Page 435 - Dust Explosions in the Process Industries

402 Dust Explosions in the Process Industries

in pure oxygen. The absorption of water from humid atmospheres by dry carbonaceous
materials was the major origin of the primary temperature rise from 70°C to 90°C.
Chamberlain and Hall (1973) discussed the various chemical and physical properties
of coals that influence their oxidizability. Continuous measurement of gases produced
during the oxidation process showed that carbon monoxide gives the earliest indication
of spontaneous heating.
Heinrich (1981) provided a nomograph from which minimum ambient air tempera-
tures for self-ignition in coal dust deposits may be determined from laboratory-scale mea-
surements of the minimum self-ignition temperatures for two powder samples of different
volume to surface ratios (see also Section 5.2.2.1).
Heemskerk (1984), using both isothermal and adiabatic test methods, investigated
the relationship between the rate of self-heating in coal piles and the oxygen content
in the atmosphere in the range 0-20 vol% oxygen. A systematic decrease of the self-
heating rate with decreasing oxygen content was found. Addition of sulfuric acid and iron
salts to coal piles stimulated self-heating. Smith, Miron, and Lazzara (1988) investigated
the effectiveness of 10 additives, applied as solutions in water, to inhibit self-heating in
deposits of a coal of high self-ignition potential, using an adiabatic heating oven. Sodium
nitrate, sodium chloride, and calcium carbonate were found to be the most effective
inhibitors, whereas sodium formate and sodium phosphate stimulated the self-heating
process.
Enemoto et al. (1987) studied the process leading to a fire in a new bag house installed
with a cyclone separator in a pneumatic transport system for pulverized coal. By using
classical Frank-Kamenetzkii-type theory and appropriate values for the thermal con-
ductivity of the very fine coal dust (2.3 pm) and the kinetic parameters, it was confirmed
that the fire was most probably caused by self-ignition in a dust deposit in the bag
house.
Bigg and Street (1989) developed a mathematical computer model for simulation of
spontaneous ignition and combustion of a bed of activated carbon granules through
which heated air was passed. The model simulated the temporal development of tem-
perature and gas species concentration. The model was validated against the experi-
mental data of Hardman, Lawn, and Street (1983) and good agreement was found.
Brooks, Svanas, and Glasser (1988) formulated a mathematical model for evaluating
the risk of spontaneous combustion in coal stock piles, using a personal computer. The
model predicts expected trends with changes in various parameters, but comprehensive
validation against experiments was not reported.
Tognotti, Petarca, and Zanelli (1988) studied self-ignition in beds of coal particles
experimentally, using various cylindrical-shaped beds of diameters 17-160 mm and
heights 10-80 mm. Theoretical thermal ignition models were used to interpret and
extrapolate the data from the small-scale experiments. Results from additional isother-
mal experiments were compared with the small-scale ignition tests. The boundary con-
ditions (Biot number) played an important part in deciding whether ignition would
occur.
Takahashi et al. (1989) simulated the temperature rise with time in a coal deposit due
to spontaneous oxidation, using a numerical computer model. The maximum temper-
ature occurred at the center of the bed when the oxygen concentration inside the bed
was not reduced due to the oxidation reaction, whereas it occurred near the bed surface
when the oxygen concentration in the bed decreased due to the consumption. The rate