Residues and energy: the second product line and the kiln bill

Every cubic metre of log that is not sawn timber is a residue: slabs, edgings, chips, sawdust, bark. Residues are a revenue line and an energy source. Energy, mainly kiln heat, is also a cost line. This lesson links the two.

How much residue, and what it earns

In the sources, a little over half of log volume leaves as sawn timber. A simulation for Norway spruce in southern Finland (Kantola 2008, one stand, not mill measurements) gave about 53 to 54% sawn timber, 27 to 28% chips and about 15% sawdust of log volume. A 2024 Polish pallet-lumber paper reports by-product shares of about 20 to 30% chips, 10 to 15% sawdust and up to 8% bark.

Residue revenue has its own cycle. Chips go to pulp, board or energy, sawdust to board or pellets, bark to energy or horticulture. When those buyers curtail, residue prices and the mill margin fall together. German wood pellet prices peaked at 760 euro per tonne in September 2022 and were about 329 euro per tonne by the end of 2023 (Holzkurier, source not specified): 329 / 760 - 1 = -57%. The portal's interpretation is that the 2022 spike cushioned sawmills and the effect reversed in 2023.

What a review of 125 studies says about residue markets

A 2026 rapid review of residue-market literature (125 studies, 68% from North America and Europe) concludes that having a technology does not create a market. Challenges reported: logistics and infrastructure in 77 studies (61.6%), competition in 48 (38.4%), supply and demand conditions in 40 (32.0%), lack of standardisation in 18 (14.4%). The practical reading (the portal's interpretation, not the authors' prescription): treat each residue stream separately, check distance to the buyer, loading, moisture and storage before choosing an end use, and agree quality specifications with buyers.

Energy: where it goes

Drying is the largest energy-consuming process in the Swedish case studies. Swedish mills use about 12% of incoming timber as heat in their own processes, mostly for drying (Applied Energy, 2011). Residues burned in kilns are not sold, so their value is the avoided energy cost. Energy is only about 5% of mill costs according to a Norwegian programme report (Olsson 2020), which weakens the incentive but not the case for measurement.

Worked example 1: heat per m³ from national totals

Swedish mills produced 16.4 million m³ of lumber a year and used about 4.9 TWh of heat for drying (Anderson and Westerlund 2011). Own calculation: 4.9 × 10⁹ kWh / 16.4 × 10⁶ m³ = 299, about 300 kWh of drying heat per m³ (averaging dried and undried product, indicative only). State-of-the-art techniques could cut about 2.9 TWh, which is 2.9 / 4.9 = 59%.

Worked example 2: heat from the water removed (own calculation, an illustration)

Take a product of 489 kg/m³ at 16% moisture (the Swedish Wood EPD product). Oven-dry mass 489 / 1.16 = 421.6 kg. Assume entry at 60% moisture (dry basis, an assumption): water removed 421.6 × (0.60 - 0.16) = 185 kg. With the conventional kiln reference of 1.6 kWh per kg of water removed (a theoretical figure including expected losses; trade source, not verified), heat is 185 × 1.6 = 296 kWh/m³. Heat demand is nearly proportional to the water removed, so starting moisture and yard drying matter.

Documented efficiency measures (North American and Norwegian sources; typical, not guaranteed)

• Variable speed drives: kiln fan paybacks of 2 to 6 years.
• Compressed air: each 2 psi pressure reduction saves about 1%; a US Oregon case saved about 1.3 million kWh a year for a 55,000 dollar project with one-year payback.
• Energy management: Norwegian mills fell from 66.8 to 53.1 kWh/m³ specific electricity between 2015 and 2020 data points, (66.8 - 53.1) / 66.8 = 20.5%; awareness alone gave 2 to 5% in the first year.

Checklist: first energy review

• One reference unit (m³ sawn at a stated moisture) plus log intake.
• Twelve months of bills, split by carrier (electricity, fossil fuel, biomass).
• Log every kiln batch: moisture in and out, run time, heat.

Key points

• Residues are a revenue line with their own cycle (pellets fell about 57% from the 2022 peak to end 2023 in the cited German series).
• In 125 reviewed studies, logistics (61.6%) and competition (38.4%) were the main obstacles to residue markets; value is stream-specific and local.
• Swedish mills used about 300 kWh of drying heat per m³ (own calculation, indicative).
• Heat demand scales with water removed: oven-dry mass × (moisture in - moisture out) × kWh per kg.
• Energy is a small share of cost in Norway (about 5%), yet measured programmes cut electricity by about 20%.