Flue gas desulphurization

Gas washer at the Allen power plant (USA)

As a flue gas desulfurization (FGD also) are flue gas cleaning process for removing sulfur compounds (SO ₂ and SO ₃ in) exhaust gases (eg. B. of power plants , waste incineration plants or large motors), respectively. The sulfur compounds are created by burning fossil fuels containing sulfur . Plants for flue gas desulfurization often with REA ( R and gas e ntschwefelungs a abbreviated nLocation).

application

A process for flue gas desulphurisation was invented in 1879 by the soda manufacturer Robert Hasenclever . Today it is used especially for power plants based on coal and heavy heating oil, in which the sulfur content is greater than one percent. When burning gas and light heating oil, desulphurization is generally not necessary.

Flue gas desulphurization has been mandatory in Germany for new hard coal power plants since 1974 and the ordinance on large combustion, gas turbine and combustion engine systems of June 1983 also required the retrofitting or decommissioning of old systems. At the beginning of the 1980s, lignite power plants initially planned to reduce sulfur dioxide emissions by adding lime to the raw lignite (also known as the dry additive process), which would have been significantly more cost-effective than conventional exhaust gas desulphurization systems. From June 1983, lignite-fired power plants in Germany also had to carry out flue gas desulphurisation, taking transitional periods into account.

Types of procedure

In power plants it is possible in principle to use technical measures to remove sulfur dioxide up to 95 percent from the flue gas. There are more than a hundred different procedures; they provide gypsum or ammonium sulfate as the end product .

Schematic structure of the absorber of a flue gas desulphurization system with lime washing

A distinction is made between regenerative and non-regenerative processes for desulfurization.

• Among the non-regenerative processes there is lime washing, which has become the most popular in the world. With the addition of "lime" (both calcium carbonate and calcium oxide ) gypsum is produced. This usually takes place in a countercurrent washer , the absorber. While the flue gas flows through the container from bottom to top, the sulfur compounds react with the washing suspension . This is finely distributed by spray levels that are attached in the upper area of ​​the absorber and trickles down into a collecting pan (sump). In the tank sump , the reaction product of sulfur dioxide ( calcium sulfite ) is oxidized to calcium sulfate (gypsum) by adding oxygen (air) :

${\displaystyle {\begin{aligned}\mathrm {2\;SO_{2}+2\;Ca(OH)_{2}} &\longrightarrow \mathrm {2\;CaSO_{3}+2\;H_{2}O} \\\mathrm {2\;CaSO_{3}+O_{2}+4\;H_{2}O} &\longrightarrow \mathrm {2\;[CaSO_{4}\cdot 2\;H_{2}O]} \\\hline \mathrm {2\;SO_{2}+2\;Ca(OH)_{2}+O_{2}+2\;H_{2}O} &\longrightarrow \mathrm {2\;[CaSO_{4}\cdot 2\;H_{2}O]} \end{aligned}}}$

After fresh lime has been added and part of the resulting gypsum has been excreted, the suspension is pumped back into the spraying levels via circulating pumps. Once the flue gas has reached the top of the scrubber, the last droplets of suspension are removed in the droplet separator and the gas can leave the system in a cleaned state. Further processes are spray absorption , dry absorption and ammonia REA ( Walther process ).

• One of the regenerative processes is the Wellman-Lord process , which is used in particular in refineries . Another method that is gaining more and more importance is carbon-based selective catalytic reduction ( CSCR ), which is based on adsorption on activated carbon . The flue gas to be cleaned preferably flows through a countercurrent moving bed adsorber from bottom to top, with the activated coke adsorbent slowly moving up from top to bottom. The loaded activated coke can be regenerated by thermal treatment in a desorption system (desorber). The rich gas obtained in this way from the desorption process, which has SO ₂ concentrations of typically 15-20% vol. can be used to produce sulfuric acid or elemental sulfur . Systems of this type are installed, for example, in waste incineration plants such as the Flingern waste incineration plant and in Chinese steelworks of the Jiangsu Shagang Group , the Anshan Iron and Steel Group and the Masteel Group.

Residue treatment

Of the approx. 7 million tons of FGD gypsum that accumulates annually in Germany, the building materials industry recycled approx. 3 million tons in 1995, so that approx. 4 million tons are sent to landfills every year . Compared with natural gypsum has FGD gypsum to be present the disadvantage of a higher residual moisture and as a dihydrate.

In the wet process for flue gas desulphurisation (this includes all processes with the exception of spray absorption and dry sorption), wastewater is produced which mainly contains the soluble components - including halogen compounds (chlorides, etc.) and also heavy metal compounds - from the fuel and the absorbent. This wastewater has to be cleaned before it is discharged into water .

literature

• Wolfgang Fritz, Heinz Kern: cleaning of exhaust gases. Legislation on emission protection, measures to prevent emissions. Mechanical, thermal, chemical and biological methods of exhaust gas cleaning. Desulphurisation and denitrification of combustion exhaust gases. Physical basics, technical implementation. 3. Edition. Vogel, Würzburg 1992, ISBN 3-8023-1454-9 .
• Hans-Günter Heitmann: Practice of power plant chemistry. 2nd Edition. Vulkan-Verlag, Essen 1997, ISBN 3-8027-2179-9 .
• Walter Kaminsky : Process for the desulphurisation of flue gas. In: Chemical Engineer Technology. 55, 9, 1983, ISSN  0009-286X , pp. 667-683.
• N. Williams, G. Srinivasan, P. Wechselblatt: Removal and recovery of SO ₂ from power plant flue gases. In: Chemical Engineer Technology. 45, 7, 1973, ISSN  0009-286X , pp. 437-441.

See also

• Exhaust gas cleaning
• Flue gas
• Flue gas denitrification
• Desulfurization
• Sulfur Emission Control Area - Regions in which the emission of sulfur and sulfur oxides by seagoing vessels is to be restricted

Web links

• Flue gas desulfurization in combustion power plants
• Desulfurization of town gas using the example of the Vienna-Simmering gasworks

Individual evidence

1. ^ W. Büchner, R. Schliebs, G. Winter, KH Büchel: Industrial Inorganic Chemistry . VCH, Weinheim 1989, ISBN 3-527-26629-1 , pp. 394 .
2. ↑ Karl Knoblauch, Ekkehard Richter, Harald Jüntgen: Application of active coke in processes of SO2 and NOx removal from flue gases . In: Fuel (=  Industrial Conversion of Coal and Carbon to Gas, Liquid and High-Value Solid Products ). tape 60 , no. 9 , September 1, 1981, ISSN  0016-2361 , pp. 832-838 , doi : 10.1016 / 0016-2361 (81) 90146-0 ( sciencedirect.com [accessed March 24, 2020]).
3. ↑ Dr. Horst Grochowski: Method and apparatus for the separation of sulfur & nitrogen oxides from a waste gas . June 6, 1977 ( google.com [accessed March 24, 2020]).
4. ↑ Dr. Horst Grochowski: moving bed reactor plant . May 12, 1990 ( google.com [accessed March 24, 2020]).
5. ↑ Waste incineration plant | Stadtwerke Düsseldorf. Retrieved March 24, 2020 . 
6. ^ WKV - Dr. Grochowski Anlagentechnik GmbH. Retrieved on March 24, 2020 (German). 
7. ^ Karl-Heinrich Grote, Jörg Feldhusen : Dubbel paperback for mechanical engineering. Springer-Verlag, 2011, ISBN 978-3-642-17306-6 , p. L52 Google Books