Article pubs.acs.org/EF
Initial Chemical Reaction Simulation of Coal Pyrolysis via ReaxFF Molecular Dynamics Mo Zheng,†,‡ Xiaoxia Li,*,† Jian Liu,†,‡ and Li Guo*,† †
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *
ABSTRACT: Mechanisms investigation of coal pyrolysis will aid efficient and clean coal conversion and utilization. However, coal pyrolysis is a complex process involving myriad coupled reaction pathways such that the deeper understanding of its mechanism is still limited even with state-of-the-art experimental approaches. In this paper, ReaxFF molecular dynamics simulation was employed to perform simulation of chemical reactions in pyrolysis of a bituminous coal model with 4976 atoms to examine the nascent decomposition mechanisms and product profiles at temperatures from 1000 to 2000 K over a 250 ps simulation period. It is found that more than 900 reactions may occur at the temperature 2000 K within the simulation period with a trajectory output interval of 12.5 ps, and a detailed chemical reaction network was obtained by further analysis of the trajectory using a newly created C++ program. The product profile evolution tendency with temperature observed in the simulation agrees well with what was obtained experimentally in the literature. In addition, the sequence of gas generation was H2O, CO2, CO, C2H6, and then CH4 consistent with experimental observations. We believe that the methodology presented in this paper offers a new and promising approach to systematically build understanding of the complex chemical reactions in thermolysis of very complicated molecular systems.
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INTRODUCTION Coal is one of combustion fuel sources and it plays an important role in the world energy resource structure. In China, about 70% of total energy consumptions are coal in 2009.1 Coal pyrolysis is the nascent decomposition pathway in the coal conversion process which accounts for up to 70% weight loss during coal transformation2 and therefore has a significant impact on coal conversion operations such as gasification, liquefaction, combustion, and carbonization, etc.3−5 Coal pyrolysis is a very complex process involving myriad coupled reaction pathways. The lack of detailed mechanistic understanding of pyrolysis chemistry is due to its complex reaction environment.6 The relevant initial rates and mechanisms are difficult to be determined experimentally since the pyrolytic reactions take place because of the free radical initiation at high temperature in an extremely short period of time, which are hard to detect and replicate in the laboratory.7 Hence it would be useful to explore the initial mechanisms of coal pyrolysis with computational approaches. Obviously the computational method for investigating coal pyrolysis mechanisms should be capable of modeling chemical reactions. Quantum mechanics (QM) modeling is capable of investigating chemical bonding with high accuracy; but unfortunately QM has rarely been applied in larger-scale models that are essential to capture the complexity of coal pyrolysis because QM is a computationally intensive and expensive method that makes it impossible to get a detailed, statistically relevant description of the coal pyrolysis reaction chemistry.8,9 Classical molecular dynamics (classical MD or MD) is applicable for large molecular system modeling but is not feasible for exploring chemical reactions in coal pyrolysis because it only describes © XXXX American Chemical Society
physical elastic collision between atoms with static bonds and fixed partial charges.10,11 In order to bridge the gap between quantum chemistry and classical molecular dynamics, a number of efforts have been dedicated for simulation of molecular systems involving chemical bonding. The Brenner reactive empirical bond order potential, first proposed in 1990,12 is based on covalent-bonding formalism with additional energy terms in force field that allows for accurate description of bond breaking for hydrocarbons. However, the Brenner potential lacks description of nonbonded interactions and is only applicable to hydrocarbons. Even after incorporating parts of nonbonded interactions, the generalized extended Brenner potential13 is still restricted to its application to solid carbon and hydrocarbon molecules and not suitable for coal pyrolysis containing oxygen, nitrogen, and sulfur elements. The Reactive Force Field (ReaxFF), a general bond order14 dependent potential by van Duin et al.,11,15,16 fully addresses the chemistry of dynamic bonds and polarization effects in addition to the conventional bonded and nonbonded interactions in classical force fields. ReaxFF has been proven to be a smooth transition from nonbonded to bonded interactions, providing a new and promising approach for molecular simulation of complex system with reactions. In ReaxFF molecular dynamics (ReaxFF MD), all connectivity-dependent interactions are bond order dependent to ensure that the energy contribution disappears upon bond breaking. Since the bond orders and partial charges of atoms Received: January 24, 2013 Revised: April 14, 2013
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dx.doi.org/10.1021/ef400143z | Energy Fuels XXXX, XXX, XXX−XXX
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Table 1. Small Molecules Contained in the Bituminous Coal Model
are updated in each MD time-step iteration,17 the connectivity of the system continuously changes over time. ReaxFF MD has been successfully applied in a number of process simulations of combustion and pyrolysis for unimolecules such as pyrolysis and combustion of n-dodecane by Wang et al.18 and 6dicyclopropane-2,4-hexyne by Sun et al.19 and also for complicated molecular systems. Salmon et al. including van Duin (the author of ReaxFF)20 recently reported ReaxFF molecular dynamics simulations of a “macromodel” for Morwell brown coal with 2692 atoms and several functional models with dozens of atoms as well, and the simulation reproduced some reactions observed in offline experiments, showing that such computation can be useful in providing molecular based kinetic models for pyrolysis processes. Fidel Castro-Marcano et al.21 coupled ReaxFF molecular dynamics with a very large-scale model of devolatilized Illinois No. 6 coal char in order to
examine the complex chemistry associated with the structural transformations and chemical reactions in char combustion and to discuss the role of sulfur within the model.22 ReaxFF MD allows direct thermolysis simulation for large-scale coal molecular models at high temperatures where the chemical reactions are driven by the energy of empirical reactive force filed. It is an approach closer to real world process in terms of taking account of multimolecular reaction environment of coal pyrolysis. However, the parameters of ReaxFF force field may not be quite accurate for larger coal molecular models as to smaller systems. Further investigation of ReaxFF MD simulation of coal pyrolysis for large-scale models would be helpful to validate its effectiveness and applicability in complex molecular systems. In this paper, the initial chemical reaction simulation of bituminous coal pyrolysis using ReaxFF molecular dynamics B
dx.doi.org/10.1021/ef400143z | Energy Fuels XXXX, XXX, XXX−XXX
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Article
will be presented to examine the nascent decomposition mechanisms and to investigate the effects of temperature on product distributions. First, the coal model construction and overview of the ReaxFF molecular simulation details will be described. Second the simulation results for various temperatures between 1000 and 2000 K will be demonstrated and discussed. Finally the findings will be summarized and conclusions made.
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COMPUTATIONAL METHODS
Coal Model Construction. The chemical structure of coal consists of various bond types and noncovalent interactions and is the basis for understanding the coal pyrolysis process. More than 130 molecular models23,24 have been reported over the last 70 years or so, indicating the complexity of coal’s structure and numerous efforts in capturing its structural features. It is a great challenge to generate a structure that could be used to describe different properties of various coal types. The Wiser model,23 introduced in the 1970s, is considered as a comprehensive and reasonable high-volatile bituminous coal model that also contains potentially reactive functional groups. The Shinn25 bituminous model is perhaps the most comprehensive 1980s representation which shows product structures for both single and two-staged liquefaction as a result of fragmenting the cross-linked initial model. The ReaxFF molecular dynamics simulation presented in this paper will start with the construction of a bituminous coal model based on the combination of Wiser and Shinn models. First, the Wiser and Shinn models were built using ChemSketch,26 and their geometry optimization of the models was carried out using the Materials Studio (MS)27 Forcite Module with the Dreiding force field. The molecular configuration was changed profoundly after they were optimized. The optimized Wiser and Shinn model structures were assembled into a cubic 3D molecular model by employing the construction function of the MS Amorphous Cell Module. By taking into account the evidence from extractability and other experiments,28−30 relatively small molecules (
