FGD System For WTE Plant

The flue gas desulfurization (FGD) system of waste to energy (WTE) power plants is mainly used to reduce the emissions of sulfur oxides (SOx), especially sulfur dioxide (SO ₂), generated during the combustion process. These gases can cause acid rain and severe air pollution, so desulfurization systems are an important part of emission control in incineration power plants.

What is FGD system? The principle of the FGD system is to remove sulfur oxides from flue gas through chemical absorption or chemical reaction. Typically, alkaline absorbents such as limestone (CaCO3), lime (CaO), or sodium hydroxide (NaOH) are used to react with SO ₂ to produce harmless solid or liquid by-products.

Generally, WET plants have special requirements for flue gas desulfurization. The smoke generated from garbage incineration has a complex composition, containing not only SO ₂ but also other harmful gases such as hydrogen chloride (HCl), hydrogen fluoride (HF), heavy metals, and dioxins. Therefore, the FGD system of incineration power plants usually needs to be used in conjunction with other purification devices (such as denitrification equipment, activated carbon adsorption systems, dust removal equipment) to ensure the removal of various pollutants.

During the process of garbage incineration, the humidity of the flue gas is high and the corrosiveness is strong. The material selection and anti-corrosion treatment of the FGD system are particularly important, and high corrosion-resistant materials (such as fiberglass and stainless steel) are often used to extend the service life of the equipment.

Ruichang – expert in FGD system

RUICHANG’s FGD system is widely used in industries such as petroleum and chemical. RUICHANG has provided engineering general contracting and supply services to dozens of domestic and foreign customers; In facilities with a capacity of over 100000 tons per year, significant innovation achievements have also been made in key equipment such as burners, and the main technical performance and indicators have reached or exceeded similar foreign products.

Why choose Ruichang

3 common-used FGD process

Common FGD methods include wet, semi dry, and dry desulfurization.

• Wet flue gas desulfurization is the most common and mature technology, especially suitable for the treatment of high humidity and highly corrosive flue gas in waste incineration power plants. The basic principle is to pass the flue gas through an absorption tower and come into contact with alkaline slurry (such as limestone slurry) for gas-liquid reaction, generating calcium sulfate (CaSO ₄), which is further oxidized to form gypsum (CaSO ₄ · 2H ₂ O).
• The semi dry flue gas desulfurization system uses a desiccant (usually hydrated lime, Ca (OH) ₂) mixed with a small amount of water or steam, sprayed into the flue gas, and removes SO ₂ through reaction. The by-product produced after the reaction is in dry powder form (usually a mixture of calcium sulfite and calcium sulfate).
• Dry flue gas desulfurization uses solid absorbents (such as dry lime powder or activated carbon) in direct contact with flue gas to remove SO ₂ through dry reactions. The byproduct generated in this method is dry particulate matter, which is subsequently collected by dust removal equipment.

Semi-dry FGD

Semi dry flue gas desulfurization (Semi dry FGD) system is a commonly used desulfurization technology in waste incineration power plants, especially suitable for small and medium-sized incineration facilities. Compared with wet flue gas desulfurization, semi dry flue gas desulfurization system has the characteristics of simple operation, less water resource consumption, and lower operation and maintenance costs, but its desulfurization efficiency is slightly lower than that of wet flue gas desulfurization.

Working principle of semi dry FGD

The basic principle of semi dry flue gas desulfurization is to spray lime slurry (Ca (OH) ₂) or other alkaline absorbent into the flue gas, and the sulfur dioxide (SO ₂) in the flue gas reacts chemically with the alkaline absorbent to produce calcium sulfite (CaSO3) and calcium sulfate (CaSO4 ₄). At the same time, the sprayed slurry will rapidly evaporate in the high-temperature flue gas, reducing the moisture in the flue gas and forming a dry solid byproduct. These by-products are ultimately captured by dust removal equipment such as bag filters or electrostatic precipitators, processed and discharged or reused.

The main steps of semi dry FGD

Flue gas pretreatment

Before entering the semi dry flue gas desulfurization system, the flue gas usually undergoes pre-treatment steps such as cooling and dust removal. This can prevent excessive flue gas temperature from affecting desulfurization efficiency, while reducing large particulate matter in the flue gas.

Absorbent preparation and spraying

1. Absorbent: The most commonly used absorbent for semi dry flue gas desulfurization systems is hydrated lime (Ca (OH) ₂), and sometimes limestone (CaCO3) or other alkaline materials are also used.
2. Slurry preparation: Lime or other absorbents are mixed with water to prepare a slurry of a certain concentration. In some systems, it is also possible to directly spray dry powder or add a small amount of water to the absorbent suspension.
3. spray drying absorber: the slurry is atomized through the nozzle and evenly sprayed into the high-temperature flue gas. The slurry reacts with SO ₂ quickly to generate solid particles of calcium sulfite and calcium sulfate. At the same time, the moisture in the flue gas evaporates rapidly, forming a dry gas-solid mixture.

Reaction and desulfurization process

During spray drying, the following chemical reactions occur between SO ₂ in flue gas and hydrated lime:

• SO2+Ca(OH)2→CaSO3+H2OSO₂ + Ca(OH)₂ → CaSO₃ + H₂OSO2​+Ca(OH)2​→CaSO3​+H2​O
• CaSO3+1/2O2→CaSO4CaSO₃ + 1/2O₂ → CaSO₄CaSO3​+1/2O2​→CaSO4​
• SO2+CaCO3→CaSO3+CO2SO₂ + CaCO₃ → CaSO₃ + CO₂SO2​+CaCO3​→CaSO3​+CO2​

The calcium sulfite (CaSO3) and calcium sulfate (CaSOx) generated by these reactions are suspended in the form of solid particles in the flue gas.

Particulate matter collection

The flue gas that has undergone desulfurization reaction contains a large amount of solid by-products (calcium sulfite, calcium sulfate, etc.), which are captured by baghouse or electrostatic precipitator (ESP). The typical equipment for dry and semi dry flue gas desulfurization is bag filter, as it can efficiently capture small particles and facilitate subsequent processing.

Advantages of Semi-dry FGD

1. Low water consumption: Compared with wet flue gas desulfurization, semi dry flue gas desulfurization has extremely low water consumption, and the sprayed slurry evaporates quickly in the flue gas without the need for wastewater treatment.
2. Simple system: Semi dry flue gas desulfurization has fewer equipment, a simple system structure, and low investment and operating costs.
3. Small footprint: Due to its simple system and small space occupation, it is suitable for waste incineration power plants with limited space.
4. No wastewater treatment problem: The semi dry process mainly produces solid by-products, which do not require the treatment of a large amount of wastewater, making it more environmentally friendly.
5. Low operation and maintenance costs: Due to the low consumption of water resources and mild corrosion problems, the maintenance requirements for semi dry flue gas desulfurization are relatively low, reducing the risk of equipment corrosion.

Is it worth to invest in semi-dry FGD system for WTE plant?

With increasingly strict global environmental standards, WTE plants are facing enormous pressure to reduce harmful gas emissions. The semi dry flue gas desulfurization system, as a relatively economical and efficient desulfurization technology, has certain investment attractiveness.

• Low cost and high efficiency: Compared with wet flue gas desulfurization, semi dry flue gas desulfurization has lower equipment investment and operating costs, making it suitable for small and medium-sized incineration plants. It consumes less water resources and does not require complex wastewater treatment systems, further reducing operation and maintenance costs.
• Strong adaptability: The semi dry process system is particularly suitable for high-temperature and high humidity flue gas generated by garbage incineration, and has relatively low requirements for site and equipment corrosion resistance, making it suitable for investment in small and medium-sized projects.
• Simplified by-product treatment: The solid by-products produced by the semi dry method are relatively dry, avoiding the processing difficulties of wet gypsum, making solid waste treatment simpler and reducing overall processing costs
• Global emission standards have been raised: for example, Europe and the United States have implemented strict smoke emission standards, while developing countries such as China have also introduced standards and regulations for SO ₂ emissions from waste incineration plants.
• Environmental subsidy policy: In many countries and regions, the government provides subsidies or tax incentives for the installation of FGD systems in waste incineration plants, which helps companies reduce their initial investment burden and improve investment returns.
• Reduce environmental risks: An effective FGD system can reduce sulfur dioxide emissions, lower fines and potential litigation costs faced by businesses for violating environmental regulations.