Flue Gas Desulphurisation System (FGD)

Engineering Equipment India, Manufacturer of Dry Flue Gas Desulphurisation (DFGD) Systems and Wet Flue Gas Desulphurisation (WFGD) Systems. EEI designed & manufactured FGD systems for the environmental norms of India and Saudi Arabia. We have expertise to Design and Manufacture Desulphurization Systems for the environmental norms of all countries.

What is Flue Gas Desulfurization?

Flue gas desulfurization (FGD) is a process of removing sulfur dioxide (SO2) from the boiler exhaust flue gas before it is released into the atmosphere. FGD will remove 95 percent of the SO2 in the flue gases.

Sulfur dioxide is one of the elements forming acid rain. Tall flue gas stacks disperse emissions by diluting the pollutants in ambient air and transporting them to other regions.

Environmental regulations regarding Sulphur Dioxide (SO2) emissions have been enacted in many countries. Indian Ministry of Environment MoEF&CC notification effective from 21st February, 2019 onwards related to emission of SPM and introduces new norms for emission of SO2, NOx and Mercury from Thermal Power Plants (TPPs). It also specifies modified limits for specific water consumption by TPPs and insists to convert existing once through based condenser cooling system to recirculation type. Different limits are specified based on capacity of power plant and year of installation.

How does Flue Gas Desulphurisation work?

The FGD technology is based on a chemical reaction that occurs when the exhaust flue gases from the coal-fired boiler come into contact with alkaline sorbents like Lime and Caustic Soda. FGD can recover sulphur, sulphuric acid or make dry gypsum from waste product.

Alkaline sorbents like Lime are used for scrubbing flue gases to remove SO2.  This reaction removes 95% of the sulphur dioxide from the flue gas and converts the limestone into Calcium Sulphite (CaSO3). Calcium sulfite when oxidized gives a by-product gypsum which used in Cement & building industry

Caustic soda is limited to smaller combustion units because of its cost but has the advantage that it forms a solution instead of slurry. It produces a solution of sodium sulfite or bisulfite, depending on the pH, which is used in the Paper & Pulp industry.

Flue Gas Desulphurization Methods:

  • Wet scrubbing method using a slurry of alkaline sorbent like limestone or lime, or seawater to scrub gases
  • Spray-dry scrubbing using similar sorbent material to adsorb slurries
  • Wet sulfuric acid process recovering sulfur in the form of commercial quality sulfuric acid
  • SNOx Flue gas desulfurization removes sulfur dioxide, nitrogen oxides and particulates from flue gases
  • Dry sorbent injection systems.

Dry Flue Gas Desulphurisation (DFGD) System

Dry Flue Gas Desulphurisation / Lime Spray Drying FGD systems LSD is a dry scrubbing process that is generally used for low-sulfur coal.

Flue gas with high sulphur content from boiler is tapped off from the last heat trap after the Economiser/ air preheaters by mixing the gas stream concurrently with lime slurry droplets for chemical treatment. The lime slurry is changed into a substance of very fine particles through spray atomizers or through dual fluid nozzles. Some of the water in the spray droplets evaporates, cooling the gas at the inlet from 300°C or higher to 160°C to 180°C, depending on the relationship between approach to saturation and removal efficiency. The Lime droplets absorb SO2 from the gas and react with the SO2 in the slurry.

The desulfurized flue gas, along with reaction products, unreacted lime, and the fly ash passes out of the dry scrubber to the Dust Collector. Waste products are collected either in a baghouse or electrostatic precipitator. Dry FGD systems have the capability of capturing a high percentage of gaseous mercury in the flue gas if the mercury is in the oxidized form.

After Desulphurisation, the emission rate confirms to the standards specified ranging from 300 to 800 ppm. Waste CaSO3, CaSO4, and calcium hydroxide are produced in a dry form. The particulate emission in the form of dust is reduced to as low as 80 mg per nm3.

The DFGD system is employed to avoid excess usage of water.

Schematic view of Dry FGD


Wet Flue Gas Desulphurisation (WFGD) System

Wet FGD system, also called a wet scrubber system, is based on the principle that limestone, in the form of slurry is brought into contact with the flue gas which absorbs the SO2 in it.

Process water requirement for a 1000MW coal based thermal powerplant is approximately 100 TPH per FGD. This employs huge quantity of water depending on scavenging of the gas.

Venturi-rod scrubbers are used for simultaneous removal of SO2 and fly ash.

A venturi scrubber is a converging section of duct which accelerates the gas stream to high velocity. When the gas passes through the venturi, water at high pressure is sprayed. At the throat which is the maximum velocity point the liquid stream is injected, the turbulence caused by the high gas velocity atomizes the liquid into small droplets. The higher the pressure drop in the venturi, the smaller the droplets and the higher the surface area. This mixture is passed through a cyclone separator with mist eliminator, clean gas is tapped off through an ID fan and the water with particle is further treated.

An Effluent Treatment Plant (ETP)/ Waste water treatment plant is suggested along with a WFGD system to treat the liquid waste prior to disposal. The wastewater treatment plant produces a small volume of waste, rich in toxic metals (including mercury) that must be disposed.

contact@ee2i.com with the following details for techno- commercial offer
Type-Dry/ Wet
Gas flow, Temperature and Pressure
Type of fuel- Coal, Agro waste, Others/ Oil/ Gas
Gas analysis – Sulphur dioxide, oxygen, nitrous oxide, carbon monoxide and carbon dioxide, solid particles
Emission rate required- sulphur dioxide, nitrous oxide and solid particles
ID fan capacity and Chimney height
Availability of water if wet-type is required.