Page 251 - Boiler_Operators_Handbook,_Second_Edition

236 Boiler Operator’s Handbook

plants where a constant movement of coal is required. practical purposes) or the more common catenary form
I’ve never had the pleasure of working a coal fired plant of hopper. The shape was developed to hold coal with-
with belt conveyors so about all I can tell you is to treat out a lot of reinforcing and structural members. Steel
the belt with care. Sudden stopping and starting of large plates were made long and literally slung, somewhat
belt conveyors tend to break the belt. like a hammock, from the building framing in the space
One coal requires careful handling on conveyors. above the burner fronts, what we call the firing aisle.
Coal from the Powder River Basin in Wyoming and The result was something like a half ellipse in shape
Montana is used throughout the country in pulver- hanging down above you with trap door openings that
ized coal fired boiler plants because of its lower sulfur were used to release the coal for feeding to the burners.
content. PRB coal, however, is very friable (it breaks up To keep track of the coal and transport it from the
easily) and it generates dust readily. After several explo- bunker to individual burners many plants have weigh
sions in conveyor systems major rebuilds were required lorries. These are mounted on tracks with wheels similar
when handling PRB to ventilate transfer sheds and other to those on a railroad car so the lorry can be moved from
areas thereby preventing flammable concentrations of under the bunker horizontally along the tracks to a posi-
airborne dust accumulation and to conveyors to reduce tion above the coal hopper of the boiler being fired. The
dust generation hot spots and sparking. Pay close at- lorry incorporates a hopper to hold the coal dropped
tention to the special instructions you normally receive from the bunker and its own drop gate to empty the
when a plant is switched to burn PRB coal. lorry hopper into the boiler hopper. The hopper on the
The typical small coal fired plant will use a front lorry is suspended from the wheels and arranged like
loader to move coal from storage to a bucket eleva- a scale so the operator can weigh each load of coal.
tor that lifts the coal from grade level to the bunkers. That way you can get an idea of your coal firing rate in
A bucket elevator can be a belt with small containers pounds per hour and track how much coal you burned
(buckets) attached to it or any number of unique ar- on a shift.
rangements of chains, connectors, and buckets that form Weigh feeders which consist of a short (up to five
a continuous and endless string of buckets to scoop up feet long) belt conveyor with the belt assembly sus-
the coal and lift it to a higher level in the plant where it’s pended so it’s weight can be consistently monitored are
dumped into the bunker. In some medium sized plants used for coal fed to pulverizers and, in a form similar to
the bucket elevator will dump the coal onto a special belt the ones used to feed bunkers, hoppers for stokers. They
conveyor that distributes the coal into the bunkers. The provide an indication of the rate at which fuel is being
belt conveyor will have a special assembly consisting of fed to the boiler.
a couple of rollers that flip the belt twice, all mounted on One rare (I’ve never seen one) but possible system
a set of rails so it can be moved along the length of the to encounter is pulverized coal storage. It would consist
belt. When the coal gets to the assembly it’s dumped as of a bunker but be covered and incorporate additional
the belt turns at the first roller and is deflected past the safety measures because the fine powdered coal readily
second turn of the belt to fall into the bunker. forms a combustible mixture when exposed to air and
Another special conveyor for coal is a Redler con- any agitation. I imagine I’ve never seen one because of
veyor. It consists of a continuous chain with metal pad- the hazards associated with them; they’re just plain rare.
dles that ride inside a rectangular metal tube. The top of When firing coal, whether on a stoker or pulverizer
the tube is eliminated at the in-feed hopper (where the (see the section on burners) a continuous supply of coal
coal is dumped or falls from a storage pile) so the pad- to the hoppers or pulverizers is always a function of the
dles can intercept the coal and start dragging it along. operator. Usually in a coal fired plant we’ll say “opera-
The tube is closed for lifting and transporting the coal tors” because it takes more than one person to keep the
horizontally past a series of gates. Each gate consists of coal moving and burning properly. You may also be
a section of the tube where the bottom can be opened to operating processing equipment that changes the size
allow the coal to fall out. The conveyor can then deliver of the coal particles or screens to actually separate out
coal to a large number of bunkers. some of the coal to provide the size and form of fuel
Okay, now it’s time to explain what a bunker is. It’s that’s required for the burner.
sort of like a day tank for coal. I can go in many boiler Coal also requires handling after it’s fired. A cer-
plants today and look up to see the bunkers are still tain amount of ash and unburned fuel (frequently called
there. That’s even when the plant hasn’t burned coal for LOI for loss on ignition) collects in the bottom of the fur-
several years. A bunker can be a concrete room (for all nace, on top of the grate, and in the dust collector at the