Bioenergy Boilers
Buildings can be heated by various bio-boilers burning pellets, briquettes, logs, chips or wood chips. They are used for heating small houses, farms and large properties, for example.
Boilers are used both as a main form of heating and as a back-up form of heating for another form of heating. Central heating boilers are boilers in which the heat energy produced is transferred, usually by water, to the heat distribution system of the same or a very nearby property. Central heating boilers can be divided into bottom, top and reverse firing boilers according to the type of combustion.
Bioenergy boilers use pellets, briquettes, chips, logs and wood chips
The pellets and briquettes are compressed to make them uniform and compact. They are also dry. They therefore have a higher calorific value than wood chips or wood shavings.
The calorific value of a fuel tells you how much heat is generated per mass of fuel during complete combustion. For solid and liquid fuels, it is usually expressed in megajoules per kilogram of fuel, MJ/kg.
The calorific value of dry wood chips with a moisture content of about 20%, expressed in loose cubes, is approximately 740-1000 kWh/i-m3 depending on the wood species, and of wood chips with the same moisture content about 900-1000 kWh/i-m3. The calorific value of wood pellets with a typical moisture content of about 8%, expressed in loose cubes, is much higher, approximately 3000 kWh/i-m3.
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Pellets
- The raw material for pellets is cutter chips, sawdust and grinding dust, which is a by-product of the carpentry and sawmill industries.
- The pellets are pressed from the chopped wood pulp into small, compact cylinders. They are 6-12 mm in diameter and 10-30 mm long. The moisture content is between 8 and 12 %.
- Pellets contain wood energy in a very dense form – one cube of pellets contains the same amount of energy as 300-330 litres of light fuel oil.
- The pellet heating system consists of a boiler, burner, transfer tube and storage silo.
- Pellets are burned in burners specially designed for pellet burning. The burner control unit regulates the operation of the fuel feed screw, the combustion air blower and the burner according to the heat demand.
- A pellet burner can be installed in a boiler specifically designed to burn pellets, but also in most wood boilers. A pellet heating system can replace, for example, an oil heating system that has reached the end of its useful life.
- Pellet stoves are also produced for pellet combustion, which can be used either as a source of supplementary heat or, in small houses, as a main source of heat. Pellets can also be burnt in a traditional fireplace if a separate combustion appliance is used.
- The storage of pellets must be carefully planned to be efficient and safe. Pellets are stored in silos indoors or outdoors, for example in bags. The silo must be completely dry, dust-tight and free of electricity
- It is very important to protect the pellet from moisture. Outside, humidity or cold will not ruin the pellets, but it should be remembered that water, caused by temperature fluctuations for example, will break down the pellets’ structure and render them unusable. It should also be borne in mind that, when placed in a silo in an outdoor area, the pellets will keep for about a year, after which their quality will deteriorate.
- The pellets are transferred from the storage to the burner by means of transfer screws. It is placed at the bottom of the silo, from where the pellets are transferred to the burner. If pellets have to be moved over longer distances, suction technology is usually a good option. With suction technology, pellets can be moved up to 30 metres. The pellet suction system is based on an efficient suction unit and a high vacuum.
- Pellets can be delivered to the customer by tanker if the order quantity is at least 4 tonnes. When designing the plot, it is important to take into account that the tanker must have access to a distance of at least 15 m from the pellet store. Pellets can also be supplied in 500 kg bulk bags.
- The pellet boiler is regularly scrubbed and ashes are removed. For some types of boilers, maintenance should be carried out every 1-2 months, for fully automatic boilers a few times a year. Boiler adjustments and cleaning of the burner, flue and boiler will also keep combustion particulate emissions to a minimum.
- It is a good idea to clean the silo every few years, as fine particles accumulate on the bottom of the silo, which can hinder pellet transport.
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Briquettes
- Briquettes are mainly made from by-products of the wood processing industry. The raw materials are dry sawdust, cutter chips and grinding dust.
- Briquettes are compressed products similar to wood pellets, but they are much larger in size than pellets. The cross-section of briquettes is usually round or square. Cylindrical briquettes range from 10 to 200 mm in length and 50 to 80 mm in diameter.
- Briquettes are made by pressing coarse wood material through a slightly conical cylinder. During burning, the briquettes lose their shape very quickly compared to pellets.
- Briquettes are burned in thermal plants together with other solid fuels. Briquettes can also be used in fireplaces and boilers instead of chips. At best, wood briquettes can have almost four times the calorific value of wet wood chips.
Wood chips, logs, and wood shavings
- According to the combustion technology used, wood-fired boilers are divided into lower, upper and reverse firing boilers.
- In an upper combustion boiler, fuel must be added in small batches at short intervals. The upper combustion boiler requires a hot water tank to store the heat produced by the boiler. The boiler required the most work from the residents.
- The lower boiler burns wood more evenly and has a longer wood replenishment interval than the upper boiler.
- In a reversed-burning boiler, wood burns in two stages. First, the wood is gasified, then the gas flows into the afterburner, where the fuel burns efficiently. The operating principle of the boiler allows clean combustion and efficient heat transfer to the boiler bed.
- Chips can be fired in all these types of boilers, wood chips only in the stoker and the bottom burner (stoker = screw feeder for wood chips).
- There are also double boilers, often designed to use oil and wood.
- The heat distribution system is usually either a radiator or underfloor heating system. The heating system usually has a storage tank to store the heat generated by the boiler. In a pan-fired boiler, at best one heating cycle and a litter of fuel is sufficient for up to 24 hours. A good wood-fired boiler has an efficiency of more than 80 % at rated output.
- Wood heating required more work from residents than other heating methods. The annual wood fuel requirement for a detached house is about 20 stack cubic metres.
- Think in advance about where you will get the fuel and where it will be stored in the house or on the plot. Chips and wood chips can be stored in an unheated warehouse.
| Types of boilers efficiencies % | Old boiler | Old boiler | Modern boiler | Modern boiler |
|---|---|---|---|---|
| Direct heating | Standby heating | Direct heating | Standby heating | |
| Double boiler | 40 | 60 | 45 | 70 |
| Upper firebox | 45 | 65 | 50 | 70 |
| Basalocattila | 50 | 70 | 60 | 80 |
| Reversible boiler | 55 | 75 | 65 | 85 |
| Front compartment and boiler | 50 | 70 | 55 | 75 |
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Tips for energy-efficient use of a pilot boiler
- When you use heat and hot water wisely, you have a significant impact on your heat demand, and therefore your consumption of spots.
- Use dry fuel. You will save 10-20% compared to using wet fuel.
- Ensure the supply of combustion air. The air inlet to the boiler room must be about twice the surface area of the flue. Follow the boiler manufacturer’s instructions for air adjustment.
- Take care of your boiler so that you don’t let the heat get to the harakiri. A one millimetre layer of soot on the heat exchange surfaces reduces heat transfer by about 5% and increases fuel consumption by 2-3 kg per heating cycle.
- You can see if the boiler is fouled by monitoring the flue gas temperature.
- The property owner is responsible for the fumigation. A chimney sweep is contracted once a year to clean the flue.
- In particular, get a well-insulated, energy-efficient storage heater for your pan-fired boiler. It will significantly improve the efficiency of heat generation.
- Insulate the boiler and heater charging pipes well. This will greatly improve the energy efficiency of the system.
- Also pay attention to the energy efficiency of the heater. Poor insulation can result in heat loss equal to the whole year’s domestic hot water demand of a small house.
- The automatic control valve (shunt) of the heat distribution system, which regulates the temperature of the supply water, quickly pays for itself compared to a manually controlled valve.
- The profitability of boiler replacement is affected by the type of boiler and the age of the boiler.
- A solar thermal system will increase the efficiency of the boiler’s heat production and reduce the need for labour, especially in summer.
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