Coalpulverizer fires and explosions have been a chronic problem at coal-fired power stations and cement plants. the extent of the problem at coal-fired power stations and utility measures being invoked to mitigate it have been documented in a series of papers presented recently at the symposium on coal pulverizers. The analogous situation in the cement industry has been described in a series of bureau of mines and portland cement association workshops and is addressed in the paper by alameddin and Luzik in this symposium volume.
As a result of the information compiled in these recent papers and symposia, a clearer picture of the pulverizer explosion hazard is emerging. Typical explosion scenarios, as specified, for example, in terms of ignition sources and fuel and oxygen concentrations, are now being identified from incident reports and from in situ measurements. Similarly, the success and limitations of alternative explosion prevention measures are being assessed from actual plant experience and test data.
the intent of this paper is to review these coal pulverizer fire and explosion incident accounts to determine their consistency with current laboratory data on coal dust flammability and explosibility .an incident database composed primarily of relevant factory mutual loss reports is used along with data on the pulverizer internal environment during normal operations and incipient fires and explosions. The incident database is presented following a brief synopsis of pulverizer design and operation.
Pulverizer Design and Operation
The three types of coal pulverizers used in the United States are:(1)ball tube mills.(2)bowl mills(also called roller-race mills or vertical spindle mills),and(3)hammermills.
Ball tube mills are rotating horizontal cylinders containing steel or special alloy balls. Coal intermingled with the balls is crushed by the impact and grinding action of the balls as the mill rotates. Hot air flowing through the ball mill carries coal particles to classifiers situated at both ends of the mill (Fig.1). Large particles rejected in the classifiers are returned to the grinding zone for further size reduction. Small particles passing through the classifier exit the mill through piping leading to burners in direct fired systems and to cyclone collectors in indirect fired (storage bin) systems.
Bowl mills are vertical cylinders containing a rotating bowl in contact with two or more spring-loaded rollers. Coal enters the top of the mill and falls through a central feed pipe leading to the bowl (Fig.2) centrifugal force displaces the raw coal toward the outer perimeter where it is crushed between the rollers and a grinding ring in the bowl.
Coal Pulverizer Fires And Explosions
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