| OCR Text |
Show The Carbon-Oxygen Reaction There are three primary heterogeneous reactions which can occur on burning carbon: C + o2-*co2, (1) 2C + 02+2CO, (2) CO + C+2C0. (3) For reactions (2) and (3) the CO will diffuse away from the particle and burn in the surrounding atmosphere. The distance at which this burning occurs has a major impact on the rate of particle weight loss under diffusional control. If the CO burns very near the surface, the effective surface reaction product is C0„. Thus for every molecule of oxygen which diffuses from the bulk, one atom of carbon will be removed. If CO burns relatively far away from the surface, the effective surface product is CO, and one molecule of oxygen will remove two atoms of carbon. Several experimental and theoretical studies have addressed the question of the relative importance of each of the three reactions and the location of the CO flame front [see for example Arthur (4), Ross and Davidson (5), Caram and Amundsen (6) and Mon and Amundsen (7)]. The most important conclusions of these studies are that CO is probably the most abundant initial product at the carbon surface, and that the location of the subsequent CO flame is a complicated function of particle size, heat transfer, and flow conditions. In general, the CO flame will move farther from the surface as particle size decreases. For particles >1 mm diam, the CO flame will be very near the surface, making CO^ the effective reaction product. For particles <50 urn diam, the CO will burn far from the particle and CO will be the effective reaction product. Between the limits of 1 mm and 50 urn, each molecule of oxygen will account for something between 1 and 2 atoms of carbon removed. Limiting Reaction Rates As discussed by Daw and Krishnan (8), the limiting mass transfer rate (regime III combustion) can be calculated from: 6 |
