The goal of a flue gas cleaning system is the reduction of emissions and the elimination of a maximum amount of pollutants from operational processes before they reach the atmosphere. The construction of a flue gas cleaning system depends primarily on the cleaning needs, the area of operation and the environment. The two main applications are in coal-fired power stations and waste incinerators. In flue gas are contained very different particulate and gaseous pollutants. For maximum removal of these substances, the flue gas has to pass through several cleaning stages. Through constant optimization of flue gas purification systems emission levels are now partly so low that they are scarcely detectable. The constant activity of the individual purification steps is controlled by continuous measurements. The main components of these systems are filtering, adsorption and absorption and catalytic conversion. The reaction products can be partly utilized even as end products. Examples include gypsum or fly ash as an additive for cement production by desulphurisation in coal fired power plants. Flue gas cleaning systems are divided into wet, dry and semi-dry systems. The wet systems can be operated free of wastewater and waste producing. The following technologies are used for flue gas cleaning: FGD technologies consist of separation of dust and particulate matter and denitrification technologies. The flue gases resulting in waste incineration process are passed through afterburner plants, electrostatic precipitators and fabric filters, mist and fume scrubber. By flue gas purification, the waste incineration can be carried out in an environmentally sound manner and climate-friendly way. Thus the acceptance of the waste incineration increases in the population and it can be used simultaneously as an option for power generation. However, a disadvantage is the high cost intensity. In addition to the nonhazardous components such as residual oxygen, water vapor and nitrogen fumes may also contain harmful substances such as hydrochloric acid, hydrofluoric acid, carbon dioxide and carbon monoxide, heavy metals, dioxins and furans, fly ash and soot, sulfur dioxide, nitrogen oxides and hydrocarbons. A portion of this material escapes as dust with different particle size or as an aerosol. The construction of a flue gas cleaning system is divided into several different stages. The typical components of most flue gas cleaning plants behind a waste incineration plant are the steps described below: dust, gas washing by HCl absorber, gas scrubbing SO2 absorber, denitrification and activated carbon adsorption.
The largest proportion of the dust in the flue gas is removed in the first stage of flue gas cleaning in a fabric filter or an electrostatic precipitator. Through the principle of filtration in fabric filters, the dust particles are deposited mainly outside of the filter material and form a so-called filter cake. With the aid of a compressed air pulse against the flow direction this filter cake is discharged into the collection hopper. When dedusting by an electrostatic filter dust is removed by means of electric charge. Electrostatically charged dust particles deposit on the collecting electrode by the action of an electric field. They are then mechanically rapped off at regular intervals and collected in a silo. The second stage of the flue gas cleaning is carried out by means of flue gas scrubbing in two sub-stages. The aim of these two sub-stages is to remove gaseous acid generators (HCI, SO2) and residual fine dust from the flue gas. First, the flue gases are cooled (quenched) and treated in countercurrent with wash water. Thereafter, in the second stage the sulfur dioxide is washed out. In a subsequent waste water treatment plant, the wastewater from the HCI level are first neutralized and then concentrated in the evaporation plant. To cool the hot dross they are then directed to the wet slag remover. In contrast, the waste water from the SO2 level reaches directly in the wet slag remover.
The final purification steps are the denitrification and activated carbon filtration. The escaping purified gas from the wet slag enters the denitrification, where it still contains nitrogen oxides. They are reconverted to elemental nitrogen and water with the addition of ammonia through a catalyst. The final stage of the flue gas cleaning - an activated carbon filter - adsorbs the residual organic still contained in the flue gas, such as halogenated hydrocarbons and dioxins and final mercury residues and other heavy metals. This adsorption is done by the addition of powdered activated carbon into the flue gas stream, where the coal is deposited together with the accumulated pollutants to the filter bags of the fabric filter again. Subsequently, the spent carbon is discharged and packed in barrels. Thus, they can be supplied to the energy recovery. Legal basis for the minimum requirements for flue gas cleaning systems are laid down in the Federal Emission Control Regulation (BImSchV) and in the Technical Instructions on Air Quality Control (TA Luft)