Reformulation of the standard equations governing internal reaction of carbon, used to predict
the Zone I−Zone II transition temperature, T
c, shows that T
c is, primarily, an inverse function
of particle radius (a). The expression obtained has the form: log(a) ∝ (1/T
c). This result is supported
by the (limited) experimental data available. The numerical values also show that, for particles
of pulverized coal size, the values of T
c are in the flame temperature range, thus possibly allowing
a discontinuous drop in reaction rate toward the end of a flame; this has relevance to increased
LOI due to staging for NOX control. The results also permit evaluation of the internal diffusion
coefficient indicating that internal reaction most probably involves only the macropores, and
total internal surface area measurements are probably irrelevant in determining reaction rates.
Transition of reaction from Zone II to Zone I during char combustion in a pulverized coal (pc)
flame is shown to result in significant and discontinuous drop, jointly, in temperature and in
reaction rate, with a correspondingly significant increase in burn-out time. In the particular case
studied, as example, the estimated increase in burn-out time is by a factor of at least 3. The
reduction in reaction rate is partly due to a (300 °C) drop in temperature, but it is jointly due to
the doubling of the (operational) activation energy associated with the Zone transition. The
probability of such Zone transition on particles in pc flames is controlled by the thermal balance
that, at the end of combustion, is shown to depend critically on the “seen” wall temperature. In
the case of a boiler, particles seeing the relatively cold walls may “extinguish”, and this is a
possible or probably cause of increased loss-on-ignition (LOI) that is increasingly being reported
for pc combustion in boilers.
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