Jonathan P. Mathews

Simulation of nanoporous carbons: a chemically constrained structure

Abstract Nanoporous carbons (NPCs) are useful in adsorptive separations and catalysis, owing to their ability to discriminate between molecules on the basis of size and shape. This property arises from their narrow pore size distribution, which is typically centred at a size corresponding to 0.5 nm. Despite this level of nanoregularity. there is no long-range order within these materials. Structural coherence dissipates to extinction at distances longer than 1–1.2 nm. For this reason, these nanoporous materials are complex solids and offer an intriguing problem in structural simulation and modelling. We show that modelling the spatial complexity of NPCs can be overcome by their chemical simplicity. Recognizing that the structures are comprised of trigonal sp2 carbon and imposing chemical and physical constraints on the possible outcomes of the simulation provide a means to surmounting the modelling problem presented by the intrinsic disorder. By this approach, models of the solid can be arrived at that ma...

Particle size dependence of coal volatile matter: is there a non-maceral-related effect?

Abstract Proximate analyses were conducted on different size fractions of pure vitrinite from two bituminous coals and demineralized vitrinite from one of the coals. CO2 surface areas were also determined, to aid in assessing the possible influence of pore diffusion of volatiles in particles of different sizes. The results implied that for comminuted coals, the particle size dependence of the measured volatile matter, after correction for effects of mineral matter, is due almost entirely to differences in maceral composition.

A Review of Microwave Coal Processing

Abstract This paper reviews the application of microwave energy in coal processing and utilisation. It brings together all of the published work on the microwave treatment of coal. The majority of the work was carried out at lab scale, although a few processes have made it to pilot or even industrial scale. The processes with more immediate promise for scale-up are identified, as are those that require further laboratory scale experiments to establish whether microwave processing should be pursued at a larger scale. The importance of dielectric properties of the materials to be processed is emphasized, and the reasons behind the need for a multi-disciplinary approach to the design and operation of electromagnetic experiments are explained. Microwave coal processing is a diverse area, which has the potential to aid in coal upgrading, cleaning and comminution, thus improving efficiency and reducing harmful emissions of coal usage. Other promising research areas include the investigation of microwave processing for coking, liquefaction, enhancing fluid flow in coal beds and coal characterisation. The paper concludes by discussing the barriers to scale-up that these processes face.

A Novel Fluidized Bed Drying and Density Segregation Process for Upgrading Low-Rank Coals

Lignite and sub-bituminous coals are attractive due to their low cost, low emissions, and high reactivity. However, these coals contain large amounts of moisture, which reduces calorific value and lowers plant efficiency. A novel fluidized bed drying process was developed that uses low-grade waste heat to reduce fuel moisture content of low-rank high-moisture coals and concurrently lowers sulfur and mercury content of dried coal through density segregation. This paper discusses quality improvement of low-rank coals by low-temperature thermal drying, describes changes in microstructure of coal particles and describes the reduction in sulfur and mercury via density segregation during thermal drying of lignite in a specially designed fluidized bed.

Breaking the limits of structural and mechanical imaging of the heterogeneous structure of coal macerals

The correlation between local mechanical (elasto-plastic) and structural (composition) properties of coal presents significant fundamental and practical interest for coal processing and for the development of rheological models of coal to coke transformations. Here, we explore the relationship between the local structural, chemical composition, and mechanical properties of coal using a combination of confocal micro-Raman imaging and band excitation atomic force acoustic microscopy for a bituminous coal. This allows high resolution imaging (10s of nm) of mechanical properties of the heterogeneous (banded) architecture of coal and correlating them to the optical gap, average crystallite size, the bond-bending disorder of sp(2) aromatic double bonds, and the defect density. This methodology allows the structural and mechanical properties of coal components (lithotypes, microlithotypes, and macerals) to be understood, and related to local chemical structure, potentially allowing for knowledge-based modeling and optimization of coal utilization processes.

Atomistic modelling insight into the structure of lignite-based activated carbon and benzene sorption behavior

Improved structure–property relationships for activated carbon were obtained by devising realistic, large-scale, structural models. Herein, an improved approach was employed to construct atomistic models of a lignite precursor of activated carbon, based on the high resolution transmission electron micrographs (HRTEM) of pyrolyzed lignite coal, in combination with experimental pore size distribution analysis of tailored lignite-based activated carbon. Benzene sorption was experimentally characterized at 303 and 318 K and resulted in 13–18% mass gain. To model the carbon structure and benzene sorption, we have devised two structures, including either micropores (4–20 A) or micro/mesopores (4–40 A). For the 303 K conditions, the predictions of the two models are consistent with experimental observations. For the micro/mesoporous model, benzene molecules sorbed in both micropores and mesopores, as the mesopores provide access to the internal part of the carbon structure, and benzene molecules would pass readily through these small mesopores to the final sorption sites in micropores of 14–18 A in size. The most favored sorption energy was −37.45 kJ mol−1, with a preferred rotation angle from 20–30°, and a second favored angle from 30–40° relative to the graphene surface. These benzene molecules were aggregated in T-shaped and parallel-displaced configurations, with a separation distance of 5.75 A from the benzene centers of mass to the carbon surface in a monolayer state. The most favored position was found to be parallel to and between two carbon surfaces, especially close to 5- or 7-membered rings.

USE OF MOLECULAR MODELING TO DETERMINE THE INTERACTION AND COMPETITION OF GASES WITHIN COAL FOR CARBON DIOXIDE SEQUESTRATION

We have utilized computational molecular modeling to generate a state-of-the-art large scale structural representation of a bituminous coal of lower bituminous rank. This structure(s) has been used to investigate the molecular forces between the bituminous coal structure (or idealized pores) and the molecular species CH{sub 4} and CO{sub 2}. We have created a new force field for these simulations and are currently carrying out molecular dynamics simulations. An initial step performed is to help define the issues with sequestration utilizing the molecular modeling approach. Once defined advanced molecular modeling techniques can be utilized in investigating sorbent and host behavior.

Reviews of Science for Science Librarians: Coal Science and Technology Research 1970–2010

A literature search utilizing the Web of Science (Science Citation Index Expanded) was used to examine historic and recent publication quantity for scientific and engineering-related coal research. Searches on the term “coal” in the title of English-language journal articles were conducted for the time spans of 1970–2000 for historic data and 2000–2010 for recent activity. The number of journal articles was 25,368 for the years 1970–2010 and 8,860 (35 percent) articles for 2000–2010. The Analyze Results tool was used to identify the most active countries, institutions, and journals.

Structural Modeling of Nanoporous Carbon: A Review of Approaches to Simulating an Aperiodic and Non-Equilibrium Solid

Nanoporous materials as a class are the subjects of intense research at present because they offer the potential for substantially new technologies in areas as diverse as electronics and medicine. Carbons derived by pyrolysis of polymeric precursors are an interesting case in point of a nanotechnological application which is already practiced.’ The separation of air has been done for over a century by brute force, and energetically intensive, method of cryogenic distillation. Zeolites (Li-X) offer an alternative means to producing oxygen via an adsorptive procedure. Nitrogen is preferentially attracted to and held the lithium cations. Nanoporous carbons (NPC) produce enriched nitrogen by a process which is quite different. Experiments show that oxygen is adsorbed substantially faster than nitrogen at pressures from 101 to 10,100 Kpa or more at room temperature.2More recently, membranes consisting of fibrous or supported forms of nanoporous carbon have been shown to display a similar preference to the transport of oxygen over nitrogen.3While much recent work has been done, the fundamental aspects of why oxygen is more rapidly transported than nitrogen remains elusive. As to whether this difference in rate is caused by a lower enthalpy barrier or an increased entropy of transition for oxygen at the narrowest constrictions of the pore structure,4 there is still considerable disagreement.