Pellet boiler

Pellet boilers are used as central heating systems for heat requirements (heating load) of more than 3.9 kW (Kilowatt). But pellet central heating systems are not only used in single or two family homes (up to 30 kW), but also for larger residential or business units with heat requirements of a few hundred kW. Pellet burner systems run most efficiently at full load and can usually be regulated down to up to 30% of its full capacity. Since the warm up phase of pellet ovens usually takes longer than for oil or gas firing systems, short burning phases have negative effects on the fuel efficiency. In order to improve energy efficiency and reduce harmful emissions, pellet ovens are usually combined with buffer systems like water tanks for example.

Similar to wood chip fuel heating systems also pellet fuel is delivered periodically and automatically from the pellet storage (for central heating systems) or the day tank (for pellet stove) according to need in the combustion chamber. With the heat generated from the heating circuit water is heated in the boiler of the pellet in pellet central heating systems. The heat distribution is the same as in other systems, which use water for heat distribution. Unlike for oil or gas heating systems, pellet heating systems demand the integration of a hot water tank in the heating system in order to reduce heat losses.

The furnace is automatically supplied with combustible material. The control technology of the system regulates the fuel input gradually in order to match the required heat output. Depending on the specific system, the supplied wood pellets are automatically ignited either with hot air blowers, or it uses a permanent ember bed in the combustion chamber.

Wood pellet heating systems work with different techniques of charging and combustion: Today there are specifically developed loading techniques for pellet combustion like drop chute firing, underfeed firing, side-fed firing or the use of a roller grate system. The method of charging and combustion of the pellet fuel is divided into 5 technologies.

For more efficiency and less pollution in the air, modern pellet heating systems control combustion either via a temperature or flame space sensor in combination with an infinitely variable input of combustion air via a suck-blow fan or a lambda probe. The hot flue gases are led into the chimney via a heat exchanger with manual or automatic cleaning of the reheating surfaces.

Pellet systems are available in different power ranges from about 3.9 Kilowatt single ovens between about 4 and 20 kW. Most systems available today have a power control over the fuel and combustion air supply, so that they can be operated at full load and at part load. Currently pellet boilers achieve a combustion efficiency of about 85-95% at full load (nominal thermal output) in thermal power operation.

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Wikipedia