Wood boilers for residential heating
Modern biomass burning residential heating systems (boilers) are constructed and designed to increase combustion efficiency, despite the fact that wood was the first fuel source for heating and reminds of traditional fireplaces.
The more heat is transferred to water and circulates inside the heating system pipelines the best is to cover the residence thermal demands.
According to EN303-5 European Standard, modern wood boilers ought to be highly efficient andclean burning. As a result, the replacement of an existing oil or gas boiler is possible, without the need to replace the other parts of the heating system (piping, valves, radiators, etc.) The possibility of automatic boiler stoking, via an auger or suction system, enhances the statement that wood boilers can easily replace existing fossil fuel systems in domestic applications.
For residential installations, three are the main boiler types:
- Wood pellet boilers
- Wood chip boilers
- Wood log and briquette boilers
Wood pellet boilers
They are the easiest type of boiler to operate at a fully automatic mode. This is due to the fuel size and its physical properties which provide wood pellets fluid flow characteristics. These characteristics make wood pellets one of the most popular renewable fuels with a constantly expanding market in Europe.
Pellets are mostly produced from sawdust, wood shavings and chips; in general from virgin (not chemically treated) wood residues and biomass. There are strict regulations regarding their quality standards in the EU affecting their size, moisture content, ash content etc.
Their production stages consist of raw material grinding, drying, pelletization, conditioning, cooling and storage. Wood pellets are compact with a glossy sheen and a high energy density; all these features make them easy to transport and store. In addition a constant automatic feeding system of wood pellets into the boiler (usually using a screw, an auger or vacuum) can be adopted.
Similarly to fossil fuel boilers, wood pellet boilers have automatic ignition system and can operate with the minimum of manual intervention. Some more sophisticated boiler designs can also automatically remove the resulting ash from the combustion chamber.
Wood chip boilers
Wood chip boilers resemble significantly with the pellet-fired ones. They are equipped also with an automatic ignition system and sometimes with an ash removal option and, as it happens with pellet boilers, they can operate constantly with the minimum of manual intervention. In order to understand the difference between these two alternatives, it is essential to focus on the fuel itself.
Wood chips are produced by certain machinery equipment, called wood chipper, which is fed with various kind and size woody and biomass bulky raw material and converts it to limited size wood chips. The exact size of the resulting chips depends on the dimensions of the blades in the chipper, but in general terms it varies between 8-15 mm. Moreover, their moisture content should not exceed 30%, they should have low ash content and minimum dust and fines.
Wood chips are less uniform in shape and in size than wood pellets and they have also less energy density. As a result they need more storage volume than the pellets, while their transportation is also not as easy. These characteristics make wood chips less popular in small residential applications than wood pellets. Even though, they are more common in larger scale residential and industrial applications (like power production plants) due to their significantly lower cost.
Wood log and briquette boilers
Log and briquette boilers are the least able to operate on a fully automatic mode due to the fuel large size. So in most of the cases the boiler feeding is made manually. At the same time, logs and briquettes have the least energy density compared to pellets and chips, thus resulting to large storage volume requirements.
Wood logs are always an attractive heating alternative when there is easy and cheap access to them (i.e. through tree pruning). On the other hand, wood briquettes are produced from sawdust compression in a process that resembles with pelletization. Like wood logs, they are usually round in shape (but because of the compression they have higher energy density than logs), with normal diameter sizes between 25-100 mm and length between 10-200 mm.
Due to their combustion conditions, wood log boilers have relatively low efficiencies. On the contrary to conventional ‘open fire’ wood boilers, modern and sophisticated wood log 'gasification' boilers can achieve very high efficiencies.
To sum up the abovementioned, each consumer should pay attention at least to the following characteristics before installing a wood-burning boiler for residential heating purposes:
- CE marking
- After sales service contract
- Accessibility to storage room volume enough for 10-15 m3 of wood pellets. For example 2m x 3m floor area and 2m height.
- Accessibility to boiler room volume of 2m x 2m x 2.5m height.
- Efficiency higher than 80%.
- CO2 emissions less than 200 mg/m3
- Self-cleaning option
EN 303-5 standard has been set for solid fuel (biomass included) heating boilers in order to define design, constructional, safety and operational requirements more
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