Variation of char reactivity during catalytic gasification with CO2 : comparison among catalytic gasification by ion-exchangeable Na, Ca, and Na/Ca mixture

doi:10.21203/rs.3.rs-1751791/v1

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Variation of char reactivity during catalytic gasification with CO2 : comparison among catalytic gasification by ion-exchangeable Na, Ca, and Na/Ca mixture

https://doi.org/10.21203/rs.3.rs-1751791/v1

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Reactivity is an important indicator to evaluate the gasification performance, and it is also the key to design and optimize the gasification process. The type and activity of the catalyst have great impacts on the gasification reactivity of the char. In this study, the catalytic behavior of ion-exchangeable Na and Ca during CO₂ gasification process and the influence mechanism of ion-exchangeable Na and Ca on the gasification reactivity of coal char were researched. The results indicated that the reactivity of Ca-Char was greater than that of Na-Char. The inactivation of Ca was mainly due to the loss and the dispersion deterioration of Ca. The increase of CO₂ concentration can not only significantly increase the catalytic activity of Na and Ca, but also enhance the sustainability of Na catalytic activity. The interaction between Na and Ca had inhibition effects on the reactivity of char during CO₂ gasification. The higher the ratio of Ca-Char, the more significant the inhibition effects.

Char

Ion-exchangeable Na and Ca

Catalytic

Gasification reactivity

Coal gasification is an important direction for efficient and clean utilization of coal [1–3]. In general, the content of alkali and alkaline earth metals (AAEMs) in low-rank coal is high, and their catalytic actions are the key to understand the gasification reactivity of coal [4^, 5]. Among them, ion-exchangeable Na and Ca are generally regarded as excellent in-situ catalysts for low-rank coal, and it is of great significance to make full use of in-situ ion-exchangeable Na or Ca for the gasification of low-rank coal [6]. Therefore, ion-exchangeable Na or Ca catalytic gasification of low-rank coals has attracted extensive attention.

Reactivity is an important indicator for evaluating gasification performance and is also the key to design and optimize the gasification process. The type and activity of the catalyst have great impacts on the gasification reactivity of the char. Lahijani et al. [7] researched the effect of alkali, alkaline earth and transition metal nitrates on CO₂ gasification reactivity of biomass char using a thermogravimetric analyzer (TGA), and the results indicated that in terms of carbon conversion, the order of catalytic effect was Na-char > Ca-char > Fe-char > K-char > Mg-char > raw char. Huang et al. [7] obtained that the order of catalytic action of the catalyst on the CO₂ gasification of biomass char is K > Na > Ca > Fe > Mg. However, Sadhwani et al. [9] reported that during CO₂ gasification of biomass char, the activity of the metals was observed in the order of: K-char > Ca-char > Na-char ≥ Mg-char. The main reason for the different sequence of catalytic activity of char may be the neglect of the changes in the catalytic activity of the catalysts throughout the gasification process. The volatilization loss of catalyst, the formation of deactivated materials, the poisoning of catalyst, and the interaction between catalyst and minerals all affect the activity of catalyst, and thus affect the gasification reactivity of char [10]. The following takes the catalytic activity of Na and Ca as an example. Yu et al. [11] used TGA to study the influence of ion-exchangeable Ca on the reactivity of char during CO₂ gasification, and found that the catalytic effect of ion-exchangeable Ca was obvious in the late stage of CO₂ gasification. Liu et al. [12] found that during CO₂ gasification, the decrease of Ca dispersion would decrease the catalytic activity of Ca. Liu et al. [13] reported that during CO₂ gasification, the addition of NaCO₃ would promote the formation of sodium-oxygen complexes, which can improve the gasification reactivity of char. At present, catalysts for char reactivity studies are mostly in the form of additives, such as the AAEM species of carbonates, halides, and other compounds. But to our knowledge, studies on the effects of ion-exchangeable Na and Ca on char reactivity are insufficient [11], especially the research on the effect of gasification conditions on catalytic effect of ion-exchanged Na and Ca.

In the actual gasification process of solid fuel, two or more catalysts usually work together. Therefore, many studies have begun to focus on the catalytic effect of bimetallic catalysts on coal gasification. Some scholars believed that the two catalysts may have synergistic effects to promote the gasification reaction of char. Bai et al. [10^, 14] reported that the coexistence of Ca and Na had better catalytic activity than that of Ca and Na alone during steam gasification. Wang et al. [15^, 16] believed that the addition of Ca during gasification suppressed the inactivation of K, thereby promoting the catalytic effect on the gasification reaction. However, some researchers [17] believed that the addition ratio of the two catalysts had an optimal value, and when the addition ratio exceeded the optimal value, it may have the opposite effect. So far, the catalytic effect of the coexistence of the two catalysts on gasification reactivity is still debated. In addition, the effect of coexistence of ion-exchangeable Na and Ca on the catalysis of CO₂ gasification reaction is seldom studied.

Therefore, the catalytic behavior of ion-exchangeable Na and Ca during CO₂ gasification process and the influence mechanism of ion-exchangeable Na and Ca on the gasification reactivity of coal char were researched. In this research, Na-Char, Ca-Char and H-Char prepared at the pyrolysis temperature of 900°C were the research objects, and the CO₂ gasification experimental research was carried out using a micro-fluidized bed reaction system. The effects of gasification temperature, CO₂ concentration, and the mixture of Na and Ca on the reactivity of char during CO₂ gasification were investigated. This study can lay an important foundation for the development of catalytic gasification.

2.1. Sample preparation

Zhundong raw coal with particle size of 106 ~ 150 µm was selected as raw materials (R-form coal). The properties of R-form coal are shown in Table 1. In order to research the behavior and action characteristics of ion-exchanged Na and Ca during gasification, ion-exchanged Na coal (Na-form coal) and ion-exchanged Ca coal (Ca-form coal) need to be prepared. The preparation of several experimental samples is shown in Fig. 1. The R-form coal sample was first mixed with 0.1 mol/L dilute hydrochloric acid (in 1 g/50 mL), and then continuously stirred for 24 hours under N₂ atmosphere. Finally, after being filtered, collected, washed repeatedly by deionized water and dried, the acid-washed coal (H-form coal) sample was obtained. The H-form coal sample was mixed with 1.5 mol/L sodium acetate and calcium acetate solutions (in 1 g/30 mL), respectively, and ion exchange was carried out for 24 hours under magnetic stirring. Since the pH value of the coal slurry solution is crucial to the formation of the ion exchange solution [18], the pH value of the solution is maintained at around 8.3 by adding sodium hydroxide or calcium hydroxide during the ion exchange process [19]. After that, the sample was washed with deionized water for several times until the solution was neutral, and collected after being filtered and dried. The Na-form coal and Ca-form coal were obtained.

Table 1

Proximate and ultimate analyses and low heating values of Zhundong coal
Proximate analysis (wt %, ad)				Ultimate analysis (wt %, ad)					Q_net,ar
M	A	V	FC^c	C	H	O^c	N	S^t	(kJ∙kg^− 1)
9.63	5.50	40.30	44.57	61.40	4.41	17.69	0.89	0.48	16119
ad, air-dried; Q_net,ar, low heating values as received; c, calculated by difference; t, total content.

The char samples of Na-Char, Ca-Char and H-Char were prepared from Na-form、Ca-form and H-form coal using fluidized-bed/fixed-bed quartz reactor, respectively. The schematic diagram of experimental system and the pyrolysis procedures were described in detail in previous studies [10]. Briefly, the furnace was heated to 900 ℃ under a continuous argon (> 99.999%) flow of 5 L/min. After the reactor reached the target temperature and stabilized, the preplaced coal samples were immediately carried into the fluidized zone by a fast gas flow (Ar) using the feeding system. The samples were pyrolyzed at a rapid heating rate (> 103 K/s) and held at 900°C for 2 min. After the end of the pyrolysis reaction, the reactor was immediately taken out of the furnace and cooled to room temperature under natural conditions. Finally, the chars were collected for further analysis. In order to prevent the reaction between char and oxidizing gas in the reactor, high-purity argon was introduced into the whole experiment. The compositions of AAEMs in the samples are shown in Table 2.

Table 2

Mass concentrations of AAEMs in the samples
AAEMs	Samples (wt %, dried basis)
	coal				char
	R-form	H-form	Na-form	Ca-form	H-Char	Na-Char	Ca-Char
Na	0.234	0.008	1.885	0.015	0.008	2.017	0.019
K	0.022	0.014	0.022	0.023	0.005	0.004	0.006
Mg	0.179	0.023	0.020	0.019	0.028	0.027	0.055
Ca	1.003	0.065	0.053	2.562	0.088	0.069	4.000

2.2. Experimental procedure

The CO₂ gasification of the char samples was performed at atmospheric pressure by a Microfluidized bed reaction analyzer (MFBRA). The MFBRA system mainly includes a fluidized-bed quartz reactor, a sample feeding system, a reaction gas supply system, and an online gas detection system. The detailed information has been found elsewhere [20^, 21]. The char samples (10 ± 0.1 mg) were placed in an instantaneous feeder, and then the reactor was heated to target temperature under argon atmosphere. When the reaction temperature and mass spectrum baseline were completely stable, argon was switched to reaction gas, and the char samples were injected into the reactor for reaction. When the baselines of mass spectra were completely stable again, reaction gas was switched to O₂/Ar for burning off the carbon remaining in the gasification experiment to achieve the carbon balance during the experiment. When the baselines of CO₂ mass spectra were completely stable again, the experimental data was saved. The relevant experimental parameters are listed in Table 3. The mixing ratios of Na-Char and Ca-Char were 30%:70%, 50%:50% and 70%:30%.

Table 3

The relevant experimental parameters
Sample	Temperature (°C)	CO₂ concentration (vol%)	Balance gas	Flow rate (L/min)
Na-Char, Ca-Char, and H-Char,	900	10, 15, 20, 30, 40	Ar	1.8
Na-Char, Ca-Char, and H-Char,	700, 750, 800, 850, 900	15
Na-Char/Ca-Char, Na-Char/H-Char, Ca-Char/H-Char	900	20

2.3. Analysis methods and data processing

During the gasification process, the gases produced by the reaction were continuously recorded in real time. The carbon conversion and the reaction rate of char were calculated by the equations:

$${\text{Q}}_{\text{V}}\text{(}\text{t}\text{)=}\frac{\text{(1-}{\text{C}}_{\text{R}}\text{)×}{\text{Q}}_{\text{F}}}{\text{1-}{\text{C}}_{\text{CO}}\text{(}\text{t}\text{+Δ}\text{t}\text{)+}{\text{C}}_{\text{CO2}}\text{(}\text{t}\text{+Δ}\text{t}\text{)+}{\text{C}}_{\text{O2}}\text{(}\text{t}\text{+Δ}\text{t}\text{)}}$$

$${\text{m}}_{\text{t}}\text{=0.5×}\frac{{\text{M}}_{\text{C}}}{{\text{V}}_{\text{M}}}\text{×}{\int }_{\text{0}}^{\text{t}}{\text{Q}}_{\text{V}}\text{(}\text{t}\text{)}\text{×}{\text{C}}_{\text{CO}}\text{(}\text{t}\text{)d}\text{t}$$

$${\text{m}}_{\text{∞}}\text{=0.5×}\frac{{\text{M}}_{\text{C}}}{{\text{V}}_{\text{M}}}\text{×}{\int }_{\text{0}}^{\text{∞}}{\text{Q}}_{\text{V}}\text{(}\text{t}\text{)}\text{×}{\text{C}}_{\text{CO}}\text{(}\text{t}\text{)d}\text{t}$$

$${\text{X}}_{\text{t}}\text{=}\frac{{\text{m}}_{\text{t}}}{{\text{m}}_{\text{∞}}}\text{=}\frac{{\int }_{\text{0}}^{\text{t}}{\text{Q}}_{\text{V}}\text{(}\text{t}\text{)}\text{×}{\text{C}}_{\text{CO}}\text{(}\text{t}\text{)d}\text{t}}{{\int }_{\text{0}}^{\text{∞}}{\text{Q}}_{\text{V}}\text{(}\text{t}\text{)}\text{×}{\text{C}}_{\text{CO}}\text{(}\text{t}\text{)d}\text{t}}$$

$${\text{R}}_{\text{t}}\text{=}\frac{\text{d}{\text{X}}_{\text{t}}}{\text{d}\text{t}}$$

Where ${\text{Q}}_{\text{F}}$is the standardized flow rate of the fluidizing gas (NL/min); ${\text{Q}}_{\text{V}}\text{(}\text{t}\text{)}$ is the standardized flow rate that is released during CO₂ gasification at time t (NL/min); ${\text{C}}_{\text{R}}$ is the volume concentration of the reacting agent in the fluidized bed (%); ${\text{C}}_{\text{O2}}\text{(}\text{t}\text{+}\text{Δ}\text{t}\text{)}$、${\text{C}}_{\text{CO}}\text{(}\text{t}\text{+}\text{Δ}\text{t}\text{)}$、${\text{C}}_{\text{CO2}}\text{(}\text{t}\text{+}\text{Δ}\text{t}\text{)}$ are the measured concentrations of each gas at time (t + Δt) (%); Δt is the delay time for measuring the released gas (s), ${\text{m}}_{\text{t}}$ and ${\text{m}}_{\text{∞}}$ represent the accumulative mass of the carbon conversion at time t and total mass (g), respectively; ${\text{M}}_{\text{C}}$ is the molecular mass of carbon (g/mol); ${\text{V}}_{\text{M}}$ is the molecular volume of gas (STP, 22.4 L/mol); ${\text{X}}_{\text{t}}$ is the carbon conversion at reaction time t (%); ${\text{R}}_{\text{t}}$ is the char reactivity at time t (s⁻¹); and ${\text{C}}_{\text{CO}}\text{(}\text{t}\text{)}$ are the volume concentrations of CO at time t (%).

From Table 2, it can be found that AAEMs of Zhundong coal are mainly Na and Ca, and the low-rank coals rich in Na and Ca are also widely used. During gasification, the interaction mechanism between Na and Ca may have the following three situations: synergistic mechanism, competitive mechanism and independent mechanism. In order to research the interaction mechanism between Na and Ca and its effect on the reactivity of char during gasification, mixtures of Na-Char and Ca-Char with different mass ratios were gasified, and the results were compared with the calculated values. The calculated values were obtained by the following equations:

$${\text{X}}_{\text{t}}^{\text{cal}}\text{(}\text{mix}\text{)}\text{=}\frac{\text{a}{\text{m}}_{\text{∞}}\text{(}\text{Na}\text{)}\text{×}{\text{X}}_{\text{t}}\text{(}\text{Na}\text{)}\text{+}{\text{b}\text{m}}_{\text{∞}}\text{(}\text{Ca}\text{)}\text{×}{\text{X}}_{\text{t}}\text{(}\text{Ca}\text{)}}{\text{a}{\text{m}}_{\text{∞}}\text{(}\text{Na}\text{)}\text{+}{\text{b}\text{m}}_{\text{∞}}\text{(}\text{Ca}\text{)}}$$

$${\text{R}}_{\text{mix}}^{\text{cal}}\text{=}\frac{\text{d}{\text{X}}_{\text{t}}^{\text{cal}}\text{(}\text{mix}\text{)}}{\text{d}\text{t}}$$

Where ${\text{X}}_{\text{t}}^{\text{cal}}\text{(mix)}$ is the carbon conversion of Na-Char and Ca-Char mixture at reaction time t (%); ${\text{R}}_{\text{mix}}^{\text{cal}}$ is the calculated reactivity value of Na-Char and Ca-Char co-gasification (s^− 1); ${\text{X}}_{\text{t}}\text{(Na)}$ and ${\text{X}}_{\text{t}}\text{(Ca)}$ are the carbon conversion of Na- and Ca-Char alone at reaction time t, respectively (%); ${\text{m}}_{\text{∞}}\text{(Na)}$ and ${\text{m}}_{\text{∞}}\text{(Ca)}$ are the total mass of Na-Char and Ca-Char, respectively (g); a and b are the mass fractions of the Na-Char and Ca-Char, respectively (%).

The contents of Na and Ca species in the char samples were obtained by inductively coupled plasma-optical emission spectroscopy (ICP-OES). In addition, the elemental analysis of the gasified chars (especially Na and Ca) was researched by scanning electron microscope combined with an energy dispersive spectrometer (SEM/EDS). The characterization method of Na and Ca dispersion in char samples has been described in detail in our previous study [10].

3.1. Effects of reaction temperature on the gasification reactivity of Na-Char and Ca-Char

The reactivity profiles of Na-Char, Ca-Char and H-Char at a function of char conversion with 15% CO2 atmosphere at different reaction temperature are shown in Fig. 2. It can be found that the gasification reactivity of H-char was much smaller than that of Na-Char and Ca-Char, especially with the decrease of gasification temperature, the reactivity of H-Char decreased significantly. H-Char hardly occur gasification reaction with CO₂ at 700 ℃, and the carbon conversion rate within the gasification time of 120 minutes was only about 1.2%. The char structure was the controlling factor limiting its reaction rate during non-catalytic gasification. The higher reactivity of Na-Char and Ca-Char was attributed to the catalytic effect of Na and Ca on the gasification reaction, which also indicated that the catalytic effect of Na and Ca during the gasification reaction of Na-Char and Ca-Char was the main controlling factor of char reactivity, and the distribution of reactivity should be closely related to the behavioral characteristics of Na and Ca. It can be found from Fig. 2 that the reactivity profiles of Na-Char and Ca-Char were similar. The reactivity increased rapidly at the beginning of the reaction, then decreased gradually with the transformation process. The maximum reactivity appeared at the beginning of the reaction. This is because at the initial stage of the reaction, the active structure content of the char was relatively high, and the content of Na or Ca in the char increased sharply, showing strong catalytic activity. However, with the conversion of char, Na and Ca began to be lost or deactivated, which gradually reduced the catalytic activity of Na and Ca. Therefore, the behavior of Na and Ca was the most critical to the reactivity profiles in the CO₂ gasification process.

From Fig. 2, it can be found that the reactivity of Na-Char and Ca-Char was sensitive to gasification temperature, especially at the gasification temperature of 700°C and 750°C, the reactivity of Na-Char and Ca-Char was very low. In addition, the reactivity of Ca-Char was higher than that of Na-Char at the gasification temperature of 700–900°C, so Ca had higher catalytic activity than Na during CO₂ gasification. This was completely different from the situation during the process of steam gasification [10], where there was a characteristic conversion value X_i when the reactivity relationship of Na-Char and Ca-Char was shifted. During the initial stage of steam gasification conversion (0 ~ X_i), the catalytic activity of Na was higher than that of Ca, while the steam gasification reactivity of Na-Char was higher than that of Ca-Char in the whole gasification conversion process at lower gasification temperatures (700°C and 750°C). This indicated that Na and Ca exhibited different catalytic properties during CO₂ and steam gasification.

In our previous study on the behavior characteristics of Na and Ca catalysts during steam gasification, it has been shown that the release of Na during the gasification process was the main factor of its inactivation, while the main cause of Ca inactivation was the deterioration of dispersion in char [10]. In order to analyze the catalytic behavior of Na and Ca during CO₂ gasification, we used the same method to characterize the release characteristics and dispersion of Na and Ca during CO₂ gasification. Figure 3 showed the retained amounts of Na and Ca in char under different conversion ratios of Na-Char and Ca-Char during CO₂ gasification at 900°C, it can be found that the content of Na and Ca in char decreased during CO₂ gasification, especially the decrease of Na content was more obvious. When the conversion rate was 50%, the release of Na reached about 56%, while the release of Ca was Only 15%. Compared with the release of Na during steam gasification, the release of Na during CO₂ gasification was relatively slow, which also made more Na catalytic medium retained in the char, thus playing a catalytic role in the gasification reaction. Therefore, the distribution of Na-Char reactivity with carbon conversion during CO₂ gasification did not show the rapid decline stage during steam gasification. The large loss of Na during steam gasification was the main reason for the rapid decline of Na-Char reactivity in the early stage, while the loss of Na during CO₂ gasification was relatively slow, and the catalytic persistence of Na during CO₂ gasification was better than that of steam gasification. When carbon reacted with gasification agent H₂O, chemisorption first occurred on the surface of coal char, forming carbon bound intermediates, oxygen-containing components and H free radical, among which H free radical was the key factor promoting the release of Na in coal char [22]. This was different from the reaction of carbon with the gasification agent CO₂ (H radicals were produced by the steam gasification reaction). During CO₂ gasification, the chemisorption of CO₂ was completed by the catalytic medium in the coal char, and the Na-O-Char intermediate structure may be formed to stabilize Na, so during CO₂ gasification, Na showed a slower volatilization rate during the catalysis process, which also enabled the Na catalytic activity to ensure better persistence [23]. Therefore, Na exhibited a slow volatilization rate during CO₂ gasification, which also enabled Na catalytic activity to ensure better persistence.

Compared with the change characteristics of Ca during steam gasification, the release behavior of Ca during CO₂ gasification was completely different. Ca can form CaCO₃ with lower Tammann temperature in coal char during CO₂ gasification [24], the fluidity of CaCO₃ is good. With the conversion of carbon, the gradually aggregated CaCO₃ in the char would be driven away from the surface of the char by the high-speed airflow, resulting in the loss of Ca during the gasification process. In addition, CaCO₃ with good fluidity can promote the contact of Ca/CO₂/Carbon and make Ca exerted a strong catalytic activity (especially at high gasification temperature), so that the gasification reactivity of Ca-Char was higher than that of Na-Char in the whole conversion process during CO₂ gasification. However, CaCO₃ is easy to be sintered and agglomerated, which affects the dispersion of Ca in char. The standard deviations of Na and Ca contents in Na-Char and Ca-Char samples with different conversion ratios with 15% CO₂ at 900 ℃ were shown in Fig. 4, which indicated the dispersity of catalysts. It can be found that the dispersion of Na in char was good during CO₂ gasification, and there was no obvious change in the dispersion with the conversion of the char, which is mainly because Na has good fluidity in the char. In contrast, the dispersity of Ca in char gradually deteriorated with the conversion of the char, mainly because in during CO₂ gasification, the Ca component in the char can form CaCO₃, which is easily sintered. Therefore, the gradual deterioration of dispersion was an important reason for Ca inactivation during CO₂ gasification.

In summary, the reactivity profiles of Na-Char and Ca-Char during CO₂ gasification was closely related to the catalytic medium Na and Ca, in which Ca had a higher catalytic activity than Na in CO₂ atmosphere, making the reactivity of Ca-char was higher than that of Na-Char in the whole conversion process. Meanwhile, the catalytic activities of Na and Ca were significantly affected by gasification temperature. The catalytic activity of Na and Ca decreased gradually during the gasification process. The loss of Na during CO₂ gasification was the main reason for the decrease of the catalytic activity, while the inactivation of Ca was mainly caused by the loss and the dispersion deterioration of Ca.

3.2. Effects of CO₂ concentrations on the gasification reactivity of Na-Char and Ca-Char

The gasification reactivity profiles of Na-Char, Ca-Char and H-Char at a function of char conversion with different CO₂ concentrations at 900℃ were shown in Fig. 5. It can be found that the reactivity of Chars, especially Na-Char and Ca-Char, gradually increased with the increase of CO₂ concentration, while CO₂ concentration had little influence on the gasification reactivity rate of H-Char. Therefore, the effect of CO₂ concentration on the reactivity of Na-Char and Ca-Char was mainly caused by the catalytic reaction of Na and Ca. The necessary condition for the occurrence of catalytic reaction was the aggregation of CO₂/Catalyst/Carbon at the interface of the char reaction. Therefore, with the increase of the concentration of CO₂ in the reaction gas, the contact opportunity of CO₂/Catalyst/Carbon increased, making Na and Ca played stronger catalytic roles in CO₂ gasification reaction. When the concentration of CO₂ was low (10% CO₂), the catalytic activity sites of Na and Ca were relatively more due to the good dispersion of alkali metals [25], so the reactivity of Na-Char and Ca-Char was relatively similar at the initial stage of the reaction. However, with the increase of CO₂ concentration, the contact chance of CO₂/Catalyst/Carbon was improved, and Ca was also promoted to form CaCO₃. The better fluidity of CaCO₃ also made the reactivity of Ca-Char higher than that of Na-Char throughout the gasification process.

With the increase of CO₂ concentration, it can also be found that the position of the maximum gasification reactivity of Ca-Char gradually moved to the right, that is, the higher the CO₂ concentration, the greater the carbon conversion rate when the maximum gasification reactivity of Ca-Char occurred. For example, the maximum gasification reactivity of Ca-Char appeared at the position of conversion rate of 0.1 at 10% CO₂ concentration, while the maximum gasification reactivity of Ca-Char shifted to the position of carbon conversion rate of 0.2 at 40% CO₂ concentration. The reaction rate of Ca-Char was slow in the low CO₂ concentration. With the carbon consuming, the concentration of Ca in char gradually increased. When there was not enough CO₂ to catalyze the reaction with active Ca, part of Ca may preferentially form CaCO₃ or other calcium compounds, and crystal growth or sintering may gradually occur. The loss of active Ca made the maximum reactivity of char appear at the initial stage of reaction. However, the reaction rate of Ca-Char was faster in the high CO₂ concentration. When CO₂ reacted in contact with the catalytic medium, new Ca components were gradually exposed. The catalytic activity of Ca gradually increased over a relatively long time, thus making the maximum reactivity of char move to the right. In addition, from the reactivity profiles of Ca-Char, it can be found that the catalytic activity of Ca can maintain good persistence under different CO₂ concentrations. Therefore, it can be inferred that the change of CO₂ concentration did not affect the catalytic mechanism of Ca.

From the reactivity profiles of Na-Char, it can be also found that with the increase of CO₂ concentration, Na-Char retained relatively high reactivity over a larger carbon conversion interval. For example, at 10% CO₂ concentration, the reactivity of Na-Char decreased rapidly after the maximum value and was close to the reactivity of H-Char when the carbon conversion rate was about 0.6. However, with the increase of CO₂ concentration, the decline rate of Na-Char reactivity slowed down, the carbon conversion rate increased gradually when Na-Char reactivity approached H-Char reactivity. It indicated that the increase of CO₂ concentration can make the catalytic activity of Na-Char more sustainable. In the catalytic gasification reaction, more CO₂ molecules were chemically adsorbed on the surface of the char, and the transmission of O was completed through the catalytic medium Na. The binding between Na and O can be closer and faster, that is, the conversion frequency of Na was higher. Meanwhile, the increase of O also made Na more stable, so the loss rate of Na during CO₂ gasification would be slowed down, which ensured the high reactivity of Na-Char during the conversion process.

3.3. Effects of Na and Ca catalyst mixtures on the gasification reactivity

The comparison of experimental and calculated gasification reactivity profile of Na-Char/Ca-Char with 20% CO₂ at 900°C were shown in Fig. 6. The experimental values of the gasification reactivity of the three different Na-Char/Ca-Char mixed chars were obviously lower than the theoretical calculated values, which indicated that the catalytic media Na and Ca had mutual inhibition effect on the gasification reactivity of the char during CO₂ gasification. Na and Ca were not independent of each other during the catalytic gasification process, but limited the catalytic activity of each other, so that the mixed char showed lower gasification reactivity.

From Fig. 6, it can be found that the reactivity of char added with 30% Ca-Char in 70% Na-Char was not only lower than the theoretical calculated value of mixed char, but also lower than the reactivity of single Na-Char. It can be found from the above that the reactivity of single Ca-Char was higher than that of single Na-Char during CO₂ gasification. In theory, the reactivity of mixed char after adding part of Ca-Char to Na-Char should be higher than that of single Na-Char. However, the above phenomenon indicated that the catalytic activity of Ca was significantly suppressed under the coexistence Na and Ca during CO₂ gasification. It can also be found that the reactivity of char added with 30% Na-Char in 70% Ca-Char was not only lower than the theoretical calculated value of mixed char, but also lower than the reactivity of single Ca-Char. Since the gasification reactivity of Ca-Char was higher than that of Na-Char, the overall reactivity of the mixed char would decrease after adding Na-Char. The addition of Na-Char can promote the formation of Na/Ca eutectic compounds. From the reactivity profile, it can be inferred that the catalytic activity of Na/Ca eutectic compounds was lower than that of Ca, so that the mixture of Na and Ca can inhibit the reactivity of char.

The difference between the experimental and calculated gasification reactivity of Na-Char/Ca-Char under three mixing ratios was shown in Table 4. Taking the reactivity at the conversion rate of 0.5 as the comparison benchmark, the difference between the experimental and calculated gasification reactivity tended to increase with the increase of the mixing ratio of Ca-Char, which indicated that the inhibition effect of Na/Ca mixture had a more obvious effect on the catalytic activity of Ca, and the inhibition of Na on the catalytic activity of Ca was the main factor of the mechanism between the two.

Table 4

Comparisons of experimental and calculated gasification reactivity of Na-Char/Ca-Char
Reactivity (R_x=0.5)	Mass ratio of Ca-Char and Na-Char in CO₂ gasification
Reactivity (R_x=0.5)	30%:70%	50%:50%	70%:30%
Experimental value	0.00134	0.00157	0.00227
Theoretical value	0.00348	0.00451	0.00553

The discrete mineral components (ash) in coal char were considered to be important factors affecting the activity of the catalytic medium, so the experimental and theoretical gasification reactivity of Na-Char and Ca-Char mixed with H-Char were analyzed in this research (see Fig. 7). From Fig. 7, it can be found that the reactivity of char added with 70% H-Char in 30% Na-Char or 30% Ca-Char was lower than the theoretical calculated value in the early stage of conversion, which indicated that the added H-Char limited Na-Char and Ca-Char in the early stage of conversion. The addition of H-Char can be considered as the increase of ash content in the mixed char, and the interaction of ash with Na or Ca reduced the catalytic activity of the catalyst. However, the reactivity of mixed char was greater than the theoretical calculated value at the later stage of conversion, indicating that the interaction of ash with Na or Ca generated inactive mineral components (such as aluminosilicates) was not the main factor inhibiting the catalytic activity. Comparing the reactivity profiles of Na-Char/H-Char and Ca-Char/H-Char, it can be found that the relationship between the experimental and the theoretical reactivity of the Na-Char/H-Char changed at the conversion rate of 0.15, while the transition position of the Ca-Char/H-Char was around 0.35. The mobility of catalytic medium was an important factor affecting the catalytic activity. After the addition of H-Char, the movement of the catalytic medium was more necessary to promote the gasification reaction of H-Char, but the addition of H-Char limited the movement of Na and Ca. With the conversion of carbon during gasification, the dispersibility of Na and Ca in the mixed char gradually became better. As a result, the reactivity of the mixed char started to be greater than the theoretical value. From the above analysis, it can be inferred that the ash content in char had a greater ability to limit the mobility of Ca than that of Na.

In conclusion, the experimental gasification reactivity of Na-Char and Ca-char mixed char was significantly lower than the theoretical calculated value, indicating that the interaction between Na and Ca had inhibition effects on the reactivity of char during CO₂ gasification. The higher the ratio of Ca-Char, the more significant the inhibition effects. In addition, the ability of ash to restrict the mobility of Ca in char was stronger than that of Na, but the interaction between catalyst and ash was not the main factor that inhibited the catalytic activity of Na and Ca. The low melting point eutectic compounds formed by Na and Ca during CO₂ gasification cannot effectively improve the catalytic activity of Ca. The catalytic activity of eutectic compounds was lower than that of Ca, which inhibited the catalytic activity of Ca.

In this study, the reaction characteristics of Na-Char, Ca-Char and H-Char during CO₂ gasification were studied by the micro-fluidized bed reaction system. The behavior characteristics of Na, Ca and Na/Ca mixture and their effect on char reactivity during CO₂ gasification were investigated. The main conclusions are as follows:

(1) During CO₂ gasification, the reactivity of Ca-Char was greater than that of Na-Char, and the catalytic activity of Ca was obviously stronger than that of Na. The loss rate of Na was slow, and its catalytic activity can maintain good continuity. The inactivation of Ca was mainly caused by the loss and the dispersion deterioration of Ca.

(2) The effect of CO₂ concentration on the reactivity of Na-Char and Ca-Char was mainly caused by the catalytic reaction of Na and Ca. The increase of CO₂ concentration can not only significantly increase the catalytic activity of Na and Ca, but also enhance the sustainability of Na catalytic activity.

(3) The interaction between Na and Ca had inhibition effects on the reactivity of char during CO₂ gasification. The higher the ratio of Ca-Char, the more significant the inhibition effects. The catalytic activity of eutectic compounds formed by Na and Ca during CO₂ gasification was lower than that of Ca, which inhibited the catalytic activity of Ca.

Acknowledgements

The authors gratefully acknowledge the China Postdoctoral Science Foundation Funded Project (2021M690824) and Heilongjiang Postdoctoral Program (Grant No.LBH-Z21133).

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Competing interests

The authors declare that they have no known conflict of interest.

Authors' contributions

Xiye Chen, Changshuai Du and Penghua Qiu conceived of the presented idea. Xiye Chen, Linyao Zhang, Zixin Jiao and Changshuai Du carried out the experiment. Xiye Chen wrote the manuscript with the support from Chang Xing. Chunhui Yang and Penghua Qiu helped supervised the project. All authors discussed the results and contributed to the final manuscript.

Authors' information

School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

Xiye Chen, Linyao Zhang, Li Liu, Zixin Jiao, Penghua Qiu

School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China

Chunhui Yang

School of Astronautics, Harbin Institute of Technology, Harbin 150001, China

Chang Xing

Technology and Engineering Center for Space Utilization, Chinese Academy of Sciences, Beijing 100094, China

Changshuai Du

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No competing interests reported.

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Variation of char reactivity during catalytic gasification with CO2 : comparison among catalytic gasification by ion-exchangeable Na, Ca, and Na/Ca mixture

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

2.1. Sample preparation

2.2. Experimental procedure

2.3. Analysis methods and data processing

3. Results And Discussion

3.1. Effects of reaction temperature on the gasification reactivity of Na-Char and Ca-Char

3.2. Effects of CO₂ concentrations on the gasification reactivity of Na-Char and Ca-Char

3.3. Effects of Na and Ca catalyst mixtures on the gasification reactivity

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

Variation of char reactivity during catalytic gasification with CO2 : comparison among catalytic gasification by ion-exchangeable Na, Ca, and Na/Ca mixture

Status:

Version 1

Abstract

Figures

1. Introduction

2. Materials And Methods

2.1. Sample preparation

2.2. Experimental procedure

2.3. Analysis methods and data processing

3. Results And Discussion

3.1. Effects of reaction temperature on the gasification reactivity of Na-Char and Ca-Char

3.2. Effects of CO2 concentrations on the gasification reactivity of Na-Char and Ca-Char

3.3. Effects of Na and Ca catalyst mixtures on the gasification reactivity

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1

3.2. Effects of CO₂ concentrations on the gasification reactivity of Na-Char and Ca-Char