What Is Flue Gas Treatment and Why Is It So Critical?

Flue gas treatment is an emission control process applied across a very wide range of industries, from coal and lignite power plants to cement kilns, and from waste incineration plants to glass and iron-steel factories. During combustion, the sulphur, chlorine, fluorine and nitrogen compounds contained in the fuel are oxidised into acidic gases, while fly ash, carbon residues and heavy metal aerosols become entrained in the gas stream. When no treatment is applied, these components become a primary trigger of acid rain, particulate pollution, ozone layer damage and respiratory diseases.

In Türkiye, under Annex 5 of the Regulation on the Control of Industrial Air Pollution (SKHKKY), a limit of 1700 mg/Nm³ SO₂ (based on 3% oxygen in the flue gas) is set for large combustion plants using liquid fuel, and a limit of 2000 mg/Nm³ SO₂ for solid-fuel plants with a thermal capacity below 50 MW. On the European Union side, the Industrial Emissions Directive (IED) and the Best Available Techniques (BAT) reference documents push these values even lower for most processes.

In addition, mechanisms such as emissions trading and the Carbon Border Adjustment Mechanism (CBAM) compel exporting industrial plants to take a stricter approach to emission control.

Modern Industrial Plant with Clean Stacks

Which Pollutants Are Controlled in Flue Gas?

Flue gas measurement and treatment systems target several parameters simultaneously, depending on the plant type. The main components monitored in standard emission measurement are:

Sulphur dioxide (SO₂): The main problematic emission in the combustion of high-sulphur fuels such as coal, fuel oil and petcoke.
HCl and HF: Seen intensively especially in the waste incineration, glass and ceramics sectors, and cause serious corrosion problems.
Nitrogen oxides (NOx): Formed at high combustion temperatures; both thermal and fuel-derived.
Particulate matter (dust, PM): Contains fly ash, carbon residues and metal particles; captured with an electrostatic precipitator or bag filter.
Heavy metals and dioxins/furans: Of critical importance especially in waste incineration, metal smelting and sludge incineration plants.

Each pollutant is controlled at a different treatment stage; a typical modern flue gas treatment line consists of a combination of particulate control, acid gas neutralisation, NOx reduction and heavy metal/dioxin capture steps.

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Comprehensive Solutions in
Core Treatment Technologies

With emission control systems designed specifically for your plant's needs, we ensure full compliance with statutory limit values while delivering high-performance, low-cost and lasting solutions.

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Wet FGD (Wet Scrubber)

Achieves 95-99% SO₂ removal with limestone or milk of lime, producing industrial-grade pure gypsum as a by-product.

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Semi-Dry (SDA)

A lime slurry is atomised into the flue gas, providing efficient neutralisation without any wastewater discharge.

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check_circle Heavy Metal and Dioxin Control
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The Role of Lime-Based Solutions in Flue Gas Treatment

Lime is the chemical backbone of flue gas treatment systems. In wet, semi-dry and dry processes alike, the neutralisation of acidic gases (SO₂, HCl, HF) relies directly on calcium-based reagents.

Hydrated Lime (Ca(OH)₂): The main reagent of dry sorbent injection and SDA systems. Its high BET surface area (typically >20 m²/g) maximises the reaction rate.
Quicklime (CaO): Used to prepare milk of lime in wet FGD plants. It also meets the high alkalinity requirement in integrated lines.
Acid Gas Remover (Special Formula): Used in plants with fluctuating and high acidic loads such as waste incineration and biomass. When dosed together with activated carbon, it also contributes to the removal of dioxins, furans and mercury.

Technical Points to Consider in Practice

Treatment performance depends not only on the chemical purity of the reagent but also on its physical properties and the process parameters:

Criterion	Optimum Value / Role
Reagent Surface Area	BET surface area of 20 m²/g and above (ideal for dry systems).
Stoichiometric Ratio	Dry: 1.8 - 2.5 \| Semi-Dry: 1.3 - 1.8
Flue Gas Temperature	140-180 °C for spray drying. At low temperatures there is a risk of acid condensation.
Filter Integration	In bag filters, the cake layer provides 10-20% additional removal.

Moreover, as of 2026, Türkiye's alignment process with the European Green Deal has increased the pressure to bring SO₂, HCl and dust limit values down to BAT reference levels in cement, iron-steel, glass, waste incineration and large combustion plants. Best practice examples include the use of high surface area lime and real-time dosing optimisation integrated with CEMS.

Frequently Asked Questions

Flue gas treatment is the process of capturing pollutants such as SO₂, HCl, HF, NOx, dust and heavy metals released from industrial combustion and process plants, using physical and chemical methods before they are emitted to the atmosphere. The goal is both to protect air quality and to comply with the emission limit values defined in the Regulation on the Control of Industrial Air Pollution and in EU directives.

Standard emission monitoring covers sulphur dioxide (SO₂), nitrogen oxides (NOx), hydrogen chloride (HCl), hydrogen fluoride (HF), particulate matter (dust), carbon monoxide (CO), heavy metals and, in waste incineration plants, dioxins/furans. A separate treatment stage is designed for each parameter.

Wet FGD is highly efficient (95-99%) but has high water consumption and a high wastewater load. Semi-dry systems (SDA) operate with a hydrated lime slurry and produce a dry ash output. Dry sorbent injection (DSI) is the lowest capital cost solution; it delivers close to 90% efficiency with high surface area hydrated lime.

Hydrated lime Ca(OH)₂ is the main reagent of dry sorbent injection and semi-dry spray dryer absorber systems. It is injected dry into the flue duct or sprayed as a suspension. Through the reaction Ca(OH)₂ + SO₂ → CaSO₃ + H₂O it neutralises acid gases and is captured as a solid salt in the bag filter.

Quicklime CaO is used particularly in wet FGD plants to produce milk of lime on site. It is reacted with controlled water in a slaking unit to obtain fresh Ca(OH)₂. This approach reduces transport costs and is suitable for cement and iron-steel plants that require high-reactivity, large-volume milk of lime.

The acid gas remover is a specially formulated lime-based product designed for applications with high and fluctuating HCl, SO₂ and HF content, such as waste incineration, biomass combustion, sludge incineration and landfill gas lines. Thanks to its high BET surface area it provides high removal efficiency at a lower stoichiometric ratio and, when dosed together with activated carbon, also contributes to dioxin and mercury removal.

Efficiency depends on the reagent's surface area, the stoichiometric ratio, the flue gas temperature, the relative humidity, the contact time and bag filter integration. In dry systems, an efficiency in the 85-90% range is typically targeted using a stoichiometric ratio of 1.8-2.5 and hydrated lime with a surface area above 20 m²/g.

For NOx control, low-NOx burners are used at the combustion source and SCR (Selective Catalytic Reduction) or SNCR (Selective Non-Catalytic Reduction) systems are used in the flue line. In these systems NOx is reduced to nitrogen gas and water vapour using a urea or ammonia reagent. Lime-based systems do not perform NOx removal.

Under Annex 5 of the Regulation on the Control of Industrial Air Pollution (SKHKKY), there is a limit of 1700 mg/Nm³ SO₂ on a 3% O₂ basis for liquid fuel, and 2000 mg/Nm³ SO₂ for solid-fuel plants below 50 MW. A threshold of 400 mg/Nm³ for NOx applies to plants with a thermal capacity of 10 MW and above. For specific values, the annexes of the SKHKKY and the relevant BAT references should be reviewed.

In waste incineration plants the flue gas is multi-component and contains HCl, HF, dioxins, mercury and heavy metals. For this reason, in addition to lime-based acid gas treatment, activated carbon injection, multi-stage bag filtration and sometimes wet stage scrubbing are used together. Sorbent (acid gas remover) products are the standard for these plants.