SCR DeNOx: The Complete Guide
Direct answer: Selective Catalytic Reduction (SCR) removes NOx from flue gas by reacting it with ammonia over a V-Mo-Ti (vanadium-based) catalyst, converting NOx to nitrogen and water. It is the workhorse NOx control technology for coal power, steel, cement, glass and waste incineration — typically delivering 80–95% NOx removal within a defined temperature window. This guide is the hub: it summarizes the whole SCR topic and links to detailed articles on every part of it.
1. What SCR does and where it is used
SCR converts nitrogen oxides (NO and NO₂, together NOx) into harmless N₂ and water using ammonia as the reducing agent:
4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O
The catalyst makes this reaction proceed at 200–420°C instead of the ~900°C that uncatalyzed thermal reduction would need. Applications:
| Industry | Typical duty |
|---|---|
| Coal-fired power | NOx compliance on high-dust flue gas |
| Steel & sintering | Sinter machine exhaust NOx |
| Cement | Kiln exhaust with high alkali dust |
| Glass & ceramics | High-temperature furnace NOx |
| Waste incineration | Municipal and hazardous waste flue gas |
| Chemical & refining | Tail gas and process vents |
2. How the catalyst works
The catalyst is titanium dioxide (TiO₂) support carrying vanadium pentoxide (V₂O₅) as the active component and tungsten or molybdenum trioxide (WO₃/MoO₃) as promoters that widen the temperature window and suppress SO₂ oxidation.
- Standard SCR: NO dominates; 4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O.
- Fast SCR: NO₂ present accelerates the reaction at lower temperature.
- Side reaction to control: SO₂ oxidation to SO₃ (target ≤1%) and NH₃ oxidation at high temperature.
Ammonia is injected upstream — in coal power as anhydrous NH₃, aqueous ammonia or urea (decomposed to NH₃) — and mixed across the duct by an ammonia injection grid (AIG).
3. Catalyst types: plate vs honeycomb
Two geometries dominate. Details in Plate vs Honeycomb.
- Plate-type: metal mesh support with rolled catalyst paste — open channels, high erosion tolerance, the standard choice for high-dust coal flue gas.
- Honeycomb: extruded homogeneous catalyst — higher specific activity per volume, preferred where dust is moderate and volume is constrained.
Both are modular: elements are assembled into steel frames (modules) stacked in the reactor, typically 2+1 or 3+1 layers with a spare layer position.
4. Key design parameters
| Parameter | Typical range | Notes |
|---|---|---|
| Operating temperature | 200–420°C | Below window: low activity + bisulfate; above: NH₃ oxidation |
| Space velocity (SV) | 2,000–8,000 h⁻¹ | Depends on gas conditions and required conversion |
| Face velocity | 4–6 m/s | Balances pressure drop and erosion |
| Channel pitch | ~6–9 mm (plate), 6–20 cells (honeycomb) | Larger pitch for high dust |
| NH₃/NOx molar ratio | 0.8–1.05 | Above 1: rising ammonia slip |
| SO₂ oxidation rate | ≤1% | Formulation-dependent; SO₃ drives bisulfate issues |
| Design NOx removal | 80–95% | Set by permit and economic trade-off |
The selection decision path is covered step-by-step in the articles below — the short version: define gas conditions (temperature, NOx, SO₂, dust, moisture), choose geometry, size volume with deactivation margin, then verify with simulation testing on the actual gas.
5. Operation: the three watched numbers
SCR health is monitored through three trends, always together:
- NOx conversion at a fixed reference load and temperature.
- NH₃ slip — the early warning of active-site loss.
- Pressure drop — the signal for plugging and mechanical issues.
The details of controlling ammonia distribution, slip and flow conditioning are in Ammonia Slip Control.
6. Deactivation: how catalysts die
| Mode | Mechanism | Reversible? | Article |
|---|---|---|---|
| Arsenic poisoning | As₂O₃ vapor blocks pores | Mostly no | Poisoning |
| Alkali (K, Na) poisoning | Neutralizes acid sites | Partially (washing) | Poisoning |
| Ammonium bisulfate | SO₃ + NH₃ deposit below ~280–320°C | Yes (thermal/cleaning) | Poisoning |
| Ash plugging & erosion | Mechanical damage | Cleaning helps; erosion is terminal | Ash & Erosion |
| Thermal sintering | High-temperature structure loss | No | Replacement |
When performance falls, the decision between regeneration and replacement follows a measured sequence — Regeneration and Replacement cover both paths.
7. Performance verification
Catalyst activity is quantified as the activity K (m/h), derived from conversion at defined space velocity. Verification runs in three tiers: laboratory characterization, simulation with the real gas, and in-situ field testing — Activity Testing - Low-Temperature SCR — 160–250°C operation, ABS control, formulation choices. - Reactor Positioning — high-dust, low-dust and tail-end layouts. - Reducing Agent Systems — anhydrous ammonia, aqueous ammonia, urea. - Catalyst Volume Calculation — K, area velocity, layer management. - Flow Distribution & CFD - SNCR vs SCR — choosing the DeNOx route, hybrids, cost curves. - Cleaning & Maintenance — sootblowing, ΔP monitoring, inspection. — uniformity, AIG tuning, field verification..
8. Quick reference: symptom → cause
| Symptom | Most likely cause | Go to |
|---|---|---|
| Conversion falls, slip rises | Chemical deactivation | Poisoning |
| Conversion falls, ΔP rises | Plugging / ash | Ash & Erosion |
| Slip high, conversion fine | AIG imbalance or over-injection | Ammonia Slip |
| Performance falls after cleaning only slightly recovers | Irreversible poisoning | Regeneration |
| ΔP doubled since commissioning | Channel erosion/plugging | Replacement |
9. The complete SCR article series
- Plate vs Honeycomb SCR — geometry choice for dust, erosion and activity.
- SCR Catalyst Poisoning — arsenic, alkali metals, SO₂/SO₃ effects.
- SCR Regeneration — when washing works and when it doesn't.
- SCR Replacement — deciding when to change, sizing the replacement.
- Ammonia Slip Control — AIG tuning, distribution, stoichiometry.
- Ash, Erosion & Mechanical Life — pitch, face velocity, flow conditioning.
- Activity Testing — K value, lab/simulation/field verification.
10. Manufacturer perspective
SCR is a systems problem, not a catalyst purchase. The same NOx target can require 20–40% more catalyst volume on a high-poisoning fuel, and a world-class catalyst behind a badly tuned AIG will slip ammonia from day one. We size from the current fuel and flue gas analysis, the measured velocity distribution and the outage window — and we recommend simulation testing with your actual gas before large orders.