Flat low-temperature two-stage flames were established on a Powling burner using rich diethyl-ether/ air or n-heptane/air mixtures, and nitrogen monoxide NO was added into the fuel-air mixtures with a concentration of 240 ppm. The temperature development and chemical-species histories, especially of
NO, nitrogen dioxide NO2 and hydrogen cyanide HCN were examined associated with an emission-spectrum
measurement from the low-temperature flames. Nitrogen monoxide was consumed in the cool-flame
region, where NO was converted to the NO2. The NO2 generated, however, fell suddenly in the cool-flame
degenerate region, in which the HCN superseded. In the blue-flame region the NO came out again and
developed accompanied with remained HCN in the post blue-flame region. The NO seeding into the
mixture intensified the blue-flame luminescence probably due to the cyanide increase

M. Furutani

M. Nose

Y. Ohta

Eurasian Chemico-Technological Journal

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*corresponding  authors. E-mail: ohta@megw.mech.nitech.ac.jpEurasian ChemTech Journal  3 (2001) 157-161 2001 al-Farabi Kazakh St. Nat. UniversityNitric Oxide Circumstances in Nitrogen-Oxide SeededLow-Temperature Powling-Burner FlamesM. Furutani, Y. Ohta* and M. NoseNagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, JapanAbstractFlat low-temperature two-stage flames were established on a Powling burner using rich diethyl-ether/air or n-heptane/air mixtures, and nitrogen monoxide NO was added into the fuel-air mixtures with aconcentration of 240 ppm. The temperature development and chemical-species histories, especially ofNO, nitrogen dioxide NO2 and hydrogen cyanide HCN were examined associated with an emission-spec-trum measurement from the low-temperature flames. Nitrogen monoxide was consumed in the cool-flameregion, where NO was converted to the NO2. The NO2 generated, however, fell suddenly in the cool-flamedegenerate region, in which the HCN superseded. In the blue-flame region the NO came out again anddeveloped accompanied with remained HCN in the post blue-flame region. The NO seeding into themixture intensified the blue-flame luminescence probably due to the cyanide increase.IntroductionIt has been indicated that some nitrogen-relatedcompounds closely take part in the ignition promo-tion/retardation phenomena.1. The NOx generation was plausible during thepreflame reactions prior to the hot ignition [1].2. The behavior of cool- and blue-flame onsets wasdifferent between the mixtures whether N2 or Arwas included as an inert diluting agent for the oxi-dizer [2].3. Hot-flame occurrence caused by mixture com-pression could be eliminated by low-volatilecompound-vapor addition in the mixture, but whenthe 21O2/79Ar synthetic air was used instead ofnatural air as oxidizer, no eliminating effect couldbe found [3].The burned gases are always included in the freshmixtures in the reciprocating piston engines. The ex-haust gas recirculation is a popular measure forreducing nitric oxide emission. On the nitrogen-oxidecontained mixtures the ignition delays has beenreported [4], but no details has been informed on thelow-temperature oxidation-reaction processes.Flat low-temperature Powling-burner flames with/without nitrogen-oxide seeding into the mixtures havebeen examined as to the emission spectrum andchemical-species history, to elucidate the effects ofnitrogen oxides on low-temperature oxidation of fuel-air mixtures in the most simple, undoubted system,and to find out ignition control procedures to realizepremixed compression-ignition (HCCI) engines.ExperimentalA modified Powling burner at atmospheric pres-sure was used to stabilize flat two-stage low-tempera-ture flames of rich diethyl-ether/air mixtures in verti-cally-flowing laminar streams. An outline is shownin Fig. 1. The details are shown in our paper for-merly published [5], though the present apparatus wasimproved for a higher-level flame stabilization. A fuel/air mixture flowed with almost uniform velocity dis-tribution from an inner burner tube surface, whichwas a porous ceramic honeycomb plate. An outerquartz shielding tube (inner diameter: 105 mm, height:172 mm) supported a stabilizing ceramic screen atthe top way-out position. The stabilizing screen wasa ceramic-fiber matrix including heaters electricallycontrolled in temperature. The low-temperature cooland blue flames are stabilized within the quartzshielding tube.Fuel used for the Powling burner was diethyl-etheror n-heptane. The equivalence ratio of mixtures wasset to be 2.0 or 3.0. The nitrogen-balanced 5000-ppmnitrogen-monoxide reference gas was supplied into thefuel/air mixtures. Pure nitrogen gas was added in theEurasian ChemTech Journal  3 (2001) 157-161Nitrogen-Oxide Seeded Low-Temperature Flames158NO-nonseeding case to keep an identical inert-gas di-lution ratio.Temperature profiles in the two-stage flames weremeasured with a type K thermocouple having 50 µmin diameter, coated with SiO2 to eliminate catalyticeffects on the wire surface.Gas samples were aspirated continuously througha quartz probe kept 105°C and then introduced into acooler kept –21°C to remove water vapor. Theconcentration of each nitrogen related compound wasdetermined by using a chemiluminescence type NOxanalyzer (Beckmann: 951) for nitrogen monoxide NO,by Salzmann absorption luminosity method (JIS K0104) for nitrogen dioxide NO2, by Zinc-reducingnaphthylethylenediamine Zn-NEDA method (JIS K0104) for nitric oxide NOx, and by Dräger tubes forhydrogen cyanide HCN respectively.Light emission was focused onto a slit of amonochrometer (Japan Spectroscopy: CT-25N) toobtain emission spectra, and was detected by a cooledphotomultiplier (Hamamatsu Photonics: R3896,190~900 nm). The slit width was set to be 50 µmexcept for the cool-flame observation with 200 µm inwidth. The spatial resolution was less than 1 mm.For the wave length higher than 620 nm, a red-colorglass filter (Toshiba: R-62) was used to eliminate thesecondary reflection.Results and DiscussionTypical low-temperature flames could be obtainedeasily by a diethyl-ether/air mixture with an equiva-lence ratio of 3.0; cool and blue flames were visibleto the naked eye and the cool-flame degeneration wasgiven as a recognizable distance between cool andblue flames [6]. The temperature development in thiscase is shown in the lower part of Fig. 2. The nitro-gen monoxide was so added to this mixture as to be240 ppm, the initial seeding NO concentration.Fig. 1. Schematics of Powling burner and gas samplingdevice.Fig. 2 Temperature development of diethyl-ether/air low-temperature flames with equivalence ratio of 2.0 and 3.0.The concentration histories of NO, NO2 and HCNare shown in Fig. 3. The abscissa is the height fromthe burner honeycomb surface. A temperature risedue to cool flame appearance could be recognized fromaround 75-mm height in Fig 2. At about 69-mm posi-tion just before the cool flame onset the NOconcentration has been slightly reduced to be 220 ppmand the amount of NO2 detected has been 16 ppm.The NO was consumed drastically in the cool flameperiod. Corresponding to the NO consumption, NO2was generated during the cool flame and showed apeak at 77-mm position within the luminous zone ofcool flame. All of the consumed NO in cool flamewas not converted to NO2. It was not until the cool-flame that the HCN appeared.The NO2 decreased steeply in the cool-flame de-generate region. At the 79-mm height the NoxEurasian ChemTech Journal  3 (2001) 157-161M. Furutani et al. 159concentration was 33 ppm. It was close to the sum ofNO (15 ppm) and NO2 (15 ppm) which were sepa-rately measured, but was far from the original NOamount of 240 ppm. The HCN accounted for a greaterpart of nitrogen-related compound in he cool-flamedegenerate region, where the amounts of NO, NO2and HCN were unperturbed.At around 87-mm height the temperature wasraised again as shown in Fig. 2; the blue flame ap-pearance. In the blue flame the HCN was consumedalmost to half. After the blue flame appeared theamount of NO increased gradually. The differencebetween the NOx concentration and the sum of NOand NO2 was a negligible one of 8 ppm at the 100-mm position.In the post blue-flame region the NO concentrationwas restored only to 54 per cent of the initial charge,while the HCN decreased to the 34 per cent of itsmaximum level. The sum of NOx and HCN did notattain to the 240 ppm; the NO initially charged.The degeneration of cool flame has led a NOconsumption/NO2 generation, which would be derivedthrough an oxidation reaction of NO:NO + HO2 → NO2 + OHIt has been reported that the nitrogen-oxide oxida-tion to the nitrogen dioxide would be promoted underhydrocarbon coexistence [7]. The nitrogen-oxide ad-dition into the mixture would result in the nitrogen-dioxide supply for the blue-flame onset. The NO andNO2 would react with organic radicals such as alkoxylradical and formed a nitrous acid or alkylnitrate inand after the blue flame. The alkylnitrate is the typi-cal one of cetane rating improvers for the hydrocar-bon fuels.It could be possible to hypothesize that the NOwould be generated from the decomposition of nitrousacid or alkylnitrate. The hypothesis could explain whythe blue-flame reactions have high fuel-decomposi-tion potency.The spectroscopic emission distribution in this caseis shown in Fig. 4. The emission spectra shown werethe ones of the middle position of each flame on thecenterline of the burner tube. The abscissa and ordi-nate are the wavelength and photomultiplier currentrespectively. The chemical species corresponding toprincipal emission peaks are indicated in the figure.The ultraviolet to visible spectrum of cool-flameemission is the one of the «Emeléus’s Cool FlameBands» overlapped with «Carbon Monoxide FlameBands». The almost all emission are originated fromthe formaldehyde.Fig. 3. NO, NO2, HCN concentration histories in diethyl-ether/air low-temperature flames with an equivalenceratio of 3.0.(1)Fig. 4. Spectral emission distribution in diethyl-ether/air low-temperature flames with an equivalence ratio of 3.0.Nitrogen-Oxide Seeded Low-Temperature FlamesEurasian ChemTech Journal  3 (2001) 157-161 160No characteristic change due to NO addition inthe mixture was found on the cool-flame emissionspectrum, through the emission was slightly intensi-fied over the ultraviolet, visible and near-infrared wavelength. Also on the blue-flame spectrum no change incharacteristic and intensity except as to a manifest-ation of near-infrared 716- and 809-nm «H2O Vibra-tion-Rotation Spectrum».Even in the NO-nonseeded low-temperature flamean emission in 388-nm wave length (equivalent to the«CN Violet System») could be found. The HCNcontent measured in the NO-nonseeded cool flamewith a 3.0 equivalence ratio was 160 ppm, whichamount was similar to the 150 ppm HCN in the NO-nonseeded cool flame shown in Fig. 3.When the equivalence ratio of the diethyl-ether/air mixture was reduced to 2.0, a thin nonluminous«Dark Zone» and a bright luminous «YellowColumn» zone [8] followed in the post blue-flameregion. Under the 2.0 equivalence ratio, the NO ad-dition up to 240 ppm in the mixture weakened theyellow-column intensity. When the NO addition wasdecreased to 180 ppm a bright luminous yellowcolumn came up again. The temperature developmentis shown in the upper part of Fig. 2. The spectralemission distribution from the low-temperature flamesof the diethyl-ether/air mixture with an equivalenceratio 2.0 is shown in Fig. 5 with/without the initialNO charge of 180 ppm. The blue-flame spectrum wasintensified in the near-infrared bandwidth. The yel-low column became brighter with the 180-ppm NOaddition.Fig. 5. Spectral emission distribution in diethyl-ether/air low-temperature flames with an equivalence ratio of 2.0.The diethyl-ether is a convenient fuel to get stabi-lized low-temperature flames. It is not, however, thenormal hydrocarbon fuel used in piston engines but theoxygenated one. n-Heptane/air stabilized low-tempera-ture flames were realized herewith with an equivalenceratio of 3.0. The NO, NO2, HCN consumption/genera-tion histories in a normal hydrocarbon low-temperatureflames with an initial content of 200-ppm NO seedingwas examined, and the results are shown in Fig. 6.The interval between cool and blue flames, i.e.,cool-flame degenerate region, are shorter than in thediethyl-ether case. The spatial resolution for gas sam-pling was narrowly permissible. The n-heptane/airmixture with the 3.0 equivalence ratio gave us a yel-low column. Characteristic with/without NO additionin the mixture did not change significantly the cool-and blue flame emission spectra and intensity, exceptthe near-infrared emission intensity of blue flame.The NO and NO2 histories of n-heptane originatedlow-temperature flames were almost identical in theM. Furutani et al.Eurasian ChemTech Journal  3 (2001) 157-161161consumption/generation trends with the diethyl-ethercases. The HCN, however, increased continuously upto the dark zone just before the yellow column, de-creased once and then increased again gradually inthe yellow column. This behavior differs from thediethyl-ether case.Prepared the diethyl-ether/air mixture with anequivalence ratio of 2.0, the HCN concentration wasmeasured in and after the blue flame under the initialNO seeding of 180 ppm, to differentiate the reasonwhether the fuel type or the equivalence ratio deter-mines the HCN history after the dark zone. Detectedwere the 140-ppm HCN in the blue flame, 32-ppmHCN in the dark zone and the 66-ppm HCN in theyellow column at the 90-mm position. It can berecognized that the HCN history after the blue flamedepends on the equivalence ratio and/or the yellowcolumn existence.ConclusionsWe have obtained the consumption/generation his-tories of NO, NO2 and HCN in low-temperaturePowling-burner flames with a relatively low contentof 240-ppm NO seeding, associated with the tempera-ture development and emission spectrum measure-ments. The features of this paper are as follows:· Initially seeded NO was consumed and NO2 wasgenerated in the cool flame.· The NO2 generated in the cool flame was consumedin the cool-flame degenerate region.· The differences caused by NO addition are due notto the initially seeded NO itself but to the productsformed through the reactions with NO. The prod-ucts would intensify the low-temperature oxidationand fuel decomposition reactions in blue flame.· The nitrogen-oxide addition into the mixture wouldresult in the nitrogen-dioxide supply for the blue-flame onset.· The information obtained using an oxigenated etheras fuel is effective to understand the low-temperatureoxidation processes of normal hydrocarbon fuels.Even in the NO-nonseeded low-temperature flamesan emission of CN system can be found, and the HCNis detected in the sampled gases. It seems that theHCN generation has no dependence on the initial NOseeding. Active oxygenate radicals, such as O atom,is necessary for the NO generation. In the low-tem-perature oxidation processes a similar generationreaction mechanism is plausible to progress.References1. Ohta, Y. and Takahashi, H., NO Generation dur-ing Compression Ignition Process, Trans. JSME(1984), 478~483, (in Japanese).2. Furutani, M., Ohta, Y. and Komatsu, K., OnsetBehavior of Low-Temperature Flames Caused byPiston Compression, JSAE Review, 9303429,(1993).3. Furutani, M. and Ohta, Y., A Novel Attempt atEliminating Piston-Compression Ignition,COMODIA 94, (1994), 189~192.4. Kane, G. P., The Effect of Nitrogen Peroxide onthe Two-Stage Ignition of Hydrocarbons, Proc.Royl. Soc. (London), A171 (1939), 251~269.5. Ohta, Y. and Takahashi, H., Temperature andPressure Effect in Cool and Blue Flames,Progress in Astronautics and Aeronautics, 88,(1983), 38~56, AIAA.6. See photos on our website: http://qube.mech.nitech.ac.jp/groups/lab/Ignition.html.7. Hori, M., Matsunaga, N., Malte, P.C. andMarinov, N.M., The Effect of Low-ConcentrationFuels on the Conversion of Nitric Oxide to Nitro-gen Dioxide, 24th Symp. (Int’l) on Combust.,(1992), 909~916, The Combustion Institute.8. Agnew, W. G. and Agnew, J. T., Visible EmissionSpectra of Two-Stage Flames of Diethyl EtherProduced in Flat-Flame Burner, Ind. Eng. Chem.,48-12 (1956), 2224~2231.Fig. 6.  NO, NO2, HCN concentration histories in n-hep-tane/air low-temperature flames with an equivalence ratioof 3.0.Received  27 November  2001.

Nitric Oxide Circumstances in Nitrogen-Oxide Seeded Low-Temperature Powling-Burner Flames

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Nitric Oxide Circumstances in Nitrogen-Oxide Seeded Low-Temperature Powling-Burner Flames

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