Wood fuel storage and boiler feeds

The key requirement for fuel storage is that it should be watertight. The moisture content of your fuel will be matched to the boiler specification so water ingress will seriously affect fuel quality and the operation of your boiler. Wood pellets which have a very low moisture content will expand if wet and can damage your fuel store.

Another important factor is vehicle access to the store for delivery: you need to minimise fuel handling as this adds to costs.

Container or hook bin

Woodchips can be delivered in a container or 'hook bin', where the container forms the fuel store and connects to the fuel extractor mechanism on delivery. This is suitable for situations where space is limited. Delivery time may be quicker than other methods.

There are only a small number of suppliers who provide hook bins. They are also relatively expensive, as at least two storage bins are required and you may have to pay permanent site rental for one bin at your supplier's depot.

Covered shed

These are cheap and easy to install and delivery is straightforward. However, the wood fuel will then need to be transferred to the boiler, which increases handling cost.

Fuel handling in large stores requires the use of mechanical handling equipment, such as a front end loader or a mechanical grab. This sort of system may be suitable if you have appropriate equipment on site.

Hoppers

A hopper is a chute with additional width and depth for storage. Above ground hoppers are relatively cheap to install.

You will need to ensure your supplier has appropriate delivery equipment, such as a scissor lift trailer or blow tanker. Delivery by bag and crane is time consuming and adds to fuel costs.

Wood pellets are usually stored in a hopper bottom store.  Providing the angle of the floor is greater than 40 degrees, pellets will flow down into the extract auger.

Wood chips with a moisture content less than 35% can also be stored in a hopper bottom incorporating a fuel extractor mechanism.

Silos

Silos are purpose-built structures for storing bulky materials. They are relatively cheap to install but specialised equipment is required for efficient delivery to them.

Where the silo is constructed underground, or has ramped access, both wood chips and pellets can be delivered by tipping into a silo. Alternatively, the delivery vehicle might have a walking floor or some kind of blower mechanism.

Blown delivery of wood pellets is now routine, and some wood chip delivery companies now offer it, allowing access to less accessible stores. However, blown delivery of chips is slow compared to tipping, and this will add to the cost; also chips can lock together during delivery which can jam feed-in augers.

Wood chips are often stored in a silo with a sloping floor, and extracted using an out feed mechanism comprising two sweeping arms which push woodchip onto the extractor. This is known as a spring loaded agitator. An alternative method of fuel extraction from a silo is by walking floor, they are the most expensive type of fuel extraction system however and are therefore suitable only for systems larger than 500kW.

Walking floors can be designed to take the weight of a delivery vehicle where chips or pellets are tipped directly onto the floor. Lighter weight versions are available where fuel is loaded onto the floor by a mechanical shovel.

Flexible silos

For smaller wood pellet installations a range of prefabricated sectional and collapsible silos manufactured from plastics, steel or fabric are available.

These are designed for installation in hard-to-access and confined spaces, and can be assembled in situ in basements or loft spaces.

They can be filled by blown delivery where one of the two hoses receives the pellets and the other provides pressure relief and recovers the dust produced to the delivery vehicle.

While these silos can be fitted with auger out feed equipment, pellets can also be extracted by a vacuum pump to the boiler's dosing silo. This requires a pressure relief hose.

Underground bunkers

These are ideal for larger installations, with easy delivery by tipping vehicle, minimising ongoing fuel delivery costs. The feasibility of underground storage will depend on ground conditions, including the water table, and on cost.

Fuel extraction is typically via walking floor or agitator.

Boiler feed

Fuel is usually fed into the boiler by either a rotary auger or a hydraulic ram-stoker.

An auger is a screw-type mechanism and by far the most common in use on automatic wood fuel boilers.

Larger systems need an automated feed using multiple augers, a cross feed auger or a hydraulic ram-stoker combined with a walking floor. Hydraulic feed mechanisms tend to be used on the larger and more expensive boilers (over 500kw).

If your combustion system will accept a wide range of chip sizes, you might consider purchase of an agitator to prevent bridging and ensure a constant feed of fuel to the boiler.

Health and safety during delivery and storage

Health and safety issues relating to wood fuel storage and handling include:

• mechanical and electrical hazards e.g. motors and drives
• enclosed spaces e.g. hoppers and silos
• respiratory problems caused by dust and fungal spores

Safety masks should be worn when moving wood chip, or else those moving the chip should be in an enclosed vehicle cab.