SNCR flue gas denitrification process

SNCR (Selective Non‑Catalytic Reduction) flue gas denitrification refers to the process where a reducing agent (typically urea solution or ammonia water) is injected into high-temperature flue gas without a catalyst. High temperature triggers the thermal decomposition of the reducing agent to generate NH₃, which undergoes a selective reduction reaction with NOx to form N₂ and H₂O.

### 1. Basic Principle

The reducing agent (urea or ammonia water) is atomized and injected into the furnace or bypass flue at a temperature of 850–1100°C (optimal range around 900–1050°C):

Taking urea as an example (thermal decomposition occurs before reaction):

(NH₂)₂CO + H₂O → 2NH₃ + CO₂

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

For direct injection of ammonia water:

NH₃ + NO + ¼O₂ → N₂ + 3/2H₂O

✅ The meaning of "selective" is the same as in SCR: NH₃ preferentially reacts with NOx instead of reacting extensively with O₂.

❌ No catalyst is used, so the temperature window for the reaction is stringent.

### 2. Selection of Reducing Agents

| Reducing Agent | Commonly Used Concentration | Characteristics |

| ---- | ---- | ---- |

| Urea Solution | 10%–50% (20%–40% in common use) | Safe, no explosion or toxic hazards; the most widely used |

| Ammonia Water | 20%–25% | Fast reaction rate but volatile and irritating |

| Liquid Ammonia | — | Rarely adopted for SNCR (stringent safety and licensing requirements) |

### 3. Typical Process Flow

Reducing agent preparation and storage (urea dissolving tank → storage tank → transfer pump)

Metering and dilution (flow rate adjusted according to load and NOx concentration)

The reducing agent is atomized by spray guns and injected into the furnace or the inlet of the cyclone separator

Commonly applied in the separators of Circulating Fluidized Bed (CFB) boilers

Reaction with NOx within the 850–1100°C temperature window

Unreacted residual NH₃ is discharged along with flue gas (accompanied by slight ammonia slip)

### 4. Key Parameters and Control

| Parameter | Typical Value | Description |

| ---- | ---- | ---- |

| Reaction Temperature Window | 850–1100°C | Too low → unreacted NH₃ (ammonia slip); too high → NH₃ oxidized into NOx |

| NSR (NH₃/NOx Molar Ratio) | 1.0–2.0 | Higher than that of SCR |

| Ammonia Slip | ≤5–10 ppm | Higher than that of SCR |

| Denitrification Efficiency | 30%–60% (typically around 40%) | Greatly affected by temperature field and mixing uniformity |

### 5. Application Scenarios

✅ Suitable for:

Circulating Fluidized Bed (CFB) boilers (the temperature at the separator perfectly matches the requirement)

Waste incinerators

Small and medium-sized coal-fired / biomass boilers

Used as a supplementary process upstream of SCR (combined process)

❌ Less suitable for:

Large pulverized coal boilers with undefined or sharply fluctuating temperature windows

Scenarios requiring ultra-low NOx emission (<50 mg/m³), where it is difficult to meet the emission standard by SNCR alone

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