A model for short residence time hydropyrolysis of single coal particles

Abstract

The chemical reactions and mass transfer processes occurring within a single coal particle during high temperature hydropyrolysis are described mathematically and the model is tested using extant experimental data. A significant feature of the model is the treatment of bulk flow due to the evolution of volatiles and the resulting balance between diffusion, bulk flow, and chemical reaction. Predictions of the variations in conversion due to changes in total pressure, hydrogen partial pressure, and particle size agree quantitatively with experimental data. The objectives of this study were to develop a model of the chemical reactions and transport processes taking place within a small coal particle after exposure to hot hydrogen at high pressures and to test the predictions using extant experimental data. Such a model provides a means for understanding the effects of process variables (principally temperature, pressure, particle size, and residence time) on the yields from a given coal. Previous work on single-particle models focuses on the various chemical reaction rates and either ignores mass transport processes completely or lumps the resistance into a film coefficient. Although such models can often be adjusted to fit data for a given particle size, the sharp increase in yield observed as particle size is decreased appears to be beyond their predictive capabilities. To illustrate the role of mass transfer by bulk flow and diffusion, the kinetics of the devolatilization process and the time temperature history of the particle have been simplified by assuming a single first-order devolatilization reaction, instantaneous heatup to an isothermal state, and a long reaction time. CONCLUSIONS AND SIGNIFICANCEA theory has been developed to describe the chemical reactions and mass transfer processes occurring in a single coal particle during hydropyrolysis. The particle is assumed to maintain a porous structure throughout decomposition, with volatiles transported by diffusion and hydrodynamic flow. Chemical reactions are described in terms of the weight loss kinetics developed by previous workers and include the primary devolatilization of the coal, secondary reactions of free radicals, and direct hydrogenation of char. Order of magnitude estimates are used to show that bulk flow and the combination of free radicals with hydrogen are rapid compared to devolatilization, while diffusion is generally slow. The corresponding mathematical simplifications in the conservation equations lead to analytical solutions for the instantaneous rate of evolution of volatiles. Total yields obtained by integrating over an idealized temperature history agree closely with experimental data for variations in pressure, hydrogen partial

A Comparison of the Gas‐Product‐Release Characteristics from Coal Pyrolysis and Hydrogasification

“…The catalytic activity sequence of each catalyst studied under a hydrogen atmosphere is as follows: K>Na>Ni>Ca. The formation rate of free radicals during the hydrogasification process is very rapid as the hydrogen supply rate is significantly lower than the free‐radical‐generating rate during the noncatalytic hydrogasification process,22 and the mass transfer rate of hydrogen becomes the main rate‐controlling step 23. The introduction of K and Na catalysts promotes the fracture of the weak chemical bonds of coal, which increases the number of active sites on coal surface.…”

Intraparticle mass transfer in coal pyrolysis

“…Finally, it is assumed that the characteristic time for mass transfer is much shorter than that for pyrolysis; therefore we need only consider the steady state mass transfer problem. The assumptions of isothermal particles and pseudo-steadystate mass transfer have been discussed by Russel et al (1979). When the conditions for using an effective particle size are satisfied (see Appendix), the particles are essentially isothermal.…”

“…At atmospheric or lower pressures, when A is small and transport occurs mainly by diffusion, the concentration profiles of the pyrolysis products are determined by the parameter B alone. Russel et al (1979) developed a model for coal hydropyrolysis which used the same flux relations as Equations ( l o ) -( 14), except that diffusion was assumed to be in the bulk regime, (on account of the high pressiire), and all binary diffusion coefficients were set equal to each other, Effective diffusivity and permeability were used, and questions of pore connectivity were not considered. In the balance equations used by these authors, the reaction rate terms were functions of the dependent variables, and the formulation resulted in a boundary value problem.…”

Kinetics of dielectric‐loss microwave degradation of polymers: Lignin

“…Kanury,22 Lee et a1.,42 and Fan et al 43 have proposed more complex chemical reaction models coupled to heat and mass transfer descriptions to account for conductivity, porosity, and heat capacity changes in wood pyrolysis. Russel et al 44 point out that when coal is pyrolyzed, there are large differences in time scales for heat transfer and volatiles escape, relative to chemical reaction. Estimation of these times for wood pyrolysis indicates that heat transfer is two orders of magnitude slower than reaction for temperatures above 4OOOC in rapid wood pyrolysis.…”