The higher the moisture content in wood fuel, the less effective it is when burned to create energy. Therefore, drying can be an important step in the wood fuel production process.
While wood pellets are dried as a part of their production process and do not require any further drying, wood residue must be dried to maximise its efficiency. Green wood waste usually has a moisture content of about 50-55%. While some modern boilers are capable of burning fuel with this level of moisture, most boilers will operate more effectively using a dryer fuel source.
To calculate the affect of moisture on the energy potential of your wood fuel, use our biomass calorific value calculator.
Natural drying can be an effective way of drying green wood in regions with a dry climate. The usual method used is to store wood in a pile under cover over the summer. In this way, moisture can drop naturally to around 30% within six months of harvest.
However, natural drying does have some disadvantages including the:
- Potential for unpredictable weather conditions
- Amount of space required
- Labour and inventory costs
- Loss of mass due to degradation
- Inconsistent final moisture content levels
Because of these factors, natural drying needs to be carefully managed.
Mechanical drying is the only method of producing fuel with consistently low moisture levels, however it does come at a higher cost than natural dying. Another major disadvantage of mechanical is the potential production of volatile organic compounds (VOCs – commonly know as ‘blue haze’) which is illegal. But blue haze can be avoided by drying using moderate temperatures.
There are several types of mechanical dryers available:
- Rotary drum dryers mix wood chips, bark or wet sawdust with hot air inside an insulated, rotating drum. They can accept a fairly wide range of particles sizes and reduce moisture content from 60% to around 15%.
- Flash dryers convey wet wood particles through a duct and mix them with a high velocity hot air stream. Flash dryers are relatively easy to operate and control and can result in very low moisture content levels. However, they require more electricity to run than rotary dryers and can only deal with fairly consistent, very small sized wood material (usually sawdust).Superheated steam dryers mix superheated steam with
- Superheated steam dryers mix superheated steam with wet wood material. The steam remains above saturation temperature and does not condense on the surface of the cooler wood material. The moisture from the wood material is driven off creating lower temperature steam. Excess steam is removed and the remainder reheated and recycled back into the dryer. This type of dryer is not common in New Zealand.