Combustion of gaseous hydrogen sulfide

C O M B U S T I O N OF GASEOUS H Y D R O G E N SULFIDE D. A. DAVIES* AND A. D. WALSH

Chemistry Department, Universily of Dundee, Dundee, Scotland The combustion of gaseous hydrogen sulfide has been studied in a static system over the temperatm'e range 230°-410°C. The existence of three pressure limits of explosion at fixed temperature (first reported by Jacovlev and Sehantarovitsch) has been confirmed. This paper deals particularly with the second explosion limit. Plots of the partial pressure of hydrogen sulfide at the explosion limit versus the partial pressure of oxygen (also at the explosion limit) are constructed for each temperature. These partial pressure plots turn out to be of two types. For the richer mixtures and the higher temperatures, the plots take the form of long straight lines of positive slope. For the weaker mixtures and the lower temperatures, the plots take the form of long straight lines of negative slope. Glows (as distinct from explosions) were found in the richer mixtures at the higher temperatures, but. not in the weaker mixtures at the lower temperatures. The plots of induction period to explosion versus pressure take one of two forms. Changing the vessel diameter has very little effect on P~. The effects of adding inert gases have been studied extensively. The plot of the sum of the partial pressures of hydrogen sulfide and oxygen versus the partial pressure of inert gas is linear with negative slope--i.e., the inert gases all inhibit. The slopes of these lines vary little with temperature and are Mmost independent of mixture strength. The latter statement, eontra~sts strongly with the evidence above of profound differences between rich and weak mixtures. An attempt is made to explain how the two types of combustion arise, and Why they have some of the distinctive properties set out above. The combustion of gaseous hydrogen sulfide has bee11 studied in a static system over the temperature range 230°-410°C. The existence at fixed temperature of three pressure limits of explosions and so of a low-pressure ignition peninsula (first reported by Jaeovlev and Schantarovitsch ~) has been confirmed. Although there has been considerable work on the slow reaction occurring at pressures between the second and third ignition limits (see Emanuel, 2 and Norrish and Zeelenberga), there has been little study of the explosion limits themselves. We have particularly studied the properties of the second explosion limit.

Reproducibility of Experiments Explosion was recognized by the occurrence of a flash of light or of a ball of light which travelled toward the reaction-vessel entry tube. Jaeovlev and Schantarovitsch * reported that * Present address: Department of Chemistry. Falkirk College of Technology, Falkirk, Scotland.

they had obtained reasonable reproducibility by first carrying out a large number of explosions in the same reaction vessel. We found this procedure to be very unsatisfactory; instead, we adopted the routine of washing our Pyrex vessel with hydrofluoric acid, followed by distilled water, before every run. This made the work very tedious, but sueeessfuI in producing a remarkably reproducible limit. A new or HF-washed vessel gave a much lower value for the second limit than that obtained with a reaction vessel which had received the "explosion treatment." For example, the value in the new or HF-treated vessel was about 17 mm Hg at 350°C for a 40% hydrogen sulfide mixture, compared with a value of about 30 mm Hg under similar conditions for the explosion-treated vessel. Further, in the latter vessel, the induction period was found to be only about 2 see, whereas in an HF-washed vessel, the corresponding value was ca. 90 sec. The matter is particularly interesting because the effect of HF treatment is evidently one of inhibition in the hydrogen sulfide-oxygen system, whereas, in the hydrogen-oxygen or

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hydrocarbon-oxygen systems, the effect is one of catalysis.4 I t may well be that: (1) the reaction deposits solid products which catalyze the reaction and, in doing so, are responsible for the lack of reproducibility in the explosion-treated vessel; and (2) H F cleans the surface and so removes the solid products. I t may be significant that deposits of solid sulfur from the slow combustion of hydrogen sulfide have been reported. 2

T h e Second Explosion Limit The explosion limits (P2) were determined over a large range of temperature and of mixture composition (410°-230°C; 55%-1.0% H2S). The determinations considerably extend the work of Jacovlev and Schantarovitsch, I which was not concerned with mixtures weaker than about 35% hydrogen sulfide. The data have been

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displayed in Fig. 1, in the form of plots of the partial pressure of hydrogen sulfide at the explosion limit versus the partial pressure of oxygen at the explosion limit, each plot being for a fixed temperature. The most striking feature of the family of isothermals thus obtained is that it shows the partial pressure plots to be of two types. For the richer mixtures and the higher temperatures, the plots take the form of long straight lines of positive slope. For the weaker mixtures and the lower temperatures,

the plots take the form of long straight lines of negative slope, reminiscent of the plots of P(H~) versus P(O~) at the second explosion limit of the hydrogen-oxygen reaction. 6 Glows

Glows (as distinct from explosions) were frequently observed at pressures just above P2. These glows never occurred in any of the mixtures associated with the plots of negative slope,

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but only in the richer mixtures. The observation of visible light emission implies that, in the slow reaction occurring in the richer mixtures at the higher temperatures, electronically excited species are present. Induction Periods Measurement of the induction period (r) to explosion, at pressures ranging from P1 to P2, also reveals the profound difference in behavior of rich and weak mixtures. See Figs. 2 and 3. In rich h~ixtures, the r/p plots are U-shaped; in weak mixtures, they possess no minimum. Thus, as an illustration, for s 40.3% mixture (which is a rich mixture) at 350°C, the induction

periods at both P1 and /'2 were of the order of minutes, whereas in the center of the explosion peninsula, r was diminished considerably. With mixtures weaker than 25% hydrogen sulfide, the r/p plot was strikingly different. Up to a pressure of about 25 mm Ha, the induction period decreased rapidly with increasing pressure, but then changed very little with further increase of pressure. Effect of Changing Vessel Diameter In agreement with earlier workers, I we find P2 to vary little as the diameter of the reaction vessel is changed. Trebling the diameter increased P2 by about 20%.

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Effects of Addition of Inert Gases It appears that there has been no previous work on the effect of inert gases on P2. I n the plots shown in Figs. 4 and 5, the ordinate is the sum of the partial pressures of hydrogen sulfide and oxygen at the limit; the abscissa

is the amount of inert gas added; 40% and 15% mixtures were taken as representative of the rich and weak mixtures, respectively. As Figs. 4 and 5 illustrate, the plot of the sum of the partial pressures of hydrogen sulfide and oxygen against the partial pressure of inert

gas added is linear and of negative slope. Very similar data apply to 40% mixtures at 380 ° and 410°C, and to 15% mixtures at 260 ° and 290°C. It can thus be seen that all the inert gases used (COs, N2, He, At) are inhibitors and that the order of their efficacy is always C02> Ne> H e > Ar. The negative slopes of Figs. 4, 5, and similar figures, are quantitative measures of the relative strengths of the various inert gases as inhibitors.

TABLE I The relative effects of inert gases on P2

Ar He N~ CO2

Determined in 15% H~S-O~ mixtures

Determined in 40% H~S-O~ mixtures

260°C

290°C

320°C

350°C

380°C

410°C

0.47 0.53 0.63 1.10

0.42 0.43 0.54 0.97

0.38 0.45 0.60 0.90

0.47 0.57 0.64 0.93

0.41 0.45 0.57 0.82

0.40 0.46 0.54 0.77

COMBUSTION OF GASEOUS HYDROGEN SULFIDE These quantitative measures are set out in Table I. For any one inert gas, the slopes change little as the temperature is raised from 260 ° to 290°C. The striking thing is that the rough constancy carries on to 350°C, to 380°C, to 410°C. In other words, it does not matter whether we are dealing with rich or with weak mixtures. This conclusion is particularly interesting, in view of the outstanding differences reported above in the behavior of rich and weak hydrogen sulfideoxygen mixtures. Discussion

Norrish and Zeelenberg, 3 from flash photolysis studies, found the reaction of hydrogen sulfideoxygen mixtures to take one of two forms, differing in their products. In the absence of a large excess of inert gas, the reaction resulted in SO2. In the presence of a large excess of inert gas, the reaction produced not S02 but $202. We shall refer to the two reaction paths as I and II, respectively. The present work also reveals two mechanisms of hydrogen sulfide combustion. The one is associated with rich hydrogen sulfide-oxygen mixtures, with the higher temperatures (see Fig. 1), and with U-shaped induction period/ pressure plots (see Figs. 2 and 3). The other is associated with weak mixtures, with the lower temperatures, and with induction period/pressure plots that resemble rectangular hyperbolae. We presume that the two mechanisms of our finding correspond, respectively, to the two reaction paths found by Norrish and Zeelenberg3 The following is an attempt to explain how the two pathways arise and why they have some of the distinctive properties given above. There is general agreement that we are dealing with a branched-chain reaction, and that. the 1° chains are 2'3 chain propagation: OH+H2S---~H20+SH,

(1)

SH+O2--~OH+SO.

(2)

These convert H2S to SO molecules. The problem is how does SO become converted into S02 or '~8202"?

There are two things an SO molecule might plausibly do. The first is to react with O2: S O + 02--->S02+ O.

(3)

481

This will be followed by 0+H~S---~0H+SH,

(4)

and (3) with (4), (1) and (2) constitute a branching-chain reaction, although (3) with (4) is not necessarily the only mode of branching. Branching may be held in check at P~ by diffusion of chain centers to the walls and destruction thereon; and at P2 by a gas-phase chain-ending reaction (see below). The second way in which SO may plausibly react is by dimerization, S O + S O + M--~$202+ M,

(5)

where M stands for any third body (including inert-gas molecules added). With molecules so small as SO, there is little doubt that the ternary reaction written will be third order7 It should be emphasized that the justification for including (5) is primarily one of plausibility. SO has a triplet (aN-) ground state, and may be thought of as having unsatisfied valencies. However, there is a large amount of evidence that a species $20 is present in the H2S-02 system, under relevant conditions, s,9 It may be that $20 is more important than S2Oe, although just how 820 is formed and just what reactions it enters into are obscure. The inclusion of Reaction (5) (and the neglect of $20) are thus tentative. To recapitulate, once SO has been formed, there are two alternative reaction pathways. One leads, via (3), to S02 as the final product. The other leads, via (5), to S2Oe. The participation of oxygen molecules in Reaction (3) means that weak mixtures will almost certainly favor (3) rather than (5). Further, in the absence of a large excess of inert gas, (3) is likely to be dominant over (5), so that S02 is the final product. In the presence of a large excess of inert gas, (5) becomes dominant over (3) and the product is $202. Reaction (5), being an association of two radicals, is likely to involve little or no activation energy. Reaction (3) is expected to have a small activation energy. One can thus understand our finding that mechanism I is favored, relative to mechanism II, by the higher temperatures. The existence of a second explosion limit must mean that a chain-ending reaction step occurs, which is of higher order ill reactant partial pressures than is the branching step with which it competes. Since there is no appreciable change

OXIDATION AND IGNITION

482

in /'2 when the vessel diameter is varied, the reaction step must take place in the gas phase. Further, the step has to be common to both the pathways mentioned above. A simple possibility would be the reaction HS+O2+M--~HS02+M,

(6)

which would compete with (2). HSO2 is a known radical found in the H2-02-S02 reaction system, s where it is sufficiently inert to be subsequently destroyed (on the walls?) without reforming an active chain center. Collecting together the various reactions, we have:

1° chains: OH+H2S---~H20+SH,

(1)

SH+O2--~0H+SO,

(2) (6)

S H + 0 2 + M / - - ~ H S 0 2 4 - M l,

SO+02--~SO~-t-O, O+H2S---~0H+SH. Pathway I Main product: SO~

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(5)

(4) Pathway I I Main products: $202, $20?

Competition between Reactions (2) and (6) accounts for the existence of a second pressure limit of explosion. At that limit, one must have /c2[SH][-02]

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Hence k2/k6= [-M']= [-H2S]+a[O2]+b[-Ar]" " .

One recognizes that the slopes in Table I, above, are the relative effieiencies of the various inert gases in bringing about Reaction (6). The reason why it does not matter whether we start with a weak or a strong mixture is simply that all the data obtained with inert gas refer to one reaction step only [-which we have identified as (6)]. Reaction (6) occurs ill the primary chains, and is not confined either to rich or to weak mixtures. I t would seem difficult to understand in any other way the figures in Table I, and the inhibiting effects of inert gases.

2. EI~IANUEL,N. M.: See references cited by N. N. Selnenov, Chemical Kinetics and Reactivity, Vol. II, translated by Bradley, Pergamon Press, 1959. 3. NORRISH, R. G. W. AND ZEELENBERG, A. P.: Proc. Roy. Soc. A240, 293 (1957). 4. CHEANEY, D. E., ])AVIES, D. A., DAVIS, A., HOARE, D. E., PROTHEROE, J., AND WALSH, A. ]).: Seventh Symposium (International) on Combustion, Butterworths, 1959.

5. DAvI~s, A. D.: Ph.D. thesis, University of Leeds, 1960. 6. Gt~ANT, G. H. AND HINSHELV.'OOD, C. N.: Proc. Roy. Soc. A141, 29 (1935). 7. (a) HOARI% D. E. AND WALSH, A. D.: Trans. Faraday Soc. 53, 1102 (1957); (b) WALSH,

A. D.: Trans. Faraday Soc. 54, 1772 (1958). 8. MESCHI, ]). J. AND MYERS, R. J.: (a) J. Amer. Chem. Soc. '?8, 6220 (1956); (b) J. Mol. Spectry. 3, 405 (1959). [). MARBDId.N', D. (~. H.: Can. J. Chem. 41, 2607

(1963).

REFERENCES 1. JACOVLEV, B. AND SCItANTAROVITSCH,P.: Acta

10. WEBSTER, P. AND WALSH, A. D.: Tenth Symposium (International) on Combustion, p. 463,

The Combustion Institute, 1965.

Physicochim. U.R.S.S. 6, 71 (1937).

COMMENTS D. R. Warren, Aeronautical Research Labs., Melbourne, Australia. I find this a most interesting paper in view of both the similarities and the differences between the hydrogen-sulfur and

hydrogen-oxygen reactions. Two questions are of particular interest: (1) You mentioned that the explosion limits were independent of vessel size. Is this true for both rich (glow region,

COMBUSTION OF GASEOUS HYDROGEN SULFIDE positive slope) and weak (negative slope) regions? and (2) In its effects in suppressing the second limit, does water show the abnormally high value of third-body efficiency, as it does in the H2/02 reaction?

Authors' Reply. As to the first question, the statement that the P2 limits are virtually unchanged by change of vessel diameter applies to both a 40% mixture at 380°C and to a 5% mixture at 260°C, i.e., to both "rich" and to "weak" mixtures. In reply to the second question, addition of water vapor lowers P2 (determined in a 40% mixture at 350°C, i.e., in a "rich" mixture) and

483

the plot of P2-vS-PH20 is linear with a negative slope of ca. 1.63. There is no evidence that water vapor behaves in any way other than as an inert gas--except that it is more effective. 1.63 should be compared with the figures in Table I; and the order of efficiency of inert gases as inhibitors seem to be

H20> CO2> N2> He> Ar.

(1)

Unfortunately, similar work on the effect of water vapor addition to a "weak" mixture has not been carried out; but, there is no evidence that the order of efficiency would not be the same as ill (1).