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For efficient, low emission combustion biomass needs to be burned fast, at a high temperature.
Does the system need to provide continuous, low level output?
Although most modern wood chip and wood pellet boilers are designed to allow modulation of the output down to perhaps 30% of maximum output, biomass is not well suited to this mode of operation and if continuous, low level output is routinely required biomass may not be the optimum choice. This also means that it is very important not to over specify biomass boilers.
Handling requirements for seasonal variations
Where seasonal operation varies between high level output for winter heating and low level output for summer hot water, a biomass boiler supplemented by a small gas boiler might offer a suitable combination. This may also allow the biomass boiler to be specified for commonly required maximum output, using the supplementary boiler to meet peak demand required for just the few coldest days of the year as well as summer hot water.
Achieving optimum firing
One way to allow optimum firing is to use an accumulator which allows the boiler to operate at maximum output to heat a large reservoir of water that is stored in an insulated container until required.
Whenever a biomass installation is planned the intended sources of fuel should always be included in the planning and the equipment specified accordingly.
If wood chips available
Where there is access to significant resources of seasoned timber residues, recycled timber or a mature local woodfuel supply chain that can readily supply wood chips at 35% moisture content or below, then specifying a suitable boiler that burns low moisture content fuel can mean:
If small round wood (SRW)or arboricultural arisings available
Alternatively, if space exists to air dry timber such as SRW, and there is the facility to chip it on site, this may be the optimum choice.
If, however, the system is intended to use arboricultural arisings from local tree surgeons, which will be delivered green and freshly chipped, then the best choice may be to choose a combustion system that is designed to burn green material and recoup the additional cost of the system in lower fuel and storage costs.
Further information about biomass fuels:
Good design of the biomass fuel store, considering delivery and extraction of the fuel, as well as access for maintenance, is very important to efficient, smooth operation of a biomass installation.
Biomass is a low energy density fuel, and consequently a large volume must be stored on site if unacceptable frequency of deliveries is to be avoided, and sufficient reserve safety margin maintained. There must therefore be a suitable area, sufficiently close to the intended site of the combustion equipment to avoid an unacceptably long fuel feed, creating increased risk of blockages or difficulties, and also accessible to the intended delivery vehicles.
The intended delivery method should also be considered. If wood chips are to be delivered by tipper truck, the most efficient method is to tip them directly into the fuel store. This may require a ramp to allow the lorry to back up to the lip of the store, or a fully or semi-underground store.
If wood is to be chipped on site, however, it may be possible to chip directly into an above ground store.
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