Biogas Plant in Zell am See
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Flue Gas Cleaning

The final unit of the incineration plant is one of the most important as it has the objective of cleaning the air pollutants produced. Each ton of incinerated waste produces solid pollutants, like dust and heavy metals, and approximately 4000 to 6000 m3 of flue gases (Bilitewski et al., 1997), mainly carbon monoxide (CO), nitrogen oxide (NOX), sulfur dioxide (SO2), nitrous oxide (N2O), hydrogen chloride (HCl) and hydrogen fluoride (HF), among others. Combinations of several individual cleaning systems are utilized to provide an overall treatment system, with the actual number of potential flue gas treatment combinations reaching the 408 (European Commission, 2006).


1 Particle removal
2 Gaseous Contaminants removal
2.1 Wet Scrubbers
2.2 Wet-Dry Scrubbers
2.3 Dry Scrubbers
3 Nitrogen Oxide removal
3.1 NOx Limit Values in different Countries
4 Stack

Particle removal

The first step in flue gas treatment consists in removing the solid particles, whose size ranges from 1 μm to 1 mm. Some of the common practices nowadays include the use of cyclones, fabric filters or electrostatic precipitators (Figure 1). "Inside the cyclone, the gas swirls around an immersed tube and the particulates are carried by inertia to the cylinder wall, from where they ext through the conical section on the bottom while the clean gas exits through the top” (Bilitewski et al., 1997). Electrostatic precipitators utilize high voltage to electrically charge the particles contained in the flue gas by making contact with ions and electrons, once the particles are charged they move toward the precipitation electrode, when the process is finished the power supply is suspended and a hammer hits the precipitator to push the dust down. Fabric filters work like a household vacuum cleaner. The raw gas passes through a filter which allows air to flow through, but it retains particle material. The particles remain in the filter until compressed air is blown in the opposite direction, cleaning the filter and causing the dust to fell down where it is collected.
Solid Particle Removers
Figure 1: Components and operation of three common solid particle removers
Electrostatic precipitators and fabric filters have a similar operational area, with the latter showing a slightly better performance for particles smaller than 1 μm (Figure 2). Cyclones, alone, cannot achieve the emission levels now applied to modern waste incinerators. Cyclones, however, can be utilized as a complement with other flue gas treatment stages (TWGComments, 2003 and Bilitewski et al., 1997).
Operational Areas of Dust Removing Processes
Figure 2: Operational Areas of Dust Removing Processes

Gaseous Contaminants removal

Once the particulate material is removed, the gaseous contaminants of the flue gas have to be removed. All the current technologies for the removal are based on either absorption or adsorption processes. By absorption processes it is understand that the flue gas is mixed with additives that react and transform the contaminant gases into nonpolluting products, while in adsorption processes the molecules of the contaminant gases attach to the surface of another material, remaining the contaminants attached and allowing the nonpolluting air to flow. The removal of the acidic and alkaline pollutants is done using gas scrubbers, these can be done in three different ways: wet, wet-dry and dry.

Wet Scrubbers

In a wet scrubber, the flue gas is fed into water, hydrogen peroxide, or/and a washing solution containing part of the reagent (e.g. sodium hydroxide solution). The reaction product is aqueous.

The scrubber solution is (in the case of water only injection) strongly acidic (typically pH 0 - 1) due to acids forming in the process of deposition. HCl and HF are mainly removed in the first stage of the wet scrubber. The effluent from the first stage is recycled many times, with small fresh water addition and a bleed from the scrubber to maintain acid gas removal efficiency. In this acidic medium, deposition of SO2 is low, so a second stage scrubber is required for its removal.

Removal of sulphur dioxide is achieved in a washing stage controlled at a pH close to neutral or alkaline (generally pH 6 - 7) in which caustic soda solution or lime milk is added. For technical reasons this removal takes place in a separated washing stage, in which, additionally, there occurs further removal of HCl and HF.

Wet-Dry Scrubbers

Wet-dry scrubbers, unlike wet scrubbers, do not saturate the flue gas stream that is being treated. This type of scrubbers spray an aqueous absorption agent into the flue gas, the water solution evaporates and the reaction products are solids which can be filtered (Thomé-Kozmiensky, 1985). Figure 3 shows the operation of a typical wet-dry scrubber.

Figure 3: Operating principle of a Wet-Dry scrubber (UBA, 2001)

Dry Scrubbers

In dry sorption processes no water dilution is utilized. Instead the flue gas passes through a fine and dry powder, the solid reaction products generated have to be later removed from the flue gas stream by a filter as shown in Figure 2.4 (Bilitewski et al., 1997 and UBA, 2001). The typical absorption agents utilized are calcium hydroxide or lime (Ca(OH)2), commonly knows as lime, and sodium bicarbonate. The ratio used with lime is typically "two or three times the stoichiometric amount of the substance to be deposited, with sodium bicarbonate the ratio is lower” (European Commission, 2006). Equations 6, 7, 8 and 9 show the typical reactions of lime with acidic components that occur in dry sorption.

Equation 6: Ca(OH)2 + SO2 → CaSO3 + H2O

Equation 7: Ca(OH)2 + SO2 ½O2→ CaSO4 + H2O

Equation 8: Ca(OH)2 + 2HCl → CaCl2 + 2H2O

Equation 9: Ca(OH)2 + 2HF → CaF2 + 2H2O

Figure 3: Operating principle of a Wet-Dry scrubber (UBA, 2001)p>

Nitrogen Oxide Removal

Nitrogen oxides are destroyed either by selective non-catalytic reduction (SNCR) or by selective catalytic reduction (SCR). SNCR applies dry urea (CO(NH2)2) or ammonia (NH3) as reductive agents directly in the furnace. At temperatures between 900 and 1050°C the reducing agents react with the nitrogen oxides to form water and nitrogen (Equations 1 and 2).

Equation 1: 4NO + 4NH3 → 4N2 + 6H2O

Equation 2: 6NO2 + 8NH3 → 7N2 + 12H2O

In SCR the reductive process takes place in a catalyst, where at temperatures between 200 and 400°C a mixture of ammonia and air reacts with the flue gas to form oxygen and water (Equation 3, 4 and 5). The SCR module must be installed after the particulate material and acidic gases are removed. SNCR can achieve reduction rates of 70 percent while SCR can achieve up to 85 percent (Bilitewski et al., 1997).

Equation 3: 4NO + 4NH3 + O2 → 4N2 + 6H2O

Equation 4: 6NO + 4NH3 → 5N2 + 6H2O

Equation 5: 2NO2 + 8NH3 + O2 → 3N2 + 6H2O

NOx Limit Values in different Countries

Limit Value
O2 Reference
mg/m3s no
(if O2 < 11%)
(> 50 MW after end of 2012)

MACT standard for large EfW

To calculate the equivalents for the different countries, please download the following Microsoft Excel File.

Fyle Type Author Rel. Date Size Download
Dr. Oliver Gohlke 03.12.2010 24,5 kB Click here to download file


Once the gas is cleaned, it is analyzed for air contaminants and then expelled through the stack. The stack has to be tall in order to disperse the exhaust gases over a greater area and reduce the concentration of the remaining pollutants to the levels required by the government.

See Also

Waste Incineration Plant Scheme
Bunker >Feeding Unit >Furnace >Boiler >Energy Generation >Flue Gas Cleaning


  • Bilitewski, B.; Härdtle, G.; Marek, K., 1997: Waste Management. Springer, Berlin, ISBN: 3-540-59210-5
  • European Commission, 2006: Integrated Pollution Prevention and Control Reference Document on the Best Available Techniques for Waste Incineration
  • TWGComments, 2003: TWG Comments on Draft 1 of Waste Incineration BREF
  • UBA, 2001: Draft of a German Report for the creation of a BREF-document "waste incineration", Umweltbundesamt

Created by (), last modified by (2010-12-03)

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