Coal Liquefaction Kinetics

Abstract

There are similarities in both the yield of volatile products and the activation energy for hydrogenation and for dissolution of bituminous coal. Pyrolysis of the same coal, while the yield of volatile products was significantly less, gave a similar activation energy.

In all three of the processes, pyrolysis, dissolution and hydrogenation both first and second order mechanism have been assumed by different investigators. The activation energies found are usually in the range of 125–210 MJ/mole regardless of the order assumed, indicating some chemical reactions are controlling the rate. It is very difficult to generalize the kinetics and mechanism of the reactions involved in direct coal liquefaction despite the similarities pointed out. Part of the reason for this is that sufficient precise work has not been done on the same coals by different investigators and to the same extent of reaction. For example, dissolution of coal may result in nearly complete “liquefaction” up to 95% of the maf coal to products soluble in tetrahydrofuran, benzene or pyridine. However, the average molecular weight of this material is typically more than 1000. Catalytic hydrogenation, batch or continuous, results in nearly as high a yield of soluble products (80–85 weight percent of maf coal). However, the average molecular weight of such products is often 300 or less and even in heavy liquid fractions species having more than 4 rings per cluster were not found.

Until the structure for a coal used in a liquefaction process can be completely defined and a complete analysis of all products obtained by the liquefaction can be carried out, we can only speculate on the reaction mechanism. Kinetic studies give valuable, although incomplete, information on the possible rate controlling reactions. Presently these reactions appear to be diffusion or physical processes at lower temperatures (< 350°C) and chemical at higher temperature (> 400°C). Although kinetics and thermodynamic calculation for any reaction are considered as separate areas of study. It has been pointed out that there is usually a correlation because reactions which are thermodynamically favored proceed at a faster rate. Others have also correlated thermodynamics and kinetics. Further work will show whether this approach can be realized for a system as complex as coal liquefaction.