Earl L. Merryman

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.

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.

Enhanced SO3 emissions from staged combustion

Staged combustion is a recognized, effective means for lowering NO x emission. Examination of the staged combustion process suggests however that the high CO levels produced in the first stage may pump a sufficient level of oxygen atoms into the second stage to result in an increased (enhanced) SO 3 formation. Experiments were carried out in a small two-stage combustor which allowed for an examination of SO 3 formation under similar single- and two-stage conditions. The experiments show that staging does cause enhanced SO 3 formation, the enhancement is of short duration, and is dependent on the air/fuel ratio of each stage and the delay interval between the first and second stages. Kinetic analysis yields a value of k 1 =7.4×10 14 cm 6 mole −2 sec −1 for the reaction SO 2 +O+M=SO 3 +M and k 2 =1.5×10 11 cm 3 mole −1 sec −1 for the reaction SO 3 +O=SO 2 +O 2 (T=1685 K). The kinetic analysis also shows that enhancement of SO 3 formation in staged combustion can occur. However, the experimental results suggest that the enhancement may only be a transient phenomenon dependent on several combustion variables.

Formation of NO2 in range-top burners

This paper describes results of a study that examined NO and NO2 formation on range-top burners and in diffusion flames. These flames were characterized by composition and temperature profiles. Range-top burner flames and pilot flames displayed qualitatively similar behavior with respect to the kinds of flame regions in which relatively high NO2/NO ratios were identified. These regions of high NO2/NO ratios were consistently either regions of low oxygen concentration or flame surfaces subjected to thermal quenching. A limited series of experiments with modified burners indicated that reduced emissions from both the RTB and pilot flames could be achieved by (1) improved primary aeration, using 50% or greater primary air, and (2) using flame geometries designed to minimize flame surface, e.g., flat-flame burners or other designs having effectively fewer distinct ports. Both NO and NO2 are readily produced in diffusion and partially premixed Bunsen-type flames, mainly in the vicinity of the hot visible zone. High NO2/NO ratios are associated with the cooler regions of the flame, as, for example, at the base of the flame in the highly diluted downstream region and in the fuel-rich regions of the flames. A simplified reaction mechanism based on CN and NH radicals being oxidized to NO followed by NO + HO2 → NO2 + OH appears to explain the high NO2/NO ratios observed. A practical implication of the study is that a burner designed with improved aeration and mixing minimization of flame surface should emit less NO2.

AEROSOL FORMATION IN PHOTOCHEMICAL SMOG

During operation of smog chambers, the gas mixture is normally well-stirred although the extent of stirring varies from chamber to chamber. Experiments conducted at Battelle’s Columbus Laboratories in a 200-liter and a 610-cu ft smog chamber have shown that stirring can decrease aerosol formation. The faster the chamber contents are stirred the greater the reduction in aerosol formation observed. Sufficiently rapid stirring can completely prevent observation of aerosol formation by light scattering. However, thermal gradients in these chambers have been adequate to maintain homogeneity after initial mixing and satisfactory chamber experiments are being performed by turning the stirring off prior to turning on the lights. The differences in aerosol formation between stirred and nonstirred chamber operation is dependent on the type of system being studied and the stirring rate. In the 200-liter chamber during dynamic operation (4-hour average residence time) aerosol formation from 1-heptene/NOx and l,3,5-tr...

A Progress Report—Fundamentals of Thermochemical Corrosion Reactions

This paper describes original work on the reactions of sulfur with oxygen in flame systems of H2 S-O2 -Ar, where the stabilized thin flame was microprobed and the quenched products of reaction were examined with a mass spectrometer. Problems in identifying reaction products are described. Although the mass spectrometer was shown capable of checking wet analyses for mixtures of gases, the composition of combustion products was not determinable accurately by this method. Preliminary kinetic data are presented for both, homogeneously and heterogeneously (catalytically) produced SO2 and SO3 .

CaO interactions in the staged combustion of coal. Second quarterly technical progress report, January 1, 1981-March 31, 1981

The CaO-FeS/sub 2/ reaction was studied as a function of temperature, reaction time, Ca/S mole ratio and particle size. All reactions were carried out under a nitrogen environment. The reactions were followed principally in terms of weight loss and SO/sub 2/ emissions. The decomposition of the pyrite produces S/sub 2/ which reacts with CaO to produce SO/sub 2/ and CaS. The extent of the reaction appears to increase monotonically between 555 and 970/sup 0/C.

Sulfur Chemistry and Its Role in Corrosion and Deposits

Sulfur in fuel oil and coal, and its resultant oxidation to SO3 during combustion, is a recognized factor in corrosion and deposits. As a step toward controlling the formation of SO3 , and eventually controlling the conditions and rate of corrosion, a program has been undertaken under ASME support to establish, among other things, the mechanism by which sulfur compounds are oxidized to SO2 and to SO3 in flames. The present paper reviews some of the basic thermodynamics and reaction kinetics pertaining to the oxidation of H2 S and SO2 , and to the SO2 –SO3 equilibrium. Included in the review are discussions of the stability of H2 S, the slow oxidation of sulfur vapor and of H2 S, the induction period (preignition) reactions leading to the fast, explosive oxidation of H2 S, and effects of additives on the explosion reaction. The heterogeneous (catalyzed) oxidation of SO2 is discussed in terms of the effects of specific catalysts on the SO2 –SO3 equilibrium.