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Biomass boiler efficiency

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.

Boiler door open

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).

  • Fuel choice:

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.