HIGH-DUST SCR DESIGN TO LIMIT IMPACT OF HIGH SULFUR OPERATION

ON AIR PREHEATER OPERATIONVolker Rummenhohl, Tackticks, LLC

2300 Englert Avenue, Suite C, Durham, NC 27713Tel: 919/602-1063; FAX: 919/484-1544;

E-mail: vrummenhohl@tackticks.com

William Ellison, Ellison Consultants4966 Tall Oaks Drive, Monrovia, Maryland 21770-9316

Tel: 301/865-5302; FAX: 301/865-5591;E-mail: ellisoncon@aol.com

Helmut Weiler, Weiler ConsultantsHofer Heide 31, D42549 Velbert, Germany

Tel: 011/49/2051-66034; Fax: 011/49/2051-66440;E-mail: weiler@aie-gmbh.de

U.S. DOE/NETL 2006 ENV. CONTS. CONF.SCR AND SNCR FOR NOX CONT.PITTSBURGH, PA, MAY 17, 2006

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• If A Unit Has Significant, Air-Preheater-Related, SO3 Problems, SCR Retrofitting Can Be Expected To Make The Situation Worse.

• If an Uncontrolled Unit Does Not Have Significant SO3 Problems, Adequate SCR Retrofit System Design and Operation Should Not Lead To Increased Problems in Boiler System Performance or Maintenance.

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INDICATION OF POTENTIAL PROBLEMS ABSENT SCR (AND ITS

CATALYTIC, SO2-TO-SO3 CONVERSION)

• Average SO3 concentration may be as high as 50 ppm, exceeding 3% of gross SO2 content.

• With unique, high iron content, e.g. western Kentucky coal: up to 10% conversion of SO2 to SO3 occurs.

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UNIT-WIDE SO2/SO3 BEHAVIOR

• An increment of SO3 generation occurs in the furnace.• Temperature-dependent, catalyzed SO2-to-SO3 conversion

occurs in the convective pass, reaching a maximum rate at 1,300oF (704oC) flue gas temperature.

• Rate of SO3 formation by SCR, increasing SO3 perhaps by 20+ ppm, is greatest at 660-750oF (350-400oC) and above.

• Below 600oF (316oC) SO3 hydrates to gaseous sulfuric acid: H2SO4(v).

• Condensation of H2SO4(v) occurs at and below the sulfuric acid dew point temperature, typically as high as 280oF (138oC).

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INFLUENCE OF SOOT BLOWING

• Low-temperature blowing/cleaning (1,100 to 1,600oF, i.e. 593 to 871oC), in removing deposits, increases the rate of SO2-to-SO3 conversion due to tube-metal surface effect.

• However, (contrariwise), presence of such ash deposits, typically iron-oxide-laden, significantly increases SO3 formation.

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INSIGHTS FROM MARCH, 1998,(MOST RECENT) DOE/FETC

CONFERENCE ON SO3

• A boiler model study showed that the condition of superheater tube surfaces radically influences catalytic SO3 formation:

– CLEAN: 20 ppm– MODERATELY FOULED: 70 ppm– HEAVILY FOULED: 32 ppm

• A large, high-SO3, electric utility unit (without SCR) achieves 60% removal of SO3 in the air preheater leading to its significant fouling (and derating). Across its exit cross-section, SO3 varies laterally from 10 to 25 ppm.

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IMPACT ON AIR PREHEATER OF H2SO4 CONDENSATION IS EXACERBATED BY:

• Air to gas-side leakage

• Displacement of flue gas into air stream

• Enhancement of corrosion due to acid-wetted ash/salt deposit.

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WITH CO-FIRING OF BIOMASS

• If more than 10% of total heat input

• Catalyst activity deterioration may be fifteen times normal

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EXTERNAL CATALYST REGENERATION, E.G.

ULTRASONIC CLEANING

• 90 TO 100% Recovery of Activity

• Max. number regenerations: Four or more

• Amount regenerated to date in Germany: 10,000 cubic meters

• Cost savings: Major

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VERY LIMITED, OVERSEAS,HIGH DUST, SCR EXPERIENCE IN

U.S.-TYPE, HIGH SULFUR, LOW ASH, COAL SERVICE

• Japan: Limited to several, early, low-efficiency, ultra-low ammonia-slip installations

• Germany….none:– All of the several high-sulfur, high-dust, SCR installations

fire ultra-high-ash, coal-cleaning middlings, (“ballast coal”), 40% ash content of which greatly mitigates the effect of SO3 and ammonia slip.

– Tail-end SCR design is commonly applied to wet-bottom boilers.

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JAPANESE SCR PRACTICES FROM 1980s IN HIGH-SULFUR, LOW-ASH,

COAL APPLICATIONS

• Limited NOx Removal Efficiency

• 10 mm Catalyst Pitch

• Maintain 330oC (625oF), Minimum, at SCR Inlet

• Ammonia Slip <1 ppm

• Soot Blowing/Periodic Washing

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GERMAN SCR DESIGN PRACTICES

• Standardized, replaceable, catalyst modules• Design catalyst-volume criteria tied to maximum

design ammonia slip (originally set as high as 5 ppm in low-sulfur service: later 2 ppm)

• Economizer bypass as necessary to maintain adequate SCR inlet temperature at low load

• Flow model test at 1:10 or 1:20 scale• Provision for a future, spare layer of catalyst• Three dimensional, two-phase flow, computer

program• Enameled steel heating plates in preheater cold-end.

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GERMAN SCR KNOW-HOW GENERALLY APPLICABLE IN HIGH-

SULFUR APPLICATIONS

• Tight control of inlet temperature• Uniform, cross-sectional, flow distribution:

– NOx mass flow: + or – 15% max.– NH3/NOx Ratio: + or – 3-5% max.– Temperature: + or – 15oC (27oF)– Partic. Mass: + or – 30% max.

• Ammonia Slip: 2 ppm average (1.5 to 3.0 ppm locally)• Minimum, catalytic, SO2-to-SO3 conversion• When appropriate, (so as to limit SO3 formation): removal of

‘excess’ catalyst!!!• Air preheater enameling• Preheater plate surface-profile: easy to blow clean• Often apply and justify the alternative, benign, tail-end, SCR

design arrangement.

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INSTRUCTIVE GUIDELINES FROM GERMANY FOR U.S. HIGH SULFUR,

LOW ASH, SCR SERVICE

• Low SO2-to-SO3 catalyst conversion rate, i.e. less than 0.3%

• Ammonia injection upstream of economizer (?)• Alkali injection upstream of air preheater• Enameled heating plates on the air preheater,

flue-gas side up to 2/3 of the plate height• Preheater corrosion protection on the casing

and cold side of the rotor structure• Optimum preheater soot-blowing means.

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CONCLUSION

• Unfettered performance of the in-place, high dust, SCR catalyst is critical for good boiler system operation and reliability.

• If functioning of the SCR catalyst surface is, or becomes, impaired, e.g. by design inadequacies or significant change in operating conditions, air preheater and fly-ash quality problems may become significant.