Flue Gas Heater in Flue Gas Desulphurisation System

Flue Gas Heater in Flue Gas Desulphurisation System

In the flue gas desulphurisation system, usually set up after the desulphurisation tower flue gas heater, its main role is to use the boiler flue gas to heat the flue gas after desulphurisation to ensure that the desulphurisation tower in the normal conduct of chemical reactions. This design can not only improve the efficiency of desulphurisation, but also prevent the temperature inside the desulphurisation tower from being too low, leading to a slowdown in the rate of desulphurisation reaction or condensation, which affects the effect of desulphurisation.

The role and working principle of flue gas heater:

Improve the efficiency of desulphurisation:

The chemical reaction in the desulphurisation process usually needs to be carried out within a certain temperature range in order to be efficient. By using boiler flue gas to heat the flue gas after desulphurisation, it can ensure that the temperature of the flue gas in the desulphurisation tower is within the optimum range (usually 50°C~80°C), so as to improve the reactivity and desulphurisation efficiency of the limestone or other desulphurising agents.

Prevention of condensation:

The flue gas after desulphurisation usually contains a certain amount of water vapour. If the temperature is too low, the water vapour will condense in the desulphurisation tower or pipeline and form acidic droplets, which will not only affect the efficiency of desulphurisation, but may also lead to equipment corrosion. The risk of corrosion is reduced by heating the flue gas to prevent water vapour condensation.

Protect downstream equipment:

The flue gas heater raises the temperature of the FGD flue gas to prevent the low temperature flue gas from entering the downstream equipment (e.g. fans, pipelines, dust collectors, etc.), protects these equipment from low temperatures, and also helps to maintain the thermal balance of the system.

Design and construction of flue gas heater:

The design of flue gas heater generally adopts the principle of heat exchanger, by exchanging heat between the boiler flue gas and the desulphurisation flue gas, so as to transfer the heat from the boiler flue gas to the desulphurised flue gas.

Common designs include:

Flue gas-flue gas heat exchanger:

In this design, the boiler flue gas and the post-desulphurisation flue gas flow through separate ducts and do not come into contact with each other, but transfer heat through the heat exchange surfaces (e.g. tube walls, fins). Since the flue gas has been desulphurised, it is usually cleaner and has a higher heat transfer efficiency, making it suitable for heat exchange.

Flue gas-water heat exchanger:

Another common design uses water as the cooling medium to transfer heat from the flue gas to the water. By heating the water, the heat is effectively transferred to the desulphurised flue gas. This design requires proper water circulation and cooling system in the system to regulate the temperature.

Workflow of flue gas heater:

Introduction of boiler flue gas:

Boiler flue gases are piped to the flue gas heater, usually high temperature flue gases (e.g., temperatures above 250°C). These flue gases exchange heat with the desulphurisation flue gases, releasing heat.

Heat transfer:

The boiler flue gas exchanges heat with the FGD flue gas through pipes or heat exchanger surfaces. The heat exchange surfaces inside the flue gas heater (e.g., pipe walls or fins) absorb heat, lowering the temperature of the boiler flue gas while raising the temperature of the FGD flue gas.

Heated FGD flue gas:

The heated FGD flue gas will enter the desulphurisation tower at a suitable temperature to complete the required chemical reaction process to ensure the desulphurisation effect.

Flue gas flow direction:

In practice, the boiler flue gas will usually have a higher temperature at the inlet end of the heater, while the temperature of the flue gas after desulphurisation is lower. Both of them maximise the heat exchange effect through reasonable design and flow direction distribution.