Wood-Chip Heating Systems - Biomass Energy Resource Center

WOOD CHIP HEATING SYSTEMS
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sweeps the silo base, discharging chips into an opening
at the center. These silo unloaders function best when
the silo is in a heated space, to avoid problems with
chips freezing to the walls of the silo.
Most systems use a series of screw augers in covered
steel troughs, either on the level or inclined, to move
fuel to the boiler. Belt conveyors and drag chains are
much less common means of transporting fuel in
institutional and commercial systems. Bucket elevators
are sometimes used in large systems for conveying
fuel vertically, but they require more maintenance than
inclined augers and are avoided in smaller systems.
Most systems have a small metering bin between the
fuel storage and the combustion chamber. It separates
the rapid fl ow of fuel being taken out of the bin from
the carefully controlled feed rate of fuel into the
combustion chamber.
The handling equipment described above requires
no intervention by site personnel. The fuel is moved
automatically from storage to combustion. Some
facilities, however, use small tractors or front-end
loaders to convey the fuel from the storage facility to
a day bin (see photo on page 31). The day bin typically
holds enough fuel so that it needs to be loaded by the
tractor operator once or twice a day, in an operation
that may take only half an hour. An auger in the
base of the day bin conveys fuel automatically to the
combustion chamber.
Although tractor-based systems require daily
operator involvement, they have been used successfully
by a variety of facilities - including small agricultural
operations, greenhouses, large industrial plants, and
a district heating system for a complex of state offi ce
buildings. The system is attractive because it reduces
capital costs. Tractor-based systems work best when
there is already appropriate staff on site to operate the
tractor on a daily basis.
The Combustion System
The furnace is the part of the combustion appliance
where burning of the solid fuel actually takes place.
(Examples of furnace confi gurations can be found in
Figures 6.1, and 6.2 in Chapter Six.) Fuel is automatically
injected into the furnace, combustion air is
added, and the fuel burns to produce heat. The hot
exhaust gases then fl ow out of the furnace area and
into the heat exchanger. As they pass through the
heat exchanger, heat is transferred to the surrounding
water (or air). The cooled exhaust gases then pass up
the chimney for discharge into the outdoor air. (See
Chapter Six for a discussion of the different generic
types of combustion systems.)
The proper conditions for complete and effi cient
combustion are achieved by:
• accurate control of the fuel feed rate;
• accurate control of the combustion air feed to
different areas within the furnace;
• the turbulence of hot gases (air, wood gases, and
water vapor);
• the ability of the furnace to maintain high
temperatures;
• the right furnace geometry, to give wood gases
enough time to burn completely, and,
• the means to prevent ash buildup.
The equipment that is used to achieve effi cient
combustion is described below.
Parts of the Combustion System
Most non-industrial systems use the last auger
of the fuel handling system, called the stoker or the
injection auger, to feed fuel to the fi re. The fuel feed can
come in at one end of the furnace, or it can be underfed
and forced up through an opening in the middle of
the grates. Some systems use a large injector fan to
blow fuel into the furnace. This approach is called
“suspension burning” because the smaller particles of
fuel burn suspended, while the heavier pieces fall to the
grates and burn there.
Most wood-chip furnaces have grates — either
sloped, stepped, or fl at — that support the burning
fuel and allow for under-fi re air to be blown up through
holes in the grate (pile burners get combustion air fed
from above). Under-fi re air dries the fuel, helps the
solid fuel on the grates to “devolatilize” or “gasify”
(changing its state from solid to gas), and aids in
burning fi xed carbon (or charcoal) on the grates. Overfi
re air, which is often preheated, is blown in from
above the grates to provide oxygen and turbulence, so
the wood gases burn completely before passing into
the heat exchanger. There are often separate fans for
under-fi re and over-fi re air.
Larger or more sophisticated systems sometimes
have moving grates. Grate movement can maintain
an even bed of fuel across the grates, giving a more
uniform and effi cient combustion. Moving grates are
also used to convey ash to the bottom of the grate area,
so that it does not build up and prevent combustion air
from reaching the fuel.
Any moving parts in the furnace area (such as
moving grate systems or under-feed augers) are
subjected to very high temperatures, and so must be
well-designed to keep them functional and prevent