Seven-effect trains, vapor recompression “heat pumps,” and plant-wide heat integration are cutting steam demand and paying back fast—if mills can keep black liquor scaling in check.
In modern kraft mills, the single biggest quiet energy hog is thickening black liquor—the spent pulping liquor that carries dissolved wood solids and inorganic salts to the recovery boiler. Engineers have a straightforward answer: push multi‑effect evaporators harder and integrate heat better. Case data and models show 7–10% energy savings from adding mechanical vapor recompression (MVR) to a seven‑effect train, and another ~12% from heat‑recovery retrofits—figures that add up in mills where black liquor thickening “consumes substantial amounts of steam” (www.mdpi.com).
The economics are compelling. In one modeled kraft line handling 15.5 kt/day dilute black liquor (≈2,500 t/d solids), a second MVR cut fresh steam use by ~263 t/day (≈10.96 t/h), lifted electricity draw by 14.4 MWh/day, and delivered ~7% primary‑energy savings—about €6,575/day fuel avoided against €2,160/day extra power, a net €4,415/day and ~2.5‑year simple payback (www.mdpi.com; www.mdpi.com).
Multi‑effect evaporation and vapor recompression
Multi‑effect evaporators (MEE—evaporators arranged in series so each “effect” reuses vapor from the previous one at lower pressure/temperature) deliver steam economy roughly in proportion to the number of effects. A seven‑stage system, for example, concentrates 16–17% black liquor to ~70–75% solids (enough for firing) with COP ≈3.3–3.5, after which industrial condensers reject the final low‑pressure vapor (www.mdpi.com).
MVR (mechanical vapor recompression—a compressor that boosts vapor pressure/temperature so it can be reused as “steam” heat) functions like a heat pump pre‑evaporator. In the 15.5 kt/day case, adding a second MVR to an existing seven‑effect train cut fresh steam ~263 t/day while raising power 14.4 MWh/day, netting the ~7% primary‑energy saving and ~2.5‑year payback noted above (www.mdpi.com; www.mdpi.com). Each added effect or MVR yields diminishing COP at part‑load, but even partial recompression improves capacity and cuts fuel use (www.mdpi.com). Overall, multi‑effect trains (4–8 effects) plus MVR can reduce black‑liquor evaporation steam requirements by roughly an order of magnitude versus single‑effect designs (www.mdpi.com; www.mdpi.com).
The trade also includes economics: industry reports stress that the marginal cost of power (from black liquor) versus steam matters when trading steam for electricity via MVR (www.mdpi.com).
Heat integration and black‑liquor preheating
Beyond the evaporators themselves, plant‑wide heat integration can preheat weak black liquor and trim evaporation duty. Hot process streams—bleaching washwater, weak wash condensates, digester blowdown condensate—can be routed through heat exchangers to warm incoming liquor. A pinch analysis (a method to balance heat sources and sinks) of a partly‑integrated kraft mill identified ~18.5 MW of steam savings (~12% of total mill steam load) via improved heat recovery; a practical retrofit centered on the heat‑exchanger network could save ~5.8 MW of steam per year at ~€0.13 M per MW saved, paying back in ~1–1.5 years (www.researchgate.net).
Many mills already reuse digester steam‑accumulator condensate to preheat boiler feedwater or black liquor, and optimizing pulp‑wash dilution reduces the water sent forward—cutting evaporator steam demand by a few percent (www.energy.gov.au; www.energy.gov.au). Downstream of the evaporators, modern recovery boilers apply condensing economizers or combustion‑air humidification to scavenge flue‑gas heat: cooling below 140 °C, even into 70–100 °C, can capture latent heat to preheat feedwater or black liquor and free low‑pressure steam for power generation—leaving more process steam for evaporation or export (www.mdpi.com; www.mdpi.com; www.researchgate.net). Even a modest preheat of weak black liquor—from ambient to 80–90 °C—can shave evaporator duty by several percent.
Fouling, scaling, and capacity loss
There’s a catch: as black liquor concentrates, inorganic salts precipitate and foul heat‑transfer surfaces. Kraft black liquor contains ~15–16% combined Na₂CO₃ and Na₂SO₄ by dry mass, and above roughly 50% solids it becomes supersaturated, forming double‑salt crystals (burkeite or Na₂CO₃·Na₂SO₄)—classic “sodium salt fouling” that drags down the overall heat‑transfer coefficient U (www.degruyter.com; www.degruyterbrill.com). Even soft, clay‑like scales cut U significantly (www.degruyterbrill.com; www.degruyter.com).
Operators respond by oversizing heat‑transfer area—sometimes by 2–3×—and installing automatic washing circuits for high‑concentration effects. Despite this, scale can develop fast enough to force process upsets. One mill with no tall‑oil side‑stream saw severe scaling around ~60% solids, while others reported little fouling until ~70% solids because tall‑oil soap/brine inhibited crystallization (www.degruyter.com). In general, all effects operating above ~50% dry solids behave like crystallizers, so design allowances for U reduction and regular washing are standard; left unchecked, scale cuts capacity and triggers shutdowns (www.degruyter.com).
Mitigation strategies and chemistry control
Mitigation strategies include wet cleaning (circulating dilute caustic or acid solutions) and mechanical scrapers during shutdowns; in practice, these washing circuits depend on accurate chemical handling, where metering hardware such as a dosing pump can be relevant in implementation. Adding tall‑oil extract (a byproduct brine) just before high‑concentration effects has proven effective at dissolving or inhibiting Na₂CO₃/Na₂SO₄ crystals and delaying scale onset; controlling liquor chemistry (pH, sulfate/carbonate ratio) helps too (www.degruyterbrill.com; www.degruyter.com). Where chemical scale‑control programs are applied, the aim mirrors the tall‑oil effect: inhibit sodium salt crystallization and keep U up (for industrial context, see scale inhibitors). Cleaning cycles still impose downtime and fresh water/chemical use, adding a hidden energy penalty because engineers must design for 100% duty at reduced U while frequent washes waste heat (www.degruyter.com; www.degruyterbrill.com).
Policy context and Indonesian upgrades
Indonesia, the world’s 8th‑largest pulp producer and 5th‑largest paper producer (infopublik.id), is pushing “green industry” measures toward 2050 net‑zero. The Ministry of Industry explicitly cites black liquor as a biomass fuel for steam and electricity, making upgrades like enhanced multi‑effect trains, MVR, and waste‑heat recovery directly aligned with national goals (infopublik.id). One Indonesian analysis notes that unutilized black liquor after evaporation represents a large missed energy opportunity; conversely, an optimized evaporator with heat integration maximizes black liquor’s bioenergy—on the order of 22 GJ per tonne pulp by burning the concentrated liquor (www.researchgate.net; www.researchgate.net).
The investment case matches policy: the MVR upgrade noted above paid back in ≈2.5 years (www.mdpi.com), and heat‑exchanger retrofits driven by pinch analysis often return capital in <2 years (www.researchgate.net).
Bottom‑line energy impacts
Upgrading black‑liquor evaporators with more effects, mechanized recompression, and comprehensive heat integration can cut steam use by ~10–20% (or more) in modern mills. Numerical studies and case data show multi‑effect+MVR yielding 7–10% energy savings (www.mdpi.com), while pinch‑driven measures can save ~12% (or ~6 MW in a 600 t/d pulp mill) (www.researchgate.net). Fouling remains the barrier: without inhibitors and cleaning, sodium‑salt scale can erase the gains (www.degruyterbrill.com).
Sources: Authoritative studies (MDPI, Nordic P&P Research J, energy‑efficiency guides) and industry analyses underpin the figures and paybacks here. Key citations include Variny (2023) and Karlsson (2020) on evaporation efficiency and scaling (www.mdpi.com; www.degruyter.com), an Australian energy guide (www.energy.gov.au; www.energy.gov.au), recovery‑boiler heat‑recovery analyses (www.mdpi.com; www.mdpi.com), pinch‑analysis casework (www.researchgate.net), and Indonesian policy/industry sources (infopublik.id).
