Page 399 - Dust Explosions in the Process Industries

368 Dust Explosions in the Process Industries

Greenwald and Wheeler (1925) and Fischer (1957) reported that coal dust flames in one-
end-open large-scalegalleries, with ignition near the closed end, accelerated up to a point
after which a high, constant flame speed was maintained during the remaining length of
the gallery. In the case of Greenwald and Wheeler, this steady flame speed was about
800 m/s, whereas Fischer reported 1040 m/s as a maximum value in his experiments.
These velocities are lower than the Chapman-Jouguetdetonationvelocities (see Section
4.5.3) that would be expected for coal dust in air. Therefore,Greenwald and Wheeler and
Fischer may have observed the kind of constant high-velocity turbulent deflagrations
described by Lindstedt and Michels (1989).However, such high-turbulence deflagrations
can be nearly as violent as proper detonations. One indication of this is that, in Fischer’s
experiments, the pressure measurement stations in the region of the gallery of the con-
stant,high flame speeds were destroyedby the explosion. Similarevidence of steady high-
speed turbulent deflagrations of dust clouds in large-scalegalleries was found by Cybulski
(1952), Bartknecht (1971), and Rae (1971).
However, both Pineau and Ronchail(l982) and Bartknecht (1971) found clear evidence
of proper dust detonations in ducts of smaller diameters. In these cases, steady flame
speeds on the order of 2000 m/s and high peak pressures on the order of 20 bar(g) were
measured, as mentioned in Section 4.4.7 and illustrated in Figure 4.64.
On this background, the contribution by Kauffman et al. (1982, 1984b) is important.
They demonstratedthat a steady detonation wave could propagate in clouds of oats and
wheat grain dust in air, in a vertical laboratory-scaleduct of square cross section 6.35 cm x
6.35 cm and length 6 m. The dust was charged into the tube at the top at a mass rate, giving
the desired dust concentrationduring gravity settling down the tube. The main dust explo-
sion was initiated by a local hydrogen/oxygen explosion at the bottom tube end.
Using a laser Schlieren technique, it was observed that the shock front was followed
closely by an induction zone, which was in turn followed by a reaction zone, as would
be expected in a proper detonation wave. The leading shock caused intense dispersion
of the particle agglomerates into an optically dense cloud of primary particles within a
few mm behind the shock front, where the particles ignited and burned. After combus-
tion, the mixture was again opticallytransparent.The combustionprocess was nearly com-
pleted 0.5 m behind the shock front, corresponding to a time interval of about 0.3 ms.
At an oats dust concentration of 250-270 g/m3, slightly lower than the stoichiometric
one of 300 g/m3,the measured detonation wave velocity was 1540ds, which is some-
what lower than the theoretical Chapman-Jouguet (C-J) velocity at stoichiometric con-
centration of 1800m/s. It would be expected, however, that the inevitable energy losses
in a dust detonation would cause the real detonation velocity to be lower than the ideal
C-J velocity. The highest measured peak pressure was about 24 bar, quite close to the
theoretical C-J pressure at stoichiometric concentration, 22.4 bar.
Kauffman et al. (1984b) also investigated the upper and lower dust concentration
limits for detonationof oats dust in airin their laboratory-scalevertical tube. They found
that, even with very vigorous ignition sources,detonationscould be initiated only within
the narrow concentrationrange of approximately200-450 g/m3.
Further important evidencedemonstratingdetonations in dust clouds in airhas been pro-
vided by Gardner, Winter, and Moore (1986). The dusts used were coals and included a
fineBritish coal fractionof 87% by mass <71 p particle size, containing 33.5% volatiles
and 3.5% moisture, and an equally fine U.S.subbituminouscoal fraction of 41.3% volatiles
and 17.3% moisture. Coarser particle size fractions of the two coals were also tested.