combustion hazard of mixing ammonia with nitric oxide

Journal of Loss Prevention in the Process Industries 16 (2003) 497–506www.elsevier.com/locate/jlp

Combustion hazard of mixing ammonia with nitric oxide

Rui Liu a,∗, David S.-K. Tinga, M. David Checkelb

a Mechanical, Automotive and Materials Engineering, University of Windsor, 401, Sunset Avenue, Windsor, Ont., Canada, N9B 3P4b Department of Mechanical Engineering, University of Alberta, Edmonton, Alta., Canada, T6G 2G8

Abstract

Premixed ammonia/nitric oxide flame was simulated using the Lindstedt 1994 and Miller–Bowman 1989 reaction mechanismsin CHEMKIN. The predicted laminar burning velocities compared well with limited measured values in the literature. The effectsof unburnt mixture temperature and pressure on laminar burning velocity, flammability limits, adiabatic flame temperature andspecies profiles were studied. The unburnt mixture temperature had a positive impact on both the laminar burning velocity and theadiabatic flame temperature, and it extended the ammonia-rich flammability limit. The pressure had a marginally negative influenceon the laminar burning velocity, while it had a slightly positive effect on the adiabatic flame temperature. 2003 Elsevier Ltd. All rights reserved.

Keywords: Ammonia; Nitric oxide; Premixed flame; Laminar burning velocity; Adiabatic flame temperature; Flammability limits; CHEMKIN

1. Introduction

Ammonia is a crucial product used in many chemicalprocesses. It is a prospective alternative fuel that wasactually utilized in Belgium in World War II because ofthe extreme shortage of diesel (Kroch, 1945). Nitricoxide usually presents itself as a by-product of high tem-perature combustion processes in air. Many studies havebeen conducted on removing NOx from burnt gas byexploiting its highly exothermic reaction with ammonia(e.g.Muzio, Maloney & Arand, 1978). Under some con-ditions, ammonia/nitric oxide mixtures pose anexplosion hazard and, with the presence of air in mix-tures of ammonia and nitric oxide, there is even a poten-tial of auto-ignition at ambient temperature. This situ-ation calls for a detailed understanding of thecombustion characteristics of ammonia/nitric oxide mix-tures. A few experimental studies focusing on this aspecthave been conducted in the past few decades(Andrews & Gray, 1964; Checkel, Ting, & Bushe, 1995;Parker & Wolfhard, 1955), leading to measurements offlame structure, laminar burning velocity, and flamm-ability limits under different conditions. While these

∗ Corresponding author. Tel.:+1-519-253-3000x2631; fax:+1-519-973-7007.

E-mail addresses: [email protected] (R. Liu); [email protected](D.S.-K. Ting).

0950-4230/$ - see front matter 2003 Elsevier Ltd. All rights reserved.doi:10.1016/S0950-4230(03)00076-7

experimental studies provide invaluable insight, there areconsiderable difficulties in burning velocity measure-ment, which may lead to up to 30% error (Andrews &Gray, 1964). Also, further details required for flowmodeling such as temperature and species profiles arenot available. This study investigates the problemnumerically, determining the laminar burning velocity,flame temperature, flammability limits and the speciesprofiles of ammonia/nitric oxide mixtures using theMiller–Bowman 1989 (Miller & Bowman, 1989) and theLindstedt 1994 (Lindstedt, Lockwood, & Selim, 1994;Lindstedt & Selim, 1994) reaction mechanisms inCHEMKIN. Values predicted with these two mech-anisms are compared with those measured in a constantvolume combustion chamber (Checkel et al., 1995).

2. Simulation

A freely propagating, adiabatic, premixed, planarflame was simulated with PREMIX (Kee et al., 2001),Sandia’s steady-state, laminar, one-dimensional flamecode. PREMIX uses a hybrid time-integrating/Newtoniteration technique to solve the steady-state mass, speciesand energy conservation equations and can be set up tosimulate a premixed propagating flame. The systems ofequations are solved using TWOPNT (Kee et al., 2001),a boundary value problem solver in the CHEMKIN 3.06

498 R. Liu et al. / Journal of Loss Prevention in the Process Industries 16 (2003) 497–506

Nomenclature

P pressureT unburnt mixture temperatureSTP standard temperature and pressure� ammonia/nitric oxide equivalence ratio�T temperature difference

package. Also built in the CHEMKIN package are atransport property processor (Kee et al., 2001) and a gas-phase interpreter providing the species transport proper-ties and processes of the chemical reaction mech-anism, respectively.

The basic requirement for the reaction mechanism isthe involvement of the necessary reactants and productswhich, in the present study, include NH3, NO, H2O andN2. Two ammonia oxidation reaction mechanisms wereapplied in this study, namely, the Miller–Bowman 1989and the Lindstedt 1994 mechanisms. The Miller–Bow-man 1989 mechanism has 20 species and 73 elementaryreactions. The Lindstedt 1994 mechanism includes twoadditional species (HNO2 and NO3) and consists of 95elementary reactions. These two ammonia oxidationmechanisms were validated by Akbar, Kaneshige,Schultz, and Shepherd (1997) against the induction timesdeduced from various experimental detonation studiesfor ammonia/air mixture.

As an initial boundary condition, the initial flow rateof the ammonia/nitric oxide mixture into the flame wasestimated as 0.04 (g/cm2 s). The pressure of the mixturewas set at 1, 2.5 or 4 atm, and the unburnt mixture tem-perature at 298, 373 or 478 K. These pressure and tem-perature settings provide nine different problem environ-ments. To solve the energy equation, a boundarycondition was set by fixing the 500 K temperature pointat a location calculated from the temperature profile. Thethickness of the stoichiometric ammonia/nitric oxideflame was estimated to be of the order of 1 mm. Calcu-lation was initiated at 20 mm upstream of the flame andthe total length of the calculation region was approxi-mately 120 mm. Initially, the unburnt mixture tempera-ture profile was estimated for 0–30 mm region, wherethe zero distance was measured from the adiabaticunburnt boundary. The temperature profile calculatedfrom the first simulation step was used to estimate thesubsequent one.

The global equation for the stoichiometric reaction ofammonia/nitric oxide is

NH3 � 1.5NO→1.25N2 � 1.5H2O. (1)

For both reaction mechanisms, the simulation was runfor ammonia/nitric oxide equivalence ratio, �, from 0.1up to values that gave diverging results which, in this

study, were in the range of 4.7–6.8, depending on thetemperature and pressure conditions.

3. Laminar burning velocity and flame temperature

This section compares simulation results from the tworeaction mechanisms with experimental results. Theeffects of temperature and pressure on laminar burningvelocity, flammability limits and adiabatic flame tem-perature are discussed using the simulation results fromthe Lindstedt 1994 mechanism. First, to clarify the defi-nition of laminar burning velocity, it is the speed atwhich the unburnt gas moves relative to and normal tothe flame front and is transformed into products underlaminar flow conditions (Glassman, 1996). For the PRE-MIX code, it is the velocity obtained at the adiabaticboundary of the flame, i.e. at the zero distance. Theflammability limits were determined as the point whereno combustion occurs. In this simulation, this corre-sponds to the point at which the solution starts divergingconsistently. The flame temperature at the end of thecomputational domain, i.e., 100 mm from the flamefront, was arbitrarily chosen as the adiabatic flame tem-perature.

3.1. Comparison of simulation and experimentallaminar burning velocities

Fig. 1 shows the laminar burning velocities predictedfrom the simulation as well as those from Checkel etal. (1995) and Parker and Wolfhard (1955) for 101 kPapressure and 298 K unburnt mixture temperature. Parkerand Wolfhard measured the laminar burning velocityusing a Bunsen cone method in which the burning velo-city was related to the inner boundary of the luminousflame front. Checkel et al. determined the laminar burn-ing velocity by measuring the pressure rise of theunburnt mixture in a constant volume bomb. Parker andWolfhard (1995) provide only one point at the stoichio-metric condition and this was approximately 15% lowerthan that of Checkel et al. (1995). On the lean side with� less than 0.6, both the Lindstedt 1994 and the Miller–Bowman 1989 mechanisms agree well with the experi-mental results. The prediction of the Lindstedt mech-

499R. Liu et al. / Journal of Loss Prevention in the Process Industries 16 (2003) 497–506

Fig. 1. Comparison of the predicted laminar burning velocities withexperimental measurements.

anism follows the experimental result until the point with� of approximately 0.8, beyond which it starts to fallshort of the experiment value. This under-predictioncontinues till � is approximately 2.5. Between � ofapproximately 2.5 and 3.5, prediction of the Lindstedtmechanism agrees fairly well with the experimentalresults. An over-prediction of the Lindstedt mechanismis observed beyond � of 3.5. The Miller–Bowmanmechanism exhibits lower predictions than Checkel etal.’s experimental burning velocity beyond � of about0.6. The maximum differences between the predictedlaminar burning velocity and the experimental result areapproximately 15% and 30% for the Lindstedt and theMiller–Bowman mechanisms, respectively. The discrep-ancy between the experimental and simulation resultscould be explained in term of systematic errors in boththe experiment and the simulation. Possible errors in theexperiment include introduction of some turbulence bythe spark electrode geometry and uncertainties in extra-polating burning velocities measured with the rapidlyrising cell pressure and temperature back to nominalconditions. Both errors would be maximized with fast-burning stoichiometric mixtures and reduced equally onthe rich and lean side by slower combustion.

The adiabatic flame temperatures of the Lindstedt andMiller–Bowman mechanisms were compared in Fig. 2.When � is less than approximately 2.5, the predictionsof the two mechanisms are found to be essentially thesame, implying similarity of the final equilibrium com-position and the heat and mass transfer properties. Asit moves farther to the rich side, the Miller–Bowmanmechanism start predicting higher adiabatic temperaturethan the Lindstedt mechanism with a maximum differ-

Fig. 2. Comparison of the predicted adiabatic flame temperaturesusing the Lindstedt 1994 and the Miller–Bowman 1989 mechanisms.

ence of approximately 200 K. Judging from the shape ofadiabatic flame temperature vs. �, the better predictioncapability as illustrated in Fig. 1, and the better conver-gence of the simulation over significantly wider range ofconditions, the Lindstedt 1994 mechanism is believed tobe superior and hence, the following discussion relatesentirely to the simulation results from the Lindstedt1994 mechanism.

To further justify the use of the Lindstedt 1994 mech-anism, the profiles of the adiabatic temperature and thespecies (NH3 and N2) concentrations are presented inFig. 3 for stoichiometric condition at standard tempera-ture and pressure (101 kPa, 298 K). The species concen-

Fig. 3. Adiabatic temperature/concentration profiles.


trations are scaled with the right axis showing the actualnumerical values to facilitate a better visualization. Bothtemperature and species concentration profiles indicateapproximately the same flame thickness of 1 mm whichagrees with the expected order of magnitude of 1 mm.The shape and the smoothness of the profiles also indi-cate the validity of the Lindstedt 1994 mechanism.

3.2. Adiabatic temperature profiles

The variations of adiabatic temperature profiles with� are shown in Fig. 4a,b for the ammonia lean and richmixtures, respectively. The temperature profiles at � of

Fig. 4. (a) Adiabatic temperature profiles for lean-to-stoichiometricammonia/nitric oxide mixtures. (b) Adiabatic temperature profiles forstoichiometric-to-rich ammonia/nitric oxide mixtures.

1.0 and 0.9 are presented in both plots for comparison.The adiabatic temperature and temperature gradient peakat ��0.9 reflect the high enthalpy of formation of theoxidizer, nitric oxide. The flame thickness increases withdecreasing � on the ammonia-lean side of (� � 0.9)and increases with increasing � for mixtures richer than0.9. The simulation shows a decreasing temperature onthe burnt side beyond the flame front for � greater than1.4, (Fig. 4b), reflecting recombination of species.

3.3. Temperature effect on laminar burning velocityand flammability limit

Checkel et al. (1995) measured sharp increases inlaminar burning velocity with increasing unburnt mix-ture temperature. This increase is expected because thenumber of molecular collisions per unit volume per unittime and the fraction of effective collisions both increasewith temperature. Fig. 5a–c show plots of laminar burn-ing velocity against � at different unburnt mixture tem-peratures but the same pressures. The maximum laminarburning velocity is predicted to be lean of stoichiometricat ��0.9. This coincides with the maximum gradient offlame temperature as shown in Fig. 4.

For the maximum predicted burning velocity(��0.9), the burning velocities at 373 and 478 K were150% and 200% of that at 298 K. The mixture tempera-ture effect on laminar burning velocity diminishes as themixture approaches the flammability limits. It can alsobe inferred from Fig. 5a–c that the laminar burning velo-city decreases with increasing pressure. This is discussedin more detail later.

At 101 kPa, the upper flammability limits for the twolower temperatures (298 and 373 K) are approximatelyat the same equivalence ratio of 4.7. As the unburnt mix-ture temperature increases to 478 K, the upper flamm-ability limit increases accordingly to ��5.3. A similartrend is observed at 253 kPa, with the upper flammabilitylimits for the two lower and the highest unburnt mixturetemperatures at � of approximately 5.7 and 6.5, respect-ively. At the highest pressure (404 kPa), the upperflammability limits for the two higher temperatures (373and 478 K) are similar, with � of approximately 6.5which is larger than the value of approximately 5.6 atthe lowest temperature. On the ammonia lean side, con-vergence was obtained under all simulation settingsinvestigated in this study, which indicates the lowerflammability limits at � no more than 0.1. This alsoimplies an independence of the lower flammability limiton the unburnt mixture temperature.

3.4. Predicted mixture temperature effect on adiabaticflame temperature

The predicted adiabatic flame temperature of the mix-ture is plotted against � as shown in Fig. 6a–c for con-


Fig. 5. (a) Laminar burning velocity at 101 kPa. (b) Laminar burning velocity at 253 kPa. (c) Laminar burning velocity at 404 kPa.

stant pressure and varied unburnt mixture temperature.The maximum flame temperature is predicted on theammonia-lean side at ��0.9 and is shown to increasewith pressure. The minimum adiabatic flame temperatureon the lean side is about 2700 K. This high “ lean-limit”flame temperature results from the exothermic break-down of nitric oxide. The global minimum adiabaticflame temperature of around 1400 K occurs on theammonia-rich side. The unburnt mixture temperature hasa slightly positive effect on the adiabatic flame tempera-ture, for which a �T increase in the unburnt mixture tem-perature results in approximately �T/5 increase in theadiabatic flame temperature when ��1.

3.5. Pressure effect on laminar burning velocity andflammability limits

Rising pressure generally decreases species diffusionbut may increase or decrease the reaction rate. So theoverall effect can be either positive or negative. The pre-dicted effect of pressure on laminar burning velocity isshown in Fig. 7a–c. A slightly negative effect of increas-ing pressure on burning velocity is observed as the press-ure rises from 101 to 253 and 404 kPa. At 298 K, theupper flammability limit is found to increase from � of

approximately 4.7 to 5.6 as the pressure varies from 101to 253 kPa, which indicates a positive effect of pressureon the upper flammability limit. Further pressureincrease from 253 to 404 kPa is found to have no effecton the upper flammability limit. A continuous variationof the upper flammability limit with pressure is found atunburnt mixture temperature of 373 K, with correspond-ing � of 4.7 (101 kPa), 5.7 (253 kPa) and 6.5 (404 kPa),respectively. At 478 K, the variation of the upperflammability limit takes a similar trend as that observedat unburnt mixture temperature of 298 K, with the upperflammability limits at � of 5.3 (101 kPa) and 6.5 (253and 404 kPa), respectively. Again, the lower flamm-ability limits for all cases are found to be no more than� of 0.1, which also indicates an independence of thelower flammability limit on the pressure.

3.6. Pressure effect on adiabatic flame temperature

The effect of pressure on the adiabatic flame tempera-ture is shown in Fig. 8a–c. Each plot can be roughlydemarcated into two regions. For the relatively highburning velocities between � of 0.2 and 1.5, increasingpressure has a positive effect on the adiabatic flame tem-perature. On the richer side, the pressure effect becomes


Fig. 6. (a) Adiabatic flame temperature of ammonia/nitric oxide mixtures at 101 kPa. (b) Adiabatic flame temperature of ammonia/nitric oxidemixtures at 253 kPa. (c) Adiabatic flame temperature of ammonia/nitric oxide mixtures at 404 kPa.

almost negligible. This shows the complexity of theammonia/nitric oxide/hydrogen/nitrogen/oxygen mech-anisms.

4. Species profiles

To investigate the dependence of the predicted speciesprofiles on the specific reaction mechanism, the profilesof OH, N2 and H2 predicted using the Lindstedt andMiller–Bowman reaction mechanisms at STP and stoi-chiometric composition are plotted in Fig. 9. The Lind-stedt mechanism seems to predict earlier generation ofthe three species than the Miller–Bowman mechanism.Also, within the region of fast chemistry characterizedby the high gradients of the three species, the Miller–Bowman mechanism predicts smaller gradients of spec-ies concentrations than the Lindstedt mechanism. Thisconfirms the aforementioned under-prediction of laminarburning velocity of the Miller–Bowman mechanism. Thediscrepancy between the predictions of the two reactionmechanisms diminishes as one moves into the region ofslow chemistry as indicated by the overlap in the tailportion of the plots. This, again, confirms the argumentmade above on the similarity of the final equilibrium

composition and the heat and mass transfer propertiesbetween the two mechanisms.

4.1. OH radical

OH is one of the most active radicals within manychemical reactions. As far as the present study is con-cerned, it consists of 41 elementary reactions of theLindstedt reaction mechanism (42%) and 31 elementaryreactions of the Miller–Bowman reaction mechanism(42%). The predicted OH profiles for different � areshown in Fig. 10a at STP using the Lindstedt reactionmechanism. Fig. 10a reveals that � has a negative effecton the OH gradient and its equilibrium concentration. Asimilar argument can also been made from the predic-tions of the Miller–Bowman mechanism as shown in Fig.10b. The negative effect of � continues for � beyondthe ones shown in Fig. 10. Both Fig. 10a,b indicate that,macroscopically, the production of OH is controlled bythe amount of nitric oxide within the ammonia/nitricoxide mixture.

Further details of the OH production were obtained inthis study through the sensitivity analysis which is abuilt-in function of the PREMIX code. The importanceof a particular elementary reaction in the OH production


Fig. 7. (a) Pressure effects on laminar burning velocity of ammonia/nitric oxide mixtures at 298 K. (b) Pressure effects on laminar burning velocityof ammonia/nitric oxide mixtures at 373 K. (c) Pressure effects on laminar burning velocity of ammonia/nitric oxide mixtures at 478 K.

was determined by identifying the magnitude of the sen-sitivity coefficient related to that elementary reaction.Five most important elementary reactions of the Lind-stedt mechanism were determined in this way at STPand stoichiometric composition. They are listed in Table1 with their maximum sensitivity coefficients along theflame path. Fig. 11 shows the variations of the sensitivityof the OH production to the five elementary reactionsalong the flame path. Two elementary reactions can befurther identified to be of extreme importance in OH pro-duction, i.e., the first and second reactions. Reaction 1has a directly positive effect on the OH productionthrough the consumption of nitric oxide. Reaction 2 hasa negative effect on OH production which acts in anindirect way by competing for the resource, nitric oxide.As mentioned above, the production of OH is observedto be macroscopically controlled by the amount of nitricoxide. The more nitric oxide is consumed in the forma-tion of N2 and H2O via the reaction with NH2, the lessit is left for the production of OH. This microscopicallygives the explanation for the macroscopic effect of NOon the formation of OH.

The temperature effect on the OH sensitivity to

elementary reactions 1 and 2 of the Lindstedt mechanismis shown in Fig. 12 at 101 kPa and stoichiometric com-position. The maximum values of the OH sensitivity toboth reactions increase with the unburnt mixture tem-perature which is consistent with temperature enhance-ment of laminar burning velocity and adiabatic flametemperature. The temperature effect on the point atwhich there is maximum OH sensitivity is ambiguous.There is a slight delay but higher peak is formed as theunburnt mixture temperature increases from 298 to 373K. However, as the unburnt mixture temperature isfurther increased to 478 K, a considerably advanced andlarger maximum OH sensitivity is observed. This agreeswith the temperature enhancement as shown in Fig. 5a.

5. Concluding remarks

The laminar burning velocity, flame temperature andflammability limits of ammonia/nitric oxide mixtureswere predicted using the Lindstedt 1994 and the Miller–Bowman 1989 reaction mechanisms in CHEMKIN.These simulations were conducted at atmospheric con-ditions and elevated temperatures and pressures.


Fig. 8. (a) Flame temperature for mixtures at 298 K. (b) Flame temperature for mixtures at 373 K. (c) Flame temperatures for mixtures at 478 K.

Fig. 9. Species profiles predicted using the Lindstedt and the Miller–Bowman mechanisms at STP and stoichiometric composition.

The Lindstedt 1994 and the Miller–Bowman 1989mechanisms agreed well on predictions of adiabaticflame temperature when the ammonia/nitric oxide equiv-alence ratio is less than 2.5. They differed in their abilityto quantitatively match experimentally measured valuesof laminar burning velocity. Both matched the experi-mental values when � was less than 0.6 and predictedslightly lower burning velocity than measured for near-stoichiometric and rich mixtures. Both the mechanismspredicted peak burning velocity for slightly lean(��0.9) mixtures. The Lindstedt 1994 mechanism pre-dicted values that were generally closer to the measuredvalues than the Miller–Bowman 1989 mechanism.

The simulation showed that adiabatic flame tempera-ture, flame temperature gradient and laminar burningvelocity peak at an ammonia-lean mixture of ��0.9.The peak flame temperature was predicted to increasemarginally with unburnt mixture temperature. This posi-tive temperature effect was somewhat more significant inthe low-burning-velocity tail portion for ammonia-richmixtures. Pressure is predicted to have a slightly positiveeffect on the adiabatic flame temperature, particularlyaround stoichiometric composition. Overall, laminarburning velocity is predicted to increase strongly withthe unburnt mixture temperature and to decrease weakly


Fig. 10. (a) Effects of � on OH concentration using the Lindstedtmechanism. (b) Effects of � on OH concentration using the Miller–Bowman mechanism.

Table 1The five most important elementary reactions of the Lindstedt mech-anism in the production of OH

Elementary reaction Maximum sensitivity coefficient

1 NH2+NO=NNH+OH 7.02 NH2+NO=N2+H2O �2.73 NH3+OH=NH2+H2O �1.44 N+OH=NO+H 1.05 NO+NNH=N2+HNO 1.0

Fig. 11. Important elementary reactions of the Lindstedt mechanismin the OH production at STP and stoichiometric composition.

Fig. 12. Temperature effects on OH sensitivity to reactions 1 and 2of the Lindstedt reaction mechanism at 101 kPa and stoichiometriccomposition.

with increasing pressure. The ammonia-rich flamm-ability limit increased with increasing unburnt mixturetemperature, but the ammonia-lean limit remained verylow at ��0.1.

The amount of nitric oxide is a controlling factor onthe OH production. Two elementary reactions of theLindstedt 1994 mechanism have significant effects onthe OH production. One of these two reactions boostsOH by consuming nitric oxide, while the other competes


for nitric oxide and has a negative effect on OH pro-duction. OH production is most sensitive to the afore-mentioned two elementary reactions at higher unburntmixture temperature.

Acknowledgements

This project was sponsored by the Natural Science andEngineering Research Council of Canada (NSERC). Thelead author gratefully acknowledges the Ontario StudentAssistance Program (OSAP) for an Ontario GraduateScholarship (OGS).

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combustion hazard of mixing ammonia with nitric oxide

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