17
Hoval STU Wood Pellet Boiler
Design Guidelines
General
The Hoval STU Wood Pellet Boiler has been designed to deliver the most flexible Biomass solution.
However as a solid fuel fired appliance the boiler must be integrated with a bulk pellet fuel store to deliver the most effective design or
solution. Therefore the design of the store and the connecting automatic pellet feed system should be considered at an early stage in the
proposed installation of the STU wood pellet boiler.
In addition in common with all solid fuel fired boilers optimum combustion conditions are achieved when the boilers are allowed to fire
as continuously as possible. The STU combustion system allows some turndown but it is important that the model selected matches the
anticipated load likely to be placed on the boiler such that excessive on/off cycling is avoided. In particular consideration should be given
to likely seasonal variation when applying the equipment to a space heating application.
As a rule the STU wood pellet boiler DOES NOT require the installation of a buffer or accumulator vessel in the boiler primary circuit.
However the inclusion of a small buffer vessel in the system design can help to reduce boiler cycling and allows more continuous operation
if significant load variation is expected. Alternatively better boiler size matching or a cascade of boilers should be considered to deliver
the required turndown.
Hoval can offer a suitably sized buffer or storage vessel to match installed boiler output to anticipated loading.
Fuel Storage
With regard to the bulk storage or wood pellet fuel store the following simple guidelines offer advice to assist in the design and sizing for
a successful installation.
A bulk wood pellet fuel store should be located as close as possible to the biomass boiler(s).
The design of the fuel store and the recovery of the pellets from the store and the subsequent transfer of the fuel to the boiler should be
kept as simple and most straightforward as possible. Tall thin storage – such as purpose design silo’s – often represent the most effective
way of storing wood pellet fuel and can lead to the best use of a given storage volume. The angle of the discharge cone from such a silo
would typically be 60º to ensure complete mass flow from the store.
A possible alternative to a storage silo would be a bespoke build bunker store. These typically require a larger floor area and are less
effective in terms of the available recoverable volume of fuel. Some profiling of the bunker base should be considered – minimum angles
of 45º should be adopted if profiling bunker – which will significantly reduce the volume available for the storage of pellet fuel.
The natural angle of repose of the fuel will vary depending on the quality of the wood pellet (see below) but will typically be greater than
45º. Again this can significantly reduce the recoverable volume available in the pellet store.
Whichever fuel store design is adopted it must be such that it:
• Keeps the fuel dry – water ingress will cause the pellets to fall apart and block handling equipment.
• Is located as close as possible to the boiler.
• Has adequate vehicle access – to allow easy bulk delivery.
• Is fitted with appropriately sized delivery pipe work.
• Bulk fuel deliveries would normally blow fuel into store via a delivery pipe.
• Delivery pipe work/bunker design should not cause undue fuel breakage during delivery.
• Is sealed to prevent possible dust nuisance.
• Conveying air used for fuel delivery needs to be filtered prior to discharge.
• Is sized to suit boiler load and likely bulk delivery payload.
• May need to be behind a fire rated wall if inside the plant room (check with Building Control).
All solids have a natural angle of repose. This is the angle formed by the fuel remaining in the bunker as it is removed from the bottom
without any outside interference, agitation or mechanical recovery. The angle is a function of the size distribution of the solid and can
vary depending on the fines or dust content of the fuel concerned. A good quality wood pellet fuel containing little of no fines will have
an angle of repose between 45º and 50º. This angle increases as the fuel quality deteriorates and can approach 90º if the fuel contains
a high percentage of fines. From this it follows that the design of the fuel store should be such to avoid ‘dead volume’ and that pellet
handling – especially during delivery – should be sympathetic to pellet quality to avoid excessive fuel degradation.
Pellet recovery from the fuel store would typically either be via Pick-up auger(s) angled into the bunker or directly from the outlet of a
coned purpose built store or silo. From the bunker or silo the wood pellet fuel is delivered to the transfer box of the stoker.
THERE SHOULD ALWAYS BE SUFFICIENT PELLETS WITHIN THE STORE – TAKING ACCOUNT OF POSSIBLE DEAD VOLUME –
TO ENSURE THAT PICK-UP AUGER(S) ARE ALWAYS COVERED WITH FUEL.
