complete-guide-on-wood-pellet-production.pdf

http://www.biofuelmachines.com/


PREFACE As wood pellets has became an affordable and optimal choice

as a substitute of fossil fuels. An increasing market demand

and correspondent knowledge information requirement are

seen in recent year, this guide is just your right resource

providing everything you should know about pellets and how

to make pellet.

! Please Note that: this guide is just for pellet making starters, it tells

everything you need to know basically.

For investors and people who want to build a

pellet factory in a industrial scale, you have to

read the complete guide to large scale production

for a deep understanding of total process and all

equipments requirement in detail.


Want to Build Your Own Wood Pellet Factory? Buy Complete Guide to Large scale Pellets Production Here Now!

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CONTENTS

1. Introduction of pellet........ 01

1.1 History and background..... 01

1.2 Four advantages from wood and biomass pellets lead to the rise in popularity

of wood pellet's market ....02

a. Price...02

b. Wide material source.....02

c. Environment friendly..02

d. Convenience......03

1.3 Industry standards....04

1.3.1 Quality pellets.....06

1.3.2 Mechanical durability.06

1.3.3 Moisture content..06

1.3.4 Quality pellet test....07

2. Introduction of pellet machine......08

2.1 Classifications...08



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2.1.1 Flat die pellet mill.....08

2.1.2 Round die pellet mill...11

2.2 Important equipments....12

2.2.1 Gearing.....12

2.2.2 Die...13

2.2.3 Die Metals....14

2.2.4 Die Temperature...16

2.2.5 Roller height and carpet...16

2.2.6 1mm gap....16

2.2.7 >1mm gap.....17

2.2.8



3

3.2.2 Moisture.....19

3.2.3 Percentage.....20

3.2.4 Lignin....20

3.2.5 Additional binders...21

3.2.6 Pressure changes.....21

3.2.7 Material binder issues....22

3.2.8 Solutions.....23

3.2.9 Power....24

3.2.10 Location..24

3.2.11 Heavy lifting equipment.24

3.2.12 Changes in other aspects...24

3.2.13 Changes in raw material particle size...25

3.2.14 Changes in raw material moisture content, pellet quality and production

rates...25

3.2.15 Changes in raw material composition and the Inclusion of binders and

lubricants .....25

3.2.16 Changes in raw material pellet mill feed rate........26

3.2.17 Changes in raw material conditioning and steam....26

3.2.18 Changes in pellet mill operating temperatures......26



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3.2.19 Changes in pellet mill die template rotation speed...27

3.2.20 Changes in pellet mill die design and metal used...27

4. Reduce the size of raw material into appropriate

particles ...........28

4.1 Chippers / Shredders......28

4.2 Hammer mill.....29

4.3 Size reduction conclusions..30

5. Dry the size-reduced material to reach required

moisture content ...........31

5.1 Rotary / Drum dryers.........31

5.2 Pipe dryers.....32

5.3 Heat source....32

5.4 Productivity increase....32



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5.5 Drying conclusions....32

6. Mix the dried material with the binder, lubricants or

other needed material ......33

7. Conditioning process to the mixed material ......35

8. Pellet mill characteristics.....38

9. Cooling.....44

10. Sieving...46

11. Pack and store the screened pellets.....48

11.1 Bags.....48

11.2 Sacks....49

11.3 Pellet tanker.......49



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11.4 Moisture protection......49

12. Common transportation machineries in pellet

plant ......50

13. Equipment and pellet mill maintenance...52

13.1 Maintenance.....52

13.2 Energy......52

13.3 Production end.......52

13.4 Material...53

13.5 Die type and metal.....53



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Introduction of pellet

1.1 History and background Pellets have been produced for over a century, by using heat and pressure small cylindrical

pellets can be produced from a variety of materials for different purposes. In the 1970s some

companies who used pellet mills to produce animal feed, started to look into producing

wood pellets as a fuel source. However

because of the cheap fossil fuels that were

available, the wood pellet market struggled

to grow. In the 21st century wood pellets

and fuel pellets in general are seeing rapid

growth, even resulting in supply problems.

Due to high oil and gas prices, and concerns

over climate change, fuel pellets are a clean, cheap heating fuel than can help to reduce global

warming. Over the last decade there have been two major factors, which have been driving

the growth of the pellet fuel market. The first is consistent rise in the cost of fossil fuels and

price instability, and the second is the increased attention of the effects of using fossil fuels

such as oil and gas on the environment. Other factors, which support the case for pellets is

they are a fuel that can be produced locally, from local wood and biomass materials. Pellet

production and distribution can produce an affordable fuel, creating local jobs while keeping

the fuels carbon footprint to a minimum. The advantages of pellets over burning logs or

briquettes are as follows.



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1 . 2 Four advantages from wood and biomass pellets lead the rising in popularity of wood pellet's market

Wood and biomass pellet fuel, as an alternative for fossil fuels, has fierce competitive and

stable price than kerosene and natural gas in many countries. And supplied by green tax

policy, wood pellet fuel is encouraged to use as a countermeasure against global warming,

energy security and rise of oil price, and it has proved to have a good effect.

As to wood and biomass pellet fuel, the raw material source is various, such as wood waste

(residual sawdust, wood shavings, wood peelings, etc.), yard debris (grass, leaves, tree sticks,

forsythia, wisteria, bushes, etc.), farm waste (corn cobs, corn stalks, straw from plants, etc.)

and other residues biomass waste. We can recycle energy from the above materials.

As a traditional heating fuel, fossil fuels are more expensive and easy to cause environmental

pollution, trees(fire logs) grow slowly and the moisture is hard to control, both above

mentioned are not good for stoves & boilers, besides the source is so limited.

Carbon-neutral is the green image and advantage of wood and biomass pellet fuel. Burn

pellet fuel only liberates CO2 which is stored during the lifetime of plant, is harmless to the

environment. But burning fossil fuels will liberate extra CO2 into atmosphere which stored

million years ago, accelerates the global warming.

a. Price

b. Wide Material

c. Environment Friendly



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Pellets are produced with uniform moisture content, shape, size, and density, which match

the needs from the automated combustion systems of the stove and boilers, and also take less

space in storage than other biomass fuels because they have a higher energy concentration.

Based on the information above, the primary features of pellets present as follows:

Density of at least 40lbs/cubic foot

Flows like a liquid, ideal for automatic systems

Can be used in Stoves and Boilers

Can be used in small and large scale applications

Easy to handle, store and transport

Improved combustion characteristics over raw material

This guide will detail the process of pellet production and the equipment required. Some of

the equipment described is essential, i.e. the pellet mill. However for other parts of the

process there are various selections of equipment to do the same job.

d.Convenience



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1.3 Industry Standards

The fuel pellet market now has several industry standards in the US and Europe, that pellet

producers need to comply with. This does depend on the size of production, and where and

if the pellets are sold on the open market. Before purchasing equipment it is recommend

researching any industry standards that may apply.

Below lines show terms of interpretation in pellet manufacturing industrial from Pellet Fuels

Institute Standard Specification for Residential/Commercial Densified Fuel October 25, 2010

bulk density the fuel mass per cubic foot of the fuel sample

diameter the average diameter of the fuel pellets in the fuel sample.

Pellet Durability Index (PDI) a standardized parameter for specifying the ability of the

fuel pellets to resist degradation caused by shipping and handling.

fines the percentage of fuel material in the fuel sample passing through a 1/8 inch

screen.

inorganic ash the percent inorganic material in the fuel sample.

length the weight percent of pellets exceeding 1.5 inches in length in the fuel

sample.

Moisture the moisture content of the as-received fuel sample.

heating value The higher heating value of the fuel sample.



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LE 1 PFI Fuel Grade Requirements

Residential/Commercial Densified Fuel Standards See Notes 1&2

Fuel Property PFI Premium Pfi Standard PFI Utility

Normative Information-Mandatory

Bulk Density, lb./cubic foot 40.0-46.0 38.0-46.0 38.0-46.0

Diameter,inches 0.230-0.285 0.230-0.285 0.230-0.285

Diameter,mm 5.84-7.25 5.84-7.25 5.84-7.25

Pellet Durability Index 96.5 95.0 95.0

Fines,%(at the mill gate) 0.50 1.0 1.0

Inorganic Ash,% 1.0 2.0 6.0

Length,%greater than 1.50 inches 1.0 1.0 1.0

Moisture,% 8.0 10.0 10.0

Chloride,ppm 300 300 300

Informative Only-Not Mandatory

Ash Fusion NA NA NA

Heating Value NA NA NA

a. The following applies to all limits in this table: For purposes of determining the fuel grade, all properties must fall at or within the specified limits listed for a particular grade. Observed

or calculated values obtained from analysis shall be rounded to the nearest unit in the last

right- hand place of the figures used in expressing the limit in accordance with ASTM E 29

Standard Practice for Using Significant Digits in Test Data to Determine Conformance with

Specifications.

b. It is the intent of these fuel grade requirements that failure to meet any fuel property requirement of a given grade does not automatically place a fuel in the next lower grade

unless it meets all requirements of the lower grade.

Table 1 Notes:



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1.3.1 Quality Pellets When we describe a quality pellet, this is a term that describes a pellet made from any

biomass material, not just wood. The reason Im stating this is because in the premium wood

pellet market a quality pellet refers to a pellet with very low ash, for example 0.3%. This book

describes all types of biomass pellet production; some of the pellets produced will have

higher ash content. We define a quality pellet through mechanical durability and moisture

content.

1.3.2 Mechanical Durability Mechanical durability simply refers to how dense the pellet is, and how well it is formed.

Pellets that are denser are of course stronger, the advantage is the pellets withstand

transportation better, and work more efficiently in the pellet burner.

When a quality pellet has exited the pellet mill, it should have a smooth surface, with little or

no cracks. If the pellet is cracking and expanding it is because there is too much moisture

within the pellet, or poor compression within the pellet mill. Once a quality pellet has cooled,

it should be like a coloring crayon. The surface of the pellet should be smooth, and have a

surface shine.

Wood pellets tend to shine more than others; the most important thing is the pellets smooth

compact state. Try tapping the pellet against a hard surface, to see if the pellet stays intact, or

if they crumble or easily crush and separate. The length of the pellet is not really that

important. However if pellets are too long (above 1 inch) they can cause damage to the auger

in the pellet burner.

1.3.3 Moisture Content The less moisture within a pellet, the more energy the pellet burner can use. However a

certain percentage of moisture is required in the pelleting process, so the target is to keep

moisture as low as possible while still creating quality pellets. Targets should be for moisture

content in the finished pellets below 10%. Pellets with more than 10% will burn, but at the

cost of efficiency.



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1.3.4 Quality Pellet Test As stated quality pellets should have moisture content below 10% and be mechanically

strong with a good density. The simplest way to test pellet quality is to place a pellet in a

glass of water, if the pellet sinks to the bottom the pellet has a high density, and was formed

under sufficient pressure. However if the pellet floats it will be a poorer quality pellet with a

lower density, lower mechanical durability and more likely to crumble and produce fines.

The second test is to take a vessel, which can hold at least 1 liter of water and weight it. Fill

the container to the top with pellets and weigh again, now fill the container with water and

weigh. Deduct the weight of the container from both measurements, and then divide the

weight of the pellets by the weight water. For quality pellets the results should be between

0.6 and 07.kg/litre, the figure may also be referred to as the pellets specific gravity. Specific

gravity is a crucial indicator that the pellets were produced under the correct pressure. Poor

quality pellets, for example with a specific gravity under 0.6 will break/crumble easily, and

produces excessive fines.



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Introduction of pellet machine

2.1 Classifications There are two types of pellet mills, one is a flat die pellet mill and the other is a round die pellet

mill. The flat die pellet mill came first and the round die was invented later. Generally flat die

pellet mills are used for small to medium scale pellet production and round die pellet mills

are used for medium to large-scale pellet production.

2.1.1 Flat Die Pellet Mills The Flat die pellet mill works on the principle of material falling from above on to the rollers,

which are rotating over the pellet mill die.



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The material is then compressed between the roller and die surface through the die holes.

Once the pellets emerge from the die a knife cuts the pellets off at a set length. Worm and

wheel drive is used for some flat die pellet mills; others are driven via bevel gears.

In some flat die pellet mills the die is stationary and the rollers are driven. In other flat die

pellet mills the die is driven and the rollers rotate as material passes between the roller and

the die.

Advantages of Flat Die Pellet Mills: Flat die pellet mills are generally easier to clean than

round die pellet mills. Quick access to the pellet mill chamber means faster die and roller

changes, which means more time in production. The compact design of the flat die pellet mill

means small, lightweight models are available, making them more suitable for small-scale

production. Another key advantage of the flat die pellet mill is visibility. If a material is



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producing poor quality pellets or no pellets at all, viewing the material during the pellet

process can give the best information on the reason why and how to correct it. With many

flat die pellet mills it is possible to see into the pellet mill chamber during the pellet process,

others have quick access doors to see into the chamber. Finally flat die pellet mills are

regarded as been more robust for pelleting problematic feedstock. Therefore flat die pellet

mills have a wider material tolerance than round die pellet mills.

Disadvantages of Flat Die Pellet Mills: Due to the principles of the flat die pellet mill, this

can result in uneven roller and die wearing. As the rollers rotate across the die surface the

inner and outer edges of the roller are covering different distances. The outside edge covers a

greater distance than the inner edge, which means the rollers slip, this slipping action can

cause increased wear. However this is not always the case, and some flat die pellet mills have

tapered rollers to correct the problem.



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2.1.2 Round Die Pellet Mills The round die pellet mill comprises of a vertical ring die with rollers on the inside, applying

pressure against the ring die.

Material is fed from a surge bin through a variable speed conditioner above the pellet mill;

the conditioned material is then fed into the door of the pellet mill. A screw auger then feeds

material into the center of the pellet mill chamber. Inside the chamber the rollers are

stationary and the die is driven, similar to a washing machine. Once in the chamber the

material is taken up by the rotating die and then compressed by the rollers.

Advantages of Round Die Pellet Mills: Firstly round die pellet mills do not suffer uneven

roller and die wear as the inner and outer edge of the roller covers the same distance. For this



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reason round die pellet mills are preferred for large scale production as the costs of roller and

die consumables are perceived to be lower. Round die pellet mills are also preferred for

large-scale production as they are considered more energy efficient. Roller slip in flat die

pellet mills causes extra friction during the pelleting process, which uses more energy during

production. However this extra friction is not totally a bad thing, as more friction results in

more heat, which can produce better pellets. All round die pellet mills come complete with a

conditioner and variable speed feeding.

Disadvantages of Round Die Pellet Mills: The first obvious disadvantage of the round die

pellet mill is its size and weight. This may not be a problem for large-scale production,

however for small- scale production this can be a major problem. Also changing rollers and

dies in a round die pellet mill is a far more labor intensive process. The dies particularly are

very large and heavy and in most cases lifting equipment is required to remove or replace

the die. Also roller adjustment is more difficult, as the majority of round die pellet mills

require manual roller adjustment, which can only be accessed by opening the pellet mill

chamber. However this is not always the case, as some now come with optional remote roller

adjustment at extra cost. Cost is another key disadvantage of the round die pellet mills, as the

machines themselves and the dies and rollers are generally more expensive than flat die

pellet mills. Finally visibility of the pellet process, as the die and roller are encased behind a

solid door, viewing the process to gain information on possible issues and correction is not

possible.

2.2 Important equipments

2.2.1 Gearing Different pellet mills are geared for different materials. Different gearing gives different

speeds to the rotating die and rollers. A low gearing means more torque but a lower speed,



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which is beneficial when processing dense materials like woods and particularly hard woods.

A low-geared pellet mill requires less binder and lubrication. A high gearing means less

torque but a higher speed, which is beneficial for low-density materials such as grasses,

straws, and animal feed products. Clearly the higher the speed, the higher the productivity;

however this can be detrimental to pellet quality. For example some woods processed in a

high-speed pellet mill will not form dense shiny pellets as the necessary pressure and heat

was not generated, as the material passes through the die too quickly. There is also a

possibility when wood is processed in a high-speed pellet mill because of the increased

density of the wood the pellet mill motor may not have the power required. This will result

in the pellet mill motor laboring and may even stall. Adding an additional binder can

sometimes compromise these issues, this can help lubricate the pellet mill and reduce

resistance, while still producing hard shiny pellets. Some pellet mills are geared in the

middle; they can therefore have a good productivity and still retain pellet quality, even for

wood pellets.

2.2.2 Die There are several different die types with regards to holes depth.

Hole Depth: The depth of the hole, illustrated by the letter L, plays an important role in

the pellet process. The deeper the die hole, the longer time the material is exposed to heat

and pressure. With more heat and pressure the lignin melts better and a harder denser

pellet is formed. However a long holes depth means more friction occurs as the material

passes through the die, so the pellet mill has to have sufficient power to push material

through the deeper die. This either means the pellet mill has a low gearing or has a larger

motor.



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Hole Pitch: The hole pitch, illustrated by the letter I, also plays an important role in the

pellet process. The pitch angle of the hole influences pellet compression and some dies

have specific angles for certain materials. There is also pitch angle as the pellet leaves the

die, this is also variable on feed stock.

Hole Width: Different dies have different sized holes, ranging from 1mm holes to 10mm

and more. For fuel pellets the most common sizes are either 6mm or 8mm, 6mm pellets

are used in stoves or 6mm or 8mm pellets are used in boilers. With a larger diameter hole

there is less friction and resistance during the pellet process. This can lead to increased

productivity; however it can also have a negative affect on pellet quality.

2.2.3 Die Metals Pellet mill dies are made from several different types of metal, each with different

advantages and disadvantages. Dies must be resistant to abrasion, strong to handle the forces

of pellet production, corrosion resistant and give a good productivity. The ideal die will be

strong with low wearing and corrosion characteristics, and give maximum productivity.

Carbon Steel Alloy: These dies are heat treated and are the strongest dies available, their



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key advantage therefore is good abrasion and breakage resistance. Their disadvantages

are in corrosion resistance and productivity. Carbon steel rusts, and surface corrosion

collects on the inside of the die holes, this corrosion can affect die blockages and

productivity. Before pellet production can begin a polishing agent must be run through

the pellet mill, for example wheat bran and vegetable oil. This will remove the surface

corrosion, then pellet production can begin. A loose oily material must also be processed

at the end of production to remain in the die before the next production cycle. If the

feedstock is left in the die, as the die cools the material forms a hard dense pellet in the

die. The moisture released as the pellet cools and dries penetrates the die surface and

corrosion forms, locking the pellet in the die. Once this has occurred drilling each hole in

the die is required to remove the material. Productivity for carbon steel dies is also lower

than with other die metals. The surface finish on the inside of the die hole affects the

resistance as the material passes through the die. Carbon steel dies partly due to the

corrosion issue generally have a rougher finish; this increases resistance as the material

passes through the die, resulting in a lower productivity. However this can have a benefit

on pellet quality producing a harder pellet.

In summary carbon steel dies may have their disadvantages with regards to corrosion

resistance and productivity, but they are the strongest dies, with the longest life and are by

far the cheapest. Carbon steel dies are seen as a good all-rounder and used for most materials,

particularly highly abrasive materials.

Stainless Steel Alloy: These dies have good corrosion and wear resistance properties.

Stainless steel dies are also used for many different types of materials. Due to their

increased corrosion resistance over carbon steel dies they have highly polished die holes

which produce a higher productivity. So the reduced resistance through the die does not

compromise pellet quality, the die depth tends to be deeper than carbon steel dies. The

key disadvantage of stainless steel dies is the increased cost.

High Chrome Alloy: These dies provide the best corrosion resistance of all the dies. The

corrosion resistance means highly polished holes are possible, even more so than



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stainless steel dies. This means pellet production start up is much easier and very high

productivities can be achieved. Again though this lack of resistance for some materials

passing through the die means a deeper die must be used. This again means a more

expensive die, due to the high chromium content and required depth.

2.2.4 Die Temperature As heat and pressure are the key factors of pellet production, die temperature is essential to

pellet quality and productivity. When starting pellet production, the material should be

slowly fed into the pellet mill to increase die temperature. If too much material is fed into the

pellet mill at start up there is a high chance of blocking the pellet mill die. This is where a

high chrome die is an advantage as the chances of blocking the die at start-up are reduced.

To get the die to optimum temperature could take between several minutes to half an hour.

Once the die is up to temperature, maximum productivity for that material will be reached.

2.2.5 Roller Height and Carpet The carpet is a thin layer of compressed material, which sits on top of the die surface. As

material goes into the pellet mill it gets pulled beneath the roller and forms a carpet. As more

material is placed in the pellet mill this is then added to the carpet. The bottom of the carpet

is then forced through the die holes, as the process continues, this results in a pellet. So for

the material to be able to form a pellet, it must first be able to form a carpet.

2.2.6 1mm Gap Generally a 1mm carpet is regarded as optimum for most materials to give the correct

balance between pellet quality, energy input and wear of equipment. Increasing the gap from

0 to 1mm increases the energy demand by 1.2 fold, however fines are reduced by up to 30%

and die and roller wear is reduced.



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2.2.7 >1mm Gap The larger the gap between the rollers and die, the greater the energy input required.

If the rollers are set above 1mm, there may not be enough pressure generated between die

and rollers. Without pressure there is no heat, no binder is released and a carpet cannot form,

thus a pellet cannot form.

2.2.8



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Preparation before starting making pellets

3.1 Introduction of the standard procedure of pelleting process This guide will follow the process step-by-step to produce quality fuel pellets, and will state

the equipment used at each stage, the principles of the process and other key points. The

process is as follows:

11 Size Reduction: Chippers/Shredders, Hammer Mills

22 Material Transportation: Fans, Cyclone Separators and Screw Augers

33 Drying: Rotary/Drum Dryers, Pipe Dryers

44 Mixing: Batch Mixers

55 Conditioning: Water and Steam Addition, Binders

66 Pellet Production: Round and Flat Die Pellet Mills

77 Sieving: Removing Fines

88 Cooling: Counter Flow Coolers

99 Pellet Transportation: Bucket Elevators

1100 Bagging and Storage: Bags, Sacks and Silos



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3.2 Preparation of Feedstock

3.2.1Material Input Regulated material input into the pellet mill is very important for many materials. During the

pellet production process moisture is released as steam. If the rate of material feed is greater

than the productivity rate of the pellet mill this can cause problems. If the steam generated

from the pellet process cannot escape the pellet chamber, the material above the roller will

absorb the steam. This can cause bridging of the input material, which will stop material feed

or affect pellet quality. If the material above the roller absorbs the steam this will increase

that materials moisture content and the characteristics of the material in the pellet mill and

pellet quality will change.

Therefore material feed rate should be regulated in tern with pellet mill productivity. At the

beginning of production the pellet mill should be fed at a lower rate and increased as the

pellet mill temperature increases. Therefore the feed rate should mirror the performance of

the pellet mill and slowly increase up to maximum productivity. As the productivity of the

pellet mill is variable based on the characteristics of the raw material, the feed rate will also

have to adjust. The target feed rate should be to have the pellet mill rollers covered but no

more.

3.2.2 Moisture The moisture content of the raw material plays an integral role in how the material performs

in the pellet mill. Too little or too much moisture and quality pellets cannot form, its also

important to keep the level of moisture within the finished pellet as low as possible, so the

pellets burn efficiently. Moisture is required to produce pressure, and therefore heat the two

essentials of pellet production.



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3.2.3 Percentage Raw material requires moisture content somewhere between 10-20% Moisture content

required is specific to that raw material. Moisture content required is specific because of the

differences in natural lignin and pressure requirements. During the pelleting process

moisture is lost as steam, if the finished pellets are dense, smooth and shiny they should have

moisture content at 10% or below.

3.2.4 Lignin Lignin is a complex carbohydrate sugar found in the cell walls of all cellulosic plant

material. Lignin is central to the plants mechanical strength, therefore the more lignin the

stronger the plant in general. So different cellulosic materials have different percentages of

lignin, for example wood has a lot and straw has a lot less, which makes sense in relation to

mechanical strength.

During pellet production lignin is what binds the material together to form a pellet, therefore

it makes sense that the more lignin present the mechanically stronger the pellet. So wood

pellets are generally harder and stronger than straw pellets. Therefore a raw material with a

higher percentage of lignin is less likely to require a binder and more likely to produce

mechanically strong pellets.

Under the heat and pressure of the pellet mill the lignin melts and binds the material

together to form a pellet. Once the pellet leaves the pellet mill the lignin begins to solidify

and cool, producing a hard dense pellet.

3.2.5 Additional Binders If sufficient lignin is not present in the raw material, additional binders can be used. Many

different binders are used in the pellet industry depending on the purpose of the pellet. For

example binders suitable for feed pellet production may not be suitable for fuel pellets as the

binder may interfere with the combustion process. Some binders are used purely to increase



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pellet density, while some are also used to improve lubrication through the pellet mill,

reducing parts wear and increasing productivity.

The percentage of binder added to the pellet mill, is dictated by its purpose, whether to

purely bind or also increase productivity. Below 10% and binder is used more for binding

properties, anything above this is to increase productivity by reducing the resistance as the

material passes through the pellet mill die. The simplest additional binders include vegetable

oil, Molasses, starch, gluten, dried distillers grain and rape cake. Many organizations

constantly experiment using different materials as pellet binders, using by products from

other processes and waste products. Generally if its sticky and oily its worth trying it as a

binder.

Mixing materials together can also address the binder issue. For example mixing wood with

straw and then placing into the pellet mill, this can create a better quality pellet than a pure

straw pellet. This can have other benefits particularly for fuel pellets. Straw pellets compared

to wood produce more corrosion and ash, and therefore create more maintenance for the

user. A wood/straw pellet would have better combustion characteristics and would extend

limited wood resources.

3.2.6 Pressure Changes As discussed to produce pellet pressure is required to compress the material. Pressure can be

altered through the type of die, die metal and roller height. Also as discussed pressure is also

dictated by the characteristics of the raw material. Therefore another way to alter pressure to

form a pellet is to alter the materials characteristics. This can be achieved through adding

water, vegetable oil or other binders into the raw material at either the conditioning stage or

in the pellet mill. Water and vegetable oil to a certain extent can both act as binders. Also the

ability to alter these parameters means one type of die can be used to process many different

materials.

Adding Water: Adding water to the pellet process can increase pressure. The additional



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pressure can help to increase the compression within the pellet mill. The increased

compression produces a denser pellet and increased pressure generates more heat, which

generates more effective lignin melting. Increased lignin melting gives better binding

qualities, which produces a denser shiny pellet. Water also helps to dissipate the lignin

through the pellet, therefore to a certain extent water can act as a binder.

Adding Vegetable Oil: Adding vegetable oil to the pellet process can reduce pressure. Some

very dense materials for example hard woods can sometimes generate too much friction and

pressure and cause excessive die wear and poor productivity. Additional oil can help to

reduce pressure and lubricate the pellet process and increase productivity. Vegetable oil can

also act as a supplementary binder for materials, which lack sufficient natural lignin.

3.2.7 Material Binder Issues

A: The material first enters the pellet mill and comes into contact with the pressing roller.

B: Some of the material will be compressed under the roller through the die holes. A material, which lacks binder, may struggle to do even this. It may display properties more

like a dry material, as the material cannot bind together under compression.



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C: Due to the lack of a binder the material may be unable to form a carpet, and again it may display similar properties to a dry material.

D: Even with sufficient moisture to give the required pressure and heat, the material may not compress at all. Generally though some compression and binding will take place, but not

to the levels required.

E: If the lack of binder is extreme the material may not form a pellet at all. In many cases small layers of compressed material will emerge, but not a complete pellet. A short pellet is

not detrimental to its purpose, however it should be checked that once the pellets cool they

do not crumble into dust too easily. If this occurs the lack of a binder needs to be addressed.

3.2.8 Solutions The above issue could be a lack of binder or an inability to release the materials natural

binder. For example the material may poses sufficient natural lignin, however if there is

insufficient moisture to generate the required pressure to melt the lignin, the material will

not bind. Some materials though do not contain sufficient lignin and here a supplementary

binder must be used. As additional binders increase production costs, it is worth

experimenting with increasing the moisture percentage to aid material binding.

3.2.9 Power Pellet production is an energy intensive process; however the ends justify the means. The

hammer mill and pellet mill demand high-energy inputs, which usually mean they require a

three-phase electricity supply. Three-phase supply is only normally found in farming and

industrial locations. However many locations do not have three phase supply and

installation can be very expensive. Before purchasing pellet production equipment suitable

power requirements must be available. If the installation of three-phase power is not possible

for logistic or cost reasons there are other solutions. Diesel generators are one possibility, or



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powering the equipment directly from a diesel engine maybe more suitable.

3.2.10 Location Ideally to save costs the plant should be situated, as close to the raw material supply as

possible, however this is obviously not always possible. Location will also be dictated by

access and a power supply if three-phase electricity is needed.

3.2.11 Heavy Lifting Equipment Pellet production equipment is very heavy, the pellet mill particularly. Therefore heavy

lifting equipment such as large forklifts is generally required. This should also be considered

when choosing a location to give sufficient access to the site.

3.2.12 Changes in other aspects Purchasing a pellet mill is actually a very small part of being able to produce wood or

biomass pellets. On the face of it making pellets in a pellet mill may seem very straight

forward, you put wood or biomass in the pellet mill and pellets come out the other end. Well,

the reality could not be further from the truth, as quality wood and biomass pellet

compression relies on a combination of principles, from material preparation to proper pellet

mill operation and maintenance. However, it is still possible for even the average

homeowner to own and operate their own pellet mill, as with every process, its just about

knowing what you have to do and when.

3.2.13 Changes in Raw Material Particle Size Before pellet compression in the pellet mill can take place, the wood, straw, grass or any

other form of biomass must be reduced in size. One fact that is not widely stated, however is

one of the most important facts in pellet production is Only a raw material of consistent



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quality can produce consistent quality pellets. Part of this consistency is the size of raw

material particles used in the pellet mill. Particles, which are too small or too large, can

severally affect pellet quality.

3.2.14 Changes in Raw Material Moisture Content, Pellet Quality

and Production Rates One of the reasons pellet fuel is so popular, is pellets have a moisture content below 10%, and

this enables the pellets to burn very efficiently, and produce virtually no smoke during

combustion. Pellet production is a high temperature process; the right moisture content will

produce the best quality pellets, reduce energy consumption and reduce pellet mill

downtime.

3.2.15 Changes in Raw Material Composition and the Inclusion of

Binders and Lubricants In pellet production every raw material behaves differently, and some materials produce

quality pellets easier than others. Depending on the equipment used, the composition of the

raw material may need to be changed to produce a quality pellets at a reasonable

productivity. Changing the composition can include adjusting particle size or moisture

content, however it may also include adding binders and lubricants to help produce higher

quality fuel pellets.

3.2.16 Changes in Raw Material Pellet Mill Feed Rate Another adjustment that is not well known to impact on pellet production is the rate of feed

into the pellet mill. Adjustments on feed rate, and maintaining a consistent feed rate can

make the key difference on how well the pellet mill operates, even if the raw material is

perfect.



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3.2.17 Changes in Raw Material Conditioning and Steam Conditioning is the pre-treatment of the raw material before it reaches the pellet mill.

Conditioning can include specific mixing techniques and the introduction of additional water

or steam. Steam can be used to pre-anneal the raw material and start the lignin melting

process. Though conditioning can have several benefits, in some cases the benefits are

negligible and in other cases it is simply not practical to use conditioning.

3.2.18 Changes in Pellet Mill Operating Temperatures Temperature is a key requirement in pellet production. Unless a certain temperature is

reached in the pellet mill natural lignin will not melt, and it is not possible to produce some

biomass pellets, for example wood pellets. However, if the temperature is too high this can

damage the pellet mill, and particular consumables such as the pellet mill die, rollers,

bearings and seals.

Changes in Pellet Mill Roller and Die Clearance

Another adjustment, which can impact heavily on how successfully, the pellet mill operates,

is the distance between the roller and die template. The roller and die are wearing

consumable parts, due to the abrasive nature and pressure of compression. The distance set

between the roller and die can impact on how much energy the pellet mill uses, the quality of

the pellet, pellet mill productivity and the amount of fines produced. Correctly setting up the

die on a pellet mill will also increase the life of the roller and die, and reduce the cost of parts.

3.2.19 Changes in Pellet Mill Die Template Rotation Speed The speeds at which the roller and die turn affect the complex relationships during pellet

compression. Some materials require a greater time under compression, and therefore

require a slower rotation speed. Also, speed and torque requirement of the pellet mill

change.



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3.2.20 Changes in Pellet Mill Die Design and Metal Used Many different forms of metal alloy are used to produce pellet mill die templates. Different

metal alloys have specific advantages and disadvantages. Using the correct alloy is critical to

reduce wear and increase production.



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Reduce the size of raw material into

appropriate particles

In this section, the raw material needs to be crushed, you can choose a hammer mill, chipper

or wood waste shredder in accordance with your raw material. Based our comparison tests

to different pellet mill and common wood & biomass materials, the pellet produced from

crushed raw material has better and consistent quality than that produced from rough raw

material. If your raw material is sawdust, or similar wood & biomass materials, you can skip

this section.

Whether the raw material is wood, grass, straw or any other type of biomass, it must be

reduced to a sufficient uniform size for the pellet mill. The general rule of thumb is the milled

material going into the pellet mill must be smaller than the die holes in the pellet mill. For

example to produce 6mm pellets the milled material must be smaller than 6mm. There are

various types of equipment to conduct size reduction; each has different abilities and

strengths. In some cases one piece of equipment is required, in other cases two are needed.

4.1 Chippers/Shredders

Raw Material: Logs, branches and material with a diameter over 1 inch

Chippers and shredders are used for size reduction of large diameter raw materials. For

example logs and branches. Chippers generally reduce material to the size of chips;

shredders can produce smaller particles.

Chippers: The conventional chipper is a rotating disc with several blades attached to the



35

surface. As the material comes into contact with the disc the blades shave off small sections

(chips) until all the material is processed.

Shredders: Shredders push the material against a rotating roller with teeth. The teeth eat into

the material and drag it against a screen. The size of the holes in the screen dictates the

particle size.

If the raw material has a diameter of over 1 inch it is recommend using a chipper/shredder

for initial size reduction. Chippers and shredders in many cases can be adjusted to give

different particle sizes however they generally cannot reduce particle size sufficiently for

pellet mills. Therefore in many cases after the chipper/shredder the material is sent into a

hammer mill for final size reduction.

4.2 Hammer Mill

Raw Material: Wood chips, straw, grasses and material with a diameter less than 1 inch

If the diameter of the raw material is less than 1 inch, the material could be processed directly

into a hammer mill, or as a finishing mill for material that has already passed through a

chipper/shredder. A hammer mill comprises of many small flailing hammer blades that

through attrition reduce particle size. Hammer mills run at speeds ranging from 3000RPM to

over 8000RPM. Particle size is dictated through the size of holes in the screen, these can range

from as little as 1mm up to 10mm or more. As hammer mills have a large range of screens to

produce various particle sizes they are the most popular choice in pellet production plants.



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4.3 Size Reduction Conclusions

The diameter of the raw material will influence the choice of chippers, shredders and

hammer mills. Hammer mills are normally the most suitable for size reduction; however

there are limitations on the size of the input material.



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Dry the size-reduced material to reach

required moisture content

The moisture content of the pellet will straightly affect the pellet quality on the burning

efficiency and the clean burning (zero smoke). According to our years` of studying and

consultation experiences, the general requirement for moisture content of wood & biomass

materials is from 10% to 15%. And by choosing and controlling on the right moisture content,

you can cut down your cost on energy consumption.

In this section, a rotary drum dryer is a good option. To produce quality pellets the moisture

percentage of the raw material will be between 10-20%. Most materials produce the highest

quality pellets with a moisture percentage around 15%, however the percentage to produce

quality pellets is specific to each raw material. If the raw material has a moisture percentage

above 15-20% it must be dried or mixed with a dryer material, this will be described in STEP

4: Mixing.

5.1 Rotary/Drum Dryers

The wet milled material is placed via screw auger into the rotary dryer. Hot air is generated

at one end of the drum; at the other end a fan pulls the hot air and material through the drum

up to a cyclone separator. The drum rotates as hot air is pulled over the material; the rotating

action produces a more effective drying process. Light dry particles are pulled through the

pipe, and heavy wet particles remain in the drum until dry.



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5.2 Pipe Dryers

The wet material is placed into the pipe dryer. Hot air is generated at one end of the pipe and

pulled through the pipe system by a fan; the material then leaves the dryer via a cyclone

separator.

5.3 Heat Source

Some dryers source their heat using oil or gas burners, however in todays market this can be

very expensive and reduces the green credentials of the biomass fuel pellet. Therefore the

cheapest solution is to generate heat through burning pellets.

5.4 Productivity Increase

Another advantage by using heat to dry the material is once the material leaves the dryer the

heat has made the material more malleable. This can therefore increase the productivity

through the pellet mill of all materials but particularly dense materials like hard woods.

The heat makes the material more malleable, and begins the process of melting the materials

natural lignin.

5.5 Drying Conclusions

Other than a pellet mill, a dryer can be the largest capital expenditure on a pellet production

plant. Drying systems are heavy and also take up quite a lot of space. Sourcing dry raw

materials saves the costs of a drying system, and can dramatically reduce the production cost

per tonne.



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Mix the dried material with the binder,

lubricants or other needed material

Due to the different characteristics of various raw materials, the pellet quality is unequal. To

improve the pellet quality or maximize the production capacity, you can add binder ( a kind

of glue, e.g., vegetable oil or rapeseed cake) to help the lignin-lacked material to compress

into pellets much easier. Whether producing a single material pellet or a mixed material

pellet, material consistence is crucial to a consistent, reliable and efficient pellet plant. Batch

mixers are used after the material has been milled and dried (if required). Through mixing

the material, a more consistent material blend is fed into the pellet mill. Mixing may occur

before or after conditioning. The graphs below illustrate the possible effects of poor material

consistency.

Inconsistent Material: Inconsistent material could be due to significant changes in moisture

percentage, sections of material with poor binding properties and changes in material

density. All of these factors affect the pellet mills ability to create quality pellets. Below is a

graph showing possible moisture changes with inconsistent material.

Pellet Mill Max (%) Material Moisture (%) Pellet Mill Min (%)

The pellet mill has certain tolerances with regards to a maximum and minimum percentage

of moisture to create a pellet. The maximum and minimum tolerances are not the same for

every material. When the material moisture percentage goes above or below the pellet mills

abilities this will either lead to pellets not forming, poor quality pellets or worst case could

block the pellet mill die.

Consistent Material: Sourcing material, which is consistent through the batch, or mixing the

material to create consistent qualities is crucial to reliable consistent pellet production. If the



40

material from graph above was mixed, a more consistent moisture percentage can be

obtained.

As the material moisture percentage consistently falls between the pellet mills abilities this

will result in more successful pellet production, with higher quality pellets and less chance of

blockages.

Mixing Conclusions:

Not all materials require mixing; some materials may already have consistent qualities. Other

materials have a low density and bind together well due to their oily properties, for instance

rape cake. Many materials do however require consistent qualities to consistently produce

quality pellets. Mixing equipment is available for small-scale production up to large-scale

industrial rolling drums and agitators.



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Conditioning process to the mixed material

By adding dry steam into the mixed material can heat and soften lignin, which helps the

mixed material to be compressed into final pellets much easier, and maintains consistent

quality of the mixed material which ensures the consistent quality of pellets. Its final purpose

is to increase the production capacity.

Please note : steam conditioning process is ONLY used in some LARGE pellet mills, if your

mill is a small plant, this process is not necessary, you can skip this section.

To produce quality pellets the raw material has to posses certain qualities, a pellet mill is

similar to a mathematical equation, the pellets that come out are only as good as the raw

material that went in. Another accurate comparison is cooking, a cake is only as good as its

ingredients and preparation, and the cook can only bake the cake. A pellet mill is basically a

cook, applying heat and pressure to the material.

Once the material has been mixed to give a good consistency, the raw material may require

other ingredients to produce pellets. Other additives can increase the productivity of the

pellet mill. To understand the conditioning process, you must first understand what qualities

the material needs to produce a pellet.

Moisture Percentage: As stated in Step 4 there are maximum and minimum moisture

percentage tolerances for a pellet mill to produce pellets. However those tolerances are

specific to the material in question, and the type of pellet mill used. As a general rule the

most common average moisture content required for producing pellets is 15%.

Adjusting Moisture Percentage: Obviously if the material has a moisture percentage which

is too high a dryer (Step 3) is required, or mixing the material with a drier raw material to



42

reach an average moisture percentage of 15%. If the moisture percentage is too low peristaltic

pumps can be used to add water into the process at variable controlled rates. A peristaltic

pump works on the principle of a rotating central cam, driven by an electric motor. As the

cam turns it compresses the pipe, flow rates are dictated by the speed of the cam and speed

adjustment is through changing voltage to the electric motor. Peristaltic pumps can also

prime themselves from several meters away. Other pumps are used including diaphragm

pumps.

Binding Qualities: Binder is the glue that holds a pellet together and produces a smooth shine;

some materials have enough lignin, which is a naturally occurring binder from within the

raw material. If the material lacks lignin to bind the material together supplement binders

can be added to create a pellet. One of the simplest binders is vegetable oil; this can also be

added via peristaltic pump into the material. Generally oily products such as rape cake and

dried distillers grain act as suitable binders.

Material Density: A pellet is formed through heat and pressure; therefore a materials density

dictates the heat and pressure within the pellet mill. Material density can influence pellet

quality and

Pellet mill productivity. Materials, which have a high density, for example hard woods

require more heat and pressure to form a pellet, therefore pellet quality can be an issue and

low pellet mill productivity. On the other hand low-density oily materials such as rape cake

and dried distillers grain produce smooth hard pellets with high pellet mill productivity.

Some materials with a low density but without oily properties struggle to compress into a

pellet, as not enough pressure and heat is generated in the pellet mill.

Steam Conditioning: As described in Binding Qualities, materials such as wood contain

enough naturally occurring lignin to act as a sufficient binder. Under the heat and pressure of



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a pellet mill the lignin melts. A pellet is then formed, and once out of the pellet mill the lignin

cools, producing a strong durable pellet, lignin is also what gives a quality pellet its surface

shine. To aid the pellet process and increase productivity large-scale pellet mills have steam

conditioners, where the material is exposed to dry steam before the material enters the pellet

mill chamber. The dry steam does not add any moisture to the material, it is purely used to

increase the temperature to aid lignin melting. After steam conditioning the material enters

the pellet mill chamber, better quality shiny pellets are produced at a higher productivity as

the material passes through the die with less resistance due to the materials softened state.

Steam conditioning is only used in large-scale pellet mills due to the added expense and

safety risks.

Increasing Productivity: Steam conditioning is one way to increase productivity of the pellet

mill; another way is to use low-density oily materials. Materials such as rape cake or dried

distillers grain can aid pellet production by reducing resistance through the pellet mill die,

while the oil content acts as a secondary binder and produces a strong shiny pellet. Adding

up to 20% of the low density oily material to the original raw feedstock can increase

productivity by up to 30%.

Pellet Mill Tolerances: Adding an oily material to the original feedstock can also increase the

tolerances of the pellet mill. The oily properties aid to the pellet process and can reduce the

frequency of blocked dies and poor pellet quality.



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Pellet Mill Characteristics

Each material has different qualities and characteristics. Therefore each material will behave

differently in the pellet mill based on its moisture, density and binding qualities. Below is

what a material should behave like, and what to aim for.

If the raw material has the correct qualities for making a pellet, you should see the following

things happen.


B: Some of the material will be compressed under the roller through the die holes.

C: Represents the material carpet that lies between the die and pressing roller. As more material is added to the pellet mill the pressing roller adds this to the carpet. The roller

therefore compresses the bottom of the carpet through the die.

D: Represents the compression of the material through the die. If the material has the



45

correct qualities heat and pressure will be produced due to friction, as the material is

compressed through the die.

E: With sufficient friction, heat and pressure a shiny dense pellet will emerge. Once this pellet cools it will dry and become hard, this is then ready for use.

Material Too Dry

A: As the dry material enters the pellet mill, due to the action of the rotating roller and die, material dust will engulf the pellet mill chamber.

B: Due to the materials low density and lack of moisture the roller will not be able to compress the material through the die.

C: Dry material will not form a carpet, as there is not enough friction to generate pressure as the material passes under the roller. The lack of pressure therefore means a lack of heat and

the lignin cannot melt to bind the material together to form a carpet.



46

D: As the dry material does not generate enough friction and pressure the material will run freely through the die without compression

E: The end result is loose dry materials freely passing through the pellet mill, and no pellets will be produced.

Solutions

To make this material form pellets, water must be added to give the material body. The more

water that is added the more pressure, to a certain extent. Slowly adding water to the

material and testing the results will eventually generate the necessary friction to produce

pressure and heat. Once dense shiny pellets are produced this will indicate the required rate of

additional water. For very dense and low lignin materials additional vegetable oil maybe

required, to lubricate the process and aid binding.

Material Body

Some materials are naturally very fine, very dusty and lack body. Material, which has a low

density, may require more moisture/binder for it to be possible to bind the material together.



47

Material Too Moist

A: As the wet material enters the pellet mill the high moisture content will produce very high pressures as the roller tries to compress the material through the die. The high pressure

will result in high temperatures and large amounts of steam will emerge from the pellet mill.

B: Due to the high pressures generated as the material is compressed through the die, high load will be placed on the pellet mill motor. If the high pressure continues this could stall the

pellet mill motor, this could also affect the bearings in the rollers.

C: Wet materials will form a carpet; again this will be releasing large amounts of steam.

D: The wet material will create high pressures as it passes through the die; this may result in the material passing through very slowly. However even though the wet material

generates high pressures the temperatures needed to release the materials natural lignin are

not reached. If the materials lignin does not melt, the material will have no binding qualities.



48

E: The pellets that exit the pellet mill will be soft and very loose, perhaps in small moist lumps. As there is still too much moisture in the pellet, the pellet will expand and release the

moisture; therefore the pellet cannot form a smooth, compact cylinder, as it should.

Blocked Die

Processing wet materials due to the pressure it generates can in cases block the die. The

material generates more resistance force when it passes through the die, than the force the

roller generates. A blocked die can also be from corrosion within the die hole. If a die has

become blocked drilling the material out of the die is required. This is a lot easier if the die is

still warm, as the material has not dried and cooled and the lignin has not set to form a hard

pellet.

Material Binder Issues




49

B: Some of the material will be compressed under the roller through the die holes. A material, which lacks a binder, may struggle to do even this. It may display properties more

like a dry material, as the materials cannot bind together under compression.

C: Due to the lack of a binder the material may be unable to form a carpet, and again it may display similar properties to a dry material.

D: Even with sufficient moisture to give the required pressure and heat, the material may not compress at all. Generally though some compression and binding will take place, but not

to the levels required.

E: If the lack of a binder is extreme the material may not form a pellet at all. In many cases small layers of compressed material will emerge, but not a complete pellet. A short pellet is

not detrimental to its purpose, however it should be checked that once the pellets cool they

do not crumble into dust too easily. If this occurs the lack of binder needs to be addressed.

Solutions

The above issue could be a lack of a binder or an inability to release the materials natural

binder. For example the material may poses sufficient natural lignin, however if there is

insufficient moisture to generate the required pressure to melt the lignin, the material will

not bind. Some materials though do not contain sufficient lignin and here a supplementary

binder must be used. As additional binders increase production costs, it is worth

experimenting with increasing the moisture percentage to aid material binding.



50

Cooling

When the pellets leave the pellet mill, they are very hot, soft and releasing moisture. Before

the pellets can be used or stored they must first be left to cool and dry. The simplest form of

pellet cooling is to spread the pellets out, and let them cool at room temperature.

Counterflow Coolers

The counterflow process is the refined technology of exposing processed product to an

upward moving stream of ambient airflow.

As the air rises through the product, it is warmed, increasing the moisture carrying capability



51

of the air. The product entering at the top of the cooling chamber is exposed to the warmest

air available within the cooler, minimizing temperature shock. The product exiting the

bottom of the unit is cooled to within 5-10 of the ambient air temperature. The product and

air have opposite flows. The gradual heat transfer greatly enhances the product quality

and reduces stressing and fines.

Cooling Tips

Hot pellets should not be left to cool and dry in a heaped pile, as the temperature in the

middle and the bottom of the pile will stay hot for quite some time, stopping the pellets from

properly cooling and hardening. The pellets should be spread out, as this will increase air

circulation around the pellets, cooling them quicker. Using fans to increase air circulation

will also aid cooling.

Once the pellets are cooled, they should be hard, smooth and in some cases have a surface

shine. One of the best ways to describe a quality pellet is like a colouring crayon. If the pellets

are not like this, brittle for example, efforts must be made to adjust the necessary parameters.

Cooling Issues

Cooling pellets is a delicate process. If the pellets are cooled too quickly the rapid release of

moisture causes stress fractures to occur. Rapid cooling also affects the lignins ability to

properly solidify to form a hard dense pellet. Therefore rapid cooling can affect pellet quality

and increase fines.



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Sieving

As the pellet mill compresses the raw material into pellets some material is not compressed

into pellets, this dust is referred to as fines. Fines are particles of material that failed to bind

to the pellet during pellet formation; this could be due to lack of pressure in the pellet mill or

a lack of binding. If the pellets produced are not mechanically strong, as the pellets move and

rub against other pellets and surfaces particles break away from the pellet, again these are

described as fines. So the performance of the pellet mill and the quality of the finished pellets

affect the percentage of fines.

Pellets are very strong with vertical pressure, but all pellets break fairly easily under

horizontal pressure. Pellet breaking is not that important, but how the pellet breaks is. When

horizontal pressure is applied to quality pellets the pellet should break cleanly. A poor

quality pellet however under horizontal pressure will break into several pieces and produce

many fine particles.



53

With regards to how important the percentage of fines is depends on the target market for

the pellets produced. Fines can interfere with the pellet feed system into the pellet

stove/boiler; this is more of an issue with small residential units. Also pellet stoves and

boilers have the feed rate and fan speed set for burning pellets. However fines burn faster

than pellets, in some pellet burners this can cause the ash to melt and sinter. In the

premium wood pellet industry the target is for fines to be less than 1% by weight. The

European CEN standard has specifications for the maximum allowance of fines, and in

many cases the percentage of fines must be stated when sold. Pellets in bags tend to have

fewer fines than those sold in bulk sacks. Pellets that are stored in silos generally have

more fines on delivery.

Pellet Mill Carpet

As stated earlier in the guide setting the pellet mill so a carpet forms over the die can

reduce the percentage of fines by up to 30%.

Grade Sieving

After pellet production pellets can be run over a vibrating screen to remove fines.

However if the pellet mill is setup properly and the material is properly compressed this

may not be required. However it must be remembered that making sure that the pellets

are mechanically strong is the most important thing, as this will reduce fines after

production.



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Pack and store the screened pellets

Because the wood pellet will not enter into the fuel process immediately, in order to keep

the wood pellet as dry as possible, to avoid the influence from water or dampness, the

packing process is necessary. To reduce your cost on labor source, a pellet Packing machine

is recommended, you can choose a semi-auto or a completely auto one in accordance your

needs.

Once the pellets have been cooled and are of sufficient quality, the pellets are ready to be

packaged, stored and sold. How the pellets are packaged and stored again depends on the

target market. Pellets must be protected from moisture and breakage. If moisture comes

into contact with the pellets, the pellets will absorb the moisture and expand, making them

useless.

11.1 Bags

The most common way to package pellets are in plastic bags that will hold between

10-20kgs of pellets. These weights are chosen so the customer can easily lift the bags to

load their pellet stove/boiler. In many cases these bags are made from a folded sheet of

plastic, which is welded at both ends. These bags can be easily stacked, and are either

sold as individual bags or a set on a pallet. These bags do serve their purpose well

however are quite wasteful. Ideally reusable plastic bags should be used with a watertight

zip, which are then returned to the pellet manufacturer. Multilayer paper bags are some

times used, as they are easy to recycle, however issues can occur with insufficient moisture

protection. Bags are mainly used for residential customers and small-scale pellet burners.

11.2 Sacks



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Large sacks or bulk sacks can carry around 1 tonne of loose pellets; these are normally for

large- scale equipment. The sacks do not normally provide moisture protection, so the

pellets are normally loaded into a silo. Pellets sold in large sacks are normally cheaper

than those sold in small bags by weight, due to the reduced materials and labour costs.

11.3 Pellet Tanker

Some customers have large silos, which feed their pellet burner. Here deliveries of pellets

can be by a pellet tanker. The tanker will arrive at the property and use air to blow the

pellets into the silo. Silos normally hold several months usage of pellets.

11.4 Moisture Protection

Exactly how the pellets are stored, packaged and sold is not that important. What is

important is that the pellets are protected from moisture. Without moisture protection the

pellets will absorb available moisture and expand, loosing mechanical strength and

rendering them useless.



56

Common transportation machineries in pellet plant

Among the above sections, the materials and the pellets need to be transported between

different equipments. To improve the production efficiency, you can use different

machines as mentioned below:

Piping system is an infrastructure conveyor corridor in a pellet plant site, it is very popular

among the above sections, such as between raw material inlet and hammer mill. A

considerate design can lower your energy consumption.

Belt conveyor is another common transportation machine, it can be used in for raw material

inlet transportation, etc. It is a great solution to reduce your labor cost.

Screw conveyor is other choice for material transportation. Because it has less weight, low

cost and easy to control.

Bucket elevator is generally used for final pellet transportation in the final stage to prevent

the damages.

Please note, the sequence of the sections and the choose of the machines may change in

accordance with different material.

Pellet transportation may be required directly after the pellet mill to be cooled, or from the

cooler to the bagging/storage facility. Screw augers, which were used for material

transportation, are not suitable. Screw augers would damage the pellets in their fragile

state. Pellets would be broken up by the screw auger, which would seriously affect pellet

quality and fines percentage.



57

Instead of screw augers, bucket elevators are used to transport the pellets from one

operation to another. A bucket elevator is simply a set of rotating buckets. The pellets are

fed via gravity down to the bucket elevator where every few seconds a bucket will emerge

from the bottom to collect a few pellets at a time, and gently move them up to the next

process.

Various sizes and speeds of bucket elevator are available, and are the preferred choice for

transporting pellets to the cooler or bagging/storage facility.



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Equipment and Pellet Mill Maintenance

The equipment used in pellet production is exposed to high pressures and temperatures,

extreme forces are placed on the equipment and proper maintenance can increase

equipment life and reduce energy demand.

13.1 Maintenance

The hammer mill and pellet mill particularly have many bearings, which reach high

temperatures during operation. Frequent checks of these bearings ar

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