Screen rejects — knots and shives flushed from brown‑stock screening — are small by mass but big in energy potential. Mills are increasingly co‑firing them, provided dewatering and debris removal don’t derail the economics.
Industry: Pulp_and_Paper | Process: Brown_Stock_Washing_&_Screening
Hidden inside the brown‑stock screening step (the cleanup after chemical pulping) is a deceptively small waste stream that can make a dent in a mill’s fuel bill. These “screen rejects” — the knots (coarse rejects) and shives (fines) that screening pulls out — are mostly woody material, bits of uncooked or partially cooked chips and bark, with sand and unavoidable trash mixed in (paperadvance.com).
In a well‑run kraft line, industry sources say the ideal knotter reject rate on wood feeds is only ~0.5% (knotter rejects). Higher levels — 2–5% of wood — point to poor wood furnish or cooking (paperadvance.com). Even so, across high‑capacity mills the volumes add up: rejects are often under 1–2% by mass of pulp, but they mount quickly in absolute tonnes.
Look at Indonesia. Mills there consume ~6.6 million t/yr of recycled paper, yielding 5–10% hydropulper rejects — roughly ~0.33–0.66 Mt/yr (researchgate.net). Virgin wood pulping typically produces fewer rejects but still some. As‑collected, these rejects are wet — often >70% water — yet the lignocellulosic fiber fraction carries “significant heating values” and “low moisture” relative to other wastes (text.123docz.net), with dried reject pellets clocking in around ~29 MJ/kg — essentially wood‑ or bark‑like (researchgate.net).
Reject composition and quantities
Screen rejects originate during brown‑stock screening (a cleanup step after digesting; kraft refers to the alkaline chemical pulping process). They are dominated by woody solids (knots and shives) with sand and trash (paperadvance.com). Even at an ideal ~0.5% knotter reject rate, higher rejects of 2–5% of wood point to cooking/furnish problems (paperadvance.com).
Because they’re fundamentally a screening stream, mills typically pair them with robust debris removal. Where coarse debris is common, continuous screens can be used for upstream interception; in pulp applications this aligns with equipment such as an automatic screen for continuous debris removal.
Recirculate, landfill, burn, or repurpose
Some mills recycle knotter rejects to the digester or refine and re‑screen to reclaim fiber, preserving yield but risking heterogeneity and over‑processing if re‑cooking never fully converts knots (paperadvance.com). One pulp study warns that each generation of knots in recirculation can spawn 17.5% additional rejects, which hurts yield (paperadvance.com).
Landfilling is possible but disfavored: the European framework pushes minimizing landfilling of organics, and Indonesian waste law similarly emphasizes reduction and energy recovery where feasible (text.123docz.net). Given the high organic heat value, landfill is wasteful. If sand or metal contamination is high, mills can separate the inorganic fraction for landfill while recovering combustibles.
Incineration in the mill is common: mills with recovery or bark boilers typically burn wood wastes — including screen rejects — to recover energy (text.123docz.net). Fully dried rejects yield ~16–20 MJ/kg (comparable to wood), and one study estimates pyrolysis‑derived bio‑oil from recycled‑paper rejects at ~77.8 MJ/kg (though that value seems high, likely on carbon basis) (researchgate.net).
Where on‑site steam capacity is limited, some mills consider pelletizing or briquetting rejects. Indonesian work on hydropulper rejects (~20% fiber, 80% HDPE plastic) produced pellets at ~29.3 MJ/kg (dry) (researchgate.net; researchgate.net). Pyrolysis of such pellets can yield ~40% bio‑oil (high heating value ~77.8 MJ/kg) plus syngas supporting ~1.08 kWh electricity per kg of reject (researchgate.net). Researchers have also explored lightweight particleboard from rejects, comparing favorably to standards (researchgate.net).
Power boiler co‑firing parameters
In practice, the go‑to route is co‑firing rejects in a power/steam boiler — often a circulating fluidized bed (CFB; a sand‑like bed that enhances mixing) or a stoker type — substituting for coal or heavy fuel oil. At typical moisture (~50% w/w), mills can co‑fire to moderate blend levels (often tens of % of fuel) without major boiler changes, as pulp boilers are designed for wet fuels like black liquor and bark. One case notes a pelletized reject with ~50% moisture “might be used as a boiler fuel by blending with coal” (vincentcorp.com).
Targeting >40% solids (≤60% moisture) and a calorific value >11 MJ/kg makes reject incineration practical (studocu.com). Mills aim to maximize dry solids (DS; the non‑water fraction), because combustion stalls once moisture exceeds ~65% (link.springer.com). In many systems, the light, fiber‑rich fraction (after grit removal) is “generally burned in the mill’s bark boiler” (text.123docz.net).
Steam‑volatility data indicate that per kg of dry waste, roughly 2–3 kWh of thermal power can be recovered (i.e. ~7–10 kWh electricity equivalent) depending on boiler efficiency, while Indonesian pellet pyrolysis suggests ~1.08 kWh/kg available from syngas alone — implying higher gross energy once char combustion is counted (researchgate.net). One North American fiberboard mill found that compacting to ≥40% solids made blending into a coal boiler technically viable, albeit usually at low blend ratios (vincentcorp.com).
Dewatering and staged handling
The hard part is water. Raw screen rejects and pulper rejects are very dilute — a newsprint mill example ran ~3% solids (97% water) — so staged dewatering is standard. A thickener+compactor might raise that to ~10–20% solids, with pressing to ~30% DS before firing (spirac.com; spirac.com). In that case, the flowrate was ~100 lps and an inlet of approx 15 l/m with 2 min per 30 min is noted (spirac.com; spirac.com).
Suppliers recommend upstream disc thickening (Bellmer AKSE F) and then screw or belt presses (AKUPRESS/AKUPAC) to reach 25–35% DS; screw presses stay automatic across wide flow swings and handle fibrous bundles (bellmer.com; spirac.com; vincentcorp.com). Typical outcomes see 5–10% DS feed raised to 30–50% DS cake; belts might deliver ~30% cake, while well‑tuned screw presses can approach 50% before boiler feed (vincentcorp.com).
Upstream physical separation reduces headaches: mills lean on screens and separators to intercept plastics and rags before presses, a role served by primary systems such as screens and other physical separation. For coarse rags (>1 mm), operators may incorporate a simple manual screen ahead of compactors to stabilize flows during dewatering.
Operational constraints and safeguards
Even at ~30% DS (70% moisture), rejects sit near the upper limit for self‑sustaining combustion, so mills co‑fire with drier fibrous fuels (link.springer.com). Wet waste (<50% solid) demands extra drying and flame‑stability control. Feed systems must manage clumping, variable composition, and debris; metals or glass can spark or damage burners, prompting magnetic and density separators — Bellmer notes heavy particle traps (Cyclones, SISAK filters) and designs tolerant of high temperatures and extreme pH (bellmer.com). Corrosion and fouling remain concerns as chlorine and other inorganics concentrate in ash, so operators may wash ashes or control feed composition.
Energy recovery can be material at mill scale. A pulp mill report notes that more than half of energy needs often come from biomass wastes (bark, rejects, sludges) burned on‑site (studocu.com). Still, rejects alone are a small volume; they mainly offset bark or auxiliary fuel usage, and the business case hinges on dewatering and any boiler modifications.
Economics of dryness
Dewatering is often the bottleneck because cheap water removal is difficult. Industry literature stresses “maximum dry content = high calorific value = maximum economic performance” (bellmer.com). Every extra percentage point of DS reduces the evaporation load in the boiler and increases net power, but drying above ~50% DS usually requires thermal drying — not typically justified for rejects. Practically, compaction and direct firing around 30–40% DS is the working limit.
Handled well — with screening/classification, mechanical dewatering, and robust feed systems — the fiber‑rich fraction can be burned for ~15–18 MJ/kg of energy (researchgate.net; link.springer.com) and avoid disposal costs. The trade‑off is straightforward: capital and energy for dewatering versus avoided fuel purchases and landfill fees.
