Via Certified Mail

Mr. Terry O'Clair

Director, Division ofAir Qualjty

North Dakota Department ofHealth

918 E. Divide Avenue

Bismarck, ND 58501-1943

Ref: 8ENF-L

UNITED STATES ENVIRONMENTAL PROTECTION AGENCy

REGION 8

1595 Wynkoop 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; Environmental Manager

1822 State Mill Road

P.O. Box 13200

",Grand Forks, North Dakota 58208-3200

Re: Transmittal ofEPA's Response to

Mirinkota's Letters Dated March 19, ,2007

and April 23, 2007, Regarding the October

2006 NOxBACTAnalysis StudyReport for

Milton R Young Station

Dear'Teny, D~vidand John:

, , .

'The United States Environmental ProtectiOn Agency" RegiorrS-(1WA) submits,this letter

and the Enclosure to respond to letters dated March 19, 2OQ7 and ApriL23", 2007 'authored by

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

.,(B&McD) BestAvailable Control Technology'(BACT) Analysis for Nrtrogen Oxides(NOx) at

Milton R. Young Station (MRYS), Units 1 and 2, prepared by B&McD forMinnkota (Minnkota

BACT,Analysisj. TIle above identified two letters were submitted byMinnkota in response1<>

letters sent by EPA to North DakotaDepartment·ofHealth (NDDli), dated .January 8 and 26,

2oo7~ and were also in response to a letter from the NDDH dated February,J;2007.

EPA appreciates the ongoing dialogue and exchange ofinformationwit6 NDDH and

Minnkota on these very complex and important issues related to the appropriate NOxcontrol

technology to be installed at MRYS Units 1 and 2. The meeting inBismarck on May· 23, 2007

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

your staffto seek a well-informed decision on this matter. The Enclosure contains a response

from EPA's consultant, Roger Christman ofEastemResearch Group, on technical issues related

to Selective Cata;!ytic Reduction (SCR) (Christman August 20, 2007 Memo). The ;Enclosure also

contamsEPA's detailed response to M.innlcota's letters dated March 19, 200~ and April 23,

2007.

Tobriefly sunimarize the Enclosure, the MinnkotaBACT Analysis is incomplete, and the

.determinations oftechnical infeasibility fonhe Iqgh-dust SCR, Low-dust SCR, Hydrocarbon

Enhanced Selective Non-Catalytic.Reduction (HE SNCR), Rebum with SNCR and Rich Reagent

Injection NOx control technologies are inC9rrect. Mitinkota has inCorrectly interpreted and

mi~pplied Chapter B ofEPA,'s "New Source Review Workshop Man.ual- Prevention of

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

..Workshop Manual) in its determin8tion that these control technologies are t~hnically infeasible.

Furthermore, MiIinkota has incorrectly utilized the results ofthe pilot testing conducted at the·

Coyote Station by the Energy & Enviromilental Research Center to find that SCR is inapplicable.

.MiI1nkota must evaluate these control technologies beyond step 2 ofthe pro<ress outlined in the

NSR WorkShop Manual. . .

The operational problems associated with the Coyote Station pilot test and the lack of

catalyst deactivation data greatly dimiIiish the usefulness ofthe teSt results in analyzing the

technical feasibility ofSCR at MRYS.EPA maintains that the resUltsofthe Coyote Station pilot

t~ do Jl()! d~1Il()~tra.t~~lIilica1 ii1f~il>ility()f ~mJOI" utility boilersfiriJlg )\IT0rtb.])a.lc<>ta

lignite. Minnkota has not provided information and/or 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 ofSCR at MRYS, EPA finds that the wealth ofexPerience in the utility industry

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

Minnkota bas not overcome this pr~mption. As.such, EPA presently finds that the ~ota

BACT NJalysis.is incomplete, • not been carried out in accordarice with Chapter B ofEPA's

NSR Workshop Manual, does not provide atl ofthe inf<:>rmation necessary for NDDH to make a

BACT determination and does not contain all ofthe information·requested by EPA·

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

entitle4 "APpropriateness ofConducting Pilot Testing ofSelective Catalytic Reduction (SCR)

Technology at Milton R Young Station Units 1.and 2, for use in a NO~ BACT Analysis" and.is

planning to review it. EPArenuUris committed to moving the BACT process forward so that

Minnk()ta can meet the deadlines specified in·theConsent Decree. Forthe BACT process to

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

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

·Minnkota to complete the Top-Down BACT analysis. for SCR apd the other control technologies

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

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

Sincerely, . fl

1Vt~bL,'9-~ Martin Hestmark, Director, Air and Toxics

Technical Enforcement Program, ~EJ .

2

.Enclosure

cc: Eddie Sierra, Deputy Assistant Regional Administrator, EeEJ

.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

(

.'

MEMORANDUM

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

EPA Region 8

SounJay Gairola, Ron Rutherford, EPAJORE HQ

FROM: Roger Christman, Eastern Research Group, Inc.

DATE: , August 20,2007

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

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

Meeting on May 23, 2007, at the Offices ofthe Division ofAir Quality, North

Dakota Department of Health (NDDH).

INTRODUCTION

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

assistance to EPA Region 8 in evaluating a series of documents prepared by Bums & McDonnell

(B&McD), and the Energy & Environmental Research Center (EERC) at the.Unlversity ofNorth

Dakota, on behalf ofMinnkota Power Cooperative, Inc. In response to the submission, in'

October of 2006, of a BACT Analysis for Milton R. Young Station (MRYS) and the appended

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

(Christman, January 2007) addressing issues raised in the BACT Analysis and the Benson paper . '

that were cited by Minnkota as rendering SCR technically infeasible at MRYS. My memo, along

with EPA Region 8 and NDDH questions and requests for additional. information were

forwarded to Minnkota.

Minnkota provided a response to the EI{G memo (Minnkota, March 19, 2007) and additional

responses and information about a month later (Minnkota, April 23, 2007). These m'aterial~ were

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

on May 23, 2007. A list of nine unresolved "core" issues were identified and were subsequently

discussed at the May me~ting. They are addressed qelow. Also, some ofthe discussion dl:lfing a

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

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

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

not discussed (e.g., popcomash).

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The nine "core" is~ues include:

1. C~nter Mine ash content variability

2. Catalyst deactivation by sodium and regeneration difficulties.

3. Downtime for catalyst cleaning and change out. .

4. Sticky ash. .

5. EE~C pilot test procedures/design.

6.. Split ait 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 th~ May meeting, B&McD representatives and Dr. Benson continued to refer to the extreme

coal variability, both in proximate. analysis and ash constituent 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 is ..

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

with none oftbe mQre than 700 other samples exhibiting an ash content gi;eater than 16%.

Examination of other data associated with the sample confirms that the ash figure is in error.

Despite having a roughly average moisture content, the heat content ofthe sample in question.is

also roughly average. Ifthe 25.5% ash figure was correct, the sample would have the lowest heat

content ofthe total data set. It does not. In conclusion, the Center Mine coal does not have.

. unusual variability in ash content and in no case has an ash content five times· higher thanPRB

·coal as B&McD has contended. .

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

variability in the "acid-to-b;:Lse ratio" ofthe ash (pages 7-9,. Minnkota, March 2007) as having'

relevance to the technical feasibility of SCR as applied to North Dakota 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 need to provide supplemental fuel.

Although the ash fusion temperature and the ash viscosity at cyclone barrel temperatures (>2,500

. F) are functions of ash composition and thus are sensitive to· variations 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 connection between his discussion of base-to~acidratio

and the functioning of anSCR system. B&McD also only mentions impact on "operation ofthe

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

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

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never heard or read anything about acid-to~base ratio of ash in any SCR document or discussion,

prior to Dr. Benson's introduction ofthe subject.

2. Catalyst deactivation by sodium and regeneration difficulties.

Dr. Benson.cited a paper by Khodayari as evidence that catalyst poisoned 'by potassium could not

be regenerated and that the same would be true of sodiuIn~poisonedcatalyst (reference 12,

Minnkota, March 2007). A careful reading ofthe paper shows that Kh04ayari itchieved 90% to

110% recovery ofthe original teactivity ofthe catalyst. Further; Khodayari's PhD dissertation

states that potassium, not total alkali, is the problemfor SCR application to· biofuel combustion.

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

'Khodayari's work as evidence that catalyst activity can be recovered by washing and will result

in reduced cost for biomass applications (Alstom, l.indated,--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 cites

potassium and phosphorus as the primary poisons associated with a range ofhio-materials that

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

relative' contributions of phosphorus, potassium and sodium. His subsequent work focused

·entireIyon phosphorus and did not address'sodium at all.

·A 2006 presentatio~ covering work conducted by the University ofUtah and REI on the

application of SCR to low-rank coals concluded that "chemical.poisoning is not important" for

low-rank coals; but masking by fly ash is a problem. The study further concluded that the

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

While Dr. Benson concludes that the sodiUIT).levels in North Dakota Lignite ash are so high as to

render SCR technically infeasible, apparently others at EERC do not share that opinion and have

researched the co-firing ofbiofuel.s with· low-rank coals (EERC, Folkedahl, 2096). The intent of

the study was to determine ifthe higher levels of alkali materials' in the biofuel would acc'eIerate

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

states: "Howevyr,.little is knoWn ofbicimass firing with low-ranked coals, such as North Dako.ta

Lignites and Powder River Basin coals." The report concludes that, although they experienced

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

In'summary, Dr. Benson misread the work of Khodayari regarding regeneration ofalkali~

poisoIl;ed catalysts. Both a careful reading .ofthe cited Khodayari paper and some of his. other

publications confirm this, as does the Alstom paper and several recent papers by the. catalyst

regeneration vendors (Full-Scale Catalyst Regeneration Experience at the Coal-fired Indiantown

·Ge~eraiion Plant, Bullock and Hartenstein, October 200J and Regeneration Advances,

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

in the U.S.- What Have'We Learned?, Cichanowicz, Muzio and Hein, 2006). 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 Uni:versity of Utah and REI

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further implicates potassium as having a greater poisoning potential than sodium. Although the

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

deactivation), Dr. Benson provides no evidence that it will render SCR technically infeasible at

MRYS.

As a final note, since 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 any installation 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 of·chemical poisons.

This is because the total exposure will be less than 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.

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 3 day turnarm;md that

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

Additionally, the boiler cleaning 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 discussions with

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

change out by providing wider than normal' platforms at each catalyst layer level, thereby

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

assumes a rather extreme short catalyst life of one 'year and ifthe change out is coordinated with

a boiler cleaning outage, the SCR unit would only contribute one or two days ofdowntime per

year..

4. Stic.ky ash.

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

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

and thus tended to accumulate 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

.soo.tblowing (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

topsofI-beams and angle-iron members that are above the catalyst layers) and aim sonic horns 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 of which remains valid. In summary, the catalyst s~ould

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have been cleaned with some frequency so that it could be continuously exposed to flue gas.

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

simulilting sootblowing was not 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 falling onto the

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

rom) was too small. S&L presenters suggested that a·l0 mmpitch might be required for North·

Dakota Lignite applications; .

In my January memo and at the May 23rd 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

S&L discussion on May 24th, Bill DePriest was asked, by an NDDHparticipant, 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 discussi9n ofthespl.it air preheater approach contained in the Minnkota March 19th

document seems a bit ambivalent, implying 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 fact 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 during the

meeting on May 2Jd

, but they did insist that BACT would not require substantial rebuild ofthe

source (boiler).

7. B&McD low dust SCR assessment.

I am riot qualified to comment on the B&McD discussion oiihe steam extraction for flue gas.

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

warranted, since ash issues should be manageable.

8. Texas Lignite comparison.

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

regard to erosion. S&L personnel 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 futUre. Also, by including a discussion on

t1?-e enforcement situation, it is clear that the requestor is looking for support for the technical

infeasibility argument. The ability to provide t1ue gas at the appropriate temperature is not

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discussed and the startup/shutdown requirements may be unreasonable. I do not expect the

exercise to produce any meaningful results.

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ENCLOSURE

RESPONSE TO,EERC AND C AND B&McD's MARCH 17,2007 LETTER

, RESPONDING TO EPA REGION VIII COMMENTS - JANUARY 8, 2007

. I. 'Introduction and Summary ofEPA's Preliminary Analysis

,EPA's January 8, 2007 Comment:

EPA ;s preliminary analysis is that th~ B&Mstudy lacks sufficient information within theBACT

analysis. Specifically. the study e~cludes Selective Catalytic iedl{ction ($CR) as a technicallyfeasible

control option at step. 2 ofthe five-step Top-Down analysis, whil~ notpresenting NOx conirol

effectiveness or catalyst activity data. As a result.' B&Mfaiis to rankSCR against i,Jther 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, EPAfound there are vendors that will offer'

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

the analysis with respect tc! SCR.

B&McD's March 13~ 2007Response:

, '

... ....The EPA's NSR Manualdoesnot specifically define (heoperatlng'parameter or performance

, measure that aparticular NOx control technique ortechnology would have to achieve 'to satisfy "a

technical assessment consideringphysiCal. chemical and (fngineering principles and/or empirical data

showing the technolflgy would not work on the emissions uni! underreview orthat unsolvable technical

difficulties wouldpreclude ih,e successful deployment ofthe technique" in orderfor said alternative to be

excludedfromfurther 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 technicalpaper. The evidentfailure ofthe

Coyote slipstream SCR'catalystfield test to avoid severe-,catalystfouling andash accumulation led Burns

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

catalyst in~tallation during operation on aNorth Dakota lignite-jired cyclone boiler. aggressive on-line

cleaning andlorjrequ?nt extendedforced outages would be needed to remove ash accumulqtion or,

remove the catalystfor cleaning in orderto restoreful/ unit output and catal)l$t NOx reduction

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

to befrequently replaced. ' '

Two monthsbetwe.en SCR catalyst maintenance activities woulc! be an extremely short operating

cycle time compared to 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 under review"for technical infeasibility purposes. Eliminating technically-infeasible'

alternatives priorto ranking thefeasible controls based upon control effectiveness. and then evaluating

the impacts'ofimplementingonlyfeasible control techniques and technologiesfollows the BACT '

process..;... '
, ,

EPA Response to MinnkotaComment Letters from

March 19,2007 and April 23, 2007

BACT Analysis at MRYS

EPA Response:

As articulated in the January 26, 2007 letter related to other widely-used NOx control

options that were eliminated' in Minnkota's BACT analysis based on technical infeasibility, 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

presumptien is based on the "technology transfer" discussion, in the NSR WQrkshop Manual,t

otherEP....guidancedOcliments,2 and the' factthat hundreds ofSCRsystems have been " ,

successfully installed on utility boilers worldwide. The NSR Workshop Man~al and other EPA,

guidance clearly places a substantial bqrden ofproofon the applicant to show that, based oli a, comparison ofthe ,physical and chemiqal characteristics ofthe exhaust gas stream under review

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

technically infeasible. EPA agrees that the NSRWorkshopManual does not specifically define, '

the operating parameter or performance measure that a partIcular NOx control 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 considerable on the part ofMinnkota and should include quantitative evidence that such: '

unresolvahle differences do exist that would render the technology unsuccessful at controlling

NOx• "

,The qualit,ative evidence provided to date by B&McD and EERC demonstrates tha,t there ,

are certain factors relevant to MRYs that will requireengirieering design considerations to make

an SCR installation workable; however,'considering each SCR system design for a specific

facility has to be customized to some extent, this situation is hardly unique. Additionally,

nothing has been provided that would demonstrate that SCR would be an ineffective means of

NOx control at MRYS. 'B&McD'sbeliefis that the pilottest results from Coyote Station show,

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

I See NSR Workshop Manual page B.19: "Unless significant differences betWeen source types exist that are

pertinent to the successful-operation ofthe control device, the cO!Jtrol option is presumed to be technically

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

2 See June 15, 1989 letter from Gary McCutchen (EPA Office ofAir Quality Planriing imd Standards, New Source

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

, po'int worth mentioning is the a'rea 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 in an effort to avoid certain

controls. For example, an applicant might say that NO" controls have been applied to a 30 and 70 MW coal boiler,

but not Ii 45 MW coal boiler; that the control technology has been applied to pUlverized and fluidized, bed units, but'

not to spreader stoke~s; or that the technology has never been applied to the particular mix of, say W90d 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 (ifthe control tecbnologybeing analyzed is an add-Qn¢ontl'ol), but also why'

that difference precludes transfer ofthat control,technology to the proposed source, The burden of proofshould be

relatively high in order to prevent circwnvention ofreasonable technology transfer by the selection ofsome slightly

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

-2..,.;
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

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

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

acknowledge that such real costs aSsociated witIi operational realities ofthe boiler and'fuel type

in an SCR application at MRYS can·be accO\:lIlted for through the costevaluation under step 40f

the BACT analysis.4 EPA does not view additional maintemmce and operational measures that

would be required at MRYS compared to oth~r facilities to be justification oftechnical

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

ofSCR for a utility 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 cat~lyst maintenance activities" every ,

two months and that this constitutes "empirical data showing the·technology would not work on

the emissions unit under review.,,5 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&M;cD'suse ofthe Coyote Station pilot testing

maintenance requirements is an accurate means ofpredicting 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 accumulation, catalyst plugging,. and pressure drop increases

occurred during the c~)Urse ofthe 'six-month test at both facilities. BaldWIn and Columbia'

Stati9ns both bum PRB coal, which is also burned at many full-scale utility boilers currently

operati'ng SCR systems with tYpical maintenance cycles. EPA believes B&McD's conclusions

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

relevant for an analysis oftechnical feasibility, and fails to consider the observati.ons on catalyst

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

tests. A comparison ofthese findings for all three facilities is presented below...

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

removed and replaceq with a new one.,,6 The narrative indicates that catalyst plugging at

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

incr~sed.,,7 .The report describes that during the rem~nder ofthe test (months 2-6), pressure

drop was highly variably but maintained within a range of 0.6 and 0.8 inches ofwater "due to the

installation offre'sh catalyst sectio~ and leaving two thirds ofthe catalysts in place that were.

3 See pages B.l9 and B.20 ofthe NSR Workshop Manual: '''Where the resolution oftechnical difficulties is a matter

ofcost, the applicant 'should consider' the technology as technically feasible." .

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

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

5 See page B.2.0 ofthe NSRWorkshop 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 MRYS.

-3-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and Apri123; 2007

BACT Analysis at MRYS

partially ph.Jgged. The gas velocitY'in the single section ofnew, ~lean catalyst was high because

of channeling, and the result pfthe high gas'flow was less deposition and accumulation. Gas

velocity has a significant impact on the ~otential for deposits to forni. However, at high gas

velocity, low NOxconversion is likely." Hence, based on B&McD's reasoning, the SCR pilot

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

, installation on PRB facilities would be infeasible, especially at the typical 36.:.month operating

cycles for major mainte~ance. In fact, the EERC report suggests that when a section of catalyst

at Baldwin was changed out 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&McD would presumably conclude that the

pilot testing at BaidwinStation showed that aSCR system, operating on PRBvoal would require

significant and frequent catalyst maIntenance and replacement and that the NOxreduction would

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

, The final test report describes the catalyst reactivity test results forcataIyst samples at 2, 4~

'and 6 months ofoperation 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 after six months, the reactivity had ,dropped to 84% ofthe reference catalyst." It

does not appear there is any real correlation between the pressure'drops observed at BaldWin

Station and the measured activity ofthe catalyst. The pressure drop did not ste~dily increase as'

the reactivity decreased during. the 4 an~ 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 ofwater. The

,most prolonged period during which the pressure drop was higher than 0.8 inches ofwater was

during the 'first two months, when no deactivation occurred. '

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

replacement within the six-month p~riodofthe test. The EERCreport 'in Appendix B' ofthe

Minnkota BACT Analysis does not provide as much information as is contained in the Fuel

Processing Technology article describing the same pilot testing. The Fuel Processing

Technology article includes a graph of pressure drop for the entiresix months oftesting, whereas

the Appendix B EERC report only shows the pressure drop graph for the first two moriths of

testing. It is unclear why this additional data was not part ofthe Append~x 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 thePuel Processing Technology article, tpere was "a rapid

increase in pressure drop across the catalyst at about 30QO h ,of operation, and aggressive pulsing

brought it down to 0.4 in. ofwater until the catalyst section was'changed out at about 3200 h.

After c!eaning the reactor '~nd replacing one catalyst section, the pressure drop was about 0.3 but

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

9 Final Report for JV Task 31 - Evaluation of Potential SCR Catalyst Blinding During Coal Combustion and Add- ,

On: Impact ofSCR Catalyst on MeI:curyOxidation in Lignite-Fired Combustions Systems, November 29; 2004,

page 41. "

-4-
EPA Response to Miririkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT.Analysis at MRYS

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

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

but rapi<lly increased to over.O.S in. ofwater within 500 h."IO Most ofthis paragraph describing

the entire 6 months ofthe test does appear in Appendix Bofthe EERC report, with the exception

ofthe first sentence•. In any c~e, 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 four 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. II Again; there does not seem t9 be'a clear

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

Station and the catalyst r:eactiv'ity testing. During the first two months, pressure drop readings

were generally between 0.4 and 0.6 inches ofwater. During months two through four, pressure

.·drop was generally higher, between 0.6 iUld 0.8 .inches ofwater. During the final two months,

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

appear that there was any .clear relationship"between degree of pres~ure drop and the level of

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

significant accumulation and plugging th~ the Baldwin Station,,,12 the catalystreactiyityfor

Baldwhl Station decreased more than at Columbia Station over the course ofthe six months of

pilot testing (84% vs.90%). Again, it does not appear that the Q1Jalitative observations from the

pilot:study for.Bald\\'in 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~ure'drop observed during

the pilot test were more pronounced and.occurred qui~ker than what was observed at the !.\V0

facilities burning PRB coal. Additiona1ly~ based on the"deposition sampling,EPA agrees there is.

evidence that sodium was a factorin the plugging 'at Coyote, Where it did not appear at the other

two facilities.' 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 .

Minnkot~'s 'March 19, 20Q71etter. 13 The results ofthe catalyst deactivation tes,ting 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 Jlily 2004) . .

11 Babcock-Hitachi K.K "Test Results ofCatalystSamples for Energy & Environmental Research Center", April,

2005, Table 3. .

12 Final Report for N Task 31 - Evaluation of Potential SCR Catalyst Bliilding During Coal Combustion and AddOn: Impact ofSCR Catalyst on Mercury Oxidation in Lignite~Fired Combustions Systems, Noveinber 29, 2004, .

'page 29. .

13 Page AI-30 ofOctober 2006 Minnkota BACT Analysis Study for MRYS indicl!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 Letters from

March 19,2007 and April 23, 2007

, BACT An'a1ysis at MRyg ,

and Columbia ~tations showed no clear correlati6n between niagnitud~ of pressure drop and the

, degree ofcatalyst deactivation at each plant. Also, the qualitative,observations on catalyst',·

plugging did not correlate to the degree of catalyst deactivation for..J3aldwin and Columbia

Stations relative to one an()ther. F~ermore, given th~t the two facilities burning PRB coal

required ,similar maintenance measur~s and schedules to Coyote Station, EPA questions how the

,maintenance activities·(and their timeframes) required during the pilot testing at Coyote Station

, 'can be significantly distinguished from the Baldwin and Columbia Stations burning PRB coal.

EPA does not agree that it is correct to defIne catalyst replacement requirements associated witli,

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

.Additionally, based on a July 2, 2004 conversatiop. between Jim Standt ofAndover

Technology Partners 'and Flemming Hansen ofRaldor Topose, the sudden build up offlyash on

the catalyst face that occurred during the piiottesting at Coyote StatiOJi was determitied to be '

caused by the accumulation of ash 'upstream ofthe catalyst and su,dden dropping ofthat

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

pilot test atCoyote Station might have been.largely due to cOl1ditions that would not exist, and/or

could be avoided thtough design, on a full-scale application ofa SCR system. This.further calls

into question the applicability ofthe pres~ure drop trendsexperi~nced during the pilottesting to

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

'Based ort the above comparison ofthe pilot testing results presented by EERC, it is unclear,

to EPAthat any definitive conclusions can be extrapolated from the results about SCR technical

feasibility and/or the timing or extent.ofSeR maintenance requirements and boiler, outages for '

these three facilities, as was done by B&McD for MRYS. Cle'arly~ 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 fuIl...scale

operation, would not be consistent with real full scale experience at ntuneious boilers' burning

PRB coal. EPA knows that there are suc~essfuI ~pplications of SCR systems to utility boilers'

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

catalyst maintenance and replacement experience from the pilot test to ,3 full-:scale

application at MRYS. .

eased on B&McD's assertions about' expected mairitenance requirements and schedules for

MRYS based on the Coyote results, a facility burning PRB would also be expected to enc'ounter

significant plugging, which would require aggressive on":line'cleaning, catalyst replacement, and

forced boiler outage periods withi]) the initial months ofinstallation. EPA knows from ,

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

been dealt with sU:ccessfuUy~ Therefore, EPA believes the pilot t~sting was either designed in a

way that does not accurately reflect full-scale SCR catalystplugging~d/ormitigation measUres.

i4 August 20, 2004 Memorandum from Jim·Staudt (Andover Technology '~artners) to Sikander Khan (EPA), "SCR

catalyst testing on ND Lignite"

,-6:'"
EPA Responseto Minnkota Comment Letters from

. March 19, 2007 and April 23, 2007

, BACT Analysis at MRYS

currently known 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, although it was mentioned in the preliminary EPAcomments,

B&Mcb h~ .not provid,ed a response to EPA's statements regarding the normally observed

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

arid how a larger pitch might have affected the test results, and how the air pulsing did not

properly represent sootblowing capabilities on a full-scale SCR application. In sumriiary, 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 catalysta;nd SCR design companiesto

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

r~quirementsand schedules. .

EERC March 19, 2007Response:

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

performance·guarantees., AfterdescYibing the characteristics ofCenter lignite, specifically issues related

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

· became evident that EPA representatives may luiv.e not indicated the levels ofs,odium and theform of

sodium in the coal. Haldor Topsoe indicated that the sodium was,a major concern and that it causes

deactivation, especially the organically associatedform.: They indicated that they had some experience

· 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 and the nature ofthe SCR pilot testing at the Coyote Station conducted by EERC.. EPA

, asked Mr. Hansen about the effects ofsodium on'SCR 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 would need to be replaced or washed/rejuvenated.

Mr. Hansen indiCated that the only experience.Haldor-Topsoe has 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 (Wf: Energies) Presque Isle

Station, which burns Powder River Basin coal with sodium levels siinilar to North Dakota

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

test coupons upstream ofthe particulatecontrol 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 .

BA~T.Analysis at MRYS·

. .

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

because the reactor became quickly plugged with flyash causing rapid iricreases in pressure drop..

. Mr. -Hansen said that the pressure drop was mainly caused by duct-wall ash accumulations .

dropping on the catalyst face. Mr. Hansen recalled that problems with the·sodium poisoning

were 'secondary compared to the ash plugging problems. H~ stated that ash plugging is' a

problem ofpilot-scale testing and would not be a real full-sqale application Issue. H~ said that he

was aware that actual operations at plants burning PRB coal perform much better than the pilot

tests would indicate.'

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

Mr. Hansen 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 of ash plugging.

Additionally, he said the pilottest d.id·n<;>t have good simulation offuli-scale sootblowing. 'Ifthe .

test were to be redone, a better effort could alSo be made to keep the catalyst face Clean. , . " .

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

determining whether SCR would be technically feasible on North Dakota lignite. Mr; Hansen

said that pilot te$tsoften have probh~msthat can be overcome at the full scale. Mr. Harisen said'

that Haldor Topsoe wouldnot"nile out bidding on a 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 indiqated by the analysis. Mr. Hansen said that there ~'lire many tools in the . . . toolkit" to address SCR technical issues. For North Dakota lignite, with its high ash and sodium

content, Mr..Hansen said that Haldor Topsoe would likely considersuch options as screens, very

good soot blowing (steam assis~ed) 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 ofabout every year.

EPA asked 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 said andlor implied

that the coal profile information and the resul~s 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 unawar~ that Haldor Topsoe sent emailsto·EPA Headquarters stating that

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

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

SCR catalysts for North Dakota lignite assuming certain criteria are met. He had also talked

with that consultaIit at that time about the fact that pilot plants have problems that full-scale SCR

systems do not. ..

II. EPA Case-Specific Review ofBurns & McDonnell & EERC Technical Feasibility

A.nalysis for Selective.Catalytic Reduction

·A. High-Dust SCR

B&M has eliminated High:-Dilst SCR as technically infeasible due to issues described

.below. B&M's·analysis utilizes a report by the Energy & Environmental Research Center

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

1. Ash Deposition Issue Raised by Burns & McDonnell:

EPA's January8, 2007 Comment:

The concern about high amounts ofash material callsing blinding ofmacro-pores'in the catalyst is

real butsolvable. Strides in catalystformulations, catalyst managementsystems, and recently, catalyst

regeneration processes, have all combif!edto lower the overall cost contribution ofboth initial and

replacement catalyst hi·modern SCR systems.

EERC March 19, 2007Response:

EPA states that the high amount 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 the problems ofhigh:·sodium coals with regard to advancedformulations and

catalyst regeneration, and he was not aware ofany work that had advanced SCR systemsfor high-sod,ium

applications..

.. Regeneration has been attempted. in Europe 'with SCR catalysts ~xposedto biomass with high·K

_(sin'Jilar in chemistry to Na), and they hadsome significant problem in recovering reactivity.

-9-
EPA Response to Minnkotil Comment Letters from

March 19, 2001 and April 23, 2007

BACT Analysis at MRYS

EPA Response: ..

EPA is also n~taware that any catalyst formulations have been·de~eloped to· deal

specifically with the issue ofsodium: H.owever, the reference cited by EERC (Khodayari and

Qdenbrand, 2001) does not indicate that the biomass plants had signific·ant problem~ jn reactivity

j recpvery. As indicated in the August 2007 Christman Memo, the Khodayari paper actually

indicates that 90:-11 0% ofthe original catalyst,activity was recoveted~

B&McD March 19, 2007 Response:

It is unclear whether EPA's claimed SCR catalyst improvements citedwould translate into ..

·preventing the catalyst layersfrom being susceptible to macro-pore blinding andplugging by calcium~

basedcompounds and the impact on the individual catalyst reaction sitesfrom sodium:'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 long currently available SCR

catalysts can survive andprovide effect~ve NOx reduction when exposed/o thejlue gas andjlyash

constituents exhaustedfrom combustion and.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 ~esponse: .

As described earlier, the burden ofproofis on Minnkot~to demonstrate SCR would be

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

· demonstrate that a technology would work for the exact type ofinstallation um,ler review,

especially when thte technology has been·well docWnented to be technically feasible for the ,

· source category under review.

EPA had asked Minnkotato look·further int~ the engineering solutioris·currently available

and determine whet~er 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

niuch consideration. Without addressing the available engineering solutions to deal with'macro-'

pore blinding and plugging, Minnkqta has not met the significant burden of proof demonstratiqn

EPA believes is required for showing SCR is nota technically feasible contr~loption for NO" at

MRYS. . ..

a. Comparison o[MRYS to SCR Systems Operating on Powder River Basin Coal

. .

EPA's January 9, 2007 Comment:

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

particularlyfor the specific constit1!ents ofconcern for catalyst deactivation, the quantity ofash in ihe ..

Center Mine coal is more than twice as high on aper MMBtu basis. However, for a cyclone boiler, only.

-10-
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 qsjly ash atfull load (less at lower loads) andfor a wall or corner-fire4

pulverized coal boiler (PC boiler), about 80% ofthe ash ends up asjly ash. The result is thatSCR

catalyst in a PC boiler PRB application is exposed to sif!lilar quantities ofthe major constituents of

qoncern (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!gh EPA does not believe this to be the case andB&Mhas notpresented any

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

nevertheless examinedB&M's arguments related to" 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 pe~ Btu basis,andthe conversion toflyash will be 30-50%for cyclone boilers and 80%for

PC boilers. As can be seenfrom this comparison, the theoretieel!flYaSh loadingfor a PC bqilerfiring

,PRB boiler (OAlblMlYfBtu) will be iTl the same range as what wouldbe expectedjor a cyclone boiler

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

exposure, and the PRB PCboiler data are extremely relevant to evaluating SCRfeasibilityfor North

Dakota lignite... .... Table 1 showsthat; on a IblMMBtu basis, total theoreticaljlyashloading at the

economizer exit ofci cyclone boilerfiring lignite would be very similar to thatofa PC boilerfiring PRB

coal.

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

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

containing partielf!. getting "caught" in a macro-pore ofthe catalyst andsubsequently reacting with S03

in thejlue gas. .The 'resulting CaSO, particle is larger in volume than the CaOpartiele and, in essence,

swells tofill the pore PRE'coal has been shown to have approximately'30% more caldum in its

ash compared to North Dakota lignite ~.Other alkaline maierials (potassium, sodium and ,

magnesium) deactivate catalysts by deactivqting individuatreaction sitesrather'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 somewhat tower than most PRE coals

andwithin the envelope ofexperience ofEuropean installations. One ofth~ refer~ncescited in the

Appendix B EERC report, Cichanowicz & Muzio (2001), places the range ofEuropean SCR ixperience

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

potential compared to other coals in use at,SCR-equipped boilers in Europe and the u.s... .....,.....

The concern about high amo'!nts ofalkaline materials in Cen!er Mine coal expressed in B&M's

BACTanalysis and described in the Appendix B EERC repor.t isreal, though not necessarily unlike

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

successful operating experience realized at cur-rent SCR installation at PRE-firedpowerplants. The

problems cited in the BACTanalysis are being dealt with atseveral NO:x'SIP Call SCR installations

today. In addition, the analysis presents no quantitative analytical iliformation (lerivedfrom the firing of

lignite coal on SCR catalyst deactivation tojustify thefinding that SCR islnfeasible.

-11-
EPA ~esponse to Mi~nkota Comment Lette~s from

March 19, 2007 and April 23, 2007

.. BACT Analysis at MRYS ,.

EERC's March 19, 2007Response:

The sodium and.ash c~ntents ojCenter lignite ·are significantly differentfrom i?RB coais. EPA

, indic:ates that the problems.ofdeposition will decrease because the cyClones take outsome ofthe ash as .

slag butforlignzte,cyclones create additional deposition problems downstream. The cyclones partition

the more refr(1ctory ash comp()nents into the slag and the more volatile components into thefly ash. The

fly ash becomes enriched in cOllJponents that vaporize anc4 subseque~tly, condense upon gas cooltng to. .

form liquidphases and small'particles that are veryreactive to gas-phase components thcifcause bonding.· . . .

, . .

EPA· discusses ash loadings and the potential impacts on SCR performance. They also ((iscuss the

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

materials (potassium, sodium, andinagnesium) and their pptential to deactivate individualsites rather

than blinding pores. . Theyfail torecoinize the formation ofcombinedphases.suchas sodium caldum

sulfates with low melting points that are·very reactiv.eand can both blockpores and deaCtivate individual

sites in catalysis. They also fail to recognize that cyclones partition the ash" increasing the proportion ·of

sodium in the smaller particles, resulting in increasedpropensityfor the formation ojthe complex sulfate .p!:lases. ' . .. ..

Tops,oe presented a paper at the u.s. Depariment ofEnergy (DOE) National Energy Technology

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

biomassfuels that contain high potassium andsodiurnfound rapid deactivation in the SCR catalyst.

Their daia suggestsodium or potassium resembles the ammoniumion and blocks access to the active

sites ~n the catalyst. They indicate that this occurs /?1ost readily where the alkali elements are in a soluble

form. Topsoe incorrectly assumes that all alkali in coaUsrio.t soluble because ofwork they did on

. potassium. Potassium is associated in the coal with clay mineralsand is notsoluble. Sodium in North

Dakota lignite is organically associated (solilbleform) andis readily availa~lelor reactions with

catalysts andflue gas species.· More recent work conducted by Beck and oth,ers (2004) e~tablished that

sodium correlates with catalyst deactivation activity and that pore condensati(Jn ofsodium is th'e major

deactivation mechan,ism.

, Sodium levels at MRYS have a multimodal distribution, a~d assllming the average sodillm level in the ash

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

EPA Response: .

EPA does not dispute,EERC's description ofthe chemical and physical fate ofsodium in I;l

cyclone boile~. Please.refer to the'A\Jgust 20,2007 Christman Memo Iterrl#2 for the response,

on the effects ofsodium on a SCR,system and the ~ppropriateness'ofusing average coal .

constituent.values when evaluathig their effect on an SCR system;

.-12-
EPA Response to Minnkota Comment Letters from

. March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS

.EPA'soriginal analysis was in response to B&McD's· quantitative comparison ofsodium

ash loading presented in the original BACT analysis. IS This comparison uses.average fuel ash

and sodium values to calculate a sodium ash leading rate of7.3 lb/MW-hr for North Dakota

lignite compared to 0.9 lb/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 PRB units, which would have a

higher ash loading rate than a cyclone unit. This information is relevant considering SCR

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

.. conversion offuel-ash to flyash for PRB-fired ~Yclori.eunits and 50% for lignite-fired cyclone

units. Using ·the 80% conversion offuel ash.to flyash for a PRB-fired PC unit'increases the

sodium loading presented by B&McD to 2.1 lb/MW-hr.· [(0.9Ib/MW-hr / 0.35) = 2.57 lb/MWhr (sodium in the fuel aSh for PRB coal) * 0.8 conversion offuel ash to flyash in a PC boiler = . 2.1 Ib/MW-hr .(0.9 Ib/MW-hr· is the sodium loading for ;PRB· coal calclllated by J3&McD on a

.cyclone boiler assuming 35% conversion offuel ash to flyash)] The other reason EPA provided

this analysis was to include the entire set ofash constituents'for the quantitative comparison. , . . ..

EPA presented this comparison with some reservations,~ince, 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 ofonly the sodium ash IQading from lignite-fired cyclone units to PRB.: .

fired cyclone units ignores the PRB-fired PC units which have the higher fuel ash to flyash .

.conversion and other·ash constituents that do effectSCR operations. On its.surface, B&McD's

.analysis portrays a sodium loading difference between North DakotaJignite and PRBcoal that is

greater than·a factor ofseven. This difference becomes less when including PRB-fired PC- units

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

· not believe there is a linear relationship between this ash loading and SCR deactivation rate.

However, since B&McD presented a theoretical sodium loaditig comparison between North

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

was significant. in the .discussion oftechnical feasibility ofSCR at MRYS, EPA wanted to

inc~ude a more complete comparison.

15. Seepage AI-29 ofOctober 2006 Minnkota BACT Analysis Study for MRYS.

16 In the March 19,2007 response (page 28), EERC disl):1isses EPA's.analysis. "Sodium levels at MRYS have a:

multimodal distribution, and assliming the average sodium level in the ash in notrealistic." Note that the analysis

presented by B&McD uses averagefuet.characteristics for North Dakota·lignite and PRB coal. . . .

-13-
EPA Response to MinnkotaComment Letters from

'March 19,2001 and April 23, 2007

BACT Analysis atMRYS

EPA acknowledge~ that the sodium content ofthe Center Mine lignite is generally higherth'lIi PRB'coals.EPi\. also recognizes that ash partitioningin a cyclone boiler and resulting'

sodium deposition' presents certain problemsthat could,pos~ engineering ~hallenges to the design

and operation ofan SCR system at MRYS. However, Minnkota has not provided any , '

quantitative data thatshows that the plugging"and deactivation ofthe SCR system would be an '

,unresolvable technical barrier and/or the ability ofthe SCR to control NOx would be,so limited

that SCR would be an ineffective contr()l option. Again, EPA thiriks that both ofthese issues

would be more appropria~ely addrl:'ssed at step 4 ofthe Top-Down BACT analysis.

, ,

The Topsoe 1999 paper referenced by EERC does observe that biomass having.C:} high,

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

'deactivation in the SCR catalyst." It is u:nclear how these soluble sodiUm or potassium

percentages 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 inappropriate to 'draw any meaningful conclusions "

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

B&McD's March 19, 2007Response:

The discussion in the EPA's pre.liminary analysis ofMinnkota's NOx BACTAnalysis atMRYS

ignores the BACTreports'statements that the firing oflignite coal in a Cyclone boilerspecifically"

designed and built to pre-dry and burn North Dakota lignite producesfine jlyash particles which will

enter the pores ofthe SCR catalyst" react with S03 in theflue gas, andform sulfates which bind other ash ' particles into the matrix. ' .

When considering the much'highersodium cimtent o/Center lignit~ coal. ash(lb/mmBtu) compared

to PRE coed, the impact on the individualcatalystreaction sitesfrom sodium-bas.edcompounds discussed

in Dr. Benson's referenced technical paper hils largely been ig"qred by the EPA's' '

commenters .

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

any cyclone boilerfiringNorth Dakota lignite appear to be unpreventablefor suc.cessful application of

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

commercial SCR installations on large coal-fired boilers with similarly high sodium levels. The iendenCy

ofs.mallsodium vapor particles to be generated during the combustion and slaggingprocesses ofa North'

Dakota lignite-fired cyclone,boiler has not been, adequately addressed by the EpA c.ommfmter'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 dataforthe Coyote slipstream

SCR catalyst.fieldtestpreclu.dedBurns & McD9nnellfrom evaluating or including it in the NOx BACT'

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

produced a recent study on the impact ofcoal type on SCR catalyst lift andperformance... ...... This '

studyfound that the Sioux plant's cyclone unit.firing the PRBlbituminous coal blend exhibited the fastest

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

economizer exitjlue gas temperatures being significantly higherthan at the othersites.

Burns & McDonnell believes that thejlue gas andjlyash conditions produced during the

, compustion and slagging processes ofa North Dakota lignite-firedcyclone boiler will be significantly

more proble~atic than the tests at Sioux Station demonstrated..~... '

EPA Response:

Please refer to the A~gust 20, Z007 Christman Memo Item #2 for the response on the

effects ofsodium on SCR. EPA's original coIilment does not largely ignore the statements in the

Minnkota BACT analysisa1;>out higher sodium content and its inipact on catalyst deactivation.

EPA's analysis looked'at the combinedpercentages 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 analysis and '

discussion above.

B&McD states that E:i>A has not adequately 'addressed the high Center Mine lignite sodium

issue and has provided no documented long-term experience at commercial SCR installations on

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

that the burd,en of proofin this'case is on Minrtkota to provide defmitivequ~titative evidence

that SCR would be technic~lly infeasible'at,MRYS due to the high number ofSCR syst~m:s

, currentlyin operation at utility boilers. Furthermore, a laclc of a full-scale long-tenncommercial

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

technically infeasible at MRYS. While EPA agrees that the sodium levels in North, Dakota

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

been applied to new fuel types with' unique characteristics over the history ofSCR installations.

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

sod,ium issue from a technical standpoint. .EPA does not believe that the qualitative information '

'presented thus far is sufficient to outweigh the vast amount pfSCR operating experience for

utility boilers to say 'that SCR would be technically ll1feasible 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 bQth the

Baldwin and Columbia Stations. "

EPA believes that the experience the utiHty industry has gained from the installations at

PRB facilities would be useful in designing an SCR systemfof a cyclone unitburning North,

-15~
EPA Response to Minnkota Comment Letters from

March 19,2007 and April23, 2007

BACT Analysis at MRYS

Dakota lignite. According to th~ pre~ehtationgiven In Bismark on May 23~ 2007 to NDDH and '

EPA. by the utility design firm, Sargent & Lundy, the successes and failures ofthe PRB

installations would pr{)vide Va,luable inforrriation for designing an SCR system pn a plant burning

North Dakota lignite. WIllIe' Sargent & Lundy indicated that getting further data from a welldesigned pilot study would enhance'their abilityto optimally desigu'a more cost-effective SCR

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

PRB installations.

EPA does not' have access to the EPRI study referenced that produced field 'data from an

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

AmereilUE Sioux Station) which is a cyclone boiler burning a PRBlbituminous blend. B&McD,

appears to imply that this result demonstrates that an SCRinstallation on acyclone boiler win

experience 'faster deactivation,than an installation on a PC-fired, boiler because ofthe highe!

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

not know how large ofa difference there was between the cyclone unit and the PC-fired units.'

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

or conclusions from this study. Furthermore, EPA is unsurehow'"this relates'exactly to the issue

ofSCR technical feasibility at MRYS. This data might be useful In trying to predict catalyst

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

feasibility of SCR at MRYS.

·b. , Engineering Solutions fOr SC!R Applications atHigh-Ash Facilities

EPA 'sjanu.ary 9, 2007Comment:

Plugging ofSCR catalysts byfly ash accumulation and bridging ofpassages in honeycomb

catalysts and betWeen plates in plate-type catalysts has' "Qeen a problemfor designers (Ind catalyst

vendors since,thefirst coal-jired ins.tallations in the'f970s. Th~primary response has been more

sootblowing" on,-line cleaning equipment, (md larger catalystpitchfor honeycomb configurations an(i ,,

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

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

Also, thefact that the MRYS wiits are brought down for planned boiler deposit cleaning at f20-day

. intervals. provides mOre opportunityfor catalyst cleaning maintenance than is available at the typical

power plant. '.

, Further demon~trating the ability ofSCR dlfsigner.s louse existing knowledge and experience

with SCR installationsfor high ashfuels, the Electric Power Research Institute (EPRl) has advised'

clients that based'on industry experience on a wide range ofSCR installations'over the years, SCR .

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

letter addressedto TXUPower, EPRIstated, "While experience specifically with Texas lignite is limited,

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

a newfuel is similarto the challengefaced by US. utilities and their SCR suppliers whenfirst installing

SCR systems on planisfiring high sulfur bituminous or PRE coals. Neithersupplier~ 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 these fuels, whichproduceflue gas properties'that are very differentjrom those in

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

rely upon their knowledge, engineeringjudgment. and experience with these other coals to desiin SCR

systems suitable for these new applkations. While some ofthefirs~ 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), none ofthese problems prevented

the systemsjrom achieving their NOx goals initially and, eventually, maintainingthem overthe expected

.life ofthe catalysts. " .

EERC's March 19, 2007Response: .

. .

The issue is·riotsirictly the high ash content ofNorthDakota lignite, it is the high sodium content of

this lignite's ash. The issuefor the application ofSCR on North Dakota lignite1iredcyclone boilers is a

. high potentialjOr ash deposition in lower-temperature convectiVe pass surfacessuch as the economizer

andprimary superheater. The high ash deposition rates require continuous cleaning withsootblowers,

. and the resulting liberated depositjragments will cause catalyst plugging.' The depositjragmenrs, along

.with the formation ofsulfatedsodium- and e.alcium-richfine particulatepluggingpores ofthe catalyst,

causesplugging ofgas paths. .

EPA Response: .

The response from EERC simply restates their position regarding the high .sodium content

ofthe Center Mine lignite and how this will cause both catalyst pore plugging and gas path

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

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

B&McD's March 19, 2007 Re~ponse:

For a North Dakota lignite-fired cyclone boiler, aggressive on-line cleaning and/orjrequent

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

cleaning in orderto'restorefull unit output qnd catalyst NOX reduction effectiveness. Either or both of

these'maintenance activities could damage the catalyst and/or require it to bejrequeritly- .

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

to be insufficient to use boiler deposit cleaning outage~ /0 perform significant catalyst cleaning or "

replacement work 'J:1'ithout delaying startup.

EPR1'sstatements do not conclusively support the technicalfeasibility ofapplying SCR wilh

.. specific technical evidence thatshowsproofofthe hypotheses. that it will be successful on Texas lignite- .

fired boilers. It also does notsuggest or imply that North Dakota lignites are similarto Texas lignites

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

a Texas lignite-fired unit would result in a different set ofchallengesfor application ofhigh·dust SCR

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

cyclone boilers in this NOx BACTanalysiSfor MRYS. Already-installed boiler and backend equipment

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

-17-
EPA Response to Minnkota Comment Letters from

March 19,2007 and April 23, 2007

BACT Analysis at MRYS .

original design engineers took to attach these situations as separate problems.. A comparison ofthe

differences between Texas andNorth Dakota lignites is included in EERC's subsequentresponse to the

$PA 's comments regarding the NSPS revisions later in this response. document. .

. EPA Response:

This response states that aggressive on-line cleaning··andlor frequent extended 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, albeit with potentially.higher than

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

the problems ~ited in the Minnkota BACT analysis appear to be. more a matter of cost than

technical feasibility. Ifindeed necessary; the extended,o~tage times could be accounted for as

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

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

The intent ofreferencing the EPRI l~tter on Texas lignite'was not to· imply that North

Dakota lignites areexacdy the same as Texas lignites. The Interit ofreferencing this letter was to

show that a major utility res~arch organization is ofthe opinion that the existing experien~e of

the SCR designers woul4 greatly help in designing SCR sy·stems for ac~ommodatingfuel types

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

.specific fuel and boUer characteristics at MRYS may present chaUenges to an SCR installation,

the significant industry 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

d;esign firm Sargent and Lundy iIi the'pre~entation described above. .

c. Oiher Ash Deposition Problems on Downstream Components

EPA 'sJanuary 9,2007 90mment:

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

conclusions related to the technicalfeasibility ofSc:R, so EPA does, not see this as a technical barriertp .

installing SCR at th,e MRYS units.

EERC March 19, 2007Response:

Not~ that it is n~t the ~sh loadings, ii is thesodium,./n ihe lignite ash thatcaus~s t~ deposition.

The sodium-related. depos~tion on SCR catalystwil!shorte,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 acknowledges that the-sodium contentmNorth Dakota lignite presents engineering

. challenges; EPA also thinks it is possible that t}:1e "sodium-related deposition on SCR catalyst--

will shorten the time between outages fo.r off-line cleani~g."As stated previously, the

.requirement for increased catalyst maintenance does not render the technology technically .

infeasible.' Any increased maintenance and unit downtime would result in added cost, which

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

conducted at Coyote Station did not provide any quantItative results tl;1at could b~ used to predict

the time periods between catalyst cleaning/replacement on 'a full-scal~ 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 with ammonia and S03 emittedfrom a high-dust SCR will exaggerate

tubular air preheater and economizer tube exteriorsurfacefouling andpluggingproble~ at any cyclone

boilerfiring thisfuel compared to bituminous or PRB coals. The tubular air preheaters and the built-in

economizers at MRYS 'ar.e not equippedwith sootblowersfor on-line gas-side surface deposit cleaning.

Noting that MRYS boilers' air preheater and economizers'would be"downstream ofthe boilerbackpass

outlets and a high-dust sci? isolating the.se components during off-line boiler deposit cleaning or SCR

cleaning outagesfrom negative itlJpacts of "diriy air- would be chalienging and expensive: Similar .

estimated amounts oftime and difficultyfor such off-line downstream deposit removals will also delay

unitstartupsfollowing such outages .

EPA's Response:

It is unclear how the tubular air preh~ater and economizer tube exterior surface fouling and

p"tugging problems would become exaggerated from the use ofan SCR system. As noted, ·in the

August 20, 2007 Christman Memo, "the SCR retrofits at New Madrid (tWo large, PRB-fired

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

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

consjdered in Minnkota'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. . Accounring fOr Catalyst Lite & PerfOrmance in Tdp;.Down BACTAnalysis

EPA's January 9, 2007.Comment: "

Finally, the conclusion reached in_ the B&MBACTcmalysis that ash deposition andcatalyst

pluggage will adversely affect SCR performance (we assume this means contro/efficiency) and catalyst

-'. life does not necessarily support a c.onclusion 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 MRYS

should be eliminated atstep 2 ofthe Top-Down BACT(lnalysis. Furthermore, the B&Manalysis presents.

no qucmtitative data related to SCRperformance and catalyst life to supportsucha conclusion...

...However, SCR performance and any real construction and operational costs are factors that would be .

accountedfor under the econ01nic impact ~tage related to cost effectiveness ofSCR (step 4) underthe' .

Top-Down BACTanalysis.

B&McD March 19,".2007Response:

The reasonsfor the. assertion ~ftechnical infeasibility ofseRforlignite-fired cyclone boilers were

presented in the MRYStation Unit J and U."U 2 NOx BACTcmalysisreports; withfurther explanation

providedherein. The EPA's preliminary analysis comments minimize the clifficulties posed by the higher

amount ofash, sodium, and sulfur in North Dakota lignite combinedwith calcium and other alkaline·

elements along with ammonia andS03 emittedfrom ahigh-dust SCR on the tWo existingNorth Dakota

lignite-firedCYclone boilerat MRYS. .

. The EPA cites no published technical literature that describes as would be the case at Milton

R. Young Station.

The evidentfailure ofthe Coyote slipstream SCR catalystfieldtest to ... ....ledBurns & McDonnell

to believe these field test.results indicate Jor technicl:ll infeasibility purpost!s. . . .

EPA Response: .

The' majority ofthis comment is addressed under EPA's response to the preliminary

summary. EPA does not discount the challe.nges 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 EERG report demonstrate that a full-scale SCR installation at MRYS,

.that in,cluded the currently available engineering design measures, would not be techniCally· .

. feasible and offer substantial NOx reductions. EPAhas pointed out some ofthe inherent

limitations in the pilot testing that might have contributed to the "failure" .ofthat test. .B&McD

and EERC's responses do not provIde much insight on whether these:tests were designed to

reasonably represent full~scale conditions, other than to say that EERC was not ai<me responsible'

for the testing's de~ign. EPA question~ the design and operation ofthe' Coyote Station pilot test.

in light of th~ lack of critical data obtained.

EPA has also not found any deflnitiveqnantitative'results, taking into accoUnt th~ ash

deposition and catalyst plugging counter'measures, that show the expected rate ofcatalyst. '.

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 hot believe the pilot testing

presented by EERC provides an accurate estimate ofwhat type of catalyst maintenance and

replacemenqvould berequired. Without such quantitative information, EPA continues to .

believe that Minnkotahas not provided sufficient evidence oftechnical ipfeasibility for SCR at

. MRYS. EPA believes SCR is in fact technically feasible at MRYS. EPA beJieves linderthe . .

-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 ofprooffor technical

infeasibility for SCR at MRYSis considerable giverithe large number ofSCR 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 Asb" Issue: "

No response given,to EPA's comment by EERC or B&McD. The topic was'similarly addressed

,in t/:ze January 2007 Christman Memo.

3. EPA Analysis of the Temperature Issue:

No response given tq 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 previously cited and explained below that are availableforfull-scale SCR ' ,'

appiications. Furthermore, EPA,Region 8 believes that there may be inherent limitations with the pilot'

'study (that was ,the basisfor bothEERC reports;' thatdraws into question the validity ofdirectly applying

those conclusions to the range o/possible SCR designs ofafull-scale application at MRYS. Statements

from three major SCR catalyst manufacturers (including Haldor Topsoe, .the catalyst manufadurer

involved in the pilot testingfor North Dakota lignite) suggest that the catalyst deposition andplugging

problems experienced during the pilot testing may be exagger.ating on apilot-scale and can be solved in a

full-scale design and application .

Other inherent proble.ms associatedwith directly applying the results ofthe EERCpilot testing at

.Coyote Station on North Dak~ia lignite include: '

1) The air pulse used to Clear deposits during the pilot test is a poor simulation ofsootblowing and

is much less effective... ... ....... ,

2) The ,6mm pitch catalyst usedforthe pilot testing at Coyote Station is much smallerthan

currently, being specifiedfor coal-jired installations. Vendors are now supplying 9mm, 10mm or even

-21-
'EPA Response to Minnkota Comment LettersJrom .

.March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

Ilmm pitch catalysts : ..

. 3) The one meier depth ofthe catalyst used in the pilot test is equivalent to '!ne layer ofcatalyst in

·a commercial instalkilion. CommerCialsystems aimerfat high remoyals haVe three layers, each about

three feet thick, and, at various times in the catalyst mi:magement cycle, incorporatefour layers. EERC's.·

test, then, ~Jnly simulated the top layer ofa'multi-layersystem This is why thefirst step in

most catalyst management schemes usually involves adding afourth layer and leaving the badly

degraded top fayer in placefor the second operatingperiod EERC's project provides no real insight

into the' effectiveness and survivability ofa second or third layer. , .

4) The scale oftesting carried out in this project is usually associatedwith observing catalyst life

prospects (or said another way, the rate ofloss in catalyst activity) rather than physicalprocesses of

. deposition or plugging. This is because the deactiva(ion mechanisms operate on a micro-scale andean .'

be accurately simulated at this smallscale, but theplugging and deposition are irifluenced by larger

processes, such asflow imbalance and countermeasures.(sootblowing,. horns, screens, bajJles; etc.) and

. are not duplicate.d on the smallscale.

For these reasons, EPA Region 8 questions the direct app/icati~n ofthe results ofthe EERCpiloi'

testing at Coyote Station and the accompanying Appendix B EERC reportfor purposes ofthe BACT'

technicalfeasil}ility analysis ofSCR.at MRYS. The experience and opinions ofthe SCR supplier that

participatedin the EERCpilot test indicate that the problems'encounteredduring the pilot. test are·not

determinate ofwhether SCR plugging experienced during the pilot test would be unresolvable by vendors.

To the contrary, statementsfrom Haldor Topsoe offering the}r SCR catalystfor installation on lignitefired boilers indicates that thefull-scale differences and engineering solutions available today provide

.ev.idence that the EERGpilot testing results should not alone be the deciding consideration when'

· evaluating SCR technicalfeasibility at MRYS. .

EERC March 19, 2007Response:

........The catalyst vendo~s, Haidor Topsoe, Cormetech, and Hitachi, were all involved in the design

OJthe pilot-scale slip stream test system ahd testingconditions..The'pulsing system'was designed with the

assistance ofthe catalyst vendors. .They providedsuggestions on the sootblowingpresSures Cindfelt it·

would be sufficient 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 instdlled, Deposition alJdplugging are significant problems that impadthe .

·reliability ofthe MRYfacility. Haldor Topsoe was the only catalyst vendor willing to provide catalystfor

the lignite tesiing. . . , '.'

The EERCpilot.,-scale testing was a collaborative effort involVing coal-fired utilities, EPR1, DOE,

and catalyst vendors.' All de~isioilS on system design, plantsfor testing,' and tes,ting conditions were .

·reviewed and agreed upon byvimdors. The EERC'comparisons ofcatalyst blindingfor/ignite and

· subbituminous.coal testing as' well as the sodium deactivation work conductedby otherresearch

organizations and catalyst vendors should ~e used in evaluating technicalfeasibility ofSCRfor MR~S.

, -':22-;-

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

- - March 19,2007 and April23, 2007

, BACT Analysis at MRYS -_

EPA Response:

The results ofthe pilot testing were examined in more deta;.l. EPA's observations on tl1e

test ~s it relates to expected catalyst maintenance and replacement requirements at MRYS aredescribed in detail under the EPA response- to the preliminary SUIIUllary.

_EERC indicates that the catalyst vendors 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, infonned EPA Region 8

that, in hindsight, the pilottesting should have been conducteddifferently. Specifically, Mr.

Hansen said that changes to the test such as increased catalyst pitch could have helped reduce the

impacts ofash plugging. Additionally, Mr. Hansen said that pilot test's air pulsingsystem did

not accurately simulate full-scale sootblowing and that ifthe test could be redone, a bett~r effort

could be made to keep the'catalyst face clean. Mr. Hansen recalledthat 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 as~ deposition would not be a problem on a full-scale application.

Mr. Hansen's statements support EPA's original comments,that there were inherent

problems with the pilottes,ting that contributed to the degree of catalyst plugging during the

,Coyote Station pilot testing. In addition, experts from Sargent & Lundy indicated that they were

disappointed in how the Coyote Stationpilot test was carried out and foun.d the results to be of

very little value for evaluating an SCR system for afacility buming North Dakota lignite. '

Based on the EERC response, the degree of catalyst plugging observed at Coyote Station

was the reason no deactivation testing onthe catalyst was conducted. Ifthe testing were' '

designed and conducted in a way that could have greatly reduced the effects-of ash deposition on

the catalyst face, presumably catalyst deactivation data would- have been obtained. As stated

previously EPA questions Minnkota~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 feasibilitY ofSCR at MRYS. Furthermore, whether 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 uilrepresentative 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 Letters from .

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

'6. EPA Analysis ofthe Burns & McDonnell's Conclusion:

EPA's January 9, 2007 Comment:

EPA notes that there is co~siderable experience in the industry to show that. use, ofSCR on North,

Dqkota lignite is technicallyfeasible. EPA has previously articulated this beliefin a number 01national'

rulemakings, including the Clean AirInterstate 'Rule (CAIR), the Best Available Retrofit Technology

(BART) Guidelines, and revisions to the New Source Performance Standards (NSPS) for Electric Utility .

Steam Generating Units (finalized on february 27, 2006)... ...... ... .... .

. .

EPA understandsjrom the EERCpiiotstudy at Coyote Station usingNorth Dakota lignite, there

was noticeable ash build up on the catalyst and concerns with catalystplugging during the test. While

EERC notes that ash deposition problems were most pronounced at Coyote Station, compared to the twp·

test/acUities burning PRB coal, it also states generally that all ofthe coals in the pilotstudy "produced

ash that hadsignificant accumulations ofash on the catalyston both macroscopic andmicroscopic

levels." Thus, the conClusions ofthe study appear to be that deposition issues might be somewhat'greater

for North Dakota lignite thanfor PRB coal; Since ~ignificant accumulations ofash on the catalyst was

also observed during pilot, testing at thosefacilities burningPRB coal and there are numerous examples

, OJPRB coal-firedpower plants. successfully operating SCRsystems, the qualitative results ofthe pilot

testingdescribed by B&Mand EERC would not appear to preclude successful operation ofSCR on a

power plant burning North Dakota lignite. The EERCstudy does not appear to quantify the effect that

the ,ash accumulation had on microscopic catalyst deae.tivation rates or overall catalyst performance.

Absentsuch information that tejJeetsthe useojcurrently C1\lailablefull-scale ash loading mitigation

designs, pilot t?stingresults may not be evidence thatsuccessful itrlP!ementation ofSCR on North Dakota

fired units' "is going to be impossible," as stated in the Appendix B.EERC report .

, The Octob?~ 1990 EPA draft New Source Revie~ Manual (NSR Manual) states that a

"demonstration oftechnicalinfeasibility is based on atechnical assessment consideringphysical,

chemical.and engineering principles, and/or empirical data showing that the technology would not work

on the emissions unit under review, or that unresiJ/vab/e technical dijjiculties ~ould preclude the· .

successful deployment ofthe technique. Physical modifications needed to resolve technical obstacles do·

, not in and ofthemselves provide ajustificationfor eliminating the control technique on the' basis of

technical infe.asibility. However, the cost ofsuch modifications can be considered in estimating cost and .

economic ,impacts, which, in turn, mayfor:m the basisforeliminating a control technology. " (Page B.20)

(Emphasis added) Based on the available information, the problems cited by B&Mfor application of

SCR at MRYS (ash depo~ition, "popcorn ash", temperature; and erosion) are resolvable,through the

engineering solutions discussed above. While sur:h engineering solutions fflay be a cost burden to

Minnkota, as noted above, step 2 is not the proper stage ofthe Top-Down BACTanalysis to accoUntfor

these costs. As described in the NSR Manual, "where the resolution oftechnical difficulties isa matter of

cost, the applicantshould consider tlte' technology as technicallyfeasible. The econo,!,icfeasibility ofa·

control alternative is reviewed in the economic impacts portion ofthe BACT-selection process." (Pages

B.19-B.2.0)· Many ofthe technicalfeasibility arguments presented in the BACTanalysis are centered on

, decreased c~ialyst life andperformance, increased maintenance and outage time, cmdplant differences

unique to MR.YS thatpreclude SCRfeasibility at MRYS. Consistent with the NSR Manual, these concerns

are incorrectly incorporated into the Top-Down BACTanalysis atstep 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 perton ofpollutantremoved a/step 4 ofthe analysis So, while EPA does·

not consider vendor guarantees 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, Minnkotaand B&D should have/t4rther explored both the

full-scale engineering solutions used to address the problems cited in their anqlysis in conjunction with

informationfrom SCR/catalyst vendors to offer performance guarantees to design and install an SCR unit

ai MRYS. The BACTanalysisfor MRYS that·was submitted relies heavily on the technical problems

. experienced during the EERCpilot testing at Coyote Station in conjunction with the fact that SCR has not

been installed on a cyclone boilerthat burns North Dakota lignite. We do not believe that this analysis

.adequately addresses the criteria·citedabove. As such, it appears that the case-specific NOx BACT

analysis submittedfor Minnkota MRYS is deficient in terms ofimproperly eliminating SCRfrom

consideration atsteJ! 2 ofthe BACTanalysisfortechnicalfeasibility.

EERC's March 19, 2007Response:

EPA did not examine the EERC report in detail. The deposition andplugging was much more

pronouncedfor the Coyote Station testing ascompared to the Columbia andBaldwin Station.. The

. pressure drop increased much more rapidly and the soot blowing was not as effective..... The most

important unresolvable technical difficulty is the impactofsodium on theformation ofbondedsulfate.

phases thatplu~ andmask the catalyst and on the ability ofsodium to dea~tivate the catalrst.

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 on 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 is true that the problems were most pronounced at the Coyote Station and the

facility experienced higher pressure 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. pilotstudy does

suggest that catalyst plugging. at a facility burning North Dakota ligiute would likely be more of

a challenge than at a facility burning PRRcoal, Ep·A can not conclude that the test is

determinative ofwhether an SCR system would not work,at MRYS. See EPA's previous

detailed analysis for more information. . .

EPA has since received the background documentsfor pr9ject participants and reviewed

quarterly progress reports. For reference, below is a summary ofsome ofthe relevant history

and background informa,ti<>u from tJIe pilotstudy.

-25-
EPA Response to Mim'lkota Comment Letters from

March 19,2007 and April 23, 2007

BACT Analysis at MRYS

Original goal~'ofthe test(2000 EERC Proposal)

.' "Determine the mechanistic and fundamental causes ofpotential blinding or

masking ofselective catalYtic reduction (SCR) catalysts d:ue 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, anel SCR manufacturers

and distributors will gain an appreciation for potential challenges facing

utiiities using low-rank coals that could cause SCR maskirig and will be able'

to adjust future systems." (Expected Results) ,

• "The test chamber will be constructed so that periodic samples o.fthe catalyst

can be removed to assess the reactivity as afunction oftime." .(EniphaSis

. added) (Task, 4 .:.. SCR Test Chamber Installation 'and Data Collection)'

• . "The endr€sultis that research is needed to :determine the true extent of' ..

potential SCR blinding for lower-rank coals. Ifindeed masking of SCR

catalystmatedal 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 tQ be amended. Other options may·

surface as result (sic).ofthis research include providing technological and

fundamental science and engineering knowledge for ma~ufacturing 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 relevantexcerpts ~aken from the final results ofthe test. (November2004

Final Report for period November 1, 2004·;June30, 2004, with two exceptIons noted

~~ .

• Baldwin Station (cyclone boiler, PRB coal) catalyst re~ctivity 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 and a: minimal amount 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'significant accumubition and pluggi.ng

than the Baldwin Station." "The six month sample from the Colunibia Station .

showed particles adhering to the surface and filling p'ores 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, NO lignite) - No catalyst testing conducted and no,

NO" data submitted. "The mostsignHicant accuniu1ation was noted for the Coyote

Station." "The,Coyote Station had some large pieces ofash 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 in the

catalyst." Describing photos ofthe catalyst inlet after two months ofoperation, "the

left side ofthe catalyst is indicative ofthe amount offouIlng present whereas the ash

present on the right side ofthe catalyst fell from the upstream flue section when the

joint wasseparated. Nev:ertheless, the entire siufac~ appears coated with ash

deposits~ and some ofthe catalyst.channels were plugged." (February 2004 Quarterly

Report - not noted inthe final report) '(Note: The right side ofthe catalyst face looks

FAR worse th~ the left side in the ph~to.)

• Study Conclusions: "The pressure drop ~cross the catalyst was found to· be the most

significant for the lignite-fired plant as. compared to the subbituminous-fired plants..

Both coals had significant,accumulations of ash on the catalyst, on both macroscopic' ,

and microscopic levels.;' ,.'

l"This study suggests careful evaluation ofeach SCR installation in applications using

subbituminous and ligriite coals. Improvements are needed to ensure technical

feasibility, especially with lignite-fired units. 'Installations involving lignite fuels will

need advanced cleaning techniques to handle the high sodium anq. 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 f8;cilities tested during the pilot study,

1. Quantitative,Data Analysis

• EERC apparently did' no catalyst reactivity testing' for the Coyote Station and

did hot monitor NO" control performance ofthe SCR system' during the test

period. (Origi.nal test proposals from 2000 included mention of

instrumentation "able to monitor whether NO" 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, bllt a

sudden dramatic increaSe likely resulting from the ash dropping from

upstream flue sections, as described in the February 2004 Quarterly report.

EPA's conversation with Haldcir Topsoe is consistent with this interPretation.

-27-
EPA Response to Minn~ota Comment Ll?tters from

'March 19, 2007 and April 23, 2007

BACT Analysis at MRYS .

Pressure drop data and comparisons to ColUmbia and Baldwin Statio~ are

therefore not meaningful. .

2. Qu~litative Catalyst Analysis

• While·Coyote Station is described as having the most significant catalyst inlet

ash accumulation, it is entirely possible that this is due in large part tothe a$h

di'~pping from upstream flue sections. . ,

• The microscopic catalyst d~scription fot Coyote Station was notentirely

unlike that ofthe Columbia Station, where both "showed particles adhering to

the surface and filling pores i~ the catalyst."

• Due to the upstream flue section 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 catalystpores), theqlialitative information does not provide

meaningful conclusiQns for comparatively asseSSing SCR technical feasibility

for North Dakota lignite and PRB coal applications. The one exception would

be that the study shows lignite at Coyote Stati~:p. showed more sodium '

deposition and less calcium compared to PRBcoal. It is unclear what the .

basis is for EERC's statement that the sample at Coyote Station "shows

, significant evidence ofsulfatlon after only 2' months ofexposure arid was·

much more pronounced than the 2-month samples for the B~ldwin and

·Columbia Stations." Even ifthis is based on visual observation ofthe SEM

images, it is not quantitatively compelling'information. As stated in EERC's:

·final report, "both coals had significant accumulations ofash on'the catalyst,

bnboth macroscopic and miCroscopi~ levels." , .

3. EPA's ConClusions ofthe EERC Pilot Study .

. • "The quantitative data provided for catalyst deactivation for Baldwin (reduced

to 84%) and Columbia (reduced to 90%) indicates that the visual observations

from the pilot study arenot meaningful ways to quantitatively represent the .

ash impacts on SCR systems.. In fact, the reactivity results show the opposite

, trend as would be expected when comparing the visual'pbservations at

Baldwin and CohuiJ.bia Station: "Columbia Station.showed more significant' ,',

accumulation and plugging than the Baldwin Station," but Columbia Station's

catalyst showed less deactIvation than BaldWin Station's catalyst. As such, it

· does not appear thRt a correlation can be made betWeen the visual . . .

'obserVations and the effects on catalyst performance. Without representative

catalyst deactivation testing or NOx control data from the Coyote Station, EPA

.-28'-
EPA Respo.nse to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

does not believe the pilotstudy provides meaningful results that could lead to

a definitive 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~ote Station· due to .

upstream flue ash dropping on.to the catalyst face.

• The catalyst analysis on both micro and J;Ilacroscopk levels does support the·

· claim by EERC that catalyst deposits will contain more sodium when exposed

to lignite comparedto PRB coal, but does not clearly support the assertion by

· EERC that SCR would not work on a lignite plant. In fact, EERC's .

conclusions.seem to question how well SCR would work atPRB plants,

which has been disproved with operating plants.

• . IIi summary, the primary goal ofthe piloftest was to determine mechanistic

and fundaniental causes ofpotential blinding or ~askingof SCR catalysts due

.. to flue gas cons.tituents in low-rank coals. In the test proposars, 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 meet·the

original goal forNorth Dakota lignite. As explained above, the plugging

: appears to be in large part, due to the upstream flue ash dropping onto. the.

catalyst face. As such, it is riot clear whether the catalyst testing would have

been conducted ifthe catalyst would have been kept cleaner. Additionally,

. EPAunderstands that the cataly~t deactivation testing still could have been

·conducted, it just would not have provided a veil'quality ·"data point" because

ofthe irregular catalyst exposure over time in light ofthe substantial ash , .. ..

.deposition on the catalyst face 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 Col~mbia Stations.

B&McD's March 19,2007Response:

... ,.. The EPA's review offered no specific documented andpublishedsubstantive demonstrated

experience ofany cyclone boilerfiring North Dakota lignite, particularly one locatedin a highdust, hot-slde application at Milton R Young Station. ·Burns & McDonnell disagrees that the lack ofSCR

catalyst deactivation datafrom the Coyote slipstream SCR catalystsfield test completely refutes the

premise ofDr. Benson referenced technicalpaper and the M.innkota NOx BACTAnalysis StudY

conclusions that SCRs are technically infeasiblefor !ignite-jired cyclone boilers.

The.EPA's NSR Manual does not.specifically define .....; in orderfor said alternative to be

excludedfrom further consideration. . .

When a BACT an~lysis is performedon a new unit, there has typically been significant conceptual

and detailed engineeringand design work already performed thatforms the bC1!is ofthe subsequent

-29-:-
EPA Response to Minnkota 'Comment-Letters from

March 19, 2007 and'April 23, 2007

, .BACT Analysis at MRYS

capital cost estimate and associated expected0 & ,Mcostsforthe particular emission control

technologies to be empl9yed orthat were considered as part ofthe overallproject effort. Performing a

BACTanalysis on an existing unitslzould notrequire similarsubstantialresources to f/evelop conceptual

designs and create technical details that determine the'estimated site-specific retrofit costsfor

overcoming significant technical challengesthat would otherwise,preclwle the'ability ofthe technique or

technl?logyfrom successfully controlling the emissions in question. The EPA's NSR Manual states: "A

source would not be.required to exper.ience extended time delays or resource penalties to allow research

to be conducted on a new technique. " .There are no cited examples'where successfulsolutions to such

challenges as posedlJY stich highly complex interactions betwe.enflyash, flue gas, and gas temper.atures

produced by lignitefiringfor SCR coal-.(ired boiler applications '

EPA Response:

Again, EPA believes that in the case ofdecid~ng on technical infeasibility for SCR at a .

utility boiler, the bUrdeIi ofproofis ont~e .applicant and is considerable. EPA's,position is. that

the hundreds of SCR installations at utility'boilers worldwide demonstrate a wealth ofexp~rience

, within the industry to address various factors over a very ,wide range ofboiler designs and fuel

types.. The qualitativeres\llts of catalyst plugging for the pilot testing,at Coyote Station do not

provide empirical 'evidence that SCR would be ineffective at controlling NO" emissions, since

there was no. quantitative data provided for eitherNO" reduction or cl:!-talyst deactivation~ As

. such, EPA is not convinced that the specific challenges presented for an SCR installation at

MRYS is a matter oftechnical infeasibility and could not be overcome with a robust suite ofash

(including sodium) mitigation ineasures and/or m~intenance measures. ' While EERC has had

verbal contact with Haldor Topsoe, Miimkota's March 17, 2007 r~sponse letter simply says that

this vendor would have concerns'about providing a performance guarantee in light ofthe speCific,

sodium content ofthe lignite burned at MRYS. It is not clear based on this statement whether

Haldor Topsoe,would or would not in Hict provide a performance gu~rantee' (or what the .

guarantee would entail) for an SCR installation at MRYS. Also, as noted previously; during

EPA's own discussion with Haldor Topsoe, Mr. Flemming HanS~n did not rule out bidding on

project that involved North Dakota lignite. .

B&McH's assertion that a BACT analysis for an exis'ting'unit "should not requir~'similar

, substantial resources to develOp conceptul:\-l designs and create technical details that determIne

the estimated site-specific retr<~-fit'costs for overcoming significant technical challenges that .

'would otherwise preclude the ability ofthe technique or technology from successfully

controlling the emissions in question" doe~ not appear to, have a basis inEPA regulations or

policy. Prevention ofSignificant Deterioration (P~D)reg1,llations make no distinction between

ijACT requirements for new and existing upits Undergoing PSD review. hi faet,EPA'sposition

has been that when it is discovered thata source avoided PSD review,but it was later determined

'that PSD permitting requirements, iIic1udingBACT, should have applied, the source would need

.to address the appropriate requirements "as ifconstruction had not yet conimenced.,,17: .

17 S'ee April 20, 1999 letter from Richard R. 'Long (EPA Region 8)to Dennis Myers (Colorado Department of

-30-
EPA Response to MinnkotaComment Letters from

March 19, i007 and April 23, 2007

BACT Analysis at MRYS

In addition, B&McD's reference to page B.IS oftb.e NSR Workshop Manual is taken out

.of context mi.d is. not relevant to their discussion on how a BACT analysis for anexistillg soUrce

should be less rigorous than fora new source. The NSR Workshop Manual citation·is a

disc;ussion ofwhether a given control technology or technique 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 installation at MRYS. hi fact,

,Minnkota's original BACT analysis contends that SCRmeets the criterion for being .

"available"}9 So, it is inappropriate for B&McD to bring upthis 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.I8 ofthe NSR Workshop Manualwould

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 it is 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.2° In conclusion, EPA

· does not believe Minnkotahas 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 at MRYS, EPA believes these would largely be a matter of eost arid

·should be examined understep 4 ofthe Top~Down BACTanalysis.

'B. ,Low-Dust SCR

Bums & McDonnell has eliminated Low.,.Dust SCR as technically infeasible due to issues

described below. EPA's response on each issue is also described below

1. EPA Analysis of the Catalyst Fouling and Deactivation Issue:

EPA's January 9, 2007Comment: .

Afew SCR systemsinstalled on coal-jired boilers have been designed with the SCR catalyst

installed after the ESP or baghouse. This approach-eliminates virtually all ofthe poisoning, plugging

and erosion prolilems that have been associatedwiih SCR cataly~t. (Christman Memo, 2/18/03) For the

same reasons noted above under the high-dust SC~ analysi~, the catalystfouling and deactivation

pro61ems may be resolvable technicalproblems that do not warrant elimination oflow-dust SCR under

,Public Health and Environment). http://epa.gov/region7/programs/artdlair/nsr/nsrmemoslamersoda.pdf

18 Page B.18 ofNSR Workshop Manual: "A control technique is considered available, within the context presented

above, ifit has reached the Iicehsing and,commercial sales stage ofdev~lopment. A source would not be required to

experience exte~ded time delays or resource penalties to allow research to be conducted ona new technique."

19 See Page A1·32 ofOctober 2006 MimikotaBAcTAnalysis. Study for MRYS. .

20 See June 15, 1989 letter from Gary McCutchen (EPA Offi<;e ofAir Quality Planning and Standards, New Source

Review Section Chief) to John Daniel(Department ofAir Pollution Control, Commonwealth ofVirginia). ..

htq):llwww.epa.govlRegion7/programslartd/air/nsr/nsrmemoslfinlbact.pdf

-31-
/3PA Response to Minnkota Comment Lettersfrom . . March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

step 2 ofthe Top-Down BACTanalysis Similarly, permit applicationsfor three recent Circulating

Fluiclized Bedprojects in North Dakota.(MDU Westmooreland Gascoyne, South Hart, .andGascoyne

500MW) did not.eliminate low-Dust SCR based on unresolvallle technk:alfeasibility issues wit" catalyst

fouling and deactivation. While there would bel!nherent costs associatedwith gas-rehe(:lt, other possible

physical modifications to the plant to accommo.dale low-dust SCR, andengineering designs to minimize' .

emd manage catalystfouling and deactivation, these factors, consistent with the NSf? Manual, should be. .

accountedfor when considering the economic impacts ofthe technology.

EERC's Marclt 19, 2007Response:

Sodium-rich aerosols are aproblem that can get through the ESP. These aerosols are known to be

, . present in high sodium coal combustionfuel gases. These species,are known to produce pyrosulfates.

These will ccmseblinding and deactivation ofthe low-dust catalyst. .

EPA Response:

As.described previously, general qualitative statements thatthe sodium-rich aerosols will

.cause blinding and deactivation do not riecessarily support the conclusion that a low-dust SCR

system would be technicallyirtfeasible at MRYS.. The effects ofthis blinding and deactivation.

should either be quantified and·used within the economic analysis undei' step 4 ofthe Top-Down '

BACT analysis, or described in sufficient detail to demonstrate that the problems associated with

catalyst blinding and deactivation are unsolvable and would.result in the unsuccessful operation

ofa low-dust SCR. As stated on page B.20 ofthe NSR Workshop Manmil, "(p)hysical

modifications' needed to resolve .technical obstl;lcles do not in and ofthemselves provide a

justification for eliminating the cOl1trol technique on the basis oftechnical infeasibility,

However, the;,.cost ofsuch modifications can be considered in'estimating cost and economic

impaCts."

B&McD's Marcll19, 2007Response: . .

The l'leedfor a major source ofheat to reheat the flue gas stream downstream ofthe ESP in a low- .

d1,lSt SCR application willrequire either an expensive natural gas supplypipeline or an unacceptable '

derate ofthe nei unit electrical output associatedwithextensive, modifications ofthe. components in steam

cycle power generation and balance-ofpkmtsystems and'boiler gas systems to supply high pressure, . .

, high temperature steam as suggestedbyihe EPA and their consultant. .The magnitude ofsuch retrofit' .

modifications and risks offailure in applying low-dust SCR technology at MRYS are much greater than ls

considered reasonable.' . ,

EPA Response:

B&McD contemplates the use of namralgas (and associated cortstrUctionof a natural gas

pipeline) and plant steimi for flue gas reheat and makes a'general statement that the "magnitude ..

ofsuch retrofit 'modifications and risks offailure in applying .low·dust SCR tec~olo~y at MRVS

-32-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

are much greater than is,considered reasonable." Absendrom B&McD's consideration for flue

gas reheat are fuel oil an~ propane (instead of natUral 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 ifit is dismissing further

analysis as unreasonable as part ofthe economical analysis under step 4 ofthe Top-Down BACT

analysis. While EPA acknowledges that the two options described by B&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.

permit actions issued by.both the North Dakota Depa.rtn1ent ofHealth and Region 8 included a

cost analysis forlow-dust SCR. B&McD did not respond to this original cominent.

, 2. EPA Analysis of.the Site Space Constraint Issue:

EPA's January 9,2007 Comment:

While the extentofthe physicalplant modifications necessary to accommodate low-dust SCR at the

MRYS is not entirely clear, the above description does not indicate that the site space constraints at

MRYS would necessarilypreclude the possible installation oflow-dust SCR. It does sound like major and

costly equipment relocation may be necessary to provide .the space necessaryfor low-dustSCB.

However. consistent witfz the NSR Manual, "physical modifications needed to resolve technical obstacles

do not in and ofthemselves provide ajustificationfor eliminating the control technique on the basis of.·

technical infeasibility." These costs should be accountedfor when estimating the .cost(md economic

, impacts oflow-dust SCR. butshouldnot be used asa basisfor eliminating the technology as technically mft~wk .

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(Jchnology at MRYS. Burns & McDonnell believes

the expected levelized tota{annual costs required fo apply low-dusi SCR technology to North Dakota

lignite-fired 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 Dakota lignite-fired· cyclone boilers are unacceptably high", more

detail should be provided within step' 4 ofthe Top~Down BACT analysis. EPA, acknQwledges

.that extensive work may be required for installation oflow-dust SCR and the costs might turn

out to be ~cceptably high, but the NSR Workshop Manual does riot allow for. dismissing a

controloptio:£!. based on unsubstantiated beliefs about costs. B&McD. should do a cost estimate

based on reasonable expected reheat fuel requirements, equipment in~tallations, plant

modifications, etc. A comprehensive analysis would ,be unneces~ary ifit is shown that costs of
EPA Response to Minnkota Comment Letters from

, March 19,2007 and April 23, 2007

BACT-Analysis at MRYS

one or two aspe,cts ofinstalling low-dust SCR (i.e. the reheat fuel costs) would be unacceptably, '

high. Since there h,ave been 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 & McDonnell's Conclusion:

,EPA's January'9, 2007 Cmnment:'

As explained above, EPA Region 8 believes that the inherent challenges with installing m1d

operatinglow-dust SCR atMRYS are'resolvable on a technical basis. Thefact that afew 10w-dustSCR

applications are currently used in practice indicates that the small-particle catalystjouling and

deactivation problem described wouldnot preclude the use oflow-dust SCR... ...;. B&D's statement's

that "low-dust SCR performance and catalystlife could be severely negatively impacted resulting in '

premature, extended, frequent outagesfor replacement" can be minimized by proper SCR design and online cleanirzg. As statedpreviously, any real costs associatedwith the construction andmaintenal'lce

required to operate the low-dust SCR should be accountedfor under the economic impacts stage ojthe

Top-Down BACTanalysis (step 4). Similarly, physical plant modifications that would be required to ,

accommodate th.e low-dust SCR (and auxiliary equipment) should be considered in the economic impact'

evaluation,rather than used as evidence oftechnicai infeasibility. ,

'B&McD's March 19, 2007 Response: '

As previously stated by the EERC, the EPAfails to recognize that lignite-fired cyclones'partition

the, ash, increasing the proportion ofsodium in the slJlallerparticles, resulting in 'increasedpropensityfor,

theformation' ofthe complex sulfate phases. The fly ash becomes enriched in components that vaporize

and, subsequently, condense upon gas cooling toform liquid'phases and small particles that are v~ry,

reactive to gas-phase components that cause bonding, blinding, and deactivation ofindividual catalyst·

reaction sites, even in a low-~ust SCR application. '

EPA Response:

B&McD appears to be saying that a significant portion ofthe sodiuni in the.flyash is in the

vapor ph~se when it enters the ESP. Therefore, these smaller sodium 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 'quantification ofthe 'flyash sodium in the vapor phase that would cause the ".

severe catalyst plugging and blinding described under the high-dust SCR portions ofthe BACt

.analysis. Presumably, if a significant portion ofthe flyash sodium 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 riot affect the catalyst. EPA is not disputing that there may be some

vapor phase flyash sodium that will bypass the ESP arid deposit on the IQw-dust SCR catalyst.

Based on B&McDcand EERC's assertions that a high-dust SCR system at MRYS would see a

sodium environment higher th~.any other SCRsystem in the world, it would seem that most ~f

-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

·and ESP. Ifthis is the case; EPA expects a very 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 Flemming Hansen of

Haldor Topsoe, Mr. Hansen indicated Haldor Topsoe did run some catalyst "test coupons" at the

.Wisconsin Electric Power(WE Energies) Presque Isle Statiop-, which burns Powder River·Basin

coal with similar sodium levels 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. 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 a great deal ofmaterial that deactivates catalyst. This

experience suggests that catalyst deactivation observed at a low-dust SCR wo~ld be less than at a

r : hi~h-dustSCR. .. .

III. Additional Information Useful to Evaluating Technical Feasibility ofSCR at MRYS

A. EPA Headquarters Believes SCR is the Correct Technology for MRYS in

Developing the Clean Air Interstate Rule (CAIR)

EPA's January 9, 20Q7 Comment:

During the preparation ofthe CAlR rule, EPA developed a scenariofor bestcontrols across coalflred 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 Utilitie.s Co. (VLC-0754),' Otter-Tail Power Co. (VLC- .

0628); Lignite Energy Council (VLC~0935); Lloyd, Gosselink, Blevins, Rochelle, Baldwin & Townsend,

P.C.jor Texas Lignite Coalition (VLC-J037); and Basin Electric Power Cooperative (VLB-0690)~­

argued that SCR technology is notfeasiblefor application on lignite-flred boilers. Some ofthese

commenters claimed that an SCRpilot test on a slipstreamfrom a boilerflring lignite. had·shown 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, as a result offlring oflignite in the boiler. One of

t~ese commenters also claimed that SCR was notfeasible for boilersflring sub-bituminous.

In its response at VIB; ofthe CAIR docket, EPA'noted that there is considerable experience in the

industry to show that use ofSCRon'sub:'bituminous coals and lignite is.technicallyfeasible. SCR is

currently'in use an severalsub-bituminous coal-flred boilers in the U8.flring Powder River Basin coal.

Regarding the USe ofSCR on boilersflring lignite, EPRIiesting'ofSCR catalyst in a slipstream at the

Martin Lake plantshowedacceptable resultsfrom Texas Lignite. FUrthermore, the technology suppliers

repor.t that SCR has been s?JCcessfully used'on lignite and brown coal boilers in Europe. From this

experi~nce, 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

Marc;h 19, ~007 and April 23, 2007

BACT Analysis at MRYS

u.s: and catalystsuppliers have indicated that they will offerperformance guarantees on these .

·applications in the u.s. . . .

In the docket information regarding this issue, one .ofthe SCR catalyst vendors, Argillon, provided·

a /ist a SCR iristallations at EGUs·in Europe that burn /ignite orsimilar rank coal, including: . '. .':

• Riedersbach installations in 1995 and 2001(hard coal/lignite)

.• Voitsberg (lignite)

• Dormagtm installations in /994..and 1996 (lignite)

EPA also noted that the commenters have not provided any details ofthe pilot test that has been

mentioned with regards to the plugging and blinding ofa cdtalys/from /ignite-generdtedfuel gases. ...

Without this information, this PClrt ofthe comment cannot be a.ddressedproperly. The EPA's ow,n ..

investigations show that the main ref;lson for this problem was ash build up on the catalyst, which is a

·probkm commonly experienced in pilot testfacilities. In thefull-scale installations, this concerlt is

· acfdressed.during the SCR design stage. The meihods·usedto avoid this problem include duct design to

promote ashfallout priorto the SCR, catalystreactor design to avoid ash build. up, and on-line cleqning

methods (sootblowers and sonic horns)..

In addition,. (hejoilowing c~talyst vendors stated that they couldprovide catalysis/SCRjorlignitefired boilers: Haldor-Topsoe Cormetec/:z Argil/on ;.. . .

Tn a memo datedAugust 20, 2004, Jim Staudt ofAndover Technology Partners to Sikander Khan of

EPA CA¥D, discussed both the EERCpilot testing results and some ofthe negative issues raised by

B&Mand documented how Haldor Topsoe was aware ofthese issues, but neverthelessfelt comfortable .

providing SCR catalyst... ... ..... .

· EERC's March 19, 2007Response:

EPA's cited experiencefrom subbituminous coals and SCR does not haVe direct app/icationto the

North Dakota lignite-fired cyclone boilers at-~YS because ofthe high sodium content ofthe lignite.

The SC;R experience withGerman. brown coals does not have direct application to ND lignite because. of .

the lower sodium levels ofthe German coals. ..

EERC spoke to Flemming Hansen ofHdldor Topsoe on Feb~ary 2, 2007 about the issues

associate.d with high, sodium North Dakota lignite. Haldor Topsoe has ?xperience·with sodium and

potassiumfrom biomassjired systems and their experience has not beim good. After describing the issues

related to sodium.......after 6000 to 7000 hours %peration with a coal having elevated levels ofsodium.

EPA Response:

EPA's pr~sentationofsomeU.S.' subbituminous coal·and German broWn coal experience·

was intended to demonstrate that SCR systems have. been 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 Letters from

March 19,2007 and April'23, 2007

, BACT Analysis at MRYS

described in the JanUary 8, 2007 comments) to conclude that SCR was technically feasible for

,lignite ~oilers in general. ' ,

As, described above, EPA's discussion with Fleriuning Hansen ofHaldor Topsoe also

revealed that despite concerns about an SCR system subject to the sodium environtilent that

would exi~ for a North Dakota lignite application, Haldor Topsoe would not rule out bidding

such a project.

"B... SCRTechnology is Presumptive BART for Cyclone Boilers in EPA's BART

Guidelines

EPA's. January 9, 2007 Comment:

EPA made a specific and informed decision in its BARTGuidelines'regarding cyclone-fired utilitY

boilers. For all other boiler types, someform ofcombustion control was deemed to be the presumptive

level ofcontrol: Howf!ver, for cyclone units burning-any,type ofcoal (bituminous, subbituminous, or

lignite)"presumptive BARTis SCR. SCR was se'ected because the technology isfeasible and cos/-

effeetive : "

B&McD's March 19,2007Response:

EPA ignored the num(j!rous comments receivedforthe draft BARTguidelines (similar to that cited

in the EPA's CAIR discusstonfor the preceding issue) about the infeasibility ofSCR applied to North,

Dakota llgnite. The unit capital cost capacityfactorfor SCR retrofits applied to cyclone boilers'py the

EPA in th~ cost-effectiveness analysis usedfor establishing presumptive BARTwas $100IkW. . '

Burns & McDonnell believes the EPA's cost,;,effe'ctiveness 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-fired cyclone boilers.

EPA Response:

B&McD contends that EPA ignored numerous comments received on the draft BART

guidelines about infe~ibility ofSCR applied to North Dakota lignite. As stated above, EPA

, weighed many different factors and concluded that SCRis technically feasible for a plant fired

on North Dakota 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 fot 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 fot MRYS.. EPA has maintained that:

Minnkota should provide a cost anaIysis for Units 1 and 2 according to step 4 ofthe Top-DowD.

BACTanalysis.'

~37-
· .

EPA Response to Mimikota Comment Letters from

March 19,2007 and April 23, 2007

BACT Analysis at MRYS

C. February 27, 2006 Final Revisions to 40 CFR Part 6.0, Subpart Da, N~w Source'

Performance Standards for Electric Utility Steam Generating Units Set Final '

NOx Limit Ba~ed on Technical Feasibility of SCR for LigDite-Fired Units.

EPA's iO",':'(Try 9,2007Comment: ,

In'its response to public comments on the February 28. 2005 draft NSPS, SubjJart 1)aproposal, '

EPA rejected comments received on NOx controlrequirementsfor lignite-fired units. Specifically, EPA

took the same position as it did in the CAIR and BARTrules, rejecting commenters' concerns that SCR

wouldnot be feasible for /ignite units. No legal challenges were made to the final rule regarding EPA's

,response to the lignite concerns... ... ..... '

B&McD's March 19,2007Response: .

Bur"s & McDonnellrejects the EPA ;s argument that the lack ofa lawsuit protesting the'

establishment ofrevisedNSPS limitsfor new electric utility steam gen.erating un.its (EUSGUs) that

require SCRsfor NOx control means that this is appropriate technologyfor a BACTretrofit on existing

lignite-fired cyclone boilers. Cyclone boilers are no longer co,mmercially available, so there are no new

cyclone boilers beingpermitted and built. Utility owners ojexisting North Dakota lignite-firedb()ilers

would not be expected te cC?mment on the. draft revisions to the NSPSproposed by rhe EPA because they

apply only to new units with different methods o/firing coal than cyclon~ boilers. It is doubtful that the'

EPA consideredfiring /ignite in new cyclone boilers would be apossihi/fty when they proposed the draft

NSPS standards..

Any recently built orpermitted lignite-fired boilers have'been predominantly cfrculatingjluidized,

bed (CFB) type andhave not incorporatedSCR technology/or NOx emissions control. 'There have, been

recenipermit application,sfiledfor COnstructing pu,lverized coal boilers burning Texas /ignite that ,

propose to include high-dust SC;R technology. However,' there are distinct differences between North

, Dakota and Texas /ignites that preclude assuming that this supports the technicalfeasibility ofapplying ,

high-dust SCR technology to e~isting cyclone boilers burning North Dakota /ignite.

EPA Response:

, B&McD contends that since no ne~cyclOIi.eunits,are being peimittedand'buil~existing ,

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 considered fIring lignite in new cycl~neswhen

proposing the draf!; NSPS st~dards. Actually, the new NSPS standards applyto affected utility

boilers that:co~ence construction, reconstru,ction, Of'modifIcation after.' February 28,2005.21

(EPA's origi~al comments erroneously said that the standards only, applied to' new units.) This

would include any existing cyclone units fIring North Dakotalignite that undergo reconstructIon

21 See 40 CFR §60.44Da(e)

-38- '
EPA Response to Miimkota Comment Letters from

.March 19, 2007 and April 23,2007

BACT Analysis at MRYS

ormodification. 'The standards th~t would apply to recons,truc~ed unjts (1.0 Ib/MWh or 0.11

Ib/MMBtuon a 30-day average) and modified units (1.4lb/MWh or 0.15 Ib/MMBtu) are well

below the BACT liinit proposed by Minnkota. Since these standards apply to modified and

reconstructed affected Units and notj\,lst new units, the exclusive construction ofnew circulating.

fluidized bed (CFB) boilers1n North Dakota and the 'differences between Texas lignite'andNorth

Dakota detailed by B&McD and EERC are not particularly relevant to the revised NSPS. EPA

does not disagree that all ofthe recently-permitted utility boilers have been CFB boilers

(although that does not rule out'the possibility ofa PC-fired unit inNorth Dakota in the future).

EPA also agrees that there have been some recently-permitted units hurning 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 differences'indicate that EPA'

, "did not consider NorthOakota lignite in revi~ing the NSPS for utility boilers.' '

EERCResponse:

Norih Dakota lignites are substantially different than Texas lignites in composition andfouling

tendencies. The followif'!g comparison shows the proximate analyses, sulfur contents and heating values

for various lignites, plus ash compositional analyses ofTexas lignites Comparison ofNorth Dakota

and Texas Lignites Table -3 summarizes ash composition analysis expressed as equivalent oxide.

The compositional analyses show that the aSh has relatively low sodium and lower cil/cium content than

North Dakota lignite. The problems ofSCR catalyst blinding due to the formation ,ofsodium calcium

'sulfates createdfrom the firing ofTexas lignites will be much less severe thanfor 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 thatthe '

Jackson South Texas lignite has a somewhat comparable ash sodium content above 3% (EPA

, assumes this is an average) compared to the average (or ~Oth percentile) value of4.4% for Center

mine coal (per the April 23, 2007 coal analysis submitted by Minnkota).· Considering the high

ash content ofthe Jackson South lignite (42% average) l:U1d low heating value (3,972 Btu/lb as

received), compared to the ash content (9.53% average) an~,heating value.(6,572 Btu/lb) of

Center Mine lignite, the Jackson South lignite ofTexas would actually have a much higher

sodium loading (on average)'ihan Center Mine lignite (by rougWy a factor of4). The Yegua

East ligriite with an ash content of 18% and a heating value of 5,761 Btu/lb 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 perBtu basis to Center Mine

',lignite.' . ' " ,

-39-
EPA Response to Minnkota Comment Letters'from .

March 19,2007 liJtd April 23, 2007

'BACT Analysis at MRYS

Notwithstanding the difference~beiween North Dakota lignite and Texas lignite, a

modified cyclone boiler firIng North Dakota lignite would~ti11 need to ~eet the most recent"

NSPS Da limit for modified' tmits, which ismuch lower than the BACT limit proposed by

Minnkota. The 'likely choice to meet thi~ limit of0.15 Ib/MMBtu would be SCR. '

D. SCR and Catalyst Vendor Information,

EPA '$ January 9, 200tComment:

As noted above, thefollowing catalyst vendors were comfortablep.roviding catalysts/SCRfo;' lignitefired boilers: Haldor-Topsoe,Cormetech, andArgillon.

In addition, ·the above-cited memo datedAugust 20, 2004, Jim Staudt ofAndover Technology

Partners to Sikander Khan ofEPA CAMD, discussed both the EERCpilot testing 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 contaetedthe Institute ofClean Air Companies website to obtain thefollowing

list of21. companies offering "SCR Systems, Equipment, Su!!pltes andServices:"

EPA Region 8 has visitedmost ofthe vendors'websites,andfound many vendors with experience

tha/would,be germane to the MRYS situation We believe it would be prudentfor Minnkota to obtain

information andproposalsfrom more ~han one vendor and involve the NDDH and.$PA}n 'theirfindings.'

EERC's. March 19, 2007Respon$e:

,EE'RC has already included some, commentsfrom discussions with Haldor Topsoe regardingtheir

, ' concerns about the sodium content in North Dakota lignite and its tendenciesfor catalyst deactivation. '

Some ofthe EPA-cited SCR catalyst vendors' websites discuss the problems ofsodium and deae,tivation,

However, EPA has notprovided a review- ofthat information.

EPA's Response:

,EPA has already disc~ssed its phone (:~nversation with FlemmingHansen ofHaldor

, Topsoe and how Mr. Hansen, while ackn9wledging some difficulty de,~ling with the sodium ,

issue with North Dakota lignite, did not dismiss that an SCR system might'be technically feasible

for aNorth Dakota lignite application, not the pos~ibi1ity that Haldor Topsoe would bid on such:·

a project. ." "

B&McD March 19, 2007Response:

EPA's listing of21 suppliers of "SCR Systems, EquiPment, Supplies, and Serv!ces~ .. " has nQt been

screened to e.liminate companies that are not active.in the U.S. coal-jired SCR market... ....EPA 's
EPA Response to MinnkotaComment.Letters from

. March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS

evidence ofSCR catalyst vendor's willingness to guarantee performance on a lignite-fired boiler is very

general in nature and doe.s not include typical qualifying statements related to (he cheni's need to adhere

to specifiedSCR design conditions, which are yet to be ~stablished .

... ....from Burns & McDonnell's experienc.e, what d vendor orsuppliersays pre-biddoes not

always match wluit they actually will offer in afirm contract..... a commercial guarantee does not mean

the system'proyided by the vendor will work long term; all it means is that basedon the language in the

contract and the amountofmoney the supplier has allocated to cover their risk or exposure, the vendor

will guarantee the system performance commercial contraCt guarantees always have afinite life,

which means the owner ofthe system only benefitsfroin the vendor's guaranteefor 3 years tYpically....

EPA's cites SCR catalyst vendorstatements lack substantive language that deals with mitigating

.the client's risks offailure to meet catalyst activity versus life guarantees, and the concerns recently

expressed by Haldor Topsoe when EERC described sodium components in thejlue·gas. In Burns &

McDonnell's opinion, SCR system vendors have not provided adequate documented andpublisheq

demonstrated catalyst experience with supporting technical information to'support these claims. The

vendors offered no specificcomptirative peer-reviewed data as proofthat currently-available catalysts .

can survive andprovide effective NOx reduction when exposed io 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 Respon~e:

EPA had originally suggested that Minnk:ota "obtain infotmation and proposals" ·from some

ofthe other companies that EPA had listed. EPA envisioned this to include more·ofthe level of

detail described by B&McD·in their March 19, 2007 response. Wliile 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 availabil.ity and the. ~echnical

'. feasibility ofa contr~l technique and could contribute to a"'determinatipn oftechnical feasibility

or technical infeasibility, depending on the circumstances." While EPA does not believe the

. vendor guarantee is the sole.factor in determining technical feasibility ofseR for MRYS, EPA

does.believe it would be useful information to obtain for making the determination. , . .

-41-
EPA Response to Minnkota Comment Letters from·

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

EPA's·RESPONSE TOEERC AND B&McD APRIL 23, 2·007 LETTER TO NDDH

COMMENTS - FEBRUARY 1, 2007 (INCLUDING RESPONSE TO u.S. EPA REGION 8.

COMMENTS - JANUARY 26, 2007): REQUEST FOR ADDITIONAL INFORMATION·

·ANi> THE NON-SCR COMMENTS FROM EPA

IV. MINNKOTA'S RESPONSES TO ADDITIONAL INFORMATION REQUESTED.

A. Center Mine Lignite Coal Ash Content and Variability

EPA's January 26, 2007Request:

Please provide data currently available on the tenter Mine coal constituents (presented in

· Minnkota's October 2006NOx BACTAnalysis), includingall av.ailabl~ data samplingforjuture and

presently-minedlireas ofthe mine. Please provide the data in terms ofaverages, 'in addition.to vqlue

ranges.

BERCApril 23, 2007 Response:

The his,torical database was obtainedfrom samples cqllect~d for "the purpose oftesting the imp~cts .

oflignite characteristiCs on plant performance. The database consists of1317 as-fired samples. .The

average, standard deviation,. maximum, minimum and selectedpercentilesfor ash, sulfur, heating value,

and selected ash constituents are includedliz Table 1.

The use ofaverage coalpropertiesfo:" design dndperformance ofcombustion and envl"on;"ental

controlsystems typically underestimates adverse problems such asfireside ash deposition, opacity, and ..

sulfur emissions. Designs ofpowersystems must considerthe challenging coal characteristics in orderto ..

develop ·designs and'operatingparameters with s.ufficientflexibility"thata(lowfor. the combustion ofcoal~. ..

that at leastrepresent the 9(jh and 1(jh percentile ·ofselected constituents ; The frequency . . .

distributions ofcomponents ofthe coal provide better understanding oj/he potential challenges....The

results ofthe analysis show that the lignite ash has a 4.4% average sodium content; however, there is q

multimodal distribution ofsodium levels in the coal ash.....The average properties as well as the

minimum and maximum values do not adequately represent the characteristics o/thefuel. As indicated in

· Figure 1, the MRYS boilers are firing some ·very high-sQdium-containing coals that can cause significant

· problems when sodium reacts with silicate, alitmrnosilicate, and sulfurspecies in the furnace and lower

temperature regions ofthe·powerplant. . ..

EPA Respons·e:

EPA has reviewed the coal data submitted by Minnkota. .EPA believes.the future coal.data .

· appears to show little change from the 2007 core samples. EPA is not sure what the basis is for·

EERC's statement that "s'odium levels have shoWn increased levels in the future coals that Win

likely contribute to increased fouling chall~:ng~s.,,22 .Th~ sodium oxide percent~ges from 2007

22 See page 10 of April 23, 2007 letter from Minnkota to EPA, DOJ, and NDDH..

-42-'
EPA Response to MinnkotaComment 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 ror making

general comparisons with other fuel types, but it is unclear how these differences can be directly

applied when making determinations on feasibility of SCR at MRYS.' '

B. High Economizer Exit Temperature

NDDH's February 1, 2007Request:

The temperature betwee,n the economizer and the air heqters is described as "much higherthan,

750 degrees". Please proyide more speqijic data on the temperatures that would be encountered at the'

location an SCR would be installed '

B&McD's April 2$, 2007 Response: , ,

The primary superheat (PSH) outletjlue gas temperaturesfor Unit 1 are provided instead ofthe

economizer outlet temperatures, Unit 1 at the MR. Young Station has a two-pass tubular aiipreheater

(APH). The eco,nomizerfor Unit 1 i~ situated between ,thefirst and secondpass ofthe air preheater. The

flue gas passes on the inside ofthe ~ubes ofthe APHoo ...:..... The configuration ofthe economizer and air ,

, preheaterphysically precludes locating a conventional high~dust SCR upstream ofthe existing air

preheater-economizer on Unit 1 in its current location. and configuration. T,he flue gas temperqture is

reduced by transferring heat to the boiler'sfeedwater and the boiler's combustion cmd drying system air

streams, respectively. In addition, temperatures at the existing air preheater outletfor Unit 1 are far

below the temperatures neededfor operation ofa conlJentional hoi-side high-dust SCR.

The Unit No. 1 boiler is shut down/or cleaning at intervals rangingjrom approximately 90 to 120

days. 'The, gas temperatures'at the, PSHoutlet vary inversely with the boiler cleanli1:zess during each

operating,run... ... Note that qt low loads and clean condition,s, the PSHoutletflue gastemperatures are

, 170-270degrees F below the acceptable range for conventional hot-side high-dust SCR, catalyst ....Unit'

1's PSHfull-loadjlue gas temperatures historically range between 840 F and 960 F, depending upon the

cleanliness ofthe boiler.....there haVe been instances when UnitJ's PSHfull-load averagejlue gas

temperatures have exceeded 1000 F and individual temperature measuring elements have exceeded 1050

F. . .

A portion ofMRYS Unit 2'boilers'feedwater heating surface is in 'a location normally occupied,by

a traditional conve1')tional economizer section in the convection'pass ofthe boiler The

configuration ofthe boiler's economizer and the air preheaterphysicallyprecludes locating a

conwintional high-dust SCR upstream ofthe existing air,preheater/economizer on Unit2 in its current

location and configuration :Unit 2 boiler is shut down for cleaning at intervals rangingjrom

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~outlet full-load

flue gas temperatures historically rangejrom 780 to /J80 F...:. there have been instances when Unit 2 's

-43-
EPA Response to Minnkota Comment Letters from

March 19, Z007 and April 23, 2007

BACT Analysis at MRYS·

average full-load economizer outletflue gas temper.atUres have exceeded 930 Fand individual

temperature measuring elements have exceeded 990 F.

EPA Response:

This inforniation that was requested by NDDH was reviewed by EPA. EPA acknowledges

that there are times when the flue gas temperatUre at the portion pfthe boiler where a high-dust .

SCR would be located (primary superheat outlet for Unit 1 and presumably the economizer exit

for Unit 2) would be higher than the temperature range typically ·suitable for SCR. .

. While B&McD seems to indicate in the March 19, 2007letter that the dual-air preheater

design suggested by ERG w:ould be problematic,it later concedes that it conceptually could

work, but would be at an additional cost. EPA agrees that sucha design would cost more thana·

typical high dust SCR application. A more refined cost estimate that 'reflects real costs

associ~ted with constructing and operating Ii split air heater design can certainly be submitted by . Minnkota. EPA believes this would be conducted under step 4 ofthe Top-Down BACT

analysis..

. ' .

. For further information on this topic, please refer to the August 2007Christman Memo

response regarding the split air,heater. ' , .

c. Low-Dust SCRFouling

Nl)DH's February 1, 2007Request: .

, It is indicated that a low dust SCR would be suhject to the samefouling problems asa high dust

SCR due the emissions ofsmall«5 micron) particies coniainbig sodium-calcium sulfurrich materials.

Please provide any existing data on the composition ofthe particulate matter exiting the electrostatic

precipitators. .

EERC Apr~123, 2007Response:

, As previousiy discussed in responsesto the ePA's January 8; 200'1 comments: the MRYS cyclone .:

boilers'fly ash becomes enriched in components that vaporize and, subsequently, cond.ense upon gas

cooling toform liquidphqses andsmallparticles/hat are very reactive to gas-phase components, '

resulting information ofcompoundS that cause bonding, blinding, and deactivation ojindividualcataiysi .

reactionsites, eve,? in a low-dust SCR application. Sodium-richaerosols can get through the

electrostatic precipitator (ESP). These aerosols are known to be present in high-:sodium coal combustion

flue gases. These spe9ies are known to produce pyrosulfates. These will cause blinding anddeactivation

ofthe low-dust catalyst. .

-44-
EPA Response to MinnkotaComment Letters from

March 19, 2007 and April 23, 2007

, BACT Analysis at MRYS

Stack sampling was conducted downstream ofthe 'BPS and set FGD on Unit 2 at the MRYplant.

The filters were examined using scanning electron microscopy andx-ray microanalysis to determine the

microscopicfeatures andchemical 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 r~viewed by EPA. While the relative

effects of vaporized particles passing through the ESP and condensing prior to a low-dust SCR

system ~ight be more pronounced compared to the non-vapor particles impacting a high-dust

environrrient, 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 in'a

high-d1,lst application would be greater than .in a low-dust application. This is substantiated by

EPA's discussion with Fleniming Hansen ofHaldor Topsoeand his experience with SCR test

coupons at the WE Energies Presque Isle Station, as discussed above. EPA acknowledges that

,there are other technical and cost issues related to low:-dust SCR that were discussed previously.

D. SCR Deactivation

EPA's Jal1:uary 26,2007 Request:

Please provide any andall data and information related to catalyst deactivation rates andNOx

, contro! effectiveness measured at the threefacilities tested during the "Ash Impacts on SCR Catalyst

, Performance" stiidy that is extensively referredto in the "EERC Report" included as Appendix B ofthe

Minnkota NOx BACTanalysis. This should include the original jJroposal,all testplans, 'all raw catalyst

deactivation andNOx control data collected during the test, and allfinal reports and conclusions that'

were producedfo~ 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 Coyote SCR test catalysts were too plugged and deactivation testing couldnot be conducted. The lack '

ofpublished catalyst activity datajor the Coyote slipstream SCR catalystfield test is a result ofthe

severity ofthe catalystfoulingfrom North DakOta lignite ash. The requested data is available; this

information has been copied, compiled, anddue tq its volume, will be submitted separately.

Information on catalyst deactivation rates andNOx reduction effectivenessforthe Baldwin and

Columbia Station slip-stream SCR catalystfield testing was determined and some ofthe information was

in the final report. EERC recently received informationfrom Hitachi on catalyst deactivation for the

-45-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACf Analysis at MRYS

catalysts atColumbia. This information is include,d inthe EERC data compiled and being submitted to

theNDDH.

Regeneration has been attempted in Europe with SCR catalysts exposed to biomass as with high

potassium (elementalsymbol K, similar in chemistry to sodium, Na), and they hadsome significant

problem.in recovering reactivity (Khodayari andOdenbrand, 2001). .

EPA Response:

EPA has reviewed the background documents. Please see EJlA;s previous detailed analysis

.related to the review ofthe pilot study documents under the section,"EPA Analysis ofBurns&

McDonnell's Conclusion" (responding to the March 19, 2007 letter response.)

B&McD's April 23, 2007Response:

.A qucmtitative analysis on the application ofthe multiple engineering solutions 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 the jlue'gas streams

emiitedfrom the boilers at MRYS over a significant length oftime to simulate real-time effects ofgas and'

.flyash constituents r;md their interactions with the catalyst. Using assumptions ofthe effectiveness of

solutions 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 not treatflue gases

producedfrom cyclone boilersfiring North Dakota lignite coal would be hypothetical. 'Such a theoretical

quantitative analysis ofEPA's claimed SCR catalyst improvements cited andwhether this.would actually

be translatable into preventing the catalyst layersfrom being susceptihle to macro-pore'blinding and

plugging by calcium-based compounds and the impact on the individual catCdyst reaction sitesfrom

sodium-based c~mpounds discussed i,! Dr. Benson's referenced technicalp'aper would be speculative.

Burns & Mc!Jonnell believe-s it would'not b~ accurate to burn lignite or use jlyash samples

collectedfrom a MRYS electrostatic precipitator w,i{h artificialflue gas to simulate catalystfouling and

blindingphenom€mon in a laboratory'- or pilot-scalete,stsetting Performing a quantitaiive analysis

. would require substantialresources to develop slip-stream SCR catalyst test equipment designs and

operational conditions to (lddress the significant technical challenges !{tat were evident in 'the reference

field demonstration testing at Coyote station. It is· unreasonable that the EPA expects that such.an indepth investigation is reqUired under a BACTanalysis.' .

Performing a BACTanalysjs 0'.' an existingunitshouldnot require substantialresources ..•. :..

-46-
EPAResponse to MinnkotaComment 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 by B&McD

and have been largely addressed elsewhere in EPA's previous responses. See the March 19,

2007 respoJJ.se to "North Dakota Industrial Commission project Evaluation of Potential SCR

Catalyst Blhiding·During Coal Combustion (from the.January 8, 2007 ERG Memo)" 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 withoutdoing additional pilot

testing. As noted previously, EPAbelieves that Minilkota 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 SCRsystem to deal with the plugging, blinding, and

'. deactivation issues, EPA is not convinced that it wou.ld be required ifaconservative design

approach is'taken. . ..

In the previously mentioned meeting with Sargent & Lundy, LLC, Mr. WilliamDePriest,

Senior Vice President and Director or Environmental Services, indIcated that an SCR system

could theoretically be designed to operate on North Dakota lignite, but indicated the design

.would likely have'to be conservative in terms ofthe catalyst design ~d sizing ofthe reactor and

could result in significantmajntenance requirements and associated outages. Sargent & Lundy

representatives were ofthe opinion that a well-run pilot test would yield valuable infoJ;1:nation

that could be used in conjunetionwith previo.us SCR e~perience to evaluat~ and design an SCR

system for·a North I;>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 it would be preferable

to have additional data related to catalystdeactivation on North Dakota lignite in order to 'select

the optimum cat~l1yst design and reactor size. It is unfortunate that the pilot testing at Coyote

Station,did not capture' this information. However, the risksinvolved in the capital and

, operational costs cannot be used as the basis for a determination oftechnical feasibility. The

NSR Workshop Manual states that "(w)here the ,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 cpnservativdy-designed SCR system at MRYS that minimizes the associated ris~s

described and includes an: anticipated catalyst operation and maintenance schedule can· be '.

estimated and acco~ted for und~r step 4 oft1le Top-Down analysis.

23 See NSR Workshop Manual page B.19-20

-47-
EPA Response to Minnkota Comment Letters from'

March 19,2007 and April 23, 2007

BACT Analysis' a~ MRYS

E. SCR Vendor'Guarantees

EPA's Janl!ary -26, 2007Request:

. . .

Although EPA's Draft 1990NSR Workshop'Manual recogniies that vendor guarantees by

themselves are not,sufficient to show that a technology isfeasible, ve,ndor guarantees can contribute to a

technicalfeasibi/ity determination in some situations. In light ofstatements EPA hasreceivedfrom three

SCR vendors (Haldor Topsoe, Cormetech, andArgillon) indicating they wouldprovide performance

guarantees on SCR lignite applications, pleqse provide additional vendor iiiformation,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" 2007Response:

BERC has already'ineluded some co;"'mentsfr9m February 2007 telephone discussiQns with

Haldor Topsoe regarding ~heir concerns about the sodium content in North Dakota /ignite and its

tendenciesfor catalyst deactivation.

EPA Response:

EPA has previously presented its own discussions with Haldor Topsoe. .- ",

B&McD's April 23,.2007 Response:

A query ofSCR catalyst vendor's willingness to guarantee peiforma1Jce on a /iinite-fired boilerto

, addrffss comments in EPA's cited documents mentioned above has not been completed Additional time is

requited. The information on, SCRperformance guaranteesfor MRYS applications has been requested;it ' will,be submitted separately when it is available. A copy ofthe information request letter and

attachments·issued to the SCR vendors' is attached (Appendix A). ' ,

:Systeinand equipment vendors have a'se/f:interesi to sell technology that they offer, and to manage

finanCial risks by specifying,the limits oftheir system design in case ofpotential operating condiiions that

, cause performance problemsthat prevent achievement ofguarantees (mdlor warranty claims .

, EPA Response:

. B&McD states obtaining this infomiation will require more time. They have s~nt a letterto

four catalyst system suppliers and five catalyst suppliers'. The letter goes into great detail about .

the worst-case nature of Cent~ Mine co~l.· During the May 23, 2007 meeting in Bismark with

Minnkota and NDDH, B&McD indicated'that'they did get some feedbackfrom a few vendors,

but were unable to disclose any written response at that time. ,EPA has still not seen any vendor

, responses to the information .,request. EPA does note: that the request seems t'to have been written•

-48-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

with a pessimistic tone that wol.i1d likely lead the vendor to believe that the requestor was not

serious about obtaining a' contract for the describedSCR project. There is also reference to

coIl'linents made by.EPA on the Minnkota MRYS BACT analysis, which.would provide further

.disincentive on the part,ofthe vendor to properly evaluate and give serious consideration to the

request. Because ofthis EPA·is suspect that the feedback might be negatively biased and not

provide a realistic response to whether a vendor wQuld be interested in bidding a performance.

guarantee on an SCR project'at MRYS. .

F., MRYSBoiler Cleaning Outages

EPA's January26, 2007Request:

The Minnkora MRYS NOx BACTanalysis includes disc~sion ofperiodic boiler outages that"are

required to address the slagging effects oftbe Center Mine lignite. Please provide historical dates'of .

thes(1 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

i1) response to items] and 2afthislist.' '. .

B&McD's April 23, 2007Response:

. Information to estimate MRYS boiler outage frequency expectedfor high-dust SCR maintenance

based upon a query.ofSCR catalyst vendors mentioned in item 2 above is notavailable at this time.

Additional time'is required . '. .

A review ofhistoric outage data shows.that over the past 10 years (1997-2.006). thetypical MRYS

Unit] boiler cleaning outage was between 67 cmd 82 hours in duration. but these can be as short as 56

hours 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 andas long as

244 hours A boilerfireside cleaning-outage is typically scheduledfor 96 hours (four days) duration. . . . .

A high-dust.SCR 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 dW'ing boilerfireside surface cleaningfrom entering the SCR reactor. Beginning the work of

removing ash accumulation or catalystfrom the SCR r(!Qctor(s) will be delayed by 48 to 60 hours after

the start oftfle outages, depending on ambient temperatures, becausethe Inside ofthereactor(s)" will be

too hotfor personnel health andsafety. Because the length ofthe boiler cleaning outages is typically less

than 4 days (96 hours), that does not leaPe 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(~), orreplacingfouled catalyst with clean spare

catalyst. is expected to require at least 144 hours (6 days). plus 60 hoW's upfrontfor allowing the reactor

to cool-down.. This is based upon the use ofmobile industrial vacuum cleaning methods ("super-sucker"

truckJ.. The· total SCR maintenance outage time will be around 200 hoW'sfrom' boilershutdown end to

-49-
EPA Response to Minnkota Com~ent Letters from

March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS

beginning ofsiartup. The frequency o/such SCR· maintena1Jce outages are expected to be required at.

least annually, or more often. . .

There were only two outages other than 111ajor scheduled overhauls during the historic ten-year .

period that appeared to be ofsufficient length (greater than lJ days) thatcould have lent themselves to

significant catalyst cleaning or replacem.ent work withOut delaying startup: one on: Unit Jr and one on

Unit 2 There are basic concerns that using short boiler cleaning outages as SCR catalyst maintenance

opportunities could be detrimental to catalyst life due to thermalstress.es andpassing through various

(Jewpoint temperatures. Bringing the very large massofceramic catalyst material and the reactor

structure down in temperature and then back up rapidly and any morefrequently than absolutely

necessary should be avoided... :... .

EPA Response:

EPA looks forward to reviewing the inforni.ation obtain~d from the vendor request. Dunng

the previously discussed phone conversation wiih Flemining Hansen of Haldor Topsoe, Mr.

Hansen gave·Ii rough estimate that an· SCR system for North Dakota lignite could be ~lesigned:

(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 (need for the cool air drat\

requires the boiler to be.shut down) and the catalyst layers rotated in about two or three days,

three days all·around.. This is· significantly less than the rougWy 200 houis-(~r 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'~Hydrocarbon-E~hanced Selective Non-Catalytic Reduction (lIE SNCR)

EPA 'sJanuary"26, 2007 ~omment:

Minnkota eliminates thistechnoiogy primarily based on thefact that it has not been appiied to

cyclone boilers or units burning /ignite or high~slagging western subbituminous coals. Thf!re are two full

scale installations ofthis technology on eastern pulverized ooal-fired boilers. There is some general .

discussion about the slaggingproblems that /ignite wouldcause the HE SNCR technology, but·it is

unclear whether the vendor ofthis technology would be. able to overcome these issues. At this point, it is

unclear wh~ther HESNCR ·is technicallyfeas.ible. Minnkota. shouldprovide a more tho;9ugh analysis on

whetherthis.control option is "available andapplicable" (as described in the NSRMa.nual) at MRYS by

examining the physical and chemical characteristics ofthe exhauststream in terms ofteqhnicalfeasibility .

and soliciting vendor information. .A quantitative analysis ofits technicalfeasibility-s.hould be provided ...

and supportedwith data.and vendorstatements.. .

-so-
EPA Response to Minnkota Comment Letters from

'March19,2007 and April 23, 2007

BACT Analysis at MRYS

B&McD's April 23, 2007 Response:

The Dctobet 200~ MRYS NOx B4CT(lflalysis was performed using technical literature m'ailable in the

public domain, proprietary vendor customer experience informatiQn, and limited engagement ofs~lected

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 lignite:fired cyclone boilerfrom the single vendor

mentioned in the October: ~006 MRYS NOx BACTAnalysis·reports has not been performed Additional"

time would be required., It is important (lfld 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 pfactl!al operating conditions and boilerfouling experience are yet tt? 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 cQnditions to be assumed before workable design,concepts hm'e been c'onfirmed.~ ..It is

interesting to note that other non-cyclone BART-elig'ble boilers burn,ing North Dakota lignite did not

..mention or evaluate HE-SNCR technology as a possible'NOx control alternative in their BARTanalyses.

"

, EPA Response: ' ,

EPA has si~ply asked MirIDkota to get the vendor's input on the facility in question.

Minnkota's original BACT an~lysis describes theoretical problems thatmight be encountered if

. HE SNCR is installed at MRYS and states that the lack of experience with cyclone boilers .

buri:ting low-rank western coal satisfies a showing that HE SNCR is technically infeasible. The

vendor who offers this technology states that it is not sensitive to fuel type or burners, so EPA

still questions whether the ,determination oftechnical infeasibility is valid without getting vendor

input orsubstantiating data.

EPA does not agree ~ith B&McD's interpretation ofthe NSR Workshop Manual.

,B&McD'states that in order for the vendoi' to develop a realistic technical basis that would .

support estimated unit.;.specif!.c HE SNCR performance, capital and operation and maintenance

costs, they would need specific customer data B&McD claims this is not required by an

applicant because the NS~ Workshop Manual states, "a source would not be required to

experience extended-time delays or resource penaltieS to allow research to be conducted on a

new technique." .This statement.is in the context o~ determining whether the technology is

"av~lable." 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 Manual made by

B&McD is only relevant ifthe 'vendor would not Qfferthe technology to acustomer based on

specific conditions ofthe plant. Minnkota can go through this process ifit believes the

~51-
EPA Response to Minnkota Comment Letter:s from

March 19, 2007 and Apri123, 2007

BACT Analysis at MRYS

technology is not available, but apparently does not wish to do so. Hence, EPA believes the

technology is "available.". IfB&McD does not believe it is "applicable," further info11I).ation

should be provided. The reasons given'byB&McD to this point (high boiler fouling conditions

and varying gas temperatures and loads) appear themselves.to be theoretical. EPA believes

.statements from vendors or more quantitative information might be helpful for making this .

determination· .

EPA does not believe that whether other ~on-cyclone fired BART applicants inclu~ed an'

analysis ofHE-SNCR is evidence, in and by' itself, that the ~echnologyis 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 power plants. ,~ , ' .

H. . Reburn with SNCR

EPA's: January 26, 2007Comment:

While variousjorms ojreburn systems were determined to be technicallyf~asible in the Minnkota .

analysis, reburn combinedwith SNCR was ruled outbased on technical infeasibility. Although..fullscale

pulverized coal installations ofgas reburn w.ith SNCR and overfire air werefou~d 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-jired boiler, especiallyfor lignite-firing, makes the applicationfor MRY .

Station's boiler injeasibie. " 'Minnkota shouldprovide a more thorough analysis.....

B&McD Response:

A detailed query ofthe potential availability and applicabilitjl,offuelreburn technology combined' .

with SNCR on a lignite-fired cyclone boilerfrom the reburn technology vendors has not been performed.

As mentioned in Appendix Al ofthe October 2006 MRYS NOx BA(:TAnalysis reports, while SNCR may ,

potentially be installed downstream ofseparated oveifire 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 a tangentially-jired boiler, wasfound [n available technical literature. Moreover,'

the very high capital and O&MCo.sts't;llready estimatedfor coal andga~ reburn technologie,s with . ,

ASOFA in Section 3 ofthe October 2006 MRYS NOx BACTAnalysisreports showed that such " . .

,alternatives were siinificantlyabove the BACTDominant Controls Cost CurvesforMRYS Unit rand '

, Unit 2. Thus, even iffuelreburn technology combinedwith,SNCR ~ere available aridapplicable to

lignitefired cyclone boilers, the average .and incremental unitcontrol co,sts ($ pertim) wouldnot be, .

'expected to be on theNOx BACTDominant-Controls Cost C1,Irvesfor MRYS Unit I andUnit 2. Such

reburn with SNCR alternatives would thus be consideredas "injerior",alJdwouldnot be considered as.

BACT Substantial resources would be required....lt is interesting to note thatother non-cyclone BARTeligible boilers burningNorth Dakota lignite did not mention or evaluate reburn with or withoutSNCR

technology as possibleNOx control alternativesiri,their BARTanalyses.

-52-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS .

EPA Response:

B&McD.has declined to perfonn an analysis ofwhether Rebum with SNCR would be

"available and applicable" because'the technology has not been "demonstrated" on cyclone

boilers. However, according to the NSR 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 delp.onstrated, it is considered technologically feasible

without going through further analysis. Hence, B&McD's logic for not doing the "available and

applicable" analysis appears to be contrary to the NSR Workshop Manual.

B&McD also makes the argument that the average and incremental cost associated with .

Rebum with SNCR would not expected t() 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, Rebl,Ull with SNCR would offer greater NOx emissions reducti<;>llS tlianjust SNCR

(assuming e~ch wo~ld 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 out that non-cyclone BART eligible'sources in North Dakota did not

provide anahalysis ofRebum with or without SNCR as a control alternative in their BART

analysis. For the reasons cited above, E~A does not agree that Reburn with SNCR can 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 ~liminates this technology because ofthe difficulty in achievingpropersubstoichiometric hoiler con,ditions necessary to get effective NOx control; As described in the application,

the problem occur~ becat!Se ofthe variable heat content ofCenter Mine lignite. While RR1 has been

.successfully demonstrated at cyclone tinits burning blends ofPowder River Basin coal with Midwestern

bituminous coal, Minnkota hqs ruledRR1 out as technically infeasible. However, there does not appear

ro be an analys.is ofwhether RR1meets the "available and appNcable" criteria described in the NSR

Manualfor elimination understep 2~ IfRR1would be less effectivefor the'reasons cited in Minnkota's

application,that would result in d lowerranking understep 3 andpossible elimination under.step 4. At'

this time, it does notseem qppropriate to eliminate RR1 understep 2. Minnkota shouldprovide.a more

thorough analysis..... . . . .' .''. .

-53-
EPA Response to Minnkota Comment Letters from

March 19,2007 and April 23,.2007.

BACT Analysis at MRYS

B&McD Response:

Since the publication ofthe MRYSNOx BACTAnalysis report; two commercial Rich Reagent .

·Injection systems have completed design, fabrication, and insta!lationin lcite 2006 at Ameren Sioux Unit

I and Unit 2. Full optimized routine operation is expected to begin by early May, 2007. Although RR1

NOx emissions reduction technology appears to satisfy the "available" criteria (as described in the NSR

Manualfor technicalfeasibility) for potential application on the cyclone boilers at MRYS, the substantive..

issue is "applicability". The expected insurmountable diffiCUlties in achieving consistent control· .

·effectiveness.;... from this technology ifapplied to modestly air-starved cyclone combustion on these

lignitefired boilers and the reasonsfor technical infeasibility ofRRl at MRYS were included in Appendix

Al ofthe C!ctdber 2006 NOx BACTAnalysis reports.

o A query ofthe single RRllicensee mentioned in the MRYS NOx BACTanalysis, and a quantitative'

analysis ofits technicalfeasibiiity for ihe expected boiler operating conditions has not been performed .

The transient variability ofthe fired lignite'coal at MRYSwas discussed in AppendiX Al ofthe October'

2006 NOx BACT.Analysis reports. This variability makes it impossible to accurately prefjictrealistica./ly

achievable NO"x emissions reductions even with sophisticated computationalfluiddynamics (CFD)

modelingofthe'MRYS boiler combustion reactions and in-furnace NOx reduction processes·associated·

with 1?RItechnology underreasonably realistic boiler operating conditions. This is due to the static(i.e.

steady-state) nature ofthe boilerfurnace operating conditions aSsumed to exist in order to simulate the

performance ofmodestlyair-stcirved cyclone. combustion on these lignitefired boilers with varying heat

and air inputs to individual cyclones usingCFD modeling techniques. Because ofthe dynamic variability

ofthe higher heating value ofthe fuel burne{J"tmd the lack ofabilitY to precisely control the actual

stoichiometry in real-time on an individual cyclone basis, RRlCFD modelpredictions would be .

unrealistic and thus these vendorstatements would not be reflective ofrealisticallyachievable NOx

. emissions reductions. Substantial resources would be required .

.EPA Response:

EPA believes that the recentinstallation ofRRI at the Sioux I and 2 cyclone wiits is also

strong evidence that the technology is "applicable" to MRYS. The NSR.Workshop Man~al .

states that '~ifthe control technology has been installed and operated successfully on the type. of

source under review, i~ is demonstrated and it is technically feasible." While the'Sioux plant'

does. not bum lignite, PRB coal is ai~o a low-ranking western coal. This indicates to EPA that

before ruling out RRI as technically infeasible, detailed analysis should be provided. . .'

.WIllIe predicting"the exact level ofNOx control for RRI and MRYS 'may not be possible, ..

EPA believes some informed estimates could be made. Further, EPA does not believe predicting

exact levels ofNOx control is necessarily crucialto whether a control option is technically

feasible. Hence, EPA does riot believe B&McD has demonstrated that RRI should be considered .

·technically infeasible: In fact, EPA believes the recent operation ofRRI at the Sioux. plant'

provides strong evidence that RRI would be technically feasible at MRYS. .

-54'-
EPA Response to Minnkota Comment Letters from

March 19, 2007 and April 23, 2007

BACT Analysis at MRYS

J. Rotating Opposed Fire Air (ROFAl

EPA's January 26, 2007 Comment:

Minnkota eliminates this technology because it "lacks demonstrated cyclone-jired experience, and thus

was considered i1Jfeasible." While a technology that has been demonstrated on the source type under

review would show it is technically ftasible, lack ofsuch demonstrationJor the specific type ofemission .

.. unit does not necessarily lead to a conclusion ofinfeasibility, Minnkota shouldprovide 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 October.2006 MRYS NOx BACTAnalysis

. reports has not been performed Burns & McDonnell has reviewed available ROFA technical literature,

attended sales presentations andparticipated in discussions with ROFA vendor personnel regarding the

application ofROFA to cyclone· boilers, incltding lignite-jired units. As an engineer and design/builder

.ofnumerous commercialseparated overjireair systems specifically designedfor cyclone-flred boilers,

.. Burns and·McDonnell believes tha! ROFA te<::hnology is notsuperior to the evaluated version ofSOFA in

the October 2006 MRYS NOx BACTAnalysis to warrantsuch a vendor query and a quantitative analysis

......... ROFA does not offer any significant advan.tages in lignite-fired cyclone-specific application of

separated overjire ,airtechnology. .

EPA :Response:

EPA does not curnbntiy have information to the contrary and will agtee that this option may

not differ substantially from ASOFA or offer greater NO" reduction on cyclone boilers. Since

. ASOFA has been 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 ofO.361b/MMBtu and the emission rate expectedfrom

installingAdvancedSepara~ed Overjire Air(ASOFA) alone (0.5131~/MMJjtu), it dppears as though the

SNCR controls will only be achieving approximately 3(JOAJ reduction. (p.3-18) AP-42 states that SNCR·

can achieve NOx reductions between 30-6Q%. Many recent PSD permit 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 uncontrolledNOxemissions (evenconsidering the ASOFA), the

IJighend ofthis range should have been considered in the BACTanalysis. Minnkota's analysis really

provides no techrzical discussion on why it ruled out higher capabilities ofSNCR control andwhy they

used the control efficiency that is proposed Using 60% control wouldyield an emissions rate of0.21

lb/MMBfI.!.for a BACTlimit with ASOFA and SNCR. compared tojhe proposed 0.36.1b(MMBtu.

-55.:..
EPA Response to Minnkota Comment Letters from

March 19, 2007 and Apr!) 23, 2007

BACT Analysis at MRYS

B&McD.'s April 23, 2007 Response:

There were pertinent rejerences in AppendixAl ojthe MRYS NOx BACTanalysis discuSsing long- ..

term examples where'SNCR has been appliedjor NDX emission control on two cyclone-jired boilers, .

achieving reductions around 30 percentfrom'high uncontrolled baseli1!e levels. There was also another

relevant cyclone boiler SNCR demonstration test listed in Appen{1ix.A2 ojthe MRYSNOx BACTanalysis'

that controlledNOx in this same 30 percentrange. NOx control effectiveness' ojSNCR is highly boilerspecific. It is important to note that PSD permit applications arejor. miw boiler~; there are no new

cyclone boilers being built, andfew new boilers (other than CFB boilers) are required to install and

operate SNCR. The citedAP-42 NO~ emissions rate reduction percentagefor SNCR application,on coal..:

fired boilers isn't applicable to the specific conditions expected at the MRYS boilers... .Burns &

McDonnell estirflt;lted the level ojemission reductions expectedfrom SNCR based on injormationfrom an

, experienced urea-based SNCR vendor. B&McD believes that 60 percent reduction olNOx emissions is '

inappropriate, and not rej1ecti~e or realistic for long-term levels ojcontrol 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 lik~ly that

,something closer,to 30% than 60% wouid be reasonable~ ,

L. Selection of 30-day Average BACT Emission Limit

EPA's January 26, 2007 Comment:'

The baseline emission rate usedfor ~alculating the proposedBACTlimit is based'on the highest,

NOx emission rate (0.884 Ib/MMBtu) during 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 ojthe historical data? Does this emission r'at~ represent a realistic upper'

bound value, or is it an outlier comparedwith the majority ofthe data? Ifihe d,ata point does not

reasonably compare with the vast majority ojthe other data>it does not make sense to set a BACTlimit

based on an outlier data point. Furthermore, the baseline emiss'ions rate usedjor calculating the 30-day

. limit'is hased on the fzighest 30-day period during actual operationsfrom 2001-2005, which included the

. longest startup event. Ifthis highest 30-day periodwas used as the baseline to calculate the 30-day

average limit, and the worst-case startup event occurred during this historic 30-dayperiod, it is not

necessary to include the recommended "startup a4iustment" '(+0. 0411b/MAmtuJ perstartup event witfz

,this limit.

B&McD's April 23"i007Response:

According to MRYS:r;nit 1 :s October 2006 BACTAnalysis, the control effectiveness calculations '

were determined based'upon the highest historical12-month rolling NOx emission summation operating'

conditionsjor MiltonRYoung Station boilers during years 2001-2005...,.MRYS operating conditions

-56-
EPA Response to Minnkot8 Comment Letters from

March 19, 2007 and April 23, 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-montkhistoricpre-control baseline period conditions. The average unitNOx

.. emission rare (lblmmBtu) from the highest historic pre-control 30-day ~aseline periodwas comparedwith

the average NOx emission rate (lblmmBtu) from th.e 12-consecutive month periodwith the highest

historic unit NOx mass emissions (pounds) reported/or actual operation during the same 5-yearlook-

.backperiod It was coincidental that the highest historical 30~day rolling NOx emission pe.riodfor MRYS

Unit 1 (December17, 2003 through January 15, 2004) was within the same highest 12-month historic .

pre-control baseline period (ending on October 30, 2004). The longest individual U1ilt 1 startup period

during actual operation between January 1,2001 andDecember 31, 2005l?ccurred during the 61-

consecutive hour periodfrom June 27-29,2005.:.... The highest historJcal30-day rollingNOx emission

periodfor MRYS Unit 2 (March 7, 2003 through April 5, 2002) was not within the same highest 12-month

historic pre-control baseline period (ending April 30, 2004). The longest individual Unit 2 startup period

during actual operation between January I, 2001 andDecember 31, 2005 occurred during the 115-

consecutive hour periodfrom October 25-30, 2001. Note that this did noUall within eithef the 30-day or

the 12-inonth highest historic rolling average NOx maSs emission periods. ... ... The selected highest

baselinepre-controI30-day rolling average NOxunif.emission ratesfor MRYS Unit 1 and Unit 2 boilers

· are considered to be realistic upper bound cases, not "outliersII•••••• The recommended 30-day Rolling,

· Avet.age unadjusted (i.e. steady-state) BACTNOx emission rates do not include the impact ofthe worstcase startup eventsfor either MRYS Unit 1 or Unit 2. The ·"startup adjustment II as described in the

· October 2006 MRYS Unit 1 and Unit 2 NOx BACTAnalysisreports is appropriate as the recommended

(unadjusted) 30-dayRollingAverage NOx Emission Rate permit limitations basically r.eflectsteady-state

rates..

EPA Response:

EPA has reyiewed B&Mc;s detailed descriptiotiof how it established the baseline emission

· rates and the startup/shutdown adjustment factor and· now understands that 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 determin~tion.

M. BACT Liinit I8-Month TrialPeriod

EPA's January 26, 2007Comment: .

EPA has concfJrns with Minnkota's proposalfor an 18-month trialperiodfor establishing afinal

BACTlimit. (p. ES-II) This proposal appears to be in direct conflict with the pr.ocessfor establishing

andpotentiallyrevisingthe NOx BACTlimits setforth in Sections V. B & D ofthe Consent Decree.

Therefore, EPA believes the proposed 18-month trialperiodfor establishing afinal BACTlimit in

Minnkota's BACTanalysis cannot be accepted

B&McD's April 26, 2007Response:

·.....Paragraph 73 ofthe referenced Consent Decree st~tes "The Settling Defendi:mts shallsubmii

anypetitionfor any Unit under this Paragraph no later than six (6) months after thefinal compliance

-57....
EPA Response to .Minnko~a Comment Letters from

March 11),2007 and April 23, 2007

BACT Analysis at MRYS

date specifiedfor that Unit in Paragraph 67".. Paragraph 67 requires that MRYS Unit 1 and. Unit 2 shall

.achieve and maintain a Phase II 30-Day RollingAverage NOx Emission Rate established·by the NDDH ..

through its NOx BACTDetermination·beginning no later than Decemb,!r 31, 2011 andDecember 31,

2010, respectively. This does notpreclude an early installation ofConsent Decree Phase IINOx

emission reduction systems with asspciaied commissioning at Milto"" R. Young Stationprior to the CD .

.compliance datesforMRYS Unit 1 and Unit 2, respectively.' This could allow Minnkota to gain· operating

experience in advance ofthe June 30,2012 andJune 30,2011 deadlinesfor submitting 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.llowing the siatedfinal

compliance dates, the applicant will'have to make Use ·ofthe petition process as described in the CD in

this circumstance, ifappropriate. .

EPA Response.:

EPA agrees this.is an acceptable appJ;oach~ .

<,..

-58-

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