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SCR Reactor Positioning: High-Dust, Low-Dust and Tail-End Layouts

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

Direct answer: The SCR reactor position is set by where the flue gas temperature matches the catalyst window. High-dust (hot side), low-dust and tail-end (wet side) layouts each trade catalyst protection against reheating cost. High-dust placement is the power-industry default; tail-end placement dominates retrofit and low-temperature applications.

High-dust (between economizer and air preheater)

The standard position: gas at 300–400°C with full fly ash load.

  • Advantages — gas is already at the right temperature, no reheat; proven at scale across thousands of units.
  • Disadvantages — catalyst faces erosion, plugging and alkali poisoning from raw ash; SO₂→SO₃ oxidation adds downstream corrosion potential.
  • Design response — plate-type or large-pitch honeycomb geometry, sootblowers, erosion-resistant edges, and a poisoning margin in the volume calculation.

Low-dust (after ESP, before FGD)

Dust is largely removed; temperature typically 300–380°C.

  • Advantages — dust-related deactivation (erosion, plugging) essentially disappears; tighter pitch and higher activity geometry become usable.
  • Disadvantages — an ESP hot enough to survive this position is expensive; SO₃ and some metals remain.
  • Best for — installations with hot-side ESPs, especially retrofits of older units.

Tail-end (after FGD and reheating)

Gas exits wet FGD at 45–60°C and is reheated to 170–320°C.

  • Advantages — cleanest gas: minimal dust, minimal SO₂, no arsenic or alkali burden; catalyst lasts longest per unit volume; ideal for low-temperature formulations.
  • Disadvantages — reheating cost is a permanent operating expense; layout requires a GGH or steam heater.
  • Best for — retrofits with space constraints, low-sulfur fuels, and plants that already reheat.

Position selection checklist

  • Temperature at the candidate position — does it sit inside the catalyst window across the load range?
  • Dust load and abrasiveness — sets geometry (plate vs honeycomb) and face velocity.
  • SO₂/SO₃ concentration — high values favor tail-end placement or upstream FGD.
  • Reheat energy cost — the deciding number for tail-end schemes.
  • Outage access — catalyst layer access and replacement logistics.

Manufacturer perspective

Position decides everything downstream of it: catalyst geometry, volume, poisoning margin and ammonia system design. We evaluate all three positions against the actual flue gas analysis and the plant's energy balance before recommending a layout.

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