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STRENGTHENING
PLANT
SAFETY
Derek Stuart, AMETEK Land,
discusses how the correct
selection of equipment and
operating criteria can reduce risk
in storage silos and mills.
Introduction
Coal provides around 90% of the energy for cement
production worldwide. It is readily available, low cost
compared to most alternatives, and offers consistent quality,
making it the preferred fuel for most cement producers. Some
European producers have used alternative fuels (AFs) in order
to reduce CO
2
emissions, but this trend has been offset by
increased coal consumption in Asia and Africa.
Risks of unwanted combustion, potentially causing injury,
damage, and downtime, occur everywhere coal is handled,
processed, or stored. Safe coal handling practices are designed
to ensure that the fuel remains intact throughout its journey
from the mine until the point at which it is ignited in the
kiln. This article examines equipment selection and operating
criteria for reducing risk in storage silos and in coal mills that
pulverise the fuel before burning.
Spontaneous heating
Of critical importance is the problem of oxidation. All coals
oxidise during storage, but sub-bituminous coals are especially
prone to self-ignition. The increasing use of sub-bituminous
coals means a greater risk of spontaneous heating and
spontaneous combustion. Good operating procedures are
designed to ensure that coal is used before it has time to self-
ignite; good housekeeping ensures that any spillage of coal
during transportation is identified and cleaned up.
When coal is exposed to the air, the volatile components
combine with oxygen in an exothermic reaction, leading to
spontaneous heating. The Arrhenius equation tells us that the rate
of oxidation approximately doubles with every 10 °C increase in
temperature, so a runaway condition can occur in which the coal
self-ignites in a process of spontaneous combustion. In addition