A study from the University of Zagreb was recently published in the scientific journal Drvna Industrija, in which researchers investigated new methods for making wood pellets suitable as a coal substitute. The reason is clear: many power plants and industrial facilities still use coal, while climate goals demand rapid alternatives. Conventional wood pellets can help, but often lack the necessary properties. Therefore, the Croatian team focused on techniques to improve this biomass.
Led by Marin Dujmović of the Faculty of Forestry and Wood Technology, the researchers compared two thermal pretreatment methods: torrefaction and steam explosion. Both processes give wood pellets a higher energy density and make them more resistant to storage problems. This brings the properties closer to those of coal, making it possible to continue using existing facilities with minimal modifications.
Torrefaction heats biomass at temperatures between 200 and 300 degrees Celsius in an oxygen-poor environment. The result is a drier, more homogeneous material that is easier to grind and burns more efficiently. The steam explosion works differently: steam is injected into the pellets under high pressure and temperature, after which a sudden drop in pressure breaks open the cell structures. This also results in a pellet with a higher energy content and improved combustion characteristics.
Measurable benefits of the treatments
Remarkably, torrefaction can halve the moisture content of wood pellets, for example, from 10 to 5 percent, while increasing the calorific yield from an average of 18 to over 25 megajoules per kilogram. Steam explosion increases the bulk density of the material from 600 to over 700 kilograms per cubic meter, making energy storage and transport more efficient. Both methods also improve the grindability of the pellets, saving energy during grinding into powder for combustion.
According to the researchers, the increased energy density is particularly important. This means that less volume is required for the same energy output. These are significant advantages for logistics and storage, especially on an industrial scale. The pellets also improve their water resistance, making them more suitable for longer storage periods.
The application of these techniques isn't new, but the Croatian research confirms their potential in real-world situations. Yet, there are challenges. Investments in new pretreatment facilities are required, and producers must anticipate higher production costs. Whether this outweighs the benefits depends on local conditions and energy prices.
International interest is growing in so-called second-generation biomass, which converts waste streams and non-edible crops into high-quality fuels. The Zagreb study aligns with this trend and offers opportunities for further scaling up. If costs can be reduced and sufficient suitable raw materials can be gathered, optimized pellets could become a realistic alternative to coal in industry and energy generation.
The publication in Drvna Industrija makes it clear that the technological foundation is in place. The next step lies with policymakers, energy companies, and investors who must facilitate the transition.
Original study in Drvna Industrija: Thermal Pre-treatments of Woody Biomass: A High-Level Overview
