The use of straw for the production of pellet

PREMISE

The use of straw for the production of pellets for biofuels is a development of the last two years. The most important experience in the EU countries is in Poland, while the government decides to give financial support to the use of biomasses – not from forest’s origin (it means practically the use of agricultural residues) – as fuels for the power stations using normally coal and dust of coals.

Poland is a country with centralized heating and power production in every city and this decision of government produced:

• From one side a research of new potential energy sources;
• From another side the possibility of new income’s opportunities for farmers.

The straw shows to have some important characteristics:

• Caloric power less than waste wood, but interesting: about 15.000 – 15.500 kcal/kg.
• Renewability every year
• Availability
• And last but not least a possibility to increase the income of the farmers

The straw types normally used in Poland as raw material for straw pellets production are:

• Wheat straw
• Rape straw
• Barley straw
• Hay
• Mixing of different straw
• Miscanthus isn’t a straw but is usable as straw.

The rice straw can also be used as raw materials for pellet production if there is the correct condition of harvest and product.

The straw is used as raw material only for pellets for industrial use while:

• The ash contents are high, about 6% (the ash can be recovered and used as fertilizer)
• The high content of chlorine normally connected with straw has a negative influence on the grids of the burners and reduces their duration.

Other important conditions necessary in order to utilize the straw to convert it into energy pellets are:

• The transportability of the straw from the field to the pellets factory
• The moisture of the straw

THE HARVEST OF STRAW

The harvest of the mentioned straw is done with specialized machines, named combine or harvester combine, which divide the straw from cereals, for example, wheat, and with the residues of wheat rape etc. produce bales or prisms as in the picture below

This bales or prisms have the advantages they can be stored and transported reducing transport and storage costs thanks to this first reduction of volume.
 
STRAW AS RAW MATERIALS FOR PELLETS

Transportable straw

The first step is to have transportable straw and it is possible with the use of combine machines for the harvest.

Straw in form to be stored

The second important step is the storage of the straw in order to maintain the correct moisture of straw.
The form of bales or prism is the most suitable to organize a storage of the straw.

Storage outside

The storage outside is done covering the straw bales or prims in order to avoid an increase of moisture due to rain and/or snow
 
The moisture of straw

The suggested moisture is about 14%, referred to wheat straw.
If the moisture is lower, it is possible to increase it by adding some liquid to the production process of pellets.
If the moisture is higher, there is the necessity to reduce it and there are two possibilities:

• Storing the straw under a roof and using the air of the environment to dry;
• Drying with an industrial process and the connected investment and energy costs.

The last preparation of straw for pelleting: the cutting

The normal pelleting process of straw starts from straw with an average length between 4-5 cm.
 The pieces of straw building the bales or the prisms have different length, normally over 20 – 25 cm.
 
The cutting of straw

First steps of cutting consists in the reduction of the length up to 4-5 cm average.

This step is realized with different machines and all derive from the agriculture straw cutter. The limit of the system is in the concept. The machine isn’t produced for a continuous industrial use, but also for agricultural, not continuous use.
A typical straw cutter for agricultural and farmers uses.

After this step starts the grinding realized with a hammer mill and the pelleting.

A second possibility is to have two grinding steps.

The first to reduce the length from average 20-25 cm, up to 4 – 5 cm. the second step mills and refines the straw for pelleting.

The air pollution

The straw cutting and grinding is a process producing a lot of dust.
Depending on the local requirement it is necessary to aspire the dust and to recover it into the process.