Automatic wood-fired heating systems

Wood is something special. Its combustion comprises three phases, as follows. First comes the drying stage at temperatures up to around 150°C during which any water contained in the wood turns to steam. Between 150° and around 600°C the process known as pyrolysis or thermal decomposition takes place whereby the gaseous compounds in the wood are liberated leaving only wood-carbon behind. From around 400° to 1300°C the actual combustion process occurs with oxidation, the addition of air (oxygen), when the gases released during pyrolysis and the wood-carbon burn. Only at this point is energy released. An accurate knowledge of the particular characteristics of the combustion process of wood permits the construction of furnaces with a high level of efficiency and the lowest emissions.

Logs
Logs are still the commonest form of wood for burning. Whoever decides to use logs has to have enough room to store the fuel and, for the most part, some connection with woodland and timber. Whoever decides to use logs is deciding in favour of predictable heating and is prepared to go to a certain amount of expenditure to purchase them.
Log supply chain – This is the oldest method of fulfilling a wood-energy requirement. It is the only one of the processes described here that in extreme situations can be achieved without any mechanical intervention.
Today, however, in the course of further technical developments, more use is made of modern work tools such as power saws, milling machines, splitters, combines cutters/splitters and trucks. The combustion process in simple stoves is becoming mechanised as well with ventilators to control the air supply and the introduction of energy storage devices. Even today, however, automation of small scale equipment is difficult and is more focused on the type of feed mechanisms required to a greater extent by larger scale installations. Burning a selection of logs that includes wood of dissimilar types easily leads to incomplete combustion or requires custom built grate systems.
A reliable and year-round supply of logs is not assured everywhere in Switzerland. Dry storage and space for preparation (splitting, chopping) are widely available in rural areas. In more densely populated areas many restrictions must be accepted.
Furthermore, main areas of application should therefore be small installations, emergency or transition heating systems. These, in the context of low energy housing (keywords “Minergie”, “log-burning stove”) are currently experiencing a significant upswing. Individual larger log-burning heating systems, subject to the right favourable conditions (cheap logs and short transport distances) can also be very cost-effective over time.
Logs are available split or in the round, in lengths of 100, 50, 33 or 25 cm in leisure markets or courtesy of local forestry operations.

Wood chips
Wood chip burners have been used successfully for years for the heating of large buildings, industrial premises and operations run by neighbourhood heating associations. But, thanks to considerable technical developments, wood chip heating systems are again suitable for properties with only small heating requirements. The smallest boiler types are available with a heat output that can be regulated from 5 kW. So they are suitable for large single family homes, house in multiple occupation and small commercial enterprises. Often neighbouring buildings are connected to a central wood chip heating system. In modern furnaces, there is no problem in burning green as well as dry wood chips.
In the case of a direct supply chain, timber is felled in the forest and processed there into wood chips then transported directly to the consumer. For greater security of supply logs can be delivered to a location where they can be stored and which is accessible even in winter (snow, ice).
The direct supply chain is cost-effective because processing costs are minimal and no intermediate storage is needed for the wood chips. The wood supplier must be able to guarantee security of supply even in the winter in conditions of poor access to the woodland. Wood chips can have a moisture content up to a maximum of 60%.
By storing felled trees that have not been defoliated for a few weeks or months in the woodland, a considerable amount of drying out can be achieved (moisture content < 50%). At the time the maximum amount of foliage and needles remains in the woodland as minerals.
In the case of an indirect supply chain, wood for burning goes into temporary storage outside the woodland in the form of wood chips. The indirect supply chain is more expensive because of the need for temporary storage of the wood chips. This can be justified where access to the woodland is not possible in winter and as a regional solution if very large quantities of wood chips have to be transported (buffer store).
Alongside the direct and indirect supply chains there is also the mixed supply chain in which both options are combined. This makes it possible to provide considerable security of supply and, because of smaller intermediate storage facilities for the wood chips, is somewhat cheaper than a fully indirect supply chain.
As the cost of felling, chopping, transport and storage depends on the volume to be processed and not the weight, the prices per unit of energy content are generally around 10% - 15% higher for softwood than for hardwood.

Pellets
Sawdust and shavings are forced under high pressure through a mesh, almost like vermicelli, to produce small cylindrical pellets between 5 and 45 mm long. Thanks to the homogenous nature of this fuel a very efficient and uniform combustion is achieved in wood pellet heating systems which produce very little ash.
Depending on the type of wood, the density of the pellets is 1.5 to 2 times higher than that of logs. 2 kg of pellets are the equivalent of 1 litre of heating oil and require around double the amount of storage space. Production of pellets requires far less energy than the whole production process might lead one to suppose. The grey energy in pellets is a maximum of 10%.
Wood pellets are produced using sawdust and shavings, which are waste products from sawing and milling operations. The raw materials used cannot contain glue, varnish or other foreign matter. Sawdust is forced at high pressure through a mesh. The heat generated by the pressure activates binder materials naturally present in the wood. This allows the pellets to keep the correct shape without the use of artificial additives and binders. Depending on the type of wood used, the specific gravity of the pellets is approx. 1.5 to 2 times greater than that of wood in log form. The small cylinders have a diameter of 5 to 7 mm and a length of 10 to 50 mm. A cubic metre of pellets weighs approx. 650 kg. Thanks to the very low moisture content (8-10%), the pellets have a very high calorific value of around 4.9 kWh/kg. Pellets are a homogenous wood fuel. Their combustion is thus highly efficient and economical and produces very little ash. Production of pellets requires far less energy than the production process might lead one to suppose. The embodied energy necessary for the production of pellets is only 2.7% relative to the energy they contain.
In contrast to other automatic wood burning central heating systems, pellet boilers with a heat output of 3 kW that can be regulated are already available. So they are particularly suitable for installation in modern single family homes which have low energy requirements («Minergie» houses).

Wood briquettes
Wood briquettes are hard pressed blocks of various uniform shapes and sizes formed under high pressure from dry untreated wood waste such as sawdust and shavings. Depending on their source material, the briquettes have different combustion characteristics. Some burn like firewood, others glow rather than burst into flames. On average it can be said that, because of the high density of the briquettes, these natural wood products have roughly the same flammability as brown coal (lignite). However, relative to brown coal, wood briquettes produce less soot, ash and sulphur during combustion and are thus more environmentally friendly and cleaner. Furthermore, the carbon footprint is neutral because the briquettes are produced from wood which is a sustainable material. During combustion, only the same amount of carbon dioxide is released into the atmosphere as is absorbed by the tree by photosynthesis as it grows. So, effectively, a wood briquette can be regarded as a piece of wood and can be used for heating in the same way.
Relative to typical wood for burning, therefore, briquettes display several advantages. In terms of volume, they have, for example, a much higher calorific value. For customers, this means that the fuel takes up less space. Depending on the quality of the briquettes, it can be said that 1 tonne of wood briquettes (the typical delivery quantity) equates to 3 – 5 cubic metres of logs but needs only about 1.5 cubic metres of space. A further advantage is the practical packaging since most briquettes come in manageable packs of around 10 kg which are stacked on a pallet. So the briquettes are protected during transport and can be easily carried to the boiler or stove.
Furthermore, there will be never be any difficulties with the briquettes as far as moisture is concerned because, as briquettes are made from largely dry timber shavings, they basically have very little residual moisture content and that should be the same in all briquettes from the same production batch. By contrast, there can be significant differences in moisture content in air-dried wood in a single delivery so that, kiln-dried products aside, a uniform dryness of the wood can never be guaranteed. If nothing else, wood briquettes produce much less ash than normal wood so that the combustion equipment needs cleaning less frequently.
Wood briquettes are produced under high pressure from dry untreated wood waste such as shavings and sawdust. The high pressure causes the lignin in the wood to change so that it binds the sawdust. The addition of a binder is therefore unnecessary. The pressure also excludes most of the air from the briquettes. This and the fact that the residual moisture in a briquette is only around 10% are reasons why they can be compared to wood with a high calorific value.