Porous char particle surface area

In contrast to the slag, char particles collected in cyclones working with moving grate installations and the cyclone material from fluid bed combustors, although free form volatiles, appear to burn well even at 800 C, which can probably be attributed to their highly porous character and consequently a large effective surface area available for reaction [13 ]. Data for fluidized bed volatile char are included in Table 2. 

Measnrements on coal particles of different sizes indicate that the burning times of both the volatiles and residue vary as the square of the initial particle diameter, which is in accord with the surface area proportionally. The porous structure of the char also exerts an effect on the burning operation as does particle temperature up to several hundred degrees above the gas temperature. 

The pore structure can markedly affect char reactivity. Most coals in general, and coal chars in particular, are highly porous and contain a polymodal pore size distribution. Pores normally are classified into macropores (>500 A in diameter), transitional pores (20-500 A in diameter), and micropores (< 20 A in diameter). Upon pyrolysis, pores in coals open up but still contain microporosity. Coal chars, in general, and lignitic chars, in particular, retain the polymodal pore distribution. The surface areas of coal chars can range between 100 and 800 m /g. Most of this surface area and therefore the active surface area resides inside the char particles so the accessibility of the reactive gases to the active sites is very important. 

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