Page 309 - Dust Explosions in the Process Industries

278 Dust Explosions in the Process Industries

0.5
PVA/30% 02 + 70% N2

0.4
I +%+
I 0 a
/ a
- 0.3 LYCOP./30% 02+ 70% N2
\ I
I
> It
t
U
0
d 0.2 I
>
L2 I
z
z
a: 'OICH. CONC. PVA
3 t!
m
0.1 PVA/AIR/'
STOICH. CONC. LYCOPODIUM -+\&
0.0
0 100 200 300 400 500
DUST CONCENTRATION [g/rn31

Figure 4.13 Burning velocities of flame of lycopodium and polyvinyl alcohol dust (<60 pm parti-
cle diameter) flames as functions of dust concentration. The dotted stoichiometric concentration lines
refer to dust in air only (Data from Kaesche-Krischer and Zehr, 1958, and Kaesche-Krischer, 7959).

for a 180 g/m3flame and 1100 K for a 500 g/m3flame. Figure 4.13 shows the measured
burning velocities as a function of the dust concentration. In the range 180-300 g/m3,
the burning velocity of lycopodium flames has a maximum value of about 0.25 ds. The
corresponding concentration range for the PVAdust was 140-220 g/m3.Figure 4.13 also
shows that an increase of the oxygen percentage in the gas from 21 for air to 30, caused
a significant increase of the measured burning velocities for both dusts, in accordance
with expectations. The photographs provided by Kaesche-Krischer and Zehr (1958)
indicate typical thicknesses of lycopodium flames of a few mm.
Kaesche-Krischer implied that the differences in the concentrationranges giving the
highest burning velocities for the two dusts were due to a higher volatile content in the
PVA than in lycopodium, assuming that the flame essentially propagates through a
homogenous mixture of volatiles and air. This is in accordance with the findings of
Hertzberg et al. (1986) for coal dust and polyethylene.
Mason and Wilson (1967) investigated laminar flames of lycopodium in air in the dust
concentrationrange 125 to 190 g/m3.When accounting for wall cooling effects in their
experiments, they arrived at maximum burning velocities similar to those found by
Kaesche-Krischer and Zehr (1958), about 0.25 ds. Mason and Wilson also measured
some temperatures in a 140 g/m3flame using a 25 pm thermocouple. At 2 mm below
the flame front, the temperature was 330-350 K; whereas 1.5mm above the flame front,
it was about 1800 K. The latter figure is in complete agreement with the temperature
measured by Kaesche-Krischer and Zehr (1958) in a 180 g/m3lycopodiudair flame.