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 Tel: 919/ ; FAX: 919/ ; William Ellison, Ellison Consultants 4966 Tall Oaks Drive, Monrovia, Maryland Tel: 301/ ; FAX: 301/ ; Helmut Weiler, Weiler Consultants Hofer Heide 31, D42549 Velbert, Germany Tel: 011/49/ ; Fax: 011/49/ ; U.S. DOE/NETL 2006 ENV. CONTS. CONF. SCR AND SNCR FOR NO X CONT. PITTSBURGH, PA, MAY 17, 2006

2 If A Unit Has Significant, Air-Preheater- Related, SO 3 Problems, SCR Retrofitting Can Be Expected To Make The Situation Worse. If an Uncontrolled Unit Does Not Have Significant SO 3 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, SO 2 - TO-SO 3 CONVERSION) Average SO 3 concentration may be as high as 50 ppm, exceeding 3% of gross SO 2 content. With unique, high iron content, e.g. western Kentucky coal: up to 10% conversion of SO 2 to SO 3 occurs.

4 UNIT-WIDE SO 2 /SO 3 BEHAVIOR An increment of SO 3 generation occurs in the furnace. Temperature-dependent, catalyzed SO 2 -to-SO 3 conversion occurs in the convective pass, reaching a maximum rate at 1,300 o F (704 o C) flue gas temperature. Rate of SO 3 formation by SCR, increasing SO 3 perhaps by 20+ ppm, is greatest at o F ( o C) and above. Below 600 o F (316 o C) SO 3 hydrates to gaseous sulfuric acid: H 2 SO 4 (v). Condensation of H 2 SO 4 (v) occurs at and below the sulfuric acid dew point temperature, typically as high as 280 o F (138 o C).

5 INFLUENCE OF SOOT BLOWING Low-temperature blowing/cleaning (1,100 to 1,600 o F, i.e. 593 to 871 o C), in removing deposits, increases the rate of SO 2 -to-SO 3 conversion due to tube-metal surface effect. However, (contrariwise), presence of such ash deposits, typically iron-oxide-laden, significantly increases SO 3 formation.

6 INSIGHTS FROM MARCH, 1998, (MOST RECENT) DOE/FETC CONFERENCE ON SO 3 A boiler model study showed that the condition of superheater tube surfaces radically influences catalytic SO 3 formation: –CLEAN:20 ppm –MODERATELY FOULED:70 ppm –HEAVILY FOULED:32 ppm A large, high-SO 3, electric utility unit (without SCR) achieves 60% removal of SO 3 in the air preheater leading to its significant fouling (and derating). Across its exit cross- section, SO 3 varies laterally from 10 to 25 ppm.

7 IMPACT ON AIR PREHEATER OF H 2 SO 4 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 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 SO 3 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 NO x Removal Efficiency 10 mm Catalyst Pitch Maintain 330 o C (625 o F), 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: –NO x mass flow: + or – 15% max. –NH 3 /NO x Ratio: + or – 3-5% max. –Temperature: + or – 15 o C (27 o F) –Partic. Mass: + or – 30% max. Ammonia Slip: 2 ppm average (1.5 to 3.0 ppm locally) Minimum, catalytic, SO 2 -to-SO 3 conversion When appropriate, (so as to limit SO 3 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 SO 2 -to-SO 3 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.