Page 339 - Boiler_Operators_Handbook,_Second_Edition

324 Boiler Operator’s Handbook

The pulverized coal burner can be as simple as a One important element of coal firing is very low air
pipe from the mill or exhauster pointed into the furnace flow purges. Operators used to wide open damper full
to a cast assembly with orifices, guide vanes, and other air flow purges for oil and gas should be aware that you
features that further mix the fuel and primary air and can blow the boiler up if you do that with coal. There
distribute it into the fire in the furnace. Over time the can be accumulations of coal or coal and ash in the boiler
coal flow can erode some of the more important parts of which a full air flow purge would lift and stir to form
the burner to destroy baffles, etc., that produce the mix a combustible (make that explosive) mixture. A purge
and, more importantly, provide that fuel-rich concentra- should be conducted at low air flows to prevent that
tion that’s needed to get the fire started and stabilized. happening. A high flow of products of combustion can
Some utility boilers are equipped with cyclone stir that stuff up and move it without hazard because
furnaces which use a pulverized coal with less size the flue gases are inert, they don’t contain any air to mix
restriction than conventional pulverized fuel burners. with the fuel.
The cyclone is a water cooled, refractory lined cylinder While I’ve had time to visit a few fluidized bed
mounted horizontally at the side of the boiler. The coal boiler plants and review descriptions of CFBs I haven’t
and air is fired at very high heat release rates within had an opportunity to spend enough time with them
the cyclone with temperatures so high that all the ash is to identify any tricks the operator should know about
melted and removed as a liquid. The flue gases exit the them. Once again your best guidance is the instruction
cyclone furnace into the boiler furnace at temperatures manual.
around 3000 degrees. The primary purpose of the cy-
clone furnace is reduced size of the boiler. Wood Burners
Modern versions of coal burning boilers are fluid- Wood burners vary from a campfire to burners
ized bed boilers and circulating fluidized bed boilers firing sander dust. On the one extreme we have large
(CFBs) where the entire furnace or the whole boiler is pieces of wood which require long retention times in the
part of the burner. The coal is introduced as solid parti- furnace and on the other we have wood so finely ground
cles into a bed that’s fluidized by the combustion air and that it burns faster than fuel oil. There are a considerable
flue gases passing up through it. Fluidizing is accom- number of different boiler, burner, and grate designs for
plished by distributing the air into the bed of coal over burning wood, wood waste, and similar fuels.
a broad area using special nozzles through refractory Wood requires some special consideration if it’s
under the bed. The solid particles seem to boil just like ‘green’ or ‘wet’ because the moisture absorbs a consider-
water in a pot as the air flows up between them. In the able amount of heat and is capable of quenching the fire
case of a circulating fluidized bed the smaller particles to the point it goes out. Dry wood from lumber opera-
are carried out of the furnace with the flue gas to be cap- tions (kiln dried) planing, sawing (of dried wood) trim-
tured and returned after they flow through the boiler. ming, and sanding burns readily and must be handled
In addition to the coal the bed is fed finely ground with care because it can easily produce an explosive at-
limestone that reacts with and absorbs the sulfur diox- mosphere during air conveying or handling operations
ide. The reacted limestone and gas leaves the boiler as that mix the fuel with air. Most wood burning boilers
part of the ash instead of emissions in the flue gas. Circu- serve industries that process that wood for such things
lating fluidized bed boilers actually allow considerable as pine chemicals and furniture.
carryover of the bed into the initial passes of the boiler The fine materials, fine sawdust (some sawdust
to prolong contact time of limestone and sulfur dioxide can be chips as large as one half inch square) and sander
plus increased fuel—air mixing. Cyclone separators act dust are typically fed to a burner similar to a pulverized
like classifiers to remove the coal and limestone particles coal burner where the material is burned in suspension
from the flue gases and return them to the furnace. like fuel oil. The furnace is usually also fitted with a
Coal firing requires consideration of the time it grate, normally water cooled because there is no layer of
takes fuel and air to mix and burn. A stoker fired boiler fuel to protect the grate from the heat of the furnace.
will hold the coal for several minutes while the heat Larger materials are usually burned in a high set
breaks each particle down, evaporating the lighter frac- spreader stoker which allows for burning of the fine par-
tions of the fuel then converting the carbon. The furnace ticles in suspension and the heavier pieces on the grate.
must be larger to hold the inventory of fuel. The fuel for A special consideration when firing wood is contami-
a coal burner has to be pulverized because the particles nation with denser solids. Material cut specifically for
have little time to burn in the furnace. firing can contain sand, rocks and dirt. Sander dust can