| The material and energy balances derived during single and multiple gas-solid reactions that take place on a single particle have been utilized to (1) model the char combustion, (2) model the gasification of char, (3) establish the conditions for ash agglomeration during combustion and gasification, and (4) verify the direct oxidation model for char combustion. The shrinking core model was employed throughout the thesis. Also, it was assumed that (1) the reacting solid particles are spherical in shape, (2) the reactions between the gases and the solids are of the first order, (3) a sharp interface exists between the solid product and the solid reactant, (4) the gas-solid reactions are confined to the interface, (5) the size of the solid reactant changes as the reaction proceeds, and (6) the ideal gas law is applicable. The reaction resistances accounted for in the development of various models include the intraparticle diffusion resistance, the intermolecular diffusion resistance, the reaction rate, and the interphase diffusion resistance. Analysis of single and multiple gas-solid reactions revealed that the particle growth has a significant influence on reaction time when the diffusion resistances control the overall rate of reaction. The solution of the unsteady-state heat balance equation showed that the particle core temperature is greatly influenced by the heat of reaction, the size of the particle, the partial pressure of the gaseous reactant, the particle growth factor, and the ambient temperature. This information was translated into the design of an ash-agglomerating coal gasifier that exclusively employs the principle of generating ash-softening temperature within the particle. When more than one reaction is involved, the ash agglomeration was found to also depend on the relative rates of reactions and relative and absolute partial pressures of the gaseous reactants. The tendency for ash agglomeration to occur under various operating conditions has been examined, and the trends have been compared with actual gasifier performance.; The reactions taking place on a single particle have been extended to the modeling of the fluidized-bed processes. The bubbling as well as nonbubbling fluidized bed were considered. The modeling procedure involves setting up gas-solid reaction equations for a single particle of different sizes; the overall rate of reaction in the bed is obtained by adding these individual rates. A variety of gas and solid contact patterns were used. For example, the pattern of solids in backmix and gases in plug flow was applied to char combustion with simultaneous sulfur removal and to the ash-agglomerating gasification process. Similarly, the pattern of solids and gases both in backmix was applied to char combustion. From the available data, it has been established that the direct oxidation model is applicable to char combustion. Using this information, it is also established that in passing through the fluidized bed, the bubbles are flushed at least once during their stay in the bed.; The fluidized-bed models presented in this thesis are capable of yielding such information as the carbon conversion efficiency, the sulfur retention, the particle size distribution in the bed, the overflow, and the carryover streams with their respective rates, the amount of carbon required in the bed corresponding to a specific feed rate, the dolomite requirement for specific sulfur reduction, and the gas flow distribution for the ash-agglomerating gasifier.; The equations, developed in the thesis for a single particle and for fluidized-bed applications are general and are applicable to most needs. They include almost all degrees of complexities at every level. These models can be extended to include any gas and solids flow patterns as well as any reaction resistances in the type of contacting pattern selected. |
