Improved generation of large-scale atomistic representations and pyrolysis/combustion simulations of Illinois coal and coal char using the ReaxFF reactive force field.

摘要

A highly automated molecular generation approach was implemented and coupled with reactive force field methods to create a new computational capability that enabled the investigation of structural transformations and chemical reactions involved in coal pyrolysis and char combustion. The present work demonstrated the applicability and utility of this new computational capability for examining at the molecular level the complex chemistry associated with coal pyrolysis and char oxidation and combustion. In this investigation, Illinois no. 6 Argonne Premium coal, the world’s most well-studied coal, was evaluated using atomistic representations of both the coal and the coal char created for this purpose. Orientation and stacking issues were also explored utilizing molecular representations of several Argonne Premium coals and an anthracite coal.;These analytical data from the literature were used to construct a large-scale coal molecular model based on an improved automated construction approach in an effort to move toward capturing the continuum structure over a large scale. The model contains 50,789 atoms within 728 diverse molecules and is the largest, most complex coal representation constructed to-date. The aromatic ring size distribution was based on multiple previously published high-resolution transmission electron microscope (HRTEM) lattice fringe micrographs and was duplicated with automated construction protocols (Fringe3D) in molecular modeling space. Additional structural data was obtained from the abundant literature assessing this Argonne Premium coal. Organic oxygen, nitrogen, and sulfur forms were incorporated primarily into the polyaromatic structures according to XPS and X-ray absorption near-edge structure spectroscopy (XANES) data. Aliphatic carbons were distributed among cross-links and pendant alkyl groups based on the combination of laser desorption ionization mass spectrometry (LDIMS), ruthenium ion catalyzed oxidation, elemental analysis, and NMR data to agree with literature data.;The ReaxFF reactive force field was used to perform pyrolysis simulations at 2000 K on the constructed large-scale molecular model for Illinois coal to examine structural modifications and reactions associated with coal pyrolysis. This high temperature enabled chemical reactions to occur within a practical simulation time. The ReaxFF simulation was performed until about 60% of the cross-links had been disrupted primarily through thermolysis. For this coal pyrolysis was mainly initiated by the release of hydroxyl groups, dehydrogenation of hydroaromatic structures, and by cleavage of heteroatom-containing cross-links. The main pyrolysis products were hydrogen, methyl, ethylene, acetylene, formaldehyde, ethynol, alkylphenols, alkylnaphthalenes and alkylnaphthols, in agreement with experimental observation.;A devolatilized Illinois no. 6 coal char atomistic representation was generated using published HRTEM lattice fringe images and Fringe3D in conjunction with Perl scripts, and coupled with the ReaxFF reactive force field. In this initial work, very high temperatures (3000-4000 K) were selected for ReaxFF simulation under stoichiometric, fuel lean and rich combustion conditions. These elevated temperatures were chosen to observe chemical reactions proceed to completion within a computationally practical simulation time. It is expected that with computational gains longer simulations at more reasonable combustion temperatures could be obtained. The char oxidation process was mainly initialized by either thermal degradation of char structure to form small fragments, that were subsequently oxidized, or by hydrogen abstraction reactions by oxygen molecules and O and OH radicals. A more rapid oxidation and combustion of the polyaromatic structures occurred at fuel lean (oxygen rich) conditions compared with fuel rich combustion. Char transitions included 6-membered ring conversion into 5- and 7-membered rings that further decomposed or reacted with mostly O and OH radicals. (Abstract shortened by UMI.).