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Steady and unsteady combustion of carbon

1973; Elsevier BV; Volume: 14; Issue: 1 Linguagem: Inglês

10.1016/s0082-0784(73)80123-7

1878-027X

L. Kurylko, Robert H. Essenhigh,

Thermochemical Biomass Conversion Processes

Dense carbon spheres, of 1.27 cm diam, were burned in oxygen and air atmospheres, suspendedin an electrically heated tube furnace, with simultaneous measurements of sample weight, diameter, and sample and gas temperatures. Under limited conditions, oscillatory reaction was found to occur, with temperature excursions of up to 100°C at periods from 10 to 100 sec. Parameters permitting oscillation were found to be: carbon porosity between 33% and 43%; pure oxygen; sphere temperatures about 1000°C; and ambient gas velocity between 0.51 and 1.0 cm/sec (STP). A finite-element model was developed to simulate combustion behavior. Results showed that CO, produced by internal reaction of carbon and oxygen, would burn entirely to CO2 inside the sphere at low temperatures (<700°C), but, at higher temperatures, an increasing fraction of CO burned in the boundary layer. At a given temperature, more CO burned in the boundary layer if the ambient was air rather than oxygen. This difference in the location of CO burn-up altered the heat balance of the burning spheres. Starting at the same sample and gas temperatures, the temperature of carbon reacting in air reached a lower steady-state value than the temperature of carbon reacting in oxygen, because of the different proportioning of the CO burn-up between the sphere and the boundary layer. The reaction rate therefore was lower burning in air than in oxygen, in spite of the zero-order reaction between oxygen and carbon, and in agreement with the previous experiments by Froberg (Ref. 1). The computations also provided a partial explanation for the oscillatory behavior.