wide variety of microstructures. Low wettability indicates that a poor bond may be expected between baked binder and coke, and this will cause high electrical resistivity. On the other extreme, high wettability may indicate an extremely porous coke whose interior pores unfilled with binder can cause high electrical resistivity. literature Cited
Abramski, C., Mackowsky, M. T., in “Handbuch der Mikroskopie in der Technik,” Hugo Freund, Ed., Vol. 2, Part 1, p. 311, Umschah Verlag, Frankfort am Main, 1959. Barrillon, E., Chim. et Ind. 98 (9), 1434 (1967). Blayden, H. E., Gibson, J., Riley, H. L., Inst. Fuel, Wartime Bull. 1945, 117-29. Bowitz, D., Eftestol, T., Selvik, R. A., “Extractive Metallurgy of Aluminum,” Gary Gerard, Ed., Vol. 2, p. 331, Interscience, New York, 1963.
Hutcheson, J. M., Jenkins, M. L., “Second Conference on Industrial Carbon and Graphite,” p. 433, Society of Chemical Industry, London, 1966. Kusakin, N. D., Vyatkin, S. E., Averina, M. V., Tsvetnye Metally 38 (lo), 65 (1965). Martin, S. W., Shea, F. L., Jr., Ind. Eng. Chem. 50, 41 (1958). Rhedey, P., Trans A I M E 239, 1084 (1967). Shea, F. L., U. S. Patent 2,775,549 (Dec. 25, 1956). Weiler, J. F., “Chemistry of Coal Utilization,” H. H. Lowry, Ed., Suppl. Vol., p. 627, Wiley, New York, 1963.
RECEIVED for review August 6, 1969 ACCEPTED December 12, 1969 Symposium on Coke and Coking Processes, Division of Fuel Chemistry, 158th Meeting, ACS, New York, N.Y., September 1969.
Thermal Cracking of Tars and Volatile Matter from Coal Carbonization W. R. Hesp and P. 1. Waters Division of Mineral Chemistry, CSIRO, Sydney, Australia Two tars (from vertical-retort and fluidized-bed carbonization) as well as volatile matter from 500” C. static-bed carbonization of coal were cracked in static-bed reactors between
500” and 1000°C. and at residence times of 1 to 440 seconds. The effects of process variables-temperature,
residence time, and composition of raw tar-on
the yields and
properties of products were investigated. Conclusions were drawn regarding the nature of chemical reactions involved and the mechanism of tar cracking. The main products of cracking were gas and carbon. The gas could be useful industrially as fuel gas or as a source of olefins for chemical syntheses; the carbon could be used as raw material for electrodes for the metallurgical industries.
COAL
TAR, the traditional source of aromatic chemicals, surface coatings, and certain liquid fuels, has been largely displaced by petroleum. Despite this, coal and coal tar are still potential sources of a wide range of chemicals on both a short- and a long-term basis. Tar is being produced in increasing amounts in Australia because of expansion of coke-oven capacity a t steelworks. There is also a continuing demand for aromatic and unsaturated aliphatic hydrocarbons by the chemical industry, for low-ash carbon by the metallurgical industry, and for high-B.t.u. gas by industry in general. From the long-term view, coal reserves are such that coal is likely to outlast petroleum as a source of hydrocarbon fuel and chemicals. This challenges researchers to find new, more economic routes of converting coal and coal tar into these commercially useful products. Several recent articles (Berber et al., 1967; Betts and Waddington, 1964; Freeman, 1965a,b; Freeman and Betts, 1964; Griffiths and Mainhood, 1967; Janardanarao et al., 1968a,b; Naugle et al., 1965; Walters and Ortuglio, 1966) as well as developments in the application of the delayed coking technique to tar-pitch processing (Review of Coal
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Tar Technology, 1968) and attempts to make electrode carbon from tar (Verkaufsverein fur Teer Erzeugnisse, 1968) indicate a current widespread interest in tars as raw materials in chemical processing. I n the current investigations, thermal cracking was regarded as supplementary to coal carbonization, since it offered a means of upgrading the commercial value of chemically complex tars by converting them into relatively simple products, such as methane- and olefin-rich gases, light aromatics, and lowash carbon, each of which has potential industrial use. The work involved the use of fluidized- and staticbed techniques. The former offered the advantage of continuous operation, but because of the temperature limit of the available fluidized-bed reactor (maximum was 700°C.) and the short vapor residence times (a matter of seconds) necessitated by high fluidizing gas flow rates, only moderate conversions were achieved (Hesp et al., 1969a). The bulk of the work was therefore carried out using static-bed reactors. Two tars were cracked, as well as primary volatile matter from coal carbonization, a t temperatures between 500” and 1000”C. and residence times between 1 and 440 seconds.
Experimental Details
Materials. The two tars used were a dry, vertical-retort, gas-works tar from the high-volatile Greta coal of New South Wales, and a low-temperature tar produced from the noncaking, high-volatile Wallarah coal, New South Wales, in a technical-scale fluidized-bed carbonizer a t 500°C. (Bowling et al., 1961). The latter coal was also used to generate volatiles in the combined carbonizationcracking tests. The reaction zone in the cracking reactor was filled with contact material consisting of -5O-mm., + 25-mm. lumps of high-temperature (900" C.) coke made from lowash coal (Stockton No. 2, New Zealand), to provide a surface for the pyrolysis reactions and for the deposition of the solid carbonaceous product of cracking. I n the combined carbonization-cracking tests, smaller lumps (-6 mm. +3 mm.) of the same coke were used as contact material. Method. The flow diagram of the static-bed apparatus used in most of the cracking tests is shown in Figure 1. The reactor was designed to suit the existing gasfired muffle furnace and recovery train of a modified B.M.A.G.A. coking unit (Reynolds and Holmes, 1946). The raw tar was preheated to about 150°C. in a graduated cylindrical glass container and fed a t the required rate (0.5 to 2.5 pounds per hour) to a mild steel evaporator (diameter 20 cm., length 20 cm.) which was heated to 500°C. by a circular gas burner. About 90% of the tar was thus flash-evaporated, a small amount of gas being formed. The tar vapors and gas entered the reactor (diameter 15 cm., length 90 cm.) and passed downward through a 60-cm. layer of contact material and then upward through a central pipe 5 cm. in diameter perforated at the lower end. The volume of the annular reaction zone was 9.6 liters. Temperatures measured a t the bottom, middle, and top of the reactor were within &lO°C. of the nominal cracking temperature.
nL
The solid carbonaceous product was deposited on the contact material and the reactor walls. The volatile products of cracking passed through a recovery train. Most of the liquid product was collected from the air- and water-cooled condensers with a small amount from the electrostatic precipitator, and the light ends were recovered from the wash oil of an oil scrubber by steam distillation. Samples were taken from the product gas a t regular intervals and after the volume had been measured with a wet-gas meter the excess gas was flared. I n the combined carbonization-cracking tests, about 100 pounds of finely ground coal (88% -3 mm.) was carbonized in a cylindrical (diameter 32.5 mm., length 65 cm.) mild steel retort heated in the gas-fired muffle furnace. The volatile matter released from the coal entered the cracking reactor, which was mounted vertically on top of the carbonizing retort and consisted of an electrically heated stainless steel tube (diameter 6 cm., length 90 cm.), the middle portion of which (length 30 cm., volume 850 ml.) was filled with -6 mm. +3-mm. particles of hightemperature coke as contact material. The products were recovered as described above. Results of Tar Cracking
Cracking experiments with the two tars were carried out in the temperature range 500" to 1000°C. a t feed flow rates of 0.5, 1.0, 1.5, and 2.0 pounds per hour. The figures give results for a tar feed rate of 1.5 pounds per hour (for full details see Hesp et al., 1 9 6 9 ~ ) . Effect of Temperature on Product Yield. The yields of main products (carbon, gas, water, and residual tar) are plotted in Figure 2. The general effect of increasing the cracking temperature was to increase the yield of gas and carbon and decrease the amount of residual tar. At low temperatures of cracking (500" to 7OO0C.), gas other than residual tar was the main product (on a weight basis); at high temperatures (800" to 1000°C.), it was carbon. Pyrolytic water originating from tar was observed 20,000 V O L T S
F E E D TANK WATER-COOLED POKERS
CONDENSER
'OIL
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GAS S A M P L E
-
I
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CONTACT
MATERIAL
( COKE)
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~
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Figure 1. Flowsheet of apparatus for tar cracking
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 2, 1970
195
a w a
500
600
700
eo o
900
w a
1000
T E M P E R A T U R E
OF
500
600
800
700
C R A C K I N G
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Figure 2. Yields of products from tar cracking at feed flow rate of 1.5 pounds per h o u r a. Vertical-retort tar
b. taw-temperature fluidized-bed tar la. Residue ( >275" C.) in raw tar
in the low-temperature, but not in the high-temperature experiments. The higher absolute amounts of light oil and liquid aromatic compounds in the low-temperature (500" t o 600" C.) products suggested that in this temperature range the long side chains of relatively large aromatic or alicyclic molecules were removed to form mainly light liquids and gaseous hydrocarbons. These reactions were presumably accompanied by cracking, dehydrogenation, and aromatization of the low-molecular weight paraffinic and naphthenic compounds. At and above 600" C., mainly aromatic compounds were present in the neutral oil of the distillable fraction. Above 600" C., cracking of tar acids became appreciable and tar bases also started to decompose, but at a slower rate than the acids. NMR spectrometry of the raw tar and the liquid products provided further evidence of changes in constitution with decomposition. The relative distribution of aromatic, benzylic, and paraffinic-naphthenic hydrogen (Figure 3) as well as their absolute amounts (calculated from their relative content and the yield of liquid product) shows that the marked increase in the relative content of aromatic hydrogen in the products with increase in temperature was due partly to the disappearance of benzylic and paraffinic-naphthenic hydrogen through cracking reactions, and partly to the formation of aromatic compounds through cyclization reactions which occurred between 700" and 800" C. Between 500" and 600" C. some of the aromatic compounds also decomposed, but not to the same extent as the paraffins and naphthenes. As indicated in Figure 3, the vertical-retort tar and cracked products are more aromatic in character than the fluidized-bed tar and its products, presumably because of the higher temperature history of the former. On a weight basis, methane was the main gaseous constituent. Its yield increased with the temperature of cracking up to 900°C. (at which point it was about one fifth of the raw tar), then decreased, indicating that it was decomposing more rapidly than it was being formed. Decomposition is due to cracking, and, to a lesser extent, 196
Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
/la. distillate (
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Figure 1 1 . Composition and properties of gas products o. From carbonization of Wollaroh coal b. From 500" C. carbonization followed by cracking of volatile matter
The variations in the volumetric and thermal yields of gaseous products obtained by the two techniques are shown in Figure 12. At the same temperature, direct carbonization of the coal yielded a much greater volume of gas than cracking of the volatile matter originating from 500" C. carbonization. At the higher temperatures, volumetric yields increased more markedly than corresponding weight yields because of the greater formatioa of the low-molecular-weight gases, HZand CO, originating from cracking and the water-gas reactions. The thermal yield of gas in the combined carbonization-cracking tests increased with cracking temperature to a maximum of 1.35 therms per 100 pounds of coal a t 8OO0C.,then declined because of a marked decrease in the calorific value of gas with further increase in the temperature. I n the purely carbonization tests, the thermal yield continuously increased with temperature up to 2.92 therms per 100 pounds of coal a t 1000"C. because over the high-temperature range the decrease in calorific value was compensated for by increased yields of gas. Discussion and Implications
Increasing the temperature of cracking increased the extent of tar conversion and the yields of gas and carbon, the main products of cracking. The residence time of the tar vapors in the reactor also proved an important process variable and affected the yields of products in a complex manner. The highest gas yields (>60% by weight on tar) were obtained a t 900" to 1000°C. a t short ( < 20 seconds) residence times. The main gaseous constituent obtained from the cracking of tars was methane, the yield of which reached a maximum around 850" to 900°C. (22.9 and 27.9% by weight, respectively, from the two tars). At higher temperatures methane and other hydrocarbon constituents decomposed or reacted, partly or entirely, with the pyrolytic water originating from the tar. Because the calorific value of the gas products gradually decreased as the temperature of cracking increased, and the thermal efficiency
-
600
J
4
I
0
: 0
30
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z
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2
m J
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Figure 12. Volumetric and thermal yields of gas from carbonization of Wallarah coal and from 500"C. carbonization followed by cracking of volatile matter ---
Carbonization Carbonization a t 500" C . plus cracking
of cracking increased up to 900'C. and then declined, tars for the production of fuel gas should not be cracked a t temperatures above 900°C. At this temperature the gas product has a calorific value of about 550 B.t.u. per cu. foot. Olefin hydrocarbons, ethylene and propylene, were also present, but their yield was generally lower ( < l o % by weight on tar) and decreased as the temperature of cracking was raised. However, in tests where residence times were very short (about 1 second), yields of ethylene were higher ( > 20% by weight on tar a t 800" C.). For carbon formation, residence times of about 2 minutes were needed in the 900" to 1000°C. temperature range, where the yield of carbon was over 60% on tar. The ash content of carbon products was higher than the Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2, 1970
201
0.5% permitted in raw materials for the manufacture of electrodes, but since the ash originated, in part a t least, from reactions between tar acids and the mild steel apparatus, it should be possible to reduce it by using stainless steel. The chemical changes occurring at the lower temperatures of cracking (500" to 700°C.) were mainly the mild cracking of high-molecular-weight hydrocarbons, the removal of side chains from alkyl-aromatic compounds and, to a lesser extent, aromatization of paraffinic and naphthenic compounds. I n the 600" to 700" C. temperature range the decomposition of tar acids started and the splitting of the hydroxyl groups from these compounds was responsible for the pyrolytic water in the products. The decomposition of aromatic rings started between 600" and 800°C. depending on the residence time of tar vapors in the reactor. Above 900°C. the cracking of lowmolecular-weight paraffin hydrocarbons and residual aromatic compounds into carbon and hydrogen, and the reactions of hydrocarbons and primary carbon with the pyrolytic water predominated. Evidence regarding the mechanism of tar cracking was obtained from tests with vertical-retort tar a t varying residence times. Two main temperature regions were discernible. Between 500" and 6OO0C., a continuous slow decomposition of the tar took place as the residence time of tar vapors in the reactor was increased, gas being formed more rapidly than carbon. Between 700" and 1000°C. the rate of tar decomposition was high and three consecutive phases became apparent: rapid decomposition of tar into gas and carbon with the former as the main product, decomposition of gas and tar to form carbon, and slow evolution of gas from the residual tar without appreciable increase in the yield of carbon. Comparison of results obtained from the two types of tar showed the effect of tar composition. The lowtemperature fluidized-bed tar, which had a larger average molecular size and a more aliphatic chemical constitution than the vertical-retort tar, gave rise to more gas and less carbon under comparable conditions. More pyrolytic water was formed from the low-temperature tar, and it participated more readily in water-gas reactions. The cracking of primary volatile matter (tar plus gas plus water) from 500°C. carbonization of coal showed trends similar to those obtained for the tars, but differences in yields were observed owing to the short residence times and the participation of water and carbonization gas in the reactions. By carbonizing the coal a t a lower temperature (500°C.) and cracking the total volatile matter (tar plus water plus gas) at a higher temperature (900" C.), the yield of gas can be increased by 50 to 80%. However, at 900" C. the yield of gas from carbonization of the coal alone was 200% higher than a t 5OO0C., because of the increased decomposition of semicoke. The material balance of the three modes of carbonization using the same coal is shown in Table I. Fluidized-bed carbonization of the coal resulted in the formation of appreciably more tar (and total volatile matter) than static-bed carbonization, and in the cracking step which followed more carbon was obtained from the fluidizedbed tar, for which the longer residence time of tar vapors in the cracking reactor rather than the technique of carbonization appeared to be responsible. The sum of the yields of final solid and volatile products was about the same in the two processes. 202
Ind. Eng. Chern. Prod. Res. Develop., Vol. 9, No. 2, 1970
Table 1. Material Balance of Carbonization and Cracking Processes Using Wallarah Coal FINAL PRODUCTS
I21
Acknowledgment
The authors thank K. McG. Bowling for helpful discussions, and D. H . Philipp, R. J. Neronowicz, R . Hamor, and other members of the staff for assistance in the experimental work. Literature Cited
Berber, J. S., Rice, R. L. Fortney, D. R., IND. ENG. CHEM.PROD. RES. DEVELOP. 6, 197 (1967). Betts, W. D., Waddington, W., Coal Tar Research Association, Rept. B/2/112/6c (1964). Bowling, K. McG., Brown, H. R., Waters, P. L., J . Inst, Fuel 36, 99 (1961). Brown, H. R., Hesp, W. R., Taylor, G. H., Carbon 4, 193 (1966). Freeman, J. W., Coal T a r Research Association Rept. 0354(B) (1965a). Freeman, J. W., C o d Tar Research Association Rept. 0356(B) (196513). Freeman, J., Betts, W. D., Coal Tar Research Association, Rept. 0323(B) (1964). Griffiths, D. M. L., Mainhood, J. S. R., Fuel 46 (3), 167 (1967). Hesp, W. R., Hamor, R., Waters, P. L., CSIRO, Division of Mineral Chemistry, Investigation Rept. 77 (1969b). Hesp, W. R., Neronowicz, R . J., Waters, P. L., CSIRO, Division of Mineral Chemistry, Investigation Rept. 76 (1969a). Hesp, W. R., Philipp, D. H., Waters, P. L., CSIRO, Division of Mineral Chemistry, Investigation Rept. 78 ( 1 9 6 9 ~ ) . Janardanarao, M., Ramacharyulu, M., Krishna, M. G., Brennstof Chern. 49 (l),10 (1968a). Janardanarao, M., Ramacharyulu, M., Krishna, M. G., Brennstof Chern. 49 (3), 65 (1968b). Naugle, B. W., Ortuglio, C., Mafrica, L., Wolfson, D. E., Bur. Mines Rept. Invest. 6625 (1965). Rev. Coal T a r Techn. 20, Part 1, 62, Ref. 116 (1968). Reynolds, D. A., Holmes, C. R., Bur. Mines Tech. Paper 685 (1946). Verkaufsverein fiu Teer Erzeugnisse, West German Patent 1,257,738 (1968). Walters, J. G., Ortuglio, C., Bur. Mines Rept. Invest. 6709 (1966). RECEIVED for review July 28, 1969 ACCEPTED November 10, 1969
