Woodfuels are of relatively low energy density, compared with fossil alternatives, and consequently large volumes are typically required to be stored and transported, both into the storage receptacle and from it to the energy conversion equipment.
Factors to consider
Biomass does not generally flow as freely as oil or natural gas.
It will usually absorb moisture if exposed to it.
It may naturally biodegrade in storage through a number of mechanisms, particularly if not absolutely dry. This will lead to loss of energy content and potentially the formation of moulds, the spores of which can be dangerous if inhaled.
A bigger store will allow larger, less frequent deliveries, a lower unit price for fuel, and more reserve in case of delays.
Design of the storage facility
The storage of biomass must be well designed and constructed for a number of functions.
It must keep the fuel in good condition, particularly protecting it from moisture.
It must also be possible to deliver the fuel into an appropriate receptacle for transport, and convey it from there to its next destination conveniently and efficiently and requiring the minimum of additional energy input.
The biomass store
The primary purpose for the store will be to retain the biomass in good condition in a convenient place for it to be transferred to the next stage of processing, combustion or energy conversion.
In many cases one of the primary concerns will be to keep it dry. This means protecting it from direct ingress of rain, but also from groundwater.
It should also be protected during delivery of additional fuel, during transfer from the store to the next stage and also during access for maintenance.
Types of store
A biomass store can be:
- A purpose designed and built structure, either above or below ground
- An adapted unit, such as a feed silo or shipping container
- An off the shelf, prefabricated unit designed for a specific range of fuels, such as pellets.
Size is important. There are many factors which will influence the optimum fuel store size but, as a general rule, bigger tends to be better. The ability to buy more at a time is likely to allow lower unit price, especially if deliveries can be scheduled to take a full load of one of the standard delivery lorries used locally. A larger store will also allow a longer period between deliveries and a greater reserve quantity when scheduling the next delivery giving flexibility in scheduling and in case of delay or delivery difficulties.
For most dry biomass stores good ventilation will be necessary to prevent the build up of condensation, allow additional drying, and to prevent the formation of moulds, the spores of which can present a serious health hazard if inhaled. The wearing of a dust mask by those working with such chips is advised.
Good air flow can also help to minimise composting of the fuel, leading to loss of energy content, and prevent the build up of excessive temperatures with the risk of fire. Advice from the US and Denmark suggests that heaps of wood chips should not be built over 8 to 10 m high for this reason.
If large stores are to be maintained then it will be necessary to ensure that the fuel is turned regularly. One mechanism that has been employed that requires minimal additional equipment is to withdraw fuel from the bottom of the store at a higher rate than is required and return the excess to the top of the pile.
Some form of drainage should be included, both in case of inadvertent ingress of water, and also if it is necessary to clean out the store, such as in case of fungal contamination.
Below vs. above ground stores
Building the store below ground may have a number of advantages, including ease of delivery from tipper type vehicles, the removal of the necessity for an additional, above ground structure, thereby potentially easing space requirements, and offering aesthetic benefits.
It is, however, likely to cost more than many above ground store designs, and it is particularly important to ensure that it does not suffer from damp from the surrounding ground. Ventilation and a drainage facility will be both harder to incorporate, and be more important.
There are particular issues with the storage of pellets, particularly concerning health and safety. There is more information about pellet stores on the separate page: