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Straw Pellets Milling Procedures: A Step-by-step Analysis

By Jack Huang    Dec 10, 2014

straw biomass

Straw biomass

Straw for Pellets fuel production

Straws is a general term for residues harvesting form grain plants, such as, rice, wheat, and corn, etc. Straw is the most important agricultural residues in Europe as quantity is concerned. According to estimation, around 23 million tones of dry straw biomass will be available each year in the 15 states. The diagram in the blow will present you the related data.

Estimated straw potential in Europe
A consertive estimation of straw potential resources in Europe

With the abundant straw available, large scale straw pellets production are possible. And now straw pellets technology is widely utilized and straw pellet mill machines are available globally which makes straw pellets are popular for heat and power production.

Main characteristics of straw

Generally, straw consists of 80-90% cell and 10-20% lumen, the inside substance, consisted of 5-10% silica and 5-15% extractives, most of which are water soluble. In the terms of molecular aspect, straw cell walls is just like woody cell walls, consisting of cellulose, hemi-celluloses, and lignin. Cellulose is aggregated in fibril which are surrounded by molecules. Lignin is the “glue” which links individual cells to form plant tissues and fibrils to form the cell wall. The straw is differentiated from the wood in terms of content of celluloses, hemi-celluloses and lignin. Straw has lower portion of cellulose and lignin but higher portion of hemi-celluloses.

Comparison of tissue composition for straw,softwoods and hardwoods
Comparison of tissue composition for straw, softwoods and hardwoods

Due to the woody like composition, straw are possible to be converted into straw pellets. However, according to the special features of straw, the pelletizing procedures of straws can be summarized as follow:

Raw materials preparation:Straws usually has a high potential of dust, which is affected by the way of harvesting. Thus, straw needs to be screened from contaminants, such as, dust that may influence straw pellet quality and other alien materials, such as, stones, metals, and those that may damage the pellet mill in the following procedures.

Grinding: Straw biomass is a tube-like or pipe-like material with a diameter of 4 to 8 mm, and wall thickness around 0.3-0.6mm. With the tube/pipe-like structure, straw can be easily processed and get the desire particle size. Generally, the grinding of straws including cutting and chopping of the straws.

Drying: Usually, straw delivered in form of air-dried bale, with the moisture around 15%, which is the proper for straw pellet production. Therefore the drying of straw pellets is often by-passed. Even if drying is needed for straw pellet production, straw requires far less drying comparing with sawdust.

test-the water content of the straw poweder by a water meter
Test the water content of straw powder to get a better moisture content for straw pellet produciton

Conditioning: Straw requires special conditioning to achieve desirable hardness. Straw conditioning includes the application of steam(and /or water) and the addition of binding agents or additives. Conditioning is required to achieve the necessary temperature and moisture to get the malleability and to melt the lignin to act as binding agent. Binding agents are needed to strengthen the hardness of the pellets and also to decrease abrasion during pelletizing.

Pelletizing: During pelletizing, it is necessary to consider the factors such as, materials moisture content, density, particle size, fiber strength, and the natural binder for they can influence the pellet quality.

straw pellet mill for high quality straw pellet produciton
Straw pellet mill for high quality straw pellet production

The common problems during pelletizing of straw pellets are pellet die blocking and breakage, overheating, high energy costs or bad pellet quality and high maintain of the pellet mill. To avoid of the mentioned problems, high quality pellet mill is needed. Generally speaking, straw requires a thicker pellet die comparing with that of sawdust to increase the compression length( the friction forces), to obtain a more favorable product quality.

Cooling and screening: Newly extruded straw pellets are hot and soft which needs to be cooled down to get a desirable hardness while at the same time fine particles will be recycled.

typical biomass pellets properties from different sources
Typical biomass pellets properities from different sources.

Straw pellets

After pelletizing, two effects are achieved: the decreased moisture content and increased bulk density. The moisture content of straw pellets are decreased from 15-20% to less than 10% while the bulk density is increased from 130 to 600 kg/m³.

General characteristics of straw pellets, wood pellets and their respective materials
General characteristics of straw pellets and wood pellets and their respective materials.
comparison of straw and straw pellets
Straw and straw pellets

Through the above comparison, it is obvious that straw pellets have a low moisture content, higher calorific value and lower ash content. After pelletizing, the heating value of straw pellet can be increased to 18-19 MK/kg while the ash content is decreased to less than 3%, which make straw pellets good for burning.

Straw pellets application

Straw pellets are widely used in daily life. Generally, straw pellets can be used as animal bedding, feed for animals, and fuel for heating for home and industry use.

1) Straw pellets as fuel

Straw pellets as fuels are usually burning at pellet boilers, pellet stoves and other pellet appliance at household. While large scale straw pellets usually co-fires with traditional fuels, such as coals, oil and others, at large scale boilers or power plant or factory, etc.

diagram of straw pellets for industrial use
Diagram of straw pellets for indsutrial use

2) As animal feed

Straw pellets can be good pellet fodder for animals. Comparing with straw, the straw pellets fodders has the advantages of cleanness, balance nutrition, easy to store and longer quality guarantee period.

straw pelletsf usedf as fodder
Straw pellets used as food

3) As animal bedding

Beside being used as fuel and animal fodders, straw pellets can also be used as animal bedding, such as horse bedding, cat litter, etc. Straw pellets used as animal bedding are popular for straw pellets are , natural products without additives and of high hygienic, they are the best products for animal bedding.

straw pellets used as animal bedding
Straw pellets used as animal bedding