Summary

Combustion of wood chips and sawdust is a very common energetic use of biomass. In general, there are three types of combustion systems for wood chips and sawdust: fixed bed combustion systems, fluidised bed combustion systems and pulverised fuel (or entrained flow) combustion systems. Wood chips can be used in fixed bed combustion systems as well as in fluidised bed combustion systems, while powdery or pulverised fuels like sawdust are usually used in pulverised fuel 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.

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).

Combustion systems for pulverised fuel are usually in the range of 500 kW_th up to 50 MW_th. They should be used for relatively dry fuels (up to 20 wt.-%).

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.

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