Process for removing mercury from flue gases

Abstract

Process for removing mercury from flue gases of high-temperature plants, in particular power stations and waste incineration plants in which a bromine compound is fed to the multistage furnace and / or the flue gas in a plant section downstream of the furnace, the temperature during contact of the bromine compound with the flue gas being at least 500° C., preferably at least 800° C. The combustion is carried out in the presence of a sulphur compound, in particular sulphur dioxide. Subsequently to the furnace, the flue gas is subjected to an optional multistage cleanup for removing mercury from the flue gas, which cleanup comprises a wet scrubber and / or a dry cleanup.

Description

[0001]This application is a continuation-in-part of application Ser. No. 10 / 202,571, filed Jul. 24, 2002, still pending.[0002]The invention relates to a process for removing mercury from flue gases of high-temperature plants, in particular power stations and waste incineration plants.BACKGROUND OF THE INVENTION[0003]Owing to the high toxicity of mercury, in particular of organically bound mercury, which is also absorbed by humans directly or indirectly via the food chain, strict limiting values exist for the legally permissible emission of mercury, for example from incineration plants and power stations. Despite the currently already low mercury concentrations of clean gas,—the half-hourly mean value currently permissible in Germany for mercury emissions from waste incineration plants is 30 μg / m3 S.T.P. dry basis (S.T.P. db),—owing to high volumetric flow rates, for example from large power stations, considerable mercury loadings are achieved, so that further reduction of the curren...

AI Technical Summary

Benefits of technology

[0008]The removal of mercury from the flue gases in a flue gas emission control system downstream of the combustion or a similar high-temperature process is critically dependent on what species of mercury is present prior to entry into the flue gas emission control system. As high a proportion as possible of ionic mercury is advantageous, since the ionic mercury is readily water soluble, that is to say it can be scrubbed out, and is readily adsorbable to a range of adsorbents. The addition of bromine or bromine compounds to the furnace causes, under the given conditions of a high-temperature process or the like, in the presence of a sulphur compound, in particular in the presence of sulphur dioxide, a substantial, essentially complete, oxidation of the mercury and therefore allows substantial removal of the mercury from flue gases.

[0011]The inventive process takes place in the presence of a sulphur compound. The addition of a bromine compound in accordance with the inventive process leads to a gas-phase reaction between mercury and bromine in the presence of sulphur dioxide. Since under the combustion processes and other high-temperature processes customary in the context of this invention, sulphur dioxide is generally formed, generally a sufficient supply of a sulphur compound is present for the inventive process. A sufficient supply in the context of this invention is present when, with addition of a bromine compound to the furnace, the content of sulphur dioxide in the flue gas upstream of the flue gas emission control system is significantly greater than zero. However, if in a combustion process sulphur dioxide is not formed, or sufficient sulphur dioxide is not formed, a sulphur compound must be fed to the process. This can be in the form of free or bound sulphur, for example sulphur granules, waste sulphuric acid or other high-sulphur wastes. In addition, in particular to decrease an excessive content of free halogens in the flue gas, a sulphur compound can also be added, if, for example, more bromine compound has been fed than is necessary to oxidize the mercury present. A sulphur compound can be added, for example, according to the process described in the patent application DE 10131464, which was unpublished at the priority date of the present application, for low-corrosion and low-emission co-combustion of high-halogenated wastes in waste incineration plants. According to this process, in the primary and / or secondary combustion chamber, sulphur or a corresponding sulphur source is added in a controlled manner. The amount of sulphur is controlled essentially in proportion to the instantaneous total halogen load introduced together with the wastes in the boiler flue gas. The added sulphur burns in the combustion chamber to form sulphur dioxide which leads within the boiler to a substantial suppression of free halogens in the boiler flue gas, which halogens are formed in the interim, and subsequently to stable halogen incorporation in the alkaline scrubber. The addition of sulphur is controlled in such a manner that the preset sulphur dioxide content in the flue gas at the boiler inlet or the preset sulphur dioxide residual content at the boiler exit, that is to say in the dirty boiler gas upstream of, for example, wet flue gas emission control, can be maintained via a simple primary control circuit in steady state operating conditions.

[0012]If specifically sodium bromide is added to the furnace, an increased consumption of sulphur dioxide is to be observed, which is due to the sulphation of the sodium bromide in the high-temperature region.

Problems solved by technology

Owing to the high toxicity of mercury, in particular of organically bound mercury, which is also absorbed by humans directly or indirectly via the food chain, strict limiting values exist for the legally permissible emission of mercury, for example from incineration plants and power stations.

Previous techniques for reduction are not sufficiently effective and, owing to their sometimes high additional capital costs and the additional consumption of operating media are relatively expensive.

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