The role of natural resins and lignin in wood pellets

Lignin, resins, extractives

Three terms that are often confused:

• Lignin — a natural polymer in the cell walls of wood, present in both softwood and hardwood. It stiffens the wood and — importantly for pellets — softens at elevated temperatures.
• Resins — substances produced mainly by coniferous trees (including pine and spruce) and stored in resin ducts.
• Extractives (Compounds that can be extracted from wood with a solvent: resins, fatty acids, terpenes, tannins and others.)Compounds that can be extracted from wood with a solvent: resins, fatty acids, terpenes, tannins and others. — a broader group that includes resins, but also, for example, the tannins found in oak.

What binds a pellet

During pelletising, the wood meal is forced through the holes of the die. Friction and pressure raise the temperature, the lignin softens and binds the particles, and once cooled the pellet hardens. This is why conditioning (steam or water before pressing) and cooling after pressing are so important — more in the article how wood pellets are made.

This leads to an important conclusion: the strength of a pellet is determined primarily by how the process is run — particle size, the moisture content of the raw material before pressing, the temperature and the degree of compression in the die — not by the species itself. Softwood and hardwood raw materials require different settings, but either can be made into durable or brittle pellets.

Effect on calorific value

Softwood has a slightly higher calorific value on a dry basis than hardwood. In the VTT data for stemwood: Scots pine 19.31 MJ/kg, Norway spruce 19.05 MJ/kg, downy birch approx. 18.6–18.7 MJ/kg; the authors link this difference to the higher lignin and resin content of softwood^[44]. The typical values cited by FAO point in the same direction (on a dry, ash-free basis: softwood 19.2, hardwood 19.0 MJ/kg)^[45].

A difference of a few per cent between species on a dry basis is real, but moisture can easily “eat it up”: every percentage point of water lowers the net calorific value as received. This is why we compare pellets by the as-received result in the test report, not by the wood species.

Additives instead of lignin?

When the raw material binds less easily, the producer may use an additive. ENplus allows additives of agricultural or forest origin — such as starch, maize or potato flour, vegetable oil or lignin from the kraft (sulphate) process — up to a total of 2.0% by mass, of which no more than 1.8% during production and no more than 0.2% as post-production additives (e.g. coating oils)^[1].

Myths about resin