Supercritical coal liquefaction

Isotope Effects in Supercritical Water Kinetic Studies of Coal Liquefaction... 

Coal liquefaction studies with Illinois No. 6 coal in supercritical water/CO systems have demonstrated that a suitable model for liquefaction includes a branch point. Thus coal is partitioned between reducing (i.e. liquefying) steps, and steps where strictly thermal reactions consume convertible sites and yield unconvertible char. 

In supercritical HjO it was found that the toluene-soluble products (TS) profile leveled off at about 50%. However in supercritical D2O, liquefaction was consistently superior to that in the protio medium, and the ultimate convertibility was about 60%. This inverse isotope effect can be explained by a model in which the limits to conversion are solely based on the competition between the kinetics of the parallel routes to TS and char, respectively. Conversion is thus limited by the kinetics of the conversion system, this limitation being even more severe in conventional donor systems which are inherently less effective for liquefaction than is water/CO. In principle the conversion of coal virtually completely to TS in a single step is feasible in a system with sufficient reducing potential. 

Amestica and Wolf (12) in a study closely related to the one described herein, measured the conversion of Illinois No. 6 coal in toluene and ethanol. Their results clearly showed that conversions increased with temperature and solvent density but were not detailed enough to show the time dependence of the conversion. However, a result important to this study was that toluene converts coal to liquids without significantly reacting itself. After reaction, 98% of the toluene used was recovered versus only 73 -85% of the ethanol in runs using it. Ethanol is a hydrogen donor and reacts extensively with the coal. While toluene probably reacts with coal to a small extent, its effect was primarily physical in nature. As such, it is a good candidate for studying the effects of a supercritical solvent on coal liquefaction kinetics since the enhancement effect of supercritical conditions is physical in nature. 

The results of the model simulation are given in Figures 6, 7 and 8. Figure 6 shows the effect of density on the kinetics of coal liquefaction at temperature of 698 K. It is very clear that conversion increases with an increase in density and it is consistent with the hypothesis made earlier on coal liquefaction with a supercritical fluid. Figure 7 shows the effect of temperature on the kinetics of coal liquefaction at density of 0.601 g/cc. It is evident that conversions are higher at higher temperatures and lower reaction times but at longer reaction times due to condensation reactions, a decrease in rate with temperature is observed. 

Coal liquefaction under supercritical (or subcritical) water condition has some advantages over organic solvents. Supercritical water is miscible with H2, CO, aromatics, and oils, which provides a unique, homogeneous reaction medium for coal liquefaction. CTL process with supercritical water is more environment-friendly than SRC-II process and has higher conversion for low-rank coal [31,32]. 

Supercritical fluid solvents have been tested for reactive extractions of liquid and gaseous fuels from heavy oils, coal, oil shale, and biomass. In some cases the solvent participates in the reactions, as in the hydrolysis of coal and heavy oils with water. Related applications include conversion of cellulose to glucose in water, dehgnincation of wood with ammonia, and liquefaction of lignin in water. 

Conclusions When coal is contacted with a non-donor supercritical fluid a part of the coal instantaneously dissolves in the. supercritical fluid. The dissolved coal undergoes liquefaction reactions which are thermal in nature resulting in toluene soluble products being formed from coal. These products can subsequently undergo retrogressive reactions yielding insoluble material. Hence the toluene solubles show a maxima in conversion with time. 

This study indicates that extraction with supercritical water could be an attractive route for liquefaction of Victorian brown coals (but probably not black Australian coals). The low cost and ready availability of the solvent (water), the relatively high H/C atomic ratios of the extracts, and also as no hydrogen or coal-drying are required, are positive factors. Higher yields can be obtained when a strong base or a hydrogen-donor is added to the water. 

Consequently, the vast majority of SCF applications are based on CO2 near the GL critical point, with a possible admixture to support the ability for solvating dipolar components. The extraction of carcinogenic aromatic hydrocarbons and their nitro derivatives from diesel particulates by CO2 + toluene or methanol SCF can serve as an example. CO2 based SCF also helps in cleaning polyethylene from undesired polymer additives. In a similar way one can consider technologies focused on so called h q)er-coal, an extremely pure and environment friendly fuel for turbines in power plants. Recently, the first power plants based on this idea are being constructed in China. The removal of pesticides from meat, decaffeinated coffee and denicotinized cigarettes are the next society-relevant applications. Noteworthy is the h q)er-oxidation with supercritical water and bitumens extraction based on supercritical toluene. The latter system is also used for the liquefaction of coal. ... 

Rinc6n, J.M. and Cruz, S. 1988. Influence of pre-swelling on the liquefaction of coal. Fuel, 67 1162-1163. Schneider, G.M., Stahl, E., and WiUce, G. 1980. Extraction with Supercritical Gases. Verlag Chemie, Weinhekn, Germany. 

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