Gareth D. Mitchell

Behavior and effect of different coal microlithotypes during gas transport for carbon dioxide sequestration into coal seams

Abstract The behavior and effect of different coal lithotypes during sequestration of carbon dioxide (CO2) into coal seams are important knowledge gaps when modeling sequestration processes and planning for field application. This paper presents the results of a laboratory study for sequestration of CO2 in a 2.5-cm diameter Pittsburgh coal sample. During the test, the sample was kept under a constant effective stress during gas uptake and CO2 storage was observed by qualitative and quantitative X-ray computerized tomography (CT) scanning. Petrographic analysis was also performed on the sample after the sequestration test to identify the microlithotypes showing different adsorption behavior. Qualitative X-ray computerized tomography (CT) and petrographic analysis showed that different microlithotypes behave differently during gas adsorption. Vitrite was observed to swell and to decrease the initial density about 5%, when CO2 diffused into the micropores. A density increase was observed in other microlithotypes present in the sample due to gas adsorption. The CT images were quantified to map the temporal and spatial variation of mass change of coal due to CO2 adsorption. These data were used to study the equilibrium isotherm parameters and to investigate the kinetics of the adsorption process for calculating the diffusivity of CO2 in different microlithotypes. Quantitative analysis of X-ray CT images indicated that the regions that were rich in inertite and clay stored higher quantities of gas in the adsorbed phase compared to other organic microlithotypes in the sample. Calculations for adsorption kinetics in different microlithotypes indicated that pore diffusion is the prime mechanism. However, only an order of magnitude difference between pore and surface diffusion coefficients suggested that both phenomena might be important for CO2 sequestration process. The calculated pore diffusion coefficients were higher in clay and inertinite regions. But, surface diffusion started to increase in vitrinite- and liptinite-rich regions, where pore diffusion was limited.

The character and origin of lacustrine source rocks in the Lower Cretaceous synrift section, Congo Basin, west Africa

Lower Cretaceous synrift lacustrine shales from the Congo Basin, west Africa, have been analyzed with sedimentological and geochemical techniques to characterize source rock quality and identify triggers for deposition of intervals richest in organic carbon. The sequence includes a lower active rift section, deposited during active faulting and subsidence, overlain by an upper late rift section, deposited during reduced faulting and subsidence.Total organic carbon (TOC) averages 2–3 wt.% throughout the active rift siliciclastic shale section, 6% in marls in the lower part of the late rift section, and 1–2% in deltaic shales in the upper part of the late rift section. Organic matter consists of mixed types I and III kerogen in the active rift shales, pure type I kerogen in the late rift marls, and a type I and III mixture in the late rift deltaic shales.Redox proxies indicate that the deep lake was relatively reducing throughout deposition of the active rift and lower late rift sections. Therefore, enhanced anoxia did not trigger deposition of the richest source rocks. Decreased sedimentation rates in the late rift do not account for the full increase in TOC nor the shift on organic matter type. The richest source rocks are associated with high rates of organic productivity and chemical sedimentation, indicating that flux of dissolved components to the rift lake, including nutrients for algae growth, was critical. We propose that reduced topography associated with the late rift was necessary for efficient cycling of plant-derived carbon into soil carbonate and ultimately the rift lake, and for enhancing chemical weathering and nutrient flux.

Residual carbon from pulverized-coal-fired boilers. 2. Morphology and physicochemical properties

The morphology and bulk physicochemical properties of residual carbon in eight fly ash samples from commercial power plants were investigated. Enriched carbon samples extracted from the bulk fly ash were characterized by high-depth-of-field optical microscopy, reflected-light microscopy, scanning electron microscopy, elemental analysis (C, H, O), and CO2 adsorption. The crystalline structure of the carbon was characterized by X-ray diffraction, optical reflectance, and high-resolution transmission electron microscopy fringe imaging. The results were compared with measurements on laboratory-generated chars in the early-to-intermediate stages of combustion. Compared with those chars, the residual carbon is of similar elemental composition, petrographic composition and surface area but higher crystallinity. The fuel-related mechanisms that can contribute to carbon carryover in boilers are discussed, including inertinite persistence, mineral matter encapsulation and char deactivation by pregraphitization, as well as the implications for utilization of residual carbon.

Deformation metamorphism of bituminous and anthracite coals from China

Abstract Tectonic displacement of coal seams in China has resulted in faulting parallel to coal bedding. Displacement along these faults caused significant comminution of the coal on the footwall contributing to various mining problems, the worst of which is catastrophic failure, or “outbursting” of the working face during mining. The granular texture and mostly unconsolidated nature of the coal suggests that faulting occurred relatively late in the coalification sequence, at a time of maximum tectonic stress. Coal samples taken on either side of the fault plane (normal and deformed coal layers) were obtained in an effort to establish what influence these tectonic stresses might have had on coal properties as well as what they might reveal about the influence of tectonic pressure on organic maturity. Sample sets were collected within coal beds from undisturbed and adjacent deformed layers, including 21 bituminous samples from the Pingdingshan coal field and nine anthracite samples from the Jiaozuo coal field, the Tieshenggou coal mine of the Yuxi coal field in Henan province, the Beijing Xishan coal field, the Baijiao mine of the Furong coal field in Sichuang province and the Baisha coal field in Hunan province, China. Results from vitrinite reflectance, proximate and ultimate analyses show some differences in reflectance, hydrogen content and nitrogen content of anthracite coal. No significant difference was found between volatile matter yields of normal and deformed coal specimens. GC measurements of the saturated hydrocarbon fraction of chloroform extracts from bituminous coals showed that lower molecular weight carbon fragments were concentrated in the deformed samples. Therefore, although changes in the gross chemical properties of the deformed coal were insignificant, some modification of the chemical structure is seen to have occurred as a result of exposure to tectonic pressure.

The nature of solids accumulated during solvent refining of coal

Abstract Solids accumulated in the reactor of a solvent-refined coal (SRC-1) pilot plant during processing of three coals were studied using optical microscopy and X-ray diffraction. A classification system was devised for each of the two groups of components: organic materials and mineral matter. The various organic components were classified by the extent of change from the original coal macerals, and by optical properties of different semi-cokes and other organic phases. Minerals were divided into four groups: those unchanged from the original coal; minerals which were physically degraded; minerals which were chemically or crystallographically transformed; and those minerals formed during processing of a subbituminous coal. Gold-tube carbonization experiments were performed on SRC to determine the conditions under which retrogressive reactions occur to form mesophase semi-coke. Autoclave experiments were designed to investigate the recrystallization of pyrite as pyrrhotites, and to determine the causes of carbonate-mineral formation in the reactor. Calcium carbonate was found to crystallize from the interaction of ion-exchangeable calcium and carbon dioxide, which are available when low-rank coals are processed.

Swelling pretreatment of coals for improved catalytic liquefaction

Abstract Two coals, a Texas lignite and a Utah high volatile C bituminous, were used to examine the effects of solvent swelling pretreatment and catalyst impregnation on conversion behaviour at 275 °C, representative of the first, low-temperature stage in a temperature-staged liquefaction reaction. Iron(II) sulphate, iron pentacarbonyl, ammonium tetrathiomolybdate, and molybdenum hexacarbonyl were used as catalyst precursors. Without swelling pretreatment, impregnation of both coals increased conversion, mainly through increased yields of preasphaltenes. Methanol, tetrahydrofuran, tetrabutylammonium hydroxide, and pyridine were used as swelling agents. In the absence of catalyst, swelling the lignite before reaction improves conversion by enhancing oil and gas yields; the effectiveness of the solvents in enhancing conversion is in the same order as their swelling ratios. Swelling with methanol or pyridine has little effect on reaction of the bituminous coal, but both tetrahydrofuran and tetrabutylammonium hydroxide treatments increase conversion as a result of higher preasphaltene yields. The combined effect of catalyst addition and swelling enhances conversion, as much as two-fold, of the lignite and increases yields of all products. On the other hand, little benefit was obtained by combining catalyst addition and swelling for the bituminous coal, though it is possible to effect changes in the relative amounts of the various products. Tetrabutylammonium hydroxide not removed from the coal after pretreatment appears to decompose to the good solvent tributylamine, suggesting the possibility of using swelling agents as ‘solvent precursors’, first swelling the coal and then thermally decomposing to a strong solvent that is emplaced inside the coal. The action of iron pentacarbonyl is sensitive to the reactive gas atmosphere used; in hydrogen it decomposes to an oxide that mainly facilitates hydrogenation of the heavy products to lighter oils, whereas in hydrogen sulphide/hydrogen mixtures the iron pentacarbonyl is sulphided and mainly facilitates depolymerization of the coal to heavy products. This finding suggests the possibility of tailoring the behaviour of the catalyst by appropriate selection of catalyst precursor and reactive atmosphere.

Optical properties of coals and liquefaction residues as indicators of reactivity

Abstract The trend of vitrinite fluorescence intensity within the bituminous rank range, peaking in high volatile A coals, is thought to be due mainly to the development of the mobile or molecular phase with thermal maturation. A strong parallelism exists between fluorescence intensity, solvent yield and the thermoplastic behaviour of coals and hydrogenated coals. Determination of the reflectance distribution of hydrogenated coals can provide a measure of competing hydrogenation and retrogressive reactions in catalysed and non-catalysed runs, conducted both dry and with a solvent vehicle. The type of vitroplast generated and the anisotropy of the unextracted products give further insight into the size and mobility of the molecules generated by hydroprocessing. The relative reactivities of vitrinite and sporinite were reversed at different rank levels. The alteration of fluorescence intensity with time is another rank-dependent property; the phenomenon is seen clearly to be due to photochemical oxidation on irradiation. Highly weathered bituminous coals respond in a unique manner, similar to those of low rank coals which lack the negative alteration component due to the presence of the mobile phase. This may be due to the creation of absorption centres and enhanced molecular rigidity through cross-linking reactions.

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.

The microscopy of mesophase formation and of anisotropic cokes produced from solvent-refined coals

Thermal processing of coal and coal products can lead to the formation of ordered carbons through the mesophase mechanism. The performance characteristics of metallurgical coke may be improved by the extent of this ordering; however, in the direct hydroliquefaction of coals the objective is to make species of low molecular weight, and coking is an undesirable retrograde reaction. This study is concerned with fundamental mechanisms of coke formation and their roles in these two processes.

THE FLUOROMETRIC CHARACTERIZATION OF ASPHALTS

ABSTRACT Fluorescence microphotometry has been used as a technique for the characterization of asphalt binder quality and the deterioration of asphalt cements. The technique employs optical microscopy for the in situ identification of asphalt binder from aggregate materials in cements, thereby eliminating the need to solvent extract the binder for evaluation. It may also obviate the possibility of solvent interaction or interference with the physical and chemical properties of asphalt binders that have developed in-service. In this research effort a standard technique for the measurement of fluorescence intensity has been developed and applied to the characterization of twelve raw asphalts. These studies were first necessary before undertaking the more difficult task of measuring asphalt binder fluorescence in cements. Analytical techniques and some of the factors which influence asphalt fluorescence have been considered. With this small sample set, relationships appear to be emerging between original asp...