The energy stored in the delivered fuel is converted
into useful energy by transferring heat from combustion to hot
water in the heat exchanger of a boiler. This process results
in some loss of heat energy, primarily to flue gases.
Biomass boilers manufactured to achieve a certain level of
combustion efficiency. Most biomass boilers
manufacturers will measure performance against the EN 303-5:2012
standard for "heating boilers, manually and automatically stoked,
nominal heat output of up to 500kW".
The heat losses and reported efficiency levels cited in
promotional and technical literature are typically in the range of
88% to 94%. However, overall boiler combustion
efficiency is somewhat different (i.e. lower). It can be
defined as:
Useful heat output per hour divided
by energy input (in the form of fuel) per hour
This is of particular interest to boiler owners and operators,
as it also considers radiation losses from the boiler to the plant
room and from unburnt fuel in the ash.
Boiler efficiency is determined and expressed as an
instantaneous value, but the nature of construction of most biomass
boilers means they have considerable heat storage capacity in water
jackets, heavy steel construction and in refractory linings.
Therefore, a reliable value for boiler efficiency can only be
determined over a period of operation.
Furthermore, other factors may also impact on boiler efficiency,
such as:
- Is the boiler designed and set up for base load or
peak loads operation?
For example a typical base load
situation would be heating a swimming pool, where the load is high
and constant and so makes it easier for the boiler to operate at
higher efficiency levels. Conversely a peak load situation
might be a school where high morning peaks are required as the
school is heated when staff and students arrive, but with reduced
performance later in the day (this causes constant cycling up and
down in output). In addition, the settings and control
optimisation of the boiler will impact upon achieved efficiency, by
promoting effective combustion and reducing issues like cycling
(the boiler constantly ramping up and down or on and off).
Biomass boilers are manufactured to
use a specific range of fuels. For example, pellets (at 10%
moisture content) to wet wood chips (at 45% to 50% moisture
content). Manufacturers often state the combustion efficiency of
their boilers for these different fuels, and that can range from
about 94% (for pellets), down to about 80% (for wet chips).
Overall, boiler related losses could be as high as 20%
and as low as 6% measured over a whole year.