United States
           Environmental Protection
           Agency
Office of Research and
Development
Washington DC 20460
EPA/625/R-96/001
February 1996
&EPA     Summary Report
           Control of NOX Emissions by
           Reburning

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                                          EPA/625/R-96/001
                                             February 1996
             Summary Report
Control of NOX Emissions by Reburning
       Center for Environmental Research Information
       National Risk Management Research Laboratory
           Office of Research and Development
          U.S. Environmental Protection Agency
                Cincinnati, Ohio 45268
                                      Printed on Recycled Paper

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                                   Notice
The information in this document has been funded wholly, or in part, by the U.S. Environ-
mental Protection Agency (EPA). This document has been subjected to EPA's peer and
administrative review and has been approved for publication as an EPA document. Mention
of trade names or commercial products does not constitute endorsement or recommenda-
tion for use.

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                                  Foreword
The U.S. Environmental Protection Agency (EPA) is charged by Congress with protecting
the Nation's land, air, and water resources. Under a mandate of national environmental
laws, the Agency strives to formulate and implement actions leading to a compatible bal-
ance between human activities and the ability of natural systems to support and nurture life.
To meet this mandate, EPA's research program is providing data and technical support for
solving environmental problems today as well as building the science knowledge base nec-
essary to manage our ecological resources wisely,  understand how pollutants affect our
health, and prevent or reduce environmental risks in the future.

The National Risk Management Research Laboratory (NRMRL) is the Agency's center for
investigation of technologies and management approaches for reducing risks from threats
to human health and the environment. NRMRL's research program focuses on methods for
the prevention and control of pollution to air, land, water, and subsurface resources; protec-
tion of water quality in public water systems; remediation of contaminated sites and ground
water; and prevention and control of indoor air pollution. The goal of this research effort is to
catalyze development and implementation of innovative, cost-effective environmental tech-
nologies; develop scientific and engineering information needed by EPA to support regula-
tory and policy decisions; and provide technical support and information transfer to ensure
effective implementation of environmental regulations and strategies.

This publication has been produced in support of NRMRL's strategic long-term research
plan. It is published and made available by EPA's Office of Research and Development to
assist the user community and to link researchers with their clients.

                            E. Timothy Oppelt, Director
                  National Risk Management Research Laboratory

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                             Acknowledgments
This report was prepared by Radian Corporation (now Radian International LLC) as a sub-
contractor to Eastern Research Group, Inc. under EPA contract 68-C3-0315, Work Assign-
ment 24. Michael L Meadows, P.E., was principal author with assistance from Benjamin P.
Kuo, Anna Roberts, and Suzette M. Puski. Greg Asbury served as Radian's Project Man-
ager. This work was done under the direction of Justice A. Manning, P.E., EPA's Center for
Environmental Research Information, with substantial assistance from Robert E. Hall, Chief,
Combustion Research Branch, National Risk Management Research Laboratory. Peer re-
viewers included Mr. Hall and Andy Miller,  EPA; John M. Pratapas and Dr. Steven F. Free-
man, Gas Research Institute. Sincere appreciation is expressed to each of these persons
for their interest, time and energy put into this report.

Appreciation is expressed to Combustion Engineering, Inc. and the Babcock & Wilcox Co.
for allowing us to use copyrighted material from their classic  publications, "Combustion
Fossil Power Systems," 4th Edition, Joseph Singer, Editor, and "Steam, Its Generation and
Use," 40th Edition, S.C. Stultz and J.B. Kitto, Editors, respectively.
                                          IV

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                                  Contents
Foreword	iii
Acknowledgments	iv

Chapter 1     Introduction	1
              Background	1
              Organization	2

Chapter 2     Theories of NO, Formation and Control by Reburn	3
              NOX Formation	3
                Thermal NOx Formation	3
                Fuel NO, Formation	 6
                Prompt NOx Formation	7
                Factors that Affect NO, Emissions	8
              Boiler Designs	8
                Tangentially-Fired Boilers	9
                Wall-Fired Boilers	11
                Cyclone-Fired Boilers	14
              Theory of NOX Emission Control by Reburn	16
                Three-Stage Combustion	16
                Main Burner Zone Heat Release Rate	17
                Lower Nitrogen Content of Reburn Fuel	17
              Operational Parameters	18
                Reburn Fuels	18
                Flue Gas Recirculation	18
                O2 Stoichiometry	19
                Residence Time	19
                Temperature	20
                Controls and Instruments	20
              Potential Application Problems	20
                Fuel Combustion Problems	20
                Boiler  Operating Problems	20
                Reburn Fuel Availability and Cost	21
                Physical Constraints	22
                Particulate Control Device Constraints	22
                Boiler  Safety	22
                Load Dispatch Range	22
              Ancillary Benefits	23

Chapter 3     Example Full-Scale Demonstrations	25
              Introduction	25
              Public Service of Colorado - Cherokee Unit 3	25
              Illinois Power Company - Hennepin Unit 1	31
              City Water, Light, and  Power - Lakeside Unit 7	34
              Wisconsin Power & Light Company - Nelson Dewey Unit 2	39
              Ohio Edison - Niles  Unit 1	41

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                          Contents (continued)
              Ladyzhin Power Station - Unit 4	43

Chapter 4    Process Economics	51
              Costing Methodology	51
                Capital Costs	51
                Operating and Maintenance Costs	53
                Busbar Cost and Cost-Effectiveness	54
              Cost Analysis	54
                Model Plants	55
                Sensitivity Analysis	55

Chapter 5    Integrated NOx Control Technologies	63
              Reburning with Low NOx Burners	63
              Reburning with SNCR	63
              Reburning with SCR	64

Chapter 6    References	67

Chapter?    Bibliography	70

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                                   Figures
2-1    Effect of Equivalence Ratio on NOx Formation	4
2-2    Effect of Equivalence Ratio on Adiabatic Combustion Temperature	5
2-3    Conversion of Fuel-Bound Nitrogen in Practical Combustors	6
2-4    Sources of NOx Emissions from Coal	7
2-5    Fuel-Bound Nitrogen-to-Nitrogen Oxide in Pulverized Coal Combustion	8
2-6    Firing Pattern in a Tangentially-Fired  Boiler	9
2-7    Burner Assembly of a Tangentially-Fired Boiler	10
2-8    Single-Wall and Opposed-Wall Type Wall-Fired Boilers	12
2-9    Typical Circular Burner	12
2-10   Cell Burner	13
2-11    Flow Pattern in an Arch-Fired Boiler	14
2-12   Cyclone Burner	15
2-13   Firing Arrangements Used with Cyclone-Fired Boilers	15
2-14   Conventional Firing and Gas-Fired Reburn Applied to a Wall-Fired Boiler	17
3-1    Cherokee Unit 3-LNB-Gas Reburn System Schematic	26
3-2    Cherokee Unit 3-Short-Term NOx Emission Data	27
3-3    Cherokee Unit 3-LNB-Gas Reburning Data	28
3-4    Cherokee Unit 3-Effect of Excess Air on NOX Emissions	29
3-5    Cherokee Unit 3-Effect of Gas Input on NOx Emissions	30
3-6    Cherokee Unit 3-Effect of Unit Load on NOx Emissions	31
3-7    Cherokee Unit 3-Long-Term NOK Emission Data	32
3-8    Hennepin Unit 1-Stacked Burners of Tangentially-Fired Boiler	33
3-9    Hennepin Unit 1-Gas Reburning Data with Coal as the Primary Fuel	35

                                         vii

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                            Figures (continued)
3-10   Hennepin Unit 1-Long-Term Gas Reburning Data	35
3-11    Lakeside Unit 7-GR-SI System Schematic	36
3-12   Lakeside Unit 7-Effect of Gas Heat Input on NOx Emissions	37
3-13   Lakeside Unit 7-Effect of Reburn Zone Stoichiometry on NOx Emissions	37
3-14   Lakeside Unit 7-Effect of Flue Gas Recirculation on NOx Emissions	38
3-15   Lakeside Unit 7-Long-Term Operation Results for NOx Reductions	39
3-16   Nelson Dewey Unit 2-Coal-Fired Reburn System Schematic	40
3-17   Nelson Dewey Unit 2-NOx Emissions vs. Unit Load - Illinois Basin Coal	41
3-18   Nelson Dewey Unit 2-NOx Emissions vs. Unit Load - Powder River
       Basin Coal	42
3-19   Niles Unit 1-Schematic of Reburn Process	44
3-20   Niles Unit 1-Variation of NOx with Reburn Stoichiometry	45
3-21    Niles Unit 1-NOX Emissions as a Function of Boiler Load	45
3-22   Ladyzhin Unit 4-Schematic of Reburn Design Arrangements	48
3-23   Ladyzhin Unit 4-NOx Emissions vs. Reburn Fuel Percentage	49
3-24   Ladyzhin Unit 4-NOx Emissions vs. Flue Gas Oxygen Content	50
3-25   Ladyzhin Unit 4-NOx Emissions vs. Boiler Load	50
4-1     Impact of Plant Characteristics on Reburn Cost Effectiveness and
       Busbar Costs for Wall-Fired Boilers	52
4-2    Impact of NOx Emission Characteristics and Heat Rate on Reburn Cost
       Effectiveness"for Wall-Fired Boilers	58
4-3    Impact of Plant Characteristics on Reburn Cost Effectiveness and Busbar
       Costs for Tangentially-Fired Boilers	59
4-4    Impact of NOx Emission Characteristics and Heat Rate on Reburn Cost
       Effectiveness for Tangentially-Fired Boilers	60
4-5    Impact of Plant Characteristics on Reburn Cost Effectiveness and Busbar
       Cost for Cyclone-Fired Boilers	60
4-6    Impact of NOx Emission Characteristics and Heat Rate on Reburn Cost
       Effectiveness"for Cyclone-Fired Boilers	61
                                         Vill

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                                  Tables
3-1     Summary of Example Reburn Installations	25
3-2     Hennepin Unit 1-Fuel Analysis Comparison	34
3-3     Nelson Dewey Unit 2-Summary of Effects of Reburning on Unit
       Operating Parameters	43
3-4     Ladyzhin Unit 4-Fuel Analyses	46
3-5     Ladyzhin Unit 4-Flow Diagram for Boiler Combustion Performance
       Model	47
3-6     Ladyzhin Unit 4-Furnace Thermal Performance Summary	47
4-1     Capital and Operating Cost Components	52
4-2     Variable O&M Unit Costs	54
4-3     Costs for Natural Gas-Fired Reburn Applied to Coal-Fired Boilers	56
5-1     Costs for SNCR Applied to Coal-Fired Boilers	65
5-2     Costs for SCR Applied to Coal-Fired Boilers	66
                                         IX

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                                             Chapter 1
                                            Introduction
Background
  The Clean Air Act Amendments of 1990 require re-
duction in emissions of nitrogen oxides (NOx) because
of NOx's contribution to acid rain formation and identifi-
cation as a precursor to ozone formation. This  report
covers NOx control employing reburning technology: a
new, effective method of controlling NOx emissions from
a wide range of stationary combustion"sources includ-
ing  large, coal-fired, utility boilers. Although reburning
potentially is applicable to either new or existing units,
this report focuses on retrofit applications on utility boil-
ers.

  NOx emission control technologies that are capable of
achieving NOX emission reductions frojn a coal-fired
boiler can be classified as either combustion modifica-
tions or post-combustion flue gas treatment. Combus-
tion modification techniques prevent the formation of NOx
during combustion or destroy the NOx formed during pri-
mary combustion. These techniques include the use of
low-NOx burners (LNBs), overfire air (OFA), and boiler
combustion optimization. Post-combustion flue gas treat-
ment reduces the NOx content of the flue gas through
techniques such as selective catalytic  reduction (SCR)
and selective noncatalytic reduction (SNCR).

  Reburning, as described in this report, is a combus-
tion modification since the formation of NO is minimized
in one portion of the boiler and a portion of the NOx that
does form, is destroyed in another.

  Unlike some other NOX control approaches, reburning
technology is applicable to a wide variety of the boilers
and, in many cases, can be  implemented within a rela-
tively short period of  time.  Reburning  is ideal for wet-
bottom (i.e.,  slagging) boilers.  The only other commer-
cially available NOx control  alternative for this type of
boiler is flue gas treatment, which is more costly per ton
of NO reduction achieved. Because of reburning's ap-
plicability to a wide variety of coal-fired combustion
sources, several demonstration projects have been un-
dertaken to gather data on reburning. As a result of such
projects, reburning technology is offered commercially
by several firms including ABB Combustion Engineer-
ing, Babcock & Wilcox (B&W), and Energy and Environ-
mental Research Corporation (EER).

  Reburning reduces NOx emissions by completing com-
bustion in three stages. In the first stage, NO formation
due to interactions between the fuel and combustion air
at high temperatures is controlled by reducing the burner
heat release rate and the amount of oxygen present. In
the second stage, additional fuel is added under reduc-
ing (oxygen-deficient) conditions to produce hydrocar-
bon radicals that react with the NOx formed in the  first
stage to produce nitrogen gas (N2). Additional combus-
tion air is added in the lower-temperature third stage  and
combustion is completed. In retrofit applications such as
discussed in Chapter 3, reburning has achieved up to
60% reduction  from baseline NOX emissions.

  The concept  for "reburning" was developed in the  late
1960s by Dr. J.O.L. Wendt,  and was first presented in
1973 at the Fourteenth Symposium  (International) on
Combustion (Wendt et. al., 1973). Japanese investiga-
tors  (Y. Takahashi, et. al.) followed up on the concept
and performed pilot-scale tests that showed promising
results, e.g., a  50% NOX reduction. Following those re-
sults, which were presented at the U.S.-Japan NOx In-
formation Exchange in Tokyo in May 1981 (Takahashi
et. al., 1981), U.S. researchers began an intensive in-
vestigation of  reburn  technology. W.S. Lanier, J.A.
Mulholland, and R.E. Hall of the U.S. Environmental Pro-
tection Agency (EPA) performed research on natural gas-
and  oil-fired reburn systems (Mulholland  and Lanier,
1985; Mulholland and Hall, 1987). At the same time EPA
sponsored tests at EER on natural gas-, oil-, and coal-
fired systems (U.S. EPA, 1985a; U.S. EPA, 1987; U.S.
EPA, 1989). This research, performed by S. B. Greene,
S. L Chen, W.  D. Clark, J. M. McCarthy, B. J. Overmoe,
M. P. Heap, D. W. Pershing, and W. R. Seeker, was later
supplemented  by the Gas Research Institute (GRI).

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  As a result of this early research, full-scale demon-
strations of natural gas reburn technology were initiated.
The first reburn demonstration, co-sponsored by EPA,
GRI, the Electric Power Research Institute (EPRI), U.S.
Department of Defense (DOE), and the Ohio Coal De-
velopment Office, was performed by ABB Combustion
Engineering on Ohio Edison's Niles No. 1 cyclone-fired
boiler. Closely following the Niles start-up, EER began a
reburn demonstration under DOE's Clean Coal Technol-
ogy Program  (CCTP) on the Illinois Power's Hennepin
No. 1 tangentially-fired boiler. This was followed by other
EER CCTP demonstrations on the City Water, Light, and
Power's Lakeside No. 7 cyclone-fired boiler and Chero-
kee No. 3 wall-fired boiler. EPA also sponsored  a gas-
fired reburn demonstration on the Ladyzhin No.  4 wet-
bottom boiler  in Ukraine. This project was performed by
ABB Combustion Engineering and, to date,  is the larg-
est boiler on which reburning has been demonstrated.
Another CCTP demo was performed by B&W on Wis-
consin Power & Light's Nelson Dewey No. 2  boiler. This
was the first  coal-fired reburn system demonstration.
Each of these tests will be described in more detail later
in this report.
Organization
  This report serves as a summary of reburning tech-
nologies that are being tested on coal-fired, utility boil-
ers and reflects on-going work in the field of reburning
systems. The data presented in this report represent an
overview of the tests occurring within the U.S. as well as
abroad. This report includes results of demonstrations
performed through mid-1994 and, necessarily, is not all-
inclusive. In  Chapter 2, the chemistry of NOX formation
in coal-fired boilers is presented along with the theoreti-
cal basis for NOx emission control through reburning.
Also in Chapter 2, an overview of various types of coal-
fired boilers  to which reburning may be applied is pro-
vided. Representative  case studies and test data for a
range of boiler types are summarized in Chapter 3. The
process economics of  retrofitting reburning to an exist-
ing boiler is  discussed in Chapter 4. The potential for
combining reburning with other NOx emission control
techniques is examined briefly in Chapter  5. A list of the
references cited in this report is contained in Chapter 6.
Finally, a bibliography of other available reports of inter-
est is presented in Chapter 7.

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                                             Chapter 2
                     Theories of NOx Formation and Control by Reburn
NOX Formation

NOX emissions from combustion devices commonly are
considered to be comprised of nitric oxide (NO) and ni-
trogen dioxide (NO2). For most combustion systems, in-
cluding coal-fired boilers, significant evidence exists to
show that NO is the predominant NOx species (over 95%
of the total).  In recent work, other forms of nitrogen ox-
ides, e.g., N2O, have been identified and are being re-
searched to characterize their contribution and their im-
portance to the need to control total NOx. N2O is of con-
cern primarily because of its impact on ozone reduction
in the stratosphere. However, for purposes of emissions
control, NOx  is defined as the sum of NO and NO, fully
converted to NO2. This corresponds to tne  output of a
chemiluminescence instrument, the most widely ac-
cepted NOX measurement technique.

The formation of NOx from a specific combustion device
is determined by a complex interaction between chemi-
cal, physical, and thermal processes occurring within the
device. To help simplify the understanding  of NOX for-
mation and assist in identifying  control strategies, NOx
typically is considered to form through three mechanisms:

  • Thermal NOX - formed by the oxidation of atmo-
   spheric nitrogen by free oxygen atoms in the higher-
   temperature regions of the combustion flame;

  •  Fuel NOX - formed from chemical reactions involv-
   ing nitrogen atoms chemically bound within the fuel
   component species; and

  •  Prompt NOX - formed by chemical reactions between
   atmospheric nitrogen and fuel-derived hydrocarbon
   radicals and subsequent oxidation.
Thermal NOX Formation

Thermal NO results from the oxidation of atmospheric
nitrogen in the higher-temperature and air-rich regions
of a combustion system. Dependent upon the type of
fuel and the air mixing profiles within the combustion
device, these regions can be a distinct fuel/air flame
(mixing) front, turbulent eddies of near-stoichiometric
composition, or a premixed*  near-stoichiometric condi-
tion. With the complex combustion processes occurring
in coal-fired boilers and their wide range of design types,
each of these situations is feasible and, in fact, may oc-
cur even within different regions of the same boiler.

The basic chemical mechanism occurring in each of
these situations has been well characterized in sub-scale
research  studies and proven in full-scale combustion
systems. During combustion at high temperatures in air-
rich regions, oxygen radicals  are formed from the disso-
ciation of atmospheric oxygen by thermal and chemical
means. These atoms react with nitrogen molecules to
start the reactions that comprise the thermal NOX forma-
tion mechanism:
        0,7! 20
        O + N, 2 NO  + N
        N + O2 ^ NO + O
        N + OH ^ NO + H
(2-1)

(2-2)


(2-3)


(2-4)
Reaction 2-2 is highly temperature dependent and oc-
curs to an appreciable extent in combustion devices of
all types but only at significant rates at temperatures
above 3200°F. The principal source of O atoms for this
reaction is dissociation of O2 (reaction 2-1),  although
other hydrocarbon/oxygen reactions can also contribute
O atoms. Reactions 2-2 and 2-3 produce approximately
the same amount of NO, with the first reaction being the
only significant source of N atoms for the reactions 2-3
and 2-4. Reaction 2-4 is generally of lower significance
in the formation scheme.
'A premixed flame exists when the reactants are mixed prior to chemical reac-
 tion.

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The major factors that influence thermal NO formation
are temperature, O atom concentrations, and residence
time. However, the mixing history of hydrocarbons from
coal with the combustion air and flue gas products con-
trols the actual profiles of temperature, stoichiometry, and
residence time distributions. If these parameters can be
changed dramatically,  thermal NOx formation is sup-
pressed or "quenched." This quenching is the basis for
several well-proven NOx control strategies.

For these reactions and the related reactions controlling
temperatures, O  and O species concentrations have
been  studied using thermochemical  equilibrium and
chemical kinetic digital computer programs. The results
from these programs, showing the importance of time,
temperature, and stoichiometry (oxygen availability), are
shown in Figures 2-1 and 2-2 (Bagwell et al., 1971).

Calculated NOx concentration as a function of the equiva-
lence  ratio* and time for 650°F combustion air preheat
* Equivalence ratio is defined as the actual fuel/oxidizer ratio divided by the sto-
 ichiometric fuel/oxidizer ratio, and is given the symbol of 0
             is depicted in Figure 2-1. The NOx formation rate is a
             maximum for slightly air-rich mixture ratios and decreases
             rapidly as the mixture becomes increasingly fuel rich.
             The rate of NO formation decreases for increasingly fuel-
             rich mixtures. The principal reason is that the available
             oxygen will react much more readily with the hydrogen
             and carbon than with the nitrogen. The decrease in oxy-
             gen atom concentration is more important than the sec-
             ondary effect of the decreasing temperature. The tem-
             perature decay is relatively slow because the excess fuel
             contributes little to the total mass.

             The NOx formed in coal-fired combustion devices is pri-
             marily a burner phenomenon, since the temperature of
             the bulk gas is too low to support significant N0x forma-
             tion. The type of burner utilized has a predominate role
             in the quantity of NO^ formed during combustion. Higher-
             intensity burners typically generate more NO than lower-
             intensity, delayed-mixing burners. Rapid mixing (produc-
             ing flame zones that are closer to an equivalence ratio
             of 1 and of higher temperature) affects the rate of NOx
             formation. This effect of mixing on NOx  formation rate is
             illustrated in Figure 2-2.
                     1000 -
                  I
                        0.6      0.7

                          Air Rich



                   A/F Stoichiometric = 16.3
0.8       0.9      1.0


     Equivalence Ratio
1.1
1.2      1.3

   Fuel Rich
                   Air Preheat = 650° F

Figure 2-1. Effect of Equivalence Ratio on NOt Formation (Bagwell, et al. 1971).

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                     3.80
                     3.75
                     3.70
                 §
                 1   3.65
                 I
                 o

                 1
                 JD

                 O
                 O
                     3.60
3.55
                 £   3.50
                 
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rates that research and development efforts are attempt-
ing to alleviate.


Fuel NOX Formation

The oxidation of fuel-bound nitrogen very often is the
principal source of NOX emissions in combustion of coal
and some fuel oils (natural gas contains negligible quan-
tities of fuel-bound nitrogen compounds). The heterocy-
clic-ring nitrogen compounds of pyridine, piperidine, and
quinoline are the most common ones found in fuel oil.
Both chain and ring nitrogen-bearing compounds  are
found in coal. The reactions involved are not so clear cut
as are reactions forming thermal NOX. One theory pro-
poses cyanide (CN) as an intermediate step, while an-
other proposes that atomic N is  released as the bonds
are broken. The rate of conversion of the fuel-bound ni-
trogen to NO is dependent on the properties of the nitro-
gen-bearing compounds as well as their rate  of evolu-
tion during combustion.

Numerous studies  have  been conducted to determine
the percent of the total fuel-bound nitrogen converted to
NO. Figure 2-3 contains data on the sensitivity of fuel-
bound nitrogen conversion to stoichiometry (oxygen
availability) for equivalence ratios ranging from 0.6 to
1.4 (Pohl and Sarofim, 1976). Other studies have con-
                                             firmed this sensitivity and also have shown that the con-
                                             version is relatively insensitive to temperature variations.

                                             During coal combustion, the burning of coal particles
                                             takes place as either volatiles released from the coal
                                             particle or as char burnout of the remaining solid mate-
                                             rial. Fuel NO can be formed in both combustion phases
                                             and is described as either volatile  NO or char NO. Re-
                                             cent research data on coal  and char oxidation show that
                                             the devolatilized nitrogen compounds  amount to the
                                             major fraction  of the NO produced from fuel-bound ni-
                                             trogen. The char-nitrogen contribution, however, cannot
                                             be neglected.

                                             The  results of one research program  (Pershing and
                                             Wendt, 1976) are shown in Figure  2-4, which illustrates
                                             the relative proportions of thermal NO and fuel NO (vola-
                                             tile NO + char  NO) produced in the combustion of coal.
                                             The findings of the  program indicate that the fuel NO
                                             comprises approximately 80% of the total NO formed in
                                             coal combustion. This illustrates the reason reducing the
                                             peak flame temperature (control of thermal NO) is rela-
                                             tively ineffective in reducing coal-fired NO emissions. The
                                             stoichiometry has a substantial impact on fuel NO for-
                                             mation. The conversion of fuel nitrogen to NOx is reduced
                                             by delaying the addition of O2 required to complete the
                                             combustion until after the  fuel-bound nitrogen has re-
                                             acted and/or until the combustion temperature has de-
   X
  o
  1
  8
  
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                 1400
                 1200
                 1000
              Q.
              Q.

              "o"

              "5   800

              g
              o
              "o
              52.  600
              O
                  400
                  200
Total NO

FuelNO(AR/O2/C02)

Calculated - NO Addition
Calculated - NH3 Addition
                                 I  ....  I  ....  I  ....  I  .  i
                     1.00       1.05       1.10        1.15       1.20       1.25       1.30

                                              Stoichiometric Ratio
Figure 2-4. Sources ofNOt Emissions from Coal (Pershing and Wendt, 1976).
creased. In this manner the fuel-bound nitrogen oxida-
tion occurs under fuel-rich conditions that favor the for-
mation of N2 and lower the conversion  rate to NOx

During one study (Singer, 1991), fuel NOx was measured
in a large  tangentially-fired coal utility  boiler. Fuel N0x
formation correlated well with the fuel oxygen-to-nitro-
gen ratio (Figure 2-5), suggesting that fuel oxygen (or
some other fuel property that correlates well with fuel
oxygen) influences the percentage of fuel nitrogen con-
verted to fuel NOX. This corresponds to previous obser-
vations that greater  levels of NOx are found in air-rich
combustion environments.

In spite of a detailed understanding of the mechanisms
for fuel-bound nitrogen conversion to NOx, the ap-
proaches used to  control thermal NOx work as well or
better on the fuel-bound nitrogen, i.e., oxygen stoichi-
ometry has a significant effect on NOx formation and tem-
perature has a lesser, but still important, effect. Thus,
two forms of NOx (fuel NOx and thermal NOx) are con-
               trolled by the same methods, but for different reasons,
               as explained in the preceding discussion.


               Prompt NOX Formation

               Prompt NOx results from the reactions of atmospheric
               nitrogen and hydrocarbon radicals during combustion.
               As opposed to the slower thermal NOx formation, prompt
               NOX formation is rapid and occurs on a time scale com-
               parable to the energy release reactions (i.e., within the
               flame). Thus, prompt NOx formation cannot be quenched
               in  the manner by which thermal NOX  formation is
               quenched. However, the contribution of prompt NOx to
               the total NO  emissions of a system is not significant
               (Bartok and Sarofim, 1991).

               Although some uncertainty exists in the detailed mecha-
               nisms for prompt NOx formation, the principal products
               of the initial reactions, hydrogen cyanide  (HCN) or CN
               radicals, are believed to be generated during combus-

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  o
      16
      14
      12
      10
  o
 O
  C    Q
  0)    O
  a>    6
                                                  I
                    I
                                    10           15           20

                                      Ratio of Coal Oxygen to Coal Nitrogen
                                25
                                    30
Figure 2-5. Fuel-Bound Nitrogen-to-Nitrogen Oxide in Pulverized Coal Combustion (Singer, 1991).
tion of the fuel, and the presence of hydrocarbon spe-
cies  is considered  to be essential for the reactions to
take place  (Glassman,  1987). The following reactions
are the most likely initiating steps for prompt NOx:
        CH+N, HHCN
        CH5+N, ^HCN +
(2-5)
(2-6)
HCN is then further reduced to form NO and other nitro-
gen oxides.

Measured levels of prompt NOx for a number of hydro-
carbon compounds in a premixed flame show that the
maximum prompt NO level is reached on the fuel-rich
side of stoichiometry (Glassman, 1987). On the fuel-lean
side of stoichiometry, few  hydrocarbon fragments are
available to react with atmospheric nitrogen to form HCN,
the precursor to prompt NOx. With increasingly fuel-rich
conditions, an increasing amount of HCN is formed, cre-
ating more NOx. However,  above an equivalence ratio
of approximately 1.4, not enough oxygen radicals are
present to react with  HCN  and form NO, so NO levels
decrease.
Factors That Affect NOX Emissions

The formation of thermal, fuel, and prompt NO in com-
bustion systems is controlled by the interplay of equiva-
lence ratio with combustion gas temperature, residence
time, and turbulence (sometimes referred to as the "three
Ts"). Of primary importance are the localized conditions
within and immediately following the primary flame zone
where most combustion reactions occur. In utility boil-
ers, the equivalence ratio and the three Ts are deter-
mined by factors associated with burner and furnace
design, fuel characteristics, and boiler operating condi-
tions. Subsequent sections of this report contain a dis-
cussion of how furnace design, fuel characteristics, and
boiler operating characteristics can influence baseline
(or uncontrolled) NOx emission rates.


Boiler Designs

A number of different furnace configurations are utilized
in coal-fired, utility boilers. Reburn NOx emission con-
trols have been applied to tangentially-fired boilers, wall-
fired boilers, and cyclone-fired boilers. Boilers can also

-------
be categorized as dry-bottom (non-slagging) boilers and
wet-bottom (slagging) boilers.

The majority of utility boilers in the U.S. are of the dry-
bottom design. In this design, the temperature in the lower
part of the furnace is kept below the initial deformation
temperature of the coal ash (from 2000°F to over 2500°F
depending upon the coal ash chemical composition and
the oxygen stoichiometry through which the ash passes)
and the ash is collected as a dry particulate. Typically,
only 20 to 30% of the total ash production is collected in
the bottom of the furnace as bottom ash; the remaining
70 to 80% leaves the boiler as fly ash entrained with the
flue gas.

In wet-bottom boilers, the temperature in the lower part
of the furnace is maintained above the fluidization tem-
perature of the ash. This temperature also depends on
the chemical composition of the ash but is typically
greater than 2400°F. The majority of the ash (60 to 80%)
is collected in the bottom of the furnace as molten slag.
This slag is removed from the furnace and quenched in
a slag tank. The remaining ash is entrained with the flue
gas leaving the boiler and is removed by particulate con-
trol equipment. Wet-bottom boilers are most frequently
used for coals with low ash fusion temperatures that
would result in  ash entering the convection portion  of
the boiler in a molten condition, creating severe slagging
conditions.
The characteristics of the boiler designs determine the
uncontrolled NOx emissions of the boiler. In particular,
the design furnace temperature and heat release rate
affect the formation of thermal NO, and fuel NO..
Tangentially-Fired Boilers

The tangentially-fired boiler is a dry-bottom boiler based
on the concept of a single flame zone within the furnace.
As shown in Figure 2-6, the fuel-air mixture in a tangen-
tially-fired boiler projects from the four corners of the fur-
nace along a line tangential to an imaginary cylinder lo-
cated along the furnace centerline (Singer, 1991). As
shown in Figure 2-7, the burners in tangentially-fired
boilers are incorporated into stacked assemblies that in-
clude several  levels of  primary air/fuel nozzles inter-
spersed with secondary air supply nozzles and warmup
guns. The burners inject stratified layers of fuel  and sec-
ondary air into a relatively low-turbulence environment.
The stratification of fuel and air creates fuel-rich regions
in an overall fuel-lean (i.e., air-rich) environment.  Before
the layers are mixed, ignition is initiated in the fuel-rich
region. Near the turbulent center fireball, cooler second-
ary air is quickly mixed with the burning fuel-rich region,
ensuring complete combustion.

The delayed mixing of fuel and combustion air reduces
local peak temperatures and thermal NO formation. In
                    Main Fuel Nozzle
                     Secondary-Air
                     Dampers
                       Burner Assembly
Figure 2-6. Firing Pattern in a Tangentially-Fired Boiler (Singer, 1991).

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<§
 M
I

I

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addition, the delayed mixing provides the fuel-nitrogen
compounds a greater residence time in the fuel-rich en-
vironment, thus reducing fuel NOx formation.

In a tangentially-fired boiler, the fuel and air nozzles tilt
vertically in concert. This tilting allows the fireball to be
moved up and  down within the furnace to control the
furnace exit gas temperature and provide superheated
steam temperature control during variations in load. Tilt-
ing the nozzles downward also reduces NOx formation
by producing more effective heat transfer to the boiler's
waterwalls.
Wall-Fired Boilers

Wall-fired boilers are characterized by multiple individual
burners located on a single wall or on opposing walls of
the furnace. These boilers can be of either the wet-bot-
tom or dry-bottom design depending on the heat release
rate in the boiler. In contrast to tangentially-fired boilers
that produce a single flame envelope, or fireball, each of
the burners in a wall-fired boiler has a relatively distinct,
high-intensity flame zone. Theses flame zones interact
with each other due to combustion gas recirculation re-
gions set up between them. Depending on the design
and location of the burners,  wall-fired boilers can be
subcategorized as either single-wall, opposed-wall type
boilers. Other variations include cell burner, vertical-fired,
arch-fired, and turbo-fired type boilers.

Single-Wall and Opposed-Wall Type Wall-Fired
Boilers

The single-wall design consists of several rows of circu-
lar-type burners mounted on either the front or rear wall
of the furnace (Figure 2-8). Opposed-wall units have cir-
cular  burners on the front and rear walls and have a
greater furnace depth.

Circular burners introduce a fuel-rich mixture of fuel and
primary air into the furnace through a central nozzle (Fig-
ure 2-9) (Stultz  and Kitto, 1992). Secondary air is sup-
plied to the burner through separate adjustable inlet air
vanes. In most circular burners, these air vanes are po-
sitioned tangentially to the burner centerline and  impart
rotation and turbulence to the secondary air. The de-
gree of air swirl, in conjunction with the  flow-shaping
contour of the burner throat, establishes a recirculation
pattern extending several burner throat diameters into
the furnace. The high level  of turbulence between the
fuel and secondary air streams promotes rapid coal vola-
tilization and creates a nearly stoichiometric combustion
mixture. Under these conditions, combustion  gas
temperatures are high and contribute to thermal and fuel
NOx formation. In addition, the high level of turbulence
causes the amount of time  available for fuel  reactions
under reducing conditions to be relatively short, thus in-
creasing the potential for formation of fuel NOx.
Unlike tangentially-fired boiler designs, the burners in
wall-fired boilers do not tilt. Superheated steam tempera-
tures are instead controlled by excess air levels, heat
input, flue gas recirculation, and/or steam attemperation.

Cell-Burner Type Wall-Fired Boilers

Cell-burner type  units consist of two or three vertically
aligned, closely spaced burners, illustrated in Figure 2-
10 (Stultz and Kitto, 1992). The cell burners are mounted
on opposing walls of the furnace. Cell-burner furnaces
have highly turbulent, compact combustion regions. This
turbulence promotes fuel-air mixing and creates a near-
stoichiometric combustion mixture. As described above,
these conditions  promote the formation of both fuel and
thermal NOX. The close spacing of the fuel nozzles gen-
erates hotter, more turbulent flames than the flames in
more widely spaced burners of other wall-fired designs.
A higher heat release rate is achieved, but at relatively
higher NOx emission levels. The high heat release rate
causes local temperatures to increase even further, caus-
ing thermal NOX  to increase due to its dependency on
local temperature.

Vertical-, Arch-, and Turbo-Fired Boilers

Vertical- and arch-fired boilers have burners that are ori-
ented downward. These boilers were developed prima-
rily to burn solid fuels that are difficult to ignite, such as
anthracite. They  have more complex firing and operat-
ing characteristics than the previously  discussed boiler
types. Anthracite burned in conventional boilers would
require supplemental fuel for ignition. These types of
boilers eliminate  that requirement.

Pulverized coal is introduced  through the nozzles, with
heated combustion air discharged  around the fuel
nozzles and through adjacent secondary ports (Figure
2-11) (Singer, 1991). Tertiary air ports are located in rows
along the front and rear walls of the lower section of the
furnace.

The units have long, looping flames directed into the
lower furnace. Delayed introduction of the tertiary air
provides the necessary air to complete combustion. The
long flames allow the heat release to be spread out over
a greater volume of the furnace, resulting in locally lower
temperatures. The  lower turbulence allows the initial
stages of combustion to occur in fuel-rich environments.
As a result, fuel NOx and thermal NOX are reduced.

Turbo-fired units have burners on opposing furnace walls
firing downward into a highly turbulent combustion cham-
ber. The turbo burners themselves are angled downward
and typically are  less turbulent than the circular burners
in opposed-wall  units. The lower combustion chamber
has highly recirculating flows that exit to the main boiler
region through a throat. The high-intensity, nearly adfa-
batic, combustion chamber region leads to high NOx for-
                                                   11

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                                                     Burner
                                                     Zone
                       Single-Wall Fired

Figure 2-8.  Single-Wall and Opposed-Wall Type Wall-Fired Boilers.
                      Opposed-Walled Fired
                                          Secondary
                                          Air
Windbox    Spin Vanes

       Lighter
                                                                                     Furnace Wall Tube
         Pulverized Coal
         and Primary Air
         from Pulverizer
                           Sliding Air      Secondary     _..  _ ._
                           Damper        Air            Pilot Tube    Impeller
                                                        Grid
                                                                                                        Furnace
                                             Swirled Air
                                             Flow Pattern
Figure 2-9.  Typical Circular Burner (Stultz and Kitto, 1992).
                                                           12

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Figure 2-10. Cell Burner (Stultz and Kitto, 1992).
                                                                13

-------
                                                                High Pressure
                                                                Jet Air

                                                                Primary Air and
                                                                Pulverized Coal

                                                                Secondary Air

                                                                    Arch
                                                                     • Tertiary Air
                                                                      Admission
                                                                "U" - Shaped
                                                                Vertical Pulverized-
                                                                Coal Flame

                                                                Furnace Enclosure
                                                                (Refractory Lined)
Flgure2-11. Flow Pattern in an Arch-Fired Boiler (Singer, 1991).
mation for coal firing but provides for good carbon utili-
zation (burnout).


Cyclone-Fired Boilers

The cyclone-fired boiler is a wet-bottom boiler design
that burns crushed,  rather than pulverized, coal. Fuel
and air are burned in horizontal cylinders, producing a
spinning, high-temperature flame (Figure 2-12) (Farzan
et at., 1991). Only a small amount of wall surface  is
present in the cylinder and this surface is partially insu-
lated by a molten slag layer. Thus, burners in cyclone-
fired boilers have a combination of high heat release rate
and low heat absorption rates, which results in very high
flame temperatures and the conversion of ash in the coal
into a molten slag. Slag collected on the burner cylinder
walls flows out of the burners, down the furnace walls,
and  into a water-filled slag tank located below the fur-
nace. The combination of high heat release rate, high
combustion temperatures, and near stoichiometric fuel/
air mixtures encourages formation of both thermal  and
fuel NOx.

Because of their slagging design, cyclone-fired boilers
are almost exclusively coal-fired, except for some units
that  were designed to also fire oil and natural gas (or
have been converted to do so). The single-wall firing and
opposed-wall firing arrangements used for cyclone fir-
ing are illustrated in Figure 2-13 (Stultz and Kitto, 1992).
For smaller boilers, sufficient firing capacity usually is
attained with cyclone burners located in only one wall.
For large units, furnace width often can be reduced by
utilizing an opposed-fired configuration.
                                                    14

-------
                                                     Tangential
                                                     Secondary
                                                     Air Inlet
           Crushed Coal and
           Primary Air
                                                                                       Tertiary Air
                                                                                         Scroll Burner


                                                                         Cyclone Barrel


                                                                         Slag Spout Opening
                                              Slag Tap
Figure 2-12. Cyclone Burner (Farzan, et al, 1991).

                                         Reheater/Superheater
                                                                UUUL
                                        Cyclon
                                        Burners
                                           Reheater/Superheater
                                                                           Opposed-Wall
                                                                           Firing
                                                               Slag Tap
                                                              Tl
                                                            \
                                           Cyclone
                                           Burners
Single-Wall Firing
                                                               V
                                                               Slag Tap
Figure 2-13.  Firing Arrangements Used with Cyclone-Fired Boilers (Stultz and Kitto, 1992).
                                                              15

-------
Theory of NOX Emission Control by
Reburn
Three-Stage Combustion

Reburn is a combustion hardware modification in which
the NO  produced in the main combustion zone is re-
duced downstream in a second combustion zone (the
reburn zone). Up to 20% of the total fuel input (on a Btu
per hour basis) is diverted from the main combustion
zone and introduced above the top row of burners to
create reducing (sub-stoichiometric) conditions in the
reburn zone. The reburn fuel (which may be natural gas,
oil, or pulverized coal) is injected  to create a fuel-rich
zone where the NOx formed in the main combustion zone
is reduced to nitrogen and water vapor. The reburn fuel
may be injected alone (natural gas or oil) or with either
air or recirculated flue gas to improve reburn fuel distri-
bution in the furnace. Combustion  of the fuel-rich com-
bustion gases leaving the reburn zone is completed by
injecting overfire air (called "completion air" when refer-
ring to reburn) in the burnout zone.  Figure 2-14 is a sim-
plified diagram of conventional firing and gas reburning
as applied to a wall-fired boiler (GRI, 1991).

In reburning, the main combustion zone operates at rela-
tively low oxygen stoichiometry (about  0.9 to 1.1), and
receives the bulk of the fuel input (80 to 90% of total
heat input). The balance of the heat input (10 to 20%) is
injected above the main combustion zone through
reburning injectors. The stoichiometry in the reburn zone
is in the range of 0.85 to 0.95. To achieve this, the reburn
fuel is injected at a stoichiometry of up to 0.4. The tem-
perature in the reburn zone must be above 1,800°F to
provide an environment  for the decomposition of the
reburn fuel.

Any unburned fuel leaving the reburn zone is then burned
to completion in the burnout zone, where completion air
(15 to 20% of  the total combustion air) is introduced.
The completion air ports are designed for adjustable air
velocities to optimize the mixing and complete  burnout
of the fuel before it exits the furnace.

The kinetics involved in the reburn zone to reduce NOX
are complex and not fully understood at the present time.
The chemical reactions involved in the reburning  pro-
cess were first proposed by J.O.L. Wendt in the late
1960s (Wendt  et al, 1973). The following discussion,
derived from a recent report on reburn published by the
U.S. Department of Energy (Farzan and Wessel, 1991),
is based on the concepts introduced in this work. The
major chemical reactions are the following:

 CH^_heat&OjLdeficiencL_>.CHj + >H (hydrocarbon radicals) (2-7)
The reaction process shown in Equation 2-7 is hydro-
carbon radical formation in the reburn zone. These hy-
drocarbon radicals are produced due to the pyrolysis of
the fuel in an oxygen-deficient, high-temperature envi-
ronment.  The hydrocarbon radicals then mix with the
combustion gases from the main combustion zone and
react with NO to form CN radicals, NH2 radicals, and
other stable products (Equations 2-8 to 2-10).
            + NO->HCN + H
N2+.CH3
                    • NH2+HCN
          + HCN->-CN+H,
                                    (2-8)

                                    (2-9)

                                   (2-10)
The CN and NH  radicals and other products can then
react with NO to form N2, thus completing the major N0x
reduction step (Equations 2-11 to 2-13).
            -NH2->N2+H2O
       NO + • CN -> N2 + CO
         NO + 2CO->N
                                   (2-11)

                                   (2-12)

                                   (2-13)
An oxygen-deficient environment is critical to these re-
actions. If O2 levels are high, the NOX reduction mecha-
nism will not occur and other reactions will predominate
(Equations 2-14 and 2-15).
       CN + O2 -> CO + NO
NH + O
           H2O
                         NO
(2-14)

(2-15)
To complete the combustion process, air must be intro-
duced above the reburn zone. Conversion of HCN and
ammonia compounds in the burnout zone may regener-
ate some of the decomposed NOx by the reactions shown
in Equations 2-16 and 2-17:
HCN + 5/4O2->NO + CO
NH3 + 5/4 O2 -» NO+ 3/2 H20
                                          (2-16)

                                          (2-17)
Although some additional NOx may be formed in the
burnout zone through these reactions, the net effect of
the reburning process is to significantly reduce the total
quantity of NOx emitted by the boiler.

The NOx may continue to be reduced by the HCN and
NH3 compounds by the reactions shown in Equations 2-
18 and 2-1 9:
HCN + 3/4O2 -» 1/2N2 +CO + 1/2H2O
                                          (2-1 8)
                                                 16

-------
                    Reheater/
                      Superheater
                 Primary Fuel-Coal
                     100%
                                              UULd
                                      Conventional Coal Firing
                   Reheater/
                      Superheater
                      Overfire Air

                 Reburn Fuel-Gas
                     ~ 20%
                  Primary Fuel-Coal
                      - 80%














*
1^












r
>
^
V.








1


1









—<
-<
_*•*•




s
y
J
\
/



/
C






_i
Burnout Zone
•• Normal Excess Air
Reburn Zone
• Slightly Fuel Rich
• NOX Reduced to N2
Primary Combustion Zone
• Reduced Firing Rate
• Low Excess Air
• Lower NOX
                                        Gas-Fired Reburning


Figure 2-14. Conventional Firing and Gas-Fired Reburn Applied to a Wall-Fired Boiler (GRI, 1991).
       NH3 + 3/4 02 -> 112 N2 + 3/2 H2O       (2-19)
Main Burner Zone Heat Release Rate

In addition to the chemical reactions resulting from three-
stage combustion, reburning also reduces the formation
of thermal NO^ due to the reduced fuel firing rate in the
main combustion zone. As discussed previously, boilers
with higher heat release rates generate relatively more
thermal NOX. By diverting 10 to 20% of the  fuel to the
reburn zone", the heat release rate and resulting thermal
NO production are reduced. This effect is most notice-
able in boilers with high burner heat release rates such
as cyclone-fired boilers, and in any type of boiler at high
unit load where the heat release rate is at its peak.


Lower Nitrogen  Content of Reburn Fuel

The reburn fuel need not be the same as the fuel used in
the primary combustion zone, although coal-fired reburn
is  under active evaluation at several installations and
has been demonstrated at the Wisconsin Power & Light
Company's Nelson Dewey Unit 2 (see Section 3) (Yagiela
et  al., 1991). To date,  natural gas has been  most fre-
                                                 17

-------
quently used as a reburn fuel for retrofit applications to
coal-fired boilers. One major advantage of natural gas
as a reburn fuel is that it has no significant nitrogen con-
tent. Fuel oil (especially distillate oil) also has a lower
nitrogen content than coal, but to date has not been stud-
ied extensively as a reburn fuel. Because of the reduced
nitrogen contents, substituting either natural gas or dis-
tillate fuel oil for a portion of the fuel input from coal (also
called "co-firing") results  in a proportional reduction in
fuel NOx emissions.


Operational Parameters

Operational parameters are those factors related to
implementing the reburn N0x control theory into an  op-
erational system. The most  significant operational  pa-
rameters that affect the performance of a reburn system
are:

  • Reburn fuel type;

  • Flue gas recirculation (FGR);

  • Fuel/O2 stoichiometry;

  • Reburn zone residence time and temperature; and

  • Controls and instrumentation.


Reburn Fuels

Theoretically, the reburn fuel can be any of three basic
fossil fuel types: coal,  natural gas, or oil, without regard
to the type of primary boiler fuel being fired. However,
as stated earlier, use of a fuel with a low nitrogen con-
tent is advantageous in minimizing fuel NOx generation.

Natural Gas

Natural gas is typically the most attractive reburn fuel
because it is effectively nitrogen-free and, therefore, pro-
vides a greater potential NOX reduction  than  a reburn
fuel with a higher nitrogen content. The replacement of
10 to 20% of the fuel input to the boiler with a  nitrogen-
free fuel  results in a comparable reduction in the fuel-
bound nitrogen component of the total boiler NOX emis-
sions. Natural gas also reacts very rapidly in the reburn
zone compared to the alternative fuels. However,  be-
cause of the relatively lower mass of natural gas, achiev-
ing good mixing of it with the flue gas in the reburn zone
is difficult. For this reason, a carrier gas such as recircu-
lated flue gas is often used to enhance mixing while main-
taining a low O2 stoichiometry.

If it is already present onsite, natural gas is the most
logical reburn fuel for existing gas-fired boilers. The rela-
tive ease of handling natural gas and installing gas-fired
reburn injectors make this an obvious candidate for boil-
ers burning other primary fuels as well. Natural gas must
be supplied via pipeline and many plants with coal-fired
or oil-fired boilers utilize natural gas as an ignition or
startup fuel, space heating, or for firing other units. How-
ever, if natural gas is not available onsite or not avail-
able in sufficient quantity, the  cost of installing a new
gas pipeline for the purpose of supplying a reburn fuel
may be economically prohibitive. Even if natural gas is
already available, the cost of natural gas may be higher
than alternative fuels on a per energy unit basis. In these
cases, an alternative reburn fuel must be evaluated.

Coal

Coal has a higher fuel-bound nitrogen level content than
natural gas but is the primary fuel at a very large num-
ber of utility boilers. Pulverized coal also has the lowest
cost per million Btu of any of the available  reburn fuels
and mixes well with the flue gas in the reburn zone. Vola-
tile coals are more effective as a reburn fuel than low-
volatile coals.

While coal may seem an obvious selection, especially
at coal-fired boilers, the use of coal as a reburn fuel may
have some significant disadvantages. The use of coal
can be difficult if the routing of coal supply pipes to the
reburn zone is restricted by work space constraints and/
or maximum fuel flow rates would be exceeded. The coal
particle size must be minimized to achieve rapid com-
bustion in the reburn zone. Some boilers,  such as cy-
clone-fired boilers, would require the addition of coal
pulverizers for the reburn fuel. Firing with pulverized coal
also requires the use of a carrier medium, which is typi-
cally heated air.  This conflicts with optimizing  NOX re-
ductions in the reburn zone which are achieved by mini-
mizing oxygen concentrations in this zone. Oxygen con-
centrations could be minimized by utilizing FGR instead
of air as a carrier gas for coal-firing in the reburn zone.
The additional costs associated with using FGR as a
carrier medium are discussed in a later section.

Fuel Oil

Fuel oil also has a higher fuel-bound nitrogen level than
natural gas but is available at a very large number of
utility boilers. Distillate fuel oil is more desirable than
heavy fuel oil since it has a lower fuel-bound nitrogen
content. Many coal-fired boilers have fuel oil available
as a supplemental or startup fuel. No full-scale utility
demonstration of NOx emission control by reburn using
fuel oil has been performed as of the writing of this docu-
ment.
Flue Gas Recirculation

Flue gas taken from just ahead of the air heater may be
injected into the reburn zone in conjunction with the
reburn fuel. The recirculated flue gas, in lieu of combus-
tion air, can be utilized as a carrier medium for the reburn
fuel to increase the penetration and mixing of the reburn
                                                   18

-------
fuel in the boiler and to cool the reburn fuel injectors.
Using FGR in the reburn zone minimizes the oxygen
concentration in the reburn zone of the boiler, which fa-
cilitates the control of O2 levels in the primary combus-
tion and burn-out zones of the boiler. FGR is also a tem-
perature-quenching strategy in which the recirculated flue
gas acts as a thermal diluent to reduce combustion tem-
peratures in the reburn zone.

The use of FGR in a reburn system differs from the tra-
ditional uses of FGR in boilers. In some coal-fired boil-
ers operating at peak boiler capacity, flue gas commonly
is readmitted through the furnace hopper or above the
windbox to control the superheated steam temperature.
However, this method of FGR does not reduce NOx emis-
sions. Windbox FGR has only a minor effect in reducing
thermal NOx and is not effective for NOx  emission con-
trol on boilers in which fuel NOx is a major contributor.

The degree of FGR  in reburn systems is variable and
depends upon the output limitation of the  forced draft
(FD) fan and minimum furnace temperatures. To maxi-
mize NOx reduction, FGR is routed through the windbox
to the reburn injectors, where temperature suppression
can occur within the reburn  zone. The effectiveness of
the technique depends on the reburn fuel and flow rate.
When burning heavier fuel oils or coal, less NOX reduc-
tion would be expected than when  burning natural gas
because of the higher nitrogen content of the fuel.

Retrofit hardware modifications to  implement FGR in-
clude new ductwork, a flue gas recirculation fan, devices
to mix flue gas with combustion air, and associated con-
trols. In addition, the FGR system itself requires a sub-
stantial maintenance program due to the high tempera-
ture environment and erosion from  entrained fly ash.

Research and development is underway to determine
the NOx capabilities  of reburn without FGR in order to
reduce the capital cost of the plant modifications needed
to implement a reburn system. These efforts are directed
toward improved reburn fuel injection methods.


O2 Stoichiometry

Typically, boilers operate at a furnace O2 Stoichiometry
in the range of 1.2 to 1.3 as  measured at the air heater
inlet. This oxygen-rich environment facilitates higher
boiler temperatures and more complete carbon burnout
in the furnace. A major factor in reducing NOx through
reburning is the precise control of stoichiometries at each
stage in a reburn system. While the stoichiometries are
different in each of the combustion zones of a boiler
employing a reburn system, the overall Stoichiometry as
measured at the air heater remains roughly the same.

With implementation of a reburn system,  the primary
combustion zone excess air is lowered to the minimum
level required to maintain flame stability. Lower primary
combustion zone stoichiometries minimize the amount
of reburn fuel necessary in the reburn zone to create a
fuel-rich condition. Low excess air in the primary com-
bustion zone also minimizes thermal NOx formation by
lowering the zone temperatures. Tests have shown that
stoichiometries in the primary combustion zone should
be maintained in the range of 1.05 to 1.15.

Considerations that limit the reduction of excess air in
the primary combustion zone include flame stability, fuel
type, burner type, and boiler rating. Primary combustion
flames can become unstable whenever stoichiometries
are lowered. Coal ash fusion temperatures are lower
under reducing (sub-stoichiometric) conditions, and if
combustion temperatures  in  a dry-bottom boiler falls
below the initial softening temperature of the ash, ex-
cessive slagging or fouling of the furnace walls occurs.
Slagging burners, such as cyclone-fired  burners, have
minimum combustion temperature requirements in or-
der to prevent solidification (freezing) of the molten slag
in the burner and lower portion of the furnace. Without
sufficient O2 in the primary combustion zone, slagging
burners  are unable to maintain adequate burner tem-
peratures due to incomplete combustion. Each furnace
should conduct a parametric testing program in order to
determine the minimum levels of  excess air in the pri-
mary combustion zone required to sustain good boiler
operation.

The reburn zone is designed to operate in a fuel-rich
environment. By injecting the remainder of the fuel input
with little or no additional combustion air, O2 stoichiom-
etries of 0.85 to 0.95 are achievable in this zone. Reburn
fuel flow rates can be affected by constraints in injector
capacity and combustion profiles in the furnace.

The final burnout zone, or completion air zone, receives
the remainder of the combustion air for the furnace. Typi-
cally, O2 stoichiometries in this zone are 1.2 or greater to
facilitate complete carbon burnout. The completion air
flow rate is often dependent on the stoichiometric condi-
tions in the previous two combustion stages.
Residence Time

A controlling factor in reducing NO emissions with reburn
is the flue gas residence time in the reburn and burnout
zones. The reburn fuel and combustion gases from the
primary combustion zone must be mixed thoroughly for
NOx reduction reactions to occur. The furnace size and
geometry determine the placement of reburn injectors
and completion air ports, which will ultimately influence
the residence time in the reburn and burnout zones. The
typical minimum residence times in the reburn and burn-
out zones for a well-mixed boiler is 0.5 second, which is-
dependent  on the degree of mixing achieved in these
zones.
                                                  19

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Temperature

The flue gas temperature in the burnout zone is an im-
portant factor for the regeneration or destruction of NOx
in  this area. High flue gas temperature promotes the
conversion of N0x compounds to N2.


Controls and Instruments

Generally the retrofit of a reburn system to an existing
boiler will  require some modifications to the boiler con-
trol system. However, investigators have shown that, with
approximate modifications, the control of that reburn sys-
tem can be automated and made fail-safe.

Additional safety sensors are required to monitor the
reburn zone. Safety equipment for burners generally rely
on flame sensing; however, the reburn injectors do not
produce a visible flame because of the low combustion
temperature and limited O2. Natural gas combustion also
does not produce a strong visible flame, which may fur-
ther contribute to the lack of a visible flame in the reburn
zone. Therefore, a reburn safety  system consists of a
comprehensive system of permissives and trips.

The permissives are a set of conditions that must be
satisfied for startup and continued operation of the reburn
system. Trips are critical boiler conditions that will trig-
ger a shut-down of the reburn system. Most of the sen-
sors required for the permissive and trip systems gener-
ally are already  in place. These sensors monitor fan op-
erating status, boiler pressure, and primary combustion
flame. Some temperature sensors may need to be added
to  the reburn zone. Boiler insurance companies have
reviewed this safety system and have determined it to
be acceptable.


Potential Application Problems

Boiler manufactures rely on a vast body of design data
in the design of a coal-fired boiler. Many interrelated pro-
cess factors must be weighed in arriving at an  optimum
boiler design for a given fuel and set of operating char-
acteristics. Existing boilers generally were not designed
with the anticipation of a future reburn system installa-
tion. As a result, the application of NOx emission control
through reburn  presents some characteristic problems
that must  be considered and overcome. The problems
include the following:

  • Fuel combustion problems;

  • Boiler operating problems;

  • Reburn fuel availability and cost;

  • Physical constraints;
  • Paniculate control device problems; and

  • Unit inflexibility.

While many of these concerns are present primarily in
retrofit application of reburn technology, they must also
be addressed in any application to a new boiler.


Fuel Combustion Problems

The existing configuration, spacing, and location of fuel
burners were designed by the boiler manufacturer to
optimize the efficiency of converting a fossil fuel's chemi-
cal energy into usable thermal energy in the steam. The
process changes required by the installation of a  reburn
system can affect the thermal efficiency of the boiler by
affecting the combustion characteristics of the fuel in a
boiler. The thermal efficiency of fuel combustion can be
measured by several parameters including unburned
carbon in the fly ash (coal-fired boilers), hydrocarbon
levels in the flue gas (oil and gas-fired boilers), and the
carbon monoxide (CO)  level in the flue gas. If  insuffi-
cient CX, is added in the burnout region of the boiler or if
insufficient time is available for the completion of com-
bustion, the levels of these  parameters would rise. This
rise would represent a loss of thermal efficiency in the
boiler and necessitate increased operating costs.


Boiler Operating Problems

In addition to  loss of thermal efficiency, the boiler may
experience other operating problems including the fol-
lowing:

  • Steam temperature control problems;

  • Increased fly ash production in slagging boilers;

  • Boiler tube corrosion;

  • Increased boiler tube slagging and fouling; and

  • Slag tapping problems.

The following is a brief overview of the characteristics of
these problems and some of the steps that can be taken
to mitigate them.

Steam Temperature Control Problems

The design of the heat transfer surfaces and of their lo-
cations in  a  boiler (tube  walls, superheaters, and
reheaters) are based on specific conditions in the boiler
such as radiation, convection, and conduction from the
primary combustion flame and hot flue gas. The installa-
tion of a reburn system can result in a major change in
these conditions.
                                                  20

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For example, diversion of 10 to 20% of the fuel from the
main combustion zone to the reburn zone reduces the
amount of heat transfer in the lower portion of the boiler
and increases the amount of heat transfer in the upper
portion. The ratio of heat transfer by radiation and con-
vection can change as well. Less heat will be transferred
to the boiler wall tubes while  more heat will be trans-
ferred in the superheat and reheat areas. This results in
changes to the superheater and reheater attemperator
flows and may destabilize steam temperature control in
the  boiler.

Increased Fly Ash Production

Increased fly ash production is a particular problem for
slagging boilers such as cyclone-fired boilers that use
coal as the reburn fuel. Typically, only 20% of the coal
ash from a cyclone-fired boiler leaves the boiler as fly
ash. The rest is collected as slag in the bottom of the
boiler. The diversion of coal from the cyclone burners to
the  reburn injectors results in the production of a higher
percentage of fly ash. This fly ash will increase the ero-
sion of tubes in the convection passes of the boiler and
of the air heater surfaces. It also increases the fly ash
load on the particulate control device, as discussed later.

Boiler Tube Corrosion

Waterwall tubes and superheater/reheater tubes may
experience increased erosion and corrosion for reasons
similar to those identified for steam control problems.
Reducing  conditions  in the reburn zone can increase
wastage or corrosion of tubes in this area. Extensive
measurements of furnace tube wall conditions before and
after reburn operation at Ohio Edison's Miles Unit 1 (114
MW, cyclone-fired boiler) and at Illinois Power Company's
Hennepin Unit 1 (71 MW, tangentially-fired boiler) have
shown tube wastage to be within normal ranges; how-
ever this issue is repeatedly raised.

Current theory holds that the tube wastage in reducing
zone of coal-fired  boilers is principally due to hydrogen
sulfide (H2S) attack from organic sulfur in the coal. In
reburn, the coal is burned in a net-oxidizing atmosphere
and all of the sulfur is oxidized. If low-sulfur fuel oil or
natural gas is used as the reburn fuel, little or no sulfur is
available to form H2S in the reburn (substoichiometric)
zone. In test at the two units identified above, the com-
bustion products near the furnace wall were tested and
no H2S was found.

Increased Boiler Tube Slagging and Fouling

Increased  flue  gas temperatures in the convection
passes, operation in reducing (substoichiometric) con-
ditions, and increased fly ash production are all factors
contributing to increased boiler tube slagging and foul-
ing conditions. Ash will adhere to boiler tube surfaces if
its temperature is above the ash  softening temperature.
As stated earlier, the ash softening temperature is a func-
tion of the ash chemical composition and is lower under
the reducing conditions found in the reburn zone.

In a dry-bottom boiler, oxidizing (above stoichiometric)
conditions and temperatures below the ash softening
temperature are maintained  at the boiler walls and in
the convection passes to minimize slagging and fouling.
Ash which does accumulate  in these areas is removed
with soot blowers. The reducing conditions in the reburn
zone and the completion of combustion later in the boiler
could result in slagging and fouling too severe for soot
blowers to handle. The potential problem of tube slagging
and fouling may occur in the  convection passes of wet-
bottom boilers as well.

While these problems remain a possibility, the tests de-
scribed in Section 3, which were conducted on full-scale
boilers, reported no discernable increase in slagging
during  reburn operation.

Slag Tapping Problems

In a wet bottom boiler, the temperatures in the lower fur-
nace must be maintained above  the ash melting tem-
perature  so that the ash can be collected as a molten
slag. Reduced temperatures in the lower furnace can
cause the slag to solidify before it can be removed. This
problem can be compounded at reduced furnace loads
when gas temperatures in the boiler are already reduced.
The combination of lower excess air and diversion of a
portion of the fuel to higher in the boiler can reduce the
primary combustion temperatures which in turn can re-
sult in  slag solidification. Generally, slag tap plugging
results in a lengthy unit outage to remove the pluggage.

While such changes in  slag behavior are possible, ad-
equate slag fluidity was maintained during the full-scale
tests on cyclone-fired boilers at Miles Unit 1 and at City
Water,  Light, and Power's Lakeside Unit 7. These tests
are summarized in Section 3.
Reburn Fuel Availability and Cost

Typically, natural gas is economically feasible as a reburn
fuel only at facilities that either already have a sufficient
natural gas supply at the site or have a gas pipeline in
very close proximity. In comparison with other NOx con-
trol alternatives, the incremental cost of utilizing a natu-
ral gas-fired reburn system can be unfavorable unless
one  of these situations exist. Also, natural gas prices
and  availability are seasonally dependent, with higher
costs and more restricted availability occurring during
the winter months. However, NOX control for ozone pre-
cursors may also be seasonally dependent, with the high-
est level of control needed during the summer months.
To determine the economic feasibility of natural gas as a
reburn fuel, the potential user must discuss annual prices
and availability with the local natural gas supplier.
                                                  21

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Limited testing has occurred with coal as a reburn fuel;
however implementation of a reburn retrofit does not af-
fect the  total quantity of coal fired significantly, only its
distribution in the furnace. If coal is used as the reburn
fuel, in some cases, reburning will require a finer coal
particle size than produced by the existing coal prepara-
tion equipment. The fine coal particle size is required to
ensure complete fuel combustion during the limited flue
gas residence time available in the reburn and burnout
zones. This could require additional capital cost for the
installation of new or additional pulverizers.


Physical Constraints

While not many limitations exist on the installation of the
equipment needed for retrofitting a reburn system on a
coal-fired boiler, some physical constraints do exist, in-
cluding:

  • Sufficient boiler height for installation of the needed
    reburn injectors and completion air ports and for ad-
    equate flue gas residence time in the reburn and
    burnout zones;

  • Sufficient room around the boiler for routing of reburn
    fuel lines, combustion air lines, reburn injectors, flue
    gas recirculation fans and ducts (if required), and
    other auxiliary equipment; and

  • Soot blowers capable of handling increased boiler
    tube slagging and fouling.

Such physical constraints must be identified and quanti-
fied  early in evaluating the feasibility of retrofitting a
reburn system on an existing boiler.


Particulate Control Device Constraints

The production of sulfur trioxide (SO ) during combus-
tion of coal is a major contributor to the conductivity of
the fly ash. When a lower sulfur fuel such as natural gas
is used as the reburn fuel, less SO3 is produced and the
resistivity of the fly ash produced generally will increase.
This increase may result in reduced particulate collec-
tion efficiency in an electrostatic precipitator. Offsetting
this effect is the reduction in ash resistivity resulting from
the higher moisture content of the flue gas produced by
combustion of natural gas. The magnitude of each ef-
fect depends on several factors including the sulfur con-
tent of the coal and the amount of reburn fuel as a frac-
tion of the total fuel input. Therefore, predicting the over-
all effect on ash resistivity that would result from a natu-
ral gas-fired reburn system is difficult prior to pilot test-
ing. However, data from the full-scale, gas-fired reburn
tests reported in Section 3  showed precipitator perfor-
mance was maintained throughout the test programs.
Thus coal-fired reburn systems, a larger percentage of
the total ash production of the boiler may leave the boiler
as fly ash. This may be especially true for slagging boil-
ers since they typically produce a relatively smaller
amount of fly ash than dry-bottom boilers. The additional
fly ash generation presents an increased load on the
particulate control device (electrostatic precipitators or
fabric filters). Modification of the particulate control de-
vice may be necessary to maintain the particulate emis-
sions  and stack  opacity within permit  limits. Likewise,
the increased volume of fly ash collected may require
modification of the fly ash handling equipment.


Boiler Safety

Current boiler safety equipment relies heavily on flame
sensing to automatically cut off fuel flow when critical
conditions occur  in a boiler. Reburn fuel injectors do  not
introduce combustion air, which eliminates the stable
visible flames that are present with the primary combus-
tion zone burners. Pulverized coal-fired reburning might
utilize air injection as a carrier media for the coal, which
may or may not produce a stable visible flame. A system
of "trips and permissives," as was discussed earlier, is
necessary to ensure safety in the reburn zone.


Load Dispatch Range

The boiler's operating load cycle  is a  major operating
parameter that affects the overall reduction of NOx emis-
sions resulting from installation of a reburn system. Gen-
erally, reburn systems operate more stably and achieve
greater NOx reductions at higher load conditions. Typi-
cally,  utility boilers do not operate at peak  loads con-
stantly. Loads vary in accordance with electrical demand.
The diversion of 10 to 20% of the fuel from the lower
furnace to the reburn injectors can result in flame insta-
bility and an increase in the unburned carbon content of
the ash. Wet-bottomed boilers will have minimum tem-
perature constraints based on ash fusion temperatures
that may limit the use of the reburn system at reduced
loads. At low loads, the amount of reburn fuel injected
may also be reduced, which could impede fuel/flue gas
mixing at the lower reburn fuel velocity and momentum.
Factors such as  these may limit the turndown range of
the boiler or the applicability of reburn for controlling NOx
emissions. Automation of the reburn system controls*
primary fuel choice (based on ash fusion temperature),
and operation with burners out of service (BOOS) can
minimize the problems associated with boiler load swings
and low-load operation.

During the full-scale demonstration  tests of reburning
discussed in Section 3, the utilities' boiler operators have
been able to find  safe and acceptable boiler control con-
ditions throughout the load ranges tested.
                                                   22

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Ancillary Benefits

The installation and operation of a natural gas-fired
reburn system for NOx control has some ancillary ben-
efits in addition to NOX reductions including:

  • Reduced emissions of acid gases (SO2 and HCI);

  • Reduced emissions of carbon dioxide;

  • Reduced fly ash loading on  the particulate control
    device; and

  • Reduced production of ash for disposal.
In comparison with coal, natural gas contains negligible
quantities of nitrogen, chlorine, and sulfur, reduced car-
bon content, and reduced incombustible material (ash).
Therefore, the replacement of 10 to 20% of the total heat
input to the boiler by natural gas would  achieve a pro-
portional reduction in the emissions of pollutants related
to these fuel components regardless of whether a reburn
system is utilized.

In addition to the environmental aspects of reducing these
constituents, the reduction in fly ash content of the flue
gas leaving the boiler would reduce the load on the par-
ticulate control device,  the erosion of boiler tubes and
air heater elements, and the power consumption of coal
handling and preparation equipment.
                                                  23

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                                             Chapter 3
                              Example Full-Scale Demonstrations
Introduction

This chapter contains five examples of full-scale dem-
onstrations of reburning to control NO  emissions from
utility boilers. Including both U.S. and foreign installa-
tions, the examples cover a wide range of boiler designs
and sizes, and two reburn fuels: natural gas and coal.
The design parameters for the example applications are
summarized in Table 3-1.
Public Service of Colorado - Cherokee
Units

Public Service of Colorado's Cherokee Unit 3 is the site
of a Round 3, Clean Coal Technology Project sponsored
by the DOE, the GRI, Colorado Interstate Gas, the EPRI,
and EER. The project sponsors tested the effectiveness
of LNBs and LNBs combined with natural gas-fired
                                   reburning (LNB gas reburn) retrofit technologies in re-
                                   ducing NOX emissions on a wall-fired boiler. The project
                                   objective was to demonstrate that the combination of
                                   gas reburning and LNB would achieve 70 to 75%  NOx
                                   reduction. Parametric testing was completed in 1993 and
                                   the unit is  currently undergoing long-term testing.  The
                                   information presented in this report on the testing at
                                   Cherokee Unit 3 was compiled from papers titled "Low
                                   NOx Burners & Gas Reburning -An Integrated Advanced
                                   NO Reduction Technology" (Sanyal et al.,  1993)  and
                                   "NO  Control by Gas Reburning in a 172 MWe Boiler"
                                   (Rindahl et al., 1994).

                                   The Unit 3 boiler is a balanced draft, 172-MW, front wall-
                                   fired unit that typically burns Colorado, low-sulfur (-0.4%
                                   S), subbituminous coal. Three other units are at the
                                   Cherokee Station. The capacity factors of the four units
                                   and swing-load conditions allowed a wide range of op-
                                   erating conditions to be tested. Originally equipped  with
Table 3-1.  Summary of Example Reburn Installations

Utility                Unit Name          Unit Size
                               Boiler Type
                               Primary Fuel
Springfield, IL
City Water, Light
& Power
Lakeside Unit 7
Wisconsin Power &  Nelson Dewey Unit 2
Light Co
33 MW       Single-wall cyclone,
            wet bottom
                   100 MW      Single-wall cyclone,
                               wet bottom
Medium sulfur, Illinois
bituminous coal
                     Reburn Fuel
Public Service
of Colorado
Illinois Power Co
Cherokee Unit 3
Hennepin Unit 1
172MW
71 MW
Single-wall-fired,
dry bottom
Tangentially-fired,
dry bottom
Western U.S., low sulfur,
subbituminous coal
High sulfur, Illinois
bituminous coal and
and natural gas
Natural gas
Natural gas
Natural gas
                               Medium sulfur, Illinois       Pulverized Coal
                               bituminous coal and Powder
                               River Basin subbituminous
                               coal
Ohio Edison
Vinnitsaenergo,
Ukraine
Niles Unit 1
Ladyzhin Unit 4
114 MW
300 MW
Single-wall cyclone,
wet bottom
Opposed-wall-fired,
wet bottom
Eastern U.S. bituminous
coal
Ukrainian bituminous coal,
and Siberian lignite and
natural gas
Natural gas
Natural gas
                                                  25

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Babcock & Wilcox (B&W) circular-type PL burners in a
four-by-four array, Unit 3 had a total design heat input of
1650 million Btu per hour (MMBtu/hr). The air pollution
control equipment included a baghouse for particulate
emissions control.

Sixteen Foster Wheeler, Internal Fuel  Staging, LNBs
replaced the original burners for the project. The boiler
had a full division wall and a radiant zone of 24 ft deep
and 42 ft wide. A schematic of the LNB-gas reburn sys-
tem tested is shown in Figure 3-1.

The LNB-gas reburn system involved a 3-stage burning
process at various stoichiometries with the first zone as
the primary burner zone. This zone was operated at 80
to 90% of the total heat input, with minimized excess air.
     Approximately 2.4 m above this zone, eight 14-cm di-
     ameter natural  gas injectors were installed for the
     reburning zone. Natural gas was injected through nozzles
     with 3.4% of the  flue gas recycled to facilitate adequate
     mixing, cool the  natural gas injectors, and disperse the
     reburn fuel. The stoichiometry in the boiler becomes fuel-
     rich at this point.  Nozzle velocities ranged from 27.5 m/s
     at 50% load to 55 m/s at full load. The flow rates of the
     reburn fuel ranged from 10 to 25% of the total heat input
     of Unit 3. The final zone was a burnout zone, with six 52-
     cm diameter injectors for OFA. The OFA injectors were
     tilted 10 degrees down to facilitate dispersion  and mix-
     ing. The design  of the OFA system facilitated carbon
     burnout in an air-rich environment.
                                  I
                  8 Gas Reburning
                  Injectors
Burnout
Zone
(SR3)
                                                             10'
                                                             5.5m
                        6 Overfire Air
                        Injectors
                                                             2.4m
                       8 Gas Reburning
                       Injectors
                                                                        16LowNO}
                                                                        Burners
Figure 3-1. Cherokee Unit 3-LNB-Gas Reburn System Schematic (Sanyal et al., 1993).
                                                   26

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Parametric tests were used to evaluate emission reduc-
tion sensitivity to operating parameters including zone
stoichiometries, gas flow rate, OFA flow rate, flue gas
recirculation rate, and load. Absolute NO emissions were
measured for each firing configuration (Figure 3-2). The
use of LNBs  alone produced NOX emission reductions
of 31 % from the baseline. The minimum NO emissions
with LNB-gas reburn corresponded to reductions of 72%
from baseline and 60% reduction from LNBs alone.

NOx  emissions increased  linearly with increasing zone
stoichiometry, with slopes varying for each case (Figure
3-3). The LNB-gas reburn tests operated at a much lower
percentage of theoretical air than the baseline and LNB
tests, resulting in lower NOX emissions. The stoichiom-
etry target for the baseline and LNB cases was an over-
all stoichiometry, while for the  reburn case it was the
LNB-gas reburn zone  stoichiometry. The baseline and
LNB data were obtained at about 20% excess air (120%
theoretical air). For LNB-gas reburn, the minimum NO
level occurs at a reburning zone stoichiometry of 88%
  theoretical air. At this point, the reburn fuel firing rate
  was 20% of the total heat input to the boiler, and the
  overall stoichiometry was normal.

  The parametric tests showed that overall excess air could
  be lower in the LNB-gas reburn cases than in either the
  baseline or the LNB cases, as seen in Figure 3-4 (Sanyal
  et al., 1993). Slagging, carbon loss, and corrosion were
  expected unless the stoichiometry in the primary burner
  zone (designated as SR, in Figure 3-1) was maintained
  above 1.05. This was accomplished by adjusting the sto-
  ichiometry in the reburn zone (SR2) and the reburn fuel
  input (Rindahl et al., 1994).

  In all cases, NOx emissions had a linear correlation with
  oxygen content. Note that the sensitivity to oxygen con-
  tent decreased for both the LNB and LNB-gas  reburn
  cases, with LNB-gas reburn exhibiting the lowest sensi-
  tivity. Minimum NOX emissions were achieved at a reburn
  zone stoichiometry of 0.88 and overall  stoichiometry in
  the range of 1.2 to 1.3 (Sanyal et al., 1993).
                   0.8
           CQ
           2

           S


           o
                   0.6
                   0.4
                   0.2
                                Baseline
       LNB


Firing Configuration
GR-LNB
Figure 3-2. Cherokee Unit 3-Short-Term NOt Emission Data (Sanyal et al., 1993).
                                                  27

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        X
       o
                0.8
               0.6
               0.4
               0.2
                                                                    I
                            Cherokee Station Unit #3
                                 147-152 MW Net
                                                  Baseline (Pre-LNB)

                                                    D
                                                                               LNB
                        LNB-Gas Reburn
                        16-23% Gas
                     "  i  i  i   i  I  i   i  i  i   I  i  i   i  i  I   i  i  i   i  I  i  i  i  i   I
80         90         100         110        120         130

                  Zone Stoichiometry (% of theoretical air)
                                                                           430
                                                                                              344
                                                                                              258
                                                                                              172
86
                                                                                   x
                                                                                  O
                                                                                         140
Figure 3-3. Cherokee Unit 3-LNB-Gas Reburning Data (Sanyal et a/., 1993).
                                                      28

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               0.8
              0.6
       ffi
        X
       O
               0.4
              0.2
                       Cherokee Station Unit #3
                       147-152 MW Net
                             I    I    I   I


                           Baseline (Pre-LNB)

                              p
                                                                    LNB
                           LNB-Gas Reburn
                           16-23% Gas
                          I
I    i   I
I	i
                                                                                       430
                                                                                       344
                                                     258
                                                                                       172
                                                                                      86
                                                             X
                                                            O
                   1234


                                         O2  Dry at Boiler Exit (%)


Figure 3-4. Cherokee Unit 3-Effect of Excess Air on NOt Emissions (Sanyal et al., 1993).
In general, NOX emissions decreased with increasing gas
heat input. The greatest incremental reductions in NO
emissions occurred at natural gas input values up to 10%
of the total fuel input to the boiler. With 10 to 20% input
from natural gas, the additional reductions in NO,, emis-
sions were marginal. The correlation between natural
gas input and NOX emissions is shown in Figure 3-5.

Natural gas also reduced SO2 and CO emissions. With
the low-sulfur coal typically  used at Cherokee, typical
SO emissions are 0.65 Ib/MMBtu. A gas heat input of
20%,  resulted in  a SO2 emissions decrease of 20% to
0.52 Ib/MMBtu, as expected by fuel substitution with
natural gas essentially free from sulfur. CO2 emissions
also are reduced because natural gas has a lower car-
bon/hydrogen ratio than coal. At a gas heat input of 20%,
the CO2 emission was reduced by 8% (Rindahl 1994).

A linear correlation was observed between unit load and
NOx emissions for all three cases (Figure 3-6). Again the
sensitivity appeared to decrease in the LNB and  LNB-
gas reburn configurations, with  LNB-gas reburn show-
ing the lowest sensitivity to unit load.
                   Overall, the parametric tests did not reveal any prob-
                   lems with the reburn retrofit. Even though carbon loss,
                   flame stability, ash fusion temperature, and steam tem-
                   perature control are parameters that are dependent on
                   the overall excess air, the short-term tests at Cherokee
                   Unit 3 demonstrated that these parameters were not
                   adversely affected by the LNB-gas reburn retrofit.

                   One concern in retrofitting the LNB-gas reburn system
                   was boiler  derating.  Boiler  heat rate is dependent on
                   carbon loss, auxiliary power needs, dry gas loss as a
                   result of excess air and temperature, and latent heat loss
                   through additional water vapor in the flue gas. Due to
                   the higher hydrogen content in natural gas, its combus-
                   tion generates more  water vapor than coal combustion
                   for the same heat input.

                   Carbon and dry gas losses were unchanged as a result
                   of the testing. A minimal increase in auxiliary power oc-
                   curred; however, this was offset by the reduced coal mill
                   power consumption due to reduced coal throughput. The
                   station staff predicted that there would be no net change
                   in power needs. Boiler efficiency for 20% natural gas
                                                  29

-------
      1
               0.8
                0.7
               0.6
               0.5
               0.4
               0.3
               0.2
              0.1
                          Baseline
                                      150 MW, 3.4 - 3.8%  02 ; SR  = 1.01 -1.20

                                   I   ....   I  i  i   i  .   I  ....   I   .
                    0             5             10             15

                                                  Gas Input (%)

Figure 3-5. Cherokee Unit 3-Effect of Gas Input on NO, Emissions (Sanyal et at., 1993).
20
                                                                                                 344
                                                                                                 301
                   258
                                                                                                215
                                                                                                 172
                                                                                                 129
                                                                                                 86
                   43
25
                                                                                                           X
                                                                                                          O
                                                       30

-------
              0.8
       S
       2
        X
       O
              0.6
              0.4
              0.2
                                                          1 I  ' '  '  ' I '  ' '  '  I '  ' ' '-
                      Cherokee Station Unit #3
                      3.4-4.1%  Qz
                                                    Baseline (Pre-LNB)
a
                           i....  i....i
                                                              LNB-Gas Reburn
        I  . t  . i I  i i  i  i I  i i  i  i
                                                                                      430
                                   344
                                   258
                                   172
                                                                                      86
                                           x
                                          O
                 80       90     100      110      120    130


                                              Load (MW)
               140
150
160
Figure 3-6. Cherokee Unit 3-Effect of Unit Load on NOt Emissions (Sanyal et ai., 1993).
firing was reduced by about 1 % due to the latent heat of
the additional flue gas moisture while the steam tem-
perature was maintained through attemperation.

Long-term testing started in April 1993. The objective of
the testing is to obtain operating data over an extended
period of time when the unit is under routine commercial
service. The long-term NO, data obtained in the first nine
months of operation are shown in Figure 3-7. The op-
eration was load-following and operated under the fol-
lowing conditions:

  • 82 to 159 MW net unit load;

  • 5 to 19% gas heat input; and

  • 2 to 6% dry O2 concentrations.

The average NOX concentration during the gas reburning-
LNB operation was 0.26 Ib/MMBtu, compared to 0.5 Ib/
MMBtu as the standard emission limit for dry bottom wall-
fired boilers (Rindahl 1994).

The gas reburning system on Cherokee Unit 3 has been
modified to eliminate flue gas recirculation to reduce
  system complexity, lower furnace exit temperature, re-
  duce operating cost, and reduce slagging. Thd OFA ports
  have been modified to optimize overfire air at low gas
  inputs. Additional tests will be conducted to verify the
  performance of the modified system. A final report on all
  testing is expected in early 1997.


  Illinois Power Company -  Hennepin Unit 1

  Hennepin Unit 1 is a Combustion Engineering, tangen-
  tially-fired, balanced draft, single furnace  boiler with a
  capacity of 71 MW. The unit is capable of achieving full
  load on either coal or natural gas. Unit 1 was the site of
  a Round 1, Clean Coal Technology Project sponsored
  by DOE, GRI, the Illinois Department of the Environment
  and Natural Resources, and EER. The objective of this
  project was to test the NOX reducing efficiencies of sev-
  eral retrofit technologies including:

    •  Natural gas as a reburn fuel (both with coal and natu-
      ral gas as the primary fuel);

    •  Bias coal/natural gas firing;
                                                  31

-------
            0.8


            0.7


            0.6
As found NOX Full load @ 3.5% excess O.
                     Average GR-LNB NOX 0.26 lb/106  Btu

                                                   82-159 MWe Net, 5-19% Gas, 2-6% O2
                   i n ii i nim n I nil ii n i nil II M i nun ii iiiiiiiiiini ii MI i li
                                                 Date
           Apr 27, 1993
                                                                                   Jan 20,1994
Figure 3-7. Cherokee Unit 3-Long-Term NOt Emission Data (Sanyal et al., 1993).
  •  Coal/gas co-firing; and

  •  Gas reburn combined with sorbent injection to re-
    duce SO2 emissions on coal-fired boilers.

The full test matrix also consisted of several baseline
performance tests for coal, gas, coal/gas co-firing, burner
turndown, and coal mill turndown. Parameters were de-
veloped from pilot-scale tests.

The burner arrangement for the Hennepin boiler is typi-
cal  of many tangentially-fired boilers. Fuel  and air are
admitted from the furnace corners in horizontal layers.
In each corner of the furnace are three pulverized coal
burners and two gas burners in an alternating stack (Fig-
ure 3-8), with the air distribution being controlled by
dampers at each compartment. This stacked arrange-
ment allows for various configurations of fuel  choice (pul-
verized coal or natural gas) and staged combustion. Each
of the corners has two levels of natural gas-fired igniters
and warm-up guns capable of supplying 1% and 5% of
the  heat input, respectively (Angello et  al., 1992).

Historically, the unit has burned Illinois bituminous coal
that was moderately high in sulfur (3% S), with 10% ash,
15% moisture, and a heating value of approximately
10,600 Btu/lb. Fuel analyses  comparing the design fuel
characteristics with pre- and  post-testing averages are
presented in Table 3-2 (Angello et al., 1992).
                                Bench- and pilot-scale studies were conducted to de-
                                velop fuel compositions and operating parameters, as
                                well as to evaluate their potential effectiveness in reduc-
                                ing NOx emissions. These studies showed that major
                                parameters of interest included oxygen stoichiometries,
                                furnace gas temperatures, furnace residence times, and
                                fuel/air  mixing. Natural gas was reported as the  most
                                effective reburn fuel, with respect to low baseline levels
                                of NOx  and limited residence time in the reburn zone.
                                Parametric testing began in 1991 with natural gas as
                                well as  coal for primary combustion fuels. The informa-
                                tion presented in this  section is the result of the para-
                                metric testing conducted with coal as  a primary com-
                                bustion fuel.  Data on  natural gas as the primary com-
                                bustion fuel is also available (May et al., 1994).

                                Baseline, uncontrolled NOx emissions firing 100% coal
                                were  approximately 550 ppm (0.75 Ib/MMBtu). Under
                                optimum conditions for NOx control, emissions were re-
                                duced by as much as 77%"from the coal-fired baseline.
                                A graph of NOx emissions and reduction versus the per-
                                centage of gas heat input is shown in Figure 3-9 at the
                                conditions that produced the best balance of performance
                                for commercial operation. Gas reburning with 18% gas
                                firing reduced NOx emissions by 60 to 70% down to 0.23
                                to 0.30  Ib/MMBtu. Even with only 10% gas firing, emis-
                                sions were reduced by 55%  to 0.34 Ib/MMBtu (Folsom
                                etal., 1993).
                                                  32

-------
                 Main Gas Burner and
                 Warm-up Guns
                                                                              Coal Burner
                        Ignitor, Gas
                                                                              Coal Burner
                Main Gas Burner and
                Warm-up Guns
                                                                              Ignitor, Gas
                                                                              Coal Burner
Figure 3-8. Hennepin Unit 1-Stacked Burners of Tangentially-Fired Boiler (Angello et at., 1992).
The data from parametric testing were analyzed to de-
termine the optimum operating conditions for achieving
the target emissions. Several parameters were estab-
lished and the nominal operating conditions for long-term
testing were:

  • Coal zone stoichiometric ratio = 1.10;

  • Reburning zone stoichiometric ratio = 0.90;

  • Burnout zone stoichiometric ratio = 1.20; and

  • Gas heat input = 18% (Keen et al., 1993).

Long-term tests were conducted in 1992, during normal
commercial service. The unit was load-cycled daily, pro-
viding a particularly severe test of the process. NOx emis-
sions measured from January 1992 to October 1992 (no
tests in May or June) showed  an average reduction of
67.3% to 0.245 Ib/MMBtu (Figure 3-10) (Folsom et al.,
1993).

A significant reduction in CO2 emissions was also mea-
sured, due to partial replacement of coal with natural
gas. The use of 18% natural gas resulted in a theoreti-
cal CO2 emissions reduction of 7.9% from the coal-fired
baseline (Keen et al., 1993).

The effect of gas reburning on the durability of the unit
was also evaluated during the  long-term test. As de-
scribed earlier, the reburning zone operates in oxygen
deficient conditions, raising concerns that tube wastage
might be accelerated due to the presence of reduced
sulfur species or fluctuating oxidizing and reducing con-
ditions. Durability evaluations were conducted through-
out the test program, including both baseline and gas
reburn-sorbent injection (GR-SI) operating periods. The
                                                   33

-------
Table 3-2.  Hennepin Unit 1 - Fuel Analysis Comparison
Parameter
Units
Original
Design
Pre-Test
Average
Post-Test
Average
Coal
Carbon
Hydrogen
Oxygen
Nitrogen
Sulfur
Moisture
Ash
HHV
Theoretical
Air Demand
%
%
%
%
%
%
%
Btu/lb
Ib air/
Ibcoal
59.16
3.97
7.46
1.04
2.82
15.99
9.56
10,632
7.999
63.14
4.28
8.50
1.21
3.05
9.06
10.76
11,353
8.510
58.52
4.06
7.65
1.11
2.97
15.07
10.18
10,583
7.955
Natural Gas
CH4
C2H6
C3H8
C4H10
C5H12

-------
                    0.8
                   0.6   -
              |
              O
                   0.4   -
                    0.2   -
                                                 8            12


                                                  Gas Heat (%)
                                                                                      -   20
                                                                            #

                                                                      40    7T
                                                                            o
                                                                            •o
                                                                            
-------
                     Overfire
                     Air
                  Natural Gas
                  15-25% of   _
                  Total Heat Input
                         Primary
                         Combustion
                         Zone
                                           Burnout
                                           Zone
                                          Reburn
                                          Zone
                                                                  Sorbent
                  Combustion Air
                                                                       Coal
                                                                       75-85% of
                                                                       Total Heat Input
Figure 3-11. Lakeside Unit 7-GR-SI System Schematic (Folsom era/., 1994).
Optimum NOX reduction was achieved at a reburn fuel
input level of 22 to 23% and reburn zone stoichiometries
between 0.90 and 0.92, as shown in Figures 3-12 and
3-13 (Folsom, 1994). The optimum NO^ reduction var-
ied between 55 and 62% depending on unit load. At all
unit loads, a reburn fuel heat input fraction of 20% or
greater  resulted in  NO  emissions of less than 0.4 Ib/
MMBtu.

As a result of the testing, a lower limit on burnout zone
stoichiometry of 1.30 was established. Under some op-
erating conditions,  burnout zone stoichiometries lower
than 1.30 resulted in flue gas CO levels exceeding 200
ppm, indicating incomplete combustion.
FOR was used to enhance the mixing of the reburn fuel
with the flue gas in the reburn zone. Within the range
tested, increasing the FGR rate improved the reduction
of NOx as shown in Figure 3-14 (Folsom, 1994).

The reburning optimization parametric testing was fol-
lowed by a series of sorbent injection parametric tests
designed to determine the optimum reagent ratio and
sorbent injection velocity. At the conclusion of these tests,
the GR-SI optimization tests were conducted to integrate
the two technologies. One modification to the initial reburn
system implemented during these tests was the replace-
ment of the fuel nozzles  used in the  parametric tests
with smaller nozzles. These smaller nozzles increased
                                                  36

-------
                              1 2 .           —.                           —	—__
                              1>£    IIII    IIII    IIII   IIII    IIII    IT
                                                                                33 MW
                                                                         -0- 25 MW
                                                                         —A- 20 MW
                                 0.0       5.0       10.0       15.0      20.0      25.0      30.0
                                                       Gas Heat Input (Percent)

Figure 3-12. Lakeside Unit 7-Effect of Gas Heat Input on NO Emissions (Folsom et a/., 1994).
                          13
                         m
                         O
                                          -S-  33 MW
                                          -O-  25 MW
                                                20 MW
                              0.0  I'll    I    I   I   I   I    I    I    I   I   I    I    I
                                                     Reburn Zone Stoichiometry
Figure 3-13. Lakeside Unit 7-Effect of Reburn Zone Stoichiometry on NOM Emissions (Folsom et al, 1994).
                                                            37

-------
                     m
                     §
                         1.2
                          1.0
                         0.8
                         0.6
                         0.4
                         0.2
                         0.0
lilt
•
•
_
m
1 1 1 1
1 1 1



1 1 1 1
1 1 1 1
D 33 MW, 23-25% Gas, SR1 =1 .1 5
O 25 MW, 22-25% Gas, SR1 =1 .1 3
A 20 MW, 22-25% Gas, SR1=1.13


-x




nrsa©
TJ^3
1 1 1 1



">
1 1 1 1



^
1 1 1 1
1 1 1 1
-1.18
-1.18
-1.18

—
•
-
1 1 1 1
                             0.0        2.5         5.0         7.5       10.0       12.5       15.0



                                                   Flue Gas Recirculation, %






Figure 3-14. Lakeside Unit 7-Effect of Flue Gas Recirculation on NOx Emissions (Folsom et al., 1994).
                                                            38

-------
the reburning; fuel penetration into the boiler and im-
proved the mixing of the fuel with the primary combus-
tion zone products. The decreased nozzle diameter re-
sulted in an additional 3 to 5 % reduction in NOx emis-
sions at all unit loads.

The results obtained during the long-term tests confirmed
that the results of the earlier tests could be maintained
during normal unit cycling service. NOX emissions mea-
sured from October 3, 1993 to June 3, 1994 show an
average reduction of 62% (Figure 3-15) (Fplsom, 1994).
The average NO emission during the period of June 5,
1993 to April 4, 1994 was 0.344 Ib/MMBtu.

Operation of the GR and GR-SI systems resulted  in a
small (0.8%) drop in the thermal efficiency of the boiler.
This drop was attributed to higher moisture of flue gas
produced by combustion of natural gas, and to a small
increase in flue gas exit temperature due to sorbent depo-
sition on the back pass heat transfer surfaces. No other
boiler operational problems associated with reburning
were experienced during the test program.

The test program team concluded that the results of the
Lakeside Unit 7 demonstration test confirmed that natu-
ral gas  reburning in a cyclone-fired furnace could main-
tain 60%  NOX reduction, consistently and reliably, with-
out significant thermal impacts on boiler performance.
                                     Wisconsin Power & Light Company -
                                     Nelson Dewey Unit 2

                                     Wisconsin Power & Light Company's (WP&L's) Nelson
                                     Dewey Generating station was the site of a Round 2,
                                     Clean Coal Technology Program sponsored by DOE,
                                     EPRI, and State of Illinois Department of Environmental
                                     and Natural Resources. B&W was the prime contractor
                                     and project manager for the project. The information pre-
                                     sented in this section was compiled from a paper titled
                                     "Update on Coal Reburning Technology for Reducing NOx
                                     in Cyclone Boilers" (Yagiela et al., 1991). The project is
                                     a unique example of the application of reburn technol-
                                     ogy using pulverized coal as a reburn fuel. Cyclone-fired
                                     boilers represent nearly 50% of WPL's coal capacity, and
                                     are responsible for almost 75% of the utility's NOx emis-
                                     sions. The objective of the project was to demonstrate
                                     that reburn could reduce NOx emissions by 50% without
                                     disrupting the reliability and operability of the boiler.

                                     The station has two 100-MW, B&W, cyclone-fired boil-
                                     ers, and  each boiler has three  9-ft diameter front-wall
                                     cyclones. Steam temperatures are 10OOT at the super-
                                     heater outlet  (1500 psig)  and 1000T at the reheater
                                     outlet. The baseline fuel fired in the demonstration was
                                     a medium-sulfur, Illinois bituminous coal. Additional tests
                                     were fired with low-sulfur, western coal from the Powder
                                     River Basin, which is now the primary fuel at the station.
            80
           70
            60
         o
        t5
        I  50
        cc
            40
           30
            20
v
V.'
                                                       NOX Reduction Goal 60%
                  Long Term GR and GR-SI Test Results
                  22-24% Gas Heat Input
            Oct4, 1993
                                                                  JuneS, 1994
Figure 3-15. Lakeside Unit 7-Long-Term Operation Results for NOf Reductions (Folsom et al., 1994).
                                                 39

-------
A pulverized coal-fired reburn system was retrofitted to
Unit 2 for the project. This installation was the first time a
full-scale unit has been retrofitted with a coal-fired reburn
system. The reburn system was developed from math-
ematical modeling of the  boiler and pilot-scale testing
conducted in B&W's Small Boiler Simulator (6 MMBtu/
hr). Results of these initial tests characterized the boiler
and were used to configure the number and locations of
reburn  burners and OFA ports in Unit 2 (Farzan et al.,
1991).  Four "S" type burners and four OFA ports were
retrofitted to Unit 2. A B&W MPS-67N pulverizer with a
dynamic classifier, rotating throat, and automatic spring
adjustment system was installed to provide the pulver-
ized coal for the reburn system (Newell  et al., 1993). A
schematic of the reburn system is presented  in Figure
3-16.
Cyclone-firing was reduced from 100% of the total fuel
input to a range of 65 to 80%, and the remaining coal
was introduced in the reburn zone downstream at sub-
stoichiometric  conditions. Temperatures  in the reburn
zone were approximately 2500°F to minimize the forma-
tion of atmospheric NOx from the addition of excess air.

NOx reductions for the firing of Illinois Basin coal ranged
from 33 to 50% over loads ranging from  40 MW to full
load at 110 MW (Figure 3-17). The test objective of 50%
reduction  in  NOx emissions was met at full load; how-
ever,  emissions "reductions diminished at loads below
80 MW. At the minimum test conditions of 40 MW, the
reduction  in  NOx  emissions was only 33%. The lower
reduction at  low loads was attributed to flame instability
of the Illinois coal at a reburn zone stoichiometry of 0.9
                                                             Furnace Enclosure
            Reburn Burners Flue
            Gas Recirculation Duct
                 Hot Primary Air
                 Fan and Motor
                                                                             B&W Dual Zone
                                                                             Overfire Air Ports
                                                                             B&W S-Type
                                                                             Reburn Burners
           B&W Cyclone Furnaces


       Gravimetric Feeder


    B&W MPS Pulverizer
Figure 3-16. Nelson Dewey Unit 2-Coal-Fired Reburn System Schematic (Newell et al., 1993).
                                                  40

-------
or less. With the returning system in operation, NOx emis-
sions as low as 250 ppm (0.34 Ib/MMBtu) were achieved.
The fuel input from the pulverized coal burners was at
34% and the reburn zone stoichiometry was 0.89.

NO reduction was enhanced when burning Powder River
Basin coal. The overall NOx reduction was greater (62%),
which was achieved at a'lower reburn fuel heat input
(30%) and a higher  reburn zone stoichiometry. The re-
ductions were consistent over the full range of loads
tested (Figure 3-18). This insensitivity to load was attrib-
uted to the flame  stability when burning Powder River
Basin coal, even at lower unit loads with a sub-stoichio-
metric environment.

Several parameters were evaluated during this reburn
retrofit demonstration to determine the effect of reburning
on the overall power plant. These parameters included
precipitator opacity, slagging and fouling, corrosion, tube
temperatures, exit gas temperatures, carbon burnout,
and hazardous air pollutants. A summary of the effects
of the reburning retrofit on the various parameters is pre-
sented in Table 3-3.  None of the evaluated parameters
were severely upset as a result of the retrofit. In some
cases, boiler performance was actually improved due to
retrofit conditions, such as a reduction  in slagging and
fouling. More importantly,  the reburn system was oper-
                                    ated automatically and the boiler controls could com-
                                    pensate for cases of a pulverized coal reburn system
                                    shutdown.

                                    As of July 1994, the pulverized coal reburn system had
                                    been in service for more than 2500 hours. Only two forced
                                    outages had occurred as a result of the retrofit. WP&L
                                    plans on continuing the firing of Powder River Basin coal
                                    in the reburn system. This system allows WP&L to meet
                                    NOX emission reduction goals while  maintaining the
                                    boiler's rating and burning low-sulfur coal to meet SO2
                                    emissions guidelines.


                                    Ohio Edison - Niles Unit 1

                                    Ohio Edison's Niles Generating Station was the site of a
                                    reburn system demonstration sponsored by Ohio Edison,
                                    EPA, GRI, EPRI, DOE, Ohio Coal Development Office,
                                    East Ohio Gas, and ABB Combustion Engineering. The
                                    information presented in this section was compiled from
                                    a paper titled "Long Term N0x Emissions Results with
                                    Natural Gas Reburning on a Coal-Fired Cyclone Boiler"
                                    (Borio  et al., 1993). Parametric and long-term testing
                                    were conducted as part of this research and develop-
                                    ment project on the feasibility of utilizing natural gas
                                    reburning to reduce NOx emissions from a cyclone-fired
                                    utility boiler.
            £
            "8
            8
             i

            I
                  700
                  600  ~
                  500   -
400  -
                  300  =
                  200
                                                ••'      	
                                   Baseline Operation
                'Reburn Operation
                                 I	T
                      •  50% Redufction @ Full Ldad

                                 i

                         ,  ,  ,   I  ,   ,   ,   I   ,   ,   ,
                                                                0.95
                                                                               -  0.78
                                                                               -  0.61
                                                                      1
                                             X
                                            i
                                                                               - 0.44
                                                                0.27
                     20
               40
60          80

Unit Load (MW)
100
120
Figure 3-17. Nelson Dewey Unit 2-NO, Emissions vs. Unit Load - Illinois Basin Coal (Newell et at., 1993).
                                                  41

-------
ouu
525
_f 450
CO
0
S> 375
d
1
5 300
X
o
225
150
f
. ' ' '
Baselir
	
	
. . . j , . ,
i 	 j 	
e Operation
r T
i 	 i

__ 	 — ,_ i
Reburn Operation
	 . 	 	
!»- — •<
i
',,,!.,.

	
- ' 50% Re
, , ,
' ' '
i 	 	 	 -j

i 	 n
i 	 i
duction @ Full
*rr.-:v_
, , ,
• . i

_
_
_______
_
_
.oad 	 z
	 *
-
, , ,
>0 40 60 80 100 12
Unit Load (MW)
                                                                                    0.82
                                                                                 -  0.66
                                                                                 -  0.51
                                                                                 -  0.36
                                                                                    0.2
Figure 3-18. Nelson Dewey UriA2-NOl Emissions vs. Unit Load - Powder River Basin Coal (Newell et a/., 1993).
Unit 1 is a 114-MW, cyclone-fired, pressurized, natural-
circulation boiler. The four cyclone burners fire eastern
bituminous coal in a single-wall fired furnace. A sche-
matic of the boiler is shown in Figure 3-19. Combustion
products  from the cyclone burners pass down through
the primary furnace-pass screen tubes. Five natural gas
injectors were installed in the lower portion of the sec-
ondary furnace. Reburn fuel is injected under sub-sto-
ichiometric conditions and allowed to react with the com-
bustion products. OFA is injected toward the top of the
secondary furnace to ensure carbon burnout. The flue
gas then  enters the boiler's convective passes.

The original design for this demonstration utilized FGR
to facilitate mixing in the reburn zone. However, during
parametric field testing, ash deposits on the  furnace's
back wall were found to be up to four times thicker than
in normal boiler operation. Although NOx emission re-
ductions were not affected, the thicker ash deposits were
an unacceptable furnace condition, and the reburn sys-
tem was redesigned to operate without FGR. "Proof-of-
performance" testing showed that operating the reburn
without FGR eliminated the ash deposition problem. The
N0x emissions were slightly  higher for the modified sys-
tem", but remained within an acceptable range of the para-
metric test results.
The original design for the reburn system operation was
for a reburn fuel heat input of 16% of total boiler heat
input at loads of 80 MW or greater. For loads of less
than 80 MW, the reburn heat input was to be proportion-
ally reduced, reaching 0% at loads of 65 MW or less.
These  design considerations for reburn fuel heat input
for loads less than 80 MW were  not applied because of
the need to maintain above the  minimum furnace tem-
perature requirements for slag tapping  in the cyclone
burners.  During the long-term testing, the reburn sys-
tem was utilized only at loads of 80 MW or greater due
to "operator judgment" on the basis of slag tapping re-
quirements.

During this testing,  the reburn section heat input was at
16% of total heat input for approximately 50% of the tests,
with the remaining tests run at between 3% and 16% of
total heat input. The reburn zone was operated with a
stoichiometry of approximately 0.94. Absolute NOx emis-
sions increased linearly with increasing  reburn stoichi-
ometries for tested  load ranges (Figure 3-20). The gen-
eral trend of greater absolute NOx emissions at higher
loads is offset by greater reductions from the baseline at
higher  loads. The reburning system effectively capped
the level of NOx emissions to 0.26 tons/hr for all loads
tested (Figure 3-21).
                                                  42

-------
Table 3-3.  Nelson Dewey Unit 2 - Summary of Effects of
          Reburning on Unit Operating Parameters
Parameter
Anticipated Results
Actual Results
NO, Emissions (Full
Load) Illinois Basin
Coal

NO, Emissions (Full
Load) Powder River
Basin Coal
Reduced 50% or more
Reduced 50% or more
Precipitator Opacity   Up 5 to 10%
Slagging/Fouling
No Change
Furnace Corrosion    No Change

Header/Tube Temps   Higher 25 to 50°F
Furnace Exit Gas
Temp

SH i RH Sprays

Carbon Carry-over
Illinois Basin Coal

Carbon Carry-over
Powder River Basin
Coal

Hazardous Air
Pollutants*
Higher by 50 to 75°F


Higher by 30%

Higher by 10 to 15%


Higher by 10 to 15%



No change
Nominal 55%
reduction
Nominal 61%
reduction
No increase
from base

Cleaner than
normal

No change

No increase
from base

Reduced by
100to150°F

50% of base

Higher by 10
to 15%

No change
No change
'Arsenic, beryllium, cadmium, chromium, lead, nickel, manganese, selenium,
 mercury, benzene, toluene, HF, and HCI.

Source:  Newell et a!., 1993
As mentioned above, the original reburn system design
involved the use of FGR to improve mixing of the reburn
fuel and combustion gases and to cool the reburn fuel
burners. The eventual long-term testing design did not
utilize FGR. As a result of this redesign, significant sav-
ings were gained in capital cost.

The original design with FGR required a windbox pen-
etration of 6 ft2 for each of the five injectors, as well as
the bending of 12 tubes out of plane. The redesign with-
out FGR required a windbox penetration of only 0.2 ft2
for each of five injectors,  and the bending of two tubes
out of plane. Water was chosen as the reburn injector
cooling medium in place of the flue gas. In addition, vari-
ous equipment such as a recirculation fan, controls, sec-
tions of ductwork, and a motor were no longer needed
for the retrofit. Elimination of FGR from the reburn sys-
tem would result in an estimated reduction in required
capital of 30%. While this retrofit was successful  in re-
ducing NOx emissions without the  use of FGR, boilers
with different flow patterns in the reburn zone may re-
quire FGR for adequate mixing in the reburn section.
Because some NOX reduction efficiency was lost in the
removal of the recirculated flue gas, attempts were made
to return to the original reduction levels. It was thought
that the natural gas reburn fuel potentially was forming
soot as it was injected into the reburn zone without dilu-
tion by recirculated flue gas or combustion air. Soot for-
mation does not reduce NOx as well as the hydroxyla-
tion reaction which forms CH radicals. Water was injected
with the reburn fuel to minimize soot formation and pro-
mote the hydroxylation reaction in the reburn zone. No
changes in NOx emissions reduction performance were
achieved, thus water was eliminated from the reburn fuel
injection.

Waterwall tube thicknesses  were measured ultrasoni-
cally before and  after the test program to  detect any
wastage. No significant increase in wastage was ob-
served. Ultrasonic measurements indicated that corro-
sion in the upper areas of the secondary  furnace were
similar to its normal patterns. The superheater did show
signs of increased wastage with the higher temperatures.
Corrosion was lowest for those metal areas  with in-
creased concentrations of chromium.

The test program has been  completed and the reburn
system was removed in August  1992. Based on the load-
cycle history of Unit 1, the annual reduction in NOX emis-
sions would be much less than the 47% achieved during
the 3-1/2 months of testing. The facility reported that the
actual  NO^ emissions reduction over the 3-1/2 month
testing period, when accounting for all hours of opera-
tion with or without reburning, was approximately 10%.
A major factor in the overall low average was minimum
ash fusion temperatures that impeded load following for
the reburn system (Kanary, 1993). Suggestions for em-
ploying the  reburn technology included  (Borio et al.,
1993):

  • Accurately  control the air/fuel mixtures  to the cy-
    clones;

  • Eliminate the need for FGR by increasing the num-
    ber of natural gas (reburn fuel) injectors;

  • Use stainless in water-cooled reburn fuel guidepipes
    to prevent the corrosion that was experienced; and

  • Use a lower fusion temperature coal to increase the
    load range at which the reburn system could oper-
    ate.
                                    Ladyzhin Power Station - Unit 4

                                    Under a joint program sponsored by EPA, and the na-
                                    tions of Russia and Ukraine, a 300-MW, opposed-wall
                                    fired, wet-bottom boiler was retrofitted with a natural gas
                                    reburn system. The objective of the test was to deter-
                                    mine the effectiveness of reburn technology in reducing
                                    NO emissions by at least 50% while minimizing any
                                                   43

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                                                       Superheat/Reheat
                                                       Corrective Passages
                      Gas
                      Recirculation
                      Fan
                               Upper Fuel
                               Injectors
                                                                 Screen Tubes
Figure 3-19. Niles Unit 1-Schematic of Reburn Process (Bono etat., 1993).
                                                            44

-------
                800
                700
                600
            OJ
           0    500
           CO
            X
           O
                400
                300
                200
                                                                  	June 1-June 12, 1992
                    0.8
         0.9
                                                      1.0              1.1
                                                 Reburn Zone Stoichiometry

Figure 3-20.  Niles Unit 1-Variation ofNOx with Reburn Stoichiometry (Borio et a/., 1993).
1.2
1.3
                       CN
                      O

                      CO
                      co
                            0.6
                            0.5  ~
0.4  -
                      &    0.3  -
                       co
                       o
                      'co
                       to
                       E
                      ULJ
                       O
0.2  -
                            0.1   -
                                  35     45     55     65     75     85     95     105    110

                                                        Load ( MW Gross)


Figure 3-21.  Niles Unit 1-NOt Emissions as a Function of Boiler Load (Borio et a/., 1993).
                                                          45

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detrimental impact from the retrofit. The information pre-
sented in this section was compiled from a paper titled
'Three-Stage Combustion (Reburning) Test Results from
a 300 MWe Boiler in the Ukraine" (LaFlesh  et al., 1993).

The boiler that was chosen as a host site is typical of at
least 300 other units in Russia and Ukraine. The boiler,
Unit 4, was located at the Ladyzhin Power Station near
Vinnitsa, Ukraine. The boiler typically fires a high vola-
tile, high ash, Ukrainian, bituminous coal (25 to 35% ash
content); a low-ash, Siberian, brown lignite coal (4 to
10% ash content); or a blend of these fuels. An analysis
of the coals is shown in Table 3-4.

Baseline NOX emissions ranged from 370 to 730 ppm
depending on various operating factors.  ABB Combus-
tion Engineering, under contract to EPA, provided a con-
ceptual reburning system design, with the Russian and
Ukrainian teams completing all other portions of the fab-
rication and testing. ABB Combustion Engineering's de-
sign was based on cold-flow modeling, computer mod-
eling, analysis of engineering drawings, and results of
the Ohio Edison Niles Unit I demonstrations program
(cited previously).
Table 3-4.  Ladyzhin Unit 4 - Fuel Analyses
Parameter
   High Volatile
Bituminous C -Donetz
Siberian Lignite
Kansko-Achinski
Proximate Analysis
Moisture, %

Volatile Matter, %

Fixed Carbon, %

Ash, %
      12.0

      22.2

      30.6

      35.2
      33.0

      29.9

      32.4

       4.7
Ultimate Analysis
Moisture, %
Carbon, %
Hydrogen, %
Sulfur, %
Oxygen, %
Nitrogen, %
LHV, Btu/lb
12.0
40.1
3.0
2.9
6.0
0.8
6,864
33.0
43.7
3.0
0.2
13.5
0.6
6,738
Critical Temperatures
Initial Deformation, °F
Softening, °F
Fusion, °F
2,190
2,440
2,520
2,320
2,350
2,398
Source: LaFlesh etal., 1993
The Ladyzhin Power Station has six 300-MW, TPP-312
boilers. These supercritical steam pressure units (3625
psig) each have 16 opposed-wall, swirl-stabilized burn-
ers and operate under slagging conditions. The slag
makes up 20 to 30% by weight of the total ash, and is
tapped at the bottom of the furnace. The fly ash is re-
moved from the flue gas by electrostatic precipitators.

A1/16-scale model was used to conduct isothermal flow
modeling of the Ladyzhin unit. The model was used to
optimize parameters such as configuration, size, loca-
tion, number, and operating values for the reburn burn-
ers and OFA injectors. Burners and OFA injectors were
assumed to be located on either the front or back wall
due to equipment obstructions on the side walls. In ad-
dition, estimates were made on the potential flue gas
velocities within the furnace.

Preliminary design configurations were modeled on a
computer in two parts. First, a reburn configuration was
evaluated independent of OFA considerations. Then, the
selected reburn configuration was tested with varying
OFA configurations. The input parameters are shown in
Table 3-5.

Parameters of interest in the analysis included  exit gas
temperature, furnace hopper gas temperature,  and fur-
nace heat absorption profile. The output of the computer
model included furnace gas temperature profiles and
furnace absorption profiles. Operational parameters such
as excess air, FGR rate, and reburn heat input were ana-
lyzed for optimal thermal performance. The values se-
lected from the computer modeling are presented in Table
3-6. A schematic of the preliminary design is shown in
Figure 3-22.

One change was made to the system after the reburn
system was designed and, thus, was independent of
considerations for the reburn retrofit. An aerodynamic
"nose" was fitted to improve a problem with heat trans-
fer in the boiler's convective section. This change does
not appear to have had any significant effect on the
reburn retrofit.

Prior to the retrofit, NOx emissions averaged 600 ppm
while at a load of 300 MW (4% O2 at economizer outlet)
and firing a blend of 90% Ukrainian coal and 10% Sibe-
rian lignite. Parametric tests were able to reduce NO
emissions to as low as 240 ppm at a reburn heat input of
15%. NOx emissions decreased as reburn heat input
percentage increased (Figure 3-23). Decreasing excess
air (shown as flue gas O? content after the economizer)
also reduced NOX emissions (Figure 3-24). The reburn
system was operated over a load range of 200 MW to
300 MW. Absolute values of NOx emissions had a linear
relation to increasing load as shown in Figure 3-25. Para-
metric testing showed that at loads of 200 MW to 300
MW, the  reburn system generally was able to reduce
NO emissions by 40 to 60% (240 to 360 ppm) from a
                                                  46

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Table 3-5.  Ladyzhin Unit 4 - Flow Diagram for Boiler Combustion Performance Model


       Inputs
Mathematical
   Model
Source:  LaFlesh et al., 1993
Table 3-6.  Ladyzhin Unit 4 - Furnace Thermal Performance Summary
Outputs
Fuel Information
• Particle Size Distribution
• Apparent Density
• Chemical Characteristics
• Ash Characteristics
Drop Tube Furnace System Information
• Char Activation Energy
• Char Frequency Factor
• Fuel Swelling Factor
• Fuel Volatile Matter
Boiler Information
• Operating Conditions








Proprietary
Computer
Code








• Temperature/Time History

Efficiency
• % Carbon Heat Loss
Profile

Performance Variables Units
Reburn Fuel Ratio %
Total Excess Air %
Burner Zone Excess Air %
Total FGR %
Reburn FGR %
Upper Furnace FGR %
Furnace Exit Gas Temp °F
Furnace Heat Absorption MMBtu
Baseline as
Found
NA
20
20
18
NA
3.2
2,028
606
Preliminary
Rebum
Case
20
20
20
18
10
3.2
2,028
609
Optimum
Reburn
Case
12
20
5
21
7.5
8.7
1,949
625
Source:  LaFlesh etal., 1993
                                                           47

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                                              ±25°
                                               Tilt
                                             ±25°
                                              Tilt
FGR Nozzles (6)


Terciary Air
(Burnout) Nozzles
(6 Front, 6 Rear)

Reburn Fuel and
FGR Injectors
(6 Front, 6 Rear)
                                                            Main Coal Burners
                                                            (8 Front, 8 Rear)
                                       Preliminary Proposal
15° X
Fixed
-15°
Fixed

•^ rvjn iiU£.£.ieo ^o;
.< Burnout Air Nozzles (5)
(5 Front, 5 Rear)
^... Reburn Fuel and
FGR Injectors
(5 Front, 5 Rear)
>Main Coal Burners
(8 Front, 8 Rear)
                                     Final Design Arrangement
Figure 3-22. Ladyzhin Unit 4-Schematic of Reburn Design Arrangements (LaFlesh et at., 1993).
                                                        48

-------
                  CM
                 o
                  o
                  O
                 O
400


350


300


250


200


150


100


 50


  0
                                                          300 MW
                                                          Baseline NOX - 600 ppm
                                  I  I I  I I I  I I  I  I I  I I I  I I I  I I  I I I  I I  I
                                        4            8            12

                                         % of Total Heat Input as Reburn Fuel
                                                        16
Figure 3-23. Ladyzhin Unit 4-NOM Emissions vs. Reburn Fuel Percentage (LaFlesh et at., 1993).
baseline of 600 ppm, with an average NOx reduction of
just over 59%.

As a slagging boiler, Ladyzhin Unit 4 experienced some
problems with maintaining fluid slag at reduced loads
when a significant fraction of the total heat input to the
boiler was directed to the reburn burners. At Ladyzhin,
slag tapping was affected at loads below 200 MW. Slag
tapping was unaffected at loads between 200 and 300
MW. Furnace operators commented that the boiler was
"more controllable" after the retrofit.

FGR was used as a carrier gas for the reburn fuel, and
to maintain burner metal temperature at 1472°F or less.
Unburned carbon in the fly ash increased 1 % after the
retrofit. CO levels were maintained at 250 ppm or less,
                               with additional reductions expected with long-term test-
                               ing.

                               Unit 4 is operating the reburn system for long-term test-
                               ing to optimize operational parameters and evaluate vari-
                               ous primary fuel compositions. Consideration is being
                               given to installing multi-fuel reburn fuel injectors in a new
                               reburn system design for Ladyzhin boiler No. 6. The de-
                               sign is being done by EER,  under contract to the EPA.
                               Partners include U.S. AID, and the U.S. Department of
                               Energy. The multi-fuel system will be capable of firing
                               natural gas, oil, or coal. This capability would be very
                               important  in Ukraine due to potential fuel shortages.
                               Ladyzhin plant personnel would like to install reburn ca-
                               pability on all six units, as funding is available.
                                                   49

-------
                 CO

                  o
                       400
                       350
                       300
                       250
                 ,3    200


                  Q.
                  a.

                  x    150
                 O
                       100
                        50
300 MW
12% of Total Boiler Heat
Input as Reburn Fuel



All Burnout Air and Reburn FGR

Dampers 100% Open
                                                  234


                                                     02% After Economizer
Figure 3-24. Ladyzhin Unit 4-NOt Emissions vs. Flue Gas Oxygen Content.



                       500
                       400
                   S   300
                   o


                   s
                   o


                   a   200
                    X
                   o





                       100
                                                       I
                                                      I
                                                             I   I   I   I
                         200
     220
240          260


      MW
280
300
Figure 3-25. Ladyzhin Unit 4-NOf Emissions vs. Boiler Load.
                                                         50

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                                              Chapter 4
                                        Process Economics
Costing Methodology

Estimates of the capital and operating costs of using the
reburning process to  reduce  NOx emissions are pre-
sented in the following section. A synopsis of the proce-
dures by which these  costs were converted to busbar
and cost-effectiveness estimates is also provided. The
cost estimation methods closely follow the procedures
used in the EPA Alternative Control Techniques (ACT)
Document — NOx Emissions  from Utility Boilers (U.S.
EPA, 1994), the general methodology contained in the
EPRI Technical Assessment Guide (TAG) (EPRI, 1986),
and the EPA's Office of Air Quality Planning and Stan-
dards (OAQPS) Costing Manual (U.S. EPA, 1990). The
general framework for handling capital and annual costs
is shown in Table 4-1. All costs, except where noted, are
presented in 1991 dollars.

Because of the limited economic data on coal-fired reburn
systems, the quantitative cost analyses are limited to gas-
fired reburn installations; however, discussions of cost
factors related to coal-fired  reburn systems are also pre-
sented.
Capital Costs

The estimated total capital cost of a reburn system in-
cludes both direct and indirect costs. Direct costs include
both costs for the basic system installation and for the
retrofit needs. Indirect costs are based on a percentage
of the direct costs and include several costs associated
with the design and engineering of the system.

Typical capital  costs for the installation of a reburn sys-
tem involve reburn fuel equipment, boiler modifications,
and particulate control device modifications (if required).
If the reburn fuel is coal, significant adjustments may be
required for the handling and preparation of the fuel, in-
cluding the addition of a pulverizer. Fuel preparation costs
are not required for natural gas-firing; however, installa-
tion of new gas supply lines can be extremely costly if
no existing gas line to the plant is available or if the exist-
ing line has inadequate capacity. Boiler modifications
include the penetration of boiler walls to install reburn
fuel injectors and OFA ports. Modification or replacement
of existing burners typically is not necessary, but may be
included in an overall NOx emission reduction program.
Additional fans and ductwork are also necessary for flue
gas recirculation and overfire air systems. Installation of
reburn systems also often includes upgrade of the boiler
control systems to include the new fuel and combustion
air controls to ensure safe start-up, shut-down, and trip
conditions. Modifications to the particulate  control de-
vices may be necessary to control the increased amount
of fly ash produced when coal is used as a reburn fuel in
a wet-bottom boiler.

Basic System Cost

The basic reburn system cost is the cost of  purchasing
and installing the system hardware directly associated
with the control technology. This cost reflects the costs
of the basic system components for a new application,
but does not include any site-specific upgrades or modi-
fications to existing equipment that may be  required to
implement the control  technology at an existing plant
(e.g., new igniters, new burner management system, and
waterwall or windbox modifications). Any reburn system
start-up/optimization tests are also included in basic sys-
tem cost. Note: The costs of purchasing and installing
any continuous emission monitoring  (CEM) equipment
that may be required for determining compliance  with
state and federal emission limits are not included in the
analysis.

The data used to estimate basic system cost were com-
piled in the ACT document (U.S. EPA, 1994) from utility
questionnaires, vendor information, published literature,
and other sources. These cost data were then compiled
in a data base, examined for general trends in capital
cost versus boiler rating, and statistically analyzed us-
ing linear regression to fit a functional form of:
               BSC = a • MWb
(4-1)
                                                  51

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Table 4-1.  Capital and Operating Cost Components
Total Capital
Cost
Total O&M
Cost
Direct Cost
Basic System
Cost
Retrofit Cost
Indirect Cost
Fixed O&M Cost
Variable O&M Cost
Basic equipment
Initial chemicals/
catalyst
Installation
Start-up/optimiza-
tion testing
Scope adders
Work area
congestion
General facilities
Engineering
Royalty fees
Project contin-
gency
Process contin-
gency
Operating labor
Maintenance labor
Supervisory labor
Maintenance
materials
Energy penalty
Chemicals/catalyst
Electricity
Water
Waste disposal
where:

  BSC = Basic system cost ($/kW)

  a    = Constant derived from regression analysis

  MW = Boiler size (MW)

  b    = Constant derived from regression analysis

The basic system cost was then derived using Equation
4-1 and the calculated values of "a" and "b".

Retrofit Cost Factor

In comparison with installation on a new unit, installation
of NOx controls on an existing boiler typically involves
additional cost categories. These additional cost catego-
ries comprise the system retrofit cost. Retrofit costs are
related to upgrades and modifications to the boiler that
are required  for the NOx control system to operate as
designed. These modifications and upgrades can  in-
clude:

  • Igniters modification or replacement;

  • Waterwall modifications;

  • Flame scanners;

  • Coal pulverizer modifications;
  •  Boiler control modifications;

  •  Burner management modifications;

  •  Coal piping modifications;

  •  Windbox modifications;

  •  Structural modifications;

  •  Asbestos removal;

  •  Insulation modifications;

  •  Electrical system modifications;

  •  Flue gas recirculation fan modifications; and

  •  Demolition.

Additional costs are incurred when accessibility is re-
stricted or work space is limited by the existing equip-
ment configuration. All of these factors are included in a
retrofit factor that is based as a percentage of the basic
system cost as presented below in Equation 4-2.
                 RC
                BSC
                                                                                                     (4-2)
                                                       where:
    RF = Retrofit factor (dimensionless);

    RC = Retrofit cost ($/kW); and

    BSC = Base system cost ($/kW).

For example, a retrofit factor of 1.3 indicates that the
retrofit cost is 30% of the basic system cost. Retrofit fac-
tors were developed based on cost data for planned or
actual reburn installations on existing utility boilers. The
cost data were also used to estimate low, medium, and
high retrofit factors for the model boiler analysis, which
are listed below:

  • A low retrofit factor of 1.0 is used for a new unit or a
    retrofit that requires minimal or no upgrades or modi-
    fications, and if no difficulties are associated with
    accessibility;

  • A medium retrofit factor is used for moderate equip-
    ment upgrades or modifications and/or if some diffi-
    culties exist that are associated with accessibility;
    and

  • A high retrofit factor indicates that extensive scope
    adders are required and/or limited accessibility and/
    or work space also may be available.

Gas-fired reburn retrofit costs are primarily due to modi-
fications and upgrading of existing equipment. Require-
ments for accessibility and work space are minimal for a
                                                   52

-------
gas-fired reburn retrofit since burners and overfire air
ports typically can be installed from inside the boiler. Coal-
fired reburn retrofits can incur significant costs associ-
ated with greater accessibility and work space require-
ments than required for  gas-fired retrofits. Gas-fired
reburn systems typically are estimated with a low to me-
dium retrofit factor while coal-fired reburn systems typi-
cally are estimated with a medium to high retrofit factor.

The total direct cost was estimated by multiplying the
basic system cost by an appropriate retrofit factor.
       TDC = BSC • RF
(4-3)
where:
  TDC = Total direct cost ($/kW);

  BSC = Basic system cost ($/kW); and

  RF  = Retrofit factor (dimensionless).

Indirect Cost Factor

The indirect cost includes the costs of general facilities,
engineering expenses, process royalty fees (if any), and
contingencies. General facilities include offices, labora-
tories, storage areas,  or other facilities required for in-
stallation or operation  of the control system. Examples
of general facilities required by installation of a reburn
system include expansion of the boiler control room to
house new computer cabinets for the boiler control sys-
tem and expansion of an analytical laboratory.

Engineering expenses include the utility's internal engi-
neering efforts as well as an architect/engineer (A&E)
contractor.  Engineering costs incurred by the  technol-
ogy vendor are included in the equipment cost and are
considered direct costs.

A process royalty fees  is a fee paid to the developer of a
patented process technology in return for permission to
use this technology. For example, a company may hold
a patent on a  unique process for reducing the volume of
flue gas  recirculation  gas required to attain adequate
mixing of the reburn  fuel and  combustion gas in the
reburn zone, and the patent-holder may charge a fee for
use of this technology. In some cases, especially where
the patent is for a specific piece of equipment, this fee
may be included in the capital cost of the equipment.

Contingencies are factors that account for the uncertainty
associated with cost estimation (project contingency) and
the maturity of the technology (process contingency).
Project contingency is assigned based on the level of
detail in the cost estimate.  The total capital cost must
include the costs of miscellaneous equipment and ma-
terials not included in  the direct cost estimate. Project
contingencies range from 5 to 50% of the direct costs,
depending  on the level of detail included in the direct
cost estimate, with lower contingencies associated with
more detailed cost estimates.  Process contingency is
based on the maturity of the technology and the number
of previous installations. Process contingency represents
unforeseen  expenses potentially incurred because of
inexperience with newer technologies. Process contin-
gencies range from 0 to over 40% of the direct costs,
with higher contingencies associated with less mature
technologies.

As shown in Equation 4-4, an indirect cost factor accounts
for the indirect costs  as a percentage of the total direct
cost:
                                                             ICF =
                                                                         1C
                                                                                                   (4-4)
                        BSC + RC

        where:

          ICF  = Indirect cost factor (dimensionless);

          1C   = Indirect costs ($/kW);

          BSC = Basic system costs ($/kW); and

          RC  = Retrofit costs ($/kW).

        For example, an indirect cost factor of 1.3 indicates that
        the indirect costs are 30% of the total direct cost (basic
        system cost plus retrofit cost). The indirect cost factors
        are based on cost data from planned and actual installa-
        tions of reburn systems on various boilers.

        Finally, the total capital cost is calculated by multiplying
        the total direct cost by the ICF.

               TCC = (BSC + RC) • ICF                (4-5)

        where:

          TCC = Total capital cost ($/kW);

          BSC = Basic system cost ($/kW);

          RC  = Retrofit cost ($/kW); and

          ICF  = Indirect cost factor (dimensionless).


        Operating and Maintenance Costs

        Operating and maintenance (O&M) costs include fixed
        and variable O&M components. Fixed O&M costs include
        operating, maintenance, and supervisory labor; mainte-
        nance  materials; and overhead. Fixed O&M costs are
        assumed to be independent of the boiler capacity factor
        (i.e., the magnitude of these costs are the same at 50%
        unit load and 100% unit load). Variable O&M costs are
        dependent on the boiler capacity factor and include any
        costs incurred from energy penalties (e.g.,  boiler effi-'
                                                  53

-------
ciency losses associated with the use of natural gas as
a reburn fuel), electrical power consumption, and waste
disposal.

Fixed costs were not included in the analysis under the
assumptions that:

  •  Very few moving  parts are needed  for gas-fired
    reburning; and

  •  Operating labor and maintenance requirements are
    expected to be very low for gas-fired reburning.

Cost rates for variable  O&M cost estimates are listed in
Table 4-2. The prices listed for coal and natural gas are
estimated national average prices for the year 2000,
based on the reference case analysis in the DOE's 1992
Annual Energy Outlook (U.S. DOE, 1992). Prices for solid
waste and electricity are listed in 1989 dollars.

The primary factor when determining variable O&M costs
for  reburn systems is the cost of the reburn fuel com-
pared to the cost of the primary fuel it replaces. This cost
is a major concern with gas reburn, as the cost of natu-
ral gas is typically $1 to $1.50 per million Btu (MMBtu)
greater than the price of coal. A small heat rate penalty
also is associated with gas reburn. However, this pen-
alty may be offset by energy savings in other areas, such
as a reduction in the energy needed to process the coal
that has been replaced by gas. The additional fuel costs
were calculated with the fuel prices listed  in Table 4-2.

Variable O&M costs also include the savings gained from
sulfur dioxide (SO2) credits because of lower SO2 emis-
sion levels when using natural gas-fired reburn on a coal-
fired boiler. The SO., emissions were calculated with typi-
cal  sulfur and calorific  content of coal (U.S. EPA 1994)
and an average AP-42 emission factor for bituminous
and subbituminous coal (U.S. EPA, 1985b). The SO2
credit was assumed to be $200/ton of SO2 (Sanyal  et
al.,  1992). The equation to determine savings from SO2
credits is:

       Savings = EF* Sulfur -MW-HR-       (4-6)

       CF-Credit- Reburn-2.19

where:

  Savings = Savings due to SO2 credits ($/yr)

  EF     = AP-42 SO Emission Factor (Ibs SO^ton
            coal/% sulfur in coal);

  Sulfur   = Sulfur (%);

  MW    = Unit size (MW);

  HR     = Boiler net heat rate (MMBtu/kWh);

  CF     = Annual capacity factor (decimal fraction);
     Credit   = SO2 credit ($/ton);

     Reburn  = Heat input of reburn fuel fired divided by to-
               tal boiler heat input (decimal fraction); and

     2.19    = Unit conversion factor.
   Table 4-2.  Variable O&M Unit Costs
Fuel
Coal
Natural gas
Solid Waste
Electricity
Cost
1.74
3.27
9.50
0.05
Unit
$/MMBtu
$/MMBtu
$/ton
$/kWh
Reference
U.S. DOE 1992
U.S. DOE 1992
EPRI 1986
EPRI 1986
   Busbar Cost and Cost-Effectiveness

   Busbar cost (mills/kWh) is defined as the sum of annu-
   alized capital costs and total O&M costs ($/yr) divided
   by the annual electrical output of the boiler (kWh/yr),
   which provides a direct indication of the cost of the reburn
   system to the utility and its customers. To convert total
   capital cost to an annualized capital charge, the total
   capital cost is multiplied by an annual capital recovery
   factor (CRF). The CRF is based on  the economic life
   over which the capital investment is amortized and the
   cost of capital (i.e., interest rate). The  CRF is calculated
   using the following equation:
           CRF =
                                                (4-7)
     i  = Interest rate (decimal fraction) [assumed to be
         0.10 (i.e., 10%)]; and

     n = Economic life of the equipment (years);

   Cost-effectiveness values indicate the total cost of a con-
   trol technology per unit of NOX removed and are calcu-
   lated by dividing the total annualized capital charge and
   O&M expense by the annual reduction in tons of N0x
   emitted from the boiler.
   Cost Analysis

   Cost estimates for a gas-fired reburn system are pre-
   sented in this section. These estimates are based on.
   systems installed on wall-, tangential-, and cyclone-fired
   boilers burning coal as the primary fuel. Limited cost
   data on natural gas-fired reburn for coal-fired boilers
54

-------
were obtained from vendor and utility responses to a
questionnaire. In response to this questionnaire, Illinios
Power submitted cost data for the reburn retrofit on the
75-MW Hennepin Unit 1 boiler; and EER  provided in-
stallation costs for retrofitting the reburn systems on the
33-MW City Water,  Light, and Power Lakeside Unit 7
boiler and the 172-MW Public Service of Colorado Chero-
kee Unit 3 boiler (U.S. EPA, 1994). A regression analy-
sis of the data showed a high degree of scatter and no
obvious costing trend. Reburn costs were based on the
Cherokee Unit 7 cost data because this unit is most in-
dicative of a typical small utility boiler. Sufficient data were
not available to  perform a cost analysis for coal-fired
reburn systems.

The economy of scale was  assumed to be 0.6 for the
gas-fired reburn basic cost algorithm. With this assump-
tion, the cost coefficients in Equation 4-1 for reburn are:

  a = 229; and

  b = -0.40.

The cost of installing a natural gas pipeline was not in-
cluded in the analysis because it is highly dependent on
site-specific parameters such as the unit's proximity to a
gas line and the difficulty of installation.

In their response to the questionnaire,  EER indicated
that the retrofit of a gas-fired reburn system would cost
10 to 20% more than a reburn system applied to a new
boiler. With this assumption, the retrofit factor was as-
sumed to be 1.15 (Jensen, 1993). However, for the sen-
sitivity analysis, the retrofit factor was varied from 1.0 to
1.6 to account for different retrofit difficulties on specific
boilers.

The indirect costs were estimated to be 40% of the total
direct cost, resulting in an indirect cost factor of 1.40 (U.S.
EPA, 1994).

Annual O&M costs included both additional fuel costs
from the higher price of natural gas versus coal, and
utility savings on SO2 credits from lower SO2 emission
levels when using natural gas-fired as the reburn fuel on
a coal-fired boiler. The analysis was conducted assum-
ing 18% of the total heat input was from natural gas. The
SCX credit was assumed to be $200 per ton of SO2, equal
to $0.24/MMBtu based on a coal-sulfur content of 1,5%
(U.S. EPA, 1994).


Model Plants

To estimate the capital cost, busbar cost,  and  cost-ef-
fectiveness of natural gas-fire reburn, a series of model
plants were developed. These model plants reflected the
projected range of size, duty cycle, retrofit difficulty, eco-
nomic life, uncontrolled NOX emissions, and controlled
NO emissions for each major boiler type.
The capital cost, busbar cost, and cost-effectiveness for
the 15 wall-, tangentially-, and cyclone-fired model boil-
ers are listed in Table 4-3. An economic life of 20 years
and a NOx reduction efficiency of 55% were assumed
for all of these boilers. The fuel price differential between
coal  and natural gas was varied from $0.50 to $2.50/
MMBtu. For the 600-MW, basefoad, wall-fired boiler, the
estimated cost-effectiveness ranges from $480 to $2,080
per ton of NOx removed. For the 100-MW, peaking, wall-
fired boiler, the estimated cost-effectiveness ranges from
$3,010 to $4,600 per ton.

Cost per ton of NOX removed with reburn was highest for
the tangentially-fired units because of the lower baseline
NOx emissions produced by this boiler type. Cost-effec-
tiveness for the tangentially-fired units ranged from $615
per ton to $2,680 per ton for the 600-MW, caseload unit,
and $3,870  per ton to $5,930 per ton for the  100-MW,
peaking unit.

Cost per ton of NOx removed was lowest for cyclone-
fired boilers  because this boiler type produces the high-
est baseline  NOx emissions. For the 600-MW, baseload,
cyclone boiler, cost-effectiveness ranged from $290 to
$1,250 per ton and for the 100-MW, peaking boiler, cost-
effectiveness ranged from $1,810 to $2,720 per ton.


Sensitivity Analysis

In addition to the model plant analysis, sensitivity analy-
ses were conducted to examine the effect of varying eight
selected  plant design and operating characteristics on
busbar cost and cost-effectiveness. The results of these
analyses are presented in two graphs for each of the
three boiler types. The eight characteristics and their ref-
erence values are:

  •  Retrofit factor (RF) -1.3;

  •  Fuel  price differential - $1.50/MMBtu;

  •  Boiler size - 400 MW;

  •  Capacity factor - 40%;

  •  Economic life - 20 years;

  •  Uncontrolled NOx emission rate:

   -   Tangentially-fired boilers - 0.7 Ib/MMBtu,

   -   Wall-fired boilers - 0.9 Ib/MMBtu, and

   -   Cyclone-fired boilers -1.5 Ib/MMBtu;

  •  NOX reduction - 55%; and

  •  Unit heat rate -11,000 Btu/KWh.
                                                  55

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Table 4-3.  Costs for Natural Gas-Fired Reburn Applied to Coal-Fired Boilers

   Plant Identification        Total Capital Cost, $/kW              Busbar Cost, mills/kWh
                              Cost-Effectiveness, $/ton
Fuel Price Differential
($/MMBtu)
Wall-Fired Boilers1
1 00 MW, Peaking"
100 MW, Baseload"
300 MW, Cycling"
300 MW, Baseload
600 MW, Baseload
Tangentially-Fired Boilers0
1 00 MW, Peaking
100 MW, Baseload
300 MW, Cycling
300 MW, Baseload
600 MW, Baseload
Cyclone-Fired Boilers"
1 00 MW, Peaking
1 00 MW, Baseload
300 MW, Cycling
300 MW, Baseload
600 MW, Baseload
0.50

58.0
58.0
38.0
38.0
29.0

58.0
58.0
38.0
38.0
29.0

58.0
58.0
38.0
38.0
29.0
1.50

58.0
58.0
38.0
38.0
29.0

58.0
58.0
38.0
38.0
29.0

58.0
58.0
38.0
38.0
29.0
2.50

58.0
58.0
38.0
38.0
29.0

58.0
58.0
38.0
38.0
29.0

58.0
58.0
38.0
38.0
29.0
0.50

8.44
1.69
2.22
1.26
1.07

8.44
1.69
2.22
1.26
1.07

8.46
1.71
2.23
1.28
1.09
1.50

10.7
3.49
4.20
3.06
2.87

10.7
3.49
4.20
3.06
2.87

10.7
3.51
4.21
3.08
2.89
2.50

12.9
5.29
6.18
4.86
4.67

12.9
5.29
6.18
4.86
4.67

13.0
5.31
6.19
4.88
4.69
0.50

3,010
753
898
562
478

3,870
968
1,150
722
615

1,810
456
543
342
291
1.50

3,800
1,560
1,700
1,360
1,280

4,900
2,000
2,190
1,750
1,650

2,290
938
1,020
823
773
2.50

4,600
2,360
2,500
2,170
2,080

5,930
3,030
3,220
2,790
2,680

2,770
1,420
1,510
1,300
1,250
•Uncontrolled NO, levels of 0.90 Ib/MMBtu and a rebum NO, reduction of 55% were used for wall-fired boilers.
"Capacity Factor: Peaking - 10%, Baseload •= 65%, and Cycling - 30%.
'Uncontrolled NO, levels of 0.70 Ib/MMBtu and a reburn NO, reduction of 55% were used for tangentially-fired boilers.
"Uncontrolled NO, levels of 1.5 Ib/MMBtu and a reburn NO, reduction of 55% were used for cyclone-fired boilers.
In each figure, the effects of the design and operating
characteristics on cost-effectiveness and busbar cost are
illustrated. Each of the curves emanating from the cen-
tral point illustrates the effect of changes in the individual
parameter on cost-effectiveness and busbar cost, while
holding the other seven characteristics constant. Thus,
each curve isolates the effect of the selected character-
istic on cost-effectiveness and busbar cost.

The effects of changes in these reference plant charac-
teristics on cost-effectiveness and busbar cost of natu-
ral gas-fire reburn applied to wall-fired boilers are shown
in Figures 4-1 and 4-2. The reference boiler's cost-effec-
tiveness and busbar cost are approximately $1,400 per
ton of NOx removed and 3.8 mills/kWh.

Of the five parameters shown in Figure 4-1, the varia-
tion of capacity factor from 10 to 70% and variation of
fuel price differential from $0.50 to $2.50/MMBtu have
the greatest impact on cost-effectiveness and busbar
cost. The cost-effectiveness value and busbar cost are
inversely related to capacity factor, and thus, as capac-
ity factor decreases, the cost-effectiveness value and
busbar cost increase. This relationship is especially no-
ticeable at low capacity factors where a decrease of 75%
in the reference plant's capacity factor (from 40% to 10%)
resulted in an increase in the cost-effectiveness value
and busbar cost of approximately 100%.

The cost-effectiveness value and busbar cost are linearly
related to fuel price differential. An increase or decrease
of $1.00/MMBtu in the fuel price differential compared to
the reference plant changed correspondingly the cost-
effectiveness and busbar cost by approximately 50%.

Variations in economic life and boiler size follow a trend
similar to capacity factor; however, cost-effectiveness and
busbar cost are not as sensitive to these variations. For
                                                      56

-------
                X
               o
                    3000
                    2500
                     500
          Retrofit Factor  1.0
      Fuel Price Diff.
             ($/MMBtu)
       Boiler Size (MW)
     Capacity Factor (%)
      Economic Life (yr)
                                                           Reference Boiler Parameters
                      Uncontrolled NOX = 0.9 Ib/MMBtu
                      NOX Reduction = 55%
                      Heat Rate = 11000 Btu/kWh
1.1
                                                                 8.17
   6.81
                                                                 1.36
1.6
0.50
100
10
5
0.83 1.17
200 300
20 30
10 15
• Retrofit Factor — B—
— i — Capacity Factor — »•
1.50 1.83
400 500
40 50
20 25
Fuel Price Diff. — A—
— Economic Life
2.17 2.50
600 700
60 70
30 35
Boiler Size
Figure 4-1. Impact of Plant Characteristics on Reburn Cost Effectiveness and Busbar Costs for Wall-Fired Boilers (U.S. EPA, 1994).
                                                        57

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                      2200'
                      2000 •
                           \
                      1000.-
       Uncontrolled NOx
               (Ib/MMBtu)
        NOX Reduction (%)
       Heat Rate (Btu/kWh)
     Reference Boiler Parameters
                                                            Retrofit Cost = 1.3
                                                            Fuel Price Diff. = 1.5 $/MMBtu
                                                            Boiler Size = 400 MW
                                                            Capacity Factor = 40%
                                                            Economic Life = 20 yrs
0.6
45.0
920C
0.7
48.3
9800
• Uncontrolled
0.8
51.7
10400
NOX -
0.9
55.0
11000
*- NOX
1.0
58.3
11600
Reduction
1.1
61.7
12200
—A— Heat Rate
1.2
65.0
12800


Figure 4-2.  Impact of NOt Emission Characteristics and Heat Rate on Reburn Cost Effectiveness for Wall-Fired Boilers (U.S. EPA, 1994).
example, a decrease of 75% in economic life (from 20 to
5 years) resulted in an increase in the plant's cost-effec-
tiveness value and busbar cost by nearly 45%. Similarly,
a decrease of 75% in the boiler size (from 400 to 100-
MW) resulted in an increase in the plant's cost-effective-
ness value and busbar cost by nearly 25%.

Variation in the retrofit factor from 1.0 to 1.6 resulted in
the smallest relative percent change in cost-effective-
ness and busbar cost. Increases of 0.1 in the retrofit fac-
tor resulted in a linear increase of approximately 6% in
the cost-effectiveness value and busbar cost.

Of the parameters shown in Figure 4-2, the variation of
uncontrolled NOX from 0.6 to 1.2 Ib/MMBtu has the great-
est impact on cost-effectiveness. Uncontrolled NOx lev-
els exhibit an inverse relationship with the cost-effective-
ness value. A 30% decrease in the reference plant's un-
controlled NOx level (0.9 to 0.6 Ib/MMBtu) resulted in an
increase in the cost-effectiveness value by 50%. Varia-
tions in the NOx reduction from 45 to 65% and heat rate
from 9,200 to *12,800 Btu/kWh have less than a 6%
change in cost-effectiveness.
The effects of the eight reference plant characteristics
on cost-effectiveness and busbar cost of natural gas-
fired reburn applied to tangentially-fired boilers are pre-
sented in Figures 4-3 and 4-4. The reference boiler's cost-
effectiveness  and busbar cost are approximately
$1,800 per ton of NO removed and 3.8 mills/kWh. The
cost-effectiveness value for natural gas-fired reburn ap-
plied to tangentially-fired boilers is somewhat mislead-
ing  in that it is generally higher than for a similar retrofit
to wall-fired boilers. This is the result of the lower uncon-
trolled NOx levels produced by tangentially-fired boilers
(i.e., the fixed capital costs must be distributed over fewer
tons of N0x). The sensitivity curves follow the same gen-
eral trends"as the same retrofit for wall-fired boilers.

The effects of eight plant characteristics on  cost-effec-
tiveness and  busbar cost of natural gas-fired reburn ap-
plied to cyclone-fired boilers are presented in Figures 4-
5 and 4-6. The reference boiler's cost-effectiveness and
busbar cost are approximately $840 per ton of  NOx re-
moved and 3.8 mills/kWh. The cost-effectiveness value
for natural gas-fired reburn applied to cyclone-fired boil-
ers is lower than a similar retrofit on wall-fired and tan-
                                                    58

-------
                    4000
                    3500
               2   3000
                o
                0)
                I

               !
               LU
               O
                    2500
                    1000
                     500
           Retrofit Factor 1.0
        Fuel Price Diff.
              ($/MMBtu)
         Boiler Size (MW)
      Capacity Factor (%)
        Economic Life (yr)
                                                            Reference Boiler Parameters
                       Uncontrolled NOx = 0.7 Ib/MMBtu
                       NOX Reduction = 55%
                       Heat Rate = 11000 Btu/kWh
1.1
1.4
1.5
                          8.47



                         •7.41



                          6.35
                                                              1.06
1.6
0.50 0.83 1.17
100 200 300
10 20 30
5 10 15
• Retrofit Factor —e
— i — Capacity Factor
1.50 1.83 2.17
400 500 600
40 50 60
20 25 30
— Fuel Price Diff. — *— Boiler Size
-*•— Economic Life
2.50
700
70
35

Figure 4-3. Impact of Plant Characteristics on Reburn Cost Effectiveness and Busbar Costs for Tangentially-Fired Boilers (U.S. EPA, 1994).
gentially-fired boilers because of higher uncontrolled NOx
levels in cyclone-fired boilers. The sensitivity curves fol-
low the same general trends as the same retrofit for wall-
fired boilers.
                                                       59

-------
1
oann _

"o
O 9400 -

1
•js onno -
1 2
•*- 1 pnn -
•ffinn -
i4nn~
1200-
i
\
\
\
N>


*=*==H






\
\
s
— jt .



I






-------
                          1400'


                          1300'


                          1200
                           500
       I
I
Reference Boiler Parameters
Retrofit Cost = 1.3
Fuel Price Diff. = 1.5 $/MMBtu
Boiler Size = 400 MW
Capacity Factor = 40%
Economic Life = 20 yrs
        Uncontrolled NO
(Ib/MMBtu)
NOv Reduction (%)
HeatRate (Btu/kWh)
0.
4J
9J
9
>00
— -
1.1
48.3
9800
Uncontrolled
1.3
51.7
10400
NOX -*
1.5
55.0
11000
1.7
58.3
11600
— NOX Reduction A
1.9 2.1
61.7 65.0
12200 12800
Heat Rate

Figure 4-6. Impact of NOX Emission Characteristics and Heat Rate on Reburn Cost Effectiveness for Cyclone-Fired Boilers (U.S. EPA, 1994).
                                                            61

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                                            Chapter 5
                             Integrated NOX Control Technologies
The examples cited in Chapter 3 demonstrated that as a
"stand alone" technology,  reburning can reduce NOx
emissions from coal-fired boilers by 40 to 60%. How-
ever, the desired degree of NOx emission reduction may
be greater than can be attained by reburning alone in
some cases. These situations may be candidates for
implementation of an integrated NOX emission  control
approach that combines reburning with another  control
technology. These other NOX emission control technolo-
gies include LNBs, SNCR and SCR.

SNCR involves injecting ammonia or urea into the flue
gas to yield nitrogen and water. The ammonia or urea
must be injected into specific high-temperature zones in
the upper furnace or convective pass for this method to
be effective. SCR involves injecting ammonia into the
flue gas in the presence of a catalyst. Selective catalytic
reduction promotes the reactions by which N0x  is con-
verted to nitrogen and water at lower temperatures than
required for SNCR.


Reburning With Low NOX Burners

The  LNB-gas reburn  retrofit at  Public Service  of
Colorado's Cherokee Unit 3 is an example of the poten-
tial for lowering NOx emissions by combining the reduc-
tions achieved through the use of low NOx burners and
reburn. As discussed in detail in Section 3, the LNBs by
themselves were able to reduce NOX emissions by 31%
from baseline conditions. The combined LNB-gas reburn
system reduced NOx emissions by 72% from baseline
emissions."
Reburning With SNCR

The SNCR process involves injecting ammonia (NH3) or
urea (CO(NH2)2) into boiler flue gas at specific tempera-
tures. The ammonia or urea reacts with NOx in the flue
gas to produce N2 and water.
For the ammonia-based SNCR process, ammonia is in-
jected into the convection passes of the boiler where the
flue gas temperature is 1,750 ± 90°F. Even though large
quantities of O^are present in the flue gas, NO is a more
effective oxidizing agent, so most of the NH3 reacts with
NO by the following mechanism:
              6NO->5N
                                    (5-1)
For Equation 5-1 to predominate over competing am-
monia reactions, the NH3 must be injected into the opti-
mum temperature zone and the ammonia must be ef-
fectively mixed with the flue gas. Even under optimum
conditions, an excess of ammonia must be provided to
achieve a high level of NOx reduction within a reason-
able time. The amount of unused ammonia is referred to
as "ammonia slip." Typical ammonia slip values, mea-
sured in the flue gas at the stack exit, are 5 to 20 parts
per million (ppm), and the maximum value usually is lim-
ited by local or state air emission regulations.

In the urea-based SNCR process, an aqueous solution
of urea is injected into the flue gas at one or more loca-
tions in the upper furnace or convective passes. The urea
reacts with NOx in the flue gas to form nitrogen, water,
and carbon dioxide (CO2). Aqueous urea has a maxi-
mum NO reduction activity at approximately 1,700 to
1,900°F. the exact reaction mechanism is not well un-
derstood because of the complexity of urea pyrolysis and
the subsequent free radical reactions; however, the over-
all reaction mechanism is:
CO(NH2)2
                   2NO +  -O2
                                                                                     2N
CO
              2H2O
(5-2)
Tests of urea-based SNCR on coal-fired boilers have
demonstrated reductions in baseline NOx emissions of
40 to 70% depending on the boiler type and urea feed.
stoichiometry (Hunt et al.,  1993; Hoffman et al.,  1993;
Nalco Fuel Tech ,1992).
                                                63

-------
Hardware requirements for SNCR processes include
reagent storage tanks, air compressors, reagent injec-
tion grids, and an ammonia vaporizer (NH3-based SNCR).
Injection equipment such as a grid system or injection
nozzles is needed at one or more locations in the upper
furnace or convective passes. A carrier gas, such as
steam or compressed air, is used to provide sufficient
velocity through the injection nozzles to ensure thorough
mixing  of the reagent and flue gas. For units that vary
loads frequently, multi-level injection is used.

To date, no full-scale demonstrations have occurred of a
combination of reburning and SNCR on  utility coal-fired
boilers. The capital cost of the combined system  antici-
pated to be approximately the sum of the costs of indi-
vidual technologies. The capital cost, busbar cost,  and
cost effectiveness of stand-alone SNCR systems for 15
wall-, tangentially-, and cyclone-fired boilers are listed in
Table 5-1  (U.S.  EPA, 1994). These are the same 15 boiler
models that  were used previously  in Table 4-3 as the
examples of reburn costs. The principal benefit to be
derived from combining SNCR and reburn technologies
would be to  increase the overall NOx reductions  with a
side benefit of reducing the total ammonia/urea consump-
tion.
Reburning With SCR

The SCR process involves injecting NH3 into boiler flue
gases in the presence of a catalyst to reduce NOx to N2
and water. The catalyst lowers the activation energy re-
quired to drive the NOx  reduction to completion, and,
therefore, decreases the temperature at which the reac-
tion occurs. The overall SCR reactions are:
4NH, + 4NO + O, -» 4N0
              6NO2->7N2
                               6H20
(5-3)

(5-4)
Undesirable reactions can occur in an SCR system, in-
cluding the oxidation of NH3 and SO2 and the formation
of sulfate salts. The reaction rates of both desired and
undesired reactions increase with increasing tempera-
ture. The optimal temperature range depends upon the
type of catalyst.

The SCR process has been demonstrated on U.S. utility
coal-fired boilers only at the pilot plant scale (Janik et
al., 1993; Huang et a!., 1993). These pilot plants treated
fuel gas from a slipstream equivalent to approximately 1
to 2 MW of generating capacity. The results indicate that
75 to 80% NOx reductions are possible with less than 20
ppm of ammonia slip.

The  hardware for an SCR system includes the catalyst
material;  the ammonia system—including a vaporizer,
storage tank, blower, valves, indicators,  and controls; the
ammonia injection grid; the SCR reactor housing (con-
taining layers of catalyst); transition ductwork; and a con-
tinuous emission monitoring system. Anhydrous or di-
lute aqueous ammonia can be used; however, aqueous
ammonia is safer to store and handle.

The capital cost of a combination of reburning and SCR
is anticipated to be approximately equivalent to the sum
of the costs of the individual technologies. The capital
cost, busbar cost, and cost effectiveness of stand-alone
SCR systems for 15 wall-, tangentially-, and cyclone-
fired boilers are listed in Table 5-2 (U.S.  EPA, 1994). The
principal benefit of combining SCR and  reburn technolo-
gies would be a higher percentage reducing the ammo-
nia  reduction of NOX  emissions  with a side benefit of
ammonia consumption relative to ammonia used in the
SCR system. Because SCR requires rigid operating con-
ditions on flue gas temperature and gas flow rate, the
operation of the SCR system  could impose operating
restrictions on the reburn system that would limit its ef-
fectiveness. The ability of the combined  systems to
produce a reduced NOx emission rate  has been tested
only in Japan and is not being actively promoted by  any
vendor at this time.
                                                 64

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Table 5-1.   Costs for SNCR Applied to Coal-Fired Boilers

    Plant Identification          Total Capital Cost, $/kW
Busbar Cost, mills/kWh
Cost-Effectiveness, $/ton
Urea cost, $/ton
Wall-Fired Boilers'
1 00 MW, Peaking"
100MW, Baseload"
300 MW, Cycling"
300 MW, Baseload
600 MW, Baseload
Tangentially-Fired Boilers'
1 00 MW, Peaking
1 00 MW, Baseload
300 MW, Cycling
300 MW, Baseload
600 MW, Baseload
Cyclone-Fired Boilers"
1 00 MW, Peaking
1 00 MW, Baseload
300 MW, Cycling
300 MW, Baseload
600 MW, Baseload
140

14
14
10
10
9

14
14
10
10
g

14
14
10
10
9
200

14
14
10
10
9

14
14
10
10
9

14
14
10
10
9
260

14
14
10
10
9

14
14
10
10
9

14
14
10
10
9
140

5.47
1.54
1.78
1.25
1.14

5.23
1.35
1.57
1.06
0.95

6.18
2.10
2.40
1.81
1.71
200

5.86
1.85
2.12
1.56
1.45

5.53
1.59
1.83
1.29
1.19

6.84
2.63
2.98
2.34
2.23
260

6.25
2.16
2.46
1.86
1.76

5.83
1.83
2.09
1.53
1.43

7.50
3.16
3.56
2.87
2.76
140

2,160
760
800
610
560

2,660
860
910
670
610

1,460
620
650
540
510
200

2,320
910
950
770
720

2,810
1,010
1,060
820
760

1,620
780
800
690
660
260

2,470
1,070
1,100
920
870

2,960
1,160
1,210
970
910

1,780
940
960
850
820
•Uncontrolled NO, levels of 0.90 Ib/MMBtu and a SNCR NO reduction of 45% were used for wall-fired boilers.
"Capacity Factor: Peaking . 10%, Baseload - 65%, and Cycling - 30%.
"Uncontrolled NO, levels of 0.70 Ib/MMBtu and a SNCR NO, reduction of 45% were used for tangentially-fired boilers.
"Uncontrolled NO, levels of 1.5 Ib/MMBtu and a SNCR NO, reduction of 45% were used for cyclone-fired boilers.

Source:   U.S. EPA, 1994
                                                                 65

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Table 5-2.  Costs for SCR Applied to Coal-Fired Boilers

    Plant Identification          Total Capital Cost, $/kW
Busbar Cost, mills/kWh
Cost-Effectiveness, Si/ton
Catalyst life (yr)
Wall-Fired Boilers'
1 00 MW, Peaking"
100MW, Baseload"
300 MW, Cycling"
300 MW, Baseload
600 MW, Baseload
Tangentially-Fired Boilers'
1 00 MW, Peaking
1 00 MW, Baseload
300 MW, Cycling
300 MW, Baseload
600 MW, Baseload
Cyclone-Fired Boilers'1
1 00 MW, Peaking
1 00 MW, Baseload
300 MW, Cycling
300 MW, Baseload
600 MW, Baseload
2

110
110
86.0
86.0
75.0

106
106
83.0
83.0
72.0

117
117
90.0
90.0
78.0
3

110
110
86.0
86.0
75.0

' 106
106
83.0
83.0
72.0

117
117
90.0
90.0
78.0
4

110
110
86.0
86.0
75.0

106
106
83.0
83.0
72.0

117
117
90.0
90.0
78.0
2

43.4
7.16
13.1
6.34
6.02

42.6
6.97
12.8
6.18
5.88

44.5
7.53
13.5
6.65
6.31
3

37.1
6.19
11.0
5.36
5.04

36.3
6.00
10.7
5.21
4.90

38.3
6.56
11.4
5.68
5.34
4

33.9
5.70
9.91
4.88
4.56

33.1
5.51
9.66
4.72
4.42

35.0
6.07
10.3
5.19
4.85
2

9,650
1,990
3,300
1,760
1,670

12,200
2,490
4,160
2,210
2,100

5,940
1,260
2.040
1,110
1,050
3

8,250
1,720
2,770
1 ,490
1,400

10,400
2,140
3,480
1,860
1,750

5,090
1,090
1,720
947
890
4

7,540
1,580
2,500
1,360
1,270

9,470
1,970
3,140
1,690
1,580

4,670
1,010
1,560
866
809
•Uncontrolled NO, levels of 0.90 Ib/MMBtu and a SCR NO, reduction of 80% were used for wall-fired boilers.
"Capacity Factor: Peaking = 10%, Baseload - 65%, and Cycling - 30%.
"Uncontrolled NO, levels ol 0.70 Ib/MMBtu and a SCR NO, reduction of 80% were used for langentially-fired boilers.
"Uncontrolled NO, levels of 1.5 Ib/MMBtu and a SCR NO, reduction of 80% were used for cyclone-fired boilers.

Source:   U.S. EPA, 1994
                                                                  66

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                                            Chapter 6
                                           References
Angello, L.C., B.A. Folsom, T.M. Sommer, J.M. Pratapas,
  and M.S. Krueger. 1992.  Field evaluation of gas
  cofiring as a viable dual fuel strategy.  Presented at
  Power-Gen '92, Orlando, FL (November).

Bagwell, FA, K.E. Rosenthal, D.P. Teixeira, Southern
  California Edison Co., and B.P. Breen,  N. Bayard de
  Volo, S.Kerho, KVB, Inc. 1971. Utility boiler operating
  modes for reduced nitric oxide emissions, No. 71-11.
  64th Annual Meeting of the Air Pollution Control Assn.,
  Atlantic City, NJ, (June-July).

Bartok, W. and A. F. Sarofim, eds. 1991. Fossil fuel com-
  bustion, Chapter 4. New York, NY: John Wiley and
  Sons, Inc.

Borio, R., R. Lewis, D. Steen, A. Lookman. 1993. Long
  term NOX emissions results with natural gas reburning
  on a coal-fired boiler. Presented at 1993 EPRI/EPA
  Joint Symposium on Stationary Combustion NOX Con-
  trol, Bat Harbour, FL (May).

Dieriex,  R. 1993. Response to  questionnaire on
  Hennepin Unit 1, Illinois Power Company to Radian
  Corporation.

EPRI (Electric Power Research Institute). 1986. TAG™
  Technical assessment guide.  EPRI P4463-SR, Vol-
  ume 1. Technical Evaluation Center, Palo Alto, CA (De-
  cember).

Energy & Environmental Research Corporation (EER),
  1996. Enhancing the Use of Coal by Gas Reburning-
  Sorbent Injection, Vol. 2 - Gas Reburning-Sorbent In-
  jection at Hennepin Unit 1,  prepared for the Depart-
  ment of Energy, Report No.  DOE/PC/79796-T38-Vol
  2. NTl'S No. DE95009448 (Available at 703/487-4650).

Farzan, H. and R. A. Wessel.  1991. Mathematical and
  experimental pilot-scale study  of coal  reburning for
  NOx control in cyclone boilers.  Topical  Report.  U. S.
  Department of Energy. Report DOE/PC/89659-2
  (June).
Farzan H., et al. 1991. Reburning scale-up methodol-
  ogy for NOx control from cyclone boilers. Presented
  at the International Power Generation Conference,
  San Diego, CA (October).

Folsom, B., C. Hong, T. Sommer, and J.M. Pratapas.
  1993. Reducing stack emissions by gas firing in coal-
  designed boilers - field evaluation. Presented at EPRI/
  EPA 1993 Joint Symposium on Stationary Combus-
  tion NOx Control, Bal Harbour, FL(May).

Folsorn, B., A. Marquez, R. Payne, R. Keen, J. Opatrny,
  T, Sommer, and H.J. Ritz. 1994. Demonstration of gas
  reburning-sorbent injection on a cyclone-fired boiler.
  Presented at the Third Annual Clean Coal Conference,
  Chicago, IL (September).

Gas Research Institute (GRI). 1991. Gas reburning tech-
  nology review. Chicago, IL (July).

Glassman, I. 1987. Combustion,  second edition, Aca-
  demic Press, Orlando, FL.

Hoffman, J.E., et al. 1993. Post combustion NOX control
  for coal fired utility boilers. Presented at the 1993 Joint
  Symposium on Stationary Combustion NOX Control.
  Miami Beach, FL (May).

Huang, C.M., et al. 1993. Status of SCR pilot plant tests
  on high sulfur coal at Tennessee Valley Authority's
  Shawnee Station. Presented at the 1993 Joint Sym-
  posium on Stationary Combustion NOx Control. Mi-
  ami Beach, FL (May).

Hunt, T., et  al. 1993. Selective non-catalytic operating
  experience using both urea and ammonia. Presented
  at the 1993 EPA/EPRI Joint Symposium on Station-
  ary Combustion NO^ Control. Bal Harbor, FL (May).

Janik,  G.,  A. Mechtenberg,  K.  Zammit,  and  E.
  Cichanowicz. 1993. Status of  post-FGR SCR pilot
  plant tests on medium sulfur coal at the New York
                                                 67

-------
  Electric and Gas Kintigh Station. Presented at the
  1993 Joint Symposium on Stationary Combustion NOx
  Control. Miami Beach, FL (May).

Jensen, A.D. 1993. Response to request for information
  on control of NOx emissions from new or modified elec-
  tric steam generating units.  Letter and attachments
  from Energy and Environmental Research (EER) Cor-
  poration to J.A. Eddinger, U.S. Environmental Pro-
  tection Agency (EPA) (February).

Kanary, D.A. 1993. Response to questionnaire on reburn
  on Niles Unit 1, Ohio Edison Company.

Keen, R.T., C.C. Hong, J.C. Opatrny, T.M. Sommer, B.A.
  Folsom, R. Payne, H.J. Ritz, J.M. Pratapas, T.J. May,
  M.S. Krueger.  1993. Enhancing the use of  coal by
  gas  reburning and sorbent  injection. Presented at
  Second Annual Clean Coal Technology Conference,
  Atlanta, GA (September).

LaFlesh,  R.C., R. Lewis, R. Hall, V. Kotler, Y. Mospan.
  1993. Three-stage combustion (reburning) test results
  from a 300 MW boiler in the Ukraine. Presented at
  the EPRI/EPA Joint Symposium on Stationary NOx
  Control, Miami Beach, FL (May).

Lisauskas, R. A. and A. H. Rawdpn. 1982. Status of NOx
  controls for Riley Stoker wall-fired and turbo-fired boil-
  ers.  Presented at the 1982 EPA-EPRI Joint Sympo-
  sium on Stationary NOx Control (November).

May, T. J., M. S. Krueger, R. T. Keen, J. C. Opatrny, C.
  C. Hong, T. M. Sommer, and B. A Folsom. 1994. Gas
  Reburning in a tangentially fired coal boiler. Presented
  at NOX Controls for Utility Boilers EPRI Workshop,
  Scottsdale, AZ (May).

Mulholland, J.A. and W.S. Lanier. 1985. Application of
  reburning for NOx control to a firetube package boiler.
  ASME's Journal of Engineering for Gas Turbines and
  Power. 107:7,739-743.

Mulholland, J.A. and R.E. Hall.  1987. Fuel oil reburning
  application for NOx control to a firetube package boiler.
  Journal of Engineering for Gas Turbines and  Power.
  109:4,207-214.

Nalco Fuel Tech. 1992. SNCR  NO,, control demonstra-
  tion, Wisconsin Electric Power Company. Valley Power
  Plant, Unit 4 (March).

Newell, R., J. Campbell, J. Wamsley, S.  Gebhart, A.
  Yagiela, G. Maringo, H. Farzan, R.  Haggard. 1993.
  Coal reburning application on a cyclone boiler. Pre-
  sented at 1993 EPRI/EPA Joint Symposium  on Sta-
  tionary Combustion NO Control, Bal Harbour,  FL
  (May).
Pohl, J.H. and A.F. Sarofim, 1976. Devolatilization and
  oxidation of coal nitrogen. Presented at the 16th Com-
  bustion Symposium (August).

Pershing, D.W.and J.O.LWendt, 1976. Pulverized coal
  combustion:  the influence of flame temperature and
  coal combustion on thermal and fuel NOx. Presented
  at the 16th Combustion Symposium, (August).

Rindahl, E.G., M.E. Light, C.C. Hong, T.M. Sommer, B.A.
  Folsom. 1994. NOx control by gas reburning in a 172
  MW coal boiler. Presented at NOx Controls for Utility
  Boilers EPRI Workshop, Scottsdale, AZ (May).

Sanyal, A.,  T.M. Sommer, B.A. Folsom, L. Angello, R.
  Payne, and  M. Ritz. 1992. Cost effective technolo-
  gies for SO2  and NOX control. In Power-Gen '92 Con-
  ference Papers, volume 3. Orlando, FL (November).

Sanyal, A., T.M. Sommer, C.C. Hong, B.A. Folsom, R.
  Payne. 1993. Low NO burners and gas reburning -
  an integrated advanceo NO  reduction technique. Pre-
  sented at the Institute of Energy/International Sym-
  posium on Combustion and Emissions Control, Uni-
  versity of Wales, College of Cardiff, UK. Energy and
  Environmental Research Corporation (September).

Singer, J. G. 1991. Combustion, fossil power systems,
  fourth edition. Windsor, CT: Combustion Engineering,
  Inc.

Stultz, S.C. and J. B. Kitto, eds. 1992. Steam, its gen-
  eration and  use. Barberton, OH: The Babcock &
  Wilcox Company.

Takahashi, Y. 1981. Development of Mitsubishi 'MACP
  in-furnace NOX removal process. Technical Review,
  Mitsubishi Heavy Industries, Inc., 18:2.

U.S. DOE (Department of Energy). 1992. Annual energy
  outlook 1992. DOE/EIA-0383(92). Office of Integrated
  Analysis and Forecasting. Washington, DC (January).

U.S. EPA (Environmental Protection Agency).  1985a.
  Bench-scale process evaluation of reburning and sor-
  bent injection for in-furnace NO/SOx reduction. EPA-
  600/7-85/012. Air and Energy Engineering Research
  Laboratory, Research Triangle Park, NC (March).

U.S. EPA (Environmental Protection Agency).  1985b.
  Compilation  of air pollutant emission factors, fourth
  edition. Office of Air Quality Planning and Standards,
  Research Triangle Park, NC (September).

U.S. EPA (Environmental Protection Agency). 1987. Pi-
  lot scale process evaluation of reburning for in-fur-
  nace NOx reduction. EPA-600/7-86/048. Air and En-
  ergy Engineering Research Laboratory, Research Tri-
  angle Park, NC (December).
                                                 68

-------
U.S. EPA (Environmental  Protection Agency). 1989.
  Bench-scale studies to identify process parameters
  controlling reburning with pulverized coal. EPA-600/
  7-89/005. Air and Energy Engineering Research Labo-
  ratory, Research Triangle Park, NC (May).

U.S. EPA (Environmental  Protection Agency). 1990.
  OAQPS Control cost manual, fourth edition, chapters
  1 and 2. EPA 450/3-90-006. Office of Air Quality and
  Planning Standards, Research Triangle  Park, NC
  (January).

U.S. EPA (Environmental Protection Agency). 1994. Al-
  ternative control techniques document — NO emis-
  sions from utility boilers.  EPA-453/R-94-023. Office
  of Air Quality and Planning Standards, Research Tri-
  angle Park, NC (March).

Wendt, J.O.L., C.V. Sternling, and M.A. Matovich. 1973.
  Reduction of sulfur trioxide and nitrogen oxides by
  secondary fuel injection. Presented at the 14th Sym-
  posium (International) on Combustion, Pittsburgh, PA:
  Combustion Institute, p. 897.

Yagiela, A.S., et al. 1991. Update on coal reburning tech-
  nology for reducing NOx in cyclone boilers. American
  Power Conference, Chicago, IL (April).
                                                  69

-------
                                             Chapter 7
                                           Bibliography
Gas Research Institute. 1993. Natural gas reburning:
  cost-effective NOx reduction for utility boilers. GRI re-
  port number GRI-93/0059. Gas Research Institute,
  8600 W. Bryn Mawr Ave., Chicago, IL 60631 (Janu-
  ary).

Gas Research Institute. 1993. Competitive analysis for
  gas-based NOx control. GRI report number GRI-93/
  0484. Gas Research Institute, Chicago, IL 60631 (No-
  vember).

Gas Research Institute. 1993. Natural gas use for NO
  control  in coal boilers.  GRI report number GRI-93/
  0404. Gas Research Institute, Chicago, IL 60631 (Sep-
  tember).

Gas Research Institute. 1995. Proceedings of the sec-
  ond international gas reburn technology workshop,
  MalmQ Sweden. Gas Research  Institute, Chicago, IL
  60631 (February).

Gavin, J.J. 1994.  Reburn projects meet goals. Power
  Generation Tech  Update, Gas  Research Institute,
  Chicago, IL 60631 (September).

Hall. R.E., R.W.  Borio,  R.D. Lewis, and R. Booth. 1991.
  Natural  gas reburning for NOX control on a cyclone-
  fired boiler. 84th Annual Meeting and Exhibition, Air &
  Waste Management Association, Vancouver, B.C.
  Canada (June).

Harding, N. S. (ed). 1993. Proceedings: Integrated natu-
  ral gas technologies into coal and oil designed boil-
  ers. EPRI-TR-103469.  Electric  Power Research In-
  stitute, 3412  Hillview Avenue, Palo Alto, CA 94303
  (June).

LaFlesh, R.C., J.L. Marion, D.P. Towle, C.Q. Maney, G.
  DeMichele, S. Pasini, S.  Batacchi, A. Piatanida, G.
  Galli, G. Mainini. 1992. Application of reburning tech-
  nologies for NOX emissions control on oil and pulver-
  ized-coal, tangentially fired boilers. ASME-91-JPGC-
  FACT-14. American Society of Mechanical Engineers,
  345 East 47th  Street,  New York,  NY  10017-2392
  (April).

May, T.J., E.G. Rindahl, T. Booker, R.T. Keen, M.E. Light,
  D.A. Engelhardt, R.Z.  Beshai, T.M.  Sommer, B.A.
  Folsom, H.J. Ritz, and J.M. Pratapas. 1994. Gas
  reburning in tangentially-, wall-, and cyclone-fired boil-
  ers, an introduction to  second  generation gas
  reburning. Third Annual Clean Coal Conference, Chi-
  cago IL (September).

Opatrny, J. C., R.T. Keen,  M.E. Light, R.Z. Beshai, T.M.
  Sommer, B.A. Folsom, H.J. Ritz, and J.M. Pratapas.
  1993. NOx control by gas reburning in coal-fired utility
  boilers. Institute of Clean Air Companies Forum '94.
  Arlington, VA (November).

Opatrny, J.C.,  C.C. Hong, and T.M. Sommer. 1994. Sec-
  ond-generation  gas reburning technology. Third An-
  nual Clean  Coal Technology Conference, Chicago, IL
  (September).

Pratapas, J.M.  and J. Bluestein. 1994.  Natural gas
  reburn: cost effective NOx  control.  Power Engineer-
  ing. 98:5,47-50.

Pratapas, J.M. 1994. Major new gas technology initia-
  tives underway at gas  research institute. IGT/EPRI
  Conference. Chicago, IL (June).

Pratapas, J.M. (Undated). Deployment of gas cofiring,
  reburning, and seasonal switching at coal and oil-fired
  boilers. Gas Research Institute, Chicago, IL 60631.
                                                   70
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