Page 290 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 259

For each run, a single particle from a 850-1000 pm sieve fraction was injected into a
reaction furnace swept with air. Experimentswere performed at five temperatures:928 K,
980 K, 1076 K, 1118 K, and 1273 K. At each temperature, two types of run were per-
formed, coal combustion and char combustion. The char particles were prepared by
injecting a coal particle into the reactor with a flowing nitrogen gas stream at the desired
temperature.After pyrolysis was completed, the char was ignited by switching the car-
rier gas from nitrogen to air.
The main conclusion drawn by Gomez and Vastola from their experiment was that
two chemical reactions compete for the oxygen surrounding the coal particle. The two
reactions are quite different in nature, one involving the carbon surface (heteroge-
neous) and the other involving the volatiles (homogeneous). The gas concentration
curves obtained for the heterogeneously oxidized char particles were considered typi-
cal for the heterogeneous reaction involving the carbon surface. Oxidation of coal par-
ticles could be heterogeneous, depending on the temperature. The gas concentration
curves obtained for heterogeneous oxidation were similar to the curves for char com-
bustion, except for an initial peak of carbon monoxide attributed to the combustion of
volatiles on the surface or within the particle at low oxygen concentrations. However,
when the coal particles ignited homogeneously, an initial pronounced peak of carbon
dioxide was detected, which was attributed to the gas phase combustion of the volatile
matter at conditions of sufficientoxygen for burning most of the carbon in the volatiles
to carbon dioxide. The initial peaks of carbon monoxide for heterogeneous coal igni-
tion and carbon dioxide for homogeneous ignition can be used to measure the pyroly-
sis time during Combustion.
Gomez and Vastola suggestedthat all the carbon in the volatiles is oxidized to carbon
monoxide or carbon dioxide. This is because methane, the most difficult hydrocarbon
to oxidize, which was detected in the volatiles of coal particles after pyrolysis in nitro-
gen, was not traced in the products from combustion in air.
If the particle burns under external diffusion control, the reaction proceeds on the
external surface of the particle at a very low oxygen concentration.The particle diame-
ter then reduces as the combustion advances, but the density of the remaining particle
mass nz at time t is the same as of the initial particle mass mo.Integration of the reaction
rate equation for this case, assuming spherical geometry, results in

(mlmo)2’3= kt (4.6)

where the global constant k embraces a number of constants and parameters.If this rela-
tionship describes the mechanism controlling the combustion process, a plot of the
power two-thirds of the reduced mass m of the particle against time, determined exper-
imentally,should result in a straightline. For char particles, Gomez and Vastola’s exper-
iments gave straight lines at gas temperatures >1100 K; whereas for coal particles,
straight Bines were found for gas temperatures >980 K.
The total combustion times, determined by both the method just described and inde-
pendent light intensity measurements,varied from 5-10 s at a gas temperature of 1300K
to 20 s at 930 K. These times are very long in the context of dust explosions and due
mainly to the large particle diameter of about 1 mm and partly to the comparativelylow
oxidizing gas temperatures in Gomez and Vastola’s experiments.