Kraft mills concentrate “black liquor” to ~65–70% solids in multi‑effect evaporators and burn it in a recovery boiler to generate high‑pressure steam and regenerate cooking chemicals — recovering ~95% of sulfur and ~80% of carbonate along the way. When production outpaces the boiler, mills face costly choices for handling excess liquor, from ultraconcentration to gasification.
In the economics of modern pulp, the recovery line is the profit center hiding in plain sight. After digestion, black liquor (BL — the spent cooking liquor with dissolved lignin and chemicals) is only ~15–20% solids. Mills concentrate it to ~65–75% solids (w/w) in multiple‑effect evaporators, then burn it in a water‑walled recovery boiler to make high‑pressure steam and regenerate cooking chemicals (ScienceDirect; MDPI). The result: roughly 3.0 MWh (≈10.8 GJ) of steam per ADt (air‑dry tonnes) of pulp from about 1.5 t of black liquor dry solids (BLDS) — and a near‑closed chemical loop (MDPI).
It’s a tight energy ballet. Thickening BL from ~17% to ~75% solids at 2,500 tDS/day takes on the order of 100–150 MW of steam, so energy integration around the boiler train is non‑negotiable (MDPI; MDPI). Utilities planning often spans evaporators and the boilerhouse; mills fit this out with ancillary packages — for example, supporting equipment for water treatment in water‑treatment ancillaries — alongside the core recovery assets.
Multi‑effect evaporation and concentration
A multiple‑effect evaporator train (typically 4–7 effects) concentrates dilute BL from ~15–20% to ~65–75% solids, maximizing heating value before firing. Mills deploy falling‑film or forced‑circulation designs (ScienceDirect). One case study processed ~15,000 t/day of ~16% solids BL to produce 2,500 t/day of dried solids at ~70% concentration (MDPI).
Viscosity is the hard limit: it rises rapidly above ~50% solids (especially for softwood BL), so mills push concentration “as high as possible” but typically cap it at ~65–70% solids to ensure stable firing (ScienceDirect; ScienceDirect). Many systems add pre‑evaporation via mechanical vapor recompression (MVR): in one retrofit, an MVR pre‑evaporator saved ~10 t/h of live steam (∼36 MW) for ~600 kW of extra power (MDPI).
Recovery boiler combustion and steam generation
Concentrated BL (≈65–70% solids) is sprayed into a water‑walled recovery furnace; organics combust to generate high‑temperature flue gas while inorganic salts form a molten smelt, mostly Na₂CO₃ and Na₂S, at the furnace bottom (ScienceDirect; ScienceDirect). Boilers commonly run high pressures: 6–9 MPa steam (HP) to the turbine, with lower‑pressure (LP) steam for process heat (ScienceDirect).
For a large modern mill, the recovery boiler yields on the order of 0.83 MWh/ADt of MP steam (≈12 bar) plus ~1.8 MWh/ADt of LP steam (≈5 bar) (MDPI). In practice, a few percent of steam is used internally — sootblowing, air preheating, liquor preheating — with ~2.5–4% of generated HP steam typically reserved just for sootblowing (ScienceDirect; ScienceDirect). Electric auxiliary load (fans, pumps, ESP, etc.) runs about 55–70 kWh per ADt of pulp (ScienceDirect).
Black liquor’s gross calorific value lands around 13–14 MJ/kg of solids (ScienceDirect) — for context, ~7 t of 15% BL are produced per tonne of pulp, containing ~1.05 t of solids (ScienceDirect). Riau pulp mill operations cite ~13.8 MJ/kg for BL energy content, which when fully recovered displaces fossil fuels (ResearchGate). Routine boilerhouse programs (chemistry control, blowdown, and monitoring) sit alongside the recovery boiler; suppliers list dedicated packages as chemicals for boilers, separate from the energy balances reported here.
Chemical recovery and causticizing loop
The smelt (Na₂CO₃/Na₂S) is dissolved to form green liquor, then causticized with limestone to regenerate white liquor (a mixture of NaOH and Na₂S used for cooking). Modern mills recover nearly all sodium salts: about 95% of sulfur (as Na₂S) and ~78–80% of carbonate are returned to cooking liquor (ScienceDirect; ScienceDirect). Put differently, ~20–22% of the Na₂CO₃ from the recovery boiler does not convert back to NaOH in causticizing (ScienceDirect).
Green liquor dregs (largely CaCO₃+Na₂CO₃) are filtered out and returned to the lime kiln; overall, the closed loop regenerates white liquor at near 100% sulfur–sodium balance and >95% of the caustic chemicals (ScienceDirect; ScienceDirect). Discussions of dosing and control for this loop often reference precise feed equipment; commercial offerings frame this as accurate chemical dosing, separate from the mass‑balance and efficiency figures cited above.
Energy and performance benchmarks
Per tonne of bone‑dry pulp, the recovery boiler uses ~4–5 GJ of black liquor energy and outputs ~3–4 GJ of useful high‑pressure steam, plus heavy flue‑gas heat. Suhr et al. (2015) note overall thermal demands of 4–5 GJ/ADt in fuel and 20–30 kWh/ADt of electricity (ScienceDirect). Separately, the recovery boiler effectively provides “free” steam: for a 1.6 million ADt/year mill, 0.83 MWh/ADt of high‑pressure steam and 1.8 MWh/ADt of low‑pressure steam were reported — roughly 2.6 MWh/ADt total (MDPI). Auxiliary demand for the recovery boiler (fans, pumps, ESP, feedwater pumps) is about 55–70 kWh/ADt (ScienceDirect).
Excess black liquor contingencies
If black liquor production exceeds recovery boiler capacity, discharge is not an option — BL is highly alkaline, high‑BOD, and treated as hazardous. Design practice aims to avoid this by right‑sizing evaporation and boiler capacity. Where necessary, mills manage excess by further concentrating into “heavy” BL for co‑firing in existing boilers or auxiliary incinerators; by temporary storage (tanks or dregs) for later burning; by export to other mills (not common); or via innovative routes (Republika).
Examples include extracting lignin and reconstituting it into a thick black liquor — Riau Andalan Pulp (RAPP) in Indonesia reported replacing about 87% of fossil fuel use this way (Republika). Research also pilots gasification of BL (or spent liquor) to syngas or hydrogen; studies of supercritical‑water gasification report significant syngas yields (ResearchGate; ResearchGate). These are not yet standard practice — the industry default is to size the recovery boiler to burn essentially all BL solids. Any remaining solids (dregs or lime mud) are handled in a lime kiln or disposed as non‑toxic ash.
Regulatory and regional context (Indonesia)
Studies note Indonesian regulations favor total internal treatment of pulp wastes. Analyses find the energy potential of BL is under‑optimized in Indonesia, arguing that fully burning BL in recovery systems is environmentally and economically preferable (ResearchGate; ResearchGate). Available “excess” liquor would represent lost energy or require capital‑intensive solutions. In practice, modern kraft mills target ~100% black liquor combustion: with well‑maintained boilers, essentially all BL is fed to the recovery furnace, with any surplus solids either re‑dissolved into green liquor (via additional causticizing) or managed through those advanced routes (ResearchGate; ScienceDirect).
Bottom line and figures in one place
State‑of‑the‑art systems concentrate BL to ~65–70% solids (ScienceDirect; MDPI), burn it to produce high‑pressure steam (∼3 MWh/ADt) and regenerate ~95% of sulfur and ~80% of carbonate chemicals (MDPI; ScienceDirect). Typical performance includes ~4–5 GJ fuel per ADt and about 60 kWh auxiliary power load, within ranges documented as 4–5 GJ/ADt fuel and 20–30 kWh/ADt electricity for overall demand, plus 55–70 kWh/ADt for recovery boiler auxiliaries (ScienceDirect; ScienceDirect). Procurement teams typically bundle utility peripherals separately from these balances (for example, water‑treatment ancillaries and accurate chemical dosing), without altering the core recovery metrics.
Sources and citations
Detailed process data are drawn from pulp‑industry references (e.g. Bajpai 2015; Cardoso 2009) and case studies (ScienceDirect; ScienceDirect; MDPI; ScienceDirect). Indonesian context is informed by national studies (e.g. Rahmat et al. 2023) (ResearchGate; ResearchGate) and industry reports (Republika). All figures and statements above are supported by these sources.
