/

Attachment 10

Initial Design Boiler Heat Input

Rating VS. As Built Boiler

Heat Input Rating
"INITIAL DESIGN" BOILER HEAT INPUT RATING

VS

"AS BUILT" BOILER HEAT INPUT RATING

George W. Bouton

January, 200 1
Experience & Background

George W. Bouton is an expert witness on [he subject of utility boiler design based on his

more than 50 years expe:ience in this area. He was graduated from Cornell University in

June 1942 with a bachelor of mechanical engineering (BME) degree and joined Babcock

& Wilcox that same month in their student engineer training program. He left B&W in

February 1943 to spend 3-1/2 years in the US Navy. He returned to B&W in September

1946 at their New York Staff Engineering Depanment. He was in the perfonnance

methods section, which had the responsibility of developing performance methods used

in the design ofB&W Boilers. This involved running circulation tests and other

performance tests on operating units and applying those results to the development of

B&W's standards. [n 1958 B&W's engineering departments were combined in

Barberton, Ohio and at that time George Bouton was named Manager, Utility Boiler

Design Engineering Technology. The Technology Section was responsible for

developing all the functional standards (heat transfer and fluid dynamics) used in the

design ofB&W boilers. He remained in this position until 1970 when he was made the

manager ofUtility Boiler Design Engineering.

George Bouton retired from B&W in May 1983 after 41 years ofservice, mostly in some

phase of utility boiler design. Since 1983 he has been doing power plant consulting for

boiler manufacturers and utilities. The boiler companies include Babcock & Wilcox,

Combustion Engineering, Riley Stoker and Zum Industries. Utility companies include

Associated Electric Cooperative, Baltimore Gas & Electric, Basin Electric, Kansas City

2
Power & Light, Minnkota Power Cooperative, Minnesota Power, Nonhem States Power

and Wisconsin Power & Light.

Boiler Design

Boiler design is based on:

I. The required heat input to the boiler.

1.. The characteristics ofthe fuel to be fired.

.J.... Specifications prepared by the purchaser or his consultant.

The required heat input to the boiler is based on the capacity specified by the utility in

terms of main steam and reheat steam flow and the associated pressures and temperatures

related to these flows as well as the feed water temperature. Using these conditions and

the boiler efliciency, either specified by the purchaser or deemed optimum by the boiler

manufacturer, the heat input to the boiler can be determined.

The characteristics ofthe fuel to be fired determine the design parameters used by the

boiler manufacturer. For example, for a given boiler heat input, the size of the furnace

will be greater when a coal with high slagging characteristics is specified.

The specifications prepared by the owner or his consultant often determine the design

parameters used by the boiler manufacturer. For example, for a given boiler heat input

and given fuel characteristics, the size of the furnace may be greater due to the

specifications than believed necessary by the boiler manufacturer.

)
The boiler specifications prepared for Milton R. Young Units I & 2 were very similar,

except for the capacity requirements, and were very comprehensive. They included:

1. The normal maximum full load steam conditions.

2. The peak maximum overload steam conditions.

3. The performance fuel and range offuels to be designed for.

4. The furnace should be sized (Q limit the maximum gas temperature leaving the

furnace and entering the superheater to 1950 of at the nonnal full load capacity. The

method for measuring this gas temperature is defined.

5. Design (safety) margins to be applied to auxiliary equipment furnished by the boiler

manufacturer, such as:

(a) The cyclone furnaces, including the coal crushers and related fuel-handling

equipment must be capable ofsupplying 125% of the nonnal full load capacity for

Unit 1 and 110% ofthe nonnal full load capacity for Unit 2. In addition, with one

cyclone furnace out ofservice, enough capacity should be available to obtain at least

100% ofthe full load normal capacity for the range offuels specified.

(b) The forced draft, induced draft and gas recirculating fans should be sized for 115% to

120% of the required maximum load capacity and }32% to 144% of the required

maximum load static pressure.

While the heat input to the boiler is fundamental to the design, it generally is not

guaranteed. The maximum capacity ofthe boiler in terms ofsteam flow, steam

temperature and feedwater flow is guaranteed. To insure that the guaranteed capacity of
f

the boiler will be met, the boiler manufacturer incorporates design margins in the

components ofthe boiler and the related auxiliary equipment supplied.

Design margins are incorporated to permit the boiler to operate satisfactorily under less

than optimum conditions. For instance, fuel input to the furnace may not be well

balanced due to equipment problems or operator error. The boiler manufacturer takes

into account typical unbalances with design margins on such parameters as furnace exit

gas temperature, flue gas flow over sections ofthe superheater, now through the

superheater tubes, etc.

The boiler manufacturer also applies capacity margins, similar to those covered in the

hailer specifications, to auxiliary equipment such as forced and induced draft fans,

recirculating fans, boiler circulating pumps, fuel preparation and burning equipment, etc.

The larger ofthe margins specified by the engineer, or normally provided by the boiler

manufacturer, would be used. The margins normally will range from 10% to as much as

40%, depending upon the type of equipment involved.

Optimum Maximum Load.

The actual maximum load (heat input) that the boiler is capable of achieving is expected

to be 100.10 or more higher than the maximum design load (heat input) due to one or more

ofthe following:

I. Margins incorporated in the boiler manufacturer's design.

5
2. Specifications prepared by the owner or his consultant that stipulate design margins

greater than those nonnally used by the boiler manufacturer.

3. Use of a fuel with more favorable characteristics than that specified by the owner or

his consultant.

4. More careful operation by the owner than that assumed as typical by the boiler

manufacturer.

In many instances the maximum load (maximum heat input) is determined by running

maximum load tests on the boiler/turbine system. This allows the limiting components in

either the boiler or turbine to be identified. Such tests may be run periodically to

determine the maximum load under different operating conditions. When burning severe

slagging and fouling fuels, such as North Dakota lignite, periodic cleaning outages may

be required to permit operation at maximum load for extended periods oftime. The

maximum load attainable immediately following a cleaning outage will be greater than

that attainable after operating for extended periods oftime at maximum loads.

"AS BUrLT" MAXIMUM HEAT INPUT

For Unit 1. the maximum design capacity (heat input) is based on operating at the 5%

overpressure condition. The design boiler heat input at this maximum load is 25 to

MKBlHr.. Considering the design margins discussed above, the boiler is expected to be

capable of operating at capacities (heat inputs) up to 10% or more higher than the design

maximum boiler heat input.

6
For Unit 2, the maximum design capacity (heat input) is based on operating at the 5%

overpressure condition. The maximum design boiler heat input at this condition is 4700

MKBlHr.. Considering the design margins discussed above, the boiler is expected to be

capable of operating at heat inputs up to 10% or more higher than the design boiler heat

input.

Due to the reasons elaborated previously, the actual maximum heat input capability

for a particular boiler design is best determined, not by the boiler manufacture's

design "targeu", but rather by the actual operation of the boiler.

P /l / / /:1~ ... /;' /..>W,4/'

George W. Bouton t'J'1l

January 5, 2001

7
Attachment 11

Expert Opinion Report, Maximum Boiler

Heat Input Capacity, Units 1 and 2,

Milton R. Young Station,

Minnkota Power Cooperative, Inc.

Center, ND 58530
Expert Opinion Report

Maximum Boiler Heat Input Capacity

Units 1 and 2

Milton R. Young Station

Minnkota Power Cooperative Inc.

Center, N.D. 58530

Robert L. Pearson, Ph. D., PE

CH2M HILL

100 Inverness Terrace East

Englewood, CO 80224
Introduction

This is an expert opinion report regarding the as designed and as built maximum heat input

capacity of Millon R Young Station. The report is an expert opinion of Dr. Robert L.

Pearson, Vice President of Utility Practice, CH2MHILL in Denver. Dr. Pearson is a

registered professional engineer with 28 years of experience in the electric utility industry.

For 19 years he was both a Senior Environmental Engineer and Administrator of

Environmental Affairs for the Public Service Company of Colorado (now Xcel Energy). He

bas also been a project manager for Radian International specializing in the electric utility

industry, and most recently he became Vice President, Utility Practice for CH2MHILL. Dr.

Pearson has been qualified as an expert witness in several administrative and judicial

proceedings including two judicial proceedings where he presented his opinion about the

operation of coal fired electric utility power plants. He bas also testified before Congress three

times on the impact of new federal envirorunental regulations on the electric utility industry.

See Attachme'nt 2 for Dr. Pearson's complete resume.

Description of the Plant

The Milton R Young Station is a two-unit lignite fired power station located at Center. North

Dakota. Unit 1. brought on line in 1970 is listed in its air permit as having 235 MW net

generation capacity with a heat input raling of 2500 millioo BTU per hour (MMBTUfhr). Unit

2 brought on line in 1977 is listed at 440 MW net generation capacity with a heat input rating

of 4696 MMBTUlhr. Both sets of numbers are based on the original anticipated fuel firing

rate of the units.

The Milton R Young Station uses North Dakota lignite coal as fuel. This coal is relatively low

in heating value (nominally 6500 BTU per pound) compared to bituminous coal used in most

U.S. power plants which has a heating value of over 1ססoo BTU per pound. Also, the

chemical composition of the ash in the lignite coal causes the ash to melt during combustion of

the coal creating a molten slag. which can cause severe fouling of the boiler tubes.

Both of these characteristics of the coal require the boiler to be designed to be able to U(ilize

this fuel. One boiler type that can burn lignite is a cyclone boiler, with the initial combustion

of the coal taking place in cyclone burners, located to the side of the main combustion zone.

The molten slag from the cyclone burner runs down to the bottom of the boiler where it is

quenched and removed for disposal. The coal combustion products enter the main combustion

chamber of the cyclone boiler where the heat is extraCled by several sets of tubes making sleam

which is used to turn a turbine generator producing power. The flue gas exits Lhe rear of the

boiler and passes into the atmosphere after being processed through the plant's air pollution

control equipment.
Original Design Considerations for Boilers

Coal fired electricity plants are designed as a system with all components specified to work

IOgeLher. For instance, the boiler is designed to provide the amount and quality of stearn the

turbine generator requires. The coal handling system is designed to provide the amount and

quality of coal the boiler requires, etc. Each equipment vendor makes a guarantee that his

equipment will operate according to specifications. The vendors need to demonstrate

compliance with their guarantees or face paying penalties to lhe plant owner. Therefore, the

vendors usually design their equipment with a margin to allow for changed circumstances in

the plant and still perform as specified. These design margins vary with the vendor and the

type of equipment but usually range from 5% to 10%.

At Ihe lime of construction of Unit 1 in the late '60's. few other power generation boilers had

been buill to burn mis fuel. Therefore, the boiler vendor, Babcock and Wilcox had to base the

design of the Unit 1 boiler on the limited infonnation available at the time. They also had 10

design the boiler to meet the purchase specifications set by Minnkota and their architect

engineer. In the case of the Milton R Young Station, the boiler design chosen to meet these

constraints contained many conservative assumptions including the anticipated heat-input

capacity of the boiler.

Comparison of Original Anticipated Heat Input to Actual as Built

Operation

Unit 1

Actual operalion of the plant has shown these original anticipated heat input assumptions LO be

very conservative. Unit 1 had an initial anticipated heat input rating of 2500 MMBTU/hr.

During a perfonnance tcst on December 2, 1983 thc unit demonstrated an actual maximum

heat input rating of 2863 MMBTUlhr., a 14% inerease over the antieipated fuel firing rate.

Figure 1 below is a plot of the maximum heat inputs for Unit 1 for each month of me first

twenty years of operation from December 1970 to January \990. The horizontal line at 2500

MMBTU/hr represents the original anticipated maximum heat input figure. Note Lhat the unit

routinely exceeded this anticipated maximum heat input figure during this period of nonnal

operation and on several occasions even exceeded 3()(x) MMBTU/hr.
Figure 1. Monthly maximum heat inputs for Unit I, December 1970 to January 1990.

Minnkota Power Cooperative

Milton R. Young Station Unlt"

Calculated URGE Bolter Heat Input

Pr1l1990

Maximum of 3.128 occurred in Jan-79

3.500 r---------.!===~===~~==!.---------,

3,000 "'

2....

l 2.000

3-

•

i 1,500 ••

1,000 ••

... ..

o

l'/'·/:/·/'/'//·,l/l/',l/'/·.l'./·/·,,"·//./',»///....///////'.,.>'///1'

In a letter dated November 20. 1995 from John T. Graves of Minnkota Power to Mr. Dana

Mount of the Division of Environmental Engineering, North Dakota Depanmenl of Health, a

comparison was made between the initial anticipated heat input figures used in the plant's

pennit to operate and actual figures measured in recent operating tests. The letter references a

performance test on May 30, 1995 lIlat demonstrated lbe actual maximum beal input rating was

3001.6 MMBTUIhr., a 20% increase over the original anticipated heat input. This figure bas

been seen or exceeded several times prior to 1990 as shown in Figure 1.

Unit2

For Unit 2, the original anticipated heat input value is 4696 MMBTUlhr. On June I, 1990

Unit 2 had a measured maximum heat input rating of 4909 MMBTU/hr., a 5% increase over

tbe original figure.

Figure 2 below is a plot of the maximum heat inputs for Unit 2 for an 18 year period from

May 1977 to March 1995. This period ends juS! before an outage in 1995 when some major

maintenance work was done on the boiler. The horizontal line al 4696 represents the

anticipated fuel firing rale maximum heat input figure. Note that this unit also frequently
exceeds this maximum heat input figure during normal operation and on several occasions has

exceeded 5000 MMBTU/hr. The maximum value is 5401 in September 1991 with a second

high value of 5177 MMBTUlhr in July 1986 and another high value of 5168 MMBTU/hr ill

December 1980.

Figure 2. Monthly maximum heat inputs for Unil2. May 1977 10 Marcb 1995.

Mlnnkola Power Cooperative

Mitton R. Young Station Unit 112

Calculated URGE Boller Heal Input

Pre 1995

Maximum of 5,401 ocaJrred In Sep-91 e.ooo ,- .b~~~~~__~;.......! ___,

.5.000 .

'.000

i 3.000

"

•

2.000

1.000 ..

o

\#~"~.j!~~~~~'l/...?~/it./-r>~/~.J'~~//...J''':/''/-r>~.?X;jC<f'-r>v~#OJ'./~OJ~~<P~<¥,ii'"~.j!o,"

For Unit 2. the November 20 lener from Mr. Graves to the Division of Environmental

Engineering listed a CEM measured maximum heat input of 6261 MMBTU/hr. This value

compares to the original anticipated heat input of 4696 MMBTU/hr. a 33 % increase.

These comparisons clearly demonstrate that both Units 1 and 2 can and do operate well above

their original anticipated maximum beat input values. The original anticipated heat levels are

very conservative and are not representative of actual operation as shown in these data and in

the referenced performance tests.

Another consideration is the way that the heat input to the boiler is currently calculated. The

heat input data shown above in Figures 1 and 2 is based on the monthly maximum power

generation level multiplied by the average energy conversion efficiency (heat ratc) for that

month. Since 1995, the permits for these units require that the continuous emission monitors
,

be used to calculate fuel input values. The CEM uses EPA Method 19 (also known as the F

factor method) to convert several Oue gas constituent concentration and flue gas flow

measurements into an estimate of heat input to the boiler. The problem is that one component

of the flue gas volume calculation uses the velocity measurements of flue gas exiting the boiler.

It is well known that flue gas flow in a dUCI exiling a boiler can be very non-uniform. If one

were to lake one or eveD a few measurements of flow velocity in Lhe dUCI using EPA Method 2

or an equivalent test method and calculate the total volumetric flow of flue gas in lhe dUCI from

these measurements. errors in flue gas volume can result These errors can be as high as 20%.

In 1999, the EPA promulgated Methods 2F, 20, and 2H tn help address these errors in

measuring stack gas velocities. These errors can result in a considerable difference between

Slack-measured power plant heat rates and combustion-calculated heat rates. An EPA

memorandum, dated AUguSl26, 1999, discussing the background and applicability of the new

methods is attached,

For the period from January 1995 to October 2000, a comparison was made of the monthly

average heat input derived from the CEM calculation of heat input compared to the calculation

based on power generation. For Unit 1 the CEM derived estimate of beat input ranged from

89% to 107% of the power generation derived estimate with an average value of97%. For

Unit 2, the comparison ranged from 95% to 121 % with an average of III %.

These comparisons demonstrate that these two methods of calculating the fuel input to the

boiler do not agree. Therefore, any use of the calculated heat input to the boiler needs to take

into account the uncertainty resulting from the disagreement belWeen the methods.

Consequence of Using Anticipated Maximum Heat Inputs in

Determining Title VPermit Condition.

The operating pennit for each unit will base the allowed pounds per hour of S02 emissions on

an emissions factor times the pounds per million BTU of maximum fuel firing capacity. This

calculation results in a maximum allowed S02 emission rate for each unit expressed in units of

pounds of 502 per bour. If the fuel firing capacities used for this S02 emission calculation are

the original anticipated fuel firing rates that are below the measured firing rates for the boiler

then the allowed 502 emission rates in pounds per hour will also be set too low. This is indeed

the case for the Milton R. Young Station.

The boilers can and do fire at a much higher firing rates than the original anticipated values as

demonstrated in this repon. The calculated pounds per hour S02 emission limits should be

calculated from the more representative measured firing rates rather than the original

anticipated firing rates. These calculated maximum 502 emission rates should also have an

allowance to account for the uncertainties in the methods of calculaling the fuel frring rate as

discussed above.
Conclusion

The Milton R. Young Station Units 1 and 2 frequently operate at heat input levels well in

excess of their original anticipated fuel-firing rate. The margin of this difference is up (Q 33 %

above original anticipated levels. Therefore, the use of the original anticipated heat input

values for the Millon R. Young boilers is not representative of actual as built operation as

demonstrated by actual performance tests and the operating data plotted in Figures I and 2.

The maximum heat input levels referenced in lbe November 20. 1995 letter from Me. Graves

are more representative of demonstrated maximum operating heat inputs and sbould be used

for the purpose of computing permit S02cmission limits for I.he units' operation.

Should the permitting aulbority continue to see the need to use these heal input numbers to

calculate emission rates that are placed into an air permit, it is our opinion that the measured

maximum heat inputs should be used for (his calculation, not the much lower original

anlicipa(ed heat inputs. The maximum values shown in Figures 1 and 2 should be used

instead. These values are 3128 MMBTUlhr for Unit 1 and 5401 MMBTUlhr for Unit 2. The

pollution emission rates stated in lhe permits should be based on lhese measured maximum

firing rates.