Run it in the right window and a recovery boiler will turn concentrated black liquor into high‑pressure steam and 95–97% chemical recovery. Drift outside, and emissions and downtime spike fast.
At the heart of a kraft mill sits the chemical recovery boiler, a continuous unit built to do two jobs at once: burn the organic portion of concentrated black liquor for steam, and reclaim sodium and sulfur as molten smelt for reuse. Modern single‑drum designs routinely handle 1,000–4,500 tonnes dry solids per day (≈100–300 MW steam) (ifrf.net) (www.babcock.com), with suppliers like Babcock & Wilcox citing capacities up to 10,000,000 lb/day (≈4,500 tpd) dry solids and steam conditions to 12.6 MPa (1,850 psi) at 510 °C (www.babcock.com).
The configuration is familiar: water‑cooled walls, an overhead convective section, and superheaters/boiler banks arranged to sidestep fouling hot spots (www.babcock.com). Liquor guns (2–16, often spaced ~3–5 m above the hearth) spray high‑solids liquor onto a sloped char bed while multi‑level air systems—now often tuned by computational fluid dynamics (CFD)—drive uniform combustion (ifrf.net) (www.valmet.com) (www.babcock.com). Advanced twists, like dual‑pressure furnaces, aim to squeeze out more power (www.babcock.com).
High solids firing (~80–85% dry solids) is now the norm because every 1% bump in solids cuts in‑furnace evaporation duty, lifting thermal efficiency; typical fuel‑to‑steam efficiency (higher heating value basis) is 60–70% (ifrf.net) (ifrf.net). In well‑run lines, chemical recovery—the fraction of sodium/sulfur re‑captured as Na₂CO₃/Na₂S—lands around ≈95–97% (www.sciencedirect.com) (www.andritz.com).
Design capacity and furnace architecture
Combustion geometry matters. Multi‑tier liquor guns improve droplet distribution; sloped floors and multiple smelt spouts stabilize bed coverage; and CFD‑shaped secondary air can eliminate dead zones and carryover (www.valmet.com) (www.babcock.com). These design choices aim to keep the upper furnace clean by completing smelt reactions in the bottom zone (ifrf.net).
Operating variables that set yield and emissions
Black liquor solids and feed rate. Operators typically fire liquor at 68–80% solids (ifrf.net). Higher solids reduce evaporation duty and raise furnace temperature (ifrf.net), but pushing too high (>80%) risks liquor “rain,” viscosity issues, and char carryover. U.S. EPA testing showed particulate emissions scale directly with black liquor solids throughput (nepis.epa.gov), while datasets indicate that at low loads (<70% of maximum continuous rating, MCR), unburned carbon (CO and Total Reduced Sulfur, TRS) rises sharply, strengthening the case for operation above ~70% load (www.researchgate.net).
Combustion air and stoichiometry. Airflow is metered for a small excess oxygen (~1–2% O₂ at the stack). Too little air fuels CO, VOCs and TRS, and risks droplet carryover. Too much air oxidizes sodium sulfide (Na₂S) to sodium sulfate (Na₂SO₄), eroding chemical yield. Regression tests showed raising excess O₂ strongly reduced TRS (nepis.epa.gov), but high O₂ correlates with more NOx and less chemical reduction. Suppliers advocate tuning to minimize CO while holding ~1–1.5% O₂ (www.valmet.com) (nepis.epa.gov), and use CFD to even out air distribution (www.valmet.com) (www.valmet.com).
Furnace temperature. A hot, uniform char bed (~850–950 °C) closes out combustion and melts inorganics to smelt; a thin/cold bed leaves carbon unburned, while hot spots can sinter ash or hasten smelt‑water reactions. Operators infer furnace health from superheater inlet temperatures and flue‑gas delta‑T, then adjust liquor gun aiming or air splits to stabilize the bed (ifrf.net).
Sulfidity and chemical balance. Sulfidity (the liquor’s Na₂S‑to‑NaOH balance) steers emissions and recovery. Higher sulfidity lowers SO₂, a trend confirmed by EPA data (nepis.epa.gov). Mills watch green‑liquor sulfidity and Total Titratable Alkali (TTA) as reduction proxies, then adjust via causticizing and make‑up dosing; stable dosing is typically executed with precise metering equipment such as a dosing pump. Target sulfidity varies by site, but tight control is central to maximizing active chemicals.
Load distribution and heat release. Hearth heat release rate and per‑gun firing limits cap “rich” operation that could freeze smelt or damage walls. Balancing the load prevents local slagging and uneven deposition, especially with chloride/potassium‑rich liquors. Unplanned upsets—carryover events that force outages—are the costliest failures to avoid (www.valmet.com) (www.valmet.com).
Reduction chemistry and carryover control
Maximizing alkali recovery means running the furnace as an efficient reduction reactor. That starts with complete combustion—measured by very low CO at the outlet—without over‑oxidizing sulfides. Suppliers note that lowering CO via tuned combustion enables lower excess O₂ and thus lower NOx (www.valmet.com).
A stable char bed—semi‑molten coke and smelt—is the second pillar. It gives sulfur time to form H₂S that’s captured as Na₂S in smelt, and ensures carbon burns out. Fluidizing the bed with high air velocity or starving it with poor air placement both undermine reduction.
Carryover is the third lever. Every kilogram of black liquor dry solids generates roughly 0.4 kg of smelt; letting droplets escape the hearth forfeits chemicals and accelerates fouling and corrosion, with risk of tube leaks (www.thefornax.com) (www.valmet.com). Operators fight carryover with good atomization, balanced air, and sootblowing.
Targets are explicit. Reduction efficiency (often inferred from green‑liquor sulfide) in modern mills runs ~90–97%, with advanced concepts targeting ~95% apparent reduction in a single pass (www.sciencedirect.com) (www.andritz.com). Deviations below ~95% should trigger checks for excess O₂, water leaks, spillback, or degrading burner/nozzle performance. Many mills use in‑situ analyzers and CFD diagnostics to fine‑tune conditions (www.valmet.com) (www.valmet.com).
Stack emissions and control benchmarks
Particulates. Electrostatic precipitators (ESPs) or baghouses capture fly ash and unburned fines. Modern recovery boilers report 10–100 mg/Nm³ (dry, 3% O₂), with conventional ESPs often at 5–10 mg/Nm³ and fabric filters below 5 mg/Nm³ (www.researchgate.net) (www.andritz.com). Best Available Techniques targets land below 0.1 kg/ADt (air‑dry tonnes) pulp—≈5–50 mg/m³ (www.andritz.com) (www.researchgate.net). Keeping ESP resistivity low and rapping effective is part of routine control.
Total Reduced Sulfur (TRS). Odorous species (H₂S, CH₃SH, etc.) collapse when combustion is complete. Modern boilers typically post <10 mg/Nm³, and historical EPA work showed that raising excess O₂ sharply cut TRS (www.researchgate.net) (nepis.epa.gov). U.S. jurisdictions such as Montana set a 17.5 ppm (≈30 mg/Nm³) daily average in the 1970s, pushing mills far lower in practice (nepis.epa.gov).
Sulfur dioxide (SO₂). Emissions depend strongly on liquor sulfidity; higher Na₂S fractions push sulfur into H₂S for recapture, while lower sulfidity lifts SO₂. Typical ranges are 100–800 mg/Nm³, though best practice and high‑sulfidity operation aim much lower (www.researchgate.net) (nepis.epa.gov).
Nitrogen oxides (NOx). With kraft liquor’s low nitrogen (~0.1% on dried liquor), uncontrolled values often sit around 200–300 mg/Nm³ (6% O₂). Air staging and tight O₂ control meet many limits, but where targets near 100 mg/Nm³ are enforced (e.g., China/EU), secondary controls are entering service. New measures (e.g., low‑NOx firing, SNCR) cite 80–95% NOx reduction, while SCR retrofits are also being applied (www.valmet.com) (www.andritz.com) (www.andritz.com). Operators avoid flue‑gas recirculation because it hurts reduction efficiency (www.andritz.com).
CO and VOCs. CO is the frontline metric for complete combustion. Good practice keeps CO <100–200 ppm. The EPA’s “controlled odor” conversions essentially eliminated visible plumes from recovery stacks, implying very low CO/VOC (nepis.epa.gov).
Benchmark ranges (dry gas, 3% O₂). Typical modern values (www.researchgate.net):
- SO₂: 100–800 mg/Nm³ (≈35–275 ppm)
- TRS: <10 mg/Nm³ (<7 ppm)
- NOx (as NO₂): 100–260 mg/Nm³ (50–125 ppm)
- Particulate (dust): 10–200 mg/Nm³
- PM: 0.1–0.5 kg/ADt (≈10–50 mg/Nm³)
Monitoring and control disciplines
Combustion controls. Maintain stable draft and track O₂ and CO continuously, with setpoints around 1–1.5% O₂ and near‑zero CO. Over‑fire air staging trims NOx without starving the hearth; fuel/air balance to each gun avoids local over‑rich pockets.
Recovery metrics instrumentation. Track green‑liquor TTA and sulfidity continuously; high TTA signals strong active alkali. Compute and trend “reduction efficiency,” with ≥95% considered good operation (www.sciencedirect.com) (www.andritz.com).
Sootblowing and cleaning. Scheduled sootblowing (steam or air) holds surfaces clean; fouling thermocouples trigger interventions. Keep ESPs tuned—an EPA case study cut particulate mass flow by 83% (6,000 ➞ 1,000 lb/day) after ESP installation and firing optimization (nepis.epa.gov).
Smelt handling. Smelt spouts and dissolving systems run best at design conditions. Continuous observation prevents plugging cascades. Cooling water quality and temperature are critical to avoid spout failures; guidance stresses high‑purity water and proper temperature control (www.thefornax.com) (www.thefornax.com). Plants achieve high‑purity cooling and make‑up water with dedicated systems, for example a demineralizer supported by water‑treatment ancillaries.
Environmental controls. Where applicable, operate auxiliary controls (e.g., TRS scrubbing on smelt tank vents, NOx SNCR/SCR). In retrofit contexts, suppliers note SCRs need very clean gas—dust <30 mg/Nm³, SO₂ <5 mg/Nm³ at ~250 °C—and elevated flue temperatures, with one case returning ~1.7‑year payback by adding 13 MW generation (www.valmet.com). Continuous emission monitoring (CEMS) underpins reporting.
Examples and performance trends
Load vs. emissions. Hourly data from a large kraft unit show that below ~70% load, CO and TRS climb, while NOx tends higher at very high loads. Above ~70%, pollutants stabilize—evidence supporting operations near design capacity (www.researchgate.net).
Chemical recovery. Reviews report ~97% pulping chemical recovery in well‑run furnaces, with design refinements (spray quality, bed depth) supporting >95% sodium/sulfur reduction. New “Lo‑alkali” concepts target ~95% reduction efficiency in a single pass (www.sciencedirect.com) (www.andritz.com). Mill‑wide sodium balances are routinely reconciled to a few percent over time.
Emission reductions. In the EPA’s 1974 “controlled odor” retrofit, optimizing combustion and adding an ESP cut stack TRS from ~400 ppm (100–700) to <10 ppm (0–25) and slashed particulates 83% (6,000 ➞ 1,000 lb/day) (nepis.epa.gov) (nepis.epa.gov).
Modern performance. With tuned operation and advanced collectors, some mills now report TRS near zero (few ppm) and particulate in single‑digit mg/Nm³ (www.andritz.com). On the energy side, incremental upgrades (power cycles, feedwater heating) have enabled surplus electricity exports; one SCR‑ready flue‑temperature retrofit delivered ~13 MW and ~1.7‑year payback (www.valmet.com).
Indonesian context. Indonesian operators report the same fundamentals. APRIL Group, Riau, tackled chloride/potassium buildup—known boiler corrosion drivers—by crystallizing and removing ~550 tons/day of electrolytic ash to protect uptime (www.sustainabilitymatters.net.au). Emission targets align with BAT levels, including recovery boiler SO₂/TRS within national limits (e.g., <0.1 kg S/ADt) (www.researchgate.net).
Operator best practices
- Optimize black liquor quality. Push evaporators to supply the highest feasible solids (typ. ≥75–80%) and watch impurities (chlorides, K⁺) via purging/pretreatment to reduce deposits and corrosion (ifrf.net) (www.sustainabilitymatters.net.au).
- Maintain combustion control. Use fast O₂/CO monitoring to hold excess air at setpoint; tune primary/secondary splits; avoid oscillations in draft or firing that spark TRS spikes or carryover (www.valmet.com).
- Rigorous monitoring. Log O₂/CO, steam flow, TTA, and reduction efficiency continuously; set alarms (e.g., CO >300 ppm, O₂ <0.5% or >2.5%); analyze hourly trends because long averages can mask load‑linked spikes (www.researchgate.net).
- Maintain heat exchange. Sootblow before heat‑flux loss mounts. Design features minimize fouling, but carryover still requires cleaning; keep feedwater heaters optimized to trim flue losses (www.babcock.com) (www.babcock.com).
- Smelt spout care. Follow cooling water quality/temperature guidance to prevent spout burnout; balance spout loads; keep dissolving tank chemistry steady; isolate compromised spouts promptly (www.thefornax.com) (www.thefornax.com).
- Emission control systems. Keep ESP/baghouse internals in spec; where NOx limits tighten, be prepared for SNCR/SCR, noting retrofit needs for very clean, warm flue gas (www.valmet.com).
Business outcomes and compliance
Chemical cost. At ~97% recovery efficiency, fresh caustic makeup is minimized; even a 1% slip is meaningful at mill scale (www.sciencedirect.com).
Energy generation. Higher liquor solids and cleaner surfaces directly lift steam/electricity; pushing from 70% to 80% solids raises steam output by cutting in‑furnace evaporation duty. Avoiding fouling preserves nameplate capacity.
Emission compliance. Reviews of Best Available Techniques list recovery boiler BAT levels around 1–4 kg(S)/ADt for SO₂ and <0.05 kg(S)/ADt for TRS, with many mills holding nearer the bottom of these bands via disciplined control and CEMS (www.researchgate.net) (www.researchgate.net).
Reliability and safety. Carryover events and smelt incidents trigger costly downtime. Minimizing carryover cuts emissions and stretches time between shutdowns—improving availability and ROI (www.valmet.com) (www.valmet.com).
In summary, the data and operating experience are consistent: holding the recovery boiler in its narrow operating window—solids, air, temperature, load, and smelt handling—delivers a rare combination of near‑complete chemical recovery and very low emissions. Hourly targets like particulate <10 mg/Nm³, TRS ≈0–10 mg/Nm³, and NOx ≈100–200 mg/Nm³ are practical reference points for operators and engineers (www.andritz.com) (www.researchgate.net), with sulfidity control, reduction efficiency, and carryover suppression as the key levers.
