Principle of SCR flue gas denitrification

SCR (Selective Catalytic Reduction) flue gas denitrification is the most widely used technology for removing nitrogen oxides (NOx) in coal-fired, gas-fired, waste incineration and other boilers at present.

## 1. Basic Principle

Under the action of a catalyst, a reducing agent (usually urea solution, liquid ammonia or aqueous ammonia) is injected into the flue gas at 300–420°C. NH₃ generated by the decomposition of the reducing agent undergoes redox reactions with NO and NO₂ in the flue gas to selectively produce N₂ and H₂O without consuming a large amount of O₂.

Main reaction (for NO, which accounts for most NOx):

4NO + 4NH₃ + O₂ → 4N₂ + 6H₂O

Side reaction (for NO₂):

2NO₂ + 4NH₃ + O₂ → 3N₂ + 6H₂O

✅ "Selective" means that NH₃ preferentially reacts with NOx instead of reacting with the abundant O₂ in flue gas.

## 2. Reducing Agents and Ammonia Generation Methods

| Reducing Agent | Common Form | Remarks |

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

| Liquid ammonia | Evaporation of anhydrous liquid ammonia | Low cost, but toxic with high explosion-proof requirements |

| Aqueous ammonia | Evaporation of 20%~25% dilute aqueous ammonia | Relatively safe with slightly higher operating cost |

| Urea | Thermal hydrolysis / pyrolysis of 50% urea solution to produce NH₃ | Safest option, high initial investment, commonly used in power plants |

## 3. Catalysts

- Types: Honeycomb, plate and corrugated styles (TiO₂ as carrier, active component V₂O₅‑WO₃/TiO₂)

- Arrangement temperature: Usually 300–420°C (after the economizer and before the air preheater)

- Service life: Generally 2–3 years, affected by fly ash abrasion, alkali metal/arsenic poisoning and sintering

## 4. Typical Process Flow

1. Reducing agent preparation (urea dissolution → hydrolysis/pyrolysis, or evaporation of aqueous ammonia / liquid ammonia)

2. Mixing of NH₃ and air → ammonia injection grid (AGC)

3. Uniform mixing with high-temperature flue gas

4. Passing through the catalyst bed in the SCR reactor → NOx is reduced to N₂

5. Outlet flue gas flows into the air preheater

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