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A cracked surface on a biomass pellet is a quality failure that shows up after the die and is often blamed on the wrong stage. The crack can form in the die when the pellet leaves the hole, in the cooler when the surface contracts faster than the core, or after the bag when the pellet absorbs moisture, and the three have different causes and different corrections. The operator who sees the crack in the bag and adjusts the die usually misses the cooling or the storage, and the crack returns on the next batch.

This page is about the cracked surface on the finished pellet, while the squeeze test, the pellet after cooling, the density, the fines and the loose pellet are covered on their own pages with their own checks. It covers the difference between a crack that forms at the die, at the cooler, and in the bag, the difference between a surface crack and a through-crack, the crack direction as a diagnostic clue, the causes of each, how to diagnose which one is acting, and what to correct.

The short answer

A cracked surface on a biomass pellet comes from the moisture at the die, the die compression ratio, the cooling rate, the drying, the storage, or the handling, and the stage where the crack forms is the first thing to establish.

The practical rule: take a sample at the die exit, at the cooler outlet, and at the bag, and look for the crack at each point. A crack that is present at the die exit is a forming problem, a crack that appears at the cooler is a cooling problem, and a crack that appears in the bag is a storage or a moisture-pickup problem, and each is corrected in a different place.

Cracks at the die, at the cooler, and in the bag

A crack at the die exit is formed as the pellet leaves the die hole. The surface of the pellet is compressed against the die wall, and as the pellet exits it expands, and a material that is too dry or a die compression ratio that is too high for the material can produce a surface that tears or splits at the exit.

A crack at the cooler outlet is formed during cooling. The surface of the pellet cools faster than the core, the surface contracts before the core, and the difference in the contraction produces a tensile stress at the surface. The crack forms when the stress exceeds the surface strength of the pellet.

A crack in the bag is formed after the cooler. The pellet picks up the moisture in the bag or in the store, the surface absorbs the water and swells, and the swelling produces a crack or a surface that flakes, and the crack appears days after the production.

The three cracks look similar in the hand and are different in the line, and the sample point is what separates them.

A surface crack, a through-crack, and the crack direction

A surface crack stays in the outer layer and does not reach the core, and it usually comes from the drying or the cooling rate. A through-crack splits the pellet across and usually comes from a severe contraction difference or a weak compaction, and the two should be checked on a broken pellet rather than on the surface alone.

The crack direction is a second clue. A longitudinal crack along the pellet axis usually points to the die exit and the expansion. A transverse or ring crack usually points to the cooling contraction. A network of fine cracks usually points to the drying that is too fast. The direction is read on the sample at the earliest stage where the crack appears, and it is combined with the sample point rather than used on its own.

The causes and how to tell them apart

Cause	What points to it	What to check
Moisture at the die too low	The pellet surface is rough or torn at the die exit, and the fines are high	The moisture at the die inlet, not at the dryer outlet
Die compression ratio too high	The pellet is dense but brittle, and the surface splits at the exit	The die specification and its definition against the material
Cooling too fast	The crack appears at the cooler outlet, and the surface is hard with a soft core	The cooling air, the residence time, and the pellet at the cooler outlet
Drying too fast or too hot	The surface is hard and the crack appears at the cooler or in the bag	The dryer temperature and the residence time
Moisture pickup in the bag or the store	The crack appears days after the production, and the bag shows condensation	The pellet temperature at the bag, the bag specification, and the store
Handling and conveying	The crack appears at the bag on the pellets that were dropped or conveyed	The conveyor, the transfer points, and the drop height

Moisture at the die

The moisture at the die sets the surface quality, and a material that is too dry produces a surface that is rough or torn at the exit.

A moisture that is too low leaves the material without the plasticising effect that helps the surface deform as the pellet leaves the hole, and the surface tears or splits. A moisture that is too high produces a soft pellet that deforms rather than cracks, so the crack at the die exit points to the dry side rather than to the wet side.

The moisture should be measured at the die inlet, not at the dryer outlet, and the target should be confirmed in a trial on the actual material.

Die compression ratio

The die compression ratio sets how much pressure the material meets, and a die that is too high in compression for the material produces a dense but brittle pellet that splits at the exit.

A die with a high compression ratio is used for a dense material that needs the pressure, and a die with the same ratio on a softer material produces a pellet that is over-compressed and cracks. The die should be matched to the material, and the compression ratio definition should be compared with the effective working length rather than with the ratio number alone.

Cooling rate

The cooling rate is the most common cause of the crack that appears at the cooler, and the crack is the result of the surface contracting before the core.

The cooling air should be reduced or the residence time increased so that the pellet is cooled gradually, and the pellet temperature at the bag is the field check. A pellet that is cooled too fast develops a hard shell and a soft core, and the crack is the visible result.

Drying

The drying affects the surface through the temperature and the rate, and a dryer that is too hot or too fast produces a hard surface that cracks later.

The dryer temperature and the residence time should be set for the material rather than for the maximum output, and the material should be checked for the surface hardness and the crack after the cooler. A dryer that scorches the surface produces a pellet that looks sound at the die and cracks at the cooler or in the bag.

Moisture pickup and storage

The crack that appears days after the production is a moisture pickup problem, and the bag and the store are the cause.

The pellet should be cooled to close to ambient before the bagging, the bag should be moisture-resistant, and the store should be dry. The bag that shows the condensation is the bag that is producing the crack, and the pellet temperature at the bag and the store condition are the check.

Handling and conveying

A pellet that is sound at the cooler and cracked at the bag is a pellet that has been damaged in the conveying or the bagging, and the crack is the result of the impact rather than of the moisture.

The conveyor and the transfer points should be checked, and the drop height should be reduced and the cushions fitted at the points where the pellet falls. A pellet that is brittle because it is over-dried or because the die is too high in compression breaks more easily, and the pellet specification should be reviewed together with the handling.

How to diagnose a cracked surface

The diagnosis starts from the sample points and moves from the bag back through the line.

The first step is to take a sample at the die exit, at the cooler outlet, and at the bag, and to look for the crack at each point. The crack that is present at the earliest point is the one that is forming in that stage, and the later cracks may be the same crack that has grown rather than a new one.

The second step is to check the surface crack against the through-crack, and the crack direction, on the sample at the earliest stage.

The third step is to run the checks in the table above in order, and to compare each value with the record from the previous good run.

The fourth step is to check the bag and the store for the condensation and the moisture, and the conveyor and the transfer points for the impact, if the earliest crack is at the bag.

What to correct and in what order

The correction follows the earliest sample point where the crack appears, and it is made one variable at a time with the pellet checked after each change.

If the crack is at the die exit, correct the moisture at the die first, then the die compression ratio, then the die condition. If the crack appears at the cooler, correct the cooling rate first, then the dryer temperature and the residence time. If the crack appears in the bag, correct the bagging temperature and the bag specification first, then the store, and then the handling.

Changing the moisture, the cooling, and the bagging at the same time makes the result impossible to attribute, and the next batch starts from an unknown baseline.

What to specify before buying a line

Before the line is specified, the surface quality requirement should be stated in the quotation and in the acceptance test, together with the material, the moisture, the pellet diameter, the test method, and the sample point. A surface requirement without the method and the sample point is not a specification, and a line that is specified without the surface requirement can be built and still produce the cracked surface.

If you are producing pellets with a cracked surface and need to trace the cause through the moisture, the die, the cooling, the drying, the storage and the handling, send the samples from the die exit, from the cooler outlet and from the bag, the moisture at the die, the die specification, the cooling and drying settings, the bag specification and the store condition, and the Shandong Changsheng Machinery team can review the line configuration and propose the correction and the acceptance test.

Bottom line

A cracked surface on a biomass pellet is formed at one of three stages, and the sample point is what separates them. A crack at the die exit is a forming problem in the moisture or the die, a crack at the cooler outlet is a cooling or a drying problem, and a crack in the bag is a moisture pickup or a handling problem. Separate a surface crack from a through-crack and read the crack direction at the earliest stage, because a surface crack and a ring crack point to the drying and the cooling, while a through-crack and a longitudinal crack point to the compaction and the die. Take a sample at each stage, find the earliest point where the crack appears, and correct the stage that produces it rather than the stage where the crack is noticed. Correct one variable at a time and check the pellet after each change, because changing the moisture, the cooling, and the bagging together makes the result impossible to attribute. State the surface requirement with the method and the sample point in the quotation and in the acceptance test, and trial the pellet on the actual material before shipment.