SCR and SNCR
High-dust, low-dust and tail-end SCR
Also known as HD SCR, LD SCR, tail-end SCR, high dust SCR.
High-dust, low-dust and tail-end describe where an SCR catalyst sits in the flue-gas path relative to upstream particulate-control equipment.
| Configuration | Position | Gas temperature | Trade-off |
|---|---|---|---|
| High-dust (HD-SCR) | Upstream of ESP / baghouse | 300–400 °C | Natural operating temperature; high catalyst pluggage and erosion |
| Low-dust (LD-SCR) | Between hot-side ESP and air heater | 300–400 °C | Cleaner gas; needs hot-side ESP upstream |
| Tail-end (TE-SCR) | Downstream of all particulate control | 130–200 °C | Cleanest gas; requires gas reheating; ABS risk |
Why high-dust dominates
Most coal-fired utility SCRs are high-dust because no flue-gas reheating is required and SCR slots cleanly between the economiser outlet and the air heater inlet at the natural process temperature. The penalty is high fly-ash loading at the catalyst inlet — hence the need for LPA screens, guard layers and active cleaning (sonic horns plus sootblowers).
Tail-end SCR niche
Tail-end SCRs are favoured where dust loading would otherwise destroy the catalyst (some WtE plants), where retrofitting onto an existing layout leaves no upstream space, or where catalyst poisons (arsenic, alkali) must be filtered out first. The reheating energy penalty is significant.
Related terms
Related terms
- Selective Catalytic ReductionSCR is the dominant NOx-control technology on industrial combustion plant. Ammonia is injected upstream of a catalyst that converts NOx to nitrogen and water.
- Catalyst pluggageCatalyst pluggage is the physical blockage of SCR catalyst channels by large-particle ash, popcorn ash or ammonium-salt deposits. It causes ΔP rise and gas-flow maldistribution.
- Ammonium bisulphateAmmonium bisulphate is a sticky low-melting deposit formed when slipped ammonia reacts with SO3 in cooling flue gas. The dominant cold-end fouling species on SCR-equipped boilers.
- Electrostatic precipitatorAn ESP removes particulate from flue gas by charging dust and collecting it on plate electrodes. Sonic horns are widely used to dislodge ash from plates and to keep hoppers from bridging.