News

Excess fines are the quality complaint that costs the most and shows up the latest. The pellet leaves the die looking intact, it passes through the cooler, and the fines appear in the bag, in the conveyor, or at the customer, and by the time the complaint arrives the material has already been produced and shipped. Fines reduce the value of the product, they raise the dust and the fire risk, and they can fail the customer’s specification even when the pellet itself is acceptable.

This page covers the causes of excess fines in wood and biomass pellets, how to diagnose which stage is producing them, how to correct each cause, how to measure the fines, and what to specify before buying a line. Shandong Changsheng Machinery supplies biomass pellet mills and complete lines to overseas buyers, and the guidance below reflects how fines problems are traced in service and where operators most often misread them.

The short answer

Fines are produced at one of four points: at the die, when the pellet is formed weak; at the cooler, when the pellet is cooled too fast or too slowly; at the screen, when the fines are not removed; and in handling, when a pellet that was acceptable at the mill breaks in the conveyor, the silo, or the bag. The cause is not always the die, and adding moisture or changing the die without finding the source often changes the amount of fines without removing the cause.

A well-formed pellet typically produces only a small percentage of fines at the mill exit, and the market specification sets the limit. If the fines at the die exit are already a significant share of the output, the die stage is the cause before any other stage is checked.

The practical rule: sample the pellet at the die exit, at the cooler outlet, at the screen outlet, and in the bag, and compare the fines at each point to find where they are produced. Then correct the stage that is producing them.

Where fines come from

Fines are small particles that have broken away from the pellet. They are produced whenever the pellet is subjected to a force it cannot withstand, and the force can come from the die, the cooling, the screening, or the handling.

At the die. A pellet that is formed weak at the die breaks as it is cut and as it falls into the discharge. The weakness can come from a moisture that is too low or too high, a die compression ratio that is too low for the material, a die that is worn, a roller gap that is too wide, or a material with a low natural binding. The fines at the die exit are the first sign, and they are visible as dust around the cutter and in the discharge.

At the cooler. A pellet that is cooled too fast develops a hard shell and a soft core, and it cracks or develops a rough, chipped surface as it cools. A pellet that is cooled too slowly stays warm and soft, and it deforms and breaks in the screen and the conveyor. The fines at the cooler outlet are the sign, and they are visible as dust in the cooler discharge and on the cooler floor.

At the screen. The screen removes the fines that are already present, but a screen that is overloaded, blinded, or undersized lets the fines through into the product. The fines in the product after the screen are the sign, and they are visible as dust in the bag and on the screen outlet.

In handling. A pellet that is acceptable at the screen can break in the conveyor, the silo, the truck, or the bag, especially if the pellet is brittle, if the handling is rough, or if the pellet is exposed to moisture. The fines at the customer are the sign, and they are visible as dust in the bag and as a complaint from the buyer.

Why the fines matter to the customer

The fines are not only a quality number. Excess fines can block the feed system of a boiler, reduce the combustion efficiency, raise the dust explosion risk in the silo and the conveying, and fail the customer’s fines limit, which can lead to a rejected load. The consequence is the reason the fines limit is written into the specification, and it is the reason the cause should be found rather than the fines screened out and shipped.

How to diagnose which stage is producing the fines

The diagnosis is made by sampling the pellet at each stage and comparing the fines, rather than by guessing from the die or the moisture.

The first sample is taken at the die exit, before the cooler. The fines here are the ones produced at the die, and they point to the moisture, the die, the roller gap, or the material.

The second sample is taken at the cooler outlet. If the fines at the cooler outlet are more than a small increment over the die exit, the cooler is a significant source, and the cooling time, the cooling air, or the pellet temperature should be checked.

The third sample is taken at the screen outlet. If the screen outlet is close to the cooler outlet but the bag is much higher, the screen is doing its job and the handling is the main source; if the screen outlet is close to the bag but both are well above the cooler outlet, the screen is not removing the fines and the screen condition, the screen size, or the screen loading should be checked.

The fourth sample is taken from the bag or the silo. The difference between the screen outlet and the bag is the fines produced in handling, and they point to the conveying, the silo, the loading, or the moisture pickup.

The four samples together show where the fines are produced, and the stage that produces the most fines is the stage to correct first.

The two moisture problems are often confused, and the sample at the die exit separates them. At the die, a moisture problem shows as dust at the cutter and a dull or dusty pellet surface. At the cooler, a moisture problem shows as dust at the cooler outlet and a pellet that looks damp, discoloured, or soft, and the moisture is picked up in the cooler rather than at the die.

Correcting the fines produced at the die

The die is the most common source of fines, and the correction depends on the cause.

If the moisture at the die is too low, the pellet is brittle and dusty, and the moisture should be raised in a conditioning stage rather than in the hopper. If the moisture is too high, the pellet is soft and deforms, and the moisture should be reduced by drying rather than by running the die hotter.

If the die compression ratio is too low for the material, the pellet does not reach the density and the durability the material can achieve, and a die with a higher compression ratio should be tried. If the die is worn, the holes have lost their shape and the pellet is formed poorly, and the die should be replaced on the wear signals rather than on a fixed hour count.

If the roller gap is too wide, the material is not pressed firmly into the die and the pellet is formed weak, and the gap should be reset to the manufacturer’s specification on every roller. If the material has a low natural binding, the die specification should be confirmed in trial, and a binder or a blend may be needed.

Correcting the fines produced in the cooler

The cooler is the second most common source of fines, and the correction depends on the cooling pattern.

If the pellet is cooled too fast, the shell hardens before the core, and the pellet cracks or develops a rough, chipped surface. The cooling air should be reduced or the cooler residence time increased, and the pellet should be cooled gradually rather than shocked with cold air.

If the pellet is cooled too slowly, it stays warm and soft, and it deforms in the screen and the conveyor. The cooling air or the cooler residence time should be increased, and the pellet should be cooled to close to ambient before it is bagged.

If the cooler is overloaded, the pellet does not get enough residence time, and the cooling should be matched to the throughput. If the cooler air is humid, the pellet can pick up moisture as it cools, and the air should be dehumidified or the cooler should be operated differently.

Correcting the fines produced at the screen

The screen removes the fines that are already present, and a screen that is not doing its job lets them into the product.

If the screen is blinded, the openings are blocked and the fines pass over the screen into the product, and the screen should be cleaned or replaced. If the screen is undersized, it cannot handle the fines load, and a larger screen should be used. If the screen is overloaded, the fines are carried over the screen by the pellet flow, and the screen should be sized for the throughput rather than for the average.

The fines that the screen removes can be returned to the pellet mill where the die and the material allow it, but the recirculation ratio should be limited and confirmed in trial, because an excessive fines return can lower the pellet durability and raise the die wear. Where the fines are contaminated or the pellet specification is tight, the fines should be sold separately rather than returned.

Correcting the fines produced in handling

A pellet that is acceptable at the screen can break in the conveyor, the silo, the truck, or the bag, and the correction is in the handling rather than in the pellet mill.

The conveyor should be designed to avoid free fall and impact, and the transfer points should be fitted with breakers or cushions. The silo should be designed to avoid the pellet falling from a height, and the loading should be controlled. The bag should be filled gently, and the pellet should be protected from moisture pickup in storage.

A pellet that is brittle because it is too dry or because the die specification is wrong will break in handling even with a good handling design, and the pellet specification should be reviewed together with the handling.

How to measure and specify fines

The fines content should be measured on a sample that has been cooled, rested, and handled in the way the product will be handled, using a standard method and a defined screen size. The handling can be simulated by dropping the sample from a defined height or by tumbling it for a defined time, and the same method should be used for every sample so that the readings are comparable.

The specification should state the fines limit, the method, the sample point, and the handling simulation, because a fines limit without the method and the sample point is not a specification.

The fines content should be measured at the same points on each run, so that the readings are comparable, and the readings should be recorded with the moisture, the die hours, and the throughput, so that the cause of a change can be found.

What to check before buying a line

Before the line is specified, the fines risk should be addressed in the quotation. The quotation should state the die specification, the cooling arrangement, the screen size, the conveying design, and the fines limit and the method that will be used in the acceptance test. A line that is specified without the cooling and the screening scope is a line that can produce fines without a remedy.

If you are producing pellets with excess fines and need to trace the cause through the die, the cooling, the screening, and the handling, send the material, the moisture, the die specification, the cooling and screening arrangement, and the fines readings at each stage, and the Shandong Changsheng Machinery team can review the line configuration and propose the correction and the acceptance test.

Bottom line

Excess fines are produced at one of four points: at the die, at the cooler, at the screen, and in handling, and the cause is not always the die. The consequence is not only a quality number: the fines can block a boiler feed system, reduce the combustion efficiency, raise the dust explosion risk, and fail the customer’s fines limit. Sample the pellet at the die exit, at the cooler outlet, at the screen outlet, and in the bag, compare the fines at each point, and correct the stage that produces the most. The die fines are corrected with the moisture, the compression ratio, the roller gap, and the die condition; the cooler fines with the cooling rate and the residence time; the screen fines with the screen condition and the size; and the handling fines with the conveying and the storage. Measure the fines with a standard method at defined points and a defined handling simulation, record the readings with the moisture and the die hours, and specify the die, the cooling, the screening, the conveying, and the fines limit in the quotation before the order.