Sourcing energy wood from forests and recycling it back

Forest felling volumes and the use of wood in energy generation have recently attracted attention at the national level. Besides various carbon sink estimates, it is known that seedling stands and young forests are suffering from lack of tending. In addition, in the large population centres of the south of Finland, subtitutes are being sought for coal and other fossil fuels, forest industries are worried about pulpwood ending up in district heating boilers, and peat should be replaced by another domestic source of fuel.

At a regional level, the types of wood fuels used in heat and power plants depend on the economic cycle and structure of forests. The consumption of wood fuels follows the fluctuations in the outdoor temperature and rainfall in the peat production season. In an economic downturn, the need for wood harvested by means of thinning increases in energy generation as the by-products flow from the wood processing industry wanes and logging residues from regeneration felling are not available before.

Thinning improves the value of the growing stock and its durability against pests and diseases. Remote sensing methods can be used to locate untended stands, and absent forest owners can be activated and informed by various means. The main problem with the utilisation of untended stands is that small-diameter trees are expensive to harvest for energy use and, on the other hand, not all forest owners can afford to tend their sprawling stands into production condition.

Until now, the equation has appeared difficult to solve, but a wood harvesting innovation that works on a continuous basis can provide a solution to the problem of untended young stands, as at least part of the costs can be covered by revenue from energy wood sales. It is estimated that the machine is most effective in the tending of dense 5-8-metre seedling stands and young forests. Maximum productivity is achieved when stems to be removed can be harvested at their full length without having to cut them into shorter pieces.

In addition to security of supply, timeliness is emphasized in the sourcing of energy wood and the related supply chain. Energy wood stocks must be produced into chips at the right time to ensure the required chip quality, and the chips must be delivered to the application sites on-demand. In addition, storage sites must be accessible by road. Capital is tied up in stocks, and this, along with quality, sets out its own requirements for the rate of turnover of stocks, not to mention that the energy wood piles drying out at the mercy of the elements are at the same time being eaten away by storage rot.

Another challenge in the chip production process is caused by the uneven workload distribution. During frosty winter months, machinery and transport vehicles are constantly in a rush, while the problem during the summer months is a lack of work. The seasonal variation could be evened out by storing chips in stacks; however, this increases the self-ignition, as well as CO2 emissions and storage losses. Competition in the fuel market is fierce and it is not worth transporting chips over long distances. Compared to industrial timber, the transport distances for energy wood are clearly shorter and the aim is always to source wood from surrounding areas.

The by-product of the use of wood in energy generation is wood ash, which can be used to improve tree growth and carbon sequestration in peatlands. The use of ash as fertilizer reduces the amount of ash in landfill sites and thus contributes to the goals of circular economy. Ash fertilisation is specially used to compensate for the lack of phosphorus, potassium and trace elements in drained of forested peatlands. The positive impact of ash on tree growth starts to show more slowly than that of artificial fertilisers, but ash can produce additional growth for up to thirty years. The annual increase in growth in well-tended forests is estimated to be between 2 ad 4 cubic metres per hectare per year.

Author: Juha Laitila, Natural Resources Institute Finland