Summary

Modern log stoves and boilers as manually operated systems for domestic use come with a capacity of between 5 - 50 kilowatts. They can be divided in two main categories: single heating wood stoves, which are for localized heating and central heating boilers, which are connected to the central heating and hot water supply system. In the case of these stoves wood still has to be fed manually. As opposed to an open fireplace, stoves are freestanding individual furnaces with closed combustion chamber that allows the regulation of the combustion air.

A popular special form is the chimney stove with a glass panel, whereby the user can watch the fire. Heat storing stoves have a high heat storage capacity. This storage capacity is the reason for the heat radiation long after combustion was completed. For central heating boilers modern log wood combustors are established. The design enables a large log wood shaft. Owing to filling intervals up to 7 hours and long stationary operating modes after the combustion started, an excellent flue gas quality and a high combustion efficiency of 90 % are possible. The most common and traditional fuel for small-scale units is log wood with a length of 30, 50 or 100 cm and a water content of 15 – 25 %. For log wood combustors two main combustion principles exist: the over-fire combustion (updraft combustion) and under-fire combustion (downdraft combustion).

For both concepts staged air combustion is possible. The effects of this combustion concept are slower fumigation and lower emissions after the charging of new log woods. Latest technology ensures that combustion is much more efficient and cleaner than before. Thanks to a lambda probe and control, there is enough oxygen for the wood to burn completely and far less air pollutants such as carbon monoxide (CO) and hydrocarbons are produced. Log gasification boilers spatially separate the releasing of light volatile particles (gasification) from the transformation of these gases into heat (combustion). To make the most efficient use of the heat produced, log boilers are combined with a hot water reservoir which releases heat into the heating system as and when required. For the operation of this central heating system, a heat storage – water based storage tank – with a capacity to receive about the energy content of log fillings is necessary. Owing to start-ups, heat losses and part load operation the thermal efficiency over one year is in the range of 65 – 75% based on fuel LHV (lower heating value).

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