Effect of High Temperature on CO₂ Gasification Kinetics of Sub-Bituminous Coal Fly Ash

Abstract

Gasification is an eco-friendly thermochemical conversion process that can use various raw materials to generate high value-added products. Coal fly ash residue from coal-based thermal power plants must be effectively managed and utilized. Therefore, this study investigates the effects of high temperatures (1100–1300 °C) on the gasification kinetics of two types of coal fly ash (KPU and LG) under isothermal CO₂ balance using a thermo-balance reactor. Three models were applied to study the reactivity of the coal fly ashes: the shrinking core model (SCM), the volume reaction model (VRM), and the random pore model (RPM). The results showed that among the three models, the SCM-based simulation was the most similar to the experimental data. We determined that low activation energy and a high pre-exponential factor achieve high gasification reactivity. With the SCM, the activation energy values for the CO₂ gasification of the KPU and LG coal fly ashes were 52.7 and 59.6 kJ/mol, respectively, and their pre-exponential factors were 1.90 × 10² and 6.51 × 10², respectively. Moreover, the high reactivity of the two fly ashes was attributed to the high reaction temperature and presence of moisture and volatile matter.

1. Introduction

Since the conclusion of the United Nations Framework Convention on Climate Change in 1992, the international community has had a shared awareness of the severity of the climate change problem and has made joint efforts to address it. Most countries have declared to achieve carbon neutrality by 2050, and global efforts to reduce greenhouse gases are continuing [1,2]. To manage climate change, the need to develop and research new sustainable and renewable energy technologies that produce fewer greenhouse gases and cause less pollution has increased. However, even with less use of fossil fuels or the development of economical energy technologies, the future demand for fossil fuels will likely still increase.

When coal is burned, it emits not only CO₂, the main culprit of climate change, but also various pollutants such as fine dust, nitrogen oxides, and sulfur oxides. Therefore, increased coal use worsens climate change and threatens public health [3,4,5]. As an active measure to reduce CO₂ emissions, the early phase-out of coal-fired power plants is ardently debated [6]. However, coal is still an important power generation source; in particular, it is an energy source that can solve the problem of intermittency, which is a drawback of new and renewable energy sources [6,7,8,9,10,11,12,13,14,15].

Unlike petroleum, coal has been widely used as an inexpensive and easily available energy source for a long time, not only for power generation, but also for transportation and heat supply. In developing countries, the demand for electricity continues to grow daily, and coal-fired power generation remains the largest source of electricity [3,16,17].

Figure 1 shows the coal-fired power generation capacity of 11 countries. According to Boom and Bust Coal 2024, published by Global Energy Monitor, China generates 268 GW, which is 70% of its total capacity (380 GW), using coal. The remaining 10 countries generate 93 GW of power generation capacity, equivalent to 25%, using coal [18,19]. Meanwhile, Indonesia, Vietnam, Japan, and Korea are generating thermal power by co-firing biomass and coal and planning to increase the proportion of biomass to 20–30% by 2025 [20].

Recently, energy supply disruptions due to the war in Ukraine have occurred, and the carbon neutral policy has shown signs of retreat as the political situation of each country overlaps [21]. European countries faced with an energy crisis due to Russia’s gas cutoff to Europe are moving to resume coal-fired power generation. Following these developments, interest in the conversion to clean energy through coal gasification and CO₂ conversion accompanying the use of fossil fuels is increasing. CO₂ reduction through efficiency improvements, high-efficiency new coal power generation technology such as the Integrated Gasification Combined Cycle (IGCC), and additional CO₂ reduction through Carbon Capture Utilization and Storage (CCUS) application have been gaining attention [9,22,23,24,25,26,27].

Gasification is an eco-friendly thermochemical conversion process that uses various raw materials and emits less sulfur, nitrogen oxide, and mercury. Synthetic gas is produced by reforming fossil fuel and natural gas, and synthetic oil, fuel, and chemicals, such as methanol or Dimethyl Ether (DME), are produced by using shale gas through the gas to liquid process [28,29,30,31,32]. Heat is supplied directly or indirectly to the gasifier, and oxygen, steam, CO₂, and air are used as oxidizing agents. However, coal can be converted to synthetic oil (coal to liquid), synthetic natural gas, and ammonia through gasification [33]. Recently, a large-scale project was underway in China to produce mono ethylene glycol from coal [34,35,36]. In addition, the interest in CO₂ gasification, which can utilize carbon dioxide, is increasing [37,38,39,40,41,42,43,44,45,46,47,48,49,50,51]. CO₂ gasification can reduce CO₂ emissions through CO₂ recycling, and it has the advantage of requiring less energy [52,53,54,55,56]. Before CO₂ and fly ash can be applied to fossil fuels, such as coal, the CO₂ must be converted to clean energy by catalytic reaction processes. For this reason, researchers have shown much interest in improving the efficiency of CO₂ reduction by using new technology. Moreover, CO₂ can be used in the transportation of pulverized coal or coal gasification by recycling corrected CO₂ gas [32,57,58,59]. Two methods exist for gasification using carbon as an oxidizing agent. One involves the reaction of CO₂ carbon gasification and steam (Equation (1)), and the other is CO₂ gasification using the Boudouard reaction [53].

$$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2 {∆H}_{1150K}^{°}={\displaystyle \frac{135.7 \mathrm{k}\mathrm{J}}{\mathrm{m}\mathrm{o}\mathrm{l}}},$$

(1)

As mentioned previously, gasification technology is recognized as a technology that can convert coal, which generates the most CO₂ per unit of energy production, into highly clean and efficient raw materials. However, in the case of coal-based thermal power plants and gasifiers, the generation of coal fly ash is inevitable, so the recycling of fixed carbon in coal fly ash is very important from the perspective of efficient use of resources. As over a million tonnes of coal fly ash is generated worldwide annually, the effective management and recycling of coal fly ash is necessary [7,16,17,60,61,62]. Coal fly ash causes various environmental problems, such as air pollution and groundwater and soil contamination from the leaching of toxic components [63,64,65,66]. Therefore, many studies are being conducted to utilize the fixed carbon content in coal fly ash [7,67,68]. Geopolymers utilizing coal fly ash can replace cement-based materials due to their excellent mechanical properties. Coal fly ash powder can be used to synthesize zeolite, silica air gel, and lightweight aggregates. It can also be used in various fields to improve natural fibers, stabilize soil, recover metals, and make ceramics [17,61,62,69,70,71,72,73].

Therefore, this study aims to investigate the CO₂ gasification kinetics of coal fly ash using thermal gravimetric analysis to determine whether the fixed carbon contained in coal fly ash and CO₂ can be recycled using gasification reactions. The shrinking core model (SCM), the volume reaction model (VRM), and the random pore model (RPM) were applied to analyze the CO₂ gasification of coal fly ash. Finally, the experimental results of this study were used to obtain data for CO₂ gasification modeling. The results of this study can help in understanding reaction kinetics, which provides valuable information on important parameters in areas such as reactor design and process analysis [74].

2. Experimental Method and Apparatus

2.1. Fly Ash Preparation

The physicochemical properties of coal fly ash are greatly affected by the rank and combustion conditions of the coal [16]. In particular, the coal fly ash produced from lignite and sub-bituminous coal has a gray color and low carbon residue [16,75]. Two types of coal fly ash from sub-bituminous coal were gasified isothermally under CO₂ gas in a thermo-balance reactor (0.055 m I.D. × 1.0 m high) at atmospheric pressure [76]. This reactor is simple and can analyze gasification kinetics economically. Therefore, in this research, the kinetics study was conducted using the thermo-balance reactor as shown in Figure 2 [77,78,79,80].

Approximately 1 g of coal fly ash sample, which is under 85 µm of fine particles, was placed in a tungsten wire-mesh basket (alumina) supported by a load cell. The CO₂ gasification reaction at high temperatures can decrease tar formation and increase carbon conversion [4,23,81,82,83]. In addition, CO₂ gasification requires a high temperature due to the endothermic reaction of the Boudouard reaction, and syngas is stably produced at high temperature gasification [84,85]. For the above reasons, the CO₂ gasification reaction temperature in this study was determined as follows. The reactor was heated to different reaction temperatures (1100, 1200, and 1300 °C) under CO₂ gas (50 mL/min), and the sample was transferred into the reactor. During the CO₂ gasification of the coal fly ash, the weight variations of the sample were recorded continuously by a personal computer. The proximate and final analyses of the fly ash are shown in

Table 1. The properties of the two coal fly ash types used in this study.

Coal		KPU Coal Fly Ash	LG Coal Fly Ash
Proximate analysis (wt.%, air-dry/dry)	Moisture	5.4 (0.0)	4.2 (0.0)
	Volatile Matter	3.0 (3.2)	1.8 (1.9)
	Fixed Carbon	51.0 (53.9)	31.3 (32.7)
	Ash	40.6 (42.9)	62.7 (65.4)
Ultimate analysis (wt.%, dry)	C	55.4	32.5
	H	0.4	0.3
	O	0.0	0.3
	N	0.4	0.5
	S	0.9	1.0
	Ash	42.9	65.4
HHV (kcal/kg, dry)		4623	2755

.

2.2. Kinetic Modeling

For kinetic modeling, the total conversion ratio x is defined as the ratio of the gasified coal fly ash weight at time t to the initial weight, as given below:

$$x=1-{\displaystyle \frac{w_i}{w_0}}$$

(2)

where w₀ is the initial mass of the coal fly ash and w_i is the mass of the coal fly ash at i time t. The rate of conversion (reactivity or reaction rate) is as follows:

$${\displaystyle \frac{dx}{dt}}=k\left(T\right)f(x)$$

(3)

where is the rate constant based on the gasification temperature T. The rate of gasification can be represented by means of the Arrhenius equation:

$$k=A \exp (-{\displaystyle \frac{E}{RT}})$$

(4)

where A, R, and E are the pre-exponential factor, universal gas constant, and activation energy, respectively. The CO₂ gasification of coal fly ash has been represented using various models [86]. In this study, three models were applied to evaluate the reactivity of the coal fly ash: the SCM, VRM, and RPM. Each model provides a different formulation of the term f(x) [87].

The SCM assumes that the reaction initially occurs at the external surface of the char and gradually moves inward. At the intermediate conversion of the solid, the core of the non-reacted solid shrinks [88]. The reaction is described as follows:

$${\displaystyle \frac{dx}{dt}}=k_{SCM}{(1-x)}^{{\displaystyle \frac{2}{3}}}$$

(5)

$$1-{\left(1-x\right)}^{{\displaystyle \frac{1}{3}}}=k_{SCM}t,$$

(6)

The VRM assumes a homogeneous reaction throughout the coal fly ash particles [89,90,91,92]. In the VRM, the reaction rate is described as follows:

$${\displaystyle \frac{dx}{dt}}=k_{VRM}(1-x)$$

(7)

$$-\ln (1-x)=k_{VRM}t,$$

(8)

The RPM considers the overlapping of the pore surfaces, which reduces the available area for the reaction [89,93,94,95,96]. The equation for this model is as follows:

$${\displaystyle \frac{dx}{dt}}=k_{RPM}(1-x)\sqrt{[1-\phi \ln \left(1-x\right)]}$$

(9)

$$\left(2\phi \right)\left[1-\phi ln{\left(1-x\right)}^{{\displaystyle \frac{1}{2}}}-1\right]=k_{RPM}t,$$

(10)

The RPM model can predict the maximum reactivity as the reaction proceeds because it simultaneously considers the effects of the pore growth during the initial stages of gasification and the destruction of pores due to the coalescence of adjacent pores. Its two parameters, k_RPM and j, are related to the pore structure of the non-reacted sample (x = 0) through the following equation:

$$\phi ={\displaystyle \frac{4\pi L_0(1-{\epsilon }_0)}{S_0^2}}$$

(11)

where S₀, L₀, and ε₀ are the pore surface area, pore length, and solid porosity, respectively. Given the variation of the carbon conversion with time, j can be obtained by differentiating Equation (12) at x = x_max as follows:

$$\phi ={\displaystyle \frac{2}{[2\ln \left(1-x_{max}\right)+1]}}$$

(12)

3. Results

3.1. Temperature Effect on Conversion

Using the experimental method described above, the results of the experiments performed more than three times were used to ensure reproducibility.

The total conversion X during gasification is defined as:

$$X={\displaystyle \frac{(W_0-W)}{(W_0-W_{ash})}}$$

(13)

where W₀ is the initial mass of the coal fly ash, W is the sample mass at any time, and W_ash is the mass of the ash in the coal fly ash measured by proximate analysis. The total conversion of the two coal fly ashes at different temperatures is shown in Figure 3.

When comparing the conversion rates of the KPU and LG coal fly ashes, both samples achieved a 100% conversion rate within one minute. The reactivity of the LG coal fly ash was faster than that of the KPU coal fly ash, which is judged to result from the compositional differences between the samples (volatile matter, fixed carbon, and ash content), as shown in Table 1 [97].

According to most studies on the CO₂ gasification characteristics of char obtained from coal or biomass, the reaction rate and carbon conversion increase with the temperature. In particular, the reaction rate of the char is influenced by the ash fusion in the coal [76,98]. However, some studies report that the reactivity of char is independent of the temperature at low ash fusion temperatures [98].

3.2. Model Plots

After the samples were loaded into the heater zone, the temperature was increased for approximately 0.1 min. This small rise in temperature is not shown in Figure 4, Figure 5 and Figure 6. The SCM, VRM, and RPM were used to interpret the conversion-time data, as shown in the figures. With the SCM in the chemical reaction-controlled regime, the relationship between [1–(1–x)^1/3] and the reaction time at 1100–1300 °C for the two coal fly ashes is shown in Figure 4.

The SCM effectively predicted the experimental data, and the correlation coefficient (R²) was not ≤0.972 for the KPU coal fly ash and not ≤0.983 for the LG fly ash. In addition, the correlation coefficient for the LG coal fly ash was higher at higher temperatures.

The VRM was used to interpret the conversion-time data, as shown in Figure 5.

The reaction constant k_VRM was determined from Equation (7). The relation between [−ln (l − x)] and reaction time t is also shown in Figure 5. The model equation of the rate constant was not in agreement with the experimental results for the given temperature range, and the R² was 0.945–0.969 for the KPU coal fly ash and 0.913–0.943 for the LG coal fly ash. When the VRM described as a reaction order was 1, the KPU coal fly ash was found to deviate from the initial 0.2 min. Thereafter, the experimental result over time (to increase the conversion) was described as well by the VRM.

The RPM has two characteristic parameters: the constant reaction rate (dx/dt) and the structure parameter (ψ). As shown in Equation (12), ψ is commonly estimated from the experimental conversion values, where the reaction rate is maximum, x_max. Unlike those in the other models, the function of 2ψ[(1 − ψln (l − x))^1/2 − 1] in the RPM increased non-linearly. The correlation coefficient (R²) was not less than 0.767 for the KPU coal fly ash and not ≤0.871 for the LG coal fly ash. Kinetic parameters such as the activation energy, pre-exponential factor, and reaction order from these results (Figure 4, Figure 5 and Figure 6) were determined using the mean reaction rate constant.

All models showed that the slope increased as the temperature increased, which is consistent with the conversion results. In addition, in the case of the SCM and VRM, the correlation coefficient (R²) was high at over 0.9 because they are linear, whereas in the case of the RPM, the correlation coefficient (R²) was relatively low at less than 0.9 because the maximum conversion of the ψ value was used. It was found that the LG coal fly ash, which has a relatively low volatile matter and high fixed carbon and ash, showed higher accuracy over all models than the KPU coal fly ash. [99]

3.3. Activation Energy and Pre-Exponential Factor

Based on the Arrhenius plot (ln k vs. 1/T) for the temperature range of 1100–1300 °C, the activation energy and pre-exponential factor were obtained, as shown in Figure 7. The activation energy of the Arrhenius plot reflects the reaction mechanism of gasification [100]. In the case of the KPU coal fly ash (Figure 7a), the activation energy values of the SCM, VRM, and RPM were 52.7, 59.9, and 93.6 kJ/mol, respectively, and their pre-exponential factors were 1.90 × 10², 1.68 × 10³, and 1.87 × 10³ 1/min, respectively.

In general, low activation energy and a high pre-exponential factor indicate high gasification reactivity, as demonstrated by the results of this study. A high pre-exponential factor compensates for reactivity, resulting in an increase in the reaction rate [87,100]. In the KPU coal fly ash, the activation energy of the SCM and VRM showed a slight difference. The RPM showed high values. The pre-exponential factor also appeared in the same order as the activation energy. However, in the case of the LG coal fly ash (Figure 7b), the activation energy values of the SCM, VRM, and RPM were 59.6, 60.2, and 89.4 kJ/mol, respectively, and their pre-exponential factors were 6.51 × 10², 3.46 × 10³, and 6.39 × 10³ 1/min, respectively. The LG coal fly ash also showed the same trend as the KPU coal fly ash.

As a result of comparing the KPU coal fly ash and the LG coal fly ash, only the VRM had a low value for the activation energy of the KPU coal fly ash, and the pre-exponential factor was high for the LG coal fly ash in the same model. Through this, it was found that the gasification reaction of the LG coal fly ash in the experimental temperature range was faster than that of the KPU coal fly ash.

Excluding the RPM, which had a relatively low correlation coefficient, the activation energy values of the SCM and VRM were similar.

In our previous study on the CO₂ gasification of two types of coal fly ash chars [36], the activation energy was in the range of 198.3–200.8 kJ/mol, similar to other studies. The kinetic parameters and CO₂ gasification conditions of the existing literature are summarized in

Table 2. Kinetic parameters and gasification conditions in literature and this study.

Ref.	Sample	Kinetic Model	Activation Energy (kJ/mol)	Pre-Exponential Factor (min−1)	Temperature (°C)	Comments
[36]	KPU coal fly ash char	VRM	198.3	2.47 × 106	900~1200	Non-isothermal
	LG coal fly ash char		200.8	8.11 × 104
[24]	ABK coal char	SCM	172.6	4.47 × 104	900~1200	Non-isothermal
	Lignite char		183.0	2.10 × 105
[44]	Bituminous coal char	SCM	239.0
[86]	Pinus densiflora for Multicaulis char	VRM SCM RPM	172 142 134		850~1050
[100]	Bituminous coal char	RPM	178.38	2.01 × 107	950
[101]	Sub-bituminous coal char	Random capillary	35.5			n = 0.57
		RPM	37.3			N = 0.56
	High volatile bituminous coal char	Random capillary	37.3			N = 0.56
		RPM	39.4			N = 0.58
[102]	Chinese anthracite char	SCM	146.4 201.2	4.51 × 103 9.78 × 105
[103]	Olive residue char	nth order model	133		800~900	N = 0.43
[104]	Waste tire char	VRM SCM RPM	191.8 197.5 197.7
[105]	Sub-bituminous coal	SCM	92.7		700~900	TGA (Isothermal)
[106]	Bio-char	VRM	78.09	330	1000	Isothermal
This work	KPU coal fly ash	SCM VRM RPM	52.7 59.9 93.6	1.90 × 102 1.68 × 103 1.87 × 103	1100~1300	Isothermal
	LG coal fly ash	SCM VRM RPM	59.6 60.2 89.4	6.51 × 102 3.46 × 103 6.39 × 102

, which shows a significant variation among the kinetic parameters reported by different researchers depending on the sample type, gasification condition, and model type used.

As shown in Table 2, most researchers analyzed CO₂ gasification after the conversion of biomass or coal to char. Other researchers reported coal char with an activation energy in the range of 92–239 kJ/mol. In contrast, we obtained a relatively low activation energy in this study, in the range 52–94 kJ/mol, because the coal fly ash that contained water and volatile matter was supplied to the reaction zone at a high temperature. In this case, since the pre-exponential factor of the coal fly ash was lower than that of the coal char, it was found that even if the temperature increased, the rate constant was not significantly affected.

Additionally, in the case of using coal fly ash of the same coal, the difference between the activation energy and pre-exponential factor is due to the effect of isothermal and non-isothermal processes [107,108].

As a result of comparing the activation energy for the CO₂ gasification reactions of char and biomass with the results in the literature, it was found that it was influenced by the type of coal and the application model. Additionally, from the results of this study, it was found that the volatile matter, fixed carbon, and ash of coal influenced the results of this study. It was found to be more greatly affected by the content. [24,36]

3.4. Experimental and Model Result Comparison

The activation energy and pre-exponential factor of the CO₂ gasification of coal fly ash was obtained for each model by applying Equations (6), (8), and (10). The conversion with respect to the experimental values and the model is shown in Figure 8 and Figure 9.

In the case of the KPU coal fly ash, the conversion below 60% by the SCM at all temperatures was higher than the experimental value, and above 80% was lower than that overall. Meanwhile, the conversion by the VRM was high compared with the experimental data. The RPM could not be compared with the model results because of large errors, as shown in Figure 8.

The modeling results for the LG coal fly ash (Figure 9) were similar to those of the KPU coal fly ash. In the SCM and VRM, the accuracy of the experimental and model values was relatively higher than that of the RPM. This is the CO₂ gasification mechanism, which shows the same result as the R² of the model plot in the previous Figure 4, Figure 5 and Figure 6.

As can be seen in Figure 3, the gasification of coal can generally be divided into pyrolysis and char gasification reactions. The pyrolysis process is the evaporation of moisture and the devolatilization of low molecular substances. The char gasification reaction is a heterogeneous reaction between solid and gas phases, where the reaction is complicated due to the internal pore structure, and the reaction rate is very slow compared to the devolatilization process. Therefore, the gasification reaction rate of char determines the reaction rate of the entire gasification reaction [109].

The reaction mechanism for the CO₂ gasification of coal is modeled based on the reversible oxygen-transfer surface reaction, described as the oxygen exchange and expansion mechanism, in the indirect gasification step. In this mechanism, the oxygen and carbon atoms of the solid are converted to CO(g) [54,110,111]. The Boudouard reaction is the rate-limiting step of CO₂ gasification, and because it is an endothermic reaction, it is greatly affected by the gasification temperature [112,113]. The details of the elemental reaction are shown in Equations (14)–(16), and the complete reaction is described by Equation (17) [57], where C* is the carbon site on the coal surface.

CO₂ + C* ↔ CO + C < O >,

(14)

CO₂ + C* ↔ C < O >,

(15)

C < O > → CO + n C*,

(16)

C + CO₂ ↔ 2CO, ∆H = 172.5 Kj/mol,

(17)

The conversion by the SCM with increasing temperature was described well. The coal fly ashes had very low fixed carbon contents, and their gasification reactions occurred in a state that contained small amounts of moisture and volatile matter. The results with the VRM were uniform with the gasification reactions for the coal fly ash particles. The RPM altered the physical structure and was not suitable. This shows that the CO₂ gasification of coal fly ash is a gasification reaction owing to the shrinkage of the particles in the SCM. Furthermore, the results obtained in this study were taken at a gasification temperature higher than those in the other literature, which was suitable for SCM.

4. Conclusions

In this study, we investigated the effects of CO₂ gasification kinetics on two types of coal fly ash under isothermal conditions. The CO₂ gasification using coal fly ash was carried out in the temperature range of 1100 °C to 1300 °C, and although there was a slight difference, it was confirmed that the experimental and calculated values were simulated better when LG coal fly ash was used than KPU coal fly ash. Although the types of coal and reaction temperatures were different, most of them applied the same model as in this study, and overall, the activation energy and pre-exponential factor showed low values for the KPU and LG coal fly ash used in this study. The SCM, VRM, and RPM were used for kinetic analysis and were compared with the experimental data, with the SCM giving the best results. The CO₂ gasification activation energy values of the two coal fly ashes were lower than those of the coal char. Therefore, because the pre-exponential factor is low, the activation energy is low, and the reaction temperature range is high, the reactivity of ash to CO₂ gasification is expected to be high. The two coal fly ashes were also highly reactive because of the high reaction temperatures and the presence of moisture and volatile matter in the CO₂ gasification.