the AR-char in the whole range process.The reactivity of the R-char (Figure 4a) or AR-char (Figure 4b) has no substantial difference, as gasification with 25% v/v and 37% v/v of CO2.Therefore, it has little effect on the carbon conversion at low concentration (below 37% v/v) under the same condition.Similar results were obtained for the CO2 gasification of Argentinean subbituminous chars [26].The gasification reactivity of R-char is higher than AR-char under the identical conditions, which is probably attributed to more alkali and alkaline earth metals in R-char than AR-char (in Table 2).However, the char activity decreases gradually along with the chemical reaction proceeding.
The specific surface should be reduced due to pores broken and carbon particles consumed.It is difficult to access to the active carbon, which results in gasification rate decreasing.Fe, K) of R-char is much more than that of AR-char, which proves that acid can remove a lot of AAMEs in R-char.The results are consistent with the consequence of Table 2.It can be seen for the two chars that the gasification rate increases with increasing temperature.Furthermore, the reaction rate of R-char is higher than that of AR-char, especially at high temperatures (1173 K, 1223 K), except that the reaction rate for two chars is similar at the low temperature (1073 K) in Figure 5. Xiong et al. [4] conducted experiments of char gasification in a PTGA and concluded that alkali metal influences on reaction rate, which could reduce gasification activation energy and improve the gasification rate.As presented in Table 2, the B/A ratio of R-char is 3.15 in ash analysis, which is much higher than that of AR-char (0.15), and the basic oxides content of R-char is much higher in comparison to the AR-char except K. Figure 6 and Figure 7 also draw a similar conclusion that AAEMs contents are much higher in R-char.This implies that the basic oxides (AAEMs) have a catalytic role rather than acid oxides on the gasification.Yang et al. [27] have studied occurrence modes and contents of AAEMs in three kinds of Zhundong coals.It was found that Na was mainly water soluble (Naw/Natotal = 50-85%), which was halite or in the form of surface bound Na + .Ca and Mg were mainly acid soluble (Caac/Catotal = 60-90%, Mgac/Mgtotal = 45-90%), which were carbonates with a little amount of sulfates.Moreover, the reaction rates of two samples are almost same at 1073 K, because temperature has significant influence on the rate of gasification rather than amount of AAEMs at low temperature.In our study, the content of K is slightly higher in ash of AR-char, since pickling hardly has effect on low K in char from Zhundong coal.However, other AAEM elements in the ash are decreasing obviously after acid pickling, which results in content of K relative increment.

Estimation of activation energy
As shown in Table 3, the coefficient of determination is higher than 0.98 for almost all cases.The activation energy calculated from free-model approach is close to that calculated from VRM and DAEM.This means that the free-model approach is feasible to calculate activation energy.Theoretically, activation energy should be constant and independent of the conversion.However, the activation energy calculated at lower conversion rate is smaller than at a higher conversion in this study.This may be attributed to part of the inactivated and lost AAEMs during the gasification [22,26].In fact, if the reaction follows the same mechanism in the whole conversion range, the activation energy should be almost the same, which are calculated at different conversions.

Compensation effect
The VRM is employed to calculate the EA and k0.As shown in Figure 8, the VRM gives a better description about the conversion among the study temperature region.The model fits better at the lower temperatures (1073 K, 1123 K) than at the higher one (1223 K).This is consistent with the finding on non-catalytic steam gasification of lignite char [28].Figure 9 illustrates the compensation effect in the study.A generalized interpretation between k0 and EA describes a linear correlation [20].Some rise of the EA is accompanied with a simultaneous increase of k0 among 10-15 min; while, some decrease of EA is compensated by decrease of k0 after 15 min.The two lines have a linear interrelationship with very good approximation (R 2 = 0.992 and 0.996).
Different carbon atoms of ground-state energy are gasified as the CO2 gasification proceeding.Nevertheless, the number of active collisions is also increased.In order for chemical reaction to happen, it is necessary for the molecules to have correct orientation and sufficient energy to overcome the activation energy barrier [5].However, not all collisions are in the correct orientation or full of energy in the chemical reaction.The EA is the least value of energy for particular chemical reaction in gasification to take place.In the Arrhenius equation, k0 is constant accounting for the crowd of rate-influencing parameters, such as molecular orientation, gaseous reactant concentration, active sites and so on.

Conclusions
The CO2 gasification reactivity of chars derived from Zhundong coal is determined by thermo-gravimetric analysis.The following conclusions are obtained from this work: (1) Carbon conversion rate increases with increasing temperature and CO2 concentration.
(2) The reactivity of AR-char is much lower than that of R-char, which identifies that alkali and alkali earth metals play a catalytic role in CO2 gasification.
(3) Free-model approach is suitable for the calculation of activation energy by compared to volumetric reaction model and distributed activation energy model.
(4) Arrhenius parameters (EA, k0) increase or decrease simultaneously, exhibiting a compensation effect of the gasification process.

Figure 1 .
Figure 1.Schematic diagram of the tubular reactor Figure 2  shows the effect of CO2 flows on R-char gasification.Experimental results indicate that the carbon conversion increases significantly with an increase in CO2 flow when it is less than 60 mL/min, whilst the conversion hardly changes when the flow is more than 60 mL/min.Combined with former, particles less than 90 μm can ignore intra pore diffusion, and flow of 80 mL/min can eliminate external diffusion.In all cases the experimental results represent average values of at least three duplicate runs.

Figure 2 .
Figure 2. Comparison of conversions and times for R-char at different CO2 flows

Figure 3 .
Comparison of conversions and times for AR-char and R-char are deliberated at different temperatures.The time scale has been truncated to better display the high temperature data.The conversion rates of both samples increase with increasing temperature from 1073 K to 1223 K.By increasing gasification temperature, R-char is quite reactive to yield high carbon conversions in reasonable time (51.59% at 1123 K/11 min), while almost complete conversion is achieved at high temperatures and prolonged reaction times (99.02% at 1173 K/18 min).It takes approximate 12 min to complete carbon conversion at higher gasification temperature (1233 K).Similarly, Skodras et al.[25] concluded that reaction time was significantly reduced with an increase in temperature and 10 min was considered adequate for complete conversion at 1123 K and 1173 K.

Figure 3 .
Figure 3.Comparison of conversions and times for AR-char and R-char at different temperatures

Figure 4 .
Figure 4. Comparison of conversions and times for R-char (a) and AR-char (b) at various CO2 concentrations

Figure 5 .Figure 6 and
Figure 5.Comparison of gasification rates and conversions for R-char and AR-char at various temperatures

Figure 6 .Figure 7 .
Figure 6.SEM morphology of R-char and EDS results of elemental analysis at selected spots

Figure 8 .
Figure 8.Comparison of conversions and times for R-char and calculated with VRM

Figure 8 .
Figure 8.Comparison of conversions and times for R-char and calculated with VRM

Figure 9 .
Figure 9. Compensation effect in isothermal CO2 gasification of R-char

Table 1 .
Proximate and ultimate analysis of Zhundong coal

Table 2 .
Ash analysis of the char samples ashed at 773 K Gasification tests are carried out in the thermo-gravimetric analyzer (NETZSCH STA 449F3).All char samples are gasified isothermally at four different temperatures from 1073 K to 1223 K under a continuous nitrogen flow of 80 mL/min.

Table 3 .
Activation energy (kJ/mol) at different conversion rates