(423i) The Influence of Solid Inertants On the Explosion Behavior of Dusts From Mechanical Waste Treatment Facilities

Conservation of natural
resources is a main target in today's lives wherefore the reusing and recycling
of waste is very important. To recycle waste material it is necessary to treat
the mass flows in special ways. In most instances the waste material passes
through mechanical treatments. Especially for the production of substitute fuel
mechanical treatment is a common treatment operation. During this mechanical
treatment the fractions get shredded, separated and prepared for further
recycling. Nevertheless, besides all those positive effects, the mechanical
treatment of waste material also generates a fraction of fine particles, which gets
dispersed in air. These occurring dusts either have to be separated with
suction cleaning or they get deposit unregulated on plant components. Anyway
all occurring dusts are fraught with the risk of dust explosions if they are
burnable and dispersed in air.

Dusts from different waste
treatment facilities were investigated in order to identify the dust explosion
potential and its influencing factors. Dusts deposited on plant components as
well as filter dusts were sampled. The reason for taking samples from deposit
dusts is that they can cause dust explosions if they get dispersed in air.
Filter dusts are relevant materials because they are dispersed in air when they
get suctioned and might form hazardous explosive atmospheres. Burnable dust
dispersed in air can cause dust explosions if the dust cloud is getting in
contact with an effective ignition source.

The explosibility
characteristics for the investigated dusts were determined by Siwek 20-Liter testing. Experiments were conducted to
identify the influence of solid inertants, as well as
the influence of their particle size and particle shape. These investigations
are done from the intention to predict the explosion behavior of dusts based on
the amount, size and shape of their inert fraction.

Because of batch wise
delivery of the recycling material and similarly input there is a variation of
composition of the treated material over time. That is why it was necessary to
check if the occurring dusts respective the explosion behavior of the occurring
dusts is altering with time. Beside of short-term fluctuations caused by
varying input material also seasonal changes were considered. The comparison of
samples taken from the same plant within different periods of time showed that
despite of changes in charge material, the explosion behavior of occurring
dusts within one operation unit in a plant is reproducible.

Solid
Inertants

A known way of reducing the
risk of dust explosions is to mix the burnable dust with a solid inertant [1]. This method is either used to generate a
resulting mixture, that is rendered non-explosible or
the admixture is done in purpose of mitigation of the consequence of a dust
explosion [2, 3]. Although there is good knowledge of solid inerting,
it is not widely used in process industries. In most applications mixing with
solid inertants can lead to unacceptable product
contamination so this procedure is only in rare cases practical from a
technical perspective.

In the case of dusts from
mechanical waste treatment facilities risk reduction due to solid inertants is an interesting approach, because of the fact,
that the dusts itself contain a huge amount of inert fraction. The inert
fraction is on average at least 40 mass percent.

To identify the influence of
inert fraction, dust samples from a mechanical sorting plant were investigated.
The ash content of the samples was increased and an inhabitation as shown in Figure
1 could be seen. On this basis it could be shown that the explosion behavior of
dust samples from different operation units in the same plant are just
differing due to the different inert fractions. So the inert fraction is within
one plant a valuable parameter for the prediction of the explosion
characteristics of the occurring dusts.

Figure 1: Pressure-time trace
for the reaction of dust sample from mechanical sorting with different inert
fractions

Particle
Size and Particle Shape

Beside the high amount of
inert particles, the fraction of coarse particles in the occurring dusts is
very high. Most of the dusts show very wide particle size distributions. For
the investigations of the explosion behavior in the 20-Liter Siwek Apparatus as well as in the modified Hartmann
Apparatus only particles beyond 500 µm were mentioned. However because of
the fact that the particle class of 500 ? 1000 µm is of the same magnitude
as the particle class smaller 500 µm, the influence of coarse particles in
the mixture was investigated.

An increased mean particle
diameter of an explosible dust caused by an increase
of particle size over the whole size distribution can significantly decrease
the inerting level [1]. Larger particles are more
difficult to disperse than smaller particles, which is another relevant aspect
of the characterization of dust explosions in mechanical waste treatment plants.
Especially, in due consideration of the high LEL (lower explosion limit) of the
outlined dust samples a restraint in dispersion properties is considerable.

In Figure 2 the notable
results are given. Already at a coarse fraction content of 5 % the explosion
characteristics switches to the characteristics of the coarse fraction.
Therefore, regardless of separation phenomena, an inhibition of the explosion
intensity can be expected because of the presence of coarse particles in the
dust samples.

Figure 2: Effect of coarse
particles on the explosion characteristics of dust samples from mechanical
waste treatment facilities

Particle
Size and Particle Shape of Solid Inertants

According to the influence of
inert fraction and particle size investigations concerning the particle size
and particle shape of solid inertants in the dust
samples were done. Therefore a sample from a document destruction plant with an
ash content of 42 % was prepared by admixture of glass beads and glass fibers
respectively. Due to this the inert fraction of the sample was raised by
5 %.

With an addition of 5 % it
could be seen that the effect of beads is nearly negligible while the admixture
of fibers has considerable effect. The added materials are of the same
equivalent diameter, which is comparable with the particle size distribution of
the original sample. Although the explosion pressure of the prepared samples is
equal the explosion intensity in terms of rate of pressure rise is strongly
weakened (cf. Figure 3). This shows the importance of the particle shape of
solid materials that act as inhibitors.    

Figure 3: Effect of the
admixture of inert material of different particle shape on the pressure rise
over time

As seen in Figures 4 and 5
the Siwek 20-Liter testing of the document
destruction dust sample with the admixture of 5 % glass beads are compared to
those of the same dust sample mixed with 5 % glass fibers. The explosion hazard
is therefore significantly magnified by the particle shape of the present inert
material.

Figure 4: Explosion pressure
as a function of dust concentration for dust from document destruction mixed
with 5 % glass beads and accordingly 5 % glass fibers

Figure 5: Rate of pressure
rise as a function of dust concentration for dust from document destruction
mixed with 5 % glass beads and accordingly 5 % glass fibers

These investigations show
that the explosion behavior of dusts from mechanical waste treatment facilities
is strongly influenced by the presence of inert material and its particle shape
as well as its particle size. Previously analyses of different dusts of
mechanical waste treatment facilities and the effect of their influencing
factors on the explosion reactions were done. This is the basis for further
investigations on the effect of particle size and shape of the ash content,
which should permit the estimation of the explosion behavior.

Further analyses with
variation of the particle size distribution and particle shape as well as the
material of the solid inertants will be done. The
investigations will be done with dusts from document destruction as well as
dusts from substitute fuel production and other mechanical waste treatment
facilities. The comparison of the influencing effects provides important
information on the characterization of these dust explosions.  

References

[1]        Amyotte, P., Pegg, M., et al. (2007). Moderation of
dust explosions. Loss Prevention in the process
industries, 20. 675-687

[2]        Chatrathi, K. &
Going, J. (2000). Dust deflagration extinction. Process
Safety Progress, 19. 146-153

[3]        Dastidar, A.
& Amyotte, P. (2002). Determination
of minimum inerting concentrations for combustible
dusts in a laboratory scale chamber. Process Safety
and Environmental Protection, 80. 287-297