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A pellet that burns poorly is not always a bad pellet. It can be a good pellet that is wet, a good pellet that is the wrong size for the appliance, a good pellet that has broken down into fines in the bag, or a pellet that was made from a feedstock the appliance was not designed for. The fault can sit in the pellet, in the storage, in the appliance, or in the way the appliance is run, and the first step is to find which of those four is the cause rather than to change the die.

This page covers the causes of poor combustion in biomass pellets, how to tell the causes apart, how to diagnose whether the fault is in the pellet, the storage, the appliance, or the operation, and what can be corrected in the pellet and what cannot. Shandong Changsheng Machinery supplies biomass pellet mills and complete lines to overseas buyers, and the guidance below reflects how combustion complaints are traced back to the pellet specification and where buyers most often misread them.

The short answer

A biomass pellet burns poorly for one of four reasons: the moisture is too high, the ash is too high or the ash fusion temperature is too low for the appliance, the fines content is too high because the pellet is weak or has broken in handling, or the pellet is the wrong size, length, or density for the appliance’s feed system and air supply.

The practical rule: check the appliance and the storage first, test the pellet for moisture, ash, ash fusion, fines, density, diameter, and length, and trace the parameter that is outside the appliance’s specification back to the mill. The moisture, the fines, the density, and the length can be corrected at the mill; the ash and the ash fusion cannot; and the appliance and the storage are corrected at the site.

What poor combustion looks like

Poor combustion shows up in different ways, and the symptom points to a different cause.

A fire that is slow to light and produces a lot of smoke at start-up usually points to a high moisture or a high fines content, and the two are separated by the moisture and the fines test on a bag sample. A high moisture pellet usually produces smoke and a fire that struggles to stay alight, while a high fines pellet usually produces sparks, a fast-burning fire, and dust in the feed.

A fire that burns with a lazy flame, a low heat output, and a heavy ash deposit usually points to a high moisture, a high ash, or a low density.

A fire that goes out in the feed auger, or a feed system that jams, usually points to a pellet that is too large or too long, or to a fines content that has compacted in the auger.

A fire that produces a lot of clinker or slag usually points to a low ash fusion temperature, and that is a feedstock property rather than a pellet mill fault. A pellet with a moderate ash can still form clinker if the ash fusion temperature is low, so the ash composition should be tested alongside the ash percentage, and the clinker problem should be checked against the ash fusion temperature rather than the ash content alone.

A fire that produces a lot of dust and sparks usually points to a high fines content, and the fines come from a weak pellet or from handling damage.

Causes in the pellet: moisture

The moisture of the finished pellet is the most common cause of poor combustion, and it is the easiest to correct at the mill and in the storage.

A pellet that leaves the cooler warm and is bagged before it has cooled can carry the moisture that condenses inside the bag, and the pellet that reaches the appliance is wetter than the pellet that was tested at the mill. A pellet that is stored in a humid place, or in a bag that is not moisture-resistant, picks up moisture in storage, and the same pellet that burned well in the first week burns poorly in the third.

The moisture should be measured on a sample taken from the bag or the silo, not only from the mill outlet, and the storage should be dry and the bag should be sealed.

Causes in the pellet: ash and ash fusion

The ash is set by the feedstock, and the pellet mill cannot remove it. A pellet that is above the ash limit of the appliance will burn poorly regardless of how well it was made, and the fault is in the feedstock and the market match rather than in the mill.

The ash fusion temperature is a separate property from the ash content. A pellet with a moderate ash can still form clinker if the ash fusion temperature is low, and a pellet with a high ash can burn cleanly if the ash fusion temperature is high enough for the appliance. The two should be tested together, and the clinker problem should be checked against the ash fusion temperature rather than the ash content alone.

A high ash reduces the heat output per unit of weight, it increases the ash handling, and it can produce clinker in an appliance that was designed for a low-ash pellet. The ash should be tested on the pellet, compared with the appliance’s limit, and the appliance and the pellet should be matched before the fuel is blamed. The appliance’s ash limit is in the manufacturer’s manual or in the local emission rules for the appliance, and it should be obtained from the appliance supplier or the installer rather than assumed.

Causes in the pellet: fines

Fines are the small particles that break away from the pellet, and a high fines content causes poor combustion because the fines burn too fast, block the air passages, and jam the feed auger.

The fines come from a weak pellet, from a pellet that is too dry or too brittle, from the cooling, and from the handling. A pellet that is acceptable at the mill can break down in the bag, the auger, or the silo, and the fines that reach the appliance are the result of the whole chain rather than of the die alone.

The fines should be measured on a sample taken from the bag or the silo, and the cause should be traced through the die, the cooling, the screening, and the handling.

Causes in the pellet: density, diameter, and length

The density, the diameter, and the length of the pellet set how the appliance feeds and how the air reaches the fuel.

A pellet that is too large or too long for the appliance’s feed system can jam the auger or the feed chute, and the appliance starves for fuel. The length is set by the cutter at the die face, and a length above the appliance’s specification can jam the auger even when the diameter is correct, so the length should be checked on the bag sample together with the diameter.

A pellet that is too small or too soft can fall through the grate or compact in the auger. The pellet diameter and the length should match the appliance’s specification, and a pellet that is outside that specification will burn poorly even when it is well made.

A low bulk density means fewer tons of energy in the same volume, and it also changes how the fuel forms a bed in the appliance, so the air distribution and the combustion can be affected even when the pellet itself is acceptable. A pellet that is too dense can affect the air distribution in the burn pot, but the jam in the auger is normally caused by the size and the length rather than by the density alone.

Causes in the storage and the handling

A pellet that was good at the mill can burn poorly after storage, and the storage and the handling should be checked before the mill is blamed.

The moisture pickup in storage is the most common, and it is caused by a warm pellet that is bagged early, a bag that is not moisture-resistant, a humid store, or a silo that is not sealed.

The fines from handling are the second, and they are caused by free fall, by rough conveying, and by repeated handling.

The contamination in storage is the third, and it comes from a dirty silo or a bag that has been opened and left open.

Causes in the appliance and the operation

The appliance and the way it is run can produce the same symptoms as a bad pellet, and they should be checked before the fuel is blamed.

The air supply and the fuel feed rate should be set for the fuel, and a low air setting with a high feed rate produces smoke and a lazy flame. The flue and the heat exchanger should be clean, because a blocked flue reduces the draught and the combustion. The grate and the burn pot should be clean, because a blocked grate prevents the air from reaching the fuel. The appliance should be set for the pellet diameter and the pellet quality that it was designed for, because an appliance that is set for a low-ash premium pellet will not run well on a high-ash industrial pellet.

How to diagnose the cause

The diagnosis moves from the outside in, rather than from the mill out.

The first step is to check the appliance. Confirm the air setting, the feed rate, the flue condition, the grate, and the burn pot against the manual. If the appliance is not set correctly, the fault is in the operation.

The second step is to check the storage and the handling. Take a sample from the bag or the silo, not from the mill, and measure the moisture and the fines. If the sample is wetter or dustier than the mill sample, the fault is in the storage or the handling.

The third step is to test the pellet. Measure the moisture, the ash, the ash fusion, the fines, the density, the diameter, and the length, and compare them with the appliance’s specification and with the market requirement. The parameter that is outside the specification points to the cause.

The fourth step is to trace the cause back to the mill. If the moisture, the fines, the density, or the length is outside the specification at the mill, the cause is in the mill and the cooling, and the mill should be corrected. If the ash or the ash fusion is outside the specification, the cause is in the feedstock, and the mill cannot correct it.

What can be corrected and what cannot

The moisture, the fines, the density, and the length can be corrected at the mill, in the cooling, in the die, and in the cutter, and the correction should be made one variable at a time with the pellet tested after each change.

The ash and the ash fusion cannot be corrected at the mill, because they are set by the feedstock. The options are to change the feedstock, to blend it with a lower-ash material, or to change the appliance or the market to one that accepts the ash.

The diameter and the length can be corrected at the mill by changing the die or the cutter, and they should be matched to the appliance rather than set by the mill.

The appliance and the storage can be corrected at the site, and they should be checked before the mill is changed.

What to specify before buying a line

Before the line is specified, the combustion requirement should be stated in the quotation and in the acceptance test. The quotation should state the moisture, the ash, the ash fusion, the fines, the density, the diameter, and the length that the pellet will meet, and the appliance or the market specification that those figures are matched to. A pellet that is specified without the combustion requirement can be well made and still burn poorly.

If you are producing pellets that burn poorly and need to trace the cause through the moisture, the ash, the ash fusion, the fines, the density, and the appliance, send the pellet test results, the appliance specification, the storage conditions, and the operating settings, and the Shandong Changsheng Machinery team can review the line configuration and propose the correction and the acceptance test.

Bottom line

A pellet that burns poorly is not always a bad pellet, and the fault can sit in the pellet, in the storage, in the appliance, or in the operation. Check the appliance and the storage first, then test the pellet for the moisture, the ash, the ash fusion, the fines, the density, the diameter, and the length, and trace the parameter that is outside the specification back to the mill. The moisture, the fines, the density, and the length can be corrected at the mill, the ash and the ash fusion cannot, and the appliance and the storage are corrected at the site. A high moisture pellet usually produces smoke and a struggling fire, a high fines pellet usually produces sparks and a fast-burning fire, and a moderate ash pellet can still form clinker if the ash fusion temperature is low, so the two pairs should be tested and separated before the fuel is blamed. State the combustion requirement with all the parameters in the quotation and in the acceptance test, and match the pellet to the appliance rather than to a general market average.