Recovering Waste Heat From Flue Gas in The Tail Section of Power Plant Boilers

Recovering Waste Heat from Boiler Flue Gases in Power Plants

The flue gas temperature from boilers in large coal-fired or gas-fired power plants typically ranges from 120°C to 150°C or even higher. This represents the largest heat loss during boiler operation, generally accounting for 5% to 10% of the total fuel calorific value. Recovering this energy is crucial for enhancing the overall efficiency of the power plant.

Flue gas heat recovery units are installed in the boiler tail flue to capture waste heat from exhaust gases. Their primary function is to reduce flue gas temperature and recover heat that would otherwise be released into the atmosphere via the stack, enabling cascaded energy utilization.

Two main technical approaches exist for power plant flue gas heat recovery, with finned tube heat exchangers playing either a core or auxiliary role.

Approach 1: Heat Recovery for Condensate/Air Heating

This traditional and widely adopted solution involves installing or retrofitting heat exchangers directly in the flue gas duct.

Low-Temperature Economizer

Function: Installed in the low-temperature flue gas duct after electrostatic precipitators or induced draft fans and before desulfurization towers. Utilizes flue gas residual heat to warm condensate exiting the steam turbine.

Core Component: Fin-tube heat exchangers are the absolute mainstay.

Working Principle: Low-temperature condensate (approx. 30-60°C) flows through the tubes while flue gas flows over the fins. The heated water returns to the thermal system, reducing the amount of steam extracted from the turbine for heating. This allows more steam to be used for power generation, thereby improving unit efficiency.

Advantages:

Directly lowers flue gas temperature and recovers heat.

Increases unit power output and lowers coal consumption per unit of electricity generated.

For gas-fired power plants, significantly improves combined-cycle efficiency.

Flue Gas Condensing Heat Recovery Unit

Function: Typically installed after the wet desulfurization tower and before the stack. Deep-cools flue gas below the water vapor dew point (approximately below 50°C).

Core Components: Typically employs highly corrosion-resistant finned tubes (e.g., 316L stainless steel).

Working Principle: Recovers not only the sensible heat of flue gas but also substantial latent heat released during water vapor condensation. Recovered heat is often utilized to warm return water in heating networks.

Advantages:

Extremely high waste heat recovery efficiency.

Can synergistically remove SOx, NOx, and dust from flue gas, achieving integrated energy conservation and environmental protection.

Option 2: Recover heat to elevate flue gas temperature entering the desulfurization tower

This approach addresses “white smoke” (plume) issues while recovering moisture.

Flue Gas-to-Flue Gas Heat Exchanger

Function: Recovers heat from high-temperature flue gas before the desulfurization tower to preheat the clean flue gas discharged after the tower.

Types:

Rotary GGH: Heat storage type, non-finned tube structure.

Shell-and-tube heat exchanger: Core component is a finned tube bundle. High-temperature flue gas heats the medium (e.g., water or thermal oil) flowing within the tubes, which then heats the clean flue gas.

Advantages:

Heating the clean flue gas increases its lift height, eliminating “white smoke” and reducing visual pollution.

Simultaneously recovers residual heat.