nox control through reburning

NOx CONTROL THROUGH REBURNING*

L. D. Smoot†a,b, S. C. Hill‡b and H. Xu§a

aChemical Engineering Department, Brigham Young University, Provo, UT 84602, U.S.A.bAdvanced Combustion Engineering Research Center, Brigham Young University, Provo, UT 84602, U.S.A.

Abstract—Reburning is a process whereby a hydrocarbon fuel is injected immediately downstream of thecombustion zone to establish a fuel-rich zone in order to convert nitric oxide to HCN. The reburning fuel can begaseous (e.g., natural gas), solid (e.g., coal char or wood) or liquid (e.g., residual oil). Typically, the amount ofreburning fuel used is 10–30% of the total fuel. This technology is practiced commercially with nitric oxidereduction levels of 35–65%, depending on the type and scale of the boiler or combustion, the primary andreburning fuels and other variables. Current research and development are suggesting several advanced reburningconcepts including injection of ammonia or urea aft of the reburning fuel injection. Nitric oxide reductions of over90% are anticipated. In this mini-review, a review of reburning technologies, measurements and mechanisms ispresented. Predictive methods for reburning are also discussed. Recent work on reburning, including developmentof a global reburning reaction rate, is summarized, and results of application of a comprehensive combustionmodel to reburning measurements are summarized.q 1998 Elsevier Science Ltd. All rights reserved.

Keywords: advanced reburning, combustion modeling, nitric oxide, NOx reduction, reburning, global model.

NOMENCLATURE

A Pre-exponential factor of the global reburning rate

b Exponent on oxygen concentration or temperature

dmm Mass mean particle size

E Activation energy of global reburning rate

f i Coefficients in Eqs (26)–(29)

ki Rate constant of reactioni

R Gas law constant

r i Rate of reactioni

385

Prog. Energy Combust. Sci.Vol. 24, pp. 385–408, 1998q 1998 Elsevier Science Ltd

Printed in Great Britain. All rights reserved0360–1285/98 $19.00

Pergamon

PII: S0360–1285(97)00022–1

CONTENTS

Nomenclature 3851. Introduction 3862. Technologies 3863. Measurements 3894. Mechanisms and Rates 3925. Predictive Methods 3946. Reburning Model Applications 398

6.1. Reaction Parameters 3996.2. Particle Size 4006.3. Reburn Gas Composition 4006.4. Injection Velocity 4036.5. Comparisons with Data 405

7. Summary 405Acknowledgments 405References 405

* This mini-review paper was presented, together with aseries of other review papers, at the Tenth Annual TechnicalConference of the Advanced Combustion Engineering ResearchCenter, held in Salt Lake City, Utah, in March 1997.

† Professor, Chemical Engineering; Director, ACERC.‡ Research Associate; Head, Combustion Computations

Laboratory.§ Graduate Research Assistant, Chemical Engineering.

T Gas temperature

t Time

Xi Concentration of speciesi

f Equivalence ratio

Abbreviations and acronyms

ABB–CE ABB Combustion Engineering Co.

ACERC Advanced Combustion Engineering ResearchCenter

B&W Babcock and Wilcox Company

CAA Clean Air Act Amendment

CARS Coherent anti-Stokes Raman spectroscopy

CCT Clean Coal Technology

CFD Computational fluid dynamics

CHEMKIN Chemical kinetics code package developed bySandia National Laboratory

1. INTRODUCTION

Nitrogen oxides (NOx) have been recognized as acidrain precursors that impose a significant threat to theenvironment. Coal combustion is a major anthropogenicsource of NOx. In coal combustion, NOx originatesmostly from nitrogen bound in the coal matrix, namely,fuel-NOx

1. Molecular nitrogen in air typically contri-butes less than 10% of overall NOx emissions from coalcombustion2. Several technologies have been demon-strated to reduce NOx emissions during fossil fuelcombustion, as summarized by Boardman and Smoot3

and others. During the past decade, work on NOx

formation and control has been substantial, includinggas-phase and coal-phase NOx formation modeling andmeasurement, demonstration of very low NOx burnertechnologies, and prediction and measurement of NOx

control through reburning. International work in NOx

formation and control has been particularly extensiveduring this decade, as noted in reviews by Miller andBowman4, Hayhurst and Lawrence5, Bowman6, andKramlich and Linak7. Among the most recent develop-ments for reducing NOx emissions from coal systems arethe reburning technologies, wherein gaseous, liquid orsolid hydrocarbon fuels are injected downstream of themain combustion zone with NO and produce HCN3,8,9.Advanced reburning is an even more recent technologywhereby ammonia, urea or similar substances areinjected aft of hydrocarbon injection for reburning tofurther reduce NOx species10. Reburning and advancedreburning technologies are commercially available

retrofit technologies, capable of providing 50%–85%NOx reduction beyond other combustion modifications.This paper provides a review of reburning work,including work at ACERC in the area of reburning.

No published review of reburning was located.However, a review chapter on reburning is reported tobe forthcoming in a new encyclopedia11. To meet therequirements of the Clean Air Act Amendments of 1990(CAA) within budget limitations and scheduling con-straints, electric utilities have considered a variety ofnovel approaches to reduce NOx emissions from power-plants. Categories of NOx control options include burnerreplacement, combustion modifications, fuel staging,fuel reburning, steam or water injection, selectivecatalytic reduction (SCR), and selective non-catalyticreduction (SNCR)12. The efficiencies and limitations ofvarious methods have been summarized by Boardmanand Smoot3. Reburning technology is not fully under-stood theoretically, but it is a successful and commer-cially available retrofit technology, capable of providing50%–70% NOx reduction beyond other combustionmodifications13. Advanced reburning technologies arereportedly capable of greater reductions.

The reduction of NO by hydrocarbons wasobserved nearly half a century ago by Patry andEngel14, and subsequently by Drummond15. This con-cept, and the termreburning, were first proposed byWendt et al.16 who noticed that, with the injection ofCH4 just downstream of the primary flame zone, upto 50% of NO was reduced, as shown in Fig. 1.Myerson17 proposed that the overall reactions for NOreduction by methane (and other hydrocarbons) could bewritten as:

2NOþ 2CH4 ¼ 2HCNþ 2H2Oþ H2 þ 88 kcal (1)

6NOþ 2CH4 ¼ 2COþ 4H2Oþ 3N2 þ 428 kcal (2)

Since this earlier work, much recent work has beencompleted in several countries. This work has includedcommercial demonstration, detailed laboratory measure-ment, and demonstration of improved and advancedreburning technologies. Also, work on understandingthe kinetics of reburning and on predicting reburningeffectiveness during turbulent fuel combustion hasbeen studied. What follows is a brief review of thisrecent work, including work in process.

2. TECHNOLOGIES

Reburning was apparently first demonstrated as apractical NOx reduction method in Japan where theconcept of reburning was first applied to a full-scaleboiler by Mitsubishi in the early 1980s, more than 50%reduction of NO being achieved18. Babcock–HitachiK.K. has also applied the technology successfully tonumerous wall-fired utility boilers in Japan19. Because ofthese successful examples and the high efficiency of thereburning technology on the reduction of NOx emissions,

386 L. D. Smootet al.

EER Energy and Environmental ResearchCorporation

GRI Gas Research Institute

HiPPS High performance power system

LDA Laser doppler anemometry

LNB Low NOx burner

MCRD Micronized coal reburning demonstration

MSW Municipal solid waste

MWe Megawatt, electric

MWt Megawatt, thermal

NYSEG New York State Electric and Gas Co.

PCGC-3 Pulverized coal gasification and combustion inthree dimensions

PETC Pittsburgh Energy Technology Center

ppm Parts per million, by volume

REI Research Engineering International Company

SCR Selective catalytic reduction

SNCR Selective non-catalytic reduction

many investigators have conducted bench-scale andpilot-scale reburning tests.

In a fossil fuel boiler configured for reburning, naturalgas, oil or coal (10–30% of total fuel)20 is injected intothe upper furnace region. The overall process occurswithin three zones of a boiler—primary zone, reburningzone, and burnout zone, as shown in Fig. 213. NOx formsin the primary combustion zone with excess air, and thenreacts with hydrocarbon in the reburning zone to formHCN and then N2. The unreacted fuel completescombustion in the burnout zone, where additional air isadded.

Laboratory experiments by Folsomet al.21 showedthat the NOx and SO2 emissions may be reduced up to60% and 20%, respectively, by reburning. Furthermore,EER22reported that in their gas reburning tests on a wall-fired boiler, approximately 6–8% reduction of CO2 wasachieved from the use of natural gas in place of coal forpart of the fuel. Several power plants in the U.S. haveemployed this technique, and some experimental data

are being collected23. According to GRI24 and Folsomet al.9, three full-scale, long-firing time evaluations ofgas reburning in coal-fired power plants had goals of60% NOx reduction on a 172 MWe wall-fired unit, 65%NOx reduction on a cyclone boiler, and 55% NOx

reduction on a 71 MWe tangentially fired unit, respec-tively (see Fig. 3). A planned gas reburning test was to be

387NOx control through reburning

Fig. 1. Effect of CH4 flowrate on NOx concentration with primary flame at 10% excess air, for a small laboratory-scale burnerwithout tertiary air addition16.

Fig. 2. Schematic of the reburning process (adapted fromPratapas and Bluestein13).

carried out during 1996 on a 330 MWe coal-firedcyclone boiler25. It has also recently been reported26

that Portugal has initiated a project to demonstrate coalas a reburning fuel in the Vado–Liguire power plant, witha goal of less than 200 mg Nm¹3 NOx emissions. EER27

has designed low NOx burners, to be used jointly with thereburning process, and is demonstrating the performanceof a second-generation gas reburning system for NOx

control, which follows the successful demonstration of agas reburning system for over 14 months.

Four demonstration projects in the Department ofEnergy’s Clean Coal Technology Program28 includereburning, as listed in Table 1. One of these uses coal asthe reburning fuel while the others use natural gas. NOx

reductions varied from 30% to over 70%, depending onfurnace type and scale, reburning fuel and simultaneoususe of low-NOx burners. In the U.S. its principalapplication has been to coal-fired boilers, while inEurope and Japan reburning applications include coal,oil and gas-fired utility boilers as well as municipal solidwaste (MSW) incinerators.

On the basis of an assumed 10% market penetration,many countries, including Britain, France, Germany,Russia and Japan, have established an internationalresearch partnership to address the question of whethergas reburning has "scale-up" limitations for large furnaceapplications23. For example, a demonstration of gasreburning on a 600 MWe coal-fired boiler at Longannet,U.K., is scheduled to be completed during 1997. It issupported by companies from Europe and the U.S., andwill be the largest scale application of gas reburning todate29. Table 2 summarizes the commercial-scalereburning demonstrations around the world. Severalvariables are associated with the reburning system andinfluence its effectiveness30, including:• reburn zone temperature;• residence time;• boiler load;• reburn fuel percentage of total boiler heat input;• reburn fuel composition;• reburn zone stoichiometry;• reburn burner stoichiometry (when coal is used as the

secondary fuel);• reburn burner pulverized coal fineness (when coal is

used as the secondary fuel);• flue gas recirculation rates to the reburn system;• reburn system spin vane and impeller (characteristics

and location);• overfire air port spin vane (characteristics and

location);• economiser outlet O2%.

Another promising concept for NOx reductioncombines aspects of reburning with concepts fromselective non-catalytic reduction (SNCR), through thejoint use of a hydrocarbon (e.g., CH4) and ammonia orurea. In this scheme, urea or ammonia is injected intothe flue gas stream downstream of the aft-hydrocarbonfuel injection point to further reduce NOx. This approachhas been labeled "advanced reburning"10,13. Thistechnique is quite attractive since it has been reportedthat up to 85% NOx reduction can be achieved and, atthe same time, the problems of carbon loss, slaggingand tube wastage may be avoided10. However, residualammonia (i.e., ammonia breakthrough or slip) is achallenge. The first observation of advancedreburning data was reported by Chenet al.31, as shownin Fig. 4.

The use of ammonia or urea in advanced reburning


Fig. 3. Results of NOx reduction of three GRI-sponsored, full-scale, long-term projects for evaluation of gas reburning in coal-

fired power plants24.

Table 1. CCT reburning demonstrations—coal-fired boilers28

Co. Boiler Reburn fuel % NO reduction Comments

B&W 60–110 MWe cyclone, coal-fired Coal 36–52%, w/bitum CompleteWisconsin 20–30% 53–62%, w/sub-bitum

EER 40–75 MWe, tangential Natural gas NOx—67% Reburn with Ca-sorbent,completeIllinois 15–25% SO2—52–80%

EER 20–33 MWe, cyclone Natural gas NOx—60–66% Reburn with Ca-sorbent,completeLakeside, Illinois 22–33% SO2—32–63%

EER 172 MWe, wall-fired Natural gas NOx— . 30% LNB Low NOx burners andreburning in processColorado 5–20% 60–73% LNB/RB

differs from its use in SNCR. In the advanced reburningscheme shown in Fig. 5, NH3 injection followshydrocarbon fuel injection to create a slightly fuel-richzone, combined with or followed by more air. The mainreactions of the advanced reburning are32:

NH3 þ OH, O, H → NH2 þ … (3)

NH2 þ NO → N2 þ H2O (4)

With increased CO present, a larger temperature windowand reduction of NH3 slip can be achieved. Chenet al.33

noted that, with reburning fuel injection, CO increases,leading to:

COþ OH → CO2 þ H (5)

H þ O2 → OHþ O (6)

Oþ H2O → OHþ OH (7)

This chain branching sequence provides additional OHradicals to initiate the NH3 oxidation sequence. In theSNCR scheme alone, NH3 is injected only into the post-combustion zone, at lower temperatures within the fuel-lean region.

In Sweden, the combination of gas reburning withSNCR has been used in an MSW incinerator34. In short-term tests, 70% reduction of NOx was achieved. In theU.S., demonstrations of advanced reburning at a tenmillion Btu hr¹1 boiler have recently been completed10.Currently, the second-generation advanced reburningtechnology is being developed, in which a specific water-soluble, inorganic promoter salt (e.g., a sodium salt) isadded to ammonia and injected into the reburning zone.Initial tests showed that 95% reduction of NO could beachieved. Like CO, the presence of Na provides moreOH radicals to initiate the NH3 oxidation sequencethrough the reaction32

[(Na)HOH] þ M ¼ (Na) þ OHþ H þ M (8)

Without an N-agent, Na has no effect on NO reductionefficiency. De Angelo and Sjoberg35 report new projectsto investigate the advanced reburning technique and themicronized coal reburning process at two NYSEG utilityboilers.

3. MEASUREMENTS

Some experimental results with gas-based reburningin the U.S. are discussed below. Marion,et al.36 haveshown that greater than 50% NOx reduction wasachieved with gas reburning in ABB–CE’s BoilerSimulation Facility, which allows for carefully con-trolled experiments at a sufficient scale to replicate thereburning process in full-scale units. Wendt and Mereb37

have completed a number of laboratory-scale, gas-basedreburning tests which consist of numerous premixed-flame cases and a few diffusion-flame cases. They alsoreported the effects of various parameters on theefficiency of reburning, including the length of thereburning zone and stoichiometric ratio. These datacould be used to evaluate predictions of reburning,though no information on particle size distribution orwall temperature profile was reported. Large-scale gasreburning tests were conducted at the Illinois PowerCompany’s Hennepin Station, which was the first gasreburning project ever demonstrated commercially for atangentially coal-fired boiler (80 MWe) in the U.S.23.Recently, ABB–CE has been involved in the applicationof reburning technologies to two coal-fired utility boilersand one oil-fired tangential boiler in the U.S. and Italy38.Bilbao et al.39 in Spain made some exploratory study ofthe effect of the operating parameters on reburningefficiency. Their lab-scale data can be used forcomparisons of the modeling predictions.

In addition, several investigations with coal or otherhydrocarbons as the reburning fuel have been conductedin the past few years, due to lower fuel costs. B&W19

demonstrated coal reburning for NOx control in a100 MWe cyclone boiler. By using Southern Indianabituminous coal, greater than 50% removal of NOx wasobtained. The research group at Tennessee River ValleyAuthority40 has planned a project called the Micronized


Table 2. Reburning commercial-scale demonstration

United States6–7 coal-fired boilers1 oil-fired boiler1 with coal as reburning fuel2–3 likely in current use4 in CCT program

Europe7 coal-fired boilers2 municipal solid waste incineratorsSeveral glass furnaces

JapanSeveral installationsCurrently inactiveDemanding NOx regulation

Fig. 4. Influence of agent injection temperature into the richzone on percent NO removed, from the first advanced reburning

data31.

Coal Reburning Demonstration (MCRD) on a 175 MWewall-fired unit. 50–60% removal of NOx was projectedthrough use of micronized (80% less than 325 mesh)eastern Kentucky and West Virginia coals. Syverudet al.41 reported that, with automobile tires as a reburningfuel, reduction of up to 40% of NOx emissions, 50% ofCO emissions, and 25% of SO2 emissions can beachieved. According toPETC Review23, a projectconducted by Wisconsin Power and Light (WP&L)showed that pulverized coal is an effective reburningfuel for reducing NOx emissions from cyclone boilers.As shown in Table 3, using bituminous coal, NOx

reduction was as high as 58% at full load. With sub-bituminous coal, the full load NOx reduction was up to62%.

Natural gas as the reburning fuel has some advantageover other fuel types since it contains no fuel-boundnitrogen, sulfur, or particulate matter, and it reacts fasterthan coal or oil. Wendt42 suggested that natural gasproduces more hydrocarbon species in the NO reductionzone. However, with coal reburning, HCN was destroyedmore rapidly, which is a key intermediate to reduce NOto N2 in fuel-rich regions. He indicated that boundnitrogen in the reburning fuel is not an issue, due to only

minor differences in the measured NO profiles, regard-less of the nitrogen content in different reburning fuels.On the basis of investigations into the reductionefficiency of different reburning fuels, Kichereret al.43

concluded that a high NOx reduction level can beachieved under the following conditions: high volatilematter of reburning fuel, long residence time, optimizedmixing conditions, and very fine grinding if solidreburning fuels are used. He indicated that the homo-geneous reduction mechanisms are more effective thanthe heterogeneous processes. However, Chen and Ma44

observed, as shown in Fig. 6, that lignite as a reburningfuel has a higher efficiency for NOx reduction than CH4or other coals in their small-scale laboratory reactor.They noted that heterogeneous reactions contributed tohigher levels of NOx reduction than homogeneousreactions under certain conditions. Moyedaet al.45 alsoconfirmed experimentally that the low-rank coals gen-erally performed in an equivalent manner to natural gas,while high-rank coals were generally less effective asreburning fuels. They found that increasing the reburningzone residence time improved the NOx reductionsachieved with all of the coals, but not for natural gas.Payneet al.46 believe that fuel volatility influences the


Fig. 5. Schematic of advanced reburning process (from Folsomet al.10).

Table 3. Wisconsin Power & Light sponsored project to demonstrate that pulverized coal is an effective reburning fuel for cycloneboiler (from PETC23)

Load (MWe) Baseline NOx Reburn NOx Percent reduction(ppm @ 3% O2) (ppm @ 3% O2)

Bituminous coal110 609 290 52.482 531 265 50.160 506 325 35.837–38 600 400 33.3

Sub-bituminous coal110 560 250 55.482 480 230 52.160 464 220 52.6

availability of fuel in the reburn zone and the evolutionof radical species; therefore, fuels with higher volatilecontent would be expected to achieve higher NOx

reduction.Another interesting study was conducted in a semi-

industrial scale (2.5 MWt) furnace with a swirl-stabilized internal fuel-staged burner, shown in Fig. 7,with different reburning fuels including two high-volatile bituminous coals, heavy fuel oil, natural gasand coke oven gas47. This work showed that the degreeof NOx reduction is independent of reburning fuel types,and that up to 89% NOx reduction can be achieved. Chenet al.48 also showed that the reburning efficiencies withethylene, isobutane and methane were similar. Bench-scale experimental results by Spliethoffet al.49 alsoshowed that, with concentrations of various hydrocarbons

between 20 and 100% in reburning fuels, no effect of fueltype on NO emission could be observed. Although thereburning efficiencies are somewhat dependent on theconcentrations and types of hydrocarbons (i.e., fueltypes), the experiments of Greulet al.50 show thatpyrolysis gas may be one of the best reburning fuels,because N-containing tars from pyrolysis gas can furtherselectively reduce nitrogen oxides in the reduction zone.

In 1992, a joint work effort was initiated as a part ofthe Phase 1 Combustion 2000 (HiPPS) program toevaluate low-NOx burner concepts. The overall conceptwas to develop a new, combined-cycle power generationsystem using state-of-the-art gas turbines in conjunctionwith innovative, low-NOx designs for coal combustion.In this project, a unique ceramic heat exchanger isintegrated with the combustor to produce hot air which is


Fig. 6. Comparison of coal, char and methane as reburning fuels simulated for a lab-scale reactor44.

Fig. 7. Schematic diagram of a novel reburning burner47.

used to drive the turbines, and use is made of state-of-the-art ash management technologies. Bench-scaleexperimentation has been conducted to determine therole of reburning to lower NOx levels in the coal-firedlaboratory flame, in conjunction with other low-NOx

technologies51.A new 29 kW, refractory-lined, pulverized coal

combustion research facility was fabricated with a16 cm diameter and a 7.3 m length, with compositerefractory walls to minimize heat losses. The burner wasmounted on the top left side of the U-shaped furnace.The long, vertical path below the burner allowed long,axial flames. This facility has been used extensively toevaluate technologies to achieve NOx emissions below0.06 lb/MM Btu using several alternative combinationsof optimized SNCR with stage, gas-stabilized axial coalflames and reburning. Results show an almost 90% NOx

reduction from the current standards51. This work hasalso explored the important parameters in the near-burner region, using a natural gas-stabilized burner. Thefinal step in this study was to determine the lowest levelsof NO that could be obtained from the optimized use of agas-stabilized, pulverized coal flame in conjunction withair staging, natural gas reburning and SNCR. Figure 8summarizes the results of a wide spectrum of testsconducted to evaluate various combinations of theseschemes in an integrated mode. These data indicate thatthe desired range of emissions can be achieved,depending on which of the various combinations oftechniques are used51.

A hardwood and softwood have also been evaluated asreburning fuels in the U-shaped furnace52. Resultsshowed that a reduction of 50–60% NO was obtainedwith approximately 10% wood heat input. There is a netreduction in CO2 and SO2 since wood is a regenerablebiofuel and contains no sulfur. These results suggest thatwood is as effective as natural gas or coal as a reburningfuel.

Measurements of the reburning process are importantto gain a fundamental understanding of this process, andfor validation of predictive techniques for NO reactionslike that described previously. Tree53 and colleagues areworking to provide detailedin situ measurements for alaboratory-scale, coal-fired, control-profile reactor(CPR) utilizing reburning and advanced reburning

technologies. The objective of this project is to developa better understanding of reburning and advancedreburning by comparing detailed combustion measure-ments of temperature, gaseous species and velocity withpredicted values, focusing primarily on improving themodels used to describe the reburning and advancedreburning reactions. PCGC-3 model predictions oftemperature, velocity and species entering the reburningand advanced reburning zones will be refined to matchupstream measurements; then comparison of the resultsof NO, NH3 and HCN measurements within thereburning zone with predicted values will be made.Mapping will include measurements at a matrix of radialand axial positions in one of the four quadrants of theCPR. Species to be mapped include CO, CO2, O2, N2,NOx, HCN and NH3. Conventional instrumentationincludes suction pyrometer for temperature, Pitot tubefor velocity and various gas analyzers for chemicalanalysis of the gaseous species. Advanced opticalinstrumentation will include laser doppler anemometry(LDA) for velocity and coherent Raman spectroscopy(CARS) for temperature. Reburning technologies to betested will include downstream natural gas injection andjoint use of natural gas and ammonia (advancedreburning). Initial reburning results in a pulverized coalflame show NO reductions of up to 65% with natural gasinjection closer to the flame zone54. Natural gas injectiondownstream led to a lower NO reduction of 35–40%.

4. MECHANISMS AND RATES

Wendt42 has recently published a review of mechan-isms governing the formation and destruction of NOx.NO reduction through the reburning-NO mechanismusually includes the interactions of HCN and NOspecies, as described in the elementary reaction stepsof reburning noted by Wendtet al.16. Under fuel-richconditions, the formation of HCN relies strongly on theconcentration of hydrocarbon species,

CHi þ NO → HCNþ … (9)

which then decays through NCO→ NH → N, as shownin Eqs. (10–12), and ultimately reaches N2 via the


Fig. 8. Strategies for NOx reduction based on measured data in 29 kW U-shaped furnace51.

reverse Zeldovich reaction, Eq. (13):

HCNþ O → NCOþ H (10)

NCOþ H → NH þ CO (11)

NH þ H → N þ H2 (12)

N þ NO → N2 þ O (13)

The above mechanisms illustrate why pure hydrogen isless effective for reburning. However, under fuel-leanconditions, hydrocarbons react with oxygen and/orhydroxyl radicals via Eq. (14) to form CO:

CHi þ O → COþ H þ … (14)

Therefore, reactants in Eqs (9), () and (14) consume CHi

competitively. A goal in reburning optimization is tomaximize the exposure of NO to CHi and minimizeCHi interaction with oxygen48.

Reliable predictions of NOx reduction with reburningdepend partly on an understanding of the controllingsequence of elementary reactions. The structure ofexisting comprehensive elementary reaction schemesfor hydrocarbon and other oxidation systems has beenpainstakingly developed over many years by a number ofdifferent groups (e.g., Miller and Bowman4), and themechanism sets are becoming larger and more reliable55.However, a reliable kinetic scheme is not easy toassemble since the collective performance of reactions ina kinetic scheme on the basis of their independentlydetermined Arrhenius parameters does not necessarilyyield the closest match to a range of experimentalresults56. Commonly used schemes of kinetic modelsinclude the MB model4, Bian’s model57, and Warnatz’smodel58. The new version 2.11 of GRI–Mech59 includesnitrogen chemistry relevant to reburning. The accuraciesof these models vary from low temperature to hightemperature, and from low pressure to high pressure, fordifferent flames.

Many researchers studying reburning mechanismsthrough elementary chemical reactions, such as Glarborgand Hadvig60, Baulchet al.61, Thorneet al.62, Li et al.63,Hura and Breent64, and Burchet al.65, found for variousreburning fuel types that two major kinetic pathwayscontrol the efficiency of reburning, i.e.:

C, CH, CH2 þ NO → HCNþ … (15)

HCNþ O, OH → N2 þ … (16)

Chagger et al.20 found experimentally that NOx isslightly reduced by the presence of SO2. On the basisof a sensitivity analysis for a 163-step elementarymechanism and experiments, Thorneet al.62 pointedout that the largest sensitivity coefficients are for thetwo reactions:

H þ O2 → OHþ O (17)

C, CH, CH2 þ NO → HCNþ … (18)

He believed that the CH2 is less important, but Chen andMa44 suggested that CH2 is as important as CH. Wendt42

included the following reaction as an important path forNOx destruction:

NOþ NHi → N2 þ products (19)

Comparing the predicted results from Baulch’selementary chemistry mechanisms with the experimentaldata, Etzkornet al.66 indicated that the current reburnmechanisms cannot describe the practical reburn processand should be improved. Stapf and Leuckel67 concludedthat current comprehensive mechanisms, such as that ofMiller and Bowman4 or Glarborg and Hadvig60, showstrong dependence on the initial oxygen content andpredict shorter global reaction time scales than arefound in the flow reactor experiments. They suggestedthat further investigations are necessary to clarifywhether the NOþ CHi pathways are sufficiently under-stood. Burchet al.68 also showed that their experimentaldata did not support the mechanism of Eqs. (9)–(14).Kristensenet al.69 investigated nitrogen chemistry inthe burnout zone of reburning, and found that the mini-mum in NO is related to the amount of CO, the oxidationof NH3 or HCN, and the reduction of NO by NH3 orHCN.

While detailed chemical kinetics of NOx have beenstudied extensively and tabulated, it is currentlyimpractical, because of unacceptable computer require-ments, to incorporate large elementary chemical kineticschemes into comprehensive combustion computermodels which include turbulent fluid dynamics, heattransfer and chemical reactions. In addition, the forma-tion and destruction rates of NOx are of the samemagnitude as the turbulent mixing rates, and conse-quently the equilibrium assumption is not adequate forcalculation of NOx concentrations70,71.

Work has been reported recently to deduce a globalreaction rate for the reburning reaction CiH j þ NO →HCN þ ...72. A method for obtaining a global chemicalreaction rate expression and its associated rate constantwas documented73, and the global reaction rate wassubsequently reported74. A reaction scheme with 254gaseous elementary reactions was used4,75in simulationsof NO formation in premixed, laminar hydrocarbon-containing flames, together with the CHEMKIN code76.De Soete77–79 had conducted extensive experiments onthe formation of nitrogen-containing pollutant species inpremixed flames. From correlation of these experimentalresults, a set of global reactions and corresponding rateconstants was determined. Methane (CH4) and ethylene(C2H4) were used as fuels, with NO, C2N2 or ammonia(NH3) being added as fuel-nitrogen sources. Though DeSoete’s experimental work was performed over twodecades ago, global rates of NO formation from fuel-nitrogen measured in those studies are still widely used inpredictions of fuel-NOx in hydrocarbon–air combustionsystems.

Rate constants for the following global reactions were


obtained by De Soete79 by correlating these laboratorydata:

HCNþ O2 → NOþ … (20)

HCNþ NO → N2 þ … (21)

The method used by Chenet al.73 for determining theglobal reburning rate was identical to that used by DeSoete77–79for global fuel-NO reactions, with one excep-tion. De Soete determined mole fractions and theirderivatives in the flow direction from test data, whereasChenet al.73 used calculated mole fraction profiles forone-dimensional, premixed flames. Otherwise, DeSoete’s method was applied to deduction of the reactionrate for the NO–CiH j reburning reaction74:∑

CiHj þ NO → HCNþ … (22)

The global fuel-NO formation pathway illustrated byFig. 9 includes the reburning reaction. Sensitivityanalyses72 indicated that reburning-NO steps wereimportant and sometimes rate determining in the forma-tion and destruction of NO in hydrocarbon-rich regionsof flames. Apparently, without the reburning-NO steps,simulations of NO formation will result in inaccurateprofiles of NO, HCN and NH3 in these fuel-richhydrocarbon flames.

oCiH j was taken to include all hydrocarbon specieswhich occur locally in the flame zone. In this work, thesespecies included C2H4, C2H3, C2H2, C2H, CH4, CH3,CH2 and CH. However, throughout the simulations usedto determine a global reburning kinetic rate72, only CHand CH2 were predicted in significant concentrations,which was also observed experimentally by Wendt andMereb37.

A method based on the simulations discussedpreviously was used to quantitatively evaluate theglobal reburning-NO pathway. From reaction (22) andFig. 9, the global reburning-NO rate,rNO

22 , in the absenceof NH3 reactions, thermal-NO and prompt-NO, wasexpressed by material balance as:

rNO22 ¼ r20 ¹

dXNO

dt¹ r21 (23)

An expression fork22 is:

k22 ¼r22

XHCXNO¼

k20XHCNXbO2

¹dXNO

dt¹ k21XHCNXNO

XHCXNO

(24)

where b is the function of the oxygen concentrationgiven by De Soete79, andXHC is the sum of all hydro-carbon radical and hydrocarbon species concentrations.In Eq. (24),k22 is a function ofk20, and hence comparisonof k22 with different k20 expressions by De Soete79 wasalso reported. Three correlations of the global reburning-NO rate expression were obtained for CH4 and C2H4

flames from this method74. Two k22 values werethose obtained from values ofk20 derived from simula-tions73 with different b values, and the thirdk22 valuewas obtained using De Soete’s experimental work79 fork22.

The correlated rate constants fromb ¼ 1.5 andb¼ f (XO2

) showed only a small difference, which meansthat the global reburning rate constants were not verydependent on the oxygen concentration in theseflames. Thek22 rate based on the simulatedk20 washigher thank22 based on the experimentalk20 (from DeSoete79).

Three morek22 correlations by Eq. (24) with NO-seeded flames were also obtained. When NO seeding wasused, the reburning-NO mechanism was dominant, andthe formation of NO through the fuel-N pathway wasthought to be less important. The global activationenergies from these expressions were within6 6% ofthe average value, and the pre-exponential factors werewithin a factor of two and a half of the average value.The global reburning rate with NO seeding exhibitedlower values than those without NO seeding.

Thek22 rate expression recommended by Chenet al.73

for a global NOx kinetic model which is a composite ofthe several expressions is:

k22 ¹ 2:7 3 106 exp ¹18800

RT

� �(25)

The quantitative evaluation of this new global rateexpression and its parameters by comparison with thedata from turbulent, particle-laden diffusion flameswith reburning is discussed subsequently.

5. PREDICTIVE METHODS

While several predictive models for formation of NOthrough modeling of thermal and fuel processes havebeen developed (e.g., Boardman and Smoot3), untilrecently there has been little published work ondestruction of NO through reburning processes.Several approaches to predict reburning-NO arebeing considered and models are now becomingavailable.

Mereb and Wendt8 simplified the detailed kineticmechanism of Glarborget al.80 for natural gas reburning,and then developed an engineering model with theassumption of partial equilibrium of some intermediatespecies:

d[NO]dt

¼ ¹ [NO][NH3] f1 ¹ [NO][CH4] f2 (26)


Fig. 9. Global NOx formation–depletion mechanism74.

d[HCN]dt

¼ ¹ [HCN]( f3 þ f4) þ [CH4] [NO] f2ÿ

þ [N2] f5 þ [NH3] f6Þ

(27)

d[N2]dt

¼ [NO][NH3] f7 ¹ [CH4][N2] f5 (28)

d[NH3]dt

¼ ¹d[NO]

dt¹

d[HCN]dt

¹ 2d[N2]

dt(29)

wheref i ¼ function ([OH], [H2O], temperature), and theCH4 concentration is calculated from partial equilibria:

CHi þ OH → CHi ¹ 1 þ H2O (30)

If initial estimates for OH and H2O are available for agiven temperature and initial measured values of CH4,NO, HCN, NH3, H2, H2O and N2 for this 1-D approach,the profiles of NO, HCN, N2 and NH3 can be computed.As shown in Fig. 10, this model provides reasonablepredictions for HCN but high values for NO and lowvalues for NH3 in the reburning zone over a widerange, provided CH4 is the reburning fuel. Recently,Wendt42 extended the model to predict N2O concentra-tions. However, this simple kinetic model cannot be inte-grated into a comprehensive computer code since noturbulence effects are included, and some measuredvalues are required.

Glarborget al. 81 developed a reduced mechanism fornitrogen chemistry in methane combustion, based onMiller and Bowman’s4 full mechanism, by usingPeters’82 systematic reduction strategy. This containeda four-step mechanism for methane oxidation plus tworate-of-production terms for HCN and NO. The modelgenerally provides a good description of formation anddestruction of nitrogen oxides compared with the fullmechanism, as shown in Fig. 11.


Fig. 10. Comparison between measured and predicted nitrogenous species for three laboratory-scale gas reburning experiments,using the model developed by Mereb and Wendt8.

Fig. 11. Comparison of results for methane combustion with NOadded in a stirred reactor as a function of fuel/air equivalenceratio (circle: experimental NO data; square: experimental HCNdata; solid line: full mechanism; dashed line: skeletal

mechanism; dotted line: reduced mechanism)81.

Starting with full mechanisms of a hydrocarbon flame,several other researchers have calculated the NOx

emissions during reburning. Liet al.63 used a PC-basedcomputer model, including 2-D furnace combustion andheat transfer, and 1-D chemical kinetics with detailedgaseous hydrocarbon and nitrogen reaction mechanisms(i.e., 43-species, 201-step NOx mechanisms) to evaluatereburning/cofiring performance without particles orturbulence interactions. After simulating four full-scale, coal-fired gas-reburning boilers, Payne andMoyeda83 indicated that the coupling of a detailedchemistry model with a simplified mixing model, basedupon the results of thermal and physical models of thefull-scale system, can reasonably predict the generalperformance trends observed in the full-scale system, butthat more accurate models that couple the complexinteractions between turbulence and chemistry whichoccur in the turbulent mixing process are needed. Huraand Breent64 investigated the effects of variation intemperature, reburn zone stoichiometry, initial NOlevels, flue gas/natural gas mixing rate, and heat losson reburning effectiveness by using Boyle’s 193-stepkinetic model and a modified CHEMKIN code with theequilibrium assumption for major species including N2,CO2, H2O, O2, NO, CO and OH. Ballesteret al.84

utilized 0.5 MWt experimental furnace measurementsand then extrapolated to a 220 MWe tangentially coal-fired boiler and a 350 MWe wall-fired boiler, using CFDmodeling with detailed NOx chemistry. The modelingresults were in reasonable agreement with reburningfield experience. A CFD model combined with sevenchemical species and four reactions in a reduced NOmodel for methane combustion has been used for anMSW incinerator by Rasmussen85. The isothermalmodeling reportedly provided a reasonable predictionof observed results. Tyson86 suggested that thesecalculational processes, assuming finite rate chemistrywith instantaneous mixing, predict a limit of 80–90%NOx reduction (with reburning), but such high NOreduction values cannot be achieved due to mixinglimitations.

None of these computational processes considered theturbulence–chemistry interactions or multidimensionaleffects on the reburning process. Chen72 also suggestedthat the reduced models derived with these varioustechniques are still too complicated to predict combus-tion systems if the models are to account for turbulenceand multidimensional effects. One solution of theproblem of incorporating effects of turbulence is tofind a reduced mechanism by using various approachesincluding global modeling, detailed mechanism reduc-tion, response modeling, chemical lumping, or statisticallumping87.

Babcock and Wilcox88 have recently incorporated theglobal reburning reaction of Chenet al.74 into their NOx

formation submodel, which itself is a component of amore general comprehensive combustion model. Theyhave recently applied this predictive method to a large-scale utility boiler fitted with a cyclone combustor. Theyfirst adjusted parameters in the NOx model to match

model predictions with observed results for a full-loadfurnace without reburning. Then, predicted NOx con-centrations with reburning were reported to be reliable atfull load. However, predictions at lower load (e.g., 75%)with reburning under-predicted the observed reduction inNOx concentration. They reported this same experiencein a few other cases with and without reburning.Fiveland88 emphasized their need to adjust the predic-tions for full load without NOx control. A commercialsoftware code, SOAPP, developed by Sargen & Lundyand EPRI, is available for the analysis of natural gasreburning in utility boilers fired by coal or oil89.

Brouweret al.90 developed a reduced mechanism (6species, 7 reactions) for the SNCR process in practicalsystems, with the influence of CO. The reagents includedammonia, cyanuric acid and urea. This simplified SNCRmechanism was derived from Miller and Bowman’smechanism4 through sensitivity analysis and curve fitting.For example, using NH3 as the reagent, the global modelwas expressed as follows, where for the reaction

NH3 þ NO → N2 þ H2Oþ H (31)

A ¼ 4.24 3 108 cm mol s K, b ¼ 5.30, and E ¼

83 600 K, and for the reaction

NH3 þ O2 → NOþ H2Oþ H (32)

A ¼ 3.50 3 105 cm mol s K, b ¼ 7.65, and E ¼

125 300 K. Hence, the rate constants (k ¼ ATbe¹E/T)are defined. This reduced mechanism was integratedinto their comprehensive CFD code, JASPER. Figure12 compares predictions from JASPER with the reducedmodel to independent, premixed ammonia data ofLyon91 from a turbulent flow reactor under isothermalconditions, and to calculations from CHEMKIN76 withthe full mechanism.


Fig. 12. Comparison of the reduced SNCR chemistry inJASPER with the full chemistry set and the data of Lyon forhomogeneous conditions (dotted line: full chemistry; solid line:JASPER calculations with reduced chemistry; triangle: data of

Lyon91)90.

In order to account for the radical chemistry effects ofCO, these authors used an empirical effective temper-ature rather than the predicted temperature in the rateexpression, i.e.,

Teff ¼ T þ S(CO) (33)

where S(CO) ¼ 17.5 3 ln ([CO]) ¹ 68.0 has beendeduced from curve fitting to match CHEMKINpredictions with the full mechanism.

Brouwer et al.90 also made experimental SNCRmeasurements in a 29 kW, refractory-lined furnace, inwhich a typical product stream flue gas was flowing with500 ppm NO, 750 ppm NH3; various levels of CO andN2 were then injected with the NH3 reagent downstream.In this work, three important parameters on SNCReffectiveness were investigated: local CO concentration,temperature quench rate, and ammonia injectionmomentum. As shown in Fig. 13, JASPER with thereduced SNCR submodel with the turbulence gave goodpredictions for this turbulent case.

This work may be the first global model for the SNCRprocess with the effect of CO. This approach is beingapplied to advanced reburning technology by the authorsin ongoing work.

In order to predict the advanced reburning process,Brouwer92 has recently extended the above work90 totreat advanced reburning through use of the set of globalreactions. The predicted results showed agreement withmeasurements, but this work has not been published.

At this center, development of an advanced reburningmodel is also in process93. Using Peters’82 method, a7-step reduced mechanism has been derived from a312-step, 50-species full mechanism, which correctlypredicts observed trends, including the effects oftemperatures, the ratio of NH3 to NO, and concentrationsof CO, O2 and H2O on NO reduction94. Incorporation ofthis 7-step mechanism into a comprehensive, three-dimensional combustion code (PCGC-3) and itsevaluation is in process93.

All of the predictive approaches noted above are forgas as a reburning fuel. So far, there is no model reportedto predict NOx emissions with coal or other solidhydrocarbon compound as the reburning fuel whereheterogeneous kinetics have been reported. In fact, manyresearchers have noticed that the heterogeneous reactionof NO with char is important in the reduction of NO fromcoal combustion processes95–97. As Guo and Hecker98

summarized, the investigations of the reaction of NOwith char involve the kinetics and mechanism95,96, theeffects of char surface area99,100, the effects of char ashand its composition, the catalytic effects of metals97,101,and the effects of feed gas composition95,102. Thereaction of NO with char has generally been reportedto be first order with respect to NO partial pressure. Guoand Hecker’s experiments showed an increase in theapparent activation energy with increasing temperature,and a decrease as the CaO content increases. A sharpshift in the activation energy has been observed, asshown in Fig. 14. This shift to higher activation energy

with increasing temperature is opposite to that expectedif a reaction is changing from chemical rate control tomass transfer control, and suggests different mechanismsor rate-determining steps at high and low temperatures.Questions concerning N2 formation, the surface com-plexes, the nature of active surface sites, and the effectsof active minerals in char are still not well understood.Also, most studies of heterogeneous reactions have beenconducted at relatively low temperatures (,1000 K)compared to those of reburning injection locations(,1500 K). Only a few investigations have been doneat high temperatures. Tenget al.96 tabulated the


Fig. 14. Arrhenius plots of the reaction constants for NO reduc-tion for three char types (from Guo and Hecker98), where NDLis North Dakota Beulah Zap lignite char, NDW is NDL washed

with HCl, and NCa is NDW reloaded with calcium oxide.

Fig. 13. Measurements and predictions of the effect of reagentjet/product stream momentum ratio on SNCR with and withoutCO at high and low quench rates. Symbols are data (square: lowquench with 0 ppm CO; circle: low quench with 750 ppm CO;triangle: high quench with 0 ppm CO; diamond: high quenchwith 750 ppm CO), and lines are results from mixing andreduced chemistry model (solid line: low quench with 0 ppmCO; dashed line: low quench with 750 ppm CO; solid line: highquench with 0 ppm CO; dotted line: high quench with 750 ppm

CO)90.

activation energies for various types of NO–carbonreactions, but the kinetic data at both low and hightemperatures are not yet sufficiently accurate. Conse-quently, reliable heterogeneous models for the reactionof NO with char or coal at high temperatures have notbeen reported.

6. REBURNING MODEL APPLICATIONS

Chen72 used PCGC-2 to initially evaluate the NOx

submodel with the global reburning reaction. Twolaboratory-scale reburning cases from Mereb andWendt8 were simulated. The predicted NO profileswere consistent with the measured results.

NOx predictions including the reburning reaction(Eq. (22)) have also been made with a comprehensivecombustion model, PCGC-3, which incorporated theNOx submodel of Fig. 993. PCGC-3 is a generalized,comprehensive, three-dimensional, steady-state com-bustion flow code for modeling turbulent combustionin practical systems103. It is the third generation ofPCGC-3 (pulverized coal gasification and combustion in3 dimensions), and incorporates advanced technology fordescribing NOx reburning, coal devolatilization, charoxidation, oil-droplet combustion, and multiple coal off-gas progress variables. It can be used to characterize non-reactive and reactive flows with entrained particles ordroplets, and is based on the general momentum, energyand mass conservation equations for turbulent systems.Details of this CFD/combustion predictive method aredocumented elsewhere104,105.

The NOx pollutant submodel incorporated in PCGC-3uses a post-processing approach to describe the forma-tion and reduction of NO in the flow field. Reactions ofNO involve thermal, fuel and reburning processes,according to Fig. 9. Formation of thermal NO isdetermined from the extended Zeldovich reactions,with two resulting rate expressions, one for fuel-richand one for fuel-lean conditions. Reactions of fuelnitrogen involve two additional species, HCN and NH3,and rate constants based on the work of severalresearchers can be selected for the fuel NO mechanismreaction rate constants. This NOx pollutant submodelincludes reduction of NO by the reburning processaccording to reaction (22).

Two other laboratory-scale, coal-fired diffusion-flamecases with gas reburning from Mereb and Wendt8 havesubsequently been simulated with PCGC-3 to furtherevaluate the reburning model93. The input conditions ofthe two cases are listed in Table 4. The diameter andlength of the cylindrical drop-tube reactor were 0.15 mand 2.12 m, respectively. The location of the reburninginjector is 0.53 m. Case 1 has a tertiary stream located at0.99 m for the addition of final combustion air. Therewas no tertiary air injection in Case 2. In Case 1 thereburning stream was methane, and in Case 2 thereburning stream consisted of methane and N2. The ratioof reburning fuel to total fuel was about 11–13% for bothcases. Coal particle size distributions and wall temper-ature profiles were not reported for either case. However,Wendt106 indicated that the wall temperatures werethought to be within 50 K of the centerline gastemperatures and that the particle size distribution was


Table 4. Input conditions for two cases with gas reburning laboratory furnace, coal-fired8

Variables Case 1 Case 2

Primary zone stoichiometry 1.1 1.1Reburning zone stoichiometry 0.9 0.88Tertiary zone stoichiometry 1.06 —

Primary streamTemperature (K) 590 590Air flow rate (kg s¹1) 5.5 3 10¹4 5.5 3 10¹4

Type of coal Utah bituminous Utah bituminousEst. mass mean particle size (mm) 80 80Mass coal/mass air 0.494 0.691

Secondary streamTemperature (K) 590 590Air flow rate (kg s¹1) 2.0793 10¹3 3.20793 10¹3

Reburning streamTemperature (K) 300 300CH4 flow rate (kg s¹1) 3.0883 10¹5 4.883 10¹5

N2 flow rate (kg s¹1) 0.00 2.2163 10¹4

Mass CH4 /(coalþ CH4) 0.125 0.114Injection location (m) 0.533 0.533

Tertiary streamTemperature (K) 590 —Air flow rate (kg s¹1) 4.6733 10¹4 —Injection location (m) 0.99 —

Est. wall temperature (K) 950a 1400a

aNo measured wall temperatures were available. Value adjusted to match temperature profile data.

a standard unclassified grind of about 70% through 200mesh. Both of these parameters are shown later to have asignificant impact on the NO profiles. The mass meanparticle diameter used herein was estimated to be about80mm for the baseline calculations. Effects of activationenergy (E), pre-exponential factor (A), particle diameter,particle size distribution, reburning gas injection velocityand reburning gas composition were considered93. Basevalues ofE ¼ 1.893 104 andA ¼ 2.723 106 were usedin these computations, essentially the same as thoserecommended by Chen72 and Chenet al.73.

6.1. Reaction Parameters

Figure 15 shows the effects of the activation energy(E) for the reburning reaction on the predicted NOconcentrations for Case 1. All other parameters remainedconstant for the simulations. The average NO concentra-tion is shown as a function of axial location in thefurnace for the cases with no reburning and threedifferent values of activation energy. The value ofactivation energy,E ¼ 1.89 3 104, results in NO

concentrations similar to those achieved with noreburning, in which methane was still injected but thereburning reaction was inactive. Reducing the value ofthe activation energy resulted in more reduction of NOby the reburning reaction. The value ofE ¼ 0.753 104

for the activation energy subsequently shows goodagreement of the measured and predicted values.

Figure 16 shows the effects of changes in the pre-exponential factor (A) for the reburning reaction on thepredicted NO concentrations for Case 1 with the value ofE ¼ 0.753 104. All other parameters remained constantin the simulations. The average NO concentration isshown as a function of axial location in the furnace forcases with no reburning for three different values of pre-exponential factors. The predicted values are alsocompared with the experimental values. Changes in thevalue of theA had a significant effect on the predictedNO concentrations, and the base value,A ¼ 2.723 106,resulted in the best agreement between measured andpredicted values.

Figure 17 shows profiles of average NO concentrationfor simulations with no reburning and with reburning for


Fig. 16. Effect of pre-exponential factor (A) on predicted NO concentrations for Case 1.

Fig. 15. Effect of activation energy (E) on predicted NO concentrations for Case 1.

two activation energies compared with the measuredvalues for Case 293. As before, the trial withoutreburning was performed by still injecting the sameamount of methane but turning off the reburningreactions.E ¼ 1.89 3 104 is the base value.E ¼ 0.753 104 is the value of the activation energy which showedthe best agreement with measured values of NOconcentration for Case 1. As before, theE base valueshows little difference with the trial without reburning,and the predicted NO concentrations obtained withE ¼

0.75 3 104 show a significantly better agreement withthe measured values. Consequently, the values ofE ¼

0.753 104 andA ¼ 2.723 106 were used in the balanceof the simulations.

6.2. Particle Size

Wall temperatures and particle size distribution werenot given by Mereb and Wendt8. The wall temperatureswere estimated from the experimental average gastemperature, but the particle size and distribution werenot reported. Therefore, the influence of particle size anddistribution on the NOx emissions was investigated. Aspecific particle size distribution was selected as baselinetrial, namely, P1 (see Table 5). The effects of mass meanparticle diameter from 64 to 96mm for the samedistribution shape for Case 2 are shown in Fig. 18. Effectson predicted NOx profiles were not very significant.

Computations were also made for five different

particle distributions at a constant mean particlediameter of 80mm, as shown in Table 5. From P1 toP5, the particle distributions become narrower andnarrower. These five sets of particle distributions wereused to simulate Case 1, and results are shown in Fig. 19.In Fig. 19(a), the location of ignition varied withchanging particle distribution. With the P1 distributioncontaining a large fraction of small particles earlierignition was observed, whereas with the P5 distributionlater ignition was observed. From Fig. 19(b), using thebroad P1 distribution, the lowest predicted NO level wasobtained, whereas with the use of the P5 distribution thehighest NO value was predicted. Smaller particles causeearlier ignition and much lower NO concentrations. Thesame observations resulted for Case 2, as shown in Fig.20. For the same mass mean particle diameter, thehighest NO was observed with the uniform particle size.Predicted NO levels were far more sensitive to thepresence of small particles than to changes in meanparticle diameter, even though only very small masspercentages of very small particles are present.

6.3. Reburn Gas Composition

Figure 21 shows the effects of changes in the masspercentage of CH4 in the reburning fuel on the predictedNO concentrations. In the N2 ¼ 100% trial, no CH4 isinjected, and the reduction in the NO concentrationresults from dilution and reduction in the gas


Fig. 17. Effect of activation energy (E) on predicted NO concentrations for Case 2.

Table 5. Selected particle size distributions for model simulations

P1 Size (mm) 1.7 6.5 40.0 105.0 195.0 dmm ¼ 80.2Mass fraction 0.0024 0.0126 0.51 0.366 0.109

P2 Size (mm) 11.8 35.3 75.0 132.0 195.0 80.2Mass fraction 0.1 0.3 0.3 0.2 0.1




temperature. Increasing the mass of CH4 to 18% resultsin significant NO reduction compared to the case withoutCH4. A further increase of the mass of CH4 to 50% doesnot show significantly lower NO concentrations,

especially at the exit of the reactor. This is consistentwith experimental observations by Chaggeret al.20, whoreported an upper limit in hydrocarbon gas percentagefor the reburning reaction.


Fig. 18. Effect of mean particle size on NOx profiles for Case 2.

Fig. 19. (a) Effect of particle distributions on gas temperature for Case 1. (b) Effect of particle distributions on NO profiles for Case 1.


Fig. 21. Effect of changes in the CH4 mass percentage in the reburning fuel on predicted NO concentrations for Case 2.

Fig. 20. Effect of particle distributions on NO profiles for Case 2.

Fig. 22. Effect of changes in the reburning injection velocity on predicted NO concentrations for Case 1.

6.4. Injection Velocity

Figure 22 shows the effects of changes in the velocityused for injection of the reburning fuel on the predicted

NO concentrations. The average NO concentration isshown as a function of axial location in the furnace forthree different injection velocities compared with themeasured values. This figure shows that as the injection


Fig. 23. (a) Comparison of measured and predicted gas temperatures as a function of axial location in the reactor for Case 1. (b)Comparison of measured and predicted CO2 and O2 concentrations as a function of axial location for Case 1. (c) Comparison of

measured and predicted average NO concentrations as a function of axial location for Case 1.

velocity is increased, NO reduction from reburning alsoincreases. This probably results because, at higherinjection velocities, the CH4 is able to penetrate furtherinto the reactor before it is consumed, which results in a

larger region where the reburning reaction has an effect.This is further supported by a reduction of the predictedNO values just upstream of the reburning injection due togreater spreading of the jet.


Fig. 24. (a) Comparison of measured and predicted temperatures as a function of axial location in the reactor for Case 2. (b)Comparison of measured and predicted CO2 and O2 concentrations as a function of axial location for Case 2. (c) Comparison of

measured and predicted average NO concentrations as a function of axial location for Case 2.

6.5. Comparisons with Data

In the following simulation, the P1 distribution (Table5) was used with a mass mean particle diameter of80mm.A andE values for these calculations were 2.723106 and 0.753 104, respectively. Figure 23 showscomparisons between measured and predicted averagetemperature, CO2, O2, and NO profiles for Case 1. Figure23(a) shows the comparison of average measured andpredicted gas temperatures as a function of axial locationin the reactor. The rapid increase and subsequentdecrease of temperature at approximately 1 m is wherethe tertiary air is injected. The predicted temperatureprofile was adjusted using a constant wall temperature tomatch the measured data, with a wall temperature of950 K giving best results. Figure 23(b) shows thecomparisons of average measured and predicted CO2

and O2 concentrations as a function of axial location inthe reactor. The measured and predicted concentrationsshow similar effluent magnitudes but somewhat differenttrends, compared with the experimental data. Thissuggests that the particle reactions occurred moreslowly in the simulations than was observed experimen-tally. Figure 23(c) shows the comparisons of averagemeasured and predicted NO concentrations as a functionof axial location in the reactor. The predicted profileshows good agreement with the measured values andindicates a rapid decay of NO just downstream of thereburning gas injection location.

Figure 24 shows comparisons of measured andpredicted average values of temperature, CO2, O2 andNO concentrations for Case 2, for a wall temperature of1450 K. Figure 24(a) shows good comparison ofaveraged measured and predicted gas temperatures as afunction of axial location in the reactor. Figure 24(b)shows comparisons of average measured and predictedCO2 and O2 concentrations as a function of axial locationin the reactor. As in Case 1, these comparisons showsimilar trends, but the predicted CO2 formation and O2

consumption are slower than those observed experimen-tally. Figure 24(c) shows the comparisons of averagemeasured and predicted NO concentrations as a functionof axial location in the reactor. This simulation wasperformed withE ¼ 0.753 104 and the pre-exponentialvalue reported by Chen72, and shows good agreementbetween measured and predicted values of NO concen-tration. This newer work93 casts doubt on the usefulnessof the activation energy reported by Chen72 for general-ized reburning computations.

7. SUMMARY

Reburning, the process whereby gaseous, liquid orsolid hydrocarbon is injected downstream of thecombustion zone to reduce NO to HCN, is a commer-cialized technology. First tested in about 1950 andnamed in 1973, it has been commercially demonstratedin very large-scale utility boilers. It has also beendemonstrated in four of the Clean Coal Technology

programs. Typically, 10–30% of the total fuel is used asthe reburn fuel.

Reburning with natural gas, without other NOx controltechnologies, can typically provide 35–65% reduction ineffluent NOx emissions. Simultaneous reductions in SOx

with addition of calcium-based sorbents, and in CO2,have been noted. Recent work suggests that jointapplication of other NOx control technologies such aslow-NOx burners, staged combustion, urea or ammoniainjection (advanced reburning) may be able to reduceemissions by up to 85%10. Even newer concepts showpromise of even greater reductions of up to 95% withoutSCR32.

Reburning technology has most commonly beenapplied to coal-fired boilers, but has also been demon-strated for oil- and gas-fired incineration systems andwaste fuels. In support of rapidly developing reburningtechnologies, substantial work on laboratory measure-ments, kinetic mechanisms and comprehensive modelinghas been reported. Published and planned profilemeasurements in laboratory reactors provide a soundbasis for model evaluation. Idealized predictions withfundamental and reduced sets of kinetic reactionsdescribe some of the observed features of reburning.Some of the first reburning predictions with comprehen-sive combustion codes, making use of reduced kineticschemes or newly reported global rate constants forreburning reactions, are being reported. Refinement,improvement and evaluation of these new predictivetools are required.

Some test results show low-rank coal chars to beparticularly effective reburning fuels, but much remainsto be done to clarify the differences in reburning fuelsand associated causes. Further research and demonstra-tion for highly efficient combined NOx reductiontechnologies and new advanced reburning technologiesare also needed, some of which was ongoing at thiswriting.

Acknowledgements—Some of this research and preparation ofthis review paper were sponsored by the Advanced CombustionEngineering Research Center. Funds for this center are receivedfrom the National Science Foundation (Engineering Educationand Centers Division), the State of Utah (Centers of Excel-lence), 37 industrial participants, and five federal agencies. Theauthors express their gratitude to all of the above, the Universityof Utah, Dr Dale Tree, Mechanical Engineering at BYU, DrWilliam Bartok, Consultant, and Mr Jerry Cole, EER. The workof Mrs Eva Black in preparing this manuscript is greatlyappreciated.

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