## 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 Study Report 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 by Minnkota 

### 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 in Bismarck 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 ofEastem Research 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 Minnkota BACT 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 MR YS.EPA maintains that the resUlts ofthe Coyote Station pilot**t~** 

### do Jl()!

### d~1Il()~tra.t~~lIilica1**ii1f~il>ility()f ~mJOI"**  

### utility boilers firiJlg 

## )\IT0rtb.])a.lc<>ta lignite. Minnkota has 

### not provided information and/or data 

### in the original Minnkota BACT. 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 MR YS, EPA finds that 

### the wealth ofexPerience in the utility industry applying SCRto 

### solid fuel utility boilers creates a presumption oftechnical feasibility at MR YS. 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·the Consent 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

·Minnkotato 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
. BrendaMorris,EPA.
. Douglas Latimer, EPA 

### Mike Owens, EPA Hans Buenning, EPA

.Jerry MacLaughlin, DOJ

3 

# *

Printedon 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 ofHealth (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 ofdocuments prepared by Bums & McDonnell (B&McD), and the Energy & Environmental Research Center (EERC) at the.Unlversity ofNorth Dakota, on behalfofMinnkota Power Cooperative, Inc. In response to the submission, in' October of2006, ofa 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 ofnine 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).

PRIVILEGED ENFORCEMENT INFORMAnON Not for Release Under FOIA or Other Information Release Statutes 

### 1**:.~**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 ofother 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 ofash 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 ofSCR 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 ofash 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~acid** ratio and the functioning ofanSCR 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

PRIVILEGED ENFORCEMltNT INFORMATION

. NoH or**R~lease** Under FOIA or Other Information Release Statutes 2never heard or read anything about**acid-to~base** ratio ofash 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~poisoned** catalyst (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 ofphosphorus, 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 ofSCR 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 ofalkali 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 ofKhodayari regarding regeneration**ofalkali~** poisoIl;ed catalysts. Both a careful reading .ofthe cited Khodayari paper and some ofhis. 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 ofSCR 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 ofUtah and REI PRIVILEGED ENFORCEMENT INFORMATION Not for Release Unde-: FOIA or Other Information Release Statutes 

### 3'.further implicates potassium as having a greater poisoning potential than sodium. Although the high sodium content ofCenter Mine '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 ofash 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 halfthetime. 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. Ifone assumes a rather extreme short catalyst life ofone 'year and ifthe change out is coordinated with a boiler cleaning outage, the SCR unit would only contribute one or two days 

### of downtime 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 ofwhich remains valid. In summary, the catalyst**s~ould** PRIVILEGED ENFORCEMENT INFORMATION Notfor Release Under FOIA or Other Information Release Statutes 

### 4

### 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** 

### ofash 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 24 th
, 

### 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 is found 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 PRIVILEGED ENFORCEMENT INFORMATION Notror Release Under.FOIA or Other Information Release Statutes 

### 5"discussed and the startup/shutdown requirements may be unreasonable. I do not expect the exercise to produce any meaningful results.

PRIVILEGED ENFORCEMENT INFORMATION Not for Release Under FOIA or Other Information Release Statutes 6.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 ;spreliminary analysis is that**th~** B&Mstudy lacks sufficient information within theBACT analysis. Specifically. the study**e~cludes** Selective Catalytic iedl{ction ($CR) as a technically feasible control option at step. 2 of thefive-step Top-Down analysis,**whil~** notpresenting NO x conirol effectiveness or catalyst activity data. As a result.'B&Mfaiis to rankSCR against i,Jther control options in terms of control effectiveness at step3, and fails' to analyze costs of SCR versus other control options at step 4. EPA questions the reasons citedby B&Mfor technical infeasibilityofSCR Furthermore. based' (In a survey ofSCR vendors described later in this 'report, EPAfoundthere are vendors that will offer' performance guarantees for this type of application. Further information shouldbe requested, to support the analysis with respect tc! SCR. 

### B&McD's March**13~** 

### 2007Response:

, '

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

, measure that aparticularNO x control technique or technology wouldhave to achieve 'to satisfy "a technical assessment consideringphysiCal. chemical and (fngineeringprinciples and/or empirical data showing the technolflgy wouldnot work on the emissions uni! underreview or that unsolvable technical difficulties wouldpreclude ih,e successful deployment of the technique" in order for saidalternative to be excludedfrom further consideration. ' '

, The severe accumulation of ash andblinding in the Coyote station slipstream SCR catalyst field test after 2 months was documented in Dr. Benson's referencedtechnicalpaper. The evident failure of the Coyote slipstream SCR'catalystfieldtest to avoidsevere-,catalystfouling andash accumulation ledBurns & McDonnell to believe that ifsuch results were routinely encounteredat afull-scale high-dust SCR catalyst**in~tallation** during operation on aNorth Dakota lignite-jiredcyclone boiler. aggressive on-line cleaning andlorjrequ?nt extendedforcedoutages would be neededto remove ash accumulqtion or, remove the catalyst for cleaning in orderto restoreful/unit outputandcatal)l$tNO x reduction effectiveness. Either or both of these maintenance activities coulddamage the catalysi and/or require it
. to be frequently replaced. ' '

Two monthsbetwe.en SCR catalyst maintenance activities woulc! be an extremely short operating cycle time comparedto 36 months typically experiencedat other $CR installations. Burns & McDonnell interpretedthese field test results as "empirical data showing the technology wouldnot work on the emissions unitunder review "for technical infeasibilitypurposes. Eliminating technically-infeasible' alternativesprior to ranking the feasible controls based upon control effectiveness. andthen evaluating the impacts'of implementingonly feasible control techniques andtechnologiesfollows 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 MR

### Ys 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 of Virginia): "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 ofproofshould 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 bejustification 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 ofcatalyst 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 of0.6 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 1 and 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 ofchanneling, 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 

### ofcatalyst 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

# a SCR 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 of0.6-0.8 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 ofpressure 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 ,ofoperation, 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 ofPotential 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 increasedto over.O.S in. ofwater within 500 h."IO Most ofthis paragraph describing the entire 6 months ofthe test does appear in Appendix B ofthe EERC report, with the exception ofthe first sentence•. In

# any**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 of pilot 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 of CatalystSamples for Energy & Environmental Research Center", April, 2005, Table 3.
. 12 Final Report for N Task 31

- Evaluation ofPotential 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~ ofpressure drop and the

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

plugging did not correlate to the degree ofcatalyst 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 ofash '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 at Coyote 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, ifapplied 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**  

# ofSCR 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/or** mitigation measUres. 

### i 4 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 beentaken 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 EPA comments, 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 companies to analyze how the sodium deposition at MRYS would affect catalyst maintenance and replacement**r~quirementsand** schedules.
. 

### EERCMarch 

### 19, 2007Response: We spoke to Flemming Hansen of Haldor Topsoe, an SCR catalyst vendor, regardingproviding performance·guarantees., AfterdescYibing the characteristics of Center lignite, specifically issues related to sodium, they were very concerned regarding guaranteeingperformance. Basedon the conversation, it

· became evident that EPA representatives may luiv.e not indicatedthe levels of s,odium andthe form of sodium in the coal. Haldor Topsoe indicatedthat the sodium was,a major concern and that itcauses deactivation, especially the organically associated form.: They indicated that they hadsome experience

· and found significantdeactivation of coupon samples after 6000 to 7000 hours of operation with a coal having elevatedlevels of sodium. '.
. 

### EPA Response:

. EPA also spoke with Flemming Hansen regarding the feasibility ofan 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'ofa higher·
. pitch catalyst, could have helped reduce, but not entirely eliminate, the impacts ofash 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
. andthat 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 consider such 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** ofcomputer flow modeling would be more. critical than the Coyote Station pilot test results in**~y** evaluation ofbidding 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 emails to· 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 consultantto 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 abouthigh amounts of ash material callsing blinding of macro-pores 'in the catalyst is real butsolvable. Strides in catalyst formulations, catalyst management systems, and recently, catalyst regenerationprocesses, have all combif!edto lower the overall cost contribution of both initial and replacement catalyst hi· modern SCR systems. EERCMarch 

### 19, 2007Response: EPA states that the high amount of ash materials causing blinding of macropores in the catalyst is real butsolvable. We spoke to Flemming Hansen of Haldor Topsoe, a catalyst vendor, regarding what has been done to deal with theproblems ofhigh:·sodium coals with regard to advancedformulations and catalyst regeneration, andhe was not aware of any work thathadadvancedSCR systems for 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 significantproblem 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, 2007Response: It is unclear whetherEPA's claimedSCR catalyst improvements citedwouldtranslate into
..

· preventing the catalyst layersfrom being susceptible to macro-pore blinding and plugging by**calcium~** basedcompounds andthe impact on the individual catalyst reaction sites from sodium:'basedcompounds· discussed in Dr. Benson's referenced technicalpaper. The EPA cites nopublishedtechnical literature
. that describes the results of testing that definitively documents how long currently available SCR catalysts can survive and provide**effect~ve** NOx reduction when exposed/o thejlue gas andjlyash constituents exhausted from combustion and.operation of any cyclone boiler firing North Dakota lignite, particularly one located in a**high~dust,** hot-side application such as wouldbe the case atMilton 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 ofproofdemonstratiqn 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 of P

RBash and CenterMine coal ash are very similar, particularly for the specific constit1!ents of concernfor catalyst deactivation, the quantity of ash in ihe
.. CenterMine coal is more than twice as high on 

# a perMMBtu 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% of the ash ends up qsjly ash atfull load (less at lower loads) andfor a wall or corner-fire4 pulverizedcoal boiler (PC boiler), about 80% of the ash ends up asjly ash. The result is thatSCR catalyst in a PC boilerP RB application is exposedto sif!lilar quantities of the majorconstituents of qoncern (alkaline material) as a catalystwouldbe exposedto atMRYS. The B&Manalysis also suggests that a linear comparison of theoretical 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 not presentedany evidence of a quantitative relationship between ash loading andSCR catalyst deactivation rates, EPA has nevertheless examinedB&M's arguments relatedto" ash loading.
.

. Table 1 below shows a comparison of the total ash loadingfor different boilerdesignsfor North' Dakota lignite, assuming a reference unit of 1.0 IblMMBtufor lignite, lignite will have twice the ash content

# on a**pe~** Btu basis,andthe conversion to flyash will be 30-50% for cyclone boilers and80% for PC boilers. As can be seen from this comparison, the theoretieel! flYaSh loadingfor a PC bqiler firing

, PRB boiler (OAlblMlYfBtu) will be iTl the same range as what wouldbe expectedjora cyclone boiler firing North Dakota lignite (0.3-0.5 IblMMBtu) Deactivation depends on the total time andmass exposure, andthe P RB PCboilerdata are extremely relevant to evaluating SCRfeasibility for North Dakota lignite
... .... Table 1 shows that; on a IblMMBtu basis, total theoretical jlyashloading at the economizer exitof ci cyclone boiler firing lignite would be very similar to thatof a PC boiler firing PRB coal. Another alkaline constituent that has been a concern'is calcium, which can react with S03 toplug
. pores as caS0 4• This has been 

### q much-studied problem andthe mechanism involves the calcium- , ' containingpartielf!. getting "caught" in a macro-pore of the catalyst andsubsequently reacting with S03 in thejlue gas. . The 'resulting CaSO, particle is larger in volume than the CaOpartiele and, in essence, swells tofill thepore PRE'coal has been shown to have approximately'30% more caldum in its ash comparedto North Dakota lignite**~.Other** alkaline maierials (potassium, sodium and , magnesium) deactivate catalysts by deactivqting individuatreaction sites rather'than the blinding by macro-poreplugging that was describedfor calcium. The average CenterMine coal ash contentfor these alkaline constituents is about 9%
.. ;.... ;.. Thispercentage is somewhattower than mostPRE coals andwithin the envelope of experience of European installations. One of**th~ refer~nces**  citedin the AppendixB EERC report, Cichanowicz & Muzio (2001), places the range of European SCR ixperience with these alkaline constituents at 8% to 15% pf ash. Thus, the coal usedat MRYS has similarpoisoning potential comparedto othercoals in use at,SCR-equipped boilers in Europe andthe 

### u.s... ..... ,..... The concern about high amo'!nts of alkaline materials in Cen!erMine coal expressed in B&M's BACTanalysis anddescribed in the Appendix B EERC repor.t is real, though not necessarily unlike operating SCR systems on PC boilers burning PRE coal. As explainedabove, we believe there is value in successfuloperating experience realizedat cur-rent SCR installation at P RE-fired powerplants. The problems cited in the BACTanalysis are being dealt with atseveralNO:x'SIP Call SCR installations today. In addition, the analysispresents no quantitative analytical iliformation (lerived from thefiring of lignite coal on SCR catalyst deactivation tojustify thefinding that SCR is lnfeasible.



### -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 different from 

### i? RB coais. EPA

, indic:ates that theproblems .of deposition will decrease because the cyClones take out some of the ash as . slag butforlignzte,cyclones create additional depositionproblems downstream. The cyclonespartition the more refr(1ctory ash comp()nents into the slag andthe more volatile components into the fly ash. The fly ash becomes enriched in cOllJponents that vaporize anc4**subseque~tly,** condense upon gas cooltng to. . form liquidphases andsmall' particles that are veryreactive to gas-phase components thcifcause bonding.·
. . .

, . . EPA· discusses ash loadings andthepotential impacts on SCRperformance. They also ((iscuss the formation of calcium sulfates that canplugpores in catalysts as well as the impacts of other alkaline.
. materials (potassium, sodium, andinagnesium) andtheirpptential to deactivate individual sites rather than blindingpores. . They fail torecoinize theformation of combined phases.suchas sodium caldum sulfates with low meltingpoints that are·very reactiv.eandcan both block pores anddeaCtivate individual sites in catalysis. They alsofail to recognize that cyclonespartition the ash" increasing theproportion ·of sodium in the smallerparticles, resulting in increased propensity for theformation ojthe complex sulfate .p!:lases. '
. .. .. Tops,oepresentedapaper at the 

### u.s. Depariment of Energy (DOE) National Energy Technology Laboratory (NETL) meeting on SCR-selective non.:catalytic reduction (SNCR) on testing conductedwith biomassfuels that contain highpotassium andsodiurnfoundrapiddeactivation in the SCR catalyst. Their daia suggest sodium orpotassium 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 of work they did on

. potassium. Potassium is associated in the coal with clay mineralsand is not soluble. 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 andoth,ers (2004)**e~tablished** that sodium correlates with catalyst deactivation activity andthatpore condensati(Jn of sodium is th'e major deactivation mechan,ism.

, Sodium levels atMRYS 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, EPA wanted 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 Minnkota Comment Letters from 'March 19,2001 and April 23, 2007 BACT Analysis atMRYS 

### EP

### A**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 problems that**could,pos~** 

### engineering**~hallenges** 

### to the design and operation of

## an SCR system at MRYS. However, Minnkota has not provided any , ' quantitative data that shows 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 of the 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'spre.liminary analysis of Minnkota's NOx BACT Analysis atMRYS ignores the BACTreports'statements that thefiring of lignite coal in a Cyclone boilerspecifically" designed and built topre-dry andburn North Dakota ligniteproduces finejlyashparticles which will enter thepores of the SCR catalyst" react with S03 in the flue gas, and form sulfates which bindotherash ' 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 individualcatalyst reaction sitesfrom sodium-bas.edcompounds discussed in Dr. Benson's referencedtechnicalpaper hils largely been ig"qredby the EPA's' ' commenters . The problems associatedwith sodium combinedwith calcium, sulfur; andother alkaline elements at any cyclone boilerfiringNorth Dakota lignite appear to be unpreventable for suc.cessful application of SCR catalysIs. There is no**full-~cale** long-term documentedandpublisheddemonstrated experience .at commercial SCR installations on large coal-firedboilers with similarly high sodium levels. The iendenCy of s.mall sodium vaporparticles to be generatedduring the combustion andslaggingprocesses of a North' Dakota lignite-firedcyclone, boilerhas 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 of publishedNOx control effectiveness andcatalyst 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 PowerResearch Institute, Inc. (EPRl) produceda recent study on the impactof coal type on SCR catalyst lift and performance
... ... ... This ' study found that the Siouxplant's cyclone unit .firing the P RBlbituminous coal blendexhibitedthefastest rate of catalyst activity degradation. Also, the higherdeactivation rates seen at this site were, due to economizer exit jlue gas temperatures being significantly higher than at the other sites. Burns & McDonnell believes that thejlue gas andjlyash conditionsproducedduring the

, compustion andslaggingprocesses of a 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 ofother alkaline materials (potassium, sodium, and magnesium) that would contribute to catalyst deactivation, instead of just 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 ofproofin 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 ofa full-scale long-tenncommercial

, SCR installation at afacility 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 ofengineering 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 unit

### burning 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~ehtation** given 

## 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 ofSCR at MRYS. ·b. , Engineering Solutions fOr SC!R Applications atHigh-Ash Facilities 

### EPA 

### 'sjanu.ary 9, 

### 2007 

### Comment: Plugging of SCR catalysts by fly ash accumulation andbridging of passages in honeycomb catalysts andbetWeenplates inplate-type catalysts has' "Qeen aproblem for 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 catalystpitch for honeycomb configurations an(i ,, widerplate spacingforplate-type configurations. These measures,' combinedwithperiodic cleaning during boileroutages have, by-and-large. reducedcatalystplugging to a manageable SCRproblem
.. ; .... Also, the fact that the MRYS wiits are broughtdownforplanned boiler deposit cleaning atf20-day

. intervals. provides mOre opportunity for catalyst cleaning maintenance than is available at the typical powerplant. '.

, Further

### demon~trating the ability ofSCR dlfsigner.s louse existing knowledge andexperience with SCR installations for high ash fuels, 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 accommodate fiiel types never before usedinpractice on afull scale. In a September 7,2006 letter addressedto TXUPower, EPRIstated, "While experience specifically with Texas lignite is limited, this shouldnotpreventsuppliers from providing an SCR system that meets'these objectives; dealing 'with a new fuel is similar to the challenge faced by US. utilities andtheir SCR suppliers when first installing SCR systems onplanisfiring high sulfur bituminous orPRE 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 thesefuels, whichproduceflue gasproperties'that are very different jrom those in the Japanese andEuropeanplants where SCR has been installedbefore. Therefore, SCR designers hadto rely upon their knowledge, engineeringjudgment. andexperience with these other coals to desiin SCR systems suitablefor these new applkations. While some of the**firs~** 

### u.s, SCR installations experienced

. problems (e.g., acceleratedcatalyst activity decay due to arsenicpoisoning in the absence of sufficient calcium in the coal, andcatalystplug gage due to largeparticle ash), none of theseproblemsprevented the systemsjrom achieving theirNOx goals initially and, eventually, maintainingthem over the expected

.life of the catalysts. "
. 

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

. . The issue is·riotsirictly the high ash content of NorthDakota lignite, it is the high sodium content of this lignite's ash. The issue for the application of SCR on North Dakota lignite1iredcyclone boilers is a

. highpotential jOr ash deposition in lower-temperature convectiVepass surfaces such as the economizer and primary superheater. The high ash deposition rates require continuous cleaning withsootblowers,

. andthe resulting liberateddepositjragments will cause catalystplugging.' The depositjragmenrs, along

. with theformation of sulfatedsodium- ande.alcium-rich fine particulatepluggingpores of the catalyst, causesplugging of gaspaths.
. 

### 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 ofavailable engineering solutions to address these problems with operation of

# an SCR at MRYS.
. .... . 

### B&McD's March 19, 2007**Re~ponse:** For a North Dakota lignite-firedcyclone boiler, aggressive on-line cleaning and/or jrequent extendedforcedoutages would be neededto remove the ash accumulation or remove the catalystfor cleaning in order to 'restore full unit output qndcatalystNOX reduction effectiveness. Either or both of these'maintenance activities coulddamage the catalyst and/or require it to bejrequeritly-
.

. replaced the**e~timated** amount of iime anddifficulty for MRYS boilers'SCR maintenance appears 

### tobe insufficient to use boiler deposit cleaning**outage~** /0perform significant catalyst cleaning or " replacement work 'J:1'ithout delaying startup. EP R1's statements do not conclusively support the technical feasibility of applying SCR wilh

.. specific technical evidence that showsproof of the hypotheses. that it will be successful on Texas lignite-
. fired boilers. It also does notsuggest or imply thatNorth Dakota lignites are similar to Texas lignites andtherefore SCR shouldalso be successful on such applications. The high ash andmoiSture content of a Texas lignite-fired unit wouldresult in a different set of challenges for application of high·dust SCR technology. Possible solutions to deal with Texas lignite are not relevant to compare to North Dakota cyclone boilers in this NOx BACTanalysiS for MRYS. Already-installedboiler andbackendequipment designs on units firing TXvs ND lignites are significantly different, reflective of the 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 separateproblems
.. 

### A comparison of the differences between Texas andNorth Dakota lignites is included in EERC's subsequent response to the $P A 's comments regarding the NSPS revisions later in this response. document.
.



### . EPA Response: This response states that aggressive on-line cleaning··andlor frequentextended 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 ofcostthan 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~ommodating** fuel 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. OiherAsh Deposition Problems on Downstream Components EPA 'sJanuary 

### 9,2007 90mment: B&M's discussion of these "other"potential accumulationproblems does not reac,h any definitive conclusions relatedto the technical feasibility of Sc:R, so EPA does, not see this as a technical barrier tp . installing SCR at th,e MRYS units. 

### EERCMarch 

### 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 content 

# m North 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 of ash, sodium, andsulfur in North Dakota lignite combinedwith calcium and other alkaline elements along with ammonia andS03 emittedfrom a high-dust SCR will exaggerate tubular airpreheater andeconomizer tube exterior surface fouling and plugging**proble~** at any cyclone boiler firing this fuel comparedto bituminous or P RB coals. The tubular airpreheaters andthe built-in economizers atMRYS 'ar.e notequippedwith sootblowers for on-line gas-side surface deposit cleaning. Noting thatMRYS boilers' airpreheater andeconomizers'would be"downstream of the boilerbackpass outlets anda high-dust 

### sci? isolating the.se components during off-line boiler deposit cleaning or SCR cleaning outages from negative itlJpacts of"diriy air- wouldbe chalienging andexpensive: Similar
. estimatedamounts of time anddifficulty for such off-line downstream deposit removals will also delay unit startups following 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.
. 

### AccounringfOr Catalyst Lite & 

### PerfOrmance in Tdp;.Down BACT Analysis EPA's January 9, 

### 2007.Comment: " Finally, the conclusion reached in_ the B&MBACTcmalysis that ash deposition andcatalyst pluggage willadversely affect SCRperformance (we assume this means contro/efficiency) andcatalyst

- '. life does notnecessarily supporta c.onclusion that 'the technology is technically infeasible andthat SCR 

### -19-EPA Response to Minnkota Comment Letters from March 19,2007 and April 23, 2007 BACT Analysis at MRYS shouldbe eliminatedat step 2 of the Top-Down BACT(lnalysis. Furthermore, the B&Manalysispresents. no qucmtitative data relatedto SCRperformance andcatalyst life to support sucha conclusion
...

... However, SCRperformance andany realconstruction andoperational costs are factors that would be
. accountedfor under the econ01nic impact**~tage** related to costeffectiveness of SCR (step 4) under the'
. Top-Down BACTanalysis. 

### B&McD March 19,".2007Response: The reasons for the. assertion**~ftechnical** infeasibility of seRfor lignite-firedcyclone boilers were presented in the MRYStation Unit Jand U."U 2 NOx BACTcmalysisreports; withfurther explanation providedherein. The EPA'spreliminary analysis comments minimize the clifficultiesposedby the higher amount of ash, sodium, andsulfur in North Dakota lignite combinedwith calcium andotheralkaline· elements along with ammonia andS03 emittedfrom

# a high-dust SCR on the tWo existingNorth Dakota lignite-fired CYclone boileratMRYS.
.

. The EPA cites nopublishedtechnical literature that describes as wouldbe the case at Milton R. Young Station. The evidentfailure of the Coyote slipstream SCR catalystfieldtest to ... ... .ledBurns & McDonnell to believe thesefield test.results indicate Jor technicl:ll infeasibilitypurpost!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. EP

## A**question~** the design and operation ofthe' Coyote Station pilot test. in light of**th~** lack ofcritical 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 ofan 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 ofcatalyst 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 ofthe 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 commentby 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, 

### 2007 Comment:

...... ....However, the conClusions reached in the Appendix B report do notaddress some of the
. engineering solutionspreviously citedandexplainedbelow that are available for full-scale SCR ' ,' appiications. Furthermore, EPA,Region 8 believes that there may be inherent limitations with thepilot' 'study (that was ,the basis for bothEERC reports;' thatdraws into question the validity of directly applying those conclusions to the range o/possible SCR designs of afull-scale application atMRYS. Statements from three majorSCR catalyst manufacturers (including Haldor Topsoe,
.the catalyst manufadurer involved in thepilottestingfor North Dakota lignite) suggest that the catalystdeposition and plugging problems experiencedduring thepilot testing may be exagger.ating on apilot-scale andcan be solved in a full-scale design andapplication
. Other inherentproble.ms associatedwith directly applying the results of the EERCpilot testing at

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

### 1) The airpulse usedto Cleardeposits during thepilot test is apoor simulation of sootblowing and is much less effective...
... ....... , 2) The ,6mmpitch catalyst usedfor thepilot testing at Coyote Station is much smaller than currently, being specified for 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 Ilmmpitch catalysts : ..

. 3) The one meier depth of the catalyst used in thepilot test is equivalent to '!ne layer of catalyst in ·a commercial instalkilion. CommerCial systems aimerf at high remoyals haVe three layers, each about threefeet thick, and, at various times in the catalyst mi:magement cycle, incorporate four layers. EERC's.· test, then,**~Jnly** simulated the top layer of a'multi-layersystem This is why the first step in most catalyst management schemes usually involves adding afourth layer and leaving the badly degraded top fayer inplace for the second operatingperiod EERC 'sprojectprovides no real insight into the' effectiveness andsurvivability of a secondor third layer. , . 4) The scale of testing carriedout in thisproject is usually associatedwith observing catalyst life prospects (or saidanother way, the rate of loss in catalyst activity) rather thanphysicalprocesses of

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

be accurately simulatedat this small scale, but theplugging anddeposition are irifluencedby larger processes, such as flow imbalance andcountermeasures. (sootblowing,. horns, screens, bajJles; etc.) and

. are not duplicate.d on the small scale. For these reasons, EPA Region 8 questions the direct**app/icati~n** of the results of the EERCpiloi' testing at Coyote Station andthe accompanyingAppendix B EERC reportforpurposes of the BACT' technical feasil}ility analysis of SCR.at MRYS. The experience andopinions of the SCR supplier that participatedin the EERCpilot test indicate that theproblems'encounteredduring thepilot. test are·not determinate of whether SCRplugging experiencedduring thepilot test would be unresolvable by vendors. To the contrary, statements from Haldor Topsoe offering the}rSCR catalyst for installation on lignitefired boilers indicates that thefull-scale differences andengineering solutions available todayprovide .ev.idence that the EERGpilot testing results shouldnot alone be the deciding consideration when'

· evaluating SCR technical feasibility atMRYS.
. 

### EERCMarch 

### 19, 2007Response:

........ The catalyst**vendo~s,** Haidor Topsoe, Cormetech, andHitachi, were all involved

### in the design 

## OJthepilot-scale slip stream test system ahdtestingconditions
.. The'pulsing system 'was designed with the assistance of the catalyst vendors
. . Theyprovidedsuggestions on the sootblowingpresSures Cindfelt it· would be sufficient to remove the deposits. The 6mmpitch was recommended by the vendors. 

### If thefirst layerplugs, the gases cannotget to'the subsequent layers,' significantdeposition was noteddownstream of the SCRcatalystsection instdlled, Deposition alJd plugging are significantproblems that impadthe
.

· reliability of the MRY facility. Haldor Topsoe was the only catalyst vendor willing toprovide catalystfor the lignite tesiing.
. . , '.' The EERCpilot.,-scale testing was a collaborative effort involVing coal-fired utilities, EP R1, DOE, andcatalyst vendors.' All**de~isioilS** on system design, plants for testing,' andtes,ting conditions were .

· reviewedandagreed upon byvimdors. The EERC'comparisons of catalyst blindingfor/ignite and

· subbituminous.coal testing as' well as the sodium deactivation workconductedby other research

organizations andcatalyst 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 

### of ash plugging. Additionally, Mr. Hansen said that pilot test's air pulsing system did not accurately simulate full-scale sootblowing and that ifthe test could be redone, a**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 ofcatalyst 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 ofcatalyst 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

- ofash 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, of SCR on North, Dqkota lignite is technically feasible. EPA haspreviously articulatedthis belief in a number

## 01national' rulemakings, including the Clean AirInterstate 'Rule (CAIR), the BestAvailable 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 andconcerns with catalystplugging during the test. While EERC notes that ash depositionproblems were mostpronouncedat Coyote Station, compared to the twp· test/acUities burning PRB coal, it also states generally that all of the coals in thepilot study "produced ash that hadsignificant accumulations of ash on the catalyston both macroscopic andmicroscopic levels." Thus, the conClusions of the study appear to be that deposition issues might be somewhat'greater for North Dakota lignite thanfor PRB coal; Since**~ignificant** accumulations of ash on the catalyst was also observedduringpilot, testing at those facilities burningPRB coal andthere are numerous examples


## , OJP RB coal-firedpowerplants. successfully operating SCRsystems, the qualitative results of thepilot testingdescribed by B&MandEERC wouldnot appear topreclude successful operation ofSCR on a powerplant burning North Dakota lignite. The EERCstudy does not appear to quantify the effect that the ,ash accumulation hadon microscopic catalyst deae.tivation rates or overall catalystperformance. Absentsuch information that tejJeetsthe useojcurrently C1\lailablefull-scale ash loading mitigation designs, pilot t?stingresults may not be evidence that successful itrlP!ementation ofSCR on North Dakota firedunits' "is going to be impossible," as statedin theAppendix B.EERC report .

, The

### Octob?~ 1990 EPA draft New Source

### Revie~ Manual (NSR Manual) states that

## a "demonstration of technicalinfeasibility is basedon 

## a technical assessment consideringphysical, chemical.andengineeringprinciples, and/or empirical data showing that the technology wouldnot work on the emissions unit under review, or that unresiJ/vab/e technical dijjiculties**~ould** preclude the·
. successful deployment of the technique. Physical modifications neededto resolve technical obstacles do·

, not in andof themselvesprovide ajustificationfor eliminating the control technique on the' basis of technical infe.asibility. However, the cost of such modifications can be considered in estimating cost and
. economic,impacts, which, in turn, may for:m the basis foreliminating a control technology. " (Page B.20) (Emphasis added) Basedon the available information, theproblems citedby B&Mforapplication of SCR atMRYS (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 notedabove, step 2 is not theproper stage of the Top-Down BACTanalysis to accoUntfor these costs. As describedin the NSR Manual, "where the resolution of technical difficulties isa matter of cost, the applicant shouldconsider tlte' technology as technically feasible. The econo,!,ic feasibility of a· control alternative is reviewed in the economic impactsportion of the BACT-selectionprocess." (Pages B.19-B.2.0)· Many of the technical feasibility argumentspresented in the BACTanalysis are centeredon

, decreased**c~ialyst** life and performance, increasedmaintenance andoutage time, cmd plant differences unique to MR.YS thatpreclude SCRfeasibility atMRYS. Consistentwith the NSR Manual, these concerns are incorrectly incorporated into the Top-Down BACTanalysis at step 2 andreally shouldbe accounted 

### -24-EPA Response to Minnkota Comment Letters from March 19,2007 and April 23, 2007 BACT Analysis at MRYS for as additional costsper ton of pollutant removeda/step 4 of the analysis So, while EPA does· not consider vendor guarantees by themselves to be determinate in making technical feasibility decisions, they are a contributingfactor in making these decisions: EPA is aWare of three catalyst vendors (HaldorTopsoe, Cormetech, andArgillion) who· wouldbe willing toprovideperformance guarantees for lignite-

. fired boilers applications.
. Basedon the criteria discussedabove, MinnkotaandB&D shouldhave/t4rther exploredboth the full-scale engineering solutions usedto address theproblems cited in their anqlysis in conjunction with information from SCR/catalyst vendors to offerperformance guarantees to design and install an SCR unit aiMRYS. The BACTanalysis for MRYS that ·was submittedrelies heavily on the technicalproblems

. experiencedduring the EERCpilot testing at Coyote Station in conjunction with thefact that SCR has not

been installedon a cyclone boiler that burns North Dakota lignite. We do not believe that this analysis

.adequately addresses the criteria·citedabove. As such, it appears that the case-specific NO x BACT analysis submittedforMinnkota MRYS is deficient in terms of improperly eliminating SCRfrom consideration atsteJ! 2 of the BACTanalysisfor technicalfeasibility. 

### EERC's March 

### 19, 2007

### Response: EPA did not examine the EERC report in detail. The deposition andplugging was much more pronouncedfor the Coyote Station testing ascomparedto the Columbia andBaldwin Station
.. The

. pressure drop increasedmuch more rapidly andthe sootblowing was notas effective
..... The most

important unresolvable technical difficulty is the impactof sodium on the formation of bondedsulfate. phases that**plu~** andmask the catalyst andon the ability of sodium 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 togetherdo not demonstrate'whether SCR would be technically feasible or infeasible at any or all ofthe.**fac~lities** 

### tested, since no quantitative 

## NOx 

### control or catalyst deactivation data was

. provided in the EERC report in Appendix B ofthe BACT analysis. While the. pilot study does

suggestthat 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 documents for pr9ject participants and reviewed quarterly progress reports. For reference, below is a summary ofsome ofthe relevant history and background informa,ti<>u from tJIe pilot study.


### -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 ofSCR 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 of time."
. (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 ofSCR 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 ofalkaline-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 was separated. 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 ofash 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 PRB coal. 

### 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;Ilacroscopklevels 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**~asking** ofSCR 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, itjust 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 offeredno specific documentedand publishedsubstantive demonstrated experience of any cyclone boiler firing North Dakota lignite, particularly one locatedin a highdust, hot-slde application at Milton R Young Station. ·Burns 

### & McDonnell disagrees that the lack ofSCR catalystdeactivation data from the Coyote slipstream SCR catalysts field test completely refutes the premise of Dr. Benson referencedtechnicalpaper andthe M.innkota NOx BACT Analysis StudY conclusions that SCRs are technically infeasible for !ignite-jiredcyclone boilers. The.EPA's NSR Manual does not.specifically define
..... ; in order for saidalternative to be excluded fromfurther consideration.
. . When a BACT**an~lysis** isperformedon a new unit, there has typically been significant conceptual anddetailed engineeringanddesign workalreadyperformedthat forms the bC1!is of the subsequent 

### -29-:-EPA Response to Minnkota'Comment-Letters from March 19, 2007 and'April 23, 2007

, .BACT Analysis at MRYS capital cost estimate andassociatedexpected0 & ,Mcostsfor theparticular emission control technologies to be empl9yedor that were consideredaspart of the overallprojecteffort. Performing a BACTanalysis on an existing unit slzouldnotrequire similar substantial resources to f/evelop conceptual designs andcreate technical details that determine the'estimatedsite-specific retrofit costsfor overcoming significant technical challenges that wouldotherwise,preclwle the 'ability of the technique or technl?logy from successfully controlling the emissions in question. The EPA's NSR Manual states: "A source wouldnot be. requiredto exper.ience extendedtime delays or resourcepenalties to allow research to be conductedon a new technique. "
.There are no citedexamples'where successfulsolutions to such challenges asposedlJYstich highly complex interactions betwe.enflyash,flue gas, andgas temper.atures producedby lignite firingfor SCR coal-.(iredboiler applications ' 

### EPA Response: 

### Again, EPA believes that in the case of**decid~ng** 

### on technical infeasibility for SCR at a . utility boiler, the bUrdeIi ofproofis on**t~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 ofcatalyst 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 Minnkota Comment 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

.ofcontext 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 matterofeost arid

· should be examined under step 4 ofthe**Top~Down** 

### BACT analysis. 'B. , Low-Dust SCR 

### Bums & McDonnell has eliminated Low.,.Dust SCR as technically infeasible due to issues describedbelow. EPA's response on each issue is also described below 1. 

### EPA Analysis ofthe Catalyst Fouling and Deactivation Issue: EPA's January 

### 9, 2007 Comment:
. Afew SCR systems installedon coal-jired boilers have been designed with the SCR catalyst installedafter the ESP or baghouse. This approach-eliminates virtually all of thepoisoning, plugging anderosionprolilems that have been associatedwiih SCR**cataly~t.** (Christman Memo, 2/18/03) For the same reasons notedabove under the high-dust**SC~ analysi~,**  the catalyst fouling anddeactivation pro61ems may be resolvable technicalproblems that do not warrantelimination of low-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 of**dev~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 Mimikota 

### BAcTAnalysis. 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 Letters from

. . March 19, 2007 and April 23, 2007 BACT Analysis at MRYS step 2 of the Top-Down BACTanalysis Similarly, permit applicationsfor three recent Circulating FluiclizedBed projects in North Dakota.(MDU Westmooreland Gascoyne, South Hart, .andGascoyne 500MW) did not. eliminate low-Dust SCR basedon unresolvallle technk:alfeasibility issues wit" catalyst fouling anddeactivation. While there would bel!nherent costs associatedwith gas-rehe(:lt, otherpossible physical modifications to theplant to accommo.dale low-dust SCR, andengineering designs to minimize'
. emdmanage catalyst fouling anddeactivation, thesefactors, consistent with the NSf? Manual, shouldbe.
. accountedfor when considering the economic impacts of the technology. 

### EERC's Marclt 

### 19, 2007Response: Sodium-rich aerosols are 

# a problem that can get through the ESP. These aerosols are known to be

, . present in high sodium coal combustion fuel gases. These species,are known toproducepyrosulfates. These will ccmseblinding anddeactivation of the 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'leed for a major source of heat to reheat theflue gas stream downstream of the ESP in a low-
. d1,lSt SCR application will require either an expensive naturalgas supplypipeline or an unacceptable ' derate of the nei unit electrical output associatedwithextensive, modifications of the. components in steam cyclepower generation andbalance-ofpkmt systems and'boiler gas systems to supply highpressure, . .
, high temperature steam as suggestedbyihe EPA and their consultant
. . The magnitude of such retrofit'
. modifications andrisks of failure in applying low-dust SCR technology atMRYS are much greater than ls considered reasonable.'
. , 

### EPA Response: B&McD contemplates the use ofnamralgas (and associated cortstrUctionofa 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 ofnatUral gas and the associated pipeline). These options would likely require a less complex analysis. Based on their response, it is unclear if

. B&McD is still eliminating low-dust SCR as technically infeasible or 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 extentof thephysicalplant 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 wouldnecessarilypreclude thepossible installation of low-dust SCR. It does soundlike major and costly equipment relocation may be necessary toprovide .the space necessary for low-dustSCB. However. consistent witfz the NSR Manual, "physical modifications needed to resolve technical obstacles do not in andof themselvesprovide ajustificationfor eliminating the control technique on the basis of.· technical infeasibility." These costs shouldbe accountedfor when estimating the .cost(md economic

, impacts of low-dust SCR. but shouldnot be usedasa basisfor eliminating the technology as technically**mft~wk** 

## . B&McD's March 19,2007Response: Significant engineering resources wouidneedto be devotedto investigate andestimate the capital costs of major boiler andairpollution equipment relocations. facility additions, and boilerhouse building modifications required to implement low-dust SCR t(Jchnology atMRYS. Burns & McDonnell believes the expectedlevelizedtota{annual costs requiredfo apply low-dusi SCR technology to North Dakota lignite-firedcyclone 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** if

### it is shown that costs ofEPA 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 explainedabove, EPA Region 

### 8 believes that the inherent challenges with installing m1d operating low-dust SCR atMRYSare'resolvable on a technical basis. Thefact that afew 10w-dustSCR applications are currently used inpractice indicates that the small-particle catalystjouling and deactivationproblem described wouldnotpreclude the use of low-dust SCR
... ... ;. B&D's statement's that "low-dust SCRperformance andcatalystlife could be severely negatively impactedresulting in ' premature, extended,frequent outages for replacement" can be minimizedbyproper SCR design andonline cleanirzg. As stated previously, any real costs associatedwith the construction andmaintenal'lce requiredto operate the low-dust SCR shouldbe accountedfor under the economic impacts stage ojthe Top-Down BACTanalysis (step 4). Similarly, physicalplant modifications that would be requiredto , accommodate th.e low-dust SCR (and auxiliary equipment) shouldbe considered in the economic impact' evaluation, rather than usedas evidence of technicai infeasibility. , 

### 'B&McD's March 19, 2007Response: ' Aspreviously statedby the EERC, the EPAfails to recognize that lignite-firedcyclones'partition the, ash, increasing theproportion of sodium in the slJlallerparticles, resulting in 'increasedpropensity for, theformation'of the complex sulfatephases. Thefly ash becomes enriched in components that vaporize and, subsequently, condense upon gas cooling to form liquid'phases andsmallparticles that are**v~ry,** reactive to gas-phase components that cause bonding, blinding, anddeactivation of individual 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, ifa 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 thepreparation of the CAlR rule, EPA developeda scenario for bestcontrols across coalflred units. in the UnitedStates basedon an assessment of boiler type,fuel type, and existing controls.
. This assessment recommended that SCR technology wouldbe the best technology for 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); andBasin Electric Power Cooperative**(VLB-0690)~­** arguedthat SCR technology is notfeasiblefor application on lignite-flred boilers. Some of these commenters claimedthat an SCRpilot test on a slipstream from a boiler flring lignite. had·shown catalyst plugging and blinding. One commenterattribuiedthisproblem to sulfates of calcium andsodium, which arepresent in theflue gas stream entering the SCR, as a result of flring of lignite in the boiler. One of**t~ese** commenters also claimedthat SCR was not feasiblefor boilers flring sub-bituminous. In its response at VIB; of the CAIR docket, EPA 'noted that there is considerable experience in the industry to show that use ofSCR on 'sub:'bituminous coals and lignite is .technically feasible. SCR is currently'in use an several sub-bituminous coal-flredboilers in the U8.flring Powder River Basin coal. Regarding the USe ofSCR on boilersflring lignite, EPRIiesting'ofSCR catalyst in a slipstream at the Martin Lakeplantshowedacceptable results from Texas Lignite. FUrthermore, the technology suppliers repor.t that SCR has been s?JCcessfully used'on lignite andbrown coal boilers in Europe. From this**experi~nce,** it can be concludedthat SCR can be usedon sub-bituminous coal andlignite 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: andcatalyst suppliers have indicatedthat they will offerperformance guarantees on these
.

·applications in the 

## u.s. . . . In the docket information regarding this issue, one 

### .of the SCR catalyst vendors, Argillon, provided· a /ist a SCR iristallations atEGUs ·in Europe that burn /ignite orsimilar rank coal, including:
. '. .':

• Riedersbach installations in 1995 and2001 (hard coal/lignite) .• Voitsberg (lignite)

• Dormagtm installations in /994
..and 1996 (lignite) EPA also notedthat the commenters have notprovidedany details of thepilot test that has been mentionedwith regards to theplugging and blinding of a cdtalys/from /ignite-generdtedfuel gases. ... Without this information, this PClrt of the comment cannot be a.ddressed properly. The EPA's ow,n .. investigations show that the main ref;lson for thisproblem was ash build up on the catalyst, which is a ·probkm commonly experienced inpilot testfacilities. In the full-scale installations, this concerlt is

· acfdressed.during the SCR design stage. The meihods·usedto avoidthisproblem include duct design to promote ash falloutprior to the SCR, catalyst reactor design to avoidash build. up, and on-line cleqning methods (sootblowers andsonic horns)
.. In addition,. (hejoilowing**c~talyst** vendors statedthat they couldprovide catalysis/SCRjorlignitefired boilers: Haldor- Topsoe Cormetec/:z Argil/on ;.. . . Tn a memo datedAugust 20, 2004, Jim Staudt of Andover Technology Partners to Sikander Khan of EPA CA¥D, discussed both the EERCpilot testing results andsome of the negative issues raisedby B&Manddocumented how Haldor Topsoe was aware of these issues, but nevertheless felt comfortable
. providing SCR catalyst...
... ..... .



### · EERC's March 

### 19, 2007Response: EPA's citedexperience from subbituminous coals andSCR does not haVe direct app/icationto the North Dakota lignite-firedcyclone boilers at-**~YS** because of the high sodium content of the lignite. The SC;R experience with German. brown coals does not have direct application to ND lignite because. of
. the lower sodium levels of the German coals.
.. EERC spoke to Flemming Hansen of Hdldor Topsoe on**Feb~ary** 2, 2007 about the issues associate.dwith high, sodium North Dakota lignite. Haldor Topsoe has ?xperience· with sodium and potassiumfrom biomassjiredsystems andtheir experience has not beim good. Afterdescribing the issues relatedto sodium
....... after 6000 to 7000 hours %peration with a coal having elevatedlevels of sodium. 

### EPA Response: EPA's

### pr~sentationofsome 

### U.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 informeddecision in its BARTGuidelines 'regarding cyclone-fired utilitY boilers. For all other boiler types, some form of combustion control was deemed to be thepresumptive level of control: Howf!ver, for cyclone units burning-any, type of coal (bituminous, subbituminous, or lignite)"presumptive BARTis SCR. SCR was se'ectedbecause the technology isfeasible andcos/- effeetive : " 

### B&McD's March 19,2007Response: EPA ignoredthe num(j!rous comments receivedforthe draft BARTguidelines (similar to that cited in the EPA's CAIR discusstonfor thepreceding issue) about the infeasibility ofSCR appliedto North, Dakota llgnite. The unit capital cost capacity factor for SCR retrofits appliedto cyclone boilers 'py the EPA in**th~** cost-effectiveness analysis used for establishingpresumptive BARTwas $100IkW.

. ' Burns & McDonnell believes the EPA's cost,;,effe'ctiveness analysis is inaccurate andsignificantly underestimates the expectedcapitalplus operating andmaintenance costs required to apply high-dustor, low-dust SCR technology to North Dakota lignite-firedcyclone 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. BACT analysis.'**~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 ofSCR for LigDite-Fired Units. EPA's 

### iO",':'(Try 9,2007 Comment: , In'its response topublic comments on the February 28. 2005 draft NSPS, SubjJart 1)aproposal, ' EPA rejectedcomments receivedon NOx control requirements for lignite-fired units. Specifically, EPA took the sameposition as it did in the CAIR andBARTrules, rejecting commenters'concerns that SCR wouldnot befeasiblefor /ignite units. No legal challenges were made to thefinal rule regarding EPA's

,response to the lignite concerns ... ... ..... ' 

### B&McD's March 19,2007Response:
. Bur"s & McDonnell rejects the EPA ;s argument that the lack of a lawsuitprotesting the' establishment of revisedNSPS limits for new electric utility steam gen.erating un.its (EUSGUs) that require SCRsfor NOx control means that this is appropriate technology for a BACTretrofit on existing lignite-firedcyclone boilers. Cyclone boilers are no longer co,mmercially available, so there are no new cyclone boilers beingpermittedandbuilt. Utility owners ojexisting North Dakota lignite-firedb()ilers wouldnot be expectedte cC?mment on the. draft revisions to the NSPSproposedby rhe EPA because they apply only to new units with different methods o/firing coal than**cyclon~** boilers. It is doubtful that the' EPA considered firing/ignite in new cyclone boilers would be apossihi/fty when theyproposedthe draft NSPS standards.. Any recently built orpermittedlignite-fired boilers have'beenpredominantly cfrculatingjluidized, bed(CFB) type andhave not incorporatedSCR technology/orNOx emissions control. 'There have, been recenipermit application,sfiled for COnstructingpu,lverizedcoal boilers burning Texas /ignite that , propose to include high-dust SC;R technology. However,' there are distinct differences between North

, Dakota and Texas /ignites thatpreclude assuming that this supports the technical feasibility of applying , 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 wouldnot 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~nes** when 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 not

### j\,lst new units, the exclusive construction ofnew circulating. fluidized bed (CFB) boilers 1n 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 ofan 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. Thefollowif'!g comparison shows theproximate analyses, sulfur contents andheating values for various lignites, plus ash compositional analyses of Texas lignites Comparison of North Dakota and Texas Lignites Table -3 summarizes ash composition analysis expressedas equivalent oxide. The compositional analyses show that the aSh has relatively low sodium and lower cil/cium content than North Dakota lignite. Theproblems of SCR catalyst blinding due to theformation ,of sodium calcium 'sulfates created from thefiring of Texas lignites will be much less severe than for 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 of5,761 Btu/lb would have a sodium loading ofroughly 60% ofaverage Center Mine lignite. As explained 

### in previous comments, EPA does not believe a strict linear comparison can be made from these loading levels, but it ,appears that some Texas lignite has 

### comparable sodium levels' on,a per Btu basis to Center Mine ',lignite.' . ' " ,

-39-EPA Response to 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 is much 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 notedabove, the following catalyst vendors were comfortablep.roviding catalysts/SCRfo;' lignitefired boilers: Haldor-Topsoe,Cormetech, andArgillon. In addition, ·the above-citedmemo datedAugust 20, 2004, Jim Staudt of Andover Technology Partners to Sikander Khan of EPA CAMD, discussed both the EERCpilot testing results andsome of the negative issues raisedby B&Manddocumented how Haldor Topsoe was aware of these issues, but' nevertheless felt comfortableproviding SCR catalyst. EPA Region 8,also contaetedthe Institute of Clean Air Companies website to obtain thefollowing list of 21. companies offering "SCR Systems, Equipment, Su!!pltes andServices:" EPA Region 8 has visitedmost of the vendors 'websites,andfound many vendors with experience tha/would,be germane to the MRYS situation We believe it wouldbeprudentfor Minnkota to obtain information and proposalsfrom more**~han** one vendor and involve the NDDHand.$PA}n 'their findings.' 

### EERC's. March 

### 19, 2007Respon$e:

,EE'RC has already includedsome, comments from discussions with Haldor Topsoe regardingtheir , ' concerns about the sodium content in North Dakota lignite and its tendenciesfor catalyst deactivation. '

Some of the EPA-citedSCR catalyst vendors' websites discuss theproblems of sodium anddeae,tivation, However, EPA has notprovideda review- of that information. 

### EPA's Response:



### ,EPA has already

### disc~ssed its phone

### (:~nversation with Flemming Hansen 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

# a North Dakotalignite 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 screenedto e.liminate companies that are notactive.in the U.S. coal-jiredSCR market...
....EPA 'sEPA Response to Minnkota Comment.Letters from

. March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS evidence ofSCR catalyst vendor's willingness to guaranteeperformance on a lignite-firedboiler is very general in nature anddoe.s not include typical qualifying statements related to (he cheni's needto adhere to specifiedSCR design conditions, which are yet to be**~stablished** .

... ... .from Burns & McDonnell's experienc.e, what d vendor orsupplier sayspre-biddoes not always match wluit they actually will offer in afirm contract..... a commercialguarantee does not mean the system'proyidedby the vendor will work long term; all it means is that basedon the language in the contract andthe amountof money the supplier has allocatedto cover their riskor exposure, the vendor willguarantee the systemperformance commercial contraCtguarantees always have a finite life, which means the owner of the system only benefitsfroin the vendor's guaranteefor 3 years tYpically
.... EPA's cites SCR catalystvendor statements lacksubstantive language that deals with mitigating

. the client 's risks of failure to meet catalyst activity versus life guarantees, andthe concerns recently expressedby Haldor Topsoe when EERC describedsodium components in thejlue·gas. In Burns & McDonnell's opinion, SCR system vendors have notprovidedadequate documentedand publisheq demonstrated catalyst experience with supporting technical information to'support these claims. The vendors offeredno specificcomptirativepeer-revieweddata asproof that currently-available catalysts
. can survive and provide effective NOx reduction when exposedio thejlue gas and jlyash constituents exhausted from combustion and operation of any cyclone boiler firing North Dakota lignite, particularly one located.in a high-dust,'**hot~side** application atMilton 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 ofa 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: Pleaseprovide data currently available on the tenterMine coal constituents (presented in

· Minnkota's October 2006NO x BACT Analysis), includingall**av.ailabl~** data samplingforjuture and presently-minedlireas of the mine. Pleaseprovide the data in terms of averages, 'in addition. to vqlue ranges. 

### BERCApril 23, 

### 2007Response: The his,torical database was obtained from samples

### cqllect~d for "thepurpose of testing the

### imp~cts . of lignite characteristiCs onplantperformance. The database consists of 1317 as-firedsamples
. .The average, standarddeviation,. maximum, minimum andselected percentiles for ash, sulfur, heating value, andselectedash constituents are includedliz Table 1. The use of average coalproperties fo:" design dnd performance of combustion and envl"on;"ental control systems typically underestimates adverseproblems such as fireside ash deposition, opacity, and
.. sulfur emissions. Designs of powersystems must consider the challenging coal characteristics in order to .. develop ·designs and'operatingparameters with s.ufficientflexibility "thata(lowfor. the combustion of**coal~.** .. that at least represent the 9(jh and 1(jhpercentile ·of selectedconstituents ; Thefrequency
. . . distributions of components of the coalprovide better understanding oj/hepotential challenges
.... The results of the analysis show that the lignite ash has 

### a 4.4% average sodium content; however, there is q multimodal distribution of sodium levels in the coal ash
..... The averageproperties as well as the minimum andmaximum 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, andsulfurspecies in thefurnace andlower

temperature regions of the·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 ofApril 23, 2007 letter from Minnkota to EPA, DOJ, and NDDH
.. 

### -42-'EPA Response to Minnkota Comment Letters from March 19, 2007 and April 23, 2007 BACT**Analysi~** at MRYS 

### compare very closely to the 2009-2010 cores samples (Ave.' = 

### 5.8% vs. 5.6%, Max = 

### 13.3% vs. 13.4%) as do the ash percentages (Ave. 

## =7.9%'vs. 7.1%, Max 

# =24.6% vs. 23.8%). As explained in 

### 'great detail above, these differences in coal characteriStics are useful ror making general comparisons with other fuel types, but it is unclear how these differences can be directly applied when making determinations on feasibility ofSCR at MRYS.' ' 

### B. High Economizer Exit Temperature NDDH's February 

### 1, 2007Request: The temperature betwee,n the economizer andthe air heqters is describedas "much higher than, 750 degrees". Pleaseproyide more speqijic data on the temperatures that wouldbe encounteredat the' location an SCR would be installed ' 

### B&McD'sApril 2$, 

### 2007Response:

, , Theprimary superheat (PSH) outletjlue gas temperatures for Unit 1 areprovided insteadof the economizer outlet temperatures, Unit 1 at the MR. Young Station has a two-pass tubular aiipreheater (APH). The eco,nomizer for Unit 1**i~** situatedbetween,the first andsecond pass of the airpreheater. The flue gaspasses on the inside of the**~ubes** of theAP Hoo
. .. : ..... The configuration of the economizer andair ,
, preheaterphysicallyprecludes locating a conventional**high~dust** SCR upstream of the existing air preheater-economizer on Unit 1 in its current location. andconfiguration. T,heflue gas temperqture is reduced by transferring heat to the boiler's feedwater and the boiler's combustion cmddrying system air streams, respectively. In addition, temperatures at the existing airpreheater outlet for Unit 1 arefar below the temperatures needed for operation of a 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' atthe, PSHoutlet vary inversely with the boiler cleanli1:zess during each operating, run... ... Note that qt low loads andclean condition,s, the PSHoutlet flue gas temperatures are

, 170-270degrees 

### F below the acceptable rangefor conventional hot-side high-dust SCR, catalyst
.... Unit'

1 's PSHfull-loadjlue gas temperatures historically range between 840 Fand 960 F, depending upon the cleanliness of the boiler
..... there haVe been instances when UnitJ's PSHfull-loadaveragejlue gas temperatures have exceeded 1000 F and individual temperature measuring elements have exceeded 1050 F.

. . Aportion of MRYS Unit 2 'boilers' feedwater heating surface is in 'a location normally occupied, by a traditional conve1')tional economizer section in the convection'pass of the boiler The configuration of the boiler 's economizer andthe airpreheaterphysicallyprecludes locating a conwintional high-dust SCR upstream of the existing air,preheater/economizer on Unit2 in its current location andconfiguration : Unit 2 boiler is shutdownfor cleaning at intervals rangingjrom approximately 80 to 110 days. : at low loads andclean conditions. boiler outlet flue 'gas temperatures are 170-270 degreesFbelow 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 exceeded930 Fand individual temperature measuring elements have exceeded990 F. 

### EPA Response: This inforniation that was requested by NDDH was reviewed by EPA. EPA acknowledges that there are times when the flue gas 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 inthe March 19, 2007 letter 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 such

### a 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 2007 Christman 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 wouldbe suhject to the same foulingproblems asa high dust SCR due the emissions of small 

# « 5 micron) particies coniainbig sodium-calcium sulfur rich materials. Pleaseprovide any existing data on the composition of theparticulate matterexiting the electrostatic precipitators.
. 

### EERC**Apr~123,** 

### 2007Response:

, Aspreviousi y discussed in responses to 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 liquid phqses andsmallparticles/hatare very reactive to gas-phase components, ' resulting information of compoundS that cause bonding, blinding, anddeactivation ojindividualcataiysi
. reactionsites, eve,? in a low-dust SCR application. Sodium-richaerosols can get through the electrostaticprecipitator (ESP). These aerosols are known to bepresent in high-:sodium coalcombustion flue gases. These spe9ies are known toproducepyrosulfates. These will cause blinding anddeactivation of the low-dust catalyst.
.


### -44-EPA Response to Minnkota Comment Letters from March 19, 2007 and April 23, 2007

, BACT Analysis at MRYS Stack sampling was conducteddownstream of the 'BPS andset FGD on Unit 2 at the MRY plant. Thefilters were examined using scanning electron microscopy andx-ray microanalysis to determine the microscopicfeatures andchemical compositions '!' •••• The chemical compositions of the analysis

, areas and points were determinedthrough the use of energy disperSive x-ray microanalysis
.... 

### EPA Response: This information that was requested by NDDH was**r~viewed** by EPA. While the relative effects ofvaporized 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 ofcatalyst'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 26,2007Request: Pleaseprovide any andall data and information relatedto catalyst deactivation rates andNO x
, contro! effectiveness measuredat the threefacilities testedduring the "Ash Impacts on SCR Catalyst

, Performance" stiidy that is extensively referredto in the "EERCReport" includedasAppendix B of the Minnkota NOx BACTanalysis. This should include the originaljJroposal,all testplans, 'all raw catalyst deactivation andNO x controldata collectedduring the test, andall final reports andconclusions that' wereproduced**fo~** the study'sparticipants
. .In addition, pleaseprovide a quantitative 'analysis of the application of the multiple engineering solutions (e.g. catalyst.rejuvenation and regeneration, acoustic horns, etc.) described by Roger Christman andEPA to address specific concerns highlightedin the BACT

, analysis andEERCReport andtheir effect on SCR catalyst deactivation rates andNO x control effectiveness at.MRYS. 

### EERC'sApril 23, 

### 2007Response: The EERC offeredto sendthem to Haldor Topsoefor evaluation; the catalyst vendor indicatedthat the Coyote SCR test catalysts were toopluggedanddeactivation testing couldnot be conducted. The lack ' of

publishedcatalyst activity datajor the Coyote slipstream SCR catalystfieldtest is a result of the severity of the catalystfoulingfrom North DakOta lignite ash. The requesteddata is available; this information has been copied, compiled, anddue tq its volume, will be submittedseparately. Information on catalyst deactivation rates andNOx reduction effectivenessfor the Baldwin and Columbia Station slip-stream SCR catalyst field testing was determinedandsome of the information was in thefinal report. EERC recently received information from Hitachi on catalyst deactivationfor the 

### -45-EPA Response to Minnkota Comment Letters from March 19, 2007 and April 23, 2007 BACf Analysis at MRYS catalysts at Columbia. This information is include,d inthe EERCdata compiledand being submittedto theNDDH. Regeneration has been attempted in Europe with SCR catalysts exposedto biomass as with high potassium (elemental symbol 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'sApril 23, 

### 2007Response: .A qucmtitative analysis on the application of the multiple engineering solutions suggested by the EPA's commenter mentionedabove has not. beenperformed Significantlymore time would be required A definitive analysis would requirepilot-scale slipstream SCR reactor testing on thejlue'gas streams emiitedfrom the boilers atMRYS over a significant length of time to simulate real-time effects of gas and'

.flyash constituents r;mdtheir interactions with the catalyst. Using assumptions of the effectiveness of solutions suggestedby EPA's commenter basedon, use of theoretical data developed from laboratory testing and/or 'empirical data gathered from other high-dustSCR installations that do not treat flue gases producedfrom cyclone boilers firing North Dakota lignite coal would be hypothetical. 'Such a theoretical quantitative analysis of EPA's claimedSCR catalyst improvements citedandwhether this .wouldactually be translatable intopreventing the catalyst layersfrom being susceptihle to macro-pore'blinding and plugging by calcium-basedcompounds andthe impact on the individual catCdyst reaction sites from sodium-based**c~mpounds** discussed i,! 

## Dr. Benson's referencedtechnicalp'aper wouldbe speculative. Burns & Mc!Jonnell believe-s it would'not**b~** accurate to burn lignite or usejlyash samples collected from a MRYS electrostaticprecipitator w,i{h artificial flue gas to simulate catalyst fouling and blindingphenom€mon in a laboratory'- orpilot-scalete,st setting Performing a quantitaiive analysis

. would require substantial resources to develop slip-stream SCR catalyst testequipment 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 substantial resources ..•. :..



## -46-EPAResponse

## to Minnkota Comment Letters·from March 19, 2007 and April 23, 2007 BACT Analysis at MRYS 

### EPA Response: Many ofthe concerns in this section have been previously presented verbatim 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 ofPotential 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 ofa 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 vendorguarantees by themselves are not,sufficient to show that a technology isfeasible, ve,ndorguarantees can contribute to a technicalfeasibi/ity determination in some situations. In light of statements EPA has receivedfrom three SCR vendors (Haldor Topsoe, Cormetech, andArgillon) indicating they would provideperformance guarantees on SCR lignite applications, pleqseprovide additionalvendor iiiformation,from multiple vendors, that Minnkota believes is more specific to MRYS them the statements receivedbyEPA andcited in EPA's analysis submittedtoNorth Dakota on January'8, 2007. We wouldexpect that the catalyst , suppliers citedabove would be among the vendors contactedby Minnkota. 

### EERC's April 23" 

### 2007Response: BERC has already'ineludedsome co;"'mentsfr9m February 2007 telephone discussiQns with Haldor Topsoe regarding**~heir** concerns about the sodium content in North Dakota /ignite and its tendencies for catalyst deactivation. 

### EPA Response: EPA has previously presented its own discussions with Haldor Topsoe
.

.

# - ", 

### B&McD'sApril 23,.2007Response: A query of SCR catalyst vendor's willingness to guaranteepeiforma1Jce on a /iinite-firedboiler to

, addrffss comments in EPA's citeddocuments mentionedabove has not been completed Additional time

## is

requited. The information on, SCRperformance guarantees for MRYS applications has been requested; 

### it ' will,be submittedseparately when it is available. A copy of the information request letter and attachments·issuedto the SCR vendors' is attached (Appendix A). ' , :Systeinandequipment vendors have a'se/f:interesi to sell technology that they offer, andto manage finanCial risks by specifying,the limits of their system design in case of potential operating condiiions that

, causeperformanceproblems thatprevent achievement of guarantees 

### (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 to have been written

., t' • 

### -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 MRYSNO x BACTanalysis includes**disc~sion** of periodic boiler outages that"are requiredto address the slagging effects oftbe Center Mine lignite. Pleaseprovide historical dates'of
. thes(1 outageperiods and their durationfor theprevious 10years. Also. pleaseprovide an estimate of

.. boiler outage frequency that would be expecteddue to SCR maintenance. bas.edon information collected i1) response to items] and2afthis list.' '.
. 

### B&McD'sApril 

### 23, 2007Response:

. Information to estimate MRYS boiler outagefrequency expected for high-dust SCR maintenance based upon a query. of SCR catalyst vendors mentioned in item 2 above is notavailable at this time. Additional time'is required
. '.
. A review of historic outage data shows. that over thepast 10years (1997-2.006). thetypicalMRYS Unit] boiler cleaning outage was between 67 cmd 82 hours in duration. but these can be as short as 56 hours andas long as 206 hours
... ... Over this same timeperiod, 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 boiler fireside cleaning-outage is typically scheduled for 96 hours (four days) duration.
. . . . A high-dust.SCR wouldneedto be isolatedfrom the gaspath between the boiler andthe inlets to the electrostaticprecipitator during boiler cleaning outages toprevent the moisture anddeposits , liberateddW'ing boiler fireside surface cleaningfrom entering the SCR reactor. Beginning the work of removing ash accumulation or catalystfrom the SCR r(!Qctor(s) will be delayedby 48 to 60 hours after the start of tfle outages, depending on ambient temperatures, becausethe Inside of thereactor(s)" will be too hotforpersonnel health andsafety. Because the length of the 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 of time anddifficulty in
. removing lignite ash accumulation from ihe SCR**reactor(~),** or replacingfouledcatalyst with clean spare catalyst. is expectedto require at least 144 hours (6 days). plus 60 hoW's upfront for allowing the reactor to cool-down
.. This is basedupon the use of mobile industrial vacuum cleaning methods ("super-sucker" truckJ.. The· total SCR maintenance outage time will be around200 hoW's from' boiler shutdown endto 

### -49-EPA Response to Minnkota**Com~ent** Letters from March 19, 2007 and April 23, 2007

. BACT Analysis at MRYS beginning of siartup. Thefrequency o/such SCR· maintena1Jce outages are expectedto be requiredat. least annually, or more often.
. . There were only two outages other than 111ajorscheduledoverhauls during the historic ten-year . period that appearedto be of sufficient length (greater than 

### lJdays) thatcouldhave lent themselves to significant catalyst cleaning or replacem.ent work withOutdelaying startup: one on: Unit Jrandone on Unit 2 There are basic concerns that using short boiler cleaning outages as SCR catalyst maintenance opportunities couldbe detrimental to catalyst life due to thermal stress.es and passing through various (Jew point temperatures. Bringing the very large mass of ceramic catalystmaterial andthe reactor structure down in temperature andthen back up rapidly andany morefrequently than absolutely necessary shouldbe 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 ofHaldor 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 ofabout 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 ofboth figures inn the context ofa cost analysis.

### G'~Hydrocarbon-**E~hanced** Selective Non-Catalytic Reduction (lIE 

### SNCR) 

### EPA 'sJanuary"26, 2007**~omment:** Minnkota eliminates this technoiogyprimarily basedon the fact that it has not been appiiedto cyclone boilers or units burning/ignite or**high~slagging** western subbituminous coals. Thf!re are twofull scale installations of this technology on easternpulverizedooal-firedboilers. There is some general
. discussion about the slaggingproblems that/ignite wouldcause the HESNCR technology, but·it is unclear whether the vendor of this technology wouldbe. able to overcome these issues. At thispoint, it is unclear**wh~ther** HESNCR ·is technically feas.ible. Minnkota. should provide a more tho;9ugh analysis on whetherthis. control option is "available andapplicable" (as described in the NSRMa.nual) atMRYS by examining thephysical andchemical characteristics of the exhauststream in terms of teqhnical feasibility
. andsoliciting vendor information. .A quantitative analysis of its technical feasibility-s.hould beprovided
... andsupportedwith data.and vendor statements.. .



# -so-EPA Response to Minnkota Comment Letters from 'March 19,2007 and April 23, 2007 BACT Analysis at MRYS 

### B&McD's April 23, 2007 Response: The Dctobet**200~** MRYS NOx B4CT(lflalysis wasperformedusing technical literature m'ailable in the public domain, proprietary vendorcustomer experience informatiQn, andlimitedengagement of**s~lected** vendorsfor capital cost, operating andmaintenance costs, andemissionsperformance estimates along

. with Burns & McDonnell's technical expertise. ,This analysis approach meets the requirements of the' EPA. 's NSR Manual. A detailed query of thepotential availability andapplicability of hydrocarbon-enhanced (HE) selective non-catalytic reduction (SNCR) on a lignite:firedcyclone boiler from the single vendor mentioned in the October:**~006** MRYSNOx BACT Analysis· reports has not beenperformed Additional" time would be required., It is important (lfld appropriate to specify the composition of the Center lignite coal andash constituents and boilerconfiguration sufficiently toprovide a soundbasisfor allowing the HE-SNCR vendors' reviews'andqualifiedresponses. HE-SNCR design conditions that are reasonably reflective pfactl!al operating conditions and boiler fouling experience are yet tt? be established The lack of detaileddevelopment of MRYS unit-specific HE-SNCR injection lance design schemes to deal with the high boiler fouling conditions andvarying boilergas temperaturescmd loads will require HE-SNCR design operating cQnditions to be assumedbefore 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 didnot

..mention or evaluate HE-SNCR technology as apossible'NOx control alternative in theirBARTanalyses.

"


### , 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 ofexperience 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 or substantiating 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 

# a customer 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. Thereasons given'by B&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 determinedto be technically**f~asible** in the Minnkota
. analysis, reburn combinedwith SNCR was ruledoutbasedon technical infeasibility. Although..full scale pulverizedcoal installations of gas reburn w.ith SNCR and overfire air were**fou~d** to exist, Minnkota did ,not analyze whether the technology combination would be "available andapplicable" to MRYS, as described in the NSR manual. Instead, the application states,**'~(t)he** lackof experience with these' combinations on a cyclone-jiredboiler, especially for lignite-firing, makes the application for MRY
. Station's boiler injeasibie. " 'Minnkota should provide a more thorough analysis
..... 

### B&McD Response: A detailed query of thepotential availability andapplicabilitjl,offuelreburn technology combined'
. with SNCR on a lignite-firedcyclone boiler from the reburn technology vendors has not beenperformed. As mentioned inAppendixAl of the October 2006 MRYSNOx BA(:T Analysis reports, while SNCR may , potentially be installeddownstream of separated oveifire a}r andvarious types offuel reburn, 'there is a . lack of demonstrated experience of this ,combination on cyclone boilers. Only one example of gas reburn
. with SNCR.. appliedto a tangentially-jired boiler, was found [n available technical literature. Moreover,' the very high capital and O&MCo.sts't;llready estimatedfor coal and**ga~** reburn technologie,s with
. , ASOFA in Section 3 of the October 2006MRYSNOx BACT Analysisreports showedthat such "
. .
, alternatives were siinificantlyabove the BACTDominant Controls Cost Curves forMRYS Unit rand '

, Unit 2. Thus, even iffuel reburn technology combinedwith,SNCR**~ere** available aridapplicable to

lignitefiredcyclone boilers, the average.andincremental unitcontrol co,sts ($per tim) wouldnot be,
. 'expected to be on theNOx BACTDominant-Controls Cost C1,IrvesforMRYS Unit I andUnit 2. Such reburn with SNCR alternatives wouldthus be consideredas "injerior",alJdwouldnot be consideredas. BACT Substantial resources wouldbe required
....lt is interesting to note thatothernon-cyclone BARTeligible boilers burning North Dakota lignite didnot mention or evaluate reburn with or withoutSNCR technology aspossibleNOx control alternatives iri,theirBARTanalyses.


### -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 

### of Rebum 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 of the difficulty in achievingproper substoichiometric hoiler con,ditions necessary to get effective NO x control; As described in the application, theproblem**occur~** becat!Se of the variable heat content of CenterMine lignite. While RR1 has been

. successfully demonstratedat cyclone tinits burning blends of Powder River Basin coal with Midwestern bituminous coal, Minnkota hqs ruledRR1 out as technically infeasible. However, there does not appear ro be an analys.is of whetherRR1 meets the "available andappNcable" criteria described in the NSR Manual for elimination under step**2~** 

### If RR1 would be less effectivefor the'reasons cited in Minnkota's application, that wouldresult in d lowerranking under step 3 and possible elimination under.step 4. At' this time, it does notseem qppropriate to eliminate RR1 understep 

### 2. Minnkota should provide
. 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 thepublication of the MRYSNOx BACT Analysis report; two commercialRich Reagent
.

·Injection systems have completeddesign, fabrication, and insta!lationin lcite 2006 atAmeren Sioux Unit I and Unit 2. Full optimizedroutine operation is expectedto begin by early May, 2007. Although RR1 NOx emissions reduction technology appears to satisfy the "available" criteria (as described in the NSR Manualfor technical feasibility)forpotential application on the cyclone boilers at MRYS, the substantive.. issue is "applicability". The expectedinsurmountable diffiCUlties in achieving consistent control·
.

· effectiveness.; ...from this technology 

### if appliedto modestly air-starvedcyclone combustion on these lignitefired boilers andthe reasons for technical infeasibility of RRlat MRYS were included in Appendix Al of the C!ctdber 2006NOx BACT Analysis reports.

o A query of the single RRllicensee mentionedin the MRYS NOx BACTanalysis, anda quantitative' analysis of its technical feasibiiity for ihe expectedboiler operating conditions has not beenperformed
. The transient variability of thefired lignite'coal atMRYSwas discussed in AppendiXAl of the October' 2006 NOx BACT.Analysis reports. This variability makes it impossible to accuratelyprefjict realistica./ly achievable NO"x emissions reductions even with sophisticatedcomputationalfluiddynamics (CFD) modelingofthe'MRYS boiler combustion reactions and in-furnace NOx reductionprocesses·associated· with 

### 1?RItechnology under reasonably realistic boiler operating conditions. This is due to the static(i.e. steady-state) nature of the boiler furnace operating conditions aSsumedto exist in order to simulate the performance ofmodestlyair-stcirvedcyclone. combustion on these lignitefiredboilers with varying heat andair inputs to individual cyclones usingCFD modeling techniques. Because of the dynamic variability of the higher heating value of thefuel burne{J"tmdthe lack of abilitYtoprecisely control the actual stoichiometry in real-time on an individual cyclone basis, RRl CFD modelpredictions wouldbe
. unrealistic andthus these vendorstatements wouldnot be reflective of realisticallyachievable 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 of

## NOx 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-jiredexperience, and thus was consideredi1Jfeasible." While a technology that has been demonstrated on the source type under review wouldshow it is technically ftasible, lack of such demonstrationJor the specific type of emission
.

.. unitdoes notnecessarily lead to a conclusion of infeasibility, Minnkota shouldprovide a more thorough analysis
.. :..... . ..



### .B&McD Response: A detailed query of thepotential availability andapplicability of ROFA technology on a lignitefired cYclone boilerfrom the single vendor mentioned in the October. 2006 MRYSNOx BACT Analysis

. reports has not beenperformed Burns & McDonnell has reviewedavailable ROF A technical literature, attendedsalespresentations and participated in discussions with ROFA vendorpersonnel regarding the application of ROF A to cyclone· boilers, incltding lignite-jired units. As an engineer anddesign/builder

.of numerous commercialseparatedoverjireair systems specifically designed for cyclone-flredboilers,

.. Burns and·McDonnell believes tha! ROF A te<::hnology is notsuperior to the evaluated version of SOFA in the October 2006MRYSNOx BACT Analysis to warrant such a vendor query anda quantitative analysis

......... ROFA does not offer any significant advan.tages in lignite-firedcyclone-specific application of separatedoverjire ,air technology.
. 

### 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, 2007 

### Comment: Based on theproposedemission rate ofO.361b/MMBtu and the emission rate expectedfrom installingAdvanced**Separa~ed** Overjire Air (ASOFA) alone**(0.5131~/MMJjtu),** itdppears 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 PSDpermit 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 of this range shouldhave been considered in the BACTanalysis. Minnkota's analysis really provides no techrzical discussion on why it ruledout higher capabilities of SNCR control andwhy they usedthe control efficiency that isproposed Using 60% control wouldyieldan emissions rate of 0.21 lb/MMBfI.!.for a BACTlimit with ASOFA andSNCR. comparedtojheproposed 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.'sApril 23, 2007

### Response: There werepertinent rejerences in AppendixAl ojthe MRYSNOx BACTanalysis discuSsing long-
.. term examples where'SNCR has been appliedjorNDX emission control on two cyclone-jiredboilers,
. achieving reductions around30percentfrom'high uncontrolledbaseli1!e levels. There was also another relevant cyclone boilerSNCR demonstration test listed in Appen{1ix.A2 ojthe MRYSNOx BACTanalysis' that controlledNOx in this same 30percentrange. NOx control effectiveness'ojSNCR is highly boilerspecific. It is important to note that PSDpermit applications arejor. miw**boiler~;** there are no new cyclone boilers being built, andfew new boilers (other than CFB boilers) are requiredto install and operate SNCR. The citedAP-42**NO~** emissions rate reductionpercentage for SNCR application,on coal..: fired boilers isn't applicable to the specific conditions expectedat the MRYS boilers
... .Burns & McDonnell estirflt;ltedthe level ojemission reductions expectedfrom SNCR basedon injormationfrom an

, experienced urea-basedSNCR vendor. B&McD believes that 60percent reduction olNOx emissions is '

inappropriate, andnot**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 of30-day Average BACT Emission Limit EPA's January 26, 2007 

### Comment:' The baseline emission rate usedfor**~alculating** theproposedBACTlimit is based'on the highest, NO x emission rate (0.884 Ib/MMBtu) during actual operations from 2001-2005. '(p

.. ES-9) Thisperiod also includes the longest reportedstqriup event during thatperiOd (61hours), How does this 30-day
. baseline compare with the rest ojthe historical data? Does this emission**r'at~** represent a realistic upper' boundvalue, or is it an outlier comparedwith the majority of the data? Ifihe d,atapoint does not reasonably compare with the vast majority ojthe other data>it does not make sense to set a BACTlimit basedon an outlierdatapoint. Furthermore, the baseline emiss'ions rate usedjor calculating the 30-day

. limit'is hasedon the fzighest 30-dayperiodduring actual operations from 2001-2005, which includedthe

. longest startup event. If this highest 30-dayperiodwas usedas the baseline to calculate the 30-day average limit, andthe worst-case startup event occurredduring this historic 30-dayperiod, it is not necessary to include the recommended "startup a4iustment" '(+0. 0411b/MAmtuJperstartup event witfz

, this limit. 

### B&McD'sApril 23"i007

### Response: According to MRYS:r;nit 1 :s October 2006BACT Analysis, the control effectiveness calculations ' were determined based'upon the highest historical12-month rolling NOx emission summation operating' conditionsjorMiltonRYoung Station boilers duringyears 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 emissionperiods foryears 2001-2005 were then comparedto the highest 12-montkhistoricpre-control baselineperiodconditions. The average unitNOx

.. emission rare (lblmmBtu)from the highest historicpre-control 30-day**~aseline** periodwas comparedwith

the average NOx emission rate (lblmmBtu)from th.e 12-consecutive monthperiodwith the highest historic unitNOx mass emissions (pounds) reported/or actual operation during the same 5-year look-

.back period It was coincidental that the highest historical**30~day** rolling NOx emissionpe.riodfor MRYS Unit 1 (December 17, 2003 through January 15, 2004) was within the same highest 12-month historic
. pre-control baselineperiod (ending on October 30, 2004). The longest individual U1ilt 1 startupperiod during actual operation between January 1,2001 andDecember31, 2005l?ccurredduring the 61- consecutive hourperiodfrom June 27-29,2005.:
. ... The highest historJcal30-day rollingNOx emission periodfor MRYS Unit 2 (March 7, 2003 throughApril 5, 2002) was not within the same highest 12-month historicpre-control baselineperiod (endingApril 30, 2004). The longest individual Unit 2 startupperiod during actual operation between January I, 2001 andDecember 31, 2005 occurredduring the 115- consecutive hourperiodfrom October 25-30, 2001. Note that this didnoUall within eithefthe 30-day or the 12-inonth highest historic rolling average NOx maSs emissionperiods.
... ... The selectedhighest baselinepre-controI30-day rolling average NOxunif.emission ratesfor MRYS Unit 1 and Unit 2 boilers

· are consideredto be realistic upper boundcases, not "outliers II •••••• The recommended30-day Rolling,

· Avet.age unadjusted (i.e. steady-state) BACTNOx emission rates do not include the impact of the worstcase startup events for eitherMRYS Unit 1 or Unit 2. The ·"startup adjustment II as described in the

· October 2006 MRYS Unit 1 and Unit 2 NOx BACT Analysis reports is appropriate as the recommended (unadjusted) 30-dayRollingAverage NOx Emission Ratepermit limitations basically r.eflectsteady-state rates.. 

### EPA Response: EPA has reyiewed B&Mc; s detailed descriptiotiofhow 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, 

### 2007 Comment:
. EPA has concfJrns with Minnkota'sproposal for an 18-month trialperiod for establishing a final BACTlimit. (p. ES-II) Thisproposal appears to be in direct conflict with thepr.ocessfor establishing and potentiallyrevisingthe NO x BACTlimits setforth in Sections V. B & D of the Consent Decree. Therefore, EPA believes theproposed 18-month trial period for establishing a final BACTlimit in Minnkota's BACTanalysis cannot be accepted 

### B&McD'sApril26, 

### 2007Response:

· .....Paragraph 73 of the referenced Consent Decree**st~tes** "The Settling Defendi:mts shall submii anypetitionforany 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 specified for that Unit in Paragraph 67 ".. Paragraph 67 requires thatMRYS Unit 1 and. Unit 2 shall .achieve andmaintain a Phase II30-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 of Consent Decree Phase IINOx emission reduction systems with asspciaiedcommissioning atMilto"" R. Young Stationprior to the CD
.

. compliance datesforMRYS Unit 1 and Unit 2, respectively.' This couldallow Minnkota to gain· operating

experience in advance of the June 30,2012 andJune 30,2011 deadlinesfor submitting apetition. lfthe
. Settling Defendants do notaccelerate the scheduleforPhase IINOx control implementation, recogn.izing·
.. that the CD apparently does not allow an 18-month trial demonstrationperiodfo.llowing the siatedfinal compliance dates, the applicant will 'have to make Use ·ofthepetitionprocess as described in the CD in this circumstance, 

### if appropriate.
. 

### EPA Response.: EPA agrees this. is 

# an acceptable**appJ;oach~** 

### .

<,..



### -58- . 

### .