Influence mechanism of alkali metals on CO2 gasification properties of metallurgical coke

Journal Pre-proofs Influence mechanism of alkali metals on CO2 gasification properties of metallurgical coke Guangwei Wang, Shan Ren, Jianliang Zhang, Xiaojun Ning, Wang Liang, Nan Zhang, Chuan Wang PII: DOI: Reference:

S1385-8947(20)30084-X https://doi.org/10.1016/j.cej.2020.124093 CEJ 124093

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Chemical Engineering Journal

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13 November 2019 7 January 2020 10 January 2020

Please cite this article as: G. Wang, S. Ren, J. Zhang, X. Ning, W. Liang, N. Zhang, C. Wang, Influence mechanism of alkali metals on CO2 gasification properties of metallurgical coke, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124093

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Influence mechanism of alkali metals on CO2 gasification properties of metallurgical coke Guangwei Wanga, Shan Renb,*, Jianliang Zhanga,c, Xiaojun Ninga,d,*, Wang Lianga, Nan Zhanga, Chuan Wang e,f a State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China b College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China c School of Chemical Engineering, The University of Queensland, St Lucia, QLD 4072, Australia d GuanZhuang campus, University of Science and Technology Beijing, Beijing 100083, China e Swerim AB, SE-971 25 Luleå, Sweden f Thermal and Flow Engineering Laboratory, Åbo Akademi University, Åbo, FI-20500 Finland. *Correspondence authors. Tel.: +86 10 62332364; fax: +86 10 62332364 E-mail addresses: [email protected] (S. Ren); [email protected] (X. Ning) ABSTRACT: In order to clarify the function mechanism of alkali metal on degradation of metallurgical coke in blast furnace, the influence of alkali metal potassium and sodium on carbon structure change was investigated by gaseous adsorption method, and also the catalytic gasification reaction behaviors of alkali metal adsorbed coke samples was further studied. The results showed that the alkali metal vapor absorbed on coke has significant degradation effect on high temperature metallurgical properties of coke, which has catalytic effect and significantly increases gasification reactivity, and the catalytic limit of potassium vapor and sodium vapor was about 5 % and 3 %, respectively. Besides, potassium adsorption could also cause more fine coke particles to be produced, further aggravating degradation of coke in blast furnace. The analysis of physicochemical structures of different samples shows that the destruction of aromatic ring structure with dense and stable carbon structure in coke by alkali metal vapor adsorption process was the essential cause of coke reactivity increase and fracture. Kinetics analysis study shows that the random pore model(RPM) has the best performance representing the catalytic gasification process, and for different samples the activation energy first increases and then decreases with the increase of alkali metal adsorption content. It was also confirmed that there existed the kinetic compensation effect in the gasification process. Key words: Metallurgical coke; Alkali metals; CO2 gasification; Catalysis; Kinetic. 1. Introduction 1

Blast furnace (BF) is the most important technology to produce molten iron, providing more than 90% of world-wide pig iron production annually in 2017. The total production of pig iron in China was 710 million tons, accounting for more than 50% of the world's total production, and more than 99% of pig iron production was from BF [1]. The modern ironmaking technology is represented by the largescale BF driven by more stringently requirements of energy conservation and environmental protection. At present, there are 20 BFs with size larger than 4000 m³ and 6 BFs over 5000 m³ in China [2]. Largescale BFs have many advantages such as high productivity, low energy consumption and less environmental impacts. However, high quality fuels (especially coke) and iron ore are required, thus, leading to increasing conflicts with the available resources of high-quality coking coal and iron ore [3]. As the only solid material in the high temperature zone of BF, the coke plays an important role on the gas and liquid permeability in the BF smelting process. With the continuous improvement of BF ironmaking process and operation, the fuel and coke consumption in BF are decreasing. The fuel rate and coke rate in domestic modern BFs have reduced to about 490kg/tHM and 330 kg/tHM, respectively [4]. The coke has the skeleton function in the high temperature zone in BF, and this function becomes more obvious with the continuous reduction of coke rate. The deterioration of coke in the BF is affected by many aspects, for instance, the mechanical damage of the furnace material to its extrusion wear, the thermal stress damage caused by heating the coke with the high temperature gas, the dissolution reaction of CO2 in the coke and the erosion of coke by liquid slag and iron below the melting zone. Both mechanical damage and chemical erosion will disintegrate the strength and size of the coke, further affecting the gas and liquid permeability in BF. In addition, the serious coke deterioration will even damage the smooth operation and the production yield of the BF [5, 6]. In order to further clarify the degradation behavior of the coke in BF, a series of studies have been carried out in recent years. Kashiwaya et al. [7] have investigated the gasification of metallurgical coke based on non-crystal and crystal ratio of carbon. The results showed that two types of carbon structure 2

in metallurgical coke could be classified by means of X-ray diffraction, and the gasification reaction could decrease the value of crystallite diameter (La). Iwanaga et al. [8] believed that coke degradation was closely related to the coke reaction index (CRI), and the coke with high CRI could cause more serious pulverization problem in BF. Yamaoka et al. [9,10] used a rotating drum to simulate the change of coke particle size caused by material extrusion friction in the BF, and they established a particle size degradation model. Lv et al. [11] studied the coke behavior in lumpy zone of BF and concluded that the degradation of small-sized coke in lumpy zone was more significant. Fang et al. [12] indicated that the change of coke strength was closely related to the carbon loss rate and the best way to protect the coke strength was to reduce the gasification of coke in the high temperature zone of the BF. Zhang [13] investigated the effects of reaction atmosphere, gasification temperature and particle size on the kinetics characteristics of metallurgical coke gasification with CO2. The results showed that H2 enhanced the reaction rate of coke and the gasification reactivity of coke increased with the decreasing of particle size and increasing of temperature. The above studies show that the deterioration of coke in the BF is mainly caused by the dissolution reaction of coke. However, the presence of alkali metal in the BF plays an important role on promoting the dissolution reaction of coke. Marsh et al. [14, 15] studied the catalytic gasification of metallurgical coke. It could be seen that the reactivity of metallurgical coke were dependent upon the anion of the potassium salt and optical texture of the metallurgical coke, and the catalysis was thought to occur by the vapor cycle mechanism. Pang et al. [16, 17] analyzed the influence of alkali metals on the gasification process of coke by alkaline solution soaking method and vapor adsorption method, and the mechanism of alkali metals on the thermal degradation of coke was studied from the perspective of the influence of coke microstructure and mineral evolution. Su et al. [18] compared the catalytic effect of alkali potassium and sodium and their co-existence on the coke gasification process, and the results showed that the effect of the co-existence was weaker than that of single alkali metal element. Wang et al. [19] further analyzed the influence of alkali potassium and 3

sodium on the degradation process of coke in the presence of water vapor, and the results showed that the catalytic effect of alkali potassium and sodium on the reaction of coke and CO2 gasification was stronger than that of coke on the reaction of water vapor. Gupta et al. [20] analyzed the mineralogy and reactivity of coke from tuyere level of a blast furnace. The coke reaction rate was shown to depend on the total amount of potassium species. The above analysis shows that it is widely recognized that alkali metal deteriorates the coke quality, and many studies have attempted to reduce the influence of potassium and sodium on the coke degradation process in the BF by controlling the alkali metal load into the BF [21]. However, the mechanism of the catalytic effect of alkali potassium and sodium on the gasification process of metallurgical coke is still unclear, especially the mechanism of alkali potassium and sodium vapor on the carbon microcrystalline structure in the coke and its influence on the CO2 gasification reaction process. In this study, the gasification behavior of the metallurgical coke under the condition of alkali-rich metal vapor is studied thoroughly. The influence of high temperature adsorption process of alkali metal vapor on the order degree of carbon microcrystalline structure in coke was analyzed, and the catalytic mechanism of alkali metal vapor on CO2 gasification process of metallurgical coke for BF was clarified, which would be helpful to enrich and improve the theoretical system of the degradation process of metallurgical coke in the BF. Thus, it can provide the theoretical basis and guidance to control the alkali metal load in the BF and improve the BF operation.

2. Methods 2.1 Raw materials and sample preparation The metallurgical coke used in this study was from Baosteel, an integrated steel plant in China. The proximate and ultimate analysis of sample are shown in Table 1. The metallurgical coke used in the BF is produced via the distillation process in the coke oven battery. For the coke used in this study, the content of volatile, fixed carbon and ash is 1.56%, 86.08% and 12.36, respectively. The ultimate analysis 4

result shows that the main component of coke is C element of 86.37%, and the contents of H, O, N and S are all less than 1%. The lower S content can reduce the sulfur load in the BF, and thus will reduce desulfurization pressure in the BF smelting process. Table 2 shows the ash composition of the coke. It can be seen that the main components of ash are SiO2 and Al2O3, and the Fe2O3 and CaO contents are also high, which are 11.30% and 11.40%, respectively. It should be also pointed out that the ash contains high contents of Na2O and K2O as well, and they are 1.04 % and 1.44 %, respectively. In order to analyze the influence of potassium and sodium metal vapor cycle enrichment on the gasification reaction performance of coke in BF, the experiment of adsorbing potassium and sodium metal vapor in coke was carried out. The experiment process was described in the previously published research paper [22, 23], and the specific scheme was shown in Table 3. In order to reach uniform adsorption of alkali metal vapor by coke samples, relatively high temperature and long time were selected in this study. The adsorption temperature of alkali metal vapor is 1573 K, the adsorption time is controlled at 120min, and the coke samples after adsorption of potassium and sodium metal vapor are cooled at the room temperature under N2 atmosphere. The samples with different alkali metal adsorption capacities were named Coke+x%M, where x were 1, 3, 5, and 7 respectively, and M represented K or Na. The samples adsorbed of alkali metal were put into sample bags and stored in a drying dish for follow-up tests. For the purpose of reducing the influence of particle size on the gasification process, the sample was ground and sieved, and samples with a particle size of 74-100 μm were selected for the thermogravimetric analysis. In order to accurately characterize the influence of alkali metal vapor adsorption process on the composition and structure of coke, various detection and analysis techniques were used to systematically analyze the different samples. The apparent morphology and structure of the sample were analyzed by SEM (FEI Quanta-450). The carbon microcrystalline structure was characterized by Xray diffraction (XRD). XRD analysis was carried by Rigaku SmartLab diffractometer with a Cu Kα radiation 5

source (d=0.1541 nm). Spectra were recorded in the 2θ range of 10 °-90 ° with a scanning speed of 0.3°/min. For comparing the XRD test results, the carbon microcrystalline structure was also tested by Raman spectroscopy at the condition of argon ion laser, 532 nm wavelength, 2 cm-1 systemic resolution and 100 cm-1-4000 cm-1 scanning. The Raman spectra at each position provided average structural information of a large number of carbon microcrystalline, and the Raman spectra was recorded at 6 locations of the samples.

2.2 Thermogravimetric analysis The CO2 gasification reaction of different samples was analyzed by a thermogravimetric thermal analyzer (HCT-3, Henven Scientific Instrument Factory, Beijing). The prepared samples (about 5mg) were loaded in a cylindrical corundum crucible (height ×diameter: 5mm×3mm) under high purity carbon dioxide atmosphere at the flow rate of 60 ml/min. In order to reduce the temperature difference between measurement and sample, the crucible was placed as close as possible to the thermocouple. The temperature-programmed thermogravimetric method was used to study the influence of alkali metal vapor adsorption on the coke gasification properties and kinetic behavior. All samples were heated from the room temperature to 1523 K at the heating rates of 3 K/min, 5 K/min, 7 K/min and 9 K/min, respectively. After some preliminary tests, the conditions of sample mass, gas flow and heating rates were selected to avoid heat transfer limitation and to minimize mass transfer effects. The gasification process curve of the different samples was automatically collected by the computer with a time interval of 1s. It was important to ensure that the tests had good repeatability, so each test was repeated at least for three times before a final result was ascertained. The CO2 gasification process of coke is a typical gas-solid heterogeneous reaction. In the gasification properties and kinetic behavior research, it is necessary to convert the gasification weightlessness data into the conversion rate. The sample gasification conversion can be calculated by the following formula:

6

x

m0  mt m0  m

(1)

where, m0 is the initial reaction mass of the sample; mt is the mass of the sample at time t ; m  is the mass of the residue after the complete reaction, corresponding to the ash content. The apparent reaction rate is calculated as the differential of conversion degree versus time, denoted as dx dt .

3. Results and discussion 3.1 Gasification behavior The non-isothermal thermogravimetric analysis method was used to study the CO2 gasification properties of coke samples with different alkali metal adsorption. The gasification conversion and reaction rate curves are shown in Fig.1. The figure shows that the shapes of different gasification curves are similar. In low temperature range, samples are preheated and with temperature rising the gasification curves change little, accordingly the values of gasification conversion rate and reaction rate close to zero. When the gasification temperature rises to about 1000 K, the gasification reaction rate constant increases gradually with the increase of temperature. The conversion curve rises rapidly to a peak, which corresponds to the initial gasification stage. As the number of gasification active sites in the coke decreases with the increase of carbon consumption, the peak gasification rate occurs, and the reaction rate decreases with the further increase of temperature. When the carbon in the coke is exhausted, the gasification reaction rate decreases to zero, and the residue is mainly ash which indicates that the gasification process of the coke ends. Comparing the gasification curves of coke with different alkali metal adsorption amounts, it can be seen that the reaction curves of the original coke correlates to the highest reaction temperature range. After the alkali metal adsorbed, the gasification curves move to the low temperature zone with the increase of alkali metal adsorption. Fig. 1(a) shows the gasification curves of samples with potassium absorbed, and it can be seen that with the increase of the adsorption amount of potassium metal vapor, both the conversion and reaction rate curves move into the low 7

temperature zone and the peak reaction rate also increases gradually. The adsorption of potassium metal vapor significantly reduces the initial gasification temperature and accelerates the progress of gasification, indicating that the adsorption of potassium metal has a catalytic effect on the coke gasification process. From Fig.1 (a), it can be also observed that when the adsorption amount of potassium metal vapor exceeds 5%, the gasification curve no longer moves further to lower temperature zone, and the curve of Coke+7%K basically coincides with the curve of Coke+5%K. This phenomenon indicates that the catalytic effect of potassium metal on coke has a limit value of about 5%. The similar phenomenon is also observed in Fig.1(b), showing that the adsorption of sodium metal vapor also has a significant catalytic effect on the gasification process of coke, but with a limit value of about 3%. In order to investigate the influence of alkali metal vapor adsorption on gasification of coke, the gasification characteristic parameters are introduced: Ti, Tf, Tm, Rmax, Rmean, S and tg [24], where Ti and Tf are the initial temperature and the final temperature respectively, Tm is the temperature of peak reaction rate, tg represents the time from the initial gasification to the complete gasification, Rmax represents the peak reaction rate, and Rmean represents the average reaction rate. The gasification characteristic parameters of different samples are listed in Table 4, showing that the values of Ti, Tm and Tf decrease with the increase of the alkali metal adsorption. The values of Ti, Tm and Tf of the original coke are 1108.5 K, 1280.5 K and 1386.3 K, respectively. The values of Ti, Tm and Tf of Coke+5%K are 1001.3 K, 1144.8 K and 1213.6 K, respectively. The reduction values reach 107.2 K, 135.7 K and 172.7 K, respectively. Meanwhile, the Ti, Tm and Tf of Coke+5%Na are 1038.5 K, 1172.2 K and 1250.1 K, respectively, the reduction values are 70.0 K, 108.3 K and 136.2 K, respectively. The values of Rmax and Rmean also increase with the increased amount of alkali metal adsorption. Comparing the gasification reaction time t of different samples, it is found that the gasification reaction time t of the coke after alkali metal adsorption is shortened, indicating that the presence of alkali metal has obvious influence 8

on the reduction of Tf than Ti. In general, the lower Ti and the faster Rmax indicate higher gasification reactivity. When comparing the influence of potassium metal and sodium metal on the gasification characteristic parameters of coke, it can be found that the characteristic temperature of potassium vapor adsorption sample is lower than that of sodium vapor adsorption sample, but the Rmax and Rmean of sodium vapor adsorption sample are larger than that of potassium vapor adsorption sample which means that the gasification reactivity of different samples could not be represented only by the values of Ti, Tf, Tm, Rmax and Rmean. In this paper, comprehensive characteristic index S is used to represent gasification reactivity of different samples. It can be found that the value of S for sodium vapor adsorption coke is larger than that of potassium vapor adsorption, which indicates that the catalytic effect of sodium metal on coke is more obvious than potassium metal. Heating rate is an important parameter influencing the gasification process. The gasification curves of Coke+3%K and Coke+3%Na samples under different heating rates (3K/min, 5K/min, 7K/min and 9K/min) are shown in Fig.2. It can be seen that with the increase of heating rate, the gasification conversion and reaction rate curves of all samples move towards high temperature zone, and correspondingly characteristic temperatures shift to higher temperature zone and the peak reaction rate increases. There are two main reasons: on one hand, when using the thermogravimetric analysis method in the laboratory, the coke is heated by the high temperature furnace radiation. Under the condition of higher heating rate, the temperature gradient inside and outside the coke increases, thus, the gasification curves move into the high temperature zone. On the other hand, with the increase of heating rate, the temperature increases faster leading to insufficient time for gasification reaction to reach equilibrium, and thus they overlap with the adjacent higher temperature reaction range, which can be called as the superposition effect. The similar findings were also observed in our previous work [25,26]. Table 5 shows the gasification parameters of Coke+3%K and Coke+3%Na samples at different heating rates. It can be seen that Ti and Tf increase when increasing the heating rate. With an increase of 9

the heating rate from 3K/min to 9K/min, the Ti and Tf of Coke+3%K and Coke+3%Na samples increase 28.6K, 21.1K and 80.6K, 80.2K, respectively. The increase value of Tf is much larger than that of Ti. At the same time, it can be found that the increased heating rate will lead to an increased peak reaction rate and a decreased gasification time tg. The higher reaction rate and shorter gasification time indicate that the gasification reactivity of different samples are improved. 3.2 Catalytic mechanism analysis The above studies show that the alkali metal adsorption has an obvious catalytic effect on the coke gasification process, and there is a catalytic limit when the potassium vapor adsorption is about 5% and the sodium vapor adsorption is about 3%. There are many explanations about the catalytic phenomenon of alkali metal, alkali metal oxide and alkali metal carbonate to coke, coal and other carbonaceous fuels. The mechanism analysis of catalytic gasification of coke mainly includes oxygen transfer theory, electrochemical theory, and intermediate theory and so on. The oxygen transfer theory [27-29] considered that the metal catalyst formed a metal-oxide intermediate in the gasification process as a carrier for oxygen transferring between gas and solid phase back and forth. In a cyclic process, the catalytic process would be finished. The electrochemical theory [30] believed that the catalytic gasification process was realized by the transfer of electrons between the different electrodes. At a temperature lower than the initial gasification temperature, the surface of the carbon particle could form a continuous liquid film, which acted as the channels of the ion diffusion. The electrons were diffused by the electrolyte in the liquid film forming a battery system to complete the gasification process. According to intermediate theory [31], the metal catalyst formed an electron donor-acceptor (EDA) complex with the aromatic carbon in the carbon. EDA had strong catalytic activity and could catalyze the reactions of C-H2, C-H2O、C-CO2. However, due to the complexity of the char structure, it is a kind of amorphous, highly complex and structurally heterogeneous material. So far, there is still lacking of unified understanding about the mechanism analysis of the catalytic gasification process. 10

In order to further clarify the mechanism of the potassium and sodium vapor on coke gasification, the physicochemical structure, especially the carbon microcrystalline structure change of the coke adsorbed potassium and sodium vapor was systematically analyzed. Previous studies showed that [32] the expansion of coke volume could be clearly observed after the adsorption of potassium vapor, and there were detached coke crumbs on the surface layer. However, there was on detached coke crumbs after sodium vapor adsorption. The same phenomenon can be also found in this study. The adsorption of alkali metal on the coke begins from the open pores and then gradually diffuses into the internal carbon matrix of the coke. The alkali metal-graphite intercalation compounds will be produced by alkali metal insetting into the graphite layer of the coke carbon matrix. During this process, a relatively large volume expansion occurs, and the internal stress of the carbon matrix increases. When the internal stress is greater than the strength of the carbon matrix, the structure of the carbon matrix will be destroyed, and consequently the coke strength decreases and lots of powder are generated. For example, when intercalation compound KC6 is formed, the volume expands by 61 %, and when intercalation compound KC60 is formed, the volume expands by 12 %. In addition, the reaction of alkali metal and mineral in coke will also cause the expansion of mineral volume and then bring about the destruction of coke structure. Fig.3 shows the SEM/EDS picture of Coke+3%K and Coke+3%Na samples. In Fig.3 (a), it can be observed that a large number of lamellar structures are formed in the dense carbon matrix after potassium vapor adsorbed. The EDS surface scanning analysis illustrates that the silicon and aluminum elements are distributed in the carbon matrix in a spot-like manner. The potassium element is evenly embedded in the carbon matrix, and the potassium element can also be found in the pores of coke. Fig.3(b) is the microstructure of coke after sodium vapor adsorption, and it can be seen that the distribution of silicon and aluminum in carbon matrix is similar to that of coke-adsorbed potassium vapor, which is unevenly distributed in spots, and the sodium element is uniformly embedded in the carbon matrix. Compared to potassium vapor, the difference of sodium vapor adsorption on coke is that 11

there is not much lamellar structure, and this explains why there is a large number of coke debris when the coke sample absorbs potassium vapor [23]. The large amount of lamellar structure makes the coke matrix structure looser and the CO2 gas react easier during the gasification process. Due to the influence of potassium vapor and sodium vapor adsorption on the pore structure of the coke, it can be obtained that the potassium vapor absorbed coke has more developed pore structure and larger reaction surface. Comparing the gasification reactivity of Coke+3%K and Coke+3%Na, it can be found that the gasification reactivity of Coke+3%Na is bigger than that of Coke+3%K, indicating that the porous and loose structure after the adsorption of potassium and sodium vapor is conducive to the gasification reaction, however, the catalytic effect on coke is not mainly caused by the volume expansion of alkali metal-graphite intercalation compound. The alkali metal-graphite intercalation compound between the carbon matrix graphite layers causes the volume expansion to destroy the strength of the coke, and meanwhile it also affects the carbon microcrystalline structure. In order to further clarify the influence of alkali metal vapor adsorption on carbon microcrystalline structure, the XRD analysis method was also used to characterize the structure changes, and the results are shown in Fig.4. It can be found that there is a significant (002) peak at 25 ° for different samples [32], which indicates the parallel and azimuthal orientation of the aromatic carbon layer in the carbon crystallites. The higher and the narrower the peak is, the more oriented the aromatic carbon layer is. It can be seen that with the increase of alkali metal vapor adsorption, the intensity of the (002) peak of the different samples decrease (indicating a increasing full width at half maximum), while the (002) peak angle shifts to the lower angle side. This phenomenon indicates that the alkali metal vapor adsorption process destroys the original regular carbon microcrystalline structure, increasing the content of amorphous heterogeneous carbon. Comparing the change of the (002) peak of the cokes adsorbed potassium metal vapor and sodium metal vapor, it can be found that the (002) peak of potassium metal vapor adsorption is significantly lower than that of 12

sodium vapor metal adsorption, which indicates that there is more destruction of the carbon microcrystalline structure in potassium vapor adsorption process. In order to quantitatively analyze the effect of alkali metal vapor adsorption on coke carbon microcrystalline structure, the XRD patterns of the different samples were also analyzed to obtain their structure parameters [33]. The interplanar distance of two aromatic layer of microcrystalline (d002), the thickness of microcrystalline (Lc) and the number of microcrystalline in a stack (N) are shown in Table 6. The table shows that with the increase of potassium metal vapor adsorption, the value of d002 gradually decreased, but the change is not significant. Compared with Coke+3%K, the d002, of original Coke increases from 0.348 nm to 0.349 nm, and the d002 of Coke+3%Na is only 0.350 nm. The value of Lc decreased with the increase of alkali metal adsorption. The Lc value of original Coke is 34.28 nm, which was similar to the average stacking thickness of the heat-treated coke in the literature [34]. While the Lc value of the Coke+3%K sample is 23.15 nm, and the Lc value of Coke+3%Na is 27.58 nm. Correspondingly, the number of layers N of the aromatic layer layers, which are calculated by the Lc value and the d002 value, also decreases with the increase of the alkali metal vapor adsorption. The reduction of the Lc and N values are related to the process of forming alkali metal-graphite intercalation compound during the alkali metal vapor adsorption. In the process of high temperature heat treatment, alkali metal atoms diffuse deep into the carbon matrix and have intercalation reaction, which disintegrates the carbon microcrystalline structure of coke and makes the carbon matrix structure become looser. In addition, the adsorption of alkali metal vapor also affects the mineral composition in the ash of coke. The ash in the original coke mainly exists in the form of SiO2 and Al2SiO5. After the alkali metal vapor adsorption, a large amount of KAlSiO4 and NaAlSiO4 minerals are formed and the mineral volume of ash expands, causing the further damage to the carbon matrix structure. In order to further clarify the influence of alkali metal vapor adsorption on the carbon microcrystalline structure in different samples, Raman spectroscopy was supplemented in this study. 13

The Raman spectra of coke with different alkali metal vapor adsorption are shown in Fig. 5. It can be seen that the Raman spectra of the different samples mainly reflect the carbon structure, with two obvious peaks appearing near the wavelength of 1350 cm-1 and 1600 cm-1, which are D peak (defect peak) and G peak (graphite peak) respectively [35]. In fact, the Raman spectra of the coke is formed by overlapping multiple sub-peaks, and the orientation parameters of the overlapping peaks can be obtained by fitting the spectra [36]. In general, the spectra can be divided into five independent vibration curves D1, D2, D3, D4 and G. D1 peak (1350 cm-1) usually represents the defective structure and is related to the in-plane vibration of carbon edge atoms, impurity atoms and sp2 carbon atoms with structural defects. G peak (1590 cm-1) is a double degenerate E2g mode of stretching vibration of carbon atoms in the laminated carbon atom of ideal graphite crystal, which is related to all paired sp2 crystal carbon atoms in aromatic ring and chain structure. D3 peak (1520 cm-1) is produced by vibration with the amorphous sp2 hybridization key carbon atoms, such as organic molecules, debris, functional groups, and poor structure of materials, etc. D4 peak (1200 cm-1) is the shoulder peak of D1 peak, which is related to sp3 or sp2-sp3 hybrid carbon atoms, C-C and C=C olefin structure stretching vibration, contributing to the increase of active sites of carbon materials. D2 peak (1620 cm-1) is the shoulder of G peak, corresponding to the E2g vibration mode of the surface graphite layer. In the process of peak splitting, D1, D2, D4 and G are using Lorentzian function, while D3 is Gaussian function. The peak fitting results of Raman spectra analyses of different samples are shown in Fig. 5. It can be found that the peak splitting results of Raman spectra of different samples are in good agreement with the detected values, indicating that the Lorentzan and Gaussian functions are applicable to this study. The peak characteristic parameters of the different samples are shown in Table 7. It can be seen that the intensity of the D1 and D3 peaks of the Coke+3%K is the largest, indicating that there are more crystal structure defects. However, the intensity of G peak is also the largest, meaning that the content of graphite crystal structure is also higher. The above analysis shows that it is difficult to judge the ordering degree of the 14

carbon microcrystalline structure of the different samples from the peak intensity comparisons of D1, D2, D3, D4 and G. Gong et al. [37,38] showed that ID1/IG, ID1+D2/IG and IG/IAll can characterize the degree of sample ordering better, the larger the ID1/IG and ID1+D2/IG, and the smaller the IG/IAll, reflecting the lower the degree of ordering of the carbon microcrystalline structure in the sample. Comparing the ID1/IG, ID1+D2/IG and IG/IAll of the different samples in Table 7, it can be found that the values of ID1/IG and ID1+D2/IG increase with the increase of alkali metal vapor adsorption, while the value of IG/IAll value decreases. The values of the original coke ID1/IG, ID1+D2/IG and IG/IAll are 2.79, 3.20 and 0.18, respectively. These values for the Coke+3%K are 3.80, 4.35 and 0.14, respectively, and these values for the Coke+3%Na are 3.73, 4.50 and 0.13, respectively. It indicates that after adsorption of alkali metal vapor at high temperature, the previously ordered carbon microcrystalline structure of the coke was destroyed by potassium and sodium atoms, and the proportion of amorphous carbon in the sample was increased, meaning that there is a lower degree of ordering for alkali metal vapor adsorbed cokes. Meanwhile, comparing the effects of potassium metal vapor adsorption and sodium metal vapor adsorption on the carbon microcrystalline structure in coke, it can be found that the effect of potassium metal vapor on the carbon ordering degree of coke is larger than that of sodium metal vapor. The above Raman analysis results of different samples further verified that when the coke contacts with alkali metal vapor under high temperature condition, the carbon matrix is invaded by potassium and sodium vapor, and the formed alkali metal-graphite intercalation compound destroys the original compact aromatic ring structure, resulting in the increase of amorphous structure in the coke sample and the decrease of the carbon microcrystalline structure ordering degree. This finding is consistent with the results of XRD analysis. The above analysis shows that after adsorption of alkali metal vapor, the ordering degree of carbon microcrystalline structure decreases and the amorphous carbon content of coke increases. In previous studies on gasification and combustion behavior of biomass char, pulverized coal, petroleum coke and 15

carbonaceous samples [37-39], it is found that there is significant correlation between the ordering degree of carbon microcrystalline structure and its reaction reactivity in different samples. The higher degree of ordering is, the lower reactivity of the sample would be. Comparing the relationship between carbon microcrystalline structure change and gasification reactivity of coke adsorbed alkali metal vapor samples, it can be found that the ordering degree of carbon microcrystalline structure in the coke matrix decrease with the increase of potassium and sodium metal vapor adsorption. These studies suggest that the potassium and sodium vapor adsorption process enhance the breakdown of the big aromatic ring systems and increase the content of amorphous density. The gasification reactivity of different samples were compared and analyzed. It can be found that the increase in the relative abundance of amorphous carbon structure from Coke, Coke+1%K, Coke+3%K, Coke+5%K to Coke+7%K was in the same order as the change in the reactivity, clearly showing the importance of carbon structure of samples to its gasification reactivity. The same phenomenon can be observed in gasification processes of Coke, Coke+1%Na, Coke+3%Na, Coke+5%Na and Coke+7%Na. According to the previous literatures [40-42], the mechanism of gasification is that the carbon in coke firstly forms the intermediate (C(O)) and (M-CO), and then the intermediate matter reacts with the gasifying agent. It was easier for the amorphous carbon structure to form the intermediate (C(O)) and (M-C-O) in the gasification process. The amount of intermediate (C(O)) and (M-C-O) increased as more active sites appeared, leading to an increase in the gasification reactivity. At the same time, the effect of potassium metal vapor and sodium metal vapor on the carbon microcrystalline structure of the different samples was also compared. Under the same adsorption amount, the effect of potassium metal vapor on the ordering degree of carbon in the coke is greater than that of sodium metal vapor, indicating that the adsorption process of potassium metal vapor has more severe damage on the big aromatic ring systems of the coke sample. While the increase of the amorphous carbon is beneficial to increase the content of intermediate (C(O)) and (M-C-O)), promoting the process of the gasification reaction. In summary, the fundamental of the catalytic action 16

of alkali metal on coke is that the alkali atom undergoes an intercalation reaction when contacting with the aromatic ring in the carbon matrix. The form of alkali metal-graphite intercalation compound destroys the aromatic ring structure, and newly produced amorphous carbon structure with high activity accelerates the progress of the gasification. At the same time, since the intercalation reaction between the potassium, sodium and carbon matrix reaches a saturated state and a new interlayer compound cannot be formed, the adding alkali metal can only be attached in the space structure of coke, but it cannot cause a further damage of the carbon microcrystalline structure of aromatic ring. Even if the adsorption amount of alkali metal vapor is increased, the gasification reaction rate cannot be accelerated, which can explain the phenomenon that the potassium metal vapor has a catalytic limit at 5%, and at 3%for the sodium vapor. 3.3 Kinetic modeling Gasification of coke has significant influence on its consumption and degradation in blast furnace. In order to clarify influence of alkali metal on reaction of coke in blast furnace, different kinetic models were used to analyze catalytic gasification process of coke through non linear fitting method. The gasification reaction of coke is a typical gas-solid heterogeneous reaction. The gasification process is that the gasifying agent CO2 diffuses through the gas phase boundary to the surface of the coke particles and reacts with carbon. The product CO diffuses into the external space through the product layer causing weight loss of the coke. From the analysis of the gasification curve of the coke with different alkali metal vapor adsorption, the main reaction process is still a single stage, and the weight loss rate peak does not appear two or more peaks. Thus, the general gas-solid reaction kinetic model can be used to characterize the coke gasification process. The formula is as follows:

dx  k  Pg , T   f  x  dt

17

(2)

where, k is the apparent gasification reaction rate constant, the value of which is affected by the temperature and the gas phase pressure of the gasification reaction system; f(x) is a conversion function that represents the reaction model of the coke gasification. Since the thermogravimetric thermal analyzer outlet is connected to the atmospheric during the coke gasification process, it can be considered that the gas phase pressure remains constant during the gasification process. The apparent reaction rate constant is mainly affected by the reaction temperature, and can be expressed by using the Arrhenius formula:

k  k0  e E RT

(3)

where, k0 is the pre-exponential factor (s-1), E is the apparent reaction activation energy (kJ/mol), and R is the gas constant (8.314J/(mol·K)). Many researchers have investigated the gasification kinetics of carbonaceous materials, and successively established a variety of kinetic models to describe gasification process. In this study, three representative gas-solid reaction kinetic models were used to analyze the kinetic behavior of coke gasification, including the random pore model (RPM) [43, 44], the unreacted core model (URCM) [45] and the volume model (VM) [46, 47]. Different kinetic models of gasification process made different assumptions about mechanism function f(α). According to the VM, RPM and URCM models introduced in the previously study[48], the kinetic parameters including: E, k0 and ψ are obtained based on the fitted gasification data at four heating rates. The kinetic parameters of different samples were investigated using the RPM, URCM and VM models by employing nonlinear least-square fitting method. The gasification kinetic parameters of the different samples are shown in Table 8. As can be seen from the table, there is a good correlation between the calculated values by different models and the experimental data, the values of R2 are all higher than 0.9901. Among them, the RPM model has the best fitting effect, and the correlation R2 exceeds 0.9953. RPM model was based on the assumption that the solid particles were porous structure. As a product of coal pyrolysis at high temperature and long 18

time, the metallurgical coke has rich pore structures, which just meets the requirements of RPM model. Moreover, through the adsorption of alkali metals at high temperature, the generation of alkali metalgraphite intercalation compound made the pore structure of coke more developed. The URCM model assumes that the solid particle in gas-solid reaction is compact in structure, and the gasification agent is difficult to diffuse into the interior of the particle. The chemical reaction could only happen on the surface of the particle, which is inconsistent with the characteristic of coke particles as porous structure. VM model assumes that chemical reaction occurs in both internal and external of the solid particles. In fact, except on the surface of coke, in the internal pore structure gasification could also happen through gasification agent diffusion. But the compact coke carbon matrix is difficult for gasification agent to diffuse into and reacts with internal carbon. The above analysis shows that for gasification of porous material like coke, the assumption of RPM is more reasonable. Fermoso et al. [49-52] also obtained a similar conclusion when studying porous carbon materials such as coal powder, biomass and coke, the characterization effect of the RPM model was better than these of URCM and VM models. The gasification activation energy of the Coke, Coke+1%K, Coke+3%K, Coke+5%K and Coke+7%K samples was 236.5kJ/mol 169.3kJ/mol, 162.1kJ/mol, 220.5kJ/mol and 227.6kJ/mol by RPM model. It can be seen that with the increase of the adsorption amount of potassium metal vapor, the gasification activation energy first decrease and then increase. When the adsorption amount of potassium metal vapor is 3%, the gasification activation energy reaches the minimum. The similar phenomenon can also be found in the gasification of coke with sodium metal vapor absorption. After coke adsorbing alkali metal vapor, the ordering degree of microcrystalline structure decreased. It required less energy when the carbon chain in the coke destroyed by CO2. As a result, the activation energy decreased with the increase of the adsorption amount of alkali metal vapor. When the adsorption amount of alkali metal vapor was further increased, a large number of alkali metal crystals would crystallize and precipitate on the pore surface of coke, which did not cause further damage to the carbon chain structure of coke, but 19

would hinder the contact between carbon and the gasification agent. Thus, the apparent gasification activation energy of coke with higher adsorption amount of potassium metal vapor increased. Tahmasebi et al. [53] also found that with the increase of microalgae and brown coal fuel content, the combustion activation energy of mixed samples first decreased and then increased, which was similar to this study. This was inconsistent with the phenomenon that the reaction rate was faster as the reaction system with lower activation energy. The main reason was that the reaction rate of gasification could be affected by both activation energy and pre-exponential factor. The lower the activation energy was the faster of the reaction rate due to the more activated molecular obtained at the same temperature. The pre-exponential factor is the number of effective collisions of the activated molecules. The bigger the pre-exponential factor is, the higher the number of the effective collision will be during the reaction process, leading to the higher reaction velocity. Table 8 shows that the increase of activation energy (E) values is accompanied by simultaneous increase of the pre-exponential factor(k0). This behavior clearly suggests that the known as "compensation effect" is present in coke gasification reaction with different alkali metals adsorbed. In the previous studies [54-56], a remarkable compensation effect is also presented between the activation energies and the pre-exponential factors for the gasification reaction of different chars. According to the study by Xie et al. [57-59], this may be due to the similar mechanisms of gasification reactions, especially for the oxygen-containing surface complexes C(O). For reactions with lower E values, it is easier for the free active carbon site to connect with CO2 to generate a C(O). However, in the meantime, the bond between C(O) becomes stronger and the char structure becomes more stable, limiting the movement of C(O) and leading to a lower k0 value. It can also be seen from Table 8 that the activation energy of different samples lies in the range of 162.1 kJ/mol-300.5 kJ/mol in this study. Wang et al. [60] have investigated the CO2 gasification kinetics of metallurgical coke(M1-coke). The results showed that the activation energy of M1-coke reacting with CO2 was 165.5 kJ/mol. The activation energy of metallurgical coke (M2-coke, M3-coke) was also 20

investigated by Guo et al. [18,61] and was found to be 126.7 kJ/mol-171.5 kJ/mol. Zhao et al. [62] studied the effects of H2O and CO2 on the kinetics of coke gasification. The results showed that the activation energy of coke gasification with CO2 was higher than that of coke gasification with H2O, and the activation energy of coke gasification reaction with CO2 was 214.8 kJ/mol. Liu et al. [52] investigated the CO2 gasification reactivity and structure characteristics of carbonaceous materials from the Corex Furnace, and the activation energies of the two coke gasification were 184.6 kJ/mol(MC-1) and 195.4 kJ/mol(MC-2), respectively. Compared with the previous research results, the activation energy of original coke gasification in this study is generally high. The main reason is that the original coke actually undergoes the high temperature treatment. However, as to the gasification activation energies of the coke with different alkali metal vapor absorption, it is determined by the leading role of the high temperature treatment and alkali metal vapor adsorption process. Meanwhile, it is also necessary to consider the influence of excessive alkali metal vapor adsorption on the crystallization process of coke surface and pores on the activation energy of gasification. The above analysis showed that the destructive effect of alkali metal vapor on the structure of aromatic carbon chain in carbon matrix greatly enhanced the gasification reactivity of coke. The existence of compensation effect indicated that the coke gasification of adsorbing alkali metal vapor was a complex gas-solid heterogeneous reaction. Moreover, the RPM, URCM and VM models have a series of assumptions about the shape and reaction process of the coke particle, which inevitably result in the error between the experimental data and the calculated values. In order to intuitively characterize the applicability of different models in the characterization of coke gasification process, the gasification kinetic parameters calculated in Table 8, the conversion rates of different samples in the gasification process were calculated, and the results are shown in Fig. 6. The line is the calculated values and dots represent experimental data. It can be seen from Fig. 6 that for different samples there are good accordance between experimental data and calculation values especially for the RPM. In order to 21

quantify the errors produced by the kinetic models in predicting the values of conversion, the deviation (DEV) between the experimental data and calculated values were calculated. The related results obtained from RPM, URCM and VM models for all the blends are summarized in Table 9. It can be seen that the lowest deviation was obtained by using RPM model. The above analyses once again indicate that the RPM has good accuracy when applying to represent the gasification of coke. Through the above analysis on the physical and chemical structure change rules and gasification characteristics of coke adsorbed on alkali metal vapor, the rule of the influence of alkali metal on coke reaction index in the blast furnace was clarified, and the harmful effect of alkali metal vapor on coke strength was further recognized. Meanwhile, the key kinetic parameters of the coke gasification for adsorbing alkali metal vapor were defined, which would be able to provide important basic data for the reaction behavior of coke in the BF.

4. Conclusion In this work, the gasification characteristics and kinetic behavior of coke with alkali metal vapor adsorption have been investigated through no-isothermal thermogravimetric analysis method. The results showed that with increase of alkali metal vapor adsorption, the gasification curves moved to low temperature zone, and the reaction rate peak values increased. The alkali metal vapor could significantly accelerate the coke gasification process. The catalytic effect of alkali metal vapor on the gasification reactivity of coke had a certain limit. When the adsorption amount of potassium metal vapor exceeded 5 % and the adsorption amount of sodium metal vapor exceeded 3 %, with further increase of adsorption amount, the characteristic parameters of coke gasification process would change little. Structure analysis of different coke samples showed that alkali metal vapor intercalated carbon structure in high temperature adsorption process and the formed alkali metal-graphite intercalation compound would destroy the stable carbon structure, which was the main reason for change of coke gasification reactivity. Kinetic analysis showed that the RPM model had the optimal performance in 22

representing the coke gasification process. The activation energies derived from the RPM for different samples lied in the range of 169.3 kJ/mol -300.5 kJ/mol, and the activation energy decreased first and then increased with the increase of alkali metal vapor adsorption. In addition, the kinetic compensation effect was confirmed to exist in the gasification process. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 51874058, 51804026 and 51604048) and Fund of Chongqing Science and Technology (cstc2019jscx- msxmX0215) for financial support. Chuan Wang would like to acknowledge the funding support from Vinnova (dnr 2017-01327).

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The list of tables: Table 1. Proximate and ultimate analyses of the original coke Table 2. Chemical composition of the ash (wt,%) Table 3. The scheme of potassium and sodium adsorbed Table 4. Gasification characteristic parameters of different samples a heating rate of 5K/min Table 5. Gasification characteristic parameters with different heating rates Table 6. Carbon microcrystalline structure parameters of different samples from XRD analysis Table 7. Carbon microcrystalline structure parameters of different samples from Raman analysis Table 8. Kinetic parameters of different samples at four different heating rates for RPM, URCM and VM Table 9. Deviation between the experimental and calculated conversion data The list of figures: Fig.1. Experimental conversion and reaction rate curves with a heating rate of 5K/min:(a) Potassiumrich samples;(b) Sodium-rich samples Fig.2. Experimental conversion and reaction rate curves with different heating rate: (a) Coke+3%K, (b) Coke+3%Na Fig.3. SEM/EDS analysis of coke after alkali metal vapor adsorbed: (a)Coke+3%K, (b)Coke+3%Na Fig.4. XRD patterns of different samples Fig.5. Raman spectra of different samples Fig.6. Experimental conversion rate curves and these calculated with different models

30

Table 1 Proximate and ultimate analyses of the original coke Sample Coke a

Proximate analysis(wt,%)

Ultimate analysis(wt,%)

FCda

Ad

Vd

Cd

Hd

Od a

Nd

Sd

86.08

12.36

1.56

86.37

0.12

0.11

0.53

0.51

Calculated by difference. FC, fixed carbon; A, ash; V, volatile matter; d, dry basis.

31

Table 2 Chemical composition of the ash (wt,%) Sample

SiO2

Al2O3

Fe2O3

CaO

Na2O

K2O

MgO

Coke

46.30

17.00

11.30

11.40

1.04

1.44

2.60

32

Table 3 The scheme of potassium and sodium adsorbed Sample

Alkali/Coke

Coke(g)

K2CO3(g)

Na2CO3(g)

CR(g)

CE(g)

CT(g)

Coke+1%K Coke+3%K Coke+5%K Coke+7%K Coke+1%Na Coke+3%Na Coke+5%Na Coke+7%Na

1% 3% 5% 7% 1% 3% 5% 7%

230 230 230 230 230 230 230 230

4.1 12.2 20.4 28.5 -----

----5.3 15.9 26.5 37.1

0.7 2.1 3.5 4.9 1.2 3.6 6.0 8.4

0.4 1.1 1.8 2.5 0.6 1.8 3.0 4.2

1.1 3.2 5.3 7.4 1.8 5.4 9.0 12.6

CR, consumption of active carbon to reduce alkali carbonate; CE, excess amount of active carbon; CT, total amount of active carbon.

33

Table 4 Gasification characteristic parameters of different samples a heating rate of 5 K/min Sample

Ti(K)

Tm(K)

Tf(K)

Rmax(s-1)

Rmean (s-1)

S×1016

tg(min)

Coke

1108.5

1280.5

1386.3

5.97E-04

2.63E-04

0.92

55.6

Coke+1%K

1060.3

1257.4

1327.3

6.27E-04

2.71E-04

1.14

53.4

Coke+3%K

1030.2

1201.1

1276.5

6.64E-04

2.94E-04

1.44

49.3

Coke+5%K

1001.3

1144.8

1213.6

7.91E-04

3.31E-04

2.15

42.5

Coke+7%K

995.6

1146.8

1208.4

7.89E-04

3.31E-04

2.18

43.0

Coke+1%Na

1080.9

1263.1

1331.1

7.35E-04

2.89E-04

1.37

50.0

Coke+3%Na

1041.7

1171.2

1248.3

8.91E-04

3.41E-04

2.24

41.3

Coke+5%Na

1038.5

1172.2

1250.1

8.92E-04

3.43E-04

2.27

42.3

Coke+7%Na

1037.9

1171.3

1250.8

8.93E-04

3.43E-04

2.27

42.6

34

Table 5 Gasification characteristic parameters with different heating rates Sample

Coke+3%K

Coke+3%Na

Heating rate (K/min)

Ti(K)

Tm(K)

Tf(K)

Rmax(s-1)

Rmean (s-1)

S×1016

tg(min)

3

1024.6

1169.8

1236.2

3.84E-04

1.69E-04

0.50

70.5

5

1030.2

1201.1

1276.5

6.64E-04

2.94E-04

1.44

49.3

7

1043.3

1220.8

1300.1

8.83E-04

4.23E-04

2.64

36.7

9

1053.2

1236.4

1316.8

11.85E-04

5.17E-04

4.19

29.3

3

1035.6

1152.8

1202.1

6.29E-04

3.03E-04

1.48

55.5

5

1041.7

1171.2

1248.3

8.91E-04

3.41E-04

2.24

41.3

7

1049.2

1187.3

1261.6

10.85E-04

4.71E-04

3.68

30.3

9

1056.6

1194.9

1282.3

13.67E-04

5.76E-04

5.50

25.1

35

Table 6 Carbon microcrystalline structure parameters of different samples from XRD analysis Sample

2θ(002) (°)

β(002) (°)

d(002) (nm)

Lc (nm)

Nmean

Coke

25.59

2.35

0.348

34.28

10.86

Coke+1%K

25.54

2.96

0.348

27.21

8.81

Coke+3%K

25.52

3.48

0.349

23.15

7.64

Coke+5%K

25.52

4.26

0.349

18.91

6.42

Coke+7%K

25.44

4.98

0.350

16.17

5.62

Coke+1%Na

25.48

2.65

0.349

30.39

9.70

Coke+3%Na

25.41

2.92

0.350

27.58

8.87

Coke+5%Na

25.07

3.22

0.353

25.31

8.16

Coke+7%Na

25.12

3.37

0.354

23.81

7.72

36

Table 7 Carbon microcrystalline structure parameters of different samples from Raman analysis Sample

ID1

ID2

ID3

ID4

IG

ID1/IG

I (D1+D2)/ IG

IG/IAll

Coke

12570

980

3315

2746

4239

2.97

3.20

0.178

Coke+1%K

13981

1776

3113

4632

4017

3.48

3.92

0.146

Coke+3%K

20108

2880

4414

4238

5286

3.80

4.35

0.143

Coke+5%K

12712

2055

3856

3007

3299

3.85

4.48

0.132

Coke+7%K

15803

3671

3238

2895

3831

4.13

5.08

0.130

Coke+1%Na

12211

2749

3004

2221

3583

3.41

4.18

0.151

Coke+3%Na

12840

2598

4423

2878

3430

3.74

4.50

0.131

Coke+5%Na

9128

2338

2318

2109

2331

3.92

4.92

0.128

37

Table 8 Kinetic parameters of different samples at four different heating rates for RPM, URCM and VM Samples

RPM E(kJ/mol)

k0(s-1)

Coke

236.5

Coke+1%K

URCM Ψ

R2

5.62E+06

0.015

169.3

1.02E+04

Coke+3%K

162.1

Coke+5%K

VM

E(kJ/mol)

k0(s-1)

R2

E(kJ/mol)

k0(s-1)

R2

0.9987

211.5

4.04E+05

0.9949

236.5

5.62E+06

0.9987

2.118

0.9993

177.5

2.78E+04

0.9990

196.9

2.41E+05

0.9948

1.13E+04

1.451

0.9958

165.7

1.81E+04

0.9959

180.4

1.04E+05

0.9914

220.5

1.68E+07

0.453

0.9966

219.4

1.39E+07

0.9941

229.9

5.20E+07

0.9946

Coke+7%K

227.6

4.36E+07

0.077

0.9894

209.1

4.85E+06

0.9889

229.9

5.74E+07

0.9893

Coke+1%Na

174.6

1.23E+04

4.175

0.9989

193.6

1.16E+05

0.9969

213.6

1.04E+06

0.9901

Coke+3%Na

300.5

5.49E+10

0.066

0.9953

264.8

1.02E+09

0.9932

303.6

7.76E+10

0.9952

Coke+5%Na

294.2

2.74E+10

0.381

0.9968

249.4

1.98E+08

0.9927

308.7

1.40E+11

0.9966

38

Table 9 Deviation between the experimental and calculated conversion data Samples Coke Coke+1%K Coke+3%K Coke+5%K Coke+7%K Coke+1%Na Coke+3%Na Coke+5%Na

DEV(x)% URCM 1.56 0.96 1.29 1.46 3.21 0.95 1.56 1.38

PRM 0.74 0.83 1.34 0.98 2.56 0.77 1.45 1.03

39

VM 0.83 1.28 2.11 1.28 2.57 1.65 1.47 1.06

0.0

(a)

Coke Coke+1％K Coke+3％K Coke+5％K Coke+7％K

0.2

0.0

0.2

0.4

(b)

Coke Coke+1％Na Coke+3％Na Coke+5％Na Coke+7％Na

0.4

-3

1.0x10

-4

8.0x10

x

x

-4

0.6

-4

6.0x10

8.0x10

0.6 dx/dt,s

dx/dt,s

-1

-1

-4

-4

4.0x10

0.8

0.8

6.0x10

-4

4.0x10

-4

2.0x10

1.0

0.0 600

-4

2.0x10

800

1000

1200

1.0

1400

0.0 600

800

600

800

1000

1200

1400

Temperature,K

Temperature,K

1000

1200

1400

600

Temperature,K

800

1000

1200

1400

Temperature,K

Fig.1. Experimental conversion and reaction rate curves with a heating rate of 5K/min: (a) Potassiumabsorption samples;(b) Sodium- absorption samples

40

(a)

0.0

3K/min 5K/min 7K/min 9K/min

0.2

0.4

(b)

0.0

3K/min 5K/min 7K/min 9K/min

0.2

0.4

-3

1.4x10

-3

1.5x10

x

-3

0.6

-4

-3

1.2x10

-1

-1

dx/dt,s

0.8

0.6

-3

1.0x10 8.0x10

-4

6.0x10

0.8

dx/dt,s

x

1.2x10

-4

9.0x10

-4

6.0x10

-4

4.0x10

-4

3.0x10 -4

2.0x10

1.0

0.0 800

900

1000

1100

1200

1300

1.0

1400

800

900

1000

0.0 900

1000

1100

1200

1300

Temperature,K

Temperature,K

1100

1200

1300

1400

900

1000

1100

1200

1300

Temperature,K

Temperature,K

Fig.2. Experimental conversion and reaction rate curves with different heating rate: (a) Coke+3%K, (b) Coke+3%Na

41

Fig.3. SEM/EDS analysis of coke after alkali metal vapor adsorbed: (a)Coke+3%K, (b)Coke+3%Na

42

1—SiO2 2—Al2SiO5 3—KAlSiO4

1

2

Intensity(a.u.)

3

1

3

1

1

31 3

1

1

31 1 1

10

20

3 31 3

1

1

2

1

3

3 1

3 1

3

3

3

1

1

1

Coke

1

30

3

Coke+1%K

3

2

40

2

3

Coke+7%K 60

1

70

10

20

Coke

3 2

2 3

Coke+1%Na

2

3

2 3

3

Coke+3%Na

2

3

2 3

Coke+5%Na

3 2

2 3

Coke+7%Na

3 2

30

2

40

2θ(°)

2θ(°)

Fig.4. XRD patterns of different samples

43

2

2 1

2

Coke+5%K

50

1

1

3

3

Coke+3%K

3

2

21

3 1

1—SiO2 2—Al2SiO5 3—NaAlSiO4

21

Intensity(a.u.)

21

50

60

70

60

Coke

Coke-1%K

Experiment

Experiment

Fitting Curve

D1

D1 G

G Intensity(a.u)

Intensity(a.u)

Fitting Curve

60

40

D2 20

D3

40

D2 D3 D4

20

D4 0

1000

1200

1400

1600

0

1800

1000

1200

-1

Coke-3%K

Coke-5%K

Experiment Fitting Curve

100

Experiment

Intensity(a.u)

80

G D2

40

D3

20

G 40

D2 D3 20

D4

D4 1000

1200

D1

Fitting Curve

60

D1

60

1800

Wavenumber(cm )

120

Intensity(a.u)

1600 -1

Wavenumber(cm )

0

1400

1400

1600

0

1800

-1

1000

1200

1400

1600 -1

Wavenumber(cm )

Wavenumber(cm )

44

1800

Coke-7%K

100

Coke-1%Na

80

Experiment

Experiment

D1

Fitting Curve 80

Fitting Curve

D1

60

Intensity(a.u)

Intensity(a.u)

60

G

40

D2

D3

G 40

D2 D3

20

20

D4

D4 0

1000

1200

1400

1600

0

1800

1000

1200

80

Coke-3%Na Experiment Fitting Curve

D1

D1 Intensity(a.u)

G 40

D2 D3

40

G 20

D4 1200

D2

D3

20

1000

1800

Coke-5%Na 60

60

0

1600

Wavenumber(cm )

Experiment Fitting Curve

Intensity(a.u)

1400 -1

-1

Wavenumber(cm )

D4 1400

1600

0

1800

1000

1200

-1

1400

1600 -1

Wavenumber(cm )

Wavenumber(cm )

Fig.5. Raman spectra of different samples

45

1800

Coke

0.0

0.2

x

0.4

0.6

3K/min 5K/min 7K/min 9K/min

RPM URCM

0.8

VM

1.0 900

1000

1100

1200

1300

1400

Temperature,K

Coke+1%K

0.0

0.2

x

0.4

0.6

3K/min 5K/min 7K/min 9K/min

RPM URCM

0.8

VM

1.0 900

1000

1200

1100

Temperature,K

46

1300

1400

Coke+3%K

0.0

0.2

x

0.4

0.6

3K/min 5K/min 7K/min 9K/min

RPM URCM

0.8

VM

1.0 900

1200

1100

1000

1300

1400

Temperature,K Coke+5%K

0.0

0.2

x

0.4

0.6

3K/min 5K/min 7K/min 9K/min

RPM URCM

0.8

VM 1.0 900

1000

1100

1200

Temperature,K

47

1300

1400

Coke+7%K

0.0

0.2

x

0.4

0.6

3K/min 5K/min 7K/min 9K/min

RPM URCM

0.8

VM 1.0 900

1000

1100

1200

1300

1400

Temperature,K Coke+1%Na

0.0

0.2

x

0.4

0.6

3K/min 5K/min 7K/min 9K/min

RPM URCM

0.8

VM 1.0 900

1000

1100

1200

Temperature,K

48

1300

1400

Coke+3%Na

0.0

0.2

x

0.4

0.6

3K/min 5K/min 7K/min 9K/min

RPM URCM

0.8

VM 1.0 900

1000

1100

1200

1300

1400

Temperature,K Coke+5%Na

0.0

0.2

x

0.4

0.6

3K/min 5K/min 7K/min 9K/min

RPM URCM

0.8

VM 1.0 900

1000

1100

1200

1300

1400

Temperature,K

Fig.6. Experimental conversion rate curves and these calculated with different models Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

49

50

Highlithts: 1) The influence of alkali metal vapor on physicochemical properties of coke was investigated. 2) CO2 gasification properties of metallurgical coke were investigated by thermogravimetric analysis method. 3) The ordering degree of carbon microstructure has significant influence on coke reactivity. 4) Three representative gas-solid reaction models were employed to describe the gasification process.

51