Filtration in coal liquefaction. Influence of filtration conditions in non-hydrogenated systems

A series of experiments has been carried out to study the effects of filtration conditions upon the rate of filtration of non-hydrogenated coal digests. The results show the dependence of cake resistivity on both the filtration temperature and pressure. Filter cakes were found to be compressible, resulting in smaller increases in rate with increasing pressure than with incompressible cakes. The filtration temperature determines the packing of residual solids in the cake which in turn affects the cake resistivity. An empirical relation has been derived between filtration temperature and resistivity. With increasing temperature there is an increase in filtration rate due to the reduced viscosity, but a reduction owing to a higher packing density of solids in the filter cake.     

Filtration in coal liquefaction. Influence of digestion conditions in the filtration of non-hydrogenated coal digests

A series of experiments has been carried out to study the way in which preparation conditions of coal digests influence filtration rate. It was shown that a relation exists between cake resistivity and digestion temperature and time, and therefore good control over digestion conditions is important for rapid filtration. Particular attention must be paid to the design of the reactor to ensure that all the material is given the same heat-treatment. The optimum residence time occurs when repolymerization of the dissolved coal commences.     

Filtration in coal liquefaction.: 2. Relation between digestion conditions and filtration rate

In systems for the liquefaction of coal by solvent extraction, removal of the undissolved solids from the liquefaction products is a fundemental part of the process. For separation of solids by filtration to be economically viable, it is essential to achieve high filtration rates. The influence of the extraction conditions, temperature, residence time and coal feed size, on the rate of filtration has been investigated. It has been shown that the rate of filtration is sensitive to the presence of a gel-like intermediate formed during the dissolution of the coal. The formation of this intermediate imposes limitations on the choice of digestion conditions, in particular the coal feed size. However, by inducing polymerization, the gel can be stabilized, thus allowing significantly improved filtration rates to be achieved.     

Filtration in coal liquefaction: 1. Influence of coal type

Commercial solvent extraction systems for coal must be able to process coals with various properties. In this study the influence of coal type upon the extraction yield of coal and the filtration of extraction products has been investigated. All the coals used gave high extraction yields in hydrogenated solvent but resultant products exhibited considerable differences in rates of filtration. This variation in filtration rates is dependent upon particle size, concentration and composition of the residual solids. No simple relation was found to relate coal type with filtration rate.     

Filtration in coal liquefaction: preparation of filter aid from filter cake

In this study the preparation of filter aid from filter cake is considered. Two methods of preparation, carbonization at 550–600 °C and combustion at 800–1000 °C, are used and the optimum conditions determined. The performance of the products is assessed as both a precoat and body feed by comparison with a commercial grade of diatomite. It is shown that efficient filter aids, with performances comparable to that of diatomite, may be produced from filter cake.     

Mesoporous carbon prepared from direct coal liquefaction residue for methane decomposition

Mesoporous carbons (MCs) were directly prepared from direct coal liquefaction residue (CLR) by KOH activation, and used as catalysts for methane decomposition. The results indicated that the prepared MCs were of a narrow pore size distribution centered at about 3.5 nm. The mineral matters in the CLR and their salts formed during KOH activation process served as templates for mesopore formation, through washing off the mineral matters and the salts occupied in the inner space of the carbon. The resultant MCs showed higher and more stable activity in methane decomposition reaction than commercial coal-based activated carbon and carbon black catalysts.     

Hierarchical porous carbons prepared from direct coal liquefaction residue and coal for supercapacitor electrodes

Hierarchical porous carbons were prepared from a coal liquefaction residue (CLR) and two coals, Shenhua (SH) coal with low and Shengli (SL) coal with high ash content, by KOH activation with the addition of some additives, and used as the electrode for supercapacitors. Two metal oxides (MgO and Al₂O₃) and three organic materials (sugar, urea and cetyltrimethylammonium bromide) were used as the additives, to investigate their effects on the structure and capacitive performance of the resultant carbons. The results show that the metal oxide and/or its salt formed by the reaction with KOH can serve as space fillers of nanopores in the carbonized carbon, while the gases produced by the decomposition of the organic additive can develop and/or widen some pores. Both help the carbon produced from CLR or the SH coal with low ash content to have additional meso- and macropores, but destroy the structure of the carbon from the SL coal with high ash content. Compared with the carbon without any additive, the optimized hierarchical porous carbon with each additive shows a smaller equivalent resistance, much higher capacitance in a wide range of charge–discharge rates and excellent cycle stability when the carbon was used as supercapacitor electrode.     

Laboratory evaluation of four coal liquefaction catalysts prepared from modified alumina supports

A blank alumina catalyst support was modified by deposition of TiO₂, ZrO₂ or carbon, and cobalt molybdate catalysts prepared from the modified supports were evaluated in a laboratory coal liquefaction microreactor unit. Performance was referenced to a commercial CoMo/alumina catalyst, Amocat 1A, which was derived from the same blank support. All catalysts possessed very similar pore structures, and thus the effect of pore structure was practically eliminated from the comparison. The effects of support modification on catalyst performance were found to be minimal.     

Rapid liquefaction of Longkou lignite coal by using a tubular reactor under methane atmosphere

Rapid liquefaction of Longkou lignite coal under methane atmosphere was studied in a novel laboratory scale tubular reactor. Experiments were performed at a temperature ranging from 400 to 800 °C, a residence time ranging from 4.5 to 11.2 s and 10–15 MPa (without catalyst). Reactions were also carried out under a nitrogen gas atmosphere at the same reaction conditions. The results indicate that there are synergistic effects between coal and methane at temperatures higher than 600 °C, and the temperature and residence time are the main factors influencing the coal conversion and products distribution. The oil yields reach a maximum of 21.97 (wt.% daf) at 750 °C during 9.0 s.     

Bioleaching of molybdenum from coal liquefaction catalyst residues

It has been shown that the bacterium Thiobacillus ferrooxidans can solubilize MoS₂ from coal liquefaction catalyst residues. The MoS₂ is formed during the liquefaction process from a molybdenum catalyst precursor. MoS₂ is insoluble; in order to be recovered and reused, it must be converted to a soluble form. T. ferrooxidans can oxidatively solubilize the molybdenum in MoS₂ to molybdate, in which form it can be recovered as a soluble or HCl extractable material. Bioleaching experiments show that with a starting cell concentration of 1.0 × 10⁷ cells ml⁻¹, or greater, a significant amount of the molybdenum in the residue was solubilized. These experiments indicate that the amount of molybdenum biologically solubilized from the liquefaction residues is dependent on inoculum size, with all strains of T. ferrooxidans tested having equal ability, and on the particle size of the residue. An important factor in the solubilization of MoS₂ by T. ferrooxidans is the inhibitory effect of molybdate. Literature reports that as little as 10 ppm molybdate is inhibitory to growth or ferrous iron oxidation. However, leachates containing in excess of 70 ppm molybdenum (equivalent to 116 ppm molybdate) were generated as a result of bioleaching of the liquefaction residue. When cells from previous leaching experiments were used to inoculate flasks containing fresh media and additional liquefaction residue, the bacteria were able to bioleach the fresh residue. Recent experiments have focused on the ability of T. ferrooxidans to produce protective agents in the leachate that minimize the inhibitory effects of molybdate. We found that production of the protective factor(s) did not depend on previous exposure of the cells to molybdenum or liquefaction residue.     

Preparation of carbon microfibers from coal liquefaction residue

Carbon microfibers (CMFs) were synthesized directly from coal liquefaction residue (CLR) by arc-jet plasma method at atmospheric pressure, and were examined using scanning electron microscopy and EDX spectroscopy. It has been found that the as-synthesized CMFs are smooth in surface and quite uniform in diameter that is smaller than 1 μm and centers at 700 nm. The possible mechanism involved in the formation process of CMFs is proposed and discussed in terms of the special chemical composition of CLR and the process parameters. This work may open a new way for direct and effective utilization of the CLR.     

Chemistry of Texas lignite liquefaction in a hydrogen-donor solvent system

To study the nature of chemical cleavage and resultant product transfer from solid lignite phase to liquid phase, autoclave (300 cm³) experiments have been carried out at pressures ranging up to 34 MPa and temperatures of 380–390 °C. The charge to the autoclave was freshly mined wet lignite, tetralin and hydrogen or helium. To obtain an indication of the reaction mechanisms underlying the liquefaction process, liquid and gas samples from the reactor at different time intervals were analysed. The gas samples were analysed by use of a multi-column, multi-valve automated gas Chromatograph, a system specially fabricated for coal-derived gas analysis. The liquid sample was filtered through Millipore filters and separate into three fractions by gel permeation chromatography. Fraction 1 is mostly colloidal carbon and high-molecular-weight species which cannot be separated on a g.c. Fractions 2 and 3 were analysed by gas chromatography — mass spectrometry (g.c.-m.s.). Fraction 2 represents the liquid products released from lignite and fraction 3 is mostly the tetralin and tetralin-derived products. Gel permeation chromatography (g.p.c.) followed by gas chromatography (g.c.) was used to devise a method for monitoring the extent of liquefaction. The production of carbon dioxide is at a maximum before the liquefaction reactions are at a significant rate. The source of carbon dioxide appears to be the carboxylic groups in lignite. The liquefaction reactions consume hydrogen from tetralin which undergoes dehydrogenation to form naphthalene. Once the lignite has undergone depolymerization, the tetralin to naphthalene conversion slows down. The continued heating of lignite conversion products in excess of tetralin does not appear to alter the molecular size distribution of the liquid product. The distillable fraction of lignite-derived liquid is composed of various alkylated phenols and aromatics and alkanes, and they are formed simultaneously.     

生物质和煤基活性炭制备过程的活化特性比较

分别以稻壳、木屑及褐煤为原料用水蒸气活化法制备了活性炭,比较了所得活性炭的吸附性能和炭化料的反应活性,探明了造成不同原料活化特性差异的原因。结果表明,活化过程中生物质原料的反应活性优于褐煤,炭化料的活化速率遵循脱灰稻壳>木材炭化料>稻壳炭化料>褐煤炭化料。通过对炭化料进行元素分析、气化反应活性分析、BET、SEM、XRD、FTIR、XPS等物理和化学性质的表征,揭示了不同原料表现出不同活化特性的原因。结果表明,在相同炭化和活化条件下,原料挥发分越高,灰分越低,炭化料有机含氧量越高,则水蒸气的活化速率越快,更容易在短时间内制备出高性能的活性炭。     

Optimization of coal liquefaction reactors

The method of Lagrangian multipliers of classical calculus is applied to obtain the optimum operating conditions of three continuous stirred tank reactors (CSTR) in series, to liquefy coal with minimum gas yield. In the formulation of the problem, the gas conversion was assumed to be the cost function. Results obtained were supported by the experimental findings in terms of liquefaction temperatures, reaction times, and conversion of coal into various liquefaction products.     

Underground gasification of coal and lignite

The underground gasification of coal and lignite is of interest when traditional coal extraction is impossible or unprofitable and also with increasing demand for thermal and/or electric power. In the Soviet Union, at six industrial Podzemgaz stations, beginning in the 1930s, more than 15 million t of coal was processed to obtain more than 50 billion m³ of gas. The South Abinsk station operated from 1955 to 1996, while the Angren station has been operating since 1963. Research on the underground gasification of coal has been largely theoretical, without close connection to industrial practice, and the results are based on mathematical modeling and data from 50–70 years ago. Obviously, Russia’s leading position in the underground gasification of coal has been lost. Russia now lags a number of countries that are making significant investments in the process. Note that, in Russia, despite the obvious benefits of underground gasification of coal, interest in the process has waned, on account of its significant deficiencies: the possibility of gas filtration to the surface; insufficient controllability of coal-bed preparation and thermal processing; the relatively low heat of combustion of the gas produced; and considerable losses of gas and coal underground. Note also the environmental impact of the underground gasification of coal, associated with the deformation of rock, its thermal, chemical, and hydrogeological changes, increase in its temperature, and active chemical pollution of groundwater. An obstacle to the adoption of the underground gasification of coal is the lack of clear ideas regarding the preparation and use of fuel gas. Recommendations for improving the process focus on the design of the underground gas generator and the gasification of the coal bed, without addressing the technology of the underground system, whose cost accounts for ~75% of the total equipment costs. The method proposed in the present work for the preparation of fuel gas from coal challenges the notion that the gas produced in underground gasification of coal should be divided into two products: gas and the tar (hydrocarbons) that forms in the preparation of the gas for combustion. If the specified temperatures are maintained, no condensation of hydrocarbons in the equipment and gas lines is observed, and dry dust removal from the gas may be employed, without complex processing of wastewater and of explosive and toxic materials. That significantly improves the economic and environmental characteristics of the process, Analysis of the results shows that the proposed approach to purifying the fuel gas produced by underground gasification of coal and lignite reduces capital costs in construction of the system by almost half; and the costs of gas production by a factor of 1.7. The time to recoup the initial investment is shortened by 41%; the yield of thermal energy is increased by 10.5%; and annual power output is increased by 151296 MW-h.     

Data on low rank coal liquefaction from DRIFT analysis

A Spanish low rank coal was hydrogenated without solvent in the presence of a molybdenum catalyst. The asphaltenes and derived residues were analysed to obtain an understanding of the behaviour of the coal under hydrogenation conditions and the influence of a previous extraction with chloroform, as well as reaction time and temperature, on the structure of the products obtained. Under the reaction conditions used, with tubing bomb reactors and an initial hydrogen pressure of 7 MPa, the data obtained for this coal show that an increase in temperature from 350 to 400 °C, and of reaction time from 5 to 60 min gives, in general, lower content of organic matter in the residues. The two variables have opposing effects on the aromatic content of the asphaltenes. Increasing reaction time reduces the aromaticity, whereas increasing the temperature will increase aromaticity. Prior extraction of the low molecular weight chloroform-soluble compounds increased conversion, and resulted in asphaltenes with higher content of aromatic- and oxygen-containing groups.     

煤直接液化技术进展

通过煤炭液化生产液体燃料油来满足我国日益增长的需求,是解决石油资源不足的有效手段之一。文中综述了国内外煤直接液化工艺及其催化剂的研究进展,并指出了当前煤直接液化技术发展的方向。     

Structural changes occurring in coal liquefaction

Wyodak and Monterey Coals have been liquefied in solvents of systematically varied composition, and the products characterized by selective oxidation. Comparison of results of treating model compounds and the parent coals indicates that liquefaction results in an increase in arylmethyl groups and in dehydrogenation of alicyclic rings to form aromatic rings.