Automated Woodfuel Boilers
Woodchip and pellet boilers can be as automatic as oil or gas boilers. The choice of whether to install a pellet or woodchip boiler will largely depend on local circumstances, for example the size of the heat load or the availability of the different fuel types. For most projects either solution will offer advantages and disadvantages, some of which are outlined in the table below:
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Wood chips systems
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Wood pellets systems
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Fuel availability
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Can be readily produced locally
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May require significant delivery distances
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Fuel quality
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Variable – must ensure boiler matches local supply
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Very standardised
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Fuel storage
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Bulky fuel
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Compact fuel
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Capital costs
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High installation costs compared to fossil-fuel alternatives
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Generally cheaper than wood chip systems although higher than fossil-fuel alternatives
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Running (fuel) costs
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Very competitive – comparable to mains gas
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Competitive – comparable to heating oil
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Embodied energy
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Very low <1%
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Low 5-10%
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Some boilers designed for wood chips can also burn pellets, however boilers designed specifically for pellets cannot generally use wood chips. A wide range of boiler systems are commercially available but all share the same basic features of a boiler, a storage facility and a feed mechanism. The diagram below shows a typical system.
Boilers
Click here for boiler diagram
Woodfuel boilers
Woodfuel boiler technology has been developed in a number of countries, particularly North America, Scandinavia, Austria and Germany. Many years of development have resulted in boiler technology being regarded as mature with reliable boiler systems readily available and able to provide heat for larger domestic properties right up to industrial scale installations and power stations.
Most heating systems are ‘wet’ i.e. central heating radiators, but warm air systems are also possible (most suitable for heating large spaces, e.g. factories). When systems are being designed it is advisable to consider the nature f the heat load (i.e. how much heat is required and when) , to identify the best system for your needs. For example, is the heat load constant (as might be expected in, for example, a swimming pool which is heated all year round 24 hours a day), or does the heat load peak and trough throughout the day and year (as may be the case in a district heating scheme serving domestic housing)?
Woodfuel boilers typically work best under a relatively constant load so, depending on the individual circumstances, if there is a fluctuating heat load it may be advisable to fit a woodfuel boiler to provide say 80% of the annual energy, with a back up boiler (either another woodfuel boiler or small gas/oil boiler) to kick-in when the heat load peaks. Advice on the best system for your needs can be sought from your local Woodfuel Information Officer during the project design stage.
Types of Woodchip Boiler
There are several types of woodchip boiler on the market and each type has its own advantages. The boilers are characterised by the type of combustion and feed mechanisms.
Stoker burners -the stoker burner is like a pressure jet oil burner. Woodchips are delivered by auger to the burner head where air is pumped into the fuel space through a ring of holes. The burner produces a vigorous flame which is fired into the boiler. This type of burner requires dry chips of a small uniform size. The size of the fire is small and an increase in fuel delivery results in an rapid increase in heat output. Stoker burners can respond more quickly to fluctuations in heat demands.
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Underfeed stoker - fuel is augered in to the base of the boiler and spreads out. These boilers are not so good at drying the fuel and so are suited more to burning dry wood chip (for example from joinery factories).
In other boiler systems woodchips are delivered automatically from the hopper into the boiler by an auger. Before the fuel enters the boiler it normally passes through an air lock to restrict excess air in the boiler and to prevent 'burnback' to the stored fuel. Once in the boiler they are immediately combusted or enter a pre-combustion drying area where some of the excess moisture within the chips can be driven-off. The chips then proceed across a moving grate to the point where combustion takes place.
In smaller systems, transfer of the woodchips from where they are processed into the hopper of the boiler can be done manually, or by tractor. For larger systems an automatic feed system is essential, there are several different systems available depending upon the type of storage and the size and grade of the woodchips. Advice should be sought on the most appropriate system in a given situation.
Pellet Boilers
Similar in principle to woodchip boilers, pellet boilers can be used for domestic scale projects and much larger projects. The principle differences between woodchip and pellet systems occur in the feed mechanisms from the fuel store to the boiler. Pellets are small, dry, uniform, and energy dense and they easily 'flow'. For this reason they are well suited to smaller and, in some cases, more sophisticated delivery mechanisms.
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Pellet System at a school
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Smaller augers can be used because there is much less chance of a blockage from irregularly sized pieces of fuel, and in some cases, it is possible to deliver pellets via a 'pump' system into the feed mechanism from, for example, an underground fuel store. This can give more flexibility in the project design with regard to the location of the fuel store in relation to the boiler.
Because of their suitability for smaller more sophisticated feed mechanisms pellets are also used in domestic stoves and space heaters for use inside the home.