What Role Do Rotary Heat Exchangers Play in Flue Gas Denitrification in Thermal Power Plants?

The mainstream denitrification method in thermal power plants is SCR (Selective Catalytic Reduction). The rotary air preheater is a key coupling device in the boiler + denitrification system, divided into high-temperature, medium-temperature, and low-temperature sections. It plays a crucial role in heat exchange between flue gas and air before and after denitrification, with five main functions:

Providing the SCR reactor with a suitable reaction temperature (its most critical function)

Optimal reaction window for SCR catalyst: 300–400℃

1. High-temperature flue gas from the boiler first passes through the high-temperature side of the rotary air preheater, with a portion of the flue gas being diverted and fed into the SCR reactor;

2. Without air preheater-based diversion and temperature control: if the flue gas temperature at the furnace outlet exceeds 450℃, the catalyst will burn; if the flue gas is directly cooled below 280℃, the catalyst will become ineffective, and ammonia escape will increase dramatically;

3. The rotary air preheater's segmented heat exchange precisely maintains the SCR inlet flue gas temperature stable at 320–380℃, ensuring the continuous and efficient denitrification reaction.

Recovering waste heat from flue gas reduces energy consumption in the denitrification system and improves boiler efficiency.

1. After denitrification, high-temperature flue gas re-enters the rotary air preheater to heat the boiler's primary and secondary air.

2. Hot air is sent into the furnace to aid combustion, reducing coal consumption and lowering power plant coal consumption.

3. Recovering heat from the flue gas reduces the inlet temperature of the desulfurization tower, alleviating the corrosion and cooling load on subsequent desulfurization equipment.

Low-Temperature Corrosion and Clogging Issues Caused by Denitrification

Side Reactions Occur After SCR Injection of NH₃:

Ammonium bisulfate is a viscous liquid in the 140~280℃ range, easily clogging heat exchange elements and corroding steel plates. The rotary air preheater is a major deposition area for ammonium bisulfate:

1. Rotary regenerative elements with compartmentalized heat exchange, combined with steam blowing and high-pressure water flushing, can periodically clean ammonium salt blockages;

2. By adjusting the heat exchange zones of the air preheater to avoid the high adhesion temperature zone of ammonium bisulfate, ash blockage and corrosion can be alleviated;

3. If the air preheater is severely blocked, flue gas resistance increases, induced draft fan power consumption increases significantly, the flue gas flow field of the denitrification system becomes turbulent, and denitrification efficiency decreases.

Stabilizing the flue gas flow field ensures uniformity of denitrification.

1. The regenerative rotor of the rotary air preheater plays a role in equalizing flue gas flow, ensuring a uniform distribution of flue gas temperature and velocity entering the SCR reactor.

2. Uneven flue gas temperature and velocity can cause localized excessive/insufficient ammonia injection: leading to increased ammonia slip and excessive NOₓ levels in some areas. A stable flow field reduces uneven ammonia injection, decreases ammonia slip, and reduces ammonium salt formation.

Isolating high and low temperature media to protect the denitrification catalyst and downstream equipment:

1. Cold-side air prevents direct contact between low-temperature cold air and the catalyst, preventing catalyst cracking and failure due to sudden cooling;

2. Low-temperature flue gas after denitrification is cooled by an air preheater before entering the electrostatic precipitator and desulfurization tower, avoiding damage to the dust collector filter bags and the anti-corrosion layer of the desulfurization absorption tower by high-temperature flue gas;

3. Reducing direct heat emissions from flue gas and lowering heat loss from chimney exhaust.