Reactions of Sodium Species in the Promoted SNCR Process

VLADIMIR M. ZAMANSKY, VITALI V. LISSIANSKI, PETER M. MALY,and LOC HO

Energy and Environmental Research Corporation, Irvine, California 92618

and

DARWIN RUSLI and WILLIAM C. GARDINER, JR.Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712

Selective Non-Catalytic Reduction (SNCR) is a well-known commercial NOx control process based on injectinga nitrogen agent into combustion products containing NO at temperatures near 1250 K. A serious limitation ofthe SNCR processes is that the temperature range over which nitrogen agents are effective is relatively narrow.In this work, we show that adding small amounts of sodium salts significantly improves the performance of theSNCR process. Parts per million levels of sodium compounds enhance NO removal and extend the effectiveSNCR temperature range in comparison with use of a nitrogen agent alone. When added in the same sodiumatom amounts, the efficiencies of different sodium compounds are similar. Kinetic modeling suggests that theperformance improvement can be explained as a homogeneous chain reaction ensuing after the sodiumcompounds are converted into NaOH. The overall result of introducing sodium compounds is conversion ofH2O and inactive HO2 radicals into reactive OH radicals, with the effective stoichiometry H2O 1 HO23 3 OH,which enhances the SNCR performance of nitrogen agents by increasing the production rate of NH2 radicals.© 1999 by The Combustion Institute

INTRODUCTION

Selective Non-Catalytic Reduction (SNCR) is awell-known commercial process for NOx controlbased on injection of a nitrogen agent (N-agent)into combustion products containing NO attemperatures near 1250 K [1, 2]. At this tem-perature, the injected N-agent reacts selectivelywith NOx to form N2. Several N-agents can beutilized in the SNCR process, including ammo-nia (NH3, Thermal DeNOx process), urea[(NH2)2CO, NOxOUT process], and cyanuricacid [(HOCN)3, RAPRENOx process]. Thechemical details vary somewhat from one agentto another, but their essential behavior is thesame: N-agents injected into combustion fluegas generate NHi radicals (NH2, NH, and N)that convert NO into molecular nitrogen. Im-plementations of SNCR processes were recentlyreviewed by Stamatakis and Sun [3] and Muzioand Quartucy [4].

A serious limitation of the SNCR processes isthat the “temperature window” over which N-agents are effective is relatively narrow. If theagent is injected at the optimum temperature,

NOx reduction is quite effective. At slightlyhigher or lower temperatures, however, NOxreduction drops off drastically. On the high-temperature side of the optimum, the agent isoxidized to NO, reducing NOx control effective-ness. On the low-temperature side, the injectedagent does not effectively react and part of itsimply passes through in the form of “ammoniaslip.” Temperature measurements in full-scaleboilers have revealed substantial temperaturestratification even at constant load conditions.As a result, it is virtually impossible to injectagents at the optimum temperature, for whichreason injection is usually done at higher tem-peratures to avoid ammonia slip.

The SNCR temperature window can be af-fected by injection of other additives togetherwith the N-agent. It has been shown [5–8] thathydrogen, hydrogen peroxide, hydrocarbons, al-cohols, carbon monoxide, and other additivescan shift the active temperature window to-wards lower temperatures. However, the sideeffects caused by such additives can includehigher CO emissions, formation of byproducts,and sometimes lower optimum NOx controlefficiencies.

Recent research [9–12] showed that some*Corresponding author. E-mail: vlissianski@eercorp.com

COMBUSTION AND FLAME 117:821–831 (1999)© 1999 by The Combustion Institute 0010-2180/99/$–see front matterPublished by Elsevier Science Inc. PII S0010-2180(98)00127-8

difficulties of the SNCR methods can be allevi-ated by injection of inexpensive, non-toxic inor-ganic compounds along with the NHi radicalprecursor. They can be added in amounts thatare small relative to the NOx concentration andto the concentration of metals commonlypresent in coal. The data [12] showed thataddition of Na2CO3 extended the SNCR tem-perature window to lower temperatures andenhanced the NOx reduction efficiencies of bothNH3 and (NH2)2CO, achieving about 90% NOxreduction at 1250 K.

The present study was undertaken in an effortto better understand the chemical basis for theinfluence of sodium compounds (the onestested being HCOONa, CH3COONa, NaNO3,and Na2CO3) on the SNCR process.

EXPERIMENT

Pilot-scale experiments on the influence ofHCOONa, CH3COONa, NaNO3, and Na2CO3on the SNCR process were conducted in aBoiler Simulator Facility (BSF). The BSF (Fig.1) has a full load firing capacity of 300 kW. It isdesigned to provide subscale simulation of theflue gas temperatures and compositions foundin conventional utility boilers. It consists of a7.0 m long vertically down-fired cylindrical ra-diant burner, horizontal convective pass, andbaghouse. The burner is constructed of eightrefractory-lined modules with 56 cm inside di-

ameter. The convective pass, also refractory-lined, contains air-cooled tube bundles to sim-ulate the superheater and reheater sections of autility boiler. Heat extraction in the radiantburner and convective pass is controlled suchthat the residence time-temperature profilematches that of a typical full scale boiler. Asuction pyrometer is used to measure furnacetemperatures. For the conditions used in theexperiments reported here the rate of temper-ature decrease in the furnace can be consideredto be approximately constant at 300 K/s.

A conventional continuous emissions moni-toring system was used for on-line analysis offlue gas composition. The NO concentrationwas measured using chemiluminescence analy-sis. The uncertainty of BSF experimental mea-surements is about 65%.

NHi radical precursors [NH3 and (NH2)2CO]and sodium compounds were injected as aque-ous solutions using a pair of twin fluid atomizerswith N2 as atomizing medium. Different sodiumcompounds (HCOONa, CH3COONa, NaNO3,and Na2CO3) were selected for testing in orderto check whether the chemical nature of theadditive would affect NO removal.

Laboratory scale experiments were per-formed in a conventional atmospheric pressureflow system with a 2 cm i.d., 50 cm long reactor.(Fig. 2) Possible contributions from wall reac-tions were checked by utilizing reactors madefrom stainless steel and quartz. Both reactorsgave similar results, indicating absence of signif-icant wall reaction contributions. To enablecorrect temperature measurements, a thermo-couple was inserted directly into the gas flow.Measurements showed that by adjusting thecurrent through each of three segments of thefurnace a uniform temperature distribution in-side the reactor could be maintained, with tem-perature variations less than 610 K along thereactor.

Sodium carbonate solutions were injectedinto an N2 stream preheated to 80–100°Cthrough an ultrasonic atomizer that provided 30mm droplets. The reactor had a vertical orien-tation with the injector situated on top of thereactor. The total gas flow rate through thereactor was maintained at 250 mL/s, whileNa2CO3 was delivered as 5% aqueous solutionat the rate of 2.5–30 mL/s. After exiting the

Fig. 1. Schematic diagram of the Boiler Simulator Facility(BSF).

822 ZAMANSKY ET AL.

reactor the stream was rapidly cooled to 0°Cand analyzed with a chemiluminescence ana-lyzer for NO/NO2 and by GC for CO2 and othergas components.

Mass spectrometric methods were used toidentify gas-phase species formed duringNa2CO3 decomposition. The products ofNa2CO3 thermal decomposition were identifiedusing a Finnigan MAT TSQ 70 mass spectrom-eter. Trace amounts of Na2CO3 were heated ona Nichrome wire until the solid or liquidNa2CO3 decomposed. Experiments were donein electron bombardment (EI) mode at ioniza-tion energies of 70, 25, and 12 V, in chemicalionization (CI) mode with CH4 buffer gas, andin thermal ionization (TI) mode. In auxiliaryexperiments the Nichrome wire was replaced bylower-melting metal wires to identify, by slowlyincreasing the heating current until the wiremelted, the effective temperatures where char-acteristic changes in the ion patterns appeared.

EXPERIMENTAL RESULTS

It had been reported previously [9–12] thatsome alkali metal compounds injected alongwith urea can substantially reduce NOx emis-sions from stationary combustion systems and

widen the temperature window of NO reduc-tion. Further natural gas firing tests in the BSFwere undertaken to investigate the chemicalreactions of HCOONa, CH3COONa, NaNO3,and Na2CO3 in flue gas containing NO, NH3,CO2, H2O, and O2. The amount of NO in themixture was 500 ppm, and NSR [(NH3)/(NO)molar ratio] values of 1.0 and 1.5 were used.The concentration of O2 in all experiments was3.9%, H2O 15.0%, CO2 7.0%, and the pressurewas 1 atm. Figures 3 and 4 present the experi-mental data on SNCR promotion. The effec-tiveness of NO reduction is defined as the ratioof NO concentration at the end of the SNCRzone to the initial NO concentration. The Y-scales on the left and right sides of Fig. 3 referto experimental data obtained with NH3 andurea injection, respectively, and Tinj is the tem-perature at which injections of N-agent andsodium compound were made. The modelingresults shown in Fig. 4 are discussed in the nextsection.

Sodium compounds are more effective pro-moters of NO removal than hydrogen, hydrogenperoxide, hydrocarbons, alcohols, carbon mon-oxide, and other additives [5–8]: Injection ofonly 5 ppm Na2CO3 widens the SNCR temper-ature window. For comparison, ethane, anotheradditive found to affect the SNCR window, is

Fig. 2. Flow system diagram.

Fig. 3. Effect of additives upon aqueous NH3 and ureaperformance as N-agents in the BSF. NSR 5 1.5, [NO]0 5500 ppm. Closed circle for injection of NH3 alone, opendiamond for CH3COONa, open square for NaNO3, opentriangle for HCOONa, open circle for Na2CO3 added, 3 forurea alone, and 3 for urea and Na2CO3. The amount ofadditives is 30 ppm.

823PROMOTED SNCR PROCESS REACTIONS

effective when present in at 100 ppm levelsunder similar conditions [8]. However, the mostimportant difference between the performanceof sodium compounds and that of other promot-ers is that the presence of sodium compounds

deepens and widens the temperature window incomparison with injection of NH3 alone, whileother additives usually just shift it toward lowertemperatures.

There are some differences in the perfor-mance of sodium compounds at T 5 1325 K,but at higher and lower temperatures theireffects are similar. It is necessary to take intoaccount that the sodium compounds tested havedifferent numbers of Na atoms and containother groups of atoms (CO3, HCOO, NO3, andCH3COO) that affect the SNCR process slightlyand account for differences in promoter perfor-mance at 1325 K. For example, the NO3

2 aniondecomposes to produce reactive species thatmay promote oxidation of NHi intermediates toNO and diminish the NO reduction perfor-mance of Na compounds. The amount of so-dium added is not critical; in the presence of 5and 15 ppm Na2CO3 in flue gas (Fig. 4) aboutthe same improvement of NO removal wasachieved.

Additives did not affect NO concentrations inthe absence of ammonia. Thus, addition ofsodium compounds enlarges the SNCR temper-ature window and does not by itself affect NOconcentrations.

Similar results on promotion with sodiumcompounds were obtained for injection of ureaas the N-agent (Fig. 3).

Figure 5 also shows the influence of Na2CO3additive on N2O emission. Na2CO3 suppressesN2O formation in comparison with the unpro-moted process. This effect, first observed exper-imentally by Ho et al. [10], was later confirmedin a modeling study by Perry and Miller [13].

The laboratory flow system was used tosearch for chemical reactions between the de-composition products of Na2CO3 and CH4, H2,CO, O2, and NO. Experiments were done at1150 K and a residence time of 2 s in mixtures of0 or 0.15% Na2CO3 1 18% H2O in N2 to which0.15% of the named flue gas components, mea-sured after passing through the reactor, wasadded. In the first set of experiments pure waterwas added while a mixture of additive and N2passed through the reactor. In the second, thegas and liquid flow conditions were identical tothe first except that the solution contained 5%Na2CO3 by mass. No changes in the additive gasconcentrations could be detected. Thus no ob-

Fig. 4. Effect of Na2CO3 additive upon aqueous NH3

performance in the BSF. NSR 5 1.5, [NO]0 5 500 ppm.Lines are calculations with the Miller and Glarborg mech-anism [18] and reactions from Table 1: the solid line is forinjection of NH3 alone, the dashed line for 5 ppm and thedotted line for 15 ppm added Na2CO3. Symbols are exper-imental data: Closed circle for injection of NH3 alone, opentriangle for 5 ppm Na2CO3 added, and open circle for 15ppm Na2CO3 added.

Fig. 5. Effect of additives on N2O formation. Closed circlefor injection of urea alone and open circle for 30 ppmNa2CO3 added.

824 ZAMANSKY ET AL.

servable chemical reaction between the decom-position products of Na2CO3 and CH4, H2, CO,and NO took place under the conditions stud-ied.

It was similarly found that the concentrationof NO stayed constant when O2 and Na2CO3were injected into the mixture at the same time.It was found, however, that Na2CO3 additive didsignificantly enhance the effectiveness of NH3as an NO control agent. Figure 6 shows theinfluence of Na2CO3 on NO removal in a mix-ture containing 270 ppm NO, 270 ppm NH3, 5%O2, and 8% H2O in N2 at 1205 K. WithoutNa2CO3 present NH3 removes about 30% of theinitial NO under the conditions of the experi-ment. Small additions of Na2CO3 promoted thiseffect. However, as the concentration ofNa2CO3 was increased, the relative effective-ness of Na2CO3 addition decreased, so thatwhen the Na2CO3 concentration reached 600ppm the final NO concentration was indepen-dent of the amount of Na2CO3. When theamount of Na2CO3 was even larger, a smallinhibiting effect was observed.

Mass spectrometric analysis was used to iden-tify species formed during Na2CO3 decomposi-tion. To correlate mass spectra with the temper-

ature at which active species are formed and/ordecomposition of Na2CO3 occurs, an Ag heat-ing wire (melting point 1235 K) was used in EImode with an ionization energy of 70 eV and inthermal ionization mode with EI 5 0, i.e., withall ions generated by thermal ionization ofNa2CO3 on the wire. The time of Ag appear-ance in the EI mass spectrum (which alsocorresponds to the maximum in total heatingcurrent before burnout) corresponds to themoment when the temperature of the wirereached 1235 K. The mass spectrum at thistemperature (Fig. 7, upper) shows CO2

1 (44),NaOH1 (40), Na2CO3

1 (106), and many otherspecies. Some of the other peaks are easilyidentifiable (H2O1 and CO1 or N2

1), whileidentification of others would require additional(i.e., high-resolution) mass analysis. These ex-periments, however, do not confirm the pres-ence of Na atoms. To find out if Na atoms arepresent in the system at 1235 K, these experi-ments were repeated with an Ag wire in thermalionization mode (Fig. 7, lower). The mass spec-trum in this mode shows Na1 (23) and specieswith m/e 5 39 (probably NaO1) and 129(Na3CO3

1). Mass spectrometric analysis ofproducts of Na2CO3 decomposition thus con-firms that the main gas-phase decompositionproducts at SNCR conditions are Na, NaOH,

Fig. 6. Dependence of NO removal efficiency in the SNCRprocess on the amount of Na2CO3 for a residence time of200 ms at 1205 K and 1 atm. Symbols represent experimen-tal data. The solid line represents simulations with thereactions and rate coefficient expressions from Table 1 andinstantaneous mixing, the dashed and dotted lines showcalculations with Na2CO3 delivery times of 100 and 400 ms.

Fig. 7. Upper: Electron bombardment mass spectrum at1235 K at an ionizing voltage of 70 V. Lower: Thermalionization mass spectrum corresponding to the moment ofAg burnout.

825PROMOTED SNCR PROCESS REACTIONS

and CO2. Proton transfer peaks in the CI massspectrum (not shown) also confirmed gas-phaseNa atoms to be present.

MODELING AND DISCUSSION

The first step in modeling the promoted SNCRprocess is selecting a chemical reaction mecha-nism for the SNCR process itself. Currently anumber of them are available for modeling theSNCR process. Four [14–18] were selected andtested at the conditions of the experiments. Themodeling codes were SENKIN [19], for calcu-lating the relative contributions of individualreactions and the sensitivity of species concen-trations to the rate coefficients of individualreactions, and ODF (for “One-DimensionalFlame”) [20] for modeling reaction in a onedimensional plug flow reactor with differentmixing regimes. The BSF data were modeledassuming a linear temperature decrease at 300K/s from the N-agent injection temperature to900 K, by which temperature reaction is practi-cally frozen.

It was found (Fig. 8) that the Miller andGlarborg [18] mechanism gives close agreementwith the results of experimental studies of theSNCR process. The Miller and Bowman [16, 17]

model is close to the experimental data on theSNCR process for low temperatures but signif-icantly overpredicts the efficiency of NO re-moval at high temperatures. Two other mecha-nisms (Bowman [14] and Dean and Bozzelli[15]) significantly overpredict the efficiency ofNO removal at the optimum temperature andshift the temperature window toward highertemperatures.

It is clear, however, that modeling the mixingof NH3 with the main gas stream in the furnacein addition to modeling the chemical reactionscan significantly affect the results. It was shown[21] that poor mixing can result in lower NOreduction in the SNCR process compared to theideal mixing case. Proper modeling of mixingwould require a combination of kinetic andgasdynamic equations, a feasible but not simpleundertaking for a detailed chemical mechanism.As a first approximation the mixing process wassimulated by assuming that mixing just retardsthe rate at which the additive is delivered to thesystem, so that the additive concentration ineach run increased with mixing time from zeroto its final value (distributed reagent injection).Calculations were done at 100 ms mixing time ofinjected N-agents, in agreement with estimatesmade using a droplet evaporation model fortypical BSF conditions. Mixing was modeled ina one-dimensional approximation, meaning thatone or several reagents were added to the mainstream over a period of time, at each moment oftime the mixing being assumed instantaneous.The main gas stream and injected substancestemperatures were assumed to be the same.

Three different mixing models were tested. Inthe first one NH3 was added at a linear rate tothe stream of NO, O2, H2O, CO2, and N2 overa period of 100 ms. This model predicted in-creased SNCR efficiency in comparison withinstantaneous mixing. The second and thirdmodels predicted a reduction in NH3 efficiencyas a result of non-instantaneous mixing. Thesecond one assumed that NO is slowly added tothe stream of NH3, O2, CO2, H2O, and N2. Thismodel is opposite to the real physical processesoccurring in the BSF, where NH3 is injected intothe main stream, but from the kinetic point ofview it provided a description of the mixingprocess close to the real one, i.e., high concen-trations of NH3 and low concentrations of NO

Fig. 8. Comparison of experimental data (symbols) withmodeling predictions for instantaneous mixing (solid lines)and distributed inverse mixing with 100 ms mixing time(dashed lines). 1, Miller and Bowman mechanism [16, 17]; 2,Miller and Glarborg [18] mechanism. NSR 5 1.5, [NO]0 5500 ppm.

826 ZAMANSKY ET AL.

in the mixing zone. The third model assumedthat a mixture of NO, O2, CO2, H2O, and N2 isslowly added to the stream of NH3 (so-calledinverse mixing). The second and third modelsgave practically identical results for tempera-ture range studied. The results reported herewere obtained using the inverse mixing model.

General considerations suggest (and are sup-ported by modeling) that the effect of mixing ismore significant at high temperatures. Indeed,at high temperatures the reaction is fast (calcu-lations show that the time required for theconcentration of NO to reach the equilibriumvalue is about 200 ms at 1270 K) and distributedreactant injection results in decreased NO re-moval efficiency. At low temperature, the timerequired to complete the reaction is longer (400ms at 1160 K) and distributed reactant injectiondoes not significantly affect the efficiency. Fig-ure 8 shows a comparison of modeling resultswith experimental data, for both instantaneousand distributed mixing models, using the mech-anisms of Miller and Bowman [16, 17] andMiller and Glarborg [18]. It demonstrates thatincluding mixing in the model significantly im-proves the agreement with experimental data atthe high temperature end of the measurementrange and practically does not affect modelingresults at the lower temperature end at all. It isalso seen that the Miller and Glarborg [18]mechanism gives a description of the SNCRprocess very close to the experimental one. Thismechanism was accordingly selected for model-ing the promoted SNCR process.

The fundamental questions addressed in thisstudy are essentially two, namely, what activespecies are formed during decomposition ofsodium-containing compounds and how theyinteract with SNCR chemistry. These questionscan only be studied by trying to describe theexperimental data using a kinetic model, eventhough most sodium reaction rate coefficients,for lack of experimental data, have to be esti-mated. It is assumed that the promoting effectof sodium species arises from homogeneousgas-phase reactions.

Since it was found that HCOONa,CH3COONa, NaNO3, and Na2CO3 have similarefficiencies, modeling was conducted under as-sumption that injected sodium compoundsquickly form NaOH, the most stable gas-phase

sodium compound, for which some kinetic in-formation is available. This approach is sup-ported by thermodynamic calculations [22, 23],which showed that significant fractional decom-position of Na2CO3 begins in the presence ofwater at temperatures slightly over 1000 Kthrough the reaction

Na2CO3~s, l ! 1 H2O~ g!3 2NaOH~ g!

1 CO2~ g!.

Literature data [24] also suggest that Na2CO3decomposes rapidly at the temperatures of theexperiments.

NaOH initiates and further participates in acatalytic chain process that increases chain cen-ter concentrations. This inference is supportedby the experimental observation that very smalladdition of sodium compounds already resultsin significant enhancement in NH3 efficiency.Further, because sodium additives are not ableto remove NO if NH3 is absent, it follows thatthe most likely effect of sodium species is toraise the concentrations of atoms and radicalsalready present in flue gas, in particular OH,rather than react directly with NO.

The reaction mechanism was composed bycombining the Na-O-H chemistry of Perry andMiller [13], who investigated the feasibility ofusing NaOH to control N2O emissions fromcombustion sources, with the N-H-O chemistryof Miller and Glarborg [18] and reactions (18)and (21). The reactions of sodium species areshown in Table 1. The rate coefficient k18 wasestimated to be 5.0 3 1013 cm3mol21s21 and therate coefficient k21 was taken from Jensen andJones [25]. Thermochemical data were takenfrom the thermodynamic tables of McBride etal. [22] except for NaO2, for which the DHf 298

0

value of 212.2 kcal/mol, derived from the D0value of 37.2 kcal/mol of Partridge et al. [30],was used. This choice of DHf 298

0 for NaO2 isbased on the conclusion of Perry and Miller [13]that higher bond dissociation energy values(55–88 kcal/mol) reported in the literature [31,32] result in NaO2 being predicted to be morestable above 1000 K than NaOH.

Preliminary calculations for the BSF condi-tions predicted that for temperatures above1250 K adding NaOH should indeed have apromoting effect on NO removal, while for

827PROMOTED SNCR PROCESS REACTIONS

temperatures below 1250 K, in contradictionwith experimental data, NaOH addition is pre-dicted to enhance NO production comparedwith NH3 alone. These results were in agree-ment with the data of Perry and Miller [13], whofound in a modeling study that NaOH additionshifts the SNCR temperature window towardshigher temperatures. Sensitivity calculations fortemperatures below 1250 K showed that amongthe 23 reactions listed in Table 1 only reactions(1) and (7) substantially affect NO removal.Reaction (1) is responsible for the decrease ofNO reduction efficiency at low temperatures,since it removes H atoms; reaction (27) en-hances NO reduction by producing OH radicals.(H and OH are important species for NOreduction because they generate NH2 radicalsin the reactions H 1 NH3 3 H2 1 NH2 andOH 1 NH3 3 H2O 1 NH2, the NH2 radicalssubsequently reacting with NO.) Thus the netdecrease in NO reduction arises because attemperatures below 1250 K the contribution ofreaction (1) is more significant than that ofreaction (27). To increase NO reduction in thepresence of NaOH we replaced (Table 1) the k1

expression estimated by Perry and Miller [13]with the one measured by Jensen and Jones [25]and increased k7 by one order of magnitude.(There are no experimental measurements ofk7; the value used by Perry and Miller [13] isonly an estimate.)

These modifications resulted in dramatic im-provement of the model performance. Figure 4shows a comparison of the model predictionsfor the promoted and unpromoted SNCR pro-cesses with experimental data. Two conclusionscan be drawn. First, the model predicts a verysmall promoting effect of Na2CO3 additive athigh temperatures and a much larger effect atlow temperatures, in qualitative agreement withthe experimental data. Second, for all tempera-tures tested, the promoting effect is not com-puted to be as significant as found experimen-tally. This is not surprising in view of the factthat most rate coefficient expressions in thesodium chemistry model are just estimates.

Figure 6 shows a comparison of model pre-dictions with experimental data obtained in theflow system. A small amount of promoter isagain predicted to significantly improve the

TABLE 1

Reactions of Na Species†

N Reactions DH2980 A n E Source

1 NaOH 1 H3 Na 1 H2O 237 1.0 3 1013 0 1.97 [25]2 NaO2 1 H3 NaOH 1 O 228 1.0 3 1014 0 0 [13]3 NaO2 1 H3 NaO 1 OH 211 5.0 3 1013 0 0 [13]4 NaO2 1 H3 HO2 1 Na 211 2.0 3 1014 0 0 [13]5 NaO2 1 O3 NaO 1 O2 227 1.0 3 1014 0 0 [13]6 NaO2 1 CO3 NaO 1 CO2 235 1.0 3 1014 0 0 [13]7 NaO2 1 OH3 NaOH 1 O2 244 2.0 3 1014 0 0 See text8 NaO 1 H3 Na 1 OH 288 2.0 3 1014 0 0 [13]9 NaO 1 O3 Na 1 O2 254 2.2 3 1014 0 0 [26]10 NaO 1 OH3 NaOH 1 O 217 2.0 3 1013 0 0 [13]11 NaO 1 OH3 Na 1 HO2 21 3.0 3 1013 0 0 [13]12 NaO 1 HO23 NaOH 1 O2 249 5.0 3 1013 0 0 [13]13 NaO 1 HO23 NaO2 1 OH 226 5.0 3 1013 0 0 [13]14 NaO 1 H23 NaOH 1 H 215 1.6 3 1013 0 0 [13]15 NaO 1 H23 Na 1 H2O 252 3.1 3 1012 0 0 [13]16 NaO 1 H2O3 NaOH 1 OH 0 1.3 3 1013 0 0 [27]17 NaO 1 NO3 Na 1 NO2 28 9.0 3 1013 0 0 [28]18 NaO 1 NH33 NaOH 1 NH2 211 5.0 3 1013 0 0 See text19 NaO 1 CO3 Na 1 CO2 262 1.0 3 1014 0 0 [13]20 Na 1 O2 1 M3 NaO2 1 M 237 1.7 3 1021 21.3 0 [29]21 Na 1 OH 1 M3 NaOH 1 M 282 1.8 3 1021 21 0 [25]22 Na 1 HO23 NaOH 1 O 216 1.0 3 1014 0 0 [13]23 Na 1 N2O3 NaO 1 N2 225 1.7 3 1014 0 3.16 [29]

† Rate coefficients are in the form k 5 ATn exp(2E/RT). Units are cm, mol, s and kcal

828 ZAMANSKY ET AL.

NO-control performance of ammonia. As theamount of additive increases, the relative pro-moting effect again diminishes. Figure 6 alsoshows that introducing a finite rate of mixing ina way similar to that used for modeling the BSFdata improves the model prediction: the pre-dicted optimal Na2CO3 amount shifts towardlarger concentrations, and NO reduction atlarge Na2CO3 concentrations is higher.

Figure 9 shows the sensitivity spectrum calcu-lated for T 5 1250 K, corresponding to theoptimum in NO reduction in the presence ofNa2CO3. Since there are many reactions in theN-O-H system that have large sensitivities in thepromoted SNCR process, and since the primaryinterest of this study is in reactions of Naspecies, the cut-off value for sensitivities ofN-O-H reactions shown in Fig. 9 is 0.1. Thecut-off value for reactions of sodium species is0.02. Figure 9 shows that there is actually onlyone reaction of sodium species that is sensitive,the assumed chain initiation reaction

NaOH1O23NaO21OH. (27)

To understand how transformation of sodiumspecies in SNCR occurs and how NaOH addi-tive promotes the SNCR process, net reactionrates for sodium reactions were calculated. (Fig.10) The calculations show that while most of the

sodium is present in the form of NaOH, some isin other species. The main conversion of sodiumgoes through the chain sequence

NaOH3NaO23Na3NaO3NaOH,

corresponding to reactions (27), (220), (217),and (16). When added together, reactions (27),(220), (217), and (16) give the net reaction

H2O1NO23NO12OH.

Since NO2 is mainly produced in the fast reac-tion

NO1HO23NO21OH,

the net action of sodium species is equivalent tothe conversion of H2O and inactive HO2 radi-cals into reactive OH radicals

H2O1HO233 OH.

Thus the net result of sodium conversionthrough the chain reaction is production of OHradicals, while NaOH itself is not consumed.Chain initiation is the rate limiting step in thissequence, and the reaction (27) responsible forit appears in the sensitivity analysis (Fig. 9). Analternative initiation step would be

NaOH1M3Na1OH1M, (221)

Fig. 9. Sensitivity analysis for NO in the promoted SNCRprocess. The amount of Na2CO3 is 10 ppm, [NO]0 5 500ppm, NSR 5 1.5, T 5 1250 K. Sensitivities less that 0.1 forN-H-O reactions are not shown. For reactions of Na speciesthe cut-off value is 0.02.

Fig. 10. Flux diagram for sodium-containing species. Widthof arrows shows relative rates of chemical reactions. Num-bers on the diagram correspond to reaction numbers inTable 1.

829PROMOTED SNCR PROCESS REACTIONS

but because of its high activation energy (40kcal/mol), it does not significantly contribute toproduction of active centers. (It does, however,become significant at higher temperatures.) Atlow temperatures the OH radicals are mainlyformed in reactions (27) and

NaO1H2O3NaOH1OH. (16)

Some amount of NH2 radicals is produced inthe reaction between NaO and NH3

NaO1NH33NaOH1NH2 (18)

but this is a minor contribution to the totalradical pool formed by the NaOH additive.

Modeling thus supports that the promotingeffect of NaOH can be attributed to enhancedradical concentrations due to reactions of so-dium species. These reactions lead to a contin-uous supply of OH radicals that can react withNH3

NH31OH3NH21H2O,

such that the efficiency of NH3 as NO removingagent, acting through

NH21NO3N21H2O,

NH21NO3NNH1OH

in the conventional SNCR manner is enhancedby sodium compounds. This effect becomesespecially pronounced at low temperatures,where generation of active centers by the N-H-O system itself slows down, and addition ofsodium compounds accelerates the radical ini-tiation process. At high temperatures, radicalproduction by sodium compounds is small com-pared to production by the N-H-O reactionsand the effect of sodium compounds on NOreduction diminishes.

It was observed experimentally (Fig. 4) thatwhen larger amounts of sodium compounds areadded, the relative efficiency of Na2CO3 addi-tion decreases. This can be understood as reac-tions (1) and (214) competing for H atoms thatwould otherwise react with O2 in the chainbranching reaction

H1O23OH1O.

Figure 11 demonstrates these ideas by showinghow Na2CO3 additive affects radical concentra-

tions. The results presented in Fig. 11 werecalculated for the conditions of flow tube exper-iments with 10 ppm Na2CO3 added. There is asignificant increase in the concentrations of OHand NH2 radicals produced in the presence ofNa2CO3 additive in comparison with the non-promoted process, and as a result the concen-tration of NO decreases much faster than withinjection of NH3 alone.

CONCLUSIONS

Experiments show that small amounts ofHCOONa, CH3COONa, NaNO3, and Na2CO3significantly enhance the performance of NH3and urea in the SNCR process. Adding just 5ppm Na2CO3 results in a significant broadeningand deepening of the SNCR temperature win-dow. Addition of larger amounts is less effec-tive, since the relative promoter effectivenessdecreases as its concentration increases. Kineticmodeling suggests that the experimental obser-vations can be explained as a homogeneouschain reaction in which sodium additives arequickly converted into NaOH, which producesOH radicals to enhance the SNCR performanceof NH3. Sodium hydroxide participates in thechain reaction NaOH 3 NaO2 3 Na 3 NaO3 NaOH with the net effect H2O 1 HO2 3 3OH. The OH radicals promote NO reduction byforming NH2 radicals, in the reaction NH3 1OH 3 NH2 1 H2O, that reduce NO. The

Fig. 11. Promotion of NO removal in presence of Na2CO3

additive. Solid lines represent calculations without Na2CO3;dashed lines with 10 ppm Na2CO3 added. T 5 1205 K.

830 ZAMANSKY ET AL.

model predicts a smaller SNCR-promoting ef-fect than was observed experimentally.

This research was supported by the U.S. Depart-ment of Energy under contract no. DE-AC22-95PC95251 (Project Manager Thomas J. Feeley),the U.S. Environmental Protection Agency undercontract no. 68D70017 (Project Manager Mar-shall Dick), and the Robert A. Welch Foundation.

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Received 27 May 1998; revised 25 August 1998; accepted 27August 1998

831PROMOTED SNCR PROCESS REACTIONS