Bark and wood residues are now core fuel streams in kraft mills, delivering steam, electricity, and sometimes export power — if operators can handle a wet, abrasive, dust‑prone material that bridges, spills, and wears out equipment.
Modern pulp mills generate large wood‑residue streams — chiefly debarking waste (“hog fuel”) including bark, plus sawdust, screening rejects, and more. Typical yields are on the order of 100–300 kg of bark per tonne of dry pulp (≈10–30% by mass) (bioresourcesbioprocessing.springeropen.com); softwoods generally yield 5–15% bark and hardwoods 5–20% (researchgate.net). For context, global chemical pulp capacity (~180 M t/yr in 2022) implies on the order of 10⁷–10⁸ t/yr of bark produced worldwide (valmet.com).
Bark differs markedly from clean wood: it contains high mineral ash (typically 5–10% ash, rich in calcium and silica) and higher fixed carbon (more lignin) than wood (researchgate.net) (researchgate.net) (frontiersin.org). It also retains high moisture (fresh debarked logs often >50% water; in one example chopped bark and chips were ~33% wet basis), which lowers net heating value (ej-energy.org). Bark’s gross calorific value on an as‑fired basis is often in the mid‑3000–4000 kcal/kg range (roughly 15–17 MJ/kg) — roughly one‑third that of coal (see Table I in the same study) — although values vary with species and moisture (ej-energy.org). Bark also corrodes equipment faster due to dust and grit from soil contamination.
Inside a kraft mill, bark and wood residues constitute the second‑largest biomass fuel after black liquor (theconiferous.com). Black liquor combustion in the recovery boiler yields on the order of 4 tonnes of steam per tonne of pulp (theconiferous.com) and often makes the mill energy‑self‑sufficient. Bark boilers supplement this: Metsä Fibre mills in Finland reported 176% energy self‑sufficiency and export excess power, thanks to burning black liquor and hog fuel (metsagroup.com). Remaining bark is therefore normally destined for energy.
Combustion for steam and electricity
By far the most common use of bark is as boiler fuel. Mills grind or chip bark into hog fuel and fire it in dedicated stoker or fluidized‑bed units to generate process steam and/or electricity in back‑pressure turbines (back‑pressure turbines convert high‑pressure steam to power while supplying lower‑pressure steam to processes). In one Indonesian pellet mill case, burning a bark–wood chip mixture (with 33% moisture) with 30% excess air produced ~35 t/h of steam in a 1×7 MW plant; fuel consumption was ~16.8 t/h (mixed chips+bark) delivering ~24.3 MW useful steam heat out of ~28.8 MW input (≈84.6% boiler thermal efficiency) (ej-energy.org) (ej-energy.org).
Another survey shows scale: bark from ~3.6 million m³ of pulpwood (wood chips and residual bark) was directed to a fluidized‑bed CHP (combined heat and power) boiler co‑fired with other fuels (mdpi.com). In practice, modern kraft pulp mills often run two boilers: the kraft recovery boiler (burning black liquor) plus a bark boiler burning wood residues. Given bark’s high ash content, designs limit maximum combustion temperature; it is mainly used for low‑pressure steam or hot‑water generation (frontiersin.org).
Even with variable moisture, burning bark yields significant net energy. In one sample (32.9% moisture), the higher heating value (HHV, a measure of energy content that includes the heat of condensation of water in the exhaust) was ~3,800 kcal/kg (≈16 MJ/kg), while coal in the same study was ~11,300 kcal/kg (about three times higher) (ej-energy.org). Using bark avoids fuel costs and landfill, and in many mills the energy from bark covers a substantial fraction of steam demand beyond the recovery boiler.
On the utilities side of these steam systems, operators typically manage condensate return quality to protect the cycle; polishing steam condensate after heat exchange cooling is a common step (condensate polisher). Within the boiler circuit, dissolved oxygen is a known corrosion driver, so oxygen removal programs are standard in steam plants even when the fuel is biomass (oxygen scavengers). Chemical feed accuracy is critical in these services, making precise metering equipment part of routine utility design (dosing pump).
Carbon profile and emission controls
Burning bark offsets CO₂ emissions: since bark is biogenic, its CO₂ output is typically considered carbon‑neutral. One study of a Chinese pulp mill found most CO₂ came from biomass (bark and black liquor) boilers, with only the lime kiln fuel being fossil gas/oil (frontiersin.org). That does not remove the need to control other emissions: particulates, NOₓ, and trace compounds. In many jurisdictions, stricter emission limits apply to biomass boilers; detailed emission data for Indonesian mills is limited, but modern biomass boilers use cyclones, ESPs (electrostatic precipitators), and flue‑gas filters to meet regulations (frontiersin.org).
Co‑firing, gasification, and advanced fuels
Some mills blend bark with other fuels. Co‑firing bark with coal in a circulating fluidized bed can improve boiler performance; one study shows modest efficiency gains from coal–bark co‑firing (especially for smaller mills) (study referenced in paper). Others explore gasification of bark to generate syngas — demonstrated for on‑site lime kiln fuel gas or for biofuel production (valmet.com) (frontiersin.org). Suppliers note that dried bark can feed gasifiers or be pelletized for fuel — for example via a belt drying line (valmet.com). Research discusses bark‑rich biofuels (e.g., Nosek et al. 2016) and highlights pyrolysis of wood waste into bio‑oil or biochar as an emerging route (Indonesian literature reviews mostly focus on palm biomass) (researchgate.net) (sciencedirect.com).
Beyond combustion, mills and wood products factories pelletize sawdust and bark for heating fuel (often for export or local heat plants). Indonesian pellet exports (from sawdust) are significant globally, though exact bark content is unclear (indonesiawoodpellet.com). Bark can also go to mulch or soil amendment: board mills in Finland have “bark mulching plants” producing horticultural mulch and panel‑board fillers from bark (paperandwood.com). Wound plaster or biochar from bark are niche products. These material uses are usually lower in volume; the bulk of bark supply goes to energy.
Bark‑fired boiler performance and economics
Modern bark boilers usually feature moving grates or fluidized beds and must handle coarse, wet fuel. Bark is often hogged (shredded) then metered by conveyors or screw feeders into the furnace. After drying in the furnace, bark combusts, producing flue gas ≈1,000–1,500 °C in the combustion zone (ej-energy.org). As noted above, a 7 MW unit in Indonesia ran at ~84.5% efficiency on a mixed bark/chip fuel (ej-energy.org), while Metsä Fibre’s mills reached 176% net energy self‑sufficiency — enough to supply district heat and export power (metsagroup.com).
From a numbers standpoint, burning 1 t of as‑fired bark (≈35–60% moisture) might produce roughly 0.6–1.0 t of steam (depending on boiler design). In the Medco example, 16.8 t/h of wet fuel produced 35 t/h steam — about 2.1 t steam per t fuel delivered (wet basis) (ej-energy.org). On a dry basis (≈11.3 MW∙h/t fuel at 3800 kcal/kg), this corresponds to about 75–80% boiler conversion of fuel HHV into steam enthalpy (ej-energy.org). Economically, utilizing bark typically pays back quickly because the fuel is essentially free (a mill byproduct) and even modest steam/power offsets expensive coal/oil. One Indonesian company (RAPP) has invested heavily in biomass power (a 20 MW CHP, plus co‑generation boilers) to cut diesel/coal use (noted in paper). A 97 MW biomass power plant (using pulp mill wood waste feed) is also under development in Riau (power-technology.com).
Operators still face ash and corrosion: bark ash is high‑alkali, raising slagging and corrosion potential, so designs temper furnace temperatures and plan for clinker removal. Particulate emissions are controlled by cyclones/ESPs. In many utilities, clean make‑up water for steam service is produced through membrane or media trains; pretreatment to reverse osmosis (RO) is common over surface or ground sources (ultrafiltration), and integrated packages are standard offerings (RO, NF, and UF systems).
Handling and conveying constraints
Bark’s physical nature makes handling particularly difficult. As a “Class 3” bulk solid (flaky, fibrous, hygroscopic), bark/hog fuel has very poor flowability (mdpi.com). Wet, stringy bark tends to form bridges and clogs, and it does not feed easily through hoppers or pneumatic lines (mdpi.com) (mdpi.com). High moisture greatly increases cohesiveness and compressibility, so that even relatively small water content causes particles to stick to each other and to surfaces — bark bunches on conveyors and screw feeders (mdpi.com) (mdpi.com).
Grit, sand, and small stones from log yards make bark abrasive: chutes, conveyors, and grinders wear rapidly. Users report that conveyor sprockets and screws require hard‑facing or ceramic liners to resist wear (bulk-online.com) (paperandwood.com). One pulp industry source notes that conveyor sprocket teeth can last over twice as long when clad with abrasion‑resistant overlays (paperandwood.com). Fine bark dust pervades equipment; it embeds in idler bearings and causes slugging on belts (flexco.co.za).
Spillage and carryback are chronic: flaky bark does not ride cleanly on belts, leading to carryback (sticky fines clinging to belt) and spillage at transfers. Because bark fuels are flammable, spilled material is a fire hazard. Mills invest in belt cleaners, skirting, and enclosed conveyors; cleated conveyor belts — used to transport uphill or steeply — are hard to seal at chutes and thus notoriously prone to spillage, and they are hard to clean (flexco.co.za). In practice, mills add multiple scrapers and brush cleaners to reduce carryback.
Given bark’s poor flow, belt conveyors (flat or inclined) and drag/slat conveyors are often preferred over pneumatic or vibrating systems. Screw conveyors can work for short drops but are prone to bridging unless steep. Many mills even use water‑flume conveyors or handle bark in wet form to mitigate dust. Where water flumes are used, the recirculating loop typically needs basic debris removal to protect pumps and nozzles (waste-water physical separation). Careful solids‑handling design — wide chutes, steep hopper walls, vibrators — is needed to feed bark reliably.
Dust and explosions remain a consideration: bark and wood fines generate combustible dust (moisture mitigates but does not eliminate risk). Facilities use extensive dust collection and strict no‑smoking policies around bark storage and conveyors. Large piles of wet bark can also self‑heat due to microbial activity: one study found ~2–4% dry‑mass loss per month in stored bark due to decomposition, especially if mixed with sludge (link.springer.com) (link.springer.com). This raises pile temperature (fire risk) and changes handling properties over time (drier/dustier on top, wetter inside).
To stabilize fuel, mills store bark in ventilated piles or silos and may pre‑dry it. Some install bark presses or belt dryers to squeeze out water; dried bark flows much better and has higher HHV (supplier example: valmet.com). In parallel, when plants draw boiler make‑up from surface sources, pretreatment steps are often applied to protect downstream membranes (ultrafiltration) or complete systems (membrane systems).
Data points and policy signals
Energy analyses confirm net gains. In the Indonesian Medco case, 7 MW cogeneration from bark/chips replaced about 4.45 MW of coal‑equivalent loss, yielding ~25 MW usable steam (ej-energy.org). Globally, many integrated pulp mills aim for overall energy self‑sufficiency by burning all their black liquor and wood residues (metsagroup.com) (frontiersin.org). Mills report >90% of bark produced in‑mill is combusted for energy (with non‑combustible contaminants screened out). Emissions data (e.g., life‑cycle studies) show that substituting bark for fossil fuel reduces CO₂ emissions by ~90% (since biogenic CO₂ is not counted) (frontiersin.org).
Policy and market signals reinforce the direction. Indonesia’s draft National Energy Policy emphasizes biomass (including wood waste) as a priority, aiming to reach ~23% renewable energy by 2025, with biomass second only to solar (fwi.or.id). PLN plans to co‑fire sawdust, wood pellets, and chips (implicitly including bark‑derived fuels) to replace 5–10% of coal in its powerplants (fwi.or.id). The pulp sector is likewise investing in biomass power; a 97 MW plant in Riau is under development using pulp mill wood waste feed (power-technology.com).
Emerging projects also re‑cast bark as a biorefinery feedstock. Studies propose combined bark and black liquor gasification to make biofuels, with pulp as co‑product (frontiersin.org) (frontiersin.org). Data also indicate that combining bark with other wastes (e.g., mill sludge) for co‑firing or densification (pellets) can improve handling and curb biodegradation losses (link.springer.com) (link.springer.com).
Bottom line and sources
Burning bark for energy dominates its utilization today. The measurable outcomes — tonnes of steam or electricity per tonne of bark, fuel cost savings, and CO₂ reductions — are large, with examples ranging from a 7 MW Indonesian unit at ~84.6% thermal efficiency to Scandinavia’s export‑power mills at 176% self‑sufficiency (ej-energy.org) (metsagroup.com). The caveat is operational: handling a wet, abrasive, poor‑flow bulk solid demands purpose‑built conveyors, wear mitigation, dust control, and in many cases drying (valmet.com) — all manageable with established equipment and practices.
Sources and supporting data: bark yields (~100–300 kg/t pulp) (bioresourcesbioprocessing.springeropen.com); composition and energy content (ej-energy.org) (researchgate.net); mill performance (steam per fuel, efficiencies) (ej-energy.org) (metsagroup.com); equipment and handling reports (valmet.com) (flexco.co.za); and regulatory/market data for Indonesia (fwi.or.id) (power-technology.com). Global capacity context (~180 M t/yr chemical pulp in 2022) appears via valmet.com.
