Home | Pellet Mill Process

• Raw Material
• Size reduction
• Drum Chipper
• Hammer mill
• Bale busters
• Rotary drum dryers
• Flash dryers
• Fluid bed
• Bed dryer
• Paddle dryer
• Furnaces:
• Hammer mill break down - rock separation:
• Double Die Pelletizer

RAW Material:

Since the demand to reduce green house gas emissions is the focus of everybody working in the energy sector many materials are considered for the production of heat and power.
There are variations of different materials as Shape, Moisture content, heat content, ash content, fume emissions….. to consider.
The cleanest and most common material used is wood fiber. There are farm crops being used for  fuel pellets such as Miscantus, straw hay and such. In Scotland farmers grow a special developed willow crop for production of pellet fuel .
http://www.ecn.nl/phyllis

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Size reduction:

As material is delivered in many forms  and shapes the proper machinery has to be selected considering , type of material, capital cost, the following process and energy consumption.

To perform the primary reduction of material we offer:

For solid pieces of wood:
In order for efficient drying the material needs to be reduced to ¼ to 3/8 “.
The most efficient way to achieve this material size is with a drum chipper and a feed system. Conventional disc chippers have difficulties producing a small consistent chip.

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Drum chipper :

Horizontal feed to reduce round logs and solid waste
Drop feed for reduction of blocks, Lilly pads and such
Drop feed with multi knife head for chip reduction
Combination design for log and trim blocks
Chip size ¼” and up
The machine is of heavy duty construction and capable to deal with up to 600 hp
The machine frame is line bored with double spherical roller bearings

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Hammer mill:

Typically for reduction of material in already broken down state as pulp mill chips, hog fuel ground up trim blocks
Hammer mills typically will perform a material reduction of 1:3. hammer mills making particles less than 8 mm need to have air flow assisting the material flow through the holes in the screen.

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Bale busters:

Agricultural product is typically dried in the field and baled.
At the pellet plant these bales are broken down by a bale buster cutting the hay into small pieces of approx. 2” long. This material cn than be conveyed to be put through a secondary knife hog.

Following the primary size reduction and with materials with elevated moisture content one or the other drying technology will reduce moisture content to the proper level.

In most drying processes heat is used to evaporate the excess water trapped in the material.
Various technologies will lend themselves for this process.

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Rotary drum dryers:

A large rotating steel drum with multiple passes, dust collection and fire depressiondirect fired

• high temperature
• short residence time
• Very reliable

We strongly recommend the use of the Earth Care Products Inc Dryers. Visit there website Earth Care Products Inc.

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Flash dryers:

Tube style construction with large and small pipes
Changing velocity - air pressure in the process

• high temperatures
• medium residence time
• mostly direct fired
• medium capital investment
• medium fire index
• difficult to control moisture content
• Lower production than drum dryers

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Fluid bed dryer:

• Vibrating conveyor trough      
• Heated air pressurized from the bottom
• Medium residence time
• Indirect heat
• Medium capital
• Can utilize waste heat form power or steam generation

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Paddle Dryer:

• Conveyor type dryer
• Bottom air for drying chip type material
• Top air for drying dust like material
• Large area for drying
• Lower temperature
• Indirect heat
• Long residence time
• Can utilize waste heat from power or steam generation

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Furnaces:

Wet fuel:

• Furnaces capable of burning up to 60% moisture in the material
• used mainly for burning bark and hog fuel in larger boilers
• the material is pushed into the furnace on a moving crate system.
• The material gets heated and moisture evaporates prior to combustion
• Emission control, caloric efficiency and capital cost are the biggest challenges.

Dry fuel:

• Dry fuel burners use Wood chips or pellets at a moisture content of 15% or less
• Agricultural pellets are often considered for this type of furnaces.
• the fuel is mechanically fed into the combustion chamber
• most furnaces are self igniting
• it can be equipped with a water boiler or air heat exchanger

Air suspended combustion:

These furnaces are equipped with an oil or gas burner to start the furnace
Once the chamber is heated up the fuel is switched to sawdust. This dust is blown into the furnace chamber creating air suspended combustion.
 

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Hammer mill break down - rock separation:

http://www.hammermills.com

The hammer mill process:

After drying the material the final break down takes place. We aim for a particle size of 3 mm for standard fuel pellets.
If pellets are reground and used in and air suspended combustion boiler often a particle size of 2 mm is required.
In order to transport the fines through the screens the assistance of air flow through the hammer mill is required.
The dried chips are conveyed to a rotary feeder ensuring a constant flow of material.
Gravity will transport the material into a vertical air stream.
Two dampers will allow controlling the velocity of the vertical air stream.
Wood chips, sawdust, rocks, metal all have different minimum velocity for vertical transport. The difference in specific weight will allow separating heavy particles from light particles.
A velocity meter and automatic controlled dampers will ensure consistent velocity in the separation chamber.

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Double Die System Pelletizer:

Double die technology history

First patent filed in 1927, 1959, 1967,……………..

Up till 2002 wood pellets were not a common fuel and wood residue was not considered a valuable commodity.  With the efforts of nations in the last few years to reduce CO2 emissions the wood pellet is becoming a new, if not the green energy world fuel.
The efforts put into making pellets guarantee a product that is consistent in heat value, consistent in ash content, consistent in moisture content, transportable similar to a liquid such as oil  and historically  around 30% cheaper than heating oil.

The principle:

This technology consists of two rotating dies that interlock. Both dies are manufactured concurrently and are offset. Each pellet chamber lines up with a plunger from the opposite die.  This arrangement reduces unproductive compression areas between the holes in a die.
The double die technology uses all areas under pressure to manufacture pellets. This configuration results in a power consumption of approx. 80 kWh per ton of pellets (conventional machines 100 – 120 kWh per ton).

The holes and the plunger are configured in a way that the material is compressed in a funnel like pre-compression chamber. In this chamber the material reaches temperatures of up to 150° C ( 300°F) prior to entering the pellet chamber. This compression results in heat and evaporation of moisture in the material. In this process the pellet material is squeezed and the lignum is softened and moisture is released.  The die makes one more revolution and moisture continues to evaporate from the pre compressed material. A new layer of material is applied and a set of pre compression rollers remove some of the air. As the material is moving through the pellet chamber, moisture continues to separate from the actual wood fibre and evaporates as soon as it reaches the relief area in the pellet chamber. The pellet is still attached and once it reaches the preset length it is cut off while moisture continues to evaporate.  At this point the pellets have reached a temperature of approx, 50° C   (120°F).  Once the pellet is produced it is picked up by the vacuum system and moved to the screening process.  The tumbling and screening process removes all the fines and sharp edges from the pellets.  The screened fines are re introduced  into the pelletizing process for reuse.  All these steps result in a pellet moisture content of approximately 7–8 % and a specific pellet density up to 90 lbs/ ft3

The illustration below shows the pre-compression zone in the PSi Double Die System where the softening of the lignum and evaporation of excess moisture occurs.  This process conditions the material for optimum pelletizing. The material was dried to 18% and an additional 3% was lost in the hammer mill process immediately prior to the pelletizer.

Double Die Technology

A further advantage of the double die system is the installation of satellite rollers. These rollers are driven and pre-compress the layer of loose material gradually increasing the density removing air from the material carpet.