Page 163 - Dust Explosions in the Process Industries

136 Dust Explosions in the Process Industries

attention should be paid to dust collecting systems and systems for storage of raw mate-
rials. Early detection of self-heating and self-ignition in deposits of dust and raw mate-
rial and adequate systems for extinctionwere recommended. It was found that automatic
injection of NH4H2P04in ducts connecting to other plant sections provided effectiveiso-
lation of the primary explosion (see Section 1.4.4).
As a result of the country’slarge peat resources, the use of peat as fuel has become a
major concern in Finland. The ignitability and explosibility of peat dust depend on the
origin and decomposition of the peat and its moisture content and particle size distribu-
tion. Weckman et al. (1981) investigatedthe possibilitiesfor reducing the fire and explo-
sion hazard in Finnish peat handling plants, with particular reference to peat power
plants. They concluded that every effort should be made to prevent ignition, but it would
also be necessary to take mitigating actions, should explosionsneverthelessbe initiated.
The recommended means of mitigation were use of pressure- (shock-)resistant equip-
ment, explosion venting, automatic explosion suppression, and isolation.

1.5.3.4
Coal Dust and Pulverized Coal (Excluding Mines)

The literature on the ignitability and explosibility of coal dust is extensive. Originally,
the objective was limited to reducing the dust explosion hazard in coal mines and asso-
ciated activities, as discussed in detail by Cybulski (1975). However, as pulverized coal
has become an increasingly important fuel both for general heat production in power
plants and other purposes (cement furnaces, for example), the coal dust explosionprob-
lem has also become an important issue in these areas.
The influences of the chemical composition,particle size, and moisturecontent of coal
dust on its ignitability and explosibility has been studied systematically since early in
the last century. Much information was collected by Nagy, Dorsett, and Cooper (1965)
and Cybulski (1975). Ignitabilityand explosibilityproperties of coal dust have also been
investigated by Carpenter and Davies (1958), Scholl (1981), Bracke (1984), Enright
(1985),Nettleton (1986),Wall et al. (1988), andWoskoboenko (1988). Torrent,Armada,
and Pedreira (1988) found a good statisticalcorrelationbetween two canonical variables
representing the explosibility properties and the chemical composition of coal dusts,
respectively. Some further data related to the combustion of coal dust are given in
Chapter 4 and Table A.l in the Appendix.
Self-heating leading to self-ignition can be a significant problem when storing coal
powder or dust in bulk. This problem and its prevention and mitigation have been dis-
cussed by Korotov and Polferov (1978); Heinrich (1981); Thatcher (1982); Chauvin,
Lodel, and Philippe (1985); Wiemann and Scholl (1985); and Braun (1987). Schlieper
(1984) was particularly concerned with self-ignition of pulverized coal during transport
by rail and road.
The extinction of coal dust explosion flames by various gaseous and pulverized solid
additiveswas studied by Rahimian,Choi, and Essenhigh (1982) in a laboratory-scalejet-
stirred reactor. Most additives tested were just thermal heat sinks; NaCl and NH4H2P04
also caused chemical reaction chain termination. Rae and Thompson (1979) investi-
gated the effectivenessof various halogenated hydrocarbons as inerting agents and sup-
pressants for coal dust explosions. However, due to the negative environmental effects
of such substances, they are currently being replaced by other extinguishing agents.