Does flue gas cleaning remove CO2?

No. Dust filters, scrubbers and catalysts target dust, sulphur dioxide, nitrogen oxides and heavy metals. Capturing carbon dioxide needs separate carbon capture equipment, which is rare in power plants today.

What happens to the gypsum from desulphurisation?

Gypsum from wet scrubbers has long been used to make plasterboard and other building materials. As coal plants close, that supply is shrinking.

What are BAT conclusions?

BAT conclusions are EU documents that describe the best available techniques for an industry and the emission levels associated with them. National authorities use them when setting permit conditions for each plant.

Do wood stoves at home need flue gas cleaning?

Small stoves are covered by product rules rather than industrial permits. Some have catalysts or can be fitted with dust separators, but the multi-stage systems described here are found in large plants.

Flue gas cleaning explained: dust, SO2, NOx and mercury

Flue gas cleaning is the set of processes that remove pollutants from a power plant’s exhaust before it leaves the stack. In a modern coal, biomass or waste plant the gas passes several stages in turn: one for dust, one for sulphur dioxide, one for nitrogen oxides and, where needed, one for mercury and other heavy metals.

What is in the exhaust

When fuel burns, it releases more than heat. Depending on the fuel, the flue gas contains:

Pollutant	Where it comes from	Why it matters
Dust (particulate matter)	ash in the fuel	harms lungs, carries heavy metals
Sulphur dioxide (SO2)	sulphur in coal and oil	acid rain, respiratory harm
Nitrogen oxides (NOx)	nitrogen in air and fuel at high temperature	smog, acidification, respiratory harm
Mercury and heavy metals	trace elements in coal and waste	toxic, accumulate in food chains
Hydrogen chloride and fluoride	chlorine and fluorine in waste and coal	acidic, corrosive
Carbon dioxide (CO2)	carbon in every fossil fuel	climate change

Flue gas cleaning deals with the first five. Carbon dioxide is a separate issue covered at the end.

Stage 1: removing dust

Electrostatic precipitators give dust particles an electrical charge as they pass between electrodes. The charged particles are drawn to collecting plates, from which they are knocked off and collected as fly ash. Precipitators handle very large gas volumes with little pressure loss.

Fabric filters, also called baghouses, push the gas through rows of filter bags. Dust stays on the fabric and is removed by pulses of air. Fabric filters catch very fine particles well and are common in biomass and waste plants.

Fly ash from coal plants has long been used in cement and concrete.

Stage 2: desulphurisation

The most widespread method is the wet limestone scrubber, known in Germany as REA. The flue gas rises through a spray of limestone slurry. Sulphur dioxide reacts with the limestone, and after oxidation the product is gypsum. Large scrubbers produce a steady stream of gypsum that has been used for plasterboard.

Alternatives include dry and semi-dry processes, in which lime or sodium bicarbonate is injected into the gas and the reacted powder is caught in the dust filter. They need less water and are often used in smaller plants and waste incinerators.

Stage 3: removing nitrogen oxides

Nitrogen oxides are partly avoided at the source with low-NOx burners and staged combustion, which keep flame temperatures and oxygen levels in check. What remains is removed chemically:

Selective catalytic reduction (SCR). Ammonia or urea is injected into the gas, which then passes over a catalyst. The nitrogen oxides are converted into nitrogen and water vapour, both harmless components of air.
Selective non-catalytic reduction (SNCR). The reducing agent is injected directly into a hot zone of the boiler, without a catalyst. It costs less to install but removes a smaller share.

Stage 4: mercury and other heavy metals

Part of the mercury is captured along the way, by the dust filter and the scrubber. Where that is not enough, activated carbon is injected into the gas. Mercury binds to the carbon, which is then caught in the filter. Waste incinerators often combine activated carbon with a fabric filter to capture dioxins as well.

The EU framework

In the EU, large combustion plants above a certain thermal input need a permit under the Industrial Emissions Directive. For each sector, the EU develops a reference document on best available techniques (BREF) through an exchange between authorities, industry and environmental organisations. Its core part, the BAT conclusions, sets emission levels associated with the best available techniques.

For large combustion plants the BAT conclusions were adopted in 2017 as Commission Implementing Decision (EU) 2017/1442. They cover dust, sulphur dioxide, nitrogen oxides, mercury and other pollutants for plants burning coal, lignite, biomass, oil and gas. National authorities had to bring permits into line within four years. The directive itself was revised in 2024.

What flue gas cleaning cannot do

Cleaning the exhaust makes combustion much less harmful to health and ecosystems, but it does not remove carbon dioxide. Capturing CO2 requires separate carbon capture equipment, which has so far been installed at only a small number of plants. The main route to lower CO2 emissions from electricity in Europe has been a change in the mix: less coal, more wind and solar. How far that has gone is shown in Europe’s electricity mix.

How it fits together

A flue gas cleaning train sits between the boiler and the stack of the plant described in how thermal power plants work. The cleaner the fuel, the simpler the train: a gas-fired plant mainly needs measures against nitrogen oxides, while coal and waste plants need every stage.

A solar module produces no flue gas at all. That is one reason why the share of solar, including small plug-in systems, matters for air quality as well as for climate.

More background is in the power and grid section.