Setting Biomass Pellet Machine Temperature by Raw Material
Why Temperature Follows Lignin Content
The purpose of temperature control on a biomass wood pellet machine is to soften the lignin in the feedstock so that it becomes the binder that holds the pellet together. Lignin softens in a broad band from about 70 to 110 degrees Celsius, and the position of a given feedstock inside that band depends on its lignin content, hardness and particle size. Softwoods generally carry the most lignin and densify most easily; hardwoods carry less lignin but resist compression; agricultural residues vary widely and often contain ash-forming minerals that change friction inside the die. Setting a single temperature for every feedstock is the most common reason a pellet line fails to repeat a good result.
Temperature Matrix by Feedstock Group
| Feedstock group | Target die temperature | Behaviour notes |
|---|---|---|
| Softwood chips: pine, cedar, fir | 75 to 85 degrees Celsius | High lignin, good adhesion, easiest group to pelletise |
| Hardwood chips: oak, beech, ash | 90 to 105 degrees Celsius | Harder particles, needs more heat to soften lignin |
| Cereal straw and similar stems | 85 to 95 degrees Celsius | Lower bulk density, higher abrasive mineral content |
| Mixed or recycled wood residue | Start at 85 and confirm by trial | Variable lignin and contamination; set per batch |
For a softwood batch, set 80 degrees Celsius first, run a ten minute trial and judge the result. If the pellet surface is smooth and hardness is normal, keep the setting. If the pellet is loose and dull, raise the set value by 5 degrees Celsius and stabilise before judging again. Work up the range in small steps; jumping directly to the top of the band risks scorching the surface of the pellet.
Adjustment Procedure When the Feedstock Changes
Every feedstock change should follow the same sequence. Empty the pressing chamber of the previous material, set the temperature to the starting value for the new group, hold the die at that temperature for five minutes without feed so the die body and rollers are uniformly warm, then introduce the new material at low feed rate. Raise the feed rate gradually while watching current draw and pellet appearance. Record the final setting together with feedstock, moisture and particle size, because the same species from a different supplier behaves differently when its moisture and fines content differ.
Hardwood needs particular care. Too low a temperature produces pellets that are loose and crack in handling; too high a temperature carbonises the material at the die face, and the carbon layer raises friction and blocks the cavity entry. Both faults show up quickly at the ammeter, so treat the current reading as a control signal rather than as an alarm.
Moisture and Temperature Interaction
Temperature cannot compensate for poor moisture control. Feedstock held between 12% and 15% moisture responds predictably to temperature; material outside that band forces extreme settings that hide the real problem. Wet feed consumes heat as steam, so the die cools at the point where the material enters and the pellets leave with soft, ragged edges. Very dry feed generates dust instead of a plastic flow, and the fines burn at the die face.
Measure moisture instead of estimating it. The oven dry method of ISO 18134-1 and ISO 18134-2 gives a reproducible result and can be quoted in a feedstock specification; ash content is determined to ISO 18122 and calorific value to ISO 18125.
Die Speed, Compression and Heat Balance
Temperature is not produced by the heater alone. Most of the heat in the pressing chamber comes from friction between the material and the die wall, so die surface speed, die compression ratio and feed rate all shift the heat balance. A die with a long compression path generates more friction heat and needs less external heating for hardwood, while a short path on a light straw feedstock may need the heater to hold the target temperature. If the machine tends to overheat in summer and underheat in winter, adjust the set value with ambient temperature rather than changing die tooling.
Keep the roller clearance and cavity entry in good condition. A worn or glazed die changes friction, so a temperature that used to work no longer produces a sound pellet. When output quality changes without any change of feedstock, inspect the die before altering the temperature setting.
Verifying Output Against ISO 17225 Classes
Operating settings should be validated against measurable product quality. Wood pellets are graded under ISO 17225-2, which uses a moisture limit of 10% or less as received (M10), fines below 1.0% (F1.0), mechanical durability of at least 97.5% (DU97.5) and ash classes of 0.7%, 1.5% and 3.0% (A0.7, A1.5, A3.0). Nominal diameter is 6 mm plus or minus 1 mm (D06) or 8 mm plus or minus 1 mm (D08), with declared length from 3.15 mm to 40 mm. Non-woody pellets such as straw pellets are graded under ISO 17225-6. Durability is measured to ISO 17831-1 and dimensions to ISO 17829.
Take samples only when the machine is at steady state, after the die reaches full working temperature, so that results describe normal production rather than a cold start.
Frequently Asked Questions
Q: What is the correct temperature for hardwood pellets?
A: The working band is 90 to 105 degrees Celsius. Hardwood has less lignin and harder particles than softwood, so it needs more heat to soften the binder. Start at 95 degrees Celsius and adjust in 5 degree steps.
Q: Can the same setting be used for straw and for pine?
A: No. Straw is worked at 85 to 95 degrees Celsius and softwood at 75 to 85 degrees Celsius. Follow the matrix, hold the die at the new value for five minutes and confirm the result with a trial run.
Q: Why does the die temperature fall when feed starts?
A: Wet feed absorbs heat and the incoming material cools the die surface. Confirm that moisture is inside the 12% to 15% band and preheat for the full five minutes before feeding.
Q: Does die compression change the temperature setting?
A: Yes. A longer compression path raises friction heat, so a hardwood die may hold temperature with less heater input. Adjust the setting on measured results, not on the die specification alone.
Q: What pellet target should the setting achieve?
A: Aim for a pellet that meets the declared class of ISO 17225-2, typically durability of at least 97.5%, fines below 1.0% and moisture of 10% or less as received, tested to ISO 17831-1, ISO 17829 and ISO 18134-1.
