Summary

Combustion of pellets is a very common energetic use of biomass. In general, there are two main types of combustion systems for wood pellets: fixed bed combustion systems and fluidised bed combustion systems.

Fixed bed combustion systems for biomass combustion systems with more than 50 kW thermal power are usually equipped with underfeed or horizontal feed systems. Installed thermal power can range from some kW up to 60 MW. Fixed bed combustion systems can be designed to use fuel with relatively high moisture contents (up to 60 wt.%). They are suitable for larger sized fuels as well as for pellets.

Fluidised bed combustion systems are usually built as bubbling fluidised bed combustion systems or as circulating fluidised bed combustion systems. Installed thermal power for fluidised bed combustion systems can range from a few MW up to 100 MW. Fluidised bed combustion systems are suitable for fuels with high moisture contents (up to 60 wt.%), depending on the technology. Fuel particle size should be below 40 to 80 mm (depending on the technology).

Due to automatic feeding and additional flue gas cleaning systems, lower emission values compared to hand fired systems can be achieved. Usually, there are two main control systems for load control and combustion control. Load control ensures that the right amount of fuel for the heat demand is supplied, while combustion control is responsible for the correct amount of excess air to ensure low emissions. Since pellets do have good transportation properties, combustion processes based on pellets are usually easy to control.

Specific investment costs for pellet combustion systems are in the range of 100 to 400 €/kW_th, depending on the technology and installed power of the system.

Storage of wood pellets should be done according to security standards to avoid danger for health and the environment.

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