The use of wood chips in district heating plants has increased steeply, since the first systems were started in the beginning of the 80's. If you look at it internationally the use of wood chips in district heating plants has reached a remarkably high extent within a relatively short period of time. Only in a few other countries, such as Sweden, Finland and Austria, is the use of wood chips in district heating plants more widely spread than in Denmark.

All the systems are built from the same main components:

• fuel store
• crane
• stoke system
• combustion chamber and boiler
• flue gas cleaning
• chimney
• ash removal

A typical wood chip plant is built around a solid fuel boiler with a stepped grate. The boiler is brick-built with fireproof walling in order to keep the combustion temperature high for use with relatively high moisture fuel.

The plants are built with a large degree of automation, meaning that the entire system of wood chips from store to grate happens by means of a computer controlled crane, which keeps track of the stock on a real time basis.

LIN-KA Maskinfabrik A/S  has recently built and started a fully automatic wood chip heating plant with an effect of 1,2 MW in Gjerild, Denmark. The pictures below are taken at Gjerild Wood Chip Heating Plant.

The hot water boiler is a combined flue pipe boiler with a smooth flue and water cooled turning chambers. The boiler is dimensioned to give full burn-out and an efficient use of the radiation heat in the flue, while the convection heat is exploited as much as possible in the subsequent fire tube sections. In the bottom the boiler is provided with a custom-made stepped grate, which ensures a complete combustion of the fuel.

The concentrated combustion means that the wood chips are soon dried and gasified, after which the actual combustion takes place. The efficient blend of fuel and combustion air means that a complete incineration is achieved and thereby very high degrees of efficiency.

The boiler is equipped with a return-water-temperature-rising-valve, this ensures an effective blend of the hot water in the return boiler water. This actions keeps the water temperature in the boiler always at a minimum of 60° C thereby preventing condensation occurring in the heat exchanger which leads to corrosion of the steel.

The wood chips from the fuel store are automatically moved by an automatic crane either to a silo or directly to the boiler. Following a signal from an O₂-meter the stoker cycle is activated. The two pushing devices will push the wood chips into the boiler by means of simultaneous conveying,  one of the devices will remain forward to keep the fuel in the flue until the other one has withdrawn. 

The length of the firing channel has been adjusted to ensure that the fuel is constantly closely packed and return-burning is avoided. 

A reporting-device in the funnel will let the crane know, when more fuel is needed.

The system is equipped with a multi-cyclone and a smoke washer for smoke cleaning. Ash removal is in the bottom by means of a sector sluice and the ashes removed by an auger from the boiler and into a fly-ash container. A silencer can be mounted in the flue between the exhaust fan and the chimney, if this is necessary in order to satisfy any environmental noise requirements.

The system is fully automatically controlled by a PLC-control unit, which at the same time monitors whether the system is constantly performing its best.