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Pellet Mill Report: A Complete Guide for Pellets Industry Beginners and Pellet Mill Buyers

By Jack Huang    May 23, 2014

 

How much do your know about pellet mill?

 

What is a pellet mill?

The definition

A pellet mill (also called pellet machine, pellet press) is a type of mill or machine which is used to make pellets from powdered materials. A pellet mill is usually consisted of pellet die and rollers. The rotating rollers or the rotating dies force the feedstock trough the die holes to form pellets. (See the picture of how the pellets are produced by a pellet mill).


The function of a pellet mill

Without any process, biomass from plants can be used to produce alternative fuel for fossil fuels. However the natural biomass with low densities, i.e. 80-150 kg/m3 for herbaceous and 150-200 kg/m3 for woody biomass, limits their application in energy production. Then the densification of these materials are needed to increase the bulk densities. Pelletizing is one of the methods to realize densification.

The function of a pellet mill is to bring the particles of the powdered materials closer so that the forces acting between the particles become stronger and stronger, which will increase the strength to the densiflied bulk materials, the pellets. The pellet mills are usually applied to produce all kinds of biomass wastes, such as, forest wastes, agricultural residues, and industries wastes into fuel pellets which are used by households or large industrial plants for heating, cooking, and power generation. Or you can use a pellet mill to make feed pellet for poultry.

The usage of wood pellets?

wood pellets

Like I said, majority of pellet mills in the market are designed for making fuel pellets, especially wood pellets. Wood pellets is a type of densified biomass fuel which can be used as an alternative fuel for fossil fuel. The wood pellets are condensed biomass cylinders usually with the diameters of 6mm or 8 mm and with the average length of no longer than 40mm, made from the by-products of forest industry, such as, wood chips, shavings, and sawdust. The condensed wood pellets are usually low in moisture content, high in heating values and combustion efficiency which make them widely used around the world.

In terms of pellet’s quality, for pellet producers, the pellets are asked to meet the quality standard in the target countries where they are sold or used. Given that different wood pellets standards are seen all across Europe and North America, a new standard called ENplus was introduced by European Pellet Council and is said to be a new benchmark substitute of all existing ones in the near future.

In Enplus standard, there are A1 and A2, respectively for residential and industrial use. Both grades have criteria stipulations on how a quality wood pellet will be, criterion covering length, ash and fine content, moisture, caloric value, etc. You can learn more from the flowing PDF files:

http://www.enplus-pellets.eu/wp-content/uploads/2012/01/ENplus_PellCert_flyer_Jan2012.pdf
http://www.pelletcouncil.eu/cms/wp-content/uploads/2011/02/ENplus-handbook-3.5.11.pdf

 

The history of pellet mill

Pellet mill have generally two types, one is called flat die pellet mill, the other is ring die pellet mill. I will talk this more later. The first pellet mill was developed in the early 1900’s , based on the plat flat dies. At the very beginning, the flat die pellet mills were used by farmer to pellet feeds. With the development of technology, the flat die pellet mills were developed to produce fuel pellets to combat the energy crises as an alternative fuel of the fossil fuels.

While the ring die pellet mill was firstly invented in the early 1930s in Texas. In the mid-1930s, the Texan workshop became the first company to introduce ring die concept to the expanding feed industry in the United States. It was in the 1970s, more and more ring die pellet mill manufactures were set up world widely. Nowadays, flat die pellet mills and ring die pellet mills with various capacities, sizes, systems and prices are provided by various pellet mill manufactures in the international market.
 

How does a pellet mill work?

 Generally, take biomass pellet mill for example, there are three stages involved in the raw material densification. In the first stage, the particles rearrange themselves to form a closely packed mass but without changing their properties too much and the energy is dissipated due to inter-particle and particle-to-wall friction. In the middle stages, the particles are forced against each other and undergo plastic and elastic deformation, which increases significantly the inner-particle contact. In the last stage, the density of the fuel pellet increasing significantly due to the high pressure casted by the pellet mill. The processing of the particles are presented in the following diagram.
pellet mill working principle
 

After going through, the three stages, the deformed and broken particles will not be able to change positions because of the decreased number of cavities and the conformity of the inter-particle. Well, it is also important to know that the densification process and the variables that govern its performance, such as the temperature, pressure, and equipment. In the following paragraphs, the different equipment are presented for your reference.
 

The classification of pellet mills

 The classification of the pellets mills can be different based on different the standards.

According to the mechanical working principles, the pellet mills can be classified into ring die pellet mills and the flat die pellet mill. The ring die pellet mills compress the feed stokes distributed over the inner surface of a rotating die ahead of each roller into trough the die holes to form pellets.

ring die pellet mill design
 

While the flat die pellet mills also can be further classified into two types, the roller-driving one which has a stationary die and the die-rotating type which has the stationary rollers. The feed stock are squeezed into the pellet die holes by the pressures caused by the interaction of the rollers and die. ( More information about the ring die and flat die pellet mills please see The Ring Die and Flat Die pellet mill Comparison).

rotating roller typerotating die type
   rotating roller type                                                                                    rotating die type
 

Based on the capacity of the pellet mills, there are small scaled pellet mills and large scaled pellet mills. The small scaled pellets mills (mostly flat die pellet mill) with the capacity of 50- 400 kg/h are usually preferred by small pellet producer and users such as households, offices, schools, etc. The large scale pellets mills are usually concerned with the pellet mill project. The large scale pellet mills (mostly ring die pellet mill) used for large pellet projects can produce at least 1- 2 tons pellet per hour per unit. In most cases, several ring die pellet mills are combined to reach higher capacity, even 20 t/h. When it comes to large pellet production, besides pellet mill, other supporting equipments are necessary, like hammer mill, dryer, cooling system, etc.

While in terms of the power sources, there are electrical pellet mills, diesel pellet mills, gasoline pellet mills and PTO pellet mills (the tractor pellet mills).


homemade pellet mills
 

When the raw materials are concerned, some pellet mills are specially designed for a given material, such as the wood pellet mill, feed pellet mill, grass pellet mills, bamboo pellet mill, even plastic pellet mill, etc. The different among these pellet machines are mainly lies compression ratio.

In recent years, a new type of mobile pellet mill is rolled out by some pellet mill manufacturers, which features in small foot print and easy moving.
 

The market status of pellet mill

 The increasing demand for wood pellets in the international market booms the pellet mill markets. The EU 2020 policy which targets for renewable energy source and greenhouse gas emissions reduction drives the wood pellet market to grow dramatically. It requires that in 2020, 20% of total energy consumption shall comes from renewable energy. Looking back at the recent years, only 10 million tones pellets are produced by about 650 pellet plants in Europe in 2009. But now the current global market volume of demand for biomass pellets is about 16 million tonnes annually and is going to triple to 46 million tons annually by 2020, which means a total market value of $8 billion (€5.75 billion). Most of the near-term growth is predicted in wood fuel pellet markets worldwide.

expected wood pellet demand
 

The urgent need for the wood pellets thrives the pellet production business. The increasing price of wood pellets makes people realize it is a chance of life time to invest in the pellet production business to make their own wood pellets. If the biomass recourse and the investment is abundant, the large scale pellet production is possible and the ring die pellet mills will be the right choice for you. And the pellet mill project design will be needed to make the whole pellet production system more scientific and efficient. When the biomass resources and investment is limited, the flat die pellet mills will be what you need to make your own pellets. Your idea matters. Please leave a message if you have any idea.