Ignition Temperature
of Coke and Air Required to Support Combustion important, however, to have a comparative measure of theignition and burning properties of cokes such as this test furnishes. For example, one can judge from the results the comparative ease with which cokes in domestic heating appliances can be ignited and what draft is required to keep the fire burning. In the survey (7) of the gas- and coke-making properties of American coals which is being carried out by the Bureau of Mines, cokes made a t carbonizing temperatures of 500” to 1100” C. (932” to 2012” F.) under carefully controlled conditions using coals covering the entire range of coking rank were available for study. Results of studies of electrical conductivity (6) and reactivity (9) were available for comparison. Data are presented in this paper on: (a) effect of carbonization temperature, (b) rank of coal, and (c) volatile-matter content of coke on the ignition temperatures and air requirements for combustion. Table I shows the source, analyses, and rank (I) of coals from which the cokes used in this studvwere obtained. Practically the entire range in rank of coals suitable for coke making is covered.
Kindling Properties
of Coke C. R . HOLMES A N D J. D. DAVIS U.S. Bureau of Mines Experiment Station, Pittsburgh, Pa.
T
HE ignition temperature of a combustible: is that temuerature to which it must be raised for active combustion to be self-sustaining (4). The temperature o b s e r v e d experimentally will vary with the conditions of the experiment so that any given method of test must be followed rigidly in order that the results may be comparative. What is wanted is a comparison of the ease of ignition of different cokes; the actual temperatures observed are defined by the conditions of the empirical method used. After the coke in the ignition test is thoroughly ignited, the air supply required to support combustion, called the “critical air blast” by Blayden, Noble, and Riley (3) and the “minimum air rates” (M. A. R.) by the Bureau of Mines, may be detmmined by cutting o f f the auxiliary heat and reducing the air supply in steps until combustion ceases. This test also is empirical. It is
0
~
Kindling properties of cokes and their draft requirements are of particular interest in the development of suitable fuels for domestic heating. These properties, as measured by methods described, vary with the carbonizing temperature a t which the coke is made and with the rank of coal used. The relation found for reactivity, volatile content of the coke, and cell space t o ignition temperature and draft required is also shown. Results are given for five coals covering the range in rank of those used for coke making. Ignition temperature and draft required increase with carbonizing temperature with approximate regularity. No consistent and simple relation of rank of coal to ignition temperature and draft was found. Some relationship between these properties and the volatile content of the coke, reactivity, and cell space could be traced, but the relationship is in no case definite over the whole carbonizing-temperature range. 484
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Apparatus The apparatus used was a modification of that employed in the Koppers R e s e a r c h Laboratory a t Pittsburgh. Figure 1 shows the s e t - u p , e x c e p t for electrical connections : The ignition t u b e was of fused s i l i c a , glazed inside, 20 inches (50.8 cm.) longand I'/z inches (3.8 cm.) inside d i a m e t e r . In the bottom was inserted a Transite plug through which passed the two terminals to the heating e l e m e n t a n d a brass tube to a d m i t air. The p l u g w a s cemented in place. A g r a t i n g of n i c k e l chromium wire gauze was connected in place as shown. The top of t h e t u b e was closed with a No. 9 rubber stopper. Three holes were bored in it, one in the center to admit the thermocouple well and one on each side for charging. A f t e r charging, one of these h o 1e s was stoppered, and the o t h e r w a s connected to a limewater seal. T h e thermocouple well was a piece of thin-walled silica tubing about */* i n c h (6.4 mm.) 0. d. and just large enough to take a f o u r - h o l e clay insulator. T h e ignition tube was ins u l a t e d for approximately the length of the heater as shown. The rest of the apparatus for drying and regulating the air consisted of a p r e s s u r e regulator, an absorption tower filled with sodium h y d r o x i d e sticks, another f i 11e d with granular a n h y FIGURE 2. ELECTRIC HEATER drous calcium chloride, a flowmeter.a manometer, and a trap bottle containing a thermometer. Figure 2 shows the electric heater: Two 1-inch (2.5-cm.) aIundum disks were bored centrally and cemented onto the ends of a 6/la-inch (8-mm.) clay tube about 6 inches (15.2 cm.) apart. Six coils of No. 20 nickel-chromium wire of 0.64 ohm per foot (2.1 ohms per meter) resistance, in series, were placed between the alundum disks, The coils were held in place and supported by clay insulators as used for thermocouple insulation. The nickel-chromium terminals were silversoldered to the brass leads entering through the Transite plug. The heater was held centrally in the ignition tube by means of a silica tube thrust through the clay tube, on top of which was cemented a erforated alundum disk as shown. The bottom end of the si&a support was thrust loosely into the end of the brass air tube. This heater gave good service throughout the
485
series of tests and had sufficient capacity for the ignition temperatures encountered. The electrical connections are shown in Figure 3: The temperature in the fuel bed was measured simultaneously with two thermocouples. One thermocouple was connected t o a Leeds & Northrup recording potentiometer. The second thermocouple was connected to a Brown controller pyrometer. This instrument, originally designed for automatic furnace control at a constant desired temperature, was modified so that it increased the furnace temperature at a constant rate. It contains a current interrupter actuated by the galvanometer pointer, and the position of the interrupter relative to the temperature scale can be adjusted to any desired point by means of a screw. In adapting the instrument to the present purpose, the adjusting screw was provided with a suitable clock-driven pulley that advanced the position of the interru ter over the temperature scale at the rate of 9' C. (16.2" Fg per minute. The comparatively small current through the galvanometer interrupter was connected t o a relay which shunted an adjustable resistance in the furnace heating circuit. Since the couple connected with the controller and that connected with the recording potentiometer occupied the same position in the fuel bed, the reading of the former checked that of the latter. Controller readings were recorded at regular intervals and later compared with the potentiometer chart.
Procedure The following procedure was adopted: The coke sample was ground to pass a 10-mesh and remain on a 20-mesh sieve, the same size used in the reactivity tests. The grate was then covered with a aj'e-inch (4.8-mm.) layer of silica brick crushed t o 8-10 mesh, the thermocouple well was put in place resting on the brick, and the measured volume (75 ml.) of coke was charged into the ignition tube alternately through the two holes in the stopper. After charging, one of the holes was stoppered and the other connected t o the limewater seal. When in readiness the test was started. Air was supplied at 7 cubic feet (200 liters) per hour. The ignition temperature was considered as that' point at which the temperature increase in the fuel bed departed rapidly from the 9" C. per minute rate. When this point was reached, the heater was turned off, Duplicate tests were run wherever possible. A limit of 10" C. (18" F.) could be obtained easily between duplicates. Following is the manner in which the minimum air rate necessary to support combustion was determined.
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TABLEI. ANALYSESOF COALS Analysis, As-Received, -ProximateMois- Volatile Fixed HydroBed Mine ture matter carbon Ash gen County Flat Top 1.2 27.9 55.0 15.9 4.6 Mary Lee Jeff erson 4.2 27.6 59.9 8.3 5.1 Flat Top Mary Lee Jefferson 1.9 33.6 57.0 7.5 5.2 Edenborn Pittsburgh Fayette 7.9 32.1 47.7 12.3 5.1 Orient No. 1 Franklin No. 6 4.6 38.8 50.6 6.0 5.7 Carbon Lower Sunnyside Columbia 6.7 5.8 Green River 10.1 36.2 47.0 Muhlenberg Green River 4.8 4.3 Pocahontas No. 4 Consolidation NO.251 0.8 15.4 79.0 M cD owell 75.8 2.1 4.9 1.3 20.8 Cranberry No . 3 Raleigh Sewell 2.4 5.2 1.9 26.5 69.2 Summerlee Fayette Sewell 5.4 5.3 Warden 1.8 35.1 57.7 Allegheny Pittsburgh
Coal No. State 7 Ala. 8 Ala. 9 Pa. 10 Ill. 19 Utah 21 Ky. 23 W.Va. 26 W . V a . 27 W.Va. 28 Pa.
OF IGNITION PROPERTIES, GREEN TABLE11. REPRODUCIBILITY RIVERCOKES
Carbonizing Temp. C. (" F.) 500 ( 932) 500 ( 932) Average
(One ooke sample at each test temperature) Min. Min. Ignition Air Carbonizing Ignition Air Temp. Rate Temp. Temp. Rate C.(" F.) Cu. f t . / h r . a C. (" F . ) C. (" P.) Cu. f t . / h r . 350 (662 1.15 900 (1652) 494 921) 3.60 356 (6731 1.15 900 (1652) 500 1932) 3.60 353 (667) 1.15 Average 497 927) 3.60
600 (1112) 600 (1112) Average
406 (763 400 (7521 403 (757)
1.60 1.50 1.55
1000 (1832) 1000 (1832) Average
550 (1022) 556 (1033) 553 (1027)
4.25 4.25 4.25
700 1292) 700 11292) Average
430 (806) 428 (802) 429 (804)
2.05 2.05 2.05
1100 (2012) 1100 (2012) Average
605 (1121)
4.60 4.75 4.68
800 (1472) 800 (1472) Average
460 (860) 456 (853) 458 (856)
2.45 2.50 2.48
............ ..
TABLE111. REPRODUCIBILITY OF IGNITION PROPERTIES OF GREENRIVER (COALNo. 21) COKES (Independent carbonization tests and sampling) Carbonizing Temp. c. ( 0 F.) 500 ( 932 500 ( 9321
Carbonisation Test No
Min.
Volatile Matter in Dry Coke P e r cent 10.9 10.8
21-15 21-13
Ignition Temp. O C. (" F . ) 353 ( 667) 353 ( 667)
600 1112) 600 11112)
21-14 21-12
416 403
{ 781)" 757)
1.45" 1.56
6.3 5.9
700 1292) 700 11292)
2 1-9 21-10
447 429
837 8O4la
2.005 2.05
2.8 2.0
800 1472) 800 11472)
2 1-5 21-8
:% I %la
2.48 2.48
1.6 1.3
900 (1652) 900 (1652)
21-2 21-1
498 ( 928)" 497 ( 927)
3.15" 3.60
0.7 0.5
1000 (1832) 1000 (1832)
21-4 21-3
562 1044)a 553 [1027)
4.05 4.25
0.4 0.3
Q
Air
Rate Cu. f t . / h r .
1.02 1.15
65;: [:2"?.",'
4.85" 1100 2012) 21-7 4.68 1100 12012) 21-11 0 Single determination, all others average of two teats.
Per Cent Ultimate Car- Nitrobon gen 71.4 1.5 76.2 1.6 77.4 1.6 65.5 1.4 72.9 1.5 66.8 1.5 86.7 1 . 1 86.8 1.7 84.6 1.6 79.0 1.6
Oxy- Sulgen fur 5.8 0.8 8.0 0.8 7.3 1.0 14.9 0.8 12.9 1.0 16.7 2.5 2.6 0.5 3.7 0.8 5.7 0.5 7.8 0.9
Rank High-volatile A High-volatile A High-volatile A High-volatile B High-volatile B Hinh-volatile C Low-volatile Low-volatile Medium-volatile High-volatile A
carbonization test a t each temperature was used; there was
no duplication of sampling. Table I11 shows the variation found when cokes from the duplicate carbonization tests were sampled and tested independently. The variations in the latter case are the wider, including, as they do, those arising from sampling and from carbonization. We might expect some lack of agreement between samples because a sized sample is used which might contain more of the soft, easily ignitable coke in the one case than in the other. The indication is that the ignition temperature test does measure approximately an inherent property of the coke; whether this property should be called ignition temperature or not is immaterial for purposes of comparison so long as it is defined by the method of test. In duplicate tests similar agreement to that of the ignition temperature tests was obtained for the air flow required to support combustion; the minimum air rate decreased with the ignition temperature.
Results of Tests Figures 4 and 5 show the ignition temperatures and minimum air rates of cokes obtained by the carbonization of several coals a t various temperatures, plotted against the carbonizing temperatures at which the cokes were made. Both the ignition temperature and minimum air rates approach straight-line functions of the carbonizing temperature; the slopes of the curves, however, vary somewhat for different coals, and the slope for a given coal in Figure 4 is not always
0.3 0.2
Sipce it was found that the coke could not be allowed to burn at the standard air rate of 7 cubic feet per hour used in the ignition temperature test because the temperature developed in the combustion zone overtaxed the temperature-measuring apparatus, and, since the combustion zone traveled up through the coke too fast for the more reactive cokes, it was necessary to cut back the air rate to a point safely above the extinguishing point. The air supply was then .decreased at such a rate that there was a uniform decrease of 5" C. (9' F.) per minute in the fuel bed temperature. The lowest air rate that would maintain this rate of temperature decrease was taken as the minimum air rate that would support combustion. The results usually agreed to within 0.2 cubic foot (5.7 liters) per hour. The flowmeter was provided with two interchangeable orifices and calibrated against a standard wet-test meter. The calibration curves for the orifices gave a change of 0.1 to 0.2 cubic foot per hour for each 0.1 inch on the flowmeter scale.
Reproducibility of Results Table I1 shows the variation between duplicate tests for the Green River series wherein samples of coke representing one
'C. 'F. CARBONIZING TEMPERATURE OF IGNITION TEMPERATURE TO CARBONFIGURE4. RELATION IZINO TEMPERATURE
INDUSTRIAL AND ENGlNEERING CHEMISTRY
APRIL, 1936
487
VOLATILE MATTER IN DRY COKE. PERCENT
FIGURE 6. RELATION OF VOLATILE MATTER IN DRYCOKE TO IGNITION TEMPERATURE
932
1112
1292
1472 1652 CARBONIZING TEMPERATURE
1832
"C. 2012'F.
FIGURE5. RELATION OF CARBONIZING TEMPERATURE TO MINIMUM AIR FLOWNECESSARY TO SUPPORT COMBUSTION the same as that for the same coal in Figure 5. We may conclude, therefore, that, while both properties are nearly proportional to the carbonizing temperature, the constants of proportionality may differ for different coals and between the properties themselves for the same coal. The order of increasing rank for the coals is: (1) Green River, (2) Illinois No..6 and Lower Sunnyside, (3) Warden, Edenborn, and Mary Lee, and (4) Summerlee, Cranberry, and Pocahontas. Classification according to rank as applied to these coals (1) is based on dry, mineral-matter-free, fixedcarbon content for coals whose beating value on the moist mineral-matter-free basis is 14,000 B. t. u. or more; for coals of lower heating value the basis is moist mineral-matter-free B. t. u. We might expect the ignition properties to vary with the rank of the coal, because in general the low-rank coals are the most reactive chemically; that is, a low ignition temperature and a low minimum air rate might be predicted for cokes from a coal of low rank. However, such a result was not obtained for the coals of this series. For example, the Warden coal gives cokes averaging a higher ignition temperature and a higher minimum air rate than the rest, and it lies midway of the rank range.
Volatile-Matter Content and Ignition Temperature of Cokes Figure 6 shows that for low- and medium-temperature cokes the elevation of the ignition temperature with decrease in volatile matter content is slow but for the high-temperature
1 572-300
\
\
. 50
cokes where the volatile content is below 2 per cent the elevation is rapid. The fact that the points do not all lie on a smooth curve indicates that some other factor (or factors) besides volatile matter influences the ignition temperature of the coke.
Ignition Temperature and Reactivity Figure 7 shows the ignition temperature of the cokes from the several coals plotted against their reactivity to carbon dioxide. For each curve the points from left to right follow the order of decreasing carbonizing temperature, beginning a t 1100" C. (2012" F.) and descending at 100" C. (180" F.) intervals to 500' C . ( 9 3 2 O F.). For cokes made a t 800" C.
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crease in cell spaces has only a slight effect in raising the ignition temperature.
Conclusions 1. For a given-size sample of coke, ignition temperatures are reproducible to 10" C. (18" F.), and the minimum air rate required to sustain combustion is reproducible to 0.2 cubic foot (5.7 liters). With coke from duplicate carbonizing tests made under the same conditions, variation between ignition temperatures may be 20" C. (36" F.), and the minimum air rate may vary accordingly. However, with carbonizing and sampling variables thus included, the test is still amply sensitive to indicate a difference in carbonizing temperature of 100" C. (180" F.) 2. For cokes made from a given coal a t different temperatures, the ignition temperature and minimum air rate increase with the carbonizing temperature with approximate regularity. 3. No consistent and simple relation of rank of coal to ignition properties of the cokes was observed. 4. The ignition temperature is nearly a straight-line function of the minimum air rate. TESTING A DOMESTIC COKE-BURNING FURNACE 5 . For high-temperature cokes the ignition temperature tends to vary regularly with the reactivity; for medium- and low-temperature cokes the rela(1472" F.) and above, the tendency is for the ignition temperationship no longer holds. ture to vary regularly with the reactivity; the Cranberry coal is a n outstanding exception. For the medium- and low-tem6. Up to 56 per cent cell space in the coke, the amount of cell space appears to have little effect on the ignition temperaperature cokes there is variation in ignition temperature corresponding to little or no variation in the reactivity. For examture. With cokes of more than 56 per cent cells, decrease in cell space is accompanied by increase in ignition temperature. ple, the reactivity of the 500" C. (932"F.), 600°C. (1112"F.), and 700" C. (1292' F.) Summerlee cokes hardly varies a t all, whereas the ignition temperaturevaries about 75" C. (135" F.). Literature Cited For the Illinois No. 6 coal the reactivity of the 500" C. Am. Soc. Testing Materials, Rept. Tech. Comm. on Nomencoke is even lower than that of the 600" or 700". The stateclature, Proc. A . S. 2'. M., 34,Part I, 475-7, 834-40 (1934). ment appearing in the literature to the effect that the ignition Am. Soc. Testing Materials, Standards for 1933, Part 11, Nontemperature varies with the reactivity (8) and is a measure metallic Materials, p. 337. Blayden, H.E.,Noble, mi., and Riley, H. L., Gas J . , 205, 201 thereof is approximately true for high-temperature cokes, as (19341. these data show, but for low-temperature cokes the relation Brown, C., FueE, 14, 14 (1935). does not hold. The writers' attention has been called to an Bunte, K.,and Windorfer, K., Gus- u. Wusserfuch, 78, 697-701, article by Bunte and Windorfer (5) on ignition temperature 720-5,737-43 (1935). and reactivity of coke appearing since the present paper was Davis, J. D., and Auvil, H. S., IND.EKG.CHEM.,27, 1196-1200 (1935). read. The methods used by Bunte and Windorfer were similar Fieldner, A. C., and Davis, J. D., Bur. Mines, Monograph 5 to those of this paper and their results are in agreement for (1934). cokes made a t high temperatures. That is, there is a rough Meleer, M., G l ~ c k a u f 66, , 1565 (1930). Reynolds, D. A., and Davis, J. D., IND.ENQ.CHEM.,Anal. Ed., relationship between reactivity and ignition temperature for 8, 33-6 (1936). these cokes, as Figure 7 shows. Bunte and Windorfer did not experiment with low-temperature cokes where the writers RECEIVED October 9, 1935. Presented before the Division of Gas and Fuel found that such a relation no longer holds. Chemistry at the 90th Meeting of the American Chemical Society, San
.
\ - - - - I
Effect of Coke Cel) Space o n Ignition Temperature Figure 8 gives the ignition temperatures of the 800" C. (1472" F.) cokes arranged in the order of decreasing percentage of cells in the corresponding cokes, cell space being determined by the standard method of the American Society for Testing Materials (3). The general trend is for increasing ignition temperature with decreasing cell spaces. The coke from coal 28 (Warden) is out of line. Ignition temperature tests for another charge of the coke substantiated the results reported here, so that clearly there are exceptions to the general trend. For cokes with cell spaces above 56 per cent, the ignition temperature increases with a decrease in cell spaces. For cokes with cell spaces below 56 per cent, the de-
Francisco, Calif., August 19 to 23, 1935. Published by permiaaion of the Director, U. S. Bureau of Mines. (Not subject to copyright.)
OXIDE XEASURED ISTO CXN OF CRUSHED DRY-ICE FROM TASK BY MEANS OF GLSS GAGE ETIiYLESE
