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.     

Dependence of carbon dioxide sorption on the petrographic composition of bituminous coals from the Czech part of the Upper Silesian Basin, Czech Republic

The effect of the rank and of the maceral composition of bituminous coal on carbon dioxide sorption capacity was studied on the basis of samples from two coal mines (Darkov, ?SM) from the Czech part of the Upper Silesian Basin. The samples from the two mines cover a small but very significant section of coalification within the transition zone between high-volatile bituminous A coal and medium-volatile bituminous coal, where porosity and sorption properties pass through their minima. The coal porous system was characterized by the micropore volume evaluated using the sorption isotherm of carbon dioxide and the volumes of meso-, macro- and coarse pores were determined by high-pressure mercury porosimetry. The micropore fraction in the coal porous system ranged between 53% and 75%. It was particularly high in coals with high vitrinite content, namely collotelinite, and also in coals with high inertinite content. The carbon dioxide sorption capacity was determined from the carbon dioxide sorption isotherms measured using a gravimetric sorption analyzer at 298 K until a relative pressure of 0.015 p/p_s, and was interpreted by characteristic parameters of the Dubinin and Langmuir equations. It was found that the adsorbed amount of CO₂ in the ?SM coal increases with the content of vitrinite and collotelinite, whereas no increase or only a slight increase was observed for the Darkov coal. The tendency of adsorption capacity to depend on maceral composition, and also to some extent on coalification, observed for the ?SM coal, may be related to higher microporosity due to the coalification process or oxidative processes leading to the formation of pseudovitrinite.     

Analysis of nitrogenous compounds in the effluent streams from a fluidized bed coal gasification reactor

A Texas lignite and a New Mexico subbituminous coal were gasified with steam and oxygen in a pilot-scale fluidized bed reactor at pressures from 770 kPa to 830 kPa, and temperatures form 795°C to 980°C. The make gas passed through a cyclone separator, and then a venturi scrubber in which condensable and water-soluble compounds were removed. The gasifier effluents (spent char, cyclone fines, tar, wastewater, and dry make gas) were analyzed for nitrogenous compounds.

For both coals, 6–12% of the nitrogen in the feed was retained in the spent char, with greater quantities being retained in the subbituminous coal char. Of the nitrogen volatilized from both coals, roughly 5% appeared in the tar, less than 0.2% appeared in the dry make gas as ammonia and NO_x, and the balance appeared in the wastewater as ammonia (60%), hydrolyzable nitrogenous compounds and possibly cyanate (10–15%), thiocyanate (1%), cyanide (0.5%), and other compounds (3–10%). The average concentration of NO_x in the dry gas was 7 ppm for lignite. No NO_x data for subbituminous coal were obtained. Reactor conditions (temperature, pressure, steam-to-carbon feed ratio) had no measurable effect on the production rates of nitrogenous compounds over the range of conditions investigated.     

WETTABILITY MEASUREMENTS ON COAL PARTICLES BY MEANS OF THE SOLIDIFICATION FRONT TECHNIQUE

Reliable methods for the characterization of the surface properties and the wettability of small particles have not been available in the past. A novel technique, based on the behaviour of the lest particles at the solidification front of certain well defined solidifying melts (matrix materials) has been developed recently in our laboratory. This method was used to determine contact angles and surface tensions of coal particles in the range of 20-250 μm. Two types of bituminous and one type of subbituminous coal were tested using thymol, biphenyl, napthalene, salol and 2-phenyl phenol as matrix materials.

The results show that contact angles on small particles can be determined to within ± 2°, i.e. to the same degree of accuracy that is typically reported for extended smooth surface by means of conventional techniques. The measurements also show that within one and the same coal sample the larger particles are somewhat more hydrophilic than the smaller ones. Surface tensions and contact angles of coal particles are of great interest in connection with coal beneficiation processes such as froth flotation and oil agglomeration.     

LOW TEMPERATURE OXIDATION OF LOW RANK COALS

The low temperature oxidation of a Montana subbituminous coal was investigated using round bottom 100 ml flasks in constant temperature baths. The experiments were carried out in normal and oxygen enriched air at 30°C, 45°C and 70°C with particle sizes ranging from 4 mesh to 100 mesh. Periodic analysis of gas samples from the flasks provided the rate data. The reactivity of the as received coal was compared with that of the same coal dried (i) in high pressure steam and (ii) in hot water

A rate equation has been proposed incorporating the effects of oxygen diffusion and surface reaction. For higher oxygen concentration and smaller particle sizes, the zero order surface reaction was found to be controlling. The temperature dependency of the reaction rate was found to be well represented by the Arrhenius equation. The activation energy varied slightly under various conditions between 15 to 20 kca)/g mole

The reactivity of the hot water dried coal was found to be similar to that of the as received coal. Steam dried coal however, was found to be much less reactive.     

Upgrading of low rank coal and woody biomass mixture by hydrothermal treatment

Attempts to produce high-grade fuel from biomass and low rank coal are important from the viewpoint of renewable energy and the utilization of unused resources. In this paper, the authors reported on the hydrothermal treatment of biomass and low rank coal at 300 °C using a bench scale continuous apparatus and a batch autoclave. The results show that coalification takes place during the hydrothermal treatment of both the low rank coal and biomass, and the upgraded solid products show similar chemical compositions, gross calorific value and effective calorific value, independent of the mixing ratio. The solid product also becomes hydrophobic and unable to re-adsorb the lost moisture. The characteristics of the solid produced by the bench scale continuous apparatus can be predicted by the results of the batch process. Thermogravimetric analysis shows that the solid product has a wide-range of molecular weight but the thermally stable heavy molecules are found more in the treated coal as opposite to the thermally unstable light molecules, more of which are found in the treated biomass. This may correlate with that the solid product of higher biomass mixing ratio has a higher volatile matter content. Polymerization is synergistically promoted during mixed hydrothermal treatment of low rank coal and biomass.     

The nature of solids accumulated during solvent refining of coal

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.     

Study of the treatment of subbituminous coals by NaOH solutions

A subbituminous coal from La Union (Chile) has been treated with aqueous NaOH solutions. The quantity of coal solubilized and the elimination of ash was found to increase with increase in hydrolysis time, temperature and NaOH concentration and with decrease in particle size. Coal solubilization reached a maximum with respect to NaOH concentration in the range studied. The largest yield of solubilized products was 28 wt%, this occurred after 8 h of hydrolysis at 80 °C using an NaOH concentration of 10 g dm⁻³ and coal having a particle size between 125 and 177 μm. In the range of conditions studied, the greatest decrease in the ash yield of the undissolved coal was ≈ 29 wt%. Under the same conditions, the total sulphur content of the coal decreased by 30 wt%.     

Upgrading of low rank coal with solvent

In this study, thermal upgrading of low-rank coal with solvent at 380–440 °C under an initial nitrogen pressure of 2 MPa was studied as a possible method for producing clean solid fuel with a high heating value and less spontaneous ignition behavior. Upgrading of Buckskin coal (USA, subbituminous coal) in the presence of t-decalin (non hydrogen-donor solvent) at 440 °C gave 11.4 wt.% of gas, 5.3 wt.% of oil and 74.1 wt.% of upgraded solid product with a small amount of water. The gaseous product consisted mainly of carbon dioxide (67 wt.%), methane, carbon monoxide, hydrogen and a trace of C₂ and C₃ hydrocarbon gases. The oil product from coal contained BTX, phenol, and their alkyl-derivatives. The heating value of the upgraded solid product from the Buckskin coal increased to 31.0 MJ/kg in dry base as compared to the heating value of wet base of the untreated raw coal, which was 19.3 MJ/kg. Spontaneous ignition behavior was greatly reduced by the upgrading. The effect of catalyst and additives on the upgrading was investigated in terms of product distribution and the quality of the solid product. Taiheiyo (Japan, subbituminous) and Yallourn (Australia, brown) coals were also studied.     

Parametric studies on devolatilization of a subbituminous coal in a reactive gas environment

A laboratory scale fixed bed coal gasifier was set up to simulate the conditions existing in the devolatilization zone of an air-blown, fixed-bed coal gasifier. Devolatilization behaviour of a subbituminous coal was evaluated in the temperature range 350 °C to 550 °C and at pressures 30, 300 and 375 psig. Three feed coal particle sizes, (−2, +1), (−4, +3) and (−9, +6)mm, were studied. The gas feed was a synthetic mixture of composition similar to that leaving the gasification zone of a fixed bed gasifier and contained 30% by volume of steam. Devolatilization runs were conducted over coal residence times of 5, 10, 20 and 30 min durations. The gas evolution rates showed a peak around 5 min from the start of a run and most of the gas evolution tapered off just under 30 min. Thirty key components in the tars were quantified and these included aliphatic and aromatic homologues, as well as sulphur and nitrogen substituted structures. The molecular weights of the tar samples showed a maximum between 300 and 500. A first order kinetic model applied to the total weight loss data yielded activation energies in the range 4 to 11 kcal mol⁻¹. Differential equations for obtaining concentration profiles for tar and gas inside the coal particle were solved numerically. From these calculations it was concluded that the pressure buildup (due to evolution of tar and gas) inside the coal particle was higher for larger particles, at a given external pressure, but decreased with external pressure. The concentration of tar inside the particle did not appear to be sensitive to low pressures (around 1 atm), but increased in the higher range of pressure (above 20 atm) and also with particle size.     

A newly proposed view on coal molecular structure integrating two concepts: Two phase and uniphase models

Concept of coal molecular structure was reexamined on the basis of detailed information concerning aliphatic moieties of coal and the reactions occurred during coalification process. Based on the observed similarity of the distribution of chain length between alkylene bridge bonds and alkyl pendant groups on coal aromatic cluster, the authors have already proposed [Nomura, M., Murata, S., Kidena, K. 2004a. Some view on the solubilities of coals toward solvents. Proceedings of 21st Annual International Pittsburgh Coal Conference, No. 12-4.] the presence of common intermediates in the coalification process which results in uniphase structure under reductive conditions and two phase structure under oxidative conditions, respectively. The present paper proposes, the concept that the reactivity of the coal aromatic moieties for common intermediates and/or the stability of common intermediates govern the reactions leading to a two phase structure or uniphase structure in more rationalized way as coalification is believed to proceed under reductive conditions, especially at its late stage. Brown and subbituminous coals have relatively many oxygen-containing functionalities on aromatic moieties so that their aromatic rings are activated for radical reactions. Common intermediate radicals, aryl alkyl radicals, from cleavage of alkyl pendant groups on aromatic rings and alkylene bridge bonds attack neighboring aromatics to form entanglement-like structure in an easy way. These structures are less soluble to organic solvents due to its entanglement character. Bituminous coals with high solubility toward CS₂/NMP solvents tend to have carbon contents from 85 to 87% (daf. basis), their aromatic moieties being so stable that intermediate radicals cited above can attack more selectively the neighboring aromatic moieties: they have less tendency to make such bridge bonds to form cross linked structures, the resulting coal molecules being not entangled like uniphase. In the case of coals having more carbon than 87% or bituminous coals with less carbon than 85%, the resulting coal molecules become less soluble toward organic solvents: In the higher rank coals, the aromatic rings are so large and so stable that their solubility toward solvent are intrinsically low. In addition to this, it is supposed that bridge bonds are cleaved to make the structure with less cross linking. Their stacking tendency should be also considered as other reason for low solubility to solvent. On the other hand, less solubility of the lower ranked bituminous coals can be rationalized partly, in an analogy with the behavior that the brown and subbituminous coals show, despite of their relatively large aromatic clusters.     

PREPARATION OF HYDROTHERMALLY TREATED LRC/WATER FUEL SLURRIES

The hot-water coal drying process is a means of thermally beneficiating and dewatering lignite and subbituminous coal for the purpose of preparing dense low-rank coal/water fuel. In hot-water coal drying, which is a form of hydrothermal treatment, low-rank coal in a water slurry is treated at elevated temperatures of 513 to 623  K and at pressures in excess of the equivalent saturated steam pressures lo minimize vaporization of the water. This produces a coal product which contains less than one-third the inherent moisture of the raw coal and which is resistant to moisture reabsorption. The hydrothermally treated coal/water slurry resulting from the process can be concentrated by mechanical means (centrifuge, pressure filter)to form a pumpable coal/water fuel with greater than 60 weight percent bone-dry solids content and with an energy content greater than 15·4 MJ/Kg (6600 Btu/lb). Hydrothermal treatment also beneficiates the coal by reducing oxygen and minerals. Over 94 percent of the energy content of the raw coal remains in the product. Low-rank coal/water fuel is typically a pseudoplastic fluid, and for some low-rank coals, the slurry is stable towards settling, without the use of additives.     

Influence of temperature on the hydrogenation of Australian Loy-Yang brown coal. 1. Oxygen removal from coal and product distribution

A study is made of the composition of the solid, liquid and gaseous fractions produced by hydrogenation of Australian Loy-Yang brown coal at temperatures ranging from 300 to 500 °C. The high oxygen content of the coal (25.5 wt%) is not found to result in a proportionally higher hydrogen consumption when compared to previously published results for a coal with approximately half the oxygen content. Oxygen is found to be removed from the coal mainly as carbon dioxide and water, most probably by decarboxylation and dehydration reactions. At temperatures up to ≈400 °C hydrogen is consumed almost solely by transference from the solvent tetralin to the coal. By this temperature both the maximum degree of conversion and the maximum oil yield are reached. The heavy oil fraction at 400 °C is composed mainly of asphaltenes and preasphaltenes. Above 400 °C hydrogen is consumed from both solvent and gas. A major part of this appears to be involved in the stabilization of decomposition products from the tetralin. The yield of pentane-soluble material is relatively constant up to 450 °C, however, at higher temperatures conversion of asphaltenes and preasphaltenes to pentane-solubles occurs in conjunction with gasification to C₁–C₃ hydrocarbons. Despite the fact hydrogen consumption and oxygen removal both increase with rising hydrogenation temperature, the H/C atomic ratio for the three heavy oil fractions decreases over the same range.     

Effects of sulfur on coal treatment in aqueous sodium hydroxide

The dissolution of Wyodak subbituminous and North Dakota lignite coals in 50% aqueous sodium hydroxide was enhanced by the addition of sulfur. The added sulfur was not incorporated into the undissolved coal residues. The effects of temperature (140–200°C), time (5–90 min), and added elemental sulfur (1–25% by weight) on dissolution were examined. Temperature affected the dissolution of subbituminous and lignite coal, while time affected the dissolution of subbituminous coal.     

煤化度对煤层气成藏特性影响规律的研究

综述了煤层气的成因类型和成气过程的研究现状,说明生物成因和热成因是煤层气的2种主要成因类型,且煤层气生成过程以热成因为主,生物成因为辅。通过分析不同煤化度煤层气的生成过程,说明在整个煤化作用过程中煤层气的生成大致经历了4个阶段,即由植物遗体转化为泥炭阶段、褐煤转型阶段、长焰煤～瘦煤转化阶段以及贫煤～无烟煤阶段。从煤化度角度阐述了不同煤化度煤层气的贮藏特性,即不同煤化度煤层气存在较大差异,特别是不同煤层气吸附、解吸特征的差异影响了其吸附成藏、解吸、渗流及产出的整个过程;煤吸附CH4的能力随着煤化度的增加呈现先急剧增加后缓慢减小的趋势。最后分析了不同煤化度煤层气存在成藏差异的原因,说明煤化学结构、煤物理结构以及成煤过程中地质条件的差异是导致不同煤化度煤层气富集成藏差异性的主要原因。     

Burn-out of pulverised coal and biomass chars☆

The burn-out of carbon in pulverised fired power stations is commercially important. Interest in the burn-out of biomass chars is growing because biomass is increasingly being co-fired with coal to reduce the carbon dioxide emissions. The significance of carbon burn-out is that it is linked with the efficiency of the plant and the suitability of the coal ash for construction purposes. Residual carbon in ash has generally increased in recent years because of the influence of the lower temperatures and slower mixing resulting from the use of low NO_x burners. The amount of unburned carbon is thus a function of the plant design and operating conditions but it is also linked to the ease of combustion of the coal and the char formed. These latter factors are related to the properties of the coal and this paper attempts to quantify the impact of certain coal and char properties on carbon-out. An approach for assessing biomass combustion performance is also discussed.     

Structural changes in humic acids during the coalification process

Humic acids from four coals, varying in rank from peat to subbituminous coal, have been characterized by elemental analysis, acidic groups, molecular weight, electrophoresis and visible, FT-i.r. and CP/MAS ¹³C n.m.r. spectroscopy. The humic acids increase in carbon content, molecular weight, condensation degree and aromaticity (f_a) with increasing maturation of the parent coals, while the oxygen content decreases with a loss of oxygen functional groups. The presence of lignin-like polymers, poly-saccharides and peptidic materials in humic acids from peat was established using i.r. and ¹³C n.m.r. spectroscopy. The structural changes observed in humic acids are in agreement with the recognized coalification theory and tend to support the hypothesis of condensation of humic acids into insoluble humin of coal.     

Origin of methane in coal beds in the initial and intermediate stages of coalification

In the initial stages of coalification, there is no dehydrogenation, and therefore the organic mass of coal cannot be the source of hydrogen for methane formation. It is possible that an explosive mixture of methane and oxygen may be formed from carbon dioxide and moisture in the coal bed as a result of bacterial activity.     

Hydrothermal Aging of Composite Materials Part 2: Matrix Aspects

The effects of hydrothermal (100°C in H₂O) aging on the epoxy matrix phase of a graphite-epoxy composite is examined in terms of chemical structure and Theological response. Infrared spectra indicate the resin system is a dicyandiamide (DICY) cured diglycyl ether of Bisphenol-A (DGEBA). Hydrothermal aging is shown to modify the resin network structure and increase the magnitude of the low-temperature β transition in short range molecular motion where T_β ≃ 45±15°C at frequencies of 3.5 to 110 Hz. In addition to network structure changes the absorbed moisture temporarily lowers the resin glass temperature so that at 100°C in water the resin exhibits a rubbery response state. Evaluation of fracture properties of aged resin shows a loss in fracture toughness and development of internal microcracks in the bulk resin during hydrothermal aging.     

The effect of water on the activity of supported palladium catalysts in the catalytic combustion of methane

Supported palladium catalysts are very active in the combustion of methane, but still little is known about the kinetic parameters. In this paper a rate expression is presented for an alumina-supported palladium oxide catalyst in the temperature range 180–515°C. Special care was taken to ensure differential conditions during the experiments. In this way, an apparent activation energy of 151±15 kJ/mol was found. The orders in methane, oxygen and water were 1.0±0.1, 0.1±0.1 and −0.8±0.2, respectively. For carbon dioxide a zero order was observed under all conditions. Inhibition by water produced during the reaction was demonstrated to cause non-differential conditions, when a dry feed was used. The rate constant that corrects for this effect could be derived.