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How to Select an SCR Catalyst

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

Direct answer: SCR catalyst selection runs through six decisions in order: (1) characterize the gas — temperature, NOx, SO₂, dust, moisture; (2) fix the temperature window and required NOx removal; (3) choose geometry (plate vs honeycomb) from dust and erosion; (4) size the catalyst volume with a deactivation margin; (5) run a poison audit from the fuel and upstream process; (6) verify by simulation testing on the actual gas. Skipping any step converts a catalyst choice into a field problem.

Step 1: Characterize the gas

Parameter Why it matters
NOx inlet and NO/NO₂ split Sets required removal and NH₃ demand
Gas temperature at catalyst Selects the catalyst formulation window
SO₂ / SO₃ Drives bisulfate risk and SO₂→SO₃ oxidation limit
Dust concentration & particle size Decides geometry and pitch
Moisture Affects acid dew point downstream
O₂ SCR needs oxygen present

Typical conventional SCR operating window is 200–420 °C; below ~200 °C choose a low-temperature formulation, and understand the bisulfate trade-off (see Low-Temperature SCR).

Step 2: Set the removal target

Design NOx removal of 80–95% is typical for permit-driven duties. Higher removal needs more volume, tighter NH₃ control and cleaner gas. The NH₃/NOx molar ratio operating range is about 0.8–1.05 — beyond 1.0 ammonia slip rises steeply.

Step 3: Choose geometry

  • Plate — wider pitch, better for high-dust and sticky ash; see Plate vs Honeycomb.
  • Honeycomb — higher surface per volume, lower volume for the same activity; fine for low-dust gas.

Face velocity, channel pitch and erosion margin follow from the dust analysis (see Ash & Erosion).

Step 4: Size the volume

Volume follows from space velocity and required conversion, then a deactivation margin is applied for the expected life (see Volume Calculation). Typical space velocity range is 2,000–8,000 h⁻¹ depending on gas and target — a design value, not a product guarantee.

Step 5: Run the poison audit

The fuel and upstream process decide catalyst life:

  • Alkali/alkaline-earth metals (biomass, waste) — see Catalyst Poisoning
  • SO₂ and ammonium bisulfate — see Ammonia Slip Control
  • Arsenic, phosphorus — fossil and some industrial fuels
  • Silica fines — erosion

The same NOx target can need 20–40% more volume on a high-poisoning fuel.

Step 6: Verify before commitment

Simulation testing on the actual flue gas (see SCR Activity Testing) confirms conversion, SO₂ oxidation and pressure drop at the design conditions. This is standard practice before large orders.

When SCR is not the right choice

  • Flue gas temperature far below the window with no reheating budget — evaluate SNCR or staging.
  • Very high SO₃ with no upstream control — catalyst poisoning dominates economics.
  • Variable fuel with unknown poison profile — resolve fuel security first.
  • Space or pressure-drop constraints that no catalyst geometry satisfies.