By Dr. Maja Berden Zrimec, researcher and content writer
Energy derived from biomass (bioenergy) is one of the most important renewable energy sources today. It is expected to play a major role in replacing fossil fuels in the global energy systems and reducing the greenhouse gas (GHG) emissions over the next decades (1). Wood is increasingly perceived as a green, renewable source of energy (2) and the industry for its processing is highly developed. Nevertheless, there is a large volume of forestry/wood residues remaining un-utilised. It is generally assumed that for every cubic meter of wood extracted from the forest, there is another cubic meter of forest residue or post-harvest waste (3–5).
Wood is already a basic energy resource for billions of people: one-third of households worldwide and two-thirds of those in Africa use wood as their main fuel for cooking, heating and boiling water (2). So called wood fuel (firewood, charcoal, pellets, wood gas, bio-oil, etc.) production is continuously rising and has reached almost 2 billion m3 in 2020 (6).
Forest or wood residues
From the global production of wood-derived biomass, 60% goes to energy generation, 20% to industrial ‘round wood’ and the remaining 20% is primary production loss that remains in-field to decay (3). The residues emerge at three stages: (1) Primary (forest) residues and waste is the residue that remains on site upon completion of harvesting for the roundwood (4) — some of it is collected for fuelwood, (2) Secondary residue is produced while processing the wood into products, while (3) Tertiary residues are formed after the end-use of wood products (1). Initial processing waste includes branch trimming and bark removal (about 12% of this material arrives at the mill), slabs/blocks/further trimmings (about 34%) and sawdust (about 12%) (3). After kiln drying, shavings (about 6%) and sawdust/trimming (about 2%) add to the total amount of waste (7). Through the value chain, app. 80% of forest tree mass is estimated to be lost as waste, with about 20% of the wood ending up in the form of kiln-dried sawn product (3).
Valorisation of the residues
Forest residuals have been traditionally used for the energy production and are still considered the largest and most important wood-based biomass source for biofuel production in the future (8). In addition, various low-volume, high-value bioproducts are becoming increasingly important to the industry. The market share of bioplastic has a great potential, and other end products are gaining interest as well, including bio-lubricants, bio-solvents, biosurfactants, enzymes, and biopharmaceuticals (8). In biorefinery approach, two trajectories are emerging: (1) gasification of biomass and biofuel production (diesel, ethanol), and (2) separation of products with high added value, the most potential biochemicals being polymers (8).
The decision on the most promising biomass conversion processes depends on many factors, including the type and quantity of available biomass, the desired end-uses, the relevant governmental policies, environmental standards and economic conditions, as well as project specific factors (9). Within the thermochemical conversion process options (combustion, pyrolysis, gasification and liquefaction), the combustion of residues in small-scale decentralized facilities can be considered the most promising present option for forest residues exploitation (9). Other options involve technological challenges and uncertain investment cost. Integrated solutions, such as anaerobic co-digestion of complementary substrates, can be appealing because of their energy potentials (9). Innovations are expected particularly in new extraction methods of hemicellulose or lignin, and in the development of new enzymes (8).
What is the hold-back?
The forestry industry has been slow to invest in technology and new market for valorising residuals due to weak market pull, high capital needs, and risk-adverse strategies among the few incumbent firms (8). That’s why the value chain for forest residues is still mainly hierarchical and rather undeveloped (8). Unpredictable and inconsistent policy landscape is the major barrier for the deployment of biorefinery technology (8). A challenge is also the lack of resources for investing in innovation of new products because most of the development is focused on the improvement of traditional effective and efficient harvesting, sawing of logs and decreased waste production (8,9). Establishing forest biorefineries also requires a different set of skills. One option is industrial symbiosis or eco-industrial parks where firms can reduce costs by cascading energy and utilising the by-products of other actors as well as avoiding transport costs (8). Luckily, more recently many new firms are competing for the biomass to valorise it into variety of products, thus value chains are shifting towards more circular and sustainable way, making the industry greener by cooperation across sectors.
- Thiffault E., Beaulieu L.D.: Using wood residues for energy as part of a bioeconomy transition. FAO.
- FAO (2022): Sustainable Forest Management (SFM) Toolbox.
- Tripathi N., Hills C.D., Singh R.S., Atkinson C.J. (2019): Biomass waste utilisation in low-carbon products: harnessing a major potential resource. Nature Partner Journals, Climate and Atmospheric Science; doi: 10.1038/s41612–019–0093–5
- Kuhn A.C. (2021): Forest residue and waste utilisation within a circular bioeconomy: Assessing non-energy utilisation alternatives in the Fort St. John timber supply area. MSc. Royal Roads University.
- Koopmans A., Koppejan J. (1997): Agricultural and Forest residues — Generation, Utilisation and Availability. FAO.
- FAOSTAT (2022): Forestry production and trade.
- FAO (2022): Global Forest Resources Assessments webpage.
- Gregg J.S., Happel M.K., Andersen N.S. Tanner A.N., Bolwig S., Klitkou A. (2020): Valorization of bio-residuals in the food and forestry sectors in support of a circular bioeconomy: A review. Journal of Cleaner Production 267: 122093. And the references within.
- d’Espiney A., Marques I.P., Pinheiro H.M. (2021): Case Study: Pathways from Forest to Energy in a Circular Economy at Lafões. In: Forest Biomas, Gonçalves A.C., Sousa C. & Malico I. (Eds.), InTech Open; doi: 10.5772/intechopen.93070