Wood Pellet Plants

To produce wood pellets in a highly automated process requires several processes in wood pellet plants. The first procedure is to reduce the raw material in size. The raw material may already be sawdust, in this case the sawdust can move straight on to drying. In cases where the raw material is wood chips or logs, size reduction will have to take place before the wood can be placed in the dryer. In the wood is already in chipped form it can be metered into a hammer mill, if not chipping will be required. In the hammer mill the hammers spin around 4000 rpm. As the hammer hits the wood chip it shatters into smaller pieces. A screen with holes of a set size then screens the particles, for wood pellet production a particle size of 5mm is generally required for quality wood pellets.

Wood Pellet Manufacturing Processes

Once the particles of a sufficient size, they are then ready to be dried down to the necessary moisture content. The two main types of dryers used are flash dryers and rotary drum dryers. To reduce the carbon footprint of the process ideally biomass should be used as the heat source for the dryer. Once the particles have been dried a fan is used to remove the particles from the dryer, and then a cyclone separator is used to separate the particles from the air stream.

For wood pellet production the pellet plants often have a steam conditioner. High temperature steam is used to condition the sawdust before it enters the pellet mill. The heat treatment of the sawdust starts the melting process of the natural lignin within the wood to bind the sawdust into pellet form. The pre-melting of the lignin before the sawdust enters the pellet mill, aids in pellet quality and also lowers pellet mill energy consumption.

Wood Pellet Mills and Wood Pellet Press

The two main pellet mill designs are the flat die pellet mill and ring die pellet mill. The flat die design is more common for small scale plants and the ring die design is more often used in large scale plants. The design of the pellet mill may involve either the roller shell rotating or the die rotating in both designs of pellet mill.

The basic principle of the pellet mill is the raw material enters and is compressed via a series of rollers through the die. The design of the holes in the die can influence pellet compression and productivity. The aim is to achieve the balance between producing pellets of a suitable quality, with the highest productivity possible while minimising energy consumption.

Wood Pellet Production and Pellet Fuel

There are different grades of wood pellets produced in pellet plants. The most common wood pellet grade produced is premium wood pellet fuel. This is produced mainly from softwood and hardwood residues from timber production. Premium pellets demand the highest prices however the timber producers are now aware of the value of their waste, and charge for the sawdust where originally they paid the pellet producers to remove the sawdust. Due to this fact pellet producers are looking for other wood resources the can use to produce wood pellet fuel. For example willow grown as short rotation coppice is being developed.

The Wood Pellet Production Guide © PelHeat Ltd - Wood Pellet Plants