Working Principle of Flue Gas Decolorization

Working Principle of Flue Gas Whitening and Dehumidification

The Nature of "White Smoke" in Flue Gas

To understand the principle of whitening and dehumidification, it's important to first clarify that "white smoke" is not smoke. It's actually:

After high-temperature, high-humidity flue gas (such as boiler and chemical exhaust) is discharged from the chimney, it comes into contact with the cooler ambient air.

The flue gas temperature drops sharply, and the water vapor it carries quickly reaches saturation and condenses into tiny droplets.

The large number of tiny droplets scatter light, forming a visible white plume.

In industry, a two-stage process of "cooling and dehumidification + heating and exhaust" is commonly used, with fin-tube heat exchangers playing a key role. The specific process is as follows:

1. Stage 1: Cooling and dehumidification (core of which is "dehydration")

Equipment: Fin-tube cooler (usually water-cooled).

Principle: High-temperature, high-humidity flue gas enters the tube or shell side of the fin-tube heat exchanger and exchanges heat with cooling water flowing on the other side. Function:

The flue gas temperature is rapidly reduced to below the acid dew point and above the water dew point (typically 30-50°C).

The majority of the water vapor in the flue gas condenses into liquid water due to the temperature drop and is discharged through the drainage system, significantly reducing the absolute humidity of the flue gas.

At the same time, some soluble pollutants (such as SO₃ and salts) in the flue gas are removed along with the condensed water, achieving initial purification.

2. Second Stage: Heating and Discharging (Core of "White Smoke Elimination")

Equipment: Fin-tube Heater (typically a flue gas reheater).

Principle: The dehydrated, low-temperature, dry flue gas enters another fin-tube heat exchanger for heat exchange with a high-temperature medium (such as raw flue gas, steam, or thermal oil).

Function:

The dry flue gas temperature is rapidly raised to 70-120°C, significantly reducing its enthalpy and relative humidity.

When the heated flue gas is discharged from the chimney, it is less likely to reach saturation during mixing with ambient air, thus preventing water vapor condensation and completely eliminating "white smoke." Fin-tube heat exchangers' core advantages in this process

Compared to conventional bare tube heat exchangers, fin-tube heat exchangers significantly improve white gas removal efficiency. These advantages are reflected in the following:

High heat transfer efficiency: The fin structure increases the heat transfer area by 3-10 times compared to bare tubes, significantly enhancing the heat transfer rate and ensuring that the flue gas temperature quickly reaches the target value (cooling or heating).

Compact equipment size: Given the same heat transfer requirements, fin-tube heat exchangers are significantly smaller than bare tube heat exchangers, saving plant floor space and equipment investment.

Suitable for high-dust flue gases: Some finned tubes (such as spiral fins and H-shaped fins) are designed with strong resistance to dust accumulation, making them suitable for treating industrial flue gases with high dust content, reducing equipment blockage and maintenance frequency.

To achieve stable white gas removal, the fin-tube heat exchanger requires precise control of two key parameters:

Outlet flue gas temperature: The cooling section must maintain a stable flue gas outlet temperature to avoid excessively low temperatures that could cause acid corrosion. The heating section must ensure that the outlet flue gas temperature is sufficiently high to prevent secondary condensation. Flue gas flow rate: It needs to match the design flow rate of the finned tube. Too low a flow rate will reduce the heat exchange efficiency, while too high a flow rate will increase the system resistance and energy consumption.