Skip to content

SNCR vs SCR: Choosing the Right DeNOx Route

Part of the SCR DeNOx: The Complete Guide — this article is one of the detailed pages in the guide.

Direct answer: SNCR (selective non-catalytic reduction) injects ammonia or urea directly into the furnace at 850–1,100°C without a catalyst; SCR (selective catalytic reduction) passes flue gas over a catalyst bed at 200–420°C. SNCR costs far less to install but typically delivers only 30–60% NOx reduction with higher ammonia slip; SCR delivers 80–95%+ at much lower slip. Many plants combine them — SNCR first, SCR polishing — to balance cost and the emission target.

The chemistry difference

Route Temperature window NOx reduction Ammonia slip Capital cost
SNCR 850–1,100°C (furnace) 30–60% 5–15 mg/Nm³ typical Low
SCR 200–420°C (reactor) 80–95%+ 1–3 mg/Nm³ with good design High
SNCR + SCR hybrid Both 80–95%+ combined Moderate Medium

When SNCR is enough

  • The emission limit is moderate (e.g., 100–200 mg/Nm³) and inlet NOx is not extreme.
  • The boiler has a stable load band where the SNCR temperature window is reliably available.
  • Capital is constrained and the plant can accept higher ammonia consumption.
  • Small industrial boilers, biomass units, and municipal waste lines.

When SCR is required

  • Ultra-low emission targets (35 mg/Nm³ and below) — SNCR cannot hold these numbers alone.
  • High baseline NOx or wide load turndown where the furnace temperature leaves the SNCR window.
  • Ammonia slip constraints — a downstream limit on slip usually forces SCR.

The hybrid pattern

SNCR upstream plus a reduced SCR layer downstream is the standard cost-optimized design: SNCR removes the cheap first 40–50%, the SCR layer polishes the remainder to target with a fraction of the full-SCR volume. The trade is that SNCR's ammonia slip consumes part of the SCR layer's capacity.

Selection checklist

  • Emission target and whether it includes ammonia slip.
  • Boiler type, load profile and the availability of the SNCR temperature window across the load range.
  • Flue gas composition at the SCR position (dust, SO₂, temperature) if SCR is contemplated.
  • Lifecycle cost: reagent consumption (SNCR uses 2–4× the NH₃/NOx ratio of SCR) vs capital.

Manufacturer perspective

SNCR and SCR are not competing religions — they are two ends of one cost curve. We size the split from the emission target and the plant's load profile: pure SNCR when the target allows, hybrid when it doesn't, full SCR when slip or ultra-low targets bind.

← Back to the SCR DeNOx: The Complete Guide