Page 224 - Materials Chemistry, Second Edition

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Municipal Solid Waste Processing; Materials Recovery Facilities 195

FIGURE 7.24 Old hammer. Compare with Figure 7.22.

shredder housing and in the shredder room or building, to cause an explosion. If a hammer strikes a
metallic object and produces a spark, the dust in the atmosphere could ignite immediately. This haz-
ard is magnified by the fact that the frictional action of the hammers and shafts produce excessive heat.
A significant danger is the inadvertent shredding of containers storing volatile or explosive
compounds such as solvents, which create explosive atmospheres. A logical protection against such
a hazard is a comprehensive inspection program to detect and remove explosive or flammable mate-
rials from entering the size reduction unit. Recent regulations governing the disposal of organic
wastes should reduce the amount of such materials entering a MRF. A single empty solvent con-
tainer is not sufficient to cause a significant explosion. A full solvent container could cause a prob-
lem if the vapors are allowed to accumulate. Dust is the most likely cause of explosions in
shredders, however (Pfeffer, 1992).
Electrical switches, controls, and lighting should be installed in explosion-proof housing and
conduit to avoid sparking (Tchobanoglous et al., 1993). Shredders should be installed in structurally
isolated rooms separate from other processing areas. In some facilities, the MSW is conveyed to a
shredder that is operated by remote control. In the event of an explosion, worker safety is enhanced
by their physical separation from the mill. Keeping the MSW moist via a fine mist of water adds
protection against dust hazards. Wetting will cause problems with liquid accumulation, treatment
and removal, however. Water applied to wastes will also affect manufacture of RDF. A good qual-
ity ventilation system will pull dust-free, filtered air into the shredder and will greatly improve the
safety of the local atmosphere (Pfeffer, 1992).
Explosion suppression devices have been manufactured which serve to reduce the force of an
explosion. Sensors, designed to detect the pressure change from an explosion, are installed within
the shredder housing. In response to a sudden pressure increase, an inert gas such as nitrogen or car-
bon dioxide is rapidly released into the chamber, thus displacing oxygen and preventing the propa-
gation of a flame. Essential to the use of fire and explosion suppressants is adequate response time.
This system works for most types of explosions but is not fast enough for supersonic releases (det-
onations) as occurs with TNT, gunpowder, and similar explosives. The added gases from the explo-
sion suppression device work to fight a fire after an explosion, also by oxygen displacement. All
shredders possess outlets for pressure buildup. Blast doors are situated in the roof above a shredder
and a blast duct is used to direct the force of the blast upward, through the blast doors.