# 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 1ICAC 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

Tab l e 2ICAC 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 O 2content 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 3ICAC 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 isreleased. When this nitrogen is released under substoichiometric, or air staged, conditions the chemical kinetic reactions promote the formation of molecular nitrogen (N 2) 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 2ICAC 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 5ICAC 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 SO 2 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 uponICAC 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