The objective of the present work is to elucidate the influence of intraparticle mass and heat transfer phenomena on the overall rate and product yields during devolatilization of a single coal particle in an inert atmosphere. To this end a mathematical model has been formulated which covers transient devolatilization kinetics and intraparticle mass and heat transport. Secondary deposition reactions of tarry volatiles also are included. These specific features of the model allow a quantitative assessment to be made of the impact of major process conditions such as the coal particle size, the ambient pressure and the heating rate on the tar, gas and total volatile yield during devolatilization. Model predictions are compared to a limited number of experimental results, both from the present work and from various literature sources.

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Effects of intraparticle heat and mass transfer during devolatilization of a single coal particle

A mathematical model is presented to simulate the multiple heterogeneous reactions with complex set of physicochemical and thermal phenomena in a moving bed of porous pellets. This model is based on both heat and mass transfer phenomena of gaseous species in a porous medium including chemical reactions at interfaces whose areas vary during the conversion. This model accounts for both the exothermic and endothermic reactions which can be equimolar or nonequimolar. Furthermore it considers simultaneously the reactions in the nonisothermal transient condition. A powerful technique based upon finite volume fully implicit approach has been implemented to solve the complicated governing equations numerically. The model has been validated by comparing with various experimental and analytical results in two cases: the single pellet scale as well as the counter current moving bed reactor.

Modeling of multiple noncatalytic gas–solid reactions in a moving bed of porous pellets based on finite volume method

https://ris.utwente.nl/ws/files/6562158/Bliek86gas.pdf

Gasification of coal-derived chars in synthesis gas mixtures under intraparticle mass-transfer-controlled conditions

Diffusion and reactions in a porous pellet are treated as transient, nonisothermal, nonisobaric processes. Continuity and energy balance equations are solved simultaneously for temperature and concentration profiles in the pellet. Conductive, convective as well as radiative heat transfer are included. The ‘dusty gas’ flux model is used to describe the transport of diffusing gases. Viscous, bulk and concentration gradient terms have been included. Structural changes with reaction are accounted by considering the effect of changing grain radius on porosity and pore diameter.



The model predictions match conversion trends for carbon gasification over a temperature range of 800 to 1100°C investigated experimentally.



On a atudia la diffusion et les raactions dans une pastille poreuse, en tant que procadas non isobares, non isorthermes et transitoires. Les aquations d'anergie et de continuita sont rasolues simultanament pour des profils connus de concentration et de temparature dans la pastille. Les transferts de chaleur conductif, convectif et radiatif sont agalement pris en compte. Le modale de flux de ‘gaz poussiareux’ est utilisa pour dacrire le transport des gaz diffusants. Les termes de gradients de concentration, d'agglomaration et de viscosita sont agalement donnas. On tient agalement compte des changements de structure avec la raaction en observant les effets produits par le changement du rayon du grain sur la porosita et le diamare du pore.



Les pradictions du modale sont en accord avec les rasultats de conversion pour la gazaification du carbone obtenus exparimentalement, a des temparatures comprises entre 800 et 1100°C.

A gas-solid reaction model ― variable temperature, pressure and structural parameters

Fully transient analysis of Fluid-Solid Non-Catalytic Reactions (FSNC) based on the Sharp Interface Model (SIM) has been carried out using the Finite Volume Method (FVM). The effects of nonlinear kinetics and relative resistances have been evaluated at the isothermal condition for particles with constant as well as varying size. The numerical solution agrees well with the analytical solution for the first order reactions and the results for non-linear kinetics compared favourably with the results reported in literature. Various types of multiple reactions like independent, parallel and consecutive reactions have also been studied. The Finite Volume Method has thus been established to be an effective numerical tool for modeling such reactions.

Mathematical Modeling of Noncatalytic Fluid-Solid Reactions—Generalized Mathematical Modeling of Fluid-Solid Non-Catalytic Reactions Using Finite Volume Method: Nonisothermal Analysis—

A method has been outlined for the calculation of the pressure gradient that can exist within the reacted shell when a spherical pellet reacts with a gas and undergoes a transport controlled topochemical reaction. It is known that pressure gradients can arise because of Knudsen flow existing in the reacted shell with small pores and the reactant gas having a different diffusivity than that of the product gas. The phenomena can be represented by a boundary value problem involving a set of partial differential equations with a moving boundary, incorporating time and positional dependence of diffusivities of the reactant and product gases. In the present work, the resulting equations have been solved numerically. A study has been made of the influence of the relevant parameters like total and Knudsen diffusivity ratios of the reactant and product gases, the porosity to tortuosity ratio of the reacted shell, the Biot modulus, the equilibrium constant of the reaction and the viscous flow parameter on the pressure build up inside the reacted shell.

T. Deb Roy

Papers

An analysis of the pressure build-up inside a reacting pellet during gas solid reactions