At a time when traditional fuels such as heating oil and natural gas have shot up in price, wood is once more gaining increasing popularity for heating purposes. It demonstrates many qualities which makes it of interest, not only as a source of energy, but also as far as protection of the environment is concerned: wood grows in our own forests and can be harvested relatively easily and at relatively low cost in terms of energy, because the cost of transport is largely nonexistent. Since, every year, more wood grows than is harvested, the supply is secure for a long time to come. Because of the large amount of forest, wood can be harvested locally, and this strengthens the local economy and secures local jobs.
Wood can be used for heating in a wide variety of forms. Wood pellets are enjoying increasing popularity. These are pressed from untreated wood waste. They are manufactured by first drying sawdust and shavings from the wood processing industry. These are then compacted under high pressure and finally cut into lengths of between 5 and 10 cm. Pellets have a heating value (energy content) of approximately 5 kWh/kg. The energy content of one kilo is equivalent to half a litre of heating oil.
In principle modern pellet-fired heating systems are designed in such a way as to give reliable and fully automated service. There is a distinction to be drawn between pellet burners (pellet stoves) and pellet-fired central heating systems. There are two types of pellet burners: one using the ambient air and the other using a boiler principle. The “air process” operates on a principle similar to that of the free-standing wood-burner. It is used principally for heating individual rooms and as supplementary or transition heating to cover peak loads. These free-standing stoves which radiate heat directly have a maximum heat output of 8 kW. Alternatively there are units designed for operation with central heating systems, which have outputs of 8 kW and more, and which can be used to heat detached or semi-detached houses, but can also supply whole apartment blocks or groups of dwellings. For this a boiler principle is employed; the burner has an integrated water jacket (heat exchanger) and boilers such as these can be successfully combined with other types of heat generation system or solar-thermal energy.
Central heating systemsusing wood pellets are very convenient to run: they are comparable with oil and gas fired heating systems both in terms of operation and servicing. Hybrid and combination systems can also be fed with other forms of firewood such as wood chips and split logs. Pellets are stored in a tank or in a storage room and fed to the boiler with a delivery system – trickle feed, airsuction or augur.
Wood is the oldest energy source known to man and, as a fuel, is on the up and up. Thanks to modern heating systems, houses, or even whole housing complexes, can be supplied with heat automatically and in an environmentally friendly way. The various wood-fired heating systems recommend themselves because of their convenience of operation; they require a minimum of effort to keep them going. And they use wood as the energy source – in the form of split logs, pellets or wood chips.
There is a variety of systems on the market when it comes to wood-fired central heating systems. Wood gasification boilers use split logs, giving high levels of efficiency and relatively low emission figures. In this case, a fan creates the right amount of draught for combustion purposes. Once the boiler is filled, it will burn for several hours at a time. Pellet-fired boilers offer the greatest efficiency and the lowest emission figures compared to other forms of wood-fired boiler. As far as convenience is concerned, they are comparable with traditional oil or gas-fired heating systems, since they are automatically fed, are fed with hot air and are self-cleaning with built-in riddles. All modern wood-fired boilers are fitted with anti-burn-back protection to prevent fire “creep” back into the storage room. Wood-chip boilers are also automatically operated, with combustion being regulated by a lambda sensor. Here too, only limited amounts of ash are produced. Wood-chip heating systems are suitable for larger installations – supplying heat to schools, swimming pools, or apartment blocks for instance. The price of woodchips is lower than that of wood pellets, although a larger store room is required. These systems are available in almost all output ranges from 4 kW to several MW.
One of the main reasons why wood is so appealing as a source of energy, is that it is CO2-neutral when burned. The amount of CO2 released when it burns corresponds exactly to the amount the tree fixed when it was growing. If the tree were left to rot in the forest, the same amount of CO2 would be released – and to no purpose. Moreover, every litre of heating oil replaced by wood saves 3 kg of CO2, which would otherwise have found its way into the atmosphere. In addition wood ash can be used as a fertilizer, which once again completes another natural cycle in the wood chain. Using wood as an energy source represents a high level of local value creation, since the purchasing power remains with the locals – and so do the jobs. Not only that, consumers can benefit from state subsidies that are available for people using sustainable raw materials.
