Drying
Green
wood waste has a moisture content of about 50% measured on a wet basis.
The water in the fuel reduces the calorific value of the fuel and so it
is often preferable to dry the fuel prior to feeding into a boiler.
To calculate the affect of moisture content on the calorific value of your fuel use our biomass calorific value calculator.
Modern boilers (such as, fluidised bed boilers) are capable of burning fuel with high moisture contents. Boilers that
are not designed specifically for high moisture content fuels will
always burn more effectively on a dry consistent fuel source.
An
effective way of drying green wood in regions with a dry climate is to
store unchipped material in a heap or pile under cover over the summer.
In this way, moisture can drop naturally to around 30% within 6 months
after harvest.
The disadvantages of natural drying are
unpredictable weather conditions, space required, labour cost, loss of
mass due to degradation, and inconsistent final moisture content.
Natural drying is not usually an option in most parts of New Zealand.
The
only method of producing fuel with consistently low moisture content is
via a mechanical drier. Often, the heat required for drying fuel can be
produced in combination with heat for kiln drying. One major issue when
drying is the production of VOC’s (blue haze) which is illegal.
Moderate temperatures are the best way to handle this.
There are three main types of direct dryers:
Rotary drum dryers
In
a rotary dryer wood chips, bark, or wet sawdust, is mixed directly with
hot air inside the rotating insulated drum. Rotary dryers can accept a
fairly wide range of particles sizes and still achieve high rates of
dryness in a single pass. Multiple pass dryers can be used to reduce
moisture content form 60% to around 15% (wet basis).
Flash dryers
Wet
biomass particles are conveyed through a duct and mixed with a high
velocity hot air stream. Flash dryers are simple, relatively easy to
operate and control but have greater electricity consumption than
rotary dryers. Because the biomass is conveyed in the air stream it
needs to have a fairly consistent, small particle size. Typical fuel
for drying in a flash dryer would be wet sawdust. Its moisture content
can be dropped right down depending on the length of drum and residence
time.
Superheated steam dryers
In
a superheated steam dryer, superheated steam is mixed with wet Biomass.
The steam remains above saturation temperature and does not condense on
the surface of the cooler biomass. Moisture from the biomass material
is driven off creating more steam, but at a lower temperature. Excess
steam is removed and the remainder reheated and recycled back into the
dryer. This type of dryer is not common in New Zealand.
Consultants working in this area are:
For more information see IEA Bioenergy: Biomass Combustion and Co-firing.