Limestone calcination porosity

The decrease in molar volume during calcination of CaC03 to CaO has the effect of increasing the porosity of the sorbent which is important to the efficient utilization of limestone. The gas-solid reaction between S02 and limestone sorbent consists of a number of steps (2.) diffusion of gaseous S02 through the pores of the calcined limestone, reaction of 02 with CaO to form calcium sulfate (CaSO,), and diffusion of S02 through the calcium sulfate product layer to react with additional CaO in the particle. Unfortunately, the sulfation reaction ... 

For the many applications of quicklime, active lime is the preferred product, hence careful control of the dissociation (calcination) process is necessary, bearing in mind that the time required for complete calcination depends on factors such as kiln temperature, stone size, and porosity of feed material. The dissociation of limestone above the decomposition temperature is a heterogeneous reaction (Figure 10.3). 

Unlike the catalytic reaction discussed above, gas-solid reactions involve the solid particle as well as the gas in the reaction. Typical examples of industrial applications include spent FCC catalyst regeneration, calcination, coal combustion, gasification, and silicon chlorination. Owing to the solid particle involvement in the reaction, significant changes in the chemical compositions and physical properties of the particles occur during the reaction. Particles reduce in size and/ or increase in porosity in some reactions like coal combustion, whereas particles increase in size and/or decrease in porosity in other reactions such as limestone sulfation. As a result, the particle properties vary unlike those particle properties in catalytic reactions. However, as with catalytic reactions, gas-solid reactions take place on the particle surface as gas reactant adsorbs to the surface. 

An important gas-solid reaction for wHich porosity decreases with time of exposure to the reactant gas is the absorption of SO2 by calcined limestone (CaO) or dolomite (CaO/MgO) to produce calcium sulfate (CaSO or CaSO./MgO), which has a larger molar volume than the reactant solid. If diffusional limitations in the pores are important, the porosity decrease will be greater near the surface of the solid particle and pores will plug, limiting access to part of the reactant solid. 

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