s

...;> Minnkota, in its November 9, 2007 response to

comments, provided evidence of sodium .and potassium

compounds that deposited downstream of the

electrostatic precipitator on Unit 1. This

indicates that the electrostatic precipitator will

not remove all .of ..the submicron sodium particles

that are generated by the combustion of Center

lignite. Given the high expected emission rate of

sodium compounds (see Table 3), it appears a

significant amount of sodium compounds, a catalyst

poison, will enter a low dust or tail gas SCR

system.

The deactivation issue for low-dust and tail-gas

SCR remains as with the high-dust· SCR. The

research, design and pilot testing needed to

develop an SCR system that will have reasonable

success makes this technology also not applicable

at this time.

Site Constraints:

The NSR Manual1 states: -Also a showing of

unresolvable technical difficulty with applying the

control would constitute a showing of technical

infeasibility (e.g. size of the unit, location of

the proposed site, and operating problems related

to specific circumstances of the source). Where

the resolution of technical difficulties is a

matter of cost, the applicant should consider the

technology feasible."

Minnkota has not indicated that the site

constraints ,are unresolvable. Therefore, this

issue appears to be an economic consideration

instead of a technical feasibility issue.

56
Tabl.e 3

HJ:STORICAL COAL EMJ:SS:ION RATE COMPAIUSON

CBN'rBR TEXAS WYOK[NG PO PA

L1:GRJ:o;rB a L1:GH1:TE b SUBBIT'CKtI!lOUS b BX'l'maNOUS b

Na20 (!$ of Ash) 4.4 0.5 1.6 0.3

CaO l!fi of Ash) 13.2 13.2 17.3 1.7

MgO (% of ash) 4.0 2.3 3.B 0.6

K20 ('is of Ash) 1.3 0.5 0.5 1.9

~a20+CaO+MgO+K20 (% of Ash) 22.9 16.5 23.2 4.5

Ash Content 9.6 12.6 5.0 13.0

Heat Value (10' Btu/ton) 13.2 15.2 17.0 25.5

PM Emission Factor (lb/ton/1% Ash) 0'.4

Cyclone Boiler 9.4 6.7 2.0 2.0

Wall/Tangentially-fired Boiler (Pulverized) 5.8 5.B 10.0 10.0

PM Emissions (lb/10' Btu)

Cyclone Boiler 6.84 5.55 0.59 1.02

Wall/Tangentially-fired Boiler (Pulverized) 4.22 4.81 2.94 5.10

Na20 Cyclone Boiler Emissions

lb/ton 3.97 0.42 0.16 O.OB

Ib/10' Btu 0.3008 0.0278 0.0094 0.0031

lb/dscf 3.05078-05 2.8164E-06 9.·62358-07 3.1276E-07

lb/wscf 2.5172E-05 2.3238E-06 8.8456E-07 2.8748E-07

Na20+CaO+MgO+K20 Cyclone Boiler Emissions

lb/ton 20.66 13 .93 2.32 1.17

lb/10' Btu 1.5655 0.92 0.14 0.05

Ib/dscf 1.5878E-04 9.2941E-05 1.39548-05 4. 6914E-06

Ib/wscf 1.31018-04 7.6686E-05 1.28268-05 4.31238-06

21
Table 3 Continued

CENTER 'l'BXAS WYOmNG PRB PA

LIG!lJ:I'l'E • L:EGNJ:'l'B 1> SUBB:t'.rtJKINOUS I> BI'l'tJM:tNOUS b

Ratio of ND Lignite Cyclone Emissions to Other

Cyclones

Na20

lb/ton 9.4 24.8 50.9

Ib/10' Btu 10.8 32.0 98.3

Ib/dscf 10.8 31.7 97.5

Ib/wscf 10.8 28.5 87.6

Na20+CaO+MgO+K20

Ib/ton 1.5 8.9 17.7

1b/10' Btu 1.7 11.5 34.1

lb/dscf 1.7 11.4 33.8

Ib/wsc,f 1.7 10.2 30.4

Comparison of ND Lignite Cyclone to Pulverized Units

Na20 Emissions

1b/ton 3.97 0.37 0.80 0.39

1b/10' Btu 0.3008 0.0240 0.0471 0.0153

Ib/dscf 3.0507E-05 2.4381E-06 4. 8117E-06 1.5638E-OG

Ib/wscf 2.5172E-05 2.0117E-06 4.4228E-06 1. 4374E-06

Na20+CaO+MgO+K20 Emissions

1b/ton 20.66 12.06 11.60 5.85

Ib/lO' Btu 1.5655 0.7933 0.6824 0.2294

Ib/dscf 1.5878E-04 8.0457E-05 6.9770E-05 2.3457E-05

lb/wscf 1.3101E-04 6.63B5E-OS 6.41309E-05 2.15613E-05

Ratio of ND Lignite Cyclone Emissions to Pulverized

Units

Na.20

Ib/ton 10.9 5.0 10.2

Ib/lO' Btu 12.5 6.4 19.7

Ib/dscf 12.5 G.3 19.5

lb/wscf 12.5 5.7 17.5

22
Tabl~ 3 Continued

OH'l'U 'l'BXAS WYOKERG PRB PA

L:tGNJ:'.rB a LZGNZ'l'B » SUBBZ'l'OIaNOtrS » BZ'1'OMJ:HOtJS »

Na20+CaO+MgO+K20

Ib/ton 1.7 1.8 3.5

Ib/lO' Btu 2.0 2.3 6.8

Ib/dscf 2.0 2.3 6.8

Ib/wscf 2.0 2.0 6.1

• Information from Minnkota's April 16, 2007 submittal.

b Data from the USGS National Coal Database and university of Wyoming.

c From AP-42, Compilation of Air Pollutant Emission Factors except for Minnkota whiCh is based on actual stack

testing.

Emission factor for tangentially and wall-fired units burning lignite is the average of the AP-42 emission

factors.

23
MinnkotaPower ~PCl COOPERATIVE, INC.

1822 Mill Road, P.O. Box 1320"0 • Grand Forks. NO 58208-3200 • Phone (701) 795·40DO

~ovember 9, 2007

Mr. Terry O'Clair

Director, Division of Air Quality

North Dakota Department of Health

918 East Divide Avenue, 2nd Floor

Bismarck, NO 58501-1947

Subject: Response to EPA's October 4,2007 Letter

Dear Mr. O'Clair:

Your Touchstone Energy- Partner

-

Enclosed, ~ease find Minnkota's comments in response to EPA's letter dated October

4, 2007. Should you have any questions concerning our comments, please contact me

at 701-7954221. .

-M~~I'-~ John T. Graves, P.E.

Environmental Manager

C: David Sogard wlo enel

Luther Kvernen wlo encl

. ·Craig Bleth wlencl

Kevin Thomas wlo encl

Stu Libby wlo encl

Tom Anseth wlo encl

Young Station File w/encl
Variations in the coal delivered and burned at MRYS have posed difficulties in maintaining

consistent combustion and slag-formation and tapping conditions in the cyclones and lower

furnaces of the boilers. Low HIA ratio correlates to undesirable cyclone operation. High

viscosity ash, related to low BIA ratio, causes more incompletely burned coal and ash particles

to be exhausted from the cyclone, which increases the amount of flyash that can create

sJagging and fouling problems in the furnace and convection passes of the boiler. This causes

furnace exit gas temperatures to increase, requiring more aggressive heat transfer surface

cleaning, which me~ns higher gas temperatures with more ash and deposit frag~ents are

exhausted from the boilert' which are problematic for potential blinding, deactivation, and

longevity of the SCR catalyst.

"Core" Issue number 2

Catalyst deactivation by sodium and regeneration difficulties:

ERG memorandum to EPA, August 20, 2007, page 3

Original Reference: EPA Region 8 January 26,2007 letter to NDDH request item number I:

[Comments regarding Khodayari's work on SCR catalyst deactivation and regeneration

due to poisoning by sodium and potassium] [ERG August 20,2007 memo, page 3]

EERC Response:

Christman appears to ignore key statements made by Khodayari in his thesis and other

publications. The following is a quote from the abstract of his PhD thesis23:

"Potassium and sodium were found to be two of the worst poisons that had significant

effects on· the activity of the SCR catalyst due to their selective chemisorption on the

BrlZlnsted acid sites."

[Comments regarding Beck's work on contributions ofphosphorus, potassium and

sodium].

"His'subsequent workfocused on phosphorus and did not address sodium at all." [ERG

August 20, 2007 memo, pa8.e 3]

EERC Response:

Beck and others focused on phosphorus because it was at a higher concentration than sodium

in the materials he was investigating.

23 See Reference number 10.

1119/2007 A-4
••• IIUniversity ofUtah and REI on application ofSCR to low-rank coals ...but masking by

jlyash is a problem. The studyfurther concluded that the poisoning potential ofthe alkali

materials ranked as: K>Na>Ca. " [ERG August 20,2007 memo., page 3]

EERC Response:

The University of Utah studies did not consider high sodium coals in their testing. They

examined lower sodium coals. We agree with the assessment that masking is a problem.

.. Cltristman brings up an important point regarding the impacts of potassium on catalyst

deactivation, but ignores its presence in lignite coals. Potassium can be as high as 2.3% in

.Center lignite. Potassium, like sodium, is prone to vaporIze in combustion systems and

condense to form small particles upon gas cooling.

----v Work was conducted at MRY Unit 1 to examine accumulations of sticky ash at 300~F

downstream ofthe ESP. Figure 1 shows a polished cross-section of a deposit that has formed

on a silicon dioxide suiface when exposed to flue gas derived from the combustion of Center

lignite. The red line shows the path of a line scan where the composition was determined using

x-ray microanalysis. Figure 1 also shows the change In chemical composition along the red . .

line beginning at the outside and moving to the silicon dioxide for selected components. The

results show levels ofpotassiu~ (K) as high as 17% and sodium (Na) as high as 16% in the

line scan. The matenal.is highly sulfated as indicated by the high levels ofsulfur (S). The

deposit shown in Figure 1 formed at about 300o

P. In addition. potassium-rich materials were

found in deposits plugging SCR catalysts at the Coyote Station as shown in Figure 2 and Table

1. Levels of potassium ranging from 2 to 8 elemental wt% were observed.

11/912007 A-5
~'I-\-:-t-ft--+f-+--t--- ._- --"

I· . ----~- ---!---- ---·---1-

+--.+--+-t---+--+---f--+-+---I-----l--_!_--+--+---I-l!'--_!_ - -.Na -

-51

-$ _ ..

-K

-Ca

'-Fe

10.0 15.0 20,0 25.0

LOCATION (urn)

Figure 1. Cross-section of a surface exposed to flue gas at about 3000 F in MRY Unit 1.

Figure 2. Ash accumulations in pores of SCR catalyst exposed to Coyote flue gases

for 4 months.

111912007 A-6
)~f

... ·l

.'

Table 1. Chemlca· • I A I . nalYSIS 0f SIt eec ed Pom. 15 SbOWDm. F'tgure 2.

Point Locations, Elemental Wt % .

Point 1 Point 2' Point 3 Point 4 Point 5

Element

Na 5.09 1.88 0.58 2.68 2.04

Mg 2,49 0.88 4.23 4.38 1.39 .. AJ 2.23 3.95 1.50 0.80 1.55

Si 4.86 1.79 4.67 .5.82 4.42

P 0.00 0,85 0.06 0.10 0.00

S 32.51 29.43 31.35 30.33 27.55

CI 0.00 0.00 0.22 0.33 0.15

K 6.64 2.54 8.74 7.85 4.64

Ca 16.08 29.69 19,42 16.91 25.20

Ti 0.00 0.11 0.64 0.16 6.02

Cr 0.00 0.00 0.00 0.00 0.00

Fe 7.76 7.52 0.00 3.44 7.37

Ba 4.99 0.00 '5.34 6.00 3.19

0 17.35 21.37 23.25· 21.20 16.49

TOTAL 100.00 100.00 100.00 100.00 100.00

.,

:':·~/ .. . .

"While Dr. Benson concludes that the sodium levels in North Dakota Lignite ash are so

high...others at EERC do not share that opinion... " [ERG August 20, 2007 memo, page 3J

EERC Response:

The cited work conducted by Folkedahl (2006) "Yas on a stoker-fired system and does not

represent the partitioning that occu,rs in a cyclone or pulverized coal-fired system. They had

significant problems during the testing because large pieces of unburned biomass kept

plugging the inlet to the slipstream syst~m.

Christman ignores work conducted by the EPA. Mussatti (2002) indicated in the "EPA Air

Pollution Control Cost Manual - Sixth Edition" the following primary mechanisms for catalyst

deactivation:

"Poisoning - Certain fuel constituents which are released during combustion act ascatal yst

poisons. Catalyst poisons include calcium oxide and'magnesium oxide, potassium,

sodium. arsenic, chlorine. fluorine. and lead. These constituents deactivate the catalyst by

diffusing into active pore sites and occupying them irreversibly. Catalyst poisoning

represents the main cause of catalyst deactivation."

11/9/2007 A-7
--?

B. Low-Dust SCR

1. EPA Analysis ofthe Catalyst Fouling and Deactivation Issue:

EPA October 4~ 2007 letter Enclosure, page 32

Original Reference: EPA Region 8 January 8. 2007 letter to NDDH:

".. .general qualitative statements that the sodium-rich aerosols will cause blinding and

deactivation do not necessarily support the conclusion that a·low-dust SCR system would

be technically infeasible at MRYS.The.effects ofthis blinding and deactivation should

either be quantified. and used within. the economic analysis under step 4 ofthe Top-Down

BACT analysis, or described in sufficient detail to demonstrate that the problems

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

Manual, U(p)hysical modifications needed to resolve technical obstacles do not in and of

themselves provide a justification for eliminating the control technique On the basis of

technical infeasibility. However, the cost ofsuch modifications can be considered in

estimating cost and economic impacts." [EPA October 4, 2007 letter Enclosure, pages 32-

33J

B&McD Response:

As EERC has described previously and elsewhere in this document, effective solutions to

prevent or remove micropore pluggage and blinding of SCR catalyst from sodium-sulfur

compounds evident in the flue gas stream exhausted from a North Dakota lignite-fired boiler,

whether installed in a conventional hot-side or cold-side application downstream of a wet.

scrubber, have not been developed by $CR catalyst vendors. Bums & McDonnell previously

discussed the estimated energy requirement~for reheating flue gas upstream of a low-dust and

t~l-end SCR in the October 2006 NOx BACT analysis and March 2007 response letters.

Propane or fuel oil will certainly be more expensive to supply than pipeline natural gas,

because of the additional over-the-road transportation requirements. These alternate fuels do

nothing to address the fundamental issues of the composition of the flue gus and flyash being

unsuitable for low-dust SCR applications. We believe these are credible arguments that

adequately describe the reasons that preclude the successful deployment of this technology on

the cyclone boilers at MRYS.

2. EPA Analysis of the Site Space ConStraint Issue:

EPA October 4, 2007 letter Enclosure, page 33

Original Reference: EPA Region 8 January 8, 2007 letter to NDDH:

1119/2007 A-28
" ,.

I'

'J

..

~/ "EPA acknowledges that extensive work may be requiredfor installation oflow-dust

SCR...based on reasonable expected reheatfuel requirements, equipment installations, plant

modifications.... " [EPA October 4, 2007 letter Enclosure, pages 33-34]

. B.&McD Response:

Burns & McDonnell believes it is appropriate to consider SCR technology for the cyclone

boilers at MRYS to be technically infe.asible an~thus eliminated it during the Top-Down

BACT analysis prior to the economic analysis step. The,extensive'plant modifications

anticipated to deploy SCRat Dnit 1 and Unit 2 at MRYS still do not resolve the catalyst

micropore plugging and blinding problems evident in the Coyote Station pilot-scale testing.

3. EPA Analysis of the Burns & McDonnell's Conclusion:

EPA October 4, 2007 letter Enclosure, page 34

Original Reference: EPA Region 8 January 8, 2007 letter to NDDH:

"B&McD appears to be saying that a significant portion ofthe sodium in the fly ash is in

the vapor phase when it enters the ESP." [EPA October 4, 2007 letter Enclosure, page 34]

. EERC Response:

EPA misinterpreted B&McD and EERC's response. B&McD and EERe do not suggest in any

of the documentation that sodium is in the vapor phase at the temperatures of the SCR.

Initially in cyclone-fired systems much of the sodium is vaporized. Vaporized sodium will

condense upon gas cool,in.g heterogeneously on entrained particles Or homogeneously to form

aerosols. The condensed material will react with other gas phase materials such as S02 and

S03 to form sulfates. There is eVide~ce ofsodium, potassium, calcium, and sulfur depositiO~ •

downstream of the ESP as illustrated in Figure 1 presented in response to ERG Core Issue

number 2.

III. Additional Information Useful to Evaluating Technical Feasibility of SCR at MRYS

A. EPA Headquarters Believes SCR is the Correct Technology ~or MRYS in

Developing the Clean Air Interstate Rule

EPA October 4, 2007 letter Enclosure, page 36

Original Reference: EPA Region 8 January 8,2007 letter to NDDH:

"...EPA's discussion with Flemming Hansen ofNaldor Topsoe also revealed that despite.

the concerns about an SCR system subjected to the sodium environment... Haldor Topsoe

would not rule out bidding such a project. " {~PA October 4, 2007 letter Enclosure, page 3n '

11/9/2007 A-29