Arthur Levy

Kinetics of Sulfur-Oxide Formation in Flames: II. Low Pressure H2S Flames

The microstructure of 1/10 and 1/20 atmosphere, lean H2S—O2—N2 flames is developed using the mass-spectrometric flame-sampling technique. The flame mechanism developed is in agreement with that determined from an earlier study on 1-atm H2S flames. The formation of SO2 appears to be primarily related to the production of SH and the ensuing oxidation steps SH + O2 = SO + OH and SO + O2 = SO2 + O. While there is some question whether SO2 formation occurs via an SO or an S2O intermediate, the present study does not give direct support to the role of S2O in the oxidation mechanism. However, the presence of significant quantities of free sulfur in the pre-flame zone may be indicative of S2O formation via SO + S → S2O, and, possibly, via the disproportionation of SO, 3SO → S2O + SO2. Kinetic analyses of some of the pre-flame reactions indicate an apparent activation energy of 17,300 calories/mole for the decomposition of H2S. The actual initiation process in the flame mechanism requires further examination. The ...

Nitrogen oxide formation in flames: The roles of NO2 and fuel nitrogen

Flat methane flames were probed in the presence and absence of nitrogen-containing compounds (referred to as fuel-N). Methylamine, pyridine and piperidine at about 120 ppm were added to the flames. The data, based on detailed NO and NO2 profiles, for flames with and without the fuel-N additives, indicate a sequence of reactions consistent with the folowing mechanism, NH and/or CH+O2=NO+OH and/or CO (1) NO+HO2=NO2+OH (2) NO2+O=NO+O2 (3) Spectroscopic data indicate that NH and CN are present in the visible flame. The NO produced from the N-containing radicals is rapidly consumed in the visible flame region by HO2 radicals, producing NO2 in accordance with step 2 of the mechanism. The NO−HO2 kinetics appear to be sufficiently rapid since NO was detected in the visible flame region only when fuel-N was added to the flames, i.e., only after saturation of Reaction 2. This is further supported by the fact that NO added to methane flames is also rapidly removed in the preflame region. The NO2 produced in the flame was subsequently converted to NO to varying degrees in a narrow reaction zone in the near postflame region where the O-atom concentration was rapidly increasing to its maximum level [Reaction (3)]. The extent to which NO2 was consumed depended on the oxygen content of the flame—complete consumption of NO2 occurring only in the fuel-rich flames. Profiles of the fuel-N compounds obtained from the probings indicate that methylamine produces more NO2 and NO in the combustion process than pyridine or piperidine. Piperidine however appeared least stable in terms of NO and NO2 produced via the preflame reactions. The relative stability of the three fuel-N compounds in the flames appeared to be pyridine, the most stable, followed by methylamine and piperidine. The fuel-N materials produce a thermally stable, as yet unidentified, intermediate during oxidation, which reacts readily with the O-atoms in the flame.

Evaluation of Arsenite-Modified Jacobs-Hochheiser Procedure.

w T h e Jacobs-Hochheiser ( J H ) method is being used to determine integrated NO:! and N O levels (after oxidation of N O to NOz) in the 1-15 pphrn range in indoor and outdoor environments. Cnder controlled experimental conditions, the absorption of small quantities of NO2 in a series of bubblers containing 0.10 or 0.25N NaOH varied considerably resulting in poor reproducibility of da ta . However, the addition of u p to 0.100/0 by weight of sodium arsenite to the absorbing solutions greatly improved the collection efficiency, the reproducibility, and the accuracy of the da ta . NO interfered with the NO2 absorption process in , the presence of arsenite; a correction factor was determined which can be used when the NO concentration is known, A COz effect on the p H of the absorbing solutions was observed and taken into account in determining NO2 levels. Water vapor or CH4 had little or no effect on the NO2 collection and analysis process. T h e J H procedure, modified with sodium arsenite and corrected for S O . appears to yield accurate and reproducible integrated SO2 values.

Transpiration Study of Magnesium Oxide

Vapor densities of magnesium‐bearing species over magnesium oxide were determined by the transpiration method. The MgO reacted with water vapor present in the carrier gas to form gaseous Mg(OH)2. The standard free energy of formation in calories per mole is given over the temperature range from 1660° to 2010°K by the equation ΔGf° = —168 600+49.8T cal mole—1. Under conditions where water vapor is minimized, a second vaporization process becomes predominant. It is concluded that this process is the congruent sublimation of MgO. A second‐law treatment of these data indicates ΔH298 = 142.5+3.3 kcal mole—1 while a third law treatment yields 145.1±0.1 kcal mole—1 using a 1Σ ground state for MgO. The ground state for gaseous MgO would appear to be Σ and most likely 1Σ. A value of 93 kcal mole—1 was obtained for D0 for MgO based upon the second‐ and third‐law treatment of the data.

Sulfur trioxide flame chemistry—H2S and cos flames

The formation and depletion of SO 3 in flat premixed hydrogen sulfide and carbonyl sulfide flames have been studied at pressures from 35 to 625 torr. Flam probings in this study indicate that the amount of SO 3 formed in a given flame system increases with increasing total pressure in accordance with the three-body reaction SO 2 +O+M=SO 3 +M. The data show that attaching hydrogen to the sulfur atom in the sulfur-bearing fuel reduces the amount of SO 3 formed in a given flame system. A depletion of SO 3 in the post-flame region of both the H 2 S and COS flames is attributed to the reaction SO 3 +O=SO 2 +O 2 . The consumption of SO 3 becomes much more noticeable below 250 torr where the lifetime of the O atoms increases. The depletion of SO 3 occurs in different regions of the two flames suggesting that different mechanisms leading to SO 3 formation are operating in each flame system. Rate constants have been determined for the reaction of SO 3 with O atoms in both the H 2 S and COS flames. The rate constant in the COS flame is expressed by k 3a =2.8×10 14 exp(−12,000/ RT ) cm 3 mole −1 sec −1 , and in the H 2 S flame by k a =6.5×10 14 exp(−10,800/ RT ) cm 3 mole −1 sec −1 . The lower activation energy value obtained from the H 2 S flame data is explained in terms of a small contribution from the SO 3 -H reaction, which implies that the latter reaction has a smaller activation energy than the SO 3 -O reaction.

A Study of Sulfur Dioxide in Photochemical Smog: II. Effect of Sulfur Dioxide on Oxidant Formation in Photochemical Smog

Sulfur dioxide, a reducing agent found in urban air, might be expected to react with the oxidizing atmosphere produced by photochemical smog. It does not, however, react directly with either ozone or nitrogen dioxide in air although these reactions can occur in solution or on surfaces. However, sulfur dioxide does react with other, less well-identified oxidants which are formed during the photochemical smog reaction process. One mechanism involves the reaction of SO2 with NO3 (or N2O5) formed as a result of the reaction of NO2 with O3. The interactions of SO2 with photochemical smog were investigated in environmental chambers. A regression analysis, carried out on the data from 23 chamber experiments with 1-butene, indicated that the effects of SO2 on oxidant production depend on the concentrations of water vapor, initial nitrogen dioxide, and sulfur dioxide. The effect also depends on the type of hydrocarbon. Sulfur dioxide was found to reduce the maximum oxidant obtained from 1-butene, 1-heptene, and 2,...

Unresolved problems in SOx, NOx, soot control in combustion

Combustion and pollution have been somewhat synonymous topics since mankinds discoveryof fire. As a result of a concerted, worldwide concern over the past fifteen or so years, for what is being emitted to the environment via the smokestack and the tailpipe, great strides have been made toward burning fuels cleanly. In line with this concern, the Combustion Institute has been devoting sessions to air pollution since the Twelfth Symposium in 1968. A great deal of progress, both in an understanding of the technical issues and the implementation of this knowledge, has taken place in this period. This paper attempts to keynote where we are today and what are the principal unanswered questions in the three key areas of SO x , NO x , and soot control in combustion. Combustion-pollution technology is probably on its firmest grounds in the area of SO x , onslightly less firm ground in the area of NO x control, and has farthest to go in soot control. Major attention in SO x technology is directed to fuel-rich combustion and the application of SO x kinetics to staged combustion, interaction of SO x with NO x control mechanisms, and a renewed interest in limestone capture of sulfur. Principal concerns of NO x control address issues of fuel nitrogen release, the nature of fuel nitrogen species, interactions of nitrogen-containing species in fuel-rich combustion, radical overshoot in rich flames, and the conversion of NO to NO 2 in gas turbines and gas burners. Problem areas in soot control are brought forth by consideration of six principal areas which make up a general mechanism for soot formation. Also addressed in the soot discussions are the role of additives, the relation of polycyclic aromatic hydrocarbon formation to soot, and the generation of soot in diesel combustion. The problem areas brought forth by our concerns for clean combustion have had a positive,synergistic effect in advancing our understanding of the combustion process.

Combustion and Photochemical Aerosols Attributable to Automobiles

An aerosol size analyzing system developed at the University of Minnesota was used to measure in situ primary aerosols produced by operation of automobiles on a chassis dynamometer and secondary aerosols produced by photochemical reactions of exhaust vapors in a 610 cu ft smog chamber. The total automotive contribution to urban aerosol concentrations is estimated using the volume emission rate of primary aerosol together with the relative concentrations of primary and secondary aerosols. The size distributions and primary aerosol concentrations determined in the laboratory agreed quite well with results obtained alongside roadways using a similar aerosol analyzer. For exhaust dilution ratios of 20/1 and 40/1 in a laboratory dilution tunnel, the predominant mode in the aerosol size distributions occurred at about 0.03 μm diameter. This size is substantially smaller than that reported in other laboratory studies. The size distribution of the diluted exhaust is sensitive to the rate of dilution at the tailpi...

A Study of Sulfur Dioxide in Photochemical Smog I. Effect of SO2 and Water Vapor Concentration in the 1-Butene/NOx/SO2 System

There is an appreciable chemical interaction between SO2 and photochemical smog which depends on the concentration of SO2 and water vapor. The rate of decay of SO2 concentration is greatly increased in the presence of photochemical smog. With 0.75 ppm SO2, a light-scattering aerosol is produced in dry systems and systems at 22 and 55% relative humidity (RH). Aerosol is not observed until after the NO2 peak has been reached and the NO concentration has fallen to a very low value. The formation of aerosol corresponds in time to the region of most rapid decrease in the SO2 profile. In systems at 65% RH or with smaller amounts of SO2, no light scattering is observed, but the percentage of SO2 disappearing is greater. In relatively dry systems the presence of SO2 results in a general slowing down of the photochemical smog reactions. In systems containing water vapor concentrations comparable to those found in the atmosphere, the inhibiting influence of SO2 on the smog reaction is less pronounced. However, the ...

The Effect of Fuel Composition on Atmospheric Aerosol Due to Auto Exhaust

Aerosols attributable to automobile exhaust can be classified as two types—primary aerosol (initially present in the exhaust) and secondary aerosol (generated photochemically from hydrocarbons and nitrogen oxides in the exhaust). In this study, investigation was made of possible effects of motor-fuel composition on the formation of these aerosols. Secondary aerosol, of principal interest in this work, was produced by irradiating auto exhaust in Battelle-Columbus’ 610 ft3 environmental chamber. A limited number of determinations of primary aerosol in diluted auto exhaust was made at the exit of a 36 ft dilution runnel. Determination of both primary and secondary aerosol was based on light-scattering measurements. Exhaust was generated with seven full-boiling motor gasolines, both leaded and nonleaded, in a 1967 Chevrolet which was not equipped with exhaust-emission control devices. Changes in fuel composition produced a maximum factor of three difference in light scattering due to primary aerosol. Aerosol ...