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

[0008] In accordance with one aspect of the present invention, it has also been discovered that a free radical scavenger, such as sodium thiosulfate in combination with an amine, is effective in stabilizing iron EDTA against oxidative degradation and in reducing the rate of Fe2+ oxidation. Further such a combination does not result in the production of salts that form if sulfite is present, such as sodium dithionate or sodium sulfate. Such salts, if they form, must be removed from the absorbent to prevent buildup of these salts as the absorbent is recycled.

[0011] Without being limited by theory, it is believed that the amine reduces the rate of oxidation of the Fe2+ EDTA absorbent. Amines effective in reducing oxidation may be primary, secondary or tertiary with pKa's in the range 2.5-10, preferably 3.9-10 and, more preferably 3.9-9.5. To prevent loss of the amine with the treated gas, the preferred amines preferably have a vapor pressure less than 1 mm Hg at 50° C. over the absorbent. Preferred amines include 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (pKa=7.5), morpholinoethanesulfonic acid (pKa=6.1), N-(2-hydroxyethyl)ethylenediamine (pKa 1=9.5, pKa 2=6.5), piperazine pKa 1=9.8, pKa 2=5.6), N-(2-hydroxyethyl)piperazine (pKa 1=9.0, pKa 2=4.5), benzimidazole (pKa 5.5), pyrazole (pKa=2.5) and N,N′-bis(2-hydroxyethyl)piperazine (pKa 1=7.8, pKa 2=3.9) and mixtures thereof.

[0020] A second embodiment for recovering mercury from a feed gas is to have two separate scrubbing steps, a first with a high percentage of Fe2+ for NOx removal (which may be in any of those ranges set out previously) and a second step with an oxidizing agent such as a ferric salt in acidic solution, potassium permanganate solution or sodium hypochlorite solution for Hg0 removal. A further advantage of having a second step utilizing an oxidizing medium is that any NO not captured in the first step will be oxidized to NO2 and further to nitrate (NO3=) or N2O5, which will dissolve in alkaline media (e.g., the oxidizing agent) and result in a higher degree of and, preferably, essentially complete removal of NOx.

[0023] In accordance with another aspect of the present invention, the NOx rich iron chelate absorbent is preferably thermally regenerated to remove the NO from the complex. It is understood that the nitric oxide (NO) that is absorbed combines with the iron chelate to from an iron chelate nitrosyl complex. Without being limited by theory, the nitrosyl group may be removed from the iron chelate absorbent, forming a NOx lean absorbent, at elevated temperature using a stripping gas to transport out the evolved NO, e.g. by steam stripping. Preferably, the pH of the absorbent at the commencement of the steam stripping process is from 4 to 6, more preferably from 5 to 6 and, most preferably 5.5 to 6.0. It has been determined that by operating the process at such pH, the formation of N,S products is minimized.

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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