Page 216 - Dust Explosions in the Process Industries

188 Dust Explosions in the Process Industries

nor death resulted and damage to surrounding structures was minimal, although large
blocks of concrete and reinforcing steel had been thrown several hundred meters from
the silo. However, the plant itself suffered substantialdamage.
The silo was full of coal 24 hours prior to the explosion. During the evening before
the explosion, 10,000 tonnes of coal were discharged. At the same time, conveying of
deep-seam coal into the silo commenced and continueduntil the explosion occurred.At
the time of the explosion,approximately 12,300tonnes of coal were in the silo, of which
7,600 tonnes were deep-seamcoal. Testing had shown that this quality of coal has a high
methane emission rate and produces a low volatile coal dust. Clouds in air of this dust
could not be ignited unless the air was mixed with methane.
The ignition source was not identified, but the following three possible sources were
considered:
Spontaneouscombustion of the stored coal.
An electrical or mechanical source.
Hot coal from the thermal dryer.
During 10 years of operation, with coal being stored in different environments for
varying lengths of time, spontaneous combustion had never presented a problem and,
consequently, was not considered a probable source of ignition. During demolition of
the damaged silo, all electrical and mechanical components were recovered and
inspected; they showed no evidence of being the ignition source. Stokes (1986)did not
exclude the remaining possibility that hot coal from the thermal dryer was the source
of ignition.

2.10.2
METHANEKOAL DUST EXPLOSION IN A COAL STORAGE SILO AT
A CEMENT WORKS IN SAN BERNARDINO COUNTY, CALIFORNIA

This incident was reported by Alameddin and Foster (1984).Afire followed by an explo-
sion occurred inside a coal silo of 900 tonnes capacity while the silo was nearly empty,
and the remaining 85 tonnes of coal were being discharged. Prior to the explosion,a hot
spot of 0.6 m x 1.0 m had been detected on the lower part of the silo wall by means of
an infrared heat detector. The hot spot originatedfrom smolderingcombustionin the coal
in the silo. This process liberated methane, carbon monoxide, and other combustible gases
from the coal. The explosion probably resulted from ignition of a mixture of combustible
gas and airborne coal dust in the space above the bulk coal by the smolderingfire or glow
when it reached the surface of the coal deposit (see Figure 1.9 in Chapter 1).
It was concluded that the supply of carbon dioxide from the top, which was used to
suppress the fire and prevent explosion, was insufficient to prevent the development of
an explosible atmosphere in the space above the bulk coal.
To prevent similar accidents in the future, it was recommended that a carbon dioxide
system be installed in both the top and bottom of the coal silo. Sufficient inerting gas
should be added for development of a slight positive pressure inside the silo. The inert-
ing gas must be of sufficient quantity to ensure a nonexplosible atmosphere above the
coal and sufficientpressure to prevent a sudden inrush of fresh air into the silo.