OCR Text |
Show reactant and product) between the particle and its surroundings, (2) diffusion of mass species through the porous solid structure, and (3) chemical reaction at the internal solid surfaces. For the purposes of this discussion, steps 2 and 3 can be lumped together under the term intrinsic chemical reaction. Step 1 is referred to as external diffusion or mass transport. For char combustion the diffusing reactant species is oxygen and the product species are primarily carbon monoxide or carbon dioxide. Other species in the bulk gas such as C0~, CO, and H~0 may also react with char, but such reactions were intentionally minimized in these experiments. Discussions in the literature [e.g., Mulcahy and Smith (2) and Laurendeau (3)] have commonly identified three special cases which arise during combustion of porous particles. As shown in Fig. 1, conditions of low intrinsic reactivity (specifically, low surface reaction rate and high pore diffusivity) allow deep penetration of oxygen into the interior of the particle with little depletion in concentration. This condition, where the intra-particle oxygen concentration is essentially equal to that in the bulk gas, is designated as regime I. As combustion proceeds, the particle size remains constant but density steadily decreases. Regime II represents the condition at which the surface reaction rate is sufficiently fast to reduce the oxygen concentration to zero at or near the particle center. In this case both particle size and density decrease with time. Still higher intrinsic reactivity leads to regime II in which all of the oxygen is consumed as soon as it reaches the outer particl surface. Under these conditions the reaction rate depends solely on the external diffusion of oxygen. The particle density remains constant, but the size steadily decreases. Regimes I-III represent ideal limits. Regime I is frequently referred to as the chemical reaction limit, regime II as the intermediate limit, and regime III as the diffusion limit. In some cases these limits are closely approached, but actual combustion usually occurs at some intermediate condition. It is important to note that combustion cannot proceed faster than any step in the reaction sequence. This constraint provides a useful standard for evaluating experimental data. 4 |