Coal conversion kinetics

The microautoclave solvent activity tests measure coal conversion in a small batch reactor under carefully controlled conditions. The tests are described as Kinetic, Equilibrium and SRT. The Kinetic and Equilibrium Tests measure coal conversion to tetrahydrofuran solubles at conditions where conversion should be monotonically related to hydrogen transfer. The Kinetic Test is performed at 399°C for 10 minutes at an 8 to 1 solvent to coal ratio. The combination of high solvent ratio and low time provide a measure of performance at essentially constant solvent composition. The measured conversion is thus related to the rate of hydrogen donation from solvent of roughly a single composition. In contrast, the Equilibrium Test is performed at 399°C for 30 minutes at a 2 to 1 solvent to coal ratio. At these conditions, hydrogen donors can be substantially depleted. Thus performance is related to hydrogen donor... 

Autoclave Results - Solvent Activity Test. The initial microautoclave work was done with tetralin and methylnaphtha-lene, using Indiana V bituminous coal (Table I). Base line data is shown in Figure 4. All three tests, Kinetic, SRT, and Equilibrium, show an increase in coal conversion with an increase in the concentration of tetralin. The Equilibrium Test shows the highest coal conversion of approximately 86 wt% of the MAF coal (based on the solubility in the tetrahydrofuran) at the 50% tetralin concentration. The Kinetic Test shows lower coal conversion. The hydrogen transferred to the coal from the tetralin in the Equilibrium Test at the 50 wt% tetralin feed concentration is approximately 0.5 wt% of the MAF coal. In the Kinetic Test 50 wt% tetralin feed concentration results in a much smaller transfer at the short reaction time of 10 minutes. 

Many studies on direct liquefaction of coal have been carried out since the 1910 s, and the effects of kinds of coal, pasting oil and catalyst, moisture, ash, temperature, hydrogen pressure, stirring and heating-up rate of paste on coal conversion, asphaltene and oil yields have been also investigated by many workers. However, few kinetic studies on their effects to reaction rate have been reported. 

If the mobile phase is present in a significant concentration, as suggested by the results of solvent extraction studies (1,8), the practical meaning of the mobile phase to coal conversion processes may be profound. In coal liquefaction, two stage processes emphasizing the mobile phase and the macromolecular structure separately could well be most economical. In devolatilization kinetics, at least two sets of kinetic parameters are necessary to model the devolatilization phenomena associated with the mobile phase and the macromolecular structure respectively since the mobile phase components devolatilize at much lower temperatures than the macromolecular structure components 0. In addition, the mobile phase appears to have a significant influence on the thermoplastic properties of coal (0 and thereby on coke quality. 

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. 

A Fundamental Chemical Kinetics Approach to Coal Conversion... 

At the present time, few, if any, details of chemical reaction mechanisms in coal conversion are known with certainty. This situation is particularly distressing in the areas of coal liquefaction and pyrolysis where chemical kinetics may strongly influence process efficiency and product quality. To improve this situation, in recent years a number of research groups have been performing chemical studies of coal and "model" compound reactions. 

This work presents the first systematic application of these methods to coal chemistry. This analysis is intended not only to suggest likely reaction mechanisms, but also to demonstrate the unique power of thermochemical kinetics methods for semi-quantitative analysis of the complex chemistry of coal conversion. 

Bond homolysis rate constants are estimated for many covalent bonds presumed to be present in coal conversion reactions. Other modes of bond breaking are examined using thermochemical kinetic methods Cl). 

Kinetic Phases of Coal Conversion in Thermal Plasma... 

Kinetic Analysis of Thermal Plasma Conversion of Coal Kinetic Features of the Major Phases of Coal Conversion in Plasma... 

The kinetics of electrochemical coal oxidation at intermediate temperature (108°C) in a slurry containing a Fe(II)/Fe(III) redox couple was studied by Jin and Botte (2010). They found that coal oxidation by Fe(III) is the limiting step. Films that grow at the surface of the coal particles limit the coal conversion. 

A kinetic model for coal conversion needs to take into account the following conversion processes ... 

Because it is decisive for the overall carbon conversion, special emphasis should be placed on the most important conversion process during coal gasification the char conversion. The intention is to establish a coal-adapted kinetic submodel for a CFD case study. 

In the design of a coal conversion reactor, it is necessary to consider not only the reaction kinetics of a single particle but also the hydrodynamics of gas and mixing of solids and the accompanying heat and mass transfer occurring in the reactor. 

As shown in Figure 3, solubilization roughly conforms to first-order kinetics, where rate = k[unconverted coal]. Rate constants of 3 x 10 and 1 x 10 min l are found for 250° and 275°C respectively, with nearly total conversion in less than 30 minutes at the higher temperature. Although negligible reaction takes place with heatup to 250°C (so-called "zero time), considerable reaction occurs in the few minutes of heatup between 250° and 275°C. During this period, solubility rises 20%, incorporation approaches its maximum extent, and the H/C ratio drops to 0.75. 

Step 1 above requires that there be bonds in the coal that are weak enough to break in appropriate numbers at conversion temperatures and times. Table I displays some kinetic data for the cleavage of benzylic bonds in a series of increasingly aromatic compounds. In accord with expectation, an extension of the aromatic system increases the ease with which the benzylic bond is broken. 

Kinetic studies ( ) of such systems Indicate that the Initial stages of liquefaction Involve conversion of the coal Into a more or less completely pyrldlne-soluble solid and thereafter Into a benzene-soluble material which Is gradually transformed Into a viscous liquid as increasing amounts of hydrogen combine with It. This process can be catalyzed by, e.g., cobalt molybdate, but proceeds rapidly even in the absence of catalysts. At 775 F (A00°C), total py-solubi1ity (and vSO per cent solubility In benzene) can be attained within less than 10 minutes. 

Karweil (12) tried to illustrate graphically the correlation between coal rank, rock temperature, and duration of heating on the basis of reaction kinetics. In Figure 18 the ordinate records the temperature, and the abscissa indicates coal rank (in terms of volatile matter and a conversion factor = Z). 

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