Wood-Chip Heating Systems - Biomass Energy Resource Center

WOOD CHIP HEATING SYSTEMS
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on the next page.)
• Should the system be of a simple low-cost design
with few extra features, or is a more sophisticated
and more costly system with added features
desirable? (See “System Sophistication and System
Cost” on page 61 and “Considerations in System
Selection” in Chapter Four.)
• What gauges or level of instrumentation is required?
• What level of spare parts for the system should be
supplied by the manufacturer?
How big should the biomass system be? How
do you determine the “correct” Btu output - and
how should the backup system be sized? These are
critical questions, and the answers depend on the
owner’s objectives for operating the plant.
There are two lines of thinking on this issue,
and they lead in opposite directions. Although this
discussion is fairly technical, it is important for the
owners to understand and, under the advice of a
mechanical engineer, to decide how the biomass
system is sized. The starting point is an accurate
number for the peak heat load of the facility
(including domestic hot water). This number is
usually provided by a mechanical engineer, based on
ASHRAE load calculations.
No matter what the objective, gross oversizing
of the biomass plant should be carefully avoided.
There is a natural tendency to design heating plants
that are oversized. But if the wood system is grossly
oversized, whether unintentionally or by design, it
will not run well, will burn too much fuel, and may
produce smoke in low-load conditions.
The owner’s primary objective may be to
minimize backup fuel use. In this case the system
should be sized to meet, or nearly meet, the facility’s
full heat load. In the coldest weather, or the period
of greatest load, the system will be running almost
constantly at full output. The backup fuel system
would not be needed to meet the load, except
perhaps for very brief periods. But since peak load
conditions usually occur only a small fraction of the
time, the rest of the time the system may run less
effi ciently.
A system with multiple fi ring rates or modulating
fuel feed will do better in low-load conditions than
will a simple on/off feed system. For these systems,
oversizing is less of a problem, since the system
Sizing the Biomass System
• How can the system be confi gured to give the best
possible match with the existing maintenance
capability?
The answers to these questions then need to be
formalized in a performance specifi cation, a written
document that tells bidding system suppliers what
is required. A performance “spec” says what the
installer’s system must be able to do, but it does not
will run effi ciently over a wide range of load conditions
throughout the heating season.
The opposing objective is to try to minimize capital
cost by installing an undersized system, compared to
the peak heat load of the facility. In this way the wood
system will run at its maximum output more of the
time. The drawback is that the backup fuel system
will be needed to boost heat output during the peak
load periods. Depending on the level of undersizing,
this can result in either a minimal backup fuel use or
a signifi cant and costly backup fuel use. Experience
over the last fi fteen years shows that signifi cantly
undersizing the system my not save much capital cost
and may signifi cantly increase the usage and cost of
backup fuel.
Sizing the system can be complicated by the
possibility of future building or load expansion. When
making sizing decisions, the owner needs to have a
realistic discussion about the likelihood and magnitude
of possible future expansions of the load on the heating
plant. There needs to be a coherent strategy that
addresses two questions: How will the system run if
we do not expand our load? and, How will we be able to
modify the system if our load does increase?
For a system with a winter seasonal heat load,
peaking in midwinter and dropping sharply in the fall
and spring, the use of more than one biomass boiler
can give effi cient operation in almost all conditions and
still meet the entire heat load with no need for burning
backup fuel. The larger of the two wood boilers
would be used in the winter season and the smaller
wood boiler could provide effi cient operation in warm
months. This approach can be attractive for a large
facility with a high summer demand for domestic hot
water that could be met by the wood system.
It is common to size the backup system to meet the
full design load of the facility. In this way the backup