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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).
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.
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, 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)
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 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
(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|
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.