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Raw Materials Requirement of Wood Pellet Mill

The popularity and reputation of wood pellets and other pellet fuels have been rising over the last twenty years. For the external part, it’s because of the limitation and high cost of traditional heating resources (such as coal, petroleum products and natural gas) as well as the environmental problems caused by their combustion. For the internal part, it’s resulted from the self advantages of wood pellets including high heat value, little pollution, low investment, wide application, valuable residuals, unlimited raw materials, etc. Therefore, in order to create a better living condition and save more energy resources, we need to know further information about the preliminary and indispensable part of wood pellets production, that is, raw materials choosing, handling and changing.

Introduction of Wood Pellets
As a kind of innovative green heating materials, wood pellets are widely used as the fuel in industrial manufacturing (biomass fuel plant, power station, wood processing factory, chemical plant, fertilizer factory, etc.), household heating and service industries (such as hotel, restaurant and bath.). They are large-density cylinders processed through chipping, crushing, drying, pelletizing, cooling, screening and packing. After the construction and form of raw materials have changed under the function of high temperature and pressure, end products are extruded out through designed dies, whether flat or ring.

Raw Materials for Wood Pellets Production
Wood pellets can be manufactured from various kinds of raw materials, from forestry residues (sawdust, shavings and little bark) to agricultural wastes (such as rice husk, corn straw, coconut shell, dry grass, etc.). Even the trees with diseases or killed by insects are put into application to produce pellets fuel. Although many kinds of categories including household garbage and waste rubber are suitable for producing wood pellets, only two kinds of them, agricultural wastes and wood residues, are frequently applied. The two main categories are listed as follows:

Agricultural wastes: Rice husk, corn core, corn stalk, peanut shell, sunflower seed shell, coconut shell, coffee bean pulp, straw, waste cigarette stem, leaf mustard, jute waste, bamboo, bagasse, tea leaf waste, wheat straw, palm kernel shell, bean stalk, dry grass, etc.

 bean stalks

bean stalk

 dry grass

dry grass

 palm kernel shell

palm kernel shell


All of the above raw materials can also be called harvest residues that can manufacture large amounts of good pellet fuels. Commonly, agricultural wastes and fruit shells are piled up and burnt directly by farmers, in which process such substances as particulates, NOx and CO are produced and released into atmosphere. This will definitely increase the burden of air purification and detoxication as well as deteriorate the existing serious environmental problems, such as green house effect, acid rain and ozone depletion. As we all know, bad air condition has great impact on human and plants. For the former, it may enter into human body through respiratory tract, digestive tract, skin, food or water, causing Chronic bronchitis, bronchial asthma, emphysema, lung cancer and other diseases. For the latter, it may harm the cells and tissues of plants, inhibiting their physiological function, development and growth.

Wood residues: The residues, such as sawdust, shavings and little bark, left behind in wood production factory, sawmill, paper manufacturing plant and construction industry; Dead trees killed by diseases or insects; Other forestry materials missed by wood-related factories before or after logging.

 sawdust

sawdust

 wood bark

wood bark

 rotten wood

rotten wood


There are several ways to distinct the categories of wood. According to species, wood is classified into coniferous (also called softwood) and broad-leaved (also called hardwood); To uses, it’s divided into stem, log and converted timber; To weight, it’s distributed into light (400kg per cubic meters), medium(500-800kg per cubic meters) and heavy (800kg per cubic meters). Here introduces the differences and comparison of hardwood and softwood which are widely used as raw materials to process wood pellets.
Mainly including birch, beech, cypress, taxus chinensis, hard maple, elm, etc., hard wood has longer combustion time compared to soft wood thanks to its higher density and lower water content. With more sufficient burning, soft wood, such as pine, redwood, cedar, fir and spruce, is generally gymnosperm with soft hand feeling and light weight. From this we can see both of the two raw materials has their own advantages.
In actual production process, the moisture of raw materials should be controlled within about 10-15%. Besides, high temperature and pressure change the structure and density of them during pelletizing, so the soft wood pellets and hard wood pellets with same weight also share the same combustion value.
In spite of the fact that many people think hardwood pellet is more environmental compared to the soft one because less ash is produced during its combustion, what really determines the ash content is cleanliness of wood pellets instead of that of raw materials. According to related statistics, some hardwood contains more ash instead. As a matter of fact, the quality of pellet mill can also influence the ash content of pellets fuel. Pelletizers play such a essential role that those with superior quality can manufacture high-class fuels whether the raw materials are soft wood or hard wood.

Comparison of ash content in different materials

Material
Ash content in % of weight
Softwood
0.3 - 0.7
Hardwood
0.5 - 2.5
Poplar and salix
1.4 - 4.5
Straw
4.0 - 6.0
Grass
2.5 - 10.0
Grain
3.0 - 4.5
Rice husks
12.0 - 18.0
Palm kernels
3.5 - 5.5


Requirements for Wood Pellet Raw Materials
What’s known universally is that raw material resources are the basic premise of products manufacturing. Unqualified materials may damage equipment or fail to produce the superb end products as desired. As for pellet mills, the water content, size and other attributes of raw materials will largely influence their producing efficiency and yield amount. Therefore, to achieve better reasonable operation, get more produced energy and obtain longer machine life, the following requirements must be paid high attention to.
Size requirement
Pellet mill has specified requirement for the size of what are fed in, so wood chipper and crusher are needed to cut raw materials into smaller logs and powder before further processing. Generally, the diameter of crushed materials should be less than 5mm. Concrete dimension depends on the expected size of produced pellets as well as the diameter of pelletizer die. Raw materials can be too big to discharge, or so small that pellets output and quality are affected.
Moisture requirement
Apart from size, pellet mill also has strict demand for the moisture of raw materials which may be so dry or so wet that pellets made from them are prone to breakage and looseness. Whether the raw materials belong to agricultural wastes or forestry sawdust, their water content should be controlled within 12%-18%. Although the needs each manufacturer can meet are different, that of the end products flowing to both domestic and foreign markets must reach below 13% which is the standard that ensures no deterioration and mildew happen.

wood pellet material moisture


Adhesive requirement
Sawdust itself is a kind of crude fiber materials with certain adhesiveness, so for most feedstock, no adhesive is needed while pelletizing. It’ll shape up naturally under the extruding force of pellets making machine. In case the materials you apply have no internal constraining force, extruders can’t make them mold or they’ll be loose even if can. Under this circumstance, raw materials should be fermented first before processing or adhesive should be added into pellet mills to make sure all procedures operate normally.
Mixing of different raw materials
Instead of only using one kind of raw materials, different categories can be mingled together. From narrow sense, new sawdust may be added to the existing one. From wide sense, the appended materials can also be agricultural stalks, fruit shells, peanut shells, straws and so on. The pellets made from wood shavings mixture are still fuel superior characteristics, while those produced from compound of sawdust and agricultural wastes are slightly inferior because quality of the latter is not so good as that of the former.
Materials with mildew
Sometimes, manufacturers may collect too many raw materials to use up before mildew and blackness appear, which will make the density and heat of compressed pellets inferior to required. Discarding of them is very wasteful, while using of them won’t reaching expected effects, so solutions to solve such phenomenon becomes one of customers’ biggest concerns. Here is an approach: Mix over 50% of fresh materials before delivering the degenerative ones into feed bin. The more mixed materials are, the higher quality finished pellets possess. Please decide the amount according to personal condition and requirements.

The five points listed above are main requirements for the raw materials of pellet mill, which together with applicable material categories constitute the yardstick for choosing and handling feedstock before and during the process of pelletizing. From all the information we can see pellets production is far more than a simple behavior of equipment purchasing, which, with no doubt, is the necessity of all followed procedures. No matter how wonderful a machine is, it can’t produce top-quality products with unqualified raw materials, so please attach more importance to the tiny but significant details of preliminary preparation.