Modern pulp mills are corralling non‑condensable gases and firing them at >850 °C to wipe out odorous TRS, while ESPs and fabric filters cut particulate to single‑digit mg/Nm³. The combination is delivering odorless stacks and meeting tough permits across markets.
Total reduced sulfur (TRS — reduced sulfur gases like H₂S and mercaptans that cause foul odors and contribute to SOₓ) hides in non‑condensable gas (NCG — process vents that don’t condense) across the kraft chemical recovery cycle. Mills are sealing those vents — from digesters, evaporators and the smelt dissolving tank (SDT) — and piping them to high‑temperature oxidation where combustion temperatures above 850 °C convert essentially all TRS to SO₂ for capture in the smelt (researchgate.net). Valmet documents an “odorless” mill case with recovery‑furnace TRS of ~0.4 mg/Nm³ (8% O₂) (docslib.org) — orders of magnitude below untreated vents that can contain TRS in the tens of thousands of ppm.
By comparison, a stringent Northern Sonoma County, CA rule caps treated NCG TRS at ~5 ppmv (≈0.1 mg/Nm³) (ww2.arb.ca.gov). Modern NCG burners routinely meet it: routing NCGs to the recovery boiler or a dedicated thermal oxidizer achieves virtually 100% TRS destruction (researchgate.net), converting sulfur to SO₂ (later captured as sulfate in the smelt).
Other configurations use Regenerative Thermal Oxidizers (RTOs) or quench‑type incinerators, often followed by caustic wet scrubbers; Andritz has supplied a quench NCG incinerator with wet scrubbing to a major Brazilian project (andritz.com). In practice, closing the chemical cycle with total NCG capture plus incineration eliminates nearly all odorous TRS, typically reducing emissions by >99% to single‑digit mg/Nm³ levels (researchgate.net) (docslib.org).
NCG collection and incineration
NCG lines consolidate digester vents, evaporator off‑gases and SDT emissions into controlled destruction points — typically the recovery boiler or a dedicated incinerator (thermal oxidizer). Where dust resistivity or scrubbing chemistry needs tight control (e.g., SO₃ or NaOH injection noted for ESP conditioning), plants rely on accurate chemical dosing; a purpose‑built dosing pump anchors consistent feed rates for those reagents.
Regulatory alignment and outcomes
Policy is converging on “near‑zero” TRS. In Indonesia, new megaprojects are designed to meet EU IPPC/BAT criteria (okipulppaper.co.id), implying single‑digit mg/Nm³ levels of TRS and SO₂, while North American and European permits now require comprehensive NCG abatement. After treatment, residual TRS is typically <1–10 mg/Nm³, and the cited Brazilian mill’s 0.4 mg/Nm³ (8% O₂) is roughly an order of magnitude below many regulatory limits (docslib.org). EPA and industry reports show properly incinerated NCGs yield negligible TRS (researchgate.net) (docslib.org).
Particulate matter control technologies
Recovery‑boiler particulate matter (PM — flyash and condensed alkali sulfates) is controlled with electrostatic precipitators (ESPs — charged plates that remove particles) or fabric filters (baghouses). Both are proven at ≥99% removal. An EPA review notes ~98% of US kraft recovery boilers use ESPs (nepis.epa.gov), and typical ESP performance removes >99% of dust (nepis.epa.gov).
Industry data (EU BREF) confirm modern devices routinely deliver cleaned gas in the single‑digit to tens of mg/Nm³; both ESPs and fabric filters “give a very high removal efficiency… in excess of 99%,” commonly yielding 5–20 mg/Nm³ under normal conditions (studylib.net). New fabric‑filter systems are often calibrated to meet even lower concentrations — well under 10 mg/Nm³ — if needed (studylib.net).
ESP and fabric filter trade‑offs
ESPs impose almost no added gas pressure drop and use electrical fields plus periodic rapping to shed dust; their efficiency can be affected by dust resistivity (often conditioned with SO₃ or NaOH injection) and rapping regimes. Fabric filters have higher pressure drop and energy use but capture ultra‑fine particulates more uniformly; BREF guidance notes they can achieve “slightly lower emissions of dust than ESPs” and are particularly efficient on very fine dust (studylib.net). Empirically, both deliver sub‑20 mg/Nm³ outlet when properly designed and maintained (studylib.net).
Modeled and field performance
U.S. regulatory analyses have modeled recovery boilers with upgraded or replaced ESPs at 0.034 g/dscm (≈30 mg/Nm³) outlet PM (nepis.epa.gov). In the field, tuned ESPs typically hold under ~10–20 mg/Nm³, while well‑maintained baghouses reach mid‑single‑digit mg levels (studylib.net). Either technology can meet strict emission caps; in Indonesia and elsewhere, pulp mill permits commonly demand PM in the low tens of mg. Older boilers almost always use ESPs, while some new or revamped units install fabric filters — especially when very low PM or extra reliability is required.
Bottom line metrics
State‑of‑the‑art recovery boilers with NCG incineration and modern PM collectors produce negligible TRS and very low PM. One Brazilian “odorless” case reports stack TRS ≈0.4 mg/Nm³ and SO₂ ≈0.1 mg/Nm³ (docslib.org), with dust emissions below typical BAT references (5–20 mg/Nm³) (studylib.net).
Sources: industry and regulatory studies provide the data above (researchgate.net) (nepis.epa.gov) (studylib.net) (docslib.org) (ww2.arb.ca.gov) (okipulppaper.co.id), including U.S. EPA pulp mill guidelines and vendor case reports. These confirm >99% control efficiencies (TRS and PM) and the specific emission outcomes cited here. (All sources are cited inline.)
Notes: mg/Nm³ is milligrams per normal cubic meter of gas (a standard concentration unit); ppmv is parts per million by volume; O₂ percentages denote oxygen‑corrected stack measurements; RTO is Regenerative Thermal Oxidizer; ESP is Electrostatic Precipitator; baghouse refers to a fabric filter system.
