I •••

Via CertifiCd Mail ..

Mr. TenyO'Clair

Director, Division ofAir Qualjty

North DakotaDepartment ofHealth

918 E. Divide Avenue

Bismarck, ND 58501-1943.

Ref: 8ENF-L

UNITED STATES ENVIRONMENTAL PROTECTION AGENCY

REGION 8

1595Wynkoop street

DENVER, CO 80202-1129

Phone 800-227-8917 .

.http://www.epa.gov/region08

OCT - 4 2007

Mr. David Sogard, General Counsel

John Grav~s~ Envirotiinental Manager

1822 StateMill Road .

P.O. Box 13200

·Grand Forks, NorthDakota 58208-3200

Re: Transmittal ofEPA's Response to

Mirinkota's Letters Dated March 19, .2007·

and April 23, 2007, Regarding the OCtober

2006 NOxBACTAnalysis Study Report fOf

. Milton R Young Station

Dear Teny, David and John:

. . . .

. .the United States Environmental ProtectiOn AgencY,.Region 8 (EPA) subinitsthis letter

and the Enclosure to respond,to l~s dated March 19, 20Q7 and April 23, 2007 m;tthored by .

Minnkota Power COOperative, Incorporated (Minnkota) regarding the Burns & McDonnell

,.(B&McD) Best Available Control TechnoI8gy{BACT) Analysis for Nitrog~n Oxides (NUx) at

Milton It. Young Station (MRYS), Units 1 and 2, prepared byB&McD for Minnkota (Minnkota

BACTAnalysis). The above identified two letters were submitted by Minnkota iIiresponse to .

letters sent byEPA to North Dakota DepartmentofHealth (NDD:a), dated 'anuary. 8 and 26,

2007~ and.were also in resp~nse to a letterfrom the NDDH dated February 1; 2007..

EPA appreciates theongoing dialogue and exchange ofinformation with NDDH and

Minnkota on these very complex and important issuesrelated to the appropriate NOx control

technology to b~ installed at MRYS Units· 1 and 2. The meeting in aismarck on May- 23, 2007

with representatives ofMinnkota was very useful. EPA also appreciat~s the efforts by you and.

your staffto seek a well-informe<l decision on thiS matter. The Enclosure contains a response

from·EPA's consUltant, Roger Christman ofEasternResearch Group, on technical issues related

to Selective Cata:Iytic Reduction(SCR) (Christman August 20,2007 Memo). The :Enclosure also

. contamsEPA's detailed response to Minnkota's letters datedMarch 19,2007 and April 23,
2007.

To briefly sutrimarizethe Enclo·sure, the MinnkotaBACT Analysis is incomplete, and the

.determinations oftechnica1 infeasibility for·the mgh-dust SCR, Low-dust SCR, Hydrocarbon .

Enhanced Selective Non-Catalytic Reductiori (HE SNCR), Reburn with SNCR and Rich Reagent

Injection NOx control technologies are incorrect. Mitinkota has inCorrectly interpreted and

tnisa,pplied Chapter B ofEPA's ''New Source Review Workshop Matiual- Prevention of

Significant Deterioration and Non-attainment Area Permitting" (Draft October 1990) (NSR

·.Workshop Manwd) in its determination that these control technologies are t~hnica1ly infeasible.

·Furthermore, Mitinkotahas incorrectly utilized the results ofthe pilottesting conducted·at the·

Coyote Station by the Energy & Enviromilental Research Centerto find that SCRis inapplicable. . .

.Minnkotamust evaluate these coptrol technologies beyond step 2 ofthe proeess outlinedin the

NSR Workshop Manual. .

The operational problems assOciated with the .Coyote Station pilot test and the .lack·of

cat3Iyst deactivation data greatly dimiIiish the usefulness ofthe test I;esults in analyzing the

technical feasibility ofSCR at MRYS.EPAmaintains thatthe resUlts ofthe Coyote Station pilot

test do not demonstrate technical infeasibility ofSeRTor utility bollers firing North Dakota

lignite. Minnkota has not provided informationandlor data in the original MinnkotaBACT.

·Analysis or in any. ofthe subsequently delivered letter responses demonstrating that the

·differences between North Dakota lignite and other solid fuels would prevent the successful

application ofSCRatMRYS, EPAfinds that the wealth ofexPerience in the utility industry

applying SeRto solid fuel utility boilers creates a presumption oftechnical feasibility at MRYS.

---------.-lMinnkOtailas not overcome thispr~t~resently finds that·th~t~--..-,~---.

BACT Analysis.is incomple~e, has noiheen carried out in accordaticewith ChapterB ofEPA's .

NSR Workshop Manual, does not provide an ofthe information necessary for NDDH to make a

BACT deteimination and does not contain all ofthe information·requestoo byEPA· ..

.EPA hasreceived NDDH's letter dated September 21, 2007 transmitting Minnkota's paper

entitled "'Approp~ness ofConducting Pilot Testing ofSelective Catalytic RedUction (SCR)

.. Technology at MiltonR Young Station Units 1 .and 2, for lise in a NOx BACT Analysis" and·is

planning to reviewit. EPA·remams committed to moving the BACT processforWard sothat

Minnkota can meet ihe deadlines specified inthe Consent Decree. Forthe BACT process to

·proceed, EPA believes NDDH·should move past the question oftechnica1 feasibility for SCR .

and the other incorrectly determined technically inf~ible control technologies and require

·Minnkota to cOmplete the Top-DownBACT analysis. for SCR apd ihe othel' controltechnologies

·in accordance with paragraphs 24 and 65· ofthe Consent Decree..Ifyou would like tQ discuss

any ofthese matters, please Call me at (303) 312-6776. . .

~tLo+-~ Martin Hestmark, Director, Air and Toxics

Technical Enforcement Program, ECEJ .

2
.EncloSure

cc: Eddie Sierra,.Deputy Assistant Regional Administrator, ECEJ

.JeffKodish, OECA

Ron Rutherford,. OECA

·Adam Kushner, OECA

Brenda Morris,EPA

Douglas Latimer, EPA

Mike Owens, EPA

Hans Buenning, EPA

.Jerry MacLaughlin, DOJ

3

*Printed on RecycledPaper
I

MEMORANDUM

TO: . Hans Buenning, Albion Carlson, Doug Latimer, Martin Hestmark, Brenda Morris,

EPA Region 8 . .

Sounjay Gairola, Ron Rutherford,EPAlORE HQ

FROM: Roger Christman,Eastern Research Group,Inc.

DATE: ..August 20,2007

SUBJECT: . Summary Discussion in Response to the Bums & McDonnell'and EERC Reports

(MiIlJl1(ota,March 19, 2007 and Minnkota, April 23, 2007) and the Subsequent

Meeting on May 23, 2007, at the Offices ofthe Division ofAir Qu'ality, North

Dakota Department ofHealth (NDDH).

INTROQUCTION

, .

ERG has been tasked by the U.S. EPA Office ofRegulatory Enforcement to provide technical

assistance-to EPA Region 8 in evaluating a series. ofdocunients prepar~d by Bums & McDonnell

(B&McD), and the'Energy & Environmental Research Center (EERC) at the ,UniversitY ofNorth

Dakota, .on behalf ofMinnkota Power Cooperative, Inc: m'respense totlie sti6missiori, in .'" '"

October of2006, ofa BACT Analysis for Milton R. Yourig Station (MRYS) and the appended

,Ash Impacts on sci? Catalyst Performance (Benson, October 2005), I prepared a memo

(Christman, January 2007) addressing issues raised in ~e BACT Analysis ~d theBenson p~per

that were cited by Minnkota as renclering. SC!t technically infe~ible at MRYS. My memo,.along

with EPA Region .8 and NDDHquestions and requests for additional. informatiori were

forwarded to Minnkota., ' ..

Minnkota provided a response to the ERG memo (Minnkota, March 19, 200?) and additional

responses' and Information about a ~onth later (Minnkota, Apri123~ 2007). These rriaterial~ were

reviewed by Region 8 personnel and ERG in preparation for'a meeting with Minnkota at NDDH

o~ May.23, 2007. A list ofnine unresolved "core" issues w~re identified and were subsequently

discussed at the May meeting. They are addressed f?elow. Also, some ofthe discussion during a

meeting 'on May 24, 2007., that included an SCR/North Dakota Lignite presentation by Sargent &

Lundy (S&L),corioborated much ofthe EPA arid ERG positions and will also be,noted in the'

discussion'here. Issues that app.ear to have been resolved prior to, or at the May 23-'meeting are

not discussed (e.g., popcorn. ash). '

PRIVILEGED ENFORCEMENT INFORMATION

Not for Release Under FOIA or Other Information Release Statutes 1
The nine "core" is~ues hiclude:

1. Center Mine. ash content variability

2. Catalyst deactivation by sodium and regeneration difficultie$.

3. Downtime for catalyst cleaning andchange·out. .

4.· Sticky ash. .

5..EE~C pilot test procedures/design.

6.. Split·air preheater.

7.. B&McD low dust SCR assessment.·

... 8. Texas Lignite comparison.

.9. SCR vendor· quote request.

DISCUSSION

1. Center Mine ash content variability.

At the May meeting, B&McD representatives and Dr. Benson continued to refer to the .extreme

coal variability, both in proximate. analysis and ash constituep.t concentrations and. continued to

refer to ash contents as high as 25.5%. An examination ofthe actual data (analyses ofmore than

700 as-fired coal samples provided by Minnkota in 2003) shows that.the coal ash content is not

particularly variable, with more than 95% ofthe samples having an ash content between 7% and

15%..Well over 50% ofthe samples fall between 8% and 11.%. The actual maximum content i~ ' .

.just under 16%, not 25.5% as'stated by B&McD. rhe higher figure is based on a single sample·

WIth none offfie more than 700 otner samples eX1immnguan~1'icornent gfeaterthan t~;·

" .

Along with the general discussion of ash variability, Dr. Benson introduced the concept of

variability in the "acid-to-b~e ratio" ofthe ash (pages7~9,.Minnkota,Marc.h ~007) as having'

relevance to thetechnical.feasibility ofSCR as applied to North D~ota Lignite.· He included

two ·figures that display·moisture and Base/Acid and an undefined "Cyclone Performance".· The

accompanying text refers to slag freezing events and the n~ed to provide ~uppl.emental fuel.

Although the ash fusion temperatufe·andthe ash viscosity at cyclone barrel temperatures (>2,500

.·F) are·functions of ash composition and thus are sensitive to· varhlti6ns in that composition with.

time; there should be no impact on that ash behavior downstream at SCR temperatures. Dr. . ..

. Benson does· not, in fact, make a direct coimection between his discussion ofbase-to-acid ratio

.and the functioning of an SCR system. B&McD also .only m~ntions impact on "operation ofthe

boilers at MRYS" (page 9) and makes .no connection specifically to an SCR system. As a final

. note, 1have been involved in one way or another with SCRsince ·the early 1970's and I have

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NoHor R~lease Under FOIA or Other Information Release Statutes 2
never heard or read anything about acid-to-base ratio of ash in any SCR document Of discussion,

prior to, Dr. Benson's introduction ofthe subject.

, '

'2. Catalys~ deactivation by'sodium and regeneration diffi~ulties.

Dr. Benson.cited a paper by'Khodayarl as evidence that catalyst ppisoned 'by potassium c~uld not

be regenerated and that the same would be true ofsodium-poisoned catalyst (reference 12',

Minnkota, March 2007)., A careful reading ofthe paper shows that Khoqayari ~chieved 90% to

'110% recovery ofthe originalteactivity ofthe catalyst. Further,' Khodayari's PhD dissertation

states that potassium" not total alkali, is the problemfor SCR application to,biofuelcombustion.

,A paper authored by engineers involved in the application ofSCR to biomass fuels cites'

'Khodayari's work as evidence that catalyst activity can'berecovered by washing and wi,ll result

in reduced cost for biomass applications (Alstom, lindated,'-2006). . ,

. . . . .

Dr: Benson further states that work by Beck (reference 16~ Minnkota, March 2007) established a

.. deactivation correlation with 'sodium in ash. Again, a careful reading'ofthe paper shows that

sodium is only mentioned 'when the discussion addresses total alkali and Beck actUally cite$

potassium and phosphorQs as the prim8l'y poisons associated with a range ofbio-materials that

are burned for energy recovery; He concludes that-additional research is needed to detennirie the

relative' contributions ofphosphorus, potassium an,d sodium. His subseque~t wo~k focused" ,

'entirely 'on phosphorus and did not address'sodium at all.

,A 2006 presentation covering work conducted by the Uniyersity ofUtah and REI on the

,application of SCR to low-rank coals concluded that "chemical,poisoning is'not irriportant" for

, I()w-raiik.' coals;out mask1ng oy nyaslils~f prooIem. 'IIre sfilOy"fUi'ttier conetutletl"'thatthe" , '

poisoning'potential ofthe alkali materials ranked as: K.>Na>Ca.

While Dr. Benson ctmcludes that the sodi~ 'levels in North Dakota Lignite ash are so high as to

, ,render sC:k technically infeasible; apparently others atEERC do'not share that opinion and have

researched the co-firing ofbiofueI,s with low-rank coal's (EERC, Folkedahl; 20(6). The intent of

the,study was to determine'ifthe higher levels of alkali materials'in the biofuel would acce~~rate '

the deactivation rate'(which Dr. Benson concludes is already mtolerably high). The final report

states: ','However"little is knoWn ofbiomass firing with low-ranked coals, such as North Dako,ta

Lignites and Powder River Basin coals." The report concludes that) a~thotigh they experienced

operating diffic,u1ties, the deactivation rate appears to be 1.7% to 2.6% every 1000 hours.

In'summary, Dr. Benson misread the work ofKhodayari regarding regeneration of.a1kalipoisoned catalysts. Both a careful reading.ofthe cited Khodayari paper and some ofhis, other ,

pUblicrttions confinn this, as does the Alst6ni paper and several recent papers by the,catalyst

regeneration vendors (Full-Scale Catalyst Regeneration Experienye at the,Coal-fired Indiantoyvn

.GeI1eration Plant, Bullockand Hartenstein, October 2003· and Regeneration Advances,

McMahon, May 2006) and a comprehensive SCR technology survey (The First 100 GW of SCR

in the 0.s.- What Have'We Learned?, Cichanowicz, Muzio and Hein, ~006). He also misread

the' Beck paper which, upon careful reading, does not shed much light on sodium poisoning, but

rather, has a major focus?n phosphorus and potassium. Work by the Uniyersity ofUtah and REI'

PRIVILEGED ENFORCEMENT INFORMATION

Not for Release Under: FOIA or Other Information Release Statutes ,3
further implicates potassium as having a greater poisoning.potential than sodium. Although the

high sodium .content ofCenter Mine 'coal'ash is an economic issue (it will contribute to catalyst

deactivatioIi); Dr. Benson provides no evidence that it will render SCR technically infeasible at

MRYS. .

As a final note,. sinc~ catalyst deactivation due to alkaline materials is a "chronic," rather than .

"acute" phenomenon, deactivation depends on the total time and mass exposure. This.

dependency is probably not linear, since the overwhelming.amount of ash in anyinstallation does

not deposit on the catalyst (it passes through the catalyst channels and ends up in the ESP

hoppers Or is emitted), but it is most appropriate to consider average values ofchemical poisons.

'Qrls is becausethe total exposure will be less tli,an the average for about halfthe time .and more

than the average about halfthe.time. The overall deactivation rate "averages out".. . .

.. 3. Downtime for catalyst cleaning and change out.

a boiler cleanirig outage, the SCR unit would only contribute one or tWo days" of doWntime per

year..

Although the downtime durations for boiler cleaning cited by Minnkota are not long enough to

permit a complete catalyst change out,they do in some cases approach the J day turnarol,1nd that

Flemming Hansen cited in the April 16, 2007 phone conversation with Region 8personnel.

Additionally, the boiler cleailing outages would afford the opportunitY for some cleaning ofthe'

catalyst four times a year; a luxury not available at most year-roUnd SCR installations. This·.

. would contribute to both catalyst'life and lower pressure drop. Also·, during discuss~ons with

S&L~ their engineers pointed outthat the system can be designed to facilitate faster catalyst "

change out by providingwider than nonnal platforms at each catalyst layer level, thereby

allowing replacement modules to be pre,,:,positioned as the spent modules are removed. If one

assumes' a ratfief extfemesfiotfcata1yst lIfe ofone "yearamt tfthe change-out iSCOOldiImtedwith

4.' Stic.ky ash.

:Much light was shed on the topic ofsticky ash during the S&L presentation and discussi<?n.

Sticky ash (in PRB applications) was described as having a steeper angle ofrep'ose (less slump)

and thus tended to accurilUlate in large amounts on structural members and subsequen,tly falling.

onto the tops ofthe catalyst layers in piles. These piles can not be readily broken down by·

·.soQtblowing (steam or horns) and eventually require shutdown for manual cleaning.. S&L's,

engineering response. has been to eliminate flat, horizontal surfaces (weld an inverted vee 'on the

tops·ofI-beams arid.angle-ironmembers that are .above the catalyst layers) and aim sonic ~oms at

areas prone to ash accumulation.. When asked directly ifsootblowing (steam or horns) would

effectively prevent plugging from the ash that is normally entrained··in the flue gas (not falling"

accumulations) the answer given was "yes".

5. EE~C pilot test procedures/design.

Page 7 ofmy previous memo (Christman, January 2007) contains a detailed discussion ofthe

. EERC test at the Coyote Station, all ofwhich remains valid. In summary, the catalyst s~ould

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NoHor Release Under FOIA or Other Information Release Statutes 4
have· been clearied:with somefrequericy so that it could be continuously exposed to fluegas~

Tests ofthe scale at Coyote'are not useful in determining plugging potential and the attempt at

. simul;:t.ting sootblowing was liot effective. By EERC's own admission (and confirmed by Hansen' .

.dUring the April 16, 2007 call)the plugging was a r~sult of ash accUmulations faIling onto the

catalyst (see "sticky ash", above). Mr. Hansen also confirmed that the catalyst pitch at Coyote (6

mm) was too small. S&L presenters suggested'that a '1 0 mm pitch might be required for North .

Dakota Lignite applications;' .'.

In my January memo and at the May 2Jd meeting, I stated that the Coyote test was'so·.flawed that

it did not provide any meaningful data bearing on the issue oftechnical feasibility.' During the .

8&1., disctission on May 24t1\ Bill DePriest was asked, by an NDDH-participant, what he thought

ofthe EERC test at.Coyote. Even though he was clearly uncomfortable criticizing the work of

another organization, he stated that the test produced no useful information.

6. Split air preheater.

The discussi(;:m ofthe .spl.it air preheater approach contained in the Minnkota March 19th

document seems a bit ambivalent, imply~ng first that it couldn't be done·and later stating that

conceptually, it could be done (page 15, Minnkota, March·2007). Further evidence that it is not

. infeasible isfound in'the Hict that the SCR retrofits a New Madrid (two large, PRB~fired cyclone

boilers) included replacement ofthe tubular air preheaters with new regenerative air preheaters .

with enamel-coated baskets. I do not believe that B&McD·indicated infeasibility d~ng the

meeting on May 23~d, but they did insist that BACT would not reqUire substantial rebuild ofthe

source (boiler).

7.B&McD low.dust 8CR assessment.

I am ~ot qualified to comment on the B&McD discussion ofthe steam'extraction forflue gas.

reheat for the low'dust application. I do not ,believe that resort to the low dust approach is

warranted, since ash issues should be manageable.

8. Texa~ Lignite comparison.

George .Offen's letter (EPRI,September 7,2006) provides some useful insights, particularly with

. regard to erosion. S&L p~rsonnel confirmed that Texas Lignite ash is more erosive that North

Dakota Lignite ·ash. This :means that the TXU pilot test currently underway should resolve any

concerns regarding erosion at a future MRYS SCR installation..

9. SCR vendor quote request..

. .

The volume'ofinformation provided in the B&McD vendor quote request will be troubling to the

vendor's sales staffsince they are not likely to invest a great deal oftime in something that is .

clearly not meant to generate a sale in the foreseeable futi.rre. Also, by including a discussion on .

the enforcement situation, it is clear that the requestor is looking for support for the technical.

infeasibility argument. The ability to provide flue gas at the appropriate temperature is not

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Notfor Release Under FOIA or Other Information Release Statutes 5
discussed and t~e startup/shutdown requirements may be unreasonable. I dQ not expect the

exercise to produce any meaningful results.

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Not for Release Under FOIA or Other Information Release Statutes 6.

ENCLOSURE

RESPONSE TO ·EERC AND C AND B&McD's MARCH17, i007 LETTER

RESPONDING TO EPA REGION VIIi COMMENTS JANUARY 8, 2007

1. Introduction au"dSu~maryofEPA's Preliminary Analysis

.EPA's January 8, 2007 Comment:. . . .

EPA is prei,minary analysis is that the' B&Mstudy lacks sufficient information within the BACT

analysis. Specifically, the study excludes Selective Catalytic R~duction($CR) as a technicallyfeasible

control option at step. 2 ofthefive-stepTop-Down analysis, while notpresentingNOx conirol

effectiveness or catalyst activity data. As a result, .B&Mfails to rankSCR against other control options

in terms ofcontrol effectiveness atstep3, andfails' to analyze costs ofSCR versus other control options at

step 4. EPA questions the reasons cited by B&Mfor technical infeasibilityofSCR Furthermore, based'

(In a survey ofSCR vendors described later in this report, EPA found there are vendors that will offer'

performance guaranteesfor this type ofapplication. Further information shouldbe requested, to support

the analysis with resp(fct to SCR

. .' .

B&McD's March 13; 2007Response:

... ,... The EPA's NSR Manual does not;pecijically define rheoperatlngpara~eter or peiformance .

measure that a particular NOx control technique ortechnology would have to achieve to satisfY "a

technicg! ass.r~s§rnent coilsidering..JJ.hx.,slcgl", chemicaland £Engineering 12rincjple§. qnd/or eml!iri~al data

showing the technology would notwork on the emissions urlii under review or that unsolvable technical

difficulties wouldpreclude the successful'deployment ofthe tech~ique" in orderfor said alternative to be

excluded[romfurther consideration. . . . . '. .

. The severe accumulation ofash and blinding in the Coyote station slipstream SCR catalystfield test

after 2 months was documented in Dr. Benson's referenced technical paper. .The evidentfailure ofthe

Coyote slipstream SCRcatalystfield test to avoid severe-.catalystfouling and ash accumulation led Burns

& McDonnell to believe that ifsuch results wereroutbiely encountered at afull-scale high-dust SCR

catalyst installation during operation on a North 'Dakota lignite-fired cyclone boiler, aggressive on-line

cleaning and/orfrequent extendedforced outages would be needed to remove ash accumulation or.

remove the catalystjor cleaning in ordertorestorefull unit output and catalJl$t NOx reduction

effectiveness. Either or both ofthese maintenance activities could damage the catalyst and/or require it .

to befrequently replaced. .

Two monthsbetwe.e.n SCR c.atalyst maintenance activities wo~k/be an extremely short operating'

cycle time comparedto 36 months typically experienced at other. $CR installations. Burns & McDonnell

interpreted thesefield test results as "empirical data showing the technology would not work on the.

emissions unit underreview" for technical infeasibility purposes. Eliminating technically':'infeasible .

alternatives prior to rankingthefeasible controls based upon control effectiveness, and then evaluating

the impacts'ofimplementingonlyfeasible control techniques and technologiesfollowsthe BACT .

process~.; ...
. ,

EPA Response to MinnkotaComment Letters from

March 19, 2(l07 and April 23, 2007

BACT Analysis at MRYS

EPA Response:

As articulated inthe Jamiary26, 2007 letter related to other widely-used NOx control ..

options that were eliminated in Minnkota's BACT analysis'based on technical ip.feasibility, EPA,

believes the Minnkota BACT analysis for SCR should begin with the presumption that SCR is

technically feasible, since the technology haS been widely demonstrated on utility boilers.. This

presumption is. based oil the "techno,togy transfer" discussion· in lhe NSR Wo.rkshop Manual,,)

. other-EPA guidancedo~timents,2andOthe' facfthat'hundreds ofSCR systems have been .

successfullyinstalled on utility boilers worldwide. ,The NSR Workshop Marillal and oth~r EPA ..

guidance clearly piacesa substantial bl!fden ofproofon the applicant to show that, based oua . comparison ofthe .:physical and chemi~al characteristics ofthe exhaust gas stream under -review .

and the sources towhich the technology had be~n previously applied, the technology would be

technically infeasible. EPA agrees that the NSR'Workshop Manual does notspecificaliy define..

the operating parameter or perfotrilance measure that a partIcular NOx contiol technology or

. technique would have to meet in order to satisfy the criterion. However, given the sheer number

, ofsuccessful SCR systems in operation at utility boilers today; EPA believes this burden ofproof

is cOD:siderable onthe part ofMinnkota and should include quantitative'evidence that such.

unresolvahle differences do exist that would renderthe technology unsuccessful at controlling

NOx• .

.The qualit.ative evidence provided to date by:B&McD and EERC demonstrates that there.

are certain factors relevant. to MRYS that will requireengirieering design considerations to make

· an SCR installation workable; however,'considering ea,ch SCR system design for a~pecifiC

facility has tobe customized to some extent, this situation is hardly unIque. Additionally,

rl9thing has beenprovided that would demonstrate that SCRwould be an ineffective means of

NOx control at MRYS. 'B&McD's beliefis that the pilottest resultsfrom Coyote Station show,

that a full-scale SCR installation at MRYS would require "aggressive on-line cleaning and/or

1 See NSR Workshop ManualpageB.19: "Unless significant differences betWeen source tYpes exist that are

pertinent to the successful,operation ofthe control device, the control option is presumed to be technically

feasible unless the source can present information to',the contrary." (Emphas,is added) . ,.' ,

2 See June 15,.1989 letter from Gary McCutchen (EPA Office of Air Quality Planning and Standards, New Source

Review Section Chief) to John Daniel (Department ofAir Pollution Control, Commonwealth 'ofVirginia): "Another

. point worth mentionjng is the area oftechnology transfer. We have heard that some applicants are attemp.ting to

· define gas streams and source types far more narrowly than common sense would dictate irian effort to avoid certain

controls. For example,an applicant might saythatNOx controls have been applied to a 30 and 70 MW coal boiler,

but not a 45 MWcoal boiler; that the control. technology has been applied to pulverized and fluidized.bed linits; but '

not to spreader stokers; or thatthe technology has never been applied to the particular mix of, say wpod and coal

planned for that unit. Such arguments should be closely scrutinized and the applicant should explain fully not only

,what is different about the gas stream (ifthecontr<>l tecbnologybeing analyzed is an add-on¢ontrol), but also why'

·that difference pre.cludes transfer ofthat controltechnology to the proposed source, .The burden of proofshould be

relatively high in order to prevent circumvention ofreasonable technology transfer by the selection ofSome slightly

different unit." http://www.epa.gov/Region7/programs/artd/air/nsr/nsnnemos/finlbact.pdf

-2-

" I,
EPA Response to Minnkota Comment Letters from

March 19,2007 and April 23, 2007

BACT Analysis at MRYS

frequent extended forced outages." While this may·or may not be the case, such conditions do

not undennine the conclusion that SCRshould still be considered technically feasible.3 EPA did

ackrtowledge that such real costs associated Withoperational realities ofthe boiler and-"fuel type

in an SCR application at MRYS can be accounted for through the costevaluationunder step 40f

the BACT analysis.4 .EPA does not view additional mainten~ce and operational measures that

would be required at MRYS· compared to other·facilities to be justificatiqn oftechriical

infeasibility. Again, the bUrden ofproofis considerable in the case ofdemonstrating infeasibility

·ofSCR for autility boiler due to the large number ofSystems currently in operation at utility

· boilers. .

B&McD has apparently interpreted the pilot testing experience at Coyote Station to mean

that an SCR installation at. MRYS would require "SCR catalyst maintenance activities" every .

two months and thatthis constitutes "empirical data showing the-technology would not work on

the emissions unit under review."s B&McD states that this time period "would be an extremely

short operating cycle time compared to 36 months typically experienced at other SCR

installations." EPA d.oes not believe B&McD's· use ofthe Coyote Station pilot testing

·.maintenance requirements is an accurate means of predicting·full-scale maintenance . .

requirements~ The EERC·report indicates that aggressive catalyst cleaning and maintenance

measures were also taken at the Baldwin and Columbia Stations during the initial months ofthe

pilot testing and that significant ash acc.umulation, catalyst plugging,. and pressure drop increases

occurred during the c(;rurse ofthe six-month test at both facilities. Baldwhi and Columbia·

Statipn~ both bum·P·Iill co~l1,· which·is also·humedat many· full-scaleutilityhoilers currently

o-perati·ng SCR systems.with typical maintenance cycles.· EPAbdieves B&McD'g conclusions

regarding the use ofthe "empirical data" from the Coyote Station pilot-scale testing is not

relevant fOf an anaiysis oftechnica:I feasibility, and fails to consider the observations on catalyst

niaintenance frequency and pressure drop trends from the Baldwin and Columbia Station pilot

tests. A comparison ofthesefindillgs for all three facilities is presented below'...

. .

.During the testing at the Baldwin Station, "(a)t 2-month intervals, a section of catalyst was.

removed and replaceCi with a new one.,,6 The nan-ative indicates that catalyst plugging at .

· Baldwin had also occurred during the first two months, as the "pressure drop steadily

·incre~ed."7 The report describes that during the remainder ofthe test (months 2-6), pressure

drop was highly variably but maintained within a range of0.6 and 0.8 inches ofwater "due to the

installation offresh catalyst section and leaving two thirds ofthe catalysts in place that were

:3 -See pages B.l9and B.20 ofthe NSR Workshop Manual: ~'Where the resolution oftechnical difficultie!! is a matter

of cost, the applicant should consider the technology as technically feasible." .

4 See page 6 of EPA Region 8 Preliminary Analysis ofBums and McDonnell BACT Analysis for Nitrogen Oxide

at Milton R. Young Station, Units 1 and 2, January 8, 2006.

5 See page B.20 ofthe NSR Workshop Manual which indicates empirical data can be used in making

determinations oftechnical feasibility.,

6 See Appendix B, Page 19 ofOctober 2006 Minnkota BACT Analysis Study for MRYS.

7 See Appendix B, Page 19 ofOctober 2006 Minnkota BACT Analysis Study for MRY8.

-3-
EPA Response to Minilkota CommentLetters from ' March 19, 2007 and April 23; 2007

BACT Analysis at MRYS

partially plugged. The gas veloCitY inthe single ~e~tion ofnew, clean catalystwas high because

of channeling, and the result pfthe high gas,flow wasless deposition and accumulation. Gas "

veloc~ty has a significant ir.npa~t ~n the ~otentlal for deposits to fomi. ~owe~,er, at high ga~

velOCIty, low NOxconverSIon IS lIkely." Hence, based on B&McD's reasomng, the SCR pIlot

testing at Baldwin would provide' empirical evidence demonstrating that a full..scale SCR '

, installation on PRB facilities wouid be infeasible, especially at the typical 36.:monthoperatini '

cycles for majormalntel1ance. IIi. fact, the EERCrepoi1: suggests that when a section ofcatalyst

at Baldwin was changed ou~ every two months to deal with the increase in pressure drop caused

,by plugging, low NOxconversion for the SCR would be likely. Based ori the'catalyst

maintenance requirements and pressure drop data, B&McDwould presumably conclude that the

pilot testing at BaidwinStation showed thataSCR system, operating()n PRB~oal would require,

significant and frequent catalyst maint~nance and replacement and tha~ the NOxreduction would

be limited, much like the results ofthe Coyote Station testing. '

The final test report describes the catalyst reactivity test results forcata:Iyst samples at 2, 4~

and 6 months of operation at Baldwin Station.9 The two month sample "had no noticeable loss

ofreactivity compared to the reference catalyst. After four months,'the reactivity was 96% ofthe

reference, and aftersixmo'nths, therea.ctivity had ,dropped to 84% ofthe reference catalyst" It

does not appear thetels any real correlation between the pressure'drops observed at Baldwin

Station and the measured activity ofthe catalyst. The pressure drop did not steadily Increase as

the reactivity decreased during. the 4 anq. 6 month intervals~ while there were some temporary

variations, the pressure drop throughout the test was in the range of 0.6-0.8 inches of water. The

most prolonged period during which the pressure drop was higher than 0.8 inches of water was

.during the 'first two mOriths, when no deactivation occurred: ' ,

For the ColUmbia Station, there was also aggressive reactor Cleaning and catalyst'

replacement within the six-month period ofthe test. The EERCreport 'in Appendix B' ofthe

Minnkota BACT Analysis does notprovide as much information as is contained in the Fuel

Processing Technology article describing the same pilot testing. ThePuelProcessing' ,

Technology article includes a graph ofpressure drop for the entire'six months oftesting, whereas'

. the Appendix BEERC report only shows the pressure drop graph for the first two months of

testing. It is unclearwhy this additional data was not part ofthe Appendjx B report,but the

entire data set does show there were significant increases in pressure drop after the first two

months ofthe test. According to the:Fuel Processing Technology article, there Was "a rapid

increase in pressure drop across ~he catalystat about 3000 h .ofoperation, and aggressive pulsing

brought it doWn to·O.4 in. ofwater until the catalyst section was changed out at about 3200h.

After cleaning the reactor and replacing one catalyst section, the pressure drop ~as about 0.3 but

8 See Appendix B, Page 19 ofOctober 2006 Minnkota BACT Analysis Study for MRYS. .

9 Final Report for JV Task J I - Evaluation ofPotential SCR Catalyst Blinding During Coal Combustion and' AddOn: Impact ofSCR Catalyst on MeI:cury Oxidation in Lignite-Fired Combustions Systems, November 29; 2004,

. page4I. '

-4-
EPA Response to Miririkota Comment Letters from

March 19, 2007 and April 23, 2007

BACTAnalysis at MRYS

. increased to over 0.6 in of:water up to about 4100 h. There 'Yasan outage at the.plant, and .

aggressive pulsing ofthe reactor was conducted; the pressure drop was brought back down to 0.3

but rapidly increased to over 0.5 in. ofwater within 500 h."IO Most ofthis paragraph describing

the entire 6 months ofthe test does appear in Appendix B ofthe EERC report; with the exception

ofthe first sentence; .In any caSe, it:appears as though significant plugging aiso occurred during.

the Columbia Station pilot test that.required aggressive on-line Cleaning and 'catalyst .

replacement. The problems first appeared at about fo"!ll' months and then reoccurred in much

shorter time intervals. These time frames, while longer than that observed at Coyote Station, are .

obviously much shorter than the 36-month operating cycle described by.B&McD for normal

SCR maintenance. . .

Additionally, Columbia Station catalyst reactivity testing showed 93% reactivity at two

months, 90% at four months and 90% at six months. 11 Agai~ there does not seem t.o be'a clear

correlation between pressure drop readings through the six months ofpilot testing at Columbia

Stationand the catalyst reactivity testing. During the first two months, pressure drop readings

weregenerallybetweenOA and 0.6 inches ofwater. During months two through four, pressure

'drop was generally higher~ between0.6 and 0.8 inches ofwater. During the final two months,

pressure drop was more varied, but generally between 0.4 and 0.6 inches ofwater. It does not

appear that there was anY .clear relationship between degree of pressure drop and the level of

re~ctivity retained by the catalyst. Furthermore, while the "Columbia Station showed more .

signifiCant accumulationandplugging th~the Baldwin Station,',12 the catalystreactiyityfor

Baldwin Station"decreased morethan at Columbia Station over the courseOnneslX iriontns of~·~~·~·._-.-.~~_.....

pilot testing (84% vs.90%). Again, itdoes notappear that the qualitative observations from the

pilot:studyfor.Baldwin andColtimbia Stations correlate well with the catalyst reactivity testing

.results, which were not included in Appendix B ofthe MinnkotaBACt analysis.

For the Coyote Station, EPA agrees that the plugging and pres~uie'drop observed during

the pilot test were more pronounced and:occurred quicker than what was observed at the two

facilities burning PRB coal. Additionally~ based on the deposition sampling, EPA agrees there is .

evidence that sodium was afactorin the plugging 'atCoyote, where itdid not appear at the other

two facilittes.· However,·EPA disagrees that these test results can lead to a conclusion that .

extended forced outages would be needed at two month intervals as stated by B&McD in· .

Minrikota's'March 19, 2007 letter~ 13 The results ofthe catalyst deactivation testing at Baldwin

. . .' . .

10 See Page 594, Fuel Processing Technology 86 (2005) 577-613, "SCR Catalyst Performance in Flue Gases· .

Derived from SUbbituminous and Lignite Coals" (Accepted July 2004) . ..

II BabCock-Hitachi K.K "Test Results ofCatalyst Samples for Energy & Environmental Rese~ch Center", April,

2005,

12 Final

Table

Report

3.

for N Task 31- Evaluation ofPotential

'.

SCR Cataiyst-Blinding During CoalCombustion and AddOn: hnpact ofseRCatalyst on Mercury Oxidation in Lignite:"Fired Combustions Systems, Noveinber 29,2004, .

~~. .

13 page AI-30 ofOctober 2006 Minnkota BACT Analysis Study for MRYS indic~tes that the anticipated catalyst

life for SCR at MRYS would be 2-12 months, "requiring extended, frequent outages for replacement."

-5-
, ' '

EPA Response to Minnkota Comment.Lett,ers from

March 19,2007 lind April 23, 2007

, BACT Anil1ysis at MRYS '

and 'Coillinbi~ ~tations showed no clea,r correlation between magnitude ofpressure drop and the

, degree of ca,talyst deactivation at each plant. Also, th~ qualitative.observations on catalyst,',·

plugging did not correlate to the degree ofcaJalyst deactivationfor..Baldwin and Columbia

Stations relative to one another. Furthermore, given th~tthe two facilitiesbtuning PRE cmil

required $imilar maintenance measures and schedules to Coyote Station~ EPA questions how the

.maintenance activities'(and their tllneframes) required during the pilot testing at Coyote Station

. 'can be significantly distinguished from the BaldwiIi and Columbia Stations burning PR.J;J coal..'

EPA does not agree that it is correct to defIne catalyst replacement requirements associated with·'

full-scale SCR at MRYS based on the pilot testing expenence at Coyote'Station. '

Additionally, based on a July 2, 2004 conversation between Jim Stalldt ofAndover .

Technology Partners and FlemmingHanse.n ofH~l1dor Topose, the sudden build up offlyash on

the catalyst face that occurred during the piiottesting.at Coyote Station was,determined to be

caused by theaccumulatjon. of ash upstream ofthe catalyst and sl\dden dropping orthat '

accumulated ash onto the catalyst fac.e. 14 Thus, the initial pressure drop experienced'during the

pilot test atCoyQte Station might have been.largely due to cOl1ditions that would notexist,and/or

could be avoided through design, on a full-scale application of a SC~ system. Thisfurther calls

intoquestion the applicability ofthe pres~ure drop trendsexperi~nced during the pilottestingto .

full..scale SCR applications and their catalyst maintenance requirements. .

Based on the above comparison orthe pilot testing results presented by EERC, it is unclear..

to EPAthat any definitive conelusions cali be extrapolated from the results about SCRtechnical

feasibility aridlof the timing or extent.of SCR maintenance requir~ments and boiler, outages for .

these three facilities, as waS done by B&McD for MRYS. Clearly, the two facilities operating on

PRB coal experienced catalyst plugging that required extensive and aggressive cleaning .

measures within timeframes that, if applied directly to anticipated maintenance at full..scale

operation, would not be consistent with real full scale experience at ntmlerous.boilers burn,ing .

PRB coal. EPA'knows that there are suc~essful ~pplications ofSCR systems to ~tility boilers

burning PRB coal. Hence,EPA does not believeit is reasonable to directly apply the

catalyst maintenance and replacement experience fronl the pilo~ test to ,a full~scale .

application atMRYS.· .

Based on B&McD;s assertions about expe~tedmairite~ancer~quireril~ntsand schedules for

MRYS based on the Coyote results, a facilitybuming PRB would also be expected to encounter

significant plugging, which would require aggressive on':linec1eaniilg, catalyst replacement, and

forced boiler outage periods within the initialmonths ofinstallation: EPA knows from

experience that the issues facing facilities burning PRB coal, while real and significant, have

been dealt with successfully. Therefore, EPAbelieves the pilot testing was either designed in a

way that does not accuratelyreflect full-scale SCR catalystpluggingandlor mitigation measures

14 August 20, 2004 Memorandum from Jim Staudt (Andover Technology 'Partners) to Sikander Khan (EPA), "SCR

catalyst testing on ND Lignite"

-6-
EPA Response.to MinnkotaComment Letters from

, March 19, 2007 and April 23, 2007

, BACT Analysis at MRYS

currently khown to exist, or that additional measures could have been.taken beyond what was

attempted'during the pilot testing to keep the catalyst face clean and mitigate the'effects of

catalyst plugging. To that end, a~thoJ1gh it was mentioned in the preliminary EPA comments;

B&Mcb h8$notprovicl~da response to EPA's statements regarding the normally observed

pronounced ash deposition during pilot ·testing, the small catalyst pitch used at Coyote Station

alid how a larger pitch might hav~affected the test results,. and how the air pulsing did not

properly represent sootblowing capabilities on a full-scale SCR application. In sumriiaiy, EPA '

does not agree with conclusions drawn from the pilot testing presented in Minnkota;s March 17,

2007 letter related to catalyst maintenance and replacement.requirements and the related forced

boiler outages to accommodate these activities. As an alternative to relying on these pilot test

results, it would be preferable for Minnkota to work with catalyst ~d SCR design companies to

analyze how the sodium deposition at MRYS would affect catalyst maintenance and replacement

requirements and schedules. " .

EERC March 19,2007Response:

. . .... . .

'We spoke to Flemming Hansen ofHaldor Topsoe, an SCR catalyst vendor, regarding providing

performance'guarantees. After,describing the characteristics ofCenierlignite, specifically issues related

to sodium, they were very concerned regarding guaranteeing performance. Based on the conversation, it

.became evident that EPA representatives may hdv.e not indicated the levels ofsodium and theform of

sodium in the coal. Haldor Topsoe indicated that the sodium wasa major concern and that it causes

~~~~~.rldeactivati~peciaH)rlhevrganicaHj"tlss(}eiaterJ-for~ky1ndieated~lhttt--therhaehom~)tperienee~~~

andfound significant deaCtivation ofcoupon samples after 6000 to 7000 hours ofoperation with a coal

having elevated levels ofsodium.

EPA Response: ,

, EPA also spoke with Flemming Hansen regarding the feasibility of an SCR installation at

MRYS andthe nature oftheSCR pilot testing at the Coyote Station condllcted by EERC., EPA

asked Mr. Hansen about the effects ofsodium onBCR catalyst performance~ Mr. Hansen said

the sodium ion can poison catalyst because it prevents the ammonia from interacting 'with it. He

also estimated that sodium catalyst poiSoning limits the life ofthe catalyst to roughly one year.

At that point, the catalyst wbuld need to be replaced'or washed/rejuvenated. '

,Mr. Hansen inditated th~t the only exp.erience"Haldor-Topsoehas had with lignite coal was

the pilot testing performed at the Coyote Station conducted by EERC. However, they did run

some catalyst ''test coupons" at the Wisconsin Electric Power (~Energies) Presque Isle

Station, which bUrns Powder River Basin coal with sodium levels similar to North Dakota

lignite. Mr. Hansen recollected that there was a significant difference in the deactivation ofthe

test coupons upstream ofthe particulate' control device,.(ESP) compared to downstream ofthe' '

ESP at Presque Isle.

-7-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007 ..

. BACTAnalysis at MRYS·

Mr. Hansen said he recalled there were problems during the Coyote Station pilot test' ,

'lJeci:uise,the reactor became quickly plugged with flyashcausiDg rapid iricreases in pressure' drop.,

, Mr. ,Hansen said that the pressure drop was mainly caused by diu::t-:wallashaccumulations, .

,dropping on the catalyst face. 'Mr. Hansen recall~d that problems with tlle,sodlum poisoning

, '~ere'secoridary compared to the ash plugging problems. H~,stated that ash plugging is,a·"

problem ofpllot~scale testing and would not be a real full,;,s¢ale application Issue. He, said thafhe

was aware, that actual operations at plants buiiling PRB coal peffonn much',better than the pilot '

tests would indicate. " " '

Mr. Hansen said that, in hindsight, the pilot test sh6uld have been performed differently.

Mr. Hatlsen stated that" changes to the pilot test at Coyote Station, 'such as the use'of a higher

pitch catalyst, could have helped reduce, but not entirely elIminate, the impacts ofash plugging:

Additionally, he said the pilottest d,id,not have good simulation offull-scale sootbloWIng. 'Ifthe '

test were to be redone, a better effort could also be made to keep the catalyst face dean.

. Mr. Hansen said the pilot testing at Coyote Station was inconclusive in terms of

determining Whether SCRwould be technically feasible on North Dakota lignite. Mr. Hansen

.said that pilot te$ts often have problemsthat can be o~ercome at the full scaie. Mr. Harisen said',

that Haldor Topsoe wouldnot'rule out bidding ona project that involved North Dakota lignite

and that they would primarily look at the computer flow modeling for the specific coal analysis

and boiler and evaluate whether there are available mitigatIon options to address,any significant '

technical issues indicated by the analysis. Mr. Hansen said that there "are niany tools in the

toolkit" to address SCR technical issues. For North Dakota lignite, with 'its high ash and sodiUm

content, Mr.,H~lJiSen said that Haldor Topsoe would likely consider,such options as screens, very

, good soot blowing (steam assisted) and a high catalyst pitch. Mr. Hansen said and/or implied

that an SCR could be designed to work with the specific characteristics ofNorth Dakota lignite

through proper reactor sizing~pitch sizing, steam soot blowing, and proper regeneration

schedule: ,He 'gave a rough estimate that the SCR system could be designed (larger) to '

, accommodate'a catalyst replacement schedule, of about every-year.

EPAasked Mr. Hansen how he would weigh the results ofthe pilot testing at Coyote ' '

, Station. Mr~ Hansen said that he would have to go back and look at the Coyote Station test ,

results and see ifthere was any useful information to consider. Mr. Hansen ~mid and/odmplied

that the coal profile, information and the results of computer flow modeling would be more, '

critical than the Coyote, Station pilot test results in ~y, evaluation 'of bidding a project With North

Dakota lignite. . ",

-8-
EPA Re~ponse to Minnkota Comment Letters from

March 19,2007 and April 23; 2007

BACT Analysis at MRYS

Mr. Hansen was unaware that Haldor 'Topsoe sent em~ils to'EPA HeadqUarters stating that

Haldor Topsoe (and two other catalyst vendors) could provide guaraIitees for the MRYS coal..

However, he sent an email t9 a consultant to EPA in 2004 statmg in general they could propose

SCR catalystsfor North Dakotalignite assuniing certain criteria are met. He had also talked

With that consult(,liit at that time about the fact that pilot plants have problems that full-scale SCR

systems do not. . . .... .

II. EPA Case-Specific Review ofBurns & McDoimell &'EERC Technical FeasibilitY

Analysis for Selective.Catalytic Reduction

'A~ High-Dust SCR

B&M has eliminated Uigh~Dust SCR as technically irlfeasible due to issues described

below. B&M'sanalysis utilizes a report by the Energy & Environmental Research Center

(EERC). EPA's response on each issue isalso presented below.

1. Ash Deposition Issue Raised by Burns & McDonnell:

EPA's January8, 2007 Comment:

The concern about high amounts ofash material causingblinding ofmacro-poresin the catalyst is

real butsolvalJle.8trides incatalystformulations, catalystmimagementsystems. and recently, catalyst

regeneration processes, have all combinedto lower the overall cost contribution ofboth initial and

replacement catalyst in modeni SCR systems. . . .

EERC March 19, 2007Response:

EPA states that the high anlOunt ofash materials causing blinding ofmacropores in the catalyst is

real butsolvable. We spoke to Flemming Hansen ofHaldor Topsoe, a catalyst vendor, regarding what

has been done to deal with theproblems ofhigh-sodium coals with regard to advancedfotmulations and

catalystregeneration, and he was not aware ofany work that had advanced SCR systemsfor hlgh-soc(tum

applications.. .

.. Regeneration h4s been attempted in Europe 'with SCR catalysts exposed/o biomass with high'K

_ (similar in chemistry to Na), andthey had some significqnt problem In recovering reactivity.

-9-
EPA Resp~nse to Minnkotit Comment Letters from·

March 19,2007 and April 23, 2007

BACT Analysis at MRYS

· B&McD March 19, 2007Response:

. It is unclear whetherEPA 's claimed SCR catalyst improvements citedwould translate i1'lto

preventing the catalyst layersfrom being susceptible to macro-pore blinding andplugging by calciumbasedcompounds andthe impact on the individual catalyst reaction sitesjromsodium:'based compounds·

discUssed in Dr. Benson's referenced technical paper.· The EPA cites no published technical literature .

that describes the results oftesting that definitively documents how longcurrently available SCR

· catalysis c(m survive andprovide effective NOx reduction when exposedio thejlue gas andjlyash

constituents exhaustedfrom combustion cmd.operation ofany cyclone boilerfiring North Dakota lignite,

particularly one located in a high-dust, hot-side application such as would be the case at Milton R. Young

Station.· .

. .EPA Response: .

As described earlier, the burden ofproofis on Minnkota to demonstrate SCR would b~

technically infeasible at MRYS. The BACT process does not require that the reviewing agency

demonstrate that a technology would work for the exact type ofinstallation under review,

especially when the technology has been well documented to be technically feasible for the

· source category under review.

EPA had asked Minnkota.t~ look ii.n1her into the engineering solutioriscurrently available

anddetertnine whet~et or not the macro-pore plugging issues described in the EERC report

could be effectively managed. It does not appear that these engineering solutions were given

.. rriuch consideration. Without addressing the available engineeririgsolutions to deal with·macfo-:·

· pore blinding and plugging, MlnnkQta has not met the significant burden ofproof demonstration

EPA believes is requiredfor showing SCR is nota technically feasible contro(option forNOx at

MRYS. ....

a. . Comparison ofMRYS to SCR Systems Operating on Powder River Basin Coal

. . .

EPA's .January 9,2007 Comment:

Even though the chemical compositions ofPRBash andCenterMine coal ash are very similar,

particularlyfor the specific constitl!ents ofconcern for catalyst deactivation, the quantity ofash in the

Center Mine coal is more than twice as high on a per MMBtit basis. However, for a cycloneboiJer, only

-lO~
EPA Response to Minnkota Comment Letters,from

, March 19,2007 and April 23, 2007

BACT Analysis at MRYS . ., .

about 30-50%'ofthe ash ends up asfly ash atfull load (less atlower loads) andfor a wall or corner-jired

pulverized coal boiler (PC boiler); about 80% ofthe ash ends up asflyash. 'The result is thatSCR

catalyst in a PC boiler PRB application is exposed to similar quem/ities ofthe major constituents of '

concern (alkaline material) as a catalystwould be exposed to atMRYS. The B&Manalysis also suggests

that a linear comparison oftheoretical total ash or aSh constituent rates to SCR catalyst deactivation

" rates can be made. AlthOl,fgh EPA does not believe this to be the case and B&Mhas notpresented any

evidence ofa quantitative relationship between ash loading and SCR catalyst deactivation rates, EPA has,

nevertheless examinedB&M's argUments related td ash loading.

, Table 1 below shows a comparison ofthe total ash loadingfor different boiler designsfor North'

Dakota lignite, assuming a reference unit of1.0 IblMMBtufor lignite, lignite will have twice the ash '

content on a per Btu basis, and the conversion to flyash will be 30-50%for cyclone boilers and 80%for

PC boilers. As can be seenfrom this comparison, the theoretiealflyash loadingfor aPC boilerfiring ,

,PRB boiler (OAlbIMMBtu) will be in the same range as what woitldbe expectedfor a cycl(1ne boiler

firing North Dakota lignite (0.3-0.5IbIMMBtu) ....;..Deactivation depends on the total time and mass

exposure, and the PRB PC boiler data are extremelyrelevant to evaluating SCRfeasibilityfor North

Dakota lignite ... :.... Table 1 shows that; on a IblMMBttl basis, total theoreticalflyashloading at the

economizer exit ofd cyclone boilerfiring lignite would be very similarto thalola PC boilerfiring PRB

coaL '

Another alkaline constituent that has been a concern is calcium, whiCh can react with S03 to plug'

pores dS caS04• This has been q much-studiedproblem and the mechanism involves the calcium- ,

~~~~~cOJ:JtainilJgp.adick.gettjl1g "(;fJJ,JgbLlnJLm£lC,ro·pore-fJithe ,catal)!.standsubsequentl)!. reacting with SO; ,

in the flue gas. ' The 'resulting CaSO4 particle is larger in volume than the CaO particle a;u!, in essence,

swells to fill the pore PRBcoal has been shown to have approximately 30% more caldum in its

, ash compared to North Dakota lignite Other alkaline materials (potassium, sodium and

magnesium) deactivate catalysts by deactivating individualreaction sites'rather.'than the blinding by

macro-pore plugging that was describedfor calcium. The average Center Mine coal ash contentfor

these alkaline constituents is about 9%..: This percentage is somewhai!Qwer than mostPRB coals

and within the envelope ofexperience ofEuropean installations; One ofthe referf!.nces cited in the

Appendix B EERC report, Cichariowicz & Muzio (2001), places the range ofEuropean SCR experience

with these alkaline constituents at 8% to 15% ofash. Thus, the coal used at MRYS has similar poisoning

potentfalcompared to othercoals in use at,SCR-equipped boilers in Europe andthe U.S... .. .., .....

The concern about high amounts ofalkaline materials in Center Mine coal expressed in B&M's

BACTanalysis and described in the AppendixB EERC repor,t isreal, though notnecessarily unlike '

operating SCR systems on PC boilers burning PRB coal. As explained above, we believe there is value in

successful operating experience realized at currentSCR instal/aNon at PRB-firedpower plants. The

problems cited in the BACTanalysis are being dealt with at severalNOx'SIP Call SCR installations

today. In addition, the analysis presents no quantitative analytical information derivedfrom thefiring of

lignite coal on SCR catalyst deactivation tojustify the finding that SCR is l1ifeasible.

-11-
. .

EPA Response to Mi",nkota Comment LetteJ;S from

March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS

EERC's March 19, 2007Response: . . .

The sodium and.ash contents ofCenter lignite are significantly differentfrom PRB coais. EPA.

, indicates thdt the problems.ofdeposition will decrease because the cyclones take out Some· ofthe ashas :

slag butfor "ligillte, cyclones create additiwuJI deposition prob.lems downstream. The cyclones partition

the more refrt;Jctoryash c(J!flP(Jnents into the slag andthe more volat!lecomponents into thejly ash. . Th(!!

jlyash becomes enriched in c011Jponents that vCiporize and.. sZ!bsequently, condense upon gas cooling to, .

form liquidphases and smaltparticles that are very reactive to ga.s-phase components thcit'cause .. bonding.· .. .

EPA· discusses ash loadings cmd the potential impdcts on SCR performance~ They also d.iscuss the

formation ofcalCium sulfates that can plugpores in catalysts as well as the impacts ofother alkaline

materials (potassium, sodium, andmagnesium) and their p,otential to deactivate individual sites rather

than blinding pores. . Theyfail ta recognize the formation ofcombinedphasessuchas sodium cafC"ium

sulfates with low meltingpoints that are·very reactive and can both blockpores and deactivate individual

sites in catalysts. They also fail to recognize that cyclones partition the ash,· increasing the prop.ortionof

sodium in the smaller particles, resulting in increasedpropensityfor theformation ofthe complex sulfate. phases. . . .. .

Tops.oepresented a paper at the u.s. DeparimentofEnergy (DOE) National Energy Technology

Laboratory (NETL) meeting on SCR-selective non..;catalytic reduction (SNCR) on testing conductedwith

biomassfuels ihat contain high potassium andsodiuJnfound rapid deactivation in·the SCR catalyst.

Their data stiggestsodiumor potassium resembles. the ammonium.ion and blocks access to the active .

sites in the catalyst. .They indicate that tMs occurs most readily where the alkali elements are in a soluble

fbrm;Topsoe incorrectly assumes that all alkali incoalisriotsol't!ble because ofwork they did on

. potassium. Potassium is asso(;iated in the coal with clay minerals and is not soluble. Sodium in North

Dakota lignite is organically associated (soluble form) and is readily available for reactions with

catalysts andflue gas spec.ies..More recent lYork conducted by Beck and others (2004) established/hat

sodium correlates with catalyst deactivation activity and that pore condensation ofsodium is th'e major

deactivation.mechanism. / .•..

.. .

, Sodizmllevels at MRYS have a multiinodal di$lribution, a~d assZ(ming the average sodiZ(m level in the ash

is not realistic... :..... . .... . .. ..

EPA Response: ..

EPA does not disputeEERC~s d~scriptioit· offuechemical and physical fate ofsodium ina

'. cyclone boiler. Please.refer to the:August 20,2007 Christman Memo Item#2 for the response .

. on the effects ofsodiuni. on a SCRsystem and the apptopriateness·ofusing average coal

constituentvalues when evaluatiD:g !heireffect on anSCR system.

-12:....
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2607

BACT Analysis atMRYS

EPA's original analysis was in response to B&McD's quantitative comparison ofsodium

ash loading presented in the original BACT analysis. 15 This comparison uses .aven~ge fuel ash

, and sodium values to calculate a sodium ash loading rate of7.31b/MW-hr :for North Dakota

lignite compared to 0.9Ib/MW-hr for a PRB-fired cyclone boiler. 16 'One reason EPA provided a "

comparison is that B&McD's analysis did not look at PC-fired PRBtinits~ whIch would have a

higher ash loading rate than a cyclone unit. ThIs infotn;lation is relevant considering SCR

systems have been installed on PRB-fired PC units. B&McD's analysis assumes 35%

'. conversion offueI-ash to flYaSh for PRB-fired cyclone units arid 50% for ligirlte-fired cyclone

. units.. Using the 80% conversion offuel ashto flyash for a PRB-fired PC unit increases the

sodium loading presented by B&McD t02.11b/MW-hr. [(0.9Ib/MW-hr / 0.35) =2.57Ib/MWhr (sodium in the fuel ash fot PRB coal) * 0.8 conversion offuel ash to flyash in aPc boiler = .

2.1Ib(MW-hr(0.9Ib/MW-hr·is the sodium loading for PRBcoal calculated by a&McD on a

cyclone boiler assuming 35% conversion offuel ash to flyash)] The other reason EPAprovided

this analysis was to include the entire Set of ash constituents'for the quantitative comparison. ,

EPA presented this comparison with some reservations, since, as stated in the comments,

EPA does not believe that a linear comparison can be made between theoretical ash constituent .

loading rates to SCR catalyst deactivation rates. Nevertheless, EPAwanted to demonstrate that

B&McD's comparison oforily the sodium ash loading from lignite-fired cyclone units to PRB..: . ,

fired cyclone unitsignores the PRB-fired PC units which have the higher fuel ash to flyash ,

.conversio!1~d()t1).e!~~hc:o~s~itllen!s,t~a!, <to effe<.;t ~_~,~ 0Jlerati~ns: ',On its_:s':lrfac~,}3~~~J?' s" .

analysis portrays a.sodium loading differen~e between North Dakot~lignite and PRBcoal that is'

greater thana factor ofseven. this difference becomes less when including PRB-fired PCunits '

(about a factor ofthree difference) and other ash constituents (virtually zero). Agairi,EPA does

. not believ.e there is Ii linear relationship between this ash loading and SCR deactivation rate. '

However, since B&McD presented a theoretical sodium loading comparison betWeen North,

, Dakota lignite and PRB coal fired cyclone units, with the apparent implication that the difference'

was significant in the.disc,ussion oftechnical feasibility ofSCR at MRYS; EPA wanted to

'include a more complete comparison.

15, See page A 1..29 ofOctober 2006 Minnkota BACT Analysis Study for MRYS.

16 In the March 19, 2007 response (page 28), EERC dist:J1isses EPA's analysis. "Sodium levels at MRYS have a

multimodal distribution, and assuming the average sodium level in the ash in not realistic." Note that the analysis

presented by B&McD uses average fuel characteristics for North Dakota lignite and PRB coal. .

-13-
EPA Response to MinnkotaCoininent Lcitte~s from .

·.March 19,2001 and April 23; 2007

BACT Arialysis atMRYS . .' . . '.' '. .

EPA acknowledge~that the'sodium contentofthe Center Mine lignite is gen~rallyhigher th&Ii PRB·coals.EP/\. also recognIzes that ash partitioning in a cyclone boiler and resultIng'

sodium·deposition·presentsceitain.problemsthatcouldpos~engineenllg ~hallenges to the design·

and operation of aD. SCR system at MRyS~ H'owever, Mi~ota haS not provided any . .

quantitatIve data thatshows that the'plugging'i,md deactivation ofthe· SCR system would.be an' .

·unresolvable technical harrier and/or the ability ofthe SCR to control NOx would be:so limited

that SCR would be aD. ineffective control option. Ag13.in, EPA thiriks that both ofthese issues

would be more appropria~ely addressed at step 4 oftlle Top-Down BACT analysis: . '. . . .

The Topsoe 1999 paper referenced byEERC does observe that biomass having.~ high.

percentage ofsoluble sodium.or potassium that was burned in an incinerator caused "rapid

·deactivation in the SCR catalyst." .It is u.nclear howthese soluble sodium or potassium .

percenti:lges compare to the alkaline makeup ofCenter.Mine lignite. Furthermore, it is unclear

what Topsoe meant by "rapid deactivation." The paper does not appear to indicate that the SCR

was inoperable or ineffective for this test. It is fnappropriatetodraw any meaningful. conclusions '.

on technical feasibility based on these qualitative statements iIi Topsoe's paper. . . . . .

B&McD's March 19~ 2007Response:

. The discussion iii the EPA's preliminary analjJsis ojMinnkota 's NOx BACTAnalysis atMRYS

ignores the BACTreports' statements that ihejiring oflignite coal in acyc!one boilerspecifically.

·designed and built to pre-dry and bu~n North Dcikota lignite producesfinejlyash particles which wiil .

enterthe poresoftheSCR catalYsi..react with S03 ifl (hejlue gas, andform sulfates which bind oiher ash

. particles into the. matrix.' .

. When considering the much higher sodium content o/Center lignite coal.ash.(lb/mmBtu) compcll'ed .

to PRE coal, the impact on tlze individual catalyst reaction sitesfrom sodium-basedcompounds discuSsed'

in Dr. Benson~s referenced technical paper has largely been ignored by the EPA;s' .

commenters .

The problems associated with sodium combined with calcium, sulfur; and other alkaline elements at

·any cyclone boilerfiring North Dakota lignite appear to be unpreventCiblefor successful application of .

SCR catalysts. There is nofull-~cale long-term documented andpublished demonstrated experience .at

commercial SCRbistallations on largecoal-jired boilers with similarly high sodium levels. .The tendency

. ofs.mall sodium vapor particles to be generated during the combustion'and slagging processes ofa North .

Dakota lignite-fired cyclone boiler has no/been. adequately addressed by/he EPA c.ommenter's . .

discussion.' . .

-14-
.. EPA Response to. Minnkota Comment Letters from .

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

The lack ofpublishedNOx control effectiveness and catalyst activity datafor the Coyote slipstream

scR catalystfield test preclu,dedBums & McDonnel/from evaluating or including it in the NOxBACT .

Analysis Study rejJortsfor the Milton R Young boilers Electric Power Research Institute, Inc. (EPRI)

. produced a recent study on the impactofcoal type on SCR catalyst lift andperformance... ..:... This

.studyfound that the SioUx phnt's cyclone unitfiring the PRBlbituminous coal blend exhibited the fastest

rate ofcatalyst activity degradation. Also, the higher deactivation rates seen at this sitewere· due to

economizer·exitflue gas temperatures being significantly higherthan at the othersites..

Bums & McDonnell believes that the flue gas andflyash conditions producedduring the

· compustion and slagging processes ofa North Dakota lignite-fired cyclone boiler will be significantly

more problematic than the tests at Sioux Station demonstrated.. ...

EPA Response:

Please refer to the August 20,-:2007 Christman Memo Item #2 for the response on the

effects of.sodium on SCR. EPA's original comment does not largely ignore the statements in the

Minnkota BACT analysis about higher sodium content and its impact on catalyst deactivation~

EPA's analysis looked·at the combined percentages of other alkaline materials (potassium,

sodium, and magnesium) that would contribute to catalyst deactivation, instead ofjust focusing

on sodium; These other constituents are ofimportance. See the expanded ahalysis and· .

discussion above.

B&McD states that E}>A has not adequately addressed high Center Mine lignite sodium

issue and has provided no·documentedlorig;,.term experience at commercial SCR·installations on

large coal-fired boilers with similarly high sodium levels. As discussed earlier, EPA believes

that the burden of proofin this·case is on Minrtkota to provide definitive qu;;mtitative evidence

that SCR would be technic~lly infeasible atMRYS due to the high number ofSCR syst~ms

· currently in operation at utility boilers. Furthermore, a lack ofa full""scale long-term commercial

· SCR installation at a facility with similar sodium levels in the fuel is not evidence that SCRis .

technically infeasible at MRYS. While EPA agreesthat the sodium levels in North. Dakota .

lignite are higher and will require aggressive mitigation measures, SCR systemshave obviously

been applie.d to new fuel types with unique characteristics over the history of SCR installation,s.

EPA has mentioned a number ofengineering solutions that could help manage the high

sodium issue from a technical standpoint. .EPA does not believe that the qualitative information .

. presented thus far is sufficient to outweigh the vast amount pf SCR operating experience for

utility boilers to say that SCR would be technically infeasible at MRYS. Due to the lack ofNOx

emission and catalyst deactivation data, the Coyote Station study does not sUPPQrt utilizing any .

definitive conClusion, other than there was significant plugging, which also appeared at both the

Baldwin and Columbia Stations. ...

EPA believes that the experience the utility industry has gained from the installationS at ..

PRB facilities would be USeful in designing an SCR systemfor a cyclone unitburning North.

-15-:-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

Dakota lignite. According 'to th~ presehtation given In Bismark on May 23~2007toNDOH and ".

EPA. by the utility design firm, Sargent & Lundy, the successes and failures ofthePRB' .

instaJlaticins would provide :'valuable infoiniation for designing an SCR system pn a plant burnIng

North Dakota lignite. 'Wliile'Sargent & Lundy indicated that getting further data from a ~ell- '

designed pilot study would enhance'theirability'to optimally design a more cost-effective SCR

system, they indicated that they would certainly draw uponthe'experience gained through,the

PRB installations. '

EPA does not have access to the EPR! study referenced that producedfield'dat~from an

"In-Situ Mini SCR Reactor" and showed that the fastest rate ofdeactivation was from the

AmerenUE Sioux Station, which is a cyclone boiler burning a PRB/bituminous blend. B&McD

appears to imply that this result demons.trates that an SCRinstallation on a cyclone boiler win

experience faster deactivation,than art installation on a PC..fired, boiler because ofthe higher

economizer flue gas exit temperatures. Since EPAdoes not have access to this data, EPA does

not know how large ofa differen~e there was between the cyclone lInit and the PC-fired units.' ,

Without access to this data, it would in approprIate for EPA to place any reliance on assumptions

or conclusions fromthis study. Furthermore, EPA is unsurehowthis relates'exactlyto the issue

of SCR technical feasibility at MRYS. This data might be useful in trying to predict catalyst

cleaning / replacement requirements, but it is not relevant to demonstratingtIie technical '

feasibility of SCR at MRYS.

. .'.

"b. .Engineering S~lutions fOr SCR Applications atHigh~AshFacilities

EPA'sJanuary 9, 2007 Co';'ment:

,Plugging ofSCR catalystsbyfly ash accumulation and bridging ofpassages in honeycomb '

catalysts and betWeen plates in plate-type catalysts h(;ls been a probletnfordesigners Cind catalyst

vendors. since'thejtrst coal-fired ins.tallations in the 1970s.Th'eprimary response has been more

sootblowing" on-line cleaning ,equipment, ctnd larger catalyst pitchfor honeycomb configurations and .

wider plate spacingfor plate-type configurations. These measures, combinedwith periodic cleaning

during boiler outages have, by-and-large, reduced c;atalyst plugging to a manageable SCR problem.. ...

Also, theidet that the MRYS units are brought down for planned boiler deposit cleaning at 120-day

, interval$ provides more opportunityfor catalyst cleaning maintenance than is available at the typical .

power plant. " .

Further d~mo~trating the ability ofSCR df;signers io use existing mowledge and experience

with SCR installatiansfor highashfuels, the. Electric Power Research Institute (EPRl) has advised'

clients that based on industry experience on a wide range ofSCR ins.tallationsOver the years, SCR. ' "

designs can accommodatefiiel types never before used in practice on afull scale. In a September 7, :2006

letter addressedto TXU Power, EPRIstated, "While experience specifically.with Texas lignite is limited,

this should notprevent'suppliersfrom providing an SCR system that meets these objectives; dealing with

a newfuel is similar to/he challengefaced by US. utilities and their SCR supplierswhenfirst installing

SCR systems on planisfiring high sulfur bituminous or PRE coals. Neithersuppliers nor utilities had

-16-

"
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23; 2007

BACT Analysis at MRYS

prior experience with thesefuels, which produceflue gas properties'that are very differentfrom those in

the Japanese andEuropean plants where SCR has been installed before. Therefore, SCR designers had to

rely upon their knowiedge, engineeringjudgment, and experience withthese other coals to design SCR

systems suitablefor these new applications. While some ofthej;rs~ u.s. SCR installations experienced.

. problems (e.g., accelerated catalyst activity decay due .to arsenic poisoning in the absence ofsufficient

calcium in the coal, and catalyst pluggage due to large particle ash), noneofthe~e problems prevented

.the systemsfrom achieving their NOx goals initially and, eventually, maintaining them over the expected

.life ofthe catalysts. 11 .

EERC's March 19, 2007 Response: .

The issue isn'ot strictly the high ash content ofNorth-Dakota lignite, it is the high sodium contentof

this lignite's ash. The issuefor the application ofSCR on North Dakotalignite,jired cycloneboilers is a

high potentialfOr ash deposition in lower-temperature convective pass suifacessuch as the economizer

andprimary superheater. The high ash deposition r.ates require continuous cleaning with-sootblowers,

. and the resulting liberated depositfragments will cause catalyst plugging. The depositfragments, along

with the formation ofsulfated sodium- and e.alcium-richfine particulate pluggingpor(!s ofthe catalyst,

causesplugging ofgas paths. . .

EPA Response: .

The response fro~EERC simply restates their position regarding the high sodium conte~t ~~~~~o~f""tT""lie~-'7"C"""e~nt~e"';:r1\Iirie IigriiteaIid howtliiswillciruse botli-catalyst porepliiggirig-iiildUgas path'

plugging. EERCJails to consider the use of available engineering solutions to address these

problems with operation ofan SCR at MRYS. . ....

B&McD'sMarch 19, 2007Response:

For a North Dakota lignite-jired cyclone boiler, aggressive on-line cleaning (mdlorfrequent

extendedforced outages would be needed to remove the ash accumulation or remove the catalystfor

cleaning in order to restore full unit output qnd catalyst NOx reduction effectiveness. Either or both of

these'maintenance activities could damage the catalyst andlor require itto befrequenHy- .

replaced the e~timated amount ofiime and difficultyfor MRYS boilers' SCR maintenance appears

to be insufficient to use boiler deposit cleaning outages to perform significant catalyst cleaning or "

replacemeniwork without delaying startup. .

EPRJ's statements do not conclusively support the technicalfeasibility ofapplying SCR wiiJi

'. specific technical evidence that showsproofofthe hypotheses, that it will be successful on Texas lignitefired boilers.. It also does notsuggest orimply that North Dakota lignites are similar to Texas lignites

and therefore SCR should also be successful on such applications. The high ash and moisture content of

a TexaS lignite-jired unit would result in a different set ofchallengesfor application ofhigh~dustSCR

technology. Possible solutions to deal with Texas lignite are notrelevant to compare to North Dakota'

cyclone boilers in this NOx BACTanalysisfor MRYS. Already-instalied boiler and backend equipment

designs on unitsfiring TXvs ND lignites are significantly difJerent, reflective ofthe approaches the

-17-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS .

origilialdesign ~ngineers took to ~tt~ch these situations as separatep~obiems. OA comparison ofthe ..

differences between Texascind North Dakota lignites is included in EERC's subsequentrespo,!ie to the' .

/iPA's comments regarding the NSPS revisions later in this response. document..

. EPA Resiumse: .

This response states that aggressive on-line cleaning"andlor frequentexiended' ol,ltages

would be required to remove ash accumulation or remove the catalyst for cleaning. The response

seems to acknowledge that the SCRcould be used at MRYS, albeitwith potentia.llyhigher than

normal maintenance requirements. This is not inconsistent with EPA's position, which is that

the problems cited in the Minnkota BACT analysis appear to be. more a matter of costthan

te.chnical feasibility.ltindeed necessary; the extended.outage times could be accounted for as

additional operational cost (ifmore frequent catalyst cleaning is necessary and/or cannot be

performed online) or accounted for, by adding additional catalyst layers to the SCR system.

The.intent ofreferencing the EPRI letter on Texas lignit~' was not to' imply that North

,Dakota iignites are exactly the same as Texas lignites. The Interit ofreferencing this letter wasto

show that a major utility res~arch organizationis bfihe opinion that the existii1gexperien<~e of

the SCR designers WQuld greatly help in designing SCR systems for accolnmodatingfuel types.

,never before used in practice on a~full scale. This underscores EPA's position that, although the

.specific fuei and boIkrcharacteristics at MRYS may present chaUenges to an SCR installation,

the significant iridustry experience with SCR for new fuel types in the past would serve'useful to.

overcome the technical challenges described by Minnkota~ This again was reiterated by the

design firm Sargent and Lundy in the'pre;sentation described above. '.

. ,

c. . Other Ash Deposition Problems on Downstream Co~ponents

EPA's January 9, 20iJ.7 C;omment:

. B&M's discussion ofthese~'other" potential accumulation problems does not reac.h any definitive

conclusions related (0 the technicalfeasibility ofSCR, so EPA does. not se~ this as atechnic,al barrier to. .

installing SCRat the MRYS units. . . . . '

EERCMarch 19, 2007Response:

Note that it is not the ash loadings, iiis thesodium,.'~ih~ lignite ash that causes th~ deposition.

, The $odium-related deposition on SCR catalyst willshorte.n the time between'outagesfor off-line cleaning~ . . ,,' ,

-18-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

EPA's Response:

EPA ac1aJ.owledges that the·sodium cont~nt in North Dakota lignite presents engineering

.challeng¢s. EPA also thinks it is possible that ~e "sodium-related deposition on SCR catalyst"

will shorten the tim~ between outages fO,r off-line cleaning." As stated previously, the

:requiremen~ for increased catalyst maintenance does not render the technology technically

infeasible.' Any increaSed maintemince and unit downtime would result in added cost, which

should be quantified at step 4 ofthe Top-down BACT analysis. Unfortunately, the piiot testing

conducted at Coyote Station did not provide any quantItative results that could b~ used to predict

the time periods between catalyst cleaning/replacement on 'a full-scale application.

B&McD's March 19,2007Response:

The higher amount ofash, sodium; and sulfur in North Dakota lignite combinedwith calcium and

. other alkaline elements along withammonia and S03 emittedfrom a high-dust SCR will exaggerate

. tubular air preheater and economizer tube exterior surfacefouling andpluggingproblem~ at any cyclone

boilerfiring thisfuel comjJared to bituminous or PRE coals. The tubular airpreheaters and the built-in

economizers at MRYS are not equipped with sootblowersfor on-line gas-side surface deposit cleaning.

NiJtingthatMRYS boilers' air preheater and ecimomizerswould be'downstream ofthe boilerbackpass

outlets and a high-dust SCR,' isolClting these components during off-line boiler deposit cleaning or SCR

cleaning outagesfrom negative impacts of "dirty air would be chaliengingand expensive: Similar .

.~~~~.effimClIed~(}ums---o.ftim~€md-aijjieultYf(}r-8ueh~ojf-line-downslFe{Jm-depos-ifepemovaf.s-wiIkH~~----_·-.-~

unit startupsfollowing such outages .....

EfA's Response:

It is unclear how the tubular air preheater and economizer tube exterior surface fouling alld

piugging problemswould become exaggerated from the use of an SCR system. As noted, ·in the

August 20, 2007 Christman Memo, "the SCRretrofits at New Madrid (two large, PRB-fired

cyclone boilers) included replacement ofthe tubular air preheaters with new regenerative air

preheaters with enamel-coated baskets." This and other alternatives should have been

consjdered in Minilkota's BACT analysis. The additional expense to clean these components.

and isolating the SCR system are described as "challenging and expensive." As discussed

, previously~ these costs should be addressed under step 4 ofthe Top-Down BACT analysis, not at.

the technical feasibility stage.

d. .. Accoun!ing fOr Catalyst Life & PerfOrmance in Tdp;.Down BACTAnalysis

EPA's January 9, 2007CiJmment:

Finally, the conclusion reached in the B&MBACTanalysis that ash deposition and catalyst

pluggage will adversely affect SCR performance (we assume this means .control e./ficiency) and catalyst

life does not necessarily support a conclUsion that'the technology is technically infeasible and that SCR

-19-
. .

EPA Response to Minnkota Comment Letters from

. . March 19,2007 and April 23, 2007

BACT Analysis at MRVS

should be eliminated at step 2 ojthe Top-Down BACT{lnalysis. Furthermore, the B&Manalysis presents,

noqucrntitative data related t.o SCR performance and catalyst life to supportsuch aconcliision... .

..~H(jwever, SCRperjormtlrice tindany real construction and operational costs arejactors that would be .

accounted/or under the economic impact stage related to cost effectiveness ojSCR (step 4) underthe· .

Top-Down BACTanalysis.

B&MdJ March 19,',2007Response:

The reasonsjor the ~ssertion ~jtechnical infeasibility ojSeRior iignite-jired cyclone boilers were

presented in the MRYStation Unit 1 and Uf?it 2 NOx BACTanalysis reports; withfUrther explanation.

providedherein. The EPA's preliminary analysis comments minimize the cfifficulties posed by the higher

amount ofaSh,sod!um, and sulfur in North Dakota lignite combinedwith calcium and other alkaline·

elements along with ammonia and S03 emittedfrom a high-dust SCR on the tWo existing North Dakota

lignite-jir¢d cyclone boilerat MRy-S; ,

The EPA cites . no published technical . literature that describes . .......as would be. . the case at Milton

R. Young Station.

. . .

·.The evidentfailure ofthe Coyote slipst~eam SCR catalystfield test to ... ....led Burns & McDonnell

to believe thesefield testresults indicate ... . ..for technic(J1 infeasiMlity purposes.

EPA Response: .

The majority ofthis coniment is addressed under EPA's response to the preliminary

summary...EPA does not discount the challenge's that Center Mine lignite might pose for an SCR

installation at MRYS. However, EPA cannot concur that the qualitative pilot testing results at .

. Coyote Station cited in the EERC report demonstrate that a rull-scale SCR installation at MRYS,

.that iQcluded the currently available engineering :design measures, would not be techniCally·· .

feasible and offer substantial NOx redil(~tions. EpAhas pointed out some ofthe inherent

limitations in the pilot testing that might have contributed to the "failure" .ofthat test. B&McD

and EE~C's responses do not provIde much insight on whether thes.etests were desigriedto

reasonably represent full-scale conditions, other than to say that EERC was notaione responsible·

.. for the testing's· de~ign. EPAquestion~ the design and operation ofthe Coyote ·Station pilot test.

in light ofthe lack of critical data obtained. .

EPA has also not found any deflnltivequautitative·results, taking intoaccoUllt th~ ash

deposition and cat,alyst plugging counter·measures, that show the expected rate of catalyst.

deactivation and off-line cleaning frequency that would be necessary for operation of an SCR

system at MRYS. As described in detail previously, EPA does not believe the pilot testing

presented by EERCprovides an accurate estimate ofwhat type of catalyst maintenance and

replacement-would be required. Without sllch quantitative information, EPA continues to

believe that Minnkota has not provided sufficient evidence oftechnical infeasibility for SCR at

. MRYS. EPA believes SCR is in fact technically feasible at MRYS. EPA be~ieves under the

-20-:
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

·Top-Down BACT analysis, any additional costs should be accounted for under step 4.

·Furthermore, as explained previously, EPA believes the burden of prooffor technical . . ..

infeasibility for SCR at MRYS is considerable giveri the large number orSCR systems currently

in operation at utility boilers worldwide with vastly different fuel types and characteristics.

Quite.simply, the record does·not support Minnkota's conclusions. '

. .

2. EPA Analysis ofthe "Popcorn Ash" Issue: '. ,

No ,response given to EPA's comment by EERC or B&McD. The topic was similarly addressed

·in tbe January 2007 Christman Memo.

3. EPA Analysis ofthe Temperature Issue:

No response given to EPA's comment by EERC or B&McD. The topic was similarly addressed '

in.the January 2007 Christman Memo.

4. EPA Analysis ofthe Erosion Issue:

No response given to EPA's comment by EERC or B&McD. The topic was similarly addressed

in the January 2007 Christman Memo. .

~~~~~~,~- ..~-~--~--.-.-.,

5. EPA Analysis ofthe Energy & Environmental Research Center Report

(Appendix B): .

EPA's January 9, 2007Comment:

. .

...... '" .However, the conClusions reached in the Appendix B report do not address some ofthe .

engineering solutions previouslyciteil and explained below that are availabk~forfull-scale SCR ' ,

appiications. Furthermore, EPA. Region 8 believes.that there may be inherentlimitations with the pilot '

study (that wasthe basisfor bothEERC reportsf that draws into question the validity ofdirectly applying

those conclusions to the range ofpossible SCR designs ofafull-scale application at MRYS. Statem.ents

from three major SCR catalyst manufacturers (including Haldor Topsoe, 'the catalyst manufacturer

, involved in the pilot testingfor North Dakota lignite) suggest that the catalyst deposition andplugging

problems experienced during the pilot testing may be exaggerating on a pilot-scale andean be solved in a

full-scale design and application .

Other inherentproblems associatedwith directly applying the results afthe EERCpilot testing at

.Coyote Station on North Dakota lignite include: ., ..

1) The airpulse used to clear deposits during the pilot test is a poorsimulation ofsootblawing and

is much less effective... .

2) The6mm pitch catalyst usedfor the pilot testing at Coyote Station is much smaller than

currently being specifiedfor coal-fired installations. Vendors are now'supplying 9mm, 10mni or even

-21-
. EPA Response to Minnkota Comment LettersJrom ..

.March 19, 2007 and April 23, 2007

BA.CT Analysis at MRYS

11mm pitch catalysts.., .:..•.....;

. 3) The. one me.ier depth ofthe catalyst used in the pilot test is equivalent to one layer ofcatalyst in

· a commercial installation. CommerCial systems aimedat high removals haVe three layers, each about

three ftet thick, alu1, at varioustimes in the catalyst managemc?1ttcycle, incorporate fourlayers. EERC's,·

· test, then,.r,mlysimitlilied the top layer ofa·multi-Iayer system.~ This is why thefirst step in

.most cqtalyst managementschemes usually involves adding afourth layer .and leaVing the badly

degraded top layer in placefor the second operatingperiod EERC's projectprovides no real insight

into the effectiveness and survivability ofa second or third layer. . .

4). The. scale ()itesting carried out in this project is usually associated with observing catalyst life

prospects (or said another way, the rate ofloss in catalyst activity) rather than physical processes of

· .deposition or plugging. This is because the deactiva(ion mechanisms·operate on a micro-scale andcan·

be accurately simulated at ihis small scale, but the plugging and deposition are influenced by larger

processes, such asflow imbalance and countermeasures. (sootblowing,. horns, screens, baffles, etc./ and

· .are not duplicated on the smallscale.· .

. For these reasons, EPA Region 8 questions the direct applicati~n ofthe results ofthe EERC pilot·'

testing at Coyote Station and the accompanying Appendix B EERC reportjorpurposes ofthe BACT

technicalfeasibility analysis ofSCR.at MRYS. The experience and opinions ofthe SCR supplier that

participatedin the EERCpilot test indicate that the problems'encountered during the pilot. test are not

determinate ofwhether SCR plugging experienced during the pilot test would be unresolvable by venddrs~

To the contrary, statementsfrom Haldor Topsoe offeringthe.ir SCR catalyst/or installation on lignitefired boilers indicates that thefull-scale differences and engineering solutions available today provide

evidence thatthe EERCpilot testing results shouldnot alone be the dec.iding considerationwhen

· e~aluating SCR technicalfeasibility at MRYS. . . ..

EERCMarch 19, 2007 Response:

... .....The catalyst vendors, Haldor Topsoe;Cormetech andHiiachi, were all involved bi thedesiin·

ofthe piiot-scaleslip,stream .testsystem and testingconditions..Thepulsing system was designed with the

assistance ofthe catalyst vendors. .They provided suggestions on the sootblowingpressures andfelt it

· .would be s.ufficient to remove the deposits~. The 6mm pitch was recommended by the vendors. Ifthe first

layer plugs, the gases cannot get to'the subsequent laYers; Significant deposition was noted downstream

ofthe SCRcatalystsection instdlledDeposition al]dplugging aresiinificant problems that impact/he .. ,

· reliability ofthe MRYfacility. Haldor Topsoe was the only catalyst vendor willirzg to provide catalystfor

the lignite tesiing. . . . . .' ," . .

The EERCpilot-scale te~ting was a collaborative effort involving coal-ji;ed utilities, EPRl, DOE, .

· and catalyst vendors. All depisions on system design, plantsfor testing; an.d testing conditions were '

.reviewed andagreed upon byvimdors.The EERCcomparisons ofcatalystblindingfodignite and .

, subbituminouscoal testing as well as the sodium deactivation work conducted by otherresearch

organizations and catalyst wmdors should be use(i in evaluating technicCilje(Jsibility ofSCRfOf' MRYS.

-22-:-
EPA Response to Minnkota Comment Letters from

, March'19, 2007 arid April 23, 2007

, BACT Analysis at MRYS ' ,

EPA Response:

, The results ofthe pilot testing were'examined in more detajl. EPA's observations on the

test ilS it relates to expected catalyst maintenance and replacement requirements at MRYS are'

described in detail under the EPA'response to the preliminary summary.

- . . . .

, EERC indicates that the catalyst vendQrs assisted in the design' ofthe air pulsing system for

the Coyote Station pilot testing and recommended the 6 mm catalyst pitch. Flemming Hansen of

Haldor Topsoe, the catalyst vendor for the Coyote Station pilot test, informed EPA Region 8'

that, in hindsight, the pilottesting should have been conducteddifferently. Specifically, Mr.

Hansen ~aid that changes to the test such as increased catalyst pitch could have helped reduce the

impacts bfash plugging. AdditiomHly, Mr. Hansen said that pilot test's air pulsing system did

not accurately simulate full-scale sootblowing and that ifthe test could be redone, a better effort

'could be made to keepthe'catalyst face clean. Mr. Hansen recalled that the Coyote Station test

reactor became 'quickly plugged with flyash'causing rapid increases in pressure loss caused

mainly by wall-duct ash accumulations dropping on the catalyst face. Mr. Hansen stated that this

type of asp depositionwould not be a problem on a full-scale appli~ation.

Mr. Hansen's statements suppbrt EPA's original comments that there wereitilierent '

problems with the pilottesting that contributed to the degree of catalyst plugging during the

~~~~,CC"0yote Station pilottesting. In addition, experts frCJm Sargent& Lundy indicated that they were

disappointed in how the Coyote Station pilot test was carried out and found the results to be of

very little value for evaluating anSCR system for a facility burning North Dakota lignite.

Based on the EERCresponse, the degree of catalyst plugging observed at Coyote Station

was the.reason no deactivation testing on:. the catalyst was conducted. Ifthe testing were'

designed and conducted in a way that could havegreatiy reduced the effects of ash deposition on

the catalyst face, presumably catalyst deactivation data would have beenobtained. As stated

previ~usly~ EPA questions Mlllnkota',s heavy reliance on the questionable results from the

Coyote Station pilot testing and cannot support using this data to draw definitive conclusions on

the feasibilityofSCR at MRYS. Furthennore, ~hether the design issues were originally

attributable to the catalyst vendor or not, EPA does not believe that a pilot test designed and

. conducted under conditions unrepresentative offull-scale engineering capabilities is a fair

measUre ofwhether the technology is technically feasible or not., Input from both Haldor Topsoe

and Sargent and Lundy support this positIon. .

-23-
EPA Response to Minnkota Comment Lett~rs from .

March 19, 2007 and Apri12J, 2007

BACT Analysis at MRYS

.. 6. . EPA Analysis ofthe Burns & McDonnell's Conclusion:

EPA'sJanuary 9, 2007 Comment: . .

EPA notes that there is co~side~able experience in ihe industry to show that.Use. ofSCR on North.

Dqkota lignite istechnicallyfeasible.· EPA has previously articulated this beliefin a number ofnational·

rulemakings, including the Clean AirInterstate ·Rzile (CAIR),the Best AvailabieReirofit TechllOlogy.

(BART) Guidelines, andrevisions to the New Source Performance Standards (NSPS) for ElectriciJtility .

··Steam Generating Units (finalized on feBruary 27,2006)... ..; ....,. .... . .. EPA understandsfrom the EERCpilotstudy at Coyote Station usrng North Dakota lignite, there

was noticeable ash build up on the catalyst.and concerns with catalyst plugging during the test. .While.

EERC notes that ash deposition problems were most pronounced at Coyote Stqtiofl, compared to the two .

testfacilities burning PRB coal, italso states generally that all ofthe coals in the pilot study "produced

ash that hadsignificant accumulations ofashon the catalyst onboth macroscopic and microscopic

levels." Thus, the conclusions ofthe study appear to be that depositionissues might be somewhatgreater

for North Dakota lignite thanfor PRB coaL Since §ignificant accumulations ofash on the· catalyst wqs

also observed during pilot testing at thosefacilities burningPRB coal and there are numerous examples

. ofPRB coal-firedpower plants. successfully operating SCRsystems, the qualitative results ofthe pilot

testing described by B&Mand EERC would not appear to preclude successful operation ofSCR on a

power plant burning North Dakota lignite. The EERG study does not appear to quantify the effect that

ihe .ash accimiulation had on microscoplccaiqlyst deactivation rates or overall catalyst performance.

Absentsuch Information that refleCts the use ofcurrently available full-scale ash loading mitigation

designs, pilot testing results may not be evidence that successful implementation ofSCR on North Dakota

firedunits: "is going to be impossible," asstatedin the Appendix B.EERC report.. . . . ' .

. The October 1990 EPA draft New Sour~e Revie~ Manual (NSR Manual) states that a "demo~sfration oftechnicalinfeasibility is based ona technical assessment consideringphysical,

chemical and engineerrrigprinciples, and/or empirical data showing that the technology would not work

.on the emissions unit under review, .orthatunresolvable technical difficulties wouldpreclude the . .

successful deploymentojthe technique. Physical modifications needed to resolve technical obstacles do .

. not in and ofthemselves provide ajustificaiionfor eliminating the conirol technique on the· basis of

. technical infeasibility. However, the cost ofsuch modificdtions can be considered in estzmating cost and .

economic impacts, which, in turn, mayform the basisfor elfminating a control technology." (Page B.20)

(Emphasis added). Based on the available information, the problems cited by B&Mfor application oj

SCR at MRYS (ash deposition, ."popcorn ash ", temperature; and erosion). are resolvablethrQugh the

engineering solutions discussed above. .While siti:h engineering solutions mciy be a cost burden to

Minnkota,. as noted aQove, step 1 is not the proper stage ofthe Top-Down .JJACTanalysis to accoUntfor

these costs; As describedin the NSR Manual, "where the resolution oftechnical difficulties isa matter of

cost, the applicant should consider the technology as technicallyfeasible. The economicfeasibility ofa·

control alternative is reviewed in the economic impacts portion ofthe BACT.selection proc(!ss." (Pages

B.19-B.2.0/· Many ofthe technicalfeasibility arguments presenied in the BACTanalysis are centered on

. decreasedca.ialyst life andperformance, increased maintenance and outage time, andplant differences

unique to MR.YS that preclude SCRfeasibility at MRYS. Consistent with the NSR Manual, these concerns

.are incorrectly incorporated into the Top-Down BACTanalysis at step 2 and really should be accounted

-24-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

for as additional costs per ton ofpollutant removed a/step 4 ofthe analysis So, while EPAdoes

not consider vendor guarantee.s by themselves to be determinate in making technicalfeasibility decisions;

they are a contributingfactor in making these decisions~ EPA is aWare ofthree catalyst vendors (HaldorTopsoe, Cormetech,. andArgillion) who would be willing to provide performance guaranteesfor lignite-

. fired boilers applications.

. Based on the criteria discussed above, MinnkotaandB&D should luwe further explored both the

full-scale engineering solutionS usedto address the problems cited in their anqlysis in conjunction with

informationJrom SCR/catalyst vendors to offer performance guarantees to design and install an SCR unit

€IiMRYS. The BACTanalysisfor MRYS thatwas submitted relies heavily on the technical problems

.experienced during the EERCpilot testing at Coyote Station in conjunction with thefact that SCR has not

been installed on a cyclone boiler that burns North Dakota lignite. We do not believe that this analysis

adequately addresses the criteria cited above. As such, itappears that the case-specific NOxBACT

analysis submittedforMinnicota MRYS is deficient in terms ofiinproperlyeliminating SCRJrom

consideration at step 2 ofthe BACTanalysisfor technicalfeasibilitjJ.

EERC's March 19,2007Response:

EPA did not examine·the EERC report in detail. The deposition andjJluggingwas much more

pronouncedfor the Coyote Station testing ascompared to the Columbia and Baldwin Station. The

. pressure drop increased much more rapidly and the s.oot blowing was not as effective..... The most

important unresolvabl(! technical difficulty is the impact.ofsodium on the formation ofbonded sulfate

~~~~~phases-cthtttfJht~and~maskth(7eatalyst-and-()n-the-ab#ity4f~()dium-lo-deae:tivate-..the.-cata/J1s1,,--------c---. ._

EPA Response:

EPA's original assessment ofsome ofthe general statements on the results ofthe pilot

testing conducted by EERC at the three power plants are.based O:Q a direct quotation from the

EERC report in Appendix B, which states that there was significant catalyst plugging at all three

test sites. While it istrue that the problems were most pronounced at the Coyote Station and the

facili-ty experienced higher pr~ssure drop during the test, the results from the three facilities taken .

together do not demonstrate· whether SCR would be technically feasible or infeasible at any or all

ofthe. fac~lities tested, since no quantitative NOx control or catalyst deactivation data was

provided in the EERC report in Appendix B ofthe BACT analysis. While the pilot study does

~uggest that catalyst plugging. at a facility burning North Dakota lignite would likely be more of

a challenge than at a facility burning PRR co~il, EPA. can not conclude that the test is

determinative ofwhether anSCR system would not work,~t MRYS. See EPA's previous

detailed analysis for more information. ,..

EPA has since received the background documentsfor project participants and reviewed

quarterly progress reports. For reference, below is a summary ofsome ofthe relevant history

and background information from the pilot study.

-25-
EPA Response to Mirinkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS. . '"

Original goals'ofthe t<~st(2000 EERC'Proposal)

'. .'

• . "Determine the mechanistic and fundamental causes ofpotential blinding or

masking ofselective catalytic reduction (SCR) catalysts due to flu~ gas

constituents in low-rank coals." (Objectives)

• "Utilities will gain scientific and engineering information-related to potential

.fouling' of SCR catalyst material and will be abl€-to negotiate guarantees and

performance criteria for SCR systems and materials, and SCR manufactllrers

. and distributors will gain an appreciation forpotential challenges facing -

utiiities using low-rank coals that could cause SCR masking and will be able

to adjust future systems." (Expected Resillts) _ . '.

• "The testchamber will be constructed so that periodic s~ples ofthe catalyst

can be removed to assess the reactivity as afunction oftime." (Eniphasis

added) (Task 4 - SCR Test Chamber Installation 'and Data Collection)

• . "The endresult is that research is needed to determine the true extent of' _

potential SCR blinding for lower~rari.k coals. Ifindeed masking of SCR

catalystmaterial is an issue~ then SCR technology may not be the "best

. available" technology for NOx control at North Dakota utility sites and the.

EPA ruling (1998 NSPS limit) may need to be amended. Other options' may

surface as result (sic).ofthis research include providing technological and

fundamental science and engineering knowledge for manufacturing catalysts

that resist blinding from low-rank coal-type ash material or designing SCR

systems that can be cleaned on-line." (Rationale for the Project)

Here are some relevant·excerpts ~akenfrom the fined results ofthe test. (November2004

Final Report for period November 1, 2004·;June30, 2004, with two exceptIonsnoted

below)

. .

• Baldwin Station (cyclone boiler, PRB coal) catalyst reactivity testing at 2 months",:"" .

100%,4 months -.:.. 96%, and 6 months - 84%. 6 month sample showed "extensive

sulfation of alkaline-earth elements present in the'deposits~" "Regions ofthe catalyst"

where all ofthe pores were blocked anciamininialamourit ofdeposit on the surface

ofthe catalyst." "Higher magnification view ofthe deposit that is filling the pore:';-

• Columbia Station (tangentialiy fired boiler, PRB coal) catalyst reactivity testing at 2

months - 93%, 4 months - 90%, and 6 months :.- 90%. (December 22,2006 email .

from Isato Morita, Hitachi Power Systems America Ltd. to Dr.. Steven Benson, .

EERC) "The Columbia Station showed more signifi~ailt accumuhliion and plugging

than the Baldwin Station." "The six month sample from the Columbia Station

showed particles adhering to the surface and filling pores in the catalyst."

-26-

. ,.
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis' at MRYS

• Coyote Station (cyclone boiler, ND lignite) - No catalyst testing conducted and no·

NOx data submitted. "The mostsigniflcant accumulation was notedfor the Coyote

Station." "The.Coyote Station had some large pieces of ash deposit material on the'

surface as well as plugging ofthe catalyst passages." "The six month sample from

the Coyote Station showed particles adhering to the surface and filling pores iIi the

catalyst." Describing photos ofthe catalyst inlet after two months ofoperation, ''the

left side ofthe catalyst is indicative ofthe amount offoullng present whereas the ash

present on the right side·ofthe catalyst fell from the upstream flue section when the

jointwas separated. Nev.ertheless, the entire surface appears coated with ash

. deposits~and some ofthe catalystchannels were plugged." (February 2004 Quarterly

Report - not noted in the final report) (Note: The right side ofthe catalyst face looks

FAR worse than the h::ft side in the photo.)

• . Study Conclusions: "The pressure drop across the catalyst was found to· be the most

significantfor the lignite-:-fired plant as. compared to the subbituminous-fired plants..

Both coals had significantaccumulations of ash on the catalyst, on bothmacroscopic'

andmicroscopic levels.;" .

. .

t"This study suggests careful evaluation of each SCR installation in applications using

subbitunlinous and lignite coals. Improvements are needed to ensure technical

~~'---.-~~~~~fi..... "easibi1ily-o-eS12eciall}'"with Jignii.~tired llilit~"..InsJallatiQn.s.invQj.YingJignjj;.eJilel~s ""w'-A!il..... l __~~~

need ~dvanced cleaning techniques to handle the high sodium and high dust loads..

aSsoCiated with burning most lignite fuels."

.. .... .

Below i!)EPA's assessment ofthese additional background documents and reports received

after the original comments were submitted on January 8; 2007 and January 26, 2007 and how to

compare the :results from the three facilities tested during the pilot study.

1. Quantitative.Data Analysis

• EERC apparently did no catalyst reactivity testing' for the Coyote Station and

did hot monitor NOx control performance ofthe SCR system during the test

period. (Original test proposals from 2000 included mention of

instrumentation "able to monitor whether NOx control "is occurring", but

EERC has not provided this data to EPA, nor. is there further evidence that it .

was ever collected.)

• Pressure drop data for Coyote Station does not show gradual increase, btlt a

sudden dramatic increaSe likely resulting from the ash dropping from

upstream flue sections, as described int-he February 2004 Quarterly report.

EPA's conversation with HaldorTbpsoe is consistent with this interPretatiOll..

-27-
EPA Response to Minnl.cota Comment Letters from

'March 19,2007 and April 23, 2007

BACT Analysis at MRYS '

,'Pressure'drop data and comparisons to Columbia and Baldwin Station~ are

therefore not meaningful. '

2: Qu~litativeCatalyst Analysis ,"

• While 'c.oyote Stationis described as having the most significant catalyst inlet

ash accumulation, it is entirely possible that this is due in large part to the ash

,dropping from upstream flue sections.

• The microscopic catalyst description for Coyote Station was notentirely

unlike that ofthe Columbia Station, where both "showed particles adhering to

the surface and filling por~s i~ the catalyst." '

". Due to the upstream fluesectiori ash contribution at Coyote Station and the

similarities between microscopic analyses presented by EERC between the '

North Dakota lignite and PRB coals (both showed particles adhering to the

,surface and filling catalyst-pores), the qualitative information does not provide

meaningful conclusions forcomparative1y assessing SCR technical feasibility

for North Dakota lignite and PRB coal applications. "The one exception would

bethat the.study shows lignite at Coyote Statio~ showedmore sodium

deposition and less calcium compared to PRB coal. It is unclear what the"

basis is (or EERC's statement that the sample at COYQte Station "shows

, , significant evidence ofsulfat"ion"after only 2 m,onthsof exposure arid was

much more pronounced than the 2':month samples for theB;;tldwin and

'Columbia Stations." Even ifthisis based on visual observation ofthe SEM

iinages, it is not quantitatively compelling'information. As stated in EERC's"

.final report, "both coals had significarit accumulations of ash on' the catalyst, ,

on both macroscopic and miCroscopic levels." ',.

", 3. EPA's ConClusions ofthe EERC pilot Stqdy ,

• The quantitativedata provided for catalyst deactivation for Baldwin (reduced,

to 84%) and. Columbia (reduced to 90%) indicates that the visual observations

from the pilot study,are not me'aningful ways to quantitatively represent the '

~h impacts on SCR systems.. In fact, the reactivity results show the opposite

, trend as. would be expected when comparing the visual'observations' at

BaldWin and Columbia Station: "Columbia Station showed more significant' ,',

accumulation and plugging than the BaldwInStation," but Columbia Station's

"catalyst showed less deactivation than BaldWin Station's catalyst. .As such, it

, does not appear that a correlation can be made ,betw~en the visual , ' '

"obserVations and the effects On catalyst performailce. Without representative

catalyst deactivation testing orNOx control:data from the Coyote Station, EPA

-28'-

, "
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

does not believe the pilot study provides meaningful results that could lead to

adefinitive conclusion that SCR would be infeasible at MRYS. '

• Th~ quantitative data on pr~ssure drop is not compelling since it is not directly

related to c~talyst performance and is suspect for the C~oteStationdue to '

upstream flue ash dropping on,to the catalyst face.' . .

• The catalystanalysis on both micro and macroscopic-levels does support the'

,claim by EERC that catalyst deposits Will contain more sodium when exposed

to.lignit~ comparedto PRB coal, but does notclearly support the assertion by

. BERC that SCRwould not work on~ lignite plant. In fact, EERC's . .

conclusions seem to question how well SCRwould work atPRB plants,

which has been disproved with operating plants.

• , IIi summary, the primary goal ofthe piloftest was to determine mechanistic

and fundamental causes'ofpotential blinding or masking ofSCR catalysts due

to flue gas cons,tituents in low-rank coals. In the test proposals, it was clear

that this included reactivity testing for the catalyst. EERC did not test the

catalyst at Coyote Station, because the catalyst was allegedly too plugged to

conduct the testing: This appears to be a failure ofthe pilot test to meetthe

originalgoal for North Dakota lignite. As explained above~ the plugging

. appears to be in largepaii, due to the upstream flue ash dropping onto the.

. catalyst face. As such, it is riot clear whether the catalyst testing would have

~~~~~~-'c-'~~~~~-,b"-"e=e#Il~c..,=b""n=du=c=t=ed=...,if"-1t"",h",,e.--"c=at=aI,YJ!twouldhave beenke12t cleaner. Additionally~~ .. ~~_~~_

EPAunderstands that the catalyst deactivation testing still could have been

conducted, it just would not have provided a very'quality "data point" because

.ofthe irregular catalyst exposure over time in light ofthe substantial.ash \ . -. . . .

deposition on the catalystface and the resulting high gas flows through the

catalyst.· This effect may have also contributed to the differences in catalyst

deactivation rates between Baldwin and Columbia Stations.

B&McD"s March 19, 2007 Response:

... ,.. The EPA's review offered no specific documented andpublished substantive demonstrated

experience '" .....ofany cyclone boilerfiring North Dako{a lignite, particularly one locatedin a highdust, hot-slde application at Milton R Young Sta#on. 'Burns & McDonnel/disagrees that the lack ofSCR

catalyst deactivation datafrom the Coyote slipstream SCR. catalystsfield te.st completely refutes (he

premise ofDr. Benson's referenced technicalpaper and the Minnkota NOx BACTAnalysis Study

conclusions that SCRs are technically infeasiblefor !ignite-jired cyclone boilers.

. '.'

The, EPA's NSR Manual does not.specifical/y define .....; in orderfor said alternative to be

excludedfrom further consideration.

When a BACTanalysis is peiformedon a new unit, there has typically been significant conceptual

and detailed engineering and design work already performed thatforms the ba:sis ofthe subsequent

-29,
EPA Respon!?e to Minnkot~ CommentLetters from'

March 19, 2007 and'April 23, 2007

,BACT Analysis at MRYS

, capital cost ,estimate and a;soci~ted expected0 & ,Mcostsfor the pwticular emission control

technologies to be employed orthat were' considered as part ofthe overallproject effort. Performing a

BACTanalysis on. an existing unit sl1ouk! notrequire'similar substantialresources to 4evelop conceptual

designs and create technical details that determine the'estimaied site-specific retrofit costsfor

overcoming significant technical challengesthat would otherwise,prechtde the'abilityofthe technique or

technl?logyfrom successfully controlling the em.issions in question. The EPA's NSR Manual states: "A

source would not be,required to exper.ienceextendediime delays orresource penalties to allow research

to be conductedon a new technique. " 'There eire no Cited examples'where successfulsolutions to such

challenges asposedl;y stich highly complex interactions b.etwe.enJlyash, Jl1,te gas, andgas temper.atures

produced by lignitefiringfor SCR coal{ired boiler applications ~. . '

EPA Response:

. . .' . .

Again, EPA believes that in the case ofdeciding on technical infeasibility for SCR at a '

utility boiler, the bUrden ofproof is on the applicant and is considerable. EPA'sposit,ion is that

the hundreds of SCR installations at utility 'boilers worldwide demonstrate awealth of-experierice

, within the industry to address various factofsover a very wide range of boiler designs and fuel

types., The qualitative resl;llts of catalyst phigging for the pilot testing at Coyote Station do not

provide empirical'evidence that SCR would be ineffective atcoIitrolling NOx emissions, since

there was no quantitative data provided for eitherNOx reduction or catalyst deactivation~ As

such, EPA is not convinced thatthe specific chaJlenges presented for aD. SCR installation at

MRYS is a matter of technic~l infeasibility and could not b~9vercome with a robust suite ofash

(including sodium) mitigation ineasures and/or maintenance measures;' While EERC has had

verbal contact with Haldor Topsoe, Minnkota's March 17,2007 r~sponse letter simply says that

this vendor would have concerns'about providing a perforri:lance guarantee in'light ofthe specific,

sodium content ofthe lignite burned at MRYS. It is not clear based on this statement whether

Raldor Topsoe would or would not in fact provide a: performance guarantee '(or whatthe ,

guarantee would entail) for ~n SCR installation at MRYS. Also, as noted previously; during

EPA's own discussion with Haldor Topsoe, Mr. Flemming Hanst:n did not rule out bidding on a

project that involved North Dakota lignite~ " , ' ,

B&McD'~ assertion thata BACT analysis for anexisting'unit "should not ~equir~slmilar

, substantial resources to develop. conceptua.l designs and create technical, details, that determIne, '

the estimated site-specific retro-fit-costs for overcoming significant technicafchallenges that

would otherwise preclude the ability ofthe technique or technology from successfully ,

controlling the emissions in question" doe~ not appearto' have a basis inEPA regulations or

policy. Prevention of Significant Deterioration (P~D)regulations make no distinction between

BACT requirements for new and existing UJ1its undergoing PSD review. In fact~EPA's position,

has been that when it is discovered that a source avoided PSD review, but it was laterdetennined

,that PSD permitting requirements, including BACT, should have applied, the source would n~ed

to address the 'appropriate requirements "as ifconstruction had not yet commenced." I? , '

17 See April 20, 1999 letter from RichardR.Long{EPA Region 8}to Dennis Myers (Colorado Department of

-30-
EPA Response to Minnkota Comment Letters from-

- March 19, .i007 and April 23, 2007

BACT Analysis at MRYS

In addition, B&McD's referellce to page·B.I8 ofthe NSRWorksh~pManual is taken out

of context mid is. not relevant to their discussion on'how a BACT anaIysis for an existing soUrce

should be less rigorous than for a new source. The NSR Workshop Manual citation' is a

discussion ofwhether a given control technology or ie'chnique should be considered "ava~lable"

_in the context ofthe technical feaSibility analysis under step 2 ofthe Top-Down: analysis. IS

There is little question ofwhether SCR is "available" for instaIiation at MRYS. hi fact; .

Minllkota's original BACT ailalysiscontends that SCR-meets the criterion for being -

"available".!9 So; it is inappropriate for B&McDto bring up this passage from the NSR

Workshop Manual. The question has been whether SCR would be "applicable" (the second

criterion under step 2). B&McD's reference on page B.18oftheNSR Workshop Manual would

only apply for a "new: technique." Given the vast experience in the utility industry. with SCR and

the EPA policy on technology transfer considerations, EPA does not believe iOs reasonable to

consider SCR as a "new technique" for MRYS, even ifthere are no SCR instaliations' .

specifically operating on cyclone boilers burning North Dakota lignite.2o In conclusion, EPA

does not believe Minnkota has satisfied the high threshold ofdemonstration that SCR would not

be technically feaSible at MRYS. While EPA-agrees there are-engineering challenges with

operating an SCRsystem atMRYS,EPAbelieves these would largely be a matter of cost and

_should be examined under step 4 ofthe Top-:Down BACT. analysis.

~--_.._----'--- :B.... .Low.,Dust SCR

BOurns & McDonriell has eliminated Low.,Dust SCR as technically infeasible due to issues

described below.. EPA's response on each issue is also described ~elow -

1~ EPA Analysis ofthe Catalyst Fouling and Deactivation Issue:

EPA's January 9, 2007Comment: .

Afew SCRsystems installedon coal-fired boilers have.been designed with theSCR catalyst

installed after the ESP or baghouse; This approach- eliminates virtually all ofthe poisoning, plugging

and erosion problems that have.been associated with SCR catalyst. '(Christman Memo, 2/18/03) For the

same reasons notedabove under the high-dust SCR analysi~, the catalystfouling anddeactivation . -

prob"lems may be resolvable technicalproblems that do not warratit elimination oflow-dust SCR under

.Public Health and Environment).http://epa.gov/region7/programs/artd/air/nsr/nsnnemos/amersoda.pdf

-18 Page RI8 ofNSR Workshop Manual: "A control technique is considered available, within the context presented

above, ifit has reached the licensing and.commercial sales stage of development. A source would not be required to

experience extended time delays or resource penalties to allow research to be conducted on a new technique."

19 See Page AI-32 ofOctober 2006 Minnkota BACTAnalysis Study for MRYS.

20 See June 15, 1989 letter from Gary McCutchen (EPA Office ofAir Quality Planning and Standards, New Source

Review Section Chief) to John Daniel(DepaTtment ofAir Pollution Control, Commonwealth ofVirginia).

htiP://www.epa.gov/Region7/programs/artd/air/nsr/nsnnemos/fmlbact.pdf

-31-
EPA Response to Minnkota Comment Letters from·

. . March 19; 2007 ~ndApriI23,2007

BAtT Analysis at MRYS. .

·step 2 ojthe .Top-:Down BACT an~lysis Similarly, permit applicationsfor three recent Circulating

Fluid.izedBedprojects in North Dakotd(MDU Westmooreland Gascoyne, South Hart, .andGascoyne·

500MW) did not.eltminate10w:.Dusi SCR basedon unresolvable .technicalfeasibility issues with catalyst

jouling and deactivation. While there would be/Inherent costs associatedwith gas-reheat, otherjJossible

·physical modifications to iheplant to accommodate low-dust SCR, and engineering ~esignsto minimize· .

and manage catalystfouling ariddeactivation, thesefactors, consistent with the NSR Manual, should be.

accol,lntedfor ·when considering ihe economic impacts of/he technology.

EERC's Marclt 19,2007Response:

. Sodium-r!ch aerosols are aproblem that can get through the ESP. These aerosols are known to be

.. present in high ~odiumcoal c0'!lbustionfuel gases.. These species are known to produce pyrosulfates.

These will causeb/inding and deactivation ofthe low-dust catalyst.

EPA Re~ponse:

As described previously, general qualitative statements thatthe scidiu~-richaerosols will

cause blinding and deactivation do not riecessarily support the conchlsion thata low-dust SCR

system would be technic~lly .infeasible at MRYS. '. The effects ofthis blinding and deactivation

·should either be quantified and·used within the economic analysis under step 4 ofthe Top-Down'

BACT analysis, or described in sufficient detail to demonstrate that the problemS ~ssociated with

catalyst blinding and deactivation are unsolvable andwouldresult in the unsuccessful operation

of a low-dust·SCR. As stated on page B.20 ofthe NSR Workshop Manual, "(p)hysical .'

modifications needed to resolve.technical obstl:lc1es do not in and ofthemselves provide a

justification for eliminating the co~tro1"technique on the basis oftechnical infeasibi1ity~

However, the cost ofsuch modifications can be considered in estimating cost and economic

impaCts."

·B&McD's March 19,2007Response:

The needfor a majorsourceofheat to reheat thejlue gas stream clownstream oithe ESP in a low- .

dustSCR application willrequire either an expensive natural gas supplypipeline onm unac<;eptable .

derate ofthe nei unit electrical output associqtedwiih extensive. modifications ofthe. components in steam

. cycle power generation and balance-ofpli:mtsystems and,boiler gas systems to supply high pressure,' .

. hightemperature steam as suggestedbythe EPA and their consultant. .The magnitude ofsuch retrofit" .

mod.ijicationsandrisks offailure in applying/ow-dust SCR technology at MRY.S are much gr~ater than Is

considell'ed reasonable. '. .' .

EPA 'Response: .

. . .

.B&McD contemplates ~he.useofnaturalgas (and associated COflstfuetionofanatural gas

pipeline) and plant steam for flue gas reheat arid makes a 'general statement that the "magnitude:

ofsuch retrofit"modificatiOJ,1s and risks offailure in applying low:-dust SCR technology atMRYS

-32-
.EPA Response to Minnkotl Comment Letters from

March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS

are much greater than is.considered reasonable." AbsenrfrOIll 8&McD's consideration for flue

gas reheat are fuel oil ait~ propane (instead ofnatUral gas and the associated pipeline). These

. options would likely require a less complex analysis. Based on their response, it is unclear if

.. B&McD is still eliminating low-dust SCR as technically infeasible or int is dismissing further

analysis as unreasonable . as part ofthe ecc:>D.omical. analysis under step 4 ofthe Top-Down BACT

analysis. While EPA acknowledges that the two options described byB&McD would require

engineering and cost estimates, EPA does not agree that a cost analysis for low·dust SCR should·

not be required. FUrthermore, as stated in EPA's original January 8, 2007 comments, recent

pennit actions issued by.both the North Dakota Department ofHealth and Region 8 included a

cost analysis for low-dust SCR. B&McD did not respond to this original corriinent. .

·2. EPA Analysis ofthe Site.SpaceConstraintIssue:

EPA's January ~,2007 Comment:

While the extentofthe physicalplant modifications necessary to accommodate low-dust SCR at the

MRYS is not entirelyclear, the above description does not indicate that the site space constraints at

MRYS would necessarily preclude the possible installation oflow-dust sci?. It does sound like major a~d

costly equipment relocation may be necessary to provide .the space necessaryfor low-dust SCR

However, consistent wlttz the NSR Manual, "physical modifications needed to resolve technical obstacles

do not in and ofthemselves provide ajusJificationforeliminating the control technique on the basis of

·~~~~~tHteehnieal-infeasibiJity~1'he8e-:c-&sts:-sh()uld-be-q~eountedf(J~hen-estima#ng4l~and-ee()n&1nie~. ~­

. impacts oflow-dust SCR, butshouldnot be used as a basisfor eliminating the technology as techfJically

mftaswk. . .

B&McD's March 19,2007Response:

Significant engineering resources wouid need to be devoted to investigate and estimate the capital

costs ofmajor boiler and air pollution·equipment relocations, facility additions, and boilerhouse building

modifications required to implement low-dust SCR t~chnoloiY at MRYS. Burns & McDonnell believes

the expected levelized tota(antzual costs required fo apply low-dust SCR technology to North Dakota

iignite-jired cyclone boilers are unacceptably high. .

EPA Response:

. IfB&McD "believes the expected levelized total annual costs required to apply low-dust

SCR technology to North Dakotalignite-frr¢d· cyclone boilers are miacceptably high", more

detail should be provided within step' 4 oftheTop~Down BACT analysis•. EPAacknowledges

.that extensive work·may be required for installation oflow-dust SCR and the costs might turn

out to be unacceptably high, but the NSR Workshop Manual does riot allow for dismissing a

control.option based on unsubstantiated beliefs ab()ut costs. B&McD should do a cost estimate

basedon reasonable expected reheat fuel requirements, equipment installations, plant .

modifications, etc. A comprehensive analysis would .be unnecessary ifit is shown that costs of

..,..33-
. EPA Response to Minnkota Comment Lettersfrom

. . March 19,2007 and April 23, 2007

. BACTAnalysis at MRYS

one or two aspe.cts ofinstalling low-dust SCR (i.e; the reheat fuel costs) would be unacceptably..

high. Since there have b~en low-dust Installations at utilitY boilers, EPA cannot concur with the

general aSsessment that the ·costs· are expected to be unacceptably high·for MRYS: ..

3. EPA Analysis ofthe Burns & McDonilell'sConcluslon:

·EPA's January··9, 2007 Co~ment:

As explained above, EPA Region 8 believes th~t the inherent challenges with installing and .

operating low-dust SCR at·MRYS are·resolvable on a technical basis. Thefact that afew low-dust SCR

applications are currently used in practice indicates that the small-pariicle catalyst.fouling and .

deactivation problem described would not preclude the use oflow-dust SCR ;. B&D's statement's

that "low-dust SCR performance and catalystlife could be severely negatively impacted r.esulting in

premature, extended, frequent outagesfor replace.ment" can be minimized by proper SCR design and online cleaning. As statedpreviously, any real costs associated with the construction and maintenance

required to operate the low-dust SCR should be accountedfor under the economic impacts stage· ofthe

Top-Down BACTanalysis (step 4). Similarly, physical plant modifications that would be required to· .

accommodate the low-dust SCR (andauxiliary equipment) should be considered in the economic impact

evaluation,rather than used as evidence oftechnicaiinfeasibility...

·B&McD's March 19, 2007Response: .

As previ(J1,Isly stated by the EERC; the EPAfdils to recognize that lignite-jired cyclonesjJartition

the. ash, increasing the proportion ofsodium in the slJlaller particles, resulting in increasedpropensityfor·

theformatiori ofthe complex sulfate phases. the.fly ash becomes enriched in components that vaporize

aruJ, subsequently, condense upon gas cooling to form liquidph(1ses and smallparticles that are very ..

reactive to gas-phase components that cause bonding, blinding, and deactivation ofindividual catalyst ..

reaction sites, even in a low-fjust SCR application.· .. .

EPA Response:

.: - . .' .'.. . '. . . . . - . . . . '. .

B&McD appears to be saying that a significant portion ofthe sodium in theflyash is irtthe

vapor phase when it enters the ESP. Therefore, these smaller sodiuin particles would not be

controlled by the· ESP and would condense: to a liquid at the temperature range for a low:"dust

SCR. This results in bonding, blinding, .and deactivation ofindividual reaction sites ofthe

catalyst. ... There is no ·qiIantificationofthe ·fly~h ·sodium in the vapor phase that would cause the

severe catalyst plugging andblindi~g described under the high:"dust SCR portions ofthe BACT

·analysis. PresumablY,if a sighificantpoi1ion ofthe flyashsodium is in vapor phase when it .

would be entering the ESP, that same portion would also be in vapor phase When entering a highdust SCR and therefore not affect the catalyst. EPAis not disputing that there may be some

vapor phaseflyash sodium that will bypass the ESP arid deposit on the lcrw-dust SCR catalyst.

Based ()n B&McD.and EERC'sasse.rtions that ahigh-dust SCR system atMRYS would see a ..

sodium environment higher th~~yother SCRsystem in the world, it would seem that most of

-34-
EPA Response to Minnkota Comment Letters from

March. 19, 2007 and April 23, 2007

BACT Analysis at MRYS

the flyash sodium is not expected to be in vapor phase at the temperature ofthe high-dust SCR

. aridESP. Ifthis is the case; EPA expects avery large portion ofthe flyash sodium would be.

removed by the ESP and not effect a low-dust SCR installation. .. .

Additionally; during EPA's April. 16, 2007 phone conversation with Flemrriing H~sen of

Haldor Topsoe, Mr. Hansen indicated Haldor Topsoe did run some catalyst "testcoupons" at the

Wisconsin Electric Power(WE Energies) Presque Isle Station, which burns Powder River·Basin

coal with similar sodium levelsto North Oakota lignite. ·Mr. Hansen recollected that there· was a

significant difference in the deactivation ofthe test coupons upstream ofthe.particulate control

device (ESP) compared to downstream ofthe ESP at Presque Isle. Mr. Hansen did not offer a

reason why or attempt to quantify the difference. The most obvious explanation for the .

difference would be that the ESP·removed Ii great deal ofmaterial that deactivates catalyst. This

experience suggeststhat catalyst deactivation observed at alow-dust SCR would be less than at a

high-dustSCR.···· ,

III. Additional Information Useful to Evaluating Technical Feasibility of SCR at MRYS

A. EPA Headquarters Believes SCR is the Correct Technology for MRYS in

Developing the Clean Air Interstate Rule (CAIR)

.. .

During the preparation ofthe CAIR rule, EPA developed a scenariofor best controls across coalfired units. in the United States based on an assessment ofboiler type, fuel type, and existing controls. .

This assessment recommended that SCR technology would be the best technologyfor the two units at

MRYS.

Several commenters -- Montana-Dakota Utilities Co. (VLC.,0754); Otter-Tail Power Co. (YLC- .

0628); Lignite Energy Council (VLC-0935); Lloyd, Gosselink, Blevins, Rochelle, Baldwin & Townsend,

P.C.for Texas Lignite Coalition (VIC-1037); and Basin Electric Power Cooperative (VLB-0690)--

argued that SCR technology is notfeasiblefor application on lignite-fired boilers. Some ofthese·

cOmmenters claimed that an SCRpilot test on a slipstreamfrom a boilerfiring lignite hadshown catalyst

plugging and blinding. One commenter attribuied this problem to sulfates ofcalcium and sodium, which

are present in the.flue gas stream entering the SCR, asa result offiring oflignite in the boiler. One of

these commenters also claimed that SCR was notfeasiblefor boilersjiring sub-bituminous.

In its response at VIB. ofthe CAfR docket. EPA·noted that there is considerable experience in the

industry to show that use ofSCRon sub~bituminouscoals and lignite is,technicallyfeasible. SCR is

currently"in use on severalsub-bituminous coal-fired boilers in the US. firing Powder River Basin coal.

Regarding the USe ofSCR on boilersjiring lignite. EPRIiesting"ofSCR catalyst in a slipstream at the

Martin Lake plant showed acceptable resultsfrom Texas Lignite. Furthermore. the technology suppliers

repor.t that SCR has been srlccessfully zisea'on lignite and brown coal boilers in Europe. From this

.experience. it can be concluded that SCR can be used on sub-bituminous coal and lignite boilers in the

-35-
" ,

EPA Response to Minnkota Comment Letters from

March 19,2007 and April 23, 2007

. BACT Analysis at MRYS

US: and catalystsuppliers have i~dicated~hat they will offerperfor~ance gu~rantees on these',

.applications in the u.s. .

In, the docket information regarding this i;sue, one .ofthe SCR catalyst vendors, Argillon, provided

a list aSCR,in'stallations at EGUs·in Eu,ropethatburnligniteor'similar rank coal, including:.,

• Riedersbach installations in 1995 and2001(hard coal/lignite)

.• 'Voitsberg (lignite)

• Dormagfm installationS in 1994,'and 1996 (lignite) .

EPA also noted that/he commeriters have not provided any details ofthe pilot test that has been

mentioned with regards to the plugging and blinding ofa catalystfrom lignite-generatedfuel gases. .'

Without this information, this part ofthe comment cannot be a.ddressedproperly. The EPA's oWfl. .

investigations show that the mainreason for this problem was ash build up on the catalyst, 'which is a

problem commonly experienced in pilot testfacilities. In thefull~scale installations, this concern is

, addressed.during the SCR design stage: The methods'usedto avoid this problem include duct design to

promote ashfallout prior to the SCR, catalystreactor design to avoid ash build, up, 'and on-line cleaning

methods (sootblowers' and sonic horns).

In addition,. thefoilowing catalyst vendors stated that they couldprovide catalysts/SCRforlignitefired boilers: Haldor-Topsoe Cormetecl:z Argilion :.. '

lit a memo datedAugust 20, 2004, Jim Staudt ofAndover Technology Partners to Sikander Khan of

EPA CAA,1D, discussed both the EERCpilot testing results and some ofthe negative issues raised by ,

B&Mand documented how Haldor Topsoe was aware ofthese issues, but neverthfllessfelt comfortable

providing SCR catalyst... ... ..... .

EERC's March 19,2007Response:

EPA's cited experiencefrom subbituminous coals cmdSCR does not haVe direCt application to the

North Dakota tignite-fired cyclone boilers a{MRYS because ofthe high sodium content ofthe lignite.

The SCR experience withGerman brown coals does not have direct appiication to ND lignite because. of .

the lower sodium levels ofthe German coals. " . ., " ".

EERC spoke to Flemming Hansen ofHaldorTopsoe ..: on February2, 2007 about ihe issues'

associated with high spdium North Dakota:lignite. Haldor Topsoe hits ?xperience with sodium and

potassiumfrom biomassfired systems and their experience has not belm good. After describing the issues

related to sodium.......after 60.00 to 7000 hours %peration with ci coal having elevated/evels ofsodium. .'

EPA Response:

EPA's presentation ofsome U.S.' subbituminous coal and Geiman broWn coal experience

was intended to demonstrate that SCRsystems 1}ave,beert shown to work worldwide on lower-,

ranked coals. While direct application ofSCR experience on these coals'may not be entirely.

appropriate for North Dakota lignite, EPA did use this information (along with the other factors

-36-

, ..
EPA Response to Minnkota Comment Lettersfrom

March 19,2007 and Aprit'23, 2007

, BACT Analysis at MRYS

described in the January 8, 2007 comments) to conclude that SCR wastechnically feasible for

lignite boilers in general. '

As described above, EPA's discussion with Fleriiming Hansen ofHaldor Topsoe also

revealed that despite concerns about ali SCR system subject to the sodium environment that

would exist for a North Dakota lignite application, Haldor Topsoe would not rule out bidding

such a project.

B. '. SCRTechnology is PresuinptiveBART for Cyclone Boilers in EPA's BART

Guidelines

EPA's January 9, 2007 Comment:

EPA made a specific and informed de(:ision in its BART dUidelinesregarding cyclone-fired utilitY

boilers. For all other boiler types, someform ofcombustion control was deemed to be ihe presumptive '

level ofcontro/; However,jor cyclone units burningany, type ofcoal (bituminous,subbituminous, or

lignite), presumptive BARTis SCR SCR was se(ected because the technology isfeasible andcosieffeCtive ~.... " '

B&McD's March 19, 2007 Response:

~~~~~~ 'KP:A Ignored the n'umerous comments ree-ewe«jor rh7rdrafrs-%~fi1TesTstmilllrtU1frat"7:ire-d~-u---~-~,-­

in the EPA's CAIR discussionfor the preceding issue) about the infeasibility ofSCR applied to North,

Dakota llgnite. The unit capital cost capaciiyfactorfor SCR retrofitsappliedto cyclone boilers'by the

EPA i.n the cost-effectiveness analysis usedforestablishingpresumptive'BARTwas $1001kW.

Burns & McDonnell believes the EPA 's cost~effeCtiveness analysis is inaccurate and significantly

underestimates the expected capital plus operating and maintenance costs required to apply high-dust or

,low-dust SCR technology, to North Dakota lignite-jired cyclone boilers.

,EPA Response:

,B&McD contends that EPA ignored numerous cornh1ents received on the draft BART

guidelines about infea~ibility ofSCR applied to North Dakota lignite. As stated above, EPA

, weighed many different factors and concluded that SCR'is technically feasible for a plant fired

on North Dalcota lignite. '

B&McD also contends that EPA underestimated the capital costs for SCR on cyclone

boilers and that total cost ,estimates in the BART guidelines are inaccurate for applying SCR to

North Dakota lignite-fired cyclone boilers. EPA is not suggesting that the cost figures used in

the BART analysis need to be used in the BACT analysis for MRYS.. EPA has maintained that

Minnkota should provide a cost analysis for Units 1 and 2 according to step 4 ofthe Top-Down

BACT analysis. . .

-37-
EPA Response to Minriko~ Comment Letters from

March 19, 2007 and Apri123, 2007

BACT Analysis at MRYS

, , ,

C. February 27, 2006 Final Revisions to 40 CFR Part 60, SUbpart Da, N~w Source'

Performance Standards for Electric Utility Steam Generating Units Set Final '

NOx Limit Based on Technical FeasibilityofSCR for Lignite-Fired Units.

EPA's jiIn':l{iry 9, 2007Comment:

In its response to public comments on the February 28,2005 draft NSPS,Subpart .(Japroposal"

EPA rejectedcomnientsreceived on NOx controlrequirementsfor lignite-fired units. Specijic;ally, EPA

took the same position as it did in the CAIR and BARTrules, rejecting commenters' concerns that SCR

would not befeasiblefor lignite units. No legalchallenges were made to thefinal rule regarding EP.A's

response to the lignite concerns... .......' ,

B&McJ)'s March 19,2007Response:.

, , '

Burns & McDonnell rejects the EPA;s argument that the lack ofa lawsuit protesting the

establishment ofrevisedNSPS limitsfor new electric utility steam generating units (EUSGUs) that

require SCRsfor NOx control mea;"s that this is appropriate technologyjor a BACTretrofit on existing

lignite-fired cyclone boilers. Cyclone boilers are no longer commercially available, so there are, no new,

cyclone boilers, beingpermitted and built. Utility owners ofexisting North Dakota lignite-Jired boilers

would not be expected te c()mment on the. draft revisions to the NSPS proposed by the EPA because they

apply only to new units with different methods offiring coalthan cyclone. boilers. It is doubtful that the '

EPA consideredjJringlignite in new cyclone boilers would be a possibllity when they proposed the draft

NSPS standards..

Anyrecently bulltor permitted lignite-jired boilers have' been predominantly ci~culatingjluidized

bed (CFB) type andhave not incorporated, SCR technologyforNOx emissions control. There havebeeTi

recenipermit applicationsfiledfor constructingpulverizedcoal boilers burning Texas lignite that ,

propose to jncludehigh-dust SCR teelmology. H9wever,. there are distinct differences betWeen North

" Dakota and Texas lignites that preclude assuming that this supports the technicalfeasibilityofapplying

high-dust SCR technology to existing cyclone boilers burning North Dakota lignite. , ' ,

EPA Response:

B&McDcontends that since no ne~cycloneunits are b~ing peimitt~d,&1d'built,.existi~g ,

utility owners in North Dakota would not be expected to comment on draft reviSIOns to the NSPS ,

proposed by EPA be.cause they apply only to, new units with different methods offiring coal than

cyclone boilers and that it was doubtful that EPA consideredfiring lignite in new cyclones when

proposing the draft NSPS standards., Actually, the new NSPS standards apply,to, affected utility,

boilers that:commence constrUc~ioi1~ reconst~ction, or modification after. Febiuary 28, 2005.21 ,

(EPA's original comments erroneously said t.hat the standards only, applied to:new Units.) This

would inClude any existing cyclone units firing North Dakota lignite that undergo reconstruction

21 See 40 CFR §60.44Da(e)

-38-
EPA Response to Minnkotll Comment Letters from

_March 19, 2007 and April 23, 2007

BACT Analysis atMRYS

or modification. -The standards th~t would apply to recon~truc~edunjts (1.0 IbIMWh or 0.11

IbIMMBtuon a 30-day average) and modified units (lA-IbIMWh or 0.15 IbIMMBtu) are well

below the BACT liinit proposed by NJ:innkota. Since these standards apply to modified and

reconstructed affected units and not jl,1st new units, the exclusive construction ofnew circulating _

fluidized bed (CFB) boilers-in North Dakota and the differences between Texas lignite-andNorth

Dakota detailed by B&McD and EERC are not parti~ublrly relevant to the revised NSPS. EPA

does not disagree that all ofthe recently-pennitted utility boilers have been CFB boilers

(although that does not rule outthepossibility ofa PC-fired unit inNorth Dakota in the future).

EPA also agrees that there have been soine recently-pennitted units burning Texas lignite that

have proposed SCR and that there are some differences in characteristics between North Dakota

and Texas lignite. EPA believes these differences might affect the design and operation of an

SCR system for the different lignites. EPA does not believe these differencesindicate that EPAdid not consider North Dakota lignite in revising the NSPS for utility boilers. -

EERCResponse:

_North Dakota lignites are substantially different than Texas lignites in composition andfouling

tendencies. The following comparison shows the proximate analyses, sulfur coritentsandheating values

for various lignites, plus ash composttionet! analyses ofTexas lignites Comparison o/North Dakota

and Texas Lignites Table 3 summarizes ashcomposition analysis expressed as equivalent oxide.

The compositional analyses show that t/:ze aSh has relativ~lylow sodium and lower calcium content than

·~-~-North~EJakota~fignite~'Fheproblems~of~SGR1JatalY9t-lJlintiing-tiue~t<'!~the-fopmati()nc.o.fs()tiiufnoC-aleium~-~-.:._.~-~-~-_.~~---- ._-~-­

sulfates createdfrom thefiringofTexas lignites will be mu~h less severe/han/or North Dakota lignites.

EPA Response:

EPA has reviewed the coal data submitted by EERC and, in general, ~grees that the sodium

content ofTexas lignites is lower than North Dakota lignites.- However, it appears that'the --

Jackson South Texas lignite has a somewhat comparable ash sodlUIil content above 3% (EPA

- assumes this is an average) compared to the average (or 59th perceiltile) value of4.4% forCeIiter

mine coal (per the April 23, 2007 coal analysis submitted by Minnk6ta). - Considering the high

ash content ofthe Jackson South lignite (42% average) and low heating value (3,972 Btullb as

received), compared to the ash content (9.53% average) andheatingvalue_(6,572 Btullb) of

Center Mine lignite, the Jackson South lignite ofTexas would actually have a much higher

sodium loading (on average)than CenterMine lignite (by roughly a factor of4). The Yegua

East lig:nite with an ash content of 18% and a heating value of 5,761 Btullb would have a sodium

loading ofroughly 60% of average Center Mine lignite. As explained in previous comments,

EPA does not believe a strict linear comparison can be made from these loading levels, but it

: -appears that some Texas lignite has comparable sodium levels- on-a per Btu basis to Center Mine

- -lignite.

-39-
EPA Response to MinnkoUl Comment Letters'from ".

March 19; 2007 and April 23, 2007

BACT Analysis at MRYS

NotWithstanding the difference~'betWeenNorthDakota lignikand Texas lignite; a

modified cyclone boiler firing North Dakota lignite would still need to meet the most recent

NSPS Da liniitfor modified'units, which is much lower than the BACT 'limit proposed by

Minnkota. The likely choice to meetthi& limito{O.15Ib/MMBtuwduldbe SCR.·

D. SCR ~nd Catalyst Vendor I~formation.

EPA's January 9, 200tComment:

As noted above, thefollowing catalyst vendors were ~omfortablep.,rovidingcataiystslSCR/or ligJ1itefired boilers: Haldor-Topsoe,Cormetech, andArgillon. . '.

. .

In addition, the above-cited memo datedAugust20, 2004, Jim Staudt ofAndover Te~hnology .

Partners to Sikander Khan ofEPA CAMD, discussed both the EERCpilot te8ting results and some ofthe

negative issues raisedby B&Mand documented how Haldor Topsoe was aware ofthese issues, but

.neverthelessfelt comfortable providing SCR catalyst. . ': .'. '. .' .

EPA Region 8.also contaCted the Instii~te ofelean Air Companies website to obtain the following

list of2I companies offering "SCR Systef11s, Equipment; $uppti"es and Services: ..

EPA Region 8 has visited most ofthe ve~dors 'websites.andfound many vendors with experience

thatwould.be germane to the MRYSsituation ..;..... Webelieve itwoitld be prudentfor Minnkota to obtain

information qndproposals.jrom more than onew!ndor and involve the NDDH andEPAin 'theirfindings.'

EERC's March 19, 2007 Response:

.EERC has already included some. commentsfrom discussions with Haldor Topsoe regarding'lheir .

. concerns about the sodium content in North Dakota lignite and its tendenciesfor catalyst deactivation..

Some ofthe. EPA-citedSCR catalyst vendors' websites discUss the problems ofsodium and deac.tivation.

However, EPA has notprovided a ri!viewofthat informaiion. .

EPA's Response:

.EPA has already disc~ssedits phone ~~nver~atioriwith FleIIimingHansen ofHaldor

Topsoe and how Mr. Hansen, while acknowledging some difficulty de.aling with the sodiuin . ...

issue with North Dakota lignite; did not dismiss that an SCR system might'be technically feasible'

for aNorth Dakota lignite application, nor the possibility that Haldor Topsoe would bid on such:'

a project: '" . . .

B&McD March 19, 2007Response:

EPA's listing of21 suppliers of "seR Systems, EquiPment, Supplies, and Services " has not been

screened to e.liminate companies that are not active in the U.S. coal-fired SCR market EPA 's

'-40-
EPA Response to Minnkota ComrtlentLetters from

, March 19, 2007 and April 23, 2007

, BACT Analysis at MRYS

evidence ofSCR catalyst vendor's willingness, to guarantee. performance on a lignite.1ired boiler is very

general in nature and does not include typical qualifying statements reiated to (he client's need to adhere,

to specified SCR design conditions, which are yet to be ~stablished...... ... '

... .•..from Burns & McDonnell's experienc.e, what d vendor orsupplier sayspre-bid does not

always match whOt they actually will offer in afirm contract..... a c.ommercial guarantee does not mean

the systemproyided by the vendor will work long term; all it means is that based'on the language in the

contract and the 'amountofmoney the supplier has allocated to cover their risk or exposure, the vendor

will guarantee the system performQ1lce commercial contraCt guarantees always have afinite life,

which means the owner ofthe system orily benefitsfrom the vendor's guarantee for 3 years ~pically....

EPA's cites SCR catalyst vendor statements lack substantive language that deais with mitigating

. the client's risks offailure to meet catalyst activiJy versus life guarantees, and the concerns recently

expressed by Haldor Topsoe when EERC described sodium components in the j/uegas. In Burns &

McDonnell's opinion, SCR system vendors have not provided adequate documented andpublishec!

demonstrated catalyst experience with supporting technical inforination to support these claims. The

vendors offered no specificcompcirative peer-reviewed data as proofthat currently-available catalysts

can survive andprovide effective NOx reduction when exposed to the jlue gas andjlyash constituents

exhaustedfrom combustion and operation ofany cyclone boilerfiring North Dakota lignite, particularly

one located in a high-dust, hot~side application at Milton R Young Station.

EPA Response:

,-~~---,._---.--,~_.._--

EPA had originally suggested that Minnkota "obtain informatioil and proposals" from SOme

ofthe other companies that EPA had listed. EPA erivisioned this to include more ofthe level of

detail described by B&McD,in their March 19,2001 response. while B&McD seems to ' ,

discount the value of a performance guarantee, EPA's NSR Workshop Manual states that

"(v)endorguarantees may provide an indication ofcommercial availability and the, technical

,feasibility of a control technique and could contribute to a'determinati()n oftechnicalfeasibility

or technical infeasibility, depending on the circumstances." While EPA does not believe the

vendor guarantee is the solefactor in determining technical feasibility of SCR for MRYS, EPA

does believe it would be 'useful information to obtain for making the determination., '

-41-
EPA Response to Minnkota Comment Lettersfrom"

March 19, 2007 lind April 23, 2007

BACT Analysis at MRYS

"EPA~s "RESPONSE .TOEERCAND B&McD APRIL 2~~ 2007 LETTER TQ ND"DH " ,

COMMENTS - FEBllUARY 1, i007 (lNCLUDINGRESPON'SE TO u.s. EPA REGiON 8"

COMMENTS - JANUARY 26,2001):" REQUEST FOR ADDITIONAL INFORMATION""

"ANi> THE NON-SCR COMMENTS FROM EPA " "

IV~" MINNKOTA'S :RESPONSES TO ADDITIONALINFORMATIONREQUESTED "

A. Center Mine Lignite Coal Ash Content and Variability

EPA's January 26,2007 Request: . . .' .."..

Please~roVidedata currently availableon the tenter Mine coal ~onstiiuehts (presented in

"Minnkota's October 2006NOx BACTAnalysis), including all av.ailable d~ta samplingforjuture and

presently-mined areas ofthe mine, Please provide the data in terms ofaverages, 'in additionio vqlue

ranges.

EERCAprii 23, 2007 Response:

The his.toriCal c/atabase was obtainedfrom samples cqlleciedfor the purpose oftesting the imp~ct; oflignite characteristics on plant performance. The database consists of13J.7 as-fired samples. "" The

average. standard deviation," maximuin, minimum and selectedpercentilesfor ash, sulfur, heating value,

and selected ash constituents are includediiz Table 1. " " ""

The use ofaverageco~lproperties/or design andperformance ofcoinbustionand en~jronm~ntal controlsystems"tYpically under"estimates adverse problemssuch asjireside ash depositioil," opacity, ahd" "" "" sulfur emissionS. Designs ofpowersystems must consider the challenging coal characteristics in" orderto " "dewlop .,/esigns and'operating parameters with sufficientflexibilitytharallowfor. the combustion ofcoals ""

thatert least represent the 9(jh arld 1(jh percentile ofselected constituents ,The frequency .' "" "

distributions a/components Qfthe coal provide better understandingofthe potentiall:hallenges~... The

results ofthe analysis show that the lignite ash has a4.4% average sodium content; however, there is ci

multimodal distribution of~odium levels in the coalash..... The average properties as well as the . "

minimum and maximum values do not adequately represent the characteristics ofthefuel. As indicated in

. Fi/Wre 1, the MRYS boilers are jiringsome very high-sodium-containing coals that can cause significant .

proble111swhen sodium .reacts with silicate, aluminosilicate, and sulfur species in thefurnace and lower

temperature regions ofthe·powerplant." . .

EPA Respons"e: "

EPA has revievved the coal data submitted by Minnkota. .EPA believesthe future coal data"

" appears to show little change from the 2007 core samples. EPA is·not sure what the basis is for

EERC's statementthat "s'odium levels have shoWn increased levels in the future coalsthat will

"likely contribute to increased fouling challeng~s.,,22 "Th~ sodium oxide percentages from 2007

22 See page 10 ofApril 23, 2007 letter from Minnkota to EPA, DOJ, and NDDH.·

-42-'
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23; 2007.

BACT Analysi~ at MRYS

compare very closely to the 2009-2010 cores samples (Ave; = 5.8% vs. 5.6%, Max =13.3% vs.

13.4%) as do the ash percentages (Ave. = 7.9%'vs. 7,1%, Max == 24.6% vs. 23.8%). As

explained in great detail above, these differences in coal characteriStics are useful tor making

general comparisons with other fuel types, but it is unclear how these differences can be directly

applied when making determinations on feasibility ofSeR atMRYS." '

B. High Economizer ExitTemperature

NDDH's February 1, 200·7Request:

The temperature between the economizer and the air heaters is described as "much higher than·

750 degrees". Please provide more spec;ijic data on the temperatures tl1Qt would be· encountered at the·

location an SCR would be installed. "

B&McD's April 23,2007 Response:

The primary superheat (PSH) outletflue gas temperaturesfor Unit I are provided instead ofthe

eco;'omizer outlet temperatures. Unit 1 at the MR. Young Station has a two-pass tUbular airpreheater

(APH). The economizerfor Unit 1 i~ situated between the first and secondpass ofthe airpreheater. The

flue gaspasses on the inside ofthe ~ubes ofthe APH... ..:..... The configuration ofthe ·economizer and air .

. preheater physically precludes locating a conventional high-dust SCR upstreatn ofthe eXisting air·

--~-P1:eheatei."ecorwmizer-OfLUniLLiJ:ilts.i:un:entlac-atiol1.JJ11dJ:f)lJJJgw...atioJ1,~TfzejlueiaS.1gmllerqllP~.g is .. _ __._.._ .

reduced by trcmsferring heat to the boiler'sfeedwater and the boiler's combustion cmd drying system air

streams; respectively. In addition, temperatures at the existing air preheate-r outletfor Unit 1 arefar

below the "temperatures neededfor operation ofa conllentionalhot-side high-dtistSCR.

The Unit No. 1 boiler is shut down/or cleaning at intervals rangingfromapproximately 90 to 120

days.. .The gas temperatures~atthe PSHoutlet vary inversely with the boiler cleanlb:zess during each

operating run Note that qt/ow loads and clean conditions, thePSH'outletjlue gastemperatures are

170-270 degrees F below the acceptable rangefor conventional hot-side high-dust SCRcatalyst .... Unit

1 's PSHfull-loadflue gas temperatures historically range between' 840F and 960 F, depending upon the

cleanliness ofthe boiler.....there have been instances when Unitl's PSHfull-load average flue gas

temperatures have exceeded 1000 F and individual temperature measuring elements have exceeded 1050

F..

.A portion ofMRYSUnit 2boilers'feedwater heating surface is iii alocation normally occupiedby

a traditional conventional econ01nizer section in the convection·pass ofthe boiler '" The

configuration ofthe boiler's economizer and the air preheater physicallyprecludes locating a

conventional high-dUst SCR upstream ofthe existing air~preheater/economizeron Unit2 in its current

location and configuration :. Unit 2 boiler is shut down for cleaning at intervals rangingfrom

approximately 80 to 110 days. . at low loads and clean conditions, boiler outletflue gas temperatures

are 170-270 degreesF below the acceptable range Unit 2's boiler-exit economizer-outletfull-load

flue gas temperatures historically range from 780 to 880 F...:.there have been instances when Unit 2's

-43-
EPA Response to Minrikota Comment Letters from

March 19, ~007 and April 23, 2007

. BACT Analysis at MRYS .

averagefull-load economizer outlet}iuegas temperatures haVe exceeded 930Fand individual

temperaturf! measuring elements have exceeded990 F. .

EPA Response:

This inforniation that was requested by NDDH was reviewed by EPA. EPA acknowledges

that there are times when the flue gas teniperatUie at the.portion pfthe boiler:where a high-dust .

SCR would be located (primary superheat outlet for Unit 1 and presUmably the economizeiexit

for Unit 2) would be higher than the temperature range typically ·suitable for SCR.

. While B&McD seems to iiidicate in the March 19, 2007Jetter that the dual-air preheater

design suggested by ERG ~ould be problematic, it later concedes thafitconceptually could

work, but would be at an additional cost. EPA agrees that such a design would cost more than a .

typical high dust SCR application. A more refined cost estimate that .·reflects real c·osts .

associated with constructing and operating a split air heater design can certainly be submitted by

Minn.kota. EPA believes this would be conducted under step 4 ofthe Top-Down BACT

analysis..

. For further information ~n this topic, please refer to the August 2007Christman Memo

response regarding the split air.heater. . . . .

C Low-Dust ~CRFouling

.. NDDH's February 1, 2007Request: .

. Itis indicated that a low dust SCR would be subject tl} the same fouling.problems as·a high dust

SCR due the emissions ofsmall« 5 micron) pariicles containbig sodium-calcium sulfur rich materials.

Please provide any existing data on the composition ofthe particulate matter exiting the electrostatic

precipitators.

EERCApril23, 2007Response:

As previously discussed in responses to the EPA's January 8, 2007 commenis} the MRYS cyclone .

b9i1ers'fly ash becomes enriched in Components that vaporize and, subsequently, condense upon gas

.cooling toform. liquidphqses and smqllpartiCles·:that are very reaCtive to.gas:'phqse components,

resultingin!ormatio':'ofcompounds ihat cause bonding; blinding, and deactivation ojindividualcatalysi

reaction sites, even in a10w-dust SCR application. Sodium-richaerosokcan get through the

electrosiatic precipitator (ESP).. These aerosols at.e known to be present in high~sodium coal combustion

flue gases. These spe~ies are known to produce pyrosulfates. These will cquse blinding anddeactivation

ofthe low-dust catalyst. . .

-44-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS

Stack sampling was conducted downstream ofthe EPSand set FGD .on Unit 2 at the MRYplant.

Theftlters were examined using scanning electron microscopy andx-ray microcmalysis to determine the

microscopicfeatures and chemical compositions , The chemical compositions ofthe analysis

. areas andpoints were determined through the use ofenergy dispersive x-ray microanalysis....

EPA Response:

This information that was requested by NDDH was reviewed by EPA. While the telative

effects ofvaporized particles' passing through the ESP and condensing.prior to a 10w-dustSCR

system .might be more pronoull<;ed compared to the non-vapor particles impacting a high-dust

environnient, EPA maintains that the ESP will remove a very substantial portion ofthe ash that

would affect·a high-dust SCR and the overall effects of catalyst'plugging and deactivation ina"

high-dl,lst application would be greater than.in a low-dust application. This is substantiated by

EPA's discussion with Flemming Hansen ofHaldor Topsoe'and his experience with SCR test

coupons at the WE Energies Presque Isle Station, as discussed above. EPA acknowledg~s that

there are other technical and cost issues related to low:-dust SCR that were discussed previously.

D. SCR DeactIvation

EPA's January 26,2007 Request:

~~~~~~.~~.~..~."Please=-provide-T:lnranrh:tlI~(nlfld-~inf()rmCltir:m~ektted~t(Teatalyst-4fleaeffv€lti€m~1'(1tt!'9'€lnd~~~~-;~-_···_--- ._".

. control effectiveness measured at the threefacilities tested during the "Ash Impacts on SCR Catalyst

, Performctnce" stiidy that is extensively referred to in the "EERC Report" included as Appendix B ofthe

Minnkota NOx BACTanalysis. This shouldinclude the original proposal, all testplans, 'all raw catalyst

deactivation andNOx control data collectedduring the test, and allfinal reports and conclusions that

were producedfor the study's participants. In addition, please provide a quantitative'analysis ofthe

application ofthe multiple engineering solutions (e.g. catalyst.rejuvenation and regeneration, acoustic

horns, etc.) described by Roger Christman and EPA to address specific concerns highlighted in the BACT

. analysis and EERC Report and their effect on SCR catalyst deactivation rates andNOx control

effectiveness at.MRYS. .

EERC's April 23, 2007Response:

" The EERC offered to send them to Haldor Topsoe for evaluation; the catalyst vendor indicated that

the Coyoie SCR test catalysts were too plugged and deactivation testing couldnot be conducted. The lack .

ofpublished catalyst activity dataior the Coyote slipstream SCR catalystfield test is a result ofthe

severitj ofthe catalystfotllingfrom North Dakota lignite ash. The requested data is available; this

information has been copied, compiled, and due to its volume, will be submitted separately.

Information on catalyst deactivation rates andNOx reduction effectivenessfor the Baldwin and

Columbia Station slip-stream SCR catalystfteld testing was determined and some ofthe information was

in the final report. EERC recently received information from Hitachi on catalyst deactivation for the

-45-
EPA Response to MinnkotaComment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYScatalysts atColumbta. This information is rnclude~ inthe EERC datacompiled and being submitted to

theNDDH. :. . :. . . .' .: . . .

Regeliera,;on has been attempted in Europe with SCR catalysts exposed to biomass as with high

potassium (elemental symbol K, similar in chemistry to sodium, Na), and they had some significant

problem in recovering reactivity (KhodayariandOdenbrand, 2001). "

,EPA Response:

EPA has reviewed the background documents. Please see EP.A;s previous detailed analysis

related to the review ofthe pilot study documents under the section,"EPA Analysis ofBums&

McDonnell's Conclusion" (responding to the March 19, 20071etter response.)

B&McD's April 23, 2007 Response:

'A qucmtitati;eanalysis on the application ofthe multiple e~gineeringsolutions suggested by the

EPA's commenter mentioned above has not been performed. Significantlymore time would be required.

A definitive analysis would require pilot-scale slipstream SCR reactor testing on thejlue'gas streams '

emittedfrom the boilers at MRYS over a significant length oftime to simula~e real-time effects ofgas and'

'flyash constituents andtheir interactions with the catalyst. Using assumptions ofthe effectiveness of

sol1,ltions suggested by EPA's commenter based on. use oftheoretical data developedfrom laboratory

testing and/or'empirical data gatheredfrom other high-dust SCR installations that do npt treatjlue gases

producedjrom cyclone boilersfiring North Dakota lignite coal would be hypothetical, Such d theoretical

quantitative analysis ofEPA's claimed SCR catalyst improvements cited andwhether this.would actually

be translatable into preventing the catalyst layersfrom being suscepiible to macro-pore'blinding and '

plugging by calcium-basedcompounds and the impact on the individual catalyst reaction sitesfrom

sodium-based compounds discussed in Dr; Benson's referenced technicalpaper would be speculative.

Burns & McOonnidl believes it wouldllOt bf; accurate to burn lignite or ~ejly~h samples' '

collectedfrom aMR,YS electrostaticprecipiiator l1(i{h artificialflue .gas to simulate c{;ltalystfouling arid

blindingphenomenon in a laboratory.:. or pilot-scalete.st setting Performing a quantitative analysis

, wo~ld require substantral resources to develop slip-stream SCR catalyst test equipment designs and'

operational conditions to'address the significant technical challenges tl;at were. evident in therefere.nce

field demonstration testing at Coyote station. It is unreasonable that the EPA expects ihat such.an indepth investigation is required under a BACTanalysis. . '

Performin8 a BACTanalysjson. an existing,unit should notrequire substantialresources...•. .~~:'

, .

-46-
. EPA·Response to Minnkota Comment Letters·from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

EPA Response:

Many ofthe concerns in this section have been previously presented verbatim byB&McD

and have been largely addressed elsewhere in EPA's previous responses. See the March 19,

2007 resp0Q.se to "North Dakota Industrial Commission project Evaluation ofPotential SCR

Catalyst Blirtding During Co~ Combustion (from the.JanUary 8~ 2007 ERG Memor and "EPA

. Analysis ofBurns & McDonnell's Conclusion." ..' .

B&McD's response indicates that a complete evaluation ofthe engineering solutions to

.address macro-pore plugging and individual catalyst site deactivation goes beyond the

requirements of a BACT analysis.and would be speculative withOlitdoing additional pilot

testing. As noted previously, EPAbelieves that Minnkota faces a high threshold for

demonstrating that a technology like .SCR, that has been widely-deployed in the same industry, is

technically infeasible. While results from a well-designed and operated pilot test would certainly

be useful in optimizing a potential SCR system to deal with the plugging, blinding, and

deactivation issues, EPA is not convinced that it would be required ifa conservative design

approach is·taken. . .

In the previously mentioned meeting with Sarge~t & Lundy, LLC, Mr. WilliamDePriest,

Senior Vice President and Director or Environmental Services, indicated that an SCR system

~~~~~c~oe-::u=ld theoreticallyJ>e .designed to operate on North Dakotalignite, but indicated the.design ._~~.__.._._

would likely have to be conservative in terms ofthe catalysf deSIgn mid sI2~rngofthereactor and

could result in significant maintenance requirements and associated outages. Sargent & Lundy

representativeswere ofthe opinion that a well-run pilot test would yield valuable information

that could be used in conjunctionwith previous SCR experience to evaluat~and design an SCR

system for-a North l)akota lignite application.. .

In EPA's view, the unanswered questions related' to North Dakota lignite described by

Sargent & Lundy would likely result in exaggerated .capital costs(conservative design) and/or

potentially result in additional operational costs to Minnkota. EPA agrees itwould be preferable

to have additional data related to catalyst deactivation on North Dakota lignite in order to'select

the optimum catalyst design and reactor size. It is unfortunate thatthe pilottesting at Coyote

Station.did not capture this information. However, the risks.involved in the capital and

operational costs cannot be used as the basis for a determination oftechnical feasibility. The

NSR Workshop Manual states that "(w)herethe resolution oftechnical difficulties is a matter of

cost, the applicant should consider'the technology as technically feasible.,,23 EPA believes that

the cost for a conservativdy-designed SCR system at MRYS that minimizes the associated ris~

described and includes an anticipated catalyst operation and maintenance schedule can· be .

estimated and accounted for under step' 4 ofthe Top-Down analysis. . .

23 See NSR Workshop Manual page 8.19-20

-47-
.EPA Response to Minnkota Comment Letters from·

March 19, 2007 and Aptil23, 2007

BACT Analysis at MRYS

K SCR Vendor Guarantees

EPA ;s Jan'!!lry 26, 2007Request:

Although EPA 'stJraft'1990NSR Workshop'Manual recogniies thai vendor guarantees by

themselves are not,sujJicient,to show that a technoloiY. isfeasible, ve,ndor guarantees can contribute to a .

technicalfeasibility determination in some situations. In light ofstatementsEPA has receivedfrom three

SCR vendors (Haldor Topsoe, Cormetech, andArgillon) indicating they wouldprovide peiformance

guarantees on SCR lignite applications, please provide additionalvendor information,from multiPle

vendors, that Minnkota believes is more specific to MRYS them the statements received byEPA and cited

in EPA's analysis submitted toNorth Dakota on January 8, 2007. We would expect that the catalyst.

suppliers cited above would be among the vendors contacted by Minnkota.

EERC's April 23" 2007 Response:

EERC has alreadyinc!uded some co;"mentsjr()m February 2007 tel~phone discussiQns with

Haldor Topsoe regarding their concerns about the sodium content in North Dakota lignite and its

tendenciesfor catalyst deactivation. .

EPA Response:

EPA has previously presented its Owll discussions with Haldoi Topso.e. .. .'

B&McD's April 23,. 2007 Response:

, .A query0;SCR catctlyst verid()r ~s willingness to guarant~e performance on a lignite-fired bOilerto '

.' address comments in EPA's cited documents mentioned above has not been completed. AdditionaltimeJ$

required The information on SCR peiformance guaranteesfor MRYS applications has been requested; it

will,be submitted separately when iUs available. A copy o/the information request letter and

. attachments,issued to the SCR vendors'is attached (Appendix A). ' . . . . ... ..

. . . ' . . ..'

,'System andequipment vendors have ase/f:interest to'sell technology that they offer, and to manage

finanCial risks by specifyinglhe limits oftheirsystem design in case ofpotential operating condiiions that

, cause peiformanceproblems that prevent achievem~ntofguarantees and/or warranty claims'" ....

, , EPA Response:

,B&McD states obtaining this infomiation will require moretim~. They have s~nt a letter to

four catalystsystem suppliers and five catalyst suppliers. TheJetter goes into great detail about

the worst-case nature ofCenter Mine coal·.' During the May 23, 2007 meeting, in Bismark with

Minnkota and NDDH, B&McD indicated' that:they did get some feedback'from a few vendors,

but were unable to disclose any written response at that time ,EPAhas still not seen any vendor

, responses to the information request. EPA does note: that the request see~s to have been written

-48-
EPA Response to Minnkota CommentLetters from

March 19,2007 and April 23, 2007

BACT Analysis at MRYS

with a pessimistic tone that would likely lead the vendor to believe that the requestor was not

serious about obtaining acontract for the described·SCR project.. There is also reference to .

cornillents made by.EPA on the Minnkota MRYS BACT analysis, which·~o\Jld provide further

.disincentive on the part.of~e vendor to properly evaluate and give serious consideration to the

request.· Because ofthis EPA·is suspect that the feedback might be negativ.ely biased and not

provide'a realistic response to whether a vendor wQuld be interested in bidding a performance.

guarantee on an SCRproject'at MRYS. . .

F. '. MRYSBoiler Cleaning Outages

EPA's January"26, 2007Request:

The Mimikota MRYS NOx BACTanalysis includes discussion ofperiodic boiler outages that are

required to address the slagging effects ofthe Center Mine lignite. Please provide historical dates of .

thesf! outage periods and their duration for the previous 10 years. Also, please provide an estimate of

· .boiler outagefrequency that would be expected due to SCR maintenance, bas.ed on information collected

in response to items 1 and 2ofthis list. . .

· B&McD'sApril 23, 2007 Response:'

. Information to estimate· MRYS boiler outage frequency expectedfor high-dust SCR maintenance

~.-.- .._.~~9fjased upon a qlIerJl:7)fieIrcutalystverTdors-mentioned-in-ite"d~above-is~not~avail-able-aHhi~ttme~---'-·.-. ~~~

Additional time"is required' .

A revieW ofhistoric outage data shows t~at over the past 10years (1997-2.006), the typical MRYS

Unit1. boiler cleaning outage was between 67 and 82 hours in duration, but these clm be as short as 56

. ha,urs and as long as 206 hours.....~Over this same time period, the typical MRYS Unit 2 boiler cleaning

. outage was between 64 and 102 hours in duration, but these can be as short as 37 hours and as long as

244 hours A boilerfireside cleaningoutage is typically scheduledfor 96 hours (four days)

duration. . .

A high-dustSCR would need to be isolatedfrom the gas path between the.boiler and the inlets to

the electrostatic precipitator during boiler cleaning outages to prevent the moisture and deposits .

liberated during boilerfireside surface cleaningfrom entering the SCR reactor. Beginning the work of

removing ash accilmulation or catalystfrom the SCR reactor(s) will be delayed by 48 to 60 hours after

·the start ofthe outages, depending on ambient temperatures, because the inside ofthe reai:tor(s)"will be

too hotfor personnel health and safety. Because the length ofthe boiler cleaning outages is typically less .

than 4 days (96 hours), that does not leave much time to do any significant catalyst cleaning or

replacement work without delaying boilerstartup. The estimatedamount oftime and difficulty in .

removing lignite ash accumulationfrom ihe SCR reactor(s), or replacingfouled catalyst with clean spare

catalyst, is expected to require at least 144 hours (6 days), plus 60 hours upfrontfor allowing the reactor

to cool-down.. This is based upon the use ofmobile industrial vacuum cleaning methods ("super-sucker"

truck). The total SCR maintenance outage time will be around 200 hoursfrom boiler shutdown end to

-49-
EPA Response to Minnkota Comment Letters from

. .March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS

beginning a/startup. rh~frequency olsuch $CRmaintena1Jce outages are expected to be required at

least annually, .or more often. .. . .. .

There were only two outagesoth~rthan majorscheduled overhauls during the hist~ric ten~y~ar ..

period that appeared to be ofsufficient length (greater than 8days) lhatcould have lent themselVes to

signijicantcatalyst cle.ariing or replacem.ent wor!rwithOutdelaying startup: one on:Unit If and one. on

Unit 2......There are basic concerns that using short boiler Cleaning outages as SCR catalyst maIntenance

opportunities could be detrimental to catalystlife due to thermalstress.es cmdpassing through various

(Jewpoint temperature~. Bringing the very ,large massofceramic catalyst material and the reactor

structure down intemperature and then back up rapidly and any morefrequently than absolutely·

necessary should be avoided... :... .

EPA Response:

EPA looks fQrward to reyiewing the infonnation obtained fro~ the vendorreq~~~t; During

the previously discussed phone conversation with Flemiliing Hansen ofHaldor Topsoe, Mr.:

Hansen gave-a rough estimate that an SCR system for NorlhDakota lignite couldbe d;esigne(il

(larger) to accommodate a catalyst replacement schedule of about every year. Mr. Hansen said

that an SCR reactor could be cooled down within 24 to 36 hours (needfor the cool air draft

requires the boiler to be.shut down) and the catalyst layers rotated in about two or three days,

.three days aflaround..This is significantly less than the roughly 200 hours-(or eight days)·

estimated by B&McD for catalyst replacement. This discrepancy would probably warrant·

further examination of both figures inn the context of a cost analysis.

G'; .Iiydrocarbon-E~hanced Selective Non-Catalytic.Reduction (HESNCR)

EPA's Janliari 26, 2007 c,omment:

Minnkota eliminates this.technoiogy irimarily based on thefact that it has not been appiied to ..

cyclone boilers or units burning lignite orhigh~slagging western. subbituminous coals. Th~re are twofull .

scale installations ofthis technology on eastern pulverized coal-fired boilers. There is some general .

discussion about the slaggingproblems that ligniie would cause the HE SNCR techn(!logy, but it is

unclear whether the vendor ofthis technology would be able to overcome these issueS. 4tthis point,·it is· .

. unclear whe.ther HE SNCR is technicallyfeasible. Minnkota, shou14provide ci more thorfJught;lnalysis· on

whetherthiscontroloption is "available andapplicf;lble" (as described in the NSRM.anual) a{MRYS by

examiningthe physical and chemical characteristics oft~e exhauststream in terms ofteqhnicalfeasibility .

and soliciting vendor information. .A qu.antitatilleanalysis ofIts techni(:alfeasibillty.$hould be proyided .

.and supported with 4ataand vendor statements..

-50-
EPA Response to Minnkota Comment Letters froll)

March 19,2007 and Apri12J;2oo7

BACT Analysis at MRYS

B&Mc:o,'s April 23, 2007 Response:

The October 200~ MRYS NOx B4CTanaiysis was performed using technical literature available in the

public domain, proprietary vendor customer experience lnformation, and limited engagement ofse'ected

vendorsfor capital cost, operating and maintenance costs, and emissions performance estimates along

· with Burns & McDonnell's technical expertise. .This analysis approach meets the requirements ofthe'

EPA. 's NSR Manual.

.' A detailed query ofthe potential availability and applicability ofhydrocarbon-enhanced (HE)

selective non-catalytic reduction (SNCR) on a tfgnite-jired cyclone boilerfrom the single vendor

mentioned in the October:,2006 MRYS NOxBACTAnalysis,reports has not been performed. Additional.'

time would be required, It is importcmtand appropriate to specify the composition ofthe Center lignite

·coal and ash constituents and boiler configuration sufficiently to provide a sound basisfor allowing the

HE-SNCR vendors' reviews'and qualified responses. HE-SNCR design conditions that are reasonably .

reflective ofactttal operating conditions and boilerfouling experience are yet to be established. The lack

ofdetailed development ofMRYS unit-specific HE-SNCR injection lance design schemes to deal with the

high boilerfouling conditions and varying boiler gas temperaturescmd loads willrequire HE-SNCR

design operating conditions to be assumed before workable design concepts hqve been confirmed.,..Itis

interesting to riote that other non-cyclone BART'-idigtble boilers burning North Dakota lignite did ~ot

·mention or evaluate HE:·SNCR technology as a possible NOx control aliernative in their BART analyses.

. EPA Response:

EPA has simply asked Minllkota to get the vendor's input on the facility in question.· .

Minnkota's original BACT analysis describes theoretical problems thatmight be encountered if

. HE SNCR is installed at MRYS and states that the lack of experience with cyclone boilers •

burning low·rank 'western coal satisfies a showing that HE SNCR is technically infeasible. The

vendor who offers this technology states thatitis notsensitive to fuel type or bumers,so EPA

still questions whether the·determination oftechnical infeasibility is valid without getting vendor

input or substantiating data.

. . '.' .

EPA does not agree ~ith B&McD's i~teIpretationofthe NSR Workshop Manual.

.B&McD'states that in order for the vendor to develop a realistic technical basis that would

support estimated unit..:.specific HE SNCR performance, capital and operation and maintemmce

costs, they would need specific customer data. B&McD claims this is not required by an

applicant because the NS~ Workshop Manlialstates, "a source would not be required to'

experience extendedtime delays or resource. penalties to allow research to be conducted on a

new technique;" This statement.is in the context o~ detennining whether the technology is

"available." According to Minnkota's ;BACT analysis, the vendor ofthis technology believes the

technology is not sensitive to fuel or burner types. As such, it would appear that, in general, this

technology is "available" for MRYS.. therefore, the reference to the NSR Manualmade by

B&McD is only relevant ifthe vendor would not ()ffer thetechnology to acustomer based on

specific conditions ofthe plant. Minnkota can go through this process ifit believes the

-51-
EPARespo~se to Minnkota C~mment Letter:sfrom

March 19, 2007 and April 23, 2007

BACT Analysisat MRYS

. .

·technology is notavailable, but appareritly does not wishto do so. Hence, EPA believesthe .

technology is "available."·. IfB&McDdoesnot believe it is "applicable," further informatIon

should be prOvided. The reaSons given·by B&McD to this point (high boiler fouling conditions

and va,ryirig gas temperatures and loadsj'appear themselves to be theoretical. EPA believes·

.statementsJrom vendors or more quantitative Information might be helpful for hUlking this .

determinatiop.

. . . . . '.' .

EPA does not believe that whether other non-cyclone fired BART applicants included an·

· analysis ofHE·SNCR isevidence~ in and by: itself, that the technology is infeasible at MRYS: If

the technology is "available," it should .be evaluated. According to statements from the vendor

in question, the technology is generally available to po.wer plants..~.. .

H. Reburn with SNCR

EPA's: Jamuiry 2.6, 2007Comment:

While various.]orms ofreburn systems were determined to be technicallYf~asible in the Minnkota .

analysis, reburn combined with SNCR was ruled out based on technical infeasibility. AlthoughJullscale

· pulverized coal installations ofgas reburn with SNCR and overfire air were found to exist, Minnkota did

not analyze whether the technology combination would be "available and applicable" to MRYS, as

described in the NSR manual. Instead, the application states, "(t)he lackofexperience with these·

combinations on a cyclone-fired boiler, especiallyfor lignite-flring, makes the applicationfor MRY·

· Station's boiler infeasible. " Minnkota shouldprOVide a more thorough analysis.....

B&McD Response:

" '., .' . .

.. A detailed query ofthepqtential availability andapplicabilitju)jfuelreburn technqlogy combined·· .

with SNCR on a lignite-fired cyclone boilerfrom the rebitrn technology vendors has not been performed .

·As mentioned in Appendix Al ofthe October 2006 MRYS NOx BA(:TAnalysis reports, whikSNCR may .

potentially be installed downstream. ofseparated oveijire a~r andvarious types offuel reburn, "there is a .

lack ofdemonstrated experience ofthis .combination on cyclone boilers. Only one example ofgas reburn .

with SNCR,applied to atangentially-fired boiler, wasfoun((l In available technical literature. Moreover..

the very high capital andO&Mco.sts't!lready estimatedfor coal and gas reburn tecknologies with . .

. ASOFA in Section 3 ofthe October 2006 MRYSNDxBACTAnalysis reports showedthat s~cJi ..

·alternatives were sig"ificantlyabove the BACTDominant Controls Cost CurvesforMRYS Unit rand .

· Unit 2. Thus, eveniffuelreburn technology com!JinedwithSNCR )vere availablea.nd.applicable to· .

lignite-fired cyclone boilers, the average .and incremental unit control co.sts ($ per ton) would not be ..

·expecte.dto be on theNOx BACTDominant-Controls Cost C~rvesforMRYS Unit 1 andUnit 2. Such

reburn with SNCR alternatives would thus be consideredas "inferior", o,!dwould not be considered tis .

. .BACT S"ubstantialresources wouldbe required..../t is interesting to note thaIother non-cyclone BARTeligible boilers burning North Dakota lignite did not mention or evaluate reburn wi/hor withoutSNCR

technology as possibleHOx control alternatives in·their BARTanalyses.

-52-
EPA Response to Minnkota Comment Letters from

.March 19,2007 andApril 23, 2007

BACT Analysis at MRYS .

EPA Response:

B~McD.has declined to perform an analysis ofwhether Rebum withSNCR would be

. "available and applicable" because·the technology haS not been"demonsirated" on cyclone

boilers. However, according.to theNSR Workshop Manual, the "available and applicable;'

analysis is only required whenever the technology in question has not been demonstrated in

practice. Ifthe technology has been deQ1onstrated, it is considered technologically feasible

without goirig through further analysis. Hence, B&McD's logic for riot doing the "available and

applicable" analysis appears to be contrary to the NSR Workshop Manual...

. B&McD also makes~he argument that the average and incrementa~ cost associated with .

Rebum with SNCR would not expected t9 be on the NOx BACT Dominant Control Cost Curves,

so they would be eliminated on that basis, ifthey were found to be technically feasible:

However, Reb1,1I11 with SNCR wouid offer greater NOx emissions reductions than just SNCR

(assuming e~ch would also include ASOFA), SO this data point on the curve would be above and

right ofthe data point for SNCR. EPA can not say with certainty that the average and. . .

incremental cost effectiveness ofRebum with SNCR would be ruled out based on·cost because

the analysis is not presente9: by B&McD.

B&McD also points Qutthatnon-cyclone BART eligible sources in North Dakota did not

provide an analysis ofRebum with or without SNC~ as a control altemati:y_~ in th~ir BA,-R~T--,-~

analysis. For the reasons cited above, EPA does not agree that Rebum with SNCRcan be ruled·

.out baSed on technological infeasibility or cost effectiveness at this time..

I. Rich Reagent Injection (RRI)

EPA's January 26, 2007 Comment: ..

. Minnkota eliminatesthis technology because ofthe difficulty in achievingproper substoichiometric 'boiler conrJitions necessary to get effective NOx control. As described in the application,

the problem occur~ because ofthe variable heat content ofCenter Mine lignite. While RRI has been

successfully demonstratedoi cyclone units burning blends ofPowder River. Basin coal with Midwestern

bituminous coal, Minnkota Ju;rs ruledRRI out as technically infeasible. However, there does not appear

(0 be an analysls ofwhether RRI meets the "available and app1icable" criteria described in the NSR

Manualfor elimination under step 2~ IfRRIwould be less effectivejor the·reasons cited in Minnkota's

application,.that would result in a·lowerrailking under step 3 andpossible elimination under.step 4. At·

this time, it does rIOt seem appropriate to eliminate RRI under step 2. Minnkota Shoilldprovidea more

thorough analysis·..... .' . . . .

-53-
EPA Response to Minnkota Comment Letters from

March 19,2007 and April 23;20(n,

, BACT Analysis at MRYS

, B&McD Response:

'Since the publication ofthe MRYS NOx BACTAnalysis report;'two commerci~l Rich Redgent ,

.Injection $ystems "'ave completed design, fabrication, and installation 'in late 2006' at Ameren Sioux Unit

I and Unit 2. Full optimized routine operation is expected to begin by early May, 2007. Although RRl

NOx emissionsreduction technology appears to satisfy the "available" criteria (as described in the NSR ,

Manualfortechnicalfeasibility)for potential application on the cyclone boilers at MRYS,' the substll.ntive.,

issue is "applicability". The expected insurmountable difficulties in achieving consistent control " '

, effectiveness.;... from this technology ifapplied to modestly air-starved cyclone combustion on these

lignite-fired boilers and the reasonsfor technical infeasibility ofRK! at MRYS were included in Appendix

Al ofthe October 2006 NOx BACTAnalysis reports.

'. A query ofthe singleRR!licensee mentioned in the MRYS NOx BACT analysis, and a quantitative'

analysis ofits technicalfeasibiiity for the expected boiler operating conditions has not beenperjormed. "

, The transient variabiliiyofthefired lignite 'coal at MRYSwas discussed /n AppendiX Al ofthe October

2006 NOx BACTAnciiysis reports. This variabilitY makes it impossible to accurately predict realistica./ly

achievable NOx emissions reductions even with sophisticated computationalfluid dynamics (CFD)

modeling'ofthe'MRYS boller cqmbzistion reactions and in-furnace NOx reduction processes·associated

with RRItechnology under reasonably realistic boiler operating conditions. This is duetd the statIc (i.e~

steady-state) nature ofthe boi~er furnace operating conditions assumed to exist in order to simulate the

performance ofmode.stlyair-sicirved cyclone. combusti(.m on theselignite-fired boilers with varying heat

and air inputs to individual cyclones usingCFD modeling techniques. Because ofthe dynamic wiriability

ofthe higher heating value ofthe fuel burnec/'tmd the lack ofabilitjJ to precisely control the actual

stoichiometry in real-time on an individual cyclone basis, R1U CFD model predictions would be

unrealistic and thus these'vendor statements would not be reflective ofreatisti.callyachievable NOX

, emissions reductions. Substantial resources would be required.... ..

'EPA Response:

,EPA believes that therecentinstallationofRRI at the Sioux 1 and 2 cyclone uriits is also

strong evidence th~t the technology is "applicable" to MRyS. the NSR,Workshop Manua1. '

states that '~if the control techriology,has been installed and operated successfully'on the type of

source under review, it is 4emonstratedand it is technically feasible." While the'Sioux plant'

does, not hum lignite, PRB coal is al~o a low-ranking western coaL This indicates to'EPA that

before ruling out RRI as technically infeasible, detailed analysisshould be provided. ' "

, ' , , .

.Wlllie predictiilg:the exaCt level ofNOx control for RRI~d MRYSmay not bepossibl~; ,'.

EPA believes'some informed estimates could be made. Further" EPA does not believe predicting,

exaCt levels ofNOx contr.ol is necessarily cruciarto whether a controloption is technically

feasible. Hence, EPAdoes riot believe B&McD has demonstrated that RRI should be considered

,technically infeasible; In fact, EPA believes therecellt operation ofRRI at the Sioux plant'

provides strong evidence that RRI would be, teclulically feasible atMRYS~ ,

-54-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

J. Rotating Opposed Fire Air (ROFA)

EPA's January 26,2007Comment:

Minnkota eliminates this techn~logy becazise it "lacks demonstrated cyclone-fired experience, and thus

was considered infeasible.;' While a technology that has been demonstrated on the source type under

review would.show it is technica!lyjeasible, lack ofsuch demonstrationforthe specific type ojemission'

'. unit does not necessarily lead to a conclUSion ofinfeasibility, Minnkotashould provide'a more thorough

analysis..:...:. '. .

B&McD Response:

A detailed query ofthe potential availability and applicability ofROFA technology on a lignitefired cyClone boilerfrom the single vendor mentioned in the October2006 MRYS NOx BACTAnalysis

. reports has not been performed. Burns & McDonnell h"as reviewed available ROFA technical literature,

attended sales presentations andparticipated in discussions with ROFA vendor personnel regarding the

application ofROFA to cyclone' boilers, inc/tiding lignite-fired units. As an engineer and design/builder

.ofnumerous commercia/separated over:fire air systemsspecijically designed/or cyclone-fired boilers,

Burns cmdMcDonnell believes that ROFA ter;:hnology is notsuperior to the evaluated version ofSOFA in

the October 2006 MRYS NOxBACTAnalysis towarrant stich a vendor query and a quantitative analysis

........, ROFA does not offer any significant advantages in lignite-fired cyclone-specific application of

separated overfire air technology.' .. .~~~

EPA Response:

EPA does not currently.have information to the contrary and will agree '.that this option may

not differ substantially from ASOPA or offer greater NO~ reduction on cyclone boilers. Since

. ASOFA has 1?een addressed elsewhere in the BACT analysis, further consideration ofROFA is

unnecessary.

K. Control Efficiency for SNCR

EPA's January26, 2007Comment:

Based on the proposed emission rate of0.36IbIMMBtu and the emission rate expectedfrom

installing Advanced Separated Over:fire Air (ASOFA) alone (O.513lb/MMBtu), it appears as though the

SNCR controls will only be achieving approximately 30% reduction. (po 3-18) AP-42 states that SNCR'

can achieve NOx reductions between 30-60%. Many recent PSD p~rmit applications also include

discussion ofSNCR being able to achieve NOx control in the 60%,range or higher. Considering that

Minnkota:s cyclone 'boiler will have high uncontrolledNOx emissions (even considering the ASOFA), the

highend ofthis range should have been considered in the BACTanalysis. Minnkota's analysis really

provides no technical discussion on whY it ruled out higher capabilities ofSNCR control and why they

used the control efficiency that is proposed. Usi~g 60% control wouldyield an emissions rate of0.21

Ib/MMBtl!for a BACTlimit with ASOFA and SNCR. compared tothe proposed 0.36.lb/MMBtu.

-55.:..
EPA Response to Minnkota Co~ment Letters from

March ]9,2007 and ApJi123, 2007 .

. BACT Analysis at MRYS .

B&McD's Apri/23, 2007Response:

There were pertrnent references in Appendix Al ofthe MRYSNOx BACTanalysis discuSsing long- ,

term examples where SNCRhas been appliedforNOx emission control on two cyclone-fired boilers, .

achieving reductions around 30 percentfrom·high uncontrolled baseline levels. There was also another

relevant cyclone boiler SNCR demonstration test listed in Appendix.A2 ofthe MRYSNOx BACTanalysiS

that controlledNOx in this same 3.0 percentrange. NOx control ef!ectivenesi ofSNCR is highly boiler-:- .

specific. It is important to note that PSJ) permit applications arefor new boilers; there are no new

cyclone boilers being built, andfew new boilers (other than CFB boilers) are required to i~tall and

operate SiVCR. The citedAP-42 NOx emissions rate reduction percentagefor SNCR application on coal~·

fired boilers isn't applicable to the specific conditions expected at the MRYS boilers.,../Jurns &

McDonnell estim(Jted the level ojemission reductions expectedfromSNCR bqsed on informationfrom an

· experienced urea-based SNCR vendor. B&fvfcD believes that 60 percent reduction o/NOx emissions is ..

inappropriate, andnot rejlecti~e or realisticfor long-term levels ofcontrol at MRYS.

EPA Response:

After further review ofthe information provided, EPA agrees that the 60% reduction cited

.at the upper end ofthe control range is likely not appropriate for MRYS. It appears likely that .

something closer to 30% than 60% would be reasonable. .

L. Selection of30-day Average BACT Emission Limit

EPA's January 26, 2007Comment:·

. .",.'.....,' .- '.

The baselli1e emission rateusedfor calculating the proposedBACTlimitis based:on the highest .

NOx emission rate (0.884IbIMMBtu)duririg actual operationsfrom 2001-2005.(p. ES-9) This period

also includes the longest reported stqriup event during that period (61hours)~ How does this 30-day .

baseline compare with the rest ofthe historical data? Does this emission rate represent a realistic upper·

bound value, or is it an outlier compared with the majority ofthe data? Jfihe d.ata point does not .

reasonably compare with the vast majority ofthe other data;"it does not make sense·to setaBACT limit

based on an outlier data point. Furthermore, the baseline emissions rate usedfor calculating the 30-day

· limitis baser! on the .highest 30-day period during actual operationsfiwn 2001-2005, which included the

· longest startup ev.ent. lfthis highest 30-day periodwas user! as the baseline to calculate the 30-day

average limit, and the worst-r:aSe startup event occurredduring this historic 30-dciyperiod,it is not

necessary to inclu4e the recommended "startup aq;ustment"{+O..D411blMMB,tu) per siartupevent with .this limit... .

B&McD's April 23,·;2007 Response:

According to MRYSUnit 1's October 2006 BACTAnalysis; the control e.!frctiveness calculations

were determined based upon the highest hlstorical12-month rollingNOx emission summation operating

conditionsfor MiltonR Young Station boilers during years 2001-2005.....MRYS operating conditions

-56-
EPA Response to MinnkotlComment Letters fro'm

March 19, 2007 and Apri123, 2007

BACT Analysis at MRYS .

during the highest historical 30-day rolling NOx emission periodsfor years 2001-2005 were then

compared to the highest 12-morUh'historic pre-control baselin.e period conditions. The average unit NGx

emission rate (lb/mmBtu) from the- highest historic pre-con"trol 30-day baseline periodwas compared with

the average NOx emission rate (lb/minBtu) from the 12-consecutive month periodwith the highest

historic unit NOx mass emissions (pounds) reportedfor actual operation during the same 5-year look-.

backperiod It waS coincidental that the highest historical30~day rolling NOx emission pe.riodfor MRYS

"Unit 1 (December 17, 2003'through January 15, '2004) was within the same highest I2-inonth historic .

pre-control baseline period (ending on October 30, 2004). The longest indiVidual Uriit 1 startup period

during actZ!al operation between January 1,2001 and December 31,2005 occurred during the 61·

consecutive hour periodfrom June 27-29, 2005 .~ .... The highest historical 30-day rolling NOx emission

.periodfor MRYS Unit 2 (March 7, 2003 through April 5, 2002) was not within the same hig~est 12-month

historic pre-control baseline period (ending April 30, 2004). The longest individual Unit 2 startup period

during actualoperation between January 1, 2001 and December 31, 2005 occurred during/he 115-

consecutive hour periodfrom October 25-30, 2001. Note that this did no(fa/l within either the 30-day or

the 12-inonth highest historic rolling average NOx mass emission periods. ... ,.. The selected highest

baseline pre-control 30-day rolling average NOx unit emission ratesfor MRYS Unit 1 and Unit 2 boilers

are considered to be realistic upper boUnd cases, not "outliers" ...... The recommended 30-day Rolling,

. Aver.age unadjusted(i.e. ste.ady-state) BACTNOx emission rates do not include the impact ofthe worstcase startup eventsfor either MRYS Unit 1 or Unit 2. The "startup adjustment~' as described In the

October 2006 MRYS Unit 1 and Unit 2 NOx BACTAnalysis reports is appropriate as the recommended

(unadjusted) 30-dayRolling Average NOx Emission Rate permit limitations basically reflect steady-state.

rates.

.EPA Response:

EPA has reviewed B&Mc;sdetailed description of how it established the baseline emission

rates and the startup/shutdown adjustment factor and· now understandsthat the startup periods

did not fall within the relevant baseline period. EPA will consider the startup adjustment concept

to the extent it is relevant to NDDH's proposed BACT determination.

M. BACT Limit IS-Month TrialPeriod

EPA's January 26,2007 Comment:"

EPA has conc.ems with Minnkota is proposalfor an 18-month trial periodfor establishing afinal

BACTlimit. (p. ES-ll) This proposal appears to be in direct conflict with the pr.ocessfor establishing

andpotentiallyrevisingthe NOx BACTlimits set/orth in Sections V. B & D ofthe Consent Decree.

Therefore, EPA believes the proposed I8-month tria/"periodfor establishingafinal BACTlimit in

Minnkota's BACTanalysis cannot be accepted

. .

B&McD's April 26, 2007 Response:

.....Paragraph 73 ofthe referencedConsent Decree states "The Settling Defendants shall submit

any petitionfor any Unit under this Paragraph no later than six (6) months after the final compliance

-57-
EPA Response to Minnkota Com"ment Letters·from

March 19, 2007 l,Uld April 23, 2007

BACT Analysis at MRYS

date specijtedfor that Unit in Paragraph 67". Paragraph 67requires that MRYS Unit land/Unit 2 shall

.achieve and maintain a Phase 1I30-Day Rolling AverageNOx EmissionRate established·bythe NDDH

through itsNOx BACTDeterminationbeginning no later than Decemb~r 31,2011 and December 31,:

20J0, respectively. This does notpreclude anearly installation ofConsent Decree Phase IINOx

emission reduction systems with associaied commissioning at Milton. R. Young Station prior to the CD . .compliance datesfor.MRYS Unit J and Unit 2, respectively.- This could allow Minnkota io gain· operating

experience in advance ofthe June 30,20JiandJune 30, 20Jldeadlinesforsubmitting a petition. lfthe .

Settling Defendants do not accelerate the schedulefor Phase IINOx control implementation; recogn.izing·.·

. that the CD apparently does not allow an 18~month trial demonstration periodfo.llowiJig the siatedjbial

. compliance dates; the applicant will 'have to make use 'ofthe petition process as de~cribedin the CD in .

. this circumstance, ifappropriate. . .. ..

EPA Respons'~: .

EPA agrees this.is an acceptable approach.

,.

-58-

. .