Kinetics and mechanism during mechanical/thermal dewatering of lignite

In order to increase the efficiency of lignite fired power stations the mechanical/thermal dewatering (German abbreviation: MTE, Mechanisch/Thermische Entwässerung, also used for ‘mechanical/thermal expression’) was developed at the University of Dortmund as an energy efficient process for the reduction of the water content of lignite prior to combustion [1-3], [Patentschrift DE 44 34 447 A1, 1994; Patent EP 0 784 660 B1; WO 96/10064, 1996; VDI-Berichte 1280 (1996) 165]. While a 25 t/h demonstration plant has been constructed at the Niederaußem power station of RWE in Germany and came into operation in 2001 [4], [Proceedings of the VGB/EPRI Conference, 2001] additional detailed research has been done on the process fundamentals at the University of Dortmund. Experiments for three different lignites from Germany, Greece and Australia are presented in this paper and it is shown, that the dewatering kinetics depending on time, temperature and pressure can be described by a new model derived from soil-mechanical fundamentals and rate-process-theory [5], [Mechanismen und Kinetik der Mechanisch/Thermischen Entwässerung von Braunkohle, 2001]. Due to differences in lignite composition the experimental determination of some model parameters for each coal is necessary. From the activation energy which is determined from experiments concerning dewatering kinetics it can be deduced, that even during secondary consolidation (creep) the drainage of water is the dominating process. The experiments also provide a clear distinction between the effect of the so-called ‘thermal dewatering’ due to heating of the lignite and the subsequent mechanical expression.     

Hydrothermal dewatering of lower rank coals. 1. Effects of process conditions on the properties of dried product

The hydrothermal dewatering of a Loy Yang Low Ash Victorian brown coal has been studied under a wide range of process conditions. The intra-particle porosity (<1 μm pore radius) determined by mercury porosimetry of the resulting dried products, a proxy for the maximum solids concentration of coal-water slurries, is reported as a function of reaction time, processing temperature, autoclave size and configuration and slurry concentration. Of these variables only the process temperature had a significant influence on the product intra-particle porosity, which decreased with increasing temperature. Other product variables were affected by a wider range of process conditions; increase in temperature and residence time and decrease in slurry concentration led to marked increase in loss of organic material to waste water, and the elemental composition of the product changed significantly with e.g. residence time. Drying of coal samples of particle sizes >50 μm gave products of constant intra-particle porosity but smaller coal particles gave products with higher intra-particle porosities as a result of agglomeration. Milling of the coal reduced the porosity of the dried coal at constant particle size.     

Hydrothermal dewatering of lower rank coals. 3. High-concentration slurries from hydrothermally treated lower rank coals

A novel method of producing a product of low intra-particle porosity (<1 μm pore radius) from highly porous Latrobe Valley raw brown coals uses a combination of hydrothermal and evaporative drying. Low porosity coal was made in three different batch autoclave systems at 320 °C for residence times as low as 10 min. Higher temperatures (up to 350 °C) increased porosity slightly but the water vapour pressure and the loss of organic material were significantly increased. Although the low and high porosity products differed dramatically in appearance and hardness, other chemical and spectroscopic properties were similar with the exception of pyrolysis-gas chromatography-mass spectrometry patterns.The relationship between intra-particle porosity and the maximum wt% dry solids concentration of aqueous slurries (for a viscosity of less than 1000 mPa s), ?_max, established by earlier workers for hydrothermally treated brown coals was found to hold for the new products and was extended to a wider range of porosities and a range of mean particle sizes (mps) (20-100 μm). A range of surfactants (anionic, cationic and neutral), which led to an increase of up to 7% in ?_max for a bituminous, Blair Athol coal, increased ?_max for products of hydrothermal or the new treatment by only 2-4%. This small increase resulted, however, in the formation of slurries of the low porosity products with ?_max's of up to 64%, similar to those obtained with high rank coals, and considered to be of commercial interest.     

The brown-coal/water system: Part 3. Thermal dewatering of brown coal

The mechanism of a process for removal of water as liquid from soft brown coals has been investigated. Coals were heated in water under pressure to temperatures of 150 to 300 °C, and the yields of dry coal, water, organic functional groups and inorganics in the product were measured. The earlier conclusions by coalification chemists on dehydration and decarboxylation under these conditions have been confirmed by the functional group analyses, and it was shown that the removal of liquid water is initiated principally by a disruption of the coal/water interactions caused by the thermal destruction of functional groups. The process is then completed by expulsion of water by the carbon dioxide evolved, and by changes in the surface wettability and shrinkage of the coal gel. At temperatures of 250–300 °C approximately three quarters of the water originally present in the coal is removed. If the coal is allowed to cool in the water after treatment at 150 °C complete reabsorption of the water occurs, but as the treatment temperature is raised progressively less reabsorption occurs. Alkali and alkaline-earth metals, chlorine and sulphur are also partly removed by the process, whereas iron, aluminium and silicon are little affected.     

Water absorption by coals: effects of pore structure and surface oxygen

The water-holding capacity of various coals from lignite to anthracite was measured and its relation to their oxygen content and pore structure was investigated. Both factors were found to play important roles in determining the water-holding capacity. Pyrolysis of a lignite at 400 °C decreased its water-holding capacity by more than two-thirds, because of the decrease in the number of hydrophilic sites. This was caused by the decomposition of oxygen-containing groups and the decrease in surface area which resulted from the plugging of small pores by volatile matter condensation, making their surfaces inaccessible to water.     

A study by density measurement of changes in pore structures of coals with heat treatment: Part 2. Micropore structure

Changes in micropore structures of six Japanese coals during heat treatment up to 1200 °C were followed by measuring densities in n-hexane, in methanol and in helium. The micropore volumes were deduced from the difference between the reciprocals of the densities in n-hexane and in methanol. The concept of the ‘crystallite’ was used in discussing the micropore structures of carbonized coals. Changes in the micropore structures were explained in terms of evolution of gases, change in the orientation of graphite-like layers, shrinkage of the structure, and so on. The different variations in micropore structures with heat treatment, between lower-rank coals and higher-rank coals, could well be accounted for by classifying the lower-rank coals and the higher-rank coals as non-graphitizing carbon and graphitizing carbon respectively. It is suggested that for coals and their carbonized products the entrance diameters of the micropores located between the crystallites are smaller than the diameter of n-hexane molecules.     

A study by density measurement of changes in pore structures of coals with heat treatment: Part 1. Macropore structure

Changes in the pore structures of six Japanese coals with heat treatment up to 1200 °C were followed both by measurement of densities in mercury and in n-hexane and by mercury porosimetry. Pore volumes deduced from the difference between the reciprocals of densities measured in mercury and in n-hexane were almost equal to pore volumes deduced from mercury penetration for all the coals heat-treated. This suggests that, for all the coals, transitional pores and micropores whose entrance diameter is larger than the diameter of the n-hexane molecule were not developed or produced by heat treatment. For caking coals, the pore volume changed sharply in the plastic stage of carbonization, with a maximum at ≈400 °C that was probably due to the appearance and disappearance of bubble structure, whereas for non-caking coal it did not show any appreciable change in the range of heat treatment temperature (HTT) studied. It is suggested that the macropore structure of carbonized coals is related to the hardness, or the binding energy between their molecules.     

Low temperature oxidation of coals: Effects of pore structure and coal composition

The low-temperature oxidation of five coals, ranging in rank from subbituminous to anthracite, was studied in the temperature range 30–250 °C, and the reaction kinetics were elucidated. The reaction rates were independent of particle diameter <1 mm. The orders of reaction for CO₂ and CO formation were 0.50 and 0.54, respectively, with respect to oxygen. Activation energies of 51.5–59.4 kJ mol^?1 were obtained for the CO₂ and CO formation reactions. The rates of formation of CO₂ and CO were correlated to the internal surface area and the oxygen contents of the coals. It was found that pores having radii >100 Å, and the oxygen-containing groups which decompose to CO₂ and/or CO, were playing important roles in low-temperature oxidation of coals.     

木质素/LDPE-EVA共混材料的力学性能及热性能

以天然热塑材料木质素和LDPE-EVA为原料,研究了不同木质素以及增容剂含量对木质素/LDPE-EVA共混材料的力学及热性能影响。结果表明,20份木质素与LDPE-EVA共混效果较好,拉伸强度达到最大（25.88 MPa）,较LDPE-EVA聚合物提高了9%,且共混物在100 ℃附近的吸热峰出现一定左移（3.6 ℃）,降解性能增加;10份增容剂LDPE-g-MAH的加入使体系拉伸强度达到35.66 MPa,较未加增容剂时提高了26.6%,100℃附近的吸热峰进一步左移（2 ℃）,降解性能小幅降低,显著提高了共混物的相容性。     

Physical and thermal properties of acid-graphite/styrene-butadiene-rubber nanocomposites

In general, carbon-based materials play a major role in today’s science and technology and are required to advance with better properties to meet new requirements or to replace existing materials. We fabricated rubber composites reinforced with 5-weight% acid-graphite. The structural, mechanical and thermal properties of these composites were studied and compared. XRD studies indicated that the structure of the acid treated pristine-graphite (acid-graphite) did not change that of pristine graphite. Tensile properties of the composites indicated higher modulus, tensile strength and elongation in comparison with composites of pristine graphite, carbon black. Also, the composites were found to be in improving tendency with thermal properties and fatigue properties. The acid-graphite was investigated for surface morphology by scanning electron microscopy (SEM) and defects or purity by Raman spectroscopy. In this article, we discuss the influence of acid-graphite on rubber with high mechanical and thermal properties.     

Pultruded fiber reinforced thermoplastic poly(methyl methacrylate) composites. Part II: Mechanical and thermal properties

A novel process has been developed to manufacture poly(methyl methacrylate) (PMMA) pultruded parts. The mechanical and dynamic mechanical properties, environmental effects, postformability of pultruded composites and properties of various fiber (glass, carbon and Kevlar 49 aramid fiber) reinforced PMMA composites have been studied. Results show that the mechanical and thermal properties (i.e. tensile strength, flexural strength and modulus, impact strength and HDT) increase with fiber content. Kevlar fiber/PMMA composites possess the highest impact strength and HDT, while carbon fiber/PMMA composites show the highest tensile strength, tensile and flexural modulus, and glass fiber/PMMA composites show the highest flexural strength. Experimental tensile strengths of all composites except carbon fiber/PMMA composites follow the rule of mixtures. The deviation of carbon fiber/PMMA composite is due to the fiber breakage during processing. Pultruded glass fiber reinforced PMMA composites exhibit good weather resistance. They can be postformed by thermoforming, and mechanical properties can be improved by postforming. The dynamic shear storage modulus (G′) of pultruded glass fiber reinforced PMMA composites increased with decreasing pulling rate, and G′ was higher than that of pultruded Nylon 6 and polyester composites.     

煤体理化性质对其孔隙结构和甲烷吸附性能影响的研究进展

煤层气主要成分为甲烷（CH₄）,其主要以吸附态形式存在于煤层中。明确煤体理化性质和煤体孔隙结构及CH₄吸附性能间构效关系,对于高效开采CH₄资源至为关键。为此,本文阐明了煤体理化性质对其孔隙结构和CH₄吸附性能的作用规律,并指出了后续研究趋势。分析表明：煤体微孔结构和其CH₄吸附容量之间呈正线性相关性;煤体介/大孔主要影响CH₄在煤层内部的吸附/扩散速率。具有墨水瓶形孔或富含镜质体的煤体通常具有较强CH₄吸附性能。煤中矿物质和水分对煤体吸附性能产生不利影响。煤中小分子有机物的抽提能够提高煤体孔隙表面积和孔容积,进而提升煤体吸附性能。为了深入研究煤体理化性质及其吸附性能的作用规律,后续需开展以下工作：研究煤体孔隙结构参数和煤体吸附/解吸性能之间的耦合作用关系;利用多重分形理论精确揭示煤体内复杂的孔隙结构信息;优化并建立考虑煤体非均质性的BET和BJH等孔隙结构参数计算模型;以煤基质表面含氧官能团在煤体孔隙内部的赋存空间为切入点,阐明煤体官能团和孔隙结构对其CH₄吸附性能的协同作用规律;从理论模拟和实验科学入手,阐明煤层中水分对煤体孔隙结构的影响;建立更为科学的含水煤体吸附性能评价方法。     

Mechanical properties,morphological structure,and thermal behavior of dynamically photocrosslinked PP/EPDM blends

A dynamically photocrosslinked polypropylene (PP)/ethylene–propylene–diene (EPDM) rubber thermoplastic elastomer was prepared by simultaneously exposing the elastomer to UV light while melt‐mixing in the presence of a photoinitiator as well as a crosslinking agent. The effects of dynamic photocrosslinking and blend composition on the mechanical properties, morphological structure, and thermal behavior of PP/EPDM blends were investigated. The results showed that after photocrosslinking, tensile strength, modulus of elasticity, and elongation at break were improved greatly. Moreover, the notched Izod impact strength was obviously enhanced compared with corresponding uncrosslinked blend. Scanning electron microscopy (SEM) morphological analysis showed that for uncrosslinked PP/EPDM blends, the cavitation of EPDM particles was the main toughening mechanism; whereas for dynamically photocrosslinked blends, shear yielding of matrix became the main energy absorption mechanism. The DSC curves showed that for each dynamically photocrosslinked PP/EPDM blend, there was a new smaller melting peak at about 152°C together with a main melting peak at about 166°C. Dynamic mechanical thermal analysis (DMTA) indicated that the compatibility between EPDM and PP was improved by dynamic photocrosslinking. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3371–3380, 2004     

Conductive PVC composites: Part 1. Physical and electrical properties

Conductive-plastic enclosures, to compete successfully in the electronics industry, must be easily and rapidly moldable into a low-cost, lightweight, structurally sound, flame-retardant, and electrically-conductive rigid part. Such a material must combine the design flexibility and moldability of a plastic with the EMI shielding properties approaching those of a metal. PVC filled with aluminum flake makes an excellent conductive composite, provided it is correctly formulated and processed. A critical concentration of metallic filler must be used; it is a function of the plastic formulation; the uniformity of the filler's dispersion; the filler type, size, and morphology; and the method by which it was compounded into the resin.     

Mechanical and thermal properties of graphite platelet/epoxy composites

Anhydride-cured diglycidyl ether of bisphenol A (DGEBA) reinforced with 2.5-5% by weight graphite platelets was fabricated. The structural, mechanical, viscoelastic and thermal properties of these composites were studied and compared. XRD studies indicated that the processing of composites did not change the original d-spacing of pure graphite. Tensile property measurements of composites indicated higher elastic modulus and tensile strength with increasing concentration of graphite platelets. The storage modulus and glass transition temperatures (T_g) of the composites also increased with increasing platelet concentration, however, the coefficient of thermal expansion decreased with the addition of graphite platelets. The thermal stability was determined using thermogravimetric analysis. The composites showed higher thermal stability in comparison with pure epoxy and increased char concentration for higher graphite concentration. The effects of reinforcement on the damage mechanisms of these composites were investigated by scanning electron microscopy.     

Mechanical and thermal properties of polypropylene/sugarcane Bagasse composites

To determine the possibility of using sugarcane bagasse (SCB) waste as reinforcing filler in the thermoplastic polymer matrix, SCB‐reinforced polypropylene (PP) composites were prepared. The PP and SCB composites were prepared by the extrusion of PP resin with 5, 10, 15, and 20 wt % of SCB filler in a corotating twin screw extruder. The extruded strands were cut into pellets and injection molded to make test specimens. These specimens were tested for physicomechanical properties such as tensile, flexural, Izod, and Charpy impact strengths, density, water absorption, and thermal characteristics, namely, heat deflection temperature (HDT), melt flow index, and thermogravimetric analysis. It was found that the flexural strength increased from 23.66 to 26.84 MPa, Izod impact strength increased from 10.499 to 13.23 Kg cm/cm, Charpy impact strength increased from 10.096 to 13.98 Kg cm/cm, and HDT increased from 45.5 to 66.5°C, with increase in filler loading from 5 to 20% in the PP matrix. However, the tensile strength and elongation decreased from 32.22 to 27.21 MPa and 164.4 to 11.20% respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3827–3832, 2007