Synergy Effect of High K-Low Ca-High Si Biomass Ash Model System on Syngas Production and Reactivity Characteristics during Petroleum Coke Steam Gasification

Abstract

The synergy effect of high K-low Ca-high Si biomass ash-based model system (BAMS) on the synthesis gas output and reaction characteristics of petroleum coke (PC) steam gasification process was studied using three biomass ash (BA) components, KCl, SiO₂, and CaCO₃, which were used as the model compounds. In the ternary model system, the steam gasification experiment of PC was conducted using a fixed bed reactor and gas phase chromatography. The synergistic effects of binary and ternary components in the ternary model system on the gasification of PC were obtained. These investigations were based on the data from the gas analysis and examined the gasification reaction process, syngas release behavior, and reaction characteristics. This study examined the effects of binary and ternary components in the ternary model system on the evolution of semi-char structure during PC gasification. This correlation revealed the synergistic effect of the model system on PC gasification. Scanning electron microscope (SEM) and Raman spectroscopy were used to characterize the structure and surface microstructure of the gasification semi-char. The results showed that the yields of different gases in the ternary model system were in H₂ > CO > CO₂. Compared with single PC gasification, the yields of H₂, CO, syngas, and carbon conversion were increased by 29.42 mmol/g, 20.40 mmol/g, 56.68 mmol/g, and 0.35, respectively. All other components in the ternary model system with high K-low Ca-high Si demonstrated catalytic effect, except for SiO₂ and the Ca-Si system, which showed inhibitory effects on syngas release and reaction features. Integrating SEM and Raman spectroscopic analyses, it was elucidated that CaCO₃ and KCl diminished the degree of graphitization in semi-char through interactions with the carbonaceous matrix. This phenomenon facilitated the gasification process and exhibited a synergistic effect. Secondly, SiO₂ will react with CaCO₃ and KCl, producing inert silicates and inactivating these compounds, leading to the decline of catalysis.

1. Introduction

The by-product of delayed coking in the petrochemical process, petroleum coke is inexpensive, non-melting, and has a high calorific value [1]. Petroleum coke has low gasification reactivity due to its high degree of carbon ordering, underdeveloped pore structure, low oxygen and mineral concentration, and these factors [2]. The yearly output of petroleum coke is on the rise, and its energy use via gasification has attracted substantial interest [3]. Petroleum coke is gasified when it reacts with various gasification agents (air, CO₂, O₂, H₂, or steam) at a specific temperature and pressure to produce crude syngas mainly composed of CO and H₂ [4]. However, the high degree of order in the microcrystalline structure, the underdeveloped pore structure, and the low amount of internal alkali (earth) metals (AAEMs) in petroleum coke result in poor reactivity [5,6,7]. Thus, increasing the gasification activity of coke from petroleum and lowering the required gasification temperature are the current objectives of petroleum coke gasification research.

In light of the continuous reduction in fossil energy resource reserves and a series of environmental problems caused by its energy utilization process, renewable energy resources represented by biomass have attracted a great deal of attention from researchers and industry [8,9,10]. Biomass combustion or gasification generates a substantial amount of biomass ash, rich in AAEMs [11]. Research indicates that biomass ash can improve petroleum coke gasification [2]. Essentially, biomass ash components and the chemical assignment forms of its components are key factors affecting biomass ash catalysis. Researchers [12] counted the chemical composition of 141 kinds of biomass ash, finding that most biomass ash’s main components were Si, K, and Ca. It has been demonstrated that the main factors influencing the catalytic effect of the metal catalyst/biomass ash coke catalytic gasification were Si, K, and Ca. Wei [13] obtained typical agricultural biomass-based leachate using a water-washing method. The findings demonstrated that the biomass leachate had a high potassium concentration (mainly water-soluble potassium, i.e., KCl), which significantly accelerated the process of gasification for coke from petroleum. Further research on the impact of soluble and insoluble biomass ash constituents on petroleum coke gasification was conducted by He et al. [14]. The findings demonstrated that the predominant forms of calcium and potassium in biomass ash were soluble and insoluble, respectively, and that the soluble components had a significantly more significant catalytic effect than the insoluble ones, indicating that the soluble components were essential to the catalytic performance of the biomass ash. However, a considerable amount of SiO₂ can be found in some biomass ashes, such as straw. When active AAEMs (potassium, calcium, etc.) are gasified, SiO₂ readily combines with them to inactivate them and reduce catalysis. Thus, a crucial component of the biomass ash catalytic process is the suppression of SiO₂.

Gasification of petroleum coke can be accelerated, lowered in temperature, and completed in a shorter period of time by using a catalyst [2,15]. Studies have shown that alkali metal elements (sodium, potassium, etc.) and alkaline earth metal elements (calcium, magnesium, etc.) can both significantly catalyze the gasification of petroleum coke at this time. However, the development of petroleum coke catalytic gasification technology is hampered by the problems associated with single-component alkali metal catalysts, including their high cost, easy deactivation, and differential catalytic effect [16,17,18,19]. The composite catalyst refers to the combination of two or more metal catalysts, which can effectively improve the gasification reaction activity [20], and optimize syngas composition [21]. Jiao et al. [21] used potassium and different transition metals to prepare composite catalysts for semi-char gasification. The outcomes demonstrated that the composite catalyst could efficiently increase both the yield of CH₄ and the gasification reaction rate and prevent the release of CO₂. In order to create composite catalysts for a coke catalytic gasification experiment, Jiang et al. [22] combined Ca (AC)₂, Ca (OH)₂, and CaCO₃ with K₂CO₃. The findings demonstrate the complementary catalytic effects of calcium-based catalyst and K₂CO₃, which prevent K₂CO₃ from deactivating during the gasification of coke. Monterroso [23] and others used FeCO₃, Na₂CO₃, and FeCO₃-Na₂CO₃ as catalysts for a coke gasification reaction, and found that compared with the single catalyst (FeCO₃ and Na₂CO₃), the corresponding H₂ yield of the FeCO₃-Na₂CO₃ composite catalyst increased by 40% and 15%, respectively, indicating that the Fe-Na composite catalyst can significantly improve the H₂ content in syngas. The combined effects of catalytic gasification can address the issue of petroleum coke’s low gasification efficiency and lower catalyst costs, large-scale land occupation, and low solid waste utilization rates.

With reference to our previous study focusing on the high K-high Ca-low Si model system [24], this study created an appropriate high K-low Ca-high Si model system (including the related monadic, binary, and ternary fractions) based on the primary occurrence forms and corresponding concentrations of K-Ca-Si in average biomass ash. The fixed bed reactor and gas chromatograph that were self-built served as the foundation for the petroleum coke steam gasification experiment. An analysis and investigation were conducted on gasification syngas’ production and reaction properties. It investigated how the model system of high K-low Ca-high Si can work in concert with binary and ternary components to enhance the evolution of the semi-char structure during the gasification of petroleum coke. This correlation revealed the ternary model system’s synergistic effect on the petroleum coke gasification reaction.

2. Experimental Section

2.1. Experimental Materials

This study used petroleum coke (PC) from Sinopec Yangzi Petrochemical Co., Ltd. (Nanjing, China). The material was filtered by a 120–180 mesh screen and dried for 24 h at 105 °C. PC’s proximate analysis and ultimate analysis are summarized in

Table 1. Proximate and ultimate analyses of PC.

Sample	Proximate Analysis (wt.%)			Ultimate Analysis (wt.%)
	FC	VM	Ash	C	H	S	N	O
PC	86.73	10.97	2.3	91.82	0.980	5.517	1.357	0.326

Note: Based on dry basis.

.

Chemicals used as sources of K, Si and Ca, respectively, such as potassium chloride, silicon dioxide and calcium carbonate, were purchased from Sinopharm group. Synthesis of a ternary system was based on 50% KCl, 35% SiO₂, and 15% CaCO₃. Determination of the molar ratio of Si-K-Ca in the ternary system was based on the chemical composition in a typical BA with high K and low Ca [12]. The catalyst is combined with PC in each loading procedure, with a constant 20% ratio of single-, binary-, and ternary catalysts. The mixture is then filtered via a mixer.

Table 2. Compositions of BAMS in different samples.

Samples	KCl	CaCO3	SiO2
I	0	0	0
I-1	0.15 g	0	0
I-2	0	0.045 g	0
I-3	0	0	0.105 g
II-1	0.15 g	0.045 g	0
II-2	0.15 g	0	0.105 g
II-3	0	0.045 g	0.105 g
III	0.15 g	0.045 g	0.105 g

lists the components of BAMS in different samples. I (PC) is used for the sample; I-1, I-2, I-3 (single catalyst); II-1, II-2, II-3 (binary catalyst); III (ternary catalyst).

2.2. Steam Gasification Experiments

A fixed-bed reactor was used for gasification experiments in this study, and Figure 1 displays the experimental equipment’s structural diagram. The detailed information could be found in our previous study [24].

The needed amount of mixture-prepared samples (Table 2) were put in a quartz tray measuring 35 mm in diameter and 12 mm in height. The temperature of the reaction zone was raised to the target temperature (900 °C) at a heating rate of 10 °C/min on the fixed bed platform in the middle of the reactor under the condition of nitrogen flow of 200 mL/min. For the gasification experiment, steam was supplied at a water flow rate of 5 mL/h (67% steam partial pressure) after being stabilized for 5 min at a constant temperature. For the gasification experiment, steam was supplied at a water flow rate of 5 mL/h (67% steam partial pressure) after being stabilized for 5 min at a constant temperature. The time interval of airbag collection is 10 min. Gas chromatography (GC with an Agilent 8860) was used to analyze the composition of gaseous products. The detector was calibrated with standard gas before beginning each experiment. Equation (1) was used for calculating the moles of different components in the exhaust gas based on the collected GC data [24].

$$n_{i,10\min }\left(\mathrm{mmol}\right)=\frac{V_{N_2}\times t}{22.4\times {\upsilon }_{N_2}}\times {\upsilon }_i\times 1000$$

(1)

where V_N2 (mL/min) is the carrier gas flow, t (min) is the gas collection time of air bag; ${\upsilon }_i$ (%) is the content of different gas components (N₂, CO, H₂, CO₂), and 22.4 (L/min) is the molar volume of standard gas. Equation (2) was used for calculating the gasification carbon conversion [24,25]:

$$C={\displaystyle \frac{m}{m_1}}$$

(2)

where m₁ (g) represents the carbon content of the feedstock and m (g) represents the mass of carbon contained in CO₂ and CO. The theoretical values were calculated by the use of Equations (3) and (4) [24].

$$N_{S\mathrm{1,2}-p}=N_{S1}+N_{S2}-N_{PC}$$

(3)

$$N_{S\mathrm{1,2},3-p}=N_{S1}+N_{S2}+N_{S3}-{2N}_{PC}$$

(4)

N represents the parameters related to gasification reaction and syngas release characteristics (carbon conversion, gas release rate, gas yield, etc.), and S represents different samples.

2.3. Semi-Char Structure Analysis

A common method for figuring out the structural properties of carbon-based materials is Raman spectroscopy because of its benefits, which include easy sample preparation, high sensitivity, and high resolution [26,27,28]. The structure of the gasification semi-char was characterized in this work using Thermo Fisher DXR laser Raman spectroscopy. The selected range of the test was 500–2000 cm⁻¹. Using an argon ion laser (wavelength and power were 514 nm and 2 nw, respectively); the analysis range and resolution of the micro-region were 1 μm and 1 cm⁻¹, respectively. PC surface micromorphology and catalyst appearance under steam gasification were characterized by JSM 7600F field emission scanning electron microscope(Japan Electronics Co., Ltd., Tokyo, Japan).

3. Results and Discussion

3.1. Influence and Synergy Effect of High K-Low Ca-High Si Model System on Gas Release Characteristics of PC Steam Gasification Process

3.1.1. Syngas Release Characteristics

The gaseous products produced during PC’s steam gasification on the K-Ca-Si catalyst are shown in Figure 2. In general, the H₂ release rates of various samples were in the order of II-1>I-1>III>II-2>I-2>I>II-3>I-3. It can be found that for the single catalyst, the H₂ release rate curve of I-2 almost coincides with the H₂ release rate curve of PC gasification alone. This suggests that in contrast to the high content of CaCO₃, the low content of CaCO₃ has almost no catalytic effect on the H₂ release rate during PC gasification. The content of CaCO₃ is an essential factor affecting its catalytic effect, as the relevant literature also demonstrates [29]. Although it has less impact than PC gasification alone, the H₂ release rate curve of I-3 is still lower. The number obtained by computing the proportional decrease in the H₂ release rate of I-3 compared to PC gasification is 14.6%. It is found that SiO₂ in the model system of high K-low Ca-high Si has a specific inhibition effect on the H₂ release rate during PC gasification. According to several studies, SiO₂ mainly reduced the gas emission rate by blocking the AAEMs that are naturally present in carbonaceous raw materials [30,31]. Still, the content of AAEMs in PC was less [32], which made the influence of SiO₂ content on the gasification process of PC insignificant. For the binary model system, the H₂ release rate of II-1 and II-2 in Figure 2 is higher than that of PC gasification alone. The H₂ release rate curve of II-1 is higher than that of II-2, which shows that the K-Ca and K-Si binary model system still shows the catalytic effect on the release rate of H₂. The catalytic performance of the K-Ca binary model system is higher than that of the K-Si binary model system. For the Ca-Si model system, the H₂ release rate is lower than that of the PC gasification alone, which indicates that the Ca-Si model system in the ternary model system has an inhibitory effect on the H₂ release rate. The release rate of H₂ in Figure 2 III for the ternary model system is higher than that of PC, which indicates that the high K-low Ca-high Si model system’s ternary model system catalyzes the release of H₂ during the PC steam gasification process.

Each sample’s CO release characteristic curve is comparable to that of H₂, and the order of the CO release rates of the various samples is similar to that of H₂. It can be found in Figure 2 that for single catalysts, the influence of each catalyst on the release rate of CO is similar to that on the release rate of H₂; that is, the amount of CaCO₃ present significantly affects how much CO is released when PC is gasified but the content of SiO₂ has no significant impact on the release of CO during the PC’s gasification. In the high K-low Ca-high Si model system, the K-Ca and K-Si binary model system has a significant catalytic effect on the release of CO. In contrast, the Ca-Si binary model system has an inhibitory effect on the release of CO. However, the K-Ca binary model system differs in that the impact of Ca content on the CO release rate is less substantial than that of H₂. For the K-Ca-Si ternary model system, the CO release rate curve in Figure 2 III is higher than that in the case of PC gasification alone, which shows that the K-Ca-Si ternary model system in the high K-low Ca-high Si model system still has a catalytic effect on the release of CO during the gasification of PC. Figure 2c shows the CO₂ emission characteristic curve of each sample during gasification. All samples exhibit a similar influence pattern on the rate of CO₂ emission during gasification as they do on H₂ and CO.

3.1.2. Synergy Effect of BAMS on Syngas Release

The theoretical gas release rate curve for each sample in the high K-low Ca-high Si model system is depicted in Figure 3 following the Equations (2)–(4).

Table 3. Key characteristic parameter of syngas release of different samples.

Samples	Tm (min) (Theoretical Value)			Rm (mmol/min) (Theoretical Value)
	H2	CO	CO2	H2	CO	CO2
I	10	10	70	0.39	0.14	0.15
I-1	20	50	50	1.06	0.63	0.36
I-2	70	70	70	0.44	0.14	0.19
I-3	10	20	40	0.38	0.14	0.14
II-1	30 (40)	30 (40)	100 (100)	1.11 (1.08)	0.67 (0.68)	0.33 (0.32)
II-2	30 (40)	30 (50)	70 (100)	0.81 (1.01)	0.42 (0.62)	0.28 (0.26)
II-3	10 (40)	40 (60)	40 (60)	0.33 (0.41)	0.14 (0.16)	0.14 (0.18)
III	30 (40)	20 (50)	70 (110)	1.02 (1.03)	0.60 (0.65)	0.26 (0.30)

T_m—maximum gas release rate time; R_m—maximum gas release rate.

displays the important gas release rate parameters for the gasification process of various samples. By comparing II-1 and II-1-T, it is discovered that, in the high K-low Ca-high Si model system, the actual H₂ release rate of the K-Ca model system is higher than the theoretical value. Still, the exact value is close to the theoretical value at each time node, which indicates that the K-Ca model system in the high K-low Ca-high Si model system has a catalytic synergistic effect. Still, the synergistic impact also weakens with the reduction in CaCO₃ content in the K-Ca model system.

Comparing the H₂ release characteristic curves of II-2 and I-1 in Figure 2, it can be found that the H₂ release rate of II-2 is generally slower than that of I-1. The peak H₂ release rate of II-2 is 0.81 mmol/min, which is also lower than that of I-1, and the peak value decreases by 23.6%, which indicates that in the high K-low Ca-high Si model system, SiO₂ can inhibit the catalytic activity of KCl. A comparison of II-3 and II-3-T shows that the Ca-Si model system of the high K-low Ca-high Si model system inhibits the release of H₂. This suggests that the catalytic activity of CaCO₃ will be ineffective when the SiO₂ content of CaCO₃ exceeds that of CaCO₃. By comparing III and II-1 in Figure 3a, it can be found that the H₂ release rate of III is lower than that of II-1, which indicates that the catalytic performance of the K-Ca-Si ternary model system in high K-low Ca-high Si model system is inferior to that of the K-Ca model system. This is primarily due to SiO₂ having a more significant concentration than CaCO₃. The remaining SiO₂ will then react with KCl and block KCl’s activity when SiO₂ and CaCO₃ react preferentially. In addition, through the comparison of III and III-T, it can be found that the overall H₂ release rate of III is lower than the theoretical value, which indicates that in the whole gasification process, the K-Ca-Si ternary model system interacts with each other, which weakens the overall catalytic effect.

Figure 3b shows the theoretical CO release rate curve of each sample. The impact of the catalysts on the rate of CO release was comparable to that of H₂: the K-Ca model system demonstrated catalytic synergy and had superior catalytic performance for CO release compared to the single catalyst. The CO release rate decreased due to SiO₂’s inhibition of KCl’s catalytic activity in the K-Si model system; on the rate of CO release, the Ca-Si model system had an inhibiting impact. The K-Ca-Si ternary model system demonstrated an inhibitory effect on the rate of CO release. However, the synergistic effect of different model systems on the CO release rate is still distinct from that on the H₂ release rate. By comparing II-1 and I-1 in Figure 3b, it can be found that the CO release rate of II-1 is higher than that of I-1 in the early and late stage of the reaction but lower than that of I-1 in the middle stage of the response. This shows that the K-Ca model system with high K-low Ca-high Si enhanced the release of CO in the early and late stages of the reaction. In addition, by comparing the theoretical and actual CO release rate curves of II-1 in Figure 3b, it can also be found that in the early and late stages of the gasification reaction, the actual CO release rate of II-1 is higher than the theoretical release rate. Still, the theoretical value is close to the actual value, which indicates that the K-Ca model system has synergistic catalysis in the early and late stages of the gasification reaction. Still, with the decrease in CaCO₃ content in the K-Ca model system, its synergistic catalysis also weakens. For the K-Ca-Si ternary model system, its catalytic effect on the CO release rate is similar to that on the H₂ release rate, and its synergistic effect on CO release is also identical to that on the H₂ release. Figure 3c shows the actual and theoretical release rates of CO₂. Similarly, the influence of binary catalysts (K-Ca, K-Si, and Ca-Si) and ternary catalyst K-Ca-Si on CO₂ release is similar to that of H₂ and CO; that is, the K-Ca binary catalyst has synergistic catalysis; the K-Si binary catalyst shows SiO₂ inhibiting KCl catalytic activity, and the K-Ca-Si ternary catalyst shows overall inhibition.

3.2. Influence and Synergy Effect of High K-Low Ca-High Si Model System on Syngas Yield in PC Steam Gasification Process

3.2.1. Syngas Yield of the PC Gasification Process

Figure 4 shows the output of different components and the total output of various products in the high K-low Ca-high Si model system. It can be found that the basic order of total gas production from other products is II-1>I-1>III>II-2>I-2>I>II-3>I-3. For single catalysts, KCl and CaCO₃ catalyze gasification synthesis gas preparation. In the case of the binary model system, it can be observed through a comparison of the total syngas yield of II-1, II-2, II-3, and I in Figure 4 that the K-Ca and K-Si model systems exhibit a catalytic effect on the syngas yield during the PC gasification process. In contrast, the Ca-Si model system exhibits an inhibitory effect. Compared to PC gasification alone, the K-Ca-Si ternary model system demonstrated a catalytic impact on the syngas yield.

In the model system of high K-low Ca-high Si, the effect of each catalyst on the yield of H₂ and CO is similar to its impact on the total yield of syngas, that is, except that the Si and Ca-Si model system has an inhibitory effect on the yield of each gas; other systems all show catalytic effect. The order of catalytic performance of each model system for H₂ yield is the following: K-Ca>K>K-Ca-Si>K-Si>Ca>Si>Ca-Si, and the absolute increment/decrement are 41.69 mmol/g, 38.62 mmol/g, 29.42 mmol/g, 19.27 mmol/g, 2.55 mmol/g, −5.18 mmol/g, and −7.41 mmol/g, respectively. The order of catalytic performance of each model system for CO yield is the following: K-Ca>K>K-Ca-Si>K-Si>Ca>Ca-Si>Si, and the absolute increment/decrement are 23.72 mmol/g, 23.07 mmol/g, 20.40 mmol/g, 12.92 mmol/g, −1.25 mmol/g, −1.35 mmol/g, and −2.29 mmol/g, respectively. The following is the order in which each model system performs catalytically for CO₂ yield: the absolute increment and decrease are 11.17 mmol/g, 9.14 mmol/g, 6.86 mmol/g, 6.11 mmol/g, 2.27 mmol/g, −3.2 mmol/g, and −3.48 mmol/g, respectively, for K-Ca>K>K-Ca-Si>K-Si>Ca>Ca-Si>Si.

3.2.2. Synergy Effect of Syngas Yield in PC Steam Gasification Process

Comparing II-1 and I-1 in Figure 4, it is observed that the total yield of syngas in II-1 is 4.2% higher than that in I-1. This shows that in the model system of high K-low Ca-high Si, the catalytic performance of the K-Ca model system for the total yield of syngas is still better than that of the single catalyst KCl. In addition, through a comparison of the actual and theoretical values of II-1 in Figure 4, it is found that the actual total syngas yield is increased by 3.7% compared with the theoretical value. Specifically, the relative increases in each gas (H₂, CO, and CO₂) compared with the theoretical value were 3.2%, 2.5%, and 3.0%, respectively. The results show that the K-Ca binary model system can synergistically promote syngas’ total yield and each gas component’s yield in the high K-low Ca-high Si model system. In the model system of high K-low Ca-high Si, the catalytic effect of the K-Si model system on the yield of syngas is not as good as that of the single catalyst KCl. As the proportion of SiO₂ in the K-Si model system increased, more SiO₂ reacted with KCl, reducing the catalytic activity of KCl. By comparing the actual and theoretical values of II-2 in Figure 4, it is found that in the high K-low Ca-high Si model system, the actual syngas yield of the K-Si model system has decreased by 16.8% in comparison with the theoretical value, and the relative reduction in CO in each gas yield is the largest, 22.6%, indicating that in this type of model system, SiO₂ mainly inhibits the catalytic ability of KCl to synthesize CO in the gasification process. When the proportion of SiO₂ in the K-Si model system increases, SiO₂ will inhibit the catalytic activity of more KCl, thereby reducing the catalytic performance of the K-Si model system. By comparing the syngas yields of II-3 and I-2, it can be found that the catalytic effect of the Ca-Si model system on the syngas yield is lower than that of CaCO₃, which indicates that SiO₂ also inhibits the catalytic activity of CaCO₃. By comparing the theoretical and actual values of II-3, it was observed that the former was more significant. This finding further indicated that SiO₂ reduced the catalytic activity of CaCO₃.

The catalytic impact of the K-Ca-Si ternary model system on the total syngas yield in the high K-low Ca-high Si model system is higher than that of the K-Si binary model system; however, it is not as excellent as that of the single catalyst KCl. In addition, through the comparison of the actual and theoretical values in Figure 4, it can be found that in the high K-low Ca-high Si catalytic model, the K-Ca-Si ternary model system presents an inhibitory effect. By comparing the relative reduction in each gas, it is found that in the high K-low Ca-high Si model system, SiO₂ mainly inhibits the catalytic ability of KCl and CaCO₃ to synthesize H₂ and CO.

3.3. Influence and Synergy Effect of High K-Low Ca-High Si Model System on Gasification Reaction Characteristics of PC

3.3.1. Influence of Gasification Reaction Characteristics of PC

Figure 5a shows the change in each sample’s carbon conversion rate of gasification reaction with time.

Table 4. Final carbon conversion of different samples.

Samples	I	I-1	I-2	I-3	II-1	II-2	II-3	III
Xf (theoretical)	0.39	0.80	0.40	0.31	0.84 (0.82)	0.63 (0.72)	0.32 (0.34)	0.74 (0.77)

shows the final carbon conversion after the gasification reaction of all samples. As Figure 5a shows, the reaction rates of samples loaded with different catalysts from small to large are II-1>I-1>III>II-2>I-2>I-3>I-3. In the model system of high K-low Ca-high Si, KCl and CaCO₃ have a catalytic effect on PC’s gasification rate. Among them, the decrease in CaCO₃ content also lessens the catalytic effect of CaCO₃ on the PC gasification rate. Furthermore, even though the high K-low Ca-high Si model system’s SiO₂ content rose, it had no additional effect on the gasification of PC. This further demonstrates that the features of the gasification reaction are not solely dependent on the SiO₂ content.

For the binary model system, in the high K-low Ca-high Si model system, the K-Ca and K-Si binary model system can catalyze the gasification reaction rate of PC. The results of the K-Si model system in the high K-low Ca-high Si model system show that with the increase in SiO₂ content in the K-Si model system, its catalytic performance decreases. Because SiO₂ has a more extensive content than CaCO₃, the Ca-Si model system in the high K-low Ca-high Si model system exhibits an inhibitory impact on the gasification rate of PC. The final carbon conversion rate improved by 89.7%, from 0.39 to 0.74, in the high K-low Ca-high Si model system. This is because the K-Ca-Si ternary model system has a catalytic effect on the gasification rate of PC.

3.3.2. Synergy Effect of Gasification Reaction Characteristics of PC

By comparing II-1 and I-1 in Figure 5a, it can be found that the carbon conversion rate curve of II-1 is higher than that of I-1 as a whole, and the final carbon conversion rate of II-1 (0.84) is also higher than that of I-1 (0.80). Compared to the single catalyst KCl, the results demonstrate that the K-Ca model system can further increase the gasification reaction rate in the high K-low Ca-high Si model system. The outcomes align with the previously mentioned patterns of gas release rate and yield. In addition, by comparing II-1 and II-1-T in Figure 5b, it can be found that the actual carbon conversion curve of the K-Ca binary model system is slightly higher than the theoretical value during the whole gasification process. Its final actual carbon conversion rate is only 2.4% higher than the theoretical value, which indicates that in the high K-low Ca-high Si model system, the K-Ca binary model system shows catalytic synergy during gasification. Still, the synergy is weakened with the reduction in CaCO₃ content. Through the comparison of the actual carbon conversion rate curve and the theoretical carbon conversion rate curve of II-2 in Figure 5b, it is found that the theoretical carbon conversion rate curve is higher than the actual carbon conversion rate curve. The results show that during the gasification process, SiO₂ will inhibit the catalytic activity of KCl, resulting in the reduction in gasification reaction rate, and with the increase in SiO₂ content, more KCl will be deactivated. As compared to the single catalyst KCl, the K-Ca-Si ternary model system performs less effectively catalytically at gasification reaction rates. Comparing the theoretical value with the actual value, it can be found that the actual carbon conversion slope of III is always lower than the theoretical value, which indicates that SiO₂ inhibits the catalytic activities of KCl and CaCO₃.

3.4. Correlation between Structural Evolution and Reaction Synergy Behavior of PC Steam Gasification in High K-Low Ca-High Si Model Systems

3.4.1. Evolution of Surface Microstructure

The SEM images of gasification semi-char loaded with different catalysts are shown in Figure 6. The surface morphology of PC during gasification is affected by various model systems in the high K-low Ca-high Si model system. Specifically, KCl will cause the smooth surface of the PC to break down and form a sheet structure, as seen from the observation. Additionally, the enlarged local view of Figure 6b shows that KCl will be uniformly attached to the PC surface, which will speed up the gasification reaction rate. In the model system of high K-low Ca-high Si, due to the reduction in CaCO₃ content, no apparent pore structure was observed on the surface of the gasification semi-char in Figure 6c, which led to the decline of its catalytic effect. Figure 6d shows the surface morphology of the gasification semi-char loaded with SiO₂. During the gasification process of PC, it was observed that the surface morphology of the semi-char loaded with SiO₂ was comparable to that of the semi-char gasified by PC alone. This suggests that SiO₂ has no noticeable effect on the surface of the PC.

Figure 6e shows the gasification semi-char supported by the K-Ca binary model system in the high K-low Ca-high Si model system. It can be found that compared with the gasification of PC alone, the surface of PC forms a sheet structure, and some pore structures can be observed. At the same time, KCl is also evenly distributed on the surface of the gasification semi-char, indicating that CaCO₃ reacts with PC to form a pore structure during gasification, and molten KCl is evenly distributed on the surface of PC, thus accelerating the gasification reaction process. Figure 6f shows the gasification semi-char loaded with a K-Si model system. By observing the local enlarged view, it can be found that the KCl particles loaded on its surface basically disappear, indicating that KCl and SiO₂ form inert silicates during the gasification reaction. Since silicates have a melting point higher than KCl and do not catalyze the gasification process, KCl’s catalytic activity becomes inactive. Figure 6g shows the gasification semi-char loaded with a Ca-Si model system. It can be found that there is no change in the surface of the gasification semi-char compared with the gasification semi-char of PC alone, indicating that the combination of SiO₂ and CaCO₃ makes CaCO₃ lose its pore-forming function, thus reducing the catalytic activity of CaCO₃.

Figure 6h shows the gasification semi-char supported by the K-Ca-Si ternary model system in the high K-low Ca-high Si model system. Its surface morphology is damaged, but it is not as damaged as the gasification semi-char supported by the single catalyst KCl and K-Ca model system. By observing the local enlarged map, it is found that the density of KCl distribution is also weaker than that of the gasification semi-char supported by the single catalyst KCl and K-Ca model system, that is, SiO₂ will react with CaCO₃ preferentially. After the amount of CaCO₃ has been transformed into calcium silicate, the remaining SiO₂ will react with KCl. After all SiO₂ has been converted to silicate, the remaining KCl then catalyzes the continuation of the PC gasification process.

3.4.2. Evolution of Carbon Structure

Degree of graphitization and amorphous carbon in gasified semi-char reflects gasification reaction properties [33]. We can better understand the catalyst’s function in the gasification process by examining the evolution of the carbon structure of gasification char. Characterization of water vapor gasification semi-char using Raman spectroscopy for structural characterization of intermediate stage semi-char was conducted. The Raman spectrum was deconvoluted into five peaks in order to determine the exact spectral parameters. The I_G/I_all value represents the degree of graphitization, and I_D3/I_G represents the content of the amorphous carbon structure. The original Raman peaks were split into several Gaussian peaks and several Lorentzian peaks to obtain more information about the carbon structure by the operation of split-peak fitting as described in [2].

Table 5. Raman band area ratios of semi-chars.

Samples	IG/Iall (AD) (TD)	ID3/IG (AD) (TD)
I	0.107	1.01
I-1	0.092 (−0.015)	1.18 (0.17)
I-2	0.100 (−0.007)	1.03 (0.02)
I-3	0.118 (0.011)	0.75 (−0.26)
II-1	0.076 (−0.031)	1.56 (0.55)
II-2	0.095 (−0.012)	1.16 (0.15)
II-3	0.109 (0.002)	0.87 (−0.14)
III	0.083 (−0.024)	1.46 (0.15)

AD—actual value difference with petroleum coke; TD—theoretical value difference with petroleum coke.

lists the Raman band area ratios of semi-chars. In the model system of high K-low Ca-high Si, through the influence of gasification semi-char supported on each catalyst on the carbon structure of gasification semi-char, it can be found that with the reduction in CaCO₃ content, the influence of the single catalyst CaCO₃ and the K-Ca binary model system on the graphitization degree decreases. The results show that in the model system of high K-low Ca-high Si, SiO₂ inhibits the catalytic activity of KCl, thus hindering the process of KCl, reducing the graphitization degree of gasification semi-char. For the Ca-Si binary model system, the graphitization degree of the Ca-Si binary model system is higher than that of PC gasification alone due to the higher SiO₂ content than CaCO₃. The K-Ca-Si ternary model system can still lower the graphitization degree of the PC gasification process, even in the high K-low Ca-high Si model system. However, because it contains more SiO₂, its effect will be impacted.

Due to its ability to promote the formation of an amorphous carbon structure, adding KCl significantly increased the steam gasification reactivity. On the other hand, adding SiO₂ will cause the steam gasification process to slow down and become less reactive. This can explain the high reactivity of PC-containing KCl during steam gasification. When CaCO₃ and KCl are mixed, they can work together to further promote carbon structure formation. SiO₂ inhibits gas release and reaction characteristics, whereas CaCO₃ and KCl have a synergistic effect in increasing the concentration of amorphous carbon. As a result, we can observe the many interactions between the various additives in the steam gasification reaction, which is crucial for increasing the reactivity.

3.5. Synergy Mechanism of BAMS with High K-Low Ca-High Si

3.5.1. Synergistic Catalysis Mechanism of K-Ca Binary System

KCl forms flowing molten KCl at high temperature. Meanwhile, CaCO₃ can destroy the carbon layer and form a large number of pore structures, which makes it easier for molten KCl to enter the PC to participate in the reaction, which increases the reaction area between KCl and PC, thus forming more active sites, which can accelerate the gasification reaction process. With the increase in CaCO₃ content in the K-Ca model system, more and more CaCO₃ can be attached to the surface of PC, which speeds up the pore-forming speed of CaCO₃, and makes more KCl enter the PC to participate in the reaction. Therefore, with the increase in CaCO₃ content, the synergistic catalytic effect of the K-Ca model system is also enhanced.

3.5.2. Mechanism of Catalytic Inhibition of SiO₂ in the K-Si/Ca-Si Binary System

During the gasification reaction, SiO₂ will combine with KCl and CaCO₃ to form inert silicate. For KCl, after KCl and SiO₂ combine to form potassium silicate, on the one hand, the content of KCl with a catalytic effect will be reduced, making less KCl participate in the catalytic reaction. On the other hand, the melting temperature of the generated potassium silicate is higher than the gasification temperature under this condition, which will cause the partially melted KCl to be wrapped by the formed potassium silicate, and then lose its catalytic activity. With the increase in SiO₂ content in the K-Si model system, the above phenomenon will become more and more serious. For CaCO₃, it is also SiO₂ that will combine with CaCO₃ to form calcium silicate, which will lose its original pore-forming function, reducing the amount of active CaCO₃, thus inhibiting the catalytic activity of CaCO₃.

3.5.3. Synergistic Catalysis Mechanism of K-Ca-Si Ternary System

Although SiO₂ can combine with KCl and CaCO₃ to form silicate, through previous analysis and some literature argumentation [34,35,36,37], it can be found that SiO₂ will preferentially combine with CaCO₃ and then with KCl, which indicates that in the K-Ca-Si ternary system, Ca not only plays a role in catalyzing the gasification of PC with KCl, but also protects the catalytic activity of KCl. For the biomass ash model material with high K-low Ca-high Si, because the content of CaCO₃ is lower than that of SiO₂, the remaining SiO₂ will still inhibit the catalytic activity of KCl after CaCO₃ is reacted by SiO₂ during the gasification reaction.

4. Conclusions

(1) In the high K-low Ca-high Si model system, the yields of different gases were in the order of H₂>CO>CO₂. In comparison with single PC gasification, the yields of H₂, CO, syngas, and carbon conversion were increased by 29.42 mmol/g, 20.40 mmol/g, 56.68 mmol/g, and 0.35, respectively.

(2) In the high K-low Ca-high Si model system, except SiO₂ and Ca-Si systems, all other catalysts showed catalytic effects on gas release and reaction characteristics. SiO₂ could inhibit gas generation and reaction rate in gasification, whereas the K-Ca binary model system demonstrated a synergistic effect.

(3) CaCO₃ can expand the pore structure of semi-char to promote the gasification reaction. At the same time, KCl will lower the graphitization degree of semi-char to strengthen the reaction, showing a synergistic effect. The K-Ca binary model system’s synergistic catalytic impact will be diminished due to SiO₂’s increased inhibition of CaCO₃ and KCl.