Pollution Engineering - January 2009 - (Page 23)

Old Fashioned CHEMISTRY The advantages of neutralizing wet ESP efﬂuent using magnesium hydroxide slurry. By STEVE LEYKAUF, Martin Marietta Magnesia Specialties lectric utilities are seeing increased pressure from regulators, environmental groups and the public to control both air and water pollutants. The higher cost, short supply, and higher level of elemental mercury concentrations of low-sulfur coal will keep them burning high-sulfur coal. The wet electrostatic precipitator (ESP) is a technology that is over a century old and has made a comeback. This is due in part to their successful use as a method for controlling SO3 gas and acidic mists in flue gas at electric power generation plants. In this process, a water mist is injected into the flue gas stream after flue gas desulfurization to capture SO3. The gas is absorbed into the water to form a sulfuric acid solution that enhances the wet ESP operation. However, this acidic effluent must be neutralized prior to discharge into regulated discharge waters or for reuse. The addition of an alkali such as magnesium hydroxide slurry – Mg(OH)2 – can effectively neutralize wet ESP effluent by providing more alkalinity per pound than other alkalis. As shown in Table 1, less Mg(OH)2 is required on a dry basis than caustic soda, hydrated Mist Water E Flue Gas Alkali FloMag H** Caustic Soda Hydrated Lime Soda Ash Lbs. Required to Neutralize One Ton of H2SO4 1190 1630 1510 2160 Total Dissolved Solids in Effluent (100% Basis) Per Ton of Acid 2460 2900 3510* 2900 * The CaSO4•2H2O would not be dissolved solids but precipitated sludge. ** FloMag is a trademark of Martin Marietta Magnesia Specialties LLC Table 1: Amount of alkali to neutralize one ton of sulfuric acid To Stack Wet ESP Flush Stream lime or soda ash to neutralize one ton of sulfuric acid. The chemical also generates less total dissolved solids than these other alkalis. In tests, one gallon of Mg(OH)2 was able to replace 1.77 gallons of 50-percent caustic soda. As shown in Figure 1, the Mg(OH)2 can be injected at the wet ESP discharge to adjust the pH to meet permitted discharge limits. The alkali buffers the solution to pH 9.0, even with an over-addition. While hydrated lime can form insoluble calcium sulfate solids, Mg(OH)2 forms soluble salts, thus lower sludge volumes. Mg(OH)2 is not regulated by the DOT and is considered to be non-toxic, nonhazardous, non-corrosive and safe for employees to handle. Mg(OH)2 naturally buffers at about 9 pH, precluding pH excursions. The rate at which neutralization occurs is more controlled than other alkalis. It is essentially a high-purity, industrial grade Milk of Magnesia. PE Steve Leykauf, sales engineer for Martin Marietta Magnesia Specialties LLC, can be reached at (574) 277-2237. Find more information about Mg(OH)2, on the product’s datasheet at http://magspecialties. com/datasheets/FloMag-pdf/FloMagH.pdf, and its MSDS sheet: www.magspecialties.com/MSDS/mgoh_slurry_flomag_h.asp. Mg(OH)2 Injection Effluent pH Control Visit www.pollutionengineering.com to electronically forward a copy of Figure 1. Treatment of wet ESP effluent with Mg(OH)2. this article to a colleague or customer. JANUARY2009 www.pollutionengineering.com 23 http://magspecialties.com/datasheets/FloMag-pdf/FloMagH.pdf http://magspecialties.com/datasheets/FloMag-pdf/FloMagH.pdf http://www.magspecialties.com/MSDS/mgoh_slurry_flomag_h.asp http://www.pollutionengineering.com http://www.pollutionengineering.com

Table of Contents Feed for the Digital Edition of Pollution Engineering - January 2009

Pollution Engineering - January 2009 Contents The Editor’s Desk EnviroNews PE Events Legal Lookout Green Connections Ten Top Technologies for 2009 Old Fashioned Chemistry Emitting Education NGWA Reports from Its Annual Meeting A Wood and a Pond Company Technical Profiles Filtration/Membrane Products Flow and Level Monitoring Equipment Classified Marketplace Advertisers Index State Rules

Pollution Engineering - January 2009

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