Systems Displaying a Shrinking Unreacted Core

The changing particle size, however, will alter the relative importance of the rates of chemical reaction and the transport of heat and mass. It is conceivable then that there may occur a switch from transport-controlled regime to kinetic-controlled regime during the reaction of nonporous particles even when conditions external to the particle remain constant throughout the reaction period. 

This effect is usually more likely when the rates of mass and heat transport depend strongly on the extent of reaction. Therefore, we will discuss this problem in more detail for the case in which the product forms an ash layer around the reactant solid. 

Gas-solid reactions of the type disucssed in this section are of obvious importance in the combustion of coal and other solid fuels. Considerable experimental work has been done on this subject and there is an extensive literature. No attempt will be made to review this work here instead the reader is referred to a recent review article by Mulcahy and Smith [45]. 

In many gas-solid reaction systems, the product solid forms an ash layer around the nonporous reactant solid  

The examples of such a system are frequently encountered in chemical and metallurgical processes, such as the reduction of metal oxides, the oxidation of metals, the roasting of ores, and the decomposition of metal compounds. The production of lime and the combustion of ashy coal are further examples. 

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