Limestone attrition

Figure 12. Variation of the steady-state attrition rate of limestone fractions with...

The abrasion/attrition resistance of dense limestones as measured by the aggregate abrasion test, are lower than many competitive aggregates, but are still acceptable for most applications. 

Arena et al. (1983) and Pis et al. (1991) studied the attrition of amorphous materials such as coal or limestone and found that the size distributions of the attrited materials were independent of the initial particle size and of most operating parameters. Ray et al. (1987a) assumed that unlike crystalline materials, amorphous materials may have some kind of natural grain size to which the degradation finally leads. 

On the other hand, a chemical reaction of the particulate material may affect the mechanical attrition resistance of the particles, as for example the sulfation of limestone sorbents in combustion processes (Anthony and Granatstein, 2001) or the different degrees of oxidation of a catalyst (Xi, 1993). 

Both devices described above were developed in order to test the friability of FCC catalysts. Nowadays the application of these or similar tests is a common procedure in the development of fluidized bed catalysts. Contractor et al. (1989), for example, used a submerged-jet test to compare the attrition resistance of newly developed VPO catalysts. In fact, such tests can be applied to any type of fluidized bed processes. Sometimes they have to be slightly modified to adapt them to the process under consideration. The drilled plate may, for example, be substituted by a porous plate if only attrition in the bed is of interest. Even temperature and pressure can be adapted. Vaux and Fellers (1981) investigated for example the friability of limestone sorbent that is used for fluidized bed combustion. By surrounding a Gwyn-type test facility with a heating system, they took thermal shock and reaction into account. 

Figure 16 Variation of the steady-state attrition-induced loss flow of limestone fractions with (u-u (). (Ray et al., 1987a.)...

Particles are the bed material employed in fluidized bed reactors and can be reactants (e.g., coal and limestone), products (e.g., polyethylene), catalysts, or inert. The choice of particle size, in general, affects the hydrodynamics, transport processes, and hence the extent of reactor conversion. Particles experience particle particle collisions, friction between particles and walls or internals, and cyclones. In some cases, the catalyst material is inherently susceptible to attrition, and special preparation to enhance the attrition resistance is required. For example, the vanadium phosphate metal oxide (VPO) catalysts developed for butane oxidation... 

The catalytic reduction of NO, is usually placed between the economizer of the boiler and the air preheater (Figure 6.18.9). The SCR catalyst used in this way is in the so-called high dust mode and the resistance of the catalyst against attrition by ash particles should be high. Downstream of the air preheater fly ash is collected by an electrostatic precipitator. By way of a heat exchanger and flue gas desulfurization (e.g., SO2 separation by scrubbing using a slurry of a sorbent, usually limestone or lime) the flue gas is passed to the stack. 

Testing of the mechanical stability of the prepared soibents is carried out by measuring the amount of elutriated material in a fluidized bed with an inner diameter of 4 cm. Superficial gas velocity in the bed is 1 m/s, gas consists of pure nitrogea A special bottom plate is designed in order to increase the amount of attrition in the bed. Batches of 25 g are used. Elutriate is collected by a cyclone, and captured on electrostaticly charged quartz wool kept in a flask. Tests are done at 20°C (cold- elutriation) and 850°C (hot-elutriation). For comparison purposes several natural limestones are also tested in this experimental set-up. Results are given in table III. 

Size distributions of solids (coal and limestone) in the feed and in the bed shoiald be considered in the evaluation of attrition and elutriation loss. Standard correlations for elutriation rate constants have been found to be inadequate for the calcijlation of solids elutriation. Data obtained from large pilot-scale FBC show large disagreement from those calciilated based on existing elutriation rate correlations. Recently, correlations for attrition and elutriation of bed particles have been proposed by Merrick and Highley (106). For larger particles, this correlation underestimates the carbon loss. 

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