Page 250 - Boiler_Operators_Handbook,_Second_Edition

Consumables 235

the valve to compensate for the pressure drop, maintain- Coal and oil require less air to burn than natural
ing the pressure and a more precise output temperature. gas and LPG for the same heat output. That’s because
Newer strategies include viscosity control. An the gases have a higher hydrogen-carbon ratio, more hy-
instrument is installed in the piping to sense the viscos- drogen in the fuel produces more water which increases
ity of the oil and there are several methods for analysis stack losses. Some hydrogen in the fuel is always desir-
ranging from vibrating a heavy wire in the oil to trap- able because it helps form coal gas which is a gas that
ping a sample and dropping a plunger through it. What- burns far more readily than solid carbon. Also, higher
ever method of sensing is used, there’s still the problem moisture content in flue gas seems to improve boiler
of response time so a viscosity controller should only be performance because water makes it possible for gas fir-
used to produce a set point for the steam pressure. ing to clean up soot that collects on boiler surfaces when
On many of the facilities that we converted to light firing oil during a gas interruption. The accumulations
oil in the 1980’s we suggested the customer retain the of soot associated with firing oil and coal are due to
fuel oil heaters. Testing at that time indicated a light oil fixed carbon that can’t be readily converted to gas so it
burner would operate cleanly and more efficiently with can burn.
a little better turndown if the oil was heated to 120°F. Another form of coal that’s being considered as a
That was, of course, a temperature still below the flash fuel is culm. That’s the waste material removed from
point of oils supplied at that time. Now, with lower sul- coal mines which contains some coal but is mixed with
fur requirements some light oils have flash points just dirt. There are several huge piles of culm around the
barely above 100°F. mines of this country. Some are big enough to supply a
The most difficult activity regarding fuel oil han- plant for several years. Modern fluidized bed boilers are
dling for operators today is keeping the system in capable of burning that material.
operating condition when it’s never used. Your SOPs As a solid, coal requires different handling meth-
should include regular operation of the system to ensure ods than oil or gas (both fluids which can be transported
it’s operational and a drill for switching to oil for each in pipelines). Every once in a while I’ll run into another
operator in the fall before the first winter interruption attempt to burn a coal and oil slurry, a mixture that han-
can be expected. I still remember one plant I was asked dles like a fluid but contains particles of solid coal. Some
to investigate where none of the operators could switch utilities manage to burn it successfully but I haven’t
over to oil. seen a successful operation in a small boiler plant.
Coal is usually delivered by truck or railroad car.
In either case they can present a serious problem in the
COAL winter if rained or snowed on with subsequent freezing.
Usually a plant with railroad supply will have a melting
Coals are commonly identified by their source, shed where the cars are heated to melt the ice so the coal
either by the area or state in which they were found or a will flow. The trucks or rail cars are dumped into a hop-
particular mine. There are three distinct classifications of per where a conveyor picks up small quantities of it and
coal, Anthracite, Bituminous, and Peat which are prin- lifts it to the bunkers or a storage pile.
cipally related to the crushing strength of the coal with Storage piles are simply piles of coal stored for burn-
anthracite being the hardest. Other criteria includes size ing. Unlike a fuel oil storage tank there’s no enclosure so
of the coal particles and characteristics that affect han- the coal is subject to degradation from weather. They also
dling and burning. Coals that are fired on grates must have a bad tendency to ignite spontaneously if left sitting
be large enough that they don’t fall through the holes in too long. When it comes time to burn, or move, the coal
the grate and have a limited portion of fines that would another conveyor can do it or it might just be handled by
fall through. you operating a little front loader. In either case the coal is
Coals that are pulverized to something close to eventually transferred to the burners.
talcum powder so they will burn in suspension (float- Conveyors come in a wide variety of sizes and
ing in the air very similar to an oil fire) are graded by styles. Many use a belt, a wide fabric reinforced rubber
how difficult it is to grind them. An operator in charge or synthetic rubber riding on rollers that shape it into a
of a coal fired boiler plant should be aware of the speci- trough that holds the coal. At some point in a belt con-
fications of the grate or burner manufacturer and boiler veyor system the belt is pulled taught by a roller that’s
manufacturer and how variations in those specifica- adjustable and the belt makes a full 180 degree turn
tions affect its firing. over the roller. Belt conveyors are used mainly in large