Diffusion, coal combustion rate

For coarse coal particles with high ash content, diffusion of oxygen through the ash layer often determines the overall combustion rate. The inner pores in the ash layer can be divided into micropores of 0.004-0.0012 /im, transition pores of 0.0012-0.03 fim and macropores of 0.03-1 fim diameter. Experiments showed that the macropores constitute the main channels for mass transfer. 

The mechanism of carbon combustion in FBC is assxjmed to be diffusion controlled in most of the modelling efforts as discussed in the section on Char-Oxygen Reaction, This is true only for large particles (> 300 microns) at high temperatures (> 1200 K). Feed coal contains a wide range of sizes, and assimiing a diffusion controlled kinetics for all particle sizes wo iLd lead to overestimation of the combustion rate. 

Char oxidation dominates the time required for complete burnout of a coal particle. The heterogeneous reactions responsible for char oxidation are much slower than the devolatilization process and gas-phase reaction of the volatiles. Char burnout may require from 30 ms to over 1 s, depending on combustion conditions (oxygen level, temperature), and char particle size and reactivity. Char reactivity depends on parent coal type. The rate-limiting step in char burnout can be chemical reaction or gaseous diffusion. At low temperatures or for very large particles, chemical reaction is the rate-limiting step. At... 

Earlier work in this laboratory has shown that combustion of chars of low and high-rank coals occurs under regime II conditions ( ), l e under circumstances where pore diffusion as well as chemical reaction exercises marked rate control Hie values of the activation energy for the Millmerran and Wandoan chars 20... 

Coal char combustion Phenomenological aspect In the regime of "low temperature," the chemical reaction rate is slow compared with the diffusion through the pores, because the O2 completely penetrates the char matrix. In this case the rate controlling the regime of char combustion is kinetically limited. 

Predictions from these models suggest that the devolatilization rate is independent of particle size. This conclusion should not, however, be applied to typical fluidized beds because the results are based on devolatilization of 0.2 mm particles while much larger coal particles are fed to commercial fluidized beds. From data on devolatilization of various coal types in the absence of combustion. La Nauze concluded that the devolatilization time is proportional to d [9]. He ascribed this to an internal diffusion controlled evolution mechanism. 

A knowledge of mass transfer is essential for the understanding of the mechanism of combustion of coal in a turbulent fluidized bed. If the kinetic rate of combustion of the fuel is known, one can estimate the burning rate using the information on the mass transfer rate. The rate of transfer of oxygen from the bulk of the bed to the particle surface, k, is often expressed as the dimensionless Sherwood number, Sh = kgdp/Dg- For diffusion to a fixed single sphere in an extensive fluid, Sherwood number may be expressed as [28, 29]... 

---