Two-stage kinetic model of primary coal liquefaction

A mathematical model enabling the calculation of reaction rate parameters for coal weight loss during liquefaction has been formulated. The model explicitly accounts for (i) product release during heatup as well as during holding at peak experimental temperature and (ii) stages of the process prior to and following the onset of extensive covalent bond scission (around 350 degrees C); the procedure thus allows the calculation of distinct activation energies for the two stages. In the initial phase of the calculation, both stage A processes (prior to the onset of extensive bond cleavage) and stage B processes (following the onset of extensive bond cleavage) have been modeled as single, irreversible, first-order processes. In the fully developed model, both stage A and stage B were considered to proceed by means of multiple parallel, independent, irreversible, first-order reactions with a Gaussian distribution of activation energies. Calculations were matched with liquefaction experiments in tetralin, carried out in a flowing-solvent reactor using seven Argonne PCSP coals and Point of Ayr (U.R) coal. For stage A processes, the single-reaction model gave activation energies between 35 and 80 kJ mol(-1). Energies of activation for the higher temperature range stage B processes (about 350-450 degrees C) were found to be significantly higher, between 124 and 238 kJ mol(-1). This result confirms the validity and necessity of describing coal liquefaction in terms of a two-stage process. For stage A, the multiple parallel independent reaction model gave mean activation energies that were only slightly larger than those calculated from the single-reaction model. Activation energies for stage B processes, however, were greater, between 160 and 275 kJ mol(-1), indicating that the single-reaction model significantly underestimates energies of activation for coal thermal breakdown. For individual coals, the standard deviation of the distribution of activation energies progressively decreased with increasing coal rank, apparently reflecting increasing structural uniformity and possibly also increasing degrees of cross-linking accompanying coal maturation. Activation energies reported in the literature for the liquefaction of coal, mostly based on isothermal kinetics, range from 18 to 358 kJ mol(-1). Qualitative agreement has been found between values from the present work and results from a multiple, parallel independent reaction scheme fitted to data from the pyrolysis of the Argonne PCSP coals.