For most energy applications low moisture content is desirable for woodfuel and biomass. This can be achieved in a number of ways.
Biomass may be dried either passively (without the use of external energy source) or actively (using external heat and/or fans).
Passive drying
Effectiveness
Passive drying is generally the cheapest, requires least additional equipment, requires the least external energy input but is the slowest. It can be used to achieve a moisture content of 25-30% in most places in the UK, and lower with care and time, however if a lower moisture content is required some active drying may be required. For most combustion systems this is sufficient, however for some gasification or pyrolysis systems, or the manufacture of pellets it may not be.
Drying wood
Wood such as small round wood (SRW), branches or brash should be:
- Stacked on bearers (typically two lengths of roundwood) to ensure the stack does not come into contact with the ground
- In a place where it is exposed to the sun and the cut ends are exposed to the prevailing wind.
The stack may be under a permanent or temporarly cover, or covered with a waterproof, or semi-permeable sheet, with the cut ends exposed, however a good airflow is the most important factor. Scoring, partial or total removal of bark will assist drying.
Drying time (seasoning time) is dependent upon many parameters of both the material to be dried (shape and size of pieces, wood density, presence of bark) and the storage conditions (method of storage and stacking, air flow, temperature, humidity).
Drying other biomass
The same basic principles apply for other forms of biomass:
- Ensure good airflow making use of natural ventilation, any prevailing airflow and any convection
- Attempt to raise the material off the ground on a structure that allows airflow
- Attempt to expose the largest surface area as possible to airflow, spreading out the material if necessary
- Design and position the store where it minimises additional ingress of water, allows any released water to drain into a suitable receptacle or sink and where it receives the most natural exposure to the sun.
Active drying
If a moisture content below that which can be achieved in the time available by passive drying alone, is required, then active drying may be necessary. Active drying requires the input of energy from an external source to speed up the process and achieve lower ultimate moisture content.
The use of external energy input usually entails additional cost, and will increase the embedded energy.
For greatest efficiency a large surface area to volume ratio is required of the material to be dried, and good air flow over as much of the surface as possible. Air flow should be increased, making as much use as possible of prevailing winds and/or convection, combined with good ventilation, together with fans or blowers, usually with heating. Moving, turning or spreading the material to maximise the surface area exposed will assist drying.
Heating can be from available excess process heat, or a dedicated heating unit. By careful design and orientation it may be possible to incorporate a significant amount of solar heating.
Blending
If there is a supply of very dry material, such as kiln dried timber or offcuts from the furniture industry, it may be appropriate to blend this with material of a higher moisture content to reduce the average figure for the batch.
Although not technically drying this may be a valid technique to ensure that a batch of fuel from a range of sources meets specification.
