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Forestland waste and residues from logging operations and forest thinning are readily available feedstocks for second generation biofuels

Summary​

Forest waste and residues serve as a significant feedstock for second generation biofuels, primarily consisting of lignocellulosic material from logging operations and forest management. The largest source comes from logging residues including tree tops, branches, and dead trees left after conventional logging operations. Environmental considerations are crucial, as forest residues play important roles in ecosystem health, nutrient cycling, and biodiversity, with guidelines recommending retaining 30%–50% of residues to maintain soil health and habitat. Urban wood waste from construction, demolition, and municipal solid waste provides additional potential biomass. Collection faces challenges including difficult terrain access, specialized equipment requirements, and economic feasibility which is dependent on biomass pricing and potential government subsidies for fire prevention.

Forest wastes and residues have been proposed as a major feedstock for the production of second generation biofuels. As with most second generation biofuel feedstocks, forest residues tend to consist primarily of lignocellulosic material which is high in cellulose, hemicellulose, and lignin. However, in contrast to agricultural waste and residues, classifying forest residues as a 'waste' product is more contentious as it serves an important function in the health of the ecosystem. Conventional biomass removals from forestland in the United States already sits around 320 million dry tons per year, with 70% made up of roundwood removal and the remainder being logging residues and other removal 1.

Forest residues in the United States​

When forest and logging residues are considered as a resource, they offer a large potential for feedstock for second generation biofuels. The largest source of currently available forest residues comes from the leftover residues of conventional logging operations 2. Logging residue material consists of tree tops, branches, dead trees, and other debris which is usually left on the site of the logging operation 1. Although not a residue, pre-commercial thinning of small diameter trees can also provides a source of biomass feedstock 2. In addition to logging residues, forest thinning offers another source of biomass feedstock whilst assisting in reducing fire risk. Due to the successful prevention of forest fires and the unfeasibility of conventional logging operations in certain areas located in the western regions of the United States, biomass can build up in areas to a point where it is overstocked and becomes a fire hazard 1.

Outside of raw forest residues that are located on the forest floor, milling residues provide another source of timber biomass that could be utilized for biofuels. A portion of mill residues are already utilized for production of pulp and paper products, provide heat for drying, and for combustion to produce on-site electricity. However, this utilization generally lies around 60% of the total mill residue generated, leaving the remaining 40% as a potential source of biofuels 1.

Urban wood wastes​

Outside of the country's forests, urban wood wastes offer an additional source of potential biomass feedstock. Although not considered a forest resource, the fact that it is timber means that it would need to undergo similar logistical and processing steps as forest residues. Urban wood waste can come from construction and demolition (C&D), as well as the wood portion of municipal solid waste (MSW) from objects such as discarded furniture or wooden packaging. As it stands, yard trimmings make up a significant portion of MSW, estimated to be around 13% of all MSW generated, of which a significant portion is recoverable 1. The wooden components of packaging and durable goods make up the second largest portion of municipal solid waste, with around 10% already recovered for recycling and 22% combusted for energy 1.

Environmental considerations​

Environmental concerns regarding the removal of forest residues primarily revolve around damage to the ecosystem, and losses of nutrients and soil organic carbon. As with other residue materials, removal of forest residues pose very little risk of causing indirect land use change (ILUC), except in the case where market conditions shift the conventionally utilized timber into biomass energy production, which would require additional harvests 1.

Logging residues tend to contain a disproportionately higher concentration of nutrients compared to the main tree stem (known as the bole). Studies have shown that removal of whole Douglas-fir trees (including branches and tops) leads to a 16% increase in biomass, but also a 65% and 83% increase in the removal of nitrogen and potassium respectively 1. However, it should be noted that in commercial logging operations, fertilizer is already applied to promote forest growth and to address existing nutrient deficiencies 1.

With regards to ecosystem and biodiversity, fallen and dead woody material provides important habitat and ecological functions for a range of species such as birds and insects 1. Dead biomass also controls water balance, water quality, soil compaction, and erosion control and nitrogen fixing in the soil 1.

General guidelines for the amount of woody material that can be sustainably removed from the forest tend to vary from region to region. For example, in Minnesota it is recommended that a minimum of 20% of logging residues be scattered on the harvest site, while in Missouri, it is recommended that 33% of biomass be retained 1. As a general rule, it is recommended that between 30%–50% of logging residues be retained depending on the slope of the land in order to maintain biodiversity and soil health 1.

Collection of forest residues​

Collection of logging residues introduces a number of challenges including reaching difficult-to-access areas, as well as the need for specialized equipment and labor. In general, estimates of recoverable logging residues take into account accessibility, and usually only include areas that are already accessible to roads, with a maximum distance from the nearest road being assigned as half a mile 2. As previously mentioned, approximately 30% of logging residues are already collected, requiring a number of extra steps to mechanically concentrate the residues and then load them onto trucks. With regard to collection of forest thinning residues, there is a significant gap in the technical feasibility of collection. Due to the sparse nature of forest thinning and lack of easy vehicle access, collection of forest thinning residues is seen as one of the most expensive sources of biomass 2.

In general, the collection of forest residues is highly dependent on the asking price of the biomass, with higher prices (or government subsidies in the case of fire prevention) resulting in a greater desire to sort and collect residues for use in biofuels, greatly increasing the recoverable supply.

Biomass availability​

Near-term availability of forestland waste and residue biomass in the United States is estimated to be 30.17 million dry tons per year.

Sources​

Footnotes​

  1. Downing, M., Eaton, L. M., Graham, R. L., Langholtz, M. H., Perlack, R. D., Turhollow, A. F., ... & Brandt, C. C. (2011). U.S. billion-ton update: Biomass supply for a bioenergy and bioproducts industry (No. ORNL/TM-2011/224). Oak Ridge National Laboratory. ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9 ↩10 ↩11 ↩12 ↩13

  2. Langholtz, M. H. (Lead). (2024). 2023 Billion-Ton report: An assessment of U.S. renewable carbon resources (ORNL/SPR-2024/3103). Oak Ridge National Laboratory. https://doi.org/10.23720/BT2023/2316165 ↩ ↩2 ↩3 ↩4