Dry flue gas cleaning

The dry flue gas cleaning is to remove pollutants caused by combustion processes used. In dry flue gas cleaning , mainly adsorptive and separating methods are used. First and foremost, the mission is intended to make a contribution to environmental protection.

Structure of dry flue gas cleaning

After the boiler and the heat exchangers , the flue gas produced during combustion enters a reactor, where, among other things, the acidic components are bound and thus removed. Following the reactor, the flue gas and the adsorbents enter the bag filter . There these are separated by surface filtration. In addition, the bag filter also serves as an adsorption surface for the flue gases. This means that when it flows through the filter cake, the remaining pollutants from the flue gas are bound.

Dry adsorption

The term adsorption is understood to mean the accumulation of molecules on the surface of solids. So that the adsorption of gases on solid surfaces is as effective as possible, adsorbing substances with the largest possible specific surface are used. Using special production processes, very porous adsorbent grains are produced which have only small dimensions in order to keep the diffusion paths for the gas molecules through the pores as short as possible. As a result, surfaces of up to 1500 m ² / g adsorbent can be achieved, since these have a high proportion of micropores.

Activated carbon in dry flue gas cleaning

A very frequently used method for cleaning both high and low concentration organically polluted flue gases is adsorption on solids. Activated carbon is used as the adsorbent, which is produced in the form of small spheres or extruded parts.

The finely ground activated carbon is added to the flue gas flow in the reactor. Due to the high specific surface area, activated carbon offers a large attack surface for the gas flowing past. The pollutants contained in the flue gas such. B. volatile heavy metals such as mercury, cadmium, thallium, selenium and arsenic, chlorinated dibenzodioxins and dibenzofurans (PCDD / PCDF), poorly volatile chlorinated hydrocarbons such as hexachlorobenzene, hexachlorocyclohexane, PCB, PAH diffuse due to a concentration gradient of the equilibrium between the gas flow and the adsorbent Gas space through the macropores and micropores into the interior of the sphere, where they are deposited on the surface. Another mechanism that leads to a separation is the so-called barrier effect . This effect is effective on small particles which because of their low mass remain on the flow lines around the grain, then hit the grain and are attracted and held by it ( Van der Waals forces ).

Chemical reaction / sodium hydrogen carbonate

Sodium hydrogen carbonate is fed into the flue gas flow in the reactor. The thermal effect of the flue gas dissociates the sodium hydrogen carbonate into sodium carbonate, water and carbon dioxide.

${\displaystyle \mathrm {NaHCO_{3(aq)}+HCl_{(aq)}\longrightarrow NaCl_{(aq)}+CO_{2(g)}+H_{2}O} }$

The dissociation temperature of dry sodium carbonate is significantly higher (approx. 850 ° C) than that of sodium hydrogen carbonate (65 ° C). In aqueous solution the reaction takes place at room temperature (20 ° C). Since the flue gas is mixed with water vapor by the oxidation (combustion), the dissociation temperature of the sodium carbonate can be reduced to approx. 165 - 180 ° C. In this temperature range the sodium carbonate loses its crystalline structure and breaks down into Na ⁺ and CO ₃ ²⁻ ions. If you look at the carbonate ions , you can see that the equilibrium is very strongly shifted to the side of CO ₂ and H ₂ O.

${\displaystyle \mathrm {2H^{+}+CO_{3}^{2-}\longrightarrow H_{2}O+CO_{2}} }$

For the reaction to take place, proton donors (H ⁺ donors) are required. B. in the form of acidic gases (HCl, H ₂ SO ₄ , HF ...) are already present. The cations (Na ⁺ ) contained in the flue gas after the reaction strive to form a stable compound and for this purpose react with the anions of the acids (Cl ^- , SO ₄ ²⁻ , F ^- ). A crystallization process takes place, whereby Na ⁺ with the anions of the acids, salts such as z. B. NaCl, NaF, Na ₂ SO ₂ forms.

Mechanical dust separation with bag filters

Filtering separators are mainly used to separate solid or liquid particles from gases. The spectrum of possible applications is wider than any other separation process. The technical spread and the economic importance of such separators are correspondingly large.

In principle, the separation takes place from a gas by means of gas filtration through a porous medium. Such media can have different structures and be made of various materials. A common feature of all filtering separators is the presence of a filter medium, which is made up of either discrete, interconnected collectors (e.g. fibers and grains) or a continuous phase with continuous cavities (e.g. perforated foil).

The gas to be cleaned is passed through the medium and, due to various mechanisms, the particles are deposited on the filter medium from a continuous phase (flue gas flow). This can be done both on the surface and inside the filter medium.

If the separation process takes place mainly inside the medium, we speak of depth or storage filtration. However, if a coherent layer (filter cake) forms on the surface of the filter medium after a short time, which then represents the actual filter medium, then we speak of cleaning or surface filtration.

Possible uses

The robust, simple and inexpensive cleaning technology, which has been the technology primarily used for fluorine and hydrogen chloride separation in the aluminum and brick industry as well as for biomass firing since the beginning of air pollution control measures in Germany, is also being used for exhaust gas cleaning through further development and new adsorbents Coal firing and waste incineration plants made accessible. The dry flue gas cleaning fulfills the requirements of the 17th BImSchV (regulation on the incineration and co-incineration of waste).

literature

• Michael Schultes: Exhaust gas cleaning. Process principles, calculation bases, process comparisons. Springer, Berlin et al. 1996, ISBN 3-540-60621-1 .
• Claus Zimmermann: 15 years of exhaust gas cleaning. Emissions regulations, measurement results, analyzes, evaluations, effects and conclusions. 4th edition, Touring-Club Schweiz, Emmen 1999, ISBN 3-908165-00-8 .