ICAC FORUM ’02

Low NOx Firing Systems and PRB Fuel;

Achieving as Low as 0.12 LB NOx/MBtu

Patrick L. Jennings, ALSTOM

Windsor, CT

Houston Texas

February 12 – 13, 2002

In the quest for meeting environmental regulations, utilities evaluate different NOx reduction

options for individual units and the system as a whole. An option available that can be included in

the mix of solutions includes utilizing Powder River Basin (PRB) coal in combination with Low

NOx firing systems. ALSTOM has demonstrated that this combination of fuel choice and firing

system is both a technically and economically viable option for many plants for NOx compliance.

INTRODUCTION

The current Clean Air Act (CAA) regulations require states to develop implementation plans (SIPs)

which address NOx emissions as part of the ozone non-attainment requirements. The EPA has

recommended NOx limits of 0.15 pounds per million Btu for utility boilers. The compliance date

was extended from the original May 2003 date to May 2004.

There is a lot of uncertainty in the electric power generation industry; the state of deregulation, the

state of electricity trading companies, New Source Review issues and future pollutant regulations.

All of these unknowns add a level of complexity to the already tough issues of pollutant compliance

strategies. This leads to the desire for companies to remain flexible and emphasize total lowest

cost compliance strategies for NOx emissions. For some Units, this can be accomplished by firing

Powder River Basin coal and installing Low NOx firing systems.

RETROFIT EXPERIENCE

ALSTOM Power’s LNCFSTM Systems

Currently ALSTOM Power has a total

of 39 Low NOx contracts that fire PRB

fuel. Of these units, only 11 were

originally designed for PRB coal.

These contracts include the entire

family of Low NOx firing systems

solutions, as shown in Table 1. The

low NOx Concentric Firing System

LNCFS TM P2 system and the LNCFS TM

Level 1 both utilize the existing

windbox and maximize the quantity of

overfire air possible within the existing 

EXISTING

WINDBOX

LNCFS

P2

LNCFS

LEVEL I

LNCFS

LEVEL II

LNCFS

LEVEL III

TFS 2000 R

SOFA

SOFA

SOFA SOFA SOFA

SOFA SOFA SOFA

END AIR VCCOFA CCOFA END AIR CCOFA CCOFA

COAL P2 COAL END AIR COAL END AIR END AIR

AUX AIR CFS COAL CFS COAL COAL

COAL P2 COAL COAL COAL COAL COAL

AUX AIR CFS CFS CFS CFS CFS

COAL P2 COAL COAL COAL COAL COAL

AUX AIR CFS CFS CFS CFS CFS

COAL P2 COAL COAL COAL COAL COAL

END AIR END AIR END AIR END AIR END AIR END AIR

Table 1
ICAC Forum ‘02 2

windbox. For maximum in furnace NOx reduction, Separated Overfire Air or SOFA is required.

SOFA is the primary NOx reduction technique of the LNCFSTM Level II, the LNCFSTM Level 3 and the

TFS 2000TM R firing systems. A typical LNCFS Level 3 system configuration is shown in Figure 1. 

Thirty-five of the contracted units firing PRB with low

NOx systems are now operational. Almost all of

the systems with Separated Overfire Air (SOFA) can

achieve NOx emissions levels of less than 0.15 LB

NOx/MBtu at Maximum Continuous Rating (MCR).

For operational and other reasons, not all of these

units are optimized for minimal NOx emissions.

These units range in size from 150 MW to 700 MW

and include both four corner and “eight corner” or

divided furnace designs. The total contracted

capacity is approximately 15,000 MW, which

represents twenty-four percent of ALSTOM’s total

installed low NOx capacity of 62,000 MW.

Technical Performance

One key issue with converting to a PRB fuel as a

NOx strategy is the ability to achieve NOx

emissions levels at MCR with margin over the

design point. This is necessary to maintain the

design point average while compensating for NOx

upsets due to load swings, mills out of service, unit

auxiliary equipment malfunction, etc. Table 2

illustrates the lowest achieved NOx emissions rate

for seven different utility boilers while firing PRB

under Low NOx conditions. These emissions rates

were achieved at the Maximum Continuous Rating while maintaining full design steam

temperatures and CO emissions below 200 PPM (@3% O2). All seven of these units can achieve

well below the EPA recommended guidelines of 0.15 LB NOx/MBtu. One unit has demonstrated

NOx emissions levels as low as 0.09 LB NOx/MBtu during steady state conditions at MCR. This is

40% below the 0.15 level. Note that this table includes boilers through all three decades of design

that account for the majority of tangentially fired boilers in service in this country.

Plant MW Boiler Contract 

Year

Lowest achieved NOx

at MCR (LB/MBtu

A 600 1970 0.12

B 200 1964 0.10

C 500 1962 0.09

D 325 1959 0.12

E 600 1975 0.11

F 200 1955 0.10

G 350 1957 0.12

As stated above, the ability to achieve low NOx emissions at steady state allows the unit to

maintain a low average over the course of an operating year to minimize total tons production. In

Figure 1

Table 2
ICAC Forum ‘02 3

the normal course of a year, operating units may have to contend with load changes, wet fuel,

mills out of service for maintenance, turbine limitations on firing, fan limitations. All of these can

affect NOx emissions. 

Table 3 * lists the top ten lowest NOx emitting generating power plants in the US for the year 2000

and their year long emissions average. Eight of the top ten lowest NOx emitters in the nation

were retrofitted with ALSTOM low NOx technology and fire Powder River Basin fuel. Two of the

following top ten are also included in Table 2.

Rank* Unit* State *

Year 2000

NOx Average

(LB/MBtu) *

ALSTOM Low

NOx Technology

and PRB

1 Polk Station #1 Florida 0.10 No

2 Labadie #1 Missouri 0.11 Yes

3 Labadie #2 Missouri 0.12 Yes

4 Labadie #3 Missouri 0.12 Yes

5 Labadie #4 Missouri 0.12 Yes

6 Rush Island #2 Missouri 0.13 Yes

7 Minnesota Valley #4 Minnesota 0.14 No

8 Rush Island #1 Missouri 0.14 Yes

9 WA Parish #7 Texas 0.15 Yes

10 Waukegan #8 Illinois 0.15 Yes

* SOURCE – AMEREN Press release July 24, 2001

Note that the above units averaged 0.15 LB NOx/MBtu or less for the year 2000. All of the plants

in Table 2 have the ability to maintain NOx emissions at the 0.15 LB NOx/MBtu level or less over

the long term. For operational reasons, not all of these units operate continuously at these levels.

NOx FACTORS; FUEL AND UNIT DESIGN

ALSTOM has retrofit over 200 units with Low NOx firing systems on fuels ranging from Lignite to

Low Volatile Eastern Bituminous. These units account for more than 62,000 MW of generating

capacity. In addition, ALSTOM has an extensive database on coal properties; this database

includes basic fuel information (Proximate, Ultimate, HHV, ash analyses) as well as proprietary

NOx generation characteristics. Our experience and data has shown that the lower the rank of

coal (all else being equal), the lower the potential NOx emissions from that fuel. There are several

reasons for this.

Coal properties directly and significantly affect the NOx emissions from coal fired power plants.

Reactivity provides a strong correlation to the NOx formation potential of a particular coal. For

U.S. coals, the coal rank, fixed carbon to volatile matter ratio (FC/VM), and coal O2 content are

some of the indices of the reactivity of a coal that are used in NOx generating calculations. Table

4 shows some key values for the coal rank of a Sub-Bituminous “C” coal and a typical Eastern

Bituminous. In addition to reactivity, nitrogen loading of the coal is another indicator of NOx

production. The higher the nitrogen weight per heat content, the greater the potential for NOx

formation due to the increased quantity of nitrogen available. As an added benefit, Powder River

Basin coals are low sulfur.

Table 3
ICAC Forum ‘02 4

Typical Sub Bituminous “C”

Typical Eastern Bituminous

(High Vol. A)

Fixed Carbon / Volatile Matter 1.0 – 1.3 1.5 – 2.0

Oxygen (% by weight) 10% – 15% 4% – 8%

Higher Heating Value (Btu/LB) 8,000 – 8,800 12,400 –13,500

Nitrogen (% by weight) 0.65 – 0.85 1.3 – 1.7

Sulfur (% by weight) < 0.6 0.8 – 2.0

Table 4

During the initial combustion process, coal is rapidly heated and devolatilizes. During this process,

organically bound nitrogen in the volatile matter is released. When this nitrogen is released under

substoichiometric, or air staged, conditions the chemical kinetic reactions promote the formation of

molecular nitrogen (N2) instead of oxides of nitrogen. As the fuel rank decreases and fuel reactivity

increases, more volatiles are released in the early part of the combustion process. This creates a

longer residence time for the kinetics to promote molecular nitrogen and hence lower NOx

emissions. In addition, laboratory data has shown that compared to Eastern Bituminous coal PRB

coal has a greater proportion of fuel nitrogen in the volatile matter. 

Figure 2 illustrates carefully controlled laboratory tests of a variety of coals under staged and

unstaged conditions. The graph compares the total percentage of fuel nitrogen that is converted to

NOx as a function of stoichiometry. Two trends are clearly evident; 1) as stoichiometry is reduced,

the nitrogen conversion to NOx

is decreased, 2) lower rank coals

convert a smaller percentage of

fuel nitrogen to NOx emissions.

Unit design factors also play a

key role in the final outlet NOx

emissions. The original design

Net Heat Input per Plan Area

(NHI/PA) is an indicator of the

Thermal NOx production. For

retrofit NOx firing systems, there

will be physical limitations to the

location of overfire air as well as

total residence time. In general,

units designed in the 1950’s

were more conservative in

furnace design with cooler, taller

furnaces. By the middle 1960’s,

economic pressure dictated reductions in capital costs, which resulted in units that were designed

with shorter, hotter furnaces. 

During the 1970’s the first large units specifically designed for PRB fuels were erected. In addition,

based on the Clean Air Act requirements and the experience of the 1960’s units, new designs for

other coals were being expanded. Unit design became larger in both total MW and on a net heat

input per plan area basis. Furnace sizes reached a maximum with furnaces approaching 55 ft

deep by 70 ft wide with four corners for a 600 MW design. In designing Low NOx retrofits, the

conservative furnace designs of the 1950’s provide the largest flexibility and easiest retrofit, while

the unit designs of the 1960’s present the most challenge for ultra low NOx emissions.

0

10

20

30

40

50

60

0 2.5

Stoichiometry

% Conversion

Low Vol

Hi Vol C

Hi Vol A

Hi Vol B

Hi Vol C/B

Sub Bit C

Figure 2
ICAC Forum ‘02 5

For units that were not designed for the sub bituminous fuels, there are areas of the boiler that will

require an analysis prior to a fuel conversion. The higher heating value of these Western fuels is

approximately thirty percent less than Eastern bituminous and twenty percent less than mid-western

bituminous. Therefore, the milling system may not have sufficient thermal or mechanical capacity

to supply the required increase in throughput. There are different options available to rectify mill

shortcomings, including mill modifications that increase air flow, duct heaters or mechanical

capacity upgrades. Depending upon the original unit design fuel and design year, there may be

modifications to pressure parts that will enhance the operation of units while firing Powder River

Basin fuels. Specifically, the spacing of the superheat and reheat sections and gas temperatures

will need to be reviewed for potential fouling or plugging issues. Continuous finned spiral

economizers may need to be reviewed for fouling potential. For hot furnaces and the higher

slagging PRB fuels, additional sootblower coverage or water cleaning of the walls can be utilized if

required. All of these issues have to be looked at on a unit by unit analysis. It is very rare that all

of these boiler sections need changes. 

Based on our experience to date, the majority of units that were originally designed for Eastern or

Mid-Western Bituminous coals can be converted to fire Sub-Bituminous “C” coals with no capacity

derate.

Powder River Basin

Powder River Basin (PRB) coal is a generic name for any of the Sub-Bituminous “C” fuels that

originate in the Powder River Basin which is located on the border of Wyoming and Montana.

Figure 3 shows the location and shape of the basin. The Powder River Basin is approximately 250

miles from North to South and approximately 95 miles East to West. There are currently 21 mines

in the PRB, 5 in Montana and 16 in Wyoming. Production from the Southern PRB of Wyoming

totaled 7.4 million tons in 1970 and has increased to 340 Million tons in 1998. A large part of

this increase was due to the inherently low sulfur emissions from these coals and the acid rain

provisions of the Clean Air Act amendments of 1990. The PRB is serviced by both the Union

Pacific (UP) and Burlington Northern Santa Fe (BNSF) rail lines.

The Powder River Basin has a range of coal characteristics within the basin itself. The basin can be

divided into three regions of similar coal properties. The largest is the Gillette Field in Eastern

Wyoming. A typical PRB fuel analysis is shown in Table 5. The mineral properties vary significantly

between the three major fields, and this leads to differences in boiler slagging and fouling

characteristics.

TYPICAL PRB FUEL ANALYSIS

(Gillette Field)

Fixed Carbon (% by weight) 33.73

Volatile Matter (% by weight) 32.04

Moisture (% by weight) 28.67

Ash (% by weight) 5.53

HHV (Btu/LB) 8650

Carbon (% by weight) 48.73

Hydrogen (% by weight) 3.51

Nitrogen (% by weight) 0.69

Sulfur (% by weight) 0.46

Oxygen (% by weight) 12.40

Figure 3

Table 5
ICAC Forum ‘02 6

ECONOMIC

PRB Utilization Economics

There are a variety of technically feasible options power plant owners can consider for their NOx

compliance plan. Powder River Basin coal provides an important and unique element to this

evaluation process as it is one of very few coal types that can consistently achieve less than 0.15

LB/MBtu NOx emission levels without the use of post-combustion treatment. As such, a PRB-fired

unit can comply with the federally mandated NOx limit without the need for post-combustion

removal, chemical injection, or the purchase of emission credits. The challenge is to balance the

associated capital and operating costs, performance impacts, and project risks of each alternative

to determine the best overall plan. 

The basic choices for NOx compliance for any given unit are:

a) Firing systems tuning and/or basic modifications.

b) Aggressive firing system modifications, possibly including SOFA systems, pulverizer and control

system modifications

c) Purchase or transfer of NOx credits

d) Selective Non-Catalytic Reduction (SNCR) system addition

e) Selective Catalytic Reduction (SCR) addition

f) Combination of any of the above choices

For units currently firing PRB fuel, the economic evaluation is relatively simple. Installed capital

costs for an aggressive low NOx firing system typically range from $5 - $12 / kW, depending upon

the unit size and specific arrangement. Operating costs generally do not significantly change from

pre-retrofit conditions. Because this option has the ability to achieve the 0.15 LB/MBtu mandated

target without additional equipment; it typically is, by far, the most cost-effective compliance

strategy.

For units firing eastern or mid-western bituminous fuels, the evaluation is more complex. As stated

earlier, an analysis must be done on a specific unit basis to determine what scope of modifications

will be required to convert the unit to PRB fuel firing while maintaining unit capacity, availability

and reliability. In addition to boiler modifications, changes to coal handling equipment, fire

control systems and other site services may be required. Under moderate-to-difficult case

conditions, the capital cost could range from $30 - $50 / kW. However this may still be the most

attractive option when alternate system operating costs are considered. As an example, SCR

capital cost can be as high as $125 / kW and have an operating cost for both the catalyst and

ammonia. In addition, the benefit of low SO2 emissions inherent with PRB coal can be a significant

cost advantage for fuel switching. It should also be noted that money invested on the boiler side

during Low NOx firing system installations often address maintenance issues that plague older

units such as mill maintenance, tilt and damper sticking, etc.

Cost of PRB – 

Last year there was a high volatility in fuel prices, including coal. The end result was that the

“reach” of PRB has increased due to the price fluctuations. PRB pricing is highly dependent upon
ICAC Forum ‘02 7

geographic location as well as access to rail or barge transportation. In a compliance strategy

based on fuel choices, the recent price fluctuations of Natural Gas far outshadowed the price

fluctuations of coal. In addition, the low sulfur content of PRB will be factored into any economic

choice.

Obviously, there are things to consider when having a NOx compliance strategy that is based on a

single regional source of fuel supply, including a limited number of suppliers and shipping issues.

These are all part of the risk management practices that have to be implemented in any low NOx

compliance strategy.

CONCLUSIONS

When a Utility company is investing in low NOx emission reduction technologies, it is important to

follow an old investment adage; “don’t put all your eggs in one basket.” One economically and

technically sound basket is the combination of sub-bituminous “C” fuel choice and low NOx firing

system technology where economically applicable. ALSTOM has the experience and has

demonstrated the capability of long-term Utility operation of Low NOx firing systems modifications

and PRB fuel to produce NOx emissions levels consistently less than 0.15 LB NOx/MBtu with

technical and economic success. 

ACKNOWLEDGEMENTS

Thanks to: 

Gary Camody – ALSTOM 

Jason McMahan – ENERMAP

REFERENCES

1) Maximize PRB Coal Usage in Conjunction with In-Furnace NOx Solutions to Minimize Cost of

NOx Compliance; Topper, J., Blue, H., Pomaranski, J., Rebula, E., Lewis, R., EPA/EPRI/DOE

Mega Symposium, 2001

2) Results from Reliant Energy, W.A. Parish 7, Achieving 0.15 lb NOx/MBtu; Gessner, T., Hoh, R.,

Ray, B., Jennings, P., Rebula, E., EPA/EPRI/DOE Mega Symposium, 1999

3) AMEREN Press Release; July 24, 2001

http://www.ameren.com/newsRoom/newsfull.asp?NewsSeqID=189

4) EPA Acid Rain Program – Preliminary Summary Emissions Reports,

http://www.epa.gov/airmarkets/emissions/prelimarp/index.html

5) EPA Acid Rain Program – NOx Raw Emissions Data,

http://www.epa.gov/airmarkets/emissions/raw/index.html