The evolution of atomistic structure and mechanical property of coke in the gasification process with CO2 and H2O at different temperatures: A ReaxFF molecular dynamics study

An atomistic coke carbon model was constructed to simulate the structural evolution in the gasification and stretching process. The coke model was placed in a box with different CO2/H2O content to investigate the evolution of the atomistic structure of coke during the gasification. It was found that different atmospheric concentrations had different effects on the structure and reaction sites of the coke model. The CO2 molecules tended to dissolve on the surface of coke and disrupt its surface structure, while H2O molecules were more likely to enter the coke model to disrupt the internal structure. For tensile simulation, it was found that CO2 and H2O had different effects on the tensile resistance of the coke model. Controlling the composition content of the reaction gas can effectively influence the tensile strength of the coke model. By revealing the behavior of coke model at the micro scale, it provides a theoretical basis for the industrial coke application process. Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) was used to conduct the molecular dynamics using the reactive force field (ReaxFF). The atomistic model of coke carbon was constructed using the well-known annealing and quenching method, and its composition is determined according to the element analysis of industrial coke. The structural evolution in the gasification with CO2/H2O and the stretching process were analyzed in detail. Molecular dynamics simulations with reactive force field (ReaxFF-MD) were used to simulate the coke dissolution reaction under CO2/H2O atmosphere and the coke stretching process. The atmosphere ratio was modified to investigate the changes in coke structure under different atmosphere conditions. The Packmol software was used to place gas and coke models into the same box. During the reaction process, the Ovito software was used to perform corresponding visualization analysis on the changes in the atomic structure of coke.


Introduction
As one of the important raw materials in the ironmaking industry, coke has been widely used due to its good reactivity and mechanical strength [1][2][3][4].The evaluation of the performance of coke is mainly related to its degree of solution loss reaction and its role as a framework for internal reactions in the blast furnaces [5][6][7].The previous studies indicate that the porosity and pore shape inside coke are the main factors affecting its strength and reactivity [8][9][10].From a molecular perspective, there is a certain amount of sp 2 bonds inside the coke, and the presence of these C-C bonds makes it easier for carbon atoms to form layered structures formed with carbon rings (6-membered ring or 5-membered ring).The layered structure is similar to the 372 Page 2 of 10 aromatic layer structure, so coke is also considered a "nano graphite material," which is connected with the aromatic carbon network [9,11,12].Researchers have found that the degree of change in coke strength can be explored by characterizing the structural changes in coke pores [8][9][10].Most of these pores are connected to the outside, even in unreacted coke [13,14].The main experimental research method is to analyze the relationship between the crystal structure and coke reaction intensity (CRI) of industrial metallurgical coke through X-ray diffraction [6].Ono et al. [15] conducted three-point bonding tests on coke samples before and after the reaction and used X-ray computed tomography to scan and analyze the fracture structure of the coke.However, these are all observations of the changes in coke structure before and after the reaction from a macro perspective, and few studies have investigated the evolution of coke structure during the reaction process from an atomic scale.
The main research method for the atomic scale of coke is to simulate the reactivity of the constructed amorphous coke model and analyze the structural transformation of the model before and after the reaction.Van et al. used ReaxFF (reactive force field) simulation to compare the dissociation and formation of chemical bonds in hydrocarbons, and the results showed that ReaxFF can indeed reproduce the energy related to the non reactive and reactive behavior of these compounds [16].Zang et al. have conducted microscopic analysis of coal materials and found the methods to control the C/H ratio and sp 2 bond content of coal coke materials, which provides an experimental basis for subsequent molecular dynamics simulations of coke [17,18].Li et al. conducted a comparative analysis of the graphitization process of amorphous carbon models and summarized the relevant simulation parameters that affect the internal structure of the model [19].Similar to the above simulation methods, Zhong et al. constructed a large molecule model of coke (molecular formula C 3717 H 1830 N 54 O 134 S 109 ) and studied its reaction performance, including calculating its internal structure such as stacking layers using ReaxFF-MD and density functional theory (DFT) [6,20].Moreover, ab initio density functional theory (DFT) was used to investigate the generic example process of the formation of carbon nanoparticles at the cooling stage of gas phase of carbon [21].The above studies all indicated that the reactivity of the coke was closely related to its structural evolution [22,23].
At present, the main way to study the mechanical properties of coke is to explore its strength of coke through experiments.However, there are almost no studies that uncover the structural changes of coke during stretching at the atomic scale.This study used ReaxFF-MD to simulate the stretching of the constructed coke model, and explored the tensile properties and changes in coke structure during the stretching process.This stretching simulation method has been well applied to the stretching process of other carbon-containing materials, including graphene [24,25].Bu et al. used this method to stretch the graphene sheets with different layers to the influence of the layers and oxidation functional groups on the tensile resistance of the graphene sheets [25].Researchers also used it to study the mechanical properties of single-layer and multi-layer graphene and its derivatives [25,26].The research results are in good agreement with the experimental measurements, indicating that the mechanical properties are closely related to the changes in structure.However, this simulation method has not yet been well applied to the strength study of coke models.
In this study, molecular dynamics simulations with reactive force field were performed to construct the molecular model of coke.Zhong et al. [9] constructed a coke model consisting of over 40,000 atoms based on the structure and elemental composition of the industrial coke.The feasibility of relevant construction methods has been proven by some research papers [9,12,27,28].Based on the coke model containing 3000 atoms generated in this study, we studied the reaction performance of coke in the CO 2 /H 2 O atmosphere and explored the microscopic process of the reaction at the atomic scale.In terms of mechanical properties, we explored the changes in mechanical properties of the coke model before and after the reaction, as well as the impact of temperature changes on tensile properties.By revealing the behavior of coke model at the micro scale, it provides a theoretical basis for the industrial coke application process.

Construction of coke model
A coke model (molecular formula C 2863 H 105 O 6 N 18 S 8 ) was constructed to perform molecular dynamics simulations of the coke stretching process.This model has the same atomic ratio as the molecular model of Pingyao coke used in Tian's article [28], which has been proven to simulate the reaction behavior of coke effectively.As shown in the Figure S1, we also compared the appearance and the internal stack structure between the two models, proving the feasibility of the model.Previous studies have investigated the effects of density and time step on the structure and reaction performance of amorphous coke models [9,12,27].Li et al. has demonstrated the applicability of the force field parameters used in this study to the amorphous carbon model when the density is 1.4 g/cm 3 and the time step is 0.2 fs.After repeated simulations and validation, we ultimately decided to set the time step and density in this study simulation to be consistent with Li's article.The specific force field parameters were provided in the supplementary materials, and these parameters have been proven to effectively characterize the reaction performance of coke [9,12].
The entire generation process of the coke model was carried out in the following four steps: (1) At the initial stage, the atoms were randomly generated anywhere inside the box by using some packages like MOLECULE in the LAMMPS installation [28], and the size of the box was determined by the model density and the total number of atoms.(2) The model was performed in NVT ensemble at 300 K (room temperature) for 10 ps to eliminate floating bonds and minimize the total energy.(3) The third step was the heating stage, where the model was heated from 300 to 3500 K in 10 ps and was performed in NVT ensemble at this temperature for 500ps.(4) The final step was the cooling step, in which the model was cooled from 3500 K to room temperature 300 K within 10 ps and was performed in NVT ensemble at 300 K for 500 ps.To understand the structural changes of the coke model at each stage, we have taken snapshots of the coke model for every 100 ps and placed them in the supplementary materials, as shown in Figure S2.

Simulations of coke dissolution reaction and mechanical properties
This study investigated the coke dissolution reaction process under different CO 2 and H 2 O atmospheres after the initial coke model was constructed.The specific reaction model is shown in Fig. 1a.In this study, the composition ratio of the atmosphere was changed to 100% CO 2 , 50% CO 2 + 50% H 2 O, and 100% H 2 O.In order to conform to the actual reaction process as much as possible, the total number of gas molecules was controlled to 500 in this study.For the simulation of coke dissolution reaction, periodic boundary conditions were set in the x and y direction of the simulation box to enable the gas molecules to react fully with the coke model.In the z direction, it was set as fixed boundary conditions to reflect the gas molecules.
For the simulation details, the time step was set to 0.1 fs, and the cutoff distance was selected to be 0.3 Å.The specific simulation parameters were consistent with Li's research, and this parameter has been proved to be able to better simulate the relevant gasification reaction of the coke model [12,19,29].The specific force field parameter code has been provided in the supplementary information.In the process of reaction, the coke model and gas molecules were performed in NPT ensemble respectively, which was to eliminate the unstable dangling bonds in the model and minimize the energy of the model.After that, the whole system was heated to 3500 K and was performed in NVT ensemble for 1000 ps at 3500 K, allowing the coke model to fully react with gas molecules.Finally, the temperature of the whole system was reduced to 300 K.
Considering that the different temperatures in the blast furnace may have an influence on the mechanical properties of the coke, the simulations were made at the temperatures of 300 K, 500 K, 1000 K, and 1500 K to investigate the effect of temperature on the models' mechanical properties.They were performed in NPT ensemble, and the time step was set at 0.1 fs in the simulations.For the simulation of coke stretching process, there are periodic boundaries in the x, y, and z directions of the box.Periodic boundaries can result in a uniform distribution of forces on the coke model in the tensile direction.Before the start of stretching, we relaxed the system at 300 K for 20 ps to bring it to equilibrium.For models stretched at 500 K, 1000 K, and 1500 K, we first heated them from 300 K to the target temperature in 20 ps under the NVT ensemble.Then the models were relaxed for 20 ps at the target temperature under NPT ensemble.The relaxed coke models were subjected to uniaxial stretching at a strain rate of 10 −5 /ps along the y-axis direction, as shown in Fig. 1b.The strain rate was determined based on references [25,[30][31][32] and tests.Referring to relevant literature, we attempted three stretching rates: 10 −4 / ps, 10 −5 /ps, and 10 −6 /ps.During the trial process, we found that the overall trends of the three rates were similar, but the ultimate fracture stress of 10 −4 /ps was significantly higher, indicating a significant deviation from other curves.To save computational resources while keeping a good accuracy, the tensile simulations of this study were performed using the strain rate of 10 −5 /ps.The stresses of all atoms were summed to get the total structural stress, and a stress-strain curve was plotted.The OVITO [33] was used to analyze the change of atomic structure and the micro mechanism of reaction process.This study focused on the changes in the coke models' structures during the stretching process, and the mechanical properties of the coke model after different reaction conditions.

Effect of solution loss reaction on mechanical properties of coke
Before the simulation of the solution loss reaction started, the unreacted gas molecules inside the box were removed to prevent adverse effects on the stretching results.The tensile strength of the unreacted coke model was calculated first, along with the coke model after the solution loss reaction in pure CO 2 atmosphere, pure H 2 O atmosphere, and 50% CO 2 + 50% H 2 O mixed atmosphere.The results are shown in Fig. 2, and the overall trend of the four curves is almost the same.In the early stage of the stretching process, the slope of the curve remains almost constant, indicating that it is currently in the stage of elastic stretching.After reaching the maximum stress during stretching, the carbon chains inside the model gradually ruptured, leading to a sharp decrease in internal stress and ultimately to a complete fracture.It can also be seen from the figure that the unreacted model has the largest tensile strength, and its maximum bearing stress is about 30 GPa.The maximum tensile strength of the coke models after reaction with an oxidation atmosphere was significantly lower than that of the non-reaction model and the maximum of the other three models is about 25 GPa.Sato et al. analyzed and estimated the tensile stress of coke cracking in coke ovens, and the results showed that coke can withstand tensile stress up to 10-20MPa [34].This is consistent with the stress results in this simulation.At the same time, the maximum tensile strength of the coke model in the atmosphere of 100% CO 2 was significantly greater than that in the atmosphere of 100% H 2 O.However, the extension platform for coke under 100% H 2 O atmosphere to reach the maximum tensile strength at break was larger.It proved that although the maximum tensile strength of coke after reaction with H 2 O was reduced, the overall plasticity was stronger, and the required change for fracture was large.The trend of stress changes proved that the addition of CO 2 is more likely to cause coke to fracture during the stretching process, which is consistent with the experimental results [15,35].
As shown in Fig. 3, the stretching process of the four coke models mentioned above was analyzed.Figure 3a,  b, c, d represents the unreacted model, 100% CO 2 atmosphere, 100% H 2 O atmosphere, and 50% CO 2 + 50% H 2 O mixed gas model respectively.The five stages represent the initial stage of stretching, the stages of reaching the strain of 10%/20%/30%/40%.As shown in Fig. 3a, at the initial stage of stretching (tensile strain < 20%), for the unreacted coke model, the colors of various parts in the structure were basically similar, which shows that the stress distribution in the coke was relatively uniform.When the tensile strength increased to 30%, some carbon bonds in the middle of the coke began to break.At this point, it can be seen from the figure that the color of the other parts of the coke has changed to blue.It proved that the stress distribution was also concentrated at the fracture location, and the stressbearing capacity of the remaining parts decreased.In addition, when the strain reached 40%, there were many carbon chains on the broken part.As shown in Fig. 3b, in the 100% CO 2 atmosphere, the fracture portion of coke often occurred on the surface of the model, and the fracture portion was relatively complete, with a large fracture surface.This corresponds to the behavior of the 100% CO 2 curve in Fig. 2, which rapidly decreases after reaching the maximum tensile stress.When the strain reaches 40%, there were many carbon chains connecting the two broken parts at the fracture site which is similar to the unreacted model.However, compared As shown in Fig. 3c, d, in an atmosphere of 100% H 2 O and 50% CO 2 + 50% H 2 O, at the initial stage of stretching, the stress distribution of the coke is more concentrated on the atoms inside, corresponding to the deep color inside the coke.When the strain level reached 40%, the fracture surface of the two models was more obvious, and the number of connected carbon chains was significantly reduced.The curves are shown in Fig. 2. The maximum stress of the curve under both conditions was smaller than that of the unreacted model and the 100% CO 2 atmosphere model.However, in Fig. 2, the downward trend of the curve for 100% H 2 O was smoother, demonstrating stronger toughness.The above results indicate that different atmosphere components have different effects on the mechanical properties of coke.The tensile resistance of the reacted coke can be adjusted by changing the reaction conditions.

Effect of solution loss reaction on chemical structure of coke
The study also investigated the microscopic influence mechanism of different atmosphere compositions on the mechanical properties of coke.As shown in Fig. 4a, the statistics were made on the gasification of carbon atoms under three atmosphere conditions.The C s atom is defined as carbon atom from coke, and the C g atom was defined as carbon atom from the CO 2 gas.It can be seen from the figure that at the initial stage of the reaction, the number of gasification C atoms generated in H 2 O atmosphere was more than that in CO 2 atmosphere, which proved that at the initial stage of the solution loss reaction, H 2 O was more likely to react with the coke model in gasification.The main reason for this phenomenon was that, as shown in Fig. 4c, the H 2 O would generate H 2 and free OH groups at high temperatures, and OH groups would adsorb on the coke surface.This would result in the formation of a CO molecule that eventually escaped from the coke surface.CO 2 gas tended to react on the surface of coke, while H 2 was more likely to enter the coke for reaction.As shown in Fig. 4b, the statistics about the amount of the C g atom in the CO 2 atmosphere and the H atom in the H 2 O atmosphere in the Z direction were carried out.The red dotted line represents the contact surface between the coke and the reaction atmosphere.The results show that C g atoms mainly gathered at the boundary of the coke at 32 Å, and the maximum distance into the coke was only about 15 Å, which indicates that the reaction of CO 2 mainly occurs at the interface of the coke and has little influence on its structure.H atoms also gathered on the surface of coke, but some H atoms infiltrated into the inside of coke, including the whole coke model.This proves that in the atmosphere of H 2 O, the internal microstructure of coke is destroyed by H atom, thus reducing the mechanical properties of coke.
As shown in Fig. 5, the specific mechanism of the reaction of CO 2 and H 2 O with coke was investigated.When CO 2 gas moved near the coke surface, C g atoms would form C-C bonds with Cs atoms, resulting in CO 2 gas molecules adsorbed on the coke surface.The CO 2 molecules would remove one of the O atoms and adsorb on the coke surface like some kind of functional group.As the reaction proceeded, the second O atom also detached, while the C g atom was completely embedded in the C s atoms on the coke surface.These free O atoms would then be adsorbed by C s atoms to generate new C s O gas.The adsorbed C g atoms then formed unstable five-member or seven-member rings with the C s atoms on the coke surface, which would lead to the fracture of the coke at the edge during the stretching process to damage the mechanical properties of the coke.
As shown in Fig. 6, the behavior of the H atom penetrating the coke was also investigated.Due to its small volume, the H 2 molecule was easier to permeate through the coke.When the H 2 molecule arrived, the H-H bond in the gas broke up to form two free H atoms.These two H atoms would be adsorbed by C s atoms inside the coke one after another, resulting in a decline in the stability of its structure, and this part was more prone to fracture during stretching.
The influence of the mixed atmosphere on coke reaction performance was also investigated through analyzing the gas products of 100% H 2 O and 50% CO 2 + 50% H 2 O models statistically.As shown in Fig. 7a, b, the rate of hydrogen generation was much greater than the rates of CO 2 and CO under both atmospheres.This indicated that the generated H 2 was easier to enter the interior of the coke and then bonded with C s atoms.Compared to the H 2 O atmosphere, the H 2 generation rate in the mixed atmosphere reached equilibrium earlier, which proved that the addition of CO 2 had an impact on the reaction rate of H 2 O.As shown in Fig. 7c, the statistics of the H 2 generation rate of the two models were counted.In the initial reaction stage, the H 2 generation rate of the 100% H 2 O model was about twice that of the mixed atmosphere model.But as the reaction proceeded, the H 2 generation rate of the mixed atmosphere gradually decreased and was less than half of the rate of the 100% H 2 O model.This indicated that when there was CO 2 in the reaction gas, it would compete with H 2 O molecules to seize the C s atoms on the coke model that can undergo solution loss reaction.This would affect the dissolution loss reaction rate of H 2 O, thus reducing the formation rate of H 2 .

Effect of temperature on the performance of coke
Considering that coke in industrial production may undergo solution loss reactions at different temperatures, the temperature was also an important factor affecting the mechanical properties of coke.As shown in Fig. 8, we carried out tensile test simulation at 300 K/500 K/1000 K/1500 K with an unreacted coke model.With the increase in temperature, the maximum tensile stress of coke decreased significantly, from about 30 GPa at 300 K to about 24 GPa at 1500 K, with a decrease of 20%.The five stages represent the initial stage of stretching, the stages of reaching the strain of 0%/10%/20%/30%/40%, as shown in Fig. 9.As shown in Fig. 9, the brighter colored parts concentrate inside the coke model.This part was also the area where the coke model ruptured later.It indicated that the increase in temperature exacerbated the concentration of internal stress in the coke model.As the temperature increased, the magnitude of stress borne by the atoms inside the coke increases, resulting in a reaction where the color of the atoms was closer to red.This indicated that an increase in temperature would increase the stress load on the coke.And with the increase in temperature, although the maximum tensile stress of the coke model decreases, the plasticity of the model increases, and the slope of the stress reduction curve is gentler.The reason for this phenomenon was that under the condition of high temperature and stretching to the same strain, the number of carbon chains connecting the two parts inside the coke increased, which shared the sudden change of stress in the broken

Conclusions
In this study, the ReaxFF-MD was used to simulate the solution loss reaction and tensile strength of the model.The results showed that the atmosphere with CO 2 /H 2 O ratios had different erosion effects on the coke model.The effect was reflected in the change of coke structure and tensile properties.CO 2 gas was more likely to erode the surface of coke, and C g atoms in CO 2 gas would replace C s atoms on the surface of coke, forming unstable fivemembered rings and seven-membered rings, which caused damage to the stability of coke structure.
In the H 2 O atmosphere, the structure of the coke model was more vulnerable to erosion, and H 2 molecules were more likely to penetrate the model and caused the C-C bond to break, thus affecting the stability of the coke structure.The tensile test of the coke model after the reaction showed that the tensile resistance of the coke decreases obviously.In addition, coke models eroded by CO 2 were more likely to fracture near the surface, while coke models eroded by H 2 O are more likely to fracture internally.With the increase in temperature, the tensile resistance of coke becomes worse, and the fracture surface is more inclined to the coke surface.The above results indicate that different atmosphere components have different effects on the mechanical properties of coke.The tensile resistance of the reacted coke can be adjusted by changing the reaction conditions.

Fig. 1 a
Fig. 1 a Reaction model of coke with different gases (CO 2 /H 2 O); b the stretching process of coke model in y direction

Fig. 2
Fig. 2 Tensile stress-strain curves of four models

Fig. 6 Fig. 7
Fig. 6 The process of solution loss reaction between H 2 O and coke: a main view of the reaction process; b enlarged micrograph