Method and apparatus for NOx and Hg removal

Abstract

A cyclic process for the removal of NOx from a NOx containing feed gas, and optionally removal mercury vapour and / or sulfur dioxide is provided. A process is also provided by which an iron chelate absorbent may be thermally regenerated.

Description

FIELD OF THE INVENTION [0001] This invention relates to a process for the recovery of NOx, and optionally mercury, from a gaseous stream. In another aspect, this invention provides a method of regenerating an iron chelate absorbent. BACKGROUND OF THE INVENTION [0002] SO2 removal is most often conducted by one of limestone slurry scrubbing, limestone addition to fluidized boilers, lime spray driers, caustic wet scrubbing or, more recently, by regenerable amine solution scrubbing. Each of these processes has properties, which tend to make them suitable for some particular applications but not others. Each specific situation is evaluated for the optimum choice of technology. [0003] NOx (mainly nitric oxide NO, with low ppmv's of NO2) removal in many cases is done by Selective Catalytic Reduction (SCR) in which the NO is reduced to N2 by ammonia over a heterogeneous catalyst at elevated temperature. Deficiencies of this process are high cost, large equipment and the toxic and flammable ...

AI Technical Summary

Benefits of technology

"The present invention relates to a method for stabilizing iron EDTA against oxidative degradation and reducing the rate of Fe2+ oxidation in an absorbent. The invention involves the use of a free radical scavenger, such as sodium thiosulfate or sodium thiosulfate in combination with an amine, to prevent the degradation of the iron chelate. The invention also involves the use of an oxygen scavenger to inhibit the oxidation of iron in the absorbent. The invention further involves the use of a pH buffer to maintain the pH of the absorbent during the absorption process. The invention provides a method for effectively stabilizing iron EDTA and reducing the rate of Fe2+ oxidation in an absorbent."

Problems solved by technology

Deficiencies of this process are high cost, large equipment and the toxic and flammable nature of the ammonia reductant.

The cost of the reagents is generally prohibitive except for the conversion of small concentrations of NOx.

The nitrates and nitrites, which are then captured in the subsequent wet scrubbing step present unacceptably high concentrations in the process effluent water in some cases.

A disadvantage of the use of iron chelates for NO capture is the high rate of oxidation of the ferrous chelate to the ferric form, which does not absorb NO.

35-37; U.S. Pat. No. 5,200,160), ascorbic acid or dithionite (K. Smith, L. Benson, S. Tseng, M. Babu and P Bergman, Proceedings of the 1992 Clean Coal Conference) is also possible but the cost of the reagent makes the process uneconomical.

Reduction by electrolysis has also been described (U.S. Pat. No. 5,320,816, U.S. Pat. No. 5,433,934 and U.S. Pat. No. 4,126,529), but the high duty required by the rapid oxidation of Fe2+ EDTA to Fe3+ EDTA again makes this alternative less desirable.

Mercury is extremely toxic, affecting the nervous system.

Since it tends to bioaccumulate into the food chain, even small concentrations can eventually cause health effects in humans and fauna.

The concentration of mercury in flue gases is generally in the range of 10 micrograms per cubic meter, so effective capture can be difficult.

Compounding this difficulty is the fact that the mercury is present both as particulates of ionic mercury(II) compounds and as a vapor of the elemental form.

Deficiencies of the preceding mercury removal methods include high cost and insufficiently low removal efficiency.

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