Low‑NOₓ burners often underdeliver, SNCR does the heavy lifting, and fabric filters are beating ESPs. Meanwhile, limestone chemistry is doing most of the SO₂ removal inside the kiln.
In the race to clean up cement, one surprise advantage is built into the kiln itself: the raw feed acts like an internal scrubber for sulfur dioxide (SO₂). But nitrogen oxides (NOₓ) and dust? Those still come down to burner design, reagent injection, and the filter at the stack.
Across plants, the playbook is converging. Low‑NOₓ burners (LNBs) reshape combustion to reduce thermal spikes. Selective non‑catalytic reduction (SNCR) injects ammonia or urea into the preheater hot zone. And baghouses are overtaking electrostatic precipitators (ESPs) on particulate. The difference between “inherent chemistry” and “add‑on technology” shows up in the numbers.
NOₓ control: combustion tweaks vs reagent injection
NOₓ (nitrogen oxides) in cement kilns forms from high‑temperature reactions of combustion air and any fuel nitrogen. The industry mixes a primary measure (combustion modification) with a secondary, post‑combustion process. Modern LNBs reshape the flame to lower peak temperatures and limit oxygen, but field performance is modest: industry data (Cembureau) puts LNB reduction at 0–30% — around ~30% only under optimal conditions with very high baselines (www.cementequipment.org), and a survey found half of retrofits delivered no measurable drop (www.cementequipment.org).
Even in well‑designed systems, LNBs typically cut NOₓ only to the order of a few hundred mg/Nm³ — e.g., 600–1000 mg/Nm³ (mg/Nm³ = milligrams per normal cubic meter) (www.cementequipment.org). That often fails to meet tightening standards. Korea’s new plants, for example, are held to ~270 ppm (≈700 mg/Nm³), and many jurisdictions now aim for 200–400 mg/Nm³ (www.mdpi.com; www.mdpi.com). In short, LNBs mainly prevent peak spikes but, under real conditions, seldom achieve double‑digit percentage cuts (www.cementequipment.org; www.mdpi.com).
SNCR tuning window and typical outcomes
That’s why most plants pair LNBs with SNCR (selective non‑catalytic reduction). SNCR injects a nitric‑oxide‑reducing reagent — typically ammonia or urea — into preheater gas at 850–1050 °C, where it reacts to form N₂ and H₂O. Properly tuned, SNCR removes roughly half the NOₓ. Korean experience shows 55–60% removal, and a recent pilot found ~55% reduction (NH₃/NOₓ ratio ~1.0) with minimal ammonia slip (www.mdpi.com; www.mdpi.com).
Across literature, SNCR removal generally ranges 30–60% (temperature control and mixing are critical). The theoretical maximum is quoted up to ~95% when combining SNCR with catalytic processes, but pure SNCR seldom exceeds ~60% in practice (www.mdpi.com; www.mdpi.com). Avoiding ammonia slip (unreacted NH₃ creating plume effects) hinges on precise location and flow control (www.mdpi.com). Plants typically meter reagent with accurate injection systems; pairing SNCR with an accurate chemical dosing pump helps maintain that tight operating window.
Adoption mirrors the performance: SNCR is used as a tail‑end control in roughly half of EU kilns and nearly all new US projects, while full SCR (selective catalytic reduction, >90% removal) remains rare due to cost (www.mdpi.com). Typical outcomes: ~30% NOₓ reduction with LNBs, versus ~50–60% with SNCR on top of that (www.cementequipment.org; www.mdpi.com). In Korea, plants using both measures now emit on the order of a few hundred mg/Nm³ of NOₓ, trending downward year‑over‑year (www.mdpi.com).
The economics follow: LNBs largely incur capital cost and minimal consumables and are often paired with staged combustion or indirect firing to push toward ~50% reductions (www.cementequipment.org). SNCR delivers the heavier NOₓ cuts but needs reagent procurement and handling — often managed via specialty chemistries for emission reduction — which can be integrated with sourcing through specialty chemical programs.
SOₓ abatement: in‑kiln “self‑scrubbing”
SOₓ (sulfur oxides) in cement kilns come chiefly from fuel sulfur and minor sulfur in raw materials. Uniquely, the kiln’s alkaline raw feed provides in‑situ scrubbing: limestone (CaCO₃) calcines to CaO, which readily reacts with SO₂ to form CaSO₃/CaSO₄ in the clinker or kiln dust. Engineering reviews report even the “least effective” kiln inherently captures ~50% of sulfur input, with 90–95% common in modern preheater/precalciner systems (www.cementequipment.org). High efficiencies especially occur when an in‑line raw mill is used (the fine CaCO₃/CaO in hot mill gases is an excellent SO₂ sorbent): in‑line mills often cut stack SO₂ by 40–60% during operation (www.cementequipment.org).
Accordingly, practical SO₂ emissions are usually low. A survey found raw SO₂ levels from 10 up to ~3500 mg/Nm³ in extreme cases (www.cementequipment.org), but on typical low‑sulfur fuels stacks are often below ~200–400 mg/Nm³. Additional process‑integrated strategies can further scrub if fuels carry more sulfur: calcined feed recirculation (the FLSmidth “DeSOx” process) removes a fraction (25–30%) by looping hot CaO into upper preheater stages (www.cementequipment.org), or alkali bypass loops (“Gas Suspended Absorption”) recycle lime‑rich dust as sorbent. Even without those, the basic chemistry acts as an internal dry scrubber.
Regulators reflect this reality. Most mature markets set Kemaratigi limits around 200–500 mg/Nm³ for SO₂ (www.globalcement.com). Indonesia has proposed or considered a cement SO₂ limit around 800 mg/Nm³ (with 400 mg/Nm³ in certain cases) (www.globalcement.com). In practice, well‑run Indonesian kilns with typical iron pyrite levels can meet such limits via inherent capture. In short, unlike coal power plants, most cement SO₂ emissions derive from raw feed sulfur, and kiln CaO locks away much of it.
Particulate control: fabric filters vs ESPs
Kiln exhaust carries entrained solids (“cement kiln dust” or CKD). Plants universally choose between fabric filters (baghouses) and ESPs (electrostatic precipitators). Modern systems reach >99% removal; stack PM is now typically single‑digit mg/Nm³. Industry surveys note today’s ceramic filters can cut to a “few milligrams per Nm³” (www.cementequipment.org), and a recent upgrade pushed one plant from ~22 mg/Nm³ down to ~1 mg/Nm³ (www.nordic-air-filtration.com). Even without such extremes, baghouse‑equipped kilns routinely hit 1–30 mg/Nm³ (www.worldcement.com).
Older ESPs typically delivered 50–100 mg/Nm³ — and, in the best cases, ~30–50 mg/Nm³ (www.worldcement.com). The trend is clear: fabric filtration is preferred, especially where limits now require <30 mg/Nm³ (and China’s “ultra‑low” regions demand <10 mg/Nm³). Baghouses handle varying moisture and alkali‑laden dust better — at the cost of higher pressure drop and maintenance — while ESPs can be cheaper at huge scales but struggle with resistivity and humidity. Many new and upgraded lines specify pulse‑jet baghouses.
Key figures: modern baghouses typically yield <30 mg/Nm³, often in single digits (www.worldcement.com); ESPs, if used, usually hover around 30–50 mg/Nm³ at best. Regulatory trends align: U.S. EPA and EU limits for cement kilns are often 10–20 mg/Nm³ (at 10% O₂). In Indonesia, cement plants typically comply with an emission limit of a few tens of mg/Nm³ (baghouse equipped) per government standards (www.cementequipment.org; www.worldcement.com; www.nordic-air-filtration.com).
Cost, compliance, and what plants actually do
Integrating LNBs and SNCR can halve or better the NOₓ output — going from 1000+ mg down to several hundred mg/Nm³ — while the CaO‑rich process chemistry inherently soaks up most SO₂ (often ~50–90%), leaving little need for separate desulfurization (www.cementequipment.org; www.mdpi.com; www.cementequipment.org). Dust is controlled to near‑negligible levels by high‑efficiency baghouses (>99% removal). These technologies are mature; plants weigh capital against operating costs (e.g., urea use for SNCR, baghouse energy and maintenance) and the limits they must meet. Reagent logistics — storage, dosing, and monitoring — ride alongside the hardware; that’s where reliable metering and procurement frameworks, including emission‑reduction chemical supply, tend to matter operationally.
Regulatory figures and sources
Typical NOₓ regulatory limits today are on the order of 200–400 mg/Nm³ (www.mdpi.com), pushing adoption of SNCR. EU‑leading plants target 200 mg/Nm³ (fact in 2018) (www.globalcement.com). Cement industry reports quote NOₓ cuts of ≤30% via LNB (www.cementequipment.org) versus ~55–60% via SNCR (www.mdpi.com). Cement SO₂ output is generally low: CEMBUREAU cites raw steel cake as 10–3500 mg/Nm³ range (www.cementequipment.org), but typical well‑mixed kilns at <500 mg. Inherent capture is high (50–95% of sulfur) (www.cementequipment.org), and an in‑line mill or recirculation system can give an additional 40–60% knockdown (www.cementequipment.org; www.cementequipment.org). PM emissions are now in the 1–30 mg/Nm³ range with modern filters (www.worldcement.com; www.cementequipment.org). Each of the above figures comes from recent industry and regulatory assessments (see references in‑line).
