Wood-Chip Heating Systems - Biomass Energy Resource Center

oiler facility. However, the backup fuel system
deserves special consideration.
Most commercial and institutional biomass plants
have oil or gas backup capability, with one or more
separate boilers that burn either oil or gas. In some
facilities the backup oil or gas burner is installed to
fi re into the biomass boiler. In a few cases, usually
industrial applications with round-the-clock staffi ng,
there is no backup fuel capability.
Backup systems are useful in a variety of circumstances:
• during periods when the heating load is too small
for the biomass boiler to run effi ciently without
smoking,
• when the wood system is shut down intentionally
for servicing,
• when the storage bin is empty,
• when an oversized wood chip unexpectedly jams an
auger and stops the fuel supply, and
• when the load on the system exceeds the wood
boiler’s capacity.
Backup systems can be manually activated or
controlled to come on automatically if the wood
fi re fails or is insuffi cient to meet the load. Institutional
applications that are not staffed at night or on
weekends usually have separate backup boilers with
automatic fi ring. In this way the backup system takes
over with no operator involvement. Some larger,
well-staffed facilities have oil or gas burners on swingout
doors in the wood boiler. These burners can be
manually positioned when there is need to use them.
Most facilities that burn biomass use the wood
boiler to supply the domestic hot water (DHW) load
along with the primary load of the building. A DHW
heat exchange water tank (or a stand-alone heat
exchanger with storage tank) is used for this purpose,
piped as a zone off the distribution system. Facilities
that do not use the biomass burner in the summer, but
still have a summer DHW load, must have a separate
water heater.
Emissions Control Systems
Institutional and commercial biomass burners
typically burn with very low levels of undesirable
stack emissions (especially compared to older wood
stoves and industrial boilers), and easily meet state
emissions standards. State air quality regulations are
usually activated according to the size of the boiler
heat exchanger. The minimum level for review or for
permitting varies from state to state. (Chapter Ten
discusses the process of checking state air quality
regulations.)
The smallest institutional and commercial biomass
systems may not be required to meet state emissions
standards, and so would need no special equipment
to reduce stack emissions. Nonetheless, most system
manufacturers routinely install devices to remove
particulates from the exhaust gases, regardless of unit
size (the exception being small agricultural systems
below 1 MMBtu). These devices, called cyclone
separators or multi-cyclones, mount between the
heat exchanger and the chimney connection. Systems
with particulate removal devices always have induced
draft fans, which create a negative pressure in the
combustion chamber and assure proper movement of
fl ue gases up the stack.
A relatively new technology called a “core separator”
has started to be used in some larger institutional
wood boiler facilities. The core separator functions
much like a multi-cyclone, but it is particularly effective
at removing very fi ne particulate matter that is of
increasing concern because of its effect on human
respiratory health.
In very large biomass plants, further levels of stack
gas cleaning may be required to meet state emissions
standards. These devices (for example, char reinjectors,
wet scrubbers, and bag houses) are almost never found
in plants of the size considered in this book. Small
facilities should avoid these devices because they are
expensive to install and maintain.
Ash Removal Systems
Ash — unburnable minerals in the fuel, mixed
with any unburned carbon — accumulates in certain
locations and must be removed regularly. Every ton of
green biomass burned produces about 25 pounds of
ash. Most of the ash accumulates in the combustion
chamber. If there are sloped or moving grates, the ash
moves to the bottom of the grates. It is important that
this ash be removed on a continuous or daily basis.
One method is removal by automatic ashing augers
(also called screws), which collect ash at low points and
move it outside the combustion chamber. Automatic
ashing is sometimes preferred in schools, since it
reduces maintenance time for staff. However, it can add
capital cost to the system.
Many plants, including some fairly large ones, use
manual ashing: the ash is raked and shoveled out of
the boiler by hand, a task that typically takes about
10-20 minutes a day. In most systems, manual ashing
can be done easily without shutting down the boiler; in
others the task is more complex and requires that the
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