Moisture control is one of the major challenges
for pellet plant operators. PSC offers a solution with an
online sensor that provides continuous moisture measurement
through out the process. Being proactive with moisture measurement
offers cost savings in the manufacturing process and eliminates
the chance of packaging wet or low quality product. It is
Process Sensors’s goal to help the mill make a high
quality pellet by providing state-of-the-art moisture measurement
with NIR technology.
Waste sawdust and wood chips are delivered to the pellet
plant by truck or rail car then stacked outside in large mounds.
A front loader is used to take the sawdust from the different
piles and load the feed system for the plant. Wet and dry
sawdust is selected by the tractor operator in an attempt
to keep a moisture balanced load heading into the dryer system.
This green wood generally has a moisture content of 20 to
50%. If the sawdust is stored outside the moisture can be
higher or lower depending on the weather. It can be a challenge
for the loader operator to keep an even mix of wet and dry
sawdust going into the feeder system.
A Process Sensors Corp. moisture meter is very helpful to
the loader operator as a sensor is mounted on the screen infeed
conveyor and a large display is mounted outside for the operator’s
The first step in the process is to screen the sawdust and
wood chip mixture to separate the fine chips from the coarse
chips. The coarse chips are ground to reduce their size for
quick and even drying. They are then remixed for temporary
After a second screening to separate the fines from the coarse,
the coarse chips are reduced in size with the hammer mill.
The hammer mill output and fines are again mixed and stored
in bins by moisture content.
The sawdust is then conveyed into a large drum dryer where
the moisture is reduced to less than 10%. The sawdust is then
conveyed into a cyclone dryer where high speed swirling air
removes additional water vapors and cools the product down.
A PSC moisture meter is typically mounted on the dryer outfeed
to ensure the moisture is approximately 10%. If the moisture
is higher than 10% the sawdust will be directed to a moist
bin. Typically, moist bins and dry bins will be blended for
the ideal moisture level going into the pellet press. Use
of PSC moisture meters on each of the bin outputs can offer
additional information to aid in the blending process.
One of the most important measurements is of the final mixture
after blending and prior to the press. A PSC moisture meter
is used to continuously measure this moisture level going
into the pellet mills. The analog output signals from the
sensor can be used to forward feed a PLC. The PLC can be programmed
to take control of the power level and speed of the press
thus creating a more consistent product. This mixture is now
ready to be pressed into pellets or sent back to the dryer
as fuel for the burner.
The processed sawdust is conveyed into the pellet press where
the pellets are formed under very high pressure. This high
pressure drives off additional water and causes the natural
wood lignites to bind the sawdust into shiny hard pellets.
After the pressing, the finished pellets go through one final
screening to recover any leftover sawdust for recycling. The
cooled and screened pellets are stored in silos while waiting
to be bagged and shipped. The standard package is a 40 lb.
moisture protected bag for household heating. A major market
is Europe, where many homes and power plants use wood pellets
as their energy source.