Wood-Chip Heating Systems - Biomass Energy Resource Center

WOOD CHIP HEATING SYSTEMS
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regulated, to some extent, by scientifi c tuning of the
system to optimize the effectiveness of the controls.
Stack temperature is also affected by ash or soot
buildup — so ash cleaning is an important way to keep
stack temperature down.
8. MOISTURE LOSSES
There are two sources of moisture in the fl ue gas:
moisture contained in the fuel and moisture produced
by the burning of the hydrogen in the fuel. In theory,
moisture losses can be reduced by pre-drying the fuel
or condensing the fl ue gas vapor in a tertiary heat
exchanger, which condenses gases and captures the
heat that is released. As a rule, neither of these methods
is practical with current biomass system technology,
particularly for systems in the size range considered
here.
9. TYPICAL PERFORMANCE OF CURRENT SYSTEMS
System manufacturers design their equipment to
minimize energy losses from each of the components
noted above, while at the same time ensuring safe,
reliable operation with minimum maintenance
requirements. Recent tests show that typical steadystate
effi ciencies of current systems are likely to be in
the 55-75% range. 3 This means that 25-45% of the
energy in the fuel is lost. (A typical oil or gas-fi red
boiler would be expected to have losses of 10-20%
under similar measurement conditions.)
The lower effi ciency of wood systems is due in part
to the nature of the fuel itself. Biomass fuel can vary
dramatically by particle size, species, and moisture
content. It is diffi cult to design a system that can deliver
high effi ciency across this range of variables. Wood
system effi ciencies can, however, be expected to rise
as manufacturers improve their products and as the
market for systems grows. It should be kept in mind
that the primary advantage of burning wood fuel lies
in its low cost — which offsets the mediocre effi ciency
— and its status as a locally produced renewable fuel.
In the following paragraphs, recent test results on
several Canadian systems 4 illustrate the effect of system
improvements on effi ciency. These units ranged in
size from .3 MMBH to 6.2 MMBH output. Despite the
wide range of design outputs, there was no clear link
between steady state effi ciency and size.
Excess air levels ranged from 185% to 71%, stack
temperatures from 579°F to 466°F, fuel moisture
contents from 49% to 39% (wet basis), and CO levels
from 614 ppm to 29 ppm. The effect on steady state
effi ciency at the high or low end of each of these
parameters is as follows:
South Shore Regional Hospital, Bridgewater, Nova Scotia
System Size: 4.0 MMBH
Manufacturer: KMW Energy Systems
Showing top-hinged hy drau li cal ly operated loading doors
for below-grade bin adjacent to boiler room.
• Reducing excess air from 185% to 71% will increase
effi ciency about 7%.
• Reducing stack temperature from 579°F to 466°F
will increase effi ciency about 5%.
• Reducing fuel moisture content from 49% to 39%
will increase effi ciency about 5%.
• Reducing carbon monoxide emissions from 614
ppm to 29 ppm will increase effi ciency by only about
0.3%.
If the poorest values in each category are used
(185% excess air, 579°F stack temperature, and 49%
moisture content), the steady state appliance effi ciency
will be 50%. At the other end of the performance
range (71% excess air, a stack temperature of 466°F,
and 39% moisture content), the system would have