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Method and apparatus for NOx and Hg removal

a technology of nox and hg, which is applied in the field of method and apparatus for nox and hg removal, can solve the problems of large equipment and the toxic and flammable nature of ammonia reductant, the inability to remove nox and hg, so as to reduce the rate of fe2+ oxidation, prevent buildup, and stabilize iron edta

Inactive Publication Date: 2008-02-21
HAKKA LEO E +3
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

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.

Method used

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  • Method and apparatus for NOx and Hg removal
  • Method and apparatus for NOx and Hg removal
  • Method and apparatus for NOx and Hg removal

Examples

Experimental program
Comparison scheme
Effect test

example 1

[0079] The DeNOx (NOx removal) performance and the rate of Fe2+ oxidation in an uninhibited 0.1 M solution of Fe2+ EDTA was measured. The absorber has 12″ of packing, the absorber and regenerator sump oil baths were kept at 60° C. and 115° C. respectively. The absorbent flow rate was 10 ml / min and the gas flow was 2.3 l / min with a composition of 5% O2 and 580 ppmv NO, 13 ppmv NO2 with the balance nitrogen. The treated gas contained undetectable NO2. The ferrous iron oxidized to Fe3+ in about 4 hours, with the % DeNOx decreasing accordingly. Addition of sodium dithionite (Na2S2O4) reducing agent increased the concentration of Fe2+ and % DeNOx. The absorbent pH dropped quickly from 5.7 to the 3-4 range. The results are given in the following table. The following is the definition of the heading of each column in the table.

Time, hoursNa2S2O4, gFe2+, %NOLean pH% DeNOx01005805.7—0.576483.9192151483.6792251953.49843381383.6764131453.657552533.15916168823.418672251093.758182251493.857491...

example 2

[0081] The experiment of Example 1 was repeated with 1.68 wt. % sulfite ion from sodium sulfite and 0.42 wt. % thiosulfate ion from sodium thiosulfate added to the absorbent. The NO and NO2 concentrations in the feed gas were 540 and 44 ppmv respectively. The feed gas flow rate was 2.3 l / min and contained 5 vol. % O2 and the absorbent rate was 10 ml / min. The treated gas contained undetectable NO2. The results are given in the table below.

Time, hoursNa2S2O5, gFe2+, %NOLean pH% DeNOx0100 5405.9—0.564255.8995%164225.9796%2N / A275.63*95%351305.6694%464375.6793%5364425.5792%651375.8193%6.5451405.8893%751435.64*92%7.5551405.7693%864335.7694%964305.8394%1051335.89*94%10.3351355.9494%

Note:

*Sulfuric acid added to maintain pH close to 5.9.

[0082] The addition of the sulfite and thiosulfate resulted in a higher and more stable % DeNOx. Sulfite was consumed by the oxidation process with the sulfite ion level decreasing to 0.68 wt. % in 4.5 hours. To maintain the presence of sulfite in the abs...

example 3

[0083] The test in Example 2 was continued with the same absorbent (i.e. the absorbent from the end of the run of Example 2), using sodium dithionite as a reducing agent reagent to maintain the Fe2+ concentration high enough to provide efficient DeNOx performance. The test data are given in the table below.

Time,hoursNa2S2O4, gFe2+, %NO% DeNOx04456611387487%1.51515590%2.51514093%3513195%41513993%5.51514193%6513494%71514991%81385291%9.51514293%10.51514193%11384692%11.51514492%

[0084] The consumption rate of dithionite reducing agent was 0.87 g / hr. The sulfite ion concentration dropped from 0.74 wt. % at time zero to 0.57 wt. 15% at 11.5 hours, while the thiosulfate concentration stayed constant at 0.34 wt. %, indicating that the dithionite was the major reagent being consumed. Sulfate ion increased from 1.66 to 1.92 wt. % over the 11.5 hour run. The absorbent pH remained essentially unchanged at pH 5.5.

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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 ...

Claims

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Application Information

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Patent Type & Authority Applications(United States)
IPC IPC(8): B01D53/56C01B21/20
CPCB01D53/507B01D53/56B01D2251/902B01D53/965B01D2251/90B01D53/64
Inventor HAKKA, LEO E.OUIMET, MICHEL A.SARLIS, JOHN NICOLASRYAN, COLIN FRANCIS
Owner HAKKA LEO E
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