Differences in how wood pellets burn

Fuel, boiler, settings

How combustion proceeds depends on three things at once: the fuel, the burner design and the settings (dose, feed time, air supply). When users say that “these pellets burn worse”, it is usually the fuel that has changed while the settings are still tuned to the previous one. This is why comparing pellets means looking at the parameters in the test reports, not just at the impression from a single season.

Volumetric dosing

Most auger feeders measure fuel by volume — a set number of revolutions in a set time. The boiler, however, needs a set amount of energy. Two parameters stand between volume and energy:

• bulk density (The mass of loose fuel in one cubic metre, including the spaces between the pellets.)The mass of loose fuel in one cubic metre, including the spaces between the pellets. — ENplus allows a range of 600–750 kg/m³^[1]. For the same volume, pellets at the upper limit weigh 25% more than those at the lower limit (calculation: 750 / 600 = 1.25);
• net calorific value as received — a minimum of 16.5 MJ/kg for all ENplus classes^[1], although actual results are often noticeably higher.

As a result, the same feeder setting can deliver noticeably different amounts of energy to the burner. Too large a dose means a high burner temperature, more unburnt fuel and a greater risk of clinker; too small a dose means a weaker flame and lower output.

Ignition and flame stability

Ignition and flame stability are most strongly affected by moisture content and fines. Damp fuel first has to evaporate its water, which uses up energy and cools the combustion zone. Dust changes the dose (fine particles fill the spaces between the pellets), and some of it is carried out of the burner by the air stream.

The wood species matters less here than is often believed. Softwood has a slightly higher calorific value on a dry basis than hardwood — according to Finnish data, for example, pine 19.31 MJ/kg, spruce 19.05 MJ/kg and birch approx. 18.6–18.7 MJ/kg — which the authors link to a higher content of lignin and resins^[44]. The observation by users that softwood pellets “light more easily” is often heard, but we have not found a study that measured this under comparable conditionsData require verification: No source measuring the ease of ignition of softwood versus hardwood pellets.

Ash and clinker

These are two different matters: how much ash is produced and how that ash behaves at high temperature. The amount is described by the ash content (A1: ≤ 0.70% on a dry basis)^[1]. The behaviour is described by the ash deformation temperature, determined according to ISO 21404 — at least 1200 °C for A1^[1],[24].

Clean stemwood usually contains little ash — typically approx. 0.3% (range 0.2–0.5%) on a dry basis for both softwood and hardwood, according to the typical values cited by FAO^[45]. At the same time, the ash deformation temperature of pine sawdust in the VTT data was 1150 °C^[44] — below the A1 minimum. This shows that clinker is decided by the specific raw material and its contamination, not by the “purity” claimed on the bag.

Symptoms and possible causes

Changing fuel in practice

We explain how to compare softwood and hardwood pellets without preconceptions in the softwood or hardwood comparison, and you will find a buying checklist in the guide how to choose wood pellets.