About wood chip combustion

Wood chip combustion has become very popular concurrently with the rising energy prices. Today, not only home-owners who have access to their own forest use wood chips as fuel but private persons by prepared chips. If you have a good intallation for this purpose it works as well as oil or pellet combustion. Keep in mind that wood chip combustion normally requires a little extra space than pellet combustion. Wood chips can be used in many different kinds and sizes of boilers. There are two different ways of wood chip combustion. You use a stoker or a so called furnace. The efficiency is high. The combustion is so efficient that 1 m³ of wood chips does not leave any liters of ash behind.
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The Furnace
In larger installations the woodchips burn on a grate that moves or in a sand bed that is upheld by a heavy air flow. The fuel can also be gasified in a specific chamber and burnt as wood-gas. In a stoker installation a small amount of wood chips is being fed at a time (or continuously) that then gets to burn under optimal circumstances. Air is being provided when needed and is normally divided into both primary and secondary air. The ash is collected within the ash part of the boiler and can then be removed manually or with a mechanical aid. Woodchips with higher moisture content often require an increased hearth temperature to aid the combustion. This can be done with the use of a ceramic jacketed furnace. From the furnace the burning gas is then transmitted to the hearth of the boiler. The largest heat and mechanical stress then occur within the end part of the furnace and inside the flue pipe between the furnace and the boiler.
The Stoker
If you have dryer woodchips or processed fuel (pellets) a so called stoker installation is then the best alternative. The woodchips are then fed from a container or from a storage with augers to the combustion part that normally only consists of a combustion cup in cast iron or high-temperature resistant steel. A woodchip stoker installation exists as either an over or under fed burner. However, the size is a little bigger than that used with the pellet burner. For woodchip combustion you should use woodchip storage with an automatic feed of the fuel via an auger. The whole process can be compared to pellet or grain combustion. The ignition of the woodchips can be made via hot air or fumes.
The fuel
Woodchips used in smaller installations should have a homogeneous particle size and should not exceed a length of 50 mm. Unknown objects like sand or metal should not exist. The moisture content decides the choice of equipment that you use. If you choose a stoker, the moisture content should not be higher than 20-25%. Generally the dryer the fuel the higher is the heating value. The best woodchip quality is reached by using branches and tops that could have been left in the forest for one year and then shed. This wood should then be kept covered until being chipped. Cold-air drying of woodchips is also possible.
The fuel storage
The storage unit should have a tight lid made of sheet metal with a switch that prevents the feed auger operating with the lid off and also that the auger automatically stops if the furnace is filled with woodchips during a power failure. The sprinkler system should have a thermostat set at approximately 70 °C. The stoker auger should be situated at the lowest point to the woodchip pocket and work as a water seal.

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