Advances in coal analyses and simulations 1989-2012

There are periodic publications that address advances in coal science. This continues that fine tradition and focuses on the author's views of the more significant advances in analytic techniques (chemical and physical) and molecular based simulation that has expanded our ability to quantify coal properties and explore behavior. During the last two decades (plus a few years) the rationalization of coal chemistry has been considerably expanded by: the additional quantification of solid state ¹³C NMR, the quantification of the lattice fringe views from coal HRTEM micrographs, evaluation of molecular weight distribution by laser desorption ionization (LDI) mass spectroscopy (MS), and the inclusion of structural diversity via the Fourier transform ion cyclotron resonance (FT-ICR) MS approach for coal extracts. Specifically, the Solum et al. combined dipolar dephasing coupled with CPMAS NMR to produce 12 carbon structural parameters was a significant advance in enhancing comparison of coals structural features. With lattice fringe extraction techniques coal HRTEM micrographs went form a hazy micrograph to the elucidation of the distribution of the aromatic lattice fringes, their size, orientation, and clustering across the rank range in coal products. Laser desorption ionization mass spectroscopy has yielded further insights into the molecular weight distributions of coal. Electrospray ionization (ESI) FT-ICR MS is expanding our understanding a coals' compositional and structural diversity by directly determining the elemental compositions of the ions of coal extracts (by accurate mass measurement alone) and capturing/visualizing the high resolution MS data with Kendrick mass defect plots vs. nominal Kendrick mass plots, heteroatom class consideration, color isoabundance plots, and van Krevelen diagrams. Physical properties and coal behavior are also further quantified by HRTEM lattice fringe images, porosity evaluation through small angle X-ray (or neutron) scattering (SAXS/SANS) approaches, and X-ray computed tomography evaluations. Specifically the HRTEM lattice fringes shows the distribution of the stacking and orientation of aromatic fringes aiding quantification of aromatic arrangements on the atomic scale. Through SAXS and SANS, particularly at U.S. National Laboratory user facilities, with contrast matching approaches the behavior of coal with gases and solvents has been elucidated. For example the pore filling of CO₂ in sequestration related studies has been captured as has changes to the pore structure with in-situ devolatilization or gasification. X-ray computed tomography has allowed the non-destructive evaluation of the physical structure of coal with resolutions in the 35 μm range with the promise of significantly higher resolution. This has permitted 3D evaluations of: the cleat structure, quantification of coal swelling/contraction with addition/removal of gases and solvents, determination and anisotropy of strains, kinetics of gas uptake, coking transitions, and impacts of microwave exposure on the cleat network. Analytical advances have resulted in the ability to quantify coal data and further constrain molecular representations. Restricting models to the average NMR parameters, while capturing a portion of the structural diversity has resulted in new computer aided tools for structure creation and evaluation. Several tools have evolved such as SIGNATURE (Stochastic generation), computer aided molecular design, with advances in the evaluation of the resulting structure (POR-pore size distribution), NMR evaluations, pair distribution modeling, etc. Building on these approaches the state-of-the-art couples HRTEM analysis (diversity of fringe lengths and orientations) to directly construct constrained molecular representations (Fringe3D) which can be manipulated in 3D structures further constrained by: multiple NMR parameters, FT-ICR data (diversity of heteroatom classes for example), SAXS and SANS data for pore size distributions (limited to a limited extent by the scale of the representation). Automated approaches have expanded the scale of representation with greater accuracy and improved ease with improvements in communication of structural information via 3D and 2D lattice structures. Thus, there has been increased utility and applicability for molecular modeling and analytical advanced to the rationalization of coal science.