Page 252 - Boiler_Operators_Handbook,_Second_Edition

Consumables 237

boiler outlet. It has to be handled back out of the plant to signed to fire wood that is sand contaminated the veloci-
be dumped in a landfill or used in cement operations. ties through the tube banks are intentionally reduced to
limit the erosive effects of the sand. Operators should also
avoid any action that produces high gas velocities (too
OTHER SOLID FUELS much excess air, over-firing) to reduce erosion damage.
Leaves are another potential source of boiler fuel
Biomass fuels can vary from firewood, the most that isn’t used as much as it could be. A principal prob-
common, to bedding which contains some unpleasant lem with leaves is they’re only available at certain times
animal waste but still burns. There are many varieties of of the year. Firing problems with leaves include an ash
wood and a considerable variation in other vegetation content greater than wood but the big one is that the
that can be burned. There are more ways to burn those fuel is tough to handle, can be messy if it gets wet, and
fuels than there are fuels and new methods of burning can be contaminated with sand and dirt. There are some
them are still being developed. systems that convert dry loose leaves to compact fuel
As with coal you have to prepare the fuel to con- packages by extruding them.
form to the specifications of the burner manufacturer so Bagasse is sugar cane after all the sugar juice has
it will burn well. They have a higher hydrogen content been squeezed out. Since I’ve never spent any appre-
so they tend to burn cleaner. The major problem with ciable amount of time in the south where the cane is
these fuels is their high moisture content, liquid water grown I have no experience with burning bagasse. I do
in the fuel cools the fire in the furnace and the vapor know that the long stringy material is tough to handle
produces high latent energy loss up the stack. The fuel’s and burn.
lower cost normally compensates for that. Other natural sources of biomass include hay, ani-
Wood can be fired in several forms, logs like on mal bedding (yes, it all burns), and corn cobs. Dried corn
a campfire, chips as large as a playing card and about itself has been used for a fuel.
one-half inch thick down to sizes rivaling sawdust and Waste paper, cardboard and similar materials that
various sizes of dust from sawing (where the dust is are contaminated, so they can’t be recycled into more
more like a chip, sometimes as big as one-quarter inch paper, are burned in trash burners but some major gov-
square) to sanding. Some of the finer and lighter materi- ernment and industrial facilities that process a lot of
als can be burned almost entirely in suspension (floating paper may have boilers fired by those fuels just so they
in the air) in a flame that is similar to an oil fire. Most of destroy the material for security purposes. Corrugated
the chip is burned on a grate although it’s common to cardboard is one of those fuels that’s very dangerous to
introduce the chips by tossing them in above the grate burn. That’s because it comes with its own air supply
where the finer dust in the fuel is burned in suspension. within the corrugations. When a corrugated cardboard
Some wood burners are dealing with raw wood is fed into a hot furnace the heat will start boiling away
which has a high moisture content and much of the en- the glues and wood to form gaseous hydrocarbons that
ergy in the fuel is used to vaporize that water. Others fire mix with the air within the corrugations. When the mix-
kiln dried wood which has less than 10% moisture and ture reaches its explosive range it explodes!
is an excellent fuel. The construction of the boiler and Hospital waste is normally burned in an incinera-
the grates are designed for the fuel to be burned and it’s tor with energy recovered by a waste heat boiler. The
usually difficult to handle a different material. A boiler purpose of the separate incinerator is to ensure that all
designed for dry fuel will probably fail to reach capac- the material is exposed to the heat of the fire so all the
ity when burning wet fuel and may not maintain igni- diseases and pathogens in the waste is destroyed. My
tion. A boiler designed for wet fuel will probably have experience with these systems is that’s not always the
problems of burning up grates due to the higher flame case. Unless the waste is mixed up so it’s all exposed
temperatures of the dry fuel. to the heat there will be unburned, and sometimes un-
A principal problem with wood firing is sand. treated, fuel in the waste. The waste heat boilers must be
When the fuel is cut, hauled, and prepared for firing a designed for high ash loads and capable of withstanding
certain amount of dirt comes with it and sand can erode occasional acid attacks because of the acids produced
boiler tubes quickly as it’s carried by the flue gas out of while firing the waste.
the furnace into the tube banks. Sander dust will always Trash burners, large boilers burning tons of gar-
contain a certain amount of flint and other sharp sands bage are considered an air pollution hazard and many
that are very damaging to the boiler. When a boiler is de- localities chose to landfill their garbage rather than burn