FLUE GAS CLEANING
Modern waste combustion had its breakthrough in the 1960s and 1970s. It was not long before plants were faced with emission requirements, leading to major investments in highly sophisticated flue gas cleaning technology to minimize impact on our environment. Many cleaning methods are available today, but lime products are included in most of them. The methods can be divided into three main groups: dry, wet-dry, and wet.
Dry Method
Lime is added to the flue gases either in the furnace or in the flue. Water is sprayed on the flue gases to cool and vaporize them. After that dry slaked lime Ca(OH)2 is added to the flue gases to bind acidic components such as HCl, HF, SO2, and SO3. A dry product is formed that is mixed with fly ash, which is captured by a downstream filter. If combustion of the waste takes place in a fluidized bed, lime can be added through a separate infeed in an unrefined form such as limestone, CaCO3, or dolomite, CaMg(CO3)2.
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Wet-dry Method
The flue gas is sprayed in a reactor with a slurry consisting of lime mixed with water. The slurry dries at the same time that it reacts with the acidic elements in the flue gases. A filter captures the particles in the flue gases most effectively. This method can use either burnt or slaked lime. The lime is used most efficiently, however, by using burnt lime and slaking it on site.
Wet Methods
In one wet method the flue gas is washed with water in a column or in a scrubber. In the other wet cleaning method, the flue gas is cooled enough for the vapor to convert from the gaseous phase to the liquid phase. This liquid phase, or condensation, contains many pollutants and must therefore be cleaned before release. Lime is used in the cleaning process to neutralize the condensed material. After that the pollutants are precipitated using a flocculent, which is disposed of later. Adding lime and active carbon substantially contributes to the separation of dioxins, mercury, and sulfur dioxides. |
