Efficient, reliable wood chip and pellet boilers
are readily available. They provide heat for a very wide range of
uses, from homes to commercial and industrial applications.
Important: before purchasing any equipment,
read Managing a successful wood fuel
installation and get expert advice.
Most systems are 'wet', i.e. they deliver hot water and central
heating via radiators, but warm air systems (suitable for heating
large spaces like factories) are also available.
Wood fuel boilers typically work best under a relatively
constant load. So, to maximise efficiency, it may be worth fitting
a wood fuel boiler that provides, say, 80% of the annual energy,
with a back-up boiler (either another wood fuel boiler or small
gas/oil boiler) to kick-in when the heat load peaks. Accumulator tanks can also help with
fluctuating loads.
Wood fuel boilers use different types of technology, usually
classified by the type of grate used. The main types of automated
boiler use either a moving grate, a plane grate or a stoker-burner,
the Carbon Trust's biomass heating guide
(pages 38-41) details the advantages and disadvantages of each
type of boiler.
Automated systems
Wood chip and pellet boiler systems can be as automated as oil
or gas boilers. A wide range of systems are commercially available
but all share the same basic features of a boiler, storage and a feed mechanism.
Wood chip boiler or wood pellet boiler?
The choice of whether to install a wood pellet or wood chip
boiler will largely depend on circumstances e.g. heat load,
availability of different fuel types and the physical
opportunities and constraints associated with the site. For
most projects what is suitable for one customer may not be
practical for another. Either solution will offer advantages and
disadvantages, some of which are outlined below:
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Wood chip systems
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Wood pellet systems
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Fuel availability
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Can be readily produced locally
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Production is more centralised, but can readily be delivered
over longer distances.
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Fuel quality
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Range of standards - must ensure boiler demands match
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, good for sites where space is limited.
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Capital costs
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High installation costs compared to fossil fuel alternatives
(but cheaper fuel costs than wood pellets, making them potentially
more economical than pellet systems overall).
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High installation costs compared to fossil fuel alternatives,
though can be cheaper than wood chip systems, particularly for
storage.
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Energy used to produce woodfuel (embodied energy)
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Very low <5%
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Low 5-10%
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| Level of input required by end user |
A woodchip system is typically more 'hands on' than a
wood pellet system and a well-designed woodchip installation will
typically need at least a visual check on the boiler at least once
a week. |
Less end user input required. |
| Best suited to (on the whole) |
Larger commercial users (typically over
100kW) |
Domestic systems or larger commercial heating systems of
over 100kW when space is at a premium or where it is too expensive
to build a store for woodchip. They can also be used in commercial
installations in urban areas since deliveries are less frequent,
and they give off slightly lower emissions - an important factor if
you live in a town or city, where emission controls tend to be
tighter than in rural areas.
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In the main, woodchip boilers are more costly to install because
the fuel feed system needs to be more heavy duty as chips are not
such a standardised fuel source as pellets. However, chips tend to
be more economical, hence they are more suitable for larger
commercial users.
Some installers may favour pellet systems over chip and vice
versa so make sure the type of system is right for your individual
circumstances, carefully examine which system is right for you
before signing any contract and seek impartial advice if in
doubt.
Some boilers designed for wood chips can also burn pellets,
however boilers designed specifically for pellets cannot generally
use wood chips.
Maintenance
Biomass boilers have greater maintenance requirements than
fossil fuelled boilers.
The boiler manufacturer's representative or boiler installer
will usually carry out an annual maintenance, including a full
internal and external inspection of the boiler, replacement of worn
components (particularly grate components on moving grate boilers),
lubrication and cleaning.
The main maintenance tasks that the user needs to do at regular
intervals are: a weekly visual inspection, emptying of the ash bin,
greasing of induced draught fan bearings and manual brushing of the
flueways. If automatic flue cleaning is installed, a significant
reduction in boiler downtime and maintenance time is possible,
reducing manual flue cleaning from a weekly to a 6-monthly
exercise.
Useful guides and articles
The Biomass Energy Centre have produced useful guides to small
and medium wood fuel systems:
The following article produced for the Scottish Farmer by
Neil Harrison will also prove helpful:
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The big biomass debate, woodchip or pellets