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High ash in biomass pellets is not a defect that the pellet mill can fix. The ash is set by the feedstock and by the contamination that enters the line with it, and the pellet mill can only compact what it is fed. What the mill can do is handle a high-ash material with the right die, the right feeder and the right wear package, and what the project can do is measure the ash early, screen the material against the end-market limit, and price the ash into the product instead of discovering it at the customer.

This page is about the ash of the finished pellet and the market that will accept it, while the assessment of a high-ash feedstock and the configuration of the line that runs it are covered separately. It covers what ash means for a biomass pellet, the causes of high ash in the feedstock and in the contamination, how to test the ash and the silica, which markets can accept higher ash, and how to correct or accept the ash. Shandong Changsheng Machinery supplies biomass pellet mills and complete lines to overseas buyers, and the guidance below reflects how ash problems are traced in service and where projects most often misjudge them.

The short answer

The ash of the finished pellet is the ash of the dry feedstock, and blending changes it in proportion to the blend ratio. The pellet mill cannot remove it, and the only ways to lower the ash of the product are to clean the feedstock of soil and sand, to blend it with a lower-ash material, or to change the feedstock.

The practical rule: test the ash and the silica on the actual material, check it against the ash limit of the end market in writing, and decide whether the cleaning or the blending can bring it below the limit before the equipment is specified. Residential heating appliances usually apply a strict limit and will reject a high-ash pellet, industrial boilers accept a higher limit with ash handling, and a pellet whose ash is well above the range accepted by the target market is the signal that the material is not suitable for that market. No equipment specification will change that.

What high ash means for a biomass pellet

Ash is the mineral residue left after combustion, and it comes from two sources that behave differently in the line: contamination from soil and sand picked up during collection and handling, and mineral matter bound in the plant structure, of which silica is the most important for wear. The first can be reduced by cleaning; the second cannot.

High ash matters in four places. It sets the ash of the finished pellet, and a pellet above the market limit can be rejected or downgraded. It accelerates the wear on the die, the rollers, and the hammer mill, because the silica and the mineral matter are abrasive, and the wear rate depends on what the ash is made of, not only on how much there is. It changes the combustion behaviour, because a high-ash fuel produces more ash in the boiler and affects the ash handling and the maintenance interval. And it is often accompanied by chlorine, potassium, and sodium, which affect the corrosion and the fouling in the boiler.

A high-ash material often carries a lower lignin and a lower binding than a clean wood, and the pellet durability can fall as the ash rises, so the durability should be tested alongside the ash rather than assumed to be unaffected.

Causes of high ash in the feedstock

The feedstock sets the ash, and the species and the plant part determine how much mineral matter is bound in the material.

Straw, rice husk, and corn stover carry a higher ash than clean wood, and the rice husk is also high in silica. The bark carries a higher ash than the wood, and a feedstock with a high bark content produces a higher-ash pellet. The energy crops and the agricultural residues each have their own ash profile, and the profile should be tested rather than assumed from the material category.

The soil and the sand are the second source, and they enter with the material during collection, storage, and handling. A material that is stored on the ground, or collected with the soil, carries a higher ash than the same material stored under cover. A destoner or a similar cleaning stage removes a substantial part of this contamination, and the achievable reduction should be confirmed by testing the cleaned material rather than assumed from the equipment specification.

Causes of high ash in the contamination

The contamination is the part of the ash that the project can control, and it should be the first target because the cleaning is cheaper than changing the feedstock.

Soil and sand from the storage and the handling. A covered storage and a paved yard reduce the soil picked up with the material, and a destoner removes the soil that is already in it.

Stones and metal from the collection. A magnetic separator removes the ferrous metal, and a screening stage removes the stones, and both protect the die and the hammer mill as well as reducing the ash.

Bark from the debarking and the collection. A high bark content raises the ash, and the bark should be separated where the market requires a low-ash pellet.

How to test and screen the ash

The ash should be tested on a representative sample of the material as received, not on a dried or cleaned subsample, and the sample should be taken across several batches because the ash can vary with the source and the season.

The silica is part of the ash composition, and it is normally determined from the ash analysis rather than from a separate sample, so the ash composition should be requested from the laboratory together with the ash percentage. The abrasiveness of the ash depends on what it is made of, and a high ash that is mostly soft mineral matter is a different problem from a lower ash that is mostly silica. The chlorine, potassium, and sodium content should be tested as well, because those elements determine the corrosion and the fouling in the boiler and often set the binding constraint in the market.

With the results, the material can be screened against the end-market specification. The ash limit differs substantially between markets, and it cannot be replaced by a general figure, because it depends on the boiler design, the emission rules in the destination country, and the individual customer’s acceptance specification. Residential heating appliances usually apply strict limits and will not accept a high-ash pellet. Industrial boilers and power generation can accept higher ash but need ash handling and may have their own limits for slagging and fouling. Export markets have their own specifications, which vary by destination.

Where the written specification is not yet available, the market category (residential, industrial, export) gives a direction, but the written limit from the end user is the only figure that governs the acceptance, and it should be obtained before the equipment is specified.

Which markets can accept higher ash

The ash of the finished pellet determines which markets will accept it, and the market should be decided before the equipment, because it sets the ash limit the project has to meet.

Residential heating appliances usually have strict ash limits and will not accept a high-ash pellet, and the residential market is normally closed to a high-ash feedstock.

Industrial boilers and power generation can accept higher ash, but they need the ash handling and they may have their own limits for slagging and fouling. The co-firing with coal is a route that some high-ash pellets take, and the acceptance depends on the boiler and on the local rules.

The export market depends on the destination specification. Some destinations accept a higher-ash industrial pellet, and some apply a strict limit that a high-ash feedstock cannot meet. The buyer should confirm the destination specification before the line is specified, rather than after the first batch is produced.

How to correct or accept the ash

The two ways to reduce the ash of the product, other than changing the feedstock, are cleaning and blending.

Cleaning removes the soil, the sand, and the stones, and it can reduce the ash of a contaminated feedstock substantially. It does not remove the silica that is bound in the plant structure, so it has a limit. A destoner and a screening stage are the usual cleaning equipment, and the achievable ash reduction should be confirmed by testing the cleaned material.

Blending with a lower-ash material reduces the ash of the product in proportion to the blend ratio. The blend ratio is limited by the ash limit of the market and by the effect of the other component on the binding, the moisture, and the cost, and the ash of the blend can be estimated by arithmetic, but the durability and the handling behaviour of the blend should be confirmed by trial.

Where neither the cleaning nor the blending can bring the ash below the limit, the material is not suitable for that market, and the options are to change the market to one that accepts the ash, to accept the price the market offers for a high-ash pellet, which is usually lower than the low-ash price but depends on the market and the contract, or to change the feedstock. The equipment specification cannot change the ash.

What to specify before buying a line

Before the line is specified, the ash should be addressed in the quotation. The quotation should state the ash and the silica of the material as tested, the ash limit of the end market in writing, the cleaning arrangement, and the ash and the durability that will be measured in the acceptance test. A line that is specified without the ash limit and the cleaning arrangement is a line that can produce a high-ash pellet without a remedy.

If you are working with a high-ash feedstock and need to test the ash and the silica, screen it against the end-market limit, and specify the die and the wear package for the abrasiveness, send the material data, the target capacity, the pellet diameter, and the intended market, and the Shandong Changsheng Machinery team can review the line configuration and propose the cleaning arrangement and the acceptance test.

Bottom line

High ash in biomass pellets is a feedstock property that the pellet mill cannot change, and the ash of the pellet is the ash of the dry feedstock. Test the ash, the silica, the chlorine, and the alkali content on the actual material, obtain the ash limit in writing from the end user, and decide whether the cleaning or the blending can bring the ash below the limit before the equipment is specified. Cleaning removes the soil and the sand but not the silica bound in the plant structure, and the blending reduces the ash in proportion to the blend ratio, and both should be confirmed by trial. The durability can fall as the ash rises, so it should be tested alongside the ash. Where neither the cleaning nor the blending can bring the ash below the limit, the material is not suitable for that market, and the options are to change the market, to accept the price the market offers for a high-ash pellet, or to change the feedstock. Specify the ash, the silica, the cleaning arrangement, and the acceptance test in the quotation, and confirm the market limit before the order is placed.