Rapid hydropyrolysis of softening coal particles—A modeling study. Part II: Model predictions and comparison with experimental results

Abstract

The mathematical model described in Part I is applied to predict CHI and tar yields in rapid hydropyrolysis of softening coal particles. Predicted effects of pressure and particle size agree with trends previously measured in a screen heater apparatus. GEORG SCHAUB SCOPEPrevious experimental results with Pittsburgh Seam bituminous coal indicate that mass transfer plays an important role in rapid hydropyrolysis. The measured effects of particle size and pressure on CHI and tar yields could not be adequately described with available kinetic models owing to their not accounting for the drastic changes in physical properties associated with softening and resolidification during hydropyrolysis of softening coals. A new hydropyrolysis model which does account for the property changes of softening coals has been formulated and presented in Part I. In the present paper the model is tested by comparing its predictions with experimental results in order to verify its underlying mechanistic concept. CONCLUSIONS AND SIGNIFICANCEMathematical representation of the global chemical reactions and mass transfer leads to predictions for particle size and pressure effects on CH4 and tar yields. The agreement of predictions with measurements, e.g., of the different effects of total pressure and Hz partial pressure on the yields, is better than previous models for softening coal (Anthony et al., 1976). This success can be attributed to the underlying mechanistic concept of diffusional transport of Hz in a material whose transient physical properties range from those of a fluid during the plastic period to those of a porous solid after resolidification. Accordingly, the coal's transient fluidity is significant, as penetration of Hz into the particle i s hindered during the plastic period due to the low Hz diffusivities in the plastic-liquid state. Diffusion is comparatively faster in the resolidified coke after pyrolysis gas evolution is nearly completed.The present experimental data base is sparse. Areas of particular interest for further experimental investigation and model validation are the rheological properties of softened coal under rapid hydropyrolysis conditions and the separate effects of inert gas pressure, Hz pressure, and particle size on individual product yields (e.g., CHI and tar).The results of this study are pertinent to coal (hydrw) pyrolysis and gasification modeling and to addressing such questions as how to maximize individual product yields or how to minimize agglomeration of softening coal in conversion processes,