# MinnkotaPower**~Pg** COOPERATIVE, INC.

1822 Mill Road' P.O. Box 13200 • Grand Forks, ND 58208·3200 • Phone (701) 795-4000 August 16, 2007

Terry O'Clair, Director Division ofAir Quality North Dakota Department of Health 918 East Divide Avenue, 2nd Floor Bismarck, ND 58501-1947
Subject: SCR Pilot Testing Dear Mr. O'Clair: YourTouchstone Energy" Partner

## ~t'J;; 

# -

Enclosed, please find a report entitled "APPROPRIATENESS OF CONDUCTING PILOTTESTING OF SELECTIVE CATALYTIC REDUCTION (SCR) TECHNOLOGY AT MILTON 

### R. YOUNG STATION UNITS 1 AND 2, FOR USE IN A NOx BACT ANALYSIS". This report is submitted response to the Department's request that Minnkota consider a pilot test of SCR catalysts, at Milton R. Young Station, to gatheradditional data for use in Minnkota's previously submitted NOx BACT analysis. This report addresses three aspects that must be considered when making a decision to pursue SCR catalyst pilot test. The aspects are: 1) the legal basis forthe testing; 2) is there a reasonable expectation thatthe results would be different than the results of previous testing; 3) could the testing be accomplished in the required amount oftime. Based upon the information presented in the enclosed report, Minnkota has decided notto pursue SCR pilot testing at Milton 

### R. Young Station, as elaborated in the document. Should you have any questions concerning the above, orthe enclosed document, please contact me by telephone at 701-795-4221 or by email atjgraves@minnkota.com.

Yours truly,

..."

.....'r,.... POWER COOPERATIVE, INC.

# ;Z::~~ John T. Graves, P.E. Environmental Manager

C: 

### David Sogard Luther Kvernen Craig Bleth Kevin Thomas Stu Libby Tom Anseth Young Station File

# APPROPRIATENESS OF CONDUCTING PILOT TESTING OF SELECTIVE CATALYTIC REDUCTION (SCR) TECHNOLOGY AT MILTON R. YOUNG STATIONUNITS 1 AND 2, FORUSE IN ANOx 

# BACT ANALYSIS

Prepared For MINNKOTA PO\VER COOPERATIVE, INC

By

Dennis L. Arfmann

Hogan & 

### Hartson StevenA. Benson Energy & 

### Environmental Research Center Carl V. Weilert Burns & 

### McDonnell

August 14, 2007

### BACKGROUND
The North Dakota Department ofHealth (NDDH) has requested that Minnkota Power Cooperative (Minnkota) consider performing a pilot test at the Milton R. Young Station (MRYS) to investigate the technical feasibility ofSelective Catalytic Reduction (SCR) technology for control ofemissions ofnitrogen oxides (NOx) from the North Dakota lignite-fired cyclone boilers at that power plant. The results ofthe pilot test would be used in the NOx BACT analysis for MRYS. Technical feasibility ofSCR application to utility boilers firing this fuel is a principal issue in the determination ofbest available control technology (BACT) for NOx emission control at MRYS. The determination ofBACT for NOxemission control at MRYS is a requirement ofthe Consent Decree (CD) filed in the case 

### UnitedStates of America andState of North Dakota v. Minnkota Power Cooperative, Inc. andSquare Butte Electric Cooperative.
Minnkota has evaluated NDDH's request and has decided not to pursue SCRpilot testing at MRYS for three reasons as elaborated in this document, below. The reasons for not conducting further pilot testing priorto the BACT determination are: 1. Pilot testing is specifically not required by the BACT determination procedures established by the U.S. Environmental Protection Agency (EPA). 2. Previous research, including previous SCR pilot testing conducted at the Coyote Station, indicates that there is not sufficient reason to believe that the pilot testing suggested by NDDH will produce results any different from those obtained from the prior pilot test. 3. The compliance schedule for installation ofNOx BACT technology at MRYS as established by the CD cannotbe met ifpilot testing ofSCR, followed by installation of SCR, is selected as the basis for BACT.
Each ofthese points is explained further in the text that follows.



### REGULATORY BASIS FORMINNKOTA'S DECISION NOT TO CONDUCT PILOT SCRTESTING
Minnkota's decision not to conductpilot testing ofSCR technology is supported on a regulatory basis for the following reasons: 1. EPA Guidance Specifically Describes When an Emission Control Technology is "Technically Feasible"
EPA has prepared a New Source Review Workshop Manual (Draft, October 1990) that explains, in detail, the evidence and analysis necessary to support a BACT determination. This process of defending a particularBACT recommendation is summarized in the Workshop Manual as follows:
The applicant's role is primarily to provide information on the various control options and, when it proposes a less stringent control option, provide a detailed**2of2~**rationale and supporting documentation foreliminating the more stringent options. [t is the responsibility ofthe permit agency to review the documentation and rationale presented and: (I) ensure that the applicant has addressed all ofthe most effective control options thatcould be applied; and (2) determine that the applicant has adequately demonstrated thatenergy, environmental, oreconomic impactsjustify any proposal to eliminate the more effective control options. Where the permit agency does not accept the basis for the proposed elimination of· acontrol option, the agency may informthe applicant ofthe need for more information regarding the control option. However, the BACT selection essentially should default to the highest level ofcontrol for which the applicant could not adequatelyjustify its elimination based on energy, environmental and economic impacts. Ifthe applicant is unable to provide to the permit agency's satisfaction an adequate demonstration for one or more control alternatives, the permit agency should proceed to establish BACT and prepare adraft permit based on the most effective control option for which an adequatejustification for rejection was not provided. [Workshop Manual at 8.53.] The Workshop Manual requires an applicant to "make a good faith. effort to compile appropriate information from available information sources, including any sources specified as necessaryby the permit agency." [ld. at B.Il]. This analysis should at least identify technologies that may be potentially transferable to the particular source, though laterthose technologies may prove technically infeasible oreconomicallycost-ineffective.
Foreach alternative emission control technique, the applicant should analyze whetherthe control technique is feasible. EPA defines a 

### technicallyfeasible alternative as one that is available (i.e., commercially available for the application in question) 

### andapplicable (i.e., whetherthe available controls may reasonably be deployed on the source in question). The Workshop Manual provides adetailed analysis ofthe availability and applicability ofacontrol technology, which bears quoting at length:
In step 2, the technical feasibility ofthe control options identified in step I is evaluated. This step should be straightforwardfor control technologies that are demonstrated-ifthe control technology has been installed and operated successfully on the type ofsource underreview, it is demonstrated and technically feasible. Forcontrol technologies that are not demonstrated in the sense indicated above,. the analysis is somewhat more involved. Two key concepts are important in determining whetheran undemonstrated technology is feasible: "availability" and "applicability." As explained in more detail below, a technology is considered "available" ifitcan be obtainedby the applicant through commercial channels or is otherwise available within the common sense meaning ofthe term. An available technology is "applicable" ifit can reasonably be installed andoperated on the source type unGerconsideration. A technology that is available and applicable is technically feasible.**30f2~**Availability in this context is further explained using the following process commonly used for bringing a control technology concept to reality as a commercial product:

• Concept stage;

• Research and patenting;

• Bench scale or laboratory testing;

• Pilot scale testing;

• Licensing and commercial demonstration; and

• Commercial sales. A control technology is considered available, within the context presented above, ifit has reached the licensing andcommercial sales stage ofdevelopment. A source would not be required to experience extended time delays or resource penalties to allow research to be conducted on a new technique. Neither is it expected that an applicant wouldbe required to experience extended trials to learn how to apply a technology on atotally new and dissimilar source type. 

### Consequently, technologies in thepilotscale testing stages 9f development would notbe consideredavailablefor BACTreview. An exception would be ifthe technology were propo1ied and permitted underthe qualifications ofan innovative control device consistent with the provision of40 CFR 52.2l(v) or, where appropriate, the applicable SIP. Commercial availability by itself, however, is not necessarily sufficientbasis for concluding a technology to be applicable and therefore technically feasible. Technical feasibility, as determined in Step 2, also means acontrol option may reasonably be deployed on or "applicable" to the source type underconstruction. Technicaljudgmenton the partofthe applicant and the review authority is to be exercised in determining whether acontrol alternative is applicable to the source type underconsideration. 

### In general, acommercially available control option will be presumed applicable ifit has been or is soon to be deployed (e.g:, is specified in a permit) on the same or a similar source type. Absent a showing ofthis type, technical feasibility would be based on examination ofthe physical and chemical characteristics ofthe pollutant-bearing gas stream and comparison to the gas streamcharacteristics ofthe source types to which the technology hadbeen applied previously. Deployment ofthe control technology on any existing source with similargas streamcharacteristics is generally sufficient basis for concluding technical feasibility barring ademonstration to the contrary. [Ed. at 8.17-8.18] (emphasis added).

The

### propermethodforshowing technical infeasibility is identifiedby the Workshop Manual as follows:**4of2~**[T]he applicant should make a factual demonstration ofinfeasibility based on commercial unavailability and/or unusual circumstances which exist with application ofthe control to the applicant's emission units. Generally, such a demonstration would involve an evaluation ofthe pollutant-bearing gas stream characteristics and the capabilities ofthe technology. Also a showing of unresolvable technical difficulty with applying the control would constitute a showingoftechnical infeasibility (e.g., size ofthe unit, location ofthe proposed site, andoperating problems related to specific circumstances ofthe source). Where the resolution oftechnical difficulties is a matter ofcost, the applicant should considerthe technology as technically feasible. The economic feasibility ofacontrol alternative is reviewed in the economic impacts portion ofthe BACT selection process. A demonstration oftechnical infeasibility is based on a technical assessment considering physical, chemical and engineering principles and/orempirical data showing that the technology would not work on the emissions unit under review, or that unresolvable technical difficulties would preclude the successful deploymentofthe technique. Physical modifications needed to resolve technical obstacles do not in and ofthemselves provide ajustification for eliminating the control technique on the basis oftechnical infeasibility. However, the cost ofsuch modifications can be considered in estimating cost and economic impacts which, in turn, may form the basis for eliminating acontrol technology (see later discussion at V.D.2). [ld. at B.19-8.20.] Each ofthe elements ofdemonstrating technical infeasibility, along with background and supporting information as necessary, must be decisively documented.

2. Following the Guidance from the Workshop Manual, Minnkotahas Decisively Documented that SCR is Technically Infeasible at Milton R. Young Station SCR has not been demonstrated on any North Dakotalignite-fired cyclone unit. ill light ofthis lack ofdemonstration, NDDH has asked Minnkotato considerconducting pilot testing ofSCRto determine the life expectancy ofa particularSCRcatalyst. However, the 1990Workshop Manual concludes that pilot testing ofan undemonstrated technology cannotbe required as a part ofa BACT analysis. The Workshop Manual addresses technical feasibility in the context ofboth demonstrated technologies (not applicable to Minnkotahere) and undemonstrated technologies (Minnkota's situation). As set forth in detail above, the key questions in determining whetherthe undemonstrated technology is feasible are whether the technology is "available" and "applicable." Most importantly, the 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. Neither is it expected that an applicant would be required to experience extended trials to learn how to apply atechnology on a totally new and dissimilar source type.**50f2~**Consequently, technologies in the pilot scale testing stages ofdevelopment would not be considered available for BACT review." [ld. at B. 18]. As Minnkota has demonstrated in its previous filings, EERC and Burns & McDonnell have demonstrated SCRcannot be reasonably installed and operated on the Milton R. Young Station. SCR is therefore unavailable and inapplicable, and NDDH may not require Minnkotato conduct extensive pilot testing as a partofaBACT analysis to experiment with the availability or applicability ofSCR to North Dakota lignite-fired cyclone boilers.
In conclusion, Minnkota has demonstrated that SCR is not technically feasible, that that SCR is not "available" under the EPA guidance, that SCR is not "applicable" under the EPA guidance, and that a pilot test is not allowed underthe EPA guidance for BACT.



### TECHNICAL BASIS FORMINNKOTA'S DECISION NOTTO CONDUCT PILOTSCR TESTING
In order to determine ifadditional pilot testing ofSCRcatalysts on lignite fueled cyclone-fired boilerwould produce similaror potentially different results from previous testing, the fouling and poisoning mechanisms must be examined. This is necessary in order to evaluate design

. changes that could be made to reduce oreliminate the fouling and poisoning ofthe SCR catalysts. Fouling and poisoning ofSCRcatalysts are affected by the following:

• Lignite properties

• Combustion processes

• Partitioning ofsodium and otherelements during combustion

• SCRcatalystcharacteristics and properties

• Chemical and physical mechanisms ofpore plugging The primary concern for lignite coals from the Northern Great Plains ofNorth America is blinding and poisoning ofSCRcatalysts because ofthe presence ofalkali and alkaline elements in the coal and their fate upon combustion. Mussatti (2002) indicatedin the "EPA Air Pollution Control Cost Manual- SixthEdition" the following primary mechanisms forcatalyst deactivation: "Poisoning

- Certain fuel constituents which are released during combustion act as catalyst poisons. Catalystpoisons include calcium oxide and magnesiumoxide, potassium, sodium, arsenic, chlorine, fluorine, and lead. These constituents deactivate the catalystby diffusing into active pore sites and occupying them irreversibly. Catalystpoisoning represents the main cause ofcatalystdeactivation." "Blinding/Plugging/Fouling

- Ammonia-sulfur salts, fly ash, and other particulate matterin the flue gas cause blinding, plugging or fouling ofthe catalyst. The particulate matter deposits on the surface and in the active pore sites ofthe catalyst. This results in adecrease**6of2~**ofthe numberofsites available for NOx reduction and an increase in t1ue gas pressure loss across the catalyst." Basedon research, development, and demonstration testing to date these challenges have not been overcome. Significant fundamental research is required to further develop NOxreduction technologies for lignite coals. 

### Lignite Properties Lignite from the Center Mine in North Dakotais partofthe Fort Union region. The Fort Union region represents the lignite bearing reserves in North America. The age oflignite in the Fort Union ranges from late Cretaceous through Eocene. The younger lignite coals have higherlevels ofoxygen as comparedto higher rankedcoals. Lignites can contain as high as 20% oxygen on a moisture and ash free basis. The most importantoxygen containing group in lignite is the carboxylic acid group accounting for up to 65% ofthe oxygen (Schobert, 1995). The carboxylic acid groups act as bonding sites for various elements such as Ba, Ca, Mg, Na, and Sr and are enriched in lignite coals as compared to bituminous coals (Schobert, 1995). As coal rank increases the oxygen functional groups are lostdue to the coalification process. The carboxylic acids act as ion exchange sites for sodium and otherelements such as Mg, Ca, Sr, and Ba that are accumulated from ground water. The abundance ofsodium and otherelements that are associated with the carboxylic acid groups in the coal are determined using chemical fractionation (Benson and Holm, 1985). The chemical fractionation results for Center lignite indicates that 76% ofthe total sodium is ion-exchangeable indicating an organic association in the coal with the remaining in a watersoluble form (Schobert, 1995). In addition to the organic and water associated impurities,lignites contain significant levels ofmineral grains. The types ofminerals present in lignite consistofpyrite, quartz, clay minerals, sulfates, carbonates, and others (Benson andothers, 1993). Lignite fired at the Milton R. Young (MRY) plant is mined from three seams at the Centermine ofthe Fort Union Region, the Kinneman Creek (KC), Hagel A (HA), and Hagel B (HB). The coal fired in the Units 1and 2 represent some blending ofthe coal through handling and delivery. The variability selectedcoal quality components for the as-fired Center ligriite is shown in Table 1. The average, maximum, and minimum as well as the percentiles were derived from analysis ofthe as-firedcoal database. The database consists of 1316 samples ofcoal collected using sampling systems on Unit 2. The elements that are largely organically associated as described above are highlighted. These elements are largely organically associated andcan contribute to the formation ofreactive fine particulate during combustion and gas cooling.**70f2~**

### Table l. 

### Center lignite composition (moisture, ash, sulfur, and heating value expressed on an as-received basis; and ash composition expressed as weight percent equivalentoxide of the ash produced at750 DC).
Moisture, Ash, Sulfur % % % Btu/lb AI20 3 BaO CaO Fe203 MgO P20 S K20 Si02 Na20 803 Average 37.13 9.64 0.98 6578 11.68 0.52 13.15 8.90 3.99 0.12 1.33 36.04 4.40 18.39 Max 39.37 25.46' 2.55 7101 15.62 1.22 23.96 24.72 7.09 1.00 2.30 55.98 13.03 34.50 Min 33.56 4.97 0.47 5852 6.31 0.20 6.80 5.38 2.30 0.01 0.16 12.30 0.55 8.12 90th Perc. 38.10 12.51 1.29 6820 14.23 0.84 19.92 12.20 5.68 0.41 1.92 45.98 8.71 26.25 10th Perc. 35.95 7.06 0.72 6359 9.58 0.37 9.73 7.08 3.00 0.04 0.61 22.70 1.99 14.35 45th Perc. 37.17 9.33 0.90 6552 11.62 0.47 12.15 8.34 3.81 0.09 1.36 36.99 4.09 17.30 55th Perc. 37.37 9.75 0.94 6592 11.97 0.50 12.74 8.68 3.97 0.10 1.44 38.36 4.61 18.02 

## * High value may be an analysis orreporting error. Sample could notbe reanalyzed to verify.
Figures 1 - 

### 3 show the frequency distributions with the lOth, 50th, and 90th percentile values for sodium, base-to-acid ratio, andash content in the as-fired database. In addition, the future Center lignite characteristics in ten-year averages for % 

### sodiumoxide in ash, base/acid ratio, and % 

### ash are plotted on the frequency distribution diagrams in Figures l-3. The future Center lignite characteristics are based on weighted averages ofanalysis conducted for drill cores. This information was used to calculate the weighted averages for to-yearspans. The frequency distribution ofthe sodiumcontent ofthe ash is shown in Figure 1. 

### The projections for the future have similar sodium levels to the upper mode ofsodiumcontents. The sodiumcontent ofthe future coals appears to be decreasing initially from the upper mode and then increasing from 2036-2064. The base-to-acid ratio shown in Figure 2 indicates coals typical oflower ash contents and high levels ofsodium, calcium, potassium, and iron relative to silicon, aluminum, and titanium. This information indicates that the future coals will have ahigher propensityon average to cause low-temperature fouling than coals fired overthe past several years. Lower ash content and higherbase-to-acidratio coals produce more low temperature fouling due to sulfate formation. Figure 3 shows the projected ash values compared to the as-fired database. The data from 2006-20l5 show similardatato the as-fired database with decreasing levels ofash in the latteryears.**8of2~**350
300

I"'- 250



### ..... ('t)



### ..... II c: 200 :>. 0 c: Q) 150 :::J 0- Q) 

# ... u. 100
50



## o-t-
.......-....:.,.-"""'T""-"'T"""-,...; EERC SB29240.CDR 2036-2045,2046-2055 Na20 in Ash

--- BNI Core Database (2003-2006) - - - 1O-yearaverages

90th Percentile: 6.74wt%



### Figure 1. 

### Sodiumoxide in ash frequency distribution for as-fireddatabase compared to 10- yearaverages and weighted Core Database values.

600

500
I"'-



### ..... ('t) 400



### ..... II c: :>. 300 0 c: Q) :::J 0- Q) 200


# ... u. 100

0**~'Y** EERC SB29241.CDR Base-ta-Acid
._---- BNI Core Database (2003-2006)

- - - 10-yearaverages

90th Percentile: 1.26wt% 2006-2015,2026-2035

2046-2055 2016-2025



### Figure 2. Base-to-acid ratio ofash frequency distribution for as-fired database compared tolO-year averages and weighted Core Database values.**90f2~**350 EERC SB29238.CDR 50th Percentile: 9.53 wt% 300 2006-2015 %Ash 2016-2025 2036-2045 -- BNI Core Database



### ...... (2003-2006) T""" 250 (W')

- - - 10-yearaverages T""" 2026-2035 II 2056-2064 c 200 2046-2055 0 c Q) 150 :::J 0' 

### u. 100



### Figure 3. Ash frequency distribution for as-fired database compared to to-yearaverages and weighted Core Oatabase values.

Combustion Processes and Partitioning The inorganic coal components undergo complex chemical and physical transformations during combustion to produce intermediate ash species in the form ofinorganic vapors, liquids, and solids. The partitioning ofthe inorganic components during combustion to form ash intermediates depends upon the association andchemical characteristics ofthe inorganic components, the physical characteristics ofthe coal particles, the physical characteristics ofthe coal minerals, and the combustion conditions. The physical transformations involved in fly ash formation illustrated in Figure 4 include 1) coalescence ofindividual mineral grains within achar particle, 2) shedding ofthe ash particles from the surface ofthe chars, 3) incomplete coalescence due to disintegration ofthe char, 4) convective transport ofash from the charsurface during devolatilization, 5) fragmentation ofthe inorganic mineral particles, 6) formation ofcenospheres, and 7) vaporization and subsequent condensation ofthe inorganic components upon gas cooling. As aresult ofthese interactions, the resulting ash composition and mass distribution as a function ofsize is also shown in Figure 4. Typical ash derived from coal combustion has a bimodal to multimodal size distribution that consists ofa submicron and supermicron size fraction as shown in Figure 

### 5. The submicron component is largely a result ofthe condensation offlamevolatilized inorganic components such as sodium. The intermediate size mode at about 

### 2 micrometers is derived from the more refractory organically associated elements such as calcium and magnesium. The larger particles at approximately 12 to 15**~lm** are derived from the mineral grains present in the coal. The larger-size particles have been called the residual ashby some



### to of262einvestigators (Sarofim and others, 1977) because these ash particles resemble, to a limited degree, the original minerals in the coal. During the gas-cooling processes in the boiler, the gasphase species condense and the liquid-phase materials solidify. The abundance ofthese gasphase and liquid materials entrained in the bulkgas flow is highly dependent upon coal composition and system operating conditions.

Remain in Vapor Phase

- reactions with flue gas components

tion 

# ."..J

0.1 10 Particle Size, J.Itrl Submicron sodium -rich aerosols

Particle Fusion o 0

## oO~o Cenosphere Fonnation Reaction Condensation i1 Coagulation Nucleation

Char or Mineral Fragmentation, Shedding, and Coalescence Vaporization

Fuel Particle Reducing Environment



### Figure4. Transformation ofinorganic components presentin coalduring combustion.

11**of2~**Condensed vapors

en Q) 

### u

a. 

### '5 c: 

### o



### .... u.. Organically associated Inorganics Coalesced minerals'other inorganics



### 0.1 2 10 -15 Particle size, Micrometers 

### Figure S. Final size distribution ofash particles produced upon combustion of lignitic coals (Benson and Laumb, 2007).
Cyclone fired systems enhance the partitioningofthe inorganic components both by high temperatures and cyclonic action. The partitioning ofsodium between the slag and coal is shown in Figure 6. The results show the sodium present in the as-fired coal ash presented with and without S03. The retention ofsulfurin the ash is an artifactofthe ashing procedure. In comparing the level ofsodium in the slag to the coal fired for the high-sodiumcases, only about halfofthe sodium is retained in the slag. The slag samples were recovered from the slag tap for the boiler. These slag samples include deposits that fall from the walls and the high-temperature convective pass.

12**of2~**14.00r-**--\;;EE..:.;R~B=2~43~.CD~R** Cl a:s (J) 12.00+---- -0 C a:s 10.00+---- (ij o () .5 8.00 Q) -0 

# d 6.00 E 4.00 o 

## en "#. 2.00
0.00 MRY MRY MRY MRY MRY MRY MRY MRY MRY MRY MRY MRY MRY MRY MRY Coal Coal Slag Coal Coal Slag Coal Coal Slag Coal Coal Slag Coal Coal Slag

111222333444555

(S03-free) (S03-free) (S03-free) (S03-free) (S03-free) Figure 6. Comparison ofsodium levels in Center lignite coal and slag samples.

The transport ofintermediate ash species (Le., inorganic vapors, liquids, and solids) to surfaces within acombustion system (combustion and airpollution control system) is a function ofthe state and size ofthe ashspecies and systemconditions such as gas flow patterns, gas velocity, and temperature. Several processes are involved in the transport ofash particles to surfaces in the combustion system. These processes have been described in detail by Raask (1985) and Rosner (1986). The primary transport mechanisms are illustrated in Figure 7. Forlarger particles inertial impaction and eddy impaction in turbulent regions are the dominant mechanisms. The small particles 

## « 1 /lm) and vapor-phase species are transported by the vapor phase and small particle diffusion. The inorganic materials are transported to heat-transfer and catalyst surfaces bydiffusion, electrophoresis, and inertial impaction. Thermophoresis is not a significant contributorto the transport ofparticles as it related to SCRmaterials. However, thermophoresis is acontributor to particle transport on heat transfer surfaces because ofthe temperature gradient.

13**of2~**WITH THERMOPHORES/S

bl DISTRIBUTION OF PARTICLE MASS LOADING

10·3 L.l.--'--'-....J.--,.J---'-........J.-::'"

.............u.-:"--'-L.J.J..':'""""-....,;;.,= 10.3 10.2 10·\ 100 10' 102 PARTICLE DIAMETER, dpCfLm) 

### Figure7. Overview ofash transport mechanisms (Rosner, 1986); (a) particle size dependence ofcaptured**fra~tion** 

### neap; (b) size distribution ofthe mainstream particle mass loading. The accumulation rate will involve a combination ofa and b. aI FRACTION CAPTURED S :'0 0

The vaporphase and small particles are characteristically rich in flame-volatilized species that condense upon gas cooling in the bulk gas or in the gas boundary layernext to the surface. The diffusion mechanisms important with respect to the transport ofvapor species and small particles to the surface ofSCRcatalysts include: l. Fickdiffusion

- molecular level. 2. Brownian diffusion

- particles suspended by a host Liquid. 3. Eddy diffusion

- turbulent systems. 

### SCR Catalyst Characteristics and Properties SCRcatalysts forcoal combustion applications consistofceramic material with high porosity that contains imbedded active catalytic sites. The NOxreduction occurs on the active surface of the catalyst. The active surface ofthe catalystconsists offlat surfaces and pores within the flat surface ofthe catalyst. The flat surfaces and pores in the catalyst are designed to provide gas contact with the active surface ofthe catalyst. The pores consistofopen macropores and micropores that occuron the surface ofthe catalyst structures. The size ofthe openings in honey comb catalyst impacts the velocity ofthe gases flowing through the catalyst. Pitch is the term used to describe the openings between the flat surfaces of the catalyst. Figure 8 illustrates the pitch, p and the wall thickness, a. Lower gas velocities will result for higher pitch numbers while small numbers will result in higher gas velocities through the catalyst. Catalyst vendors have methods to identify the properpitch for specific fuel types and ash characteristics to minimize bridging or plugging where the dominant transport mechanism is inertial impaction and eddy impaction for the large particles. Selecting a pitch that



### 140f26Uwill minimize the transport ofsmall particles and vapor phase species is not as straightforward. High pitch catalysts will result in lowervelocity gas flow through the catalyst and will create a thicker boundary layer resulting in an increased potential for small particle transport. Catalyst geometry plays asignificant impact on gas flow patterns through the catalyst. Evaluation ofthe literature and information provided by catalyst vendors indicate that the geometries ofcatalysts have not been improved to minimize small particle and vapor phase deposition on the surface and within the pores ofcatalyst (Hansen, 2007)

.



### . Figure.

### 8. Catalystpitch (Mussatti, 2002).

An example ofthe macropore structure is shown in Figure 9 for an actual surface for a honeycomb type catalyst. The openings in the catalysts create porosity and surface area to expose the reactive components ofthe catalyst to gas phase components. These openings are prone to filling with small particles and large particles through the transport processes of diffusion and impaction, respectively. Once these particles are transported to the surfaces they will be held in place by weakelectrostatic and van del' Waals forces and they will react with vapor phase sulfuroxide species. 

### Chemical and physical mechanisms ofpore plugging The most significant problems that limit the successful application ofSCRcatalysts to lignite coal is the formation oflow-temperature sodium-calcium-magnesium sulfates that will form on the surfaces ofcatalysts and the carryoverofdeposits that will plug the catalyst openings, resulting in increased pressure drop and decreased efficiency. Studies conducted at Coyote station (Benson and others, 2005) found significantblinding and filling ofthe pores with small particles as sQown in Figure 9. Rapid filling ofthe pores occurred with the catalyst pitch of6 mm. These materials were subjected to airpulsing that did not remove the materials from the catalyst. The characteristics ofthe materials filling the pore were rich in sodium andcalcium sulfates. A high magnification image ofthe materials coating the catalyst and filling the pore is shown in Figure 10. This image shows the importance ofthe very small particles less than I micrometerand their impact on the bonding ofthe deposited materials. These small particles readily interact with gas phase S02 and SO} expanding to create a bonded matrix as shown in Figure II that completely fills the pores ofthe catalyst with acontinuous bonded phase. Ahigh magnification cross-section is shown in Figure 12 ofahighly sulfated bonded matrix.

15**of2~**Diffusion transport, particles <5Jlm and vapors Rebound

Diffusion transport, particles <5Jlm and vapors Inertial impaction transport, Particles >5Jlm

Stick Catalyst Surface

Inertial impaction transport, Particles >5Jlm 

### Figure 9. Scanning electron microscope secondary electron image ofthe macropore structure ofan SCR catalyst with transport process descriptions.**16of2~**

### Figure 10. High magnification secondary electron image ofash filling pores and covering catalysts.

Figure 11. Scanningelectron microscopy image ofash materials filling thepores of catalystexposed to flue gases derived from the firing ofBeulah lignite atCoyote Stations (Benson and others, 2005).

17**of2~**

### Figure 12. High magnification image ofmaterials typical ofash filling pores that has sulfated and bonded.

Pyrosulfates are present in ash deposits and will likely form in SCRcatalysts. Melting points for pyrosulfates between 535° and 770°F have been reported in the literature (Singer, 1981). Much ofthe past work has focused on the formation ofthese phases on tube surfaces. These species contribute to the corrosion ofheat-transfer surfaces in coal-firedpowerplants. The exact melting pointdepends on the relative amounts ofsodium and/or potassium. The melting points ofselected pyrosulfate phases are shown in Table 2. The formation ofpyrosulfates involves the following processes l. Formation ofsulfates such as NaZS04 and KzS04 2. Conversion ofSOz to S03 in the bulk gas phase

- catalytically active surface such as an SCR catalyst

- SOz 

### + V2 Oz 

### -+ S03 3.Pyrosulfate formation

- NaZS04 

### + S03 

### -+ NaZSZ07 Adiagram depictingthe physical and chemical reactions involved in the filling and plugging of catalyst pores is shown in Figure 13 (Benson and others, 2005).**180f2~**

### Table 2. Melting Points ofSelected Pyrosulfate Compounds 

### A. Compound 

### Temperature, °C Temperature, of

K3 Fe(S04h 618 1144 K

3Al(S04h 654 1209 KFe(S04h 694 1281 Na3Fe(S04h 624 1155 Na3A1(S04h 646 1195 NaFe(S04h 690 1274 Na2S207 401 754 K2 S2 07 300 572 (K1.5Nao.5)S20 7 279 535

SulfurOxides Carbon Dioxide

Reactions with SO:3 and CO2

Deterioration (NH3 and NOxcannot react In mlcropore on catalyst surface) No Deterioration Vet (NH, and NOxcan react In mlcropore on catalyst surface)

__- Partides
- Silicates and Other FlyAsh Particles __- Partides
- Silicates and Other FlyAsh Particles '--_-- Matrix Material
- PartiallySulfated Alkali and Alkaline-Earth Oxides 

# -+14% Expansion**~--** Matrix Material
- FullySulfated Alkali and Alkaline-Earth Oxides Particles
- Silicates Small Particles.--......

_ : • Nat Mg, Ca, K**~.**



### Figure 13. Overall mechanismofSCR Catalystblinding (modified after Pritchard and others (1995».

19**of2~**

### Sodium-related catalyst deactivation mechanisms Alkali elements such as sodium and potassium are known to cause catalyst deactivation. Prichard and others (1995) indicate that the alkali elements can interact with the catalyst active sites and cause them to lose their activity as shown in Figure 14. They indicate that the degree ofdeactivation is dependent upon the concentration ofalkali elements and the form ofthe alkali. Based on the evaluations conducted and/or reported by Cormetech (Prichard and others, 1995), Topsoe (1999), EPRI (Broske, 2005), and EPA (Mussatti, 2002) the water soluble form ofthe alkali is the problematic form since these alkali species are highly mobile allowing them to migrate into the catalyst. Cormetech (Prichard and others, 1995) incorrectly indicate the following:
"Forthe subjectcataLyst in coaLjiredapplications, itis ourexperience that this type of poisoning has a small impact, since mostofthe aLkaLine metaLs in the coaL ash are notwater soLubLe

. .. This comment is very surprising and is likely indicative ofcatalyst vendor's level of understanding oflignite coals. Much work has been conducted for decades on the solubility of alkali and alkaline earth elements in coal ash. Sodium in Center lignite is associated in the groundwater and with the organic matrix ofthe coal (Benson and Holm, 1985; Schobert, 1995). Very little ofthe sodium is associated with the mineral fraction ofthe coal such as clays. During combustion, the sodiumin the water soluble and organic association will vaporize and become dispersed in the gas stream and react with sulfur oxides to form soluble sodium sulfates species. Combustion Engineering conducted an analysis ofthe impact ofsoluble and organically associated sodium with fouling potential (Singer, 198\). The results ofthe analysis are summarized in Table 3. The soluble alkali shown for lignite and subbituminous coals is very high, with North Dakotalignite and Montana high-sodium subbituminous coals having the highest available sodium for low-temperature fouling processes.



### H 

# I 

# ?**--Me-~O-** 

### -Me-- Fresh Catalyst 

### Na(K) 

## I 

# P --Me--O- -Me-- Alkaline Deterioration 

### Figure 14. Alkaline (alkali) SCR catalyst deterioration mechanism (Prichard and others, 1995).**200f2~**

### Table 3. Comparative Alkali Content Analysis for U.S. Coals (Singer, 1981)

Rank Lignite Sub8 Lignite Lignite hv8b hvAb Lignite Region NO MT TX**(~egua)** TX(Wilcox) UT PA TX(Wilcox) HHV. Btu/lb, Dry Basis 10640 12130 7750 9710 12870 13200 8420 AshComposition (0/0) SiO. 20.0 33.9 62.1 52.3 52.5 51.1 57.9 AI.O, 9.1 11.4 15.1 17.4 18.9 30.7 21.8 Fep, 10.3 10.6 3.5 5.3 1.1 10.0 3.9 CaO 22.4 21.0 6.2 9.4 13.2 1.6 7.1 MgO 6.4 2.7 0.7 3.2 1.3 0.9 2.1 Nap 5.0 5.8 3.6 0.9 3.8 0.4 0.7 K.O 0.5 1.6 1.9 1.2 0.9 1.7 0.8 TiO 0.4 0.7 0.9 1.2 1.2 2.0 1.1 503 21.9 12.0 6.1 9.6 6.2 1.4 4.4 FoulingPotential Sllvllre High High Moderate Moderate Low Low

----

- ------.._---- ..._--

LbAsh/106 Btu, DryBasis 9.0 4.6 43.3 20.1 7.9 10.2 34.4 Acetic-Acid-Soluble Sodium (Na. ppm) 3960 2660 9650 1030 1120 250 340 Potassium (K. ppm) 1230 85 85 no Alkali in Ash. % WI. Nap 5.0 5.8 3.6 0.9 3

..8 0.4 0.7 K.O 0.5 1.6 1.9 1.2 0.9 1.7 0.6 Equiv. Sol. Alkali in Ash, 0/0Wt. ofAsh Nap 5.58 6.45 3.68 0.71 1.49 0.15 0.16 K.O 0.44 0.04 0.06 0.05 0/0Sol. AlkaliofTotal (Equiv. Sol. Na.O)/ (Na.Oin Ash) 112 111 108 79 39 38 23 (Lbs Sol. Na)/ (106 BtuFired) 0.374 0.221 1.245 0.106 0.087 0.018 .040 (Lbs Sol. Na)/(Ib Ash! 106 BtuFired) 0.044 0.048 0.223 0.005 0.014 0.002 0.001

The following summarizes the state-of-the-art relative to the application ofSCR for use in high sodium lignite combustion systems such as at the MRY plant.

• Coal properties o Center lignite coal contains organically associated sodium, magnesium and calcium o The levels ofsodium in the Center lignite will remain constant and will likely increase in the future years

• Ash formation o Sodium vaporizes during combustion to form very small particles o Sodium is in asoluble form



### 21**of2~**• Catalyst properties o Catalysts have high porosity in the form ofmacropores and micropores that have active components dispersed within them allowing for NOx reduction reaction o Pitch ofthe catalyst appears to be the only change in geometry available to minimize deposition o Catalyst are prone to deactivation because ofthe presence ofsoluble sodium

• Ash transport in SCR catalyst o Ash particle transport mechanisms are dependent upon gas velocity o Particles greaterthan approximately 10 micrometers are transported to the surfaces ofcatalysts by inertial impaction mechanisms o Vapor phase and small particles are transported through diffusion processes into the pores ofthe catalyst across the stagnant gas boundary layer next to the surface o Small ash particles rich in sodium, calcium, and sulfur infiltrate deep into the catalyst pores causing blinding and plugging and possibly deactivation The following are changes thatcould be made in future pilot testing and, based on the previously presented information, the impacts ofthose changes that could reasonably be anticipated to occur.

• Test catalysts with larger openings (greater Pitch) Impacts: The current recommendation (forthe coal-fired industry in general) by catalyst vendors is to use catalysts that have a larger pitch. Using catalysts with a larger pitch will likely decrease the velocity through the catalyst and decrease the impaction rates ofthe larger particles. However, this does not address the key problemofvapor phase and small particle diffusion into the pores ofthe catalysts that is associated with the combustion ofhigh sodium Center lignite. Bydecreasing the velocity, the gas boundary layer may be largerresulting in an increase in the diffusion processes resulting in increased deposition ofthe smallerparticles into the pores. In addition, decreasing gas velocity will decrease the scrubbing action ofimpacting particles that aids in removing deposited ash particles.

• Test catalyst withmodified pore structure thatdoes not accumulate small particles

. Impacts: The use ofthese catalysts should have apositive impact on reducing

catalyst poisoning and pore plugging. However, discussions with catalyst vendors and literature searches indicate that little orno work has been conductedon such formulations. Most ofthe work being conducted by catalyst vendors is focused on reducing S03 production and enhancements for mercury oxidation. As aresult, these catalysts are not available.

• Enhanced cleaning technologies for removal ofparticles in the pores ofcatalysts Impacts: The use ofacoustic horns appears to clean the surfaces as well as sootblowers on subbituminous and bituminous fueled boilers. However, this

22**of2~**technology has not been demonstrated on lignite fueled boilers. There is significant uncertainty in the ability ofacoustic horns to clean the ash particles from the pores of the catalyst. There does not appear to be acatalyst material that has shown significantly improvementover the past catalyst relative to the performance in high sodiumcontaining system. The specific question was asked to Flemming Hansen (Hansen, 2007) as to whether improvements have been made in catalysts over the ones tested at Coyote station that warrant further testing. He indicated that no formulation ordesign changes have been made to improve theirperformance in high sodiumcontaining flue gases. The testing at Coyote station indicated that the macropores and the catalyst openings were plugged. The blinding ofthe catalyst from the Coyote station testing was so severe that the catalyst vendor could not make deactivation measurements. Cleaning the catalyst and measuring deactivation was not performed based on recommendation ofthe catalyst vendor. The reason for not cleaning and conducting the measurements were basedon concerns expressed by the vendor that the results ofsuch deactivation testing aftercleaning would not provide meaningful data on deactivation rates. At the present time, there is not sufficientevidence to indicate that further pilot testing, even though itcould be significantly reconfigured from previous testing, would yield significantly different results from thatoftesting already conducted at the Coyote station. Additionally, there is nothing to suggest that further testing would necessarily result in catalyst samples being made available that would allow for the measurement ofdeactivation rates.



### SCHEDULE BASIS FOR MINNKOTA'S DECISION NOT TO CONDUCT PILOT SCR TESTING The CD requires that compliance with the "3D-day RollingAverage NOx Emission Rate" established as BACT in accordance with the CD be achieved for Unit 2 at MRYS not laterthan December 31, 2010. For the hypothetical case in which an SCR pilot test Were conducted to potentially establish the technical basis for SCR application to the Units at MRYS, the following tasks would need to be completed priorto this date: 1. Design the SCRpilot test protocol and obtain concurrence ofNDDH and EPA 2. Design·, procure and install the pilot SCR 3. Conduct the SCR pilot testing and evaluate the results 4. Conduct a technical feasibility analysis ofthe boiler modifications that would be necessary for Units 1and 2 to allow reduction offlue gas temperatures (which are currently well above the required range for SCR operation, as previously documented in Minnkota's submittals to NDDH) while maintaining the ability to dry the lignite as required for combustion. 5. Revise the BACT analysis based on the results ofSteps 3 and 4 (hypothetically assuming that they concluded it was technically possible to install SCR) to establish the revised "3D-day Rolling Average NOxEmission Rate" based on the hypothetical use ofSCR.



### 23**of2~**6. Receive the final determination from NDDH regarding the "3D-day Rolling Average NOx Emission Rate" representing BACT in accordance with the CD. 7. Incorporate the results ofsteps 3 through 6 (hypothetically assuming that they concluded it was technically possible to install SCR and established an SCR-based value for the "3D-day Rolling Average NOxEmission Rate") into a procurement specification for the SCR system in accordance with RUS requirements. 8. Procure the SCR systemon a "furnish and erect" basis, obtaining a vendor guarantee that assured compliance with the BACT emission limit as set forth in Step 6. 9. Design, fabricate, and erect the SCR systems and the necessary boiler modifications, using the Contractorselectedin Step 8. lO. Start-up and commission the SCR system for Unit 2 and achieve stable performance to allow continuous compliance with the BACT emission limit as set forth in Step 6. Based on Burns & McDonnell's evaluation, the time required to complete these tasks would be approximately 65 months from initiation ofTask 

### 1. Thus it would not be possible to comply with the CD date certain ofDecember 31, 2D10 for compliance with the "3D-day Rolling Average NOx. Emission Rate" established as BACT in accordance with the CD. The estimate of65 months duration to complete this process is based on the following assumptions for the duration ofthe individual steps: Step 1is assumed to require two months. It should be noted thatthe EPA and its consultants have claimed that the previous SCR pilot testing at Coyote was fundamentally flawed. Thus it is essentialthat this step be thoroughly and carefully conducted and documented in orderto assure the "buy in" ofthe interested parties. Step 2 is estimated to require six months. Step 3 is estimated to require 24 months. It is believed that the collection of 18 months ofdata will be necessary to properly evaluate the technical feasibility ofSCR at MRYS. Due to the operating characteristics ofthe Units and basedon priorexperience with pilot testing, it is believed that the duration ofpilot testing needed to acquire 18 months ofdata will be 24 months. Step 4 is estimated to require six months, based on consultation with the boiler OEM. It should be noted that although Step 4 is assumed to be performed concurrently with Steps 1 through 3, all othersteps must be conducted sequentially. Also, it should be noted that the subsequent sequence is based on the hypothetical assumption that the results ofSteps 3 and 4 are favorable to the technical feasibility ofSCR. Steps 5 and 6 are estimated to require two months each. Because no SCR vendorhas experience with SCR operation on North Dakota lignite, the determination ofan emission rate corresponding to the hypothetical use ofSCR will require the dissemination ofthe pilot test results from the catalyst vendors (which are assumed to be active participants in the pilot SCR program) to the SCR system suppliers. This is because the establishment ofaNOx. emission rate representing BACT will require at least a preliminary indication from the system suppliers that such a level can be guaranteed. It is assumed that the system suppliers will guarantee the**240f2~**emission rate established in Step 6, and will otherwise respond favorably to the procurement process in Step 8. The development ofthe technical specifications for the SCR procurement contract (Step 7) is estimated to require two months. The specifications must incorporate not only the results ofthe pilot SCR testing from Step 3 but the boiler modifications identified in the studyconducted in Step 4. Again, the sequence assumes that it will be possible to find a feasible way of**mod~fying** the boilers to simultaneously achieve the flue gas temperature range required for SCR operation while continuing to achieve properpre-drying ofthe lignite. The period for bidding and award ofthe SCRcontract (Step 8) is estimated to be four months. Typical bid period for acomplex "furnish and erect" SCR retrofit contract such as this will be three months. One month is assumed for evaluation and negotiation. ForStep 9 it has been assumed that the period from award ofthe SCRcontract to the completion oferection and tie-in will be 20 months. It is anticipated that the duration ofconstmction ofan SCR system in North Dakota will require somewhat longer than would be expected for other parts ofthe country due to the severity ofthe winter. Also, depending upon the complexityof the required boiler modifications, the constmction period can be expected to be extended compared to a "typical" SCR project schedule which does not include the need for extensive boiler modifications. The startup, commissioning and tuning ofthe SCR system (Step 10) has been estimated to require three months. It shouldbe noted that concerns with the characteristics ofthe fly ash are anticipated to extend the time that would routinely be required for this step. Also, unlike most retrofit SCRprojects completed to date in the U.S., this installation would be subject to a "notto exceed" emission limitation on a 30-day rolling average. Assurance that the limitcould be continuously met would require more than the usual extent ofoperational tuning.



### References Benson, S.A.; Laumb, J.D.; Crocker, C.R.; Pavlish, J.H. SCR Catalyst Performance in Flue Gases Derived from Subbituminous and Lignite Coals. 

### Fuel Process. Techno!. 2005, 86, 577- 613. Benson, S.A.;-Laumb, M.L., Ash Behaviorin Combustion Systems, Energy Generation Conference, Bismarck, NO, 2007. Benson, S.A.; Holm, P.L. Comparison ofInorganic Constituents in Three Low-Rank Coals, Ind. Eng. Chern. Prod. Res. Dev. 

### 1985,24, 145. Benson, S.A.; Jones, M.L.; Harb, J.N. Ash Formation and Deposition-Chapter 4. In 

### Fundamentals ofCoal Combustionfor Clean and Efficient Use; Smoot, L.D., Ed.; Elsevier: Amsterdam, London, New York, Tokyo, 1993; pp 299-373.

25**of2~**Broske, D., On-Line Guidelines for Operation and Maintenance ofSelective Catalytic Reduction Systems, 2005 Edition, EPRI, May 2005. Hansen, F. Personal Communication, Feb 2007. Mussatti, D.C., EPA Air Pollution Control Cost Manual- Sixth Edition. Section 4, Chapter2, Selective Catalytic Reduction., EPN452/B-02-001, 2002 Raask, E. Mineral Impurities in Coal Combustion; Hemisphere Publishing Company: Washington, 1985. Rosner, D.E., Transport Processes in Chemically Reacting Flow Systems, Butterworths, Boston, MA 1986. Sarofim, A.F.; Howard, 1.8.; Padia, A.S. The Physical Transformation ofthe Mineral Matterin Pulverized Coal under SimulatedCombustion Conditions. Combust. Sci. Techno!. 1977, /6, 187-204. Schobert, 

### RR Coal Science and Technology 23: Lignites of NorthAmerica; Elsevier, 1995. Singer, 

### J.G. Combustion Fossil PowerSystems; Combustion Engineering: Windsor, CT, 1981. Topsoe. SCR Catalyst Improvements, 1999, www.netl.doe.gov/ publications/proceedings/99/99scr-sncr/enevolds.pdf(accessed in Feb 2007).**26of2~**