I
GEORGE J. MULLANEY
I ...
Research Laboratory, General Electric
Air Pollution
Co., Schenectady, N. Y.
Nitrogen Oxide Formation in Autoignition of Liquid Fuel Sprays Lowering maximum flame temperature of internal combustion engines may reduce nitric oxide formation in their exhaust gases
N I T R O G E N OXIDES play an important part in air pollution even though they are minor constituents in the exhaust of internal combustion engines. Less than 1 p.p.m. of nitrogen dioxide together with similar quantities of oxidized gasoline fractions, ozone, and the presence of sunlight can form irritating and lachrymating pollutants (6). Both Otto and Diesel cycle machines produce in their exhaust systems nitrogen oxide concentrations of 1000 to 2000 p.p.m. ( I ) . Little is known about the complex flame in the rapid chemical reaction following autoignition. However, basic reaction kinetic data (2, 5, 73) on nitrogen-oxygen reactions obtained in simpler systems can be applied in a limited sense to studies of autoignition flames. In the work described here, formation of nitric oxide in the complex flame following autoignition of liquid fuel sprays was explored to find a way of suppressing its production in engines. A shock tube technique ( 8 ) was used to provide a source of stationary, high pressure-high temperature air. Several liquid fuels were sprayed into the zone of compressed air after reflection of the incident shock wave from the tube end. Maximum duration of high temperature
Figure 1 . During the time interval for high temperature reaction, average pressure was 250 p.s.i.a.
a
following autoignition was about 25 msec. Quenching of the nitric oxide reaction occurred when the rarefaction wave arrived in the combustion zone a t the driven end of the shock tube. The period of rapid cooling caused by the expansion wave was also about 25 msec., and the quenching rate was estimated to be 15' per msec. A colorimetric technique ( 7 7 ) was used to measure the nitrogen dioxide resulting from the nitric oxide which formed initially in the high temperature flame.
Experimental The shock tube, fuel injection system, and most of the experimental apparatus have been described previously (8, 9). After reducing the air pressure in the driven tube to the required level, which varied from 6.0 to 2.4 inches of mercury, the driver section was slowly loaded with high pressure air. The experiment was initiated by pressure breaking of a scored metal diaphragm a t 440 p.s.i.g. Careful scoring resulted in a precision of diaphragm breaking of i5yo of the design pressure. Fuel injection occurred after a selected time delay.
The signal for fuel injection was initiated by passing the incident shock wave over a pressure transducer, one third the way along the driven tube from the driver section. The liquid fuel spray was injected into a zone of high pressure-high temperature air a t the end of the driven tube after the incident shock wave had reflected from the tube end. For some of the experimental work the ignition lag was long enough (over 2 msec.) so that maximum penetration of the liquid spray occurred before the beginning of rapid chemical reaction. Both earlier work (3) and high speed photographs of the spray taken during these experiments showed that final penetration distances were the same (about 15 cm.) for a given fuel, when fuel quantity was changed by varying the initial fuel pressure over a 4 to 1 range. Although considerable differences in kinematic viscosity existed between fuels, there was no evidence of significant increases in final penetration with increased kinematic viscosity for the intermittent sprays used in the shock tube tests. Evaporation time of the liquid sprays was about 2.5 msec. for all the fuels investigated. Because penetration and evaporation
looSHOCK R E F L E C T I O N FROM END O F T U B E
SECOND SHOCK R E FLECTION
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VOL. 52, NO. 6
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2 L
C,H2,
w
- AIR
MIXTURES
E
3 I-
a a W
2
2800
W
IW
5 2700
= 17 ATMOSPHERE Figure 2. Using an initial air temperature of 950" K. when fuel i s injected, the maximum flame temperature i s over 2600" K.
a
J
CI
I-
2600
a m 5
2 r,
2500
P = I ATMOSPHERE
d
2
X
a 2
2400
800
900
IO00
1200
1100
A I R TEMPERATURE AT F U E L INJECT1 ON,"K.
times of the various fuel sprays were the same, it was felt that nitrogen oxide formation could be compared directly. An estimate of the size of the combustion zone (375 cc.) was useful in making approximate calculations of the over-all fuel-air ratio. The size was also helpful in calculating the concentration of nitric oxide formed in the combustion zone corresponding to the diluted sample taken for measuring nitrogen dioxide. A copy of a typical pressure record from a high-response pressure gage in the combustion zone of the shock tube is shown in Figure 1. The time of fuel injection, duration of high temperature, and the beginning of rapid cooling due to the arrival of the expansion wave are also indicated. The initial cooling rate can be increased by decreasing the total length of the shock tube (4) but either the duration of high temperaturehigh pressure gas is shortened or its quantity is reduced. The secondary pressure pulse and temperature pulse were so small that a large evacuated tank and second diaphragm on the driver tube (4)was not necessary. Gas samples (13 liters) were withdrawn by opening a manual valve in the combustion zone of the shock tube a few seconds after autoignition of the liquid fuel spray. A second sample gave a nitrogen dioxide yield which was 20'% of the first. T h e 13-liter sample flask contained glass beads to provide mixing action when the sample was shaken. Yield of nitrogen dioxide was determined by withdrawing a small quantity of processed gas from the 13-liter flask. The smaller sample flask was usually
530
45 ml. and it contained 10 mi. of absorbing reagent. After allowing 15 minutes for color development, absorbance of the colored reagent was measured with a Beckman DU spectrophotometer at 550 mp, using an optical light path of 1 cm. T h e calibration described by Saltzman ( I 7) was used for these experiments.
perature for a stoichiometric C,Hn,-air mixture is above 2600' K. (Figure 2) when air temperature before fuel injection is 950' K. and air pressure is 17 atmospheres. Combustion in the autoignition of liquid fuel sprays undoubtedly takes place with inadequate mixing (7). Because degree and uniformity of mixing is unknown, nitric oxide equilibrium and reaction kinetic calculations based on the assumption of premixed gases cannot represent the actual autoignition process for a liquid fuel spray. However, such an analysis for lean flames might indicate maximum yields of nitric oxide and trends to be exprcted with temperature and time. The calculated equilibrium nitric oxide for a lean, premixed adiabatic C,Hsn-air flame (Figure 3) indicates the upper limit of nitric oxide concentration which may be approached in the combustion gas. Equivalence ratio varies from 0.5 at 1900' K. to 0.85 a t 2500' K. on this plot. Equilibrium nitric oxide for lean flames does not change rapidly with pressure. As noted earlier, the reaction kinetics for decomposition and formation of nitric oxide a t high temperatures (2000" to 3000" K.) has been investigated by several authors in recent years. Zeldovich's experimental results (73) and those reported later are in fair agreement. The over-all rate equation for the nitrogen-oxygen reaction is giben by Fenimore (activation energy is the average of that found by Zeldovich and Fenimore) :
Results and Discussion
For a premixed gas, the calculated
(70) maximum adiabatic flame tem-
INJECTION, 9 5 O O K . Ps 17 ATMOSPHERES
EQUIL I BR I UM
A t = 2 5 MILLISECONDS A t = IO MILLISECONDS
1900
2100
2300
2500
TEMPERATURE,-OK. Figure 3. For lean flames, calculated nitric oxide yield depends on reaction time available
INDUSTRIAL AND ENGINEERING CHEMISTRY
NITROGEN OXIDE FORMATION 50
SHOCK TUBE COOLING RATE
IOo/o FORMATION
302o 2 0 o/'
Figure 4. If yield of nitric oxide has not reached e q u i I i b r i um during the reaction time interval, it continues to form during the quench period
FORMATION
------
-
30
IO
O/o
FORMATION
TEMPERATURE AT FUEL INJECTION 950°K PRESSU E 17 ATM
5
I
I
I
TEMPERATURE Equation form is
-
1 expressed in
N O = NO,,,,
K.
integrated
(:I:>
(2)
N O is concentration of nitric oxide, and NO,,,,, is equilibrium concentration of nitric oxide where
2K k2 = 2 X 1012 6-B2,0001RT and 0 2 is concentration of oxygen, moles per liter; R, universal gas constant, 1.99 calories/' C. mole; T , temperature, ' K.; and At, time interval a t temperature, seconds. Using Equation 2, calculations for two intervals of time a t high temperature (Figure 3 ) indicate the approximate nitric oxide yields to be expected in the
5
U
ln
z
'""1
l S 0 0CTA NE
200
z 0 a u n
c
a U
1 0 0 ~
c W
a
e-
/
ln
experiments. Here it is assumed that the nitrogen-oxygen reaction proceeds in the complex diffusion flame in the same manner as in the simpler systems where the nitrogen fixation reaction has been studied ( 2 , 73). The rising pressure (Figure 1) during the time interval available for high temperature reaction in the shock tube leads to a n error of +15% in calculating the amount of nitric oxide formed. Equation 1 may also be used to derive an approximate relationship between the cooling rate and the amount of nitric oxide formed (or decomposed) during quench. For average values over a given temperature interval
I
ETHYL
/. /
ALCOHOL
50 ISOPROPYL ALCOHOL
z W W
g o k z
where AT is temperature interval over which average values are substituted, and ANO/NO is fractional change in nitric oxide. A high quench rate is desirable in a laboratory experiment to reduce the amount of products lost or gained during the cooling process. Starting initially with a nitric oxide composition considerably less than equilibrium, nitric oxide formation will continue during the cooling period until equilibrium a t a new lower temperature is obtained. Decomposition will then begin. Calculations based on nonequilibrium nitric oxide yields shown in Figure 3 indicate that a maximum increase of 20 to 30y0 of nitric oxide could be expected (Figure 4) for the cooling rate obtained with this shock tube. The amounts of nitrogen dioxide obtained during autoignition of several liquid fuel sprays in the shock tube is presented in Figure 5. Judging from the measurements of the spray penetration mentioned earlier, nitric oxide concentration in the combustion zone during autoignition would be 6 times the measured quantities. Test points for 1-propanol indicate the degree of reproducibility obtained. The lower yield of nitrogen dioxide for some alcohols is a t first somewhat surprising. The adiabatic flame temperature for ethyl alcohol (12) is nearly the same as that for a paraffin hydrocarbon, although the stoichiometric fuel-air ratio for ethyl alcohol is 1.66 times that of n-heptane. The differences in nitric oxide formation between various fuels are attributed to ignition lag. Earlier studies of autoignition (9) had shown that large
I
0
20
60 80 100 120 FUEL QUANTITY INJECTED, MlLLtGRAMS Figure 5. In the shock tube experiment, the measured 40
amount of nitrogen dioxide formed depended on the fuel used. Experimental points are for 1 -propanol
.5
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I l l 1 1500
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AIR PRESSURE 2 5 0 F!S.I I
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1100 1000 900 AIR TEMPERATURE,
800
700 650
OK..
Figure 6. Difference in ignition l a g between fuels becomes less as initial air temperature increases VOL. 52,
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JUNE 1960
531
Acknowledgment
0 0 0
0 0
0
FUEL N-HEPTANE TEMPERATURE AT FUEL INJECTION 1100% PRESSURE 17 ATM,
DURATION OF FLAME GAS TEMPERATURE, MILLISECONDS Figure 7. For a given fuel, amount of nitrogen dioxide formed depends on the time interval for high temperature reaction
differences in ignition lag could be expected for various fuels (Figure 6). The long ignition delay for ethyl alcohol is noteworthy as well as the fact that at temperatures before fuel injection higher than 1000” K . ignition lag tends to become the same for all the fuels studied. Ignition lag for 2-propanol is similar to ethyl alcohol but 1-propanol is between ethyl alcohol and iso-octane. For the apparatus used in this work, combustion of a liquid fuel spray with a somewhat shorter ignition lag-e.g., n-heptane-would provide high temperature gases for a maximum time of about 25 msec. before beginning of the quench period. IYith the time of injection and the quench time fixed, a liquid fuel spray with a long ignition lag such as 2-propanol would react and provide a high temperature interval of about 10 msec. when air temperature before fuel injection is 950” K . and air pressure is 17 atm. Ignition lag reduced the time interval a t high temperature during combustion. I t also provided more time for fuel-air mixing and therefore because over-all composition of the reacting mixture is lean, it may have lowered the maximum effective flame temperature after autoignition. Thus, another approach was tried for investigating the effect of exposure time at high temperature on nitric oxide formation. By using a high air temperature before fuel injection (1100” K.) to ensure a short ignition lag and changing the time of fuel injection the duration of the combustion process could be controlled. As noted earlier, the quench time was fixed by the geometry of the shock tube. For a fixed quantity of fuel injected (54 mg. of n-heptane), the nitrogen dioxide yield increased as the time interval available for the high temperature combustion gas to form nitric oxide was increased (Figure 7). By applying the data in Figure 3, these results suggest that a n effective temperature of 2100”
532
to 2200” K. !vas obtained in the combustion zone of the shock tube and that less than 2070 of the equilibrium nitric oxide concentration is formed in the time intervals used. Formation of nitric oxide in the diffusion flame resulting from the autoignition of a liquid fuel spray appears to depend on the reactions for oxidation of nitrogen a t high temperatures. Because of the exponential dependence of nitric oxide formation rate on temperature: the maximum temperature is probably the most important factor to be considered in studies of nitric oxide abatement. These results also apply to Otto cycle engines.
.4. J. Nerad initiated the work reported here. Helpful discussions Jvith G. E. Moore, W. E. Kaskan, and C. P. Fenimore are gratefully acknowledged. l i t e r a t u r e Cited (I) Elliott, M., Nebel, G., Pounds, F., J . Air Pollution Control Assoc. 5 , 103 (1955). (2) Fenimore, C. P., Jones, G. W., J . Phys. Chem. 61, 654 (1957). (3;) Giffen, E., Muraszew, -4., “Atomization of Liquid Fuels,” p. 176, John M’iley, New York, 1953. (4) Glick, H., Squire, N., Hertzberg, A.: 5th Symp. on Combustion, pp. 393-402, Reinhold, New York, 1955. (5) Glock, H., Klein, J., Squire, S . , J . Chem. Phys. 27, 850-7 (1957). (6) Haagen-Smit, ‘4.J.: IND.ENG.CHEM. 44, 1342 (1952). (7) Jost, W., “Explosion and Combustion Processes, in Gases,” p. 589, McGrawHill, New York, 1946. (8) Mullaney, G. J., IND.ENG.CHEM.50, 53-8 (1958). (9) Zbid., 51, 779-82 (1959). (10) Powell, H., Schaffer, A., Suciu, S., “Properties of Combustion Gases. System, C,,H,,-Air,” McGraw-Hill, New York, 1956. (11) Saltzman, B., Anal. Chem. 26, 1949-55 (1954). , (12) Tawde, N., Laud, B., 6th Symp. on Combustion, p. 143, Reinhold, New York, 1957. (13) Zeldovich, J., Acta Physarochzm., L‘.R.S.S. 21, 577 (1946). RECEIVED for review November 2, 1959 ACCEPTED FEBRUARY 29, 1960 Division of Water and Wastes, Symposium on Air Pollution, 136th Meeting, ACS, Atlantic City, N. J., September 13-18, 1959. \ - - -
CORRESPONDENCE
Basic Raw Materials in the Petrochemical Industry SIR: As part of a Symposium on Plant Costs and Economics in the Chemical Process Industry you published a n article on “Basic R a w Materials in the Petrochemical Industry” by Oscar A. Colten of Shell Chemical Corp. [IND. ENG.CHEM.51, No. 9, 983 (1959)l. A discussion of the cost of acetylene included reference to both the Sachsse and SBA processes. T h e figures given were misleading as to the competitive position of the two processes. W e have discussed this matter with the author and find that 1. H e inadvertently compared the investment of a n 80-ton-per-day Sachsse unit with a 90-ton-per-day SBA unit. 2. H e used different costs for the oxygen unit, which in either case could be a ”purchased” unit as it is not an integral
INDUSTRIAL AND ENGINEERING CHEMISTRY
part of either design. The cost of the oxygen unit as reported in the Kellogg article on the SBA process is SOYc more than the cost of the oxygen unit mentioned in the Forbath article on the Sachsse process. Because the oxygen units required are practically identical, a fairer comparison of the two acetylene processes would have assigned equal investment costs to the oxygen units. IVith one oxygen unit representing 35% of the total investment. an error of nearly 15yofavoring the Sachsse process was therefore introduced in Mr. Colten’s article in addition to the error resulting from the 12.5y0 difference in plant sizes.
J. L. PATTON M. \V. Kellogg Co. New York, N . Y .
