i I
years, research on gasification has been markedly intensified because of the widespread interest production. Although emphasis has been placed o n process development, many workers the need for reliable kinetic data on which to base process design calculations. As a result, amount of valuable fundamental data has been accumulated. T h e purpose of this symposium gether some of this new information on the theoretical aspects of gasification a n d combustion. e voluminous literature on the reactions of carbon with oxygen, steam, a n d carbon dioxide, there sing lack of agreement among various investigators regrading even such basic points as the mechanism of the various heterogeneous reactions. Moreover, the available kinetic data are largely empirical relationships, applicable only to the particular carbon sample, apparatus, and/or experimental procedure employed. In the past, too Little attention has been paid to the properties of the carbon itself a n d to the possible influence of such factors as inorganic impurities, previous temperature history of the sample, previous exposure to so-called inert atmospheres, a n d changes in the surface chemistry of the carbon resulting from the reaction under investigation. Fortunately, the importance of most of these variables is becoming more generally recognized, a n d this should lead to a more fundamental understanding of both the mechanism a n d kinetics of the various reactions. Although not in themselves complete answers to either the mechanism or kinetics of the various gasification reactions, several of the papers of the present symposium indicate rather clearly the lines along which future research should b e directed. It is the hope of those responsible for organizing the symposium that the papers a n d discussion will stimulate further research on the gasification a n d combustion reactions so that ultimately reliable kinetic data for these important reactions will become generally available.
C. C. WRIGHT
ode of Energy Release in Combustion of Carbon J. R. Arthur and J. A. Bleach BRITISH COAL UTILISATION RESEARCH ASSOCIATION LEATHERHEAD, SURREY, ENGLAND
The work was undertaken with a view to gaining further information about the chemical reactions that occur in fuel beds. Relatively simple solid fuels (such as charcoal a n d graphite) were reacted, as single particles, with air a t temperat u r e s i n the range 800' to 1000° C. a n d at various pressures from atmospheric down to 2 cm. of mercury. Particular attention was paid to the conditions u n d e r which a blue glow was observed around the carbon particle. A critical rate of air flow exists (at given conditions of temperature a n d pressure) above which the glow i s visible. T h e appearance of the glow does not correspond with a n y sharp change i n the analyses of the combustion products. T h e intensity of the glow is increased b y pretreatment of the fuel with sodium carbonate or cupric chloride. Addition of sufficient water vapor to the combustion air quenches out the glow, the quantity required being much higher for charcoal than for graphite. T h e proportion of carbon monoxide in the combustion products increases a s the pressure is decreased i n the above range.
1028
T h e glow is associated with the secondary oxidation of carbon monoxide, a n d its presence or absence gives information about the mode of dissipation of the heat of this reaction. The observations are also important because they are relevant to considerations of the reaction between freshly formed carbon dioxide a n d carbon which can occur i n fuel beds.
UEL beds that are in active combustion evolve large quantities of carbon monoxide. The mode of formation of this gas is uncertain, there being two main possible contributing reactions 1 / 2 0 2 +co (1) coz 4- +2co (2)
c+
c
There is considerable evidence ( 2 , 5 , 20) that oxygen reacts with solid carbonaceous fuels a t high temperatures ( > 1000' C.) mainly according t o Equation 1. If Equation 2 is t o make a significant contribution t o the carbon monoxide t h a t is evolved from the bed, carbon dioxide must first be formed according t o Equation 3.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
of Solid Fuels Part of the symposium, “T bustion and Gasification,” p sions of Gas and Fuel Chemis organic Chemistry at the Diam AMERICANCHEMICAL Socr pers from this symposium will issue of INDUSTRIAL AND E
H
Aspects of Comed before the Bidd Physical and Inilee Meeting of the ther group of pablished in the July ERING CHEMISTRY
ENERGY RELEASE IN COMBUSTION OF CARBON J. R. Arthur and J. A. Bleach
................................
1028
REACTIONS OF ARTIFICIAL GRAPHITE Earl A. Gnlbransen and Kenneth F. Andrew
.........................
1034
KINETICS OF CARBON GASIFICATION BY STEAM G. E. Goring, G. P. Curran, R. P. Tarbox, and E. Gorin
....................
CATALYTIC REVERSE SHIFT REACTION
L. W. Barkley, T. E. Corrigan, H. W. Wainwright, and A. E. Sands
, , ,
.,..,
, , ,
..
1051
, ,
1066
GASIFICATION AT ELEVATED PRESSURES WilhelmGnmz.
.......................................
GASIFICATION IN A VORTEX REACTOR
M. A. Elliott, H. Perry, J. Jonakin, R. C. Corey, and M. L. Khullar
+
...............
GO ‘/ZO2--i, coz (3) If, as is often stated, Reaction 3 is indeed followed by 2, then the latter reaction occurs under different conditions from those obtaining in laboratory studies-for example, t h e carbon dioxide is freshly formed. There is considerable reason t o believe t h a t such carbon dioxide might be more reactive than the normal gas because of a n absence of equal energy distribution between the various degrees of freedom. An abnormality of this kind is supported by studies of t h e carbon monoxide flame spectrum. A phenomenon which is related t o these considerations is the blue glow or corona which can be observed under suitable conditions around a burning carbon particle. This phenomenon was commented on by Davis and Hottel(9) who have published photographs of the glow. Parker and Hottel (19) did not, however, consider t h e phenomenon t o be of basic importance as their other evidence led them t o conclude t h a t the burning of solid fuels at high temperatures was not effected by primary carbon monoxide formation. I n experiments in which carbons were burned in the presence of inhibitors of t h e combustion of carbon monoxide, Arthur ( 5 ) observed t h a t the glow was extinguished and t h a t the carbon monoxide t o carbon dioxide ratio in t h e product gases was very high compared with this ratio when t h e inhibitors were absent. The glow was therefore identified with a stage of t h e reaction GO Cog. This view is strongly supported by a n analysis of the spectrum of the glow (81). This paper records some further observations governing the appearance and intensity of the glow. I n t h e present experiments, carbon particles have been burned in t h e temperature range 800’ t o 1000”C. and a t pressures between 2 and 76 cm. of mercury. -f
May 1952
1071
1074
The main apparatus was simple and similar t o t h a t used in previous work ( 5 ) . It allowed for close observation of a burning carbon particle and for varying the absolute pressure a t which air or other oxygen-nitrogen mixtures could be drawn through the reaction vessel; these gases were not preheated. The essential features are shown in Figure 1A. A piece of carbon (usually in the form of a tube) was burned in a quartz tube mounted in a Nichrome wire-wound furnace, and a glass window a t the one end of t h e quartz tube allowed end-on observation of the carbon. The blue glow was best observed through a filter of cobalt blue glass. A side arm at the inlet t o the quartz tube connected through a control t a p t o a flow gage and drying tubes containing magnesium perchlorate and another led t o a mercury manometer. The exit of the quartz tube ended in a socket which fitted into a glass cone through which was mounted a sheathed 36 SWG Chromel-Alumel thermocouple. The junction of the latter was in the center of the carbon tube. A side arm from the cone led through the sampling device t o an oil pump. Taps TI, T4, and TSallowed samples of gas t o be withdrawn with minimum disturbance of the condition of an experiment, Taps TZand Ta were turned off simultaneousIy,,as nearly as possible, during sampling, after which T I and 2‘4 were closed; all gab was then passing through Ts. This simple technique allowed samples t o be withdrawn at the pressure (PI)indicated by t h e mercury manometer. For example, the data of Table I relate t o a run in which the volumes (V,) of all the samples were measured at atmospheric pressure after withdrawal ; the corresponding pressures (Pz) were evaluated by Boyle’s law, the capacities (Vz)of the collecting vesbels being known. The values of PI and PZagree closely. Differences of pressure were obtained by adjusting a variable Ieak through T6.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1029
Fuel Gasification After the gas samples had been brought t o atmospheric pressure over mercury, they were analyzed in a Haldane apparatus. Air or nitrogen was used as diluent gas when the sample was less than 21 cc.; air was used where the sample contained insufficient oxygen t o burn the carbon monoxide. Gasification rates -
(2)
were calculated, if required, from the percentage
constituents of carbon dioxide, carbon monoxide, and oxygen in the exit gases using the relation
where Poz is the percentage of oxygen in t h e inlet gas and F is rate of flow of gas into t h e furnace in cc. per minute a t standard temperature and pressure.
PI,Cm. Hg.
Table I. Gas Sampling Data vz,cc. vi, c o . Pz (Calcd.),Cm. Hg.
coal. The proximate analyses of the latt.er two fuels and one of the graphites are given in Table 11.
Table 11. Analyses of Fuels Fixed carbon Volatile matter Ash, Moisture
Graphite, % 99.7 0.1 0.1 0.1
Electrode Carbon, % 99.3 0.1 0.5 0.1
Charcoal, % 86.9 6.8 4.1 2.2
The fourth fuel was a spectroscopically pure graphite. A determination of its ash content in these laboratories gave an average value of 0.026%. The electrode carbon was used for the greater part of the work a t reduced pressures, and a complete analysis of the ash of this material showed that the main impurities were silicon, boron, and iron. The analysis of the ash of the charcoal, which was used particularly in the experiments involving water vapor addition to the air combustion, showed that the main impurities were silicon, calcium, and sulfur.
Experimental Results
A second apparatus, which was used t o observe the influence of added salts and water vapor on the glow a t normal pressures, is illustrated in Figure 1B. A carbon particle was suspended in a quartz tube (2.8-em. diameter) which stood upright in a vertical furnace. The particle, which could be observed from above the top open end of the quartz tube, was burned in a Eteady air stream which was introduced from the bottom of the quartz tube. I n this case also the air was dried by magnesium perchlorate. A measured portion of the air stream passed into the quartz tube could earlier be diverted t o pass through two water bubblers contained in Dewar vessels for the introduction of water vapoi.
THERM0
\'
GLASS
THERMOCOUPLE
SAMPLING VESSEL (VOL y)
Table 111. Gas Analyses at Different Times at Two Working Pressures A
VACUUM PUMP
B
Figure 1. Apparatus Used for Work (A) at Reduced Pressures, (B) at Normal Pressures The particle -;as suspended in the furnace by the thermocouple which measured and controlled its temperature. The temperature control was of the simple photocell amplifier type described by Hirst and Cannon (fS). The thermocouple (28 SWG Chromel-Alumel wires) was threaded through the center of the particle which was generally used in the form of a cylinder, 1.0 em. in diameter and 1.5 em. long. If required, the particle-thermocouple assembly could be supported from a balance arm so t h a t the rate of gasification could be measured directly. Four different carbonized materials were used-namely, two artificial graphites, a n electrode carbon, and compressed char-
1030
Relation between Gasification Rate and Time. As a preliminary step in a n investigation of the effect of pressure on the gasification rate, the variation of the latter as a tube of carbon burned out was investigated. Runs were carried out at air flow rates of 270 cc. per minute (measured a t standard temperature and pressure) and a carbon temperature of 850' C., using pressures of 40 and 2 cm. of mercury, respectively. Samples of the exit gases were collected a t 30-minute intervals for 4l/2 hours in the first case and each 45 minutes for B1/4 hours in the second. The gasification rates calculated from the gas composition are plotted against time in Figure 2. Both curves have the same form, and little change in gasification rate is observed for the first 2 hours in both cases. The gasification rates were substantially the same for the two pressures at corresponding times, a result in agreement with the investigations to be described later. The area under the curves of Figure 2B was 7.62 grams; the measured weight of carbon consumed was 7.59 grams. The actual analyses obtained in the runs are given in Table 111. The nitrogen figures were obtained by difference. An oxygen balance based on the oxygen content of the entry air and on the free and combined oxygen content in the exit gas mixtures showed no discrepancies that could not be attributed t o experimental errors.
Time Hour$ COz 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
40 Cm. H g . CO 0%
NZ 78.28 77,95 77.62 77.85 77.40 78.99 79,30 79.27 79.40
Time, ___ Hours COz 0.25 1.00 1.15 2.00 3.26 4.00 4.75 6 50 6.25
2 Cm. Hg. CO 0 2
Nz 73.53 75,98 77.93 74.89 72.87 78.48 78 80 78.80 78.32
Influence of Pressure Variation a t Constant Mass Velocity. It is apparent from the analyses given in Table I11 that the proportion of carbon monoxide in the sampled gases was substantially higher at 2 em. than a t 40 cm. of mercury pressure. A systematic variation of pressure over the range 76 to 2 cm. of mercury (temperature, 850" C.; flow rate, 270 cc. per minute a t standard temperature and pressure) gave the result shown in Figure 3. The run was confined t o that period of time over which the burning rate would be substantially constant (Figure 2 ) . There is a continuous increase in the ratio of carbon monoyide t o carbon
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
Fuel Gasificationtherefore be due to either radical (Cu or C1) alone. The behavior of t h e glow obtained from carbon impregnated with copper chloride, including its fading and apparent movement into t h e carbon during the combustion of the latter, was i n all respects parallel t o t h a t obtained from the untreated material. No glow could be obtained from a block of refractory material which had been soaked in a solution of the salt, so t h a t i t must exert its effect by participating in the oxidation process. Inhibitors of t h e reaction CO 1 / 2 0 2 -+- Cot, such as phosphorus oxychloride, were found t o suppress completely t h e glow obtained from carbon treated with copper chloride. ATTEMPTS TO RELATE APPEARANCE OF THE LUMINESCENCE TO EXITGAS C m POSITION. If the glow were associated with the combustion of carbon monoxide, it might be expected t h a t the ratio of carbon monoxide t o carbon dioxide in the exit gases would show a discontinuity Figure 2. Relation between Gasification Rate and Time when the glow became visible. In an Air flow rate, 270 cc./min. (standard temperature and pressure); temperature, 850° C. experiment designed t o investigate this effect, air was passed at 500 cc. per minProperties of the Blue Glow. When a n oxygen-containing ute through a carbon tube heated t o 850' C. at pressures bemixture was passed over the carbon tube heated t o incandescence, tween 3 and 16 cm. of mercury. Gas samples were withdrawn a blue glow was observed for flow rates above a certain value. every 10 minutes for 80 minutes (this period being short enough The intensity of this luminescence grew with increase in flow rate to obviate changes due to consumption of the carbon), and oband the color appeared as a dark blue a t t h e lower rates, changing servations of t h e luminescence were made at each sampling t o a brilliant light blue at a high mass velocity. I n the absence time. The results are shown in Figure 7 . There was a very of any simple method of assessing the intensity of the glow in the marked pale blue glow visible at the pressures indicated, but presence of the hot carbon, attention was confined t o estimating the carbon monoxide t o carbon dioxide ratio varied continuously the critical flow rate (C.F.R.) a t which t h e luminescence could through the transition region. There was no evidence therejust be observed. The ease of estimation of this cutout point fore of a correlation between the appearance of t h e glow and varied with t h e conditions of temperature and pressure, low the analyses of the exit gases, temperatures (700' C.) and high (near atmospheric) pressures being particularly difficult. However, i t was found possible t o investigate the dependence of the C.F.R. on temperature and pressure in such a way as t o demonstrate clearly t h e general form of t h e curves. The latter are shown in Figure 5. A striking result was t h e rapid increase in C.F.R. with decrease in pressure in t h e range 5 t o 10 cm. of mercury. The overlapping at low pressure is due to experimental errors Eiuch as t h e time effect (described previously) and the inaccuracy of visual observation. A glow could be obtained even at very low pressures by t h e use of oxygen instead of air, but the temperatures were very high and uncontrolled under these conditions. An important intensity change in the glow was observed as the 01 I I \ I I I I carbon burned out. The conditions were first chosen so t h a t 0 IO 20 30 40 50 60 70 t h e glow was just visible. It was observed t h a t the glow faded PRESSURE, cms. HQ as t h e carbon was consumed. When the pressure was increased, Figure 3. Carbon Monoxide-Carbon Dioxide t h e glow reappeared, but gradually faded again. The pressure Ratio as Function of Pressure was stepped up several times in this way. Figure 6 shows the Air flow rate, 270 cc./min.; temperature 850' C. pressure adjustments, t h e time being measured from the commencement of t h e combustion. At A , B, C, D, and E no glow was visible; i t had faded after t h e previous increase in pressure. EFFECTSOF WATERVAPORON THE GLOWAT ATMOSPHERIC Observations made with the second apparatus, where the glow PRESSURES. Exploratory experiments showed t h a t entrainment could be observed from above, showed t h a t the glow appeared t o of small amounts of water vapor in t h e air stream quenched out recede into t h e carbon surface during t h e course of combustion. t h e blue glow with all four of the fuels. Most of t h e subsequent This was particularly noticeable during t h e initial stages of comwork was done with the spectroscopically pure graphite and the bustion of spheres of the charcoal. The glow was generally somecompressed charcoal. This combination gave the maximum varwhat easier t o see using carbons of high ash content, such as iation in impurity content (1: 160). charcoal, than with low-ash graphites. The most interesting feature of the results was t h a t t h e critical EFFECTOF INORGANIC SALTSON THE GLOW. Spectroscopic concentration of water required for suppression of the glow was analysis of t h e glow showed not only t h e carbon monoxide flame greater for the charcoal than the graphite by a factor of 20 t o 130. spectrum but also faint bands thought t o be due t o copper chloTable IV gives the critical water vapor concentrations (yoby ride. Pieces of carbon pretreated with cupric chloride gave a volume of the entry air) required for extinction of the blue glow much stronger glow than the untreated material, and sodium for cylinders of graphite and charcoal burning a t t h e temperacarbonate also gave an increased intensity. Tests with sodium tures indicated; air was supplied t o them at the rate of 500 cc. chloride and copper sulfate did not show this effect, which cannot per minute.
dioxide as the pressure is reduced, the amounts of the two gaseb being approximately equal a t the lowest pressures examined. Relation between Gasification Rate and Pressure. The gasification rates were calculated from the run described in the preceding section and are plotted in Figure 4. It will be observed t h a t the gasification rate is substantially independent of pressure in the range indicated.
+
M a y 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
1031
-Fuel
Gasification
The two temperature ranges do not coincide, but over each of them carbon was being gasified a t comparable rates, Po was very large and insensitive to temperature variations in the range given, whereas P, increarjed steadily with temperature, tending t o flatten off a t the highest temperatures used. I
x-
?%
L 10
20 30 4 0 PRESSURE, m y
Figure 4. Independence of Rate of Gasification on Pressure Air flow rate, 270 cc./min.; 850' C.
temperature,
A variation of the air quantity fed t o each of the fuels a t a given temperature led t o parallel variations in Po and P,. Table V shows a few results for the graphite a t 1000' C.
Table IV.
Effect of Temperature on Critical Water Vapor Concentration ( P ) Graphite
Charcoal
P,, %-
Temp., O C . 867 925 922 1025
0.021 0.058 0.102 0.190
Pc, % '
Temp.. C. 708 750 792 807
of oxygen, at 2 em. of mercury, significant amounts of oxygen escape reaction at all sampling times. The main factor contributing to the insensitivity is the development of the reactivity of the carbon as it burns due to "etching" of the internal surfaces. Much other evidence of appreciable internal burning of fuel particles has been obtained in these laboratories ( 1 )and a striking demonstration of the movement of gas into the interior of burning particles has been given by the pressures measured by Crone and Bowring (8). The sampling of relatively large quantities of carbon monoxide at a working pressure of 2 cm. of mercury, which is large by comparison with the premures used in carbon filament experiments ( B O ) , is probably mainly due t o the high stieaming velocities used a t the low pressures. The decreaking residence time together with the increase in mean free path would cleaily tend t o stabilize the primary formed carbon monoxide. The inhibition of carbon monoxide combustion as the pressure is lowered means that more oxygen becomes available for carbon gasification. However, the gasification rate was independent of pressure so that some opposing influence must be a t work. On the evidence presented this cannot be identified with certainty, but a possibility arises fiom the following considerations. Experimental evidence is accumulating for the belief t h a t the carbon-oxygen and wkt carbon monoxide-oxygen reactions have some common feature in their mechanism. For example, Arthur and Bowring ( 4 ) and also Letort and Martin ( 1 8 ) have observed that inorganic catalysts of the carbon-oxygen reaction increase both the carbon dioxide-carbon monoxide ratio in the product gases and the rate of gasification of carbon. The former authors also found some evidence, that a t 600" C., there was a parallel between the extents t o which these effects were observed.
2.99 2.96 2.87 2 85
700-
Table V.
Effect of Air Rate on P ,
(Temperature, 1000° C . )
Air Rate, Cc./XIin. 500
600-
Po 0.10 0.28 0.54
750
1000
i
sooU
w
With both fuels, P remained constant for a large proportion of the life of each particle, although the intensity of the glow appeared visibly t o fade. Some observations were made on the rate of gasification of the graphite (0.1% ash) under conditions where ( a ) the glow was present and ( b ) it was extinguished by 1% by volume of water vapor in the air stream. Particles were continuously weighed as they burned for these experiments. At 1000" C. and with an air flow rate of 1500 cc. per minute, the gasification rates were identical in the presence and absence of the water vapor. This result was confirmed by analysis of the exhaust gases which contained a very high ratio of carbon dioxide t o monoxide. An attempt was made t o measure the concentration of carbon dioxide (introduced with the air) required t o quench the glow. The amounts required were so large that they could only be measured approximately because of the incipient cooling effect of this gas. It appeared t h a t about 17% carbon dioxide (by volume of the air) was required with the spectroscopically pure graphite a t 1050' C. in an air stream of 1000 cc. per minute.
Discussion The rates of gasification of carbon are insensitive t o the amounts of carbon present during about the first 2 hours of the runs represented in Table I11 and Figure 2. Although this arises in part a t 40 em. of mercury from the extensive depletion
1032
2400-
LL
s 0
2 300-I
Q
0
E200a
U
loot 01 0
I
IO
I
20
I
30
I
I
40 50 PRESSURE, c m s . H g .
I
60
I
70
Figure 5. Relation between Critical Flow Rate for Appearance of the Glow and Pressure Moreover, it has been found ( 3 ) that phosphorus oxychloride, a powerful inhibitor of the carbon monoxide-oxygen reaction, also retards the solid-gas reaction under certain conditions. From much earlier work Baker ( 6 ) considered that intensive drying slowed down the solid-gas reaction, and Bangham and Stafford ( 7 ) found t h a t carbon subjected t o the influence of atomic hydrogen is subsequently more reactive toward oxygen. Again, the recent work of Jones and Townend ( 1 6 )admittedly under very much milder conditions than those used in the present work,
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
4-Fuel has shown t h e importance of moisture in the formation of peroxides on coal and carbon surfaces These several observations suggest t h a t some elementary hydrogen-containing spepies (simpler than water) participates not only in the carbon monoxide-oxygen reaction but also in t h e solid-gas reaction, and this provides a clue to the pressureindependence of the, gasification rates of carbon. For, on this basis, any change which slows down the carbon monoxide-carbon dioxide reaction may be expected t o reduce the concentration of the necessary hydrogen-containing species which are normally maintained in sufficient supply for the solid-gas reaction by t h e branching cycle of t h e gas phase reaction.
I-1
i;
I
'z
Ol
I
I
Figure 6.
I
2 3 TIME IN HOUR+
I
,
4
I
5
Appearance and Fading of the Glow
No glow was visible at A, B, C , P,and E
The observations relating to the blue glow support the view t h a t it is associated with the formation of carbon dioxide from carbon monoxide and it is clear that its presence or absence when large quantities of carbon dioxide are being formed is due t o a variation i n the manner in which the energy of the reaction CO COzis dissipated. The emmision of heat and light from a carbon monoxide flame is recognized t o be a complex process; Kondratjew ( 1 6 ) and Gaydon ( 1 1 ) have identified the band spectrum of burning carbon monoxide as due t o an emission from excited carbon dioxide, and more recently Hornbeck (14) has shown t h a t in carbon monoxide-oxygen explosions the SchumannRunge bands of oxygen are observed. The excitation of the oxygen was attributed t o the reaction
*
*cos*+ 0 2 --+ cos + 0 2 *
Garner (10) and Kondratjew ( 1 7 ) have found that the emission of infrared and visible radiation respectively from carbon monoxide flames is diminished when water vapor is present. The decrease in the infrared emission is attributed by Garner t o the deactivation of vibrationally excited carbon dioxide molecules. Kondratjew states t h a t the reduction of light emission by moisture cannot be explained quantitatively in terms of quenching of electronic4ly excited carbon dioxide but must be due t o a change in the mechanism of combustion. Both these effects were probably operative with the carbon in the present case although only visual observation of t h e glow was made. The observation t h a t the copper chloride bands appear in the spectrum of the glow is not unexpected in view of Gaydon's earlier observations ( 1 2 ) of the ease of excitation of these bands in carbon monoxide flames. [Other bands may also be observed, depending on the nature of the inorganic impuritieb in t h e carbon ( $ I ) ] . It seems clear t h a t the excitation of copper, chloride depends on t h e prior formation of energized carbon dioxide since i t is shown here t h a t the very strong glow obtained from carbon treated with cupric chloride was quenched by phosphorus oxychloride addition t o the oxidizing gas. May 1952
Gasification-
So far discussion of the glow phenomena has been confined t o those observations which are mainly a confirmation of earlier work with carbon monoxide flames where the carbon dioxide was not freshly formed and where carbon surfaces were absent. Some of the other observations, however, are peculiar t o solid fuel systems in which the carbon monoxide is being produced and burned in close proximity t o a carbon surface. Thus evidence was obtained t h a t although the glow faded as the particles burned away, the amount of water vapor required to suppress the glow remained constant as long as the gasification rate of carbon was approximately constant. This seems t o imply that some of the water quenches excited carbon dioxide in a region invisible t o the eye. Of course, when the gasification rate of carbon falls considerably, the glow is obviously expected t o fade. The increase in pressure from low values (Figure 6) presumably increased the intensity of the glow due t o the promotion of the carbon dioxide-carbon monoxide ratio in the product gases (Figure 3)-that is, the secondary reaction goes more nearly ' A similar explanation may be applied t o underrequired to produce a visible of mercury (Figure 2), though it is evident that the rate of increase of C.F.R. with pressure does not parallel the slower change of the carbon monoxide-carbon dioxide ratio in the eame pressure range. I n the absence of detailed spectroscopic studies, no satisfactory reason can be given for the result t h a t the quantity of water vapor required t o suppress the glow was widely different (Table IV) for the charcoal and t h e graphite. The relative values of t h e hydrogen contents of the two fuels do not provide a basis for accounting for the differences.
.
I
I
> x L
BLUE CLbW
"t BLUECLOW
0
I I 5 IO PRESSURE, cmr. H g
*
I
15
Figure 7. Carbon Monoxide-Carbon Dioxide Ratio at Various Pressures between 3 and 16 Cm. Hg Air flow rate, 500 cc./min.; carbon temperature, 850° C.
The different variations of Poand P, with temperature (Table IV) is expected: With the highly reactive charcoal, the rate of gasification is mainly controlled by oxygen velocity rather than by temperature even at quite low temperatures (500' C.); with the graphite, the temperature continues t o be of importance u p t o about 1000O C. The increase in P, and P, with air velocity (Table V) is probably due merely t o the increased rate of gasification of carbon (and hence of excited carbon dioxide formation).
Conclusions The glow is associated with the oxidation of carbon monoxide. Whether it is visible or not when large quantities of carbon dioxide are being formed depends on t h e way in which the heat of reaction is dissipated. The concentration of water vapor is significant in determining the presence of the glow. The air velocity and its pressure are important variables in determining the completeness of burning carbon monoxide.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1033
-Puel
GasificationAcknowledgment
The Authors are grateful to the director-general of British Coal Utilisation Research Association for permission t o present this paper.
Literature Cited Adama, A. M., and Kramers, W. J., private communication. Arthur, J. R., Nature, 157,732 (1946). iirthur, J. R., Trans. Faradag Soc., 47, 164 (1951). Arthur, J. R., and Bowring, J. R., IND.ENG.CHEM.,43, 528 (1951).
Arthur, J. R., and Bowring, J. R., J . Chem. Soc., 1949, Sl. Baker, H. B., J. Chem. Soc., 1885, 349. Bangham, D. H., and Stafford, J., Ibid., 1925,1085. Crone, H. G., and Bowring, J. R., Symposium on Combustion of Carbon, Nancy (1949). Davis, H., and Hottel, H. C., IND. EKG.CHEM., 26,889 (1934).
Garner, W. E., et al., J. Chem. Soc., 1929, 1123; 1930, 2037; 1932, 129; 1935, 144. Gaydon, -4. G., PTOC. Roy. Sac. (London),A.176,505 (1940). Gaydon, A. G., "Spectroscopy and Combustion Theory,'' p. 79, London, Chapman and Hall, 1948. Hirst, W., and Cannon, C. G., J . S c i . Inatiuments, 20, 129 (1943). Hornbeck, G., "Third Symposium on Combustion. Flame, and Explosion Phenomena," p. 501, Baltimore, WilliaJns and Wilkins, 1949. Jones, R.E., and Townend, D. T. A., J . Soc. Chem. Ind., 68, 197 (1949). Kondratjew, V., 2. Physik, 63,322 (1930). Kondratjew, V., et al., J . Phys. Chem. (U.S.S.R.), 11, 331 (1938). Letort, hl., and Martin, J., Bull. soc. chim. France, 1947, 400. Parker, A. S., and Hottel, H. C., IND.ENG. CHEW,28, 1334 (1936). Strickland-Constable, R. F., Trans. Faraday Soc., 40, 333 (1944). Whittingham, G., Fuel, 19,244 (1950). RECEIVED for review July 31,1951.
ACCEPTED M a r c h 8,1952.
Reactions of Artificial Graphite Kinetics of Oxidation of Artificial Graphite at Temperatures of 425" to 575" C. and Pressures of 0.15 to 9.8 Cm. of Mercury of Oxygen Earl A. Gulbransen and Kenneth F. Andrew WESTINGHOUSE RESEARCH LABORATORIES, EAST PITTSBURGH, PA.
A systematic study has been made of the chemical reaction of p u r e artificial graphite with oxygen at temperatures of 425" to 5 7 5 " C. a n d at pressures of 0.15 to 9.8 cm. of mercury of oxygen a n d with carbon dioxide at temperatures of 500' to 900' C. a n d 7.6 cm. of mercury of carbon dioxide. T h e naturg of the resulting surface oxides has been investigated. T h e oxidation data are correlated with the fundamental postulates of the activated state theory of chemical reactions on surfaces. Oxidation rate data can b e fitted to the empirical CtZ, where K a n d C a r e constants a n d equation W = K t t is the time. T h e initial rate constant, K, follows a n exponential ratelaw as a function of the temperature, K = Z e - E I R T . A n energy of activation of 36,700 calories per mole is calculated for E. As a function of the pressure K follows the equation K = A BP. T h e effect of pretreatment on the rate of reaction with oxygen has been investigated. T h e formation of a surface oxide with oxygen a t 500 O C.is a gradual a n d not a simultaneous process. Surface roughness as determined by the adsorption of krypton a t liquid nitrogen temperatures can b e correlated to the value calculated from
the extent of the surface oxide formation. Heating to 950" C. increases the surface roughness, as does oxidation at 500' C. Oxidation at room temperature decreases the surface roughness. T h e absolute value for the reaction rate of graphite is determined from surface roughness measurements a n d kinetic data. T h e fundamental postulates of the activated state theory of chemical reactions on surfaces are discussed. Theoretical rate expressions for the oxidation of graphite a r e compared with experimental rates of reaction. Two adsorption processes, immobile adsorption with dissociation and mobile adsorption, are shown to b e possibly the rate-controlling processes for the oxidation of pure graphite. T h e reaction of graphite with carbon dioxide a t 500' C. a n d 7.6 cm. of mercury of carbon dioxide corresponds to the formation of one fortieth or less of a monolayer of surface oxide. Elemental iron greatly accelerates this reaction. Preoxidation is found to change the initial rate of reaction with only a minor effect on the long-term reaction. Results show that the surface oxide that is observed on degassing is not a preliminary step in the reaction.
T
order to avoid limitation of the reaction by transport of the reacting gas to the surface and the reaction products away from the surface, to avoid temperatures where the reduction of carbon dioxide by carbon becomes feasible thermodynamically, to avoid conditions where the slow heterogeneous wall reaction becomes important (28) and t o avoid the tip of the low pressure explosion peninsula of carbon monoxide and oxygen (28). Slthough a number of methods may be used for the study of the reaction kinetics, it occurred to the authors that a sensitive balance operating in a high vacuum system (14, 15) would be particularly appropriate for studying the oxidation kinetics on strip specimens of pure graphite.
+
+
HE primary chemical reaction of graphite with oxygen is of interest from both a scientific and a technical point of view. If the direct chemical reaction can be studied by a suitable choice of material, method, and experimental conditions of temperature, pressure, and pretreatment. fundamental information can be obtained on the kinetics and mechanism of the reaction, including the nature of the primary reaction product. Such information is important from a technical point of view in t h e design and use of equipment and the choice of conditions for burning solid commercial fuels. To study the primary reaction a t normal pressures it is necessary t o study the reaction a t temperatures we11 below 700" C., in 1034
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 5
