COKE REACTIVITY Determination by a Modified Ignition Point Method -
J. J. S. SEBASTIAN AND M. A. MAYERS Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa.
An apparatus has been designed and a method developed for the determination of absolute reaction rates between coke and oxygen by a modification of the usual ignition point method, which requires the measurement of the time rate of temperature rise of the coke at the ignition point. The apparatus is simple and easy t o operate : the relations between reaction rate and temperature calculated from the results are reproducible and characteristic of the cokes, and can be used t o measure their reactivities. The reactivities of fifteen cokes made from
different coals by widely differing methods of carbonization have been determined. The reactivity characteristic of each coke may be specified either by a line on a plot of the Arrhenius equation, or by the two parameters b and E appearing in the equation, whose values usually decrease with increasing reactivity. In general, the reactivity is found t o increase with decreasing temperature of carbonization of the coal. Investigation of the effect of the kind of coal and of carbonization conditions on the reactivity of coke is being continued.
T
determinations of reactivity of cokes ranging from low-temperature cokes prepared under high vacuum to commercial high-temperature cokes are also given. These data illustrate the type of results to be expected and the magnitude of the differences among different cokes.
HE reactivity of coke is one of its important characteristics which may influence its behavior in blast furnaces, gas producers, asd water gas generators, as well as in grate fires and heating boilers (6). Many methods of testing this characteristic have been proposed, involving different arrangements of apparatus, oxidizing atmospheres, and criteria of reactivity, but all those suggested hitherto fail to satisfy certain essedtial conditions di&&sed in a previous paper ( 7 ) . In the present paper the absolute reaction rate of a unit amount of the coke with oxygen a t unit pressure is used as the criterion of reactivity, The reaction rates of each coke over a range of temperatures can be specified by two parameters, the constants in the Arrhenius equation relating reaction rate and temperature, so that a compound number consisting of these two parameters can be used to describe the reactivity of a coke. A modified method of ignition point determination, involving the measurement of the time rate of rise of coke temperature a t the ignition point, was shown in the earlier paper ( 7 ) to permit the calculation of the absolute reaction rate. In common with other measurements of ignition point, this test has the following advantages: (a) The test is fast and easily carried out; (b) the apparatus is simple and does not require high-temperature equipment; ( c ) the coke to be tested is not exposed to temperatures above that a t which it was made, which would change its characteristics while they were being measured; and (d) no allowance need be made for diffusion phenomena'since the reaction rates are so low that the limiting effect of diffusion is negligible. Since to these features the modified method adds the important characteristic that absolute reaction rates can be calculated, making possible an exact comparison between different cokes, the test should be useful as a routine determination of reactivity. This paper describes the apparatus developed for the performance of this test and the experimental verification of the analysis on which the test is based. Results of a number of 1118
Theoretical The method of determining the rate of reaction of carbonized materials with oxygen described in this paper is based on a n analysis given previously (7). Consider the heat balance in the sample of coke whose ignition point is being determined. Referred to unit weight of coke, dT m (Tf - T) dt where c = sp. heat of coke c-=
- n(T - T.) 4-H p p
(1)
t = time T , To,T / = temp. of coke, oxidizing gas stream, and furnace, respectively m = coefficient of heat transfer between furnace and sample n = coefficient of heat transfer between sample and oxidizing gas stream H = heat of combustion p = abs. reaction rate per unit pressure of oxygen p = oxygen partial pressure
If the ignition point is taken as the point a t which the line representing the coke temperature plotted against time crosses the furnace temperature line, andif the apparatusisso arranged that the oxidizing gas temperature is always equal to the coke temperature when it reaches the sample, then a t the ignition temperature,
T
=
To = Tf
(2)
(3)
OCTOBER, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
where, as shown in Figure 1, a is the angle between the tangent to the curve representingcoke temperature at the ignition point, and the time axis.
1119
Apparatus and Materials The apparatus (Figure 2 ) consists of two parallel trains in the same furnace. Each train contains the following units: A is a gasometer of &gallon (19-liter) capacity, supplyin the reaction tubes with oxygen or air; B is a mioropressurer e e f t o r to rovide a constant gas flow; C is a drying tube filled with Angydrone (magnesium perchlorate); D is a flowmeter; E is a reaction tuhe fitted into a hole drilled in F, B vertical cast-iron cylinder heated by a Nichrome winding. The reaction tube contains E I-inch (2.5-cm.) layer of granular sample resting on a platinum gauze plug. The sample has a volume of about 1.3 cc. and weighs ahout 0.75 gram. The tip of a Chromel-Alumel thermocouple extends into the sample t o whatever height is desired. A ground-glass joint at the hottom of the reaction tuhe connects to E glass tuhe with two horizontal aide arms which serve *s out-
FIQQRF. 1. TYPICAL TEMPERATVR=TIME Guans (DATAFROM EXPERIMENTS 57a AND 5%)
0
F
sample in 0IYE.B" A = Ssmple in sir 0 = Furn~~e
This method of determining reaction rates is free of errors due to Musion because the velocity of the chemical reaction required to produce enough heat to satisfy Equation 2 is so small that diffusion does not limit the rate. Moreover, by making two determinations differing in some detail, such as the heating rate of the furnace or the concentration of oxygen in the gas stream (e. g., using air in one case and pure oxygen in another), different values of p a t different ignition temperatures can be secured which can he correlated by the Arrhenius equation for the dependence of reaction rates on temperature: F log. p = b' - I
RT
whew 1'
R b', E
tubes are supported so that the sarnges are in the center of the iron block, lengthwise. A third thermocouple, to meamre the furnace temperature, extends from the hottom into the center of the furnace block. A rotary switch, 0,connects any one of the three thermocounles with a wtentiometer. facilitating rmid
(4)
= ahs. temp. = gas canstant = empirical constants
Thus, if the logarithms of the reaction rates are plotted against the inverse absolute ignition ternperature, and Equations 2,3, and 4 are satisfied, a straight line, whose slope is E/R, should result. This criterion supplies the final test as to whether the theoretical conditions are satisfied. The lines determined by Equation 4, easily identified by the two constants b' and E, are specific characteristics of each coke and permit the comparison of the reaction rates of different cokes at the same temperature or the determination of the temperatures at which different cokes have the same reaction rates towards oxygen. Since l / p tan a is proportional to p (Equation 3), i t may he substituted for p in Equation 4, and the resulting equstion will differ only in the constant 6'. In most of what follows, except where otherwise specified, correlations will be given in terms of l l p tan a-i. e., 1 E log-tana = b - __ (5) P 4.575 T since this eliminates tlieintrodnction of the factor C/H whose value is in some doubt and which changes from one coke to another. For cnnvenience Equation 5 has been changed to common logarithm which merely introduce a constant [actor in 6.
AhoGt 5 wunds (3.3 k . 1 of Fairmont coke marke&d in Pittssuhsequently ground to' the various required sizes. The othe;
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INDUSTRIAL AND ENGINEERING CHEMISTRY
cokes, described in Table 11, were tested in the standard size -40 +60 mesh, which was adopted to secure precision of measurement as described below.
cient oxygen is supplied to maintain theconcentration throughout the column practically uniform at its initial value. In order to determine whether the gas stream was removing heat from the coke, the temperature difference between the Method of Testing center of the furnace block and the coke sample was measured The procedure in making a run is as follows: A sample of as the furnace was heated, first with and then without gas flow. coke is measured into a small glass beaker which is a duplicate I n the former case a layer of -12 +20 mesh pumice was used of the space within the reaction tube. The beaker is tapped instead of coke to prevent the evolution of heat by oxidation; and additional coke added until a space exactly one inch long in the latter case 12 +20 mesh Fairmont coke was used, the is filled to maximum bulk density. This sample is then poured stationary atmosphere around the coke preventing its oxidainto the reaction tube, tion. The temperature reaaings taken which is then tapped and in each 3-minute interval were plotted tamped until the coke against time, and from the curves fills exactly the same drawn the actual temperature differvolume in the reaction ences were read in each 5-minute intube; this v o l u m e is terval. Figure 3 shows that the temgaged by a scratched perature differences, plotted against mark on the tamping the furnace temperature, are the same rod. Two reaction tubes, with and w i t h o u t g a s flow within filled according to the 1.5' C. a t all temperatures except procedure described, are 370' C. Furthermore, the temperathen fixed in place, gas ture differences were identical whether connections are made, oxygen a t 50 cc. or air a t 85 cc. per and the air and oxygen minute, a comparatively high flow are started flowing and rate, was passed through the sample. adjusted to their fixed The coke temperature is always below rates. The current to that of the furnace, the difference bet h e f u r n a c e is then tween them decreasing a t higher temturned on, and frequent peratures. These results together indicate that no heat is lost from the readings are made of the temperatures of b o t h coke to the gas stream, so that the latter must have assumed the coke temcokes and of the furnace, p e r a t u r e before reaching the coke s t a r t i n g f a r enough charge, satisfying condition 2. below the ignition point FIQURE2. DIAGRAMMATIC LAYOUT OF APPARATUS The difference in temperature beso that at least five readtween the furnace and the coke charge ings have been obtained wKen one value of coke temperature well above the furnace a t a nearly constant temperature slightly above the ignition temperature was studied in order to determine whether coke temperature is reached. The usual period between readings is one minute, but for slow heating rates this may be increased. temperature was a sensitive indicator of heat release. Under these conditions the coke temperature was slightly higher than In each case the temperature of the coke in oxygen is read first, the furnace temperature. At any constant temperature of the followed 15 seconds afterwards by the temperature of the coke furnace the temperature differencebetween furnace and sample in air, and 15 secondslater by the furnace temperature. The 15was the same as in runs in which the furnace temperature insecond intervals between each reading are taken into consideracreased continuously; with small increases in furnace temperation when the data are plotted, or the results are calculated. For Equations 1to 4 to apply to the experimental data, the arrangement oi apparatus must be such as t o satisfy the following conditions:
-
1. Sufficient oxygen must be supplied by the oxidizing gas stream so that its concentration throughout the coke coIumn is effectively constant at the ignition point. 2. The gas stream must reach the temperature of the coke charge before it comes in contact with it, so that T , = T . 3. The heat evolved must be immediately indicated by temperature rise and not be dissipated. 4. The temperature of the coke should be uniform when the ignition oint is reached. 5. T i e curves representing coke and furnace temperatures must cross sharply enough to permit precise determination of the ignition temperature and of the time rate of increase of coke temperature.
I-
I
1
1
I
I
I
I
350 40 450 500 550 00 650 250 &%"ACE TEM8ERATURE IN D E G R E B C.
]
FIQUFCE 3. TEMPERATURE DIFFERENCES BETWEEN CENFURNACE BLOCKAND REACTION TUBESDURING HEATING
TER OF
The first condition was satisfied by determining ignition points on the same sample of coke with increasing rates of gas flow through the reaction tubes. When the oxygen flow was increased from a rate of 5 cc. to above 50 cc. per minute, the observed ignition point decreased continuously up to the rate of 50 CC. per minute, above which the ignition point remained constant. I n the same way the ignition point in air remained constant above a flow rate of 85 cc. per minute. These values were adopted as standard for all subsequent work, since the constancy of observed ignition point indicates that suffi-
0 = With gas flow
A = Without ga8 flow
ture, the coke temperature increased 1.6 times as far because of increasing reaction rate. Thus, it is apparent that the evolution of heat is immediately indicated by rise. in temperature. The axial uniformity of the temperature wlthin the mass of the sample was determined in successive experiments with the thermocouple extending 0.02, 0.25, and 0.50 inch (0.5, 6.4, and 12.7 mm.), respectively, into the granular charge Thi
Courtesy, The K o p p e r s Company BL.%ST
FURNACE USING HIGH-TEMPERATURE COKE
it was found difficult to check the ignition points obtained in results of theseexperiments are plotted in Figure 4 for -40 +60 successive runs under apparently the same conditions. In mesh coke, and similar results were obtained for two other the light of the results shown in Figure 5 it was ascertained coke sizes. The values plotted are averages of a t least two that the apparent lack of precision was caused by differences experiments under each condition and were obtained in oxyin heating rate due to small changes in the voltage of the gen. Figure 4 shows that for temperatures up to the ignition laboratory power supply. An increase in the supply voltage point the temperature of the coke mass is nearly uniform, the causedanincrease of the rate of heating which, in turn, inlargest variation observed being 3" C . This satisfies condicreased the observed ignition point. At the higher ignition tion 4. The fact that above the ignition point considerably temperature d T / d t was increased because of its connection larger temperature differences develop does not affect the with the reaction rate, so that both the ignition point and analysis. dT/dt increased with increase of heating rate and supply Radial uniformity of the coke temperature was assured by voltage. This fact makes possible the adjustment of experichoice of reaction tube diameter. Previous work (1) showed mental conditions to secure sharply defined ignition points. that no temperature difference exceeding 2" C. can exist in a These results emphasize the important characteristic of this tube 8 mm. in diameter containing coke heated a t the rates test that strict reproducibility of the observed quantities is used in these experiments. It was found that the sharpness of the crossing point-i. e., not required; the relation between them, represented by the Arrhenius equation, is reproduced even when these quantities the precision of the determination of the ignition temperavary over a wide ranEe. ture-could be controlled by regulation of the rate of heating. In general, to secure a given preciFigure 5 shows that the data for the three sizes fall on nearly parallel lines, sion, unreactive cokes r e q u i r e a the smallest size corresponding to the higher rate of heating than reactive top line. These differences can be ones, but, as shown in the next attributed to the differences in surparagraph, a quantity characteristic face exposed to reaction by the differof each coke is determined whatever the rate of heating; adjustment of ent sizes, and the smallest size shows this variable is dictated solely by the highest reaction rate, as was to considerations of convenience. be expected. The ratios of the reacThe final test of the method is tion rates, whose logarithms are given the correlation of the results obtained by the differences along the ordinates on the same sample of coke under between the curves, are not precisely proportional to the differences of area different experimental conditions by means of the Arrhenius equation. exposed, calculated from the screen For this purpose Fairmont coke of size. However, with a porous matethree different particle sizes was rial this cannot be expected, and reactested in both air and oxygen, and tion rates must be referred to units of weight or volume of a given size of with heating rates of the furnace material in a given state of packing. varying between 6" and 25" C. per It is to be observed that here, as in minute. The results of these tests all cases where only physical differare shown in Figure 5. Regardless of the particular value of ignition ences occur between different samples, the several lines have approxipoint observed, the points always FIGURE 4. COKETEMPERATURES AT VARImately the same slope and differ only fall on or close to the line representOUS POSITIONS WITHIN THE SAMPLE FOR in position. There is considerable ing the reaction rate for each coke DIFFERENT FURNACE TEMPERATURES difference among the three series of size. This fact e x p l a i n e d some Coke size, -40 f 6 0 mesh: data from experiments tests in the degree of scattering of the a,pparently discordant results ob29 t o 32 points about the lines. The serious tained early in the development when A = Sample in oxygen 0 = Furnace 0
t
1121
INDUSTRIAL AND ENGINEERING CHEMISTRY
1122
VOL. 29, NO. 10
determined either by construction or by the use of a Bausch and Lomb tangent meter. It was found that data obtained this way were subject to considerable variation, depending on the judgment of the observer; to eliminate this factor, a n analytical method was developed by H. G. Landau of this laboratory. In this method the five readings of coke temperature nearest the ignition point are fitted by a second-degree equation of the form T
A’
=
+ B’s + c
(22
- 2)
(6)
by the method of least squares. Similarly, the five corresponding readings of furnace temperature are fitted by a straight line of the form
658
597
560
527
496
DEGREES CfNTlGRADE
by the same method, where x represents the time coordinate of unit length, h is the interval between readings, starting from zero closest to the ignition point, and k is the fraction of an interval by which the reading of furnace t e m p e r a t u r e follows the reading of coke temperature. Then the interval a t the ignition point, 20, is found by equating (6) and (7), resulting in a seconddegree equation
468
FIQURE 5. REACTIVITY OF THREQSIZESOF FAIRMONT COKE
-
A = -12 f20 mesh -20 $40 mash 0 = -40 +BO mesh
C/
lack of precision for the largest size coke may be attributed to the poor sampling of temperature by the thermocouple tip in this heterogeneous system where a single particle is several times as large as the thermocouple junction. The calculated probable error of a single observation for the -40 +60mesh series is 5.0 per cent; for the -20 +40 mesh size it is 5.8 per cent, and for the - 12 +20 mesh size it is about 20 per cent. The probable error for the -40 +60 mesh series appears reasonable for a heterogeneous m a t e r i a1 such as coke. In order to secure this precision, all s u b s e q u e n t tests were run on coke sized through 40 on 60 U. S. Standard sieves. Figure 6 shows that the reaction of coke with oxygen is of the first order in accordance with the findings of other investigators (6) and as assumed in Equation 1. The upper full line represents data obtained in pure oxygen; the lower one represents those obtained in air with the same coke. The lines have the same slope and differ by the constant value of 0.675. Equations 3 and 5 show that, if the reaction is of order n, this difference would be n times the logarithm of the ratio of the partial pressures of oxygen in air and pure oxygen. The measured value, 0.675, is the logarithm of 4.73 = 1.00/0.212, Since the concentration of oxygen in atmospheric air is 0.209, it is evident that n, the order of the reaction, is unity within the limits of error. In conformity with this result it is observed that, when the test results are corrected for the oxygen concentration according to Equation 5, the points fall on the dashed line, a collinear extension of the upper line.
Calculation of Results When this method was first developed, graphical interpolation was used to determine the i g n i t i o n p o i n t , and tan a waa
(B’
- B)
20
= (A
Bk
- A‘ -
+ PC) - CZC* ( 8 )
which can easily be solved by slide rule. With this value of xo, the igniFIQURE6. EFFBCTOF OXYQEN tion temperature is found by substiCONCENTRATION ON REACTIVITY OF -12 +20 MESH FAIRMONT tuting in Equation 7, and tan a = d T / d t is calculated by substituting COKE in 0 = Oxygen = Air 0 = Air oorrected for oxygen conoentration A
IOOO/T-T 900 800
700
800
500
450
IN DEGREES KELVIN 400
350
300
250
200
DEGREES CENTIGRADE
FIGURE 7. REACTIVITY CHARACTERISTICS OF EXPERIMENTAL COKES Size, -40 4-80 mesh; identifying numbers refer t o Table 11. Symbol Atmosphere Heating Rate, O C./Min. 0 Oxygen 6 A Oxygen 14 V Oxygen 25 rn Air 14 25 0 Air
OCTOBER, 1937
INDUSTRIAL AND ENGINEERING CHEMISTRY
about 1 1 0 per cent. The error involved can be eliminated by the use of l / p tan Q instead of p for specifying the quality of coke until the values of c and H a r e known for each fuel.
TABLE I. SAMPLECALCULATION OF IQNITION POINT AND dT/dt" t
I
10
-2
11
-1
12
0
( h = 1 min., k T AT 7.14 0.99 8.13
9.21
13
1
10.37
14
2
12.22
z
47.07
=
1.0s 1.16 1.85
0.50) A2T 0.09
Tf 8.47 9.23
0.08
10.03
0.69
10.81 11.54 50.08
5.08
A Tr
Reactivities of Representative Cokes
0.76 0.80
This investigation showed that the proposed method of testing gave satisfactory reproducibility of the relation between reaction rate and temperature, and the absolute values of reaction rate are of the order of magnitude expected from the results of earlier workers (4). It remained to determine whether the reactivity lines shown in Figure 6, or the corresponding reactivity numbers specified by constants b and E, were unique characteristics of individual cokes. For this purpose fifteen different kinds of coke prepared by various methods a t widely differing coking temperatures and coking rates were tested.
0.78 0.73
3.07
\\\ \
-0.80. F KaU.? 01
02
1123
.
KI
5
the order of magnitude of the reaction rates. Figure 7 shows graphically the results obtained
VOL. 29, NO. 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
1124
TABLE
sample NO.
E121 E81
Ell0 E03 P200 P258 0201 506
J48.49 K1 K2
K3
B1 B2
F
111. REACTIVITY CHARACTERISTTICG OF 15 C O K E S
- r ~ r ~ m e t e r a of Reactivity Lineb. EOUBb‘, EquActivation tian 5 lion 4 enerxy. E
6.41 11.38 9.22 11.00 7.50
7.90 10.90 5.76 7.02 0.24 10.81
11.29 9.01 9.14 12.02
7.48 18.90 13.93 19.41 9.97 10.89 17.80 5.96 10.24 7.07 17.59 18.09 13.44 13.74 20.37
14,980 33.160 25,120 32 800 18$40 20,170 33 070 10:800 14,500 12.700 32 930 36:360 26 220 25:5SO 38,790
havior in use, appear in the upper right-hand portions of Fignres 7 and 8, and their characteristic curves have the smaller slopes. Thus, a t low temperatures the more reactive cokes have higher reaction rates; hut as the temperature is increased. the reaction rates of the less reactive cokes increase m o r e r a p i d l y and may eventually surpass those of the more reactive cokes if the latter do not change their c h a r a c t e r i s t i c s a t h i g h e r temperatures. In conformity with this result, the values of both b and E are, in general, smaller for the more reactive cokes than for the less reactive ones, and cokes rated in order of increasing values of b and E will be approximately in the order of decreasing reactivity. The reactivity of the experimental cokes in the order of decreasing reaction rate at 500” C. is (a) highv a c u u m c o k e , (b) t h e 700°C. cokes from Edenborn and Pocahontas coals, with the cokes prepared a t lower heating rates showing higher reactivity, and (e) the 1000°C.cokes from Edenborn and Orient coak with the cokes prepared a t lower heating rates showing l o w e r reactivity. Of the commercial cokes, that from the top section of the Knowles oven was the most reactive, followed a t a great distance by the blast furnace cokes, the lower sections of the Knowles coke and finally by F a i r m o n t coke, the least r e a c t i v e coke tested. One of the blast f u r n a c e cokes, B,, termed by the operators R
Isnition ~i~~ Rate Point at Tern . Kie dT/& = d&dl 24.5” C./Min. at ,500- C.
379 453 42s 429 38.3 404 487 270 236 300 491 508 479 449 525
14s 100 132 210 200 159 36 490 3300 450 32 20 40 81
11.3
Paaction Rate at 500” C.
0.100 0.068 0.089 0.145 0.138 0.107 0.024 0.3 2.23 0.30 0.021 0.019 0.021 0.055 0.0077
good coke, which permitted operation of a furnace a t a greater rate of production of pig iron than that obtainable with coke Bi under the same conditions, showed about twice the rate of reaction of coke B, over the entire temperature range. These data are presented by way of illustration of the p o s s i b i l i t i e s of the m e t h o d and are not intended as a study of the factors affecting the reactivity of coke. This will be the subject of a special investigation which is now under way.
Literature Cited (1) Burke. S c h u m a n n , and Perry, “Physios of Cod Carbonization,” Am. Gas Assoo., 1930. (2) Ficidner, A. C., and Davis, J. U., U. S. Bur. Mines, Monograph 5, 34 (1934). (3) Jucttner, €3.. Coal Re search Lsb., Contrib. 8 ( 1 9 3 4 ) ; I ~ ~ . ECimna., ~a. 26, 1115 (1934). (4) ICreulen, D. J. W.. Brenn stolf-Chm., 10, 12&31. 148-53 (1929).
(5) Mayem, M.A,, Am. Inst. Mining Met. Engrs., Tech. Pub. 771 (1937); Trana. Am. Soc. Mech. Enurs.. 59, 279-88 (1937). (.6.) Mavera. M.A.. Chem. Reo..
14, 31-62 (1934). (7) Mayers, M.A,, 15th C w . de d i m . ind., Brussels. 1935. 667-76: Carneeie Iast: Tech. .Coal Rsearch Lab., Contrib. 36 (1936).
(8) Terros. E., et al.. Anyaw. Chenc.. 48, 17-21 (1935). (8) Warren, W.B., IND.ENU. CXEM., 27, 72 (1935); unpublished data. R ~ c a i v i n June 3, 1937. Presented before the Division of Gas end PuclChernistry st the 93rd Meeting of the Ameiicnn Chsmicai Society. Chapel Hill, N. C.. .April 12 t o 15, 1937.
