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Automated boiler with feed

Wood chip and pellet boilers inc. automated systems

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:

 

Wood chip systems

Wood pellet systems

Fuel availability

Can be readily produced locally

Production is more centralised, but can readily be delivered over longer distances.

Fuel quality

Range of standards  - must ensure boiler demands match local supply

Very standardised

Fuel storage

Bulky fuel

Compact fuel, good for sites where space is limited.

Capital costs

High installation costs compared to fossil fuel alternatives (but cheaper fuel costs than wood pellets, making them potentially more economical than pellet systems overall).

High installation costs compared to fossil fuel alternatives, though can be cheaper than wood chip systems, particularly for storage.

Energy used to produce woodfuel (embodied energy)

Very low <5%

Low 5-10%

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.

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:

For businesses and public sector organisations, the Carbon Trust have published Biomass heating: a practical guide for potential users which covers the issues, technology and processes involved in choosing and installing a heating system running on woodfuel and other biomass products.