Kraft pulping’s worst smells are being torched to near‑zero, and dust is being knocked down to single‑digit mg/Nm³ with proven hardware. The twist: the “stink” is also fuel.
For all its forest‑to‑fiber alchemy, the kraft process has two unavoidable byproducts: odorous reduced sulfur gases and fine particulate dust. Mills are answering with a one‑two: capture and incinerate the gases, then strip out the particles with electrostatic precipitators or fabric filters.
The results are striking. One mill reported stack total reduced sulfur (TRS, a measure of reduced sulfur gases like H₂S and methyl mercaptan) at just 0.4 mg/Nm³ (8% O₂), effectively ≈99% destruction (docslib.org). At a large Finnish mill, average recovery‑boiler TRS ran below 1 mg/Nm³ (researchgate.net).
TRS control via NCG collection and incineration
Kraft pulping emits odorous reduced sulfur gases primarily in non‑condensable gas streams (NCG, vented gases that do not condense) from digesters, flash tanks and washers. Best practice is to capture all NCG and incinerate them in high‑temperature units—typically the recovery boiler (the sodium‑chemical recovery furnace), the lime kiln, or a dedicated incinerator—where combustion oxidizes nearly all TRS to SO₂ and then to sodium sulfate (Na₂SO₄).
In one documented case, commissioning NCG collection and a recovery‑boiler burner drove stack TRS to 0.4 mg/Nm³ (8% O₂), underscoring how “oxidation and sulphur conversion from gas phase to ash in [the] recovery boiler is extremely efficient,” with SO₂ largely captured as Na₂SO₄ (docslib.org). Measurements at a large Finnish mill showed an average TRS below 1 mg/Nm³ (researchgate.net), consistent with EU/BAT (Best Available Techniques) studies indicating single‑digit mg/Nm³ is routine.
Collection is comprehensive. Following EU/BAT guidance, mills route both concentrated and dilute malodorous streams—CNCG (concentrated NCG) and DNCG (dilute NCG)—from blow tanks, evaporators and washers to the furnace. APRIL’s Kerinci mill, for example, “collect[ed] and incinerate[d]” both in its recovery and lime kilns (aprilasia.com), with monitoring showing TRS “well below” BAT reference levels (aprilasia.com).
There’s an energy angle too. Andritz notes that capturing NCG “can yield a ‘fuel’ with significant heating value,” highlighting that incineration recovers energy (andritz.com). Babcock & Wilcox engineers systems specifically to “burn dilute and concentrated NCGs in a recovery or power boiler” safely, as part of a wider emissions‑control portfolio (babcock.com).
Design rules matter: high black liquor solids (>70%) and 5–10% Na₂CO₃ in fly ash help ensure the recovery boiler handles excess SO₂ without corrosion (researchgate.net).
When incineration isn’t possible
If direct incineration is infeasible due to layout or safety constraints, wet or caustic scrubbing of NCG can reduce TRS. Alkali scrubbing has been reviewed by the U.S. EPA as a control option in such cases (noting added system complexity). In these scrubbing setups, accurate chemical dosing is a practical requirement; mills typically deploy precise feed equipment such as a dosing pump to manage caustic addition.
Particulate control: ESPs and baghouses
Particulate matter (PM) in pulp mills arises from combustion units—recovery boilers, power boilers, lime kilns—and from fiber lines. Two main technologies dominate: electrostatic precipitators (ESPs, which charge and collect particles electrically) and fabric filters (baghouses, which filter flue gas through fabric bags). Both can exceed 99% removal.
ESPs are the veteran choice on high‑volume, high‑temperature flue streams. Modern designs remove >99% of fly ash, with Babcock reporting ESP efficiencies often >95–99% (babcock.com). APRIL Asia equipped recovery and power boiler stacks with ESPs and complies with Indonesian limits (230 mg/Nm³ for boilers) (aprilasia.com), noting that a modern ESP on a new recovery boiler cut total and specific dust markedly (aprilasia.com). In practice, a well‑operated ESP can routinely drive stack dust into the single‑digit mg/Nm³ range, and typical recovery‑boiler averages often land around 5–20 mg/Nm³ under normal operation (aprilasia.com).
Fabric filters shine when legal limits are extremely tight. A TAPPI/PEERS (2017) study found sub‑10 mg/Nm³ (6% O₂) emissions are attainable on recovery boilers, even with their ultra‑fine, monodisperse dust, if a sorbent is carefully dosed upstream to agglomerate particles (research.aalto.fi). With that approach, life‑cycle costs can undercut an ESP where strict <10 mg/Nm³ limits apply; without sorbents, ESPs remain simpler (research.aalto.fi). In these cases, the choice and handling of the upstream sorbent—a specialty chemical used to bind fine particles—becomes a central design decision, a role often served by solutions in the category of chemical specialty for emission reductions.
Comparative outcomes and selection factors
Both ESPs and baghouses meet regulatory targets when sized and operated correctly. ESPs offer robustness and low pressure drop, which is why many existing boilers use them. Baghouses yield more consistent removal of fine and corrosive dust. Empirical outcomes show ESP‑equipped recovery boilers often averaging 5–20 mg/Nm³, while switching to a fabric filter can push below 10 mg/Nm³ (as modeling indicates), with maintenance and chemistry considerations to manage (aprilasia.com; research.aalto.fi).
In practice, large combustion stacks almost always deploy ESPs (veteran tech with proven 99%+ efficiency, babcock.com), while smaller lines or new ultra‑clean systems may use baghouses. Ultimately, technology choice depends on required emission level, fuel/ash properties, and capital/maintenance budgets. Modern control systems routinely achieve single‑digit mg/Nm³ PM with either ESP or baghouse (aprilasia.com; research.aalto.fi).
Source notes and key data
Authoritative industry and regulatory sources on pulp mill emissions, including mill sustainability reports and technical conferences, underpin the data cited here. Key points and URLs: TRS after NCG incineration (~0.4 mg/Nm³, docslib.org); average recovery boiler TRS (<1 mg/Nm³, researchgate.net); Indonesian particulate limits (230–350 mg/Nm³, aprilasia.com); fabric‑filter performance targets (<10 mg/Nm³ at 6% O₂, research.aalto.fi); energy recovery from NCG combustion (andritz.com); and ESP performance and NCG‑burning solutions (babcock.com). These support the analysis of emission control outcomes.
