Wood chips are introduced for combustion on the grate in the combustion chamber that is often situated immediately below
the boiler. The most common type of grate in wood chip-fired systems in district heating plants is the step grate/inclined grate
and the chain grate/travelling grate.
For both grate types, the primary air that is needed for the combustion is supplied from underneath the grate and passed up through the grate.
The step grate has the advantage that wood chips are turned upside down when tumbling down the “steps”, which increases the air mixing and burnout.
The travelling grate is known from coal-fired systems. There the wood chips lie without moving in a uniform layer, whose thickness is controlled by a sliding
gate. During combustion the grate and the chips move towards the ash chute.
Air for combustion is introduced by two air fans in the form of primary and secondary air (see Chapter 6). For the combustion of moist wood chips, the
combustion chamber has refractory linings round the walls. This insulation ensures a high combustion temperature and suspended arches radiating heat to
the wood chips. The amount and the design of the lining are factors of great importance to the combustion quality during the combustion of wet fuels. When firing with dry fuels, e.g. wood pellets, the lining is of no benefit to the combustion quality. Rather the opposite, since the
combustion temperature will be too high, thereby risking soot in the flue gas and grate slagging. Therefore, the type of fuel and its water content should be determined before choosing installation.