Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.1 Role of Sulfides in the Sequestration of Mercury by B&W Wet Scrubbers 2007 OLI User Conference, Morristown, NJ Behrooz Ghorishi Advisory Engineer

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.2 McDermott International, Inc. The Babcock & Wilcox Company Power-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

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.3 Over 135 Years of Excellence Research Center located in Alliance, OH, since 1947

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.4 Typical Supercritical Boiler and Environmental Equipment for Pulverized Coal with >2% Sulfur Complete system provided by B&W Hg 0  Hg 2+ + Hg P Hg 0 Hg P Hg 2+

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.5 B&W Wet Scrubber

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.6 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

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.7 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

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.8 The B&W Road to Sulfide Why? BaghouseWet Scrubber No Re-emission Electro static Precipitator (ESP) Wet Scrubber Re-emission from boiler

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.9 The B&W Postulate BaghouseWet Scrubber No Re-emission ESPWet Scrubber Re-emission from boiler Trace conc. H 2 S No H 2 S H 2 S g → H 2 S aq H 2 S aq → H + + HS - HS - + Hg 2+ ⇌ HgS  + H +

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.10 Discovery of the role of H 2 S in B&W’s pilot-scale facility ESP power must have destroyed H 2 S in the flue gas 3O 3 +H 2 S →SO 2 + H 2 O + 3O 2

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.11 B&W Postulate (continued) Precipitation of Hg 2+ as HgS retards reactions between Hg 2+ and any reductant. ○Potential reductants proposed by B&W  Sulfites  Transition metals and Sn, … ○Very low H 2 S sufficient to influence re-emission.  H 2 1 ppm is 1000x larger than the Hg B&W Pilot scale tests supported basic postulate ○Control method by H 2 S captured in US Patent 6,284,199 ○Method requires a gas injection grid

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.12 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 H 2 S ○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

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.13 Successful Field-scale testing of NaHS; Endicott Station 55 MW (Ohio Bit.), limestone, in-situ forced oxidation wet FGD system gph corresponds to ppm H 2 S on flue gas

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.14 Successful Field-scale testing of NaHS; Mt. Storm Station 563 MW (Med S East. Bit.), limestone, in-situ forced oxidation wet FGD system 15 gph of reagent corresponding to 1 ppm H 2 S in the flue gas

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.15 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

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.16 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      

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.17 Simulation of a wet scrubber using the OLI Model A typical bituminous or subbituminous flue gas spiked with 2 ppb HgCl 2 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 NaHSO 3 ; soluble chloride by CaCl 2 Fe 2+, Fe 3+, Cu +, Cu 2+ and other metals by their respective chlorides

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.18 In the absence of sulfides, how does re-emission occur? Two influential parameters: O 2 and S(IV)

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.19 Hg 0 re-emission mechanism in the absence of sulfide; role of O 2 and S(IV) O 2vap ⇌ O 2aq O 2aq + 2SO 3 2- ⇌ 2SO 4 2- HSO 3 - ⇌ SO H + 2H 2 O ⇌ O 2aq + 2H 2aq H 2aq + Hg 2+ ⇌ 2H + + Hg 0 aq Hg 0 aq ⇌ Hg 0 vap HSO H 2 O + Hg 2+ → SO H + + Hg 0 vap Suppression of Redox formation of H 2 by O 2 Consumption of O 2 by S(IV) Note: The global re-emission reaction does not reveal the role of O 2 and S(IV) Note: Hg in slurry is in ppb/ppt range, miniscule formation of H 2 is required to trigger re-emission Re-emission of Hg 0 (Redox reaction)

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.20 Minor effect of metallic ions on Hg 0 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 2Fe H + ⇌ 2Fe 3+ + H 2aq Hg 2+ + H 2aq ⇌ Hg 0 aq + 2H + Hg 0 aq ⇌ Hg 0 vap Fe 2+ + Hg 2+ → 2Fe 3+ + Hg 0 vap Promotion of Hg Redox reactions

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.21 Effect of ppm H 2 S or NaHS Formation of HgS prevents re-emission of Hg 0 (not shown on the graph)

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.22 Interesting and complex interaction of metal ions, sulfides and pH Scavenging of sulfides by metal ions Fe 2+ (and not Fe 3+ ) + S 2 2- (and not S 2- ) ⇌ FeS 2  However at low pH, Fe 2+ is converted to inactive Fe 3+ 2Fe H + ⇌ 2Fe 3+ + H 2aq S 2 2- is produced from S 2- via a number of Redox reaction

Proprietary and Confidential © 2007 The Babcock & Wilcox Company. All rights reserved. The Babcock & Wilcox Company.23 Conclusions  Importance of sulfide chemistry in determining Hg control by wet scrubber  Importance of flue gas O 2 and soluble sulfite, S(IV) on Hg re-emission  Complex interactions of oxidizable metallic ions, pH, and sulfides on prevention of Hg 0 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