For the gasification reaction to take place within the char’s pores, the reacting gas must enter the pores. If the availability of the gas is so limited that it is entirely consumed by the reaction on the outer surface of the char, gasification is restricted to the external surface area. This can happen because of the limita­tion of the mass transfer of gas to the char surface. We can illustrate using the example of char gasification in CO2:

C + CO2 ^ 2CO (5.58)

Here, the CO2 gas has to diffuse to the char surface to react with the active carbon sites. The diffusion, however, takes place at a finite rate. If the kinetic rate of this reaction is much faster than the diffusion rate of CO2 to the char surface, all of the CO2 gas molecules transported are consumed on the external surface of the char, leaving none to enter the pores and react on their surfaces. As the overall reaction is controlled by diffusion, it is called the diffusion — or mass-transfer-controlled regime of reaction.

On the other hand, if the kinetic rate of reaction is slow compared to the transport rate of CO2 molecules, then the CO2 will diffuse into the pores and react on their walls. The reaction in this situation is “kinetically controlled.”

Diffusion rate >> kinetic rate [Kinetic control reaction] (, ,„)

Diffusion rate << kinetic rate [ Diffusion control reaction ]

Between the two extremes lie intermediate regimes. The relative rates of chemical reaction and diffusion determine the gas concentration profile in the vicinity of the char particle; how the reaction progresses; and how char size, pore distribution, reaction temperature, char gas relative velocity, and so forth,

influence overall char conversion. Figure 5.9 shows how the concentration profile of CO2 around the particle changes with temperature. With a rise in the surface temperature, the kinetic rate increases and therefore the overall reaction moves from the kinetic to the diffusion-controlled regime, resulting in less reaction within the pores.

The overall gasification rate of char particles, Q, when both mass transfer and kinetic rates are important, may be written as

p

Q = — ^^kg Carbon/m2.s (5.60)

—— 1—-

fym Rc

where Pg is the concentration in partial pressure (bar) of the gasifying agent outside the char particle, hm is the mass transfer rate (kg carbon/(m2bar. s)) to the surface, and Rc is the kinetic rate of reaction: kg carbon/(m2bar. s).