DRAFT, 2/06/07 

EERC Response 

Roger Christman, Eastern Research Group, Inc. on Review and Critiquie of the 

Burns&McDonnell, NOx BACT Analysis for the Milton R. Young Station Operated by 

Minnkota Power (October 2006). 

EPA Preliminary Analysis of B&M NOx BACT Analysis (January 8, 2007. 

INTRODUCTION 

ERG was asked to review the U.S. EPA Office of Regulatory Enforcement ot Provide 

technical assistance to EPA Region 8 in evaluating the Report on the B4st Available 

Control Technology (BACT) Analysis for Milton R. Roung Station Units 1 & 2, prepared 

by Burns & McDonnell and the appended Ash impacts on SCR Catalyst Perfomance 

(Benson, 2005) prepared by the EERC. 

REVIEW RESULTS 

Christman indicates the reports on ash impacts on SCR performance include a substantial 

amount of “chaff” that has no bearing on the SCR issue. Specifically he indicates that 

what happens in the boiler has no relevance to what happens in lower temperatures of the 

boiler downstream of the economizer. However, what happens inside the boiler defines 

the flux of material that will be carried back into the lower temperature regions of the 

furnace. Of specific importance in the details presented was a discussion of how a 

cyclone partitions the ash between fly ash and slag and how these processes for lignite 

coals can enhance lower temperatures reactions where SCRs are operated. The 

combustion processes of importance that take place in the boiler that are critical to ash 

behavior downstream include: vaporization and condensation processes related to ash 

formation, ash particle formation, ash deposition, and soot blowing cycles. An 

understanding of what happens in the boiler defines the flux of materials (vapors, 

particles, deposit fragments) that will be transported through the boiler with the flue 

gases. These phenomena are especially important for lignite since the low temperature 

ash deposition processes related to alkali and alkaline earth elements are much more 

challenging for high sodium lignite coals as compared to other coals. 

GENERAL BACKGROUND 

Cyclone fired units – Christman indicates that the cyclones are important even though his 

initial statements indicated what happens in the boiler is not important and the cyclones 

are part of the boiler. The chemical composition of the ashes derived from lignite and 
subbituminous coals have similarities in the types of elements present. However, the 

abundance of elements in that ash between most subbituminous coals and lignite are 

different. Shown in Figure 1 is the abundance of sodium and calcium presented as 

equivalent oxides and ash content of primarily subbituminous coals. There are several 

additional coals including bituminous coals such as Illinois #6, Utah Blind Canyon 

(Wasatch Basin), and Shoshone (Green River Formation); blends; and Beulah lignite 

(Fort Union of North Dakota). Based on these analyses only the Spring Creek 

subbituminous coal has a high level of sodium that is typical of North Dakota lignite. 

Spring Creek coal is known to cause significant fouling problems and is a coal that was 

indicated to cause problems associated with sodium deactivation of SCR catalyst coupons 

at a Michigan power plant as indicated by Flemming Hansen (Personal Communication 

February 2007). All of the other PRB coals have sodium oxide levels of less that 2 to 

3%. Figure 2 shows the distribution of sodium oxide in Center lignite. The results of the 

analysis show that the sodium content is on average of 4.4% however, there is a 

multimodal distribution of sodium levels in the coal ash. Using average compositions 

can be misleading related to ash deposition problems. As indicated in Figure 2 the boiler 

is firing some very high sodium containing coals that can cause significant problems. In 

addition, the sodium in the Center lignite is organically associated or sometime referred 

to as water or acid soluble components and based on publications on Haldor Topsoe’s 

website causes catalyst deactivation (T. Slabiak and S. Enevoldsen, Topsoe’s Corrugatedtype DeNOx Catalysts – Characteristics and Operating Experience, 

http://www.topsoe.com/site.nsf/vALLWEBDOCID/KVOO-5PGF7F/$file/Cat%20-

%20Topsoe's%20corrugated-type%20DeNOx%20Catalysts.pdf). On December 6, 1999 

Dr. Heinz Gutberlet and Jürgen Brandenstein from PreussenElektra Engineering of 

Germany visited the EERC to discuss sodium reactions with SCR catalysts. They 

expressed their concern regarding deactivation. Latter that year they presented their work 

on sodium deactivated catalysts from coals that contain higher soluble sodium (Gutberlet, 

NETL SCR 2000 Conference). In discussions with Haldor Topsoe has indicated that 

soluble sodium is the worst kind. 

Christman indicates for a typical cyclone that about 30% of the ash ends up as fly ash and 

the remaining ash ends up as slag. This is not the case for lignite where about 45 to 50 

percent of the ash forming components end up as fly ash. Because more of the ash 

forming components in the lignite is organically associated more of the species end up in 

the fly ash. Christman does not discuss the enrichment of sodium and other volatile 

species in the fly ash from cyclone fired systems. The species that end up in the flyash 

for a cyclone as compared to pc wall or tangential unit are enriched in the inorganic or 

ash species that are more volatile such as sodium. In wall or corner fired systems the 

enrichment of sodium in the fly ash is not as significant because of the dilution of the 

other ash forming components. The ash loading may be less but the melting point and 

the stickiness of the fly ash is greater for the cyclone units as compared to the pc units. 

This creates more deposits in the boiler and downstream surfaces. 

The SCR catalysts sections that were exposed to the flue gas from Coyote Stations were 

completely plugged. EERC offered to send them to Haldor Topsoe and they indicated 

that they were too plugged and deactivation testing could not be conducted. This 
discussion was held between Jason Laumb of the EERC and Flemming Hansen of Haldor 

Topsoe. 

Figure 1. Ash Content (as received) and sodium and calcium expressed as equivalent 

oxides in the ash in coals from EERC’s database. 

Figure 2. Distribution of sodium (expressed as equivalent oxide) in ash derived from the 

coals in the as-fired database. 

0

50

100

150

200

250

300

350

<0.5

0.5-1

1-2

2-3

3-4

4-5

5-6

6-7

7-8

8-9

9-10

10-11

11-12

12-13

13-14

Frequency, N=1317

Na2O in ash

10th Percentile:

1.99 w t%

50th 

Percentile:

 4.4 w t%

55th 

Percentile:

4.61 w t%

45th 

Percentile:

4.09 w t%

90th Percentile:

6.74 w t%

Comparison of Coal Data (CaO sorted)

0.00

5.00

10.00

15.00

20.00

25.00

30.00

Illinois #6

Utah Blind

Coal Creek

Peabody Big Sky

Dietz

North Decker 2

Wyodak

Colstrip

Sherco

North Decker 1

Spring Creek

BT-Co. Blend

Beulah

Jacobs Ranch

Sarpy Creek

Rochelle

Buckskin

Cordero

Black Thunder

Smith-Roland

CIT EB

Nanticoke PRB

Caballo Rojo

Eagle Butte

Shoshone

Caballo

North Antelope

Dry Fork

Antelope

Belle Ayr

Raw Hide

Coal

Mass %

CaO Na2O ASH
1. No comment 

2. Center Lignite is extremely variable in ash content and the constituents that make 

up the ash. Christman focuses attention on averages. Typically it is not the 

average coal that causes ash related problems and it is those excursions that 

impact the formation of difficult to remove deposits on surfaces exposed to flue 

gases. Figure 3 and 4 show actual data from the MRY plant on lignite variability 

that shows the extreme variability in both the ash content and base-to-acid ratio 

(B/A=(Na2O+MgO+CaO+K2O+FeO)/(SiO2+Al2O3+TiO2)). 

Figure 3. Moisture free ash content of as-fired coal samples showing variability. 

Figure 3. Base-to-Acid ratio of as-fired coal samples showing variability. 

0

5

10

15

20

25

30

35

40

45

081898 11081898 13081898 15081898 17 120298 01120298 09120298 17121598 07121598 07121598 07121598 07122698 07122698 05122798 19122798 17122798 15011699 09011699 09011699 09040699 09040699 15040699 21040799 03050499 15050699 05050599 07050599 09050499 17050699 21071199 21071099 05071299 09071099 11071299 21071099 17071399 09

Date

Ash (moisture free)

5400

5600

5800

6000

6200

6400

6600

6800

7000

7200

Cyclone Performnace

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

081898 11081898 15081898 19081898 23120298 09120298 19120198 23121598 21121598 23121698 01121698 03122798 05122798 05122798 05122798 05011699 01011699 03011699 05040699 07040699 15040699 23040799 07050499 21050699 13050599 17050599 21050599 11071099 15071399 09071199 01071499 05071199 17071099 05071299 13

Date

Base/Acid

5400

5600

5800

6000

6200

6400

6600

6800

7000

7200

Cyclone Performance
Figure 5. Ash contents of selected subbituminous coals (as-received basis). 

SPECIFIC ISSUES 

SCR Applicabiltiy

High Dust SCR 

Catalyst Deactivation

Christman ignores the primary deactivation mechanism associated with lignite and that is 

sodium. He focused on the impacts of calcium. Sodium is much higher in Center lignite 

than nearly all PRB coals with the exception of Spring Creek a Northern PRB coal. 

Sodium combines with sulfur and calcium to produce low melting point sulfate phases 

that plug pores and is much more aggressive in plugging pores that just calcium sulfate 

alone. Christman rightly brings up the other problem of sodium and that is by 

deactivating individual reaction sites. Christman appears to miss the major problem and 

that is sodium. 

Regeneration has been attempted in Europe with SCR catalysts exposed to biomass as 

with high K (similar in chemistry to Na) and they had some significant problem in 

recovering reactivity. 

Catalyst Erosion

Christman appears to have misunderstood the problem with ash deposition and 

reentraining the deposit fragments that can cause erosion. The primary superheater and 

economizer in cyclone fired boilers are prone to significant fouling that requires frequent 

sootblowing. This deposition problem is not related to the partitioning that occurs in 

cyclones. The species that are in the fly ash have more calcium and sodium than the bulk 

% Ash

0.00

2.00

4.00

6.00

8.00

10.00

12.00

PRB Blend

Colstrip

BT-Co. Blend

Peabody Big Sky

North Decker 1

North Antelope

North Decker 2

Wyodak

CIT EB

Eagle Butte

Caballo Rojo

Belle Ayr

Coal Creek

Smith-Roland
coal ash. This results in more sticky materials that create dense deposits these lower 

temperature convective pass heat transfer surfaces. Related to ash deposition for high 

sodium lignites cyclones make it worse not better as Christman appears to think. It is 

not the ash content that counts, it is the propensity to produce deposits on the convective 

pass heat exchanges surfaces and when that material is soot blown it is entrained and the 

entrained deposit fragment can cause erosion. The entrained deposits are greater due to 

the high ash deposition propensity of the Center lignite it is not the ash content 

Catalyst Plugging

The catalysts that were tested at Coyote station were completely plugged and deposits 

were forming at the exit of the catalyst. These deposits were highly consolidated and 

bonded by sulfates and are shown in Figure 6. The ash that was responsible for the bulk 

of the problem was sodium, calcium and sulfur rich fine particles. That cause severe 

bonding after 2 to 6 months of operation. 

Figure 6. Plugging of SCR catalyst and deposits formed downstream of catalyst section 

after removal. 

High economizer outlet temperature 

No Comment 

Low Dust SCR 

North Dakota Industrial Commission project Evaluation of Potential SCR Catalyst 

Blinding During Coal Combustion

The SCR catalysts from Otter Tail Power’s Coyote Station were too plugged to run a test 

on deactivation. Flemming Hansen did not see any point in trying to get values of 

catalyst activity. The aim of the project was to determine the rate of loss of catalytic 

activity. However, the challenges of plugging and blinding due to deposit formation 

limited our ability to obtain activity data. 

Regarding the design and operation of the slip stream system – the design and operation 

of the slip stream reactor was conducted with input from Cormatech, Hitachi, and Haldor 
Topsoe. They all approved the design of the reactor, sootblowing system, and operating 

parameters. They also provided the catalyst. The vendor selected the pitch of the 

catalysts. 

CONCLUSIONS 

The conclusions do not address the high sodium levels but appears to focus again on the 

lower calcium levels. None of the sodium issues have been resolved. Sodium is a 

problem in the formation of deposits and in deactivation as indicated by several 

investigators. The catalyst vendors have not addressed them. Mr. Christman curiously 

did not address the challenges of sodium even though it is the major problem identified in 

the reports submitted. 

EPA REGION 8 Preliminary Analysis of Burns & McDonnell BACT Analysis for 

Nitrogen Oxide at Milton R. Youn Station, Units 1 and 2 

January 8, 2007 

I. Introduction and Summary of EPA’s Preliminary Analysis 

a. Summary of EPA’s Preliminary Analysis 

We spoke to Haldor Topsoe and after describing the issues related to sodium they 

were very concerned regarding guaranteeing performance. It was evident that EPArepresentatives may have not indicated the levels of sodium and the form of sodium in 

the coal. They indicated that the sodium in the lignite was a major concern and that it 

causes deactivation, especially the organically associated form. They indicated that they 

had some experience and found significant deactivation of in coupon samples after 6000 

to 7000 hours of operation with a coal having elevated levels of sodium. 

b. Relevant Language from Consent Decree 

II. EPA Case-Specific Review of Burns & McDonnell & EERC Technical 

Feasibility Analysis for Selective Catalytic Reduction 

a. High Dust SCR

EPA Analysis of the Ash Deposition Issue:

EPA states that the high amount of ash materials causing blinding of macro-pores 

in the catalyst is real but solvable. Strides in catalyst formulations, catalyst management 

system, and recently, catalyst regeneration processes, have all combined to lower the 

overall cost of both initial and replacement catalyst in modern SCR systems. 

We spoke to a catalyst vendor regarding what has been done to deal with the 

problems of high sodium coals regarding advanced formulations, catalyst regeneration 
and he was not aware of any work that had advanced SCR systems for high sodium 

applications. 

Comparison of MRYS to SCR Systems Operating on Power River Basin coals

 The sodium and ash contents of Center lignite are significantly different from 

PRB coals. EPA indicates that the problems of deposition will decrease because the 

cyclones take out some of the ash but for lignites removing some of the ash but for 

lignites cyclones create additional deposition problems downstream. The cyclones 

partition the more refractory ash components into the slag and the more volatile 

components into the fly ash. The fly ash becomes enriched in components that vaporize 

and subsequently condense upon gas cooling to form liquid phases and small particles 

that are very reactive to gas phase components that cause bonding. 

 EPA discusses ash loadings and the potential impacts on SCR performance. They 

also discuss the formation of calcium sulfates the can plug pores in catalysts and also 

discuss the impacts of other alkaline materials (potassium, sodium, and magnesium) and 

their potential to deactivate individual sites rather than blinding pores. They fail to 

recognize the formation of combined phases such as sodium calcium sulfates those low 

melting points that are very reactive and can both block pores and deactivate individual 

sites in catalysts. They also fail to recognize that cyclones partition the ash increasing the 

proportion of sodium in the smaller particles resulting in increased propensity for the 

formation of the complex sulfate phases. Topsoe presented at the DOE NETL meeting 

on SCR-SNCR on (http://www.netl.doe.gov/publications/proceedings/99/99scrsncr/enevolds.pdf) testing conducted with biomass fuels that contain high potassium and 

sodium and found rapid deactivation in the SCR catalyst. They indicate that the sodium 

or potassium resembles the ammonium ion and blocks access to the active sites. They 

indicate that this occurs most readily where the alkali elements are soluble. In North 

Dakota lignite most of the sodium is organically associated and is in a soluble form. 

Topsoe goes on to indicate that the alkali in coal is not soluble but they only looked at 

potassium. Potassium is associated in the coal with clay minerals and is not soluble. 

More recently work conducted by Beck and others, 2004 (J. Beck, J. Brandenstein, S. 

Unterberger, K. Hein, Applied Catalysis B: Environmental 49(2004)15-25) established 

that sodium correlates with catalyst deactivation activity and that pore condensation of 

sodium is the major deactivation mechanism. 

Engineering Solutions for SCR Applications at High Ash Facilities

 

 The issue is not high ash it is high sodium. The issue is a high potential for ash 

deposition in lower temperature convective pass surfaces such as the economize and 

primary superheater that require continuous cleaning and the resulting deposit fragments 

will cause plugging in addition to the reactions of the sodium and calcium rich fine 

particulate plugging pores of the catalyst. 

Other Ash Deposition Problems on Downstream Components
 Note that it is not the ash loadings it is the sodium that causes the deposition 

Popcorn Ash issue Raised by Burns & McDonnell

EPA Analysis of the Erosion Issue

EPA Analysis of the Energy &Environmental Research Center Report (Appendix B):

EPA believes that there may be inherent limitations with the pilot study. The following 

will address EPA’s comments. The following vendors were involved in the design and 

testing that included Haldor Topsoe, Cometech, and Hitachi. 

1) The pulsing system was designed with the assistance of the catalyst vendors. They 

provided suggestions on the sootblowing pressures and felt it would be sufficient to 

remove the deposits. 

2) The 6 mm pitch was recommended by the vendors. 

3) If the first layer plugs the gases cannot get to the subsequent layers. 

4) Deposition and plugging are significant problems that impact the reliability of the 

MRY facility. 

Haldor Topsoe was the only catalyst vendor willing to provide catalyst for the lignite 

testing. 

We agree that the EERC pilot scale testing should not alone be the deciding 

considerations when evaluating SCR technical feasibility at MRYS. The EERC 

comparisons of catalyst blinding for lignite and subituminous coal testing as well as the 

sodium deactivation work conducted by Haldor Topsoe (Slabiak and Envoldsen, ), 

PreussenElektra Engineering (Gutberlet and others, ), J. Beck and others (2004) 

EPA Analysis of Burns & McDonnell Conclusions

 EPA did not examine the report in detail. The deposition and plugging was much 

more pronounced for the Coyote Station testing as compared to the Columbia and 

Baldwin Station. The pressure drop increased much more rapidly and the soot blowing 

was not as effective. The conclusions made by EPA that the accumulations are similar 

are incorrect. 

 The most important unresolvable technical difficulty is the impact of sodium on 

the formation of bonded sulfate phases that plug and mask the catalyst and on the ability 

of sodium to deactivate the catalyst. 
Low Dust SCR 

Sodium-rich aerosols are a problem that can get through the ESP. These aerosol are 

known to be present in high sodium coal combustion flue gases. These species are 

known to produce pyrosulfates. 

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

MRYS 

a. EPA Headquarters Believes SCR is the Correct Technology for 

MRYS in Developing the Clean Air Interstate Rule (CAIR)

The experience from subbituminous coals and SCR does not have direct 

application to the lignite coals because of the high sodium content of the lignite. The 

experience with German brown coals does not have direct application to ND lignite 

because of the lower sodium levels. 

We spoke to Flemming Hansen of Haldor Topsoe on February 2, 2007 about the 

issues associated with high sodium North Dakota lignite. They have experience with 

sodium and potassium from biomass fired systems and their experience has not been 

good. After describing the issues related to sodium he was very concerned regarding 

guaranteeing performance. It was evident he was not aware of the details of the levels of 

sodium and the form of sodium in the coal when he was asked before. He indicated that 

the sodium in the lignite was a major concern and that it causes deactivation, especially 

the acid soluble form. He indicated that they had some experience and found significant 

deactivation of SCR coupon samples after 6000 to 7000 hours of operation with a coal 

having elevated levels of sodium. 

 b. 

 c. 

d. SCR and Catalyst Vendor Information

Some of the websites discuss the problems of sodium and deactivation. However, 

EPA has not provided a review of that information.