Granulation is the process of processing materials into pellets, which, as a rule, have a cylindrical shape with a diameter of 6 to 12 mm and a length approximately two times the diameter.
What types of biomass can be used for the production of pellets: wood waste, sawdust and shavings, sunflower husks, macuh, croutons, alfalfa, sainfoin, cake, straw, bran and others are especially suitable.
Granulation is performed in order to obtain a fuel pellet, which has the following advantages:
- environmentally friendly fuel. The ash content is not more than 3%, as a result of which, in the process of burning granules, CO2 is released into the atmosphere in the same amount as was absorbed by the plant during its growth.
- less degree of spontaneous combustion.
- high and constant bulk density, which makes it possible to effectively use vehicles and storage facilities.
- Low humidity, which together with high density provide a high calorific value. When one ton of pellets burns, approximately 5000 kW/ h of heat is released, which is one and a half times more than for firewood.
Granulation is also used to produce granules from mixed fodders. Advantages of using granulated feed are as follows:
- canning of feed.
- impossibility of separation of the combined mixture.
- reduction of the surface exposed to microbiological organisms.
- The ability to control the accuracy of the proportions of ingredients.
- In the process of granulation, thermal treatment is used, which kills harmful microbes, including salmonella.
- save of carotene.
Feature of obtaining fuel pellets
The moisture content of the feedstock is optimum for pressing from 6 to 12%. At higher humidity, the density of the granule decreases, and as a result, its strength decreases.
The binding agent in the wood granule is lignin, an amorphous polymer that is released by pressure and temperature. It is he who performs the role of “glue” in the formation of granules.
The physical characteristics of raw materials directly depend on the equipment of the line, the selection of additional equipment and matrix. If the raw material has a high humidity, it is necessary to pre-dry it; if the raw material is a large fraction, it requires preliminary grinding.
Raw materials are crushed in a crusher, then it is loaded with a cyclone and a sluice into the hopper, from where it is fed evenly into the mixer, where it is moistened with water or steam to the humidity necessary for granulation and mixed intensively with a stirrer (moisture contributes to the formation of strong granules).
From the mixer the moistened raw material through the separator of ferromagnetic impurities is discharged into the press. In the compression chamber, the raw material is tightened between the rotating matrix and the pressing rollers and forced into the radial holes of the matrix, where the formation of granules under the action of high pressure.
The granules ejected from the holes encounter a fixed knife and break off. By turning the knife on the axis, it is possible to adjust the length of the breakable granules. The broken granules fall down and through the sleeve of the casing are removed from the press.
Blades delivering materials under the rolls are made: one triangular (it only feeds half the product layer under the roll), and the second one is rectangular (it feeds the rest of the product under the second roll), so the arrangement of the blades and rolls must be preserved when developing the press.
The granules coming out of the press have a high temperature and are unstable, so they are transported to the cooling column. Here, through the layer of granules, the air is sucked in by the fan, which cools the granules and simultaneously sucks a part of the ungranulated raw material into the cyclone. During cooling, the moisture content of the granules decreases due to the evaporation of moisture, and physical and chemical changes occur in the granules. As a result, they acquire the necessary hardness, humidity and temperature.
From the cooling column, as it is being filled, the pellets are sent for sorting, where the separation of the granules from granules occurs. Granules are discharged through the discharge neck, and the crumb is sucked into the cyclone and then sent along with the raw material for repeated pressing.