Page 217 - Dust Explosions in the Process Industries

Case Histories 189

2.1 0.3
GAS AND DUST EXPLOSION IN A PULVERIZED COAL
PRODUCBION/COMBUSTIONPLANT IN A CEMENT FACTORY
IN LbiGERDORF, FEDERAL REPUBLIC OF GERMANY,
IN OCTOBER 1980

According to Patzke (198l),who described this explosion accident,the explosion occurred
while coal of about 30% volatiles was milled at a rate of 55 tonnes per hour. The startup
of the cement burner plant followed a compulsory break of at least 20 minutes of the ani
operation to allow all airborne dust to settle out. A few seconds after the main gas
been opened, there was a violent explosion. The probable reason was a failure in the
em for electric ignition of the gas. Within the period of 6 seconds before the gas valve
was reclosed automatically, about 1m3 of gas had been discharged to the atmosphere of
the hot combustion chamber and become mixed with the air to form an explosible gas
cloud. The temperature of the walls of the chamber was sufficientlyhigh to ignite the gas,
and a gas explosion resulted. The blast and Name jet from this comparatively mild initial
explosion was vented into the milling system, where a large, turbulent dust cloud was gen-
erated and ignited, resulting in a violent secondary dust explosion.
Various parts of the milling plant, some unvented and some vented, had been designed
to withstand the pressure generated in an extensive dust explosion. Furthermore, a pas-
sive device for explosion isolation ofthe type shown in Figure 1.82 in Chapter 1had been
installed upstream of an electrostatic dust filter.
Apaxt from the deformation of some explosion vent doors, the dip tubes of two
cyclones, and the coal feeder upstream of the mill, the plant had been able to withstand
the explosion without damage. The passive explosion isolation device effectively
tected the electrostatic filter from becoming involved in the system.

2.1 0.4
FURTHER EXPLOSION AND FIRE INCIDENTS INVOLVING COAL

Andersson (1988) gave a step-by-step account of the process of extinction of a smoldering
fire in a 50 m3coal dust silo in Arvika, Sweden, in August 1988. It was necessary to pay
attention to the risk of explosion of combustible gases driven out of the coal by the heat
from the fire.
First, gaseous carbon dioxide was loaded into the silo at the top to build up a lid of
inert atmosphere immediately above the coal deposit. Then, all the coal was discharged
carefully through the exit at the silo bottom. In this particular case, supply of carbon diox-
ide at xhe silo bottom was considered superfluous.
Wibbelhoff (1981)described a dust explosion in a coal dust burner plant of a cement
works in the Federal Republic of Germany, in March 1981. Prior to the explosion, an
electrical fault had caused failure of an air blower. The explosion occurredjust after restart
of the repaired blower. During the period in which the blower was out of operation, dust
had accumulated on the hot surfaces inside the furnace and ignited; and as soon as the
blower was restarted, the glowing and burning dust deposits were dispersed into a dust
cloud that exploded immediately.