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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: [email protected]
William Ellison, Ellison Consultants4966 Tall Oaks Drive, Monrovia, Maryland 21770-9316
Tel: 301/865-5302; FAX: 301/865-5591;E-mail: [email protected]
Helmut Weiler, Weiler ConsultantsHofer Heide 31, D42549 Velbert, Germany
Tel: 011/49/2051-66034; Fax: 011/49/2051-66440;E-mail: [email protected]
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.