Mechanism of Combustion of Individual Particles of Solid Fuels

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March, 1931

INDUSTRIAL AND ENGINEERING CHEMISTRY

277

Mechanism of Combustion of Individual Particles of Solid Fuels’” David F. Smith3 and Austin Gudmundsen3 PITTSBURGH EXPERIMENT STATION, U. S. BUREAUOF MINES, PITTSBURGH, PA.

A new approach has been made t o a fundamental UNDAMENTAL data of particle and furnace gases, study of the composition of solid fuel. The effect of (c) temperature of furnace on the rate of comparticle size, air velocity, temperature, and humidity bustion of single pargases and furnace walls, and on the rate of burning of carbon particles has been ( d ) humidity of furnace atticles of solid fuel, burned in investigated under carefully controlled conditions. suspension, are meager. The mosphere. The information The method involves bringing accurately shaped and few investigations tbus far sought was the effect of the weighed spheres of carbon to a known temperature in made on the subject are of foregoing items on (1) the a n inert gas. suddenly blasting the particle with a n general interest because they rate of burning of the fuel air stream of known velocity and temperature for a deal with the c o m p o s i t e particles and ( 2 ) the tempersvery short interval, quickly quenching the particle in effects of many variables. ture of the particles while inert gas, again weighing t o determine the amount of Most noteTYorthy of these inburning. To control item (b) reaction, and obtaining careful records of particle vestigations are the two conthe particles were held stasurface temperature during the combustion. In all ducted a t the Pittsburgh Extionary and the flow of air cases, the specific surface-reaction rate increases with periment Station of the U. S. through the furnace was reguincreasing particle surface temperature. This temBureau of Mines. ‘The first lated. The holding of parperature coefficient and the specific-surface reaction was a study of the relative ticles s t a t i o n a r y also perrate are enormously larger for the smaller particles flammability of clouds of coal mitted their temperatures to than for the larger particles. The rate, of course, and of semi-coke dust, (d), and be measured most readily. increases with increasing air velocity. the second was a study of the The results obtained were accurately reproducible Description of Apparatus effect of p a r t i c l e size and and throw considerable light on the mechanism of furnace temperature on the one of the most important industrial reactions-the The apparatus used contime of burning of pulverized combustion of carbon. sists of thefollowingelenients: coal, semicoke, and activated (1) a furnace tube of clear charcoal ( I ). I n the latter investigation, wherein particles of the dust were photographed as silica glass, (2) electric-heating elements, (3) mechanism they burned in still air, it was clearly indicated that the process for introducing particles into the furnace, (4)air-conditionof combustion of coal particles could be separated into two ing apparatus, ( 5 ) an air flowmeter, (6) a pyrometer for distinct stages: first, the distillation and burning of the indicating particle temperature, and (7) a rotating-drum volatile matter; second, the burning of the residual carbon, camera. FURNACE-The furnace proper (Figure 1) consists of a or coke. An examination of the photographs of burning coal particles in still air a t 1000° C. clearly shows this divi- clear silica-glass tube 1/2 inch (1.3 cm.) inside diameter and sion. The time required to burn the fixed carbon was ap- 48 inches (122 cm.) in length. A branch is provided near proximately 90 per cent of the total burning time for the the top end to permit exit of the furnace gases when the top coal particles. As the burning of the fixed carbon is so portion of the tube is closed. The electrical resistance heatimportant in the matter of time required, a study of the ing element is in three sections. The top section comprises inch (1.3 cm.) mechanism of combustion of the fixed carbon was considered a double spiral winding, the tape being fundamental and was made the subject of the present in- in width and 0.01 inch (0.25 mm.) in thickness. This is secured to the inside of an alundum sleeve 3/4 inch (19 mm.) vestigation. In any furnace burning powdered fuel the burning processes inside diameter and 10 inches (25.4 cm.) in length. Windows are complicated by many variables, the major ones of which 3 / ~inch (9.5 mm.) in diameter are provided near the top of may be listed as follows: this section to permit an unobstructed view through the clear silica-glass furnace tube. The double spiral winding (1) Physical and chemical characteristics of the fuel. permits the windows in the alundum tube to be diametri(2) Composition of furnace atmosphere. cally opposite each other without interfering with the re(3) Relative velocity of fuel particle and furnace gases. sistance tape. The window construction is shown in Figure 1, (4) Proximity of adjacent particles and surfaces. (5) Temperature of furnace atmosphere and of adjacent section A-A . particles and surfaces. The middle and bottom sections of the heating element (6) Character and amounts of intermediate and ultimate products of thermal decomposition and of products of combus- are 12 and 15 inches (30 and 38 cm.) in length, respectively. tion. Each comprises a single spiral winding, the tape being inch (3 mm.) wide and 0.02 inch (0.5 mm.) thick. These I n the present investigation, which deals with the com- elements are also secured to the inside of alundum sleeves bustion of substantially pure carbon, the controllable vari- 3/4 inch (1.9 cm.) in diameter. Each of the heating eleables relate to (a) size of fuel particle, (b) relative velocity ments is independently controlled. A maximum of 25 amperes a t 20 volts is required for the top section, 6 amperes a t Received November 20, 1930. Presented before the Division of Gas and Fuel Chemistry at the 80th Meeting of the American Chemical 110 volts for the middle, and 10 amperes at 110 volts for Society. Cincinnati, Ohio, September 8 to 12, 1930. the bottom section. A transformer controls the 20-volt * Published by permission of the Director, U. S. Bureau of Mines, circuit, and rheostats are used in the 110-volt circuits. Carnegie Institute of Technology, and the Mining Advisory Board. (Not Thermocouples are embedded in the alundum tubes of subject t o copyright.) a Present address, A. 0. Smith Corp., Milwaukee, Wis. the bottom and middle sections. The temperature a t the

F

P

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INDUSTRIAL AND ENGINEERING CHEMISTRY

top section is indicated by a thermocouple inside the furnace tube a t a point just below the windows. The furnace assembly is encased in a sheet-metal shell, 6 inches (15 cm.) in diameter and 40 inches (102 cm.) long, and the space between the heating elements and the shell is packed with Sil-0-Cel. The construction permits the silica-glass furnace tube to be readily withdrawn or to be adjusted to insure a suitable clear portion at the windows, The bottom portion of the tube is filled with a bed of broken porcelain to insure a rapid rate of heat transfer from the heating elements to the gas passing through the tube.

n

Vol. 23, No. 3

property is utilized in securing the supporting wire to the particle, the object being to prevent too intimate thermal contact between the particle and the wire. AIR-MEASURING AND CONDITIONING APPARATus-Figure 2 is a diagrammatic sketch of the furnace and auxiliaries. The air-drying train consists of two sulfuric acid bubblers and a trap. The air saturator consists of a water bubbler and a trap. The orifice meter measures only dry air or nitrogen. MEASUREMENT O F PARTICLE TEMPERATURE-hl Optical pyrometer is used for measuring the particle temperature. It is placed opposite the furnace windows, as shown in Figure 2, and may be readily sighted on a particle suspended inside the furnace. FUELUSED AND METHODO F PREPARATION-The fuel is electrode carbon of the kind used in arc carbons for projection lanterns, and is of the following composition: %

-

Moisture Volatile matter Fixed carbon Ash Specific weight, mg. per

0.0 0.5

98.9 0.6

cu. mm.

1.36

Of the forms of nearly pure carbon available, it was thought that in respect to uniformity of compoiition and density this material is perhaps the best for fundamental studies. The carbons were 5 mm. in diameter and 6 inches (15 cm.) long. Figure 3 shows the apparatus for making accurately turned carbon spheres. It consists of a steel ball race adjustably mounted on a base plate, and a steel grinding:disk with a Carborundum face. The disk is secured to the spindle of a small bench drill press, and the base plate with the adjustable race is mounted on the bed plate of the press.

hi

Optical pyrometer

Figure 1-Furnace for Studying the Mechan i s m of Combustion of Individual Particles of Solid Fuel

MECHANISMFOR INTRODUCING FUEL PARTICLE-The mechanism used for introducing the fuel particle into the furnace is shown in Figure 1 a t the top of the furnace proper. A slender porcelain stem 5 mm. in diameter and 12 inches (30 cm.) long is suspended from a cross member which slides on fixed, vertical guides. The stem, to which a fuel particle may be attached through its supporting mechanism, is lowered into the furnace, being kept in alignment by a porcelain sleeve which is fitted in the top of the furnace tube. An adjustable stop on the vertical guides permits the accurate placement of the particle in alignment with the furnace windows. SUPPORTING MECHANISM FOR FUELPARTICLE-A carbon sphere with its supporting mechanism is shown in Figure 1. A hole 0.2 mm. in diameter is drilled into the particle to a depth slightly past the center. A platinum wire 0.1 mm. in diameter and 3 cm. long is cemented in the hole with a material known as “Insalute.” This material is a pasty mixture when applied, and when dried slowly and fired a t 1000° C. it produces a solid, porcelain-like substance. However, when it is dried quickly in a flame, the structure is very porous and would appear to be a poor heat conductor. This

I Air saturator

Air d n i n g train

Figure 2-Apparatus for Studying the Mechanism of Combustion of Individual Particles of Solid Fuel

Roughly shaped carbon spheres are introduced into the ball race, which is then adjusted to place the particles in contact with the plane surface of the rotating grinding disk. By holding the race eccentric with respect to the center line of the grinding disk, the balls are given a combined rotating and sliding action against the grinding face, with the result that accurately sized spheres are produced. The spheres are drilled and the supporting wires cemented in, as previously described. To remove all moisture and volatile matter, they are then placed in the experimental furnace in an atmosphere of nitrogen and maintained a t a

March, 1931

INDUSTRIAL AND ENGINEERING CHEAIISTRY

temperature of 1000" C. for 30 minutes. Each particle is weighed and filed in an individual glass vial. Description of Experiments PRELIMINARY EXPERIMENTS-It was obvious a t the beginning of the investigation that the influence of the supporting mechanism on the burning of supported fuel particles might be important. A detailed study of this item was undertaken ($), with the following conclusions:

(1) The size of wire influences the time required t o ignite the particle. (2) The smallest wire practicable is 0.1 mm. in diameter. Wire smaller than this is so flexible that the particle cannot be held in position a t the high air velocities. (3) W!th furnace temperatures below 900' C. the initial period of ignition exerts a marked influence on the burning time of the particles and interpretation of results is diliicult. (4) The supporting mechanism may have a serious influence on the burning of the particle in the last stages, where the particle is small compared with the diameter of the wire. ( 5 ) The minimum size of particle that may be studied with reproducible results is approximately 1.0 mm. in diameter

METHOD-A method that minimized the difficulties introduced by the supporting mechanism was finally adopted. The particles were not completely burned in one operation, but were consumed in stages by a process of intermittent burning. At the termination of the last stage the particle was still of appreciable size compared with the wire diameter. To reduce the time for ignition at the beginning of the burning period the particle was first heated to furnace temperature in an atmosphere of nitrogen. This method gave reproducible results for furnace temperatures of 900" C. and above. The apparatus used is that shown in Figure 2. It will be noted that either dry or wet air, or nitrogen, may be passed through the furnace. As the nitrogen was not absolutely pure, the experimental method involved a correction for loss in weight of the particle while it was in a nitrogen atmosphere A typical experiment using dry air will now be described. With the furnace temperature regulated to the desired value, nitrogen is passed through the furnace tube a t a velocity equal to the contemplated air velocity. A weighed fuel particle is introduced into the furnace and permitted to remain for 30 seconds. It is then transferred from the hot zone into the cool stem a t the top of the furnrtce tube, remaining there 1 minute, It is then withdrawn and weighed. The loss in weight determines the effect of small amounts of oxygen in the nitrogen supply. The correction is small, being of the order of 0.1 to 0.2 mg. for a particle weighing 70 mg. The particle is again introduced into the furnace, with nitrogen passing through as before. Thirty seconds are allowed for it to reach furnace temperature. The nitrogen is then shut off and air admitted at the same rate. These events are practically simultaneous because a single movement of the two-way stopcock switches from nitrogen t o air. The air may remain on for 15 to 30 seconds. Another movement of the two-way cock switches from air to nitrogen, which quenches the burning. The particle is immediately withdrawn from the hot zone into the cool stem a t the top, allowed to cool for 1minute, and then withdrawn and weighed. The reduction in weight, corrected for nitrogen losses, is recorded. The process is repeated until the diameter of the particle is reduced to about 1.0 mm. The surface temperature of the particle is determined a t the middle of each burning period. The optical pyrometer is focused on the particle while the particle is coming up to temperature in the nitrogen, so that a very quick observation can be made when the particle burns. In the course of the investigation supported electrodecarbon spheres weighing 2 to 70 mg. were burned with both

dry and wet air (saturated a t 24" C.) and a t furnace temperatures of 850°, goo", 950", and 1000" C. At the last temperature the air velocities ranged from 3 to 40 feet (91 em. to 12.2 meters) per second. The experiments were performed by the intermittent burning method described. The data obtained for each furnace temperature and air velocity (for both wet and dry air) were (a) loss in weight for small time intervals (15 to 30 seconds) as the particles were reduced in weight from 70 mg. to a minimum of 2 mg., and (b) temperature of the particle surface during the period of burning. From these data the following relationships have been derived: (I) The effect of particle size on (a) surface temperature acquired by the particle and ( b ) specific surface-reaction rate (for each furnace temperature and for various air velocities). (2) The effect of surface temperature of particle on specific surface-reaction rate (for various particle sizes and air velocities). (3) The effect of air velocity on the specific surface-reaction rate (for various particle sizes and surface temperatures).

I n addition to the foregoing experiments a method of preparing fixed carbon particles with known ash content and a technic for testing the same Tvere developed. (See Appendix.) Experimental Data

A condensed summary of the data is given in Table I. The original data were plotted as curves to show graphically the manner in which the carbon particles lose weight over a burning period comprising seyeral stages. Only a few of these curves are presented here.

CarbNndurn disc

. %* * i

I

'

1"xZOthds.

1

L&l Fiqure 3-Device

for Making Carbon Spheres

Figure 4, for a furnace temperature of 1000" C. and for a moderate air velocity, shows how pure-carbon spheres lose weight and how the temperature varies when burned in wet and dry air. It will be observed from curves 1 and 2 that four 30-second burning periods were employed. The two points indicated a t the end of the curves show the total loss in weight when particles were burned in but one 120-second interval. The close agreement of the results for intermittent and one-step burning is evident. The surface temperature of the particles for intermittent and for one-step burning is also plotted, the latter curves being practically straight lines. Data for plotting the surface-area curves were calculated from the weight curves. It was assumed that the particles remained spherical throughout the burning period. Observation has proved the assumption to be justifiable within close limits, particularly for moderate air velocities. At high air velocities a particle tends to burn flat a t the bottom, but it is considered that the error thus introduced in computing surface area from weight is small.

INDUSTRIAL AND ENGINEERING CHEMISTRY

280

FUR-

Table I-Summary ~ E L O C I T Yow

AIR I FUENACE

NACE TEMP.

- 27 c. 1000

Wet air"

Fret per se6.b 4.63 4.78

of Experimental Data

0

69.8 51.3 35.6 21.8 9.5

MQ. 69.8 52.0 37.7 24.5 12.8

1000 1234 1262 1292

1000 1308 1336 1360

..

ture difference for dry- and wet-air conditions became less as the size of the particle decreased. An increase in the air velocity had a similar effect. On considering the foregoing data it should be remembered that the same quantity of air per unit time is introduced into the furnace for either dry- or wet-air conditions. Dry air is metered in both cases. As the volume of mixture is greatest in the case of wet air, the velocity through the furnace tube is slightly higher for wet air than for dry.

0

68.6 37.6 14.8 2.5

68.6 40.0 19.2 4.9

1000 1335 1370 1404

1000 1385 1405

Computation of Specific Surface-Reaction Rates and Comparison of Results

30 50 70

71.2 32.8 14.9 2.9

71.2 35.2 17.3 5.3

1000 1360 1382 1404

1000 1425 1446 1480

To compare results for the various air velocities, furnace temperatures, and sizes of particle, the data were reduced to a basis of specific surface-reaction rates. The term "specific surface-reaction rate" or "reaction rate" used herein is defined as the rate of removal of carbon per unit area of surface. The units employed are milligrams per square millimeter per second. The reaction rate may be expressed

WEIGHTOB TEMP.OF P A R T I TIME NET PARTICLE CLE SURFACE AQTER IGNI-

TION

Sec.

30 60 90 120 14.87

15.34

30 60 90 28.68

950

850

a s

b 1

0

j 2

Mg.

WFt air

2 O

c. ..

Wet air

a

c.

..

43.59

44.95

0 15 35 50

70.0 43.1 18.3 4.6

70.0 45.0 21.7 8.8

1000 1391 1426

1000 1445 1486

4.46

4.60

0 30 60 90 120

62.7 42.8 26.3 13.4 3.8

62.7 44.4 29.2 16.6 6.6

950 1250 1270 1290 1310

950 1300 1315 1330 1348

7.69

900

29.56

7.92

Vol. 23, No. 3

0

..

..

30 60 90 120

61.7 38.5 20.5 7.4 0.2

61.7 41.3 24.1 10.9 2.2

950 1280 1302 1322

950 1330 1345 1265

..

..

14.32

14.75

0 30 60 90

58.7 30.1 10.3 0.3

58.7 33.0 14.1 2.5

950 1318 1348

959 1345 1360

41.92

43.25

0 30 50 70

77.1 31.8 12.2 1.2

77.1 34.0 14.5 2.0

950 1375 1408

950 1390 1415

4.27

4.41

0 30 60 90 120

57.8 39.6 24.6 12.5 3.5

57.8 40.5 26.0 14.2 5.2

900 1220 1240 1260

900 1280 1295 1310

.. .. ..

.. ..

..

13.72

14.50

0 30 60 90

57.5 31.8 13.2 2.3

57.5 33.4 15.5 4.0

900 1268 1295 1322

900 1340 1368 1392

26.45

27.26

0 30 60 90

74.7 39.1 14.3 0.9

74.7 39.7 15.7 3.2

900 1300 1320

900 1340 1370

40.18

45.45

0 20 40 60 75

75.1 47.5 23.8 9.5 2.6

75.1 48.3 25.0 10.7 3.7

900 1302 1335 1370

900 1335 1360 1380

4.63

4.78

0 30 60 90 120 140

67.0 50.6 35.5 22.9 12.0 5.1

67.0 63.6 40.1 27.1 14.8 7.1

850 1160 1170 1180 1190

850 1210 1230 1248 1262

-

..

..

..

..

..

..

irated over water at 24' C. obtain meters per second multiply by 0.305.

For purposes of comparison Figures 5 and 6 are presented. Figure 5, which is for the same furnace temperature as Figure 4 but for higher air velocity, shows the increase in rate of burning due to increasing air velocity. Figure 6 shows the effect of lower furnace temperature on the rate of burning and on the surface temperature of the particles. It will be seen that in this case there is a large difference in results for the intermittent and single-step burning. Delayed ignition at the low furnace temperature used could cause this. It was concluded that reliable results could not be obtained by this method for furnace temperatures below 900" c. An examination of all the data for all the furnace temperatures and air velocities used shows that electrode-carbon spheres lose weight more rapidly when burned in dry air. The surface temperature of a particle, however, is higher for wet air. In all cases it was observed that this tempera-

as = -1 _dm

k a

A dt

where dm representsa small loss in weight in time dt, when the mean surface area is A . As it did not seem practicable to obtain equations for each of the numerous weight-time curves, a more direct method was sought. It appeared that fairly accurate results could be obtained by using finite values of At and the corresponding values of Am from the curves. As the weight-time curves have little curvature, fairly large values of At were possible. Also, because the area-time curves have almost no curvature, the mean surface area over the time interval At could be obtained with considerable accuracy The specific surface-reaction rates, as functions of the mean area of the particles, are plotted in Figures 7 to 10, inclusive. The surface temperature of the particles is also plotted against mean area. The curves include the results for furnace temperatures of 1000", 950°, goo", and 850" C., and for air velocities (on a basis of 24" C.) between 1 and 11 feet (30 to 335 cm.) per second. Data for both dry and wet air are included. From these curves relationship of reaction rate to surface temperature of particle was determined and these data are plotted in Figure 11 for dry air, and in Figure 12 for wet air. Curves are plotted for particles having surface areas of 10, 20, 30, and 50 sq. mm. In each case the effect of temperature on reaction rate is shown for the several air velocities used. By a moderate extrapolation of the curves in Figures 11 and 12, the relationships between reaction rate and air velocity were determined for each of the particle sizes for a number of surface temperatures. The relationships are shown in Figure 13 for dry air and Figure 14 for wet air. Velocities are given on a basis of air a t 24" C., because these data are derived from tests with various furnace temperatures.

.

Combined Effect of Particle Area, Air Velocity, Temperature, and Humidity

The reaction-rate curves in Figures 7 to 10, inclusive, clearly indicate an increase in specific rate of burning as the particles become smaller, for both dry- and wet-air conditions. I n the region of smallest area the curves become very steep, indicating an enormous reaction rate for very small particles. It will also be noted that the curves for dry and wet air consistently tend to converge in the region of small areas, indicating that the effect of moisture on reaction rate is not pronounced in the case of the small particles.

INDUSTRIA11 A N D ENGINEERING CHEMISTRY

March, 1931

281

9

3 P E

0

2 cc

E

3

BURTISG TIME. SECONDS

Figure 4 Figure 5 Relationship of Weight and Surface Temperature t o T i m e of Burning.

The general effect of increasing air velocity on reaction rate is evidenced by a shifting of the curves upward for each increase in air velocity. As a rule the shape of the curves is similar, but a t 1000" C. furnace temperature as the velocity increases the reaction-rate curves begin to rise sooner than for the lower velocities. The critical velocity in the furnace tube a t this temperature is approximately 40 feet (12 meters) per second, so that flow of air past the particle is turbulent in the case of the highest (44 feet or 13.4 meters per second), air velocity. The effect is seen in Figure 7 by comparing reaction-rate curves 2 and 8. The tendency is not so pronounced for a furnace temperature of 950" C., and it has disappeared altogether a t 900" C. The temperature curves in Figures 7 to 10, inclusive, are numbered to correspond to their respective reaction-rate curves. It may be observed that the temperature difference between dry- and wet-air conditions has a tendency to decrease as the air velocity increases, and is also different for the various furnace temperatures. This tendency is shown in Table 11. Table 11-Effect

of Air Velocity an Surface-Temperature Differences for Wet and Dry Air

~~

~

SURF~

-

FURNACE TEIIPERATLRE AT

1 O

c.

1000 930 900

C TEMPERATURE E DIFFERENCE FOR

DRYAIR

L O W E S T AIR VELOCITY

50 sq. mm. O

10 sq. mm.

c.

I 1

A T HIGHEST AIR VELOCITY

50 sq. mm.

c. 45 45

~~

WET A N D

20 40

10 sq. mm. O

c. 70 10 20

It appears that the surface-temperature difference between wet and dry air is consistently smallest for the highest air velocity. The relationship between surface temperature and area is observed to be substantially a straight line in all cases. This is in accord with the previous observation that the temperature-time curves and the area-time curves, in Figures

Figure 6 Electrode-Carbon Spheres

4 to 6, approximate straight lines. On the assumption that the surface temperature bears a direct relationship to the surface area, the curves of reaction rate to area were investigated to determine if a logarithmic relationship existed between reaction rate and temperature, but in no case was this observed to be so; the relationship appears to be of a more complicated order. EFFECT O F SURFACE TEMPERATURE-The CUrVeS in Figures 11 and 12 show the effect of surface temperature of particles on the reaction rate for several particle sizes and several air velocities for dry and wet air, respectively. It will be seen that the curves are arranged in groups. For example, curves 13, 14, 15, and 16 in Figure 11 are all for the same particle size but for different velocities. It is shown in Figure 11, for dry air, that an increase in surface temperature results in an increase in reaction rate for all the particle sizes. It will be further observed that this positive temperature coefficient is most pronounced for the small particles, also a t the higher surface temperatures. The highest surface temperatures were produced when the furnace temperature and air velocities are highest, and when the particle is smallest. A very marked increase in reaction rate for the 10- and 20-sq. mm. particles (curves 4 and 8) is noted for a small increase in temperature in the region of highest temperatures. The largest positive temperature coefficients are all associated with high air velocity, small particle size, and high surface temperature. The curves in Figure 12 for wet air are quite different from those in Figure 11 for dry air. The effect of air velocity on the temperature coefficient is more pronounced. For example, in Figure 12, curve 1, for a 10-sq. mm. surface area and the lowest air velocity, indicates a pronounced negative temperature coefficient; curve 2 for a higher velocity shows only a small negative coefficient; curve 3 for the next highest velocity shows a small positive temperature coefficient; and curve 4 for the maximum velocity shows a very pronounced positive temperature coefficient. A similar, though much less pronounced tendency is shown by curves 5, 6, 7, and 8 for the 30-sq. mm. surface area.

282

INDUSTRIAL A N D ENGINEERING CHEMISTRY

NEAS AREA

Vol. 23, No. 3

OF 3PHEP.E. S C A R E MILLIIIFIERS

Figure 8

Relationship of Reaction Rate and Surface T e m perature to Size of Particle, at Various Air Velocities. Electrode-Carbon Spheres

Figure 9

EFFECT OF AIR VELocrTY-The effect of air velocity on reaction rate for dry and wet air is shown in Figures 13 and 14. The data in each curve are for constant surface temperature and particle size, which permits observation of the effect of air velocity only. I n Figure 13, for dry air, it is seen that a small increase in air velocity in the low-velocity range produces a larger increase in reaction rate than does a

similar increase in the high-velocity range. It is important to note that the reaction rate is highest for small particles, even though the surface temperature be the same as that for the large ones. This holds for surface temperatures of both 1300" and 1400" C. It would indicate that size of particle itself has an effect on reaction rate, independently of other variables, I n other words, for the same air velocity

March, 1931

I N D U S T R I A L A N D ENGINEERING CHEMISTRY

TEIlPER.ATURE OF PARTICLLSLXFACE 'C.

283

TFXPER4TCRF. O f PARTICLESCRF.XE

'c.

Figure 11-Dry Air Figure 12-Wet Air Relationship of Reaction Rate t o Surface Temperature of Particles for Various Air Velocities and Size Particles. Electrode-Carbon Spheres

and surface temperature a small particle burns faster than a larger one. In Figure 14, for wet air, the effect of air velocity is shown to be somewhat different than for dry air. I n the case of small particles (10-sq. mm. area) an increase in air velocity markedly increases the reaction rate when the surface temperature is 1400" C. For a surface temperature of 1450" C. the effect is even greater. EFFECTOF MOISTIJRE-A full explanation of the effect of moisture on the mechanism of the reactions involved in this problem will require further work. However, the following explanation of the effect on surface temperature and reaction rate is offered as a reasonable hypothesis consistent with the observed facts: It will be assumed that the mechanism of the oxygencarbon reaction is that suggested in the "carbon-complex" theory proposed by Rhead and Wheeler (5) and substantiated by Langmuir ( 3 ) . According to this theory the initially formed surface complex of carbon and oxygen breaks down to form carbon dioxide as the evident primary product of combustion, with but small amounts of carbon monoxide. Any large amount of carbon monoxide present is assumed to result from the reduction of carbon dioxide in the adsorbed gas layer on the carbon surface. It may be considered, then, that a t the bottom of a spherical particle of carbon the adsorbed layer, or film, is composed largely of carbon dioxide, oxygen, and nitrogen. The carbon ' dioxide on contacting with the carbon surface from bottom to top would have a good opportunity of being reduced to carbon monoxide. The longer the time of contact, other things being equal, the more carbon monoxide will be formed. As the reduction of C 0 2to 2CO results in an increase in volume of gases adjacent to the surface, and as the film thickness is a constant for a given mass velocity of the gas past the particle, carbon monoxide must be swept from the surface. As

a CO-COzdry air mixture lean in carbon monoxide is known to burn very slowly, it is possible that secondary combustion takes place at some distance above the particle, where the heat liberated would have little effect on raising its temperature. With water vapor present, however, the steam-carbon reactions will take place. The most probable reactions are C

+ HzO = CO + Hz

and

CO

+ HzO = HZ+ COn

As a mixture of carbon monoxide and hydrogen burns many times faster than dry carbon monoxide alone, it is probable that much of the heat of combustion from the burning of this mixture would be liberated adjacent to the particle surface and would thus be effective in raising its temperature. Even if the steam-carbon reactions did not occur, we might still explain the more rapid burning of carbon monoxide adjacent to the particle from the known catalytic effect of water on this reaction. However, in later experiments, where wet air saturated at 0" C. instead of a t 24" C. was used in the furnace, the high surface temperatures previously observed when using air saturated a t 24" C. were not found; in fact, there was very little difference between results for dry air and air saturated a t 0" C. This would indicate that small amounts of moisture do not produce the same result as do larger quantities. If the shielding effect of the secondary reactions, using wet air, is considered, the foregoing explanations might also account for the lower reaction rate with moist air. Not only would the burning of the carbon monoxide and hydrogen mixture adjacent to the particle tend to prevent the diffusion of oxygen into the carbon surface, but there is also a possibility that the mechanism of the steam-carbon reaction itself may be such as to inhibit the diffusion of oxygen to the carbon surface. The explanation is suggested from the experiments

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of Thiele and Haslam (6) on the mechanism of the steamcarbon reactions. They propose that a stable surface com, , complex, is formed in the plex, similar to the so-called CO steam-carbon reactions. The shielding effect of this stable film might a c c o u n t for the smaller reaction rate with wet

air. As both the steam-carbon r e a c t i o n s and the reaction C COe = 2CO are affected by time of contact, we might

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ticle size. It is enormously larger for a small particle than for a large one. (2) The temperature of the surface increases as the particle is reduced in size. (3) At the lower velocities, below about 4 feet (122 cm.) per second as measured a t 24" C., the reaction rate increases rapidly with increase in velocity, independent of other variables. This decreases continually as the velocity increases until in the high-velocity range a large increase in velocity increases the reaction rate only slightly. (4) Size of particle, independent of other variables, has an effect on specific surface-reaction rate; that is, for the same air velocity and the same surface temperature, a small particle has a higher specific surfacereaction rate than does a larger one. (5) Carbon spheres of surface area 70 to 10 sq. mm. burn faster in dry air than in moist air. (6) The surface temperature is higher when the particles are burned in moist air. (7) An increase in surface temperature, for particles burned in dry air, increases the reaction ratethat is, the temperature coefficient is positive. The maximum temperature coefficient is associated with small particle size, high air velocity, and high surface temperature. (8) An increase in surface temperature for particles burned in wet air at low velocities decreases the reaction rate, but for higher velocities the reaction rate increases with rise in surface temperature. (9) A mechanism explaining the observed effects of water vapor on the combustion of electrode-carbon spheres burned in wet air is suggested.

VELOCITY OF AIR (24'C I. FEET P E R SECOND

Figure 13-Dry Air Relationshi of Reaction Rate t o Air Velocity for Various Surface T e m peratures a n f Size Particles. Electrode-Carbon Spheres. Furnace T e m perature, f r o m 850° to 1000' C.

expect any factor that reduces the time of contact also to reduce the surface-temperature difference for dry and wet air. It is of interest to note, therefore, that the surfacetemperature difference was smaller for the high air velocities and for the small particles, both of which factors reduce the time of contact. '

Conclusions

It is possible that in some respects the data obtained in this work may be characteristic of the particular methods and apparatus used. To the extent that this is true the data may thus be lacking somewhat in fundamental significance. It is thought, however, that when interpreted in the light of the conditions under which they were obtained, the conclusions are in the main reliable. As the writers are not likely to continue this work, it seemed desirable to publish the results, together with such interpretation as opportunity has permitted, although the latter has not been all that might be desired. The work is being continued in the Bureau of Mines by other investigators. It is planned to extend the study to higher temperatures, different furnace atmospheres, and other forms of carbon. It is hoped, therefore, that others interested in this field may be able to use some of the information here presented as a basis for further study. The following conclusions relating to the burning of electrode-carbon spheres in dry and moist air may be set down as a result of this investigation: (1) The specific surface-reaction rate for carbon spheres burned in dry and moist air is a complex function of the par-

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APPENDIX

Preparation a n d Testing of Fuels w i t h Known Ash C o n t e n t

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PREPARATION-&fixing. Electrode c a r b o n is g r o u n d to pass through a 200-mesh s c r e e n . A quantity of a known ash to give a definite ash content is added and the mixture is thoroughly ground. A high-temperature coal-tar pitch is used as a binder in the proportion of 25 parts pitch t o 100 parts coal plus ash. The whole is thoroughly mixed by grinding in a m o r t a r . This is important because a coherent product may not be obtained unless a complete mixing is effected. After this dry mixing, just enough light oil is added to moisten the batch, and this is further mixed until a uniform, moist product is obtained. Molding. The mold used is a thick-walled steel tube a / l ~inch (4.8 mm.) bore and l l / t inches (38 mm.) long. The fuel mixture is packed Bill w m * c in the mold by tamping lightly. A Figure 15-Relationship of loose-fitting steel plunger 2 inches Weight a n d Surface Tempera(5 cm.) in length is then pressed ture t o T i m e of Burning Elecinto the mold with the aid of a vise. trode-Carbon Spheres (with 12 Per Cent Ash). Ash-Sof-, With the mold in the vise and a tening Temperature, 1190' C. m o d e r a t e pressure applied, it is heated until the pitch melts. Pressure is then applied to about the limit of strength of the plunger. On relieving the pressure somewhat, the mold is then heated t o redness. On cooling, a sleeve is placed over the open end of the mold, which is again placed in the vise and heated red hot while the pellet is pressed out. If the pellet is pressed out with the mold cold, it will stick and be shattered from the excessive pressure required. Pellets made by this method are as firm liCOYili

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INDUSTRIAL AND ENGINEERING CHEMISTRY

and hard as the original sticks of electrode carbon, and may be shaped into spheres just as readily. BURNING THE PARTIcLEs-On burning particles of fuel having an appreciable ash content it was a t once evident that the accumulation of ash on the particle surface affects the rate of burning. The intermittent burning method of testing is therefore more difficult than for ash-free fuels. First, a comparison of results must be made between particles of identical initial dimensions. This does not apply t o ash-free fuels, because interpolation of the weight-time curves can be made between different particle sizes. The accumulation of ash on the surface of the particle prevents this in the former case. The technic illustrated by the following typical experiment has been used with seemingly satisfactory results, although sufficient data have not been obtained t o warrant any but the most general conclusions: A particle of initial weight 64 mg. was burned in three steps each of 30 seconds duration. After each step the ash was carefully removed before weighing the particle; therefore, a clean carbon surface was presented a t the beginning of each 30-second burning period. A second particle bf identical initial dimension was then burned for 60 seconds, and another for 90 seconds. Before weighing these particles after the burning period, the ash adhering to the surface was removed, so that the loss in weight during the burning interval represents carbon plus ash. On plotting these data, the weight-time and temperature-time curves appear as shown in Figure 15. DIsCuSsION-It will be seen, by comparing curves 1 and 2 in Figure 15, that accumulation of ash on the particle surface does slow down the rate of burning. It will also be observed that

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the surface temperature of the particle decreases with time instead of increasing as when a pure carbon particle burns. The nature of the ash accumulation will no doubt have an effect. For example, in the foregoing experiments with a fuel having a fusible ash (1190' C.) the ash accumulation consisted of very loosely packed, small sphere clusters of fused ash over the particle surface. There were numerous bare spaces between the ash spheres that still permitted burning to proceed a t a fairly rapid rate. When particles having a refractory ash (1600' C.) were burned, the ash was not fused, but adhered t o the surface as a dense blanket which slowed down the rate of combustion very materially in the latter stages. In fact, when a particle weighing 70 mg. was burned in one step to a point where it appeared to be completely consumed and only the ash husk remained, examination disclosed a considerable core of unburned carbon.

Literature Cited (1) Griffin, H. K.,Adams, J. R., and Smith, D . F . , IND. Ewc. CHEM.,21, 808 (1929). (2) Gudmundsen, Austin, "Mechanism of Combustion of Individual Particles of Solid Fuels," Thesis, Carnegie Institute of Technology, June, 1930. (3) Langmuir, Irving, J . Am. Chem. Soc., 37, 1154 (1915). (4) Reed, Donald, "Relative Flammability of Powdered Low-Temperature Coke and Coal," Thesis, Carnegie Institute of Technology, 1928. (5) Rhead, T.F. E., and Wheeler, R. V., J . Chcm. Soc., 97, 2178 (1910); 99, 1140 (1911); 101, 836, 846 (1912); 103, 461, 1210 (1913). (6) Thiele, E. W., and Haslam, R. T . , "Mechanism of the Steam-Carbon Reactions," Thesis, Massachusetts Institute of Technology, 1927.

Amount of Lubricating Oil Burned in the Gasoline Engine' Clarke C. Minter and Wm. J. Finn THETEXASCOYLPANY, BAYONNE, N. J.

INCE control of mixture ratio by exhaust-gas analysis a thinner film on the cylinder walls, with less accumulation has become quite common, a question often arises on the top of the piston. The relation between viscosity and regarding the error introduced into the calculations the amount of burning obviously cannot be applied to oil 3 by the combustion of an unknown amount of lubricating oil. on account of its gasoline content. That this error is not large is obvious when the ratio of oil Carbon Dioxide in Exhaust Using Hydrogen as Fuel consumed to gasoline consumed is considered, but it is PROPERTIES OF LUBRICATIIG clear also that all the oil consumed is not burned. Since COMPOSITION O F EXHA-CST OILS no data have ever been published showing how much oil OIL H?in excess Air in excess Viscosity actually undergoes combustion in the engine, the writers :;$,' Flash Saybolt at 100" F. coz 0 2 COZ 0 2 settled the point for the conditions of their experiments by running an engine on hydrogen and determining the amount F. Sec. % % % % of carbon dioxide in the exhaust gas. 0.1 0.1 0.3 10.2 0.9242 460 1178 0.1 0.2 0.3 3.7 Commercial electrolytic hydrogen was used in the experi0.0 0.2 6.0 0.1 0.5 3.4 ment. It was found by analysis that carbon dioxide and carbon monoxide were not present, The manufacturers 0.0 0.1 0.2 5.4 0.9340 370 318 0.0 0.0 0.2 4.5 state that very slight traces of hydrocarbons might be present 0.2 0.1 0.1 5.8 0.1 0.0 6.0 0.2 owing to the breaking down of lubricating oil in the com0.3 5.7 0.0 0.0 pressors. 0.2 5,s The hydrogen was led into the intake pipe of a single0.8 3.2 0.9160' 245 937 0 4 4.2 cylinder research engine, which was free from carbon deposits. The engine was lightly loaded and run a t 600 r. p. m. with a a Iso-Vis, contained 10 to 12 per cent of heavy ends of gasoline. spark advance of 22 degrees. The cooling water was kept at 100' C . The fresh oil was put in the cleaned crankcase It was observed that the temperature of the exhaust when of the engine and run on hydrogen until the engine and oil running the engine on hydrogen was about 100' C. lower than attained normal operating temperatures. The exhaust gas when operating on gasoline under approximately the same sample was sampled and analyzed in an Orsat apparatus. conditions. For this reason it is possible that slightly more The results for several ratios of hydrogen to air are shown lubricating oil would be burned when operating on gasoline in the accompanying table. because the oil film in the cylinder would be exposed to A negligible quantity of oil appears to be burned when hy- higher temperatures. drogen is in excess. An oil of low viscosity shows less burning These conclusions obviously should not be applied a t much than an oil of high viscosity, doubtless because less of the higher engine speeds, where the oil consumption sometimes former gets into the cylinders on account of the formation of increases greatly with a possible increase in the amount of oil burned. 1 Received October 22, 1930.

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