OCR Text |
Show temperature and oxygen concentration in an element of mixing and reacting fluid. The basic flow and property conditions for the calculations are given in Table I, and time scales for mixing inferred from flow velocities, typical dimensions, and turbulence time scales. This calculation and the others given here assume that the primary air/coal input mixes directly with the recirculation zone products, followed after finite delay by finite rate addition of secondary air. It is also possible to use this model with other combinations of coal - air - combustion product mixing sequences. The primary coal-air stream is assigned an exponential mixing time scale of 10 milliseconds with combustion products from the recirculation zone. The combination of heat addition and initial reaction of the fine fraction of the pulverized coal drive the calculated temperature rapidly upward. Also, dilution by combustion products and oxygen depletion by combustion drives the oxygen concentration downward. In this case, with a delay time of 20 ms arbitrarily assigned before secondary air mixing occurs, oxygen depletion to zero local concentration is predicted, at about 9 ms. Subsequent coal reaction consists of endothermic gasification, driving the fluid temperature downward. This could suggest local sooting, depending upon the rate and uniformity of volatile mixing with reactive gases. In this treatment, locally homogeneous equilibrium gas chemistry is assigned. At 20 ms, rapid introduction of secondary air (10 ms e-fold time) drives a transient temperature overshoot, as the locally fuel rich gas is combusted. Subsequently, however, the dilution by cold secondary air is faster than combustion heat release, which is by this time controlled by a combination of heterogeneous oxidation of small char particles, and pyrolysis of larger particles. In this case, the temperature drop predicted is small enough that extinction is probably not an issue. At longer times, as secondary air mixing is completed and combustion proceeds, the product temperature increases. -10- .^V/7AVCO E V E R E TT |