Page 289 - Dust Explosions in the Process Industries

258 Dust Explosions in the Process Industries

E
FLAME

J
-Y
-.--

EDGE OF

Figure 4.3 Combustion of a condensed fuel particle in a forced convective oxidizing gas flow, the
theoretical model and coordinate system (From Fernandez-Pello, 1982).
where

m is the remaining particle mass at time t;
Y is the particle radius at time t;
is the thermal conductivity of the oxidizing gas;
C is the mean specific heat of the reaction products;
p is the density of the particle;
Re is the particle Reynolds number, referred to the velocity and viscosity of the oxidiz-
ing gas upstream of the particle;
fi andf, are functions of a mass transfer number B, a normalized energy species func-
tion G, and the angular coordinate 0.
The predicted dependence of the overall particle regression rate, or the Nusselt
number, on the Reynolds and mass transfer numbers was in qualitative agreement with
semi-empirical correlations based on experiments with polymethyl methacrylate par-
ticles burning in mixtures of oxygen and nitrogen. Quantitative comparison between
theory and experiments was difficult because of different definitions of the mass trans-
fer number B and difference between theoretical and experimental environment con-
ditions. However, it appeared that the theoretical analysis predicts higher (by a factor
of approximately 2) mass burning rates than those observed experimentally.The choice
of the thermophysical properties of the fuel and oxidizer used in the theory and the ide-
alized assumptionsimplicit in the theoretical analysis could explain the quantitativedis-
agreement with the experiments.The predicted variation of the particle radius with time
is of the form r:” - P/, - t.
Unless the total specific surface area (N2adsorption) of the particles exceeds about
100 m2/g,clouds of pure carbon dust, such as graphite, in air at atmospheric pressure
are unlikely to represent a significant explosionhazard in practice. Therefore, coals con-
taining volatiles are of greater practical interest. However, the volatiles complicate the
ignition and combustion mechanisms, and the picture is less clear than for pure carbon
combustion.
Gomez and Vastola (1985) compared the ignition and combustion of single coal and
char particles in an isothermal flow reactor, by measuring the concentrationsof CO and
CO, in the downstream gas flow as functions of time. A subbituminouscoal containing
22% moisture, 4.6% ash, 33.8% volatiles, and 39.6% fixed carbon was used in the study.