Cymraeg

Boilers

Woodchip boilers have been developed over the last 40 years to become sophisticated and reliable automatic devices. Although they are relatively uncommon in the UK, there are many thousands of installations across Europe and America providing heat for everything from isolated farmhouses to large public buildings and whole towns via district heating systems.

Boiler technology depends on scale, with small installations (up to 100 kW) generally using a stoker burner, whilst medium scale (100 - 1000 kW) plants tend to use moving grates. Large plants used for industrial heat or CHP use more complex systems such as fluidised beds and rotating grates, similar to those used in large coal-burning plants.

Stoker burners

Small wood chip boilers typically use a stoker burner in which fuel is fed by auger into a cast iron burner head. A small fan blows air under and over the fuel to provide primary and secondary air producing a large flame, which passes through the boiler, transferring heat to the water. The heat from chips burning in the middle dries fuel entering the head. At the end, only ash remains and is gradually pushed off into a bin.




Fuel quality is critical for good performance. Moisture content should be less than 30% and the particle size should be consistent to maintain the fire. Since the amount of burning fuel is small, the stoker can respond quickly to load changes and does not produce much heat once the boiler enters 'slumber' mode when the water reaches the set temperature.

An alternative system feeds fuel into the burner head from below rather than the side but the principle is similar with fresh fuel drying in the centre, surrounded by a ring of burning chips, and ash gradually falling off the outer rim.

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