Low emission operation of automatic wood boilers operated in cascades

Feb 2023


Low emission operation of automatic wood boilers operated in cascades


Energy wood including forestry wood residues, wood residues from industry and urban waste wood is mainly used for heat and partly for combined heat and power.
Today, mainly combustion systems with automated wood boilers are applied to provide central heat for buildings and district heating, often in the size range from 100 kW up to more than 10 MW.
Compared to wood stoves and log wood boilers, automated boilers are beneficial since the continuous operation enables improved combustion conditions and an operation with controlled air and fuel input. However, in most applications for heat supply, the heat demand is varying which may lead to an unsteady boiler operation with frequent starts and stops or boiler operation at very low load. Such conditions can lead to increased emissions due to non-ideal combustion or due to non-ideal operation of the flue gas cleaning systems such as electrostatic precipitators (ESP). Generally a thermal energy storage is recommended in the system to minimize non-ideal operation conditions such as start-up and low-load operation.

This report concerns installation of cascades of three or more usually identical serial devices of smaller wood boilers as an alternative to industrial boilers (in the range of 200 kW to 2 MW), and how their operation affects emissions. Cascades of smaller boilers can be used where space is more limited (e.g. in residential buildings), and they will allow for production to be less affected by periodic maintenance, as not all boilers will need to be shut down for this. The number of starts can be strongly reduced. Conversely, a cascaded system allows for variation in the total heat production in a wide load range, e.g., by operation of one single boiler during the summer period.
The study confirms the benefit of a heat storage tank and the recommendation of one hour of storage capacity for two-thirds of the capacity installed of the multiple boilers. With up to one hour storage capacity, operation can be improved by smoothing pronounced load peaks.

This report is based on the research project “Holzkessel Kaskadenanlagen mit Speicher (HoKaSpe)” (Wood Boiler Cascade Systems with Heat Storage), carried out by Verenum AG and the Lucerne University of Applied Sciences and Arts. The project received financial support of the Swiss Federal Office of Energy. The translation to English was supported by IEA Bioenergy Task 32 (Biomass Combustion).


Flow diagram of the model of cascaded boilers, connected with a heat storage tank.