Page 626 - Dust Explosions in the Process Industries

Research and Development 593

minimum laser beam power required for igniting dust clouds by the heat absorbed by a
solid target heated by the laser beam. The variable parameters included the laser beam
diameter, the duration of the irradiation, the target material (combustible or noncom-
bustible), and the type of dust (starch,lycopodium,lignite, sulphur,ABS, and aluminum).
Gieras and Klemens (1994) studied the critical conditions for ignition of single coal
particles in air and in air-containingmethane. They also investigated the critical condi-
tions for ignition of clouds of coal dust in air and methane/air.
In the past, the minimum hot-surface temperaturefor ignition of a dust cloud has often
been regarded as if it were a universal constant for a given cloud. Consequently, results
from small-scale laboratory tests were often applied directly to industrial plant design.
However, it has been known for some time (see Section 5.5 in Chapter 5) that minimum
ignition temperatures of dust clouds vary significantly with scale. This was confirmed
by Wolanski (1992).
Ignition of dust clouds by small burning metal particles (impact sparks,metal sparks)
generated by mechanical impact has been discussed in Section 5.4 in Chapter 5. This is
a complex process, and comprehensive, practically useful theories do not seem to be
within sight. Such theories must comprise several complex subprocesses.The first is the
generation and initial heating of the metal particle by the impact. The second is the igni-
tion of the flying hot particle and the subsequent burning process. The third is the heat
transfer to the dust cloud, which ultimately determines whether ignition occurs or not.
Ignition of dust cloudsby electric and electrostatic spark dischargesbetween two metal
electrodes is another complex topic. Such sparks can be generatedin a number of ways.
such a.sin switches,by various failures in electric circuits, and by discharge of static elec-
tricity. Some approximate theories for ignition of dust clouds by electric sparks are
reviewed in Section 5.3 in Chapter 5. The variables of the electric spark ignition prob-
lem include voltage and current characteristics across the spark gap, spark gap geome-
try, and electrode material, as well as dust cloud variables. The variables include particle
material and particle size and shape distributions, dust moisture content, dust concen-
tration, and the dynamic state of the dust cloud with respect to the spark gap.
Xu and Lin (1993)performed a semi-quantitativetheoreticalanalysis of electric spark
ignition of dust clouds. They proposed a strategyfor calculatingminimum ignition ener-
gies, in terms of the lowest energy capable of establishing self-sustainedlaminar flame
propagation in the dust cloud. This strategy is the same as the one proposed previously
by Wemens and Wojcicki (see Chapter 5). Bobkov et al. (1996) analyzed electrostatic
discharge processes and ignition of dust clouds by electrostaticdischarges theoretically,
by means of mathematical catastrophetheory.Although the analysis confirmedmeasured
trends, qualitatively, it also confirmed the substantial complexity of these kinds of
processes and that one most probably has to rely on experimentalinvestigationin the fore-
seeable future. Dahn, Reyes, and Kashani (1993) reviewed some published work on
electric spark ignition of dust clouds and confirmed the dramatic influence of the com-
bination of capacitance and resistance of the experimental discharge circuit on the min-
imum capacitorenergy required for ignition.Wieczorek and Zalosh (1998)confirmedthe
earlier important findings(see Chapter 5 and Figure 1.40in Chapter 1)that the minimum
ignition energy (MIE) of dust clouds depends strongly on the dischargetime of the elec-
tric sparks used and the movement or turbulence of the dust cloud in the spark region.
Lorenz and Schiebler (2001a) presented the results from a comprehensive, detailed
experimental and theoretical investigation of the energy transfer processes taking place