ammonia conversion and nox formation in laminar coflowing nonpremixed methane-air flames

Ammonia Conversion and NOx Formation in LaminarCoflowing Nonpremixed Methane-Air Flames

NEAL SULLIVAN†, ANKER JENSEN, and PETER GLARBORGDepartment of Chemical Engineering, Technical University of Denmark, DK-2800, Lyngby, Denmark

MARCUS S. DAY, JOSEPH F. GRCAR*, and JOHN B. BELLCenter for Computational Sciences and Engineering, Lawrence Berkeley National Laboratory, Berkeley, CA

94720, USA

CHRISTOPHER J. POPECombustion Research Facility, Sandia National Laboratories, Livermore, CA 94551, USA

and

ROBERT J. KEEEngineering Division, Colorado School of Mines, Golden, CO 80401, USA

This paper reports on a combined experimental and modeling investigation of NOx formation in laminar,ammonia-seeded, nitrogen-diluted, methane diffusion flames. The methane-ammonia mixture is a surrogate forbiomass fuels, which contain significant fuel-bound nitrogen. The experiments use flue-gas sampling to measurethe concentration of stable species in the exhaust gas. The computations use adaptive mesh refinement (AMR)to capture fine-scale features of the flame. The model includes a detailed chemical mechanism, differentialdiffusion, buoyancy, and radiative losses. The model shows good agreement with the measurements over the fullrange of experimental NH3 seeding amounts. As more NH3 is added, a greater percentage is converted to N2

rather than to NO. The simulation results are analyzed to trace the changes in NO formation mechanisms withincreasing amounts of ammonia in the fuel. © 2002 by The Combustion Institute

INTRODUCTION

The dependence on combustion for meetingworld energy demands has given rise to manyharmful side effects, such as photochemicalsmog and “acid rain,” due in part to the emis-sions of the nitrogen oxides NO and NO2,collectively termed NOx. With increasing gov-ernmental regulation of pollutant emissions, thecontrol and reduction of NOx is not only anenvironmental matter, but a financial one aswell [1]. The problem is exacerbated by a desireto use a combination of national coal reservesand biomass power supplies to alleviate oildependence. Coal and biomass fuels may con-tain significant amounts of chemically boundnitrogen—as much as 2% by mass. The nitrog-enous gases that are released from these solidfuels during pyrolysis are converted in the flame

to either N2 or NOx, depending on the localcombustion conditions.

The chemical form taken by volatile fuel-nitrogen has little influence on the overall con-version rate to NO [2–7]. Typically, the nitro-gen-containing compounds form either HCN orNH3. Although the fraction of each speciesremains the subject of research, ammonia istypically present in higher concentrations thanother fuel-nitrogen species in volatiles frombiomass feedstocks [7, 8]. Thus, we primarilyuse an ammonia-seeded fuel in the presentstudy, but for a few experiments we use hydro-gen cyanide seeding for comparison.

The main chemical formation routes of NOx

in flames are well established [9]. At tempera-tures exceeding 1800 K, atmospheric nitrogenoxidizes to NO through the thermal (or Zeldov-ich) mechanism. In the prompt (or Fenimore)mechanism, NO forms in the flame zonethrough reactions between molecular nitrogenand hydrocarbon radicals. In the ammoniamechanism (shown in Fig. 1, adapted from

*Corresponding author. E-mail: [email protected]†Currently at ITN Energy Systems, 8130 Shaffer Parkway,Littleton, CO 80127 USA.

COMBUSTION AND FLAME 131:285–298 (2002)© 2002 by The Combustion Institute 0010-2180/02/$–see front matterPublished by Elsevier Science Inc. PII S0010-2180(02)00413-3

Miller and Bowman [9]), fuel-nitrogen speciespresent as NH3 undergo hydrogen abstractionreactions. Each resulting NHi radical then par-ticipates in one of two subsequent reactionmechanisms: oxidation leading to NO forma-tion, or conversion to N2 through reactions thatadditionally consume NO.

Like most solid fuels, volatiles from biomassare burned in non-premixed flames. Many pre-vious studies of NOx formation in diffusionflames focus on the relative roles of thermal andprompt formation routes under a variety ofconditions [10–13]. However, NOx from thecombustion of biomass and other solid fuels isdominated by the conversion of significantamounts of fuel-bound nitrogen; thermal andprompt mechanisms are comparatively insignif-icant. Despite its importance, the formation ofNOx from fuel-nitrogen in non-premixed flameshas seen comparatively little investigation.

In the present work we consider an axisym-metric, laminar, coflowing, non-premixed flamedepicted in Fig. 2. This represents a reasonablycomplete model system for studying NOx for-mation during the combustion of volatiles fromsolid fuels. It affords evaluation of productionpathways in a realistic flow field without intro-ducing models for turbulence or compromisingthe fidelity of the chemical kinetics representa-tion.

Early models of this system assumed negligi-ble axial diffusion (i.e., the “boundary layer”assumption). Burke and Schumann [14] demon-strated that such a model could accurately pre-

dict diffusion flame heights. Roper et al. [15, 16]and Gordon et al. [17, 18] generalized thisapproach into a well-established flame analysistool. Miller and Kee [19] combined the bound-ary layer assumption with detailed reaction ki-netics to simulate a laminar, hydrogen-air, non-premixed flame. Interestingly, they found thatflame-zone radical concentrations may exceedequilibrium values by more than an order ofmagnitude. Super-equilibrium atomic O con-centrations may strongly affect NOx formation[13, 20].

A non-premixed flame forms where there is abalance between chemical reactions and diffu-sive and convective transport. For lower-speedflows, where axial diffusion is comparable to theadvective transport, Chung and Law [21] dem-onstrated that the boundary layer assumptionleads to poor predictions for flame height andshape of diffusion flames. The experimentalwork of Ishizuka and Sakai [22] and numericalstudies by Takagi and Xu [23] indicated thataxial diffusion plays a key role in the diffusionflame’s structure.

The work of Smooke et al. [24, 25] wasapparently the first to successfully model thecoflowing laminar non-premixed flame withoutmaking use of the boundary layer approxima-tion. The 2-D model has served well as a tool toprovide significant insight into the structure ofdiffusion flames [24–31], including NO forma-

Fig. 1. Ammonia oxidation mechanism [50].

Fig. 2. Schematic of the laminar, coflowing, non-premixedflame.

286 N. SULLIVAN ET AL.

tion [12]. However, no study of fuel-nitrogeneffects has yet been performed.

Nishioka et al. [32] correlated flame structurefrom simulations of axisymmetric, coflowing,non-premixed flames with that of one-dimen-sional, opposed-flow, non-premixed flamesthrough a “representative diffusion time.” Theability to capture the physics of this 2-D prob-lem through a series of one-dimensional calcu-lations represented a significant breakthroughand decrease in complexity. However, in a sub-sequent study, Zhu et al. [33] found that thiscorrelation breaks down when considering NOx

formation, indicating that emission characteris-tics from 2-D flames are different from those of1-D flames.

In this study, we investigate a laminar, ammo-nia-enriched, methane-air non-premixed flamethrough both experiment and computation. Thefocus of the work is to understand how thefuel-nitrogen routes for NO formation differfrom those of flames without fuel-nitrogen. Thesimulations provide steady, axisymmetric spa-tially resolved species and thermal profiles fordetailed analysis of the NO production path-ways. Experimental data are used to validateglobal predictions of the model, and to providea basis for comparing discrepancies betweendifferent detailed chemical mechanisms.

EXPERIMENTAL SETUP

The flame depicted in Fig. 2 is established in aquartz reactor of dimensions � � 1 mm, rf � 6mm, rox � 14 mm, and height 760 mm. Longentrance lengths are provided for the inlet gasesso that fully developed flow is established in thefuel and oxidizer tubes upstream of the fuelnozzle. The two streams are mixed through en-trainment and diffusion, and for suitable fuel andoxidizer flow rates, a stable flame is produced.

The fuel is a mixture of methane and nitrogenat flow rates of 150 and 220 mL/min, respec-tively. Ammonia is added to the fuel stream inamounts ranging from 0 to 1000 ppm of thetotal fuel-oxidizer inflow. In selected experi-ments with fuel-N doping levels below 150 ppm,the NH3 is replaced with HCN to observe theeffects of fuel-N speciation on NO formation.The oxidizer is a mixture of research grade

oxygen and nitrogen at flow rates of 840 mL/minand 3160 mL/min, respectively (21% oxygen).The nitrogen dilution in the fuel stream resultsin a relatively cool flame, where peak tempera-tures rise only slightly above the 1800 K re-quired for significant thermal NO production.Flue gases from the reactor are dehumidifiedthrough a water trap upstream of Hartmann &Braun gas analyzers for measurement of O2,NO, CO2, and CO concentration.

THE COMPUTATIONAL MODEL

The flame is assumed to be axisymmetric so themodel has two spatial dimensions, axial z andradial r. The computational domain extends 11cm downstream from z � 0 at the exit edge ofthe fuel tube. This is shorter than the 76-cmquartz reactor, but it is much longer than theflame as indicated by peak temperature. Inflowboundary conditions specify fully developedlaminar pipe flow, or co-annular flow, for thefuel and oxidizer, respectively (Refuel � 41, Reox� 127). No-slip conditions apply along the outerwall and at the fuel tube edge. A non-reflectingoutflow boundary is enforced at z � 11 cm. Theouter wall of the domain has a fixed piecewise-linear temperature profile obtained by experi-mental measurement (rising from 500 K at theinlet to 800 K at 6 cm, then dropping to 300 Kat 50 cm). The fuel tube edge is assumed to beat 300 K. All gases enter the system at standardtemperature and pressure.

We use the computational approach pre-sented by Day and Bell [34]. This consists of atime-dependent, low Mach number model thatincludes buoyancy, mixture-averaged differen-tial diffusion [35] in both spatial coordinates,and optically thin (emission-only) radiation[36]. The numerical discretization is based on arobust projection formulation that accommo-dates large density contrasts. The algorithmuses an operator-split treatment of stiff reactionterms. The basic computational approach isembedded in an adaptive mesh refinement(AMR) framework that uses structured hierar-chical grids to concentrate mesh points near theflame sheet. The integration algorithm sub-cycles in time and preserves the discrete conser-vation properties of the underlying, single-grid

287AMMONIA CONVERSION IN NON-PREMIXED FLAMES

algorithm. Details of the discretization and im-plementation are discussed in Day and Bell [34].

The flow is “ignited” by choosing initial, t � 0,temperature conditions that have a hot zone(T � 2000 K) across the fuel and oxidizerinterface at z � 0. The solution then evolves intime until steady-state conditions are achieved.Convergence is monitored by the axial profile ofthe radially integrated NO profile.

The simulations are performed using two dif-ferent chemical mechanisms. The first is a mech-anism proposed by Glarborg et al. consisting of 66species and 447 reactions [37]. It contains theoxidation of C1 and C2 hydrocarbons, HCN, andNH3, with a subset describing interactions be-tween hydrocarbons and nitrogenous species. Thesecond is the GRI 3.0 mechanism containing 53species and 325 chemical reactions [38].

COMPARISON OF DATA ANDSIMULATION RESULTS

In Fig. 3, we present experimental and com-puted values for the flue-gas NO concentrationfor various amounts of ammonia in the fuelstream. The experimental values have beencorrected for the water removal upstream of thegas analyzers by assuming stoichiometric H2Oproduction in the flame zone.

With no ammonia added to the fuel stream,

the methane-air non-premixed flame is found toproduce 25 ppm of NO. This level increases to297 ppm NO in flue gases when the inflowcontains 1000 ppm NH3. While the flue gas NOconcentration increases with ammonia addition,the conversion rate of NH3 to NO decreases fromover 50% at [NH3]in � 100 ppm to less than 30%at [NH3]in � 800 ppm. Similar behavior has beenobserved in previous fuel-nitrogen studies [3].

Experiments with NH3 replaced by HCNshow no difference in NO formation. This is inagreement with results from the literature,which indicate that the speciation of the volatilenitrogen compounds does not have a significanteffect on the NO yield, both in lean premixedflames and in diffusion flames [7].

Results from the simulation are shown forthree amounts of fuel ammonia: 0, 500 ppm, and1000 ppm NH3 of the combined fuel-oxidizerinflow of 4.37 liter/min. As a convenience forsubsequent discussion, Fig. 3 also includes dataderived from a reduced model and from simula-tions incorporating alternative chemistry mech-anisms that are discussed later.

Generally, the agreement between experimentand modeling is good at all three ammonia con-centrations simulated. The Glarborg et al. mech-anism overall provides the more accurate andconsistent NO predictions. The model results us-ing the GRI 3.0 mechanism overpredict the fluegas NO concentration for the ammonia-enrichedflames by as much as 30% in the 1000 ppm case.We examine the source of these differences later.

ANALYSIS OF COMPUTATIONALRESULTS

Flame Structure

The results in this section apply to all the flamesinvestigated. The NH3 content of the fuelstream is found to have no significant effect onthe overall flame structure in the computationalmodel. General agreement with previous stud-ies (particularly [12] and others cited below)and the comparison in the previous section withexperimentally determined flue gas NO concen-tration provide a measure of model validation.The following discussion is based on the modelresults using the Glarborg et al. mechanism.

Fig. 3. Measured and computed flue-gas NO concentra-tions versus NH3 seeding as a fraction of the total fuel-airinflow (ammonia is added only to the fuel stream). Errorbars on the experimental data represent �6� (� is the SD).The reduced-model curve and the GRI data are discussedlater.


Figure 4a shows temperature in the lowerpart of the computational domain. The flame islifted �0.1 cm above the edge of the fuel tube,and is anchored at an ignition point located atz � 0.1 cm, r � 0.7 cm. Based on peak temper-ature, the flame length extends to z � 3.6 cm(Tmax � 1847 K). Figure 4b shows heat releaseand superimposed convection streamlines thatreveal entrainment of oxidizer fluid into the fuelstream. From the ignition zone, a rich premixedflame extends on the fuel side, while a leanpremixed flame burns on the oxidizer side. Anon-premixed flame lies between the two, lo-cated roughly at the stoichiometric line. Thistriple-flame structure has been noted in previ-ous studies [26, 27].

A weakly endothermic region lies betweenthe rich premixed flame and the non-premixedflame. This is the result of two hydrocarbondissociation reactions,

C2Hn�1(�M)º C2Hn � H(�M), n � 2, 4.

Endothermicity at the flame tip because of acety-lene formation has been observed in the flamessimulated by Ern et al. [27]. The chemical mech-anism used in that work did not include ethylene,which increases the area and magnitude of theendothermic zone in this simulation. The for-mation of these species has been experimentallyconfirmed by Gordon et al. [39].

Reaction path analysis reveals the carbonoxidation system shown in Fig. 5. The Appendixdescribes how the diagram is generated fromthe simulation. The overall chemical pathwaysare similar to those observed in premixed flamesimulations [40, 41]. Although there is signifi-cant activity involving C2 hydrocarbons, the bulkof the methane is converted through the C1 path.

Ammonia Oxidation and NOx Formation

Ammonia-Free Flame

Nitrogen reaction pathways for the ammonia-free flame are shown in Fig. 6. The most impor-tant mechanism for NO formation in this flameis prompt NO, initiated by the reaction CH�N23HCN � N. Thermal NO, initiated by N2 � O3 N � NO, is much less important, as theflame’s peak temperature is barely high enoughto support this mechanism. The relative contri-butions (77% vs. 17%) of these two reactions tothe path N2 3 N indicate the relative impor-tance of the prompt and thermal mechanisms inthis flame. In addition to prompt NO andthermal NO, mechanisms involving N2O yield aconsiderable amount of NO. The nitrous oxideis formed mostly from the reaction N2 � O � M3 N2O � M, but the sequence N2 � H3 NNHand NNH � O 3 N2O � H also contributes.

Fig. 4. a) Temperature (K) and b) heat release (ergs/cm3

sec) with superimposed advection streamlines. A non-linearscale is used for the heat release contours to highlight thetriple flame structure and the endothermic zone.

Fig. 5. Carbon reaction paths (Glarborg et al. mechanism).The thickness of an arrow indicates the quantity (mol/s) ofatomic carbon moving through the path; only paths at least4% of the greatest are shown. Table 1 gives the percent ofeach path as a result of various reactions.


Subsequently, N2O is largely recycled to N2, buta minor fraction reacts with H or O to form NO.

As can be seen from Fig. 6, the bulk of theNO is formed from oxidation of atomic nitro-gen, with significant contribution from oxidation

of HNO and NH. Interestingly, most of the N isnot formed directly from N2, but rather isderived from NH that in turn is formed fromcarbon-bearing species originating in HCN. Fig-ure 7 shows the mole fraction and the netproduction of NO in the lower part of thecomputational domain. The right side reveals atwo-layer structure which extends from the ig-nition zone to nearly the flame tip and remainsroughly parallel to the bulk convection. ThatNO is produced on the lean side of the flameand is consumed on the rich side has beenobserved in previous non-premixed flame stud-ies [13, 20, 42]. Transverse to the flow, nitrogenatoms in the species NO, HCN, NCO, NH, N,and HNO, move back and forth between thelayers of NO production and consumption. This“recycling” can be observed as the closed loop inFig. 6. The consumption of NO diffusing towardthe fuel stream occurs through two reactions:

NO � HCCONO � CH2

º

º

HCN � CO2,HCN � OH.

Some of the HCN produced in these reactionsaccumulates in the fuel stream and flows up-ward toward the flame tip. Above z � 3 cm,diffusion from the production zone to the con-sumption layer must compete with the upwardfluid advection. The remaining HCN is con-verted to NO, which then peaks in concentra-tion along the centerline just above the flame

TABLE 1

Composition of the Carbon Reaction Paths Depicted in Fig. 5

(100) CH4 3 CH3 (29) C2H3 3 C2H2 (17) C2H2 3 HCCO (9) HCCO 3 CO (4) CH2 3 C2H4

68% � H 65% �M 100% �O 57% �H 100% �CH3

26% �OH 15% �H (16) CH3 3 CH2O 26% �O2 (4) CH2 3 CH2O(77) CO 3 CO2 13% �CH3 98% �O (8) CH2 (S) 3 CH2 74% �OH

98% �OH (28) C2H5 3 C2H4 (15) CH3 3 C2H6 60% � N2 24% �CO2

(50) HCO 3 CO 99% �M 100% �CH3 29% �H2O (4) C2O 3 CO78% �M (20) CH2OH 3 CH2O (15) C2H6 3 C2H5 (7) CH3 3 CH2(S) 58% �O2

(47) CH2O 3 HCO 83% �M 64% �H 79% �OH 26% �OH52% �OH 13% � O2 20% �OH 20% �H (4) CH3 3 C2H4

34% �H (20) CH3 3 CH4 11% � CH3 (5) HCCO 3 CH2(S) 98% �CH2

10% �CH3 53% �H�M (13) CH3 3 C2H5 100% �H (4) CO2 3 CO(33) C2H4 3 C2H3 23% �CH2O 100% �CH3 (5) HCCO 3 C2O 32% �H

80% �H (18) CH3 3 CH2OH 100% �OH 31% �CH2(S)19% �OH 100% �OH 25% �CH2

Paths are listed by decreasing amount of atomic carbon (mol/s) moving through them; only paths at least 4% of the greatestappear here and in the figure. The percent of each path because of various reactions is shown; only contributions of at least10% are included.

Fig. 6. Nitrogen reaction paths for the flame without am-monia seeding (Glarborg et al. mechanism). Note thecontinuous recycling of nitrogen: starting in the form of NO,it passes through carbon-bearing species to NH and finallyback to NO. The thickness of an arrow indicates thequantity (mol/s) of atomic nitrogen moving through thepath; only paths at least 0.8% of the greatest are shown.Table 2 gives the percent of each path as a result of variousreactions.


tip. Beyond this location, NO diffuses across thedomain, and is carried out the flue.

Comparison of this mechanism with theprompt mechanism from Miller and Bowman[9] reveals interesting differences. While theinitiation reaction is identical, production ofNO by the HNO route is absent in [9]. Virtuallyall of the atomic nitrogen is converted to NO,with negligible N2 formation. This is likely be-cause of the low NO concentration in thisammonia-free flame, minimizing the effect ofthe N � NO 3 N2 � O reaction. Finally,extensive recycling reactions through cyano-spe-cies are captured in the Glarborg et al. mecha-nism. In view of Fig. 6, these reactions play asignificant role and lead to a more complexcollection of reactions paths than the promptmechanism shown in [9].

NO Formation in the Ammonia-ContainingFlame

Figure 8 displays the NO concentration and netproduction rate with 1000 ppm NH3 added tothe fuel stream. Generally, nitrogen species

concentrations and production rates increase byan order of magnitude, as does the final, flue gasNO concentration (from 25 ppm to 297 ppm).In comparison with Fig. 7a, a much differentNO field is observed. The peak in concentrationshifts from the area around the flame tip to theignition region at the flame base. (Planar laser-induced fluorescence images appear in a subse-quent paper [43].) NO formation in the ignitionregion is an order of magnitude higher than ataxial positions z � 1 cm.

From Figs. 7b and 8b, whether or not ammo-nia is added, it is clear that net NO productionis on the lean side of the flame zone and net NOconsumption is on the rich side. Diffusion to thefuel stream causes the NO concentration toreach nearly a third of its peak value along thecenterline. Notice that here, the reactions withHCCO and CH2 are not strong enough tosignificantly consume the fuel-side NO profile;the pool of hydrocarbons for these reactions hasnot changed from that of the ammonia-freeflame. In Fig. 8a, a mild NO reduction zone isevident in the interior of the flame just belowthe flame tip from reactions with HCCO.

Fig. 7. NO a) mole fraction and b) net production rate(mole/cm3 sec) for the ammonia-free methane-air flame.

Fig. 8. NO a) mole fraction and b) net production rate(mole/cm3s) for the 1000 ppm ammonia-seeded flame.


The nitrogen reaction paths are shown in Fig.9. Despite the complexity of this diagram, com-parison of the NO formation routes with theammonia oxidation mechanism in Fig. 1 showsexcellent agreement. It is noteworthy, however,that the NNH route shown in Fig. 1 is insignif-icant in this flame. As seen in the reactionpathways, atomic nitrogen both produces andconsumes NO. In fuel rich zones, atomic nitro-gen consumes NO through the reaction N �NO3 N2 � O, while in fuel lean zones, atomicnitrogen is oxidized to form NO through reac-tions with OH. In the ignition region, the ma-jority of the atomic nitrogen is oxidized to NOwhile less than 20% forms N2, consuming only afraction of NO in the process.

Comparison of the NOx formation pathwayswith and without ammonia present in the fuelstream (Figs. 9 and 6, respectively) reveals somesimilarities. In both cases, the bulk of the NO isformed either from HNO reactions with H andOH, or from N reactions with OH. The NOrecycling reactions through cyano species areidentical. The prompt-NOx chemistry of Fig. 6 is

also active in the ammonia-containing flame.However, the production of HCN and N fromfuel-nitrogen routes in the ignition region aretwo orders of magnitude higher than that fromthe prompt route, limiting the significance ofprompt NOx chemistry in the ammonia-contain-ing flame.

Finally, the conversion of atomic nitrogen toN2 through reactions with NO is insignificant inthe flame without ammonia, while it is quitesignificant in the ammonia-containing flame.This is because of the comparatively low NOconcentration in the pure methane-air flame.The activity of the N � NO3 N2 � O reactionincreases with increasing ammonia concentra-tion resulting in the non-linear ammonia con-version rate observed in Fig. 3.

Reduced Model Analysis of the DecliningEfficiency of NO Production

The results presented here and in previousstudies [3] show that NO production increaseswith declining efficiency as NH3 is added to the

TABLE 2

Composition of the Nitrogen Reaction Paths Depicted in Fig. 6 for the Flame withoutAmmonia Seeding

(100) N2 3 NNH (9) N 3 NO (3) CN 3 NCO (2) N2 3 N (1) HCN 3 CH3CN51% �H�O2 80% �OH 66% �OH 77% �CH 100% �CH3

48% �H 11% �O2 31% �O2 17% �O (1) CH3CN 3 CH2CN(96) NNH 3 N2 (8) NO 3 HCN (3) NO 3 HCNO (2) HCN 3 HOCN 87% �H

91% �O2 57% �HCCO 68% �CO 100% �OH 12% �OH(26)NO2 3 NO 22% �CH2 32% �H (2) H2CN 3 HCN (1) N2O 3 NH

77% �H (6) NH 3 N (3) NH 3 HNO 100% �M 99% �H18% �O 68% �H 96% �OH (2) N2 3 HCN (1) CN 3 HCN

(24) NO 3 NO2 32% �OH (2) NH 3 NO 100% �CH 39% �CH4

66% �O�M (5) NCO 3 NH 98% �O (1) N 3 H2CN 33% �C2H4

33% �HO2 100% �H (2) HCN 3 CN 100% �CH3 (0.9) NO 3 N(15) HNO 3 NO (5) HCN 3 NCO 49% �H (1) NNH 3 N2O 61% �C

49% �H 100% �O 44% �OH 100% �O 38% �CH45% �OH (4) HNCO 3 NH2 (2) HCNO 3 NO (1) NCO 3 NO (0.8) NO 3 CN

(14) N2 3 N2O 100% �H 79% �OH 99% �O 55% �C2H100% �O�M (3) NH2 3 NH 21% �H (1) N2O 3 NO 44% �C

(14) N2O 3 N2 55% �H (2) HOCN 3 NCO 76% �H (0.8) NH2 3 NH3

96% �H 40% �OH 96% �H 23% �O 42% �H�M(12) NO 3 HNO (3) NCO 3 HNCO (2) NO 3 HONO (1) HCN 3 NH 41% �H2O

80% �H�M 70% �H2O 100% �OH�M 100% �O 15% �H2

11% �H�N2 21% �H2 (2) HONO 3 NO2 (1) CH2CN 3 CN

91% �OH 98% �O

Paths are listed by decreasing amount of atomic nitrogen (mol/s) moving through them; only paths at least 0.8% of thegreatest appear here and in the figure. The strongest paths (above the line in the table) are opposed and net to weaker paths.The percent of each path because of various reactions is shown; only contributions of at least 10% are listed.


fuel stream. It is worth noting that this is asecond-order effect, so it cannot be explained bysensitivity analysis, which relies on first-orderderivatives. Also, except at very high fuel-N levels[44] it is not observed in premixed flames [3], so itcannot be explained by studying the chemicalmechanism alone. Evidently the declining effi-ciency of NO production depends on chemistry-fluid interactions in non-premixed flames. A re-duced model that more simply describes theconversion of NH3 to NO can identify the reac-tions involved. The following analysis, which sim-plifies the entire chemical-fluid simulation in away that preserves second-order information, may

be useful in elucidating the behavior of tracespecies in other reacting flows. The mass conser-vation equation for the k-th species is

��Yk

�t� � � ��Yk � � � �Dk�Yk � Mkk

in which � is the mass density, Yk is the massfraction, � is the velocity, Dk is a mixture-averageddiffusion coefficient, Mk is the molar mass, and k

is the molar rate of formation. The latter is given by

k � �i

�vi,k�r� � vi,k

� f ��ki� f � �

lCl

vi,l� f �

� ki�r� �

lCl

vi,l�r��

in which ki is the rate constant of the i-threaction, i,k is the stoichiometric coefficient ofthe k-th species in the i-th reaction, Cl is themolar concentration of the l-th species, and thesuperscripts (f) and (r) indicate forward or re-verse.

Suppose the reacting fluid is in steady-stateso that the time derivatives vanish, and sup-pose further the fluid is in a chamber withvanishing composition gradients normal toany orifices. Then the conservation equationscan be integrated over the volume of thechamber to leave

�out

�n� � ��Yk��in

�n� � ��Yk��vol

Mkk

where n� is the exterior unit normal and theorifices have been segregated into outflow andinflow surfaces. If each species in some group ofspecies is subject to a multiplicative perturba-tion, �k � 1, which is uniform throughout thechamber, and if other quantities remain fixed,then the multiplicative perturbations can bepulled outside of the concentrations, densities,and integrals to leave the following equation.

��out

�n� � � ��Yk��k � ��in

�n� � � ��Yk��k � �i �Mk�vi,k

�r� � vi,k� f ��

vol

ki� f ��

lCl

vi,l� f ��

l�l

vi,l� f �

� �i �Mk�vi,k

�r� � vi,k� f ��

vol

ki�r��

lCl

vi,l�r��

l�l

vi,l�r�

(1)

The quantities in square brackets are fixed andcan be determined from the simulation. Uponreplacing these quantities by specific numbers,

there results a system of algebraic equationsthat can be solved for the multiplicative pertur-bations, �k. In the present situation the species

Fig. 9. Nitrogen reaction paths for the 1000 ppm ammonia-seeded flame (Glarborg et al. mechanism). The thickness ofan arrow indicates the quantity (mol/s) of atomic nitrogenmoving through the path; only paths at least 4% of thegreatest are shown. Table 3 gives the percent of each path asa result of various reactions.


of interest are those except N2 that containnitrogen. They are present in such small quan-tities that fluctuations in them would not beexpected to alter the velocity and temperaturefields. That is, the rate constants ki

(f) and ki(r) are

insensitive to the nitrogen species, other thanN2, so the model reduction described aboveapplies to them.

To apply the reduced model’s equations,(1), we evaluate the quantities in squarebrackets from the simulation with 1000 ppmof NH3 flowing in. The equation for ammoniais set aside, and the variable �NH3 is treated asa free parameter. Altogether there are 23equations and variables for the nitrogen spe-cies besides N2 and NH3. The equations aresolved using Mathematica [46] to express �k

for the remaining nitrogen species as a func-tion of �NH3. The exact analytic formulasfound by Mathematica are quite complicated.However, the following low-order Pade ap-proximation for the NO dependent variable isalmost indistinguishable from the analytic so-lution.

�NO �2.902�NH3

2 � 4.084�NH3� 0.017

�NH3

2 � 4.125�NH3� 1.844

(2)

The reduced model can be used in a variety ofways to obtain information about the chemical-fluid system. The expression

��out

�n� � � ��YNO��NO

is the reduced model’s prediction for theamount of NO flowing out of the reactionchamber. The independent variable, �NH3, canbe similarly scaled to the desired physical units.A graph of this prediction is shown in Fig. 3along with the experimental data. Note that thereduced model matches the predictions of thereacting flow simulation at 1000 ppm NH3, as itshould, since this is where the reduced modelwas derived. Moreover, the good agreementwith experimental data down past 500 ppm NH3

is remarkable given the extent of consolidationin creating the reduced model. This indicates

TABLE 3

Composition of the Nitrogen Reaction Paths Depicted in Fig. 9 for the Flame withAmmonia Seeding

(100) NO2 3 NO (40) NO 3 HNO (17) NO 3 N2 (12) NO 3 HCNO (9) N 3 H2CN79% �H 74% �H�M 58% �N 61% �CO 100% �CH3

15% �O 11% �H�N2 23% �NH2 38% �H (9) HOCN 3 NCO(89) NO 3 NO2 10% �HCO 16% �NH (11) CN 3 NCO 96% �H

63% �M (33) N 3 NO (16) NH 3 NO 61% �OH (8) HCN 3 HOCN36% �OH 78% �OH 96% �O 36% �O2 100% �OH

(74) NH3 3 NH2 13% �O2 (16) N2O 3 N2 (11) NO 3 HONO (7) NH2 3 HNO65% �OH (32) NO 3 HCN 86% �H 100% �OH�M 100% �O28% �H 51% �HCCO 10% �M (11) HONO 3 NO2 (5) NCO 3 NO

(72) NH2 3 NH 26% �CH2 (14) HNCO 3 NH2 89% �OH 99% �O60% �H (21) NH 3 HNO 100% �H (11) HCNO 3 NO (5) CH2CN 3 CN35% �OH 94% �OH (13) NCO 3 HNCO 74% �OH 99% �O

(70) HNO 3 NO (20) NCO 3 NH 68% �H2O 25% �O (5) HCN 3 CH3CN49% �H 100% �H 19% �H2 (10) HCN 3 CN 100% �CH3

39% �OH (20) NNH 3 N2 (12) N2 3 NNH 48% �H (4) HCN 3 NH(48) NH 3 N 71% �O2 53% �H�O2 44% �OH 100% �O

72% �H 18% � 46% �H (10) N 3 N2 (4) CH3CN 3 CH2CN27% �OH (18) HCN 3 NCO (12) H2CN 3 HCN 96% �NO 86% �H

100% �O 100% �M (10) NO 3 N2O 13% �OH97% �NH (4) NH2 3 N2

(10) NH 3 N2O 93% �NO99% �NO

The strongest paths (above the line in the table) are opposed and net to weaker paths. Paths are listed by decreasing amountof atomic nitrogen (mol/s) moving through them; only paths at least 4% of the greatest appear here and in the figure. Thepercent of each path because of various reactions is shown; only contributions of at least 10% are listed.


that the assumptions underlying the reducedmodel remain valid for relatively large changesin NH3. Near 0 ppm NH3 the ammonia chem-istry is insignificant, so the reduced model ofNH3 oxidation fails to predict NO productionthere.

The reduced model identifies the reactionsresponsible for the declining efficiency of NOproduction. Information about individual reac-tions is readily available from the reduced mod-el’s formulas. For the 75 reactions in the Glar-borg et al. mechanism that consume NO(forward and reverse directions are consideredseparate), Fig. 10 (left) plots their consumptionof NO as a function of NH3 seeding. Fivereactions can be seen to accelerate their con-sumption of NO with increased seeding; theyare the reactions in which NO combines withanother nitrogen species. In order of decliningstrength they are:

N � NO3 N2 � O,

NH � NO3 N2O � H,

NH � NO3 N2 � OH,

NH2 � NO3 N2 � H2O,

NH2 � NO3 NNH � OH.

For each of these reactions, increased NH3seeding increases the concentrations of bothreactants, which by mass action kinetics, qua-dratically increases the reaction’s rate of

progress. Figure 10 (right) shows that theamount of NO consumed by these five reactionsis significant because it is comparable to theflame’s net production. If the consumption bythese five reactions is added back to the net NOproduction, then the figure shows the resultingquantity varies linearly with NH3 seeding. Thusthese reactions account for the non-linearity ofthe NH3 to NO conversion.

Comparison of NOx Formation BetweenMechanisms

Reaction pathway analysis of simulations usingthe GRI 3.0 mechanism provide similar resultsto those shown in Figs. 6 and 9 for the Glarborget al. mechanism, although some differences areevident. The most significant difference be-tween the mechanisms is the level of conversionof NH to HNO. In the Glarborg et al. mecha-nism, this occurs through reaction with hydroxylradical, with about 20% of the NH being con-verted to HNO. Using GRI 3.0, two additionalNH 3 HNO conversion routes exist:

Rx. 195 NH � H2Oº HNO � H2

k � 2 � 1013 exp(13850/RT)

Rx. 278 NH � CO2º HNO � CO

k � 1 � 1013 exp(14350/RT)

Reaction numbers are given in the order listedin the GRI 3.0 mechanism. These two addi-

Fig. 10. (left) The reduced model’s predictions of NO consumption by individual (forward or reverse) reactions in theGlarborg et al. mechanism. The 15 strongest consumers are identified in the legend (center). Five reactions (solid curves)accelerate their consumption of NO with increased NH3 seeding. (right) When the amount of NO consumed by thesereactions is restored to the net NO production, the total has a linear variation with the amount of NH3 seeding.


tional reactions increase the NH 3 HNO con-version by a factor of two, which comes at thecost of the NH 3 N conversion. For bothmechanisms, virtually all HNO is converted toNO, while atomic nitrogen may produce orconsume NO depending upon local combustionconditions. This favoring of HNO over N leadsto the higher NO concentrations predicted bythe GRI 3.0 mechanism.

The validity of the rate expressions for thetwo reactions, above, has been called into ques-tion in previous studies [47]. To assess theclaims suggested in the reference, three simula-tions were performed using the GRI 3.0 mech-anism with reactions 195 and 278 removed;results are included in Fig. 3 as the modifiedGRI mechanism. With 1000 ppm of ammonia inthe fuel stream, the agreement between simula-tion and experiment improves significantly. Thisdramatic effect is not observed in the ammonia-free flame; the removal leads to a slight increase(5 ppm) in flue gas NO concentration. Nearly allatomic nitrogen and nitroxyl are oxidized to NO(Fig. 6) so that conversion of NH to eitherspecies in ammonia-free flames yields the sameNO production.

Further comparison of mechanisms using thePREMIX flame code [48] reveals that the re-moval of these reactions results in only a 3%decrease in NO concentration. This implies thatthe strong effect of these reactions may belimited to non-premixed flames.

CONCLUSIONS

In this study, the conversion of volatile fuel-nitrogen species is investigated through a seriesof experiments and computations involving alaminar, coflowing, non-premixed flame. In oneseries of experiments, increasing concentrationsof ammonia are added to the fuel stream of amethane-air flame. The conversion efficiency ofammonia to NO is found to decrease from over50% at low ammonia concentration to less than30% at higher ammonia concentration. Themeasured data are compared with simulationresults for three different ammonia concentra-tions using two different chemical mechanisms.Better agreement between experiment and

model is found using the Glarborg et al. mech-anism.

A two-zone NOx formation structure is ob-served in the ammonia-free flame. At low axialpositions, NOx forms primarily via the promptmechanism, with significant production throughthe HNO intermediate. NO is also formed inminor amounts through N2O and by the thermalmechanism. Thermal NO formation is limitedbecause of the low temperatures (1800 K) foundin this flame. Significant NO consumption oc-curs on the fuel side of the flame throughreactions with hydrocarbon radicals. The result-ing HCN is advected upward along the axis, andconverted back to NO as it crosses at the flametip. In the ammonia-seeded case, much moreNO is produced than can be consumed by theavailable CH2 and HCCO. Moreover, for thelarger ammonia seeding rates, a greater fractionof the NO produced is converted to N2.

Simulations using both the Glarborg et al.and the GRI 3.0 chemical mechanisms matchexperimental flue gas measurements quite well.The Glarborg et al. mechanism produces moreaccurate and consistent results; its performanceis attributed to the differences in the HNO-formation reactions and the more completeNO-recycling chemistry.

The work of Sullivan, Glarborg and Jensen wassupported by the research program CHEC (Com-bustion and Harmful Emission Control), which isco-funded by the Danish power companies Elsamand Energi E2, and by the Danish Ministry ofEnergy.

The work of Day, Grcar and Bell was supportedby the Applied Mathematical Sciences Program ofthe DOE Office of Mathematics, Information,and Computational Sciences, under contract DE-AC03-76SF00098.

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Received 19 December 2001; revised 4 June 2002; accepted 7June 2002

APPENDIX

Reaction Path Diagrams

The reaction path diagrams appearing in thispaper are generated using the steady 2D solu-tions directly. Automatically generated reactionpath diagrams have appeared elsewhere. Forexample, Warnatz et al. [49] discuss “integral


reaction flow analysis” with edge weights inte-grated over a region of space, as in this paper, orover an interval of time. We wish to acknowl-edge the help of Prof. D. G. Goodwin in formu-lating the conserved-scalar approach used here.

Only conserved scalars provide a consistentmeasure of the exchange of material amongspecies because of chemical reactions. For twospecies, s1 and s2, let ni,j(s1, s2) � 0 be the rate(mol/cm3s) at which atoms of type j are trans-ferred from s1 to s2 as a result of reaction i. Thetotal transfer of j from s1 to s2 is then i �volni,j(s1, s2) (mol/s), where the sum is overallreactions. In our diagrams the integral is overthe whole computational domain; however, itmay be useful to restrict the integration to somepart of the flame determined by auxiliary fieldquantities, such as mixture fraction, tempera-ture, or characteristics of the flow field. Theresulting sum represents the weight of the paths1 3 s2, and its value determines the directionand relative thickness the arrow between the

species in the reaction path diagram for elementj. The data needed to evaluate these weights isreadily available from the simulation and fromthe chemical mechanism’s CHEMKIN database.

A minor difficulty arises when the CHEMKINdescription of chemical reactions is inadequateto infer ni,j(s1, s2) uniquely. For example, theCHEMKIN input “N2H2 � NH � NNH �NH2” may mean that an H atom leaves N2H2 toform NNH, or that an N atom leaves to formNH2. Most ambiguities in the present study’sCHEMKIN mechanisms are resolved by sup-posing that the reactions shift the fewest possi-ble atoms. The remaining cases (four, for thenitrogen chemistry) are resolved by shifting thespecies fragment of lower atomic weight. Reac-tions whose species contain many atoms of thesame kind account for the ambiguities; theymake relatively small contributions to the pathdiagram’s arrows, so resolving the ambiguitiesdifferently leaves the figures unchanged.


ammonia conversion and nox formation in laminar coflowing nonpremixed methane-air flames

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