Effect of Ash on Combustion Characteristics of Carbons Y. OSHIMAAND Y. FUKUDA, Imperial University, Tokyo, Japan
I
N THEIR e a r l i e r work ( l ) the authors reported
the influence of the ash naturally occurring in carbonaceous materials on their rea+ tivity to carbon dioxide and t h e i r combustibility in air a t 900" c. It was found that, in the case of charcoal, both reactivity and combustibility were d e c r e a s e d when the ash had been removed by e x t r a c t i n g with hydrofluoric and h y d r o chloric acids, the Same removal of ash from coke adversely affected its r e a c t i v i t y and combustibility. It was c o n c l u d e d that the ash in c h a r c o a l could be regarded as a in the reaction Of with dioxide or air, while the coke
A weighing method of measuring the combustion characteristics qf carbonaceous materials is described, and the results are analyzed graphiCallY. Definitions Of ignition temperature, combusfibility, total rate of combustion, and direct combustion velocily are given. ~hash in charcoalis conducive to pronounced catalytic lowering of the ignition temperature, while the ~ m d ~ ~ d velocity i o n after ignition is almost unaffected by the ash. On the other hand, coke ash retards more or less the ignitibility rather fhan catalyzes it. The cause of the different effects between the coke ash and charcoal ash is discussed. The catalytic behavior qf charcoal ash is observed a!so when certain inorganic and organic salts are added. Moreover, the catalytic effectiveness is shown to depend o n the metallic as well as the negative component of the salt.
ash was catalytically inactive, at least in the gross effect, and was m e r e l y an i m p u r i t y 01 diluent of carbon a t the surface. The cause of these different beOf a s h was s h o w n to be attributable to the outstanding differences in distribution of ash as s h o r n by x-ray radiographic examination. The Present Paper reports res u l t s f r o m a continuation of this study and includes data on the effect of the natural ash in carbonaceous m a t e r i a l s upon their ignitibility and combustion velocity, and, in addition, some new data s h o w i n g t h e effect of added agents on reactivity and combustibility. P a r t of this has already been published in Japan (2).
K
SPRING
BALANCE
bulb, A . The lower end, C, of the spring carries, by means of platinum fiber, an iron core, D,
~(2~cm.o &in do ifa m~ e,t t";e r;i)S,surrounded ~s ~~ & ~ ~ ' ~ ~ ~
2im;f&
~ a t ~ ? u ~ $ ~ , l f ~ ~ ~ T
and 0.5 cm. deep, serves as the sample container. Thermocouple J is inserted just below this container. The s p r i n g - c o r e unit is suspendedina one-piecevertica~g~ass container tube with an air-tight
~ t ~ & f ~~ ~ $' ~ i , ~ g ~$ tion in the tube. To the lower
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F:cooling ki y i ~~~a,Yh",r,"i,"a~,~$~~ jacket, F; around F and ~
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c,O,~o~~p~~'~$~;r~ &
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magnetic forceon the core. The iron core was kept throughout in a fixed position by means of the solenoid t h a t balanced any displacement caused by the change in weight of the sample s u s p e n d e d . The lower end, C, of the spring was observed constantly through a reading microscope, M , and, whenever there was any displacement, the solenoid current was adjusted to restore C to its original position. The record of the solenoid current thus supplied, when converted b means of a current-weight conversion curve, showed the czange in weight of the sample with a sensitivity of 0.1 t o 0.2 mg. The carbon sample was ground to pass a 250-mesh Tyler sieve, and a definite amount was weighed into the sample container and transferred to the reaction tube. A stream of air flowing at a constant rate of 92 cc. per minute was admitted through the tubing, G, into the reaction tube, and the furnace was heated at a constant rate of 6" C. per minute, while the exit gases are led away from L. The weight changes of the sample were determined continuously.
The data thus obtained showed the combustion loss a t successive stages during combustion. The area of the burning surface is practically constant, in spite of its continual downward travel, because the sectional area of the sample container is uniform throughout the entire depth. The experiment was repeated using a fresh portion of the same sample under the same conditions of experiment as above except that a stream of nitrogen was substituted for air; the result showed the loss of volatile matter a t various stages of combustion. The results of these measurements may be plotted as a DETERMINATION OF COMBUSfunction of temperature. A typical case is that shown in TION CHARACTERISTICS Figure 2, where curves a. and an illustrate, respectively, A P P A R A T U SA N D METHOD the combustion loss and loss of volatile matter. It is Clear OF MEASUREMENT. The ap- that the earlier courses of the two curves practically coincide; paratus employed for determin- during that period the evolution of moisture is complete a t ing the combustion characteris- about 150' C., followed by a period with little or no weight tics of carbonaceous m a t e r i a l change, until the point is reached a t which the two curves (Figure 1) was the same as was separate. From this point curve a. is bent abruptly upward to describe a n almost linear course of combustion, used in the previous work (1): while curve CY,, indicates that the evolution of the volatile A fine helical spring of fused matter becomes considerable only a t more elevated temperasilica is suspended from a platinum hook, B, sealed in a glass ture. 212
~
~
~
February, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
Such was generally the case with the majority of carbonaceous materials tested by the authors. Sometimes the combustion curve was concave downward just before the beginning of combustion, suggesting the fixation or the chemosorption of oxygen by carbonaceous material at this heating stage. Gradually this was carried to a maximum, eventually leading up to combustion as in the ordinary case mentioned. The amount of oxygen thus fixed varied from an amount hardly perceptible up to several per cent of the
213
a point, 0, on the temperature axis so that it cuts mm’, an,a=,aa’, and ?VOW’,,respectively, at A , B, C, D, and D’. The amount of the volatile loss, say z, incorporated in the total combustion loss, represented by AC, now can be determined. Since the total carbonaceous content, AD, when it is heated in the atmosphere of nitrogen, is divided up into volatile part A B and fixed part BD at temperature 0, the volatile loss 2: associated with the corresponding unburned residue CD a t this temperature during combustion may be represented approximately by the following formula: AB
x = DC.-C D By measuring off CE equal to z thus found, we obtain a point, E , showing the direct combustion loss; similar operations along the entire burning region of the curve furnish the direct combustion curve sought. Part of it is indicated by EE‘ in Figure 2. Figure 3 illustrates a typical example of the direct combustion curve thus obtained. At first there is a period of slow combustion during which the rate of the oxidation is gradually increased from one that is hardly perceptible to a maximum; this is followed by combustion proceeding a t almost a constant velocity with little or no regard to the temperature 150’C. 0 rise until the greater part of the combustibles is exhausted. 4 Temperature This constancy of the combustion velocity was almost CURVEOF WEIGHTLoss 13s.TEMPERA- perfect throughout the burning region with carbonaceous FIQURE 2. TYPICAL TURE material of high-temperature production. In carbon of lowweight of the carbon according to the variety of carbon; temperature origin, the combustion velocity declined graduit is attributed to the formation of the well-known inter- ally in the later stage of combustion where the evolution of volatile matter becomes considerable. This is attributed, mediate complex, C;,O,. DISCUSSIOX OF COMBUSTION CHARACTERISTICS. Under the on one hand, to the difficulty with which air reaches the conditions of the above experiment where the area of the burning surface because of burning surface, as well as the rate of the air stream, was kept the upward rush of evolved constant, the weight (TV) of the carbon material may be ex- volatile matter and, on the other hand, to the thermal pressed as a function of temperature (0) and time (t) : s h r i n k a g e of t h e w h o l e mass of the carbon sample, = (e, t ) thus decreasing the effecFurther, since the temperature is in itself varying with t h e , tive surface area exposed to the current of air. e = IL(O Extrapolating the linear burning region of the curve From these relations the following differential equation to the temperature axis, we may be obtained (cf. citation 3) : obtain the point, T, locating the ignition temperature. Combustion may be where dW/dt = total rate of combustion with respect to temp. assumed to start substantially from this point; the FIGURE3. TYPICAL DIRECT and time COMBUSTION CURVE DERIVED dW/dt = rate of combustion at constant temp. p r e c e d i n g period of slow FROM FIGURE 2 bW/&3 = temp. coefficient of combustion loss c o m b u s t i o n m a y be redO/dt = rate of temp. rise with time garded as an i n d u c t i o n period during which the oxidation is gradually accelerated Evidently the total rate of combustion, d V / d t , is repre- up to combustion. sented here by the inclination of the burning region of curve The experiments have indicated that the ignition temaain Figure 2, while bW/bt corresponds to the combustibility perature differed greatly according to the variety of carbon, studied in the previous investigation (1). whereas it was practically unaffected by variation in the rate It is possible that the weight loss indicated by curve a. of air flow within wide limits. On the contrary, the direct will include, besides the direct combustion loss a t the surface, combustion velocity was influenced considerably by the air the loss due to the volatile matter which would be evolved factor rather than by variation in carbon. Also, it was inconcurrently from the bulk of the same sample. Although dicated that the lower ignition temperature was always asthe volatile matter may catch fire when it is brought into con- sociated with the shorter period of induction, suggesting the tact with the ambient atmosphere, still it is the volatile loss possible mechanism involved in the initiation of the combusso far as the weight change is concerned. Therefore, in order tion. to rule out this loss from the direct combustion loss, a graphic method was developed on Figure 2. EFFECTOF NATURAL ASH ON COMBUSTION Horizontal lines WOW‘,,mm’, and aa‘ are drawn to locate, respectively, the total amount of the sample taken, the The fundamental idea in attacking the present problem moisture, and the ash contents. A vertical line is drawn from was the same as in the previous study of reactivity and com-
w
214'
INDUSTRIAL AND ENGINEERING CHEMISTRY
bustibility (I)-namely, the comparison of data obtained with de-ashed carbon to those of the original material. The materials used were the same as those previously employedmetallurgical coke 11, special coke I1 (a low-temperature product), high-temperature charcoal I, and low-temperature charcoal 11, together with the same samples de-ashed by the combined treatment of the carbon with hydrofluoric and hydrochloric acids. The ash content of the carbon before and after treatment is as follows: SAMPLE Metallurgical coke I1 S ecial coke I1 d g h - t e m p . charcoal I Low-temp. chsrooal 11
AMOUNTO F ASH (DRYBABIE) Before extn. After extn.
%
%
14.15 15.05 1.61 4.33
3.91 4.88 0.32
3.32
EXPERIMENTAL RESULTS. The combustion experiments and the volatile matter tests already described were conducted with both the de-ashed and original material using 0.3000 gram of sample. The results for the four varieties of carbonaceous materials are shown in Figure 4. It is evident that ash removal causes little or no variation in the volatile curve of the metallurgical coke or of the two charcoals. The case is somewhat different with the special coke, where both moisture and volatile matter are more or less increased after the treatment, although the original shape of the curve was almost unaffected. These variations may be ascribed to the greater percentage content of carbon, moisture, and volatile matter in the deashed sample than in the original coke; the extent of the moisture and volatile evolution is dependent upon the ad-
Vol. 27, No. 2
sorption characteristics and the volatile content of the sample, per se, as well as the amount and nature of the ash removed. Thus, in the case of the special coke with abundant moisture and volatile matter, a correspondingly large amount of these materials will be evolved with the increment of the carbon content in the de-ashed sample, while the circumstances are just the reverse for the metallurgical coke. Since the volatile characteristics are the indication of the thermal decomposition characteristics which, in turn, are closely associated with the chemical nature of the carbonaceous materials, the foregoing will suggest a t once that there can be no great essential variation in the carbon itself due to the treatment of ash. Accordingly, any variation in the combustion characteristics occurring after the ash treatment may safely be ascribed merely t o the decrease in ash. Returning now to the combustion curves of Figure 4, it is indicated that, after the removal of ash, the curve is always shifted t o the right or the left of the original curve, while its essential form is almost unaffected in each case. This may be considered more in detail by means of the direct combustion curves shown in Figure 5 which were obtained from Figure 4 according to the graphic method developed in Figure 2. The first noticeable point is that for both cokes, the direct combustion curves are shifted more or less to the lower temperature side after the removal of ash, while in the charcoals they are shifted far to the higher temperature side, accompanied on all occasions by a corresponding lowering or elevation in the ignition temperature :
February, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY r
SAMPLE
-
I
Original, To O
Metallurgical coke I1
c.
554 340 410 332
S ecial coke I1
&ph-temp. charcoal I Low-ternp. charcoal I1
~
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~ TEMPERATURE ~ ~ ~ ~
De-ashed, T
* c.
549 321 499 418
Difierence, To
c.
-T
+-89+519 -86
It will be inferred that the presence of ash in charcoal is conducive, in silu, t o the catalytic lowering of the ignition temperature, while the coke ash is not. These two different effects of ash on the parent carbon are parallel to their effect on the reactivity and combustibility ( 1 ) and can be similarly explained. Thus the catalytic nature of the ash in charcoal may be due to its extremely minute state of dispersion which is one of the most important factors in a heterogeneous catalyst because of the great development of active centers and of the interface between catalyst and reactants. On the other hand, the coke ash, owing to its macroscopic granular state, will absorb some part of the heat to be developed during the induction period which might otherwise go to raise the surface temperature. I n other words, the coke ash may be regarded as a cooling agent rather than a catalyst, retarding the ignitibility more or less by its presence. The extent of this may be the combined effect of the thermal conductivity and thermal capacity of both ash and carbon along with many other thermochemical factors. No available data on the chemical differences in ash between coke and charcoal could be found to give any reason for the two different behaviors of the ash towards the ignition temperature (cf. citation 1 ) . S p a r t from the variation in the ignition temperature thus far considered, some slight variations are also manifested in the direct combuption curve after the ash treatment.
First, in all the de-ashed samples except that of the metallurgical coke, the slight fixation of oxygen is always indicated during the induction period according to the manner already described. This may be explained by assuming that, in the absence of ash in these carbons, the fixed oxygen is accumulated in the form of C,O, a t this period, whereas in the presence of ash, the C,Ou formed is decomposed by the possible catalysis of ash. This explanation may appear somewhat contradictory in the case of the special coke, where the ash was not entitled to be a catalyst. However, unpublished data of the authors using the selectively de-ashed carbon have shown that, even in the case of coke, some slight catalysis was attributable t o the hydrochloric-acid-soluble component of ash which was not manifested when the total ash had been removed, as in the present case, because of the opposite effect of the hydrofluoric-acid-soluble part of the ash. The only exceptional case of the metallurgical coke with no h a t i o n of oxygen will show that here the carbon in itself has no tendency to absorb oxygen, perhaps owing to its far-reaching graphitic nature. Other anomalies associated with the ash treatment are that the rate of direct combustion a t the linear burning region is slightly increased in all de-ashed cases, whereby the clear distinctions between coke and charcoal, such as is found in the ignitibility studies, do not exist. Undoubtedly this vi11 point t o the fact that the cinder zone of ash formed on the burning surface with the progress of combustion would be much less in the de-ashed samples than in the original, and accordingly the accessibility of oxygen to the burning surface will be comparatively greater in the de-ashed material, resulting in the correspondingly greater rate of direct combustion. Here the prominent catalysis by the ash in charcoal as in the ignitibility experiments could not be expected because the interaction of carbon with oxygen must in itself be almost instantaneous under the drastic temperature conditions existing there.
EFFECTOF
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.
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FIGURE5. DIRECTCOMBUSTION CIJRVES
215
CATALYSTS ON COhlBUSTION
With a view to obtaining additional evidence on the catalytic lowering of ignition temperature as found above, the following experiment was undertaken in order to show the effect of a number of added catalysts on the combustion characteristics of carbon. ~ I A T E R I A L s . A carbon black produced by the channel process in Formosa was used because it is nearly free from ash (0.12 per cent on the dry basis). The catalysts most extensively employed were the salts of alkali metals which were all commercially pure products and not specially purified before the experiment, since the carbon black was itself by no means free of impurities, The entire list of the catalysts used is as follows: sodium carbonate (anhydrous), sodium hydroxide, sodium acetate, sodium tartrate, sodium arsenate (crystalline), potassium carbonate, potassium hydroxide, potassium chloride, potassium borate, tripotassium phosphate, potassium acetate, potassium tartrate (neutral), lithium carbonate, lithium nitrate, lithium sulfate, lithium chloride, lithium acetate, zinc chloride, manganese sulfate, and lead acetate. The carbon black (50 grams) was first washed with 1.5 liters of distilled water. The mixture was shaken for 8 hours at room temperature. After filtering off the liquid, the carbon was dried at 105' to 110" C. Catalysts were then incorporated into the sample according to the following procedure: Two grams of the
216
Vol. 27, No. 2
INDUSTRIAL AND ENGINEERING CHEhlISTRY
$00
500
400
300
600
700
Tempernlure &)
FIGURE7 . EFFECTOF CATALYSTS ON DIRECT COMBUSTION CURVES
s u g g e s t i n g the decomposition of the added salts or their possible interaction with carbon. Furthermore, the direct combustion curves in Figure 7 show that the catalysts cause cons i d e r a b l e contraction of the induction period with corresponding lowering in the ignition temperature, while the rate of direct combustion is almost parallel to the original curve in each case. I n s h o r t , the behavior of the added catalyst towards the combustion characteristics of carbon is almost the same as that of the natural ash in charcoal, as described in the foregoing. This was also true with the remaining catalysts employed; therefore only the numerical data on t h e i g n i t i o n temperature, along with those of moisture, volatile matter, and ash content, are shown in Table I. In Table I the moisture and volatile matter are taken to be the weight loss up to 150" and to 150-900" C., respectively, in the volatile curve, while the ash content is the incombustible residue in the total combustion curve (all on the dry basis). From Table I the effectiveness of the added catalyst may be estimated roughly. Potassium Temperature ( o c . ) salts seem to be the best and those of sodium second best. At the same time, their efficiency FIGURE6. EFFECTOF CATALYSTS ON COMBUSTION AND VOLATILE MATTEREXPERINENTS is governed by the negative constituent of the salt: these range from the most effective hydroxwashed carbon black were weighed into a cylindrical weighing ide, through carbonate, acetate, and tartrate, down to the bottle and then moistened with 5 cc. of a 2 per cent solution of comparatively ineffective chloride and sulfate. catalyst (water 100 parts, catalyst 2 parts). This mixture was carefully dried at 100" C., and the charge, consisting of carbon OF ADDEDM A T E R I 4 L S ON COMBUSTION T.4BLE I. EFFECT with about 5 per cent of its weight of catalyst, resulted. With VOLATILE SUBSTANCE ADDED TO IQNXTION lithium carbonate, owing to its slight solubility, fourfold imCARBON BLACK TEMP. MOIBTURE MATTER A m pregnation with 5 cc. of a 0.5 per cent solution was required. % %
c.
EXPERIMENTAL RESULTS. The same combustion experiments together with the volatile tests were conducted as described in the foregoing, and the results analyzed by the same graphic method. The amount of sample was 0.1000 gram in each case. Figure 6 shows the results obtained with the carbon without catalysts and with potassium chloride, carbonate, and hydroxide added. Comparison of these graphs shows that, after the addition of catalysts, combustion curve a0 is markedly displaced in each case to lower temperatures. There are also some essential variations in the volatile matter curve, especially in the higher temperature region,
None (original carbon black) Sodium carbonate Sodium hydroxide Sodium acetate Sodium tartrate Sodium arsenate Potassium carbonate Potassium hydroxide Potassium chloride Potassium borate Potassium phosphate Potassium acetate Potassium tartrate Lithium carbonate Lithium nitrate Lithium sulfate Lithium chloride Lithium acetate Zinc chloride Manganese sulfate Lead acetate
507 305 299 303 303 358 286 278 403 439 354 295 285 373 318 427 366 288 416 418 282
%
2.7 5.8 8.7 6.2 5.8 4.3 7.3 4.5 3.7 4.1 5.4 7.7 4.4 4.7 11.7 4.3 20.2 9.6 7.7 10.2 1.5
5.5 10.8 14.5 11.9 10.4 6.7 11.6 11.5 2 180 . 3
0.12
6.0 8,6 9.8 12.4 12.2 13.1 8.3 10.4 9.2 10.7 7.3
3.8 4.2 2.4 6.2 2.8 5.4 5.2 2.2 1.6 2.8 2.4
4.8 6.6 5.2 4.8 2.0 5.2 4.8 6 . 04 3
February, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
Moreover, since the lower ignition temperature is always associated with the more contracted period of induction, as is noticeable from Figure 7 , it is clear that the catalyst is promoting the slow oxidation preceding the combustion. Also the fact that added catalyst has practically nothing to do with the combustion velocity can be attributed to the same cause as was given for the similar phenomenon in the case of charcoal. In addition, the data on the ash content in Table I show that the added catalysts are retained in the carbon almost in their original form up to the final stage of combustion.
217
This perhaps renders untenable the idea attributing the catalysis involved to the metallic vapor from the salt added.
LITERATURE CITED (1) Oshima, Y . , and Fukuda, I-., J . SOC.Chem. Ind. Japan, 34, 238-40B (1931); Fuel, 11, 135 (1932). (2) Oshima, Y., and Fukuda, Y . . 3. SOC.Chem. Ind. Japan, 35, 199B (1932) ; 37,184B (1934). (3) Ibid., 36, 246B (1933). RSCEIVXDSeptember 29, 1534. Presented before the Division of Gas and Fuel Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934.
Removal of Copper Ions from Water by Sodium Aluminate C. J. BROCKMAN, University of Georgia, Athens, Ga.
P
REVIOUS papers were concerned with the removal of copper ions from water by means of the alum floc ( 2 ) and by the ferric floc (1) under circumstances of p H control such as are met in municipal water treatment p l a n k Copper sulfate is added in many instances t o control the growth of algae which impart unpleasant tastri and odora to the water. In the case of the alum floc it is possible to remove considerable quantities of copper as the sulfate in the treatment process, provided the filtered water possesses a pH of 6.3 or above. The pH range is somewhat larger in the case of the ferric floc, the lower value being 3.8 for 100 per cent removal. In each case the copper salt is removed, posaibly as a n insoluble basic sulfate, a t a pH of 7.0 and above without the addition of any coagulating agent. I n continuing this work it was of interest to study the effect which the use of the
