Removal of sulphur dioxide and halogens

[1, UBA, 2001] Sulphur dioxide and gaseous halogens are cleaned from flue-gases by the injection of chemical or physical sorption agents, which are brought into contact with the flue-gas. According to technique, the reaction products are dissolved or dry salts.

Dry systems:

In dry sorption processes the absorption agent (usually lime or sodium bicarbonate) is fed into the reactor as a dry powder. The dose rate of reagent may depend on the temperature as well as on reagent type. With lime this ratio is typically two or three times the stoichiometric amount of the substance to be deposited, with sodium bicarbonate the ratio is lower. This is required to ensure emission limits are complied with over a range of inlet concentrations. The reaction products generated are solid and need to be deposited from the flue-gas as dust in a subsequent stage, normally a bag filter.

The overdose of lime (or other reagent) leads to a corresponding increase in the amount of residues, unless reagent recirculation is carried out, when the un-reacted fraction can be recirculated and the stoichiometric ratio reduced accordingly.

If there is no pre-deposition stage (e.g. electrostatic precipitator), particles are removed with the used reagent and reaction products. The cake of reagent that forms on fabric filters gives effective contact between flue-gas and absorbent.

Plumes are rarely visible with this technique.

Figure 2.44: Schematic diagram of a dry FGT system with reagent injection to the FG pipe and downstream bag filtration

Semi-wet systems:

These are also called semi-dry processes. In the spray absorption, the absorption agent is injected either as suspension or solution into the hot flue-gas flow in a spray reactor (see Figure 2.45).

This type of process utilises the heat of the flue-gas for the evaporation of the solvent (water).

The reaction products generated are solid and need to be deposited from the flue-gas as dust in a subsequent stage e.g. bag filter. These processes typically require overdoses of the sorption agent of 1.5 to 2.5.

Here, the fabric filter is also an important part of the process. Plumes are also rarely visible with this technique.

Figure 2.45: Operating principle of a spray absorber [1, UBA, 2001]

A system which falls between the normal dry and semi-wet systems is also applied. This is sometimes known as a flash-dry system. (Alstom 2003) These systems re-inject into the inlet flue-gas a proportion of the solids collected on a bag filter. Water is added at a controlled rate to the collected fly ash and reagent to ensure that it remains free flowing and not prone to stickiness or scaling. No contact tower or slurry handling is required (cf. semi-wet systems) and no effluents are produced (cf. wet systems).

The recycling of reagent reduces demand for reagent and the amount of solid residue produced.

Stoichiometric ratios in the range of 1.5 to 2 are common. Recycling of reagent can also be applied to dry and semi-wet systems.

Wet systems:

Wet flue-gas cleaning processes use different types of scrubber design. For example:

 jet scrubbers

 rotation scrubbers

 venturi scrubbers

 dry tower scrubbers

 spray scrubbers

 packed tower scrubbers.

The scrubber solution is (in the case of water only injection) strongly acidic (typically pH 0 - 1) due to acids forming in the process of deposition. HCl and HF are mainly removed in the first stage of the wet scrubber. The effluent from the first stage is recycled many times, with small fresh water addition and a bleed from the scrubber to maintain acid gas removal efficiency. In this acidic medium, deposition of SO2 is low, so a second stage scrubber is required for its removal.

Removal of sulphur dioxide is achieved in a washing stage controlled at a pH close to neutral or alkaline (generally pH 6 - 7) in which caustic soda solution or lime milk is added. For technical reasons this removal takes place in a separated washing stage, in which, additionally, there occurs further removal of HCl and HF.

If the treated waste contains bromine and iodine, these elements can be deposited from the flue-gas flow if waste containing sulphur is combusted simultaneously. In addition to sulphur compounds, water-soluble salts of bromine and iodine will form, which can be deposited through the wet SO2 flue-gas cleaning processes. Additionally, the deposition of elementary bromine and iodine may be improved by specific employment of reductive washing stages (sulphite solution, bisulphite solution). In any case, it is important to be aware of which wastes contain iodine or bromine.

If lime milk or limestone is used as a neutralising agent in the wet flue-gas cleaning stages, sulphate (as gypsum), carbonates and fluorides will accumulate as water-insoluble residues.

These substances may be removed to reduce the salt load in the waste water and hence reduce the risk of encrustation within the scrubbing system. Residues of the cleaning process (e.g.

gypsum) can be recovered. When using a caustic soda solution there is no such risk because the reaction products are water-soluble. If NaOH is used, CaCO3 may form (depending upon water hardness), which will again lead to deposits within the scrubber. These deposits need to be removed periodically by acidification.

The diagram below shows a typical 2 stages wet scrubbing system. The number of scrubbing stages usually varies between 1 and 4 with multiple stages being incorporated in each vessel:

1 Rohgas 2 Sprühabsorber 3 Sorptionsmittelzugabe 4 Gewebefilter 6 Reingas

Figure 2.46: Diagram of a 2 stage wet scrubber with upstream de-dusting

Waste water from wet scrubbers:

To maintain scrubbing efficiency and prevent clogging in the wet scrubber system, a portion of the scrubber liquor must be removed from the circuit as waste water. This waste water must be subjected to special treatment (neutralisation, precipitation of heavy metals), before discharge or use internally. Mercury removal is given special attention. Volatile Hg compounds, such as HgCl2, will condense when flue-gas is cooled, and dissolve in the scrubber effluent. The addition of reagents for the specific removal of Hg provides a means for removing it from the process.

In some plants, the waste water produced is evaporated in the incineration plant by spraying it back into the flue-gas as a quench in combination with a dust filter.