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Fossil Fuels
Reductive Gaseous (H2/NH3) Desulfurization and Gasification of High-Sulfur Petroleum Coke via ReaxFF MD Simulations Qifan Zhong, Yu Zhang, Sharmin Shabnam, Jin Xiao, Adri C.T. van Duin, and Jonathan P. Mathews Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01425 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019
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Energy & Fuels
Reductive Gaseous (H2/NH3) Desulfurization and Gasification of High-Sulfur Petroleum Coke via ReaxFF MD Simulations Qifan Zhong*,†,‡,, Yu Zhang‡,§,, Sharmin Shabnamd, Jin Xiao*,†, Adri C. T. van Duind, Jonathan P. Mathews‡ †School of Metallurgy and Environment, Central South University, Changsha 410083, China ‡ EMS Energy Institute, and Leone Family Department of Energy and Mineral Engineering, & dDepartment of Mechanical and Nuclear Engineering, The Pennsylvania State University, University Park, PA 16802, United States; §College of Geoscience and Surveying, China University of Mining & Technology, Beijing 100083, China. ABSTRACT: The use of high sulfur petroleum cokes (petcoke) as raw material in the carbon industry requires an effective desulfurization process. Hydro-desulfurization (HDS) and NH3 gaseous desulfurization are the most effective approaches. However, the S/N removal and the gasification ability of hydro- and NH3 desulfurization have not been well explored. Here, petcoke transitions were examined using the reactive force field (ReaxFF) simulation approach at a constant volume and temperature (3000 K for 250 ps). The S/N removal and transformations in HDS were: thiophenic-S → C1-4S, H1-2S → H2S; pyrrolic and pyridinic N → C1-4N → HCN. Given the large H2 production from NH3 decomposition, the S/N removal and transformations in NH3 desulfurization were similar to that of HDS. However, NH-compounds (NH3) directly bonded with C atoms in petcoke, adding to the coke yield but adding an additional heteroatom challenge with utilization after NH3 desulfurization. The produced C1-4N (CN, mostly) and H2, was transformed into HxC1-4N (080 wt.%), very low ash (< 1 wt.%), and acceptable (low) S content10. However, with the decreasing quality of crude oil, more petcoke are produced with high S content, which also results in a fragile and porous structure11 that can be easily crushed. This would increase the coal tar pitch requirements (adhesive use)12 and is thus uneconomic as a high mechanical strength is necessary13. Thus, high-S petcoke these
authors contribute equally. 1
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cannot be used as a feedstock for anode production because of its high-S content and poor mechanical properties. Currently, there is no economic desulfurization approach for high-S petcokes that reach the desired levels3, 6, 8. Hydro-desulfurization (HDS) has been used for desulfurization and gasification in oil refining for decades14, 15. Sulfone, thioether, mercaptan, and other simple thiophenes are removed effectively16, 17. Hydrogen has also been explored for petcoke desulfurization where the contact efficiency (hydrogen to S) and the reaction temperature are the key factors6, 11, 18. Powdered petcoke and reaction within a fluidized bed can both improve the desulfurization rate and content efficiency. The optimum temperature for HDS of petcoke is ~1000 K17. Zhong et al.11 found that ~80% of the S removal occurred within 120 min for Qingdao petcoke (with 6.5 wt.% S and particle size being 10,000 atoms) in a reactive simulation based mainly on bond length/bond order.42-45 Equation 1 shows the energy potential calculation. Esystem = Ebond + Eover + Eangle + Etors + ECoulomb + EvdWaals (1) The accuracy and products of the ReaxFF simulation directly depend on the force field file. Here, the ReaxFF-CHONSSi force field38 was used in the ADF software package with similar settings for comparison with previous work40. A 0.3 bond order cutoff was used for molecule recognition and 0.001 bond order cutoff for valency and torsion angles in the NVT system. The cell sizes were 130 × 130 × 130 Å with densities of ~0.008 and 0.067 g/cm3 gases (~5,000 H2/NH3 molecules) for H2 and NH3 respectively. The high gaseous density/pressure was used for accelerating reactions and providing gas-rich environments40. Before the reactive simulation, these systems were energy-optimized using the conjugate gradient method and equilibrated via low-temperature (100 K) ReaxFF simulations (time step of 0.25 fs) to prevent chemical reactions from occurring during equilibration. Then, the temperature of these equilibrated systems was ramped up to react temperature (2000, 2500, and 3000 K) at a rate of ~50 K/ps with the system states. We applied ReaxFF simulations with a time step of 0.25 fs and prevented chemical reactions from occurring during equilibration36, 37, 42. To accelerate the reaction kinetics, simulation temperatures were set higher than typical reaction conditions (usually ~ 1073 K)46. The temperature was controlled using Berendsen thermostat. The reactive simulations were performed for 250 ps with a time-step of 0.25 fs (Supplementary Material, Fig. S1) and damping constant of 100 fs. With the same simulation strategy, the S/N removal mechanisms could be summarized and compared in different reaction conditions such as pyrolysis38, combustion40, 3
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gasification41, and etc. The brief description of force field parameters used were explained in previous work9, 38, 40, 41, and the codes used for simulation and result analysis were shown in the manual47. 3. RESULTS AND DISCUSSION 3.1. Comparison of decomposition behavior for simulations at 2000, 2500, and 3000 K As shown in Fig. 1a, ~40 additional molecules were present at the end of 250 ps at 2000 K. Upon heating to 2500 K, ~100 additional molecules were present at the end of the simulation. However, when the temperature increased to 3000 K, over 800 molecules were present at 250 ps. A large number of molecules decomposed in the 2500 K and 3000 K simulations. At the end of the 250 ps simulations, the total number of molecules increased to ~800 and ~4000 at 2500 K and 3000 K respectively. This difference is mostly caused by ammonia decomposition (Equation 2)11, 18. 2NH3 ⇌ N2 + 3H2 (2)
Figure. 1. Total number of molecules in a) HDS and b) NH3 desulfurization ReaxFF simulations for 250 ps at 2000, 2500, and 3000 K. 3.2. HDS ReaxFF Simulation As shown in Fig. 2, petcoke model was subjected to ReaxFF simulations for 250 ps at 2000, 2500, and 3000 K with ~5,000 H2 molecules. Fig. 2a shows that the C5+ molecules changed after 250 ps. Few C5+ molecules were left at the end of the 250 ps simulation, indicating that much more abundant conversion occurs at 3000 K simulation. By contrast, no extensive HDS reactions occurred in the 2000 and 2500 K ReaxFF simulations, as shown in the snapshots in Figs. 2b–c. The simulation at 3000 K will be discussed in the following parts.
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Energy & Fuels
Figure. 2. Snapshots of the petcoke model in HDS ReaxFF simulations: a) 0 ps to 250 ps at 3000 K, and 250 ps at b) 2000 K at 250 ps, and c) 2500 K at 250 ps with C5+ molecules shown in balls (C atoms are green, H grey, O red, N blue, and S yellow). In the pyrolysis simulations of coal48-50 or petcoke38, 40, compounds with
