proprietary and confidential © 2007 the babcock & wilcox company. all rights reserved. the...

© 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.1Proprietary and Confidential

Role of Sulfides in the Sequestration of Mercury by B&W Wet Scrubbers

Role of Sulfides in the Sequestration of Mercury by B&W Wet Scrubbers

2007 OLI User Conference, Morristown, NJ2007 OLI User Conference, Morristown, NJ

Behrooz GhorishiAdvisory Engineer


McDermott International, Inc.

The Babcock & Wilcox CompanyPower-generating systems and equipment For utilities and industry

J. Ray McDermott — Design / construction for deepwater and subsea production of oil / gas

BWX Technologies — Supplier of nuclear products; management and operation of government facilities


Over 135 Years of Excellence

Research Center located in Alliance, OH, since 1947


Typical Supercritical Boiler and Environmental Equipment for Pulverized Coal with >2% SulfurComplete system provided by B&W

Hg0 Hg2+ + HgP

Hg0

HgP

Hg2+


B&W Wet Scrubber


Motivation for This Study

• Regulations for Hg control on existing and new coal fired power plants

• Potentially high cost of control

• Implementation of Hg control using wet scrubbers


B&W Strategy • Develop low cost solutions using existing emission control

equipment

• Wet scrubber showed greatest promise.○ Oxidized Hg easily captured ○ Issue: Some captured, oxidized Hg converts to elemental Hg in

the wet scrubber and is “re-emitted”

• B&W focused much of its research on mitigating re-emission from wet scrubbers (discovery of the role of sulfides)

• Filed tests: importance of understanding complex electrolytic and Redox reactions in the slurry, modeling needed


The B&W Road to Sulfide

Why?

Baghouse Wet Scrubber

No Re-emission

Electro static

Precipitator (ESP)Wet Scrubber

Re-emission

from boiler

from boiler


The B&W Postulate

Baghouse Wet Scrubber

No Re-emission

ESP Wet Scrubber

Re-emission

from boiler

from boiler

Trace conc. H2S

Trace conc. H2S

Trace conc. H2S No H2S

H2Sg → H2Saq

H2Saq → H+ + HS- HS- + Hg2+ HgS⇌ + H+


Discovery of the role of H2S in B&W’s pilot-scale facility

ESP power must have destroyed H2S in the flue gas

3O3 +H2S →SO2 + H2O + 3O2


B&W Postulate (continued)

• Precipitation of Hg2+ as HgS retards reactions between Hg2+ and any reductant.

○ Potential reductants proposed by B&W Sulfites Transition metals and Sn, …

○ Very low H2S sufficient to influence re-emission. H2S @ 1 ppm is 1000x larger than the Hg

• B&W Pilot scale tests supported basic postulate○ Control method by H2S captured in US Patent 6,284,199

○ Method requires a gas injection grid


First Process improvement• Add aqueous sulfide donor (NaHS) to suction of

recirculation pumps○ Avoids need for gas injection grid

○ Spray header acts as distributor of H2S

○ Sulfide readily available for reaction at gas-liquid interface

○ Very little residual soluble sulfide remains in scrubber

• This concept is captured in US Patents 6,503,470 and 7,037,474; Marketed as AbsorptionPlus(Hg)TM

• The “NaHS solution” method was field-tested○ Success stories

○ Lessons learned


Successful Field-scale testing of NaHS; Endicott Station

55 MW (Ohio Bit.), limestone, in-situ forced oxidation wet FGD system0.07-2.9 gph corresponds to 0.06-2 ppm H2S on flue gas


Successful Field-scale testing of NaHS; Mt. Storm Station

563 MW (Med S East. Bit.), limestone, in-situ forced oxidation wet FGD system15 gph of reagent corresponding to 1 ppm H2S in the flue gas

0

5

10

15

20

25

30C

on

ce

ntr

ati

on

in

μg

/dscm

oxidized Hg

elemental Hg

FGD inlet FGD outlet

test w/oadditive

test w/oadditive

test withadditive

test withadditive

OHM data with SCR in by-pass

In the absence of the additive:

Hg0 control of -15% (re-emission of 15%)Total Hg control across FGD =71%

In the Presence of the additive:Hg0 control of 30% (re-emission was suppressed)Total Hg control across FGD =78%


Other Field tests Raise New Questions

• Cinergy’s 1300 MW Zimmer Station○ Thiosorbic® Lime (mag-enhanced) ex-situ oxidation○ Hg removal of only 51%,

• Re-emission was not prevented• Higher soluble sulfites?• Higher metallic ions?

• Power Plant A, 500 MW○ Significant fly ash penetration from ESP○ Precipitation of NaHS by metallic ions was observed○ Re-emission was prevented, However, higher reagent

injection rates were needed


Our Mission: To improve our understanding of the fundamentals involved in preventing Re-emission

• Bench-scale laboratory studies

• Basic Chemical Equilibrium Modeling ○ OLI – A fully functional commercial aqueous

electrolytic equilibrium model

• Field experience + Equilibrium electrolytic modeling led to some interesting findings


Simulation of a wet scrubber using the OLI Model

• A typical bituminous or subbituminous flue gas spiked with 2 ppb HgCl2

• Inlet gas temperature of 300 ºF

• L/G of 70 gal/1000 acf

• Slurry of 17% gypsum

• Limestone added to adjust desired pH

• Soluble S(IV) by NaHSO3; soluble chloride by CaCl2

• Fe2+, Fe3+, Cu+, Cu2+ and other metals by their respective chlorides


In the absence of sulfides, how does re-emission occur? Two influential parameters: O2 and S(IV)

0

10

20

30

40

50

60

70

80

90

100

O2 concentration in the flue gas, %

% in

let

Hg

2+ e

mit

ted

as

Hg

0

added S(IV) in mM as NaHSO3

none1

5

10 50 100 300 500

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2 3 4 5 6 7 8 9 10


Hg0 re-emission mechanism in the absence of sulfide; role of O2 and S(IV)

O2vap O⇌ 2aq

O2aq + 2SO32- 2SO⇌ 4

2-

HSO3- SO⇌ 3

2- + H+

2H2O O⇌ 2aq + 2H2aq

H2aq + Hg2+ 2H⇌ + + Hg0aq

Hg0aq Hg⇌ 0

vap

------------------------------------------HSO3

- + H2O + Hg2+ → SO42- + 3H+ + Hg0

vap

Suppression of Redox formation of H2 by O2

Consumption of O2 by S(IV)

Note: The global re-emission reaction does not reveal the role of O2 and S(IV)

Note: Hg in slurry is in ppb/ppt range, miniscule formation of H2 is required to trigger re-emission

Re-emission of Hg0 (Redox reaction)


Minor effect of metallic ions on Hg0 re-emission, shown for Fe (same behavior for Cu)

Only those transition metal ions capable of being oxidized to a higher state are active in promoting Hg re-emission

2Fe2+ + 2H+ 2Fe⇌ 3+ + H2aq

Hg2+ + H2aq Hg⇌ 0

aq + 2H+

Hg0

aq Hg⇌ 0vap

--------------------------------------2Fe2+ + Hg2+ → 2Fe3+ + Hg0

vap

Promotion of Hg Redox reactions


Effect of 0.01-100 ppm H2S or NaHSFormation of HgS prevents re-emission of Hg0 (not shown on the graph)

0

10

20

30

40

50

60

70

80

90

100

0 500 1000 1500 2000 2500 3000 3500 4000

Fe2+ added, ppm

% p

reci

pat

ed a

s H

gS

pH=6

pH=4

pH=2

No re-emissionregardless of pH


Interesting and complex interaction of metal ions, sulfides and pH

Scavenging of sulfides by metal ionsFe2+ (and not Fe3+) + S2

2- (and not S2-) FeS⇌ 2

However at low pH, Fe2+ is converted to inactive Fe3+

2Fe2+ + 2H+ 2Fe⇌ 3+ + H2aq

S22- is produced from S2- via a number of Redox reaction


Conclusions

Importance of sulfide chemistry in determining Hg control by wet scrubber

Importance of flue gas O2 and soluble sulfite, S(IV) on Hg re-emission

Complex interactions of oxidizable metallic ions, pH, and sulfides on prevention of Hg0 re-emission

Information on the level of these parameters in wet FGDs + detailed wet scrubber chemistry knowledge is necessary to implement Hg control in wet scrubbers

proprietary and confidential © 2007 the babcock & wilcox company. all rights reserved. the...

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