Cement kilns can push 500–1,500 ppmv of NOx at the stack without controls, but two very different toolkits—combustion tweaks and reagent injection—set the tone for compliance. Add limestone’s near-total SO₂ “self-scrubbing” and fabric filters’ near-99% dust capture, and the abatement strategy becomes a study in chemistry, temperature windows, and maintenance.
Cement producers don’t get gentle baselines. Unabated, kiln NOx (nitrogen oxides, NO + NO₂) often clocks in at 500–1,500 ppmv—about 670–2,000 mg/Nm³—before controls (cementequipment.org). That forces a choice between primary combustion control—low‑NOx burners and staging—or post‑combustion chemistry through SNCR (Selective Non‑Catalytic Reduction), which injects ammonia or urea into hot preheater gas.
The split is stark: low‑NOx burners are simpler, “plug‑in” swaps that typically shave 20–30% NOx (up to roughly 40–47% in best cases) (nepis.epa.gov), with a capital tag around €0.15–0.35 million for a 3,000 tpd (tonnes per day) kiln (cementequipment.org). SNCR, by contrast, leans on reagent injection at 870–1,150 °C to deliver bigger cuts—roughly 30–70%—at the cost of ongoing operations and some ammonia slip (nepis.epa.gov).
While the NOx story is about trade‑offs, the SO₂ story often isn’t: limestone’s calcium chemistry scrubs fuel‑borne SO₂ in‑process, sometimes nearly to zero at the stack. On particulates, modern plants overwhelmingly rely on baghouses (fabric filters) or ESPs (electrostatic precipitators), with baghouses typically delivering the lowest outlets.
NOx reduction technologies: performance and costs
Low‑NOx burners reshape flame and stage air/fuel to curb thermal NOx formation. Industry and EPA data put their average reduction at about 20–30% (up to ~40–47% at the high end) (nepis.epa.gov). They are relatively simple retrofits—often a drop‑in burner change—costing on the order of €0.15–0.35 million for a 3,000 tpd kiln (cementequipment.org) and require no reagent.
SNCR (Selective Non‑Catalytic Reduction) injects ammonia or aqueous urea into the preheater/precalciner gas, targeting an 870–1,150 °C window for optimal reactions (nepis.epa.gov). Typical NOx removal lands around 30–70% (nepis.epa.gov), with one case halving a kiln from ~1,200 to 600 mg/Nm³ (cementequipment.org). EPA reviews cite ~30–50% for SNCR alone, rising to ~65–75% if combined with staged combustion or low‑NOx burners (nepis.epa.gov), whereas burners alone rarely exceed ~30% (nepis.epa.gov).
SNCR is OPEX‑heavy—about 65–85% of annual cost is reagent and operations (nepis.epa.gov)—and can produce ammonia slip (unreacted ammonia) of a few ppm to ~10 ppm if not optimized (nepis.epa.gov). Plants typically manage reagent accuracy with metering equipment; for example, integrating an accurate dosing pump into the SNCR injection train for ammonia or urea control. Many operators standardize on urea solutions akin to DEF; supply considerations often overlap with products such as AdBlue when specifying reagent logistics.
The business decision is blunt: low capital, modest reductions with low‑NOx burners, versus higher removal (often roughly double) via SNCR but with reagent cost and control complexity (nepis.epa.gov; nepis.epa.gov). In jurisdictions with tight limits (e.g., <800 mg/Nm³), many new kilns default to SNCR—often on staged‑combustion lines—while older lines first push burner tuning and flue‑gas recirculation. Where chemical optimization is needed, plants may look to emission‑reduction specialty chemicals to fine‑tune performance without straying outside the SNCR temperature window.
SO₂ control via inherent alkaline scrubbing
SO₂ in cement comes from sulfur in fuel and raw feed. The kiln’s alkaline matrix—limestone (CaCO₃) transforming to CaO—naturally scrubs much of it. Technical sources note that SO₂ from fuels is largely retained: CaO reacts with SO₂ to form CaSO₃/CaSO₄ (gypsum‑type solids), which recycle in‑process (cementequipment.org). Reviews describe a “high‑temperature SO₂ cycle” in modern preheater/precalciner kilns where most fuel‑derived sulfur is oxidized then captured on calcium, with one noting that after combustion “most of the SO₂ was absorbed in the calciner generating alkali sulfates” (researchgate.net). In practice, this inherent scrubbing can be nearly 100% for fuel‑borne sulfur—plants can fire high‑sulfur coal or petcoke yet see little SO₂ at the stack (cementequipment.org).
Raw‑feed sulfur is trickier. About 40–70% of raw sulfur (e.g., pyritic limestone) can oxidize to SO₂ in preheater exhaust and escape if not re‑captured (researchgate.net). Some SO₂ re‑adsorbs when it meets fresh CaO, especially with the raw mill in service; one source notes raw‑mill dust removal can adsorb ~80% of SO₂ (cementequipment.org).
When internal capture isn’t enough—say, during mill downtime—plants use in‑process sorbent injection or end‑of‑pipe scrubbing. Slaked lime (Ca(OH)₂) injection in the preheater at about a 3–6 molar Ca/S ratio is reported to remove ~60–80% of SO₂ spikes (cementequipment.org), typically metered with precise feed equipment similar to a dosing pump. For high concentrations (≫1,000 mg/Nm³), dry or wet scrubbers are deployed; venturi or wet scrubbers can cut SO₂ by ~90%. At Holcim Untervaz, reported inlet >2,000 mg/Nm³ dropped to <200 mg/Nm³ under a 90% reduction scheme (cementequipment.org).
Dust capture: baghouse and ESP performance
Cement plants rely on fabric filters (baghouses) and ESPs to rein in particulate (clinker dust, ash, raw meal fines). Baghouses generally achieve ≥99% removal, delivering outlet dust around 10–50 mg/Nm³, with sub‑10 mg/Nm³ routinely reported in practice (cementequipment.org). They capture submicron particles and heavy metals effectively and hold efficiency under varying gas conditions when filter media are properly selected; filter bags typically need replacement every 3–5 years.
ESPs can match high removal on very large gas flows but are more sensitive to gas resistivity, temperature, and flow distribution. They may require higher gas temperatures to avoid condensation and careful field balancing. In general, baghouses have the edge for ultra‑low outlets under variable conditions and, for comparable dust loads, have been reported to cost about 20–40% less in capital than ESPs and to use less energy once installed (cementequipment.org). Many plants mix and match—an ESP on raw‑mill dedusting with a final stack baghouse is common practice.
Regulatory context: Indonesia thresholds
Indonesia is tightening cement emission limits. One survey lists a proposed SO₂ cap of ~800 mg/Nm³ (400 mg for newer plants) (globalcement.com) and a proposed dust limit of ~50 mg/Nm³ (current 80 mg) (globalcement.com). In practice, the kiln’s inherent alkalinity often keeps fuel‑SO₂ very low, while baghouses readily meet sub‑50 mg/Nm³ when maintained (cementequipment.org). Any residual compliance gap—e.g., raw‑sulfur‑driven SO₂ or trace particulates—tends to be closed via the measures above (lime injection, scrubbing, baghouse upkeep).
Sources and references
Cited data draw from engineering reports and reviews. For NOx control, U.S. EPA analyses report “23–47%” NOx cuts from low‑NOx burners and “30–70%” from SNCR (nepis.epa.gov; nepis.epa.gov). EPA fact sheets note SNCR alone gives ~30–50% removal (rising to 65–75% with combustion staging) (nepis.epa.gov). CaO/SO₂ chemistry in the kiln is documented in technical literature (e.g., “SO₂ from fuels is therefore completely retained” in a precalciner) (cementequipment.org; researchgate.net). Dust control efficacy comes from process descriptions (e.g., baghouse outlet “below 50 mg, even ~10 mg”) (cementequipment.org). Indonesian standards cited are from industry surveys (globalcement.com; globalcement.com). All sources are cited inline.
- Battye, R., Walsh, S., Lee‑Greco, J., “NOx Control Technologies for the Cement Industry: Final Report” (EPA‑457/R‑00‑002), U.S. EPA, Sept. 2000.
- U.S. EPA, “Air Pollution Control Technology Fact Sheet: Selective Non‑Catalytic Reduction (SNCR)” (EPA‑452/F‑03‑031), Feb. 2004.
- Edwards, P., “Global Cement Emissions Standards”, Global Cement Magazine, 21 Feb. 2014.
- Wang, J., Fang, J., Wang, L., “The Sulfur Cycle Mechanism in the Whole Process of Cement Manufacturing: a Review”, Chinese Journal of Environmental Engineering, Dec. 2018.
- CEM Equipment (Infinity for Cement), “Emissions from cement kilns”, cementequipment.org (no date).
- CEM Equipment (Infinity for Cement), “Comparison of Performance of Electrostatic Precipitator and Bag Dust Collector”, cementequipment.org (no date).
- CEM Equipment (Infinity for Cement), “NOx reduction techniques”, cementequipment.org (no date).
