Why is Flue Gas Desulfurization Technology Needed?
During industrial combustion processes, whether in coal-fired boilers, cement kilns, or steel sintering systems, large amounts of sulfur-containing flue gas are inevitably produced, with sulfur dioxide (SO₂) being the most significant pollutant. After being released into the atmosphere, this gas reacts with moisture and oxygen in the air to form sulfuric acid mist or sulfate particles, a major source of acid rain
Against this backdrop, industrial enterprises face two core issues: on the one hand, how to achieve stable emissions that meet environmental regulatory requirements, and on the other hand, how to choose an economical solution that balances investment costs, operating expenses, and treatment efficiency across different desulfurization technologies.
Driven by this demand, flue gas desulfurization (FGD) technology has become a key component of industrial flue gas treatment systems. Among them, the two most common routes—dry FGD and wet FGD—have become the key options that companies must compare when selecting technologies.
What is Dry FGD (Dry flue gas desulfurization)?
Dry flue gas desulfurization (DGD) is an environmentally friendly treatment method that uses solid or dry powder absorbents to remove sulfur dioxide (SO₂) from flue gas. Its core feature is that it achieves the absorption and transformation of pollutants through gas-solid reactions without using large amounts of water. Therefore, it is widely used in flue gas treatment systems for small and medium-sized boilers, waste incinerators, and some industrial kilns.
In typical dry flue gas desulfurization processes, commonly used absorbents include slaked lime (Ca(OH)₂) or sodium bicarbonate (NaHCO₃). These absorbents are injected into the flue or reactor in a dry powder or semi-dry state. After sufficient contact with the high-temperature flue gas, a chemical reaction occurs, absorbing SO₂ in the flue gas and converting it into solid salt byproducts, such as calcium sulfate or sulfite, thereby achieving the purpose of desulfurization.
In the dry FGD system, sulfur dioxide in the flue gas is gradually fixed by a solid absorbent and transformed into stable solid products. These reaction products, along with fly ash, are then captured by the dust removal system, thus achieving pollutant removal. The entire process does not require a complex liquid circulation system, and therefore does not generate large amounts of wastewater, which is one of the key characteristics of dry desulfurization technology.
Compared to wet desulfurization systems, dry desulfurization systems have a simpler structure and require less floor space, making them suitable for space-constrained or small- to medium-sized industrial projects. Furthermore, because they eliminate the need for complex slurry circulation and wastewater treatment systems, their operation and maintenance are simpler, thereby reducing overall operating costs to some extent.
Currently, dry flue gas desulfurization technology is widely used in waste incineration, small industrial boilers, and some retrofit projects. It offers significant advantages, particularly in land- and investment-sensitive engineering scenarios, making it a crucial industrial flue gas desulfurization solution.
What is Wet FGD?
Wet Flue Gas Desulfurization (Wet FGD) is currently the most widely used and technologically mature flue gas desulfurization method in the industrial field. It is mainly used to remove sulfur dioxide (SO₂) from combustion flue gas, and is particularly suitable for high-emission industrial scenarios such as large coal-fired power plants, cement production lines, and steel sintering.
Its core principle is to use limestone slurry (CaCO₃) or lime slurry (Ca(OH)₂) as the absorption medium, allowing the flue gas to come into full contact with the slurry inside the absorption tower. When flue gas containing SO₂ enters the absorption tower, it undergoes a gas-liquid reaction with the sprayed slurry. SO₂ is rapidly absorbed and converted into sulfite, which is then further converted into stable calcium sulfate (CaSO₄·2H₂O) under the action of oxidizing air, which is the common industrial byproduct gypsum.
Due to sufficient gas-liquid contact and stable reaction conditions, wet desulfurization typically achieves high desulfurization efficiency, generally reaching 95% or even higher, making it the mainstream choice in areas with strict emission standards. Simultaneously, this process also allows for the resource utilization of byproducts, such as using the generated gypsum in building material production.
However, wet desulfurization systems are typically more complex, requiring supporting systems such as slurry preparation, circulating pumps, demisters, and wastewater treatment systems. Therefore, their investment costs, floor space requirements, and operation and maintenance costs are relatively high. Nevertheless, due to their high efficiency and stability, they remain one of the most reliable technologies for large-scale industrial flue gas treatment.
Dry FGD vs Wet FGD Comparative Analysis
| Comparison Parameter | Dry FGD | Wet FGD |
|---|---|---|
| Desulfurization efficiency | Moderate (70–90%) | High (95–99%) |
| Investment cost | Low | High |
| Footprint (space requirement) | Small | Large |
| Wastewater generation | None (dry process, no liquid effluent) | Yes (requires wastewater treatment) |
| O&M complexity | Simple / low maintenance | Relatively complex (corrosion, scaling, slurry handling) |
| Applicable scale | Small to medium-sized industrial plants | Large-scale industrial & utility power plants |
Overall, Wet FGD (wet flue gas desulfurization) achieves high desulfurization efficiency, typically exceeding 95%, thanks to its efficient and stable gas-liquid contact reaction mechanism. Therefore, it is more suitable for large-scale industrial plants with stringent emission requirements and is considered a standard solution in heavy industry. In contrast, Dry FGD (dry flue gas desulfurization) uses a gas-solid reaction method, resulting in a simpler system structure. It eliminates the need for complex slurry circulation and wastewater treatment systems, making it more economical in terms of investment costs, floor space, and operation and maintenance. It is more suitable for small and medium-sized industrial boilers or projects with limited space. Ultimately, the core difference between the two technologies lies in the trade-off between the “high efficiency and complex system” of the wet approach and the “economic simplification and flexible application” of the dry approach.
How Should Different Factories Choose FGD Systems?
In practical applications, the choice between dry and wet desulfurization processes is mainly determined by many factors, including flue gas scale, local emission standards, site conditions, and investment and operating costs. The applicable technical routes also differ significantly among different types of factories.
Applications Suitable for Dry FGD
Due to its relatively simple structure, compact footprint, and lower investment cost, Dry FGD systems are generally more suitable for small- to medium-scale industrial applications or projects that are sensitive to system complexity. For example, small industrial boilers and distributed heating systems usually generate relatively low flue gas volumes and face less stringent emission pressures, making Dry FGD system sufficient to meet basic desulfurization requirements.
Dry FGD is also well-suited for small-scale waste incineration projects, where limited installation space and budget constraints are important considerations. In addition, for plants with restricted space or retrofit projects, Dry FGD is often preferred because of its flexible equipment layout and high level of system integration.
Applications Suitable for Wet FGD
Compared with Dry FGD systems, Wet FGD systems are better suited for large-scale industrial applications with high emission intensity, especially in regions or industries with stricter environmental regulations.
For example, large coal-fired power plants typically generate massive flue gas volumes with high SO₂ concentrations, requiring high-efficiency Wet FGD systems to achieve stable and compliant emissions. In cement production lines, rotary kilns and other kiln systems usually operate continuously with relatively stable emission conditions, allowing Wet FGD systems to provide more reliable and efficient desulfurization performance. Similarly, in steel sintering plants, where flue gas composition is more complex and pollutant concentrations are higher, Wet FGD has also become the mainstream choice.
