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to the obvious safety concerns, spontaneous heating and
spontaneous combustion result in loss of material, so that
coal which has been paid for is no longer available to burn
in the kiln.
It takes as little as 1.4 kg of pulverised coal in
28 m
3
of air to form an explosive mixture. Since a large
cement kiln burns more than 10 kg/sec. of coal, the safe
burning of pulverised coal necessitates strict adherence
to planned operating sequences. The greatest risk of fire
occurs when the pulveriser is shut down under load, as
this leaves a large amount of finely-divided fuel inside
a hot mill. The large surface area of the pulverised coal
and the high temperature inside the mill lead to rapid
oxidation of the coal. This results in further heat buildup
and the potential for a fire. If the mill is restarted without
first removing the hot coal, an explosion can occur when
particles are suspended and exposed to the air. Even in
routine mill shutdowns, there is a danger that any residual
coal left within the mill will oxidise and may explode as
the mill is restarted. To prevent a coal fire, the mills can
be made inert with a steam deluge when an unexpected
shutdown occurs, or when there is a high risk of a coal fire.
Available detection techniques
Several techniques are available that can monitor the
effects of spontaneous heating and give early warning
that a dangerous condition could occur. They work by
detecting one of the tell-tale signs of oxidation: either
heat buildup or the emission of carbon monoxide (CO)
gas. The choice of the most appropriate monitoring
technique depends on both the measurement location
and on the degree of risk. More sophisticated
–
and
therefore more expensive
–
monitors are appropriate
where low-rank coals pose an increased likelihood of
spontaneous combustion.
Thermocouples are widely used to detect the heat
buildup from oxidation or early-stage mill fire, but
they have limited sensitivity and discrete sensors have
difficulty monitoring the whole volume of the mill. It
also takes time for sufficient heat to build up within
the mill to give a detectable increase in temperature.
Experience shows that thermocouples do not provide
a reliable indication that a hazardous condition is
developing.
Carbon monoxide detection is ideal for the detection
of spontaneous oxidation in silos and pulverisers.
Large amounts of CO are produced by the inefficient
oxidation associated with spontaneous combustion.
Ambient air contains a very low concentration of CO
(usually well below 10 ppm), so a significantly higher
concentration provides a sensitive and fast indication
that unwanted oxidation is occurring. The technique
can only be used in enclosed spaces, because the
wind will quickly disperse any gas emissions before a
measurable concentration can accumulate.
Installation location
For monitoring storage areas, the sample point
should be located towards the top of the silo so
that it measures the gas in the headspace above the
coal. In most cases, a single sample point is sufficient
because natural convection will ensure the gases in
the headspace are well-mixed. Very large or highly
asymmetric storage volumes may require additional
sample points to ensure adequate coverage.
In mill applications, the best measurement point is
usually at the exit of the classifier. The continuous flow
of coal and air through the mill ensures a representative
sample of the gases in the mill and the predictable
flow direction allows the use of an abrasion shield to
prevent excessive wear on a sample probe. Even though
the inside of the classifier is a hazardous area, sample
probes are simple devices with no electrical connection
therefore no special precautions are needed.
Determining alarm levels
One of the biggest challenges in configuring a Millwatch
system is the determination of suitable alarm levels. A
CO concentration greater than 250 ppm can be seen
during mill startup, but in normal operation the CO
concentration is in the region of 10 ppm. Millwatch
analysers offer two independent alarm points, so alarm
levels can be set high during startup and lower in
normal operation. Careful assessment of the alarm level
is important. Obviously, it is important to ensure the
alarm level is not set too high, otherwise there can be a
significant time delay before the system responds to a
potentially hazardous condition. However, an excessively
low alarm level will lead to nuisance alarms, which will
undermine the usefulness of the monitoring system.
Verifying correct operation
It is important to verify correct operation of the sensor,
otherwise a negative indication can give a false sense
of security. For example, a sensor can become plugged
by coal dust so that it is no longer exposed to the silo
atmosphere. Electrochemical sensors are compact and
sensitive, but they give zero output when they fail
and this means a faulty sensor will indicate a low CO
concentration. In-situ sensors are compact and easy to
install, but blockage is difficult to detect, and calibration
checks are usually performed manually by a technician
with a bottle of calibration gas. Extractive analysers
require additional hardware for installation, as the sensor
is mounted remotely from the sample point. However,
they can detect a blocked sample probe by sensing
a reduction in sample flow. They are also generally
equipped with an automatic calibration mechanism.
Successful co-monitoring
HouShi power plant is a 4200 MW electricity generating
plant in China’s Fujian province, operated by the
Huayang Group. It supplies electricity to the city of
ZhangZhou and the surrounding area. There are seven
electricity generating units at the site, each of which
is rated for 600 MW. In 2011, the plant operators
decided to add CO monitors to the five coal mills in
Unit 1, supplementing their existing temperature and