Wood-Chip Heating Systems - Biomass Energy Resource Center

Murray Farms
Greenhouse, Penacook,
New Hampshire
Size: 2.5 MMBH
Manufacturer: Chiptec
Wood Energy System
Greenhouses are a good
match with woodchip
heating systems
because they have very
high heat loads.
them from burning out quickly. Otherwise, they can
add to the costs of maintenance and replacement
parts. The experience of operators who have run a
particular brand or type of system will best predict the
lifetime and maintenance requirements of these system
components.
Some systems, typically larger ones, use watercooled
grates when very high temperatures in the
primary combustion zone could otherwise warp or
deteriorate the grates. Some two-chamber systems (see
Chapter Six) water-cool the parts of the combustor
that are in contact with the grates, rather than cooling
the grates themselves. Water cooling is generally
considered an expensive feature, and is rarely used in
systems of smaller the type considered in this guide.
The furnace is lined with high-temperature
“refractory,” a material that refl ects some heat back
to the fuel and keeps the furnace at an even high
temperature. The refractory also protects the material
of the walls and base of the furnace from the high
temperatures of the combustion zone.
Combustion Controls
The conditions for effi cient biomass combustion
are set by controlling the rates at which fuel and
combustion air are fed to the fi re. The simplest
systems have on/off fuel feed. When the boiler water
temperature or steam pressure drops below a set value,
this type of system turns on and supplies fuel (and
combustion air) to the fi re until the water temperature
or steam pressure is brought back up to its set value.
Then the system shuts off the fuel feed system and
combustion air. These simpler systems usually have
timed fuel injection cycles during the period when the
boiler temperature is being brought back up. The timed
“on” and “off ” periods can be set manually to adjust
for different kinds of fuel or different seasons.
Systems with on/off controls usually have an idle
mode so they can hold a fi re during periods when there
is little or no load. In this mode a small amount of fuel
is fed to the fi re periodically, and the combustion air
fans are turned on to keep the coals from burning out.
The weakness of the simple on/off control strategy
is that it does not respond well to varying loads. If the
feed cycles are set for effi cient combustion during
midwinter conditions when there is a heavy heating
load, the combustion may be ineffi cient and smoky at
times when there is a much lower load (at certain times
of day or in warmer months).
The “turn-down ratio” characterizes a system’s
ability to burn effi ciently over a broad range of loads,
such as heating loads from fall to midwinter. This
ratio compares the full rated output of the boiler to
the lowest boiler output at which effi cient combustion
is still achieved. For example, a system with a full
output rating of 3.0 MMBtu and a 3:1 turn-down ratio
would be able to maintain its parameters for effi cient
combustion at varying loads from 1.0 to 3.0 MMBtu.
More sophisticated systems use a control strategy
with multiple, separate fi ring modes. Controlling
how the system switches back and forth between the
different fi ring modes (such as low, medium, and high)
WOOD CHIP HEATING SYSTEMS
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