Page 293 - Dust Explosions in the Process Industries

262 Dust Explosions in the Process Industries

BROWN COAL 148% VOLATILES)
PARTICLES
BLACK COAL 130.5% VOLATILES)

ANTHRACITE 15% VOLATILES)

0 10 20 30 40 50 60 70 80 90 100
VOLUME PERCENT 02IN 02/Nz MIXTURE

Figure 4.4 Influence of volatile content in coal and oxygen concentration in gas on the maximum
center-to-center distance between particles for the ignition of a 700 pm diameter coal particle by a
burning neighbor particle of the same size, at zero gravity (From Cieras, Klemens, and Wojcicki, 7 98.5).
distance D,,, for the second particle to become ignited by the first one increased sys-
tematically with the volatile content of the coal and the oxygen content of the gas, as
shown in Figure 4.4. It was also found that D,,, was proportional to the particle diam-
eter in the range 300-1200 pm investigated. For anthracite and coke in air, ignition of
the second particle did not take place unless the particles were nearly touching, whereas
particles of the coal of the highest volatile content in air could be separated by up to about
two to three particle diameters.
In Figure 4.5, the relative flame radius, Rfias observed on 48 fr/s movie photos, has
been plotted as a function of time. Rfis defined as the ratio between the radius of the appar-
ent flame around the particle and that of the original particle. Figure 4.5 shows that the
time required to reach the maximum flame radius decreased and the maximum flame

6- rBROWN COAL (48% VOLATILES)
rBLACK COAL (30% VOLATILESI

./.-

l0-0-
2 -/.dANTHRACITE (5% VOLATILES)
I I YWI I I I I
0 100 200 300 400 500 600 700 800 900
TIME lrnsl

Figure 4.5 Change of relative flame radius Rfwith time during combustion of a 700 pm diameter
coal particle at zero gravity (From Cieras et al., 198s).