Professional Articles

Coal Handling

Coal Storage and Oxygen Access

Coal can be stored in properly designed Bunkers, Silos or Piles. Decisive factors in determining the type of storage to be used are initial capital investment, and pay back period. However, in addition to the economics, the factors spontaneous combustion, self heating, and minimizing the flow of air through the stack should be assessed. The storage process as well as the configuration of the coal storage are significant in evaluating these factors.
This paper focuses on the importance of limiting oxygen access to the stored coal, and is illustrated by calculations which predict possible self heating scenarios of a multiple coal silo storage facility at Helsinki Energy, Finland.
The primary cause of self heating is the exothermic oxidation of coal at low temperatures. The rate of oxidation is slow below 40 ºC but accelerates by a factor of 1.8 thereafter, adding that the critical temperature above which the oxidation and self heating process becomes selfsustaining is about at 50 ºC for lignites and 70 ºC to 80 ºC for bituminous coals. In general, the oxidation increases at ten times its usual rate as the temperature rises from 30 ºC to 100 ºC [3]. Heating can only occur where there is sufficient oxygen, the coal is dry and the heat balance is in favour of heat retention. With the number and variety of variables it is clear that a prediction of self heating is a complex matter. However, by adapting a controlled storage process, avoiding particle segregation and limiting air flow, a prediction is viable.