Effect of longwall face advance rate on spontaneous heating process in the gob area - CFD modelling

A commercial CFD software programme, FLUENT, was used to study the oxidation process of coal in the mined-out longwall (gob) area. A three-dimensional, single-phase model with a continuously advancing longwall face has been developed. For the model, the gob longwall area was designed on the basis of the actual longwall panel operating in the Ostrava-Karviná Coal Mines (OKD, Czech Republic). The behaviour of the coal to oxygen was modelled using the results arising mainly from the former laboratory-scale experiments with Czech bituminous coals. Basically, the technique of pulse flow calorimetry and measurements at a continuous airflow reactor were applied during the laboratory investigations. In the contribution, the main focus was to understand the effect of the longwall face advancing speed on the oxidation heat production as well as evolution of the gases in the gob area. Simultaneously, the effect of coal crushing in the mined-out area on the spontaneous heating process was examined.Numerical simulations confirmed the existence of a “favourable” zone for the onset and development of the spontaneous heating process in the gob area. The location and the maximal temperature reached in the “favourable” zone were found to be significantly affected by the advancing rate of the coalface. The slower the advancing rate is, the higher the maximal temperature and smaller the depth of the “favourable” zone in the gob area are. When the rate drops to a certain “critical” value, spontaneous heating turns to flammable combustion of the coal. The value of the “critical” advancing rate was confirmed to increase if the grain size of the coal left in the gob decreases. Numerical examinations of carbon monoxide concentrations then proved that small incidents of spontaneous heating could occur in the gob area that need not be detected in the airflow of the longwall tail gate.     

Comparison of textural information from argon(87 K) and nitrogen(77 K) physisorption

Argon(87 K) and nitrogen(77 K) adsorption of three types of porous samples (microporous, microporous-mesoporous and mesoporous) was evaluated. The shapes of isotherms, specific surface areas, mesopore-, micropore- and net pore volumes, pore-size distributions (PSD) and positions of micropore and mesopore PSD’s maxima were compared to ascertain the credibility of individual adsorbates for the texture information estimation. The shapes of adsorption isotherms for Ar and N₂ of all samples are similar and the adsorbed amounts at relative pressure x = 0.975 differ slightly. For mesoporous samples some differences are observed between specific mesopore surface areas derived from nitrogen and argon isotherms. Radii of pore size maxima from Ar(87 K) PSD’s are on average systematically lower than from N₂(77 K) for all samples. The Saito-Foley approach for ZSM-5 samples gives consistently lower mean pore radius from N₂(77 K) adsorbate than from Ar(87 K). This difference probably arises from the multiplication factor, Ω, in the Saito-Foley equation which includes physical properties (magnetic susceptibility, polarizability etc. at 77 and 87 K) and is not easy to obtain with sufficient precision. The use of both adsorbates Ar(87 K) and N₂(77 K) possesses some advantages as well some disadvantages and the comparison of textural properties of individual samples must be evaluated with respect to adsorbate.     

Electrochemical investigation of Al–Li/Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FePO<Subscript>4</Subscript> cells in oligo(ethylene glycol) dimethyl ether/LiPF<Subscript>6</Subscript>

1 M LiPF₆ dissolved in oligo(ethylene glycol) dimethyl ether with a molecular weight, 500 g mol⁻¹ (OEGDME500, 1 M LiPF₆), was investigated as an electrolyte in experimental Al–Li/LiFePO₄ cells. More than 60 cycles were achieved using this electrolyte in a Li-ion cell with an Al–Li alloy as an anode sandwiched between two Li _x FePO₄ electrodes (cathodes). Charging efficiencies of 96–100% and energy efficiencies of 86–89% were maintained during 60 cycles at low current densities. A theoretical investigation revealed that the specific energy can be increased up to 15% if conventional LiC₆ anodes are replaced by Al–Li alloy electrodes. The specific energy and the energy density were calculated as a function of the active mass per electrode surface (charge density). The results reveal that for a charge density of 4 mAh cm⁻² about 160 mWh g⁻¹ can be reached with Al–Li/LiFePO₄ batteries. Power limiting diffusion processes are discussed, and the power capability of Al–Li/LiFePO₄ cells was experimentally evaluated using conventional electrolytes.     

Statistical prediction of fracture parameters of concrete and implications for choice of testing standard

This article shows how the fracture energy of concrete, as well as other fracture parameters such as the effective length of the fracture process zone, critical crack-tip opening displacement and the fracture toughness, can be approximately predicted from the standard compression strength, maximum aggregate size, water-cement ratio, and aggregate type (river or crushed). A database, consisting of 238 test data, is extracted from the literature and tabulated, and approximate mean prediction formulae calibrated by this very large data set are developed. A distinction is made between (a) the fracture energy, G_f, corresponding to the area under the initial tangent of the softening stress-separation curve of cohesive crack model, which governs the maximum loads of structures and is obtained by the size effect method (SEM) or related methods (Jenq-Shah two-parameter method and Karihaloo's effective crack model, ECM) and (b) the fracture energy, G_F, corresponding to the area under the complete stress-separation curve, which governs large postpeak deflections of structures and is obtained by the work-of-fracture method (WFM) proposed for concrete by Hillerborg. The coefficients of variation of the errors in the prediction formulae compared to the test data are calculated; they are 17.8% for G_f and 29.9% for G_F, the latter being 1.67 times higher than the former. Although the errors of the prediction formulae taking into account the differences among different concretes doubtless contribute significantly to the high values of these coefficients of variation, there is no reason for a bias of the statistics in favor of G_f or G_F. Thus, the statistics indicate that the fracture energy based on the measurements in the maximum load region is much less uncertain than that based on the measurement of the tail of the postpeak load-deflection curve. While both G_f and G_F are needed for accurate structural analysis, it follows that if the testing standard should measure, for the sake of simplicity, only one of these two fracture energies, then G_f is preferable.     

Energetische Nutzung von Biomasse – Verfahren zur Herstellung von Synthesegas,Wasserstoff, Methanol und flüssigen Brennstoffen

Die Verfahren zur Aufbesserung der Brennstoffeigenschaften von Biomasse orientieren sich weitgehend an der Vergasung oder Verflüssigung von Braunkohle. Die hydrierende Umsetzung der in der Biomasse vorliegenden Biopolymere (Cellulosen, Lignin und Lignocellulosen) erfordern vor dem hydrierenden Angriff den chemischen Abbau der komplexen Biopolymer-Matrix. Man unterscheidet pyrolytische Verfahren (350 bis 500°C, drucklos) von den unter Druck durchgeführten hydrolytischen, solvolytischen und extraktiven Verfahren. Die relativ hohen Investitionskosten der Iysierenden Verfahren erfordern wegen der “Economy of scale” eine Mindestanlagengröße von mehreren hundert MW. Die Biomassevergasung und die darauf aufbauende Wasserstoff und Methanol-Erzeugung aus Biomasse sind vom Investitionsaufwand her gesehen auch für kleinere Anlagen geeignet. In nächster Zukunft dürfte sich die energetische Biomassenutzung allerdings auf die direkte Verbrennung in Biomasse-Heizkraftwerken bzw. die Vergasung und Verstromung des Gases in Brennstoffzellen beschränken.     

Regio‐ and Enantioselective Sequential Dehalogenation of rac‐1,3‐Dibromobutane by Haloalkane Dehalogenase LinB

The hydrolytic dehalogenation of rac‐1,3‐dibromobutane catalyzed by the haloalkane dehalogenase LinB from Sphingobium japonicum UT26 proceeds in a sequential fashion: initial formation of intermediate haloalcohols followed by a second hydrolytic step to produce the final diol. Detailed investigation of the course of the reaction revealed favored nucleophilic displacement of the sec‐halogen in the first hydrolytic event with pronounced R enantioselectivity. The second hydrolysis step proceeded with a regioselectivity switch at the primary position, with preference for the S enantiomer. Because of complex competition between all eight possible reactions, intermediate haloalcohols formed with moderate to good ee ((S)‐4‐bromobutan‐2‐ol: up to 87 %). Similarly, (S)‐butane‐1,3‐diol was formed at a maximum ee of 35 % before full hydrolysis furnished the racemic diol product.     

CLA isomers in milk fat from cows fed diets with high levels of unsaturated fatty acids

The concentrations of CLA isomers were determined by Ag⁺-HPLC in the milk fat of cows fed a control diet consisting of hay ad libitum and 15 kg of fodder beets or this diet supplemented with oilseeds containing either high levels of oleic acid (rapeseed), linoleic acid (sunflower seed), or α-linolenic acid (linseed). Highly significant (P≤0.001) correlations were found between the daily intakes of oleic acid and the concentration of the CLA isomer trans-7,cis-9 in milk fat; of linoleic acid and the CLA isomers trans-10,trans-12, trans-9,trans-11, trans-8,trans-10, trans-7,trans-9, trans-10,cis-12, cis-9,trans-11, trans-8,cis-10, and trans-7,cis-9; and of α-linolenic acid and the CLA isomers trans-12,trans-14, trans-11,trans-13, cis,trans/trans,cis-12,14, trans-11,cis-13, and cis-11,trans-13. CLA concentrations were also determined in the milk fat of cows grazing in the lowlands (600–650 m), the mountains (900–1210 m), and the highlands (1275–2120 m). The concentrations of many isomers were highest in milk fat from the highlands, but only three CLA isomers (cis-9,trans-11, trans-11,cis-13, and trans-8,cis-10) showed a nearly linear increase with elevation. Therefore, these three CLA isomers, and particularly the CLA isomer trans-11,cis-13, the second-most important CLA in milk fat from cows grazing at the three altitudes, could be useful indicators of milk products of Alpine origin.     

Titania supported Co-Mn-Al oxide catalysts in total oxidation of ethanol

Catalytic activity of the Co-Mn-Al mixed oxide catalysts (Co:Mn:Al molar ratio of 4:1:1) supported over titania was examined in total oxidation of ethanol. The prepared catalysts were characterized by chemical analysis (AAS), surface area measurements, and temperature programmed techniques (TPR, TPD). In ethanol oxidation, the catalysts activity gradually increased with increasing active phase content. Low concentration of Co-Mn-Al oxides in the catalyst negatively affected formation of reaction byproducts: carbon monoxide production steeply increased when Co + Mn metals concentration were lower than 5 wt.%. On the other hand, formation of the second main reaction intermediate, acetaldehyde was limited, when acidity of the catalyst was not high, i.e. concentration of Co-Mn metals over titania was low.     

Zirconocene silanolate complexes and their heterogeneous siliceous analogues as catalysts for phenylsilane dehydropolymerization

The efficient catalytic dehydropolymerization of phenylsilane by homogeneous zirconocene bissilanolates ([{(c-C₅H₉)₇Si₈O₁₂O}₂Zr(η⁵-C₅H₅)₂] (1a); [{(c-C₅H₉)₇Si₈O₁₂O}₂Zr(η⁵-C₅H₄Bu)₂] (1b) [{(c-C₅H₉)₇Si₇O₉(OSiMe₃)O₂}Zr(η⁵-C₅H₅)₂] (4); [{(Me₃CO)₃SiO}₂Zr(η⁵-C₅H₅)₂] (5)) and chlorosilanolates ([{(c-C₅H₉)₇Si₈O₁₂O}ZrCl(η⁵-C₅H₄Bu)₂] (2); ([{(c-C₅H₉)₇Si₇O₉O₃}Zr₂Cl(η⁵-C₅H₅)₄] (3a); [{(c-C₅H₉)₇Si₇O₉O₃}Zr₂Cl(η⁵-C₅H₄Bu)₄] (3b)) has been demonstrated. The presence of at least one silanol ligand in the zirconocene moiety was found essential for high catalytic performance. Solid state structure of complex 1a was determined by single crystal X-ray diffraction analysis. A series of nine zirconocene-siliceous catalysts were prepared by grafting of zirconocene moiety onto silica using three general methods: (a) reaction of [(η⁵-C₅H₅)₂ZrCl₂] with silica in the presence of NEt₃; (b) reaction of [(η⁵-C₅H₅)₂ZrMe₂] with silica; (c) reaction of solely [(η⁵-C₅H₅)₂ZrCl₂] with silica. Supported catalysts were characterized by ICP-MS, FT-IR, TGA and selected examples by XPS analysis. Those catalysts prepared by method (a) and (b) were found efficient in the phenylsilane polymerization although a higher Zr/monomer ratio had to be used in comparison with homogeneous analogues. The low concentration of residual silanol groups in supported catalysts was found essential for their high catalytic performance. Advantageous reusability of supported catalysts was demonstrated using SiO₂₍₅₀₀₎/Cp₂ZrCl₂/NEt₃(5.8). The catalytic performance was retained in three consecutive cycles producing polymers with almost identical properties.