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Combustion of hydrogen and hydrazine with nitrous oxide and nitric oxide: Flame speeds and flammability limits of ternary mixtures at sub-atmospheric pressures
COMBUSTION OF HYDROGEN AND HYDRAZINE WITH N I T R O U S O X I D E AND N I T R I C O X I D E : F L A M E S P E E D S AND FLAMMABILITY LIMITS OF TERNARY MIXTURES AT S U B - A T M O S P H E R I C PRESSURES P. GRAY, R. MACKINVEN AND D. B. SMITH* Department of Physical Chemistry. University of Leeds, Leeds 2 Flame speeds relative to the burnt gas h:ive been measured at sub-atmospheric prcssur¢~, over the whole range of flammability lbr the ternary systems hydrazine plus nitrous oxide plus nitric oxide and hF.lrogen plus nitrous oxide plus nitric oxide, and for a number of hydrogen plus nitrous oxide plus nitrogen mixtnres. Measurements were made in a closed vessel, using a schlieren technique in conjunction with a rotating.drum camera. Composition limits of flammability (at 50 mm and 70 mm of mercury) have al~ been determined. In ~he combustion of hydrogen supported by the: mixed oxides of nitrogen the maximum flame speed (SH = 3470 cm see" i) occurred in a slightly fuel-rich mixture ~53 per cent hydrogen) of hydrogen plus nitrous oxide from 'vhich nitric oxide was absent. Adding nitric oxide had ¢lTecls more like adding an inert diluent than a second oxidant. Flame speeds were reduced and, at the pressures used, binary mixtures of nitric oxide with hydrogen could not be ignited. Composition limits of flammability for hydrogen plus nitrous oxide plus nitrogen and for hydrogen plus nitrous oxide plus nitric oxide were very similar, This b~haviour results from two simultaneous roles of nitric oxide-as an inhibitor and as an oxidant-that lend to eanceI out, In the combustion of hydrazine supported by mixtures of nitric oxide and nitrous oxide, the rever~ circumstance was found, nitric oxide supporting combustion better than nitrous oxide and maximum flame speeds (SB = 3600 cm sec- ~)occurred in a fuel.rich (50 per cent hydrazine) mixture of hydrazine and nitric oxide from which nitrous oxide was ,'~bscnt, In lean ternary mixtures, the mixed oxides enhanced one another's reactivity, flame speeds being greatest when both were present. This synergistic behaviour resembles that found in ammonia systems, but its prominence is diminished by the occurrence of hydrazine decomposition along with oxidation, The same competition between decomposition and oxidation may account for the unusually fuel-rich location ~,f maximum flame speed in hydrazine-nitric oxide mixtures.
Introduction A SYSTEMATIC study has recently been undertaken 1-3 of the combustion of binary systemsof hydrazine and ammonia and hydrogen with oxygen, nitrous oxide and nitric oxide. More recently, this work has been extended3"'* to ternary systems. When ternary mixtures are burned, two offdations occur simultaneously and they may be expected to interact. An example of this interaction was found in the hydrogen plus oxygen plus nitrous oxide system, studied4 by Gray, Mackinven and Smith. Often, however, ternary * I'rescnt address: Departing:hiof Furl S~:ien¢¢,The Universily. Leeds 2. 217
systems behave as if there were no interactions, The composition limits of flammability then obey the rule5 of L¢ Chatelier: the flammability limits, represented on a triangular diagram, are straight linesjoining the limitsfor the constituent binary systems. Correspondingly, flame speeds and temperatures have maxima in one of the binary systems, and decrease monotonically through the ternary mixtures to the other binary system. A striking example of interaction between two combustions was found3 for the ammonia plus nitrous oxide plus nitric oxide system, where the flame speeds of ternary mixtures were faster than those of either binary system. This unusual behaviour was explained in terms of the
2t8
P, (;RAY. R, MA('KI,XVii% AND I). B. SMIIII
reactions of the NHz radical (from ammonia) with the oxides of nhrogen. If this cxpl,matiow~ is correct, similar behaviour would be expected for the hydro cine plus nitrous oxide p!us nitric oxide systelr, since hydrazine can act as ,'1 still more ready source of NH2 radicals than ammonia. The present work was undertaken partly to throw more light ~n the role of nitrou:; oxide and nitric oxide acting as joint oxidants in the combustion of hydrazine. The combustion of hydrazine is complicated by the possible importance of prior decomposition of fuel and the subsequent oxidation of the hydrogen and ammonia produced in this decomposition, A study of the hydrogen plus nitrous oxide plus nitric oxide system is therefore likely to throw additional light on the mechanism of Iwdrazine combustion in flames. Accordingly. flame speeds for the ~ernary systems hydrazine plus nitrous oxide plus nitric oxide and hydrogen plus nitrous oxide plus nitric oxide have been measured at low pressure in a closed vessel at an initial temperature of 60'~C. The technique has the advantages that it allows easy control of initial conditions of temperature and pressure, and avoids the difficulty of accurately metering vaporized fuels. such as hydrazine. Furthermore, as this technique was used in the earlier studies t-4. it allows direct comparisons to be made with them.
Experimental Materials Hydrogen, nitrous oxide, nitric oxide and nitrogen were taken from cylinders. Nitrous oxide and nitric oxide were fractionated in vacuo, using liquid nitrogen as refrigerant; nitric oxide was condensed on to pellets of sodium hydroxide to remove traces of nitrogen dioxide. Anhydrous hydrazine (iodate analysis corresponding to 100.1 per cent NzH¢) was prepared by refluxing 95 per cent hydrazine (supplied by EastmanKodak Ltd) over sodium hydroxide for three hours in an atmosphere of nitrogen at 10 to 15 cm of mercury, followed by distillation at the same pressure.
Apparatus and procedure Measurement of flame speeds--Flame speeds
(88) relative to the burnt gas were measured in
Vol. II
a cylinder combustion vessel (20 on long; 20 cm i.d,). A schliercn lechniqtJc in conjunction with a rotating-drunl camera was used to record the progress of the flame front, Details of the method have been given previously". For the hydrogen plus nitrous oxide plus nitric oxide system, an initial pressure of 70 mm of mercury was used. For the hydrazine plus nitrous oxide plus nitric oxide system, the initial pressure was 40 mm. and a few me,'|surements were made at 30 mm of mercury. Above 40 ram. combustion was too violent for the present glass combustion vessel. The earlier measurements t on hydrazine combustion were also made at 40 mm pressure, Results were displayed as flame speed contours against composition on a triangular diagram. The standard deviation of an individual measurement was probably four per cent. but contours were probably located to better than two per cent, Pressures before and after ignition were measured in all experiments, Determ#latDn of .flanmtability limi;, The composition limits of flammability for upward propagation at 50 mm and 70 mm of mercury were determined in a vertical tube {length 70 cm. diameter ca, 7.5 cm), Successful propagation up the whole length of the tube constituted flammability. Outside the lean limit, partial ignitions in which the flame expired before reaching the top were sometimes observed. Details of the apparatus and procedure have been given'~previously.
Calculations oJ'flame temperatures, burnt-gas compositions andflame speeds relative to unburnt gas--The present values of Sn may be converted into flame speeds {Su) relative to unburnt gas. if the expansion ratio (Po PtJ) is known. The density of the burnt gas depends on ils composition and on the final flame temperature. Calculations of the equilibrium burnt gas composition and adiabatic flame temperatures have been made for selected mixture compositions. The method of Damkfhler and Edse~Ldescribed by Gaydon and Wolfl~ard"7. was used fo~' these calculations. The thermodynamic properties required were taken from standard compilations 7. s. In near-stoichiometrie mixtures, combustion is complete and reliable values for Tn and So are obtained. For mixtures with compositions far
IIYI)RAZINli ('OMIIUSTIONIN TliRNARY MIXTURi'~S
June 1!167
from stoichiomctric, non-equilibrium conditions do nol permit such calculations to be made with confidence. Results
Composition limits offlammability H ydrooen plus nitrous oxide plus nitric oxideThe composition limits of flammability for H2 + N20 + NO mixtures at 50 mm and 70 mm of mercury in a tube of 7.5 cm diameter are displayed in Figure I,
219
not support a flame, and in the present apparatus, no H 2 + NO mixtures could be ignited.
Hydrooen plus nitrous oxide plus nitrooen (H2 + NzO + Nz)---The composition limits of flammability for mixtures of hydrogen and nitrous oxide diluted with nitrogen at 50 mm and 70 mm of mercury in a tube of 6.8 cm diameter arc displayed in Figure 2. H2
Stoicl
;tric
N20
N20
NO
Flca;l~i! I, Conlposilion lintit-s of I]amnlability ~. :,ubatmospheric pressures lbr ternary mixtures of I lz + N20 + NO. (Outer lines refer to 70 m,-. pressure and inner to 50 111111.Da.,,hed line .ioin~ all stoichiomelric composilions)
For fuel-rich mixtures, the flames were vigorous, instantaneous and yellow in coiour. For lean mixtures, slower flames, yellow-green to bright-green in coiour, were propagated, and partial ignitions sometimes occurred outside the flammability limits. '[he limits for the binary H2 + N20 system occurred at 4.5 and 68.5 per cent H2 at 70 mm, ,'rod 7.0 and 62.0 per cent H2 at 50 mm of mercury. The range of flammability narrowed almost linearly as the nitric oxide concentration was increased. The tip of the combustible range occurred at a composition of 9.0 per cent H2 plus 20.0 per cent N20 plus 71,0 per cent NO at 70 mm, and 9.0 per cent H2 plus 23.0 per cent N 20 plus 68.0 pet"cent NO at 50 mm of mercury. Mixtures containing more NO than this would
Nz
I:l(;t]Ri~2. Composition limits of t]amrnabilily at sub-atmos-. pheric pressures Ibr ternary mixtures ot' H~ + N~O + N,, (Ottler lines refer to 70 mm pressure ~md inner to 50 ram, [),tshed line joins all stoichiometric compositions)
For fuel-rich mixtures, flames were rapid, vigorous and white in colour. As the nitrogen concentration was increased, the flames became less luminous. For fuel-lean mixtures, much slower flames, yellow-white in colour could be propagated, and partial ignitions sometimes occurred outside the flammability limits. With high nitrogen concentration, the flames were almost non-luminous and very difficult to observe, The limits for the binary H2 + N20 system* occurred at 7.0 and 67.0 per cent !'12 at 70 mm of mercury, and 8.0 and 62.0 per cent at 50 mm of mercury. The flammable region again narrowed linearly with increasir,g diluent con, centtation, The tip of the combustible range occurred at a composition of 7'5 per cent H~ * The Ilammability limit tube was slitbtly wider for the nitric oxide mixture~(diameler 7,5 cm c~)nlpar~'dwith 6,8 cm Ibr thal with: Ih¢ nitrogen mixtures).
220
Vol. II
P, (ilIA¥. R, MA('KINVI N AND I). P. SMI'III
plus 19,0 per cent N20 plus 73.5 per cent N 2 at 70 mm. and 8.0 per cent H2 plus 22,0 per cent N20 plus 70.0 per cent N2 at 50 mm ofmercttry,
Laminar flame speeds: effect q/'composition Hydrogen plus nitrous oxide plus nitric oxide~ The dependence of flame speed on composition for the combustion of hydrogen with nitrous oxide and nitric oxide over the whole range of flammability is shown in Figure 3. The measurements were made at an initial temperature of 62" + 2'~C. and an initial pressure of 70 mm of mercury. The flame speed contours reflected the form of the flammability limits, Flame speeds fell continuously as nitrous oxide was replaced by nitric oxide. The maximum flame speed (SB = 3470 cm sec -j) occurred in a slightly fuel-rich 153'i/o H2) H2 + N20 mixture, containing no nitric oxide. At the stoichiometric composition, S~ was 3440 cm sec- ~. It was not possible to ignite H2 + NO mixtures under the same experimental conditions.
the addition of only 20 per cent nitric oxide, 1he decrease was more gradual for mixtures conraining equimohtr hydrogen and nitrous oxide with added nitric oxide [H2 + N20 + xNO, dei~oted by line bb' on Figure 3]. Mixtures with more than 53 per :enl nitric oxide would not support a flan,c, and a mixture with composition 23.5 per cent hydrogen plus 23'5 per cent nitrous oxide plus 53 per cent nitric oxide burned with a fl~me speed of 520 cm sec- 1, The flame speeds of these latter mixturcs arc displayed in Figure 4, 4 o Added NO
J
b3
H2 t.I.
-'
2"/10
N20
' 4'o ' Diluent ( NO or N2). %
6'o
FIGL'Rli 4. Effi:ct ot" nitric oxide illld nilrog¢ll till the Ilamc speed (Sx, ¢m ~ec-=) tot stoichionletruc mixtures of H, with N20: ilame speed, Ill a total prcssUl'e of 71) mm of Hg. its a funclion ot' nitric oxide or nilrogelt ¢otlcentrafion
b
NO"'b'
i==Gu~ 3. Dependence of burning ve!ocity (Sn, cm sec-~) on composition in ternary mixtures H2 + NzO + NO: contours join compositions giving equal values of S~. (Dashed line aa' joins all stoichiometric compositions and dashed line bb' all compositions H2 + xN20 + (I - x)NO)
where they are compared with flame speeds in mixtures in which nitrogen replaces nitric oxide (H z + NzO + xN2), The results for the two systems show great similarity. After the addition of ca, 15 per cent of the third constituent {NO or N2). the flame speeds decreased linearly with increasing concentrations of either nitric oxide or nitrogen. The differences in SH for the two systems were about 360 cm sec- 1 (400 cm see- 1 at 15 per cent and 330 cm sec-t at 50 per cent of the third constituent),
Hydrazine plus nitrous oxide plus nitric oxide Flame speeds for stoichiometric ternary mixtures [with compositions H2 + xN20 + (1 - x)NO, denoted by line aa' on Figure 3] decreased rapidly with inereaslng nitric oxide ~:oncontration, failing to 2000 cm see-~ after
(N2H4 + N20 + NO)--The dependence of flame speed on composition is shown in Figure 5, The measurements were made at an initial temperature of 62° + 2°C and an initial pressure of 40 mm of mercury. In addition, a few flame
Jvne 1967
tlYI)RAZINE COMBUSTION IN TF~RNARYMIXTURES
speeds were measured at an initial pressure of 30 mm of mercury. These results were ca. 5 per cent lower than those at 40 ram. The fastest flame occurred in a very rich N2H4 + NO binary mixture, containing 50 per cent NzH,, for which Sa was 3600 cm sec-~. A stoichiometric N~H,, + NO mixture (containing 33.3 per cent N2H,,) had a burning velocity of 3150 cm sec- ~.For the N2H,, + NzO system, the maximum flame speed (SB = 2400 cm sec-~) occurred at stoichiometric composition 133'3 per cent N2H,d.
N2H~,
Np
No
]:l(ilJRf! 5. Dependence of burning velocity (Su, cm scc J} on composilion in ternary mixturesofN.,ll.~ + N,O + NO: ¢ontotll's join compositions giving equal valtlcs of SIj. (Dashed line joil,:s all ~,toichioinctric ¢onipositions}
For ternary mixtures on the rich side of stoichiometric, replacement of NO by N20 generally caused a decrease in flame speed. For stoichiometric and lean mixtures, however, as NO was replaced by N zO. flame speeds increased to a maximum value and then decreased again, For stoichiometric mixtures, the maximum flame speed (Sn = 3 400 cm see -~) occurred at a composition close to 0.33 N2H,, + 0.22 N~O + 0.44 NO.
Fh,,ne temperatures, burnt oas compositions, and flanw speeds relative to the unburnt ~ias Calculations of the equilibrium burnt gas composition were based on the assuml":ons that combustion was complete and that any
221
excess reactant (nitrous oxide or nitric oxide) decomposed into its elements. Support for the view that complete combustion occurred comes from the measurement of pressure changes for combustion. Although pressure measurements alone cannot reveal whether excess nitric oxide remains, chemical investigation after explosion showed no sign of nitric oxide, whereas the presence of oxygen was revealed by the artificial addition of nitric oxide to the product gases, causing the formation of nitrogen dioxide. This indicates that when nitric oxide was initially present at high concentration, it was almost entirely decomposed during the reaction. For stoichiometric ternary mixtures of hydrogen with nitrous and nitric oxides (denoted by line aa' on Figure 3), flame temperatures increased monotonically throughout the measured range from the value of 271ff~K in H~ + N20 to 275ffK for a mixture with 20 per cent NO. For mixtures equimolar in H2 and Nee with compositions denoted by the line bb' on Figure 3, the calculated flame temperatures increased slightly to 2720"K with the addition of ten per cent NO, and then decreased very slowly, having a value of 271ffK at a composition 35 per cent Hz + 35 per cent N20 + 30 per cent NO. For stoichiometric mixtures of hydrazine with nitrous and nitric oxides, the flame temperatures increased linearly as NzO was replaced by NO, from the value of 2655°K in N,H4 + N,O mixtures to 2745°K in N2H,, + NO mixtures. For binary N2H,~ + NO mixtures, flame temperatures decreased as the hydrazine concentration was increased, falling to a value of 2645°K at an equimolar composition. Calculations of burnt gas compositions and adiabatic flame temperatures for these and other compositions are shown in Table 1. The derived values of flame speed relative to the unburnt gas (Stj) are also included. Discussion Both the ternary systems studied here show: marked departures from expectations based on the Le Chatelier rule s for the combustion of one fuel supported by two oxidants, in the combustion of hydrazine, the two oxidants ( !!
+ NO
N2H4 + N20 40 m m H g
70 m m H g
Ha+ NzO
System
66-7 33-3 ---.. -.....
33-3 33-3 33-3 40 41-6 50
m
50 80 40 30 45 40 35
--------
50 20 50 50 45 4O 35
N20
N2H.~
H2
burnt
-33"3 66-7 60 58-4 50
-~ -10 20 10 20 30
NO
equilibrium
Original composition. mole per cent
flame temperatures,
+ NO
TABLE 1. A d i a b a t i c
Su 379 89 366 234 337 285 226 164 237 245 252 247 238
3440 855 3120 2000 3050 2550 2000 2 400 3 290 3 150 3460 3 480 3550
2655 2700 _ ;4_ 2725 2715 2645
2440 2730 2750 2720 2715 2710
i 27i0
K
SB
Flame temp.
Flame .~peed, CtPI ,~('£'- 1
40-7 30-7
41-0
53-5 48"3 41 "8
44-1 59-5 41"3 38"1 44.3 44.8 44"9
N,
23-4 24"2 24-6 25-1 24-8 23-0
30-2 13-2 31-2 31 "6 28-6 26-4 23-7
H,O
7.6 ,'.I-7 !0" I 14-6 15-8 24-2
8-5 0'4 9"3 10-0 0"3~ 4-7 3"4
H:
+ NO
and
6-f) 0.4 6.3 7-1 5-1 4-3 3-6 5-5 7-2 9-1 10-4 10-6 9-8
2-6 3.0 3.4 I-4 1-2 0"25
H
2-9 20.6 3"i 3.4 5-4 8-2 t 1.7
O.
i~ioh, p e r C('~//
21 2-7 3.,'4 2-I 1-8 0-5
2-3 i -6 2-3 2-7 2.9o 3-6 4.3
O
N2H. t + H.O
Final composition.
g a s c o m p o s i t i o n s a n d f l a m e s p e e d s f o r H2 + N 2 0 Initial t e m p e r a t u r e . 62 C
4-7 4-4 2,4
6. t
4.3 5. !
5-1 2-5 5-5 f,- i 64) 6-3 6,5
OH
+ NO
1.0 1,0 1.1 0-7 0-6 0.25
11-8 I-9 1.0 141 1-3 I-6 1.9
NO
mixtures.
,<
June 1967
IIYDRAZINECOMBUSTIONIN 1ERNAR¥MIXTURES
assist one another; while in the combustion of hydrogen, nitric oxide is apparently no better than an inert diluent.
ttydrotlen plus nitrous oxide plus nitric oxide (H, + N20 + NO) Previous burning velocity work 3 'Jon this system is confined to the binary systems H2 + N20 and H2 + NO. although composition limits of flammability for the binary and ternary mix. turcs have been reported {see below). The present results for flame speeds in H2 + N20 mixtures agree with earlier results in the same apparatus, obtained by Armitage and Gray 3. Comparison with the burning velocities at Iatm pressure, measured by Parker and Wolfhard t° and by Kozachenko and Skachov tt, confirms the conclusion that Sv is independent of pressure over a considerable pressure range, implying7' 8 an effective overall reaction order n of two. Flame speeds in binary H2 + NO mixtures burned at pressures from I to t0 atm have been reported ~ by Adams and Stocks, who found values for Sv around 60 to 80 cm sec-t somewhat dependent on pressure. Composition limits of flammability at 1 atm for the H2 + N 2 0 + NO system have been determined by Scott, Van Dolah and Zabetakis for upward i~ropagationtz in a tube of 5 cm diameter, and by Van der Wai for downward propagation '3 in a 1.5 cm tube. These results. together with the present measurements, show a continuous variation from the relatively easy inflammation at I arm in a wide tube, through the more difficult downward propagation in a narrower tube, to the lowest flammability at low pressure. Both the flame speed contours and flammability limits for ternary mixtures reveal that the combustion of hydrogen supported by nitrous oxide and nitric oxide deviates from the 'normal' behaviour expected for a fuel burning with two oxidants. At 1 arm, the rich limit shows that nitrous oxide and nitric oxide are antagonistic oxidants. At low pressures the region of flammability at the upper limit narrows very sharply as the nitric oxide concentration is increased, and nitric oxide appears no better than an inert diluent, despite its oxygen content and endothermic nature. Support for this view comes
223
from the experiments on the H 2 + N20 + N 2 system, The small differences in the ranges of flammability when nitric oxide is replaced by nitrogen are wholly understandable in terms of the small differences in the experimental conditions* and in the physical properties of nitric oxide and nitrogen. The view that nitric oxide is merely a diluent has, however, to be modified in the light of several observations. The present work indicates that nitric oxide does not survive the flame unchanged, but reacts with hydrogen. Secondly, the flammability limits, determined by Scott et al, show that at 1 atm nitric oxide is not chemically inert. Thirdly. the flame speeds for H2 + N20 + xNO mixtures are consistently higher by ca. 350 cm sec- t than those for H2 + N20 + xN: mixtures, although they decrease at almost the same rate with increasing concentrations of third constituent, Thus, nitric oxide is not chemically inert in these flames. It is possible to assign a tentative role to nitric oxide in these flames. It is well known, from work on flow discharge systems and flame studies, that nitric oxide is an effective catalyst for the recombination of hydrogen and hydroxyl radicals. Day, Dixon-Lewis, Sutton and Walton t4 found that for H2 + 02 + Na mixtures, addition of traces of nitric oxide rapidly decreased the burning velocity. For the H2 + N20 + NO system, early work ts on spontaneous ignition by Fenimore and Kelso showed no inhibition was caused by the addition of 1'5 per cent nitric oxide, but a more complete study t6 by Destriau and Laffitte revealed thai the spontaneous ignition of Hz + NzO mixtures was markedly inhibited by nitric oxide. At the much higher temperatures used in the present work, however, the nitric oxide can react in the flame. Merely by increasing the exothermicity, it increases the flame tempera. tures, and hence the flame speed. This reactivity is the explanation of the observed differences in flame speed between H2 + N20 + xNO and H2 + N20 + xN2 mixtures. Thus nitric oxide has a dual role: it acts simultaneously as an oxidant and an inhibitor. * As Sl'atcd above, the~ results rdcr to a somewhat narrower tube,
224
P. GRAY, R, MA('KINVliN AND D. tl. SMIIlt
It is the cancellation of these two effects that explains the resemblance to the inert diluent. nitrogen. Hydrazine plus nittvus oxide plus nitric oxide (N2H4 + N20 + NO) There are no references to previous measurements of flame speeds for ternary mixtures of N2H4 + N20 + NO. For the two extreme binary systems, N2H4 + NzOand N2H4 + NO. flame speeds have been determined I by Gray and Lee using the same technique and initial conditions as in the present work. Our results agree well with these earlier measurements. The small differences between flame speeds measured at 30 mm of mercury and those at 40 mm can probably be attributed to the decrease in flame temperature at the lower pressure. There appear to be no reported values for burning velocities at 1 atm pressure for any of the hydrazine systems; so the variation of flame speed with pressure is not known over a wide pressure range, The present results illustrate the well known fact 7 that nitric o~ide may be a very good or a very poor oxidant, depending on the fuel. For hydrogen, carbon monoxide and hydrocarbons, nitric oxide is very poor {although nitrous oxide is good}. For ammonia and hydrazine on the other hand, nitric oxide is an excellent oxidant; flame speeds and temperatures are higher for nitric cxide than for nitrous oxide mixtures. For fuels of the first type, flame speeds are low, as no efficient reaction exists between the fuel and the oxidant that can break the very strong N--O bond, Ammonia and hydrazine, on the other hand, readily form NH2 and NH radicals; NH2 radicals, and probably NH radicals also, react 23 efficiently with nitric oxide, rupturing the N--O bond, and so flame speeds are high for mixtures of these fuels with nitric oxide. One of the striking features of the behaviour of nitric oxide towards hydrazine is the very fuel-rich composition (50 per cent N,H,,) at which the maximum flame speed occurs. Expressed in terms of the reduced richness
R -{F/O)/{{F/O)+ (F./Ol,,o,~h. }
Vol, ~1
where !: and O are Ihe pcrcemages of fuel and oxidant respectively, the maximum occurs at R = 0,67. The calculated flame temperature is a maximum near the stoicl'aometric composition {R = 0.5) and decreases in rich mixtures. When ammonia replaces hydrazine, the maximum flame speed in the NH3 + NO system occurs at R -- 0.55. FortheNH 3 + O2and NH3 + N2Osystems, Ausloos and Van Tiggelen 17 have explained the location of the maximum flame speeds in slightly lean mixtures in terms of the need for an excess of oxidant to oxidize the fuel and to allow for the formation of a small concentration of nitric oxide. Gaydon and Wolfhard. on the other hand, ascribed 7 the occurrence of the maximum {calculatedl flame temperature at a slightly lean composition for the NH~ + 02 system to the exothermicity of lhe fuel [AH~(NH3) = - II kcal molc-1], whereas the occurrence of the maximum tcmperature at a slightly rich composition for the C2H,, + Oz system was ascribed I0 the endothermic nature of ethylene {AH~ = + 12.5 kcal mole- 1). Although explanations of thcse types are not inconsistent with the position of the maximum flame speed in NH.~ + NO mixtures, it seems impossible that they can explain the very marked displacement of the maximum observed for NzH,, + NO mixtures. An explanation of this behaviour must be sought in the chemistry of the system, and a possible explanation lies in the simultaneous pyrolysis and oxidation of the fuel. The question of prior or ::imultaiieous pyrolysis and oxidation has been examined for the N2H,, + O2 system. Murray and Hall TM. Hall and Wolfhard ~'J. and Ausloos and Van Tiggelen 17 have concluded that extensive or complete decomposition of hydrazine occurs prior to oxidation, whereas Gray and Lee I and Sawyer (from adiabatic flow reactor studies2° around 1000~K) suggested that simultaneous pyrolysis and oxidation occurs. When the oxidant reacts equaiiy readily with the fuel or its decomposition products {e,g. N20 with N2H,, or with NH3 + ½H2 + ~N2), measurements of flame speeds alone do not reveal the importance of prior or simultaneous decomposition. But if the oxidant reacts less
June 1967
HvD~azlr~liC:OMBUS'HONE~"rti~tN^a¥MtX'rURt!S 225 readily with the decomposition products, flame The most striking feature of the NH~ +N20 speed anomalies might be expected. From the + NO system is the enhanced combustion study J of the NH 3 q- H z + NO system by when both oxidants are acting together. The Armitage and Gray. it is known that nitric oxide maximum flame speed occurs in an equimolar is a poor oxidant towards hydrogen plus mixture, containing INH3: INzO: INO. In ammonia mixtures, the hydrogen appearing to the N2H,~ + NzO + NO system, basically simiact merely as an inert di!uent. Since nitric oxide lar behaviour is observed, but it is less marked. reacts only slowly with the decomposition For stoichiometric compositions, flame temproducts of hydrazine, maximum flame speeds peratures increase steadily as N 20 is replaced by are found when excess fuel is present, if total NO; flame speeds, on the other hand, increase decomposition occurred before oxidation, it is with increasing NO concentration until the unlikely that flame speeds up to 3600 cm secoxidant proportions are ca. 1N20: 2NO, and would be observed, and the maximum flame then deere',~se as further NO is added. speed would be expected near the 45 per cent An explap.atior., of the synergistic behaviour fuel composition (the position of the maximum of the ammonia system was given by Armitage flame speed for NH3 + NO mixtures), Thereand Gray. As N~O is replaced by NO, flame fore the conclusion is that simultaneous pyrolysis temperatures increase, reactions generally proand oxidation of the fuel are occurring in the ceed more rapitiiy, and flame speeds are encombustion of hydrazine with nitric oxide. hanced. However, nitric oxide only reacts One of the aims of the present work was a efficiently with N l-lz radicals comparison of the combustions of hydrazine NH z + N O - - } N 2 + H + O H and of ammonia, supported by nitrous oxide and nitric oxide, The most obvious difference and wh,~n the quotient NO: NHz exceeds unity between the two systems is in their flame speeds. which is eventually caused by the replacement With hydrazine as h~el.the maximum flame speed of NzO by NO. the excess nitric oxide cannot is 3600 em sec-L, whereas with ammonia, the react in this way, but only via slower reactions. maximum is 1260 cm sec- . Although hydraTherefore flame speeds diminish as further zinc {gast is more cndothermic than ammonia nitric oxide is added. [Att/IN2H,O, I = + 22 kcal mole- i; AH/{NH3) The basic similarity in the behaviour of the = - I i kcal mole-i], ~he difference in flame hydrazine and ammonia indicates that an speed is not explained in terms of different explanation in terms of the reactions of NH., flame temperatures, since flame lemperatures radicals is probably correct. It is noteworthy, for hydrazine combustion are only slightly however, that the behaviour is much less greater than for their ammonia equivalents, e.g. marked in the hydrazine system, although for combustion with nitric oxide, the difference hydrazine should act as a more ready source of is only 35 deg. Tnm,,~, is 2745~K for hydrazine NH~ radicals than ammonia does. This may be and 2710"K for ammonia. The temperature explained by the fact that the synergistic levelling effect is caused by dissociation,in the behaviour of the hydrazine system is partially burnt gas. The difference in flame speed is due masked by the pyrolysis of the fuel occurring to the higher chemical reactivity of hydrazine. Hydrazine has a bond zl strength D{H:N--N H2) simultaneously with its oxidation. This effect is not important in the ammonia system where of 57 to 60 kcal mole- ~, whereas D(H....NHz) is the homogeneous decomposition of the fuel over 100 kcal mole" t Also hydrazine is much occurs far less readily, more susceptible to radical attack z~ than is ammonia: the activation energies for hydrogen abstraction by methyl radicals are 5.0 and 9.8 W~; thank the S.R.C. for maintenance awards kcal mole -~ respectively. Although there is to R.M. and D.B,S. disagreement over its actual value, the bond strength DIH--N~Ha)is considerably lesszz (Received December I966} than 100 kcal mole- i
P. (;RAY. R. MAt KINVI!NAND D. II. SMI'Hi
226
References ' GRAY,P. and LEt!,J. C. Se~','nth ,~rml,o,~it#, (hll~,r, alimut/) on ('mllbtt,sliofl. p 6I. Brtlcrworlhs: London {1959) 2 A.M)REw, D, G, R. and GRAY, P. ('onlhustiml & Flanw, 8, 113 (1964) 3 ARMITAGI'~.J. W. and GRAY. P. (.'oIII/~lI.Ltil#l~ Flulll¢. 9. 173(1965) 4 GRAY, P,, MACKINVI-N.R. and SMIIII, D. B. ('t#~lhllSlbn & Fhmw, II, t09 lt967). Lli CiiATI,'LIER, H, Ann..4Ji,'~,, Paris, S~r. VIII, 19. 39:1 (1891) *' DAMKOIILI~r,G. and Eosll, R. Z, Lh,kmwhem. 49, 17;'~
(1943) GAv~n, A. G. and WOLFilARI). Ii. G, Flames: Their Structure. Radiation and Temperature. 2nd ed. Chapman and Hall: London (1960) s LEWIS, B. and yon El.hi/, G. Cotnhtlslhm, F/ames and Explosions t~'Guses. 2nd ed. Academic I'rcss: New York (1961) ADams, G. K. and StocKs, G. W. Roy. hlst.fi'an¢, iYtmh,, 13, 483 (1958) Io PARKER,W, G. and WOLI:IIArO, tl. G. Fourth Symlmsium (International) on Combustion, p 420. Williams and Wilkins: Baltimore (1953) ~ KOZACIIENKO,L, S, and SKACIIt)V,G. I. Zh. prikl. M,'kh. wkh. Fi=. 93 (1960)
Vol. II
~:~SCOTT.F. E.. VAN DOI.AII. R. W. and ZAaE'rAKJS.M. G. Sixth .~rmlm.sitlm (Inlerll{mmla]) .ll ('omhuslion. p 540. Reinhold: N~:w York 11957) i~ VA~ I}IR WAI. M. (J. Rcc. /)'lit'. chilli. ]~d)'s-B,s. 5.~. 97 (1934) 14 DAY. M. J.. DIxo~-LI!wls. G.. SH'Ion. M. M. and WAI.IoN. M. T. t:'olnhllMiollt~ F/mile, 10, 200 I I %6) I.~ FENIMORI~.C. P. alld Klil.SO.J. R..1. Alll('r. f/lt't/I. Sot'. 71. 371)6(1949) t~, DESHtlAti. M, and LAI:FIIII;.P. ('.R. Acrid, ScJ., Paris, 2~1, 3022 (1960) iv Ausl.O()s. P. and VAy TIta;l!l.l~, A. Bull. Soc, chim. Bt'lg. (d). 433 (1951) i~ MURRAY. R. C. arid |ial,i., A. R. TrllllS. FarilthO' Soc. 47, 743 (1951) t.~ HAl.L.A.R. and Wt)I.HtARD.H. G. Trlltl.i'. Ftlr¢ldtly .~oc. 52, 1520 (1956) 2o SAWYER. R. F. Princeton University. DeparLmenl of Aerospace and Mechanical Sciences, Tech. Rep. No. 761 21 GRAY. P. alld "[IIYNNI~, j. C. J. TrOlls. Fartldar Soc, 60. 1047 (1964) '" l:lStll':r, i, P. and ililaJlt. G. A. Nalur<',/.mid. 208, 1199 (1965) "~ FliNIMORli, ('. P. Ch¢llli.~l/'.r #f i'remi.wd l'hlllles. Pergamon : Oxford (1964)
Introduction A SYSTEMATIC study has recently been undertaken 1-3 of the combustion of binary systemsof hydrazine and ammonia and hydrogen with oxygen, nitrous oxide and nitric oxide. More recently, this work has been extended3"'* to ternary systems. When ternary mixtures are burned, two offdations occur simultaneously and they may be expected to interact. An example of this interaction was found in the hydrogen plus oxygen plus nitrous oxide system, studied4 by Gray, Mackinven and Smith. Often, however, ternary * I'rescnt address: Departing:hiof Furl S~:ien¢¢,The Universily. Leeds 2. 217
systems behave as if there were no interactions, The composition limits of flammability then obey the rule5 of L¢ Chatelier: the flammability limits, represented on a triangular diagram, are straight linesjoining the limitsfor the constituent binary systems. Correspondingly, flame speeds and temperatures have maxima in one of the binary systems, and decrease monotonically through the ternary mixtures to the other binary system. A striking example of interaction between two combustions was found3 for the ammonia plus nitrous oxide plus nitric oxide system, where the flame speeds of ternary mixtures were faster than those of either binary system. This unusual behaviour was explained in terms of the
2t8
P, (;RAY. R, MA('KI,XVii% AND I). B. SMIIII
reactions of the NHz radical (from ammonia) with the oxides of nhrogen. If this cxpl,matiow~ is correct, similar behaviour would be expected for the hydro cine plus nitrous oxide p!us nitric oxide systelr, since hydrazine can act as ,'1 still more ready source of NH2 radicals than ammonia. The present work was undertaken partly to throw more light ~n the role of nitrou:; oxide and nitric oxide acting as joint oxidants in the combustion of hydrazine. The combustion of hydrazine is complicated by the possible importance of prior decomposition of fuel and the subsequent oxidation of the hydrogen and ammonia produced in this decomposition, A study of the hydrogen plus nitrous oxide plus nitric oxide system is therefore likely to throw additional light on the mechanism of Iwdrazine combustion in flames. Accordingly. flame speeds for the ~ernary systems hydrazine plus nitrous oxide plus nitric oxide and hydrogen plus nitrous oxide plus nitric oxide have been measured at low pressure in a closed vessel at an initial temperature of 60'~C. The technique has the advantages that it allows easy control of initial conditions of temperature and pressure, and avoids the difficulty of accurately metering vaporized fuels. such as hydrazine. Furthermore, as this technique was used in the earlier studies t-4. it allows direct comparisons to be made with them.
Experimental Materials Hydrogen, nitrous oxide, nitric oxide and nitrogen were taken from cylinders. Nitrous oxide and nitric oxide were fractionated in vacuo, using liquid nitrogen as refrigerant; nitric oxide was condensed on to pellets of sodium hydroxide to remove traces of nitrogen dioxide. Anhydrous hydrazine (iodate analysis corresponding to 100.1 per cent NzH¢) was prepared by refluxing 95 per cent hydrazine (supplied by EastmanKodak Ltd) over sodium hydroxide for three hours in an atmosphere of nitrogen at 10 to 15 cm of mercury, followed by distillation at the same pressure.
Apparatus and procedure Measurement of flame speeds--Flame speeds
(88) relative to the burnt gas were measured in
Vol. II
a cylinder combustion vessel (20 on long; 20 cm i.d,). A schliercn lechniqtJc in conjunction with a rotating-drunl camera was used to record the progress of the flame front, Details of the method have been given previously". For the hydrogen plus nitrous oxide plus nitric oxide system, an initial pressure of 70 mm of mercury was used. For the hydrazine plus nitrous oxide plus nitric oxide system, the initial pressure was 40 mm. and a few me,'|surements were made at 30 mm of mercury. Above 40 ram. combustion was too violent for the present glass combustion vessel. The earlier measurements t on hydrazine combustion were also made at 40 mm pressure, Results were displayed as flame speed contours against composition on a triangular diagram. The standard deviation of an individual measurement was probably four per cent. but contours were probably located to better than two per cent, Pressures before and after ignition were measured in all experiments, Determ#latDn of .flanmtability limi;, The composition limits of flammability for upward propagation at 50 mm and 70 mm of mercury were determined in a vertical tube {length 70 cm. diameter ca, 7.5 cm), Successful propagation up the whole length of the tube constituted flammability. Outside the lean limit, partial ignitions in which the flame expired before reaching the top were sometimes observed. Details of the apparatus and procedure have been given'~previously.
Calculations oJ'flame temperatures, burnt-gas compositions andflame speeds relative to unburnt gas--The present values of Sn may be converted into flame speeds {Su) relative to unburnt gas. if the expansion ratio (Po PtJ) is known. The density of the burnt gas depends on ils composition and on the final flame temperature. Calculations of the equilibrium burnt gas composition and adiabatic flame temperatures have been made for selected mixture compositions. The method of Damkfhler and Edse~Ldescribed by Gaydon and Wolfl~ard"7. was used fo~' these calculations. The thermodynamic properties required were taken from standard compilations 7. s. In near-stoichiometrie mixtures, combustion is complete and reliable values for Tn and So are obtained. For mixtures with compositions far
IIYI)RAZINli ('OMIIUSTIONIN TliRNARY MIXTURi'~S
June 1!167
from stoichiomctric, non-equilibrium conditions do nol permit such calculations to be made with confidence. Results
Composition limits offlammability H ydrooen plus nitrous oxide plus nitric oxideThe composition limits of flammability for H2 + N20 + NO mixtures at 50 mm and 70 mm of mercury in a tube of 7.5 cm diameter are displayed in Figure I,
219
not support a flame, and in the present apparatus, no H 2 + NO mixtures could be ignited.
Hydrooen plus nitrous oxide plus nitrooen (H2 + NzO + Nz)---The composition limits of flammability for mixtures of hydrogen and nitrous oxide diluted with nitrogen at 50 mm and 70 mm of mercury in a tube of 6.8 cm diameter arc displayed in Figure 2. H2
Stoicl
;tric
N20
N20
NO
Flca;l~i! I, Conlposilion lintit-s of I]amnlability ~. :,ubatmospheric pressures lbr ternary mixtures of I lz + N20 + NO. (Outer lines refer to 70 m,-. pressure and inner to 50 111111.Da.,,hed line .ioin~ all stoichiomelric composilions)
For fuel-rich mixtures, the flames were vigorous, instantaneous and yellow in coiour. For lean mixtures, slower flames, yellow-green to bright-green in coiour, were propagated, and partial ignitions sometimes occurred outside the flammability limits. '[he limits for the binary H2 + N20 system occurred at 4.5 and 68.5 per cent H2 at 70 mm, ,'rod 7.0 and 62.0 per cent H2 at 50 mm of mercury. The range of flammability narrowed almost linearly as the nitric oxide concentration was increased. The tip of the combustible range occurred at a composition of 9.0 per cent H2 plus 20.0 per cent N20 plus 71,0 per cent NO at 70 mm, and 9.0 per cent H2 plus 23.0 per cent N 20 plus 68.0 pet"cent NO at 50 mm of mercury. Mixtures containing more NO than this would
Nz
I:l(;t]Ri~2. Composition limits of t]amrnabilily at sub-atmos-. pheric pressures Ibr ternary mixtures ot' H~ + N~O + N,, (Ottler lines refer to 70 mm pressure ~md inner to 50 ram, [),tshed line joins all stoichiometric compositions)
For fuel-rich mixtures, flames were rapid, vigorous and white in colour. As the nitrogen concentration was increased, the flames became less luminous. For fuel-lean mixtures, much slower flames, yellow-white in colour could be propagated, and partial ignitions sometimes occurred outside the flammability limits. With high nitrogen concentration, the flames were almost non-luminous and very difficult to observe, The limits for the binary H2 + N20 system* occurred at 7.0 and 67.0 per cent !'12 at 70 mm of mercury, and 8.0 and 62.0 per cent at 50 mm of mercury. The flammable region again narrowed linearly with increasir,g diluent con, centtation, The tip of the combustible range occurred at a composition of 7'5 per cent H~ * The Ilammability limit tube was slitbtly wider for the nitric oxide mixture~(diameler 7,5 cm c~)nlpar~'dwith 6,8 cm Ibr thal with: Ih¢ nitrogen mixtures).
220
Vol. II
P, (ilIA¥. R, MA('KINVI N AND I). P. SMI'III
plus 19,0 per cent N20 plus 73.5 per cent N 2 at 70 mm. and 8.0 per cent H2 plus 22,0 per cent N20 plus 70.0 per cent N2 at 50 mm ofmercttry,
Laminar flame speeds: effect q/'composition Hydrogen plus nitrous oxide plus nitric oxide~ The dependence of flame speed on composition for the combustion of hydrogen with nitrous oxide and nitric oxide over the whole range of flammability is shown in Figure 3. The measurements were made at an initial temperature of 62" + 2'~C. and an initial pressure of 70 mm of mercury. The flame speed contours reflected the form of the flammability limits, Flame speeds fell continuously as nitrous oxide was replaced by nitric oxide. The maximum flame speed (SB = 3470 cm sec -j) occurred in a slightly fuel-rich 153'i/o H2) H2 + N20 mixture, containing no nitric oxide. At the stoichiometric composition, S~ was 3440 cm sec- ~. It was not possible to ignite H2 + NO mixtures under the same experimental conditions.
the addition of only 20 per cent nitric oxide, 1he decrease was more gradual for mixtures conraining equimohtr hydrogen and nitrous oxide with added nitric oxide [H2 + N20 + xNO, dei~oted by line bb' on Figure 3]. Mixtures with more than 53 per :enl nitric oxide would not support a flan,c, and a mixture with composition 23.5 per cent hydrogen plus 23'5 per cent nitrous oxide plus 53 per cent nitric oxide burned with a fl~me speed of 520 cm sec- 1, The flame speeds of these latter mixturcs arc displayed in Figure 4, 4 o Added NO
J
b3
H2 t.I.
-'
2"/10
N20
' 4'o ' Diluent ( NO or N2). %
6'o
FIGL'Rli 4. Effi:ct ot" nitric oxide illld nilrog¢ll till the Ilamc speed (Sx, ¢m ~ec-=) tot stoichionletruc mixtures of H, with N20: ilame speed, Ill a total prcssUl'e of 71) mm of Hg. its a funclion ot' nitric oxide or nilrogelt ¢otlcentrafion
b
NO"'b'
i==Gu~ 3. Dependence of burning ve!ocity (Sn, cm sec-~) on composition in ternary mixtures H2 + NzO + NO: contours join compositions giving equal values of S~. (Dashed line aa' joins all stoichiometric compositions and dashed line bb' all compositions H2 + xN20 + (I - x)NO)
where they are compared with flame speeds in mixtures in which nitrogen replaces nitric oxide (H z + NzO + xN2), The results for the two systems show great similarity. After the addition of ca, 15 per cent of the third constituent {NO or N2). the flame speeds decreased linearly with increasing concentrations of either nitric oxide or nitrogen. The differences in SH for the two systems were about 360 cm sec- 1 (400 cm see- 1 at 15 per cent and 330 cm sec-t at 50 per cent of the third constituent),
Hydrazine plus nitrous oxide plus nitric oxide Flame speeds for stoichiometric ternary mixtures [with compositions H2 + xN20 + (1 - x)NO, denoted by line aa' on Figure 3] decreased rapidly with inereaslng nitric oxide ~:oncontration, failing to 2000 cm see-~ after
(N2H4 + N20 + NO)--The dependence of flame speed on composition is shown in Figure 5, The measurements were made at an initial temperature of 62° + 2°C and an initial pressure of 40 mm of mercury. In addition, a few flame
Jvne 1967
tlYI)RAZINE COMBUSTION IN TF~RNARYMIXTURES
speeds were measured at an initial pressure of 30 mm of mercury. These results were ca. 5 per cent lower than those at 40 ram. The fastest flame occurred in a very rich N2H4 + NO binary mixture, containing 50 per cent NzH,, for which Sa was 3600 cm sec-~. A stoichiometric N~H,, + NO mixture (containing 33.3 per cent N2H,,) had a burning velocity of 3150 cm sec- ~.For the N2H,, + NzO system, the maximum flame speed (SB = 2400 cm sec-~) occurred at stoichiometric composition 133'3 per cent N2H,d.
N2H~,
Np
No
]:l(ilJRf! 5. Dependence of burning velocity (Su, cm scc J} on composilion in ternary mixturesofN.,ll.~ + N,O + NO: ¢ontotll's join compositions giving equal valtlcs of SIj. (Dashed line joil,:s all ~,toichioinctric ¢onipositions}
For ternary mixtures on the rich side of stoichiometric, replacement of NO by N20 generally caused a decrease in flame speed. For stoichiometric and lean mixtures, however, as NO was replaced by N zO. flame speeds increased to a maximum value and then decreased again, For stoichiometric mixtures, the maximum flame speed (Sn = 3 400 cm see -~) occurred at a composition close to 0.33 N2H,, + 0.22 N~O + 0.44 NO.
Fh,,ne temperatures, burnt oas compositions, and flanw speeds relative to the unburnt ~ias Calculations of the equilibrium burnt gas composition were based on the assuml":ons that combustion was complete and that any
221
excess reactant (nitrous oxide or nitric oxide) decomposed into its elements. Support for the view that complete combustion occurred comes from the measurement of pressure changes for combustion. Although pressure measurements alone cannot reveal whether excess nitric oxide remains, chemical investigation after explosion showed no sign of nitric oxide, whereas the presence of oxygen was revealed by the artificial addition of nitric oxide to the product gases, causing the formation of nitrogen dioxide. This indicates that when nitric oxide was initially present at high concentration, it was almost entirely decomposed during the reaction. For stoichiometric ternary mixtures of hydrogen with nitrous and nitric oxides (denoted by line aa' on Figure 3), flame temperatures increased monotonically throughout the measured range from the value of 271ff~K in H~ + N20 to 275ffK for a mixture with 20 per cent NO. For mixtures equimolar in H2 and Nee with compositions denoted by the line bb' on Figure 3, the calculated flame temperatures increased slightly to 2720"K with the addition of ten per cent NO, and then decreased very slowly, having a value of 271ffK at a composition 35 per cent Hz + 35 per cent N20 + 30 per cent NO. For stoichiometric mixtures of hydrazine with nitrous and nitric oxides, the flame temperatures increased linearly as NzO was replaced by NO, from the value of 2655°K in N,H4 + N,O mixtures to 2745°K in N2H,, + NO mixtures. For binary N2H,~ + NO mixtures, flame temperatures decreased as the hydrazine concentration was increased, falling to a value of 2645°K at an equimolar composition. Calculations of burnt gas compositions and adiabatic flame temperatures for these and other compositions are shown in Table 1. The derived values of flame speed relative to the unburnt gas (Stj) are also included. Discussion Both the ternary systems studied here show: marked departures from expectations based on the Le Chatelier rule s for the combustion of one fuel supported by two oxidants, in the combustion of hydrazine, the two oxidants ( !!
+ NO
N2H4 + N20 40 m m H g
70 m m H g
Ha+ NzO
System
66-7 33-3 ---.. -.....
33-3 33-3 33-3 40 41-6 50
m
50 80 40 30 45 40 35
--------
50 20 50 50 45 4O 35
N20
N2H.~
H2
burnt
-33"3 66-7 60 58-4 50
-~ -10 20 10 20 30
NO
equilibrium
Original composition. mole per cent
flame temperatures,
+ NO
TABLE 1. A d i a b a t i c
Su 379 89 366 234 337 285 226 164 237 245 252 247 238
3440 855 3120 2000 3050 2550 2000 2 400 3 290 3 150 3460 3 480 3550
2655 2700 _ ;4_ 2725 2715 2645
2440 2730 2750 2720 2715 2710
i 27i0
K
SB
Flame temp.
Flame .~peed, CtPI ,~('£'- 1
40-7 30-7
41-0
53-5 48"3 41 "8
44-1 59-5 41"3 38"1 44.3 44.8 44"9
N,
23-4 24"2 24-6 25-1 24-8 23-0
30-2 13-2 31-2 31 "6 28-6 26-4 23-7
H,O
7.6 ,'.I-7 !0" I 14-6 15-8 24-2
8-5 0'4 9"3 10-0 0"3~ 4-7 3"4
H:
+ NO
and
6-f) 0.4 6.3 7-1 5-1 4-3 3-6 5-5 7-2 9-1 10-4 10-6 9-8
2-6 3.0 3.4 I-4 1-2 0"25
H
2-9 20.6 3"i 3.4 5-4 8-2 t 1.7
O.
i~ioh, p e r C('~//
21 2-7 3.,'4 2-I 1-8 0-5
2-3 i -6 2-3 2-7 2.9o 3-6 4.3
O
N2H. t + H.O
Final composition.
g a s c o m p o s i t i o n s a n d f l a m e s p e e d s f o r H2 + N 2 0 Initial t e m p e r a t u r e . 62 C
4-7 4-4 2,4
6. t
4.3 5. !
5-1 2-5 5-5 f,- i 64) 6-3 6,5
OH
+ NO
1.0 1,0 1.1 0-7 0-6 0.25
11-8 I-9 1.0 141 1-3 I-6 1.9
NO
mixtures.
,<
June 1967
IIYDRAZINECOMBUSTIONIN 1ERNAR¥MIXTURES
assist one another; while in the combustion of hydrogen, nitric oxide is apparently no better than an inert diluent.
ttydrotlen plus nitrous oxide plus nitric oxide (H, + N20 + NO) Previous burning velocity work 3 'Jon this system is confined to the binary systems H2 + N20 and H2 + NO. although composition limits of flammability for the binary and ternary mix. turcs have been reported {see below). The present results for flame speeds in H2 + N20 mixtures agree with earlier results in the same apparatus, obtained by Armitage and Gray 3. Comparison with the burning velocities at Iatm pressure, measured by Parker and Wolfhard t° and by Kozachenko and Skachov tt, confirms the conclusion that Sv is independent of pressure over a considerable pressure range, implying7' 8 an effective overall reaction order n of two. Flame speeds in binary H2 + NO mixtures burned at pressures from I to t0 atm have been reported ~ by Adams and Stocks, who found values for Sv around 60 to 80 cm sec-t somewhat dependent on pressure. Composition limits of flammability at 1 atm for the H2 + N 2 0 + NO system have been determined by Scott, Van Dolah and Zabetakis for upward i~ropagationtz in a tube of 5 cm diameter, and by Van der Wai for downward propagation '3 in a 1.5 cm tube. These results. together with the present measurements, show a continuous variation from the relatively easy inflammation at I arm in a wide tube, through the more difficult downward propagation in a narrower tube, to the lowest flammability at low pressure. Both the flame speed contours and flammability limits for ternary mixtures reveal that the combustion of hydrogen supported by nitrous oxide and nitric oxide deviates from the 'normal' behaviour expected for a fuel burning with two oxidants. At 1 arm, the rich limit shows that nitrous oxide and nitric oxide are antagonistic oxidants. At low pressures the region of flammability at the upper limit narrows very sharply as the nitric oxide concentration is increased, and nitric oxide appears no better than an inert diluent, despite its oxygen content and endothermic nature. Support for this view comes
223
from the experiments on the H 2 + N20 + N 2 system, The small differences in the ranges of flammability when nitric oxide is replaced by nitrogen are wholly understandable in terms of the small differences in the experimental conditions* and in the physical properties of nitric oxide and nitrogen. The view that nitric oxide is merely a diluent has, however, to be modified in the light of several observations. The present work indicates that nitric oxide does not survive the flame unchanged, but reacts with hydrogen. Secondly, the flammability limits, determined by Scott et al, show that at 1 atm nitric oxide is not chemically inert. Thirdly. the flame speeds for H2 + N20 + xNO mixtures are consistently higher by ca. 350 cm sec- t than those for H2 + N20 + xN: mixtures, although they decrease at almost the same rate with increasing concentrations of third constituent, Thus, nitric oxide is not chemically inert in these flames. It is possible to assign a tentative role to nitric oxide in these flames. It is well known, from work on flow discharge systems and flame studies, that nitric oxide is an effective catalyst for the recombination of hydrogen and hydroxyl radicals. Day, Dixon-Lewis, Sutton and Walton t4 found that for H2 + 02 + Na mixtures, addition of traces of nitric oxide rapidly decreased the burning velocity. For the H2 + N20 + NO system, early work ts on spontaneous ignition by Fenimore and Kelso showed no inhibition was caused by the addition of 1'5 per cent nitric oxide, but a more complete study t6 by Destriau and Laffitte revealed thai the spontaneous ignition of Hz + NzO mixtures was markedly inhibited by nitric oxide. At the much higher temperatures used in the present work, however, the nitric oxide can react in the flame. Merely by increasing the exothermicity, it increases the flame tempera. tures, and hence the flame speed. This reactivity is the explanation of the observed differences in flame speed between H2 + N20 + xNO and H2 + N20 + xN2 mixtures. Thus nitric oxide has a dual role: it acts simultaneously as an oxidant and an inhibitor. * As Sl'atcd above, the~ results rdcr to a somewhat narrower tube,
224
P. GRAY, R, MA('KINVliN AND D. tl. SMIIlt
It is the cancellation of these two effects that explains the resemblance to the inert diluent. nitrogen. Hydrazine plus nittvus oxide plus nitric oxide (N2H4 + N20 + NO) There are no references to previous measurements of flame speeds for ternary mixtures of N2H4 + N20 + NO. For the two extreme binary systems, N2H4 + NzOand N2H4 + NO. flame speeds have been determined I by Gray and Lee using the same technique and initial conditions as in the present work. Our results agree well with these earlier measurements. The small differences between flame speeds measured at 30 mm of mercury and those at 40 mm can probably be attributed to the decrease in flame temperature at the lower pressure. There appear to be no reported values for burning velocities at 1 atm pressure for any of the hydrazine systems; so the variation of flame speed with pressure is not known over a wide pressure range, The present results illustrate the well known fact 7 that nitric o~ide may be a very good or a very poor oxidant, depending on the fuel. For hydrogen, carbon monoxide and hydrocarbons, nitric oxide is very poor {although nitrous oxide is good}. For ammonia and hydrazine on the other hand, nitric oxide is an excellent oxidant; flame speeds and temperatures are higher for nitric cxide than for nitrous oxide mixtures. For fuels of the first type, flame speeds are low, as no efficient reaction exists between the fuel and the oxidant that can break the very strong N--O bond, Ammonia and hydrazine, on the other hand, readily form NH2 and NH radicals; NH2 radicals, and probably NH radicals also, react 23 efficiently with nitric oxide, rupturing the N--O bond, and so flame speeds are high for mixtures of these fuels with nitric oxide. One of the striking features of the behaviour of nitric oxide towards hydrazine is the very fuel-rich composition (50 per cent N,H,,) at which the maximum flame speed occurs. Expressed in terms of the reduced richness
R -{F/O)/{{F/O)+ (F./Ol,,o,~h. }
Vol, ~1
where !: and O are Ihe pcrcemages of fuel and oxidant respectively, the maximum occurs at R = 0,67. The calculated flame temperature is a maximum near the stoicl'aometric composition {R = 0.5) and decreases in rich mixtures. When ammonia replaces hydrazine, the maximum flame speed in the NH3 + NO system occurs at R -- 0.55. FortheNH 3 + O2and NH3 + N2Osystems, Ausloos and Van Tiggelen 17 have explained the location of the maximum flame speeds in slightly lean mixtures in terms of the need for an excess of oxidant to oxidize the fuel and to allow for the formation of a small concentration of nitric oxide. Gaydon and Wolfhard. on the other hand, ascribed 7 the occurrence of the maximum {calculatedl flame temperature at a slightly lean composition for the NH~ + 02 system to the exothermicity of lhe fuel [AH~(NH3) = - II kcal molc-1], whereas the occurrence of the maximum tcmperature at a slightly rich composition for the C2H,, + Oz system was ascribed I0 the endothermic nature of ethylene {AH~ = + 12.5 kcal mole- 1). Although explanations of thcse types are not inconsistent with the position of the maximum flame speed in NH.~ + NO mixtures, it seems impossible that they can explain the very marked displacement of the maximum observed for NzH,, + NO mixtures. An explanation of this behaviour must be sought in the chemistry of the system, and a possible explanation lies in the simultaneous pyrolysis and oxidation of the fuel. The question of prior or ::imultaiieous pyrolysis and oxidation has been examined for the N2H,, + O2 system. Murray and Hall TM. Hall and Wolfhard ~'J. and Ausloos and Van Tiggelen 17 have concluded that extensive or complete decomposition of hydrazine occurs prior to oxidation, whereas Gray and Lee I and Sawyer (from adiabatic flow reactor studies2° around 1000~K) suggested that simultaneous pyrolysis and oxidation occurs. When the oxidant reacts equaiiy readily with the fuel or its decomposition products {e,g. N20 with N2H,, or with NH3 + ½H2 + ~N2), measurements of flame speeds alone do not reveal the importance of prior or simultaneous decomposition. But if the oxidant reacts less
June 1967
HvD~azlr~liC:OMBUS'HONE~"rti~tN^a¥MtX'rURt!S 225 readily with the decomposition products, flame The most striking feature of the NH~ +N20 speed anomalies might be expected. From the + NO system is the enhanced combustion study J of the NH 3 q- H z + NO system by when both oxidants are acting together. The Armitage and Gray. it is known that nitric oxide maximum flame speed occurs in an equimolar is a poor oxidant towards hydrogen plus mixture, containing INH3: INzO: INO. In ammonia mixtures, the hydrogen appearing to the N2H,~ + NzO + NO system, basically simiact merely as an inert di!uent. Since nitric oxide lar behaviour is observed, but it is less marked. reacts only slowly with the decomposition For stoichiometric compositions, flame temproducts of hydrazine, maximum flame speeds peratures increase steadily as N 20 is replaced by are found when excess fuel is present, if total NO; flame speeds, on the other hand, increase decomposition occurred before oxidation, it is with increasing NO concentration until the unlikely that flame speeds up to 3600 cm secoxidant proportions are ca. 1N20: 2NO, and would be observed, and the maximum flame then deere',~se as further NO is added. speed would be expected near the 45 per cent An explap.atior., of the synergistic behaviour fuel composition (the position of the maximum of the ammonia system was given by Armitage flame speed for NH3 + NO mixtures), Thereand Gray. As N~O is replaced by NO, flame fore the conclusion is that simultaneous pyrolysis temperatures increase, reactions generally proand oxidation of the fuel are occurring in the ceed more rapitiiy, and flame speeds are encombustion of hydrazine with nitric oxide. hanced. However, nitric oxide only reacts One of the aims of the present work was a efficiently with N l-lz radicals comparison of the combustions of hydrazine NH z + N O - - } N 2 + H + O H and of ammonia, supported by nitrous oxide and nitric oxide, The most obvious difference and wh,~n the quotient NO: NHz exceeds unity between the two systems is in their flame speeds. which is eventually caused by the replacement With hydrazine as h~el.the maximum flame speed of NzO by NO. the excess nitric oxide cannot is 3600 em sec-L, whereas with ammonia, the react in this way, but only via slower reactions. maximum is 1260 cm sec- . Although hydraTherefore flame speeds diminish as further zinc {gast is more cndothermic than ammonia nitric oxide is added. [Att/IN2H,O, I = + 22 kcal mole- i; AH/{NH3) The basic similarity in the behaviour of the = - I i kcal mole-i], ~he difference in flame hydrazine and ammonia indicates that an speed is not explained in terms of different explanation in terms of the reactions of NH., flame temperatures, since flame lemperatures radicals is probably correct. It is noteworthy, for hydrazine combustion are only slightly however, that the behaviour is much less greater than for their ammonia equivalents, e.g. marked in the hydrazine system, although for combustion with nitric oxide, the difference hydrazine should act as a more ready source of is only 35 deg. Tnm,,~, is 2745~K for hydrazine NH~ radicals than ammonia does. This may be and 2710"K for ammonia. The temperature explained by the fact that the synergistic levelling effect is caused by dissociation,in the behaviour of the hydrazine system is partially burnt gas. The difference in flame speed is due masked by the pyrolysis of the fuel occurring to the higher chemical reactivity of hydrazine. Hydrazine has a bond zl strength D{H:N--N H2) simultaneously with its oxidation. This effect is not important in the ammonia system where of 57 to 60 kcal mole- ~, whereas D(H....NHz) is the homogeneous decomposition of the fuel over 100 kcal mole" t Also hydrazine is much occurs far less readily, more susceptible to radical attack z~ than is ammonia: the activation energies for hydrogen abstraction by methyl radicals are 5.0 and 9.8 W~; thank the S.R.C. for maintenance awards kcal mole -~ respectively. Although there is to R.M. and D.B,S. disagreement over its actual value, the bond strength DIH--N~Ha)is considerably lesszz (Received December I966} than 100 kcal mole- i
P. (;RAY. R. MAt KINVI!NAND D. II. SMI'Hi
226
References ' GRAY,P. and LEt!,J. C. Se~','nth ,~rml,o,~it#, (hll~,r, alimut/) on ('mllbtt,sliofl. p 6I. Brtlcrworlhs: London {1959) 2 A.M)REw, D, G, R. and GRAY, P. ('onlhustiml & Flanw, 8, 113 (1964) 3 ARMITAGI'~.J. W. and GRAY. P. (.'oIII/~lI.Ltil#l~ Flulll¢. 9. 173(1965) 4 GRAY, P,, MACKINVI-N.R. and SMIIII, D. B. ('t#~lhllSlbn & Fhmw, II, t09 lt967). Lli CiiATI,'LIER, H, Ann..4Ji,'~,, Paris, S~r. VIII, 19. 39:1 (1891) *' DAMKOIILI~r,G. and Eosll, R. Z, Lh,kmwhem. 49, 17;'~
(1943) GAv~n, A. G. and WOLFilARI). Ii. G, Flames: Their Structure. Radiation and Temperature. 2nd ed. Chapman and Hall: London (1960) s LEWIS, B. and yon El.hi/, G. Cotnhtlslhm, F/ames and Explosions t~'Guses. 2nd ed. Academic I'rcss: New York (1961) ADams, G. K. and StocKs, G. W. Roy. hlst.fi'an¢, iYtmh,, 13, 483 (1958) Io PARKER,W, G. and WOLI:IIArO, tl. G. Fourth Symlmsium (International) on Combustion, p 420. Williams and Wilkins: Baltimore (1953) ~ KOZACIIENKO,L, S, and SKACIIt)V,G. I. Zh. prikl. M,'kh. wkh. Fi=. 93 (1960)
Vol. II
~:~SCOTT.F. E.. VAN DOI.AII. R. W. and ZAaE'rAKJS.M. G. Sixth .~rmlm.sitlm (Inlerll{mmla]) .ll ('omhuslion. p 540. Reinhold: N~:w York 11957) i~ VA~ I}IR WAI. M. (J. Rcc. /)'lit'. chilli. ]~d)'s-B,s. 5.~. 97 (1934) 14 DAY. M. J.. DIxo~-LI!wls. G.. SH'Ion. M. M. and WAI.IoN. M. T. t:'olnhllMiollt~ F/mile, 10, 200 I I %6) I.~ FENIMORI~.C. P. alld Klil.SO.J. R..1. Alll('r. f/lt't/I. Sot'. 71. 371)6(1949) t~, DESHtlAti. M, and LAI:FIIII;.P. ('.R. Acrid, ScJ., Paris, 2~1, 3022 (1960) iv Ausl.O()s. P. and VAy TIta;l!l.l~, A. Bull. Soc, chim. Bt'lg. (d). 433 (1951) i~ MURRAY. R. C. arid |ial,i., A. R. TrllllS. FarilthO' Soc. 47, 743 (1951) t.~ HAl.L.A.R. and Wt)I.HtARD.H. G. Trlltl.i'. Ftlr¢ldtly .~oc. 52, 1520 (1956) 2o SAWYER. R. F. Princeton University. DeparLmenl of Aerospace and Mechanical Sciences, Tech. Rep. No. 761 21 GRAY. P. alld "[IIYNNI~, j. C. J. TrOlls. Fartldar Soc, 60. 1047 (1964) '" l:lStll':r, i, P. and ililaJlt. G. A. Nalur<',/.mid. 208, 1199 (1965) "~ FliNIMORli, ('. P. Ch¢llli.~l/'.r #f i'remi.wd l'hlllles. Pergamon : Oxford (1964)