Page 304 - Dust Explosions in the Process Industries

Propagation of Flames in Dust Clouds 273

Table 4.4 Size distributions of five metal powders used in flame propagation experiments

*AI urn in u m/magnesium alloy.
Source: Alekseev and Sudakova, 1983.

Experiments in closed bombs give pressurerise ratios up to 12.5 for explosions of alu-
minum dust in air (BIA/BVS/IES, 1987).For ideal adiabatic expansion and assuming a
specific heat ratio of 1.4, this gives expansionratios of up to 6.1, and accordingto equa-
tion (4.ls),the radial flame speed is then 6.l times the radial burning velocity.The burn-
ing velocity correspondingto a flame speed of 2.5 m/s is then about 0.4 m/s; that is, close
to the value found in laminar burner experiments for aluminum flames.
Jarosinski et al. (1987) determinedthe quenching distance for laminar flames in air of
aluminumflakes of thickness0.1 pm and average diameter 15pm and atomized aluminum
particles of average diameter 8 pm. The smallest quenching distance found for both
dusts was 10 mm. This occurred in the dust concentration range 700-1000 g/m3.

4.2.3.2
Coal Dusts

In a comprehensive survey of a number of investigationson the propagation of laminar
pulverized coal dust/air flames, Smoot and Horton (1977) discussed factors influencing
experimentally determined burning velocities, flame temperatures, and flame thick-
nesses. Most experimentsare performed by stabilizingdust flames in burners of various
kinds. Due to heat losses by radiation from the hot dust particles and conduction, typi-
cal stabilized burner flames have temperatures lower than the adiabatic flame tempera-
ture. In principle, heat losses can be avoided by using burners of very large diameters
or equipped with walls having temperatureand emissivityprofiles matching those of the
flame. However, according to Smoot and Horton, the use of such devices had not been
reported up to the time of their survey (1977).
Smoot and Horton found large differences in burning velocities observed by various
investigators that could not be explained in terms of variations in dust properties or dust
concentration.They consideredincompletedispersion of fine cohesive dusts as the main
source of error (see Chapter 3). Figure 4.10 illustrateshow improved dispersionof a fine
coal dust increases burning velocity by 50%and even more. Somemain conclusionsfrom
the survey of Smoot and Horton are given in Table 4.5.
Horcton, Goodson, and Smoot (1977), investigating flat, laminar coal dust flames,
found that the peak burning velocities for a 9 pm (mass average particle size) Pittsburgh
coal dust in air was about 0.33 ds, whereas a coarser fraction of the same coal (33 pm
mass average fraction)gave peak velocities of about 0.22 ds. A similar influence of par-
ticle size was found for a Pocahontas coal.