This tutorial on power plant technologies elaborates on approaches to deal with flue gases and how pollutants within those are reduced.
5. Flue gas treatment
In order to abide to regulations that limit the emission of pollutants in big power plants, the flue gas is treated before being released into the environment. The application of relevant secondary measures serves to limit the emission of sulphur and nitrogen oxides, as well as dust:
- Dust precipitation
Settlement chamber, cyclone separator, electrostatic separator - Desulphurization
Flue gas desulphurization plant - Denitrification
DeNOx (Denitrification plan)
Dust precipitation
Dust is the solid, non-combustible remainder of the fuel. It is carried along with the flue gas. In a dust precipitation facility, the different aggregate conditions and thus different densities of flue gas and dust are used to remove the dust.
In a settlement chamber the flue gas is conducted through a series of chamber, thereby forcing the flus gas to change its direction several times. The dust, which is just carried along by the flue gas will thus settle at the bottom of the chambers and has to be removed mechanically.
When gas set in rotation, the heavy components are forced to the outside. The cyclone separator applies this principle. Dust rotates at the outer wall of the cyclone separator and slowly falls down, while the flue gas is able to exit the separator through the middle.
Within an electrostatic separator flue gas that contains dust is guided through an electric field. The electric field is created by the application of high voltage between two electrodes. In the electric field, electrons move into one direction, while attaching themselves to the dust particles. Thus, the electrically charged dust is deflected by the electric field and will gather at one of the two electrodes, the so-called separation electrode. The dust has to be removed from there mechanically.
Desulphurization
The wet scrubbing method with limestone has established itself for the desulphurization process. Limestone and water are mixed and the resulting suspension is sprayed onto the flue gas in the washing tower. The gaseous sulphur dioxide is dissolved.
During this process sulphur dioxide and limestone form carbon dioxide. By fanning oxygen into the lower part of the washing tower, gypsum starts to accrue.
Denitrification
In hard coal power plants primary measures are not sufficient to comply with regulatory thresholds for nitrogen oxides. The SCR-method (selective catalytic reaction) is the most frequent form in large combustion plants. The reaction of the nitrogen oxides with ammonia at 300 to 400 °C, aided by a catalyst (mainly titanium oxide) results in the removal of the pollutant. The products of the reaction are water and nitrogen.
In the best case, the ammonia used during the process is entirely consumed.
