Jiazhuang Tian,Qiuyun Mao,Zihan You,Qifan Zhong
Investigation of Petroleum Coke Gasification with CO2/H2O Mixtures and S/N Removal Mechanism via ReaxFF MD Simulation
The removal of environmentally harmful S/N is crucial for utilization of high-S petroleum coke (petcoke) as fuels. Gasification of petcoke enables enhanced desulfurization and denitrification efficiency. Herein, petcoke gasification with the mixture of two effective gasifiers (CO2 and H2O) was simulated via reactive force field molecular dynamics (ReaxFF MD). The synergistic effect of the mixed agents on gas production was revealed by altering the CO2/H2O ratio. It was determined that the rise in H2O content could boost gas yield and accelerate desulfurization. Gas productivity reached 65.6% when the CO2/H2O ratio was 3:7. During the gasification, pyrolysis occurred first to facilitate the decomposition of petcoke particles and S/N removal. Desulfurization with the CO2/H2O gas mixture could be expressed as thiophene-S → S → COS → CHOS, thiophene-S → S → HS → H2S. The N-containing components experienced complicated mutual reactions before being transferred into CON, H2N, HCN, and NO. Simulating the gasification process on a molecular level is helpful in capturing the detailed S/N conversion path and reaction mechanism.
Investigation of Petroleum Coke Gasification with CO2/H2O Mixtures and S/N Removal Mechanism via ReaxFF MD Simulation

The removal of environmentally harmful S/N is crucial for utilization of high-S petroleum coke (petcoke) as fuels. Gasification of petcoke enables enhanced desulfurization and denitrification efficiency. Herein, petcoke gasification with the mixture of two effective gasifiers (CO2 and H2O) was simulated via reactive force field molecular dynamics (ReaxFF MD). The synergistic effect of the mixed agents on gas production was revealed by altering the CO2/H2O ratio. It was determined that the rise in H2O content could boost gas yield and accelerate desulfurization. Gas productivity reached 65.6% when the CO2/H2O ratio was 3:7. During the gasification, pyrolysis occurred first to facilitate the decomposition of petcoke particles and S/N removal. Desulfurization with the CO2/H2O gas mixture could be expressed as thiophene-S → S → COS → CHOS, thiophene-S → S → HS → H2S. The N-containing components experienced complicated mutual reactions before being transferred into CON, H2N, HCN, and NO. Simulating the gasification process on a molecular level is helpful in capturing the detailed S/N conversion path and reaction mechanism.

Petroleum coke (petcoke) is a valuable oil refinery product derived from delayed coking of carbon–hydrogen compounds (such as petroleum asphalt and residual oil). As fuel, it is competitive because of the price advantage, extraordinary heat value (30.25–34.91 MJ/kg), high carbon (>80 wt %), and low ash (<1.5 wt %) content. Nonetheless, due to the continuous decline in crude oil quality in recent years, petcoke high in S (>4 wt %) and N (∼1 wt %) content gradually dominates in the application of petcoke. The extensive S and N elements in these petcoke are attributed to a large quantity of SOx and NOx emission and can aggravate environmental pollution. As a result, the direct energy use of petcoke is doomed. Although efforts such as thermal desulfurization and alkali metal compound desulfurization have been made, technological and economic obstacles remain. Therefore, a productive and environmentally benign technique must be adopted to facilitate the further practice of petcoke. Gasification is an efficient and facile industrial technology for the transformation of carbonaceous materials into synthetic gas (syngas) by agents such as air, oxygen, and water. Its raw materials range broadly from coal to lignocellulosic wastes such as marc and pistachio exocarp. During the process, S and N elements migrate to the gas phase as H2S, NH3, and HCN, among others, to reduce pollution. This provides a promising methodology for the utilization of petcoke and the removal of the stubborn thiophene-S, pyrrole-N, and pyridine-N. Rana et al. employed supercritical water (SCW) in petcoke gasification due to its enhanced solubility and mass transfer properties. Under the effect of this single agent, H2 yields multiplied by 74% as the SCW and the petcoke system was heated from 350 to 650 °C. Gasification agents including H2, O2, and NH3 were also discussed in the corresponding research. By mixing various gasification agents such as CO2, O2, H2O, etc., enhanced syngas heat value, optimized gasifier utilization, and suppressed S- and N-containing gas emission can be achieved. Kislov et al. combined experiments with numerical simulation in the case of coal gasification by a mixture of air and CO2. It was determined that the addition of CO2 and H2O into the air increased the caloric value of the gaseous product. Recently, Sharma et al. investigated the Indian biomass gasification under an air-steam environment and achieved an enhanced H2 productivity compared to that of air. Microcrystalline structure, pore structure, and particle size have been considered to have a dramatic influence on petcoke gasification reactivity. For example, Huo et al. studied the correlation between the influencing factors of gasification reactivity. With a large particle size (∼250 μm) and high temperature (>1273 K), the petcoke gasification rate decreased due to the effect of pore diffusion. Zhou et al. compared the gasification reactivity of petcoke and semicoke, a solid fuel derived from heat treatment of petcoke. It was found that the low graphitization degree of semicoke resulted in a more disordered microcrystalline structure and thus exposed more active sites to facilitate the reaction. The transformation mechanism of the crucial intermediate products, such as OH and H radicals, is crucial to gasification characteristics. However, the lack of detection methods for such products hinders the further understanding for petcoke gasification. To achieve a comprehensive understanding of the petcoke gasification mechanism and conversion path, research on a molecular level needs to be conducted. A reactive force field molecular dynamics (ReaxFF MD) approach is capable of handling the mechanism study of intricate hydrocarbon systems such as petcoke. Castro-Marcano et al. conducted a ReaxFF MD simulation for Illinois No. 6 coal char combustion research. The reaction system involved a 7458 atom char model and up to 14000 O2 molecules. It was found that the 6-membered rings in char would be transferred into 5- and 7-membered rings. The decomposed small molecules would be oxidized by O and OH radicals. Bhoi et al. constructed a ReaxFF MD system to explore the combustion and pyrolysis behaviors of brown coal. The system used was 100 times faster than quantum mechanics methods. Thus, as an atomic level calculation, large-scale (over 10000 atoms) ReaxFF MD simulation is practical in expense while retaining efficiency and accuracy. Herein, a Qingdao petcoke model from a previous study (C1648H772O59N24S47) was used to conduct the ReaxFF MD simulation. A mixture of CO2/H2O was introduced as the petcoke gasification agent. The productivity and component of the gaseous products were investigated. The existing forms of S/N elements and their transformation paths during gasification were investigated in the reaction mechanism study. The results of this work can help establish the connection between mechanism and productivity in petcoke gasification with a CO2/H2O mixture.

The gasification system involved a typical Qingdao high-S petcoke model (C1648H772O59N24S47) and 4000 CO2/H2O molecules in total. In the previous work, the petcoke model was well constructed and rationalized by X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), X-ray powder diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM). The petcoke model consisted of 56 hydrocarbon molecules, stacked, curved, and planar. The element content, functional groups, aromaticity, stacking extent, etc., of the model corresponded to the actual Qingdao petcoke.

The potential function (eq 1) of ReaxFF covers bond energy (Ebond), overcoordinated atom in the energy contribution (Eover), three-body valence angle strain energy (Eangle), angle strain energy (Eangle), penalty energy (Epen), torsion conjugation energy (Econj), four-body torsional angle strain energy (Etors), Coulomb interaction (ECoulomb), and nonbonded van der Waals interaction (EvdWaals). The ReaxFF MD simulation was carried out using the ReaxFF–CHONSSi force field in the Amsterdam Density Functional (ADF) software package. The constructed petcoke model was located at the center of the 146 × 146 × 146 Å cell. The bond order cutoff was selected as 0.3 for graphs and 0.001 for valency angles and torsion angles. The temperature was 3000 K to ensure that the reaction would be completed within an acceptable time. The time step was selected as 0.25 ps to better observe the transformation of S and N elements. To explore the relevance and regularity between petcoke gasification productivity and the CO2/H2O ratio, the simulations were carried out at 3000 K with the ratio of CO2 and H2O being its only variable (CO2/H2O ratio was 3:7, 4:6, 5:5, 6:4, and 7:3).

Figure 1 presents the separation and dispersion of the petcoke particle and its fragments under different CO2/H2O ratios, as exemplified by the 5:5 ratio. To exclude the interference of CO self-reactions, CxOy molecules such as C2O2 were ignored.Figure 2a–e shows the evolution of the total molecular number and mass of the raw materials (CO2 and H2O) and gaseous products. A dramatic increase in gaseous products was observed before their generation rate slowed down, while the total number of CO2 and H2O molecules exhibited an opposite behavior. This tendency was closely connected with the decomposition of H2O. It was found that a considerable amount of Hx and HxOy (Figure S1) molecules was produced at the early stage of gasification due to the rapid decomposition of H2O under extremely high temperatures. These molecules were consumed quickly, resulting in a decelerated generation of all gas products. Consequently, as shown in Figure 2a–e, a tendency of gentle decline emerged after the rapid growth in product quantity. After this smooth transition stage, many smaller product molecules appeared due to the sufficient decomposition of petcoke particles aided by abundant reductive agents. Hence, the number of gas molecules increased at the final stage, despite the consumption of reductive agents. Meanwhile, both the raw material and gaseous product curves moved down. The total gas product molecules decreased from 1500 (Figure 2a) to less than 1000 (Figure 2e) at the end of the simulation. Such results were similar to those in the previous report in which adding CO2 into the CO2/H2O mixture was found to be capable of decelerating the gasification reaction. As shown in Figure 2f–j, the tendency of the single molecular mass curve was identical to that of the molecular number. Differently, as the CO2/H2O ratio increased from 3:7 (Figure 2f) to 7:3 (Figure 2j), the CO2/H2O curve increased owing to the larger molar mass of CO2. At the final stage of gasification, the molecular masses of all groups were close to each other (all reached 12000), while the molecular number decreased as the CO2 content in gasification agent increased. Therefore, it could be inferred that the increase of the CO2/H2O ratio would generate a higher carbon content in gaseous products. Such a phenomenon is adverse to the petcoke gasification process because products such as CO, CH4, and H2 (syngas) are favorable for energy applications. Figure 3 demonstrates the comparison of molecular number and molecular mass during gasification aided by H2O, CO2, and their mixture. The positive effect that H2O has on gasification is further confirmed. Figure 3a,c shows that, as the reaction proceeded, the number of nongas molecules increased while their molecular mass decreased, indicating the decomposition of the petcoke molecule. Noticeably, in Figure 3a,c, the CO2/H2O gasification curves were located in the middle of H2O gasification (CO2/H2O ratio 0:10) and CO2 gasification (CO2/H2O ratio 10:0). This phenomenon is further demonstrated in Figure 3b,d. The curves for CO2/H2O gasification moved up as CO2 content increased. Although all seven groups reached an equilibrium state in the last 10 ps of the reaction, Figure 3c indicates that the more H2O participates in gasification, the faster the equilibrium state was achieved. For the H2O gasification system, the reaction was almost completed at 170 ps. Moreover, Figure 4 depicts the distribution of gas, tar, and coke during different gasification stages. It was clear that a higher CO2/H2O ratio resulted in an incomplete gasification extent. The gas content dropped from 66 to 57% while coke multiplied by over two times as the CO2/H2O ratio increased from 3:7 (Figure 3a) to 7:3 (Figure 3e). Based on the findings mentioned above, it could be verified that H2O was more helpful in boosting the extent of gasification.

The molecular number of S/N-containing gas and main product species (CxHyOz and CxHy) are presented in Figure 5. In all five graphs, the number of product molecules grew over time. Despite the CO2/H2O ratio, the molecular number of all four major product types ended up close. The most produced molecule species was CxHyOz (70–80 molecules), and the least was CxHy (∼10 molecules), while the final numbers of S- and N-containing molecules were around 30 and 20, respectively. This implied that desulfurization and denitrification reactions could complete within 250 ps regardless of CO2/H2O ratio. Nevertheless, from Figure 5a–e, the initial appearance of S-containing gas seemed to be delayed as the CO2/H2O ratio increased. Figure 5a (CO2/H2O ratio 3:7) shows that desulfurization was half completed at 150 ps and approached equilibrium at 200 ps. In stark contrast, in Figure 5e (CO2/H2O ratio 7:3), the number of S-containing molecules only started to increase prominently after 150 ps. In the other three groups (CO2/H2O ratios 4:6, 5:5, and 6:4), S-containing products appeared between 80 and 100 ps. It could be concluded that the generation rate of S-containing gas was slower when the CO2/H2O ratio was 7:3 compared to when it was 3:7. Figure 6 shows the positive effect of H2O on the evolution of S-containing gas as CO2/H2O gradually increased. When the CO2/H2O ratio was 3:7 (Figure 6a), a considerable amount of four kinds of S-containing gas species (CxHyOzS, CxOyS, HxS, and CxHyS) were generated before 100 ps. Comparatively, in Figure 6e, none of the S-containing species exceeded 5 molecules before 150 ps under a CO2/H2O ratio of 7:3. Apart from accelerating the generation of S-containing gas, the decrease of the CO2/H2O ratio also influenced the percentage of the H-abundant HxS species. The number of HxS produced at the end of the reaction was 17 and 10 as the CO2/H2O ratio increased from 3:7 to 7:3. From Figure 6a–c, the HxS curves exhibited a dramatic downward trend but remained unchanged in Figure 6d,e because of the limited S atoms (47 in total). This indicates that the predominance of HxS species in S-containing products was replaced by the high carbon-containing CxOyS. Therefore, the higher the CO2/H2O ratio, the more the amount of high carbon-containing gaseous products. Figure 7 presents the change of N-containing gas under different CO2/H2O ratios. In stark contrast to S-containing gas, the amount of N-containing gas had a mere correlation with the CO2/H2O ratio. From Figure 5 and Figure 7, it was found that the initiation of the denitrification reaction was not delayed as desulfurization was. From Figure 7a–e, N-containing gas emerged around 75 ps. Furthermore, no obvious fluctuation in the generation rate of N-containing gas was observed as the CO2/H2O ratio increased from 3:7 to 7:3. The major existing form of N element in the product was CxHyOzN (x included 0). Not less than 5 CxHyOzN molecules were produced at 250 ps in all five groups, with the highest being 14 (Figure 7a), which dramatically surpassed the production of HxN. Notice that the petcoke model applied in this research was limited in scale, especially for the N element (only 24 N atoms in total). Consequently, detailed regularity for the transformation of the N element was difficult to observe. The above findings imply that in petcoke CO2/H2O gasification, a low CO2/H2O ratio is preferred for S removal. A high H2O proportion in the gasifier helps to improve the extent of gasification and the conversion of environmentally hazardous S, but the CO2/H2O ratio has a mere influence on N removal.

To obtain a comprehensive understanding of the synergistic effect of the CO2/H2O mixture in petcoke gasification, the reaction path needs to be analyzed. To ensure an equal influence of H2O and CO2, results of 5:5 CO2/H2O ratio simulation were chosen for the mechanism exploration. Figure 8 demonstrates the distribution of all S-containing gas produced in the simulation. The removed S atoms mostly existed in the form of CxOyS, HxS, and CxHyOzS under the reductive gasification environment. COS, H2S, and CHOS dominated among all the S-containing products produced in gasification. From Figure 8b, it was found that both the number of S and HS decreased after reaching their peaks. Differently, the number of H2S increased rapidly at the last 30 ps of the reaction. Such a difference indicates that the generation of H2S involved S and HS. Additionally, in Figure 8b, the product S emerged dramatically ahead of HS, implying the transformation from S to HS. The inferred relationship among S, HS, and H2S could be expressed as eq 2. Figure 8c shows that the COS (Figure 8c) fluctuated several times. It was determined that the conversion from COS to CHOS (eq 3) was responsible for the fluctuation of COS and the rapid increase of CHOS. During carbon-based material gasification, pyrolysis often takes place before or concurrent with other reactions involving gasifiers. Large raw material particles undergo complicated pyrolysis reactions and break down to produce tar, inorganic gas, and other volatile products. In petcoke gasification by the CO2/H2O mixture, the S element was found to detach first in much smaller molecules and via pyrolysis. Under high temperatures, these S-containing small molecules continued to decompose and produce S atoms. This process paved the way for the production of major S-containing gas such as CHOS, COS, and H2S because S atoms could easily react with CO and H. As is shown in Figure 9a, in a typical formation of CHOS, large S-containing particles broke down gradually before the eventual detachment of thiophene-S at 124 ps. Shortly after 15.25 ps, the S atom was reduced by the CO molecule to form COS. At 153.25 ps, the COS molecule was transformed to CHOS by H in the H2O molecule. The COS molecules involved in this process acted as both the major product of gasification and an important intermediate to generate CHOS. Similarly, during the formation of H2S presented in Figure 9b, a single S atom was generated during pyrolysis. The S atom combined with CO to form COS at 136.5 ps. At 141.75 ps, the CO in COS was substituted by the more competitive H atoms to produce HS and, finally, H2S. Nevertheless, in Figure 10, N atoms were distributed in various kinds of gas molecules including CxOyN, H1–2N, H0–1CN, CHON and NO. The variety in N-containing products stemmed from the distinctive N transfer behavior compared with S. In Figure 10a, the predominant CON presented a rapidly increasing trend from 150 to 200 ps. There were 8 CON molecules at the end of gasification, while no more than 3 molecules were produced for C2ON, C2O2N, and CHON each. However, the evolution of N-containing products such as CON and CHON did not show the similar correlation with each other as COS and CHOS did. For example, in Figure 10c, CHN was first generated at around 140 ps, and its quantity began to decline at 220 ps. Nevertheless, the number of CN experienced 2 peaks and maintained an increasing tendency at 250 ps. Therefore, it could be speculated from Figure 10 that pyrolysis and mutual conversion were attributed to the variety of N-containing products rather than the reductive CO, H, and OH. And because the number of CON far exceeded other N-containing gas products, most N atoms would eventually exist in the form of CON. The detailed N removal path is shown in Figure 11. The complexity of N transformation varies among each path. In Figure 11a, CON detached from petcoke particle as a whole at 118 ps. It was quickly stabilized with the aid of CO and O within 8.25 ps. However, in Figure 11b, distinct from the generation path of H2S (Figure 9b), H2N appeared directly via pyrolysis. Instead of combining one H atom to form NH3 molecule each time, H2N reacted with OH to produce HN. Then the HN reacted with O2 and C2O successively to produce CON. Furthermore, Figure 11c depicts a much more complicated denitrification path. Most of the main N-containing gaseous products were involved, including HCN, CHON, CON, CO2N, etc. In detail, the N atom came out in the form of HCN. The HCN reacted with HO and H successively. During the transformation process, CH2ON, CHON, and CON were generated. At 242.5 ps, the CON molecule reacted with another gasification product, CHO2, and was finally converted into CN. The denitrification paths could also be expressed as eq 4–6. Overall, CO2/H2O ratio impacts on desulfurization more than denitrification. This phenomenon can be explained by two main factors. On the one hand, the S-removal path was simpler than N-removal due to its dependence on the reductive CO and H. S element mostly detached from petcoke via pyrolysis as a single atom. Then the S atoms reacted with the reductive CO and H. As a result, almost all the S-containing gas products belonged to one of the CxOyS, CxHyOzS, or HxS or species. In comparison, single N atoms were merely observed to be separated from the petcoke particle. N-containing products such as CON could directly originate from petcoke pyrolysis. Besides, through reacting with CO, H and other gasification products, including CHO2, denitrification products could be converted from one to another. On the other hand, major gasification agents could react with each other. As gasifiers, instead of directly participating in the reaction, the CO2/H2O mixture aided petcoke gasification by generating the reductive CO, Hx, and OH. Interactions among these agents could not be neglected. Two typical reactions were the water gas shift reaction (eq 7) and the steam-gasification reaction (eq 8). In both reactions, the active H2O was reduced to produce the reductive H2, while CO2 acted as the product. Consequently, the more the H2O, the more the reducing agents were generated. According to the previous analysis, H/H2 was able to react with COS to generate H2S or CHOS during desulfurization (Figure 8b). Hence, the increase in H2O content could generate the increase of Hx and therefore accelerate the S removal process.

The regularity of petcoke CO2/H2O gasification under different CO2/H2O ratios was explored via ReaxFF MD simulations. The simulation results revealed the influence of the CO2/H2O ratio on the gasification, desulfurization, and denitrification extent. A higher H2O content in gasifiers dramatically facilitated gasification and desulfurization, while denitrification was not significantly influenced. By increasing H2O content, the remaining coke in the product was reduced from 28.5 to 12.7%, and the gas yield increased from 56.8 to 65.6%. For desulfurization, thiophene-S experienced more than 100 ps to detach from solid particles in the form of S atoms. The single S atoms combined with either CO or H to produce COS and HS. The abundant HS and COS were further transformed into H2S and CHOS by the reductive H. The denitrification mechanism was found diversified. Apart from reacting with CO2 and H2O, interconversion among N-containing products occurred frequently. With the aid of H and CO, CHON was transferred into CON and H2N into HN. Non-N/S-containing gas such as C2O and CHO2 could also react with NO and CON to produce CON and CN, respectively. Direct reduction of N-containing gases by H and CO was infrequent. Consequently, the N-removal process was little affected by the CO2/H2O ratio and the composition of N-containing gas was complicated. The results of this study provide constructive guidance for the future application of petcoke gasification with mixed agents.