Page 672 - Dust Explosions in the Process Industries

Research and Development 639

The European standardizationorganizationCEN has issued a series of new standards
for testing dust ignitability and explosibility (CEN, April 2001, 2003a, 2003b, 2003c,
2003d). ASTM (2001) in the United States is annually bringing an updated review of
their standards.

9.4.4
DETERMINING THE LIMITS OF FLAME PROPAGATION:
OBLEM OF THE SCALE OF THE EXPERIMENT

Reliable assessment of whether or not a given dust cloud can propagate a self-sustained
ame constitutes the basic test objective in the assessment of both introductory dust
explosibility/nonexplosibility and the minimum explosive dust concentration (MEC)
and limiting oxygen concentration for inerting (LOC). However, great care must be
exercised in designing such tests. A basic inherent problem is that, near the limits, self-
sustained flame propagation cannot be established unless a considerable amount of
energy is supplied to initiate flame propagation. Hence, if the volume of the experi-
mental dust cloud is too small, it is difficult to assess whether observed flame propaga-
tion is truly independentof the ignition source. Some results by Cashdollaret al. (1992)
and Cashdollar and Chatrathi (1992) are of fundamental significancein this context. They
found that clouds in air, at normal ambient conditions, of an anthracite coal dust of 8%
volatile matter did not show self-sustainedflame propagation in a 1m3test chamber, even
when exposed to a 30 kJ chemical ignitor. However, in a 20 liter chamber, fully devel-
oped explosions were generated even with a 5 kJ chemical ignitor. The reason for this
could be that, in the smaller chamber, the initial combustion and expansion of the dust
cloud was directly supported by the ignition source.The pressure and temperaturein the
unburned cloud ahead of the flame would then have increased significantly above ambi-
ent when the flame eventually propagated without support from the ignition source.
Consequently, the self-sustainedflame propagation, if any, would then occur in an adi-
abatically precompressed dust cloud, rather than in a cloud of normal ambient temper-
ature and pressure. These results suggest that great care must be exercised whenever
comparatively small chambers, in particular, closed ones, are used to determine any
explosionlimit. Going, Chatrathi, and Cashdollar (1998,2000) carried out a further par-
allel experimental determination of MEC and LQC in a 20 liter spherical vessel and the
standard (ISQ) 1 m3vessel. The results confirmedthat the limits determinedin the 20 liter
vessel vary significantlywith the energy of the pyrotechnical ignition source used. A 2.5 W
ignition source gave the closest match to the data obtained in the I m3 vessel.
Wiemann (1996) reported on controlled real-plant-condition dust explosion expexC
ments in ajet anill in a coal power plant. The overall conclusion was that borderlines drawn
between conditions of dust concentration, dust moisture, oxygen concentration of the
atmosphere, ignition energy, and so forth that produce and do not produce explosions
are generally more liberal than corresponding borderlines based on laboratory experi-
ments. In particular. the LOC was significantlyhigher in the actual mill explosionexper-
iments than in closed-bomb laboratory experiments.
Matsuda and Itagaki (1994) compared dust explosions in a 30 liter explosion bomb
with explosions in a 1 m3vessel. They found that the range of explosive concentrations
in the 30 liter vessel were considerably wider than those in the 1m3vessel for the same dust.