A pellet boiler is a heating system that burns wood pellets. Pellet boilers are used in central heating systems for heat requirements (heating load) from 3.9 kW (kilowatt) to 1 MW (megawatt) or more. Pellet central heating systems are used in single family homes, and in larger residential, commercial, or institutional applications. Pellet boiler systems run most efficiently at full load and can usually be regulated down to 30% of full load. Since the warm up phase of pellet boilers 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 boilers are usually combined with buffer systems, such as insulated water tanks.
Operation method
Similar to wood chip heating systems, pellet fuel is delivered periodically and automatically from the pellet storage (for central heating systems) or the day tank (for a pellet stove) according to need in the combustion chamber. With the heat generated from the burning pellets, circuit water is heated in the boiler of the pellet. The heat distribution is the same as in other systems, which use water for heat distribution. Unlike oil or gas heating systems, pellet heating systems require the integration of a hot water tank in the heating system in order to reduce heat losses.
Burning unit
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 from a permanent ember bed in the combustion chamber.
Wood pellet heating systems work with different techniques of charging and combustion. 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.
Drop shaft burner - Pellets slide down a drop chute into a burn pot. This way the combustion area is exactly defined, and combustion can therefore be exactly controlled. With this technology the least ash remains compared with other systems and can be carried out of the burn pot by cleaning mechanisms. This type of charging is usually used for Pellet stoves. Roller grate system - The pellets fall from above onto several slow-turning steel plates with small clearances. A skimmer cleans the clearance spaces with every revolution so that the ash can fall down unhindered and combustion air can be fed through upwards. Underfeed burner - The pellets are pressed from below into a burn plate by an auger where they are burnt and the ash that remains falls over the edge of the plate into the ash pan below. Side-fed firing or retorte burner - This technology functions similarly to the underfeed firing except that the fuel is pressed via an auger onto the burn plate from the side. At the same time, the burn plate as well as the air supply can be configured to match partial performances. Suction technology The 100% burnback security of the drop-chute system is combined with a solid metal rotary valve and prevents coating of the cyclone or suction turbine with dust. The narrow and high cyclone cleans the return air of dust and thereby guarantees maximum service life of the turbine. The variable control firebed possible with the drop principle, together with the high combustion chamber temperatures, makes modulation characteristics possible.
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.
Power range and efficiency
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.