Sample conditioning system for removing selenium from a gas stream

Abstract

A sample conditioning system removes selenium from a flue gas sample to provide more accurate measure of mercury in the gas stream. Ammonia or another basic reagent is added to the sampled gas stream to increase the pH of the condensate, and thereby ensuring the removal of hydrogen selenide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more readily understood in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to FIG. 1, a schematic block diagram is provided of the present invention. A sample flue gas stream 10 containing selenium, selenium compounds, sulfur and/or sulfur compounds is drawn into the system and passes through a mercury reduction module 20, which converts oxidized forms of mercury in the sample gas to elemental mercury. For example, propane or other reducing agents can be added to the sample flue gas stream 10, which consume available oxygen and other oxidizing agents. The result is a reductive environment capable of catalytically reducing chemical compounds found in the gas stream. The resulting reduced sample gas stream 25 also has been found to typically contain hydrogen selenide, hydrogen sulfide and other reduced compounds.

Ammonia or another basic reagent 30 is injected into the reduced sample gas stream 25 to increase its pH. For example, gas-phase ammonia can be injected through a number of injectors at the outlet of the reduction module 20. The gas stream then enters a condensing module 40 or scrubber that removes moisture, condensibles, and soluble gases. The increased pH of the sample gas stream enables the condensing module 40 to remove virtually all of the hydrogen selenide present in the sample gas stream. In addition to hydrogen selenide, the resulting condenser effluent 45 also typically contains hydrogen sulfide and other soluble compounds. However, elemental mercury is not soluble in water, and therefore remains in the gas stream. The scrubbed gas stream is then processed by a mercury detector 50 (e.g., a CVAA detector) to measure elemental mercury.

The above disclosure sets forth a number of embodiments of the present invention described in detail with respect to the accompanying drawings. Those skilled in this art will appreciate that various changes, modifications, other structural arrangements, and other embodiments could be practiced under the teachings of the present invention without departing from the scope of this invention as set forth in the following claims.

Claims

1. A method for conditioning a flue gas stream to remove selenium prior to measuring mercury in the gas stream, said method comprising: adding a basic reagent to the gas stream to increase solubility of hydrogen selenide in water; andremoving moisture and soluble gases, including hydrogen selenide, from the gas stream.

2. The method of claim 1 wherein the basic reagent comprises ammonia.

3. The method of claim 1 further comprising adding a reducing agent to the gas stream to consume available oxidizing agents in the gas stream.

4. The method of claim 3 wherein the reducing agent comprises propane.

5. The method of claim 1 wherein moisture and soluble gases are removed from the gas stream by a condenser.

6. A method for measuring mercury in a flue gas stream comprising: adding a reducing agent to the gas stream to consume available oxidizing agents in the gas stream and convert oxidized forms of mercury in the gas stream to elemental mercury;adding a basic reagent to the gas stream to increase solubility of hydrogen selenide in water;removing moisture and soluble gases, including hydrogen selenide, from the gas stream; andmeasuring elemental mercury in the gas stream.

7. The method of claim 6 wherein the basic reagent comprises ammonia.

8. The method of claim 6 wherein the reducing agent comprises propane.

9. The method of claim 6 wherein moisture and soluble gases are removed from the gas stream by a condenser.

10. An apparatus for conditioning a flue gas stream to remove selenium prior to measuring mercury in the gas stream, said apparatus comprising: a mercury reduction module adding a reducing agent to the gas stream to consume available oxidizing agents in the gas stream and convert oxidized forms of mercury in the gas stream to elemental mercuryan injector adding a basic reagent to the reduced gas stream to increase solubility of hydrogen selenide in water; anda condensing module removing moisture and soluble gases, including hydrogen selenide, from the gas stream.

11. The apparatus of claim 10 wherein the basic reagent comprises ammonia.

12. The apparatus of claim 10 wherein the reducing agent comprises propane.