Catalytic Kinetics and Mechanisms of KCl with Different Concentrations on Gasification of Coal Char

Abstract

In this work, the influence of KCl concentration on the gasification characteristics of lignite coal char between 800–1100 °C is studied through the experiments of temperature programmed gasification and isothermal gasification using a thermogravimetric analyzer. The gasification kinetics, characteristic parameters, and the reaction mechanism of catalytic gasification is explored using the random pore model (RPM). In view of temperature programmed gasification, the gasification rate of coal char is relatively slow when the temperature is below 700 °C, and only when the temperature is higher than 700 °C, the gasification starts to accelerate. Results show that with the existence of a catalyst, the temperature required for gasification reaction is reduced and the gasification reaction rate is increased. Furthermore, the higher concentration of KCl leads to the shorter half reaction time, the higher gasification rate, and the stronger catalysis. In addition, the activation energy of AW-char (the char from acid-washed coal) is the highest, while the activation energy and the energy level required for the gasification reaction are reduced by adding KCl. Based on the analysis of the catalytic mechanism, it is found that the unified mechanism of catalytic gasification of alkali and alkaline earth metals is applicable for the KCl catalysis on coal char gasification.

1. Introduction

Coal gasification and pyrolysis are both important processes of coal conversion. It takes oxygen, water vapor, carbon dioxide or hydrogen as a gasification agent, and the process of coal conversion to specific products occurs under high temperature. The gasification of coal char with CO₂ and water vapor as gasifiers plays an important role in thermal power generation and chemical production, which is a research subject of great concern [1,2,3,4,5].

As a kind of energy, there are two important goals in coal gasification. One is to increase the proportion of ash-free components in coal converted into coal gas, and the other is to produce coal gas containing as much energy as possible. Coal gasification is divided into two processes: first, coal pyrolysis and then, coal char gasification. Coal pyrolysis is a rapid spontaneous process, and coal char gasification process is a limited rate process. Therefore, coal char gasification is considered to be the decisive process of the whole gasification reaction.

In the study of catalytic gasification, Kazi et al. [6] studied the effect of cheap catalysts, such as Ca and Fe, on coal gasification in CO₂ atmosphere, and found that the addition of Ca produced more reactive sites in coal char, thus effectively improving the gasification activity. However, the addition of Fe did not have much catalytic effect on coal gasification. Mei et al. [7] investigated the mechanism of Na₂CO₃ catalytic coal gasification, and results showed that at 700 °C, Na₂CO₃ was first inactivated to produce inert sodium aluminosilicate (Na_1.55Al_1.55Si_0.45O₄). With the increase of temperature and Na₂CO₃ concentration, many different types of sodium aluminum silicate were produced, among which sodium aluminosilicate ((Na₂O)_0.33NaAlSiO₄) was proved to be the most stable mineral in gasification reaction. Zhang et al. [8] discovered that the catalyst of Na and Fe displayed an effect on gas products and can reduce the activation energy of reaction. A similar conclusion was obtained by Zhou et al. [9], that FeCl₃ had the best catalytic activity among iron species catalysts in the gasification of petroleum coke. In our previous work (unpublished research), the existence of KCl, CaCl₂, and NiCl₂ increased the gasification rate and changed the reactivity profiles. For KCl, a consistent conclusion was drawn by Encinar [10].

Based on our previous work on the effect of metal catalysts, it is known that K-based catalyst effectively improves the gasification reaction rate and the gasification yield. Therefore, in this study, KCl is selected as the catalyst to study the effect of concentrations on the gasification reaction, including gasification characteristics, kinetic analysis, and the reaction mechanism of catalytic gasification. The acid-washed sample is donated as AW, and the corresponding coal char is donated as AW-char.

2. Materials and Methods

2.1. Sample Preparation

A lignite coal, collected from Inner Mongolia of China, is used to obtain coal char. Firstly, the coal samples are ground and sieved (a particle size smaller than 75 μm). Secondly, acid-washing procedure (HCl and HF) is applied to the coal sample to realize demineralization. The proximate and ultimate analysis of the raw and AW coal samples are listed in

Table 1. Proximate and ultimate analysis of raw and AW coal.

Coal Sample	Proximate Analysis (wt.%, ad)				Ultimate Analysis (wt.%, daf)
	Mad	Ad	Vd	Vdaf	C	Hdaf	Hd	N	O a	S
RAW	29.14	13.00	41.92	48.19	71.32	4.13	3.59	1.36	22.4	0.79
AW	10.82	0.52	45.47	45.71	70.29	5.37	5.34	1.46	22.37	0.51

^a By difference.

. The main function of acid washing on coal is to remove minerals. Silicon dioxide can be dissolved in HF, while other metal elements can be dissolved in HC1. Thus, the ash content (A_d) in coal is greatly reduced from 13% to 0.52% after acid-washing procedure. At the same time, the decrease of volatile (V_daf) and moisture (M_ad) in coal indicates that acid washing has a certain destructive effect on unstable groups in coal, such as fat long chain and aromatic side chain. Furthermore, it has a certain destructive effect on the hydrophilic oxygen-containing functional groups in coal, and this judgment is consistent with the decrease of oxygen content in elemental analysis.

The catalyst, KCl, is mixed with the AW using the immersion method, and the mixtures with different weight dosage of K are denoted as AW + 3% K, AW + 6% K, and AW + 9% K, respectively. The weight dosage is of the dry basis. Thus, the effect of K concentration on gasification can be realized.The details of the acid-washing procedure and the hybrid approach are both described in our previous study [11].

The preparation of coal char samples is completed in a box-type atmosphere furnace (HMX1600-30, Shanghai, China). Different coal samples are heated from room temperature to 1000 °C at the rate of 10 °C/min and retained for 30 min, then cooled to room temperature naturally. The precision of controlling temperature is ±1 °C. The whole process is completed under the protection of N₂ atmosphere, and the N₂ flow rate is 0.5 L/min. Three parallel tests are conducted for each sample.

2.2. TGA Experiments

The gasification experiments are carried out by a thermogravimetric analyzer (STA449F3, Netzsch, Selb, Germany). The precision of controlling temperature is ±0.3 °C, and the resolution of the balance is 0.1 μg. Non-isothermal method (temperature programmed) and isothermal method are both conducted. The sample used is about 15 mg in each trial. For the non-isothermal method, the whole process is under CO₂ atmosphere and the flow rate keeps 70 mL/min. Then, heat the coal sample from room temperature to 105 °C and leave for about 20 min to move the moisture in the coal. After the mass is stable, the temperature rises to 980 °C at the rate of 10 °C/min. For the isothermal method, the target temperatures are 800 °C, 900 °C, 1000 °C, and 1100 °C. The sample is heated up from room temperature to the target temperature at the heating rate of 20 °C/min in N₂ atmosphere with a flow rate of 70 mL/min. To ensure that the sample mass does not change again, the flow rate is maintained for 90 min. After this pyrolysis procedure, the isothermal gasification of coal char is activated by turning on CO₂ flow (70 mL/min) and kept at target temperature till completing gasification. Three parallel tests are conducted for each sample.

2.3. Random Pore Model (RPM)

Random pore model is used to calculate the kinetic parameters of coal char isothermal gasification. Its derivation and explanation are as follows. The kinetic equation of gasification reaction is expressed as:

$$r=\frac{dX}{dt}=k\left(T,P_{C{O}_2}\right)f\left(X\right)$$

(1)

where $k$ is the apparent gasification reaction rate, depending on the partial pressure of gasifier CO₂ ($P_{C{O}_2}$) and the temperature (T); f(X) describes the change of physical or chemical properties of the sample during the gasification reaction. Since the concentration change of CO₂ can be ignored, the apparent gasification reaction rate (r) depends on the temperature, which can be parameterized by Arrhenius formula as in Equation (2):

$$k=Aexp\left(-\frac{E}{RT}\right)$$

(2)

where A is the pre-exponential factor; E is the activation energy (kJ/mol); R is universal gas constant (8.314 J/(mol·K)).

The RPM takes into account the overlap of pore structure, which reduces the effective reaction area. The reaction rate is expressed as:

$$\frac{dX}{dt}=k\left(1-X\right)\sqrt{1-\psi \ln \left(1-X\right)}$$

(3)

where $\psi$ is a parameter related to the pore structure of the unreacted coal sample, and it is calculated by:

$$\psi =\frac{2}{2\ln \left(1-X_{max}\right)+1}$$

(4)

where $X_{max}$ is the maximum of the gasification reaction rate.

After separating the variables and integrating of Equation (3), the linearized solution is expressed as:

$$k t=\left(\frac{2}{\psi }\right)\times \left(\sqrt{1-\psi \ln \left(1-X\right)}-1\right)$$

(5)

3. Results and Discussion

3.1. TGA (Temperature Programmed)

The gasification characteristic curve of coal char under the condition of temperature programmed is shown in Figure 1. Since the experimental results have strong stability and repeatability, only one of the intermediate data is selected as the final result and presented in Figure 1. According to Kabir’s research [6], the gasification rate of coal char below 700 °C is very slow, and the same phenomenon in this experiment is observed. When the temperature is lower than 700 °C, the weight loss rate of coal char is very small, and the gasification activity of coal char is very weak. Therefore, in the subsequent isothermal gasification experiments, the initial gasification temperature is set at 800 °C. It is worth noting that the reaction rate of AW-char sample is very small through the whole heating gasification stage. When the temperature reaches the final temperature, there is still 88.8% coal char residue. It can be seen obviously that the catalyst of KCl added promotes the gasification reaction. For the samples of AW + 3%K, AW + 6%K, and AW + 9%K, the gasification reaction increases dramatically at 890 °C, 830 °C, and 750 °C, respectively. The addition of KCl can effectively promote the gasification of coal char, and the weight loss rate of coal char reaches 74.3% with 3% K added at the temperature of 980 °C. With the increasing of the KCl concentration, the catalytic effect is reflected in two aspects. One is to reduce the temperature required for gasification reaction, the other is to increase the gasification rate of coal char. It can be seen from Figure 1 that the gasification reaction rate after adding 9% K is smaller than that of 6%, which is mainly because 9% K accelerates the gasification reaction of coal char from about 750 °C, which is lower than 830 °C when the concentration of K is 6%. In addition to catalyst, temperature is also an important factor affecting the gasification reaction. The higher the temperature is, the faster the gasification reaction is.

3.2. Gasification Characteristics

The characteristic curves and parameters of the coal char gasification which vary over time from 800 °C to 1100 °C are shown in Figure 2 and

Table 2. Effects of KCl concentration on the characteristic parameters of char gasification.

Sample	Temperature (°C)	Half Reaction Time (τ0.5, min)	Reactivity Index (RI)	Gasification Rate (min−1)
AW	800	460.30	0.0011	0.0011
	900	56.20	0.0089	0.0087
	1000	12.20	0.0410	0.0415
	1100	2.85	0.1754	0.1845
AW + 3%K	800	106.15	0.0047	0.0051
	900	14.55	0.0344	0.0445
	1000	1.85	0.2703	0.4212
	1100	1.45	0.3448	0.5265
AW + 6%K	800	62.30	0.0080	0.0071
	900	12.30	0.0407	0.0490
	1000	2.95	0.1695	0.2476
	1100	1.35	0.3704	0.5469
AW + 9%K	800	7.85	0.0637	0.0761
	900	2.30	0.2174	0.2833
	1000	1.20	0.4167	0.6585
	1100	0.80	0.6250	1.0259

. Similarly, only one of the intermediate data is selected and shown in Figure 2. It reveals that KCl has an obvious promoting effect on coal gasification, and the duration time of gasification is greatly shortened. For example, the total gasification time is cut down from 900 min to 200 min at 800 °C, and from 5 min to 2.5 min at 1100 °C. The higher the concentration of KCl is, the shorter the half reaction time is. Furthermore, the gasification rate is higher, under better catalytic conditions with higher KCl concentration. Under the condition of the final gasification temperature at 1100 °C, the effect of KCl concentration on the gasification rate still could be observed significantly. It indicates that the temperature and catalysis are both the main factors at the temperature below 1100 °C.

In addition, the relationship between gasification rate and carbon conversion rate is usually analyzed to explore the reaction mechanism of coal char gasification, as shown in Figure 3.

The gasification rate increases first and then decreases with the increase of carbon conversion rate. A maximum gasification reaction rate existed, and it is because in the early stage of gasification reaction, the sealed pores in coal char are gradually opened as the reaction proceeds. Then, the flow of gasifier and gasification products is accelerated, and thus the contact between gasifier and coal particle surface is more sufficient. Therefore, the gasification rate is increased first. When the carbon conversion rate is relatively high, the amount of coal char is reduced and meanwhile, the effective surface area that can participate in the reaction is reduced, and thereby resulting in the decrease of the reaction rate. Under the same condition of carbon conversion rate, the reaction rate of coal char with different concentration of KCl is different. The higher the concentration is, the higher the gasification rate is, which is consistent with the previous conclusion.

3.3. Kinetic Analysis

3.3.1. Reaction Rate

Based on previous unpublished work, RPM model is applied to calculate the constant k of gasification reaction rate, as shown in Figure 4 (Product of reaction rate constant k and time t vs. Time t) and

Table 3. Summary of kinetic and empirical parameters of the RPM model.

Sample	T (°C)	RPM
		k (s−1)	R2	Ψ
AW	800	0.00002111	0.9885	2.1117
	900	0.0001739	0.9888	2.1555
	1000	0.0008424	0.9905	2.0497
	1100	0.00348	0.9962	3.0219
AW + 3% K	800	0.00009308	0.9787	2.1155
	900	0.0007373	0.9529	2.1145
	1000	0000596	0.9616	4.6752
	1100	0.0096	0.9805	2.4181
AW + 6% K	800	0.0001476	0.9888	2.1406
	900	0.0008754	0.9592	2.1109
	1000	0.00418	0.9669	2.1479
	1100	0.008	0.9950	6.1389
AW + 9% K	800	0.0013	0.9610	2.1697
	900	0.00502	0.9711	2.2405
	1000	0.01134	0.9782	2.4483
	1100	0.02037	0.9869	2.2305

. The slope of the fitting line represents the reaction rate constant k. Fitting results show that higher slope corresponds to relatively higher temperature, i.e., higher reaction rate. At the same temperature, the reaction rate constant k of the samples with KCl adding is greater than that of AW-char, which indicates that the catalyst promotes the gasification reaction. For the same sample, the reaction rate constant k varies greatly at different temperatures. Taking AW char as an example, it increases from 0.00002111/s at 800 °C to 0.00348/s at 1100 °C, with a high growth rate of 164 times. It can be seen that the effect of temperature on gasification rate is extremely obvious.

As expected, results reveal that at the same gasification temperature, the reaction rate of AW-char is the lowest, and the reaction rate increases several times after adding KCl. Furthermore, the reaction rate increases with the increase of KCl concentration.

3.3.2. Kinetic Parameters

Arrhenius plot (Natural logarithm of reaction rate constant ln k vs. Reciprocal of temperature1/T) is applied to obtain the activation energy (E) and the pre-exponential factor (A), and the linear relationship and the calculation results are displayed in Figure 5 and

Table 4. Summary of the kinetic and empirical parameters of the RPM model.

Sample	RPM
	E (kJ/mol)	A (s−1)	R2
AW	207.53	2.79 × 105	0.9990
AW + 3% K	198.31	4.89 × 105	0.9527
AW + 6% K	167.46	2.35 × 104	0.9777
AW + 9% K	112.22	4.24 × 102	0.9796

. It needs to be pointed out that this pattern is only applicable to the chemical reaction control area, not to the diffusion control area. The four temperature points used in this experiment can be well fitted into a straight line, indicating that chemical reaction control is the main factor below 1100 °C. Therefore, all four temperature points will be used to calculate the activation energy. The activation energy of AW-char is the highest, which can be reduced from 207.53 kJ/mol to 198.31 kJ/mol, 167.46 kJ/mol, and 112.22 kJ/mol by adding 3%, 6%, and 9% K, respectively. It can be clearly seen that increasing the concentration of K⁺ can significantly reduce the activation energy required for the reaction, and meanwhile, the marginal effect of reducing the activation energy is greater. The lower the activation energy is, the higher the gasification reaction activity is, and the easier the reaction will occur, which is of great significance for guiding the directional gasification of coal. Due to the existence of catalysts, a bridge is formed between gasifier and carbon atoms in coal, which makes the reaction easier.

It is found from Figure 5 that at the temperature below 1000 °C, the gasification reaction of coal char is mainly controlled by the chemical reaction on the surface of coal particle. When the temperature is 1100 °C, the growth rate of gasification reaction tends to slow down with the increasing of temperature, which indicates that the gasification of coal char is controlled by both chemical reaction and diffusion reaction (the transfer of gasification agent between pores). Therefore, the effect of catalyst on coal char gasification is most obvious when the temperature is lower than 1000 °C, and weakens when the temperature is higher than 1000 °C. The catalyst mainly plays a role in the chemical reaction. By continuously forming and breaking chemical bonds between carbon atoms of coal char and gasifier, the energy level required for gasification reaction is reduced and the activation energy is reduced.

3.4. Mechanism of Catalytic Gasification

The role of catalysts in coal gasification has been studied for more than 100 years, and the catalytic mechanism is also being explored. So far, many mechanisms of catalytic gasification have been proposed, such as oxygen transfer mechanism [12,13], reaction intermediate mechanism [14], and electrochemical mechanism [14], etc. For the gasification experiment in this study, the coal sample is first pyrolyzed in N₂ atmosphere at 1000 °C to produce coal char, therefore, the phase of metal chloride in the char has been changed. Thus, it is the new phase that plays a catalytic role in the gasification process.

However, the phase of catalyst KCl in char did not change after pyrolysis of coal, as the XRD results shown in our previous work [11]. Based on the analysis of the functional group structure of the char, the content of aliphatic hydrocarbon in the pyrolysis char is very small, while the content of aromatic structure is very high, and the residual oxygen element mainly exists in the form of -OH. Therefore, it can be reasonably inferred that the oxygen element in the char mainly exists in the form of phenolic hydroxyl, which is easy to combine with alkali metal and alkaline earth metal. Therefore, the catalytic mechanism can be explained by the unified mechanism of catalytic gasification of alkali and alkaline earth metals, which is proposed by Chen and Yang [15]. After combining K and char, the adjacent carbon atom is positively charged due to the influence of conjugation. The vaporizer CO₂ or H₂O would be preferentially adsorbed on the carbon atom with a positive charge to form C-O compounds. Therefore, the presence of C-O-K clusters promotes the formation of C-O compounds between adjacent carbon atoms and oxygen, and plays a catalytic role in gasification. In other words, oxygen complexes are generated by the alkali metal in the form of oxide and the graphite structure in char, and then decomposed continuously to produce CO. Thus, the catalytic mechanism of KCl in char is explained, and it is still the K⁺ that plays a catalytic role.

4. Conclusions

In this study, the experiments of temperature programmed gasification and isothermal gasification of coal char are carried out to study the influence of gasification temperature and catalyst concentration on the gasification characteristics. The characteristic parameters and gasification kinetics of gasification reaction are analyzed and discussed. Finally, the reaction mechanism of catalytic gasification of coal char is explored. The following conclusions are obtained:

• Different catalyst concentration has different influence on the gasification of coal char. By increasing the concentration of KCl, the catalytic effect can be enhanced by shortening the half reaction time and increasing the gasification rate.
• The activation energy of AW-char is the highest, which can be reduced to some extent by adding KCl. The chemical bond between the metal cation and the carbon atom, as well as the gasifier in the coal char, is formed and broken continuously, so as to reduce the energy level required for the gasification reaction and achieve the effect of reducing the activation energy.
• The unified mechanism of catalytic gasification of alkali and alkaline earth metals is applicable for the KCl catalysis on coal char gasification.