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HIGH-DUST SCR DESIGN TO LIMIT IMPACT OF HIGH SULFUR OPERATION ON AIR PREHEATER OPERATION

HIGH-DUST SCR DESIGN TO LIMIT IMPACT OF HIGH SULFUR OPERATION ON AIR PREHEATER OPERATION. Volker Rummenhohl, Tackticks, LLC 2300 Englert Avenue, Suite C, Durham, NC 27713 Tel: 919/602-1063; FAX: 919/484-1544; E-mail: vrummenhohl@tackticks.com William Ellison, Ellison Consultants

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HIGH-DUST SCR DESIGN TO LIMIT IMPACT OF HIGH SULFUR OPERATION ON AIR PREHEATER OPERATION

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  1. HIGH-DUST SCR DESIGN TO LIMIT IMPACT OF HIGH SULFUR OPERATION ON AIR PREHEATER OPERATION Volker Rummenhohl, Tackticks, LLC 2300 Englert Avenue, Suite C, Durham, NC 27713 Tel: 919/602-1063; FAX: 919/484-1544; E-mail: vrummenhohl@tackticks.com William Ellison, Ellison Consultants 4966 Tall Oaks Drive, Monrovia, Maryland 21770-9316 Tel: 301/865-5302; FAX: 301/865-5591; E-mail: ellisoncon@aol.com Helmut Weiler, Weiler Consultants Hofer 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

  2. 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.

  3. 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.

  4. 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).

  5. 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.

  6. 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.

  7. 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.

  8. WITH CO-FIRING OF BIOMASS • If more than 10% of total heat input • Catalyst activity deterioration may be fifteen times normal

  9. 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

  10. VERY LIMITED, OVERSEAS,HIGH DUST, SCR EXPERIENCE INU.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.

  11. 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

  12. 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.

  13. 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.

  14. 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.

  15. 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.

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