Heat: kiln drying and residues as fuel

Where the energy goes

A sawmill has four energy consumers: electricity (log yard, debarking, saw line, conveyors, extraction, compressed air), thermal energy (almost entirely kiln drying, plus building heat and, in cold climates, log thawing), diesel for mobile equipment, and wood residues that are both by-product and fuel. In mills with kilns, heat is usually the largest single flow. A Swedish study of wood-industry case companies (Energy, 2019) found drying to be the largest energy-consuming and emitting process.

How large is the drying share? Published statements depend on the boundary:

Use these as a range, not as a value for your mill.

Heat demand is proportional to water removed

Wet wood has to lose water. A reference figure from FPInnovations (Canada, trade article 2011, a theoretical value including expected losses) is about 1.6 kWh per kg of water for a conventional heat-and-vent kiln. Define moisture content (MC) here as water mass divided by oven-dry mass. Then:

• oven-dry mass = mass at 16% MC / 1.16
• water removed = oven-dry mass x (MC in - MC out)
• heat = water removed x 1.6 kWh/kg

Worked example (illustration with stated assumptions, not a measurement)

A planed softwood product of 489 kg/m3 at 16% MC (the Swedish Wood declaration basis) has an oven-dry mass of 489 / 1.16 = 421.6 kg per m3.

• Entry at 60% MC, exit at 16%: water removed 421.6 x 0.44 = 185.5 kg; heat 185.5 x 1.6 = 297 kWh/m3, about 1,068 MJ (at 3.6 MJ per kWh).
• Entry at 100% MC: water 421.6 x 0.84 = 354 kg; heat about 567 kWh/m3.

The range brackets the 435-480 kWh/m3 measured in a Serbian beech model (Europe), though species and thickness differ. Starting moisture and pre-drying matter.

Drying technology: heat versus electricity

• A Hungarian plant case on 28 mm oak (Europe, 2026): convective drying took 20-25 days with about 1,560 MJ/m3 of heat and 45 kWh/m3 of electricity; vacuum drying took 4-5 days with about 1,000 MJ/m3 and 176 kWh/m3. This shifts energy from heat to electricity. It is one plant; no capital cost was found.
• Dehumidification kilns can use up to 50% less energy than heat-and-vent kilns but run on electricity (Canadian trade source). Whether they pay depends on electricity price relative to residue-based heat.

Residues as the kiln fuel

Roughly 40-45% of log volume leaves as residues in the US studies (cubic recovery 0.55-0.60), and many mills fire kilns with their own bark, sawdust and shavings. In the north-eastern US hardwood inventory, on-site wood fuel supplied about 74% of the thermal energy. The same inventory reports 5,800 MJ of thermal and 608 MJ of electrical energy per m3 of planed dry lumber, counted from incoming logs (a second table in the same source shows 5,400 and 597 MJ, so expect a few percent of uncertainty). In kWh these are 5,800 / 3.6 = 1,611 kWh thermal and 608 / 3.6 = 169 kWh electrical.

Applying 74%: wood fuel supplied 1,611 x 0.74 = 1,192 kWh per m3 and other fuels about 1,611 - 1,192 = 419 kWh. This thermal figure is far above softwood totals and unexplained; do not transfer it to a softwood mill.

A Finnish sawmill (2013) runs a bark-only heating plant that supplies all its heat and steam. The portal reads this as a standard Nordic design principle: size the boiler to the drying load first, sell what is left.

Checklist: a first look at kiln heat

• Fix the reference: kWh per m3 of dried output at a stated final MC.
• Log initial and final MC of each batch.
• Compute kWh per kg of water removed and compare with 1.6 kWh/kg.

Key points

• Heat is usually the largest energy flow; published drying shares range from about two-thirds to 80%, depending on boundary (mostly North American hardwood).
• Heat is roughly proportional to the water removed: water = oven-dry mass x (MC in - MC out), with a 1.6 kWh/kg reference.
• Vacuum drying moved energy from heat to electricity in one Hungarian plant case; results are plant-specific.
• Residues often cover most kiln heat (74% of thermal energy in the north-eastern US hardwood survey), so saving heat frees residues for sale.
• Convert units explicitly: 3.6 MJ per kWh.