Wood fires have been used as a local heat source for thousands of years, progressing from open pit to semi-open pit (a fireplace) to enclosed pit (a stove).
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| A microprocessor controlled woodlog stove with downdraught combustion and separate chamber where secundary combustion takes place (Courtesy of Fröling, Austria) |
The interest in using wood for heating purposes is increasing. Besides heating, wood-burning appliances are also used for cooking, for producing a pleasant atmosphere, and for interior decoration. Domestic wood-burning appliances include fireplaces, fireplace inserts, heat storing stoves, pellet stoves and burners, central heating furnaces and boilers for wood logs and wood chips, and different kinds of automatic wood chip and pellet appliances.
Over-fire boilers are commonly used to burn logs and are relatively inexpensive. In such a boiler, a fuel batch is placed on a grate and the whole batch burns at the same time. The stove or boiler is normally equipped with a primary air inlet under the grate and a secondary air inlet above the fuel batch, into the gas combustion zone. Wood is fed from above and ashes are removed from a door below the grate. These boilers work on the principle of natural draught and, as the fuel bed is cooled by fresh fuel, the initial CO-emissions can be relatively high.
Very strict emission limits in some countries have made it necessary to introduce down-draught boilers. Here, unburned wood gases released by wood placed on a ceramic grate are forced by a fan to flow downward through holes in the grate. Air is introduced below the grate in the secondary combustion chamber, where the gases flow along ceramic tunnels, and final combustion takes place at high temperatures. By using lambda control probes to measure and control flue gas oxygen concentration, staged air combustion, and even fuzzy-logic control, very low emissions are achieved.
Naturally, down-draught boilers are much more expensive than conventional boilers.
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| Two wood pellet boilers, used to heat a school in Denmark |
A recent innovation in space heating is automatic pellet combustion.
The excellent handling properties of pellets mean that the fuel is gaining popularity rapidly in Sweden, Denmark, and Austria. In other countries, the interest in pellet burners is just awakening. Pellet burners are of special interest since they can replace an oil burner in an existing oil-fired boiler.
If the burner-boiler combination is well designed, efficiencies over 90% can be achieved at nominal thermal output. At part load, and varying load, the efficiency decreases but for the best burners efficiencies over 86% have been obtained.
