Catalytic adsorbents for mercury removal from flue gas and methods of manufacture therefor

Abstract

The present invention provides catalytic adsorbents formed from doping activated carbon with a dispersed halide salt. The catalytic adsorbents provided herein are stable and harmless at room temperature, yet allow for chemical adsorption at elevated temperatures typical of those for flue gas streams. The present invention also provides methods of manufacturing the doped activated carbon adsorbents.

Description

TECHNICAL FIELD [0001] The present invention relates generally to catalytic adsorbents for use in the removal of mercury from flue gas streams and methods of manufacturing such catalytic adsorbents. BACKGROUND OF THE INVENTION [0002] The toxicity of mercury to humans and the environment has long been known. It is known for example that mercury exposure can cause neurological damage in humans. A particularly devastating example of the harmful effects of mercury occurred in Minamata, Japan in the 1950's where organic mercury byproducts of acetaldehyde production were discharged into the local bay. The byproducts were consumed and metabolized by fish. By consuming fish in the bay, wide spread neurological damage and birth defects among the local population were reported. [0003] Coals used for generating electric power often contain about 0.1 ppm mercury. In the United States alone, about 50 tons of mercury are discharged as vapor in stack gas every year. Through chemical and biological...

AI Technical Summary

Benefits of technology

[0016] The present invention provides catalytic adsorbents in which a halide salt is dispersed on activated carbon and the oxidation catalytic activity of the activated carbon promotes the formation of mercury halide. At the same time, the adsorbent qualities of activated carbon retain the mercury halides thus formed. The present invention recognizes that while the halide salts are stable and harmless at room temperature, these doped activated carbon compounds form mercury halogen compounds at elevated temperatures typical of those found in flue gas streams, and in the presence of reactive components typical of flue gas. These mercury halogen compounds are retained on the surface of the activated carbon. Moreover, the increased adsorbent capacity and faster rate of adsorption result in a need for smaller quantities of adsorbent relative to an undoped activated carbon formed from the same starting material.

[0019] The present invention also provides methods of manufacturing such doped activated carbon adsorbents that are both economical and safe. The catalytic adsorbents of the present invention can be made from a variety of methods. In one embodiment, the catalytic adsorbents can be formed by placing an activated carbon in an aqueous solution containing a halide salt to form a mixture, stirring the mixture until a homogeneous slurry is formed and drying the activated carbon such that water from the aqueous solution evaporates and the halide salt is dispersed on the surface of the activated carbon.

[0021] The catalytic adsorbents of the present invention are suitable for use in the removal of mercury from a gas stream containing an oxidant and / or acidic gases at an elevated temperature such as a flue gas stream exiting a boiler or combustion process. In this process, the catalytic adsorbents of the present invention are injected into the flue gas stream for an in-flight mode of mercury capture. As discussed above, the dopant is inert with respect to the mercury at room temperature. At flue gas temperatures and in the presence of the activated carbon, oxidant and / or acidic gases, however, the dopant effectively removes mercury from the flue gas stream. The mercury is retained on the activated carbon in the form of mercury halogen compounds and can be separated from the flue gas stream together with the flyash.

Problems solved by technology

The toxicity of mercury to humans and the environment has long been known.

It is known for example that mercury exposure can cause neurological damage in humans.

A particularly devastating example of the harmful effects of mercury occurred in Minamata, Japan in the 1950's where organic mercury byproducts of acetaldehyde production were discharged into the local bay.

By consuming fish in the bay, wide spread neurological damage and birth defects among the local population were reported.

Through chemical and biological processes, this mercury can become concentrated in fish by many thousand fold, thereby entering human food supplies at harmful levels.

The effort to remove trace mercury from air, water, natural gas, and other industrial streams has a long history, however; removing mercury from coal burning flue gas streams is a very different problem.

Prior art techniques for removing mercury from air or hydrocarbons at room temperature generally have limited relevance to removing mercury from flue gas streams.

Mercury has a high atomic weight and adsorption temperature is a significant issue.

(the temperature of many flue gas streams), however, physical adsorption is no longer able to hold down the volatile elemental mercury.

In addition, sufficient contact time with rapidly moving flue gas streams is another issue for mercury removal.

The demand on reactivity and reaction kinetics by flue gas cleaning can not be properly tested by conventional packed beds.

Conventional packed beds are insufficient for flue gas cleaning because the volume of flue gas is so large, the cost for compressing it to push it through a packed bed is prohibitive.

Further issues relating to the removal of mercury from flue gas include the small, yet potentially toxic, concentration levels of mercury in the flue gas streams.

Bromine gas is known to be highly toxic by inhalation, ingestion or skin contact.

HBr is also known to be corrosive.

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